Ankyrins are essential for synapse development in the mouse outer retina

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 distinct 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 post-synaptic neurons such as bipolar cells 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 which then affects connectivity to their bipolar targets and disrupts in vivo retinal responses. In summary, our findings uncovered a new role for Ankyrins in setting up the early connections between horizontal cells and photoreceptors required for proper circuit assembly and normal visual function.