Melatonin receptor heterodimerization in a photoreceptor-like cell line endogenously expressing melatonin receptors

PURPOSE: Melatonin signaling plays an important role in the modulation of retinal physiology and photoreceptor viability during aging. In this study, we investigated whether 661W cells—a photoreceptor-like cell that endogenously expresses melatonin receptor type 1 (MT(1)) and melatonin receptor type 2 (MT(2)) receptors—represent a useful model for studying the biology of heterodimerization and signaling of MT(1/2) receptors. METHODS: 661W cells were cultured, and MT(1)/MT(2) heterodimerization in 661W cells was assessed with proximity ligation assay. MT(2) was removed from the 661W cells using the MT(2)-CRISPR/Cas9 system. Melatonin receptor signaling was investigated by measuring cAMP levels and activation of the AKT-FoxO1 pathway. RESULTS: The results demonstrated that heterodimerization of MT(1) and MT(2) receptors occurs in 661W cells. The pathways activated by MT(1)/MT(2) heterodimer (MT(1/2h)) in 661W cells are similar to those previously reported in mouse photoreceptors. Disruption of the heterodimer formation by genetically ablating MT(2) from 661W cells abolished the activation of melatonin signaling in these cells. CONCLUSIONS: The data indicated that in 661W cells, MT(1) and MT(2) receptors are functional only when they are associated in a heteromeric complex, as occurs in mouse photoreceptors. 661W cells represent a useful model for studying the mechanism underlying MT(1)/MT(2) heterodimerization.