Mip1 associates with both the Mps1 kinase and actin, and is required for cell cortex stability and anaphase spindle positioning

Discovery of pyrazolo[3,4-b]pyridine derivatives as novel and potent Mps1 inhibitors for the treatment of cancer

Monopolar spindle kinase 1 (Mps1) is a key element of the mitotic checkpoint and clinically evaluated as a target in the treatment of aggressive tumors. With this aim, a set of pyrazolo[3,4-b]pyridine-based compounds as new Mps1 inhibitors was investigated through a multidisciplinary approach, based on virtual screening, chemical synthesis and biological evaluation. One of the representative compounds, 31, exhibited strong kinase inhibitory potency against Mps1 with an IC₅₀ value of 2.596 nM and significantly inhibited proliferation of cancer cells, especially MDA-MB-468 and MV4-11 cells. Compound 31 also displayed reasonable kinome selectivity against a panel of 606 wild-type kinases at 1 μM. Moreover, compound 31 exhibited suitable preclinical pharmacokinetic parameters and a promising pharmacodynamic profile. Further, compound 31 showed good antitumor efficacy in MDA-MB-468 xenograft model with no obvious toxicity. Overall, compound 31 was identified as a potential Mps1 inhibitor for cancer therapy and deserve further research.

SPECC1L binds the myosin phosphatase complex MYPT1/PP1β and can regulate its distribution between microtubules and filamentous actin

The subcellular localization, activity , and substrate specificity of the serine/threonine protein phosphatase 1 catalytic subunit (PP1_cat) is mediated through its dynamic association with regulatory subunits in holoenzyme complexes. While some functional overlap is observed for the three human PP1_cat isoforms, they also show distinct targeting based on relative preferences for specific regulatory subunits. A well-known example is the preferential association of MYPT1 with PP1β in the myosin phosphatase complex. In smooth muscle, MYPT1/PP1β counteracts the muscle contraction induced by phosphorylation of the light chains of myosin by the myosin light chain kinase. This phosphatase complex is also found in nonmuscle cells, where it is targeted to both myosin and nonmyosin substrates and contributes to regulation of the balance of cytoskeletal structure and motility during cell migration and division. Although it remains unclear how MYPT1/PP1β traffics between microtubule- and actin-associated substrates, our identification of the microtubule- and actin-binding protein SPECC1L in both the PP1β and MYPT1 interactomes suggests that it is the missing link. Our validation of their association using coimmunoprecipitation and proximity biotinylation assays, together with the strong overlap that we observed for the SPECC1L and MYPT1 interactomes, confirmed that they exist in a stable complex in the cell. We further showed that SPECC1L binds MYPT1 directly and that it can impact the balance of the distribution of the MYPT1/PP1β complex between the microtubule and filamentous actin networks.

Autocleavage of separase suppresses its premature activation by promoting binding to cyclin B1

Accurate chromosome segregation requires timely activation of separase, a protease that cleaves cohesin during the metaphase-to-anaphase transition. However, the mechanism that maintains the inactivity of separase prior to this event remains unclear. We provide evidence that separase autocleavage plays an essential role in this process. We show that the inhibition of separase autocleavage results in premature activity before the onset of anaphase, accompanied by the formation of chromosomal bridges and spindle rocking. This deregulation is attributed to the reduced binding of cyclin B1 to separase that occurs during the metaphase-to-anaphase transition. Furthermore, when separase is mutated to render the regulation by cyclin B1 irrelevant, which keeps separase in securin-binding form, the deregulation induced by autocleavage inhibition is rescued. Our results reveal a physiological role of separase autocleavage in regulating separase, which ensures faithful chromosome segregation.

Cytospin-A Regulates Colorectal Cancer Cell Division and Migration by Modulating Stability of Microtubules and Actin Filaments

Simple Summary

In this study, we report the effects of depleting cytospin-A (CYTSA), also known as the sperm antigen with calponin homology and coiled-coil domain (SPECC1L) protein, on the proliferation and migration of colorectal cancer (CRC) cells. Mutations in this protein have been previously linked to different developmental disorders. In our studies, depletion of CYTSA in various CRC cells led to significant decreases in proliferation, increases in cell death, and increased formation of multinucleated cells. Knocking down CYTSA also led to severe inhibition of CRC cell migration and invasion. These effects could be related to a significant decrease in the stability of microtubules and alterations in polymerized actin filaments in CYTSA depleted CRC cells. Our studies, for the first time, provide evidence suggesting that targeting CYTSA may be a novel therapeutic strategy for patients with CRC.

Abstract

Proteins that interact with cytoskeletal elements play important roles in cell division and are potentially important targets for therapy in cancer. Cytospin-A (CYTSA), a protein known to interact with actin and microtubules, has been previously described to be important in various developmental disorders, including oblique facial clefting. We hypothesized that CYTSA plays an important role in colorectal cancer (CRC) cell division. The effects of CYTSA depletion on CRC cell proliferation were analyzed using cell growth assays, microscopic analyses of live and fixed cells, and time-lapse imaging. CYTSA depletion led to inhibition of cell proliferation, significant increases in CRC cell death, and accumulation of doublet cells during and following cell division. Depletion of CYTSA also resulted in strong inhibition of CRC cell migration and invasion. Mechanistically, CYTSA depletion resulted in significant decreases in the stability of microtubules and altered polymerization of actin filaments in CRC cells. Finally, bioinformatic analyses were performed to determine the correlation between CYTSA expression and survival of patients with CRC. Interestingly, a strong correlation between high CYTSA expression and poor survival was observed in the TCGA adenocarcinoma data set but not in an independent data set. Since inhibiting CYTSA significantly reduces CRC cell proliferation, migration, and invasion, targeting CYTSA may be a potential novel therapeutic option for patients with metastatic CRC.

A New Quantitative Cell-Based Assay Reveals Unexpected Microtubule Stabilizing Activity of Certain Kinase Inhibitors, Clinically Approved or in the Process of Approval

Agents able to modify microtubule dynamics are important anticancer drugs. The absence of microtubules resulting from drug-induced depolymerization is easy to detect. However the detection of a stabilized microtubule network needs specific assays since there is not a significant visual difference between normal and stabilized microtubule networks. Here, we describe a quantitative cell-based assay, suitable for automation, which allows the detection of stabilized microtubules without the need of microscopic examination. The rationale of this assay is based on the drug-induced resistance of the microtubule network to the depolymerizing agent combretastatin A4 and the subsequent detection of the residual microtubules by immunoluminescence. Using this assay to screen a kinase inhibitor library allowed the selection of seven known kinase inhibitors: selonsertib, masatinib, intedanib, PF0477736, SNS-314 mesylate, MPI0479605, and ponatinib. The yet undescribed ability of these inhibitors to stabilize cellular microtubules was confirmed using additional markers of stable microtubules and time-lapse video-microscopy to track individual microtubules in living cells. None of the compounds interacted, however, directly with tubulin. By employing other inhibitors of the same kinases, which have structurally unrelated scaffolds, we determined if the microtubule stabilizing effect was due to the inhibition of the targeted kinase, or to an off-target effect. Many of these inhibitors are clinically approved or currently assayed in phase 2 or phase 3 clinical trials. Their microtubule-stabilizing effect may account for their therapeutic effect as well as for some of their adverse side effects. These results indicate also a possible repurposing of some of these drugs.

Role of E2Fs and mitotic regulators controlled by E2Fs in the epithelial to mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is a complex cellular process in which epithelial cells acquire mesenchymal properties. EMT occurs in three biological settings: development, wound healing and fibrosis, and tumor progression. Despite occurring in three independent biological settings, EMT signaling shares some molecular mechanisms that allow epithelial cells to de-differentiate and acquire mesenchymal characteristics that confer cells invasive and migratory capacity to distant sites. Here we summarize the molecular mechanism that delineates EMT and we will focus on the role of E2 promoter binding factors (E2Fs) in EMT during tumor progression. Since the E2Fs are presently undruggable due to their control in numerous pivotal cellular functions and due to the lack of selectivity against individual E2Fs, we will also discuss the role of three mitotic regulators and/or mitotic kinases controlled by the E2Fs (NEK2, Mps1/TTK, and SGO1) in EMT that can be useful as drug targets.

Impact statement

The study of the epithelial to mesenchymal transition (EMT) is an active area of research since it is one of the early intermediates to invasion and metastasis—a state of the cancer cells that ultimately kills many cancer patients. We will present in this review that besides their canonical roles as regulators of proliferation, unregulated expression of the E2F transcription factors may contribute to cancer initiation and progression to metastasis by signaling centrosome amplification, chromosome instability, and EMT. Since our discovery that the E2F activators control centrosome amplification and mitosis in cancer cells, we have identified centrosome and mitotic regulators that may represent actionable targets against EMT and metastasis in cancer cells. This is impactful to all of the cancer patients in which the Cdk/Rb/E2F pathway is deregulated, which has been estimated to be most cancer patients with solid tumors.

Protein kinase TTK promotes proliferation and migration and mediates epithelial-mesenchymal transition in human bladder cancer cells

AIM

To investigate the expression level of TTK in bladder cancer, and its role in the proliferation and migration. To investigate the relationship between TTK and epithelial-mesenchymal transition (EMT).

PATIENTS AND METHODS

We compared the expression level of TTK between human bladder cancer tissues and normal bladder epithelial tissues from 70 patients using immunohistochemistry, qRT-PCR and western blotting. Subsequently, we conducted cell viability and cell migration experiments to investigate the effect of TTK on bladder cancer cells. Furthermore, we used qRT-PCR to detect the biomarkers of EMT to examine the relationship between TTK and EMT.

RESULTS

The expression level of TTK was significantly higher in bladder cancer tissues as compared to the adjacent noncancerous tissues (P < 0.001). The qRT-PCR, immunohistochemistry, and western blotting also showed the same trend. Furthermore, cell viability and cell migration assays showed that TTK promoted proliferation and migration of human bladder cancer cells, and mediated EMT.

CONCLUSION

This study showed that high expression of TTK can promote proliferation and migration, and might mediate the EMT process in human bladder cancer cells.

Whole-genome duplication increases tumor cell sensitivity to MPS1 inhibition

Several lines of evidence indicate that whole-genome duplication resulting in tetraploidy facilitates carcinogenesis by providing an intermediate and metastable state more prone to generate oncogenic aneuploidy. Here, we report a novel strategy to preferentially kill tetraploid cells based on the abrogation of the spindle assembly checkpoint (SAC) via the targeting of TTK protein kinase (better known as monopolar spindle 1, MPS1). The pharmacological inhibition as well as the knockdown of MPS1 kills more efficiently tetraploid cells than their diploid counterparts. By using time-lapse videomicroscopy, we show that tetraploid cells do not survive the aborted mitosis due to SAC abrogation upon MPS1 depletion. On the contrary diploid cells are able to survive up to at least two more cell cycles upon the same treatment. This effect might reflect the enhanced difficulty of cells with whole-genome doubling to tolerate a further increase in ploidy and/or an elevated level of chromosome instability in the absence of SAC functions. We further show that MPS1-inhibited tetraploid cells promote mitotic catastrophe executed by the intrinsic pathway of apoptosis, as indicated by the loss of mitochondrial potential, the release of the pro-apoptotic cytochrome c from mitochondria, and the activation of caspases. Altogether, our results suggest that MPS1 inhibition could be used as a therapeutic strategy for targeting tetraploid cancer cells.

Mps1 kinase regulates tumor cell viability via its novel role in mitochondria

Targeting mitotic kinase monopolar spindle 1 (Mps1) for tumor therapy has been investigated for many years. Although it was suggested that Mps1 regulates cell viability through its role in spindle assembly checkpoint (SAC), the underlying mechanism remains less defined. In an endeavor to reveal the role of high levels of mitotic kinase Mps1 in the development of colon cancer, we unexpectedly found the amount of Mps1 required for cell survival far exceeds that of maintaining SAC in aneuploid cell lines. This suggests that other functions of Mps1 besides SAC are also employed to maintain cell viability. Mps1 regulates cell viability independent of its role in cytokinesis as the genetic depletion of Mps1 spanning from metaphase to cytokinesis affects neither cytokinesis nor cell viability. Furthermore, we developed a single-cycle inhibition strategy that allows disruption of Mps1 function only in mitosis. Using this strategy, we found the functions of Mps1 in mitosis are vital for cell viability as short-term treatment of mitotic colon cancer cell lines with Mps1 inhibitors is sufficient to cause cell death. Interestingly, Mps1 inhibitors synergize with microtubule depolymerizing drug in promoting polyploidization but not in tumor cell growth inhibition. Finally, we found that Mps1 can be recruited to mitochondria by binding to voltage-dependent anion channel 1 (VDAC1) via its C-terminal fragment. This interaction is essential for cell viability as Mps1 mutant defective for interaction fails to main cell viability, causing the release of cytochrome c. Meanwhile, deprivation of VDAC1 can make tumor cells refractory to loss of Mps1-induced cell death. Collectively, we conclude that inhibition of the novel mitochondrial function Mps1 is sufficient to kill tumor cells.

SPECC1L deficiency results in increased adherens junction stability and reduced cranial neural crest cell delamination

Cranial neural crest cells (CNCCs) delaminate from embryonic neural folds and migrate to pharyngeal arches, which give rise to most mid-facial structures. CNCC dysfunction plays a prominent role in the etiology of orofacial clefts, a frequent birth malformation. Heterozygous mutations in SPECC1L have been identified in patients with atypical and syndromic clefts. Here, we report that in SPECC1L-knockdown cultured cells, staining of canonical adherens junction (AJ) components, β-catenin and E-cadherin, was increased, and electron micrographs revealed an apico-basal diffusion of AJs. To understand the role of SPECC1L in craniofacial morphogenesis, we generated a mouse model of Specc1l deficiency. Homozygous mutants were embryonic lethal and showed impaired neural tube closure and CNCC delamination. Staining of AJ proteins was increased in the mutant neural folds. This AJ defect is consistent with impaired CNCC delamination, which requires AJ dissolution. Further, PI3K-AKT signaling was reduced and apoptosis was increased in Specc1l mutants. In vitro, moderate inhibition of PI3K-AKT signaling in wildtype cells was sufficient to cause AJ alterations. Importantly, AJ changes induced by SPECC1L-knockdown were rescued by activating the PI3K-AKT pathway. Together, these data indicate SPECC1L as a novel modulator of PI3K-AKT signaling and AJ biology, required for neural tube closure and CNCC delamination.

Harnessing the microRNA pathway for cardiac regeneration

Mounting evidence over the last few years has indicated that the rate of cardiomyocyte proliferation, and thus the extent of cardiac renewal, is under the control of the microRNA network. Several microRNAs (e.g. miR-1) regulate expansion of the cardiomyocyte pool and its terminal differentiation during the embryonic life; some not only promote cardiomyocyte proliferation but also their de-differentiation towards an embryonic cell phenotype (e.g. the miR-302/367 cluster); a few others are involved in the repression of cardiomyocyte proliferation occurring suddenly after birth (e.g. the miR-15 family); others again are not physiologically involved in the regulation of cardiomyocyte turnover, but nevertheless are able to promote cardiomyocyte proliferation and cardiac regeneration when delivered exogenously (e.g. miR-199a-3p). With a few exceptions, the molecular mechanisms underlying the pro-proliferative effect of these microRNAs, most of which appear to act at the level of already differentiated cardiomyocytes, remain to be thoroughly elucidated. The possibility of harnessing the miRNA network to achieve cardiac regeneration paves the way to exciting therapeutic applications. This could be achieved by either administering miRNA mimics or inhibitors, or transducing the heart with viral vectors expressing miRNA-encoding genes.

Characterization of novel MPS1 inhibitors with preclinical anticancer activity

Monopolar spindle 1 (MPS1), a mitotic kinase that is overexpressed in several human cancers, contributes to the alignment of chromosomes to the metaphase plate as well as to the execution of the spindle assembly checkpoint (SAC). Here, we report the identification and functional characterization of three novel inhibitors of MPS1 of two independent structural classes, N-(4-{2-[(2-cyanophenyl)amino][1,2,4]triazolo[1,5-a]pyridin-6-yl}phenyl)-2-phenylacetamide (Mps-BAY1) (a triazolopyridine), N-cyclopropyl-4-{8-[(2-methylpropyl)amino]-6-(quinolin-5-yl)imidazo[1,2-a]pyrazin-3-yl}benzamide (Mps-BAY2a) and N-cyclopropyl-4-{8-(isobutylamino)imidazo[1,2-a]pyrazin-3-yl}benzamide (Mps-BAY2b) (two imidazopyrazines). By selectively inactivating MPS1, these small inhibitors can arrest the proliferation of cancer cells, causing their polyploidization and/or their demise. Cancer cells treated with Mps-BAY1 or Mps-BAY2a manifested multiple signs of mitotic perturbation including inefficient chromosomal congression during metaphase, unscheduled SAC inactivation and severe anaphase defects. Videomicroscopic cell fate profiling of histone 2B-green fluorescent protein-expressing cells revealed the capacity of MPS1 inhibitors to subvert the correct timing of mitosis as they induce a premature anaphase entry in the context of misaligned metaphase plates. Hence, in the presence of MPS1 inhibitors, cells either divided in a bipolar (but often asymmetric) manner or entered one or more rounds of abortive mitoses, generating gross aneuploidy and polyploidy, respectively. In both cases, cells ultimately succumbed to the mitotic catastrophe-induced activation of the mitochondrial pathway of apoptosis. Of note, low doses of MPS1 inhibitors and paclitaxel (a microtubular poison) synergized at increasing the frequency of chromosome misalignments and missegregations in the context of SAC inactivation. This resulted in massive polyploidization followed by the activation of mitotic catastrophe. A synergistic interaction between paclitaxel and MPS1 inhibitors could also be demonstrated in vivo, as the combination of these agents efficiently reduced the growth of tumor xenografts and exerted superior antineoplastic effects compared with either compound employed alone. Altogether, these results suggest that MPS1 inhibitors may exert robust anticancer activity, either as standalone therapeutic interventions or combined with microtubule-targeting chemicals.

The novel actin/focal adhesion-associated protein MISP is involved in mitotic spindle positioning in human cells

Accurate mitotic spindle positioning is essential for the regulation of cell fate choices, cell size and cell position within tissues. The most prominent model of spindle positioning involves a cortical pulling mechanism, where the minus end-directed microtubule motor protein dynein is attached to the cell cortex and exerts pulling forces on the plus ends of astral microtubules that reach the cortex. In nonpolarized cultured cells integrin-dependent, retraction fiber-mediated cell adhesion is involved in spindle orientation. Proteins serving as intermediaries between cortical actin or retraction fibers and astral microtubules remain largely unknown. In a recent genome-wide RNAi screen we identified a previously uncharacterized protein, MISP (C19ORF21) as being involved in centrosome clustering, a process leading to the clustering of supernumerary centrosomes in cancer cells into a bipolar mitotic spindle array by microtubule tension. Here, we show that MISP is associated with the actin cytoskeleton and focal adhesions and is expressed only in adherent cell types. During mitosis MISP is phosphorylated by Cdk1 and localizes to retraction fibers. MISP interacts with the +TIP EB1 and p150^glued, a subunit of the dynein/dynactin complex. Depletion of MISP causes mitotic arrest with reduced tension across sister kinetochores, chromosome misalignment and spindle multipolarity in cancer cells with supernumerary centrosomes. Analysis of spindle orientation revealed that MISP depletion causes randomization of mitotic spindle positioning relative to cell axes and cell center. Together, we propose that MISP links microtubules to the actin cytoskeleton and focal adhesions in order to properly position the mitotic spindle.

The MPS1 family of protein kinases

MPS1 protein kinases are found widely, but not ubiquitously, in eukaryotes. This family of potentially dual-specific protein kinases is among several that regulate a number of steps of mitosis. The most widely conserved MPS1 kinase functions involve activities at the kinetochore in both the chromosome attachment and the spindle checkpoint. MPS1 kinases also function at centrosomes. Beyond mitosis, MPS1 kinases have been implicated in development, cytokinesis, and several different signaling pathways. Family members are identified by virtue of a conserved C-terminal kinase domain, though the N-terminal domain is quite divergent. The kinase domain of the human enzyme has been crystallized, revealing an unusual ATP-binding pocket. The activity, level, and subcellular localization of Mps1 family members are tightly regulated during cell-cycle progression. The mitotic functions of Mps1 kinases and their overexpression in some tumors have prompted the identification of Mps1 inhibitors and their active development as anticancer drugs.

Centriole assembly and the role of Mps1: defensible or dispensable?

The Mps1 protein kinase is an intriguing and controversial player in centriole assembly. Originally shown to control duplication of the budding yeast spindle pole body, Mps1 is present in eukaryotes from yeast to humans, the nematode C. elegans being a notable exception, and has also been shown to regulate the spindle checkpoint and an increasing number of cellular functions relating to genomic stability. While its function in the spindle checkpoint appears to be both universally conserved and essential in most organisms, conservation of its originally described function in spindle pole duplication has proven controversial, and it is less clear whether Mps1 is essential for centrosome duplication outside of budding yeast. Recent studies of Mps1 have identified at least two distinct functions for Mps1 in centriole assembly, while simultaneously supporting the notion that Mps1 is dispensable for the process. However, the fact that at least one centrosomal substrate of Mps1 is conserved from yeast to humans down to the phosphorylation site, combined with evidence demonstrating the exquisite control exerted over centrosomal Mps1 levels suggest that the notion of being essential may not be the most important of distinctions.