It has been a decade or so since the first induced pluripotent stem (iPS) cells were produced. Researchers discovered a recipe by which ordinary, limited, adult somatic cells could be reprogrammed into a state near identical to that of embryonic stem cells, meaning they are pluripotent and can then in principle be used to produce any of the cell types in the body. Doing so in practice requires researchers to establish a suitable methodology to guide cellular differentiation in the right direction, only accomplished at this point for a fraction of all possible cell types. The early attempts at induced pluripotency worked, and were easy to set up, but were also comparatively inefficient. Since then researchers have produced considerable improvement in the methodologies used, and along the way have explored other facets of this reprogramming process. One of the most intriguing aspects of induced pluripotency is that it appears to produce a form of cellular rejuvenation, a sweeping reset and repair of many forms of damage.
There are many open questions regarding this incompletely explored cellular rejuvenation achieved through induced pluripotency: how it works at the detail level; exactly which types of damage are repaired and which are not; how it relates to the equivalent process that occurs in the early development of the embryo. How do old gamete cells, laden with the molecular damage of aging, produce young offspring who lack that damage? Somewhere in there, rejuvenation happens. Is there any way to adapt this process of rejuvenation for use in therapies? It seems unwise to, for example, apply pluripotency reprogramming methods directly to a patient. This sounds a lot like opening the door to a high risk of uncontrolled cellular replication, or cancer in other words. Nonetheless, that experiment was recently carried out in mice, more or less, so we'll likely hear more about the risks in the years ahead. It is possible that such an approach will in the end fall into the same ballpark as stem cell therapies when it comes to overall degree of risk, though it is worth noting that, when performed improperly, stem cell therapies can also result in cancer, and considerable amount of work has gone into minimizing that outcome in those therapies that have made it to widespread clinical availability.
There are other possibilities when it comes to using the rejuvenation that occurs during the induced pluripotency process, however. Take a population of cells that are damaged and dysfunctional in an old individual, for example. Obtain a sample, create an induced pluripotent lineage from that sample, and then apply a suitable recipe to differentiate the pluripotent cells back into the original cell type. Do these recreated cells now behave as though they are younger, and can thus form the basis for a cell therapy to replace the old cells in the patient? Researchers here demonstrate that this is in fact the case for the stem cells responsible for generating blood:
When we are young, our blood stem cells produce an even and well-balanced number of red and white blood cells according to need. As we age, however, the capacity of the blood stem cells to produce the number of blood cells we need declines. "This type of age-related change can have major consequences as it can lead to an imbalance in stem cell production. For example, a reduced production of immune cells or excessive production of other types of cells can be a precursor to leukaemia."
A fundamental question was whether blood stem cells age differently within a single individual or whether all blood stem cells are equally affected by advancing age. In an initial stage, it was therefore important to genetically mark old blood stem cells, to enable the identification and tracking of those most affected by age. In the next step, these traceable cells were reprogrammed to another type of stem cell - known as iPS cells, which can generate all cells in an individual and not only blood cells. When the cells are reprogrammed, their identity is "re-set"; when these reprogrammed iPS cells formed new blood stem cells, the researchers observed that the re-set had entailed a rejuvenation of the cells. "We found that there was no difference in blood-generating capacity when we compared the reprogrammed blood stem cells with healthy blood stem cells from a young mouse. This is, as far as we know, the first time someone has directly succeeded in proving that it is possible to recreate the function of young stem cells from a functionally old cell.ˮ
The research team's studies have also thereby shown that many age-related changes in the blood system cannot be explained by mutations in the cells' DNA. If the changes depended on permanent damage at the DNA level, the damage would still be present after the re-set. Instead, epigenetic changes appear to underlie the decline in function associated with advancing age.
While age-related diseases evidently can arise due to changes that compromise or alter the function of mature effector cells, this is harder to reconcile with organs such as the blood, that rely on inherently short-lived effector cells in need of continuous replenishment. Rather, accumulating data have suggested that the de novo production of subclasses of haematopoietic cells shifts in an age-dependent manner, akin to that seen during more narrow time windows in early development. These findings have to a large extent also challenged the classically defining criteria of haematopoietic stem cells (HSCs) as a homogenous population of cells with differentiation capacity for all haematopoietic lineages. Rather, the differentiation capacity of HSCs might be more appropriately defined by a continuous multilineage haematopoietic output, but which might not necessarily include the production of all types of blood cells at all points in time.
The mechanisms that drive ageing at both the organismal and cellular level have attracted significant attention as they represent prime targets for intervention. An increased function of aged cells by 'young'-associated systemic factors has been proposed. Whether such approaches indeed reflect rejuvenation at a cellular level or rather stimulate cells less affected by age is mostly unclear. This concern applies also to previous studies approaching the prospects of reversing cellular ageing by somatic cell reprogramming, which have typically failed to distinguish between functionally versus merely chronologically aged cells.
Here we approach these issues by genetic barcoding of young and aged HSCs that allows for evaluations, at a clonal level, of their regenerative capacities following transplantation. This allows us to establish that ageing associates with a decrease of HSC clones with lymphoid potential and an increase of clones with myeloid potential. We generate induced pluripotent stem (iPS) lines from functionally defined aged HSC clones, which we next evaluate from the perspective of their blood-forming capacity following re-differentiation into HSCs by blastocyst/morula complementation. Our experiments reveal that all tested iPS clones, including such that were originally completely devoid of T-cell and/or B-cell potential, perform similar to young HSCs both in steady-state and when forced to regenerate lymphomyeloid haematopoiesis in secondary transplantations. This regain in function coincides with transcriptional features shared with young rather than aged HSCs. Thereby, we provide direct support to the notion that several functional aspects of HSC ageing can be reversed to a young-like state.