ABSTRACT
Synapses are fundamental units of communication in the brain. The prototypical synapse-organizing complex neurexin-neuroligin mediates synapse development and function and is central to a shared genetic risk pathway in autism and schizophrenia. Neurexin's role in synapse development is thought to be mediated purely by its protein domains, but we reveal a requirement for a rare glycan modification. Mice lacking heparan sulfate (HS) on neurexin-1 show reduced survival, as well as structural and functional deficits at central synapses. HS directly binds postsynaptic partners neuroligins and LRRTMs, revealing a dual binding mode involving intrinsic glycan and protein domains for canonical synapse-organizing complexes. Neurexin HS chains also bind novel ligands, potentially expanding the neurexin interactome to hundreds of HS-binding proteins. Because HS structure is heterogeneous, our findings indicate an additional dimension to neurexin diversity, provide a molecular basis for fine-tuning synaptic function, and open therapeutic directions targeting glycan-binding motifs critical for brain development.
Subject(s)
Heparitin Sulfate/metabolism , Neural Cell Adhesion Molecules/metabolism , Synapses/metabolism , Amino Acid Sequence , Animals , Calcium-Binding Proteins , Cell Adhesion Molecules, Neuronal/antagonists & inhibitors , Cell Adhesion Molecules, Neuronal/genetics , Cell Adhesion Molecules, Neuronal/metabolism , Drosophila , Drosophila Proteins/antagonists & inhibitors , Drosophila Proteins/genetics , Drosophila Proteins/metabolism , Female , Glycopeptides/analysis , Heparitin Sulfate/chemistry , Humans , Membrane Proteins , Mice , Mice, Inbred C57BL , Nerve Tissue Proteins , Neural Cell Adhesion Molecules/antagonists & inhibitors , Neural Cell Adhesion Molecules/genetics , Neurons/cytology , Neurons/metabolism , Protein Binding , RNA Interference , RNA, Small Interfering/metabolism , Rats , Sequence AlignmentABSTRACT
Gas2-like proteins harbour putative binding sites for both the actin and the microtubule cytoskeleton and could thus mediate crosstalk between these cytoskeletal systems. Family members are highly conserved in all metazoans but their in vivo role is not clear. The sole Drosophila Gas2-like gene, CG3973 (pigs), was recently identified as a transcriptional target of Notch signalling and might therefore link cell fate decisions through Notch activation directly to morphogenetic changes. We have generated a null mutant in CG3973 (pigs): pigs(1) mutants are semi-viable but adult flies are flightless, showing indirect flight muscle degeneration, and females are sterile, showing disrupted oogenesis and severe defects in follicle cell differentiation, similar to phenotypes seen when levels of Notch/Delta signalling are perturbed in these tissues. Loss of Pigs leads to an increase in Notch signalling activity in several tissues. These results indicate that Gas2-like proteins are essential for development and suggest that Pigs acts downstream of Notch as a morphogenetic read-out, and also as part of a regulatory feedback loop to relay back information about the morphogenetic state of cells to restrict Notch activation to appropriate levels in certain target tissues.