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Perturbed neuronal Ca2+ homeostasis is implicated in age-related cognitive impairment and Alzheimer’s disease (AD). With advancing age neurons encounter increased oxidative stress and impaired energy metabolism, which compromise the function of proteins that control membrane excitability and subcellular Ca2+ dynamics. Toxic forms of amyloid β-peptide (Aβ) may induce Ca2+ influx into neurons by inducing membrane-associated oxidative stress or by forming an oligomeric pore in the membrane, thereby rendering neurons vulnerable to excitotoxicity and apoptosis. AD-causing mutations in the β-amyloid precursor protein and presenilins may compromise normal of these proteins in the plasma membrane and endoplasmic reticulum, respectively. Emerging knowledge of the actions of Ca2+ upstream and downstream of Aβ provide opportunities to develop novel preventative and therapeutic interventions for AD.
Neurons use Ca2+ signals to control membrane excitability, to trigger release of a neurotransmitter, to mediate activity-dependent changes in gene expression and modulate neuronal growth, differentiation and transition to apoptosis1. Neuronal Ca2+ signaling involves an intricate interplay between Ca2+ influx across plasma membrane through voltage-gated Ca2+ channels, NMDA receptors and TRP (transient receptor potential) channels, and Ca2+ release from intracellular Ca2+ stores via inositol triphosphate receptor (IP3R) and ryanodine receptor (RyR) channels in the endoplasmic reticulum (ER). Intracellular Ca2+ release via IP3R is triggered by second messenger IP3 which is produced following activation of metabotropic receptors coupled to phospholipase C. Neuronal RyR function primarily as Ca2+-activated Ca2+ channels which further amplify Ca2+ signals originating from other sources. Mitochondria also play an important role in shaping neuronal Ca2+ signaling by utilizing potent mitochondrial Ca2+ uptake mechanisms. Ca2+ uptake into mitochondria plays an important role in neuronal physiology by stimulating mitochondrial metabolism and increasing mitochondrial energy production. Excessive Ca2+ uptake into mitochondria can lead to opening of a permeability transition pore (PTP) and apoptosis2. Owing to its importance for neuronal function, Ca2+ signaling in neurons is tightly compartmentalized and regulated within signaling microdomains which involve, for example, functional coupling between voltage-gated Ca2+ channels and intracellular Ca2+ release channels, or between ER Ca2+ release and Ca2+ uptake into mitochondria.
The major risk factor for Alzheimer’s disease (AD) is advancing age; in the most common sporadic form of AD the individuals first manifest symptoms when they are in their 7th or 8th decades of life. But even those who inherit a disease-causing mutation in the β-amyloid precursor protein (APP) or one of the presenilins (PS1 and PS2) remain asymptomatic into their fourth or fifth decades3. Age-related alterations in specific Ca2+-regulating systems in brain cells have been reported including: elevated intracellular Ca2+ levels; enhanced Ca2+ influx through voltage-dependent Ca2+ channels; impaired ability of mitochondria to buffer or cycle Ca2+; perturbed Ca2+ regulation in ryanodine and IP3-sensitive Ca2+ stores. Gene array and proteomic analyses suggest dysregulation of the expression of an array of Ca2+-handling systems during aging4–7. Many of the alterations in Ca2+-handling described in normal aging can be reproduced by subjecting neurons to oxidative and metabolic stress in culture or in vivo, suggesting important contributions of these fundamental aging processes to the dysregulation of neuronal Ca2+ regulation in AD described below. Moreover, studies of brain tissue samples obtained from brains of AD patients and animal models of AD have revealed significant alterations in levels of proteins and genes directly involved in neuronal Ca2+ signaling8–10. Many of the latest advances in understanding the roles of perturbed cellular Ca2+ handling in AD pathogenesis are described in the remainder of this article.
Amyloid plaques, a histological hallmark of AD, are comprised of extracellular aggregates of the amyloid β-peptide (Aβ) a 40–42 amino acid peptide generated by successive enzymatic cleavages of APP by β- and γ-secretases (Fig. 1). Aβ is believed to be a pivotal mediator of neuronal degeneration and impaired cognitive function in AD3,11. Adverse effects of Aβ on synaptic function and neuronal survival are mediated primarily by soluble protein oligomers12. Aβ interaction with the plasma membrane results in elevated intracellular (cytoplasmic) Ca2+ concentrations ([Ca2+]i) and increased vulnerability of the neurons to excitotoxicity13. Oligomeric forms of Aβ42 cause Ca2+-mediated toxicity in cultured cells14. Degenerative changes occur in neurites associated with Aβ deposits in APP mutant mice, suggesting the involvement of Ca2+-mediated Aβ neurotoxicity in vivo15. In addition to increasing the production of Aβ, amyloidogenic processing of APP may perturb neuronal Ca2+ homeostasis by decreasing the production of a secreted form of APP (sAPPα) that activates K+ channels16, and by generating an APP intracellular domain that affects ER Ca2+ release by regulating the expression of genes involved in Ca2+ homeostasis (Fig. 1)17.
One mechanism by which Aβ may cause Ca2+ influx is by inserting into the plasma membrane and forming ion-conducting pores18 (Fig. 1). Neurotoxic forms of Aβ are oligomers that share structural and functional homology with pore-forming bacterial toxins and the cytotoxic lymphocyte protein perforin19. Interestingly, the ability of Aβ to associate with membranes and form channels is enhanced by exposure of phosphatidylserine on the cell surface20. Because cell surface exposure of phosphatidylserine is usually indicative of apoptotic or energy-deprived cells, it is possible that age-related mitochondrial impairments may increase surface phosphatydylserine levels in affected neurons and thereby facilitate Aβ-mediated pore formation, Ca2+ influx and cell death (Fig. 1); indeed, neurons with reduced cytosolic ATP levels and elevated surface phosphatidylserine are particulary vulnerable to Aβ toxicity21. The surface exposure of phosphatidylserine may also result from activation of a Ca2+-sensitive phospholipid scrambalase 1 (PLSCR1) which mediates a rapid trans-bilayer reorganization of plasma membrane phospholipids22 (Fig. 1).
A different mechanism by which Aβ perturbs neuronal Ca2+ homeostasis is by inducing membrane lipid peroxidation11. During peptide oligomer formation Aβ generates hydrogen peroxide, a process enhanced by iron (Fe2+) and copper (Cu+)23,24. Hydrogen peroxide is then converted to hydroxyl radical which initiates lipid peroxidation resulting in the generation of toxic lipid aldehydes such a 4-hydroxynonenal which impair the function of ion-motive ATPases, and glutamate and glucose transporters resulting in Ca2+ overload, synaptic dysfunction, neuronal degeneration and cognitive impairment11,25. Particularly striking, is the ability of Aβ to increase the vulnerability of neurons to excitotoxicity mediated by the N-methyl-D-aspartate (NMDA) receptor11,13. Because excessive and sustained Ca2+ elevations induce free radical production (by altering mitochondrial oxidative phosphorylation and activating oxygenases) it is likely that perturbed Ca2+ homeostasis contributes to the increased oxidative stress in neurons in AD, resulting in a self-amplifying cascade of free radical- and Ca2+-mediated degenerative processes (Fig. 1). Lesser amounts of Aβ may also be toxic to neurons. For example, exposure of rat organotypic hippocampal slice cultures to picomolar concentrations of Aβ oligomers caused the loss of dendritic spines and decreased numbers of electrophysiologically active synapses; the spine loss was reversible and required NMDA receptor activity26. Aβ oligomers cause an increase in NMDA receptor activity, which may require direct association between Aβ oligomers and NR1 subunit of the NMDAR. On the other hand, Aβ oligomers may suppress activity of presynaptic P/Q-type voltage-gated Ca2+ channels27. Aβ also blocks the response of α7-containing nicotinic acetylcholine receptors (nAChRs) in hippocampal neurons28 and directly evokes sustained nAChR-mediated increases in presynaptic Ca2+ levels29 suggesting a mechanism for impairment of cholinergic signaling in AD.
Presenilins (PS1 and the structurally and functionally related PS2) are integral membrane proteins. The holoprotein form of presenilins is located in the ER. Both PS1 and PS2 holoproteins undergo endoproteolysis in the cytosolic loop between the 6th and 7th transmembrane domains, resulting in the generation of amino-terminal and carboxy-terminal fragments, which remain associated with each other. Cleaved presenilins assemble with nicastrin, Aph-1 and Pen-2, exit the ER and translocate into the Golgi apparatus and eventually to plasma membrane. A mature complex of cleaved presenilins, nicastrin, Aph-1 and Pen-2 possesses aspartyl protease activity and functions as the γ-secretase enzyme that cleaves APP to generate Aβ3,11. Many mutations in presenilins that cause familial (dominantly inherited) AD (FAD) increase the production of the long aggregation-prone form of Aβ (Aβ42) or reduce the production of a short soluble form Aβ40, and therefore one way in which presenilin mutations may perturb neuronal Ca2+ homeostasis is by elevating Aβ42:Aβ40 ratio and activating the Aβ oligomer-mediated mechanisms described above.
Additional roles for presenilins in modulating Ca2+ homeostasis are suggested by data linking two other presenilin substrate cleavage products, the AICD and the Notch intracellular domain (NICD), to Ca2+-mediated neuroplasticity and cell death. AICD may translocate to the nucleus and therein regulate the expression of genes encoding proteins involved in Ca2+ homeostasis17. Notch, a membrane receptor activated by cell surface-associated ligands such as Jagged and Delta, plays fundamental roles in regulating the proliferation and differentiation of neural progenitor cells in the developing and adult brain30. Upon ligand binding, γ-secretase cleaves Notch to release the NICD which translocates to the nucleus where it regulates gene transcription. Recent findings suggest potential roles for Notch and NICD in synaptic plasticity, learning and memory and Ca2+-mediated cell death31.
A γ-secretase-independent connection between presenilins and Ca2+ signaling was initially suggested in Ca2+ imaging experiments with fibroblasts from FAD patients containing PS1-A246E mutation32. It was then shown that cultured neural cells expressing AD PS1 mutations exhibit increased amounts of Ca2+ released from the ER when exposed to ligands that stimulate IP3 production or activation of RyR33,34. Similar results were obtained in Ca2+ imaging experiments with Xenopus oocytes injected with cRNA encoding PS1-M146V and PS2-N141I FAD mutants35, in experiments with synaptosomes and cortical neurons from PS1-M146V mutant mice36,37, and in hippocampal neurons from PS2-N141I transgenic mice38. In vitro and in vivo studies demonstrated that exaggerated ER Ca2+ signaling resulting from FAD mutations in presenilins leads to sensitization of PS-FAD neurons to Aβ and excitotoxic cell death via a Ca2+-dependent mehanism involving excessive Ca2+ release from the ER36–39.
Biochemical and functional interactions have been uncovered between presenilins and several ER Ca2+-regulating proteins including ryanodine receptors40, sorcin41, the myristoylated calcium-binding protein calmyrin42 and calsenilin43. Presenilins may also modulate SERCA Ca2+ pump activity44. Presenilin-2 has been reported to associate with IP3R and to enhance IP3R activity45. Specific effects of FAD mutants PS1-M146V and PS2-N141I on sensitivity of IP3R1 to activation by IP3 have been recently discovered in patch-clamp experiments46. A significant increase in ryanodine receptor expression levels has been reported in brains from PS1 mutant mice47, an alteration that increases as the mice age, providing a potential link between AD pathogenesis and aging.
While the studies described above suggest that FAD mutations in presenilins act by altering a normal function of other Ca2+-regulating proteins, recent findings indicated that presenilins themselves may play a direct role in Ca2+ signaling. It is well established known that the ER membrane is “leaky” for Ca2+, but the exact identity of the putative “Ca2+ leak channel” was previously unknown. Recent results suggest that presenilins function as ER Ca2+ leak channels in cells, and that a balance between SERCA Ca2+ pump activity and presenilin-mediated passive Ca2+ leak determines the steady-state resting ER Ca2+ levels in cells48. The ER Ca2+ leak function of presenilins does not involve γ-secretase activity and is not supported by a cleaved form of presenilins; instead many FAD mutations in presenilins result in “loss of function” for ER Ca2+ leak activity48,49 resulting in excessive Ca2+ accumulation in the ER (Fig. 1). Although most tested FAD mutants in presenilinscompromiseed its ER Ca2+ leak function, the PS1-ΔE9 mutant was unique and appeared to act as a “gain of function” leading to “superleaky” channels48. The “gain of Ca2+ leak function” phenotype of PS1-ΔE9 mutant is consistent with an earlier observation of elevated basal Ca2+ levels in neuronal cells transfected with PS1-ΔE9 expression construct50 Thus, the cells expressing PS1-ΔE9 mutants are expected to be exposed to constitutively elevated cytosolic Ca2+ levels and partially depleted ER. This is in contrast to cells expressing “loss of ER leak function” PS FAD mutants, which are expected to have normal steady-state cytosolic Ca2+ levels and overloaded ER. Interestingly, the PS1-ΔE9 mutation is associated with a unique cotton wool plaques and spastic paraparesis clinical phenotype (CWP/SP), which is not observed for most other FAD PS1 mutations51. It will be very important to determine if other FAD mutations in PS1 associated with CWP/SP phenotype may also be associated with “gain of function” for the ER Ca2+ leak activity. If such a correlation is established, it would support a causal connection between ER Ca2+ dyshomeostasis and Aβ pathology in AD.
Neurofibrillary tangles, the most overt manifestation of cytoskeletal abnormalities in AD, consist of intracellular fibrillar aggregates of hyperphosphorylated forms of the microtubule-associated protein tau11. Tau is normally located in axons where it maintains microtubules in a polymerized state, but in AD tau dissociates from microtubules resulting in microtubule depolymerization and the accumulation of tau in the cell body. Studies of AD patient brain tissue samples suggest an association between elevated [Ca2+]i and neurofibrillary pathology. For example, neurons prone to neurofibrillary tangle formation are enriched in type II calcium/calmodulin-dependent protein kinase52, and calpains (Ca2+-dependent proteases that cleave cytoskeletal proteins) are elevated in vulnerable neuronal populations early in the disease process53. Overactivation of glutamate receptors in hippocampal neurons can cause Ca2+-mediated changes in tau and microtubules similar to those seen in neurofibrillary tangles72 suggesting a possible cause-effect relationship between aberrant increases in [Ca2+]i and tangle formation (Fig. 2). In addition, Ca2+ can cause AD-like tau phosphorylation and intracellular Aβ accumulation in neurons54. Conversely, tau mutations that cause tangle formation in frontotemporal lobe dementia alter the function of voltage-dependent Ca2+ channels in a manner that increases Ca2+ influx55 and may contribute to the cell death process in this disease.
The placement of Aβ at the apex of the amyloid cascade hypothesis belies the fact there must be changes that occur during aging and AD that result in increased production and aggregation of Aβ. Evidence suggests that Ca2+ may be such an upstream factor. Environmental factors that inhibit amyloidogenesis (caloric restriction, cognitive stimulation and antioxidants) stabilize neuronal Ca2+ homeostasis, whereas factors that enhance amyloidogenesis disrupt Ca2+ homeostasis. In addition to these kinds of circumstantial evidence, direct evidence that Ca2+ influences APP processing has been reported. Exposure of cultured neurons to Ca2+ ionophores increases their production of Aβ56, as do conditions such as ischemia that cause sustained elevations of [Ca2+]i57. On the other hand, physiological Ca2+ transients (as occur during LTP, for example) increase α-secretase cleavage of APP and may thereby decrease Aβ production58,59.
Studies of patients with mild cognitive impairment and AD suggest that synaptic dysfunction and degeneration may occur relatively early in the disease process, and studies of AD mouse models uniformly support this tenet11. Synaptic terminals are particularly vulnerable to Ca2+-mediated degeneration because they experience repeated bouts of Ca2+ influx and have unusually high energy requirements to support their ion-homeostatic and signaling systems. APP is actively transported to presynaptic terminals and considerable evidence suggests that Aβ is produced and accumulated primarily in synaptic regions60. Aβ can directly disrupt Ca2+ homeostasis in synaptic terminals by causing membrane-associated oxidative stress11. Consistent with a major role for Aβ in synaptic damage in AD are data showing loss of dendritic spines in dendrites associated with Aβ deposits in APP mutant mice61. Aβ also causes down-regulation of expression of calcineurin, a Ca2+-activated phosphatase known to play fundamental roles in synaptic plasticity62. Aβ oligomers caused a rapid decrease in membrane expression of NMDA and EphB2 receptors, followed by abnormal dendritic spine morphology and degeneration of spines63. The latter effects of Aβ are prevented by treatment with an NMDA receptor antagonist suggesting a major role for Ca2+ influx in the dendritic dystrophy. Moreover, Aβ immunotherapy prevented synaptic dysfunction and restores cognitive function in a mouse model of AD64.
Electrophysiological analyses of synaptic activity in hippocampal slices from APP and PS1 mutant mice have revealed abnormalities in several aspects of Ca2+-mediated synaptic function. APP mutant mice exhibit abnormal excitatory neuronal activity and compensatory remodeling of inhibitory circuits in the hippocampus65. Expression of mutant PS1 in cultured hippocampal neurons results in a significant depression of the amplitude of evoked AMPA and NMDA receptor-mediated synaptic currents, and a lower frequency of spontaneous miniature synaptic currents66. Aβ impairs spike-timing-dependent synaptic potentiation at excitatory synapses on neocortical layer 2/3 cortical pyramidal cells in APP mutant mice, which was associated with a decrease in AMPA but not NMDA receptor-mediated currents67. Aβ may also perturb Ca2+ handling in neural stem cells resulting in impaired hippocampal neurogenesis and a compromised ability to form and integrate new neurons from endogenous stem cells68.
PS1 mutations have a local adverse effect on synaptic Ca2+ regulation that may contribute to mitochondrial dysfunction and synaptic degeneration in AD. Thus, synaptosomes from PS1 mutant transgenic mice which exhibit enhanced elevations of cytoplasmic Ca2+ levels following exposure to depolarizing agents, Aβ, and a mitochondrial toxin compared with synaptosomes from nontransgenic mice and mice overexpressing wild-type PS139. Two-photon imaging studies revealed a 10-fold enhancement in RyR-mediated Ca2+ release in spines of PS1-M146V mutant-expressing mice, indicating a major alteration in synaptic ER Ca2+ handling in this AD model (Beth Stutzmann, personal communication). Agents that buffer cytoplasmic Ca2+ or that prevent Ca2+ release from the ER protected synaptosomes against the adverse effect of PS1 mutations.
Polymorphisms in the apolipoprotein E (ApoE) gene affect one’s risk for late onset AD. Of the three isoforms (E2, E3 and E4), E3 is the most common and E2 the least common. The three isoforms differ at residues 112 and 158; E3 has Cys-112 and Arg-158, whereas E4 has arginine in both positions and E2 has cysteine in both positions. Inheritance of the allele for E4 isoform is associated with increased risk and earlier age of onset of the sporadic AD whereas E2 reduces risk69. Several studies indicate a potential link between ApoE and synaptic Ca2+ signaling. As mentioned above, ApoE2, but not ApoE4, can inhibit Aβ association with phosphatydylserine in the membrane, providing a potential explanation for protective effects of ApoE2 in AD. A different line of research demonstrated that application of low levels of ApoE4 to cultured neurons induces NMDAR-mediated Ca2+ influx and causes neuronal toxicity70. In addition, it was recently demonstrated that reelin can activate neuronal NMDAR via a src-family tyrosine kinase (SFK)-mediated mechanism and that reelin association with ApoE receptor 2 (ApoER2) was necessary for activation of NMDAR71. Marked changes in reelin expression levels were observed in brains from AD patients and AD mouse models72,73 further implicating a potential importance of reelin signaling pathway in AD.
Differential production and deposition of Aβ and the resulting disruption of Ca2+ homeostasis is one likely determinant of selective neuronal vulnerability because neurons in brain regions with high Aβ loads (entorhinal cortex, hippocampus, inferior parietal cortex) degenerate, whereas neurons in regions with little or no Aβ accumulation (cerebellum, striatum, motor cortex) typically do not4. However, it is clear that there are additional factors at work because within a vulnerable brain region (in the presence of similar amounts of Aβ) some neurons degenerate in AD, whereas others do not. Populations of neurons that degenerate in AD typically express high levels of NMDA receptors and have relatively low levels of some Ca2+-binding proteins compared to resistant neurons2. Although hippocampal dentate and CA1 neurons each express NMDA receptors, the dentate neurons express high amounts of calbindin, whereas CA1 neurons do not. Experimental findings suggest that calbindin buffers Ca2+ loads and protects neurons against excitotoxicity; in the hippocampus calbindin-positive neurons are relatively preserved in AD patients with moderate plaque and tangle content, but in severe cases the calbindin-positive pyramidal cells are also lost, suggesting the possibility that calbindin protects neurons in the early stages of AD74. Basal forebrain cholinergic neurons may become depleted of calbindin during aging, which may increase their vulnerability to degeneration in AD75. However, in the entorhinal cortex calbindin- and parvalbumin-positive non-principal neurons exhibit degenerative changes early in AD, whereas calretinin- and calbindin-positive pyramidal neurons are relatively preserved76. Changes in the expression of glutamate receptors may also contribute to altered neuronal Ca2+ handling in AD; as AD progresses the levels of NR1/2B subunits in hippocampal neurons decrease, while the NR2A subunit levels remain unchanged77. Other factors that may contribute to selective neuronal vulnerability in AD by perturbing Ca2+ homeostasis are neuron-specific differences in energy metabolism, antioxidant systems and neurotrophic factor support4.
Because aging is the major risk factor for AD, it follows that interventions that counteract the aging process would protect neurons against Ca2+ dysregulation and AD (Fig. 3). Epidemiological and experimental evidence suggests that exercise, dietary energy restriction and cognitive stimulation may retard aging processes and protect against AD11. Indeed, environmental enrichment78, exercise79 and dietary energy restriction80 suppress the disease process and enhance cognitive performance in mouse models of AD. These beneficial environmental factors may act, in part, by inducing the expression of neurotrophic factors such as BDNF, that stabilize neuronal Ca2+ homeostasis81. Antioxidants and cellular energy-promoting agents might also be expected to stabilize neuronal Ca2+ homeostasis and protect against AD. Because increased Aβ production and accumulation at synapses is of major importance in AD pathogenesis, treatments that reduce Aβ production or enhance its clearance from the brain are being vigorously pursued. One of the most promising anti-Aβ approaches that is currently being tested in patients is immunization with Aβ or treatment with purified Aβ antibodies64. By removing Aβ from the brain, immunization would be expected to prevent or reverse Aβ-induced neuronal Ca2+ dysregulation.
Drugs that inhibit β- or γ-secretases are another viable approach for reducing Aβ production and associated Ca2+-mediated neurotoxicity82. Drugs that target specific Ca2+-regulating systems (downstream of age- and Aβ-related disruption of Ca2+ homeostasis) provide another approach. Indeed, the only drug thus far shown to slow disease progression in AD patients is the NMDA receptor open channel blocker memantine83. Beneficial effects have also been reported in AD clinical trials of Dimebon84, a drug that has been claimed to stabilize Ca2+ signaling by blocking NMDAR and voltage-gated Ca2+ channels85. As with other major age-related diseases (cardiovascular disease, diabetes and cancers) risk reduction for AD may be achievable by dietary moderation and exercise combined with dietary supplements (omega-3 fatty acids and folic acid, for example). For individuals at high risk for AD (ApoE4 genotype and family history, for example) prophylactic approaches may be prescribed including anti-inflammatory drugs and immunization.
The ability of neurons to regulate the influx, efflux and subcellular compartmentalization of Ca2+ is compromised in AD as the result of age-related oxidative stress and metabolic impairment in combination with disease-related accumulation of Aβ oligomers. Aβ may promote cellular Ca2+ overload by inducing membrane-associated oxidative stress and by forming pores in the membrane. Mutant forms of presenilins that cause many cases of early-onset FAD cause ER Ca2+ overload, apparently by impairing the normal ER Ca2+ leak channel function of the presenilins. Synapses are particularly sensitive to the adverse effects of Aβ and presenilin mutations, and environmental factors and therapeutic agents that promote synaptic Ca2+ homeostasis may be effective in preventing and treating AD. Key remaining questions include: does perturbed Ca2+ occur early in the AD process and contribute to altered APP processing and Aβ production?; is there a cause-and-effect connection between abnormal neuronal Ca2+ signaling and amyloid plaque accumulation in AD brains?; what is a connection between “amyloid toxicity” and “Ca2+ toxicity” in AD?; what are the weakest links in the various cellular Ca2+-regulating systems in AD?; what is a role played by mitochondria in AD pathogenesis?; how, at the molecular and cellular levels, do risk factors for AD impact neuronal Ca2+ homeostasis?; can specific Ca2+-regulating mechanisms be targeted for therapeutic intervention? Answering these and related questions will clarify a possibile role of abnormal Ca2+ signaling in AD pathogenesis and may open a door to development of new classes of therapeutic agents targeting neuronal Ca2+ signaling pathways.
Neurons possess multiple defenses against the Ca2+-destabilizing forces of aging and AD-specific pathogenic abnormalities (Fig. 3). Activity-dependent neurotrophic factor signaling plays a major role in stabilizing neuronal Ca2+ homeostasis as is evident from the abilities of BDNF, NGF, bFGF and others to protect neurons against excitotoxic, oxidative and metabolic insults relevant to AD81. The neurotrophic factors protect against sustained elevations of [Ca2+]i by modifying the expression of Ca2+-binding proteins, glutamate receptor subunits, antioxidant enzymes and mitochondrial membrane-stabilizing proteins such as Bcl-2. In addition, transcription factors that mediate adaptive stress responses, including NF-κB and Nrf-2 may be activated oxidative and metabolic stress resulting in the up-regulation of antioxidant and phase 2 enzymes. Two organelles in which it is particularly important to maintain Ca2+ regulation are the ER and mitochondria. Evidence suggests that the ER is under stress in neurons affected in AD and may contribute to perturbed cellular Ca2+ homeostasis86. Three proteins that have been shown to stabilize ER Ca2+ homeostasis and protect neurons against insults relevant to AD are Bcl-2 which stabilizes membranes and GRP-78 (glucose-regulated protein 78) which guards against protein misfolding and Herp (homocysteine-inducible ER protein87. Depletion of Herp by RNA interference sensitizes neural cells to apoptosis induced by ER stress, whereas Herp overexpression promotes survival by a mechanism involving stabilization of ER Ca2+ levels, preservation of mitochondrial function and suppression of caspase 3 activation. Mitochondrial Ca2+ handling may be impaired in AD. For example, cyclical fluctuations in mitochondrial membrane potential, which are mediated by Ca2+ and likely represent coupling of membrane potential to ATP production, are reduced in AD cybrid cells88. Mitochondrial uncoupling proteins (UCPs) may play important roles in stabilizing mitochondrial and cellular Ca2+ homeostasis as suggested by studies showing that UCP-4 stabilizes total cellular Ca2+ homeostasis (including ER and plasma membrane systems) which is associated with reduced mitochondrial oxidative stress and resistance of neurons to death89.
We would like to thank our colleagues and collaborators for insightful discussions that help us formulate many ideas expressed in this article. In particular, we would like to thank Joachim Herz, Gang Yu and Bart De Strooper for productive collaboration, Frank La Ferla, Beth Stutzmann, and Kevin Foskett for sharing their unpublished results with us, Sam Gandy, Harvey B. Pollard and Zaven Khachaturian for stimulating discussions. We also would like to sincerely apologize to many scientists working in this field whose interesting work we could not cite due to space limitations. We also thank K. C. Alexander for preparing Figures 1 and and2.2. I.B. is a holder of Carla Cocke Francis Professorship in Alzheimer’s Research and supported by the McKnight Neuroscience of Brain Disorders Award, the Alzheimer’s Association Research Grant IIRG-06-24703, and NINDS grants R01 NS38082 and R01 NS056224. MM is supported by the Intramural Research Program of the National Institute on Aging.