Driving lymphoid potential in multipotent hematopoietic progenitors by linker histones Abstract Functionally impaired hematopoietic stem and progenitor cells (HSPCs) often display differentiation skewed toward the myeloid lineage, underlying disease states such as myeloproliferation, inflammation and cancer1-3. The molecular mechanisms responsible for the myeloid versus lymphoid decision within the multipotent HSPCs remains poorly understood, and intervention strategies to boost lymphopoiesis are limited. Chromatin organizes as DNA wrapping around the core nucleosomes, with linker histones binding to the nucleosome dyad. Linker histone binding stabilizes nucleosomes, compacts chromatin, reduces accessibility, and is enriched in heterochromatic regions. We have generated a doxycycline (Dox) inducible H1.0 overexpression transgene (iH1.0). Using this mouse model, we discovered that H1.0 overexpression in HSPCs leads to dramatically expanded lymphoid biased and committed progenitors as well as more mature lymphocytes in circulation. Based on the strong lymphoid differentiation potential of H1.0+ HSPCs, we aim to define a molecular pathway that regulates the lineage output of multipotent HSPCs by linker histones via three specific aims. Aim 1 will test the hypothesis that either a specific linker histone isoform, i.e. H1.0, or the abundance of total H1, drive the lymphoid potential of multipotent HSPCs, by examining the lineage potential of various H1 null and re-expression models. Aim 2 will address how lineage specificality is accomplished, as H1s do not have sequence specificity. Our preliminary data show that H1.0 overexpression leads to sharply reduced chromatin accessibility at the Hepatic Leukemia Factor (Hlf) gene, and reduced Hlf mRNA expression. As Hlf has strong myeloid-promoting effect, we will test whether linker histone promotes lymphoid potency by restricting chromatin accessibility at this myeloid- specifying factors to reduce its expression. As our preliminary data show that H1.0 protein undergoes aspartyl protease-dependent turnover, Aim 3 will test the hypothesis that inhibiting aspartyl proteases prevents the drop in H1.0 protein and sustains lymphoid differentiation potency. The effects of several inhibitors of the HIV protease, an aspartyl protease, on the endogenous H1.0 and lymphopoiesis will be examined. This proposal will yield novel insights on how lymphoid fate specification is controlled by nucleosome/chromatin compaction via linker histones, and approaches to adjust the hematopoietic lineage output for therapeutic gains.