Direct Interaction Between CD34⁺ Hematopoietic Stem Cells and Mesenchymal Stem Cells Reciprocally Preserves Stemness

Background/Objectives: A specialized microenvironment in the bone marrow, composed of stromal cells including mesenchymal stem cells (MSCs), supports hematopoietic stem cell (HSC) self-renewal, and differentiation bands play an important role in leukemia development and progression. The reciprocal direct interaction between MSCs and CD34⁺ HSCs under physiological and pathological conditions is yet to be fully characterized. Methods: Here, we established a direct co-culture model between MSCs and CD34⁺ HSCs or MSCs and acute myeloid leukemia cells (THP-1, Molm-13, and primary cells from patients) to study heterocellular communication. Results: Following MSCs-CD34⁺ HSCs co-culture, the expression of adhesion markers N-Cadherin and connexin 43 increased in both cell types, forming gap junction channels. Moreover, the clonogenic potential of CD34⁺ HSCs was increased. However, direct contact of acute myeloid leukemia cells with MSCs reduced the expression levels of connexin 43 and N-Cadherin in MSCs. The impairment in gap junction formation may potentially be due to a defect in the acute myeloid leukemia-derived MSCs. Interestingly, CD34⁺ HSCs and acute myeloid leukemia cell lines attenuated MSC osteoblastic differentiation upon prolonged direct cell–cell contact. Conclusions: In conclusion, under physiological conditions, connexin 43 and N-Cadherin interaction preserves stemness of both CD34⁺ HSCs and MSCs, a process that is compromised in acute myeloid leukemia, pointing to the possible role of gap junctions in modulating stemness.