Reduction of PALS1/Nok disrupts retinal lamination through altered cell positioning while preserving photoreceptor self-organization capacity

The vertebrate neural retina is composed of several neuronal types that precisely organize into layers, with photoreceptors facing the outer surface and the projection neurons, retinal ganglion cells, at the innermost layer. This organization, essential for its function, is established during early development through a complex process involving cell-cell interactions such as adhesion. In the case of photoreceptors, two adhesion complexes, based on the adhesive proteins Cadherin2 and Crumbs, appear essential for their correct localization at the outer nuclear layer (ONL). We here aimed at better characterizing the role of the scaffolding protein PALS1, a central component of the Crumbs complex. Through a validated pals1a/nok morpholino knockdown strategy in zebrafish embryos, we demonstrate that its reduced expression causes photoreceptor progenitors to initially disperse as actively migrating cells, to then coalesce into cell groups around the central retina. They eventually start polarizing, forming rosette-like structures with the apical border towards the inside. Conversely, in organoids derived from uncommitted neuroepithelial retinal progenitors, PALS1 deficiency causes an inversion of their localization from internal rosette-like structures to an organized superficial layer. In both conditions, photoreceptors show signs of polarization, with apical borders towards the inside of rosettes in wild-type organoids, to surface-directed apical borders in morphants. Altogether, our results support previous observations of the pivotal function of the Crumbs complex in ONL formation, but also indicate that either the Crumbs complex, or PALS1 itself, are central for the delicate balance in differential cell adhesion partly responsible for retinal lamination.