Developmental control of rod photoreceptor number via a light-dependent retrograde pathway from intrinsically photosensitive retinal ganglion cells

Photoreception, a form of sensory experience, is essential for normal development of the mammalian visual system. Detecting photons during development is a prerequisite for visual system function - from shaping vision’s first synapse and maturation of retinal vascular networks, to transcriptional establishment and maturation of cell types within the visual cortex. Consistent with this theme, we find that the lighting environment regulates developmental rod photoreceptor apoptosis via OPN4-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs). Using a combination of genetics, sensory environment manipulations, and computational approaches, we establish a molecular pathway in which light-dependent glutamate release from ipRGCs is detected via a transiently expressed kainate receptor (GRIK3) in rod precursors localized to the inner retina. Communication between ipRGCs and nascent inner retinal rods appears to be mediated by transient hybrid neurites projecting from ipRGCs that sense light before eye-opening. These structures, previously referred to as outer retinal dendrites (ORDs), span the ipRGC-rod precursor distance over the first postnatal week and contain the machinery for sensory detection (melanopsin, OPN4) and anterograde neurotransmitter release (Synaptophysin and VGLUT2). Computational and histological assessment of human mid-gestation development reveal conservation of several hallmarks of an ipRGC-to-rod precursor pathway, including displaced rod precursors, transient GRIK3 expression in the rod lineage, and the presence of ipRGCs with putative neurites projecting deep into the developing human retina. Thus, this analysis defines a retinal retrograde signaling pathway that links the sensory environment to rod precursors via ipRGC photoreceptors, allowing the visual system to adapt to distinct lighting environments prior to eye-opening.