Date Published: December 26, 2011
Publisher: Impact Journals LLC
Author(s): Antonello Lorenzini, Lauren S. Fink, Thomas Stamato, Claudio Torres, Christian Sell.
We have examined the tolerance of the spindle assembly checkpoint (SAC), as measured by the appearance of tetraploid cells in the presence of a microtubule inhibitor, in a series of primary cell strains derived from species with diverse lifespan and body size. We find that the integrity of the SAC varies among these species. There is a robust correlation between the integrity of the SAC and body size, but poor correlation with longevity and parameters of species development (i.e., time of female fertility, gestation length, and postnatal growth rate). The results suggest that fidelity of the SAC co-evolved more closely with the number of mitoses needed to reach adulthood than with species lifespan.
Rodent and human tissues are histologically indistinguishable, yet the rate of tumor formation differs significantly between the species. A higher rate of transformation in vitro (10−5 to 10−6 in rodents compared to 10−9 to 10−10 in humans) and a higher rate of tumor formation in vivo suggest that the rodent genome is inherently less stable [1, 2].
In order to examine the relative stability of the SAC in species with differing lifespans, we first confirmed the original observations that rodent cells were unable to maintain a G2 arrest in the presence of a microtubule inhibitor. Because the original observations were made using immortalized cell lines, it is important to verify that primary strains display the same difference in SAC stability. We examined primary fibroblast strains from mouse, dog, rabbit, cow, little brown bat, and human for their ability to maintain the SAC in the presence of the microtubule inhibitor colcemid. Cells were treated with colcemid while in log phase growth and analyzed for DNA content at 48 and 72 hours post-treatment. Flow cytometry analysis indicated, consistent with previous reports [12, 13], that mouse cells were unable to maintain a stable G2 arrest in the presence of colcemid. In addition, we found that the stability of the SAC varied widely across species (Table 1 and Figure 2). Following mitotic block, cells from some species, including mouse, rabbit, and little brown bat, displayed an increased percentage of cells containing greater than 4N DNA content compared with human cells. The increased polyploidy in these cell types in the presence of colcemid suggests that these cells continue to cycle, while human cells remain arrested in mitosis. In contrast, when DNA content was measured in dog and cow fibroblasts following colcemid treatment, there was no significant increase in polyploid DNA content, suggesting that fibroblasts from these species did not progress through the cell cycle.
Consistent with previous work , we find that stability of the SAC varies among species and that human cells have a very stable SAC compared with mouse cells. Interestingly, the percentage of cells with ˃4N DNA content, which presumably reflects the inherent stability of the SAC, varies significantly in untreated cells and correlates strongly with the mass of the organism from which the cell strain was derived. The correlation with species lifespan however, is poor, suggesting that the stability of the SAC is related to body mass more strongly than to species lifespan. From an evolutionary viewpoint, it is tempting to speculate that cellular mechanisms ensuring proper cell division may be more relevant for species that require a greater number of mitoses in a relatively short life span, cow for example, while other maintenance mechanisms may be more relevant when fewer mitoses are required over a longer life span, for example in the little brown bat (Figure 1). The high correlation between SAC stringency and species body size suggests that the SAC is more relevant to organisms that require a greater number of cell divisions, i.e., organisms with large body size.