In principle any cell state can be reprogrammed into another cell state – it is a matter of figuring out the machinery involved, which remains no small task even now in this age of revolutionary progress in the tools of biotechnology. Some cell state changes are more plausible and easily discovered since they correspond, nearly or exactly, to transitions that already take place in at least some circumstances and species. So skin cells can be turned into the induced pluripotent stem cells that are near identical to embryonic stem cells, and which can then differentiate into another cell type, such as a neuron. Alternatively those skin cells can be converted directly to entirely different cell types without going through the pluripotent stage, via forms of transdifferentiation.
Senescent cells are those that have entered a state of growth arrest in response to damage, a toxic environment, or hitting the replication limit that exists for all somatic cells. Senescent cells do not replicate, and they either remain in this state indefinitely, in a tiny minority of cases, or self-destruct, in the vast majority of cases. They never return to replication. But when we say that the state of cellular senescence is irreversible, we mean that it is observed to be irreversible in the normal run of things in our tissues, just as skin cells don’t randomly turn into induced pluripotent stem cells in the normal run of things in our tissues. Once researchers can start tinkering with cell programming and the controlling levers of cell state, however, all the rules are there to be broken. Senescent cells can be made to replicate once more, given the right modification.
The presence of growing numbers of lingering senescent cells is one of the root causes of aging. In recent years there has been an explosion of interest in developing therapies to prevent the contribution of cellular senescence to aging – and to turn it back to generate rejuvenation in the old. This is enormously gratifying to advocacy groups such as the SENS Research Foundation and Methuselah Foundation, and advocates such as Aubrey de Grey, who have been trying to create this surge of investment and progress since just after the turn of the century. The primary therapeutic approach to senescent cells is to selectively destroy them. It is simple, it absolutely gets rid of all the problems, whether known or yet to be cataloged, and it is shown to extend life and reverse measures of aging in mice.
Should we be interested in reversal of senescence as an approach, however? Senescent cells are generally senescent for a reason, and that reason either involves their age and amount of replication, or it involves internal damage that can be harmful to the surrounding tissues. That includes cells with DNA damage that causes them to be potentially cancerous. The relationship of cell damage to outcomes such as cancer is a numbers game: simply re-enabling replication in all senescent cells will probably raise the risk of issues down the line. However, the harms done by senescent cells due to the characteristics of their state are also significant. These cells cause harm through their signaling profile, a mix of secreted molecules that generate chronic inflammation, fibrosis, and all sorts of other woes in surrounding tissue. Turning off senescence and enabling replication in senescent cells should be a considerable improvement over leaving them as they are, provided that it does in fact prevent their damaging signaling. This is true, at least, in the short term, but I think it a poor second best to their destruction over the long term. These are not high-quality cells; on average they will bear a burden of damage and dysfunction that is distinct from whether or not they are senescent. Cancer is definitely one of the concerns.
A new way to rejuvenate old cells in the laboratory, making them not only look younger, but start to behave more like young cells, has been discovered. A team has discovered a new way to rejuvenate inactive senescent cells. Within hours of treatment the older cells started to divide, and had longer telomeres – the ‘caps’ on the chromosomes which shorten as we age. This discovery builds on earlier findings showed that a class of genes called splicing factors are progressively switched off as we age. The team found that splicing factors can be switched back on with chemicals called resveratrol analogues. The chemicals caused splicing factors, which are progressively switched off as we age to be switched back on, making senescent cells not only look physically younger, but start to behave more like young cells and start dividing.
The discovery has the potential to lead to therapies which could help people age better, without experiencing some of the degenerative effects of getting old. Most people by the age of 85 have experienced some kind of chronic illness, and as people get older they are more prone to stroke, heart disease, and cancer. “This is a first step in trying to make people live normal lifespans, but with health for their entire life. Our data suggests that using chemicals to switch back on the major class of genes that are switched off as we age might provide a means to restore function to old cells. When I saw some of the cells in the culture dish rejuvenating I couldn’t believe it. These old cells were looking like young cells. It was like magic. I repeated the experiments several times and in each case the cells rejuvenated. I am very excited by the implications and potential for this research.”
As we age, our tissues accumulate senescent cells which are alive but do not grow or function as they should. These old cells lose the ability to correctly regulate the output of their genes. This is one reason why tissues and organs become susceptible to disease as we age. When activated, genes make a message that gives the instructions for the cell to behave in a certain way. Most genes can make more than one message, which determines how the cell acts. Splicing factors are crucial in ensuring that genes can perform their full range of functions. One gene can send out several messages to the body to perform a function – such as the decision whether or not to grow new blood vessels – and the splicing factors make the decision about which message to make. As people age, the splicing factors tend to work less efficiently or not at all, restricting the ability of cells to respond to challenges in their environment. Senescent cells, which can be found in most organs from older people, also have fewer splicing factors.
“This demonstrates that when you treat old cells with molecules that restore the levels of the splicing factors, the cells regain some features of youth. They are able to grow, and their telomeres are now longer, as they are in young cells. Far more research is needed now to establish the true potential for these sort of approaches to address the degenerative effects of ageing.”
Messenger RNA (mRNA) processing has been implicated as a key determinant of lifespan. Splicing factor expression is dysregulated in the peripheral blood of aging humans, where they are the major functional gene ontology class whose transcriptpatterns alter with advancing age and in senescent primary human cells of multiple lineages. Splicing factor expression is also an early determinant of longevity in mouse and man, and in both species these changes are likely to be functional, since they are associated with alterations in splice site usage for many genes. Recent data suggests that modification of the levels of SFA-1, a core component of the spliceosome, influences lifespan in C. elegans through interaction with TORC1 machinery.
The splicing process is regulated on two levels. Firstly, constitutive splicing is carried out by the core spliceosome, which recognises splice donor and acceptor sites that define introns and exons. However, fine control of splice site usage is orchestrated by a complex interplay between splicing regulator proteins such as the Serine Arginine (SR) class of splicing activators and the heterogeneous ribonucleoprotein (hnRNP) class of splicing repressors. Other aspects of information transfer from DNA to protein, such as RNA export and mRNA stability are also influenced by splicing factors. Intriguingly, in addition to their splicing roles, many splicing factors have non-canonical additional functions regulating processes relevant to ageing. For example, hnRNPK, hnRNPD and hnRNPA1have been shown to have roles in telomere maintenance and hnRNPA2/B1 is involved in maintenance of stem cell populations.
Splicing factor expression is known to be dysregulated in senescent cells of multiple lineages and it is now well established that the accumulation of senescent cells is a direct cause of multiple aspects of both ageing and age-related disease in mammals. These observations suggest that an interrelationship may exist between well characterised mechanisms of ageing, such as cellular senescence, and the RNA splicing machinery where the mechanistic relationship to ageing remains largely correlational.
In contrast to the situation with core spliceosomal proteins such as SFA-1, perturbation of a single splicing regulator by standard molecular techniques such as knockdown or overexpression is unlikely to be informative for assessment of effects on ageing and cell senescence, since ageing is characterised by co-ordinate dysregulation of large modules of splicing factors. Thus experimental tools capable of co-ordinately modulating the expression of multiple components simultaneously are required to address the potential effects of the dysregulation of large numbers of splicing factors that we note during the ageing process. Small molecules such as resveratrol have been reported to influence splicing regulatory factor expression. Unfortunately, resveratrol has multiple biological effects, and thus a ‘clean’ assessment of the effects of moderation of splicing factor levels on cell physiology cannot be achieved using this compound alone.
We have overcome this limitation through development of a novel library of resveratrol-related compounds (resveralogues) which are all capable of either directly or indirectly influencing the expression of multiple splicing factors of both SRSF and HNRNP subtypes. Treatment of senescent human fibroblasts from different developmental lineages with any of these novel molecules shifts expression patterns of multiple splicing factors to those characteristic of much younger cells. This change occurs regardless of cell cycle traverse and is associated with a marked decrease in key biochemical and molecular biomarkers of senescence without any significant alteration in levels of apoptosis. Elevated splicing factor expression is also associated with elongation of telomeres, and in growth permissive conditions, these previously senescent populations show significant increases in growth fraction and in absolute cell number, indicating cell cycle re-entry.
This article was originally published at Fight Aging and has been republished under Creative Commons.