Purification and characterization of human truncated prorenin

ER ribosomal-binding protein 1 regulates blood pressure and potassium homeostasis by modulating intracellular renin trafficking

BACKGROUND

Genome-wide association studies (GWASs) have linked RRBP1 (ribosomal-binding protein 1) genetic variants to atherosclerotic cardiovascular diseases and serum lipoprotein levels. However, how RRBP1 regulates blood pressure is unknown.

METHODS

To identify genetic variants associated with blood pressure, we performed a genome-wide linkage analysis with regional fine mapping in the Stanford Asia-Pacific Program for Hypertension and Insulin Resistance (SAPPHIRe) cohort. We further investigated the role of the RRBP1 gene using a transgenic mouse model and a human cell model.

RESULTS

In the SAPPHIRe cohort, we discovered that genetic variants of the RRBP1 gene were associated with blood pressure variation, which was confirmed by other GWASs for blood pressure. Rrbp1- knockout (KO) mice had lower blood pressure and were more likely to die suddenly from severe hyperkalemia caused by phenotypically hyporeninemic hypoaldosteronism than wild-type controls. The survival of Rrbp1-KO mice significantly decreased under high potassium intake due to lethal hyperkalemia-induced arrhythmia and persistent hypoaldosteronism, which could be rescued by fludrocortisone. An immunohistochemical study revealed renin accumulation in the juxtaglomerular cells of Rrbp1-KO mice. In the RRBP1-knockdown Calu-6 cells, a human renin-producing cell line, transmission electron and confocal microscopy revealed that renin was primarily retained in the endoplasmic reticulum and was unable to efficiently target the Golgi apparatus for secretion.

CONCLUSIONS

RRBP1 deficiency in mice caused hyporeninemic hypoaldosteronism, resulting in lower blood pressure, severe hyperkalemia, and sudden cardiac death. In juxtaglomerular cells, deficiency of RRBP1 reduced renin intracellular trafficking from ER to Golgi apparatus. RRBP1 is a brand-new regulator of blood pressure and potassium homeostasis discovered in this study.

An alternative transcript of the rat renin gene can result in a truncated prorenin that is transported into adrenal mitochondria

Characterization of the local renin-angiotensin system in the rat adrenal zona glomerulosa indicated a dual targeting of renin both to the secretory pathway and mitochondria. To investigate the transport of renin into mitochondria, we constructed a series of amino-terminal deletion variants of preprorenin. One of these variants, lacking the complete signal sequence for the endoplasmic reticulum and 10 amino acids of the profragment, was transported efficiently into isolated mitochondria. The transport was further shown to be dependent on mitochondrial membrane potential and ATP synthesis. Analysis of adrenal RNA revealed the existence of 2 renin transcripts. While one of the transcripts corresponds to the known full-length transcript, the other one lacks exon 1; instead, exon 2 is preceded by a domain of 80 nucleotides originating from intron 1. This domain, as well as the following region of intron 1 being excised, shows all essential sequence elements defining an additional, so-far-unknown exon. The second mRNA possibly derives from an additional transcription start in intron 1 and an alternative splicing process. Translation of this mRNA could result in a truncated prorenin representing a cytosolic form of renin, which is required for transport into mitochondria. This truncated prorenin corresponds exactly to the deletion variant being imported into mitochondria in vitro.

The ovarian renin-angiotensin system in reproductive physiology

The identification of the presence of prorenin, renin, angiotensinogen, angiotensin-converting enzyme, angiotensin II (Ang II), and Ang II receptors in the ovary suggests that there is a functional ovarian renin-angiotensin system (RAS). It could play a significant role in such areas of ovarian physiology as follicular development, steroidogenesis, oocyte maturation, ovulation, and follicle atresia. Expression of the ovarian RAS is regulated by gonadotropins. Ang II, a bioactive octapeptide of RAS, has important effects as a paracrine/autocrine regulator at different stages of the reproductive cycle. Ang II modulates ovarian steroidogenesis and formation of the corpus luteum and also stimulates oocyte maturation and ovulation via Ang II receptors on granulosa cells. In addition, increasing evidence demonstrates that Ang II is a major factor in regulating the function of atretic follicles. In any physiologic system, aberrations result in the development of pathologic states. Disturbances in the ovarian RAS can be the cause or the result of such reproductive disorders as polycystic ovary syndrome, ovarian hyperstimulation syndrome, ovarian tumors, and ectopic pregnancy. Data support the concept of an active and regulated RAS in ovarian follicles. Species differences observed in the expression of ovarian RAS suggest varying functional roles among species with respect to ovarian physiology.

Regulation of utero-placental prorenin

Prorenin (Pro) is synthesized in a number of human utero-placental tissues, including chorion, decidua, villous placenta and probably mesenchymal cells. The release of Pro from these extra-renal tissues follows new protein synthesis and appears to utilize the constitutive secretory pathway. Unlike processing in the kidney, very little of the Pro is subsequently cleaved to the smaller product (active renin). Primary signals which regulate Pro include protein hormones and peptides (relaxin, endothelin, hCG), amines (epinephrine, norepinephrine, and related beta adrenergic agents), and eicosanoids. These agents increase the mRNA for prorenin at a time before peak secretory effects are noted. Other extracellular signals have negative regulatory effects. These include angiotensin, endotoxin and cytokines (TNF-alpha and interleukin-1 B). There is also evidence that glucocorticoid receptor activation has an inhibitory effects on Pro release in placenta. Second messengers involved in the regulation of Pro include cyclic AMP and protein kinase A (PKA), protein kinase C (PKC), and calcium. The possible biological effect(s) of the extracellular Pro are unknown but may be due to direct generation of angiotensin I. Since angiotensin-peptides have a number of trophic effect on both vascular and non-vascular tissues, regulation of utero-placental Pro by autocrine, paracrine or endocrine signalling may be critical in normal fetal and/or placental development.

Prorenin secretion from villous placenta: regulation by cyclic AMP and angiotensin

Renin synthesis and secretion from human chorion and decidua have previously been shown to be stimulated by agents which increase cellular cyclic adenosine monophosphate (cAMP). We have now used organ culture of villous placenta, incubated for periods up to 72 h, to investigate the cellular regulation of renin in this tissue. The placental tissues release renin (92-96% in the form of prorenin) and human chorionic gonadotrophin (hCG), but not prolactin. We found that cholera toxin and forskolin markedly stimulate the synthesis and release of renin in a time-dependent manner. This stimulation was potentiated by phosphodiesterase inhibitors and inhibited by an angiotensin II agonist, sar-1-angiotensin II. The inhibitory action of the angiotensin agonist on renin release was blocked by sar-1-leu-8-angiotensin II, a selective angiotensin receptor antagonist. The potential for stimulation of renin expression by cyclic AMP-regulated elements is supported by the dramatic (two-orders of magnitude) increase in renin release observed with cholera and forskolin at 72 h. There are several possible candidates for primary signals for adenylyl cyclase-coupled renin secretion from the placenta, including relaxin and epinephrine. The extremely low concentration of renin in term villous placenta may be related to activation of negative regulatory elements on the renin gene. We propose that angiotensin II is one negative regulator of this system.