Polo-like kinase-1 (Plk1) performs multiple essential functions during the cell cycle. Here we show that human Plk1-deficient cells are unable to separate their centrosomes, fail to form a bipolar spindle, and undergo a Mad2/BubR1-dependent prometaphase arrest. However, electron microscopy demonstrates that kinetochore-microtubule interactions can be established in cells lacking Plk1. In addition, co-depletion of Plk1 and survivin allows mitotic exit. This indicates that Plk1 depletion does not prevent microtubule attachment, but specifically interferes with the generation of tension, as a consequence of a failure to form a bipolar spindle. Moreover, we find that after silencing of the spindle assembly checkpoint, degradation of cyclin B1 is unaffected in cells lacking Plk1. These data indicate that activation of the anaphase promoting complex or cyclosome (APC/C)-Cdc20 complex that is under control of the spindle assembly checkpoint does not require Plk1 activity. Finally, we find that translocation of chromosome passengers and initiation of cleavage furrow ingression is unaffected in cells depleted of Plk1. Thus, our data confirm an important role of Plk1 in bipolar spindle formation, and also demonstrate that Plk1 is dispensable for APC/C-Cdc20 activation and the initiation of cytokinesis.

CD84 is a member of the CD2 subset of the immunoglobulin superfamily of cell surface receptors. Several members of this family are involved in the activation of T cells and NK cells. Although CD84 was originally cloned from a human B cell line cDNA library, very little is known regarding its biology on primary human leukocytes. We investigated the expression and biochemistry of CD84 on human B cells. CD84 was expressed on B cells in peripheral blood, spleen and cord blood. Two populations of splenic B cells could be resolved, CD84lo and CD84hi. CD84hi B cells represented a subset of memory B cells as demonstrated by increased cell size, co‐expression of the memory B cell‐specific marker CD27, somatically mutated Ig variable region genes, and increased proliferation compared to CD84lo B cells. CD84 became rapidly phosphorylated on tyrosine residues following ligation with a specific monoclonal antibody and recruited the cytoplasmic adaptor proteins SAP and EAT‐2. The ability of CD84 to undergo tyrosine phosphorylation and to recruit these SH2 domain‐containing proteins suggests it may function in the activation of B cells, particularly memory cells, and its signal transduction pathway may utilize SAP and/or EAT‐2. Thus, investigation of expression and function of CD84 and CD27 is likely to contribute to a greater understanding of the development and biology of memory B cells in normal and immunocompromised hosts.

Polo, the founding member of the family of polo-like kinases (Plks) was identified in a Drosophila screen for mutants affecting spindle pole behavior.(1) Several mutants showed defects at their spindle poles and were hence named after the magnetic poles of the earth or geo-magnetic phenomena associated with them, like Polo and Aurora. Currently, the conserved family of Plks consists of many members throughout various species. Multiple Plks are present in mammalian cells (Plk1, Plk2/Snk, Plk3/Fnk/Prk, and Plk4/Sak) and Xenopus (Plx1-3), whereas in other species only one member has been identified, like Polo in Drosophila, Cdc5 in budding yeast and Plo1 in fission yeast. Plks are now viewed as important regulators of multiple functions before and during the mitotic cell division. In this review, we will focus our attention on human Plk1 and its family members Plk2-4 and the many roles they play during mitosis. Furthermore, we will describe the currently knowledge of the regulation of these functions.

T cell factor / lymphocyte enhancer factor (Tcf/Lef) transcription factors complex with the transcriptional co-activator beta-catenin to transduce Wnt signals in a variety of developmental systems. The prototypic family member Tcf-1 is highly expressed in T lineage cells. Tcf1-/- mice are defective in cell cycling of early thymocyte stages. Here, we show that the interaction of beta-catenin with Tcf-1 is required for full thymocyte development. This interaction may be established by signals mediated by Wnt1 and Wnt4, leading to increased Tcf-dependent transcriptional activity in thymocytes, as demonstrated in Tcf-LacZ reporter mice. Transduction of fetal thymocytes with Wnt1 and Wnt4 results in increased survival in an in vitro cell culture system. Retroviral expression of soluble Wnt receptor mutants that block Wnt signaling inhibits thymocyte development. These results imply an important role for the Wnt cascade in thymocyte development.

Cell surface receptors belonging to the CD2 subset of the Ig superfamily of molecules include CD2, CD48, CD58, 2B4, signaling lymphocytic activation molecule (SLAM), Ly9, CD84, and the recently identified molecules NTB-A/Ly108/SLAM family (SF) 2000, CD84H-1/SF2001, B lymphocyte activator macrophage expressed (BLAME), and CRACC (CD2-like receptor-activating cytotoxic cells)/CS-1. Some of these receptors, such as CD2, SLAM, 2B4, CRACC, and NTB-A, contribute to the activation and effector function of T cells and NK cells. Signaling pathways elicited via some of these receptors are believed to involve the Src homology 2 (SH2) domain-containing cytoplasmic adaptor protein SLAM-associated protein (SAP), as it is recruited to SLAM, 2B4, CD84, NTB-A, and Ly-9. Importantly, mutations in SAP cause the inherited human immunodeficiency X-linked lymphoproliferative syndrome (XLP), suggesting that XLP may result from perturbed signaling via one or more of these SAP-associating receptors. We have now studied the requirements for SAP recruitment to CD84 and lymphocyte activation elicited following ligation of CD84 on primary and transformed human T cells. CD84 was found to be rapidly tyrosine phosphorylated following receptor ligation on activated T cells, an event that involved the Src kinase Lck. Phosphorylation of CD84 was indispensable for the recruitment of SAP, which was mediated by Y262 within the cytoplasmic domain of CD84 and by R32 within the SH2 domain of SAP. Furthermore, ligating CD84 enhanced the proliferation of anti-CD3 mAb-stimulated human T cells. Strikingly, this effect was also apparent in SAP-deficient T cells obtained from patients with XLP. These results reveal a novel function of CD84 on human lymphocytes and suggest that CD84 can activate human T cells via a SAP-independent mechanism.