Phosphorylation of α-tubulin by protein kinase C stimulates microtubule dynamics in human breast cells

The Tubulin Code, from Molecules to Health and Disease

Microtubules are essential dynamic polymers composed of α/β-tubulin heterodimers. They support intracellular trafficking, cell division, cellular motility, and other essential cellular processes. In many species, both α-tubulin and β-tubulin are encoded by multiple genes with distinct expression profiles and functionality. Microtubules are further diversified through abundant posttranslational modifications, which are added and removed by a suite of enzymes to form complex, stereotyped cellular arrays. The genetic and chemical diversity of tubulin constitute a tubulin code that regulates intrinsic microtubule properties and is read by cellular effectors, such as molecular motors and microtubule-associated proteins, to provide spatial and temporal specificity to microtubules in cells. In this review, we synthesize the rapidly expanding tubulin code literature and highlight limitations and opportunities for the field. As complex microtubule arrays underlie essential physiological processes, a better understanding of how cells employ the tubulin code has important implications for human disease ranging from cancer to neurological disorders.

Tau, microtubule dynamics, and axonal transport: New paradigms for neurodegenerative disease

The etiology of Tauopathies, a diverse class of neurodegenerative diseases associated with the Microtubule Associated Protein (MAP) Tau, is usually described by a common mechanism in which Tau dysfunction results in the loss of axonal microtubule stability. Here, we reexamine and build upon the canonical disease model to encompass other Tau functions. In addition to regulating microtubule dynamics, Tau acts as a modulator of motor proteins, a signaling hub, and a scaffolding protein. This diverse array of functions is related to the dynamic nature of Tau isoform expression, post-translational modification (PTM), and conformational flexibility. Thus, there is no single mechanism that can describe Tau dysfunction. The effects of specific pathogenic mutations or aberrant PTMs need to be examined on all of the various functions of Tau in order to understand the unique etiology of each disease state.

Phosphomimetic Mutation at Ser165 of α-Tubulin Promotes the Persistence of GTP Caps in Microtubules

Protein kinase C (PKC)-mediated phosphorylation of α-tubulin at Ser165 or expression of phosphomimetic (S165D)-α-tubulin stimulates microtubule (MT) polymerization (Cytoskeleton 2014, 71, 257-272). Ser165 lies near the interface between adjacent αβ-tubulin heterodimers and helix H8, which contains Glu254, the catalytic residue in α-tubulin that hydrolyzes the exchangeable GTP in β-tubulin (β:GTP) and triggers MT depolymerization. It was hypothesized that S165D, a phosphomimetic variant of α-tubulin, perturbs the alignment of α:Glu254 with respect to β:GTP, thereby impairing its hydrolysis. Molecular simulations were performed with cryoEM structures of MTs (PDB ID: 3J6E) in which phosphomimetic S165D or control S165N had been substituted. Unlike native and S165N structures, the distance between S165D and α:Glu254 increased by 0.6 Å, while the distance between α:Glu254 and β:GTP decreased by 0.4 Å. Rotation of β:GTP by 4 Å occurred in the S165D variant, whereas β:GTP in the S165N control was unchanged from the native structure. Additionally, the S165D variant exhibited an altered pattern of H-bonding to β:GTP, including the loss of three H-bonds. The significance of these findings to β:GTP hydrolysis was analyzed in MCF-10A human breast cells treated with an antibody that detects GTP-bound tubulin. Compared with controls, GTP-tubulin signals were at higher levels in cells that ectopically expressed S165D-α-tubulin (TUBA1C) or had been treated with PKC activator DAG-lactone to induce phosphorylation of Ser165 in native α-tubulin. These findings support a model whereby conformational changes induced by Ser165 phosphorylation alter the spatial relationship between β:GTP and α:Glu254, thereby slowing GTP hydrolysis and promoting GTP caps.

Posttranslational modifications of the cytoskeleton

The cytoskeleton plays important roles in many essential processes at the cellular and organismal levels, including cell migration and motility, cell division, and the establishment and maintenance of cell and tissue architecture. In order to facilitate these varied functions, the main cytoskeletal components-microtubules, actin filaments, and intermediate filaments-must form highly diverse intracellular arrays in different subcellular areas and cell types. The question of how this diversity is conferred has been the focus of research for decades. One key mechanism is the addition of posttranslational modifications (PTMs) to the major cytoskeletal proteins. This posttranslational addition of various chemical groups dramatically increases the complexity of the cytoskeletal proteome and helps facilitate major global and local cytoskeletal functions. Cytoskeletal proteins undergo many PTMs, most of which are not well understood. Recent technological advances in proteomics and cell biology have allowed for the in-depth study of individual PTMs and their functions in the cytoskeleton. Here, we provide an overview of the major PTMs that occur on the main structural components of the three cytoskeletal systems-tubulin, actin, and intermediate filament proteins-and highlight the cellular function of these modifications.

FKBP25 Regulates Meiotic Apparatus During Mouse Oocyte Maturation

FK506 binding proteins 25 (FKBP25) has been shown to function in ribosome biogenesis, chromatin organization, and microtubule stability in mitosis. However, the role of FKBP25 in oocyte maturation has not been investigated. Here, we report that oocytes with FKBP25 depletion display abnormal spindle assembly and chromosomes alignment, with defective kinetochore-microtubule attachment. Consistent with this finding, aneuploidy incidence is also elevated in oocytes depleted of FKBP25. Importantly, FKBP25 protein level in old oocytes is significantly reduced, and ectopic expression of FKBP25 could partly rescue the aging-associated meiotic defects. In addition, by employing site-specific mutagenesis, we identify that serine 163 is a major, if not unique, phosphorylation site modulating the action of FKBP25 on meiotic maturation. In summary, our data indicate that FKBP25 is a pivotal factor for determining oocyte quality, and may mediate the effects of maternal aging on female reproduction.

Intrinsic and Extrinsic Factors Affecting Microtubule Dynamics in Normal and Cancer Cells

Microtubules (MTs), highly dynamic structures composed of α- and β-tubulin heterodimers, are involved in cell movement and intracellular traffic and are essential for cell division. Within the cell, MTs are not uniform as they can be composed of different tubulin isotypes that are post-translationally modified and interact with different microtubule-associated proteins (MAPs). These diverse intrinsic factors influence the dynamics of MTs. Extrinsic factors such as microtubule-targeting agents (MTAs) can also affect MT dynamics. MTAs can be divided into two main categories: microtubule-stabilizing agents (MSAs) and microtubule-destabilizing agents (MDAs). Thus, the MT skeleton is an important target for anticancer therapy. This review discusses factors that determine the microtubule dynamics in normal and cancer cells and describes microtubule–MTA interactions, highlighting the importance of tubulin isoform diversity and post-translational modifications in MTA responses and the consequences of such a phenomenon, including drug resistance development.

Evaluation of active Rac1 levels in cancer cells: A case of misleading conclusions from immunofluorescence analysis

A large number of aggressive cancer cell lines display elevated levels of activated Rac1, a small GTPase widely implicated in cytoskeleton reorganization, cell motility, and metastatic dissemination. A commonly accepted methodological approach for detecting Rac1 activation in cancer cells involves the use of a conformation-sensitive antibody that detects the active (GTP-bound) Rac1 without interacting with the GDP-bound inactive form. This antibody has been extensively used in fixed cell immunofluorescence and immunohistochemistry. Taking advantage of prostate and pancreatic cancer cell models known to have high basal Rac1-GTP levels, here we have established that this antibody does not recognize Rac1 but rather detects the intermediate filament protein vimentin. Indeed, Rac1-null PC3 prostate cancer cells or cancer models with low levels of Rac1 activation still show a high signal with the anti-Rac1-GTP antibody, which is lost upon silencing of vimentin expression. Moreover, this antibody was unable to detect activated Rac1 in membrane ruffles induced by epidermal growth factor stimulation. These results have profound implications for the study of this key GTPase in cancer, particularly because a large number of cancer cell lines with characteristic mesenchymal features show simultaneous up-regulation of vimentin and high basal Rac1-GTP levels when measured biochemically. This misleading correlation can lead to assumptions about the validity of this antibody and inaccurate conclusions that may affect the development of appropriate therapeutic approaches for targeting the Rac1 pathway.

The Tubulin Code in Microtubule Dynamics and Information Encoding

Microtubules are non-covalent mesoscale polymers central to the eukaryotic cytoskeleton. Microtubule structure, dynamics, and mechanics are modulated by a cell's choice of tubulin isoforms and post-translational modifications, a "tubulin code," which is thought to support the diverse morphology and dynamics of microtubule arrays across various cell types, cell cycle, and developmental stages. We give a brief historical overview of research into tubulin diversity and highlight recent progress toward uncovering the mechanistic underpinnings of the tubulin code. As a large number of essential pathways converge upon the microtubule cytoskeleton, understanding how cells utilize tubulin diversity is crucial to understanding cellular physiology and disease.

Bone marrow mesenchymal stem cells modified by angiogenin-1 promotes tissue repair in mice with oxygen-induced retinopathy of prematurity by promoting retinal stem cell proliferation and differentiation

Retinopathy has become one of the major factors that lead to blindness worldwide. Although many clinical therapies are concerned about such disease, most of them focus on symptoms alleviation. In this study, we aim to investigate whether coculture retinal stem cells (RSCs) with bone marrow mesenchymal stem cells transfected with angiogenin-1 (Ang-1-BMSCs) affects the damaged retinal tissue of oxygen-induced retinopathy of prematurity (OIR-ROP) mice. After OIR-ROP mouse model establishment, Ang-1-BMSCs, RSCs, and OIR-ROP retinal tissues were cocultured in a a transwell chamber. RSCs proliferation and the expression of Ang-1, insulin-like growth factor-1 (IGF-1) in the supernatant of RSCs, as well as β-tubulin and protein kinase C (PKC) expression were evaluated. Finally, the repair of OIR-ROP mice retinal tissues was observed by injecting Ang-1-BMSCs + RSCs. In the OIR-ROP mouse model, RSCs cocultured with OIR-ROP retinal tissues could be induced to differentiate into cells expressing β-tubulin and PKC and promote the expression of Ang-1 and IGF-1. coculture of Ang-1-BMSCs further enhanced the proliferation and differentiation of RSCs by promoting the expression of Ang-1 and IGF-1. Coculture of RSCs + Ang-1-BMSCs induced differentiation of Ang-1-BMSCs through interaction among intercellular factors and restored the damaged retinal tissue of OIR-ROP mice. Collectively, our study provided evidence that coculture of Ang-1-BMSCs and RSCs could promote the proliferation and differentiation of RSCs and improve the treatment for the damaged retina tissue of OIR-ROP mice.

Phosphorylation state of Ser165 in α-tubulin is a toggle switch that controls proliferating human breast tumors

Engineered overexpression of protein kinase Cα (PKCα) is known to phosphorylate Ser165 in α-tubulin resulting in stimulated microtubule dynamics and cell motility, and activation of an epithelial-mesenchymal transition (EMT) in non-transformed human breast cells. Here it is shown that endogenous phosphorylation of native α-tubulin in two metastatic breast cell lines, MDA-MB-231-LM2-4175 and MDA-MB-468 is detected at PKC phosphorylation sites. α-Tubulin mutants that simulated phosphorylated (S165D) or non-phosphorylated (S165 N) states were stably expressed in MDA-MB-231-LM2-4175 cells. The S165D-α-tubulin mutant engendered expression of the EMT biomarker N-cadherin, whereas S165 N-α-tubulin suppressed N-cadherin and induced E-cadherin expression, revealing a 'cadherin switch'. S165 N-α-tubulin engendered more rapid passage through the cell cycle, induced shorter spindle fibers and exhibited more rapid proliferation. In nude mice injected with MDA-MB-231-LM2-4175 cells, cells expressing S165 N-α-tubulin (but not the S165D mutant) produced hyper-proliferative lung tumors with increased tumor incidence and higher Ki67 expression. These results implicate the phosphorylation state of Ser165 in α-tubulin as a PKC-regulated molecular switch that causes breast cells to exhibit either EMT characteristics or hyper-proliferation. Evaluation of genomic databases of human tumors strengthens the clinical significance of these findings.

CD30 Characterizes Polylobated Lymphocytes and Disease Progression in HTLV-1-Infected Individuals

Purpose: Although expression of CD30 is reported in a subset of adult T-cell leukemia/lymphoma cases, its clinicopathologic significance is poorly understood. We aimed to characterize CD30-positive cells and clarify their tumorigenic role in human T-cell lymphotropic virus type 1 (HTLV-1)-infected cells.Experimental Design: CD30-positive peripheral blood mononuclear cells from individuals with differing HTLV-1 disease status were characterized, and the role of CD30 signaling was examined using HTLV-1-infected cell lines and primary cells.Results: CD30-positive cells were detected in all samples examined, and the marker was coexpressed with both CD25 and CD4. This cell population expanded in accordance with disease progression. CD30-positive cells showed polylobation, with some possessing "flower cell" features, active cycling, and hyperploidy. CD30 stimulation of HTLV-1-infected cell lines induced these features and abnormal cell division, with polylobation found to be dependent on the activation of PI3K. The results thus link the expression of CD30, which serves as a marker for HTLV-1 disease status, to an active proliferating cell fraction featuring polylobation and chromosomal aberrations. In addition, brentuximab vedotin, an anti-CD30 monoclonal antibody conjugated with auristatin E, was found to reduce the CD30-positive cell fraction.Conclusions: Our results indicate that CD30-positive cells act as a reservoir for tumorigenic transformation and clonal expansion during HTLV-1 infection. The CD30-positive fraction may thus be a potential molecular target for those with differing HTLV-1 disease status. Clin Cancer Res; 24(21); 5445-57. ©2018 AACR.

The prolyl isomerase FKBP25 regulates microtubule polymerization impacting cell cycle progression and genomic stability

FK506 binding proteins (FKBPs) catalyze the interconversion of cis-trans proline conformers in proteins. Importantly, FK506 drugs have anti-cancer and neuroprotective properties, but the effectors and mechanisms underpinning these properties are not well understood because the cellular function(s) of most FKBP proteins are unclear. FKBP25 is a nuclear prolyl isomerase that interacts directly with nucleic acids and is associated with several DNA/RNA binding proteins. Here, we show the catalytic FKBP domain binds microtubules (MTs) directly to promote their polymerization and stabilize the MT network. Furthermore, FKBP25 associates with the mitotic spindle and regulates entry into mitosis. This interaction is important for mitotic spindle dynamics, as we observe increased chromosome instability in FKBP25 knockdown cells. Finally, we provide evidence that FKBP25 association with chromatin is cell-cycle regulated by Protein Kinase C phosphorylation. This disrupts FKBP25-DNA contacts during mitosis while maintaining its interaction with the spindle apparatus. Collectively, these data support a model where FKBP25 association with chromatin and MTs is carefully choreographed to ensure faithful genome duplication. Additionally, they highlight that FKBP25 is a MT-associated FK506 receptor and potential therapeutic target in MT-associated diseases.

Tubulin Post-Translational Modifications and Microtubule Dynamics

Microtubules are hollow tube-like polymeric structures composed of α,β-tubulin heterodimers. They play an important role in numerous cellular processes, including intracellular transport, cell motility and segregation of the chromosomes during cell division. Moreover, microtubule doublets or triplets form a scaffold of a cilium, centriole and basal body, respectively. To perform such diverse functions microtubules have to differ in their properties. Post-translational modifications are one of the factors that affect the properties of the tubulin polymer. Here we focus on the direct and indirect effects of post-translational modifications of tubulin on microtubule dynamics.

The Effect of SIN1 and Microtubules on Insulin Induced PKC ζ Activation

Background

Protein kinase C zeta (PKC ζ) plays an important role in insulin induced glycometabolism and insulin receptor (IR) associated signaling pathways. The full activation of PKC ζ depends on its translocation from cytosol to membrane and phosphorylation at Thr410. However, the mechanism of PKC ζ activation remains elusive. In this study, the effect of SIN1 and microtubules on insulin-induced PKC ζ activation was investigated.

Material/Methods

HepG2 cells were stimulated with insulin for co-immunoprecipitation (co-IP) assay. The immunocomplex was captured by using anti-PKC ζ, anti-SIN1 or anti-FLAG antibodies and was subjected to western blotting analysis for detecting PKC ζ, SIN1, and β-tubulin protein expression level. The cells were intervened by small interfering RNA (siRNA) that targeted exon regions of SIN1. Then the glucose uptake ratio after cells were stimulated by insulin was measured. The PKC ζ insulin receptor levels in the membranes were analyzed. Cells stained with anti-PKC ζ, anti-SIN1 antibodies and probed with molecular probes were observed by immunofluorescence confocal microscopy.

Results

SIN1 interacted and co-located with PKC ζ by pleckstrin homology (PH) domain. Downregulation of SIN1 severely impaired PKC ζ translocation and phosphorylation induced by insulin. PKC ζ co-immunoprecipitated with β-tubulin at different intervals upon insulin stimulus, and the activation of PKC ζ was affected by paclitaxel and nocodazole.

Conclusions

PKC ζ translocated from cytosol to membrane depending on SIN1, which suggested that PKC ζ may be activated directly by PI3K and the reaction probably carried out on microtubules in HepG2 cells.

Role of LPS-elicited signaling in triggering gastric mucosal inflammatory responses to H. pylori: modulatory effect of ghrelin

Infection with Helicobacter pylori is a primary culprit in the etiology of gastric disease, and its cell-wall lipopolysaccharide (LPS) is recognized as a potent endotoxin responsible for triggering a pattern of the mucosal inflammatory responses. The engagement by the LPS of gastric mucosal Toll-like receptor 4 (TLR4) leads to initiation of signal transduction events characterized by the activation of mitogen-activated protein kinase (MAPK) cascade, induction of phosphoinositide-specific phospholipase C (PLC)/protein kinase C (PKC)/phosphatidylinositol 3-kinase (PI3K) pathway, and up-regulation in Src/Akt. These signaling events in turn exert their influence over H. pylori-elicited excessive generation of NO and PGE2 caused by the disturbances in nitric oxide synthase and cyclooxygenase isozyme systems, increase in epidermal growth factor receptor transactivation, and the induction in matrix metalloproteinase-9 (MMP-9) release. Interestingly, the extent of gastric mucosal inflammatory response to H. pylori is influenced by a peptide hormone, ghrelin, the action of which relays on the growth hormone secretagogue receptor type 1a (GHS-R1a)-mediated mobilization of G-protein dependent transduction pathways. Yet, the signals triggered by TLR-4 activation as well as those arising through GHS-R1a stimulation converge at MAPK and PLC/PKC/PI3K pathways that form a key integration node for proinflammatory signals generated by H. pylori LPS as well as for those involved in modulation of inflammation by ghrelin. Hence, therapeutic targeting these signals' convergence and integration node could provide a novel and attractive opportunities for developing more effective treatments of H. pylori-related gastric disease.

Post-translational modifications of Annexin A2 are linked to its association with perinuclear nonpolysomal mRNP complexes

Various post‐translational modifications (PTMs) regulate the localisation and function of the multifunctional protein Annexin A2 (AnxA2). In addition to its various tasks as a cytoskeletal‐ and membrane‐associated protein, AnxA2 can function as a trans‐acting protein binding to cis‐acting sequences of specific mRNAs. In the present study, we have examined the role of Ser25 phosphorylation in subcellular localisation of AnxA2 and its interaction with mRNP complexes. Subcellular fractionation and confocal microscopy of rat neuroendocrine PC12 cells showed that Ser25‐phosphorylated AnxA2 (pSer25AnxA2) is absent from the nucleus and mainly localised to the perinuclear region, evidently associating with both membranes and cytoskeletal elements. Perinuclear targeting of AnxA2 was abolished by inhibition of protein kinase C activity, which resulted in cortical enrichment of the protein. Although oligo(dT)‐affinity purification of mRNAs revealed that pSer25AnxA2 associates with nonpolysomal, translationally inactive mRNP complexes, it displayed only partial overlap with a marker of P‐bodies. The phosphorylated protein is present as high‐molecular‐mass forms, indicating that it contains additional covalent PTMs, apparently triggered by its Ser25 phosphorylation. The subcellular distributions of these forms clearly differ from the main form of AnxA2 and are also distinct from that of Tyr23‐phosphorylated AnxA2. Immunoprecipitation verified that these high‐molecular‐mass forms are due to ubiquitination and/or sumoylation. Moreover, these results indicate that Ser25 phosphorylation and ubiquitin/SUMO1 conjugation of AnxA2 promote its association with nonpolysomal mRNAs, providing evidence of a possible mechanism to sequester a subpopulation of mRNAs in a translationally inactive and transport competent form at a distinct subcellular localisation.

Helicobacter pylori-induced changes in microtubule dynamics conferred by α-tubulin phosphorylation on Ser/Tyr mediate gastric mucosal secretion of matrix metalloproteinase-9 (MMP-9) and its modulation by ghrelin

Regulation of matrix metalloproteinase-9 (MMP-9) secretion in response to proinflammatory challenge remains under a strict control of factors that affect the stability dynamics of the major cytoskeleton polymeric structures, microtubules (MTs). In this study, we report that H. pylori LPS-elicited induction gastric mucosal MMP-9 secretion is accompanied by the enhancement in MT stabilization as evidenced by the increase in α-tubulin acetylation and detyrosination while the modulatory influence of hormone, ghrelin, is associated with MT destabilization and reflected in a decrease in α-tubulin acetylation and detyrosination. Further, we reveal that the LPS-induced enhancement in MT stabilization and up-regulation in MMP-9 secretion as well as the modulatory influence of ghrelin occur with the involvement of PKCδ and SFK. The LPS effect is reflected in a marked increase in PKCδ-mediated α-tubulin phosphorylation on Ser, while the modulatory effect of ghrelin on MT dynamics and MMP-9 secretion is manifested by the SFK-dependent phosphorylation of α-tubulin on Tyr. Moreover, the changes in α-tubulin phosphorylation and MT stabilization dynamics occur in concert with the Golgi recruitment and activation of PKD2 and Arf-GEF. The findings demonstrate that the enhancement in gastric mucosal MMP-9 secretion in response to H. pylori and its modulation by ghrelin are the result of changes in MT dynamics conferred by PKCδ/SFK- mediated α-tubulin Ser/Tyr phosphorylation.

Phosphorylation of Cdc42 effector protein-4 (CEP4) by protein kinase C promotes motility of human breast cells

Cdc42 effector protein-4 (CEP4) was recently identified by our laboratory to be a substrate of multiple PKC isoforms in non-transformed MCF-10A human breast cells. The significance of phosphorylated CEP4 to PKC-stimulated motility of MCF-10A cells was evaluated. Single site mutants at Ser residues embedded in potential PKC consensus sites (Ser(18), Ser(77), Ser(80), and Ser(86)) were individually replaced with Asp residues to simulate phosphorylation. Following expression in weakly motile MCF-10A cells, the S18D and S80D mutants each promoted increased motility, and the double mutant (S18D/S80D) produced a stronger effect. MS/MS analysis verified that Ser(18) and Ser(80) were directly phosphorylated by PKCα in vitro. Phosphorylation of CEP4 severely diminished its affinity for Cdc42 while promoting Rac activation and formation of filopodia (microspikes). In contrast, the phosphorylation-resistant double mutant S18A/S80A-CEP4 blocked CEP4 phosphorylation and inhibited motility of MCF-10A cells that had been stimulated with PKC activator diacylglycerol lactone. In view of the dissociation of phospho-CEP4 from Cdc42, intracellular binding partners were explored by expressing each CEP4 double mutant from a tandem affinity purification vector followed by affinity chromatography, SDS-PAGE, and identification of protein bands evident only with S18D/S80D-CEP4. One binding partner was identified as tumor endothelial marker-4 (TEM4; ARHGEF17), a guanine nucleotide exchange factor that is involved in migration. In motile cells expressing S18D/S80D-CEP4, knockdown of TEM4 inhibited both Rac activation and motility. These findings support a model in which PKC-mediated phosphorylation of CEP4 at Ser(18) and Ser(80) causes its dissociation from Cdc42, thereby increasing its affinity for TEM4 and producing Rac activation, filopodium formation, and cell motility.