Molecular mechanism of juxtaglomerular cell hyperplasia: a unifying hypothesis

Proliferation does not contribute to murine models of renin cell recruitment

Aim

Renin cells are essential for regulation of blood pressure and fluid‐electrolyte homeostasis. During homeostatic threat, the number of renin cells in the kidney increases, a process termed as recruitment. It has been proposed that recruitment occurs by proliferation, yet no systematic studies have been performed. We sought to determine the extent to which proliferation contributes to the recruitment process.

Methods

Mice were subjected to recruitment before analysing the renin cells’ cell cycle. For acute threats, we subjected SV129 and C57Bl6 mice to a low sodium diet plus captopril. Tissue sections from treated mice were co‐stained for proliferation markers (Ki67, PCNA, pH3 and BrdU) and renin. Chronic recruitment was studied in deletion models of aldosterone synthase and angiotensinogen through co‐immunostaining and counting mitotic figures in periodic acid‐Schiff‐stained sections. Finally, RNA‐seq of renin cells isolated from recruited mice was performed to study mitotic signature.

Results

Mice subjected to low salt and captopril displayed increases in renin cell number (312 ± 40 in controls to 692 ± 85 in recruited animals, P<.0001), 10‐fold increases in renin mRNA and fourfold increases in circulating renin. Co‐staining these kidney sections for proliferation markers revealed negligible proliferation of renin cells (<2%), indistinguishable from control animals. Similarly, chronic models of recruitment—aldosterone synthase KO and angiotensinogen KO—had negligible proliferation. Additionally, the transcriptome of recruited renin cells revealed overall downregulation of mitotic pathways when compared to proliferative cell lines.

Conclusion

Acute and chronic physiological threats to homeostasis produced a distinct increase in renin‐synthesizing cells, but we found no evidence to suggest the involvement of proliferation.

Mesenchymal stem cells in obesity: insights for translational applications

Obesity is now a major public health problem worldwide. Lifestyle modification to reduce the characteristic excess body adiposity is important in the treatment of obesity, but effective therapeutic intervention is still needed to control what has become an obesity epidemic. Unfortunately, many anti-obesity drugs have been withdrawn from market due to adverse side effects. Bariatric surgery therefore remains the most effective therapy for severe cases, although such surgery is invasive and researchers continue to seek new control strategies for obesity. Mesenchymal stem cells (MSCs) are a major source of adipocyte generation, and studies have been conducted into the potential roles of MSCs in treating obesity. However, despite significant progress in stem cell research and its potential applications for obesity, adipogenesis is a highly complex process and the molecular mechanisms governing MSC adipogenesis remain ill defined. In particular, successful clinical application of MSCs will require extensive identification and characterization of the transcriptional regulators controlling MSC adipogenesis. Since obesity is associated with the incidence of multiple important comorbidities, an in-depth understanding of the relationship between MSC adipogenesis and the comorbidities of obesity is also necessary to evaluate the potential of effective and safe MSC-based therapies for obesity. In addition, brown adipogenesis is an attractive topic from the viewpoint of therapeutic innovation and future research into MSC-based brown adipogenesis could lead to a novel breakthrough. Ongoing stem cell studies and emerging research fields such as epigenetics are expected to elucidate the complicated mechanisms at play in MSC adipogenesis and develop novel MSC-based therapeutic options for obesity. This review discusses the current understanding of MSCs in adipogenesis and their potential clinical applications for obesity.

Juxtaglomerular Cell Phenotypic Plasticity

Renin is the first and rate-limiting step of the renin-angiotensin system. The exclusive source of renin in the circulation are the juxtaglomerular cells of the kidney, which line the afferent arterioles at the entrance of the glomeruli. Normally, renin production by these cells suffices to maintain homeostasis. However, under chronic stimulation of renin release, for instance during a low-salt diet or antihypertensive therapy, cells that previously expressed renin during congenital life re-convert to a renin-producing cell phenotype, a phenomenon which is known as “recruitment”. How exactly such differentiation occurs remains to be clarified. This review critically discusses the phenotypic plasticity of renin cells, connecting them not only to the classical concept of blood pressure regulation, but also to more complex contexts such as development and growth processes, cell repair mechanisms and tissue regeneration.

Mesenchymal Stem Cells and Metabolic Syndrome: Current Understanding and Potential Clinical Implications

Metabolic syndrome is an obesity-based, complicated clinical condition that has become a global epidemic problem with a high associated risk for cardiovascular disease and mortality. Dyslipidemia, hypertension, and diabetes or glucose dysmetabolism are the major factors constituting metabolic syndrome, and these factors are interrelated and share underlying pathophysiological mechanisms. Severe obesity predisposes individuals to metabolic syndrome, and recent data suggest that mesenchymal stem cells (MSCs) contribute significantly to adipocyte generation by increasing the number of adipocytes. Accordingly, an increasing number of studies have examined the potential roles of MSCs in managing obesity and metabolic syndrome. However, despite the growing bank of experimental and clinical data, the efficacy and the safety of MSCs in the clinical setting are still to be optimized. It is thus hoped that ongoing and future studies can elucidate the roles of MSCs in metabolic syndrome and lead to MSC-based therapeutic options for affected patients. This review discusses current understanding of the relationship between MSCs and metabolic syndrome and its potential implications for patient management.

Comparison of risk factors for acute worsening renal function in heart failure patients with and without preserved ejection fraction

OBJECTIVE

We compared the risk factors for acute worsening renal function (AWRF) in patients with acute decompensated heart failure with preserved ejection fraction (HFpEF) versus those with reduced ejection fraction (HFrEF).

METHODS

We retrospectively studied 181 consecutive patients. AWRF was defined as a rise in serum creatinine of ≥0.3 mg/dL from admission to day 3. Potential risk factors of AWRF were identified in univariate analyses; then logistic regression analysis with backward stepwise selection was performed.

RESULTS

In the present study of limited sample size, 46% had HFpEF (EF≥50%) and 54% had HFrEF (EF<50%). In the HFpEF group, history of hypertension (odds ratio [OR] 32.46, 95% CI 2.39-440.12, P=0.009), the increased serum potassium value at admission (OR 4.61, 95% CI 1.14-18.73, P=0.032), and the pretreatment with calcium channel blocker (OR 8.52, 95% CI 1.21-60.09, P=0.032) were independent risk factors (defined as P<0.05 and OR>1.01) for AWRF. In contrast, diastolic blood pressure at admission (OR 1.07, 95% CI 1.02-1.13, P=0.004) was the sole independent risk factor for AWRF in the HFrEF group.

CONCLUSIONS

Hypertension was associated with AWRF in both HFpEF and HFrEF patients. A history of hypertension was more important than elevated blood pressure at admission as a risk factor for AWRF in HFpEF, whereas the reverse was observed for HFrEF. Among antihypertensive drugs, pretreatment with calcium channel blocker was an independent risk factor for AWRF in HFpEF, but not in HFrEF.

Classical Renin-Angiotensin system in kidney physiology

The renin-angiotensin system has powerful effects in control of the blood pressure and sodium homeostasis. These actions are coordinated through integrated actions in the kidney, cardiovascular system and the central nervous system. Along with its impact on blood pressure, the renin-angiotensin system also influences a range of processes from inflammation and immune responses to longevity. Here, we review the actions of the "classical" renin-angiotensin system, whereby the substrate protein angiotensinogen is processed in a two-step reaction by renin and angiotensin converting enzyme, resulting in the sequential generation of angiotensin I and angiotensin II, the major biologically active renin-angiotensin system peptide, which exerts its actions via type 1 and type 2 angiotensin receptors. In recent years, several new enzymes, peptides, and receptors related to the renin-angiotensin system have been identified, manifesting a complexity that was previously unappreciated. While the functions of these alternative pathways will be reviewed elsewhere in this journal, our focus here is on the physiological role of components of the "classical" renin-angiotensin system, with an emphasis on new developments and modern concepts.

Adult renal mesenchymal stem cell-like cells contribute to juxtaglomerular cell recruitment

The renin-angiotensin-aldosterone system (RAAS) regulates BP and salt-volume homeostasis. Juxtaglomerular (JG) cells synthesize and release renin, which is the first and rate-limiting step in the RAAS. Intense pathologic stresses cause a dramatic increase in the number of renin-producing cells in the kidney, termed JG cell recruitment, but how this occurs is not fully understood. Here, we isolated renal CD44(+) mesenchymal stem cell (MSC)-like cells and found that they differentiated into JG-like renin-expressing cells both in vitro and in vivo. Sodium depletion and captopril led to activation and differentiation of these cells into renin-expressing cells in the adult kidney. In summary, CD44(+) MSC-like cells exist in the adult kidney and can differentiate into JG-like renin-producing cells under conditions that promote JG cell recruitment.

Mesenchymal stem cells differentiate into renin-producing juxtaglomerular (JG)-like cells under the control of liver X receptor-alpha

Renin is a key enzyme for cardiovascular and renal homeostasis and is produced by highly specialized endocrine cells in the kidney, known as juxtaglomerular (JG) cells. The nature and origin of these cells remain as mysteries. Previously, we have shown that the nuclear hormone receptor liver X receptor-alpha (LXRalpha) is a major transcriptional regulator of the expression of renin, c-myc, and other genes involved with growth/differentiation. In this study we test the hypothesis that LXRalpha plays an important role not only in renin expression but also in renin-containing cell differentiation, specifically from the mesenchymal stem cell (MSC), which may be the origin of the JG cell. Indeed, our data demonstrated that LXRalpha activation by its ligands or cAMP stimulated renin gene expression in both murine and human MSCs. Furthermore, sustained cAMP stimulation of murine MSCs overexpressing LXRalpha led to their differentiation into JG-like cells expressing renin and alpha-smooth muscle actin. These MSC-derived JG-like cells contained renin in secretory granules and released active renin in response to cAMP. In conclusion, the activation of LXRalpha stimulates renin expression and induces MSCs differentiation into renin-secreting, JG-like cells. Our results suggest that the MSC may be the origin of the juxtaglomerular cell and provide insight into novel understanding of pathophysiology of the renin-angiotensin system.