Semaphorin 3f and post-embryonic regulation of retinal progenitors.

Neural progenitors produce specific cell types that form the circuits of the nervous system. Extrinsic signals regulate both progenitor proliferation and the production of specific neuron types. Where progenitors reside within a progenitor niche determines to which of these signals they are exposed, and thus likely has important consequences on the progeny they produce. Little is known, however, of the signals that govern progenitor location within the niche. Here we show that a member of the Class III family of secreted Semaphorins, Semaphorin3fa (Sema3fa), is required for the orderly arrangement of progenitors with a niche present in the periphery of the larval and adult retina of zebrafish, the ciliary marginal zone (CMZ). CMZ progenitors express mRNAs for various Sema3 receptors, including for nrp2a, nrp2b and plxna1. Loss of Sema3fa in a previously characterized CRISPR/Cas9-generated sema3fa mutant allele (sema3faca304) results in a reduced eye size in juvenile fish, implicating Sema3fa in the ongoing production of retinal cells by the CMZ. Larval progenitors show altered cell cycle parameters, and the spatial organization of functionally distinct progenitors is disrupted, as is the generation of retinal cell types in the appropriate proportions and numbers. Our data support a model whereby Sema3fa secreted by CMZ progenitors reduces adhesive interactions to allow for smooth progression of progenitors through the niche, ensuring progenitors receive the correct recipe of extrinsic signals to secure the proper generation of new retinal circuits.