Deciphering the molecular mechanisms in photoreceptor wiring

Project Summary Proper transmission of visual information relies on photoreceptors forming appropriate synaptic connections during development. Nearly all retinal diseases that lead to blindness are caused by loss of photoreceptors connections. Thus, elucidating the molecular mechanisms that mediate proper photoreceptor connectivity may lead to better therapies to treat patients with retinal diseases. During development, photoreceptors first synapse selectively to horizontal cells, where the dendrites of horizontal cells synapse to cone photoreceptors and the axon connects to rod photoreceptors. Cones and rods then synapse to their respective bipolar target. Cones synapse to cone bipolars and rods to rod bipolars. The molecular mechanisms that guide selective wiring of the different photoreceptors to their distinct synaptic partners remains poorly understood. Our data shows the L1 cell adhesion molecule Neurofascin (Nfasc) is localized to the synaptic layer during development and expressed in rods, horizontal cells, and rod bipolars. Moreover, we find disruption of Nfasc results in rod synaptic defects and abnormal rod-driven visual responses. As Nfasc is known to mediate adhesive interactions between neurons, we propose Nfasc is a key molecule mediating selective connectivity of rods to horizontal cells and then to rod bipolars. In addition, we find other cell adhesion molecules that are known to work alongside Nfasc (i.e. Caspr, Cntn1, Nrcam), to be expressed in the complementary cone pathway. Thereby, we hypothesize that restricted expression of cell adhesion molecules mediates selective wiring of the different photoreceptors to their respective targets. To test our hypothesis, we will mouse transgenics, in vivo genetic manipulations, and single neuron labeling approaches to identify the key molecular interactions that guide photoreceptors to synapse selectively to different partners. The proposed research will elucidate the adhesive molecular interactions that instruct selective wiring of photoreceptors to horizontal cells (Aim 1) and to bipolar neurons (Aim 2). Through these experiments, we will uncover the molecular mechanisms involved in complex wiring of neural circuits during development. This knowledge will be necessary to develop new strategies to restore vision in those with retinal diseases.