==== Front Int J Mol Sci Int J Mol Sci ijms International Journal of Molecular Sciences 1422-0067 MDPI 33271784 10.3390/ijms21239153 ijms-21-09153 Review The Role of Mitochondrial Calcium Homeostasis in Alzheimer’s and Related Diseases Ryan Kerry C. Ashkavand Zahra Norman Kenneth R. * Department of Regenerative and Cancer Cell Biology, Albany Medical College, Albany, NY 12208, USA; ryank8@amc.edu (K.C.R.); ashkavz@amc.edu (Z.A.) * Correspondence: normank@amc.edu; Tel.: +1-518-262-2529 01 12 2020 12 2020 21 23 915329 10 2020 26 11 2020 © 2020 by the authors.2020Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Calcium signaling is essential for neuronal function, and its dysregulation has been implicated across neurodegenerative diseases, including Alzheimer’s disease (AD). A close reciprocal relationship exists between calcium signaling and mitochondrial function. Growing evidence in a variety of AD models indicates that calcium dyshomeostasis drastically alters mitochondrial activity which, in turn, drives neurodegeneration. This review discusses the potential pathogenic mechanisms by which calcium impairs mitochondrial function in AD, focusing on the impact of calcium in endoplasmic reticulum (ER)–mitochondrial communication, mitochondrial transport, oxidative stress, and protein homeostasis. This review also summarizes recent data that highlight the need for exploring the mechanisms underlying calcium-mediated mitochondrial dysfunction while suggesting potential targets for modulating mitochondrial calcium levels to treat neurodegenerative diseases such as AD. mitochondriacalciumneurodegenerationAlzheimer’s diseaseMCUROSpresenilin ==== Body 1. Introduction With a rapidly aging population, it is expected that neurodegenerative disorders will become an increasingly pressing health issue, intensifying the urgency for effective treatment. Most neurological disorders are chronic and incurable, and their debilitating effects can continue for decades. There are approximately 50 million people living worldwide with dementia and this number is expected to reach 82 million in 2030 and 152 million by 2050 (WHO.int) [1]. Alzheimer’s disease (AD), the most common neurodegenerative disease [1], is the sixth leading cause of death [2]. In the United States alone, approximately 5 million people currently suffer from the disease, and these numbers are projected to triple by the year 2050 [2]. AD is characterized by gradual cognitive decline and memory loss, synaptic dysfunction, and neuronal death. Despite decades of research, there is no effective therapy for AD and the cause of AD remains unclear, especially due to its complex etiology. The primary histopathological hallmarks of AD are neurofibrillary tangles composed of the microtubule-associated protein tau and extracellular deposition of senile plaques composed of amyloid-beta (Abeta) peptides. Most AD research has focused on investigating the amyloid hypothesis, which proposes that aberrant Abeta production is the core cause of AD, and that all other dysfunctions observed in AD, including hyperphosphorylated tau tangles, are a consequence of Abeta neurotoxicity. Abeta plaques on their own have been shown to interfere with synaptic function and cause extensive neuronal death, especially within the cortical and hippocampal regions of the brain that are responsible for learning and memory [3]. In support of Abeta being the causative agent of AD, genetic mutations in genes encoding presenilin 1 (PSEN1), presenilin 2 (PSEN2), and the amyloid precursor protein (APP), which lead to nearly all early-onset familial AD (FAD) cases, are all involved in Abeta processing. Abeta peptides of varying lengths are generated from APP by the consecutive cleavage by beta-secretase and gamma-secretase. Mutations in the genes encoding PSEN1 and PSEN2, which underlie roughly 70% of all familial AD cases [4], form the catalytic component of the gamma-secretase complex. Increased production of the more toxic, aggregation-prone Abeta42 species relative to Abeta40, which results in the generation of extracellular amyloid fibrils and plaques, is thought to be the primary cause of AD [3]. Therapeutic strategies for treating AD have focused on reducing the burden of Abeta plaques by either preventing Abeta production or by promoting its clearance. However, gamma-secretase inhibitors have been a failure clinically, and Abeta plaque load is not strongly correlated with dementia onset or severity [5], which has prompted reconsideration of the amyloid hypothesis [6,7,8,9]. There is also considerable evidence that Abeta oligomers, as opposed to the more highly aggregated Abeta fibrils, are the primary cytotoxic species and pathological agent of AD. This soluble form has been shown to damage synapses absent Abeta plaque formation [10,11]. Additionally, oligomeric Abeta isolated from AD patients was sufficient to cause synaptic dysfunction and memory loss in mice [12]. It remains unclear whether selectively targeting Abeta oligomers will show more success in clinical trials. Indeed, the use of immunotherapies targeting Abeta, some of which have been shown to bind oligomeric Abeta with high affinity, have no effect on disease progression [8,13]. Other studies have shown that overexpression of Abeta peptides in mice, while simulating the Abeta pathology observed in AD, does not result in similar synaptic loss and memory impairment [14]. Additionally, neurodegeneration induced by presenilin mutations can occur in the absence of Abeta production [15,16]. Mutations in other components of the gamma-secretase complex (APH-1, PEN-2, and nicastrin) have not been implicated in FAD, implying APP and presenilin mutations do not cause FAD merely by altering Abeta production [17]. Furthermore, many other symptoms (e.g., increased inflammation, altered calcium signaling, mitochondrial dysfunction, oxidative damage) appear to arise independently of any Abeta involvement as they often precede Abeta plaque formation, and are also more strongly correlated with cognitive decline [18,19]. Collectively, this suggests that Abeta production is not necessarily the proximate cause of AD and that additional pathological factors must be explored to develop viable therapies. 2. Calcium Dysregulation in AD The connection between calcium and AD was observed several decades ago [20], but recent data have increased support for this hypothesis, and strongly implicate a role for calcium in AD. The calcium hypothesis of AD states that disruptions to neuronal calcium signaling underlie not only amyloid plaque deposition but a series of molecular changes within the neuron that cause neuronal dysfunction [21]. During neurotransmission, a rise in intracellular calcium following membrane depolarization transmits the signal to synapses. Calcium signaling in neurons is, therefore, crucial for neurotransmission and for maintaining synaptic plasticity and generating long-term potentiation (LTP), which forms the basis of learning and memory through the progressive strengthening of synapses [22,23]. Calcium signaling also regulates neuronal metabolism and energy production which is necessary to sustain synaptic transmission [24]. Unsurprisingly, disruptions to calcium signaling have debilitating consequences on neuronal function. Evidence for calcium dysregulation in AD was initially found over 25 years ago in fibroblast cells isolated from AD patients, which showed enhanced endoplasmic reticulum (ER) calcium uptake and ER calcium release [25,26]. Further studies in AD mouse models have supported the ubiquitous involvement of calcium dysregulation in AD, linking it to memory loss and increased toxicity of Abeta peptides [27,28,29]. Other studies have shown that elevated ER or cytoplasmic calcium increases Abeta production by triggering phosphorylation of APP and tau, indicating intracellular calcium dysregulation exacerbates amyloidosis and tau pathology [30,31]. Emilsson and Jazin also showed that mRNA expression of genes involved in calcium regulation is altered in AD brains [32]. The results from this study further suggest that ER calcium channel activity is elevated in AD [32]. Many familial AD PSEN mutations are also linked to dysregulated calcium signaling [33]. In addition to PSEN1/2′s role as the catalytic subunit of gamma-secretase, PSEN1/2 regulates ER calcium stores, a function that is notably gamma-secretase independent, demonstrating that PSEN1 and PSEN2′s impact on neuronal function extends beyond Abeta generation [34,35]. PSEN1/2 are transmembrane proteins present on most endomembranes but predominate on the ER membrane, where they have been found to physically interact with ER calcium channels, including the two main ER calcium release channels, inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) [28,36,37]. In line with this, mice carrying FAD mutations show altered activity of both IP3Rs and RyRs. Presenilin mutations have been documented to increase either the expression or sensitivity of RyRs and IP3Rs, leading to exaggerated RyR- and IP3R-mediated calcium release in response to various agonists [28,36,38,39,40]. Loss of PSEN1 function has also been shown in Xenopus oocytes to increase the activity of smooth endoplasmic reticulum Ca2+ ATPase (SERCA), which is responsible for pumping calcium into the ER to maintain cytosolic calcium levels [29]. Elevated SERCA activity in turn leads to overloading of ER calcium stores and a compensatory release of ER calcium [29]. PSEN itself may act as a passive ER calcium leak channel, which similarly results in ER calcium overfilling and exaggerated ER calcium release [35]. Notably, this elevated ER calcium release also precedes Abeta pathology and stimulates Abeta formation [29]. This is consistent with a previous study showing that altered calcium signaling in fibroblasts derived from asymptomatic FAD families was a strong predictor of future disease development [41]. Moreover, several other studies demonstrate that FAD mutations in the genes encoding PSEN1 or PSEN2 result in higher basal levels of calcium in cortical and hippocampal neurons due to excessive ER calcium release [29,36,38,39]. Elevated cytosolic calcium, in turn, impedes induction of long-term potentiation (LTP) [42]. There is also evidence that FAD mutations in APP similarly cause increased intracellular calcium concentrations independent of Abeta involvement [43,44]. Abeta itself has also been shown to derive some of its cytotoxic effects from promoting cytosolic calcium influx [45]. Other studies in cultured cortical neurons showed that oligomeric Abeta promoted ER calcium release, which led to cell death [46]. Altogether, the data strongly suggest that exaggerated ER calcium release and disruptions to calcium homeostasis play an important role in AD pathogenesis. 3. Mitochondria and ER Crosstalk in Presenilin Mutants, and in Sporadic AD Both ER and mitochondria are dynamic organelles that are actively moving within the cell and make transient contacts to facilitate crosstalk between the two organelles [47]. The mitochondria-associated membranes (MAMs) are regions where the ER closely associates with the outer mitochondrial membrane (OMM). These MAMs contain a specialized distribution of phospholipids and proteins to enable ER–mitochondria communication [48]. MAMs regulate a variety of processes including mitochondrial metabolism and energy production, the ER stress response, lipid synthesis, and apoptosis signaling [48]. Calcium homeostasis is also highly dependent on MAM function (Figure 1). IP3R and RyR localization is concentrated at the MAMs to promote the rapid uptake of calcium into the mitochondria [49] (Figure 1). MAMs are also enriched in voltage-dependent anion channels (VDACs), an ion channel on the OMM that regulates the transportation of a variety of ions and metabolites into and out of the mitochondria, and is primarily responsible for calcium uptake into the mitochondria across the OMM [50,51] (Figure 1). There is growing evidence that ER–mitochondrial communication is perturbed in AD. It has been shown in neurons of both sporadic and familial AD patients and an AD mouse model that there are increased ER–mitochondria contact points and expression of MAM-associated proteins, including IP3Rs, RyRs, and VDACs [50]. Abeta exposure to hippocampal neurons was also shown to increase ER–mitochondrial contact and promote transfer of calcium from the ER into the mitochondria [50]. Similarly, cells expressing the ε4 allele of apolipoprotein E (APOE4), which is considered a major risk factor for developing sporadic AD [52], show upregulated MAM activity and ER–mitochondrial communication [53], further suggesting disruption of MAM function is a common characteristic observed in AD. PSEN1/2 localization is also concentrated at the MAMs [54] (Figure 1). Accordingly, PSEN1 and PSEN2 FAD mutations have been shown to alter lipid and phospholipid synthesis, lipid exchange, and calcium transfer between the ER and mitochondria [55,56]. Moreover, presenilin and APP FAD mutations have been shown to increase the number of ER–mitochondria contact sites [57]. In Caenorhabditis elegans, which has a highly conserved presenilin ortholog but does not produce Abeta peptides [58], presenilin loss promotes ER-to-mitochondria calcium uptake, suggesting that presenilin alters ER–mitochondria calcium transfer via an Abeta-independent mechanism [59]. This study further demonstrated that the function of presenilin in mediating ER–mitochondrial calcium signaling is independent of gamma-secretase activity. However, there is also evidence that changes to MAM function or connectivity may alter or increase gamma-secretase activity due to presenilin enrichment at the MAM, and this may in turn promote pathologic Abeta42 generation [60]. Considering MAMs are involved in processes that are frequently disrupted in AD, including calcium homeostasis, it is likely the altered MAM distribution and functions are involved in AD pathogenesis. The increased ER–mitochondrial communication observed across multiple AD models has especially important implications on mitochondrial function. Indubitably, neurons are highly sensitive to mitochondrial defects and alterations in mitochondrial oxidative respiration. 4. Mitochondrial Calcium and AD Neurons are particularly reliant on mitochondria for energy generation to sustain synaptic transmission. The brain accounts for only 2% of human body weight, but uses roughly 20% of the body’s oxygen supply [61], The vast majority of neuronal ATP is produced through mitochondrial oxidative respiration [62,63], and neurons use this ATP primarily to generate the ionic gradients necessary for synaptic transmission [64]. Mitochondria are highly mobile organelles, and their subcellular localization impacts their ability to provide ATP to various cellular compartments [65]. In neurons, mitochondria are predominantly localized to the synapses to meet the energy demand at these sites [64]. Preserving mitochondrial quality and the mitochondrial trafficking network is thus especially important for neurons, as both are required to maintain synaptic plasticity and ultimately the learning and memory process [66,67]. Consequently, neurons are highly sensitive to mitochondrial defects and alterations in mitochondrial oxidative respiration [64]. Mitochondrial dysfunction is a common feature across neurodegenerative diseases, including AD [66,67]. In fact, mitochondrial dysfunction is thought to be one of the main drivers of the disease [68,69]. Intracellular calcium greatly impacts mitochondrial function. Indeed, calcium plays a direct role in stimulating enzymes of the tricarboxylic acid (TCA) cycle and electron transport chain leading to increased oxidative phosphorylation [70,71,72,73] (Figure 1). Mitochondria, in turn, regulate cellular calcium signaling by sequestering and buffering cytosolic calcium. As mentioned, the positioning of the ER calcium channels at the MAMs facilitates calcium transfer into the mitochondria. Selective transport of calcium into the matrix across the inner mitochondrial membrane (IMM) is accomplished by the highly calcium selective mitochondrial calcium uniporter (MCU) protein complex [74,75,76] (Figure 1). The MCU complex is composed of four core components: the pore forming MCU protein, an auxiliary subunit EMRE (essential for MCU regulator) and the MICU gatekeepers, MICU1 and MICU2/3 [77,78,79]. The MCU complex regulates calcium uptake into the matrix primarily through the MICU1 and MICU2/3 proteins that sense calcium through their conserved calcium-binding EF hand domains [80]. MICU1 and MICU2 are widely expressed in most mammalian tissues, whereas MICU3 is expressed only in skeletal muscles and the CNS [81]. MICU2 and MICU3 have similar structure and function in regulating MCU complex activity [82,83]. Recently, MICU2 has been shown bind to MICU1 and together these proteins allow for gatekeeper activity. Specifically, elevation in cytosolic calcium promotes calcium binding to the EF hands of the MICU1-MICU2 heterodimer, enabling MICU1 to facilitate MCU activity allowing calcium entry into the mitochondria [84,85]. MICU1 activation is also influenced by the activity and expression of cytosolic calcium binding proteins, which also have EF-hand domains that compete with MICU1 for calcium binding [86]. By promoting calcium uptake when cytosolic levels are high, mitochondria buffer calcium to maintain intracellular calcium homeostasis. This uptake is energetically favorable due to the negative membrane potential generated by transport of H+ across the IMM by the electron transport chain, making mitochondria well suited for this task. The mitochondria at synapses are important not only for ATP delivery but also for tightly regulating calcium concentration at the synapses for effective neurotransmission [87]. Considering the abundant evidence implicating intracellular calcium dysregulation in AD, it is likely that regulation of intracellular calcium through mitochondrial calcium buffering is a factor in this process. Calcium signaling also plays a direct role in mitochondrial activity [88]. Mitochondrial calcium levels significantly impact mitochondrial activity and cellular ATP supply. Calcium uptake into the mitochondria increases oxidative phosphorylation by stimulating the activity of the F1F0-ATP synthase and enzymes within the TCA cycle, specifically alpha-ketoglutarate dehydrogenase, isocitrate dehydrogenase, and pyruvate dehydrogenase [88]. Mitochondrial activity is thus highly sensitive to calcium levels. Evidence suggests that along with increased ER–mitochondrial communication and elevated cytosolic calcium, mitochondrial calcium is elevated in AD. Due to MICU1 and MICU2/3 calcium-sensing and gating properties, increased cytosolic calcium can promote mitochondrial calcium uptake [74]. Multiple studies have also reported that expression of cytosolic calcium binding proteins calmodulin, calbindin D28K, and parvalbumin is reduced in AD patients and AD models, which would presumably free up calcium to bind MICU1 and MICU2/3 and activate MCU [89,90,91,92]. Therefore, it is unsurprising that mitochondrial calcium is elevated in AD models. Moreover, Abeta exposure has been shown to increase mitochondrial calcium levels in cortical neurons that promotes neurodegeneration, which can be reversed by blocking MCU [50]. There is evidence that the oligomeric form of Abeta produces calcium-permeable pores in the mitochondrial membrane, which promotes calcium uptake, suggesting that Abeta’s toxicity may result in part from its ability to disrupt mitochondrial calcium homeostasis [93,94]. Similarly, an additional study demonstrated that Abeta oligomers increased mitochondrial calcium levels by promoting ER calcium release, resulting in mitochondrial dysfunction [95]. Furthermore, a separate study looking at a mouse model with FAD mutations in PSEN1 and APP showed that these animals had elevated mitochondrial calcium [96]. Mitochondrial calcium levels are also increased in C. elegans presenilin mutants, which can be abrogated by preventing ER calcium release or mitochondrial calcium uptake, suggesting a conserved role for presenilin in maintaining mitochondrial calcium homeostasis by regulating ER calcium release at the MAMs [59]. Importantly, reduction of mitochondrial calcium levels in the C. elegans presenilin mutants restores neuronal function. What are the consequences of elevated mitochondrial calcium? Although mitochondrial calcium homeostasis can be restored through calcium efflux pathways, which are regulated primarily through the sodium–calcium exchanger NCLX [97], excessive calcium uptake or impairments to calcium efflux can overwhelm mitochondrial calcium capacity. Mitochondrial calcium overload, which when combined with other stressors such as oxidative damage, results in the formation and opening of the mitochondrial permeability transition pore (mPTP) [98] (Figure 1). Although the molecular components of the mPTP are still under debate, the activity of the mPTP is known to span both the OMM and IMM, and its opening induces calcium efflux from the matrix. However, prolonged opening of the mPTP leaves the mitochondria open to the osmotic influx of cytosolic solutes and water, which causes the matrix to swell and rupture. Cytochrome c is also released from the mitochondria from prolonged mPTP opening, leading to the initiation of apoptosis. This process has been observed in several AD mouse models. In the aforementioned study showing elevated mitochondrial calcium in AD mice, high mitochondrial calcium correlated with the induction of apoptosis, whereas neurons containing mitochondria with normal calcium concentrations did not undergo apoptosis [96]. Another study showed that Abeta42 exposure could cause neuronal apoptosis by promoting mitochondrial calcium overload and stimulating the opening of the mPTP [99]. Excessive mitochondrial uptake in such contexts would present a particular problem for aged mitochondria, which are particularly vulnerable to high calcium levels, as they have lower calcium buffering capacities and are more susceptible to calcium overload [100]. 5. Calcium-Induced Changes to Mitochondrial Activity Promote ROS Production Elevated mitochondrial calcium has also been shown to disrupt neuronal function through the elevated production of reactive oxygen species (ROS) [101]. Mitochondrial oxidative respiration is a major source of ROS, as ROS are produced as a byproduct of the reduction of oxygen [102]. The energy used to pump H+ across the mitochondria’s inner membrane space is generated by a series of electron transfer reactions along the electron transport chain where each electron transport chain complex has increasingly greater reduction potential, the final electron acceptor being oxygen. Although electrons transferred sequentially along the electron transport chain will react at the end with oxygen to produce water, electrons may also be passed to oxygen prematurely, resulting in partial reduction of oxygen to superoxide anion (O2•−). This superoxide radical is a major source of oxidants and free radicals in the cell, as it reacts to produce hydrogen peroxide, which again reacts to produce the highly toxic hydroxyl radical (·OH). Although ROS at low levels provide important cellular functions by acting as signaling molecules, excessive ROS are toxic to the cell. As a site of superoxide generation, mitochondria are exposed to an especially high level of ROS, which can damage mitochondrial DNA and proteins over time, leading to severe mitochondrial damage and inefficiency. Dysfunctional mitochondria, in turn, generate greater levels of ROS; in this way, oxidative stress and mitochondrial dysfunction are closely linked. It has long been theorized that the aging process is caused in part by the progressive accumulation of ROS-induced cellular damage, both as a result of increased ROS generated by an inefficient electron transport chain or failure of antioxidant systems [103]. If the collective damage to proteins, DNA, and organelles induced by ROS is too great, the cell will undergo apoptosis. Like mitochondrial dysfunction, oxidative stress plays a significant role in the pathogenesis of AD [104,105]. In vivo and in vitro studies show a direct relationship between oxidative stress and AD [105,106,107]. Furthermore, elevated ROS is highly correlated with the early stages of AD [108] and precedes Abeta plaque formation [109]. Oxidative stress can also promote tau hyperphosphorylation and fibril formation [110]. In a variety of AD animal models, high ROS levels have been shown to cause extensive neuronal death and cognitive decline [111]. Neurons are especially vulnerable to oxidative damage induced by mitochondrial respiration due to their especially high energy and oxygen demand, which in turn produces greater relative levels of ROS [98]. Their high lipid content also makes them vulnerable to lipid peroxidation by ·OH [98]. It has also been proposed that the regions of the brain that first undergo neurodegeneration in AD occur due to their increased susceptibility to oxidative damage, which can result from greater energy demands or higher basal levels of ROS required for signaling [112]. Elevated mitochondrial calcium levels have been associated with increased oxidative stress in AD models. Impairment to NCLX function that prevents mitochondrial calcium efflux in a mouse AD model resulted in excessive mitochondrial calcium and oxidative stress, which in turn led to amyloid and tau pathology and ultimately neuronal death [113]. Rescue of NCLX function was able to prevent cognitive defects in these animals. Buildup of mitochondrial ROS as a result of mitochondrial calcium overload also triggered apoptosis in a separate AD mouse model [99]. Mitochondrial calcium influx has also been shown to induce mitochondrial damage by stimulating oxidative phosphorylation, increasing the amount of ROS generated as a byproduct (Figure 1). Abeta oligomers also damage neurons by elevating lipid peroxidation and oxidative stress, further supporting the idea that oxidative damage underlies AD pathogenesis [114,115,116]. High mitochondrial calcium, fragmentation of mitochondria, oxidative stress and neuronal death have also been observed in APP/PS1 transgenic mice and in the brains of AD patients prior to Abeta formation [33,99,117]. Fibroblasts isolated from FAD patients showed both elevated oxidative respiration and ROS, which could be blocked either by preventing mitochondrial calcium uptake or by reducing respiration [94]. Similarly, in astrocytes differentiated from iPSCs derived from FAD patients bearing PSEN1 mutations, the mutant astrocytes showed higher oxygen consumption that resulted in greater ROS levels [118]. Notably, this alteration in metabolism was gamma-secretase independent. In C. elegans, neuronal and behavior defects resulting from presenilin dysfunction was dependent on elevated mitochondrial calcium-induced oxidative respiration and concomitant ROS generation [119]. Indeed, by limiting ER calcium release or mitochondrial calcium uptake in the C. elegans presenilin mutants reduced mitochondrial generated ROS and neurodegeneration. Furthermore, the neuronal defects observed in the C. elegans presenilin mutants were suppressed by treating these animals with the mitochondrial directed antioxidant, MitoTEMPO [94]. These studies indicate that aberrant mitochondrial activity induced by altered calcium signaling may be a mechanism by which FAD mutations generate ROS and lead to neuronal dysfunction. Therefore, this initial mitochondrial hyperactivity caused by aberrant mitochondrial calcium influx may in turn accelerate mitochondrial impairment, resulting in reduced ATP generation insufficient for neuronal synaptic transmission. Taken together, these data indicate that altered mitochondrial calcium homeostasis in FAD models leads to mitochondrial dysfunction and ROS production that promotes neurodegeneration. 6. Mitochondrial Function in Protein Homeostasis The preservation of a highly functional proteome is vital for the aging nervous system. Indeed, protein misfolding and aggregation is a common manifestation observed in neurodegenerative diseases [120,121]. Therefore, the integrity of the protein homeostasis (proteostasis) network is critical for cell function and survival and is particularly important for non-dividing cells such as the neurons of the central nervous system. Insults that affect proteostasis include oxidative stress [122]. Accordingly, perturbations in mitochondrial function resulting in elevated production of ROS will greatly impact the maintenance of the proteome. Proteins that are irreversibly oxidized need to be removed by the proteostasis network; however, as organismal aging occurs, the efficiency of the proteostasis network diminishes, leading to protein misfolding and aggregation. In C. elegans, a clear link between altered calcium uptake into the mitochondria and the impact this has on proteostasis has been demonstrated. Mutations in the gene encoding presenilin in C. elegans result in elevated ER to mitochondrial calcium transfer. The elevation in mitochondrial calcium results in mitochondrial hyperactivity, namely increased oxidative phosphorylation and ROS production that promotes neuronal dysfunction [119]. The elevated oxidative stress caused by increased mitochondrial calcium leads to proteostasis defects in the presenilin mutants that can be rescued by inhibiting mitochondrial calcium uptake or by treating the animals with antioxidants [123] (Figure 1). The two main pathways involved in proteome maintenance are the ubiquitin-proteasome system and the autophagy-lysosome system. While the ubiquitin-proteasome activity appeared normal in the C. elegans presenilin mutants, there was a clear reduction in the formation of autophagosomes suggesting a defect in the autophagy-lysosome system [123]. Findings from FAD mouse and cell culture models also demonstrate that calcium dysregulation is linked to defects in autophagy [124]. Interestingly, the nutrient and energy sensing mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway is a central inhibitor of autophagy and has been reported to be upregulated in AD as well as other neurodegenerative disorders [125,126,127,128,129]. However, the mechanism underlying the upregulation of mTORC1 signaling in these disorders is not clear. Nevertheless, these studies highlight the importance of mitochondrial calcium homeostasis and the impact it can have on proteostasis. 7. Mitochondrial Calcium-Dependent Subcellular Localization and Transport As mentioned, trafficking and localization of mitochondria is essential for neuronal function. Mitochondria are extremely dynamic organelles undergoing constant fission and fusion events and they migrate along microtubule tracks to ensure their localization throughout the neuron. Principally, the fission and fusion events are regulated by three large GTPases, namely mitofusins (Mfn1/2), optic atrophy1 (OPA1) and dynamin related protein 1 (Drp1) and the trafficking along microtubules is mediated by the kinesin and dynein motors through the action of the adaptor protein Milton/Trak and the atypical Rho GTPase Miro [130,131,132]. These pathways are interconnected and are required to balance mitochondrial shape, morphology and function. Given the critical role of mitochondria in neurons, impaired mitochondrial dynamics is increasingly implicated in neurodegenerative diseases [133,134,135]. Additionally, mitochondrial fission and fusion dynamics have been associated in neurotransmitter release and terminal axon branching by regulating cytosolic calcium levels [136]. Trafficking of mitochondria is regulated in part by calcium and is mediated via Miro 1 and Miro 2 [137]. Miro proteins are transmembrane proteins that are localized to the OMM and contain two-conserved EF-hand calcium binding domains flanked by two Rho-like GTPase domains [133,138,139]. Miro interacts with Milton/Trak to link mitochondria to kinesin and dynein to allow for trafficking along the microtubule cytoskeleton [140,141]. When cytosolic calcium levels increase, calcium binds to the Miro EF-hands creating a conformational change that disengages Miro and the mitochondria from the microtubule network [132,142]. Consequently, an increase in calcium can pause the trafficking of mitochondria along the microtubules. It has also been demonstrated that Miro can influence mitochondrial trafficking by directly mediating mitochondrial calcium uptake via the MCU complex. In this study, the authors found that increased mitochondrial matrix calcium influx inversely correlated with mitochondrial trafficking speed [143]. Thus, cytosolic as well as mitochondrial calcium levels, can influence the localization and transport of mitochondria. In addition to mediating mitochondrial trafficking and mitochondrial calcium uptake, Miro proteins have also been implicated in several other mitochondrial functions, including IMM and OMM organization, mitochondrial fission, ER–mitochondrial contacts, and mitophagy. In both Drosophila and mammalian cells, Miro was shown to promote ER–mitochondrial interactions [144,145]. Moreover, in Drosophila, Miro was shown to facilitate ER to mitochondria calcium signaling [144,146]. In C. elegans and mammalian cells, Miro was shown to mediate mitochondrial fission upon an increase in cytosolic calcium that is independent of the conical Drp1 fission machinery [147,148]. Notably in mammalian cells, this fission event facilitates the turnover of mitochondria through the process of mitophagy, which is the selective removal of defective mitochondria by autophagy [147]. Furthermore, another study using human induced pluripotent stem cell-derived neurons from Parkinson’s disease patients found that targeted loss of Miro enables the removal of damaged mitochondria by mitophagy and promotes neuronal fitness [149]. It is unclear whether the trafficking function of Miro is required to mediate these additional activities of Miro (e.g., ER–mitochondria contact, fission or mitophagy). Nonetheless, these data highlight the pleiotropic role Miro and calcium have on mediating mitochondrial localization and function. 8. Conclusions The influx of calcium into the mitochondria has long been established as a key regulator of many cellular homeostatic processes that range from energy production to cell death and necrosis [150,151]. The recent molecular discovery of the MCU complex components has provided critical insight into the role mitochondrial calcium influx has in energy production under increased work load but also, paradoxically, in promoting disease, such as neurodegeneration [152,153]. For example, studies in mice and C. elegans AD models have shown that there is an increase in mitochondrial calcium levels that can be reduced by blocking the MCU complex [99,119]. Critically, it was demonstrated in the mouse AD model that the high levels of mitochondrial calcium precede neuronal dysfunction and that this dysfunction can be rescued by inhibiting the MCU complex and blocking mitochondrial calcium uptake [99]. Similar observations have been made studying other neurodegenerative diseases [154,155,156]. For instance, recent work in zebrafish and Drosophila models of Parkinson’s disease have discovered elevated mitochondrial calcium levels that when reduced, could alleviate neurodegeneration [146,157]. These data highlight the significance mitochondrial calcium homeostasis has in neurodegeneration and point to the importance of understanding the mechanisms mediating mitochondrial calcium influx and activity. Indeed several recent review articles have suggested that targeting the MCU complex could be a potential therapeutic target for treating neurodegenerative diseases [158,159,160]. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Author Contributions Conceptualization, K.C.R., Z.A. and K.R.N.; writing—original draft preparation K.C.R. and Z.A.; writing—review and editing, K.C.R., Z.A. and K.R.N. All authors have read and agreed to the published version of the manuscript. Funding This work is funded by NIH grants AG064175 and GM088213. Conflicts of Interest The authors declare no conflict of interest. Figure 1 Schematic of familial Alzheimer’s Disease (FAD) mutations facilitating endoplasmic reticulum (ER)–mitochondrial calcium transfer. Increased calcium release from the IP3 receptor (IP3R) and Ryanodine receptors (RyR) at the ER mitochondrial-associated membranes (MAMs) is taken up through the voltage-dependent ion channel (VDAC) on the mitochondrial outer membrane and the calcium-selective mitochondrial calcium uniporter (MCU) on the mitochondrial inner membrane. Increased mitochondrial calcium stimulates oxidative phosphorylation, leading to increased reactive oxygen species (ROS) generation, which promotes oxidative stress, mitochondrial permeable transition pore (mPTP) opening and apoptosis, protein misfolding and proteostatic collapse, and neurodegeneration. ==== Refs References 1. WHO 10 Facts on Dementia WHO Geneva, Switzerland 2017 2. Hebert L.E. Weuve J. Scherr P.A. Evans D.A. Alzheimer disease in the United States (2010–2050) estimated using the 2010 census Neurology 2013 80 1778 1783 10.1212/WNL.0b013e31828726f5 23390181 3. Haass C. Selkoe D.J. Soluble protein oligomers in neurodegeneration: Lessons from the Alzheimer’s amyloid β-peptide Nat. Rev. Mol. Cell Biol. 2007 8 101 112 10.1038/nrm2101 17245412 4. Dai M.-H. Zheng H. Zeng L.-H. Zhang Y. The genes associated with early-onset Alzheimer’s disease Oncotarget 2018 9 15132 10.18632/oncotarget.23738 29599933 5. Price J.L. Morris J.C. Tangles and plaques in nondemented aging and “preclinical” Alzheimer’s disease Ann. Neurol. Off. J. Am. Neurol. Assoc. Child Neurol. Soc. 1999 45 358 368 10.1002/1531-8249(199903)45:3<358::AID-ANA12>3.0.CO;2-X 6. Golde T.E. Schneider L.S. Koo E.H. Anti-Aβ therapeutics in Alzheimer’s disease: The need for a paradigm shift Neuron 2011 69 203 213 10.1016/j.neuron.2011.01.002 21262461 7. Holmes C. Boche D. Wilkinson D. Yadegarfar G. Long-term effects of Aβ42 immunisation in Alzheimer’s disease: Follow-up of a randomised, placebo-controlled phase I trial Lancet 2008 372 216 223 10.1016/S0140-6736(08)61075-2 18640458 8. Doody R.S. Raman R. Farlow M. Iwatsubo T. Vellas B. Joffe S. Kieburtz K. He F. Sun X. Thomas R.G. A phase 3 trial of semagacestat for treatment of Alzheimer’s disease N. Engl. J. Med. 2013 369 341 350 10.1056/NEJMoa1210951 23883379 9. Chételat G. Aβ-independent processes—Rethinking preclinical AD Nat. Rev. Neurol. 2013 9 123 124 10.1038/nrneurol.2013.21 23399647 10. Sengupta U. Nilson A.N. Kayed R. The role of amyloid-β oligomers in toxicity, propagation, and immunotherapy EBioMedicine 2016 6 42 49 10.1016/j.ebiom.2016.03.035 27211547 11. Mucke L. Masliah E. Yu G.-Q. Mallory M. Rockenstein E.M. Tatsuno G. Hu K. Kholodenko D. Johnson-Wood K. McConlogue L. High-level neuronal expression of Aβ1–42 in wild-type human amyloid protein precursor transgenic mice: Synaptotoxicity without plaque formation J. Neurosci. 2000 20 4050 4058 10.1523/JNEUROSCI.20-11-04050.2000 10818140 12. Shankar G.M. Li S. Mehta H.T. Gracia-Munoz A. Shepardson N.E. Smith I. Brett F.M. Farrell M.A. Rowan M.J. Lemer C.A. Amyloid-β protein dimers isolated directly from Alzheimer’s brains impair synaptic plasticity and memory Nat. Med. 2008 14 837 842 10.1038/nm1782 18568035 13. Panza F. Lozupone M. Dibello V. Greco A. Daniele A. Serpia D. Logroscino G. Imbimbo B.P. Are antibodies directed against amyloid-β (Aβ) oligomers the last call for the Aβ hypothesis of Alzheimer’s disease? Future Med. 2019 11 3 6 10.2217/imt-2018-0119 30702009 14. Kim J. Chakrabarty P. Hanna A. March A. Dickson D.W. Borchelt D.R. Golde T. Janus C. Normal cognition in transgenic BRI2-Aβ mice Mol. Neurodegener. 2013 8 1 12 10.1186/1750-1326-8-15 23281774 15. Tanzi R.E. Bertram L. Twenty years of the Alzheimer’s disease amyloid hypothesis: A genetic perspective Cell 2005 120 545 555 10.1016/j.cell.2005.02.008 15734686 16. Saura C.A. Choi S.-Y. Beglupoulos V. Malkani S. Zhang D. Shankaranarayana Rao B.S. Chatarji S. Kelleher R.J. 3rd Kendel E.R. Duff K. Loss of presenilin function causes impairments of memory and synaptic plasticity followed by age-dependent neurodegeneration Neuron 2004 42 23 36 10.1016/S0896-6273(04)00182-5 15066262 17. Wu L. Rosa-Neto P. Hsiung G.R. Sadovnik D.A. Masellis M. Black S.E. Jia J. Gauthier S. Early-onset familial Alzheimer’s disease (EOFAD) Can. J. Neurol. Sci. 2012 39 436 445 10.1017/S0317167100013949 22728850 18. Bekris L.M. Yu C.E. Bird T.D. Tsuang D.W. Genetics of Alzheimer disease J. Geriatr. Psychiatry Neurol. 2010 23 213 227 10.1177/0891988710383571 21045163 19. Morris G.P. Clark I.A. Vissel B. Inconsistencies and controversies surrounding the amyloid hypothesis of Alzheimer’s disease Acta Neuropathol. Commun. 2014 2 135 10.1186/s40478-014-0135-5 25231068 20. Khachaturian Z.S. Introduction and overview Ann. N. Y. Acad. Sci. 1989 568 1 4 10.1111/j.1749-6632.1989.tb12485.x 21. Alzheimer’s Association Calcium Hypothesis Workgroup Khachaturian Z.S. Calcium hypothesis of Alzheimer’s disease and brain aging: A framework for integrating new evidence into a comprehensive theory of pathogenesis Alzheimer’s Dement. 2017 13 178 182.e17 28061328 22. Morris R.G. Long-term potentiation and memory Philos. Trans. R. Soc. Lond. Ser. B Biol. Sci. 2003 358 643 647 10.1098/rstb.2002.1230 12740109 23. Kawamoto E.M. Vivar C. Camandola S. Physiology and pathology of calcium signaling in the brain Front. Pharmacol. 2012 3 61 10.3389/fphar.2012.00061 22518105 24. Brini M. Cali T. Ottolini D. Carafoli E. Neuronal calcium signaling: Function and dysfunction Cell. Mol. Life Sci. 2014 71 2787 2814 10.1007/s00018-013-1550-7 24442513 25. Peterson C. Gibson G. Blass J. Altered calcium uptake in cultured skin fibroblasts from patients with Alzheimer’s disease N. Engl. J. Med. 1985 312 1063 3982463 26. Ito E. Oka K. Etcheberrigaray R. Nelson T.J. McPhie D.L. Tofel-Grehl B. Gibson G.E. Alkon D.L. Internal Ca2+ mobilization is altered in fibroblasts from patients with Alzheimer disease Proc. Batl. Acad. Sci. USA 1994 91 534 538 10.1073/pnas.91.2.534 27. LaFerla F.M. Calcium dyshomeostasis and intracellular signalling in Alzheimer’s disease Nat. Rev. Neurosci. 2002 3 862 872 10.1038/nrn960 12415294 28. Smith I.F. Hitt B. Green K.N. Oddo S. Laferla F.M. Enhanced caffeine-induced Ca2+ release in the 3xTg-AD mouse model of Alzheimer’s disease J. Neurochem. 2005 94 1711 1718 10.1111/j.1471-4159.2005.03332.x 16156741 29. Green K. Demuro A. Akbari Y. Hitt B.D. Smith I.F. Parker I. LaFerla F.M. SERCA pump activity is physiologically regulated by presenilin and regulates amyloid beta production J. Cell Biol. 2008 181 1107 1116 10.1083/jcb.200706171 18591429 30. Querfurth H.W. Selkoe D.J. Calcium ionophore increases amyloid. beta. Peptide production by cultured cells Biochemistry 1994 33 4550 4561 10.1021/bi00181a016 8161510 31. Pierrot N. Ferrao Santos S. Feyt C.H. Morel M. Brion J.-P. Octave J.-N. Calcium-mediated transient phosphorylation of tau and amyloid precursor protein followed by intraneuronal amyloid-β accumulation J. Biol. Chem. 2006 281 39907 39914 10.1074/jbc.M606015200 17085446 32. Emilsson L. Saetre P. Jazin E. Alzheimer’s disease: mRNA expression profiles of multiple patients show alterations of genes involved with calcium signaling Neurobiol. Dis. 2006 21 618 625 10.1016/j.nbd.2005.09.004 16257224 33. Supnet C. Bezprozvanny I the dysregulation of intracellular calcium in Alzheimer disease Cell Calcium. 2010 47 183 189 10.1016/j.ceca.2009.12.014 20080301 34. Wang Y. Greig N.H. Yu Q.-S. Mattson M.P. Presenilin-1 mutation impairs cholinergic modulation of synaptic plasticity and suppresses NMDA currents in hippocampus slices Neurobiol. Aging 2009 30 1061 1068 10.1016/j.neurobiolaging.2007.10.009 18068871 35. Tu H. Nelson O. Bezprozvanny A. Wang Z. Lee S.-F. Hao Y.-H. Serneels L. De Strooper B. Yu G. Bezprozvanny I. Presenilins form ER Ca2+ leak channels, a function disrupted by familial Alzheimer’s disease-linked mutations Cell 2006 126 981 993 10.1016/j.cell.2006.06.059 16959576 36. Cheung K.-H. Mei L. Mak D.O. Hayashi I. Iwatsubo T. Kang D.E. Foskett J.K. Gain-of-function enhancement of IP3 receptor modal gating by familial Alzheimer’s disease–linked presenilin mutants in human cells and mouse neurons Sci. Signal. 2010 3 ra22 10.1126/scisignal.2000818 20332427 37. Cheung K.-H. Shineman D. Muller M. Cardenas C. Mei L. Yang J. Tomita T. Iwatsubo T. Lee V.M. Foskett J.K. Mechanism of Ca2+ disruption in Alzheimer’s disease by presenilin regulation of InsP3 receptor channel gating Neuron 2008 58 871 883 10.1016/j.neuron.2008.04.015 18579078 38. Chan S.L. Mayne M. Holden C.P. Geiger J.D. Mattson M.P. Presenilin-1 mutations increase levels of ryanodine receptors and calcium release in PC12 cells and cortical neurons J. Biol. Chem. 2000 275 18195 18200 10.1074/jbc.M000040200 10764737 39. Lee S.Y. Hwang D.Y. Kim Y.K. Lee J.W. Shin I.C. Oh K.W. Lee M.K. Lim J.S. Yoon D.Y. Hwang S.J. PS2 mutation increases neuronal cell vulnerability to neurotoxicants through activation of caspase-3 by enhancing of ryanodine receptor-mediated calcium release FASEB J. 2006 20 151 153 16394273 40. Stutzmann G.E. Caccamo A. Laferla F.M. Parker I. Dysregulated IP3 signaling in cortical neurons of knock-in mice expressing an Alzheimer’s-linked mutation in presenilin1 results in exaggerated Ca2+ signals and altered membrane excitability J. Neurosci. 2004 24 508 513 10.1523/JNEUROSCI.4386-03.2004 14724250 41. Etcheberrigaray R. Hirashima N. Nee L. Prince J. Govoni S. Racchi M. Tanzi R.E. Alkon D.L. Calcium responses in fibroblasts from asymptomatic members of Alzheimer’s disease families Neurobiol. Dis. 1998 5 37 45 10.1006/nbdi.1998.0176 9702786 42. Briggs C.A. Chakroborty S. Stutzmann G.E. Emerging pathways driving early synaptic pathology in Alzheimer’s disease Biochem. Biophys. Res. Commun. 2017 483 988 997 10.1016/j.bbrc.2016.09.088 27659710 43. Rojas G. Cardenas A.M. Fernandez-Olivares P. Shimahara T. Segura-Aguilar J. Caviedes R. Caviedes P. Effect of the knockdown of amyloid precursor protein on intracellular calcium increases in a neuronal cell line derived from the cerebral cortex of a trisomy 16 mouse Exp. Neurol. 2008 209 234 242 10.1016/j.expneurol.2007.09.024 17976585 44. Leissring M.A. Murphy M.P. Mead T.R. Akbari Y. Sugarman M.C. Jannatipour P. Anliker B. Muller S. Saftig P. De Strooper B. A physiologic signaling role for the γ-secretase-derived intracellular fragment of APP Proc. Natl. Acad. Sci. USA 2002 99 4697 4702 10.1073/pnas.072033799 11917117 45. Birnbaum J. Bali J. Rajendran L. Nitsch R.M. Tackenberg C. Calcium flux-independent NMDA receptor activity is required for A β oligomer-induced synaptic loss Cell Death Dis. 2015 6 e1791 10.1038/cddis.2015.160 26086964 46. Resende R. Ferreiro E. Pereira C. de Oliveira C.R. Neurotoxic effect of oligomeric and fibrillar species of amyloid-beta peptide 1-42: Involvement of endoplasmic reticulum calcium release in oligomer-induced cell death Neuroscience 2008 155 725 737 10.1016/j.neuroscience.2008.06.036 18621106 47. Lackner L.L. Voeltz G.K. The mechanisms and functions of interorganelle interactions Mol. Biol. Cell 2017 28 703 704 10.1091/mbc.e16-11-0799 28292913 48. Pinton P. Mitochondria-Associated Membranes (MAMs) and Pathologies Cell Death Dis. 2018 9 413 10.1038/s41419-018-0424-1 29549303 49. Rizzuto R. Pinton P. Carrington W. Fay F.S. Fogarty K.E. Lifshitz L.M. Tuft R.A. Pozzan T. Close contacts with the endoplasmic reticulum as determinants of mitochondrial Ca2+ responses Science 1998 280 1763 1766 10.1126/science.280.5370.1763 9624056 50. Hedskog L. Pinho C.M. Filadi R. Ronnback A. Hertwig L. Wiehager B. Larssen P. Gellhaar S. Sandebring A. Westerlund M. Modulation of the endoplasmic reticulum–mitochondria interface in Alzheimer’s disease and related models Proc. Natl. Acad. Sci. USA 2013 110 7916 7921 10.1073/pnas.1300677110 23620518 51. Fujimoto M. Hayashi T. New insights into the role of mitochondria-associated endoplasmic reticulum membrane Int. Rev. Cell Mol. Biol. 2011 73 117 52. Roses M. Allen D. Apolipoprotein E alleles as risk factors in Alzheimer’s disease Annu. Rev. Med. 1996 47 387 400 10.1146/annurev.med.47.1.387 8712790 53. Tambini M.D. Pera M. Kanter E. Yang H. Guardia-Laguarta C. Holtzman D. Sulzer D. Area-Gomez E. Schon E.A. ApoE4 upregulates the activity of mitochondria-associated ER membranes EMBO Rep. 2016 17 27 36 10.15252/embr.201540614 26564908 54. Area-Gomez E. de Groof A.J. Boldogh I. Bird T.D. Gibson G.E. Koehler C.M. Yu W.H. Duff K.E. Yaffe M.P. Pon L.A. Presenilins are enriched in endoplasmic reticulum membranes associated with mitochondria Am. J. Pathol. 2009 175 1810 1816 10.2353/ajpath.2009.090219 19834068 55. Area-Gomez E. Del Camen Lara Castillo M. Tambini M.D. Guardia-Laguarta C. De Groof A.J. Madra M. Ikenouchi J. Umeda M. Bird T.D. Sturley S. Upregulated function of mitochondria-associated ER membranes in Alzheimer disease EMBO J. 2012 31 4106 4123 10.1038/emboj.2012.202 22892566 56. Zampese E. Fasolato C. Kipanyula M.J. Bortolozzi M. Pozzan T. Pizzo P. Presenilin 2 modulates endoplasmic reticulum (ER)–mitochondria interactions and Ca2+ cross-talk Proc. Natl. Acad. Sci. USA 2011 108 2777 2782 10.1073/pnas.1100735108 21285369 57. Del Prete D. Suski J.M. Oules B. Debayle D. Gay A.S. Lacas-Gervais S. Bussiere R. Bauer C. Pinton P. Paterlini-Brechot P. Localization and processing of the amyloid-β protein precursor in mitochondria-associated membranes J. Alzheimer’s Dis. 2017 55 1549 1570 10.3233/JAD-160953 27911326 58. Daigle I. Li C. Apl-1, a Caenorhabditis elegans gene encoding a protein related to the human beta-amyloid protein precursor Proc. Natl. Acad. Sci. USA 1993 90 12045 12049 10.1073/pnas.90.24.12045 8265668 59. Sarasija S. Norman K.R. A γ-secretase independent role for presenilin in calcium homeostasis impacts mitochondrial function and morphology in Caenorhabditis elegans Genetics 2015 201 1453 1466 10.1534/genetics.115.182808 26500256 60. Schreiner B. Hedskog L. Weihager H. Ankarcrona M. Amyloid-β peptides are generated in mitochondria-associated endoplasmic reticulum membranes J. Alzheimer’s Dis. 2015 43 369 374 10.3233/JAD-132543 25096627 61. Mink J.W. Blumenschine R.J. Adams D.B. Ratio of central nervous system to body metabolism in vertebrates: Its constancy and functional basis Am. J. Physiol. Regul. Integr. Comp. Physiol. 1981 241 R203 R212 10.1152/ajpregu.1981.241.3.R203 62. Ames A. III CNS energy metabolism as related to function Brain Res. Rev. 2000 34 42 68 10.1016/S0165-0173(00)00038-2 11086186 63. Sokoloff L. The metabolism of the central nervous system in vivo Handbook of Physiology, Section I Neurophysiology Washington, DC, USA 1960 Volume 3 1843 1864 64. Harris J.J. Jolivet R. Attwell D. Synaptic energy use and supply Neuron 2012 75 762 777 10.1016/j.neuron.2012.08.019 22958818 65. Schwarz T.L. Mitochondrial trafficking in neurons Cold Spring Harb. Perspect. Biol. 2013 5 a011304 10.1101/cshperspect.a011304 23732472 66. Swerdlow R.H. Burns J.M. Khan S.M. The Alzheimer’s disease mitochondrial cascade hypothesis: Progress and perspectives Biochim. Biophys. Acta (BBA) Mol. Basis Dis. 2014 1842 1219 1231 10.1016/j.bbadis.2013.09.010 24071439 67. Todorova V. Blokland A. Mitochondria and synaptic plasticity in the mature and aging nervous system Curr. Neuropharmacol. 2017 15 166 173 10.2174/1570159X14666160414111821 27075203 68. Cenini G. Voos W. Mitochondria as potential targets in Alzheimer disease therapy: An update Front. Pharmacol. 2019 10 902 10.3389/fphar.2019.00902 31507410 69. Moreira P.I. Carvalho C. Zhu X. Smith M.A. Perry G. Mitochondrial dysfunction is a trigger of Alzheimer’s disease pathophysiology Biochim. Biophys. Acta (BBA) Mol. Basis Dis. 2010 1802 2 10 10.1016/j.bbadis.2009.10.006 70. Balaban R.S. The role of Ca2+ signaling in the coordination of mitochondrial ATP production with cardiac work Biochim. Biophys. Acta (BBA) Bioenerg. 2009 1787 1334 1341 10.1016/j.bbabio.2009.05.011 71. Denton R.M. Regulation of mitochondrial dehydrogenases by calcium ions Biochim. Biophys. Acta (BBA) Bioenerg. 2009 1787 1309 1316 10.1016/j.bbabio.2009.01.005 72. Carafoli E. The fateful encounter of mitochondria with calcium: How did it happen? Biochim. Biophys. Acta (BBA) Bioenerg. 2010 1797 595 606 10.1016/j.bbabio.2010.03.024 73. Ivannikov M.V. Macleod G.T. Mitochondrial free Ca2+ levels and their effects on energy metabolism in Drosophila motor nerve terminals Biophys. J. 2013 104 2353 2361 10.1016/j.bpj.2013.03.064 23746507 74. Mishra J. Jhun B.S. Hurst S. O-Uchi J. Csordas G. Sheu S.-S. The Mitochondrial Ca 2+ uniporter: Structure, function, and pharmacology Pharmacology of Mitochondria Springer Berlin/Heidelberg, Germany 2017 129 156 75. Baughman J.M. Perocchi F. Girgis H.S. Plovanich M. Belcher-Trimme C.A. Sancak Y. Bao X. Strittmatter L. Goldberger O. Bogorad R.L. Integrative genomics identifies MCU as an essential component of the mitochondrial calcium uniporter Nature 2011 476 341 345 10.1038/nature10234 21685886 76. De Stefani D. Raffaello A. Teardo E. Szabo I. Rizzuto R. A forty-kilodalton protein of the inner membrane is the mitochondrial calcium uniporter Nature 2011 476 336 340 10.1038/nature10230 21685888 77. Kamer K.J. Mootha V.K. The molecular era of the mitochondrial calcium uniporter Nat. Rev. Mol. Cell Biol. 2015 16 545 553 10.1038/nrm4039 26285678 78. Pallafacchina G. Zanin S. Rizzuto R. Recent advances in the molecular mechanism of mitochondrial calcium uptake F1000Research 2018 7 10.12688/f1000research.15723.1 79. Patron M. Checchetto V. Raffaello A. Teardo E. Vecellio Reane D. Mantoan M. Granatiero V. Szabo I. De Stefani D. Rizzuto R. MICU1 and MICU2 finely tune the mitochondrial Ca2+ uniporter by exerting opposite effects on MCU activity Mol. Cell 2014 53 726 737 10.1016/j.molcel.2014.01.013 24560927 80. Perocchi F. Gohil V.M. Girgis H.S. Bao X.R. McCombs J.E. Palmer A.E. Mootha V.K. MICU1 encodes a mitochondrial EF hand protein required for Ca2+ uptake Nature 2010 467 291 296 10.1038/nature09358 20693986 81. Plovanich M. Bogorad R.L. Sancak Y. Kamer K.J. Strittmatter L. Li A.A. Girgis H.S. Kuchimanchi S. De Groot J. Speciner L. MICU2, a paralog of MICU1, resides within the mitochondrial uniporter complex to regulate calcium handling PLoS ONE 2013 8 e55785 10.1371/journal.pone.0055785 23409044 82. Xing Y. Wang M. Wang J. Nie Z. Wu G. Yang X. Shen Y. Dimerization of MICU proteins controls Ca2+ influx through the mitochondrial Ca2+ uniporter Cell Rep. 2019 26 1203 1212.e4 10.1016/j.celrep.2019.01.022 30699349 83. Patron M. Granatiero V. Espino J. Rizzuto R. De Stefani D. MICU3 is a tissue-specific enhancer of mitochondrial calcium uptake Cell Death Differ. 2019 26 179 195 10.1038/s41418-018-0113-8 29725115 84. Wang C. Jacewicz A. Delgado B.D. Baradaran R. Long S.B. Structures reveal gatekeeping of the mitochondrial Ca2+ uniporter by MICU1-MICU2 eLife 2020 9 e59991 10.7554/eLife.59991 32667285 85. Fan M. Zhang J. Tsai C.-W. Orlando B.J. Rodriguez M. Xu Y. Liao M. Tsao M.-F. Feng L. Structure and mechanism of the mitochondrial Ca2+ uniporter holocomplex Nature 2020 582 1 5 10.1038/s41586-020-2309-6 32494073 86. Schwaller B. Cytosolic Ca2+ buffers Cold Spring Harb. Perspect. Biol. 2010 2 a004051 10.1101/cshperspect.a004051 20943758 87. Pivovarova N.B. Andrews S.B. Calcium-dependent mitochondrial function and dysfunction in neurons FEBS J. 2010 277 3622 3636 10.1111/j.1742-4658.2010.07754.x 20659161 88. Jouaville L. Pinton P. Bastianutto C. Rutter G.A. Rizzuto R. Regulation of mitochondrial ATP synthesis by calcium: Evidence for a long-term metabolic priming Proc. Natl. Acad. Sci. USA 1999 96 13807 13812 10.1073/pnas.96.24.13807 10570154 89. Riascos D. de Leon D. Baker-Neigh A. Nicholas A. Yukhananov R. Bu J. Wu C.-K. Geula C. Age-related loss of calcium buffering and selective neuronal vulnerability in Alzheimer’s disease Acta Neuropathol. 2011 122 565 10.1007/s00401-011-0865-4 21874328 90. McLachlan D.R. Wong L. Bergeron C. Baimbridge K.G. Calmodulin and calbindin D28K in Alzheimer disease Alzheimer Dis. Assoc. Disord. 1987 1 171 179 10.1097/00002093-198701030-00009 2840101 91. Ahmadian S.S. Rezvanian A. Peterson M. Weintraub S. Bigio E.H. Mesulam M.M. Geula C. Loss of calbindin-D28K is associated with the full range of tangle pathology within basal forebrain cholinergic neurons in Alzheimer’s disease Neurobiol. Aging 2015 36 3163 3170 10.1016/j.neurobiolaging.2015.09.001 26417681 92. Ali F. Baringer S.L. Neal A. Cho E.Y. Kwan A.C. Parvalbumin-Positive Neuron Loss and Amyloid-β Deposits in the Frontal Cortex of Alzheimer’s Disease-Related Mice J. Alzheimer’s Dis. 2019 1 17 10.3233/JAD-181190 93. Lashuel H.A. Hartley D. Petre B.M. Walz T. Lansbuty P.T. Jr. Neurodegenerative disease: Amyloid pores from pathogenic mutations Nature 2002 418 291 10.1038/418291a 94. Shirwany N.A. Payette D. Xie J. Guo Q. The amyloid beta ion channel hypothesis of Alzheimer’s disease Neuropsychiatr. Dis. Treat. 2007 3 597 19300589 95. Ferreira I.L. Ferreiro E. Schmidt J. Cardoso J.M. Pereira C.M. Carvalho A.L. Oliveira C.R. Rego A.C. Aβ and NMDAR activation cause mitochondrial dysfunction involving ER calcium release Neurobiol. Aging 2015 36 680 692 10.1016/j.neurobiolaging.2014.09.006 25442114 96. Sanz-Blasco S. Valero R.A. Rodriguez-Crespo I. Villalobos C. Nunez L. Mitochondrial Ca2+ overload underlies Aβ oligomers neurotoxicity providing an unexpected mechanism of neuroprotection by NSAIDs PLoS ONE 2008 3 e2718 10.1371/journal.pone.0002718 18648507 97. Boyman L. Williams G.S. Khananshvili D. Sekler I. Lederer W.J. NCLX: The mitochondrial sodium calcium exchanger J. Mol. Cell. Cardiol. 2013 59 205 213 10.1016/j.yjmcc.2013.03.012 23538132 98. Kim G.H. Kim J.E. Rhie S.J. Yoon S. The role of oxidative stress in neurodegenerative diseases Exp. Neurobiol. 2015 24 325 10.5607/en.2015.24.4.325 26713080 99. Calvo-Rodriguez M. Hou S.H. Synder A.C. Kharitonova E.K. Russ A.N. Das S. Fan Z. Muzikansky A. Gracia-Alloza M. Serrano-Pozo A. Increased mitochondrial calcium levels associated with neuronal death in a mouse model of Alzheimer’s disease Nat. Commun. 2020 11 1 17 10.1038/s41467-020-16074-2 31911652 100. Panel M. Ghaleh B. Morin D. Mitochondria and aging: A role for the mitochondrial transition pore? Aging Cell 2018 17 e12793 10.1111/acel.12793 29888494 101. Brookes P.S. Yoon Y. Robotham J.L. Anders M.W. Sheu S.-S. Calcium, ATP, and ROS: A mitochondrial love-hate triangle Am. J. Physiol. Cell Physiol. 2004 287 C817 C833 10.1152/ajpcell.00139.2004 15355853 102. Zorov D.B. Juhaszova M. Sollott S.J. Mitochondrial reactive oxygen species (ROS) and ROS-induced ROS release Physiol. Rev. 2014 94 909 950 10.1152/physrev.00026.2013 24987008 103. Balaban R. Nemoto S. Finkel T. Mitochondria, oxidants, and aging Cell 2005 120 483 495 10.1016/j.cell.2005.02.001 15734681 104. Birnbaum J.H. Wanner D. Geitl A.F. Saake A. Kundig T. Hock C. Nitsch R.M. Tackenberg C. Oxidative stress and altered mitochondrial protein expression in the absence of amyloid-β and tau pathology in iPSC-derived neurons from sporadic Alzheimer’s disease patients Stem Cell Res. 2018 27 121 130 10.1016/j.scr.2018.01.019 29414602 105. Pohanka M. Alzheimer s disease and oxidative stress: A review Curr. Med. Chem. 2014 21 356 364 10.2174/09298673113206660258 24059239 106. Harrison F.E. Allard J. Bixier R. Usoh C. Li L. May J.M. McDonald M.P. Antioxidants and cognitive training interact to affect oxidative stress and memory in APP/PSEN1 mice Nutr. Neurosci. 2009 12 203 218 10.1179/147683009X423364 19761651 107. Tönnies E. Trushina E. Oxidative stress, synaptic dysfunction, and Alzheimer’s disease J. Alzheimer’s Dis. 2017 57 1105 1121 10.3233/JAD-161088 28059794 108. de Leeuw F.A. Peeters C.F.W. Kester M.I. Harms A.C. Struys E.A. Hankemeier T. van Vlijmen H. van der Lee S. van Dujin C.M. Scheltens P. Blood-based metabolic signatures in Alzheimer’s disease Alzheimer’s Dement. Diagn. Assess. Dis. Monit. 2017 8 196 207 10.1016/j.dadm.2017.07.006 109. Su B. Wang X. Nunomura A. Moreira P.I. Lee H.G. Perry G. Smith M.A. Zhu X. Oxidative stress signaling in Alzheimer’s disease Curr. Alzheimer Res. 2008 5 525 532 10.2174/156720508786898451 19075578 110. Liu Q. Smith M.A. Avila J. DeBernardis J. Kansal M. Takeda A. Zhu X. Nunomura A. Honda K. Moreira P. Alzheimer-specific epitopes of tau represent lipid peroxidation-induced conformations Free Radic. Biol. Med. 2005 38 746 754 10.1016/j.freeradbiomed.2004.11.005 15721985 111. LaFerla F.M. Green K.N. Animal models of Alzheimer disease Cold Spring Harb. Perspect. Med. 2012 2 a006320 10.1101/cshperspect.a006320 23002015 112. Wang X. Michaelis E. Selective neuronal vulnerability to oxidative stress in the brain Front. Aging Neurosci. 2010 2 1 13 10.3389/fnagi.2010.00012 20552041 113. Jadiya P. Kolmetzky D.W. Tomar D. Di Meco A. Lombardi A.A. Lambert J.P. Luongo T.S. Ludtmann M.H. Pratico D. Elord J.W. Impaired mitochondrial calcium efflux contributes to disease progression in models of Alzheimer’s disease Nat. Commun. 2019 10 3885 10.1038/s41467-019-11813-6 31467276 114. Tamagno E. Bardini P. Guglielmotto M. Danni O. Tabaton M. The various aggregation states of β-amyloid 1–42 mediate different effects on oxidative stress, neurodegeneration, and BACE-1 expression Free Radic. Biol. Med. 2006 41 202 212 10.1016/j.freeradbiomed.2006.01.021 16814100 115. Butterfield D.A. Castegna A. Lauderback C.M. Drake J. Evidence that amyloid beta-peptide-induced lipid peroxidation and its sequelae in Alzheimer’s disease brain contribute to neuronal death Neurobiol. Aging 2002 23 655 664 10.1016/S0197-4580(01)00340-2 12392766 116. Alberdi E. Sanchez-Gomez M.V. Cavaliere F. Perez-Samartin A. Zugaza J.L. Trullas R. Domercq M. Matute C. Amyloid β oligomers induce Ca2+ dysregulation and neuronal death through activation of ionotropic glutamate receptors Cell Calcium 2010 47 264 272 10.1016/j.ceca.2009.12.010 20061018 117. Calvo-Rodriguez M. Bacskai B.J. High mitochondrial calcium levels precede neuronal death in vivo in Alzheimer’s disease Cell Stress 2020 4 187 10.15698/cst2020.07.226 32656500 118. Oksanen M. Peterson A.J. Naumenko N. Puttonen K. Lehtonen S. Gubert Olive M. Shakrizyanova A. Leskela S. Sarajarvi T. Viitanen M. PSEN1 mutant iPSC-derived model reveals severe astrocyte pathology in Alzheimer’s disease Stem Cell Rep. 2017 9 1885 1897 10.1016/j.stemcr.2017.10.016 119. Sarasija S. Norman K.R. Role of presenilin in mitochondrial oxidative stress and neurodegeneration in Caenorhabditis elegans Antioxidants 2018 7 111 10.3390/antiox7090111 120. Kaushik S. Cuervo A.M. Proteostasis and aging Nat. Med. 2015 21 1406 1415 10.1038/nm.4001 26646497 121. Labbadia J. Morimoto R.I. The biology of proteostasis in aging and disease Annu. Rev. Biochem. 2015 84 435 464 10.1146/annurev-biochem-060614-033955 25784053 122. Di Domenico F. Head E. Butterfield D.A. Perluigi M. Oxidative stress and proteostasis network: Culprit and casualty of Alzheimer’s-like neurodegeneration Adv. Geriatr. 2014 2014 527518 10.1155/2014/527518 123. Ashkavand Z. Sarasija S. Ryan K.C. Laboy J.T. Norman K.R. Corrupted ER-mitochondrial calcium homeostasis promotes the collapse of proteostasis Aging Cell 2020 19 e13065 10.1111/acel.13065 31714672 124. Fedeli C. Filadi R. Rossi A. Mammucari C. Pizzo P. PSEN2 (presenilin 2) mutants linked to familial Alzheimer disease impair autophagy by altering Ca2+ homeostasis Autophagy 2019 15 2044 2062 10.1080/15548627.2019.1596489 30892128 125. Bhaskar K. Miller M. Chludzinski A. Herrup K. Zagorski M. Lamb B.T. The PI3K-Akt-mTOR pathway regulates Aβ oligomer induced neuronal cell cycle events Mol. Neurodegener. 2009 4 1 18 10.1186/1750-1326-4-14 19126211 126. Caccamo A. Maldonado M.A. Majumder S. Medina D.X. Holbein W. Magri A. Oddo S. Naturally secreted amyloid-β increases mammalian target of rapamycin (mTOR) activity via a PRAS40-mediated mechanism J. Biol. Chem. 2011 286 8924 8932 10.1074/jbc.M110.180638 21266573 127. Kim J. Guan K.-L. mTOR as a central hub of nutrient signalling and cell growth Nat. Cell Biol. 2019 21 63 71 10.1038/s41556-018-0205-1 30602761 128. Liu G.Y. Sabatini D.M. Author Correction: mTOR at the nexus of nutrition, growth, ageing and disease Nat. Rev. Mol. Cell Biol. 2020 21 246 10.1038/s41580-020-0219-y 32005970 129. Tramutola A. Lanzillota C. Perluigi M. Butterfield D.A. Oxidative stress, protein modification and Alzheimer disease Brain Res. Bull. 2017 133 88 96 10.1016/j.brainresbull.2016.06.005 27316747 130. Hirokawa N. Niwa S. Tanaka Y. Mwolecular motors in neurons: Transport mechanisms and roles in brain function, development, and disease Neuron 2010 68 610 638 10.1016/j.neuron.2010.09.039 21092854 131. Chan D.C. Mitochondrial fusion and fission in mammals Annu. Rev. Cell Dev. Biol. 2006 22 79 99 10.1146/annurev.cellbio.22.010305.104638 16704336 132. MacAskill A.F. Rinholm J.E. Twelevetrees A.E. Arancibia-Carcamo I.L. Muir J. Fransson A. Aspenstrom P. Atwell D. Kittler J.T. Miro1 is a calcium sensor for glutamate receptor-dependent localization of mitochondria at synapses Neuron 2009 61 541 555 10.1016/j.neuron.2009.01.030 19249275 133. Kerr J.S. Adriaanese B.A. Greig N.H. Mattson M.P. Cader M.Z. Bohr V.A. Fang E.F. Mitophagy and Alzheimer’s disease: Cellular and molecular mechanisms Trends Neurosci. 2017 40 151 166 10.1016/j.tins.2017.01.002 28190529 134. Kirichok Y. Krapivinsky G. Clapham D.E. The mitochondrial calcium uniporter is a highly selective ion channel Nature 2004 427 360 364 10.1038/nature02246 14737170 135. Schon E.A. Przedborski S. Mitochondria: The next (neurode)generation Neuron 2011 70 1033 1053 10.1016/j.neuron.2011.06.003 21689593 136. Lewis T.L. Kwon S.-K. Lee A. Shaw R. Polleux F. MFF-dependent mitochondrial fission regulates presynaptic release and axon branching by limiting axonal mitochondria size Nat. Commun. 2018 9 1 15 10.1038/s41467-018-07416-2 29317637 137. Saotome M. Safiliulina D. Szabadkai G. Das S. Fransson A. Aspenstrom P. Rizzuto R. Hajnoczky G. Bidirectional Ca2+ -dependent control of mitochondrial dynamics by the Miro GTPase Proc. Natl. Acad. Sci. USA 2008 105 20728 20733 10.1073/pnas.0808953105 19098100 138. Fransson Å. Ruusala A. Aspenström P. The atypical Rho GTPases Miro-1 and Miro-2 have essential roles in mitochondrial trafficking Biochem. Biophys. Res. Commun. 2006 344 500 510 10.1016/j.bbrc.2006.03.163 16630562 139. Fransson Å. Ruusala A. Aspenström P. Atypical Rho GTPases have roles in mitochondrial homeostasis and apoptosis J. Biol. Chem. 2003 278 6495 6502 10.1074/jbc.M208609200 12482879 140. Brickley K. Stephenson F.A. Trafficking kinesin protein (TRAK)-mediated transport of mitochondria in axons of hippocampal neurons J. Biol. Chem. 2011 286 18079 18092 10.1074/jbc.M111.236018 21454691 141. Glater E.E. Megeath L.J. Stowers R.S. Schawaz T.L. Axonal transport of mitochondria requires milton to recruit kinesin heavy chain and is light chain independent J. Cell Biol. 2006 173 545 557 10.1083/jcb.200601067 16717129 142. Wang X. Schwarz T.L. The mechanism of Ca2+ -dependent regulation of kinesin-mediated mitochondrial motility Cell 2009 136 163 174 10.1016/j.cell.2008.11.046 19135897 143. Chang K.T. RNiescier F. Min K.-T. Mitochondrial matrix Ca2+ as an intrinsic signal regulating mitochondrial motility in axons Proc. Natl. Acad. Sci. USA 2011 108 15456 15461 10.1073/pnas.1106862108 21876166 144. Lee S. Lee K.-S. Huh S. Liu S. Lee D.-Y. Hong S.H. Yu K. Lu B. Polo kinase phosphorylates miro to control ER-mitochondria contact sites and mitochondrial Ca2+ homeostasis in neural stem cell development Dev. Cell 2016 37 174 189 10.1016/j.devcel.2016.03.023 27093086 145. Modi S. Lopez-Domenech G. Halff E.F. Covill-Cooke C. Ivankovic D. Melandri D. Arancibia- Carcamo I.L. Burden J.J. Lowe A.R. Kittler J.T. Miro clusters regulate ER-mitochondria contact sites and link cristae organization to the mitochondrial transport machinery Nat. Commun. 2019 10 1 15 10.1038/s41467-019-12382-4 30602773 146. Lee K.-S. Huh S. Lee S. Wu Z. Kim A.-K. Kang H.-Y. Lu B. Altered ER–mitochondria contact impacts mitochondria calcium homeostasis and contributes to neurodegeneration in vivo in disease models Proc. Natl. Acad. Sci. USA 2018 115 E8844 E8853 10.1073/pnas.1721136115 30185553 147. Nemani N. Carvalho E. Tomar D. Dong Z. Ketschek A. Breves S.L. Jana F. Worth A.M. Heffler J. Palaniappan P. MIRO-1 determines mitochondrial shape transition upon GPCR activation and Ca2+ stress Cell Rep. 2018 23 1005 1019 10.1016/j.celrep.2018.03.098 29694881 148. Fu H. Zhou H. Yu X. Xu J. Zhou J. Meng X. Zhao J. Zhou Y. Chisholm A.D. XU S. Wounding triggers MIRO-1 dependent mitochondrial fragmentation that accelerates epidermal wound closure through oxidative signaling Nat. Commun. 2020 11 1 13 10.1038/s41467-020-14885-x 31911652 149. Hsieh C.-H. Shaltouki A. Gonzalez A.E. Battencourt da Cruz A. Burbulla L.F. St Lawrence E. Schule B. Krainc D. Palmer T.D. Wang X. Functional impairment in miro degradation and mitophagy is a shared feature in familial and sporadic Parkinson’s disease Cell Stem Cell 2016 19 709 724 10.1016/j.stem.2016.08.002 27618216 150. Giorgi C. Marchi S. Pinton P. Publisher Correction: The machineries, regulation and cellular functions of mitochondrial calcium. Nature reviews Mol. Cell Biol. 2018 19 746 10.1038/s41580-018-0066-2 151. Granatiero V. de Stefani D. Rizzuto R. Mitochondrial calcium handling in physiology and disease Mitochondrial Dynamics in Cardiovascular Medicine Springer Berlin/Heidelberg, Germany 2017 25 47 152. Pan X. Liu J. Nguyen T. Liu C. Sun J. Teng Y. Fergusson M.M. Rovira I.I. Allen M. Springer D.A. The physiological role of mitochondrial calcium revealed by mice lacking the mitochondrial calcium uniporter Nat. Cell Biol. 2013 15 1464 1472 10.1038/ncb2868 24212091 153. Liao Y. Dong Y. Cheng J. The function of the mitochondrial calcium uniporter in neurodegenerative disorders Int. J. Mol. Sci. 2017 18 248 10.3390/ijms18020248 154. Ham S.J. Lee D. Yoo H. Jun K. Shin H. Chung J. Decision between mitophagy and apoptosis by Parkin via VDAC1 ubiquitination Proc. Natl. Acad. Sci. USA 2020 117 4281 4291 10.1073/pnas.1909814117 32047033 155. Ludtmann M.H.R. Kostik M. Horne A. Gandhi S. Sekler I. Abramov A.Y. LRRK2 deficiency induced mitochondrial Ca 2+ efflux inhibition can be rescued by Na+ /Ca2+ /Li+ exchanger upregulation Cell Death Dis. 2019 10 1 10 10.1038/s41419-019-1469-5 30890692 156. Verma M. Callio J. Otero P.A. Sekler I. Wills Z.P. Chu C.T. Mitochondrial calcium dysregulation contributes to dendrite degeneration mediated by PD/LBD-associated LRRK2 mutants J. Neurosci. 2017 37 11151 11165 10.1523/JNEUROSCI.3791-16.2017 29038245 157. Soman S.K. Bazala M. Keatinge M. Bandmann O. Kuznicki J. Restriction of mitochondrial calcium overload by mcu inactivation renders a neuroprotective effect in zebrafish models of Parkinson’s disease Biol. Open 2019 8 bio044347 10.1242/bio.044347 31548178 158. Woods J.J. Wilson J.J. Inhibitors of the mitochondrial calcium uniporter for the treatment of disease Curr. Opin. Chem. Biol. 2020 55 9 18 10.1016/j.cbpa.2019.11.006 31869674 159. Dey K. Bazala M.A. Kuznicki J. Targeting mitochondrial calcium pathways as a potential treatment against Parkinson’s disease Cell Calcium 2020 16 102216 10.1016/j.ceca.2020.102216 160. Venugopal A. Iyer M. Balasubramanian V. Vellingiri B. Mitochondrial calcium uniporter as a potential therapeutic strategy for Alzheimer’s disease Acta Neuropsychiatr. 2020 32 65 71 10.1017/neu.2019.39 31556366