The extracellular matrix in selective decussation of retinal ganglion cell axons: β2 laminins regulate the ipsilateral projection.

In binocular animals, retinal ganglion cell (RGC) axons selectively decussate at the optic chiasm. Selective decussation is directed by the radial glia and midline neurons in the optic chiasm. Radial glia attach to the underlying pial basement membrane (PBM) that is rich in β2 laminins. Here, we asked whether, and if so, how, β2 laminins in the PBM impact the selective decussation of RGC axons. β2 laminins are expressed in the PBM during the peak and late phases of RGC axon growth, and their deletion results in the fragmentation of the PBM and dysmorphic radial glia that lack properly elaborated endfeet. Genetic deletion of the β2 laminin subunit increased the proportion of non-decussating RGC axons in the ipsilateral tract. Consistent with the increase in the proportion of ipsilateral axons, we found persistent expression of the ipsilateral guidance cue EphrinB2 in radial glia during the late phase of axonal decussation and an increased number of ipsilateral RGCs in the ventrotemporal retina. These results suggest that in the optic chiasm β2 laminins are required for the integrity of the PBM and radial glia organization, and that this in turn may influence the production of EphrinB2. Our results also suggest that β2 laminins in the retina are necessary for neurogenesis and the laterality fate of ipsilateral RGCs, a function possibly mediated by their role in maintaining the attachment of retinal progenitor cells (RPCs) to the inner limiting membrane (ILM).