The TrkC-PTPσ complex governs synapse maturation and anxiogenic avoidance via synaptic protein phosphorylation.

The precise organization of pre- and postsynaptic terminals is crucial for normal synaptic function in the brain. In addition to its canonical role as a neurotrophin-3 receptor tyrosine kinase, postsynaptic TrkC promotes excitatory synapse organization through interaction with presynaptic receptor-type tyrosine phosphatase PTPσ. To isolate the synaptic organizer function of TrkC from its role as a neurotrophin-3 receptor, we generated mice carrying TrkC point mutations that selectively abolish PTPσ binding. The excitatory synapses in mutant mice had abnormal synaptic vesicle clustering and postsynaptic density elongation, more silent synapses, and fewer active synapses, which additionally exhibited enhanced basal transmission with impaired release probability. Alongside these phenotypes, we observed aberrant synaptic protein phosphorylation, but no differences in the neurotrophin signaling pathway. Consistent with reports linking these aberrantly phosphorylated proteins to neuropsychiatric disorders, mutant TrkC knock-in mice displayed impaired social responses and increased avoidance behavior. Thus, through its regulation of synaptic protein phosphorylation, the TrkC-PTPσ complex is crucial for the maturation, but not formation, of excitatory synapses in vivo.

α-Synuclein Preformed Fibrils Bind to ß-Neurexins and Impair ß-Neurexin-Mediated Presynaptic Organization.

Synucleinopathies form a group of neurodegenerative diseases defined by the misfolding and aggregation of α-synuclein (α-syn). Abnormal accumulation and spreading of α-syn aggregates lead to synapse dysfunction and neuronal cell death. Yet, little is known about the synaptic mechanisms underlying the α-syn pathology. Here we identified ß-isoforms of neurexins (ß-NRXs) as presynaptic organizing proteins that interact with α-syn preformed fibrils (α-syn PFFs), toxic α-syn aggregates, but not α-syn monomers. Our cell surface protein binding assays and surface plasmon resonance assays reveal that α-syn PFFs bind directly to ß-NRXs through their N-terminal histidine-rich domain (HRD) at the nanomolar range (KD: ~500 nM monomer equivalent). Furthermore, our artificial synapse formation assays show that α-syn PFFs diminish excitatory and inhibitory presynaptic organization induced by a specific isoform of neuroligin 1 that binds only ß-NRXs, but not α-isoforms of neurexins. Thus, our data suggest that α-syn PFFs interact with ß-NRXs to inhibit ß-NRX-mediated presynaptic organization, providing novel molecular insight into how α-syn PFFs induce synaptic pathology in synucleinopathies such as Parkinson's disease and dementia with Lewy bodies.

SorCS1 inhibits amyloid-ß binding to neurexin and rescues amyloid-ß-induced synaptic pathology.

Amyloid-ß oligomers (AßOs), toxic peptide aggregates found in Alzheimer's disease, cause synapse pathology. AßOs interact with neurexins (NRXs), key synaptic organizers, and this interaction dampens normal trafficking and function of NRXs. Axonal trafficking of NRX is in part regulated by its interaction with SorCS1, a protein sorting receptor, but the impact of SorCS1 regulation of NRXs in Aß pathology was previously unstudied. Here, we show competition between the SorCS1 ectodomain and AßOs for ß-NRX binding and rescue effects of the SorCS1b isoform on AßO-induced synaptic pathology. Like AßOs, the SorCS1 ectodomain binds to NRX1ß through the histidine-rich domain of NRX1ß, and the SorCS1 ectodomain and AßOs compete for NRX1ß binding. In cultured hippocampal neurons, SorCS1b colocalizes with NRX1ß on the axon surface, and axonal expression of SorCS1b rescues AßO-induced impairment of NRX-mediated presynaptic organization and presynaptic vesicle recycling and AßO-induced structural defects in excitatory synapses. Thus, our data suggest a role for SorCS1 in the rescue of AßO-induced NRX dysfunction and synaptic pathology, providing the basis for a novel potential therapeutic strategy for Alzheimer's disease.

Synaptic Organizers in Alzheimer's Disease: A Classification Based on Amyloid-ß Sensitivity.

Synaptic pathology is one of the major hallmarks observed from the early stage of Alzheimer's disease (AD), leading to cognitive and memory impairment characteristic of AD patients. Synaptic connectivity and specificity are regulated by multiple trans-bindings between pre- and post-synaptic organizers, the complex of which exerts synaptogenic activity. Neurexins (NRXs) and Leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs) are the major presynaptic organizers promoting synaptogenesis through their distinct binding to a wide array of postsynaptic organizers. Recent studies have shown that amyloid-ß oligomers (AßOs), a major detrimental molecule in AD, interact with NRXs and neuroligin-1, an NRX-binding postsynaptic organizer, to cause synaptic impairment. On the other hand, LAR-RPTPs and their postsynaptic binding partners have no interaction with AßOs, and their synaptogenic activity is maintained even in the presence of AßOs. Here, we review the current evidence regarding the involvement of synaptic organizers in AD, with a focus on Aß synaptic pathology, to propose a new classification where NRX-based and LAR-RPTP-based synaptic organizing complexes are classified into Aß-sensitive and Aß-insensitive synaptic organizers, respectively. We further discuss how their different Aß sensitivity is involved in Aß vulnerability and tolerance of synapses for exploring potential therapeutic approaches for AD.

IgSF21 promotes differentiation of inhibitory synapses via binding to neurexin2α.

Coordinated development of excitatory and inhibitory synapses is essential for higher brain function, and impairment in this development is associated with neuropsychiatric disorders. In contrast to the large body of accumulated evidence regarding excitatory synapse development, little is known about synaptic adhesion and organization mechanisms underlying inhibitory synapse development. Through unbiased expression screens and proteomics, we identified immunoglobulin superfamily member 21 (IgSF21) as a neurexin2α-interacting membrane protein that selectively induces inhibitory presynaptic differentiation. IgSF21 localizes postsynaptically and recruits axonal neurexin2α in a trans-interaction manner. Deleting IgSF21 in mice impairs inhibitory presynaptic organization, especially in the hippocampal CA1 stratum radiatum, and also diminishes GABA-mediated synaptic transmission in hippocampal CA1 neurons without affecting their excitatory synapses. Finally, mice lacking IgSF21 show a sensorimotor gating deficit. These findings suggest that IgSF21 selectively regulates inhibitory presynaptic differentiation through interacting with presynaptic neurexin2α and plays a crucial role in synaptic inhibition in the brain.Molecular mechanisms regulating the development of inhibitory synapses are poorly understood. Here the authors show that IgSF21 interacts with neurexin2α to induce presynaptic differentiation of inhibitory synapses, and that mice lacking IgSF21 exhibit deficits in inhibitory synaptic transmission.

Amyloid-ß Oligomers Interact with Neurexin and Diminish Neurexin-mediated Excitatory Presynaptic Organization.

Alzheimer's disease (AD) is characterized by excessive production and deposition of amyloid-beta (Aß) proteins as well as synapse dysfunction and loss. While soluble Aß oligomers (AßOs) have deleterious effects on synapse function and reduce synapse number, the underlying molecular mechanisms are not well understood. Here we screened synaptic organizer proteins for cell-surface interaction with AßOs and identified a novel interaction between neurexins (NRXs) and AßOs. AßOs bind to NRXs via the N-terminal histidine-rich domain (HRD) of ß-NRX1/2/3 and alternatively-spliced inserts at splicing site 4 of NRX1/2. In artificial synapse-formation assays, AßOs diminish excitatory presynaptic differentiation induced by NRX-interacting proteins including neuroligin1/2 (NLG1/2) and the leucine-rich repeat transmembrane protein LRRTM2. Although AßOs do not interfere with the binding of NRX1ß to NLG1 or LRRTM2, time-lapse imaging revealed that AßO treatment reduces surface expression of NRX1ß on axons and that this reduction depends on the NRX1ß HRD. In transgenic mice expressing mutated human amyloid precursor protein, synaptic expression of ß-NRXs, but not α-NRXs, decreases. Thus our data indicate that AßOs interact with NRXs and that this interaction inhibits NRX-mediated presynaptic differentiation by reducing surface expression of axonal ß-NRXs, providing molecular and mechanistic insights into how AßOs lead to synaptic pathology in AD.

Emerging roles of the neurotrophin receptor TrkC in synapse organization.

Tropomyosin-receptor-kinase (Trk) receptors have been extensively studied for their roles in kinase-dependent signaling cascades in nervous system development. Synapse organization is coordinated by trans-synaptic interactions of various cell adhesion proteins, a representative example of which is the neurexin-neuroligin complex. Recently, a novel role for TrkC as a synapse organizing protein has been established. Post-synaptic TrkC binds to pre-synaptic type-IIa receptor-type protein tyrosine phosphatase sigma (PTPσ). TrkC-PTPσ specifically induces excitatory synapses in a kinase domain-independent manner. TrkC has distinct extracellular domains for PTPσ- and NT-3-binding and thus may bind both ligands simultaneously. Indeed, NT-3 enhances the TrkC-PTPσ interaction, thus facilitating synapse induction at the pre-synaptic side and increasing pre-synaptic vesicle recycling in a kinase-independent fashion. A crystal structure study has revealed the detailed structure of the TrkC-PTPσ complex as well as competitive modulation of TrkC-mediated synaptogenesis by heparan sulfate proteoglycans (HSPGs), which bind the same domain of TrkC as PTPσ. Thus, there is strong evidence supporting a role for the TrkC-PTPσ complex in mechanisms underlying the fine turning of neural connectivity. Furthermore, disruption of the TrkC-PTPσ complex may be the underlying cause of certain psychiatric disorders caused by mutations in the gene encoding TrkC (NTRK3), supporting its role in cognitive functions.