Quantitative morphological analysis of proliferating and nonproliferating subpopulations of IMR-90 fibroblasts during aging in vitro

Isolation of Live Premature Senescent Cells Using FUCCI Technology

Cellular senescence plays an important role in diverse biological processes such as tumorigenesis and organismal aging. However, lack of methods to specifically identify and isolate live senescent cells hampers the precise understanding of the molecular mechanisms regulating cellular senescence. Here, we report that utilization of fluorescent ubiquitination-based cell cycle indicator (FUCCI) technology allows isolation of live premature senescent cells induced by doxorubicin treatment. Exposure of human foreskin fibroblasts (HFFs) to a low dose of doxorubicin led to cellular senescent phenotypes including formation of γ-H2AX and 53BP1 foci indicative of DNA damage, decreased cell proliferation and increased senescence-associated β-galactosidase (SA-β-gal) activity. Importantly, doxorubicin-induced senescent cells were arrested at S/G2/M phases of cell cycle which can be reported by a construct encoding a fragment of hGeminin fused with monomeric Azami-Green (mAG-hGeminin). Flow cytometric sorting of GFP⁺ cells from doxorubicin-treated HFFs carrying mAG-hGeminin reporter enabled isolation and enrichment of live senescent cells in the culture. Our study develops a novel method to identify and isolate live premature senescent cells, thereby providing a new tool to study cellular senescence.

The concept of telomeric non-reciprocal recombination (TENOR) applied to human fibroblasts grown in serial cultures: concordance with genealogical data

Since the discovery of the limited life span of human fibroblasts some 50 years ago, many genealogical studies have been undertaken to describe growth kinetics of fibroblasts in serial cultures by their individual division behavior. It is now accepted that proliferation capacities of human fibroblasts strongly depend on their telomere lengths and integrity. Telomeres shorten with each replication round, and there is a direct correlation between cell division capacity and telomere lengths; that is, the consumption of disposable telomeric DNA repeats during cell divisions progresses until critically short telomeres determining the replicative senescence of the cells are present. Recently, we have suggested that telomeres in fibroblasts can also become elongated during DNA replication by telomeric non-reciprocal recombination (TENOR). Here we discuss genealogical data collected over the last decades as well as more recent findings on the telomere-driven replicative senescence process, and we summarize both to give an integrated picture.

Heterogeneity of dimer excision in young and senescent human dermal fibroblasts

We have examined the relationship between nucleotide excision of the main UV-induced photoproduct, the cyclobutane pyrimidine dimer and in vitro cellular senescence. An in situ semiquantitative immunocytochemical assay has demonstrated that, following a UV-C dose of 15 J m-2, young human dermal fibroblasts maintained in a high level of serum are more efficient than senescent fibroblasts in the removal of dimers. However, in G0-arrested cultures (serum-starved), young fibroblasts are compromised in their ability to remove dimers and are significantly less efficient than senescent cells in this process. Supplementation of the culture medium with 0.1 mm deoxyribonucleosides enhances the removal of dimers in both young and senescent fibroblasts in proliferating or serum-starved cells. These data indicate that overall there is a modest but significant reduction in nucleotide excision of dimer photoproducts in cells as they age in vitro. In addition, G0-arrested young cells exhibit reduced removal of dimers, although this can be complemented by deoxyribonucleoside addition. In addition, this in situ assay has revealed heterogeneity in both susceptibility to UV-C-induced damage and excision. Overall, we provide evidence of reduced UV-induced damage excision in senescent compared with young fibroblasts, and demonstrate modulation of these processes in young and senescent cells under specific growth conditions.

Analysis of UV-induced damage and repair in young and senescent human dermal fibroblasts using the comet assay

A major cause of ageing is thought to be the accumulation of damage to macromolecules. Accumulation to DNA damage in cells therefore presupposes that aged cells are unable to repair this damage. We have used the in vitro model of cellular ageing to test the idea that senescent cells are deficient in some aspect of DNA repair. Using the alkaline single cell gel electrophoresis assay (comet assay), we have determined the responses of young and senescent human dermal fibroblasts to DNA damage caused by exposure to UVC light. At low doses of UVC, senescent cells generate smaller comets than young cells whilst at medium doses the situation is reversed. At high doses, young and senescent cells respond similarly to one another. Time course experiments revealing repair of DNA damage show that senescent cells generate larger comets than young cells at early stages of repair suggesting that either senescent cells bear more damage per genome than do young cells or that senescent cells are more efficient at excising bulky adducts from DNA. Cells maintained in low levels of serum irrespective of age are less able to repair DNA damage compared with cells maintained in high levels of serum, and furthermore young and senescent cells maintained in high levels of serum are equally able to repair DNA damage. Our data, therefore, reveal both age-dependent and age-independent responses to UV-induced DNA damage. Use of the comet assay highlights the heterogeneity of cellular responses to genotoxic stress.

Effect of dietary high doses of vitamin E on the cell size of T and B lymphocyte subsets in young and old CBA mice

Using anti-CD5 and anti-SIgM fluorescent monoclonal antibodies, four subsets of lymphocytes can be distinguished in CBA mice, SIgM+ (T2) and SIgM- (T1) T lymphocytes and, CD5+ (B1) and CD5- (B2) B lymphocytes. L3T4 anti-CD4 and Lyt2 anti-CD8 positivities delineate two major T lymphocyte subsets. The cell size of these subsets has been evaluated by their forward light scatter in flow cytometry after cell fixation. The mean cell size of the different subsets differs, according to subset, age and vitamin E treatment. Globally, there is an age-related increase in size for all subsets. In vitamin-E treated young animals, all subsets are smaller, and the percentages of the biggest B1 and B2 cells decrease. In old mice, the vitamin-E effect is far more variable. B2 cells tend to increase in size but the percentage of the biggest cells diminishes. On the contrary, there is a marked expansion of the large B1 cells. No effect is discernible on CD5+ T lymphocytes, but L3T4 and Lyt2 subpopulations increase in size. This study is a retrospective one and the mechanisms affecting cell size are speculative. Since the lymphocyte cell size was measured after fixation in an hypertonic medium devised for human blood processing, we cannot differentiate a real size modification from a differential volume resistance to experimental conditions. Whatever the case, the changes in cell volume argue for age-related changes in cell membrane permeability and volume homeostasis. For some subsets, cell activation and consequent size increase must also be considered. As far as vitamin E has marked anti-oxidant properties, its effect on cell size provides indirect evidence for a role of free radicals in the observed changes and gives support to the oxidant stress theory of ageing in immune senescence.