Sites on the cytoplasmic region of phospholamban involved in interaction with the calcium-activated ATPase of the sarcoplasmic reticulum

Arrhythmia mechanisms in the failing heart

BACKGROUND

Heart failure (HF) claims over 200,000 lives annually in the United States alone. Approximately 50% of these deaths are sudden and unexpected, and presumably the consequence of lethal ventricular tachyarrhythmias. Electrical remodeling that occurs at the cellular and tissue network levels predisposes patients with HF to malignant arrhythmias. Our limited understanding of fundamental arrhythmia mechanisms has hampered the development of effective treatment strategies for these patients.

METHODS AND CONCLUSIONS

In this review, we outline recent advances in our understanding of arrhythmia mechanisms in the failing heart, highlighting various aspects of remodeling of ion channels, calcium handling proteins, and gap junction-related molecules. As will be discussed, these changes promote the prolongation of the action potential, the enhancement of spatio-temporal gradients of repolarization, the formation of calcium-mediated triggers and conduction abnormalities, all of which combine to form an electrophysiological substrate that is ripe for the genesis of lethal arrhythmias and sudden cardiac death.

Phospholamban Interacts with HAX-1, a Mitochondrial Protein with Anti-apoptotic Function

Phospholamban (PLN) is a key regulator of Ca(2+) homeostasis and contractility in the heart. Its regulatory effects are mediated through its interaction with the sarcoplasmic reticulum Ca(2+)-ATPase, (SERCA2a), resulting in alterations of its Ca(2+)-affinity. To identify additional proteins that may interact with PLN, we used the yeast-two-hybrid system to screen an adult human cardiac cDNA library. HS-1 associated protein X-1 (HAX-1) was identified as a PLN-binding partner. The minimal binding regions were mapped to amino acid residues 203-245 for HAX-1 and residues 16-22 for PLN. The interaction between the two proteins was confirmed using GST-HAX-1, bound to the glutathione-matrix, which specifically adsorbed native PLN from human or mouse cardiac homogenates, while in reciprocal binding studies, recombinant His-HAX-1 bound GST-PLN. Kinetic studies using surface plasmon resonance yielded a K(D) of approximately 1 muM as the binding affinity for the PLN/HAX-1 complex. Phosphorylation of PLN by cAMP-dependent protein kinase reduced binding to HAX-1, while increasing concentrations of Ca(2+) diminished the PLN/HAX-1 interaction in a dose-dependent manner. HAX-1 concentrated to mitochondria, but upon transient co-transfection of HEK 293 cells with PLN, HAX-1 redistributed and co-localized with PLN at the endoplasmic reticulum. Analysis of the anti-apoptotic function of HAX-1 revealed that the presence of PLN enhanced the HAX-1 protective effects from hypoxia/reoxygenation-induced cell death. These findings suggest a possible link between the Ca(2+) handling by the sarcoplasmic reticulum and cell survival mediated by the PLN/HAX-1 interaction.

Anti-oxidant gene expression imbalance, aging and Down syndrome

The expression of copper zinc superoxide dismutase (SOD1), manganese superoxide dismutase (SOD2), glutathione peroxidase (GPx), and catalase (CAT) genes have been detected in human skin fibroblast cells for 2 year normal child (control), 50 year old normal male and female and a 1 year old Down Syndrome (DS) male and female with established trisomy karyotype using the RT-PCR technique. Differential expression of these genes is quantified individually against a beta-Actin gene that has been employed as an internal control. The immunoblotting of cell lysate proteins with polyclonal antibodies exhibit SOD1 (16 kD), SOD2 (40 kD), GPx (23 and 92 kD), CAT (64 kD), and Actin (43 kD) as translational products. The results demonstrate that the enhancement in the level of mRNAs encoding SOD1 in DS male and female, as well as aged male and female are 51, 21, 31 and 50% respectively compared to the normal child (control). In SOD2, DS male and female display higher (176%) and lower (26%) levels of expression whereas aged male and female exhibit enhanced levels of expression (66 and 119%) respectively compared to the control. This study demonstrates that DS affects the female less than the male whereas in the aging process, the female is more prone to oxidative damage than the male. These results not only indicate that the level of GPx mRNA is constant except in DS male, which shows a downward regulation but that even CAT mRNA is upward regulated in aged as well as in DS males and females. These disproportionate changes in anti-oxidant genes, which are incapable of coping with over expressed genes, may contribute towards the aging process, dementia and Down syndrome.

Ion channels as novel therapeutic targets in heart failure

Electrophysiological remodeling in heart failure (HF) is characterized by major changes in ion channel function and expression that alter the electrical phenotype and predispose to the development of lethal ventricular tachyarrhythmias. In this article, we provide a review of our current understanding of HF-induced ion channel dysfunction by highlighting changes in potassium and sodium currents, pumps, and exchangers as well as calcium handling proteins. We further relate these changes in ion channel function to abnormalities in impulse generation, conduction, and repolarization with a view towards identifying potentially novel targets for anti-arrhythmic therapy for this public health epidemic.

Conformational changes within the cytosolic portion of phospholamban upon release of Ca-ATPase inhibition

Phospholamban (PLB) is a major target of the beta-adrenergic cascade in the heart, functioning to modulate contractile force by altering the rate of calcium re-sequestration by the Ca-ATPase. Functionally, inhibition by PLB binding is manifested by shifts in the calcium dependence of Ca-ATPase activation toward higher calcium levels; phosphorylation of PLB by PKA reverses the inhibitory action of PLB. To investigate structural changes in the cytoplasmic portion of PLB that result from either the phosphorylation of PLB by cAMP-dependent protein kinase (PKA) or calcium binding to the Ca-ATPase, we have used frequency-domain fluorescence spectroscopy to measure the spatial separation and conformational heterogeneity between N-(1-pyrenyl)maleimide, covalently bound to a single cysteine (Cys(24)) engineered near the membrane surface of the transmembrane domain of PLB, and Tyr(6) in the cytosolic domain. Irrespective of calcium activation of the Ca-ATPase or phosphorylation of Ser(16) in PLB by PKA, we find that PLB remains tightly associated with the Ca-ATPase in a well-defined conformation. However, calcium activation of the Ca-ATPase induces an increase in the overall dimensions of the cytoplasmic portion of bound PLB, whereas PLB phosphorylation results in a more compact structure, consistent with increased helical content induced by a salt link between phospho-Ser(16) and Arg(13). Thus, enzyme activation of the Ca-ATPase may occur through different mechanisms: calcium binding to high-affinity sites within the Ca-ATPase functions to overcome conformational constraints imposed by PLB on the N-domain of the Ca-ATPase; alternatively, phosphorylation stabilizes the backbone fold of PLB to release inhibitory interactions with the Ca-ATPase.

Characterization of the Interaction between the N-Terminal Extension of Human Cardiac Troponin I and Troponin C†

The N-terminal extension of cardiac troponin I (TnI) is bisphosphorylated by protein kinase A in response to beta-adrenergic stimulation. How this signal is transmitted between TnI and troponin C (TnC), resulting in accelerated Ca(2+) release, remains unclear. We recently proposed that the unphosphorylated extension interacts with the N-terminal domain of TnC stabilizing Ca(2+) binding and that phosphorylation prevents this interaction. We now use (1)H NMR to study the interactions between several N-terminal fragments of TnI, residues 1-18 (I1-18), residues 1-29 (I1-29), and residues 1-64 (I1-64), and TnC. The shorter fragments provide unambiguous information on the N-terminal regions of TnI that interact with TnC: I1-18 does not bind to TnC whereas the C-terminal region of unphosphorylated I1-29 does bind. Bisphosphorylation greatly weakens this interaction. I1-64 contains the phosphorylatable N-terminal extension and a region that anchors I1-64 to the C-terminal domain of TnC. I1-64 binding to TnC influences NMR signals arising from both domains of TnC, providing evidence that the N-terminal extension of TnI interacts with the N-terminal domain of TnC. TnC binding to I1-64 broadens NMR signals from the side chains of residues immediately C-terminal to the phosphorylation sites. Binding of TnC to bisphosphorylated I1-64 does not broaden these NMR signals to the same extent. Circular dichroism spectra of I1-64 indicate that bisphosphorylation does not produce major secondary structure changes in I1-64. We conclude that bisphosphorylation of cardiac TnI elicits its effects by weakening the interaction between the region of TnI immediately C-terminal to the phosphorylation sites and TnC either directly, due to electrostatic repulsion, or via localized conformational changes.

A structural model of the complex formed by phospholamban and the calcium pump of sarcoplasmic reticulum obtained by molecular mechanics

Phospholamban (PLN) is an intrinsic membrane protein of 52 amino acids that modulates the activity of the reticular Ca(2+) ion pump. We recently solved the three-dimensional structure of chemically synthesized, unphosphorylated, monomeric PLN (C41F) by high-resolution nuclear magnetic resonance spectroscopy in chloroform/methanol. The structure is composed of two alpha-helical regions connected by a beta turn (Type III). We used this structure and the crystallographic structure of the sarcoplasmic reticulum calcium pump (SERCA) recently determined by Toyoshima and co-workers and modeled into its E(2) form by Stokes (1KJU) or by Toyoshima (1FQU). We applied restrained and unrestrained energy optimizations and used the AMBER molecular mechanics force field to model the complex formed between PLN and the pump. The results indicate that transmembrane helix 6 (M6) of the SERCA pump is energetically favored, with respect to the other transmembrane helices, as the PLN binding partner within the membrane and is the only one of these helices that also permits contact between the N-terminal residues of PLN and the critical cytosolic binding loop region of the pump. This result is in agreement with published biochemical data and with the predictions of previous mutagenesis work on the membrane sector of the pump. The model reveals that PLN does not span the entire width of the membrane, that is, its hydrophobic C-terminal end is located near the center of the transmembrane region of the SERCA pump. The model also shows that interaction with M6 is stabilized by additional contacts made by PLN to M4. The contact between the N-terminal portion of PLN and the pump is stabilized by a number of salt and hydrogen-bond bridges, which may be abolished by phosphorylation of PLN. The contacts between the cytosolic portions of PLN and the pump are only observed in the E(2) conformation of the pump. Our model of the complex also offers a plausible structural explanation for the preference of protein kinase A for phosphorylation of Ser16 of PLN.

Phosphorylation induces a conformational transition near the lipid-water interface of phospholamban reconstituted with the Ca-ATPase

We have measured conformational changes of phospholamban (PLB) induced both by its interaction with the SR Ca-ATPase and by phosphorylation of Ser-16 by cAMP-dependent protein kinase (PKA) using an engineered PLB having a single cysteine (Cys-24) derivatized with the fluorophore 2-(4'-maleimidylanilino)naphthalene-6-sulfonic acid (ANSmal). This modified mutant PLB is fully functional when co-reconstituted with the affinity-purified Ca-ATPase in liposomes. ANSmal emission properties and its solvent accessibility indicate that Cys-24 is in an aqueous environment outside the membrane. Fluorescence quenching and time-resolved anisotropy measurements of ANSmal-PLB demonstrate distinct structures for PLB in the free and Ca-ATPase-bound state. Both solvent exposure and probe motions of ANSmal are enhanced upon interaction of PLB with the Ca-ATPase. This conformational transition entails conversion of free PLB in a conformation which is insensitive to one which is sensitive to the phosphorylation state of PLB. Upon phosphorylation of Ca-ATPase-bound PLB, a decreased level of solvent exposure of ANSmal is observed, suggesting that the amino acid sequence of PLB near the lipid-water interface acts as a conformational switch in response to the phosphorylation of PLB. A longer correlation time, resolved by anisotropy measurements, corresponding to polypeptide chain fluctuations, is substantially restricted by interaction of PLB with the Ca-ATPase. This restriction is not reversed by phosphorylation of PLB, indicating that the region around Cys-24 near the lipid-water interface does not undergo dissociation from the Ca-ATPase. These results suggest that the phosphorylation by PKA induces a redistribution of PLB-Ca-ATPase protein contacts to relieve the inhibitory effect of PLB for the activation of calcium transport.

Comparable levels of Ca-ATPase inhibition by phospholamban in slow-twitch skeletal and cardiac sarcoplasmic reticulum

Alterations in expression levels of phospholamban (PLB) relative to the sarcoplasmic reticulum (SR) Ca-ATPase have been suggested to underlie defects of calcium regulation in the failing heart and other cardiac pathologies. To understand how variation in PLB expression relative to that of the Ca-ATPase can modulate calcium transport, we have investigated the inhibition of the Ca-ATPase by PLB in native SR membranes from slow-twitch skeletal and cardiac muscle and in reconstituted proteoliposomes. Quantitative immunoblotting in combination with affinity-purified protein standards was used to measure protein concentrations of PLB and of the Ca-ATPase. Functional inhibition of the Ca-ATPase was determined from both the calcium concentrations for half-maximal activation (Ca(1/2)) and the shift in the calcium concentrations following release of PLB inhibition (i.e., (Delta)Ca(1/2)) by incubation with monoclonal antibodies against PLB, which are equivalent to phosphorylation of PLB by cAMP-dependent protein kinase. We report that equivalent levels of PLB inhibition and antibody-induced activation ((Delta)Ca(1/2) = 0.25 +/- 0.02 microM) are observed in SR membranes from slow-twitch skeletal and cardiac muscle, where molar stoichiometries of PLB expressed per Ca-ATPase vary, respectively, from 0.9 +/- 0.1 to 4.1 +/- 0.8. Similar levels of inhibition to those observed in isolated SR vesicles were observed using reconstituted proteoliposomes following co-reconstitution of affinity-purified Ca-ATPase with PLB. These results indicate that total expression levels of one PLB per Ca-ATPase result in full inhibition of the Ca-ATPase and, based on the measured K(D) (140 +/- 30 microM), suggests one PLB complexed with two Ca-ATPase molecules is sufficient for full inhibition of activity. Therefore, the excess PLB expressed in the heart over that required for inhibition suggests a capability for graded responses of the Ca-ATPase activity to endogenous kinases and phosphatases that modulate the level of phosphorylation necessary to relieve inhibition of the Ca-ATPase by PLB.

Structure of the 1-36 N-terminal fragment of human phospholamban phosphorylated at Ser-16 and Thr-17

The structure of a 36-amino-acid-long N-terminal fragment of human phospholamban phosphorylated at Ser-16 and Thr-17 and Cys-36-->Ser mutated was determined from nuclear magnetic resonance data in aqueous solution containing 30% trifluoroethanol. The peptide assumes a conformation characterized by two alpha-helices connected by an irregular strand, which comprises the amino acids from Arg-13 to Pro-21. The proline is in a trans conformation. The two phosphate groups on Ser-16 and Thr-17 are shown to interact preferably with the side chains of Arg-14 and Arg-13, respectively. The helix comprising amino acids 22 to 35 is well determined (the rmsd for the backbone atoms, calculated for a family of 24 nuclear magnetic resonance structures is 0.69 +/- 0.28 A). The structures of phosphorylated and unphosphorylated phospholamban are compared, and the effect of the two phosphate groups on the relative spatial position of the two helices is examined. The packing parameters Omega (interhelical angle) and d (minimal interhelical distance) are calculated: in the case of the phosphorylated phospholamban, Omega = 100 +/- 35 degrees and d = 7.9 +/- 4.6 A, whereas for the unphosphorylated peptide the values are Omega = 80 +/- 20 degrees and d = 7.0 +/- 4.0 A. We conclude that 1) the phosphorylation does not affect the structure of the C terminus between residues 21 and 36 and 2) the phosphorylated phospholamban has more loose helical packing than the nonphosphorylated.

Alzheimer's disease: role of aging in pathogenesis

Alzheimer's disease (AD) is characterized by intraneuronal fibrillary tangles, plaques, and cell loss. Brain lesions in both sporadic AD (SAD) and familial AD (FAD) are the same, and in the same distribution pattern, as those in individuals with Down syndrome (DS) and in smaller numbers in nondemented older individuals. Dementia onset is around 40 years for DS, 40-60 years for FAD, and usually over 60 years for SAD. The different categories of AD may be due to processes that augment to different degrees the innate cellular aging rate, that is, mitochondrial superoxide radical (SO) formation. Thus, they increase the rate of accumulation of AD lesions. This lowers the age of onset into the dementia ranges associated with DS, FAD, and SAD, and concomitantly shortens life spans. Faster aging lowers AD onset age by decreasing the onset age for neurofibrillary tangle formation and neuronal loss, and the age when brain intercellular H2O2 can activate microglial cells. The early AD onset in DS is attributed to a defective mitochondrial complex 1. The proteins associated with FAD and their normal counterparts undergo proteolytic processing in the endoplasmic reticulum (ER). The mutated compounds increase the ratio of betaA42 to betaA40 and likely also down-regulate the ER calcium (Ca2+) buffering activity. Decreases in ER Ca2+ content should increase the mitochondrial Ca2+ pool, thus enhancing SO formation. SAD may be due to increased SO formation caused by mutations in the approximately 1000 genes involved in mitochondrial biogenesis and function. The hypothesis suggests measures to prevent and treat.

Oligomeric interactions between phospholamban molecules regulate Ca-ATPase activity in functionally reconstituted membranes

Phospholamban (PLB) is a major target of the beta-adrenergic cascade in the heart, and functions as an endogenous inhibitor of Ca-ATPase transport activity. To identify whether oligomeric interactions between PLB molecules are involved in regulating Ca-ATPase transport activity, we have investigated functional interactions between PLB and the Ca-ATPase in proteoliposomes of purified PLB functionally co-reconstituted with the SERCA2a isoform of the Ca-ATPase isolated from cardiac sarcoplasmic reticulum (SR). The calcium sensitivity of this reconstituted preparation and functional stimulation by cAMP-dependent protein kinase (PKA) are virtually identical to those of the Ca-ATPase in cardiac SR microsomes, ensuring the functional relevance of this reconstituted preparation. Interactions between PLB molecules were measured following covalent modification of the single lysine (i.e., Lys(3)) in PLB isolated from cardiac SR membranes with fluorescein isothiocyanate (FITC) prior to co-reconstitution with the Ca-ATPase. FITC modification of PLB does not interfere with the ability of PLB to inhibit the Ca-ATPase, since FITC-PLB co-reconstituted with the Ca-ATPase exhibits a similar calcium dependence of Ca-ATPase activation to that observed in native SR membranes. Thus, the functional arrangement of PLB with the Ca-ATPase is not modified by FITC modification. Using changes in the anisotropy of FITC-PLB resulting from fluorescence resonance energy transfer (FRET) between proximal PLB molecules to measure the average size and spatial arrangement of FITC chromophores, we find that PLB self-associates to form oligomers whose spatial arrangement with respect to one another is in agreement with earlier suggestions that PLB exists predominantly as a homopentamer. The inability of PKA to activate PLB following covalent modification with FITC permits functional interactions between PLB molecules associated with the Ca-ATPase activation to be identified. A second-order loss of Ca-ATPase activation by PKA is observed as a function of the fractional contribution of FITC-PLB, indicating that PKA-dependent activation of two PLB molecules within a quaternary complex containing the Ca-ATPase is necessary for activation of the Ca-ATPase. We suggest that the requirement for activation of two PLB molecules by PKA represents a physiological mechanism to ensure that activation of the Ca-ATPase following beta-adrenergic stimulation in the heart only occurs above a threshold level of PKA activation.

Cytoplasmic interactions between phospholamban residues 1-20 and the calcium-activated ATPase of the sarcoplasmic reticulum

Phospholamban regulates the activity of the calcium-activated ATPase (CaATPase) of cardiac sarcoplasmic reticulum. Equilibrium fluorescence studies have shown that the N-terminal cytoplasmic region of phospholamban (residues 1-20, domain 1) causes a decrease in the intrinsic tryptophan fluorescence of the CaATPase. The interaction of phospholamban residues 1-20 with the CaATPase also results in spectral changes for the extrinsic chromophore FITC covalently attached to the cytoplasmic region of the calcium pump. The fluorescence changes for both reporter groups correlate with a dissociation constant of approximately 40 microM for the complex between phospholamban residues 1-20 and the CaATPase. Complex formation is notably weaker when phospholamban 1-20 is titrated into the CaATPase in the presence of calcium, with altered conformational effects resulting from binding. The interaction of domain 1 of phospholamban with the CaATPase is also reduced upon phosphorylation of phospholamban 1-20 at Ser-16. This region of phospholamban 1-20 is shown by isotope-edited NMR study to be involved in interaction with the CaATPase. Binding of the phosphorylated peptide is not abolished, however, indicating that phospholamban 1-20 remains associated with the CaATPase even after phosphorylation. The data provide direct evidence for the interaction between the cytoplasmic regions of phospholamban and the pump, and are discussed in the context of the mechanism for inhibition of cardiac CaATPase activity by phospholamban.

Phospholamban remains associated with the Ca2+- and Mg2+-dependent ATPase following phosphorylation by cAMP-dependent protein kinase

We have used fluorescence and spin-label EPR spectroscopy to investigate how the phosphorylation of phospholamban (PLB) by cAMP-dependent protein kinase (PKA) modifies structural interactions between PLB and the Ca(2+)- and Mg(2+)-dependent ATPase (Ca-ATPase) that result in enzyme activation. Following covalent modification of N-terminal residues of PLB with dansyl chloride or the spin label 4-isothiocyanato-2,2,6,6-tetramethylpiperidine-N-oxyl ('ITC-TEMPO'), we have co-reconstituted PLB with affinity-purified Ca-ATPase isolated from skeletal sarcoplasmic reticulum (SR) with full retention of catalytic function. The Ca(2+)-dependence of the ATPase activity of this reconstituted preparation is virtually identical with that observed using native cardiac SR before and after PLB phosphorylation, indicating that co-reconstituted sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase 1 (SERCA1) and PLB provide an equivalent experimental model for SERCA2a-PLB interactions. Phosphorylation of PLB in the absence of the Ca-ATPase results in a greater amplitude of rotational mobility, suggesting that the structural linkage between the transmembrane region and the N-terminus is destabilized. However, whereas co-reconstitution with the Ca-ATPase restricts the amplitude of rotational motion of PLB, subsequent phosphorylation of PLB does not significantly alter its rotational dynamics. Thus structural interactions between PLB and the Ca-ATPase that restrict the rotational mobility of the N-terminus of PLB are retained following the phosphorylation of PLB by PKA. On the other hand, the fluorescence intensity decay of bound dansyl is sensitive to the phosphorylation state of PLB, indicating that there are changes in the tertiary structure of PLB coincident with enzyme activation. These results suggest that PLB phosphorylation alters its structural interactions with the Ca-ATPase by inducing structural rearrangements between PLB and the Ca-ATPase within a defined complex that modulates Ca(2+)-transport function.

Sarco(endo)plasmic reticulum calcium pumps: recent advances in our understanding of structure/function and biology (review)

This review examines the structure and function of the sarco(endo)plasmic reticulum calcium pump (SERCA1a) in the light of the recent publication of the 2.6 A resolution structure of this protein, and looks at the increasing awareness of the key role played by SERCAs in calcium signalling. The roles played by the calcium pump isoforms, SERCA1a/b, SERCA2a/b and SERCA3a/b/c in cellular function are discussed, and the modulation of SERCA activity by phospholamban, sarcolipin and other modulatory influences is examined. The recent discoveries of human SERCA mutations leading to disease states is reviewed, and the insights into SERCA function using transgenic approaches are outlined.

Alzheimer's disease: A hypothesis on pathogenesis

Alzheimer's disease (AD) is the major cause of dementia. It is a systemic disorder whose major manifestations are in the brain. AD cases can be categorized into two groups on the basis of the age of onset-before or after about age 60. The majority of cases, 90-95 percent, are in the late onset category. Early onset cases are largely, if not all, familial (FAD). These are caused by mutations in the genes for the amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2). In contrast late onset cases are mainly sporadic. The disorder is characterized by intraneuronal fibrillary tangles, plaques, and cell loss. The brain lesions in both early and late-onset AD are the same, and in the same distribution pattern, as those seen in individuals with Down's syndrome (DS) and in smaller numbers in normal older individuals. Extensive studies of AD have yet to result in a generally accepted hypothesis on the pathogenesis of the disorder. Major emphasis has been placed on the role of amyloid, the neurotoxin formed by the action of free radicals on preamyloid. The observation that AD lesions are frequently present in normal older individuals prompted the hypothesis that AD is the result of faster than normal aging of the neurons associated with it. This hypothesis provides plausible explanations for FAD and AD. FAD is associated with mutations in APP, PS1, and PS2. These substances, along with their normal counterparts, undergo proteolytic processing in the endoplasmic reticulum (ER). The mutated compounds, aside from increasing the ratio of βA42 to βA40, may down-regulate the calcium buffering activity of the ER in a manner akin to one or more of the many compounds known to do so. Decreases in the ER calcium pool would cause compensatory increases in other calcium pools, particularly in mitochondria. Increases in mitochondrial calcium levels are associated with enhanced formation of superoxide radical formation, and hence of the rate of aging. SAD may be caused by nuclear and/or mitochondrial DNA mutations beginning early in life that enhance mitochondrial superoxide radical formation in the neurons associated with the disorder. The above explanations for FAD and AD are suggestive of measures to prevent and for treatment.