The Role of Gja1 Gap Junctions in the Bone Marrow Niche.

Abstract The interplay between the stroma and hematopoietic progenitors within the bone marrow niche is critical for the homeostatic regulation of both mesenchymal and blood lineages. Gap junctions play an important role in the communication between hematopoietic supportive cells of the fetal liver and bone marrow stroma. Targeted deletion of the gap junction protein Gja1 (connexin 43) demonstrated a requirement of Gja1 for placental and cardiac function leading to neonatal lethality. Surprisingly, Gja-1 heterozygous animals demonstrated no steady state hematopoietic defects, suggesting that either Gja-1 is not critical for hematopoietic stromal cell communication or that Gja-1 hemizygosity does not functionally alter gap junctions. We have identified an N-ethyl-N-nitrosourea (ENU)-induced strain (Gja1JRT) with a dominant negative mutation in Gja1 which phenocopies the human autosomal dominant disorder oculodentodigital dysplasia (ODDD). ODDD is characterized by a variety of developmental abnormalities including syndactyly, enamel hypoplasia, craniofacial abnormalities and cardiac dysfunction. In addition to these phenotypes, Gja-1JRT mice exhibit decreased bone mass and mechanical strength as well as alterations in hematopoietic progenitor frequencies, abnormalities not previously reported in human ODDD patients (Development132: 4375–4386, 2005). We are currently performing a longitudinal analysis of 2, 4, 8 and 12 month old Gja-1JRT animals, examining hematopoietic including osteoclastic and mesenchymal/osteoblast progenitors to evaluate the dynamic interplay between these two tissues. To date, we have observed alterations in hematopoietic parameters including increased peripheral blood cell number and increased myeloid colony number at 2 months of age. At 4 months of age, Gja-1JRT mice demonstrated a significant decrease in overall clonogenic (CFU-C) cells, while some peripheral blood cell parameters remained elevated. Despite the increases in differentiated myeloid cells and their precursors at 2 months of age, analysis of hematopoietic-derived bone resorbing osteoclasts showed a reduction in osteoclasts at 2 months of age but not at later time points. Analysis of the mesenchymal compartment of the bone marrow demonstrated increased total mesenchymal progenitors (CFU-F) and osteoprogenitors (CFU-O) at 8 and 12 months of age. Thus, alterations in hematopoietic and osteoclastic cell frequencies preceded alterations in the bone marrow mesenchymal compartment in the Gja-1JRT mice. In addition to a complete longitudinal analysis, we will report on the alterations in trabecular bone, which create the niche microenvironment, and the impact of these changes on the hematopoietic stem cell frequency.