Aging is the best predictor of osteoporosis1,3
. Aging-associated changes in metabolism and hormonal balance have been shown to play important roles in the progression of osteoporosis13,14
. Indeed, supplemental intake of calcium (1000–1200 mg) and vitamin D (700–800 IU) is currently recommended to prevent and, perhaps, treat osteoporosis5,6
. In addition, appropriate and persistent exercise has been shown to increase bone density and reduce the risk of hip fracture in older women15
. Pharmacologic interventions2,5,6
, including anabolic agents such as hormone replacement, bisphosphonates, calcitonin, raloxifene, teriparatide and alternative therapies are currently being used in osteoporosis prevention and treatment. Although these strategies are at least partially effective, recent evidence indicates that the loss of functional osteogenic progenitor cells with age might be responsible for age-related osteoporosis. Specially, the proliferative capacity of osteogenic progenitor cells derived from the bone marrow of old mice is substantially lower than those from young ones7
. A significant age-dependent decline of cells with osteogenic potential has also been observed in human bone marrow16,17
. Additionally, alkaline phosphatase-positive osteogenic progenitor colonies (CFU-ALP) and total fibroblastic colonies (CFU-F) of mice, rats and human BMSCs have also shown declined osteogenic and growth potential with age18,19,20
To determine whether the senescence of BMSCs contributes to the accelerated bone mass loss, we compared the in vitro
differentiation capacity of BMSCs isolated from young (1–2 months old) vs.
old mice (20–24 months old). Our results indicate that the differentiation capability of young BMSCs is significantly higher than those of old BMSCs. This is in agreement with recent findings that mesenchymal stem cells derived from mice, Wistar rats, rhesus monkeys and humans show declined viability and capacity for differentiation with age.10,11,12,17,19
. The molecular mechanism(s) underlying BMSCs aging are unclear. Cumulative DNA damage has been reported to affect cell aging21
. Upon DNA damage (i.e., double strand breaks), phosphorylated H2AX accumulates at sites of DNA lesions, which triggers a DNA damage checkpoint response22
. 53BP1 is a DNA damage checkpoint response mediator and plays an important role in DNA damage checkpoint response and damage repair23
. Our results show that there is a good agreement between BMSCs aging and 53BP1 and γ-H2AX up-regulation. However, we cannot conclude yet whether cumulative DNA damage is the cause of BMSCs aging and loss of function.
By analyzing bone sections from transplanted mice, we have found that more young BMSCs than old BMSCs appeared in the tissue sections, suggesting that the migratory ability of BMSCs may decline with age. This is supported by parallel observations that the migration of endothelial progenitor cells is significantly reduced in older humans and mice24,25
. It has also been shown that the migratory ability of human BMSCs significantly declines with age26
. Such aged-related impaired migration has also been shown in human lung, skin and dermal fibroblasts27,28
. The inability of BMSCs from aged individuals to migrate throughout the body may impede the distribution of the BMSCs and later regeneration of various tissues and organs which might, in the present case, lead to osteoporosis.
It is well established that, compared with young individuals, old animals have significant reductions of osteogenic activity in bone, including the expression and production osteocalcin29
. Our results showed that transplantation of BMSCs from young mice leads to a dramatic increase of AP and osteocalcin activities in bone tissue. On the other hand, the transplantation of BMSCs from old animals had no such effect. These results suggest that young BMSCs can actively migrate, and differentiate into a variety of cells, including osteoblasts. However, old BMSCs may somehow lose such capability. These observations are in agreement with several recent findings. First, with increasing age, murine bone marrow derived stem cells lose their ability to adhere, proliferate, and undergo chondrogenic and osteogenic differentiation10
. Second, stem cells isolated from old animals were found to engender less bone formation when subcutaneously implanted in porous hydroxyapatite scaffolds than stem cells recovered from young ones30
. Finally, a negative correlation has been reported between in vitro
osteogenic activity of CD105+ human MSCs and cell donor age31
Using Micro-CT imaging it has been established that, during the development of age-related osteoporosis, there are substantial decreases of bone to total tissue ratio, trabecular thickness, trabecular number, and cortical thickness while the values for trabecular separation are increased significantly32,33
. In fact, these bone physical characteristics have been used as indicators for diagnosis and treatment of osteoporosis34,35
. Trabecular architecture parameter assessments using CT or MRI as a bone mineral density-independent determinant of bone strength are able to separate patients with and without osteoporotic fractures better than with BMD alone, and parameters of trabecular architecture are also more sensitive to treatment effects than BMD36
. Equally important, trabecular architecture parameters and microstructure measurements using micro-CT have proven to be a very good tool to evaluate osteogenic activity and new bone formation37,38
. We find that, simply by transplantation of BMSCs from young animals, it is possible to restore almost all aspects of bone microstructure in aged animals. However, transplantation of BMSCs from old animals had little or no effect on bone microstructure in similar animals. Similar morphological changes have been shown to reflect osteogenic activities39
Surprisingly, we observed that recipients of BMSCs from young animals had significantly increased life spans. It is not clear how administration of young BMSCs leads to increased longevity but it is possible that the transplanted young BMSCs may enhance cell/tissue regeneration and thereby slow down the aging process. This potential influence is based on the following evidence. First, EGFP positive cells were found in many vital organs, including skin, heart, spleen, lung, liver, bone marrow, etc. Second, studies have shown that transplants of young mesenchymal stem cells were able to restore cardiac angiogenic function and also improve functional outcomes in old mice after a myocardial infarction40
. Third, similar transplants of young mesenchymal stem cells in old female mice were also reported to postpone age-related reproductive failure and improve offspring survival41
. In addition, bone marrow transplantation from young donors was also found to be able to rejuvenate the B-cell lineage in aged mice42
. Finally, myofiber-associated satellite cell transplantation has been found to be effective in the prevention of age-related sarcopenia43
. Overall, these observations support the idea that young BMSCs transplantation might extend life span by improving the functions of multiple vital organs, including lung, heart, and immune system, albeit through presently unknown mechanisms.
In conclusion, the transplantation of BMSCs from young animals effectively restores bone microstructure and density in aged animals. As an unexpected consequence, old mice receiving BMSCs from young animals also have longer life spans compared to control mice or mice with transplanted BMSCs from old mice. These findings provide a potential link between stem cell activities and aging. However, further studies are needed to determine mechanisms through which BMSCs transplantation restores bone function and prolongs life.