Elucidating the role of translational control in hematopoietic stem cell quiescence - PROJECT SUMMARY Hematopoietic stem cells (HSCs) maintain blood production throughout an organism’s lifespan. Thus, HSCs must balance differentiation with self-renewal to protect stemness. To this end, HSCs are largely quiescent. Quiescence protects HSCs from genotoxic insults and functional exhaustion. Understanding the molecular mechanisms controlling HSC quiescence will yield valuable insights into mechanisms underlying hematopoietic disorders. While post-transcriptional regulation is increasingly recognized as important for hematopoietic cell fate specification, its role in HSC quiescence remains poorly understood. Preliminary data indicate that the post- transcriptional regulator DDX6 is important to maintain HSC quiescence. DDX6, an RNA helicase, orchestrates translational suppression and mRNA sequestration in cytoplasmic condensates known as P-bodies. Notably, Ddx6 knockout mice have normal mature blood cell populations but exhibit loss of HSC quiescence. Accordingly, Ddx6−⁄− HSCs exhibit increased proliferation and mitochondrial numbers, which results in diminished fitness during serial, competitive transplants. Mechanistically, initial analysis of DDX6-targeted transcripts in P-bodies revealed an enrichment for untranslated mRNAs encoding crucial regulators involved in exiting quiescence. Together, these data lead to our central hypothesis that Ddx6-mediated RNA processing is pivotal in protecting HSC quiescence and function. Aim 1 will test the hypothesis that Ddx6 is required for in situ stress hematopoiesis by challenging Ddx6−⁄− HSCs in vivo using regenerative and infectious stressors. Aim 2 will elucidate the mRNAs translationally suppressed by Ddx6 in HSCs and characterize the HSC translatome in situ both with and without Ddx6 deletion. Additionally, we will investigate the functional role of Ddx6 targeted transcripts in vivo, specifically Myc. The overall goal of this project is to elucidate a new mechanism controlling HSC function at the molecular and cellular levels and to advance strategies for treating hematologic diseases. This fellowship application is sponsored by Bruno Di Stefano, Ph.D., an expert in post-transcriptional gene regulation in stem cells, and Katherine King, M.D., Ph.D., a physician-scientist and expert in hematopoietic stem cells, who will provide close guidance throughout the fellowship period. The training plan includes strategies to 1) Learn from accomplished scientists and physician-scientists that will advise the applicant through her training goals; 2) Undergo rigorous scientific training in hematopoiesis and gene regulation; 3) Experience opportunities to improve scientific communication skills and expand professional networks; 4) Advance the applicant’s clinical training, especially in hematology. The clinical and scientific training environment at Baylor College of Medicine is within the Texas Medical Center, the largest medical research complex in the world. This environment is ideal to foster the applicant’s scientific and clinical growth toward a career as a physician-scientist investigating the role of post-transcriptional regulation in hemopoietic stem cell function and dysfunction.
