Abstract
Purpose :
Establishing proper retinal circuitry remains a central issue in visual system development. How neurons find their correct target remains largely unknown. The first synapse in the retina provides a simple and experimentally tractable system to explore the basis of synaptic specificity. In the Outer Plexiform Layer (OPL), rod and cone photoreceptors synapse selectively with rod bipolars and cone bipolars, respectively.
Methods :
To identify the mechanism of neuron specificity, we used our published RNA sequencing data to identify potential interactions found in synaptic partners. In our sequencing data, we found two members of the L1 cell adhesion family to be differentially expressed in the rod pathway compared to the cone pathway. Neurofascin (Nfasc) is enriched in both rods and rod bipolars, whereas Nrcam is highly expressed in cones and cone bipolars. Based on their unique expression pattern, we propose adhesive interactions between these homophilic adhesion proteins mediate appropriate matching of photoreceptors to their respective bipolar targets. To test this, we used a CRISPR-based genetic approach to disrupt Nfasc in rods and rod bipolars.
Results :
We found that disruption of Nfasc leads to OPL defects affecting rod connectivity but not cone connectivity. Rods synapse to the axon of horizontal cells and to the dendrites of rod bipolars, and cones connect to the dendrites of horizontal cells and to the dendrites of cone bipolars. With loss of Nfasc, we find disorganization in the processes from horizontal cells and rod bipolars but not of cones nor cone bipolars. Furthermore, ectopic expression of Nrcam in rods results in rod terminals projecting deeper into the OPL and failing to stop at the rod layer.
Conclusions :
These data suggests Nfasc plays an important role in rod synapse formation. Future work will focus on testing if Nrcam has a similar role as Nfasc but in cone connectivity. In summary, this work will elucidate the molecular pathways responsible for rod and cone specificity in retinal circuitry.
This is a 2020 ARVO Annual Meeting abstract.