Two Studies Lead to a New Paradigm for Nephrogenesis

Researchers at Cincinnati Children’s discovered in 2016 that Notch signaling plays an early, critical role in nephrogenesis. Now, a follow-up study shows that Notch signaling is continuously required for nephron formation during differentiation, revising a decade-old understanding of Notch signaling’s role. Here, principal investigator Joo-Seop Park, PhD, talks about what these discoveries mean and how they could impact the development of new therapies for kidney disease.

Describe the initial Notch signaling study, and what it told you about the role of Notch signaling in nephrogenesis.
Park: All nephron tubules are derived from mesenchymal nephron progenitor cells, which disappear around birth. Once these cells are gone, no new nephrons can be formed. Researchers have long known that, whenever nephron progenitor cells decide to form a nephron, they turn off a gene called Six2. In the initial study (Chung et al., 2016), we first tested if nephron progenitor cells can form a nephron without turning off Six2 and found that the answer is no, definitively showing that downregulation of Six2 is essential for nephrogenesis. Then, we tried to determine which signaling pathway is responsible for turning off Six2 and found that it was Notch signaling. This finding showed a novel role of Notch signaling in making nephrons.

How does the follow-up study build on what you learned?
Park: Different parts of a nephron perform unique physiological functions. It was previously thought that Notch signaling regulates nephron segmentation, promoting the formation of a certain segment and repressing the formation of other segments. In our initial study, removing Notch genes in undifferentiated nephron progenitor cells blocked nephron formation before the process of nephron segmentation started. Therefore, in our new study (Chung et al., 2017), we genetically manipulated Notch signaling in differentiating nephron progenitor cells that are undergoing nephron segmentation. When we blocked Notch signaling, no nephron was formed, suggesting that Notch signaling is continuously required for nephron formation during differentiation.

When we activated Notch signaling in differentiating nephron progenitors, properly segmented nephrons still formed, strongly refuting the previous model that Notch signaling promotes the formation of the proximal tubules and represses the formation of the distal tubules. Furthermore, we found that Notch signaling regulates the expression of key transcription factors known to be required for differentiation of nephron progenitors. Taken together, these results suggest that Notch signaling primes nephron progenitors for differentiation rather than directing their cell fates into proximal tubules. This new study revised a decade-old model for the role of Notch signaling in nephron formation.

What are the potential clinical applications?
Park: There is currently a focus on generating functional nephrons in vitro for potential cell replacement therapy. Our finding suggests that Notch signaling efficiently triggers nephron tubule formation, without biasing the formation of a specific segment of the nephron. Our discovery provides a new paradigm for nephrogenesis and brings us closer to generating functional nephrons, a potential treatment for kidney disease.

What are the next steps for your Notch signaling research?
Park: Our lab is interested in how nephron segmentation is regulated. Although this is one of the most important subjects in developmental nephrology, it is currently understudied. Having established that Notch signaling is required for the formation of all nephron segments, we are investigating other signaling pathways that potentially regulate mammalian nephron segmentation.

Both Notch signaling studies appeared in the journal Development.