3012 – DEEPLY QUIESCENT HEMATOPOIETIC STEM CELLS MAINTAIN FUNCTION BY GPRASP2-MEDIATED CELL-SURFACE RECEPTOR DEGRADATION

Abstract

The hematopoietic stem cell (HSC) pool is highly heterogeneous, including a subset of HSCs that rarely contribute to homeostatic hematopoiesis but can be recruited to cycle under stress. Such deeply quiescent HSCs with a low division history perform best when transplanted. However, few tools exist to isolate and interrogate mechanisms regulating the balance between quiescence and activation required to maintain HSC integrity. We recently reported Gprasp2 (G-protein Coupled Receptor (GPCR)-associated Sorting Protein 2) as an HSC regulator during transplantation. Involved in post-endosomal sorting to the lysosome, GPRASP2 is HSC-enriched and heterogeneously expressed in single HSCs. Further, low Gprasp2 (Gprasp2low) expressing HSCs are transcriptionally programmed for lineage-specific differentiation and cell cycling relative to Gprasp2high HSCs. Using our Gprasp2-reporter mouse, we serially transplanted Gprasp2high/low HSCs and found that Gprasp2high HSCs have slower repopulation kinetics with balanced reconstitution and increased, prolonged blood output compared to Gprasp2low HSCs. Single Gprasp2high/low HSC transplantation confirms Gprasp2high clones with delayed yet robust, balanced blood output. Proteomic profiling reveals Gprasp2high HSCs are programmed for quiescence, confirmed by assaying in vivo cycling kinetics. Prospectively, elevated GPRASP2 maintains HSC quiescence by limiting GPCR cell-surface availability via targeted lysosomal degradation. Assessed GPCR candidates show decreased cell surface expression on Gprasp2high HSCs and increased expression on Gprasp2low HSCs. Gprasp2 is hierarchically restricted and heterogeneously expressed in human HSCs, and human Gprasp2high/low HSCs are transcriptionally distinct. Cumulatively, GPRASP2 marks a subset of quiescent, durable repopulating HSCs that preserve function by limiting GPCR cell-surface availability.