Nuclear γ-tubulin associates with nucleoli and interacts with tumor suppressor protein C53

Regulation of microtubule nucleation in glioblastoma cells by ARF GTPase-activating proteins GIT1 and GIT2 and protein kinase C

BACKGROUND

G protein-coupled receptor kinase-interacting proteins (GITs) function as GTPase-activating proteins (GAPs) for small GTPases of the ADP-ribosylation factor (Arf) family. While GIT proteins (GIT1 and GIT2) regulate both cell migration and microtubule organization, their corresponding regulatory mechanisms in glioblastoma cells remain largely unknown. To further investigate their role in microtubule modulation, we examined the function of GITs in microtubule nucleation and the involvement of protein kinase C (PKC) in this process.

METHODS

Glioblastoma cell lines with depleted GIT protein levels were generated using shRNA lentiviral vectors. The cellular localization of GITs was visualized by immunofluorescence microscopy, microtubule nucleation was analyzed using time-lapse imaging, and cell migration was assessed through a wound healing assay. Phosphomimetic and non-phosphorylatable variants of GIT2 were prepared by site-directed mutagenesis. Immunoprecipitation, pull-down experiments, and kinase assays in the presence of PKC inhibitors were used to study protein interactions.

RESULTS

Both GIT1 and GIT2 associate with proteins of the γ-tubulin ring complexes (γTuRCs), the primary microtubule nucleators, and localize to centrosomes. Depletion of GIT2 enhances centrosomal microtubule nucleation and has a more pronounced, yet opposite, effect on this process compared to GIT1. In contrast, the depletion of both GIT1 and GIT2 similarly affects cell migration. The N-terminal ArfGAP domain of GIT2 associates with centrosomes, regulates microtubule nucleation, and is phosphorylated by PKC, which modulates this process. We identified serine 46 (S46) on the ArfGAP domain as a PKC phosphorylation site and demonstrated that phosphorylation of GIT2 at S46 promotes microtubule nucleation.

CONCLUSIONS

We propose that GIT2 phosphorylation provides a novel regulatory mechanism for microtubule nucleation in glioblastoma cells, contributing to their invasive properties.

γ-tubulin mediates DNA double-strand break repair

Double-strand breaks (DSBs) in DNA pose a critical threat to genomic integrity, potentially leading to the onset and progression of various diseases, including cancer. Cellular responses to such lesions entail sophisticated repair mechanisms primarily mediated by non-homologous end joining (NHEJ) and homologous recombination (HR). Interestingly, the efficient recruitment of repair proteins and completion of DSB repair likely involve complex, inter-organelle communication and coordination of cellular components. In this study, we report a role of γ-tubulin in DSB repair. γ-tubulin is a major microtubule nucleation factor governing microtubule dynamics. We show that γ-tubulin is recruited to the site of DNA damage and is required for efficient DSB repair via both NHEJ and HR. Suppression of γ-tubulin impedes DNA repair and exacerbates DNA damage accumulation. Furthermore, γ-tubulin mediates the mobilization and formation of DNA damage foci, which serve as repair centers, thereby facilitating the recruitment of HR and NHEJ repair proteins on damaged chromatin. Finally, pharmacological inhibition of γ-tubulin enhances the cytotoxic effect of DNA-damaging agents, consistent with the DNA repair function of γ-tubulin, and underscoring the potential of its therapeutic intervention in cancer therapy.

Integrating molecular, biochemical, and immunohistochemical features as predictors of hepatocellular carcinoma drug response using machine-learning algorithms

Introduction

Liver cancer, particularly Hepatocellular carcinoma (HCC), remains a significant global health concern due to its high prevalence and heterogeneous nature. Despite the existence of approved drugs for HCC treatment, the scarcity of predictive biomarkers limits their effective utilization. Integrating diverse data types to revolutionize drug response prediction, ultimately enabling personalized HCC management.

Method

In this study, we developed multiple supervised machine learning models to predict treatment response. These models utilized classifiers such as logistic regression (LR), k-nearest neighbors (kNN), neural networks (NN), support vector machines (SVM), and random forests (RF) using a comprehensive set of molecular, biochemical, and immunohistochemical features as targets of three drugs: Pantoprazole, Cyanidin 3-glycoside (Cyan), and Hesperidin. A set of performance metrics for the complete and reduced models were reported including accuracy, precision, recall (sensitivity), specificity, and the Matthews Correlation Coefficient (MCC).

Results and Discussion

Notably, (NN) achieved the best prediction accuracy where the combined model using molecular and biochemical features exhibited exceptional predictive power, achieving solid accuracy of 0.9693 ∓ 0.0105 and average area under the ROC curve (AUC) of 0.94 ∓ 0.06 coming from three cross-validation iterations. Also, found seven molecular features, seven biochemical features, and one immunohistochemistry feature as promising biomarkers of treatment response. This comprehensive method has the potential to significantly advance personalized HCC therapy by allowing for more precise drug response estimation and assisting in the identification of effective treatment strategies.

UFMylation: a ubiquitin-like modification

Post-translational modifications (PTMs) add a major degree of complexity to the proteome and are essential controllers of protein homeostasis. Amongst the hundreds of PTMs identified, ubiquitin and ubiquitin-like (UBL) modifications are recognized as key regulators of cellular processes through their ability to affect protein-protein interactions, protein stability, and thus the functions of their protein targets. Here, we focus on the most recently identified UBL, ubiquitin-fold modifier 1 (UFM1), and the machinery responsible for its transfer to substrates (UFMylation) or its removal (deUFMylation). We first highlight the biochemical peculiarities of these processes, then we develop on how UFMylation and its machinery control various intertwined cellular processes and we highlight some of the outstanding research questions in this emerging field.

The Post-Translational Role of UFMylation in Physiology and Disease

Ubiquitin-fold modifier 1 (UFM1) is a newly identified ubiquitin-like protein that has been conserved during the evolution of multicellular organisms. In a similar manner to ubiquitin, UFM1 can become covalently linked to the lysine residue of a substrate via a dedicated enzymatic cascade. Although a limited number of substrates have been identified so far, UFM1 modification (UFMylation) has been demonstrated to play a vital role in a variety of cellular activities, including mammalian development, ribosome biogenesis, the DNA damage response, endoplasmic reticulum stress responses, immune responses, and tumorigenesis. In this review, we summarize what is known about the UFM1 enzymatic cascade and its biological functions, and discuss its recently identified substrates. We also explore the pathological role of UFMylation in human disease and the corresponding potential therapeutic targets and strategies.

γ-Tubulin in microtubule nucleation and beyond

Microtubules composed of αβ-tubulin dimers are dynamic cytoskeletal polymers that play key roles in essential cellular processes such as cell division, organelle positioning, intracellular transport, and cell migration. γ-Tubulin is a highly conserved member of the tubulin family that is required for microtubule nucleation. γ-Tubulin, together with its associated proteins, forms the γ-tubulin ring complex (γ-TuRC), that templates microtubules. Here we review recent advances in the structure of γ-TuRC, its activation, and centrosomal recruitment. This provides new mechanistic insights into the molecular mechanism of microtubule nucleation. Accumulating data suggest that γ-tubulin also has other, less well understood functions. We discuss emerging evidence that γ-tubulin can form oligomers and filaments, has specific nuclear functions, and might be involved in centrosomal cross-talk between microtubules and microfilaments.

Screening of co-pathogenic genes of non-alcoholic fatty liver disease and hepatocellular carcinoma

Background

Non-alcoholic fatty liver disease (NAFLD) is a risk factor for hepatocellular carcinoma (HCC). However, its carcinogenic mechanism is still unclear, looking for both diseases’ transcriptome levels, the same changes as we are looking for NAFLD may provide a potential mechanism of action of HCC. Thus, our study aimed to discover the coexisting pathogenic genes of NAFLD and HCC.

Methods

We performed a variance analysis with public data for both diseases. At the same time, weighted gene correlation network analysis (WGCNA) was used to find highly correlated gene modules in both diseases. The darkturquoise gene module was found to be highly correlated with both diseases. Based on the diagnosis related module genes and the differential genes of the two diseases, we constructed diagnostic and prognostic models by logistic regression, univariate Cox regression, and LASSO regression. Public datasets verified the results. Meanwhile, we built a competing endogenous RNA (ceRNA) network based on the model genes and explored the related pathways and immune correlation involved in the two diseases by using Gene Ontology, Kyoto Encyclopedia of Genes and Genomes, and gene set enrichment analyses. Immunohistochemistry was used to verify the different expression of ABCC5 and TUBG1 among the normal liver, NAFLD, and HCC tissues. Sodium palmitate/sodium oleate was used to establish high-fat cell models, and Real Time Quantitative Polymerase Chain Reaction (RT-qPCR) was used to verify the messenger RNA (mRNA) expression of ABCC5 in lipidization cells.

Results

A total of 26 upregulated genes and 87 downregulated genes were found using limma package identification analysis. According to WGCNA, the darkturquoise gene module was highly correlated with the prognosis of both diseases. The coexisting genes acquired by the two groups were only three central genes, that is, ABCC5, DHODH and TUBG1. The results indicated that the diagnostic and prognostic models constructed by ABCC5 and TUBG1 genes had high accuracy in both diseases. The results of immunohistochemistry showed that ABCC5 and TUBG1 were significantly overexpressed in NAFLD and HCC tissues compared with normal liver tissues. The Oil Red O staining and triglyceride identified the successful construction of HepG2 and LO2 high-fat models using PA/OA. The results of RT-qPCR showed that the lipidization of LO2 and HepG2 increased the mRNA expression of ABCC5.

Conclusions

The gene model constructed by ABCC5 and TUBG1 has high sensibility and veracity in the diagnosis of NAFLD as well as the diagnosis and prognosis of HCC. ABCC5 and TUBG1 may play an important role in the development of NAFLD to HCC. In addition, lipidization could upregulate the mRNA expression of ABCC5 in HCC.

C53 Interacting with UFM1-Protein Ligase 1 Regulates Microtubule Nucleation in Response to ER Stress

ER distribution depends on microtubules, and ER homeostasis disturbance activates the unfolded protein response resulting in ER remodeling. CDK5RAP3 (C53) implicated in various signaling pathways interacts with UFM1-protein ligase 1 (UFL1), which mediates the ufmylation of proteins in response to ER stress. Here we find that UFL1 and C53 associate with γ-tubulin ring complex proteins. Knockout of UFL1 or C53 in human osteosarcoma cells induces ER stress and boosts centrosomal microtubule nucleation accompanied by γ-tubulin accumulation, microtubule formation, and ER expansion. C53, which is stabilized by UFL1, associates with the centrosome and rescues microtubule nucleation in cells lacking UFL1. Pharmacological induction of ER stress by tunicamycin also leads to increased microtubule nucleation and ER expansion. Furthermore, tunicamycin suppresses the association of C53 with the centrosome. These findings point to a novel mechanism for the relief of ER stress by stimulation of centrosomal microtubule nucleation.

Dysregulation of Microtubule Nucleating Proteins in Cancer Cells

Simple Summary

The dysfunction of microtubule nucleation in cancer cells changes the overall cytoskeleton organization and cellular physiology. This review focuses on the dysregulation of the γ-tubulin ring complex (γ-TuRC) proteins that are essential for microtubule nucleation. Recent research on the high-resolution structure of γ-TuRC has brought new insight into the microtubule nucleation mechanism. We discuss the effect of γ-TuRC protein overexpression on cancer cell behavior and new drugs directed to γ-tubulin that may offer a viable alternative to microtubule-targeting agents currently used in cancer chemotherapy.

Abstract

In cells, microtubules typically nucleate from microtubule organizing centers, such as centrosomes. γ-Tubulin, which forms multiprotein complexes, is essential for nucleation. The γ-tubulin ring complex (γ-TuRC) is an efficient microtubule nucleator that requires additional centrosomal proteins for its activation and targeting. Evidence suggests that there is a dysfunction of centrosomal microtubule nucleation in cancer cells. Despite decades of molecular analysis of γ-TuRC and its interacting factors, the mechanisms of microtubule nucleation in normal and cancer cells remains obscure. Here, we review recent work on the high-resolution structure of γ-TuRC, which brings new insight into the mechanism of microtubule nucleation. We discuss the effects of γ-TuRC protein dysregulation on cancer cell behavior and new compounds targeting γ-tubulin. Drugs inhibiting γ-TuRC functions could represent an alternative to microtubule targeting agents in cancer chemotherapy.

Cyclin-Dependent Kinase 5 Regulatory Subunit Associated Protein 3: Potential Functions and Implications for Development and Disease

Cyclin-dependent kinase 5 (CDK5) regulatory subunit associated protein 3 (CDK5RAP3, also named as C53 or LZAP) was initially identified as a binding protein of CDK5 activator p35. To date, CDK5RAP3 has been reported to interact with a range of proteins involved in cellular events ranging from cell cycle, apoptosis, and invasion to UFMylation modification and endoplasmic reticulum stress. Owing to its crucial roles in cellular processes, CDK5RAP3 is demonstrated to be not only an active participant in embryonic and mammalian tissue development, but also a key regulator in the onset and progress of human cancers such as head and neck squamous cell carcinoma, gastric cancer, hepatocellular cancer, lung cancer, kidney cancer and breast cancer. Notwithstanding, the detailed function of CDK5RAP3 and its mechanism remain poorly defined. Here, we briefly described a history of the discovery of CDK5RAP3, and systematically overviewed its gene structural and distribution features. We also focused on the known functions of this protein and its implications for embryogenesis and tissue development, as well as diseases especially carcinoma. This review may facilitate to understand the molecular and functional basis of CDK5RAP3 and its association with development and disease, and provide a reasonable idea for novel therapeutic opportunities targeting CDK5RAP3.

The γ-tubulin meshwork assists in the recruitment of PCNA to chromatin in mammalian cells

Changes in the location of γ-tubulin ensure cell survival and preserve genome integrity. We investigated whether the nuclear accumulation of γ-tubulin facilitates the transport of proliferating cell nuclear antigen (PCNA) between the cytosolic and the nuclear compartment in mammalian cells. We found that the γ-tubulin meshwork assists in the recruitment of PCNA to chromatin. Also, decreased levels of γ-tubulin reduce the nuclear pool of PCNA. In addition, the γ-tubulin C terminus encodes a PCNA-interacting peptide (PIP) motif, and a γ-tubulin–PIP-mutant affects the nuclear accumulation of PCNA. In a cell-free system, PCNA and γ-tubulin formed a complex. In tumors, there is a significant positive correlation between TUBG1 and PCNA expression. Thus, we report a novel mechanism that constitutes the basis for tumor growth by which the γ-tubulin meshwork maintains indefinite proliferation by acting as an opportune scaffold for the transport of PCNA from the cytosol to the chromatin.

Corvaisier et al discover that γ-tubulin and replication protein PCNA forms a complex and that this facilitates recruitment of PCNA to chromatin both during cell division and during the DSB repair response. They identify a PCNA binding motif in γ-tubulin, which when mutated affects replication fork progression, providing insights into the role of the nuclear γ-tubulin meshwork.

Cyanidin 3-glucoside modulated cell cycle progression in liver precancerous lesion, in vivo study

BACKGROUND

Cyanidin-3-O-glucoside (cyan) exhibits antioxidant and anticancer properties. The cell cycle proteins and antimitotic drugs might be promising therapeutic targets in hepatocellular carcinoma.

AIM

To investigate the effect of cyan administration on cell cycle in hepatic precancerous lesion (PCL) induced by diethylnitrosamine/2-acetylaminofluorene (DEN/2-AAF) in Wistar rats.

METHODS

In vivo, DEN/2-AAF-induced hepatic PCL, rats were treated with three doses of cyan (10, 15, and 20 mg/kg/d, for four consecutive days per week for 16 wk). Blood and liver tissue samples were collected for measurement of the followings; alpha fetoprotein (AFP) liver function and RNA panel differential expression was evaluated via real time polymerase chain reaction. Histopathological examination of liver sections stained with H&E and immunohistochemical study using glutathione S-transferase placental (GSTP) and proliferating cell nuclear antigen (PCNA) antibodies were assessed.

RESULTS

Cyan administration mitigated the effect of DEN/2-AFF induced PCL, decreased AFP levels, and improved liver function. Remarkably, treatment with cyan dose dependently decreased the long non-coding RNA MALAT1 and tubulin gamma 1 mRNA expressions and increased the levels of miR-125b, all of which are involved in cell cycle and mitotic spindle assembly. Of note, cyan decreased GSTP foci percent area and PCNA positively stained nuclei.

CONCLUSION

Our results indicated that cyan could be used as a potential therapeutic agent to inhibit liver carcinogenesis in rat model via modulation of cell cycle.

Cdk5rap3 is essential for intestinal Paneth cell development and maintenance

Intestinal Paneth cells are professional exocrine cells that play crucial roles in maintenance of homeostatic microbiome, modulation of mucosal immunity, and support for stem cell self-renewal. Dysfunction of these cells may lead to the pathogenesis of human diseases such as inflammatory bowel disease (IBD). Cdk5 activator binding protein Cdk5rap3 (also known as C53 and LZAP) was originally identified as a binding protein of Cdk5 activator p35. Although previous studies have indicated its involvement in a wide range of signaling pathways, the physiological function of Cdk5rap3 remains largely undefined. In this study, we found that Cdk5rap3 deficiency resulted in very early embryonic lethality, indicating its indispensable role in embryogenesis. To further investigate its function in the adult tissues and organs, we generated intestinal epithelial cell (IEC)-specific knockout mouse model to examine its role in intestinal development and tissue homeostasis. IEC-specific deletion of Cdk5rap3 led to nearly complete loss of Paneth cells and increased susceptibility to experimentally induced colitis. Interestingly, Cdk5rap3 deficiency resulted in downregulation of key transcription factors Gfi1 and Sox9, indicating its crucial role in Paneth cell fate specification. Furthermore, Cdk5rap3 is highly expressed in mature Paneth cells. Paneth cell-specific knockout of Cdk5rap3 caused partial loss of Paneth cells, while inducible acute deletion of Cdk5rap3 resulted in disassembly of the rough endoplasmic reticulum (RER) and abnormal zymogen granules in the mature Paneth cells, as well as loss of Paneth cells. Together, our results provide definitive evidence for the essential role of Cdk5rap3 in Paneth cell development and maintenance.

Non-Canonical Functions of the Gamma-Tubulin Meshwork in the Regulation of the Nuclear Architecture

Simple Summary

The appearance of a cell is connected to its function. For example, the fusiform of smooth muscle cells is adapted to facilitate muscle contraction, the lobed nucleus in white blood cells assists with the migratory behavior of these immune cells, and the condensed nucleus in sperm aids in their swimming efficiency. Thus, changes in appearance have been used for decades by doctors as a diagnostic method for human cancers. Here, we summarize our knowledge of how a cell maintains the shape of the nuclear compartment. Specifically, we discuss the role of a novel protein meshwork, the gamma-tubulin meshwork, in the regulation of nuclear morphology and as a therapeutic target against cancer.

Abstract

The nuclear architecture describes the organization of the various compartments in the nucleus of eukaryotic cells, where a plethora of processes such as nucleocytoplasmic transport, gene expression, and assembly of ribosomal subunits occur in a dynamic manner. During the different phases of the cell cycle, in post-mitotic cells and after oncogenic transformation, rearrangements of the nuclear architecture take place, and, among other things, these alterations result in reorganization of the chromatin and changes in gene expression. A member of the tubulin family, γtubulin, was first identified as part of a multiprotein complex that allows nucleation of microtubules. However, more than a decade ago, γtubulin was also characterized as a nuclear protein that modulates several crucial processes that affect the architecture of the nucleus. This review presents the latest knowledge regarding changes that arise in the nuclear architecture of healthy cells and under pathological conditions and, more specifically, considers the particular involvement of γtubulin in the modulation of the biology of the nuclear compartment.

Choreography of the centrosome

More than a century ago, the centrosome was discovered and described as “the true division organ of the cell”. Electron microscopy revealed that a centrosome is an amorphous structure or pericentriolar protein matrix that surrounds a pair of well-organized centrioles. Today, the importance of the centrosome as a microtubule-organizing center and coordinator of the mitotic spindle is questioned, because centrioles are absent in up to half of all known eukaryotic species, and various mechanisms for acentrosomal microtubule nucleation have been described. This review recapitulates the known functions of centrosome movements in cellular homeostasis and discusses knowledge gaps in this field.

Tubulin: Structure, Functions and Roles in Disease

Highly conserved α- and β-tubulin heterodimers assemble into dynamic microtubules and perform multiple important cellular functions such as structural support, pathway for transport and force generation in cell division. Tubulin exists in different forms of isotypes expressed by specific genes with spatially- and temporally-regulated expression levels. Some tubulin isotypes are differentially expressed in normal and neoplastic cells, providing a basis for cancer chemotherapy drug development. Moreover, specific tubulin isotypes are overexpressed and localized in the nuclei of cancer cells and/or show bioenergetic functions through the regulation of the permeability of mitochondrial ion channels. It has also become clear that tubulin isotypes are involved in multiple cellular functions without being incorporated into microtubule structures. Understanding the mutations of tubulin isotypes specifically expressed in tumors and their post-translational modifications might help to identify precise molecular targets for the design of novel anti-microtubular drugs. Knowledge of tubulin mutations present in tubulinopathies brings into focus cellular functions of tubulin in brain pathologies such as Alzheimer’s disease. Uncovering signaling pathways which affect tubulin functions during antigen-mediated activation of mast cells presents a major challenge in developing new strategies for the treatment of inflammatory and allergic diseases. γ-tubulin, a conserved member of the eukaryotic tubulin superfamily specialized for microtubule nucleation is a target of cell cycle and stress signaling. Besides its microtubule nucleation role, γ-tubulin functions in nuclear and cell cycle related processes. This special issue “Tubulin: Structure, Functions and Roles in Disease” contains eight articles, five of which are original research papers and three are review papers that cover diverse areas of tubulin biology and functions under normal and pathological conditions.

Regulation of Microtubule Nucleation in Mouse Bone Marrow-Derived Mast Cells by Protein Tyrosine Phosphatase SHP-1

The antigen-mediated activation of mast cells initiates signaling events leading to their degranulation, to the release of inflammatory mediators, and to the synthesis of cytokines and chemokines. Although rapid and transient microtubule reorganization during activation has been described, the molecular mechanisms that control their rearrangement are largely unknown. Microtubule nucleation is mediated by γ-tubulin complexes. In this study, we report on the regulation of microtubule nucleation in bone marrow-derived mast cells (BMMCs) by Src homology 2 (SH2) domain-containing protein tyrosine phosphatase 1 (SHP-1; Ptpn6). Reciprocal immunoprecipitation experiments and pull-down assays revealed that SHP-1 is present in complexes containing γ-tubulin complex proteins and protein tyrosine kinase Syk. Microtubule regrowth experiments in cells with deleted SHP-1 showed a stimulation of microtubule nucleation, and phenotypic rescue experiments confirmed that SHP-1 represents a negative regulator of microtubule nucleation in BMMCs. Moreover, the inhibition of the SHP-1 activity by inhibitors TPI-1 and NSC87877 also augmented microtubule nucleation. The regulation was due to changes in γ-tubulin accumulation. Further experiments with antigen-activated cells showed that the deletion of SHP-1 stimulated the generation of microtubule protrusions, the activity of Syk kinase, and degranulation. Our data suggest a novel mechanism for the suppression of microtubule formation in the later stages of mast cell activation.

Microtubular and Nuclear Functions of γ-Tubulin: Are They LINCed?

γ-Tubulin is a conserved member of the tubulin superfamily with a function in microtubule nucleation. Proteins of γ-tubulin complexes serve as nucleation templates as well as a majority of other proteins contributing to centrosomal and non-centrosomal nucleation, conserved across eukaryotes. There is a growing amount of evidence of γ-tubulin functions besides microtubule nucleation in transcription, DNA damage response, chromatin remodeling, and on its interactions with tumor suppressors. However, the molecular mechanisms are not well understood. Furthermore, interactions with lamin and SUN proteins of the LINC complex suggest the role of γ-tubulin in the coupling of nuclear organization with cytoskeletons. γ-Tubulin that belongs to the clade of eukaryotic tubulins shows characteristics of both prokaryotic and eukaryotic tubulins. Both human and plant γ-tubulins preserve the ability of prokaryotic tubulins to assemble filaments and higher-order fibrillar networks. γ-Tubulin filaments, with bundling and aggregating capacity, are suggested to perform complex scaffolding and sequestration functions. In this review, we discuss a plethora of γ-tubulin molecular interactions and cellular functions, as well as recent advances in understanding the molecular mechanisms behind them.

Cellular senescence is associated with reorganization of the microtubule cytoskeleton

Senescent cells undergo structural and functional changes that affect essentially every aspect of cell physiology. To date, the impact of senescence on the cytoskeleton is poorly understood. This study evaluated the cytoskeleton in two independent cellular models of kidney epithelium senescence. Our work identified multiple senescence-related alterations that impact microtubules and filamentous actin during interphase. Both filamentous systems reorganized profoundly when cells became senescent. As such, microtubule stability increased during senescence, making these filaments more resistant to disassembly in the cold or by nocodazole. Microtubule stabilization was accompanied by enhanced α-tubulin acetylation on lysine 40 and the depletion of HDAC6, the major deacetylase for α-tubulin lysine 40. Rho-associated kinase Rock1 is an upstream regulator that modulates key properties of the cytoplasmic cytoskeleton. Our research shows that Rock1 concentrations were reduced significantly in senescent cells, and we revealed a mechanistic link between microtubule stabilization and Rock1 depletion. Thus, Rock1 overexpression partially restored the cold sensitivity of microtubules in cells undergoing senescence. Additional components relevant to microtubules were affected by senescence. Specifically, we uncovered the senescence-related loss of the microtubule nucleating protein γ-tubulin and aberrant formation of γ-tubulin foci. Concomitant with the alterations of microtubule and actin filaments, senescent cells displayed functional changes. In particular, cell migration was impaired significantly in senescent cells. Taken together, our study identified new senescence-associated deficiencies of the microtubule and actin cytoskeleton, provided insights into the underlying molecular mechanisms and demonstrated functional consequences that are important to the physiology and function of renal epithelial cells.

CDK5RAP3, a UFL1 substrate adaptor, is crucial for liver development

Protein modification by ubiquitin and ubiquitin-like proteins (UBLs) regulates numerous biological functions. The UFM1 system, a novel UBL conjugation system, is implicated in mouse development and hematopoiesis. However, its broad biological functions and working mechanisms remain largely elusive. CDK5RAP3, a possible ufmylation substrate, is essential for epiboly and gastrulation in zebrafish. Herein, we report a crucial role of CDK5RAP3 in liver development and hepatic functions. Cdk5rap3 knockout mice displayed prenatal lethality with severe liver hypoplasia, as characterized by delayed proliferation and compromised differentiation. Hepatocyte-specific Cdk5rap3 knockout mice suffered post-weaning lethality, owing to serious hypoglycemia and impaired lipid metabolism. Depletion of CDK5RAP3 triggered endoplasmic reticulum stress and activated unfolded protein responses in hepatocytes. We detected the in vivo interaction of CDK5RAP3 with UFL1, the defined E3 ligase in ufmylation. Notably, loss of CDK5RAP3 altered the ufmylation profile in liver cells, suggesting that CDK5RAP3 serves as a novel substrate adaptor for this UBL modification. Collectively, our study identifies CDK5RAP3 as an important regulator of ufmylation and suggests the involvement of ufmylation in mammalian development.

The transmembrane adaptor protein NTAL limits mast cell chemotaxis toward prostaglandin E2

Chemotaxis of mast cells is one of the crucial steps in their development and function. Non–T cell activation linker (NTAL) is a transmembrane adaptor protein that inhibits the activation of mast cells and B cells in a phosphorylation-dependent manner. Here, we studied the role of NTAL in the migration of mouse mast cells stimulated by prostaglandin E2 (PGE2). Although PGE2 does not induce the tyrosine phosphorylation of NTAL, unlike IgE immune complex antigens, we found that loss of NTAL increased the chemotaxis of mast cells toward PGE2. Stimulation of mast cells that lacked NTAL with PGE2 enhanced the phosphorylation of AKT and the production of phosphatidylinositol 3,4,5-trisphosphate. In resting NTAL-deficient mast cells, phosphorylation of an inhibitory threonine in ERM family proteins accompanied increased activation of β1-containing integrins, which are features often associated with increased invasiveness in tumors. Rescue experiments indicated that only full-length, wild-type NTAL restored the chemotaxis of NTAL-deficient cells toward PGE2. Together, these data suggest that NTAL is a key inhibitor of mast cell chemotaxis toward PGE2, which may act through the RHOA/ERM/β1-integrin and PI3K/AKT axes.

γ-Tubulin⁻γ-Tubulin Interactions as the Basis for the Formation of a Meshwork

In cytoplasm, protein γ-tubulin joins with various γ-tubulin complex proteins (GCPs) to form a heterotetramer γ-tubulin small complex (γ-TuSC) that can grow into a ring-shaped structure called the γ-tubulin ring complex (γ-TuRC). Both γ-TuSC and γ-TuRC are required for microtubule nucleation. Recent knowledge on γ-tubulin with regard to its cellular functions beyond participation in its creation of microtubules suggests that this protein forms a cellular meshwork. The present review summarizes the recognized functions of γ-tubulin and aims to unite the current views on this protein.

γ-tubulin as a signal-transducing molecule and meshwork with therapeutic potential

Knowledge of γ-tubulin is increasing with regard to the cellular functions of this protein beyond its participation in microtubule nucleation. γ-Tubulin expression is altered in various malignancies, and changes in the TUBG1 gene have been found in patients suffering from brain malformations. This review recapitulates the known functions of γ-tubulin in cellular homeostasis and discusses the possible influence of the protein on disease development and cancer.

γ-Tubulin has a conserved intrinsic property of self-polymerization into double stranded filaments and fibrillar networks

γ-Tubulin is essential for microtubule nucleation and also plays less understood roles in nuclear and cell-cycle-related functions. High abundancy of γ-tubulin in acentrosomal Arabidopsis cells facilitated purification and biochemical characterization of large molecular species of γ-tubulin. TEM, fluorescence, and atomic force microscopy of purified high molecular γ-tubulin forms revealed the presence of linear filaments with a double protofilament substructure, filament bundles and aggregates. Filament formation from highly purified γ-tubulin free of γ-tubulin complex proteins (GCPs) was demonstrated for both plant and human γ-tubulin. Moreover, γ-tubulin associated with porcine brain microtubules formed oligomers. Experimental evidence on the intrinsic ability of γ-tubulin to oligomerize/polymerize was supported by conservation of α- and β-tubulin interfaces for longitudinal and lateral interactions for γ-tubulins. STED (stimulated emission depletion) microscopy of Arabidopsis cells revealed fine, short γ-tubulin fibrillar structures enriched on mitotic microtubular arrays that accumulated at polar regions of acentrosomal spindles and the outer nuclear envelope before mitosis, and were also present in nuclei. Fine fibrillar structures of γ-tubulin representing assemblies of higher order were localized in cell-cycle-dependent manner at sites of dispersed γ-tubulin location in acentrosomal plant cells as well as at sites of local γ-tubulin enrichment after drug treatment. Our findings that γ-tubulin preserves the capability of prokaryotic tubulins to self-organize into filaments assembling by lateral interaction into bundles/clusters help understanding of the relationship between structure and multiple cellular functions of this protein species and suggest that besides microtubule nucleation and organization, γ-tubulin may also have scaffolding or sequestration functions.

Characterization of gamma-tubulin filaments in mammalian cells

Overexpression of γ-tubulin leads to the formation of filaments, but nothing is known about such filaments with regard to possible presence in cells, structure and probable dynamics. Here, we used mammalian cell lines to investigate the ability of γ-tubulin to form filaments. We found that γ-tubulin produces fibers called γ-tubules in a GTP-dependent manner and that γ-tubules are made up of pericentrin and the γ-tubulin complex proteins 2, 3, 5 and 6. Furthermore, we noted that the number of cells with cytosolic γ-tubules is increased in non-dividing cells. Our experiments showed that γ-tubules are polar structures that have a low regrowth rate compared to microtubules. Also, we observed that γ-tubules were disassembled by treatment with cold, colcemid, citral dimethyl acetal, dimethyl fumarate or mutation of γ-tubulin GTPase domain, but were increased in number by treatment with taxol or by stable expression of the γ-tubulin^1-333 GTPase domain. Our results demonstrate that γ-tubulin forms filaments, and such assembly is facilitated by the GTPase domain of γ-tubulin.

Regulation of microtubule nucleation mediated by γ-tubulin complexes

The microtubule cytoskeleton is critically important for spatio-temporal organization of eukaryotic cells. The nucleation of new microtubules is typically restricted to microtubule organizing centers (MTOCs) and requires γ-tubulin that assembles into multisubunit complexes of various sizes. γ-Tubulin ring complexes (TuRCs) are efficient microtubule nucleators and are associated with large number of targeting, activating and modulating proteins. γ-Tubulin-dependent nucleation of microtubules occurs both from canonical MTOCs, such as spindle pole bodies and centrosomes, and additional sites such as Golgi apparatus, nuclear envelope, plasma membrane-associated sites, chromatin and surface of pre-existing microtubules. Despite many advances in structure of γ-tubulin complexes and characterization of γTuRC interacting factors, regulatory mechanisms of microtubule nucleation are not fully understood. Here, we review recent work on the factors and regulatory mechanisms that are involved in centrosomal and non-centrosomal microtubule nucleation.

Differential expression of human γ-tubulin isotypes during neuronal development and oxidative stress points to a γ-tubulin-2 prosurvival function

γ-Tubulins are highly conserved members of the tubulin superfamily essential for microtubule nucleation. Humans possess 2 γ-tubulin genes. It is thought that γ-tubulin-1 represents a ubiquitous isotype, whereas γ-tubulin-2 is found predominantly in the brain, where it may be endowed with divergent functions beyond microtubule nucleation. The molecular basis of the purported functional differences between γ-tubulins is unknown. We report discrimination of human γ-tubulins according to their electrophoretic and immunochemical properties. In vitro mutagenesis revealed that the differences in electrophoretic mobility originate in the C-terminal regions of the γ-tubulins. Using epitope mapping, we discovered mouse monoclonal antibodies that can discriminate between human γ-tubulin isotypes. Real time quantitative RT-PCR and 2-dimensional-PAGE showed that γ-tubulin-1 is the dominant isotype in fetal neurons. Although γ-tubulin-2 accumulates in the adult brain, γ-tubulin-1 remains the major isotype in various brain regions. Localization of γ-tubulin-1 in mature neurons was confirmed by immunohistochemistry and immunofluorescence microscopy on clinical samples and tissue microarrays. Differentiation of SH-SY5Y human neuroblastoma cells by all-trans retinoic acid, or oxidative stress induced by mitochondrial inhibitors, resulted in upregulation of γ-tubulin-2, whereas the expression of γ-tubulin-1 was unchanged. Fractionation experiments and immunoelectron microscopy revealed an association of γ-tubulins with mitochondrial membranes. These data indicate that in the face of predominant γ-tubulin-1 expression, the accumulation of γ-tubulin-2 in mature neurons and neuroblastoma cells during oxidative stress may denote a prosurvival role of γ-tubulin-2 in neurons.-Dráberová, E., Sulimenko, V., Vinopal, S., Sulimenko, T., Sládková, V., D'Agostino, L., Sobol, M., Hozák, P., Křen, L., Katsetos, C. D., Dráber, P. Differential expression of human γ-tubulin isotypes during neuronal development and oxidative stress points to γ-tubulin-2 prosurvival function.

Gamma-tubulin coordinates nuclear envelope assembly around chromatin

The cytosolic role of γ-tubulin as a microtubule organizer has been studied thoroughly, but its nuclear function is poorly understood. Here, we show that γ-tubulin is located throughout the chromatin of demembranated Xenopus laevis sperm and, as the nucleus is formed, γ-tubulin recruits lamin B3 and nuclear membranes. Immunodepletion of γ-tubulin impairs X. laevis assembly of both the lamina and the nuclear membrane. During nuclear formation in mammalian cell lines, γ-tubulin establishes a cellular protein boundary around chromatin that coordinates nuclear assembly of the daughter nuclei. Furthermore, expression of a γ-tubulin mutant that lacks the DNA-binding domain forms chromatin-empty nuclear like structures and demonstrate that a constant interplay between the chromatin-associated and the cytosolic pools of γ-tubulin is required and, when the balance between pools is impaired, aberrant nuclei are formed. We therefore propose that the nuclear protein meshwork formed by γ-tubulin around chromatin coordinates nuclear formation in eukaryotic cells.

Human TUBG2 gene is expressed as two splice variant mRNA and involved in cell growth

In mammals, γ-tubulin, a key protein in microtubule nucleation, is encoded by two genes, TUBG1 and TUBG2. Human TUBG1 and TUBG2 mRNA are expressed ubiquitously and predominantly in preimplantation embryos and the brain, respectively, but specific detection of γ-tubulin2 protein expression is difficult due to their high sequence similarity. Here, we describe a protocol for differential detection of two human γ-tubulins by western blotting. In several cancer cell lines and the brain, expression of γ-tubulin2 along with γ-tubulin1 and a novel TUBG2 splice variant are identified. Contribution of ectopically expressed γ-tubulin2 in cancer growth was determined by depletion of γ-tubulin2.

GIT1/βPIX signaling proteins and PAK1 kinase regulate microtubule nucleation

Microtubule nucleation from γ-tubulin complexes, located at the centrosome, is an essential step in the formation of the microtubule cytoskeleton. However, the signaling mechanisms that regulate microtubule nucleation in interphase cells are largely unknown. In this study, we report that γ-tubulin is in complexes containing G protein-coupled receptor kinase-interacting protein 1 (GIT1), p21-activated kinase interacting exchange factor (βPIX), and p21 protein (Cdc42/Rac)-activated kinase 1 (PAK1) in various cell lines. Immunofluorescence microscopy revealed association of GIT1, βPIX and activated PAK1 with centrosomes. Microtubule regrowth experiments showed that depletion of βPIX stimulated microtubule nucleation, while depletion of GIT1 or PAK1 resulted in decreased nucleation in the interphase cells. These data were confirmed for GIT1 and βPIX by phenotypic rescue experiments, and counting of new microtubules emanating from centrosomes during the microtubule regrowth. The importance of PAK1 for microtubule nucleation was corroborated by the inhibition of its kinase activity with IPA-3 inhibitor. GIT1 with PAK1 thus represent positive regulators, and βPIX is a negative regulator of microtubule nucleation from the interphase centrosomes. The regulatory roles of GIT1, βPIX and PAK1 in microtubule nucleation correlated with recruitment of γ-tubulin to the centrosome. Furthermore, in vitro kinase assays showed that GIT1 and βPIX, but not γ-tubulin, serve as substrates for PAK1. Finally, direct interaction of γ-tubulin with the C-terminal domain of βPIX and the N-terminal domain of GIT1, which targets this protein to the centrosome, was determined by pull-down experiments. We propose that GIT1/βPIX signaling proteins with PAK1 kinase represent a novel regulatory mechanism of microtubule nucleation in interphase cells.

The Arabidopsis mitogen-activated protein kinase 6 is associated with γ-tubulin on microtubules, phosphorylates EB1c and maintains spindle orientation under nitrosative stress

Stress-activated plant mitogen-activated protein (MAP) kinase pathways play roles in growth adaptation to the environment by modulating cell division through cytoskeletal regulation, but the mechanisms are poorly understood. We performed protein interaction and phosphorylation experiments with cytoskeletal proteins, mass spectrometric identification of MPK6 complexes and immunofluorescence analyses of the microtubular cytoskeleton of mitotic cells using wild-type, mpk6-2 mutant and plants overexpressing the MAP kinase-inactivating phosphatase, AP2C3. We showed that MPK6 interacted with γ-tubulin and co-sedimented with plant microtubules polymerized in vitro. It was the active form of MAP kinase that was enriched with microtubules and followed similar dynamics to γ-tubulin, moving from poles to midzone during the anaphase-to-telophase transition. We found a novel substrate for MPK6, the microtubule plus end protein, EB1c. The mpk6-2 mutant was sensitive to 3-nitro-l-tyrosine (NO2 -Tyr) treatment with respect to mitotic abnormalities, and root cells overexpressing AP2C3 showed defects in chromosome segregation and spindle orientation. Our data suggest that the active form of MAP kinase interacts with γ-tubulin on specific subsets of mitotic microtubules during late mitosis. MPK6 phosphorylates EB1c, but not EB1a, and has a role in maintaining regular planes of cell division under stress conditions.

γ-Tubulin complexes in microtubule nucleation and beyond

Tremendous progress has been made in understanding the functions of γ-tubulin and, in particular, its role in microtubule nucleation since the publication of its discovery in 1989. The structure of γ-tubulin has been determined, and the components of γ-tubulin complexes have been identified. Significant progress in understanding the structure of the γ-tubulin ring complex and its components has led to a persuasive model for how these complexes nucleate microtubule assembly. At the same time, data have accumulated that γ-tubulin has important but less well understood functions that are not simply a consequence of its function in microtubule nucleation. These include roles in the regulation of plus-end microtubule dynamics, gene regulation, and mitotic and cell cycle regulation. Finally, evidence is emerging that γ-tubulin mutations or alterations of γ-tubulin expression play an important role in certain types of cancer and in other diseases.

Overexpression and Nucleolar Localization of γ-Tubulin Small Complex Proteins GCP2 and GCP3 in Glioblastoma

The expression, cellular distribution, and subcellular sorting of the microtubule (MT)-nucleating γ-tubulin small complex (γTuSC) proteins, GCP2 and GCP3, were studied in human glioblastoma cell lines and in clinical tissue samples representing all histologic grades of adult diffuse astrocytic gliomas (n = 54). Quantitative real-time polymerase chain reaction revealed a significant increase in the expression of GCP2 and GCP3 transcripts in glioblastoma cells versus normal human astrocytes; these were associated with higher amounts of both γTuSC proteins. GCP2 and GCP3 were concentrated in the centrosomes in interphase glioblastoma cells, but punctate and diffuse localizations were also detected in the cytosol and nuclei/nucleoli. Nucleolar localization was fixation dependent. GCP2 and GCP3 formed complexes with γ-tubulin in the nucleoli as confirmed by reciprocal immunoprecipitation experiments and immunoelectron microscopy. GCP2 and GCP3 depletion caused accumulation of cells in G2/M and mitotic delay but did not affect nucleolar integrity. Overexpression of GCP2 antagonized the inhibitory effect of the CDK5 regulatory subunit-associated tumor suppressor protein 3 (C53) on DNA damage G2/M checkpoint activity. Tumor cell GCP2 and GCP3 immunoreactivity was significantly increased over that in normal brains in glioblastoma samples; it was also associated with microvascular proliferation. These findings suggest that γTuSC protein dysregulation in glioblastomas may be linked to altered transcriptional checkpoint activity or interaction with signaling pathways associated with a malignant phenotype.

Therapeutic Targeting of Nuclear γ-Tubulin in RB1-Negative Tumors

In addition to its cytosolic function, γ-tubulin is a chromatin-associated protein. Reduced levels of nuclear γ-tubulin increase the activity of E2 promoter-binding factors (E2F) and raise the levels of retinoblastoma (RB1) tumor suppressor protein. In tumor cells lacking RB1 expression, decreased γ-tubulin levels induce cell death. Consequently, impairment of the nuclear activity of γ-tubulin has been suggested as a strategy for targeted chemotherapy of RB1-deficient tumors; thus, tubulin inhibitors were tested to identify compounds that interfere with γ-tubulin. Interestingly, citral increased E2F activity but impaired microtubule dynamics while citral analogues, such citral dimethyl acetal (CDA), increased E2F activity without affecting microtubules. The cytotoxic effect of CDA on tumor cells was attenuated by increased expression of either RB1 or γ-tubulin, and increased by reduced levels of either RB1 or γ-tubulin. Mechanistic study, in silico and in vitro, demonstrated that CDA prevents GTP binding to γ-tubulin and suggested that the FDA-approved drug dimethyl fumarate is also a γ-tubulin inhibitor. Finally, in vivo growth of xenograft tumors carrying defects in the RB1 signaling pathway were inhibited by CDA treatment. These results demonstrate that inhibition of γ-tubulin has the potential to specifically target tumor cells and may aid in the design of safer and more efficient chemotherapeutic regimes.

IMPLICATIONS

The in vivo antitumorigenic activity of γ-tubulin inhibitors paves the way for the development of a novel broad range targeted anticancer therapy that causes fewer side effects.

Shaping of interphase chromosomes by the microtubule network

It is well established that microtubule dynamics play a major role in chromosome condensation and localization during mitosis. During interphase, however, it is assumed that the metazoan nuclear envelope presents a physical barrier, which inhibits interaction between the microtubules located in the cytoplasm and the chromatin fibers located in the nucleus. In recent years, it has become apparent that microtubule dynamics alter chromatin structure and function during interphase as well. Microtubule motor proteins transport several transcription factors and exogenous DNA (such as plasmid DNA) from the cytoplasm to the nucleus. Various soluble microtubule components are able to translocate into the nucleus, where they bind various chromatin elements leading to transcriptional alterations. In addition, microtubules may apply force on the nuclear envelope, which is transmitted into the nucleus, leading to changes in chromatin structure. Thus, microtubule dynamics during interphase may affect chromatin spatial organization, as well as transcription, replication and repair.

Emerging microtubule targets in glioma therapy

Major advances in the genomics and epigenomics of diffuse gliomas and glioblastoma to date have not been translated into effective therapy, necessitating pursuit of alternative treatment approaches for these therapeutically challenging tumors. Current knowledge of microtubules in cancer and the development of new microtubule-based treatment strategies for high-grade gliomas are the topic in this review article. Discussed are cellular, molecular, and pharmacologic aspects of the microtubule cytoskeleton underlying mitosis and interactions with other cellular partners involved in cell cycle progression, directional cell migration, and tumor invasion. Special focus is placed on (1) the aberrant overexpression of βIII-tubulin, a survival factor associated with hypoxic tumor microenvironment and dynamic instability of microtubules; (2) the ectopic overexpression of γ-tubulin, which in addition to its conventional role as a microtubule-nucleating protein has recently emerged as a transcription factor interacting with oncogenes and kinases; (3) the microtubule-severing ATPase spastin and its emerging role in cell motility of glioblastoma cells; and (4) the modulating role of posttranslational modifications of tubulin in the context of interaction of microtubules with motor proteins. Specific antineoplastic strategies discussed include downregulation of targeted molecules aimed at achieving a sensitization effect on currently used mainstay therapies. The potential role of new classes of tubulin-binding agents and ATPase inhibitors is also examined. Understanding the cellular and molecular mechanisms underpinning the distinct behaviors of microtubules in glioma tumorigenesis and drug resistance is key to the discovery of novel molecular targets that will fundamentally change the prognostic outlook of patients with diffuse high-grade gliomas.

Centrosomal nucleolin is required for microtubule network organization

Nucleolin is a pleiotropic protein involved in a variety of cellular processes. Although multipolar spindle formation has been observed after nucleolin depletion, the roles of nucleolin in centrosome regulation and functions have not been addressed. Here we report using immunofluorescence and biochemically purified centrosomes that nucleolin co-localized only with one of the centrioles during interphase which was further identified as the mature centriole. Upon nucleolin depletion, cells exhibited an amplification of immature centriole markers surrounded by irregular pericentrin staining; these structures were exempt from maturation markers and unable to nucleate microtubules. Furthermore, the microtubule network was disorganized in these cells, exhibiting frequent non-centrosomal microtubules. At the mature centriole a reduced kinetics in the centrosomal microtubule nucleation phase was observed in live silenced cells, as well as a perturbation of microtubule anchoring. Immunoprecipitation experiments showed that nucleolin belongs to protein complexes containing 2 key centrosomal proteins, γ-tubulin and ninein, involved in microtubule nucleation and anchoring steps. Altogether, our study uncovered a new role for nucleolin in restricting microtubule nucleation and anchoring at centrosomes in interphase cells.

The nuclear localization of γ-tubulin is regulated by SadB-mediated phosphorylation

γ-Tubulin is an important cell division regulator that arranges microtubule assembly and mitotic spindle formation. Cytosolic γ-tubulin nucleates α- and β-tubulin in a growing microtubule by forming the ring-shaped protein complex γTuRC. Nuclear γ-tubulin also regulates S-phase progression by moderating the activities of E2 promoter-binding factors. The mechanism that regulates localization of γ-tubulin is currently unknown. Here, we demonstrate that the human Ser/Thr kinase SadB short localizes to chromatin and centrosomes. We found that SadB-mediated phosphorylation of γ-tubulin on Ser(385) formed chromatin-associated γ-tubulin complexes that moderate gene expression. In this way, the C-terminal region of γ-tubulin regulates S-phase progression. In addition, chromatin levels of γ-tubulin were decreased by the reduction of SadB levels or expression of a non-phosphorylatable Ala(385)-γ-tubulin but were enhanced by expression of SadB, wild-type γ-tubulin, or a phosphomimetic Asp(385)-γ-tubulin mutant. Our results demonstrate that SadB kinases regulate the cellular localization of γ-tubulin and thereby control S-phase progression.

Synergistic antitumor cytotoxic actions of ascorbate and menadione on human prostate (DU145) cancer cells in vitro: nucleus and other injuries preceding cell death by autoschizis

Scanning (SEM) and transmission electron microscopy (TEM) were used to characterize the cytotoxic effects of ascorbate (VC), menadione (VK3), or a VC:VK3 combination on a human prostate carcinoma cell line (DU145) following a 1-h vitamin treatment and a subsequent 24-h incubation in culture medium. Cell alterations examined by light and electron microscopy were treatment-dependent with VC + VK3 >VK3 > VC > Sham. Oxidative stress-induced damage was found in most organelles. This report describes injuries in the tumor cell nucleus (chromatin and nucleolus), mitochondria, endomembranes, lysosomal bodies (autophagocytoses) and inclusions. Morphologic alterations suggest that cytoskeleton damage is likely responsible for the superficial cytoplasmic changes, including major changes in cell shape and size and the self-excising phenomena. Unlike apoptotic bodies, the excised pieces contain ribonucleoproteins, but not organelles. These deleterious events cause a progressive, significant reduction in the tumor cell size. During nuclear alterations, the nuclei maintain their envelope during chromatolysis and karyolysis until cell death, while nucleoli undergo a characteristic segregation of their components. In addition, changes in fat and glycogen storage are consistent the cytotoxic and metabolic alterations caused by the respective treatments. All cellular ultrastructural changes are consistent with cell death by autoschizis and not apoptosis or other kinds of cell death.

Caspase-mediated cleavage of C53/LZAP protein causes abnormal microtubule bundling and rupture of the nuclear envelope

Apoptotic nucleus undergoes distinct morphological and biochemical changes including nuclear shrinkage, chromatin condensation and DNA fragmentation, which are attributed to caspase-mediated cleavage of several nuclear substrates such as lamins. As most of active caspases reside in the cytoplasm, disruption of the nuclear-cytoplasmic barrier is essential for caspases to reach their nuclear targets. The prevailing proposed mechanism is that the increase in the permeability of nuclear pores induced by caspases allows the caspases and other apoptotic factors to diffuse into the nucleus, thereby resulting in the nuclear destruction. Here, we report a novel observation that physical rupture of the nuclear envelope (NE) occurs in the early stage of apoptosis. We found that the NE rupture was caused by caspase-mediated cleavage of C53/LZAP, a protein that has been implicated in various signaling pathways, including NF-κB signaling and DNA damage response, as well as tumorigenesis and metastasis. We also demonstrated that C53/LZAP bound indirectly to the microtubule (MT), and expression of the C53/LZAP cleavage product caused abnormal MT bundling and NE rupture. Taken together, our findings suggest a novel role of C53/LZAP in the regulation of MT dynamics and NE structure during apoptotic cell death. Our study may provide an additional mechanism for disruption of the nuclear-cytoplasmic barrier during apoptosis.

Fully synthetic polymer vesicles for intracellular delivery of antibodies in live cells

There is an emerging need both in pharmacology and within the biomedical industry to develop new tools to target intracellular mechanisms. The efficient delivery of functionally active proteins within cells is potentially a powerful research strategy, especially through the use of antibodies. In this work, we report on a nanovector for the efficient encapsulation and delivery of antibodies into live cells with no significant loss of cell viability or any deleterious effect on cell metabolic activity. This delivery system is based on poly[2-(methacryloyloxy)ethyl phosphorylcholine]-block-[2-(diisopropylamino)ethyl methacrylate] (PMPC-PDPA), a pH-sensitive diblock copolymer that self-assembles to form nanometer-sized vesicles, also known as polymersomes, at physiological pH. Polymersomes can successfully deliver relatively high antibody payloads within different types of live cells. We demonstrate that these antibodies can target their respective epitope showing immunolabeling of γ-tubulin, actin, Golgi protein, and the transcription factor NF-κB in live cells. Finally, we demonstrate that intracellular delivery of antibodies can control specific subcellular events, as well as modulate cell activity and proinflammatory processes.

Nuclear LSm8 affects number of cytoplasmic processing bodies via controlling cellular distribution of Like-Sm proteins

We show that depletion of nuclear Like-Sm 8 (LSm8) dramatically increases processing body (P-body) number, provide the explanation that LSm8 acts via the alteration of the nuclear–cytoplasmic distribution of LSm2–7 proteins, and propose a model that P-bodies form via self-organization.

Processing bodies (P-bodies) are dynamic cytoplasmic structures involved in mRNA degradation, but the mechanism that governs their formation is poorly understood. In this paper, we address a role of Like-Sm (LSm) proteins in formation of P-bodies and provide evidence that depletion of nuclear LSm8 increases the number of P-bodies, while LSm8 overexpression leads to P-body loss. We show that LSm8 knockdown causes relocalization of LSm4 and LSm6 proteins to the cytoplasm and suggest that LSm8 controls nuclear accumulation of all LSm2–7 proteins. We propose a model in which redistribution of LSm2–7 to the cytoplasm creates new binding sites for other P-body components and nucleates new, microscopically visible structures. The model is supported by prolonged residence of two P-body proteins, DDX6 and Ago2, in P-bodies after LSm8 depletion, which indicates stronger interactions between these proteins and P-bodies. Finally, an increased number of P-bodies has negligible effects on microRNA-mediated translation repression and nonsense mediated decay, further supporting the view that the function of proteins localized in P-bodies is independent of visible P-bodies.

γ-Tubulin 2 nucleates microtubules and is downregulated in mouse early embryogenesis

γ-Tubulin is the key protein for microtubule nucleation. Duplication of the γ-tubulin gene occurred several times during evolution, and in mammals γ-tubulin genes encode proteins which share ∼97% sequence identity. Previous analysis of Tubg1 and Tubg2 knock-out mice has suggested that γ-tubulins are not functionally equivalent. Tubg1 knock-out mice died at the blastocyst stage, whereas Tubg2 knock-out mice developed normally and were fertile. It was proposed that γ-tubulin 1 represents ubiquitous γ-tubulin, while γ-tubulin 2 may have some specific functions and cannot substitute for γ-tubulin 1 deficiency in blastocysts. The molecular basis of the suggested functional difference between γ-tubulins remains unknown. Here we show that exogenous γ-tubulin 2 is targeted to centrosomes and interacts with γ-tubulin complex proteins 2 and 4. Depletion of γ-tubulin 1 by RNAi in U2OS cells causes impaired microtubule nucleation and metaphase arrest. Wild-type phenotype in γ-tubulin 1-depleted cells is restored by expression of exogenous mouse or human γ-tubulin 2. Further, we show at both mRNA and protein levels using RT-qPCR and 2D-PAGE, respectively, that in contrast to Tubg1, the Tubg2 expression is dramatically reduced in mouse blastocysts. This indicates that γ-tubulin 2 cannot rescue γ-tubulin 1 deficiency in knock-out blastocysts, owing to its very low amount. The combined data suggest that γ-tubulin 2 is able to nucleate microtubules and substitute for γ-tubulin 1. We propose that mammalian γ-tubulins are functionally redundant with respect to the nucleation activity.