Interplay between microtubule dynamics and intracellular organization

Saccharin enhances neurite extension by regulating organization of the microtubules

AIMS

In the present study, we found that saccharin, an artificial calorie-free sweetener, promotes neurite extension in the cultured neuronal cells. The purposes of this study are to characterize the effect of saccharine on neurite extension and to determine how saccharin enhances neurite extension.

MAIN METHODS

The analyses were performed using mouse neuroblastoma N1E-115 cells and rat pheochromocytoma PC12 cells. Neurite extension was evaluated by counting the cells bearing neurites and measuring the length of neurites. Formation, severing and transportation of the microtubules were evaluated by immunostaining and western blotting analysis.

KEY FINDINGS

Deprivation of glucose increased the number of N1E-115 cells bearing long processes. And the effect was inhibited by addition of glucose. Saccharin increased the number of these cells bearing long processes in a dose-dependent manner and total neurite length and longest neurite length in each cell. Saccharin also had a similar effect on NGF-treated PC12 cells. Saccharin increased the amount of the microtubules reconstructed after treatment with nocodazole, a disruptor of microtubules. The effect of saccharin on microtubule reconstruction was not influenced by dihydrocytochalasin B, an inhibitor of actin polymerization, indicating that saccharin enhances microtubule formation without requiring actin dynamics. In the cells treated with vinblastine, an inhibitor of microtubule polymerization, after microtubule reorganization, filamentous microtubules were observed more distantly from the centrosome in saccharin-treated cells, indicating that saccharin enhances microtubule severing and/or transportation.

SIGNIFICANCE

These results suggest that saccharin enhances neurite extension by promoting microtubule organization.

Moonlighting microtubule-associated proteins: regulatory functions by day and pathological functions at night

The sensing, integrating, and coordinating features of the eukaryotic cells are achieved by the complex ultrastructural arrays and multifarious functions of the cytoskeletal network. Cytoskeleton comprises fibrous protein networks of microtubules, actin, and intermediate filaments. These filamentous polymer structures are highly dynamic and undergo constant and rapid reorganization during cellular processes. The microtubular system plays a crucial role in the brain, as it is involved in an enormous number of cellular events including cell differentiation and pathological inclusion formation. These multifarious functions of microtubules can be achieved by their decoration with proteins/enzymes that exert specific effects on the dynamics and organization of the cytoskeleton and mediate distinct functions due to their moonlighting features. This mini-review focuses on two aspects of the microtubule cytoskeleton. On the one hand, we describe the heteroassociation of tubulin/microtubules with metabolic enzymes, which in addition to their catalytic activities stabilize microtubule structures via their cross-linking functions. On the other hand, we focus on the recently identified moonlighting tubulin polymerization promoting protein, TPPP/p25. TPPP/p25 is a microtubule-associated protein and it displays distinct physiological or pathological (aberrant) functions; thus it is a prototype of Neomorphic Moonlighting Proteins. The expression of TPPP/p25 is finely controlled in the human brain; this protein is indispensable for the development of projections of oligodendrocytes that are responsible for the ensheathment of axons. The nonphysiological, higher or lower TPPP/p25 level leads to distinct CNS diseases. Mechanisms contributing to the control of microtubule stability and dynamics by metabolic enzymes and TPPP/p25 will be discussed.

Cyclin-dependent kinases regulate splice-specific targeting of dynamin-related protein 1 to microtubules

The splice isoform Drp1-x01 promotes mitochondrial fission and is regulated by Cdk phosphorylation-dependent changes in microtubule association.

Fission and fusion reactions determine mitochondrial morphology and function. Dynamin-related protein 1 (Drp1) is a guanosine triphosphate–hydrolyzing mechanoenzyme important for mitochondrial fission and programmed cell death. Drp1 is subject to alternative splicing of three exons with previously unknown functional significance. Here, we report that splice variants including the third but excluding the second alternative exon (x01) localized to and copurified with microtubule bundles as dynamic polymers that resemble fission complexes on mitochondria. A major isoform in immune cells, Drp1-x01 required oligomeric assembly and Arg residues in alternative exon 3 for microtubule targeting. Drp1-x01 stabilized and bundled microtubules and attenuated staurosporine-induced mitochondrial fragmentation and apoptosis. Phosphorylation of a conserved Ser residue adjacent to the microtubule-binding exon released Drp1-x01 from microtubules and promoted mitochondrial fragmentation in a splice form–specific manner. Phosphorylation by Cdk1 contributed to dissociation of Drp1-x01 from mitotic microtubules, whereas Cdk5-mediated phosphorylation modulated Drp1-x01 targeting to interphase microtubules. Thus, alternative splicing generates a latent, cytoskeletal pool of Drp1 that is selectively mobilized by cyclin-dependent kinase signaling.

Microtubule dynamics in neuronal morphogenesis

Microtubules (MTs) are essential for neuronal morphogenesis in the developing brain. The MT cytoskeleton provides physical support to shape the fine structure of neuronal processes. MT-based motors play important roles in nucleokinesis, process formation and retraction. Regulation of MT stability downstream of extracellular cues is proposed to be critical for axonogenesis. Axons and dendrites exhibit different patterns of MT organization, underlying the divergent functions of these processes. Centrosomal positioning has drawn the attention of researchers because it is a major clue to understanding neuronal MT organization. In this review, we focus on how recent advances in live imaging have revealed the dynamics of MT organization and centrosome positioning during neural development.

Actin acting at the Golgi

The organization, assembly and remodeling of the actin cytoskeleton provide force and tracks for a variety of (endo)membrane-associated events such as membrane trafficking. This review illustrates in different cellular models how actin and many of its numerous binding and regulatory proteins (actin and co-workers) participate in the structural organization of the Golgi apparatus and in trafficking-associated processes such as sorting, biogenesis and motion of Golgi-derived transport carriers.

Recruitment of EB1, a master regulator of microtubule dynamics, to the surface of the Theileria annulata schizont

The apicomplexan parasite Theileria annulata transforms infected host cells, inducing uncontrolled proliferation and clonal expansion of the parasitized cell population. Shortly after sporozoite entry into the target cell, the surrounding host cell membrane is dissolved and an array of host cell microtubules (MTs) surrounds the parasite, which develops into the transforming schizont. The latter does not egress to invade and transform other cells. Instead, it remains tethered to host cell MTs and, during mitosis and cytokinesis, engages the cell's astral and central spindle MTs to secure its distribution between the two daughter cells. The molecular mechanism by which the schizont recruits and stabilizes host cell MTs is not known. MT minus ends are mostly anchored in the MT organizing center, while the plus ends explore the cellular space, switching constantly between phases of growth and shrinkage (called dynamic instability). Assuming the plus ends of growing MTs provide the first point of contact with the parasite, we focused on the complex protein machinery associated with these structures. We now report how the schizont recruits end-binding protein 1 (EB1), a central component of the MT plus end protein interaction network and key regulator of host cell MT dynamics. Using a range of in vitro experiments, we demonstrate that T. annulata p104, a polymorphic antigen expressed on the schizont surface, functions as a genuine EB1-binding protein and can recruit EB1 in the absence of any other parasite proteins. Binding strictly depends on a consensus SxIP motif located in a highly disordered C-terminal region of p104. We further show that parasite interaction with host cell EB1 is cell cycle regulated. This is the first description of a pathogen-encoded protein to interact with EB1 via a bona-fide SxIP motif. Our findings provide important new insight into the mode of interaction between Theileria and the host cell cytoskeleton.

The apicomplexan parasite Theileria can reprogram the cell it infects, inducing uncontrolled proliferation and clonal expansion. This is brought about by the schizont, which resides free in the host cell cytoplasm. As the schizont never leaves the cell to infect other cells, it can only persist provided it is distributed over the two daughter cells each time the host cell divides. This is achieved by interacting dynamically with microtubules (MTs) that form part of the host cell mitotic apparatus. How MTs are recruited to the schizont surface is not known. MTs are highly dynamic, undergoing continuous cycles of growth and shrinkage that is regulated to a large extent by an array of proteins, called +TIPs, that associate with the free plus-ends of MTs. End-binding protein 1 (EB1) is a master regulator and central adaptor that mediates MT plus-end tracking of potentially all other +TIPs. We established that a schizont surface protein, p104, provides a docking site for EB1, which critically depends on a consensus SxIP motif, present in p104. These finding provides important new insight into the complex interaction of the transforming schizont with host cell MTs. To our knowledge, p104 is the first pathogen-derived protein identified so far to join the SxIP family of EB1-binding proteins.

MAP/microtubule affinity-regulating kinases, microtubule dynamics, and spermatogenesis

During spermatogenesis, spermatids derived from meiosis simultaneously undergo extensive morphological transformation, to become highly specialized and metabolically quiescent cells, and transport across the seminiferous epithelium. Spermatids are also transported back-and-forth across the seminiferous epithelium during the epithelial cycle until they line up at the luminal edge of the tubule to prepare for spermiation at stage VIII of the cycle. Spermatid transport thus requires the intricate coordination of the cytoskeletons in Sertoli cells (SCs) as spermatids are nonmotile cells lacking the ultrastructures of lamellipodia and filopodia, as well as the organized components of the cytoskeletons. In the course of preparing this brief review, we were surprised to see that, except for some earlier eminent morphological studies, little is known about the regulation of the microtubule (MT) cytoskeleton and the coordination of MT with the actin-based cytoskeleton to regulate spermatid transport during the epithelia cycle, illustrating that this is a largely neglected area of research in the field. Herein, we summarize recent findings in the field regarding the significance of actin- and tubulin-based cytoskeletons in SCs that support spermatid transport; we also highlight specific areas of research that deserve attention in future studies.

A non-canonical mode of microtubule organization operates throughout pre-implantation development in mouse

In dividing animal cells, the centrosome, comprising centrioles and surrounding pericentriolar-material (PCM), is the major interphase microtubule-organizing center (MTOC), arranging a polarized array of microtubules (MTs) that controls cellular architecture. The mouse embryo is a unique setting for investigating the role of centrosomes in MT organization, since the early embryo is acentrosomal, and centrosomes emerge de novo during early cleavages. Here we use embryos from a GFP::CETN2 transgenic mouse to observe the emergence of centrosomes and centrioles in embryos, and show that unfocused acentriolar centrosomes first form in morulae (~16-32-cell stage) and become focused at the blastocyst stage (~64-128 cells) concomitant with the emergence of centrioles. We then used high-resolution microscopy and dynamic tracking of MT growth events in live embryos to examine the impact of centrosome emergence upon interphase MT dynamics. We report that pre-implantation mouse embryos of all stages employ a non-canonical mode of MT organization that generates a complex array of randomly oriented MTs that are preferentially nucleated adjacent to nuclear and plasmalemmal membranes and cell-cell interfaces. Surprisingly, however, cells of the early embryo continue to employ this mode of interphase MT organization even after the emergence of centrosomes. Centrosomes are found at MT-sparse sites and have no detectable impact upon interphase MT dynamics. To our knowledge, the early embryo is unique among proliferating cells in adopting an acentrosomal mode of MT organization despite the presence of centrosomes, revealing that the transition to a canonical mode of interphase MT organization remains incomplete prior to implantation.

Regulatable transgenic mouse models of Alzheimer disease: onset, reversibility and spreading of Tau pathology

Accumulation of amyloidogenic proteins such as Tau is a hallmark of neurodegenerative diseases including Alzheimer disease and fronto-temporal dementias. To link Tau pathology to cognitive impairments and defects in synaptic plasticity, we created four inducible Tau transgenic mouse models with expression of pro- and anti-aggregant variants of either full-length human Tau (hTau40/ΔK280 and hTau40/ΔK280/PP) or the truncated Tau repeat domain (Tau(RD)/ΔK280 and Tau(RD)/ΔK280/PP). Here we review the histopathological features caused by pro-aggregant Tau, and correlate them with behavioral deficits and impairments in synaptic transmission. Both pro-aggregant Tau variants cause Alzheimer-like features, including synapse loss, mis-localization of Tau into the somatodendritic compartment, conformational changes and hyperphosphorylation. However, there is a clear difference in the extent of Tau aggregation and neurotoxicity. While pro-aggregant full-length hTau40/ΔK280 leads to a 'pre-tangle' pathology, the repeat domain Tau(RD)/ΔK280 causes massive formation of neurofibrillary tangles and neuronal loss in the hippocampus. However, both Tau variants cause co-aggregation of human and mouse Tau and similar functional impairments. Thus, earlier Tau pathological stages and not necessarily neurofibrillary tangles are critical for the development of cognitive malfunctions. Most importantly, memory and synapses recover after switching off expression of pro-aggregant Tau. The rescue of functional impairments correlates with the rescue of most Tau pathological changes and most strikingly the recovery of synapses. This implies that tauopathies as such are reversible, provided that amyloidogenic Tau is removed. Therefore, our Tau transgenic mice may serve as model systems for in vivo validation of therapeutic strategies and drug candidates with regard to cognition and synaptic function.

Multi-step down-regulation of the secretory pathway in mitosis: a fresh perspective on protein trafficking

The secretory pathway delivers proteins synthesized at the rough endoplasmic reticulum (RER) to various subcellular locations via the Golgi apparatus. Currently, efforts are focused on understanding the molecular machineries driving individual processes at the RER and Golgi that package, modify and transport proteins. However, studies are routinely performed using non-dividing cells. This obscures the critical issue of how the secretory pathway is affected by cell division. Indeed, several studies have indicated that protein trafficking is down-regulated during mitosis. Moreover, the RER and Golgi apparatus exhibit gross reorganization in mitosis. Here I provide a relatively neglected perspective of how the mitotic cyclin-dependent kinase (CDK1) could regulate various stages of the secretory pathway. I highlight several aspects of the mitotic control of protein trafficking that remain unresolved and suggest that further studies on how the mitotic CDK1 influences the secretory pathway are necessary to obtain a deeper understanding of protein transport.

Viral interactions with microtubules: orchestrators of host cell biology?

Viral interaction with the microtubule (MT) cytoskeleton is critical to infection by many viruses. Most data regarding virus–MT interaction indicate key roles in the subcellular transport of virions/viral genomic material to sites of replication, assembly and egress. However, the MT cytoskeleton orchestrates diverse processes in addition to subcellular cargo transport, including regulation of signaling pathways, cell survival and mitosis, suggesting that viruses, expert manipulators of the host cell, may use the virus–MT interface to control multiple aspects of cell biology. Several lines of evidence support this idea, indicating that specific viral proteins can modify MT dynamics and/or structure and regulate processes such as apoptosis and innate immune signaling through MT-dependent mechanisms. Here, the authors review general aspects of virus–MT interactions, with emphasis on viral mechanisms that modify MT dynamics and functions to affect processes beyond virion transport. The emerging importance of discrete viral protein–MT interactions in pathogenic processes indicates that these interfaces may represent new targets for future therapeutics and vaccine development.

Hypertonic stress promotes autophagy and microtubule-dependent autophagosomal clusters

Osmotic homeostasis is fundamental for most cells, which face recurrent alterations of environmental osmolality that challenge cell viability. Protein damage is a consequence of hypertonic stress, but whether autophagy contributes to the osmoprotective response is unknown. Here, we investigated the possible implications of autophagy and microtubule organization on the response to hypertonic stress. We show that hypertonicity rapidly induced long-lived protein degradation, LC3-II generation and Ptdlns3K-dependent formation of LC3- and ATG12-positive puncta. Lysosomotropic agents chloroquine and bafilomycin A 1, but not nutrient deprivation or rapamycin treatment, further increased LC3-II generation, as well as ATG12-positive puncta, indicating that hypertonic stress increases autophagic flux. Autophagy induction upon hypertonic stress enhanced cell survival since cell death was increased by ATG12 siRNA-mediated knockdown and reduced by rapamycin. We additionally showed that hypertonicity induces fast reorganization of microtubule networks, which is associated with strong reorganization of microtubules at centrosomes and fragmentation of Golgi ribbons. Microtubule remodeling was associated with pericentrosomal clustering of ATG12-positive autolysosomes that colocalized with SQSTM1/p62 and ubiquitin, indicating that autophagy induced by hypertonic stress is at least partly selective. Efficient autophagy by hypertonic stress required microtubule remodeling and was DYNC/dynein-dependent as autophagosome clustering was enhanced by paclitaxel-induced microtubule stabilization and was reduced by nocodazole-induced tubulin depolymerization as well as chemical (EHNA) or genetic [DCTN2/dynactin 2 (p50) overexpression] interference of DYNC activity. The data document a general and hitherto overlooked mechanism, where autophagy and microtubule remodeling play prominent roles in the osmoprotective response.

A novel method for purification of polymerizable tubulin with a high content of the acetylated isotype

Tubulin can be acetylated/deacetylated on Lys40 of the α-subunit. Studies of the post-translational acetylation/deacetylation of tubulin using biochemical techniques require tubulin preparations that are enriched in AcTubulin (acetylated tubulin) and (for comparison) preparations lacking AcTubulin. Assembly–disassembly cycling of microtubules gives tubulin preparations that contain little or no AcTubulin. In the present study we demonstrated that this result is owing to the presence of high deacetylating activity in the extracts. This deacetylating activity in rat brain homogenates was inhibited by TSA (Trichostatin A) and tubacin, but not by nicotinamide, indicating that HDAC6 (histone deacetylase 6) is involved. TSA showed no effect on microtubule polymerization or depolymerization. We utilized these properties of TSA to prevent deacetylation during the assembly–disassembly procedure. The effective inhibitory concentration of TSA was 3 μM in the homogenate and 1 μM in the subsequent cycling steps. By comparison with immunopurified AcTubulin, we estimated that ~64% of the tubulin molecules in the three cycled preparations were acetylated. The protein profiles of these tubulin preparations, as assessed by SDS/PAGE and Coomassie Blue staining, were identical to that of a preparation completely lacking AcTubulin obtained by assembly–disassembly cycles in the absence of TSA. The tyrosination state and in vitro assembly–disassembly kinetics were the same regardless of the degree of acetylation.

Nek7 kinase accelerates microtubule dynamic instability

The NIMA-related kinases (NRK or Nek) are emerging as conserved and crucial regulators of mitosis and cilia formation. The microtubule (MT) network has long been suspected as a major target of the Neks. However, the underlying mechanism remains unclear. Using the PlusTipTracker software, recently developed by the Danuser group, we followed the consequences of alterations in Nek7 levels on MT dynamic instability. siRNA-mediated downregulation of Nek7 in HeLa cells resulted in lower speeds of MT growth and catastrophe, reduction of the relative time spent in catastrophe, and considerably lowered the overall MT dynamicity. Co-expression of Nek7 with the siRNA treatment rescued the MT phenotypes, while ectopic overexpression of Nek7 yielded inverse characteristics compared to Nek7 downregulation. MT dynamics in mouse embryonic fibroblasts derived from targeted null mutants for Nek7 recapitulated the siRNA downregulation phenotypes. Precise MT dynamic instability is critical for accurate shaping of the mitotic spindle and for cilium formation, and higher MT dynamicity is associated with tumorigenicity. Thus, our results can supply a mechanistic explanation for Nek involvement in these processes.

High-content screening and analysis of the Golgi complex

High-content screening (HCS) as a methodological tool has evolved relatively recently, largely driven by the demand for in depth spatial and temporal information from intact cells exposed to a range of chemical and/or genomic perturbations. The technology is based around automated fluorescence microscopy in combination with advanced imaging processing and analysis tools, which together can provide quantitative information as a first-level description of complex cellular events. HCS and high-content analysis are particularly powerful when combined with perturbation techniques such as RNA interference (RNAi), as this allows large families of genes to be interrogated with respect to a biological pathway or process of interest. In this methodology chapter, we describe an approach by which HCS can be applied to study the morphological state of the Golgi complex in cultured mammalian cells. We provide a detailed protocol for the highly parallel downregulation of gene activity using RNAi in 384-well plates and describe an automated image analysis routine that could be used to quantify Golgi complex in a genome-wide RNAi context.

ARHGAP21 protein, a new partner of α-tubulin involved in cell-cell adhesion formation and essential for epithelial-mesenchymal transition

Cell-cell adhesions and the cytoskeletons play important and coordinated roles in cell biology, including cell differentiation, development, and migration. Adhesion and cytoskeletal dynamics are regulated by Rho-GTPases. ARHGAP21 is a negative regulator of Rho-GTPases, particularly Cdc42. Here we assess the function of ARHGAP21 in cell-cell adhesion, cell migration, and scattering. We find that ARHGAP21 is localized in the nucleus, cytoplasm, or perinuclear region but is transiently redistributed to cell-cell junctions 4 h after initiation of cell-cell adhesion. ARHGAP21 interacts with Cdc42, and decreased Cdc42 activity coincides with the appearance of ARHGAP21 at the cell-cell junctions. Cells lacking ARHGAP21 expression show weaker cell-cell adhesions, increased cell migration, and a diminished ability to undergo hepatocyte growth factor-induced epithelial-mesenchymal transition (EMT). In addition, ARHGAP21 interacts with α-tubulin, and it is essential for α-tubulin acetylation in EMT. Our findings indicate that ARHGAP21 is a Rho-GAP involved in cell-cell junction remodeling and that ARHGAP21 affects migration and EMT through α-tubulin interaction and acetylation.

Exploitation of Cellular Cytoskeletons and Signaling Pathways for Cell Entry by Kaposi's Sarcoma-Associated Herpesvirus and the Closely Related Rhesus Rhadinovirus

As obligate intracellular pathogens, viruses depend on the host cell machinery to complete their life cycle. Kaposi’s sarcoma-associated herpes virus (KSHV) is an oncogenicvirus causally linked to the development of Kaposi’s sarcoma and several other lymphoproliferative malignancies. KSHV entry into cells is tightly regulated by diverse viral and cellular factors. In particular, KSHV actively engages cellular integrins and ubiquitination pathways for successful infection. Emerging evidence suggests that KSHV hijacks both actin and microtubule cytoskeletons at different phases during entry into cells. Here, we review recent findings on the early events during primary infection of KSHV and its closely related primate homolog rhesus rhadinovirus with highlights on the regulation of cellular cytoskeletons and signaling pathways that are important for this phase of virus life cycle.

Enhancement of tubulin polymerization by Cl(-)-induced blockade of intrinsic GTPase

In growing neurite of neuronal cells, it is suggested that α/β-tubulin heterodimers assemble to form microtubule, and assembly of microtubule promotes neurite elongation. On the other hand, recent studies reveal importance of intracellular Cl(-) in regulation of various cellular functions such as cell cycle progression, differentiation, cell migration, and elongation of neurite in neuronal cells. In this study, we investigated effects of Cl(-) on in vitro tubulin polymerization. We found that efficiency of in vitro tubulin polymerization (the number of microtubule) was higher (3 to 5-fold) in Cl(-)-containing solutions than that in Cl(-)-free solutions containing Br(-) or NO(3)(-). On the other hand, GTPase activity of tubulin was lower (2/3-fold) in Cl(-)-containing solutions than that in Cl(-)-free solutions containing Br(-) or NO(3)(-). Efficiency of in vitro tubulin polymerization in solutions containing a non-hydrolyzable analogue of GTP (GpCpp) instead of GTP was much higher than that in the presence of GTP. Effects of replacement of GTP with GpCpp on in vitro tubulin polymerization was weaker in Cl(-) solutions (10-fold increases) than that in Br(-) or NO(3)(-) solutions (20-fold increases), although the efficiency of in vitro tubulin polymerization in Cl(-) solutions containing GpCpp was still higher than that in Br(-) or NO(3)(-) solutions containing GpCpp. Our results suggest that a part of stimulatory effects of Cl(-) on in vitro tubulin polymerization is mediated via an inhibitory effect on GTPase activity of tubulin, although Cl(-) would also regulate in vitro tubulin polymerization by factors other than an inhibitory effect on GTPase activity.

Fractal properties of macrophage membrane studied by AFM

Complexity of cell membrane poses difficulties to quantify corresponding morphology changes during cell proliferation and damage. We suggest using fractal dimension of the cell membrane to quantify its complexity and track changes produced by various treatments. Glutaraldehyde fixed mouse RAW 264.7 macrophage membranes were chosen as model system and imaged in PeakForce QNM (quantitative nanomechanics) mode of AFM (atomic force microscope). The morphology of the membranes was characterized by fractal dimension. The parameter was calculated for set of AFM images by three different methods. The same calculations were done for the AFM images of macrophages treated with colchicine, an inhibitor of the microtubule polymerization, and microtubule stabilizing agent taxol. We conclude that fractal dimension can be additional and useful parameter to characterize the cell membrane complexity and track the morphology changes produced by different treatments.