Mesenchymal stem cell (MSC) transplantation has been tested in animal and clinical fracture studies. We have developed a bone-seeking compound, LLP2A-Alendronate (LLP2A-Ale) that augments MSC homing to bone. The purpose of this study was to determine whether treatment with LLP2A-Ale or a combination of LLP2A-Ale and MSCs would accelerate bone healing in a mouse closed fracture model and if the effects are sex dependent. A right mid-femur fracture was induced in two-month-old osterix-mCherry (Osx-mCherry) male and female reporter mice. The mice were subsequently treated with placebo, LLP2A-Ale (500 µg/kg, IV), MSCs derived from wild-type female Osx-mCherry adipose tissue (ADSC, 3 × 105, IV) or ADSC + LLP2A-Ale. In phosphate buffered saline-treated mice, females had higher systemic and surface-based bone formation than males. However, male mice formed a larger callus and had higher volumetric bone mineral density and bone strength than females. LLP2A-Ale treatment increased exogenous MSC homing to the fracture gaps, enhanced incorporation of these cells into callus formation, and stimulated endochondral bone formation. Additionally, higher engraftment of exogenous MSCs in fracture gaps seemed to contribute to overall fracture healing and improved bone strength. These effects were sex-independent. There was a sex-difference in the rate of fracture healing. ADSC and LLP2A-Ale combination treatment was superior to on callus formation, which was independent of sex. Increased mobilization of exogenous MSCs to fracture sites accelerated endochondral bone formation and enhanced bone tissue regeneration.

In this study, we engineered mesenchymal stem cells (MSCs) to over-express basic fibroblast growth factor (bFGF) and evaluated its effects on fracture healing. Adipose-derived mouse MSCs were transduced to express bFGF and green fluorescence protein (ADSC bFGF -GFP). Closed-femoral fractures were performed with osterix-mCherry reporter mice of both sexes. The mice received 3 3 10 5 ADSCs transfected with control vector or bFGF via intramuscular injection within or around the fracture sites. Mice were euthanized at days 7, 14, and 35 to monitor MSC engraftment, osteogenic differentiation, callus formation, and bone strength. Compared to ADSC culture alone, ADSC bFGF increased bFGF expression and higher levels of bFGF and vascular endothelial growth factor (VEGF) in the culture supernatant for up to 14 days. ADSC bFGF treatment increased GFP-labeled MSCs at the fracture gaps and these cells were incorporated into the newly formed callus. quantitative reverse transcription polymerase chain reaction (qRT-PCR) from the callus revealed a 2-to 12-fold increase in the expression of genes associated with nervous system regeneration, angiogenesis, and matrix formation. Compared to the control, ADSC bFGF treatment increased VEGF expression at the periosteal region of the callus, remodeling of collagen into mineralized callus and bone strength. In summary, MSC bFGF accelerated fracture healing by increasing the production of growth factors that stimulated angiogenesis and differentiation of MSCs to osteoblasts that formed new bone and accelerated fracture repair. This novel treatment may reduce the time required for fracture healing. STEM CELLS TRANSLATIONAL MEDICINE 2017;6:1880-1893 SIGNIFICANCE STATEMENTMesenchymal stem cells engineered to express basic fibroblast growth factor may provide a cell-based treatment for fracture repair that provides an environment rich in stem cells, growth factors, and bone matrix proteins over a short time, thereby promoting bone regeneration.

Prolonged glucocorticoid (GC) administration causes secondary osteoporosis (GIOP) and non-traumatic osteonecrosis. LLP2A-Ale is a novel bone seeking compound that recruits mesenchymal stem cells to the bone surface, stimulates bone formation, and increases bone mass. The purpose of this study was to determine if treatment with LLP2A-Ale alone or in combination with parathyroid hormone (PTH) could prevent or treat GIOP in a mouse model. Four month old male Swiss-Webster mice were randomized to a prevention study with placebo, GC (day 1 – 28), and GC + LLP2A-Ale (IV, day 1) or a treatment study with placebo, GC (day 1 – 56), GC + LLP2A-Ale (IV, day 28), GC + PTH, and GC + LLP2A-Ale + PTH (day 28 – 56). Mice were sacrificed on day 28 (prevention study) or on day 56 (treatment study). The study endpoints included bone mass, bone strength, serum markers of bone turnover (P1NP and CTX-I) and angiogenesis (VEGF-A), surface-based bone turnover, and blood vessel density. LLP2A-Ale prevented GC-induced bone loss and increased mechanical strength in the vertebral body (day 28 and 56) and femur (day 56). LLP2A-Ale, PTH, and LLP2A-Ale + PTH treatment significantly increased the mineralizing surface, bone formation rate, mineral apposition rate, double labeled surface and serum P1NP level on day 56. LLP2A-Ale and PTH treatment increased femoral blood vessel density and LLP2A-Ale increased serum VEGF-A on day 28. Therefore, LLP2A-Ale monotherapy could be a potential option to both prevent and treat GC-induced osteoporosis and bone fragility.

The role of the progesterone receptor (PR) in the regulation of sexual dimorphism in bone has yet to be determined. Here we utilized genetic fate mapping and western blotting to demonstrate age-dependent PR expression in the mouse femoral metaphysis and diaphysis. To define sex-dependent and osteoblast stage-specific effects of PR on bone acquisition, we selectively deleted PR at different stages of osteoblast differentiation. We found that when Prx1-Cre mice were crossed with PR floxed mice to generate a MSC conditional KO model (Prx1; PRcKO), the mutant mice developed greater trabecular bone volume with higher mineral apposition rate and bone formation. This may be explained by increased number of MSCs and greater osteogenic potential, particularly in males. Age-related trabecular bone loss was similar between the Prx1; PRcKO mice and their WT littermates in both sexes. Hormone deficiency during the period of rapid bone growth induced rapid trabecular bone loss in both the WT and the Prx1; PRcKO mice in both sexes. No differences in trabecular bone mass was observed when PR was deleted in mature osteoblasts using Bglap- Cre. Also, there were no differences in cortical bone mass in all three PRcKO mice. In conclusion, PR inactivation in early osteoprogenitor cells but not in mature osteoblasts influenced trabecular bone accrual in a sex dependent manner. PR deletion in osteoblast lineage cells did not affect cortical bone mass.

Augmenting peak bone mass is a promising strategy to prevent osteoporosis. A mouse model of global progesterone receptor (PR) ablation showed increased bone mass through a sex-dependent mechanism. Cre-Lox recombination was used to generate a mouse model of osteoprogenitor specific PR inactivation, which recapitulated the high bone mass phenotype. In this work, we employed RNA sequencing analysis to evaluate sex-independent and sex-dependent differences in gene transcription of osteoprogenitors of wild type and PR conditional knockout mice. PR deletion caused marked sex hormone-dependent changes in gene transcription in male mice as compared to wild-type controls. These transcriptional differences revealed dysregulation in pathways involving immunomodulation, osteoclasts, bone anabolism, extracellular matrix interaction, and matrix interaction. These results identified many potential mechanisms that may explain our observed high bone mass phenotype with sex differences when PR was selectively deleted in the MSCs.