Role of secretin peptide family and their receptors in the hypothalamic control of energy homeostasis.

Secretin family of peptide hormones is a group of structurally related brain-gut peptides that exert their functions via interactions with the class B1 G protein-coupled receptors (GPCRs). Recent researches of these peptides and receptors in metabolism have been an area of intense focus for the development of promising drug targets as therapeutic potentials for metabolic disorders. The fact that agonists of GLP-1, a member in the family, have already started being used as therapeutics clearly indicates the importance and relevance of further research on the clinical applications of these peptides. This review aims to provide an overview of the current understanding regarding the importance of this family of peptides as well as their receptors in metabolism with special focus on their actions in the hypothalamus.

Latent membrane protein 1 suppresses RASSF1A expression, disrupts microtubule structures and induces chromosomal aberrations in human epithelial cells.

Epstein-Barr virus (EBV) infection is closely associated with nasopharyngeal carcinoma (NPC) and can be detected in early premalignant lesions of nasopharyngeal epithelium. The latent membrane protein 1 (LMP1) is an oncoprotein encoded by the EBV and is believed to play a role in transforming premalignant nasopharyngeal epithelial cells into cancer cells. RASSF1A is a tumor-suppressor gene commonly inactivated in many types of human cancer including NPC. In this study, we report a novel function of LMP1, in down-regulating RASSF1A expression in human epithelial cells. Downregulation of RASSF1A expression by LMP1 is dependent on the activation of intracellular signaling of NF-kappaB involving the C-terminal activating regions (CTARs) of LMP1. LMP1 expression also suppresses the transcriptional activity of the RASSF1A core promoter. RASSF1A stabilizes microtubules and regulates mitotic events. Aberrant mitotic spindles and chromosome aberrations are reported phenotypes in RASSF1A inactivated cells. In this study, we observed that LMP1 expression in human epithelial cells could induce aberrant mitotic spindles, disorganized interphase microtubules and aneuploidy. LMP1 expression could also suppress microtubule dynamics as exemplified by tracking movements of the growing tips of microtubules in live cells by transfecting EGFP-tagged EB1 into cells. The aberrant mitotic spindles and interphase microtubule organization induced by LMP1 could be rescued by transfecting RASSF1A expression plasmid into cells. Downregulation of RASSF1A expression by LMP1 may facilitate its role in transformation of premalignant nasopharyngeal epithelial cells into cancer cells.

Secretin: a pleiotrophic hormone.

Secretin holds a unique place in the history of endocrinology and gastrointestinal physiology, as it is the first peptide designated as a hormone. During the last century since its first discovery, the hormonal effects of secretin in the gastrointestinal tract were extensively studied, and its principal role in the periphery was found to stimulate exocrine secretion from the pancreas. Recently, a functional role of secretin in the brain has also been substantiated, with evidence suggesting a possible role of secretin in embryonic brain development. Given that secretin and its receptors are widely expressed in multiple tissues, this peptide should therefore exhibit pleiotrophic functions throughout the body. The present article reviews the current knowledge on the central and peripheral effects of secretin as well as its therapeutic uses.

Endogenous release of secretin from the hypothalamus.

Previous studies demonstrated that secretin could be released from the cerebellum, where it exerts a facilitatory action on the GABAergic inputs into the Purkinje neurons. In the present article, we provide evidence of the endogenous release of secretin in the hypothalamus and the mechanisms underlying this release. Incubation of the hypothalamic explants with KCl induces the release of secretin to 4.35 +/- 0.45-fold of the basal level. This K+-induced release was tetrodotoxin and cadmium sensitive, suggesting the involvement of voltage-gated sodium and calcium channels. The use of specific blockers further revealed the involvement of L-, N-, and P-type high voltage-activated (HVA) calcium channels. Results present in the current article provide further and more solid evidence of the role of secretin as a neuropeptide in the mammalian central nervous system.

Pharmacological Actions of Glucagon-Like Peptide-1, Gastric Inhibitory Polypeptide, and Glucagon.

Glucagon family of peptide hormones is a group of structurally related brain-gut peptides that exert their pleiotropic actions through interactions with unique members of class B1 G protein-coupled receptors (GPCRs). They are key regulators of hormonal homeostasis and are important drug targets for metabolic disorders such as type-2 diabetes mellitus (T2DM), obesity, and dysregulations of the nervous systems such as migraine, anxiety, depression, neurodegeneration, psychiatric disorders, and cardiovascular diseases. The current review aims to provide a detailed overview of the current understanding of the pharmacological actions and therapeutic advances of three members within this family including glucagon-like peptide-1 (GLP-1), gastric inhibitory polypeptide (GIP), and glucagon.

Endogenous release and multiple actions of secretin in the rat cerebellum.

Previous studies demonstrated that secretin could modulate synaptic transmission in the rat cerebellum. In the present report, we provide evidence for the endogenous release of secretin in the cerebellum and further characterize the actions of secretin in this brain area. First, to show that secretin is released endogenously, blocks of freshly dissected cerebella were challenged with a high concentration of KCl. Incubation with KCl almost doubled the rate of secretin release. This KCl-induced release was sensitive to tetrodotoxin and cadmium suggesting the involvement of voltage-gated sodium and calcium channels. The use of specific channel blockers further revealed that L-type and P/Q-type calcium channels underlie both basal and KCl-evoked secretin release. In support of this, depolarization of Purkinje neurons in the presence of NMDA, group II mGluR and cannabinoid CB1 receptor blockers resulted in increased inhibitory postsynaptic current frequency. Second, we found that the previously reported facilitatory action of secretin on GABAergic inputs to Purkinje neurons is partly dependent on the release of endogenous glutamate. In the presence of CNQX, an AMPA/kainate receptor antagonist, the facilitatory effect of secretin on GABA release was significantly reduced. In support of this idea, application of AMPA, but not kainate receptor agonist, facilitated GABA release from inhibitory terminals, an action that was sensitive to AMPA receptor antagonists. These data indicate that a direct and an indirect pathway mediate the action of secretin in the basket cell-Purkinje neuron synapse. The results provide further and more solid evidence for the role of secretin as a neuropeptide in the mammalian CNS.

Expression and spatial distribution of secretin and secretin receptor in human cerebellum.

The expression and spatial distribution of secretin and its receptor in human cerebellum were investigated by in situ hybridization and immunohistochemical techniques. Secretin mRNAs are found in Purkinje cells whereas secretin receptor transcripts are present in Purkinje cells and basket cells in the molecular cell layer. In addition, secretin-immunoreactivities are localized in both the soma and dendrites of Purkinje cells. These data are the first demonstration of the spatial distribution of secretin and its receptor in distinct neurons within the human cerebellum. The cellular localizations of this ligand-receptor pair are consistent with the proposed actions of secretin in the cerebellum of rodents and hence suggest that secretin also serves specific neural functions in the human cerebellum.

Characterization of an additional molt inhibiting hormone-like neuropeptide from the shrimp Metapenaeus ensis.

We have identified a second form of the type-II neuropeptide encoding a molt inhibiting hormone-like (MeeMIH-B) neuropeptide. MeeMIH-B showed only a 70% amino acid identity to the MIH-A (formerly MIH) isolated from the same species, suggesting a possible different function of the deduced neuropeptide. Like other neuropeptide members of the CHH family, the MIH-B gene consists of three exons separated by two introns. The levels of MIH-B mRNA transcript in the eyestalk decrease in the initial phase of gonad maturation and increase towards the end of maturation. The drop in MIH-B level suggests an inhibitory role for this neuropeptide in the initiation of vitellogenesis. MIH-B transcripts can also be detected in the brain, thoracic ganglion and ventral nerve cord. Together with the CHH-B peptide that we have previously described, this is the second peptide of the CHH family that can also be identified in the ventral nerve cord and in the XOSG complex. A recombinant MIH-B was produced and a polyclonal antibody against rMIH-B was subsequently generated. Specific anti-rMIH-B antiserum recognized the presence of MIH-B in the sinus gland, X-organs, as well as a giant neuron of the ventral nerve cord. Injection of rMIH-B delayed the molting cycle of the maturing female. Taken together, the results of this study suggest that a drop in MIH-B level may be required for the delay in the molting of the maturing females.

Molecular cloning and mRNA distribution of pituitary adenylate cyclase-activating polypeptide (PACAP)/PACAP-related peptide in the lungfish.

In this article, we report the isolation of a full-length cDNA clone encoding pituitary adenylate cyclase-activating polypeptide (PACAP)/PACAP-related peptide (PRP) from lungfish Protopterus dolloi. When comparing the deduced amino acid sequences, the lungfish PACAP was found to be highly conserved with other vertebrates; however, the PRP shares only lower levels of sequence identity with known PRP sequences. Consistently in phylogenetic analysis, the lungfish PRP, similar to sturgeon PRP, fails to cluster with other PRPs. In addition to the full-length clone, another cDNA encoding a short precursor that lacks the first 32 amino acids of the PRP was also isolated. Interestingly, similar isoforms were also identified in several nonmammalian vertebrates, and it was suggested that exon skipping of PRP/PACAP transcripts was a mechanism that regulated the expression ratio of PACAP to PRP in nonmammalian vertebrates. By real-time PCR, both long and short PRP/PACAP transcripts were found almost exclusively in the brain, and the short isoform is the more abundant transcript (3.7 times more), indicating that PACAP is the major product produced in lungfish brain. The expression patterns of lungfish and previously studied frog PRP/PACAP suggest that the PRP/PACAP gene in the tetrapod lineage may first express in the central nervous system; in the process of evolution, the functions of these peptides diversified and were later found in other tissues.

Secretin controls anion secretion in the rat epididymis in an autocrine/paracrine fashion.

There is growing evidence that secretin, the first hormone discovered in our history, has functions in the brain other than in the gastrointestinal tract. This article reports for the first time that secretin and its receptor mRNAs are produced in distinct cell types within the epididymis. To test if secretin affects electrolyte transport in the epididymis, we measured short-circuit current (Isc) in cultured epididymal epithelia and found secretin dose-dependently stimulated Isc. Ion substitution experiments and use of pharmacological agents inferred that the stimulated Isc is a result of concurrent electrogenic chloride and bicarbonate secretion. It is further shown that secretin and pituitary adenylate cyclase-activating polypeptide (PACAP) function via totally different mechanisms: 1) PACAP works only from the apical side of the epithelium to stimulate chloride and not bicarbonate secretion, while secretin acts on the apical and basolateral sides to stimulate chloride and bicarbonate secretion. 2) the stimulation by PACAP but not secretin requires local prostaglandin synthesis. By immunocytochemical staining, secretin is localized in the principal cells of the initial segment and caput epididymidis, whereas secretin receptor is present in the principal cells of the proximal as well as the distal part of the epididymis. This pattern of distribution appears to be consistent with the idea that secretin is secreted by the proximal epididymis and acts on the proximal and distal epididymis in an autocrine and paracrine fashion. Its function is to control secretion of electrolytes and water.

Function and cellular localization of farnesoic acid O-methyltransferase (FAMeT) in the shrimp, Metapenaeus ensis.

The isoprenoid methyl farnesoate (MF) has been implicated in the regulation of crustacean development and reproduction in conjunction with eyestalk molt inhibiting hormones and ecdysteroids. Farnesoic acid O-methyltransferase (FAMeT) catalyzes the methylation of farnesoic acid (FA) to produce MF in the terminal step of MF synthesis. We have previously cloned and characterized the shrimp FAMeT. In the present study, recombinant FAMeT (rFAMeT) was produced for bioassay and antiserum generation. FAMeT is widely distributed in shrimp tissues with the highest concentration observed in the ventral nerve cord. Interestingly, an additional larger protein in the eyestalk also showed immunoreactivity to anti-FAMeT serum. FAMeT was localized in the neurosecretory cells of the X-organ-sinus gland complex of the eyestalk. As shown by RT-PCR, FAMeT mRNA is constitutively expressed throughout the molt cycle in the eyestalk and the ventral nerve cord. To show that our cloned gene product had FAMeT activity, we demonstrated that expressed rFAMeT gene product catalyzed the conversion of FA to MF in a radiochemical assay. The ubiquitous distribution of FAMeT suggests that this enzyme is involved in physiological processes in addition to gametogenesis, oocyte maturation and development and metamorphosis of the shrimp. We hypothesize that FAMeT directly or indirectly (through MF) modulates the reproduction and growth of crustaceans by interacting with the eyestalk neuropeptides as a consequence of its presence in the neurosecretory cells of the X-organ-sinus gland.