Selecting the optimal cell for kidney regeneration: fetal, adult or reprogrammed stem cells

OCT4 induces long-lived dedifferentiated kidney progenitors poised to redifferentiate in 3D kidney spheroids

Upscaling of kidney epithelial cells is crucial for renal regenerative medicine. Nonetheless, the adult kidney lacks a distinct stem cell hierarchy, limiting the ability to long-term propagate clonal populations of primary cells that retain renal identity. Toward this goal, we tested the paradigm of shifting the balance between differentiation and stemness in the kidney by introducing a single pluripotency factor, OCT4. Here we show that ectopic expression of OCT4 in human adult kidney epithelial cells (hKEpC) induces the cells to dedifferentiate, stably proliferate, and clonally emerge over many generations. Control hKEpC dedifferentiate, assume fibroblastic morphology, and completely lose clonogenic capacity. Analysis of gene expression and histone methylation patterns revealed that OCT4 represses the HNF1B gene module, which is critical for kidney epithelial differentiation, and concomitantly activates stemness-related pathways. OCT4-hKEpC can be long-term expanded in the dedifferentiated state that is primed for renal differentiation. Thus, when expanded OCT4-hKEpC are grown as kidney spheroids (OCT4-kSPH), they reactivate the HNF1B gene signature, redifferentiate, and efficiently generate renal structures in vivo. Hence, changes occurring in the cellular state of hKEpC following OCT4 induction, long-term propagation, and 3D aggregation afford rapid scale-up technology of primary renal tissue-forming cells.

Successful Introduction of Human Renovascular Units into the Mammalian Kidney

BACKGROUND

Cell-based therapies aimed at replenishing renal parenchyma have been proposed as an approach for treating CKD. However, pathogenic mechanisms involved in CKD such as renal hypoxia result in loss of kidney function and limit engraftment and therapeutic effects of renal epithelial progenitors. Jointly administering vessel-forming cells (human mesenchymal stromal cells [MSCs] and endothelial colony-forming cells [ECFCs]) may potentially result in in vivo formation of vascular networks.

METHODS

We administered renal tubule-forming cells derived from human adult and fetal kidneys (previously shown to exert a functional effect in CKD mice) into mice, alongside MSCs and ECFCs. We then assessed whether this would result in generation of "renovascular units" comprising both vessels and tubules with potential interaction.

RESULTS

Directly injecting vessel-forming cells and renal tubule-forming cells into the subcutaneous and subrenal capsular space resulted in self-organization of donor-derived vascular networks that connected to host vasculature, alongside renal tubules comprising tubular epithelia of different nephron segments. Vessels derived from MSCs and ECFCs augmented in vivo tubulogenesis by the renal tubule-forming cells. In vitro coculture experiments showed that MSCs and ECFCs induced self-renewal and genes associated with mesenchymal-epithelial transition in renal tubule-forming cells, indicating paracrine effects. Notably, after renal injury, renal tubule-forming cells and vessel-forming cells infused into the renal artery did not penetrate the renal vascular network to generate vessels; only administering them into the kidney parenchyma resulted in similar generation of human renovascular units in vivo.

CONCLUSIONS

Combined cell therapy of vessel-forming cells and renal tubule-forming cells aimed at alleviating renal hypoxia and enhancing tubulogenesis holds promise as the basis for new renal regenerative therapies.

The multiple functions of melatonin in regenerative medicine

Melatonin research has been experiencing hyper growth in the last two decades; this relates to its numerous physiological functions including anti-inflammation, oncostasis, circadian and endocrine rhythm regulation, and its potent antioxidant activity. Recently, a large number of studies have focused on the role of melatonin in the regeneration of cells or tissues after their partial loss. In this review, we discuss the recent findings on the molecular involvement of melatonin in the regeneration of various tissues including the nervous system, liver, bone, kidney, bladder, skin, and muscle, among others.

Heterogeneity of Stem Cells: A Brief Overview

Stem cells possess the extraordinary capacity of self-renewal and differentiation to various cell types, thus to form original tissues and organs. Stem cell heterogeneity including genetic and nongenetic mechanisms refers to biological differences amongst normal and stem cells originated within the same tissue. Cell differentiation hierarchy and stochasticity in gene expression and signaling pathways may result in phenotypic differences of stem cells. The maintenance of stemness and activation of differentiation potential are fundamentally orchestrated by microenvironmental stem cell niche-related cellular and humoral signals.

Renal lineage cells as a source for renal regeneration

The mammalian kidney is a highly complex organ, composed of various cell types within a unique structural framework. Nonetheless, in recent years, giant leaps in our understanding of nephrogenesis and the origin of new cells in the adult kidney have resulted in novel routes to regenerate damaged nephrons. While several strategies can be envisioned to achieve this aim, one common theme is the reliance on renal lineage cells, as extrarenal cells, such as bone marrow-derived cells, have been shown to be devoid of renal differentiation capacity. Herein, we will present the main motivation for the pursuit for cell-based therapies, which is the ever growing problem of chronic kidney disease (CKD), and discuss different strategies toward replenishing the damaged renal parenchyma. These include transplantation of fetal kidney grafts or fetal kidney stem cells, directed differentiation of pluripotent stem cells into kidney epithelia, establishment of renal progenitors from the adult kidney, and genetic reprogramming of mature kidney cells into a progenitor state. Taken together with novel techniques recapitulating the three-dimensional developmental environment, these advances are expected to take the field into a new era, bringing us closer than ever to the day when kidney stem cell-based therapy becomes a viable therapeutic option.

Peroxisome proliferator-activated receptor gamma (PPARγ) is central to the initiation and propagation of human angiomyolipoma, suggesting its potential as a therapeutic target

Angiomyolipoma (AML), the most common benign renal tumor, can result in severe morbidity from hemorrhage and renal failure. While mTORC1 activation is involved in its growth, mTORC1 inhibitors fail to eradicate AML, highlighting the need for new therapies. Moreover, the identity of the AML cell of origin is obscure. AML research, however, is hampered by the lack of in vivo models. Here, we establish a human AML‐xenograft (Xn) model in mice, recapitulating AML at the histological and molecular levels. Microarray analysis demonstrated tumor growth in vivo to involve robust PPARG‐pathway activation. Similarly, immunostaining revealed strong PPARG expression in human AML specimens. Accordingly, we demonstrate that while PPARG agonism accelerates AML growth, PPARG antagonism is inhibitory, strongly suppressing AML proliferation and tumor‐initiating capacity, via a TGFB‐mediated inhibition of PDGFB and CTGF. Finally, we show striking similarity between AML cell lines and mesenchymal stem cells (MSCs) in terms of antigen and gene expression and differentiation potential. Altogether, we establish the first in vivo human AML model, which provides evidence that AML may originate in a PPARG‐activated renal MSC lineage that is skewed toward adipocytes and smooth muscle and away from osteoblasts, and uncover PPARG as a regulator of AML growth, which could serve as an attractive therapeutic target.

Evidence of In Vitro Preservation of Human Nephrogenesis at the Single-Cell Level

During nephrogenesis, stem/progenitor cells differentiate and give rise to early nephron structures that segment to proximal and distal nephron cell types. Previously, we prospectively isolated progenitors from human fetal kidney (hFK) utilizing a combination of surface markers. However, upon culture nephron progenitors differentiated and could not be robustly maintained in vitro. Here, by culturing hFK in a modified medium used for in vitro growth of mouse nephron progenitors, and by dissection of NCAM⁺/CD133⁻ progenitor cells according to EpCAM expression (NCAM⁺/CD133⁻/EpCAM⁻, NCAM⁺/CD133⁻/EpCAM^dim, NCAM⁺/CD133⁻/EpCAM^bright), we show at single-cell resolution a preservation of uninduced and induced cap mesenchyme as well as a transitioning mesenchymal-epithelial state. Concomitantly, differentiating and differentiated epithelial lineages are also maintained. In vitro expansion of discrete stages of early human nephrogenesis in nephron stem cell cultures may be used for drug screening on a full repertoire of developing kidney cells and for prospective isolation of mesenchymal or epithelial renal lineages for regenerative medicine.

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mNPEM enables in vitro preservation of human renal embryonic CM and epithelia

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EpCAM allows further dissection of expanded NCAM⁺CD133⁻ early nephric population

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Single-cell analysis unveils a continuous lineage hierarchy in nephrogenesis and WT

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Splice isoform switching confirms a unified MET hierarchy in nephrogenesis and WT

Dissecting Stages of Human Kidney Development and Tumorigenesis with Surface Markers Affords Simple Prospective Purification of Nephron Stem Cells

When assembling a nephron during development a multipotent stem cell pool becomes restricted as differentiation ensues. A faulty differentiation arrest in this process leads to transformation and initiation of a Wilms’ tumor. Mapping these transitions with respective surface markers affords accessibility to specific cell subpopulations. NCAM1 and CD133 have been previously suggested to mark human renal progenitor populations. Herein, using cell sorting, RNA sequencing, in vitro studies with serum-free media and in vivo xenotransplantation we demonstrate a sequential map that links human kidney development and tumorigenesis; In nephrogenesis, NCAM1⁺CD133⁻ marks SIX2⁺ multipotent renal stem cells transiting to NCAM1⁺CD133⁺ differentiating segment-specific SIX2⁻ epithelial progenitors and NCAM1⁻CD133⁺ differentiated nephron cells. In tumorigenesis, NCAM1⁺CD133⁻ marks SIX2⁺ blastema that includes the ALDH1⁺ WT cancer stem/initiating cells, while NCAM1⁺CD133⁺ and NCAM1⁻CD133⁺ specifying early and late epithelial differentiation, are severely restricted in tumor initiation capacity and tumor self-renewal. Thus, negative selection for CD133 is required for defining NCAM1⁺ nephron stem cells in normal and malignant nephrogenesis.

Fetal kidney stem cells ameliorate cisplatin induced acute renal failure and promote renal angiogenesis

AIM: To investigate whether fetal kidney stem cells (fKSC) ameliorate cisplatin induced acute renal failure (ARF) in rats and promote renal angiogenesis.

METHODS: The fKSC were isolated from rat fetuses of gestation day 16 and expanded in vitro up to 3^rd passage. They were characterized for the expression of mesenchymal and renal progenitor markers by flow cytometry and immunocytochemistry, respectively. The in vitro differentiation of fKSC towards epithelial lineage was evaluated by the treatment with specific induction medium and their angiogenic potential by matrigel induced tube formation assay. To study the effect of fKSC in ARF, fKSC labeled with PKH26 were infused in rats with cisplatin induced ARF and, the blood and renal tissues of the rats were collected at different time points. Blood biochemical parameters were studied to evaluate renal function. Renal tissues were evaluated for renal architecture, renal cell proliferation and angiogenesis by immunohistochemistry, renal cell apoptosis by terminal deoxynucleotidyl transferase nick-end labeling assay and early expression of angiogenic molecules viz. vascular endothelial growth factor (VEGF), hypoxia-inducible factor (HIF)-1α and endothelial nitric oxide synthase (eNOS) by western blot.

RESULTS: The fKSC expressed mesenchymal markers viz. CD29, CD44, CD73, CD90 and CD105 as well as renal progenitor markers viz. Wt1, Pax2 and Six2. They exhibited a potential to form CD31 and Von Willebrand factor expressing capillary-like structures and could be differentiated into cytokeratin (CK)18 and CK19 positive epithelial cells. Administration of fKSC in rats with ARF as compared to administration of saline alone, resulted in a significant improvement in renal function and histology on day 3 (2.33 ± 0.33 vs 3.50 ± 0.34, P < 0.05) and on day 7 (0.83 ± 0.16 vs 2.00 ± 0.25, P < 0.05). The infused PKH26 labeled fKSC were observed to engraft in damaged renal tubules and showed increased proliferation and reduced apoptosis (P < 0.05) of renal cells. The kidneys of fKSC as compared to saline treated rats had a higher capillary density on day 3 [13.30 ± 1.54 vs 7.10 ± 1.29, capillaries/high-power fields (HPF), P < 0.05], and on day 7 (21.10 ± 1.46 vs 15.00 ± 1.30, capillaries/HPF, P < 0.05). In addition, kidneys of fKSC treated rats had an up-regulation of angiogenic proteins hypoxia-inducible factor-1α, VEGF and eNOS on day 3 (P < 0.05).

CONCLUSION: Our study shows that fKSC ameliorate cisplatin induced ARF in rats and promote renal angiogenesis, which may be an important therapeutic mechanism of these stem cells in the disease.

The lymph node as a new site for kidney organogenesis

The shortage of organs for kidney transplantation has created the need to develop new strategies to restore renal structure and function. Given our recent finding that the lymph node (LN) can serve as an in vivo factory to generate or sustain complex structures like liver, pancreas, and thymus, we investigated whether it could also support kidney organogenesis from mouse renal embryonic tissue (metanephroi). Here we provide the first evidence that metanephroi acquired a mature phenotype upon injection into LN, and host cells likely contributed to this process. Urine-like fluid-containing cysts were observed in several grafts 12 weeks post-transplantation, indicating metanephroi transplants' ability to excrete products filtered from the blood. Importantly, the kidney graft adapted to a loss of host renal mass, speeding its development. Thus, the LN might provide a unique tool for studying the mechanisms of renal maturation, cell proliferation, and fluid secretion during cyst development. Moreover, we provide evidence that inside the LN, short-term cultured embryonic kidney cells stimulated with the Wnt agonist R-Spondin 2 gave rise to a monomorphic neuron-like cell population expressing the neuronal 200-kDa neurofilament heavy marker. This finding indicates that the LN might be used to validate the differentiation potential of candidate stem cells in regenerative nephrology.

Kidney stem cells in development, regeneration and cancer

The generation of nephrons during development depends on differentiation via a mesenchymal to epithelial transition (MET) of self-renewing, tissue-specific stem cells confined to a specific anatomic niche of the nephrogenic cortex. These cells may transform to generate oncogenic stem cells and drive pediatric renal cancer. Once nephron epithelia are formed the view of post-MET tissue renal growth and maintenance by adult tissue-specific epithelial stem cells becomes controversial. Recently, genetic lineage tracing that followed clonal evolution of single kidney cells showed that the need for new cells is constantly driven by fate-restricted unipotent clonal expansions in varying kidney segments arguing against a multipotent adult stem cell model. Lineage-restriction was similarly maintained in kidney organoids grown in culture. Importantly, kidney cells in which Wnt was activated were traced to give significant clonal progeny indicating a clonogenic hierarchy. In vivo nephron epithelia may be endowed with the capacity akin to that of unipotent epithelial stem/progenitor such that under specific stimuli can clonally expand/self renew by local proliferation of mature differentiated cells. Finding ways to ex vivo preserve and expand the observed in vivo kidney-forming capacity inherent to both the fetal and adult kidneys is crucial for taking renal regenerative medicine forward. Some of the strategies used to achieve this are sorting human fetal nephron stem/progenitor cells, growing adult nephrospheres or reprogramming differentiated kidney cells toward expandable renal progenitors.

Reprogramming somatic cells to a kidney fate

Recent years have challenged the view that adult somatic cells reach a state of terminal differentiation. Although the ultimate example of this, somatic cell nuclear transfer, has not proven feasible in human beings, dedifferentiation of mature cell types to a more primitive state, direct reprogramming from one mature state to another, and the reprogramming of any adult cell type to a pluripotent state via enforced expression of key transcription factors now all have been shown. The implications of these findings for kidney disease include the re-creation of key renal cell types from more readily available and expandable somatic cell sources. The feasibility of such an approach recently was shown with the dedifferentiation of proximal tubule cells to nephrogenic mesenchyme. In this review, we examine the technical and clinical challenges that remain to such an approach and how new reprogramming approaches also may be useful for kidney disease.

Cell therapy for kidney injury: different options and mechanisms--kidney progenitor cells

BACKGROUND

Since no specific or radical treatments have yet been established for acute kidney injury (AKI), the development of cell transplantation therapy using renal progenitors is desirable as a new therapeutic option for AKI. The recent advances in developmental biology, stem cell biology, and nephrology have led to an increased availability of renal progenitors from multiple sources.

SUMMARY

Four main sources of renal progenitors have been described so far: isolation from (1) embryonic or (2) adult kidneys, (3) directed differentiation of pluripotent stem cells such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs), and (4) cellular reprogramming of fully differentiated adult renal cells. Renal progenitors from adult kidneys may not be equivalent to those from embryonic kidneys, and they contain several different cell populations identified by various methods. The methods used for the directed differentiation of ESCs/iPSCs and reprogramming of differentiated adult renal cells into renal progenitors have not been fully established. The therapeutic effects of progenitor cell transplantation in AKI animal models have been examined in a small number of reports using renal progenitors from adult kidneys, while no reports have described the therapeutic potential of renal progenitors from other sources.

KEY MESSAGES

Renal progenitor transplantation might provide a novel therapeutic strategy for AKI. Further research efforts toward the clinical application of this strategy are needed, including a detailed characterization of embryonic or adult renal progenitors and the development of in vitro expansion methods and therapeutically effective transplantation methods for these cell types. More experience and knowledge should be accumulated regarding the directed differentiation of pluripotent stem cells and cellular reprogramming to generate renal progenitor cells.

Differentiation of human pluripotent stem cells into nephron progenitor cells in a serum and feeder free system

Objectives

Kidney disease is emerging as a critical medical problem worldwide. Because of limited treatment options for the damaged kidney, stem cell treatment is becoming an alternative therapeutic approach. Of many possible human stem cell sources, pluripotent stem cells are most attractive due to their self-renewal and pluripotent capacity. However, little is known about the derivation of renal lineage cells from human pluripotent stem cells (hPSCs). In this study, we developed a novel protocol for differentiation of nephron progenitor cells (NPCs) from hPSCs in a serum- and feeder-free system.

Materials and Methods

We designed step-wise protocols for differentiation of human pluripotent stem cells toward primitive streak, intermediate mesoderm and NPCs by recapitulating normal nephrogenesis. Expression of key marker genes was examined by RT-PCR, real time RT-PCR and immunocytochemistry. Each experiment was independently performed three times to confirm its reproducibility.

Results

After modification of culture period and concentration of exogenous factors, hPSCs can differentiate into NPCs that markedly express specific marker genes such as SIX2, GDNF, HOXD11, WT1 and CITED1 in addition to OSR1, PAX2, SALL1 and EYA1. Moreover, NPCs possess the potential of bidirectional differentiation into both renal tubular epithelial cells and glomerular podocytes in defined culture conditions. In particular, approximately 70% of SYN-positive cells were obtained from hPSC-derived NPCs after podocytes induction. NPCs can also form in vitro tubule-like structures in three dimensional culture systems.

Conclusions

Our novel protocol for hPSCs differentiation into NPCs can be useful for producing alternative sources of cell replacement therapy and disease modeling for human kidney diseases.

Immature Dental Pulp Stem Cells Showed Renotropic and Pericyte-Like Properties in Acute Renal Failure in Rats

Acute renal failure (ARF) is a common renal disease that can lead to high mortality. Recovery from ARF occurs with the replacement of necrotic tubular cells by functional tubular epithelial cells and the normalization of microvascular endothelial cell function in the peritubular capillaries. Conventional therapeutic techniques are often ineffective against ARF. Hence, stem cell therapies, which act through multiple trophic and regenerative mechanisms, are encouraging. We investigated the homing of human immature dental pulp stem cells (IDPSCs) after endovenous (EV) or intraperitoneal (IP) injection, in immunocompetent Wistar rats with ARF induced by intramuscular injection of glycerol, without the use of immunosuppression. The cells, which had been cryopreserved for 6 years, were CD105(+), CD73(+), CD44(+), and partly, STRO-1(+) and CD146(+), and presented unaltered mesoderm differentiation potential. The presence of these cells in the tubular region of the kidney and in the peritubular capillaries was demonstrated. These cells accelerate tubular epithelial cell regeneration through significant increase of Ki-67-immunoreactive cells in damaged kidney. Flow cytometry analysis confirmed that IDPSCs home to the kidneys (EV 34.10% and IP 33.25%); a lower percentage of cells was found in the liver (EV 19.05% and IP 9.10%), in the muscles (EV 6.30% and IP 1.35%), and in the lungs (EV 2.0% and IP 1.85%). After infusion into rat, these cells express pericyte markers, such as CD146(+), STRO-1(+), and vascular endothelial growth factor (VEGF(+)). We found that IDPSCs demonstrate renotropic and pericyte-like properties and contributed to restore renal tubule structure in an experimental rat ARF model.

Engineering kidneys from simple cell suspensions: an exercise in self-organization

Increasing numbers of people approaching and living with end-stage renal disease and failure of the supply of transplantable kidneys to keep pace has created an urgent need for alternative sources of new organs. One possibility is tissue engineering of new organs from stem cells. Adult kidneys are arguably too large and anatomically complex for direct construction, but engineering immature kidneys, transplanting them, and allowing them to mature within the host may be more feasible. In this review, we describe a technique that begins with a suspension of renogenic stem cells and promotes these cells' self-organization into organ rudiments very similar to foetal kidneys, with a collecting duct tree, nephrons, corticomedullary zonation and extended loops of Henle. The engineered rudiments vascularize when transplanted to appropriate vessel-rich sites in bird eggs or adult animals, and show preliminary evidence for physiological function. We hope that this approach might one day be the basis of a clinically useful technique for renal replacement therapy.

Identification of human nephron progenitors capable of generation of kidney structures and functional repair of chronic renal disease

Identification of tissue-specific renal stem/progenitor cells with nephrogenic potential is a critical step in developing cell-based therapies for renal disease. In the human kidney, stem/progenitor cells are induced into the nephrogenic pathway to form nephrons until the 34 week of gestation, and no equivalent cell types can be traced in the adult kidney. Human nephron progenitor cells (hNPCs) have yet to be isolated. Here we show that growth of human foetal kidneys in serum-free defined conditions and prospective isolation of NCAM1(+) cells selects for nephron lineage that includes the SIX2-positive cap mesenchyme cells identifying a mitotically active population with in vitro clonogenic and stem/progenitor properties. After transplantation in the chick embryo, these cells-but not differentiated counterparts-efficiently formed various nephron tubule types. hNPCs engrafted and integrated in diseased murine kidneys and treatment of renal failure in the 5/6 nephrectomy kidney injury model had beneficial effects on renal function halting disease progression. These findings constitute the first definition of an intrinsic nephron precursor population, with major potential for cell-based therapeutic strategies and modelling of kidney disease.

Epoxyeicosanoids promote organ and tissue regeneration

Epoxyeicosatrienoic acids (EETs), lipid mediators produced by cytochrome P450 epoxygenases, regulate inflammation, angiogenesis, and vascular tone. Despite pleiotropic effects on cells, the role of these epoxyeicosanoids in normal organ and tissue regeneration remains unknown. EETs are produced predominantly in the endothelium. Normal organ and tissue regeneration require an active paracrine role of the microvascular endothelium, which in turn depends on angiogenic growth factors. Thus, we hypothesize that endothelial cells stimulate organ and tissue regeneration via production of bioactive EETs. To determine whether endothelial-derived EETs affect physiologic tissue growth in vivo, we used genetic and pharmacological tools to manipulate endogenous EET levels. We show that endothelial-derived EETs play a critical role in accelerating tissue growth in vivo, including liver regeneration, kidney compensatory growth, lung compensatory growth, wound healing, corneal neovascularization, and retinal vascularization. Administration of synthetic EETs recapitulated these results, whereas lowering EET levels, either genetically or pharmacologically, delayed tissue regeneration, demonstrating that pharmacological modulation of EETs can affect normal organ and tissue growth. We also show that soluble epoxide hydrolase inhibitors, which elevate endogenous EET levels, promote liver and lung regeneration. Thus, our observations indicate a central role for EETs in organ and tissue regeneration and their contribution to tissue homeostasis.

Chromatin-modifying agents reactivate embryonic renal stem/progenitor genes in human adult kidney epithelial cells but abrogate dedifferentiation and stemness

Recent studies have suggested that epigenetic modulation with chromatin-modifying agents can induce stemness and dedifferentiation and increase developmental plasticity. For instance, valproic acid (VPA), a histone deacetylase (HDAC) inhibitor, has been shown to promote self-renewal/expansion of hematopoietic stem cells and facilitate the generation of induced pluripotent stem cells (iPSCs). Previously, we observed that downregulation of embryonic renal stem/progenitor genes in the adult kidney was associated, at least in part, with epigenetic silencing. Therefore, we hypothesized that VPA may alter the expression of these genes and reprogram mature human adult kidney epithelial cells (hKEpCs) to a stem/progenitor-like state. Here, using quantitative RT-PCR and flow cytometry [fluorescence-activated cell sorting (FACS)] analysis, we show in VPA-treated primary cultures of human adult and fetal kidney significant reinduction of the renal stem/progenitor markers SIX2, OSR1, SALL1, NCAM, and PSA-NCAM. Robust SIX2 mRNA re-expression was confirmed at the protein level by western blot and was associated with epigenetic changes of the histones at multiple sites of the SIX2 promoter leading to gene activation, significantly increased acetylation of histones H4, and methylation of lysine 4 on H3. Furthermore, we could demonstrate synergistic effects of VPA and Wnt antagonists on SIX2 and also OSR1 reinduction. Nevertheless, VPA resulted in upregulation of E-CADHERIN and reduction in VIMENTIN, preventing the skewing of hKEpCs towards a more replicative mesenchymal state required for clonogenic expansion and acquisition of stem cell characters, altogether inducing cell senescence at early passages. These results demonstrating that chromatin-modifying agents prevent dedifferentiation of hKEpCs have important clinical implications as they may limit ex-vivo self-renewal/expansion and possibly the in vivo renal regenerative capacity initiated by dedifferentiation.

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.

Illustration of extensive extracellular matrix at the epithelial-mesenchymal interface within the renal stem/progenitor cell niche

BACKGROUND

Stem/progenitor cells are promising candidates to treat diseased renal parenchyma. However, implanted stem/progenitor cells are exposed to a harmful atmosphere of degenerating parenchyma. To minimize hampering effects after an implantation investigations are in progress to administer these cells within an artificial polyester interstitum supporting survival. Learning from nature the renal stem/progenitor cell niche appears as a valuable model. At this site epithelial stem/progenitor cells within the collecting duct ampulla face mesenchymal stem/progenitor cells. Both cell types do not have close contact but are separated by a wide interstitium.

METHODS

To analyze extracellular matrix in this particular interstitium, special contrasting for transmission electron microscopy was performed. Kidneys of neonatal rabbits were fixed in solutions containing glutaraldehyde (GA) or in combination with cupromeronic blue, ruthenium red and tannic acid.

RESULTS

GA revealed a basal lamina at the ampulla and a bright but inconspicuously looking interstitial space. In contrast, GA containing cupromeronic blue exhibits numerous proteoglycan braces lining from the ampulla towards the interstitial space. GA containing ruthenium red or tannic acid demonstrates clouds of extracellular matrix protruding from the basal lamina of the ampulla to the surface of mesenchymal stem/progenitor cells.

CONCLUSIONS

The actual data show that the interstitium between epithelial and mesenchymal stem/progenitor cells contains much more and up to date unknown extracellular matrix than earlier observed by classical GA fixation.

Detrimental effects of rat mesenchymal stromal cell pre-treatment in a model of acute kidney rejection

Mesenchymal stromal cells (MSC) have shown immunomodulatory and tissue repair potential including partial tolerance induction by pre-treatment of donor-specific cells in a rat heart transplantation model. Very recently, we could show that autologous MSC attenuated ischemia reperfusion injury in a highly mismatched donor–recipient rat kidney transplant model. Therefore, we investigated donor-specific MSC pre-treatment in this rat kidney transplantation model to study whether graft function could be improved, or if tolerance could be induced. Donor- and recipient-type MSC or phosphate buffered saline (PBS) as a control was injected i.v. 4 days before kidney transplantation. Mycophenolate mofetil immunosuppression (20mg/kg body weight) was applied for 7 days. Kidney grafts and spleens were harvested between days 8 and 10 and analyzed by quantitative RT-PCR and immunohistology. In addition, creatinine levels in the blood were measured and serum was screened for the presence of donor-specific antibodies. Surprisingly, application of both donor- and recipient-specific MSC resulted in enhanced humoral immune responses verified by intragraft B cell infiltration and complement factor C4d deposits. Moreover, signs of inflammation and rejection were generally enhanced in both MSC-treated groups relative to PBS control group. Additionally, pre-treatment with donor-specific MSC significantly enhanced the level of donor-specific antibody formation when compared with PBS- or recipient MSC-treated groups. Pre-treatment with both MSC types resulted in a higher degree of kidney cortex tissue damage and elevated creatinine levels at the time point of rejection. Thus, MSC pre-sensitization in this model impairs the allograft outcome. Our data from this pre-clinical kidney transplantation model indicate that pre-operative MSC administration may not be optimal in kidney transplantation and caution must be exerted before moving forward with clinical studies in order to avoid adverse effects.