Indeed, the randomization of anterior-posterior postcrossing trajectories observed in B3gnt1, ISPD, and dystroglycan mutants has not been reported in either Slit or Robo mutants

but is seen in Sema3B/Npn2/Plexin-A1 and Wnt4/Fzd3 mutants ( Lyuksyutova et al., 2003; Nawabi et al., 2010; Zou et al., 2000), suggesting that dystroglycan may organize additional floor plate or basement membrane-associated axon guidance cues. Interestingly, consistent with our observation of axonal guidance defects in B3gnt1, ISPD, and dystroglycan mutants, postmortem analysis of a patient with Walker-Warburg syndrome, a severe form of dystroglycanopathy, revealed a reduction Protein Tyrosine Kinase inhibitor of the spinal cord lateral funiculus ( Kanoff et al., 1998). Together, these findings suggest that defects in axon guidance cue signaling, including Slit-Robo signaling, are contributing factors in the pathology of human patients with dystroglycanopathies. In addition to guiding axonal projections at the floor plate through interactions with Slit, we find that

glycosylated dystroglycan controls axon guidance through a second, distinct mechanism: organization of basement membrane ECM components. Although the role of ECM proteins in regulating axonal growth and guidance has been well documented in vitro, an understanding of how these molecules regulate specific axon guidance events in vivo is lacking. In Drosophila, Laminin A is required for guidance of ocellar photoreceptor axons but not the neighboring mechanosensory bristle axons, demonstrating that different neuronal populations can have distinct ECM Selleckchem PLX3397 requirements for axonal guidance in vivo ( García-Alonso et al., 1996). Throughout

the mammalian nervous system, glycosylated dystroglycan localized near the endfeet of radial neuroepithelial cells serves as an essential scaffold for ECM proteins, including laminin, perlecan, and collagen IV, to form the basement membrane. The axons that form the dorsal funiculus, ventrolateral funiculus, and descending hindbrain projections extend along the basal surface of the developing hindbrain and spinal cord, in direct apposition to the basement membrane ( Figure S6E). The coincident disorganization of these axon tracts and the check disruption of the basement membrane components laminin, perlecan, and collagen IV in B3gnt1, ISPD, and dystroglycan mutants strongly suggests that development of these axonal projections requires dystroglycan to organize the ECM-rich basement membrane as a growth and guidance substrate. Recent work has also implicated the basement membrane in coordinating the localization of axon guidance cues, including draxin in the developing spinal cord ( Islam et al., 2009) and collagen IV-dependent localization of Slit in the optic tectum ( Xiao et al., 2011).