Much of the constant signaling that takes place between cells is carried via microvesicles and exosomes, membrane-bound packages of molecules. Researchers are finding that the contents of vesicles change in characteristic ways with advancing age, one of the many cellular reactions to rising levels of molecular damage and environmental stress. Some of these changes might be useful as a marker of cellular senescence, one of the more important changes in cell state associated with age. It should also be possible to use suitably formed vesicles to adjust cell behavior in situ, such as to spur greater regeneration. Perhaps these vesicles are harvested from young cells, or perhaps they might be manufactured directly. Many of the current class of widely used cell therapies might in theory be replaced by delivery of vesicles, as the cell therapies achieve their beneficial results via signaling, not other cell activities.
Another of the more important changes in cell state that occurs with age is the decline in stem cell activity. Stem cells are responsible for providing a supply of somatic cells for tissue maintenance and regeneration, and the progressive loss of that supply contributes to the gradual failure of tissue and organ function in later life. There is ample evidence to suggest that, at least in the stem cell populations most studied to date, such as those supporting skeletal muscle tissue, this is at least as much a problem of signaling as it is a problem of damage to the cells themselves. The stem cells react to the state of damage and behavior of other cells in the niche that supports them, as reflected in the signal molecules they receive. The current consensus in the scientific community is that this response to the damage of aging evolved to reduce cancer risk, one part of the current human life span as a balance between death by cancer versus death by slowly declining tissue function.
As research community interest in vesicle signaling picks up, we should expect to see more in the way of research results such as the one below, in which scientists find that delivery of vesicles from young niche cells can restore more youthful function to aged hematopoietic stem cells, the population resident in bone marrow and responsible for generating blood and immune cells. It seems plausible that we stand at the verge of an important shift in focus for the field of regenerative medicine, a change based on an improved understanding of how cells influence one another via signaling processes, and the identification of which of these signals are important determinants of the changes in regeneration and stem cell activity that occur over the course of aging.
Donor age is one of the major concerns in Bone Marrow Transplantation (BMT). Studies on murine system have demonstrated that aged marrow harbors increased pool of hematopoietic stem cells (HSCs) exhibiting myeloid bias and having compromised competitive repopulating ability. Aged HSCs also exhibit multiple epigenome and transcriptome changes. DNA damage, replication stress, and ribosomal stress have been shown to cause aging of HSCs. Age-associated changes in human HSCs were similar to those observed in mouse HSCs, suggesting that hematopoietic aging is an evolutionarily conserved process. A retrospective study done in BMT patients showed age as the only donor trait associated with their overall and disease-free survival.
Since donor age is such an important concern in BMT, it might be argued that the upper limit of donor age may be lowered. However, patients having an older individual as the sole matched donor could be denied access to this potentially life-saving treatment. To overcome this impediment, efforts are being made to rejuvenate aged HSCs to improve their performance. Here we report a novel finding that a brief exposure of aged HSCs to young mesenchymal stromal cells (MSCs) rejuvenates them via intercellular transfer of microvesicles (MVs) containing "youth signals". We also demonstrate that intercellular transfer of aged exosomes carrying negative regulators of autophagy causes aging of HSCs. Our data are relevant in both allogenic as well as autologous transplantations involving older individuals as donors and recipients, respectively.
Rejuvenation of aged HSCs prior to transplantation could expand the donor cohort and also help older individuals undergoing autologous stem cell therapy. Application of the MSCs as well as MVs in clinical BMT/SCT might be logistically straightforward, since they can be cryopreserved as "ready-to-use" reagents. Use of pharmacological compounds to rejuvenate aged stem cells in general, and aged HSCs in particular, is being pursued to gain clinical advantage. However, most pharmacological compounds could show off-target effects and they also regulate diverse processes and pathways. Therefore, use of clinical grade "cellular products" in manipulating HSCs, albeit expensive, would be a safer approach than the direct application of pharmacological tools on them.
Reduced autophagy is associated with aging, whereas stimulation of autophagy is speculated to have anti-aging effects. Aged HSCs having high autophagy levels are known to preserve their regenerative capacity. Here we provide a direct evidence for this hypothesis. We demonstrate that young MSCs transfer autophagy initiating mRNAs to the aged HSCs via intercellular transfer of MVs, leading to their rejuvenation. ATG-7 is a critical component of the autophagy pathway and has been shown to be essential for the maintenance of human CD34+ HSCs. Here we show that young MSCs and their MVs transfer Atg7 to aged HSCs.
FOXO3a has been linked to longevity in multiple population studies. FOXO3a is known to stimulate autophagy in primary mouse renal cells. Similarly, FOXO3a inhibition or depletion prevents autophagy induction by starvation in vivo in mouse muscle, confirming a strong link between transcription factors of the FOXO family and autophagy. We found that aged HSCs treated with young MVs show high levels of FOXO3. In the light of these reports, our data clearly demonstrate that direct transfer of MVs containing autophagy-inducing mRNAs seems to be one of the important mechanisms involved in rejuvenation of aged HSCs by young MSCs.
Myeloid bias of HSCs has been considered as a hallmark of their aging. This has been attributed to an accumulation of myeloid-biased HSCs in the aged marrow. In transplantation between old and young individuals, microenvironment-mediated myeloid skewing has been demonstrated. Here, we report a novel finding that myeloid bias of aged HSCs could also be a non-cell-autonomous process involving intercellular communication mechanisms. We demonstrate that aged MVs contain higher levels of Itga2b, which is a myeloid commitment marker, whereas young MVs contain higher levels of IL7r, which is a lymphoid commitment marker. Importantly, we show that partitioning of these mRNAs depends upon the levels of activated AKT in the stromal cells.
Thus, a continuous transfer of aged MVs containing Itga2b to the HSCs could impose a myeloid bias in them, and this coupled with their low levels of apoptosis, could lead to the accumulation of aged HSCs in the marrow. Our data strongly suggest that the lineage bias of HSCs could be dictated by the mRNA profile of the MVs transferred to them, which in turn depends on the signaling mechanisms prevailing in the stromal cells. This aspect needs further investigation. Nonetheless, our findings have certainly added a new dimension to the existing academic debate.