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Calcium, ageing, and neuronal vulnerability in Parkinson's disease
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Calcium, ageing, and neuronal vulnerability in Parkinson’s disease D James Surmeier
Parkinson’s disease is a common neurodegenerative disorder of unknown cause. There is no cure or proven strategy for slowing the progression of the disease. Although there are signs of pathology in many brain regions, the core symptoms of Parkinson’s disease are attributable to the selective degeneration of dopaminergic neurons in the substantia nigra pars compacta. A potential clue to the vulnerability of these neurons is their increasing reliance on Ca²+ channels to maintain autonomous activity with age. This reliance could pose a sustained metabolic stress on mitochondria, accelerating cellular ageing and death. The Ca²+ channels underlying autonomous activity in dopaminergic neurons are closely related to the L-type channels found in the heart and smooth muscle. Systemic administration of isradipine, a dihydropyridine blocker of L-type channels, forces dopaminergic neurons in rodents to revert to a juvenile, Ca²+-independent mechanism to generate autonomous activity. More importantly, reversion confers protection against toxins that produce experimental parkinsonism, pointing to a potential neuroprotective strategy for Parkinson’s disease with a drug class that has been used safely in human beings for decades. These studies also suggest that, although genetic and environmental factors can hasten its onset, Parkinson’s disease stems from a distinctive neuronal design common to all human beings, making its appearance simply a matter of time.
Introduction Parkinson’s disease is a common neurodegenerative disease strongly associated with ageing.1 The cardinal motor symptoms of Parkinson’s disease are bradykinesia, rigidity, and tremor.1,2 Parkinson’s disease has no cure and nothing is known to slow its progression.3,4 Several regions of the brain display signs of pathology in Parkinson’s disease,5 but the motor symptoms of the disease are clearly linked with the degeneration and death of dopamine neurons in the substantia nigra pars compacta (SNc).6,7 The clinical efficacy of levodopa—a dopamine precursor—is testament to the centrality of these neurons in Parkinson’s disease. Why dopamine neurons are preferentially lost in Parkinson’s disease is not clear. Perhaps the most widely held theory suggests that the cause is dopamine itself. There is evidence that oxidation of cytosolic dopamine (and its metabolites) leads to the production of damaging free radicals.8 However, there are many reasons to doubt that this type of cellular stress has a key role in normal ageing and Parkinson’s disease. For example, there is considerable regional variability in the vulnerability of dopamine neurons in Parkinson’s disease, with some classes showing no signs of loss at all.9–13 Moreover, administration of levodopa (which increases dopamine levels) in patients with Parkinson’s disease does not accelerate disease progression,14 suggesting that, at normal cytosolic concentrations, dopamine is not a substantial source of reactive oxidative products and stress. If not dopamine, then what? Several lines of study suggest that mitochondrial and proteasome dysfunction are involved in Parkinson’s disease.15–17 However, why SNc dopamine neurons should be any more vulnerable to this type of dysfunction than other neurons is not clear. Similarly, genetic studies have identified several potential determinants of Parkinson’s disease but have http://neurology.thelancet.com Vol 6 October 2007
Lancet Neurol 2007; 6: 933–38 Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA (D J Surmeier PhD) Correspondence to: Prof D James Surmeier, Department of Physiology, Feinberg School of Medicine, Northwestern University, 303 E Chicago Avenue, Chicago, IL 60611, USA [email protected]
not provided many clues as to selective vulnerability, at least not to this point. None of the genes linked to familial forms of Parkinson’s disease are preferentially expressed by SNc dopamine neurons, suggesting that these neurons display some epigenetic feature that increases the cellular effect of polymorphisms or mutations.
SNc dopamine neurons and calcium The physiology of SNc dopamine neurons might hold the key needed to unlock this mystery. Unlike most neurons in the brain, SNc dopamine neurons are autonomously active; that is, they generate action potentials at a clock-like 2–4 Hz in the absence of synaptic input. In this respect, they are much like cardiac pacemakers. Although they display considerably more complex activity patterns in vivo than do cardiac cells,18 the foundation on which the activity is built is the same. Even more unusual than their autonomous activity is how it is generated. Whereas most neurons use channels that allow Na+ ions across the membrane, SNc dopamine neurons rely on L-type Ca²+ channels.19-23 The L-type channels underlying pacemaking in SNc dopamine neurons have a pore-forming Cav1.3 subunit, rather than the cardiac Cav1.2 subunit.24 Cav1.3 channels constitute roughly a quarter of the L-type channels in the brain,25 have a low affinity for dihydropyridines, and open at more hyperpolarised membrane potentials than do Cav1.2 channels.26–28 The reliance on Cav1.3 Ca²+ channels to drive pacemaking could cause problems. Ca²+ is central to a wide variety of cellular processes ranging from the regulation of enzyme activity and gene expression to programmed cell death. Neurons use Ca²+ entry as a way of monitoring their activity and interactions with other neurons. Befitting its importance to cellular function, there is a complex homoeostatic network inside neurons that allows transient increases in free Ca²+ concentration 933
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but tries to maintain basal levels that are 10 000 times lower (about 50–100 nmol/L) than found in the extracellular space (1–2 mmol/L). Because of its steep concentration gradient, Ca²+ flows into cells readily through pores, like L-type Ca²+ channels, but has to be pumped out through slow membrane transporters that consume energy stored as ATP. In most neurons, the opening of Ca²+ channels is a rare event, occurring mainly during very brief action potentials. As a result, it is easy for a small number of Ca²+ ions to enter the cell, report to intracellular signalling complexes, and then be rapidly extruded back across the plasma membrane at little metabolic cost. But in SNc dopamine neurons, where Ca²+ channels are open much of the time (because they are responsible for pacemaking), the magnitude of the Ca²+ influx seems to be much larger and the burden to the cell much greater.29 Because of the slow kinetics of the plasma membrane transporters and their restriction to the cellular surface, Ca²+ entering neurons must be rapidly sequestered either in organelles lying below the plasma membrane or through ionic interactions with mobile buffering proteins before being escorted out of the cell. Mitochondria and the endoplasmic reticulum (ER) are the principal organelles involved in sequestering Ca²+ in neurons. The ER uses high-affinity ATP-dependent transporters to take Ca²+ from the cytoplasm into the ER lumen. As this store fills up, cytosolic Ca²+ can trigger the opening of ER Ca²+ channels that let the Ca²+ ions flow back into the cytoplasm.30 These channels are often found in close apposition to mitochondria and their opening creates a region of high local Ca²+ concentration that drives influx of Ca²+ into the matrix of mitochondria through Ca²+ uniporters.31 Accumulation of Ca²+ in the mitochondrial matrix again comes at an energetic cost, since it dissipates the electrochemical gradient created by respiratory metabolism along the electron transport chain. Through a poorly understood process, matrix Ca²+ is returned to the cytoplasm where it is either pumped out across the plasma membrane or taken up again by the ER. In most neurons, this interaction is episodic, but in SNc dopamine neurons it must be sustained, going on all the time, consuming ATP produced by oxidative phosphorylation in mitochondria.
Calcium, ageing, and Parkinson’s disease Although the demand on oxidative phosphorylation posed by the reliance on Ca²+ channels for pacemaking is not lethal, it might accelerate the ageing of SNc dopamine neurons. One of the oldest and most popular theories of ageing is that it is a direct consequence of accumulated mitochondrial DNA (mtDNA) damage produced by reactive oxygen species and related free radicals generated in the course of oxidative phosphorylation.32,33 A corollary of this hypothesis is that the rate of ageing is directly related to the metabolic rate. By extension, the reliance of SNc dopamine neurons on a metabolically expensive 934
strategy should mean that they age more rapidly than would other types of neuron. Histological estimates of normal ageing-related cell death are consistent with this hypothesis, suggesting that SNc dopamine neurons are lost at a substantially higher rate (5–10% per decade) than many other types of neurons (some of which show no appreciable loss over a six to seven decade span).34 Functional measures of the nigrostriatal system also decline with normal ageing,35,36 although not as rapidly, presumably because of the capacity of the remaining neurons to compensate.37 There are other signs that SNc dopamine neurons age more rapidly than do other neurons. Perhaps the most compelling is that cells in the SNc have higher than normal levels of mtDNA mutations and have diminished mitochondrial complex I function, both signs of ageing.33,38–40 Another supporting observation is the resilience of SNc dopamine neurons that express Ca²+-buffering proteins, which effectively sequester Ca²+without using ATP; expression of the Ca²+-buffering protein calbindin diminishes vulnerability to mitochondrial toxins41 and seems to confer resistance to the agents at work in Parkinson’s disease.10,11,42 Is Parkinson’s disease thus ageing related, without a pathogen or causative agent other than the physiology of SNc dopamine neurons (figure 1)? This theory predicts that everyone should get Parkinson’s disease if they live long enough. Why, then, do some people develop symptoms in their 50s and others in their 70s, or not at all? Over the course of a lifetime, small differences in the rate of cellular ageing could have a substantial effect on the time at which cell loss reaches the threshold necessary for the emergence of Parkinson’s symptoms (figure 1).37 These differences might arise from genetic mutations or polymorphisms that modestly change the efficiency of mitochondrial oxidative phosphorylation and the rate of generation of reactive oxygen species. Indeed, some mtDNA polymorphisms are associated with a higher incidence of Parkinson’s disease, whereas others are associated with a lower incidence.43–47 Mutations in nuclear genes that produce small alterations in mitochondrial function might have the same sort of effect on cellular ageing and on the probability of developing Parkinson’s disease within a normal lifespan. Mutations in the PARK7 (DJ-1), LRRK2, and PINK1 genes increase the risk of developing Parkinson’s disease, and each gene product is associated with mitochondria.15,48 However, none of these mutations result in a profound mitochondrial or cellular phenotype in mouse models,15 suggesting that their effect on SNc dopamine neurons is slow to develop. Mutations in other nuclear genes associated with Parkinson’s disease—eg, PARK2 and SNCA—have been linked to mitochondrial stress also, albeit indirectly.16 The rate of ageing and cell loss could also be accelerated by secondary factors arising from exposure to environmental toxins that compromise mitochondrial function or that of cellular systems dealing with the consequences of oxidative stress, like the ubiquitin-proteasome system or autophagic http://neurology.thelancet.com Vol 6 October 2007
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vacuoles.16,49,50 These factors might in and of themselves be fairly innocuous but, because of their potential to synergise with the intrinsic vulnerabilities of SNc dopamine neurons, might seem causal, giving Parkinson’s disease a confusing, multifactorial clinical profile. In this model, the rate of cell loss is directly related to the accumulation of cellular defects with ageing (figure 1). In other words, ageing-related deficits in organelle function build upon one another, further increasing stress and the likelihood of cell death. This positive feedback gives our hypothetical ageing plot its non-linear, downwardly curving shape (figure 1). Nominally extrinsic events, like neuromelanin release or inflammation engaged by neuronal death, could provide more positive feedback, leading to a steeper slope as the symptomatic threshold is crossed.50,51 However, when viewed piecemeal, this kind of relation could give the appearance of healthy and diseased states. At early, presymptomatic time points, the rate of cell loss seems to be slow and linear, whereas, at later symptomatic time points, the rate of cell loss seems to be greater (figure 1). Differences of precisely this sort could be used—mistakenly, in my view—to distinguish normal ageing-related cell loss from that found in Parkinson’s disease, providing impetus to extrinsic disease theories. Without longitudinal estimates of SNc dopamine cell numbers in individuals that develop Parkinson’s, it is impossible to exclude the possibility that rapid cell loss in symptomatic patients simply reflects the culmination of a decades long process of accelerated ageing, not the late onset of disease. Can the Ca²+-mediated cellular ageing hypothesis account for the vulnerability of other cell types in Parkinson’s disease? One other region of the brain that has cellular losses paralleling those of the SNc is the locus coeruleus.52 The noradrenergic neurons of the locus coeruleus are very similar to SNc dopamine neurons in several respects. Like SNc dopamine neurons, noradrenergic neurons in the locus coeruleus are autonomous pacemakers.53 More importantly, this pacemaking depends on Ca²+ channels and is likely to create a very similar metabolic stress. Another common feature is their enormous axonal terminal field.54,55 Neurons in both the locus coeruleus and the SNc support more than 100 times as many synapses as do cortical pyramidal neurons. Aside from the additional metabolic burden this poses, these terminal regions might create a mitochondrial sink, thereby lowering mitochondrial mass in the somatodendritic region and the ability to cope with the metabolic demands of Ca²+-dependent pacemaking.56 Lastly, like SNc dopamine neurons, noradrenergic neurons in the locus coeruleus accumulate neuromelanin in autophagic vacuoles, a sign that these neurons are experiencing oxidative stress that is exacerbated by cytosolic dopamine.50 The release of neuromelanin from dying neurons could itself prove toxic to neighbouring neurons, adding to the inflammatory stress triggered by cell death.51
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Figure 1: Hypothetical models of Parkinson’s disease (A) An ageing model where SNc dopamine cell loss is gradual is compared with a conventional disease agent model. The relative proportion of surviving SNc dopamine neurons is plotted as a function of age. (B) Variation in cellular ageing could predispose individuals to early onset of Parkinson’s disease. The dark blue line represents the rate of loss expected from normal ageing; the red line is the rate of loss in a population in which rate of ageing is raised because of genetic or environmental factors. The green line estimates the rate of cell loss in other neuronal populations that do not have high basal metabolic rates. (C) Fitting straight lines to the early and late phases of the ageing model gives the appearance of slow (“normal”) and fast (“diseased”) SNc dopamine cell loss.
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Figure 2: Schematic summarising the key events in the ageing model of Parkinson’s disease At the left, a schematic of an SNc dopamine neuron showing that Ca2+ entry during pacemaking creates a demand on oxidative phosphorylation, leading to increased production of reactive oxygen species and cellular damage associated with ageing (damage is denoted by the red circles). This is recapitulated at the right in the form of a flow diagram. Genetic polymorphisms or mutations and environmental toxins could alter the generation or effect of generation of reactive oxygen species on cellular ageing.
Staving off Parkinson’s disease If Parkinson’s disease is a consequence of the accelerated ageing of neurons that rely heavily on Ca²+ channels (figure 2), then reducing this dependence should delay the onset of the disease and slow its progression. This might be possible with orally deliverable drugs shown to be safe in human beings.24 In mice, the reliance of SNc dopamine neurons on Ca²+ channels to drive pacemaking is developmentally regulated. Young neurons are autonomously active but generate this activity with channels used by pacemakers in the brain that do not succumb in Parkinson’s disease. These channels allow Na+ ions to cross the membrane, not Ca²+ ions. This juvenile mechanism is retained in adult SNc dopamine neurons, but becomes latent. Surprisingly, sustained block of the L-type Cav1.3 Ca²+ channels that underlie normal adult pacemaking re-engages the juvenile mechanism. Rejuvenated SNc dopamine neurons spike at perfectly normal rates, reflecting the strong homoeostatic pressure on this parameter of cellular function.57 Moreover, treated mice have no obvious motor, learning, or cognitive deficits, suggesting that network activity controlling episodic activity is substantially unchanged.22,58 More importantly, although the effect on mitochondrial oxidative stress can only be inferred at this point, switching the pacemaking mechanism of SNc dopamine neurons made them resistant to mitochondrial toxins used to generate animal models of Parkinson’s disease, just like young SNc dopamine neurons. Is there evidence that this strategy might work in human beings to prevent or slow Parkinson’s disease? 936
Ca²+ channel blockers are commonly used in clinical practice, creating a potential database that could be mined for answers. For example, such drugs have been used for decades to treat hypertension. The Ca²+ channel blockers approved for use in hypertension fall into one of five pharmacological categories.59 The most commonly used are verapamil and diltiazem. In a recent epidemiological study of the potential effect of use of Ca²+ channel blockers on the evolution of Parkinson’s disease,60 about two-thirds of the patients were treated with one of these two drugs. No relation between use of such drugs and the incidence of Parkinson’s disease was found. However, neither verapamil nor dilitazem are potent blockers of the Cav1.3 channels that underlie pacemaking in SNc dopamine neurons. Other nominal Ca²+ channel blockers—flunarizine and cinnarizine— worsen the symptoms of Parkinson’s, but this effect is attributable to their antagonism of dopamine receptors, not their block of Ca²+ channels.61 By contrast, retrospective examination of patients treated for hypertension with dihydropyridines—the class of drug used to induce reversion of the SNc dopamine neuron pacemaking and neuroprotection in mice—revealed a lower than expected incidence of Parkinson’s disease.62 Although this finding is encouraging, it is not a substitute for a controlled clinical trial. In moving forward, there are several issues that need to be considered. One is the choice of drug. In the absence of a selective Cav1.3 Ca²+ channel antagonist, dihydropyridines offer the best therapeutic options. Dihydropyridines are more selective blockers of L-type channels than are other Ca²+ channel blockers approved for human use, and have good brain bioavailability.63 However, most members of this drug class, including nimodipine and nifedipine, are more potent blockers of Cav1.2 than of Cav1.3 channels.26 This is also true of isradipine,64 but at the depolarised membrane potentials traversed by pacemaking SNc dopamine neurons, it more potently blocks pacemaking than either nimodipine or nifedipine (unpublished data). At the doses used to treat hypertension, isradipine has fairly minor side-effects, making it attractive clinically.65 However, in patients with Parkinson’s disease, in whom autonomic dysfunction, especially orthostatic hypotension, is pronounced, these side-effects might pose more of a problem.66 What is not clear at this point is how much dihydropyridine is enough to confer protection. In our animal studies, isradipine was administered with subcutaneous, timed-release pellets. The daily release from these pellets was roughly ten times the maximum daily human oral dose recommended for treating hypertension (20 mg/day). Pharmacokinetic studies are underway to map the relation between subcutaneous dose, serum and brain concentrations of isradipine, and neuroprotection in mouse models, to provide a dosing guide for human beings. Because of species-specific differences in absorption and hepatic clearance, the serum and brain concentrations of http://neurology.thelancet.com Vol 6 October 2007
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isradipine needed to protect SNc dopamine neurons in mice could be achievable in human beings within a dose range known to be safe and well tolerated. Within this dose range, there seems to be little effect on brain function, despite L-type channels being implicated in a broad array of functions, including forms of synaptic plasticity thought to underlie learning and memory.67 This lack of effect on brain function could be due to the voltage-dependence of the L-type channel block by dihydropyridines,68 an especially striking feature of Cav1.3 Ca²+ channels.69 If higher concentrations of dihydropyridines are required for protection, then there is a compelling need for a more selective Cav1.3 channel antagonist. Most of the cardiovascular and peripheral effects of dihydropyridines are attributable to Cav1.2 channels.25 However, the differences in the dihydropyridine binding pocket between Cav1.2 and Cav1.3 subunits are very small, making variations on this structure unlikely to yield highly selective antagonists. Combination therapies involving drugs that have the potential to reduce the deleterious consequences of oxidative phosphorylation should also be explored, despite disappointing results from early clinical trials with them.4 Irrespective of the strategic details, if the ageing model of Parkinson’s disease is correct, neuroprotective therapy should begin well before the appearance of symptoms, at a stage where altering the rate of cell loss can have the biggest effect on the timing of the threshold crossing and the emergence of symptoms—that is, the earlier, the better.
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Conflicts of interest Northwestern University has applied for a use patent in the USA for the targeting of Cav1.3 channels in the treatment of Parkinson’s disease. However, isradipine, the drug discussed in this report, is off-patent in the USA and in Europe.
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Acknowledgments The work described was funded by grants from the US National Institutes of Health. NIH had no role in the preparation of this report.
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References 1 Dauer W, Przedborski S. Parkinson’s disease: mechanisms and models. Neuron 2003; 39: 889–909. 2 Hoehn MM, Yahr MD. Parkinsonism: onset, progression and mortality. Neurology 1967; 17: 427–42. 3 Fahn S, Sulzer D. Neurodegeneration and neuroprotection in Parkinson disease. NeuroRx 2004; 1: 139–54. 4 Hung AY, Schwarzschild MA. Clinical trials for neuroprotection in Parkinson’s disease: overcoming angst and futility? Curr Opin Neurol 2007; 20: 477-83. 5 Braak H, Ghebremedhin E, Rub U, Bratzke H, Del Tredici K. Stages in the development of Parkinson’s disease-related pathology. Cell Tissue Res 2004; 318: 121–34. 6 Hornykiewicz O. Dopamine (3-hydroxytyramine) and brain function. Pharmacol Rev 1966; 18: 925–64. 7 Riederer P, Wuketich S. Time course of nigrostriatal degeneration in parkinson’s disease. A detailed study of influential factors in human brain amine analysis. J Neural Transm 1976; 38: 277–301. 8 Greenamyre JT, Hastings TG. Parkinson’s—divergent causes, convergent mechanisms. Science 2004; 304: 1120–22. 9 Kish SJ, Shannak K, Hornykiewicz O. Uneven pattern of dopamine loss in the striatum of patients with idiopathic Parkinson’s disease. Pathophysiologic and clinical implications. N Engl J Med 1988; 318: 876–80.
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28
29
30 31
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Ito H, Goto S, Sakamoto S, Hirano A. Calbindin-D28k in the basal ganglia of patients with parkinsonism. Ann Neurol 1992; 32: 543–50. Damier P, Hirsch EC, Agid Y, Graybiel AM. The substantia nigra of the human brain. II. Patterns of loss of dopamine-containing neurons in Parkinson’s disease. Brain 1999; 122: 1437–48. Saper CB, Sorrentino DM, German DC, de Lacalle S. Medullary catecholaminergic neurons in the normal human brain and in Parkinson’s disease. Ann Neurol 1991; 29: 577–84. Matzuk MM, Saper CB. Preservation of hypothalamic dopaminergic neurons in Parkinson’s disease. Ann Neurol 1985; 18: 552–55. Fahn S. Does levodopa slow or hasten the rate of progression of Parkinson’s disease? J Neurol 2005; 252 (suppl 4): 37–42. Abou-Sleiman PM, Muqit MM, Wood NW. Expanding insights of mitochondrial dysfunction in Parkinson’s disease. Nat Rev Neurosci 2006; 7: 207–19. Moore DJ, West AB, Dawson VL, Dawson TM. Molecular pathophysiology of Parkinson’s disease. Annu Rev Neurosci 2005; 28: 57–87. Duke DC, Moran LB, Kalaitzakis ME, et al. Transcriptome analysis reveals link between proteasomal and mitochondrial pathways in Parkinson’s disease. Neurogenetics 2006; 7: 139–48. Tepper JM, Paladini CA, Celada P. GABAergic control of the firing pattern of substantia nigra dopaminergic neurons. Adv Pharmacol 1998; 42: 694–99. Grobaski KC, Ping H, daSilva HM, Bowery NG, Connelly ST, Shepard PD. Responses of rat substantia nigra dopamine-containing neurones to (-)-HA-966 in vitro. Br J Pharmacol 1997; 120: 575–80. Ping HX, Shepard PD. Blockade of SK-type Ca²+-activated K+ channels uncovers a Ca2+-dependent slow afterdepolarization in nigral dopamine neurons. J Neurophysiol 1999; 81: 977–84. Ping HX, Shepard PD. Apamin-sensitive Ca(²+)-activated K+ channels regulate pacemaker activity in nigral dopamine neurons. Neuroreport 1996; 7: 809–14. Bonci A, Grillner P, Mercuri NB, Bernardi G. L-Type calcium channels mediate a slow excitatory synaptic transmission in rat midbrain dopaminergic neurons. J Neurosci 1998; 18: 6693–703. Puopolo M, Raviola E, Bean BP. Roles of subthreshold calcium current and sodium current in spontaneous firing of mouse midbrain dopamine neurons. J Neurosci 2007; 27: 645–56. Chan CS, Guzman JN, Ilijic E, et al. ‘Rejuvenation’ protects neurons in mouse models of Parkinson’s disease. Nature 2007; 447: 1081–86. Striessnig J, Koschak A, Sinnegger-Brauns MJ, et al. Role of voltage-gated L-type Ca²+ channel isoforms for brain function. Biochem Soc Trans 2006; 34: 903–09. Xu W, Lipscombe D. Neuronal Ca(V)1.3alpha(1) L-type channels activate at relatively hyperpolarized membrane potentials and are incompletely inhibited by dihydropyridines. J Neurosci 2001; 21: 5944–51. Olson PA, Tkatch T, Hernandez-Lopez S, et al. G-protein-coupled receptor modulation of striatal CaV1.3 L-type Ca²+ channels is dependent on a Shank-binding domain. J Neurosci 2005; 25: 1050–62. Scholze A, Plant TD, Dolphin AC, Nurnberg B. Functional expression and characterization of a voltage-gated CaV1.3 (alpha1D) calcium channel subunit from an insulin-secreting cell line. Mol Endocrinol 2001; 15: 1211–21. Wilson CJ, Callaway JC. Coupled oscillator model of the dopaminergic neuron of the substantia nigra. J Neurophysiol 2000; 83:3084-100. Berridge MJ. Neuronal calcium signaling. Neuron 1998; 21: 13–26. Rizzuto R, Pozzan T. Microdomains of intracellular Ca²+: molecular determinants and functional consequences. Physiol Rev 2006; 86: 369–408. Harman D. The biologic clock: the mitochondria? J Am Geriatr Soc 1972; 20: 145–47. Wallace DC. A mitochondrial paradigm of metabolic and degenerative diseases, aging, and cancer: a dawn for evolutionary medicine. Annu Rev Genet 2005; 39: 359–407. Stark AK, Pakkenberg B. Histological changes of the dopaminergic nigrostriatal system in aging. Cell Tissue Res 2004; 318: 81–92. Ishikawa T, Dhawan V, Kazumata K, et al. Comparative nigrostriatal dopaminergic imaging with iodine-123-beta CIT-FP/SPECT and fluorine-18-FDOPA/PET. J Nucl Med 1996; 37: 1760–65.
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40
41
42
43
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45
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47
48
49
50 51
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Backman L, Ginovart N, Dixon RA, et al. Age-related cognitive deficits mediated by changes in the striatal dopamine system. Am J Psychiatry 2000; 157: 635–37. Zigmond MJ, Abercrombie ED, Berger TW, Grace AA, Stricker EM. Compensations after lesions of central dopaminergic neurons: some clinical and basic implications. Trends Neurosci 1990; 13: 290–96. Bender A, Krishnan KJ, Morris CM, et al. High levels of mitochondrial DNA deletions in substantia nigra neurons in aging and Parkinson disease. Nat Genet 2006; 38: 515–17. Kraytsberg Y, Kudryavtseva E, McKee AC, Geula C, Kowall NW, Khrapko K. Mitochondrial DNA deletions are abundant and cause functional impairment in aged human substantia nigra neurons. Nat Genet 2006; 38: 518–20. Swerdlow RH, Parks JK, Miller SW, et al. Origin and functional consequences of the complex I defect in Parkinson’s disease. Ann Neurol 1996; 40: 663–71. McMahon A, Wong BS, Iacopino AM, Ng MC, Chi S, German DC. Calbindin-D28k buffers intracellular calcium and promotes resistance to degeneration in PC12 cells. Brain Res Mol Brain Res 1998; 54: 56–63. German DC, Manaye KF, Sonsalla PK, Brooks BA. Midbrain dopaminergic cell loss in Parkinson’s disease and MPTP-induced parkinsonism: sparing of calbindin-D28k-containing cells. Ann NY Acad Sci 1992; 648: 42–62. Ghezzi D, Marelli C, Achilli A, et al. Mitochondrial DNA haplogroup K is associated with a lower risk of Parkinson’s disease in Italians. Eur J Hum Genet 2005; 13: 748–52. van der Walt JM, Nicodemus KK, Martin ER, et al. Mitochondrial polymorphisms significantly reduce the risk of Parkinson disease. Am J Hum Genet 2003; 72: 804–11. Shoffner JM, Brown MD, Torroni A, et al. Mitochondrial DNA variants observed in Alzheimer disease and Parkinson disease patients. Genomics 1993; 17: 171–84. Huerta C, Castro MG, Coto E, et al. Mitochondrial DNA polymorphisms and risk of Parkinson’s disease in Spanish population. J Neurol Sci 2005; 236: 49–54. Pyle A, Foltynie T, Tiangyou W, et al. Mitochondrial DNA haplogroup cluster UKJT reduces the risk of PD. Ann Neurol 2005; 57: 564–67. Zhang L, Shimoji M, Thomas B, et al. Mitochondrial localization of the Parkinson’s disease related protein DJ-1: implications for pathogenesis. Hum Mol Genet 2005; 14: 2063–73. Betarbet R, Canet-Aviles RM, Sherer TB, et al. Intersecting pathways to neurodegeneration in Parkinson’s disease: Effects of the pesticide rotenone on DJ-1, alpha-synuclein, and the ubiquitinproteasome system. Neurobiol Dis 2006; 22: 404–20. Sulzer D. Multiple hit hypotheses for dopamine neuron loss in Parkinson’s disease. Trends Neurosci 2007; 30: 244–50. Whitton PS. Inflammation as a causative factor in the aetiology of Parkinson’s disease. Br J Pharmacol 2007; 150: 963–76.
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German DC, Manaye KF, White CL 3rd, et al. Disease-specific patterns of locus coeruleus cell loss. Ann Neurol 1992; 32: 667–76. Williams JT, North RA, Shefner SA, Nishi S, Egan TM. Membrane properties of rat locus coeruleus neurones. Neuroscience 1984; 13: 137–56. Arbuthnott GW, Wickens J. Space, time and dopamine. Trends Neurosci 2007; 30: 62–69. Moore RY, Bloom FE. Central catecholamine neuron systems: anatomy and physiology of the norepinephrine and epinephrine systems. Annu Rev Neurosci 1979; 2: 113–68. Liang CL, Wang TT, Luby-Phelps K, German DC. Mitochondria mass is low in mouse substantia nigra dopamine neurons: implications for Parkinson’s disease. Exp Neurol 2007; 203: 370–80. Paladini CA, Robinson S, Morikawa H, Williams JT, Palmiter RD. Dopamine controls the firing pattern of dopamine neurons via a network feedback mechanism. Proc Natl Acad Sci USA 2003; 100: 2866–71. Grace AA, Bunney BS. The control of firing pattern in nigral dopamine neurons: single spike firing. J Neurosci 1984; 4: 2866–76. Eisenberg MJ, Brox A, Bestawros AN. Calcium channel blockers: an update. Am J Med 2004; 116: 35–43. Ton TG, Heckbert SR, Longstreth WT Jr, et al. Calcium channel blockers and beta-blockers in relation to Parkinson’s disease. Parkinsonism Relat Disord 2007; 13: 165–69. Brucke T, Wober C, Podreka I, et al. D2 receptor blockade by flunarizine and cinnarizine explains extrapyramidal side effects. ASPECT study. J Cereb Blood Flow Metab 1995; 15: 513–18. Rodnitzky RL. Can calcium antagonists provide a neuroprotective effect in Parkinson’s disease? Drugs 1999; 57: 845–49. Kupsch A, Sautter J, Schwarz J, Riederer P, Gerlach M, Oertel WH. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity in non-human primates is antagonized by pretreatment with nimodipine at the nigral, but not at the striatal level. Brain Res 1996; 741: 185–96. Koschak A, Reimer D, Huber I, et al. alpha 1D (Cav1.3) subunits can form l-type Ca²+ channels activating at negative voltages. J Biol Chem 2001; 276: 22100–06. Fitton A, Benfield P. Isradipine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in cardiovascular disease. Drugs 1990; 40: 31–74. Oka H, Yoshioka M, Onouchi K, et al. Characteristics of orthostatic hypotension in Parkinson’s disease. Brain 2007; 130: 2425–32 Calabresi P, Pisani A, Mercuri NB, Bernardi G. Post-receptor mechanisms underlying striatal long-term depression. J Neurosci 1994; 14: 4871–81. Bean BP. Pharmacology of calcium channels in cardiac muscle, vascular muscle, and neurons. Am J Hypertens 1991; 4: 406S–11S. Helton TD, Xu W, Lipscombe D. Neuronal L-type calcium channels open quickly and are inhibited slowly. J Neurosci 2005; 25: 10247–51.
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Calcium, ageing, and neuronal vulnerability in Parkinson’s disease D James Surmeier
Parkinson’s disease is a common neurodegenerative disorder of unknown cause. There is no cure or proven strategy for slowing the progression of the disease. Although there are signs of pathology in many brain regions, the core symptoms of Parkinson’s disease are attributable to the selective degeneration of dopaminergic neurons in the substantia nigra pars compacta. A potential clue to the vulnerability of these neurons is their increasing reliance on Ca²+ channels to maintain autonomous activity with age. This reliance could pose a sustained metabolic stress on mitochondria, accelerating cellular ageing and death. The Ca²+ channels underlying autonomous activity in dopaminergic neurons are closely related to the L-type channels found in the heart and smooth muscle. Systemic administration of isradipine, a dihydropyridine blocker of L-type channels, forces dopaminergic neurons in rodents to revert to a juvenile, Ca²+-independent mechanism to generate autonomous activity. More importantly, reversion confers protection against toxins that produce experimental parkinsonism, pointing to a potential neuroprotective strategy for Parkinson’s disease with a drug class that has been used safely in human beings for decades. These studies also suggest that, although genetic and environmental factors can hasten its onset, Parkinson’s disease stems from a distinctive neuronal design common to all human beings, making its appearance simply a matter of time.
Introduction Parkinson’s disease is a common neurodegenerative disease strongly associated with ageing.1 The cardinal motor symptoms of Parkinson’s disease are bradykinesia, rigidity, and tremor.1,2 Parkinson’s disease has no cure and nothing is known to slow its progression.3,4 Several regions of the brain display signs of pathology in Parkinson’s disease,5 but the motor symptoms of the disease are clearly linked with the degeneration and death of dopamine neurons in the substantia nigra pars compacta (SNc).6,7 The clinical efficacy of levodopa—a dopamine precursor—is testament to the centrality of these neurons in Parkinson’s disease. Why dopamine neurons are preferentially lost in Parkinson’s disease is not clear. Perhaps the most widely held theory suggests that the cause is dopamine itself. There is evidence that oxidation of cytosolic dopamine (and its metabolites) leads to the production of damaging free radicals.8 However, there are many reasons to doubt that this type of cellular stress has a key role in normal ageing and Parkinson’s disease. For example, there is considerable regional variability in the vulnerability of dopamine neurons in Parkinson’s disease, with some classes showing no signs of loss at all.9–13 Moreover, administration of levodopa (which increases dopamine levels) in patients with Parkinson’s disease does not accelerate disease progression,14 suggesting that, at normal cytosolic concentrations, dopamine is not a substantial source of reactive oxidative products and stress. If not dopamine, then what? Several lines of study suggest that mitochondrial and proteasome dysfunction are involved in Parkinson’s disease.15–17 However, why SNc dopamine neurons should be any more vulnerable to this type of dysfunction than other neurons is not clear. Similarly, genetic studies have identified several potential determinants of Parkinson’s disease but have http://neurology.thelancet.com Vol 6 October 2007
Lancet Neurol 2007; 6: 933–38 Department of Physiology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA (D J Surmeier PhD) Correspondence to: Prof D James Surmeier, Department of Physiology, Feinberg School of Medicine, Northwestern University, 303 E Chicago Avenue, Chicago, IL 60611, USA [email protected]
not provided many clues as to selective vulnerability, at least not to this point. None of the genes linked to familial forms of Parkinson’s disease are preferentially expressed by SNc dopamine neurons, suggesting that these neurons display some epigenetic feature that increases the cellular effect of polymorphisms or mutations.
SNc dopamine neurons and calcium The physiology of SNc dopamine neurons might hold the key needed to unlock this mystery. Unlike most neurons in the brain, SNc dopamine neurons are autonomously active; that is, they generate action potentials at a clock-like 2–4 Hz in the absence of synaptic input. In this respect, they are much like cardiac pacemakers. Although they display considerably more complex activity patterns in vivo than do cardiac cells,18 the foundation on which the activity is built is the same. Even more unusual than their autonomous activity is how it is generated. Whereas most neurons use channels that allow Na+ ions across the membrane, SNc dopamine neurons rely on L-type Ca²+ channels.19-23 The L-type channels underlying pacemaking in SNc dopamine neurons have a pore-forming Cav1.3 subunit, rather than the cardiac Cav1.2 subunit.24 Cav1.3 channels constitute roughly a quarter of the L-type channels in the brain,25 have a low affinity for dihydropyridines, and open at more hyperpolarised membrane potentials than do Cav1.2 channels.26–28 The reliance on Cav1.3 Ca²+ channels to drive pacemaking could cause problems. Ca²+ is central to a wide variety of cellular processes ranging from the regulation of enzyme activity and gene expression to programmed cell death. Neurons use Ca²+ entry as a way of monitoring their activity and interactions with other neurons. Befitting its importance to cellular function, there is a complex homoeostatic network inside neurons that allows transient increases in free Ca²+ concentration 933
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but tries to maintain basal levels that are 10 000 times lower (about 50–100 nmol/L) than found in the extracellular space (1–2 mmol/L). Because of its steep concentration gradient, Ca²+ flows into cells readily through pores, like L-type Ca²+ channels, but has to be pumped out through slow membrane transporters that consume energy stored as ATP. In most neurons, the opening of Ca²+ channels is a rare event, occurring mainly during very brief action potentials. As a result, it is easy for a small number of Ca²+ ions to enter the cell, report to intracellular signalling complexes, and then be rapidly extruded back across the plasma membrane at little metabolic cost. But in SNc dopamine neurons, where Ca²+ channels are open much of the time (because they are responsible for pacemaking), the magnitude of the Ca²+ influx seems to be much larger and the burden to the cell much greater.29 Because of the slow kinetics of the plasma membrane transporters and their restriction to the cellular surface, Ca²+ entering neurons must be rapidly sequestered either in organelles lying below the plasma membrane or through ionic interactions with mobile buffering proteins before being escorted out of the cell. Mitochondria and the endoplasmic reticulum (ER) are the principal organelles involved in sequestering Ca²+ in neurons. The ER uses high-affinity ATP-dependent transporters to take Ca²+ from the cytoplasm into the ER lumen. As this store fills up, cytosolic Ca²+ can trigger the opening of ER Ca²+ channels that let the Ca²+ ions flow back into the cytoplasm.30 These channels are often found in close apposition to mitochondria and their opening creates a region of high local Ca²+ concentration that drives influx of Ca²+ into the matrix of mitochondria through Ca²+ uniporters.31 Accumulation of Ca²+ in the mitochondrial matrix again comes at an energetic cost, since it dissipates the electrochemical gradient created by respiratory metabolism along the electron transport chain. Through a poorly understood process, matrix Ca²+ is returned to the cytoplasm where it is either pumped out across the plasma membrane or taken up again by the ER. In most neurons, this interaction is episodic, but in SNc dopamine neurons it must be sustained, going on all the time, consuming ATP produced by oxidative phosphorylation in mitochondria.
Calcium, ageing, and Parkinson’s disease Although the demand on oxidative phosphorylation posed by the reliance on Ca²+ channels for pacemaking is not lethal, it might accelerate the ageing of SNc dopamine neurons. One of the oldest and most popular theories of ageing is that it is a direct consequence of accumulated mitochondrial DNA (mtDNA) damage produced by reactive oxygen species and related free radicals generated in the course of oxidative phosphorylation.32,33 A corollary of this hypothesis is that the rate of ageing is directly related to the metabolic rate. By extension, the reliance of SNc dopamine neurons on a metabolically expensive 934
strategy should mean that they age more rapidly than would other types of neuron. Histological estimates of normal ageing-related cell death are consistent with this hypothesis, suggesting that SNc dopamine neurons are lost at a substantially higher rate (5–10% per decade) than many other types of neurons (some of which show no appreciable loss over a six to seven decade span).34 Functional measures of the nigrostriatal system also decline with normal ageing,35,36 although not as rapidly, presumably because of the capacity of the remaining neurons to compensate.37 There are other signs that SNc dopamine neurons age more rapidly than do other neurons. Perhaps the most compelling is that cells in the SNc have higher than normal levels of mtDNA mutations and have diminished mitochondrial complex I function, both signs of ageing.33,38–40 Another supporting observation is the resilience of SNc dopamine neurons that express Ca²+-buffering proteins, which effectively sequester Ca²+without using ATP; expression of the Ca²+-buffering protein calbindin diminishes vulnerability to mitochondrial toxins41 and seems to confer resistance to the agents at work in Parkinson’s disease.10,11,42 Is Parkinson’s disease thus ageing related, without a pathogen or causative agent other than the physiology of SNc dopamine neurons (figure 1)? This theory predicts that everyone should get Parkinson’s disease if they live long enough. Why, then, do some people develop symptoms in their 50s and others in their 70s, or not at all? Over the course of a lifetime, small differences in the rate of cellular ageing could have a substantial effect on the time at which cell loss reaches the threshold necessary for the emergence of Parkinson’s symptoms (figure 1).37 These differences might arise from genetic mutations or polymorphisms that modestly change the efficiency of mitochondrial oxidative phosphorylation and the rate of generation of reactive oxygen species. Indeed, some mtDNA polymorphisms are associated with a higher incidence of Parkinson’s disease, whereas others are associated with a lower incidence.43–47 Mutations in nuclear genes that produce small alterations in mitochondrial function might have the same sort of effect on cellular ageing and on the probability of developing Parkinson’s disease within a normal lifespan. Mutations in the PARK7 (DJ-1), LRRK2, and PINK1 genes increase the risk of developing Parkinson’s disease, and each gene product is associated with mitochondria.15,48 However, none of these mutations result in a profound mitochondrial or cellular phenotype in mouse models,15 suggesting that their effect on SNc dopamine neurons is slow to develop. Mutations in other nuclear genes associated with Parkinson’s disease—eg, PARK2 and SNCA—have been linked to mitochondrial stress also, albeit indirectly.16 The rate of ageing and cell loss could also be accelerated by secondary factors arising from exposure to environmental toxins that compromise mitochondrial function or that of cellular systems dealing with the consequences of oxidative stress, like the ubiquitin-proteasome system or autophagic http://neurology.thelancet.com Vol 6 October 2007
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vacuoles.16,49,50 These factors might in and of themselves be fairly innocuous but, because of their potential to synergise with the intrinsic vulnerabilities of SNc dopamine neurons, might seem causal, giving Parkinson’s disease a confusing, multifactorial clinical profile. In this model, the rate of cell loss is directly related to the accumulation of cellular defects with ageing (figure 1). In other words, ageing-related deficits in organelle function build upon one another, further increasing stress and the likelihood of cell death. This positive feedback gives our hypothetical ageing plot its non-linear, downwardly curving shape (figure 1). Nominally extrinsic events, like neuromelanin release or inflammation engaged by neuronal death, could provide more positive feedback, leading to a steeper slope as the symptomatic threshold is crossed.50,51 However, when viewed piecemeal, this kind of relation could give the appearance of healthy and diseased states. At early, presymptomatic time points, the rate of cell loss seems to be slow and linear, whereas, at later symptomatic time points, the rate of cell loss seems to be greater (figure 1). Differences of precisely this sort could be used—mistakenly, in my view—to distinguish normal ageing-related cell loss from that found in Parkinson’s disease, providing impetus to extrinsic disease theories. Without longitudinal estimates of SNc dopamine cell numbers in individuals that develop Parkinson’s, it is impossible to exclude the possibility that rapid cell loss in symptomatic patients simply reflects the culmination of a decades long process of accelerated ageing, not the late onset of disease. Can the Ca²+-mediated cellular ageing hypothesis account for the vulnerability of other cell types in Parkinson’s disease? One other region of the brain that has cellular losses paralleling those of the SNc is the locus coeruleus.52 The noradrenergic neurons of the locus coeruleus are very similar to SNc dopamine neurons in several respects. Like SNc dopamine neurons, noradrenergic neurons in the locus coeruleus are autonomous pacemakers.53 More importantly, this pacemaking depends on Ca²+ channels and is likely to create a very similar metabolic stress. Another common feature is their enormous axonal terminal field.54,55 Neurons in both the locus coeruleus and the SNc support more than 100 times as many synapses as do cortical pyramidal neurons. Aside from the additional metabolic burden this poses, these terminal regions might create a mitochondrial sink, thereby lowering mitochondrial mass in the somatodendritic region and the ability to cope with the metabolic demands of Ca²+-dependent pacemaking.56 Lastly, like SNc dopamine neurons, noradrenergic neurons in the locus coeruleus accumulate neuromelanin in autophagic vacuoles, a sign that these neurons are experiencing oxidative stress that is exacerbated by cytosolic dopamine.50 The release of neuromelanin from dying neurons could itself prove toxic to neighbouring neurons, adding to the inflammatory stress triggered by cell death.51
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Figure 1: Hypothetical models of Parkinson’s disease (A) An ageing model where SNc dopamine cell loss is gradual is compared with a conventional disease agent model. The relative proportion of surviving SNc dopamine neurons is plotted as a function of age. (B) Variation in cellular ageing could predispose individuals to early onset of Parkinson’s disease. The dark blue line represents the rate of loss expected from normal ageing; the red line is the rate of loss in a population in which rate of ageing is raised because of genetic or environmental factors. The green line estimates the rate of cell loss in other neuronal populations that do not have high basal metabolic rates. (C) Fitting straight lines to the early and late phases of the ageing model gives the appearance of slow (“normal”) and fast (“diseased”) SNc dopamine cell loss.
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Figure 2: Schematic summarising the key events in the ageing model of Parkinson’s disease At the left, a schematic of an SNc dopamine neuron showing that Ca2+ entry during pacemaking creates a demand on oxidative phosphorylation, leading to increased production of reactive oxygen species and cellular damage associated with ageing (damage is denoted by the red circles). This is recapitulated at the right in the form of a flow diagram. Genetic polymorphisms or mutations and environmental toxins could alter the generation or effect of generation of reactive oxygen species on cellular ageing.
Staving off Parkinson’s disease If Parkinson’s disease is a consequence of the accelerated ageing of neurons that rely heavily on Ca²+ channels (figure 2), then reducing this dependence should delay the onset of the disease and slow its progression. This might be possible with orally deliverable drugs shown to be safe in human beings.24 In mice, the reliance of SNc dopamine neurons on Ca²+ channels to drive pacemaking is developmentally regulated. Young neurons are autonomously active but generate this activity with channels used by pacemakers in the brain that do not succumb in Parkinson’s disease. These channels allow Na+ ions to cross the membrane, not Ca²+ ions. This juvenile mechanism is retained in adult SNc dopamine neurons, but becomes latent. Surprisingly, sustained block of the L-type Cav1.3 Ca²+ channels that underlie normal adult pacemaking re-engages the juvenile mechanism. Rejuvenated SNc dopamine neurons spike at perfectly normal rates, reflecting the strong homoeostatic pressure on this parameter of cellular function.57 Moreover, treated mice have no obvious motor, learning, or cognitive deficits, suggesting that network activity controlling episodic activity is substantially unchanged.22,58 More importantly, although the effect on mitochondrial oxidative stress can only be inferred at this point, switching the pacemaking mechanism of SNc dopamine neurons made them resistant to mitochondrial toxins used to generate animal models of Parkinson’s disease, just like young SNc dopamine neurons. Is there evidence that this strategy might work in human beings to prevent or slow Parkinson’s disease? 936
Ca²+ channel blockers are commonly used in clinical practice, creating a potential database that could be mined for answers. For example, such drugs have been used for decades to treat hypertension. The Ca²+ channel blockers approved for use in hypertension fall into one of five pharmacological categories.59 The most commonly used are verapamil and diltiazem. In a recent epidemiological study of the potential effect of use of Ca²+ channel blockers on the evolution of Parkinson’s disease,60 about two-thirds of the patients were treated with one of these two drugs. No relation between use of such drugs and the incidence of Parkinson’s disease was found. However, neither verapamil nor dilitazem are potent blockers of the Cav1.3 channels that underlie pacemaking in SNc dopamine neurons. Other nominal Ca²+ channel blockers—flunarizine and cinnarizine— worsen the symptoms of Parkinson’s, but this effect is attributable to their antagonism of dopamine receptors, not their block of Ca²+ channels.61 By contrast, retrospective examination of patients treated for hypertension with dihydropyridines—the class of drug used to induce reversion of the SNc dopamine neuron pacemaking and neuroprotection in mice—revealed a lower than expected incidence of Parkinson’s disease.62 Although this finding is encouraging, it is not a substitute for a controlled clinical trial. In moving forward, there are several issues that need to be considered. One is the choice of drug. In the absence of a selective Cav1.3 Ca²+ channel antagonist, dihydropyridines offer the best therapeutic options. Dihydropyridines are more selective blockers of L-type channels than are other Ca²+ channel blockers approved for human use, and have good brain bioavailability.63 However, most members of this drug class, including nimodipine and nifedipine, are more potent blockers of Cav1.2 than of Cav1.3 channels.26 This is also true of isradipine,64 but at the depolarised membrane potentials traversed by pacemaking SNc dopamine neurons, it more potently blocks pacemaking than either nimodipine or nifedipine (unpublished data). At the doses used to treat hypertension, isradipine has fairly minor side-effects, making it attractive clinically.65 However, in patients with Parkinson’s disease, in whom autonomic dysfunction, especially orthostatic hypotension, is pronounced, these side-effects might pose more of a problem.66 What is not clear at this point is how much dihydropyridine is enough to confer protection. In our animal studies, isradipine was administered with subcutaneous, timed-release pellets. The daily release from these pellets was roughly ten times the maximum daily human oral dose recommended for treating hypertension (20 mg/day). Pharmacokinetic studies are underway to map the relation between subcutaneous dose, serum and brain concentrations of isradipine, and neuroprotection in mouse models, to provide a dosing guide for human beings. Because of species-specific differences in absorption and hepatic clearance, the serum and brain concentrations of http://neurology.thelancet.com Vol 6 October 2007
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isradipine needed to protect SNc dopamine neurons in mice could be achievable in human beings within a dose range known to be safe and well tolerated. Within this dose range, there seems to be little effect on brain function, despite L-type channels being implicated in a broad array of functions, including forms of synaptic plasticity thought to underlie learning and memory.67 This lack of effect on brain function could be due to the voltage-dependence of the L-type channel block by dihydropyridines,68 an especially striking feature of Cav1.3 Ca²+ channels.69 If higher concentrations of dihydropyridines are required for protection, then there is a compelling need for a more selective Cav1.3 channel antagonist. Most of the cardiovascular and peripheral effects of dihydropyridines are attributable to Cav1.2 channels.25 However, the differences in the dihydropyridine binding pocket between Cav1.2 and Cav1.3 subunits are very small, making variations on this structure unlikely to yield highly selective antagonists. Combination therapies involving drugs that have the potential to reduce the deleterious consequences of oxidative phosphorylation should also be explored, despite disappointing results from early clinical trials with them.4 Irrespective of the strategic details, if the ageing model of Parkinson’s disease is correct, neuroprotective therapy should begin well before the appearance of symptoms, at a stage where altering the rate of cell loss can have the biggest effect on the timing of the threshold crossing and the emergence of symptoms—that is, the earlier, the better.
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Conflicts of interest Northwestern University has applied for a use patent in the USA for the targeting of Cav1.3 channels in the treatment of Parkinson’s disease. However, isradipine, the drug discussed in this report, is off-patent in the USA and in Europe.
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Acknowledgments The work described was funded by grants from the US National Institutes of Health. NIH had no role in the preparation of this report.
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References 1 Dauer W, Przedborski S. Parkinson’s disease: mechanisms and models. Neuron 2003; 39: 889–909. 2 Hoehn MM, Yahr MD. Parkinsonism: onset, progression and mortality. Neurology 1967; 17: 427–42. 3 Fahn S, Sulzer D. Neurodegeneration and neuroprotection in Parkinson disease. NeuroRx 2004; 1: 139–54. 4 Hung AY, Schwarzschild MA. Clinical trials for neuroprotection in Parkinson’s disease: overcoming angst and futility? Curr Opin Neurol 2007; 20: 477-83. 5 Braak H, Ghebremedhin E, Rub U, Bratzke H, Del Tredici K. Stages in the development of Parkinson’s disease-related pathology. Cell Tissue Res 2004; 318: 121–34. 6 Hornykiewicz O. Dopamine (3-hydroxytyramine) and brain function. Pharmacol Rev 1966; 18: 925–64. 7 Riederer P, Wuketich S. Time course of nigrostriatal degeneration in parkinson’s disease. A detailed study of influential factors in human brain amine analysis. J Neural Transm 1976; 38: 277–301. 8 Greenamyre JT, Hastings TG. Parkinson’s—divergent causes, convergent mechanisms. Science 2004; 304: 1120–22. 9 Kish SJ, Shannak K, Hornykiewicz O. Uneven pattern of dopamine loss in the striatum of patients with idiopathic Parkinson’s disease. Pathophysiologic and clinical implications. N Engl J Med 1988; 318: 876–80.
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28
29
30 31
32 33
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Ito H, Goto S, Sakamoto S, Hirano A. Calbindin-D28k in the basal ganglia of patients with parkinsonism. Ann Neurol 1992; 32: 543–50. Damier P, Hirsch EC, Agid Y, Graybiel AM. The substantia nigra of the human brain. II. Patterns of loss of dopamine-containing neurons in Parkinson’s disease. Brain 1999; 122: 1437–48. Saper CB, Sorrentino DM, German DC, de Lacalle S. Medullary catecholaminergic neurons in the normal human brain and in Parkinson’s disease. Ann Neurol 1991; 29: 577–84. Matzuk MM, Saper CB. Preservation of hypothalamic dopaminergic neurons in Parkinson’s disease. Ann Neurol 1985; 18: 552–55. Fahn S. Does levodopa slow or hasten the rate of progression of Parkinson’s disease? J Neurol 2005; 252 (suppl 4): 37–42. Abou-Sleiman PM, Muqit MM, Wood NW. Expanding insights of mitochondrial dysfunction in Parkinson’s disease. Nat Rev Neurosci 2006; 7: 207–19. Moore DJ, West AB, Dawson VL, Dawson TM. Molecular pathophysiology of Parkinson’s disease. Annu Rev Neurosci 2005; 28: 57–87. Duke DC, Moran LB, Kalaitzakis ME, et al. Transcriptome analysis reveals link between proteasomal and mitochondrial pathways in Parkinson’s disease. Neurogenetics 2006; 7: 139–48. Tepper JM, Paladini CA, Celada P. GABAergic control of the firing pattern of substantia nigra dopaminergic neurons. Adv Pharmacol 1998; 42: 694–99. Grobaski KC, Ping H, daSilva HM, Bowery NG, Connelly ST, Shepard PD. Responses of rat substantia nigra dopamine-containing neurones to (-)-HA-966 in vitro. Br J Pharmacol 1997; 120: 575–80. Ping HX, Shepard PD. Blockade of SK-type Ca²+-activated K+ channels uncovers a Ca2+-dependent slow afterdepolarization in nigral dopamine neurons. J Neurophysiol 1999; 81: 977–84. Ping HX, Shepard PD. Apamin-sensitive Ca(²+)-activated K+ channels regulate pacemaker activity in nigral dopamine neurons. Neuroreport 1996; 7: 809–14. Bonci A, Grillner P, Mercuri NB, Bernardi G. L-Type calcium channels mediate a slow excitatory synaptic transmission in rat midbrain dopaminergic neurons. J Neurosci 1998; 18: 6693–703. Puopolo M, Raviola E, Bean BP. Roles of subthreshold calcium current and sodium current in spontaneous firing of mouse midbrain dopamine neurons. J Neurosci 2007; 27: 645–56. Chan CS, Guzman JN, Ilijic E, et al. ‘Rejuvenation’ protects neurons in mouse models of Parkinson’s disease. Nature 2007; 447: 1081–86. Striessnig J, Koschak A, Sinnegger-Brauns MJ, et al. Role of voltage-gated L-type Ca²+ channel isoforms for brain function. Biochem Soc Trans 2006; 34: 903–09. Xu W, Lipscombe D. Neuronal Ca(V)1.3alpha(1) L-type channels activate at relatively hyperpolarized membrane potentials and are incompletely inhibited by dihydropyridines. J Neurosci 2001; 21: 5944–51. Olson PA, Tkatch T, Hernandez-Lopez S, et al. G-protein-coupled receptor modulation of striatal CaV1.3 L-type Ca²+ channels is dependent on a Shank-binding domain. J Neurosci 2005; 25: 1050–62. Scholze A, Plant TD, Dolphin AC, Nurnberg B. Functional expression and characterization of a voltage-gated CaV1.3 (alpha1D) calcium channel subunit from an insulin-secreting cell line. Mol Endocrinol 2001; 15: 1211–21. Wilson CJ, Callaway JC. Coupled oscillator model of the dopaminergic neuron of the substantia nigra. J Neurophysiol 2000; 83:3084-100. Berridge MJ. Neuronal calcium signaling. Neuron 1998; 21: 13–26. Rizzuto R, Pozzan T. Microdomains of intracellular Ca²+: molecular determinants and functional consequences. Physiol Rev 2006; 86: 369–408. Harman D. The biologic clock: the mitochondria? J Am Geriatr Soc 1972; 20: 145–47. Wallace DC. A mitochondrial paradigm of metabolic and degenerative diseases, aging, and cancer: a dawn for evolutionary medicine. Annu Rev Genet 2005; 39: 359–407. Stark AK, Pakkenberg B. Histological changes of the dopaminergic nigrostriatal system in aging. Cell Tissue Res 2004; 318: 81–92. Ishikawa T, Dhawan V, Kazumata K, et al. Comparative nigrostriatal dopaminergic imaging with iodine-123-beta CIT-FP/SPECT and fluorine-18-FDOPA/PET. J Nucl Med 1996; 37: 1760–65.
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50 51
938
Backman L, Ginovart N, Dixon RA, et al. Age-related cognitive deficits mediated by changes in the striatal dopamine system. Am J Psychiatry 2000; 157: 635–37. Zigmond MJ, Abercrombie ED, Berger TW, Grace AA, Stricker EM. Compensations after lesions of central dopaminergic neurons: some clinical and basic implications. Trends Neurosci 1990; 13: 290–96. Bender A, Krishnan KJ, Morris CM, et al. High levels of mitochondrial DNA deletions in substantia nigra neurons in aging and Parkinson disease. Nat Genet 2006; 38: 515–17. Kraytsberg Y, Kudryavtseva E, McKee AC, Geula C, Kowall NW, Khrapko K. Mitochondrial DNA deletions are abundant and cause functional impairment in aged human substantia nigra neurons. Nat Genet 2006; 38: 518–20. Swerdlow RH, Parks JK, Miller SW, et al. Origin and functional consequences of the complex I defect in Parkinson’s disease. Ann Neurol 1996; 40: 663–71. McMahon A, Wong BS, Iacopino AM, Ng MC, Chi S, German DC. Calbindin-D28k buffers intracellular calcium and promotes resistance to degeneration in PC12 cells. Brain Res Mol Brain Res 1998; 54: 56–63. German DC, Manaye KF, Sonsalla PK, Brooks BA. Midbrain dopaminergic cell loss in Parkinson’s disease and MPTP-induced parkinsonism: sparing of calbindin-D28k-containing cells. Ann NY Acad Sci 1992; 648: 42–62. Ghezzi D, Marelli C, Achilli A, et al. Mitochondrial DNA haplogroup K is associated with a lower risk of Parkinson’s disease in Italians. Eur J Hum Genet 2005; 13: 748–52. van der Walt JM, Nicodemus KK, Martin ER, et al. Mitochondrial polymorphisms significantly reduce the risk of Parkinson disease. Am J Hum Genet 2003; 72: 804–11. Shoffner JM, Brown MD, Torroni A, et al. Mitochondrial DNA variants observed in Alzheimer disease and Parkinson disease patients. Genomics 1993; 17: 171–84. Huerta C, Castro MG, Coto E, et al. Mitochondrial DNA polymorphisms and risk of Parkinson’s disease in Spanish population. J Neurol Sci 2005; 236: 49–54. Pyle A, Foltynie T, Tiangyou W, et al. Mitochondrial DNA haplogroup cluster UKJT reduces the risk of PD. Ann Neurol 2005; 57: 564–67. Zhang L, Shimoji M, Thomas B, et al. Mitochondrial localization of the Parkinson’s disease related protein DJ-1: implications for pathogenesis. Hum Mol Genet 2005; 14: 2063–73. Betarbet R, Canet-Aviles RM, Sherer TB, et al. Intersecting pathways to neurodegeneration in Parkinson’s disease: Effects of the pesticide rotenone on DJ-1, alpha-synuclein, and the ubiquitinproteasome system. Neurobiol Dis 2006; 22: 404–20. Sulzer D. Multiple hit hypotheses for dopamine neuron loss in Parkinson’s disease. Trends Neurosci 2007; 30: 244–50. Whitton PS. Inflammation as a causative factor in the aetiology of Parkinson’s disease. Br J Pharmacol 2007; 150: 963–76.
52 53
54 55
56
57
58 59 60
61
62 63
64
65
66 67
68 69
German DC, Manaye KF, White CL 3rd, et al. Disease-specific patterns of locus coeruleus cell loss. Ann Neurol 1992; 32: 667–76. Williams JT, North RA, Shefner SA, Nishi S, Egan TM. Membrane properties of rat locus coeruleus neurones. Neuroscience 1984; 13: 137–56. Arbuthnott GW, Wickens J. Space, time and dopamine. Trends Neurosci 2007; 30: 62–69. Moore RY, Bloom FE. Central catecholamine neuron systems: anatomy and physiology of the norepinephrine and epinephrine systems. Annu Rev Neurosci 1979; 2: 113–68. Liang CL, Wang TT, Luby-Phelps K, German DC. Mitochondria mass is low in mouse substantia nigra dopamine neurons: implications for Parkinson’s disease. Exp Neurol 2007; 203: 370–80. Paladini CA, Robinson S, Morikawa H, Williams JT, Palmiter RD. Dopamine controls the firing pattern of dopamine neurons via a network feedback mechanism. Proc Natl Acad Sci USA 2003; 100: 2866–71. Grace AA, Bunney BS. The control of firing pattern in nigral dopamine neurons: single spike firing. J Neurosci 1984; 4: 2866–76. Eisenberg MJ, Brox A, Bestawros AN. Calcium channel blockers: an update. Am J Med 2004; 116: 35–43. Ton TG, Heckbert SR, Longstreth WT Jr, et al. Calcium channel blockers and beta-blockers in relation to Parkinson’s disease. Parkinsonism Relat Disord 2007; 13: 165–69. Brucke T, Wober C, Podreka I, et al. D2 receptor blockade by flunarizine and cinnarizine explains extrapyramidal side effects. ASPECT study. J Cereb Blood Flow Metab 1995; 15: 513–18. Rodnitzky RL. Can calcium antagonists provide a neuroprotective effect in Parkinson’s disease? Drugs 1999; 57: 845–49. Kupsch A, Sautter J, Schwarz J, Riederer P, Gerlach M, Oertel WH. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurotoxicity in non-human primates is antagonized by pretreatment with nimodipine at the nigral, but not at the striatal level. Brain Res 1996; 741: 185–96. Koschak A, Reimer D, Huber I, et al. alpha 1D (Cav1.3) subunits can form l-type Ca²+ channels activating at negative voltages. J Biol Chem 2001; 276: 22100–06. Fitton A, Benfield P. Isradipine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in cardiovascular disease. Drugs 1990; 40: 31–74. Oka H, Yoshioka M, Onouchi K, et al. Characteristics of orthostatic hypotension in Parkinson’s disease. Brain 2007; 130: 2425–32 Calabresi P, Pisani A, Mercuri NB, Bernardi G. Post-receptor mechanisms underlying striatal long-term depression. J Neurosci 1994; 14: 4871–81. Bean BP. Pharmacology of calcium channels in cardiac muscle, vascular muscle, and neurons. Am J Hypertens 1991; 4: 406S–11S. Helton TD, Xu W, Lipscombe D. Neuronal L-type calcium channels open quickly and are inhibited slowly. J Neurosci 2005; 25: 10247–51.
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