Overview

Effects of Simulated Weightlessness on the Repair of Lower-Limb Bone Fractures and on the Number of Bone-Derived Stem Cells

Principal Investigator:
Ronald J. Midura, Ph.D.

Organization:
The Cleveland Clinic

During prolonged spaceflight, microgravity reduces astronauts capacity to form new bone tissue and diminishes bone mass, leading to increased skeletal fragility. Dr. Ronald J. Midura is employing imaging to examine bone formation and fracture healing during simulated weightlessness. These studies will include an analysis of the presence and functionality of bone-forming marrow progenitor cell populations. In addition, he is researching the efficacy of two FDA-approved bone disease treatments as practical countermeasures to prevent bone loss and promote fracture healing during simulated weightlessness.

NASA Taskbook Entry

Technical Summary

Specific Aims

Assess the rate and extent of fracture healing in osteotomized rats in simulated weightlessness versus normal gravity.
Assess the numbers of progenitor cell colony-forming units (CFU) and osteogenic cells per CFU in bone marrow and periosteum tissues from osteotomized rats in simulated weightlessness versus normal gravity.
Determine the rate and extent of fracture healing in osteotomized rats in simulated weightlessness when treated with PTH or bisphosphonate therapy.
Assess the numbers of progenitor cell CFU and osteogenic cells per CFU in bone marrow and periosteal tissues from osteotomized rats in simulated weightlessness undergoing either PTH or bisphosphonate therapy.

In Year 1, rats experiencing normal gravity (weight-bearing or WB) exhibited a spontaneous repair of cortical bone trauma within a five-week healing period. Intermittent PTH therapy enhanced this WB repair process by:

increasing the rate of hard callus formation and peak hard callus volume compared to vehicle-control rats;
increasing bone formation rates (BFR); and
increasing the number of osteoprogenitors within bone marrow tissue.

In Year 2, rats experiencing hindlimb unloading (non-weight-bearing or NWB) exhibited a greatly diminished level of spontaneous repair of bone trauma as compared to the WB group within the same healing period. This was reflected in a diminished maximum hard callus volume and a rate of hard callus formation that were both only ~20 percent of those of the WB group. Reacquisition of a WB status equal in time to that of the period of simulated weightlessness prior to bone trauma (NWB-WB group) did not substantially improve the bone-healing response. In this case, this group exhibited a maximum hard callus volume that was only 34 percent of, and a rate of hard callus formation that was only 28 percent of, the WB group. Both NWB and NWB-WB groups exhibited low bending strength values at their respective callus sites by five-weeks post-op (0.28 0.05 N/m) in stark contrast to the bending strength of the WB callus (1.75 0.35 N/m). Histology revealed that 46 percent of the NWB group fibulae were reconnected by a fibrous soft tissue union (non-union), while all of the WB group specimens were reconnected by a bony tissue union. Lastly, NWB reduced the number of marrow progenitor cells and osteoprogenitors by 90 percent from the levels exhibited by WB bone specimens.

In Year 3, intermittent PTH therapy (80 ug/kg BW) was shown to partially reverse the deleterious effects of NWB on cortical bone trauma healing. Only 12.5 percent of NWB-PTH specimens were classified as non-unions (a 73 percent decrease from that of NWB) and 50-75 percent increases in maximum hard callus volume and hard callus formation rates were observed as compared to NWB specimens. PTH therapy increased BFR by ~2-fold over those exhibited by NWB rats. PTH therapy enhanced the numbers of osteoprogenitor cells by ~4-fold as compared to those measured from NWB specimens. We are currently assessing bending strength and histology of these specimens. Altogether, intermittent PTH therapy partially restores select aspects of bone healing under simulated microgravity. The grant was extended for an extra six months and during this time NWB rats were treated with alendronate (bisphosphonate). Currently, the results from this trial are under analysis and results will be forthcoming.

Findings

Results indicate that cortical bone fracture healing is impaired in hindlimb unloaded rats. The implication of these findings is that bone trauma repair in astronauts on long space missions would likely be compromised and presents a potential threat to mission effectiveness and astronaut health. Further, these findings also suggest that astronauts who return from extended spaceflight missions are at continued risk for impaired bone trauma repair after returning to a normal gravity environment even up to a period of time equal to that of the space mission.
Results indicate that extended exposure to hindlimb unloading results in drastic reductions in both total marrow progenitor cells and osteoprogenitors. Thus, a simulated microgravity situation appears to cause a reduction in the number of preosteoblasts that would be needed to generate a functional osteoblast population necessary to repair bone trauma. These findings provide a plausible mechanism of action underlying this impaired fracture-healing response.
Results indicate that intermittent PTH therapy improves fracture healing under chronic hindlimb unloading, though it does not normalize fracture healing to WB levels. The dramatic reduction in the number of fracture non-unions resulting from PTH therapy suggests that this treatment is a viable candidate to be tested as a countermeasure to offset any potential deleterious effects of weightlessness on bone trauma healing.
Results indicate that PTH therapy significantly increased the number of functional osteogenic progenitor cells in bone under chronic hindlimb unloading. PTH therapys increase in the number of osteoprogenitors correlated with its improvements in fracture healing in NWB rats strongly suggests a cause and effect between these parameters and strengthens our hypothesis that a decrease in progenitor cells represents a mechanism of action underlying the impairment to fracture healing. If correct, then the implications of these findings are that marrow progenitor cell populations may be altered in astronauts on long space missions and might manifest in deficiencies in musculoskeletal tissue repair after trauma.

Given that the project has reached is closure date, our remaining plans are to complete the remaining data assessments and submit manuscripts for submission to peer-reviewed publications.

Earth Applications

The research impact of findings from this project suggest that impaired bone healing during and after an extended exposure to simulated weightlessness should manifest relevance to the underlying causes of impaired bone healing in patients experiencing paralysis, chronic immobility or extended bed rest. Our data indicating that intermittent PTH therapy can counteract the impairment of bone healing under a NWB situation may offer a potential treatment for augmenting bone healing in these Earth-bound, NWB patients. Our studies found that PTH therapy significantly increased the prevalence of osteogenic progenitor cells within bone marrow tissue over that measured in vehicle-control rats (normal gravity group). Further, PTH therapy increased the prevalence of osteogenic progenitor cells within bone marrow tissue four-fold over that measured in vehicle-control hindlimb suspended rats (chronic non-weight-bearing). Coupled with the observations that PTH therapy improved fracture healing in hindlimb suspended rats, these findings suggest that an underlying mechanism of action of PTH in its anabolic activity for enhancing bone fracture repair lies at the level of increasing the pool size of functional bone cells derived from osteogenic precursor cells. These same findings suggest that PTH therapy should aid in the healing of recalcitrant bone fractures here on Earth, particularly in patients experiencing paralysis, chronic immobility or extended bed rest.

This project's funding ended in 2007