Probing the coordination of cell cycle progression and differentiation in hematopoietic stem cells

摘要

ABSTRACT Hematopoietic stem cells (HSCs) generate all mature blood cells in adults. They remain quiescent at homeostasis but when needed, can reenter the cell cycle to either self-renew or differentiate and replenish the hematopoietic system. While the pathways governing HSC self-renewal are just being uncovered, what dictates HSC differentiation remains largely elusive. Recent studies indicate that residence in discrete phases of the cell cycle potentiates differentiation in embryonic and adult stem cells of different origins. However, the impact of cell cycle residence on HSC differentiation has not been directly explored. Here, we report the hematopoietic phenotypes elicited upon genetic inactivation of the chromatin modifier Sin3B. Sin3B serves as a scaffold protein that tethers repressive activities to discrete loci by interacting with sequence-specific transcription factors and histone modifiers. Through its ability to modulate transcription of cell cycle genes Sin3B restricts progression through the early phases of the cell cycle. Sin3B-deleted HSCs retain their self- renewal capacities, but they are unable to properly differentiate. The appropriate expression of lineage specification transcription factors suggests that alterations of the chromatin landscape likely dictate the defective differentiation in Sin3B-/- HSCs. Thus, Sin3B inactivation provides a unique opportunity to study the processes linking cell cycle progression with the generation of a transcriptional environment permissive for HSC differentiation. Our central hypothesis is that the ability of HSCs to differentiate is restricted to a discrete window within the early phase of the cell cycle. We will test the possibility that loss of Sin3B induces spurious progression through early stages of the cell cycle, which is incompatible with proper HSC differentiation, due to alterations in the chromatin landscape. We propose here to: Establish whether Sin3B-dependent restriction of cell cycle progression enables HSC differentiation (aim 1). Using cell cycle reporters, lineage tracing tools and single cell transcriptomic analyses, we will leverage the phenotypes elicited upon Sin3B inactivation to determine the functional relationship between cell cycle progression and differentiation in HSCs; and Determine the molecular bases for Sin3B-dependent differentiation in HSCs (aim 2).Through chromatin accessibility assays and genome-wide mapping of histone marks, we will determine the chromatin features that enable HSCs to respond to pro-differentiation stimuli during cell cycle progression. Based on our preliminary data, we will test the hypothesis that specific signaling pathways we found altered in Sin3B-/- HSCs, including the tonic interferon response, modulate the ability of HSCs to differentiate. Our proposed work will define an understudied molecular link between cell cycle progression and hematopoietic differentiation, pointing to potential approaches to modulate HSC expansion and differentiation for therapeutic purposes.