The kidney contains a multitude of functional units called nephrons that are built during fetal development from a mesenchymal progenitor population. Although stem-like in many aspects, these progenitors differentiate en masse before or shortly after birth. Variability in the timing of this event determines nephron endowment, with low nephron number being correlated with serious health consequences including hypertension, glomerulosclerosis, and chronic kidney disease in adulthood. The mechanisms that lead to the depletion of CM cells remain elusive. Because the progenitors contribute to their own niche we hypothesized that cell intrinsic changes within the CM dictate the collective exit from the niche. To test this hypothesis, we established a new transplantation assay of nephron progenitors to evaluate engraftment of old and young progenitors into the same young niche. We discovered proliferation rates, exit rates, adhesion properties and prolonged niche residence a were inversely correlated with age. Importantly, a few old progenitors remain in the niche for up to 7 days post engraftment, a net gain of 50% to their lifespan, but only if completely surrounded by young neighbors. We will test three hypotheses: (1) changes in Fgf20 (reduced with age) and/or mTor signaling (increased with age) are driving exit from the niche. (2) We will also test the hypothesis that age-dependent epigenetic changes lower the bar for mesenchymal to epithelial transition. (3) We posits that progressive changes in the transcriptome and translatome drive niche exit. We performed unbiased transcriptome profiling of single nephron progenitors and identified distinct age-dependent transcriptional signatures, showing changes in the translation machinery. We propose to complete the analysis by examining the changes in the translatome. The engraftment assay will determine which manipulations of candidate genes delays niche exit. We will use animal models to test of this opens the door to increasing nephron endowment in situ.

Public Health Relevance

During mammalian development, a specialized group of mesodermal progenitor cells called the metanephric mesenchyme (MM) build metanephric kidneys with a large surplus of nephrons Although stem-cell like in many respects, these embryonic progenitors differentiate en masse before or shortly after birth when nephron numbers reach a species-appropriate limit termed 'nephron endowment', and are not found in adult kidney. Nephron endowment may be determined by the lifespan of renal progenitors; low nephron numbers have been linked to hypertension and other illnesses. It is unclear at present whether progenitor lifespan is regulated by progenitor-extrinsic signals (progenitor being 'evicted') or intrinsic mechanisms (progenitor 'quitting'). A better understanding of how intrinsic and extrinsic cues regulate progenitor longevity is critical for achieving our long-term goal of increasing nephron endowment in premature babies and other at-risk infants. We have recently established that two proteins, Fibroblast growth factors FGF20 and FGF9, are necessary and sufficient to maintain embryonic kidney progenitors in vivo and in vitro. In this application, we seek to leverage this new understanding of the progenitor niche into practical knowledge about the regulation of nephron endowment. We developed a transplantation assay in which we move progenitors from an old niche to a young one and ask if they can engraft. Whereas young progenitors engraft well, only a few old ones do, and only when in contact with young cells as opposed to their own 'class mates'. We will investigate how the young cells 'rejuvenate' the old progenitors by interrogating the molecular networks controlling progenitor aging in their niche. We will evaluate the impact of intrinsic and extrinsic factors on progenitor lifespan and nephron endowment, and aim to learn how to manipulate the network to gain more nephrons. In addition, as the field optimizes conditions for long-term expansion of nephron progenitors in vitro, we will use our engraftment method to assess their ability to make functioning organs and investigate how to best utilize them to expand the repair capacity of the adult organ. These aims have practical implications for both the young and the aging human population. Lacking adult stem cells, the kidney has only a limited repair capacity, and repeated injury exacerbated by conditions such as diabetes can lead to end stage renal disease (ESRD). This risk is greatest for those with low nephron number (such as premature babies). The only two available treatments for ESRD are dialysis, a stopgap measure with significant impact on quality of life, and kidney transplantation, which is limited by organ donation that chronically lags behind demand. Understanding the regulatory network involved in maintenance of competent embryonic renal epithelial progenitors will pave the road to cultivation of progenitors derived from induced pluripotent stem cells (iPS) or from direct reprogramming of adult fibroblasts.

National Institute of Health (NIH)
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Research Project (R01)
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Kidney Molecular Biology and Genitourinary Organ Development (KMBD)
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Hoshizaki, Deborah K
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Cincinnati Children's Hospital Medical Center
United States
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Volovelsky, Oded; Nguyen, Thi; Jarmas, Alison E et al. (2018) Hamartin regulates cessation of mouse nephrogenesis independently of Mtor. Proc Natl Acad Sci U S A 115:5998-6003
Volovelsky, Oded; Kopan, Raphael (2016) Making new kidneys: On the road from science fiction to science fact. Curr Opin Organ Transplant 21:574-580