Alternative splicing event modifying ADGRL1/latrophilin-1 cytoplasmic tail promotes both opposing and dual cAMP signaling pathways

The adhesion G protein-coupled receptor ADGRL1/latrophilin-1 (LPHN1) stabilizes synapse formation through heterophilic interactions. A growing consensus is pointing to the role of LPHN1 in modulating intracellular levels of cAMP, although conflicting data exist. Variants of LPHN1 resulting from alternative splicing differ at multiple sites, two of which, designated as SSA and SSB, modify extracellular and intracellular receptor regions, respectively. While SSA splicing modulates receptor-ligand affinity, the function of SSB splicing remains elusive. Here, we explored the role of SSB in an attempt to unify current findings on LPHN1 signaling pathways by testing SSB-containing and SSB-deficient receptor variants in signaling paradigms involving interaction with their ligands neurexin and FLRT. cAMP competitive binding assays revealed that cells expressing either receptor variant exhibited a ligand-dependent decrease in the forskolin-induced cAMP accumulation. Surprisingly, the expression of SSB-containing LPHN1 promoted both constitutive and ligand-dependent cAMP production, whereas SSB-deficient LPHN1 did not. Pertussis toxin treatment unveiled a constitutive coupling to G_αi/o for SSB-containing LPHN1 while abrogating the ligand-mediated activation of G_αs . Importantly, neither receptor variant increased the intracellular concentration of Ca²⁺ nor MAP kinase activation in the presence of ligands. These results suggest that SSB splicing selectively affects the duality of LPHN1 signaling toward opposing cAMP pathways.