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Disruption of microtubules activates the spindle checkpoint, of which BubR1 is a major component. Our early studies show that BubR1 haplo‐insufficiency results in enhanced mitotic slippage in vitro and tumorigenesis in vivo. Objective: Given that both MAPKs/ERKs and MEK play an important role during mitosis, we investigated whether there existed regulatory relationship between the MAPK signalling pathway and BubR1. Method and Results: Here, we have demonstrated that BubR1 deficiency is correlated with enhanced activation of MEK and ERKs after disruption of microtubule dynamics. Specifically, treatment with nocodazole and paclitaxel resulted in hyper‐activation of ERKs and MEK in BubR1(+/–) murine embryonic fibroblasts (MEF) compared to that of wild‐type MEFs. This enhanced activation of ERKs and MEK was at least partly responsible for more successful proliferation completion when cells were treated with nocodazole. BubR1 knockdown via RNAi resulted in enhanced activation of ERKs and MEK in HeLa cells, correlating with inhibition of PP1, a negative regulator of MEK. Moreover, when BubR1 was partially inactivated due to premature missegregation of chromosomes after Sgo1 depletion, phosphorylation of ERKs and MEK was enhanced in mitotic cells; in contrast, little, if any activated ERKs and MEK were detected in mitotic cells induced by nocodazole. Furthermore, BubR1, activated ERKs and activated MEK all localized to spindle poles during mitosis, and also, the proteins physically interacted with each other. Conclusion: Our studies suggest that there exists a cross‐talk between spindle checkpoint components and ERKs and MEK and that BubR1 may play an important role in mediating the cross‐talk.

Perforin and Fas are the two main pathways by which cytotoxic T lymphocytes (CTLs) mediate target cell lysis in vitro. The perforin pathway is predominantly used by CD8+ cells, which comprise the majority of CTLs. The Fas pathway has been demonstrated to be the principal cytolytic mechanism in CD4+ CTLs. CTLs have been shown to play an important role in allograft rejection. In this study, we examined the relevance of perforin and Fas to allograft rejection by transplanting pancreatic islets from fully allogeneic C3H/HeJ (C3H) or Fas-deficient C3H/lpr donors into perforin-deficient (P0) mice or controls with intact perforin genes (P2).P0 or P2 mice that were rendered diabetic with streptozotocin at 300 mg/kg i.p. received approximately 350 islets obtained from C3H or C3H/lpr donors by in situ collagenase digestion and Ficoll density centrifugation of the pancreas. Four groups of animals were studied: C3H to P2 (group 1), C3H to P0 (group 2), lpr to P0 (group 3), and syngeneic P2 to P2 (group 4). Graft survival monitored by blood sugar levels was compared among the groups. At the time of rejection (blood sugar300 mg/100 ml), grafts were harvested and analyzed by histopathology, immunocytochemistry, and reverse transcriptase-polymerase chain reaction. Primary splenic T cells of the recipients, harvested at the time of rejection, were tested for cytotoxicity against 51Cr-labeled donor cells.The mean graft survival for groups 1, 2, and 3 was 10.2+/-1.4, 12.2+/-6.0, and 13.2+/-0.8 days, respectively. Syngeneic grafts survived indefinitely. Rejecting grafts from all groups (1, 2, and 3) showed an intense infiltration by both CD4+ and CD8+ cells and complete islet destruction. Reverse transcriptase-polymerase chain reaction revealed granzyme B in rejecting grafts from all three groups.Perforin and Fas pathways alone or in combination are not required for islet rejection, suggesting that these pathways may not play a crucial role in allograft rejection.