Cryptides: functional cryptic peptides hidden in protein structures

Peptidergic hormones, neurotransmitters, and neuromodulators are extracellular signaling molecules that play central roles in physiological signal transmissions between various cells, tissues, and organs. These factors are primarily translated as inactive precursor proteins according to the genetic information. These precursor proteins are then cleaved by various proteases including signal peptidases and processing enzymes to produce matured bioactive factors. During these processes, various fragmented peptides are also produced from the same precursor proteins. Such fragmented peptides may have various unexpected biological activities that have not been identified yet because these peptides are considered to be produced and released along with mature factors at the same secretary pathways. Recently, we found that various fragmented peptides of mitochondrial proteins that are produced during the maturation processes, such as fragments of cytochrome c oxidase, activate neutrophils whose functions are distinct from their parent proteins. These findings suggest the existence of many different functional peptides whose functions have not been identified yet. These unidentified peptides may play a variety of roles in various regulatory mechanisms, and therefore, they are expected to provide novel regulatory and signaling mechanisms, "Peptide World".     

TRH-like peptides in prostate gland and other tissues

This minireview is aimed to recapitulate the occurrence of TRH-like peptides in the prostate gland and other tissues and to discuss their known functions in the organism. The hypothalamic thyrotropin-releasing hormone (TRH) was the first chemically defined hypophyseotropic hormone with the primary structure pGLU-HIS-PRO.NH2. However, the presence of extrahypothalamic TRH-immunoreactive peptides was reported in peripheral tissues including the gastrointestinal tract, placenta, neural tissues, male reproductive system and certain endocrine tissues. It was supposed that this TRH immunoreactivity can partially originate from TRH-homologous peptides and that these peptides have significant cross-reactions with the antibody specific against authentic TRH. This assumption was confirmed by the identification of prostatic TRH immunoreactivity as pyroGLU-GLU-PRO.NH2 using fast atom bombardment mass spectrometry and gas phase sequence analysis. TRH-like peptides are characterized by substitution of the basic amino acid histidine (related to authentic TRH) for neutral or acidic amino acids, such as glutamic acid, phenylalanine, glutamine or tyrosine. The physiological role of TRH-like peptides in peripheral tissues is not precisely known, but they possess a C-terminal amide group which is characteristic for many biologically active peptides. The occurrence of these peptides in the male reproductive system can influence male fertility. They are also closely related to circulating thyroid and steroid hormones. There might be an important connection of TRH-like peptides to the prostatic local autocrine/paracrine network mediated by extrahypothalamic TRH immunoreactivity corresponding to TRH-like peptides and extrapituitary thyrotropin (TSH) immunoreactivity also found in the prostatic tissue. A similar system of intraepithelial lymphocyte hormonal regulation due to the local paracrine network of TRH/TSH has been described in the gastrointestinal tract. The local network of TRH-like peptides/TSH may be involved in possible regulation of prostatic growth.     

Embryology of the diffuse neuroendocrine system and its relationship to the common peptides.

The diffuse neuroendocrine system is constituted by the cells, now more than 40 in number, of the central and peripheral divisions of the amine precursor uptake and decarboxylation (APUD) series. At one time presumed to be derived from a common "neural" ancestor, all are now deemed to be "neuroendocrine-programmed," arising either in the embryonic epiblast itself or in one of its principal descendants. The APUD cells produce more than 35 physiologically active peptides and a small number of equally active amines. Within the last 3 years, 17 of these peptides have been identified jointly in endocrine cells and in neuronal cell bodies or processes. Sharing in this way a neural and an endocrine location and site of production, they are called the "common peptides." The diffuse neuroendocrine system is to be regarded as a third division of the nervous system, whose products suppress, amplify, or modulate the activities of the other two divisions. The relationship of its products to the cells and processes of these two divisions is currently the object of intensive inquiry.     

Evolutionary significance of the phylogenetic distribution of the mammalian hypothalamic releasing hormones

The hypophysiotrophic hormones isolated from the mammalian hypothalamus are distributed throughout the nervous system of vertebrate species. Although their role in regulating pituitary hormone secretion in mammals is clear, a similar function in lower species has not been established. Thyrotropin-releasing hormone is unable to stimulate thyroid function in amphibia and fish, despite being present in the hypothalamus and brain of these species of high concentration. The tripeptide is also found in high concentration in frog skin, a tissue derived from (or programed by) primitive neuroectoderm that is also a rich source of other peptides structurally related to neural peptides located in mammalian brain and gut. Luteinizing hormone-releasing hormone (LHRH) is able to activate gonadotropin secretion in submammalian species but there is evidence that the LHRH material present in avian, reptilian, and piscine brain is not identical to the mammalian decapeptide. An LHRH-like material present in frog sympathetic ganglia appears to function as a neurotransmitter in this location. Somatostatin is present in high concentrations in the hypothalamus, brain, pancreas, and gastrointestinal tract of all vertebrates and chromatographically is identical to the mammalian material, suggesting that this peptide is an "ancient" molecule with an important role in neuronal pancreatic and digestive function. The hypothalamic releasing hormones are part of a family of neural peptides that have a widespread anatomic and phylogenetic distribution and form a diffuse neuroendocrine system. It an material, suggesting that this peptide is an "ancient" molecule with an important role in neuronal pancreatic and digestive function. The hypothalamic releasing hormones are part of a family of neural peptides that have a widespread anatomic and phylogenetic distribution and form a diffuse neuroendocrine system. It an material, suggesting that this peptide is an "ancient" molecule with an important role in neuronal pancreatic and digestive function. The hypothalamic releasing hormones are part of a family of neural peptides that have a widespread anatomic and phylogenetic distribution and form a diffuse neuroendocrine system. It appears likely that the releasing hormones initially arose with a neurocrine or paracrine function, and that only later in evolution did they acquire the role of regulating adenohypophysial secretion.     

Local regulation within the female reproductive system and upon embryonic implantation: identification of cells expressing proenkephalin A

The detection of proenkephalin A (PEA) mRNA and encoded peptides in various regions of the female reproductive system raised the possibility that opioid peptides might act as local regulators within this system. Assignment of a specific role for locally synthesized enkephalins has been hampered, however, by the unknown identity of the cells that produce PEA. Using in situ hybridization analysis we have now identified the cell types that express PEA mRNA in the reproductive system of female mice. In the ovary, PEA mRNA was localized primarily in theca cells of preovulatory follicles, and to a lesser extent, in follicular granulosa cells. In the oviducts, where PEA mRNA is most abundant, expression was confined to the secretory and ciliated epithelium of the mucosa. In the uterus, the site of PEA mRNA expression was the deep glandular layer of the endometrium. When pregnancy ensues, and upon decidual transformation, PEA expression by the same uterine cells was dramatically elevated. Elevated levels of PEA mRNA were detected predominantly in the vicinity of the implantation site, suggesting that signaling by the implanted embryo play a role in stimulating PEA expression. Based on these results, possible physiological roles for PEA-encoded peptides as autocrine/paracrine regulators within the female reproductive system are suggested.     

Natriuretic peptides in fish physiology

Natriuretic peptides exist in the fishes as a family of structurally-related isohormones including atrial natriuretic peptide (ANP), C-type natriuretic peptide (CNP) and ventricular natriuretic peptide (VNP); to date, brain natriuretic peptide (or B-type natriuretic peptide, BNP) has not been definitively identified in the fishes. Based on nucleotide and amino acid sequence similarity, the natriuretic peptide family of isohormones may have evolved from a neuromodulatory, CNP-like brain peptide. The primary sites of synthesis for the circulating hormones are the heart and brain; additional extracardiac and extracranial sites, including the intestine, synthesize and release natriuretic peptides locally for paracrine regulation of various physiological functions. Membrane-bound, guanylyl cyclase-coupled natriuretic peptide receptors (A- and B-types) are generally implicated in mediating natriuretic peptide effects via the production of cyclic GMP as the intracellular messenger. C- and D-type natriuretic peptide receptors lacking the guanylyl cyclase domain may influence target cell function through G(i) protein-coupled inhibition of membrane adenylyl cyclase activity, and they likely also act as clearance receptors for circulating hormone. In the few systems examined using homologous or piscine reagents, differential receptor binding and tissue responsiveness to specific natriuretic peptide isohormones is demonstrated. Similar to their acute physiological effects in mammals, natriuretic peptides are vasorelaxant in all fishes examined. In contrast to mammals, where natriuretic peptides act through natriuresis and diuresis to bring about long-term reductions in blood volume and blood pressure, in fishes the primary action appears to be the extrusion of excess salt at the gills and rectal gland, and the limiting of drinking-coupled salt uptake by the alimentary system. In teleosts, both hypernatremia and hypervolemia are effective stimuli for cardiac secretion of natriuretic peptides; in the elasmobranchs, hypervolemia is the predominant physiological stimulus for secretion. Natriuretic peptides may be seawater-adapting hormones with appropriate target organs including the gills, rectal gland, kidney, and intestine, with each regulated via, predominantly, either A- or B-type (or C- or D-type?) natriuretic peptide receptors. Natriuretic peptides act both directly on ion-transporting cells of osmoregulatory tissues, and indirectly through increased vascular flow to osmoregulatory tissues, through inhibition of drinking, and through effects on other endocrine systems.     

Immunolocalisation of vasoactive intestinal peptide and substance P in the developing gut of Dicentrarchus labrax (L.)

This study was carried out on the sea bass (Dicentrarchus labrax) to follow, during development, the appearance and distribution of substance P (SP) and vasoactive intestinal peptide (VIP), which act on gut motility. The results suggest that SP and VIP play an important role as neuromodulators, influencing the motility of the digestive tract starting from the early stages of gut development, even prior to exotrophic feeding. In the peptidergic nervous system, the appearance of immunoreactivity to SP began at the rectum and followed a distal to proximal gradient, whereas for VIP, it began proximally and progressed along a proximal to distal gradient. The two peptides also appeared in gut epithelial cells. In some regions, all the cells were positive. From this distribution of positive cells, we conclude that these peptides may also have other roles, besides being neurotransmitters in the enteric nervous system and hormones of the gastro-entero-pancreatic system. VIP and SP might have paracrine and/or autocrine activity in the physiological maturation of the gut epithelium, as it has already been hypothesised for other peptides.     

Mass spectrometric characterization and physiological actions of VPNDWAHFRGSWamide, a novel B type allatostatin in the crab, Cancer borealis

The neural networks in the crustacean stomatogastric ganglion are modulated by neuroactive substances released locally into the neuropil of the stomatogastric ganglion and by circulating hormones released by neuroendocrine structures including the pericardial organs. Using nanoscale liquid chromatography coupled to electrospray ionization quadrupole-time-of-flight mass spectrometry, we have identified and sequenced a novel B type allatostatin (CbAST-B1), VPNDWAHFRGSWamide, present in the pericardial organs of the crabs, Cancer borealis, and Cancer productus. We describe the physiological actions of CbAST-B1 on the pyloric rhythm of the stomatogastric ganglion of the crab, Cancer borealis. CbAST-B1 reduces the pyloric network frequency in a dose-dependent manner. The effect of bath-applied CbAST-B1 depends on the preceding physiological state of the preparation. Surprisingly, despite marked amino-acid sequence dissimilarity between the novel CbAST-B1 and the A type allatostatin family of peptides (AST-A), the physiological effects of CbAST-B1 are similar to those of AST-A.     

Molecular evolution of the neurohypophysial hormones: The active peptides of a primitive bony fish Polypterus bichir

Neurohypophysial hormones have been so far identified in Neopterygii and Crossopterygii but not in species of the bird sub-class of bony fishes, the Palaeopterygii. Isolation and chemical characterization of the active principles of a primitive bony fish, Polypterus bichir, have been performed. Isotocin (Ser(4)-Ile(8)-oxytocin) and arginine vasotocin (Arg(8)-oxytocin) have been identified. Because the same peptides were found in the recent Neopterygii, it can be deduced that neurohypophysial hormones have displayed a peculiar stability in the course of the evolution of bony fishes. However isotocin and vasotocin are replaced by oxytocin and vasopressins in mammals and therefore might be regarded as "old" molecules.     

Drosophila melanogaster myotropins have unique functions and signaling pathways

Drosophila melanogaster TDVDHVFLRFamide (DMS), SDNFMRFamide, and pEVRFRQCYFNPISCF (FLT) represent three structurally distinct peptide families. Each peptide decreases heart rate albeit with different magnitudes and time-dependent responses. DMS and FLT are expressed in the crop and decrease crop motility; however, SDNFMRFamide expression and effect on the crop has not been reported. These data suggest the peptides have different physiological roles. The peptides have non-overlapping expression patterns in neural tissue, which suggests different mechanisms regulate their synthesis and release. The structures, expression patterns, and activities of the myotropins suggest they have important but different roles in biology and different signaling pathways.     

Hormones of the Cardiovascular System

For the past few decades, several hormones secreted by myocardium and blood vessel walls and regulate various physiological functions have been identified. They include natriuretic hormones, endothelins, proteins related to the parathyroid hormone, adrenomedullin and others. Therefore, the heart and blood vessels, apart from their main function, blood circulation, also perform important endocrine function, i.e., they are an organ controlling various physiological processes including hemodynamics, skeletal growth, reproductive function, immunity, etc.     

Identification of novel MAGE-A6- and MAGE-A12-derived HLA-A24-restricted cytotoxic T lymphocyte epitopes using an in silico peptide-docking assay

Many cancer-testis antigen genes have been identified; however, few human leukocyte antigen (HLA)-A24-restricted cytotoxic T cell (CTL) epitope peptides are available for clinical immunotherapy. To solve this problem, novel tools increasing the efficacy and accuracy of CTL epitope detection are needed. In the present study, we utilized a highly active dendritic cell (DC)-culture method and an in silico HLA-A24 peptide-docking simulation assay to identify novel CTL epitopes from MAGE-A6 and MAGE-A12 antigens. The highly active DCs, called ??-type-1 DCs, were prepared using a combination of maturation reagents to produce a large amount of interleukin-12. Meanwhile, our HLA-A24 peptide-docking simulation assay was previously demonstrated to have an obvious advantage of accuracy over the conventional prediction tool, bioinformatics and molecular analysis section. For CTL induction assays, peripheral blood mononuclear cells derived from six cases of HLA-A24⁺ melanoma were used. Through CTL induction against melanoma cell lines and peptide-docking simulation assays, two peptides (IFGDPKKLL from MAGE-A6 and IFSKASEYL from MAGE-A12) were identified as novel CTL epitope candidates. Finally, we verified that the combination of the highly active DC-culture method and HLA-A24 peptide-docking simulation assay might be tools for predicting CTL epitopes against cancer antigens.     

Does the natriuretic peptide system exist throughout the animal and plant kingdom?

Natriuretic peptides (NPs) and their receptors have been identified in vertebrate species ranging from elasmobranchs to mammals. Atrial, brain and ventricular NP (ANP, BNP and VNP) are endocrine hormones secreted from the heart, while C-type NP (CNP) is principally a paracrine factor in the brain and periphery. In elasmobranchs, only CNP is present in the heart and brain and it functions as a circulating hormone as well as a paracrine factor. Four types of NP receptors are cloned in vertebrates. NPR-A and NPR-B are guanylyl cyclase-coupled receptors, whereas NPR-C and NPR-D have only a short cytoplasmic domain. NPs are hormones important for volume regulation in mammals, while they act more specifically for Na(+) regulation in fishes. The presence of NP and its receptor has also been suggested in the most primitive vertebrate group, cyclostomes, and its molecular identification is in progress. The presence of ANP or its mRNA has been reported in the hearts and ganglia of various invertebrate species such as mollusks and arthropods using either antisera raised against mammalian ANP or rat ANP cDNA as probes. Immunoreactive ANP has also been detected in the unicellular Paramecium and in various species of plants including Metasequoia. Furthermore, the N-terminal prosegments of ANP, whose sequences are scarcely conserved even in vertebrates, have also been detected by the radioimmunoassay for human ANP prosegments in all invertebrate and plant species examined including Paramecium. Although these data are highly attractive, the current evidence is too circumstantial to be convincing that the immunoreactivity truly originates from ANP and its prosegments in such diverse organisms. The caution that has to be exercised in identification of vertebrate hormones from phylogenetically distant organisms is discussed.     

Effects of pro-opiomelanocortin-derived peptides on plasma levels of glucagon, insulin and glucose

Pro-opiomelanocortin (POMC) is a prohormone for several peptides including corticotropin, melanocyte stimulating hormones and beta-endorphin. POMC-derived peptides have been demonstrated in many tissues, including the hypothalamus and the endocrine pancreas, which play important roles in the control of plasma levels of glucagon, insulin and glucose. This article reviews the present knowledge concerning in vitro and in vivo effects of POMC-derived peptides on glucagon, insulin and glucose levels involving several possible mechanisms: direct effects on the endocrine pancreas (including endocrine, paracrine and peptidergic regulation) and glucose production, and indirect effects involving the hypothalamus, the autonomic nervous system and the adrenal gland.     

Gut hormone secreting cells--unsolved problems

The gut endocrine cells generally are bipolar paraneurons, extending from the epithelial base to the luminal surface. The cells, also known as the basal-granulated cells, produce peptidic and amine ++ signal substances to release them in response to the luminal stimuli. The substances are released not only in an endocrine fashion, but also, and more importantly, in a paracrine way, thus stimulating nervous, muscular, secretory and other elements in the vicinity. Attention should be paid also to other secretions of the gut endocrine cells, ATP and chromogranins (acidic carrier proteins) which are co-released with the peptides and amines. Various gut hormones have been demonstrated to be released into the gut lumen, besides into the blood and tissue spaces. The possible mechanism of this partial exocrine function of the endocrine cells remains to be explored.     

Food-derived peptides and intestinal functions

Many researchers have reported that food proteins and their peptides expressed a variety of functions in the body, including a reduction of blood pressure, modulation of immune cell functions, and regulation of nerve functions. However, food-derived proteins and peptides also play important roles in the intestinal tract before being absorbed. For example, some of the proteins and peptides can regulate the activity of digestive enzymes in the intestinal tract, thereby modulating the nutrient absorption in the intestines. These proteins and peptides have been used for functional foods with blood glucose- and blood cholesterol-lowering effects. Enhancement of the intestinal calcium absorption by casein-derived peptides is another example, such peptides being used as functional food ingredients. We have recently observed that certain milk peptides might stimulate the calcium transporter in intestinal epithelial cells. Carnosine, a dipeptide contained in skeletal muscles, was observed to suppress the secretion of inflammatory cytokines by intestinal epithelial cells that had been exposed to oxidative stress. Understanding the behavior of dietary proteins and peptides in the intestines is important for designing functional foods with physiological functions.     

The complex understanding of Annexin A1 phosphorylation

Annexin A1 (ANXA1) is the first characterized member of the annexins superfamily. It binds the cellular membrane phospholipids in Ca^2 + regulated manner. Annexin A1 has been found in several tissues and many physiological roles as hormones secretion, vesiculation, inflammatory response, apoptosis and differentiation have been shown. Its subcellular localization and binding with many partner proteins are altered accordingly with its physiological role. The Annexin A1 membrane localization is crucial for binding to receptors, suggesting a paracrine and juxtacrine extracellular action. Annexin A1 is subjected to several post-translational modifications. In particular the protein is phosphorylated on several residues both on the N-terminal functional domain and on the C-terminus core. Different kinases have been identified as responsible for the phosphorylation status of selective residues. The specific change in the phosphorylation status on the different sites alters ANXA1 localization, binding properties and functions. This review shows the physiological relevance of the ANXA1 phosphorylation leading to the conclusion that numerous and different roles of Annexin A1 could be associated with different phosphorylations to alter not only intracellular localization and bindings to its partners but also the extracellular receptor interactions.     

Identification of target ligands of CORYNE in Arabidopsis by phage display library

CORYNE (CRN) plays important roles in stem cell division and differentiation of shoot apical meristem (SAM) in Arabidopsis thaliana. The cytoplasmic kinase domain of CRN has been cloned and expressed in Escherichia coli, and further purified by two consecutive steps of affinity chromatography. By using this purified CRN as a ligand, a 12-mer random-peptide library was used to determine the specific amino acid sequences binding with the recombinant CRN molecule. After four rounds of biopanning, positive phage clones were isolated and sequenced, and further tested by enzyme linked immunosorbent assay for their binding ability and specificity. Two positive clones that specifically bind to the intracellular protein kinase domain of CRN have been identified. Alignment of these peptides and the kinase-associated protein phosphatase (KAPP) shows high similarity, indicating that KAPP might interact with the cytoplasmic kinase domain of CRN and negatively regulate the CLV signal. Our current study would be helpful to better understand the CLV3 signal pathway.     

What is the role of endothelin system?

Endothelins are a family of three peptides of 21 amino acids with strong vasoconstrictor effects. The three peptides are encoded by three different genes and derived from precursors (" big endothelins") which are cleaved by metalloproteases, named endothelin-converting enzyme. Two receptors have been cloned, ET-A and ET-B which bind the three endothelins with various affinities. The diverse expression pattern of the endothelin system (ET) components is associated with a complex pharmacology and its counteracting physiological actions. New modulators of the ET system have been described : retinoic acid, leptin, prostaglandins, hypoxia. Endothelins can be considered as regulators working in paracrine and autocrine fashion in a variety of organs in different cellular types. The ET system has beneficial and detrimental roles in mammals. The different components have been shown to be essential for a normal embryonic and neonatal development, for renal homeostasis and maintenance of basal vascular tone. They are involved in physiological and tumoral angiogenesis. They affect the physiology and pathophysiology of the liver, muscle, skin, adipose tissue and reproductive tract. The endothelin system participates in the development of atherosclerosis as well as pulmonary hypertension, and mediates cardiac remodeling in heart failure. Elaboration of new animal models (knock-out, pathophysiological models em leader ) will allow the clear genetic dissection of physiological and pathophysiological roles of the endothelin system.     

Gastric intrinsic factor: The gastric and small intestinal stages of cobalamin absorption. A personal journey

Intrinsic factor (IF) was first identified as a component of the gastric mucosa that reacted with an extrinsic factor, later discovered to be vitamin B12 (VB12). IF has been extensively characterized, and its cloned cDNA used to produce sufficient IF to produce high quality antibodies, and to elucidate its 3-dimensional structure bound to cobalamin (Cbl, VB12). The absorption of the IF–Cbl complex involves internalization by endocytosis, incorporation into multivesicular/lysosomal bodies, release of Cbl by lysosomal proteolysis and pH effects, with subsequent binding to transcobalamin (TC). Hereditary IF deficiency is rare, consistent with the need for IF to absorb Cbl, a vitamin essential for cell replication. When mutations occur, they are most often associated with loss of function, but some mutations occur outside the coding region. The IF-mediated intestinal uptake of Cbl has been harnessed for use as a transporter for peptides, proteins and even nanoparticles. Nanoparticle (NP) technology has produced Cbl-coated NPs that can incorporate peptides (insulin, IgG) that can be absorbed orally to function as hormones and antibodies in rodent models, but these systems are not yet ready for clinical use.