دانلود رایگان مقاله حذف مشروط IGF-I در استخوان و تسریع پیوند استخوانی

ABSTRACT

This study evaluated the effects of deficient IGF-I expression in osteocytes on fracture healing. Transgenic mice with conditional knockout (cKO) of Igf1 in osteocytes were generated by crossing Dmp1-Cre mice with Igf1 flox mice. Fractures were created on the mid-shaft of tibia of 12-week-old male cKO mice and wild-type (WT) littermates by three-point bending. At 21 and 28 days post-fracture healing, the increases in cortical bone mineral density, mineral content, bone area, and thickness, as well as sub-cortical bone mineral content at the fracture site were each greater in cKO calluses than in WT calluses. There were 85% decrease in the cartilage area and N2-fold increase in the number of osteoclasts in cKO calluses at 14 days post-fracture, suggesting a more rapid remodeling of endochondral bone. The upregulation of mRNA levels of osteoblast marker genes (cbfa1, alp, Opn, and Ocn) was greater in cKO calluses than in WT calluses. μ-CT analysis suggested an accelerated bony union of the fracture gap in cKO mice. The Sost mRNA level was reduced by 50% and the Bmp2 mRNA level was increased 3-fold in cKO fractures at 14 days post-fracture, but the levels of these two mRNAs in WT fractures were unchanged, suggesting that the accelerated fracture repair may in part act through the Wnt and/or BMP signaling. In conclusion, conditional deletion of Igf1 in osteocytes not only did not impair, but unexpectedly enhanced, bony union of the fracture gap. The accelerated bony union was due in part to upregulation of the Wnt and BMP2 signaling in response to deficient osteocyte-derived IGF-I expression, which in turn favors intramembranous over endochondral bone repair.

4. Discussion

The total economic burden to the United States alone is approximately one billion dollars per year as a consequence of bone fractures. Accordingly, scientific advances to shorten the fracture healing time would have important consequences on both economy and patient morbidity. There is an abundance of information to suggest that the anabolic growth factor, IGF-I, would accelerate fracture healing [16,19,21, 22] and that osteocytes play an essential role in fracture healing [6,8,15]. We undertook the present study to test the hypothesis that osteocytederived IGF-I plays an enhancing role in the fracture repair process by evaluating the consequence of conditional deletion of Igf1 in osteocytes on the healing of tibial fractures in mice. Of all the disciplines of biology, the one that produces the most unexpected results is genetics. This study with the osteocyte Igf1 cKO mice fulfilled that prophecy. Accordingly, we did not find an impaired fracture healing in our cKO mice as expected. Even more surprising was the finding that the fracture healing as assessed by the valid criterion (i.e., bony union of the fracture gap) was actually accelerated. These surprising results were reproducible and were confirmed in repeat experiments. Bony union is a valid histological surrogate of fracture healing. Thus, it appears that conditional deletion of Igf1 in osteocytes not only did not impede, but in fact unexpectedly accelerated, fracture healing. We should also note that although IGF-I is known to be essential for normal cartilage growth and development, the reduced formation of the cartilaginous callus and the accelerated fracture repair seen in cKO mutants were probably not due to reduction in local production of chondrocyte-derived IGF-I in fracture calluses, since there was no apparent reduction in the IGF-I expression in fracture callus chondrocytes between osteocyte Igf1 cKO mutants and corresponding WT littermates (Suppl. Fig. S1B).