Intermediate filaments: from cell architecture to nanomechanics

Intermediate filaments (IFs) constitute a major structural element of animal cells. They build two distinct systems, one in the nucleus and one in the cytoplasm. In both cases, their major function is assumed to be that of a mechanical stress absorber and an integrating device for the entire cytoskeleton. In line with this, recent disease mutations in human IF proteins indicate that the nanomechanical properties of cell-type-specific IFs are central to the pathogenesis of diseases as diverse as muscular dystrophy and premature ageing. However, the analysis of these various diseases suggests that IFs also have an important role in cell-type-specific physiological functions.     

Biomechanical properties of intermediate filaments: from tissues to single filaments and back

The animal cell cytoskeleton consists of three interconnected filament systems: actin-containing microfilaments (MFs), microtubules (MTs), and the lesser known intermediate filaments (IFs). All IF proteins share a common tripartite domain structure and the ability to assemble into 8-12 nm wide filaments. Electron microscopy data suggest that IFs are built according to a completely different plan from that of MFs and MTs. IFs are known to impart mechanical stability to cells and tissues but, until recently, the biomechanical properties of single IFs were unknown. However, with the discovery of naturally occurring micrometer-wide IF bundles and the development of new methodologies to mechanically probe single filaments, it is now possible to propose a more unified view of IF biomechanics. Unlike MFs and MTs, single IFs can now be described as flexible, extensible and tough, which has important implications for our understanding of cell and tissue mechanics. Furthermore, the molecular mechanisms at play when IFs are deformed point toward a pivotal role for them in mechanotransduction.     

An epithelium-type cytoskeleton in a glial cell: astrocytes of amphibian optic nerves contain cytokeratin filaments and are connected by desmosomes

In higher vertebrates the cytoskeleton of glial cells, notably astrocytes, is characterized (a) by masses of intermediate filaments (IFs) that contain the hallmark protein of glial differentiation, the glial filament protein (GFP); and (b) by the absence of cytokeratin IFs and IF-anchoring membrane domains of the desmosome type. Here we report that in certain amphibian species (Xenopus laevis, Rana ridibunda, and Pleurodeles waltlii) the astrocytes of the optic nerve contain a completely different type of cytoskeleton. In immunofluorescence microscopy using antibodies specific for different IF and desmosomal proteins, the astrocytes of this nerve are positive for cytokeratins and desmoplakins; by electron microscopy these reactions could be correlated to IF bundles and desmosomes. By gel electrophoresis of cytoskeletal proteins, combined with immunoblotting, we demonstrate the cytokeratinous nature of the major IF proteins of these astroglial cells, comprising at least three major cytokeratins. In this tissue we have not detected a major IF protein that could correspond to GFP. In contrast, cytokeratin IFs and desmosomes have not been detected in the glial cells of brain and spinal cord or in certain peripheral nerves, such as the sciatic nerve. These results provide an example of the formation of a cytokeratin cytoskeleton in the context of a nonepithelial differentiation program. They further show that glial differentiation and functions, commonly correlated with the formation of GFP filaments, are not necessarily dependent on GFP but can also be achieved with structures typical of epithelial differentiation; i.e., cytokeratin IFs and desmosomes. We discuss the cytoskeletal differences of glial cells in different kinds of nerves in the same animal, with special emphasis on the optic nerve of lower vertebrates as a widely studied model system of glial development and nerve regeneration.     

The bacterial cytoskeleton: an intermediate filament-like function in cell shape

Various cell shapes are encountered in the prokaryotic world, but how they are achieved is poorly understood. Intermediate filaments (IFs) of the eukaryotic cytoskeleton play an important role in cell shape in higher organisms. No such filaments have been found in prokaryotes. Here, we describe a bacterial equivalent to IF proteins, named crescentin, whose cytoskeletal function is required for the vibrioid and helical shapes of Caulobacter crescentus. Without crescentin, the cells adopt a straight-rod morphology. Crescentin has characteristic features of IF proteins including the ability to assemble into filaments in vitro without energy or cofactor requirements. In vivo, crescentin forms a helical structure that colocalizes with the inner cell curvatures beneath the cytoplasmic membrane. We propose that IF-like filaments of crescentin assemble into a helical structure, which by applying its geometry to the cell, generates a vibrioid or helical cell shape depending on the length of the cell.     

Cytoskeletal proteins connecting intermediate filaments to cytoplasmic and nuclear periphery

Intermediate filaments (IFs), together with microtubules and microfilaments build up the cytoskeleton of most eukaryotic cells. Cytoplasmic IFs form a dense filament network radiating from the nucleus and extending to the plasma membrane. The association between the cytoplasmic and nuclear surfaces appears to provide a continuous link important for the organisation of the cytoplasm, for cellular communication, and possibly for the transport into and out of the nucleus. Cytoplasmic IFs approach the nuclear surface, thin fibrils seem to connect the IFs with the nuclear pore complexes and a direct interaction of cytoplasmic IFs with the nuclear lamin B has been observed by in vitro binding studies. However, none of the components that cross-link IFs to the nucleus has been unambiguously identified. Furthermore, if a direct interaction between cytoplasmic IFs and the nuclear lamin B occurs in vivo, the question of how cytoplasmic IFs get access to the nuclear interior remains to be resolved. The association of IFs with the plasma membranes involves different components, some of which are cell type specific. Two specialised complexes in epithelial cells: the desmosome and the hemidesmosome, serve as attachment sites for keratin filaments. Desmoplakin is considered as the cross-linking component of IFs to the desmosomal plaque, whereas BPAG1 (bullous pemphigoid antigen) would cross-link IFs at the hemidesmosomal plaque. In other cell types the modality of how IFs are anchored to the plasma membrane is less well understood. It involves different components such as the spectrin based membrane skeleton, ankyrin, myosin, plectin and certainly many other still unravelled partners. Association between the IFs and cellular membranes plays an important role in determining cell shape and tissue integrity. Thus, the identification and characterisation of the components involved in these interactions will be crucial for understanding the function of intermediate filaments.     

Intermediate filament-like cytoskeleton of Caulobacter crescentus

Eukaryotic cytoskeleton consists of three main types of filaments: actin microfilaments, microtubules and intermediate filaments (IFs). Actin and tubulin-like proteins are also found in bacteria where they perform diverse cytoskeletal functions. IFs, however, are considered to be a characteristic constituent of metazoan cells only, where they (among other functions) are involved in determination and maintenance of cell shape and cellular integrity. Surprisingly, a coiled coil-rich protein called crescentin was recently shown to play a key role in determining the complex curved and helical cell shapes of the bacterium Caulobacter crescentus, and to exhibit several characteristic properties of animal IF proteins. First, the arrangement of the coiled coil domains of crescentin closely resembles the tripartite molecular architecture of IF proteins. Second, crescentin also possesses the defining biochemical property of IF proteins to assemble into 10-nm-wide filaments in vitro without cofactors. Furthermore, crescentin forms a higher-order helical structure in vivo, which is localized asymmetrically along the concave side of the cell. In close association with the cell membrane, the crescentin structure promotes the helical growth of the cell and thereby determines a curved or a helical shape, depending on the length of the cell. The unexpected finding of an IF-like element in a bacterium raises several interesting questions concerning, for example, the molecular mechanisms whereby complex and asymmetric cell shapes are generated by different bacteria, or the functional and evolutionary relatedness of crescentin to animal IF proteins.     

Small heat shock proteins and the cytoskeleton: An essential interplay for cell integrity?

The cytoskeleton is a highly complex network of three major intracellular filaments, microfilaments (MFs), microtubules (MTs) and intermediate filaments (IFs). This network plays a key role in the control of cell shape, division, functions and interactions in animal organs and tissues. Dysregulation of the network can contribute to numerous human diseases. Although small HSPs (sHSPs) and in particular HSP27 (HSPB1) or αB-crystallin (HSPB5) display a wide range of cellular properties, they are mostly known for their ability to protect cells under stress conditions. Mutations in some sHSPs have been found to affect their ability to interact with cytoskeleton proteins, leading to IF aggregation phenotypes that mimick diseases related to disorders in IF proteins (i.e. desmin, vimentin and neuro-filaments). The aim of this review is to discuss new findings that point towards the possible involvement of IFs in the cytoprotective functions of sHSPs, both in physiological and pathological settings, including the likelihood that sHSPs such as HSPB1 may play a role during epithelial-to-mesenchymal transition (EMT) during fibrosis or cancer progression. This article is part of a Directed Issue entitled: Small HSPs in physiology and pathology.     

Conserved segments 1A and 2B of the intermediate filament dimer: their atomic structures and role in filament assembly

Intermediate filaments (IFs) are key components of the cytoskeleton in higher eukaryotic cells. The elementary IF 'building block' is an elongated coiled-coil dimer consisting of four consecutive alpha-helical segments. The segments 1A and 2B include highly conserved sequences and are critically involved in IF assembly. Based on the crystal structures of three human vimentin fragments at 1.4-2.3 A resolution (PDB entries 1gk4, 1gk6 and 1gk7), we have established the molecular organization of these two segments. The fragment corresponding to segment 1A forms a single, amphipatic alpha-helix, which is compatible with a coiled-coil geometry. While this segment might yield a coiled coil within an isolated dimer, monomeric 1A helices are likely to play a role in specific dimer-dimer interactions during IF assembly. The 2B segment reveals a double-stranded coiled coil, which unwinds near residue Phe351 to accommodate a 'stutter'. A fragment containing the last seven heptads of 2B interferes heavily with IF assembly and also transforms mature vimentin filaments into a new kind of structure. These results provide the first insight into the architecture and functioning of IFs at the atomic level.     

Softness, strength and self-repair in intermediate filament networks

One cellular function of intermediate filaments is to provide cells with compliance to small deformations while strengthening them when large stresses are applied. How IFs accomplish this mechanical role is revealed by recent studies of the elastic properties of single IF protein polymers and by viscoelastic characterization of the networks they form. IFs are unique among cytoskeletal filaments in withstanding large deformations. Single filaments can stretch to more than 3 times their initial length before breaking, and gels of IF withstand strains greater than 100% without damage. Even after mechanical disruption of gels formed by crossbridged neurofilaments, the elastic modulus of these gels rapidly recovers under conditions where gels formed by actin filaments are irreversibly ruptured. The polyelectrolyte properties of IFs may enable crossbridging by multivalent counterions, but identifying the mechanisms by which IFs link into bundles and networks in vivo remains a challenge.     

The nanomechanical properties of rat fibroblasts are modulated by interfering with the vimentin intermediate filament system

The contribution of the intermediate filament (IF) network to the mechanical response of cells has so far received little attention, possibly because the assembly and regulation of IFs are not as well understood as that of the actin cytoskeleton or of microtubules. The mechanical role of IFs has been mostly inferred from measurements performed on individual filaments or gels in vitro. In this study we employ atomic force microscopy (AFM) to examine the contribution of vimentin IFs to the nanomechanical properties of living cells under native conditions. To specifically target and modulate the vimentin network, Rat-2 fibroblasts were transfected with GFP-desmin variants. Cells expressing desmin variants were identified by the fluorescence microscopy extension of the AFM instrument. This allowed us to directly compare the nanomechanical response of transfected and untransfected cells at high spatial resolution by means of AFM. Depending on the variant desmin, transfectants were either softer or stiffer than untransfected fibroblasts. Expression of the non-filament forming GFP-DesL345P mutant led to a collapse of the endogenous vimentin network in the perinuclear region that was accompanied by localized stiffening. Correlative confocal microscopy indicates that the expression of desmin variants specifically targets the endogenous vimentin IF network without major rearrangements of other cytoskeletal components. By measuring functional changes caused by IF rearrangements in intact cells, we show that IFs play a crucial role in mechanical behavior not only at large deformations but also in the nanomechanical response of individual cells.     

APC binds intermediate filaments and is required for their reorganization during cell migration

Intermediate filaments (IFs) are components of the cytoskeleton involved in most cellular functions, including cell migration. Primary astrocytes mainly express glial fibrillary acidic protein, vimentin, and nestin, which are essential for migration. In a wound-induced migration assay, IFs reorganized to form a polarized network that was coextensive with microtubules in cell protrusions. We found that the tumor suppressor adenomatous polyposis coli (APC) was required for microtubule interaction with IFs and for microtubule-dependent rearrangements of IFs during astrocyte migration. We also show that loss or truncation of APC correlated with the disorganization of the IF network in glioma and carcinoma cells. In migrating astrocytes, vimentin-associated APC colocalized with microtubules. APC directly bound polymerized vimentin via its armadillo repeats. This binding domain promoted vimentin polymerization in vitro and contributed to the elongation of IFs along microtubules. These results point to APC as a crucial regulator of IF organization and confirm its fundamental role in the coordinated regulation of cytoskeletons.     

A multi-scale approach to understand the mechanobiology of intermediate filaments

The animal cell cytoskeleton consists of three interconnected filament systems: actin microfilaments, microtubules and the lesser known intermediate filaments (IFs). All mature IF proteins share a common tripartite domain structure and the ability to assemble into 8–12 nm wide filaments. At the time of their discovery in the 1980s, IFs were only considered as passive elements of the cytoskeleton mainly involved in maintaining the mechanical integrity of tissues. Since then, our knowledge of IFs structure, assembly plan and functions has improved dramatically. Especially, single IFs show a unique combination of extensibility, flexibility and toughness that is a direct consequence of their unique assembly plan. In this review we will first discuss the mechanical design of IFs by combining the experimental data with recent multi-scale modeling results. Then we will discuss how mechanical forces may interact with IFs in vivo both directly and through the activation of other proteins such as kinases.     

Co-expression of cytokeratins and neurofilament proteins in a permanent cell line: cultured rat PC12 cells combine neuronal and epithelial features

The cytoskeleton of the rat cultured cell line PC12, which is widely used in cell biology as a model system for neuron-like differentiation, displays an unusual combination of intermediate-sized filaments (IFs). As determined by electron microscopy, immunolocalization, and biochemical analyses, these cells contain, in addition to neurofilaments, an extended meshwork of bundles of cytokeratin IFs comprising cytokeratins A and D, equivalent to human cytokeratin polypeptides Nos. 8 and 18, irrespective of whether they are grown in the presence or absence of nerve growth factor. The two IF systems differ in their fibrillar arrays, the neurofilaments being concentrated in perinuclear aggregates similar to those found in certain neuroendocrine tumors of epithelial origin. We conclude that PC12 cells permanently co-express IFs of both the epithelial and the neuronal type and thus present an IF combination different from those of adrenal medulla cells and pheochromocytomas, i.e., the putative cells of origin of the line PC12. The IF cytoskeleton of PC12 cells resembles that of various neuroendocrine tumors derived from epithelial cells. The results show that the development of a number of typical neuronal differentiation features is compatible with the existence of an epithelial type IF cytoskeleton, i.e., cytokeratins. The implications of these findings concerning the validity of the PC12 cell line as a model for neuronal differentiation and possible explanations of the origin of cells with this type of IF co-expression are discussed.     

植物细胞中间纤维角蛋白性质的分析

角蛋白是植物细胞中间纤维的主要成分。应用选择性抽提和生物化学技术,分离纯化了豌豆根尖细胞58-、52 kD、白菜子叶52kD和胡萝卜悬浮细胞64kD角蛋白,测定了它们的氨基酸组成,结果表明上述角蛋白与动物细胞中间纤维角蛋白的氨基酸组成有较大的相似性。比较了动、植物细胞角蛋白的肽谱,结果显示它们之间存在较大的差异,但是植物细胞间角蛋白的肽谱比较一致,这提示它们属于同一蛋白家族,为植物中间纤维及其角蛋白的存在提供了新的论据。     

The mechanical properties of hydrated intermediate filaments: insights from hagfish slime threads

Intermediate filaments (IFs) impart mechanical integrity to cells, yet IF mechanics are poorly understood. It is assumed that IFs in cells are as stiff as hard alpha-keratin, F-actin, and microtubules, but the high bending flexibility of IFs and the low stiffness of soft alpha-keratins suggest that hydrated IFs may be quite soft. To test this hypothesis, we measured the tensile mechanics of the keratin-like threads from hagfish slime, which are an ideal model for exploring the mechanics of IF bundles and IFs because they consist of tightly packed and aligned IFs. Tensile tests suggest that hydrated IF bundles possess low initial stiffness (E(i) = 6.4 MPa) and remarkable elasticity (up to strains of 0.34), which we attribute to soft elastomeric IF protein terminal domains in series with stiffer coiled coils. The high tensile strength (180 MPa) and toughness (130 MJ/m(3)) of IF bundles support the notion that IFs lend mechanical integrity to cells. Their long-range elasticity suggests that IFs may also allow cells to recover from large deformations. X-ray diffraction and congo-red staining indicate that post-yield deformation leads to an irreversible alpha-->beta conformational transition in IFs, which leads to plastic deformation, and may be used by cells as a mechanosensory cue.     

Changes in cell morphology and cytoskeletal organization are induced by human mitotic checkpoint gene, Bub1

The mitotic spindle checkpoint prevents the onset of anaphase and subsequent cell division until chromosomes are properly aligned on a bipolar spindle. Thus, it regulates the cell division cycle by keeping cells with defective spindles from leaving mitosis. The budding uninhibited by benzimidazole (Bub1) is a key component of mitotic checkpoint. Bub1 encodes a serine/threonine kinase required for mitotic spindle checkpoint function. The regulation of cell morphology in eukaryotic cells is a complex process involving major components of the cytoskeleton including actin microfilaments, microtubules, and intermediate filaments (IFs). Here we show that Bub1 directly affects the structural integrity of IFs. Constitutive expression of Bub1 caused disappearance of filamentous vimentin, a type III IF, and consequently changed cell morphology. Expression of kinase domain—deleted Bub1 induced neither morphological change nor disappearance of vimentin. These observations suggest that Bub1 not only regulates the cell cycle, but also may be involved in the cytoskeletal control in interphase cells.     

Hierarchical Structure Controls Nanomechanical Properties of Vimentin Intermediate Filaments

Intermediate filaments (IFs), in addition to microtubules and microfilaments, are one of the three major components of the cytoskeleton in eukaryotic cells, playing a vital role in mechanotransduction and in providing mechanical stability to cells. Despite the importance of IF mechanics for cell biology and cell mechanics, the structural basis for their mechanical properties remains unknown. Specifically, our understanding of fundamental filament properties, such as the basis for their great extensibility, stiffening properties, and their exceptional mechanical resilience remains limited. This has prevented us from answering fundamental structure-function relationship questions related to the biomechanical role of intermediate filaments, which is crucial to link structure and function in the protein material''s biological context. Here we utilize an atomistic-level model of the human vimentin dimer and tetramer to study their response to mechanical tensile stress, and describe a detailed analysis of the mechanical properties and associated deformation mechanisms. We observe a transition from alpha-helices to beta-sheets with subsequent interdimer sliding under mechanical deformation, which has been inferred previously from experimental results. By upscaling our results we report, for the first time, a quantitative comparison to experimental results of IF nanomechanics, showing good agreement. Through the identification of links between structures and deformation mechanisms at distinct hierarchical levels, we show that the multi-scale structure of IFs is crucial for their characteristic mechanical properties, in particular their ability to undergo severe deformation of ≈300% strain without breaking, facilitated by a cascaded activation of a distinct deformation mechanisms operating at different levels. This process enables IFs to combine disparate properties such as mechanosensitivity, strength and deformability. Our results enable a new paradigm in studying biological and mechanical properties of IFs from an atomistic perspective, and lay the foundation to understanding how properties of individual protein molecules can have profound effects at larger length-scales.     

Regulation of the association of alpha 6 beta 4 with vimentin intermediate filaments in endothelial cells

The adhesion of microvascular endothelial cells to their underlying basement membrane is important for the maintenance of vascular integrity. Most integrins function in endothelial cell adhesion by forming a transmembrane link between their basement membrane ligand and the actin microfilament cytoskeleton. The alpha 6 beta 4 laminin-binding integrin, however, associates with vimentin intermediate filaments (IFs) in microvascular endothelial cells and therefore is likely to uniquely contribute to the barrier function of the endothelium. In this study, we examined the regulation of alpha 6 beta 4-vimentin IF association. We first tested the requirement for alpha 6 beta 4-laminin interactions and actin microfilament assembly. We found that alpha 6 beta 4 associated with vimentin IFs when cells were adherent to either laminin 5 or fibronectin, indicating that this association can occur independent of alpha 6 beta 4-ligand interactions. Additionally, we found that alpha 6 beta 4 was associated with vimentin IFs prior to cell spreading, indicating that changes in the microfilament cytoskeleton associated with changes in cell shape are also not required. Thus, although the association of alpha 6 beta 4 with vimentin IFs may strengthen cell adhesion by providing endothelial cells with an additional transmembrane linkage between the basement membrane and the cytoskeleton, this association is not itself regulated by alpha 6 beta 4-mediated adhesion. Finally, we tested the role of plectin in the association of alpha 6 beta 4 with vimentin IFs. Plectin is known to bind in vitro to both IFs and the beta 4 cytoplasmic domain (beta 4 tail), suggesting that it may be important for this linkage. Therefore, we generated deletion mutants of the beta 4 tail and compared the ability of alpha 6 beta 4 containing these deletions to associate with vimentin IFs. We targeted the two regions of the beta 4 tail known to bind to plectin IN VITRO: the N-terminal and C-terminal plectin binding sites. We found that deletion of the N-terminal binding site inhibited the association of alpha 6 beta 4 with vimentin IFs. Thus, plectin-beta 4 tail interactions may play an important role in connecting alpha 6 beta 4 with vimentin IFs and may prove to be important targets in the regulation of this association in endothelial cells.     

Role of phosphorylation on the structural dynamics and function of types III and IV intermediate filaments

Phosphorylation of types III and IV intermediate filaments (IFs) is known to regulate their organization and function. Phosphorylation of the amino-terminal head domain sites on types III and IV IF proteins plays a key role in the assembly/disassembly of IF subunits into 10 nm filaments, and influences the phosphorylation of sites on the carboxyl-terminal tail domain. These phosphorylation events are largely under the control of second messenger-dependent protein kinases and provide the cells a mechanism to reorganize the IFs in response to the changes in second messenger levels. In mitotic cells, Cdk1, Rho kinase, PAK1 and Aurora-B kinase are believed to regulate vimentin and glial fibrillary acidic protein phosphorylation in a spatio-temporal manner. In neurons, the carboxyl-terminal tail domains of the NF-M and NF-H subunits of heteropolymeric neurofilaments (NFs) are highly phosphorylated by proline-directed protein kinases. The phosphorylation of carboxyl-terminal tail domains of NFs has been suspected to play roles in forming cross-bridges between NFs and microtubules, slowing axonal transport and promoting their integration into cytoskeleton lattice and, in doing so, to control axonal caliber and stabilize the axon. The role of IF phosphorylation in disease pathobiology is discussed.     

Isomin: a novel cytoplasmic intermediate filament protein from an arthropod species

Background  

The expression of intermediate filaments (IFs) is a hallmark feature of metazoan cells. IFs play a central role in cell organization and function, acting mainly as structural stress-absorbing elements. There is growing evidence to suggest that these cytoskeletal elements are also involved in the integration of signalling networks. According to their fundamental functions, IFs show a widespread phylogenetic expression, from simple diblastic animals up to mammals, and their constituent proteins share the same molecular organization in all species so far analysed. Arthropods represent a major exception in this scenario. Only lamins, the nuclear IF proteins, have so far been identified in the model organisms analysed; on this basis, it has been considered that arthropods do not express cytoplasmic IFs.