Control of renin secretion from kidneys with renin cell hyperplasia

Sex-Specific Causal Relations between Steroid Hormones and Obesity-A Mendelian Randomization Study

Steroid hormones act as important regulators of physiological processes including gene expression. They provide possible mechanistic explanations of observed sex-dimorphisms in obesity and coronary artery disease (CAD). Here, we aim to unravel causal relationships between steroid hormones, obesity, and CAD in a sex-specific manner. In genome-wide meta-analyses of four steroid hormone levels and one hormone ratio, we identified 17 genome-wide significant loci of which 11 were novel. Among loci, seven were female-specific, four male-specific, and one was sex-related (stronger effects in females). As one of the loci was the human leukocyte antigen (HLA) region, we analyzed HLA allele counts and found four HLA subtypes linked to 17-OH-progesterone (17-OHP), including HLA-B*14*02. Using Mendelian randomization approaches with four additional hormones as exposure, we detected causal effects of dehydroepiandrosterone sulfate (DHEA-S) and 17-OHP on body mass index (BMI) and waist-to-hip ratio (WHR). The DHEA-S effect was stronger in males. Additionally, we observed the causal effects of testosterone, estradiol, and their ratio on WHR. By mediation analysis, we found a direct sex-unspecific effect of 17-OHP on CAD while the other four hormone effects on CAD were mediated by BMI or WHR. In conclusion, we identified the sex-specific causal networks of steroid hormones, obesity-related traits, and CAD.

Renin cells in homeostasis, regeneration and immune defence mechanisms

An accumulating body of evidence suggests that renin-expressing cells have developed throughout evolution as a mechanism to preserve blood pressure and fluid volume homeostasis as well as to counteract a number of homeostatic and immunological threats. In the developing embryo, renin precursor cells emerge in multiple tissues, where they differentiate into a variety of cell types. The function of those precursors and their progeny is beginning to be unravelled. In the developing kidney, renin-expressing cells control the morphogenesis and branching of the renal arterial tree. The cells do not seem to fully differentiate but instead retain a degree of developmental plasticity or molecular memory, which enables them to regenerate injured glomeruli or to alter their phenotype to control blood pressure and fluid-electrolyte homeostasis. In haematopoietic tissues, renin-expressing cells might regulate bone marrow differentiation and participate in a circulating leukocyte renin-angiotensin system, which acts as a defence mechanism against infections or tissue injury. Furthermore, renin-expressing cells have an intricate lineage and functional relationship with erythropoietin-producing cells and are therefore central to two endocrine systems - the renin-angiotensin and erythropoietin systems - that sustain life by controlling fluid volume and composition, perfusion pressure and oxygen delivery to tissues. However, loss of the homeostatic control of these systems following dysregulation of renin-expressing cells can be detrimental, with serious pathological events.

Endocrine functions of the renal interstitium

This review aims to summarize the knowledge about the sensor and endocrine response functions of resident interstitial cells of the kidney. By the production of renin, erythropoietin and arachidonate metabolites (medullipin) subsets of renal interstitial fibroblasts and pericytes in different kidney zones play a central role in salt, blood pressure and oxygen homeostasis of the body. Common to these endocrine functions is that their regulation mainly occurs by (de)recruitment of active cells.

Renin-Angiotensin-Aldosterone System Inhibition Increases Podocyte Derivation from Cells of Renin Lineage

Because adult podocytes cannot proliferate and are therefore unable to self-renew, replacement of these cells depends on stem/progenitor cells. Although podocyte number is higher after renin-angiotensin-aldosterone system (RAAS) inhibition in glomerular diseases, the events explaining this increase are unclear. Cells of renin lineage (CoRL) have marked plasticity, including the ability to acquire a podocyte phenotype. To test the hypothesis that RAAS inhibition partially replenishes adult podocytes by increasing CoRL number, migration, and/or transdifferentiation, we administered tamoxifen to Ren1cCreERxRs-tdTomato-R CoRL reporter mice to induce permanent labeling of CoRL with red fluorescent protein variant tdTomato. We then induced experimental FSGS, typified by abrupt podocyte depletion, with a cytopathic antipodocyte antibody. RAAS inhibition by enalapril (angiotensin-converting enzyme inhibitor) or losartan (angiotensin-receptor blocker) in FSGS mice stimulated the proliferation of CoRL, increasing the reservoir of these cells in the juxtaglomerular compartment (JGC). Compared with water or hydralazine, RAAS inhibition significantly increased the migration of CoRL from the JGC to the intraglomerular compartment (IGC), with more glomeruli containing RFP⁺CoRL and, within these glomeruli, more RFP⁺CoRL. Moreover, RAAS inhibition in FSGS mice increased RFP⁺CoRL transdifferentiation in the IGC to phenotypes, consistent with those of podocytes (coexpression of synaptopodin and Wilms tumor protein), parietal epithelial cells (PAX 8), and mesangial cells (α8 integrin). These results show that in the context of podocyte depletion in FSGS, RAAS inhibition augments CoRL proliferation and plasticity toward three different glomerular cell lineages.

Inducible deletion of connexin 40 in adult mice causes hypertension and disrupts pressure control of renin secretion

Genetic loss-of-function defects of connexin 40 in renal juxtaglomerular cells are associated with renin-dependent hypertension. The dysregulation of renin secretion results from an intrarenal displacement of renin cells and an interruption of the negative feedback control of renin secretion by blood pressure. It is unknown whether this phenotype is secondary to developmental defects of juxtaglomerular renin cells due to connexin 40 malfunction, or whether acute functional defects of connexin 40 in the normal adult kidney can also lead to a similar dysregulation of renin secretion and hypertension. To address this question, we generated mice with an inducible deletion of connexin 40 in the adult kidney by crossing connexin 40-floxed mice with mice harboring a ubiquitously expressed tamoxifen-inducible Cre recombinase. Tamoxifen treatment in these mice strongly reduced connexin 40 mRNA and protein expression in the kidneys. These mice displayed persistent hypertension with renin expression shifted from the media layer of afferent arterioles to juxtaglomerular periglomerular cells. Control of renin secretion by the perfusion pressure was abolished in vitro, whereas in vivo plasma renin concentrations were increased. Thus, interruption of the connexin 40 gene in the adult kidney produced very similar changes in the renin system as had embryonic deletion. Hence, impairments of connexin 40 function in the normal adult kidney can cause renin-dependent hypertension.

The water channel aquaporin-1 contributes to renin cell recruitment during chronic stimulation of renin production

Both the processing and release of secretory granules involve water movement across granule membranes. It was hypothesized that the water channel aquaporin (AQP)1 directly contributes to the recruitment of renin-positive cells in the afferent arteriole. AQP1−/− and AQP1+/+ mice were fed a low-salt (LS) diet [0.004% (wt/wt) NaCl] for 7 days and given enalapril [angiotensin-converting enzyme inhibitor (ACEI), 0.1 mg/ml] in drinking water for 3 days. There were no differences in plasma renin concentration at baseline. After LS-ACEI, plasma renin concentrations increased markedly in both genotypes but was significantly lower in AQP1−/− mice compared with AQP1+/+ mice. Tissue renin concentrations were higher in AQP1−/− mice, and renin mRNA levels were not different between genotypes. Mean arterial blood pressure was not different at baseline and during LS diet but decreased significantly in both genotypes after the addition of ACEI; the response was faster in AQP1−/− mice but then stabilized at a similar level. Renin release after 200 μl blood withdrawal was not different. Isoprenaline-stimulated renin release from isolated perfused kidneys did not differ between genotypes. Cortical tissue norepinephrine concentrations were lower after LS-ACEI compared with baseline with no difference between genotypes. Plasma nitrite/nitrate concentrations were unaffected by genotype and LS-ACEI. In AQP1−/− mice, the number of afferent arterioles with recruitment was significantly lower compared with AQP1+/+ mice after LS-ACEI. We conclude that AQP1 is not necessary for acutely stimulated renin secretion in vivo and from isolated perfused kidneys, whereas recruitment of renin-positive cells in response to chronic stimulation is attenuated or delayed in AQP1−/− mice.