Apela Regulates Fluid Homeostasis by Binding to the APJ Receptor to Activate Gi Signaling. Deng C et al. (2015) Apela (APJ early endogenous ligand, also known as elabela or toddler) is a recently discovered peptide hormone. Based on genetic studies in zebrafish, apela was found to be important for endoderm differentiation and heart development during embryogenesis. Although common phenotypes of apela and APJ-null zebrafish during embryonic development suggested that apela interacts with the APJ receptor, kinetics of apela binding to APJ and intracellular signaling pathways for apela remain unknown. The role of apela in adults is also uncertain. Using a chimeric apela ligand, we showed direct binding of apela to APJ with high affinity (Kd = 0.51 nm) and the ability of apelin, the known peptide ligand for APJ, to compete for apela binding. Apela, similar to apelin, acts through the inhibitory G protein pathway by inhibiting forskolin-stimulated cAMP production and by inducing ERK1/2 phosphorylation. In adult rats, apela is expressed exclusively in the kidney, unlike the wide tissue distribution of apelin. In vivo studies demonstrated the ability of apela to regulate fluid homeostasis by increasing diuresis and water intake. Dose-response studies further indicated that apela induces 2- and 5-fold higher maximal responses than apelin in ERK1/2 phosphorylation and diuresis/water intake, respectively. After designing an apela antagonist, we further demonstrated the role of endogenous ligand(s) in regulating APJ-mediated fluid homeostasis. Our results identified apela as a potent peptide hormone capable of regulating fluid homeostasis in adult kidney through coupling to the APJ-mediated Gi signaling pathway.//////////////////
Apela Regulates Fluid Homeostasis by Binding to the APJ Receptor to Activate Gi Signaling. Deng C et al. (2015) Apela (APJ early endogenous ligand, also known as elabela or toddler) is a recently discovered peptide hormone. Based on genetic studies in zebrafish, apela was found to be important for endoderm differentiation and heart development during embryogenesis. Although common phenotypes of apela and APJ-null zebrafish during embryonic development suggested that apela interacts with the APJ receptor, kinetics of apela binding to APJ and intracellular signaling pathways for apela remain unknown. The role of apela in adults is also uncertain. Using a chimeric apela ligand, we showed direct binding of apela to APJ with high affinity (Kd = 0.51 nm) and the ability of apelin, the known peptide ligand for APJ, to compete for apela binding. Apela, similar to apelin, acts through the inhibitory G protein pathway by inhibiting forskolin-stimulated cAMP production and by inducing ERK1/2 phosphorylation. In adult rats, apela is expressed exclusively in the kidney, unlike the wide tissue distribution of apelin. In vivo studies demonstrated the ability of apela to regulate fluid homeostasis by increasing diuresis and water intake. Dose-response studies further indicated that apela induces 2- and 5-fold higher maximal responses than apelin in ERK1/2 phosphorylation and diuresis/water intake, respectively. After designing an apela antagonist, we further demonstrated the role of endogenous ligand(s) in regulating APJ-mediated fluid homeostasis. Our results identified apela as a potent peptide hormone capable of regulating fluid homeostasis in adult kidney through coupling to the APJ-mediated Gi signaling pathway.//////////////////
In the course of a search for genes closely related to the vasopressin receptor gene, O'Dowd et al. (1993) found a
putative receptor protein related to the angiotensin receptor. The AGTRL1 gene, which they designated
APJ, was cloned using PCR with a set of degenerate primers based on a conserved transmembrane domain found in members
of the G protein-coupled receptor (GPCR) gene family .
The human APJ receptor functions as an efficient alternative
co-receptor for a number of human immunodeficiency virus type 1 and simian immunodeficiency virus
strains.
NCBI Summary:
This gene encodes a member of the G protein-coupled receptor gene family. The encoded protein is related to the angiotensin receptor, but is actually an apelin receptor that inhibits adenylate cyclase activity and plays a counter-regulatory role against the pressure action of angiotensin II by exerting hypertensive effect. It functions in the cardiovascular and central nervous systems, in glucose metabolism, in embryonic and tumor angiogenesis and as a human immunodeficiency virus (HIV-1) coreceptor. Two transcript variants resulting from alternative splicing have been identified. [provided by RefSeq, Jul 2009]
General function
Receptor
Comment
Cellular localization
Plasma membrane
Comment
Ovarian function
Steroid metabolism, Luteinization
Comment
Apelin (APLN) and Apelin Receptor (APLNR) in Human Ovary: Expression, Signaling, and Regulation of Steroidogenesis in Primary Human Luteinized Granulosa Cells. Roche J et al. (2016) Apelin (APLN) is a recently discovered adipokine involved in the regulation of various metabolic functions. Its receptor, APLNR, is expressed in reproductive tissues, however, its role in human ovarian cells is unknown. In this study we identified APLN and APLNR in human ovarian follicles and analyzed their expression in granulosa cells and follicular fluid obtained from obese and non-obese patients, with or without polycystic ovary syndrome (PCOS). We also investigated the effect of APLN on steroidogenesis in cultured human luteinized granulosa cells (hGCs) from non-obese patients without PCOS. Using RT-PCR and immunoblotting we found that APLN and APLNR were expressed in hGCs and cumulus and theca cells. We confirmed these data immunohistochemically, and observed that APLNR and APLN are present in human oocytes at different stages of follicular development. In patients with PCOS, we observed that follicular fluid APLN concentration and granulosa cell APLN and APLNR mRNA expression was higher than that observed in control patients. In cultured hGCs from non-obese patients without PCOS, insulin-like growth factor (IGF)-1 increased APLNR expression, and recombinant human (rh) APLN (APLN-13 and APLN-17) increased both basal and IGF1-induced steroid secretion. These effects on steroid production were reversed when cultured in the presence of ML221, an APLNR antagonist, which was associated with an increased 3β-hydrosteroid dehydrogenase (HSD3B) protein concentration. We showed that these effects were dependent on the activation of the AKT and MAPK3/1 pathways using pharmacological inhibitors. Our results show that APLN and APLNR are present in human ovarian cells, and APLN increases IGF1-induced steroidogenesis in granulosa cells through an increase in HSD3B protein expression and activation of the MAPK3/1 and Akt pathways. Therefore, APLN and APLNR may play a role in human follicular development and the pathogenesis of PCOS.//////////////////
Expression regulated by
Comment
Ovarian localization
Cumulus, Granulosa, Theca, Luteal cells
Comment
O'Carrolla A, et al.(2000) reported the distribution of mRNA encoding B78/apj, the rat homologue of the human APJ
receptor, and its endogenous ligand apelin in brain and peripheral tissues.
They cloned the rat APJ receptor, which we term B78/apj, and have mapped the mRNA
distribution of both the receptor and its natural ligand apelin in rat tissues. Northern blot analysis
showed a similar pattern of expression for B78/apj and apelin mRNAs with hybridising transcripts
seen in the lung, heart, skeletal muscle, kidney, brain and liver. In situ hybridisation histochemistry
studies revealed intense B78/apj gene expression in the parenchyma of the lung, a sub-population of
glomeruli in the kidney, the corpora lutea of the ovary and isolated cells of the anterior lobe of the
pituitary.
Apelin and APJ receptor expression in granulosa and theca cells during different stages of follicular development in the bovine ovary: Involvement of apoptosis and hormonal regulation. Shimizu T et al. In the mammalian ovary, the microvasculature in the thecal layer of follicles is associated with follicular development. Apelin and its receptor, APJ, are expressed in the tissues and organs which include the vasculature. The aims of the present study were to examine the mRNA expression of apelin and the APJ receptor in granulosa cells and theca tissue of bovine follicles and the effects of steroid hormone and gonadotrophins on the expression of these genes in cultured granulosa cells and theca cells. The expression of apelin mRNA was not found in the granulosa cells of bovine follicles. The expression of the APJ gene was increased in granulosa cells of estrogen-inactive dominant follicles. The expression of apelin mRNA increased in theca tissues of estrogen-inactive dominant follicles. APJ expression in theca tissues increased with follicle growth. Progesterone stimulated the expression of APJ mRNA in the cultured granulosa cells. FSH stimulated the expression of APJ mRNA in the cultured granulosa cells. LH induced the expression of apelin and APJ receptor mRNAs in cultured theca cells. Taken together, our data indicate that the APJ receptor in granulosa cells and both apelin and the APJ receptor in theca tissues are expressed in bovine ovary, that APJ in granulosa cells may be involved in the appearance of the cell apoptosis, and that LH stimulates the expression of apelin and APJ genes in theca cells.
Follicle stages
Corpus luteum
Comment
Expression and localization of apelin and its receptor APJ in the bovine corpus luteum during the estrous cycle and prostaglandin F2alpha-induced luteolysis. Shirasuna K et al. Angiogenesis, changes in blood flow, and extracellular matrix remodeling are the processes associated with the development and demise of the bovine corpus luteum (CL) during the estrous cycle. APJ (putative receptor protein related to angiotensin type 1 receptor) is a G-protein-coupled receptor, and its ligand, apelin, has been identified as a novel regulator of blood pressure and as an angiogenic factor. We hypothesized that the apelin-APJ system is involved in luteal function. This study investigated whether apelin-APJ exists in bovine CL and determined their expression profiles and localization during luteal phase and prostaglandin F(2)(alpha) (PGF(2)(alpha))-induced luteolysis. During the luteal phase, apelin mRNA expression increased from early to late CL and decreased in regressed CL. APJ mRNA expression increased from early to mid-CL and remained elevated in late and regressed CL. Apelin and APJ proteins were immunohistochemically detected only in the smooth muscle cells of intraluteal arterioles during the luteal phase. PGF(2)(alpha) stimulated apelin and APJ mRNA expression at 0.5-2 and 2 h respectively, and then the mRNA expression of apelin-APJ was inhibited from 4 h during PGF(2)(alpha)-induced luteolysis. Additionally, apelin mRNA and protein were stimulated at 1 h after PGF(2)(alpha) injection only in the periphery of mid- but not early CL. The present study indicated that the apelin-APJ was localized in the smooth muscle cells of intraluteal arterioles, and responded to PGF(2)(alpha) at the periphery of mid-CL in the cow. Thus, the apelin-APJ system may be involved in the maturation of CL and the luteolytic cascade as a regulator of intraluteal arterioles in cow.