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

Redox aspects of cytotoxicity and anti-neuroinflammatory profile of chloroquine and hydroxychloroquine in serum-starved BV-2 microglia

Chloroquine (CQ) and hydroxychloroquine (HCQ) have long been used worldwide to treat and prevent human malarias. However, these 4-aminoquinolines have also shown promising potential in treating chronic illnesses with an inflammatory component, including neurological diseases. Given the current demand for serum avoidance during pharmacological testing and modeling of some pathologies, we compared cytotoxicities of CQ and HCQ in both serum-deprived and -fed murine BV-2 microglia. Furthermore, we assessed the anti-neuroinflammatory potential of both compounds in serum-deprived cells. Under both conditions, CQ showed higher cytotoxicity than HCQ. However, the comparable MTT-assay-derived data measured under different serum conditions were associated with disparate cytotoxic mechanisms of CQ and HCQ. In particular, under serum starvation, CQ mildly enhanced secondary ROS, mitochondrial hyperpolarization, and decreased phagocytosis. However, CQ promoted G1 phase cell cycle arrest and mitochondrial depolarization in serum-fed cells. Under both conditions, CQ fostered early apoptosis. Additionally, we confirmed that both compounds could exert anti-inflammatory effects in microglia through interference with MAPK signaling under nutrient-deprivation-related stress. Nevertheless, unlike HCQ, CQ is more likely to exaggerate intracellular prooxidant processes in activated starved microglia, which are inefficiently buffered by Nrf2/HO-1 signaling pathway activation. These outcomes also show HCQ as a promising anti-neuroinflammatory drug devoid of CQ-mediated cytotoxicity.

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.

The Atypical Cyclin-Dependent Kinase 5 (Cdk5) Guards Podocytes from Apoptosis in Glomerular Disease While Being Dispensable for Podocyte Development

Cyclin-dependent kinase 5 (Cdk5) is expressed in terminally differentiated cells, where it drives development, morphogenesis, and survival. Temporal and spatial kinase activity is regulated by specific activators of Cdk5, dependent on the cell type and environmental factors. In the kidney, Cdk5 is exclusively expressed in terminally differentiated glomerular epithelial cells called podocytes. In glomerular disease, signaling mechanisms via Cdk5 have been addressed by single or combined conventional knockout of known specific activators of Cdk5. A protective, anti-apoptotic role has been ascribed to Cdk5 but not a developmental phenotype, as in terminally differentiated neurons. The effector kinase itself has never been addressed in animal models of glomerular disease. In the present study, conditional and inducible knockout models of Cdk5 were analyzed to investigate the role of Cdk5 in podocyte development and glomerular disease. While mice with podocyte-specific knockout of Cdk5 had no developmental defects and regular lifespan, loss of Cdk5 in podocytes increased susceptibility to glomerular damage in the nephrotoxic nephritis model. Glomerular damage was associated with reduced anti-apoptotic signals in Cdk5-deficient mice. In summary, Cdk5 acts primarily as master regulator of podocyte survival during glomerular disease and—in contrast to neurons—does not impact on glomerular development or maintenance.

Apoptotic Bodies: Mechanism of Formation, Isolation and Functional Relevance

In the final stages of apoptosis, apoptotic cells can generate a variety of membrane-bound vesicles known as apoptotic extracellular vesicles (ApoEVs). Apoptotic bodies (ApoBDs), a major subset of ApoEVs, are formed through a process termed apoptotic cell disassembly characterised by a series of tightly regulated morphological steps including plasma membrane blebbing, apoptotic membrane protrusion formation and fragmentation into ApoBDs. To better characterise the properties of ApoBDs and elucidate their function, a number of methods including differential centrifugation, filtration and fluorescence-activated cell sorting were developed to isolate ApoBDs. Furthermore, it has become increasingly clear that ApoBD formation can contribute to various biological processes such as apoptotic cell clearance and intercellular communication. Together, recent literature demonstrates that apoptotic cell disassembly and thus, ApoBD formation, is an important process downstream of apoptotic cell death. In this chapter, we discuss the current understandings of the molecular mechanisms involved in regulating apoptotic cell disassembly, techniques for ApoBD isolation, and the functional roles of ApoBDs in physiological and pathological settings.

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.

Defining the role of cytoskeletal components in the formation of apoptopodia and apoptotic bodies during apoptosis

During apoptosis, dying cells undergo dynamic morphological changes that ultimately lead to their disassembly into fragments called apoptotic bodies (ApoBDs). Reorganisation of the cytoskeletal structures is key in driving various apoptotic morphologies, including the loss of cell adhesion and membrane bleb formation. However, whether cytoskeletal components are also involved in morphological changes that occur later during apoptosis, such as the recently described generation of thin apoptotic membrane protrusions called apoptopodia and subsequent ApoBD formation, is not well defined. Through monitoring the progression of apoptosis by confocal microscopy, specifically focusing on the apoptopodia formation step, we characterised the presence of F-actin and microtubules in a subset of apoptopodia generated by T cells and monocytes. Interestingly, targeting actin polymerisation and microtubule assembly pharmacologically had no major effect on apoptopodia formation. These data demonstrate apoptopodia as a novel type of membrane protrusion that could be formed in the absence of actin polymerisation and microtubule assembly.

Mechanical Stretching induces the apoptosis of parametrial ligament Fibroblasts via the Actin Cytoskeleton/Nr4a1 signalling pathway

The anatomical positions of pelvic floor organs are maintained mainly by ligaments and muscles. Long-term excessive mechanical tension stimulation of pelvic floor tissue beyond the endurance of ligaments or muscles will lead to the occurrence of pelvic organ prolapse (POP). In addition, cytoskeletal reconstitution is a key process by which cells respond to mechanical stimulation. The aim of the present study was to investigate the protective effect of actin cytoskeleton to resist mechanical stretching (MS)-induced apoptosis in parametrial ligament fibroblasts (PLFs) and the underlying mechanisms. MS provided by a four‑point bending device could significantly induce apoptosis of PLFs from non-POP patients, which exhibited an apoptosis rate close to that of PLFs from POP patients, and the apoptosis rate was higher following latrunculin A (Lat-A, a potent inhibitor of actin) treatment. In addition, Nr4a1 and Bax expression was increased while Bcl-2 and caspase-3 expression was clearly decreased after treatment with MS and Lat-A. However, the apoptosis induced by MS was reduced when the expression of Nr4a1 was downregulated by siRNA. These outcomes reveal a novel mechanism that links the actin cytoskeleton and apoptosis in PLFs by Nr4a1; this mechanism will provide insight into the clinical diagnosis and treatment of POP.

Competitive Cell Death Interactions in Pulmonary Infection: Host Modulation Versus Pathogen Manipulation

In the context of pulmonary infection, both hosts and pathogens have evolved a multitude of mechanisms to regulate the process of host cell death. The host aims to rapidly induce an inflammatory response at the site of infection, promote pathogen clearance, quickly resolve inflammation, and return to tissue homeostasis. The appropriate modulation of cell death in respiratory epithelial cells and pulmonary immune cells is central in the execution of all these processes. Cell death can be either inflammatory or anti-inflammatory depending on regulated cell death (RCD) modality triggered and the infection context. In addition, diverse bacterial pathogens have evolved many means to manipulate host cell death to increase bacterial survival and spread. The multitude of ways that hosts and bacteria engage in a molecular tug of war to modulate cell death dynamics during infection emphasizes its relevance in host responses and pathogen virulence at the host pathogen interface. This narrative review outlines several current lines of research characterizing bacterial pathogen manipulation of host cell death pathways in the lung. We postulate that understanding these interactions and the dynamics of intracellular and extracellular bacteria RCD manipulation, may lead to novel therapeutic approaches for the treatment of intractable respiratory infections.

Parkin-mediated mitophagy and autophagy flux disruption in cellular models of MERRF syndrome

Mitochondrial diseases are considered rare genetic disorders characterized by defects in oxidative phosphorylation (OXPHOS). They can be provoked by mutations in nuclear DNA (nDNA) or mitochondrial DNA (mtDNA). MERRF (Myoclonic Epilepsy with Ragged-Red Fibers) syndrome is one of the most frequent mitochondrial diseases, principally caused by the m.8344A>G mutation in mtDNA, which affects the translation of all mtDNA-encoded proteins and therefore impairs mitochondrial function. In the present work, we evaluated autophagy and mitophagy flux in transmitochondrial cybrids and fibroblasts derived from a MERRF patient, reporting that Parkin-mediated mitophagy is increased in MERRF cell cultures. Our results suggest that supplementation with coenzyme Q₁₀ (CoQ), a component of the electron transport chain (ETC) and lipid antioxidant, prevents Parkin translocation to the mitochondria. In addition, CoQ acts as an enhancer of autophagy and mitophagy flux, which partially improves cell pathophysiology. The significance of Parkin-mediated mitophagy in cell survival was evaluated by silencing the expression of Parkin in MERRF cybrids. Our results show that mitophagy acts as a cell survival mechanism in mutant cells. To confirm these results in one of the main affected cell types in MERRF syndrome, mutant induced neurons (iNs) were generated by direct reprogramming of patients-derived skin fibroblasts. The treatment of MERRF iNs with Guttaquinon CoQ₁₀ (GuttaQ), a water-soluble derivative of CoQ, revealed a significant improvement in cell bioenergetics. These results indicate that iNs, along with fibroblasts and cybrids, can be utilized as reliable cellular models to shed light on disease pathomechanisms as well as for drug screening.

Death for life: a path from apoptotic signaling to tissue-scale effects of apoptotic epithelial extrusion

Apoptosis plays a crucial role in clearing old or critically compromised cells, and actively maintains epithelial homeostasis and epithelial morphogenesis during embryo development. But how is the apoptotic signaling pathway able to orchestrate such complex and dynamic multi-cellular morphological events at the tissue scale? In this review we collected the most updated knowledge regarding how apoptosis controls different cytoskeletal components. We describe how apoptosis can control epithelial homeostasis though epithelial extrusion, a highly orchestrated process based on high- order actomyosin structures and on the coordination between the apoptotic and the neighboring cells. Finally, we describe how the synergy among forces generated by multiple apoptotic cells can shape epithelia in embryo development.

ROCK1 but not LIMK1 or PAK2 is a key regulator of apoptotic membrane blebbing and cell disassembly

Many cell types are known to undergo a series of morphological changes during the progression of apoptosis, leading to their disassembly into smaller membrane-bound vesicles known as apoptotic bodies (ApoBDs). In particular, the formation of circular bulges called membrane blebs on the surface of apoptotic cells is a key morphological step required for a number of cell types to generate ApoBDs. Although apoptotic membrane blebbing is thought to be regulated by kinases including ROCK1, PAK2 and LIMK1, it is unclear whether these kinases exhibit overlapping roles in the disassembly of apoptotic cells. Utilising both pharmacological and CRISPR/Cas9 gene editing based approaches, we identified ROCK1 but not PAK2 or LIMK1 as a key non-redundant positive regulator of apoptotic membrane blebbing as well as ApoBD formation. Functionally, we have established an experimental system to either inhibit or enhance ApoBD formation and demonstrated the importance of apoptotic cell disassembly in the efficient uptake of apoptotic materials by various phagocytes. Unexpectedly, we also noted that ROCK1 could play a role in regulating the onset of secondary necrosis. Together, these data shed light on both the mechanism and function of cell disassembly during apoptosis.

Disassembly of dying cells in diverse organisms

Programmed cell death (PCD) is a conserved phenomenon in multicellular organisms required to maintain homeostasis. Among the regulated cell death pathways, apoptosis is a well-described form of PCD in mammalian cells. One of the characteristic features of apoptosis is the change in cellular morphology, often leading to the fragmentation of the cell into smaller membrane-bound vesicles through a process called apoptotic cell disassembly. Interestingly, some of these morphological changes and cell disassembly are also noted in cells of other organisms including plants, fungi and protists while undergoing 'apoptosis-like PCD'. This review will describe morphologic features leading to apoptotic cell disassembly, as well as its regulation and function in mammalian cells. The occurrence of cell disassembly during cell death in other organisms namely zebrafish, fly and worm, as well as in other eukaryotic cells will also be discussed.

Binucleate germ cells in Caenorhabditis elegans are removed by physiological apoptosis

Cell death plays a major role during C. elegans oogenesis, where over half of the oogenic germ cells die in a process termed physiological apoptosis. How germ cells are selected for physiological apoptosis, or instead become oocytes, is not understood. Most oocytes produce viable embryos when apoptosis is blocked, suggesting that physiological apoptosis does not function to cull defective germ cells. Instead, cells targeted for apoptosis may function as nurse cells; the germline is syncytial, and all germ cells appear to contribute cytoplasm to developing oocytes. C. elegans has been a leading model for the genetics and molecular biology of apoptosis and phagocytosis, but comparatively few studies have examined the cell biology of apoptotic cells. We used live imaging to identify and examine pre-apoptotic germ cells in the adult gonad. After initiating apoptosis, germ cells selectively export their mitochondria into the shared pool of syncytial cytoplasm; this transport appears to use the microtubule motor kinesin. The apoptotic cells then shrink as they expel most of their remaining cytoplasm, and close off from the syncytium. Shortly thereafter the apoptotic cells restructure their microtubule and actin cytoskeletons, possibly to maintain cell integrity; the microtubules form a novel, cortical array of stabilized microtubules, and actin and cofilin organize into giant cofilin-actin rods. We discovered that some apoptotic germ cells are binucleate; the binucleate germ cells can develop into binucleate oocytes in apoptosis-defective strains, and appear capable of producing triploid offspring. Our results suggest that the nuclear layer of the germline syncytium becomes folded during mitosis and growth, and that binucleate cells arise as the layer unfolds or everts; all of the binucleate cells are subsequently removed by apoptosis. These results show that physiological apoptosis targets at least two distinct populations of germ cells, and that the apoptosis machinery efficiently recognizes cells with two nuclei.

Many germ cells die by apoptosis during the development of animal oocytes, including more than half of all germ cells in the model system C. elegans. How individual germ cells are selected for apoptosis, or survival, is not known. Here we study the cell biology of apoptosis. The C. elegans gonad is a syncytium, with nearly 1000 germ “cells” connected to a shared, core cytoplasm. Once apoptosis is initiated, germ cells selectively transport their mitochondria into the gonad core, apparently using the microtubule motor protein kinesin. The apoptotic cells next constrict, expelling most of their remaining cytoplasm into the core, and close off from the gonad core. The microtubule and actin cytoskeletons are remodeled and stabilized, presumably to maintain the integrity of the dying cell. The apoptotic cells form giant cofilin-actin rods, similar to rods described in stressed cultured cells and in human myopathies and neuropathies such as Alzheimer’s and Huntington’s disease. We show that some germ cells are binucleate; these cells appear to form during germline morphogenesis, and are removed by apoptosis. These results demonstrate heterogeneity between oogenic germ cells, and show that the apoptosis machinery efficiently recognizes and removes cells with two nuclei.

Caspase-mediated cleavage of the centrosomal proteins during apoptosis

The centrosome is the major microtubule-organizing center and plays important roles in intracellular transport, cellular morphology, and motility. In mitotic cells, centrosomes function as spindle poles to pull a set of chromosomes into daughter cells. In quiescent cells, primary cilia are originated from the centrosomes. Given its involvement in various cellular processes, it is little surprising that the organelle would also participate in apoptotic events. However, it remains elusive how the centrosome changes in structure and organization during apoptosis. Apoptosis, a programmed cell death, is required for homeostatic tissue maintenance, embryonic development, stress responses, etc. Activation of caspases generates a cascade of apoptotic pathways, explaining much of what happens during apoptosis. Here, we report the proteolytic cleavage of selected centrosomal proteins in apoptotic cells. SAS-6, a cartwheel component of centrioles, was specifically cleaved at the border of the coiled-coil domain and the disordered C-terminus. Pericentrin, a scaffold of pericentriolar material, was also cleaved during apoptosis. These cleavages were efficiently blocked by the caspase inhibitors. We propose that the caspase-dependent proteolysis of the centrosomal proteins may destabilize the configuration of a centrosome. Loss of centrosomes may be required for the formation of apoptotic microtubule networks, which are essential for apoptotic fragmentation. This work demonstrates the first centrosomal targets by caspases during apoptosis.

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.

Self-Assembling Ability Determines the Activity of Enzyme-Instructed Self-Assembly for Inhibiting Cancer Cells

Enzyme-instructed self-assembly (EISA) represents a dynamic continuum of supramolecular nanostructures that selectively inhibits cancer cells via simultaneously targeting multiple hallmark capabilities of cancer, but how to design the small molecules for EISA from the vast molecular space remains an unanswered question. Here we show that the self-assembling ability of small molecules controls the anticancer activity of EISA. Examining the EISA precursor analogues consisting of an N-capped d-tetrapeptide, a phosphotyrosine residue, and a diester or a diamide group, we find that, regardless of the stereochemistry and the regiochemistry of their tetrapeptidic backbones, the anticancer activities of these precursors largely match their self-assembling abilities. Additional mechanistic studies confirm that the assemblies of the small peptide derivatives result in cell death, accompanying significant rearrangement of cytoskeletal proteins and plasma membranes. These results imply that the diester or diamide derivatives of the d-tetrapeptides self-assemble pericellularly, as well as intracellularly, to result in cell death. As the first case to correlate thermodynamic properties (e.g., self-assembling ability) of small molecules with the efficacy of a molecule process against cancer cells, this work provides an important insight for developing a molecular dynamic continuum for potential cancer therapy, as well as understanding the cytotoxicity of pathogenic assemblies.

Disassembly of the Dying: Mechanisms and Functions

The disassembly of an apoptotic cell into subcellular fragments, termed apoptotic bodies (ApoBDs), is a hallmark of apoptosis. Although the generation of ApoBDs is generally understood as being stochastic, it is becoming increasingly clear that ApoBD formation is a highly regulated process involving distinct morphological steps and molecular factors. Functionally, ApoBDs could facilitate the efficient clearance of apoptotic material by surrounding phagocytes as well as mediate the transfer of biomolecules including microRNAs and proteins between cells to aid in intercellular communications. Therefore, the formation of ApoBDs is an important process downstream from apoptotic cell death. We discuss here the mechanisms and functions of apoptotic cell disassembly.

Enzyme-Regulated Supramolecular Assemblies of Cholesterol Conjugates against Drug-Resistant Ovarian Cancer Cells

We report that phosphotyrosine–cholesterol conjugates effectively and selectively kill cancer cells, including platinum-resistant ovarian cancer cells. The conjugate increases the degree of noncovalent oligomerization upon enzymatic dephosphorylation in aqueous buffer. This enzymatic conversion also results in the assembly of the cholesterol conjugates inside and outside cells and leads to cell death. Preliminary mechanistic studies suggest that the formed assemblies of the conjugates not only interact with actin filaments and microtubules but also affect lipid rafts. As the first report of multifaceted supramolecular assemblies of cholesterol conjugates against cancer cells, this work illustrates the integration of enzyme catalysis and self-assembly of essential biological small molecules on and inside cancer cells as a promising strategy for developing multifunctional therapeutics to treat drug-resistant cancers.

Cytotoxic Tumor-Targeting Peptides From In Vivo Phage Display

We previously utilized an in vivo peptide phage display selection technique, which included the use of detergent elution of phage from excised tumor, to obtain tumor-targeting phage with the ability to extravasate the vasculature and bind directly to prostate tumor tissue. It is hypothesized that this same in vivo phage selection technique can be used to functionally select for molecules that not only bind to cancer cells but also kill them. Here we analyzed two different in vivo phage display selected phage clones, G1 and H5, retrieved from PC-3 human prostate carcinoma xenografted tumors. First, cell de-attachment as an endpoint criterion for apoptosis and cell cycle was examined. After 2.5 hours incubation with G1 phage, PC-3 cell attachment was reduced by 23.8% and the percent of cell population in M phase reduced by 32.1%. In comparison, PC-3 cells incubated with H5 phage had a reduction of 25.0% cell attachment and 33.6% of cell population in M phase. These changes in combination with elevated caspase activation within cells in M phase, and no significant changes to G1/G0 or S phase cell populations suggest that the cytotoxic phages are targeting actively dividing PC-3 cells. Microscopic studies were also performed to further analyze the nature of cytotoxicity of these two phage clones. It was found that G1 phage induced and co- localized with tubulin based projections within apoptotic cells, while H5 phage did not. These phage may form the foundation for a new class of targeted prostate cancer therapeutic agents.

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.

Immunohistochemical and ultrastructural study of the lamellae of oocytes in atretic follicles in relation to different processes of cell death

Atresia is the process through which non-selectable oocytes are eliminated; it involves apoptosis and/or autophagy. This study used immunohistochemical and ultrastructural techniques to characterize the lamellae present in the cytoplasm of oocytes in follicles in the process of atresia in prepubertal and adult Wistar rats. The results indicate that the lamellae are positive to tubulin and myosin immunodetection under light and electron microscopy. Labeling is greater with anti-tubulin and lesser with anti-myosin. Our observations indicate that lamellae are present in oocytes at the initial antral stage in prepubertal rats; that is, from day 14 post-birth to adult age. We were able to determine that the increase in altered lamellae principally occurs in the apoptotic cells rather than in the autophagic cells.

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.

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.

Linking cytoarchitecture to metabolism: sarcolemma-associated plectin affects glucose uptake by destabilizing microtubule networks in mdx myofibers

BACKGROUND

Duchenne muscular dystrophy (DMD) is one of the most frequent forms of muscular disorders. It is caused by the absence of dystrophin, a core component of the sarcolemma-associated junctional complex that links the cytoskeleton to the extracellular matrix. We showed previously that plectin 1f (P1f), one of the major muscle-expressed isoforms of the cytoskeletal linker protein plectin, accumulates at the sarcolemma of DMD patients as well as of mdx mice, a widely studied animal model for DMD.Based on plectin's dual role as structural protein and scaffolding platform for signaling molecules, we speculated that the dystrophic phenotype observed after loss of dystrophin was caused, at least to some extent, by excess plectin. Thus, we hypothesized that elimination of plectin expression in mdx skeletal muscle, while probably resulting in an overall more severe phenotype, may lead to a partial phenotype rescue. In particular, we wanted to assess whether excess sarcolemmal plectin contributes to the dysregulation of sugar metabolism in mdx myofibers.

METHODS

We generated plectin/dystrophin double deficient (dKO) mice by breeding mdx with conditional striated muscle-restricted plectin knockout (cKO) mice. The phenotype of these mice was comparatively analyzed with that of mdx, cKO, and wild-type mice, focusing on structural integrity and dysregulation of glucose metabolism.

RESULTS

We show that the accumulation of plectin at the sarcolemma of mdx muscle fibers hardly compensated for their loss of structural integrity. Instead, it led to an additional metabolic deficit by impairing glucose uptake. While dKO mice suffered from an overall more severe form of muscular dystrophy compared to mdx or plectin-deficient mice, sarcolemmal integrity as well as glucose uptake of their myofibers were restored to normal levels upon ablation of plectin. Furthermore, microtubule (MT) networks in intact dKO myofibers, including subsarcolemmal areas, were found to be more robust than those in mdx mice. Finally, myotubes differentiated from P1f-overexpressing myoblasts showed an impairment of glucose transporter 4 translocation and a destabilization of MT networks.

CONCLUSIONS

Based on these results we propose that sarcolemma-associated plectin acts as an antagonist of MT network formation in myofibers, thereby hindering vesicle-mediated (MT-dependent) transport of glucose transporter 4. This novel role of plectin throws a bridge between extra-sarcomeric cytoarchitecture and metabolism of muscle fibers. Our study thus provides new insights into pathomechanisms of plectinopathies and muscular dystrophies in general.

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.

The rotavirus enterotoxin (NSP4) promotes re-modeling of the intracellular microtubule network

Expression of the rotavirus enterotoxin (NSP4) in transfected monkey kidney cells was found to result in a dramatic re-modeling of the microtubule (MT) network. This important centrosome organized cytoskeletal element was dissolved by expression of NSP4 and re-formed in a ring array at the periphery of the cell, similar to that seen following normal virus infection. Site directed mutagenesis of the N-linked glycosylation sites in NSP4 was employed to show that glycosylation of NSP4 was not required for it to promote changes in the MT network. This result together with experiments using conventional inhibitors indicated that NSP4's ability to cause elevation of intracellular calcium levels was also not necessary to effect the changes in the MT network. Use of the centrosome function inhibitor nocodazole demonstrated that NSP4 based remodeling of the MT network was dominant over the normal organizational role of the centrosome. Finally the remodeling of the MT network was shown not to be linked to cellular apoptosis or necrosis. The potential importance of this newly recognised role for NSP4 in the overall process of intracellular pathogenesis by rotaviruses is discussed.

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.

BASP1 promotes apoptosis in diabetic nephropathy

Apoptosis contributes to the development of diabetic nephropathy (DN), but the mechanisms that lead to diabetes-induced cell death are not fully understood. Here, we combined a functional genomics screen for cDNAs that induce apoptosis in vitro with transcriptional profiling of renal biopsies from patients with DN. Twelve of the 138 full-length cDNAs that induced cell death in human embryonic kidney cells matched upregulated mRNA transcripts in tissue from human DN. Confirmatory screens identified induction of BASP1 in tubular cross sections of human DN tissue. In vitro, apoptosis-inducing conditions such as serum deprivation, high concentrations of glucose, and proinflammatory cytokines increased BASP1 mRNA and protein in human tubular epithelial cells. In normal cells, BASP1 localized to the cytoplasm, but in apoptotic cells, it colocalized with actin in the periphery. Overexpression of BASP1 induced cell death with features of apoptosis; conversely, small interfering RNA (siRNA)-mediated knockdown of BASP1 protected tubular cells from apoptosis. Supporting possible involvement of BASP1 in renal disease other than DN, we also observed significant upregulation of renal BASP1 in spontaneously hypertensive rats and a trend toward increased tubulointerstitial BASP1 mRNA in human hypertensive nephropathy. In summary, a combined functional genomics approach identified BASP1 as a proapoptotic factor in DN and possibly also in hypertensive nephropathy.

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.

Interaction of CDK5RAP2 with EB1 to track growing microtubule tips and to regulate microtubule dynamics

Mutations in cdk5rap2 are linked to autosomal recessive primary microcephaly, and attention has been paid to its function at centrosomes. In this report, we demonstrate that CDK5RAP2 localizes to microtubules and concentrates at the distal tips in addition to centrosomal localization. CDK5RAP2 interacts directly with EB1, a prototypic member of microtubule plus-end tracking proteins, and contains the basic and Ser-rich motif responsible for EB1 binding. The EB1-binding motif is conserved in the CDK5RAP2 sequences of chimpanzee, bovine, and dog but not in those of rat and mouse, suggesting a function gained during the evolution of mammals. The mutation of the Ile/Leu-Pro dipeptide within the motif abolishes EB1 interaction and plus-end attachment. In agreement with the mutational analysis, suppression of EB1 expression inhibits microtubule tip-tracking of CDK5RAP2. We have also found that the CDK5RAP2-EB1 complex regulates microtubule dynamics and stability. CDK5RAP2 depletion by RNA interference impacts the dynamic behaviors of microtubules. The CDK5RAP2-EB1 complex induces microtubule bundling and acetylation when expressed in cell cultures and stimulates microtubule assembly and bundle formation in vitro. Collectively, these results show that CDK5RAP2 targets growing microtubule tips in association with EB1 to regulate microtubule dynamics.

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.

Tau dephosphorylation and microfilaments disruption are upstream events of the anti-proliferative effects of DADS in SH-SY5Y cells

Garlic organosulphur compounds have been successfully used as redox anti-proliferative agents. In this work, we dissect the effects of diallyl disulphide (DADS) focusing on the events upstream of cell cycle arrest and apoptosis induced in neuroblastoma SH-SY5Y cells. We demonstrate that DADS is able to cause early morphological changes, cytoskeleton oxidation, microfilaments reduction and depolymerization of microtubules. These events are attenuated in cells stably overexpressing the antioxidant enzyme SOD1, suggesting that superoxide plays a crucial role in destabilizing cytoskeleton. Moreover, we evidence that the main microtubules-associated protein Tau undergoes PP1-mediated dephosphorylation as demonstrated by treatment with okadaic acid as well as by immunoreaction with anti-Tau-1 antibody, which specifically recognizes its dephosphorylated forms. Tau dephosphorylation is inhibited by the two-electron reductants NAC and GSH ester but not by SOD1. The inability of DADS to induce apoptosis in neuroblastoma-differentiated cells gives emphasis to the anti-proliferative activity of DADS, which can be regarded as a promising potent anti-neuroblastoma drug by virtue of its widespread cytoskeleton disrupting action on proliferating cells.

Leptosphaeria maculans elicits apoptosis coincident with leaf lesion formation and hyphal advance in Brassica napus

Programmed cell death, with many of the morphological markers of apoptosis, is increasingly recognized as an important process in plant disease. We have investigated the involvement and potential role of apoptosis during the formation of leaf lesions by the fungus Leptosphaeria maculans on susceptible Brassica napus cv. Westar. There were no signs of host cell damage until 7 to 8 days postinoculation (dpi), when trypan-blue-stained leaf mesophyll cells were first detected. Hyphae were visible in the intercellular spaces of the inoculated area from 5 dpi and were associated with trypan-blue-stained cells at 8 to 9 dpi. Hallmarks of apoptosis, observed coincident with or immediately prior to the formation of leaf lesions at 8 to 10 dpi, included membrane shrinkage of the mesophyll cell cytoplasm, loss of cell to cell contact in mesophyll cells, and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling of nuclei in apparently "healthy" tissue immediately adjacent to dead areas. Hyphae were highly branched and prolific in the "healthy" tissue immediately adjacent to dead areas 9 to 10 dpi, and formed pycnidia inside dead areas 11 to 12 dpi. Coinfiltration of the tetrapeptide caspase inhibitor Ac-DEVD-CHO with spores of the pathogen significantly suppressed development of leaf lesions but did not affect fungus viability. We hypothesize that L. maculans elicits apoptosis as a dependent component of pathogenesis in susceptible B. napus, and that the fungus uses apoptotic cells as a source of nutrition for reproduction and further growth.

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.

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.