310 Stainless steel capillary coil tubing chemical component ,The role of dystrophin glycoprotein complexes in the mechanotransduction of muscle cells

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Dystrophin is the main protein of the dystrophin-glycoprotein complex (DGC) in skeletal muscle and cardiomyocytes. Dystrophin binds the actin cytoskeleton to the extracellular matrix (ECM). The rupture of the connection between the extracellular matrix and the intracellular cytoskeleton can have devastating consequences for the homeostasis of skeletal muscle cells, leading to a number of muscular dystrophies. In addition, loss of functional DGCs leads to progressive dilated cardiomyopathy and premature death. Dystrophin acts as a molecular spring and DHA plays a key role in maintaining the integrity of the sarcolemma. Moreover, evidence is accumulating linking DGC to mechanistic signaling, although this role remains poorly understood. This review article aims to provide a modern view of DGCs and their role in mechanotransduction. We first discuss the complex relationship between muscle cell mechanics and function, and then review recent research on the role of the dystrophin glycoprotein complex in mechanotransduction and maintenance of muscle cell biomechanical integrity. Finally, we review the current literature to understand how DGC signaling intersects with mechanosignaling pathways to highlight potential future intervention points, with a particular focus on cardiomyopathy.
Cells are in constant communication with their microenvironment, and a two-way dialogue between them is necessary for the interpretation and integration of biomechanical information. Biomechanics controls key subsequent events (eg, cytoskeletal rearrangements) by controlling the overall cellular phenotype in space and time. Central to this process in cardiomyocytes is the costal region, the region where the sarcolemma connects to a sarcomere composed of integrin-talin-vinculin and dystrophin-glycoprotein (DGC) complexes. Attached to the intracellular cytoskeleton, these discrete focal adhesions (FAs) propagate a cascade of biomechanical and biochemical cellular changes that control differentiation, proliferation, organogenesis, migration, disease progression, and more. The conversion of biomechanical forces into biochemical and/or (epi)genetic changes is known as mechanotransduction1.
The integrin transmembrane receptor 2 has long been known to anchor the extracellular matrix in cells and mediate both internal and external signaling. In parallel with integrins, DGCs bind the ECM to the cytoskeleton, establishing a critical link between the outside and inside of the cell3. Full-length dystrophin (Dp427) is primarily expressed in cardiac and skeletal muscle, but is also observed in tissues of the central nervous system, including the retina and Purkinje tissue4. Mutations in integrins and DGC are thought to be the causes of muscular dystrophy and progressive dilated cardiomyopathy (DCM) (Table 1)5,6. In particular, DMD mutations encoding the central dystrophin protein DGCs cause Duchenne muscular dystrophy (DMD)7. DGC is composed of several subcomplexes including α- and β-dystroglycan (α/β-DG), sarcoglycan-sarcospan, syntrophin, and dystrophin8.
Dystrophin is a cytoskeletal protein encoded by DMD (Xp21.1-Xp22) that plays a central role in maintaining DGC. DGC maintains the integrity of the sarcolemma, the plasma membrane of striated muscle tissue. Dystrophin further attenuates damage caused by contraction by acting as a molecular spring and molecular scaffold9,10. Full-length dystrophin has a molecular weight of 427 kDa, however, due to the many internal promoters in DMD, there are several naturally occurring truncated isoforms, including Dp7111.
Accessory proteins have been shown to be localized to dystrophin, including true mechanotransducers such as neuronal nitric oxide synthase (nNOS), Yes-associated protein (YAP), and caveolin-3, thus representing important components of cellular signaling. Compounds 12, 13, 14. In addition to adhesion, a cellular mechanism associated with interactions between cells and the matrix, formed by integrins and their downstream targets, these two complexes represent the interface between the “inside” and “outside” of the cell. Protecting these focal adhesions from abnormal destruction is critical to cell behavior and survival. In addition, data support that dystrophin is a modulator of mechanosensitive ion channels, including stretch-activated channels, especially L-type Ca2+ channels and TRPC 15 channels.
Although dystrophin is important for the homeostatic function of striated muscle cells, the precise supporting mechanisms are less clear, especially the role of dystrophin and its ability to act as a mechanosensor and mechanical protector. Due to the loss of dystrophin, several unanswered questions have arisen, including: are mechanosensitive proteins such as YAP and AMPK mislocated to the sarcolemma; Are there crosstalk with integrins, circumstances that can lead to abnormal mechanotransduction? All of these features may contribute to the severe DCM phenotype seen in patients with DMD.
In addition, the association of changes in cellular biomechanics with the overall DMD phenotype has important clinical implications. DMD is an X-linked muscular dystrophy affecting 1:3500–5000 men, characterized by early loss of mobility (<5 years) and progressive DCM with a significantly worse prognosis than DCM of other etiologies16,17,18.
The biomechanics of dystrophin loss has not been fully described, and here we review the evidence supporting the notion that dystrophin does indeed play a mechanoprotective role, i.e. maintaining the integrity of the sarcolemma, and is critical in mechanotransduction. In addition, we reviewed the evidence suggesting important crosstalk with integrins, specifically binding laminin α7β1D in striated muscle cells.
Insertions and deletions are responsible for a large number of mutations in DMD, with 72% of mutations being caused by such mutations19. Clinically, DMD presents in infancy (≤5 years) with hypotension, positive Gower’s sign, delayed progression of age-related changes, mental retardation, and skeletal muscle atrophy. Respiratory distress has historically been the leading cause of death in DMD patients, but improved supportive care (corticosteroids, continuous positive airway pressure) has increased life expectancy in these patients, and the median age of DMD patients born after 1990 is 28.1 years 20 ,21. . However, as patient survival increases, the prognosis of progressive DCM is significantly worse compared to other cardiomyopathies16, leading to end-stage heart failure, which is currently the leading cause of death, accounting for approximately 50% of DMD deaths17,18.
Progressive DCM is characterized by increased left ventricular dilatation and compliance, ventricular thinning, increased fibrofatty infiltration, decreased systolic function, and increased frequency of arrhythmias. The degree of DCM in patients with DMD is almost universal in late adolescence (90% to 18 years of age), but is present in approximately 59% of patients by 10 years of age8,22. Addressing this issue is critical as left ventricular ejection fraction has been steadily declining at a rate of 1.6% per year23.
Cardiac arrhythmias are common in patients with DMD, especially sinus tachycardia and ventricular tachycardia, and are the cause of sudden cardiac death22. Arrhythmias are the result of fibrofatty infiltration, especially in the subbasal left ventricle, which impairs the return circuitry as well as [Ca2+]i processing dysfunction and ion channel dysfunction24,25. Recognition of the clinical cardiac presentation is critical, as early treatment strategies may delay the onset of severe DCM.
The importance of treating cardiac dysfunction and skeletal muscle morbidity is shown in an interesting study that used a mouse model of DMD called mdx26 to study the effects of improving skeletal muscle tissue without addressing the underlying cardiac problems present in DMD. Here, the authors demonstrated a paradoxical 5-fold increase in cardiac dysfunction after improvement in skeletal muscle, and mice had a significant reduction in ejection fraction26. Improved skeletal muscle function allows higher physical activity to put more strain on the myocardium, making it more susceptible to general dysfunction. This highlights the importance of treating DMD patients in general and cautions against skeletal muscle therapy alone.
DGCs perform several additional functions, namely, provide structural stability to the sarcolemma, become a molecular scaffold that acts as a signaling link, regulate mechanosensitive ion channels, the core of costal mechanotransduction, and participate in the transmission of lateral force in the region of the ribs (Fig. 1b). . Dystrophin plays a central role in this ability, and due to the presence of many internal promoters, there are several different isoforms, each playing a different role in different tissues. Differential tissue expression of different dystrophin isoforms supports the notion that each isoform plays a different role. For example, cardiac tissue expresses the full length (Dp427m) as well as the shorter Dp71m isoform of dystrophin, while skeletal tissue only expresses the first of the two. Observation of the role of each subtype can reveal not only its physiological function, but also the pathogenesis of muscular dystrophy.
Schematic representation of the full-length dystrophin (Dp427m) and the smaller, truncated Dp71 isoform. Dystrophin has 24 spectrin repeats separated by four loops, as well as an actin-binding domain (ABD), a cysteine-rich (CR) domain, and a C-terminus (CT). Key binding partners have been identified, including microtubules (MTs) and the sarcolemma. There are many isoforms of Dp71, Dp71m refers to the muscle tissue and Dp71b refers to the nervous tissue isoform. In particular, Dp71f refers to the cytoplasmic isoform of neurons. b The dystrophin-glycoprotein complex (DHA) is located in the sarcolemma as a whole. Biomechanical forces switch between ECM and F-actin. Note potential crosstalk between DGCs and integrin adhesion, Dp71 may play a role in focal adhesions. Created with Biorender.com.
DMD is the most common muscular dystrophy and is caused by mutations in DMD. However, to fully appreciate our current understanding of the role of anti-dystrophin, it is important to place it in the context of DGC as a whole. Thus, the other constituent proteins will be briefly described. The protein composition of DGC began to be studied in the late 1980s, with particular attention to dystrophin. Koenig27,28, Hoffman29 and Ervasti30 made an important discovery by identifying dystrophin, a 427 kDa protein in striated muscle31.
Subsequently, other subcomplexes were shown to be associated with dystrophin, including sarcoglycan, transsyn, dystrophin subcomplex, dysbrevin, and syntrophins8, which together constitute the current DGC model. This section will first disseminate the evidence for the role of the DGC in mechanosensory perception while examining the individual components in detail.
The full-length dystrophin isoform present in striated muscle tissue is Dp427m (e.g. “m” for muscle to distinguish it from the brain) and is a large rod-shaped protein with four functional domains located under the cardiomyocyte sarcolemma, especially in the costal region 29, 32. Dp427m, encoded by the DMD gene on Xp21.1, consists of 79 exons generated at 2.2 megabases and is thus the largest gene in our genome8.
Several internal promoters in DMD produce multiple truncated dystrophin isoforms, some of which are tissue specific. Compared to Dp427m, Dp71m is significantly truncated and lacks a spectrin repeat domain or an N-terminal ABD domain. However, Dp71m retains the C-terminal binding structure. In cardiomyocytes, the role of Dp71m is unclear, but it has been shown to localize in T tubules, suggesting that it may help regulate the excitation-contraction coupling 33,34,35. To our knowledge, the recent discovery of Dp71m in cardiac tissue has received little attention, but some studies suggest that it is associated with stretch-activated ion channels, and Masubuchi suggested that it may play a role in the regulation of nNOS33. , 36. In doing so, Dp71 has received significant attention in neurophysiology and platelet research, areas that may provide insight into a role in cardiomyocytes37,38,39.
In nervous tissue, the Dp71b isoform is predominantly expressed, with 14 isoforms reported38. Deletion of Dp71b, an important regulator of aquaporin 4 and Kir4.1 potassium channels in the central nervous system, has been shown to alter blood-brain barrier permeability40. Given the role of Dp71b in ion channel regulation, Dp71m may play a similar role in cardiomyocytes.
The presence of DGC in the costal ganglia immediately indicates a role in mechanotransduction, and indeed it has been shown to co-localize with integrin-talin-vinculin complexes 41 . Moreover, given that the costal segment is a focus for transverse mechanotransduction, the localization of Dp427m here highlights its role in protecting cells from damage caused by contraction. Further, Dp427m interacts with actin and the microtubule cytoskeleton, thereby completing the connection between the intracellular environment and the extracellular matrix.
The N-terminus containing actin-binding domain 1 (ABD1) consists of two calmodulin homology domains (CH) that are required for interaction with F-actin and anchoring the γ-actin isoform to the sarcolemma42,43. Dystrophin may contribute to the overall viscoelasticity of cardiomyocytes by attaching to the subsarcolemmal cytoskeleton, and its localization in the costal ganglia supports its involvement in mechanotransduction as well as mechanoprotection44,45.
The central core domain consists of 24 spectrin-like repeat proteins, each of which is approximately 100 amino acid residues in length. The spectrin repeats are interspersed with four hinge domains, giving the protein flexibility and a high degree of extensibility. Dystrophin spectrin repeats can unfold within a physiological range of forces (15-30 pN) extending from 21 nm to 84 nm, forces achievable for myosin contraction 46 . These features of the spectrin repeat domain allow dystrophin to act as a molecular shock absorber.
The central rod of Dp427m ensures its localization in the sarcolemma, in particular, through hydrophobic and electrostatic interactions with phosphatidylserine 47,48. Interestingly, the central core of dystrophin interacts differently with sarcolemma phospholipids in skeletal and cardiac tissues, possibly reflecting different spring patterns. critical, while skeletal muscles are also associated with R10-R1249.
Binding to the γ-actin cytoskeleton requires the ABD2 spectrin repeat 11–17 region, which consists of basic amino acid residues and differs from the F-actin-binding CH domain. Microtubules interact directly with the core domain of dystrophin, this interaction requires residues of spectrin repeats 4-15 and 20-23, and the presence of ankyrin B is required to prevent the formation of microtubules at this site. Tubes are absent 50,51,52. A gap between microtubules and dystrophin has been shown to exacerbate DMD pathology by increasing reactive oxygen species (X-ROS).
The CR domain via ankyrin B is another anchor for sarcolemmal phospholipids52. Ankyrin-B and ankyrin-G are required for rib localization of dystrophin/DGC, and their absence results in a diffuse sarcolemmal pattern of DGC52.
The CR domain contains a WW binding domain that interacts directly with the PPxY binding motif of β-DG. By attaching to the dystrophin-glycan complex, dystrophin completes the link between the inside and outside of the cell54. This connection is critical for striated muscle, as evidenced by the fact that disruption of the connection between the ECM and the interior of the cell leads to life-limiting muscular dystrophy.
Finally, the CT domain is a highly conserved region that forms a coiled helix and is critical for binding to α-dystrobrevin and α1-,β1-syntrophins55,56. α-dystrobrevin binds to the CT domain of dystrophin and provides additional resistance to dystrophin in the sarcolemma57.
During embryonic and fetal development, Utrophin is widely expressed in various tissues, including endothelial cells, nervous tissue, and striated muscle tissue58. Utrophin is expressed by UTRN located on chromosome 6q and is a dystrophin autolog with 80% protein homology. During development, utrophin is localized in the sarcolemma but is markedly suppressed in postnatal striated muscle tissue, where it is replaced by dystrophin. After birth, the localization of utrophin is limited to tendons and neuromuscular junctions of skeletal muscles58,59.
Utrophin binding partners are broadly similar to those of dystrophins, although some key differences have been described. For example, dystrophin interacts with β-DG through its WW domain, which is stabilized by the ZZ domain (named for its ability to bind two zinc ions) within its CT region, where cysteic acid residues 3307-3354 are especially important for this interaction60. , 61. Utrophin also binds to β-DG via the WW/ZZ domain, but the exact residues supporting this interaction differ from dystrophin residues (3307–3345 in dystrophin and 3064–3102 in utrophin) 60,61. Importantly, utrophin’s binding to β-DG was approximately 2-fold lower compared to dystrophin 61. Dystrophin has been reported to bind to F-actin via spectrin repeats 11–17, while similar sites in utrophin cannot bind to F-actin, even at high concentrations, but can interact through their CH-domains. Action 62,63,64. Finally, unlike dystrophin, utrophin cannot bind to microtubules51.
Biomechanically, utrophin spectrin repeats have a distinct unfolding pattern compared to dystrophin65. Utrophin-spectrin repeats deployment at higher forces, similar to titin but not dystrophin65. This is consistent with its localization and role in the transmission of rigid elastic force at tendon junctions, but may make utrophin less suitable to act as a molecular spring in buffering forces induced by contraction 65 . Taken together, these data suggest that mechanotransduction and mechanobuffering capabilities may be altered in the presence of utrophin overexpression, especially given different binding partners/mechanisms, however this requires further experimental study.
From a functional point of view, the fact that utrophin is believed to have similar effects to dystrophin makes it a potential treatment target for DMD66,67. In fact, some DMD patients have been shown to overexpress utrophin, possibly as a compensatory mechanism, and the phenotype has been successfully restored in a mouse model with utrophin overexpression 68 . While upregulation of utrophin is a likely therapeutic strategy, consideration of the formal and functional difference between utrophin and dystrophin and the utility of inducing this overexpression with proper localization along the sarcolemma makes the long-term strategy of utrophin still unclear. Notably, female carriers show a mosaic pattern of utrophin expression, and the ratio between dystrophin and utrophin may influence the degree of dilated cardiomyopathy in these patients,69 although murine models of carriers have shown. .
The dystroglycan subcomplex consists of two proteins, α- and β-dystroglycan (α-, β-DG), both transcribed from the DAG1 gene and then post-translationally cleaved into two component proteins 71 . α-DG is highly glycosylated in the extracellular aspect of DGCs and interacts directly with proline residues in laminin α2 as well as with agrin72 and picaculin73 and the CT/CR region of dystrophin73,74,75,76. O-linked glycosylation, especially of serine residues, is required for its interaction with the ECM. The glycosylation pathway includes many enzymes whose mutations lead to muscular dystrophy (see also Table 1). These include the O-mannosyltransferase POMT2, fucutin and fucutin-related protein (FKRP), two ribitol phosphotransferases that add tandem ribitol phosphates to the core glycan, and the LARGE1 protein that adds xylose and glucose. Linear uronic acid polysaccharide, also known as the matrix glycan at the end of the glycan77. FKRP is also involved in the development and maintenance of the ECM, and mutations in it lead to decreased expression of laminin α2 and α-DG77,78,79. In addition, FKRP can also direct the formation of the basal lamina and the cardiac extracellular matrix through glycosylated fibronectin 80.
β-DG contains a PPxY binding motif that directly localizes and sequesters YAP12. This is an interesting finding as it implies that DGC regulates the cardiomyocyte cell cycle. α-DH in neonatal cardiomyocytes interacts with agrin, which promotes heart regeneration and DGC76 lysis due to cell maturation. As cardiomyocytes mature, aggrin expression decreases in favor of laminin, which is thought to contribute to cell cycle arrest76. Morikawa12 showed that double knockdown of dystrophin and salvador, a negative regulator of YAP, leads to hyperproliferation of cardiomyocytes in the infarct-causing rumen. This led to the exciting idea that YAP manipulation could be of clinical value in preventing tissue loss after myocardial infarction. Thus, agrin-induced DGC lysis could represent an axis that allows for YAP activation and is a potential pathway for cardiac regeneration.
Mechanically, α- and β-DG are required to maintain interaction between the sarcolemma and the basal layer 81 . Both α-DG and α7 integrins contribute to force generation in the costal ganglion, and loss of α-DG causes separation of the sarcolemma from the basal lamina, leaving skeletal muscle tissue vulnerable to contraction-induced damage. As previously described, the dystroglycan complex regulates the overall turnover of DGCs, where binding to cognate ligand laminin results in tyrosine phosphorylation of the PPPY-binding motif of β-DG892. Tyrosine phosphorylation here promotes dystrophin disassembly, which flips the DGC complex. Physiologically, this process is highly regulated, which is absent in muscular dystrophy82, although the underlying mechanisms that control this process are not fully understood.
Cyclic stretch has been shown to activate the ERK1/2 and AMPK pathways through the dystrophin complex and related protein plectin83. Together, plectin and dystroglycan are required not only to act as a scaffold, but also to participate in mechanotransduction, and knockdown of plectin leads to a decrease in the activity of ERK1/2 and AMPK83. Plectin also binds to cytoskeletal intermediate filament desmin, and desmin overexpression has been shown to improve the disease phenotype in mdx:desmin and mdx mice, a DMD84 double knockout mouse model. By interacting with β-DG, plectin indirectly binds DGC to this component of the cytoskeleton. In addition, dystroglycan interacts with growth factor receptor-binding protein 2 (Grb2), which is known to be involved in cytoskeletal rearrangements85. Ras activation by integrin has been shown to be mediated through Grb2, which may provide a potential pathway for crosstalk between integrins and DGC86.
Mutations in the genes involved in α-DH glycosylation lead to the so-called muscular dystrophy. Dystroglycanopathies show clinical heterogeneity but are mainly caused by a disruption in the interaction between α-DG and laminin α277. Dystrophiglicanoses caused by primary mutations in DAG1 are generally extremely rare, probably because they are embryonic lethal87, thus confirming the need for cellular association with ECM. This means that most dystrophic glycan diseases are caused by secondary protein mutations associated with glycosylation. For example, mutations in POMT1 cause the extremely severe Walker-Warburg syndrome, which is characterized by anencephaly and markedly shortened life expectancy (less than 3 years)88. However, FKRP mutations predominantly manifest as limb-girdle muscular dystrophy (LGMD), which is usually (but not always) relatively mild. However, mutations in FKRP have been shown to be a rare cause of WWS89. Many mutations have been identified in FKRP, of which the founder mutation (c.826>A) most commonly causes LGMD2I90.
LGMD2I is a relatively mild muscular dystrophy whose pathogenesis is based on disruption of the connection between the extracellular matrix and the intracellular cytoskeleton. Less clear is the relationship between genotype and phenotype in patients with mutations in these genes, and indeed this concept is applicable to other DSC proteins. Why do some patients with FKRP mutations show a disease phenotype consistent with WWS while others have LGMD2I? The answer to this question may lie in i) which step of the glycosylation pathway is affected by the mutation, or ii) the degree of hypoglycosylation at any given step. Hypoglycosylation of α-DG may still allow some degree of interaction with the ECM resulting in a milder overall phenotype, while dissociation from the basement membrane increases the severity of the disease phenotype. Patients with LGMD2I also develop DCM, although this is less documented than DMD, motivating the urgency of understanding these mutations in the context of cardiomyocytes.
The sarcospan-sarcoglycan subcomplex promotes the formation of DHA and interacts directly with β-DH. There are four unidirectional sarcoglycans in cardiac tissue: α, β, γ, and δ91. It has recently been described that a c.218C>T missense mutation in exon 3 of the SGCA gene and a partial heterozygous deletion in exons 7–8 cause LGMD2D92. However, in this case, the authors did not evaluate the cardiac phenotype.
Other groups have found that SGCD in porcine93 and mouse94 models results in reduced protein expression in the sarcoglycan subcomplex, disrupting the overall structure of DGCs and leading to DCM. In addition, 19% of all patients with SGCA, SGCB, or SGCG mutations were reported to have dilated cardiomyopathy, and 25% of all patients also required respiratory support95.
Recessive mutations in sarcoglycan (SG) δ result in a reduction or complete absence of sarcoglycan complexes and hence DGC in cardiac tissue and are responsible for LGMD and its associated DCM96. Interestingly, dominant-negative mutations in SG-δ are specific to the cardiovascular system and are the cause of familial dilated cardiomyopathy97. SG-δ R97Q and R71T dominant-negative mutations have been shown to be stably expressed in rat cardiomyocytes without significant impairment of total DGC98. However, heart cells carrying these mutations are more susceptible to sarcolemma damage, permeability, and mechanical dysfunction under mechanical stress, consistent with the DCM98 phenotype.
Sarcospan (SSPN) is a 25 kDa tetraspanin localized in the sarcoglycan subcomplex and is believed to serve as a protein scaffold99,100. As a protein scaffold, SSPN stabilizes the localization and glycosylation of α-DG99,101. Overexpression of SSPN in mouse models has been found to increase binding between muscle and laminin 102 . In addition, SSPN has been shown to interact with integrins, suggesting the degree of crosstalk between the two rib commissures, DGC, and the integrin-talin-vinculin glycoprotein structure100,101,102. Knockdown of SSPN also resulted in an increase in α7β1 in mouse skeletal muscle.
A recent study showed that sarcospan overexpression enhances maturation and glycosylation of α-DG in cardiac tissue independently of galactosylaminotransferase 2 (Galgt2) knockdown in an mdx mouse model of DMD, thereby alleviating disease phenotype 101. Increased glycosylation of the dystroglycan complex may enhance interaction with the ECM, thereby most mitigating the disease. Moreover, they have shown that sarcospan overexpression reduces the interaction of β1D integrin with DGCs, highlighting a possible role for sarcospan in the regulation of integrin complexes101.
Syntrophins are a family of small (58 kDa) proteins that localize to DGCs, do not themselves have intrinsic enzymatic activity, and serve as molecular adapters103,104. Five isoforms (α-1, β-1, β-2, γ-1 and γ-2) have been identified showing tissue-specific expression, with the α-1 isoform predominantly expressed in striated muscle tissue 105 . Syntrophins are important adapter proteins that facilitate communication between dystrophin and signaling molecules, including neuronal nitric oxide synthase (nNOS) in skeletal muscle106. α-syntrophin interacts directly with the dystrophin 16-17 spectrin repeat domain, which in turn binds to the nNOS106,107 PDZ-binding motif.
Syntrophins also interact with dystrobrevin via the PH2 and SU binding domains, and they also interact with the actin cytoskeleton 108 . Indeed, syntrophins seem to play a particularly important role in the regulation of cytoskeletal dynamics, and the α and β isoforms are able to directly interact with F-actin 108 and thus likely play a role in the regulation of tensegrity and the biomechanics of the cellular effect. In addition, syntrophins have been shown to regulate the cytoskeleton through Rac1109.
Modulating syntrophin levels can restore function, and a recent study using mini-dystrophin showed that the ΔR4-R23/ΔCT construct was able to restore α-syntrophin as well as other DGC proteins to levels comparable to WT mdx cardiomyocytes.
In addition to their role in the regulation of the cytoskeleton, syntrophins are also well documented in the regulation of ion channels 111,112,113. The PDZ-binding motif of syntrophins regulates the cardiac voltage-dependent Nav1.5111 channel, which plays a key role in establishing cardiac excitability and conduction. Interestingly, in the mdx mouse model, Nav1.5 channels were found to be downregulated and cardiac arrhythmias were found in the animals 111 . In addition, a family of mechanosensitive ion channels, the transient receptor potential channel (TRPC), has been shown to be regulated by α1-syntrophin in cardiac tissue 113 and TRPC6 inhibition has been shown to improve arrhythmias in the DMD112 mouse model. Increased TRPC6 activity in DMD has been reported to result in cardiac arrhythmias, which are relieved when combined with PKG 112 . Mechanically, dystrophin depletion promotes a stretch-induced influx of [Ca2+]i that acts upstream of TRPC6 to activate it, as shown in cardiomyocytes and vascular smooth muscle cells112,114. Hyperactivation of TRPC6 to stretch makes it a major mechanosensor and potential therapeutic target in DMD112,114.
Loss of dystrophin leads to lysis or marked suppression of the entire DGC complex, with subsequent loss of many mechanoprotective and mechanotransduction functions, resulting in the catastrophic phenotype seen in striated muscle tissue in DMD. Therefore, it may be reasonable to consider that RSKs work in concert and that individual components are dependent on the presence and functioning of other components. This is especially true for dystrophin, which appears to be required for assembly and localization of the sarcolemma complex in cardiomyocytes. Each component plays a unique role in contributing to overall stabilization of the sarcolemma, localization of key accessory proteins, regulation of ion channels and gene expression, and the loss of a single protein in the DGC leads to dysregulation of the entire myocardium.
As shown above, many DGC proteins are involved in mechanotransduction and signaling, and dystrophin is particularly suited to this role. If DGC is located in the ribs, this confirms the opinion that it participates in mechanotransduction along with integrins. Thus, DGCs physically undergo anisotropic force transfer and participate in mechanosensory and cytoskeletal rearrangement of the intracellular microenvironment, consistent with the tensegrity model. In addition, Dp427m buffers incoming biomechanical forces by expanding spectrin repeats within its central core domain, thereby acting as a mechanoprotector by maintaining a 25 pN unwinding force over an extended 800 nm range. By splitting, dystrophin is able to “buffer” the force of contraction-relaxation produced by cardiomyocytes10. Given the diversity of proteins and phospholipids that interact with spectrin repeat domains, it is interesting to speculate whether spectrin repeat unwinding alters the binding kinetics of mechanosensitive proteins in a manner similar to that of talin116,117,118. However, this has not yet been determined and further investigation is required.