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Peter Koulen; The synaptic clustering protein Vesl-1/Homer 1 mediates protection of retinal neurons. Invest. Ophthalmol. Vis. Sci. 2013;54(15):3263.
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© ARVO (1962-2015); The Authors (2016-present)
Optic neuropathies resulting in the acute or gradual loss of visual function share cell death of retinal ganglion cells (RGCs) as a defining characteristic. Specifically, loss of RGCs due to programmed cell death in degenerative retinopathies is preceded by L-Glutamate mediated excitotoxicity, subsequent intracellular calcium dyshomeostasis and calciotoxicity. The Vesl-1/Homer 1 (H1) protein, as a member of the Homer protein family of synaptic clustering proteins, controls the intracellular free calcium ion concentration by interaction with intracellular calcium release channels and their upstream G protein signaling cascades. The present study tested the hypothesis that modulation of the H1 protein concentration in RGCs leads to protection against excitotoxicity.
RGCs and retinal explants were isolated and cultured from adult C57BL/6J mice and cellular viability in response to chronic L-Glutamate-mediated toxicity was measured using RGC-specific immunohistochemistry and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assays. The concentration of H1 proteins in RGCs and organotypic cultures was modulated using H1 isoform-specific expression vectors and small interfering RNA and measured using microfluorimetry. Changes in the intracellular free calcium ion concentration were determined using calcium imaging and pharmacological stimulation of intracellular calcium release.
Both the long and the short isoform of H1, Vesl-1L/Homer 1c (H1c) and Vesl-1S/Homer 1a (H1a), respectively, are expressed by RGCs. Overexpression of H1a increased RGC viability in both isolated cells and organotypic cultures by 37% and 46%, respectively. This was accompanied by attenuation of calcium release from inositol-1,4,5-trisphosphate (IP3)-sensitive stores by 29% and 38%, respectively. Overexpression of H1c or siRNA mediated knockdown of H1a resulted in increased calcium release and decreased RGC viability.
H1 isoforms differentially modulate the gain of intracellular calcium release allowing the control of cellular toxicity affecting intracellular calcium levels with H1a as a dominant-negative regulator generating RGC protection. This mechanism of action represents a novel target for therapeutic intervention in optic neuropathies.
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