Functional genetic targeting of embryonic kidney progenitor cells ex vivo

From ex ovo to in vitro: xenotransplantation and vascularization of mouse embryonic kidneys in a microfluidic chip

Organoids are emerging as a powerful tool to investigate complex biological structures in vitro. Vascularization of organoids is crucial to recapitulate the morphology and function of the represented human organ, especially in the case of the kidney, whose primary function of blood filtration is closely associated with blood circulation. Current in vitro microfluidic approaches have only provided initial vascularization of kidney organoids, whereas in vivo transplantation to animal models is problematic due to ethical problems, with the exception of xenotransplantation onto a chicken chorioallantoic membrane (CAM). Although CAM can serve as a good environment for vascularization, it can only be used for a fixed length of time, limited by development of the embryo. Here, we propose a novel lab on a chip design that allows organoids of different origin to be cultured and vascularized on a CAM, as well as to be transferred to in vitro conditions when required. Mouse embryonic kidneys cultured on the CAM showed enhanced vascularization by intrinsic endothelial cells, and made connections with the chicken vasculature, as evidenced by blood flowing through them. After the chips were transferred to in vitro conditions, the vasculature inside the organoids was successfully maintained. To our knowledge, this is the first demonstration of the combination of in vivo and in vitro approaches applied to microfluidic chip design.

Engineering physiological environments to advance kidney organoid models from human pluripotent stem cells

During embryogenesis, the mammalian kidney arises because of reciprocal interactions between the ureteric bud (UB) and the metanephric mesenchyme (MM), driving UB branching and nephron induction. These morphogenetic processes involve a series of cellular rearrangements that are tightly controlled by gene regulatory networks and signaling cascades. Here, we discuss how kidney developmental studies have informed the definition of procedures to obtain kidney organoids from human pluripotent stem cells (hPSCs). Moreover, bioengineering techniques have emerged as potential solutions to externally impose controlled microenvironments for organoid generation from hPSCs. Next, we summarize some of these advances with major focus On recent works merging hPSC-derived kidney organoids (hPSC-kidney organoids) with organ-on-chip to develop robust models for drug discovery and disease modeling applications. We foresee that, in the near future, coupling of different organoid models through bioengineering approaches will help advancing to recreate organ-to-organ crosstalk to increase our understanding on kidney disease progression in the human context and search for new therapeutics.

Generation of novel in vitro flexible kidney organoid model to investigate the role of extracellular vesicles in induction of nephrogenesis

BACKGROUND

During kidney organogenesis, metanephric mesenchyme (MM) and ureteric bud (UB) interact reciprocally to form nephrons. Signaling stimuli involved in these interactions include Wnts, growth factors and nano/micro particles. How UB and MM are interacting is not completely understood. Our study investigated the signaling and communication via extracellular vesicles (EVs) during nephrogenesis. Embryonic day (E) 11.5 mouse kidney UB and MM produce very low number of primary cells that have limited ability for proliferation in culture. Such limitations obstruct studying the role of EVs in induction of nephrogenesis. These issues necessitate to generate a nephrogenesis model allowing to study the comprehensive role of EVs during nephrogenesis.

RESULTS

Our study generated a UB derived cell line-based in vitro flexible model of nephrogenesis allowing expandable cell culturing, in addition to performing characterization, tracking and blocking of EVs. UB cell line aggregation with E11.5 MM cells induced the formation of segmented nephrons. Most efficient nephrogenesis was obtained by the co-culturing of 30,000 cells of UB cell line with 50,000 MM cells. Results revealed that both the UB and the MM secrete EVs during nephrogenesis. UB cell line derived EVs were characterized by their size, morphology and expression of markers (CD63, TSG101, CD9 and CD81). Furthermore, proteomics data of UB cell line-derived EVs revealed large number of proteins involved in nephrogenesis-related signaling pathways. Palmitoylated GFP-tagged EVs from UB cell line were found in the nephron formation zone in the developing kidney organoid. UB cell line derived EVs did not induce nephrogenesis in MM cells but significantly contributed to the survival and nephrogenesis-competency of MM cells. The secretion of EVs was continuously inhibited during the ongoing nephrogenesis by the knockdown of RalA and RalB gene expression using short hairpin RNAs. This inhibition partially impaired the ability of UB cell line to induce nephrogenesis. Moreover, impaired nephrogenesis was partially rescued by the addition of EVs.

CONCLUSION

Our study established a novel in vitro flexible model of nephrogenesis that solved the limitations of primary embryonic kidney cells and mouse embryonic stem cell kidney organoids for the EV research. EVs were found to be an integral part of nephrogenesis process. Video Abstract.

Production of kidney organoids arranged around single ureteric bud trees, and containing endogenous blood vessels, solely from embryonic stem cells

There is intense worldwide effort in generating kidney organoids from pluripotent stem cells, for research, for disease modelling and, perhaps, for making transplantable organs. Organoids generated from pluripotent stem cells (PSC) possess accurate micro-anatomy, but they lack higher-organization. This is a problem, especially for transplantation, as such organoids will not be able to perform their physiological functions. In this study, we develop a method for generating murine kidney organoids with improved higher-order structure, through stages using chimaeras of ex-fetu and PSC-derived cells to a system that works entirely from embryonic stem cells. These organoids have nephrons organised around a single ureteric bud tree and also make vessels, with the endothelial network approaching podocytes.

FGF8 induces chemokinesis and regulates condensation of mouse nephron progenitor cells

Kidneys develop via iterative branching of the ureteric epithelial tree and subsequent nephrogenesis at the branch points. Nephrons form in the cap mesenchyme as the metanephric mesenchyme (MM) condenses around the epithelial ureteric buds (UBs). Previous work has demonstrated that FGF8 is important for the survival of nephron progenitor cells (NPCs), and early deletion of Fgf8 leads to the cessation of nephron formation, which results in post-natal lethality. We now reveal a previously unreported function of FGF8. By combining transgenic mouse models, quantitative imaging assays and data-driven computational modelling, we show that FGF8 has a strong chemokinetic effect and that this chemokinetic effect is important for the condensation of NPCs to the UB. The computational model shows that the motility must be lower close to the UB to achieve NPC attachment. We conclude that the FGF8 signalling pathway is crucial for the coordination of NPC condensation at the UB. Chemokinetic effects have also been described for other FGFs and may be generally important for the formation of mesenchymal condensates.

Computational modelling of nephron progenitor cell movement and aggregation during kidney organogenesis

During early kidney organogenesis, nephron progenitor (NP) cells move from the tip to the corner region of the ureteric bud (UB) branches in order to form the pretubular aggregate, the early structure giving rise to nephron formation. NP cells derive from metanephric mesenchymal cells and physically interact with them during the movement. Chemotaxis and cell-cell adhesion differences are believed to drive the cell patterning during this critical period of organogenesis. However, the effect of these forces to the cell patterns and their respective movements are known in limited details. We applied a Cellular Potts Model to explore how these forces and organizations contribute to directed cell movement and aggregation. Model parameters were estimated based on fitting to experimental data obtained in ex vivo kidney explant and dissociation-reaggregation organoid culture studies. Our simulations indicated that optimal enrichment and aggregation of NP cells in the UB corner niche requires chemoattractant secretion from both the UB epithelial cells and the NP cells themselves, as well as differences in cell-cell adhesion energies. Furthermore, NP cells were observed, both experimentally and by modelling, to move at higher speed in the UB corner as compared to the tip region where they originated. The existence of different cell speed domains along the UB was confirmed using self-organizing map analysis. In summary, we saw faster NP cell movements near aggregation. The applicability of Cellular Potts Model approach to simulate cell movement and patterning was found to be good during for this early nephrogenesis process. Further refinement of the model should allow us to recapitulate the effects of developmental changes of cell phenotypes and molecular crosstalk during further organ development.

Dissecting nephron morphogenesis using kidney organoids from human pluripotent stem cells

During kidney development the emergence of complex multicellular shapes such as the nephron (the functional unit of the kidney) rely on spatiotemporally coordinated developmental programs. These involve gene regulatory networks, signaling pathways and mechanical forces, that work in concert to shape and form the nephron(s). The generation of kidney organoids from human pluripotent stem cells now represent an unprecedented experimental set up to study these processes. Here we discuss the potential applications of kidney organoids to advance our knowledge of how mechanical forces and cell fate specification spatiotemporally interact to orchestrate nephron patterning and morphogenesis in humans. Progress in innovative techniques for quantifying and perturbing these processes in a controlled manner will be crucial. A mechanistic understanding of the multicellular dynamic processes occurring during nephrogenesis will pave the way to unveil new mechanisms of human kidney disease.

Deciphering the minimal quantity of mouse primary cells to undergo nephrogenesis ex vivo

BACKGROUND

Tissue organoids derived from primary cells have high potential for studying organ development and diseases in numerous organs. They recreate the morphological structure and mimic the functions of given organ while being compact in size, easy to produce, and suitable for use in various experimental setups.

RESULTS

In this study we established the number of cells that form mouse kidney rudiments at E11.5, and generated renal organoids of various sizes from the mouse primary cells of the metanephric mesenchyme (MM). We investigated the ability of renal organoids to undergo nephrogenesis upon Wnt/ β-catenin pathway-mediated tubule induction with a GSK-3 inhibitor (BIO) or by initiation through the ureteric bud (UB). We found that 5000 cells of MM cells are necessary to successfully form renal organoids with well-structured nephrons as judged by fluorescent microscopy, transmission electron microscopy (TEM), and quantitative Polymerase Chain Reaction (qPCR). These mouse organoids also recapitulated renal secretion function in the proximal tubules.

CONCLUSIONS

We show that a significant decrease of cells used to generate renal mouse organoids in a dissociation/re-aggregation assay, does not interfere with development, and goes toward 3Rs. This enables generation of more experimental samples with one mouse litter, limiting the number of animals used for studies.

Mouse Embryonic Stem Cell-Derived Ureteric Bud Progenitors Induce Nephrogenesis

Generation of kidney organoids from pluripotent stem cells (PSCs) is regarded as a potentially powerful way to study kidney development, disease, and regeneration. Direct differentiation of PSCs towards renal lineages is well studied; however, most of the studies relate to generation of nephron progenitor population from PSCs. Until now, differentiation of PSCs into ureteric bud (UB) progenitor cells has had limited success. Here, we describe a simple, efficient, and reproducible protocol to direct differentiation of mouse embryonic stem cells (mESCs) into UB progenitor cells. The mESC-derived UB cells were able to induce nephrogenesis when co-cultured with primary metanephric mesenchyme (pMM). In generated kidney organoids, the embryonic pMM developed nephron structures, and the mESC-derived UB cells formed numerous collecting ducts connected with the nephron tubules. Altogether, our study established an uncomplicated and reproducible platform to generate ureteric bud progenitors from mouse embryonic stem cells.

Kidney Regeneration in Later-Stage Mouse Embryos via Transplanted Renal Progenitor Cells

BACKGROUND

The limited availability of donor kidneys for transplantation has spurred interest in investigating alternative strategies, such as regenerating organs from stem cells transplanted into animal embryos. However, there is no known method for transplanting cells into later-stage embryos, which may be the most suitable host stages for organogenesis, particularly into regions useful for kidney regeneration.

METHODS

We demonstrated accurate transplantation of renal progenitor cells expressing green fluorescent protein to the fetal kidney development area by incising the opaque uterine muscle layer but not the transparent amniotic membrane. We allowed renal progenitor cell-transplanted fetuses to develop for 6 days postoperatively before removal for analysis. We also transplanted renal progenitor cells into conditional kidney-deficient mouse embryos. We determined growth and differentiation of transplanted cells in all cases.

RESULTS

Renal progenitor cell transplantation into the retroperitoneal cavity of fetuses at E13-E14 produced transplant-derived, vascularized glomeruli with filtration function and did not affect fetal growth or survival. Cells transplanted to the nephrogenic zone produced a chimera in the cap mesenchyme of donor and host nephron progenitor cells. Renal progenitor cells transplanted to conditional kidney-deficient fetuses induced the formation of a new nephron in the fetus that is connected to the host ureteric bud.

CONCLUSIONS

We developed a cell transplantation method for midstage to late-stage fetuses. In vivo kidney regeneration from renal progenitor cells using the renal developmental environment of the fetus shows promise. Our findings suggest that fetal transplantation methods may contribute to organ regeneration and developmental research.

Simple 3D culture of dissociated kidney mesenchyme mimics nephron progenitor niche and facilitates nephrogenesis Wnt-independently

Kidney mesenchyme (KM) and nephron progenitors (NPs) depend on WNT activity, and their culture in vitro requires extensive repertoire of recombinant proteins and chemicals. Here we established a robust, simple culture of mouse KM using a combination of 3D Matrigel and growth media supplemented with Fibroblast Growth Factor 2 (FGF2) and Src inhibitor PP2. This allows dissociated KM to spontaneously self-organize into spheres. To reassess the requirement of WNT activity in KM self-organization and NPs maintenance, cells were cultured with short pulse of high-dose GSK3β inhibitor BIO, on a constant low-dose or without BIO. Robust proliferation at 48 hours and differentiation at 1 week were observed in cultures with high BIO pulse. Importantly, dissociated KM cultured without BIO, similarly to that exposed to constant low dose of BIO, maintained NPs up to one week and spontaneously differentiated into nephron tubules at 3 weeks of culture. Our results show that KM is maintained and induced to differentiate in a simple culture system. They also imply that GSK3β/WNT-independent pathways contribute to the maintenance and induction of mouse KM. The robust and easy 3D culture enables further characterization of NPs, and may facilitate disease modeling when applied to human cells.

Virus as Renal Functional Genetic Tools

Viral vectors enable efficient transfection of ectopic DNA into hard to transfect cells. Viral vectors are normally used to obtain permanent modification of target cells, and tissues expect for the cases where integrase-deficient viruses are used. Here we describe a method to stably transfect metanephric mesenchyme cells isolated from the murine embryonic kidney at day E11.5. Using this method, it is possible to transfect hard to transfect cells and successfully evade host tissue immune response. Due to these advantages, this method has become one of the most frequently used in generating stable cell line, manipulation of tissues, and gene therapy.

Experimental Tubulogenesis Induction Model in the Mouse

Kidney development and induction of tubulogenesis have been studied for almost seven decades. The experimental setup of metanephric mesenchyme induction ex vivo allows to control the environment, to perform cellular manipulations, and to learn about renal development. Since the establishment of the ex vivo kidney culture technique in 1953, the method was modified to suit well the progress in biological and medical fields and still today present many advantages over the traditional in vivo studies.

Embryonic Stem Cells Derived Kidney Organoids as Faithful Models to Target Programmed Nephrogenesis

The kidney is a complex organ that is comprised of thousands of nephrons developing through reciprocal inductive interactions between metanephric mesenchyme (MM) and ureteric bud (UB). The MM undergoes mesenchymal to epithelial transition (MET) in response to the signaling from the UB. The secreted protein Wnt4, one of the Wnt family members, is critical for nephrogenesis as mouse Wnt4^−/− mutants fail to form pretubular aggregates (PTA) and therefore lack functional nephrons. Here, we generated mouse embryonic stem cell (mESC) line lacking Wnt4 by applying the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated systems 9 (Cas9). We describe here, differentiation of the wild type and Wnt4 knockout mESCs into kidney progenitors, and such cells induced to undergo nephrogenesis by the mouse E11.5 UB mediated induction. The wild type three-dimensional (3D) self-organized organoids depict appropriately segmented nephron structures, while the Wnt4-deficient organoids fail to undergo the MET, as is the case in the phenotype of the Wnt4 knockout mouse model in vivo. In summary, we have established a platform that combine CRISPR/Cas9 and kidney organoid technologies to model kidney development in vitro and confirmed that mutant organoids are able to present similar actions as in the in vivo studies.

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.

The multisystemic functions of FOXD1 in development and disease

Transcription factors (TFs) participate in a wide range of cellular processes due to their inherent function as essential regulatory proteins. Their dysfunction has been linked to numerous human diseases. The forkhead box (FOX) family of TFs belongs to the "winged helix" superfamily, consisting of proteins sharing a related winged helix-turn-helix DNA-binding motif. FOX genes have been extensively present during vertebrates and invertebrates' evolution, participating in numerous molecular cascades and biological functions, such as embryonic development and organogenesis, cell cycle regulation, metabolism control, stem cell niche maintenance, signal transduction, and many others. FOXD1, a forkhead TF, has been related to different key biological processes such as kidney and retina development and embryo implantation. FOXD1 dysfunction has been linked to different pathologies, thereby constituting a diagnostic biomarker and a promising target for future therapies. This paper aims to present, for the first time, a comprehensive review of FOXD1's role in mouse development and human disease. Molecular, structural, and functional aspects of FOXD1 are presented in light of physiological and pathogenic conditions, including its role in human disease aetiology, such as cancer and recurrent pregnancy loss. Taken together, the information given here should enable a better understanding of FOXD1 function for basic science researchers and clinicians.

Exosomes as secondary inductive signals involved in kidney organogenesis

The subfraction of extracellular vesicles, called exosomes, transfers biological molecular information not only between cells but also between tissues and organs as nanolevel signals. Owing to their unique properties such that they contain several RNA species and proteins implicated in kidney development, exosomes are putative candidates to serve as developmental programming units in embryonic induction and tissue interactions. We used the mammalian metanephric kidney and its nephron-forming mesenchyme containing the nephron progenitor/stem cells as a model to investigate if secreted exosomes could serve as a novel type of inductive signal in a process defined as embryonic induction that controls organogenesis. As judged by several characteristic criteria, exosomes were enriched and purified from a cell line derived from embryonic kidney ureteric bud (UB) and from primary embryonic kidney UB cells, respectively. The cargo of the UB-derived exosomes was analysed by qPCR and proteomics. Several miRNA species that play a role in Wnt pathways and enrichment of proteins involved in pathways regulating the organization of the extracellular matrix as well as tissue homeostasis were identified. When labelled with fluorescent dyes, the uptake of the exosomes by metanephric mesenchyme (MM) cells and the transfer of their cargo to the cells can be observed. Closer inspection revealed that besides entering the cytoplasm, the exosomes were competent to also reach the nucleus. Furthermore, fluorescently labelled exosomal RNA enters into the cytoplasm of the MM cells. Exposure of the embryonic kidney-derived exosomes to the whole MM in an ex vivo organ culture setting did not lead to an induction of nephrogenesis but had an impact on the overall organization of the tissue. We conclude that the exosomes provide a novel signalling system with an apparent role in secondary embryonic induction regulating organogenesis.

Renal carcinoma/kidney progenitor cell chimera organoid as a novel tumorigenesis gene discovery model

Three-dimensional (3D) organoids provide a new way to model various diseases, including cancer. We made use of recently developed kidney-organ-primordia tissue-engineering technologies to create novel renal organoids for cancer gene discovery. We then tested whether our novel assays can be used to examine kidney cancer development. First, we identified the transcriptomic profiles of quiescent embryonic mouse metanephric mesenchyme (MM) and of MM in which the nephrogenesis program had been induced ex vivo. The transcriptome profiles were then compared to the profiles of tumor biopsies from renal cell carcinoma (RCC) patients, and control samples from the same kidneys. Certain signature genes were identified that correlated in the developmentally induced MM and RCC, including components of the caveolar-mediated endocytosis signaling pathway. An efficient siRNA-mediated knockdown (KD) of Bnip3, Gsn, Lgals3, Pax8, Cav1, Egfr or Itgb2 gene expression was achieved in mouse RCC (Renca) cells. The live-cell imaging analysis revealed inhibition of cell migration and cell viability in the gene-KD Renca cells in comparison to Renca controls. Upon siRNA treatment, the transwell invasion capacity of Renca cells was also inhibited. Finally, we mixed E11.5 MM with yellow fluorescent protein (YFP)-expressing Renca cells to establish chimera organoids. Strikingly, we found that the Bnip3-, Cav1- and Gsn-KD Renca-YFP+ cells as a chimera with the MM in 3D organoid rescued, in part, the RCC-mediated inhibition of the nephrogenesis program during epithelial tubules formation. Altogether, our research indicates that comparing renal ontogenesis control genes to the genes involved in kidney cancer may provide new growth-associated gene screens and that 3D RCC-MM chimera organoids can serve as a novel model with which to investigate the behavioral roles of cancer cells within the context of emergent complex tissue structures.

Editor’s Choice: Chimeras between embryonic kidney cells and renal carcinoma cells serve as a novel model to assay the roles of co-regulated genes in kidney development and renal carcinogenesis.

Troy/TNFRSF19 marks epithelial progenitor cells during mouse kidney development that continue to contribute to turnover in adult kidney

During kidney development, progressively committed progenitor cells give rise to the distinct segments of the nephron, the functional unit of the kidney. Similar segment-committed progenitor cells are thought to be involved in the homeostasis of adult kidney. However, markers for most segment-committed progenitor cells remain to be identified. Here, we evaluate Troy/TNFRSF19 as a segment-committed nephron progenitor cell marker. Troy is expressed in the ureteric bud during embryonic development. During postnatal nephrogenesis, Troy⁺ cells are present in the cortex and papilla and display an immature tubular phenotype. Tracing of Troy⁺ cells during nephrogenesis demonstrates that Troy⁺ cells clonally give rise to tubular structures that persist for up to 2 y after induction. Troy⁺ cells have a 40-fold higher capacity than Troy^- cells to form organoids, which is considered a stem cell property in vitro. In the adult kidney, Troy⁺ cells are present in the papilla and these cells continue to contribute to collecting duct formation during homeostasis. The number of Troy-derived cells increases after folic acid-induced injury. Our data show that Troy marks a renal stem/progenitor cell population in the developing kidney that in adult kidney contributes to homeostasis, predominantly of the collecting duct, and regeneration.

New insights into precursors of renal endothelium

The kidney vasculature is extremely complex, yet, despite recent progress, our understanding of how the renal vascular system develops is limited. By using advanced tissue engineering techniques and in vivo and in vitro depletion of specific populations of endothelial cell precursors, Halt et al. have identified a CD146-expressing precursor as an important player in the development of the renal vasculature.

Endothelial marker-expressing stromal cells are critical for kidney formation

Kidneys are highly vascularized and contain many distinct vascular beds. However, the origins of renal endothelial cells and roles of the developing endothelia in the formation of the kidney are unclear. We have shown that the Foxd1-positive renal stroma gives rise to endothelial marker-expressing progenitors that are incorporated within a subset of peritubular capillaries; however, the significance of these cells is unclear. The purpose of this study was to determine whether deletion of Flk1 in the Foxd1 stroma was important for renal development. To that end, we conditionally deleted Flk1 (critical for endothelial cell development) in the renal stroma by breeding-floxed Flk1 mice (Flk1fl/fl ) with Foxd1cre mice to generate Foxd1cre; Flk1fl/fl (Flk1ST-/- ) mice. We then performed FACsorting, histological, morphometric, and metabolic analyses of Flk1ST-/- vs. control mice. We confirmed decreased expression of endothelial markers in the renal stroma of Flk1ST-/- kidneys via flow sorting and immunostaining, and upon interrogation of embryonic and postnatal Flk1ST-/- mice, we found they had dilated peritubular capillaries. Three-dimensional reconstructions showed reduced ureteric branching and fewer nephrons in developing Flk1ST-/- kidneys vs.

CONTROLS

Juvenile Flk1ST-/- kidneys displayed renal papillary hypoplasia and a paucity of collecting ducts. Twenty-four-hour urine collections revealed that postnatal Flk1ST-/- mice had urinary-concentrating defects. Thus, while lineage-tracing revealed that the renal cortical stroma gave rise to a small subset of endothelial progenitors, these Flk1-expressing stromal cells are critical for patterning the peritubular capillaries. Also, loss of Flk1 in the renal stroma leads to nonautonomous-patterning defects in ureteric lineages.

Novel fixed z-direction (FiZD) kidney primordia and an organoid culture system for time-lapse confocal imaging

Tissue, organ and organoid cultures provide suitable models for developmental studies, but our understanding of how the organs are assembled at the single-cell level still remains unclear. We describe here a novel fixed z-direction (FiZD) culture setup that permits high-resolution confocal imaging of organoids and embryonic tissues. In a FiZD culture a permeable membrane compresses the tissues onto a glass coverslip and the spacers adjust the thickness, enabling the tissue to grow for up to 12 days. Thus, the kidney rudiment and the organoids can adjust to the limited z-directional space and yet advance the process of kidney morphogenesis, enabling long-term time-lapse and high-resolution confocal imaging. As the data quality achieved was sufficient for computer-assisted cell segmentation and analysis, the method can be used for studying morphogenesis ex vivo at the level of the single constituent cells of a complex mammalian organogenesis model system.

Summary: Time-lapse confocal imaging of organoids and embryonic tissues through fixed z-direction culture allows long-term single-cell resolution live imaging of tissue growth and morphogenesis.

cGMP production of patient-specific iPSCs and photoreceptor precursor cells to treat retinal degenerative blindness

Immunologically-matched, induced pluripotent stem cell (iPSC)-derived photoreceptor precursor cells have the potential to restore vision to patients with retinal degenerative diseases like retinitis pigmentosa. The purpose of this study was to develop clinically-compatible methods for manufacturing photoreceptor precursor cells from adult skin in a non-profit cGMP environment. Biopsies were obtained from 35 adult patients with inherited retinal degeneration and fibroblast lines were established under ISO class 5 cGMP conditions. Patient-specific iPSCs were then generated, clonally expanded and validated. Post-mitotic photoreceptor precursor cells were generated using a stepwise cGMP-compliant 3D differentiation protocol. The recapitulation of the enhanced S-cone phenotype in retinal organoids generated from a patient with NR2E3 mutations demonstrated the fidelity of these protocols. Transplantation into immune compromised animals revealed no evidence of abnormal proliferation or tumor formation. These studies will enable clinical trials to test the safety and efficiency of patient-specific photoreceptor cell replacement in humans.

Wnt Signaling in Renal Cell Carcinoma

Renal cell carcinoma (RCC) accounts for 90% of all kidney cancers. Due to poor diagnosis, high resistance to the systemic therapies and the fact that most RCC cases occur sporadically, current research switched its focus on studying the molecular mechanisms underlying RCC. The aim is the discovery of new effective and less toxic anti-cancer drugs and novel diagnostic markers. Besides the PI3K/Akt/mTOR, HGF/Met and VHL/hypoxia cellular signaling pathways, the involvement of the Wnt/β-catenin pathway in RCC is commonly studied. Wnt signaling and its targeted genes are known to actively participate in different biological processes during embryonic development and renal cancer. Recently, studies have shown that targeting this pathway by alternating/inhibiting its intracellular signal transduction can reduce cancer cells viability and inhibit their growth. The targets and drugs identified show promising potential to serve as novel RCC therapeutics and prognostic markers. This review aims to summarize the current status quo regarding recent research on RCC focusing on the involvement of the Wnt/β-catenin pathway and how its understanding could facilitate the identification of potential therapeutic targets, new drugs and diagnostic biomarkers.

Beyond Turing: mechanochemical pattern formation in biological tissues

Background

During embryogenesis, chemical (morphogen) and mechanical patterns develop within tissues in a self-organized way. More than 60 years ago, Turing proposed his famous reaction-diffusion model for such processes, assuming chemical interactions as the main driving force in tissue patterning. However, experimental identification of corresponding molecular candidates is still incomplete. Recent results suggest that beside morphogens, also tissue mechanics play a significant role in these patterning processes.

Results

Combining continuous finite strain with discrete cellular tissue models, we present and numerically investigate mechanochemical processes, in which morphogen dynamics and tissue mechanics are coupled by feedback loops. We consider three different mechanical cues involved in such feedbacks: strain, stress, and compression. Based on experimental results, for each case, we present a feedback loop spontaneously creating robust mechanochemical patterns. In contrast to Turing-type models, simple mechanochemical interaction terms are sufficient to create de novo patterns.

Conclusions

Our results emphasize mechanochemical processes as possible candidates controlling different steps of embryogenesis. To motivate further experimental research discovering related mechanisms in living tissues, we also present predictive in silicio experiments.

Reviewers

Reviewer 1 - Marek Kimmel; Reviewer 2 - Konstantin Doubrovinski (nominated by Ned Wingreen); Reviewer 3 - Jun Allard (nominated by William Hlavacek).

CD146(+) cells are essential for kidney vasculature development

The kidney vasculature is critical for renal function, but its developmental assembly mechanisms remain poorly understood and models for studying its assembly dynamics are limited. Here, we tested whether the embryonic kidney contains endothelial cells (ECs) that are heterogeneous with respect to VEGFR2/Flk1/KDR, CD31/PECAM, and CD146/MCAM markers. Tie1Cre;R26R(YFP)-based fate mapping with a time-lapse in embryonic kidney organ culture successfully depicted the dynamics of kidney vasculature development and the correlation of the process with the CD31(+) EC network. Depletion of Tie1(+) or CD31(+) ECs from embryonic kidneys, with either Tie1Cre-induced diphtheria toxin susceptibility or cell surface marker-based sorting in a novel dissociation and reaggregation technology, illustrated substantial EC network regeneration. Depletion of the CD146(+) cells abolished this EC regeneration. Fate mapping of green fluorescent protein (GFP)-marked CD146(+)/CD31(-) cells indicated that they became CD31(+) cells, which took part in EC structures with CD31(+) wild-type ECs. EC network development depends on VEGF signaling, and VEGF and erythropoietin are expressed in the embryonic kidney even in the absence of any external hypoxic stimulus. Thus, the ex vivo embryonic kidney culture models adopted here provided novel ways for targeting renal EC development and demonstrated that CD146(+) cells are critical for kidney vasculature development.

Mechanochemical symmetry breaking in Hydra aggregates

Tissue morphogenesis comprises the self-organized creation of various patterns and shapes. Although detailed underlying mechanisms are still elusive in many cases, an increasing amount of experimental data suggests that chemical morphogen and mechanical processes are strongly coupled. Here, we develop and test a minimal model of the axis-defining step (i.e., symmetry breaking) in aggregates of the Hydra polyp. Based on previous findings, we combine osmotically driven shape oscillations with tissue mechanics and morphogen dynamics. We show that the model incorporating a simple feedback loop between morphogen patterning and tissue stretch reproduces a wide range of experimental data. Finally, we compare different hypothetical morphogen patterning mechanisms (Turing, tissue-curvature, and self-organized criticality). Our results suggest the experimental investigation of bigger (i.e., multiple head) aggregates as a key step for a deeper understanding of mechanochemical symmetry breaking in Hydra.

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.