Role of ?-actinin in Cav1.2 Function Abstract L-type Ca2+ channels are tightly coupled to gene transcription, control neuronal excitability, and mediate multiple forms of synaptic plasticity. Yet its cell biology and regulation is remarkably poorly understood. Our long term interest is to determine the molecular mechanisms that regulate the L-type channel Cav1.2 (e.g., Science 293, 98; Science 293, 2205; PNAS 103, 7500; Neuron 78, 483), which is the most prevalent L-type channel in brain and heart. We recently found that ?-actinin binds directly to the IQ motif of the central pore- forming Cav1.2 subunit ?11.2 and augments its surface localization (Neuron 78, 483). We now identify three point mutations in the IQ motif that individually impair ?-actinin binding. Our preliminary electrophysiological and surface labeling data suggest that impairing ?-actinin binding to the IQ motif decreases surface expression and, unexpectedly, also channel open probability (Po).
Aim 1 is the very first comprehensive analysis of trafficking kinetics of WT and point mutated Cav1.2 through the ER-Golgi-TGN secretory pathway and endocytic recycling and degradation pathway by surface biotinylation, N-glycosylation analysis and colocalization with respective markers like BiP and Rab5.
Aim 2 is to determine and compare current density, gating currents, and single channel currents to test whether the ?-actinin binding - deficient point mutations of Cav1.2 have decreased Po. Our structural analysis is guiding charge reversal experiments for unequivocal (!) assignment of deficits in surface expression and Po in the Cav1.2 mutants to loss of ?-actinin binding. We found that Ca2+ influx specifically through Cav1.2 leads to displacement of ?-actinin from the IQ motif by Ca2+/calmodulin and in parallel to run down of Cav1.2 via endocytosis and reduction in Po.
Aim 3 is to unravel the interplay between ?-actinin and calmodulin at the IQ motif to precisely define the molecular mechanisms of how Ca2+/calmodulin displaces ?-actinin and leads to endocytosis and run down of Cav1.2.
Aim 4 will test first the role of ?-actinin in neuronal Cav1.2 functions including Ca2+ influx into spines and regulation of gene expression via NFAT and then why ?-actinin association and, fittingly, surface expression of Cav1.2 is increased in rodent models of senility and Alzheimer?s disease (AD). Increased Cav1.2 channel activity contributes to senile symptoms and AD (e.g., Science 272, 1017; Science 243, 809). Cav1.2 also plays an important role in PTSD and dysregulation of Cav1.2 leads to autism spectrum disorders (Cell 119, 19-31). Thus our work on the functional interplay of Cav1.2 with ?-actinin and calmodulin is of high significance for understanding and ultimately treatment of these brain diseases. On a broader perspective it is of physiological importance as it will define important aspects that govern the functional availability of Cav1.2 with its manifold functions in neurons and beyond including learning and memory.
The L-type Ca2+ channel Cav1.2 is in several ways the most important influx pathway for Ca2+ into neurons. Ca2+ in turn is a potent regulator of neuronal functions including neuronal excitability and gene expression. We recently discovered that the structural protein ?-actinin is the main docking protein that anchors Cav1.2 at defined subcellular regions and especially at postsynaptic sites where neurons receive input from other neurons. Excessive Ca2+ influx via Cav1.2 has been implicated in brain diseases such as Alzheimer?s disease (AD), posttraumatic stress disorder (PTSD), and autism. Prolonged Ca2+ influx leads to displacement of ?-actinin and triggers removal of Cav1.2 from the cell surface. This mechanism curbs influx of Ca2+ into neurons to prevent toxic effects by excessive Ca2+ influx. We have evidence that this mechanism is dysfunct in AD. The excessive Ca2+ influx than can cause neuronal damage leading to loss of neurons. We will define the precise functional interplay between Cav1.2 and ?-actinin to better understand surface targeting of Cav1.2 in health anddiseases including AD and PTSD.
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