Inhibitory CCK+ basket synapse defects 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 Dystroglycans 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.

The many roles of dystroglycan in nervous system development and function

“…Interestingly, Briatore et al 109 performed dystrophin and Dag1 immunostaining in Purkinje neurons of the dystrophin‐deficient mdx mouse and showed that the extracellular α‐Dag1 maintains proper localization at inhibitory synapses, while the transmembrane β‐Dag1 is undetected. Another disconnect between dystrophin and Dag1 has been observed in the hippocampus, where deletion of DAG1 results in abnormal perisomatic targeting of CCK+ basket interneurons 74 . CCK+ basket interneurons in the mdx hippocampus show the same axonal mistargeting phenotype 87 .…”

“…74,87 Glycosylation is required for neuronal migration and axon guidance, as mutations in genes within the glycosylation pathway result in a near-complete loss of glycosylation phenocopy loss of DAG1 itself. 34,69,74,85,132 However, whether glycosylation is required for Dag1 function at synapses remains unclear. In one study, DAG1 T190M mutants with significant hypoglycosylation show normal pyramidal neuron innervation by CCK+ basket interneurons in the hippocampus.…”

“…This results in the cortical migration phenotypes that resemble type II lissencephaly. 27,[64][65][66]70,74 In contrast, when deletion of DAG1 is restricted to newly born pyramidal neurons using NEX Cre , cortical migration is normal. 27,75,76 This demonstrates that the neuronal migration phenotype in DAG1 mutants is non-cell autonomous, with neuroepithelial cells playing the primary role as a scaffold to support proper neuronal migration.…”

“…77 ) Conversely, mice expressing truncated Dag1 lacking the intracellular domain but with normal alphasubunit glycosylation (Dag1 Δcyto ) exhibit normal cellular migration throughout the central nervous system. 27,35,74,78…”

“…34,35 Since the initial discovery of a role for Dag1 in axon guidance in the spinal cord, additional work has identified the importance of Dag1 for axon guidance in the internal capsule, lateral olfactory tract, anterior commissure, and optic chiasm, supporting a conserved role for Dag1 in axon guidance across multiple systems. 35,[74][75][76]85 The internal capsule is comprised of descending corticothalamic axons (CTAs) and ascending thalamocortical axons (TCAs). A third population of "corridor cells" acts as an intermediate target to facilitate the guidance of CTAs and TCAs as they navigate through the ventral forebrain.…”