Chronic kidney disease (CKD) is a major global public health problem. In the US, about 11% of adults have CKD as of 2012, and CKD accounts for $41 billion in Medicare expenditures (17%). When patients with CKD progress to end-stage renal disease (ESRD), the options for treatment are limited to dialysis and kidney transplantation. Dialysis is associated with significant morbidity and mortality, and kidney transplantation is limited by the supply of organs as well as the need for patients to take immunosuppressive medications for the rest of their lives. There is a need for new, innovative therapies to treat CKD and ESRD. One promising approach is to rebuild or repair cells, tissues, or organs to restore proper function. This exciting new area of medicine has been termed “Regenerative Medicine.”
We have been working for the last seven years on developing strategies to differentiate human pluripotent stem cells, particularly human embryonic stem (ES) cells and human induced pluripotent stem (iPS) cells, into cells of the kidney lineage for the purposes of kidney regeneration and kidney disease modeling. We believe that the successful derivation of functional kidney cells and structures from human pluripotent stem cells will have an enormous impact on a variety of clinical and translational applications, including kidney tissue bioengineering to replace lost kidney tissue, renal assist devices to treat acute and chronic kidney injury, drug toxicity screening, screening for novel therapeutic agents, and human kidney disease modeling.
Our primary goal was to develop a highly efficient, chemically defined method of differentiating human pluripotent stem cells into kidney tissue. The normal kidney consists of approximately one million nephrons (the functional units of the kidney). During normal kidney development, nephron progenitor cells (NPCs) give rise to nearly all the epithelial cells of nephrons. Nephrons are highly complex structures with multiple segments, each of which performs a set of specific physiologic functions of the kidney such as salt and water regulation and waste product elimination. While previous studies, including work from our own lab, have demonstrated the ability to generate NPCs from human pluripotent stem cells, efficiencies have been low. Furthermore, while these NPCs have been able to differentiate into rudimentary structures of the nephron, none of the prior studies have demonstrated the ability to form a complete, mature nephron from NPCs.
We hypothesized that a much higher efficiency of NPC generation and formation of kidney units could be achieved by following nature’s normal differentiation pathway. We therefore set out to establish a differentiation protocol that would mimic the stages of nephron formation as closely as possible. Our approach in recapitulating the steps of kidney development as precisely as possible resulted in a highly robust recipe for generation of kidney organoids. To our knowledge, this is the most efficient method for generating complex kidney structures from human pluripotent stem cells. The ability to do this using induced pluripotent stem cells, which are derived from skin or blood cells of patients, allows creation of kidney tissue without ethical concerns and allows the tissue to be “personalized”, that is, generated from a particular patient. If in the future the tissue is re-implanted back into the patient, the immune response may then be very limited since the tissue will be recognized as self.
Finally, we tested our nephron organoids for the ability to model human kidney development and drug toxicity to the kidneys. Kidney development is an important medical topic since it has been increasingly recognized that individuals can be born with fewer functional kidney units and these patients are plagued by an increased chance of hypertension and kidney disease in later life. By altering the environment of the NPC-derived renal vesicles with drugs that are known to affect kidney development, we found that the proximal tubule structures are greatly affected. This finding indicated that the nephron organoids are usable for the study of human kidney development, for which no “ex vivo” models currently exist. With this model system we have a tool to evaluate potential therapeutic agents.
In addition, we tested nephron organoids for drug toxicity. The kidney organoids were treated with the nephrotoxicants gentamicin and cisplatin. Both nephrotoxicants induced segment-specific injury to nephron structures within organoids in a pattern that is consistent with what is observed in the clinical setting. Given the individual variation in drug sensitivity in humans, the generation of these nephron organoids from human iPSCs would enable drug testing in a patient-specific manner.
Kidneys are the most commonly transplanted organs, but demand far outweighs supply. While the human kidney does have the capacity to repair itself after injury, it is not able to regenerate new nephrons, the individual functional units that make up the kidney. Human pluripotent stem cells are the only human cells we can grow in the laboratory with the potential to generate new functional kidney tissue. Previously, researchers have been able to differentiate pluripotent stem cells into heart, liver, pancreas, or nerve cells by adding certain chemicals, but it has been challenging to turn these stem cells into kidney. Using normal kidney development as a roadmap, we developed the most efficient method for converting human pluripotent stem cells into kidney stem cells that will give rise to nearly all the functional cells of the kidney. These kidney stem cells organize into mature kidney structures that resemble the structures found in a normal human kidney. This gives us hope that, one day, we might be able to create kidney tissues that could function in a human patient and would be 100% immunocompatible with that patient.
Ryuji Morizane, MD, PhD
Postdoctoral fellow, Renal Division, Brigham and WOmen's Hospital
Albert Q. Lam, MD
Associate Physician, Renal Division, Brigham and Women's Hospital
Joseph V. Bonventre, MD, PhD
Chief, Renal Division, Brigham and Women's Hospital