The role of extracellular matrix in axon routing

Project Summary/Abstract Vision, in general, and stereopsis, in particular, depends on the selective decussation of retinal ganglion cell axons and their termination in an orderly topographic manner. Axonal decussation is directed by the radial glia and midline line neurons at the optic chiasm. The radial glia and midline neurons attach to the pial basement membrane (PBM), a cell adherent, highly organized extracellular matrix sheet that forms the basal surface of the brain including the chiasm. A key component of the PBM are laminins. In other brain regions laminins containing the 2 subunit (2 laminins) provide environmental cues that control tissue and cellular organization including polarity, proliferation, and differentiation. We hypothesize that 2 laminins in the PBM of the chiasm function to polarize the radial glia and midline neurons. This polarization, in turn, regulates the spatial organization of guidance cues thereby regulating axon decussation. Consistent with this hypothesis, we found that loss of 2 laminins from the PBM decreased axonal decussation resulting in an increased ipsilateral projection. We will critically test this hypothesis in three aims. In Aim 1 we will determine the role of 2 laminins in the control of axonal guidance cue expression during the early, peak, and late phases of axon growth. In Aim 2, we will determine which laminin receptors are required for the polarity of radial glial cells and axon decussation using a combination of in vivo and in vitro approaches. Our in vivo experiments will employ cell specific deletion of laminin receptors, while our in vitro experiments will employ a technically innovative organotypic approach using function blocking techniques. In Aim 3, we will determine how 2 laminins control the radial glia and midline neuron cell polarity using sparse labeling of the cells and high-resolution microscopy. The experiments proposed here will define the role of the β2 laminins during optic chiasm and tract formation. The findings from this work will pave the way to exploit cell-matrix interactions for the development of rational strategies to promote the guided growth and decussation of regenerating retinal ganglion cell axons.