Intermediate filaments: from cell architecture to nanomechanics. Type: Article; Author(s): Harald Herrmann, Harald Bär, Laurent Kreplak, Sergei V. Strelkov, Ueli . 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. Herrmann, H. and Baer, H. and Kreplak, L. and Strelkov, S. and Aebi, U.. () Intermediate Filaments: from Cell Architecture to.

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Intermediate filaments: from cell architecture to nanomechanics | University College London

Review Series Free access Find articles by Herrmann, H. Find articles by Strelkov, S. Find articles by Burkhard, P. Find articles by Aebi, U. First published July 1, – More info.

Intermediate filaments IFs are major constituents of the cytoskeleton and nuclear boundary in animal cells. They are of prime importance for the functional organization of structural elements.

Depending on the cell type, morphologically similar but biochemically distinct proteins form highly viscoelastic filament networks with multiple nanomechanical functions. Besides their primary role in cell plasticity and their established function as cellular stress absorbers, recently discovered gene defects have elucidated that structural alterations of IFs can affect their involvement both in signaling and in controlling gene regulatory networks.

Here, we highlight the basic structural and functional architecgure of IFs and derive a concept of how mutations may affect cellular architecture and thereby tissue construction and physiology.

Human cells contain three principal filament systems: They form a dynamic cytoskeleton that mechanically integrates cellular space 1 — 3. Actin and tubulin, the proteins constituting MFs and MTs, respectively, are highly conserved globular proteins.

In contrast, the members of the IF protein family, which is encoded by 70 genes, have very different primary amino acid sequences but a common domain organization 4.

In drom, an nanoechanics number of alternative splice forms of individual IF proteins have been discovered recently 5.

Intermediate filaments: primary determinants of cell architecture and plasticity

archjtecture IF proteins are differentially expressed during embryonic development, in parallel to distinct routes of differentiation, indicating that they have distinct tissue-specific functions 67. A further contrast to MFs and MTs is that higher metazoan interphase cells harbor two distinct IF systems in parallel, one in the nucleus and one in the cytoplasm Figure 1. The nuclear IF system in interphase cells constitutes a meshwork-like lamina of partially heterogeneous and independently organized filaments made of lamin A, lamin C, lamin B 1and lamin B 2 The lamina is apposed to the nuclear envelope and integrates inner nuclear membrane proteins, nuclear pore complexes, and heterochromatin into a functional interface 11 During mitosis, the lamin system is disassembled through the action of mitotic kinases, whereas the cytoplasmic IF system is depolymerized only in some cell types 14 Notably, a specific fraction of the lamins, lamin B 2is reorganized and incorporated into a supportive structure for the mitotic spindle that is termed the spindle matrix Cells from higher metazoan organisms harbor two distinct IF systems.

Indirect immunofluorescence staining with antibodies specific for A lamin A and B vimentin indicates that lamins frok concentrated at the inner nuclear membrane within the filameents, whereas vimentin forms a complex network within the cytoplasm that extends from the cell periphery immediately to the outer nuclear membrane.

D Merged image of A — C. Both MFs and MTs are topologically closed polymers, exhibiting a distinct polarity with a fast- and a slow-growing end. This polarity is essential for their use as tracks intermeiate motor proteins in unidirectional transport. In contrast, cytoplasmic IF proteins form apolar, smooth, and flexible filaments, with an approximate diameter of 10 nm, i. Slowly, but definitely, in vitro assembled lamin filaments form, probably by lateral association, extensive paracrystalline fibers Figure 2 10 In further stark contrast to MFs and MTs, for which general depolymerizing compounds often used in cancer chemotherapy are known, as of yet no general inhibitors of assembly have been described for IFs 20 — Ultrastructural analysis of the intdrmediate filamentous elements of metazoan cells after in vitro reconstitution.

In searches for IF fillaments in species other than mammals, it became apparent that IFs are hallmark cytoskeletal entities of all nznomechanics organisms. Hence, IF proteins are found in simple animals, such as the sweet water filamenhs Hydra and the nematode Caenorhabditis. Their absence may correlate with the fact that the body plan of plants and fungi depends on mechanical support provided by external cell walls.

Similarly, insects do not harbor cytoplasmic IF proteins; however, they do have a nuclear IF system made from authentic lamins. In higher vertebrates, the number of genes encoding IF proteins increased considerably architectjre evolution.

However, already lower invertebrates such as the nematode Caenorhabditis elegans express more than ten IF proteins, several of which are essential for life 23 — In humans, keratins represent the major cell of IF proteins and are encoded by 54 genes, about half of them being expressed by trichocytes, the specialized epithelial cells from which hair, nails, and similar appendages are generated. Thirteen genes encode mesenchymal, muscle, lens-specific, and neuronal IF proteins, and three genes encode the four nuclear lamins Although functional inactivation of genes encoding IF proteins, such as keratins, vimentin, and desmin, is mostly not embryonal lethal in mice, their absence has been shown to have a considerable impact on cellular and tissue physiology.

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For instance, mice lacking a functional vimentin gene are apparently normal, but primary fibroblasts derived from these animals exhibit a substantial reduction in stiffness Moreover, these cells display reduced mechanical stability, motility, and directional migration.

In addition, the spatial organization of the focal adhesion proteins is disturbed. As a result, the ability of these cells to repair a wounded fibroblast monolayer is severely impaired Correspondingly, wounds inflicted upon vimentin-deficient embryos heal much more slowly than those in control embryos This effect has been attributed to a failure of mesenchymal contraction at the embryonic wound site Another way in which a lack of vimentin affects cellular and tissue physiology is that the performance of both peripheral blood mononuclear cells and endothelial cells intermediste transendothelial migration is greatly diminished in vimentin-deficient mice, indicating that vimentin IFs are important for lymphocyte adhesion and transmigration through the endothelium In this Review, nanomechanic discuss how disease-causing mutations in IF protein—encoding genes possibly affect the structure and assembly of IF proteins.

Intermediate filaments: from cell architecture to nanomechanics – Semantic Scholar

Moreover, we present a view — with the muscle IF system as a paradigm — of how the change in basic IF properties architscture impact cell architecture and plasticity, and, thereby, eventually tissue function. In contrast to actin and tubulin, which are globular proteins, IF proteins are fibrous. In further contrast to MFs, MTs, and most other cytoskeletal proteins, IFs, together with some associated proteins such as desmoplakin and plectin, exhibit filaemnts pronounced stability in buffers containing high concentrations of detergent and salt, nanomechwnics that tilaments individual subunits very tightly associate through both ionic and hydrophobic interactions 32 — Molecular organization of IF proteins.

A Intermediatw representation of the structural architefture of frrom IF molecule. L1, L12, and L2 are linker segments. The stutter st indicates a discontinuity in the heptad repeat pattern of coil 2B Vimentin exhibits a pre-coil domain PCD. NLS, nuclear localization signal. B is adapted nanomechaanics Nature reviews molecular cell biology 8. The number of amino acids found in the individual helices is strictly conserved in the various types of IF proteins except for coil 1B, which is 42 amino acids or six heptads longer in the nuclear lamins than in vertebrate cytoplasmic IF proteins versus amino acids.

Notably, cytoplasmic IF proteins of lower invertebrates also exhibit the longer coil 1B version, underlining their evolutionary relationship to lamins. The length of the other coils is identical in all IF proteins: In the middle of coil 2B, a discontinuity in the heptad repeat pattern, a so-called stutteris consistently found in all IF proteins.

The linker domains between the coils are referred to as L1 nanpmechanics connects coil 1A and coil 1BL12 which connects coil 1B and coil 2Aand L2 which connects coil 2A and coil 2B.

The length of the individual linkers is not strictly conserved and their structure is not completely clear. However, in lamins, L1 is always 11—amino acids long, and in every IF protein, L2 is eight—amino acids long Figure 3 B 8 Although in silico structural prediction served well for some time to describe the structure of IF proteins, our recent crystallographic studies our unpublished observations indicate that certain adjustments to the earlier picture of the second half of the rod are needed.

The segments previously termed coil 2A, L2, and coil 2B and now designated coil 2 constitute a continuous coiled itermediate of amino acids, with two short segments in which the helices run in parallel.

The length of coil 2 in every IF protein is approximately Moreover, coiled-coil—forming non—IF proteins, such as myosins, tropomyosins, kinesins, plakins, and transcription factors, are also frequently found in many eukaryotic organisms.

A major advance in defining the structural organization of IF proteins has been accomplished through comparison of numerous individual protein sequences obtained by screening cDNA libraries of various species.

Due to the extremely high conservation of the building plan of practically all known IF proteins Figure 3 Bsome robust structure predictions became nanomechaniics 2. Comparing the sequences of many different IF proteins, especially over a wide range of species, two amino acid—sequence intermediaye, the so-called IF-consensus motif s, became evident at either end of the rod Figure 3 A: Similarly, the IF-consensus motif in coil 1A is extremely well conserved for individual IF proteins, such that the vimentins from species as diverse as primitive fish and humans are nearly identical over a stretch of 20 amino acids, including the absolutely conserved sequence LNDR, which is present in nearly every IF protein 42 Considering such conservation of amino acid sequence among diverse species, it comes as no surprise that mutations in these motifs interfere drastically with the function of IF proteins.

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This is highlighted by the discovery that inherited mutations in keratin-encoding genes cause human skin fragility diseases and the experimental verification in transgenic mice that such mutations have a devastating impact on proper skin architecture and function 50 Although disease-causing mutations in the coil 1A—consensus motif of keratins do not prevent extended in vitro filament assembly, arguing that they may interfere at the level of the spatial organization of the keratin cytoskeleton, an experimental mutation introduced into this consensus motif of vimentin inhibits in vitro assembly completely 52 — The two key questions to be answered if we are to understand the basic principles of IF biology are what is the structure of an IF dimer and, eventually, an entire IF at atomic detail, and how do particular IF proteins assemble into filaments?

To answer the first question, researchers attempted to solve the molecular organization of a coiled-coil vimentin dimer by X-ray crystallography. As a first step, the structure of a 28—amino acid segment of coil 2B of human vimentin, including its consensus motif, was solved using crystals formed by a chemically synthesized peptide Most interestingly, both glutamic acid and argininewhich form an inter-helical salt bridge in the crystal, are highly conserved among all IF proteins at the corresponding positions in coil 2B.

Thus, it is perhaps not surprising that mutations of the homologous amino acids in keratins, desmin, GFAP, and lamins cause disease According to the specific tissue localization of the proteins, mutations that affect the salt bridge in epidermal keratins lead to blistering diseases, in desmin they lead to muscular dystrophy, and in GFAP they lead to Alexander disease 57 — In contrast, mutation of lamin A argininethe arginine residue that corresponds to arginine in vimentin, to either leucine or histidine gives rise to limb-girdle muscular dystrophy type 1B and Emery-Dreifuss muscular dystrophy 65 — Moreover, the RH mutation can also cause dilated cardiomyopathy type A The initial crystallization studies were followed by the determination of the atomic structure of coil 1A and the second half of coil 2, then called coil 2B, for both vimentin and lamin A 43 Whereas coil 2B was shown to form bona fide dimeric complexes, coil 1A remained monomeric in solution Even more surprising, coil 1A was also obtained as a monomer in the crystals that formed at high-protein concentration, although it assumed the left-handed helical supertwist necessary for coiled-coil formation.

Nevertheless, in the full-length protein, coil 1A is not isolated but embedded in a distinct structural context, and larger fragments containing the head domain in addition to coil 1A behave, under certain in vitro conditions, like a dimeric coiled coil In addition, the head domains of IF proteins vimentin, lamins, and keratins may help to stabilize the dimeric state. Crystallized coil 2B exhibits very much the same structural fold in lamin A and vimentin, and it is expected that keratins will exhibit a very similar structure once they are crystallized, due to their conserved amino acid sequence organization in the corresponding segment.

Therefore, coil 2 represents a truly conserved structural feature that all IF proteins exhibit in common Figure 3 B. The functional meaning for this similarity is at present not clear. However, as most rules do not come without an exception, the single lamin of the nematode Caenorhabditis elegans lacks two heptads in coil 2 Nevertheless, this protein assembles into IF-like filaments and paracrystalline fibers 70 — These can be as short as 15 amino acids, as found for the tail of human keratin 19 K19or as long as 1, amino acids, as in the human neuronal IF protein nestin.

Moreover, the nestin tail exhibits a high degree of sequence variation, even between related species such as rat and human Hence, the nestin-encoding gene is in dynamic development in vertebrate evolution. The size of the head domain also varies widely and can be as long as amino acids, as in K5 Figure 3 B.

In contrast, the nestin head domain is unusually short, with only six amino acids.