posted on 2023-08-30, 14:08authored byHelen R. McRobie
The grey squirrel (Sciurus carolinensis) has wildtype and melanic (dark) colour morphs.
Melanism is associated with variations in the melanocortin-1 receptor (MC1R) gene in a
number of species. The MC1R protein is a G-protein coupled receptor, predominantly
expressed in melanocytes, where it is a key regulator of pigment production. To investigate
the genetic and molecular basis of melanism, the MC1R genes of the wildtype and melanic
grey squirrel were sequenced. The wildtype (MC1R-wt) and melanic (MC1RΔ24) variants of
the MC1R were then functionally characterised in a cell-based assay.
The MC1R gene of the grey squirrel was found to have a 24 base pair (bp) deletion associated
with melanism. The MC1R is typically activated by its agonist, the alpha-melanocyte
stimulating hormone (α-MSH), which stimulates dark pigment production by raising
intracellular cAMP levels. Conversely, the MC1R is inactivated by its inverse agonist, the
agouti signalling protein (ASIP), which stops dark pigment production by lowering
intracellular cAMP levels. To investigate the effects that the 24 bp deletion have on receptor
function, MC1R-wt and MC1RΔ24 genes were transfected into HEK293 cells. Cells
expressing either MC1R-wt or MC1RΔ24 were stimulated with α-MSH or ASIP and
intracellular cAMP levels were measured. Unstimulated MC1RΔ24 cells showed higher basal
activity than the MC1R-wt cells. Both MC1R-wt and MC1RΔ24 cells responded to α-MSH
with a concentration-dependent increase in intracellular cAMP. However, while the MC1Rwt
cells responded to ASIP with a concentration-dependent decrease in intracellular cAMP,
MC1RΔ24 cells responded with an increase in cAMP.
Melanism in the grey squirrel is associated with a 24 bp deletion in the MC1R. Cells
expressing MC1RΔ24 have higher basal levels of cAMP than MC1R-wt cells. ASIP acts as
an inverse agonist to the MC1R-wt but as an agonist to the MC1RΔ24. As MC1RΔ24 cells
have higher levels of cAMP, and higher levels of cAMP lead to dark pigment production, the
24 bp deletion is the likely molecular cause of melanism in the grey squirrel.