Research Highlights

Life-supporting Functional Kidney Replacement by Integration of Embryonic Metanephros-bladder Composite Tissue Transplants Kinoshita Y, Kobayashi E, Matsui K, et al. Kidney Int. 2025;107(6):1051–1063. Although deceased and living organ donation provide the only clinically practical transplant options, novel approaches to organ replacement—xenotransplantation, bioprinting, recellularization of scaffolds, cellular reprogramming, and transdifferentation—are tantalizingly close to clinical application. Embryonic tissue is an important potential source of organs for transplantation, including the kidney, where the metanephros is a viable allograft source. Previous studies have shown that the metanephros can successfully integrate with the host urinary tract upon transplantation and mature to produce urine-like fluid.1,2 However, the smaller size of metanephroi (compared with adult kidneys) limited functionality of the graft and survival of recipient animals.2 In this study, Kinoshita et al3 developed a technically impressive strategy for transplanting up to 20 metanephroi into a host, to evaluate whether transplanting increased tissue mass can enhance graft function and host survival (Figure 1). Metanephros-bladder (MNB) composites were obtained from fetal Lewis (LEW) rats, and the bladder of each MNB was divided. MNB integrated grafts were created by suturing together the divided bladders from 2 individual MNB composites. MNB-integrated grafts were transplanted onto the abdominal aorta of adult NOG mice or syngeneic LEW rats. The host animals received either 3, 4, or 5 MNB-integrated grafts, corresponding to 12, 16, or 20 metanephroi, respectively. Three weeks after transplantation, the left native kidney was removed, and urinary tract reconstruction was performed by anastomosing the left native ureter to the bladders of the MNB grafts. In LEW rats who received MNB graft transplants, the right native kidney was removed 8–14 wk after transplantation.FIGURE 1.: Workflow of the experimental design. Created with Biorender.Increasing the number of transplanted metanephroi was found to increase host survival: rats transplanted with 12 metanephroi survived >1 wk, whereas 20 metanephroi led to survival past 1 mo. In addition, rats transplanted with 20 metanephroi were able to stabilize serum creatinine levels, serum pH, and regulate electrolyte balance, after receiving postoperative hydration (5% glucose) after a right native kidney nephrectomy. This is in contrast with sham-operated controls which demonstrated persistently rising serum creatinine measurements and did not survive beyond 1 wk. The novel technique of transplanting multiple MNB-integrated grafts provided increased luminal space for urine accumulation, and therein facilitated successful urinary tract reconstruction. Single-cell RNA-sequencing of MN samples at 3 wk (compared with healthy control kidney tissue) identified 18 distinct renal cell groups. Trajectory analysis of proximal tubule segment 2 cells in the transplanted metanephros showed an advanced pseudotime stage, mirroring that of adult controls. Expression of maturation markers for individual nephron segments was comparable between adult kidneys and the transplanted metanephros, and GeneOntology analysis revealed developmental transitions. Together, these findings indicate that the development of the metanephros resembled that of a healthy adult kidney. These results were reinforced through ultrastructural analysis of the transplanted metanephros. At 8 wk after transplantation, there was nephron segment growth, podocyte foot process development, and microvilli and basolateral infoldings equivalent to those seen in healthy adult kidneys. Renin-producing cells were present and appropriately localized to the juxtaglomerular apparatus, and the vitamin D hydroxylase CYP27B1 was expressed at a level which matched adult controls. Despite showing development analogous to adult kidneys, the size of the transplanted metanephros remained smaller than controls. The mass of renal tissue in rats transplanted with 20 metanephroi reached 60% of the mass of 2 adult kidneys, and the thin descending limbs were undersized. This impeded the development of an adequate countercurrent multiplier system, leading to an impaired urine concentrating ability. The metanephroi were also able to establish glomerular and peritubular capillary networks through integration with the host vasculature. Interestingly, host blood vessels were observed to penetrate the metanephros from multiple anatomical locations, including the capsule, rather than through a single renal artery. Rats transplanted with 20 metanephroi had 50 000 glomeruli (60% of the number present in control kidneys), which enabled survival beyond 1 mo; however, the creatinine clearance was only 15% that of healthy adult kidneys. The authors establish that increasing the mass of transplanted embryonic kidney tissue improves its functionality, aids urinary tract reconstruction through expanding the amount of luminal space available, and ultimately enhances host survival—to a point. This study is the first to show that transplanted metanephroi can mature to transcriptomically and structurally resemble a healthy adult kidney. There are still substantial differences in terms of functional maturation, and this remains a crucial challenge in the field that cannot simply be overcome by mass of embryonic kidney. Regardless, these findings support the potential suitability of embryonic kidney tissue for generating functional transplant grafts and are an important step toward realizing embryonic tissue transplantation in clinical settings. Bioprinting of Bespoke Islet-specific Niches to Promote Maturation of Stem-cell Derived Islets Kim M, Cho S, Hwang DG, et al. Nat Commun. 2025;16:1430. The endocrine pancreas demonstrates an extensive, dense, and fenestrated microvasculature, supplying 10%–15% of arterial blood supplied to islets despite occupying only 1%–2% of total pancreatic mass.1 Islets comprised many endocrine cell types including the predominant β cells, followed by α cells and δ cells (which differ in proportion and orientation in mammals), as well as intra-islet endothelial cells which make up the complex capillary network and allow for efficient oxygen and nutrition delivery to intra-islet cells, as well as dissemination of hormones released by islet endocrine cells into the systemic circulation. The extracellular matrix (ECM) provides a further structural framework that preserves islet morphology and microenvironment, and facilitates insulin secretion, supporting both islet differentiation and survival. Islet transplantation provides restoration of true metabolic control, protection from life-threatening hypoglycemia, and insulin independence (the latter in select cases). However, widespread application of this procedure is restricted in part by donor availability. Stem cell-derived islets (SC-islets) provide a renewable resource for transplantation, solving the limitation imposed by deceased donation, but they do not wholly replicate primary β-cell function in terms of differentiation and lineage specificity, transcriptional phenotype, as well as insulin secretagogue capacity.2 Further protocol refinements have improved SC-islet functionality, and maturation of SC-islets has been shown to continue following long-term engraftment in vivo.3 Kim et al4 have now leveraged bioprinting to create islet-specific niches that augment SC-islet differentiation and maturation (Figure 1). They achieve this goal by performing proteomic analysis of endocrine pancreas ECM (pdECM) and comparing with collagen. Although pdECM contained abundant collagen (particular type VI), there were additional unique components including proteoglycans and glycoproteins which are crucial to structural organization and support of cellular signaling. This was emphasized further by the gene ontology molecular function analysis demonstrating response to stress, development, and biological regulation pathways far exceeding that provided by collagen. These features were decimated by a decellularization process akin to (but more stringent than) islet isolation for transplantation.FIGURE 1.: Improving the function of SC-islets through ECM composition. A, Compositional differences between pdECM and collagen. B, The beneficial effect of ECM on SC-derived islet function. C, Creation of HICA-V composites to facilitate SC-islet differentiation. BM, basement membrane; EC, endothelial cell; ECM, extracellular matrix; FGF, fibroblast growth factor; GO, gene ontology; GSIS, glucose-stimulated insulin secretion; HICA-V, human islet-like cellular aggregates-vasculatures; pdECM, endocrine pancreas extracellular matrix; SC-islets, stem cell–derived islets. Created with Biorender.The authors then used this information to synthesize an islet-sympathetic niche that approximated pdECM. Human embryonic stem cells were differentiated into β cells using a standardized protocol5 and then co-cultured with supplemented proteins, resulting in higher insulin secretion, in addition to greater Ca2+ influx, and stable INS expression. Co-culture of SC-islets with endothelial cells also supported metabolic maturation markers. Bioprinting was subsequently used to develop a sophisticated vasculature/pdECM/islet 3D network (denoted human islet-like cellular aggregates-vasculatures [HICA-V]). In particular, the rheological behavior (essentially viscosity and elasticity) of pdECM—unlike collagen—changes under shear stress conditions, and this could only be replicated by incorporating basement membrane proteins. The bioprinting also faithfully replicated the dense vascular network of the endocrine pancreas, not only allowing for colocalization of endothelial cells and SC-islets, but also providing reproducible vessel size. Expression of cell adhesion molecules, fibroblast growth factor, insulin secretion, and glucose-stimulated insulin secretion was improved in the context of appropriate geometrical cues, compared with either randomly dispersed SC-islets and endothelial cells or aggregates. Although the authors did not explore the ability of HICA-V to restore physiological glucose concentrations in vivo, they exposed cultures in vitro to high glucose with/without a sodium-glucose transport protein 2 inhibitor and metformin. Drug exposure maintained CD31 and INS expression, limited proinflammatory cytokine profiles, and improved glucose-stimulated insulin secretion profile. These data highlight the importance of the microenvironment in supporting cell development, proliferation, and survival, particularly for vulnerable and highly specialized cell populations such as islets, in addition to facilitating stem cell maturation. Manipulation of ECM composition appears critical to this function, regulating calcium influx and glucose responsiveness, as well as spatial fidelity, although additional studies are required to validate functional maturity. The field of islet transplantation has focused on encapsulation for cytoprotection and metabolic support, but perhaps the failure of this technology to translate to clinical application despite decades of research belies the importance of ECM, together with intact vasculature. This study reflects the true complexity of organ replacement—HICA-V did not contain pericytes per se, nor neuronal connections which remain fundamental to hormonal output. However, can more simplistic constructs suffice to bridge the donor gap?