Monday, February 10, 2025
2/10/2025
Proper synaptic function relies on a dedicated assembly of pre- and postsynaptic proteins. Neurexin1 is a principal presynaptic organizing protein essential for synaptic function. Mammalian neurexin1 has three isoforms (α, β, and γ). While neurexin1α and β have laminin/neurexin/sex hormone (LNS) domains, 1γ lacks LNS domains. This is important because LNS domains of neurexin1 bind to post-synaptic partners such as neuroligins, which recruit proteins for proper synaptic function. Deleting neurexin1α globally or neurexin1 α and β in hippocampal neurons impairs postsynaptic receptor activation without affecting presynaptic function at CA3-CA1 synapses. Thus, whether and how neurexin1 organizes presynaptic terminals is unclear. Recently, we made the surprising discovery that all three neurexin1 isoforms are covalently attached to a complex glycan, heparan sulfate (HS). We generated a Nrxn1ΔHS mouse line to block HS attachment to all three neurexin1 isoforms and found severe deficits in presynaptic function and synapse ultrastructure. In this proposal, we have generated a Nrxn1-/- mouse line to delete all three isoforms and found similar presynaptic deficits at CA3-CA1 synapses in Nrxn1ΔHS and Nrxn1-/-. These findings now suggest that the neurexin1 protein backbone alone is insufficient for presynaptic function and that additional factors, which act through HS, are involved in presynaptic development. Furthermore, the presynaptic deficit observed in Nrxn1-/- was not found in neurexin1 α and β mutants, raising the exciting possibility that neurexin1γ is sufficient to maintain presynaptic function. Thus, our central hypothesis is that the HS modification has a specific role in neurexin1-mediated presynaptic assembly and function and is essential for the roles of neurexin1γ and a novel HS-binding growth factor pleiotrophin in presynaptic assembly and function. To test this hypothesis, we will examine the specific role of HS in neurexin1-mediated pre- and post-synaptic assembly (Aim 1), the sufficiency of neurexin1γ and its HS in presynaptic assembly and function (Aim 2), and the role of pleiotrophin in presynaptic assembly and function (Aim 3) in mouse brains. To understand the synaptic assembly within an intact brain circuit, we will utilize innovative Expansion Microscopy to visualize synaptic proteins in the higher order at individual synapses. Given the critical role of neurexin1 in synapse development, the expected outcome of this work will provide novel molecular insights into presynaptic development and function. Mutations in neurexin1 and abnormalities in the biosynthesis of HS have been linked to schizophrenia and autism. Therefore, our work will shed light on how mutations in neurexin and heparan sulfate biosynthetic enzymes affect synapse development and function in autism and schizophrenia and may lead to the development of novel therapeutic strategies for these diseases.
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