N-Terminal Acetylation of Melanophore-Stimulating Hormone in the Pars intermedia of <i>Xenopus laevis </i>Is a Physiologically Regulated Process

Abstract

The N-terminal acetylation of melanophore-stimulating hormone (MSH) increases the melanotropic potency of the peptide. This modification may be important in amphibians, where MSH causes skin darkening during adaptation to black background. This study examines the acetylation status of the peptide in the toad Xenopus laevis under different conditions of background adaptation. Acetylated and nonacetylated α-MSH were analyzed by high-performance liquid chromatography and quantified by radioimmunoassay. The acetylation status of α-MSH was analyzed in tissue, in plasma and in media obtained from in vitro incubation of neurointermediate lobe tissue. Nonacetylated MSH is the major form of α-MSH in tissue from both black- and white-background-adapted animals. In plasma of black-adapted animals only acetylated α-MSH could be detected. Plasma MSH levels of white-adapted animals were barely detectable. Analysis of peptides secreted during in vitro incubations of neurointermediate lobe tissue from black-adapted animals showed that the relative contribution of α-MSH to the immunoreactive profile was considerably enhanced, which supports the concept that acetylation of MSH in Xenopus is associated with the secretory process. Acetylation capacity of tissue from white-adapted animals was much lower and only after several days on black background was full capacity acquired. It is suggested that de novo biosynthesis of acetylation enzymes may be necessary for the acquisition of the acetylation capacity. Transfer of black animals to white background caused a rapid decrease in acetylation capacity, which suggests that factors involved in the rapid inhibition of secretion might also regulate acetylation. This finds support in the observation that dopamine was effective in partially inhibiting the acetylation capacity of this tissue. It is concluded that the acetylation of MSH in the pars intermedia of X. laevis is a regulated process related to physiological parameters concerning background color.

Immunocytochemical Localization and Biochemical Characterization of α‐Melanocyte‐Stimulating Hormon in the Brain of the Rainbow Trout, Salmo gairdneri

“…Since acetylation of a-MSH markedly enhances the behavioural activity of the neuropeptide (45), it is tempting to speculate that the acetylation system might represent a regulatory point to modulate the bioactivity of the peptide. Consistent with (55,56). A t the moment, t h e function of a-MSH-related peptides in t h e fish brain remains unknown.…”

Why Are Several Inhibitory Transmitters Present in the Innervation of Pituitary Melanotrophs?

“…It is thus possible that an explanation for the redundancy of the innervation may yet be found at the postsynaptic level in some differential effects of the transmitters on other functional parameters in the melano trophs, for example, gene expression, or synthesis or posttranslational processing of the secretory products. GABA, for in stance, has been observed to inhibit expression of the proopio melanocortin gene [37], And dopamine has been seen to inhibit the synthesis of proopiomelanocortin [38], and also the acetyla tion of both [Vendorphin [39,40] and «-melanocyte stimulating hormone [41,42]. But conjectures on possible differential ef fects at the postsynaptic level must confront the indications that the intracellular signals set in motion by the three transmitters in the melanotrophs appear, from current evidence, from me lanotrophs and other cells, to be similar, each involving re ductions in cyclic AMP and cytosolic Ca2+; and furthermore, that these same intracellular signals have been implicated in the regulation of gene expression and posttranslationai processing [43,44].…”

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

“…In melanotrope cells of mammals, acetylation of endorphins is a common feature (Mains & Eipper 1981, Dores 1988) and about 90% of the synthesized endorphins are N-acetylated (Zakarian 8c Smyth 1982 (Martens et al 1981, Verbürg-van Kemenade et al 1987, Dores et al 1991, Maruthainar et al 1992) that in black-adapted animals the main tissue form of melanotro¬ pin is the non-acetylated peptide, i.e. desacetyl a-MSH.…”

“…, Verbürg-van Kemenade et al 1987, Rouillé et al 1989, Dores et al 1991, Jenks et al 1993). This spatio-temporal difference in acetylation of the two peptides suggests that there are separate acetylation enzymes for endorphins and melanotropins.Physiological relevance of the release of melanotropins In Xenopus on a black background, the melanotropes are extremely active in synthesizing and releasing high amounts of a-MSH…”