Regionally Restricted Hox Function in Adult Bone Marrow Multipotent Mesenchymal Stem/Stromal Cells

Abstract

Summary Posterior Hox genes (Hox9-13) are critical for patterning the limb skeleton along the proximodistal axis during embryonic development. Here we show that Hox11 paralogous genes, which developmentally pattern the zeugopod (radius/ulna and tibia/fibula), remain regionally expressed in the adult skeleton. Using Hoxa11eGFP reporter mice, we demonstrate expression exclusively in multi-potent mesenchymal stromal cells (MSCs) in the bone marrow of the adult zeugopod. Hox-positive cells express PDGFRα and CD51, are marked by LepR-Cre, exhibit CFU-F activity and tri-lineage differentiate in vitro. Loss of Hox11 function leads to fracture repair defects, including reduced cartilage formation and delayed ossification. Hox mutant cells are defective in osteoblastic and chondrogenic differentiation in tri-lineage differentiation experiments and these defects are zeugopod-specific. In the stylopod (humerus and femur) and sternum, BM-MSCs express other regionally restricted Hox genes and femur fractures heal normally in Hox11 mutants. Together, our data supports regional Hox expression and function in skeletal MSCs.

Skeletal Stem Cells: Origins, Functions, and Uncertainties

“…Interestingly, Hoxa11 is expressed with the same regional specificity in adult mice and is associated with SSCs [53]. A GFP-knockin genetic approach was used to show that Hoxa11+ cells appear early in the developing limb bud in the outer perichondrium around cartilaginous rudiments.…”

“…As the zeugopod anlage develops, Hoxa11 expression is restricted to the outer perichondrial stromal cells outside the osteoblast layer that surrounds the developing elements [54, 55]. Hox11 persists in the periosteum and also within the BM throughout adulthood [53]. Marrow Hoxa11+ cells are detected within a non-hematopoietic, non-endothelial stromal cell population that is positive for PDGFRα, CD51, and LepR and capable of tri-lineage differentiation and self-renewal ex vivo.…”

“…Marrow Hoxa11+ cells are detected within a non-hematopoietic, non-endothelial stromal cell population that is positive for PDGFRα, CD51, and LepR and capable of tri-lineage differentiation and self-renewal ex vivo. When transplanted into a fracture callus, these cells contribute to cartilage and bone while maintaining their regional specificity[53]. When Hox11 genes were mutated (Hoxa, b and c11), fracture healing in these zeugopod-derived elements was defective while healing the humerus was normal.…”

“…When Hox11 genes were mutated (Hoxa, b and c11), fracture healing in these zeugopod-derived elements was defective while healing the humerus was normal. In addition, SSCs from mutants were unable to undergo proper differentiation to give rise cell types essential for injury repair, and exhibited skewed adipogenic differentiation ex vivo [53]. Additionally, BM cells positive for Hoxa11EGFP, PDGFRα, and CD51 showed a 3-fold enrichment of clonogenic potential compared with PDGFRα+ CD51+ cells.…”

HOX transcription factors and the prostate tumor microenvironment

“…The subsequent differentiation of HSCs along different lineages and ultimately to mature blood cells is also dependent on distinct patterns of HOX gene expression [16] . Other adult processes that are known to be at least partly dependent on HOX genes include the menstrual cycle [17] and the differentiation of mesenchymal stem cells [18] . Over the last 20 years it has become increasing clear that HOX genes are also very highly dysregulated, and usually strongly over expressed in a wide range of haematological and solid malignancies compared to the cells form which these cancers originate [19,20] .…”