A synthetic HS structure selectively impairs the morphology and function of excitatory synapse by disrupting neurexin1 interactions

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Excitatory and inhibitory synapses are the two major fundamental units of neuronal communication in the brain. The imbalance between excitatory and inhibitory synapses (E/I imbalance) is a leading mechanism underlying mental illness. Heparan sulfate (HS), a complex polysaccharide frequently implicated in mental disorders, is an emergent player in synaptic function. Yet, it remains unclear whether and how HS plays a preferential role in excitatory versus inhibitory synapses. This question is further complicated by the structural complexity of HS and the combined effects of both HS glycans and their attached proteoglycans. To address this challenge, we developed a platform that combines synthetic chemistry and synaptic biology to dissect the role of pure HS glycans in synapse development. As proof of principle, we assessed the effects of a synthetic dodecasaccharide (12-mer-19) and its non-sulfated counterpart (12-mer-NAc) on excitatory and inhibitory synapses in primary rat hippocampal neuron cultures. Unexpectedly, we found that 12-mer-19 selectively impaired the morphology and function of excitatory but not inhibitory synapses. Mechanistically, 12-mer-19 interferes with the interaction between neurexin1 and its partners at excitatory synapses, but has little effect on neurexin1's partner at inhibitory synapses. Moreover, 12-mer-NAc didn't have such effects, highlighting the importance of sulfated groups. Our results suggest that extracellular complex glycans may have a selective yet underappreciated role in excitatory synapses, perhaps contributing to the E/I imbalance. Moreover, current studies lay a foundation for future work to dissect the contribution of specific heparan sulfate structures to synaptic morphology and function.

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