Summary Incorrect attachment of kinetochore microtubules is the leading cause of chromosome missegregation in cancers. The highly conserved chromosomal passenger complex (CPC), containing mitotic kinase Aurora B as a catalytic subunit, ensures faithful chromosome segregation through destabilizing incorrect microtubule attachments and promoting bi-orientation of chromosomes on the mitotic spindle. It is unknown whether CPC dysfunction affects chromosome segregation fidelity in cancers and, if so, how. Here we show that heterochromatin protein 1 (HP1) is an essential CPC component required for full Aurora B activity. HP1 binding to the CPC becomes particularly important when Aurora B phosphorylates kinetochore targets to eliminate erroneous microtubule-attachments. Remarkably, a reduced proportion of HP1-bound to CPC is widespread in cancers, which causes an impairment in Aurora B activity. These results indicate that HP1 is an essential modulator for CPC function, and identify a molecular basis for chromosome segregation errors in cancer cells.

The cyclin B-dependent protein kinase Cdk1 is a master regulator of mitosis and phosphorylates numerous proteins on the minimal consensus motif Ser/Thr-Pro (S/T-P). At least in several proteins, however, not well-defined motifs lacking a Pro in the +1 position, referred herein to as non-S/T-P motifs, have been shown to be phosphorylated by Cdk1. Here we show that non-S/T-P motifs in fact form consensus sequences for Cdk1 and probably play roles in mitotic regulation of physiologically important proteins. First, we show, by in vitro kinase assays, that previously identified non-S/T-P motifs all harbour one or more C-terminal Arg/Lys residues essential for their phosphorylation by Cdk1. Second, using Arg/Lys-scanning oriented peptide libraries, we demonstrate that Cdk1 phosphorylates a minimal sequence S/T-X-X-R/K and more favorable sequences (P)-X-S/T-X-[R/K]2–5 as its non-S/T-P consensus motifs. Third, on the basis of these results, we find that highly conserved linkers (typically, T-G-E-K-P) of C2H2 zinc finger proteins and a nuclear localization signal-containing sequence (matching P-X-S-X-[R/K]5) of the cytokinesis regulator Ect2 are inhibitorily phosphorylated by Cdk1, well accounting for the known mitotic regulation and function of the respective proteins. We suggest that non-S/T-P Cdk1 consensus motifs identified here may function to regulate many other proteins during mitosis.

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In vertebrates, unfertilized eggs are arrested at metaphase of meiosis II by Mos and Emi2, an inhibitor of the APC/C ubiquitin ligase. In Xenopus, Cdk1 phosphorylates Emi2 and both destabilizes and inactivates it, whereas Mos recruits PP2A phosphatase to antagonize the Cdk1 phosphorylation. However, how Cdk1 phosphorylation inhibits Emi2 is largely unknown. Here we show that multiple N-terminal Cdk1 phosphorylation motifs bind cyclin B1-Cdk1 itself, Plk1, and CK1δ/ε to inhibit Emi2. Plk1, after rebinding to other sites by self-priming phosphorylation, partially destabilizes Emi2. Cdk1 and CK1δ/ε sequentially phosphorylate the C-terminal APC/C-docking site, thereby cooperatively inhibiting Emi2 from binding the APC/C. In the presence of Mos, however, PP2A-B56β/ε bind to Emi2 and keep dephosphorylating it, particularly at the APC/C-docking site. Thus, Emi2 stability and activity are dynamically regulated by Emi2-bound multiple kinases and PP2A phosphatase. Our data also suggest a general role for Cdk1 substrate phosphorylation motifs in M phase regulation.

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There are thousands of known cellular phosphorylation sites, but the paucity of ways to identify kinases for particular phosphorylation events remains a major roadblock for understanding kinase signaling. To address this, we here develop a generally applicable method that exploits the large number of kinase inhibitors that have been profiled on near-kinomewide panels of protein kinases. The inhibition profile for each kinase provides a fingerprint that allows identification of unknown kinases acting on target phosphosites in cell extracts. We validate the method on diverse known kinase-phosphosite pairs, including histone kinases, EGFR autophosphorylation, and Integrin β1 phosphorylation by Src-family kinases. We also use our approach to identify the previously unknown kinases responsible for phosphorylation of INCENP at a site within a commonly phosphorylated motif in mitosis (a non-canonical target of Cyclin B-Cdk1), and of BCL9L at S915 (PKA). We show that the method has clear advantages over in silico and genetic screening.

Previous work showed a link between Tie2+ nucleus pulposus progenitor cells (NPPC) and disc degeneration. However, NPPC remain difficult to maintain in culture. Here, we report whole tissue culture (WTC) combined with fibroblast growth factor 2 (FGF2) and chimeric FGF (cFGF) supplementation to support and enhance NPPC and Tie2 expression. We also examined the role of PI3K/Akt and MEK/ERK pathways in FGF2 and cFGF-induced Tie2 expression. Young herniating nucleus pulposus tissue was used. We compared WTC and standard primary cell culture, with or without 10 ng/mL FGF2. PI3K/Akt and MEK/ERK signaling pathways were examined through western blotting. Using WTC and primary cell culture, Tie2 positivity rates were 7.0 ± 2.6% and 1.9 ± 0.3% (p = 0.004), respectively. Addition of FGF2 in WTC increased Tie2 positivity rates to 14.2 ± 5.4% (p = 0.01). FGF2-stimulated expression of Tie2 was reduced 3-fold with the addition of the MEK inhibitor PD98059 (p = 0.01). However, the addition of 1 μM Akt inhibitor, 124015-1MGCN, only reduced small Tie2 expression (p = 0.42). cFGF similarly increased the Tie2 expression, but did not result in significant phosphorylation in both the MEK/ERK and PI3K/Akt pathways. WTC with FGF2 addition significantly increased Tie2 maintenance of human NPPC. Moreover, FGF2 supports Tie2 expression via MEK/ERK and PI3K/Akt signals. These findings offer promising tools and insights for the development of NPPC-based therapeutics.

After the discovery of functionally superior Tie2-positive nucleus pulposus (NP) progenitor cells, new methods were needed to enable mass culture and cryopreservation to maintain these cells in an undifferentiated state with high cell yield. We used six types of EZSPHERE® dishes, which support spheroid-forming colony culture, and examined NP cell spheroid-formation ability, number, proliferation, and mRNA expression of ACAN, COL1A2, COL2A1, and ANGPT1. Six different types of cryopreservation solutions were examined for potential use in clinical cryopreservation by comparing the effects of exposure time during cryopreservation on cell viability, Tie2-positivity, and cell proliferation rates. The spheroid formation rate was 45.1% and the cell proliferation rate was 7.75 times using EZSPHERE® dishes. The mRNA levels for COL2A1 and ANGPT1 were also high. In cryopreservation, CryoStor10 (CS10) produced ≥90% cell viability and a high proliferation rate after thawing. CS10 had a high Tie2-positive rate of 12.6% after culturing for 5 days after thawing. These results suggest that EZSPHERE enabled colony formation in cell culture without the use of hydrogel products and that CS10 is the best cryopreservation medium for retaining the NP progenitor cell phenotype and viability. Together, these data provide useful information of NP cell-based therapeutics to the clinic.