Jessica Y. S. Chu

Phenotypes developed in secretin receptor-null mice indicated a role for secretin in regulating renal water reabsorption.

ABSTRACT Aquaporin 2 (AQP2) is responsible for regulating the concentration of urine in the collecting tubules of the kidney under the control of vasopressin (Vp). Studies using Vp-deficient Brattleboro rats, however, indicated the existence of substantial Vp-independent mechanisms for membrane insertion, as well as transcriptional regulation, of this water channel. The Vp-independent mechanism(s) is clinically relevant to patients with X-linked nephrogenic diabetes insipidus (NDI) by therapeutically bypassing the dysfunctional Vp receptor. On the basis of studies with secretin receptor-null (SCTR−/−) mice, we report here for the first time that mutation of the SCTR gene could lead to mild polydipsia and polyuria. Additionally, SCTR−/− mice were shown to have reduced renal expression of AQP2 and AQP4, as well as altered glomerular and tubular morphology, suggesting possible disturbances in the filtration and/or water reabsorption process in these animals. By using SCTR−/− mice as controls and comparing them with wild-type animals, we performed both in vivo and in vitro studies that demonstrated a role for secretin in stimulating (i) AQP2 translocation from intracellular vesicles to the plasma membrane in renal medullary tubules and (ii) expression of this water channel under hyperosmotic conditions. The present study therefore provides information for at least one of the Vp-independent mechanisms that modulate the process of renal water reabsorption. Future investigations in this direction should be important in developing therapeutic means for treating NDI patients.

Secretin as a neurohypophysial factor regulating body water homeostasis

Hypothalamic magnocellular neurons express either one of the neurohypophysial hormones, vasopressin or oxytocin, along with different neuropeptides or neuromodulators. Axonal terminals of these neurons are generally accepted to release solely the two hormones but not others into the circulation. Here, we show that secretin, originally isolated from upper intestinal mucosal extract, is present throughout the hypothalamo–neurohypophysial axis and that it is released from the posterior pituitary under plasma hyperosmolality conditions. In the hypothalamus, it stimulates vasopressin expression and release. Considering these findings together with our previous findings that show a direct effect of secretin on renal water reabsorption, we propose here that secretin works at multiple levels in the hypothalamus, pituitary, and kidney to regulate water homeostasis. Findings presented here challenge previous understanding regarding the neurohypophysis and could provide new concepts in treating disorders related to osmoregulation.

Central and Peripheral Administration of Secretin Inhibits Food Intake in Mice through the Activation of the Melanocortin System

Secretin (Sct) is released into the circulation postprandially from the duodenal S-cells. The major functions of Sct originated from the gastrointestinal system are to delay gastric emptying, stimulate fluid secretion from pancreas and liver, and hence optimize the digestion process. In recent years, Sct and its receptor (Sctr) have been identified in discrete nuclei of the hypothalamus, including the paraventricular nucleus (PVN) and the arcuate nucleus (Arc). These nuclei are the primary brain sites that are engaged in regulating body energy homeostasis, thus providing anatomical evidence to support a functional role of Sct in appetite control. In this study, the effect of Sct on feeding behavior was investigated using wild-type (wt), Sct−/−, and secretin receptor-deficient (Sctr−/−) mice. We found that both central and peripheral administration of Sct could induce Fos expression in the PVN and Arc, suggesting the activation of hypothalamic feeding centers by this peptide. Consistent with this notion, Sct was found to increase thyrotropin-releasing hormone and melanocortin-4 receptor (Mc4r) transcripts in the PVN, and augment proopiomelanocortin, but reduces agouti-related protein mRNA expression in the Arc. Injection of Sct was able to suppress food intake in wt mice, but not in Sctr−/− mice, and that this effect was abolished upon pretreatment with SHU9119, an antagonist for Mc4r. In summary, our data suggest for the first time that Sct is an anorectic peptide, and that this function is mediated by the melanocortin system.

RFWD3–Mdm2 ubiquitin ligase complex positively regulates p53 stability in response to DNA damage

In unstressed cells, the tumor suppressor p53 is maintained at low levels by ubiquitin-mediated proteolysis mainly through Mdm2. In response to DNA damage, p53 is stabilized and becomes activated to turn on transcriptional programs that are essential for cell cycle arrest and apoptosis. Activation of p53 leads to accumulation of Mdm2 protein, a direct transcriptional target of p53. It is not understood how p53 is protected from degradation when Mdm2 is up-regulated. Here we report that p53 stabilization in the late phase after ionizing radiation correlates with active ubiquitination. We found that an E3 ubiquitin ligase RFWD3 (RNF201/FLJ10520) forms a complex with Mdm2 and p53 to synergistically ubiquitinate p53 and is required to stabilize p53 in the late response to DNA damage. This process is regulated by the DNA damage checkpoint, because RFWD3 is phosphorylated by ATM/ATR kinases and the phosphorylation mutant fails to stimulate p53 ubiquitination. In vitro experiments suggest that RFWD3 is a p53 E3 ubiquitin ligase and that RFWD3–Mdm2 complex restricts the polyubiquitination of p53 by Mdm2. Our study identifies RFWD3 as a positive regulator of p53 stability when the G1 cell cycle checkpoint is activated and provides an explanation for how p53 is protected from degradation in the presence of high levels of Mdm2.

An indispensable role of secretin in mediating the osmoregulatory functions of angiotensin II

Fluid balance is critical to life and hence is tightly controlled in the body. Angiotensin II (ANGII), one of the most important components of this regulatory system, is recognized as a dipsogenic hormone that stimulates vasopressin (VP) expression and release. However, detailed mechanisms regarding how ANGII brings about these changes are not fully understood. In the present study, we show initially that the osmoregulatory functions of secretin (SCT) in the brain are similar to those of ANGII in mice and, more important, we discovered the role of SCT as the link between ANGII and its downstream effects. This was substantiated by the use of two knockout mice, SCTR(-/-) and SCT(-/-), in which we show the absence of an intact SCT/secretin receptor (SCTR) axis resulted in an abolishment or much reduced ANGII osmoregulatory functions. By immunohistochemical staining and in situ hybridization, the proteins and transcripts of SCT and its receptor are found in the paraventricular nucleus (PVN) and lamina terminalis. We propose that SCT produced in the circumventricular organs is transported and released in the PVN to stimulate vasopressin expression and release. In summary, our findings identify SCT and SCTR as novel elements of the ANGII osmoregulatory pathway in maintaining fluid balance in the body.

Multiple Actions of Secretin in the Human Body

The discovery of secretin initiated the field of endocrinology. Over the past century, multiple gastrointestinal functions of secretin have been extensively studied, and it was discovered that the principal function of this peptide in the gastrointestinal system is to facilitate digestion and to provide protection. In view of the late identification of secretin and the secretin receptor in various tissues, including the central nervous system, the pleiotropic functions of secretin have more recently been an area of intense focus. Secretin is a classical hormone, and recent studies clearly showed secretins involvement in neural and neuroendocrine pathways, although the neuroactivity and neural regulation of its release are yet to be elucidated. This chapter reviews our current understanding of the pleiotropic actions of secretin with a special focus on the hormonal and neural interdependent pathways that mediate these actions.

Vasopressin-independent mechanisms in controlling water homeostasis

The maintenance of body water homeostasis depends on the balance between water intake and water excretion. In the kidney, vasopressin (Vp) is a critical regulator of water homeostasis by controlling the insertion of aquaporin 2 (AQP2) onto the apical membrane of the collecting duct principal cells in the short term and regulating the gene expression of AQP2 in the long term. A growing body of evidence from both in vitro and in vivo studies demonstrated that both secretin and oxytocin are involved as Vp-independent mechanisms regulating the renal water reabsorption process, including the translocation and expression of AQP2. This review focuses on how these two hormones are potentially involved as Vp-independent mechanisms in controlling water homeostasis.

Secretin: A Pleiotrophic Hormone

Abstract:  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

Abstract:  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.

Transmembrane peptides as unique tools to demonstrate the in vivo action of a cross-class GPCR heterocomplex

Angiotensin (ANGII) and secretin (SCT) share overlapping, interdependent osmoregulatory functions in brain, where SCT peptide/receptor function is required for ANGII action, yet the molecular basis is unknown. Since receptors for these peptides (AT1aR, SCTR) are coexpressed in osmoregulatory centers, a possible mechanism is formation of a cross‐class receptor heterocomplex. Here, we demonstrate such a complex and its functional importance to modulate signaling. Association of AT1aR with SCTR reduced ability of SCT to stimulate cyclic adenosine monophosphate (cAMP), with signaling augmented in presence of ANGII or constitutively active AT1aR. Several transmembrane (TM) peptides of these receptors were able to affect their conformation within complexes, reducing receptor BRET signals. AT1aR TM1 affected only formation and activity of the heterocomplex, without effect on homomers of either receptor, and reduced SCT‐stimulated cAMP responses in cells expressing both receptors. This peptide was active in vivo by injection into mouse lateral ventricle, thereby suppressing water‐drinking behavior after hyperosmotic shock, similar to SCTR knockouts. This supports the interpretation that active conformation of AT1aR is a key modulator of cAMP responses induced by SCT stimulation of SCTR. The SCTR/AT1aR complex is physiologically important, providing differential signaling to SCT in settings of hyperosmolality or food intake, modulated by differences in levels of ANGII.—Lee, L. T. O., Ng, S. Y. L., Chu, J. Y. S., Sekar, R., Harikumar, K. G., Miller, L. J., Chow, B. K. C. Transmembrane peptides as unique tools to demonstrate the in vivo action of a cross‐class GPCR heterocomplex. FASEB J. 28, 2632–2644 (2014). www.fasebj.org