Peptide nanocarriers co-delivering an antisense oligonucleotide and photosensitizer elicit synergistic cytotoxicity

Combination therapies demand co-delivery platforms with efficient entrapment of distinct payloads and specific delivery to cells and possibly organelles. Herein, we introduce the combination of two therapeutic modalities, gene and photodynamic therapy, in a purely peptidic platform. The simultaneous formation and cargo loading of the multi-micellar platform is governed by self-assembly at the nanoscale. The multi-micellar architecture of the nanocarrier and the positive charge of its constituent micelles offer controlled dual loading capacity with distinct locations for a hydrophobic photosensitizer (PS) and negatively charged antisense oligonucleotides (ASOs). Moreover, the nuclear localization signal (NLS) sequence built-in the peptide targets PS + ASO-loaded nanocarriers to the nucleus. Breast cancer cells treated with nanocarriers demonstrated photo-triggered enhancement of radical oxygen species (ROS) associated with increased cell death. Besides, delivery of ASO payloads resulted in up to 90 % knockdown of Bcl-2, an inhibitor of apoptosis that is overexpressed in more than half of all human cancers. Simultaneous delivery of PS and ASO elicited synergistic apoptosis to an extent that could not be reached by singly loaded nanocarriers or the free form of the drugs. Both, the distinct location of loaded compounds that prevents them from interfering with each other, and the highly efficient cellular delivery support the great potential of this versatile peptide platform in combination therapy.

Advancing the Design of Artificial Nano-organelles for Targeted Cellular Detoxification of Reactive Oxygen Species

Artificial organelles (AnOs) are in the spotlight as systems to supplement biochemical pathways in cells. While polymersome-based artificial organelles containing enzymes to reduce reactive oxygen species (ROS) are known, applications requiring control of their enzymatic activity and cell-targeting to promote intracellular ROS detoxification are underexplored. Here, we introduce advanced AnOs where the chemical composition of the membrane supports the insertion of pore-forming melittin, enabling molecular exchange between the AnO cavity and the environment, while the encapsulated lactoperoxidase (LPO) maintains its catalytic function. We show that H2O2 outside AnOs penetrates through the melittin pores and is rapidly degraded by the encapsulated enzyme. As surface attachment of cell-penetrating peptides facilitates AnOs uptake by cells, electron spin resonance revealed a remarkable enhancement in intracellular ROS detoxification by these cell-targeted AnOs compared to nontargeted AnOs, thereby opening new avenues for a significant reduction of oxidative stress in cells.

FAP Targeting of Photosensitizer-Loaded Polymersomes for Increased Light-Activated Cell Killing

As current chemo- and photodynamic cancer therapies are associated with severe side effects due to a lack of specificity and to systemic toxicity, innovative solutions in terms of targeting and controlled functionality are in high demand. Here, we present the development of a polymersome nanocarrier equipped with targeting molecules and loaded with photosensitizers for efficient uptake and light-activated cell killing. Polymersomes were self-assembled in the presence of photosensitizers from a mixture of nonfunctionalized and functionalized PDMS-b-PMOXA diblock copolymers, the latter designed for coupling with targeting ligands. By encapsulation inside the polymersomes, the photosensitizer Rose Bengal was protected, and its uptake into cells was mediated by the nanocarrier. Inhibitor of fibroblast activation protein α (FAPi), a ligand for FAP, was attached to the polymersomes' surface and improved their uptake in MCF-7 breast cancer cells expressing relatively high levels of FAP on their surface. Once internalized by MCF-7, irradiation of Rose Bengal-loaded FAPi-polymersomes generated reactive oxygen species at levels high enough to induce cell death. By combining photosensitizer encapsulation and specific targeting, polymersomes represent ideal candidates as therapeutic nanocarriers in cancer treatment.

Impact of a brochure and empathetic physician communication on patients' perception of breast biopsies

PURPOSE

This study investigated the effect of an intervention designed to reduce patients' emotional distress associated with breast biopsy.

METHODS

125 breast biopsy patients receiving standard of care (control group, CG) were compared to 125 patients (intervention group, IG) who received a brochure with information prior to the biopsy and were biopsied by physicians trained in empathic communication. Anxiety was assessed by the State-Anxiety Inventory (STAI-S) at four time points (pre- and post-procedural, pre- and post-histology). All participants completed pre- and post-procedural questionnaires addressing worries, pain and comprehension. We evaluated the impact of the intervention on STAI-S levels using a log-transformed linear mixed effects model and explored patients' and physicians' perceptions of the procedure descriptively.

RESULTS

Post-procedural and post-histology timepoints were associated with 13% and17% lower with STAI-S levels than at the pre-procedural timepoint on average. The histologic result had the strongest association with STAI-S: malignancy was associated with 28% higher STAI-S scores than a benign finding on average. Across all time points, the intervention did not affect patient anxiety. Nevertheless, IG participants perceived less pain during the biopsy. Nearly all patients agreed that the brochure should be handed out prior to breast biopsy.

CONCLUSION

While the distribution of an informative brochure and a physician trained in empathic communication did not reduce patient anxiety overall, we observed lower levels of worry and perceived pain regarding breast biopsy in the intervention group. The intervention seemed to improve patient's understanding of the procedure. Moreover, professional training could increase physicians' empathic communication skills.

TRIAL REGISTRATION NUMBER

NCT02796612 (March 19, 2014).

A DNA-Micropatterned Surface for Propagating Biomolecular Signals by Positional on-off Assembly of Catalytic Nanocompartments

Signal transduction is pivotal for the transfer of information between and within living cells. The composition and spatial organization of specified compartments are key to propagating soluble signals. Here, a high-throughput platform mimicking multistep signal transduction which is based on a geometrically defined array of immobilized catalytic nanocompartments (CNCs) that consist of distinct polymeric nanoassemblies encapsulating enzymes and DNA or enzymes alone is presented. The dual role of single entities or tandem CNCs in providing confined but communicating spaces for complex metabolic reactions and in protecting encapsulated compounds from denaturation is explored. To support a controlled spatial organization of CNCs, CNCs are patterned by means of DNA hybridization to a microprinted glass surface. Specifically, CNC-functionalized DNA microarrays are produced where individual reaction compartments are kept in close proximity by a distinct geometrical arrangement to promote effective communication. Besides a remarkable versatility and robustness, the most prominent feature of this platform is the reversibility of DNA-mediated CNC-anchoring which renders it reusable. Micropatterns of polymer-based nanocompartment assemblies offer an ideal scaffold for the development of the next generation responsive and communicative soft-matter analytical devices for applications in catalysis and medicine.

Inverting glucuronidation of hymecromone in situ by catalytic nanocompartments

Glucuronidation is a metabolic pathway that inactivates many drugs including hymecromone. Adverse effects of glucuronide metabolites include a reduction of half-life circulation times and rapid elimination from the body. Herein, we developed synthetic catalytic nanocompartments able to cleave the glucuronide moiety from the metabolized form of hymecromone in order to convert it to the active drug. By shielding enzymes from their surroundings, catalytic nanocompartments favor prolonged activity and lower immunogenicity as key aspects to improve the therapeutic solution. The catalytic nanocompartments (CNCs) consist of self-assembled poly(dimethylsiloxane)-block-poly(2-methyl-2-oxazoline) diblock copolymer polymersomes encapsulating β-glucuronidase. Insertion of melittin in the synthetic membrane of these polymersomes provided pores for the diffusion of the hydrophilic hymecromone-glucuronide conjugate to the compartment inside where the encapsulated β-glucuronidase catalyzed its conversion to hymecromone. Our system successfully produced hymecromone from its glucuronide conjugate in both phosphate buffered solution and cell culture medium. CNCs were non-cytotoxic when incubated with HepG2 cells. After being taken up by cells, CNCs produced the drug in situ over 24 hours. Such catalytic platforms, which locally revert a drug metabolite into its active form, open new avenues in the design of therapeutics that aim at prolonging the residence time of a drug.

A self-assembling peptidic platform to boost the cellular uptake and nuclear delivery of oligonucleotides

The design of non-viral vectors that efficiently deliver genetic materials into cells, in particular to the nucleus, remains a major challenge in gene therapy and vaccine development. To tackle the...

Membrane protein channels equipped with a cleavable linker for inducing catalysis inside nanocompartments

Precisely timed initiation of reactions and stability of the catalysts are fundamental in catalysis. We introduce here an efficient closing-opening method for nanocompartments that contain sensitive catalysts and so achieve a controlled and extended catalytic activity. We developed a chemistry-oriented approach for modifying a pore-forming membrane protein which allows for a stimuli-responsive pore opening within the membrane of polymeric nanocompartments. We synthesized a diol-containing linker that selectively binds to the pores, blocking them completely. In the presence of an external stimulus (periodate), the linker is cleaved allowing the diffusion of substrate through the pores to the nanocompartment interior where it sets off the in situ enzymatic reaction. Besides the precise initiation of catalytic activity by opening of the pores, oxidation by periodate guarantees the cleavage of the linker under mild conditions. Accordingly, this kind of responsive nanocompartment lends itself to harboring a large variety of sensitive catalysts such as proteins and enzymes.

Clustering of catalytic nanocompartments for enhancing an extracellular non-native cascade reaction

Compartmentalization is fundamental in nature, where the spatial segregation of biochemical reactions within and between cells ensures optimal conditions for the regulation of cascade reactions. While the distance between compartments or their interaction are essential parameters supporting the efficiency of bio-reactions, so far they have not been exploited to regulate cascade reactions between bioinspired catalytic nanocompartments. Here, we generate individual catalytic nanocompartments (CNCs) by encapsulating within polymersomes or attaching to their surface enzymes involved in a cascade reaction and then, tether the polymersomes together into clusters. By conjugating complementary DNA strands to the polymersomes' surface, DNA hybridization drove the clusterization process of enzyme-loaded polymersomes and controlled the distance between the respective catalytic nanocompartments. Owing to the close proximity of CNCs within clusters and the overall stability of the cluster architecture, the cascade reaction between spatially segregated enzymes was significantly more efficient than when the catalytic nanocompartments were not linked together by DNA duplexes. Additionally, residual DNA single strands that were not engaged in clustering, allowed for an interaction of the clusters with the cell surface as evidenced by A549 cells, where clusters decorating the surface endowed the cells with a non-native enzymatic cascade. The self-organization into clusters of catalytic nanocompartments confining different enzymes of a cascade reaction allows for a distance control of the reaction spaces which opens new avenues for highly efficient applications in domains such as catalysis or nanomedicine.

Artificial Melanogenesis by Confining Melanin/Polydopamine Production inside Polymersomes

Melanin and polydopamine are potent biopolymers for the development of biomedical nanosystems. However, applications of melanin or polydopamine-based nanoparticles are limited by drawbacks related to a compromised colloidal stability over long time periods and associated cytotoxicity. To overcome these hurdles, a novel strategy is proposed that mimics the confinement of natural melanin in melanosomes. Melanosome mimics are developed by co-encapsulating the melanin/polydopamine precursors L-DOPA/dopamine with melanogenic enzyme Tyrosinase within polymersomes. The conditions of polymersome formation are optimized to obtain melanin/polydopamine polymerization within the cavity of the polymersomes. Similar to native melanosomes, polymersomes containing melanin/polydopamine show long-term colloidal stability, cell-compatibility, and potential for cell photoprotection. This novel kind of artificial melanogenesis is expected to inspire new applications of the confined melanin/polydopamine biopolymers.

Peptide-Assisted Nucleic Acid Delivery Systems on the Rise

Concerns associated with nanocarriers' therapeutic efficacy and side effects have led to the development of strategies to advance them into targeted and responsive delivery systems. Owing to their bioactivity and biocompatibility, peptides play a key role in these strategies and, thus, have been extensively studied in nanomedicine. Peptide-based nanocarriers, in particular, have burgeoned with advances in purely peptidic structures and in combinations of peptides, both native and modified, with polymers, lipids, and inorganic nanoparticles. In this review, we summarize advances on peptides promoting gene delivery systems. The efficacy of nucleic acid therapies largely depends on cell internalization and the delivery to subcellular organelles. Hence, the review focuses on nanocarriers where peptides are pivotal in ferrying nucleic acids to their site of action, with a special emphasis on peptides that assist anionic, water-soluble nucleic acids in crossing the membrane barriers they encounter on their way to efficient function. In a second part, we address how peptides advance nanoassembly delivery tools, such that they navigate delivery barriers and release their nucleic acid cargo at specific sites in a controlled fashion.

Agreement in breast lesion assessment and final BI-RADS classification between radial and meander-like breast ultrasound

BACKGROUND

This study prospectively investigates the agreement between radial (r-US) and meander-like (m-US) breast ultrasound with regard to lesion location, lesion size, morphological characteristics and final BI-RADS classification of individual breast lesions.

METHODS

Each patient of a consecutive, unselected, mixed collective received a dual ultrasound examination.

RESULTS

The agreement between r-US and m-US for lesion location ranged from good (lesion to mammilla distance ICC 0.64; lesion to skin distance ICC 0.72) to substantial (clock-face localization κ 0.70). For lesion size the agreement was good (diameter ICC 0.72; volume ICC 0.69), for lesion margin and architectural distortion it was substantial (κ 0.68 and 0.70, respectively). Most importantly, there was a substantial agreement (κ 0.76) in the final BI-RADS classification between r-US and m-US.

CONCLUSIONS

Our recent comparison of radial and meander-like breast US revealed that the diagnostic accuracy of the two scanning methods was comparable. In this study, we observe a high degree of agreement between m-US and r-US for the lesion description (location, size, morphology) and final BI-RADS classification. These findings corroborate that r-US is a suitable alternative to m-US in daily clinical practice. Trial registration NCT02358837. Registered January 2015, retrospectively registered https://clinicaltrials.gov/ct2/results?cond=&term=NCT02358837&cntry=&state=&city=&dist =.

DNA-tethered Polymersome Clusters as Nanotheranostic Platform

Nanotheranostics combine the use of nanomaterials and biologically active compounds to achieve diagnosis and treatment at the same time. To date, severe limitations compromise the use of nanotheranostic systems as potent nanomaterials are often incompatible with potent biomolecules. Herein we emphasize how a novel type of polymersome clusters loaded with active molecules can be optimized to obtain an efficient nanotheranostic platform. Polymersomes loaded with enzymes and specific dyes, respectively and exposing complementary DNA strands at their external surface formed clusters by means of DNA hybridization. We describe factors at the molecular level and other conditions that need to be optimized at each step of the cluster formation to favor theranostic efficiency.

From spherical compartments to polymer films: exploiting vesicle fusion to generate solid supported thin polymer membranes

Solid supported polymer membranes as scaffold for the insertion of functional biomolecules provide the basis for mimicking natural membranes. They also provide the means for unraveling biomolecule-membrane interactions and engineering platforms for biosensing. Vesicle fusion is an established procedure to obtain solid supported lipid bilayers but the more robust polymer vesicles tend to resist fusion and planar membranes rarely form. Here, we build on vesicle fusion to develop a refined and efficient way to produce solid supported membranes based on poly(dimethylsiloxane)-poly(2-methyl-2-oxazoline) (PMOXA-b-PDMS-b-PMOXA) amphiphilic triblock copolymers. We first create thiol-bearing polymer vesicles (polymersomes) and anchor them on a gold substrate. An osmotic shock then provokes polymersome rupture and drives planar film formation. Prerequisite for a uniform amphiphilic planar membrane is the proper combination of immobilized polymersomes and osmotic shock conditions. Thus, we explored the impact of the hydrophobic PDMS block length of the polymersome on the formation and the characteristics of the resulting solid supported polymer assemblies by quarz crystal microbalance with dissipation monitoring (QCM-D), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). When the PDMS block is short enough, attached polymersomes restructure in response to osmotic shock, resulting in a uniform planar membrane. Our approach to rapidly form planar polymer membranes by vesicle fusion brings many advantages to the development of synthetic planar membranes for bio-sensing and biotechnological applications.

Catalytic polymersomes to produce strong and long-lasting bioluminescence

Here, we introduce an artificial bioluminescent nanocompartment based on the encapsulation of light-producing enzymes, luciferases, inside polymersomes. We exploit nanocompartmentalization to enhance luciferase stability in a cellular environment but also to positively modulate enzyme kinetics to achieve a long-lasting glow type signal. These features pave the way for expanding bioluminescence to nanotechnology-based applications.

Immobilization of arrestin-3 on different biosensor platforms for evaluating GPCR binding

G protein-coupled receptors (GPCRs) are a large and ubiquitous family of membrane receptors of great pharmacological interest. Cell-based assays are the primary tool for assessing GPCR interactions and activation but their design and intrinsic complexity limit their application. Biosensor-based assays that directly and specifically report GPCR-protein binding (e.g. arrestin or G protein) could provide a good alternative. We present an approach based on the stable immobilization of different arrestin-3 proteins (wild type, and two mutants, mutant X (arrestin-3 I386A) and mutant Y (arrestin-3 R393E)) via histidine tags on NTA(Ni2+)-coated sensors in a defined orientation. Using biolayer interferometry (BLI), surface plasmon resonance (SPR), and quartz crystal microbalance with dissipation (QCM-D), we were able to follow the interaction between the different arrestin-3 proteins and a representative GPCR, jumping spider rhodopsin-1 (JSR1), in a label-free manner in real-time. The interactions were quantified as binding affinity, association and dissociation rate constants. The combination of surface-based biosensing methods indicated that JSR1 showed the strongest binding to arrestin mutant Y. Taken together, this work introduces direct label-free, biosensor-based screening approaches that can be easily adapted for testing interactions of proteins and other compounds with different GPCRs.

Urotherapist activities in caring for patients with pelvic floor disorders: a prospective single-center observational study

Purpose

Patients with pelvic floor disorders are growing in number. The aim of this study is to outline the main activities of a urotherapist, an advanced nurse practitioner, in the care of patients with pelvic floor disorders and to evaluate patient satisfaction with the service urotherapists provide.

Methods

The prospective single-center observational study was carried out from July 2016 to June 2018. Parameters used to assess the urotherapist activities included the number of consultations, type of counselling, time frame of consultations and therapy and patient satisfaction. In a subgroup of 38 patients, satisfaction with the urotherapy sessions was evaluated by a questionnaire.

Results

Totally, 1709 patients were examined by urogynecologists. Five hundred and fourteen (30%) with chronic pelvic floor disorders were subsequently referred to a urotherapist. Of these patients, 60% were at least 65 years old. The most common pelvic floor disorders (221 patients; 43%) were an overactive bladder, recurrent urinary tract infections, chronic cystitis and pelvic pain syndrome; the second most common pelvic floor disorder was pelvic organ prolapsed (151 patients; 29%). Of the study subgroup comprising 38 patients, 32 (84%) returned the patient satisfaction questionnaire. All 32 patients specified their level of agreement with the urotherapist’s professional competence, empathy, temporal availability and quality of advice as “agree to strongly agree.”

Conclusions

Management by a urotherapist was highly appreciated. The role of the urotherapist as a care coordinator, their level of autonomy and barriers to the implementation in primary care requires further exploration.

Electronic supplementary material

The online version of this article (10.1007/s00404-020-05810-0) contains supplementary material, which is available to authorized users.

Peptide-Based Nanoassemblies in Gene Therapy and Diagnosis: Paving the Way for Clinical Application

Nanotechnology approaches play an important role in developing novel and efficient carriers for biomedical applications. Peptides are particularly appealing to generate such nanocarriers because they can be rationally designed to serve as building blocks for self-assembling nanoscale structures with great potential as therapeutic or diagnostic delivery vehicles. In this review, we describe peptide-based nanoassemblies and highlight features that make them particularly attractive for the delivery of nucleic acids to host cells or improve the specificity and sensitivity of probes in diagnostic imaging. We outline the current state in the design of peptides and peptide-conjugates and the paradigms of their self-assembly into well-defined nanostructures, as well as the co-assembly of nucleic acids to form less structured nanoparticles. Various recent examples of engineered peptides and peptide-conjugates promoting self-assembly and providing the structures with wanted functionalities are presented. The advantages of peptides are not only their biocompatibility and biodegradability, but the possibility of sheer limitless combinations and modifications of amino acid residues to induce the assembly of modular, multiplexed delivery systems. Moreover, functions that nature encoded in peptides, such as their ability to target molecular recognition sites, can be emulated repeatedly in nanoassemblies. Finally, we present recent examples where self-assembled peptide-based assemblies with "smart" activity are used in vivo. Gene delivery and diagnostic imaging in mouse tumor models exemplify the great potential of peptide nanoassemblies for future clinical applications.

The rise of bio-inspired polymer compartments responding to pathology-related signals

Self-organized nano- and microscale polymer compartments such as polymersomes, giant unilamellar vesicles (GUVs), polyion complex vesicles (PICsomes) and layer-by-layer (LbL) capsules have increasing potential in many sensing applications. Besides modifying the physicochemical properties of the corresponding polymer building blocks, the versatility of these compartments can be markedly expanded by biomolecules that endow the nanomaterials with specific molecular and cellular functions. In this review, we focus on polymer-based compartments that preserve their structure, and highlight the key role they play in the field of medical diagnostics: first, the self-assembling abilities that result in preferred architectures are presented for a broad range of polymers. In the following, we describe different strategies for sensing disease-related signals (pH-change, reductive conditions, and presence of ions or biomolecules) by polymer compartments that exhibit stimuli-responsiveness. In particular, we distinguish between the stimulus-sensitivity contributed by the polymer itself or by additional compounds embedded in the compartments in different sensing systems. We then address necessary properties of sensing polymeric compartments, such as the enhancement of their stability and biocompatibility, or the targeting ability, that open up new perspectives for diagnostic applications.

A self-assembling amphiphilic peptide nanoparticle for the efficient entrapment of DNA cargoes up to 100 nucleotides in length

To overcome the low efficiency and cytotoxicity associated with most non-viral DNA delivery systems we developed a purely peptidic self-assembling system that is able to entrap single- and double-stranded DNA of up to 100 nucleotides in length. (HR)3gT peptide design consists of a hydrophilic domain prone to undergo electrostatic interactions with DNA cargo, and a hydrophobic domain at a ratio that promotes the self-assembly into multi-compartment micellar nanoparticles (MCM-NPs). Self-assembled (HR)3gT MCM-NPs range between 100 to 180 nm which is conducive to a rapid and efficient uptake by cells. (HR)3gT MCM-NPs had no adverse effects on HeLa cell viability. In addition, they exhibit long-term structural stability at 4 °C but at 37 °C, the multi-micellar organization disassembles overtime which demonstrates their thermo-responsiveness. The comparison of (HR)3gT to a shorter, less charged H3gT peptide indicates that the additional arginine residues result in the incorporation of longer DNA segments, an improved DNA entrapment efficiency and an increase cellular uptake. Our unique non-viral system for DNA delivery sets the stage for developing amphiphilic peptide nanoparticles as candidates for future systemic gene delivery.

Array based real-time measurement of fluid viscosities and mass-densities to monitor biological filament formation

Liquid mass density and viscosity are fundamental characteristics of fluids. Their quantification by means of classical viscosity and density meters has several drawbacks: (i) the liquid-density and the viscosity cannot be measured simultaneously, (ii) sample volumes in the mL-range are consumed, (iii) the measurements cannot be multiplexed, and, (iv) the quantifications are time-consuming (minutes). Nano-mechanical transducers promise to overcome these limitations. We use fully clamped, gold coated silicon-nitride membranes with a thickness of 200 nm to measure liquid viscosity and density of samples of 1 μL volumes residing above the membrane in a miniature well. Photo-thermal actuation is used to excite the membrane, and an optical deflection system measures the response. From the response spectra, the eigenfrequency (f) and the quality (Q) factor are extracted and used to determine liquid density and viscosity by applying a three-point calibrated, simplified lumped model. We tested the system using calibrated solutions with viscosities in the range of 1-219 mPa s and mass densities between 998 kg m-3 and 1235 kg m-3. Real-time measurements were performed that characterize the polymerization of G-actin to F-actin filaments. The method presented promises to overcome the aforementioned limitations and thereby enables the real-time characterization of sub-μL sample volumes in a multiplexed manner.

Biomolecules Turn Self-Assembling Amphiphilic Block Co-polymer Platforms Into Biomimetic Interfaces

Biological membranes constitute an interface between cells and their surroundings and form distinct compartments within the cell. They also host a variety of biomolecules that carry out vital functions including selective transport, signal transduction and cell-cell communication. Due to the vast complexity and versatility of the different membranes, there is a critical need for simplified and specific model membrane platforms to explore the behaviors of individual biomolecules while preserving their intrinsic function. Information obtained from model membrane platforms should make invaluable contributions to current and emerging technologies in biotechnology, nanotechnology and medicine. Amphiphilic block co-polymers are ideal building blocks to create model membrane platforms with enhanced stability and robustness. They form various supramolecular assemblies, ranging from three-dimensional structures (e.g., micelles, nanoparticles, or vesicles) in aqueous solution to planar polymer membranes on solid supports (e.g., polymer cushioned/tethered membranes,) and membrane-like polymer brushes. Furthermore, polymer micelles and polymersomes can also be immobilized on solid supports to take advantage of a wide range of surface sensitive analytical tools. In this review article, we focus on self-assembled amphiphilic block copolymer platforms that are hosting biomolecules. We present different strategies for harnessing polymer platforms with biomolecules either by integrating proteins or peptides into assemblies or by attaching proteins or DNA to their surface. We will discuss how to obtain synthetic structures on solid supports and their characterization using different surface sensitive analytical tools. Finally, we highlight present and future perspectives of polymer micelles and polymersomes for biomedical applications and those of solid-supported polymer membranes for biosensing.

Length Scale Matters: Real-Time Elastography versus Nanomechanical Profiling by Atomic Force Microscopy for the Diagnosis of Breast Lesions

Real-time elastography (RTE) is a noninvasive imaging modality where tumor-associated changes in tissue architecture are recognized as increased stiffness of the lesion compared to surrounding normal tissue. In contrast to this macroscopic appraisal, quantifying stiffness properties at the subcellular level by atomic force microscopy (AFM) reveals aggressive cancer cells to be soft. We compared RTE and AFM profiling of the same breast lesion to explore the diagnostic potential of tissue elasticity at different length scales. Patients were recruited from women who were scheduled for a biopsy in the outpatient breast clinic of the University Hospital Basel, Switzerland. RTE was performed as part of a standard breast work-up. Individual elastograms were characterized based on the Tsukuba elasticity score. Additionally, lesion elasticity was semiquantitatively assessed by the strain ratio. Core biopsies were obtained for histologic diagnosis and nanomechanical profiling by AFM under near-physiological conditions. Bulk stiffness evaluation by RTE does not always allow for a clear distinction between benign and malignant lesions and may result in the false assessment of breast lesions. AFM on the other hand enables quantitative stiffness measurements at higher spatial, i.e., subcellular, and force resolution. Consequently, lesions that were false positive or false negative by RTE were correctly identified by their nanomechanical AFM profiles as confirmed by histological diagnosis. Nanomechanical measurements can be used as unique markers of benign and cancerous breast lesions by providing relevant information at the molecular level. This is of particular significance considering the heterogeneity of tumors and may improve diagnostic accuracy compared to RTE.

Inter- and Intra-Observer Agreement in Ultrasound BI-RADS Classification and Real-Time Elastography Tsukuba Score Assessment of Breast Lesions

Our aim was to prospectively evaluate inter- and intra-observer agreement between Breast Imaging Reporting and Data System (BI-RADS) classifications and Tsukuba elasticity scores (TSs) of breast lesions. The study included 164 breast lesions (63 malignant, 101 benign). The BI-RADS classification and TS of each breast lesion was assessed by the examiner and twice by three reviewers at an interval of 2 months. Weighted κ values for inter-observer agreement ranged from moderate to substantial for BI-RADS classification (κ = 0.585-0.738) and was substantial for TS (κ = 0.608-0.779). Intra-observer agreement was almost perfect for ultrasound (US) BI-RADS (κ = 0.847-0.872) and TS (κ = 0.879-0.914). Overall, individual reviewers are highly self-consistent (almost perfect intra-observer agreement) with respect to BI-RADS classification and TS, whereas inter-observer agreement was moderate to substantial. Comprehensive training is essential for achieving high agreement and minimizing the impact of subjectivity. Our results indicate that breast US and real-time elastography can achieve high diagnostic performance.

Biopores/membrane proteins in synthetic polymer membranes

BACKGROUND

Mimicking cell membranes by simple models based on the reconstitution of membrane proteins in lipid bilayers represents a straightforward approach to understand biological function of these proteins. This biomimetic strategy has been extended to synthetic membranes that have advantages in terms of chemical and mechanical stability, thus providing more robust hybrid membranes.

SCOPE OF THE REVIEW

We present here how membrane proteins and biopores have been inserted both in the membrane of nanosized and microsized compartments, and in planar membranes under various conditions. Such bio-hybrid membranes have new properties (as for example, permeability to ions/molecules), and functionality depending on the specificity of the inserted biomolecules. Interestingly, membrane proteins can be functionally inserted in synthetic membranes provided these have appropriate properties to overcome the high hydrophobic mismatch between the size of the biomolecule and the membrane thickness.

MAJOR CONCLUSION

Functional insertion of membrane proteins and biopores in synthetic membranes of compartments or in planar membranes is possible by an appropriate selection of the amphiphilic copolymers, and conditions of the self-assembly process. These hybrid membranes have new properties and functionality based on the specificity of the biomolecules and the nature of the synthetic membranes.

GENERAL SIGNIFICANCE

Bio-hybrid membranes represent new solutions for the development of nanoreactors, artificial organelles or active surfaces/membranes that, by further gaining in complexity and functionality, will promote translational applications. This article is part of a Special Issue entitled: Lipid order/lipid defects and lipid-control of protein activity edited by Dirk Schneider.

Actin ADP-ribosylation at Threonine148 by Photorhabdus luminescens toxin TccC3 induces aggregation of intracellular F-actin

Intoxication of eukaryotic cells by Photorhabdus luminescens toxin TccC3 induces cell rounding and detachment from the substratum within a few hours and compromises a number of cell functions like phagocytosis. Here, we used morphological and biochemical procedures to analyse the mechanism of TccC3 intoxication. Life imaging of TccC3-intoxicated HeLa cells transfected with AcGFP-actin shows condensation of F-actin into large aggregates. Life cell total internal reflection fluorescence (TIRF) microscopy of identically treated HeLa cells confirmed the formation of actin aggregates but also disassembly of F-actin stress fibres. Recombinant TccC3 toxin ADP-ribosylates purified skeletal and non-muscle actin at threonine148 leading to a strong propensity to polymerize and F-actin bundle formation as shown by TIRF and electron microscopy. Native gel electrophoresis shows strongly reduced binding of Thr148-ADP-ribosylated actin to the severing proteins gelsolin and its fragments G1 and G1-3, and to ADF/cofilin. Complexation of actin with these proteins inhibits its ADP-ribosylation. TIRF microscopy demonstrates rapid polymerization of Thr148-ADP-ribosylated actin to curled F-actin bundles even in the presence of thymosin β4, gelsolin or G1-3. Thr148-ADP-ribosylated F-actin cannot be depolymerized by gelsolin or G1-3 as verified by TIRF, co-sedimentation and electron microscopy and shows reduced treadmilling as indicated by a lack of stimulation of its ATPase activity after addition of cofilin-1.

Contributions of the lower dimer to supramolecular actin patterning revealed by TIRF microscopy

Two distinct dimers are formed during the initial steps of actin polymerization. The first one, referred to as the 'lower dimer' (LD) was discovered many years ago by means of chemical crosslinking. Owing to its transient nature, a biological relevance had long been precluded when, using LD-specific antibodies, we detected LD-like contacts in actin assemblies that are associated with the endolysosomal compartment in a number of different cell lines. Moreover, immunofluorescence showed the presence of LD-related structures at the cell periphery of migrating fibroblasts, in the nucleus, and in association with the centrosome of interphase cells. Here, we explore contributions of the LD to the assembly of supramolecular actin structures in real time by total internal reflection fluorescence (TIRF) microscopy. Our data shows that while LD on its own cannot polymerize under filament forming conditions, it is able to incorporate into growing F-actin filaments. This incorporation of LD triggers the formation of X-shaped filament assemblies with barbed ends that are pointing in the same direction in the majority of cases. Similarly, an increased frequency of junction sites was observed when filaments were assembled in the presence of oxidized actin. This data suggests that a disulfide bridge between Cys374 residues might stabilize LD-contacts. Based on our findings, we propose two possible models for the molecular mechanism underlying the supramolecular actin patterning in LD-related structures.

FHOD1 is a combined actin filament capping and bundling factor that selectively associates with actin arcs and stress fibers

Formins are actin polymerization factors that are known to nucleate and elongate actin filaments at the barbed end. In the present study we show that human FHOD1 lacks actin nucleation and elongation capacity, but acts as an actin bundling factor with capping activity toward the filament barbed end. Constitutively active FHOD1 associates with actin filaments in filopodia and lamellipodia at the leading edge, where it moves with the actin retrograde flow. At the base of lamellipodia, FHOD1 is enriched in nascent, bundled actin arcs as well as in more mature stress fibers. This function requires actin-binding domains located N-terminally to the canonical FH1-FH2 element. The bundling phenotype is maintained in the presence of tropomyosin, confirmed by electron microscopy showing assembly of 5 to 10 actin filaments into parallel, closely spaced filament bundles. Taken together, our data suggest a model in which FHOD1 stabilizes actin filaments by protecting barbed ends from depolymerization with its dimeric FH2 domain, whereas the region N-terminal to the FH1 domain mediates F-actin bundling by simultaneously binding to the sides of adjacent F-actin filaments.

The nanomechanical signature of breast cancer

Cancer initiation and progression follow complex molecular and structural changes in the extracellular matrix and cellular architecture of living tissue. However, it remains poorly understood how the transformation from health to malignancy alters the mechanical properties of cells within the tumour microenvironment. Here, we show using an indentation-type atomic force microscope (IT-AFM) that unadulterated human breast biopsies display distinct stiffness profiles. Correlative stiffness maps obtained on normal and benign tissues show uniform stiffness profiles that are characterized by a single distinct peak. In contrast, malignant tissues have a broad distribution resulting from tissue heterogeneity, with a prominent low-stiffness peak representative of cancer cells. Similar findings are seen in specific stages of breast cancer in MMTV-PyMT transgenic mice. Further evidence obtained from the lungs of mice with late-stage tumours shows that migration and metastatic spreading is correlated to the low stiffness of hypoxia-associated cancer cells. Overall, nanomechanical profiling by IT-AFM provides quantitative indicators in the clinical diagnostics of breast cancer with translational significance.

Neuronal profilin isoforms are addressed by different signalling pathways

Profilins are prominent regulators of actin dynamics. While most mammalian cells express only one profilin, two isoforms, PFN1 and PFN2a are present in the CNS. To challenge the hypothesis that the expression of two profilin isoforms is linked to the complex shape of neurons and to the activity-dependent structural plasticity, we analysed how PFN1 and PFN2a respond to changes of neuronal activity. Simultaneous labelling of rodent embryonic neurons with isoform-specific monoclonal antibodies revealed both isoforms in the same synapse. Immunoelectron microscopy on brain sections demonstrated both profilins in synapses of the mature rodent cortex, hippocampus and cerebellum. Both isoforms were significantly more abundant in postsynaptic than in presynaptic structures. Immunofluorescence showed PFN2a associated with gephyrin clusters of the postsynaptic active zone in inhibitory synapses of embryonic neurons. When cultures were stimulated in order to change their activity level, active synapses that were identified by the uptake of synaptotagmin antibodies, displayed significantly higher amounts of both isoforms than non-stimulated controls. Specific inhibition of NMDA receptors by the antagonist APV in cultured rat hippocampal neurons resulted in a decrease of PFN2a but left PFN1 unaffected. Stimulation by the brain derived neurotrophic factor (BDNF), on the other hand, led to a significant increase in both synaptic PFN1 and PFN2a. Analogous results were obtained for neuronal nuclei: both isoforms were localized in the same nucleus, and their levels rose significantly in response to KCl stimulation, whereas BDNF caused here a higher increase in PFN1 than in PFN2a. Our results strongly support the notion of an isoform specific role for profilins as regulators of actin dynamics in different signalling pathways, in excitatory as well as in inhibitory synapses. Furthermore, they suggest a functional role for both profilins in neuronal nuclei.

An antiparallel actin dimer is associated with the endocytic pathway in mammalian cells

The dynamic rearrangement of the actin cytoskeleton plays a key role in several cellular processes such as cell motility, endocytosis, RNA processing and chromatin organization. However, the supramolecular actin structures involved in the different processes remain largely unknown. One of the less studied forms of actin is the lower dimer (LD). This unconventional arrangement of two actin molecules in an antiparallel orientation can be detected by chemical crosslinking at the onset of polymerization in vitro. Moreover, evidence for a transient incorporation of LD into growing filaments and its ability to inhibit nucleation of F-actin filament assembly implicate that the LD pathway contributes to supramolecular actin patterning. However, a clear link from this actin species to a specific cellular function has not yet been established. We have developed an antibody that selectively binds to LD configurations in supramolecular actin structures assembled in vitro. This antibody allowed us to unveil the LD in different mammalian cells. In particular, we show an association of the antiparallel actin arrangement with the endocytic compartment at the cellular and ultrastructural level. Taken together, our results strongly support a functional role of LD in the patterning of supramolecular actin assemblies in mammalian cells.

Actin: from structural plasticity to functional diversity

This article addresses the multiple activities of actin. Starting out with the history of actin's discovery, purification and structure, it emphasizes the close relation between structure and function. In this context, we also point to unconventional actin conformations. Their existence in living cells is not yet well documented, however, they seem to play a special role in the supramolecular patterning that underlies some of the physiological functions of actin. Conceivably, such conformations may contribute to actin's diverse activities in the nucleus that are poorly understood so far.

Spatial organization acts on cell signaling: how physical force contributes to the development of cancer

Cells constantly encounter physical forces and respond to neighbors and circulating factors by triggering intracellular signaling cascades that in turn affect their behavior. The mechanisms by which cells transduce mechanical signals to downstream biochemical changes are not well understood. In their work, Salaita and coworkers show that the spatial organization of cell surface receptors is crucial for mechanotransduction. Consequently, force modulation that disrupts the mechanochemical coupling may represent a critical step in cancerogenesis.

Peptide nanoparticles serve as a powerful platform for the immunogenic display of poorly antigenic actin determinants

The role of actin in transcription and RNA processing is now widely accepted but the form of nuclear actin remains enigmatic. Monomeric, oligomeric or polymeric forms of actin seem to be involved in nuclear functions. Moreover, uncommon forms of actin such as the "lower dimer" have been observed in vitro. Antibodies have been pivotal in revealing the presence and distribution of different forms of actin in different cellular locations. Because of its high degree of conservation, actin is a poor immunogen and only few specific actin antibodies are available. To unravel the mystery of less common forms of actin, in particular those in the nucleus, we chose to tailor monoclonal antibodies to recognize distinct forms of actin. To increase the immune response, we used a new approach based on peptide nanoparticles, which are designed to mimic an icosahedral virus capsid and allow the repetitive, ordered display of a specific epitope on their surface. Actin sequences representing the highly conserved "hydrophobic loop," which is buried in the filamentous actin filament, were grafted onto the surface of nanoparticles by genetic engineering. After immunization with "loop nanoparticles," a number of monoclonal antibodies were established that bind to the hydrophobic loop both in vitro and in situ. Immunofluorescence studies on cells revealed that filamentous actin filaments were only labeled once the epitope had been exposed. Our studies indicate that self-assembling peptide nanoparticles represent a versatile platform that can easily be customized to present antigenic determinants in repetitive, ordered arrays and elicit an immune response against poor antigens.

Ca2+ -dependent interaction of S100A1 with F1-ATPase leads to an increased ATP content in cardiomyocytes

S100A1, a Ca(2+)-sensing protein of the EF-hand family that is expressed predominantly in cardiac muscle, plays a pivotal role in cardiac contractility in vitro and in vivo. It has recently been demonstrated that by restoring Ca(2+) homeostasis, S100A1 was able to rescue contractile dysfunction in failing rat hearts. Myocardial contractility is regulated not only by Ca(2+) homeostasis but also by energy metabolism, in particular the production of ATP. Here, we report a novel interaction of S100A1 with mitochondrial F(1)-ATPase, which affects F(1)-ATPase activity and cellular ATP production. In particular, cardiomyocytes that overexpress S100A1 exhibited a higher ATP content than control cells, whereas knockdown of S100A1 expression decreased ATP levels. In pull-down experiments, we identified the alpha- and beta-chain of F(1)-ATPase to interact with S100A1 in a Ca(2+)-dependent manner. The interaction was confirmed by colocalization studies of S100A1 and F(1)-ATPase and the analysis of the S100A1-F(1)-ATPase complex by gel filtration chromatography. The functional impact of this association is highlighted by an S100A1-mediated increase of F(1)-ATPase activity. Consistently, ATP synthase activity is reduced in cardiomyocytes from S100A1 knockout mice. Our data indicate that S100A1 might play a key role in cardiac energy metabolism.

Raver1 is an integral component of muscle contractile elements

Raver1, a ubiquitously expressed protein, was originally identified as a ligand for metavinculin, the muscle-specific isoform of the microfilament-associated protein vinculin. The protein resides primarily in the nucleus, where it colocalises and may interact with polypyrimidine-tract-binding protein, which is involved in alternative splicing processes. During skeletal muscle differentiation, raver1 translocates to the cytoplasm and eventually targets the Z-line of sarcomeres. Here, it colocalises with metavinculin, vinculin and alpha-actinin, all of which have biochemically been identified as raver1 ligands. To obtain more information about the potential role of raver1 in muscle structure and function, we have investigated its distribution and fine localisation in mouse striated and smooth muscle, by using three monoclonal antibodies that recognise epitopes in different regions of the raver1 protein. Our immunofluorescence and immunoelectron-microscopic results indicate that the cytoplasmic accumulation of raver1 is not confined to skeletal muscle but also occurs in heart and smooth muscle. Unlike vinculin and metavinculin, cytoplasmic raver1 is not restricted to costameres but additionally represents an integral part of the sarcomere. In isolated myofibrils and in ultrathin sections of skeletal muscle, raver1 has been found concentrated at the I-Z-I band. A minor fraction of raver1 is present in the nuclei of all three types of muscle. These data indicate that, during muscle differentiation, raver1 might link gene expression with structural functions of the contractile machinery of muscle.

Tracking down the different forms of nuclear actin

Actin is a rather uncommitted protein with a high degree of structural plasticity: it can adopt a variety of structural states, depending on the specific ionic conditions or the interaction with ligand proteins. These interactions lock actin into a distinct conformation, which specifies the oligomeric or polymeric form it can assume. The interplay between monomeric, oligomeric and polymeric forms is used by the cell to execute an enormous variety of motility processes, such as lamellipodium formation during locomotion or intracellular transport of vesicles. In these cytoplasmic events, monomeric G-actin and filamentous F-actin are the prevalent forms. However, there might be other structural states of actin in cells that have so far not received the attention they deserve. Here, we propose that specific, "unconventional" actin conformations might contribute especially to the multitude of functions executed by actin in the nucleus. We present evidence for the existence of different forms of nuclear actin, taken from studies with selected antibodies.

Distinct subcellular location of the Ca2+-binding protein S100A1 differentially modulates Ca2+-cycling in ventricular rat cardiomyocytes

Calcium is a key regulator of cardiac function and is modulated through the Ca2+-sensor protein S100A1. S100 proteins are considered to exert both intracellular and extracellular functions on their target cells. Here we report the impact of an increased intracellular S100A1 protein level on Ca2+-homeostasis in neonatal ventricular cardiomyocytes in vitro. Specifically, we compare the effects of exogenously added recombinant S100A1 to those resulting from the overexpression of a transduced S100A1 gene. Extracellularly added S100A1 enhanced the Ca2+-transient amplitude in neonatal ventricular cardiomyocytes (NVCMs) through a marked decrease in intracellular diastolic Ca2+-concentrations ([Ca2+]i). The decrease in [Ca2+]i was independent of sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) activity and was probably the result of an increased sarcolemmal Ca2+-extrusion through the sodium-calcium exchanger (NCX). At the same time the Ca2+-content of the sarcoplasmic reticulum (SR) decreased. These effects were dependent on the uptake of extracellularly added S100A1 protein and its subsequent routing to the endosomal compartment. Phospholipase C and protein kinase C, which are tightly associated with this subcellular compartment, were found to be activated by endocytosed S100A1.

By contrast, adenoviral-mediated intracellular S100A1 overexpression enhanced the Ca2+-transient amplitude in NVCMs mainly through an increase in systolic [Ca2+]i. The increased Ca2+-load in the SR was based on an enhanced SERCA2a activity while NCX function was unaltered. Overexpressed S100A1 colocalized with SERCA2a and other Ca2+-regulatory proteins at the SR, whereas recombinant S100A1 protein that had been endocytosed did not colocalize with SR proteins. This study provides the first evidence that intracellular S100A1, depending on its subcellular location, modulates cardiac Ca2+-turnover via different Ca2+-regulatory proteins.

Cell polarity and epithelial oncogenesis

Pathologists have long recognized that tumour formation in epithelia leads to disruption of normal epithelial cell polarity. Despite this, few studies have taken advantage of new information on the biogenesis of cell polarity to analyse the process of epithelial oncogenesis. Recent studies of epithelial cell lines now indicate that the pattern of breakdown of polarity during oncogenesis may reflect the way in which normal epithelial cells achieve polarity. These results suggest not only a novel way to study the development of polarity in vitro, but also new ideas for the early detection of cancer.

Extracellular S100A1 protein inhibits apoptosis in ventricular cardiomyocytes via activation of the extracellular signal-regulated protein kinase 1/2 (ERK1/2)

S100A1 is a Ca2+-binding protein of the EF-hand type that belongs to the S100 protein family. It is specifically expressed in the myocardium at high levels and is considered to be an important regulator of cardiac contractility. Because the S100A1 protein is released into the extracellular space during ischemic myocardial injury, we examined the cardioprotective potential of the extracellular S100A1 protein on ventricular cardiomyocytes in vitro. In this report we show that extracellularly added S100A1 protein is endocytosed into the endosomal compartment of neonatal ventricular cardiomyocytes via a Ca2+-dependent clathrin-mediated process. S100A1 uptake protects neonatal ventricular cardiomyocytes from 2-deoxyglucose and oxidative stress-induced apoptosis in vitro. S100A1-mediated anti-apoptotic effects involve specific activation of the extracellular signal-regulated kinase 1/2 (ERK1/2) pro-survival pathway, including activation of phospholipase C, protein kinase C, mitogen-activated protein kinase kinase 1, and ERK1/2. In contrast, neither transsarcolemmal Ca2+ influx via the L-type channel nor protein kinase A activity seems to take part in the S100A1-mediated signaling pathway. In conclusion, this study provides evidence for the S100A1 protein serving as a novel cardioprotective factor in vitro. These findings warrant speculation that injury-dependent release of the S100A1 protein from cardiomyocytes may serve as an intrinsic mechanism to promote survival of the myocardium in vivo.

Actin's propensity for dynamic filament patterning

Actin, through its various forms of assembly, provides the basic framework for cell motility, cell shape and intracellular organization in all eukaryotic cells. Many other cellular processes, for example endocytosis and cytokinesis, are also associated with dynamic changes of the actin cytoskeleton. Important prerequisites for actin's functional diversity are its intrinsic ability to rapidly assemble and disassemble filaments and its spatially and temporally well-controlled supramolecular organization. A large number of proteins that interact with actin, collectively referred to as actin-binding proteins (ABPs), carefully orchestrate different scenarios. Since its isolation in 1994 [Machesky, L.M. et al. (1994) J. Cell Biol. 127, 107-115], the Arp2/3 complex containing the actin-related proteins Arp2 and Arp3 has evolved to be one of the main players in the assembly and maintenance of many actin-based structures in the cell (for review see [Borths, E.L. and Welch, M.D. (2002) Structure 10, 131-135; May, R.C. (2001) Cell Mol. Life Sci. 58, 1607-1626; Pollard, T.D. et al. (2000) Rev. Biophys. Biomol. Struct. 29, 545-576; Welch, M.D. (1999) Trends Cell Biol. 11, 423-427]). In particular, when it comes to the assembly of the intricate branched actin network at the leading edge of lamellipodia, the Arp2/3 complex seems to have received all the attention in recent years. In parallel, but not so much in the spotlight, several reports showed that actin on its own can assume different conformations [Bubb, M.R. et al. (2002) J. Biol. Chem. 277, 20999-21006; Schoenenberger, C.-A. et al. (1999) Microsc. Res. Tech. 47, 38-50; Steinmetz, M.O. et al. (1998) J. Mol. Biol. 278, 793-811; Steinmetz, M.O. et al. (1997) J. Cell Biol. 138, 559-574; Millonig, R., Salvo, H. and Aebi, U. (1988) J. Cell Biol. 106, 785-796] through which it drives its supramolecular patterning, and which ultimately generate its functional diversity.

Polymerization, three-dimensional structure and mechanical properties of Ddictyostelium versus rabbit muscle actin filaments

To assess more systematically functional differences among non-muscle and muscle actins and the effect of specific mutations on their function, we compared actin from Dictyostelium discoideum (D-actin) with actin from rabbit skeletal muscle (R-actin) with respect to the formation of filaments, their three-dimensional structure and mechanical properties. With Mg(2+) occupying the single high-affinity divalent cation-binding site, the course of polymerization is very similar for the two types of actin. In contrast, when Ca(2+ )is bound, D-actin exhibits a significantly longer lag phase at the onset of polymerization than R-actin. Crossover spacing and helical screw angle of negatively stained filaments are similar for D and R-F-actin filaments, irrespective of the tightly bound divalent cation. However, three-dimensional helical reconstructions reveal that the intersubunit contacts along the two long-pitch helical strands of D-(Ca)F-actin filaments are more tenuous compared to those in R-(Ca)F-actin filaments. D-(Mg)F-actin filaments on the other hand exhibit more massive contacts between the two long-pitch helical strands than R-(Mg)F-actin filaments. Moreover, in contrast to the structure of R-F-actin filaments which is not significantly modulated by the divalent cation, the intersubunit contacts both along and between the two long-pitch helical strands are weaker in D-(Ca)F-actin compared to D-(Mg)F-actin filaments. Consistent with these structural differences, D-(Ca)F-actin filaments were significantly more flexible than D-(Mg)F-actin. Taken together, this work documents that despite being highly conserved, muscle and non-muscle actins exhibit subtle differences in terms of their polymerization behavior, and the three-dimensional structure and mechanical properties of their F-actin filaments which, in turn, may account for their functional diversity.

Structure, assembly, and dynamics of actin filaments in situ and in vitro

Actin, though highly conserved, exhibits a myriad of diverse functions, most of which ultimately depend on its intrinsic ability to rapidly assemble and disassemble filamentous structures. Many organisms synthesize multiple actin isoforms even within the same cell. Tissue-specific expression patterns and tight developmental regulation as well as a high conservation across species emphasize the functional importance of isoforms. The detailed knowledge of the structure, assembly, and dynamic behavior of actin provides important pieces in solving the puzzle of how the different isoforms can be so versatile despite their extremely high sequence identity.