Biosynthesis, processing, and control of release of melanotropic peptides in the neurointermediate lobe of Xenopus laevis.

Abstract

A B S T R A C T The neurointermediate lobes of dark-adapted toads Xenopus laeviswere incubated for 30 rain in [aH]arginine and then "chased" for various time periods. By use of this pulse-chase paradigm there were detected 10 trichloroacetic acid (TCA)-precipitable peptides separated on acid-urea polyacrylamide gels and one TCA-soluble peptide separated by high-voltage electrophoresis (pH 4.9) with melanotropic activity. Each of these peptides had a different degree of melanocyte stimulating hormone (MSH) activity as revealed by the Anolis skin bioassay. Three of these TCA-precipitable peptides comigrated with ACTH, fl-lipotrophin, and ot-MSH on acid-urea gels. Evidence suggesting a precursor-product mode of biosynthesis of the melanotropic peptides is presented. 7 of the 10 TCA-precipitable peptides and the one TCA-soluble peptide with melanotropic activity were released into the medium. The half-time of release of the TCA-precipitable peptides was about 2 h, whereas the half-time of TCA-soluble peptide release was about 30 min. The release of these peptides was inhibited by 5 x 10 -5 M dopamine. Dopamine inhibition of release did not appear to affect the biosynthesis of the melanotropic peptides, but did appear to enhance the degradation of the newly synthesized TCA-soluble peptide in the tissue. White adaptation of the toads greatly decreased the biosynthesis of all of the TCA-precipitable melanotropic peptides.

The development of the pars intermedia and its role in the regulation of dermal melanophores in the larvae of the amphibian Xenopus laevis

“…On the basis of the identification of newly synthesized peptides in adults by Martens et al (1982a), we propose that terminal products in larval Xenopus are a y,-MSH-like peptide, des-Na-acetyl-cl-MSH, a-MSH, two CLIPS, and two endorphin-like peptides. In larvae, adaptation to a black background induced higher rates of biosynthesis, an observation also reported for adults (Whur and Weatherhead, 1971;Jenks et al, 1977;Loh and Gainer, 1977). From stage 51 onward, and including the adult stage, peak II, representing des-N-a-acetyl-ol-MSH, constituted a major tissue product, but was very low in the medium.…”

“…Several investigators studying Xenopus laevis established that in this species the pars intermedia produces several different melanotropic substances. Analysis of biosynthetic events in neurointermediate lobes of X. laevis showed that the production of melanotropins has the characteristics of a precursor-product relationship (Loh and Gainer, 1977;Jenks and van Overbeeke, 1980). In vitro and in vivo studies by Martens et al (1980Martens et al ( , 1982aMartens et al ( -c, 1983 revealed that in Xenopus the biosynthesis commences with the production of a prohormone, pro-opiomelanocortin (POMC) which is subsequently processed to yield three melanotropic peptides, namely, a-MSH, des-N-a-acetyl-ol-MSH, and a y,-MSH-like peptide, as well as several other peptides, including two forms of a corticotropin-like intermediate peptide (CLIP) and two peptides structurally related to the endogenous opiate B-endorphin.…”

Regulation of melanotropin release from the pars intermedia of the amphibian Xenopus laevis: Evaluation of the involvement of serotonergic, cholinergic, or adrenergic receptor mechanisms

“…Biochemical (Loh and Gainer, 1977;Martens et al, 1982) and morphological studies Weatherhead and Whur, 1972) have shown that the cells of the pars intermedia of black-background-adapted toads are extremely active in biosynthesis and release of melanotropins. In contrast, white-backgroundadapted animals have low biosynthetic rates in these cells and, in V~'VO, release of melanotropins is under tonic inhibition.…”

“…For amphibians, the initial indication of P-adrenergic stimulation of MSH secretion from Rnna pipiens: (Bower et al, 1974) has recently been extended to a second amphibian species, Rana ridibunda (Tonon et al, 1983). XII the aquatic toad Xenopus laevis, the p-adren ergic receptor' agonist isopro'terenol has been reported to inhibit the release of &WI and other peptides derived from the common prohormone pro-o,piomelanoj cortin (Loh and Gainer, 1977). This observation appears to be in agreement with a report that CAMP inhibits M!SH release in this species , although it has earlier been reported'that cGMP is exl tremely potent in stimulating release of MSH from Xenopus neurointermediate lobes (Jenks, 1977).…”

Biosynthesis of melanotropins and endorphins by the lead-haematoxylin positive cells in pars intermedia of the cichlid teleost Sarotherodon mossambicus

“…Such results are largely in agreement with those of in vitro experiments done with pars intermedia lobes or cell lines of higher vertebrate species, in which it has been shown that M SH , ACTH and endorphins orig inate from a common precursor (Roberts and Herbert, 1977a, b;Eipper. 1979, 1980;Mains el al., 1977;Nakanishi el al., 1979Nakanishi el al., , 1980Loh and Gainer, 1977;Jenks et al, 1979;Crine el al., 1980). In addition, our results add to the increasing amount of evidence that points towards melanotropic and endorphin-like peptides as final products of precursor processing in the pars intermedia cells Eipper, 1979, 1980;Gianoulakis et al, 1979;Loh, 1979;Crine et al, 1980).…”