Macula densa sensing and signaling mechanisms of renin release

Inhibition of circulating dipeptidyl-peptidase 3 by procizumab in experimental septic shock reduces catecholamine exposure and myocardial injury

BACKGROUND

Dipeptidyl peptidase 3 (DPP3) is a ubiquitous cytosolic enzyme released into the bloodstream after tissue injury, that can degrade angiotensin II. High concentrations of circulating DPP3 (cDPP3) have been associated with worse outcomes during sepsis. The aim of this study was to assess the effect of Procizumab (PCZ), a monoclonal antibody that neutralizes cDPP3, in an experimental model of septic shock.

METHODS

In this randomized, open-label, controlled study, 16 anesthetized and mechanically ventilated pigs with peritonitis were randomized to receive PCZ or standard treatment when the mean arterial pressure (MAP) dropped below 50 mmHg. Resuscitation with fluids, antimicrobial therapy, peritoneal lavage, and norepinephrine was initiated one hour later to maintain MAP between 65-75 mmHg for 12 h. Hemodynamic variables, tissue oxygenation indices, and measures of organ failure and myocardial injury were collected. Organ blood flow was assessed using isotopic assessment (99mtechnetium albumin). cDPP3 activity, equilibrium analysis of the renin-angiotensin system and circulating catecholamines were measured. Tissue mRNA expression of interleukin-6 and downregulation of adrenergic and angiotensin receptors were assessed on vascular and myocardial samples.

RESULTS

PCZ-treated animals had reduced cDPP3 levels and required less norepinephrine and fluid than septic control animals for similar organ perfusion and regional blood flow. PCZ-treated animals had less myocardial injury, and higher PaO2/FiO2 ratios. PCZ was associated with lower circulating catecholamine levels; higher circulating angiotensin II and higher angiotensin II receptor type 1 myocardial protein expression, and with lower myocardial and radial artery mRNA interleukin-6 expression.

CONCLUSIONS

In an experimental model of septic shock, PCZ administration was associated with reduced fluid and catecholamine requirements, less myocardial injury and cardiovascular inflammation, along with preserved angiotensin II signaling.

Excess renin is attributed to the combination of forward and backward failure in HFrEF

AIMS

Regulation of the renin-angiotensin system (RAS) in heart failure (HF) with reduced ejection fraction (HFrEF) still raises questions, as a large proportion of patients show normal renin levels despite manifest disease. Experimental venous congestion results in reduced renal perfusion pressure and stimulates renin secretion. We hypothesized that excess renin levels are mainly a result of right ventricular failure as a sequalae of left ventricular dysfunction. The study aimed to link right ventricular function (RVF) with renin levels and to investigate further contributors to excess RAS activation.

METHODS AND RESULTS

Three hundred thirty-two chronic HFrEF patients undergoing routine ambulatory care were consecutively enrolled in a prospective, registry-based, observational study. Laboratory parameters, including cardiac-specific markers renin, aldosterone, and N-terminal pro-brain natriuretic peptide (NT-proBNP), echocardiographic examination (n = 247), and right heart catheterization (n = 85), were documented. The relationship between renin and its respective parameters was analysed. Renin concentration was not associated with the New York Heart Association class or NT-proBNP. Systolic blood pressure, systemic vascular resistance, serum sodium, aldosterone, and lactate dehydrogenase were associated with increased renin levels (P < 0.035 for all). Renin levels similarly increased with worsening of RVF parameters such as fractional area change, tricuspid annular plane systolic excursion, tissue Doppler imaging, and inferior vena cava diameter (P < 0.011 for all), but not with pulmonary pressure. Excess renin levels were observed when worsening RVF was combined with reduced renal perfusion {625 μIU/mL [interquartile range (IQR): 182-1761] vs. 67 μIU/mL [IQR: 16-231], P < 0.001}, which was associated with worse survival.

CONCLUSIONS

While unrelated to classical indices of HF severity, circulating renin levels increase with the worsening of RVF, especially in the combined presence of forward and backward failure. This might explain normal renin levels in HFrEF patients but also excess renin levels in poor haemodynamic conditions.

Molecular Characterization of Juxtaglomerular Cell Tumors: Evidence of Alterations in MAPK-RAS Pathway

Juxtaglomerular cell tumor (JGCT) is a rare neoplasm, part of the family of mesenchymal tumors of the kidney. Although the pathophysiological and clinical correlates of JGCT are well known, as these tumors are an important cause of early-onset arterial hypertension refractory to medical treatment, their molecular background is unknown, with only few small studies investigating their karyotype. Herein we describe a multi-institutional cohort of JGCTs diagnosed by experienced genitourinary pathologists, evaluating clinical presentation and outcome, morphologic diversity, and, importantly, the molecular features. Ten JGCTs were collected from 9 institutions, studied by immunohistochemistry, and submitted to whole exome sequencing. Our findings highlight the morphologic heterogeneity of JGCT, which can mimic several kidney tumor entities. Three cases showed concerning histologic features, but the patient course was unremarkable, which suggests that morphologic evaluation alone cannot reliably predict the clinical behavior. Gain-of-function variants in RAS GTPases were detected in JGCTs, with no evidence of additional recurrent genomic alterations. In conclusion, we present the largest series of JGCT characterized by whole exome sequencing, highlighting the putative role of the MAPK-RAS pathway.

Renin in critically ill patients

The renin-angiotensin system (RAS) constitutes one of the principal mechanisms to maintain hemodynamic and fluid homeostasis. However, most research until now on RAS primarily focuses on its relationship with hypertension and its role in critically ill hypotensive populations is not well understood. With the approval of angiotensin II (Ang II) in the United States and Europe, following a phase 3 randomized controlled trial showing efficacy in catecholamine-resistant vasodilatory shock, there is growing interest in RAS in critically ill patients. Among the fundamental components of RAS, renin acts as the initial stimulus for the entire system. In the context of hypotension, its release increases in response to low blood pressure sensed by renal baroreceptors and attenuated negative Ang II feedback loop. Thus, elevated renin could reflect disease severity and predict poor outcomes. Studies investigating this hypothesis have validated the prognostic accuracy of renin in various critically ill populations, with several reports indicating its superiority to lactate for mortality prediction. Accordingly, renin reduction has been used to assess the effectiveness of Ang II administration. Furthermore, renin holds potential to identify patients who might benefit from Ang II treatment, potentially paving the way for personalized vasopressor management. Despite these promising data, most available evidence is derived from retrospective analysis and necessitates prospective confirmation. The absence of a rapid, point-of-care and reliable renin assay presents another hurdle to its integration into routine clinical practice. This narrative review aims to describe the current understanding and future directions of renin as a biomarker during resuscitation of critically ill patients.

Sodium-glucose cotransporter 2 inhibitors: are they ready for prime time in the management of lupus nephritis?

PURPOSE OF REVIEW

Lupus nephritis is a common complication of systemic lupus erythematosus and is associated with significant morbidity and mortality. The utility of sodium-glucose cotransporter 2 (SGLT2) inhibitors in the management of lupus nephritis is currently uncertain. Here, we summarize the rationale for their use among patient with lupus nephritis.

RECENT FINDINGS

SGLT2 inhibitors were initially developed as antihyperglycemic agents. They have since been shown to have additional, profound effects to slow the progression of chronic kidney disease and lessen the long-term risks of cardiovascular disease in large clinic trials of patients with chronic kidney disease, with and without diabetes, as well as in patients with and without proteinuria. Patients with recent exposure to immunosuppression were excluded from these trials due to concern for risk of infection. In the few, small trials of patients with lupus nephritis, SGLT2 inhibitors were found to be well tolerated. They have been shown to reduce proteinuria and to have modest beneficial effects on blood pressure and BMI among patients with lupus nephritis. They have not been shown to influence disease activity.

SUMMARY

SGLT2 inhibitors may have a role in mitigating the chronic renal and cardiovascular effects of lupus nephritis. They should be introduced after kidney function has been stabilized with appropriate immunosuppression, in conjunction with angiotensin-converting enzyme inhibitors or angiotensin receptor blockers. They currently have no role in active disease.

Neuronally differentiated macula densa cells regulate tissue remodeling and regeneration in the kidney

Tissue regeneration is limited in several organs, including the kidney, contributing to the high prevalence of kidney disease globally. However, evolutionary and physiological adaptive responses and the presence of renal progenitor cells suggest an existing remodeling capacity. This study uncovered endogenous tissue remodeling mechanisms in the kidney that were activated by the loss of body fluid and salt and regulated by a unique niche of a minority renal cell type called the macula densa (MD). Here, we identified neuronal differentiation features of MD cells that sense the local and systemic environment and secrete angiogenic, growth, and extracellular matrix remodeling factors, cytokines and chemokines, and control resident progenitor cells. Serial intravital imaging, MD nerve growth factor receptor and Wnt mouse models, and transcriptome analysis revealed cellular and molecular mechanisms of these MD functions. Human and therapeutic translation studies illustrated the clinical potential of MD factors, including CCN1, as a urinary biomarker and therapeutic target in chronic kidney disease. The concept that a neuronally differentiated key sensory and regulatory cell type responding to organ-specific physiological inputs controls local progenitors to remodel or repair tissues may be applicable to other organs and diverse tissue-regenerative therapeutic strategies.

Renin Levels and Angiotensin II Responsiveness in Vasopressor-Dependent Hypotension

OBJECTIVES

The relationship between renin levels, exposure to renin-angiotensin system (RAS) inhibitors, angiotensin II (ANGII) responsiveness, and outcome in patients with vasopressor-dependent vasodilatory hypotension is unknown.

DESIGN

We conducted a single-center prospective observational study to explore whether recent RAS inhibitor exposure affected baseline renin levels, whether baseline renin levels predicted ANGII responsiveness, and whether renin levels at 24 hours were associated with clinical outcomes.

SETTING

An academic ICU in Melbourne, VIC, Australia.

PATIENTS

Forty critically ill adults who received ANGII as the primary agent for vasopressor-dependent vasodilatory hypotension who were included in the Acute Renal effects of Angiotensin II Management in Shock study.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

After multivariable adjustment, recent exposure to a RAS inhibitor was independently associated with a relative increase in baseline renin levels by 198% (95% CI, 36-552%). The peak amount of ANGII required to achieve target mean arterial pressure was independently associated with baseline renin level (increase by 46% per ten-fold increase; 95% CI, 8-98%). Higher renin levels at 24 hours after ANGII initiation were independently associated with fewer days alive and free of continuous renal replacement therapy (CRRT) (-7 d per ten-fold increase; 95% CI, -12 to -1).

CONCLUSIONS

In patients with vasopressor-dependent vasodilatory hypotension, recent RAS inhibitor exposure was associated with higher baseline renin levels. Such higher renin levels were then associated with decreased ANGII responsiveness. Higher renin levels at 24 hours despite ANGII infusion were associated with fewer days alive and CRRT-free. These preliminary findings emphasize the importance of the RAS and the role of renin as a biomarker in patients with vasopressor-dependent vasodilatory hypotension.

Active plasma renin concentration throughout healthy and complicated pregnancy: a systematic review and meta-analysis

BACKGROUND

Pregnancy is characterized by profound circulatory changes and compensatory adjustments in the renin-angiotensin-aldosterone system (RAAS). Differences in regulatory response may antedate or accompany vascular complicated pregnancy. We performed a systematic review and meta-analysis to delineate the trajectory of active plasma renin concentration (APRC) in healthy pregnancy and compare this to complicated pregnancy.

METHODS

We performed a systematic review and meta-analysis on APRC during normotensive and hypertensive pregnancies, using PubMed (NCBI) and Embase (Ovid) databases. We included only studies reporting measurements during pregnancy together with a nonpregnant reference group measurement. Risk of bias was assessed with QUIPS. Ratio of the mean (ROM) and 95% confidence intervals (CI) of APRC values between pregnant and nonpregnant women were estimated for predefined intervals of gestational age using a random-effects model. Meta-regression was used to analyze APRC over time.

RESULTS

In total, we included 18 studies. As compared to nonpregnant, APRC significantly increased as early as the first weeks of healthy pregnancy and stayed increased throughout the whole pregnancy (ROM 2.77; 95% CI 2.26-3.39). APRC in hypertensive complicated pregnancy was not significantly different from nonpregnancy (ROM 1.32; 95% CI 0.97-1.80).

CONCLUSION

Healthy pregnancy is accompanied by a profound rise in APRC in the first trimester that is maintained until term. In hypertensive complicated pregnancy, this increase in APRC is not observed.

Physiological Mechanisms of Dietary Salt Sensing in the Brain, Kidney, and Gastrointestinal Tract

Excess dietary salt (NaCl) intake is strongly correlated with cardiovascular disease and is a major contributing factor to the pathogenesis of hypertension. NaCl-sensitive hypertension is a multisystem disorder that involves renal dysfunction, vascular abnormalities, and neurogenically-mediated increases in peripheral resistance. Despite a major research focus on organ systems and these effector mechanisms causing NaCl-induced increases in arterial blood pressure, relatively less research has been directed at elucidating how NaCl is sensed by various tissues to elicit these downstream effects. The purpose of this review is to discuss how the brain, kidney, and gastrointestinal tract sense NaCl including key cell types, the role of NaCl versus osmolality, and the underlying molecular and electrochemical mechanisms.

SGLT2 inhibition promotes glomerular repopulation by cells of renin lineage in experimental kidney disease

AIM

Sodium glucose co-transporter-2 (SGLT2) inhibitors stimulate renal excretion of sodium and glucose and exert renal protective effects in patients with (non-)diabetic chronic kidney disease (CKD) and may as well protect against acute kidney injury (AKI). The mechanism behind this kidney protective effect remains unclear. Juxtaglomerular cells of renin lineage (CoRL) have been demonstrated to function as progenitors for multiple adult glomerular cell types in kidney disease. This study assesses the impact of SGLT2 inhibition on the repopulation of glomerular cells by CoRL and examines their phenotypic commitment.

METHODS

Experiments were performed in Ren1cre-tdTomato lineage-trace mice. Either 5/6 nephrectomy (5/6NX) modeling CKD or bilateral ischaemia reperfusion injury (bIRI) mimicking AKI was applied, while the SGLT2 inhibitor empagliflozin (10 mg/kg) was administered daily via oral gavage for 14 days.

RESULTS

Both 5/6NX and bIRI-induced kidney injury increased the number of glomerular CoRL-derived cells. SGLT2 inhibition improved kidney function after 5/6NX, indicated by decreased blood creatinine and urea levels, but not after bIRI. In line with this, empagliflozin in 5/6NX animals resulted in less glomerulosclerosis, while it did not affect histopathological features in bIRI. Treatment with empagliflozin resulted in an increase in the number of CoRL-derived glomerular cells in both 5/6NX and bIRI conditions. Interestingly, SGLT2 inhibition led to more CoRL-derived podocytes in 5/6NX animals, whereas empagliflozin-treated bIRI mice presented with increased levels of parietal epithelial and mesangial cells derived from CoRL.

CONCLUSION

We conclude that SGLT2 inhibition by empagliflozin promotes CoRL-mediated glomerular repopulation with selective CoRL-derived cell types depending on the type of experimental kidney injury. These findings suggest a previously unidentified mechanism that could contribute to the renoprotective effect of SGLT2 inhibitors.

Transient inhibition of sodium-glucose cotransporter 2 after ischemia/reperfusion injury ameliorates chronic kidney disease

Sodium-glucose cotransporter 2 (SGLT2) inhibitor, dapagliflozin (Dapa), exhibited nephroprotective effects in patients with chronic kidney disease (CKD). We assessed the efficacy of short-term Dapa administration following acute kidney injury (AKI) in preventing CKD. Male Wistar rats were randomly assigned to Sham surgery, bilateral ischemia for 30 minutes (abbreviated as IR), and IR + Dapa groups. Daily treatment with Dapa was initiated just 24 hours after IR and maintained for only 10 days. Initially, rats were euthanized at this point to study early renal repair. After severe AKI, Dapa promptly restored creatinine clearance (CrCl) and significantly reduced renal vascular resistance compared with the IR group. Furthermore, Dapa effectively reversed the mitochondrial abnormalities, including increased fission, altered mitophagy, metabolic dysfunction, and proapoptotic signaling. To study this earlier, another set of rats was studied just 5 days after AKI. Despite persistent renal dysfunction, our data reveal a degree of mitochondrial protection. Remarkably, a 10-day treatment with Dapa demonstrated effectiveness in preventing CKD transition in an independent cohort monitored for 5 months after AKI. This was evidenced by improvements in proteinuria, CrCl, glomerulosclerosis, and fibrosis. Our findings underscore the potential of Dapa in preventing maladaptive repair following AKI, emphasizing the crucial role of early intervention in mitigating AKI long-term consequences.

Hyporeninemic hypoaldosteronism in RMND1-related mitochondrial disease

BACKGROUND

RMND1 is a nuclear gene needed for proper function of mitochondria. A pathogenic gene will cause multiple oxidative phosphorylation defects. A renal phenotype consisting of hyponatremia, hyperkalemia, and acidosis is frequently reported, previously considered to be due to aldosterone insensitivity.

METHODS

Clinical features and pathophysiology of three patients will be reported. DNA of these patients was subjected to exome screening.

RESULTS

In the first family, one pathogenic heterozygous and one highly probable heterozygous mutation were detected. In the second family, a homozygous pathogenic mutation was present. The electrolyte disbalance was not due to aldosterone insensitivity but to low plasma aldosterone concentration, a consequence of low plasma renin activity. This disbalance can be treated. In all three patients, the kidney function declined. In the first family, both children suffered from an unexplained arterial thrombosis with dire consequences.

CONCLUSIONS

Hyporeninemic hypoaldosteronism is the mechanism causing the electrolyte disbalance reported in patients with RMND1 mutations, and can be treated.

The Functional Role of microRNAs and mRNAs in Diabetic Kidney Disease: A Review

Diabetes is a group of diseases marked by poor control of blood glucose levels. Diabetes mellitus (DM) occurs when pancreatic cells fail to make insulin, which is required to keep blood glucose levels stable, disorders, and so on. High glucose levels in the blood induce diabetic effects, which can cause catastrophic damage to bodily organs such as the eyes and lower extremities. Diabetes is classified into many forms, one of which is controlled by hyperglycemia or Diabetic Kidney Disease (DKD), and another that is not controlled by hyperglycemia (nondiabetic kidney disease or NDKD) and is caused by other factors such as hypertension, hereditary. DKD is associated with diabetic nephropathy (DN), a leading cause of chronic kidney disease (CKD) and end-stage renal failure. The disease is characterized by glomerular basement membrane thickening, glomerular sclerosis, and mesangial expansion, resulting in a progressive decrease in glomerular filtration rate, glomerular hypertension, and renal failure or nephrotic syndrome. It is also represented by some microvascular complications such as nerve ischemia produced by intracellular metabolic changes, microvascular illness, and the direct impact of excessive blood glucose on neuronal activity. Therefore, DKD-induced nephrotic failure is worse than NDKD. MicroRNAs (miRNAs) are important in the development and progression of several diseases, including diabetic kidney disease (DKD). These dysregulated miRNAs can impact various cellular processes, including inflammation, fibrosis, oxidative stress, and apoptosis, all of which are implicated during DKD. MiRNAs can alter the course of DKD by targeting several essential mechanisms. Understanding the miRNAs implicated in DKD and their involvement in disease development might lead to identifying possible therapeutic targets for DKD prevention and therapy. Therefore, this review focuses specifically on DKD-associated DN, as well as how in-silico approaches may aid in improving the management of the disease.

The role of renin-angiotensin system in sepsis-associated acute kidney injury: mechanisms and therapeutic implications

PURPOSE OF REVIEW

This review aims to explore the relationship between the renin angiotensin system (RAS) and sepsis-associated acute kidney injury (SA-AKI), a common complication in critically ill patients associated with mortality, morbidity, and long-term cardiovascular complications. Additionally, this review aims to identify potential therapeutic approaches to intervene with the RAS and prevent the development of AKI.

RECENT FINDINGS

Recent studies have provided increasing evidence of RAS alteration during sepsis, with systemic and local RAS disturbance, which can contribute to SA-AKI. Angiotensin II was recently approved for catecholamine resistant vasodilatory shock and has been associated with improved outcomes in selected patients.

SUMMARY

SA-AKI is a common condition that can involve disturbances in the RAS, particularly the canonical angiotensin-converting enzyme (ACE) angiotensin-II (Ang II)/angiotensin II receptor 1 (AT-1R) axis. Increased renin levels, a key enzyme in the RAS, have been shown to be associated with AKI and may also guide vasopressor therapy in shock. In patients with high renin levels, angiotensin II administration may reduce renin concentration, improve intra-renal hemodynamics, and enhance signaling through the angiotensin II receptor 1. Further studies are needed to explore the role of the RAS in SA-AKI and the potential for targeted therapies.

The alternative renin-angiotensin system in critically ill patients: pathophysiology and therapeutic implications

The renin-angiotensin system (RAS) plays a crucial role in regulating blood pressure and the cardio-renal system. The classical RAS, mainly mediated by angiotensin I, angiotensin-converting enzyme, and angiotensin II, has been reported to be altered in critically ill patients, such as those in vasodilatory shock. However, recent research has highlighted the role of some components of the counterregulatory axis of the classical RAS, termed the alternative RAS, such as angiotensin-converting Enzyme 2 (ACE2) and angiotensin-(1-7), or peptidases which can modulate the RAS like dipeptidyl-peptidase 3, in many critical situations. In cases of shock, dipeptidyl-peptidase 3, an enzyme involved in the degradation of angiotensin and opioid peptides, has been associated with acute kidney injury and mortality and preclinical studies have tested its neutralization. Angiotensin-(1-7) has been shown to prevent septic shock development and improve outcomes in experimental models of sepsis. In the context of experimental acute lung injury, ACE2 activity has demonstrated a protective role, and its inactivation has been associated with worsened lung function, leading to the use of active recombinant human ACE2, in preclinical and human studies. Angiotensin-(1-7) has been tested in experimental models of acute lung injury and in a recent randomized controlled trial for patients with COVID-19 related hypoxemia. Overall, the alternative RAS appears to have a role in the pathogenesis of disease in critically ill patients, and modulation of the alternative RAS may improve outcomes. Here, we review the available evidence regarding the methods of analysis of the RAS, pathophysiological disturbances of this system, and discuss how therapeutic manipulation may improve outcomes in the critically ill.

Physiological role of hydrogen sulfide in the kidney and its therapeutic implications for kidney diseases

For over three centuries, hydrogen sulfide (H2S) has been known as a toxic and deadly gas at high concentrations, with a distinctive smell of rotten eggs. However, studies over the past two decades have shown that H2S has risen above its historically notorious label and has now received significant scientific attention as an endogenously produced gaseous signaling molecule that participates in cellular homeostasis and influences a myriad of physiological and pathological processes at low concentrations. Its endogenous production is enzymatically regulated, and when dysregulated, contributes to pathogenesis of renal diseases. In addition, exogenous H2S administration has been reported to exhibit important therapeutic characteristics that target multiple molecular pathways in common renal pathologies in which reduced levels of renal and plasma H2S were observed. This review highlights functional anatomy of the kidney and renal production of H2S. The review also discusses current understanding of H2S in renal physiology and seeks to lay the foundation as a new targeted therapeutic agent for renal pathologies such as hypertensive nephropathy, diabetic kidney disease and water balance disorders.

Renal Oxygen Demand and Nephron Function: Is Glucose a Friend or Foe?

The kidneys and heart work together to balance the body's circulation, and although their physiology is based on strict inter dependence, their performance fulfills different aims. While the heart can rapidly increase its own oxygen consumption to comply with the wide changes in metabolic demand linked to body function, the kidneys physiology are primarily designed to maintain a stable metabolic rate and have a limited capacity to cope with any steep increase in renal metabolism. In the kidneys, glomerular population filters a large amount of blood and the tubular system has been programmed to reabsorb 99% of filtrate by reabsorbing sodium together with other filtered substances, including all glucose molecules. Glucose reabsorption involves the sodium-glucose cotransporters SGLT2 and SGLT1 on the apical membrane in the proximal tubular section; it also enhances bicarbonate formation so as to preserve the acid-base balance. The complex work of reabsorption in the kidney is the main factor in renal oxygen consumption; analysis of the renal glucose transport in disease states provides a better understanding of the renal physiology changes that occur when clinical conditions alter the neurohormonal response leading to an increase in glomerular filtration pressure. In this circumstance, glomerular hyperfiltration occurs, imposing a higher metabolic demand on kidney physiology and causing progressive renal impairment. Albumin urination is the warning signal of renal engagement over exertion and most frequently heralds heart failure development, regardless of disease etiology. The review analyzes the mechanisms linked to renal oxygen consumption, focusing on sodium-glucose management.

ZO-1 expression in normal human macula densa: Immunohistochemical and immunofluorescence investigations

The macula densa (MD) is an anatomical structure having a plaque shape, placed in the distal end of thick ascending limb of each nephron and belonging to juxtaglomerular apparatus (JGA). The aim of the present investigation is to investigate the presence of ZO-1, a specific marker of tight juncions (TJs), in MD cells. Six samples of normal human renal tissue were embedded in paraffin for ZO-1 expression analysis by immunohistochemical and immunofluorescence techniques. We detected ZO-1 expression in the apical part of cell membrane in MD cells by immunohistochemistry. In addition, ZO-1 and nNOS expressions (a specific marker of MD) were colocalized in MD cells providing clear evidence of TJs presence in normal human MD. Since ZO-1 is responsible for diffusion barrier formation, its presence in the MD supports the existence of a tubulomesangial barrier that ensures a regulated exchange between MD and JGA effectors in renal and glomerular haemodynamic homeostasis.

Dietary sodium enhances the expression of SLC4 family transporters, IRBIT, L-IRBIT, and PP1 in rat kidney: Insights into the molecular mechanism for renal sodium handling

The kidney plays a central role in maintaining the fluid and electrolyte homeostasis in the body. Bicarbonate transporters NBCn1, NBCn2, and AE2 are expressed at the basolateral membrane of the medullary thick ascending limb (mTAL). In a previous study, NBCn1, NBCn2, and AE2 are proposed to play as a regulatory pathway to decrease NaCl reabsorption in the mTAL under high salt condition. When heterologously expressed, the activity of these transporters could be stimulated by the InsP3R binding protein released with inositol 1,4,5-trisphosphate (IRBIT), L-IRBIT (collectively the IRBITs), or protein phosphatase PP1. In the present study, we characterized by immunofluorescence the expression and localization of the IRBITs, and PP1 in rat kidney. Our data showed that the IRBITs were predominantly expressed from the mTAL through the distal renal tubules. PP1 was predominantly expressed in the TAL, but is also present in high abundance from the distal convoluted tubule through the medullary collecting duct. Western blotting analyses showed that the abundances of NBCn1, NBCn2, and AE2 as well as the IRBITs and PP1 were greatly upregulated in rat kidney by dietary sodium. Co-immunoprecipitation study provided the evidence for protein interaction between NBCn1 and L-IRBIT in rat kidney. Taken together, our data suggest that the IRBITs and PP1 play an important role in sodium handling in the kidney. We propose that the IRBITs and PP1 stimulates NBCn1, NBCn2, and AE2 in the basolateral mTAL to inhibit sodium reabsorption under high sodium condition. Our study provides important insights into understanding the molecular mechanism for the regulation of sodium homeostasis in the body.

Novel Concepts in Nephron Sodium Transport: A Physiological and Clinical Perspective

The kidneys play a critical role in maintaining total body sodium (Na⁺) balance across a wide range of dietary intake, accomplished by a concerted effort involving multiple Na⁺ transporters along the nephron. Furthermore, nephron Na⁺ reabsorption and urinary Na⁺ excretion are closely linked to renal blood flow and glomerular filtration such that perturbations in either of them can modify Na⁺ transport along the nephron, ultimately resulting in hypertension and other Na⁺-retentive states. In this article, we provide a brief physiological overview of nephron Na⁺ transport and illustrate clinical syndromes and therapeutic agents that affect Na⁺ transporter function. We highlight recent advances in kidney Na⁺ transport, particularly the role of immune cells, lymphatics, and interstitial Na⁺ in regulating Na⁺ reabsorption, the emergence of potassium (K⁺) as a regulator of Na⁺ transport, and the evolution of the nephron to modulate Na⁺ transport.

Etiology and Management of Edema: A Review

The development of peripheral edema can often pose a significant diagnostic and therapeutic challenge for practitioners due to its association with a wide variety of underlying disorders ranging in severity. Updates to the original Starling's principle have provided new mechanistic insights into edema formation. Additionally, contemporary data highlighting the role of hypochloremia in the development of diuretic resistance provide a possible new therapeutic target. This article reviews the pathophysiology of edema formation and discusses implications for treatment.

SGLT2 inhibitors in the treatment of type 2 cardiorenal syndrome: Focus on renal tubules

The pathogenesis of type 2 cardiorenal syndrome (CRS) is mostly associated with reduced cardiac output, increased central venous pressure (CVP), activation of the renin-angiotensin-aldosterone system (RAAS), inflammation, and oxidative stress. As a drug to treat diabetes, sodium-glucose transporter 2 inhibitor (SGLT2i) has been gradually found to have a protective effect on the heart and kidney and has a certain therapeutic effect on CRS. In the process of chronic heart failure (CHF) leading to chronic renal insufficiency, the renal tubular system, as the main functional part of the kidney, is the first to be damaged, but this damage can be reversed. In this review, we focus on the protective mechanisms of SGLT2i targeting renal tubular in the treatment of CRS, including natriuresis and diuresis to relieve renal congestion, attenuate renal tubular fibrosis, improve energy metabolism of renal tubular, and slow tubular inflammation and oxidative stress. This may have beneficial effects on the treatment of CRS and is a direction for future research.

Chloride and Cardiorenal Interactions in Heart Failure

The cardiorenal literature has long been dominated by a sodium-centric view. However, mechanisms affecting sodium homeostasis in patients with heart failure (HF) commonly lead to concurrent changes in the serum levels of chloride as well. There is a growing body of evidence on a strong link between low serum chloride levels and adverse outcomes in HF, which might be even more potent than that of sodium. Maladaptive neurohormonal activation and unresponsiveness to diuretics have been proposed as potential mechanisms to explain this phenomenon. In parallel with accumulating evidence on the predictive value of chloride in various HF populations, the limited available interventional studies that were aimed at increasing serum chloride levels have also shown promising results. Ongoing studies are designed to elucidate the role of chloride as a key cardiorenal connector and whether hypochloremia represents a modifiable risk factor (i.e., target of therapy) or a mere marker of disease severity and poor prognosis.

Emerging mechanisms involving brain Kv7 channel in the pathogenesis of hypertension

Hypertension is a prevalent health problem inducing many organ damages. The pathogenesis of hypertension involves a complex integration of different organ systems including the brain. The elevated sympathetic nerve activity is closely related to the etiology of hypertension. Ion channels are critical regulators of neuronal excitability. Several mechanisms have been proposed to contribute to hypothalamic-driven elevated sympathetic activity, including altered ion channel function. Recent findings indicate one of the voltage-gated potassium channels, Kv7 channels (M channels), plays a vital role in regulating cardiovascular-related neurons activity, and the expression of Kv7 channels is downregulated in hypertension. This review highlights recent findings that the Kv7 channels in the brain, blood vessels, and kidneys are emerging targets involved in the pathogenesis of hypertension, suggesting new therapeutic targets for treating drug-resistant, neurogenic hypertension.

Principles of human and mouse nephron development

The mechanisms underlying kidney development in mice and humans is an area of intense study. Insights into kidney organogenesis have the potential to guide our understanding of the origin of congenital anomalies and enable the assembly of genetic diagnostic tools. A number of studies have delineated signalling nodes that regulate positional identities and cell fates of nephron progenitor and precursor cells, whereas cross-species comparisons have markedly enhanced our understanding of conserved and divergent features of mammalian kidney organogenesis. Greater insights into the complex cellular movements that occur as the proximal-distal axis is established have challenged our understanding of nephron patterning and provided important clues to the elaborate developmental context in which human kidney diseases can arise. Studies of kidney development in vivo have also facilitated efforts to recapitulate nephrogenesis in kidney organoids in vitro, by providing a detailed blueprint of signalling events, cell movements and patterning mechanisms that are required for the formation of correctly patterned nephrons and maturation of physiologically functional apparatus that are responsible for maintaining human health.

Scattered Tubular Cells Markers in Macula Densa of Normal Human Adult Kidney

Background: The scattered tubular cells (STCs) are a population of resident progenitor tubular cells with expansion, self-renewal and epithelial differentiation abilities. Although these cells are localized within the proximal (PTs) and distal (DTs) tubules in a normal adult kidney, their presence has never been demonstrated in human macula densa (MD). The purpose of the present study is to describe the presence of STCs in MD using specific markers such as prominin-1 (CD133), cytokeratin 7 (KRT7) and vimentin (VIM). Methods: We analyzed two sets of three consecutive serial sections for each sample. The first sections of each set were immunostained for nNOS to identify MD, the second sections were immune-stained for CD133 (specific STCs marker) while the third sections were analyzed for KRT7 (another STCs specific marker) and VIM (that stains the basal pole of the STCs) in the first and second sets, respectively, in order to study the co-expression of KRT7 and VIM with the CD133 marker. Results: CD133 was localized in some MD cells and in the adjacent DT cells. Moreover, CD133 was detected in the parietal epithelial cells of Bowman’s capsule and in some proximal tubules (PT). KRT7-positive cells were identified in MD and adjacent DT cells, while KRT7 positivity was mostly confined in both DT and collecting ducts (CD) in the other areas of the renal parenchyma. CD133 and KRT7 were co-expressed in some MD and adjacent DT cells. Some of the latter cells were positive both for CD133 and VIM. CD133 was always localized in the apical part of the cells, whereas the VIM expression was evident only in the cellular basal pole. Although some cells of MD expressed VIM or CD133, none of them co-expressed VIM and CD133. Conclusions: The presence of STCs was demonstrated in human adult MD, suggesting that this structure has expansion, self-renewal and epithelial differentiation abilities, similar to all other parts of renal tubules.

The Proximal Tubule as the Pathogenic and Therapeutic Target in Acute Kidney Injury

BACKGROUND

In 2004, the term acute kidney injury (AKI) was introduced with the intention of broadening our understanding of rapid declines in renal function and to replace the historical terms of acute renal failure and acute tubular necrosis (ATN). Despite this evolution in terminology, the mechanisms of AKI have stayed largely elusive with the pathophysiological concepts of ATN remaining the mainstay in our understanding of AKI.

SUMMARY

The proximal tubule (PT), having the highest mitochondrial content in the kidney and relying heavily on oxidative phosphorylation to generate ATP, is vulnerable to ischaemic insults and mitochondrial dysfunction. Histologically, pathological changes in the PT are more consistent than changes to the glomeruli or the loop of Henle in AKI. Physiologically, activation of tubuloglomerular feedback due to PT dysfunction leads to an increase in preglomerular afferent arteriole resistance and a reduction in glomerular filtration. Pharmacologically, frusemide - a drug commonly used in the setting of oliguric AKI - is actively secreted by the PT and its diuretic effect is compromised by its failure to be secreted into the urine and thus be delivered to its site of action at the loop of Henle in AKI. Increases in the urinary, but not plasma biomarkers, of PT injury within 1 h of shock suggest that the PT as the initiation pathogenic target of AKI.

KEY MESSAGE

Therapeutic agents targeting specifically the PT epithelial cells, in particular its mitochondria - including amino acid ergothioneine and superoxide scavenger MitoTEMPO - show great promises in ameliorating AKI.

Spatial and single-cell transcriptome analysis reveals changes in gene expression in response to drug perturbation in rat kidney

The kidney is a complex organ that consists of various types of cells. It is occasionally difficult to resolve molecular alterations and possible perturbations that the kidney experiences due to drug-induced damage. In this study, we performed spatial and single-cell transcriptome analysis of rat kidneys and constructed a precise rat renal cell atlas with spatial information. Using the constructed catalogue, we were able to characterize cells of several minor populations, such as macula densa or juxtaglomerular cells. Further inspection of the spatial gene expression data allowed us to identify the upregulation of genes involved in the renin regulating pathway in losartan-treated populations. Losartan is an angiotensin II receptor antagonist drug, and the observed upregulation of the renin pathway-related genes could be due to feedback from the hypotensive action of the drug. Furthermore, we found spatial heterogeneity in the response to losartan among the glomeruli. These results collectively indicate that integrated single-cell and spatial gene expression analysis is a powerful approach to reveal the detailed associations between the different cell types spanning the complicated renal compartments.

Anticipated pharmacological role of Aviptadil on COVID-19

Vasoactive intestinal peptide (VIP) is a neuropeptide that is produced by the lymphoid cells and plays a major role in immunological functions for controlling the homeostasis of the immune system. VIP has been identified as a potent anti-inflammatory factor, in boosting both innate and adaptive immunity. Since December 2019, SARS-Cov-2 was found responsible for the disease COVID-19 which has spread worldwide. No specific therapies or 100% effective vaccines are yet available for the treatment of COVID-19. Drug repositioning may offer a strategy and several drugs have been repurposed, including lopinavir/ritonavir, remdesivir, favipiravir, and tocilizumab. This paper describes the main pharmacological properties of synthetic VIP drug (Aviptadil) which is now under clinical trials. A patented formulation of vasoactive intestinal polypeptide (VIP), named RLF-100 (Aviptadil), was developed and finally got approved for human trials by FDA in 2001 and in European medicines agency in 2005. It was awarded Orphan Drug Designation in 2001 by the US FDA for the treatment of acute respiratory distress syndrome and for the treatment of pulmonary arterial hypertension in 2005. Investigational new drug (IND) licenses for human trials of Aviptadil was guaranteed by both the US FDA and EMEA. Preliminary clinical trials seem to support Aviptadil's benefit. However, such drugs like Aviptadil in COVID-19 patients have peculiar safety profiles. Thus, adequate clinical trials are necessary for these compounds.

Aldosterone-Regulated Sodium Transport and Blood Pressure

Aldosterone is a major mineralocorticoid steroid hormone secreted by glomerulosa cells in the adrenal cortex. It regulates a variety of physiological responses including those to oxidative stress, inflammation, fluid disruption, and abnormal blood pressure through its actions on various tissues including the kidney, heart, and the central nervous system. Aldosterone synthesis is primarily regulated by angiotensin II, K⁺ concentration, and adrenocorticotrophic hormone. Elevated serum aldosterone levels increase blood pressure largely by increasing Na⁺ re-absorption in the kidney through regulating transcription and activity of the epithelial sodium channel (ENaC). This review focuses on the signaling pathways involved in aldosterone synthesis and its effects on Na⁺ reabsorption through ENaC.

A new view of macula densa cell protein synthesis

Macula densa (MD) cells, a chief sensory cell type in the nephron, are endowed with unique microanatomic features including a high density of protein synthetic organelles and secretory vesicles in basal cell processes ("maculapodia") that suggest a so far unknown high rate of MD protein synthesis. This study aimed to explore the rate and regulation of MD protein synthesis and their effects on glomerular function using novel transgenic mouse models, newly established fluorescence cell biology techniques, and intravital microscopy. Sox2-tdTomato kidney tissue sections and an O-propargyl puromycin incorporation-based fluorescence imaging assay showed that MD cells have the highest level of protein synthesis within the kidney cortex followed by intercalated cells and podocytes. Genetic gain of function of mammalian target of rapamycin (mTOR) signaling specifically in MD cells (in MD-mTORgof mice) or their physiological activation by low-salt diet resulted in further significant increases in the synthesis of MD proteins. Specifically, these included both classic and recently identified MD-specific proteins such as cyclooxygenase 2, microsomal prostaglandin E2 synthase 1, and pappalysin 2. Intravital imaging of the kidney using multiphoton microscopy showed significant increases in afferent and efferent arteriole and glomerular capillary diameters and blood flow in MD-mTORgof mice coupled with an elevated glomerular filtration rate. The presently identified high rate of MD protein synthesis that is regulated by mTOR signaling is a novel component of the physiological activation and glomerular hemodynamic regulatory functions of MD cells that remains to be fully characterized.NEW & NOTEWORTHY This study discovered the high rate of protein synthesis in macula densa (MD) cells by applying direct imaging techniques with single cell resolution. Physiological activation and mammalian target of rapamycin signaling played important regulatory roles in this process. This new feature is a novel component of the tubuloglomerular cross talk and glomerular hemodynamic regulatory functions of MD cells. Future work is needed to elucidate the nature and (patho)physiological role of the specific proteins synthesized by MD cells.

Morphology and cyclooxygenase-2 and renin expression in the kidney of young spontaneously hypertensive rats

The kidneys play an important role in blood pressure regulation under normal and pathological conditions. We examined the histological changes and expression patterns of cyclooxygenase-2, renin, and (pro)renin receptor (PRR) in the renal cortex of prehypertensive spontaneously hypertensive rats (SHRs) and Wistar Kyoto rats (WKYs). Moreover, blood pressure and plasma urea, creatinine, angiotensin II, and angiotensin (1-7) levels were measured. The results showed that both strains had similar blood pressure and plasma urea and creatinine levels. The glomerular area, basement membrane thickness, collagen fiber content, and arterial wall thickness were greater in SHRs than in WKYs. By immunohistochemistry, cyclooxygenase-2 was localized in the macula densa and renal tubules of both strains. In SHRs, cyclooxygenase-2 was detected in a larger number of tubules, and the cortical expression of cyclooxygenase-2 was also increased. In both strains, PRR and renin were localized in the tubular epithelium and juxtaglomerular cells, respectively. In SHRs, PRR immunolocalization was increased in the glomerulus. The cortical expression of immature renin was markedly increased in SHRs compared to that in WKYs, while renin was significantly decreased. These changes were associated with higher plasma angiotensin II levels and lower plasma angiotensin (1-7) levels in SHRs. The results indicate that the kidneys of SHRs showed morphological changes and variations in cortical expression patterns of PRR, cyclooxygenase-2, and renin before the development of hypertension.

Association between admission hemoglobin level and prognosis in patients with type 2 diabetes mellitus

BACKGROUND

Anaemia is common in patients with chronic kidney disease (CKD) and is a major risk factor that contributes to mortality in such patients. Type 2 diabetes mellitus (T2DM) is one of the leading causes of CKD. The association between admission hemoglobin levels and renal damage in patients with T2DM remains unclear.

AIM

To evaluate the relationship between admission hemoglobin levels and prognosis in patients with T2DM.

METHODS

We performed a retrospective analysis of 265 consecutive patients presenting with T2DM between 2011 and 2015. The composite endpoint was end-stage renal disease or a 50% reduction in the estimated glomerular filtration rate.

RESULTS

In multivariable-adjusted Cox proportional hazards models (adjusting for demographic factors, traditional risk factors, lipids), the adjusted hazard ratios (HRs) for the highest and middle tertiles compared to the lowest tertile of hemoglobin were 0.82 (95%CI: 0.11-6.26, P = 0.8457) and 0.28 (95%CI: 0.09-0.85, P = 0.0246), respectively. However, after further adjustment for glycaemia control, hemoglobin was positively related to the risk of the composite endpoint (HR: 1.05, 95%CI: 0.14-8.09, P = 0.9602) when the highest tertile was compared to the lowest tertile of hemoglobin. We found a U-shaped relationship between hemoglobin levels and the composite endpoint. The curve tended to reach the lowest level at an optimal hemoglobin level.

CONCLUSION

Among patients with T2DM, a U-shaped relationship was observed between hemoglobin levels and renal damage. A lower admission hemoglobin level (hemoglobin < 13.3 g/dL) is an independent predictor of renal damage.

Knockout of Macula Densa Neuronal Nitric Oxide Synthase Increases Blood Pressure in db/db Mice

[Figure: see text].

Spatial transcriptional mapping of the human nephrogenic program

Congenital abnormalities of the kidney and urinary tract are among the most common birth defects, affecting 3% of newborns. The human kidney forms around a million nephrons from a pool of nephron progenitors over a 30-week period of development. To establish a framework for human nephrogenesis, we spatially resolved a stereotypical process by which equipotent nephron progenitors generate a nephron anlage, then applied data-driven approaches to construct three-dimensional protein maps on anatomical models of the nephrogenic program. Single-cell RNA sequencing identified progenitor states, which were spatially mapped to the nephron anatomy, enabling the generation of functional gene networks predicting interactions within and between nephron cell types. Network mining identified known developmental disease genes and predicted targets of interest. The spatially resolved nephrogenic program made available through the Human Nephrogenesis Atlas (https://sckidney.flatironinstitute.org/) will facilitate an understanding of kidney development and disease and enhance efforts to generate new kidney structures.

Role of Renal Sympathetic Nerve Activity in Volatile Anesthesia's Effect on Renal Excretory Function

Regulation of fluid balance is pivotal during surgery and anesthesia and affects patient morbidity, mortality, and hospital length of stay. Retention of sodium and water is known to occur during surgery but the mechanisms are poorly defined. In this study, we explore how the volatile anesthetic sevoflurane influences renal function by affecting renal sympathetic nerve activity (RSNA). Our results demonstrate that sevoflurane induces renal sodium and water retention during pediatric anesthesia in association with elevated plasma concentration of renin but not arginine-vasopressin. The mechanisms are further explored in conscious and anesthetized ewes where we show that RSNA is increased by sevoflurane compared with when conscious. This is accompanied by renal sodium and water retention and decreased renal blood flow (RBF). Finally, we demonstrate that renal denervation normalizes renal excretory function and improves RBF during sevoflurane anesthesia in sheep. Taken together, this study describes a novel role of the renal sympathetic nerves in regulating renal function and blood flow during sevoflurane anesthesia.

Serum renin and major adverse kidney events in critically ill patients: a multicenter prospective study

BACKGROUND

Preliminary studies have suggested that the renin-angiotensin system is activated in critical illness and associated with mortality and kidney outcomes. We sought to assess in a larger, multicenter study the relationship between serum renin and Major Adverse Kidney Events (MAKE) in intensive care unit (ICU) patients.

METHODS

Prospective, multicenter study at two institutions of patients with and without acute kidney injury (AKI). Blood samples were collected for renin measurement a median of 2 days into the index ICU admission and 5-7 days later. The primary outcome was MAKE at hospital discharge, a composite of mortality, kidney replacement therapy, or reduced estimated glomerular filtration rate to ≤ 75% of baseline.

RESULTS

Patients in the highest renin tertile were more severely ill overall, including more AKI, vasopressor-dependence, and severity of illness. MAKE were significantly greater in the highest renin tertile compared to the first and second tertiles. In multivariable logistic regression, this initial measurement of renin remained significantly associated with both MAKE as well as the individual component of mortality. The association of renin with MAKE in survivors was not statistically significant. Renin measurements at the second time point were also higher in patients with MAKE. The trajectory of the renin measurements between time 1 and 2 was distinct when comparing death versus survival, but not when comparing MAKE versus those without.

CONCLUSIONS

In a broad cohort of critically ill patients, serum renin measured early in the ICU admission is associated with MAKE at discharge, particularly mortality.

Prevention of hemorrhage-induced renal vasoconstriction and hypoxia by angiotensin II type 1 receptor antagonism in pigs

Angiotensin II (ANG II) is a potent vasoconstrictor and may reduce renal blood flow (RBF), causing renal hypoxia. Hypotensive hemorrhage elevates plasma ANG II levels and is associated with increased risk of acute kidney injury. We hypothesized that ANG II antagonism prevents renal vasoconstriction and hypoxia caused by hemorrhage. Pigs were anaesthetized, surgically prepared, and randomized to intravenous losartan (1.5 mg·kg^-1·h^-1, n = 8) or an equal volume of intravenous Ringer acetate (vehicle-treated, n = 8). Hemorrhage was induced by continuous aspiration of blood to reach and sustain mean arterial pressure of <50 mmHg for 30 min. Plasma ANG II levels, hemodynamics and oxygenation were assessed 60 min prehemorrhage, 30-min after the start of hemorrhage, and 60 min posthemorrhage. Erythropoietin mRNA was analyzed in cortical and medullary tissue sampled at the end of the experiment. Hypotensive hemorrhage increased plasma ANG II levels and decreased RBF and oxygen delivery in both groups. Losartan-treated animals recovered in RBF and oxygen delivery, whereas vehicle-treated animals had persistently reduced RBF and oxygen delivery. In accordance, renal vascular resistance increased over time post hemorrhage in vehicle-treated animals but was unchanged in losartan-treated animals. Renal oxygen extraction rate and cortical erythropoietin mRNA levels increased in the vehicle group but not in the losartan group. In conclusion, ANG II antagonism alleviates prolonged renal vasoconstriction and renal hypoxia in a large animal model of hypotensive hemorrhage.

Vascular control of kidney epithelial transporters

A major pathway in hypertension pathogenesis involves direct activation of ANG II type 1 (AT1) receptors in the kidney, stimulating Na+ reabsorption. AT1 receptors in tubular epithelia control expression and stimulation of Na+ transporters and channels. Recently, we found reduced blood pressure and enhanced natriuresis in mice with cell-specific deletion of AT1 receptors in smooth muscle (SMKO mice). Although impaired vasoconstriction and preserved renal blood flow might contribute to exaggerated urinary Na+ excretion in SMKO mice, we considered whether alterations in Na+ transporter expression might also play a role; therefore, we carried out proteomic analysis of key Na+ transporters and associated proteins. Here, we show that levels of Na+-K+-2Cl- cotransporter isoform 2 (NKCC2) and Na+/H+ exchanger isoform 3 (NHE3) are reduced at baseline in SMKO mice, accompanied by attenuated natriuretic and diuretic responses to furosemide. During ANG II hypertension, we found widespread remodeling of transporter expression in wild-type mice with significant increases in the levels of total NaCl cotransporter, phosphorylated NaCl cotransporter (Ser71), and phosphorylated NKCC2, along with the cleaved, activated forms of the α- and γ-epithelial Na+ channel. However, the increases in α- and γ-epithelial Na+ channel with ANG II were substantially attenuated in SMKO mice. This was accompanied by a reduced natriuretic response to amiloride. Thus, enhanced urinary Na+ excretion observed after cell-specific deletion of AT1 receptors from smooth muscle cells is associated with altered Na+ transporter abundance across epithelia in multiple nephron segments. These findings suggest a system of vascular-epithelial in the kidney, modulating the expression of Na+ transporters and contributing to the regulation of pressure natriuresis.NEW & NOTEWORTHY The use of drugs to block the renin-angiotensin system to reduce blood pressure is common. However, the precise mechanism for how these medications control blood pressure is incompletely understood. Here, we show that mice lacking angiotensin receptors specifically in smooth muscle cells lead to alternation in tubular transporter amount and function. Thus, demonstrating the importance of vascular-tubular cross talk in the control of blood pressure.

Single-cell RNA sequencing reveals the mesangial identity and species diversity of glomerular cell transcriptomes

Molecular characterization of the individual cell types in human kidney as well as model organisms are critical in defining organ function and understanding translational aspects of biomedical research. Previous studies have uncovered gene expression profiles of several kidney glomerular cell types, however, important cells, including mesangial (MCs) and glomerular parietal epithelial cells (PECs), are missing or incompletely described, and a systematic comparison between mouse and human kidney is lacking. To this end, we use Smart-seq2 to profile 4332 individual glomerulus-associated cells isolated from human living donor renal biopsies and mouse kidney. The analysis reveals genetic programs for all four glomerular cell types (podocytes, glomerular endothelial cells, MCs and PECs) as well as rare glomerulus-associated macula densa cells. Importantly, we detect heterogeneity in glomerulus-associated Pdgfrb-expressing cells, including bona fide intraglomerular MCs with the functionally active phagocytic molecular machinery, as well as a unique mural cell type located in the central stalk region of the glomerulus tuft. Furthermore, we observe remarkable species differences in the individual gene expression profiles of defined glomerular cell types that highlight translational challenges in the field and provide a guide to design translational studies.

The molecular identity of renal glomerular cells is poorly characterized and rodent glomerulopathy models translate poorly to humans. Here, the authors show molecular signatures of glomerulus-associated cells using single cell RNA sequencing and highlight differences between mouse and human cells.

Localization of angiotensin II type 1 receptor gene expression in rodent and human kidneys

The kidneys are an important target for angiotensin II (ANG II). In adult kidneys, the effects of ANG II are mediated mainly by ANG II type 1 (AT₁) receptors. AT₁ receptor expression has been reported for a variety of different cell types within the kidneys, suggesting a broad spectrum of actions for ANG II. Since there have been heterogeneous results in the literature regarding the intrarenal distribution of AT₁ receptors, this study aimed to obtain a comprehensive overview about the localization of AT₁ receptor expression in mouse, rat, and human kidneys. Using the cell-specific and high-resolution RNAscope technique, we performed colocalization experiments with various cell markers to specifically discriminate between different segments of the tubular and vascular system. Overall, we found a similar pattern of AT₁ mRNA expression in mouse, rat, and human kidneys. AT₁ receptors were detected in mesangial cells and renin-producing cells. In addition, AT₁ mRNA was found in interstitial cells of the cortex and outer medulla. In rodents, late afferent and early efferent arterioles expressed AT₁ receptor mRNA, but larger vessels of the investigated species showed no AT₁ expression. Tubular expression of AT₁ mRNA was species dependent with a strong expression in proximal tubules of mice, whereas expression was undetectable in human tubular cells. These findings suggest that the (juxta)glomerular area and tubulointerstitium are conserved expression sites for AT₁ receptors across species and might present the main target sites for ANG II in adult human and rodent kidneys.NEW & NOTEWORTHY Angiotensin II (ANG II) type 1 (AT₁) receptors are essential for mediating the effects of ANG II in the kidneys. This study aimed to obtain a comprehensive overview about the cell-specific localization of AT₁ receptor expression in rodent and human kidneys using the novel RNAscope technique. We found that the conserved AT₁ receptor mRNA expression sites across species are the (juxta)glomerular areas and tubulointerstitium, which might present main target sites for ANG II in adult human and rodent kidneys.

A new view of macula densa cell microanatomy

Although macula densa (MD) cells are chief regulatory cells in the nephron with unique microanatomical features, they have been difficult to study in full detail due to their inaccessibility and limitations in earlier microscopy techniques. The present study used a new mouse model with a comprehensive imaging approach to visualize so far unexplored microanatomical features of MD cells, their regulation, and functional relevance. MD-GFP mice with conditional and partial induction of green fluorescent protein (GFP) expression, which specifically and intensely illuminated only single MD cells, were used with fluorescence microscopy of fixed tissue and live MD cells in vitro and in vivo with complementary electron microscopy of the rat, rabbit, and human kidney. An elaborate network of major and minor cell processes, here named maculapodia, were found at the cell base, projecting toward other MD cells and the glomerular vascular pole. The extent of maculapodia showed upregulation by low dietary salt intake and the female sex. Time-lapse imaging of maculapodia revealed highly dynamic features including rapid outgrowth and an extensive vesicular transport system. Electron microscopy of rat, rabbit, and human kidneys and three-dimensional volume reconstruction in optically cleared whole-mount MD-GFP mouse kidneys further confirmed the presence and projections of maculapodia into the extraglomerular mesangium and afferent and efferent arterioles. The newly identified dynamic and secretory features of MD cells suggest the presence of novel functional and molecular pathways of cell-to-cell communication in the juxtaglomerular apparatus between MD cells and between MD and other target cells.NEW & NOTEWORTHY This study illuminated a physiologically regulated dense network of basal cell major and minor processes (maculapodia) in macula densa (MD) cells. The newly identified dynamic and secretory features of these microanatomical structures suggest the presence of novel functional and molecular pathways of cell-to-cell communication in the juxtaglomerular apparatus between MD and other target cells. Detailed characterization of the function and molecular details of MD cell intercellular communications and their role in physiology and disease warrant further studies.

Connexin Signaling in the Juxtaglomerular Apparatus (JGA) of Developing, Postnatal Healthy and Nephrotic Human Kidneys

Our study analyzed the expression pattern of different connexins (Cxs) and renin positive cells in the juxtaglomerular apparatus (JGA) of developing, postnatal healthy human kidneys and in nephrotic syndrome of the Finnish type (CNF), by using double immunofluorescence, electron microscopy and statistical measuring. The JGA contained several cell types connected by Cxs, and consisting of macula densa, extraglomerular mesangium (EM) and juxtaglomerular cells (JC), which release renin involved in renin-angiotensin- aldosteron system (RAS) of arterial blood pressure control. During JGA development, strong Cx40 expression gradually decreased, while expression of Cx37, Cx43 and Cx45 increased, postnatally showing more equalized expression patterning. In parallel, initially dispersed renin cells localized to JGA, and greatly increased expression in postnatal kidneys. In CNF kidneys, increased levels of Cx43, Cx37 and Cx45 co-localized with accumulations of renin cells in JGA. Additionally, they reappeared in extraglomerular mesangial cells, indicating association between return to embryonic Cxs patterning and pathologically changed kidney tissue. Based on the described Cxs and renin expression patterning, we suggest involvement of Cx40 primarily in the formation of JGA in developing kidneys, while Cx37, Cx43 and Cx45 might participate in JGA signal transfer important for postnatal maintenance of kidney function and blood pressure control.

COVID-19 Usurps Host Regulatory Networks

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection causes coronavirus disease 2019 (COVID-19). SARS-CoV-2 binds the angiotensin-converting enzyme 2 (ACE2) on the cell surface and this complex is internalized. ACE2 serves as an endogenous inhibitor of inflammatory signals associated with four major regulator systems: the renin-angiotensin-aldosterone system (RAAS), the complement system, the coagulation cascade, and the kallikrein-kinin system (KKS). Understanding the pathophysiological effects of SARS-CoV-2 on these pathways is needed, particularly given the current lack of proven, effective treatments. The vasoconstrictive, prothrombotic and pro-inflammatory conditions induced by SARS-CoV-2 can be ascribed, at least in part, to the activation of these intersecting physiological networks. Moreover, patients with immune deficiencies, hypertension, diabetes, coronary heart disease, and kidney disease often have altered activation of these pathways, either due to underlying disease or to medications, and may be more susceptible to SARS-CoV-2 infection. Certain characteristic COVID-associated skin, sensory, and central nervous system manifestations may also be linked to viral activation of the RAAS, complement, coagulation, and KKS pathways. Pharmacological interventions that target molecules along these pathways may be useful in mitigating symptoms and preventing organ or tissue damage. While effective anti-viral therapies are critically needed, further study of these pathways may identify effective adjunctive treatments and patients most likely to benefit.

SGLT2 inhibitors - a potential treatment for Alport syndrome

Alport syndrome is a rare genetic disease that results in disordered basement membrane type IV collagen resulting in occular and auditory defects as well of progressive kidney disease. Although no 'cure' currently exists, therapeutic blockade of the renin-angiotensin-aldosterone system can slow the progression to end-stage kidney disease (ESKD). Clinical trials for treatments in preventing chronic kidney disease have largely been negative over the last two decades until recent trials have shown positive cardiovascular and renal outcomes of sodium-glucose co-transporter-2 (SGLT2) inhibitors in patients with diabetes mellitus. Although marketed as medications for Type 2 diabetes, SGLT2 inhibitors have been found to have additional properties that are nephroprotective which makes them a potential candidate for treatment for those with other forms of progressive kidney disease. This review discusses the evidence for the use of SGLT2 inhibitors as a potential treatment in Alport syndrome that may slow the progression of chronic kidney disease and prevent patients reaching ESKD.

Normal human macula densa morphology and cell turnover: A histological, ultrastructural, and immunohistochemical investigation

The aim was to analyze the morphology of normal human macula densa (MD), evaluate the cells that may be responsible for its turnover, and collect quantitative data. Of four samples of normal human renal tissue, two were embedded in resin to measure the longitudinal extension and examine the ultrastructure of the MD, the other two were embedded in paraffin to study apoptosis and cell proliferation. The MD is composed of a monolayer tissue about 40 μm long, which includes 35-40 cells arranged in overlapping rows. Ultrastructurally, MD cells show two polarized portions: an apical end, with sensory features, and a basolateral aspect, with paracrine function. MD cells are connected apically by tight junctions, with/without adherens junctions, which form a barrier between the distal tubule lumen and the interstitium. Cells in degeneration, often associated with macrophages, and undifferentiated cells were found in the MD and adjacent distal tubule. A filamentous mat previously described in proximal tubule scattered tubular cells (STCs) was detected in the basal cytoplasm in undifferentiated cells. The tissue was consistently negative for the proliferation marker Ki67 and for the apoptotic markers caspase-3 and caspase-9. This work confirms our earlier morphological findings and provides new data: (a) MD cells display both apical adherens and tight junctions, the latter forming a tubulo-mesangial barrier; (b) the MD is a monolayer made up of about 40 cells arranged in rows; (c) the simultaneous presence of degenerating (8-13%) and undifferentiated (4-13%) cells reminiscent of STCs suggests a non-negligible cell turnover.

Pannexin 1 channels in renin-expressing cells influence renin secretion and blood pressure homeostasis

Kidney function and blood pressure homeostasis are regulated by purinergic signaling mechanisms. These autocrine/paracrine signaling pathways are initiated by the release of cellular ATP, which influences kidney hemodynamics and steady-state renin secretion from juxtaglomerular cells. However, the mechanism responsible for ATP release that supports tonic inputs to juxtaglomerular cells and regulates renin secretion remains unclear. Pannexin 1 (Panx1) channels localize to both afferent arterioles and juxtaglomerular cells and provide a transmembrane conduit for ATP release and ion permeability in the kidney and the vasculature. We hypothesized that Panx1 channels in renin-expressing cells regulate renin secretion in vivo. Using a renin cell-specific Panx1 knockout model, we found that male Panx1 deficient mice exhibiting a heightened activation of the renin-angiotensin-aldosterone system have markedly increased plasma renin and aldosterone concentrations, and elevated mean arterial pressure with altered peripheral hemodynamics. Following ovariectomy, female mice mirrored the male phenotype. Furthermore, constitutive Panx1 channel activity was observed in As4.1 renin-secreting cells, whereby Panx1 knockdown reduced extracellular ATP accumulation, lowered basal intracellular calcium concentrations and recapitulated a hyper-secretory renin phenotype. Moreover, in response to stress stimuli that lower blood pressure, Panx1-deficient mice exhibited aberrant "renin recruitment" as evidenced by reactivation of renin expression in pre-glomerular arteriolar smooth muscle cells. Thus, renin-cell Panx1 channels suppress renin secretion and influence adaptive renin responses when blood pressure homeostasis is threatened.

MicroRNA-132 regulates salt-dependent steady-state renin levels in mice

The body's salt and fluid balance is regulated by the renin-angiotensin-aldosterone system. Generation of prostaglandin-E2 (PGE2) in a cyclo-oxygenase-2 (COX-2)-dependent manner in the macula densa, the salt-sensing cells of the kidney, plays a dominant role in renin regulation. Here we show that miR-132 directly targets Cox-2 and affects subsequent PGE2 and renin levels. MiR-132 is induced and reduced by low- and high salt treatment, respectively, in a p38- and ERK1/2-independent and CREB- and salt inducible kinase-dependent manner. Silencing of miR-132 in mice increases macula densa COX-2 expression and elevates PGE2 and renin levels, which are abrogated by the selective COX-2-inhibitor Celecoxib. Furthermore, a low or high salt diet induces and reduces macula densa miR-132 expression, while low salt diet combined with silencing miR-132 further increases renin levels. Taken together, we demonstrate a posttranscriptional regulatory role for salt-dependent miR-132 in fine-tuning the steady-state levels of renin.

Cellular plasticity: A mechanism for homeostasis in the kidney

Cellular plasticity is a topical subject with interest spanning a wide range of fields from developmental biology to regenerative medicine. Even the nomenclature is a subject of debate, and the underlying mechanisms are still under investigation. On top of injury repair, cell plasticity is a constant physiological process in adult organisms and tissues, in response to homeostatic challenges. In this review we discuss two examples of plasticity for the maintenance of homeostasis in the renal system-namely the renin-producing juxtaglomerular cells (JG cells) and cortical collecting duct (CCD) cells. JG cells show plasticity through recruitment mechanisms, answering the demand for an increase in renin production. In the CCD, cells appear to have the ability to transdifferentiate between principal and intercalated cells to help maintain the highly regulated solute transport levels of that segment. These two cases highlight the complexity of plasticity processes and the role they can play in the kidney.

Effects of Sodium Reduction on Energy, Metabolism, Weight, Thirst, and Urine Volume: Results From the DASH (Dietary Approaches to Stop Hypertension)-Sodium Trial

Two recent studies challenged traditional paradigms of mammalian sodium physiology, suggesting that sodium reduction might cause weight gain by altering metabolism. This new theory has important implications for population-wide dietary recommendations. However, these observations have not been confirmed. In the DASH (Dietary Approaches to Stop Hypertension)-Sodium trial, 412 adults with systolic blood pressure of 120 to 159 mm Hg and diastolic blood pressure of 80 to 95 mm Hg not taking antihypertensive medications were randomly assigned to the DASH diet or a control diet (parallel design). On their assigned diet, participants randomly consumed each of the 3 sodium levels for 4 weeks (crossover design). Participants were provided all meals but could drink noncaloric beverages (eg, water) freely. Throughout the trial, energy intake was adjusted to maintain weight constant. The 3 sodium levels (at 2100 kcal/day) were: low (1150 mg of Na/day), medium (2300 mg of Na/day), and high (3450 mg of Na/day). Energy intake, weight, self-reported thirst, and 24-hour urine volume were assessed after each period. Participants were 57% women and 57% black; mean age was 48 years [SD, 10]). Among those assigned the control, mean weight increased slightly with higher sodium but not among those assigned DASH. Energy intake did not vary across sodium levels in either diet (P-trends ≥0.36). Higher sodium resulted in more thirst (P-trends <0.001 on both diets) and higher urine volume (suggesting higher fluid intake) during the control diet (P-trend=0.007). Reducing sodium did not increase energy requirements to maintain stable weights but did decrease thirst and urine volume (control diet only), findings consistent with the traditional understanding of mammalian sodium physiology. Clinical Trial Registration URL: http://www.clinicaltrials.gov. Unique identifier: NCT00000608.