Intermediate Filaments: Structure and Assembly

Perception and response of skeleton to mechanical stress

Mechanical stress stands as a fundamental factor in the intricate processes governing the growth, development, morphological shaping, and maintenance of skeletal mass. The profound influence of stress in shaping the skeletal framework prompts the assertion that stress essentially births the skeleton. Despite this acknowledgment, the mechanisms by which the skeleton perceives and responds to mechanical stress remain enigmatic. In this comprehensive review, our scrutiny focuses on the structural composition and characteristics of sclerotin, leading us to posit that it serves as the primary structure within the skeleton responsible for bearing and perceiving mechanical stress. Furthermore, we propose that osteocytes within the sclerotin emerge as the principal mechanical-sensitive cells, finely attuned to perceive mechanical stress. And a detailed analysis was conducted on the possible transmission pathways of mechanical stress from the extracellular matrix to the nucleus.

Thermosensory Spiking Activity of Proteinoid Microspheres Cross-Linked by Actin Filaments

Actin, found in all eukaryotic cells as globular (G) or filamentous (F) actin, undergoes polymerization, with G-actin units changing shape to become F-actin. Thermal proteins, or proteinoids, are created by heating amino acids (160-200 °C), forming polymeric chains. These proteinoids can swell in an aqueous solution at around 50 °C, producing hollow microspheres filled with a solution, exhibiting voltage spikes. Our research explores the signaling properties of proteinoids, actin filaments, and hybrid networks combining actin and proteinoids. Proteinoids replicate brain excitation dynamics despite lacking specific membranes or ion channels. We investigate enhancing conductivity and spiking by using pure actin, yielding improved coordination in networks compared with individual filaments or proteinoids. Temperature changes (20 short-peptide supramolecular C to 80 °C) regulate conduction states, demonstrating external control over emergent excitability in protobrain systems. Adding actin to proteinoids reduces spike timing variability, providing a more uniform feature distribution. These findings support theoretical models proposing cytoskeletal matrices for functional specification in synthetic protocell brains, promoting stable interaction complexity. The study concludes that life-like signal encoding can emerge spontaneously within biological polymer scaffolds, incorporating abiotic chemistry.

Deciphering vimentin assembly: Bridging theoretical models and experimental approaches

The intricate assembly process of vimentin intermediate filaments (IFs), key components of the eukaryotic cytoskeleton, has yet to be elucidated. In this work, we investigated the transition from soluble tetrameric vimentin units to mature 11-nm tubular filaments, addressing a significant gap in the understanding of IF assembly. Through a combination of theoretical modeling and analysis of experimental data, we propose a novel assembly sequence, emphasizing the role of helical turns and gap filling by soluble tetramers. Our findings shed light on the unique structural dynamics of vimentin and suggest broader implications for the general principles of IF formation.

Switchable microscale stress response of actin-vimentin composites emerges from scale-dependent interactions

The mechanical properties of the mammalian cell regulate many cellular functions and are largely dictated by the cytoskeleton, a composite network of protein filaments, including actin, microtubules, and intermediate filaments. Interactions between these distinct filaments give rise to emergent mechanical properties that are difficult to generate synthetically, and recent studies have made great strides in advancing our understanding of the mechanical interplay between actin and microtubule filaments. While intermediate filaments play critical roles in the stress response of cells, their effect on the rheological properties of the composite cytoskeleton remains poorly understood. Here, we use optical tweezers microrheology to measure the linear viscoelastic properties and nonlinear stress response of composites of actin and vimentin with varying molar ratios of actin to vimentin. We reveal a surprising, nearly opposite effect of actin-vimentin network mechanics compared to single-component networks in the linear versus nonlinear regimes. Namely, the linear elastic plateau modulus and zero-shear viscosity are markedly reduced in composites compared to single-component networks of actin or vimentin, whereas the initial response force and stiffness are maximized in composites versus single-component networks in the nonlinear regime. While these emergent trends are indicative of distinct interactions between actin and vimentin, nonlinear stiffening and longtime stress response appear to both be dictated primarily by actin, at odds with previous bulk rheology studies. We demonstrate that these complex, scale-dependent effects arise from the varied contributions of network density, filament stiffness, non-specific interactions, and poroelasticity to the mechanical response at different spatiotemporal scales. Cells may harness this complex behavior to facilitate distinct stress responses at different scales and in response to different stimuli to allow for their hallmark multifunctionality.

Structural determinants of intermediate filament mechanics

Intermediate filaments (IFs) are integral to the cell cytoskeleton, supporting cellular mechanical stability. Unlike other cytoskeletal components, the detailed structure of assembled IFs has yet to be resolved. This review highlights new insights, linking the complex IF hierarchical assembly to their mechanical properties and impact on cellular functions. While we focus on vimentin IFs, we draw comparisons to keratins, showcasing the distinctive structural and mechanical features that underlie their unique mechanical responses.

A Comprehensive Analysis of Non-Desmosomal Rare Genetic Variants in Arrhythmogenic Cardiomyopathy: Integrating in Padua Cohort Literature-Derived Data

Arrhythmogenic cardiomyopathy (ACM) is an inherited myocardial disease at risk of sudden death. Genetic testing impacts greatly in ACM diagnosis, but gene-disease associations have yet to be determined for the increasing number of genes included in clinical panels. Genetic variants evaluation was undertaken for the most relevant non-desmosomal disease genes. We retrospectively studied 320 unrelated Italian ACM patients, including 243 cases with predominant right-ventricular (ARVC) and 77 cases with predominant left-ventricular (ALVC) involvement, who did not carry pathogenic/likely pathogenic (P/LP) variants in desmosome-coding genes. The aim was to assess rare genetic variants in transmembrane protein 43 (TMEM43), desmin (DES), phospholamban (PLN), filamin c (FLNC), cadherin 2 (CDH2), and tight junction protein 1 (TJP1), based on current adjudication guidelines and reappraisal on reported literature data. Thirty-five rare genetic variants, including 23 (64%) P/LP, were identified in 39 patients (16/243 ARVC; 23/77 ALVC): 22 FLNC, 9 DES, 2 TMEM43, and 2 CDH2. No P/LP variants were found in PLN and TJP1 genes. Gene-based burden analysis, including P/LP variants reported in literature, showed significant enrichment for TMEM43 (3.79-fold), DES (10.31-fold), PLN (117.8-fold) and FLNC (107-fold). A non-desmosomal rare genetic variant is found in a minority of ARVC patients but in about one third of ALVC patients; as such, clinical decision-making should be driven by genes with robust evidence. More than two thirds of non-desmosomal P/LP variants occur in FLNC.

Are the Head and Tail Domains of Intermediate Filaments Really Unstructured Regions?

Intermediate filaments (IFs) are integral components of the cytoskeleton which provide cells with tissue-specific mechanical properties and are involved in a plethora of cellular processes. Unfortunately, due to their intricate architecture, the 3D structure of the complete molecule of IFs has remained unresolved. Even though most of the rod domain structure has been revealed by means of crystallographic analyses, the flanked head and tail domains are still mostly unknown. Only recently have studies shed light on head or tail domains of IFs, revealing certainsecondary structures and conformational changes during IF assembly. Thus, a deeper understanding of their structure could provide insights into their function.

The host cytoskeleton: a key regulator of early HIV-1 infection

Due to its central role in cell biology, the cytoskeleton is a key regulator of viral infection, influencing nearly every step of the viral life cycle. In this review, we will discuss the role of two key components of the cytoskeleton, namely the actin and microtubule networks in early HIV-1 infection. We will discuss key contributions to processes ranging from the attachment and entry of viral particles at the cell surface to their arrival and import into the nucleus and identify areas where further research into this complex relationship may yield new insights into HIV-1 pathogenesis.

Type III intermediate filaments in redox interplay: key role of the conserved cysteine residue

Intermediate filaments (IFs) are cytoskeletal elements involved in mechanotransduction and in the integration of cellular responses. They are versatile structures and their assembly and organization are finely tuned by posttranslational modifications. Among them, type III IFs, mainly vimentin, have been identified as targets of multiple oxidative and electrophilic modifications. A characteristic of most type III IF proteins is the presence in their sequence of a single, conserved cysteine residue (C328 in vimentin), that is a hot spot for these modifications and appears to play a key role in the ability of the filament network to respond to oxidative stress. Current structural models and experimental evidence indicate that this cysteine residue may occupy a strategic position in the filaments in such a way that perturbations at this site, due to chemical modification or mutation, impact filament assembly or organization in a structure-dependent manner. Cysteine-dependent regulation of vimentin can be modulated by interaction with divalent cations, such as zinc, and by pH. Importantly, vimentin remodeling induced by C328 modification may affect its interaction with cellular organelles, as well as the cross-talk between cytoskeletal networks, as seems to be the case for the reorganization of actin filaments in response to oxidants and electrophiles. In summary, the evidence herein reviewed delineates a complex interplay in which type III IFs emerge both as targets and modulators of redox signaling.

Keratin isoform shifts modulate motility signals during wound healing

Keratin intermediate filaments form strong mechanical scaffolds that confer structural stability to epithelial tissues, but the reason this function requires a protein family with 54 isoforms is not understood. During skin wound healing, a shift in keratin isoform expression alters the composition of keratin filaments. How this change modulates cellular function to support epidermal remodeling remains unclear. We report an unexpected effect of keratin isoform variation on kinase signal transduction. Increased expression of wound-associated keratin 6A, but not of steady-state keratin 5, potentiated keratinocyte migration and wound closure without compromising epidermal stability by activating myosin motors. This pathway depended on isoform-specific interaction between intrinsically disordered keratin head domains and non-filamentous vimentin shuttling myosin-activating kinases. These results substantially expand the functional repertoire of intermediate filaments from their canonical role as mechanical scaffolds to include roles as isoform-tuned signaling scaffolds that organize signal transduction cascades in space and time to influence epithelial cell state.

Impact of MG132 induced-proteotoxic stress on αB-crystallin and desmin phosphorylation and O-GlcNAcylation and their partition towards cytoskeleton

Small Heat Shock Proteins are considered as the first line of defense when proteostasis fails. Among them, αB-crystallin is expressed in striated muscles in which it interacts with desmin intermediate filaments to stabilize them, maintaining cytoskeleton's integrity and muscular functionalities. Desmin is a key actor for muscle health; its targeting by αB-crystallin is thus crucial, especially in stress conditions. αB-crystallin is phosphorylated and O-GlcNAcylated. Its phosphorylation increases consecutively to various stresses, correlated with its recruitment for cytoskeleton's safeguarding. However, phosphorylation as unique signal for cytoskeleton translocation remains controversial; indeed, O-GlcNAcylation was also proposed to be involved. Thus, there are still some gaps for a deeper comprehension of how αB-crystallin functions are finely regulated by post-translational modifications. Furthermore, desmin also bears both post-translational modifications; while desmin phosphorylation is closely linked to desmin intermediates filaments turnover, it is unclear whereas its O-GlcNAcylation could impact its proper function. In the herein paper, we aim at identifying whether phosphorylation and/or O-GlcNAcylation are involved in αB-crystallin targeting towards cytoskeleton in proteotoxic stress induced by proteasome inhibition in C2C12 myotubes. We demonstrated that proteotoxicity led to αB-crystallin's phosphorylation and O-GlcNAcylation patterns changes, both presenting a dynamic interplay depending on protein subfraction. Importantly, both post-translational modifications showed a spatio-temporal variation correlated with αB-crystallin translocation towards cytoskeleton. In contrast, we did not detect any change of desmin phosphorylation and O-GlcNAcylation. All together, these data strongly support that αB-crystallin phosphorylation/O-GlcNAcylation interplay rather than changes on desmin is a key regulator for its cytoskeleton translocation, preserving it towards stress.

Vimentin regulates mitochondrial ROS production and inflammatory responses of neutrophils

The intermediate filament vimentin is present in immune cells and is implicated in proinflammatory immune responses. Whether and how it supports antimicrobial activities of neutrophils is not well established. Here, we developed an immortalized neutrophil model to examine the requirement of vimentin. We demonstrate that vimentin restricts the production of proinflammatory cytokines and reactive oxygen species (ROS), but enhances phagocytosis and swarming. We observe that vimentin is dispensable for neutrophil extracellular trap (NET) formation, degranulation, and inflammasome activation. Moreover, gene expression analysis demonstrated that the presence of vimentin was associated with changes in expression of multiple genes required for mitochondrial function and ROS overproduction. Treatment of wild-type cells with rotenone, an inhibitor for complex I of the electron transport chain, increases the ROS levels. Likewise, treatment with mitoTEMPO, a SOD mimetic, rescues the ROS production in cells lacking vimentin. Together, these data show vimentin regulates neutrophil antimicrobial functions and alters ROS levels through regulation of mitochondrial activity.

Unraveling Desmin's Head Domain Structure and Function

Understanding the structure and function of intermediate filaments (IFs) is necessary in order to explain why more than 70 related IF genes have evolved in vertebrates while maintaining such dramatically tissue-specific expression. Desmin is a member of the large multigene family of IF proteins and is specifically expressed in myocytes. In an effort to elucidate its muscle-specific behavior, we have used a yeast two-hybrid system in order to identify desmin's head binding partners. We described a mitochondrial and a lysosomal protein, NADH ubiquinone oxidoreductase core subunit S2 (NDUFS2), and saposin D, respectively, as direct desmin binding partners. In silico analysis indicated that both interactions at the atomic level occur in a very similar way, by the formation of a three-helix bundle with hydrophobic interactions in the interdomain space and hydrogen bonds at R16 and S32 of the desmin head domain. The interactions, confirmed also by GST pull-down assays, indicating the necessity of the desmin head domain and, furthermore, point out its role in function of mitochondria and lysosomes, organelles which are disrupted in myopathies due to desmin head domain mutations.

IOP and glaucoma damage: The essential role of optic nerve head and retinal mechanosensors

There are many unanswered questions on the relation of intraocular pressure to glaucoma development and progression. IOP itself cannot be distilled to a single, unifying value, because IOP level varies over time, differs depending on ocular location, and can be affected by method of measurement. Ultimately, IOP level creates mechanical strain that affects axonal function at the optic nerve head which causes local extracellular matrix remodeling and retinal ganglion cell death - hallmarks of glaucoma and the cause of glaucomatous vision loss. Extracellular tissue strain at the ONH and lamina cribrosa is regionally variable and differs in magnitude and location between healthy and glaucomatous eyes. The ultimate targets of IOP-induced tissue strain in glaucoma are retinal ganglion cell axons at the optic nerve head and the cells that support axonal function (astrocytes, the neurovascular unit, microglia, and fibroblasts). These cells sense tissue strain through a series of signals that originate at the cell membrane and alter cytoskeletal organization, migration, differentiation, gene transcription, and proliferation. The proteins that translate mechanical stimuli into molecular signals act as band-pass filters - sensing some stimuli while ignoring others - and cellular responses to stimuli can differ based on cell type and differentiation state. Therefore, to fully understand the IOP signals that are relevant to glaucoma, it is necessary to understand the ultimate cellular targets of IOP-induced mechanical stimuli and their ability to sense, ignore, and translate these signals into cellular actions.

The thermodynamics of neurodegenerative disease

The formation of protein aggregates in the brain is a central aspect of the pathology of many neurodegenerative diseases. This self-assembly of specific proteins into filamentous aggregates, or fibrils, is a fundamental biophysical process that can easily be reproduced in the test tube. However, it has been difficult to obtain a clear picture of how the biophysical insights thus obtained can be applied to the complex, multi-factorial diseases and what this means for therapeutic strategies. While new, disease-modifying therapies are now emerging, for the most devastating disorders, such as Alzheimer's and Parkinson's disease, they still fall well short of offering a cure, and few drug design approaches fully exploit the wealth of mechanistic insights that has been obtained in biophysical studies. Here, I attempt to provide a new perspective on the role of protein aggregation in disease, by phrasing the problem in terms of a system that, under constant energy consumption, attempts to maintain a healthy, aggregate-free state against the thermodynamic driving forces that inexorably push it toward pathological aggregation.

Identification of genomic regions and pathways associated with traits related to rumen acidosis in feedlot Nellore cattle

There may be an increased risk of metabolic disorders, such as rumen acidosis, in cattle fed high-concentrate diets, particularly those from Bos taurus indicus genotypes, which have shown to be more sensitive to ruminal acidification. Therefore, this study aimed to estimate (co)variance components and identify genomic regions and pathways associated with ruminal acidosis in feedlot Nellore cattle fed high-concentrate diets. It was utilized a dataset containing a total of 642 Nellore bulls that were genotyped from seven feedlot nutrition studies. The GGP Indicus 35k panel was used with the single step genome-wide association study methodology in which the effects of the markers were obtained from the genomic values estimated by the GBLUP model. A bivariate model to estimate genetic correlations between the economically important traits and indicator traits for acidosis was used. The traits evaluated in this study that were nutritionally related to rumen acidosis included average daily gain (ADG), final body weight, time spent eating (TSE), time spent ruminating, rumenitis score (RUM), rumen absorptive surface area (ASA), rumen keratinized layer thickness (KER) and hot carcass weight (HCW). The identified candidate genes were mainly involved in the negative or non-regulation of the apoptotic process, salivary secretion, and transmembrane transport. The genetic correlation between HCW and ASA was low positive (0.27 ± 0.23), and between ADG and ASA was high moderate (0.58 ± 0.59). A positive genetic correlation between RUM and all performance traits was observed, and TSE correlated negatively with HCW (-0.33 ± 0.21), ASA (-0.75 ± 0.48), and KER (-0.40 ± 0.27). The genetic association between economically important traits and indicator traits for acidosis suggested that Nellore cattle may be more sensitive to acidosis in feedlot systems.

Filament structure and subcellular organization of the bacterial intermediate filament-like protein crescentin

The protein crescentin is required for the crescent shape of the freshwater bacterium Caulobacter crescentus (vibrioides). Crescentin forms a filamentous structure on the inner, concave side of the curved cells. It shares features with eukaryotic intermediate filament (IF) proteins, including the formation of static filaments based on long and parallel coiled coils, the protein's length, structural roles in cell and organelle shape determination and the presence of a coiled coil discontinuity called the "stutter." Here, we have used electron cryomicroscopy (cryo-EM) to determine the structure of the full-length protein and its filament, exploiting a crescentin-specific nanobody. The filament is formed by two strands, related by twofold symmetry, that each consist of two dimers, resulting in an octameric assembly. Crescentin subunits form longitudinal contacts head-to-head and tail-to-tail, making the entire filament non-polar. Using in vivo site-directed cysteine cross-linking, we demonstrated that contacts observed in the in vitro filament structure exist in cells. Electron cryotomography (cryo-ET) of cells expressing crescentin showed filaments on the concave side of the curved cells, close to the inner membrane, where they form a band. When comparing with current models of IF proteins and their filaments, which are also built from parallel coiled coil dimers and lack overall polarity, it emerges that IF proteins form head-to-tail longitudinal contacts in contrast to crescentin and hence several inter-dimer contacts in IFs have no equivalents in crescentin filaments. Our work supports the idea that intermediate filament-like proteins achieve their shared polymerization and mechanical properties through a variety of filament architectures.

CRISPR screen for protein inclusion formation uncovers a role for SRRD in the regulation of intermediate filament dynamics and aggresome assembly

The presence of large protein inclusions is a hallmark of neurodegeneration, and yet the precise molecular factors that contribute to their formation remain poorly understood. Screens using aggregation-prone proteins have commonly relied on downstream toxicity as a readout rather than the direct formation of aggregates. Here, we combined a genome-wide CRISPR knockout screen with Pulse Shape Analysis, a FACS-based method for inclusion detection, to identify direct modifiers of TDP-43 aggregation in human cells. Our screen revealed both canonical and novel proteostasis genes, and unearthed SRRD, a poorly characterized protein, as a top regulator of protein inclusion formation. APEX biotin labeling reveals that SRRD resides in proximity to proteins that are involved in the formation and breakage of disulfide bonds and to intermediate filaments, suggesting a role in regulation of the spatial dynamics of the intermediate filament network. Indeed, loss of SRRD results in aberrant intermediate filament fibrils and the impaired formation of aggresomes, including blunted vimentin cage structure, during proteotoxic stress. Interestingly, SRRD also localizes to aggresomes and unfolded proteins, and rescues proteotoxicity in yeast whereby its N-terminal low complexity domain is sufficient to induce this affect. Altogether this suggests an unanticipated and broad role for SRRD in cytoskeletal organization and cellular proteostasis.

Vimentin cage - A double-edged sword in host anti-infection defense

Vimentin, a type III intermediate filament, reorganizes into what is termed the 'vimentin cage' in response to various pathogenic infections. This cage-like structure provides an envelope to key components of the pathogen's life cycle. In viral infections, the vimentin cage primarily serves as a scaffold and organizer for the replication factory, promoting viral replication. However, it also occasionally contributes to antiviral functions. For bacterial infections, the cage mainly supports bacterial proliferation in most observed cases. These consistent structural alterations in vimentin, induced by a range of viruses and bacteria, highlight the vimentin cage's crucial role. Pathogen-specific factors add complexity to this interaction. In this review, we provide a thorough overview of the functions and mechanisms of the vimentin cage and speculate on vimentin's potential as a novel target for anti-pathogen strategies.

Biological and Physical Performance Markers for Early Detection of Cognitive Impairment in Older Adults

Dementia is a major cause of poor quality of life, disability, and mortality in old age. According to the geroscience paradigm, the mechanisms that drive the aging process are also involved in the pathogenesis of chronic degenerative diseases, including dementia. The dissection of such mechanisms is therefore instrumental in providing biological targets for interventions and new sources for biomarkers. Within the geroscience paradigm, several biomarkers have been discovered that can be measured in blood and that allow early identification of individuals at risk of cognitive impairment. Examples of such markers include inflammatory biomolecules, markers of neuroaxonal damage, extracellular vesicles, and DNA methylation. Furthermore, gait speed, measured at a usual and fast pace and as part of a dual task, has been shown to detect individuals at risk of future dementia. Here, we provide an overview of available biomarkers that may be used to gauge the risk of cognitive impairment in apparently healthy older adults. Further research should establish which combination of biomarkers possesses the highest predictive accuracy toward incident dementia. The implementation of currently available markers may allow the identification of a large share of at-risk individuals in whom preventive interventions should be implemented to maintain or increase cognitive reserves, thereby reducing the risk of progression to dementia.

Neurofilament Light Protein Rod Domain Exhibits Structural Heterogeneity

Neurofilaments are neuron-specific proteins that belong to the intermediate filament (IFs) protein family, with the neurofilament light chain protein (NFL) being the most abundant. The IFs structure typically includes a central coiled-coil rod domain comprised of coils 1A, 1B, and 2, separated by linker regions. The thermal stability of the IF molecule plays a crucial role in its ability for self-association. In the current study, we investigated the thermal stability of NFL coiled-coil domains by analyzing a set of recombinant domains and their fusions (NFL1B, NFL1A+1B, NFL2, NFL1B+2, and NFLROD) via circular dichroism spectroscopy and differential scanning calorimetry. The thermal stability of coiled-coil domains is evident in a wide range of temperatures, and thermal transition values (Tm) correspond well between isolated coiled-coil domains and full-length NFL. NFL1B has a Tm of 39.4 °C, and its' fusions, NFL1A+1B and NFL1B+2, have a Tm of 41.9 °C and 41.5 °C, respectively. However, in the case of NFL2, thermal denaturation includes at least two thermal transitions at 37.2 °C and 62.7 °C. These data indicate that the continuous α-helical structure of the coil 2 domain has parts with varied thermal stability. Among all the NFL fragments, only NFL2 underwent irreversible heat-induced denaturation. Together, these results unveil the origin of full-length NFL's thermal transitions, and reveal its domains structure and properties.

Novel Genetic and Biochemical Insights into the Spectrum of NEFL-Associated Phenotypes

Background

NEFL encodes for the neurofilament light chain protein. Pathogenic variants in NEFL cause demyelinating, axonal and intermediate forms of Charcot-Marie-Tooth disease (CMT) which present with a varying degree of severity and somatic mutations have not been described yet. Currently, 34 different CMT-causing pathogenic variants in NEFL in 174 patients have been reported. Muscular involvement was also described in CMT2E patients mostly as a secondary effect. Also, there are a few descriptions of a primary muscle vulnerability upon pathogenic NEFL variants.

Objectives

To expand the current knowledge on the genetic landscape, clinical presentation and muscle involvement in NEFL-related neurological diseases by retrospective case study and literature review.

Methods

We applied in-depth phenotyping of new and already reported cases, molecular genetic testing, light-, electron- and Coherent Anti-Stokes Raman Scattering-microscopic studies and proteomic profiling in addition to in silico modelling of NEFL-variants.

Results

We report on a boy with a muscular phenotype (weakness, myalgia and cramps, Z-band alterations and mini-cores in some myofibers) associated with the heterozygous p.(Phe104Val) NEFL-variant, which was previously described in a neuropathy case. Skeletal muscle proteomics findings indicated affection of cytoskeletal proteins. Moreover, we report on two further neuropathic patients (16 years old girl and her father) both carrying the heterozygous p.(Pro8Ser) variant, which has been identified as 15% somatic mosaic in the father. While the daughter presented with altered neurophysiology,neurogenic clump feet and gait disturbances, the father showed clinically only feet deformities. As missense variants affecting proline at amino acid position 8 are leading to neuropathic manifestations of different severities, in silico modelling of these different amino acid substitutions indicated variable pathogenic impact correlating with disease onset.

Conclusions

Our findings provide new morphological and biochemical insights into the vulnerability of denervated muscle (upon NEFL-associated neuropathy) as well as novel genetic findings expanding the current knowledge on NEFL-related neuromuscular phenotypes and their clinical manifestations. Along this line, our data show that even subtle expression of somatic NEFL variants can lead to neuromuscular symptoms.

Defining a timeline of colon pathologies after keratin 8 loss: rapid crypt elongation and diarrhea are followed by epithelial erosion and cell exfoliation

Keratins are epithelial intermediate filament proteins that play a crucial role in cellular stress protection, with K8 being the most abundant in the colon. The intestinal epithelial-specific K8-deficient mouse model (K8flox/flox;Villin-Cre) exhibits characteristics of inflammatory bowel disease, including diarrhea, crypt erosion, hyperproliferation, and decreased barrier function. Nevertheless, the order in which these events occur and whether they are a direct cause of K8 loss or a consequence of one event inducing another remains unexplored. Increased knowledge about early events in the disruption of colon epithelial integrity would help to understand the early pathology of inflammatory and functional colon disorders and develop preclinical models and diagnostics of colonic diseases. Here, we aimed to characterize the order of physiological events after Krt8 loss by utilizing K8flox/flox;Villin-CreERt2 mice with tamoxifen-inducible Krt8 deletion in intestinal epithelial cells, and assess stool analysis as a noninvasive method to monitor real-time gene expression changes following Krt8 loss. K8 protein was significantly decreased within a day after induction, followed by its binding partners, K18 and K19 from day 4 onward. The sequential colonic K8 downregulation in adult mice leads to immediate diarrhea and crypt elongation with activation of proliferation signaling, followed by crypt loss and increased neutrophil activity within 6-8 days, highlighting impaired water balance and crypt elongation as the earliest colonic changes upon Krt8 loss. Furthermore, epithelial gene expression patterns were comparable between colon tissue and stool samples, demonstrating the feasibility of noninvasive monitoring of gut epithelia in preclinical research utilizing Cre-LoxP-based intestinal disease models.NEW & NOTEWORTHY Understanding the order in which physiological and molecular events occur helps to recognize the onset of diseases and improve their preclinical models. We utilized Cre-Lox-based inducible keratin 8 deletion in mouse intestinal epithelium to characterize the earliest events after keratin 8 loss leading to colitis. These include diarrhea and crypt elongation, followed by erosion and neutrophil activity. Our results also support noninvasive methodology for monitoring colon diseases in preclinical models.

Impairment of Assembly of the Vimentin Intermediate Filaments Leads to Suppression of Formation and Maturation of Focal Contacts and Alteration of the Type of Cellular Protrusions

Cell migration is largely determined by the type of protrusions formed by the cell. Mesenchymal migration is accomplished by formation of lamellipodia and/or filopodia, while amoeboid migration is based on bleb formation. Changing of migrational conditions can lead to alteration in the character of cell movement. For example, inhibition of the Arp2/3-dependent actin polymerization by the CK-666 inhibitor leads to transition from mesenchymal to amoeboid motility mode. Ability of the cells to switch from one type of motility to another is called migratory plasticity. Cellular mechanisms regulating migratory plasticity are poorly understood. One of the factors determining the possibility of migratory plasticity may be the presence and/or organization of vimentin intermediate filaments (VIFs). To investigate whether organization of the VIF network affects the ability of fibroblasts to form membrane blebs, we used rat embryo fibroblasts REF52 with normal VIF organization, fibroblasts with vimentin knockout (REF-/-), and fibroblasts with mutation inhibiting assembly of the full-length VIFs (REF117). Blebs formation was induced by treatment of cells with CK-666. Vimentin knockout did not lead to statistically significant increase in the number of cells with blebs. The fibroblasts with short fragments of vimentin demonstrate the significant increase in number of cells forming blebs both spontaneously and in the presence of CK-666. Disruption of the VIF organization did not lead to the significant changes in the microtubules network or the level of myosin light chain phosphorylation, but caused significant reduction in the focal contact system. The most pronounced and statistically significant decrease in both size and number of focal adhesions were observed in the REF117 cells. We believe that regulation of the membrane blebbing by VIFs is mediated by their effect on the focal adhesion system. Analysis of migration of fibroblasts with different organization of VIFs in a three-dimensional collagen gel showed that organization of VIFs determines the type of cell protrusions, which, in turn, determines the character of cell movement. A novel role of VIFs as a regulator of membrane blebbing, essential for manifestation of the migratory plasticity, is shown.

Neurofilaments in health and Charcot-Marie-Tooth disease

Neurofilaments (NFs) are the most abundant component of mature neurons, that interconnect with actin and microtubules to form the cytoskeleton. Specifically expressed in the nervous system, NFs present the particularity within the Intermediate Filament family of being formed by four subunits, the neurofilament light (NF-L), medium (NF-M), heavy (NF-H) proteins and α-internexin or peripherin. Here, we review the current knowledge on NF proteins and neurofilaments, from their domain structures and their model of assembly to the dynamics of their transport and degradation along the axon. The formation of the filament and its behaviour are regulated by various determinants, including post-transcriptional (miRNA and RBP proteins) and post-translational (phosphorylation and ubiquitination) modifiers. Altogether, the complex set of modifications enable the neuron to establish a stable but elastic NF array constituting the structural scaffold of the axon, while permitting the local expression of NF proteins and providing the dynamics necessary to fulfil local demands and respond to stimuli and injury. Thus, in addition to their roles in mechano-resistance, radial axonal outgrowth and nerve conduction, NFs control microtubule dynamics, organelle distribution and neurotransmission at the synapse. We discuss how the studies of neurodegenerative diseases with NF aggregation shed light on the biology of NFs. In particular, the NEFL and NEFH genes are mutated in Charcot-Marie-Tooth (CMT) disease, the most common inherited neurological disorder of the peripheral nervous system. The clinical features of the CMT forms (axonal CMT2E, CMT2CC; demyelinating CMT1F; intermediate I-CMT) with symptoms affecting the central nervous system (CNS) will allow us to further investigate the physiological roles of NFs in the brain. Thus, NF-CMT mouse models exhibit various degrees of sensory-motor deficits associated with CNS symptoms. Cellular systems brought findings regarding the dominant effect of NF-L mutants on NF aggregation and transport, although these have been recently challenged. Neurofilament detection without NF-L in recessive CMT is puzzling, calling for a re-examination of the current model in which NF-L is indispensable for NF assembly. Overall, we discuss how the fundamental and translational fields are feeding each-other to increase but also challenge our knowledge of NF biology, and to develop therapeutic avenues for CMT and neurodegenerative diseases with NF aggregation.

Direct observation of oriented behavior of actin filaments interacting with desmin intermediate filaments

BACKGROUND

Associations between actin filaments (AFs) and intermediate filaments (IFs) are frequently observed in living cells. The crosstalk between these cytoskeletal components underpins cellular organization and dynamics; however, the molecular basis of filamentous interactions is not fully understood. Here, we describe the mode of interaction between AFs and desmin IFs (DIFs) in a reconstituted in vitro system.

METHODS

AFs (rabbit skeletal muscle) and DIFs (chicken gizzard) were labeled with fluorescent dyes. DIFs were immobilized on a heavy meromyosin (HMM)-coated collodion surface. HMM-driven AFs with ATP hydrolysis was assessed in the presence of DIFs. Images of single filaments were obtained using fluorescence microscopy. Vector changes in the trajectories of single AFs were calculated from microscopy images.

RESULTS

AF speed transiently decreased upon contact with DIF. The difference between the incoming and outgoing angles of a moving AF broadened upon contact with a DIF. A smaller incoming angle tended to result in a smaller outgoing angle in a nematic manner. The percentage of moving AFs decreased with an increasing DIF density, but the speed of the moving AFs was similar to that in the no-desmin control. An abundance of DIFs tended to exclude AFs from the HMM-coated surfaces.

CONCLUSIONS

DIFs agitate the movement of AFs with the orientation. DIFs can bind to HMMs and weaken actin-myosin interactions.

GENERAL SIGNIFICANCE

The study indicates that apart from the binding strength, the accumulation of weak interactions characteristic of filamentous structures may affect the dynamic organization of cell architecture.

Lamins as structural nuclear elements through evolution

Lamins are nuclear intermediate filament proteins with important, well-established roles in humans and other vertebrates. Lamins interact with DNA and numerous proteins at the nuclear envelope to determine the mechanical properties of the nucleus, coordinate chromatin organization, and modulate gene expression. Many of these functions are conserved in the lamin homologs found in basal metazoan organisms, including Drosophila and Caenorhabditis elegans. Lamin homologs have also been recently identified in non-metazoans, like the amoeba Dictyostelium discoideum, yet how these proteins compare functionally to the metazoan isoforms is only beginning to emerge. A better understanding of these distantly related lamins is not only valuable for a more complete picture of eukaryotic evolution, but may also provide new insights into the function of vertebrate lamins.

How do disordered head domains assist in the assembly of intermediate filaments?

The dominant structural feature of intermediate filament (IF) proteins is a centrally located α-helix. These long α-helical segments become paired in a parallel orientation to form coiled-coil dimers. Pairs of dimers further coalesce in an anti-parallel orientation to form tetramers. These early stages of intermediate filament assembly can be accomplished solely by the central α-helices. By contrast, the assembly of tetramers into mature intermediate filaments is reliant upon an N-terminal head domain. IF head domains measure roughly 100 amino acids in length and have long been understood to exist in a state of structural disorder. Here, we describe experiments favoring the unexpected idea that head domains self-associate to form transient structural order in the form of labile cross-β interactions. We propose that this weak form of protein structure allows for dynamic regulation of IF assembly and disassembly. We further offer that what we have learned from studies of IF head domains may represent a simple, unifying template for understanding how thousands of other intrinsically disordered proteins help to establish dynamic morphological order within eukaryotic cells.

Transverse Fluctuations Control the Assembly of Semiflexible Filaments

The kinetics of the assembly of semiflexible filaments through end-to-end annealing is key to the structure of the cytoskeleton, but is not understood. We analyze this problem through scaling theory and simulations, and uncover a regime where filaments' ends find each other through bending fluctuations without the need for the whole filament to diffuse. This results in a very substantial speedup of assembly in physiological regimes, and could help with understanding the dynamics of actin and intermediate filaments in biological processes such as wound healing and cell division.

Effects of truncations in the N- and C-terminal domains of filensin on filament formation with phakinin in cell-free conditions and cultured cells

Filensin and phakinin are lens fiber cell-specific proteins that constitute the beaded filaments (BFs) that are critical for maintaining lens transparency. In the Shumiya cataract rat, filensin 94 kDa undergoes N- and C-terminal proteolytic processing to give a transient 50 kDa fragment and a final 38 kDa fragment, just before opacification. To characterize the effects of this processing on filensin function, recombinant proteins representing the two filensin fragments, termed Fil(30-416) and Fil(30-369), respectively, were examined. Fil(30-416) lacks the N-terminal 29 amino acids and the C-terminal 248 amino acids. Fil(30-369) lacks the N-terminal 29 residues and the C-terminal 295 residues. In cell-free assembly characterized by electron microscopy, filensin and Fil(30-416) co-polymerized with phakinin and formed rugged, entangled filaments, whereas Fil(30-369) formed only aggregates. In cultured SW-13 and MCF-7 cells expressing fluorescent fusion proteins, filensin and Fil(30-416) co-polymerized with phakinin and formed cytoplasmic sinuous filaments with different widths, while Fil(30-369) gave aggregates. Therefore, while truncation of the N-terminal 29 amino acids did not affect filament formation, truncation of the C-terminal 295 but not the 248 residues resulted in failure of filament formation. These results indicate that the tail B region (residues 370-416) of rat filensin is essential for filament formation with phakinin. Truncation of the tail B region by proteolytic processing in the cataract rat lens might interfere with BF formation and thereby contribute to opacification.

Biomarkers of aging in frailty and age-associated disorders: State of the art and future perspective

According to the Geroscience concept that organismal aging and age-associated diseases share the same basic molecular mechanisms, the identification of biomarkers of age that can efficiently classify people as biologically older (or younger) than their chronological (i.e. calendar) age is becoming of paramount importance. These people will be in fact at higher (or lower) risk for many different age-associated diseases, including cardiovascular diseases, neurodegeneration, cancer, etc. In turn, patients suffering from these diseases are biologically older than healthy age-matched individuals. Many biomarkers that correlate with age have been described so far. The aim of the present review is to discuss the usefulness of some of these biomarkers (especially soluble, circulating ones) in order to identify frail patients, possibly before the appearance of clinical symptoms, as well as patients at risk for age-associated diseases. An overview of selected biomarkers will be discussed in this regard, in particular we will focus on biomarkers related to metabolic stress response, inflammation, and cell death (in particular in neurodegeneration), all phenomena connected to inflammaging (chronic, low-grade, age-associated inflammation). In the second part of the review, next-generation markers such as extracellular vesicles and their cargos, epigenetic markers and gut microbiota composition, will be discussed. Since recent progresses in omics techniques have allowed an exponential increase in the production of laboratory data also in the field of biomarkers of age, making it difficult to extract biological meaning from the huge mass of available data, Artificial Intelligence (AI) approaches will be discussed as an increasingly important strategy for extracting knowledge from raw data and providing practitioners with actionable information to treat patients.

Co-expression of distinct L1 retrotransposon coiled coils can lead to their entanglement

L1 (LINE1) non-LTR retrotransposons are ubiquitous genomic parasites and the dominant transposable element in humans having generated about 40% of their genomic DNA during their ~ 100 million years (Myr) of activity in primates. L1 replicates in germ line cells and early embryos, causing genetic diversity and defects, but can be active in some somatic stem cells, tumors and during aging. L1 encodes two proteins essential for retrotransposition: ORF2p, a reverse transcriptase that contains an endonuclease domain, and ORF1p, a coiled coil mediated homo trimer, which functions as a nucleic acid chaperone. Both proteins contain highly conserved domains and preferentially bind their encoding transcript to form an L1 ribonucleoprotein (RNP), which mediates retrotransposition. However, the coiled coil has periodically undergone episodes of substantial amino acid replacement to the extent that a given L1 family can concurrently express multiple ORF1s that differ in the sequence of their coiled coils. Here we show that such distinct ORF1p sequences can become entangled forming heterotrimers when co-expressed from separate vectors and speculate on how coiled coil entanglement could affect coiled coil evolution.

Calpain-2 protein influences chikungunya virus replication and regulates vimentin rearrangement caused by chikungunya virus infection

Chikungunya fever (CHIF), a vector-borne disease transmitted mainly by Aedes albopictus and Aedes aegypti, is caused by Chikungunya virus (CHIKV) infection. To date, it is estimated that 39% of the world's population is at risk of infection for living in countries and regions where CHIKV is endemic. However, at present, the cellular receptors of CHIKV remains not clear, and there are no specific drugs and vaccines for CHIF. Here, the cytotoxicity of calpain-2 protein activity inhibitor III and specific siRNA was detected by MTT assays. The replication of CHIKV was detected by qPCR amplification and plaque assay. Western blot was used to determine the level of the calpain-2 protein and vimentin protein. Immunofluorescence was also operated for detecting the rearrangement of vimentin protein. Our results indicated that calpain-2 protein activity inhibitor III and specific siRNA might suppress CHIKV replication. Furthermore, CHIKV infection led to vimentin remodeling and formation of cage-like structures, which could be inhibited by the inhibitor III. In summary, we confirmed that calpain-2 protein influenced chikungunya virus replication and regulated vimentin rearrangement caused by chikungunya virus infection, which could be important for understanding the biological significance of CHIKV replication and the future development of antiviral strategies.

O-GlcNAcylation regulates neurofilament-light assembly and function and is perturbed by Charcot-Marie-Tooth disease mutations

The neurofilament (NF) cytoskeleton is critical for neuronal morphology and function. In particular, the neurofilament-light (NF-L) subunit is required for NF assembly in vivo and is mutated in subtypes of Charcot-Marie-Tooth (CMT) disease. NFs are highly dynamic, and the regulation of NF assembly state is incompletely understood. Here, we demonstrate that human NF-L is modified in a nutrient-sensitive manner by O-linked-β-N-acetylglucosamine (O-GlcNAc), a ubiquitous form of intracellular glycosylation. We identify five NF-L O-GlcNAc sites and show that they regulate NF assembly state. NF-L engages in O-GlcNAc-mediated protein-protein interactions with itself and with the NF component α-internexin, implying that O-GlcNAc may be a general regulator of NF architecture. We further show that NF-L O-GlcNAcylation is required for normal organelle trafficking in primary neurons. Finally, several CMT-causative NF-L mutants exhibit perturbed O-GlcNAc levels and resist the effects of O-GlcNAcylation on NF assembly state, suggesting a potential link between dysregulated O-GlcNAcylation and pathological NF aggregation. Our results demonstrate that site-specific glycosylation regulates NF-L assembly and function, and aberrant NF O-GlcNAcylation may contribute to CMT and other neurodegenerative disorders.

Uncovering the role of cytoskeleton proteins in the formation of neutrophil extracellular traps

Neutrophils are a type of lymphocyte involved in innate immune defense. In response to specific stimuli, these phagocytic cells undergo a unique form of cell death, NETosis, during which they release neutrophil extracellular traps (NETs) composed of modified chromatin structures decorated with cytoplasmic and granular proteins. Multiple proteins and pathways have been implicated in the formation of NETs. The cytoskeleton, an interconnected network of filamentous polymers and regulatory proteins, plays a crucial role in resisting deformation, transporting intracellular cargo, and changing shape during movement of eukaryotic cells. It may also have evolved to defend eukaryotic organisms against infection. Recent research focuses on understanding the mechanisms underlying NETs formation and how cytoskeletal networks contribute to this process, by identifying enzymes that trigger NETosis or interact with NETs and influence cellular behavior through cytoskeletal dynamics. An enhanced understanding of the complex relationship between the cytoskeleton and NET formation will provide a framework for future research and the development of targeted therapeutic strategies, and supports the notion that the long-lived cytoskeleton structures may have a lasting impact on this area of research.

RNF26 binds perinuclear vimentin filaments to integrate ER and endolysosomal responses to proteotoxic stress

Proteotoxic stress causes profound endoplasmic reticulum (ER) membrane remodeling into a perinuclear quality control compartment (ERQC) for the degradation of misfolded proteins. Subsequent return to homeostasis involves clearance of the ERQC by endolysosomes. However, the factors that control perinuclear ER integrity and dynamics remain unclear. Here, we identify vimentin intermediate filaments as perinuclear anchors for the ER and endolysosomes. We show that perinuclear vimentin filaments engage the ER-embedded RING finger protein 26 (RNF26) at the C-terminus of its RING domain. This restricts RNF26 to perinuclear ER subdomains and enables the corresponding spatial retention of endolysosomes through RNF26-mediated membrane contact sites (MCS). We find that both RNF26 and vimentin are required for the perinuclear coalescence of the ERQC and its juxtaposition with proteolytic compartments, which facilitates efficient recovery from ER stress via the Sec62-mediated ER-phagy pathway. Collectively, our findings reveal a scaffolding mechanism that underpins the spatiotemporal integration of organelles during cellular proteostasis.

Desmin Reorganization by Stimuli Inducing Oxidative Stress and Electrophiles: Role of Its Single Cysteine Residue

The type III intermediate filament proteins vimentin and GFAP are modulated by oxidants and electrophiles, mainly through perturbation of their single cysteine residues. Desmin, the type III intermediate filament protein specific to muscle cells, is critical for muscle homeostasis, playing a key role in sarcomere organization and mitochondrial function. Here, we have studied the impact of oxidants and cysteine-reactive agents on desmin behavior. Our results show that several reactive species and drugs induce covalent modifications of desmin in vitro, of which its single cysteine residue, C333, is an important target. Moreover, stimuli eliciting oxidative stress or lipoxidation, including H2O2, 15-deoxy-prostaglandin J2, and CoCl2-elicited chemical hypoxia, provoke desmin disorganization in H9c2 rat cardiomyoblasts transfected with wild-type desmin, which is partially attenuated in cells expressing a C333S mutant. Notably, in cells lacking other cytoplasmic intermediate filaments, network formation by desmin C333S appears less efficient than that of desmin wt, especially when these proteins are expressed as fluorescent fusion constructs. Nevertheless, in these cells, the desmin C333S organization is also protected from disruption by oxidants. Taken together, our results indicate that desmin is a target for oxidative and electrophilic stress, which elicit desmin remodeling conditioned by the presence of its single cysteine residue.

Influence of phosphorylation on intermediate filaments

The cytoskeleton of eukaryotes consists of actin filaments, microtubules and intermediate filaments (IF). IFs, in particular, are prone to pronounced phosphorylation, leading to additional charges on the affected amino acids. In recent years, a variety of experiments employing either reconstituted protein systems or living cells have revealed that these altered charge patterns form the basis for a number of very diverse cellular functions and processes, including reversible filament assembly, filament softening, network remodeling, cell migration, interactions with other protein structures, and biochemical signaling.

Intracellular Vimentin Regulates the Formation of Classical Swine Fever Virus Replication Complex through Interaction with NS5A Protein

Vimentin (VIM), an indispensable protein, is responsible for the formation of intermediate filament structures within cells and plays a crucial role in viral infections. However, the precise role of VIM in classical swine fever virus (CSFV) infection remains unclear. Herein, we systematically investigated the function of VIM in CSFV replication. We demonstrated that both knockdown and overexpression of VIM affected CSFV replication. Furthermore, we observed by confocal microscopy the rearrangement of cellular VIM into a cage-like structure during CSFV infection. Three-dimensional (3D) imaging indicated that the cage-like structures were localized in the endoplasmic reticulum (ER) and ringed around the double-stranded RNA (dsRNA), thereby suggesting that VIM was associated with the formation of the viral replication complex (VRC). Mechanistically, phosphorylation of VIM at serine 72 (Ser72), regulated by the RhoA/ROCK signaling pathway, induced VIM rearrangement upon CSFV infection. Confocal microscopy and coimmunoprecipitation assays revealed that VIM colocalized and interacted with CSFV NS5A. Structurally, it was determined that amino acids 96 to 407 of VIM and amino acids 251 to 416 of NS5A were the respective important domains for this interaction. Importantly, both VIM knockdown and disruption of VIM rearrangement inhibited the localization of NS5A in the ER, implying that VIM rearrangement recruited NS5A to the ER for VRC formation. Collectively, our results suggest that VIM recruits NS5A to form a stable VRC that is protected by the cage-like structure formed by VIM rearrangement, ultimately leading to enhanced virus replication. These findings highlight the critical role of VIM in the formation and stabilization of VRC, which provides alternative strategies for the development of antiviral drugs. IMPORTANCE Classical swine fever (CSF), caused by classical swine fever virus (CSFV), is a highly infectious disease that poses a significant threat to the global pig industry. Therefore, gaining insights into the virus and its interaction with host cells is crucial for developing effective antiviral measures and controlling the spread of CSF. Previous studies have shown that CSFV infection induces rearrangement of the endoplasmic reticulum, leading to the formation of small vesicular organelles containing nonstructural protein and double-stranded RNA of CSFV, as well as some host factors. These organelles then assemble into viral replication complexes (VRCs). In this study, we have discovered that VIM recruited CSFV NS5A to form a stable VRC that was protected by a cage-like structure formed by rearranged VIM. This enhanced viral replication. Our findings not only shed light on the molecular mechanism of CSFV replication but also offer new insights into the development of antiviral strategies for controlling CSFV.

Lamin Filament Assembly Derived from the Atomic Structure of the Antiparallel Four-Helix Bundle

The nucleoskeletal protein lamin is primarily responsible for the mechanical stability of the nucleus. The lamin assembly process requires the A11, A22, and ACN binding modes of the coiled-coil dimers. Although X-ray crystallography and chemical cross-linking analysis of lamin A/C have provided snapshots of A11 and ACN binding modes, the assembly mechanism of the entire filament remains to be explained. Here, we report a crystal structure of a coil 2 fragment, revealing the A22 interaction at the atomic resolution. The structure showed detailed structural features, indicating that two coiled-coil dimers of the coil 2 subdomain are separated and then re-organized into the antiparallel-four-helix bundle. Furthermore, our findings suggest that the ACN binding mode between coil 1a and the C-terminal part of coil 2 when the A11 tetramers are arranged by the A22 interactions. We propose a full assembly model of lamin A/C with the curvature around the linkers, reconciling the discrepancy between the in situ and in vitro observations. Our model accounts for the balanced elasticity and stiffness of the nuclear envelopes, which is essential in protecting the cellular nucleus from external pressure.

Polarity signaling balances epithelial contractility and mechanical resistance

Epithelia maintain a functional barrier during tissue turnover while facing varying mechanical stress. This maintenance requires both dynamic cell rearrangements driven by actomyosin-linked intercellular adherens junctions and ability to adapt to and resist extrinsic mechanical forces enabled by keratin filament-linked desmosomes. How these two systems crosstalk to coordinate cellular movement and mechanical resilience is not known. Here we show that in stratifying epithelia the polarity protein aPKCλ controls the reorganization from stress fibers to cortical actomyosin during differentiation and upward movement of cells. Without aPKC, stress fibers are retained resulting in increased contractile prestress. This aberrant stress is counterbalanced by reorganization and bundling of keratins, thereby increasing mechanical resilience. Inhibiting contractility in aPKCλ^-/- cells restores normal cortical keratin networks but also normalizes resilience. Consistently, increasing contractile stress is sufficient to induce keratin bundling and enhance resilience, mimicking aPKC loss. In conclusion, our data indicate that keratins sense the contractile stress state of stratified epithelia and balance increased contractility by mounting a protective response to maintain tissue integrity.

CaaX-less lamins: Lophotrochozoa provide a glance at the playground of evolution

Nuclear lamins are the main components of the nuclear lamina in many eukaryotes. They are members of the intermediate filament (IF) protein family. Lamins differ from cytoplasmic IF proteins by the presence of a nuclear localisation sequence (NLS) and a C-terminal tetrapeptide, the CaaX motif. The CaaX motif is target of post-translational modifications including isoprenylation, proteolytic processing, and carboxyl-methylation. These modifications, in conjunction with the NLS, direct lamins to the inner nuclear membrane where they assemble into filaments. Lamins lacking a CaaX motif are unable to associate independently with nuclear membranes and remain in the nucleoplasm. So far, three species have been reported to exclusively express CaaX-less lamins. All three belong to the lophotrochozoan lineage. To find out whether they represent rare exceptions, we analysed lamins of representatives of 17 lophotrochozoan phyla. Here we report that all four clades of Rotifera as well as individual taxa of Mollusca and Annelida lack CaaX-lamins, but express lamins with alternative C-termini. Of note, the respective mollusc and annelid groups occupy very different phylogenetic ranks. Most of these alternative C-termini are rich in aromatic residues. A possible function of these residues in membrane association is discussed. Alternative splicing of terebellid lamin transcripts gives rise to two lamin variants, one with a CaaX motif and one with an alternative C-terminus. A similar situation is found in Arenicolidae, Opheliidae, Capitellidae, and Echiura. This points a way, how the switch from lamins carrying a CaaX motif to lamins with alternative C-termini may have occurred.

Small GTPase Cdc42, WASP, and scaffold proteins for higher-order assembly of the F-BAR domain protein

The higher-order assembly of Bin-amphiphysin-Rvs (BAR) domain proteins, including the FCH-BAR (F-BAR) domain proteins, into lattice on the membrane is essential for the formation of subcellular structures. However, the regulation of their ordered assembly has not been elucidated. Here, we show that the higher ordered assembly of growth-arrested specific 7 (GAS7), an F-BAR domain protein, is regulated by the multivalent scaffold proteins of Wiskott-Aldrich syndrome protein (WASP)/neural WASP, that commonly binds to the BAR domain superfamily proteins, together with WISH, Nck, the activated small guanosine triphosphatase Cdc42, and a membrane-anchored phagocytic receptor. The assembly kinetics by fluorescence resonance energy transfer monitoring indicated that the GAS7 assembly on liposomes started within seconds and was further increased by the presence of these proteins. The regulated GAS7 assembly was abolished by Wiskott-Aldrich syndrome mutations both in vitro and in cellular phagocytosis. Therefore, Cdc42 and the scaffold proteins that commonly bind to the BAR domain superfamily proteins promoted GAS7 assembly.

A Functional Yeast-Based Screen Identifies the Host Microtubule Cytoskeleton as a Target of Numerous Chlamydia pneumoniae Proteins

Bacterial pathogens have evolved intricate ways to manipulate the host to support infection. Here, we systematically assessed the importance of the microtubule cytoskeleton for infection by Chlamydiae, which are obligate intracellular bacteria that are of great importance for human health. The elimination of microtubules in human HEp-2 cells prior to C. pneumoniae infection profoundly attenuated the infection efficiency, demonstrating the need for microtubules for the early infection processes. To identify microtubule-modulating C. pneumoniae proteins, a screen in the model yeast Schizosaccharomyces pombe was performed. Unexpectedly, among 116 selected chlamydial proteins, more than 10%, namely, 13 proteins, massively altered the yeast interphase microtubule cytoskeleton. With two exceptions, these proteins were predicted to be inclusion membrane proteins. As proof of principle, we selected the conserved CPn0443 protein, which caused massive microtubule instability in yeast, for further analysis. CPn0443 bound and bundled microtubules in vitro and co-localized partially with microtubules in vivo in yeast and human cells. Furthermore, CPn0443-transfected U2OS cells had a significantly reduced infection rate by C. pneumoniae EBs. Thus, our yeast screen identified numerous proteins encoded using the highly reduced C. pneumoniae genome that modulated microtubule dynamics. Hijacking of the host microtubule cytoskeleton must be a vital part of chlamydial infection.

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

Biomolecular condensation involving the cytoskeleton

Biomolecular condensation of proteins contributes to the organization of the cytoplasm and nucleoplasm. A number of condensation processes appear to be directly involved in regulating the structure, function and dynamics of the cytoskeleton. Liquid-liquid phase separation of cytoskeleton proteins, together with polymerization modulators, promotes cytoskeletal fiber nucleation and branching. Furthermore, the attachment of protein condensates to the cytoskeleton can contribute to cytoskeleton stability and organization, regulate transport, create patterns of functional reaction containers, and connect the cytoskeleton with membranes. Surface-bound condensates can exert and buffer mechanical forces that give stability and flexibility to the cytoskeleton, thus, may play a large role in cell biology. In this review, we introduce the concept and role of cellular biomolecular condensation, explain its special function on cytoskeletal fiber surfaces, and point out potential definition and experimental caveats. We review the current literature on protein condensation processes related to the actin, tubulin, and intermediate filament cytoskeleton, and discuss some of them in the context of neurobiology. In summary, we provide an overview about biomolecular condensation in relation to cytoskeleton structure and function, which offers a base for the exploration and interpretation of cytoskeletal condensates in neurobiology.

O-GlcNAcylation regulates neurofilament-light assembly and function and is perturbed by Charcot-Marie-Tooth disease mutations

The neurofilament (NF) cytoskeleton is critical for neuronal morphology and function. In particular, the neurofilament-light (NF-L) subunit is required for NF assembly in vivo and is mutated in subtypes of Charcot-Marie-Tooth (CMT) disease. NFs are highly dynamic, and the regulation of NF assembly state is incompletely understood. Here, we demonstrate that human NF-L is modified in a nutrient-sensitive manner by O-linked-β-N-acetylglucosamine (O-GlcNAc), a ubiquitous form of intracellular glycosylation. We identify five NF-L O-GlcNAc sites and show that they regulate NF assembly state. Interestingly, NF-L engages in O-GlcNAc-mediated protein-protein interactions with itself and with the NF component α-internexin, implying that O-GlcNAc is a general regulator of NF architecture. We further show that NF-L O-GlcNAcylation is required for normal organelle trafficking in primary neurons, underlining its functional significance. Finally, several CMT-causative NF-L mutants exhibit perturbed O-GlcNAc levels and resist the effects of O-GlcNAcylation on NF assembly state, indicating a potential link between dysregulated O-GlcNAcylation and pathological NF aggregation. Our results demonstrate that site-specific glycosylation regulates NF-L assembly and function, and aberrant NF O-GlcNAcylation may contribute to CMT and other neurodegenerative disorders.

Anticancer activity of glycoalkaloids from Solanum plants: A review

Cancer is still one of the main causes of death worldwide. For this reason, new compounds that have chemotherapeutic potential have been identified. One such group of substances is Solanaceae glycoalkaloids (GAs). They are natural compounds produced by plants widely used in traditional medicine for healing many disorders. Among others, GAs exhibit significant antitumor properties, for example, a strong inhibitory effect on cancer cell growth. This activity can result in the induction of tumor cell apoptosis, which can occur via different molecular pathways. The molecular mechanisms of the action of GAs are the subject of intensive research, as improved understanding could lead to the development of new cancer therapies. The genetic basis for the formation of neoplasms are mutations in protooncogenes, suppressors, and apoptosis-controlling and repair genes; therefore, substances with antineoplastic properties may affect the levels of their expression or the levels of their expression products. Therapeutic compounds can be applied separately or in combination with other drugs to increase the efficiency of cancer therapy; they can act on the cell through various mechanisms at different stages of carcinogenesis, inducing the process of apoptosis, blocking cell proliferation and migration, and inhibiting angiogenesis. This review summarizes the newest studies on the anticancer properties of solanine (SN), chaconine (CH), solasonine (SS), solamargine (SM), tomatine (TT) and their extracts from Solanum plants.

The N-Terminal Part of the 1A Domain of Desmin Is a Hot Spot Region for Putative Pathogenic DES Mutations Affecting Filament Assembly

Desmin is the major intermediate filament protein of all three muscle cell types, and connects different cell organelles and multi-protein complexes such as the cardiac desmosomes. Several pathogenic mutations in the DES gene cause different skeletal and cardiac myopathies. However, the significance of the majority of DES missense variants is currently unknown, since functional data are lacking. To determine whether desmin missense mutations within the highly conserved 1A coil domain cause a filament assembly defect, we generated a set of variants with unknown significance and systematically analyzed the filament assembly using confocal microscopy in transfected SW-13, H9c2 cells and cardiomyocytes derived from induced pluripotent stem cells. We found that mutations in the N-terminal part of the 1A coil domain affect filament assembly, leading to cytoplasmic desmin aggregation. In contrast, mutant desmin in the C-terminal part of the 1A coil domain forms filamentous structures comparable to wild-type desmin. Our findings suggest that the N-terminal part of the 1A coil domain is a hot spot for pathogenic desmin mutations, which affect desmin filament assembly. This study may have relevance for the genetic counselling of patients carrying variants in the 1A coil domain of the DES gene.