The SALM Family of Adhesion-like Molecules Forms Heteromeric and Homomeric Complexes

Abstract

Synaptic adhesion-like molecules (SALMs) are a newly discovered family of adhesion molecules that play roles in synapse formation and neurite outgrowth. The SALM family is comprised of five homologous molecules that are expressed largely in the central nervous system. SALMs 1-3 contain PDZ-binding domains, whereas SALMs 4 and 5 do not. We are interested in characterizing the interactions of the SALMs both among the individual members and with other binding partners. In the present study, we focused on the interactions formed by the five SALM members in rat brain and heterologous cells. In brain, we found that SALMs 1-3 strongly co-immunoprecipitated with each other, whereas SALMs 4 and 5 did not, suggesting that SALMs 4 and 5 mainly form homomeric complexes. In heterologous cells transfected with SALMs, co-immunoprecipitation studies showed that all five SALMs form heteromeric and homomeric complexes. We also determined if SALMs could form trans-cellular associations between transfected heterologous cells. Both SALMs 4 and 5 formed homophilic, but not heterophilic associations, whereas no trans associations were formed by the other SALMs. The ability of SALM4 to form trans interactions is due to its extracellular N terminus because chimeras of SALM4 N terminus and SALM2 C terminus can form trans interactions, whereas chimeras of SALM2 N terminus and SALM4 C terminus cannot. Co-culture experiments using HeLa cells and rat hippocampal neurons expressing the SALMs showed that SALM4 is recruited to points of contact between the cells. In neurons, these points of contact were seen in both axons and dendrites.Critical steps in the development of the nervous system, including cell migration, neurite outgrowth, growth cone guidance, and synapse formation, depend on cellular interactions mediated by adhesion molecules (1-4). A number of adhesion molecules that are essential to the proper development of the nervous system have been identified (5-7). The importance of these molecules is illustrated in cases of dysfunctional adhesion molecules that have been associated with specific disorders in humans, including SLITRK1 in Tourette syndrome (8) and neuroligin in autism (9, 10). Whereas our understanding of the role of adhesion molecules in neuronal development is rapidly increasing, little is known about their functions, and it is likely that many remain to be identified.Synaptic adhesion-like molecules (SALMs) 2 are a recently identified class of adhesion molecules that are highly enriched in brain (11-13). All five members of the SALM family contain extracellular leucine-rich repeats (LRR), an immunoglobulin C2-like (IgC2) domain, a fibronectin type III (FNIII) domain, a transmembrane domain, and a cytoplasmic C-terminal tail. SALMs 1-3 contain a C-terminal PDZ-binding domain (PDZ-BD), which associates with the PSD-95 family of proteins (11-13). SALMs are expressed early in the developing nervous system and persist into adulthood. They are present at the synaptic membrane as well as at non-synaptic locations. SALM1 overe...

Structure of the LINGO-1–Anti-LINGO-1 Li81 Antibody Complex Provides Insights into the Biology of LINGO-1 and the Mechanism of Action of the Antibody Therapy

“…4D and 5B) may also contribute to interactions between LINGO-1 and its ligands and coreceptors. This type of situation where the same interface can be used for multiple interactions such as ligand binding and multimer formation has been reported for other LRR-containing proteins (Nose et al, 1992;Karaulanov et al, 2006;Seabold et al, 2008;Kajander et al, 2011).…”

Dileucine and PDZ-binding Motifs Mediate Synaptic Adhesion-like Molecule 1 (SALM1) Trafficking in Hippocampal Neurons

“…Labeled sections were postfixed in 2% glutaraldehyde, followed by 1% osmium tetroxide and then 1% uranyl acetate during dehydration, and embedded in epon resin. This was followed by removal of glass using hydrofluoric acid and further processing for electron microscopy, as described previously (15,18).…”

“…Antibodies-Production of SALM polyclonal antiserum was described previously (15). Briefly, a SALM1 N-terminal antibody was generated in rabbits using a synthetic peptide directed toward amino acids 384 -397 (Covance, Denver, PA).…”

“…In the present study, we studied the surface expression of SALM2 and -3 and asked if deletion of their PDZ-BDs also led to a reduction of surface expression; SALM4 and -5, which naturally lack PDZ-BDs, are abundantly expressed on the cell surface in heterologous cells (15). Only SALM1 had reduced surface expression when its PDZ-BD was removed.…”

“…SALMs also differ in their associational properties. SALM4 and -5 form homophilic intercellular interactions, whereas SALM1 to -3 do not, and binding partners for SALM1 to -3 remain unidentified (15). Co-immunoprecipitation studies showed that SALMs can form both homomeric and heteromeric complexes in neurons and heterologous cells, but individual SALMs show specific preferences (15).…”