Chromatin Remodeling Subunit Brm Regulates Hematopoietic Stem Cell Self-Renewal By Limiting Intracellular Valine Levels
Authors: Naidu Samisubbu R , Capitano Maegan L. , Cooper Scott , Huang Xinxin , Broxmeyer Hal E. 
Published: November 2019

Chromatin remodeling complexes facilitate gene expression and control cell fate decisions. The ATPase subunit of chromatin remodeling complex BRG1 is essential for stem cell function, but the role of its paralog Brm remains essentially unknown. To assess a role(s) for Brm in hematopoietic cell regulation in vivo, we studied hematopoietic stem (HSCs) and progenitor cells (HPCs) in bone marrow (BM) of Brm -/- vs. wildtype (WT) control mice. While BM from Brm -/- mice contain increased numbers of rigorously-defined phenotypic populations of long- and short-term repopulating HSCs and granulocyte macrophage progenitors (GMPs) and increased numbers and cycling status of functional HPC (assessed by CFU-GM, BFU-E, and CFU-GEMM colony assays), they were defective in self-renewal capacity upon in vivo serial transplantation using congenic mice (CD45.2+ donor cells, CD45.1+ competitor cells, and F1 (CD45.2+/CD45.1+) recipient mice). Increased numbers of HSCs from Brm-/- BM failed to show competitive advantage over wild type (WT) control BM cells in primary (1°) transplantation in lethally irradiated mice (based on month 4 donor cell chimerism and phenotypically defined HSC numbers). Moreover, 2° and 3° engraftment at 4 months post transplantation each, a measure of HSC self-renewal capacity, revealed much reduced engraftment of donor Brm -/- BM cell chimerism and HSC numbers compared to the extensive 2° and 3° engraftment of control WT BM. No significant differences in myeloid/lymphoid ratios were noted in 1° or 2° engrafted mice, suggesting no differentiation lineage bias of donor Brm -/- BM cells. This demonstrates a critical role for Brm in controlling in vivo self-renewal of mouse BM HSCs. Valine [(2S)-2 amino-3 methylbutanoic acid (C5H11N02)] is implicated in hematopoietic regulation, since depleting dietary valine permitted non-myeloablative mouse HSC transplantation (Taya et. al. Science 354:1152-1155, 2016). Metabolic analysis of lineage negative (lin-) cells demonstrated that valine, but not leucine, levels were very highly elevated in Brm -/- BM cells, thus linking intracellular valine levels with Brm expression. Exogenously added valine significantly increased basal oxygen consumption rates of both total WT BM and WT lin- cells, but not of total or lin-Brm -/- BM cells in vitro (via Seahorse machine analysis). To study effects of valine on HPCs, we assessed the addition of exogenously added valine on mouse BM and human cord blood (CB) cells cultured in the presence of cytokines with either non-dialyzed or dialyzed fetal bovine serum (FBS). Valine, but not leucine, dose-dependently enhanced HPC (CFU-GM, BFU-E, and CFU-GEMM) colony formation and secondary replating capacity of cytokine stimulated CFU-GM and CFU-GEMM derived colonies of normal mouse BM cells in vitro in presence of non-dialyzed FBS; additional enhanced valine effects were noted when dialyzed FBS (lacking valine and other amino acids) was used. Valine also enhanced mouse BM HPC survival in vitro in context of delayed addition of growth factors, and cytokine stimulated (SCF, FL, TPO) ex-vivo expansion of normal mouse BM HSCs and HPCs. Valine enhancement of the above noted functional mouse BM HPC assays in the presence of dialyzed FBS was also apparent with low density and CD34+ purified CB cells, demonstrating that valine effects are not species specific. Our results suggest that valine is an enhancing factor for HSC maintenance of self-renewal capacity and HPC proliferation, and that Brm gene expression limits intracellular valine levels, thereby controlling HSC self-renewal and HPC proliferation. This information is of potential use for future translation to modulate self-renewal of HSCs and survival and proliferation of HPCs for clinical advantage.

Disclosures
No relevant conflicts of interest to declare.