engleski

In vivo Fate Mapping identifies Mesenchymal Progenitor Cells

Adult mesenchymal progenitor cells have enormous potential for use in regenerative medicine. However, the true identity of the progenitors in vivo and its progeny has not been precisely defined. We hypothesize that cells expressing smooth muscle a-actin promoter (aSMA) directed transgene represent mesenchymal progenitors of adult bone tissue. By combining complementary colors in combination with transgenes activating at mature stages of the lineage we confirmed the ability of isolated aSMA+ cells to progress from a progenitor to mature osteoblasts and adipocytes. In order to determine whether aSMA promoter can identify cells with progenitor activity in vivo, we generated aSMACreERT2 transgenic mice and we characterized its expression by crossing it with the Ai9 reporter transgenic line to generate aSMACreERT2/Ai9 (SMA9) mice. In vivo lineage tracing experiments using new bone formation model confirmed the osteogenic phenotype of aSMA+ cells. The labeling pattern of SMA9 cells defined a small population of cells present in primary spongiosa or within the periosteum two days after tamoxifen induction. No cells expressing SMA9 reporter were detected within the bone matrix. Interestingly, 17 days after the tamoxifen activation we found numerous osteoblast and osteocytes labeled by the transgene. Similarly, in fracture healing model aSMA+ cells served as a pool of fibrocartilage and skeletal progenitors. One week following injury we could observe the expansion of SMA9+ cells within bone marrow and periosteum. Interestingly, we detected also the presence of SMA9+ expressing cells in newly formed chondrogenic areas of the fracture callus, and a numerous areas of osteoblasts. We observed that the osteoblasts derived from SMA9 progenitor cells showed intense deposition of new bone, indicated by the tight correlation between calcein label (green) and tdTomato (red) positive cells. To expression of SMA9+ cells co-localized to Col2.3emd expressing mature osteoblasts and osteocytes. This finding was a direct evidence for terminal differentiation of SMA9+ cells into mature osteoblast lineage cells. Our findings provide a novel in vivo identification of defined population of mesenchymal progenitor cells with active role in bone remodeling and regeneration.

mesenchymal cells; differentiation; osteoprogenitor cells; regeneration

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