The biosynthesis, intracellular transport, and packaging of melanocyte-stimulating peptides in the amphibian pars intermedia

Fetal and Neonatal HPA Axis

Stress is an integral part of life. Activation of the hypothalamus-pituitary-adrenal (HPA) axis in the adult can be viewed as mostly adaptive to restore homeostasis in the short term. When stress occurs during development, and specifically during periods of vulnerability in maturing systems, it can significantly reprogram function, leading to pathologies in the adult. Thus, it is critical to understand how the HPA axis is regulated during developmental periods and what are the factors contributing to shape its activity and reactivity to environmental stressors. The HPA axis is not a passive system. It can actively participate in critical physiological regulation, inducing parturition in the sheep for instance or being a center stage actor in the preparation of the fetus to aerobic life (lung maturation). It is also a major player in orchestrating mental function, metabolic, and cardiovascular function often reprogrammed by stressors even prior to conception through epigenetic modifications of gametes. In this review, we review the ontogeny of the HPA axis with an emphasis on two species that have been widely studied-sheep and rodents-because they each share many similar regulatory mechanism applicable to our understanding of the human HPA axis. The studies discussed in this review should ultimately inform us about windows of susceptibility in the developing brain and the crucial importance of early preconception, prenatal, and postnatal interventions designed to improve parental competence and offspring outcome. Only through informed studies will our public health system be able to curb the expansion of many stress-related or stress-induced pathologies and forge a better future for upcoming generations.

Development and programming of the hypothalamus-pituitary-adrenal axis

The fetal hypothalamus-pituitary-adrenal (HPA) axis plays a critical role in fetal development and physiology in utero: appropriate function of the fetal HPA axis is critical for preparation of the fetus for birth and survival in postnatal life. Because of the critical importance of appropriate physiological regulation of HPA activity in postnatal life, there has been intense interest in the possibility that fetal or neonatal stressors can permanently "program" the axis to hyperrespond or hyporespond to stimuli. This is a review of the literature relevant to normal development and "programming" of the HPA axis.

Intracisternal granules in the adipokinetic cells of locusts are not degraded and apparently function as supplementary stores of secretory material

The intracisternal granules in locust adipokinetic cells appear to represent accumulations of secretory material within cisternae of the rough endoplasmic reticulum. An important question is whether these granules are destined for degradation or represent stores of (pro)hormones. Two strategies were used to answer this question. First, cytochemistry was applied to elucidate the properties of intracisternal granules. The endocytic tracers horseradish peroxidase and wheat-germ agglutinin-conjugated horseradish peroxidase were used to facilitate the identification of endocytic, autophagic, and lysosomal organelles, which may be involved in the degradation of intracisternal granules. No intracisternal granules could be found within autophagosomes, and granules fused with endocytic and lysosomal organelles were not observed, nor could tracer be found within the granules. The lysosomal enzyme acid phosphatase was absent from the granules. Second, biochemical analysis of the content of intracisternal granules revealed that these granules contain prohormones as well as hormones. Prohormones were present in relatively higher amounts compared with ordinary secretory granules. Since the intracisternal granules in locust adipokinetic cells are not degraded and contain intact (pro)hormones it is concluded that they function as supplementary stores of secretory material.

Translocon-associated protein TRAP delta and a novel TRAP-like protein are coordinately expressed with pro-opiomelanocortin in Xenopus intermediate pituitary

In the intermediate pituitary gland of Xenopus laevis, the expression levels of the prohormone pro-opiomelanocortin (POMC) can be readily manipulated. When the animal is placed on a black background, the gene for POMC is actively transcribed, whereas on a white background the gene is virtually inactive. In this study, we characterized two genes whose transcript levels in the intermediate pituitary are regulated in coordination with that for POMC. One of these codes for a protein homologous to translocon-associated protein TRAP delta, a subunit of a transmembrane protein complex located at the site where nascent secretory proteins enter the endoplasmic reticulum (ER). Both Xenopus and mice were found to express an alternatively spliced transcript that gives rise to a previously unknown version of the TRAP delta protein. The product of the second gene is a novel and highly conserved protein with structural similarity to glycoprotein gp25L, a constituent of another translocon-associated protein complex. A database search revealed the existence of a novel family of gp25L-related proteins whose members occur throughout the animal kingdom. Together, our data imply that (i) the group of ER proteins surrounding translocating polypeptide chains may be far more complex than previously expected, and (ii) a number of the accessory components of the translocon participate in early steps of prohormone biosynthesis.

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.

Secretory granule formation. The morphologist's view

A deeper understanding of the regulated exocytic pathway, and for that matter the constitutive exocytic pathway, will depend on our ability to characterize the proteins in the vesicle membranes. Characterizing the protein composition of secretory granule membrane has proven to be a formidable task, and as far as I know, the work done to date has not told us a great deal about the mechanisms involved in sorting the contents of regulated secretory granules, or bringing about constitutive or regulated fusion with the plasma membrane. Without knowing a great deal more about the membranes, there seems to be little prospect of real further progress in understanding the key properties of the regulated exocytic pathway.

Morphology of the pars intermedia and the melanophore-stimulating cells in Xenopus laevis in relation to background adaptation

The melanophore-stimulating hormone (MSH) cells of the amphibian pars intermedia secrete the peptide alpha-melanophore-stimulating hormone (alpha-MSH), which induces pigment dispersion in dermal melanophores. The purpose of the present study was to determine (1) whether prolonged activation of the secretory activity of the pars intermedia is associated with hypertrophy, hyperplasia, or both and (2) whether the MSH cells function as a homogeneous or heterogeneous population in meeting the physiological demand for MSH. The demand for MSH was manipulated by adapting animals for at least 3 weeks to white, two shades of grey, or black backgrounds. Morphometric analysis showed that the intermediate lobe volume was positively correlated with the degree of pigment dispersion in the melanophores. The number of MSH cells per lobe was not affected by the degree of pigment dispersion. Therefore, we conclude that enlargement of the tissue associated with MSH cell activation involves hypertrophy rather than hyperplasia. Ultrastructural examination indicated that the majority of MSH cells in black-adapted animals are biosynthetically active, whereas the cells of white-adapted animals are relatively inactive and show granule storage. The pars intermedia of grey-adapted toads contained both active and inactive cells, indicating that MSH cells respond as a heterogeneous cell population in meeting the endocrine demand imposed by background.

Condensation-sorting events in the rough endoplasmic reticulum of exocrine pancreatic cells

In guinea pig exocrine pancreatic cells intracisternal granules (ICGs) occur at a low frequency within the lumen of the RER. By starving and refeeding guinea pigs or injecting them in CoCl2 solution, the number of these granules is greatly increased. We show here that ICGs contain the complete set of secreted pancreatic digestive enzymes and proenzymes. Two other soluble proteins in the lumen of the RER, GRP 78/BiP and protein disulphide isomerase (PDI), are specifically excluded from ICGs. The formation of ICGs, which occurs without acidification of the RER cisternae, is therefore a sorting event involving the cocondensation of a complete set of secretory enzymes and proenzymes, which for brevity we refer to collectively as the zymogens. With the exception of approximately 50% of the RNase, the zymogens in ICGs are covalently cross-linked by intermolecular disulphide bonds. The synthesis of all three resident ER cisternal proteins--PDI, GRP 78/BiP, and GRP 94--with the carboxy-terminal sequence KDEL, is induced in response to the accumulation of massive amounts of misfolded secretory protein in the ICGs in the lumen of the RER. After injection of rats with large doses of parachlorophenylalanine-methylester, crystals form in the lumen of the RER. We show that these crystals appear to be a lattice of amylase with the other zymogens incorporated between the layers. Both GRP 78/BiP and PDI are excluded from these crystals. The formation of these amylase crystals within the RER and the inclusion of other zymogens is, therefore, also a sorting event. These data establish that in exocrine pancreatic cells zymogens can cocondense in the RER into either amorphous aggregates or crystals that exclude other soluble RER proteins. This demonstrates that cocondensation is a mechanism capable of sorting zymogens within the secretory pathway.

In vivo biosynthesis of melanotropins and related peptides in the pars intermedia of Xenopus laevis

To study in vivo biosynthesis of pars intermedia peptides in Xenopus laevis, [3H]lysine was administered by an osmotic minipump via a cannula inserted near the pituitary gland. Following extraction of the neurointermediate lobe, high-performance liquid chromatography was used to separate the newly synthesized peptides. In black-background adapted animals, [3H]lysine was incorporated into a number of peptides. The elution characteristics of these peptides corresponded exactly with those of peptides synthesized during in vitro incubation of neurointermediate lobes, and which were identified as des-N alpha-acetyl-alpha-MSH, a gamma-MSH-like peptide, two corticotropin-like intermediate lobe peptides, and two forms of endorphin. In white-background adapted Xenopus, practically no synthesis of pars intermedia peptides occurred. Transfer of black-adapted toads to a white background at the beginning of infusion led to storage of newly synthesized peptides. When such animals were maintained on a white background for 10 days, des-N alpha-acetyl-alpha-MSH, but not alpha-MSH, was present in the pars intermedia; this supports the notion that des-N alpha-acetyl-alpha-MSH constitutes the "storage form" of alpha-MSH.

Biosynthesis of pairs of peptides related to melanotropin, corticotropin and endorphin in the pars intermedia of the amphibian pituitary gland

This study concerns the biosynthesis of a number of peptides in the neurointermediate lobe of the pituitary gland of the aquatic toad, Xenopus laevis. Using pulse-chase incubations in vitro and high-performance liquid chromatographic analysis, it could be shown that these peptides are synthesized through processing of a prohormone, pro-opiomelanocortin; all peptides were released into the incubation medium. On the basis of electrophoretic analysis, selective amino acid incorporation and immunoprecipitation, as well as peptide mapping by high-performance liquid chromatography, the peptides were classified into three distinct groups: two related to melanocyte-stimulating hormone (melanotropin), two related to adrenocorticotropic hormone (corticotropin) and two endorphin-like peptides. Using tryptic and chymotryptic maps of synthetic alpha-melanotropin and des-Ac alpha N-alpha-melanotropin as references, one of the melanotropin-like peptides was identified as des-Ac alpha N-alpha-melanotropin; the other one represents neither alpha-melanotropin nor any other known melanotropic peptide. The two peptides that were immunologically related to corticotropin had characteristics consistent with a structures resembling a peptide previously named 'corticotropin-like intermediate lobe peptide', corticotropin-(18-39). The two endorphin-like peptides, although highly related, do not have the same primary structure. In view of the apparent structural differences between the two peptides in each group, the possible occurrence of two prohormones is discussed.

Ultrastructural cytochemistry of atrial muscle cells. VIII. Radioautographic study of synthesis and migration of proteins

Sections of atrial cardiocytes from young rats were subjected to radioautography after a single intravenous injection of L-leucine-4,5 3H to identify the sites of synthesis and to follow the migration of newly-formed proteins. As early as 2 min after injection of L-leucine 3H, the label was highest in the rough endoplasmic reticulum (RER), suggesting that cisternal ribosomes are sites of protein synthesis. By 5 min, most of the label had migrated from the RER to the Golgi complex. Some label had migrated from the RER to the Golgi complex. Some label was already present over specific granules by 2 min but the peak was reached at 1 h. By 4 h, the label over the specific granules had diminished, possibly indicating a release of newly-synthetized secretory material outside the cell. The label over myofilaments and Z-bands was relatively high at most time intervals, suggesting an early and important incorporation of leucine into the contractile and structural proteins of these organelles. The label over the cytosol was initially high and increased even further at 5 and 20 min but decreased to a very low level at 4 h. In contrast, the label over the cell surface rose continuously and peaked at 4 h. The pattern of increment of the label over the cell surface suggests that the newly-formed proteins of these sites are also synthetized in the RER, pass through the Golgi complex and are transported in the cytosol before reaching their destination.

Control of the pars intermedia of the lizard, Anolis carolinensis. II. Ultrastructure of the intact intermediate lobe

The ultrastructure of the intermediate lobe of the hypophysis was studied in Anolis carolinensis with the use of a threefold aldehyde fixative. Lizards with a brown skin were selected. The possibility of two types of secretory cells is discussed; neither cell type is innervated. Type I cells are rarely found and contain dense granules approximately 0.3 micron in diameter; Type II cells vary widely in secretory activity. Most of the Type II cells contain a large number of dense secretory granules (up to about 1.3 micron in diameter) almost filling the cytoplasm. Rough endoplasmic reticulum (RER), Golgi apparatus and mitochondria are poorly developed. Only some of these cells show signs suggesting a high secretory activity, namely a well developed RER, Golgi apparatus and numerous mitochondria. In these cells the RER sometimes forms large intracisternal droplets (up to 7 micron in diameter). Two of the animals exhibited a more uniform, high secretory activity. Large (about 2 micron in diameter), pale vacuoles, probably of extracellular character, were found mostly in the vicinity of the perivascular septum. Their role in the release of MSH is discussed. The present data, which are discussed with reference to earlier findings (Forbes, 1972), form the morphological basis for an experimental study on regulation of MSH release (Larsson et al., 1979).

Ultrastructural radioautography of the incorporation of tritiated leucine by the rat adrenal medulla in vivo

Sections of tissues from the adrenal medullae of young rats were subjected to radioautography after a single intravenous injection of L-leucine 4, 5(3)H to identify the sites of synthesis and follow the migration of newly-formed proteins in both adrenaline-storing (A) and noradrenaline-storing (N) cells. As early as 2 min after injection of leucine 3H, the label was highest in the rough endoplasmic reticulum (RER) of A and N cells, suggesting that cisternal ribosomes are sites of protein synthesis. By 5 and 10 min, much of the label had migrated from the RER into the Golgi complex of both cell types. Some label was already present over the secretory granule matrix (chromogranins) by 2 min but the peak was reached at 1 h in both A and N cells. By 4 h, the label over the secretory granules had diminished, indicating a release of newly-synthetized chromogranins outside the cells. The label over the hyaloplasm was relatively high at 2 min but it decreased rapidly to low levels. In contrast, the label over the cell surface continually increased to reach the highest levels among all organelles at 4 h in both cell types. The pattern of increment of the label over the cell surface suggests that the newly-formed proteins of these sites are also synthetized in the RER, pass through the Golgi complex and are transported in the hyaloplasm, before reaching the surface of A and N cells.

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

The neurointermediate lobes of dark-adapted toads Xenopus laevis were incubated for 30 min in [3H]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, beta-lipotrophin, and alpha-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.

Synthesis and intracellular transport of proteins in the exocrine pancreas of the frog (Rana esculenta). II. An in vitro study of the transport process and the influence of temperature

Frog pancreatic tissue was pulse-labelled in vitro with 3H-leucine and protein transport was studied in exocrine cells by electron microscope autoradiography. The proteins appeared to be synthesized in the RER and transported to the secretory granules along a similar route and with the same velocity as previously described under in vitro conditions. Evidence was obtained for the involvement of the vesicular and tubular elements at the periphery of the Golgi system in transferring protein from the RER to the Golgi cisternae. Kinetics of the release of newly synthesized proteins from the RER and their appearance in the condensing vacuoles are discussed and related to results reported from other tissues. The transport velocity in this poikilothermic system was studied in relation to the incubation temperature and compared with results reported from its mammalian counterpart. At temperatures between 20 and 30 degrees C intracellular protein transport occurs faster in the frog than in the Guinea pig pancreas. At higher temperature the transport process was severely disturbed in the frog.

Electron microscope studies on the intermediate lobe of the embryonic mouse

The development of the intermediate lobe of the hypophysis was studied in the embryonic C3H mouse; at least four glands from embryos of every gestational day from 15 to 19 were examined. In the 16 day-old embryo prospective secretory cells proliferate at the centre of the intermediate lobe anlage. At the same stage cylindrical cells bordering the hypophyseal cleft begin to reorganize into marginal cells. By the end of fetal life marginal cells are well differentiated. In the 17 day-old embryo a few granular inclusions appear in some centrally located cells. Secretory cells increase in number during the following two embryonic days. Some of these cells contain polymorphic populations of granular and vesicular inclusions by gestational day 19. The possibility of a dual formation of secretory inclusions is discussed. The result implies that the onset of granule-formation by these cells is not contemporaneous with the start of production of melanophore-expanding substances, the presence of which has been detected by earlier biological assays.

Effect of thyreotrope-releasing hormone (TRH) on prolactin turnover in culture

A kinetic study of the influence of thyreotrope-releasing hormone (TRH) on prolactin turnover and synthesis by a new rat pituitary prolactin cell line (SD1) has been performed by means of pulse-chase experiments. After a 10-min [3H]leucine pulse, the chase was carried out in the presence or absence of TRH (54 nM), cycloheximide (3.6 X 10(-5)M) and/or [14C]-proline. The prolactin content of the cells in the medium was estimated using a radioimmunoassay technique. The specific radioactivity of prolactin in the medium was estimated after its isolation by disc gel electrophoresis. This kinetic study demonstrated, firstly, a rapid intracellular transit of newly synthesized prolactin (15 + 10 min or less); secondly, the existence of at least two intracellular prolactin pools; thirdly, a rapid effect of TRH on release of stored prolactin, which is independent of de novo protein synthesis, and fourthly, a delayed stimulating effect of TRH on prolactin synthesis.

Dynamics of cytoplasmic membranes in guinea pig pancreatic acinar cells. I. Synthesis and turnover of membrane proteins

The rate of synthesis and the turnover of cytoplasmic membrane proteins were determined in the acinar cells of guinea pig pancreas with the aim of investigating the mechanisms by which the intracellular transport of secretion products occurs. These cells are highly specialized toward protein secretion. By means of in vitro pulse-chase experiments and in vivo double-labeling experiments, using radioactive L-leucine as the tracer, it was found that the turnover of secretory proteins is much faster than that of all membranes involved in their transport (rough and smooth microsome and zymogen granule membranes). Sodium dodecyl sulfate-polyacrylamide disk gel electrophoresis of membrane proteins revealed that in each of these membranes there is a marked heterogeneity of turnover; generally the high molecular weight polypeptides have a shorter half-life than the low molecular weight polypeptides. These data indicate that the membranes participating in the intracellular transport of secretory proteins are not synthesized concomitantly with the latter. Rather, they are probably reutilized in several successive secretory cycles. The possible relevance of these findings to other secretory systems is discussed.

Hormone secretion by cells dissociated from rat anterior pituitaries

A new procedure has been developed for dissociating anterior pituitary tissue and producing a viable suspension of single cells. The procedure involves incubation of small tissue blocks in 1 mg/ml trypsin (15 min), followed by incubation in 8 microg/ml neuraminidase and 1 mM EDTA (15 min), followed by mechanical dispersion. Cell yields are approximately 55%, based on recovered DNA. By electron microscopy five types of secretory cells (somatotrophs, mammotrophs, thyrotrophs, gonadotrophs, and corticotrophs) plus endothelial and follicular cells can be identified and are morphologically well preserved up to 20 h after dissociation. Throughout this period, the cells incorporate linearly [(3)H]leucine into protein for up to 4 h at a rate 90% greater than hemipituitaries, and they synthesize, transport intracellularly, and release the two major pituitary secretory products, growth hormone and prolactin. Immediately after dissociation the cells' ability to respond to secretogogues (high K(+) and dibutyryl cyclic AMP) is impaired, but after a 6-12-h culture period, the cells apparently recover and discharge 24% and 52%, respectively, of their content of prelabeled growth hormone over a 3-h period in response to these two secretogogues. This represents a stimulation of 109% and 470% over that released by cells incubated in control medium. The results demonstrate that function and morphologic integrity are preserved in this cell system. Therefore it is suitable for the study of various aspects of pituitary secretion and its control.