A role for CK2 upon interkinetic nuclear migration in the cell cycle of retinal progenitor cells

Can tumor treating fields induce DNA damage and reduce cell motility in medulloblastoma cell lines?

OBJECTIVE

Medulloblastoma (MB) is the most common malignant pediatric brain tumor and accounts for approximately 20% of all pediatric CNS tumors. Current multimodal treatment is associated with a 70%-90% 5-year survival rate; however, the prognosis for patients with tumor dissemination and recurrent MB remains poor. The majority of survivors exhibit long-term neurocognitive complications; thus, more effective and less toxic treatments are critically needed. Tumor treating fields (TTFields) are low-intensity, alternating electric fields that disrupt cell division through physical interactions with key molecules during mitosis. Side effects from TTField therapy are minimal, making it an ideal candidate for MB treatment.

METHODS

To determine if TTFields can be an effective treatment for MB, the authors conducted an in vitro study treating multiple MB cell lines. Three MB molecular subgroups (SHH [sonic hedgehog], group 3, and group 4) were treated for 24, 48, and 72 hours at 100, 200, 300, and 400 kHz. Combinatorial studies were conducted with the small-molecule casein kinase 2 inhibitor CX-4945.

RESULTS

TTFields reduced MB cell growth with an optimal frequency of 300 kHz, and the most efficacious treatment time was 72 hours. Treatment with TTFields dysregulated actin polymerization and corresponded with a reduction in cell motility and invasion. TTFields also induced DNA damage (γH2AX, 53BP1) that correlated with an increase in apoptotic cells. The authors discovered that CX-4945 works synergistically with TTFields to reduce MB growth. In addition, combining CX-4945 and TTFields increased the cellular actin dysregulation, which correlated with a decrease in MB migration.

CONCLUSIONS

The findings of this study demonstrate that TTFields may be a novel and less toxic method to treat patients with MB.

Ouabain-Na+/K+-ATPase Signaling Regulates Retinal Neuroinflammation and ROS Production Preventing Neuronal Death by an Autophagy-Dependent Mechanism Following Optic Nerve Axotomy In Vitro

Ouabain is a classic Na+K+ATPase ligand and it has been described to have neuroprotective effects on neurons and glial cells at nanomolar concentrations. In the present work, the neuroprotective and immunomodulatory potential of ouabain was evaluated in neonatal rat retinal cells using an optic nerve axotomy model in vitro. After axotomy, cultured retinal cells were treated with ouabain (3 nM) at different periods. The levels of important inflammatory receptors in the retina such as TNFR1/2, TLR4, and CD14 were analyzed. We observed that TNFR1, TLR4, and CD14 were decreased in all tested periods (15 min, 45 min, 24 h, and 48 h). On the other hand, TNFR2 was increased after 24 h, suggesting an anti-inflammatory potential for ouabain. Moreover, we showed that ouabain also decreased Iba-1 (microglial marker) density. Subsequently, analyses of retrograde labeling of retinal ganglion cells (RGC) were performed after 48 h and showed that ouabain-induced RGC survival depends on autophagy. Using an autophagy inhibitor (3-methyladenine), we observed a complete blockage of the ouabain effect. Western blot analyses showed that ouabain increases the levels of autophagy proteins (LC3 and Beclin-1) coupled to p-CREB transcription factor and leads to autophagosome formation. Additionally, we found that the ratio of cleaved/pro-caspase-3 did not change after ouabain treatment; however, p-JNK density was enhanced. Also, ouabain decreased reactive oxygen species production immediately after axotomy. Taken together, our results suggest that ouabain controls neuroinflammation in the retina following optic nerve axotomy and promotes RGC neuroprotection through activation of the autophagy pathway.

Protein kinase CK2: a potential therapeutic target for diverse human diseases

CK2 is a constitutively active Ser/Thr protein kinase, which phosphorylates hundreds of substrates, controls several signaling pathways, and is implicated in a plethora of human diseases. Its best documented role is in cancer, where it regulates practically all malignant hallmarks. Other well-known functions of CK2 are in human infections; in particular, several viruses exploit host cell CK2 for their life cycle. Very recently, also SARS-CoV-2, the virus responsible for the COVID-19 pandemic, has been found to enhance CK2 activity and to induce the phosphorylation of several CK2 substrates (either viral and host proteins). CK2 is also considered an emerging target for neurological diseases, inflammation and autoimmune disorders, diverse ophthalmic pathologies, diabetes, and obesity. In addition, CK2 activity has been associated with cardiovascular diseases, as cardiac ischemia-reperfusion injury, atherosclerosis, and cardiac hypertrophy. The hypothesis of considering CK2 inhibition for cystic fibrosis therapies has been also entertained for many years. Moreover, psychiatric disorders and syndromes due to CK2 mutations have been recently identified. On these bases, CK2 is emerging as an increasingly attractive target in various fields of human medicine, with the advantage that several very specific and effective inhibitors are already available. Here, we review the literature on CK2 implication in different human pathologies and evaluate its potential as a pharmacological target in the light of the most recent findings.

A Journey through the Cytoskeleton with Protein Kinase CK2

Substrate pleiotropicity, a very acidic phosphorylation consensus sequence, and an apparent uncontrolled activity, are the main features of CK2, a Ser/Thr protein kinase that is required for a plethora of cell functions. Not surprisingly, CK2 appears to affect cytoskeletal structures and correlated functions such as cell shape, mechanical integrity, cell movement and division. This review outlines our current knowledge of how CK2 regulates cytoskeletal structures, and discusses involved pathways and molecular mechanisms.

Interaction of tubulin and protein kinase CK2 in Trypanosoma equiperdum

A polypeptide band with an apparent molecular weight of 55,000 was phosphorylated in vitro in whole-cell lysates of Trypanosoma equiperdum. This band corresponds to tubulin as demonstrated by immunoprecipitation of the phosphorylated polypeptide from T. equiperdum extracts when anti-α and anti-β tubulin monoclonal antibodies were employed. A parasite protein kinase CK2 was in charge of modifying tubulin given that common mammalian CK2 inhibitors such as emodin and GTP, hindered the phosphorylation of tubulin and exogenously added casein. Interestingly, a divalent cation-dependent translocation of the T. equiperdum tubulin and the CK2 responsible for its phosphorylation was noticed, suggesting a direct interaction between these two proteins. Additionally, this fraction of tubulin and its kinase coeluted using separations based on parameters as different as charge (DEAE-Sepharose anion-exchange chromatography) and size (Sephacryl S-300 gel filtration chromatography). Analyses by non-denaturing polyacrylamide gel electrophoresis and immunoblot of the purified and radioactively labeled fraction containing both tubulin and the CK2 enzyme, established the phosphorylation of a single band that was recognized by anti-CK2 α-subunit and anti-tubulin antibodies. All these findings revealed a physical association between a pool of tubulin and a CK2 in T. equiperdum.

Growth arrest and morphological changes triggered by emodin on Trypanosoma cruzi epimastigotes cultivated in axenic medium

Emodin is an anthraquinone obtained from Rheum palmatum rootstocks. Here we tested the cytotoxic effects of emodin on Trypanosoma cruzi epimastigotes, as well as the morphological changes that were induced by this compound in the parasite. Emodin was permeable and blocked in vitro cell division of T. cruzi epimastigotes in axenic medium, causing growth arrest in a dose-dependent but reversible manner. Emodin-exposed epimastigotes underwent duplication of organelles, such as the nucleus, kinetoplast and flagellum, but were incapable of completing cytokinesis. Neither elongation of the parasite body nor appearance of the regular longitudinal cleavage furrow was displayed, suggesting that emodin is most likely affecting components of the parasite cytoskeleton. Moreover, drug-treated parasites acquired alterations such as protuberances, folds and indentations on their membrane surface. Since emodin has been shown to be a potent protein kinase CK2 inhibitor, and we have previously described an association between tubulin and CK2 in T. cruzi epimastigotes (De Lima et al. Parasitology132, 511-523, 2006), we also measured the indirect effect of the drug on tubulin. Incubation of epimastigotes with axenic medium containing emodin hindered the endogenous phosphorylation of tubulin in whole-cell parasite extracts. All our results suggested that the parasite CK2 may be important for the maintenance of the morphology and for the regulation of mitosis-cytokinesis transition in T. cruzi epimastigotes.

Inhibition of casein kinase 2 blocks G2/M transition in early embryo mitosis but not in oocyte meiosis in mouse

Casein kinase 2 (CK2) is a highly conserved, ubiquitously expressed serine/threonine protein kinase with hundreds of substrates. The role of CK2 in the G₂/M transition of oocytes, zygotes, and 2-cell embryos was studied in mouse by enzyme activity inhibition using the specific inhibitor 4, 5, 6, 7-tetrabromobenzotriazole (TBB). Zygotes and 2-cell embryos were arrested at G₂ phase by TBB treatment, and DNA damage was increased in the female pronucleus of arrested zygotes. Further developmental ability of arrested zygotes was reduced, but that of arrested 2-cell embryos was not affected after releasing from inhibition. By contrast, the G₂/M transition in oocytes was not affected by TBB. These results indicate that CK2 activity is essential for mitotic G₂/M transition in early embryos but not for meiotic G₂/M transition in oocytes.

Progenitor genealogy in the developing cerebral cortex

The mammalian cerebral cortex is characterized by a complex histological organization that reflects the spatio-temporal stratifications of related stem and neural progenitor cells, which are responsible for the generation of distinct glial and neuronal subtypes during development. Some work has been done to shed light on the existing filiations between these progenitors as well as their respective contribution to cortical neurogenesis. The aim of the present review is to summarize the current views of progenitor hierarchy and relationship in the developing cortex and to further discuss future research directions that would help us to understand the molecular and cellular regulating mechanisms involved in cerebral corticogenesis.

Cell rounding in cultured human astrocytes and vascular endothelial cells upon inhibition of CK2 is mediated by actomyosin cytoskeleton alterations

Protein kinase CK2 participates in a wide range of cellular events, including the regulation of cellular morphology and migration, and may be an important mediator of angiogenesis. We previously showed that in the retina, CK2 immunolocalizes mostly to vascular endothelium and astrocytes in association with the cytoskeleton. Additionally, CK2 inhibitors significantly reduced retinal neovascularization and stem cell recruitment in the mouse model of oxygen-induced proliferative retinopathy. We have also shown that CK2 and F-actin co-localized in actin stress fibers in microvascular endothelial cells, and that highly specific CK2 inhibitors caused cell rounding in astrocytes and microvascular endothelial cells, which was alleviated by serum that promotes spreading by Rho/Rho-kinase (RhoK) activation of myosin II. Therefore, we examined a possible role of CK2 in the regulation of actin-myosin II-based contractility. Treatment with CK2 inhibitors correlated with disassembly of actomyosin stress fibers and cell shape changes, including cytoplasmic retraction and process formation that were similar to those occurring during astrocyte stellation. Low doses of specific inhibitors of kinases (RhoK and MLCK) that phosphorylate myosin light chain (MLC) enhanced the effect of suboptimal CK2 inhibition on cell shape. Such striking stellation-like alteration was accompanied by decreased level of phospho-MLC, thus implying a CK2 role in regulation of actomyosin cytoskeleton. Our results suggest an important role of CK2 in the control of cell contractility and motility, which may account for suppressing effect of CK2 inhibition on retinal neovascularization. Together, our data implicate protein kinase CK2 for the first time in stellation-like morphological transformation.

Interkinetic nuclear migration: a mysterious process in search of a function

During interkinetic nuclear migration (INM), the nuclei in many epithelial cells migrate between the apical and basal surfaces, coordinating with the cell cycle, and undergoing cytokinesis at the apical surface. INM is observed in a wide variety of tissues and species. Recent advances in time-lapse microscopy have provided clues about the mechanisms and functions of INM. Whether actin or microtubules are responsible for nuclear migration is controversial. How mitosis is initiated during INM is poorly understood, as is the relationship between the cell cycle and nuclear movement. It is possible that the disagreements stem from differences in the tissues being studied, since epithelia undergoing INM vary greatly in terms of cell height and cell fates. In this review we examine the reports addressing the mode and mechanisms that regulate INM and suggest possible functions for this dramatic event.

Regulation of interkinetic nuclear migration by cell cycle-coupled active and passive mechanisms in the developing brain

A hallmark of neurogenesis in the vertebrate brain is the apical-basal nuclear oscillation in polarized neural progenitor cells. Known as interkinetic nuclear migration (INM), these movements are synchronized with the cell cycle such that nuclei move basally during G1-phase and apically during G2-phase. However, it is unknown how the direction of movement and the cell cycle are tightly coupled. Here, we show that INM proceeds through the cell cycle-dependent linkage of cell-autonomous and non-autonomous mechanisms. During S to G2 progression, the microtubule-associated protein Tpx2 redistributes from the nucleus to the apical process, and promotes nuclear migration during G2-phase by altering microtubule organization. Thus, Tpx2 links cell-cycle progression and autonomous apical nuclear migration. In contrast, in vivo observations of implanted microbeads, acute S-phase arrest of surrounding cells and computational modelling suggest that the basal migration of G1-phase nuclei depends on a displacement effect by G2-phase nuclei migrating apically. Our model for INM explains how the dynamics of neural progenitors harmonize their extensive proliferation with the epithelial architecture in the developing brain.

Treatment of cultured human astrocytes and vascular endothelial cells with protein kinase CK2 inhibitors induces early changes in cell shape and cytoskeleton

Ubiquitous protein kinase CK2 is a key regulator of cell migration, proliferation and tumor growth. CK2 is abundant in retinal astrocytes, and its inhibition suppresses retinal neovascularization in a mouse retinopathy model. In human astrocytes, CK2 co-distributes with GFAP-containing intermediate filaments, which implies its association with cytoskeleton. Contrary to astrocytes, CK2 is co-localized in microvascular endothelial cells (HBMVEC) with microtubules and actin stress fibers, but not with vimentin-containing intermediate filaments. Specific CK2 inhibitors (TBB, TBI, TBCA and DMAT) and nine novel CK2 inhibiting compounds (TID43, TID46, Quinolone-7, Quinolone-39, FNH28, FNH62, FNH64, FNH68 and FNH74) were tested at 10-200 μM for their ability to induce morphological alterations in cultured human astrocytes (HAST-40), and HBMVEC (For explanation of the inhibitor names, see "Methods" section). CK2 inhibitors caused dramatic changes in shape of cultured cells with effective inhibitor concentrations between 50 and 100 μM. Attached cells retracted, acquired shortened processes, and eventually rounded up and detached. CK2 inhibitor-induced morphological alterations were completely reversible and were not blocked by caspase inhibition. However, longer treatment or higher inhibitor concentration did cause apoptosis. The speed and potency of the CK2 inhibitors effects on cell shape and adhesion were inversely correlated with serum concentration. Western analyses showed that TBB and TBCA elicited a significant (about twofold) increase in the activation of p38 and ERK1/2 MAP kinases that may be involved in cytoskeleton regulation. This novel early biological cell response to CK2 inhibition may underlie the anti-angiogenic effect of CK2 suppression in the retina.

Platelet activating factor blocks interkinetic nuclear migration in retinal progenitors through an arrest of the cell cycle at the S/G2 transition

Nuclear migration is regulated by the LIS1 protein, which is the regulatory subunit of platelet activating factor (PAF) acetyl-hydrolase, an enzyme complex that inactivates the lipid mediator PAF. Among other functions, PAF modulates cell proliferation, but its effects upon mechanisms of the cell cycle are unknown. Here we show that PAF inhibited interkinetic nuclear migration (IKNM) in retinal proliferating progenitors. The lipid did not, however, affect the velocity of nuclear migration in cells that escaped IKNM blockade. The effect depended on the PAF receptor, Erk and p38 pathways and Chk1. PAF induced no cell death, nor a reduction in nucleotide incorporation, which rules out an intra-S checkpoint. Notwithstanding, the expected increase in cyclin B1 content during G2-phase was prevented in the proliferating cells. We conclude that PAF blocks interkinetic nuclear migration in retinal progenitor cells through an unusual arrest of the cell cycle at the transition from S to G2 phases. These data suggest the operation, in the developing retina, of a checkpoint that monitors the transition from S to G2 phases of the cell cycle.