Glycosylation of Cblns attenuates their receptor binding

Cbln1 is the prototype of a family (Cbln1-Cbln4) of secreted glycoproteins and is essential for normal synapse structure and function in cerebellum by bridging presynaptic Nrxn to postsynaptic Grid2. Here we report the effects of glycosylation on the in vitro receptor binding properties of Cblns. Cbln1, 2 and 4 harbor two N-linked glycosylation sites, one at the N-terminus is in a region implicated in Nrxn binding and the second is in the C1q domain, a region involved in Grid2 binding. Mutation (asparagine to glutamine) of the N-terminal site, increased neurexin binding whereas mutation of the C1q site markedly increased Grid2 binding. These mutations did not influence subunit composition of Cbln trimeric complexes (mediated through the C1q domain) nor their assembly into hexamers (mediated by the N-terminal region). Therefore, glycosylation likely masks the receptor binding interfaces of Cblns. As Cbln4 has undetectable Grid2 binding in vitro we assessed whether transgenic expression of wild type Cbln4 or its glycosylation mutants rescued the Cbln1-null phenotype in vivo. Cbln4 partially rescued and both glycosylation mutants completely rescued ataxia in cbln1-null mice. Thus Cbln4 has intrinsic Grid2 binding that is attenuated by glycosylation, and glycosylation mutants exhibit gain of function in vivo.