The therapeutic application of potassium gated ATP channels (K-ATP) in Parkinson's disease arises from their ubiquitous expression in the basal ganglia. Regulation of these channels evokes cell hyperpolarization in order to prevent cell excitability. In the mitrochondria, they play a role in translating the metabolic state of the neuron. This week's journal club discussed an article suggesting that K-ATP channels are necessary for the selective vulnerability of dopamine neurons in the substantia nigra pars compacta (SNpc) relative to the ventral tegmental area (VTA). Liss et al demonstrate this theory using mitrochondrial complex I inhibitors rotenone and MPP+, both neurotoxins commonly used in developing Parkinson's disease models in rodents.
Rotenone and MPP+ are known to selectively degenerate dopamine neurons of the SNpc, leaving the VTA dopamine neurons primarily in tact. Liss et al suggest that this phenomenon is due to differential mitochondrial uncoupling (or, disruption of metabolism). Extensive uncoupling with the application of FCCP resulted in activation of K-ATP channels in both the SNpc and VTA. Mild uncoupling with FCCP did not activate K-ATP channels in either region.
"Notably, however, mild uncoupling inverted the response of K-ATP channels to complex I inhibition: in this case, VTA DA neurons, but not SN DA neurons, were hyperpolarized and functionally silenced due to K-ATP channel activation. In the presence of 50 nM FCCP, none of the SN DA neurons was significantly affected by 100 nM rotenone (Fig. 5a,b, left; perforatedpatch recording in 50 nM FCCP: 2.33 ± 0.29 Hz; FCCP + rotenone: 1.92 ± 0.36, n ¼ 6; P ¼ 0.40) or 10 mM MPP+ (data not shown). In contrast, the presence of 50 nM FCCP sensitized K-ATP channels of VTA DA neurons to complex I inhibition (Fig. 5a,b, right; 50 nM FCCP: 2.4 ± 0.55 Hz; FCCP + rotenone: 0 ± 0 Hz, n ¼ 6; P ¼ 0.0075)."
"Stereological analysis of all SN pars compacta neurons in hematoxylin-eosin counterstained sections demonstrated genuineMPTP-induced neuronal death in wildtype mice and confirmed the complete rescue of SN neurons in the Kir6.2-/- mice (Fig. 6d, middle panel; Kir6.2+/+ SN: control, 11,882 ± 222; post MPTP, 8,061 ± 632, P ¼ 0.029; Kir6.2 -/- SN: control, 12,288 ± 231; post-MPTP, 12,619 ± 223; P ¼ 0.36; n ¼ 3 each)." ** Kir6.2 -/- mice are a genetic strain not expressing a unit of the K-ATP channel necessary for activation. This means that blocking the channel's activity prevented SN DA neurons from being lost.I want to see some apoptosis markers in these SNpc DA neurons due to K-ATP activity. The comaprison of SNpc and VTA DA neurons is an invaluable resource for identifying mechanisms of the selective degeneration that marks Parkinson's disease. Because the VTA DA neuron population is so identifiably unaffected by most neurotoxins from which Parkinson's models are developed, the selectivity of the models and the degree of neural degeneration is not only measurable but comparable to many cellular mechanisms of the disease itself. Uncoupling of the mitochondria speaks to selective metabolic toxicity, and a new target for neuroprotective therapies.
** This was a very complex article using six different mouse strains/treatment groups and analyzing the cell viability using electrophysiology, histology and RT-PCR -- I am reciting only the briefest summary which does not to justice to the extensive work done (although my critique is long-winded, I was impressed with these studies).
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