Maintaining proteostasis under mechanical stress
2021; Springer Nature; Volume: 22; Issue: 8 Linguagem: Inglês
10.15252/embr.202152507
ISSN1469-3178
AutoresJörg Höhfeld, Thomas Benzing, Wilhelm Bloch, Dieter O. Fürst, Sebastian Gehlert, Michael Hesse, Bernd Hoffmann, Thorsten Hoppe, Pitter F. Huesgen, Maja Köhn, Waldemar Kolanus, Rudolf Merkel, Carien M. Niessen, Wojciech Pokrzywa, Markus M. Rinschen, Dagmar Wachten, Bettina Warscheid,
Tópico(s)Mitochondrial Function and Pathology
ResumoReview26 July 2021Open Access Maintaining proteostasis under mechanical stress Jörg Höhfeld Corresponding Author Jörg Höhfeld [email protected] orcid.org/0000-0003-4403-0757 Institute for Cell Biology, Rheinische Friedrich-Wilhelms University Bonn, Bonn, Germany Search for more papers by this author Thomas Benzing Thomas Benzing Department II of Internal Medicine and Center for Molecular Medicine Cologne (CMMC), University of Cologne, Cologne, Germany Search for more papers by this author Wilhelm Bloch Wilhelm Bloch orcid.org/0000-0003-1786-8853 Institute of Cardiovascular Research and Sports Medicine, German Sport University, Cologne, Germany Search for more papers by this author Dieter O Fürst Dieter O Fürst Institute for Cell Biology, Rheinische Friedrich-Wilhelms University Bonn, Bonn, Germany Search for more papers by this author Sebastian Gehlert Sebastian Gehlert Institute of Cardiovascular Research and Sports Medicine, German Sport University, Cologne, Germany Department for the Biosciences of Sports, Institute of Sports Science, University of Hildesheim, Hildesheim, Germany Search for more papers by this author Michael Hesse Michael Hesse Institute of Physiology I, Life & Brain Center, Medical Faculty, Rheinische Friedrich-Wilhelms University, Bonn, Germany Search for more papers by this author Bernd Hoffmann Bernd Hoffmann Institute of Biological Information Processing, IBI-2: Mechanobiology, Forschungszentrum Jülich, Jülich, Germany Search for more papers by this author Thorsten Hoppe Thorsten Hoppe orcid.org/0000-0002-4734-9352 Institute for Genetics, Cologne Excellence Cluster on Cellular Stress Responses in Aging-Associated Diseases (CECAD) and CMMC, University of Cologne, Cologne, Germany Search for more papers by this author Pitter F Huesgen Pitter F Huesgen orcid.org/0000-0002-0335-2242 Central Institute for Engineering, Electronics and Analytics, ZEA3, Forschungszentrum Jülich, Jülich, Germany CECAD, University of Cologne, Cologne, Germany Search for more papers by this author Maja Köhn Maja Köhn orcid.org/0000-0001-8142-3504 Institute of Biology III, Faculty of Biology, and Signalling Research Centres BIOSS and CIBSS, Albert-Ludwigs-University Freiburg, Freiburg, Germany Search for more papers by this author Waldemar Kolanus Waldemar Kolanus LIMES-Institute, Rheinische Friedrich-Wilhelms University Bonn, Bonn, Germany Search for more papers by this author Rudolf Merkel Rudolf Merkel orcid.org/0000-0003-3178-3282 Institute of Biological Information Processing, IBI-2: Mechanobiology, Forschungszentrum Jülich, Jülich, Germany Search for more papers by this author Carien M Niessen Carien M Niessen Department of Dermatology and CECAD, University of Cologne, Cologne, Germany Search for more papers by this author Wojciech Pokrzywa Wojciech Pokrzywa orcid.org/0000-0002-5110-4462 International Institute of Molecular and Cell Biology, Warsaw, Poland Search for more papers by this author Markus M Rinschen Markus M Rinschen orcid.org/0000-0002-9252-1342 Department of Biomedicine and Aarhus Institute of Advanced Studies, Aarhus University, Aarhus, Denmark Department of Medicine, University Medical Center Hamburg-Eppendorf, Hamburg, Germany Search for more papers by this author Dagmar Wachten Dagmar Wachten orcid.org/0000-0003-4800-6332 Institute of Innate Immunity, University Hospital Bonn, Bonn, Germany Search for more papers by this author Bettina Warscheid Bettina Warscheid orcid.org/0000-0001-5096-1975 Institute of Biology II, Faculty of Biology, and Signalling Research Centres BIOSS and CIBSS, Albert-Ludwigs-University Freiburg, Freiburg, Germany Search for more papers by this author Jörg Höhfeld Corresponding Author Jörg Höhfeld [email protected] orcid.org/0000-0003-4403-0757 Institute for Cell Biology, Rheinische Friedrich-Wilhelms University Bonn, Bonn, Germany Search for more papers by this author Thomas Benzing Thomas Benzing Department II of Internal Medicine and Center for Molecular Medicine Cologne (CMMC), University of Cologne, Cologne, Germany Search for more papers by this author Wilhelm Bloch Wilhelm Bloch orcid.org/0000-0003-1786-8853 Institute of Cardiovascular Research and Sports Medicine, German Sport University, Cologne, Germany Search for more papers by this author Dieter O Fürst Dieter O Fürst Institute for Cell Biology, Rheinische Friedrich-Wilhelms University Bonn, Bonn, Germany Search for more papers by this author Sebastian Gehlert Sebastian Gehlert Institute of Cardiovascular Research and Sports Medicine, German Sport University, Cologne, Germany Department for the Biosciences of Sports, Institute of Sports Science, University of Hildesheim, Hildesheim, Germany Search for more papers by this author Michael Hesse Michael Hesse Institute of Physiology I, Life & Brain Center, Medical Faculty, Rheinische Friedrich-Wilhelms University, Bonn, Germany Search for more papers by this author Bernd Hoffmann Bernd Hoffmann Institute of Biological Information Processing, IBI-2: Mechanobiology, Forschungszentrum Jülich, Jülich, Germany Search for more papers by this author Thorsten Hoppe Thorsten Hoppe orcid.org/0000-0002-4734-9352 Institute for Genetics, Cologne Excellence Cluster on Cellular Stress Responses in Aging-Associated Diseases (CECAD) and CMMC, University of Cologne, Cologne, Germany Search for more papers by this author Pitter F Huesgen Pitter F Huesgen orcid.org/0000-0002-0335-2242 Central Institute for Engineering, Electronics and Analytics, ZEA3, Forschungszentrum Jülich, Jülich, Germany CECAD, University of Cologne, Cologne, Germany Search for more papers by this author Maja Köhn Maja Köhn orcid.org/0000-0001-8142-3504 Institute of Biology III, Faculty of Biology, and Signalling Research Centres BIOSS and CIBSS, Albert-Ludwigs-University Freiburg, Freiburg, Germany Search for more papers by this author Waldemar Kolanus Waldemar Kolanus LIMES-Institute, Rheinische Friedrich-Wilhelms University Bonn, Bonn, Germany Search for more papers by this author Rudolf Merkel Rudolf Merkel orcid.org/0000-0003-3178-3282 Institute of Biological Information Processing, IBI-2: Mechanobiology, Forschungszentrum Jülich, Jülich, Germany Search for more papers by this author Carien M Niessen Carien M Niessen Department of Dermatology and CECAD, University of Cologne, Cologne, Germany Search for more papers by this author Wojciech Pokrzywa Wojciech Pokrzywa orcid.org/0000-0002-5110-4462 International Institute of Molecular and Cell Biology, Warsaw, Poland Search for more papers by this author Markus M Rinschen Markus M Rinschen orcid.org/0000-0002-9252-1342 Department of Biomedicine and Aarhus Institute of Advanced Studies, Aarhus University, Aarhus, Denmark Department of Medicine, University Medical Center Hamburg-Eppendorf, Hamburg, Germany Search for more papers by this author Dagmar Wachten Dagmar Wachten orcid.org/0000-0003-4800-6332 Institute of Innate Immunity, University Hospital Bonn, Bonn, Germany Search for more papers by this author Bettina Warscheid Bettina Warscheid orcid.org/0000-0001-5096-1975 Institute of Biology II, Faculty of Biology, and Signalling Research Centres BIOSS and CIBSS, Albert-Ludwigs-University Freiburg, Freiburg, Germany Search for more papers by this author Author Information Jörg Höhfeld *,1, Thomas Benzing2, Wilhelm Bloch3, Dieter O Fürst1, Sebastian Gehlert3,4, Michael Hesse5, Bernd Hoffmann6, Thorsten Hoppe7, Pitter F Huesgen8,9, Maja Köhn10, Waldemar Kolanus11, Rudolf Merkel6, Carien M Niessen12, Wojciech Pokrzywa13, Markus M Rinschen14,15, Dagmar Wachten16 and Bettina Warscheid17 1Institute for Cell Biology, Rheinische Friedrich-Wilhelms University Bonn, Bonn, Germany 2Department II of Internal Medicine and Center for Molecular Medicine Cologne (CMMC), University of Cologne, Cologne, Germany 3Institute of Cardiovascular Research and Sports Medicine, German Sport University, Cologne, Germany 4Department for the Biosciences of Sports, Institute of Sports Science, University of Hildesheim, Hildesheim, Germany 5Institute of Physiology I, Life & Brain Center, Medical Faculty, Rheinische Friedrich-Wilhelms University, Bonn, Germany 6Institute of Biological Information Processing, IBI-2: Mechanobiology, Forschungszentrum Jülich, Jülich, Germany 7Institute for Genetics, Cologne Excellence Cluster on Cellular Stress Responses in Aging-Associated Diseases (CECAD) and CMMC, University of Cologne, Cologne, Germany 8Central Institute for Engineering, Electronics and Analytics, ZEA3, Forschungszentrum Jülich, Jülich, Germany 9CECAD, University of Cologne, Cologne, Germany 10Institute of Biology III, Faculty of Biology, and Signalling Research Centres BIOSS and CIBSS, Albert-Ludwigs-University Freiburg, Freiburg, Germany 11LIMES-Institute, Rheinische Friedrich-Wilhelms University Bonn, Bonn, Germany 12Department of Dermatology and CECAD, University of Cologne, Cologne, Germany 13International Institute of Molecular and Cell Biology, Warsaw, Poland 14Department of Biomedicine and Aarhus Institute of Advanced Studies, Aarhus University, Aarhus, Denmark 15Department of Medicine, University Medical Center Hamburg-Eppendorf, Hamburg, Germany 16Institute of Innate Immunity, University Hospital Bonn, Bonn, Germany 17Institute of Biology II, Faculty of Biology, and Signalling Research Centres BIOSS and CIBSS, Albert-Ludwigs-University Freiburg, Freiburg, Germany *Corresponding author. Tel: +49 228 735308; Fax: +49 228 735302; E-mail: [email protected] EMBO Reports (2021)22:e52507https://doi.org/10.15252/embr.202152507 See the Glossary for abbreviations used in this article. PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Abstract Cell survival, tissue integrity and organismal health depend on the ability to maintain functional protein networks even under conditions that threaten protein integrity. Protection against such stress conditions involves the adaptation of folding and degradation machineries, which help to preserve the protein network by facilitating the refolding or disposal of damaged proteins. In multicellular organisms, cells are permanently exposed to stress resulting from mechanical forces. Yet, for long time mechanical stress was not recognized as a primary stressor that perturbs protein structure and threatens proteome integrity. The identification and characterization of protein folding and degradation systems, which handle force-unfolded proteins, marks a turning point in this regard. It has become apparent that mechanical stress protection operates during cell differentiation, adhesion and migration and is essential for maintaining tissues such as skeletal muscle, heart and kidney as well as the immune system. Here, we provide an overview of recent advances in our understanding of mechanical stress protection. Glossary ABD actin-binding domain AJ adherens junction CASA chaperone-assisted selective autophagy CIM critical illness myopathy CMA chaperone-mediated autophagy ECM extracellular matrix FA focal adhesion FAK focal adhesion kinase HSP heat shock protein ICU intensive care unit Ig immunoglobulin mTORC1 mechanistic target of rapamycin complex I PDZ PSD-95/Dlg1/ZO-1 domain SD slit diaphragm sHSP small heat shock protein TPR tetratricopeptide repeat TSC tuberous sclerosis complex UCS UNC-45/CRO1/She4p domain UnDOx unfolded domain oxidation UPS ubiquitin–proteasome system Introduction In a living organism, the integrity of the cellular protein network is under constant threat. Increased temperature, the generation of reactive oxygen species or the accumulation of aberrant proteins lead to heat, oxidative and proteotoxic stress, which shifts the conformational equilibrium of proteins towards partially unfolded and misfolded states prone to aggregation (Richter et al, 2010; Dahl et al, 2015; Sala et al, 2017; Hipp et al, 2019). Protein aggregation can interfere with essential cellular processes causing severe pathology including neurodegeneration and dementia (Douglas & Dillin, 2010; Choe et al, 2016; Woerner et al, 2016; Hipp et al, 2019). To protect the proteome against stressful conditions, the cell employs protein folding and degradation systems. These highly regulated systems recognize partially unfolded and misfolded conformers and promote refolding or, in case of terminal damage, mediate disposal (Fig 1A) (Buchberger et al, 2010; Balchin et al, 2016; Cohen-Kaplan et al, 2016; Dikic, 2017; Klimek et al, 2017; Nillegoda et al, 2018). Key players in stress protection are molecular chaperones and their regulatory cochaperones. They sense protein unfolding based on the exposure of hydrophobic surfaces that are otherwise buried in the native structure (Kim et al, 2013b; Dahiya & Buchner, 2019; Rosenzweig et al, 2019). Chaperone binding prevents aggregation and directs the non-native client onto specific folding or degradation pathways. Protein degradation can be mediated by the ubiquitin–proteasome system (UPS) or selective autophagy (Cohen-Kaplan et al, 2016; Dikic, 2017; Höhfeld & Hoppe, 2018). In both cases, degradation is usually initiated by the attachment of a ubiquitin-derived degradation signal onto the client, based on the activity of client-specific ubiquitin conjugation systems (Fig 1A) (Khaminets et al, 2016; Gatica et al, 2018). Depending on the selected degradation pathway, the generated degradation signal is recognized either by ubiquitin receptors in the proteasome or by autophagic ubiquitin adaptors that facilitate an association of the ubiquitylated client with phagophore membranes leading to autophagosome formation and client degradation in autolysosomes. An extensive body of work has elucidated the adaptation of cellular folding and degradation machineries in response to heat, oxidative and proteotoxic stress (Richter et al, 2010; Vihervaara & Sistonen, 2014; Dahl et al, 2015; Sala et al, 2017; Nillegoda et al, 2018; Dahiya & Buchner, 2019; Hipp et al, 2019; Rosenzweig et al, 2019). In contrast, the impact of mechanical stress on these machineries became only recently apparent, when force-regulated proteostasis systems were identified and shown to be essential for cell and tissue homeostasis (Janiesch et al, 2007; Arndt et al, 2010; Gazda et al, 2013; Ulbricht et al, 2013, 2015; Kathage et al, 2017; Rinschen et al, 2017b; Donkervoort et al, 2020). Figure 1. Protecting the proteome against mechanical stress relies on protein folding and degradation systems that recognize force-unfolded proteins (A) Stress protection systems comprise chaperone/cochaperone complexes and specialized E2/E3 ubiquitin conjugation systems, which recognize unfolded proteins under stress conditions to facilitate their refolding or their degradation by the proteasome and autophagic/lysosomal pathways. Ubiquitylation can be assisted by molecular chaperones or can proceed through a direct recognition of unfolded proteins by quality control E3 ubiquitin ligases. Stress-induced signalling pathways regulate the activity of the involved folding and degradation systems. (B) Cellular systems are permanently exposed to a wide variety of mechanical signals. After recognition and transmission through cell–cell and cell–matrix contacts as well as cytoskeletal systems, these signals induce a variety of specific cell responses. (C) Mechanical forces can trigger the unfolding of mechanosensory proteins such as talin, which links integrin-containing adhesion complexes in the plasma membrane (PM) to the actin cytoskeleton (ECM—extracellular matrix). Download figure Download PowerPoint Mechanical stress is a highly relevant physiological stimulus Cells in multicellular organisms are permanently exposed to mechanical forces, which are either passively applied or generated inside the cell (Discher et al, 2009; Irianto et al, 2016; Hu et al, 2017; Wolfenson et al, 2019) (Fig 1B). Indeed, the ability to generate and respond to mechanical forces is a basic requirement for the development, life and survival of organisms (Alon & Dustin, 2007; Johnson et al, 2007; Moore et al, 2010; DuFort et al, 2011; Hoffman et al, 2011; Mammoto et al, 2013; Yusko & Asbury, 2014). To stand and walk, to take a breath, or to pump blood through the body depends on force generation in skeletal and cardiac muscle. Cells in the skin, the cardio-vascular system and the kidney must withstand mechanical forces under touch and during blood circulation and filtration. Immune cells are subjected to mechanical forces when they attach to blood vessel walls upon exit from the vasculature and during migration to sites of infection and inflammation. Finally, the division, migration, functionality and differentiation of cells rely on their ability to sense the mechanical properties of their surrounding such as tissue elasticity and matrix stiffness (Engler et al, 2006; Gilbert et al, 2010; Hersch et al, 2013; Wolfenson et al, 2019). Impairment of these processes results in diverse diseases, including myopathies, heart and kidney failure, leukocyte adhesion deficiencies and cancer (Etzioni, 2007; DuFort et al, 2011; Hoffman et al, 2011; Yang et al, 2015; Ayad et al, 2019; Maurer & Lammerding, 2019). The actin cytoskeleton is centrally involved in generating and responding to mechanical forces (Small & Resch, 2005; Johnson et al, 2007; Luo et al, 2013; Schiller & Fässler, 2013; Gautel & Djinović-Carugo, 2016; Schiffhauer et al, 2016). Comprised of globular actin molecules, which polymerize into a dynamic filament network, it provides shape and mechanical stability to cells, generates mechanical force and drives cell migration. Actin-anchoring sites are of particular importance for force generation and force transduction. At the plasma membrane, proteins of the integrin and cadherin families together with their interaction partners form cell adhesions that link the actin cytoskeleton to the extracellular matrix and neighbouring cells (Fig 1B and C) (Schiller & Fässler, 2013; Irianto et al, 2016; Schiffhauer et al, 2016). At the nucleo-cytoskeletal interface, the actin cytoskeleton contacts the nuclear envelope to impact genome functions (Simon & Wilson, 2011; Isermann & Lammerding, 2013; Graham & Burridge, 2016; Cho et al, 2017; Nava et al, 2020). Furthermore, actin filaments are anchored to the constituents of the blood filtration barrier at the slit diaphragm in the kidney (Huber & Benzing, 2005; New et al, 2014), and in skeletal and cardiac muscle, actin anchoring is mediated by Z-discs, which limit the minimal contractile units of muscle, the sarcomeres (Fig 2) (Frank et al, 2006). Mechanical forces, externally applied through blood pressure and movement or internally generated through actomyosin contraction, accumulate at anchoring sites leading to a high stress concentration. Figure 2. The UNC-45-containing chaperone system mediates the folding and assembly of myosin in muscle sarcomeres (A) The sarcomere represents the smallest contractile unit of striated muscles. It is repetitively arranged in tubular myofibrils, numerous bundles of which form the muscle fibre. Z-discs limit the sarcomere on both sides and mediate the anchoring of actin thin filaments. Myosin thick filaments are intercalated between the actin filaments and are connected at the M-line. The I-band is the region that contains exclusively actin filaments. (B) Actin and filamin crosslink actin filaments within the Z-disc. In addition, filamin interacts with integrin molecules in the sarcolemma. (C) Schematic representation of the domain structure of the cochaperone UNC-45. UCS, UNC-45/CRO1/She4p domain and TPR, tetratricopeptide repeat. (D) Oligomeric UNC-45 coordinates the activity of HSP70 and HSP90 chaperone proteins during the folding and assembly of myosin filaments. (E) The UNC-45 oligomer provides a molecular scaffold for enforcing the regular spacing of folded myosin head domains in the myosin thick filament. Download figure Download PowerPoint Force-induced protein unfolding Many constituents of actin-anchoring sites undergo force-induced unfolding (Hu et al, 2017; Saini & Discher, 2019). One such protein is vinculin, which links the actin cytoskeleton to integrin complexes at cell–matrix adhesions (focal adhesions, FAs) and to cadherin complexes at cell–cell junctions (adherens junctions, AJs) (Atherton et al, 2016; Goldmann, 2016). Mechanical force disrupts auto-inhibitory interactions between the head and tail domains of vinculin and in this way stimulates binding to other anchoring components and actin filaments (Atherton et al, 2016). At FAs, vinculin interacts with talin, which is also a mechanosensitive protein (Fig 1C) (Austen et al, 2015; Ringer et al, 2017; Lemke et al, 2019; Fischer et al, 2021). Remarkably, mechanical forces do not just induce a domain rearrangement, but instead trigger the loss of tertiary and secondary structural elements within talin (del Rio et al, 2009; Yao et al, 2014a, 2016). Talin possesses an amino-terminal integrin binding domain connected to a rod-like structure formed by 13 α-helical bundles, the last of which contacts actin. When talin is stretched in cells, tensile forces are exerted linearly along the molecule (Fig 1C) (Ringer et al, 2017). The mechanical properties of talin domains were analysed in biochemical experiments using magnetic tweezers, total internal reflection fluorescence and atomic force microscopy (del Rio et al, 2009; Yao et al, 2014a, 2016). These experiments demonstrated that 12 out of the 13 helical bundles undergo force-induced unfolding, leading to a disruption of helix–helix interactions and the complete unfolding of individual helices. In turn, buried vinculin binding sites become exposed. Talin unfolding thus contributes to the transduction of mechanical signals. Importantly, the force required for the unfolding of different helical bundles varies between 5 and 20 pN (Yao et al, 2016). This suggests that the talin rod acts as a force buffer. Integrin–actin contacts are maintained by talin over a significant force range through the subsequent unfolding of single helical bundles (Fig 1C). Another proposed mechanosensor and interactor of various FA proteins (i.e. vinculin and FAK) is the stretch-sensitive adaptor protein p130Cas (Sawada et al, 2006). Following early FA maturation, p130Cas is recruited to FAs, unfolds due to applied tension and exposes up to 15 formerly hidden phosphorylation sites (Sawada et al, 2006). Phosphorylated p130Cas engages in different mechanosensitive signalling cascades, in agreement with a role of the protein as a central hub for force transmission. Moreover, the interaction of p130Cas with vinculin has been proposed to stabilize an open conformation of vinculin, thereby promoting talin binding and FA stability (Janoštiak et al, 2014). Finally, p130Cas has been shown to be important for actin reorientation in response to cyclic mechanical stretch (Niediek et al, 2012). Importantly, mechanical forces are transduced not only at the cell–matrix interface but also at the cell–cell contacts through AJs. AJs contain α-catenin as a main mechanosensor (Huveneers & de Rooij, 2013; Yao et al, 2014b; Ladoux et al, 2015). α-catenin physically couples the cadherin complex, the core component of AJs, to the actin cytoskeleton. Loss of α-catenin function disrupts epithelial integrity, the ability of cells to undergo coordinated cell movements and the ability to reorient actin fibres under cyclic strain (Noethel et al, 2018). α-catenin comprises three evolutionary conserved regions: an N-terminal region that binds to β-catenin, a central M-region and a C-terminal actin-binding domain (ABD). Opening of the ABD under tension results in enforced actin binding and facilitates dimerization of the ABD to promote actin bundling (Buckley et al, 2014; Ishiyama et al, 2018). In addition, also the M-region of α-catenin undergoes a conformational change when stretched to reveal a formerly hidden binding site for vinculin (Yonemura et al, 2010; Choi et al, 2012; Yao et al, 2014b). The recruitment of vinculin is thought to strengthen the AJ-actin link and seems to enhance the mechanosensitivity of α-catenin (Noethel et al, 2018). A function as mechanotransducer and force buffer was also assigned to filamin, which binds actin along stress fibres, at focal adhesions and at the sarcomeric Z-disc (Fig 2B) (Frank et al, 2006; Ehrlicher et al, 2011; Nakamura et al, 2011, 2014). Filamin is a large homodimeric protein comprised of two rods with terminal actin and integrin binding sites, respectively, on opposing ends. This domain arrangement enables the protein to crosslink and anchor actin filaments. The filamin rod is formed by 24 immunoglobulin (Ig) domains, some of which engage in pairwise interactions (Nakamura et al, 2011). Single-molecule mechanical measurements demonstrated that the Ig domain pair 20-21 of filamin functions as an auto-inhibited force-activatable mechanosensor (Lad et al, 2007; Rognoni et al, 2012). In the inhibited state, the amino-terminal ß-strand of domain 20 folds onto domain 21. Mechanical force disrupts this interaction, leading to rod extension and the exposure of binding sites in domain 21 for integrins and other partner proteins. Molecular dynamics simulation suggests that also other Ig domains of filamin are sensitive to mechanical force (Kesner et al, 2010). Thereby, unfolding of the amino-terminal ß-strand seems to represent a common initial step, resulting in metastable states similar to those observed upon thermal unfolding (Kesner et al, 2010). The gigantic myofilament protein titin (Mr: ˜3,800 kDa) spans the muscle sarcomere from the Z-disc to the M-band and confers elasticity and passive force under stretch (Linke & Hamdani, 2014). Titin regions localized in the I-band adjacent to the Z-disc function as molecular springs that extend and condense as the muscle stretches and contracts. At low forces, regions containing tandemly arranged Ig domains elongate due to the straightening of interdomain linkers, whereas at higher forces, intrinsically disordered structures are extended (Linke & Hamdani, 2014; Mártonfalvi et al, 2014). As already mentioned above, talin unfolding is observed at forces between 5 and 20 pN (Yao et al, 2016). Unfolding of the Ig domain 20–21 pair of filamin is triggered at forces between 2 and 4 pN (Rognoni et al, 2012), and conformational transitions of titin occur upon stretching with forces as low as 5 pN (Mártonfalvi et al, 2014). To put this into perspective, one should note that the contraction of a single actin–myosin unit generates a force of 3–4 pN (Finer et al, 1994). Thus, cytoskeleton components constantly undergo unfolding reactions under physiological conditions. Indeed, studies with cultured cells illustrate the prevalence and significance of force-induced protein unfolding in the cellular context. In one approach, the accessibility of cysteine residues was monitored (Johnson et al, 2007). Cysteines are often buried within tertiary and/or quaternary structure, and their force-induced exposure is therefore a readout for a significant loss of structural elements. Red blood cells were subjected to fluid shear stress to determine the force-induced unfolded proteome, and also adherent mesenchymal stem cells were analysed, in which traction forces are generated based on actomyosin contraction (Johnson et al, 2007). In both cell types, mechanical stress caused unfolding of the actin-associated scaffolding protein spectrin, confirming previous biochemical studies on the mechanosensitive properties of the cytoskeleton protein (Rief et al, 1999; Altmann et al, 2002; Johnson et al, 2007). Moreover, in adherent mesenchymal stem cells traction forces resulted in the unfolding of non-muscle myosin and filamin (Johnson et al, 2007). The latter was also at the focus of another study, in which a fluorescence resonance energy transfer (FRET)-based sensor was constructed to visualize the disruption of the Ig domain 20–21 pair of filamin in adherent mammalian cells (Nakamura et al, 2014). Maximal opening of filamin occurred predominantly at the cell edge and in protruding areas following pharmacological stimulation of cell spreading and migration. Taken together, the described findings demonstrate that cells of a multicellular organism are challenged continuously by mechanical protein unfolding. Even in the absence of externally applied force, the tension generated inside adherent and migrating cells is sufficient to alter the conformation of abundant cytoskeleton proteins, leading to a significant loss of structural elements. This sets the stage for a critical involvement of protein folding and degradation machineries, which recognize force-unfolded proteins during mechanical stress protection. Proteostasis machineries involved in mechanical stress protection Molecular chaperones are defined by the ability to bind and stabilize unfolded proteins (see Fig 1A) (Hartl et al, 2011; Dahiya & Buchner, 2019; Rosenzweig et al, 2019). This enables them to assist in the folding, sorting and degradation of proteins, often in cooperation with regulating cochaperones. Small heat shock proteins (sHSPs), members of the HSP70 and HSP90 chaperone families as well as diverse cochaperones of HSP70 and HSP90, have been linked to mechanical stress protection (Arndt et al, 2010; Sarparanta et al, 2012; Gazda et al, 2013; Ulbricht et al, 2013; Adriaenssens et al, 2017; Jacko et al, 2020). The HSP90 chaperone system is essential for muscle assembly in worms, fish, mice and men (Janiesch et al, 2007; Du et al, 2008; Gaiser et al, 2011; Donlin et al, 2012; Gazda et al, 2013; Echeverría et al, 2
Referência(s)