Today I'll point out progress towards an as yet unrealized category of stem cell treatments involving the wholesale replacement of entire stem cell populations and their niches, to remove age-related damage and sustain tissue maintenance for the long term. This will become an essential component for any future rejuvenation toolkit. From a stem cell perspective, rejuvenation has two components: firstly revert the root causes of signaling changes in blood and tissues that result in stem cell populations becoming less active; secondly, replace the stem cells themselves, scores of different types in different locations, to clear out damaged cells. The root cause of signaling changes in old individuals is, collectively, all of the forms of damage listed in the SENS proposals for rejuvenation treatments - a lot of work is yet to be accomplished there to reach even the initial goals of prototype treatments across the board. Nonetheless, it is still the case that replacement of aged stem cell populations with undamaged, pristine stem cells created from the patient's own cells is an important target for future development in the stem cell field.
Most stem cell therapies in use today are actually far removed from this goal: the transplanted cells do not live long, and do not integrate with recipient tissues. They achieve beneficial effects through a temporary alteration of the signaling environment that spurs regeneration and reduces inflammation. In effect the transplant temporarily overrules the evolved reaction to being aged and damaged and puts sleeping cells back to work - but without fixing that low level damage. So there can be some degree of rebuilding of worn tissues and organs, but the causes of aging are still present and continue to cause harm: cross-links, mitochondrial mutations, and so forth.
There are exceptions to the outcome of benefits through signaling mechanisms, however, and these exceptions include types of therapy in which cells are transplanted into the brain. Some of the earliest stem cell transplants trialed in humans aimed to treat Parkinson's disease, for example, and at least some of the transplanted cells survived and integrated into the brains of patients for the long term. This is still a considerable distance removed from a controlled repopulation of stem cell niches in all of the right places and with cells that will pick up tissue maintenance activities in exactly the right ways, but it is a step in the right direction. In the research materials linked below, scientists report on further progress along these lines, and that they were able to create new stem cell niches in brain tissue seems like an important advance:
Although brains - even adult brains - are far more malleable than we used to think, they are eventually subject to age-related illnesses, like dementia, and loss of cognitive function. Someday, though, we may actually be able to replace brain cells and restore memory. Recent work hints at this possibility with a new technique of preparing donor neural stem cells and grafting them into an aged brain. The team took neural stem cells and implanted them into the hippocampus - which plays an important role in making new memories and connecting them to emotion - of an animal model, essentially enabling them to regenerate tissue.
"We're very excited to see that the aged hippocampus can accept grafted neural stem cells as superbly as the young hippocampus does and this has implications for treating age-related neurodegenerative disorders. It's interesting that even neural stem cell niches can be formed in the aged hippocampus." The team found that the neural stem cells engrafted well onto the hippocampus in the young animal models (which was expected) as well as the older ones that would be, in human terms, about 70 years old. Not only did these implanted cells survive, they divided several times to make new cells. "They had at least three divisions after transplantation. So the total yield of graft-derived neurons and glia (a type of brain cell that supports neurons) were much higher than the number of implanted cells, and we found that in both the young and aged hippocampus, without much difference between the two. What was really exciting is that in both old and young brains, a small percentage of the grafted cells retained their 'stemness' feature and continuously produced new neurons."
This is called creating a new 'niche' of neural stem cells, and these niches seemed to be functioning well. "They are still producing new neurons at least three months after implantation, and these neurons are capable of migrating to different parts of the brain. Next, we want to test what impact, if any, the implanted cells have on behavior and determine if implanting neural stem cells can actually reverse age-related learning and memory deficits. That's an area that we'd like to study in the future."
As clinical application of neural stem cell (NSC) grafting into the brain would also encompass aged people, critical evaluation of engraftment of NSC graft-derived cells in the aged hippocampus has significance. We examined the engraftment and differentiation of alkaline phosphatase-positive NSCs expanded from the postnatal subventricular zone (SVZ), 3 months after grafting into the intact young or aged rat hippocampus. Graft-derived cells engrafted robustly into both young and aged hippocampi. Although most graft-derived cells pervasively migrated into different hippocampal layers, the graft cores endured and contained graft-derived neurons.
The results demonstrate that advanced age of the host at the time of grafting has no major adverse effects on engraftment, migration, and differentiation of grafted subventricular zone-neural stem cells (SVZ-NSCs) in the intact hippocampus, as both young and aged hippocampi promoted excellent engraftment, migration, and differentiation of SVZ-NSC graft-derived cells in the present study. Furthermore, SVZ-NSC grafts showed ability for establishing neurogenic niches in non-neurogenic regions, generating new neurons for extended periods after grafting. This phenomenon will be beneficial if these niches can continuously generate new neurons and glia in the grafted hippocampus, as newly generated neurons and glia are expected to improve, not only the microenvironment, but also the plasticity and function of the aged hippocampus. Overall, these results have significance because the potential application of NSC grafting for treatment of neurodegenerative disorders at early stages of disease progression and age-related impairments would mostly involve aged persons as recipients.