Spindle assembly checkpoint genes reveal distinct as well as overlapping expression that implicates MDF-2/Mad2 in postembryonic seam cell proliferation in Caenorhabditis elegans

Background

The spindle assembly checkpoint (SAC) delays anaphase onset by inhibiting the activity of the anaphase promoting complex/cyclosome (APC/C) until all of the kinetochores have properly attached to the spindle. The importance of SAC genes for genome stability is well established; however, the roles these genes play, during postembryonic development of a multicellular organism, remain largely unexplored.

Results

We have used GFP fusions of 5' upstream intergenic regulatory sequences to assay spatiotemporal expression patterns of eight conserved genes implicated in the spindle assembly checkpoint function in Caenorhabditis elegans. We have shown that regulatory sequences for all of the SAC genes drive ubiquitous GFP expression during early embryonic development. However, postembryonic spatial analysis revealed distinct, tissue-specific expression of SAC genes with striking co-expression in seam cells, as well as in the gut. Additionally, we show that the absence of MDF-2/Mad2 (one of the checkpoint genes) leads to aberrant number and alignment of seam cell nuclei, defects mainly attributed to abnormal postembryonic cell proliferation. Furthermore, we show that these defects are completely rescued by fzy-1(h1983)/CDC20, suggesting that regulation of the APC/C^CDC20 by the SAC component MDF-2 is important for proper postembryonic cell proliferation.

Conclusion

Our results indicate that SAC genes display different tissue-specific expression patterns during postembryonic development in C. elegans with significant co-expression in hypodermal seam cells and gut cells, suggesting that these genes have distinct as well as overlapping roles in postembryonic development that may or may not be related to their established roles in mitosis. Furthermore, we provide evidence, by monitoring seam cell lineage, that one of the checkpoint genes is required for proper postembryonic cell proliferation. Importantly, our research provides the first evidence that postembryonic cell division is more sensitive to SAC loss, in particular MDF-2 loss, than embryonic cell division.     

Use of C-banding for identifying radiation induced micronuclei

Differentiation of micronuclei (MN) caused by ionizing radiation from those caused by chemicals is a crucial step for managing treatment of individuals exposed to radiation. MN in binucleated lymphocytes in peripheral blood are widely used as biomarkers for estimating dose of radiation, but they are not specific for ionizing radiation. MN induced by ionizing radiation originate predominantly as a result of chromosome breaks (clastogenic action), whereas MN caused by chemical agents are derived from the loss of entire chromosomes (aneugenic action). C-banding highlights centromeres, which might make it possible to distinguish radiation induced MN, i.e., as a byproduct of acentric fragments, from those caused by the loss of entire chromosomes. To test the use of C-banding for identifying radiation induced MN, a blood sample from a healthy donor was irradiated with 3 Gy of Co-60 gamma rays and cultured. Cells were harvested and dropped onto slides, divided into a group stained directly with Giemsa and another processed for C banding, then stained with Giemsa. The frequency of MN in 500 binucleated cells was scored for each method. In preparations stained with Giemsa directly, the MN appeared as uniformly stained structures, whereas after C banding, some MN exhibited darker regions corresponding to centromeres that indicated that they were not derived from acentric fragments. The C-banding technique enables differentiation of MN from acentric chromosomal material. This distinction is useful for improving the specificity of the MN assay as a biomarker for ionizing radiation.     

Stability and sequence-specific DNA binding of activation-labile mutants of the human glucocorticoid receptor

The stability and DNA-binding properties of activation-labile (act1) human glucocorticoid receptors (hGRs) from the glucocorticoid-resistant mutant 3R7.6TG.4 were investigated. These receptors are able to bind reversibly associating ligands with normal affinity and specificity, but become unstable during attempted activation to the DNA binding form [Harmon et al. (1984) J. Steroid Biochem. 21, 227-236]. Affinity labeling and immunochemical analysis demonstrated that act1 receptors are not preferentially proteolyzed during attempted activation. In addition, analysis of binding to calf thymus DNA showed that after loss of ligand, act1 receptors retain the ability to bind to DNA nonspecifically. A 370 bp MMTV promoter fragment containing multiple GREs and an upstream 342 bp fragment lacking GRE sequences were used to assess the binding of act1 hGR to specific DNA sequences. Immunoadsorption of hGR-DNA complexes after incubation with 32P-end-labeled fragments showed that both normal and act1 hGR bound selectively to the GRE-containing fragment in an activation-dependent manner. Binding of both normal and act1 hGRs could be blocked with a synthetic oligonucleotide containing a perfect palindromic GRE, but not with an oligonucleotide in which the GRE was replaced by an ERE. Analogous results were obtained for normal and act1 hGR activated in the absence of ligand, or after incubation with the glucocorticoid antagonist RU 38486. These results suggest that sequence-specific binding of the hGR does not require the presence of bound ligand and suggest a role for the ligand in trans-activation of hormonally responsive genes.