Contribution of human amniotic fluid stem cells to renal tissue formation depends on mTOR

Fetal Reprogramming of Nutrient Surplus Signaling, O-GlcNAcylation, and the Evolution of CKD

ABSTRACT

Fetal kidney development is characterized by increased uptake of glucose, ATP production by glycolysis, and upregulation of mammalian target of rapamycin (mTOR) and hypoxia-inducible factor-1 alpha (HIF-1 α ), which (acting in concert) promote nephrogenesis in a hypoxic low-tubular-workload environment. By contrast, the healthy adult kidney is characterized by upregulation of sirtuin-1 and adenosine monophosphate-activated protein kinase, which enhances ATP production through fatty acid oxidation to fulfill the needs of a normoxic high-tubular-workload environment. During stress or injury, the kidney reverts to a fetal signaling program, which is adaptive in the short term, but is deleterious if sustained for prolonged periods when both oxygen tension and tubular workload are heightened. Prolonged increases in glucose uptake in glomerular and proximal tubular cells lead to enhanced flux through the hexosamine biosynthesis pathway; its end product-uridine diphosphate N -acetylglucosamine-drives the rapid and reversible O-GlcNAcylation of thousands of intracellular proteins, typically those that are not membrane-bound or secreted. Both O-GlcNAcylation and phosphorylation act at serine/threonine residues, but whereas phosphorylation is regulated by hundreds of specific kinases and phosphatases, O-GlcNAcylation is regulated only by O-GlcNAc transferase and O-GlcNAcase, which adds or removes N-acetylglucosamine, respectively, from target proteins. Diabetic and nondiabetic CKD is characterized by fetal reprogramming (with upregulation of mTOR and HIF-1 α ) and increased O-GlcNAcylation, both experimentally and clinically. Augmentation of O-GlcNAcylation in the adult kidney enhances oxidative stress, cell cycle entry, apoptosis, and activation of proinflammatory and profibrotic pathways, and it inhibits megalin-mediated albumin endocytosis in glomerular mesangial and proximal tubular cells-effects that can be aggravated and attenuated by augmentation and muting of O-GlcNAcylation, respectively. In addition, drugs with known nephroprotective effects-angiotensin receptor blockers, mineralocorticoid receptor antagonists, and sodium-glucose cotransporter 2 inhibitors-are accompanied by diminished O-GlcNAcylation in the kidney, although the role of such suppression in mediating their benefits has not been explored. The available evidence supports further work on the role of uridine diphosphate N -acetylglucosamine as a critical nutrient surplus sensor (acting in concert with upregulated mTOR and HIF-1 α signaling) in the development of diabetic and nondiabetic CKD.

Amniotic fluid stem cells ameliorate cisplatin-induced acute renal failure through induction of autophagy and inhibition of apoptosis

Background

We have recently demonstrated that amniotic fluid stem cells (AFSC) express renal progenitor markers and can be differentiated in vitro into renal lineage cell types, viz, juxtaglomerular and renal proximal tubular epithelial-like cells. Here, we have evaluated the therapeutic efficacy of AFSC in a cisplatin-induced rat model of acute renal failure (ARF) and investigated the underlying mechanisms responsible for their renoprotective effects.

Methods

ARF was induced in Wistar rats by intra-peritoneal injection of cisplatin (7 mg/kg). Five days after cisplatin injection, rats were randomized into two groups and injected with either AFSC or normal saline intravenously. On days 8 and 12 after cisplatin injection, the blood biochemical parameters, histopathological changes, apoptosis and expression of pro-apoptotic, anti-apoptotic, and autophagy-related proteins in renal tissues were studied in both groups of rats. To further confirm whether the protective effects of AFSC on cisplatin-induced apoptosis were dependent on autophagy, chloroquine, an autophagy inhibitor, was administered by the intra-peritoneal route.

Results

Administration of AFSC in ARF rats resulted in improvement of renal function and attenuation of renal damage as reflected by significant decrease in blood urea nitrogen, serum creatinine levels, tubular cell apoptosis as assessed by Bax/Bcl2 ratio, and expression of the pro-apoptotic proteins, viz, PUMA, Bax, cleaved caspase-3, and cleaved caspase-9, as compared to the saline-treated group. Furthermore, in the AFSC-treated group as compared to the saline-treated group, there was a significant increase in the activation of autophagy as evident by increased expression of LC3-II, ATG5, ATG7, Beclin1, and phospho-AMPK levels with a concomitant decrease in phospho-p70S6K and p62 expression levels. Chloroquine administration led to significant reduction in the anti-apoptotic effects of the AFSC therapy and further deterioration in the renal structure and function caused by cisplatin.

Conclusion

AFSC led to amelioration of cisplatin-induced ARF which was mediated by inhibition of apoptosis and activation of autophagy. The protective effects of AFSC were blunted by chloroquine, an inhibitor of autophagy, highlighting that activation of autophagy is an important mechanism of action for the protective role of AFSC in cisplatin-induced renal injury.

Amniotic Fluid Cells, Stem Cells, and p53: Can We Stereotype p53 Functions?

In recent years, great interest has been devoted to finding alternative sources for human stem cells which can be easily isolated, ideally without raising ethical objections. These stem cells should furthermore have a high proliferation rate and the ability to differentiate into all three germ layers. Amniotic fluid, ordinarily discarded as medical waste, is potentially such a novel source of stem cells, and these amniotic fluid derived stem cells are currently gaining a lot of attention. However, further information will be required about the properties of these cells before they can be used for therapeutic purposes. For example, the risk of tumor formation after cell transplantation needs to be explored. The tumor suppressor protein p53, well known for its activity in controlling Cell Prolif.eration and cell death in differentiated cells, has more recently been found to be also active in amniotic fluid stem cells. In this review, we summarize the major findings about human amniotic fluid stem cells since their discovery, followed by a brief overview of the important role played by p53 in embryonic and adult stem cells. In addition, we explore what is known about p53 in amniotic fluid stem cells to date, and emphasize the need to investigate its role, particularly in the context of cell tumorigenicity.

Generation of Functional Kidney Organoids In Vivo Starting from a Single-Cell Suspension

Novel methods in developmental biology and stem cell research have made it possible to generate complex kidney tissues in vitro that resemble whole organs and are termed organoids. In this chapter we describe a technique using suspensions of fully dissociated mouse kidney cells to yield organoids that can become vascularized in vivo and mature and display physiological functions. This system can be used to produce fine-grained human-mouse chimeric organoids in which the renal differentiation potential of human cells can be assessed. It can also be an excellent method for growing chimeric organoids in vivo using human stem cells, which can differentiate into specialized kidney cells and exert nephron-specific functions. We provide detailed methods, a brief discussion of critical points, and describe some successfully implemented examples of the system.

Ex vivo live cell tracking in kidney organoids using light sheet fluorescence microscopy

Screening cells for their differentiation potential requires a combination of tissue culture models and imaging methods that allow for long-term tracking of the location and function of cells. Embryonic kidney re-aggregation in vitro assays have been established which allow for the monitoring of organotypic cell behaviour in re-aggregated and chimeric renal organoids. However, evaluation of cell integration is hampered by the high photonic load of standard fluorescence microscopy which poses challenges for imaging three-dimensional systems in real-time over a time course. Therefore, we employed light sheet microscopy, a technique that vastly reduces photobleaching and phototoxic effects. We have also developed a new method for culturing the re-aggregates which involves immersed culture, generating organoids which more closely reflect development in vivo. To facilitate imaging from various angles, we embedded the organoids in a freely rotatable hydrogel cylinder. Endpoint fixing and staining were performed to provide additional biomolecular information. We succeeded in imaging labelled cells within re-aggregated kidney organoids over 15 hours and tracking their fate while simultaneously monitoring the development of organotypic morphological structures. Our results show that Wt1-expressing embryonic kidney cells obtained from transgenic mice could integrate into re-aggregated chimeric kidney organoids and contribute to developing nephrons. Furthermore, the nascent proximal tubules that formed in the re-aggregated tissues using the new culture method displayed secretory function, as evidenced by their ability to secrete an organic anion mimic into the tubular lumen.

Amniotic fluid cells: current progress and emerging challenges in renal regeneration

Amniotic fluid (AF) contains a heterogeneous population of cells that have been identified to possess pluripotent and progenitor-like characteristics. These cells have been applied in various regenerative medicine applications ranging from in vitro cell differentiation to tissue engineering to cellular therapies for different organs including the heart, the liver, the lung, and the kidneys. In this review, we examine the different methodologies used for the derivation of amniotic fluid stem cells and renal progenitors, and their application in renal repair and regeneration. Moreover, we discuss the recent achievements and newly emerging challenges in our understanding of their biology, their immunoregulatory characteristics, and their paracrine-mediated therapeutic potential for the treatment of acute and chronic kidney diseases.

The presence of human mesenchymal stem cells of renal origin in amniotic fluid increases with gestational time

Background

Established therapies for managing kidney dysfunction such as kidney dialysis and transplantation are limited due to the shortage of compatible donated organs and high costs. Stem cell-based therapies are currently under investigation as an alternative treatment option. As amniotic fluid is composed of fetal urine harboring mesenchymal stem cells (AF-MSCs), we hypothesized that third-trimester amniotic fluid could be a novel source of renal progenitor and differentiated cells.

Methods

Human third-trimester amniotic fluid cells (AFCs) were isolated and cultured in distinct media. These cells were characterized as renal progenitor cells with respect to cell morphology, cell surface marker expression, transcriptome and differentiation into chondrocytes, osteoblasts and adipocytes. To test for renal function, a comparative albumin endocytosis assay was performed using AF-MSCs and commercially available renal cells derived from kidney biopsies. Comparative transcriptome analyses of first, second and third trimester-derived AF-MSCs were conducted to monitor expression of renal-related genes.

Results

Regardless of the media used, AFCs showed expression of pluripotency-associated markers such as SSEA4, TRA-1-60, TRA-1-81 and C-Kit. They also express the mesenchymal marker Vimentin. Immunophenotyping confirmed that third-trimester AFCs are bona fide MSCs. AF-MSCs expressed the master renal progenitor markers SIX2 and CITED1, in addition to typical renal proteins such as PODXL, LHX1, BRN1 and PAX8. Albumin endocytosis assays demonstrated the functionality of AF-MSCs as renal cells. Additionally, upregulated expression of BMP7 and downregulation of WT1, CD133, SIX2 and C-Kit were observed upon activation of WNT signaling by treatment with the GSK-3 inhibitor CHIR99201. Transcriptome analysis and semiquantitative PCR revealed increasing expression levels of renal-specific genes (e.g., SALL1, HNF4B, SIX2) with gestational time. Moreover, AF-MSCs shared more genes with human kidney cells than with native MSCs and gene ontology terms revealed involvement of biological processes associated with kidney morphogenesis.

Conclusions

Third-trimester amniotic fluid contains AF-MSCs of renal origin and this novel source of kidney progenitors may have enormous future potentials for disease modeling, renal repair and drug screening.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0864-7) contains supplementary material, which is available to authorized users.

An Update on the Therapeutic Potential of Stem Cells

The seeming setbacks noted for stem cells underscore the need for experimental studies for safe and efficacious application to patients. Both clinical and experimental researchers have gained valuable knowledge on the characteristics of stem cells, and their behavior in different microenvironment. This introductory chapter focuses on adult mesenchymal stem cells (MSCs) based on the predominance in the clinic. MSCs can be influenced by inflammatory mediators to exert immune suppressive properties, commonly referred to as "licensing." Interestingly, while there are questions if other stem cells can be delivered across allogeneic barrier, there is no question on the ability of MSCs to provide this benefit. This property has been a great advantage since MSCs could be available for immediate application as "off-the-shelf" stem cells for several disorders, tissue repair and gene/drug delivery. Despite the benefit of MSCs, it is imperative that research continues with the various types of stem cells. The method needed to isolate these cells is outlined in this book. In parallel, safety studies are needed; particularly links to oncogenic event. In summary, this introductory chapter discusses several potential areas that need to be addressed for safe and efficient delivery of stem cells, and argue for the incorporation of microenvironmental factors in the studies. The method described in this chapter could be extrapolated to the field of chimeric antigen receptor T-cells (CAR-T). This will require application to stem cell hierarchy of memory T-cells.

Characterisation of Cultured Mesothelial Cells Derived from the Murine Adult Omentum

The human omentum has been long regarded as a healing patch, used by surgeons for its ability to immunomodulate, repair and vascularise injured tissues. A major component of the omentum are mesothelial cells, which display some of the characteristics of mesenchymal stem/stromal cells. For instance, lineage tracing studies have shown that mesothelial cells give rise to adipocytes and vascular smooth muscle cells, and human and rat mesothelial cells have been shown to differentiate into osteoblast- and adipocyte-like cells in vitro, indicating that they have considerable plasticity. However, so far, long-term cultures of mesothelial cells have not been successfully established due to early senescence. Here, we demonstrate that mesothelial cells isolated from the mouse omentum could be cultured for more than 30 passages. While epithelial markers were downregulated over passages in the mesothelial cells, their mesenchymal profile remained unchanged. Early passage mesothelial cells displayed clonogenicitiy, expressed several stem cell markers, and up to passage 5 and 13, respectively, could differentiate along the adipogenic and osteogenic lineages, demonstrating stem/progenitor characteristics and differentiation potential.

Rapamycin-Induced Hypoxia Inducible Factor 2A Is Essential for Chondrogenic Differentiation of Amniotic Fluid Stem Cells

Amniotic fluid stem (AFS) cells represent a major source of donor cells for cartilage repair. Recently, it became clear that mammalian target of rapamycin (mTOR) inhibition has beneficial effects on cartilage homeostasis, but the effect of mTOR on chondrogenic differentiation is still elusive. Therefore, the objectives of this study were to investigate the effects of mammalian target of rapamycin complex 1 (mTORC1) modulation on the expression of SOX9 and on its downstream targets during chondrogenic differentiation of AFS cells. We performed three-dimensional pellet culturing of AFS cells and of in vitro-expanded, human-derived chondrocytes in the presence of chondrogenic factors. Inhibition of mTORC1 by rapamycin or by small interfering RNA-mediated targeting of raptor (gene name, RPTOR) led to increased AKT activation, upregulation of hypoxia inducible factor (HIF) 2A, and an increase in SOX9, COL2A1, and ACAN abundance. Here we show that HIF2A expression is essential for chondrogenic differentiation and that AKT activity regulates HIF2A amounts. Importantly, engraftment of AFS cells in cell pellets composed of human chondrocytes revealed an advantage of raptor knockdown cells compared with control cells in their ability to express SOX9. Our results demonstrate that mTORC1 inhibition leads to AKT activation and an increase in HIF2A expression. Therefore, we suggest that mTORC1 inhibition is a powerful tool for enhancing chondrogenic differentiation of AFS cells and also of in vitro-expanded adult chondrocytes before transplantation.

SIGNIFICANCE

Repair of cartilage defects is still an unresolved issue in regenerative medicine. Results of this study showed that inhibition of the mammalian target of rapamycin complex 1 (mTORC1) pathway, by rapamycin or by small interfering RNA-mediated targeting of raptor (gene name, RPTOR), enhanced amniotic fluid stem cell differentiation toward a chondrocytic phenotype and increased their engrafting efficiency into cartilaginous structures. Moreover, freshly isolated and in vitro passaged human chondrocytes also showed redifferentiation upon mTORC1 inhibition during culturing. Therefore, this study revealed that rapamycin could enable a more efficient clinical use of cell-based therapy approaches to treat articular cartilage defects.

Functional Human Podocytes Generated in Organoids from Amniotic Fluid Stem Cells

Generating kidney organoids using human stem cells could offer promising prospects for research and therapeutic purposes. However, no cell-based strategy has generated nephrons displaying an intact three-dimensional epithelial filtering barrier. Here, we generated organoids using murine embryonic kidney cells, and documented that these tissues recapitulated the complex three-dimensional filtering structure of glomerular slits in vivo and accomplished selective glomerular filtration and tubular reabsorption. Exploiting this technology, we mixed human amniotic fluid stem cells with mouse embryonic kidney cells to establish three-dimensional chimeric organoids that engrafted in vivo and grew to form vascularized glomeruli and tubular structures. Human cells contributed to the formation of glomerular structures, differentiated into podocytes with slit diaphragms, and internalized exogenously infused BSA, thus attaining in vivo degrees of specialization and function unprecedented for donor stem cells. In conclusion, human amniotic fluid stem cell chimeric organoids may offer new paths for studying renal development and human podocyte disease, and for facilitating drug discovery and translational research.

Cell-based therapy for kidney disease

The prevalence of renal disease continues to increase worldwide. When normal kidney is injured, the damaged renal tissue undergoes pathological and physiological events that lead to acute and chronic kidney diseases, which frequently progress to end stage renal failure. Current treatment of these renal pathologies includes dialysis, which is incapable of restoring full renal function. To address this issue, cell-based therapy has become a potential therapeutic option to treat renal pathologies. Recent development in cell therapy has demonstrated promising therapeutic outcomes, in terms of restoration of renal structure and function impaired by renal disease. This review focuses on the cell therapy approaches for the treatment of kidney diseases, including various cell sources used, as well recent advances made in preclinical and clinical studies.

Organ In Vitro Culture: What Have We Learned about Early Kidney Development?

When Clifford Grobstein set out to study the inductive interaction between tissues in the developing embryo, he developed a method that remained important for the study of renal development until now. From the late 1950s on, in vitro cultivation of the metanephric kidney became a standard method. It provided an artificial environment that served as an open platform to study organogenesis. This review provides an introduction to the technique of organ culture, describes how the Grobstein assay and its variants have been used to study aspects of mesenchymal induction, and describes the search for natural and chemical inducers of the metanephric mesenchyme. The review also focuses on renal development, starting with ectopic budding of the ureteric bud, ureteric bud branching, and the generation of the nephron and presents the search for stem cells and renal progenitor cells that contribute to specific structures and tissues during renal development. It also presents the current use of Grobstein assay and its modifications in regenerative medicine and tissue engineering today. Together, this review highlights the importance of ex vivo kidney studies as a way to acquire new knowledge, which in the future can and will be implemented for developmental biology and regenerative medicine applications.

Cell-cell interactions driving kidney morphogenesis

The mammalian kidney forms via cell-cell interactions between an epithelial outgrowth of the nephric duct and the surrounding nephrogenic mesenchyme. Initial morphogenetic events include ureteric bud branching to form the collecting duct (CD) tree and mesenchymal-to-epithelial transitions to form the nephrons, requiring reciprocal induction between adjacent mesenchyme and epithelial cells. Within the tips of the branching ureteric epithelium, cells respond to mesenchyme-derived trophic factors by proliferation, migration, and mitosis-associated cell dispersal. Self-inhibition signals from one tip to another play a role in branch patterning. The position, survival, and fate of the nephrogenic mesenchyme are regulated by ECM and secreted signals from adjacent tip and stroma. Signals from the ureteric tip promote mesenchyme self-renewal and trigger nephron formation. Subsequent fusion to the CDs, nephron segmentation and maturation, and formation of a patent glomerular basement membrane also require specialized cell-cell interactions. Differential cadherin, laminin, nectin, and integrin expression, as well as intracellular kinesin and actin-mediated regulation of cell shape and adhesion, underlies these cell-cell interactions. Indeed, the capacity for the kidney to form via self-organization has now been established both via the recapitulation of expected morphogenetic interactions after complete dissociation and reassociation of cellular components during development as well as the in vitro formation of 3D kidney organoids from human pluripotent stem cells. As we understand more about how the many cell-cell interactions required for kidney formation operate, this enables the prospect of bioengineering replacement structures based on these self-organizing properties.

Intercellular protein expression variability as a feature of stem cell pluripotency

The expression of pluripotent stem cell protein markers, self-renewal, the potential to differentiate in cell types of all three germlines and teratoma formation in nude mice form the spectrum of the stringent pluripotency criteria for human stem cells. Currently, intercellular variability is discussed as an additional putative defining property of pluripotent stem cells. In future, it will be of relevance to clarify the genesis of intercellular variability for each stem cell line/population before its application in basic science or therapy. Furthermore, for a better understanding of stemness it will be indispensable to separately investigate the issue of intercellular variability for each feature of pluripotency.

mTORC1 is essential for early steps during Schwann cell differentiation of amniotic fluid stem cells and regulates lipogenic gene expression

Schwann cell development is hallmarked by the induction of a lipogenic profile. Here we used amniotic fluid stem (AFS) cells and focused on the mechanisms occurring during early steps of differentiation along the Schwann cell lineage. Therefore, we initiated Schwann cell differentiation in AFS cells and monitored as well as modulated the activity of the mechanistic target of rapamycin (mTOR) pathway, the major regulator of anabolic processes. Our results show that mTOR complex 1 (mTORC1) activity is essential for glial marker expression and expression of Sterol Regulatory Element-Binding Protein (SREBP) target genes. Moreover, SREBP target gene activation by statin treatment promoted lipogenic gene expression, induced mTORC1 activation and stimulated Schwann cell differentiation. To investigate mTORC1 downstream signaling we expressed a mutant S6K1, which subsequently induced the expression of the Schwann cell marker S100b, but did not affect lipogenic gene expression. This suggests that S6K1 dependent and independent pathways downstream of mTORC1 drive AFS cells to early Schwann cell differentiation and lipogenic gene expression. In conclusion our results propose that future strategies for peripheral nervous system regeneration will depend on ways to efficiently induce the mTORC1 pathway.

Regeneration and bioengineering of the kidney: current status and future challenges

The prevalence of chronic kidney disease continues to outpace the development of effective treatment strategies. For patients with advanced disease, renal replacement therapies approximate the filtration functions of the kidney at considerable cost and inconvenience, while failing to restore the resorptive and endocrine functions. Allogeneic transplantation remains the only restorative treatment, but donor shortage, surgical morbidity and the need for lifelong immunosuppression significantly limit clinical application. Emerging technologies in the fields of regenerative medicine and tissue engineering strive to address these limitations. We review recent advances in cell-based therapies, primordial allografts, bio-artificial organs and whole-organ bioengineering as they apply to renal regeneration. Collaborative efforts across these fields aim to produce a bioengineered kidney capable of restoring renal function in patients with end-stage disease.

Cell therapy for kidney injury: different options and mechanisms--mesenchymal and amniotic fluid stem cells

BACKGROUND

Acute kidney injury (AKI) is emerging as a public health problem in developing and developed countries. It affects up to 7% of hospitalized patients, with a higher prevalence in critical care units. Despite major advances in preventive strategies and support measures, the mortality rate among patients remains higher than 50%. Several pharmacological approaches to improve renal function and survival after an AKI episode have been largely unsuccessful in clinical practice.

SUMMARY

Stem cell-based therapy has provided new hopes of innovative interventions to enhance the limited capability of kidney regeneration in AKI. An important target for cell therapy is represented by tubular epithelial cells which after acute ischemic or toxic insults undergo dysfunction and detachment. Among adult stem cells, mesenchymal stromal/stem cells (MSC) are an attractive therapeutic tool by virtue of their unique biological properties, tropism for damaged tissues, and proregenerative capacity. In the present review, we discuss the mechanisms underlying the renoprotective effects of therapies with stem cells of different origins in preclinical models of AKI by evaluating new modalities by which MSC interact with damaged cells via the release of soluble factors and exosomes/microvesicles. Several biological effects, including antiapoptotic, promitogenic, immunomodulatory, and anti-inflammatory activities, have been analyzed in renal tissue of AKI animals receiving stem cell treatments. The mechanisms of stem cell homing and engraftment to sites of tissue damage have also been discussed.

KEY MESSAGES

The translation of preclinical data on stem cells into effective and safe new modalities of care is still limited, and further studies are needed before their application in patients with AKI.

Reforming the kidney starting from a single-cell suspension

BACKGROUND

Chronic kidney disease affects 5-7% of people worldwide. The increasing number of patients and the shortage of transplantable organs create an imperative need to develop new methods for generating kidney tissue.

SUMMARY

Recent advances in our understanding of the developmental biology of the kidney, along with the establishment of novel methodologies in the field of regenerative medicine, have created significant potential for kidney regeneration. These advances incorporate both transplantation of metanephric primordia into adult recipients and construction of 'fetal' kidney tissue from suspensions of single cells of metanephric origin. This paper examines these approaches in the context of organ regeneration.

KEY MESSAGES

The use of transplants of metanephric origin has the advantage over undifferentiated stem cells of already being committed to a renal developmental program. Although several technical difficulties remain to be overcome, the validation of these systems in preclinical models of renal disease will be of decisive importance in the coming years.

Stem cells from fetal membranes and amniotic fluid: markers for cell isolation and therapy

Stem cell therapy is in constant need of new cell sources to conceive regenerative medicine approaches for diseases that are still without therapy. Scientists drew the attention toward amniotic membrane and amniotic fluid stem cells, since these sources possess many advantages: first of all as cells can be extracted from discarded foetal material it is inexpensive, secondly abundant stem cells can be obtained and finally, these stem cell sources are free from ethical considerations. Many studies have demonstrated the differentiation potential in vitro and in vivo toward mesenchymal and non-mesenchymal cell types; in addition the immune-modulatory properties make these cells a good candidate for allo- and xenotransplantation. This review offers an overview on markers characterisation and on the latest findings in pre-clinical or clinical setting of the stem cell populations isolated from these sources.

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.

Reconsidering pluripotency tests: do we still need teratoma assays?

The induction of teratoma in mice by the transplantation of stem cells into extra-uterine sites has been used as a read-out for cellular pluripotency since the initial description of this phenomenon in 1954. Since then, the teratoma assay has remained the assay of choice to demonstrate pluripotency, gaining prominence during the recent hype surrounding human stem cell research. However, the scientific significance of the teratoma assay has been debated due to the fact that transplanted cells are exposed to a non-physiological environment. Since many mice are used for a result that is heavily questioned, it is time to reconsider the teratoma assay from an ethical point of view. Candidate alternatives to the teratoma assay comprise the directed differentiation of pluripotent stem cells into organotypic cells, differentiation of cells in embryoid bodies, the analysis of pluripotency-associated biomarkers with high correlation to the teratoma forming potential of stem cells, predictive epigenetic footprints, or a combination of these technologies. Each of these assays is capable of addressing one or more aspects of pluripotency, however it is essential that these assays are validated to provide an accepted robust, reproducible alternative. In particular, the rapidly expanding number of human induced pluripotent stem cell lines, requires the development of simple, affordable standardized in vitro and in silico assays to reduce the number of animal experiments performed.

Amniotic fluid as a source of pluripotent and multipotent stem cells for organ regeneration

PURPOSE OF REVIEW

Amniotic fluid, due to its contact to the fetus during development, is considered an important diagnostic tool to evaluate the health status of the fetus during pregnancy. However, amniotic fluid also contains a heterogeneous cellular population that can be safely collected by amniocentesis and easily cultured. Many different cell types have been found within amniotic fluid and currently some of them are being tested for their possible use for cellular therapy.

RECENT FINDINGS

Potential of pluripotent and multipotent cells isolated from the amniotic fluid has been tested and in-vitro differentiations toward various cell types have been successfully performed. Furthermore, in-vivo studies are highlighting the benefits and mechanisms of amniotic fluid cells for therapy, with particular focus on kidney and lung diseases.

SUMMARY

Amniotic fluid may represent a precious source for easily and safely retrievable cell types that may be used for regenerative medicine purposes.

Stem cells derived from neonatal mouse kidney generate functional proximal tubule-like cells and integrate into developing nephrons in vitro

We have recently shown that kidney-derived stem cells (KSCs) isolated from the mouse newborn kidney differentiate into a range of kidney-specific cell types. However, the functionality and integration capacity of these mouse KSCs remain unknown. Therefore, the main objectives of this study were (1) to determine if proximal tubule-like cells, generated in vitro from KSCs, displayed absorptive function typical of proximal tubule cells in vivo, and (2) to establish whether the ability of KSCs to integrate into developing nephrons was comparable with that of metanephric mesenchyme (MM), a transient population of progenitor cells that gives rise to the nephrons during kidney organogenesis. We found that proximal tubule-like cells generated in vitro from mouse KSCs displayed megalin-dependent absorptive function. Subsequently, we used a chimeric kidney rudiment culture system to show that the KSCs could generate proximal tubule cells and podocytes that were appropriately located within the developing nephrons. Finally, we compared the ability of KSCs to integrate into developing kidneys ex vivo with that of metanephric mesenchyme cells. We found that KSCs integrated into nascent nephrons to a similar extent as metanephric mesenchyme cells while both were excluded from ureteric bud branches. Our analysis of the behavior of the two cell types shows that some, but not all KSC characteristics are similar to those of the MM.

Chondrogenic differentiation of amniotic fluid stem cells and their potential for regenerative therapy

Chronic articular cartilage defects are the most common disabling conditions of humans in the western world. The incidence for cartilage defects is increasing with age and the most prominent risk factors are overweight and sports associated overloading. Damage of articular cartilage frequently leads to osteoarthritis due to the aneural and avascular nature of articular cartilage, which impairs regeneration and repair. Hence, patients affected by cartilage defects will benefit from a cell-based transplantation strategy. Autologous chondrocytes, mesenchymal stem cells and embryonic stem cells are suitable donor cells for regeneration approaches and most recently the discovery of amniotic fluid stem cells has opened a plethora of new therapeutic options. It is the aim of this review to summarize recent advances in the use of amniotic fluid stem cells as novel cell sources for the treatment of articular cartilage defects. Molecular aspects of articular cartilage formation as well as degeneration are summarized and the role of growth factor triggered signaling pathways, scaffolds, hypoxia and autophagy during the process of chondrogenic differentiation are discussed.

Amniotic fluid-derived stem cells for cardiovascular tissue engineering applications

Recent research has demonstrated that a population of stem cells can be isolated from amniotic fluid removed by amniocentesis that are broadly multipotent and nontumorogenic. These amniotic fluid-derived stem cells (AFSC) could potentially provide an autologous cell source for treatment of congenital defects identified during gestation, particularly cardiovascular defects. In this review, the various methods of isolating, sorting, and culturing AFSC are compared, along with techniques for inducing differentiation into cardiac myocytes and endothelial cells. Although research has not demonstrated complete and high-yield cardiac differentiation, AFSC have been shown to effectively differentiate into endothelial cells and can effectively support cardiac tissue. Additionally, several tissue engineering and regenerative therapeutic approaches for the use of these cells in heart patches, injection after myocardial infarction, heart valves, vascularized scaffolds, and blood vessels are summarized. These applications show great promise in the treatment of congenital cardiovascular defects, and further studies of isolation, culture, and differentiation of AFSC will help to develop their use for tissue engineering, regenerative medicine, and cardiovascular therapies.

Shedding new light on neurodegenerative diseases through the mammalian target of rapamycin

Neurodegenerative disorders affect a significant portion of the world's population leading to either disability or death for almost 30 million individuals worldwide. One novel therapeutic target that may offer promise for multiple disease entities that involve Alzheimer's disease, Parkinson's disease, epilepsy, trauma, stroke, and tumors of the nervous system is the mammalian target of rapamycin (mTOR). mTOR signaling is dependent upon the mTORC1 and mTORC2 complexes that are composed of mTOR and several regulatory proteins including the tuberous sclerosis complex (TSC1, hamartin/TSC2, tuberin). Through a number of integrated cell signaling pathways that involve those of mTORC1 and mTORC2 as well as more novel signaling tied to cytokines, Wnt, and forkhead, mTOR can foster stem cellular proliferation, tissue repair and longevity, and synaptic growth by modulating mechanisms that foster both apoptosis and autophagy. Yet, mTOR through its proliferative capacity may sometimes be detrimental to central nervous system recovery and even promote tumorigenesis. Further knowledge of mTOR and the critical pathways governed by this serine/threonine protein kinase can bring new light for neurodegeneration and other related diseases that currently require new and robust treatments.

In vivo maturation of functional renal organoids formed from embryonic cell suspensions

The shortage of transplantable organs provides an impetus to develop tissue-engineered alternatives. Producing tissues similar to immature kidneys from simple suspensions of fully dissociated embryonic renal cells is possible in vitro, but glomeruli do not form in the avascular environment. Here, we constructed renal organoids from single-cell suspensions derived from E11.5 kidneys and then implanted these organoids below the kidney capsule of a living rat host. This implantation resulted in further maturation of kidney tissue, formation of vascularized glomeruli with fully differentiated capillary walls, including the slit diaphragm, and appearance of erythropoietin-producing cells. The implanted tissue exhibited physiologic functions, including tubular reabsorption of macromolecules, that gained access to the tubular lumen on glomerular filtration. The ability to generate vascularized nephrons from single-cell suspensions marks a significant step to the long-term goal of replacing renal function by a tissue-engineered kidney.

Applications of amniotic membrane and fluid in stem cell biology and regenerative medicine

The amniotic membrane (AM) and amniotic fluid (AF) have a long history of use in surgical and prenatal diagnostic applications, respectively. In addition, the discovery of cell populations in AM and AF which are widely accessible, nontumorigenic and capable of differentiating into a variety of cell types has stimulated a flurry of research aimed at characterizing the cells and evaluating their potential utility in regenerative medicine. While a major focus of research has been the use of amniotic membrane and fluid in tissue engineering and cell replacement, AM- and AF-derived cells may also have capabilities in protecting and stimulating the repair of injured tissues via paracrine actions, and acting as vectors for biodelivery of exogenous factors to treat injury and diseases. Much progress has been made since the discovery of AM and AF cells with stem cell characteristics nearly a decade ago, but there remain a number of problematic issues stemming from the inherent heterogeneity of these cells as well as inconsistencies in isolation and culturing methods which must be addressed to advance the field towards the development of cell-based therapies. Here, we provide an overview of the recent progress and future perspectives in the use of AM- and AF-derived cells for therapeutic applications.

Development of embryonic stem cells in recombinant kidneys

Embryonic stem cells (ESC) are self-renewing and can generate all cell types during normal development. Previous studies have begun to explore fates of ESCs and their mesodermal derivatives after injection into explanted intact metanephric kidneys and neonatal kidneys maturing in vivo. Here, we exploited a recently described recombinant organ culture model, mixing fluorescent quantum dot labeled mouse exogenous cells with host metanephric cells. We compared abilities of undifferentiated ESCs with ESC-derived mesodermal or non-mesodermal cells to contribute to tissue compartments within recombinant, chimeric metanephroi. ESC-derived mesodermal cells downregulated Oct4, a marker of undifferentiated cells, and, as assessed by locations of quantum dots, contributed to Wilms’ tumor 1-expressing forming nephrons, synaptopodin-expressing glomeruli, and organic ion-transporting tubular epithelia. Similar results were observed when labeled native metanephric cells were recombined with host cells. In striking contrast, non-mesodermal ESC-derived cells strongly inhibited growth of embryonic kidneys, while undifferentiated ESCs predominantly formed Oct4 expressing colonies between forming nephrons and glomeruli. These findings clarify the conclusion that ESC-derived mesodermal cells have functional nephrogenic potential, supporting the idea that they could potentially replace damaged epithelia in diseased kidneys. On the other hand, undifferentiated ESCs and non-mesodermal precursors derived from ESCs would appear to be less suitable materials for use in kidney cell therapies.

CD117(+) amniotic fluid stem cells: state of the art and future perspectives

Broadly multipotent stem cells can be isolated from amniotic fluid by selection for the expression of the membrane stem cell factor receptor c-Kit, a common marker for multipotential stem cells. They have clonogenic capability and can be directed into a wide range of cell types representing the three primary embryonic lineages. Amniotic fluid stem cells maintained for over 250 population doublings retained long telomeres and a normal karyotype. Clonal human lines verified by retroviral marking were induced to differentiate into cell types representing each embryonic germ layer, including cells of adipogenic, osteogenic, myogenic, endothelial, neuronal and hepatic lineages. AFS cells could be differentiate toward cardiomyogenic lineages, when co-cultured with neonatal cardiomyocytes, and have the potential to generate myogenic and hematopoietic lineages both in vitro and in vivo. Very recently first trimester AFS cells could be reprogrammed without any genetic manipulation opening new possibilities in the field of fetal/neonatal therapy and disease modeling. In this review we are aiming to summarize the knowledge on amniotic fluid stem cells and highlight the most promising results.

Amniotic fluid stem cells to study mTOR signaling in differentiation

The protein kinase mTOR is the central player within a pathway, which is known to be involved in the regulation of e.g., cell size, cell cycle, apoptosis, autophagy, aging and differentiation. mTOR activity responds to many signals, including cellular stress, oxygen, nutrient availability, energy status and growth factors. Deregulation of this enzyme is causatively involved in the molecular development of monogenic human diseases, cancer, obesity, type 2 diabetes or neurodegeneration. Recently, mTOR has also been demonstrated to control stem cell homeostasis. A more detailed investigation of this new mTOR function will be of highest relevance to provide more explicit insights into stem cell regulation in the near future. Different cellular tools, including adult stem cells, embryonic stem cells or induced pluripotent stem cells could be used to investigate the role of mTOR in mammalian stem cell biology. Here we discuss the potential of amniotic fluid stem cells to become a promising cellular model to study the role of signaling cascades in stem cell homeostasis.

Amniotic fluid stem cells: future perspectives

The existence of stem cells in human amniotic fluid was reported for the first time almost ten years ago. Since this discovery, the knowledge about these cells has increased dramatically. Today, amniotic fluid stem (AFS) cells are widely accepted as a new powerful tool for basic research as well as for the establishment of new stem-cell-based therapy concepts. It is possible to generate monoclonal genomically stable AFS cell lines harboring high proliferative potential without raising ethical issues. Many different groups have demonstrated that AFS cells can be differentiated into all three germ layer lineages, what is of relevance for both, the scientific and therapeutical usage of these cells. Of special importance for the latter is the fact that AFS cells are less tumorigenic than other pluripotent stem cell types. In this paper, we have summarized the current knowledge about this relatively young scientific field. Furthermore, we discuss the relevant future perspectives of this promising area of stem cell research focusing on the next important questions, which need to be answered.

Renal differentiation of amniotic fluid stem cells: perspectives for clinical application and for studies on specific human genetic diseases

BACKGROUND

Owing to growing rates of diabetes, hypertension and the ageing population, the prevalence of end-stage renal disease, developed from earlier stages of chronic kidney disease, and of acute renal failure is dramatically increasing. Dialysis and preferable renal transplantation are widely applied therapies for this incurable condition. However these options are limited because of morbidity, shortage of compatible organs and costs. Therefore, stem cell-based approaches are becoming increasingly accepted as an alternative therapeutic strategy.

DESIGN

This review summarizes the current findings on the nephrogenic potential of amniotic fluid stem (AFS) cells and their putative implications for clinical applications and for studies on specific human genetic diseases.

RESULTS

Since their discovery in 2003, AFS cells have been shown to be pluripotent with the potential to form embryoid bodies. Compared to adult stem cells, induced pluripotent stem cells or embryonic stem cells, AFS cells harbour a variety of advantages, such as their high differentiation and proliferative potential, no need for ectopic induction of pluripotency and no somatic mutations and epigenetic memory of source cells, and no tumourigenic potential and associated ethical controversies, respectively.

CONCLUSIONS

Recently, the results of different independent studies provided evidence that AFS cells could indeed be a powerful tool for renal regenerative medicine.

Human amniotic fluid stem cells as a model for functional studies of genes involved in human genetic diseases or oncogenesis

Besides their putative usage for therapies, stem cells are a promising tool for functional studies of genes involved in human genetic diseases or oncogenesis. For this purpose induced pluripotent stem (iPS) cells can be derived from patients harbouring specific mutations. In contrast to adult stem cells, iPS cells are pluripotent and can efficiently be grown in culture. However, iPS cells are modulated due to the ectopic induction of pluripotency, harbour other somatic mutations accumulated during the life span of the source cells, exhibit only imperfectly cleared epigenetic memory of the source cell, and are often genomically instable. In addition, iPS cells from patients only allow the investigation of mutations, which are not prenatally lethal. Embryonic stem (ES) cells have a high proliferation and differentiation potential, but raise ethical issues. Human embryos, which are not transferred in the course of in vitro fertilization, because of preimplantation genetic diagnosis of a genetic defect, are still rarely donated for the establishment of ES cell lines. In addition, their usage for studies on gene functions for oncogenesis is hampered by the fact the ES cells are already tumorigenic per se. In 2003 amniotic fluid stem (AFS) cells have been discovered, which meanwhile have been demonstrated to harbour the potential to differentiate into cells of all three germ layers. Monoclonal human AFS cell lines derived from amniocenteses have a high proliferative potential, are genomically stable and are not associated with ethical controversies. Worldwide amniocenteses are performed for routine human genetic diagnosis. We here discuss how generation and banking of monoclonal human AFS cell lines with specific chromosomal aberrations or monogenic disease mutations would allow to study the functional consequences of disease causing mutations. In addition, recently a protocol for efficient and highly reproducible siRNA-mediated long-term knockdown of endogenous gene functions in AFS cells was established. Since AFS cells are not tumorigenic, gene modulations not only allow to investigate the role of endogenous genes involved in human genetic diseases but also may help to reveal putative oncogenic gene functions in different biological models, both in vitro and in vivo. This concept is discussed and a "proof of principle", already obtained via modulating genes involved in the mammalian target of rapamycin (mTOR) pathway in AFS cells, is presented.

Amniotic fluid stem cell-based models to study the effects of gene mutations and toxicants on male germ cell formation

Male infertility is a major public health issue predominantly caused by defects in germ cell development. In the past, studies on the genetic regulation of spermatogenesis as well as on negative environmental impacts have been hampered by the fact that human germ cell development is intractable to direct analysis in vivo. Compared with model organisms including mice, there are fundamental differences in the molecular processes of human germ cell development. Therefore, an in vitro model mimicking human sperm formation would be an extremely valuable research tool. In the recent past, both human embryonic stem (ES) cells and induced pluripotent stem (iPS) cells have been reported to harbour the potential to differentiate into primordial germ cells and gametes. We here discuss the possibility to use human amniotic fluid stem (AFS) cells as a biological model. Since their discovery in 2003, AFS cells have been characterized to differentiate into cells of all three germ layers, to be genomically stable, to have a high proliferative potential and to be non-tumourigenic. In addition, AFS cells are not subject of ethical concerns. In contrast to iPS cells, AFSs cells do not need ectopic induction of pluripotency, which is often associated with only imperfectly cleared epigenetic memory of the source cells. Since AFS cells can be derived from amniocentesis with disease-causing mutations and can be transfected with high efficiency, they could be used in probing gene functions for spermatogenesis and in screening for male reproductive toxicity.

siRNA-mediated RNA interference in embryonic kidney organ culture

In principle, treatment of embryonic kidneys growing in organ culture with short interfering RNA (siRNA) offers a powerful means of investigating molecular function quickly and cheaply. Experiments using this approach have yielded significant new data, but they have also highlighted important limitations. Here, we briefly describe the published successes and limitations and present detailed instructions for two methods of siRNA treatment. The first method applies siRNA to intact cultured kidneys; this method is the quicker and easier of the two, but it is the one most affected by problems of siRNA uptake by certain renal tissues. The second method reduces kidney rudiments to a suspension of single cells, applies siRNA at that stage, when the cells are highly accessible, and then reaggregates the kidney; this method is more time-consuming but suffers less from problems of limited uptake. As well as proving instructions for the methods, we provide a brief discussion of necessary controls.

Current thoughts on the therapeutic potential of stem cell

Stem cells are considered as potential therapy for inflammatory disorders, tissue repair, and gene delivery, among others. The heterogeneity of a disease and the underlying disorder of a patient bring up the question on the method by which stem cells should be delivered. This summary discusses potential complex interactions among mediators at sites to tissue insults with stem cells. The chapter selects mesenchymal stem cells (MSCs) as a model, although the discussion is relevant to all stem cells. The review examines how MSCs and their differentiated cells can develop cross communication with soluble factors and cells within the region of tissue damage. Inflammatory cytokines, IL-1, TNFα, and TGFβ are selected to explain how they can affect the responses of MSCs, while predisposing the stem cells to oncogenic event. By understanding the varied functions of MSCs, one will be able to intervene to form a balance in functions, ultimately to achieve safety and efficient application. Cytokines can affect the expression of pluripotent genes such as REST and Oct-4. REST is a critical gene in the decision of a cell to express or repress neural genes. Since cytokines can affect microRNAs, the review incorporates this family of molecules as mediators of cytokine effects. IFNγ, although an inflammatory mediator, is central to the expression of MHC-II on MSCs. Therefore, it is included to discuss its role in the transplantation of stem cells across allogeneic barrier. In summary, this chapter discusses several potential areas that need to be addressed for safe and efficient delivery of stem cells, and argue for the incorporation of microenvironmental factors in the studies.

Dissociation of embryonic kidney followed by re-aggregation as a method for chimeric analysis

This chapter presents three methods for re-constructing mouse foetal kidney tissue from simple suspensions of cells. These techniques are very useful for a number of purposes: (1) they allow the production of fine-grained chimaeras in which cell autonomy of mutations can be tested, (2) they provide an environment that allows the renal differentiation potential of stem cells to be assessed, and (3) they are an excellent system in which to study the mechanisms of self-organization. Each of the methods described here begins with disaggregation of embryonic mouse kidneys, followed by re-aggregation and culture; the main differences are in the culture methods, each of which has advantages for particular purposes.

Mammalian target of rapamycin: a signaling kinase for every aspect of cellular life

The mammalian (or mechanistic) target of rapamycin (mTOR) is an evolutionarily conserved serine-threonine kinase that is known to sense the environmental and cellular nutrition and energy status. Diverse mitogens, growth factors, and nutrients stimulate the activation of the two mTOR complexes mTORC1 and mTORC2 to regulate diverse functions, such as cell growth, proliferation, development, memory, longevity, angiogenesis, autophagy, and innate as well as adaptive immune responses. Dysregulation of the mTOR pathway is frequently observed in various cancers and in genetic disorders, such as tuberous sclerosis complex or cystic kidney disease. In this review, I will give an overview of the current understanding of mTOR signaling and its role in diverse tissues and cells. Genetic deletion of specific mTOR pathway proteins in distinct tissues and cells broadened our understanding of the cell-specific roles of mTORC1 and mTORC2. Inhibition of mTOR is an established therapeutic principle in transplantation medicine and in cancers, such as renal cell carcinoma. Pharmacological targeting of both mTOR complexes by novel drugs potentially expand the clinical applicability and efficacy of mTOR inhibition in various disease settings.

Integration potential of mouse and human bone marrow-derived mesenchymal stem cells

Mesenchymal stem cells (MSCs) are a multipotent cell population which has been described to exert renoprotective and regenerative effects in experimental models of kidney injury. Several lines of evidence indicate that MSCs also have the ability to contribute to nephrogenesis, suggesting that the cells can be employed in stem cell-based applications aimed at de novo renal tissue generation. In this study we re-evaluate the capacity of mouse and human bone marrow-derived MSCs to contribute to the development of renal tissue using a novel method of embryonic kidney culture. Although MSCs show expression of some genes involved in renal development, their contribution to nephrogenesis is very limited in comparison to other stem cell types tested. Furthermore, we found that both mouse and human MSCs have a detrimental effect on the ex vivo development of mouse embryonic kidney, this effect being mediated through a paracrine action. Stimulation with conditioned medium from a mouse renal progenitor population increases the ability of mouse MSCs to integrate into developing renal tissue and prevents the negative effects on kidney development, but does not appear to enhance their ability to undergo nephrogenesis.

Tuberin and PRAS40 are anti-apoptotic gatekeepers during early human amniotic fluid stem-cell differentiation

Embryoid bodies (EBs) are three-dimensional multicellular aggregates allowing the in vitro investigation of stem-cell differentiation processes mimicking early embryogenesis. Human amniotic fluid stem (AFS) cells harbor high proliferation potential, do not raise the ethical issues of embryonic stem cells, have a lower risk for tumor development, do not need exogenic induction of pluripotency and are chromosomal stable. Starting from a single human AFS cell, EBs can be formed accompanied by the differentiation into cells of all three embryonic germ layers. Here, we report that siRNA-mediated knockdown of the endogenous tuberous sclerosis complex-2 (TSC2) gene product tuberin or of proline-rich Akt substrate of 40 kDa (PRAS40), the two major negative regulators of mammalian target of rapamycin (mTOR), leads to massive apoptotic cell death during EB development of human AFS cells without affecting the endodermal, mesodermal and ectodermal cell differentiation spectrum. Co-knockdown of endogenous mTOR demonstrated these effects to be mTOR-dependent. Our findings prove this enzyme cascade to be an essential anti-apoptotic gatekeeper of stem-cell differentiation during EB formation. These data allow new insights into the regulation of early stem-cell maintenance and differentiation and identify a new role of the tumor suppressor tuberin and the oncogenic protein PRAS40 with the relevance for a more detailed understanding of the pathogenesis of diseases associated with altered activities of these gene products.

The potential use of stem cells derived from human amniotic fluid in renal diseases

Amniotic fluid (AF) contains a variety of cell types derived from fetal tissues that can easily grow in culture. These cells can be obtained during amniocentesis for prenatal screening of fetal genetic diseases, usually performed during the second trimester of pregnancy. Of particular interest, some expanded sub-populations derived from AF cells are capable of extensive self-renewal and maintain prolonged undifferentiated proliferation, which are defining properties of stem cells. These human AF stem cells (hAFSCs) exhibit multilineage potential and can differentiate into the three germ layers. They have high proliferation rates and express mesenchymal and embryonic markers, but do not induce tumor formation. In this study, hAFSCs derived from amniocentesis performed at 16–20 weeks of pregnancy were isolated, grown in culture, and characterized by flow cytometry and by their potential ability to differentiate into osteogenic, adipogenic, and chondrogenic lineages. After 4–7 passages, 5 × 10⁵ hAFSCs were inoculated under the kidney capsule of Wistar rats that were subjected to an experimental model of chronic renal disease, the 5/6 nephrectomy model (Nx). After 30 days, Nx rats treated with hAFSCs displayed significant reductions in blood pressure, proteinuria, macrophages, and α-smooth muscle actin expression compared with Nx animals. These preliminary results suggest that hAFSCs isolated and expanded from AF obtained by routine amniocentesis can promote renoprotection in the Nx model. Considering that the AF cells not used for fetal karyotyping are usually discarded, and that their use does not raise ethical issues, they may represent an alternative source of stem cells for cell therapy and regenerative medicine.