Differential acetylation of pro-opiomelanocortin-derived peptides in the pituitary gland of Xenopus laevis in relation to background adaptation

Abstract

Immunocytochemical analysis revealed the presence of acetylated endorphins in both melanotropes and corticotropes of the pituitary gland of Xenopus laevis. Chemical acetylation studies to determine the steady-state level of acetylated versus non-acetylated endorphins showed that virtually all endorphins are acetylated in both melanotropes and corticotropes. Apparently Xenopus is unique among vertebrates as non-acetylated endorphins are major end-products in the distal lobe of all other vertebrate species studied thus far. The dynamics of endorphin biosynthesis in melanotrope cells using pulse-chase analysis coupled to immunoaffinity chromatography revealed that processing of pro-opiomelanocortin to produce N-terminal-acetylated endorphins is very rapid. To determine the effect of long-term background adaptation on acetylation status of endorphins and alpha-MSH-related peptides, Xenopus laevis were adapted for 3 or 6 weeks to either a black or a white background. In both physiological states the major intracellular form of alpha-MSH-related peptides in melanotropes was desacetyl alpha-MSH while the major endorphin-related peptide was alpha, N-acetyl-beta-endorphin[1-8]. In the medium of superfused neuro-intermediate lobes of black background-adapted animals the major form of secreted melanotropins and endorphins was alpha-MSH and alpha, N-acetyl-beta-endorphin[1-8] respectively. We conclude that there is a marked spatio-temporal difference in acetylation of melanotropin and endorphins, with rapid intracellular acetylation of endorphins while melanotropin is acetylated at the time of its exocytosis. In the medium of superfused neurointermediate lobes of white background-adapted animals the amount of desacetyl alpha-MSH was much higher than in the medium of lobes of black-adapted animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of pro-opiomelanocortin and its peptide end products in the brain and hypophysis of the aquatic toad, Xenopus laevis

“…As to the antiserum against nonacetylated endorphins it should be noted that it recognizes both the acetylated and non-acetylated forms of endorphin (Van Strien et al 1995a), whereas acetylated endorphins are specifically recognized by the anti-acetylated endorphin serum. Consequently, the endorphins demonstrated in the hypophysis are acetylated as they react with both the anti-endorphin sera whereas the endorphins found in the brain appear non-acetylated as they do not react with the antiserum raised against acetylated endorphin.…”

“…The a-MSH peptide causes dispersion of the black pigment melanin in dermal melanophores (Jenks and van Zoest 1990;Jenks et al 1993), resulting in darkening of the toads skin. In addition to a-MSH, Xenopus melanotropes produce various other POMC peptides with, up to now, unknown functions, such as g-MSH and acetylated endorphin (Martens et al 1983(Martens et al , 1985Martens 1987;Deen et al 1991;Van Strien et al 1995a,b, 1996a.…”

Melanotrope Cell Heterogeneity in the Pars Intermedia of Amphibiansa

“…8 In X. laevis , it has been demonstrated that the pars intermedia of black‐adapted toads exhibits greater activity than does that of white‐adapted animals. 28–30 These observations strongly suggest that LD melanotrope cells, with an intense secretory activity, a high acetylation rate, and an elevated POMC mRNA content, are actively participating in the process of skin color adaptation. In our laboratory, we tested the effect of background color adaptation on the heterogeneous pattern of the melanotrope population.…”

About a snail, a toad, and rodents: animal models for adaptation research

“…A phenomenon unique to amphibians is the posttranslational acetylation reaction to form mature N-acetylated αMSH, which increases the bioactivity of the peptide ∼10-fold and is associated with the exocytotic release of the peptide (Martens et al, 1981b; Van Strien et al, 1995b). In Xenopus , the final processing product of the C-terminal region of POMC is N -acetyl-β-endorphin(1-8; Van Strien et al, 1993).…”