David P. Hoogerheide

α-Synuclein Shows High Affinity Interaction with Voltage-dependent Anion Channel, Suggesting Mechanisms of Mitochondrial Regulation and Toxicity in Parkinson Disease

Background: The intrinsically disordered protein α-synuclein, a hallmark of Parkinson disease, is involved in mitochondrial dysfunction in neurodegeneration and directly interacts with mitochondria. Results: α-Synuclein regulates VDAC permeability; α-synuclein toxicity in yeast depends on VDAC. Conclusion: α-Synuclein both blocks VDAC and translocates via this channel across the mitochondrial outer membrane. Significance: (Patho)physiological roles of monomeric α-synuclein may originate from its interaction with VDAC. Participation of the small, intrinsically disordered protein α-synuclein (α-syn) in Parkinson disease (PD) pathogenesis has been well documented. Although recent research demonstrates the involvement of α-syn in mitochondrial dysfunction in neurodegeneration and suggests direct interaction of α-syn with mitochondria, the molecular mechanism(s) of α-syn toxicity and its effect on neuronal mitochondria remain vague. Here we report that at nanomolar concentrations, α-syn reversibly blocks the voltage-dependent anion channel (VDAC), the major channel of the mitochondrial outer membrane that controls most of the metabolite fluxes in and out of the mitochondria. Detailed analysis of the blockage kinetics of VDAC reconstituted into planar lipid membranes suggests that α-syn is able to translocate through the channel and thus target complexes of the mitochondrial respiratory chain in the inner mitochondrial membrane. Supporting our in vitro experiments, a yeast model of PD shows that α-syn toxicity in yeast depends on VDAC. The functional interactions between VDAC and α-syn, revealed by the present study, point toward the long sought after physiological and pathophysiological roles for monomeric α-syn in PD and in other α-synucleinopathies.

Conductance hysteresis in the voltage-dependent anion channel

Hysteresis in the conductance of voltage-sensitive ion channels is observed when the transmembrane voltage is periodically varied with time. Although this phenomenon has been used in studies of gating of the voltage-dependent anion channel, VDAC, from the outer mitochondrial membrane for nearly four decades, full hysteresis curves have never been reported, because the focus was solely on the channel opening branches of the hysteresis loops. We studied the hysteretic response of a multichannel VDAC system to a triangular voltage ramp the frequency of which was varied over three orders of magnitude, from 0.5xa0mHz to 0.2xa0Hz. We found that in this wide frequency range the area encircled by the hysteresis curves changes by less than a factor of three, suggesting broad distribution of the characteristic times and strongly non-equilibrium behavior. At the same time, quasi-equilibrium two-state behavior is observed for hysteresis branches corresponding to VDAC opening. This enables calculation of the usual equilibrium gating parameters, gating charge and voltage of equipartitioning, which were found to be almost insensitive to the ramp frequency. To rationalize this peculiarity, we hypothesize that during voltage-induced closure and opening the system explores different regions of the complex free energy landscape, and, in the opening branch, follows quasi-equilibrium paths.