| The evolution of the melanocortin receptor (MCR) gene family has been dictated by two genome duplication events (2R hypothesis). The gnathostomes are thus predicted to possess a minimum of four MCR genes. Previous studies on cartilaginous fish have shown evidence for the presence of only three MCRs. The purpose of this thesis is to functionally express the MC5R from the spiny dogfish (Squalus acanthias) and the putative MC2R and MC3R from the elephant shark (Callorhinchus milii); these receptors have not yet been fully characterized. In this study, SacMC5R was able to be expressed in CHO cells without the presence of an accessory protein, like MRAP. This receptor showed a preference for ACTH over alpha-MSH, and, even though SacMC5R showed no requirement for MRAP, co-transfection with mouse MRAP led to an increase in receptor sensitivity to ACTH. CmiMC3R was also found to have the highest sensitivity to ACTH. This receptor was able to be expressed without MRAP, but receptor activity was significantly enhanced in the presence of mouse MRAP. Functional analysis of CmiMC2R showed that this receptor was able to be trafficked to the cell surface and functionally expressed without MRAP; furthermore, the presence of MRAP did not appear to enhance activation. It was also found that while CmiMC2R had the greatest affinity for ACTH, it was also able to be stimulated by all of the MSHs. Analysis of the MCRs in these species suggests that cartilaginous fish have secondarily lost a MCR gene. The relationship between the MC2R and MC5R in cartilaginous fish is still unresolved. |
