A molecular trajectory of α-actinin activation

α-Actinin-4 recruits Shp2 into focal adhesions to potentiate ROCK2 activation in podocytes

α-Actinin-4 is crucial in the regulation of Shp2 FA targeting to enhance ROCK2-mediated actomyosin contractility and thereby strengthening cell adhesion and cytoskeletal architecture in podocytes.

Cell–matrix adhesions are mainly provided by integrin-mediated focal adhesions (FAs). We previously found that Shp2 is essential for FA maturation by promoting ROCK2 activation at FAs. In this study, we further delineated the role of α-actinin-4 in the FA recruitment and activation of Shp2. We used the conditional immortalized mouse podocytes to examine the role of α-actinin-4 in the regulation of Shp2 and ROCK2 signaling. After the induction of podocyte differentiation, Shp2 and ROCK2 were strongly activated, concomitant with the formation of matured FAs, stress fibers, and interdigitating intracellular junctions in a ROCK-dependent manner. Gene knockout of α-actinin-4 abolished the Shp2 activation and subsequently reduced matured FAs in podocytes. We also demonstrated that gene knockout of ROCK2 impaired the generation of contractility and interdigitating intercellular junctions. Our results reveal the role of α-actinin-4 in the recruitment of Shp2 at FAs to potentiate ROCK2 activation for the maintenance of cellular contractility and cytoskeletal architecture in the cultured podocytes.

Binding Dynamics of α-Actinin-4 in Dependence of Actin Cortex Tension

Mechanosensation of cells is an important prerequisite for cellular function, e.g., in the context of cell migration, tissue organization, and morphogenesis. An important mechanochemical transducer is the actin cytoskeleton. In fact, previous studies have shown that actin cross-linkers such as α-actinin-4 exhibit mechanosensitive properties in their binding dynamics to actin polymers. However, to date, a quantitative analysis of tension-dependent binding dynamics in live cells is lacking. Here, we present a, to our knowledge, new technique that allows us to quantitatively characterize the dependence of cross-linking lifetime of actin cross-linkers on mechanical tension in the actin cortex of live cells. We use an approach that combines parallel plate confinement of round cells, fluorescence recovery after photobleaching, and a mathematical mean-field model of cross-linker binding. We apply our approach to the actin cross-linker α-actinin-4 and show that the cross-linking time of α-actinin-4 homodimers increases approximately twofold within the cellular range of cortical mechanical tension, rendering α-actinin-4 a catch bond in physiological tension ranges.

The "Stressful" Life of Cell Adhesion Molecules: On the Mechanosensitivity of Integrin Adhesome

Cells have evolved into complex sensory machines that communicate with their microenvironment via mechanochemical signaling. Extracellular mechanical cues trigger complex biochemical pathways in the cell, which regulate various cellular processes. Integrin-mediated focal adhesions (FAs) are large multiprotein complexes, also known as the integrin adhesome, that link the extracellular matrix (ECM) to the actin cytoskeleton, and are part of powerful intracellular machinery orchestrating mechanotransduction pathways. As forces are transmitted across FAs, individual proteins undergo structural and functional changes that involve a conversion of chemical to mechanical energy. The local composition of early adhesions likely defines the regional stress levels and determines the type of newly recruited proteins, which in turn modify the local stress distribution. Various approaches have been used for detecting and exploring molecular mechanisms through which FAs are spatiotemporally regulated, however, many aspects are yet to be understood. Current knowledge on the molecular mechanisms of mechanosensitivity in adhesion proteins is discussed herein along with important questions yet to be addressed, are discussed.

α-Actinin Induces a Kink in the Transmembrane Domain of β3-Integrin and Impairs Activation via Talin

Integrin-mediated signaling is crucial for cell-substrate adhesion and can be triggered from both intra- and extracellular interactions. Although talin binding is sufficient for inside-out activation of integrin, other cytoplasmic proteins such as α-actinin and filamin can directly interfere with talin-mediated integrin activation. Specifically, α-actinin plays distinct roles in regulating αIIbβ3 versus α5β1 integrin. It has been shown that α-actinin competes with talin for binding to the cytoplasmic tail of β3-integrin, whereas it cooperates with talin for activating integrin α5β1. In this study, molecular dynamics simulations were employed to compare and contrast molecular mechanisms of αIIbβ3 and α5β1 activation in the presence and absence of α-actinin. Our results suggest that α-actinin impairs integrin signaling by both undermining talin binding to the β3-integrin cytoplasmic tail and inducing a kink in the transmembrane domain of β3-integrin. Furthermore, we showed that α-actinin promote talin association with β1-integrin by restricting the motion of the cytoplasmic tail and reducing the entropic barrier for talin binding. Taken together, our results showed that the interplay between talin and α-actinin regulates signal transmission via controlling the conformation of the transmembrane domain and altering natural response modes of integrins in a type-specific manner.

Mechanosensitive Conformation of Vinculin Regulates Its Binding to MAPK1

Extracellular matrix stiffness sensing by living cells is known to play a major role in a variety of cell mechanobiological processes, such as migration and differentiation. Various membrane and cytoplasmic proteins are involved in transmitting and transducing environmental signals to biochemical cascades. Protein kinases play a key role in regulating the activity of focal adhesion proteins. Recently, an interaction between mitogen-activated protein kinase (MAPK1) and vinculin was experimentally shown to mediate this process. Here, we adopt a molecular modeling approach to further investigate this interaction and its possible regulatory effects. Using a combination of data-driven flexible docking and molecular dynamics simulations guided by previous experimental studies, we predict the structure of the MAPK1-vinculin complex. Furthermore, by comparing the association of MAPK1 with open versus closed vinculin, we demonstrate that MAPK1 exhibits preferential binding toward the open conformation of vinculin, suggesting that the MAPK1-vinculin interaction is conformationally selective. Finally, we demonstrate that changes in the size of the D3-D4 cleft provide a structural basis for the conformational selectivity of the interaction.

Looking "Under the Hood" of Cellular Mechanotransduction with Computational Tools: A Systems Biomechanics Approach across Multiple Scales

Signal modulation has been developed in living cells throughout evolution to promote utilizing the same machinery for multiple cellular functions. Chemical and mechanical modules of signal transmission and transduction are interconnected and necessary for organ development and growth. However, due to the high complexity of the intercommunication of physical intracellular connections with biochemical pathways, there are many missing details in our overall understanding of mechanotransduction processes, i.e., the process by which mechanical signals are converted to biochemical cascades. Cell-matrix adhesions are mechanically coupled to the nucleus through the cytoskeleton. This modulated and tightly integrated network mediates the transmission of mechanochemical signals from the extracellular matrix to the nucleus. Various experimental and computational techniques have been utilized to understand the basic mechanisms of mechanotransduction, yet many aspects have remained elusive. Recently, in silico experiments have made important contributions to the field of mechanobiology. Herein, computational modeling efforts devoted to understanding integrin-mediated mechanotransduction pathways are reviewed, and an outlook is presented for future directions toward using suitable computational approaches and developing novel techniques for addressing important questions in the field of mechanotransduction.

The fifth sense: Mechanosensory regulation of alpha-actinin-4 and its relevance for cancer metastasis

Metastatic cancer cells invading through dense tumor stroma experience internal and external forces that are sensed through a variety of mechanosensory proteins that drive adaptations for specific environments. Alpha-actinin-4 (ACTN4) is a member of the α-actinin family of actin crosslinking proteins that is upregulated in several types of cancers. It shares 86% protein similarity with α-actinin-1, another non-muscle ACTN isoform, which appears to have a more modest role, if any, in cancer progression. While they share regulatory mechanisms, such as phosphorylation, calcium binding, phosphatidyl inositol binding, and calpain cleavage, α-actinin-4 exhibits a unique mechanosensory regulation that α-actinin-1 does not. This behavior is mediated, at least in part, by each protein's actin-binding affinity as well as the catch-slip-bond behavior of the actin binding domains. We will discuss currently known modes of ACTN4 regulation, their interactions, and how mechanosensation may provide major therapeutic targeting potential for cancer metastasis.

Transient mechanical strain promotes the maturation of invadopodia and enhances cancer cell invasion in vitro

Cancer cell invasion is influenced by various biomechanical forces found within the microenvironment. We have previously found that invasion is enhanced in fibrosarcoma cells when transient mechanical stimulation is applied within an in vitro mechano-invasion assay. This enhancement of invasion is dependent on cofilin (CFL1), a known regulator of invadopodia maturation. Invadopodia are actin-rich structures present in invasive cancer cells that are enzymatically active and degrade the surrounding extracellular matrix to facilitate invasion. In this study, we examine changes in gene expression in response to tugging on matrix fibers. Interestingly, we find that integrin β3 expression is downregulated and leads to an increase in cofilin activity, as evidenced by a reduction in its Ser3 phosphorylation levels. As a result, invadopodia lengthen and have increased enzymatic activity, indicating that transient mechanical stimulation promotes the maturation of invadopodia leading to increased levels of cell invasion. Our results are unique in defining an invasive mechanism specific to the invasive process of cancer cells that is triggered by tugging forces in the microenvironment, as opposed to rigidity, compression or stretch forces.

Fibronectin binding protein and Ca2+ play an access key role to mediate pathogenesis in Mycobacterium tuberculosis: An overview

The anomalous distribution of adhesive proteins throughout on the cell surface of the Mycobacterium tuberculosis H₃₇ Rv and their contribution in cell surface adhesion and host-pathogen interaction remain elusive. The completion of M. tuberculosis H₃₇ Rv genome sequence analysis gives some interesting information about polymorphic GC-rich repetitive sequence (PGRS) subfamily of M. tuberculosis that encodes fibronectin binding proteins (FnBP), which have been extensively studied, but the function in the pathogenesis of most of these proteins remains unknown and unclear. This review addresses the M. tuberculosis entry mechanism in the host cell. In particular, an effort has been made to focus on several aspects, (a) association of FnBP encodes by PE_PGRS protein family of M. tuberculosis during host-pathogen interactions. (b) Effect of calcium ions in and outside of the host cell is overriding to maintenance of calcium trafficking in phagocytosis. Furthermore, FnBP may be a potential source of antigenic variation that participating in evoking immune response. M. tuberculosis entry mechanism does not have a major influence alone, involvement of calcium ions, perhaps shed light on host-pathogen interaction relationship, and could open up new avenues for development of novel drug by targeting M. tuberculosis FnBP and blockade of selective adhesions could be useful for therapeutics.

Increased α-Actinin-2 Expression in the Atrial Myocardium of Patients with Atrial Fibrillation Related to Rheumatic Heart Disease

OBJECTIVES

Atrial fibrosis, a marker of atrial structural remodeling, plays a critical role in atrial fibrillation (AF). α- Actinin-2 is associated with structural remodeling related to stretching. The transforming growth factor-β1 (TGF-β1)/Smad pathway plays an important role in atrial fibrosis. We investigated the effects of the TGF-β1/Smad signaling pathway on α-actinin-2 in atrial fibrosis in patients with AF.

METHODS

Forty-one right atrial specimens obtained from patients with rheumatic heart disease (RHD) were divided into a chronic (c)AF group, i.e. RHD + cAF (n = 29), and a sinus rhythm group, i.e. RHD + sinus rhythm (n = 12). Patients with congenital heart disease (CHD) and sinus rhythm who underwent heart surgery served as controls (n = 10). Fibrosis was assessed by histological examination, and expression of α-actinin-2, TGF-β1 and Smad2/phosphorylated Smad2 (p-Smad2) was evaluated by immunohistochemistry, quantitative real-time PCR and Western blotting. In rat atrial fibroblasts treated with TGF-β1, the collagen content was measured using hydroxyproline detection, and α-actinin-2 and p-Smad2 were evaluated by semiquantitative reverse-transcription PCR and Western blotting.

RESULTS

The histology results revealed a significant increase in atrial fibrosis in AF patients. The collagen content, mRNA and protein expression levels of α-actinin-2 and the components of the TGF-β1/Smad signaling pathway were significantly gradually increased in the CHD + sinus rhythm, RHD + sinus rhythm and RHD + cAF groups (p < 0.05). The mRNA and protein levels of α-actinin-2 and TGF-β1 in RHD patients were positively correlated with the collagen volume fraction. A positive correlation between the expression of α-actinin-2 and TGF-β1 was also observed. In rat atrial fibroblasts treated with TGF-β1, the collagen content was greater than that in the control group (p < 0.05), and the expression levels of α- actinin-2 and p-Smad2 were also upregulated (p < 0.05).

CONCLUSIONS

α-Actinin-2 expression was increased in the atrial tissues of patients with AF secondary to RHD. α-Actinin-2 was upregulated via the TGF-β1/Smad pathway in atrial fibroblasts, which suggests that it may be involved in TGF-β1/Smad pathway-induced atrial fibrosis in patients with AF.

Dynamic Regulation of α-Actinin's Calponin Homology Domains on F-Actin

α-Actinin is an essential actin cross-linker involved in cytoskeletal organization and dynamics. The molecular conformation of α-actinin's actin-binding domain (ABD) regulates its association with actin and thus mutations in this domain can lead to severe pathogenic conditions. A point mutation at lysine 255 in human α-actinin-4 to glutamate increases the binding affinity resulting in stiffer cytoskeletal structures. The role of different ABD conformations and the effect of K255E mutation on ABD conformations remain elusive. To evaluate the impact of K255E mutation on ABD binding to actin we use all-atom molecular dynamics and free energy calculation methods and study the molecular mechanism of actin association in both wild-type α-actinin and in the K225E mutant. Our models illustrate that the strength of actin association is indeed sensitive to the ABD conformation, predict the effect of K255E mutation--based on simulations with the K237E mutant chicken α-actinin--and evaluate the mechanism of α-actinin binding to actin. Furthermore, our simulations showed that the calmodulin domain binding to the linker region was important for regulating the distance between actin and ABD. Our results provide valuable insights into the molecular details of this critical cellular phenomenon and further contribute to an understanding of cytoskeletal dynamics in health and disease.

Synaptopodin couples epithelial contractility to α-actinin-4-dependent junction maturation

A novel tension-sensitive junctional protein, synaptopodin, can relay biophysical input from cellular actomyosin contractility to induce biochemical changes at cell–cell contacts, resulting in structural reorganization of the junctional complex and epithelial barrier maturation.

The epithelial junction experiences mechanical force exerted by endogenous actomyosin activities and from interactions with neighboring cells. We hypothesize that tension generated at cell–cell adhesive contacts contributes to the maturation and assembly of the junctional complex. To test our hypothesis, we used a hydraulic apparatus that can apply mechanical force to intercellular junction in a confluent monolayer of cells. We found that mechanical force induces α-actinin-4 and actin accumulation at the cell junction in a time- and tension-dependent manner during junction development. Intercellular tension also induces α-actinin-4–dependent recruitment of vinculin to the cell junction. In addition, we have identified a tension-sensitive upstream regulator of α-actinin-4 as synaptopodin. Synaptopodin forms a complex containing α-actinin-4 and β-catenin and interacts with myosin II, indicating that it can physically link adhesion molecules to the cellular contractile apparatus. Synaptopodin depletion prevents junctional accumulation of α-actinin-4, vinculin, and actin. Knockdown of synaptopodin and α-actinin-4 decreases the strength of cell–cell adhesion, reduces the monolayer permeability barrier, and compromises cellular contractility. Our findings underscore the complexity of junction development and implicate a control process via tension-induced sequential incorporation of junctional components.

Probing the ATP-induced conformational flexibility of the PcrA helicase protein using molecular dynamics simulation

Helicases are enzymes that unwind double-stranded DNA (dsDNA) into its single-stranded components. It is important to understand the binding and unbinding of ATP from the active sites of helicases, as this knowledge can be used to elucidate the functionality of helicases during the unwinding of dsDNA. In this work, we investigated the unbinding of ATP and its effect on the active-site residues of the helicase PcrA using molecular dynamic simulations. To mimic the unbinding process of ATP from the active site of the helicase, we simulated the application of an external force that pulls ATP from the active site and computed the free-energy change during this process. We estimated an energy cost of ~85 kJ/mol for the transformation of the helicase from the ATP-bound state (1QHH) to the ATP-free state (1PJR). Unbinding led to conformational changes in the residues of the protein at the active site. Some of the residues at the ATP-binding site were significantly reoriented when the ATP was pulled. We observed a clear competition between reorientation of the residues and energy stabilization by hydrogen bonds between the ATP and active-site residues. We also checked the flexibility of the PcrA protein using a principal component analysis of domain motion. We found that the ATP-free state of the helicase is more flexible than the ATP-bound state.

The talin dimer structure orientation is mechanically regulated

Formation of a stable cell-substrate contact can be regulated by mechanical force, especially at the focal adhesion. Individual proteins that make up the focal adhesions, such as talin, can exhibit mechanosensing. We previously described one mode of talin mechanosensing in which the vinculin-binding site of talin is exposed after force-induced stretch of a single talin rod domain. Here, we describe a second mode of talin mechanosensing in which the talin dimer itself can adopt different orientations in response to mechanical stimulation. Using molecular dynamics models, we demonstrate that the C-terminus region of the talin dimer is flexible mainly at the linker between the dimerization helices and the nearby actin-binding helical bundle. Our molecular dynamics simulations reveal two possible orientations of the talin dimer at its C-terminus. The extracellular matrix (ECM)-bound integrins cross-linked by talin can be forced apart leading to an elongated orientation of the talin dimer, and the ECM-bound integrins can be forced together by the ECM producing a collapsed orientation of the talin dimer. Formation of the elongated orientation is shown to be more favorable. Switching between the two talin dimer orientations constitutes a mode of mechanosensing.

Multiscale mechanobiology: computational models for integrating molecules to multicellular systems

Mechanical signals exist throughout the biological landscape. Across all scales, these signals, in the form of force, stiffness, and deformations, are generated and processed, resulting in an active mechanobiological circuit that controls many fundamental aspects of life, from protein unfolding and cytoskeletal remodeling to collective cell motions. The multiple scales and complex feedback involved present a challenge for fully understanding the nature of this circuit, particularly in development and disease in which it has been implicated. Computational models that accurately predict and are based on experimental data enable a means to integrate basic principles and explore fine details of mechanosensing and mechanotransduction in and across all levels of biological systems. Here we review recent advances in these models along with supporting and emerging experimental findings.

On the activation of integrin αIIbβ3: outside-in and inside-out pathways

Integrin αIIbβ3 is a member of the integrin family of transmembrane proteins present on the plasma membrane of platelets. Integrin αIIbβ3 is widely known to regulate the process of thrombosis via activation at its cytoplasmic side by talin and interaction with the soluble fibrinogen. It is also reported that three groups of interactions restrain integrin family members in the inactive state, including a set of salt bridges on the cytoplasmic side of the transmembrane domain of the integrin α- and β-subunits known as the inner membrane clasp, hydrophobic packing of a few transmembrane residues on the extracellular side between the α- and β-subunits that is known as the outer membrane clasp, and the key interaction group of the βA domain (located on the β-subunit head domain) with the βTD (proximal to the plasma membrane on the β-subunit). However, molecular details of this key interaction group as well as events that lead to detachment of the βTD and βA domains have remained ambiguous. In this study, we use molecular dynamics models to take a comprehensive outside-in and inside-out approach at exploring how integrin αIIbβ3 is activated. First, we show that talin's interaction with the membrane-proximal and membrane-distal regions of integrin cytoplasmic-transmembrane domains significantly loosens the inner membrane clasp. Talin also interacts with an additional salt bridge (R734-E1006), which facilitates integrin activation through the separation of the integrin's α- and β-subunits. The second part of our study classifies three types of interactions between RGD peptides and the extracellular domains of integrin αIIbβ3. Finally, we show that the interaction of the Arg of the RGD sequence may activate integrin via disrupting the key interaction group between K350 on the βA domain and S673/S674 on the βTD.

Actin reorganization through dynamic interactions with single-wall carbon nanotubes

Single-wall carbon nanotubes (SWCNTs) have been widely used for biological applications in recent years, and thus, it is critical to understand how these inert nanomaterials influence cell behavior. Recently, it has been observed that cellular phenotypes such as proliferation, force generation and growth change upon SWCNT treatment, and SWCNTs directly affect the organization and redistribution of the actin cytoskeleton. However, the interactions between SWCNTs and actin at the molecular level or how this interaction changes actin structure remain largely unknown. Here, we investigated direct interaction of actin with SWCNT using all-atom molecular dynamics simulations and NIR spectroscopy of actin-dispersed SWCNTs. Actin can stably bind to the SWCNT surfaces via hydrophobic interactions but still allows nanotubes to slide and rotate on the actin surface. Our results establish several nanoscale conformational changes for the actin-SWCNT complexes, and we suggest these changes likely induce reorganization of actin filaments observed at larger scales.

Integrin cytoplasmic tail interactions

Integrins are heterodimeric cell surface adhesion receptors essential for multicellular life. They connect cells to the extracellular environment and transduce chemical and mechanical signals to and from the cell. Intracellular proteins that bind the integrin cytoplasmic tail regulate integrin engagement of extracellular ligands as well as integrin localization and trafficking. Cytoplasmic integrin-binding proteins also function downstream of integrins, mediating links to the cytoskeleton and to signaling cascades that impact cell motility, growth, and survival. Here, we review key integrin-interacting proteins and their roles in regulating integrin activity, localization, and signaling.

Protein conformation as a regulator of cell–matrix adhesion

Conformational changes within proteins play key roles in the regulation of cell–matrix adhesion. We discuss the mechanisms involved in conformational regulation, including mechanical signals, posttranslational modifications and intrinsically disordered proteins.

Mechanotransduction pathways linking the extracellular matrix to the nucleus

Cells contain several mechanosensing components that transduce mechanical signals into biochemical cascades. During cell-ECM adhesion, a complex network of molecules mechanically couples the extracellular matrix (ECM), cytoskeleton, and nucleoskeleton. The network comprises transmembrane receptor proteins and focal adhesions, which link the ECM and cytoskeleton. Additionally, recently identified protein complexes extend this linkage to the nucleus by linking the cytoskeleton and the nucleoskeleton. Despite numerous studies in this field, due to the complexity of this network, our knowledge of the mechanisms of cell-ECM adhesion at the molecular level remains remarkably incomplete. Herein, we present a review of the structures of key molecules involved in cell-ECM adhesion, along with an evaluation of their predicted roles in mechanical sensing. Additionally, specific binding events prompted by force-induced conformational changes of each molecule are discussed. Finally, we propose a model for the biomechanical events prominent in cell-ECM adhesion.

The interaction of vinculin with actin

Vinculin can interact with F-actin both in recruitment of actin filaments to the growing focal adhesions and also in capping of actin filaments to regulate actin dynamics. Using molecular dynamics, both interactions are simulated using different vinculin conformations. Vinculin is simulated either with only its vinculin tail domain (Vt), with all residues in its closed conformation, with all residues in an open I conformation, and with all residues in an open II conformation. The open I conformation results from movement of domain 1 away from Vt; the open II conformation results from complete dissociation of Vt from the vinculin head domains. Simulation of vinculin binding along the actin filament showed that Vt alone can bind along the actin filaments, that vinculin in its closed conformation cannot bind along the actin filaments, and that vinculin in its open I conformation can bind along the actin filaments. The simulations confirm that movement of domain 1 away from Vt in formation of vinculin 1 is sufficient for allowing Vt to bind along the actin filament. Simulation of Vt capping actin filaments probe six possible bound structures and suggest that vinculin would cap actin filaments by interacting with both S1 and S3 of the barbed-end, using the surface of Vt normally occluded by D4 and nearby vinculin head domain residues. Simulation of D4 separation from Vt after D1 separation formed the open II conformation. Binding of open II vinculin to the barbed-end suggests this conformation allows for vinculin capping. Three binding sites on F-actin are suggested as regions that could link to vinculin. Vinculin is suggested to function as a variable switch at the focal adhesions. The conformation of vinculin and the precise F-actin binding conformation is dependent on the level of mechanical load on the focal adhesion.

The interface between a cell and its substrate is strengthened by the formation of focal adhesions. In this study molecular dynamics simulations are used to explore the connectivity of one focal adhesion forming protein, vinculin, and the cytoskeletal filament, F-actin. The simulations demonstrate: (1) that vinculin can link along F-actin at these focal adhesions when it adopts an open conformation, (2) that the vinculin tail (Vt) can bind F-actin at its barbed-end preventing actin polymerization, (3) that vinculin can adopt two open conformations, and (4) that the second open conformation is necessary for vinculin to cap the actin filament. The results suggest that vinculin can act as a variable switch, changing its shape and the nature of its interaction with F-actin depending on the level of stress seen at a focal adhesion. Under the highest stress vinculin would adopt the open II conformation and link anywhere on F-actin, even its barbed-end. Under less stress vinculin could adopt the open I conformation and bind along F-actin. And under minimal stress vinculin could adopt its closed conformation. This variability allows for vinculin to truly function as the cell's mechanical reinforcing agent.

Localized lipid packing of transmembrane domains impedes integrin clustering

Integrin clustering plays a pivotal role in a host of cell functions. Hetero-dimeric integrin adhesion receptors regulate cell migration, survival, and differentiation by communicating signals bidirectionally across the plasma membrane. Thus far, crystallographic structures of integrin components are solved only separately, and for some integrin types. Also, the sequence of interactions that leads to signal transduction remains ambiguous. Particularly, it remains controversial whether the homo-dimerization of integrin transmembrane domains occurs following the integrin activation (i.e. when integrin ectodomain is stretched out) or if it regulates integrin clustering. This study employs molecular dynamics modeling approaches to address these questions in molecular details and sheds light on the crucial effect of the plasma membrane. Conducting a normal mode analysis of the intact αllbβ3 integrin, it is demonstrated that the ectodomain and transmembrane-cytoplasmic domains are connected via a membrane-proximal hinge region, thus merely transmembrane-cytoplasmic domains are modeled. By measuring the free energy change and force required to form integrin homo-oligomers, this study suggests that the β-subunit homo-oligomerization potentially regulates integrin clustering, as opposed to α-subunit, which appears to be a poor regulator for the clustering process. If α-subunits are to regulate the clustering they should overcome a high-energy barrier formed by a stable lipid pack around them. Finally, an outside-in activation-clustering scenario is speculated, explaining how further loading the already-active integrin affects its homo-oligomerization so that focal adhesions grow in size.

Focal adhesions are complex, dynamic structures of multiple proteins that act as the cell's mechanical anchorage to its surrounding. Integrins are proteins linking the cell inner and outer environments, which act as a bridge that crosses the cell membrane. Integrins respond to mechanical loads exerted to them by changing their conformations. Several diseases, such as atherosclerosis and different types of cancer, are caused by altered function of integrins. Essential to the formation of focal adhesions is the process of integrin clustering. Bidirectional integrin signaling involves conformational changes in this protein, clustering, and finally the assembly of a large intracellular adhesion complex. Integrin clustering is defined as the interaction of integrins to form lateral assemblies that eventually lead to focal adhesion formation. The effect of the plasma membrane on formation of integrin clusters has been largely neglected in current literature; subsequently some apparently contradictory data has been reported by a number of researchers in the field. Using a molecular dynamics modeling approach, a computational method that simulates systems in a full-atomic scale, we probe the role of the plasma membrane in integrin clustering and hypothesize a clustering scenario that explains the relationship between integrin activation and focal adhesion growth.

Vinculin, cell mechanics and tumour cell invasion

The focal adhesion protein, vinculin, is important for transmitting mechanical forces and orchestrating mechanical signalling events. Deregulation of vinculin results in altered cell adhesion, contractility, motility and growth, all of which are important processes in cancer metastasis. This review summarises recent reports on the role of vinculin in cellular force generation and signalling, and discusses implications for a role of vinculin in promoting cancer cell migration in 3D environments.