Researchers here investigate one of the more complex cell signaling mechanisms, providing evidence to link age-related changes in this mechanism to the development of osteoporosis, the loss of bone mass and strength that occurs with age. There is a growing interest in the research community in alterations to cell signaling that occur with aging. A great deal of this focuses on the changing amounts of various molecules found in the bloodstream, and arises from parabiosis studies that link the circulatory systems of old and young animals, producing benefits to measures of health in the older individual. Researchers have been isolating specific molecules of interest, showing that levels change with aging, and that reversal of those changes produces benefits in old animals.
These changes are most likely secondary reactions to forms of cell and tissue damage that accumulate over a lifespan, but in turn they are proximate causes for a range of undesirable outcomes that include the characteristic decline in stem cell activity that occurs with age. The signals that cells pass between one another are complex and varied. It isn't just a matter of releasing specific molecules into the bloodstream and surrounding tissues. For example, cells also create and emit vesicles, membrane-enclosed packages of molecular machinery that are used for a wide variety of purposes. Just as simple signals vary with age, so too do the number and contents of these vesicles, and here researchers provide evidence to suggest that there is to be found one of the proximate contributing causes to the loss of bone that occurs with age:
The capability of stem cells to regenerate tissue by differentiating into specialized cells has been shown to decrease with age. One organ that is notably affected by this loss in stem cell functionality is the skeleton. Bone is a highly dynamic organ that is constantly remodeled and maintained by the coordinated activity of bone forming osteoblasts and bone excavating osteoclasts. This balance is particularly important at older age, as too high osteoclast activity versus too few osteoblasts is considered to give rise to lower bone strength. The molecular mechanisms by which the imbalance is caused in the elderly are still incompletely understood. However, it is clear, that after skeletal maturation a constant number of mesenchymal stem cells (MSCs) and a reduced number of mature osteoblasts are observed with increasing age. This indicates that the functionality or the osteogenic commitment of MSCs might be impaired. Supportingly, the numbers of pre-osteoblasts, pre-osteoclasts and osteoclasts do not change with age per unit bone length, at least in elderly rats. However, a strong decline of mature osteoblasts has been described, as well as impaired osteoblastogenesis in age associated osteoporosis. This supports the hypothesis that impaired osteoblastogenesis contributes to age-related bone loss and loss of mechanical strength.
Since it has been proposed recently that the systemic environment of young versus elderly individuals can influence stem and progenitor cell functionality in different tissues, such as bone repair, specific focus is put on secreted circulating factors, in particular with regard to extracellular vesicles (EVs), small vesicles released by many if not all cell types. The cargo of EVs, consisting of proteins, mRNAs and non-coding RNAs, including miRNAs, is selectively packaged and delivered to specific recipient cells over short and long distances.
In the present study we set out to determine whether circulating factors, in particular human plasma-derived EVs from the elderly, contribute to the age-dependent loss of stem cell functionality. We observed that vesicles isolated from young donors enhance osteoblastogenesis in vitro compared to elderly-derived EVs. While searching for factors mediating this donor-age-dependent vesicular effect, we identified Galectin-3 to be enriched in EVs from young individuals. Indeed, we found that increased levels of Galectin-3 have a positive impact on the osteogenic differentiation capacity of MSCs and that extracellular vesicles enriched in Galectin-3 enhance osteoblastogenesis of MSCs. We elucidated its molecular mechanism of action by showing that this protein protects β-Catenin from degradation and that its Serine-96 (S96) phosphorylation site is crucial to mediate this effect. Finally, we demonstrated that cell-penetrating peptides fused to a 13 amino acid sequence, mimicking Galectin-3's Serine-96 phosphorylation site, are able to enhance osteoblastogenesis.