The advent of human organoids—3D culture systems derived from stem cells—has made it possible to re-create the architecture and physiology of human organs in great detail, complementing animal models for studying disease.
Derivation of kidney tissue poses unique challenges, though. Kidneys have sophisticated architecture and comprise two embryologically distinct progenitor tissues:
- The ureteric bud (UB) drives the growth and radial organization of the developing fetal kidney, forming the ureter and collecting ducts (CDs)
- The metanephric mesenchyme (MM) is induced by UB-derived signals to form nephrons
Early successes in creating kidney organoids focused on MM tissues, and the organoids contained multiple nephron components. However, they lacked important fundamentals of renal development, such as branching morphogenesis, probably because they lacked UB structures.
Now, Brigham and Women’s Hospital researchers have reported an efficient method for generating functional UB and CD organoids. These tissues demonstrate a range of normal features, including developmental stages, growth and morphology, and cell fate determination. Another important advance is that cells derived from these organoids demonstrate ion transport physiology similar to what is found in the normal kidney. Joseph V. Bonventre, MD, PhD, chief of the Division of Renal Medicine and founding chief of the Division of Engineering in Medicine, and colleagues describe their accomplishments in Nature Biotechnology.
Generating UB Organoids
The researchers created a strategy for differentiating human progenitor stem cells into UB organoids at unprecedented efficiency, without requiring cell sorting or purification.
The UB organoids progressed through typical stages of ureteric bud epithelial development. They manifested complex three-dimensional morphogenesis, including bifurcative branching with high levels of similarity to what happens in normal kidney development, as the research team verified using extensive molecular characterization.
Moreover, when tested in chimeric fetal kidney explant culture using these human cells mixed with mouse developing kidney cells, organoid-derived cells were able to integrate into the UB tips of the progenitor niche.
Generating CD Epithelia
The organoids differentiated into CD epithelia at >95% efficiency, as judged from the results of single-cell RNA sequencing. They represented the inner medullary CD at >85% efficiency.
Functional Human Cell Lines
The CD epithelium is the final site of physiologic modification of urinary composition and serves as the structural system for urinary drainage. The two major functional cell types are:
- Principal cells (PCs), which regulate sodium, potassium, and water homeostasis in the human body
- Intercalated cells (ICs), which maintain acid–base equilibrium
Using the CD organoids, the research team:
- Established PC cell lines that exhibited robust sodium reabsorption mediated by the epithelial sodium channel (ENaC)
- Demonstrated a method to reliably derive these PC cells to ICs in a controlled manner in response to FOXI1 expression
New Directions for Research
The ability to generate UB and CD tissues that recapitulate normal features and function creates many exciting opportunities. These tissues could greatly advance the tissue engineering of more complex kidney structures. They will also advance our understanding of inherited and acquired disorders involving the CD.
For example, lithium-induced injury to the CD cells results in the inability of the kidney to retain water (nephrogenic diabetes insipidus) and the development of chronic kidney disease are of particular interest in psychiatry where lithium is used often, but the exact pathobiological mechanisms are unknown.
Another important implication is the potential to identify novel drug targets. PCs are already targets of two widely used drug classes (ENaC antagonists and mineralocorticoid receptor antagonists), so functional CD cells are expected to have a major influence in advancing pharmacologic discovery and investigation.