Biosynthesis of a gamma 3-melanotropin-like peptide in the pars intermedia of the amphibian pituitary gland

V-ATPase-mediated granular acidification is regulated by the V-ATPase accessory subunit Ac45 in POMC-producing cells

The regulation of the V-ATPase, the proton pump mediating intraorganellar acidification, is still elusive. We find that excess of the neuroendocrine V-ATPase accessory subunit Ac45 reduces the intragranular pH and consequently disturbs prohormone convertase activation and prohormone processing. Thus, Ac45 represents the first V-ATPase regulator.

The vacuolar (H⁺)-ATPase (V-ATPase) is an important proton pump, and multiple critical cell-biological processes depend on the proton gradient provided by the pump. Yet, the mechanism underlying the control of the V-ATPase is still elusive but has been hypothesized to involve an accessory subunit of the pump. Here we studied as a candidate V-ATPase regulator the neuroendocrine V-ATPase accessory subunit Ac45. We transgenically manipulated the expression levels of the Ac45 protein specifically in Xenopus intermediate pituitary melanotrope cells and analyzed in detail the functioning of the transgenic cells. We found in the transgenic melanotrope cells the following: i) significantly increased granular acidification; ii) reduced sensitivity for a V-ATPase-specific inhibitor; iii) enhanced early processing of proopiomelanocortin (POMC) by prohormone convertase PC1; iv) reduced, neutral pH–dependent cleavage of the PC2 chaperone 7B2; v) reduced 7B2-proPC2 dissociation and consequently reduced proPC2 maturation; vi) decreased levels of mature PC2 and consequently reduced late POMC processing. Together, our results show that the V-ATPase accessory subunit Ac45 represents the first regulator of the proton pump and controls V-ATPase-mediated granular acidification that is necessary for efficient prohormone processing.

A cell-specific transgenic approach in Xenopus reveals the importance of a functional p24 system for a secretory cell

The p24alpha, -beta, -gamma, and -delta proteins are major multimeric constituents of cycling endoplasmic reticulum-Golgi transport vesicles and are thought to be involved in protein transport through the early secretory pathway. In this study, we targeted transgene overexpression of p24delta2 specifically to the Xenopus intermediate pituitary melanotrope cell that is involved in background adaptation of the animal and produces high levels of its major secretory cargo proopiomelanocortin (POMC). The transgene product effectively displaced the endogenous p24 proteins, resulting in a melanotrope cell p24 system that consisted predominantly of the transgene p24delta2 protein. Despite the severely distorted p24 machinery, the subcellular structures as well as the level of POMC synthesis were normal in these cells. However, the number and pigment content of skin melanophores were reduced, impairing the ability of the transgenic animal to fully adapt to a black background. This physiological effect was likely caused by the affected profile of POMC-derived peptides observed in the transgenic melanotrope cells. Together, our results suggest that in the early secretory pathway an intact p24 system is essential for efficient secretory cargo transport or for supplying cargo carriers with the correct protein machinery to allow proper secretory protein processing.

Biosynthesis and processing of the N-terminal part of proopiomelanocortin in Xenopus laevis: characterization of gamma-MSH peptides

The aim of this study was to determine the terminal products of processing of the N-terminal part of proopiomelanocortin (POMC) in pituitary melanotrope cells of Xenopus laevis. Biosynthetic in vitro labelling studies showed that POMC is rapidly processed to form N-terminal peptides with an estimated molecular mass of 18 kDa, 9 kDa and 4 kDa. All peptides were released into the medium, indicating that they are processing end products. An antiserum was raised against the synthetic N-terminal eight amino acids of the putative Xenopus gamma-MSH which is present in the N-terminal part of POMC. With immunocytochemistry we demonstrated that gamma-MSH-immunoreactive material in the pituitary gland is restricted to the pars intermedia. A radioimmunoassay in combination with reversed-phase HPLC revealed the presence of at least two gamma-MSH-like peptides. Complete purification followed by electrospray ionization mass spectrometry and amino acid sequence determination showed that these peptides are gamma 1-MSH and glycosylated gamma 3-MSH. The amounts of these gamma-MSH peptides were low compared to the other POMC-derived peptides, alpha-MSH and beta-endorphin. Only 10% of POMC is processed into gamma-MSH peptides and the 4 kDa peptide, leaving the 18 kDa and 9 kDa peptides as the major end products.

Analysis of autofeedback mechanisms in the secretion of pro-opiomelanocortin-derived peptides by melanotrope cells of Xenopus laevis

The secretion of most pituitary hormones is under the control of feedback mechanisms. The feedback control of alpha-melanophore-stimulating hormone (alpha-MSH) from melanotrope cells is controversial. The possible existence of an autofeedback exerted by alpha-MSH or other POMC-derived peptides on melanotrope cells of the amphibian Xenopus laevis has been investigated. alpha-MSH or its potent agonist 4-norleucine,7-D-phenylalanine-alpha-MSH has no effect on the release of radiolabeled POMC-derived peptides or immunoreactive beta-endorphin from superfused neurointermediate pituitary lobes. Melanin concentrating hormone, previously reported to have an alpha-MSH-like effect on melanophores, did not affect alpha-MSH secretion. Neurointermediate lobe superfusate, which contains a mixture of POMC-derived peptides, failed to affect the secretory activity of melanotropes. It is concluded that in X. laevis the secretory activity of melanotropes is not under the control of short-term autofeedback mechanisms involving alpha-MSH or other POMC-derived peptides.

Co-localization of tyrosine hydroxylase, GABA and neuropeptide Y within axon terminals innervating the intermediate lobe of the frog Rana ridibunda

Possible co-existence of gamma-aminobutyric acid (GABA), catecholamines, and neuropeptide Y (NPY) in the same nerve terminals of the frog intermediate lobe was investigated by immunocytochemistry at the electron microscopic level. Co-localization of GABA and tyrosine hydroxylase (TH) was studied by using a double immunogold labeling procedure. Co-localization of glutamate decarboxylase (GAD) and NPY was studied by combining, respectively, the peroxidase-antiperoxidase method and a radioimmunocytochemical labeling procedure. Catecholamines and GABA were systematically co-localized in nerve endings of the pars intermedia. Most of the NPY-immunoreactive fibers also contained GAD-like immunoreactivity. However, a few NPY-positive nerve terminals were not immunoreactive for GAD. These data provide evidence for co-existence of a regulatory peptide (NPY) and several neurotransmitters (i.e., GABA and catecholamines) within the same axon terminals in the intermediate lobe. Since GABA, dopamine, and NPY have all been shown to inhibit the activity of frog melanotrope cells, the present findings suggest that these neuroendocrine factors may interact either at the pre-synaptic or post-synaptic level.

A slow and a fast secretory compartment of POMC-derived peptides in the neurointermediate lobe of the amphibian Xenopus laevis

1. Peptide release from the neurointermediate lobe of Xenopus laevis has been studied using dual pulse-chase incubation, superfusion and HPLC techniques. 2. Lobes release pulse-labelled material in two phases, the first phase lasting about 6 hr, the second persisting up to 14 hr. 3. In both phases similar, POMC-derived peptides are released. Their release can be inhibited by dopamine. 4. When release during the first phase is inhibited, newly synthesized peptides are shunted into the second release pathway. 5. It is concluded that the neurointermediate lobe contains two release compartments. The possible locations of these compartments within melanotrope cells have been discussed.

An NPY-like peptide may function as MSH-release inhibiting factor in Xenopus laevis

This study demonstrates the presence of a rich plexus of neuropeptide Y (NPY) immunoreactive fibers in the hypothalamus and in the intermediate lobe of the pituitary of Xenopus laevis. During superfusion of neurointermediate lobe tissue, synthetic NPY induces a rapid, powerful and dose-dependent inhibition of in vitro release of MSH, endorphin and other proopiomelanocortin (POMC) derived peptides. Therefore, NPY undoubtedly is one of the growing number of neuropeptides that are likely involved in control of the amphibian MSH cells. Although a number of stimulatory neuropeptides have been found, this is the first neuropeptide to apparently function through an inhibitory mechanism. In that a 2-hr treatment with NPY did not influence POMC biosynthesis, nor processing of this prohormone to smaller peptides, we conclude that the primary action of NPY is a direct effect on the secretory process of the MSH cell.

Effect of tunicamycin on biosynthesis, processing and release of proopiomelanocortin-derived peptides in the intermediate lobe of the frog Rana ridibunda

The intermediate lobe of the pituitary gland synthesizes a glycoprotein, proopiomelanocortin (POMC), which is cleaved by specific proteolytic enzymes to generate several hormonal peptides. The purpose of the present study was to examine the possible role of the carbohydrate moiety in the synthesis, intracellular processing and release of POMC-derived peptides in frog (Rana ridibunda) intermediate lobe cells. In vitro incorporation of [3H]-labelled glucosamine gave rise to three major radioactive products. Trypsin digestion of each of these glycopeptides gave a single glucosamine-labelled tryptic fragment with identical chromatographic characteristics. We conclude that Rana POMC is glycosylated in only one site (its gamma-MSH region) and that intracellular processing of this prohormone gives rise to smaller glycopeptides including glycosylated gamma-MSH. Treatment with the antibiotic tunicamycin (10 micrograms/ml, 6 hr) inhibited the glycosylation of POMC but did not significantly alter the neosynthesis of the peptide moiety of the precursor. Pulse-chase experiments combined with high-performance liquid chromatography analysis of the peptides derived from POMC revealed that inhibition of glycosylation by tunicamycin had no effect on the enzymatic cleavage of the precursor nor on the release of mature peptides. Thus, it is concluded that, in the frog, glycosylation of POMC has no influence on the biosynthesis, processing and release of intermediate lobe hormones.

Immunocytochemical identification of the proopiocortin-producing cells in the chum salmon pituitary with antisera to endorphin and NH2-terminal peptide of salmon proopiocortin

The proopiocortin-containing cells were identified immunocytochemically in the chum salmon pituitary using specific antibodies raised against NH2-terminal peptide (sNPP) and COOH-terminal peptide, endorphin (sEP), of salmon proopiocortin. Immunoreactivity for both sNPP and sEP was observed in the same cells, melanotrops, in the pars intermedia. In the pars distalis, on the other hand, corticotrops were stained only with antibody to sNPP but not with that to sEP. The present results indicate that proopiocortin or a precursor molecule for NH2-terminal peptide and endorphin is biosynthesized in both melanotrops of the pars intermedia and corticotrops of the pars distalis. However, the absence of immunoreactivity of corticotrops with sEP antibody suggests that the processing of the precursor molecule in the pars distalis differs significantly from that in the pars intermedia in the chum salmon pituitary as the processings established in the two lobes in the mammalian pituitaries.

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

The ontogenesis of biosynthesis of pro-opiomelanocortin (POMC)-related peptides in the pars intermedia of Xenopus laevis tadpoles was studied. The results were related to the capacity of the animal to adapt to background color through regulation of pigment dispersion in dermal melanophores. Using immunocytochemical techniques with antisera to alpha-melanophore-stimulating hormone (alpha-MSH), it was revealed that this peptide first appeared at developmental stage 37/38, just prior to the animal's ability to adapt to background. It was shown that pigment dispersion in melanophores between stages 33 and 39 was not dependent on melanotropins of pituitary origin. Using in vitro biosynthetic studies it was possible to follow POMC biosynthetic activity, its processing and the release of peptides from stage 48 onward. Among the newly synthesized peptides observed were a gamma 3-MSH-like peptide, des-N-alpha-acetyl-alpha-MSH, alpha-MSH, and two endorphin-like peptides. By stage 57 a biosynthetic pattern almost identical to that of the adult pars intermedia had evolved. It was concluded that stage 39/40 is a critical stage in the simultaneous development of a number of the components involved in the neuroendocrine control of background adaptation.

An investigation of N-terminal pro-opiocortin peptides in the rat pituitary

Extracts of neurointermediate lobe (NIL) and anterior lobe (AL) of the rat pituitary, and material released from perfused rat pars distalis (PD) and pars intermedia (PI) cells were gel chromatographed and monitored using three antisera, each recognizing different regions of the non-corticotropin (ACTH)-lipotropin (LPH) portion of pro-opiocortin (POC). Two peaks (termed N-POC I) which emerged close to the elution position of rat beta-LPH were detected. The first peak was reduced significantly in the PI. Two smaller N-POC fragments which eluted near beta-endorphin were detected only in extracts and secretions of intermediate lobe tissue. One peak cross-reacted in the gamma 3-melanotropin (MSH) assay (N-POC III) whereas the other peak possessed amino (N)-terminal N-POC immunoreactivity (N-POC II). The results demonstrated differences in the distribution and nature of N-POC peptides released and extracted from the PD and PI of the rat pituitary, and suggest that the enzymic processing of N-POC is different in the two pituitary lobes.

Immunocytochemical evidence of intragranular acetylation of alpha-MSH in the melanotrophic cells of the rabbit

Melanotrophic cells of the pars intermedia of the rabbit were studied at the electron-microscopic level by means of the immuno-gold technique with the use of antisera against gamma 3-MSH and alpha-MSH. Both antibodies labelled all secretory vesicles stored in the peripheral cytoplasm, but secretory vesicles and intrasaccular condensations in the Golgi area reacting for gamma 3-MSH were not labelled with the antibody against alpha-MSH. Since this antibody appears to recognise only the acetylated form of alpha-MSH, the present observations suggest that acetylation occurs at a stage subsequent to the Golgi packaging, during maturation of the secretory vesicles. Thus, the morphological evidence supports biochemical data in favour of intragranular processing of opiomelanocortin in melanotrophic cells.

Biosynthesis of two structurally different pro-opiomelanocortins in the pars intermedia of the amphibian pituitary gland

This study reports the biosynthesis of two forms of pro-opiomelanocortin in the pars intermedia of the pituitary gland of the African clawed toad Xenopus laevis. The two forms could be resolved by dodecyl sulphate gel electrophoresis on a 9-16% acrylamide gradient and their molecular weights were 38200 and 37300. Incubation of neurointermediate lobes with [3H]glucosamine followed by tryptic digestion of the newly synthesized glycoproteins, revealed that both prohormones have only one glycosylated site, namely within the region corresponding to gamma 3-melanotropin. Biosynthesis of proteins in lobes treated with tunicamycin to prevent glycosylation again resulted in the production of two pro-opiomelanocortins (Mr 35000 and 34200), indicating that the two forms differ in their primary structure. This notion was corroborated by the results of tryptic mapping of the newly synthesized prohormones. The maps showed that the primary structures of the two forms of pro-opiomelanocortin differ in at least two parts of the molecules, one part concerning the endorphin region.