Structural basis of SALM3 dimerization and synaptic adhesion complex formation with PTPσ

Alternative splicing controls teneurin-3 compact dimer formation for neuronal recognition

Neuronal network formation is facilitated by recognition between synaptic cell adhesion molecules at the cell surface. Alternative splicing of cell adhesion molecules provides additional specificity in forming neuronal connections. For the teneurin family of cell adhesion molecules, alternative splicing of the EGF-repeats and NHL domain controls synaptic protein-protein interactions. Here we present cryo-EM structures of the compact dimeric ectodomain of two teneurin-3 isoforms that harbour the splice insert in the EGF-repeats. This dimer is stabilised by an EGF8-ABD contact between subunits. Cryo-EM reconstructions of all four splice variants, together with SAXS and negative stain EM, reveal compacted dimers for each, with variant-specific dimeric arrangements. This results in specific trans-cellular interactions, as tested in cell clustering and stripe assays. The compact conformations provide a structural basis for teneurin homo- and heterophilic interactions. Altogether, our findings demonstrate how alternative splicing results in rearrangements of the dimeric subunits, influencing neuronal recognition and likely circuit wiring.

Crystallization and low-resolution structure solution of the SALM3-PTPσ synaptic adhesion complex

Synaptic adhesion molecules are major organizers of the neuronal network and play a crucial role in the regulation of synapse development and maintenance in the brain. Synaptic adhesion-like molecules (SALMs) and leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-PTPs) are adhesion protein families with established synaptic function. Dysfunction of several synaptic adhesion molecules has been linked to cognitive disorders such as autism spectrum disorders and schizophrenia. A recent study of the binding and complex structure of SALM3 and PTPσ using small-angle X-ray scattering revealed a 2:2 complex similar to that observed for the interaction of human SALM5 and PTPδ. However, the molecular structure of the SALM3-PTPσ complex remains to be determined beyond the small-angle X-ray scattering model. Here, the expression, purification, crystallization and initial 6.5 Å resolution structure of the mouse SALM3-PTPσ complex are reported, which further verifies the formation of a 2:2 trans-heterotetrameric complex similar to the crystal structure of human SALM5-PTPδ and validates the architecture of the previously reported small-angle scattering-based solution structure of the SALM3-PTPσ complex. Details of the protein expression and purification, crystal optimization trials, and the initial structure solution and data analysis are provided.

SALM4 negatively regulates NMDA receptor function and fear memory consolidation

Many synaptic adhesion molecules positively regulate synapse development and function, but relatively little is known about negative regulation. SALM4/Lrfn3 (synaptic adhesion-like molecule 4/leucine rich repeat and fibronectin type III domain containing 3) inhibits synapse development by suppressing other SALM family proteins, but whether SALM4 also inhibits synaptic function and specific behaviors remains unclear. Here we show that SALM4-knockout (Lrfn3^-/-) male mice display enhanced contextual fear memory consolidation (7-day post-training) but not acquisition or 1-day retention, and exhibit normal cued fear, spatial, and object-recognition memory. The Lrfn3^-/- hippocampus show increased currents of GluN2B-containing N-methyl-D-aspartate (NMDA) receptors (GluN2B-NMDARs), but not α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors (AMPARs), which requires the presynaptic receptor tyrosine phosphatase PTPσ. Chronic treatment of Lrfn3^-/- mice with fluoxetine, a selective serotonin reuptake inhibitor used to treat excessive fear memory that directly inhibits GluN2B-NMDARs, normalizes NMDAR function and contextual fear memory consolidation in Lrfn3^-/- mice, although the GluN2B-specific NMDAR antagonist ifenprodil was not sufficient to reverse the enhanced fear memory consolidation. These results suggest that SALM4 suppresses excessive GluN2B-NMDAR (not AMPAR) function and fear memory consolidation (not acquisition).

Proper synaptic adhesion signaling in the control of neural circuit architecture and brain function

Trans-synaptic cell-adhesion molecules are critical for governing various stages of synapse development and specifying neural circuit properties via the formation of multifarious signaling pathways. Recent studies have pinpointed the putative roles of trans-synaptic cell-adhesion molecules in mediating various cognitive functions. Here, we review the literature on the roles of a diverse group of central synaptic organizers, including neurexins (Nrxns), leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs), and their associated binding proteins, in regulating properties of specific type of synapses and neural circuits. In addition, we highlight the findings that aberrant synaptic adhesion signaling leads to alterations in the structures, transmission, and plasticity of specific synapses across diverse brain areas. These results seem to suggest that proper trans-synaptic signaling pathways by Nrxns, LAR-RPTPs, and their interacting network is likely to constitute central molecular complexes that form the basis for cognitive functions, and that these complexes are heterogeneously and complexly disrupted in many neuropsychiatric and neurodevelopmental disorders.