The dystroglycan protein complex provides a link between the cytoskeleton and the extracellular matrix (ECM). Defective O-glycosylation of a-dystroglycan (a-DG) severs this link leading to muscular dystrophies named dystroglycanopathies. These are characterized not only by muscle degeneration, but also by brain and ocular defects. In brain and retina, a-DG and ECM molecules are enriched around blood vessels where they may be involved in localizing the inwardly rectifying potassium channel, Kir4.1, and aquaporin channel, AQP4, to astrocytic endfeet. To investigate in vivo the role of ECM ligand-binding to glycosylated sites on a-DG in the polarized distribution of these channels, we used the Large myd mouse, an animal model for dystroglycanopathies. We found that Kir4.1 and AQP4 are lost from astrocytic endfeet in brain whereas significant labeling for these channels is detected at similar cell domains in retina. Furthermore, while both a-and b1-syntrophins are lost from perivascular astrocytes in brain, labeling for b1-syntrophin is found in retina of the Large myd mouse. These findings show that while ligand-binding to the highly glycosylated isoform of a-DG in concert with a-and b1-syntrophins is crucial for the polarized distribution of Kir4.1 and AQP4 to functional domains in brain, distinct mechanisms may contribute to their localization in retina.

Several recent studies have shown that neuroligin 2 (NL2), a component of the cell adhesion neurexins-neuroligins complex, is localized postsynaptically at hippocampal and other inhibitory synapses throughout the brain. Other studies have shown that components of the dystroglycan complex are also localized at a subset of inhibitory synapses and are coexpressed with NL2 in brain. These data prompted us to undertake a comparative study between the localization of NL2 and the dystroglycan complex in the rodent retina. First, we determined that NL2 mRNA is expressed both in the inner and in the outer nuclear layers. Second, we found that NL2 is localized both in the inner and in the outer synaptic plexiform layers. In the latter, the horseshoe-shaped pattern of NL2 and its extensive colocalization with RIM2, a component of the presynaptic active zone at ribbon synapses, argue that NL2 is localized presynaptically at photoreceptor terminals. Third, comparison of NL2 and the dystroglycan complex distribution patterns reveals that, despite their coexpression in the outer plexiform layer, they are spatially segregated within distinct domains of the photoreceptor terminals, where NL2 is selectively associated with the active zone and the dystroglycan complex is distally distributed in the lateral regions. Finally, we report that the dystroglycan deficiency in the mdx(3cv) mouse does not alter NL2 localization in the outer plexiform layer. These data show that the NL2- and dystroglycan-containing complexes are differentially localized in the presynaptic photoreceptor terminals and suggest that they may serve distinct functions in retina.