Recent research at IRIC helps better understand how the cellular nucleus reassembles after mitosis. In eukaryotes, chromosomes are contained in a nucleus delineated by the nuclear envelope. This membrane is composed of two lipid bilayers associated with proteins that play structural roles and control exchanges between the nucleus and the cytoplasm. In animals including humans, the nuclear envelope is dissolved in mitosis to allow the assembly of a microtubule-based spindle that mechanically separates chromosomes. At the end of cell division, nuclear envelopes reassemble around two sets of segregated chromosomes as daughter cells enter interphase. How the nuclear envelope reassembles after mitosis is not completely understood.
The team led by Vincent Archambault at IRIC wanted to better understand the roles of a particular phosphatase enzyme known as PP2A-B55, in the cell cycle. PP2A-B55 was known to be essential from the transition from mitosis to interphase by removing phosphate groups from other proteins. However, its precise cellular functions were unknown. To identify genes that function with PP2A-B55 in the cell cycle, the researchers used the fruit fly Drosophila melanogaster. They looked for genomic deletions and mutations that induce lethal developmental defects in embryos where the activity of PP2A-B55 is already lowered. In this context, they found that weakening genes that encode proteins of the nuclear envelope is particularly deleterious. To investigate the molecular functions of PP2A-B55 during the reassembly of the nuclear envelope, Haytham Mehsen, a PhD student, used an approach combining genetics, biochemistry, molecular biology and imaging. He found that PP2A-B55 must dephosphorylate BAF, a small protein that binds DNA after mitosis and contributes to initiate the assembly of the nuclear envelope. If BAF is not dephosphorylated, it fails to fulfill this function. Results from this study also suggest that PP2A-B55 regulates other proteins in the complex process of exit from mitosis, which the Archambault lab will continue to investigate.
As genes and proteins have been largely conserved between animal species during evolution, molecular studies in the Drosophila model help understand various biological processes that are disrupted in human disease. Defects in cell division can lead to cancers. Nuclear envelope abnormalities in humans are associated with cancers, laminopathies and progeria.
This study was done in collaboration with researchers at the University of Windsor and the University of North Carolina at Chapel Hill. It is published in the December 3 issue of the Journal of Cell Biology.
Figure: Nuclei entering mitosis in a syncytial Drosophila embryo. Immunofluorescence analysis reveals microtubules in green, centrosomes and the nuclear envelope in red, and DNA in blue.