LIM domain proteins in cell mechanobiology

Cracked actin filaments as mechanosensitive receptors

Actin filament networks are exposed to mechanical stimuli, but the effect of strain on actin filament structure has not been well established in molecular detail. This is a critical gap in understanding because the activity of a variety of actin-binding proteins has recently been determined to be altered by actin filament strain. We therefore used all-atom molecular dynamics simulations to apply tensile strains to actin filaments and find that changes in actin subunit organization are minimal in mechanically strained, but intact, actin filaments. However, a conformational change disrupts the critical D-loop to W-loop connection between longitudinal neighboring subunits, which leads to a metastable cracked conformation of the actin filament whereby one protofilament is broken prior to filament severing. We propose that the metastable crack presents a force-activated binding site for actin regulatory factors that specifically associate with strained actin filaments. Through protein-protein docking simulations, we find that 43 evolutionarily diverse members of the dual zinc-finger-containing LIM-domain family, which localize to mechanically strained actin filaments, recognize two binding sites exposed at the cracked interface. Furthermore, through its interactions with the crack, LIM domains increase the length of time damaged filaments remain stable. Our findings propose a new molecular model for mechanosensitive binding to actin filaments.

Autoinhibition and relief mechanisms for MICAL monooxygenases in F-actin disassembly

MICAL proteins represent a unique family of actin regulators crucial for synapse development, membrane trafficking, and cytokinesis. Unlike classical actin regulators, MICALs catalyze the oxidation of specific residues within actin filaments to induce robust filament disassembly. The potent activity of MICALs requires tight control to prevent extensive damage to actin cytoskeleton. However, the molecular mechanism governing MICALs' activity regulation remains elusive. Here, we report the cryo-EM structure of MICAL1 in the autoinhibited state, unveiling a head-to-tail interaction that allosterically blocks enzymatic activity. The structure also reveals the assembly of C-terminal domains via a tripartite interdomain interaction, stabilizing the inhibitory conformation of the RBD. Our structural, biochemical, and cellular analyses elucidate a multi-step mechanism to relieve MICAL1 autoinhibition in response to the dual-binding of two Rab effectors, revealing its intricate activity regulation mechanisms. Furthermore, our mutagenesis study of MICAL3 suggests the conserved autoinhibition and relief mechanisms among MICALs.

Molecular simulation approaches to probing the effects of mechanical forces in the actin cytoskeleton

In this article we give our perspective on the successes and promise of various molecular and coarse-grained simulation approaches to probing the effect of mechanical forces in the actin cytoskeleton.

The roles of FHL2 as a mechanotransducer for cellular functions in the mechanical environment

The cell has multiple mechanisms for sensing and responding to dynamic changes in the mechanical environment. In the process, intracellular signaling is activated to modulate gene expression. Recent studies have shown that multifunctional signaling molecules that link intracellular force and gene expression are important for understanding cellular functions in the mechanical environment. This review discusses recent studies on one of the mechanotransducers, Four-and-a-half LIM domains 2 (FHL2), which localizes to focal adhesions (FAs), actin cytoskeleton, and nucleus. FHL2 localizes to FAs and the actin cytoskeleton in the cell on stiff substrate. In this situation, intracellular tension of F-actin by Myosin II is critical for FHL2 localization to FAs and actin stress fibers. In the case, a conserved phenylalanine in each LIM domain is responsible for its localization to F-actin. On the other hand, lower tension of F-actin in the cell on a soft substrate causes FHL2 to be released into the cytoplasm, resulting in its localization in the nucleus. At the molecular level, phosphorylation of specific tyrosine in FHL2 by FAK, non-receptor tyrosine kinase, is critical to nuclear localization. Finally, by binding to transcription factors, FHL2 modulates gene expression for cell proliferation as a transcriptional co-factor. Thus, FHL2 is involved in mechano-sensing and -transduction in the cell in a mechanical environment.

Mechanosensitive FHL2 tunes endothelial function

Endothelial tissues are essential mechanosensors in the vasculature and facilitate adaptation to various blood flow-induced mechanical cues. Defects in endothelial mechanoresponses can perturb tissue remodelling and functions leading to cardiovascular disease progression. In this context, the precise mechanisms of endothelial mechanoresponses contributing to normal and diseased tissue functioning remain elusive. Here, we sought to uncover how flow-mediated transcriptional regulation drives endothelial mechanoresponses in healthy and atherosclerotic-prone tissues. Using bulk RNA sequencing, we identify novel mechanosensitive genes in response to healthy unidirectional flow (UF) and athero-prone disturbed flow (DF). We find that the transcription as well as protein expression of Four-and-a-half LIM protein 2 (FHL2) are enriched in athero-prone DF both in vitro and in vivo. We then demonstrate that the exogenous expression of FHL2 is necessary and sufficient to drive discontinuous adherens junction morphology and increased tissue permeability. This athero-prone phenotype requires the force-sensitive binding of FHL2 to actin. In turn, the force-dependent localisation of FHL2 to stress fibres promotes microtubule dynamics to release the RhoGEF, GEF-H1, and activate the Rho-ROCK pathway. Thus, we unravelled a novel mechanochemical feedback wherein force-dependent FHL2 localisation promotes hypercontractility. This misregulated mechanoresponse creates highly permeable tissues, depicting classic hallmarks of atherosclerosis progression. Overall, we highlight crucial functions for the FHL2 force-sensitivity in tuning multi-scale endothelial mechanoresponses.

DNA Electrotransfer Regulates Molecular Functions in Skeletal Muscle

Background

Tissues, such as skeletal muscle, have been targeted for the delivery of plasmid DNA (pDNA) encoding vaccines and therapeutics. The application of electric pulses (electroporation or electrotransfer) increases cell membrane permeability to enhance plasmid delivery and expression. However, the molecular effects of DNA electrotransfer on the muscle tissue are poorly characterized.

Materials and Methods

Four hours after intramuscular plasmid electrotransfer, we evaluated gene expression changes by RNA sequencing. Differentially expressed genes were analyzed by gene ontology (GO) pathway enrichment analysis.

Results

GO analysis highlighted many enriched molecular functions. The terms regulated by pulse application were related to muscle stress, the cytoskeleton and inflammation. The terms regulated by pDNA injection were related to a DNA-directed response and its control. Several terms regulated by pDNA electrotransfer were similar to those regulated by pulse application. However, the terms related to pDNA injection differed, focusing on entry of the plasmid into the cells and intracellular trafficking.

Conclusion

Each muscle stimulus resulted in specific regulated molecular functions. Identifying the unique intrinsic molecular changes driven by intramuscular DNA electrotransfer will aid in the design of preventative and therapeutic gene therapies.

Arp2/3 complex- and formin-mediated actin cytoskeleton networks facilitate actin binding protein sorting in fission yeast

While it is well-established that F-actin networks with specific organizations and dynamics are tightly regulated by distinct sets of associated actin-binding proteins (ABPs), how ABPs self-sort to particular F-actin networks remains largely unclear. We report that actin assembly factors Arp2/3 complex and formin Cdc12 tune the association of ABPs fimbrin Fim1 and tropomyosin Cdc8 to different F-actin networks in fission yeast. Genetic and pharmacological disruption of F-actin networks revealed that Fim1 is preferentially directed to Arp2/3-complex mediated actin patches, whereas Cdc8 is preferentially targeted to formin Cdc12-mediated filaments in the contractile ring. To investigate the role of Arp2/3 complex- and formin Cdc12-mediated actin assembly, we used four-color TIRF microscopy to observe the in vitro reconstitution of ABP sorting with purified proteins. Fim1 or Cdc8 alone bind similarly well to filaments assembled by either assembly factor. However, in 'competition' reactions containing both actin assembly factors and both ABPs, ∼2.0-fold more Fim1 and ∼3.5-fold more Cdc8 accumulates on Arp2/3 complex branch points and formin Cdc12-assembled actin filaments, respectively. These findings indicate that F-actin assembly factors Arp2/3 complex and formin Cdc12 help facilitate the recruitment of specific ABPs, thereby tuning ABP sorting and subsequently establishing the identity of F-actin networks in fission yeast.

The role of prickle proteins in vertebrate development and pathology

Prickle is an evolutionarily conserved family of proteins exclusively associated with planar cell polarity (PCP) signalling. This signalling pathway provides directional and positional cues to eukaryotic cells along the plane of an epithelial sheet, orthogonal to both apicobasal and left-right axes. Through studies in the fruit fly Drosophila, we have learned that PCP signalling is manifested by the spatial segregation of two protein complexes, namely Prickle/Vangl and Frizzled/Dishevelled. While Vangl, Frizzled, and Dishevelled proteins have been extensively studied, Prickle has been largely neglected. This is likely because its role in vertebrate development and pathologies is still being explored and is not yet fully understood. The current review aims to address this gap by summarizing our current knowledge on vertebrate Prickle proteins and to cover their broad versatility. Accumulating evidence suggests that Prickle is involved in many developmental events, contributes to homeostasis, and can cause diseases when its expression and signalling properties are deregulated. This review highlights the importance of Prickle in vertebrate development, discusses the implications of Prickle-dependent signalling in pathology, and points out the blind spots or potential links regarding Prickle, which could be studied further.

Characterization of early and late events of adherens junction assembly

Cadherins are transmembrane adhesion receptors. Cadherin ectodomains form adhesive 2D clusters through cooperative trans and cis interactions, whereas its intracellular region interacts with specific cytosolic proteins, termed catenins, to anchor the cadherin-catenin complex (CCC) to the actin cytoskeleton. How these two types of interactions are coordinated in the formation of specialized cell-cell adhesions, adherens junctions (AJ), remains unclear. We focus here on the role of the actin-binding domain of α-catenin (αABD) by showing that the interaction of αABD with actin generates actin-bound CCC oligomers (CCC/actin strands) incorporating up to six CCCs. The strands are primarily formed on the actin-rich cell protrusions. Once in cell-cell interface, the strands become involved in cadherin ectodomain clustering. Such combination of the extracellular and intracellular oligomerizations gives rise to the composite oligomers, trans CCC/actin clusters. To mature, these clusters then rearrange their actin filaments using several redundant pathways, two of which are characterized here: one depends on the α-catenin-associated protein, vinculin and the second one depends on the unstructured C-terminus of αABD. Thus, AJ assembly proceeds through spontaneous formation of trans CCC/actin clusters and their successive reorganization.

The PDLIM family of actin-associated proteins and their emerging role in membrane trafficking

The PDZ and LIM domain (PDLIM) proteins are associated with the actin cytoskeleton and have conserved in roles in metazoan actin organisation and function. They primarily function as scaffolds linking various proteins to actin and its binding partner α-actinin via two conserved domains; an N-terminal postsynaptic density 95, discs large and zonula occludens-1 (PDZ) domain, and either single or multiple C-terminal LIN-11, Isl-1 and MEC-3 (LIM) domains in the actinin-associated LIM protein (ALP)- and Enigma-related proteins, respectively. While their role in actin organisation, such as in stress fibres or in the Z-disc of muscle fibres is well known, emerging evidence also suggests a role in actin-dependent membrane trafficking in the endosomal system. This is mediated by a recently identified interaction with the sorting nexin 17 (SNX17) protein, an adaptor for the trafficking complex Commander which is itself intimately linked to actin-directed formation of endosomal recycling domains. In this review we focus on the currently understood structural basis for PDLIM function. The PDZ domains mediate direct binding to distinct classes of PDZ-binding motifs (PDZbms), including α-actinin and other actin-associated proteins, and a highly specific interaction with the type III PDZbm such as the one found in the C-terminus of SNX17. The structures of the LIM domains are less well characterised and how they engage with their ligands is completely unknown. Despite the lack of experimental structural data, we find that recently developed machine learning-based structure prediction methods provide insights into their potential interactions and provide a template for further studies of their molecular functions.

Actin-dependent recruitment of AGO2 to the zonula adherens

Adherens junctions are cadherin-based structures critical for cellular architecture. E-cadherin junctions in mature epithelial cell monolayers tether to an apical actomyosin ring to form the zonula adherens (ZA). We have previously shown that the adherens junction protein PLEKHA7 associates with and regulates the function of the core RNA interference (RNAi) component AGO2 specifically at the ZA. However, the mechanism mediating AGO2 recruitment to the ZA remained unexplored. Here, we reveal that this ZA-specific recruitment of AGO2 depends on both the structural and tensile integrity of the actomyosin cytoskeleton. We found that depletion of not only PLEKHA7, but also either of the three PLEKHA7-interacting, LIM-domain family proteins, namely LMO7, LIMCH1, and PDLIM1, results in disruption of actomyosin organization and tension, as well as disruption of AGO2 junctional localization and of its miRNA-binding ability. We also show that AGO2 binds Myosin IIB and that PLEKHA7, LMO7, LIMCH1, and PDLIM1 all disrupt interaction of AGO2 with Myosin IIB at the ZA. These results demonstrate that recruitment of AGO2 to the ZA is sensitive to actomyosin perturbations, introducing the concept of mechanosensitive RNAi machinery, with potential implications in tissue remodeling and in disease.

The PMA phorbol ester tumor promoter increases canonical Wnt signaling via macropinocytosis

Activation of the Wnt pathway lies at the core of many human cancers. Wnt and macropinocytosis are often active in the same processes, and understanding how Wnt signaling and membrane trafficking cooperate should improve our understanding of embryonic development and cancer. Here, we show that a macropinocytosis activator, the tumor promoter phorbol 12-myristate 13-acetate (PMA), enhances Wnt signaling. Experiments using the Xenopus embryo as an in vivo model showed marked cooperation between the PMA phorbol ester and Wnt signaling, which was blocked by inhibitors of macropinocytosis, Rac1 activity, and lysosome acidification. Human colorectal cancer tissue arrays and xenografts in mice showed a correlation of cancer progression with increased macropinocytosis/multivesicular body/lysosome markers and decreased GSK3 levels. The crosstalk between canonical Wnt, focal adhesions, lysosomes, and macropinocytosis suggests possible therapeutic targets for cancer progression in Wnt-driven cancers.

PDZ and LIM Domain-Encoding Genes: Their Role in Cancer Development

PDZ-LIM family proteins (PDLIMs) are a kind of scaffolding proteins that contain PDZ and LIM interaction domains. As protein-protein interacting molecules, PDZ and LIM domains function as scaffolds to bind to a variety of proteins. The PDLIMs are composed of evolutionarily conserved proteins found throughout different species. They can participate in cell signal transduction by mediating the interaction of signal molecules. They are involved in many important physiological processes, such as cell differentiation, proliferation, migration, and the maintenance of cellular structural integrity. Studies have shown that dysregulation of the PDLIMs leads to tumor formation and development. In this paper, we review and integrate the current knowledge on PDLIMs. The structure and function of the PDZ and LIM structural domains and the role of the PDLIMs in tumor development are described.

The PMA Phorbol Ester Tumor Promoter Increases Canonical Wnt Signaling Via Macropinocytosis

Activation of the Wnt pathway lies at the core of many human cancers. Wnt and macropinocytosis are often active in the same processes, and understanding how Wnt signaling and membrane trafficking cooperate should improve our understanding of embryonic development and cancer. Here we show that a macropinocytosis activator, the tumor promoter Phorbol 12-myristate 13-acetate (PMA), enhances Wnt signaling. Experiments using the Xenopus embryo as an in vivo model showed marked cooperation between the PMA phorbol ester and Wnt signaling, which was blocked by inhibitors of macropinocytosis, Rac1 activity, and lysosome acidification. Human colorectal cancer tissue arrays and xenografts in mice showed a correlation of cancer progression with increased macropinocytosis/multivesicular body/lysosome markers and decreased GSK3 levels. The crosstalk between canonical Wnt, focal adhesions, lysosomes, and macropinocytosis suggests possible therapeutic targets for cancer progression in Wnt-driven cancers.

The Impact of ETV6-NTRK3 Oncogenic Gene Fusions on Molecular and Signaling Pathway Alterations

Chromosomal translocations creating fusion genes are common cancer drivers. The oncogenic ETV6-NTRK3 (EN) gene fusion joins the sterile alpha domain of the ETV6 transcription factor with the tyrosine kinase domain of the neurotrophin-3 receptor NTRK3. Four EN variants with alternating break points have since been detected in a wide range of human cancers. To provide molecular level insight into EN oncogenesis, we employed a proximity labeling mass spectrometry approach to define the molecular context of the fusions. We identify in total 237 high-confidence interactors, which link EN fusions to several key signaling pathways, including ERBB, insulin and JAK/STAT. We then assessed the effects of EN variants on these pathways, and showed that the pan NTRK inhibitor Selitrectinib (LOXO-195) inhibits the oncogenic activity of EN2, the most common variant. This systems-level analysis defines the molecular framework in which EN oncofusions operate to promote cancer and provides some mechanisms for therapeutics.

Cracked actin filaments as mechanosensitive receptors

Actin filament networks are exposed to mechanical stimuli, but the effect of strain on actin filament structure has not been well-established in molecular detail. This is a critical gap in understanding because the activity of a variety of actin-binding proteins have recently been determined to be altered by actin filament strain. We therefore used all-atom molecular dynamics simulations to apply tensile strains to actin filaments and find that changes in actin subunit organization are minimal in mechanically strained, but intact, actin filaments. However, a conformational change disrupts the critical D-loop to W-loop connection between longitudinal neighboring subunits, which leads to a metastable cracked conformation of the actin filament, whereby one protofilament is broken prior to filament severing. We propose that the metastable crack presents a force-activated binding site for actin regulatory factors that specifically associate with strained actin filaments. Through protein-protein docking simulations, we find that 43 evolutionarily-diverse members of the dual zinc finger containing LIM domain family, which localize to mechanically strained actin filaments, recognize two binding sites exposed at the cracked interface. Furthermore, through its interactions with the crack, LIM domains increase the length of time damaged filaments remain stable. Our findings propose a new molecular model for mechanosensitive binding to actin filaments.

Live Cells Imaging and Comparative Phosphoproteomics Uncover Proteins from the Mechanobiome in Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite and the causative agent of amoebiasis in humans. This amoeba invades human tissues by taking advantage of its actin-rich cytoskeleton to move, enter the tissue matrix, kill and phagocyte the human cells. During tissue invasion, E. histolytica moves from the intestinal lumen across the mucus layer and enters the epithelial parenchyma. Faced with the chemical and physical constraints of these diverse environments, E. histolytica has developed sophisticated systems to integrate internal and external signals and to coordinate cell shape changes and motility. Cell signalling circuits are driven by interactions between the parasite and extracellular matrix, combined with rapid responses from the mechanobiome in which protein phosphorylation plays an important role. To understand the role of phosphorylation events and related signalling mechanisms, we targeted phosphatidylinositol 3-kinases followed by live cell imaging and phosphoproteomics. The results highlight 1150 proteins, out of the 7966 proteins within the amoebic proteome, as members of the phosphoproteome, including signalling and structural molecules involved in cytoskeletal activities. Inhibition of phosphatidylinositol 3-kinases alters phosphorylation in important members of these categories; a finding that correlates with changes in amoeba motility and morphology, as well as a decrease in actin-rich adhesive structures.

Adherens junction: the ensemble of specialized cadherin clusters

The cell-cell connections in adherens junctions (AJs) are mediated by transmembrane receptors, type I cadherins (referred to here as cadherins). These cadherin-based connections (or trans bonds) are weak. To upregulate their strength, cadherins exploit avidity, the increased affinity of binding between cadherin clusters compared with isolated monomers. Formation of such clusters is a unique molecular process that is driven by a synergy of direct and indirect cis interactions between cadherins located at the same cell. In addition to their role in adhesion, cadherin clusters provide structural scaffolds for cytosolic proteins, which implicate cadherin into different cellular activities and signaling pathways. The cluster lifetime, which depends on the actin cytoskeleton, and on the mechanical forces it generates, determines the strength of AJs and their plasticity. The key aspects of cadherin adhesion, therefore, cannot be understood at the level of isolated cadherin molecules, but should be discussed in the context of cadherin clusters.

Biochemical and mechanical regulation of actin dynamics

Polymerization of actin filaments against membranes produces force for numerous cellular processes, such as migration, morphogenesis, endocytosis, phagocytosis and organelle dynamics. Consequently, aberrant actin cytoskeleton dynamics are linked to various diseases, including cancer, as well as immunological and neurological disorders. Understanding how actin filaments generate forces in cells, how force production is regulated by the interplay between actin-binding proteins and how the actin-regulatory machinery responds to mechanical load are at the heart of many cellular, developmental and pathological processes. During the past few years, our understanding of the mechanisms controlling actin filament assembly and disassembly has evolved substantially. It has also become evident that the activities of key actin-binding proteins are not regulated solely by biochemical signalling pathways, as mechanical regulation is critical for these proteins. Indeed, the architecture and dynamics of the actin cytoskeleton are directly tuned by mechanical load. Here we discuss the general mechanisms by which key actin regulators, often in synergy with each other, control actin filament assembly, disassembly, and monomer recycling. By using an updated view of actin dynamics as a framework, we discuss how the mechanics and geometry of actin networks control actin-binding proteins, and how this translates into force production in endocytosis and mesenchymal cell migration.

Expression of the phagocytic receptors αMβ2 and αXβ2 is controlled by RIAM, VASP and Vinculin in neutrophil-differentiated HL-60 cells

Activation of the integrin phagocytic receptors CR3 (α_Mβ₂, CD11b/CD18) and CR4 (α_Xβ₂, CD11c/CD18) requires Rap1 activation and RIAM function. RIAM controls integrin activation by recruiting Talin to β₂ subunits, enabling the Talin-Vinculin interaction, which in term bridges integrins to the actin-cytoskeleton. RIAM also recruits VASP to phagocytic cups and facilitates VASP phosphorylation and function promoting particle internalization. Using a CRISPR-Cas9 knockout approach, we have analyzed the requirement for RIAM, VASP and Vinculin expression in neutrophilic-HL-60 cells. All knockout cells displayed abolished phagocytosis that was accompanied by a significant and specific reduction in ITGAM (α_M), ITGAX (α_X) and ITGB2 (β₂) mRNA, as revealed by RT-qPCR. RIAM, VASP and Vinculin KOs presented reduced cellular F-actin content that correlated with αM expression, as treatment with the actin filament polymerizing and stabilizing drug jasplakinolide, partially restored α_M expression. In general, the expression of α_X was less responsive to jasplakinolide treatment than α_M, indicating that regulatory mechanisms independent of F-actin content may be involved. The Serum Response Factor (SRF) was investigated as the potential transcription factor controlling α_Mβ₂ expression, since its coactivator MRTF-A requires actin polymerization to induce transcription. Immunofluorescent MRTF-A localization in parental cells was primarily nuclear, while in knockouts it exhibited a diffuse cytoplasmic pattern. Localization of FHL-2 (SRF corepressor) was mainly sub-membranous in parental HL-60 cells, but in knockouts the localization was disperse in the cytoplasm and the nucleus, suggesting RIAM, VASP and Vinculin are required to maintain FHL-2 close to cytoplasmic membranes, reducing its nuclear localization and inhibiting its corepressor activity. Finally, reexpression of VASP in the VASP knockout resulted in a complete reversion of the phenotype, as knock-ins restored α_M expression. Taken together, our results suggest that RIAM, VASP and Vinculin, are necessary for the correct expression of α_Mβ₂ and α_Xβ₂ during neutrophilic differentiation in the human promyelocytic HL-60 cell line, and strongly point to an involvement of these proteins in the acquisition of a phagocytic phenotype.

Phosphorylation of the small heat shock protein HspB1 regulates cytoskeletal recruitment and cell motility

The small heat shock protein HspB1, also known as Hsp25/27, is a ubiquitously expressed molecular chaperone that responds to mechanical cues. Uniaxial cyclic stretch activates the p38 mitogen-activated protein kinase (MAPK) signaling cascade and increases the phosphorylation of HspB1. Similar to the mechanosensitive cytoskeletal regulator zyxin, phospho-HspB1 is recruited to features of the stretch-stimulated actin cytoskeleton. To evaluate the role of HspB1 and its phosphoregulation in modulating cell function, we utilized CRISPR/Cas9-edited HspB1-null cells and determined they were altered in behaviors such as actin cytoskeletal remodeling, cell spreading, and cell motility. In our model system, expression of WT HspB1, but not nonphosphorylatable HspB1, rescued certain characteristics of the HspB1-null cells including the enhanced cell motility of HspB1-null cells and the deficient actin reinforcement of stretch-stimulated HspB1-null cells. The recruitment of HspB1 to high-tension structures in geometrically constrained cells, such as actin comet tails emanating from focal adhesions, also required a phosphorylatable HspB1. We show that mechanical signals activate posttranslational regulation of the molecular chaperone, HspB1, and are required for normal cell behaviors including actin cytoskeletal remodeling, cell spreading, and cell migration.

Role of Muscle LIM Protein in Mechanotransduction Process

The induction of protein synthesis is crucial to counteract the deconditioning of neuromuscular system and its atrophy. In the past, hormones and cytokines acting as growth factors involved in the intracellular events of these processes have been identified, while the implications of signaling pathways associated with the anabolism/catabolism ratio in reference to the molecular mechanism of skeletal muscle hypertrophy have been recently identified. Among them, the mechanotransduction resulting from a mechanical stress applied to the cell appears increasingly interesting as a potential pathway for therapeutic intervention. At present, there is an open question regarding the type of stress to apply in order to induce anabolic events or the type of mechanical strain with respect to the possible mechanosensing and mechanotransduction processes involved in muscle cells protein synthesis. This review is focused on the muscle LIM protein (MLP), a structural and mechanosensing protein with a LIM domain, which is expressed in the sarcomere and costamere of striated muscle cells. It acts as a transcriptional cofactor during cell proliferation after its nuclear translocation during the anabolic process of differentiation and rebuilding. Moreover, we discuss the possible opportunity of stimulating this mechanotransduction process to counteract the muscle atrophy induced by anabolic versus catabolic disorders coming from the environment, aging or myopathies.

Analysis of the Drosophila Ajuba LIM protein defines functions for distinct LIM domains

The Ajuba LIM protein Jub mediates regulation of Hippo signaling by cytoskeletal tension through interaction with the kinase Warts and participates in feedback regulation of junctional tension through regulation of the cytohesin Steppke. To investigate how Jub interacts with and regulates its distinct partners, we investigated the ability of Jub proteins missing different combinations of its three LIM domains to rescue jub phenotypes and to interact with α-catenin, Warts and Steppke. Multiple regions of Jub contribute to its ability to bind α-catenin and to localize to adherens junctions in Drosophila wing imaginal discs. Co-immunoprecipitation experiments in cultured cells identified a specific requirement for LIM2 for binding to Warts. However, in vivo, both LIM1 and LIM2, but not LIM3, were required for regulation of wing growth, Yorkie activity, and Warts localization. Conversely, LIM2 and LIM3, but not LIM1, were required for regulation of cell shape and Steppke localization in vivo, and for maximal Steppke binding in co-immunoprecipitation experiments. These observations identify distinct functions for the different LIM domains of Jub.

Bioinformatics Profiling and Experimental Validation of 4 Differentially-Expressed LIM Genes in the Course of Colorectal-Adenoma-Carcinoma

BACKGROUND LIM domain proteins play crucial roles in tumors by interacting with diverse proteins. However, their roles in the course of colorectal mucosa-adenoma-carcinoma remain unclear. This study aimed to depict their dynamic expression profiles and elucidate their potential functions in this transition course. MATERIAL AND METHODS Differentially-expressed LIM proteins (DELGs) in paired adenomas, carcinomas, and mucosae were identified using the GEO dataset (GSE 117606) and validated by immunohistochemistry using our tissue microarray. Kaplan-Meier survival analysis, WGCNA, module-trait analysis, and KEGG enrichment were conducted. The correlation of DELGs expression levels with immune infiltration was assessed using the ESTIMATE package and TISCH database. The role of DELGs of interest was validated using cell proliferation, migration, and invasion assays. RESULTS Four DELGs were identified - LMO3, FHL1, NEBL, and TGFB1I1 - all of which were of significance in prognosis. Module-trait correlation and KEGG enrichment revealed their involvement in cancer-related signaling. Immunohistochemistry showed gradual downregulation of LMO3 but upregulation of NEBL in the mucosa-adenoma-carcinoma sequence. The opposite expression patterns were observed for FHL1 and TGFB1I1 in tumor epithelium and mesenchyme. High expression levels of the DELGs were correlated with increased infiltration of NK, NKT, and macrophages, except for NEBL. Importantly, LMO3 inhibited proliferation, migration, and invasion of colon epithelial cells. CONCLUSIONS This study identified 4 differentially-expressed LIM genes - LMO3, FHL1, TGFB1I1, and NEBL - and revealed they were involved in the mucosa-adenoma-carcinoma sequence via regulating cancer-related pathways, influencing epigenetic field, or affecting immune infiltration. Our findings provide new insights into the roles of LIM proteins in the course of mucosa-adenoma-carcinoma.

Cellular force-sensing through actin filaments

The actin cytoskeleton orchestrates cell mechanics and facilitates the physical integration of cells into tissues, while tissue-scale forces and extracellular rigidity in turn govern cell behaviour. Here, we discuss recent evidence that actin filaments (F-actin), the core building blocks of the actin cytoskeleton, also serve as molecular force sensors. We delineate two classes of proteins, which interpret forces applied to F-actin through enhanced binding interactions: 'mechanically tuned' canonical actin-binding proteins, whose constitutive F-actin affinity is increased by force, and 'mechanically switched' proteins, which bind F-actin only in the presence of force. We speculate mechanically tuned and mechanically switched actin-binding proteins are biophysically suitable for coordinating cytoskeletal force-feedback and mechanical signalling processes, respectively. Finally, we discuss potential mechanisms mediating force-activated actin binding, which likely occurs both through the structural remodelling of F-actin itself and geometric rearrangements of higher-order actin networks. Understanding the interplay of these mechanisms will enable the dissection of force-activated actin binding's specific biological functions.

Powering morphogenesis: multiscale challenges at the interface of cell adhesion and the cytoskeleton

Among the defining features of the animal kingdom is the ability of cells to change shape and move. This underlies embryonic and postembryonic development, tissue homeostasis, regeneration, and wound healing. Cell shape change and motility require linkage of the cell's force-generating machinery to the plasma membrane at cell-cell and cell-extracellular matrix junctions. Connections of the actomyosin cytoskeleton to cell-cell adherens junctions need to be both resilient and dynamic, preventing tissue disruption during the dramatic events of embryonic morphogenesis. In the past decade, new insights radically altered the earlier simple paradigm that suggested simple linear linkage via the cadherin-catenin complex as the molecular mechanism of junction-cytoskeleton interaction. In this Perspective we provide a brief overview of our current state of knowledge and then focus on selected examples highlighting what we view as the major unanswered questions in our field and the approaches that offer exciting new insights at multiple scales from atomic structure to tissue mechanics.

Allosteric regulation controls actin-bundling properties of human plastins

Plastins/fimbrins are conserved actin-bundling proteins contributing to motility, cytokinesis and other cellular processes by organizing strikingly different actin assemblies as in aligned bundles and branched networks. We propose that this ability of human plastins stems from an allosteric communication between their actin-binding domains (ABD1/2) engaged in a tight spatial association. Here we show that ABD2 can bind actin three orders of magnitude stronger than ABD1, unless the domains are involved in an equally strong inhibitory engagement. A mutation mimicking physiologically relevant phosphorylation at the ABD1-ABD2 interface greatly weakened their association, dramatically potentiating actin cross-linking. Cryo-EM reconstruction revealed the ABD1-actin interface and enabled modeling of the plastin bridge and domain separation in parallel bundles. We predict that a strong and tunable allosteric inhibition between the domains allows plastins to modulate the cross-linking strength, contributing to remodeling of actin assemblies of different morphologies defining the unique place of plastins in actin organization.

PINCH1 Promotes Fibroblast Migration in Extracellular Matrices and Influences Their Mechanophenotype

Cell migration performs a critical function in numerous physiological processes, including tissue homeostasis or wound healing after tissue injury, as well as pathological processes that include malignant progression of cancer. The efficiency of cell migration and invasion appears to be based on the mechano-phenotype of the cytoskeleton. The properties of the cytoskeleton depend on internal cytoskeletal and external environmental factors. A reason for this are connections between the cell and its local matrix microenvironment, which are established by cell-matrix adhesion receptors. Upon activation, focal adhesion proteins such as PINCH1 are recruited to sites where focal adhesions form. PINCH1 specifically couples through interactions with ILK, which binds to cell matrix receptors and the actomyosin cytoskeleton. However, the role of PINCH1 in cell mechanics regulating cellular motility in 3D collagen matrices is still unclear. PINCH1 is thought to facilitate 3D motility by regulating cellular mechanical properties, such as stiffness. In this study, PINCH1 wild-type and knock-out cells were examined for their ability to migrate in dense extracellular 3D matrices. Indeed, PINCH1 wild-type cells migrated more numerously and deeper in 3D matrices, compared to knock-out cells. Moreover, cellular deformability was determined, e.g., elastic modulus (stiffness). PINCH1 knock-out cells are more deformable (compliable) than PINCH1 wild-type cells. Migration of both PINCH1−/− cells and PINCH1fl/fl cells was decreased by Latrunculin A inhibition of actin polymerization, suggesting that actin cytoskeletal differences are not responsible for the discrepancy in invasiveness of the two cell types. However, the mechanical phenotype of PINCH1−/− cells may be reflected by Latrunculin A treatment of PINCH1fl/fl cells, as they exhibit resembling deformability to untreated PINCH1−/− cells. Moreover, an apparent mismatch exists between the elongation of the long axis and the contraction of the short axis between PINCH1fl/fl cells and PINCH1−/− cells following Latrunculin A treatment. There is evidence of this indicating a shift in the proxy values for Poisson’s ratio in PINCH1−/− cells compared with PINCH1fl/fl cells. This is probably attributable to modifications in cytoskeletal architecture. The non-muscle myosin II inhibitor Blebbistatin also reduced the cell invasiveness in 3D extracellular matrices but instead caused a stiffening of the cells. Finally, PINCH1 is apparently essential for providing cellular mechanical stiffness through the actin cytoskeleton, which regulates 3D motility.

Impact of Treadmill Interval Running on the Appearance of Zinc Finger Protein FHL2 in Bone Marrow Cells in a Rat Model: A Pilot Study

Although the benefits of physical exercise to preserve bone quality are now widely recognized, the intimate mechanisms leading to the underlying cell responses still require further investigations. Interval training running, for instance, appears as a generator of impacts on the skeleton, and particularly on the progenitor cells located in the bone marrow. Therefore, if this kind of stimulus initiates bone cell proliferation and differentiation, the activation of a devoted signaling pathway by mechano-transduction seems likely. This study aimed at investigating the effects of an interval running program on the appearance of the zinc finger protein FHL2 in bone cells and their anatomical location. Twelve 5-week-old male Wistar rats were randomly allocated to one of the following groups (n = 6 per group): sedentary control (SED) or high-intensity interval running (EX, 8 consecutive weeks). FHL2 identification in bone cells was performed by immuno-histochemistry on serial sections of radii. We hypothesized that impacts generated by running could activate, in vivo, a specific signaling pathway, through an integrin-mediated mechano-transductive process, leading to the synthesis of FHL2 in bone marrow cells. Our data demonstrated the systematic appearance of FHL2 (% labeled cells: 7.5%, p < 0.001) in bone marrow obtained from EX rats, whereas no FHL2 was revealed in SED rats. These results suggest that the mechanical impacts generated during high-intensity interval running activate a signaling pathway involving nuclear FHL2, such as that also observed with dexamethasone administration. Consequently, interval running could be proposed as a non-pharmacological strategy to contribute to bone marrow cell osteogenic differentiation.

Endothelial Mechanosensors for Atheroprone and Atheroprotective Shear Stress Signals

Vascular endothelial cells (ECs), derived from the mesoderm, form a single layer of squamous cells that covers the inner surface of blood vessels. In addition to being regulated by chemical signals from the extracellular matrix (ECM) and blood, ECs are directly confronted to complex hemodynamic environment. These physical inputs are translated into biochemical signals, dictating multiple aspects of cell behaviour and destination, including growth, differentiation, migration, adhesion, death and survival. Mechanosensors are initial responders to changes in mechanical environments, and the overwhelming majority of them are located on the plasma membrane. Physical forces affect plasma membrane fluidity and change of protein complexes on plasma membrane, accompanied by altering intercellular connections, cell-ECM adhesion, deformation of the cytoskeleton, and consequently, transcriptional responses in shaping specific phenotypes. Among the diverse forces exerted on ECs, shear stress (SS), defined as tangential friction force exerted by blood flow, has been extensively studied, from mechanosensing to mechanotransduction, as well as corresponding phenotypes. However, the precise mechanosensors and signalling pathways that determine atheroprone and atheroprotective phenotypes of arteries remain unclear. Moreover, it is worth to mention that some established mechanosensors of atheroprotective SS, endothelial glycocalyx, for example, might be dismantled by atheroprone SS. Therefore, we provide an overview of the current knowledge on mechanosensors in ECs for SS signals. We emphasize how these ECs coordinate or differentially participate in phenotype regulation induced by atheroprone and atheroprotective SS.

Structural Aspects of LIMK Regulation and Pharmacology

Malfunction of the actin cytoskeleton is linked to numerous human diseases including neurological disorders and cancer. LIMK1 (LIM domain kinase 1) and its paralogue LIMK2 are two closely related kinases that control actin cytoskeleton dynamics. Consequently, they are potential therapeutic targets for the treatment of such diseases. In the present review, we describe the LIMK conformational space and its dependence on ligand binding. Furthermore, we explain the unique catalytic mechanism of the kinase, shedding light on substrate recognition and how LIMK activity is regulated. The structural features are evaluated for implications on the drug discovery process. Finally, potential future directions for targeting LIMKs pharmacologically, also beyond just inhibiting the kinase domain, are discussed.