Research into exosome signaling has grown in recent years. Arguably the bulk of signaling between cells is transported via varieties of extracellular vesicle, collections of molecules packaged within a membrane. Exosomes are one such type of vesicle. An originating cell generates exosomes, releasing them to the environment, and other cells accept them as they arrive. The contents of an accepted exosome then go on to influence cell machinery and activities. The beneficial effects of most stem cell therapies are mediated by signaling rather than by any other actions of the transplanted cells, and thus in principle it should be possible to do away with the cells and base a therapy on the signals alone. In the near term that might be accomplished by harvesting exosomes from cell cultures, while in the long term manufacturing and delivering specific desired signal molecules directly will probably emerge as the dominant approach.
The research noted here is carried out in cell cultures only, but it is an interesting example of the degree of influence over cell behavior that might be obtained through delivery of exosomes. If cells in many tissue types can be encouraged to greater regeneration and greater resilience to adversity through exosomes harvested from stem cells, then this is enough, no doubt, to support a wide range of potential therapies. Juvena Therapeutics is one example of a company that is mining this sort of cell signaling to pull out therapeutics. In the years ahead a great many other similar ventures will arise.
At this point, even given two decades of experimentation with stem cell therapies, it remains something of an open question as how great of a benefit can be provided by regenerative therapies that work around underlying damage. "Putting cells back to work" might be the motto, but this happens without any deliberate attempt to repair the accumulation of damage that is present old tissues. How much of that damage will be fixed by telling cells to work harder? Certainly issues caused by too few active cells seem amenable to treatment via simple therapies that override cell instructions, but we know that at least some forms of molecular damage at the root of aging, such as persistent cross-links and a few varieties of metabolic waste, cannot be effectively repaired even by youthful and active cells.
Skin undergoes physiological changes as a consequence of the aging process. There are two basic types of skin aging, i.e., intrinsic and extrinsic aging. Intrinsic aging is genetically determined, which indicates that it occurs inevitably as time passes. Many studies have suggested epigenetic changes and post-translational mechanisms are more important pathways of intrinsic aging rather than genetic influence. On the other hand, extrinsic aging occurs by external factors such as smoking, air pollution, and unbalanced nutrition. Among them, UV exposure is the most important cause of extrinsic aging. Therefore, the skin damages induced by UV exposure is called "photoaging". Photoaging is characterized by irregular pigmentation, dryness, sallowness, roughness, premalignant lesions, and skin cancer. Intrinsic skin aging, in contrast, is characterized by a loss of elasticity and fine wrinkles rather than deep wrinkles due to photoaging.
Fibroblasts are the primary cell types constituting the dermis and are responsible for the synthesis of structural components such as procollagen and elastic fibers. Fibroblasts lose their capacities for proliferation and synthesis of collagen, the major extracellular matrix (ECM) constituent of the skin dermis, with aging. On the other hand, the expression of various types of matrix-degrading metalloproteinase (MMP) is upregulated in the aged fibroblasts. Age changes the number and proliferation of dermal fibroblasts, reduces collagen synthesis and repair, and accelerates degradation of the existing skin matrix by MMPs, thereby reducing the regenerative capacity of skin.
Stem cells have been widely used for skin regeneration. Recently, it has been demonstrated in several preclinical and clinical studies that the transplantation of mesenchymal stem cells (MSCs) contributes to wound repair and regeneration. However, paracrine actions of the transplanted stem cells are believed to play a crucial role in the therapeutic effects. Many studies have reported that stem cells secrete several cytokines which promote the proliferation of dermal fibroblasts and the synthesis of ECM molecules. In addition, there have been advances in exploring the roles of exosomes secreted from stem cells in these paracrine actions. Exosomes are small membrane lipid vesicles secreted by most cell types (30-120 nm in diameter).
Exosomes contain functional messenger RNAs (mRNAs) and microRNAs (miRNAs), as well as several proteins, that originate from the host cells. Several evidences have also been revealed that the presence of several classes of long noncoding RNAs (lncRNAs) in exosomes. As lncRNAs have the function to induce epigenetic modifications by binding to specific genomic loci and recruiting epigenetic regulators such as chromatin remodeling complexes, exosomes secreted from one cell may also induce epigenetic modifications in recipient cells.
We previously demonstrated the stimulatory effects of human induced pluripotent stem cell-conditioned medium (iPSC-CM) on the proliferation and migration of dermal fibroblasts. Herein, we hypothesized that the iPSCs-CM contained exosomes and the human induced pluripotent stem cells-derived exosomes (iPSC-Exo) played a key role in these effects of iPSC-CM. To address this hypothesis, we isolated exosomes from iPSC-CM and examined their effects on several cellular responses associated with skin aging, as well as the proliferation and migration in human dermal fibroblasts (HDFs).
To induce photoaging and natural senescence, HDFs were irradiated by UV and subcultured for over 30 passages, respectively. The expression level of certain mRNAs was evaluated by quantitative real-time PCR (qPCR). Senescence-associated-β-galactosidase (SA-β-Gal) activity was assessed as a marker of natural senescence. As a result, we found that exosomes derived from human iPSCs (iPSCs-Exo) stimulated the proliferation and migration of HDFs under normal conditions. Pretreatment with iPSCs-Exo inhibited the damages of HDFs and overexpression of matrix-degrading enzymes caused by UV irradiation. The iPSCs-Exo also increased the expression level of collagen type I in the photo-aged HDFs. In addition, we demonstrated that iPSCs-Exo significantly reduced the expression level of SA-β-Gal and matrix-degrading enzymes and restored the collagen type I expression in senescent HDFs. Taken together, it is anticipated that these results suggest a therapeutic potential of iPSCs-Exo for the treatment of skin aging.