Cellular maintenance and repair of the kidney

miRNA and mRNA Signatures in Human Acute Kidney Injury Tissue

Acute kidney injury (AKI) is an important contributor to the development of chronic kidney disease (CKD). There is a need to understand molecular mediators that drive recovery and progression to CKD. In particular, the regulatory role of miRNAs in AKI is poorly understood. Herein, miRNA and mRNA sequencing were performed on biobanked human kidney tissues obtained during the routine care of subjects with a diagnosis of AKI, minimal change disease, or on nephrectomy tissue with no known kidney disease. mRNA analysis revealed that nephrectomy tissues exhibited an injury signature similar to that of AKI which was not identified in minimal change disease samples. The transcriptomic signature of human AKI was enriched in pathways involved in cell adhesion, epithelial-to-mesenchymal transition, and cell cycle arrest (eg, CDH6, ITGB6, CDKN1A). In AKI, up-regulation of miR-146a, miR-155, miR-142, and miR-122 was associated with pathways involved in immune cell recruitment, inflammation, and epithelial-to-mesenchymal transition. miR-122 and miR-146 were associated with down-regulation of DDR2 and IGFBP6, which are genes involved in the recovery and progression of kidney disease. These data provide integrated miRNA signatures that complement mRNA and other epigenetic data available in kidney atlases.

Forsythiaside A suppresses renal fibrosis and partial epithelial-mesenchymal transition by targeting THBS1 through the PI3K/AKT signaling pathway

Renal fibrosis is a key feature of chronic kidney disease (CKD) progression, whereas no proven effective anti-fibrotic treatments. Forsythiaside A (FTA), derived from Forsythia suspense, has been found to possess nephroprotective properties. However, there is limited research on its anti-fibrotic effects, and its mechanism of action remains unknown. This study aimed to investigate the suppressive effects of FTA on renal fibrosis and explore the underlying mechanisms. In vitro, we established a HK2 cell model induced by transforming growth factor β1 (TGF-β1), and in vivo, we used a mice model induced by unilateral ureteral obstruction (UUO). CCK-8 assay, qRT-PCR, Western blotting, immunofluorescence, flow cytometry, histological staining, immunohistochemistry, TUNEL assay, RNA transcriptome sequencing, and molecular docking were performed. The results showed that FTA (40 μM or 80 μM) treatment improved cell viability and suppressed TGF-β1-induced fibrotic changes and partial epithelial-mesenchymal transition (EMT). Furthermore, FTA treatment reversed the activation of the PI3K/AKT signaling pathway, and THBS1 was identified as the target gene. We found that THBS1 knockdown suppressed the activation of the PI3K/AKT signaling pathway and reduced the fibrosis and partial EMT-related protein level. Conversely, THBS1 overexpression activated the PI3K/AKT signaling pathway and exacerbated renal fibrosis and partial EMT. In vivo, mice were administered FTA (30 or 60 mg/kg) for 2 weeks, and the results demonstrated that FTA administration significantly mitigated tubular injury, tubulointerstitial fibrosis, partial EMT, and apoptosis. In conclusion, FTA inhibited renal fibrosis and partial EMT by targeting THBS1 and inhibiting activation of the PI3K/AKT pathway.

Fisetin ameliorates fibrotic kidney disease in mice via inhibiting ACSL4-mediated tubular ferroptosis

Kidney fibrosis is the hallmark of chronic kidney disease (CKD) progression, whereas no effective anti-fibrotic therapies exist. Recent evidence has shown that tubular ferroptosis contributes to the pathogenesis of CKD with persistent proinflammatory and profibrotic responses. We previously reported that natural flavonol fisetin alleviated septic acute kidney injury and protected against hyperuricemic nephropathy in mice. In this study, we investigated the therapeutic effects of fisetin against fibrotic kidney disease and the underlying mechanisms. We established adenine diet-induced and unilateral ureteral obstruction (UUO)-induced CKD models in adult male mice. The two types of mice were administered fisetin (50 or 100 mg·kg-1·d-1, i.g.) for 3 weeks or 7 days, respectively. At the end of the experiments, the mice were euthanized, and blood and kidneys were gathered for analyzes. We showed that fisetin administration significantly ameliorated tubular injury, inflammation, and tubulointerstitial fibrosis in the two types of CKD mice. In mouse renal tubular epithelial (TCMK-1) cells, treatment with fisetin (20 μM) significantly suppressed adenine- or TGF-β1-induced inflammatory responses and fibrogenesis, and improved cell viability. By quantitative real-time PCR analysis of ferroptosis-related genes, we demonstrated that fisetin treatment inhibited ferroptosis in the kidneys of CKD mice as well as in injured TCMK-1 cells, as evidenced by decreased ACSL4, COX2, and HMGB1, and increased GPX4. Fisetin treatment effectively restored ultrastructural abnormalities of mitochondrial morphology and restored the elevated iron, the reduced GSH and GSH/GSSG as well as the increased lipid peroxide MDA in the kidneys of CKD mice. Notably, abnormally high expression of the ferroptosis key marker ACSL4 was verified in the renal tubules of CKD patients (IgAN, MN, FSGS, LN, and DN) as well as adenine- or UUO-induced CKD mice, and in injured TCMK-1 cells. In adenine- and TGF-β1-treated TCMK-1 cells, ACSL4 knockdown inhibited tubular ferroptosis, while ACSL4 overexpression blocked the anti-ferroptotic effect of fisetin and reversed the cytoprotective, anti-inflammatory, and anti-fibrotic effects of fisetin. In summary, we reveal a novel aspect of the nephroprotective effect of fisetin, i.e. inhibiting ACSL4-mediated tubular ferroptosis against fibrotic kidney diseases.

Advances in understanding and treating diabetic kidney disease: focus on tubulointerstitial inflammation mechanisms

Diabetic kidney disease (DKD) is a serious complication of diabetes that can lead to end-stage kidney disease. Despite its significant impact, most research has concentrated on the glomerulus, with little attention paid to the tubulointerstitial region, which accounts for the majority of the kidney volume. DKD's tubulointerstitial lesions are characterized by inflammation, fibrosis, and loss of kidney function, and recent studies indicate that these lesions may occur earlier than glomerular lesions. Evidence has shown that inflammatory mechanisms in the tubulointerstitium play a critical role in the development and progression of these lesions. Apart from the renin-angiotensin-aldosterone blockade, Sodium-Glucose Linked Transporter-2(SGLT-2) inhibitors and new types of mineralocorticoid receptor antagonists have emerged as effective ways to treat DKD. Moreover, researchers have proposed potential targeted therapies, such as inhibiting pro-inflammatory cytokines and modulating T cells and macrophages, among others. These therapies have demonstrated promising results in preclinical studies and clinical trials, suggesting their potential to treat DKD-induced tubulointerstitial lesions effectively. Understanding the immune-inflammatory mechanisms underlying DKD-induced tubulointerstitial lesions and developing targeted therapies could significantly improve the treatment and management of DKD. This review summarizes the latest advances in this field, highlighting the importance of focusing on tubulointerstitial inflammation mechanisms to improve DKD outcomes.

miRNA and mRNA Signatures in Human Acute Kidney Injury Tissue

Acute kidney injury (AKI) is an important contributor to the development of chronic kidney disease (CKD). There is a need to understand molecular mediators that drive either recovery or progression to CKD. In particular, the role of miRNA and its regulatory role in AKI is poorly understood. We performed miRNA and mRNA sequencing on biobanked human kidney tissues obtained in the routine clinical care of patients with the diagnoses of AKI and minimal change disease (MCD), in addition to nephrectomized (Ref) tissue from individuals without known kidney disease. Transcriptomic analysis of mRNA revealed that Ref tissues exhibited a similar injury signature to AKI, not identified in MCD samples. The transcriptomic signature of human AKI was enriched with genes in pathways involved in cell adhesion and epithelial-to-mesenchymal transition (e.g., CDH6, ITGB6, CDKN1A ). miRNA DE analysis revealed upregulation of miRNA associated with immune cell recruitment and inflammation (e.g., miR-146a, miR-155, miR-142, miR-122). These miRNA (i.e., miR-122, miR-146) are also associated with downregulation of mRNA such as DDR2 and IGFBP6 , respectively. These findings suggest integrated interactions between miRNAs and target mRNAs in AKI-related processes such as inflammation, immune cell activation and epithelial-to-mesenchymal transition. These data contribute several novel findings when describing the epigenetic regulation of AKI by miRNA, and also underscores the importance of utilizing an appropriate reference control tissue to understand canonical pathway alterations in AKI.

Downregulation of Swine Leukocyte Antigen Expression Decreases the Strength of Xenogeneic Immune Responses towards Renal Proximal Tubular Epithelial Cells

Xenotransplantation reemerged as a promising alternative to conventional transplantation enlarging the available organ pool. However, success of xenotransplantation depends on the design and selection of specific genetic modifications and on the development of robust assays allowing for a precise assessment of tissue-specific immune responses. Nevertheless, cell-based assays are often compromised by low proliferative capacity of primary cells. Proximal tubular epithelial cells (PTECs) play a crucial role in kidney function. Here, we generated immortalized PTECs (imPTECs) by overexpression of simian virus 40 T large antigen. ImPTECs not only showed typical morphology and phenotype, but, in contrast to primary PTECs, they maintained steady cell cycling rates and functionality. Furthermore, swine leukocyte antigen (SLA) class I and class II transcript levels were reduced by up to 85% after transduction with lentiviral vectors encoding for short hairpin RNAs targeting β2-microglobulin and the class II transactivator. This contributed to reducing xenogeneic T-cell cytotoxicity (p < 0.01) and decreasing secretion of pro-inflammatory cytokines such as IL-6 and IFN-γ. This study showed the feasibility of generating highly proliferative PTECs and the development of tissue-specific immunomonitoring assays. Silencing SLA expression on PTECs was demonstrated to be an effective strategy to prevent xenogeneic cellular immune responses and may strongly support graft survival after xenotransplantation.

Cell Profiling of Acute Kidney Injury to Chronic Kidney Disease Reveals Novel Oxidative Stress Characteristics in the Failed Repair of Proximal Tubule Cells

Chronic kidney disease (CKD) is a major public health issue around the world. A significant number of CKD patients originates from acute kidney injury (AKI) patients, namely "AKI-CKD". CKD is significantly related to the consequences of AKI. Damaged renal proximal tubular (PT) cell repair has been widely confirmed to indicate the renal prognosis of AKI. Oxidative stress is a key damage-associated factor and plays a significant role throughout the development of AKI and CKD. However, the relationships between AKI-CKD progression and oxidative stress are not totally clear and the underlying mechanisms in "AKI-CKD" remain indistinct. In this research, we constructed unilateral ischemia-reperfusion injury (UIRI)-model mice and performed single-nucleus RNA sequencing (snRNA-seq) of the kidney samples from UIRI and sham mice. We obtained our snRNA-seq data and validated the findings based on the joint analysis of public databases, as well as a series of fundamental experiments. Proximal tubular cells associated with failed repair express more complete senescence and oxidative stress characteristics compared to other subgroups. Furthermore, oxidative stress-related transcription factors, including Stat3 and Dnmt3a, are significantly more active under the circumstance of failed repair. What is more, we identified abnormally active intercellular communication between PT cells associated with failed repair and macrophages through the APP-CD74 pathway. More notably, we observed that the significantly increased expression of CD74 in hypoxia-treated TECs (tubular epithelial cells) was dependent on adjacently infiltrated macrophages, which was essential for the further deterioration of failed repair in PT cells. This research provides a novel understanding of the process of AKI to CKD progression, and the oxidative stress-related characteristics that we identified might represent a potentially novel therapeutic strategy against AKI.

The microplastics exposure induce the kidney injury in mice revealed by RNA-seq

Microplastics (MPs) may pollute drinking water, accumulate in the food chain, and release toxic chemicals that may cause a variety of diseases. The detrimental effects of MPs on kidney injury and fibrosis under long-term accumulation have not been fully documented. In this study, mice were exposed to MPs with three different diameters (80 nm, 0.5 µm, and 5 µm) to investigate the detrimental influences of MPs on the kidney. The results showed that MPs of different diameters caused varying degrees of injury to the murine kidney. MPs exposure can induce an inflammatory response, oxidative stress, and cell apoptosis in the kidney and induce kidney injury, which ultimately promotes kidney fibrosis. Furthermore, transcriptome data revealed that chronic exposure to MPs could alter the expressions of multiple genes related to immune response (80 nm) and circadian rhythm (0.5 µm, and 5 µm). Overall, our data provide new evidence and potential research for investigating the harm of MPs to kidney of mammals and even humans.

Modeling Podocyte Ontogeny and Podocytopathies with the Zebrafish

Podocytes are exquisitely fashioned kidney cells that serve an essential role in the process of blood filtration. Congenital malformation or damage to podocytes has dire consequences and initiates a cascade of pathological changes leading to renal disease states known as podocytopathies. In addition, animal models have been integral to discovering the molecular pathways that direct the development of podocytes. In this review, we explore how researchers have used the zebrafish to illuminate new insights about the processes of podocyte ontogeny, model podocytopathies, and create opportunities to discover future therapies.

Age-related glomerular histogenesis in inbred indigenous rabbit (Oryctolagus cuniculus): A morphological, morphometrical, and immunohistochemical study with emphasis on Lgr5-positive cells

Although the regeneration of renal glomeruli and nephrons after injuries especially in adult mammals is not possible, understanding normal glomerular histogenesis is important. Here, we sought to study the morphometrical and histological development of the normal renal glomeruli of rabbits from birth until postnatal day 40. Moreover, we immunohistochemically evaluated the extent and rate of the Lgr5 expression in the immature renal stem/progenitor cells. The untreated, clinically healthy inbred indigenous rabbits (from Duhok city of Iraqi Kurdistan) were sacrificed at postnatal days 1, 10, 15, 30, and 40. After being processed and embedded in paraffin, rabbit anti-human Lgr5 as a primary antibody and rabbit ImmunoCruz LSAB as a staining kit were used for the immunohistochemical detection of Lgr5^+ve cells. For normal histology, hematoxylin and eosin were used. The peak generation and regression of renal corpuscles were at postnatal days 10, and 40, respectively, with 50% decrease. The glomeruli diameter significantly increased (1.3-fold, p = 0.001), whereas the Bowman's space diameter decreased (50%, p < 0.0001) from postnatal day 1-40. The immature nephrons were seen only in one-day postnatal rabbits. While the superficial glomeruli were compact and small, the juxtamedullary glomeruli were larger and segmented. The formation and development of the juxtaglomerular apparatus were documented at postnatal days 30 and 40 only. Our data revealed highly expressed Lgr5 protein at postnatal day one, and the expression level decreased gradually with advancing age. It was moderately expressed on day 10 and mildly expressed on day 15, whereas no expression was recorded on days 30 and 40 postnatally. Our study provides evidence that the Lgr5 gene, within multipotent stem cells and their lineage progeny, was activated within newly formed glomeruli throughout the early postnatal stages of nephrogenesis.

Identification of functional pathways for regenerative bioactivity of selected renal cells

Background

Selected renal cells (SRC) are in Phase II clinical trials as a kidney-sourced, autologous, tubular epithelial cell-enriched cell-based therapy for chronic kidney disease (CKD). In preclinical studies with rodent models of CKD, SRC have been shown to positively modulate key renal biomarkers associated with development of the chronic disease condition.

Methods

A comparative bioinformatic analysis of transcripts specifically enriched or depleted in SRC component sub-populations relative to the initial, biopsy-derived cell source was conducted.

Results

Outcomes associated with therapeutically relevant bioactivity from a systematic, genome-wide transcriptomic profiling of rodent SRC are reported. Key transcriptomic networks and concomitant signaling pathways that may underlie SRC mechanism of action as manifested by reparative, restorative, and regenerative bioactivity in rodent models of chronic kidney disease are identified. These include genes and gene networks associated with cell cycle control, transcriptional control, inflammation, ECM–receptor interaction, immune response, actin polymerization, regeneration, cell adhesion, and morphogenesis.

Conclusions

These data indicate that gene networks associated with development of the kidney are also leveraged for SRC regenerative bioactivity, providing evidence of potential mechanisms of action.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02713-6.

Inhibition of HDAC3 protects against kidney cold storage/transplantation injury and allograft dysfunction

Cold storage/rewarming is an inevitable process for kidney transplantation from deceased donors, which correlates closely with renal ischemia-reperfusion injury (IRI) and the occurrence of delayed graft function. Histone deacetylases (HDAC) are important epigenetic regulators, but their involvement in cold storage/rewarming injury in kidney transplantation is unclear. In the present study, we showed a dynamic change of HDAC3 in a mouse model of kidney cold storage followed by transplantation. We then demonstrated that the selective HDAC3 inhibitor RGFP966 could reduce acute tubular injury and cell death after prolonged cold storage with transplantation. RGFP966 also improved renal function, kidney repair and tubular integrity when the transplanted kidney became the sole life-supporting graft in the recipient mouse. In vitro, cold storage of proximal tubular cells followed by rewarming induced remarkable cell death, which was suppressed by RGFP966 or knockdown of HDAC3 with shRNA. Inhibition of HDAC3 decreased the mitochondrial pathway of apoptosis and preserved mitochondrial membrane potential. Collectively, HDAC3 plays a pathogenic role in cold storage/rewarming injury in kidney transplantation, and its inhibition may be a therapeutic option.

Single-nuclear transcriptomics reveals diversity of proximal tubule cell states in a dynamic response to acute kidney injury

A single acute kidney injury event increases the risk of progression to chronic kidney disease (CKD). Combining single-nucleus RNA sequencing with genetic tracing of injured proximal tubule cells identified a spatially dynamic, evolving injury response following ischemia–reperfusion injury. Failed proximal tubule repair leads to the persistence of a profibrotic, proinflammatory Vcam1⁺/Ccl2⁺ cell type exhibiting a senescence-associated secretory phenotype and a marked transcriptional activation of NF-κB and AP-1 pathway signatures, but no signs of G₂/M cell cycle arrest. Insights from this study can inform strategies to improve renal repair and prevent CKD progression.

Acute kidney injury (AKI), commonly caused by ischemia, sepsis, or nephrotoxic insult, is associated with increased mortality and a heightened risk of chronic kidney disease (CKD). AKI results in the dysfunction or death of proximal tubule cells (PTCs), triggering a poorly understood autologous cellular repair program. Defective repair associates with a long-term transition to CKD. We performed a mild-to-moderate ischemia–reperfusion injury (IRI) to model injury responses reflective of kidney injury in a variety of clinical settings, including kidney transplant surgery. Single-nucleus RNA sequencing of genetically labeled injured PTCs at 7-d (“early”) and 28-d (“late”) time points post-IRI identified specific gene and pathway activity in the injury–repair transition. In particular, we identified Vcam1⁺/Ccl2⁺ PTCs at a late injury stage distinguished by marked activation of NF-κB–, TNF-, and AP-1–signaling pathways. This population of PTCs showed features of a senescence-associated secretory phenotype but did not exhibit G₂/M cell cycle arrest, distinct from other reports of maladaptive PTCs following kidney injury. Fate-mapping experiments identified spatially and temporally distinct origins for these cells. At the cortico-medullary boundary (CMB), where injury initiates, the majority of Vcam1⁺/Ccl2⁺ PTCs arose from early replicating PTCs. In contrast, in cortical regions, only a subset of Vcam1⁺/Ccl2⁺ PTCs could be traced to early repairing cells, suggesting late-arising sites of secondary PTC injury. Together, these data indicate even moderate IRI is associated with a lasting injury, which spreads from the CMB to cortical regions. Remaining failed-repair PTCs are likely triggers for chronic disease progression.

Anti-fibrotic effects of synthetic TGF-β1 and Smad oligodeoxynucleotide on kidney fibrosis in vivo and in vitro through inhibition of both epithelial dedifferentiation and endothelial-mesenchymal transitions

Kidney fibrosis is a common process of various kidney diseases leading to end-stage renal failure irrespective of etiology. Myofibroblasts are crucial mediators in kidney fibrosis through production of extracellular matrix (ECM), but their origin has not been clearly identified. Many study proposed that epithelial and endothelial cells become myofibroblasts by epithelial dedifferentiation and endothelial-mesenchymal transition (EndoMT). TGF-β1/Smad signaling plays a crucial role in partly epithelial-mensencymal transition (EMT) and EndoMT. Thus, we designed the TGF-β1/Smad oligodeoxynucleotide (ODN), a synthetic short DNA containing complementary sequence for Smad transcription factor and TGF-β1 mRNA. Therefore, this study investigated the anti-fibrotic effect of synthetic TGF-β1/Smad ODN on UUO-induced kidney fibrosis in vivo model and TGF-β1-induced in vitro model. To examine the effect of TGF-β1/Smad ODN, we performed various experiments to evaluate kidney fibrosis. The results showed that UUO induced inflammation, ECM accumulation, epithelial dedifferentiation and EndoMT processes, and tubular atrophy. However, synthetic TGF-β1/Smad ODN significantly suppressed UUO-induced fibrosis. Furthermore, synthetic ODN attenuated TGF-β1-induced epithelial dedifferentiation and EndoMT program via blocking TGF-β1/Smad signaling. In conclusion, this study demonstrated that administration of synthetic TGF-β1/Smad ODN attenuates kidney fibrosis, epithelial dedifferentiation, and EndoMT processes. The findings propose the possibility of synthetic ODN as a new effective therapeutic tool for kidney fibrosis.

Wnt signaling mediates new nephron formation during zebrafish kidney regeneration

Zebrafish kidneys use resident kidney stem cells to replace damaged tubules with new nephrons: the filtration units of the kidney. What stimulates kidney progenitor cells to form new nephrons is not known. Here, we show that wnt9a and wnt9b are induced in the injured kidney at sites where frizzled9b- and lef1-expressing progenitor cells form new nephrons. New nephron aggregates are patterned by Wnt signaling, with high canonical Wnt-signaling cells forming a single cell thick rosette that demarcates: domains of cell proliferation in the elongating nephron; and tubule fusion where the new nephron plumbs into the distal tubule and establishes blood filtrate drainage. Pharmacological blockade of canonical Wnt signaling inhibited new nephron formation after injury by inhibiting cell proliferation, and resulted in loss of polarized rosette structures in the aggregates. Mutation in frizzled9b reduced total kidney nephron number, caused defects in tubule morphology and reduced regeneration of new nephrons after injury. Our results demonstrate an essential role for Wnt/frizzled signaling in adult zebrafish kidney development and regeneration, highlighting conserved mechanisms underlying both mammalian kidney development and kidney stem cell-directed neonephrogenesis in zebrafish.

Summary: Adult zebrafish kidneys induce Wnt signaling to generate new nephrons from resident kidney progenitor cells, highlighting how embryonic morphogens are reactivated in adult organs to drive regeneration.

Tubular epithelial C1orf54 mediates protection and recovery from acute kidney injury

Acute kidney injury (AKI) incidence among hospitalized patients is increasing steadily. Despite progress in prevention strategies and support measures, AKI remains correlated with high mortality, particularly among ICU patients, and no effective AKI therapy exists. Here, we investigated the function in kidney ischaemia‐reperfusion injury (IRI) of C1orf54, a newly identified protein encoded by an open reading frame on chromosome 1. C1orf54 expression was high in kidney and low in heart, liver, spleen, lung and skeletal muscle in healthy mice, and in the kidney, C1orf54 was expressed in tubular epithelial cells (TECs), but not in glomeruli. C1orf54 expression was markedly decreased on Day 1 after kidney IRI and then gradually recovered to baseline levels by Day 7. Notably, relative to wild‐type mice, C1orf54‐knockout mice exhibited impaired TEC proliferation and delayed recovery after kidney IRI, which led to deteriorated renal function and increased mortality. Conversely, adenovirus‐mediated C1orf54 overexpression promoted TEC proliferation and ameliorated kidney pathology, which resulted in accelerated renal repair and improved renal function. Mechanistically, C1orf54 was found to promote TEC proliferation through PI3K/AKT signalling. Thus, C1orf54 holds considerable potential as a therapeutic target in kidney IRI.

Modeling Renal Disease "On the Fly"

Detoxification is a fundamental function for all living organisms that need to excrete catabolites and toxins to maintain homeostasis. Kidneys are major organs of detoxification that maintain water and electrolyte balance to preserve physiological functions of vertebrates. In insects, the renal function is carried out by Malpighian tubules and nephrocytes. Due to differences in their circulation, the renal systems of mammalians and insects differ in their functional modalities, yet carry out similar biochemical and physiological functions and share extensive genetic and molecular similarities. Evolutionary conservation can be leveraged to model specific aspects of the complex mammalian kidney function in the genetic powerhouse Drosophila melanogaster to study how genes interact in diseased states. Here, we compare the human and Drosophila renal systems and present selected fly disease models.

Intraglomerular crosstalk elaborately regulates podocyte injury and repair in diabetic patients: insights from a 3D multiscale modeling study

Podocytes are mainly involved in the regulation of glomerular filtration rate (GFR) under physiological condition. Podocyte depletion is a crucial pathological alteration in diabetic nephropathy (DN) and results in a broad spectrum of clinical syndromes such as protein urine and renal insufficiency. Recent studies indicate that depleted podocytes can be regenerated via differentiation of the parietal epithelial cells (PECs), which serve as the local progenitors of podocytes. However, the podocyte regeneration process is regulated by a complicated mechanism of cell-cell interactions and cytokine stimulations, which has been studied in a piecemeal manner rather than systematically. To address this gap, we developed a high-resolution multi-scale multi-agent mathematical model in 3D, mimicking the in situ glomerulus anatomical structure and micro-environment, to simulate the podocyte regeneration process under various cytokine perturbations in healthy and diabetic conditions. Our model showed that, treatment with pigment epithelium derived factor (PEDF) or insulin-like growth factor-1 (IGF-1) alone merely ameliorated the glomerulus injury, while co-treatment with both cytokines replenished the damaged podocyte population gradually. In addition, our model suggested that continuous administration of PEDF instead of a bolus injection sustained the regeneration process of podocytes. Part of the results has been validated by our in vivo experiments. These results indicated that amelioration of the glomerular stress by PEDF and promotion of PEC differentiation by IGF-1 are equivalently critical for podocyte regeneration. Our 3D multi-scale model represents a powerful tool for understanding the signaling regulation and guiding the design of cytokine therapies in promoting podocyte regeneration.

Chimeric maternal cells in offspring do not respond to renal injury, inflammatory or repair signals

Maternal microchimerism (MMc) can persist for years in a child, and has been implicated in the pathogenesis of chronic inflammatory autoimmune diseases. Chimeric cells may either contribute to disease by acting as immune targets or expand in response to signals of injury, inflammation or repair. We investigated the role of maternal cells in tissue injury in the absence of autoimmunity by quantifying MMc by quantitative PCR in acute and chronic models of renal injury: (1) reversible acute renal injury, inflammation and regeneration induced by rhabdomyolysis and (2) chronic injury leading to fibrosis after unilateral ureteral obstruction. We found that MMc is common in the mouse kidney. In mice congenic with their mothers neither acute nor chronic renal injury with fibrosis influenced the levels or prevalence of MMc. Maternal cells expressing MHC antigens not shared by offspring (H2(b/d)) were detected at lower levels in all groups of homozygous H2(b/b) or H2(d/d) offspring, with or without renal injury, suggesting that partial tolerance to low levels of alloantigens may regulate the homeostatic levels of maternal cells within tissues. Maternal cells homozygous for H2(b) were lost in H2(b/d) offspring only after acute renal failure, suggesting that an inflammatory stimulus led to loss of tolerance to homozygous maternal cells. The study suggests that elevated MMc previously found in association with human autoimmune diseases may not be a response to non-specific injury or inflammatory signals, but rather a primary event integral to the pathogenesis of autoimmunity.

Beneficial Effects of Systemically Administered Human Muse Cells in Adriamycin Nephropathy

Multilineage-differentiating stress-enduring (Muse) cells are nontumorigenic endogenous pluripotent-like stem cells that can be collected from various organs. Intravenously administered Muse cells have been shown to spontaneously migrate to damaged tissue and replenish lost cells, but the effect in FSGS is unknown. We systemically administered human bone marrow-derived Muse cells without concurrent administration of immunosuppressants to severe combined immune-deficient (SCID) and BALB/c mouse models with adriamycin-induced FSGS (FSGS-SCID and FSGS-BALB/c, respectively). In FSGS-SCID mice, human Muse cells preferentially integrated into the damaged glomeruli and spontaneously differentiated into cells expressing markers of podocytes (podocin; 31%), mesangial cells (megsin; 13%), and endothelial cells (CD31; 41%) without fusing to the host cells; attenuated glomerular sclerosis and interstitial fibrosis; and induced the recovery of creatinine clearance at 7 weeks. Human Muse cells induced similar effects in FSGS-BALB/c mice at 5 weeks, despite xenotransplant without concurrent immunosuppressant administration, and led to improvement in urine protein, creatinine clearance, and plasma creatinine levels more impressive than that in the FSGS-SCID mice at 5 weeks. However, functional recovery in FSGS-BALB/c mice was impaired at 7 weeks due to immunorejection, suggesting the importance of Muse cell survival as glomerular cells in the FSGS kidney for tissue repair and functional recovery. In conclusion, Muse cells are unique reparative stem cells that preferentially home to damaged glomeruli and spontaneously differentiate into glomerular cells after systemic administration. Introduction of genes to induce differentiation is not required before Muse cell administration; thus, Muse cells may be a feasible therapeutic strategy in FSGS.

Selective activation of epidermal growth factor receptor in renal proximal tubule induces tubulointerstitial fibrosis

Epidermal growth factor receptor (EGFR) has been implicated in the pathogenesis of diabetic nephropathy and renal fibrosis; however, the causative role of sustained EGFR activation is unclear. Here, we generated a novel kidney fibrotic mouse model of persistent EGFR activation by selectively expressing the EGFR ligand, human heparin-binding EGF-like growth factor (hHB-EGF), in renal proximal tubule epithelium. hHB-EGF expression increased tyrosine kinase phosphorylation of EGFR and the subsequent activation of downstream signaling pathways, including ERK and AKT, as well as the profibrotic TGF-β1/SMAD pathway. Epithelial-specific activation of EGFR was sufficient to promote spontaneous and progressive renal tubulointerstitial fibrosis, as characterized by increased collagen deposition, immune cell infiltration, and α-smooth muscle actin (α-SMA)-positive myofibroblasts. Tubule-specific EGFR activation promoted epithelial dedifferentiation and cell-cycle arrest. Furthermore, EGFR activation in epithelial cells promoted the proliferation of α-SMA⁺ myofibroblasts in a paracrine manner. Genetic or pharmacologic inhibition of EGFR tyrosine kinase activity or downstream MEK activity attenuated the fibrotic phenotype. This study provides definitive evidence that sustained activation of EGFR in proximal epithelia is sufficient to cause spontaneous, progressive renal tubulointerstitial fibrosis, evident by epithelial dedifferentiation, increased myofibroblasts, immune cell infiltration, and increased matrix deposition.-Overstreet, J. M., Wang, Y., Wang, X., Niu, A., Gewin, L. S., Yao, B., Harris, R. C., Zhang, M.-Z. Selective activation of epidermal growth factor receptor in renal proximal tubule induces tubulointerstitial fibrosis.

Bone mesenchymal stem cells ameliorate ischemia/reperfusion-induced damage in renal epithelial cells via microRNA-223

Background

Recent studies have indicated that microRNA-223 (miR-223) plays a role in the tissue-protective effect of mesenchymal stem cells (MSCs). NLR family-pyrin domain containing 3 (NLRP3) was reported to affect a renal ischemia/reperfusion (I/R) injury by exerting a direct effect on the renal tubular epithelium. Therefore, we investigated how miR-223 and NLRP3 might function in kidneys exposed to conditions of ischemia and subsequent reperfusion.

Methods

Hypoxia/reoxygenation (H/R) murine renal tubular epithelial cells (RTECs) were cocultured with either MSCs or hypoxia-pretreated MSCs (htMSCs), after which the RTECs were examined for their viability and evidence of apoptosis. Next, miR-223 expression in the MSCs was downregulated to verify that MSCs protected RTECs via the transport of miR-223. Kidney I/R KM/NIH mouse models were created and used for in vivo studies.

Results

The results showed that coculture with MSCs significantly increased the viability of RTECs and decreased their rates of apoptosis. The levels of hepatocyte growth factor (HGF), insulin-like growth factor-1 (IGF-1), transforming growth factor beta (TGF-β), and vascular endothelial growth factor (VEGF) in samples of coculture supernatants were higher than those in samples of non-coculture supernatants. A bioinformatics analysis revealed a targeting relationship between miR-223 and NLRP3. A dual luciferase assay showed that miR-223 inhibited NLRP3 expression. The htMSCs displayed a protective function associated with an upregulation of miR-223 as induced by Notch1 and the downregulation of NLRP3. Conversely, inhibition of miR-223 impeded the protective effect of MSCs. In the I/R mouse models, injection of either MSCs or htMSCs ameliorated the damage to kidney tissue, while suppression of miR-223 expression in MSCs reduced their protective effect on mouse kidneys.

Conclusions

Our results demonstrate that miR-223 and NLRP3 play important roles in the treatment of renal tissue injuries with transplanted MSCs.

Transcription Factor Trps1 Promotes Tubular Cell Proliferation after Ischemia-Reperfusion Injury through cAMP-Specific 3',5'-Cyclic Phosphodiesterase 4D and AKT

Trichorhinophalangeal 1 (Trps1) is a transcription factor essential for epithelial cell morphogenesis during kidney development, but the role of Trps1 in AKI induced by ischemia-reperfusion (I/R) remains unclear. Our study investigated Trps1 expression during kidney repair after acute I/R in rats and explored the molecular mechanisms by which Trps1 promotes renal tubular epithelial cell proliferation. Trps1 expression positively associated with the extent of renal repair after I/R injury. Compared with wild-type rats, rats with knockdown of Trps1 exhibited significantly delayed renal repair in the moderate I/R model, with lower GFR levels and more severe morphologic injury, whereas rats overexpressing Trps1 exhibited significantly accelerated renal repair after severe I/R injury. Additionally, knockdown of Trps1 inhibited and overexpression of Trps1 enhanced the proliferation of renal tubular epithelial cells in rats. Chromatin immunoprecipitation sequencing assays and RT-PCR revealed that Trps1 regulated cAMP-specific 3',5'-cyclic phosphodiesterase 4D (Pde4d) expression. Knockdown of Trps1 decreased the renal protein expression of Pde4d and phosphorylated Akt in rats, and dual luciferase analysis showed that Trps1 directly activated Pde4d transcription. Furthermore, knockdown of Pde4d or treatment with the phosphatidylinositol 3 kinase inhibitor wortmannin significantly inhibited Trps1-induced tubular cell proliferation in vitro Trps1 may promote tubular cell proliferation through the Pde4d/phosphatidylinositol 3 kinase/AKT signaling pathway, suggesting Trps1 as a potential therapeutic target for kidney repair after I/R injury.

BMP-7 Signaling and its Critical Roles in Kidney Development, the Responses to Renal Injury, and Chronic Kidney Disease

Chronic kidney disease (CKD) is a significant health problem that most commonly results from congenital abnormalities in children and chronic renal injury in adults. The therapeutic potential of BMP-7 was first recognized nearly two decades ago with studies demonstrating its requirement for kidney development and ability to inhibit the pathogenesis of renal injury in models of CKD. Since this time, our understanding of CKD has advanced considerably and treatment strategies have evolved with the identification of many additional signaling pathways, cell types, and pathologic processes that contribute to disease progression. The purpose of this review is to revisit the seminal studies that initially established the importance of BMP-7, highlight recent advances in BMP-7 research, and then integrate this knowledge with current research paradigms. We will provide an overview of the evolutionarily conserved roles of BMP proteins and the features that allow BMP signaling pathways to function as critical signaling nodes for controlling biological processes, including those related to CKD. We will discuss the multifaceted functions of BMP-7 during kidney development and the potential for alterations in BMP-7 signaling to result in congenital abnormalities and pediatric kidney disease. We will summarize the renal protective effects of recombinant BMP-7 in experimental models of CKD and then propose a model to describe the potential physiological role of endogenous BMP-7 in the innate repair mechanisms of the kidneys that respond to renal injury. Finally, we will highlight emerging clinical approaches for applying our knowledge of BMP-7 toward improving the treatment of patients with CKD.

Insights into kidney stem cell development and regeneration using zebrafish

Kidney disease is an escalating global health problem, for which the formulation of therapeutic approaches using stem cells has received increasing research attention. The complexity of kidney anatomy and function, which includes the diversity of renal cell types, poses formidable challenges in the identification of methods to generate replacement structures. Recent work using the zebrafish has revealed their high capacity to regenerate the integral working units of the kidney, known as nephrons, following acute injury. Here, we discuss these findings and explore the ways that zebrafish can be further utilized to gain a deeper molecular appreciation of renal stem cell biology, which may uncover important clues for regenerative medicine.

Selected renal cells modulate disease progression in rodent models of chronic kidney disease via NF-κB and TGF-β1 pathways

AIM

Identification of mechanistic pathways for selected renal cell (SRC) therapeutic bioactivity in rodent models of chronic kidney disease.

MATERIALS & METHODS

In vivo and in vitro functional bioassays applied to investigate regenerative outcomes associated with delivery of SRC to diseased rodent kidney.

RESULTS

In vivo, SRC reduces chronic infiltration by monocytes/macrophages. SRC attenuates NF-κB and PAI-1 responses while simultaneously promoting host tubular cell expansion through trophic cues. In vitro, SRC-derived conditioned media attenuates TNF-α-induced NF-κB response, TGF-β-mediated PAI-1 response and increases expression of transcripts associated with cell cycle regulation. Observed bioactive responses were from vesicle and nonvesicle-associated factors, including specific miRNAs.

CONCLUSION

We identify a paracrine mechanism for SRC immunomodulatory and trophic cues on host renal tissues, catalyzing long-term functional benefits in vivo.

Urinary mitochondrial DNA is a biomarker of mitochondrial disruption and renal dysfunction in acute kidney injury

Recent studies show the importance of mitochondrial dysfunction in the initiation and progression of acute kidney injury (AKI). However, no biomarkers exist linking renal injury to mitochondrial function and integrity. To this end, we evaluated urinary mitochondrial DNA (UmtDNA) as a biomarker of renal injury and function in humans with AKI following cardiac surgery. mtDNA was isolated from the urine of patients following cardiac surgery and quantified by quantitative PCR. Patients were stratified into no AKI, stable AKI, and progressive AKI groups based on Acute Kidney Injury Network (AKIN) staging. UmtDNA was elevated in progressive AKI patients and was associated with progression of patients with AKI at collection to higher AKIN stages. To evaluate the relationship of UmtDNA to measures of renal mitochondrial integrity in AKI, mice were subjected to sham surgery or varying degrees of ischemia followed by 24 h of reperfusion. UmtDNA increased in mice after 10-15 min of ischemia and positively correlated with ischemia time. Furthermore, UmtDNA was predictive of AKI in the mouse model. Finally, UmtDNA levels were negatively correlated with renal cortical mtDNA and mitochondrial gene expression. These translational studies demonstrate that UmtDNA is associated with recovery from AKI following cardiac surgery by serving as an indicator of mitochondrial integrity. Thus UmtDNA may serve as valuable biomarker for the development of mitochondrial-targeted therapies in AKI.

Kidney Regeneration in Adult Zebrafish by Gentamicin Induced Injury

The kidney is essential for fluid homeostasis, blood pressure regulation and filtration of waste from the body. The fundamental unit of kidney function is the nephron. Mammals are able to repair existing nephrons after injury, but lose the ability to form new nephrons soon after birth. In contrast to mammals, adult fish produce new nephrons (neonephrogenesis) throughout their lives in response to growth requirements or injury. Recently, lhx1a has been shown to mark nephron progenitor cells in the adult zebrafish kidney, however mechanisms controlling the formation of new nephrons after injury remain unknown. Here we show our method for robust and reproducible injury in the adult zebrafish kidney by intraperitoneal (i.p.) injection of gentamicin, which uses a noninvasive visual screening process to select for fish with strong but nonlethal injury. Using this method, we can determine optimal gentamicin dosages for injury and go on to demonstrate the effect of higher temperatures on kidney regeneration in zebrafish.

Improved Structure and Function in Autosomal Recessive Polycystic Rat Kidneys with Renal Tubular Cell Therapy

Autosomal recessive polycystic kidney disease is a truly catastrophic monogenetic disease, causing death and end stage renal disease in neonates and children. Using PCK female rats, an orthologous model of autosomal recessive polycystic kidney disease harboring mutant Pkhd1, we tested the hypothesis that intravenous renal cell transplantation with normal Sprague Dawley male kidney cells would improve the polycystic kidney disease phenotype. Cytotherapy with renal cells expressing wild type Pkhd1 and tubulogenic serum amyloid A1 had powerful and sustained beneficial effects on renal function and structure in the polycystic kidney disease model. Donor cell engraftment and both mutant and wild type Pkhd1 were found in treated but not control PCK kidneys 15 weeks after the final cell infusion. To examine the mechanisms of global protection with a small number of transplanted cells, we tested the hypothesis that exosomes derived from normal Sprague Dawley cells can limit the cystic phenotype of PCK recipient cells. We found that renal exosomes originating from normal Sprague Dawley cells carried and transferred wild type Pkhd1 mRNA to PCK cells in vivo and in vitro and restricted cyst formation by cultured PCK cells. The results indicate that transplantation with renal cells containing wild type Pkhd1 improves renal structure and function in autosomal recessive polycystic kidney disease and may provide an intra-renal supply of normal Pkhd1 mRNA.

Heme Oxygenase-1 Regulates Myeloid Cell Trafficking in AKI

Renal ischemia-reperfusion injury is mediated by a complex cascade of events, including the immune response, that occur secondary to injury to renal epithelial cells. We tested the hypothesis that heme oxygenase-1 (HO-1) expression, which is protective in ischemia-reperfusion injury, regulates trafficking of myeloid-derived immune cells in the kidney. Age-matched male wild-type (HO-1(+/+)), HO-1-knockout (HO-1(-/-)), and humanized HO-1-overexpressing (HBAC) mice underwent bilateral renal ischemia for 10 minutes. Ischemia-reperfusion injury resulted in significantly worse renal structure and function and increased mortality in HO-1(-/-) mice. In addition, there were more macrophages (CD45(+) CD11b(hi)F4/80(lo)) and neutrophils (CD45(+) CD11b(hi) MHCII(-) Gr-1(hi)) in HO-1(-/-) kidneys than in sham and HO-1(+/+) control kidneys subjected to ischemia-reperfusion. However, ischemic injury resulted in a significant decrease in the intrarenal resident dendritic cell (DC; CD45(+)MHCII(+)CD11b(lo)F4/80(hi)) population in HO-1(-/-) kidneys compared with controls. Syngeneic transplant experiments utilizing green fluorescent protein-positive HO-1(+/+) or HO-1(-/-) donor kidneys and green fluorescent protein-negative HO-1(+/+) recipients confirmed increased migration of the resident DC population from HO-1(-/-) donor kidneys, compared to HO-1(+/+) donor kidneys, to the peripheral lymphoid organs. This effect on renal DC migration was corroborated in myeloid-specific HO-1(-/-) mice subjected to bilateral ischemia. These mice also displayed impaired renal recovery and increased fibrosis at day 7 after injury. These results highlight an important role for HO-1 in orchestrating the trafficking of myeloid cells in AKI, which may represent a key pathway for therapeutic intervention.

Renal stem cell reprogramming: Prospects in regenerative medicine

Stem cell therapy is a promising future enterprise for renal replacement in patients with acute and chronic kidney disease, conditions which affect millions worldwide and currently require patients to undergo lifelong medical treatments through dialysis and/or organ transplant. Reprogramming differentiated renal cells harvested from the patient back into a pluripotent state would decrease the risk of tissue rejection and provide a virtually unlimited supply of cells for regenerative medicine treatments, making it an exciting area of current research in nephrology. Among the major hurdles that need to be overcome before stem cell therapy for the kidney can be applied in a clinical setting are ensuring the fidelity and relative safety of the reprogrammed cells, as well as achieving feasible efficiency in the reprogramming processes that are utilized. Further, improved knowledge about the genetic control of renal lineage development is vital to identifying predictable and efficient reprogramming approaches, such as the expression of key modulators or the regulation of gene activity through small molecule mimetics. Here, we discuss several recent advances in induced pluripotent stem cell technologies. We also explore strategies that have been successful in renal progenitor generation, and explore what these methods might mean for the development of cell-based regenerative therapies for kidney disease.

Maternal protein-energy malnutrition during early pregnancy in sheep impacts the fetal ornithine cycle to reduce fetal kidney microvascular development

This paper identifies a common nutritional pathway relating maternal through to fetal protein-energy malnutrition (PEM) and compromised fetal kidney development. Thirty-one twin-bearing sheep were fed either a control (n=15) or low-protein diet (n=16, 17 vs. 8.7 g crude protein/MJ metabolizable energy) from d 0 to 65 gestation (term, ∼145 d). Effects on the maternal and fetal nutritional environment were characterized by sampling blood and amniotic fluid. Kidney development was characterized by histology, immunohistochemistry, vascular corrosion casts, and molecular biology. PEM had little measureable effect on maternal and fetal macronutrient balance (glucose, total protein, total amino acids, and lactate were unaffected) or on fetal growth. PEM decreased maternal and fetal urea concentration, which blunted fetal ornithine availability and affected fetal hepatic polyamine production. For the first time in a large animal model, we associated these nutritional effects with reduced micro- but not macrovascular development in the fetal kidney. Maternal PEM specifically impacts the fetal ornithine cycle, affecting cellular polyamine metabolism and microvascular development of the fetal kidney, effects that likely underpin programming of kidney development and function by a maternal low protein diet.—Dunford, L. J., Sinclair, K. D., Kwong, W. Y., Sturrock, C., Clifford, B. L., Giles, T. C., Gardner, D. S.. Maternal protein-energy malnutrition during early pregnancy in sheep impacts the fetal ornithine cycle to reduce fetal kidney microvascular development.

Kidney injury, stem cells and regeneration

PURPOSE OF REVIEW

The review summarizes the most recent advances in stem cell and regenerative approaches to treat kidney injury, and highlights areas of active controversy. Over the past year, a number of findings have been reported that have brought this field much closer to clinical translation.

RECENT FINDINGS

Recent progress in regenerative nephrology includes the directed differentiation of embryonic stem cells to kidney fates, understanding the proliferative capacity of tubules after injury, the use of mesenchymal stem cells for kidney disease and tissue engineering approaches to renal replacement. Controversies persist, however, including whether adult epithelial stem cells exist at all, the best therapeutic strategy for the treatment of kidney injury and how to use mesenchymal stem cells optimally for the prevention of acute kidney injury.

SUMMARY

Although recent progress in kidney regeneration is very encouraging, current controversies must be resolved before clinical breakthroughs can occur.

Emerging roles of hematopoietic cells in the pathobiology of diabetic complications

Diabetic complications encompass macrovascular events, mainly the result of accelerated atherosclerosis, and microvascular events that strike the eye (retinopathy), kidney (nephropathy), and nervous system (neuropathy). The traditional view is that hyperglycemia-induced dysregulated biochemical pathways cause injury and death of cells intrinsic to the organs affected. There is emerging evidence that diabetes compromises the function of the bone marrow (BM), producing a stem cell niche-dependent defect in hematopoietic stem cell mobilization. Furthermore, dysfunctional BM-derived hematopoietic cells contribute to diabetic complications. Thus, BM cells are not only a victim but also an accomplice in diabetes and diabetic complications. Understanding the underlying molecular mechanisms may lead to the development of new therapies to prevent and/or treat diabetic complications by specifically targeting these perpetrators.

Kidney regeneration: common themes from the embryo to the adult

The vertebrate kidney has an inherent ability to regenerate following acute damage. Successful regeneration of the injured kidney requires the rapid replacement of damaged tubular epithelial cells and reconstitution of normal tubular function. Identifying the cells that participate in the regeneration process as well as the molecular mechanisms involved may reveal therapeutic targets for the treatment of kidney disease. Renal regeneration is associated with the expression of genetic pathways that are necessary for kidney organogenesis, suggesting that the regenerating tubular epithelium may be "reprogrammed" to a less-differentiated, progenitor state. This review will highlight data from various vertebrate models supporting the hypothesis that nephrogenic genes are reactivated as part of the process of kidney regeneration following acute kidney injury (AKI). Emphasis will be placed on the reactivation of developmental pathways and how our understanding of the resulting regeneration process may be enhanced by lessons learned in the embryonic kidney.

Promotion of cell proliferation by clusterin in the renal tissue repair phase after ischemia-reperfusion injury

Renal repair begins soon after the kidney suffers ischemia-reperfusion injury (IRI); however, its molecular pathways are not fully understood. Clusterin (Clu) is a chaperone protein with cytoprotective functions in renal IRI. The aim of this study was to investigate the role of Clu in renal repair after IRI. IRI was induced in the left kidneys of wild-type (WT) C57BL/6J (B6) vs. Clu knockout (KO) B6 mice by clamping the renal pedicles for 28–45 min at the body temperature of 32°C. The renal repair was assessed by histology and confirmed by renal function. Gene expression was examined using PCR array. Here, we show that following IRI, renal tubular damage and Clu expression in WT kidneys were induced at day 1, reached the maximum at day 3, and significantly diminished at day 7 along with normal function, whereas the tubular damage in Clu KO kidneys steadily increased from initiation of insult to the end of the experiment, when renal failure occurred. Renal repair in WT kidneys was positively correlated with an increase in Ki67+ proliferative tubular cells and survival from IRI. The functions of Clu in renal repair and renal tubular cell proliferation in cultures were associated with upregulation of a panel of genes that could positively regulate cell cycle progression and DNA damage repair, which might promote cell proliferation but not involve cell migration. In conclusion, these data suggest that Clu is required for renal tissue regeneration in the kidney repair phase after IRI, which is associated with promotion of tubular cell proliferation.

Early treatment with xenon protects against the cold ischemia associated with chronic allograft nephropathy in rats

Chronic allograft nephropathy (CAN) is a common finding in kidney grafts with functional impairment. Prolonged hypothermic storage-induced ischemia-reperfusion injury is associated with the early onset of CAN. As the noble gas xenon is clinically used as an anesthetic and has renoprotective properties in a rodent model of ischemia-reperfusion injury, we studied whether early treatment with xenon could attenuate CAN associated with prolonged hypothermic storage. Exposure to xenon enhanced the expression of insulin growth factor-1 (IGF-1) and its receptor in human proximal tubular (HK-2) cells, which, in turn, increased cell proliferation. Xenon treatment before or after hypothermia-hypoxia decreased cell apoptosis and cell inflammation after reoxygenation. The xenon-induced HK-2 cell proliferation was abolished by blocking the IGF-1 receptor, mTOR, and HIF-1α individually. In the Fischer-to-Lewis rat allogeneic renal transplantation model, xenon exposure of donors before graft retrieval or recipients after engraftment enhanced tubular cell proliferation and decreased tubular cell death and cell inflammation associated with ischemia-reperfusion injury. Compared with control allografts, xenon treatment significantly suppressed T-cell infiltration and fibrosis, prevented the development of CAN, and improved renal function. Thus, xenon treatment promoted recovery from ischemia-reperfusion injury and reduced susceptibility to the subsequent development of CAN in allografts.

Met activation is required for early cytoprotection after ischemic kidney injury

Renal proximal tubule epithelial cells express high levels of the hepatocyte growth factor receptor Met, and both the receptor and ligand are upregulated after ischemic injury. Activation of the Met receptor after hepatocyte growth factor stimulation in vitro promotes activities involved in kidney repair, including cell survival, migration, and proliferation. However, characterizing the in vivo role of these signaling events in proximal tubule responses to kidney injury has been difficult because global Met knockout results in embryonic lethality due to placental and liver abnormalities. Here, we used γGT-Cre to knockout Met receptor expression selectively in the proximal tubules of mice (γGT-Cre;Met(fl/fl)). The kidneys of these mice developed normally, but exhibited increased initial tubular injury, tubular cell apoptosis, and serum creatinine after ischemia/reperfusion compared with γGT-Cre;Met(+/+) kidneys. These changes in γGT-Cre;Met(fl/fl) mice correlated with a selective reduction in PI3K/Akt activation in response to injury and subsequent decreases in inhibitory phosphorylation of the proapoptotic factor Bad and activating phosphorylation of the ribosomal regulatory protein p70-S6 kinase. Moreover, tubular cell proliferation after ischemia/reperfusion was delayed in γGT-Cre;Met(fl/fl) mice. In conclusion, this study identifies Met-dependent phosphoinositide 3-kinase activation in proximal tubules as a critical determinant of initial tubular cell survival and reparative proliferation after ischemic injury.

Intravenous renal cell transplantation with SAA1-positive cells prevents the progression of chronic renal failure in rats with ischemic-diabetic nephropathy

Diabetic nephropathy, the most common cause of progressive chronic renal failure and end-stage renal disease, has now reached global proportions. The only means to rescue diabetic patients on dialysis is renal transplantation, a very effective therapy but severely limited by the availability of donor kidneys. Hence, we tested the role of intravenous renal cell transplantation (IRCT) on obese/diabetic Zucker/SHHF F1 hybrid (ZS) female rats with severe ischemic and diabetic nephropathy. Renal ischemia was produced by bilateral renal clamping of the renal arteries at 10 wk of age, and IRCT with genetically modified normal ZS male tubular cells was given intravenously at 15 and 20 wk of age. Rats were euthanized at 34 wk of age. IRCT with cells expressing serum amyloid A had strong and long-lasting beneficial effects on renal function and structure, including tubules and glomeruli. However, donor cells were found engrafted only in renal tubules 14 wk after the second infusion. The results indicate that IRCT with serum amyloid A-positive cells is effective in preventing the progression of chronic kidney disease in rats with diabetic and ischemic nephropathy.

Reconstitution of kidney side population cells after ischemia-reperfusion injury by self-proliferation and bone marrow-derived cell homing

The aim of this study was to examine the contribution of side population (SP) cells from kidney and bone marrow for reconstitution of kidney SP pools after ischemia-reperfusion injury (IRI). The SP and non-SP cells in kidneys following IRI were isolated and serially assessed by fluorescence-activated cell sorting. The apoptosis, proliferation, phenotype, and paracrine actions of SP cells were evaluated in vitro and in vivo. Results indicated that the SP cells from ischemic kidney were acutely depleted within one day following renal IRI and were progressively restored to baseline within 7 days after IRI, through both proliferation of remaining kidney SP cells and homing of bone marrow-derived cells to ischemic kidney. Either hypoxia or serum deprivation alone increased apoptosis of SP cells, and a combination of both further aggravated it. Furthermore, hypoxia in vivo and in vitro induced the increase in the secretion of vascular endothelial growth factor, insulin-like growth factor 1, hepatocyte growth factor, and stromal cell-derived factor-1 α in kidney SP but not non-SP cells. In summary, these results suggest that following renal IRI, kidney SP cells are acutely depleted and then progressively restored to baseline levels by both self-proliferation and extrarenal source, that is, bone marrow-derived cell homing.

NADPH oxidase as a therapeutic target for oxalate induced injury in kidneys

A major role of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase family of enzymes is to catalyze the production of superoxides and other reactive oxygen species (ROS). These ROS, in turn, play a key role as messengers in cell signal transduction and cell cycling, but when they are produced in excess they can lead to oxidative stress (OS). Oxidative stress in the kidneys is now considered a major cause of renal injury and inflammation, giving rise to a variety of pathological disorders. In this review, we discuss the putative role of oxalate in producing oxidative stress via the production of reactive oxygen species by isoforms of NADPH oxidases expressed in different cellular locations of the kidneys. Most renal cells produce ROS, and recent data indicate a direct correlation between upregulated gene expressions of NADPH oxidase, ROS, and inflammation. Renal tissue expression of multiple NADPH oxidase isoforms most likely will impact the future use of different antioxidants and NADPH oxidase inhibitors to minimize OS and renal tissue injury in hyperoxaluria-induced kidney stone disease.

Restitutio ad integrum: a dream or a real possibility?

The subject of organ regeneration has attracted substantial investigative attention and has been extensively reviewed. Therefore, I shall focus on several only recently emerged issues and on those aspects of stem cell-mediated regeneration which, although are important in my opinion, have nevertheless evaded the radar of scientific pursuit. Specifically, I shall describe the recent work on the prominence of local lineage-restricted stem cells, as opposed to the bone marrow-derived or circulating ones, in regeneration. This will be followed by an attempt to re-interpret a bulk of published data on the beneficial effects of cell therapy with the focus on the secretome of stem cells. Multiple factors that conspire to cause insufficient or failed regeneration in adult mammals will be screened with emphasis placed on the mechanical forces, senescence and exhaustion, each leading to phenotypical switch and/or stem cell incompetence. Finally, I shall enumerate several potential pathways to induce or restore stem cell competence. Although a significant amount of work has been performed in the non-renal field, I would hope that some of the mechanisms and concepts discussed herein will eventually trickle into kidney regeneration.

Endothelin-1 and the kidney--beyond BP

Since its discovery over 20 years ago endothelin-1 (ET-1) has been implicated in a number of physiological and pathophysiological processes. Its role in the development and progression of chronic kidney disease (CKD) is well established and is an area of ongoing intense research. There are now available a number of ET receptor antagonists many of which have been used in trials with CKD patients and shown to reduce BP and proteinuria. However, ET-1 has a number of BP-independent effects. Importantly, and in relation to the kidney, ET-1 has clear roles to play in cell proliferation, podocyte dysfunction, inflammation and fibrosis, and arguably, these actions of ET-1 may be more significant in the progression of CKD than its prohypertensive actions. This review will focus on the potential role of ET-1 in renal disease with an emphasis on its BP-independent actions.

EGFR activity is required for renal tubular cell dedifferentiation and proliferation in a murine model of folic acid-induced acute kidney injury

Proliferation of dedifferentiated intrinsic renal tubular cells has been recognized to be the major cellular event that contributes to renal repair after acute kidney injury (AKI). However, the underlying mechanism that initiates renal tubular dedifferentiation in vivo remains unexplored. Here we investigated whether epidermal growth factor receptor (EGFR) mediates this process in a murine model of folic acid (FA)-induced AKI using waved-2 mice that have reduced tyrosine kinase activity of EGFR and gefitinib, a specific EGFR inhibitor. Administration of FA for 48 h induced EGFR phosphorylation in the kidney of wild-type mice, but this was inhibited in waved-2 mice and wild-type mice given gefitinib. Compared with wild-type mice, waved-2 mice and wild-type mice treated with gefitinib had increased renal dysfunction, histologic damage, and tubular cell apoptosis after FA administration. PAX2, a dedifferentiation marker, and proliferating cell nuclear antigen, a proliferating marker, were highly expressed in renal tubular cells in wild-type mice; however, their expression was largely inhibited in the kidney of waved-2 mice. Inhibition of EGFR with gefitinib also blocked FA-induced expression of these two proteins in wild-type mice. Moreover, FA exposure resulted in phosphorylation of AKT, a downstream signaling molecule of the phosphatidylinositol 3-kinases pathway associated with renal epithelial proliferation in wild-type mice, and its phosphorylation was totally suppressed in waved-2 mice and wild-type mice given gefitinib. Taken together, these results suggest that EGFR activation is essential for initiation of renal tubular cell dedifferentiation and proliferation after AKI.