Connexin-43 gap junctions are involved in multiconnexin-expressing stromal support of hemopoietic progenitors and stem cells.

Gap junctions (GJs) provide for a unique system of intercellular communication (IC) allowing rapid transport of small molecules from cell to cell. GJs are formed by a large family of proteins named connexins (Cxs). Cx43 has been considered as the predominantly expressed Cx by hematopoietic-supporting stroma. To investigate the role of the Cx family in hemopoiesis, we analyzed the expression of 11 different Cx species in different stromal cell lines derived from murine bone marrow (BM) or fetal liver (FL). We found that up to 5 Cxs are expressed in FL stromal cells (Cx43, Cx45, Cx30.3, Cx31, and Cx31.1), whereas only Cx43, Cx45, and Cx31 were clearly detectable in BM stromal cells. In vivo, the Cx43-deficient 14.5- to 15-day FL cobblestone area-forming cells (CAFC)-week 1-4 and colony-forming unit contents were 26%-38% and 39%-47% lower than in their wild-type counterparts, respectively. The reintroduction of the Cx43 gene into Cx43-deficient FL stromal cells was able to restore their diminished IC to the level of the wild-type FL stromal cells. In addition, these Cx43-reintroduced stromal cells showed an increased support ability (3.7-fold) for CAFC-week 1 in normal mouse BM and 5-fold higher supportive ability for CAFC-week 4 in 5-fluorouracil-treated BM cells as compared with Cx43-deficient FL stromal cells. These findings suggest that stromal Cx43-mediated IC, although not responsible for all GJ-mediated IC of stromal cells, plays a role in the supportive ability for hemopoietic progenitors and stem cells. (Blood. 2000;96:498-505)

Seeding efficiency of primitive human hematopoietic cells in nonobese diabetic/severe combined immune deficiency mice: implications for stem cell frequency assessment.

Nonobese diabetic/severe combined immune deficiency (NOD/SCID) mouse repopulating cells (SRC) have been proposed to represent a more primitive human stem cell subset than the cobblestone area-forming cell (CAFC) week (wk) 6 or the long-term culture-initiating cell (LTC-IC) wk 5 on the basis of their difference in frequency, phenotype, transfectibility, and multilineage outgrowth potential in immunodeficient recipients. We have assessed the percentage of various progenitor cell populations (colony-forming cell [CFC] and CAFC subsets) contained in unsorted NOD/SCID BM nucleated cells (nc), human umbilical cord blood (UCB) nc, bone marrow (BM) nc, peripheral blood stem cells (PBSC), and CD34(+) selected UCB nc, seeding in the BM and spleen of NOD/SCID mice within 24 hours after transplantation. The seeding efficiency of NOD/SCID BM CAFC wk 5 was median (range) in the spleen 2.9% (0.7% to 4.0%) and in the total BM 8.7% (2.0% to 9.2%). For human unsorted UCB nc, BM nc, PBSC, and CD34(+) UCB cells, the seeding efficiency for CAFC wk 6 in the BM of NOD/SCID mice was 4.4% (3.5% to 6.3%), 0.8% (0.3% to 1.7%), 5.3% (1. 4% to 13.6%), and 4.4% (3.5% to 6.3%), respectively. Using flow cytometry, the percentage CD34(+) UCB cells retrieved from the BM of sublethally or supralethally irradiated NOD/SCID mice was 2.3 (1.4 to 2.8) and 2.5 (1.6 to 2.7), respectively. Because we did not observe any significant differences in the seeding efficiencies of the various stem cell subsets, it may be assumed that the SRC seeding efficiency in NOD/SCID mice is similarly low. Our data indicate that the seeding efficiency of a graft can be of great influence when assessing stem cell frequencies in in vivo repopulation assays.