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

• Published in: The Journal of biological chemistry 2012-02, Vol.287 (7), p.4470-4484
• Main Authors: Seabold, Gail K., Wang, Philip Y., Petralia, Ronald S., Chang, Kai, Zhou, Arthur, McDermott, Mark I., Wang, Ya-Xian, Milgram, Sharon L., Wenthold, Robert J.
• Format: Article
• Language: English
• Subjects: Amino Acid Motifs; Amino Acid Sequence; Axons - metabolism; Axons - ultrastructure; Cell Adhesion; Cell Adhesion Molecules; Cell Adhesion Molecules, Neuronal - genetics; Cell Adhesion Molecules, Neuronal - metabolism; Dendrites - metabolism; Dendrites - ultrastructure; Dileucine Motif; ER Retention; Glycosylation; HeLa Cells; Hippocampus - cytology; Hippocampus - metabolism; Humans; Nerve Tissue Proteins - genetics; Nerve Tissue Proteins - metabolism; Neurite Outgrowth; Neurobiology; Neurodevelopment; PDZ Domains; Protein Targeting; Protein Transport - physiology; Sequence Deletion; Synapse Formation
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M111.279661

Synaptic adhesion-like molecules (SALMs) are a family of cell adhesion molecules involved in neurite outgrowth and synapse formation. Of the five family members, only SALM1, -2, and -3 contain a cytoplasmic C-terminal PDZ-binding motif. We have found that SALM1 is unique among the SALMs because deletion of its PDZ-binding motif (SALM1ΔPDZ) blocks its surface expression in heterologous cells. When expressed in hippocampal neurons, SALM1ΔPDZ had decreased surface expression in dendrites and the cell soma but not in axons, suggesting that the PDZ-binding domain may influence cellular trafficking of SALMs to specific neuronal locations. Endoglycosidase H digestion assays indicated that SALM1ΔPDZ is retained in the endoplasmic reticulum (ER) in heterologous cells. However, when the entire C-terminal tail of SALM1 was deleted, SALM1 was detected on the cell surface. Using serial deletions, we identified a region of SALM1 that contains a putative dileucine ER retention motif, which is not present in the other SALMs. Mutation of this DXXXLL motif allowed SALM1 to leave the ER and enhanced its surface expression in heterologous cells and neurons. An increase in the number of protrusions at the dendrites and cell body was observed when this SALM1 mutant was expressed in hippocampal neurons. With electron microscopy, these protrusions appeared to be irregular, enlarged spines and filopodia. Thus, enrichment of SALM1 on the cell surface affects dendritic arborization, and intracellular motifs regulate its dendritic versus axonal localization. Background: The SALMs are neuronal cell adhesion molecules. Results: Deletion of the SALM1 PDZ-binding motif or mutation of a dileucine motif affects ER retention and surface expression. Conclusion: Enhanced SALM1 surface causes formation of elongated processes. Significance: SALMs can regulate neuronal morphology and may be involved in developmental disorders like autism.