Mps1Mph1 Kinase Phosphorylates Mad3 to Inhibit Cdc20Slp1-APC/C and Maintain Spindle Checkpoint Arrests

The spindle checkpoint is a mitotic surveillance system which ensures equal segregation of sister chromatids. It delays anaphase onset by inhibiting the action of the E3 ubiquitin ligase known as the anaphase promoting complex or cyclosome (APC/C). Mad3/BubR1 is a key component of the mitotic checkpoint complex (MCC) which binds and inhibits the APC/C early in mitosis. Mps1^Mph1 kinase is critical for checkpoint signalling and MCC-APC/C inhibition, yet few substrates have been identified. Here we identify Mad3 as a substrate of fission yeast Mps1^Mph1 kinase. We map and mutate phosphorylation sites in Mad3, producing mutants that are targeted to kinetochores and assembled into MCC, yet display reduced APC/C binding and are unable to maintain checkpoint arrests. We show biochemically that Mad3 phospho-mimics are potent APC/C inhibitors in vitro, demonstrating that Mad3p modification can directly influence Cdc20^Slp1-APC/C activity. This genetic dissection of APC/C inhibition demonstrates that Mps1^Mph1 kinase-dependent modifications of Mad3 and Mad2 act in a concerted manner to maintain spindle checkpoint arrests.

When cells divide they need to ensure that a complete copy of their genetic material is transmitted to both daughter cells. Cells have evolved many controls to ensure that every division is carried out with very high fidelity. The spindle checkpoint is one such control, which acts as a surveillance system during mitosis. Defects in this checkpoint control lead to unequal segregation of DNA/chromosomes, termed aneuploidy, which is responsible for human birth defects and is very common in tumour cells.

The molecular components of the spindle checkpoint, identified initially through yeast genetics, include several protein kinases. Surprisingly few of their substrates have been identified. Here we identify the checkpoint protein Mad3 as an important substrate of the Mps1^Mph1 kinase. We show that Mps1^Mph1-dependent modification of Mad3 and Mad2 acts to delay cell division in situations where the genetic material would not be equally inherited by daughter cells. This delay enables the cell to correct any problems within the division machinery and thus avoid aneuploidy.

Embryonic stem cells as a source of differentiated neural cells for pharmacological screens

The process of bringing a new pharmacologically active drug to market is laborious, time consuming, and costly. From drug discovery to safety assessment, new methods are constantly sought to develop faster and more efficient procedures to eliminate drugs from further investigation because of their limited effectiveness or high toxicity. Because in vitro cell assays are an important arm of this discovery process, it is therefore somewhat unsurprising that there is an emerging contribution of embryonic stem (ES) cell technology to this area. This technology utilizes the in vitro differentiation of ES cells into somatic cell target populations that, when coupled to the use of "lineage selection" protocols, allows for the production of infinite numbers of pure populations of the desired cells for both bioactivity and toxicological screens. Unlike the use of transformed cell lines, ES-derived cells remain karyotypically normal and therefore better reflect the potential responses of cells in vivo, and when selected are more homogeneous than those obtained using primary cultures. In this chapter we discuss the use of ES cell-derived somatic cells in pharmacological screens, with particular emphasis on neural cells, and describe the methods and protocols associated with the development of ES cell-derived neural cell assays.

Epigenetic regulation of mammalian pericentric heterochromatin in vivo by HP1

We developed a model system whereby HP1 can be targeted to pericentric heterochromatin in ES cells lacking Suv(3)9h1/2 histone methyltransferase (HMTase) activities. HP1 so targeted can reconstitute tri-methylated lysine 9 of histone H3 (Me(3)K9H3) and tri-methylated lysine 20 of histone H4 (Me(3)K20H4) at pericentric heterochromatin, indicating that HP1 can regulate the distribution of these histone modifications in vivo. Both homo- and hetero-typic interactions between the HP1 isotypes were demonstrated in vivo as were HP1 interactions with the ESET/SETDB1 HMTase and the ATRX chromatin remodelling enzyme. We conclude that HP1 not only "deciphers" the histone code but can also "encode it".

Highly Efficient mRNA-Based Gene Transfer in Feeder-Free Cultured H9 Human Embryonic Stem Cells

Several protocols have been described for virus-based gene transfer in human embryonic stem (hES) cells, while efficient non-viral methods are currently non-existing. In this study, we investigated the efficiency of mRNA-based gene transfer in feeder-free cultured H9 hES cells, based on electroporation of in vitro transcribed mRNA encoding the enhanced green fluorescent protein (EGFP). Optimisation of culture and electroporation conditions for feeder-free cultured H9 hES cells resulted a highly pure, transgene-expressing (90% positive cells) H9 hES cell population.

Loss of Heterochromatin Protein 1 (HP1) chromodomain function in mammalian cells by intracellular antibodies causes cell death

The chromodomain (CD) is a highly conserved motif present in a variety of animal and plant proteins, and its probable role is to assemble a variety of macromolecular complexes in chromatin. The importance of the CD to the survival of mammalian cells has been tested. Accordingly, we have ablated CD function using two single-chain intracellular Fv (scFv) fragments directed against non-overlapping epitopes within the HP1 CD motif. The scFv fragments can recognize both CD motifs of HP1 and Polycomb (Pc) in vitro and, when expressed intracellularly, interact with and dislodge the HP1 protein(s) from their heterochromatin localization in vivo. Mouse and human fibroblasts expressing anti-chromodomain scFv fragments show a cell-lethal phenotype and an apoptotic morphology becomes apparent soon after transfection. The mechanism of cell death appears to be p53 independent, and the cells are only partly rescued by incubation with the wide spectrum caspase inhibitor Z-VAD fmk. We conclude that expression of anti-chromodomain intracellular antibodies is sufficient to trigger a p53-independent apoptotic pathway that is only partly dependent on the known Z-VAD-inhibitable caspases, suggesting that CD function is essential for cell survival.

The Ki-67 protein interacts with members of the heterochromatin protein 1 (HP1) family: a potential role in the regulation of higher-order chromatin structure

The expression of the nuclear protein Ki-67 (pKi-67) is strictly correlated with cell proliferation. Because of this, anti-Ki-67 antibodies can be used as operational markers to estimate the growth fraction of human neoplasia in situ. For a variety of tumours, the assessment of pKi-67 expression has repeatedly been proven to be of prognostic value for survival and tumour recurrence, but no cellular function has yet been ascribed to the Ki-67 protein. This study shows that a C-terminal domain of pKi-67 (Kon21) is able to bind to all three members of the mammalian heterochromatin protein 1 (HP1) family in vitro and in vivo. This interaction can be manipulated in living cells, as evidenced by ectopic expression of GFP-tagged HP1 proteins in HeLa cells, which results in a dramatic relocalization of endogenous pKi-67. Taken together, the data presented in this study suggest a role for pKi-67 in the control of higher-order chromatin structure.