Oligodendrocyte precursor cells (OPCs) exist in abundance throughout the developing and adult central nervous system (CNS), where they evenly distribute and form tiled networks with their elaborate processes. It is known that OPC tiling is established and maintained by principles of self-avoidance and contact-mediated repulsion. However, it is unknown what signalling mechanisms OPCs use to form and maintain these adaptive networks, nor do we understand why OPCs form these process networks throughout the CNS.

Using RNA-sequencing analyses complemented by whole-mount RNA labelling in zebrafish we screened candidate genes with known roles in self-avoidance and contact mediated repulsion for enriched expression in OPCs. We identified the dscam gene family (consisting of dscama, dscamb, and dscaml1), as most highly expressed by OPCs but not in mature oligodendrocytes. Moreover, we observed that all OPCs co-express all three dscam genes.

To test for roles of dscams in OPC tiling we used injection-mediated CRISPR knockdown of dscamb and dscaml1 in transgenic zebrafish that have their entire OPC population fluorescently labelled. These ‘CRISPants’ exhibit a striking disruption of OPC network tiling with reduced process density and large areas that are not interspersed by OPC processes. We also observed less regular spacing of OPC somas with both clumping of somas and larger regions devoid of OPCs. In addition to these population effects, high-resolution analysis revealed that OPCs in dscamb/l1 CRISPants exhibit thicker, more highly branched and frequently clumped processes. Timelapse analysis of OPC process dynamics showed that they also explore smaller territories than wildtype controls.

Together, our data reveal a key role for dscam signalling in the formation of tiled OPC networks and normal OPC process morphology. In our ongoing work we are generating cell type specific and sparse manipulations of dscam to test whether this phenotype represents an OPC autonomous deficit, or one that is mediated by intercellular signalling between neighbouring OPCs. Furthermore, we will present data investigating how dscam-mediated disruption of OPC tiling may affect their roles in patterning a myelinated CNS.