Characterization and fate of telomerase-expressing epithelia during kidney repair

Immune-mediated tubule atrophy promotes acute kidney injury to chronic kidney disease transition

Incomplete repair after acute kidney injury can lead to development of chronic kidney disease. To define the mechanism of this response, we compared mice subjected to identical unilateral ischemia-reperfusion kidney injury with either contralateral nephrectomy (where tubule repair predominates) or contralateral kidney intact (where tubule atrophy predominates). By day 14, the kidneys undergoing atrophy had more macrophages with higher expression of chemokines, correlating with a second wave of proinflammatory neutrophil and T cell recruitment accompanied by increased expression of tubular injury genes and a decreased proportion of differentiated tubules. Depletion of neutrophils and T cells after day 5 reduced tubular cell loss and associated kidney atrophy. In kidney biopsies from patients with acute kidney injury, T cell and neutrophil numbers negatively correlated with recovery of estimated glomerular filtration rate. Together, our findings demonstrate that macrophage persistence after injury promotes a T cell- and neutrophil-mediated proinflammatory milieu and progressive tubule damage.

Regenerating tubular epithelial cells of the kidney

Acute tubular injury accounts for the most common intrinsic cause for acute kidney injury. Normally, the tubular epithelium is mitotically quiescent. However, upon injury, it can show a brisk capacity to regenerate and repair. The scattered tubular cell (STC) phenotype was discovered as a uniform reaction of tubule cells triggered by injury. The STC phenotype is characterized by a unique protein expression profile, increased robustness during tubular damage and increased proliferation. Nevertheless, the exact origin and identity of these cells have been unveiled only in part. Here, we discuss the classical concept of renal regeneration. According to this model, surviving cells dedifferentiate and divide to replace neighbouring lost tubular cells. However, this view has been challenged by the concept of a pre-existing and fixed population of intratubular progenitor cells. This review presents a significant body of previous work and animal studies using lineage-tracing methods that have investigated the regeneration of tubular cells. We review the experimental findings and discuss whether they support the progenitor hypothesis or the classical concept of renal tubular regeneration. We come to the conclusion that any proximal tubular cell may differentiate into the regenerative STC phenotype upon injury thus contributing to regeneration, and these cells differentiate back into tubular cells once regeneration is finished.

Determining lineage relationships in kidney development and disease

The lineage relationships of cells provide information about the origins of component cell types during development and repair as well as the source of aberrant cells during disease. Genetic approaches to lineage tracing applied in the mouse have revealed much about how the mammalian kidney forms, including the identification of key progenitors for the nephrons and stromal compartments. Inducible Cre systems have also facilitated lineage tracing studies in the postnatal animal that illustrate the changes in cellular fate that can occur during kidney injury. With the advent of single-cell transcriptional profiling and trajectory analyses, predictions of cellular relationships across development are now being made in model systems, such as the mouse, as well as in human fetal kidney. Importantly, these approaches provide predictions of lineage relationships rather than definitive evidence. Although genetic approaches to the study of lineage have not previously been possible in a human setting, the application of CRISPR-Cas9 gene editing of pluripotent stem cells is beginning to teach us about human lineage relationships.

Targeted Delivery of Therapeutics to Urological Cancer Stem Cells

Urological cancer refers to cancer in organs of the urinary system and the male reproductive system. It mainly includes prostate cancer, bladder cancer, renal cancer, etc., seriously threatening patients' survival. Although there are many advances in the treatment of urological cancer, approved targeted therapies often result in tumor recurrence and therapy failure. An increasing amount of evidence indicated that cancer stem cells (CSCs) with tumor-initiating ability were the source of treatment failure in urological cancer. The development of CSCstargeted strategy can provide a possibility for the complete elimination of urological cancer. This review is based on a search of PubMed, Google scholar and NIH database (http://ClinicalTrials.gov/) for English language articles containing the terms: "biomarkers", "cancer stem cells", "targeting/targeted therapy", "prostate cancer", bladder cancer" and "kidney cancer". We summarized the biomarkers and stem cell features of the prostate, bladder and renal CSCs, outlined the targeted strategies for urological CSCs from signaling pathways, cytokines, angiogenesis, surface markers, elimination therapy, differentiation therapy, immunotherapy, microRNA, nanomedicine, etc., and highlighted the prospects and future challenges in this research field.

Targeting Strategies for Renal Cancer Stem Cell Therapy

Renal cell carcinoma (RCC) is an intractable genitourinary malignancy that accounts for approximately 4% of adult malignancies. Currently, there is no approved targeted therapy for RCC that has yielded durable remissions, and they remain palliative in intent. Emerging evidence has indicated that renal tumorigenesis and RCC treatment-resistance may originate from renal cancer stem cells (CSCs) with tumor-initiating capacity (CSC hypothesis). A better understanding of the mechanism underlying renal CSCs will help to dissect RCC heterogeneity and drug treatment efficiency, to promote more personalized and targeted therapies. In this review, we summarized the stem cell characteristics of renal CSCs. We outlined the targeting strategies and challenges associated with developing therapies that target renal CSCs angiogenesis, immunosuppression, signaling pathways, surface biomarkers, microRNAs and nanomedicine. In conclusion, CSCs are an important role in renal carcinogenesis and represent a valid target for treatment of RCC patients.

Kidney Cells Regeneration: Dedifferentiation of Tubular Epithelium, Resident Stem Cells and Possible Niches for Renal Progenitors

A kidney is an organ with relatively low basal cellular regenerative potential. However, renal cells have a pronounced ability to proliferate after injury, which undermines that the kidney cells are able to regenerate under induced conditions. The majority of studies explain yielded regeneration either by the dedifferentiation of the mature tubular epithelium or by the presence of a resident pool of progenitor cells in the kidney tissue. Whether cells responsible for the regeneration of the kidney initially have progenitor properties or if they obtain a “progenitor phenotype” during dedifferentiation after an injury, still stays the open question. The major stumbling block in resolving the issue is the lack of specific methods for distinguishing between dedifferentiated cells and resident progenitor cells. Transgenic animals, single-cell transcriptomics, and other recent approaches could be powerful tools to solve this problem. This review examines the main mechanisms of kidney regeneration: dedifferentiation of epithelial cells and activation of progenitor cells with special attention to potential niches of kidney progenitor cells. We attempted to give a detailed description of the most controversial topics in this field and ways to resolve these issues.

Genetic Susceptibility to Chronic Kidney Disease - Some More Pieces for the Heritability Puzzle

Chronic kidney disease (CKD) is a major global health problem with an increasing prevalence partly driven by aging population structure. Both genomic and environmental factors contribute to this complex heterogeneous disease. CKD heritability is estimated to be high (30-75%). Genome-wide association studies (GWAS) and GWAS meta-analyses have identified several genetic loci associated with CKD, including variants in UMOD, SHROOM3, solute carriers, and E3 ubiquitin ligases. However, these genetic markers do not account for all the susceptibility to CKD, and the causal pathways remain incompletely understood; other factors must be contributing to the missing heritability. Less investigated biological factors such as telomere length; mitochondrial proteins, encoded by nuclear genes or specific mitochondrial DNA (mtDNA) encoded genes; structural variants, such as copy number variants (CNVs), insertions, deletions, inversions and translocations are poorly covered and may explain some of the missing heritability. The sex chromosomes, often excluded from GWAS studies, may also help explain gender imbalances in CKD. In this review, we outline recent findings on molecular biomarkers for CKD (telomeres, CNVs, mtDNA variants, sex chromosomes) that typically have received less attention than gene polymorphisms. Shorter telomere length has been associated with renal dysfunction and CKD progression, however, most publications report small numbers of subjects with conflicting findings. CNVs have been linked to congenital anomalies of the kidney and urinary tract, posterior urethral valves, nephronophthisis and immunoglobulin A nephropathy. Information on mtDNA biomarkers for CKD comes primarily from case reports, therefore the data are scarce and diverse. The most consistent finding is the A3243G mutation in the MT-TL1 gene, mainly associated with focal segmental glomerulosclerosis. Only one GWAS has found associations between X-chromosome and renal function (rs12845465 and rs5987107). No loci in the Y-chromosome have reached genome-wide significance. In conclusion, despite the efforts to find the genetic basis of CKD, it remains challenging to explain all of the heritability with currently available methods and datasets. Although additional biomarkers have been investigated in less common suspects such as telomeres, CNVs, mtDNA and sex chromosomes, hidden heritability in CKD remains elusive, and more comprehensive approaches, particularly through the integration of multiple -"omics" data, are needed.

Telomerase Mediates Lymphocyte Proliferation but Not the Atherosclerosis-Suppressive Potential of Regulatory T-Cells

Supplemental Digital Content is available in the text.

Objective—

Atherosclerosis is an age-related disease characterized by systemic oxidative stress and low-grade inflammation. The role of telomerase and telomere length in atherogenesis remains contentious. Short telomeres of peripheral leukocytes are predictive for coronary artery disease. Conversely, attenuated telomerase has been demonstrated to be protective for atherosclerosis. Hence, a potential causative role of telomerase in atherogenesis is critically debated.

Approach and Results—

In this study, we used multiple mouse models to investigate the regulation of telomerase under oxidative stress as well as its impact on atherogenesis in vitro and in vivo. Using primary lymphocytes and myeloid cell cultures, we demonstrate that cultivation under hyperoxic conditions induced oxidative stress resulting in chronic activation of CD4⁺ cells and significantly reduced CD4⁺ T-cell proliferation. The latter was telomerase dependent because oxidative stress had no effect on the proliferation of primary lymphocytes isolated from telomerase knockout mice. In contrast, myeloid cell proliferation was unaffected by oxidative stress nor reliant on telomerase. Telomerase reverse transcriptase deficiency had no effect on regulatory T-cell (T_reg) numbers in vivo or suppressive function ex vivo. Adoptive transfer of telomerase reverse transcriptase^–/– T_regs into Rag2^–/– ApoE^–/– (recombination activating gene 2/apolipoprotein E) double knockout mice demonstrated that telomerase function was not required for the ability of T_regs to protect against atherosclerosis. However, telomere length was critical for T_reg function.

Conclusions—

Telomerase contributes to lymphocyte proliferation but plays no major role in T_reg function, provided that telomere length is not critically short. We suggest that oxidative stress may contribute to atherosclerosis via suppression of telomerase and acceleration of telomere attrition in T_regs.

Telomere attrition, kidney function, and prevalent chronic kidney disease in the United States

Background

Telomere length is an emerging novel biomarker of biologic age, cardiovascular risk and chronic medical conditions. Few studies have focused on the association between telomere length (TL) and kidney function.

Objective

We investigated the association between TL and kidney function/prevalent chronic kidney disease (CKD) in US adults.

Methods

The National Health and Nutrition Examination Survey (NHANES) participants with measured data on kidney function and TL from 1999 to 2002 were included. Estimated glomerular filtration rate (eGFR) was based on CKD Epidemiology Collaboration (CKD-EPI) equation. Urinary albumin excretion was assessed using urinary albumin-creatinine ratio (ACR). We used multivariable adjusted linear and logistic regression models, accounting for the survey design and sample weights.

Results

Of the 10568 eligible participants, 48.0% (n=5020) were men. Their mean age was 44.1 years. eGFR significantly decreased and ACR significantly increased across increasing quarters of TL (all p<0.001). The association between TL and kidney function remained robust even after adjusting for potential confounding factors, but the association between TL and ACR was only borderline significant (β-coefficient= -0.012, p=0.056).

Conclusion

The association of kidney function with a marker of cellular senescence suggests an underlying mechanism influencing the progression of nephropathy.

Comparing adult renal stem cell identification, characterization and applications

Despite growing interest and effort, a consensus has yet to be reached in regards to the identification of adult renal stem cells. Organ complexity and low turnover of renal cells has made stem cell identification difficult and lead to the investigation of multiple possible populations. In this review, we summarize the work that has been done toward finding and characterizing an adult renal stem cell population. In addition to giving a general overview of what has been done, we aim to highlight the variation in methods and outcomes. The methods used to locate potential stem cell populations can vary widely, but even within the relatively standard practice of BrdU labeling of slowly dividing cells, there are significant differences in protocols and results. Additional diversity exists in cell marker profiles and apparent differentiation potential seen in potential stem cell sources. Cataloging the variety of methods and outcomes seen so far may help to streamline future investigation and stear the field toward consensus. But even without firmly defined populations, the application of renal stem cells holds tantalizing potential. Populations of highly proliferative, multipotent cells of renal origin show the ability to engraft in injured kidneys, mitigate functional loss and occasionally show the ability to generate nephrons de novo. The progress toward regenerative medicine applications is also summarized.

Neonatal vascularization and oxygen tension regulate appropriate perinatal renal medulla/papilla maturation

Congenital medullary dysplasia with obstructive nephropathy is a common congenital disorder observed in paediatric patients and represents the foremost cause of renal failure. However, the molecular processes regulating normal papillary outgrowth during the postnatal period are unclear. In this study, transcriptional profiling of the renal medulla across postnatal development revealed enrichment of non-canonical Wnt signalling, vascular development, and planar cell polarity genes, all of which may contribute to perinatal medulla/papilla maturation. These pathways were investigated in a model of papillary hypoplasia with functional obstruction, the Crim1(KST264/KST264) transgenic mouse. Postnatal elongation of the renal papilla via convergent extension was unaffected in the Crim1(KST264/KST264) hypoplastic renal papilla. In contrast, these mice displayed a disorganized papillary vascular network, tissue hypoxia, and elevated Vegfa expression. In addition, we demonstrate the involvement of accompanying systemic hypoxia arising from placental insufficiency, in appropriate papillary maturation. In conclusion, this study highlights the requirement for normal vascular development in collecting duct patterning, development of appropriate nephron architecture, and perinatal papillary maturation, such that disturbances contribute to obstructive nephropathy.

C-kit(+) cells isolated from developing kidneys are a novel population of stem cells with regenerative potential

The presence of tissue specific precursor cells is an emerging concept in organ formation and tissue homeostasis. Several progenitors are described in the kidneys. However, their identity as a true stem cell remains elusive. Here, we identify a neonatal kidney-derived c-kit(+) cell population that fulfills all of the criteria as a stem cell. These cells were found in the thick ascending limb of Henle's loop and exhibited clonogenicity, self-renewal, and multipotentiality with differentiation capacity into mesoderm and ectoderm progeny. Additionally, c-kit(+) cells formed spheres in nonadherent conditions when plated at clonal density and expressed markers of stem cells, progenitors, and differentiated cells. Ex vivo expanded c-kit(+) cells integrated into several compartments of the kidney, including tubules, vessels, and glomeruli, and contributed to functional and morphological improvement of the kidney following acute ischemia-reperfusion injury in rats. Together, these findings document a novel neonatal rat kidney c-kit(+) stem cell population that can be isolated, expanded, cloned, differentiated, and used for kidney repair following acute kidney injury. These cells have important biological and therapeutic implications.

Kidney regeneration and stem cells

The kidney has the capacity to recover from ischemic and toxic insults. Although there has been debate about the origin of cells that replace injured epithelial cells, it is now widely recognized that intrinsic surviving tubular cells are responsible for the repair. On the other hand, the cells, which have stem cell-like characteristics, have been isolated in the kidney using various methods, but it remains unknown if these stem cells actually exist in the adult kidney and if they are involved in kidney regeneration. This review will focus on the pathophysiology of kidney regeneration and the contribution of renal stem cells. We also discuss possible therapeutic applications to kidney disease.

Stem cells: potential and challenges for kidney repair

Renal damage resulting from acute and chronic kidney injury poses an important problem to public health. Currently, patients with end-stage renal disease rely solely on kidney transplantation or dialysis for survival. Emerging therapies aiming to prevent and reverse kidney damage are thus in urgent need. Although the kidney was initially thought to lack the capacity for self-repair, several studies have indicated that this might not be the case; progenitor and stem cells appear to play important roles in kidney repair under various pathological conditions. In this review, we summarize recent findings on the role of progenitor/stem cells on kidney repair as well as discuss their potential as a therapeutic approach for kidney diseases.

Regenerative medicine for the kidney: stem cell prospects & challenges

The kidney has key roles in maintaining human health. There is an escalating medical crisis in nephrology as growing numbers of patients suffer from kidney diseases that culminate in organ failure. While dialysis and transplantation provide life-saving treatments, these therapies are rife with limitations and place significant burdens on patients and healthcare systems. It has become imperative to find alternative ways to treat existing kidney conditions and preemptive means to stave off renal dysfunction. The creation of innovative medical approaches that utilize stem cells has received growing research attention. In this review, we discuss the regenerative and maladaptive cellular responses that occur during acute and chronic kidney disease, the emerging evidence about renal stem cells, and some of the issues that lie ahead in bridging the gap between basic stem cell biology and regenerative medicine for the kidney.

Proximal tubular cells contain a phenotypically distinct, scattered cell population involved in tubular regeneration

Regeneration of injured tubular cells occurs after acute tubular necrosis primarily from intrinsic renal cells. This may occur from a pre-existing intratubular stem/progenitor cell population or from any surviving proximal tubular cell. In this study, we characterize a CD24-, CD133-, and vimentin-positive subpopulation of cells scattered throughout the proximal tubule in normal human kidney. Compared to adjacent 'normal' proximal tubular cells, these CD24-positive cells contained less cytoplasm, fewer mitochondria, and no brush border. In addition, 49 marker proteins are described that are expressed within the proximal tubules in a similar scattered pattern. For eight of these markers, we confirmed co-localization with CD24. In human biopsies of patients with acute tubular necrosis (ATN), the number of CD24-positive tubular cells was increased. In both normal human kidneys and the ATN biopsies, around 85% of proliferating cells were CD24-positive - indicating that this cell population participates in tubular regeneration. In healthy rat kidneys, the novel cell subpopulation was absent. However, upon unilateral ureteral obstruction (UUO), the novel cell population was detected in significant amounts in the injured kidney. In summary, in human renal biopsies, the CD24-positive cells represent tubular cells with a deviant phenotype, characterized by a distinct morphology and marker expression. After acute tubular injury, these cells become more numerous. In healthy rat kidneys, these cells are not detectable, whereas after UUO, they appeared de novo - arguing against the notion that these cells represent a pre-existing progenitor cell population. Our data indicate rather that these cells represent transiently dedifferentiated tubular cells involved in regeneration.

Kidney injury molecule-1: more than just an injury marker of tubular epithelial cells?

Regardless of the original causes and etiology, the progression to renal function declines follows a final common pathway associated with tubulointerstitial injury, in which the proximal tubular epithelial cells (PTEC) are instrumental. Kidney injury molecule-1 (KIM-1) is an emerging biomarker, and its expression and release are induced in PTEC upon injury. KIM-1 plays the role as a double-edged sword and implicates in the process of kidney injury and healing. Expression of KIM-1 is also associated with tubulointerstitial inflammation and fibrosis. More importantly, KIM-1 expressing PTEC play the role as the residential phagocytes, contribute to the removal of apoptotic cells and facilitate the regeneration of injured tubules. The precise mechanism of KIM-1 and its sheded ectodomain on restoration of tubular integrity after injury is not fully understood. Other than PTEC, macrophages (Mø) also implicate in tubular repair. Understanding the crosstalk between Mø and the injured PTEC is essential for designing appropriate methods for controlling the sophisticated machinery in tubular regeneration and healing. This article will review the current findings of KIM-1, beginning with its basic structure, utility as a biomarker, and possible functions, with focus on the role of KIM-1 in regeneration and healing of injured PTEC.

Association between kidney function and telomere length: the heart and soul study

BACKGROUND

Telomere attrition is a novel risk factor for cardiovascular disease. Studies of telomere length in relation to kidney function are limited. We explored the association of kidney function with telomere length and telomere shortening.

METHODS

The Heart and Soul Study is a longitudinal study of patients with stable coronary heart disease. Measures of baseline kidney function included: serum creatinine, creatinine-derived estimated glomerular filtration rate (eGFR(CKD-EPI)), 24-hour urine measured creatinine clearance, cystatin C, cystatin C-derived estimated glomerular filtration rate (eGFRcys) and urine albumin to creatinine ratio. Telomere length was measured from peripheral blood leukocytes at baseline (n = 954) and 5 years later (n = 608). Linear regression models were used to test the association of kidney function with (i) baseline telomere length and (ii) change in telomere length over 5 years.

RESULTS

At baseline, mean eGFR(CKD-EPI) was 72.6 (±21.5) ml/min/1.73 m(2), eGFRcys was 71.0 (±23.1) ml/min/1.73 m(2) and ACR was 8.6 (±12.3) mg/g. Only lower baseline eGFR(CKD-EPI) was associated with shorter baseline telomere length (9.1 (95% CI 1.2-16.9) fewer base pairs for every 5 ml/min/1.73 m(2) lower eGFR(CKD-EPI)). Lower baseline eGFR(CKD-EPI) (and all other measures of kidney function) predicted more rapid telomere shortening (10.8 (95% CI 4.3-17.3) decrease in base pairs over 5 years for every 5 ml/min/1.73 m(2) lower eGFR(CKD-EPI)). After adjustment for age, these associations were no longer statistically significant.

CONCLUSIONS

In patients with coronary heart disease, reduced kidney function is associated with (i) shorter baseline telomere length and (ii) more rapid telomere shortening over 5 years; however, these associations are entirely explained by older age.

Kidney protection and regeneration following acute injury: progress through stem cell therapy

Acute kidney injury (AKI) is a common clinical entity with high morbidity and mortality rates and ever increasing medical costs. A large number of patients who are hospitalized with morbidities such as diabetes, vascular disease, or chronic kidney disease are at high risk to develop AKI due to ischemic and nephrotoxic insults. The pathophysiology of ischemic and toxic forms of AKI is complex and includes tubular and vascular cell damage and inflammation. Given the seriousness of this essentially therapy-resistant complication, treatment beyond supportive measures and renal replacement therapy is urgently needed. Recent stem cell research has shown promising results, and cell therapy-based interventions are advancing into clinical trials. An example is our phase 1 clinical trial (NCT00733876) in which cardiac surgery patients at high risk of postoperative AKI were treated safely with allogeneic mesenchymal stem cells. Together with the introduction of biomarkers for an earlier and specific AKI diagnosis, currently tested stem cell-based therapies are expected to provide an entirely new class of diagnostic and therapeutic tools.

Mouse model of ischemic acute kidney injury: technical notes and tricks

Renal ischemia-reperfusion leads to acute kidney injury (AKI), a major kidney disease associated with an increasing prevalence and high mortality rates. A variety of experimental models, both in vitro and in vivo, have been used to study the pathogenic mechanisms of ischemic AKI and to test renoprotective strategies. Among them, the mouse model of renal clamping is popular, mainly due to the availability of transgenic models and the relatively small animal size for drug testing. However, the mouse model is generally less stable, resulting in notable variations in results. Here, we describe a detailed protocol of the mouse model of bilateral renal ischemia-reperfusion. We share the lessons and experiences gained from our laboratory in the past decade. We further discuss the technical issues that account for the variability of this model and offer relevant solutions, which may help other investigators to establish a well-controlled, reliable animal model of ischemic AKI.

Renal stem cells: fact or science fiction?

The kidney is widely regarded as an organ without regenerative abilities. However, in recent years this dogma has been challenged on the basis of observations of kidney recovery following acute injury, and the identification of renal populations that demonstrate stem cell characteristics in various species. It is currently speculated that the human kidney can regenerate in some contexts, but the mechanisms of renal regeneration remain poorly understood. Numerous controversies surround the potency, behaviour and origins of the cell types that are proposed to perform kidney regeneration. The present review explores the current understanding of renal stem cells and kidney regeneration events, and examines the future challenges in using these insights to create new clinical treatments for kidney disease.