Date Published: September 20, 2016
Publisher: Public Library of Science
Author(s): Hans K. H. Ng, Kaleeckal G. Harikumar, Laurence J. Miller, Billy K. C. Chow, Giovanna Valenti.
The involvement of secretin (SCT) and secretin receptor (SCTR) in regulating body water homeostasis is well established. Identified as one of the vasopressin (Vp)-independent mechanisms in fluid balance, SCT regulates aquaporin 2 (AQP2) in the kidney distal collecting duct cells through activating intracellular cAMP production. This ability to bypass Vp-mediated water reabsorption in kidney implicates SCT’s potential to treat nephrogenic diabetes insipidus (NDI). Research on NDI in the past has largely been focused on the searching for mutations in vasopressin receptor 2 (AVPR2), while the functional relationship between SCTR, AVPR2 and NDI remains unclear. Here, we demonstrate the interaction between SCTR and AVPR2 to modulate cellular signaling in vitro. Interestingly, we show in this report that upon heteromer formation with SCTR, R137H, a NDI-causing AVPR2 mutant that is defective in trafficking to cell surface, can functionally be rescued. Our data may provide an explanation for this clinically mild case of NDI, and insights into the pathological development of NDI in the future.
Water homeostasis is one of the most tightly regulated physiological events in the human body . In addition to the well-recognized Vp axis, the existence of Vp-independent mechanisms in regulating water reabsorption is confirmed [2–12]. Among these, SCT was discovered to be a neurohypophysial factor secreted alongside Vp in the posterior pituitary to control fluid balance by stimulating Vp expression and release from the hypothalamic paraventricular nucleus . SCT also stimulates water reabsorption in the kidney via activating the cAMP signaling pathway and subsequently AQP2 trafficking in the kidney distal collecting duct cells . X-linked NDI is a form of NDI caused by AVPR2 gene mutation on the X chromosome, and the condition is characterized by very low urine osmolality plus marked increase in urine output . Over 170 different mutations were discovered leading to various degree of impairment in kidney’s responsiveness to Vp stimulation . There is no known cure for the disease; NDI patient management relies primarily on diuretics to reduce glomerular filtration rate, and supplemented by tightly controlled intake of sodium and water . A handful of novel treatment strategies for NDI are currently under investigation. Notably, the vasopressin 1a receptor antagonist SR 49059 was reported as being effective [15, 16]. In the past, elucidation of the molecular mechanism of the disease focused heavily on AVPR2 [17, 18]. Most studies were based on clinical reports of AVPR2 mutations, followed by cloning and functional characterization of the mutants [19–21]. However, these studies were conducted in heterologous system expressing only the mutants [22–24]. There are recent evidences showing G protein-coupled receptors (GPCRs) function as monomer and oligomers, with oligomerization of GPCRs modulating a number of receptor physiologies from cellular signaling cascade to receptor trafficking [25, 26]. In light of SCT’s role in regulating body fluid, SCT was suggested as a potential treatment option . As SCTR and AVPR2 are co-localized in the kidney distal tubules , in this report, we studied potential heteromer formation between SCTR with Vp receptors. We found that SCTR specifically hetero-oligomerizes with AVPR2, but not with AVPR1b. Both SCTR and AVPR2 primarily utilize the cAMP signaling pathway, but SCTR is also known to signaling through the calcium-IP3 pathway [28, 29]. We therefore investigated the functional consequences of receptor oligomer formation, and found that the interaction between SCTR and AVPR2 elicits differential receptor functions in vitro. Interestingly, we show here that upon heteromer formation with SCTR, R137H, a NDI-causing AVPR2 mutant that is defective in trafficking to cell surface, can functionally be rescued. Our data may provide an explanation for this clinically mild case of NDI, and insights into the pathological development of NDI in the future.
The signaling modification observed in the present study illustrates the importance of GPCR heteromer in affecting cellular physiology. More notably, our data exemplifies a second instance of cross-class GPCR heteromer formation in addition to the previously elucidated SCTR-AT1aR heteromer . Since both AT1aR and AVPR2 pose key roles in regulation of body water, heteromer formation with these receptors implicates SCTR as an important partner in fluid balance.