Stem cells reside in specialized niches that support their long-term maintenance. Disruption of the niche, for example due to aging, injury, or genetic mutations, can lead to declines in stem cells that result in an ability to maintain and repair tissues. Compared to our understanding of the stem cells that maintain and repair skeletal tissues, we know much less about the cell types that constitute the niche for skeletal stem cells. In this proposal, I have taken an innovative single-cell transcriptomic approach to characterize potential niche cells in the zebrafish face. Preliminary data have uncovered molecular signatures of putative niche cells, including homologs of genes that when mutated cause craniofacial malformations in humans: WNT5A (Robinow Syndrome), HGD (Alkaptonuria, aka ?black bone disease?), and PAH (maternal Phenylketonuria). This analysis suggests a role for Wnt5a as a niche-derived paracrine factor for skeletal stem cell maintenance, and an unexpected local role for phenylalanine catabolism, outside the liver, in protecting craniofacial skeletal tissues.
The aims outlined in this proposal leverage powerful genomic, genetic, and high-resolution imaging approaches to test that niche cells are located in the outer periosteum in both fish and mammals (Aim 1), that niche cells and Wnt5a secretion act to maintain skeletal stem cells in the inner periosteum (Aim 2), and that efficient breakdown of phenylalanine in niche cells is critical for craniofacial skeletal health (Aim 3). Completion of these aims will inform how defects in the niche result in both developmental defects in the face (such as in Robinow Syndrome) and a failure to maintain the facial skeleton (such as with the severe arthritis seen in Alkaptonuria). These findings may also lead to future therapies aimed at better maintaining and repairing the skeleton through modulating the niche. The project and mentorship plan outlined in this proposal were designed to lay the groundwork for my career goal of obtaining a position as a tenure-track Assistant Professor at a top-tier academic research institution. During the K99 phase, I will receive mentorship in zebrafish biology from Gage Crump and crucial mouse training from my co-mentor Yang Chai. Regular interactions with clinician-scientists Shoji Yano and Kathryn Moseley, experts in Phenylketonuria and metabolic diseases and members of my advisory committee, will allow me to connect findings to human health. Career development activities at USC will prepare me for the transition to an independent faculty position during the R00 phase. As USC hosts one of the most experienced communities of craniofacial and skeletal biologists in the country, there are few better places to conduct this research and acquire the training to achieve my career goals.

Public Health Relevance

Stem cells reside in specialized microenvironments or ?niches? that maintain stem cell function for the growth, maintenance, and repair of tissues. My project investigates a novel population of cells in the vertebrate face that may function as a niche to support skeletal stem cells. As several of the genes enriched in this niche cause craniofacial malformations when mutated in humans, my work will provide new insights into how proper niche function ensures normal development and maintenance of the head skeleton.

National Institute of Health (NIH)
National Institute of Dental & Craniofacial Research (NIDCR)
Career Transition Award (K99)
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NIDR Special Grants Review Committee (DSR)
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Frieden, Leslie A
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University of Southern California
Other Basic Sciences
Schools of Medicine
Los Angeles
United States
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