Tetraspanin CD82 Regulates Hematopoietic Stem and Progenitor Cell Quiescence and Regeneration

The significant cellular demand of the hematopoietic system is maintained by a rare pool of tissue-specific, hematopoietic stem and progenitor cells (HSPCs) that are primarily found in a quiescent state. Upon hemopoietic stresses, such as significant bleeding, overwhelming infection, and myelosuppressive therapy, HSPCs undergo rapid cell cycle activation, but ultimately must return to quiescence to prevent exhaustion of the hematopoietic system. Emerging evidence from our laboratory suggests that the tetraspanin CD82 plays a critical role in the regulation of HSPC quiescence and activation. Tetraspanins are membrane scaffold proteins with the ability to modulate signaling through the formation of tetraspanin-enriched microdomains, which organize membrane signaling receptors and intracellular signaling molecules critical for propagating downstream signaling. Previous data from our laboratory identified a role for CD82 in HSPC quiescence, where we find a reduction in long term-HSCs in global CD82KO mice, resulting from increased HSPC activation and a reduction of quiescent G ₀ cells. In the present study, we test the hypothesis that CD82 expression promotes HSPC return to quiescence following hematopoietic stress, by regulating the activation of TGF-β signaling. To investigate the impact of CD82 expression on hematopoietic regeneration under stress, we treated WT and CD82KO mice with 2 doses of 200mg/kg of chemotherapy agent 5-FU and measured overall survival. Interestingly, we find that CD82KO mice have significantly longer overall survival compared to their WT counterparts. Moreover, we find an increase in peripheral blood HSPCs in CD82KO mice during the early recovery period from 5FU treatment suggestive of enhanced cell activation upon stress and effective regeneration post injury. Mechanistically, the multifunctional cytokine TGF-β plays an essential role in supporting HSPC quiescence and activation, as one of the most potent inhibitors of HSPC growth both in vitroand in vivo. To investigate whether CD82 plays a role in TGF-β signaling, we stimulated WT and CD82KO HSPCs with TGF-β and measured nuclear translocation of SMAD2/3, a downstream effector of TGF-β activation. Confocal imaging demonstrates that CD82KO HSPCs have decreased nuclear translocation of SMAD2/3 upon TGF-β activation, consistent with reduced TGF-β signaling. Similarly, primary human CD34 ⁺HSPCs sorted for low and high CD82 expression also have decreased and increased SMAD2/3 nuclear translocation, respectively. Moreover, gene expression analysis of cell cycle regulators identified the reduced expression of p57, a downstream gene target of TGF-β signaling in HSPCs harvested from CD82KO mice. Currently, we are investigating the impact of the CD82 scaffold on TGF-β signaling of HSPCs localized within the context of the bone marrow niche, by analyzing bone marrow sections from mice injected with HSPC-like cells differentially expressing CD82. Together, these data suggest that CD82 regulates HSPC quiescence and activation through modulation of TGFβ signaling. Future studies will focus on investigating how CD82 modulates local TGFβ signaling within the niche.