Midkines, in the Neural Stem Cell Niche, during Developmental and Regenerative Neurogenesis and Their Regulation by the Circadian Clock in the Retina of Zebrafish.

Abstract

In the retina of adult teleosts, stem cells are sustained in two specialized niches: the ciliary marginal zone (CMZ) and the microenvironment surrounding adult Müller glia. Recently, Müller glia were identified as retinal stem cells responsible for neuronal regeneration. In a screen to discover secreted molecules that regulate neuronal regeneration in the retina, we identified midkine-b (mdkb). Midkine is a highly conserved pleiotropic, heparin-binding growth-factor. The zebrafish genome encodes two midkine genes: midkine-a (mdka) and mdkb. Expression and function of Midkines in the vertebrate retina are largely unknown. My research shows that zebrafish mdka and mdkb are expressed in distinct patterns in developing, mature and regenerating retina, suggesting different functions for the two molecules. In the developing zebrafish retina, mdka is expressed in the CMZ and mdkb in newly postmitotic cells, suggesting these molecules may sequentially regulate aspects of retinal neurogenesis. In the juvenile/adult retina, mdka is expressed in presumptive Müller glia at the retinal margin, cells at the origin of the rod photoreceptor lineage, and in horizontal cells. Following selective death of photoreceptors in the adult retina, mdka and mdkb are co-expressed in horizontal cells, proliferating Müller glia and their neurogenic progeny. The retina entrains the circadian clock to changes in the light/dark cycle and is characterized by numerous biological processes that follow a circadian rhythm. Expression of Mdka in horizontal cells is regulated by the circadian clock, with increased expression during subjective day. Expression of mdkb is weakly modulated by the circadian clock, increasing during subjective night in horizontal cells. The two midkin es show therefore asynchronous circadian regulation, suggesting different biological activities at distinct circadian times. Expression of mdkb in horizontal cells during the subjective night, similar to the regenerating retina, suggests a role in persistent neurogenesis. In conclusion, Mdka and Mdkb are molecular components in the retinal stem cell compartments during developmental, regenerative and growth-associated neurogenesis suggesting they function as autocrine/paracrine signaling molecules and sequentially regulate different aspects of neurogenesis in the zebrafish retina. These data establish the foundation for future studies to investigate functional roles of these molecules in retinal neurogenesis.