Enhancing retinal photoreceptor neurite outgrowth with small molecules and growth factors.

Retinal degenerative diseases are among the leading causes of blindness worldwide. Photoreceptor cell replacement therapy holds promise but remains limited by poor donor cell survival, neurite elongation, and synaptic integration with the host retina. Here, we investigated how small molecules and the retinal microenvironment influence neurite outgrowth of photoreceptor precursor (PRP) cells. Treatment with the RhoA kinase (ROCK) inhibitor Y-27632 significantly increased both the proportion of PRPs bearing neurites and the mean neurite length. Similarly, taurine and retinal conditioned medium (RCM) enhanced neurite extension. RNA-seq analysis of Y-27632-treated PRPs revealed upregulation of genes related to bone morphogenetic protein 4 (BMP4) signaling and phototransduction. Inhibition of BMP4 reduced neurite elongation induced by taurine and RCM, supporting BMP4 pathway involvement in this process. Finally, PRPs seeded on degenerated retinal explants from Royal College of Surgeons (RCS) rats exhibited enhanced neurite extension, an effect further amplified by a collagen matrix mimicking the outer plexiform layer (OPL). These findings demonstrate that neurite elongation in PRPs involves both ROCK and BMP4 pathways and highlight the critical roles of small molecules and a three-dimensional retinal microenvironment in promoting photoreceptor integration.