Recapitulating kidney development: Progress and challenges

Autologous adipose-derived mesenchymal stem cell therapy reverses detrusor underactivity: open clinical trial

BACKGROUND

Detrusor underactivity is a disease that can cause chronic urinary tract infection, urinary tract infection, urinary retention and kidney failure and has no effective treatment in traditional medicine. The present research evaluated the effects of cell therapy with adipose tissue-derived stem cells on the treatment of detrusor underactivity in men.

METHODS

Nine male patients diagnosed with a clinical and urodynamic diagnosis of detrusor underactivity were evaluated and underwent two transplants via cystourethroscopy, with 2 × 10⁶ cells/transplant, performed by intravesical injection at five points on the bladder body above the vesical trigone.

RESULTS

Cell therapy increased the maximum flow from 7.22 ± 1.58 to 13.56 ± 1.17, increased the mean flow from 3.44 ± 0.74 to 5.89 ± 0.45, increased the urinated volume from 183.67 ± 49.28 to 304.78 ± 40.42 and reduced the residual volume in the uroflowmetry exam from 420.00 ± 191.41 to 118.33 ± 85.51; all of these changes were significant (p < 0.05). There were also significant increases (p < 0.05) in maximum flow (from 7.78 ± 0.76 to 11.56 ± 1.67), maximum detrusor pressure (from 20.22 ± 8.29 to 41.56 ± 5.75), urinary volume (from 244 ± 27.6 to 418.89 ± 32.73) and bladder contractility index (from 44.33 ± 4.85 to 100.56 ± 8.89) in the pressure flow study. Scores on the International Consultation on Incontinence Questionnaire decreased from 11.44 ± 1.43 to 3.78 ± 0.78 after cell therapy, which indicates an improvement in quality of life and a return to daily activities. No complications were observed in the 6-month follow-up after cell therapy. Before treatment, all patients performed approximately five intermittent clean catheterizations daily. After cell therapy, 7/9 patients (77.78%) did not need catheterizations, and the number of catheterizations for 2/9 patients (22.28%) was reduced to two catheterizations/day.

CONCLUSIONS

The results indicate that stem cell therapy led to improvements in voiding function. Cell therapy with adipose tissue-derived stem cells is safe and should be considered a new therapeutic option for the treatment of detrusor underactivity. Trial registration ISRCTN, ISRCTN23909398; Registered 15 March 2021-Retrospectively registered, https://doi.org/10.1186/ISRCTN23909398.

The "3Ds" of Growing Kidney Organoids: Advances in Nephron Development, Disease Modeling, and Drug Screening

A kidney organoid is a three-dimensional (3D) cellular aggregate grown from stem cells in vitro that undergoes self-organization, recapitulating aspects of normal renal development to produce nephron structures that resemble the native kidney organ. These miniature kidney-like structures can also be derived from primary patient cells and thus provide simplified context to observe how mutations in kidney-disease-associated genes affect organogenesis and physiological function. In the past several years, advances in kidney organoid technologies have achieved the formation of renal organoids with enhanced numbers of specialized cell types, less heterogeneity, and more architectural complexity. Microfluidic bioreactor culture devices, single-cell transcriptomics, and bioinformatic analyses have accelerated the development of more sophisticated renal organoids and tailored them to become increasingly amenable to high-throughput experimentation. However, many significant challenges remain in realizing the use of kidney organoids for renal replacement therapies. This review presents an overview of the renal organoid field and selected highlights of recent cutting-edge kidney organoid research with a focus on embryonic development, modeling renal disease, and personalized drug screening.

Tubuloid culture enables long-term expansion of functional human kidney tubule epithelium from iPSC-derived organoids

Kidney organoids generated from induced pluripotent stem cells (iPSC) have proven valuable for studies of kidney development, disease, and therapeutic screening. However, specific applications have been hampered by limited expansion capacity, immaturity, off-target cells, and inability to access the apical side. Here, we apply recently developed tubuloid protocols to purify and propagate kidney epithelium from d7+18 (post nephrogenesis) iPSC-derived organoids. The resulting 'iPSC organoid-derived (iPSCod)' tubuloids can be exponentially expanded for at least 2.5 mo, while retaining expression of important tubular transporters and segment-specific markers. This approach allows for selective propagation of the mature tubular epithelium, as immature cells, stroma, and undesirable off-target cells rapidly disappeared. iPSCod tubuloids provide easy apical access, which enabled functional evaluation and demonstration of essential secretion and electrolyte reabsorption processes. In conclusion, iPSCod tubuloids provide a different, complementary human kidney model that unlocks opportunities for functional characterization, disease modeling, and regenerative nephrology.

A Concise Review on Electrospun Scaffolds for Kidney Tissue Engineering

Chronic kidney disease is one of the deadliest diseases globally and treatment methods are still insufficient, relying mostly on transplantation and dialysis. Engineering of kidney tissues in vitro from induced pluripotent stem cells (iPSCs) could provide a solution to this medical need by restoring the function of damaged kidneys. However, implementation of such approaches is still challenging to achieve due to the complexity of mature kidneys in vivo. Several strategies have been defined to obtain kidney progenitor endothelial and epithelial cells that could form nephrons and proximal tube cells, but these lack tissue maturity and vascularisation to be further implemented. Electrospinning is a technique that has shown promise in the development of physiological microenvironments of several tissues and could be applied in the engineering of kidney tissues. Synthetic polymers such as polycaprolactone, polylactic acid, and poly(vinyl alcohol) have been explored in the manufacturing of fibres that align and promote the proliferation and cell-to-cell interactions of kidney cells. Natural polymers including silk fibroin and decellularised extracellular matrix have also been explored alone and in combination with synthetic polymers promoting the differentiation of podocytes and tubular-specific cells. Despite these attempts, further work is still required to advance the applications of electrospun fibres in kidney tissue engineering and explore this technique in combination with other manufacturing methods such as bioprinting to develop more organised, mature and reproducible kidney organoids.

Organs-on-chip technology: a tool to tackle genetic kidney diseases

Chronic kidney disease (CKD) is a major healthcare burden that takes a toll on the quality of life of many patients. Emerging evidence indicates that a substantial proportion of these patients carry a genetic defect that contributes to their disease. Any effort to reduce the percentage of patients with a diagnosis of nephropathy heading towards kidney replacement therapies should therefore be encouraged. Besides early genetic screenings and registries, in vitro systems that mimic the complexity and pathophysiological aspects of the disease could advance the screening for targeted and personalized therapies. In this regard, the use of patient-derived cell lines, as well as the generation of disease-specific cell lines via gene editing and stem cell technologies, have significantly improved our understanding of the molecular mechanisms underlying inherited kidney diseases. Furthermore, organs-on-chip technology holds great potential as it can emulate tissue and organ functions that are not found in other, more simple, in vitro models. The personalized nature of the chips, together with physiologically relevant read-outs, provide new opportunities for patient-specific assessment, as well as personalized strategies for treatment. In this review, we summarize the major kidney-on-chip (KOC) configurations and present the most recent studies on the in vitro representation of genetic kidney diseases using KOC-driven strategies.

Photo-crosslinked hyaluronic acid hydrogel as a biomimic extracellular matrix to recapitulate in vivo features of breast cancer cells

2D cell culture is widely utilized to develop anti-cancer drugs and to explore the mechanisms of cancer tumorigenesis and development. However, the findings obtained from 2D culture often fail to provide guidance for clinical tumor treatments since it cannot precisely replicate the features of real tumors. 3D tumor models capable of recapitulating native tumor microenvironments have been proved to be a promising alternative technique. Herein, we constructed a breast tumor model from novel hyaluronic acid (HA) hydrogel which was prepared through photocrosslinking of methacrylated HA. The hydrogel was used as a biomimetic extracellular matrix to incubate MCF-7 cells. It was found that methacrylation degree had great effects on hydrogel's microstructure, mechanical performances, and liquid-absorbing and degradation abilities. Optimized hydrogel exhibited highly porous morphology, high equilibrium swelling ratio, suitable mechanical properties, and hyaluronidase-responsive degradation behavior. The results demonstrated that the HA hydrogel facilitated MCF-7 cell proliferation and growth in an aggregation manner. Furthermore, 3D-cultured MCF-7 cells not only up-regulated the expression of VEGF, bFGF and interleukin-8 but exhibited greater invasion and tumorigenesis capabilities compared with 2D-cultured cells. Therefore, the HA hydrogel is a reliable substitute for tumor model construction.

Function and Regulation of the Epithelial Na+ Channel ENaC

The Epithelial Na⁺ Channel, ENaC, comprised of 3 subunits (αβγ, or sometimes δβγENaC), plays a critical role in regulating salt and fluid homeostasis in the body. It regulates fluid reabsorption into the blood stream from the kidney to control blood volume and pressure, fluid absorption in the lung to control alveolar fluid clearance at birth and maintenance of normal airway surface liquid throughout life, and fluid absorption in the distal colon and other epithelial tissues. Moreover, recent studies have also revealed a role for sodium movement via ENaC in nonepithelial cells/tissues, such as endothelial cells in blood vessels and neurons. Over the past 25 years, major advances have been made in our understanding of ENaC structure, function, regulation, and role in human disease. These include the recently solved three-dimensional structure of ENaC, ENaC function in various tissues, and mutations in ENaC that cause a hereditary form of hypertension (Liddle syndrome), salt-wasting hypotension (PHA1), or polymorphism in ENaC that contributes to other diseases (such as cystic fibrosis). Moreover, great strides have been made in deciphering the regulation of ENaC by hormones (e.g., the mineralocorticoid aldosterone, glucocorticoids, vasopressin), ions (e.g., Na⁺ ), proteins (e.g., the ubiquitin-protein ligase NEDD4-2, the kinases SGK1, AKT, AMPK, WNKs & mTORC2, and proteases), and posttranslational modifications [e.g., (de)ubiquitylation, glycosylation, phosphorylation, acetylation, palmitoylation]. Characterization of ENaC structure, function, regulation, and role in human disease, including using animal models, are described in this article, with a special emphasis on recent advances in the field. © 2021 American Physiological Society. Compr Physiol 11:1-29, 2021.

A coordinated progression of progenitor cell states initiates urinary tract development

The kidney and upper urinary tract develop through reciprocal interactions between the ureteric bud and the surrounding mesenchyme. Ureteric bud branching forms the arborized collecting duct system of the kidney, while ureteric tips promote nephron formation from dedicated progenitor cells. While nephron progenitor cells are relatively well characterized, the origin of ureteric bud progenitors has received little attention so far. It is well established that the ureteric bud is induced from the nephric duct, an epithelial duct derived from the intermediate mesoderm of the embryo. However, the cell state transitions underlying the progression from intermediate mesoderm to nephric duct and ureteric bud remain unknown. Here we show that nephric duct morphogenesis results from the coordinated organization of four major progenitor cell populations. Using single cell RNA-seq and Cluster RNA-seq, we show that these progenitors emerge in time and space according to a stereotypical pattern. We identify the transcription factors Tfap2a/b and Gata3 as critical coordinators of this progenitor cell progression. This study provides a better understanding of the cellular origin of the renal collecting duct system and associated urinary tract developmental diseases, which may inform guided differentiation of functional kidney tissue.

Regenerative treatments for kidney diseases: The closest and fastest strategies to solving related medical and economic problems

Recent advances in developmental biology and stem cell biology have led to the increased availability of extrarenal stem cells, including mesenchymal/stromal stem cells (MSCs), renal stem or progenitor cells isolated from embryonic and adult kidneys, and kidney lineage cells or tissues generated from human pluripotent stem cells (hPSCs), such as human embryonic stem cells and human-induced pluripotent stem cells. Regenerative medicine strategies for kidney diseases are largely categorized into the transplantation of reconstructed kidney organs and cell therapies. Reconstruction is being attempted by hPSC-derived kidney lineage cells with various strategies, such as self-organization, interspecies blastocyst complementation, utilization of a xenogeneic organ niche, decellularization and repopulation, and 3D bioprinting. However, cell therapies using extrarenal stem cells, such as MSCs, and renal stem or progenitor cells derived from embryonic and adult kidneys or differentiated from hPSCs have been investigated in animal models of both acute kidney injury and chronic kidney disease. Indeed, multiple clinical trials using MSCs, bone marrow stem cells, and kidney-derived cells have already been carried out. This review summarizes the current status and future perspective of kidney regenerative medicine strategies and discusses the closest and fastest strategies to solving the medical and economic problems associated with kidney diseases.

Culture and analysis of kidney tubuloids and perfused tubuloid cells-on-a-chip

Advanced in vitro kidney models are of great importance to the study of renal physiology and disease. Kidney tubuloids can be established from primary cells derived from adult kidney tissue or urine. Tubuloids are three-dimensional multicellular structures that recapitulate tubular function and have been used to study infectious, malignant, metabolic, and genetic diseases. For tubuloids to more closely represent the in vivo kidney, they can be integrated into an organ-on-a-chip system that has a more physiological tubular architecture and allows flow and interaction with vasculature or epithelial and mesenchymal cells from other organs. Here, we describe a detailed protocol for establishing tubuloid cultures from tissue and urine (1-3 weeks), as well as for generating and characterizing tubuloid cell-derived three-dimensional tubular structures in a perfused microfluidic multi-chip platform (7 d). The combination of the two systems yields a powerful in vitro tool that better recapitulates the complexity of the kidney tubule with donor-specific properties.

In Vivo Assessment of Size-Selective Glomerular Sieving in Transplanted Human Induced Pluripotent Stem Cell-Derived Kidney Organoids

BACKGROUND

The utility of kidney organoids in regenerative medicine will rely on the functionality of the glomerular and tubular structures in these tissues. Recent studies have demonstrated the vascularization and subsequent maturation of human pluripotent stem cell-derived kidney organoids after renal subcapsular transplantation. This raises the question of whether the glomeruli also become functional upon transplantation.

METHODS

We transplanted kidney organoids under the renal capsule of the left kidney in immunodeficient mice followed by the implantation of a titanium imaging window on top of the kidney organoid. To assess glomerular function in the transplanted human pluripotent stem cell-derived kidney tissue 1, 2, and 3 weeks after transplantation, we applied high-resolution intravital multiphoton imaging through the imaging window during intravenous infusion of fluorescently labeled low and high molecular mass dextran molecules or albumin.

RESULTS

After vascularization, glomerular structures in the organoid displayed dextran and albumin size selectivity across their glomerular filtration barrier. We also observed evidence of proximal tubular dextran reuptake.

CONCLUSIONS

Our results demonstrate that human pluripotent stem cell-derived glomeruli can develop an appropriate barrier function and discriminate between molecules of varying size. These characteristics together with tubular presence of low molecular mass dextran provide clear evidence of functional filtration. This approach to visualizing glomerular filtration function will be instrumental for translation of organoid technology for clinical applications as well as for disease modeling.

Differentiating Induced Pluripotent Stem Cells into Renal Cells: A New Approach to Treat Kidney Diseases

Renal disease is a major issue for global public health. Despite some progress in supportive care, the mortality rates among patients with this condition remain alarmingly high. Studies in pursuit of innovative strategies to treat renal diseases, especially stimulating kidney regeneration, have been developed. In this field, stem cell-based therapy has been a promising area. Induced pluripotent stem cell-derived renal cells (iPSC-RCs) represent an interesting source of cells for treating kidney diseases. Advances in regenerative medicine using iPSC-RCs and their application to the kidney are discussed in this review. Furthermore, the way differentiation protocols of induced pluripotent stem cells into renal cells may also be applied for the generation of kidney organoids is also described, contributing to studies in renal development, kidney diseases, and drug toxicity tests. The translation of the differentiation methodologies into animal model studies and the safety and feasibility of renal differentiated cells as a treatment for kidney injury are also highlighted. Although only few studies were published in this field, the results seem promising and support the use of iPSC-RCs as a potential therapy in the future.

A novel ADPKD model using kidney organoids derived from disease-specific human iPSCs

Autosomal dominant polycystic kidney disease (ADPKD) is a hereditary disorder which manifests progressive renal cyst formation and leads to end-stage kidney disease. Around 85% of cases are caused by PKD1 heterozygous mutations, exhibiting relatively poorer renal outcomes than those with mutations in other causative gene PKD2. Although many disease models have been proposed for ADPKD, the pre-symptomatic pathology of the human disease remains unknown. To unveil the mechanisms of early cytogenesis, robust and genetically relevant human models are needed. Here, we report a novel ADPKD model using kidney organoids derived from disease-specific human induced pluripotent stem cells (hiPSCs). Importantly, we found that kidney organoids differentiated from gene-edited heterozygous PKD1-mutant as well as ADPKD patient-derived hiPSCs can reproduce renal cysts. Further, we demonstrated the possibility of ADPKD kidney organoids serving as drug screening platforms. This newly developed model will contribute to identifying novel therapeutic targets, extending the field of ADPKD research.

Kidney Organoids and Tubuloids

In the past five years, pluripotent stem cell (PSC)-derived kidney organoids and adult stem or progenitor cell (ASC)-based kidney tubuloids have emerged as advanced in vitro models of kidney development, physiology, and disease. PSC-derived organoids mimic nephrogenesis. After differentiation towards the kidney precursor tissues ureteric bud and metanephric mesenchyme, their reciprocal interaction causes self-organization and patterning in vitro to generate nephron structures that resemble the fetal kidney. ASC tubuloids on the other hand recapitulate renewal and repair in the adult kidney tubule and give rise to long-term expandable and genetically stable cultures that consist of adult proximal tubule, loop of Henle, distal tubule, and collecting duct epithelium. Both organoid types hold great potential for: (1) studies of kidney physiology, (2) disease modeling, (3) high-throughput screening for drug efficacy and toxicity, and (4) regenerative medicine. Currently, organoids and tubuloids are successfully used to model hereditary, infectious, toxic, metabolic, and malignant kidney diseases and to screen for effective therapies. Furthermore, a tumor tubuloid biobank was established, which allows studies of pathogenic mutations and novel drug targets in a large group of patients. In this review, we discuss the nature of kidney organoids and tubuloids and their current and future applications in science and medicine.

In Search of Imprints Left by the Impairment of Nephrogenesis

Clinical aspects dealing with the impairment of nephrogenesis in preterm and low birth weight babies were intensely researched. In this context it was shown that quite different noxae can harm nephron formation, and that the morphological damage in the fetal kidney is rather complex. Some pathological findings show that the impairment leads to changes in developing glomeruli that are restricted to the maturation zone of the outer cortex in the fetal human kidney. Other data show also imprints on the stages of nephron anlage including the niche, the pretubular aggregate, the renal vesicle, and comma- and S-shaped bodies located in the overlying nephrogenic zone of the rodent and human kidneys. During our investigations it was noticed that the stages of nephron anlage in the fetal human kidney during the phase of late gestation have not been described in detail. To contribute, these stages were recorded along with corresponding images. The initial nephron formation in the rodent kidney served as a reference. Finally, the known imprints left by the impairment in both specimens were listed and discussed. In sum, the relatively paucity of data on nephron formation in the fetal human kidney during the late phase of gestation is a call to start with intense research so that concepts for a therapeutic prolongation of nephrogenesis can be designed.

Human kidney organoids: progress and remaining challenges

Kidney organoids are regarded as important tools with which to study the development of the normal and diseased human kidney. Since the first reports of human pluripotent stem cell-derived kidney organoids 5 years ago, kidney organoids have been successfully used to model glomerular and tubular diseases. In parallel, advances in single-cell RNA sequencing have led to identification of a variety of cell types in the organoids, and have shown these to be similar to, but more immature than, human kidney cells in vivo. Protocols for the in vitro expansion of stem cell-derived nephron progenitor cells (NPCs), as well as those for the selective induction of specific lineages, especially glomerular podocytes, have also been reported. Although most current organoids are based on the induction of NPCs, an induction protocol for ureteric buds (collecting duct precursors) has also been developed, and approaches to generate more complex kidney structures may soon be possible. Maturation of organoids is a major challenge, and more detailed analysis of the developing kidney at a single cell level is needed. Eventually, organotypic kidney structures equipped with nephrons, collecting ducts, ureters, stroma and vascular flow are required to generate transplantable kidneys; such attempts are in progress.

Mechanisms of Nephrogenesis Revealed by Zebrafish Chemical Screen: Prostaglandin Signaling Modulates Nephron Progenitor Fate

Nephron development involves the creation of discrete segment populations that are specialized to fulfill unique physiological roles. As such, renal function is reliant on the proper execution of segment patterning programs. Despite the central importance of nephron segmentation, the genetic mechanisms that regulate this process are far from understood, in large part due to the experimental complexities and cost of interrogating these events in the mammalian metanephros. For this reason, forward genetics utilizing phenotypic screening in the zebrafish pronephros provides an avenue to gain novel insights about the mechanisms of nephron segmentation in the vertebrate kidney. Discoveries from zebrafish can highlight possible conserved pathways and provide a useful starting point for reverse genetic analyses with other animal models or in vitro approaches. In this review, we discuss the results of a novel chemical screen using the zebrafish to identify segmentation regulators. Through this screen, we identified for the first time that prostaglandin signaling can modulate nephron segmentation, and that it is normally requisite during development to mitigate segment fate choice in the embryonic kidney. We briefly discuss how these discoveries relate to current knowledge about nephron segmentation. Finally, we explore the possible implications of these findings for understanding renal ontogeny and disease, and how this knowledge may be useful for ongoing research initiatives that are aimed at deciphering how to build or rebuild the human kidney.

Iroquois transcription factor irx2a is required for multiciliated and transporter cell fate decisions during zebrafish pronephros development

The genetic regulation of nephron patterning during kidney organogenesis remains poorly understood. Nephron tubules in zebrafish are composed of segment populations that have unique absorptive and secretory roles, as well as multiciliated cells (MCCs) that govern fluid flow. Here, we report that the transcription factor iroquois 2a (irx2a) is requisite for zebrafish nephrogenesis. irx2a transcripts localized to the developing pronephros and maturing MCCs, and loss of function altered formation of two segment populations and reduced MCC number. Interestingly, irx2a deficient embryos had reduced expression of an essential MCC gene ets variant 5a (etv5a), and were rescued by etv5a overexpression, supporting the conclusion that etv5a acts downstream of irx2a to control MCC ontogeny. Finally, we found that retinoic acid (RA) signaling affects the irx2a expression domain in renal progenitors, positioning irx2a downstream of RA. In sum, this work reveals new roles for irx2a during nephrogenesis, identifying irx2a as a crucial connection between RA signaling, segmentation, and the control of etv5a mediated MCC formation. Further investigation of the genetic players involved in these events will enhance our understanding of the molecular pathways that govern renal development, which can be used help create therapeutics to treat congenital and acquired kidney diseases.