Cyclic mechanical distension regulates renin gene transcription in As4.1 cells

Piezo2 expression and its alteration by mechanical forces in mouse mesangial cells and renin-producing cells

The kidney plays a central role in body fluid homeostasis. Cells in the glomeruli and juxtaglomerular apparatus sense mechanical forces and modulate glomerular filtration and renin release. However, details of mechanosensory systems in these cells are unclear. Piezo2 is a recently identified mechanically activated ion channel found in various tissues, especially sensory neurons. Herein, we examined Piezo2 expression and regulation in mouse kidneys. RNAscope in situ hybridization revealed that Piezo2 expression was highly localized in mesangial cells and juxtaglomerular renin-producing cells. Immunofluorescence assays detected GFP signals in mesangial cells and juxtaglomerular renin-producing cells of Piezo2^GFP reporter mice. Piezo2 transcripts were observed in the Foxd1-positive stromal progenitor cells of the metanephric mesenchyme in the developing mouse kidney, which are precursors of mesangial cells and renin-producing cells. In a mouse model of dehydration, Piezo2 expression was downregulated in mesangial cells and upregulated in juxtaglomerular renin-producing cells, along with the overproduction of renin and enlargement of the area of renin-producing cells. Furthermore, the expression of the renin coding gene Ren1 was reduced by Piezo2 knockdown in cultured juxtaglomerular As4.1 cells under static and stretched conditions. These data suggest pivotal roles for Piezo2 in the regulation of glomerular filtration and body fluid balance.

TRPV4 participates in pressure-induced inhibition of renin secretion by juxtaglomerular cells

KEY POINTS

Increase in blood pressure in the renal afferent arteriole is known to induce an increase in cytosolic calcium concentration ([Ca²⁺ ]_i ) of juxtaglomerular (JG) cells and to result in a decreased secretion of renin. Mechanical stimulation of As4.1 JG cells induces an increase in [Ca²⁺ ]_i that is inhibited by HC067047 and RN1734, two inhibitors of TRPV4, or by siRNA-mediated repression of TRPV4. Inhibition of TRPV4 impairs pressure-induced decrease in renin secretion. Compared to wild-type mice, Trpv4^-/- mice present increased resting plasma levels of renin and aldosterone and present a significantly altered pressure-renin relationship. We suggest that TRPV4 channel participates in mechanosensation at the juxtaglomerular apparatus.

ABSTRACT

The renin-angiotensin system is a crucial blood pressure regulation system. It consists of a hormonal cascade where the rate-limiting enzyme is renin, which is secreted into the blood flow by renal juxtaglomerular (JG) cells in response to low pressure in the renal afferent arteriole. In contrast, an increase in blood pressure results in a decreased renin secretion. This is accompanied by a transitory increase in [Ca²⁺ ]_i of JG cells. The inverse relationship between [Ca²⁺ ]_i and renin secretion has been called the 'calcium paradox' of renin release. How increased pressure induces a [Ca²⁺ ]_i transient in JG cells, is however, unknown. We observed that [Ca²⁺ ]_i transients induced by mechanical stimuli in JG As4.1 cells were completely abolished by HC067047 and RN1734, two inhibitors of TRPV4. They were also reduced by half by siRNA-mediated repression of TRPV4 but not after repression or inhibition of TRPV2 or Piezo1 ion channels. Interestingly, the stimulation of renin secretion by the adenylate cyclase activator forskolin was totally inhibited by cyclic stretching of the cells. This effect was mimicked by stimulation with GSK1016790A and 4αPDD, two activators of TRPV4 and inhibited in the presence of HC067047. Moreover, in isolated perfused kidneys from Trpv4^-/- mice, the pressure-renin relationship was significantly altered. In vivo, Trpv4^-/- mice presented increased plasma levels of renin and aldosterone compared to wild-type mice. Altogether, our results suggest that TRPV4 is involved in the pressure-induced entry of Ca²⁺ in JG cells, which inhibits renin release and allows the negative feedback regulation on blood pressure.

Control of renin [corrected] gene expression

Renin, as part of the renin-angiotensin system, plays a critical role in the regulation of blood pressure, electrolyte homeostasis, mammalian renal development, and progression of fibrotic/hypertrophic diseases. Renin gene transcription is subject to complex developmental and tissue-specific regulation. Initial studies using the mouse As4.1 cell line, which has many characteristics of the renin-expressing juxtaglomerular cells of the kidney, have identified a proximal promoter region (-197 to -50 bp) and an enhancer (-2,866 to -2,625 bp) upstream of the Ren-1(c) gene, which are critical for renin gene expression. The proximal promoter region contains several transcription factor binding sites including a binding site for the products of the developmental control genes Hox. The enhancer consists of at least 11 transcription factor binding sites and is responsive to various signal transduction pathways including cAMP, retinoic acid, endothelin-1, and cytokines, all of which are known to alter renin mRNA levels. Furthermore, in vivo models have validated several of these key components found within the proximal promoter region and the enhancer as well as other key sites necessary for renin gene transcription.

Characterization of the genomic structure and function of regions influencing renin and angiogenesis in the SS rat

Impaired regulation of renin in Dahl salt-sensitive rats (SS/JRHsdMcwi, SS) contributes to attenuated angiogenesis in this strain. This study examined angiogenic function and genomic structure of regions surrounding the renin gene using subcongenic strains of the SS and BN/NHsdMcwi (BN) rat to identify important genomic variations between SS and BN involved in angiogenesis. Three candidate regions on Chr 13 were studied: two congenic strains containing 0.89 and 2.62 Mb portions of BN Chr 13 that excluded the BN renin allele and a third strain that contained a 2.02 Mb overlapping region that included the BN renin allele. Angiogenesis induced by electrical stimulation of the tibialis anterior muscle was attenuated in the SS compared with the BN. Congenics carrying the SS renin allele had impaired angiogenesis, while strains carrying the BN renin allele had angiogenesis restored. The exception was a congenic including a region of BN genome 0.4 Mb distal to renin that restored both renin regulation and angiogenesis. This suggests that there is a distant regulatory element in the BN capable of restoring normal regulation of the SS renin allele. The importance of ANG II in the restored angiogenic response was demonstrated by blocking with losartan. Sequencing of the 4.05 Mb candidate region in SS and BN revealed a total of 8,850 SNPs and other sequence variants. An analysis of the genes and their variants in the region suggested a number of pathways that may explain the impaired regulation of renin and angiogenesis in the SS rat.

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 ).

Intracardiac and intrarenal renin-angiotensin systems: mechanisms of cardiovascular and renal effects

The renin-angiotensin system (RAS) is a hormonal system that controls body fluid volume, blood pressure, and cardiovascular function in both health and disease. Various tissues, including the heart and kidneys, possess individual locally regulated RASs. In each RAS, the substrate protein angiotensinogen is cleaved by the peptidases renin and angiotensin-converting enzyme to form the biologically active product angiotensin II, which acts as an intracrine cardiac and renal hormone. The components of each RAS, including aldosterone (ALDO), may be produced locally and/or may be delivered by or sequestered from the circulation. Overactivity of the cardiac RAS has been associated with cardiac diseases, including cardiac hypertrophy due to volume and/or pressure overload, heart failure, coronary artery disease with myocardial infarction, and hypertension. Overactivity of the renal RAS has been associated with various kidney diseases, including nephropathies and renal artery stenosis. The principal effects of an overactive RAS include the generation of reactive oxygen species, which leads to "oxidative stress," activation of the nuclear transcription factor kappaB, and stimulation of pathways and genes that promote vasoconstriction, endothelial dysfunction, cell hypertrophy, fibroblast proliferation, inflammation, excess extracellular matrix deposition, atherosclerosis, and thrombosis. It has been suggested that oxidative stress is the central mechanism underlying the pathogenesis of RAS-related and ALDO-related chronic cardiovascular and renal tissue injury and of cardiac arrhythmias and conduction disturbances.

Transcriptional Regulation of Renin

Renin, as a component of the renin-angiotensin system, plays important roles in the regulation of blood pressure, electrolyte homeostasis, and mammalian renal development. Transcription of renin genes is subject to complex developmental and tissue-specific regulation. Progress has been made recently in elucidating the molecular mechanisms involved in renin gene expression. Using mouse As4.1 cells, which have many features characteristic of the renin-expressing juxtaglomerular cells of kidney, a proximal promoter region (−197 to −50 bp) and an enhancer (−2866 to −2625 bp) have been identified in the mouse renin gene, Ren-1 c , that are critical for its expression. The proximal promoter region contains at least 7 transcription factor-binding sites, including a binding site for the products of Hox , developmental control genes. The enhancer consists of at least 11 transcription factor-binding sites and is responsive to various signal transduction pathways, including cAMP, retinoic acid, endothelin-1, and cytokines, to alter renin mRNA levels. Sequence highly homologous to the mouse enhancer is also found in the human and rat renin genes. How these regulatory regions function in vivo will be the focus of future study.

Enhancer-dependent inhibition of mouse renin transcription by inflammatory cytokines

Inflammatory cytokines have been shown to inhibit renin gene expression in the kidney in vivo and the kidney tumor-derived As4.1 cell line. In this report, we show that cytokines oncostatin M (OSM), IL-6, and IL-1beta inhibit transcriptional activity associated with 4.1 kb of the mouse renin 5'-flanking sequence in As4.1 cells. The 242-bp enhancer (-2866 to -2625 bp) is sufficient to mediate the observed inhibitory effects. Sequences within the enhancer required for inhibition by each of these cytokines have been determined by deletional and mutational analysis. Results indicate that a 39-bp region (CEC) containing a cAMP-responsive element, an E-box, and a steroid receptor-binding site, previously identified as the most critical elements for enhancer activity, is sufficient for the inhibition induced by IL-1beta. However, mutation of each of the three component sites does not abolish the inhibition by IL-1beta, suggesting that the target(s) of cytokine action may not be the transcription factors binding directly to these sites. This CEC region is also critical, but not sufficient, for the inhibition mediated by OSM and IL-6. These data suggest that the direct target of the associated cytokines may be coactivators interacting with transcription factors binding at the enhancer. Finally, we show that OSM treatment caused a 17-fold increase in promoter activity when only 2,625 bp of the Ren-1(c) flanking sequence were tested, in which the enhancer is not present. Three regions including -2625 to -1217 bp, the HOX.PBX binding site at -60 bp, and -59 to +6 bp have been found to contribute to this induction.

Endothelin-1 increases calcium and attenuates renin gene expression in As4.1 cells

Endothelin-1 (ET-1) is a potent vasoconstrictor and blood pressure modulator. Renin secretion from juxtaglomerular (JG) cells is crucial for blood pressure and electrolyte homeostasis and has been shown to be modulated by ET-1; however, the cellular and molecular mechanism of this regulation is not clear. The purpose of this study was to gain a better understanding of the cellular and molecular pathways activated by ET-1 by using a renin-producing cell line As4.1. ET-1 caused an increase in As4.1 cell intracelluar Ca(2+) concentration ([Ca(2+)](i)) mediated by the ET(A) receptor as its antagonist, BQ-123, abolished the response. The nitric oxide donor nitroprusside, but not 8-bromo-cGMP, reduced the time necessary for successive ET-1 responses. Endothelin-3 had no effect on [Ca(2+)](i). ET-1 dose dependently increased total inositol phosphates with an EC(50) of 2.1 nM. ET-1 reduced renin mRNA by 68% independently of changes in message decay. With the use of a renin-luciferase reporter system in As4.1 cells, ET-1 reduced luciferase activity by 51%, suggesting that renin gene transcription is directly modified by ET-1.

Calcium-dependent activation of phospholipase C by mechanical distension in renin-expressing As4.1 cells

One of the major physiological regulators for the production and release of renin from the kidney is blood pressure. The juxtaglomerular (JG) cells, located primarily at the afferent arterioles leading to the glomerulus, are thought to be the baroreceptor of the kidney and adjust their ability to secrete renin in an inverse relationship to changes in pressure (mechanical force). The characteristics of JG cells that allow them to sense and respond to changes in mechanical force at the cellular level are not clear. By use of a renin-expressing clonal cell line (As4.1) as a model for JG cells, it was the purpose of this paper to identify cellular pathways that are activated by mechanical distension. Fura 2-labeled As4.1 cells were mechanically probed to observe changes of intracellular calcium concentration ([Ca(2+)](i)). Mechanical distension of As4.1 cells resulted in an influx of Ca(2+) to the cytosol, mediated by stretch-activated ion channels and dependent on the presence of extracellular Ca(2+). Furthermore, cyclic mechanical distension elevated total inositol phosphates (IP) in As4.1 cells. This response was also dependent on the presence of extracellular Ca(2+), and the addition of U-73122, a phospholipase C (PLC) antagonist, significantly attenuated the increase of IP. Taken together, these findings demonstrate the calcium-dependent activation of PLC and the subsequent increase of IP and [Ca(2+)](i) to be a potentially important pathway for the modality of pressure sensing by renin-expressing cells in response to mechanical stimulation.