The E92K Melanocortin 1 Receptor Mutant Induces cAMP Production and Arrestin Recruitment but Not ERK Activity Indicating Biased Constitutive Signaling

BACKGROUND: The melanocortin 1 receptor (MC1R) constitutes a key regulator of melanism. Consequently, many naturally-occurring MC1R mutations are associated with a change in color. An example is the Glu-to-Lys substitution found at position II:20/2.60 in the top of transmembrane helix II which has been identified in melanic mice and several other species. This mutation induces a pronounced increase in MC1R constitutive activity suggesting a link between constitutive activity and melanism which is corroborated by the attenuation of α-melanocyte stimulating hormone (αMSH) induced activation. However, the mechanism by which the mutation induces constitutive activity is currently not known. METHODOLOGY/PRINCIPAL FINDINGS: Here we characterize the constitutive activity, cell surface expression and internalization of the mouse mutant, Mc1r E92K. As previously reported, only positively charged residues at position II:20/2.60 induced an increase in constitutive activity as measured by cAMP accumulation and CREB activation. Furthermore, the mutation induced a constitutive recruitment of β-arrestin. This phenomenon is only observed in MC1R, however, as the equivalent mutations in MC2-5R had no effect on receptor signaling. Interestingly, the mutation did not induce constitutive ERK1/2 phosphorylation or increase the internalization rate indicating the constitutive activity to be biased. Finally, to identify regions of importance for the increased constitutive activity of Mc1r E92K, we employed a chimeric approach and identified G102 and L110 in the extracellular loop 1 to be selectively important for the constitutive activity as this, but not αMSH-mediated activation, was abolished upon Ala substitution. CONCLUSIONS/SIGNIFICANCE: It is concluded that the E92K mutation induces an active conformation distinct from that induced by αMSH and that the extracellular loop 1 is involved in maintaining this conformational state. In turn, the results suggest that in MC1R, which lacks an extracellular loop 2, the first extracellular loop may play a more prominent role during receptor activation than in general

The G Protein-Coupled Receptor GPR17: Overview and Update

The GPR17 receptor is a G protein-coupled receptor (GPCR) that seems to respond to two unrelated families of endogenous ligands: nucleotide sugars (UDP, UDP-galactose, and UDP-glucose) and cysteinyl leukotrienes (LTD4 , LTC4 , and LTE4 ), with significant affinity at micromolar and nanomolar concentrations, respectively. This receptor has a broad distribution at the level of the central nervous system (CNS) and is found in neurons and in a subset of oligodendrocyte precursor cells (OPCs). Unfortunately, disparate results emerging from different laboratories have resulted in a lack of clarity with regard to the role of GPR17-targeting ligands in OPC differentiation and in myelination. GPR17 is also highly expressed in organs typically undergoing ischemic damage and has various roles in specific phases of adaptations that follow a stroke. Under such conditions, GPR17 plays a crucial role; in fact, its inhibition decreases the progression of ischemic damage. This review summarizes some important features of this receptor that could be a novel therapeutic target for the treatment of demyelinating diseases and for repairing traumatic injury

Pharmacological Properties and Biological Functions of the GPR17 Receptor, a Potential Target for Neuro-Regenerative Medicine

In 2006, cells heterologously expressing the "orphan" receptor GPR17 were shown to acquire responses to both uracil nucleotides and cysteinyl-leukotrienes, two families of signaling molecules accumulating in brain or heart as a result of hypoxic/traumatic injuries. In subsequent years, evidence of GPR17 key role in oligodendrogenesis and myelination has highlighted it as a "model receptor" for new therapies in demyelinating and neurodegenerative diseases. The apparently contrasting evidence in the literature about the role of GPR17 in promoting or inhibiting myelination can be due to its transient expression in the intermediate stages of differentiation, exerting a pro-differentiating function in early oligodendrocyte precursor cells (OPCs), and an inhibitory role in late stage maturing cells. Meanwhile, several papers extended the initial data on GPR17 pharmacology, highlighting a "promiscuous" behavior of this receptor; indeed, GPR17 is able to respond to other emergency signals like oxysterols or the pro-inflammatory cytokine SDF-1, underlying GPR17 ability to adapt its responses to changes of the surrounding extracellular milieu, including damage conditions. Here, we analyze the available literature on GPR17, in an attempt to summarize its emerging biological roles and pharmacological properties