Molecular dissection of zyxin function reveals its involvement in cell motility

The role of zyxin in signal transduction and its relationship with diseases

This review highlighted the pivotal role of zyxin, an essential cell focal adhesions protein, in cellular biology and various diseases. Zyxin can orchestrate the restructuring and dynamic alterations of the cellular cytoskeleton, which is involved in cell proliferation, adhesion, motility, and gene transcription. Aberrant zyxin expression is closely correlated with tumor cell activity and cardiac function in both tumorigenesis and cardiovascular diseases. Moreover, in fibrotic and inflammatory conditions, zyxin can modulate cellular functions and inflammatory responses. Therefore, a comprehensive understanding of zyxin is crucial for deciphering signal transduction networks and disease pathogenesis. Investigating its role in diseases holds promise for novel avenues in early diagnosis and therapeutic strategies. Nevertheless, targeting zyxin as a therapeutic focal point presents challenges in terms of specificity, safety, drug delivery, and resistance. Nonetheless, in-depth studies on zyxin and the application of precision medicine could offer new possibilities for personalized treatment modalities.

Zyxin is important for the stability and function of podocytes, especially during mechanical stretch

Podocyte detachment due to mechanical stress is a common issue in hypertension-induced kidney disease. This study highlights the role of zyxin for podocyte stability and function. We have found that zyxin is significantly up-regulated in podocytes after mechanical stretch and relocalizes from focal adhesions to actin filaments. In zyxin knockout podocytes, we found that the loss of zyxin reduced the expression of vinculin and VASP as well as the expression of matrix proteins, such as fibronectin. This suggests that zyxin is a central player in the translation of mechanical forces in podocytes. In vivo, zyxin is highly up-regulated in patients suffering from diabetic nephropathy and in hypertensive DOCA-salt treated mice. Furthermore, zyxin loss in mice resulted in proteinuria and effacement of podocyte foot processes that was measured by super resolution microscopy. This highlights the essential role of zyxin for podocyte maintenance in vitro and in vivo, especially under mechanical stretch.

Quantitative proteomic analysis of human serum using tandem mass tags to predict cardiovascular risks in patients with psoriasis

Although biomarker candidates associated with psoriasis have been suggested, those for predicting the risk of cardiovascular disease (CVD) early in patients with psoriasis are lacking. We aimed to identify candidate biomarkers that can predict the occurrence of CVD in psoriasis patients. We pursued quantitative proteomic analysis of serum samples composed of three groups: psoriasis patients with and those without CVD risk factors, and healthy controls. Age/Sex-matched serum samples were selected and labeled with 16-plex tandem mass tag (TMT) and analyzed using liquid chromatography-mass spectrometry and subsequent verification with ELISA. Of the 184 proteins that showed statistical significance (P-value < 0.05) among the three groups according to TMT-based quantitative analysis, 98 proteins showed significant differences (> 2.0-fold) between the psoriasis groups with and without CVD risk factors. Verification by ELISA revealed that caldesmon (CALD1), myeloid cell nuclear differentiation antigen (MNDA), and zyxin (ZYX) levels were significantly increased in the psoriasis group with CVD risk factors. Further network analysis identified pathways including integrin signaling, which could be related to platelet aggregation, and actin cytoskeleton signaling. Three novel candidates (MNDA, ZYX, and CALD1) could be potential biomarkers for predicting CVD risks in psoriasis patients. We expect these biomarker candidates can be used to predict CVD risk in psoriasis patients in clinical settings although further studies including large validation are needed.

Synaptic branch stability is mediated by non-enzymatic functions of MEC-17/αTAT1 and ATAT-2

Microtubules are fundamental elements of neuronal structure and function. They are dynamic structures formed from protofilament chains of α- and β-tubulin heterodimers. Acetylation of the lysine 40 (K40) residue of α-tubulin protects microtubules from mechanical stresses by imparting structural elasticity. The enzyme responsible for this acetylation event is MEC-17/αTAT1. Despite its functional importance, however, the consequences of altered MEC-17/αTAT1 levels on neuronal structure and function are incompletely defined. Here we demonstrate that overexpression or loss of MEC-17, or of its functional paralogue ATAT-2, causes a delay in synaptic branch extension, and defective synaptogenesis in the mechanosensory neurons of Caenorhabditis elegans. Strikingly, by adulthood, the synaptic branches in these animals are lost, while the main axon shaft remains mostly intact. We show that MEC-17 and ATAT-2 regulate the stability of the synaptic branches largely independently from their acetyltransferase domains. Genetic analyses reveals novel interactions between both mec-17 and atat-2 with the focal adhesion gene zyx-1/Zyxin, which has previously been implicated in actin remodelling. Together, our results reveal new, acetylation-independent roles for MEC-17 and ATAT-2 in the development and maintenance of neuronal architecture.

Calcium Electroporation Reduces Viability and Proliferation Capacity of Four Uveal Melanoma Cell Lines in 2D and 3D Cultures

Simple Summary

Calcium electroporation (CaEP) is an innovative anti-tumor treatment modality that induces cell death by introducing supraphysiological concentrations of calcium into cells with a limited effect on normal cells. The objective of the present study is to assess the effect of CaEP in uveal melanoma (UM) cell lines in comparison to electrochemotherapy (ECT) with bleomycin using 2D monolayer cell cultures as well as 3D tumor spheroid models in four different UM cell lines. The morphological changes of the spheroids, the cell viability, growth rate as well as the cytotoxic effect of electroporation (EP) with calcium chloride and bleomycin were evaluated with various drug concentrations. The results of CaEP and ECT both suggest a comparable dose-dependent reduction in cell viability and proliferation rate in all tested 2D cell lines and 3D tumor spheroids. These data point out that CaEP is an established anticancer treatment causing cell death by ATP depletion in in vitro and in vivo, representing an efficient alternative therapy with a lower cytotoxic potency for the local UM tumor control.

Abstract

Electrochemotherapy (ECT) is the combination of transient pore formation following electric pulse application with the administration of cytotoxic drugs, which enhances the cytotoxic effect of the applied agent due to membrane changes and permeabilization. Although EP represents an established therapeutic option for solid malignancies, recent advances shift to the investigation of non-cytotoxic agents, such as calcium, which can also induce cell death. The present study aims to evaluate the cytotoxic effect, the morphological changes in tumor spheroids, the effect on the cell viability, and the cell-specific growth rate following calcium electroporation (CaEP) in uveal melanoma (UM) 2D monolayer cell cultures as well as in 3D tumor spheroid models. The experiments were conducted in four cell lines, UM92.1, Mel270, and two primary UM cell lines, UPMD2 and UPMM3 (UPM). The 2D and 3D UM cell cultures were electroporated with eight rectangular pulses (100 µs pulse duration, 5 Hz repetition frequency) of a 1000 V/cm pulse strength alone or in combination with 0.11 mg/mL, 0.28 mg/mL, 0.55 mg/mL or 1.11 mg/mL calcium chloride or 1.0 µg/mL or 2.5 µg/mL bleomycin. The application of calcium chloride alone induced an ATP reduction only in the UM92.1 2D cell cultures. Calcium alone had no significant effect on ATP levels in all four UM spheroids. A significant decrease in the intracellular adenosine triphosphate (ATP) level was documented in all four 2D and 3D cell cultures for both CaEP as well as ECT with bleomycin. The results suggest a dose-dependent ATP depletion with a wide range of sensitivity among the tested UM cell lines, control groups, and the applied settings in both 2D monolayer cell cultures and 3D tumor spheroid models. The colony formation capacity of the cell lines after two weeks reduced significantly after CaEP only with 0.5 mg/mL and 1.1 mg/mL, whereas the same effect could be achieved with both applied bleomycin concentrations, 1.0 µg/mL and 2.5 µg/mL, for the ECT group. The specific growth rate on day 7 following CaEP was significantly reduced in UM92.1 cell lines with 0.5 and 1.1 mg/mL calcium chloride, while Mel270 showed a similar effect only after administration of 1.1 mg/mL. UM92.1 and Mel270 spheroids exhibited lower adhesion and density after CaEP on day three in comparison to UPM spheroids showing detachment after day 7 following treatment. CaEP and bleomycin electroporation significantly reduce cell viability at similar applied voltage settings. CaEP may be a feasible and inexpensive therapeutic option for the local tumor control with fewer side effects, in comparison to other chemotherapeutic agents, for the treatment of uveal melanoma. The limited effect on normal cells and the surrounding tissue has already been investigated, but further research is necessary to clarify the effect on the surrounding tissue and to facilitate its application in a clinical setting for the eye.

O-GlcNAc transferase promotes the nuclear localization of the focal adhesion-associated protein Zyxin to regulate UV-induced cell death

Zyxin is a zinc-binding phosphoprotein known to regulate cell migration, adhesion, and cell survival. Zyxin also plays a role in signal transduction between focal adhesions and the nuclear compartment. However, the mechanism of Zyxin shuttling to nucleus is still unclear. Here, we identify that the GlcNAc transferase (O-linked GlcNAc [O-GlcNAc] transferase) can O-GlcNAcylate Zyxin and regulate its nuclear localization. We show that O-GlcNAc transferase O-GlcNAcylates Zyxin at two residues, serine 169 (Ser-169) and Ser-246. In addition, O-GlcNAcylation of Ser-169, but not Ser-246, enhances its interaction with 14-3-3γ, which is a phosphoserine/threonine-binding protein and is reported to bind with phosphorylated Zyxin. Furthermore, we found that 14-3-3γ could promote the nuclear localization of Zyxin after Ser-169 O-GlcNAcylation by affecting the function of the N-terminal nuclear export signal sequence; functionally, UV treatment increases the O-GlcNAcylation of Zyxin, which may enhance the nuclear location of Zyxin. Finally, Zyxin in the nucleus maintains homeodomain-interacting protein kinase 2 stability and promotes UV-induced cell death. In conclusion, we uncover that the nuclear localization of Zyxin can be regulated by its O-GlcNAcylation, and that this protein may regulate UV-induced cell death.

Zyxin Is Involved in Fibroblast Rigidity Sensing and Durotaxis

Focal adhesions (FAs) are specialized structures that enable cells to sense their extracellular matrix rigidity and transmit these signals to the interior of the cells, bringing about actin cytoskeleton reorganization, FA maturation, and cell migration. It is known that cells migrate towards regions of higher substrate rigidity, a phenomenon known as durotaxis. However, the underlying molecular mechanism of durotaxis and how different proteins in the FA are involved remain unclear. Zyxin is a component of the FA that has been implicated in connecting the actin cytoskeleton to the FA. We have found that knocking down zyxin impaired NIH3T3 fibroblast's ability to sense and respond to changes in extracellular matrix in terms of their FA sizes, cell traction stress magnitudes and F-actin organization. Cell migration speed of zyxin knockdown fibroblasts was also independent of the underlying substrate rigidity, unlike wild type fibroblasts which migrated fastest at an intermediate substrate rigidity of 14 kPa. Wild type fibroblasts exhibited durotaxis by migrating toward regions of increasing substrate rigidity on polyacrylamide gels with substrate rigidity gradient, while zyxin knockdown fibroblasts did not exhibit durotaxis. Therefore, we propose zyxin as an essential protein that is required for rigidity sensing and durotaxis through modulating FA sizes, cell traction stress and F-actin organization.

A Layered View on Focal Adhesions

Simple Summary

The cytoskeleton is a network of protein fibres within cells that provide structure and support intracellular transport. Focal adhesions are protein complexes associated with the outer cell membrane that are found at the ends of specialised actin fibres of this cytoskeleton. They mediate cell adhesion by connecting the cytoskeleton to the extracellular matrix, a protein and sugar network that surrounds cells in tissues. Focal adhesions also translate forces on actin fibres into forces contributing to cell migration. Cell adhesion and migration are crucial to diverse biological processes such as embryonic development, proper functioning of the immune system or the metastasis of cancer cells. Advances in fluorescence microscopy and data analysis methods provided a more detailed understanding of the dynamic ways in which proteins bind and dissociate from focal adhesions and how they are organised within these protein complexes. In this review, we provide an overview of the advances in the current scientific understanding of focal adhesions and summarize relevant imaging techniques. One of the key insights is that focal adhesion proteins are organised into three layers parallel to the cell membrane. We discuss the relevance of this layered nature for the functioning of focal adhesion.

Abstract

The cytoskeleton provides structure to cells and supports intracellular transport. Actin fibres are crucial to both functions. Focal Adhesions (FAs) are large macromolecular multiprotein assemblies at the ends of specialised actin fibres linking these to the extracellular matrix. FAs translate forces on actin fibres into forces contributing to cell migration. This review will discuss recent insights into FA protein dynamics and their organisation within FAs, made possible by advances in fluorescence imaging techniques and data analysis methods. Over the last decade, evidence has accumulated that FAs are composed of three layers parallel to the plasma membrane. We focus on some of the most frequently investigated proteins, two from each layer, paxillin and FAK (bottom, integrin signalling layer), vinculin and talin (middle, force transduction layer) and zyxin and VASP (top, actin regulatory layer). Finally, we discuss the potential impact of this layered nature on different aspects of FA behaviour.

Anti-Adhesion Behavior from Ring-Strain Amine Cyclic Monolayers Grafted on Silicon (111) Surfaces

In this manuscript, a series of amine tagged short cyclic molecules (cyclopropylamine, cyclobutylamine, cyclopentylamine and cyclohexylamine) were thermally grafted onto p-type silicon (111) hydride surfaces via nucleophilic addition. The chemistries of these grafting were verified via XPS, AFM and sessile droplet measurements. Confocal microscopy and cell viability assay was performed on these surfaces incubated for 24 hours with triple negative breast cancer cells (MDA-MB 231), gastric adenocarcinoma cells (AGS) endometrial adenocarcinoma (Hec1A). All cell types had shown a significant reduction when incubated on these ring-strain cyclic monolayer surfaces than compared to standard controls. The expression level of focal adhesion proteins (vinculin, paxilin, talin and zyxin) were subsequently quantified for all three cell types via qPCR analysis. Cells incubate on these surface grafting were observed to have reduced levels of adhesion protein expression than compared to positive controls (collagen coating and APTES). A potential application of these anti-adhesive surfaces is the maintenance of the chondrocyte phenotype during in-vitro cell expansion. Articular chondrocytes cultured for 6 days on ring strained cyclopropane-modified surfaces was able to proliferate but had maintained a spheroid/aggregated phenotype with higher COL2A1 and ACAN gene expression. Herein, these findings had help promote grafting of cyclic monolayers as an viable alternative for producing antifouling surfaces.

The β-Carboline Harmine Induces Actin Dynamic Remodeling and Abrogates the Malignant Phenotype in Tumorigenic Cells

Numerous studies have shown that alteration of actin remodeling plays a pivotal role in the regulation of morphologic and phenotypic changes leading to malignancy. In the present study, we searched for drugs that can regulate actin polymerization and reverse the malignant phenotype in cancer cells. We developed a cell-free high-throughput screening assay for the identification of compounds that induce the actin polymerization in vitro, by fluorescence anisotropy. Then, the potential of the hit compound to restore the actin cytoskeleton and reverse the malignant phenotype was checked in EWS-Fli1-transformed fibroblasts and in B16-F10 melanoma cells. A β-carboline extracted from Peganum harmala (i.e., harmine) is identified as a stimulator of actin polymerization through a mechanism independent of actin binding and requiring intracellular factors involved in a process that regulates actin kinetics. Treatment of malignant cells with non-cytotoxic concentrations of harmine induces the recovery of a non-malignant cell morphology accompanied by reorganization of the actin cytoskeleton, rescued cell–cell adhesion, inhibition of cell motility and loss of anchorage-independent growth. In conclusion, harmine induces the reversion of the malignant phenotype by a process involving the modulation of actin dynamics and is a potential anti-tumor agent acting principally through a non-cytotoxic process.

Identification of new binding proteins of focal adhesion kinase using immunoprecipitation and mass spectrometry

Focal adhesion kinase (FAK) is a 125 kDa protein recruited as a participant in focal adhesion dynamics and serves as a signaling scaffold for the assembly and subsequent maturation of focal contact. Identification of new FAK binding proteins could reveal potential signaling targets and contribute to further development of therapeutic drugs in the treatment of colon cancer. Here, we applied a functional proteomic strategy to identify proteins that interact with FAK in human colon cancer cell line HCT-116. Proteins were targeted by coimmunoprecipitation with an anti-FAK antibody and resolved on 1D-SDS-PAGE. The gel was excised, reduced, alkylated, and trypsin digested. Tryptic peptides were separated by nano-LC-MS/MS by an LTQ-Orbitrap-Velos spectrometer. We identified 101 proteins in the immunocomplex under epithelial growth factor (EGF) stimulation. Three proteins, zyxin, nesprin-1, and desmoplakin, were discovered and validated using reciprocal immunoprecipitation and Western blot analysis. Then, we sought to study the biological relevance of these proteins by siRNA transfection of HCT-116 cells. According to the results, zyxin might play a central role as an upstream regulator to mediate critical cancer-related signaling pathways. Zyxin and nesprin-1 depletion significantly impaired cell migration and invasion capabilities. Additionally, we performed ELISA assays on serum samples from patients with colon cancer instead of cell models to quantify the protein levels of zyxin and nesprin-1. Our results suggested that zyxin and nesprin-1 are not only promising therapeutic targets but also potential diagnostic biomarkers for colon cancer.

The role of zyxin in regulation of malignancies

Focal adhesions are highly dynamic multi-protein complexes found at the cell surface and effectively link the cell's internal cytoskeleton to a complex mixture of macromolecules known as the extracellular matrix and mediate transmission of signals from the extracellular matrix to the nucleus. Zyxin is one of the key focal adhesion proteins and is also found to shuttle in the nucleus. Although the mechanism of shuttling to the nucleus unclear, it moves out from the nucleus through a leucine-rich nuclear export signal sequence. It is known to contribute to fundamental cellular activities such as cell migration, adhesion and proliferation by interacting with a variety of cellular proteins. It is also linked with a number of cancers such as melanoma, hepatocellular carcinoma, oral squamous-cell carcinoma, Ewing sarcoma and prostate cancer. However, in many cases, the precise mechanisms by which the absence or presence of zyxin contributes to cancer progression or suppression is unknown. Thus, more work is required to gain insights into how zyxin modulates cellular functions in relationship to cancer. This review summarises the role of zyxin in cancer, with an emphasis on conflicting roles in prostate cancer.

Calcium electroporation for treatment of sarcoma in preclinical studies

Calcium electroporation (CaEP) describes the use of electric pulses (electroporation) to transiently permeabilize cells to allow supraphysiological doses of calcium to enter the cytosol. Calcium electroporation has successfully been investigated for treatment of cutaneous metastases in a clinical study. This preclinical study explores the possible use of calcium electroporation for treatment of sarcoma.

A normal murine muscle cell line (C2C12), and a human rhabdomyosarcoma cell line (RD) were used in the undifferentiated and differentiated state. Electroporation was performed using 8 pulses of 100 μs at 600–1000 V/cm; with calcium (0, 0.5, 1, and 5 mM). Viability was examined by MTS assay, intracellular calcium levels were measured, and expression of plasma membrane calcium ATPase (PMCA) was investigated using western blotting. Calcium/sodium exchanger (NCX1), ryanodine receptor (RyR1) expression and cytoskeleton structure (zyxin/actin) were assessed by immunofluorescence. CaEP efficiency on RD tumors was tested in vivo in immuno-deficient mice.

CaEP was significantly more efficient in RD than in normal cells. Intracellular Ca²⁺ levels after CaEP increased significantly in RD, whereas a lower increase was seen in normal cells. CaEP caused decreased expression of PMCA and NCX1 in malignant cells and RyR1 in both cell lines whereas normal cells exhibited increased expression of NCX1 after CaEP. Calcium electroporation also affected cytoskeleton structure in malignant cells.

This study showed that calcium electroporation is tolerated significantly better in normal muscle cells than sarcoma cells and as an inexpensive and simple cancer treatment this could potentially be used in connection with sarcoma surgery for local treatment.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Distinct focal adhesion protein modules control different aspects of mechanotransduction

Focal adhesions (FAs) are macromolecular complexes that regulate cell adhesion and mechanotransduction. By performing fluorescence recovery after photobleaching (FRAP) and fluorescence loss after photoactivation (FLAP) experiments, we found that the mobility of core FA proteins correlates with their function. Structural proteins such as tensin, talin and vinculin are significantly less mobile in FAs than signaling proteins such as FAK (also known as PTK2) and paxillin. The mobilities of the structural proteins are directly influenced by substrate stiffness, suggesting that they are involved in sensing the rigidity of the extracellular environment. The turnover rates of FAK and paxillin, as well as kindlin2 (also known as FERMT2), are not influenced by substrate stiffness. By using specific Src and FAK inhibitors, we reveal that force-sensing by vinculin occurs independently of FAK and paxillin phosphorylation. However, their phosphorylation is required for downstream Rac1-driven cellular processes, such as protrusion and cell migration. Overall, we show that the FA is composed of different functional modules that separately control mechanosensing and the cellular mechano-response.

Delineating the Tes Interaction Site in Zyxin and Studying Cellular Effects of Its Disruption

Focal adhesions are integrin-based structures that link the actin cytoskeleton and the extracellular matrix. They play an important role in various cellular functions such as cell signaling, cell motility and cell shape. To ensure and fine tune these different cellular functions, adhesions are regulated by a large number of proteins. The LIM domain protein zyxin localizes to focal adhesions where it participates in the regulation of the actin cytoskeleton. Because of its interactions with a variety of binding partners, zyxin has been proposed to act as a molecular scaffold. Here, we studied the interaction of zyxin with such a partner: Tes. Similar to zyxin, Tes harbors three highly conserved LIM domains of which the LIM1 domain directly interacts with zyxin. Using different zyxin variants in pull-down assays and ectopic recruitment experiments, we identified the Tes binding site in zyxin and showed that four highly conserved amino acids are crucial for its interaction with Tes. Based upon these findings, we used a zyxin mutant defective in Tes-binding to assess the functional consequences of abrogating the zyxin-Tes interaction in focal adhesions. Performing fluorescence recovery after photobleaching, we showed that zyxin recruits Tes to focal adhesions and modulates its turnover in these structures. However, we also provide evidence for zyxin-independent localization of Tes to focal adhesions. Zyxin increases focal adhesion numbers and reduces focal adhesion lifetimes, but does so independent of Tes. Quantitative analysis showed that the loss of interaction between zyxin and Tes affects the process of cell spreading. We conclude that zyxin influences focal adhesion dynamics, that it recruits Tes and that this interaction is functional in regulating cell spreading.

Selective phosphorylation during early macrophage differentiation

The differentiation of macrophages from monocytes is a tightly controlled and complex biological process. Although numerous studies have been conducted using biochemical approaches or global gene/protein profiling, the mechanisms of the early stages of differentiation remain unclear. Here we used SILAC-based quantitative proteomics approach to perform temporal phosphoproteome profiling of early macrophage differentiation. We identified a large set of phosphoproteins and grouped them as PMA-regulated and non-regulated phosphoproteins in the early stages of differentiation. Further analysis of the PMA-regulated phosphoproteins revealed that transcriptional suppression, cytoskeletal reorganization and cell adhesion were among the most significantly activated pathways. Some key involved regulators of these pathways are mTOR, MYB, STAT1 and CTNNB. Moreover, we were able to classify the roles and activities of several transcriptional factors during different differentiation stages and found that E2F is likely to be an important regulator during the relatively late stages of differentiation. This study provides the first comprehensive picture of the dynamic phosphoproteome during myeloid cells differentiation, and identifies potential molecular targets in leukemic cells.

A guide to mechanobiology: Where biology and physics meet

Cells actively sense and process mechanical information that is provided by the extracellular environment to make decisions about growth, motility and differentiation. It is important to understand the underlying mechanisms given that deregulation of the mechanical properties of the extracellular matrix (ECM) is implicated in various diseases, such as cancer and fibrosis. Moreover, matrix mechanics can be exploited to program stem cell differentiation for organ-on-chip and regenerative medicine applications. Mechanobiology is an emerging multidisciplinary field that encompasses cell and developmental biology, bioengineering and biophysics. Here we provide an introductory overview of the key players important to cellular mechanobiology, taking a biophysical perspective and focusing on a comparison between flat versus three dimensional substrates. This article is part of a Special Issue entitled: Mechanobiology.

Zyxin antagonizes the FERM protein expanded to couple F-actin and Yorkie-dependent organ growth

BACKGROUND

Coordinated multicellular growth during development is achieved by the sensing of spatial and nutritional boundaries. The conserved Hippo (Hpo) signaling pathway has been proposed to restrict tissue growth by perceiving mechanical constraints through actin cytoskeleton networks. The actin-associated LIM proteins Zyxin (Zyx) and Ajuba (Jub) have been linked to the control of tissue growth via regulation of Hpo signaling, but the study of Zyx has been hampered by a lack of genetic tools.

RESULTS

We generated a zyx mutant in Drosophila using TALEN endonucleases and used this to show that Zyx antagonizes the FERM-domain protein Expanded (Ex) to control tissue growth, eye differentiation, and F-actin accumulation. Zyx membrane targeting promotes the interaction between the transcriptional co-activator Yorkie (Yki) and the transcription factor Scalloped (Sd), leading to activation of Yki target gene expression and promoting tissue growth. Finally, we show that Zyx's growth-promoting function is dependent on its interaction with the actin-associated protein Enabled (Ena) via a conserved LPPPP motif and is antagonized by Capping Protein (CP).

CONCLUSIONS

Our results show that Zyx is a functional antagonist of Ex in growth control and establish a link between actin filament polymerization and Yki activity.

Sensing the local environment: actin architecture and Hippo signalling

The Hippo network is a major conserved growth suppressor pathway that participates in organ size control during development and prevents tumour formation during adult homeostasis. Recent evidence has implicated the actin cytoskeleton as a link between tissue architecture and Hippo signalling. In this review, we will consider the evidence and models proposed for the regulation of Hippo signalling by actin dynamics and structure. We cover aspects of signalling regulation by mechanotransduction, cytoskeletal tethering and the spatial reorganization of signalling components. We also examine the physiological and pathological contexts in which these mechanisms are relevant.

The conserved LIM domain-containing focal adhesion protein ZYX-1 regulates synapse maintenance in Caenorhabditis elegans

We describe the identification of zyxin as a regulator of synapse maintenance in mechanosensory neurons in C. elegans. zyx-1 mutants lacked PLM mechanosensory synapses as adult animals. However, most PLM synapses initially formed during development but were subsequently lost as the animals developed. Vertebrate zyxin regulates cytoskeletal responses to mechanical stress in culture. Our work provides in vivo evidence in support of such a role for zyxin. In particular, zyx-1 mutant synaptogenesis phenotypes were suppressed by disrupting locomotion of the mutant animals, suggesting that zyx-1 protects mechanosensory synapses from locomotion-induced forces. In cultured cells, zyxin is recruited to focal adhesions and stress fibers via C-terminal LIM domains and modulates cytoskeletal organization via the N-terminal domain. The synapse-stabilizing activity was mediated by a short isoform of ZYX-1 containing only the LIM domains. Consistent with this notion, PLM synaptogenesis was independent of α-actinin and ENA-VASP, both of which bind to the N-terminal domain of zyxin. Our results demonstrate that the LIM domain moiety of zyxin functions autonomously to mediate responses to mechanical stress and provide in vivo evidence for a role of zyxin in neuronal development.

LIM proteins in actin cytoskeleton mechanoresponse

The actin cytoskeleton assembles into branched networks or bundles to generate mechanical force for critical cellular processes such as establishment of polarity, adhesion, and migration. Stress fibers (SFs) are contractile actomyosin structures that physically couple to the extracellular matrix through integrin-based focal adhesions (FAs), thereby transmitting force into and across the cell. Recently, LIN-11, Isl1, and MEC-3 (LIM) domain proteins have been implicated in mediating this cytoskeletal mechanotransduction. Among the more well-studied LIM domain adapter proteins is zyxin, a dynamic component of both FAs and SFs. Here we discuss recent research detailing the mechanisms by which SFs adjust their structure and composition to balance mechanical forces and suggest ways that zyxin and other LIM domain proteins mediate mechanoresponse.

Zyxin regulates cell migration and differentiation in EMT during chicken AV valve morphogenesis

During heart valve development, epithelial-mesenchymal transformation (EMT) is a key process for valve formation. EMT leads to the generation of mesenchymal cells that will eventually become the interstitial cells (fibroblasts) of the mature valve. During EMT, cell architecture and motility change markedly; significant changes are also observed in various signaling pathways. Here we systematically examined the expression, localization, and function of zyxin, a focal adhesion protein, in EMT during atrioventricular (AV) valve morphogenesis. Expression and localization studies showed that zyxin was expressed in the AV canal region during crucial stages of valve development. An in vitro 3D collagen gel culture system was used to determine zyxin function either after siRNA gene knockdown or after overexpression. Our studies revealed that zyxin overexpression inhibited endocardial cell migration and cell differentiation and also led to a decrease in the number of migrating mesenchymal cells. Moreover, correlative cytoskeletal changes were apparent in response to both overexpression and knockdown treatments. Thus, zyxin appears to play a role as a regulator of cell migration and differentiation during EMT in chicken AV valve formation.

Rigidity-patterned polyelectrolyte films to control myoblast cell adhesion and spatial organization

In vivo, cells are sensitive to the stiffness of their micro-environment and especially to the spatial organization of the stiffness. In vitro studies of this phenomenon can help to better understand the mechanisms of the cell response to spatial variations of the matrix stiffness. In this work, we design polelyelectrolyte multilayer films made of poly(L-lysine) and a photo-reactive hyaluronan derivative. These films can be photo-crosslinked through a photomask to create spatial patterns of rigidity. Quartz substrates incorporating a chromium mask are prepared to expose selectively the film to UV light (in a physiological buffer), without any direct contact between the photomask and the soft film. We show that these micropatterns are chemically homogeneous and flat, without any preferential adsorption of adhesive proteins. Three groups of pattern geometries differing by their shape (circles or lines), size (form 2 to 100 μm) or interspacing distance between the motifs are used to study the adhesion and spatial organization of myoblast cells. On large circular micropatterns, the cells form large assemblies that are confined to the stiffest parts. Conversely, when the size of the rigidity patterns is subcellular, the cells respond by forming protrusions. Finally, on linear micropatterns of rigidity, myoblasts align and their nuclei drastically elongate in specific conditions. These results pave the way for the study of the different steps of myoblast fusion in response to matrix rigidity in well-defined geometrical conditions.

Zyxin-VASP interactions alter actin regulatory activity in zyxin-VASP complexes

Cell-cell and cell-substrate adhesions are sites of dramatic actin rearrangements and where actin-membrane connections are tightly regulated. Zyxin-VASP complexes localize to sites of cell-cell and cell-substrate adhesion and function to regulate actin dynamics and actin-membrane connections at these sites. To accomplish these functions, zyxin recruits VASP to cellular sites via proline-rich binding sites near zyxin's amino terminus. While the prevailing thought has been that zyxin simply acts as a scaffold protein for VASP binding, the identification of a LIM domain-VASP interaction could complicate this view. Here we assess how zyxin-VASP binding through both the proline rich motifs and the LIM domains alters specific VASP functions. We find that neither individual interaction alters VASP's actin regulatory activities. In contrast, however, we find that full-length zyxin dramatically reduces VASP-mediated actin bundling and actin assembly. Taken together, these results suggest a model where zyxin-VASP complexes occur in complex organizations with suppressed actin regulatory activity.

Control of actin dynamics by allosteric regulation of actin binding proteins

The regulated assembly and organization of actin filaments allows the cell to construct a large diversity of actin-based structures specifically suited to a range of cellular processes. A vast array of actin regulatory proteins must work in concert to form specific actin networks within cells, and spatial and temporal requirements for actin assembly necessitate rapid regulation of protein activity. This chapter explores a common mechanism of controlling the activity of actin binding proteins: allosteric autoinhibition by interdomain head-tail interactions. Intramolecular interactions maintain these proteins in a closed conformation that masks protein domains needed to regulate actin dynamics. Autoinhibition is typically relieved by two or more ligand binding and/or posttranslational modification events that expose key protein domains. Regulation through multiple inputs permits precise temporal and spatial control of protein activity to guide actin network formation.

Zyxin emerges as a key player in the mechanotransduction at cell adhesive structures

Actin stress fiber (SF), focal adhesion (FA) and adherens junction (AJ) are known structures whose formation and development are mechanical force-dependent. At these structures, actin is actively polymerized, which in turn contributes the development of these structures. Recently, we reported that actin polymerization at FAs is facilitated by mechanical forces, which was critically dependent on the force-induced recruitment of the LIM protein zyxin to FAs. Zyxin enhances actin polymerization with the aid of Ena/VASP proteins. Both zyxin and Ena/VASP proteins are localized not only to FAs but also to AJs and SFs, facilitating actin polymerization at these structures. We discuss here the possibility that zyxin is a common mechanotransducer element regulating actin polymerization at FAs, AJs and SFs.

Stretch-induced actin remodeling requires targeting of zyxin to stress fibers and recruitment of actin regulators

Mechanical stimulation induces zyxin-dependent actin cytoskeletal reinforcement. Stretch induces MAPK activation, zyxin phosphorylation, and recruitment to actin stress fibers, independent of p130Cas. Zyxin's C-terminal LIM domains are required for stretch-induced targeting to stress fibers, and zyxin's N-terminus is necessary for actin remodeling.

Reinforcement of actin stress fibers in response to mechanical stimulation depends on a posttranslational mechanism that requires the LIM protein zyxin. The C-terminal LIM region of zyxin directs the force-sensitive accumulation of zyxin on actin stress fibers. The N-terminal region of zyxin promotes actin reinforcement even when Rho kinase is inhibited. The mechanosensitive integrin effector p130Cas binds zyxin but is not required for mitogen-activated protein kinase–dependent zyxin phosphorylation or stress fiber remodeling in cells exposed to uniaxial cyclic stretch. α-Actinin and Ena/VASP proteins bind to the stress fiber reinforcement domain of zyxin. Mutation of their docking sites reveals that zyxin is required for recruitment of both groups of proteins to regions of stress fiber remodeling. Zyxin-null cells reconstituted with zyxin variants that lack either α-actinin or Ena/VASP-binding capacity display compromised response to mechanical stimulation. Our findings define a bipartite mechanism for stretch-induced actin remodeling that involves mechanosensitive targeting of zyxin to actin stress fibers and localized recruitment of actin regulatory machinery.

Actin dynamics associated with focal adhesions

Cell-matrix adhesion plays a major role during cell migration. Proteins from adhesion structures connect the extracellular matrix to the actin cytoskeleton, allowing the growing actin network to push the plasma membrane and the contractile cables (stress fibers) to pull the cell body. Force transmission to the extracellular matrix depends on several parameters including the regulation of actin dynamics in adhesion structures, the contractility of stress fibers, and the mechanosensitive response of adhesion structures. Here we highlight recent findings on the molecular mechanisms by which actin assembly is regulated in adhesion structures and the molecular basis of the mechanosensitivity of focal adhesions.

Ajuba is required for Rac activation and maintenance of E-cadherin adhesion

A Rac–PAK1–Ajuba feedback loop stabilizes cadherin complexes via coordination of spatiotemporal signaling with actin remodeling at cell–cell contacts.

Maintenance of stable E-cadherin–dependent adhesion is essential for epithelial function. The small GTPase Rac is activated by initial cadherin clustering, but the precise mechanisms underlying Rac-dependent junction stabilization are not well understood. Ajuba, a LIM domain protein, colocalizes with cadherins, yet Ajuba function at junctions is unknown. We show that, in Ajuba-depleted cells, Rac activation and actin accumulation at cadherin receptors was impaired, and junctions did not sustain mechanical stress. The Rac effector PAK1 was also transiently activated upon cell–cell adhesion and directly phosphorylated Ajuba (Thr172). Interestingly, similar to Ajuba depletion, blocking PAK1 activation perturbed junction maintenance and actin recruitment. Expression of phosphomimetic Ajuba rescued the effects of PAK1 inhibition. Ajuba bound directly to Rac·GDP or Rac·GTP, but phosphorylated Ajuba interacted preferentially with active Rac. Rather than facilitating Rac recruitment to junctions, Ajuba modulated Rac dynamics at contacts depending on its phosphorylation status. Thus, a Rac–PAK1–Ajuba feedback loop integrates spatiotemporal signaling with actin remodeling at cell–cell contacts and stabilizes preassembled cadherin complexes.

Zyxin is a critical regulator of the apoptotic HIPK2-p53 signaling axis

HIPK2 activates the apoptotic arm of the DNA damage response by phosphorylating tumor suppressor p53 at serine 46. Unstressed cells keep HIPK2 levels low through targeted polyubiquitination and subsequent proteasomal degradation. Here we identify the LIM domain protein Zyxin as a novel regulator of the HIPK2-p53 signaling axis in response to DNA damage. Remarkably, depletion of endogenous Zyxin, which colocalizes with HIPK2 at the cytoskeleton and in the cell nucleus, stimulates proteasome-dependent HIPK2 degradation. In contrast, ectopic expression of Zyxin stabilizes HIPK2, even upon enforced expression of its ubiquitin ligase Siah-1. Consistently, Zyxin physically interacts with Siah-1, and knock-down of Siah-1 rescues HIPK2 expression in Zyxin-depleted cancer cells. Mechanistically, our data suggest that Zyxin regulates Siah-1 activity through interference with Siah-1 dimerization. Furthermore, we show that endogenous Zyxin coaccumulates with HIPK2 in response to DNA damage in cancer cells, and that depletion of endogenous Zyxin results in reduced HIPK2 protein levels and compromises DNA damage-induced p53 Ser46 phosphorylation and caspase activation. These findings suggest an unforeseen role for Zyxin in DNA damage-induced cell fate control through modulating the HIPK2-p53 signaling axis.

Cell migration: an overview

Cell migration is a fundamental process that controls morphogenesis and inflammation. Its deregulation causes or is part of many diseases, including autoimmune syndromes, chronic inflammation, mental retardation, and cancer. Cell migration is an integral part of the cell biology, embryology, immunology, and neuroscience fields; as such, it has benefited from quantum leaps in molecular biology, biochemistry, and imaging techniques, and the emergence of the genomic and proteomic era. Combinations of these techniques have revealed new and exciting insights that explain how cells adhere and move, how the migration of multiple cells are coordinated and regulated, and how the cells interact with neighboring cells and/or react to changes in their microenvironment. This introduction provides a primer of the molecular and cellular insights, particularly the signaling networks, which control the migration of individual cells as well as collective migrations. The rest of the chapters are devoted to describe in detail some of the most salient technical advances that have illuminated the field of cell migration in recent years.

[The cytoskeletal protein zyxin--a universal regulator of cell adhesion and gene expression]

The attachment of a cell to an extracellular matrix or the surface of another cells affects not only the cell motility, but also gene expression. In view of this, an important problem is to establish the molecular mechanisms of signal transduction from the receptors of cell adhesion to the nucleus, in particular, to identify and investigate the protein transducers of these signals. One of these transducers, the LIM domain protein zyxin, is predominantly localized at the sites of cell adhesion, where it participates in the assembly of actin filaments. Owing to its location near the inner surface of the membrane, zyxin can interact with the transmembrane receptors of some signaling cascades and affect the signal transduction from the extracellular ligands of these receptors. Furthermore, under particular conditions, zyxin moves from the sites of cell contacts to the nucleus, where it directly participates in the regulation of gene expression. Of particular interest is the function of zyxin as a possible coordinator of gene expression and morphogenetic movements in embryogenesis. The published data discussed in the present review indicate the important role of zyxin in transmitting information from the regions of cell contacts to the genetic apparatus of the cell.

Zyxin controls migration in epithelial-mesenchymal transition by mediating actin-membrane linkages at cell-cell junctions

Development is punctuated by morphogenetic rearrangements of epithelial tissues, including detachment of motile cells during epithelial-mesenchymal transition (EMT). Dramatic actin rearrangements occur as cell-cell junctions are dismantled and cells become independently motile during EMT. Characterizing dynamic actin rearrangements and identifying actin machinery driving these rearrangements is essential for understanding basic mechanisms of cell-cell junction remodeling. Using immunofluorescence and live cell imaging of scattering MDCK cells we examine dynamic actin rearrangement events during EMT and demonstrate that zyxin-VASP complexes mediate linkage of dynamic medial actin networks to adherens junction (AJ) membranes. A functional analysis of zyxin in EMT reveals its role in regulating disruption of actin membrane linkages at cell-cell junctions, altering cells' ability to fully detach and migrate independently during EMT. Expression of a constitutively active zyxin mutant results in persistent actin-membrane linkages and cell migration without loss of cell-cell adhesion. We propose zyxin functions in morphogenetic rearrangements, maintaining collective migration by transducing individual cells' movements through AJs, thus preventing the dissociation of individual migratory cells.

Synemin interacts with the LIM domain protein zyxin and is essential for cell adhesion and migration

Synemin is a unique cytoplasmic intermediate filament protein for which there is limited understanding of its exact cellular functions. The single human synemin gene encodes at least two splice variants named alpha-synemin and beta-synemin, with the larger alpha-synemin containing an additional 312 amino acid insert within the C-terminal tail domain. We report herein that, by using the entire tail domain of the smaller beta-synemin as the bait in a yeast two-hybrid screen of a human skeletal muscle cDNA library, the LIM domain protein zyxin was identified as an interaction partner for human synemin. The synemin binding site in human zyxin was subsequently mapped to the C-terminal three tandem LIM-domain repeats, whereas the binding site for zyxin within beta-synemin is within the C-terminal 332 amino acid region (SNbetaTII) at the end of the long tail domain. Transient expression of SNbetaTII within mammalian cells markedly reduced zyxin protein level, blocked localization of zyxin at focal adhesion sites and resulted in decreased cell adhesion and increased motility. Knockdown of synemin expression with siRNAs within mammalian cells resulted in significantly compromised cell adhesion and cell motility. Our results suggest that synemin participates in focal adhesion dynamics and is essential for cell adhesion and migration.

The neurogenic basic helix-loop-helix transcription factor NeuroD6 concomitantly increases mitochondrial mass and regulates cytoskeletal organization in the early stages of neuronal differentiation

Mitochondria play a central role during neurogenesis by providing energy in the form of ATP for cytoskeletal remodelling, outgrowth of neuronal processes, growth cone activity and synaptic activity. However, the fundamental question of how differentiating neurons control mitochondrial biogenesis remains vastly unexplored. Since our previous studies have shown that the neurogenic bHLH (basic helix–loop–helix) transcription factor NeuroD6 is sufficient to induce differentiation of the neuronal progenitor-like PC12 cells and that it triggers expression of mitochondrial-related genes, we investigated whether NeuroD6 could modulate the mitochondrial biomass using our PC12-ND6 cellular paradigm. Using a combination of flow cytometry, confocal microscopy and mitochondrial fractionation, we demonstrate that NeuroD6 stimulates maximal mitochondrial mass at the lamellipodia stage, thus preceding axonal growth. NeuroD6 triggers remodelling of the actin and microtubule networks in conjunction with increased expression of the motor protein KIF5B, thus promoting mitochondrial movement in developing neurites with accumulation in growth cones. Maintenance of the NeuroD6-induced mitochondrial mass requires an intact cytoskeletal network, as its disruption severely reduces mitochondrial mass. The present study provides the first evidence that NeuroD6 plays an integrative role in co-ordinating increase in mitochondrial mass with cytoskeletal remodelling, suggestive of a role of this transcription factor as a co-regulator of neuronal differentiation and energy metabolism.

A zyxin head-tail interaction regulates zyxin-VASP complex formation

Zyxin is an adhesion protein that regulates actin assembly by binding to VASP family members through N-terminal proline-rich motifs. Evidence suggests that zyxin's C-terminal LIM domains function as a negative regulator of zyxin-VASP complexes. Zyxin LIM domains access to binding partners is negatively regulated by an unknown mechanism. One possibility is that zyxin LIM domains mediate a head-tail interaction, blocking interactions with other proteins. Such a mechanism might prevent both zyxin-VASP complexes activity and LIM domain access. In this report, the effect of LIM domains on zyxin-VASP complex assembly is defined. We find that zyxin LIM domains associate with zyxin's VASP binding sites, preventing zyxin from binding to PKA-phosphorylated VASP. Unphosphorylated VASP overcomes the head-tail interaction, a result of a direct interaction with the LIM domain region. Zyxin, like a growing number of actin regulators, is controlled by intramolecular interactions.

Mechanical forces facilitate actin polymerization at focal adhesions in a zyxin-dependent manner

We examined the effects of mechanical forces on actin polymerization at focal adhesions (FAs). Actin polymerization at FAs was assessed by introducing fluorescence-labeled actin molecules into permeabilized fibroblasts cultured on fibronectin. When cell contractility was inhibited by the myosin-II inhibitor blebbistatin, actin polymerization at FAs was diminished, whereas α5β1 integrin remained accumulated at FAs. This suggests that actin polymerization at FAs depends on mechanical forces. To examine the action of mechanical forces more directly, the blebbistatin-treated cells were subjected to a sustained uniaxial stretch, which induced actin polymerization at FAs. These results demonstrate the novel role of mechanical forces in inducing actin polymerization at FAs. To reveal the molecular mechanism underlying the force-induced actin polymerization at FAs, we examined the distribution of zyxin, a postulated actin-regulatory protein. Actin-polymerizing activity was strong at zyxin-rich FAs. Accumulation of zyxin at FAs was diminished by blebbistatin, whereas uniaxial stretching of the cells induced zyxin accumulation. Displacing endogenous zyxin from FAs by expressing the FA-targeting region of zyxin decreased the force-induced actin polymerization at FAs. These results suggest that zyxin is involved in mechanical-force-dependent facilitation of actin polymerization at FAs.

The LIM and SH3 domain protein family: structural proteins or signal transducers or both?

LIM and SH3 Protein 1 (LASP-1) was initially identified from a cDNA library of metastatic axillary lymph nodes (MLN) more than a decade ago. It was found to be overexpressed in human breast and ovarian cancer and became the first member of a newly defined LIM-protein subfamily of the nebulin group characterized by the combined presence of LIM and SH3 domains. LASP2, a novel LASP1-related gene was first identified and characterized in silico. Subsequently it proved to be a splice variant of the Nebulin gene and therefore was also termed LIM/nebulette. LASP-1 and -2 are highly conserved in their LIM, nebulin-like and SH3 domains but differ significantly at their linker regions. Both proteins are ubiquitously expressed and involved in cytoskeletal architecture, especially in the organization of focal adhesions. Here we present the first systematic review to summarize all relevant data concerning their domain organization, expression profiles, regulating factors and function. We compile evidence that both, LASP-1 and LASP-2, are important during early embryo- and fetogenesis and are highly expressed in the central nervous system of the adult. However, only LASP-1 seems to participate significantly in neuronal differentiation and plays an important functional role in migration and proliferation of certain cancer cells while the role of LASP-2 is more structural. The increased expression of LASP-1 in breast tumours correlates with high rates of nodal-metastasis and refers to a possible relevance as a prognostic marker.

Cytoskeleton-interacting LIM-domain protein CRP1 suppresses cell proliferation and protects from stress-induced cell death

Members of the cysteine-rich protein (CRP) family are actin cytoskeleton-interacting LIM-domain proteins known to act in muscle cell differentiation. We have earlier found that CRP1, a founding member of this family, is transcriptionally induced by UV radiation in human diploid fibroblasts [M. Gentile, L. Latonen, M. Laiho, Cell cycle arrest and apoptosis provoked by UV radiation-induced DNA damage are transcriptionally highly divergent responses, Nucleic Acids Res. 31 (2003) 4779-4790]. Here we show that CRP1 is induced by growth-inhibitory signals, such as increased cellular density, and cytotoxic stress induced by UV radiation or staurosporine. We found that high levels of CRP1 correlate with differentiation-associated morphology towards the myofibroblast lineage and that expression of ectopic CRP1 suppresses cell proliferation. Following UV- and staurosporine-induced stresses, expression of CRP1 provides a survival advantage evidenced by decreased cellular death and increased cellular metabolic activity and attachment. Our studies identify that CRP1 is a novel stress response factor, and provide evidence for its growth-inhibitory and cytoprotective functions.

Akt phosphorylation of zyxin mediates its interaction with acinus-S and prevents acinus-triggered chromatin condensation

Zyxin, a focal adhesion molecule, contains LIM domains and shuttles between the cytoplasm and the nucleus. Nuclear zyxin promotes cardiomyocyte survival, which is mediated by nuclear-activated Akt. However, the molecular mechanism of how zyxin antagonizes apoptosis remains elusive. Here, we report that zyxin binds to acinus-S, a nuclear speckle protein inducing apoptotic chromatin condensation after cleavage by caspases, and prevents its apoptotic action, which is regulated by Akt. Akt binds and phosphorylates zyxin on serine 142, leading to its association with acinus. Interestingly, 14-3-3gamma, but not zeta isoform selectively, triggers zyxin nuclear translocation, which is Akt phosphorylation dependent. Zyxin is also a substrate of caspases, but Akt phosphorylation is unable to prevent its apoptotic cleavage. Expression of zyxin S142D, a phosphorylation mimetic mutant, diminishes acinus proteolytic cleavage and chromatin condensation; by contrast, wild-type zyxin or unphosphorylated S142A mutant fails. Thus, Akt regulates zyxin/acinus complex formation in the nucleus, contributing to suppression of apoptosis.

Quantitative proteomic analysis of human breast epithelial cells with differential telomere length

Telomeres play important functional roles in cell proliferation, cell cycle regulation, and genetic stability, in which telomere length is critical. In this study, quantitative proteome comparisons for the human breast epithelial cells with short and long telomeres (184-hTERTL vs. 184-hTERTS and 90P-hTERTL vs. 90P-hTERTS), resulting from transfection of the human telomerase reverse transcriptase (hTERT) gene, were performed using cleavable isotope-coded affinity tags. More than 2000 proteins were quantified in each comparative experiment, with approximately 77% of the proteins identified in both analyses. In the cells with long telomeres, significant and consistent alterations were observed in metabolism (amino acid, nucleotide, and lipid metabolism), genetic information transmission (transcription and translation regulation, spliceosome and ribosome complexes), and cell signaling. Interestingly, the DNA excision repair pathway is enhanced, while integrin and its ligands are downregulated in the cells with long telomeres. These results may provide valuable information related to telomere functions.

Overexpression of LASP-1 mediates migration and proliferation of human ovarian cancer cells and influences zyxin localisation

LIM and SH3 protein 1 (LASP-1), initially identified from human breast cancer, is a specific focal adhesion protein involved in cell proliferation and migration. In the present work, we analysed the effect of LASP-1 on biology and function of human ovarian cancer cell line SKOV-3 using small interfering RNA technique (siRNA). Transfection with LASP-1-specific siRNA resulted in a reduced protein level of LASP-1 in SKOV-3 cells. The siRNA-treated cells were arrested in G(2)/M phase of the cell cycle and proliferation of the tumour cells was suppressed by 60-90% corresponding to around 70% of the cells being transfected successfully as seen by immunofluorescence. Moreover, transfected tumour cells showed a 40% reduced migration. LASP-1 silencing is accompanied by a reduced binding of the LASP-1-binding partner zyxin to focal contacts without changes in actin stress fibre and microtubule organisation or focal adhesion morphology as observed by immunofluorescence. In contrast, silencing of zyxin is not influencing cell migration and had neither influence on LASP-1 expression nor actin cytoskeleton and focal contact morphology suggesting that LASP-1 is necessary and sufficient for recruiting zyxin to focal contacts. The data provide evidence for an essential role of LASP-1 in tumour cell growth and migration, possibly through influencing zyxin localization.

Myopodin-mediated suppression of prostate cancer cell migration involves interaction with zyxin

Myopodin was identified as a tumor suppressor gene that is frequently deleted in aggressive prostate cancer. Expression of myopodin protein suppresses both tumor growth and metastasis in vitro and in vivo. In the present study employing a yeast two-hybrid system, we found that zyxin, a molecule known to regulate cell motility and migration, binds with myopodin with high affinity. The binding between zyxin and myopodin seems to be direct. Screening of a series of myopodin deletion mutants and peptide competition analyses revealed that myopodin is bound by zyxin at a site located within the sequence of the 19 amino acids at the myopodin COOH terminus. Importantly, this is the same region where the tumor suppressor activity of myopodin is located. The motility and invasion suppression activity of myopodin were significantly weakened in myopodin mutants lacking this sequence. Thus, our studies suggest that zyxin may be a critical functional regulator of myopodin.

Drosophila alpha-actinin in ovarian follicle cells is regulated by EGFR and Dpp signalling and required for cytoskeletal remodelling

alpha-Actinin is an evolutionarily conserved actin filament crosslinking protein with functions in both muscle and non-muscle cells. In non-muscle cells, interactions between alpha-actinin and its many binding partners regulate cell adhesion and motility. In Drosophila, one non-muscle and two muscle-specific alpha-actinin isoforms are produced by alternative splicing of a single gene. In wild-type ovaries, alpha-actinin is ubiquitously expressed. The non-muscle alpha-actinin mutant Actn(Delta233), which is viable and fertile, lacks alpha-actinin expression in ovarian germline cells, while somatic follicle cells express alpha-actinin at late oogenesis. Here we show that this latter population of alpha-actinin, termed FC-alpha-actinin, is absent from the dorsoanterior follicle cells, and we present evidence that this is the result of a negative regulation by combined Epidermal growth factor receptor (EGFR) and Decapentaplegic signalling. Furthermore, EGFR signalling increased the F-actin bundling activity of ectopically expressed muscle-specific alpha-actinin. We also describe a novel morphogenetic event in the follicle cells that occurs during egg elongation. This event involves a transient repolarisation of the basal actin fibres and the assembly of a posterior beta-integrin-dependent adhesion site accumulating alpha-actinin and Enabled. Clonal analysis using Actn null alleles demonstrated that although alpha-actinin was not necessary for actin fibre formation or maintenance, the cytoskeletal remodelling was perturbed, and Enabled did not localise in the posterior adhesion site. Nevertheless, epithelial morphogenesis proceeded normally. This work provides the first evidence that alpha-actinin is involved in the organisation of the cytoskeleton in a non-muscle tissue in Drosophila.

Novel identification of zyxin upregulations in the motile phenotype of hepatocellular carcinoma

Genome-wide copy number aberrations are common in hepatocellular carcinoma, although the precise genetic events underlying disease progression remain poorly defined. Previous work from our group has indicated several regional chromosomal gains such as chromosome 7q34-q36 that are associated with advanced metastatic tumors. Although the distal chromosome 7q gains have also been implicated in the progression of other malignancies, information on underlying targeted genes is limited. In this study, we have examined the chromosome 7q34-q36 region for involved gene(s) (or genes of interest). An integrated array-based comparative genomic hybridization and transcriptional mapping analyses has enabled us to identify a single candidate, zyxin on chromosome 7q34-q36. This array-derived finding was supported by quantitative reverse transcription-polymerase chain reaction, which also indicated common upregulations of zyxin in hepatocellular carcinoma tumors compared to their corresponding nonmalignant liver tissue (17/52 cases; 33%). Although there was no correlation between zyxin expression and tumor stagings, there was a significant increase in messenger RNA levels in hepatocellular carcinoma cases that presented with multifocal disease (211.5 +/- 936.9-fold) compared to those with solitary lesions (3.5 +/- 6.3-fold). Moreover, recurrence after resection was common in cases that displayed zyxin overexpressions in the initial resected tumor (P = 0.05). Functional examination of zyxin by small interfering RNA-mediated knockdown in Hep3B cell line indicated a significant inhibition on cell migration through porous membrane (P = 0.002) and invasion through matrigel-coated membrane (P = 0.005). In summary, mapping of chromosome 7q34-q36 has led to the identification of frequent zyxin overexpressions in hepatocellular carcinoma, and a potential role for zyxin in conferring a motile phenotype.

Mechanical forces alter zyxin unbinding kinetics within focal adhesions of living cells

The formation of focal adhesions that mediate alterations of cell shape and movement is controlled by a mechanochemical mechanism in which cytoskeletal tensional forces drive changes in molecular assembly; however, little is known about the molecular biophysical basis of this response. Here, we describe a method to measure the unbinding rate constant k(OFF) of individual GFP-labeled focal adhesion molecules in living cells by modifying the fluorescence recovery after photobleaching (FRAP) technique and combining it with mathematical modeling. Using this method, we show that decreasing cellular traction forces on focal adhesions by three different techniques--chemical inhibition of cytoskeletal tension generation, laser incision of an associated actin stress fiber, or use of compliant extracellular matrices--increases the k(OFF) of the focal adhesion protein zyxin. In contrast, the k(OFF) of another adhesion protein, vinculin, remains unchanged after tension dissipation. Mathematical models also demonstrate that these force-dependent increases in zyxin's k(OFF) that occur over seconds are sufficient to quantitatively predict large-scale focal adhesion disassembly that occurs physiologically over many minutes. These findings demonstrate that the molecular binding kinetics of some, but not all, focal adhesion proteins are sensitive to mechanical force, and suggest that force-dependent changes in this biophysical parameter may govern the supramolecular events that underlie focal adhesion remodeling in living cells.

Genetic ablation of zyxin causes Mena/VASP mislocalization, increased motility, and deficits in actin remodeling

Focal adhesions are specialized regions of the cell surface where integrin receptors and associated proteins link the extracellular matrix to the actin cytoskeleton. To define the cellular role of the focal adhesion protein zyxin, we characterized the phenotype of fibroblasts in which the zyxin gene was deleted by homologous recombination. Zyxin-null fibroblasts display enhanced integrin-dependent adhesion and are more migratory than wild-type fibroblasts, displaying reduced dependence on extracellular matrix cues. We identified differences in the profiles of 75- and 80-kD tyrosine-phosphorylated proteins in the zyxin-null cells. Tandem array mass spectrometry identified both modified proteins as isoforms of the actomyosin regulator caldesmon, a protein known to influence contractility, stress fiber formation, and motility. Zyxin-null fibroblasts also show deficits in actin stress fiber remodeling and exhibit changes in the molecular composition of focal adhesions, most notably by severely reduced accumulation of Ena/VASP proteins. We postulate that zyxin cooperates with Ena/VASP proteins and caldesmon to influence integrin-dependent cell motility and actin stress fiber remodeling.

Silencing of LASP-1 influences zyxin localization, inhibits proliferation and reduces migration in breast cancer cells

LIM and SH3 protein (LASP-1), initially identified from human breast cancer, is a specific focal adhesion protein involved in cell migration. LASP-1 is an actin binding protein, which also interacts with the proline-rich domains of zyxin, a scaffolding protein required for cell movement and gene transcription. In the present work, we analyzed the effect of LASP-1 on different human breast cancer cell lines. Transfection with LASP-1-specific siRNA resulted in a reduced protein level of LASP-1 in BT-20 and MCF-7 cell lines. The siRNA-treated cells were arrested in G2/M phase of cell cycle, and proliferation of the tumor cells was suppressed by 30-50% corresponding to around 50% of the cells being transfected successfully as seen by immunofluorescence. In addition, tumor cells showed a 50% reduced migration after siRNA treatment, while overexpression of LASP-1 in non-tumor PTK-2 cells, which do not express endogenous LASP-1, resulted in a significant increase in cell motility. LASP-1 silencing is accompanied with a reduced binding of the of LASP-1 binding partner zyxin to focal contacts without changes in actin stress fiber organization as observed in immunofluorescence experiments. The data provide evidence for an essential role of LASP-1 in tumor cell growth and migration, possibly by influencing the localization of zyxin.