Use of LiCl in phospholipase C assays masks the impaired functionality of a mutant angiotensin II receptor

KISS1R: Hallmarks of an Effective Regulator of the Neuroendocrine Axis

Kisspeptin (KP) is now well recognized as a potent stimulator of gonadotropin-releasing hormone (GnRH) secretion and thereby a major regulator of the neuroendocrine-reproductive axis. KP signals via KISS1R, a G protein-coupled receptor (GPCR) that activates the G proteins Gαq/11. Modulation of the interaction of KP with KISS1R is therefore a potential new therapeutic target for stimulating (in infertility) or inhibiting (in hormone-dependent diseases) the reproductive hormone cascade. Major efforts are underway to target KISS1R in the treatment of sex steroid hormone-dependent disorders and to stimulate endogenous hormonal responses along the neuroendocrine axis as part of in vitro fertilization protocols. The development of analogs modulating KISS1R signaling will be aided by an understanding of the intracellular pathways and dynamics of KISS1R signaling under normal and pathological conditions. This review focuses on KISS1R recruitment of intracellular signaling (Gαq/11- and β-arrestin-dependent) pathways that mediate GnRH secretion and the respective roles of rapid desensitization, internalization, and recycling of resensitized receptors in maintaining an active population of KISS1R at the cell surface to facilitate prolonged KP signaling. Additionally, this review summarizes and discusses the major findings of an array of studies examining the desensitization of KP signaling in man, domestic and laboratory animals. This discussion highlights the major effects of ligand efficacy and concentration and the physiological, developmental, and metabolic status of the organism on KP signaling. Finally, the potential for the utilization of KP and analogs in stimulating and inhibiting the reproductive hormone cascade as an alternative to targeting the downstream GnRH receptor is discussed.

Inhibition of adipocytogenesis by canonical WNT signaling in human mesenchymal stem cells

The WNT signaling pathway plays important roles in the self-renewal and differentiation of mesenchymal stem cells (MSCs). Little is known about WNT signaling in adipocyte differentiation of human MSCs. In this study, we tested the hypothesis that canonical and non-canonical WNTs differentially regulate in vitro adipocytogenesis in human MSCs. The expression of adipocyte gene PPARγ2, lipoprotein lipase, and adipsin increased during adipocytogenesis of hMSCs. Simultaneously, the expression of canonical WNT2, 10B, 13, and 14 decreased, whereas non-canonical WNT4 and 11 increased, and WNT5A was unchanged. A small molecule WNT mimetic, SB-216763, increased accumulation of β-catenin protein, inhibited induction of WNT4 and 11 and inhibited adipocytogenesis. In contrast, knockdown of β-catenin with siRNA resulted in spontaneous adipocytogenesis. These findings support the view that canonical WNT signaling inhibits and non-canonical WNT signaling promotes adipocytogenesis in adult human marrow-derived mesenchymal stem cells.

The initial inositol 1,4,5-trisphosphate response induced by histamine is strongly amplified by Ca(2+) release from internal stores in smooth muscle

We have studied the Ca(2+)-dependence and wortmannin-sensitivity of the initial inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) response induced by activation of either histamine or muscarinic receptors in smooth muscle from guinea pig urinary bladder. Activation of H(1) receptors with histamine (100 microM) produced a significant elevation in Ins(1,4,5)P(3) levels with only 5s stimulation and in the presence of external Ca(2+). However, this response was abolished fully by either the prolonged absence of external Ca(2+) or the depletion of internal Ca(2+) stores with thapsigargin (100nM) or ryanodine (10 microM). In contrast, the same conditions only slightly reduced the initial Ins(1,4,5)P(3) response induced by carbachol. The prolonged incubation of smooth muscle in 10 microM wortmannin to inhibit type III PI 4-kinase abolished both the early histamine-evoked Ins(1,4,5)P(3) and Ca(2+) responses. Conversely, wortmannin did not alter Ca(2+) release induced by carbachol, despite a partial reduction of its Ins(1,4,5)P(3) response. Collectively, these data indicate that the detectable histamine-induced increase in Ins(1,4,5)P(3) is more the consequence of Ca(2+) release from internal stores than a direct activation of phospholipase C by H(1) receptors. In addition, the effect of wortmannin implies the existence of a Ca(2+)-dependent amplification loop for the histamine-induced Ins(1,4,5)P(3) response in smooth muscle.

Requirement of membrane-proximal amino acids in the carboxyl-terminal tail for expression of the rat AT1a angiotensin receptor

A series of deletion mutants was created to analyze the function of the membrane-proximal region of the cytoplasmic tail of the rat type 1a (AT1a) angiotensin receptor. In transiently transfected COS-7 cells, the truncated mutant receptors showed a progressive decrease in surface expression, with no major change in binding affinity for the peptide antagonist, [Sar1,Ile8]angiotensin II. In parallel with the decrease in receptor expression, a progressive decrease in angiotensin II-induced inositol phosphate responses was observed. Alanine substitutions in the region 307-311 identified the highly conserved phenylalanine309 and adjacent lysine residues as significant determinants of AT1a receptor expression.