The function of CLSTN3 (calsyntenin 3, Ensembl gene identifier ENSG00000139182) is as follows. Postsynaptic adhesion molecule that binds to presynaptic neurexins to mediate both excitatory and inhibitory synapse formation (PubMed:25352602). Promotes synapse development by acting as a cell adhesion molecule at the postsynaptic membrane, which associates with both neurexin-alpha and neurexin-beta proteins at the presynaptic membrane (PubMed:25352602). Regulates the balance between excitatory and inhibitory synapses by inhibiting formation of excitatory parallel- fiber synapses and promoting formation of inhibitory synapses in the same neuron (By similarity). May also be involved in ascorbate (vitamin C) uptake via its interaction with SLC23A2/SVCT2 (PubMed:34673103). Complex formation with APBA2 and APP, stabilizes APP metabolism and enhances APBA2-mediated suppression of beta-APP40 secretion, due to the retardation of intracellular APP maturation (Probable) (PubMed:12972431). {ECO:0000250|UniProtKB:Q99JH7, ECO:0000269|PubMed:25352602, ECO:0000269|PubMed:34673103, ECO:0000305|PubMed:12972431}. [Isoform CLSTN3beta]: Adipose-specific isoform that plays a key role in adaptive thermogenesis. Facilitates the efficient use of stored triglyceride by promoting multilocular morphology of thermogenic adipocytes: acts by inhibiting the activity of CIDEA and CIDEC on lipid droplets, thereby preventing lipid droplet fusion and facilitating lipid utilization. May also participate in adaptive thermogenesis by promoting sympathetic innervation of thermogenic adipose tissue: acts by driving secretion of neurotrophic factor S100B from brown adipocytes, stimulating neurite outgrowth from sympathetic neurons. .