Osmolarity-induced renin secretion from kidneys: evidence for readily releasable renin pools

Human Transplant Kidneys on Normothermic Machine Perfusion Display Endocrine Activity

Normothermic machine perfusion (NMP) is an alternative to hypothermic machine perfusion (HMP) for donor kidney preservation before transplantation. Contrary to HMP, NMP allows for functional assessment of donor kidneys because normothermic conditions allow for metabolic activity. The kidneys are key producers of hormones. Yet, it remains unknown whether donor kidneys during NMP display endocrine functions.

Methods

Fifteen donor kidneys were subjected to HMP followed by 2 h of NMP before transplantation. NMP perfusate was collected at 3 time points (0, 1, 2 h) for the measurements of prorenin/renin, erythropoietin (EPO), and vitamin D, and urine samples were collected at 1 h and 2 h for urodilatin measurement. Fifteen HMP perfusate samples were collected for the same measurements.

Results

Kidneys on NMP secreted significantly more prorenin, renin, EPO, and active vitamin D than during HMP. EPO and vitamin D secretion remained stable during 2 h of NMP, whereas the prorenin release rate increased and renin release rate decreased after 1 h. Donation after brain death kidneys secreted more vitamin D and less EPO during NMP than donation after circulatory death kidneys. Twelve donor kidneys produced urine during NMP and released detectable levels of urodilatin. Kidneys exhibited a large variation in hormone release rates. No significant differences were found in hormone release capacity between delayed graft function (DGF) and non-DGF kidneys, and no significant correlations were found between hormone release rates and the duration of DGF or 1-mo posttransplant serum creatinine levels.

Conclusions

Human transplant kidneys display endocrine activity during NMP. To explore whether correlations exist between hormone release rates and posttransplant kidney function, large numbers of kidneys are required.

Parallel regulation of renin and lysosomal integral membrane protein 2 in renin-producing cells: further evidence for a lysosomal nature of renin secretory vesicles

The protease renin is the key enzyme in the renin-angiotensin system (RAS) that regulates extracellular volume and blood pressure. Renin is synthesized in renal juxtaglomerular cells (JG cells) as the inactive precursor prorenin. Activation of prorenin by cleavage of the prosegment occurs in renin storage vesicles that have lysosomal properties. To characterize the renin storage vesicles more precisely, the expression and functional relevance of the major lysosomal membrane proteins lysosomal-associated membrane protein 1 (LAMP-1), LAMP-2, and lysosomal integral membrane protein 2 (LIMP-2) were determined in JG cells. Immunostaining experiments revealed strong coexpression of renin with the LIMP-2 (SCARB2), while faint staining of LAMP-1 and LAMP-2 was detected in some JG cells only. Stimulation of the renin system (ACE inhibitor, renal hypoperfusion) resulted in the recruitment of renin-producing cells in the afferent arterioles and parallel upregulation of LIMP-2, but not LAMP-1 or LAMP-2. Despite the coregulation of renin and LIMP-2, LIMP-2-deficient mice had normal renal renin mRNA levels, renal renin and prorenin contents, and plasma renin and prorenin concentrations under control conditions and in response to stimulation with a low salt diet (with or without angiotensin-converting enzyme (ACE) inhibition). No differences in the size or number of renin vesicles were detected using electron microscopy. Acute stimulation of renin release by isoproterenol exerted similar responses in both genotypes in vivo and in isolated perfused kidneys. Renin and the major lysosomal protein LIMP-2 are colocalized and coregulated in renal JG cells, further corroborating the lysosomal nature of renin storage vesicles. LIMP-2 does not appear to play an obvious role in the regulation of renin synthesis or release.

Renin release: sites, mechanisms, and control

In the adult organism, systemically circulating renin almost exclusively originates from the juxtaglomerular cells in the afferent arterioles of the kidneys. These cells share similarities with pericytes and myofibro-blasts. They store renin in a vesicular network and granules and release it in a regulated fashion. The release mode of renin is not understood; in particular, the involvement of SNARE proteins is unknown. Renin release is acutely increased via the cAMP signaling pathway, which is triggered mainly by catecholamines and other G(s)-coupled agonists, and is inhibited by calcium-related pathways that are commonly activated by vasoconstrictors. Renin release from juxtaglomerular cells is directly modulated in an inverse fashion by the blood pressure inside the afferent arterioles and by the chloride content in the tubule fluid at the macula densa segment of the distal tubule. Renin release is stimulated by nitric oxide and by prostanoids released by neighboring endothelial and macula densa cells. Steady-state renin concentrations in the plasma are determined essentially by the number of renin-producing cells in the afferent arterioles, which changes in parallel with challenges to the renin-angiotensin-aldosterone system.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Hypotonicity-induced Renin exocytosis from juxtaglomerular cells requires aquaporin-1 and cyclooxygenase-2

The mechanism by which extracellular hypotonicity stimulates release of renin from juxtaglomerular (JG) cells is unknown. We hypothesized that osmotically induced renin release depends on water movement through aquaporin-1 (AQP1) water channels and subsequent prostanoid formation. We recorded membrane capacitance (C(m)) by whole-cell patch clamp in single JG cells as an index of exocytosis. Hypotonicity increased C(m) significantly and enhanced outward current. Indomethacin, PLA(2) inhibition, and an antagonist of prostaglandin transport impaired the C(m) and current responses to hypotonicity. Hypotonicity also increased exocytosis as determined by a decrease in single JG cell quinacrine fluorescence in an indomethacin-sensitive manner. In single JG cells from COX-2(-/ -) and AQP1(-/ -) mice, hypotonicity increased neither C(m) nor outward current, but 0.1-muM PGE(2) increased both in these cells. A reduction in osmolality enhanced cAMP accumulation in JG cells but not in renin-producing As4.1 cells; only the former had detectable AQP1 expression. Inhibition of protein kinase A blocked the hypotonicity-induced C(m) and current response in JG cells. Taken together, our results show that a 5 to 7% decrease in extracellular tonicity leads to AQP1-mediated water influx in JG cells, PLA(2)/COX-2-mediated prostaglandin-dependent formation of cAMP, and activation of PKA, which promotes exocytosis of renin.

Intrarenal renin-angiotensin system

Phylogenetically the renin-angiotensin system (RAS) is an ancient regulatory system which has attracted the attention of researchers for about a century. As a result of their efforts, different types of RAS inhibitors are now widely used as therapeutic medicines. The scientific enthusiasm toward RAS remains undiminished and new findings and discoveries are to be expected. Early investigators described the role of RAS in the local control of renal hemodynamics. This correlated well with the morphology of juxtaglomerular apparatus (JGA). Recently developed imaging techniques has allowed for in vivo visualization of cellular functions and the use of molecular biological tools have shed new light on the morphology and physiology of renal RAS, especially in connection with the tubular system. RAS has gained recognition to be more than just an endocrine regulatory system for regulating hemodynamics and water/salt metabolism. RAS is a local tissue and/or cellular regulator with a wide range of effects exerted via various receptors. Local RAS is crucially involved in basic physiological processes like ontogenesis and cell proliferation as well as pathophysiological conditions such as inflammation and tissue fibrosis. These findings may open new frontiers for novel therapeutic approaches. This review focuses only on some specific - less discussed and recently described or hypothesized - morphological and functional aspects of intrarenal RAS, including in vivo imaging of RAS, its effects on juxtaglomerular apparatus and possible cooperative mechanisms among various local renal RAS systems.

Regulation of renin release by local and systemic factors

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.

Succinate receptor GPR91 provides a direct link between high glucose levels and renin release in murine and rabbit kidney

Diabetes mellitus is the most common and rapidly growing cause of end-stage renal disease in developed countries. A classic hallmark of early diabetes mellitus includes activation of the renin-angiotensin system (RAS), which may lead to hypertension and renal tissue injury, but the mechanism of RAS activation is elusive. Here we identified a paracrine signaling pathway in the kidney in which high levels of glucose directly triggered the release of the prohypertensive hormone renin. The signaling cascade involved the local accumulation of succinate and activation of the kidney-specific G protein-coupled metabolic receptor, GPR91, in the glomerular endothelium as observed in rat, mouse, and rabbit kidney sections. Elements of signal transduction included endothelial Ca2+, the production of NO and prostaglandin (PGE2), and their paracrine actions on adjacent renin-producing cells. This GPR91 signaling cascade may serve to modulate kidney function and help remove metabolic waste products through renal hyperfiltration, and it could also link metabolic diseases, such as diabetes, or metabolic syndrome with RAS overactivation, systemic hypertension, and organ injury.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Wilms' tumor protein (-KTS) modulates renin gene transcription

Renin plays a crucial role in the control of various physiological processes such as blood pressure and body fluid homeostasis. Here, we show that a splice variant of the Wilms' tumor protein lacking three amino acids WT1(-KTS) suppresses renin gene transcription. Using bioinformatics tools, we initially predicted that a WT1-binding site exists in a regulatory region about 12 kb upstream of the renin promoter; this was confirmed by reporter gene assays and gel shift experiments in heterologous cells. Co-expression of Wt1 and renin proteins was found in rat kidney sections, mouse kidney blood vessels, and a cell line derived from the juxtaglomerular apparatus that produces renin. Knockdown of WT1 protein by siRNA significantly increased the cellular renin mRNA content, while overexpression of WT1(-KTS) reduced renin gene expression in stable and transiently transfected cells. A mutant WT1(-KTS) protein found in Wilms' tumors failed to suppress renin gene reporter activity and endogenous renin expression. Our findings show that renin gene transcription is regulated by the WT1(-KTS) protein and this may explain findings in patients with WT1 gene mutations of increased plasma renin and hypertension.