MicroRNA199b regulates mouse hematopoietic stem cells maintenance

The preservation of hematopoieticstem cell pool in bone marrow (BM) is crucial for sustained hematopoiesisin adults. Studies assessing adult hematopoieticstem cells functionality had been shown that for example loss of quiescence impairs hematopoieticstem cells maintenance. Although, miR‐199b is frequently down‐regulated in acute myeloid leukemia, its role in hematopoieticstem cells quiescence, self‐renewal and differentiation is poorly understood. Our laboratory investigated the role of miR‐199b in hematopoieticstem and progenitor cells (HSPCs) fate using miR‐199b‐5p global deletion mouse model. Characterization of miR‐199b expression pattern among normal HSPC populations revealed that miR‐199b is enriched in LT‐HSCsand reduced upon myeloablativestress, suggesting its role in HSCsmaintenance. Indeed, our results reveal that loss of miR‐199b5p results in imbalance between long‐term hematopoieticstem cells (LT‐HSCs), short‐term hematopoieticstem cells (ST‐HSCs) and multipotentprogenitors (MMPs) pool. We found that during homeostasis, miR‐199b‐null HSCshave reduced capacity to maintain quiescent state and exhibit cell‐cycle deregulation. Cell cycle analyses showed that attenuation of miR‐199b controls HSCspool, causing defects in G1‐S transition of cell cycle, without significant changes in apoptosis. This might be due to increased differentiation of LT‐HSCs into MPPs. Indeed, cell differentiation assay in vitro showed that FACS‐sorted LT‐HSCs(LineagenegSca1posc‐Kitpos CD48neg CD150pos) lacking miR‐199b have increased differentiation potential into MPP in the presence of early cytokines. In addition, differentiation assays in vitro in FACS‐sorted LSK population of 52 weeks old miR‐199b KO mice revealed that loss of miR‐199b promotes accumulation of GMP‐like progenitors but decreases lymphoid differentiation, suggesting that miR199b may regulate age‐related pathway. We used noncompetitive repopulation studies to show that overall BM donor cellularitywas markedly elevated in the absence of miR‐199b among HSPCs, committed progenitors and mature myeloid but not lymphoid cell compartments. This may suggest that miR‐199b‐null LT‐HSC render enhanced self‐renewal capacity upon regeneration demand yet promoting myeloid reconstitution. Moreover, when we challenged the self‐renewal potential of miR‐199b‐null LT‐HSC by a secondary BM transplantation of unfractionatedBM cells from primary recipients into secondary hosts, changes in PB reconstitution were dramatic. Gating for HSPCspopulations in the BM of secondary recipients in 24 weeks after BMT revealed that levels of LT‐HSC were similar between recipients reconstituted with wild‐type and miR‐199b‐KO chimeras, whereas miR‐199b‐null HSCscontributed relatively more into MPPs. Our data identify that attenuation of miR‐199b leads to loss of quiescence and premature differentiation of HSCs. These findings indicate that loss of miR‐199b promotes signals that govern differentiation of LT‐HSC to MPP leading to accumulation of highly proliferativeprogenitors during long‐term reconstitution. Hematopoieticregeneration via repopulation studies also revealed that miR‐199b‐deficient HSPCshave a lineage skewing potential toward myeloid lineage or clonalmyeloid bias, a hallmark of aging HSCs, implicating a regulatory role for miR‐199b in hematopoietic aging