News Physiol Sci 16: 214-217, 2001;
1548-9213/01 $5.00
News in Physiological Sciences, Vol. 16, No. 5, 214-217,
October 2001
© 2001 Int. Union Physiol. Sci./Am. Physiol. Soc.
ATP-Sensitive Potassium Channels in Dopaminergic Neurons: Transducers of Mitochondrial Dysfunction
Birgit Liss and
Jochen Roeper
MRC Anatomical Neuropharmacology Unit, University of Oxford, OX1 3TH Oxford, UK
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Abstract
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ATP-sensitive potassium (KATP) channels directly couple the metabolic state of a cell to its electrical activity. Dopaminergic midbrain neurons express alternative types of KATP channels mediating their differential response to mitochondrial complex I inhibition. Because reduced complex I activity is present in Parkinson's Disease, differential KATP channel expression suggests a novel candidate mechanism for selective dopaminergic degeneration.
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Introduction
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Dopaminergic neurons play a crucial role in a variety of brain functions such as reward, working memory, and voluntary movement. The latter is severely affected in Parkinson's Disease (PD), a common neurodegenerative movement disorder that affects >1% of the elderly population. The pathological hallmark of PD is the selective degeneration of a subpopulation of dopaminergic midbrain neurons, mainly in the substantia nigra pars compacta (SNpc). This highlights one of the enigmas of PD: why are some dopaminergic neurons highly vulnerable to the degenerative process but others remain largely unaffected? It suggests the presence of relevant differences in gene expression that determine the differential vulnerability of single dopaminergic neurons. In some rare familial forms of PD, the mutated genes have recently been identified (e.g., 
-synuclein, parkin). However, for the common form of sporadic PD, the molecular disease mechanisms remain unclear. Metabolic stress has been identified as an important trigger factor for the neurodegenerative process of PD. In particular, reduced activity of the mitochondrial respiratory chain complex I (CXI, also known as NADH:ubiquinone reductase) of ~40% has been consistently found in PD patients. Cybrid studies indicate that the CXI defect in PD has a genetic component and may arise from mutations in the mitochondrial DNA. In these studies, mitochondria from PD patients were fused with carrier cells that possessed no mitochondria (15). However, the pathophysiological downstream events that are induced by CXI inhibition and lead to the selective dopaminergic degeneration are still unclear.
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The KATP channel
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In this context of metabolic dysfunction in PD, ATP-sensitive potassium (KATP) channels are of special interest, because their open probability directly depends on the metabolic state of a cell. KATP channels are closed at high ATP-to-ADP ratios and open in response to decreased ATP and increased ADP levels. By this mechanism, KATP channel activity exerts a powerful control mechanism of cellular excitability.
The KATP channel was first discovered by Akinori Noma in cardiac myocytes (12) and was subsequently found to be expressed in many other cell types, e.g., in cardiac and smooth muscle, pancreatic beta cells, and various brain regions. KATP channels are octameric proteins consisting of two different types of subunits: members of the Kir6 inwardly rectifying potassium channel family and sulfonylurea receptor (SUR) subunits, which are members of the ATP-binding cassette transporter superfamily (Fig. 1
). In functional channels, four pore-forming Kir6 subunits are joined together with four regulatory SUR subunits. At present, two members of the Kir6 family have been cloned, Kir6.1 and Kir6.2, and two SUR isoforms have been identified, SUR1 and SUR2. A variety of SUR1 and SUR2 splice variants have been described, with SUR2A and SUR2B being the most important. In heterologous expression systems, these different subunit combinations give rise to functional KATP channels with different biophysical, pharmacological, and metabolic properties. Kir6.2 in combination with SUR1 subunits form KATP channels with properties very similar to those described in pancreatic beta cells. Kir6.2 and SUR2A pairs resemble the KATP channels of cardiac and skeletal muscle, and SUR2B in combination with Kir6.1 subunits generates KATP channels that possess properties reminiscent of those studied in smooth muscle. Structure-and-function aspects of the KATP channel have been intensively studied (for recent reviews, see Refs. 1, 2, and 16).
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FIGURE 1. ATP-sensitive potassium (KATP) channels are octameric proteins made up of 4 sulfonylurea receptor subunits (SUR1, SUR2A, or SUR2B) and 4 pore-forming inwardly rectifying potassium channel subunits (Kir6.1 or Kir6.2). Two intracellular nucleotide-binding folds (NBF-1 and NBF-2) are located on the SUR subunits.
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KATP channel subunits are also widely expressed throughout different brain regions, as indicated by sulfonylurea binding studies and in situ hybridization data. Moreover, a variety of different neuronal populations possess somatodendritic KATP currents with diverging biophysical and pharmacological properties (e.g., in hippocampal pyramidal neurons, locus coerulus and dorsal vagal neurons, striatal interneurons, hypothalamic neurons, and GABAergic and dopaminergic substantia nigra neurons), and there is also evidence for functional presynaptic KATP channels. In accordance with the functional and pharmacological diversity, the molecular makeup of neuronal KATP channels also appears to be not homogeneous. With a combined approach of electrophysiological patch-clamp and single-cell mRNA expression profiling techniques, different combinations of coexpressed KATP channel subunits have been identified. This suggests the presence of molecularly distinct neuronal KATP channels that might reflect different functional roles in the brain. Here we would like to focus on the potential role of molecularly identified KATP channel subtypes expressed in dopaminergic SNpc neurons in the context of their selective vulnerability in PD.
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KATP channels in dopaminergic neurons: function and molecular composition
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The functional role of KATP channels is best understood for the pancreatic beta cells where these channels couple blood glucose levels to insulin secretion. Thus beta cell KATP channels are essential in peripheral glucose sensing. By analogy, neuronal KATP channels, in particular in the hypothalamus, are involved in central glucose sensing and neuroendocrine control of glucose homeostasis (7, 11). In a more general sense, neuronal KATP channels could act as energy control elements, adapting electrical activity and in turn neuronal ATP consumption to the delicate metabolic state of neurons. KATP channel-mediated membrane hyperpolarization will reduce neuronal activity and neurotransmitter release and thus could counteract calcium overload and excitotoxicity. This mechanism could be important in pathophysiological situations like ischemia or epilepsy. In addition, activation of KATP channels before an insult initiates adaptive responses (preconditioning) that have strong neuroprotective effects (5). The molecular mechanisms of preconditioning are unclear; however, protein kinase C-mediated phosphorylation of Kir6.2 has recently been proposed.
In this context, KATP channels in dopaminergic neurons might have an additional role. It has been shown that dopaminergic terminals directly innervate brain arterioles. Thus dopaminergic neurons might not only control neuronal activity but also the perfusion of their axonal target areas (6). Under physiological conditions, dopaminergic midbrain neurons show spontaneous action potential firing and, at least in in vitro brain slices, most KATP channels are closed (Fig. 2A). The activation of KATP channels leads to a hyperpolarization of the dopaminergic neurons, accompanied by a complete loss of their normal pacemaker activity (Fig. 2B). However, the KATP channel response to metabolic stress is not uniform within the population of dopaminergic SNpc neurons. In acute mouse brain slices, a partial inhibition of CXI by rotenone only activates KATP channels in a subpopulation of ~40% of dopaminergic neurons with a half-maximal effective concentration (EC50) of 15 nM. Combined single-cell RT-PCR experiments demonstrated that these highly responsive dopaminergic neurons express the KATP channel subunits SUR1 and Kir6.2 (9). In contrast, the other population of dopaminergic SNpc neurons possess a >100-fold higher EC50 (2 µM) for rotenone-induced KATP channel activation, indicative of a significantly lower sensitivity to CXI inhibition. These neurons, which maintain their pacemaker activity during partial CXI inhibition, express the other SUR isoform, SUR2B, in combination with Kir6.2 mRNA (9). It is important to note that the molecular mechanisms defining metabolic sensitivity of KATP channels are still not clear. However, recombinant SUR1/Kir6.2-containing KATP channels also show a higher sensitivity to metabolic stress compared with SUR2A/Kir6.2-mediated channels (2). This demonstrates that the alternative expression of SUR subunits is a major determinant of metabolic sensitivity of KATP channels. In addition, other intracellular factors like phosphoinositol phosphates (e.g., PIP2), G proteins, or protein kinases could modulate the metabolic sensitivity of KATP channels (3, 8, 17). The rotenone EC50 of 15 nM for activating SUR1-containing KATP channels and inducing complete loss of activity would correspond to a degree of CXI inhibition similar to that found in brains of PD patients (15) and suggests the possibility of KATP channel activation in PD. By analogy, the activity of SUR2B-expressing dopaminergic neurons might not be perturbed in PD.
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FIGURE 2. Schematic illustration of the effect of complex I (CXI) inhibition in single dopaminergic neurons. A: under physiological control conditions, dopaminergic neurons show pacemaker activity and most KATP channels are closed. A patch-clamp recording (current clamp) of spontaneous activity of a dopaminergic neuron in an in vitro mouse brain slice preparation is shown at left. B: in the presence of the CXI inhibitor rotenone, SUR1/Kir6.2-containing KATP channels activate. That leads to a membrane hyperpolarization of the dopaminergic neuron, which is accompanied by a complete loss of spontaneous activity. Current-clamp recording of the same neuron as in A but in the presence of the CXI blocker rotenone (100 nM) is shown at left.
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KATP channels in the weaver mouse: a genetic model of dopaminergic degeneration
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Studies of dopaminergic midbrain neurons in the weaver mouse, a genetic mouse model of dopaminergic degeneration similar to that in PD (13), support the idea of KATP channel activation as a neuroprotective strategy. The weaver mouse is characterized by a point mutation in the pore region of a G protein-coupled inwardly rectifying potassium channel (GIRK2). As a homomer, the mutated GIRK2 channel lost its potassium selectivity and can be constitutively active, resulting in a chronic, depolarizing sodium influx into dopaminergic SNpc neurons. The chronic sodium overload is expected to further enhance ATP consumption of the Na+-K+-ATPase, which already is the major single consumer of neuronal ATP. We demonstrated for dopaminergic neurons in homozygous weaver mice that SUR1/Kir6.2-containing KATP channels are tonically activated in direct consequence to the activity of mutated GIRK2 channels in the weaver mouse. This partially compensates for the chronic depolarization of the dopaminergic weaver neurons. Single-cell RT-PCR experiments revealed the complete absence of SUR2B-expressing dopaminergic neurons in weaver mice (9, 10). These results give the first evidence that alternative SUR1 expression and activation of KATP channels might be an important event in the process of dopaminergic neurodegeneration.
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Selective dopaminergic degeneration by CXI inhibition
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The status of CXI inhibition and its potential downstream consequences like KATP channel activation are still unresolved in PD. Here, neurotoxicological animal models of PD, in which the mitochondrial CXI is chronically inhibited, provide further insights. One classic neurotoxicological rodent model of PD is the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse (14). 1-Methyl-4-phenyl-pyridinium (MPP+), the neurotoxic metabolite of MPTP, is selectively transported into dopaminergic neurons via the dopamine transporter (DAT). Once in the cell, MPP+ blocks the CXI and finally triggers selective dopaminergic degeneration, similar to that found in PD. Alternatively, MPP+ can be sequestered into intracellular vesicles via the vesicular monoamine transporter (VMAT2). Because the toxic intracellular MPP+ concentration depends on the relative expression levels of these two dopamine transporters, the pattern of selective vulnerability might simply reflect differential DAT/VMAT2 expression ratios in the dopaminergic midbrain system. Thus the MPTP model can not fully address two central questions: 1) are dopaminergic neurons more vulnerable to CXI inhibition per se compared with other neurons? and 2) is there differential vulnerability to CXI inhibition within the dopaminergic midbrain population, and is this correlated with the pattern of neurodegeneration? In this context, a new neurotoxicological PD model, developed by Bertabet et al. (4), is of high relevance. Chronic brain infusion of low doses of the CXI inhibitor rotenone induced Parkinsonism and dopaminergic degeneration similar to that of PD in rats. Because there is no selective mechanism for rotenone uptake, all neurons throughout the brain are expected to be exposed to the same degree of partial CXI inhibition. Importantly, this rotenone model is highly selective for dopaminergic neurons. Moreover, dopaminergic neurons showed a pattern of differential sensitivity to rotenone-induced cell death, similar to that found in PD. This rotenone model of PD demonstrates that dopaminergic neurons possess important intrinsic differences in responses to CXI inhibition that are crucial for death or survival. Differential gene expression between dopaminergic subpopulations is likely to determine these critical differences in the pathophysiological response to CXI inhibition. One promising candidate that is differentially expressed in dopaminergic neurons and directly senses CXI inhibition is the KATP channel. In the rotenone model, the free toxin concentration is in the range of 20–30 nM. This corresponds to the degree of CXI inhibition found in PD (15) but might not substantially impair ATP synthesis in brain mitochondria. It is, however, sufficient to activate SUR1/Kir6.2-containing KATP channels in dopaminergic neurons. Consequently, a subpopulation of dopaminergic neurons might tonically hyperpolarize and reduce their physiological spontaneous activity (Fig. 2
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If and how KATP channel-mediated changes in electrical activity finally decide about death or survival of dopaminergic neurons is unknown. However, one can speculate that in PD a chronic reduction of neuronal activity might not be primarily neuroprotective by reducing ATP consumption but might lead to a reduced expression of activity-dependent genes that promote survival, such as neurotrophins. In this scenario, transient KATP channel activation is a short-term neuroprotective response to metabolic stress, but chronic KATP channel activity could have fatal consequences for the dopaminergic neuron.
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Acknowledgments
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We apologize to all of our colleagues whose work we could not cite appropriately due to the strict referencing limitations of the journal.
This work was supported by the Medical Research Council and the Deutsche Forschungsgemeinschaft. B. Liss was supported by a Blaschko Visiting Research Scholarship and is a Todd-Bird Junior Research Fellow at New College, Oxford. J. Roeper is the Monsanto Senior Research Fellow at Exeter College, Oxford.
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References
|
|---|
-
Aguilar-Bryan L and Bryan J. Molecular biology of adenosine triphosphate-sensitive potassium channels. Endocr Rev 20: 101–135, 1999.[Abstract/Free Full Text]
-
Ashcroft FM and Gribble FM. Correlating structure and function in ATP-sensitive K+ channels. Trends Neurosci 21: 288–294, 1998.[ISI][Medline]
-
Baukrowitz T, Schulte U, Oliver D, Herlitze S, Krauter T, Tucker SJ, Ruppersberg JP, and Fakler B. PIP2 and PIP as determinants for ATP inhibition of KATP channels. Science 282: 1141–1144, 1998.[Abstract/Free Full Text]
-
Betarbet R, Sherer TB, MacKenzie G, Garcia-Osuna M, Panov AV, and Greenamyre JT. Chronic systemic pesticide exposure reproduces features of Parkinson's disease. Nat Neurosci 3: 1301–1306, 2000.[ISI][Medline]
-
Blondeau N, Plamondon H, Richelme C, Heurteaux C, and Lazdunski M. K(ATP) channel openers, adenosine agonists and epileptic preconditioning are stress signals inducing hippocampal neuroprotection. Neuroscience 100: 465–474, 2000.[ISI][Medline]
-
Krimer LS, Muly EC 3rd, Williams GV, and Goldman-Rakic PS. Dopaminergic regulation of cerebral cortical microcirculation. Nat Neurosci 1: 286–289, 1998.[ISI][Medline]
-
Levin BE, Dunn-Meynell AA, and Routh VH. Brain glucose sensing and body energy homeostasis: role in obesity and diabetes. Am J Physiol Regulatory Integrative Comp Physiol 276: R1223–R1231, 1999.[Abstract/Free Full Text]
-
Lin YF, Jan YN, and Jan LY. Regulation of ATP-sensitive potassium channel function by protein kinase A-mediated phosphorylation in transfected HEK293 cells. EMBO J 19: 942–955, 2000.[ISI][Medline]
-
Liss B, Bruns R, and Roeper J. Alternative sulfonylurea receptor expression defines metabolic sensitivity of K-ATP channels in dopaminergic midbrain neurons. EMBO J 18: 833–846, 1999.[ISI][Medline]
-
Liss B, Neu A, and Roeper J. The weaver mouse gain-of-function phenotype of dopaminergic midbrain neurons is determined by coactivation of wvGirk2 and K-ATP channels. J Neurosci 19: 8839–8848, 1999.[Abstract/Free Full Text]
-
Miki T, Liss B, Minami K, Shiuchi T, Saraya A, Kashima Y, Horiuchi M, Ashcroft F, Minokoshi Y, Roeper J, and Seino S. ATP-sensitive potassium channels in hypothalamic neurons are essential for the maintenance of glucose homeostasis. Nat Neurosci 4: 507–512, 2001.[ISI][Medline]
-
Noma A. ATP-regulated K+ channels in cardiac muscle. Nature 305: 147–148, 1983.[Medline]
-
Patil N, Cox DR, Bhat D, Faham M, Myers RM, and Peterson AS. A potassium channel mutation in weaver mice implicates membrane excitability in granule cell differentiation. Nat Genet 11: 126–129, 1995.[ISI][Medline]
-
Przedborski S and Jackson-Lewis V. Mechanisms of MPTP toxicity. Mov Disord 13: 35–38, 1998.
-
Schapira AH, Gu M, Taanman JW, Tabrizi SJ, Seaton T, Cleeter M, and Cooper JM. Mitochondria in the etiology and pathogenesis of Parkinson's disease. Ann Neurol 44: S89–S98, 1998.[ISI][Medline]
-
Seino S. ATP-sensitive potassium channels: a model of heteromultimeric potassium channel/receptor assemblies. Annu Rev Physiol 61: 337–362, 1999.[ISI][Medline]
-
Shyng SL and Nichols CG. Membrane phospholipid control of nucleotide sensitivity of KATP channels. Science 282: 1138–1141, 1998.[Abstract/Free Full Text]
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Top
Introduction
The KATP channel
