Microtubules: forgotten players in the apoptotic execution phase

Inhalation of hydrogen gas mitigates sevoflurane-induced neuronal apoptosis in the neonatal cortex and is associated with changes in protein phosphorylation

Inhalation of hydrogen (H2) gas is therapeutically effective for cerebrovascular diseases, neurodegenerative disorders, and neonatal brain disorders including pathologies induced by anesthetic gases. To understand the mechanisms underlying the protective effects of H2 on the brain, we investigated the molecular signals affected by H2 in sevoflurane-induced neuronal cell death. We confirmed that neural progenitor cells are susceptible to sevoflurane and undergo apoptosis in the retrosplenial cortex of neonatal mice. Co-administration of 1-8% H2 gas for 3 h to sevoflurane-exposed pups suppressed elevated caspase-3-mediated apoptotic cell death and concomitantly decreased c-Jun phosphorylation and activation of the c-Jun pathway, all of which are induced by oxidative stress. Anesthesia-induced increases in lipid peroxidation and oxidative DNA damage were alleviated by H2 inhalation. Phosphoproteome analysis revealed enriched clusters of differentially phosphorylated proteins in the sevoflurane-exposed neonatal brain that included proteins involved in neuronal development and synaptic signaling. H2 inhalation modified cellular transport pathways that depend on hyperphosphorylated proteins including microtubule-associated protein family. These modifications may be involved in the protective mechanisms of H2 against sevoflurane-induced neuronal cell death.

Extracellular and Intracellular Skeletons: How Do They Involve in Apoptosis

Apoptosis plays a key role in removing abnormal or senescent cells, maintaining the overall health of the tissue, and coordinating individual development. Recently, it has been discovered that the intracellular cytoskeleton plays a role in the apoptotic process. In addition, the regulatory role of extracellular matrix (ECM) fibrous proteins, which can be considered as the extracellular skeleton, in the process of apoptosis is rarely summarized. In this review, we collect the latest knowledge about how fibrous proteins inside and outside cells regulate apoptosis. We describe how ECM fibrous proteins participate in the regulation of death receptor and mitochondrial pathways through various signaling cascades mediated by integrins. We then explore the molecular mechanisms by which intracellular intermediate filaments regulate cell apoptosis by regulating death receptors on the cell membrane surface. Similarly, we report on novel supporting functions of microtubules in the execution phase of apoptosis and discuss their formation mechanisms. Finally, we discuss that the polypeptide fragments formed by caspase degradation of ECM fibrous proteins and intracellular intermediate filament act as local regulatory signals to play different regulatory roles in apoptosis.

Involvement of the Actin Machinery in Programmed Cell Death

Programmed cell death (PCD) depicts a genetically encoded and an orderly mode of cellular mortality. When triggered by internal or external stimuli, cells initiate PCDs through evolutionary conserved regulatory mechanisms. Actin, as a multifunctional cytoskeleton protein that forms microfilament, its integrity and dynamics are essential for a variety of cellular processes (e.g., morphogenesis, membrane blebbing and intracellular transport). Decades of work have broadened our knowledge about different types of PCDs and their distinguished signaling pathways. However, an ever-increasing pool of evidences indicate that the delicate relationship between PCDs and the actin cytoskeleton is beginning to be elucidated. The purpose of this article is to review the current understanding of the relationships between different PCDs and the actin machinery (actin, actin-binding proteins and proteins involved in different actin signaling pathways), in the hope that this attempt can shed light on ensuing studies and the development of new therapeutic strategies.

Chemical Investigation and Screening of Anti-Proliferative Activity on Human Cell Lines of Pure and Nano-Formulated Lavandin Essential Oil

Lavandin essential oil (LEO), a natural sterile hybrid obtained by crossbreeding L. angustifolia × L. latifolia, is mainly composed by active components belonging to the family of terpenes endowed with relevant anti-proliferative activity, which can be enhanced by proper application of nanotechnology. In particular, this study reports the chemical characterization and the screening of the anti-proliferative activity on different human cell lines of pure and nano-formulated lavandin essential oil (EO). LEO and its formulation (NanoLEO) were analyzed by HS/GC-MS (Headspace/Gas Chromatography-Mass Spectrometry) to describe and compare their chemical volatile composition. The most abundant compounds were linalool and 1,8-cineole (LEO: 28.6%; 27.4%) (NanoLEO: 60.4%; 12.6%) followed by α-pinene (LEO: 9.6%; NanoLEO: 4.5%), camphor (LEO: 6.5%; NanoLEO: 7.0%) and linalyl acetate (LEO: 6.5%; NanoLEO: 3.6%). The cytotoxic effects of LEO and NanoLEO were investigated on human neuroblastoma cells (SHSY5Y), human breast adenocarcinoma cells (MCF-7), human lymphoblastic leukemia cells (CCRF CEM), human colorectal adenocarcinoma cells (Caco-2) and one normal breast epithelial cell (MCF10A) by the MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide)-assay. Caco-2, MCF7 and MCF10A normal cells resulted more resistant to the treatment with LEO, while CCRF-CEM and SHSY5Y cells were more sensitive. The antiproliferative effect of LEO resulted amplified when the essential oil was supplied as nanoformulation, mainly in Caco-2 cells. Scanning and transmission electron microscopy investigations were carried out on Caco-2 cells to outline at ultrastructural level possible affections induced by LEO and NanoLEO treatments.

The Golgi Apparatus May Be a Potential Therapeutic Target for Apoptosis-Related Neurological Diseases

Increasing evidence shows that, in addition to the classical function of protein processing and transport, the Golgi apparatus (GA) is also involved in apoptosis, one of the most common forms of cell death. The structure and the function of the GA is damaged during apoptosis. However, the specific effect of the GA on the apoptosis process is unclear; it may be involved in initiating or promoting apoptosis, or it may inhibit apoptosis. Golgi-related apoptosis is associated with a variety of neurological diseases including glioma, Alzheimer’s disease (AD), Parkinson’s disease (PD), and ischemic stroke. This review summarizes the changes and the possible mechanisms of Golgi structure and function during apoptosis. In addition, we also explore the possible mechanisms by which the GA regulates apoptosis and summarize the potential relationship between the Golgi and certain neurological diseases from the perspective of apoptosis. Elucidation of the interaction between the GA and apoptosis broadens our understanding of the pathological mechanisms of neurological diseases and provides new research directions for the treatment of these diseases. Therefore, we propose that the GA may be a potential therapeutic target for apoptosis-related neurological diseases.

Methods for monitoring the progression of cell death, cell disassembly and cell clearance

Cell death through apoptosis, necrosis, necroptosis and pyroptosis, as well as the clearance of dead cells are crucial biological processes in the human body. Likewise, disassembly of dying cells during apoptosis to generate cell fragments known as apoptotic bodies may also play important roles in regulating cell clearance and intercellular communication. Recent advances in the field have led to the development of new experimental systems to identify cells at different stages of cell death, measure the levels of apoptotic cell disassembly, and monitor the cell clearance process using a range of in vitro, ex vivo and in vivo models. In this article, we will provide a comprehensive review of the methods for monitoring the progression of cell death, cell disassembly and cell clearance.

Steroidal glycoalkaloids from Solanum nigrum target cytoskeletal proteins: an in silico analysis

Background

Solanum nigrum (black nightshade; S. nigrum), a member of family Solanaceae, has been endowed with a heterogeneous array of secondary metabolites of which the steroidal glycoalkaloids (SGAs) and steroidal saponins (SS) have vast potential to serve as anticancer agents. Since there has been much controversy regarding safety of use of glycoalkaloids as anticancer agents, this area has remained more or less unexplored. Cytoskeletal proteins like actin play an important role in maintaining cell shape, synchronizing cell division, cell motility, etc. and along with their accessory proteins may also serve as important therapeutic targets for potential anticancer candidates. In the present study, glycoalkaloids and saponins from S. nigrum were screened for their interaction and binding affinity to cytoskeletal proteins, using molecular docking.

Methods

Bioactivity score and Prediction of Activity Spectra for Substances (PASS) analysis were performed using softwares Molinspiration and Osiris Data Explorer respectively, to assess the feasibility of selected phytoconstituents as potential drug candidates. The results were compared with two standard reference drugs doxorubicin hydrochloride (anticancer) and tetracycline (antibiotic). Multivariate data obtained were analyzed using principal component analysis (PCA).

Results

Docking analysis revealed that the binding affinities of the phytoconstituents towards the target cytoskeletal proteins decreased in the order coronin>villin>ezrin>vimentin>gelsolin>thymosin>cofilin. Glycoalkaloid solasonine displayed the greatest binding affinity towards the target proteins followed by alpha-solanine whereas amongst the saponins, nigrumnin-I showed maximum binding affinity. PASS Analysis of the selected phytoconstituents revealed 1 to 3 violations of Lipinski’s parameters indicating the need for modification of their structure-activity relationship (SAR) for improvement of their bioactivity and bioavailability. Glycoalkaloids and saponins all had bioactivity scores between −5.0 and 0.0 with respect to various receptor proteins and target enzymes. Solanidine, solasodine and solamargine had positive values of druglikeness which indicated that these compounds have the potential for development into future anticancer drugs. Toxicity potential evaluation revealed that glycoalkaloids and saponins had no toxicity, tumorigenicity or irritant effect(s). SAR analysis revealed that the number, type and location of sugar or the substitution of hydroxyl group on alkaloid backbone had an effect on the activity and that the presence of α-L-rhamnopyranose sugar at C-2 was critical for a compound to exhibit anticancer activity.

Conclusion

The present study revealed some cytoskeletal target(s) for S. nigrum phytoconstituents by docking analysis that have not been previously reported and thus warrant further investigations both in vitro and in vivo.

Tubulin couples death receptor 5 to regulate apoptosis

Activation of death receptor 5 (DR5) to induce apoptosis in cancer cells is an attractive strategy for cancer therapy. However, many tumor cell lines and primary tumors are resistant to DR5 targeted agents including recombinant tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) and anti-DR5 agonistic antibodies. Here we identify tubulin proteins - primarily consisting of α and β subunits folded into microtubule polymers - as a crucial modulator of DR5 mediated apoptosis. Using affinity purification coupled with mass spectrometry, we found that DR5 interacts with both α- and β-tubulin proteins in cancer cells. Pharmacological disruption of microtubules increased DR5 protein expression and subsequently sensitized the cells to TRAIL-induced apoptosis. Similar results were observed by selectively silencing tubulin transcript using small RNA interference. We also demonstrate that tubulin/microtubule blockade augments TRAIL induced apoptosis by stabilizing DR5 protein. Together, our results link the tubulin/microtubule network to the stringent regulation of DR5 mediated apoptosis, which could lead to potential therapeutic strategies to enhance cancer therapy efficacy.

Dynamic Reorganization of the Cytoskeleton during Apoptosis: The Two Coffins Hypothesis

During apoptosis, cells undergo characteristic morphological changes in which the cytoskeleton plays an active role. The cytoskeleton rearrangements have been mainly attributed to actinomyosin ring contraction, while microtubule and intermediate filaments are depolymerized at early stages of apoptosis. However, recent results have shown that microtubules are reorganized during the execution phase of apoptosis forming an apoptotic microtubule network (AMN). Evidence suggests that AMN is required to maintain plasma membrane integrity and cell morphology during the execution phase of apoptosis. The new “two coffins” hypothesis proposes that both AMN and apoptotic cells can adopt two morphological patterns, round or irregular, which result from different cytoskeleton kinetic reorganization during the execution phase of apoptosis induced by genotoxic agents. In addition, round and irregular-shaped apoptosis showed different biological properties with respect to AMN maintenance, plasma membrane integrity and phagocyte responses. These findings suggest that knowing the type of apoptosis may be important to predict how fast apoptotic cells undergo secondary necrosis and the subsequent immune response. From a pathological point of view, round-shaped apoptosis can be seen as a physiological and controlled type of apoptosis, while irregular-shaped apoptosis can be considered as a pathological type of cell death closer to necrosis.

Caspases rule the intracellular trafficking cartel

During apoptosis, caspases feast on several hundreds of cellular proteins to orchestrate rapid cellular demise. Indeed, caspases are known to get a taste of every cellular process in one way or another, activating some, but most often shutting them down. Thus, it is not surprising that caspases proteolyze proteins involved in intracellular trafficking with particularly devastating consequences for this important process. This review article focuses on how caspases target the machinery responsible for smuggling goods within and outside the cell.

Calcium-independent disruption of microtubule dynamics by nanosecond pulsed electric fields in U87 human glioblastoma cells

High powered, nanosecond duration, pulsed electric fields (nsPEF) cause cell death by a mechanism that is not fully understood and have been proposed as a targeted cancer therapy. Numerous chemotherapeutics work by disrupting microtubules. As microtubules are affected by electrical fields, this study looks at the possibility of disrupting them electrically with nsPEF. Human glioblastoma cells (U87-MG) treated with 100, 10 ns, 44 kV/cm pulses at a frequency of 10 Hz showed a breakdown of their interphase microtubule network that was accompanied by a reduction in the number of growing microtubules. This effect is temporally linked to loss of mitochondrial membrane potential and independent of cellular swelling and calcium influx, two factors that disrupt microtubule growth dynamics. Super-resolution microscopy revealed microtubule buckling and breaking as a result of nsPEF application, suggesting that nsPEF may act directly on microtubules.

Emerging roles of apoptotic microtubules during the execution phase of apoptosis

Apoptosis is a genetically programmed energy-dependent process of cell demise, characterized by specific morphological and biochemical events in which the activation of caspases has an essential role. During apoptosis the cytoskeleton participates actively in characteristic morphological rearrangements of the dying cell. This reorganisation has been assigned mainly to actinomyosin ring contraction, while microtubule and intermediate filaments are depolymerized at early stages of apoptosis. However, recent reports have showed that microtubules are reformed during the execution phase of apoptosis organizing an apoptotic microtubule network (AMN). AMN is organized behind plasma membrane, forming a cortical structure. Apoptotic microtubules repolymerization takes place in many cell types and under different apoptotic inducers. It has been hypothesized that AMN is critical for maintaining plasma membrane integrity and cell morphology during the execution phase of apoptosis. AMN disorganization leads apoptotic cells to secondary necrosis and the release of potential toxic molecules which can damage neighbor cells and promotes inflammation. Therefore, AMN formation during physiological apoptosis or in pathological apoptosis induced by anti-cancer treatments is essential for tissue homeostasis and the prevention of additional cell damage and inflammation.

Stabilization of apoptotic cells: generation of zombie cells

Apoptosis is characterized by degradation of cell components but plasma membrane remains intact. Apoptotic microtubule network (AMN) is organized during apoptosis forming a cortical structure beneath plasma membrane that maintains plasma membrane integrity. Apoptotic cells are also characterized by high reactive oxygen species (ROS) production that can be potentially harmful for the cell. The aim of this study was to develop a method that allows stabilizing apoptotic cells for diagnostic and therapeutic applications. By using a cocktail composed of taxol (a microtubule stabilizer), Zn²⁺ (a caspase inhibitor) and coenzyme Q₁₀ (a lipid antioxidant), we were able to stabilize H460 apoptotic cells in cell cultures for at least 72 h, preventing secondary necrosis. Stabilized apoptotic cells maintain many apoptotic cell characteristics such as the presence of apoptotic microtubules, plasma membrane integrity, low intracellular calcium levels and mitochondrial polarization. Apoptotic cell stabilization may open new avenues in apoptosis detection and therapy.

Intermediate filament: structure, function, and applications in cytology

Intermediate filament (IF) constitutes an important cytoskeletal component in nearly all the vertebrate cells. IFs are present both in the cytoplasm and in the nucleus. They play an important role in providing mechanical strength of the cell and tissue, growth and regeneration, cell survival and apoptosis, and finally cell migration. IFs are also expressed differentially in different body tissues. Therefore, judicious use of IF may provide the diagnosis and confirmation of different malignancies. This is particularly helpful in the diagnosis of metastatic malignant tumor from an unknown primary. Expression of IFs particularly cytokeratin and vimentin is also related to prognosis of tumors. In this review, we have discussed the basic structure, dynamics, distribution of IF in cells, and its role in diagnosis of cytology. Possible prognostic roles of IF are also discussed.

A cellular genome-wide association study reveals human variation in microtubule stability and a role in inflammatory cell death

Interindividual variation was screened for inflammatory cell death—pyroptosis. Natural variation in expression of the tubulin isoform TUBB6 or experimental manipulation of expression altered microtubule stability and susceptibility of cells to pyroptosis. Diversity in microtubule stability regulates pyroptosis and likely other human traits.

Pyroptosis is proinflammatory cell death that occurs in response to certain microbes. Activation of the protease caspase-1 by molecular platforms called inflammasomes is required for pyroptosis. We performed a cellular genome-wide association study (GWAS) using Salmonella typhimurium infection of human lymphoblastoid cell lines as a means of dissecting the genetic architecture of susceptibility to pyroptosis and identifying unknown regulatory mechanisms. Cellular GWAS revealed that a common human genetic difference that regulates pyroptosis also alters microtubule stability. An intergenic single-nucleotide polymorphism on chromosome 18 is associated with decreased pyroptosis and increased expression of TUBB6 (tubulin, β 6 class V). TUBB6 is unique among tubulin isoforms in that its overexpression can completely disrupt the microtubule network. Cells from individuals with higher levels of TUBB6 expression have lower microtubule stability and less pyroptosis. Reducing TUBB6 expression or stabilizing microtubules pharmacologically with paclitaxel (Taxol) increases pyroptosis without affecting the other major readout of caspase-1 activation, interleukin-1β secretion. The results reveal a new role for microtubules and possibly specific tubulin isoforms in the execution of pyroptosis. Furthermore, the finding that there is common diversity in TUBB6 expression and microtubule stability could have broad consequences for other microtubule-dependent phenotypes, diseases, and pharmacological responses.

Morphological Features of Organelles during Apoptosis: An Overview

An apoptotic program leading to controlled cell dismantling implies perturbations of nuclear dynamics, as well as changes affecting the organelle structure and distribution. In human cancer cells driven to apoptosis by different stimuli, we have recently investigated the morphological properties of several organelles, including mitochondria, lysosomes, endoplasmic reticulum and Golgi apparatus. In this review, we will discuss the body of evidence in the literature suggesting that organelles are generally relocated and/or degraded during apoptosis, irrespectively of the apoptogenic stimulus and cell type.

Apoptotic microtubules delimit an active caspase free area in the cellular cortex during the execution phase of apoptosis

Apoptotic microtubule network (AMN) is organized during apoptosis, forming a cortical structure beneath plasma membrane, which has an important role in preserving cell morphology and plasma membrane permeability. The aim of this study was to examine the role of AMN in maintaining plasma membrane integrity during the execution phase of apoptosis. We demonstrated in camptothecin-induced apoptosis in H460 cells that AMN delimits an active caspase free area beneath plasma membrane that permits the preservation of cellular cortex and transmembrane proteins. AMN depolymerization in apoptotic cells by a short exposure to colchicine allowed active caspases to reach the cellular cortex and cleave many key proteins involved in plasma membrane structural support, cell adhesion and ionic homeostasis. Cleavage of cellular cortex and plasma membrane proteins, such as α-spectrin, paxilin, focal adhesion kinase (FAK), E-cadherin and integrin subunit β4 was associated with cell collapse and cell detachment. Otherwise, cleavage-mediated inactivation of calcium ATPase pump (PMCA-4) and Na⁺/Ca²⁺ exchanger (NCX) involved in cell calcium extrusion resulted in calcium overload. Furthermore, cleavage of Na⁺/K⁺ pump subunit β was associated with altered sodium homeostasis. Cleavage of cell cortex and plasma membrane proteins in apoptotic cells after AMN depolymerization increased plasma permeability, ionic imbalance and bioenergetic collapse, leading apoptotic cells to secondary necrosis. The essential role of caspase-mediated cleavage in this process was demonstrated because the concomitant addition of colchicine that induces AMN depolymerization and the pan-caspase inhibitor z-VAD avoided the cleavage of cortical and plasma membrane proteins and prevented apoptotic cells to undergo secondary necrosis. Furthermore, the presence of AMN was also critical for proper phosphatidylserine externalization and apoptotic cell clearance by macrophages. These results indicate that AMN is essential to preserve an active caspase free area in the cellular cortex of apoptotic cells that allows plasma membrane integrity during the execution phase of apoptosis.

Apoptotic microtubule network organization and maintenance depend on high cellular ATP levels and energized mitochondria

Microtubule cytoskeleton is reformed during apoptosis, forming a cortical structure beneath plasma membrane, which plays an important role in preserving cell morphology and plasma membrane integrity. However, the maintenance of the apoptotic microtubule network (AMN) during apoptosis is not understood. In the present study, we examined apoptosis induced by camptothecin (CPT), a topoisomerase I inhibitor, in human H460 and porcine LLCPK-1α cells. We demonstrate that AMN was organized in apoptotic cells with high ATP levels and hyperpolarized mitochondria and, on the contrary, was dismantled in apoptotic cells with low ATP levels and mitochondrial depolarization. AMN disorganization after mitochondrial depolarization was associated with increased plasma membrane permeability assessed by enhancing LDH release and increased intracellular calcium levels. Living cell imaging monitoring of both, microtubule dynamics and mitochondrial membrane potential, showed that AMN persists during apoptosis coinciding with cycles of mitochondrial hyperpolarization. Eventually, AMN was disorganized when mitochondria suffered a large depolarization and cell underwent secondary necrosis. AMN stabilization by taxol prevented LDH release and calcium influx even though mitochondria were depolarized, suggesting that AMN is essential for plasma membrane integrity. Furthermore, high ATP levels and mitochondria polarization collapse after oligomycin treatment in apoptotic cells suggest that ATP synthase works in "reverse" mode during apoptosis. These data provide new explanations for the role of AMN and mitochondria during apoptosis.

Proteomics analysis of chinese hamster ovary cells undergoing apoptosis during prolonged cultivation

The degradation of environmental conditions, such as nutrient depletion and accumulation of toxic waste products over time, often lead to premature apoptotic cell death in mammalian cell cultures and suboptimal protein yield. Although apoptosis has been extensively researched, the changes in the whole cell proteome during prolonged cultivation, where apoptosis is a major mode of cell death, have not been examined. To our knowledge, the work presented here is the first whole cell proteome analysis of non-induced apoptosis in mammalian cells. Flow cytometry analyses of various activated caspases demonstrated the onset of apoptosis in Chinese hamster ovary cells during prolonged cultivation was primarily through the intrinsic pathway. Differential in gel electrophoresis proteomic study comparing protein samples collected during cultivation resulted in the identification of 40 differentially expressed proteins, including four cytoskeletal proteins, ten chaperone and folding proteins, seven metabolic enzymes and seven other proteins of varied functions. The induction of seven ER chaperones and foldases is a solid indication of the onset of the unfolded protein response, which is triggered by cellular and ER stresses, many of which occur during prolonged batch cultures. In addition, the upregulation of six glycolytic enzymes and another metabolic protein emphasizes that a change in the energy metabolism likely occurred as culture conditions degraded and apoptosis advanced. By identifying the intracellular changes during cultivation, this study provides a foundation for optimizing cell line-specific cultivation processes, prolonging longevity and maximizing protein production.

Growth inhibition of colorectal carcinoma by lentiviral TRAIL-transgenic human mesenchymal stem cells requires their substantial intratumoral presence

Colorectal carcinoma (CRC) constitutes a common malignancy with limited therapeutic options in metastasized stages. Mesenchymal stem cells (MSC) home to tumours and may therefore serve as a novel therapeutic tool for intratumoral delivery of antineoplastic factors. Tumour necrosis factor (TNF)-related apoptosis inducing ligand (TRAIL) which promises apoptosis induction preferentially in tumour cells represents such a factor. We generated TRAIL-MSC by transduction of human MSC with a third generation lentiviral vector system and analysed their characteristics and capacity to inhibit CRC growth. (1) TRAIL-MSC showed stable transgene expression with neither changes in the defining MSC characteristics nor signs of malignant transformation. (2) Upon direct in vitro coculture TRAIL-MSC induced apoptosis in TRAIL-sensitive CRC-cell lines (DLD-1 and HCT-15) but also in CRC-cell lines resistant to soluble TRAIL (HCT-8 and SW480). (3) In mixed subcutaneous (s.c.) xenografts TRAIL-MSC inhibited CRC-tumour growth presumably by apoptosis induction but a substantial proportion of TRAIL-MSC within the total tumour cell number was needed to yield such anti-tumour effect. (4) Systemic application of TRAIL-MSC had no effect on the growth of s.c. DLD-1 xenografts which appeared to be due to a pulmonary entrapment and low rate of tumour integration of TRAIL-MSC. Systemic TRAIL-MSC caused no toxicity in this model. (5) Wild-type MSC seemed to exert a tumour growth-supporting effect in mixed s.c. DLD-1 xenografts. These novel results support the idea that lentiviral TRAIL-transgenic human MSC may serve as vehicles for clinical tumour therapy but also highlight the need for further investigations to improve tumour integration of transgenic MSC and to clarify a potential tumour-supporting effect by MSC.

Human mesenchymal stem cells are sensitive to abnormal gravity and exhibit classic apoptotic features

The aim of the present study was to investigate the effects of abnormal gravity on human mesenchymal stem cells (hMSCs). Strong magnetic field and magnetic field gradient generate a magnetic force that can add to or subtract from the gravitational force. In this study, this is defined as a high-magneto-gravitational environment (HMGE). The HMGE provides three apparent gravity levels, i.e. hypogravity (μg), hypergravity (2g) and normal gravity with strong magnetic field (1g) conditions. After hMSCs were subject to HMGE for 12 h, the proliferation, morphology, structure and apoptosis were investigated. Results showed that the proliferation of hMSCs was inhibited under μg condition. The abnormal gravity induced morphologic characteristics of apoptosis cells, such as cell shrinkage, membrane blebbing, nuclear chromatin condensation and margination, decreased cell viability, and increased caspase-3/7 activity. The rate of apoptosis under μg condition is up to 56.95%. The F-actin stress fibers and microtubules were disrupted under abnormal gravity condition. Under μg-condition, the expression of p53 at mRNA and protein levels was up-regulated more than 9- and 6 folds, respectively. The Pifithrin-α, an specific inhibitor of p53, inhibited the apoptosis and prevented the disruption of cytoskeleton induced by abnormal gravity. These results implied that hMSCs were sensitive to abnormal gravity and exhibited classic apoptotic features, which might be associated with p53 signaling.

Expression of targeting protein for Xenopus kinesin-like protein 2 is associated with progression of human malignant astrocytoma

In humans, the targeting protein for Xenopus kinesin-like protein 2 (TPX2) is a cell cycle-associated protein, and altered TPX2 expression has been found in various malignancies. However, the contribution of TPX2 expression to astrocytoma progression is unclear. The aim of this study was to investigate TPX2 expression in human astrocytoma samples and cell lines. TPX2 protein expression was detected in the nucleus of astrocytoma tissues by immunohistochemistry and immunofluorescence staining. Real-time PCR and Western blot analysis showed that the expression levels of TPX2 were higher in high-grade astrocytoma tissues and cell lines than that in low-grade astrocytoma tissues and normal cell lines. Immunohistochemical analysis of tumor tissues from 52 patients with astrocytoma showed that TPX2 over-expression was significantly associated with decreased patient survival. In addition, down-regulation of the TPX2 gene by RNA interference inhibited proliferation of U87 cells. TPX2 gene silencing also increased early-stage apoptosis in U87 cells. Western blotting and real-time PCR showed changes in the protein and mRNA expression of Aurora A, Ran, p53, c-Myc and cyclin B1 in U87 cells that had been transfected with pSUPER/TPX2/siRNA. These data suggest that TPX2 expression is associated with the progression of malignant astrocytoma.

Proteome analysis of microtubule-associated proteins and their interacting partners from mammalian brain

The microtubule (MT) cytoskeleton is essential for a variety of cellular processes. MTs are finely regulated by distinct classes of MT-associated proteins (MAPs), which themselves bind to and are regulated by a large number of additional proteins. We have carried out proteome analyses of tubulin-rich and tubulin-depleted MAPs and their interacting partners isolated from bovine brain. In total, 573 proteins were identified giving us unprecedented access to brain-specific MT-associated proteins from mammalian brain. Most of the standard MAPs were identified and at least 500 proteins have been reported as being associated with MTs. We identified protein complexes with a large number of subunits such as brain-specific motor/adaptor/cargo complexes for kinesins, dynein, and dynactin, and proteins of an RNA-transporting granule. About 25% of the identified proteins were also found in the synaptic vesicle proteome. Analysis of the MS/MS data revealed many posttranslational modifications, amino acid changes, and alternative splice variants, particularly in tau, a key protein implicated in Alzheimer's disease. Bioinformatic analysis of known protein-protein interactions of the identified proteins indicated that the number of MAPs and their associated proteins is larger than previously anticipated and that our database will be a useful resource to identify novel binding partners.

Src-family kinase signaling, actin-mediated membrane trafficking and organellar dynamics in the control of cell fate: lessons to be learned from the adenovirus E4orf4 death factor

Evidence has accumulated that there are different modes of regulated cell death, which share overlapping signaling pathways. Cytoskeletal-dependent inter-organellar communication as a result of protein and lipid trafficking in and out of organelles has emerged as a common, key issue in the regulation of cell death modalities. The movement of proteins and lipids between cell compartments is believed to relay death signals in part through modifications of organelles dynamics. Little is known, however, regarding how trafficking is integrated within stress signaling pathways directing organelle-specific remodeling events. In this review, we discuss emerging evidence supporting a role for regulated changes in actin dynamics and intracellular membrane flow. Based on recent findings using the adenovirus E4orf4 death factor as a probing tool to tackle the mechanistic underpinnings that control alternative modes of cell death, we propose the existence of multifunctional platforms at the endosome-Golgi interface regulated by SFK-signaling. These endosomal platforms could be mobilized during cell activation processes to reorganize cellular membranes and promote inter-organelle signaling.

Increase in cytosolic calcium maintains plasma membrane integrity through the formation of microtubule ring structure in apoptotic cervical cancer cells induced by trichosanthin

This study investigates the role of dysregulated cytosolic free calcium ([Ca(2+)]c) homeostasis on microtubule (MT) ring structure in apoptotic cervical cancer (HeLa) cells induced by trichosanthin (TCS), a type I ribosome inactivating protein (RIP). The TCS-induced decrease in cell viability was significantly enhanced in combination with the specific calcium chelator, EGTA-AM. Sequestration of [Ca(2+)]c markedly disrupted the special MT ring structure. Furthermore, TCS tended to increase LDH release, whereas no significant differences were observed until 48 h of the treatment. In contrast, combined addition of EGTA-AM or colchicine (an inhibitor of tubulin polymerization) significantly reinforced LDH release. The data suggest that TCS-elevated [Ca(2+)]c maintains plasma membrane integrity via the formation of the MT ring structure in apoptotic HeLa cells.

Actin and RIG-I/MAVS signaling components translocate to mitochondria upon influenza A virus infection of human primary macrophages

Influenza A virus is one of the most important causes of respiratory infection. During viral infection, multiple cell signaling cascades are activated, resulting in the production of antiviral cytokines and initiation of programmed cell death of virus-infected cells. In the present study, we have used subcellular proteomics to reveal the host response to influenza A infection at the protein level in human macrophages. Macrophages were infected with influenza A virus, after which the cytosolic and mitochondrial cell fractions were prepared and analyzed by using two-dimensional electrophoresis for protein separation and mass spectrometry for protein identification. In cytosolic proteomes, the level of several heat shock proteins and fragments of cytoskeletal proteins was clearly up-regulated during influenza A virus infection. In mitochondrial proteomes, simultaneously with the expression of viral proteins, the level of intact actin and tubulin was highly up-regulated. This was followed by translocation of the components of antiviral RNA recognition machinery, including RIG-I (retinoic acid-inducible protein I), TRADD (TNFR1-associated death domain protein), TRIM25 (tripartite motif protein 25), and IKKepsilon (inducible IkappaB kinase), onto the mitochondria. Cytochalasin D, a potent inhibitor of actin polymerization, clearly inhibited influenza A virus-induced expression of IFN-beta, IL-29, and TNF-alpha, suggesting that intact actin cytoskeleton structure is crucial for proper activation of antiviral response. At late phases of infection mitochondrial fragmentation of actin was seen, indicating that actin fragments, fractins, are involved in disruption of mitochondrial membranes during apoptosis of virus-infected cells. In conclusion, our results suggest that actin network interacts with mitochondria to regulate both antiviral and cell death signals during influenza A virus infection.

Dynamic release of nuclear RanGTP triggers TPX2-dependent microtubule assembly during the apoptotic execution phase

During apoptosis, the interphase microtubule network is dismantled then later replaced by a novel, non-centrosomal microtubule array. These microtubules assist in the peripheral redistribution of nuclear fragments in the apoptotic cell; however, the regulation of apoptotic microtubule assembly is not understood. Here, we demonstrate that microtubule assembly depends upon the release of nuclear RanGTP into the apoptotic cytoplasm because this process is blocked in apoptotic cells overexpressing dominant-negative GDP-locked Ran (T24N). Actin-myosin-II contractility provides the impetus for Ran release and, consequently, microtubule assembly is blocked in blebbistatin- and Y27632-treated apoptotic cells. Importantly, the spindle-assembly factor TPX2 (targeting protein for Xklp2), colocalises with apoptotic microtubules, and siRNA silencing of TPX2, but not of the microtubule motors Mklp1 and Kid, abrogates apoptotic microtubule assembly. These data provide a molecular explanation for the assembly of the apoptotic microtubule network, and suggest important similarities with the process of RanGTP- and TPX2-mediated mitotic spindle formation.

A role for actin in regulating apoptosis/programmed cell death: evidence spanning yeast, plants and animals

Achieving an understanding of how apoptosis/PCD (programmed cell death) is integrated within cellular responses to environmental and intracellular signals is a daunting task. From the sensation of a stimulus to the point of no return, a programme of cell death must engage specific pro-death components, whose effects can in turn be enhanced or repressed by downstream regulatory factors. In recent years, considerable progress has been made in our understanding of how components involved in these processes function. We now know that some of the factors involved in PCD networks have ancient origins that pre-date multicellularity and, indeed, eukaryotes themselves. A subject attracting much attention is the role that the actin cytoskeleton, itself a cellular component with ancient origins, plays in cell death regulation. Actin, a key cellular component, has an established role as a cellular sensor, with reorganization and alterations in actin dynamics being a well known consequence of signalling. A range of studies have revealed that actin also plays a key role in apoptosis/PCD regulation. Evidence implicating actin as a regulator of eukaryotic cell death has emerged from studies from the Animal, Plant and Fungal Kingdoms. Here we review recent data that provide evidence for an active, functional role for actin in determining whether PCD is triggered and executed, and discuss these findings within the context of regulation of actin dynamics.

Dynamic relocation of nuclear proteins during the execution phase of apoptosis

In the apoptotic program of controlled cell dismantling, the most characteristic nuclear changes involve chromatin, which condenses and often collapses against the nuclear envelope in the form of crescents. A severe reorganization also occurs in ribonucleoprotein (RNP)-containing structures which are involved in the synthesis and processing of transcripts: already during early apoptosis, the nucleoplasmic RNPs (namely, perichromatin fibrils, perichromatin granules, and interchromatin granules) coalesce in the interchromatin space where they associate with segregated nucleolar components, to ectopically form fibro-granular heterogeneous clusters. This was found to occur in cell systems in vivo and in cultured cell lines, after different apoptogenic stimuli. These RNP aggregates we have called heterogeneous ectopic RNP-derived structures (HERDS) move from the nucleus to the cytoplasm, and may be found in apoptotic bodies, in late apoptosis. Immunolabeling experiments demonstrated that several other proteins which are normally located inside the nucleus also move into the cytoplasm, during apoptosis, independently from HERDS. Apoptotic cells have been suggested to be a powerful source of nuclear auto-antigens, which are produced by the partial proteolytic or nucleolytic cleavage of a wide variety of nuclear substrates. In the presence of defective phagocytosis (or when massive apoptosis overwhelms the clearance capability of the tissue scavenger cells), the disposal of apoptotic cells becomes insufficient and unphagocytosed late apoptotic cells may accumulate in the tissue where they may be engulfed by antigen-presenting cells (such as dendritic cells); an autoimmune response may thus be elicited, by which apoptosis-derived auto-antigens are recognized and presented to the immune system.

Cytoskeleton and apoptosis

Apoptosis is a genetically programmed and physiological mode of cell death that leads to the removal of unwanted or abnormal cells. Cysteine-proteases called caspases are responsible for the apoptotic execution phase which is characterized by specific biochemical events as well as morphological changes. These changes, which lead to the orderly dismantling of the apoptotic cell, include cell contraction, dynamic membrane blebbing, chromatin condensation, nuclear disintegration, cell fragmentation followed by phagocytosis of the dying cell. They involve major modifications of the cytoskeleton which are largely mediated by cleavage of several of its components by caspases. For example, dynamic membrane blebbing is due to the increased contractility of the acto-myosin system following myosin light chain (MLC) phosphorylation. MLC phosphorylation is a consequence of the cleavage of a Rho GTPase effector, the kinase ROCK I, by caspase-3. This cleavage induces a constitutive kinase activity by removal of an inhibitory domain. Chromatin condensation is facilitated by the processing of lamins by caspases. Collapse of the cytokeratin network is mediated by cleavage of keratin 18. On another hand, the actin cytoskeleton rearrangement needed in the phagocyte for engulfment of the dying cell is due to the activation of the small GTPase Rac, a GTPase of the Rho family that induces actin polymerisation and formation of lamellipodia. In addition to mediating the morphological modifications of the apoptotic cell, several proteins of the cytoskeleton such as actin and keratins are also involved in the regulation of apoptotic signaling.

Extensive structural remodeling of the injured spinal cord revealed by phosphorylated MAP1B in sprouting axons and degenerating neurons

Spinal cord injury (SCI) results in loss of sensory and motor function because injured axons do not regenerate and neurons that die are not replaced. Nevertheless, there is evidence for spontaneous reorganization of spared pathways (i.e. sprouting) that could be exploited to improve functional recovery. The extent of morphological remodeling after spinal cord injury is, however, not understood. We have previously shown that a phosphorylated form of microtubule-associated protein-1B, MAP1B-P, is expressed by growing axons, but is detected in intact adult SC in fibers exhibiting a somatotopic distribution of myelinated sensory fibers. We now demonstrate that after adult SCI, MAP1B-P is up-regulated in other classes of axons. We used immunohistochemistry to show changing levels and distributions of MAP1B-P after a right thoracic hemisection of adult rat spinal cord. MAP1B-P labeling suggests rearrangements of the axonal circuitry that go well beyond previous descriptions. MAP1B-P-positive fibers are present in ectopic locations in gray matter in both dorsal and ventral horns and around the central canal. Double staining reveals that primary sensory and descending serotonergic and corticospinal axons are MAP1B-P positive. In white matter, high MAP1B-P expression is found on terminal enlargements near the injury, reflecting retraction of transected axons. MAP1B-P also accumulates in pre-apoptotic neuronal somata axotomized by the lesion, indicating association of MAP1B-P not only with axon extension and retraction, but also with neuronal degeneration. Finally, we provide evidence that MAP1B phosphorylation is correlated with activation of JNK MAP-kinase, providing a step towards unraveling the mechanisms of regulation of this plasticity-related cytoskeletal protein.

Trichosanthin-induced specific changes of cytoskeleton configuration were associated with the decreased expression level of actin and tubulin genes in apoptotic Hela cells

Trichosanthin (TCS) possesses a broad spectrum of biological and pharmacological activities, including anti-cancer activities through apoptosis pathway. However, little is known about the effects of TCS on the cytoskeleton configuration and expression of actin and tubulin genes in Hela cell apoptosis. In the present study, apoptotic cytoskeleton structures were observed by confocal immunofluorescence microscopy, absolute amounts of actin and tubulin subunit mRNAs were determined by quantitative real-time PCR assays (QRT-PCR). Our results showed that the execution phase of cell apoptosis was a highly coordinated process of cellular reorganization, depolymerized microfilaments (MFs) accumulated in the coarsened cytoplasm and apoptotic bodies, followed by the formation of a ring microtubule (MT) structure beneath the plasma membrane. Importantly, apoptosis occurred by a suppression of actin and tubulin subunit gene expression. In particular, a rapid decrease in the amounts of gamma-actin mRNA preceded that of beta-actin; alpha- and beta-tubulin mRNAs were subsequently down-regulated in the later stage of Hela cell apoptosis. These results suggested that the execution of Hela cell apoptosis induced by TCS accompanied the specific changes of cytoskeleton configuration and, significantly, decreased the expression level of actin and tubulin subunit genes in different stages.

Intermediate filament scaffolds fulfill mechanical, organizational, and signaling functions in the cytoplasm

Intermediate filaments (IFs) are cytoskeletal polymers whose protein constituents are encoded by a large family of differentially expressed genes. Owing in part to their properties and intracellular organization, IFs provide crucial structural support in the cytoplasm and nucleus, the perturbation of which causes cell and tissue fragility and accounts for a large number of genetic diseases in humans. A number of additional roles, nonmechanical in nature, have been recently uncovered for IF proteins. These include the regulation of key signaling pathways that control cell survival, cell growth, and vectorial processes including protein targeting in polarized cellular settings. As this discovery process continues to unfold, a rationale for the large size of this family and the context-dependent regulation of its members is finally emerging.

Dual roles of intermediate filaments in apoptosis

New roles have emerged recently for intermediate filaments (IFs), namely in modulating cell adhesion and growth, and providing resistance to various forms of stress and to apoptosis. In this context, we first summarize findings on the IF association with the cell response to mechanical stress and growth stimulation, in light of growth-related signaling events that are relevant to death-receptor engagement. We then address the molecular mechanisms by which IFs can provide cell resistance to apoptosis initiated by death-receptor stimulation and to necrosis triggered by excessive oxidative stress. In the same way, we examine IF involvement, along with cytolinker participation, in sequential caspase-mediated protein cleavages that are part of the overall cell death execution, particularly those that generate new functional IF protein fragments and uncover neoantigen markers. Finally, we report on the usefulness of these markers as diagnostic tools for disease-related aspects of apoptosis in humans. Clearly, the data accumulated in recent years provide new and significant insights into the multiple functions of IFs, particularly their dual roles in cell response to apoptotic insults.

The apoptotic microtubule network preserves plasma membrane integrity during the execution phase of apoptosis

It has recently been shown that the microtubule cytoskeleton is reformed during the execution phase of apoptosis. We demonstrate that this microtubule reformation occurs in many cell types and under different apoptotic stimuli. We confirm that the apoptotic microtubule network possesses a novel organization, whose nucleation appears independent of conventional gamma-tubulin ring complex containing structures. Our analysis suggests that microtubules are closely associated with the plasma membrane, forming a cortical ring or cellular "cocoon". Concomitantly other components of the cytoskeleton, such as actin and cytokeratins disassemble. We found that colchicine-mediated disruption of apoptotic microtubule network results in enhanced plasma membrane permeability and secondary necrosis, suggesting that the reformation of a microtubule cytoskeleton plays an important role in preserving plasma membrane integrity during apoptosis. Significantly, cells induced to enter apoptosis in the presence of the pan-caspase inhibitor z-VAD, nevertheless form microtubule-like structures suggesting that microtubule formation is not dependent on caspase activation. In contrast we found that treatment with EGTA-AM, an intracellular calcium chelator, prevents apoptotic microtubule network formation, suggesting that intracellular calcium may play an essential role in the microtubule reformation. We propose that apoptotic microtubule network is required to maintain plasma membrane integrity during the execution phase of apoptosis.