ERK Signaling Is Essential for Macrophage Development

Macrophages depend on colony stimulating factor 1 (also known as M-CSF) for their growth and differentiation, but the requirements for intracellular signals that lead to macrophage differentiation and function remain unclear. M-CSF is known to activate ERK1 and ERK2, but the importance of this signaling pathway in macrophage development is unknown. In these studies, we characterized a novel model of Erk1 -/- Erk2 flox/flox Lyz2 Cre/Cre mice in which the ERK2 isoform is deleted from macrophages in the background of global ERK1 deficiency. Cultures of M-CSF-stimulated bone marrow precursors from these mice yielded reduced numbers of macrophages. Whereas macrophages developing from M-CSF-stimulated bone marrow of Erk2 flox/flox Lyz2 Cre/Cre mice showed essentially complete loss of ERK2 expression, the reduced number of macrophages that develop from Erk1 -/- Erk2 flox/flox Lyz2 Cre/Cre bone marrow show retention of ERK2 expression, indicating selective outgrowth of a small proportion of precursors in which Cre-mediated deletion failed to occur. The bone marrow of Erk1 -/- Erk2 flox/flox Lyz2 Cre/Cre mice was enriched for CD11b + myeloid cells, CD11b hi Gr-1 hi neutrophils, Lin - c-Kit + Sca–1 + hematopoietic stem cells, and Lin - c-Kit + CD34 + CD16/32 + granulocyte-macrophage progenitors. Culture of bone marrow Lin - cells under myeloid-stimulating conditions yielded reduced numbers of monocytes. Collectively, these data indicate that the defect in production of macrophages is not due to a reduced number of progenitors, but rather due to reduced ability of progenitors to proliferate and produce macrophages in response to M-CSF-triggered ERK signaling. Macrophages from Erk1 -/- Erk2 flox/flox Lyz2 Cre/Cre bone marrow showed reduced induction of M-CSF-regulated genes that depend on the ERK pathway for their expression. These data demonstrate that ERK1/ERK2 play a critical role in driving M-CSF-dependent proliferation of bone marrow progenitors for production of macrophages.