Ankyrins are essential for synaptic integrity of photoreceptors in the mouse outer retina

Abstract

Retinal circuit assembly relies on the precise timing and positioning of key molecules between neuronal partners to mediate proper synapse formation. In the outer retina, horizontal cells are important interneurons that make the first contacts to photoreceptors and begin to segregate visual information into two distinct pathways by selectively forming synapses to the different types of photoreceptors. Dendrites of horizontal cells synapse exclusively to cone photoreceptors whereas the axon terminal synapses to rod photoreceptors. Failure to properly form these early connections disrupts the downstream connectivity of other postsynaptic neurons and leads to abnormal visual function. Although these early events are critical for proper synapse development, little is known about the molecular mechanisms that establish horizontal cell to photoreceptor connectivity during development. In the present study, we performed single-cell RNA sequencing and uncovered new molecules that are highly expressed in horizontal cells. These include different members of the cytoskeletal scaffolding family of Ankyrins that are known to form specialized regions within neurons by recruiting different molecules to the membrane and linking them to the cytoskeleton. Specifically, we found Ankyrin-B to be highly expressed in horizontal cells at early time points and Ankyrin-G to be expressed at later stages. Loss of both Ankyrin-B and Ankyrin-G leads to synaptic defects between horizontal cells and photoreceptors and disrupts in vivo retinal responses. In summary, our findings uncovered a new role for Ankyrins in mediating synaptic connectivity between horizontal cells and photoreceptors required for normal visual function. SIGNIFICANCE STATEMENTIn the mammalian retina, the first synapse between photoreceptors and their downstream targets begins to separate visual information into two distinct pathways. During retinal development, photoreceptors first make contacts to horizontal cells in a temporal- and spatial-specific manner. Although this initial contact is critical for synaptogenesis, little is known about the key molecules responsible for selective wiring of horizontal cells to photoreceptors. In this study, we performed single-cell RNA sequencing and identified the family of cytoskeletal scaffolding proteins Ankyrins to be differentially expressed in horizontal cells. Loss of Ankyrins impairs synaptic connectivity between horizontal cells and their photoreceptor partners leading to abnormal visual responses. Taken together, our work uncovered a new function of Ankyrins at photoreceptor synapses.