Preservation of Eumelanin Hair Pigmentation in Proopiomelanocortin-Deficient Mice on a Nonagouti (a/a) Genetic Background

Abstract

The original strain of proopiomelanocortin (POMC)-deficient mice (Pomc Ϫ/Ϫ ) was generated by homologous recombination in 129X1/SvJ (A w /A w )-derived embryonic stem cells using a targeting construct that deleted exon 3, encoding all the known functional POMC-derived peptides including ␣MSH, from the Pomc gene. Although these Pomc Ϫ/Ϫ mice exhibited adrenal hypoplasia and obesity similar to the syndrome of POMC deficiency in children, their agouti coat color was only subtly altered. To further investigate the mechanism of hair pigmentation in the absence of POMC peptides, we studied wild-type (Pomc ϩ/ϩ ), heterozygous (Pomc ϩ/Ϫ ), and homozygous (Pomc Ϫ/Ϫ ) mice on a nonagouti (a/a) 129;B6 hybrid genetic background. All three genotypes had similar black fur pigmentation with yellow hairs behind the ears, around the nipples, and in the perianal area characteristic of inbred C57BL/6 mice. Histologic and electron paramagnetic resonance spectrometry examination demonstrated that hair follicles in back skin of Pomc Ϫ/Ϫ mice developed with normal structure and eumelanin pigmentation; corresponding molecular analyses, however, excluded local production of ␣MSH and ACTH because neither Pomc nor putative Pomc pseudogene mRNAs were detected in the skin. Thus, 129;B6 Pomc null mutant mice produce abundant eumelanin hair pigmentation despite their congenital absence of melanocortin ligands. These results suggest that either the mouse melanocortin receptor 1 has sufficient basal activity to trigger and sustain eumelanogenesis in vivo or that redundant nonmelanocortin pathway(s) compensate for the melanocortin deficiency. Whereas the latter implies feedback control of melanogenesis, it is also possible that the two mechanisms operate jointly in hair follicles. (Endocrinology 146: M ELANIN BIOSYNTHESIS IS initiated both in vitro andin vivo from the obligatory hydroxylation of ltyrosine to l-dihydroxyphenylalanine (L-DOPA), catalyzed by tyrosinase (EC 1.14.18.1) (1-3). Once L-DOPA is formed, further steps of melanogenesis consisting of a series of oxidoreduction reactions and intramolecular transformations can occur spontaneously and at varying rates, depending on local concentrations of hydrogen ions, metal cations, reducing agents, thiols, and oxygen (1). Eumelanogenesis involves the transformation of dopaquinone to leukodopachrome, followed by a series of oxidoreduction reactions with production of the intermediates dihydroxyindole and dihydroxyindole carboxylic acid, which undergo polymerization to form eumelanin (1,4,5). Pheomelanogenesis also starts with dopaquinone, which is conjugated to cysteine or glutathione to yield cysteinyldopa and glutathionyldopa for further transformation into pheomelanin (1,5,6).Although tyrosinase activity is the rate-limiting step among the melanogenesis-related enzymes (1,7,8), pigmentation is under complex genetic control regulated by more than 150 alleles representing more than 90 gene loci (3, 9 -14). Protein products of these loci include enzymes, structural proteins, transcriptional ...

Differential expression of HPA axis homolog in the skin

“…In addition, (Yaswen et al, 1999) clearly demonstrated the defect in skin pigmentation in POMC-knockout (POMC-KO) mice. The latter study, however, showed that this effect is dependent on the genotype of the mice used for testing, e.g., in C57BL6/J mice, a pigmentary effect of the POMC-KO is absent, likely due to the fact that this strain is recessive for agouti protein (Slominski et al, 2005a). Other cutaneous abnormalities in POMC-KO mice (including those of hair follicle cycling and the hair follicle immune system (Paus et al, 2006) still remain to be systematically explored.…”

Mouse coat color mutations: From fancy mice to functional genomics

“…That Pomc1 knockout mice are brownish rather than yellow suggests that the Mc1r receptor can function without the ␣-MSH ligand or that Mc1r has another melanocytespecific ligand yet to be isolated. In fact, mice that carry the Pomc1 mutation on a mixed 129;C57BL/6J background (the latter inbred strain is black due to the presence of the a mutation) produce abundant eumelanin, despite the absence of ␣-MSH (Slominski et al, 2005). Interestingly, human hair is not banded, and the ASP protein does not appear to play a role in human pigmentation, although involvement in skin pigmentation of Africans has been proposed recently (Bonilla et al, 2005).…”

Characterization of Melanin Radicals in Paraffin-embedded Malignant Melanoma and Nevus Pigmentosus Using X-band EPR and EPR Imaging

“…Thus, non-invasive EPRI can provide detailed information regarding the location and concentration of paramagnetic species in naturally occurring biomedical samples, such as melanin radicals in MM. Melanin radicals in MM have been studied using EPR methods in various ways, such as using low temperature and synthetic melanin, 7 animal models, [8][9][10][11] paraffin-embedded human melanoma, 12 and human specimens. 13 Melanin radicals in MM and NP were also compared using CW EPR.…”

“…3) and the spectral pattern was similar to that of previously reported pheomelanin-related radicals. 8,17 The low and high field signal intensities were markedly deferent (asymmetric pattern), suggesting that the radicals were immobilized. 8 The low and high field peak distance was approximately 3.2 mT.…”