The ureteric bud is an essential embryonic structure for the development of the kidneys, and its defects are attributed to the majority of developmental defects in the kidney. Scientists have attempted to create the ureteric bud from stem cells, but with limited results, hampering our understanding of kidney development and related diseases.

A new study by the Kenji Osafune laboratory reports how iPS cell technology can be used to produce ureteric bud organoids that show the necessary characteristics leading to formation of the kidney collecting ducts, which is responsible for collecting and passing urine. The report also goes on to show how the organoids can be used to study multicystic dysplastic kidney (MCDK).

Congenital anomalies of the kidney and urinary tract (CAKUT) are present from birth and estimated to occur in at least one of every 500 births. The effects of CAKUT can be extreme and obvious, like the absence of a kidney, or milder but still serious, like the obstruction of urine flow. MCDK is one type of CAKUT. Normally affecting only one of the two kidneys, the unhealthy kidney has irregular cysts and no function, eventually disappearing from the body, while the healthy kidney grows abnormally large to compensate.

Because CAKUT describe congenital disorders, understanding the normal developmental course of the kidneys is necessary to understand how the pathology develops. However, says Osafune, there are no effective ways to recapitulate kidney growth in the laboratory.

"The ureteric bud generated from human stem cells show limited developmental ability and limited branching. Branching is critical for the kidney to take its shape and exchange fluids."

The successful derivation of ureteric bud organoids with branching ability in the new study comes from modifying previous work done by the lab.

"We have divided our differentiation protocol into multiple stages. We found that prolonging the retinoic acid exposure and reducing the Matrigel content in some stages were key for the branching," explains Dr. Shin-Ichi Mae, a specially-appointed professor in the Osafune lab.

The ureteric bud organoids expressed the tip and trunk domains, epithelial polarity and tubular lumen seen in normal ureteric bud development. Further, a transcriptome analysis confirmed gene expressions consistent of ureteric bud.

Separating and culturing the tips from the organoids resulted in new ureteric bud organoids, confirming the repeated branching potential, which is crucial for kidney development. Further, trunk cells could produce tip cells, showing that both cell types could be the source of repeated ureteric bud branching. The organoids could also be differentiated into progenitor cells for the collecting duct.

To determine if their ureteric bud organoids can be used to study CAKUT, the researchers forced a mutation in the HNF1β gene, which is associated with MCDK, by using the CRISPR-Cas9 gene editing system in iPS cells. The resulting ureteric bud organoids showed fewer branching regions, smaller tip regions and lost polarity compared with organoids made from iPS cells without the mutation.

Osafune says that with added improvement to the protocol, the derived ureteric bud organoids will contribute greatly to the study and treatment of CAKUT.

"The branching of the organoids is very good, but the collecting ducts are immature. Once we optimize the differentiation conditions, I expect these organoids will contribute to our understanding of kidney development and congenital diseases," he says.

• doi: 10.1016/j.celrep.2020.107963