Skeletal muscle intermediate filaments form a stress-transmitting and stress-signaling network

Time-of-day effects on ex vivo muscle contractility following short-term satellite cell ablation

Muscle isometric torque fluctuates according to time-of-day with such variation owed to the influence of circadian molecular clock genes. Satellite cells (SCs), the muscle stem cell population, also express molecular clock genes with several contractile-related genes oscillating in a diurnal pattern. Currently, limited evidence exists regarding the relationship between SCs and contractility, although long-term SC ablation alters muscle contractile function. Whether there are acute alterations in contractility following SC ablation and with respect to the time-of-day is unknown. We investigated whether short-term SC ablation affected contractile function at two times of day and whether any such alterations led to different extents of eccentric contraction-induced injury. Using an established mouse model to deplete SCs, we characterized muscle clock gene expression and ex vivo contractility at two times-of-day (morning: 0700 and afternoon: 1500). Morning-SC+ animals demonstrated ∼25%-30% reductions in tetanic/eccentric specific forces and, after eccentric injury, exhibited ∼30% less force-loss and ∼50% less dystrophinnegative fibers versus SC- counterparts; no differences were noted between Afternoon groups (Morning-SC+: -5.63 ± 0.61, Morning-SC-: -7.93 ± 0.61; N/cm2; P < 0.05) (Morning-SC+: 32 ± 2.1, Morning-SC-: 64 ± 10.2; dystrophinnegative fibers; P < 0.05). As Ca++ kinetics underpin force generation, we also evaluated caffeine-induced contracture force as an indirect marker of Ca++ availability and found similar force reductions in Morning-SC+ vs. SC- mice. We conclude that force production is reduced in the presence of SCs in the morning but not in the afternoon, suggesting that SCs may have a time-of-day influence over contractile function.NEW & NOTEWORTHY Muscle isometric torque fluctuates according to time-of-day with such variation owed to molecular clock regulation. Satellite cells (SCs) have recently demonstrated diurnal characteristics related to muscle physiology. In our work, force production was reduced in the presence versus absence of SCs in the morning but, not in the afternoon. Morning-SC+ animals, producing lower force, sustained lesser degrees of injury versus SC- counterparts. One potential mechanism underpinning lower forces produced appears to be lower calcium availability.

Traumatic muscle injury

Traumatic muscle injury represents a collection of skeletal muscle pathologies caused by trauma to the muscle tissue and is defined as damage to the muscle tissue that can result in a functional deficit. Traumatic muscle injury can affect people across the lifespan and can result from high stresses and strains to skeletal muscle tissue, often due to muscle activation while the muscle is lengthening, resulting in indirect and non-contact muscle injuries (strains or ruptures), or from external impact, resulting in direct muscle injuries (contusion or laceration). At a microscopic level, muscle fibres can repair focal damage but must be completely regenerated after full myofibre necrosis. The diagnosis of muscle injury is based on patient history and physical examination. Imaging may be indicated to eliminate differential diagnoses. The management of muscle injury has changed within the past 5 years from initial rest, immobilization and (over)protection to early activation and progressive loading using an active approach. One challenge of muscle injury management is that numerous medical treatment options, such as medications and injections, are often used or proposed to try to accelerate muscle recovery despite very limited efficacy evidence. Another challenge is the prevention of muscle injury owing to the multifactorial and complex nature of this injury.

Inter-set stretch: A potential time-efficient strategy for enhancing skeletal muscle adaptations

Time is considered a primary barrier to exercise adherence. Therefore, developing time-efficient resistance training (RT) strategies that optimize muscular adaptations is of primary interest to practitioners. A novel approach to the problem involves combining intensive stretch protocols with RT. Conceivably, integrating stretch into the inter-set period may provide an added stimulus for muscle growth without increasing session duration. Mechanistically, stretch can regulate anabolic signaling via both active and passive force sensors. Emerging evidence indicates that both lengthening contractions against a high load as well as passive stretch can acutely activate anabolic intracellular signaling pathways involved in muscle hypertrophy. Although longitudinal research investigating the effects of stretching between RT sets is limited, some evidence suggests it may in fact enhance hypertrophic adaptations. Accordingly, the purpose of this paper is threefold: (1) to review how the active force of a muscle contraction and the force of a passive stretched are sensed; (2) to present evidence for the effectiveness of RT with inter-set stretch for muscle hypertrophy (3) to provide practical recommendations for application of inter-set stretch in program design as well as directions for future research.

Protein degradation-associated mechanisms that are affected in Alzheimer´s disease

Alzheimer's disease (AD) is the most common type of dementia associated with age-related neurodegeneration. Alteration of several molecular mechanisms has been correlated with the progression of AD. In recent years, dysregulation of proteostasis-associated pathways has emerged as a potential risk factor for neurodegenerative diseases. This review investigated the ubiquitin-proteasome system, lysosome-associated degradation, endoplasmic-reticulum-associated degradation, and the formation of advanced glycation end products. These pathways involved in proteostasis have been reported to be altered in AD, suggesting that their study may be critical for identifying new biomarkers and target molecules for AD.

Plectin-Mediated Intermediate Filament Functions: Why Isoforms Matter

This essay focuses on the role of plectin and its various isoforms in mediating intermediate filament (IF) network functions. It is based on previous studies that provided comprehensive evidence for a concept where plectin acts as an IF recruiter, and plectin-mediated IF networking and anchoring are key elements in IF function execution. Here, plectin’s global role as modulator of IF functionality is viewed from different perspectives, including the mechanical stabilization of IF networks and their docking platforms, contribution to cellular viscoelasticity and mechanotransduction, compartmentalization and control of the actomyosin machinery, connections to the microtubule system, and mechanisms and specificity of isoform targeting. Arguments for IF networks and plectin acting as mutually dependent partners are also given. Lastly, a working model is presented that describes a unifying mechanism underlying how plectin–IF networks mechanically control and propagate actomyosin-generated forces, affect microtubule dynamics, and contribute to mechanotransduction.

Cytoskeleton and Associated Proteins: Pleiotropic JNK Substrates and Regulators

This review extensively reports data from the literature concerning the complex relationships between the stress-induced c-Jun N-terminal kinases (JNKs) and the four main cytoskeleton elements, which are actin filaments, microtubules, intermediate filaments, and septins. To a lesser extent, we also focused on the two membrane-associated cytoskeletons spectrin and ESCRT-III. We gather the mechanisms controlling cytoskeleton-associated JNK activation and the known cytoskeleton-related substrates directly phosphorylated by JNK. We also point out specific locations of the JNK upstream regulators at cytoskeletal components. We finally compile available techniques and tools that could allow a better characterization of the interplay between the different types of cytoskeleton filaments upon JNK-mediated stress and during development. This overview may bring new important information for applied medical research.

Biochemical and structural basis of the passive mechanical properties of whole skeletal muscle

Passive mechanical properties of whole skeletal muscle are not as well understood as active mechanical properties. Both the structural basis for passive mechanical properties and the properties themselves are challenging to determine because it is not clear which structures within skeletal muscle actually bear passive loads and there are not established standards by which to make mechanical measurements. Evidence suggests that titin bears the majority of the passive load within the single muscle cell. However, at larger scales, such as fascicles and muscles, there is emerging evidence that the extracellular matrix bears the major part of the load. Complicating the ability to quantify and compare across size scales, muscles and species, definitions of muscle passive properties such as stress, strain, modulus and stiffness can be made relative to many reference parameters. These uncertainties make a full understanding of whole muscle passive mechanical properties and modelling these properties very difficult. Future studies defining the specific load bearing structures and their composition and organization are required to fully understand passive mechanics of the whole muscle and develop therapies to treat disorders in which passive muscle properties are altered such as muscular dystrophy, traumatic laceration, and contracture due to upper motor neuron lesion as seen in spinal cord injury, stroke and cerebral palsy.

LINCing Nuclear Mechanobiology With Skeletal Muscle Mass and Function

Skeletal muscle demonstrates a high degree of adaptability in response to changes in mechanical input. The phenotypic transformation in response to mechanical cues includes changes in muscle mass and force generating capabilities, yet the molecular pathways that govern skeletal muscle adaptation are still incompletely understood. While there is strong evidence that mechanotransduction pathways that stimulate protein synthesis play a key role in regulation of muscle mass, there are likely additional mechano-sensitive mechanisms important for controlling functional muscle adaptation. There is emerging evidence that the cell nucleus can directly respond to mechanical signals (i.e., nuclear mechanotransduction), providing a potential additional level of cellular regulation for controlling skeletal muscle mass. The importance of nuclear mechanotransduction in cellular function is evident by the various genetic diseases that arise from mutations in proteins crucial to the transmission of force between the cytoskeleton and the nucleus. Intriguingly, these diseases preferentially affect cardiac and skeletal muscle, suggesting that nuclear mechanotransduction is critically important for striated muscle homeostasis. Here we discuss our current understanding for how the nucleus acts as a mechanosensor, describe the main cytoskeletal and nuclear proteins involved in the process, and propose how similar mechanoresponsive mechanisms could occur in the unique cellular environment of a myofiber. In addition, we examine how nuclear mechanotransduction fits into our current framework for how mechanical stimuli regulates skeletal muscle mass.

New roles for desmin in the maintenance of muscle homeostasis

Desmin is the primary intermediate filament (IF) of cardiac, skeletal, and smooth muscle. By linking the contractile myofibrils to the sarcolemma and cellular organelles, desmin IF contributes to muscle structural and cellular integrity, force transmission, and mitochondrial homeostasis. Mutations in desmin cause myofibril misalignment, mitochondrial dysfunction, and impaired mechanical integrity leading to cardiac and skeletal myopathies in humans, often characterized by the accumulation of protein aggregates. Recent evidence indicates that desmin filaments also regulate proteostasis and cell size. In skeletal muscle, changes in desmin filament dynamics can facilitate catabolic events as an adaptive response to a changing environment. In addition, post-translational modifications of desmin and its misfolding in the heart have emerged as key determinants of homeostasis and disease. In this review, we provide an overview of the structural and cellular roles of desmin and propose new models for its novel functions in preserving the homeostasis of striated muscles.

Selective association of desmin intermediate filaments with a phospholipid layer in droplets

Desmin, an intermediate filament protein expressed in muscle cells, plays a key role in the integrity and regulation of the contractile system. Furthermore, the distribution of desmin in cells and its interplay with plasma and organelle membranes are crucial for cell functions; however, the fundamental properties of lipid-desmin interactions remain unknown. Using a water-in-oil method for a limited space system in vitro, we examined the distribution of desmin in three types of phospholipid droplets: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and 1,2-dioleoyl-sn-glycero-3-phosphoserine (DOPS). When fluorescent-labeled desmin was observed for 60 min after desmin assembly was initiated by adding 25 mM KCl, desmin accumulated on both the DOPE and DOPS layers; however, it did not accumulate on the DOPC layer of droplets. An increase in salt concentration did not moderate the accumulation. The initial form of either oligomer or mature filament affected the accumulation on each lipid layer. When liposomes were included in the droplets, desmin was associated with DOPE but not on DOPC liposomes. These results suggest that desmin has the potential for association with phospholipids concerning desmin form and lipid shape. The behavior and composition of living membranes may affect the distribution of desmin networks.

In Full Force. Mechanotransduction and Morphogenesis during Homeostasis and Tissue Regeneration

The interactions of form and function have been the focus of numerous studies in the context of development and more recently regeneration. Our understanding on how cells, tissues and organs sense and interpret external cues, such as mechanical forces, is becoming deeper as novel techniques in imaging are applied and the relevant signaling pathways emerge. These cellular responses can be found from bacteria to all multicellular organisms such as plants and animals. In this review, we focus on hemodynamic flow and endothelial shear stress during cardiovascular development and regeneration, where the interactions of morphogenesis and proper function are more prominent. In addition, we address the recent literature on the role of extracellular matrix and fibrotic response during tissue repair and regeneration. Finally, we refer to examples where the integration of multi-disciplinary approaches to understand the biomechanics of cellular responses could be utilized in novel medical applications.

Identification and characterization of Fbxl22, a novel skeletal muscle atrophy-promoting E3 ubiquitin ligase

Muscle-specific E3 ubiquitin ligases have been identified in muscle atrophy-inducing conditions. The purpose of the current study was to explore the functional role of F-box and leucine-rich protein 22 (Fbxl22), and a newly identified splice variant (Fbxl22-193), in skeletal muscle homeostasis and neurogenic muscle atrophy. In mouse C₂C₁₂ muscle cells, promoter fragments of the Fbxl22 gene were cloned and fused with the secreted alkaline phosphatase reporter gene to assess the transcriptional regulation of Fbxl22. The tibialis anterior muscles of male C57/BL6 mice (12-16 wk old) were electroporated with expression plasmids containing the cDNA of two Fbxl22 splice variants and tissues collected after 7, 14, and 28 days. Gastrocnemius muscles of wild-type and muscle-specific RING finger 1 knockout (MuRF1 KO) mice were electroporated with an Fbxl22 RNAi or empty plasmid and denervated 3 days posttransfection, and tissues were collected 7 days postdenervation. The full-length gene and novel splice variant are transcriptionally induced early (after 3 days) during neurogenic muscle atrophy. In vivo overexpression of Fbxl22 isoforms in mouse skeletal muscle leads to evidence of myopathy/atrophy, suggesting that both are involved in the process of neurogenic muscle atrophy. Knockdown of Fbxl22 in the muscles of MuRF1 KO mice resulted in significant additive muscle sparing 7 days after denervation. Targeting two E3 ubiquitin ligases appears to have a strong additive effect on protecting muscle mass loss with denervation, and these findings have important implications in the development of therapeutic strategies to treat muscle atrophy.

Growing Old Too Early: Skeletal Muscle Single Fiber Biomechanics in Ageing R349P Desmin Knock-in Mice Using the MyoRobot Technology

Muscle biomechanics relies on active motor protein assembly and passive strain transmission through cytoskeletal structures. The desmin filament network aligns myofibrils at the z-discs, provides nuclear–sarcolemmal anchorage and may also serve as memory for muscle repositioning following large strains. Our previous analyses of R349P desmin knock-in mice, an animal model for the human R350P desminopathy, already depicted pre-clinical changes in myofibrillar arrangement and increased fiber bundle stiffness. As the effect of R349P desmin on axial biomechanics in fully differentiated single muscle fibers is unknown, we used our MyoRobot to compare passive visco-elasticity and active contractile biomechanics in single fibers from fast- and slow-twitch muscles from adult to senile mice, hetero- or homozygous for the R349P desmin mutation with wild type littermates. We demonstrate that R349P desmin presence predominantly increased axial stiffness in both muscle types with a pre-aged phenotype over wild type fibers. Axial viscosity and Ca2+-mediated force were largely unaffected. Mutant single fibers showed tendencies towards faster unloaded shortening over wild type fibers. Effects of aging seen in the wild type appeared earlier in the mutant desmin fibers. Our single-fiber experiments, free of extracellular matrix, suggest that compromised muscle biomechanics is not exclusively attributed to fibrosis but also originates from an impaired intermediate filament network.

Desmin prevents muscle wasting, exaggerated weakness and fragility, and fatigue in dystrophic mdx mouse

KEY POINTS

Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix. Deletion of the desmin gene in mdx mice [double knockout (DKO) mice] induces marked muscle weakness and fatigue resistance compared to mdx mice. Muscle fragility (higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice. By contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy. Desmin cDNA transfer with adeno-associated virus in newborn mdx mice reduced muscle weakness. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic muscle.

ABSTRACT

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease caused by dystrophin deficiency. Desmin, similar to dystrophin, is associated with costameric structures bridging sarcomeres to the extracellular matrix that contributes to muscle function. In the present study, we attempted to provide further insight into the roles of desmin, for which the expression is increased in the muscle from the mouse mdx DMD model. We show that a deletion of the desmin gene (Des) in mdx mice [double knockout (DKO) mice, mdx:desmin-/-] induces a marked muscle weakness; namely, a reduced absolute maximal force production and increased fatigue compared to that in mdx mice. Fragility (i.e. higher susceptibility to contraction-induced injury) was also aggravated in DKO mice compared to mdx mice, despite the promotion of supposedly less fragile muscle fibres in DKO mice, and this worsening of fragility was related to a decreased muscle excitability. Moreover, in contrast to mdx mice, the DKO mice did not undergo muscle hypertrophy, as indicated by smaller and fewer fibres, with a reduced percentage of centronucleated fibres, potentially explaining the severe muscle weakness. Notably, Desmin cDNA transfer with adeno-associated virus in newborn mdx mice improved specific maximal force normalized to muscle weight. Overall, desmin plays important and beneficial roles in muscle wasting, performance and fragility in dystrophic mdx mice, which differ, at least in part, from those observed in healthy muscle.

AAV-mediated cardiac gene transfer of wild-type desmin in mouse models for recessive desminopathies

Mutations in the human desmin gene cause autosomal-dominant and recessive cardiomyopathies and myopathies with marked phenotypic variability. Here, we investigated the effects of adeno-associated virus (AAV)-mediated cardiac wild-type desmin expression in homozygous desmin knockout (DKO) and homozygous R349P desmin knockin (DKI) mice. These mice serve as disease models for two subforms of autosomal-recessive desminopathies, the former for the one with a complete lack of desmin protein and the latter for the one with solely mutant desmin protein expression in conjunction with protein aggregation pathology in striated muscle. Two-month-old mice were injected with either a single dose of 5 × 10¹² AAV9-hTNT2-mDes (AAV-Des) vector genomes or NaCl as control. One week after injection, mice were subjected to a forced swimming exercise protocol for 4 weeks. Cardiac function was monitored over a period of 15 month after injection and before the mice were sacrificed for biochemical and morphological analysis. AAV-mediated cardiac expression of wild-type desmin in both the homozygous DKO and DKI backgrounds reached levels seen in wild-type mice. Notably, AAV-Des treated DKO mice showed a regular subcellular distribution of desmin as well as a normalization of functional and morphological cardiac parameters. Treated DKI mice, however, showed an aberrant subcellular localization of desmin, unchanged functional cardiac parameters, and a trend toward an increased cardiac fibrosis. In conclusion, the effect of a high-dose AAV9-based desmin gene therapy is highly beneficial for the heart in DKO animals, but not in DKI mice.

Increased expression of desmin and vimentin reduces bladder smooth muscle contractility via JNK2

Bladder dysfunction is associated with the overexpression of the intermediate filament (IF) proteins desmin and vimentin in obstructed bladder smooth muscle (BSM). However, the mechanisms by which these proteins contribute to BSM dysfunction are not known. Previous studies have shown that desmin and vimentin directly participate in signal transduction. In this study, we hypothesized that BSM dysfunction associated with overexpression of desmin or vimentin is mediated via c-Jun N-terminal kinase (JNK). We employed a model of murine BSM tissue in which increased expression of desmin or vimentin was induced by adenoviral transduction to examine the sufficiency of increased IF protein expression to reduce BSM contraction. Murine BSM strips overexpressing desmin or vimentin generated less force in response to KCl and carbachol relative to the levels in control murine BSM strips, an effect associated with increased JNK2 phosphorylation and reduced myosin light chain (MLC20 ) phosphorylation. Furthermore, desmin and vimentin overexpressions did not alter BSM contractility and MLC20 phosphorylation in strips isolated from JNK2 knockout mice. Pharmacological JNK2 inhibition produced results qualitatively similar to those caused by JNK2 knockout. These findings suggest that inhibition of JNK2 may improve diminished BSM contractility associated with obstructive bladder disease.

Skeletal muscle: A review of molecular structure and function, in health and disease

Decades of research in skeletal muscle physiology have provided multiscale insights into the structural and functional complexity of this important anatomical tissue, designed to accomplish the task of generating contraction, force and movement. Skeletal muscle can be viewed as a biomechanical device with various interacting components including the autonomic nerves for impulse transmission, vasculature for efficient oxygenation, and embedded regulatory and metabolic machinery for maintaining cellular homeostasis. The "omics" revolution has propelled a new era in muscle research, allowing us to discern minute details of molecular cross-talk required for effective coordination between the myriad interacting components for efficient muscle function. The objective of this review is to provide a systems-level, comprehensive mapping the molecular mechanisms underlying skeletal muscle structure and function, in health and disease. We begin this review with a focus on molecular mechanisms underlying muscle tissue development (myogenesis), with an emphasis on satellite cells and muscle regeneration. We next review the molecular structure and mechanisms underlying the many structural components of the muscle: neuromuscular junction, sarcomere, cytoskeleton, extracellular matrix, and vasculature surrounding muscle. We highlight aberrant molecular mechanisms and their possible clinical or pathophysiological relevance. We particularly emphasize the impact of environmental stressors (inflammation and oxidative stress) in contributing to muscle pathophysiology including atrophy, hypertrophy, and fibrosis. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Developmental Biology > Developmental Processes in Health and Disease Models of Systems Properties and Processes > Cellular Models.

A Balance Between Intermediate Filaments and Microtubules Maintains Nuclear Architecture in the Cardiomyocyte

Rationale: Mechanical forces are transduced to nuclear responses via the linkers of the nucleoskeleton and cytoskeleton (LINC) complex, which couples the cytoskeleton to the nuclear lamina and associated chromatin. While disruption of the LINC complex can cause cardiomyopathy, the relevant interactions that bridge the nucleoskeleton to cytoskeleton are poorly understood in the cardiomyocyte, where cytoskeletal organization is unique. Furthermore, while microtubules and desmin intermediate filaments associate closely with cardiomyocyte nuclei, the importance of these interactions is unknown. Objective: Here, we sought to determine how cytoskeletal interactions with the LINC complex regulate nuclear homeostasis in the cardiomyocyte. Methods and Results: To this end, we acutely disrupted the LINC complex, microtubules, actin, and intermediate filaments and assessed the consequences on nuclear morphology and genome organization in rat ventricular cardiomyocytes via a combination of super-resolution imaging, biophysical, and genomic approaches. We find that a balance of dynamic microtubules and desmin intermediate filaments is required to maintain nuclear shape and the fidelity of the nuclear envelope and lamina. Upon depletion of desmin (or nesprin [nuclear envelope spectrin repeat protein]-3, its binding partner in the LINC complex), polymerizing microtubules collapse the nucleus and drive infolding of the nuclear membrane. This results in DNA damage, a loss of genome organization, and broad transcriptional changes. The collapse in nuclear integrity is concomitant with compromised contractile function and may contribute to the pathophysiological changes observed in desmin-related myopathies. Conclusions: Disrupting the tethering of desmin to the nucleus results in a loss of nuclear homeostasis and rapid alterations to cardiomyocyte function. Our data suggest that a balance of forces imposed by intermediate filaments and microtubules is required to maintain nuclear structure and genome organization in the cardiomyocyte.

The MyoRobot technology discloses a premature biomechanical decay of skeletal muscle fiber bundles derived from R349P desminopathy mice

Mutations in the Des gene coding for the muscle-specific intermediate filament protein desmin lead to myopathies and cardiomyopathies. We previously generated a R349P desmin knock-in mouse strain as a patient-mimicking model for the corresponding most frequent human desmin mutation R350P. Since nothing is known about the age-dependent changes in the biomechanics of affected muscles, we investigated the passive and active biomechanics of small fiber bundles from young (17–23 wks), adult (25–45 wks) and aged (>60 wks) heterozygous and homozygous R349P desmin knock-in mice in comparison to wild-type littermates. We used a novel automated biomechatronics platform, the MyoRobot, to perform coherent quantitative recordings of passive (resting length-tension curves, visco-elasticity) and active (caffeine-induced force transients, pCa-force, ‘slack-tests’) parameters to determine age-dependent effects of the R349P desmin mutation in slow-twitch soleus and fast-twitch extensor digitorum longus small fiber bundles. We demonstrate that active force properties are not affected by this mutation while passive steady-state elasticity is vastly altered in R349P desmin fiber bundles compatible with a pre-aged phenotype exhibiting stiffer muscle preparations. Visco-elasticity on the other hand, was not altered. Our study represents the first systematic age-related characterization of small muscle fiber bundle preparation biomechanics in conjunction with inherited desminopathy.

Eccentric Muscle Contractions: Risks and Benefits

Eccentric contractions, characterized by the lengthening of the muscle-tendon complex, present several unique features compared with other types of contractions, which may lead to unique adaptations. Due to its specific physiological and mechanical properties, there is an increasing interest in employing eccentric muscle work for rehabilitation and clinical purposes. However, unaccustomed eccentric exercise is known to cause muscle damage and delayed pain, commonly defined as “Delayed-Onset Muscular Soreness” (DOMS). To date, the most useful preventive strategy to avoid these adverse effects consists of repeating sessions involving submaximal eccentric contractions whose intensity is progressively increased over the training. Despite an increased number of investigations focusing on the eccentric contraction, a significant gap still remains in our understanding of the cellular and molecular mechanisms underlying the initial damage response and subsequent adaptations to eccentric exercise. Yet, unraveling the molecular basis of exercise-related muscle damage and soreness might help uncover the mechanistic basis of pathological conditions as myalgia or neuromuscular diseases. In addition, a better insight into the mechanisms governing eccentric training adaptations should provide invaluable information for designing therapeutic interventions and identifying potential therapeutic targets.

Deciphering Nuclear Mechanobiology in Laminopathy

Extracellular mechanical stimuli are translated into biochemical signals inside the cell via mechanotransduction. The nucleus plays a critical role in mechanoregulation, which encompasses mechanosensing and mechanotransduction. The nuclear lamina underlying the inner nuclear membrane not only maintains the structural integrity, but also connects the cytoskeleton to the nuclear envelope. Lamin mutations, therefore, dysregulate the nuclear response, resulting in abnormal mechanoregulations, and ultimately, disease progression. Impaired mechanoregulations even induce malfunction in nuclear positioning, cell migration, mechanosensation, as well as differentiation. To know how to overcome laminopathies, we need to understand the mechanisms of laminopathies in a mechanobiological way. Recently, emerging studies have demonstrated the varying defects from lamin mutation in cellular homeostasis within mechanical surroundings. Therefore, this review summarizes recent findings highlighting the role of lamins, the architecture of nuclear lamina, and their disease relevance in the context of nuclear mechanobiology. We will also provide an overview of the differentiation of cellular mechanics in laminopathy.

On the role of tubulin, plectin, desmin, and vimentin in the regulation of mitochondrial energy fluxes in muscle cells

Mitochondria perform a central role in life and death of the eukaryotic cell. They are major players in the generation of macroergic compounds and function as integrated signaling pathways, including the regulation of Ca²⁺ signals and apoptosis. A growing amount of evidence is demonstrating that mitochondria of muscle cells use cytoskeletal proteins (both microtubules and intermediate filaments) not only for their movement and proper cellular positioning, but also to maintain their biogenesis, morphology, function, and regulation of energy fluxes through the outer mitochondrial membrane (MOM). Here we consider the known literature data concerning the role of tubulin, plectin, desmin and vimentin in bioenergetic function of mitochondria in striated muscle cells, as well as in controlling the permeability of MOM for adenine nucleotides (ADNs). This is of great interest since dysfunctionality of these cytoskeletal proteins has been shown to result in severe myopathy associated with pronounced mitochondrial dysfunction. Further efforts are needed to uncover the pathways by which the cytoskeleton supports the functional capacity of mitochondria and transport of ADN(s) across the MOM (through voltage-dependent anion channel).

Clathrin plaques and associated actin anchor intermediate filaments in skeletal muscle

Clathrin plaques are stable features of the plasma membrane observed in several cell types. They are abundant in muscle, where they localize at costameres that link the contractile apparatus to the sarcolemma and connect the sarcolemma to the basal lamina. Here, we show that clathrin plaques and surrounding branched actin filaments form microdomains that anchor a three-dimensional desmin intermediate filament (IF) web. Depletion of clathrin plaque and branched actin components causes accumulation of desmin tangles in the cytoplasm. We show that dynamin 2, whose mutations cause centronuclear myopathy (CNM), regulates both clathrin plaques and surrounding branched actin filaments, while CNM-causing mutations lead to desmin disorganization in a CNM mouse model and patient biopsies. Our results suggest a novel paradigm in cell biology, wherein clathrin plaques act as platforms capable of recruiting branched cortical actin, which in turn anchors IFs, both essential for striated muscle formation and function.

Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise

One of the most striking adaptations to exercise is the skeletal muscle hypertrophy that occurs in response to resistance exercise. A large body of work shows that a mammalian target of rapamycin complex 1 (mTORC1)-mediated increase of muscle protein synthesis is the key, but not sole, mechanism by which resistance exercise causes muscle hypertrophy. While much of the hypertrophy signaling cascade has been identified, the initiating, resistance exercise-induced and hypertrophy-stimulating stimuli have remained elusive. For the purpose of this review, we define an initiating, resistance exercise-induced and hypertrophy-stimulating signal as "hypertrophy stimulus," and the sensor of such a signal as "hypertrophy sensor." In this review we discuss our current knowledge of specific mechanical stimuli, damage/injury-associated and metabolic stress-associated triggers, as potential hypertrophy stimuli. Mechanical signals are the prime hypertrophy stimuli candidates, and a filamin-C-BAG3-dependent regulation of mTORC1, Hippo, and autophagy signaling is a plausible albeit still incompletely characterized hypertrophy sensor. Other candidate mechanosensing mechanisms are nuclear deformation-initiated signaling or several mechanisms related to costameres, which are the functional equivalents of focal adhesions in other cells. While exercise-induced muscle damage is probably not essential for hypertrophy, it is still unclear whether and how such muscle damage could augment a hypertrophic response. Interventions that combine blood flow restriction and especially low load resistance exercise suggest that resistance exercise-regulated metabolites could be hypertrophy stimuli, but this is based on indirect evidence and metabolite candidates are poorly characterized.

Imbalances in protein homeostasis caused by mutant desmin

AIMS

We investigated newly generated immortalized heterozygous and homozygous R349P desmin knock-in myoblasts in conjunction with the corresponding desminopathy mice as models for desminopathies to analyse major protein quality control processes in response to the presence of R349P mutant desmin.

METHODS

We used hetero- and homozygous R349P desmin knock-in mice for analyses and for crossbreeding with p53 knock-out mice to generate immortalized R349P desmin knock-in skeletal muscle myoblasts and myotubes. Skeletal muscle sections and cultured muscle cells were investigated by indirect immunofluorescence microscopy, proteasomal activity measurements and immunoblotting addressing autophagy rate, chaperone-assisted selective autophagy and heat shock protein levels. Muscle sections were further analysed by transmission and immunogold electron microscopy.

RESULTS

We demonstrate that mutant desmin (i) increases proteasomal activity, (ii) stimulates macroautophagy, (iii) dysregulates the chaperone assisted selective autophagy and (iv) elevates the protein levels of αB-crystallin and Hsp27. Both αB-crystallin and Hsp27 as well as Hsp90 displayed translocation patterns from Z-discs as well as Z-I junctions, respectively, to the level of sarcomeric I-bands in dominant and recessive desminopathies.

CONCLUSIONS

Our findings demonstrate that the presence of R349P mutant desmin causes a general imbalance in skeletal muscle protein homeostasis via aberrant activity of all major protein quality control systems. The augmented activity of these systems and the subcellular shift of essential heat shock proteins may deleteriously contribute to the previously observed increased turnover of desmin itself and desmin-binding partners, which triggers progressive dysfunction of the extrasarcomeric cytoskeleton and the myofibrillar apparatus in the course of the development of desminopathies.

Biomechanical response of skeletal muscle to eccentric contractions

The forced lengthening of an activated skeletal muscle has been termed an eccentric contraction (EC). This review highlights the mechanically unique nature of the EC and focuses on the specific disruption of proteins within the cell known as cytoskeletal proteins. The major intermediate filament cytoskeletal protein, desmin, has been the focus of work in this area because changes to desmin occur within minutes of ECs and because desmin has been shown to play both a mechanical and biologic role in a muscle's response to EC. It is hoped that these types of studies will assist in decreasing the incidence of muscle injury in athletes and facilitating the development of new therapies to treat muscle injuries.

A rim-and-spoke hypothesis to explain the biomechanical roles for cytoplasmic intermediate filament networks

Textbook images of keratin intermediate filament (IF) networks in epithelial cells and the functional compromization of the epidermis by keratin mutations promulgate a mechanical role for this important cytoskeletal component. In stratified epithelia, keratin filaments form prominent radial spokes that are focused onto cell-cell contact sites, i.e. the desmosomes. In this Hypothesis, we draw attention to a subset of keratin filaments that are apposed to the plasma membrane. They form a rim of filaments interconnecting the desmosomes in a circumferential network. We hypothesize that they are part of a rim-and-spoke arrangement of IFs in epithelia. From our review of the literature, we extend this functional role for the subplasmalemmal rim of IFs to any cell, in which plasma membrane support is required, provided these filaments connect directly or indirectly to the plasma membrane. Furthermore, cytoplasmic IF networks physically link the outer nuclear and plasma membranes, but their participation in mechanotransduction processes remain largely unconsidered. Therefore, we also discuss the potential biomechanical and mechanosensory role(s) of the cytoplasmic IF network in terms of such a rim (i.e. subplasmalemmal)-and-spoke arrangement for cytoplasmic IF networks.

Molecular Regulation of Exercise-Induced Muscle Fiber Hypertrophy

Skeletal muscle hypertrophy is a widely sought exercise adaptation to counteract the muscle atrophy of aging and disease, or to improve athletic performance. While this desired muscle enlargement is a well-known adaptation to resistance exercise training (RT), the mechanistic underpinnings are not fully understood. The purpose of this review is thus to provide the reader with a summary of recent advances in molecular mechanisms-based on the most current literature-that are thought to promote RT-induced muscle hypertrophy. We have therefore focused this discussion on the following areas of fertile investigation: ribosomal function and biogenesis, muscle stem (satellite) cell activity, transcriptional regulation, mechanotransduction, and myokine signaling.

Adaptations to Endurance and Strength Training

The capacity for human exercise performance can be enhanced with prolonged exercise training, whether it is endurance- or strength-based. The ability to adapt through exercise training allows individuals to perform at the height of their sporting event and/or maintain peak physical condition throughout the life span. Our continued drive to understand how to prescribe exercise to maximize health and/or performance outcomes means that our knowledge of the adaptations that occur as a result of exercise continues to evolve. This review will focus on current and new insights into endurance and strength-training adaptations and will highlight important questions that remain as far as how we adapt to training.

---Mechanosensitivity of aged muscle stem cells

During aging, skeletal muscle tissue progressively declines in mass, strength, and regenerative capacity. Decreased muscle stem cell (MuSC) number and impaired function might underlie the aging-related muscle wasting and impaired regenerative capacity. As yet, the search for factors that regulate MuSC fate and function has revealed several biochemical factors within the MuSC niche that may be responsible for the decline in MuSC regenerative capacity. This decline cannot be explained by environmental factors solely, as the MuSC potential to regenerate muscle tissue is not reversed by changing the biochemical MuSC niche composition. Here we discuss the likeliness that during physical exercise, MuSCs within their niche are subjected to mechanical loads, in particular pressure and shear stress, as well as associated deformations. We postulate that these physical cues are involved in the activation and differentiation of MuSCs as these cells contain several transmembrane sensor proteins that have been shown to be mechanosensitive in other cell types, that is, endothelial cells and osteoprogenitors. We will specifically address age-related changes in mechanosensing in MuSCs and their niche. Insight in the physical cues applied to the MuSCs in vivo, and how these cues affect MuSC fate and function, helps to develop new therapeutic interventions to counterbalance age-related muscle loss. This requires an approach combining two- and three-dimensional live cell imaging of MuSCs within contracting muscle tissue, mathematical finite element modeling, and cell biology. © 2017 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:632-641, 2018.

αB-crystallin is a sensor for assembly intermediates and for the subunit topology of desmin intermediate filaments

Mutations in the small heat shock protein chaperone CRYAB (αB-crystallin/HSPB5) and the intermediate filament protein desmin, phenocopy each other causing cardiomyopathies. Whilst the binding sites for desmin on CRYAB have been determined, desmin epitopes responsible for CRYAB binding and also the parameters that determine CRYAB binding to desmin filaments are unknown. Using a combination of co-sedimentation centrifugation, viscometric assays and electron microscopy of negatively stained filaments to analyse the in vitro assembly of desmin filaments, we show that the binding of CRYAB to desmin is subject to its assembly status, to the subunit organization within filaments formed and to the integrity of the C-terminal tail domain of desmin. Our in vitro studies using a rapid assembly protocol, C-terminally truncated desmin and two disease-causing mutants (I451M and R454W) suggest that CRYAB is a sensor for the surface topology of the desmin filament. Our data also suggest that CRYAB performs an assembly chaperone role because the assembling filaments have different CRYAB-binding properties during the maturation process. We suggest that the capability of CRYAB to distinguish between filaments with different surface topologies due either to mutation (R454W) or assembly protocol is important to understanding the pathomechanism(s) of desmin-CRYAB myopathies.

Age-related Differences in Dystrophin: Impact on Force Transfer Proteins, Membrane Integrity, and Neuromuscular Junction Stability

The loss of muscle strength with age has been studied from the perspective of a decline in muscle mass and neuromuscular junction (NMJ) stability. A third potential factor is force transmission. The purpose of this study was to determine the changes in the force transfer apparatus within aging muscle and the impact on membrane integrity and NMJ stability. We measured an age-related loss of dystrophin protein that was greatest in the flexor muscles. The loss of dystrophin protein occurred despite a twofold increase in dystrophin mRNA. Importantly, this disparity could be explained by the four- to fivefold upregulation of the dystromir miR-31. To compensate for the loss of dystrophin protein, aged muscle contained increased α-sarcoglycan, syntrophin, sarcospan, laminin, β1-integrin, desmuslin, and the Z-line proteins α-actinin and desmin. In spite of the adaptive increase in other force transfer proteins, over the 48 hours following lengthening contractions, the old muscles showed more signs of impaired membrane integrity (fourfold increase in immunoglobulin G-positive fibers and 70% greater dysferlin mRNA) and NMJ instability (14- to 96-fold increases in Runx1, AchRδ, and myogenin mRNA). Overall, these data suggest that age-dependent alterations in dystrophin leave the muscle membrane and NMJ more susceptible to contraction-induced damage even before changes in muscle mass are obvious.

Titin force enhancement following active stretch of skinned skeletal muscle fibres

In actively stretched skeletal muscle sarcomeres, titin-based force is enhanced, increasing the stiffness of active sarcomeres. Titin force enhancement in sarcomeres is vastly reduced in mdm, a genetic mutation with a deletion in titin. Whether loss of titin force enhancement is associated with compensatory mechanisms at higher structural levels of organization, such as single fibres or entire muscles, is unclear. The aim of this study was to determine whether mechanical deficiencies in titin force enhancement are also observed at the fibre level, and whether mechanisms compensate for the loss of titin force enhancement. Single skinned fibres from control and mutant mice were stretched actively and passively beyond filament overlap to observe titin-based force. Mutant fibres generated lower contractile stress (force divided by cross-sectional area) than control fibres. Titin force enhancement was observed in control fibres stretched beyond filament overlap, but was overshadowed in mutant fibres by an abundance of collagen and high variability in mechanics. However, titin force enhancement could be measured in all control fibers and most mutant fibres following short stretches, accounting for ∼25% of the total stress following active stretch. Our results show that the partial loss of titin force enhancement in myofibrils is not preserved in all mutant fibres and this mutation likely affects fibres differentially within a muscle. An increase in collagen helps to reestablish total force at long sarcomere lengths with the loss in titin force enhancement in some mutant fibres, increasing the overall strength of mutant fibres.

Deficiency of the intermediate filament synemin reduces bone mass in vivo

While the type IV intermediate filament protein, synemin, has been shown to play a role in striated muscle and neuronal tissue, its presence and function have not been described in skeletal tissue. Here, we report that genetic ablation of synemin in 14-wk-old male mice results in osteopenia that includes a more than 2-fold reduction in the trabecular bone fraction in the distal femur and a reduction in the cross-sectional area at the femoral middiaphysis due to an attendant reduction in both the periosteal and endosteal perimeter. Analysis of serum markers of bone formation and static histomorphometry revealed a statistically significant defect in osteoblast activity and osteoblast number in vivo. Interestingly, primary osteoblasts isolated from synemin-null mice demonstrate markedly enhanced osteogenic capacity with a concomitant reduction in cyclin D1 mRNA expression, which may explain the loss of osteoblast number observed in vivo. In total, these data suggest an important, previously unknown role for synemin in bone physiology.

Effects of aging, exercise, and disease on force transfer in skeletal muscle

The loss of muscle strength and increased injury rate in aging skeletal muscle has previously been attributed to loss of muscle protein (cross-sectional area) and/or decreased neural activation. However, it is becoming clear that force transfer within and between fibers plays a significant role in this process as well. Force transfer involves a secondary matrix of proteins that align and transmit the force produced by the thick and thin filaments along muscle fibers and out to the extracellular matrix. These specialized networks of cytoskeletal proteins aid in passing force through the muscle and also serve to protect individual fibers from injury. This review discusses the cytoskeleton proteins that have been identified as playing a role in muscle force transmission, both longitudinally and laterally, and where possible highlights how disease, aging, and exercise influence the expression and function of these proteins.