Browsing by Author "Engle, Andrew"

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    Megakaryocytes: Regulators of Bone Mass and Hematopoiesis
    (Office of the Vice Chancellor for Research, 2016-04-08) Alvarez, Marta B.; Xu, LinLin; Himes, Evan R.; Chitteti, Brahmananda R.; Cheng, Yinghua; Engle, Andrew; Olivos, David; Childress, Paul; Srour, Edward F.; Kacena, Melissa A.
    Emerging evidence demonstrates that megakaryocytes (MK) play a key role in regulating skeletal homeostasis and hematopoiesis. Recent reports show that MK reside in close proximity to hematopoietic stem cells (HSC). Genetic depletion of MK resulted in mitotic activation of HSC suggesting that MK maintain HSC quiescence. Other studies demonstrated that following irradiation, surviving MK migrate to endosteal surfaces where osteoblast (OB) lineage cells dramatically increase and promote engraftment of transplanted HSC. Here we investigated if MK directly impact hematopoiesis or whether they indirectly support HSC function through their interaction with OB-lineage cells. Our data suggests that LSK (Lin-Sca+CD117+, an enriched HSC population) co-cultured with MK and OB generate significantly higher numbers of colony forming cells (HSC function) compared to LSK cocultured with either MK or OB alone. The functionality of this in vitro data was confirmed in vivo with transplantation studies which showed increased engraftment in mice transplanted with LSK cells co-cultured with OB and MK compared to LSK cells co-cultured with OB alone. To test if loss of MK negatively impacts osteoblastogenesis, we generated conditional knockout mice where cMpl, the receptor for the main MK growth factor, thrombopoietin (TPO), was deleted in MK (cMplfl/fl x PF4Cre). Unexpectedly, these mice exhibited a 10-fold increase in platelet numbers, megakaryocytosis, a dramatic expansion of phenotypically defined hematopoietic precursors, and a remarkable 20-fold increase in the bone volume fraction. Collectively, these data indicate that while MK modulate HSC function, this activity is in part mediated through interactions with OB and suggest a complex role for TPO and MK in HSC regulation. While work is needed to further elucidate mechanisms, understanding the coordinated interaction between MK, OB, HSC, and TPO/Mpl should inform the development of novel treatments to enhance HSC recovery following myelosuppressive injuries, as well as bone loss diseases, such as osteoporosis.
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    Thrombopoietin: A Novel Bone Healing Agent
    (Office of the Vice Chancellor for Research, 2013-04-05) Engle, Andrew; Bemenderfer, Thomas; Bethel, Monique; Millikan, Patrick D.; Wessel, Alexander R.; Cheng, Ying-Hua; Wilhite, Jonathan H.; Chu, Tien-Min Gabriel; Kacena, Melissa A.
    Critical-size defects in bones do not heal spontaneously and usually require the use of grafts. Unfortunately, grafts have several limitations. To improve bone formation, many clinicians now use bone morphogenetic proteins (BMP), particularly in spinal fusion, fracture healing, and in critical-size defect regeneration. However, multiple side effects of BMP treatment have been uncovered including increased incidence of cancer. Thus, there is great interest in alternatives that allow for safe and effective bone regeneration. Here we show the ability of thrombopoietin (TPO), the main megakaryocyte growth factor, to heal critical-size femoral defects rodents. 5mm or 4mm segmental defects were created in the femur of Long Evans rats or C57BL/6 mice, respectively. The defects were filled with a novel bioabsorbable scaffold which was loaded with recombinant human TPO, BMP-2, or saline, and held stable by a retrograde 1.6 mm intramedullary Kirschner wire (rats) or 23G needle (mice). Xrays were taken every 3 weeks in rats and weekly in mice. Animal were sacrificed at 15 weeks, at which time micro-computed tomography (μCT) and histological analyses were performed. The results observed in mice and rats were similar. The saline control group did not show bridging callus at any time. Both the BMP-2 and TPO groups healed the defect, although bridging callus was evident at earlier times in the BMP-2 groups. However, the TPO groups showed a much more remodeled and physiologic contour on both Xray and μCT. μCT and histological analysis confirms that compared to BMP-2, TPO-treated specimens have a thicker cortex but smaller diameter and smoother contour. TPO appears to restore the original bone contour by stimulating osteoblastogenesis, allowing for periosteal bridging and stabilization to occur, while simultaneously stimulating osteoclast formation. Thus, TPO may serve as a novel bone healing agent.