Secretoneurin stimulates the production and release of luteinizing hormone in mouse L{beta}T2 gonadotropin cells

Function of secretoneurin in regulating the expression of reproduction-related genes in ovoviviparous black rockfish (Sebastes schlegelii)

Secretoneurin (SN), a conserved peptide derived from secretogranin-2 (scg2), also known as secretogranin II or chromogranin C, plays an important role in regulating gonadotropin in the pituitary, which affects the reproductive system. This study aimed to clarify the mode of action of scg2 in regulating gonad development and maturation and the expression of mating behavior-related genes. Two scg2 cDNAs were cloned from the ovoviviparity teleost black rockfish (Sebastes schlegelii). In situ hybridization detected positive scg2 mRNA signals in the telencephalon and hypothalamus, where sgnrh and kisspeptin neurons were reported to be located and potentially regulated by scg2. In vivo, intracerebral ventricular injections of synthetic black rockfish SNa affected brain cgnrh, sgnrh, kisspeptin1, pituitary lh and fsh and gonad steroidogenesis-related gene expression levels with sex dimorphism. In vitro, a similar effect was found in primary cultured brain and pituitary cells. Thus, SN could contribute to the regulation of gonadal development, as well as reproductive behaviors, including mating and parturition.

Effects of Secretoneurin and Gonadotropin-Releasing Hormone Agonist on the Spawning of Captive Greater Amberjack (Seriola dumerili)

The greater amberjack (Seriola dumerili), a pelagic marine species with a global distribution, has considerable worldwide potential as an aquaculture species. However, difficulties have been encountered in inducing spontaneous spawning in cultured fish stocks. In this study, we analysed the key regulatory factors, secretoneurin (SN) and gonadotropin-releasing hormone (GnRH), in greater amberjack. Active peptides of SN and GnRH, SdSNa, and SdGnRH, respectively, were obtained by comparative analysis of homologous proteins from different species. Amino acid substitutions of the SdGnRH decapeptide at position 6 with a dextrorotatory (D) amino acid and at position 10 with an ethylamide group yielded a super-active agonist (SdGnRHa). The injection of SdSNa and SdGnRHa elevated luteinizing hormone, thyroid-stimulating hormone, and oxytocin levels in the sera of sexually mature fish, whereas it reduced the level of follicle-stimulating hormone. Furthermore, in response to the SdSNa and SdGnRHa injections, we detected an increase in the expression of genes associated with oocyte development and spermatogenesis. We established that the greater amberjack cultured along the southern coast of China reached sexual maturity at three years of age, and its reproductive season extended from February to April. Spawning of the cultured greater amberjack was successfully induced with a single injection of SdGnRHa/SdSN/DOM/HCG. Our findings indicate that similar to GnRHa, SNa is a potential stimulator of reproduction that can be used to artificially induce spawning in marine fish.

Neuroendocrine Control of Reproduction in Teleost Fish: Concepts and Controversies

During the teleost radiation, extensive development of the direct innervation mode of hypothalamo-pituitary communication was accompanied by loss of the median eminence typical of mammals. Cells secreting follicle-stimulating hormone and luteinizing hormone cells are directly innervated, distinct populations in the anterior pituitary. So far, ∼20 stimulatory and ∼10 inhibitory neuropeptides, 3 amines, and 3 amino acid neurotransmitters are implicated in the control of reproduction. Positive and negative sex steroid feedback loops operate in both sexes. Gene mutation models in zebrafish and medaka now challenge our general understanding of vertebrate neuropeptidergic control. New reproductive neuropeptides are emerging. These include but are not limited to nesfatin 1, neurokinin B, and the secretoneurins. A generalized model for the neuroendocrine control of reproduction is proposed. Hopefully, this will serve as a research framework on diverse species to help explain the evolution of neuroendocrine control and lead to the discovery of new hormones with novel applications. Expected final online publication date for the Annual Review of Animal Biosciences, Volume 10 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Secretoneurin is a secretogranin-2 derived hormonal peptide in vertebrate neuroendocrine systems

Secretogranin-2 (SCG2) is a large precursor protein that is processed into several potentially bioactive peptides, with the 30-43 amino acid central domain called secretoneurin (SN) being clearly evolutionary conserved in vertebrates. Secretoneurin exerts a diverse array of biological functions including regulating nervous, endocrine, and immune systems in part due to its wide tissue distribution. Expressed in some neuroendocrine neurons and pituitary cells, SN is a stimulator of the synthesis and release of luteinizing hormone from both goldfish pituitary cells and the mouse LβT2 cell line. Neuroendocrine, paracrine and autocrine signaling pathways for the stimulation of luteinizing hormone release indicate hormone-like activities to regulate reproduction. Mutation of the scg2a and scg2b genes using TALENs in zebrafish reduces sexual behavior, ovulation, oviposition, and fertility. A single injection of the SNa peptide enhanced reproductive outcomes in scg2a/scg2b double mutant zebrafish. Evidence in goldfish suggests a new role for SN to stimulate food intake by actions on other feeding-related neuropeptides. Expression and regulation of the Scg2a precursor mRNA in goldfish gut also supports a role in feeding. In rodent models, SN has trophic-like properties promoting both neuroprotection and neuronal plasticity and has chemoattractant properties that regulate neuroinflammation. Data obtained from several cellular models suggest that SN binds to and activates a G-protein coupled receptor (GPCR), but a bona fide SN receptor protein needs to be identified. Other signaling pathways for SN have been reported which provides alternatives to the GPCR hypothesis. These include AMP-activated protein kinase (AMPK), extracellular signal-regulated kinases (ERK), mitogen-activated protein kinase (MAPK)and calcium/calmodulin-dependent protein kinase II in cardiomyocytes, phosphatidylinositol 3-kinase (PI3K) and Akt/Protein Kinase B (AKT, and MAPK in endothelial cells and Janus kinase 2/signal transducer and activator of transcription protein (JAK2-STAT) signaling in neurons. Some studies in cardiac cells provide evidence for cellular internalization of SN by an unknown mechanism. Many of the biological functions of SN remain to be fully characterized, which could lead to new and exciting applications.

Multimodal hypothalamo-hypophysial communication in the vertebrates

The vertebrate pituitary is arguably one of the most complex endocrine glands from the evolutionary, anatomical and functional perspectives. The pituitary plays a master role in endocrine physiology for the control of growth, metabolism, reproduction, water balance, and the stress response, among many other key processes. The synthesis and secretion of pituitary hormones are under the control of neurohormones produced by the hypothalamus. Under this conceptual framework, the communication between the hypophysiotropic brain and the pituitary gland is at the foundation of our understanding of endocrinology. The anatomy of the connections between the hypothalamus and the pituitary gland has been described in different vertebrate classes, revealing diverse modes of communication together with varying degrees of complexity. In this context, the evolution and variation in the neuronal, neurohemal, endocrine and paracrine modes will be reviewed in light of recent discoveries, and a re-evaluation of earlier observations. There appears to be three main hypothalamo-pituitary communication systems: 1. Diffusion, best exemplified by the agnathans; 2. Direct innervation of the adenohypophysis, which is most developed in teleost fish, and 3. The median eminence/portal blood vessel system, most conspicuously developed in tetrapods, showing also considerable variation between classes. Upon this basic classification, there exists various combinations possible, giving rise to taxon and species-specific, multimodal control over major physiological processes. Intrapituitary paracrine regulation and communication between folliculostellate cells and endocrine cells are additional processes of major importance. Thus, a more complex evolutionary picture of hypothalamo-hypophysial communication is emerging. There is currently little direct evidence to suggest which neuroendocrine genes may control the evolution of one communication system versus another. However, studies at the developmental and intergenerational timescales implicate several genes in the angiogenesis and axonal guidance pathways that may be important.

Targeted mutation of secretogranin-2 disrupts sexual behavior and reproduction in zebrafish

The luteinizing hormone surge is essential for fertility as it triggers ovulation in females and sperm release in males. We previously reported that secretoneurin-a, a neuropeptide derived from the processing of secretogranin-2a (Scg2a), stimulates luteinizing hormone release, suggesting a role in reproduction. Here we provide evidence that mutation of the scg2a and scg2b genes using TALENs in zebrafish reduces sexual behavior, ovulation, oviposition, and fertility. Large-scale spawning within-line crossings (n = 82 to 101) were conducted. Wild-type (WT) males paired with WT females successfully spawned in 62% of the breeding trials. Spawning success was reduced to 37% (P = 0.006), 44% (P = 0.0169), and 6% (P < 0.0001) for scg2a ^-/- , scg2b ^-/- , and scg2a ^-/- ;scg2b ^-/- mutants, respectively. Comprehensive video analysis indicates that scg2a ^-/- ;scg2b ^-/- mutation reduces all male courtship behaviors. Spawning success was 47% in saline-injected WT controls compared to 11% in saline-injected scg2a ^-/- ;scg2b ^-/- double mutants. For these mutants, spawning success increased 3-fold following a single intraperitoneal (i.p.) injection of synthetic secretoneurin-a (P = 0.0403) and increased 3.5-fold with injection of human chorionic gonadotropin (hCG). Embryonic survival at 24 h remained on average lower in scg2a ^-/- ;scg2b ^-/- fish compared to WT injected with secretoneurin-a (P < 0.001). Significant reductions in the expression of gonadotropin-releasing hormone 3 in the hypothalamus, and luteinizing hormone beta and glycoprotein alpha subunits in the pituitary provide evidence for disrupted hypothalamo-pituitary function in scg2a and scg2b mutant fish. Our results indicate that secretogranin-2 is required for optimal reproductive function and support the hypothesis that secretoneurin is a reproductive hormone.

Spexin and a Novel Cichlid-Specific Spexin Paralog Both Inhibit FSH and LH Through a Specific Galanin Receptor (Galr2b) in Tilapia

Spexin (SPX) is a 14 amino acid peptide hormone that has pleiotropic functions across vertebrates, one of which is involvement in the brain-pituitary-gonad axis of fish. SPX(1) has been identified in each class of vertebrates, and a second SPX (named SPX2) has been found in some non-mammalian species. We have cloned two spexin paralogs, designated as Spx1a and Spx1b, from Nile tilapia (Oreochromis niloticus) that have varying tissue distribution patterns. Spx1b is a novel peptide only identified in cichlid fish, and is more closely related to Spx1 than Spx2 homologs as supported by phylogenetic, synteny, and functional analyses. Kisspeptin, Spx, and galanin (Gal) peptides and their corresponding kiss receptors and Gal receptors (Galrs), respectively, are evolutionarily related. Cloning of six tilapia Galrs (Galr1a, Galr1b, Galr2a, Galr2b, Galr type 1, and Galr type 2) and subsequent in vitro second-messenger reporter assays for Gα_s, Gα_q, and Gα_i suggests that Gal and Spx activate Galr1a/Galr2a and Galr2b, respectively. A decrease in plasma follicle stimulating hormone and luteinizing hormone concentrations was observed with injections of Spx1a or Spx1b in vivo. Additionally, application of Spx1a and Spx1b to pituitary slices decreased the firing rate of LH cells, suggesting that the peptides can act directly at the level of the pituitary. These data collectively suggest an inhibitory mechanism of action against the secretion of gonadotropins for a traditional and a novel spexin paralog in cichlid species.

Knockout of the Gnrh genes in zebrafish: effects on reproduction and potential compensation by reproductive and feeding-related neuropeptides

Gonadotropin-releasing hormone (GNRH) is known as a pivotal upstream regulator of reproduction in vertebrates. However, reproduction is not compromised in the hypophysiotropic Gnrh3 knockout line in zebrafish (gnrh3-/-). In order to determine if Gnrh2, the only other Gnrh isoform in zebrafish brains, is compensating for the loss of Gnrh3, we generated a double Gnrh knockout zebrafish line. Surprisingly, the loss of both Gnrh isoforms resulted in no major impact on reproduction, indicating that a compensatory response, outside of the Gnrh system, was evoked. A plethora of factors acting along the reproductive hypothalamus-pituitary axis were evaluated as possible compensators based on neuroanatomical and differential gene expression studies. In addition, we also examined the involvement of feeding factors in the brain as potential compensators for Gnrh2, which has known anorexigenic effects. We found that the double knockout fish exhibited upregulation of several genes in the brain, specifically gonadotropin-inhibitory hormone (gnih), secretogranin 2 (scg2), tachykinin 3a (tac3a), and pituitary adenylate cyclase-activating peptide 1 (pacap1), and downregulation of agouti-related peptide 1 (agrp1), indicating the compensation occurs outside of Gnrh cells and therefore is a noncell autonomous response to the loss of Gnrh. While the differential expression of gnih and agrp1 in the double knockout line was confined to the periventricular nucleus and hypothalamus, respectively, the upregulation of scg2 corresponded with a broader neuronal redistribution in the lateral hypothalamus and hindbrain. In conclusion, our results demonstrate the existence of a redundant reproductive regulatory system that comes into play when Gnrh2 and Gnrh3 are lost.

Knockout of Gnrh2 in zebrafish (Danio rerio) reveals its roles in regulating feeding behavior and oocyte quality

Many studies on Gnrh1, and the teleost Gnrh3, have elucidated the roles of these peptides in reproductive regulation. However, the role of the midbrain population of Gnrh, Gnrh2, has long been a mystery, despite its ubiquitous conservation in all jawed vertebrates except rodents. Previous behavioral studies in sparrows, musk shrews, mice, zebrafish, and goldfish show that Gnrh2 administrations both increase spawning behaviors and decrease feeding behaviors, suggesting a role of this peptide in metabolism regulation along with the canonical role in regulating reproduction. In order to more deeply explore the roles of Gnrh2, we used a cyprinid teleost, zebrafish, which has 2 forms of Gnrh, Gnrh2 and Gnrh3, to generate a knockout zebrafish line which contains a frameshift mutation and subsequent disruption of the coding for the functional Gnrh2 peptide. We examined differences in reproduction, feeding, growth, and mobility in this line, and discovered major differences in feeding and growth parameters, suggesting that Gnrh2 is a potent anorexigen in zebrafish. Additionally, there were no differences in mobility except for increased distances swam during feeding periods. There were no major differences in reproductive success, however, female gnrh2^-/- zebrafish exhibited smaller oocytes and increased embryo mortality, indicating slightly decreased oocyte quality. Additionally, there were changes in the expression levels of many feeding, growth, and reproductive neuropeptides in gnrh2^-/- zebrafish. Taken together, these findings suggest a role for Gnrh2 in controlling satiation in zebrafish along with a minor role in maintaining optimal oocyte quality in females.

Chromogranin A: its known and possible roles in obstetrics and gynecology

Chromogranin A (CgA) is a prohormone initially extracted from the adrenal medulla, however, increased quantities of CgA are secreted by a wide array of human tissues in the course of a variety of disorders. This protein exhibits a number of interesting endocrine and non-endocrine functions. Here we briefly review the possible involvements of CgA in the areas covered by obstetrics and gynecology. Our account indicates the need to verify its association with the intrapartum fetal stress, and the involvement of CgA in the pathomechanisms of obstetric disorders related to placental dysfunction, as well as in the pathogenesis of endometrioid endometrial cancer as a hormonally regulated malignancy.

Characterization of the EM66 Biomarker in the Pituitary and Plasma of Healthy Subjects With Different Gonadotroph Status and Patients With Gonadotroph Tumor

Granins and their derived-peptides are useful markers of secretion from normal and tumoral neuroendocrine cells. The need to identify new diagnostic markers for neuroendocrine tumors, including pituitary tumors prompted us to determine plasma levels of the secretogranin II-derived peptide EM66 in healthy volunteers with different gonadotroph status and to evaluate its usefulness as a circulating marker for the diagnosis of gonadotroph tumor. Using a radioimmunoassay, we determined plasma EM66 concentrations in healthy men and women volunteers in different physiological conditions in relation with the gonadotroph function. Our results revealed that in men, in women with or without contraception, in pregnant or post-menopausal women, plasma EM66 concentrations are not significantly different, and did not show any correlation with gonadotropin levels. In addition, stimulation or inhibition tests of the gonadotroph axis had no effect on EM66 levels, whatever the group of healthy volunteers investigated while gonadotropin levels showed the expected variations. Immunohistochemical experiments and HPLC analysis showed the occurrence of EM66 in pituitary gonadotroph, lactotroph and corticotroph tumors but not in somatotroph tumor. In patients with gonadotroph or lactotroph tumor, plasma EM66 levels were 1.48 (0.82-4.38) ng/ml and 2.49 (1.19-3.54) ng/ml, respectively. While median value of EM66 was significantly lower in patients with gonadotroph tumor compared to healthy volunteers [2.59 (0.62-4.95) ng/ml], plasma EM66 concentrations were in the same range as normal values and did not show any correlation with gonadotropin levels. These results show that plasma EM66 levels are independent of the activity of the gonadotroph axis in healthy volunteers and, while EM66 levels are reduced in gonadotroph tumors, plasma EM66 does not provide a helpful marker for the diagnosis of these tumors.

Identification and functional characterization of two Secretogranin II genes in orange-spotted grouper (Epinephelus coioides)

Secretoneurin (SN) is an important stimulator of pituitary luteinizing hormone (LH) synthesis and secretion in goldfish. It is unknown whether this neuropeptide performs the same role in other fish species. In this study, the full-length cDNAs encoding Secretogranin IIa (SgIIa) and b (SgIIb) were cloned from the brain of orange-spotted grouper. Sequence analysis showed that a 34-amino acid SN peptide (SNa) is present in SgIIa proprotein, and a 33-amino acid SN peptide (SNb) is present in SgIIb proprotein. The two SN peptides share a low degree of similarity but contain the signature YTPQ-X-LA-X₇-EL sequence. Real-time PCR showed that two SgII genes are mainly expressed in the brain and pituitary. During ovarian development, the expression levels of two SgII genes in the hypothalamus and pituitary were significantly reduced at the stage when the ovary contained full-grown oocytes. The biological functions of the two SN peptides were further investigated in vitro and in vivo. Both SN peptides could significantly elevate the mRNA levels of Gonadotropin-Releasing Hormone 1 (GnRH1) and 3 (GnRH3) in the hypothalamic fragments and upregulated the expression of Follicle-Stimulating Hormone beta (FSHb) and Luteinizing Hormone beta (LHb) in the pituitary cells. The stimulatory effects on the expression of GnRHs and Gonadotropins were also observed after intraperitoneal injection of SN peptides. Our study indicated that the SgII/SN system has stimulatory effects on the reproductive axis of orange-spotted grouper.

Secretoneurin-A inhibits aromatase B (cyp19a1b) expression in female goldfish (Carassius auratus) radial glial cells

In the teleost brain, radial glial cells (RGCs) are the main macroglia and are stem-like progenitors that express key steroidogenic enzymes, including the estrogen-synthesizing enzyme, aromatase B (cyp19a1b). As a result, RGCs are integral to neurogenesis and neurosteroidogenesis, however little is known about the regulatory factors and signaling mechanisms that control these functions. A potential new role of the secretogranin II-derived neuropeptide secretoneurin A (SNa) in the control of goldfish (Carassius auratus) RGC function is the subject of this study. Immunohistochemistry revealed a close neuroanatomical relationship between RGCs and soma of SNa-immunoreactive magnocellular and parvocellular neurons in the preoptic nucleus of female goldfish. Five hours following intracerebroventricular injection of 0.2ng/g SNa cyp19a1b mRNA levels were decreased by 86% (P<0.05) in the hypothalamus and by 88% (P<0.05) in the telencephalon. In vitro, 24 h incubation with 500nM SNa decreased cyp19a1b mRNA by 51% (P<0.05) in cultured RGCs. These data provide evidence that SNa can regulate aromatase expression in goldfish RGCs. By regulating neuroestrogen production in RGCs SNa may therefore be implicated in the control of major estrogen-dependent functions of the preoptic region such as reproductive behavior and osmoregulation.

Granin-derived peptides

The granin family comprises altogether 7 different proteins originating from the diffuse neuroendocrine system and elements of the central and peripheral nervous systems. The family is dominated by three uniquely acidic members, namely chromogranin A (CgA), chromogranin B (CgB) and secretogranin II (SgII). Since the late 1980s it has become evident that these proteins are proteolytically processed, intragranularly and/or extracellularly into a range of biologically active peptides; a number of them with regulatory properties of physiological and/or pathophysiological significance. The aim of this comprehensive overview is to provide an up-to-date insight into the distribution and properties of the well established granin-derived peptides and their putative roles in homeostatic regulations. Hence, focus is directed to peptides derived from the three main granins, e.g. to the chromogranin A derived vasostatins, betagranins, pancreastatin and catestatins, the chromogranin B-derived secretolytin and the secretogranin II-derived secretoneurin (SN). In addition, the distribution and properties of the chromogranin A-derived peptides prochromacin, chromofungin, WE14, parastatin, GE-25 and serpinins, the CgB-peptide PE-11 and the SgII-peptides EM66 and manserin will also be commented on. Finally, the opposing effects of the CgA-derived vasostatin-I and catestatin and the SgII-derived peptide SN on the integrity of the vasculature, myocardial contractility, angiogenesis in wound healing, inflammatory conditions and tumors will be discussed.

Stimulatory effect of the secretogranin-ll derived peptide secretoneurin on food intake and locomotion in female goldfish (Carassius auratus)

Secretoneurin (SN) is a conserved peptide derived by proteolytic processing from the middle domain of the ∼600 amino acid precursor secretogranin-II (SgII). Secretoneurin is widely distributed in secretory granules of endocrine cells and neurons and has important roles in reproduction as it stimulates luteinizing hormone release from the pituitary. A potential new role of SN in goldfish feeding is the subject of this study. Firstly, we established that acute (26 h; p<0.0001) and short-term (72 h; p=0.016) fasting increased SgIIa precursor mRNA levels 1.25-fold in the telencephalon, implicating SN in the control of feeding. Secondly, we determined that intracerebroventricular injections of the type A SN (SNa; 0.2 and 1 ng/g BW) increased food intake and locomotor behavior by 60 min. Fish injected with the lower and higher doses of SNa (0.2 and 1 ng/g) respectively exhibited significant 1.77- and 2.58-fold higher food intake (p<0.0001) than the saline-injected control fish. Locomotor behavior was increased by 1.35- and 2.26-fold for 0.2 ng/g SNa (p=0.0001) and 1 ng/g SNa (p<0.0001), respectively. Injection of 1 ng/g SNa increased mRNA levels of hypothalamic neuropeptide Y 1.36-fold (p=0.038) and decreased hypothalamic cocaine-and amphetamine-regulated transcript by 33% (p=0.01) at 2h and 5h post-injection, respectively. These data suggest interactions of SNa with stimulatory and inhibitory pathways of food intake control in fish.

The secretogranin-II derived peptide secretoneurin modulates electric behavior in the weakly pulse type electric fish, Brachyhypopomus gauderio

Secretoneurin (SN) in the preoptic area and pituitary of mammals and fish has a conserved close association with the vasopressin and oxytocin systems, members of a peptide family that are key in the modulation of sexual and social behaviors. Here we show the presence of SN-immunoreactive cells and projections in the brain of the electric fish, Brachyhypopomus gauderio. Secretoneurin colocalized with vasotocin (AVT) and isotocin in cells and fibers of the preoptic area. In the rostral pars distalis of the pituitary, many cells were both SN and prolactin-positive. In the hindbrain, at the level of the command nucleus of the electric behavior (pacemaker nucleus; PN), some of SN-positive fibers colocalized with AVT. We also explored the potential neuromodulatory role of SN on electric behavior, specifically on the rate of the electric organ discharge (EOD) that signals arousal, dominance and subordinate status. Each EOD is triggered by the command discharge of the PN, ultimately responsible for the basal EOD rate. SN modulated diurnal basal EOD rate in freely swimming fish in a context-dependent manner; determined by the initial value of EOD rate. In brainstem slices, SN partially mimicked the in vivo behavioral effects acting on PN firing rate. Taken together, our results suggest that SN may regulate electric behavior, and that its effect on EOD rate may be explained by direct action of SN at the PN level through either neuroendocrine and/or endocrine mechanisms.

Subtype-specific parafollicular localization of the neuropeptide manserin in the rat thyroid gland

The thyroid gland is an endocrine organ which is involved in metabolism, neuroexcitability, body growth and development. The thyroid gland is also involved in the regulation of calcium metabolism, which is not yet fully understood. In this study, we investigated the localization of the granin-derived neuropeptide, manserin, in the adult rat thyroid gland. Manserin immunoreactivity was detected in thyroid follicular epithelial cells. Intense manserin signals were also detected in some, but not all, parafollicular cells, indicating that parafollicular manserin may be subtype-specific. These results indicate that thyroid manserin may play pivotal roles in parafollicular cells and follicular epithelial cells such as in calcium metabolism and/or thyroid hormone secretion.

Is secretoneurin a new hormone?

Numerous small potentially bioactive peptides are derived from the selective processing of the ~600 amino acid secretogranin II (SgII) precursor, but only the 31-42 amino acid segment termed secretoneurin (SN) is well-conserved from sharks to mammals. Both SNa and SNb paralogs have been identified in some teleosts, likely arising as a result of the specific genome duplication event in this lineage. Only one copy of the putative lamprey SgII (188 amino acids) could be identified which gives rise to a divergent agnathan SN that contains the signature YTPQ-X-LA-X(7)-EL sequence typical of the central core of all known SN peptides. In rodent models, SN has regulatory effects on neuroinflammation and neurotransmitter release, and possesses therapeutic potential for the induction of angiogenesis. The wide distribution of SN in neuroendocrine neurons and pituitary cells suggests important endocrine roles. The clearest example of the endocrine action of SN is the stimulatory effects on pituitary luteinizing hormone release from goldfish pituitary and mouse LβT2 gonadotroph cells, indicative of an important role in reproduction. Several lines of evidence suggest that the SN receptor is most likely a G-protein coupled protein. Microarray analysis of SN effects on dispersed goldfish pituitary cells in vitro reveals novel SN actions that include effects on genes involved in notch signaling and the guanylate cyclase pathway. Intracerebroventricular injection of SN increases feeding and locomotory behaviors in goldfish. Given that SgII appeared early in vertebrate evolution, SN is an old peptide with emerging implications as a new multifunctional hormone.

The extended granin family: structure, function, and biomedical implications

The chromogranins (chromogranin A and chromogranin B), secretogranins (secretogranin II and secretogranin III), and additional related proteins (7B2, NESP55, proSAAS, and VGF) that together comprise the granin family subserve essential roles in the regulated secretory pathway that is responsible for controlled delivery of peptides, hormones, neurotransmitters, and growth factors. Here we review the structure and function of granins and granin-derived peptides and expansive new genetic evidence, including recent single-nucleotide polymorphism mapping, genomic sequence comparisons, and analysis of transgenic and knockout mice, which together support an important and evolutionarily conserved role for these proteins in large dense-core vesicle biogenesis and regulated secretion. Recent data further indicate that their processed peptides function prominently in metabolic and glucose homeostasis, emotional behavior, pain pathways, and blood pressure modulation, suggesting future utility of granins and granin-derived peptides as novel disease biomarkers.