p38α Is Required for Proper Proliferation of Hematopoietic Stem Cells during Stress

Hematopoietic stem cells (HSCs) are capable of self-renewal and multilineage-differentiation during lifespan. HSCs are maintained in a quiescent state to avoid cellular senescence. Previous reports utilizing pharmacological inhibitors or shRNAs against p38MAPK suggest a pivotal role of p38MAPK-pRb-Ink4a signaling in induction of HSC senescence by hematological stress or chronological aging. However, no genetic evidence exists for p38MAPK-mediated cellular senescence in vivo. Here we report unexpected roles of the dominant isoform of p38MAPK family, p38α, in adult hematological system. p38MAPK has four isozymes, α, β, γ and δ. Among them, p38α isozyme was highly expressed in various bone marrow hematopoietic cells, and the expression level of p38α in HSCs was higher than differentiated cells (p<0.01). Phosphorylation of p38MAPK was mainly observed in multipotent progenitors but not in HSCs in steady-state hematopoiesis, in addition, physiological aging (1 year old mouse bone marrow) did not affect phosphorylation status of p38MAPK in steady state. In contrast, p38MAPK was phosphorylated in HSCs after transplantation or 5-FU treatment. Mean fluorescence index (MFI) of phosphorylation of p38MAPK in HSCs is significantly higher at day 3 post 5-FU treatment (250 mg/kg) than steady-state. MFI of phosphorylation of p38MAPK in HSCs was higher at day 1 post transplantation than steady state, and returned to normal at day 7 post transplantation. These results showed phosphorylation of p38α was immediately induced after hematopoietic demand.

p38α-deficient embryos die due to defective erythropoiesis in a non-cell-autonomous manner. Thus, we used a conditional knockout model; CAG-CreERT2:p38α^fl/fl mouse to analyze the effects of p38α on adult hematopoiesis and HSCs. Expression level of p16^Ink4a, one of the cellular senescence markers, was not significantly different between p38α-deficient mice and wild-type mice. Treatment of p38α-deficient mice with 5-FU exhibited defective recovery of hematopoiesis, and the survival rate were lower in p38α-deficient mice than wild-type (42.9%, N=7, p38α-deficient mice, vs 100%, wild-type, N=6, p<0.05). Loss of p38α in HSCs showed a defective transplantation capacity. Inducible loss of p38α in bone marrow chimera resulted in a gradual loss of peripheral blood chimerism of p38α-deficient cells. In addition, short-term BrdU incorporation assay showed that the cell cycle progression of p38α-deficient HSCs was suppressed (BrdU positive rate; 3.5±2.2%, N=9, p38α-deficient cells vs 6.5±2.6%, N=5, wild-type, p<0.05). Therefore, hematopoietic function was obviously lowered in p38α-deficient HSCs during hematopoietic stresses. These observations collectively support the requirement of p38α for proper proliferation of HSCs during stress hematopoiesis.