CDB25:0003923 LRRTM2 — NRXN1

ABSTRACT: We identify the leucine-rich repeat transmembrane protein LRRTM2 as a key regulator of excitatory synapse development and function. LRRTM2 localizes to excitatory synapses in transfected hippocampal neurons, and shRNA-mediated knockdown of LRRTM2 leads to a decrease in excitatory synapses without affecting inhibitory synapses. LRRTM2 interacts with PSD-95 and regulates surface expression of AMPA receptors, and lentivirus-mediated knockdown of LRRTM2 in vivo decreases the strength of evoked excitatory synaptic currents. Structure-function studies indicate that LRRTM2 induces presynaptic differentiation via the extracellular LRR domain. We identify Neurexin1 as a receptor for LRRTM2 based on affinity chromatography. LRRTM2 binds to both Neurexin 1alpha and Neurexin 1beta, and shRNA-mediated knockdown of Neurexin1 abrogates LRRTM2-induced presynaptic differentiation. These observations indicate that an LRRTM2-Neurexin1 interaction plays a critical role in regulating excitatory synapse development.

ABSTRACT: Leucine-rich repeat transmembrane neuronal proteins (LRRTMs) were recently found to instruct presynaptic and mediate postsynaptic glutamatergic differentiation. In a candidate screen, here we identify neurexin-1beta lacking an insert at splice site 4 (-S4) as a ligand for LRRTM2. Neurexins bind LRRTM2 with a similar affinity but distinct code from the code for binding neuroligin-1 (the predominant form of neuroligin-1 at glutamate synapses, containing the B splice site insert). Whereas neuroligin-1 binds to neurexins 1, 2, and 3 beta but not alpha variants, regardless of insert at splice site 4, LRRTM2 binds to neurexins 1, 2, and 3 alpha and beta variants specifically lacking an insert at splice site 4. We further show that this binding code is conserved in LRRTM1, the family member linked to schizophrenia and handedness, and that the code is functional in a coculture hemisynapse formation assay. Mutagenesis of LRRTM2 to prevent binding to neurexins abolishes presynaptic inducing activity of LRRTM2. Remarkably, mutagenesis of neurexins shows that the binding face on neurexin-1beta (-S4) is highly overlapping for the structurally distinct LRRTM2 and neuroligin-1 partners. Finally, we explore here the interplay of neuroligin-1 and LRRTM2 in synapse regulation. In neuron cultures, LRRTM2 is more potent than neuroligin-1 in promoting synaptic differentiation, and, most importantly, these two families of neurexin-binding partners cooperate in an additive or synergistic manner. Thus, we propose a synaptic code hypothesis suggesting that neurexins are master regulators of the cooperative activities of LRRTMs and neuroligins.

ABSTRACT: Growing genetic evidence is converging in favor of common pathogenic mechanisms for autism spectrum disorders (ASD), intellectual disability (ID or mental retardation) and schizophrenia (SCZ), three neurodevelopmental disorders affecting cognition and behavior. Copy number variations and deleterious mutations in synaptic organizing proteins including NRXN1 have been associated with these neurodevelopmental disorders, but no such associations have been reported for NRXN2 or NRXN3. From resequencing the three neurexin genes in individuals affected by ASD (n = 142), SCZ (n = 143) or non-syndromic ID (n = 94), we identified a truncating mutation in NRXN2 in a patient with ASD inherited from a father with severe language delay and family history of SCZ. We also identified a de novo truncating mutation in NRXN1 in a patient with SCZ, and other potential pathogenic ASD mutations. These truncating mutations result in proteins that fail to promote synaptic differentiation in neuron coculture and fail to bind either of the established postsynaptic binding partners LRRTM2 or NLGN2 in cell binding assays. Our findings link NRXN2 disruption to the pathogenesis of ASD for the first time and further strengthen the involvement of NRXN1 in SCZ, supporting the notion of a common genetic mechanism in these disorders.

ABSTRACT: Leucine-rich repeat transmembrane neuronal proteins (LRRTMs) function as postsynaptic organizers that induce excitatory synapses. Neurexins (Nrxns) and heparan sulfate proteoglycans have been identified as presynaptic ligands for LRRTMs. Specifically, LRRTM1 and LRRTM2 bind to the Nrxn splice variant lacking an insert at the splice site 4 (S4). Here, we report the crystal structure of the Nrxn1β-LRRTM2 complex at 3.4 Å resolution. The Nrxn1β-LRRTM2 interface involves Ca2+-mediated interactions and overlaps with the Nrxn-neuroligin interface. Together with structure-based mutational analyses at the molecular and cellular levels, the present structural analysis unveils the mechanism of selective binding between Nrxn and LRRTM1/2 and its modulation by the S4 insertion of Nrxn.