Towards restoring proper chromosome segregation and preventing ageing

The natural progressive dysfunction of most living organisms—ageing—has captured the attention of several studies, with the intention to develop rejuvenation strategies. Evidence is emerging of a positive correlation between natural ageing and chromosomal instability (CIN). In this issue of EMBO Reports, Barroso‐Vilares et al [1] now show a link between ageing and the erroneous assembly of the apparatus required for a proper cellular division. They compare this mechanism in young and naturally aged human cells and describe a strategy to delay age‐related CIN.

Chromosomal instability leads to aneuploidy and contributes to ageing. A study in this issue shows that increased chromosome mis‐segregation rates and senescent phenotypes in old‐aged cells can be reduced by genetic or pharmacological enhancement of kinesin‐13 activity.

Ageing is a considerable risk factor for many diseases, including progeroid syndromes, pulmonary fibrosis, cardiovascular disease, diabetes and cancer 2. Ageing is associated with cellular senescence, a cell‐intrinsic mechanism that arises in the presence of different types of stress such as certain DNA lesions, oncogene activation or exposure to reactive oxygen species 3. These stressors induce signalling networks that result in a temporal cell cycle arrest to allow the repair of the damaged site in the genome. In severe situations, a shift in the cellular metabolism induces the establishment of full senescence. Although previously considered an irreversible process, there is evidence showing that the ageing process is plastic, and rejuvenation strategies have been recently developed in order to target ageing cell traits and renew aged cells 4. Of these, clearance of senescent cells is a promising approach to renew tissues and extend lifespan 5. A recent study from the Logarinho group following cell divisions in young and old human cells using live‐cell imaging revealed increased chromosome segregation errors in elderly cells that provoked senescence 6. Despite the increasing evidence of a direct link between aneuploidy (the wrong number of chromosomes) and age, the analysis of the inaccurate underlying mitotic process is still incomplete, and a challenge in the field remains how to rescue the mitotic decline in elderly cells.

At the beginning of mitosis, key components of the mitotic machinery—the centrosomes—localise at opposite sides of the cell and nucleate the spindle microtubules that will search for chromosomes to attach to. To ensure accurate chromosome segregation during mitosis, a crucial step is the formation of correct chromosome‐microtubule attachments. Proper segregation occurs when both sister chromatids are attached to microtubules emanating from opposite centrosomes (bioriented), ready to be pulled apart. However, during the process of chromosome capture by the spindle microtubules, some chromosomes may fail in the process of biorientation. Although a dedicated apparatus (the mitotic checkpoint) is able to detect errors and delay progression to restore correct attachment, some erroneous attachment types can slip through undetected. Specifically, when a single chromatid becomes attached to microtubules emanating from both centrosomes, a robust microtubule depolymerase activity is required to detach microtubules, allowing another attempt at biorientation (Fig 1). In this issue of EMBO reports, Barroso‐Vilares et al 1 report that human fibroblasts from elderly healthy Caucasian males show an increased number of over‐stable chromosome‐microtubule attachments, compared with neonatal human fibroblasts, indicating an imbalance in the dynamics of microtubule growth and depolymerisation. This could lead to an increased number of incorrect chromosome‐microtubule attachments. To test this hypothesis, the authors assessed the efficiency of error correction in elderly cells by introducing small molecules that transiently disturb chromosome‐microtubule attachment and induce chromosome segregation errors. By live‐cell imaging, they demonstrated that elderly cells are less able to correct the errors than their young counterparts. Accordingly, experiments using fluorescence in situ hybridisation (FISH) to count chromosome numbers in the recently created daughter cells confirmed the higher chromosome mis‐segregation rates for elderly cells. A tight control of microtubule attachment to chromosomes is essential during mitosis, together with a mechanism able to release improper attachment and restore the mitotic spindle fidelity. The relationship between these mechanisms and genome instability in human cancer cells has been carefully described by the Compton group 7. The ability of the cell to release faulty attachments relies on kinesin‐13 proteins, which promote microtubule depolymerisation and turnover, allowing the formation of new chromosome‐microtubule attachments. Decreased levels of kinesin‐13 proteins underlie the reduced error correction in chromosomally unstable cancer cells and are one of the main causes of their high rate of chromosome mis‐segregation. Interestingly, among other genes involved in mitotic regulation, one such kinesin, the mitotic centromere‐associated kinesin (MCAK), was downregulated in elderly cells studied by Barroso‐Vilares et al 1. This suggested that aged cells are less efficient in correcting erroneous chromosome‐microtubule attachments due to insufficient MCAK activity.

Figure 1. A small molecule can be used to rescue age‐associated CIN and delay cellular ageing in elderly human cells.

Blue panel—Cartoon of a healthy young cell going into mitosis. Proper kinetochore‐microtubule (KT‐MT) attachments are established in metaphase and result in proper chromosome segregation. Daughter cells are diploid and exhibit normal proliferation. Red panel—Cartoon of an elderly cell going into mitosis. An increased number of KT‐MT attachments are established, together with a higher rate of erroneous attachment. This leads to an improper chromosome segregation, with lagging chromosomes that are enclosed in micronuclei (MN). When the content of micronuclei comes into contact with the cytosol, the DNA‐sensing receptor cGAS initiates an innate immune response, which culminates in the expression of senescence‐associated genes and contributes to a full‐senescence phenotype. Yellow panel—Barroso‐Vilares and colleagues 1 used a small molecule to modulate the activity of MCAK kinesin in elderly cells; this increases the turnover of microtubules and re‐establishes proper KT‐MT attachments. Therefore, age‐associated chromosome instability and the consequent senescent phenotype are delayed.

Impaired error correction can be rescued in chromosomally unstable cancer cell lines by overexpressing MCAK. Barroso‐Vilares et al 1 showed that this approach can also reduce chromosome segregation errors in elderly cells; however, overexpression of proteins is not easily applicable to delay ageing in organisms. A recently developed method to restore kinesin‐13 function relies on the small molecule UMK57, which potentiates the activity of MCAK and rescues chromosome segregation errors in the chromosomally unstable cancer cells 8. Barroso‐Vilares et al 1 now demonstrated that a low dose of UMK57 can also rescue age‐associated CIN without affecting normal mitotic progression of both old and young cells. The consequent reduction of aneuploidy impacted the downstream cellular signalling pathways and delayed the establishment of a full‐senescence phenotype (Fig 1). This is in agreement with an earlier study from the Logarinho group 6 showing that improved mitotic fitness ameliorates the accumulation of senescence phenotypes in elderly cells. Chromosomally unstable cancer cells treated with UMK57 rapidly display adaptive resistance and regain high chromosome mis‐segregation rates 8. Barroso‐Vilares and colleagues 1 now describe how the same treatment in aged cells can improve the mitotic fidelity of the population even after a prolonged time. This opens up the possibility to use a small molecule potentiator to slow down the senescence caused by chromosome mis‐segregation and aneuploidy. The next step would be to ascertain the validity of this approach in whole organisms.

Another characteristic of old‐aged aneuploid cells and a marker for chromosome instability are micronuclei. These are formed from lagging chromosomes that fail to rejoin the primary nucleus after a chromosome segregation error. Micronuclei are a source of DNA that can be released into the cytosol and start a cascade of pro‐inflammatory responses that culminate in the appearance of the senescence‐associated secretory phenotype (SASP). As a consequence, global and local modifications in DNA organisation take place, together with the upregulation of genes involved in the secretion of inflammatory factors and those involved in preventing the propagation of damaged cells. In this scenario, inhibition of micronuclei formation may prevent SASP acquisition and full‐senescence development. Indeed, the treatment with UMK57 decreased the number of micronuclei in elderly cells and reduced the number of senescent cells (Fig 1) 1.

An interesting observation that emerged from the study of Barroso‐Vilares et al 1 regards changes in chromatin remodelling during senescence. A fundamental player in mitosis is the centromere, the condensed region of the chromosome on which the kinetochore proteins assemble to form the attachment sites for microtubules of the mitotic spindle. A universal hallmark of early senescence is the decondensation of peri‐ and centromeric satellite heterochromatin, seen in both naturally aged human cells and fibroblasts from patients with Hutchinson‐Gilford progeria syndrome (HGPS), a rare genetic disorder characterised by the appearance of age‐related phenotypes in childhood. In elderly cells, the level of the centromeric histone variant CENP‐A is lower than in young healthy cells, while another centromeric protein involved in kinetochore assembly, CENP‐C, is upregulated. Given recent evidence about the role of kinetochore size for proper chromosome segregation 9, the authors hypothesised that the distension of the centromere in elderly cells may provide a larger platform for more microtubule occupancy that could also contribute to an impaired ability to correct erroneous chromosome‐microtubule attachments. It will be interesting to investigate this from a molecular point of view and further characterise the kinetochore architecture in young, elderly and HGPS human cells.

Due to the emerging links between ageing and aneuploidy, developing strategies to maintain proper chromosome segregation over lifetime is of great relevance. This could allow delaying the accumulation of senescence traits and improve the lifespan of individuals. Another possible application could be in the treatment of Hutchinson‐Gilford progeria syndrome. As recently reported, fibroblasts from an 8‐year‐old patient with progeria displayed high levels of senescence biomarkers, together with a significantly increased level of chromosome segregation errors compared with young fibroblasts 6. As for naturally aged human fibroblasts, the re‐establishment of a downregulated mitotic transcription factor was able to rescue the mitotic defects, aneuploidy and senescence in these progeria‐derived cells. It will be interesting to see whether the rescue of the mitotic fidelity via UMK57 treatment is also relevant in human fibroblasts from donors who exhibit premature ageing such as HGPS. This would provide a more feasible way of delaying senescence onset, with the potential for disease treatment and consequently lifespan extension. Future studies could further investigate other mitotic dysfunctions typical of HGPS cultured cells, such as delays in cytokinesis progression, nuclear assembly and progression through the cell cycle 10, for a full understanding of the disease and of the natural course of ageing.

EMBO Reports (2020) 21: e50322