The GTPase-activating protein-related domain of neurofibromin interacts with MC1R and regulates pigmentation-mediated signaling in human melanocytes.

The melanocortin 1 receptor (MC1R) is a G-protein coupled receptor (GPCR) which plays a major role in controlling melanogenesis. A large body of evidence indicates that GPCRs are part of large protein complexes that are critical for their signal transduction properties. Among proteins which may affect MC1R signaling, neurofibromin (Nf1), a GTPase activating protein (GAP) for Ras, is of special interest as it regulates adenylyl cyclase activity and ERK signaling, two pathways involved in MC1R signaling. Moreover, mutations in this gene encoding Nf1 are responsible for neurofibromatosis type I, a disease inducing hyperpigmented flat skin lesions. Using co-immunoprecipitation and Bioluminescence Resonance Energy Transfer experiments we demonstrated a physical interaction of Nf1 with MC1R. In particular, the GAP domain of Nf1 directly and constitutively interacts with MC1R in melanocytes. Pharmacologic and genetic approaches revealed that the GAP activity of Nf1 is important to regulate intracellular signaling pathways involved in melanogenesis and, consequently, melanogenic enzyme expression and melanin production. These finding shed new light on the understanding and cure of skin pigmentation disorders.

Physical interaction between neurofibromin and serotonin 5-HT6 receptor promotes receptor constitutive activity.

Active G protein-coupled receptor (GPCR) conformations not only are promoted by agonists but also occur in their absence, leading to constitutive activity. Association of GPCRs with intracellular protein partners might be one of the mechanisms underlying GPCR constitutive activity. Here, we show that serotonin 5 hydroxytryptamine 6 (5-HT6) receptor constitutively activates the Gs/adenylyl cyclase pathway in various cell types, including neurons. Constitutive activity is strongly reduced by silencing expression of the Ras-GTPase activating protein (Ras-GAP) neurofibromin, a 5-HT6 receptor partner. Neurofibromin is a multidomain protein encoded by the NF1 gene, the mutation of which causes Neurofibromatosis type 1 (NF1), a genetic disorder characterized by multiple benign and malignant nervous system tumors and cognitive deficits. Disrupting association of 5-HT6 receptor with neurofibromin Pleckstrin Homology (PH) domain also inhibits receptor constitutive activity, and PH domain expression rescues 5-HT6 receptor-operated cAMP signaling in neurofibromin-deficient cells. Furthermore, PH domains carrying mutations identified in NF1 patients that prevent interaction with the 5-HT6 receptor fail to rescue receptor constitutive activity in neurofibromin-depleted cells. Further supporting a role of neurofibromin in agonist-independent Gs signaling elicited by native receptors, the phosphorylation of cAMP-responsive element-binding protein (CREB) is strongly decreased in prefrontal cortex of Nf1+/- mice compared with WT mice. Moreover, systemic administration of a 5-HT6 receptor inverse agonist reduces CREB phosphorylation in prefrontal cortex of WT mice but not Nf1+/- mice. Collectively, these findings suggest that disrupting 5-HT6 receptor-neurofibromin interaction prevents agonist-independent 5-HT6 receptor-operated cAMP signaling in prefrontal cortex, an effect that might underlie neuronal abnormalities in NF1 patients.