Polycation induced actin bundles

Sweat Proteomics in Cystic Fibrosis: Discovering Companion Biomarkers for Precision Medicine and Therapeutic Development

In clinical routine, the diagnosis of cystic fibrosis (CF) is still challenging regardless of international consensus on diagnosis guidelines and tests. For decades, the classical Gibson and Cooke test measuring sweat chloride concentration has been a keystone, yet, it may provide normal or equivocal results. As of now, despite the combination of sweat testing, CFTR genotyping, and CFTR functional testing, a small fraction (1–2%) of inconclusive diagnoses are reported and justifies the search for new CF biomarkers. More importantly, in the context of precision medicine, with a view to early diagnosis, better prognosis, appropriate clinical follow-up, and new therapeutic development, discovering companion biomarkers of CF severity and phenotypic rescue are of utmost interest. To date, previous sweat proteomic studies have already documented disease-specific variations of sweat proteins (e.g., in schizophrenia and tuberculosis). In the current study, sweat samples from 28 healthy control subjects and 14 patients with CF were analyzed by nanoUHPLC-Q-Orbitrap-based shotgun proteomics, to look for CF-associated changes in sweat protein composition and abundance. A total of 1057 proteins were identified and quantified at an individual level, by a shotgun label-free approach. Notwithstanding similar proteome composition, enrichment, and functional annotations, control and CF samples featured distinct quantitative proteome profiles significantly correlated with CF, accounting for the respective inter-individual variabilities of control and CF sweat. All in all: (i) 402 sweat proteins were differentially abundant between controls and patients with CF, (ii) 68 proteins varied in abundance between F508del homozygous patients and patients with another genotype, (iii) 71 proteins were differentially abundant according to the pancreatic function, and iv) 54 proteins changed in abundance depending on the lung function. The functional annotation of pathophysiological biomarkers highlighted eccrine gland cell perturbations in: (i) protein biosynthesis and trafficking, (ii) CFTR proteostasis and membrane stability, and (iii) cell-cell adherence, membrane integrity, and cytoskeleton crosstalk. Cytoskeleton-related biomarkers were of utmost interest because of the consistency between variations observed here in CF sweat and variations previously documented in other CF tissues. From a clinical stance, nine candidate biomarkers of CF diagnosis (CUTA, ARG1, EZR, AGA, FLNA, MAN1A1, MIA3, LFNG, SIAE) and seven candidate biomarkers of CF severity (ARG1, GPT, MDH2, EML4 (F508del homozygous), MGAT1 (pancreatic insufficiency), IGJ, TOLLIP (lung function impairment)) were deemed suitable for further verification.

Polycation-Mediated Transfection: Mechanisms of Internalization and Intracellular Trafficking

Polyplex-mediated gene transfection is now in its' fourth decade of serious research, but the promise of polyplex-mediated gene therapy has yet to fully materialize. Only approximately one in a million applied plasmids actually expresses. A large part of this is due to an incomplete understanding of the mechanism of polyplex transfection. There is an assumption that internalization must follow a canonical mechanism of receptor mediated endocytosis. Herein, we present arguments that untargeted (and most targeted) polyplexes do not utilize these routes. By incorporating knowledge of syndecan-polyplex interactions, we can show that syndecans are the "target" for polyplexes. Further, it is known that free polycations (which disrupt cell-membranes by acid-catalyzed hydrolysis of phospholipid esters) are necessary for (untargeted) endocytosis. This can be incorporated into the model to produce a novel mechanism of endocytosis, which fits the observed phenomenology. After membrane translocation, polyplex containing vesicles reach the endosome after diffusing through the actin mesh below the cell membrane. From there, they are acidified and trafficked toward the lysosome. Some polyplexes are capable of escaping the endosome and unpacking, while others are not. Herein, it is argued that for some polycations, as acidification proceeds the polyplexes excluding free polycations, which disrupt the endosomal membrane by acid-catalyzed hydrolysis, allowing the polyplex to escape. The polyplex's internal charge ratio is now insufficient for stability and it releases plasmids which diffuse to the nucleus. A small proportion of these plasmids diffuse through the nuclear pore complex (NPC), with aggregation being the major cause of loss. Those plasmids that have diffused through the NPC will also aggregate, and this appears to be the reason such a small proportion of nuclear plasmids express mRNA. Thus, the structural features which promote unpacking in the endosome and allow for endosomal escape can be determined, and better polycations can be designed.

Lysozyme-induced suppression of enzymatic and motile activities of actin-myosin: Impact of basic proteins

Electrostatic interactions between actin filaments and myosin molecules, which are ubiquitous proteins in eukaryotes, are crucial for their enzymatic activity and motility. Nonspecific electrostatic interactions between proteins are unavoidable in cells; therefore, it is worth exploring how ambient proteins, such as polyelectrolytes, affect actin-myosin functions. To understand the effect of counterionic proteins on actin-myosin, we examined ATPase activity and sliding velocity via actin-myosin interactions in the presence of the basic model protein hen egg lysozyme. In an in vitro motility assay with ATP, the sliding velocity of actin filaments on heavy meromyosin (HMM) decreased with increasing lysozyme concentrations. Actin filaments were completely stalled at a lysozyme concentration above 0.08 mg/mL. Lysozyme decreased the ATP hydrolysis rate of the actin-HMM complex but not that HMM alone. Co-sedimentation assays revealed that lysozyme enhanced the binding of HMM to actin filaments in the presence of ATP. Additionally, lysozyme could bind to actin and myosin filaments. The inhibitory effect of poly-l-lysine, histone mixture, and lactoferrin on the motility of actin-myosin was higher than that of lysozyme. Thus, nonspecific electrostatic interactions of basic proteins are involved in the bundling of actin filaments and modulation of essential functions of the actomyosin complex.

Gelator length precisely tunes supramolecular hydrogel stiffness and neuronal phenotype in 3D culture

The brain is one of the softest tissues in the body with storage moduli (G') that range from hundreds to thousands of pascals (Pa) depending upon the anatomic region. Furthermore, pathological processes such as injury, aging and disease can cause subtle changes in the mechanical properties throughout the central nervous system. However, these changes in mechanical properties lie within an extremely narrow range of moduli and there is great interest in understanding their effect on neuron biology. We report here the design of supramolecular hydrogels based on anionic peptide amphiphile nanofibers using oligo-L-lysines of different molecular lengths to precisely tune gel stiffness over the range of interest and found that G' increases by 10.5 Pa for each additional lysine monomer in the oligo-L-lysine chain. We found that small changes in storage modulus on the order of 70 Pa significantly affect survival, neurite growth and tyrosine hydroxylase-positive population in dopaminergic neurons derived from induced pluripotent stem cells. The work reported here offers a strategy to tune mechanical stiffness of hydrogels for use in 3D neuronal cell cultures and transplantation matrices for neural regeneration.

Electromechanics of polarized cell growth

One of the most challenging questions in cell and developmental biology is how molecular signals are translated into mechanical forces that ultimately drive cell growth and motility. Despite an impressive body of literature demonstrating the importance of cytoskeletal and motor proteins as well as osmotic stresses for cell developmental mechanics, a host of dissenting evidence strongly suggests that these factors per se cannot explain growth mechanics even at the level of a single tip-growing cell. The present study addresses this issue by exploring fundamental interrelations between electrical and mechanical fields operating in cells. In the first instance, we employ a simplified but instructive model of a quiescent cell to demonstrate that even in a quasi-equilibrium state, ion transport processes are conditioned principally by mechanical tenets. Then we inquire into the electromechanical conjugacy in growing pollen tubes as biologically relevant and physically tractable developmental systems owing to their extensively characterized growth-associated ionic fluxes and strikingly polarized growth and morphology. A comprehensive analysis of the multifold stress pattern in the growing apices of pollen tubes suggests that tip-focused ionic fluxes passing through the polyelectrolyte-rich apical cytoplasm give rise to electrokinetic flows that actualize otherwise isotropic intracellular turgor into anisotropic stress field. The stress anisotropy can be then imparted from the apical cytoplasm to the abutting frontal cell wall to induce its local extension and directional cell growth. Converging lines of evidence explored in the concluding sections attest that tip-focused ionic fluxes and associated interfacial transport phenomena are not specific for pollen tubes but are also employed by a vast variety of algal, plant, fungal and animal cells, rendering their cytoplasmic stress fields essentially anisotropic and ultimately instrumental in cell shaping, growth and motility.

Histones bundle F-actin filaments and affect actin structure

Histones are small polycationic proteins complexed with DNA located in the cell nucleus. Upon apoptosis they are secreted from the cells and react with extracellular polyanionic compounds. Actin which is a polyanionic protein, is also secreted from necrotic cells and interacts with histones. We showed that both histone mixture (histone type III) and the recombinant H2A histone bundles F-actin, increases the viscosity of the F-actin containing solution and polymerizes G-actin. The histone-actin bundles are relatively insensitive to increase of ionic strength, unlike other polycation, histatin, lysozyme, spermine and LL-37 induced F-actin bundles. The histone-actin bundles dissociate completely only in the presence of 300–400 mM NaCl. DNA, which competes with F-actin for histones, disassembles histone induced actin bundles. DNase1, which depolymerizes F- to G-actin, actively unbundles the H2A histone induced but slightly affects the histone mixture induced actin bundles. Cofilin decreases the amount of F-actin sedimented by low speed centrifugation, increases light scattering and viscosity of F-actin-histone mixture containing solutions and forms star like superstructures by copolymerizing G-actin with H2A histone. The results indicate that histones are tightly attached to F-actin by strong electrostatic and hydrophobic forces. Since both histones and F-actin are present in the sputum of patients with cystic fibrosis, therefore, the formation of the stable histone-actin bundles can contribute to the pathology of this disease by increasing the viscosity of the sputum. The actin-histone interaction in the nucleus might affect gene expression.

Interactions of histatin-3 and histatin-5 with actin

Background

Histatins are histidine rich polypeptides produced in the parotid and submandibular gland and secreted into the saliva. Histatin-3 and −5 are the most important polycationic histatins. They possess antimicrobial activity against fungi such as Candida albicans. Histatin-5 has a higher antifungal activity than histatin-3 while histatin-3 is mostly involved in wound healing in the oral cavity. We found that these histatins, like other polycationic peptides and proteins, such as LL-37, lysozyme and histones, interact with extracellular actin.

Results

Histatin-3 and −5 polymerize globular actin (G-actin) to filamentous actin (F-actin) and bundle F-actin filaments. Both actin polymerization and bundling by histatins is pH sensitive due to the high histidine content of histatins. In spite of the equal number of net positive charges and histidine residues in histatin-3 and −5, less histatin-3 is needed than histatin-5 for polymerization and bundling of actin. The efficiency of actin polymerization and bundling by histatins greatly increases with decreasing pH. Histatin-3 and −5 induced actin bundles are dissociated by 100 and 50 mM NaCl, respectively. The relatively low NaCl concentration required to dissociate histatin-induced bundles implies that the actin-histatin filaments bind to each other mainly by electrostatic forces. The binding of histatin-3 to F-actin is stronger than that of histatin-5 showing that hydrophobic forces have also some role in histatin-3- actin interaction. Histatins affect the fluorescence of probes attached to the D-loop of G-actin indicating histatin induced changes in actin structure. Transglutaminase cross-links histatins to actin. Competition and limited proteolysis experiments indicate that the main histatin cross-linking site on actin is glutamine-49 on the D-loop of actin.

Conclusions

Both histatin-3 and −5 interacts with actin, however, histatin 3 binds stronger to actin and affects actin structure at lower concentration than histatin-5 due to the extra 8 amino acid sequence at the C-terminus of histatin-3. Extracellular actin might regulate histatin activity in the oral cavity, which should be the subject of further investigation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-017-0078-0) contains supplementary material, which is available to authorized users.

Actin and DNA Protect Histones from Degradation by Bacterial Proteases but Inhibit Their Antimicrobial Activity

Histones are small polycationic proteins located in the cell nucleus. Together, DNA and histones are integral constituents of the nucleosomes. Upon apoptosis, necrosis, and infection – induced cell death, histones are released from the cell. The extracellular histones have strong antimicrobial activity but are also cytotoxic and thought as mediators of cell death in sepsis. The antimicrobial activity of the cationic extracellular histones is inhibited by the polyanionic DNA and F-actin, which also become extracellular upon cell death. DNA and F-actin protect histones from degradation by the proteases of Pseudomonas aeruginosa and Porphyromonas gingivalis. However, though the integrity of the histones is protected, the activity of histones as antibacterial agents is lost. The inhibition of the histone’s antibacterial activity and their protection from proteolysis by DNA and F-actin indicate a tight electrostatic interaction between the positively charged histones and negatively charged DNA and F-actin, which may have physiological significance in maintaining the equilibrium between the beneficial antimicrobial activity of extracellular histones and their cytotoxic effects.

Tropomyosin as a Regulator of Actin Dynamics

Tropomyosin is a major regulatory protein of contractile systems and cytoskeleton, an actin-binding protein that positions laterally along actin filaments and modulates actin-myosin interaction. About 40 tropomyosin isoforms have been found in a variety of cytoskeleton systems, not necessarily connected with actin-myosin interaction and contraction. Involvement of specific tropomyosin isoforms in the regulation of key cell processes was shown, and specific features of tropomyosin genes and protein structure have been investigated with molecular biology and genetics approaches. However, the mechanisms underlying the effects of tropomyosin on cytoskeleton dynamics are still unclear. As tropomyosin is primarily an F-actin-binding protein, it is important to understand how it interacts both with actin and actin-binding proteins functioning in muscles and cytoskeleton to regulate actin dynamics. This review focuses on biochemical data on the effects of tropomyosin on actin assembly and dynamics, as well as on the modulation of these effects by actin-binding proteins. The data indicate that tropomyosin can efficiently regulate actin dynamics via allosteric conformational changes within actin filaments.

Interaction of the core fragments of the LL-37 host defense peptide with actin

Host defense peptides are effector molecules of the innate immunity that possess antimicrobial and health-promoting properties. Due to their potential therapeutic activities, host defense peptides are being explored as alternatives for antibiotics. The human LL-37 and its shorter, cost-effective, bactericidal core peptide derivates have been suggested for their therapeutic potential. Bacteria evade host defense peptides by proteolytic inactivation. Actin released from necrotized cells and abundant in infected sites was shown to bind and protect LL-37 from microbial proteolytic degradation, and to enable the peptide's antimicrobial action despite the presence of the proteases. Here, we characterized the interactions of the 10-13 residues long LL-37 core peptides with actin. We show that the LL-37 core peptides associate with actin with a lower affinity than that of LL-37. Their association with actin, which is very ionic strength sensitive, is mainly based on electrostatic interactions. Likewise, the antimicrobial activity against Escherichia coli of the minimal antimicrobial peptide KR-12 but not FK-13 nor LL-37 is also very sensitive to salts. In addition, the antimicrobial activity of the FK-13 core peptide is protected by actin against the tested bacterial proteases in a similar manner to that of LL-37, supporting its potential for therapeutic use.

Uncovering protein polyamination by the spermine-specific antiserum and mass spectrometric analysis

The polyamines spermidine and spermine, and their precursor putrescine, have been shown to play an important role in cell migration, proliferation, and differentiation. Because of their polycationic property, polyamines are traditionally thought to be involved in DNA replication, gene expression, and protein translation. However, polyamines can also be covalently conjugated to proteins by transglutaminase 2 (TG2). This modification leads to an increase in positive charge in the polyamine-incorporated region which significantly alters the structure of proteins. It is anticipated that protein polyamine conjugation may affect the protein-protein interaction, protein localization, and protein function of the TG2 substrates. In order to investigate the roles of polyamine modification, we synthesized a spermine-conjugated antigen and generated an antiserum against spermine. In vitro TG2-catalyzed spermine incorporation assays were carried out to show that actin, tubulins, heat shock protein 70 and five types of histone proteins were modified with spermine, and modification sites were also identified by liquid chromatography and linear ion trap-orbitrap hybrid mass spectrometry. Subsequent mass spectrometry-based shotgun proteomic analysis also identified 254 polyaminated sites in 233 proteins from the HeLa cell lysate catalyzed by human TG2 with spermine, thus allowing, for the first time, a global appraisal of site-specific protein polyamination. Global analysis of mouse tissues showed that this modification really exists in vivo. Importantly, we have demonstrated that there is a new histone modification, polyamination, in cells. However, the functional significance of histone polyamination demands further investigations.

Actin enables the antimicrobial action of LL-37 peptide in the presence of microbial proteases

Host defense peptides play an important host-protective role by their microcidal action, immunomodulatory functions, and tissue repair activities. Proteolysis is a common strategy of pathogens used to neutralize host defense peptides. Here, we show that actin, the most abundant structural protein in eukaryotes, binds the LL-37 host defense peptide, protects it from degradation by the proteases of Pseudomonas aeruginosa and Porphyromonas gingivalis, and enables its antimicrobial activity despite the presence of the proteases. Co-localization of LL-37 with extracellular actin was observed in necrotized regions of samples from oral lesions. Competition assays, cross-linking experiments, limited proteolysis, and mass spectrometry revealed that LL-37 binds by specific hydrophobic interactions to the His-40-Lys-50 segment of actin, located in the DNase I binding loop. The integrity of the binding site of both LL-37 and actin is a prerequisite to the binding. Our results demonstrate that actin, presumably released by dead cells and abundant in infected sites, might be utilized by the immune system to enhance spatio-temporal immunity in an attempt to arrest infection and control inflammation.

Synthetic polyamine BPA-C8 inhibits TGF-β1-mediated conversion of human dermal fibroblast to myofibroblasts and establishment of galectin-1-rich extracellular matrix in vitro

Cancer-associated fibroblasts (CAFs) play a role in the progression of malignant tumors. They are formed by conversion of fibroblasts to smooth muscle α-actin-positive (SMA-positive) myofibroblasts. Polyamines are known to change the arrangement of the actin cytoskeleton by binding to the anionic actin. We tested the effect of the synthetic polyamine BPA-C8 on the transition of human dermal fibroblasts to myofibroblasts induced either by TGF-β1 alone or by TGF-β1 together with adhesion/growth-regulatory galectin-1. Pre-existing CAFs, myofibroblasts from pancreatitis, and rat smooth muscle cells were also exposed to BPA-C8. BPA-C8 impaired myofibroblast formation from activated fibroblasts, but it had no effect on cells already expressing SMA. BPA-C8 also reduced the occurrence of an extracellular matrix around the activated fibroblasts. The reported data thus extend current insights into polyamine activity, adding interference with tumor progression to the tumor-promoting processes warranting study.

Polyelectrolyte properties of filamentous biopolymers and their consequences in biological fluids

Anionic polyelectrolyte filaments are common in biological cells. DNA, RNA, the cytoskeletal filaments F-actin, microtubules, and intermediate filaments, and polysaccharides such as hyaluronan that form the pericellular matrix all have large net negative charge densities distributed over their surfaces. Several filamentous viruses with diameters and stiffnesses similar to those of cytoskeletal polymers also have similar negative charge densities. Extracellular protein filaments such collagen, fibrin and elastin, in contrast, have notably smaller charge densities and do not behave as highly charged polyelectrolytes in solution. This review summarizes data that demonstrate generic counterion-mediated effects on four structurally unrelated biopolymers of similar charge density: F-actin, vimentin, Pf1 virus, and DNA, and explores the possible biological and pathophysiological consequences of the polyelectrolyte properties of biological filaments.

Disulfide cross-linked antiparallel actin dimer

Oxidation of actin monomer (G-actin) with copper o-phenanthroline resulted in a rapid, high yield of disulfide cross-linked dimer. The cross-link is due to an intermolecular disulfide bond between actin Cys374 of each molecule, resulting in a tail-to-tail, i.e., antiparallel, actin dimer. Analytical ultracentrifugation profiles of G-actin can be ascribed to the existence of actin monomers with very little, if any, dimer. Thus, actin dimers are not energetically favorable, indicating that cross-linked dimers are formed during random diffusional collisions. On the other hand, a similar oxidation of actin polymer (F-actin) resulted in a much lower yield of the cross-linked actin dimer that showed no sign of leveling off. Therefore, it is proposed that the cross-linked dimer from actin polymer is due to collisional complexes of actin monomers that are in equilibrium with the polymer during actin treadmilling. These results account for the reported observation that during the early stages of actin polymerization (where the actin monomer concentration is high) cross-linked antiparallel actin dimers are formed in relatively high yield whereas none are formed at later stages of polymerization. These findings raise questions concerning the validity of the antiparallel actin dimer model of in vitro actin polymerization that is based on the assumption that the ability to form cross-linked actin dimers implies the existence of stable dimers.

LL-37 induces polymerization and bundling of actin and affects actin structure

Actin exists as a monomer (G-actin) which can be polymerized to filaments) F-actin) that under the influence of actin-binding proteins and polycations bundle and contribute to the formation of the cytoskeleton. Bundled actin from lysed cells increases the viscosity of sputum in lungs of cystic fibrosis patients. The human host defense peptide LL-37 was previously shown to induce actin bundling and was thus hypothesized to contribute to the pathogenicity of this disease. In this work, interactions between actin and the cationic LL-37 were studied by optical, proteolytic and surface plasmon resonance methods and compared to those obtained with scrambled LL-37 and with the cationic protein lysozyme. We show that LL-37 binds strongly to CaATP-G-actin while scrambled LL-37 does not. While LL-37, at superstoichiometric LL-37/actin concentrations polymerizes MgATP-G-actin, at lower non-polymerizing concentrations LL-37 inhibits actin polymerization by MgCl₂ or NaCl. LL-37 bundles Mg-F-actin filaments both at low and physiological ionic strength when in equimolar or higher concentrations than those of actin. The LL-37 induced bundles are significantly less sensitive to increase in ionic strength than those induced by scrambled LL-37 and lysozyme. LL-37 in concentrations lower than those needed for actin polymerization or bundling, accelerates cleavage of both monomer and polymer actin by subtilisin. Our results indicate that the LL-37-actin interaction is partially electrostatic and partially hydrophobic and that a specific actin binding sequence in the peptide is responsible for the hydrophobic interaction. LL-37-induced bundles, which may contribute to the accumulation of sputum in cystic fibrosis, are dissociated very efficiently by DNase-1 and also by cofilin.

Actin bundling and polymerisation properties of eukaryotic elongation factor 1 alpha (eEF1A), histone H2A-H2B and lysozyme in vitro

Elongation factor 1 alpha (eEF1A) is a positively charged protein which has been shown to interact with the actin cytoskeleton. However, to date, a specific actin binding site within the eEF1A sequence has not been identified and the mechanism by which eEF1A interacts with actin remains unresolved. Many protein-protein interactions occur as a consequence of their physicochemical properties and actin bundle formation has been shown to result from non-specific electrostatic interaction with basic proteins. This study investigated interactions between actin, eEF1A and two other positively charged proteins which are not regarded as classic actin binding proteins (namely lysozyme and H2A-H2B) in order to compare their actin organising effects in vitro. For the first time using atomic force microscopy (AFM) we have been able to image the interaction of eEF1A with actin and the subsequent bundling of actin in vitro. Interestingly, we found that eEF1A dramatically increases the rate of polymerisation (45-fold above control levels). We also show for the first time that H2A-H2B has remarkably similar effects upon actin bundling (relative bundle size/number) and polymerisation (35-fold increase above control levels) as eEF1a. The presence of lysozyme resulted in bundles which were distinct from those formed due to eEF1A and H2A-H2B. Lysozyme also increased the rate of actin polymerisation above the control level (by 10-fold). Given the striking similarities between the actin bundling and polymerisation properties of eEF1A and H2A-H2B, our results hint that dimerisation and electrostatic binding may provide clues to the mechanism through which eEF1A-actin bundling occurs.