You might recall a paper published last year in which the authors reported that cellular senescence is a primary cause of declining capacity in liver regeneration with age. Here the obvious next step is taken, and a senolytic therapy is tested for its ability to restore the capacity of the aging liver to regenerate by clearing senescent cells. Improvement in all sorts of measures that normally decline with age is the expected result of senolytic treatment at this point, given the extensive evidence accumulated to date. Senescent cells are a cause of aging and age-related decline, they actively maintain a dysfunction state of metabolism via their secretions, and thus of course getting rid of them helps. Though, as noted in this paper, things are never as simple as we might hope them to be.
Many tissues, including the liver, heart and limbs possess a limited regenerative capacity in newborn or young mice, but which is lost upon maturation. As far as we are aware, misregulation of senescence has not been causally linked to such loss of regenerative capacity. Here, we first assessed the dynamic patterns of senescence markers during liver regeneration in young and adult mice. Although a transient p53-independent increase in p21 is well described following partial hepatectomy, its precise functions remain unclear, and loss of p21 does not seem to adversely impact regeneration in young animals. However, in models of advanced aging and severe liver damage, aberrantly expressed senescence markers, including p21 and p16Ink4a have been reported to impede liver regeneration. For example, in models of liver fibrosis, a robust senescence response is induced primarily in the stellate cells, which serves to limit fibrosis. Other models of severe liver damage induce a pronounced p21-expressionin hepatocytes, which results in decreased regeneration, senescence,and senescence-spreading.
We find that following partial hepatectomy, the senescence markers p21, p16Ink4a, and p19Arf become dynamically expressed at an age when regenerative capacity decreases. In addition, we demonstrate that treatment with a senescence-inhibiting drug improves regenerative capacity, through targeting of aberrant p21 expression. Surprisingly, we also find that the senescence marker p16Ink4a is expressed in a different cell-population to p21, and is unaffected by senescence targeting. This work suggests that senescence may initially develop as a heterogeneous cellular response, and that treatment with senolytic drugs may aid in promoting organ regeneration.
Senolytic treatment is increasingly shown to have beneficial effects in enhancing tissue function and alleviating disease symptoms in a variety of tissues. However, in many cases, the specific cellular targets or molecular mediators in vivo remain to be identified. Our study suggests that p21-positive cells may be a primary target. This is supported by the findings that protection from apoptosis is a main function of p21, including in senescent cells, and many senolytics, including the one used here, work by blocking anti-apoptotic pathways. In addition,as p21 functions to protect cells from damage, prolonged loss of p21 in aging mice predisposes to cancer through loss of this cytoprotective effect.
Interestingly, our study suggests that a one-time removal of p21 positive cells using a senolytic has a beneficial effect on regeneration, but probably without the long-term consequences of p21-loss. Surprisingly however, we see no effect of senolytic treatment on the increased expression of p16Ink4a that is present prior to hepatectomy, and which becomes detectable at the same stage as the decrease in regenerative capacity. Many studies show how targeting p16Ink4a expression has beneficial effects on aged and damaged tissue. However, in most cases, this also results in reduction of p21 and p19Arf, making it difficult to discern specific effects of each gene.
As p16Ink4a and p21 are expressed in different cell populations in our study, this hints that p16Ink4a, at this level of expression at least, may have beneficial effects in the liver also. However, why senolytic treatment seems to eliminate p16Ink4a positive cells in other contexts and not here remains unknown, but probably relates to the level of p16Ink4a-expression or co-expression with other senescence genes, as p16Ink4a levels become increasingly higher with age. Perhaps with advanced age or chronic damage, p21 and p16Ink4a become co-expressed, and at higher levels in the same cell types, resulting in a full-senescence response, and what we witness here is an early stage in a cumulative and progressive decline that becomes more complex over time.