Abnormal CCK/CB1R axon targeting in mouse models of dystroglycanopathy

ABSTRACT

Dystroglycan (Dag1) is a cell adhesion molecule that links the extracellular matrix to the actin cytoskeleton, and is critical for normal muscle and brain development. Mutations in Dag1 or the genes required for its functional glycosylation result in dystroglycanopathy, which is characterized by a wide range of phenotypes including muscle weakness, brain defects, and cognitive impairments. Whereas Dystroglycan’s role in muscle and early brain development are well defined, much less is known about its role at later stages of neural circuit development including synapse formation and refinement. Recent work has found that selective deletion of Dag1 from pyramidal neurons leads to a loss of presynaptic CCK+ inhibitory neurons (INs) early in development. In this study, we investigated how IN development is affected in multiple mouse models of dystroglycanopathy. Widespread forebrain deletion of Dag1 or Pomt2, which is required for Dystroglycan glycosylation, recapitulates brain phenotypes seen in severe forms of dystroglycanopathy. CCK+ INs were present in Dag1 and Pomt2 mutant mice, but their axons failed to properly target the somatodendritic compartment of pyramidal neurons in the hippocampus. In contrast, CCK+ IN axon targeting was largely normal in mouse models of mild dystroglycanopathy with partially reduced Dystroglycan glycosylation (B4gat1, Fkrp). Furthermore, the intracellular domain of Dystroglycan appears to be dispensable for CCK+ IN axon targeting. Collectively, these data show that synaptic defects are a hallmark of severe dystroglycanopathy.