α2δ-4 Is Required for the Molecular and Structural Organization of Rod and Cone Photoreceptor Synapses

Abstract

α₂δ-4 is an auxiliary subunit of voltage-gated Ca_v1.4 L-type channels that regulate the development and mature exocytotic function of the photoreceptor ribbon synapse. In humans, mutations in the CACNA2D4 gene encoding α₂δ-4 cause heterogeneous forms of vision impairment in humans, the underlying pathogenic mechanisms of which remain unclear. To investigate the retinal function of α₂δ-4, we used genome editing to generate an α₂δ-4 knock-out (α₂δ-4 KO) mouse. In male and female α₂δ-4 KO mice, rod spherules lack ribbons and other synaptic hallmarks early in development. Although the molecular organization of cone synapses is less affected than rod synapses, horizontal and cone bipolar processes extend abnormally in the outer nuclear layer in α₂δ-4 KO retina. In reconstructions of α₂δ-4 KO cone pedicles by serial block face scanning electron microscopy, ribbons appear normal, except that less than one-third show the expected triadic organization of processes at ribbon sites. The severity of the synaptic defects in α₂δ-4 KO mice correlates with a progressive loss of Ca_v1.4 channels, first in terminals of rods and later cones. Despite the absence of b-waves in electroretinograms, visually guided behavior is evident in α₂δ-4 KO mice and better under photopic than scotopic conditions. We conclude that α₂δ-4 plays an essential role in maintaining the structural and functional integrity of rod and cone synapses, the disruption of which may contribute to visual impairment in humans with CACNA2D4 mutations.SIGNIFICANCE STATEMENT In the retina, visual information is first communicated by the synapse formed between photoreceptors and second-order neurons. The mechanisms that regulate the structural integrity of this synapse are poorly understood. Here we demonstrate a role for α₂δ-4, a subunit of voltage-gated Ca²⁺ channels, in organizing the structure and function of photoreceptor synapses. We find that presynaptic Ca²⁺ channels are progressively lost and that rod and cone synapses are disrupted in mice that lack α₂δ-4. Our results suggest that alterations in presynaptic Ca²⁺ signaling and photoreceptor synapse structure may contribute to vision impairment in humans with mutations in the CACNA2D4 gene encoding α₂δ-4.

Keywords: Ca2+ channel; photoreceptor; retina; ribbon synapse; synaptogenesis.