A COMBINED CELL-SURFACE PROTEOMIC AND FUNCTIONAL FRAMEWORK IDENTIFIES IGLON5 AS A REGULATOR OF DIRECTION-SELECTIVE CIRCUITS

Cell adhesion molecules (CAMs) are thought to guide neural circuit assembly by directing selective synaptic connections, yet the molecular “connectivity code” organizing synapses in intact circuits remains poorly defined. Moreover, most functional studies of CAMs rely on gross anatomical phenotypes, potentially missing critical circuit dysfunction. To overcome these limitations, we conducted a comprehensive survey of cell-surface proteins across the retina’s inner plexiform layer (IPL) and established an alternative framework for functionally characterizing circuit dysfunction.

Using in situ cell-surface proteome extraction via extracellular labeling (iPEEL) in multiple retinal cell types stratifying at distinct IPL sublayers, we profiled cell-surface proteins across the IPL. Integration of these datasets with cell-type-specific transcriptomic profiles revealed circuit-specific CAM codes that instruct synaptic interactions. Among several previously uncharacterized candidates, we identified IgLON5, an immunoglobulin superfamily CAM recently implicated in autoimmune neurodegeneration.

IgLON5 is enriched in starburst amacrine cells (SACs), which are essential for direction-selective (DS) computation. Despite this enrichment, SAC lamination and overall morphology within the IPL remained intact in an IgLON5 loss-of-function mouse model. To assess functional consequences, we performed high-throughput ex-vivo retinal calcium imaging. Compared with controls, direction-selective ganglion cells (DSGCs) exhibited weaker DS responses. These deficits were accompanied by marked alterations in DSGC receptive field organization, including reduced surround strength and enlarged center size, consistent with diminished SAC-mediated input. Such changes are predicted to impair processing of naturalistic motion stimuli.

Together, this IPL cell-surface proteome map reveals molecular mechanisms of sublamina-specific circuit assembly and provides a resource for decoding CAM-based synaptic specificity.