Loss of miR-199b Impairs Active Cycling of Hematopoietic Stem Cells during Steady-State Hematopoiesis

Several recent studies have showed that dysregulation of microRNA (miRNA) expression in hematopoietic stem cells (HSC) can affect self-renewal of HSCs, and indicated a role for miRNAs in development of acute myeloid leukemia (AML). We and others have reported a significant down-regulation of miR-199b in AML patients. Recently we found that miR-199b is enriched in long-term hematopoietic stem cells (LT-HSC), suggesting that miR199-b may regulate HSCs function. Therefore, to understand the physiologic role of miR-199 in hematopoiesis during homeostasis, we evaluated various hematopoietic stem and progenitor cells (HSPC) populations in mice harboring genetic deletion of miR199b-5p using CRISPR/Cas method. We found that ablation of miR199b resulted in markedly increased frequencies of primitive HSC and MPPs, and analyses of distribution pattern in myeloid progenitor populations showed reduced numbers of common myeloid progenitors (CMPs) biased toward granulocyte-monocyte (GMPs) linage with no changes in megakaryocytic-erythroid progenitors (MEPs). The elevated numbers of HSC and MPPs may indicate that increased proportion of HSC population is actively cycling, thus we analyzed LSK populations for expression of proliferation marker Ki67 along with DNA staining. We found that miR-199b deletion reduces proportion of primitive HSC and MPPs in cell-cycle, which may affect HSC cell self-renewal. Futher cell-cycle analyses revealed that miR-199b null HSCs leave G0 faster to accumulate in G1, but rather do not progress into mitosis, which was recovered upon 5-fluorouracil-induced cytokine burst. These results indicate that loss of miR-199b increases cell cycle duration. To verify that the absence of miR-199b influences proliferation of HSCs we pulsed miR-199b KO and WT mice with BrdU for 16 hours. We found the difference in the cell cycle distribution between HSCs and progenitors, namely reduction of BrdU-positive HSC and MPPs and progression of GMP compartment. These results show that miR-199b deletion decreases HSC active cell cycle by prolonging cell cycle transition during steady-state hematopoiesis and promotes proliferation of myeloid cells. Because quiescent cells only become susceptible to 5-FU during hematopoietic stress, driving them into cycle, we injected 5-FU into miR-199 KO and WT mice once per week until hematopoietic failure occurred. We found that miR199-b KO mice died soon after two subsequent injections, most likely due to the faster HSC exhaust as compared to WT mice. These results show that loss of miR199b produces HSC with reduced quiescence and prolonged cell cycle, however upon stress these cells progress into cell cycle, making them more susceptible for 5-FU treatment. These results demonstrate that miR-199b intrinsically regulates active cycling of HSCs. CFU-S assays showed that miR-199b KO donors showed decreased colonies in spleen, suggesting that miR-199b deletion affects short-term repopulation. In long-term repopulation assay, we observed a significant reduction of HSCs compartment, but elevated numbers of MPPs in host mice transplanted with BM from miR-199 KO mice. This data indicates that loss of miR-199b causes defects in HSC self-renewal and alters HSCs reconstitution potential. To identify potential miR-199b targets in HSCs under steady-state hematopoiesis, we performed a gene profiling in SLAM-HSCs. mRNA levels of several putative miR-199b targets were markedly elevated in miR-199b KO HSCs. These genes are known to be involved in cell adhesion, cell cycle, transcription regulation and chromatin remodeling including Klf12, Tox3 and Cdk18. Our findings reveal a novel functional role for miR-199b in governing HSC maintenance.