Dystroglycan regulates visual circuit development

Abstract

Purpose: Human patients with dystroglycanopathy lack functionally glycosylated dystroglycan, and exhibit defects in visual system function. The molecular basis for these defects is largely unknown. This study defines the neuropathological basis of these defects by examining neuronal migration and axon guidance using a mouse model of dystroglycanopathy and identifies a novel role for dystroglycan in regulating visual circuit development.

Methods: Mice with a mutation in isoprenoid synthesis domain (ISPD^L79*) were utilized as a model of severe dystroglycanopathy. Retinal development was assessed at early (e13), middle (e16), and late (P0) timepoints. Cryosections of eyes from mutant mice and age-matched littermates were stained and imaged to study cell migration. Intraretinal axon guidance was examined in cryosections and flatmount preparations, and guidance of retinal ganglion cell (RGC) axons through the optic chiasm was examined in wholemount preparations. ISPD^L79* mutants exhibit perinatal lethality. Therefore, to examine postnatal dendritic stratification within the retina and innervation of retinal recipient regions, we generated mice in which dystroglycan is conditionally deleted in the visual system (DG^F/-;Six3^cre). Cryosections of eyes from mutant mice and control littermates were stained for markers of RGCs and amacrine cells.

Results: We find that dystroglycan is required for proper neuronal migration and axon guidance in the embryonic retina. While retinal development in ISPD^L79* mutants appears normal at e13 (n=4), we observe a progressive breakdown of the inner limiting membrane beginning at e16 (n=3) and continuing at P0 (n=5). This is accompanied by the appearance of mis-migrating neurons that form ectopic clusters in the vitreous space and defects in axonal fasciculation within the optic fiber layer. RGC axons in ISPD^L79* mutants also exhibit significant defects in their trajectory in the optic chiasm as early as e13, exhibiting inappropriate ipsilateral projections and stalling within the chiasm. In the postnatal retina of DG^F/-;Six3^cre mutants (n=3), loss of dystroglycan leads to focal disruptions in the laminar positioning and dendritic stratification of amacrine cells and RGCs.

Conclusions: Using two distinct genetic models, we identify a critical requirement for dystroglycan in the maintenance of the inner limiting membrane and the regulation of neuronal migration and axon guidance during visual circuit development.