Identification of renin progenitors in the mouse bone marrow that give rise to B-cell leukaemia

Tcf21 as a Founder Transcription Factor in Specifying Foxd1 Cells to the Juxtaglomerular Cell Lineage

Renin is crucial for blood pressure regulation and electrolyte balance, and its expressing cells arise from Foxd1+ stromal progenitors. However, the factors guiding these progenitors toward the renin-secreting cell fate are not fully understood. Tcf21, a basic helix-loop-helix (bHLH) transcription factor, is essential in kidney development. Utilizing Foxd1 Cre/+ ;Tcf21 f/f and Ren1 dCre/+ ;Tcf21 f/f mouse models, we investigated the role of Tcf21 in the differentiation of Foxd1+ progenitor cells into juxtaglomerular (JG) cells. Immunostaining and in-situ hybridization demonstrated significantly fewer renin-positive areas and altered renal arterial morphology in Foxd1 Cre/+ ;Tcf21 f/f kidneys compared with controls, indicating Tcf21's necessity for renin cell emergence. However, Tcf21 inactivation in renin-expressing cells ( Ren1 dCre/+ ;Tcf21 f/f ) did not recapitulate this phenotype, suggesting Tcf21 is dispensable once renin cell identity is established. Integrated analysis of single-cell RNA sequencing (scRNA-seq) and single-cell assay for transposase-accessible chromatin sequencing (scATAC-seq) on GFP+ cells (stromal lineage) from E12, E18, P5, and P30 Foxd1 Cre/+ ;Rosa26 mTmG control kidneys revealed that Tcf21 expression peaks at embryonic day 12, crucial for early JG cell specification. Subsequent analyses confirmed Tcf21's critical role in early kidney development, with expression declining as development progresses. Our results highlight the temporal and spatial dynamics of Tcf21, showing its importance in the early specification of Foxd1+ cells into JG cells. These findings provide new insights into the molecular mechanisms governing JG cell differentiation and underscore Tcf21's pivotal role in kidney development. The data suggest that Tcf21 expression in Foxd1+ progenitors is essential for the specification of renin-expressing JG cells, but once renin cell identity is assumed, Tcf21 becomes redundant.

NEW & NOTEWORTHY

This manuscript provides novel insights into the role of Tcf21 in the differentiation of Foxd1+ cells into JG cells. Utilizing integrated scRNA-seq and scATAC-seq, the study reveals that Tcf21 expression is crucial during early embryonic stages, with its peak at embryonic day 12. The findings demonstrate that inactivation of Tcf21 leads to fewer renin-positive areas and altered renal arterial morphology, underscoring the importance of Tcf21 in the specification of renin-expressing JG cells and kidney development.

The contribution of the AT1 receptor to erythropoiesis

The renin-angiotensin system (RAS) comprises a broad set of functional peptides and receptors that play a role in cardiovascular homeostasis and contribute to cardiovascular pathologies. Angiotensin II (Ang II) is the most potent peptide hormone produced by the RAS due to its high abundance and its strong and pleiotropic impact on the cardiovascular system. Formation of Ang II takes place in the bloodstream and additionally in tissues in the so-called local RAS. Of the two Ang II receptors (AT1 and AT2) that Ang II binds to, AT1 is the most expressed throughout the mammalian body. AT1 expression is not restricted to cells of the cardiovascular system but in fact AT1 protein is found in nearly all organs, hence, Ang II takes part in several modulatory physiological processes one of which is erythropoiesis. In this review, we present multiple evidence supporting that Ang II modulates physiological and pathological erythropoiesis processes trough the AT1 receptor. Cumulative evidence indicates that Ang II by three distinct mechanisms influences erythropoiesis: 1) stimulation of renal erythropoietin synthesis; 2) direct action on bone marrow precursor cells; and 3) modulation of sympathetic nerve activity to the bone marrow. The text highlights clinical and preclinical evidence focusing on mechanistic studies using rodent models.

Cells of the renin lineage promote kidney regeneration post-release of ureteral obstruction in neonatal mice

AIM

Ureteral obstruction leads to significant changes in kidney renin expression. It is unclear whether those changes are responsible for the progression of kidney damage, repair, or regeneration. In the current study, we aimed to elucidate the contribution of renin-producing cells (RPCs) and the cells of the renin lineage (CoRL) towards kidney damage and regeneration using a model of partial and reversible unilateral ureteral obstruction (pUUO) in neonatal mice.

METHODS

Renin cells are progenitors for other renal cell types collectively called CoRL. We labeled the CoRL with green fluorescent protein (GFP) using genetic approaches. We performed lineage tracing to analyze the changes in the distribution of CoRL during and after the release of obstruction. We also ablated the RPCs and CoRL by cell-specific expression of Diphtheria Toxin Sub-unit A (DTA). Finally, we evaluated the kidney damage and regeneration during and after the release of obstruction in the absence of CoRL.

RESULTS

In the obstructed kidneys, there was a 163% increase in the renin-positive area and a remarkable increase in the distribution of GFP+ CoRL. Relief of obstruction abrogated these changes. In addition, DTA-expressing animals did not respond to pUUO with increased RPCs and CoRL. Moreover, reduction in CoRL significantly compromised the kidney's ability to recover from the damage after the release of obstruction.

CONCLUSIONS

CoRL play a role in the regeneration of the kidneys post-relief of obstruction.

Determinants of renin cell differentiation: a single cell epi-transcriptomics approach

Rationale

Renin cells are essential for survival. They control the morphogenesis of the kidney arterioles, and the composition and volume of our extracellular fluid, arterial blood pressure, tissue perfusion, and oxygen delivery. It is known that renin cells and associated arteriolar cells descend from FoxD1 + progenitor cells, yet renin cells remain challenging to study due in no small part to their rarity within the kidney. As such, the molecular mechanisms underlying the differentiation and maintenance of these cells remain insufficiently understood.

Objective

We sought to comprehensively evaluate the chromatin states and transcription factors (TFs) that drive the differentiation of FoxD1 + progenitor cells into those that compose the kidney vasculature with a focus on renin cells.

Methods and Results

We isolated single nuclei of FoxD1 + progenitor cells and their descendants from FoxD1 ^cre/+ ; R26R-mTmG mice at embryonic day 12 (E12) (n _cells =1234), embryonic day 18 (E18) (n _cells =3696), postnatal day 5 (P5) (n _cells =1986), and postnatal day 30 (P30) (n _cells =1196). Using integrated scRNA-seq and scATAC-seq we established the developmental trajectory that leads to the mosaic of cells that compose the kidney arterioles, and specifically identified the factors that determine the elusive, myo-endocrine adult renin-secreting juxtaglomerular (JG) cell. We confirm the role of Nfix in JG cell development and renin expression, and identified the myocyte enhancer factor-2 (MEF2) family of TFs as putative drivers of JG cell differentiation.

Conclusions

We provide the first developmental trajectory of renin cell differentiation as they become JG cells in a single-cell atlas of kidney vascular open chromatin and highlighted novel factors important for their stage-specific differentiation. This improved understanding of the regulatory landscape of renin expressing JG cells is necessary to better learn the control and function of this rare cell population as overactivation or aberrant activity of the RAS is a key factor in cardiovascular and kidney pathologies.

Overexpression of notch signaling in renin cells leads to a polycystic kidney phenotype

Polycystic kidney disease (PKD) is an inherited disorder that results in large kidneys, numerous fluid-filled cysts, and ultimately end-stage kidney disease. PKD is either autosomal dominant caused by mutations in PKD1 or PKD2 genes or autosomal recessive caused by mutations in the PKHD1 or DZIP1L genes. While the genetic basis of PKD is known, the downstream molecular mechanisms and signaling pathways that lead to deregulation of proliferation, apoptosis, and differentiation are not completely understood. The Notch pathway plays critical roles during kidney development including directing differentiation of various progenitor cells, and aberrant Notch signaling results in gross alternations in cell fate. In the present study, we generated and studied transgenic mice that have overexpression of an intracellular fragment of mouse Notch1 ('NotchIC') in renin-expressing cells. Mice with overexpression of NotchIC in renin-expressing cells developed numerous fluid-filled cysts, enlarged kidneys, anemia, renal insufficiency, and early death. Cysts developed in both glomeruli and proximal tubules, had increased proliferation marks, and had increased levels of Myc. The present work implicates the Notch signaling pathway as a central player in PKD pathogenesis and suggests that the Notch-Myc axis may be an important target for therapeutic intervention.

Serial intravital imaging captures dynamic and functional endothelial remodeling with single-cell resolution

Endothelial cells are important in the maintenance of healthy blood vessels and in the development of vascular diseases. However, the origin and dynamics of endothelial precursors and remodeling at the single-cell level have been difficult to study in vivo owing to technical limitations. Therefore, we aimed to develop a direct visual approach to track the fate and function of single endothelial cells over several days and weeks in the same vascular bed in vivo using multiphoton microscopy (MPM) of transgenic Cdh5-Confetti mice and the kidney glomerulus as a model. Individual cells of the vascular endothelial lineage were identified and tracked owing to their unique color combination, based on the random expression of cyan/green/yellow/red fluorescent proteins. Experimental hypertension, hyperglycemia, and laser-induced endothelial cell ablation rapidly increased the number of new glomerular endothelial cells that appeared in clusters of the same color, suggesting clonal cell remodeling by local precursors at the vascular pole. Furthermore, intravital MPM allowed the detection of distinct structural and functional alterations of proliferating endothelial cells. No circulating Cdh5-Confetti⁺ cells were found in the renal cortex. Moreover, the heart, lung, and kidneys showed more significant clonal endothelial cell expansion compared with the brain, pancreas, liver, and spleen. In summary, we have demonstrated that serial MPM of Cdh5-Confetti mice in vivo is a powerful technical advance to study endothelial remodeling and repair in the kidney and other organs under physiological and disease conditions.

A primitive type of renin-expressing lymphocyte protects the organism against infections

The hormone renin plays a crucial role in the regulation of blood pressure and fluid-electrolyte homeostasis. Normally, renin is synthesized by juxtaglomerular (JG) cells, a specialized group of myoepithelial cells located near the entrance to the kidney glomeruli. In response to low blood pressure and/or a decrease in extracellular fluid volume (as it occurs during dehydration, hypotension, or septic shock) JG cells respond by releasing renin to the circulation to reestablish homeostasis. Interestingly, renin-expressing cells also exist outside of the kidney, where their function has remained a mystery. We discovered a unique type of renin-expressing B-1 lymphocyte that may have unrecognized roles in defending the organism against infections. These cells synthesize renin, entrap and phagocyte bacteria and control bacterial growth. The ability of renin-bearing lymphocytes to control infections-which is enhanced by the presence of renin-adds a novel, previously unsuspected dimension to the defense role of renin-expressing cells, linking the endocrine control of circulatory homeostasis with the immune control of infections to ensure survival.

Deciphering the Identity of Renin Cells in Health and Disease

Hypotension and changes in fluid-electrolyte balance pose immediate threats to survival. Juxtaglomerular cells respond to such threats by increasing the synthesis and secretion of renin. In addition, smooth muscle cells (SMCs) along the renal arterioles transform into renin cells until homeostasis has been regained. However, chronic unrelenting stimulation of renin cells leads to severe kidney damage. Here, we discuss the origin, distribution, function, and plasticity of renin cells within the kidney and immune compartments and the consequences of distorting the renin program. Understanding how chronic stimulation of these cells in the context of hypertension may lead to vascular pathology will serve as a foundation for targeted molecular therapies.

Renin-angiotensin system and cancer: epidemiology, cell signaling, genetics and epigenetics

Day by day, the health and economical burden of cancer increases globally. Indeed it can be considered that there is ''cancer pandemic''. Blocking the renin-angiotensin system (RAS) by angiotensin-converting enzyme (ACE) inhibitors (ACEI) or angiotensin-receptor blockers (ARB) are widely used measures to treat hypertension and heart failure. It has been recently suggested the activation and blocking of RAS has been associated with various types of cancer in epidemiological and experimental studies. Various studies have shown that RAS blockage is protective in some cancers. However, although fewer, contradictory data also showed that RAS blockage is either not related or adversely related to cancer. Although the reasons for these findings are not exactly known, different types of receptors and effectors in RAS may account for these findings. In the current review, we summarize the different RAS receptors and cancer development with regard to epidemiology, and pathogenesis including cell signaling pathways, apoptosis, genetic and epigenetic factors.

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.

Renin-Expressing Cells Require β1-Integrin for Survival and for Development and Maintenance of the Renal Vasculature

Juxtaglomerular cells are crucial for blood pressure and fluid-electrolyte homeostasis. The factors that maintain the life of renin cells are unknown. In vivo, renin cells receive constant cell-to-cell, mechanical, and neurohumoral stimulation that maintain their identity and function. Whether the presence of this niche is crucial for the vitality of the juxtaglomerular cells is unknown. Integrins are the largest family of cell adhesion molecules that mediate cell-to-cell and cell-to-matrix interactions. Of those, β1-integrin is the most abundant in juxtaglomerular cells. However, its role in renin cell identity and function has not been ascertained. To test the hypothesis that cell-matrix interactions are fundamental not only to maintain the identity and function of juxtaglomerular cells but also to keep them alive, we deleted β1-integrin in vivo in cells of the renin lineage. In mutant mice, renin cells died by apoptosis, resulting in decreased circulating renin, hypotension, severe renal-vascular abnormalities, and renal failure. Results indicate that cell-to-cell and cell-to-matrix interactions via β1-integrin is essential for juxtaglomerular cells survival, suggesting that the juxtaglomerular niche is crucial not only for the tight regulation of renin release but also for juxtaglomerular cell survival-a sine qua non condition to maintain homeostasis.

Beyond the Paradigm: Novel Functions of Renin-Producing Cells

The juxtaglomerular renin-producing cells (RPC) of the kidney are referred to as the major source of circulating renin. Renin is the limiting factor in renin-angiotensin system (RAS), which represents a proteolytic cascade in blood plasma that plays a central role in the regulation of blood pressure. Further cells disseminated in the entire organism express renin at a low level as part of tissue RASs, which are thought to locally modulate the effects of systemic RAS. In recent years, it became increasingly clear that the renal RPC are involved in developmental, physiological, and pathophysiological processes outside RAS. Based on recent experimental evidence, a novel concept emerges postulating that next to their traditional role, the RPC have non-canonical RAS-independent progenitor and renoprotective functions. Moreover, the RPC are part of a widespread renin lineage population, which may act as a global stem cell pool coordinating homeostatic, stress, and regenerative responses throughout the organism. This review focuses on the RAS-unrelated functions of RPC - a dynamic research area that increasingly attracts attention.

Pathobiological Interactions of Local Bone Marrow Renin-Angiotensin System and Central Nervous System in Systemic Arterial Hypertension

Circulating renin-angiotensin system (RAS) and local paracrin-autocrin-intracrin tissue-based RAS participate in numerous pathobiological events. Pro-inflammatory, pro-fibrotic, and pro-thrombotic consequences associated with local RAS activation have been detected at cellular and molecular level. Regenerative progenitor cell therapy in response to RAS modulating pharmacotherapy has emerged as an adjunct in the context of endothelial cell injury and regeneration to improve regeneration of the vascular endothelium. Local hematopoietic bone marrow (BM) RAS symbolizes the place of cross-interaction between vascular biology and cellular events from embryogenesis to definitive hematopoiesis underlying vascular atherosclerosis. The BM microenvironment also contains Mas receptors, which control the proliferative role of Ang 1-7 on hematopoietic stem cells. Ang 1-7 is produced from Ang-II or Ang-I with the help of ACE2. Various tissues and organs also have an effect on the RAS system. The leukocytes contain and synthesize immunoreactive angiotensinogen species capable of producing angiotensin in the basal state or after incubation with renin. The significance of RAS employment in atherosclerosis and hypertension was indicated by novel bidirectional Central Nervous System (CNS) RAS-BM RAS communications. Myeloid cells generated within the context of hematopoietic BM RAS are considered as the initiators and decision shapers in atherosclerosis. Macrophages in the atherosclerotic lesions contain angiotensin peptides by which RAS blockers inhibit monocyte activation and adherence. Furthermore, vascular biology in relation to inflammation and neoplasia is also affected by local tissue RAS. The purpose of this article is to outline interactions of circulating and local angiotensin systems, especially local bone marrow RAS, in the vascular pathobiological microenvironment of CNS.

Juxtaglomerular Cell Tumor: Reviewing a Cryptic Cause of Surgically Correctable Hypertension

Juxtaglomerular cell tumor (JGCT), or reninoma, is a typically benign neoplasm generally affecting adolescents and young adults due to modified smooth muscle cells from the afferent arteriole of the juxtaglomerular apparatus. Patients experience symptoms related to hypertension and hypoka-lemia due to renin-secretion by the tumor. MRI, PET, CT, and renal vein catheterizations can be used to capture JGCTs, with laparoscopic ultrasonography being most cost-efective. Surgical removal is the best option for management; electrolyte imbalances are a potential complication which may be assuaged via pre-surgical administration of aliskiren, a renin inhibitor. Considering the vast etiology for hypertension and rarity of JGCT, the diagnosing physician must have a high index of suspicion for JGCT. Early recognition and management can help prevent cardiovascular or pregnancy complications and fatalities, vascular invasion and metastasis, improve quality of life, and limit socioeconomic liabilities. Herein we review the epidemiology, genetics, histopathol-ogy, clinical presentation, and management of this rare condition. The impact of genetics on prognosis warrant further research.

The Role of the Local Bone Marrow Renin-Angiotensin System in Multiple Myeloma

Objective:

Angiotensin II promotes growth and angiogenesis via type 1 receptors (AGTR1) in certain tumors. In this study, we examine the bone marrow AGTR1 expression in multiple myeloma (MM) and its relationship with the regulation of angiogenesis and prognostic factors.

Materials and Methods:

Bone marrow AGTR1 mRNA levels of 39 MM patients and 15 healthy controls were analyzed with quantitative RT-PCR. Immunohistochemical staining of the tissue vascular endothelial growth factor (VEGF), CD34, and factor VIIIrAg (fVIIIrAg) was used to assess bone marrow angiogenesis.

Results:

Bone marrow samples of the patients showed increased VEGF, fVIIIrAg, and CD34 staining and higher AGTR1 expression levels when compared to controls. Patients with severe-diffuse bone marrow infiltration showed higher bone marrow VEGF, fVIIIrAg, CD34, and AGTR1 mRNA levels when compared to other patients.

Conclusion:

AGTR1 expression was found positively correlated with plasma β2-microglobulin level and patients with increased AGTR1 expression showed increased bone marrow CD34 levels.

EXPRESSION OF THE RENIN-ANGIOTENSIN SYSTEM COMPONENTS IN ONCOLOGIC DISEASES

The literature devoted to changes in the expression of the renin-angiotensin system (RAS) proteins of cancer cells was analyzed. The dynamics of RAS protein expression in malignant tumors and the possible role of epigenetic mechanisms in these processes are briefly reviewed. Through research of the epigenetic mechanisms in cancer, principally new techniques for their correction based on the use of selective regulatory systems of covalent modification of histone proteins (for example, deacetylase inhibitor) and microRNA synthesis technologies have been developed. Literature data show promising pharmacological correction of epigenetic modification of chromatin in the treatment of cancer.

Leukemia development initiated by deletion of RBP-J: mouse strain, deletion efficiency and cell of origin

Conditional deletion of RBP-J, the major transcriptional effector of Notch signaling, specifically within renin-expressing cells leads to the development of B-cell leukemia. However, the influence of contributing factors such as mouse strain, cell of origin and Cre recombinase copy number are unknown. In this study, we compared RBP-J deletion efficiency using one versus two copies of Cre recombinase. Further, we compared the incidence and timing of leukemia development in two unique strains of mice, C57BL/6 and 129/SV, as well as at different B-cell developmental stages. We found that animals expressing two copies of Cre recombinase developed B-cell leukemia at an earlier age and with more fulminant disease, compared with control animals and animals expressing one copy of Cre recombinase. In addition, we found a difference in leukemia incidence between C57BL/6 and 129/SV mouse strains. Whereas deletion of RBP-J in renin-expressing cells of C57BL/6 mice leads to the development of B-cell leukemia, 129/SV mice develop dermatitis with a reactive, myeloproliferative phenotype. The difference in phenotypes is explained, in part, by the differential expression of extra-renal renin; C57BL/6 mice have more renin-expressing cells within hematopoietic tissues. Finally, we found that deletion of RBP-J in Mb1- or CD19-expressing B lymphocytes does not result in leukemia development. Together, these studies establish that renin progenitors are vulnerable cells for neoplastic transformation and emphasize the importance of genetic background on the development of inflammatory and malignant conditions.This article has an associated First Person interview with the first author of the paper.

Changes in cell fate determine the regenerative and functional capacity of the developing kidney before and after release of obstruction

Congenital obstructive nephropathy is a major cause of chronic kidney disease (CKD) in children. The contribution of changes in the identity of renal cells to the pathology of obstructive nephropathy is poorly understood. Using a partial unilateral ureteral obstruction (pUUO) model in genetically modified neonatal mice, we traced the fate of cells derived from the renal stroma, cap mesenchyme, ureteric bud (UB) epithelium, and podocytes using Foxd1Cre, Six2Cre, HoxB7Cre, and Podocyte.Cre mice respectively, crossed with double fluorescent reporter (membrane-targetted tandem dimer Tomato (mT)/membrane-targetted GFP (mG)) mice. Persistent obstruction leads to a significant loss of tubular epithelium, rarefaction of the renal vasculature, and decreased renal blood flow (RBF). In addition, Forkhead Box D1 (Foxd1)-derived pericytes significantly expanded in the interstitial space, acquiring a myofibroblast phenotype. Degeneration of Sine Oculis Homeobox Homolog 2 (Six2) and HoxB7-derived cells resulted in significant loss of glomeruli, nephron tubules, and collecting ducts. Surgical release of obstruction resulted in striking regeneration of tubules, arterioles, interstitium accompanied by an increase in blood flow to the level of sham animals. Contralateral kidneys with remarkable compensatory response to kidney injury showed an increase in density of arteriolar branches. Deciphering the mechanisms involved in kidney repair and regeneration post relief of obstruction has potential therapeutic implications for infants and children and the growing number of adults suffering from CKD.

Super-enhancers maintain renin-expressing cell identity and memory to preserve multi-system homeostasis

Renin cells are crucial for survival - they control fluid-electrolyte and blood pressure homeostasis, vascular development, regeneration, and oxygen delivery to tissues. During embryonic development, renin cells are progenitors for multiple cell types that retain the memory of the renin phenotype. When there is a threat to survival, those descendants are transformed and reenact the renin phenotype to restore homeostasis. We tested the hypothesis that the molecular memory of the renin phenotype resides in unique regions and states of these cells' chromatin. Using renin cells at various stages of stimulation, we identified regions in the genome where the chromatin is open for transcription, mapped histone modifications characteristic of active enhancers such as H3K27ac, and tracked deposition of transcriptional activators such as Med1, whose deletion results in ablation of renin expression and low blood pressure. Using the rank ordering of super-enhancers, epigenetic rewriting, and enhancer deletion analysis, we found that renin cells harbor a unique set of super-enhancers that determine their identity. The most prominent renin super-enhancer may act as a chromatin sensor of signals that convey the physiologic status of the organism, and is responsible for the transformation of renin cell descendants to the renin phenotype, a fundamental process to ensure homeostasis.

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.

Persistent and inducible neogenesis repopulates progenitor renin lineage cells in the kidney

Renin lineage cells (RLCs) serve as a progenitor cell reservoir during nephrogenesis and after renal injury. The maintenance mechanisms of the RLC pool are still poorly understood. Since RLCs were also identified as a progenitor cell population in bone marrow we first considered that these may be their source in the kidney. However, transplantation experiments in adult mice demonstrated that bone marrow-derived cells do not give rise to RLCs in the kidney indicating their non-hematopoietic origin. Therefore we tested whether RLCs develop in the kidney through neogenesis (de novo differentiation) from cells that have never expressed renin before. We used a murine model to track neogenesis of RLCs by flow cytometry, histochemistry, and intravital kidney imaging. During nephrogenesis RLCs first appear at e14, form a distinct population at e16, and expand to reach a steady state level of 8-10% of all kidney cells in adulthood. De novo differentiated RLCs persist as a clearly detectable population through embryogenesis until at least eight months after birth. Pharmacologic stimulation of renin production with enalapril or glomerular injury induced the rate of RLC neogenesis in the adult mouse kidney by 14% or more than three-fold, respectively. Thus, the renal RLC niche is constantly filled by local de novo differentiation. This process could be stimulated consequently representing a new potential target to beneficially influence repair and regeneration after kidney injury.

Chronic Stimulation of Renin Cells Leads to Vascular Pathology

Experimental or spontaneous genomic mutations of the renin-angiotensin system or its pharmacological inhibition in early life leads to renal abnormalities, including poorly developed renal medulla, papillary atrophy, hydronephrosis, inability to concentrate the urine, polyuria, polydipsia, renal failure, and anemia. At the core of such complex phenotype is the presence of unique vascular abnormalities: the renal arterioles do not branch or elongate properly and they have disorganized, concentric hypertrophy. This lesion has been puzzling because it is often found in hypertensive individuals whereas mutant or pharmacologically inhibited animals are hypotensive. Remarkably, when renin cells are ablated with diphtheria toxin, the vascular hypertrophy does not occur, suggesting that renin cells per se may contribute to the vascular disease. To test this hypothesis, on a Ren1^c-/- background, we generated mutant mice with reporter expression (Ren1^c-/-;Ren1^c-Cre;R26R.mTmG and Ren1^c-/-;Ren1^c-Cre;R26R.LacZ) to trace the fate of renin^null cells. To assess whether renin^null cells maintain their renin promoter active, we used Ren1^c-/-;Ren1^c-YFP mice that transcribe YFP (yellow fluorescent protein) directed by the renin promoter. We also followed the expression of Akr1b7 and miR-330-5p, markers of cells programmed for the renin phenotype. Contrary to what we expected, renin^null cells did not die or disappear. Instead, they survived, increased in number along the renal arterial tree, and maintained an active molecular memory of the myoepitheliod renin phenotype. Furthermore, null cells of the renin lineage occupied the walls of the arteries and arterioles in a chaotic, directionless pattern directly contributing to the concentric arterial hypertrophy.

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.

Identification of cardiac hemo-vascular precursors and their requirement of sphingosine-1-phosphate receptor 1 for heart development

The cardiac endothelium plays a crucial role in the development of a functional heart. However, the precise identification of the endocardial precursors and the mechanisms they require for their role in heart morphogenesis are not well understood. Using in vivo and in vitro cell fate tracing concomitant with specific cell ablation and embryonic heart transplantation studies, we identified a unique set of precursors which possess hemogenic functions and express the stem cell leukemia (SCL) gene driven by its 5' enhancer. These hemo-vascular precursors give rise to the endocardium, atrioventricular cushions and coronary vascular endothelium. Furthermore, deletion of the sphingosine-1-phosphate receptor 1 (S1P1) in these precursors leads to ventricular non-compaction cardiomyopathy, a poorly understood condition leading to heart failure and early mortality. Thus, we identified a distinctive population of hemo-vascular precursors which require S1P1 to exert their functions and are essential for cardiac morphogenesis.

Novel Functions of Renin Precursors in Homeostasis and Disease

Renin progenitors appear early and are found in multiple tissues throughout the embryo. Besides their well known role in blood pressure and fluid homeostasis, renin progenitors participate in tissue morphogenesis, repair, and regeneration, and may integrate immune and endocrine responses. In the bone marrow, renin cells offer clues to understand normal and neoplastic hematopoiesis.

Vascular versus tubular renin: role in kidney development

Renin, the key regulated enzyme of the renin-angiotensin system regulates blood pressure, fluid-electrolyte homeostasis, and renal morphogenesis. Whole body deletion of the renin gene results in severe morphological and functional derangements, including thickening of renal arterioles, hydronephrosis, and inability to concentrate the urine. Because renin is found in vascular and tubular cells, it has been impossible to discern the relative contribution of tubular versus vascular renin to such a complex phenotype. Therefore, we deleted renin independently in the vascular and tubular compartments by crossing Ren1(c fl/fl) mice to Foxd1-cre and Hoxb7-cre mice, respectively. Deletion of renin in the vasculature resulted in neonatal mortality that could be rescued with daily injections of saline. The kidneys of surviving mice showed the absence of renin, hypertrophic arteries, hydronephrosis, and negligible levels of plasma renin. In contrast, lack of renin in the collecting ducts did not affect kidney morphology, intra-renal renin, or circulating renin in basal conditions or in response to a homeostatic stress, such as sodium depletion. We conclude that renin generated in the renal vasculature is fundamental for the development and integrity of the kidney, whereas renin in the collecting ducts is dispensable for normal kidney development and cannot compensate for the lack of renin in the vascular compartment. Further, the main source of circulating renin is the kidney vasculature.

B-1 cells produce insulin and abrogate experimental streptozotocin-induced diabetes

The participation of B-1 cells in a murine model of spontaneous diabetes has been recently reported. Here, we describe the role of B-1 cells in streptozotocin (STZ) induced diabetes in mice. We demonstrated that XID (B-1 cell-deficient) mice are more susceptible to STZ treatment than WT mice, as evidenced by their higher blood glucose level in response to STZ. Unexpectedly, the XID mice that were i.p. transferred with purified B-1 cells, either before or after the STZ treatment, did not develop diabetes. These cell transfers provided long-lasting protection for the XID mice against STZ-induced diabetes, suggesting that B-1 cells play an important role in the experimental diabetes pathobiology. We also showed that B-1 cell culture supernatants were able to regulate the blood glucose level of the diabetic XID mice, and we identified insulin-producing cells when B-1 cells were differentiated in B-1 cell-derived phagocyte in vitro. These findings provide a novel role for B-1 cells in metabolic processes, presenting a new mechanism to explain the pathogenesis of diabetes and a possible therapeutical target.