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Review
. 2017 Jun 18;7(3):891-944.
doi: 10.1002/cphy.c160033.

Overview of the Muscle Cytoskeleton

Christine A Henderson  1   2 , Christopher G Gomez  1   2 , Stefanie M Novak  1   2 , Lei Mi-Mi  1   2 , Carol C Gregorio  1   2
Affiliations
Review

Overview of the Muscle Cytoskeleton

Christine A Henderson et al. Compr Physiol. .

Abstract

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

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Figures

Figure 1
Figure 1
(A) Schematic representation of a cardiac sarcomere (lacking nebulin) illustrating the three major filament systems: actin-based thin filaments (gray), myosin-based thick filaments (blue), and titin (pink). The lateral boundaries of the sarcomere are the Z-discs. The I-bands surrounds the Z-disc and is a region where thin filaments are not superimposed by thick filaments. The A-band region contains thin filaments and thick filaments. The M-band falls within the H-zone, where thick filaments do not interdigitate with thick filaments. (B) Electron micrograph of skeletal muscle sarcomere. (C) Enlarged view of the M-band region. The M-band is composed of a series of three to five electron-dense M-lines: M6′, M4′, M1, M4, and M6. [Part A modified, with permission, from (255); Parts B and C modified, with permission, from (9).]
Figure 2
Figure 2
Z-discs define the lateral edge of the sarcomere, and also participate in numerous cellular processes including signal transduction and protein turnover. Abbreviations: FAK, focal adhesion kinase; γ-filamin, also known as Filamin C; FHL, four-and-a-half LIM domains protein; ERK, extracellular signal-regulated kinase; MLP, muscle LIM protein; ALP, actin-associated LIM protein; PKCε, protein kinase C epsilon; MuRF, muscle-ring-finger protein; ENH, enigma-homolog protein; NFAT, nuclear factor of activated T-cells; MAFbx, muscle atrophy F-box (striated muscle-specific E3 ubiquitin ligase) protein; GATA4, GATA sequence-binding zinc-finger transcription factor 4. [Fig. modified, with permission, from (175).]
Figure 3
Figure 3
Schematic representation of the intermediate filament (IF) scaffold in striated muscle. The IF scaffold, predominantly composed of desmin (yellow), links the entire contractile apparatus to the sarcolemma and other organelles, such as the nucleus, mitochondria, lysosomes, and potentially the sarcoplasmic reticulum (SR). Desmin interacts with many other proteins including synemin, paranemin, syncoilin, and myospryn. Keratins (K8/K19) link the contractile apparatus to the sarcolemma and interact with the dystrophin-dystroglycan (DG) complex. Overall, the IF scaffold helps maintain the integrity of muscle cytoarchitecture and provide mechanical strength to the cell. Abbreviations: MLP, striated muscle-specific LIM protein; SG, sarcoglycan. [Fig. modified, with permission, from (88).]
Figure 4
Figure 4
MLP (muscle LIM protein) is a functionally diverse, multicompartment protein. MLP interacts with β1 spectrin, zyxin, and integrin-linked kinase (ILK) in costameres and plays a role in force transmission. MLP also binds to α-actinin to help stabilize the Z-disc. At the intercalated discs, MLP binds to N-RAP. MLP acetylated by HDAC4 [histone acetyltransferases (HATs) and deacetylases] and PCAF (P300/CBP-associated factor) enhance calcium sensitivity and increase contractile function. In addition, MLP and cofilin form a complex and regulate actin dynamics. MLP is an important stretch sensor. The MLP/titin/telethonin (T-Cap) complex plays a key role in stretch-induced signaling. MLP translocates to the nucleus and interacts with transcription factors, which regulate myogenesis [e.g., MyoD, myogenin and MRF4 (muscle-specific regulatory factor 4)]. [Fig. reprinted, with permission, from (84).]
Figure 5
Figure 5
Schematic representation of the titin domain structure and localization of its binding partners in striated muscle. Titin is a huge protein that spans half a sarcomere from the Z-disc to the M-line region. The N-terminal region of titin inserts into the Z-disc, and many of the interaction in this region contribute to mechanosensing, structural integrity, and force transmission. I-band titin contains elastic elements, which play a critical role in passive tension. The A-band region binds to myosin and MyBPc, linking the myosin-based thick filaments to titin. M-band titin is important to both structural support and signaling. Abbreviations: sAnk1, small-ankyrin-1 isoform; FHL1 and 2, four-and-a-half-LIM-domain protein-1 and -2; PKG and PKA, protein kinase-G and -A; MARPs, muscle ankyrin repeat proteins; CARP, cardiac ankyrin repeat protein; ankrd-2/Arpp, ankyrin repeat domain 2; DARP, diabetes-related ankyrin repeat protein; S100A1, S100 calcium-binding protein A1; MyBPC, myosin-binding protein-C; MURF-1 and MURF-2, muscle-specific RING-finger protein-1 and -2; FN3, fibronectin type 3 like domain; Ig-like, immunoglobulin-like domain; N2-bus, N2-B-unique sequence; PEVK, titin region rich in proline (P), glutamate (E), valine (V), and lysine (K). The following titin-binding proteins were not discussed in this review: HSP27, heat shock protein-27; Smyd2, SET and MYND domain-containing protein-2; mHSP90, methylated heat shock protein-90; Nbr1, neighbor of BRCA1 gene-1; Bin-1, bridging integrator-1 [see (105) for discussion of these proteins]. [Fig. reprinted, with permission, from (105).]
Figure 6
Figure 6
Schematic representation of nebulin domain structure and localization of binding partners in striated muscle. Nebulin is a large protein that interacts with a multitude of sarcomeric proteins including: capZ, titin, myopalladin, α-actinin, and desmin at its C-terminus in the Z-disc; tropomyosin, troponins, myosin, calmodulin, actin, and myosin-binding protein C (MyBPC) along its 22 seven-module super-repeats (blue); and tropomodulin at its N-terminus, though this interaction is likely transient. These protein interactions have given rise to two similar yet distinct functional models—as a molecular ruler and as an actin stabilizer. Archvillin is not discussed in this review. [Fig. modified, with permission, from (330).]
Figure 7
Figure 7
Schematic drawing of thin and thick filament interactions in striated muscle highlighting the major myosin regulatory proteins. Muscle contraction is dependent on the interactions between myosin-based thick filament via the head domain and actin-based thin filament. Thick filament regulatory proteins—myosin essential light chain (ELC), myosin regulatory light chain-2 (MLC2v), and myosin-binding protein C (MyBP-C)—control muscle contraction. MyBP-C interactions with actin, the myosin rod domain, MLC2v, and titin are depicted. The dashed circle is a magnified view highlighting (i) MyBP-C interaction with MLC2v located in the neck domain of myosin, (ii) the actin and MgATP-binding sites located within the myosin head domain, and (iii) MLC2v phosphorylation (Ser14/15) site important in promoting actin-myosin interactions. Abbreviations: Tm, tropomyosin; TnT, Troponin T; TnI, Troponin I; TnC, Troponin C. [Fig. modified, with permission, from (633).]
Figure 8
Figure 8
The sarcomeric M-band contains components important for mechanosensing, proteosomal degradation, actin dynamics, metabolism, and signal transduction. Myomesin is a key structural protein of the M-band. MURFs (muscle-specific ring finger protein) are multifunctional proteins that ubiquitinate certain myofibrillar proteins, play a key role in muscle atrophy and regulate hypertrophic signaling. Obscurin interacts with ankyrin and anchors the sarcomere to the sarcoplasmic reticulum; ankyrin and obscurin also sequester PP2A (protein phosphatase 2A) to the M-band. FHLs (four-and-a-half LIM proteins) bind to titin’s N2B spring region and activate downstream signaling pathways, thus serving as an important mechanosensor that triggers hypertrophy in response to strain. FHL2 also docks important metabolic enzymes such as the metabolic enzymes muscle-specific M-CK (creatine kinase), AK (adenylate kinase), and PFK (phosphofructokinase). M-CK anchors the glycolytic enzyme β/α-enolase to the M-band. The muscle isoform of AMPD (adenosine monophosphate deaminase) works with M-CK and AK to monitor local ATP levels. Other proteins identified at the M-band, but not discussed in this review include SmyD1, SCPL-1 (Caenorhabditis elegans), UNC-82 (C. elegans), p62, rhoA, CRIK, and active ROCK1. [Fig. reprinted, with permission, from (277).]
Figure 9
Figure 9
(Right) Longitudinal view of myosin (blue), myomesin (red) and titin (green). The M-band is composed of a series of electron-dense M-lines: M4, M1, and M4′ (see Fig. 1C for an electron micrograph of M-lines). Myomesin family members form antiparallel homodimers through interactions called M-bridges between the C-terminal immunoglobulin domain (labeled 13), and bind to myosin at the N-terminal domain. (Left) Cross-sectional view highlighting myomesin forming an antiparallel dimer. Myomesin acts as a thick filament cross-linking protein. [Fig. reprinted, with permission, from (9).]
Figure 10
Figure 10
Tropomyosin positions on the surface of F-actin in the presence (green) and absence (red) of myosin. Ten actin-pairs (alternately colored blue and cyan) are shown with the pointed end facing up. Two tropomyosin α-helical chains form coiled-coils that interact with the positively charged groove of actin filaments and form dimers that span seven actin monomers. Tropomyosin regulates interactions between actin-based thin filaments and myosin-based thick filaments to control cross-bridge cycling. Depicted in ribbon representation are tropomyosin coiled-coils in either in the troponin and myosin-free (red), or the myosin head (S1)-decorated (green). Tropomyosin residue 125 is shown in black as a reference point, highlighting the relative sliding between the positions. Scale equals 50Å Actin is numbered -1 to 8. [Fig. reprinted, with permission, from (523).]
Figure 11
Figure 11
Ribbon structure of globular actin in the ADP-bound state. Actin is an asymmetrical protein composed of four subdomains (subdomain 1 shown in purple, subdomain 2 shown in green, subdomain 3 shown in red, and subdomain 4 shown in yellow) connected via two “hinge” strands. The representation is oriented with the pointed (minus end) at the top and the barbed (plus end) at the bottom. ADP is shown in stick representation bound in the cleft. Shown in cyan in stick representation is tetramethylrhodamine-5-maleimide (TMR), a fluorescent probe that inhibits actin polymerization. [Fig. reprinted, with permission, from (534).]
Figure 12
Figure 12
CapZ dynamics at the barbed end of F-actin. (A) CapZ has two subunits: α1 and β1 each with a tentacle that binds one terminal actin. Tightly capped F-actin has a low actin off rate. (B) Following mechanical stimulation (to simulate exercise), the β tentacle undergoes a structural change via post-translation modification (PTM) including phosphorylation on serine-204 and acetylation on lysine-199. The β tentacle shifts off the terminal actin, which increases actin monomer exchange. Regulation of actin dynamics at the barbed end may also play a key role in both skeletal and cardiac hypertrophy. [Fig. modified, with permission, from (381).]
Figure 13
Figure 13
Tropomyosin and the troponin complex regulate striated muscle contraction. Each tropomyosin (orange chain) molecule is associated with one troponin complex [TnI (inhibitory-blocks myosin binding to actin; green), TnC (binds calcium; red barbells), and TnT (binds tropomyosin; blue)] and seven actin monomers. In the relaxed state tropomyosin blocks the myosin-binding site on actin. TnC is weakly bound to TnI; TnI binds to actin (TnI-actin binding) and inhibits myosin from binding to actin. Following the release of calcium (Ca2+), calcium binds to TnC and a patch of residues in the N-terminal domain of TnC is exposed and the interaction of TnC with TnI is enhanced. TnI then dissociates its inhibitory region from actin, and forms a complex with TnT and tropomyosin. Following the conformational change in the troponin complex, tropomyosin shifts and the myosin head binds to actin. [Fig. reprinted, with permission, from (642).]
Figure 14
Figure 14
Schematic drawing of the cardiac cross-bridge cycle. Thin-filaments are shown with actin, tropomyosin (Tm) and the troponin (Tn) complex with the Ca2+-binding unit (cTnC) in pink, the Tm-binding unit (cTnT) in blue, and the inhibitory unit (cTnI) in light green. Thick-filament cross-bridges (XB) are shown with myosin heavy chain (MHC; figure illustrating one MHC) in red, myosin light chains (LC) in green, along with myosin-binding protein C (MyBP-C) in purple and titin in orange. Cross-bridges are initially in a rest state (1) where they are weakly bound and do not generate force. Cross-bridges enter a transition state (2) determined by the on (kCa) and off rates (kCa-1) for Ca2+ exchange with cTnC. During this transition state, cross-bridges are weakly bound (kXB-1) and do not generate force. In the active state (3), the cTnT-dependent shift of Tm from its blocking position on actin filaments allows strong cross-bridge binding (kXB) and induces cooperative activation of the thin filament (e.g., increase Ca2+ affinity of cTnC; kCa-XB-1). In the active state (4) with loss of bound Ca2+, the cooperative mechanisms allow a population of cross-bridges to remain active and force generating (kCa-XB). Mechanical feedback termed shortening-induced deactivation (kvel) will transition active cross-bridges back to the resting state. [Fig. modified, with permission, from (265).]
Figure 15
Figure 15
Myosin-binding proteins (MyBP). (A) Schematic drawing of MyBP domain organization. MyBPs are composed of a series of immunoglobulin (Igl-like in pink) and fibronectin type III (Fn3 in green) repeat domains. Domains termed C1 through C10 and a 105-residue linker between C1 and C2 termed the MyBP-C motif (in blue) make up the core structure of MyBP-C isoforms. Cardiac MyBP-C has the addition of an eight IgI-like domain termed C0, a unique amino acid sequence—LAGGGRRIS—insertion (in light blue) in the MyBP-C motif, and a 28 amino acid insertion (in dark pink) in the C5 domain. Slow skeletal MyBP-C differs from the fast isoform with an extended Pro/Ala-rich region at the N-terminus. MyBP-H is the smallest isoform with four domains similar to C7 through C10 of MyBP-C and a unique Pro/Ala-rich linker (in black) region. (B) Example electron micrograph of frog skeletal muscle showing MyBP-C transverse stripes located in the C-Zone. [Part A modified, with permission, from (167); Part B modified, with permission, from (406).]
Figure 16
Figure 16
Schematic representation of costameric proteins, which bidirectionally link the extracellular matrix to the sarcomere. There are two major components of the costamere: the vinculin-talin-integrin complex and the dystrophin glycoprotein complex (DGC). The DGC includes dystrophin, sarcoglycans, α/β dystroglycans, dystrobrevin and syntrophin. Additional integrin-associated proteins include melusin, FAK (focal adhesion kinase), ILK (integrin-linked kinase, PINCH (particularly interesting new cysteine-histidine-rich protein), and kindlin. [Fig. reprinted, with permission, from (292).]
Figure 17
Figure 17
Schematic representation of the dystrophin associated protein complex in muscle. The three subcomplexes are shown: the dystroglycan subcomplex (blue), the dystrobrevin:syntrophin subcomplex (red) and the sarcoglycan:sarcospan subcomplex (green). Also indicated are the muscular dystrophies caused due to defects or deficiencies of proteins within the dystrophin associated protein complex. Abbreviations: BMD, Becker muscular dystrophy; CMD1C-1D, congenital muscular dystrophy type 1C-1D; DMD, Duchenne muscular dystrophy; FCMD, Fukuyama; CMD, LGMD2C-2F, limb-girdle muscular dystrophy type 2C-2F; LAMA2, laminin alpha 2 chain or merosin-deficient muscular dystrophy; MEB, muscle-eye-brain disease; WWS, Walker–Warburg syndrome. [Fig. reprinted, with permission, from (583).]
Figure 18
Figure 18
Structural organization and molecular components of the intercalated disc (ICD). Low-magnification transmission electron micrograph (A) and schematic drawing of cardiac myocardium (B) exhibit characteristic step-like structures of intercalated discs (A, arrowheads) formed through syncytial interconnection of rod-shaped cardiomyocytes. (C and D) Higher magnification view of areas enclosed in A and B, respectively, show three specialized substructures of intercalated discs—fascia adherens (adherens junction), desmosome (desmosomal junction, arrowhead), and gap junction. The ends of gap junction here connect to two adherens junctions (arrows). (D) Molecular components of ICD substructures not only serve as mechanical and electrochemical coupling platforms between adjacent cardiomyocytes, but also interact with major cytoskeletal filament systems (e.g., actin and microtubule cytoskeletons). The following proteins are not discussed in texts: p120, p150, EB1, and protein 4.3). [Parts A and C reprinted, with permission, from (626); B and D reprinted, with permission, from (38).]
Figure 19
Figure 19
Nuclear lamins. (A) Schematic drawing of nuclear lamins and their nearby protein interactions. Nuclear lamins localizes underneath the inner nuclear membrane where they directly bind lamina-associated proteins (e.g., emerin, nesprin-1, and nesprin-2). Nesprin-1 and emerin both interact with nuclear actin and mediate the cortical actin cytoskeleton assembly at the nuclear envelope. The integral membrane protein MAN1 allows lamins to associate with transcription factors (e.g., SMAD), while SUN1/2 allows lamins to associate with microtubules and anchors nesprin-2 to the nuclear envelope. Interactions with barrier-to-autointegration factor (BAF) and lamin B receptor (LBR), as well as directly to chromatin, allows lamins to influence chromatin organization and gene expression. (B) Electron micrograph of the nuclear lamina composed of lamin intermediate filaments and associated proteins that extend between the nuclear pore complexes (NPCs). [Parts A and B modified, with permission, from (80).]
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