Membrane channel connexin 32 maintains Lin-/c-kit+ hematopoietic progenitor cell compartment:: Analysis of the cell cycle

Membrane channel connexin (Cx) forms gap junctions that are implicated in the homeostatic regulation of multicellular systems; thus, hematopoietic cells were assumed not to express Cxs. However, hematopoietic progenitors organize a multicellular system during the primitive stage; thus, the aim of the present study was to determine whether Cx32, a member of the Cx family, may function during the primitive steady-state hematopoiesis in the bone marrow ( BM). First, the numbers of mononuclear cells in the peripheral blood and various hematopoietic progenitor compartments in the BM decreased in Cx32-knockout ( KO) mice. Second, on the contrary, the number of primitive hematopoietic progenitor cells, specifically the Lin (-)/c-kit(+)/Scal(+) fraction, the KSL progenitor cell compartment, also increased in Cx32-KO mice. Third, expression of Cx32 was detected in Lin(-)/c- kit(+) hematopoietic progenitor cells of wild-type mice (0.27% in the BM), whereas it was not detected in unfractionated wildtype BM cells. Furthermore, cell-cycle analysis of the fractionated KSL compartment from Cx32-KO BM showed a higher ratio in the G(2)/M fraction. Taken together, all these results imply that Cx32 is expressed solely in the primitive stem cell compartment, which maintains the stemness of the cells, i.e., being quiescent and noncycling; and once Cx32 is knocked out, these progenitor cells are expected to enter the cell cycle, followed by proliferation and differentiation for maintaining the number of peripheral blood cells.