Monensin-resistant LLC-PK1 cell mutants are affected in recycling of the adenylate cyclase-stimulating vasopressin V2-receptor

Regulation of the vasopressin V2 receptor by vasopressin in polarized renal collecting duct cells

Binding of arginine-vasopressin (AVP) to its V2 receptor (V2R) in the basolateral membrane of principal cells induces Aquaporin-2-mediated water reabsorption in the kidney. To study the regulation of the V2R by dDAVP in a proper model, a polarized renal cell line stably-expressing V2R-GFP was generated. Labeled AVP-binding studies revealed an equal basolateral vs. apical membrane distribution for V2R-GFP and endogenous V2R. In these cells, GFP-V2R was expressed in its mature form and localized for 75% in the basolateral membrane and for 25% to late endosomes/lysosomes. dDAVP caused a dose- and time-dependent internalization of V2R-GFP, which was completed within 1 h with 100 nM dDAVP, was prevented by coincubation with a V2R antagonist, and which reduced its half-life from 11.5 to 2.8 h. Semiquantification of the V2R-GFP colocalization with E-cadherin (basolateral membrane), early endosomal antigen-1 (EEA-1) and lysosome-associated membrane protein-2 (LAMP-2) in time revealed that most dDAVP-bound V2R was internalized via early endosomes to late endosomes/lysosomes, where it was degraded. The dDAVP-internalized V2R did not recycle to the basolateral membrane. In conclusion, we established the itinerary of the V2R in a polarized cell model that likely resembles the in vivo V2R localization and regulation by AVP to a great extent.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.

Agonist and antagonist-dependent internalization of the human vasopressin V2 receptor

In this report we demonstrate that in HEK293 cells stably expressing the human V2 vasopressin receptor, ligand-induced internalization of the hormone receptor occurs via the clathrin-dependent pathway. Studies of receptor trafficking either by direct visualization of the V2 receptor by confocal microscopy or binding experiments show a rapid internalization (half-time 6-7 min). Blocking of the clathrin-dependent pathway by hypertonic sucrose increased vasopressin-induced cellular cAMP production and decreased the desensitization of the V2 receptor-adenylyl cyclase system. Thus, internalization appears to be a major regulatory mechanism terminating vasopressin action in HEK293 cells. Two antagonists of the vasopressin V2 receptor exerted different effects on receptor internalization, as determined by confocal fluorescence microscopy. The nonpeptidic antagonist OPC31260 did not induce any visible receptor internalization, whereas the peptidic antagonist d(CH2)5[D-Tyr(Et)2,Val4,Lys8,Tyr-NH29]VP induced a slow but substantial receptor internalization. These results suggest that long-term treatment with peptidic V2 receptor antagonists might lead to desensitization.

A functional bipartite nuclear localisation signal in the cytokine interleukin-5

Interleukin (IL)-5 is central in regulating eosinophilia in allergic disease and parasitic infections. We have identified a bipartite nuclear localisation signal (NLS) within amino acids 95-111 of human IL-5 (hIL-5), also present in mouse IL-5 (mIL-5). hIL-5 and mIL-5 were labelled fluorescently, and nuclear uptake subsequent to membrane binding and internalisation by intact receptor expressing cells visualised and quantified using confocal laser scanning microscopy. hIL-5 and mIL-5 were shown to be transported to the nucleus in in vivo and in vitro nuclear protein import assays. The hIL-5 NLS was able to target a heterologous protein to the nucleus both in vivo and in vitro. Mutations within the proximal arm of the NLS abrogated nuclear targeting activity, confirming its bipartite nature. The results imply a nuclear signalling role for IL-5 additional to pathways linked to the membrane receptor system.