Kinetics and characterization of initially regenerating proximal tubules in S3 segment in response to various degrees of acute tubular injury

Characterization of macrophages in ischemia-reperfusion injury-induced acute kidney injury based on single-cell RNA-Seq and bulk RNA-Seq analysis

Acute kidney injury (AKI) is a complex disease, with macrophages playing a vital role in its progression. However, the mechanism of macrophage function remains unclear and strategies targeting macrophages in AKI are controversial. To address this issue, we used single-cell RNA-seq analysis to identify macrophage sub-types involved in ischemia-reperfusion-induced AKI, and then screened for associated hub genes using intersecting bulk RNA-seq data. The single-cell and bulk RNA-seq datasets were obtained from the Gene Expression Omnibus (GEO) database. Screening of differentially-expressed genes (DEGs) and pseudo-bulk DEG analyses were used to identify common hub genes. Pseudotime and trajectory analyses were performed to investigate the progression of cell differentiation. CellChat analysis was performed to reveal the crosstalk between cell clusters. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses were used to identify enriched pathways in the cell clusters. Immunofluorescence and RT-PCR were preformed to validate the expression of the identified hub genes. Four hub genes, Vim, S100a6, Ier3, and Ccr1, were identified in the infiltrated macrophages between normal samples and those 3 days after ischemia-reperfusion renal injury (IRI); all were associated with the progression of IRI-induced AKI. Increased expression of Vim, S100a6, Ier3, and Ccr1 in infiltrated macrophages may be associated with inflammatory responses and may mediate crosstalk between macrophages and renal tubular epithelial cells under IRI conditions. Our results reveal that Ier3 may be critical in AKI, and that Vim, S100a6, Ier3, and Ccr1 may act as novel biomarkers and potential therapeutic targets for IRI-induced AKI.

Great potential of renal progenitor cells in kidney: From the development to clinic

The mammalian renal organ represents a pinnacle of complexity, housing functional filtering units known as nephrons. During embryogenesis, the depletion of niches containing renal progenitor cells (RPCs) and the subsequent incapacity of adult kidneys to generate new nephrons have prompted the formulation of protocols aimed at isolating residual RPCs from mature kidneys and inducing their generation from diverse cell sources, notably pluripotent stem cells. Recent strides in the realm of regenerative medicine and the repair of tissues using stem cells have unveiled critical signaling pathways essential for the maintenance and generation of human RPCs in vitro. These findings have ushered in a new era for exploring novel strategies for renal protection. The present investigation delves into potential transcription factors and signaling cascades implicated in the realm of renal progenitor cells, focusing on their protection and differentiation. The discourse herein elucidates contemporary research endeavors dedicated to the acquisition of progenitor cells, offering crucial insights into the developmental mechanisms of these cells within the renal milieu and paving the way for the formulation of innovative treatment modalities.

Post-mortem assessment of vimentin expression as a biomarker for renal tubular regeneration following acute kidney injury

BACKGROUND

Acute kidney injury (AKI) is a common cause of morbidity and mortality. It mainly targets the renal tubular epithelium with pathological changes, referred to as acute tubular injury. The latter is followed by a regenerative response that is difficult to visualize on routine hematoxylin and eosin (H&E) stains. In this study, we examined the regenerative capacity of renal tubules by correlating vimentin (VIM) immunohistochemical (IHC) expression and pathological findings of AKI and renal tubular regeneration (RTR) on H&E.

METHODS

We reviewed 23 autopsies performed in the clinical setting of AKI and RTR. VIM expression was scored in the renal cortical tubular epithelium using a statistical cutoff ≥ 3% for high expression and < 3% for low expression.

RESULTS

Of the 23 kidney tissues examined, seven (30.4%) had low VIM expression, and 16 (69.6%) had high VIM expression. Kidney tissues with evidence of AKI and RTR had significantly higher VIM expression. Renal peritubular microenvironment features showing regenerative changes on H&E were associated with high VIM expression. In the univariate model, kidney tissues with RTR were 18-fold more likely to have high VIM expression.

CONCLUSIONS

In conclusion, our findings suggest that VIM could serve as an IHC marker for RTR following AKI. However, correlation with H&E findings remains critical to excluding chronic tubular damage. Collectively, our preliminary results pave the way for future studies including a larger sample size to validate the use of VIM as a reliable biomarker for RTR.

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.

Phosphorus Localization and Its Involvement in the Formation of Concentrated Uranium in the Renal Proximal Tubules of Rats Exposed to Uranyl Acetate

Although the kidneys comprise a critical target of uranium exposure, the dynamics of renal uranium distribution have remained obscure. Uranium is considered to function physiologically in the form of uranyl ions that have high affinity for phosphate groups. The present study applied microbeam-based elemental analysis to precisely determine the distribution of phosphorus and uranium in the kidneys of male Wistar rats exposed to uranium. One day after a single subcutaneous injection of uranyl acetate (2 mg/kg), areas of concentrated phosphorus were scattered in the S3 segments of the proximal tubule of the kidneys, whereas the S3 segments in control rats and in rats given a lower dose of uranium (0.5 mg/kg) contained phosphorus without concentrated phosphorus. Areas with concentrated phosphorus contained uranium 4- to 14-fold more than the mean uranium concentration (126–472 vs. 33.1 ± 4.6 μg/g). The chemical form of uranium in the concentrated phosphorus examined by XAFS was uranium (VI), suggesting that the interaction of uranyl ions with the phosphate groups of biomolecules could be involved in the formation of uranium concentration in the proximal tubules of kidneys in rats exposed to uranium.

Inhibition of mTORC1 signaling protects kidney from irradiation-induced toxicity via accelerating recovery of renal stem-like cells

Background

Irradiation-induced kidney damage is inevitable during radiotherapeutic practice, which limits effective radiotherapy doses on tumor treatment. In the present study, the role of mTOR complex 1 (mTORC1) signaling was investigated in irradiation-induced renal injuries.

Methods

Mice were exposed to 8.0-Gy X-ray of total body irradiation and subsequently treated with rapamycin. Changes of renal morphology were assessed by hematoxylin and eosin staining. Expression of pS6 and CD133 was detected via immunostaining. Cellular apoptosis and proliferation were measured by TUNEL, caspase-3 and BrdU staining. Activation of mTORC1, TGF-β and NF-κB signaling pathways was determined through western blot analysis.

Results

Our data displayed that irradiation disrupted the structures of renal corpuscles and tubules and decreased the density of CD133⁺ renal stem-like cells, which were related with increasing cellular apoptosis and decreasing cell proliferation post exposure. Activation of mTORC1, TGF-β and NF-κB signaling pathways was determined in irradiated renal tissues, which were inhibited by rapamycin treatment. Application of rapamycin after irradiation decreased cellular apoptosis and increased autophagy and cell proliferation in renal tissues. The density of CD133⁺ renal stem-like cells was significantly increased in irradiated kidneys after rapamycin treatment. The morphology of irradiated renal corpuscles and tubules was gradually recovered upon rapamycin treatment.

Conclusions

These findings indicate that inhibition of mTORC1 signaling by rapamycin ameliorates irradiation-induced renal toxicity mediated by decreasing cellular apoptosis and increasing CD133⁺ renal stem-like cells.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0963-5) contains supplementary material, which is available to authorized users.

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.

Uranium XAFS analysis of kidney from rats exposed to uranium

The kidney is the critical target of uranium exposure because uranium accumulates in the proximal tubules and causes tubular damage, but the chemical nature of uranium in kidney, such as its chemical status in the toxic target site, is poorly understood. Micro-X-ray absorption fine-structure (µXAFS) analysis was used to examine renal thin sections of rats exposed to uranyl acetate. The U LIII-edge X-ray absorption near-edge structure spectra of bulk renal specimens obtained at various toxicological phases were similar to that of uranyl acetate: their edge position did not shift compared with that of uranyl acetate (17.175 keV) although the peak widths for some kidney specimens were slightly narrowed. µXAFS measurements of spots of concentrated uranium in the micro-regions of the proximal tubules showed that the edge jump slightly shifted to lower energy. The results suggest that most uranium accumulated in kidney was uranium (VI) but a portion might have been biotransformed in rats exposed to uranyl acetate.

The role of long-term label-retaining cells in the regeneration of adult mouse kidney after ischemia/reperfusion injury

Background

Label-retaining cells (LRCs) have been recognized as rare stem and progenitor-like cells, but their complex biological features in renal repair at the cellular level have never been reported. This study was conducted to evaluate whether LRCs in kidney are indeed renal stem/progenitor cells and to delineate their potential role in kidney regeneration.

Methods

We utilized a long-term pulse chase of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells in C57BL/6J mice to identify renal LRCs. We tracked the precise morphological characteristics and locations of BrdU⁺LRCs by both immunohistochemistry and immunofluorescence. To examine whether these BrdU⁺LRCs contribute to the repair of acute kidney injury, we analyzed biological characteristics of BrdU⁺LRCs in mice after ischemia/reperfusion (I/R) injury.

Results

The findings revealed that the nuclei of BrdU⁺ LRCs exhibited different morphological characteristics in normal adult kidneys, including nuclei in pairs or scattered, fragmented or intact, strongly or weakly positive. Only 24.3 ± 1.5 % of BrdU⁺ LRCs co-expressed with Ki67 and 9.1 ± 1.4 % of BrdU⁺ LRCs were positive for TUNEL following renal I/R injury. Interestingly, we found that newly regenerated cells formed a niche-like structure and LRCs in pairs tended to locate in this structure, but the number of those LRCs was very low. We found a few scattered LRCs co-expressed Lotus tetragonolobus agglutinin (LTA) in the early phase of injury, suggesting differentiation of those LRCs in mouse kidney.

Conclusions

Our findings suggest that LRCs are not a simple type of slow-cycling cells in adult kidneys, indicating a limited role of these cells in the regeneration of I/R injured kidney. Thus, LRCs cannot reliably be considered stem/progenitor cells in the regeneration of adult mouse kidney. When researchers use this technique to study the cellular basis of renal repair, these complex features of renal LRCs and the purity of real stem cells among renal LRCs should be considered.

Acquired resistance to rechallenge injury after acute kidney injury in rats is associated with cell cycle arrest in proximal tubule cells

Rats that have recovered from severe proximal tubule (PT) injury induced by uranyl acetate (UA), a toxic stimulus, developed resistance to subsequent UA treatment. We investigated cell cycle status and progression in PT cells in relation to this acquired resistance. Fourteen days after pretreatment with saline (vehicle group) or UA [acute kidney injury (AKI) group], rats were injected with UA or lead acetate (a proliferative stimulus). Cell cycle status (G0/G1/S/G2/M) was analyzed by flow cytometry. The expression of cell cycle markers, cyclin-dependent kinase inhibitors, and phenotypic markers were examined by immunohistochemistry. Cell cycle status in PT cells in the AKI group was comparable to those of the vehicle group. However, more early G1-phase cells (cyclin D1- or Ki67-) and p21+ or p27+ cells were found in the PT of the AKI group than in that of the vehicle group. UA induced G1 arrest and inhibited S phase progression with earlier dedifferentiation and less apoptosis in PT cells of the AKI group. Lead acetate induced proliferation without dedifferentiation but with delayed G0-G1 transition and inhibited S phase progression in PT cells in the AKI group. Sustained p21 and increased p27 expression in PT cells were found in the AKI group in response to UA and lead acetate. PT cells in the AKI group inhibited cell cycle progression by enhanced G1 arrest, probably via p21/p27 modulation as an injury or proliferation response, resulting in cytoresistance to rechallenge injury.

Increasing Proliferation of Intrinsic Tubular Cells after Renal Ischemia-reperfusion Injury in Adult Rat

The kidney is capable of regeneration following injury. However, whether renal stem/progenitor cells contribute to the repair process after injury, as well as the origin of the cells that repair and replace damaged renal tubule cells remains debated. Therefore, better understanding of the repair process will be critical to developing new strategies for the treatment of acute renal failure. Using an ischemia-reperfusion injury mode and an immunocytochemistry method, we counted the number of BrdU-positive cells in damged regions at different durations of reperfusion. We found that BrdU, a cell proliferative marker, was mainly incorporated in the tubular cells of both medulla and cortex 1 day after reperfusion. The number of BrdU-positive cells reached a peak at 3 days and lasted for two months after injury. BrdU-positive cells were barely found in the renal glomerulus and the parietal layer of Bowman's capsule after injury, and only a few were found in the intersititium. PAX2, an embryonic renal marker, was also increased in renal tubule cells. Confocal images show that BrdU-positive cells co-expressed PAX2, but not the activated form of caspase-3, a cell death marker. Our data suggest that renal stem-like cells or dedifferentiation of surviving renal tubular cells in both the medulla and cortex may predominantly contribute to the repair process after renal ischemia-reperfusion injury in rat.

Diverse Cell Populations Involved in Regeneration of Renal Tubular Epithelium following Acute Kidney Injury

Renal tubular epithelium has the capacity to regenerate, repair, and reepithelialize in response to a variety of insults. Previous studies with several kidney injury models demonstrated that various growth factors, transcription factors, and extracellular matrices are involved in this process. Surviving tubular cells actively proliferate, migrate, and differentiate in the kidney regeneration process after injury, and some cells express putative stem cell markers or possess stem cell properties. Using fate mapping techniques, bone marrow-derived cells and endothelial progenitor cells have been shown to transdifferentiate into tubular components in vivo or ex vivo. Similarly, it has been demonstrated that, during tubular cell regeneration, several inflammatory cell populations migrate, assemble around tubular cells, and interact with tubular cells during the repair of tubular epithelium. In this review, we describe recent advances in understanding the regeneration mechanisms of renal tubules, particularly the characteristics of various cell populations contributing to tubular regeneration, and highlight the targets for the development of regenerative medicine for treating kidney diseases in humans.

Adult stem-like cells in kidney

Human pluripotent cells are promising for treatment for kidney diseases, but the protocols for derivation of kidney cell types are still controversial. Kidney tissue regeneration is well confirmed in several lower vertebrates such as fish, and the repair of nephrons after tubular damages is commonly observed after renal injury. Even in adult mammal kidney, renal progenitor cell or system is reportedly presents suggesting that adult stem-like cells in kidney can be practical clinical targets for kidney diseases. However, it is still unclear if kidney stem cells or stem-like cells exist or not. In general, stemness is defined by several factors such as self-renewal capacity, multi-lineage potency and characteristic gene expression profiles. The definite use of stemness may be obstacle to understand kidney regeneration, and here we describe the recent broad findings of kidney regeneration and the cells that contribute regeneration.

Cellular localization of uranium in the renal proximal tubules during acute renal uranium toxicity

Renal toxicity is a hallmark of uranium exposure, with uranium accumulating specifically in the S3 segment of the proximal tubules causing tubular damage. As the distribution, concentration and dynamics of accumulated uranium at the cellular level is not well understood, here, we report on high-resolution quantitative in situ measurements by high-energy synchrotron radiation X-ray fluorescence analysis in renal sections from a rat model of uranium-induced acute renal toxicity. One day after subcutaneous administration of uranium acetate to male Wistar rats at a dose of 0.5 mg uranium kg(-1) body weight, uranium concentration in the S3 segment of the proximal tubules was 64.9 ± 18.2 µg g(-1) , sevenfold higher than the mean renal uranium concentration (9.7 ± 2.4 µg g(-1) ). Uranium distributed into the epithelium of the S3 segment of the proximal tubules and highly concentrated uranium (50-fold above mean renal concentration) in micro-regions was found near the nuclei. These uranium levels were maintained up to 8 days post-administration, despite more rapid reductions in mean renal concentration. Two weeks after uranium administration, damaged areas were filled with regenerating tubules and morphological signs of tissue recovery, but areas of high uranium concentration (100-fold above mean renal concentration) were still found in the epithelium of regenerating tubules. These data indicate that site-specific accumulation of uranium in micro-regions of the S3 segment of the proximal tubules and retention of uranium in concentrated areas during recovery are characteristics of uranium behavior in the kidney.

A high ratio of G1 to G0 phase cells and an accumulation of G1 phase cells before S phase progression after injurious stimuli in the proximal tubule

Proximal tubule (PT) cells can proliferate explosively after injurious stimuli. To investigate this proliferative capacity, we examined cell cycle status and the expression of cyclin-dependent kinase inhibitor p27, a G1 phase mediator, in PT cells after a proliferative or injurious stimulus. Rats were treated with lead acetate (proliferative stimulus) or uranyl acetate (UA; injurious stimulus). Isolated tubular cells were separated into PT and distal tubule (DT) cells by density-gradient centrifugation. Cell cycle status was analyzed with flow cytometry by using the Hoechst 33342/pyronin Y method. Most PT and DT cells from control rats were in G0/G1 phase, with a higher percentage of PT cells than DT cells in G1 phase. Lead acetate and UA administration promoted the G0-G1 transition and the accumulation of G1 phase cells before S phase progression. In PT cells from rats treated with lead acetate or a subnephrotoxic dose of UA, p27 levels increased or did not change, possibly reflecting G1 arrest. In contrast, p27 became undetectable before the appearance of apoptotic cells in rats treated with a nephrotoxic dose of UA. The decrease in p27 might facilitate rapid cell cycling. The decreased number of p27-positive cells was associated with PT cell proliferation in renal tissues after a proliferative or injurious stimulus. The findings suggest that a high ratio of G1 to G0 phase cells and a rapid accumulation of G1 phase cells before S phase progression in the PT is a biological strategy for safe, timely, and explosive cell proliferation in response to injurious stimuli.

Regenerative medicine for the kidney: renotropic factors, renal stem/progenitor cells, and stem cell therapy

The kidney has the capacity for regeneration and repair after a variety of insults. Over the past few decades, factors that promote repair of the injured kidney have been extensively investigated. By using kidney injury animal models, the role of intrinsic and extrinsic growth factors, transcription factors, and extracellular matrix in this process has been examined. The identification of renal stem cells in the adult kidney as well as in the embryonic kidney is an active area of research. Cell populations expressing putative stem cell markers or possessing stem cell properties have been found in the tubules, interstitium, and glomeruli of the normal kidney. Cell therapies with bone marrow-derived hematopoietic stem cells, mesenchymal stem cells, endothelial progenitor cells, and amniotic fluid-derived stem cells have been highly effective for the treatment of acute or chronic renal failure in animals. Embryonic stem cells and induced pluripotent stem cells are also utilized for the construction of artificial kidneys or renal components. In this review, we highlight the advances in regenerative medicine for the kidney from the perspective of renotropic factors, renal stem/progenitor cells, and stem cell therapies and discuss the issues to be solved to realize regenerative therapy for kidney diseases in humans.

Megalin in acute kidney injury: foe and friend

The kidney proximal tubule is a key target in many forms of acute kidney injury (AKI). The multiligand receptor megalin is responsible for the normal proximal tubule uptake of filtered molecules, including nephrotoxins, cytokines, and markers of AKI. By mediating the uptake of nephrotoxins, megalin plays an essential role in the development of some types of AKI. However, megalin also mediates the tubular uptake of molecules implicated in the protection against AKI, and changes in megalin expression have been demonstrated in AKI in animal models. Thus, modulation of megalin expression in response to AKI may be an important part of the tubule cell adaption to cellular protection and regeneration and should be further investigated as a potential target of intervention. This review explores current evidence linking megalin expression and function to the development, diagnosis, and progression of AKI as well as renal protection against AKI.

Uranium dynamics and developmental sensitivity in rat kidney

Renal toxicity is the principal health concern after uranium exposure. Children are particularly vulnerable to uranium exposure; with contact with depleted uranium in war zones or groundwater contamination the most likely exposure scenarios. To investigate renal sensitivity to uranium exposure during development, we examined uranium distribution and uranium-induced apoptosis in the kidneys of neonate (7-day-old), prepubertal (25-day-old) and adult (70-day-old) male Wistar rats. Mean renal uranium concentrations increased with both age-at-exposure and exposure level after subcutaneous administration of uranium acetate (UA) (0.1-2 mg kg(-1) body weight). Although less of the injected uranium was deposited in the kidneys of the two younger rat groups, the proportion of the peak uranium content remaining in the kidneys after 2 weeks declined with age-at-exposure, suggesting reduced clearance in younger animals. In situ high-energy synchrotron radiation X-ray fluorescence analysis revealed site-specific accumulation of uranium in the S3 segment of the proximal tubules, distributed in the inner cortex and outer stripe of the outer medulla. Apoptosis and cell loss in the proximal tubules increased with age-at-exposure to 0.5 mg kg(-1) UA. Surprisingly, prepubertal rats were uniquely sensitive to uranium-induced lethality from the higher exposure levels. Observations of increased apoptosis in generating/re-generating tubules particularly in prepubertal rats could help to explain their high mortality rate. Together, our findings suggest that age-at-exposure and exposure level are important parameters for uranium toxicity; uranium tends to persist in developing kidneys after low-level exposures, although renal toxicity is more pronounced in adults.

Dephosphorylated Ser985 of c-Met is associated with acquired resistance to rechallenge injury in rats that had recovered from uranyl acetate-induced subclinical renal damage

BACKGROUND

We previously reported that rats that had recovered from mild proximal tubule (PT) injury induced by a sub-toxic dose of uranyl acetate (UA) showed partial resistance to a subsequent nephrotoxic dose of UA in association with reduced renal dysfunction and accelerated PT proliferation. We demonstrated that this resistance may involve hepatocyte growth factor (HGF)/c-Met signaling. Here, we examined whether primary cultured tubular cells derived from this model had acquired sensitivity to HGF.

METHODS

Tubular cells were isolated by collagenase digestion from rat kidneys after recovery from UA-induced mild PT injury and were cultured for 48 h. Their survival and proliferation were examined using the MTS assay/5-bromo-2'-deoxyuridine labeling or MTS assay, respectively, and their migration was assayed using wound-healing and cell scattering assays, with/without HGF. HGF/c-Met signaling was assayed using phospho-specific antibodies.

RESULTS

HGF-stimulated cultured tubular cells from UA-treated rats showed better survival after UA exposure and higher proliferation and migration than cells from vehicle-treated rats. Furthermore, HGF induced higher phosphorylation of c-Met (Tyr1234/1235) and of its major downstream signals (AKT and extracellular signal-regulated kinase 1/2) with maintained dephosphorylation of Ser985 as a negative regulator of HGF/c-Met signaling in the tubular cells of UA-treated rats compared to those of vehicle-treated rats. Immunohistochemically, dephosphorylated Ser985 was confirmed in PT cells in vivo.

CONCLUSIONS

These results suggest that elevated sensitivity to HGF, via dephosphorylated Ser985 of c-Met of tubular cells that had recovered from mild tubular injury, may be associated with cytoprotection, accelerated proliferation and migration.

Different modes of renal proximal tubule regeneration in health and disease

Tissues are equipped with reasonable strategies for repair and regeneration and the renal proximal tubule (PT) is no exception. New information has become available on the mode of PT regeneration in mammals. Unlike the intestinal epithelium with a high rate of turnover maintained by the stem cell system, the kidney has low turnover under normal physiological conditions. The PT seems to be maintained physiologically by hyperplasia, a regenerating system with self-renewal of mature tubular cells. This mode of regeneration is advantageous for effective replenishment of randomly isolated and eliminated tubular cells by self-renewal of adjacent cells. On the other hand, it has been suggested that dedifferentiation of mature tubular cells plays a role in regeneration after acute kidney injury. Recent studies employing genetic labeling and DNA-labeling techniques have confirmed that the proliferation of preexisting injured mature tubular cells contributes mainly to PT regeneration in ischemic reperfusion injury. This mode of regeneration is beneficial with regard to the rapid reparation of focally injured tubules often induced by ischemic reperfusion injury. What happens, however, when the PT is homogeneously injured with almost no remaining surviving cells Is the PT equipped with another backup regeneration system, e.g., the stem cell system Is it possible that certain types of renal injuries evoke a stem cell response whereas others do not This review focuses on all three possible modes of tissue regeneration (compensatory hyperplasia, dedifferentiation and stem cell system) in mammals and their involvement in PT regeneration in health and disease.

Distinct bone morphogenetic proteins activate indistinguishable transcriptional responses in nephron epithelia including Notch target genes

Endogenous Bone Morphogenetic Protein (BMP) signaling plays a significant role in the kidney's recovery from acute injury and exogenous administration of BMP7 has therapeutic potential in numerous rodent models of renal injury and disease. However, in the healthy kidney endogenous BMP7 ligand is vigorously counteracted by extracellular antagonists such as USAG1 and CHRDL1. Little is known about the degree of BMP signaling and the ligands driving it in the healthy adult kidney. In this study we characterize basal BMP signaling in the healthy tubular nephron, and show that BMP2 is expressed in proximal nephron epithelial cells. Comparative gene profiling of proximal tubule cell responses to BMP2 and BMP7 does not reveal any qualitative difference, suggesting that identical BMP gene targets may be activated in healthy and injured organs. Interestingly, our gene profiling analysis shows that BMP signaling activates a number of Notch regulated transcription factors, including HEY1. As in other biological systems, HEY1 functions as a negative feedback regulator of BMP2 expression in the proximal tubule. In summary, this work reveals endogenous BMP signaling patterns in the healthy human and mouse kidneys, and identifies novel gene targets, some of which are involved in the complex regulation of BMP signaling in the adult kidney.

Age-related decline in label-retaining tubular cells: implication for reduced regenerative capacity after injury in the aging kidney

Recovery after acute kidney injury is impaired in the elderly, but the precise mechanism for such age-related incompetence remains unclear. By in vivo bromodeoxyuridine (BrdU) labeling, renal progenitor cells (label-retaining cells; LRCs) were identified in tubules of normal rat kidney and were shown to be the origin of proliferating cells after injury. In the present study, the involvement of LRCs in the age-related decline of tubular recovery after injury was examined. After 1 wk of BrdU labeling followed by a 2-wk chase period, ischemia-reperfusion injury was induced in 7-wk-, 7-mo-, and 12-mo-old rats. Age-related decreases in DNA synthesis and cell proliferation in renal tubules after injury were found. The number of LRCs also significantly declined with age. At 24 h after reperfusion, the number of LRCs significantly increased in all ages of rats tested. There was no significant difference in the ratio of LRC division among rats of different ages. The area of the rat endothelial cell antigen (RECA)-1-positive capillary network declined with age. When renal tubules isolated from rats treated with BrdU label were cocultured with human umbilical vein endothelial cells (HUVEC), the number of LRCs significantly increased compared with tubules cultured without HUVEC. These data suggest that the reduced capacity of tubular regeneration in the aging kidney is partly explained by the shortage of LRC reserves. The size of the LRC pool might be regulated by the surrounding peritubular capillary network.

Intra-renal slow cell-cycle cells contribute to the restoration of kidney tubules injured by ischemia/reperfusion

Renal epithelial cells damaged by ischemia/reperfusion (I/R) can be restored by timely and appropriate treatment. Recent studies have reported that intra renal adult kidney stem cells contribute to the restoration of tubules damaged by I/R. Here, we determined the role of adult tubular cells in the restoration of damaged tubules. We labeled slow cell-cycle cells (SCCs) with 5-bromo-2'-deoxyuridine (BrdU) and investigated their location in the kidneys as well as their contribution to the restoration of tubular cells damaged by I/R injury in mice. Thirty minutes of bilateral ischemia resulted in severe disruption of tubular epithelial cells along with a decline in renal function. The post-ischemic disruption of tubular epithelial cells was most severe in the S3 segment of the outer stripe of the outer medulla. Damaged tubules demonstrated gradual recovery of renal function over time. BrdU-labeled SCCs were mainly observed in tubules located at the junction of cortex and outer medulla, as well as in the inner medulla. The tubular SCCs expressed functional tubule cell markers such as Na/K-ATPase, Na-K-Cl cotransporter-2, and aquaporin 1 and 2. BrdU-labeled SCCs survived I/R injury and proliferated. These results demonstrate that SCCs present in tubules contribute to the restoration of tubular epithelial cells injured by I/R.

Treatment with valsartan stimulates endothelial progenitor cells and renal label-retaining cells in hypertensive rats

OBJECTIVE

The pathogenesis of hypertension is dependent on tissue angiotensin (Ang) II, which induces cardiovascular and renal remodeling. The presence of label-retaining cells (LRCs) as renal stem cells has been reported in nephrotubulus. We examined effects of treatment with valsartan on endothelial progenitor cells (EPCs) and renal LRCs in stroke-prone spontaneously hypertensive rats (SHR-SP).

METHODS

SHR-SP were salt-loaded and treated with hydralazine or valsartan. Peripheral blood mononuclear cells (MNCs) were cultured to assess EPC colony formation and migration. LRCs were labeled for 1 week with bromodeoxyuridine (BrdU) and were detected after a 2-week chase period. We measured expression of c-kit and Pax-2 mRNAs in renal medulla.

RESULTS

Colony formation and migration of EPCs were suppressed in salt-loaded SHR-SP. Treatment with valsartan markedly stimulated these EPC functions. There was no difference in the number of renal LRCs in normotensive Wistar-Kyoto rats and SHR-SP. Treatment with valsartan significantly improved renal tubular degeneration and increased the number of LRCs in renal medulla from salt-loaded SHR-SP. Treatment with valsartan significantly increased expression of c-kit and Pax-2 mRNAs in renal medulla from salt-loaded SHR-SP.

CONCLUSION

These findings suggest that ARBs have cardiovascular and renal protective effects through an antioxidative action that stimulates ECP function and increases the number of the self-repairing renal LRCs.

Exosomes/microvesicles as a mechanism of cell-to-cell communication

Microvesicles (MVs) are circular fragments of membrane released from the endosomal compartment as exosomes or shed from the surface membranes of most cell types. An increasing body of evidence indicates that they play a pivotal role in cell-to-cell communication. Indeed, they may directly stimulate target cells by receptor-mediated interactions or may transfer from the cell of origin to various bioactive molecules including membrane receptors, proteins, mRNAs, microRNAs, and organelles. In this review we discuss the pleiotropic biologic effects of MVs that are relevant for communication among cells in physiological and pathological conditions. In particular, we discuss their potential involvement in inflammation, renal disease, and tumor progression, and the evidence supporting a bidirectional exchange of genetic information between stem and injured cells. The transfer of gene products from injured cells may explain stem cell functional and phenotypic changes without the need of transdifferentiation into tissue cells. On the other hand, transfer of gene products from stem cells may reprogram injured cells to repair damaged tissues.

Immunohistochemical study of heat shock protein 27 with respect to survival and regeneration of proximal tubular cells after uranyl acetate-induced acute tubular injury in rats

This study examined the possible role of heat shock protein 27 (HSP27) expression in the survival and regeneration of proximal tubule (PT) cells after acute tubular injury. Rats were injected with a low (0.2 mg/kg) or high (4 mg/kg) dose of uranyl acetate (UA) to induce renal injury. Renal tissues were immunostained for HSP27, focal adhesion kinase (FAK), and bromodeoxyuridine (BrdU), and stained by the TUNEL method. Low-dose UA induced focal PT depletion in the proximal three-quarters of the S3 segment. Here, cells became sporadically positive for cytoplasmic HSP27 in association with FAK+, and almost all BrdU+ early regenerating cells were positive for HSP27 from days 2 to 3. High-dose UA induced severe PT depletion in the proximal three-quarters of S3, and a small number of PT cells became positive for HSP27 as early as day 2. BrdU+, early regenerating cells were restricted to the distal quarter of S3 from days 2 to 3, with or without HSP27 staining and with FAK. In both groups, HSP+ PT cells and BrdU+ cells peaked in number at day 5. The PT cells showed reduced HSP27 accumulation by day 7 as they differentiated, but remained immunopositive for FAK. TUNEL+ apoptotic cells were immunonegative for both HSP27 and FAK. Cytoplasmic HSP27 accumulation in PT cells seems to contribute to PT survival and transition from PT cell proliferation to differentiation. When PT cells are severely impaired, distinct cells in the distal areas of S3 could undergo cell cycle progression without HSP27 accumulation.

Paracrine/endocrine mechanism of stem cells on kidney repair: role of microvesicle-mediated transfer of genetic information

PURPOSE OF REVIEW

The mechanism of stem cell-induced kidney repair remains controversial. Engraftment of bone marrow-derived stem cells is considered a rare event and several studies point to paracrine/endocrine processes. This review focuses on microvesicle-mediated transfer of genetic information between stem cells and injured tissue as a paracrine/endocrine mechanism.

RECENT FINDINGS

The following findings support a bidirectional exchange of genetic information between stem and injured cells: microvesicles shuttle defined patterns of mRNA and microRNA, are actively released from embryonic and adult stem cells and are internalized by a receptor-mediated mechanism in target cells; transcripts delivered by microvesicles from injured cells may reprogram the phenotype of stem cells to acquire specific features of the tissue; transcripts delivered by microvesicles from stem cells may induce dedifferentiation of cells surviving injury with cell cycle reentry and tissue self-repair.

SUMMARY

Transfer of genetic information from injured cells may explain stem cell functional and phenotypic changes without the need for transdifferentiation into tissue cells. On the contrary, transfer of genetic information from stem cells may redirect altered functions in target cells suggesting that stem cells may repair damaged tissues without directly replacing parenchymal cells.

Acquired resistance to rechallenge injury in rats recovered from subclinical renal damage with uranyl acetate--Importance of proliferative activity of tubular cells

Animals recovered from acute renal failure are resistant to subsequent insult. We investigated whether rats recovered from mild proximal tubule (PT) injury without renal dysfunction (subclinical renal damage) acquire the same resistance. Rats 14 days after recovering from subclinical renal damage, which was induced by 0.2 mg/kg of uranyl acetate (UA) (sub-toxic dose), were rechallenged with 4 mg/kg of UA (nephrotoxic dose). Fate of PT cells and renal function were examined in response to nephrotoxic dose of UA. All divided cells after sub-toxic dose of UA insult were labeled with bromodeoxyuridine (BrdU) for 14 days then the number of PT cells with or without BrdU-labeling was counted following nephrotoxic dose of UA insult. Rats recovered from subclinical renal damage gained resistance to nephrotoxic dose of UA with reduced renal dysfunction, less severity of peak damage (necrotic and TUNEL+ apoptotic cells) and accelerated PT cell proliferation, but with earlier peak of PT damage. The decrease in number of PT cells in the early phase of rechallenge injury with nephrotoxic UA was more in rats pretreated with sub-toxic dose of UA than vehicle pretreated rats. The exaggerated loss of PT cells was mainly caused by the exaggerated loss of BrdU+ divided cells. In contrast, accelerated cell proliferation in rats recovered from sub-toxic dose of UA was observed mainly in BrdU- non-divided cells. The findings suggest that rats recovered from subclinical renal damage showed partial acquired resistance to nephrotoxic insult. Accelerated recovery with increased proliferative activity of non-divided PT cells after subclinical renal damage may mainly contribute to acquired resistance.

Experimental Models of Acute Renal Failure and Erythropoietin: What Evidence of a Direct Effect?

The kidney can achieve a structural and functional recovery after the damage induced by ischemia and reperfusion. This is due to the regeneration of epithelial tubular cells, the intervention of immature cells mainly localized in the medulla, and a small number of bone marrow-derived stem cells. In many instances, however, recovery is delayed or does not occur at all. The mechanisms allowing the renal cells to de-differentiate still need to be clarified in order to find a therapeutic approach that can amplify this ability and then stop the fibroid involution and the progression toward renal failure. Several authors have hypothesized a protective effect of EPO against ischemic and cytotoxic renal damage and observed that patients precociously treated with EPO showed a slower progression of renal failure. EPO has been demonstrated to have proliferative and anti-apoptotic effects in ischemia-reperfusion models in the brain and cell cultures. Moreover, EPO can mobilize stem cells and increase the plasmatic levels and the renal expression of VEGF. These effects seem to be dose-dependent and could be due to the activation of signal transduction systems, like Jak and STAT. In the presence of high doses of exogenous EPO or during the treatment with long-acting EPO-like molecules, non-specific receptors may be activated through a low-affinity link. Further investigations are needed to determine new therapeutic applications for EPO and other analogous hormones. Very long-acting molecules or molecules with cyto-protective but no erythropoietic effect may represent useful tools in the study of the molecular mechanisms underlying EPO's action and may have a rapid and safe therapeutic application.

The long-term label retaining population of the renal papilla arises through divergent regional growth of the kidney

Long-term pulse chase experiments previously identified a sizable population of BrdU-retaining cells within the renal papilla. The origin of these cells has been unclear, and in this work we test the hypothesis that they become quiescent early during the course of kidney development and organ growth. Indeed, we find that BrdU-retaining cells of the papilla can be labeled only by pulsing with BrdU from embryonic (E) day 11.25 to postnatal (P) day 7, the approximate period of kidney development in the mouse. BrdU signal in the cortex and outer medulla is rapidly diluted by cellular proliferation during embryonic development and juvenile growth, whereas cells within the papilla differentiate and exit the cell cycle during organogenesis. Indeed, by E17.5, little or no active proliferation can be seen in the distal papilla, indicating maturation of this structure in a distal-to-proximal manner during organogenesis. We conclude that BrdU-retaining cells of the papilla represent a population of cells that quiesce during embryonic development and localize within a region of the kidney that matures early. We therefore propose that selective papillary retention of BrdU arises through a combination of regionalized slowing of, and exit from, the cell cycle within the papilla during the period of ongoing kidney development, and extensive proliferative growth of the juvenile kidney resulting in dilution of BrdU below the detection level in extra-papillary regions.

Cell division and phenotypic regression of proximal tubular cells in response to uranyl acetate insult in rats

BACKGROUND

We examined whether dedifferentiation is necessary for cell division of proximal tubule (PT) cells after acute PT injury.

METHODS

Rats were injected with a low (0.2 mg/kg) or high (4 mg/kg) dose of uranyl acetate (UA) to induce acute PT injury. Proliferating PT cells were labelled with bromodeoxyuridine (BrdU) before sacrifice. Renal tissues were examined by double labelling of BrdU and megalin, aquaporin 1 (AQP1), Na(+)-K(+)ATPase or vimentin, and by immunoelectron microscopy for BrdU+ cells.

RESULTS

Under normal conditions, BrdU+ PT cells were positive for the PT phenotype (megalin-, AQP1- and Na(+)-K(+)ATPase positive and vimentine negative, a mesenchymal marker). Low-dose UA induced focal PT injury, and BrdU+ initially proliferating PT cells were found in the proximal three quarters of the S3 segment of nephron as early as 12 h, which maintained the PT phenotype and were vimentin negative. Proliferating PT cells showed low expression of the PT cell protein phenotype from Day 2 to Day 5 with vimentin expression from Day 2. High-dose UA induced severe PT injury in the proximal three quarters of the S3 segment by Day 5. BrdU+ initially proliferating PT cells, which were found in distal areas of the S3 segment as early as Day 2, showed low expression of the PT protein phenotype but were vimentin positive. Immunoelectron microscopy showed mature PT morphology for BrdU+ PT cells in control rats. BrdU+ initially proliferating PT cells showed a relatively mature phenotype after low-dose UA in- sult but an immature phenotype after high-dose UA insult.

CONCLUSIONS

PT cells can initiate cell division without de- differentiation after mild PT injury by low-dose UA insult.

HIV-1 expression induces tubular cell G2/M arrest and apoptosis

Human renal biopsy studies suggest the presence of HIV-1 and associated signs of injury in renal tubular epithelial cells. Because renal epithelial cells lack conventional HIV-1 receptors, the modus operandi of HIV-1 in the induction of tubular cell injury remains a mystery. In the present study, we evaluated the role of HIV-1 gene expression in human proximal tubular cell apoptosis and cell cycle progression. HIV-1- or vector-transduced cells were assayed for cellular injury and cell cycle defect. HIV-1-transduced cells showed the progressive loss of viability in a time-dependent manner. Similarly, HIV-1-transduced cells showed greater apoptosis when compared with vector-transduced cells. A higher number of HIV-1 expressing cells showed cell cycle arrest at G2/M phase and enhanced tubular cell expression of phospho-p53(ser15), phospho-cdc-2(Tyr 15), and phospho-chk-2 (Thr 68). These findings suggest that in addition to the activation of apoptotic pathway, HIV-1-induced G2/M arrest may also contribute to tubular cell injury.

Acute kidney injury: stimulation of repair

PURPOSE OF REVIEW

The kidney has the ability to restore the structural and functional integrity of the proximal tubule, which undergoes extensive epithelial cell death via necrosis and apoptosis after a prolonged ischaemic insult. This review focuses on the recent advances in this area, and discusses the possible therapeutic interventions that might be derived from these insights.

RECENT FINDINGS

Interest has recently been focused on the possible role of bone marrow originating stem cells in endogenous repair of the injured tubule, the identification of a resident population of progenitor cells in the kidney, and the potential therapeutic role of growth factors including erythropoietin and hepatocyte growth factor to stimulate these processes.

SUMMARY

Advances in the understanding of the early processes that initiate and control the proliferation of surviving tubular epithelium and vascular structures are ready to be translated into clinical trials in acute kidney injury.

Regulation of dedifferentiation and redifferentiation in renal proximal tubular cells by the epidermal growth factor receptor

Repair of injured renal epithelium is thought to be mediated by surviving renal proximal tubular cells (RPTC) that must dedifferentiate to allow the proliferation and migration necessary for epithelial regeneration. RPTC then redifferentiate to restore tubular structure and function. Current models suggest that epidermal growth factor receptor (EGFR) activation is required for dedifferentiation characterized by enhanced vimentin expression, decreased N-cadherin expression, spindle morphology, and loss of apical-basal polarity after injury. Because an in vitro model of RPTC redifferentiation has not been reported, and the mechanism(s) of redifferentiation has not been determined, we used rabbit RPTC in primary cultures to address these issues. H2O2 induced the dedifferentiated phenotype that persisted >48 h; redifferentiation occurred spontaneously in the absence of exogenous growth factors after 72 to 120 h. Phosphorylation of two tyrosine residues of EGFR increased 12 to 24 h, peaked at 24 h, and declined to basal levels by 48 h after injury. EGFR inhibition during dedifferentiation restored epithelial morphology and apical-basal polarity, and it decreased vimentin expression to control levels 24 h later. In contrast, exogenous epidermal growth factor addition increased vimentin expression and potentiated spindle morphology. p38 mitogen-activated protein kinase (MAPK) and transforming growth factor (TGF)-beta receptor inhibitors did not affect redifferentiation after H2O2 injury. Similar results were observed in a mechanical injury model. These experiments represent a new model for the investigation of RPTC redifferentiation after acute injury and identify a key regulator of redifferentiation: EGFR, independent of p38 MAPK and the TGF-beta receptor.

Proliferation capacity of the renal proximal tubule involves the bulk of differentiated epithelial cells

We investigated the proliferative capacity of renal proximal tubular cells in healthy rats. Previously, we observed that tubular cells originate from differentiated cells. We now found 1) by application of bromo-deoxyuridine (BrdU) for 14 days and costaining for BrdU, and the G(1)-phase marker cyclin D1 that the bulk of cells in the S3 segment of juvenile rats were involved in proliferation; 2) that although the proliferation rate was about 10-fold higher in juvenile rats compared with adult rats, roughly 40% of S3 cells were in G(1) in both groups; 3) that after a strong mitotic stimulus (lead acetate), proliferation was similar in juveniles and adults; 4) that there was a high incidence of cyclin D1-positive cells also in the healthy human kidney; and 5) by labeling dividing cells with BrdU for 2 days before the application of lead acetate and subsequent costaining for BrdU and cell cycle markers, that, although a strong mitotic stimulus does not abolish the period of quiescence following division, it shortens it markedly. Thus the capacity of the proximal tubule to rapidly recruit cells into division relies on a large reserve pool of cells in G(1) and on the shortening of the obligatory period of quiescence that follows division.

Kidney Epithelial Cells

Kidney tubules are an essential component of an organism's blood clearance mechanism, recovering essential metabolites from glomerular filtration by active transport. Tubules are subject to injury, usually as the result of ischemia-reperfusion events that damage the polarized tubular cell layer that coats the tubule basement membrane, causing dysfunction and necrosis that is often associated with acute renal failure. However, tubules are capable of self-repair, forming new proximal tubular cells to replace failing or necrotic cells. The origin of the progenitor cells that give rise to new tubular cells is unknown. At one extreme, it is possible that all or a fraction of tubular cells can undergo a form of dedifferentiation and subsequent mitosis to form new tubular cells, or alternatively, it is possible that tubular regeneration follows the stem cell/transit-amplifying cell paradigm described for more rapidly regenerating organ systems. Regardless of the mechanism employed to generate new tubular cells, human tubular cells are readily grown in primary cultures and can recapitulate many of the metabolic, endocrine, and immunological properties attributable to endogenous renal proximal tubules when engrafted into bioartificial devices.