Expression of RESP18 in peptidergic and catecholaminergic neurons

Regulated Endocrine-Specific Protein-18, an Emerging Endocrine Protein in Physiology: A Literature Review

Regulated endocrine-specific protein-18 (RESP18), a novel 18-kDa protein, was first identified in neuroendocrine tissue. Subsequent studies showed that Resp18 is expressed in the adrenal medulla, brain, pancreas, pituitary, retina, stomach, superior cervical ganglion, testis, and thyroid and also circulates in the plasma. Resp18 has partial homology with the islet cell antigen 512, also known as protein tyrosine phosphatase, receptor type N (PTPRN), but does not have phosphatase activity. Resp18 might serve as an intracellular signal; however, its function is unclear. It is regulated by dopamine, glucocorticoids, and insulin. We recently reported that the targeted disruption of the Resp18 locus in Dahl salt-sensitive rats increased their blood pressure and caused renal injury. The aim of the present review was to provide a comprehensive summary of the reported data currently available, especially the expression and proposed organ-specific function of Resp18.

RESP18 is involved in the cytotoxicity of dopaminergic neurotoxins in MN9D cells

RESP18 (Regulated endocrine-specific protein, 18 kDa) was first identified as a dopaminergic drugs-regulated intermediate pituitary transcript. RESP18 protein is a unique endoplasmic reticulum (ER) resident protein. Its functions in the brain especially in the nervous system disorders remain unknown. ER stress (ERS) has been proved to be one of the important pathogenesis of neurodegenerative diseases, including Parkinson's disease (PD). Here, we explored the association of RESP18 and ERS in cell models of PD. Dopaminergic neurotoxin 1-methyl-4-phenyl-pyridinium ion (MPP⁺), 6-hydroxydopamine (6-OHDA), and rotenone evoked dramatic MN9D cell death. The transcriptional expressions of RESP18 and two ERS markers--binding immunoglobulin protein/glucose-regulated protein 78 (BiP/GRP78) and CCAAT/enhancer-binding protein homologous protein (CHOP) manifested differential changes in MN9D cells treated with MPP⁺, 6-OHDA, and rotenone. The RESP18 protein levels increased in MPP⁺ and 6-OHDA-treated cells, but did not change in the cells treated with rotenone, while the protein levels of ER molecular chaperone heat shock protein 90 kDa beta member 1/glucose-regulated protein 94 (HSP90B1/GRP94) and BiP in the cells were up-regulated by MPP⁺ and 6-OHDA, respectively. Salubrinal, an ERS inhibitor, significantly reduced MPP⁺ and 6-OHDA-induced cell death. Moreover, ERS inducer--thapsigargin and tunicamycin, decreased the expression of RESP18, which is different from the changes of BiP, GRP94, and CHOP. Silencing RESP18 expression with Lenti-shRNA alleviated MPP⁺-induced cell death, while over-expression of RESP18 resulted in aggravated cell death induced by MPP⁺ and 6-OHDA. Taken together, our results suggest that RESP18 is involved in the cytotoxicity of dopaminergic neurotoxins.

Regulated endocrine-specific protein 18 (RESP18) is localized to and regulated in A-like cells and G-cells in rat stomach

The regulated endocrine-specific protein 18 (RESP18) has previously been localized to different endocrine cells and neurons, in particular the pituitary gland and hypothalamus. It is found in the lumen of the endoplasmic reticulum and is degraded at the post-ER pre-Golgi compartment, and a role in processing of secreted peptides has been hypothesized. The present study examines localization of RESP18 in the gastrointestinal mucosa of rats by immunohistochemistry, and expression and regulation in response to hypergastrinemia induced by acid inhibition (pantoprazole), gastrin antagonism (YF476), fasting-refeeding and octreotide by mRNA measurements. RESP18 was mainly found in the gastric mucosa, but could also be detected in a few, scattered cells in the lower small intestine and in colon. In the antral mucosa, all RESP18 immunoreactivity was localized to ghrelin-producing A-like cells and gastrin-producing G-cells. In the corpus mucosa, a significant fraction, but not all of the RESP18 immunoreactive cells, were A-like cells. In both antrum and corpus, Resp18 mRNA seemed to vary similarly with the activation of the A-like cells, and in the antrum also with stimulation of the G-cells. This study demonstrates, for the first time, the localization of RESP18 to specific neuroendocrine cells of the gastrointestinal mucosa and that it seems to be regulated synchronously with the peptides secreted from these cells. This suggests that Resp18 may indeed have a functional role in the synthesis or storage of these gastrointestinal peptides.

Endocrine and developmental effects in Atlantic salmon (Salmo salar) exposed to perfluorooctane sulfonic or perfluorooctane carboxylic acids

In this study, we have investigated the effect of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) on endocrine signalling, growth and development in Atlantic salmon (Salmo salar) embryos and larvae. Expression of genes related to the hypothalamic-pituitary-thyroid (HPT) axis, growth-hormone/insulin-like growth factor (GH/IGF) axis and the steroid hormone axis were used as indicators of endocrine disruption. We also studied bone development in larvae, both by observing skeletal structure formation and by investigating expression of genes involved in ossification process. Atlantic salmon embryos, kept in plastic tanks at 5-7°C, were exposed to 100 μg/L PFOA or PFOS from egg stage for a period of 52 days, followed by one-week recovery period. Sampling was performed at day 21, 35, 49 and 56 representing age 549, 597, 679 and 721 dd (dd or day degrees = number of days × temperature in degree Celsius:°C). Note that day 56 or 721 dd is the end of the 1-week recovery period. Larvae were divided into designated head and body regions for the purpose of gene expression analysis, except for genes that regulate ossification that were analyzed in whole larvae. Expression of thyroid receptor α and β (TRα and TRβ), thyroid-stimulating hormone β (TSHβ), T(4) outer-ring deiodinase (T(4)ORD), growth hormone (GH), insulin-like growth factor-I and II (IGF-I and II), insulin-like growth factor I receptor (IGF-IR), and estrogen receptor α and β (ERα and ERβ) were investigated using quantitative PCR. Both PFOS and PFOA exposure produced non-significant alterations in larvae weight (except after the recovery period when a decrease was observed), while larvae length was unaffected. PFOS and PFOA exposure produced body- and head region-specific alterations in expression of all the investigated gene transcripts. Expression of IGF-I and IGF-IR paralleled that of GH, indicating that perturbation of GH expression is a possible end point for disruption of the GH-IGF axis. We did not observe developmental changes related to angiogenesis, ossification and chondrogenesis after exposure to PFOS and PFOA. Transcriptional abnormalities may serve as indicators of chronic exposure, although the concrete mechanisms causing the observed effects remain ambiguous. The implications of these findings for the complete lifecycle, including other developmental and/or reproductive damage, are areas of future study.

Requirement of regulated endocrine-specific protein-18 for development and expression of regulated endocrine-specific protein-18 isoform c in mice

Regulated endocrine-specific protein-18 (RESP18) is distributed mainly in the peripheral endocrine and neuroendocrine tissues. The expression of RESP18 protein is regulated by physiological factors, such as blood glucose or dopaminergic drugs, but its functions remain unclear. In this study, to explore the biological functions of RESP18 in vivo, we generated RESP18 heterozygous deficient mice, and further found RESP18 was essential for embryonic development. In addition, we cloned a new isoform of mouse RESP18 by reverse transcription-polymerase chain reaction (RT-PCR), and denominated it as RESP18-c. Mouse RESP18-c, by skipping exon4 (43 bp in length), encodes a shorter protein of 120 amino acid residues. The distribution of RESP18-c mRNA is similar with that of RESP18 mRNA in the peripheral tissues and brains of mice.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

The novel kinase peptidylglycine alpha-amidating monooxygenase cytosolic interactor protein 2 interacts with the cytosolic routing determinants of the peptide processing enzyme peptidylglycine alpha-amidating monooxygenase

The cytosolic domain of the peptide-processing integral membrane protein peptidylglycine alpha-amidating monooxygenase (PAM; EC 1.14. 17.3) contains multiple signals determining its subcellular localization. Three PAM cytosolic interactor proteins (P-CIPs) were identified using the yeast two hybrid system (Alam, M. R., Caldwel, B. D., Johnson, R. C., Darlington, D. N., Mains, R. E., and Eipper, B. A. (1996) J. Biol. Chem. 271, 28636-28640); the partial amino acid sequence of P-CIP2 suggested that it was a protein kinase. In situ hybridization and immunocytochemistry show that P-CIP2 is expressed widely throughout the brain; PAM and P-CIP2 are expressed in the same neurons. Based on subcellular fractionation, the 47-kDa P-CIP2 protein is mostly cytosolic. P-CIP2 is a highly selective kinase, phosphorylating the cytosolic domain of PAM, but not the corresponding region of furin or carboxypeptidase D. Although P-CIP2 interacts with stathmin, it does not phosphorylate stathmin. Site-directed mutagenesis, phosphoamino acid analysis, and use of synthetic peptides demonstrate that PAM-Ser(949) is the major site phosphorylated by P-CIP2. Based on both in vitro binding experiments and co-immunoprecipitation from cell extracts, P-CIP2 interacts with PAM proteins containing the wild type cytosolic domain, but not with mutant forms of PAM whose trafficking is disrupted. P-CIP2, through its highly selective phosphorylation of a key site in the cytosolic domain of PAM, appears to play a critical role in the trafficking of this protein.

Kalirin, a multifunctional PAM COOH-terminal domain interactor protein, affects cytoskeletal organization and ACTH secretion from AtT-20 cells

The production and regulated secretion of bioactive peptides require a series of lumenal enzymes to convert inactive precursors into bioactive peptides plus several cytosolic proteins to govern granule formation, maturation, translocation, and exocytosis. Peptidylglycine alpha-amidating monooxygenase (PAM), an enzyme essential for biosynthesis of many peptides, is an integral membrane protein with trafficking information in both its lumenal and cytosolic domains. Kalirin, a PAM cytosolic domain interactor protein with spectrin-like repeats and GDP/GTP exchange factor activity for Rac1, is expressed with PAM in neurons but is not expressed in the anterior pituitary or AtT-20 corticotrope cells. Expression of Kalirin alters the cytoskeletal organization of Chinese hamster ovary and AtT-20 cells expressing membrane PAM. Expression of membrane PAM also alters cytoskeletal organization, demonstrating the presence of endogenous proteins that can mediate this effect. Significant amounts of both PAM and Kalirin fractionate with cytoskeletal elements. Since cytoskeletal organization is critical for exocytosis, constitutive-like and regulated secretions were evaluated. Whereas the constitutive-like secretion of adrenocorticotropic hormone (ACTH) is increased by expression of membrane PAM, regulated secretion is eliminated. Expression of Kalirin in AtT-20 cells expressing membrane PAM restores stimulated secretion of ACTH. Thus, Kalirin or its homologue may be essential for regulated secretion, and the PAM-Kalirin interaction may coordinate intragranular with cytosolic events.