Striated muscles, including cardiac muscle, skeletal muscle, or insect flight muscles, are highly organized tissues specifically designed to produce precise forces and movements for a diverse set of functions. Striated muscle cells (myocytes) comprising these tissues all share the remarkable ability to both produce and adapt to highly variable microenvironmental forces. During myocyte contraction, ATP-driven myosin motors on the thick filament of the sarcomere, the contractile unit of myocytes, cyclically interact with actin filaments to induce filament sliding and force-generating sarcomere shortening (Powers et al., 2021). For these molecular-level processes to translate to cell- and tissue-level force production, sarcomere-generated forces must be transmitted from the sarcomere, to the intermediate filaments of the cytoskeleton and costamere, and outward to the extracellular matrix (ECM) and surrounding cells. Similarly, forces generated from outside the cell (e.g., passive stretch, ECM stiffness) are transmitted into and distributed throughout the cell, putatively through the same molecular structures.

Both intracellular and extracellular forces ultimately reach the myocyte nucleus, where they can be processed into biological responses that mediate the structural or functional adaptation of the cell through a process called nuclear mechanosignaling. The nucleus of striated muscle cells, among many other cell types (Maurer and Lammerding, 2019), is now considered to be a key mechanosensor, reacting to forces at the nucleus-cytoskeleton interface and mediating downstream biological responses. Factors such as nuclear envelope structure, intracellular nuclear positioning, and nuclear deformation can influence the mechanosensing ability and response to mechanical forces (Jabre et al., 2021). Therefore, myocytes and their nuclei must maintain a mechanical homeostasis during muscle contractions and deformations to preserve healthy muscle structure and function.

However, this homeostasis can be disrupted by defects in the ability of the myocyte nucleus to sense the mechanical forces and/or by abnormal mechanical forces imposed onto the nucleus. Myocytes under persistent abnormal mechanical forces, such as chronically altered contractility or abnormal strains imposed during passive muscle stretch, can trigger a myriad of biological responses that often lead to maladaptive cell remodeling or hypertrophy, contractile dysfunction, and cell death (Dupont et al., 2011; Kresh and Chopra, 2011). In mammalian hearts, for example, prolonged high load or deposition of extracellular matrix (ECM) can lead to cardiac hypertrophy, cardiomyopathy, and heart failure (Cook et al., 2014; Lyon et al., 2015; Yadav et al., 2021). Moreover, skeletal muscle cells with defective proteins that mediate the structural integrity of the nucleoskeleton or the ability of the nucleus to sense mechanical loading can lead to muscular dystrophy phenotypes (Bianchi et al., 2018).

Elucidating the role of nuclear mechanosignaling in mechanosensitive genetic signaling pathways and their involvement in muscular diseases is currently a major research area, but there are still many unknown mechanisms about the transduction of intracellular and extracellular forces into downstream gene expression via nuclear mechanosignaling. Here, we briefly review our current understanding of mechanisms by which mechanical forces and nuclear mechanosensing influence gene regulatory programs underlying pathological remodeling and dysfunction of striated muscle cells.

In healthy striated muscle cells, the pseudo-crystalline structure of myofilaments, sarcomeres, and myofibrils establishes a highly organized and uniquely anisotropic subcellular architecture. This subcellular organization is, in part, maintained by rapid and spatially controlled protein turnover (Wood et al., 2022) of large macromolecular complexes, which together provide mechanical linkages between chromatin, the cytoskeletal and sarcomere network, and the extracellular matrix (Figure 1). At the cell membrane-ECM interface, the cell connection to the ECM is supported by transmembrane protein complexes called costameres. As depicted in Figure 1, the costamere is a molecular complex that consists of integrin, vinculin, integrin-linked kinase, ankyrin, filamin C, talin, desmin, dystrophin, the dystrophin-glycoprotein complex (DGC), among many others (Bang et al., 2022). The costamere also helps connect the Z-disk of the sarcomere to the sarcolemma and the ECM, providing a structural and mechanical link between the intracellular contractile machinery and the extracellular environment. Deeper into the cell interior, a framework of microtubules, intermediate filaments, and F-actin maintain the connection of the Z-disks throughout the cytoskeleton and to the nucleoskeleton.

The correct crosstalk between cytoskeleton and nucleoskeleton is necessary for proper mechanosignaling in straited muscle cells. Muscle-specific LIM domain protein (MLP) has been shown to be a stretch sensor in cardiac muscle in both mice and human (Knöll et al., 2002). MLP KO mice showed severe DCM phenotype and mRNA profile. Cardiomyocytes from these mice indicate significant disruption in their cardiomyocyte cytoarchitecture and myofibril arrays (Arber et al., 1997). It is thought that MLP-TCAP binding is necessary for proper cardiac muscle mechanotransduction. Loss of MLP-TCAP changes intrinsic elastic properties of titin, which further affects proper mechanotransduction from the ECM to the nucleus (Arber et al., 1997; Knöll et al., 2002). Another example of cytoskeleton-nucleoskeleton crosstalk is desmin [please see (Capetanaki et al., 2015; Hnia et al., 2015; Tsikitis et al., 2018) for more comprehensive reviews on desmin in muscle diseases]. Desmin interfilament cytoskeleton network connects with the lamin nucleoskeleton either directly through the nuclear pores or indirectly through plectin-nesprin3-LINC complex (Georgatos et al., 1987; Lockard and Bloom, 1993; Frock et al., 2006). This crosstalk is essential for propagating mechanical stimuli from ECM to the nucleus. Depletion of either desmin or Nesprin-3 causes nuclear envelope deformation and chromatin disorganization (also discussed in Section 4, below). Moreover, desmin-KD cardiomyocytes showed an altered expression profile that has significant cardiac dysfunction, DCM, hypertrophy signatures (Heffler et al., 2020). Patients with dysfunctional desmin mutations showed granulofilamentous accumulations and sandwich formations and characteristic z-disk deformity (Vrabie et al., 2005; Claeys et al., 2008). These abnormal structures might affect cytoskeleton and nucleoskeleton crosstalk. Furthermore, in addition to nuclear mechanical regulation, desmin also serves as a signaling platform to help its binding partners in significant cellular functions like DNA repair (with MLH1), endocytosis pathway (with ITSN1), and calcium hemostasis (with S100A1) (Hnia et al., 2015).

The primary connection between the nucleus and cytoskeleton in striated muscles is a protein complex called the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, which comprise many proteins that have been associated with cardiomyopathies, demonstrating its critical role in maintaining striated muscle function [please see (Stroud et al., 2014; Stroud, 2018; Bang et al., 2022) for reviews on this subject]. As illustrated in Figure 1, the LINC complex is part of the molecular connections between the nuclear lamina and the cytoskeletal/sarcomeric network, which is facilitated (in part) by nesprins at the outer nuclear membrane and SUN domain proteins (SUN1/2) at the inner nuclear membrane. In the inner nucleus, chromatin interacts directly with the LINC complex via emerin, lamins, and many other proteins. In particular, the interaction between lamin and desmin in cardiomyocytes is critical for normal heart function. A lack of lamin A/C expression in mice cardiomyocytes has been shown to exhibit disorganized desmin filaments and paranuclear regions where the desmin filaments are detached from the nucleus (Nikolova et al., 2004). Many Lamin A/C mutations also show different degrees of disruption on cytoskeleton structures in both human and mice models (Sebillon et al., 2003; Mounkes et al., 2005; Lanzicher et al., 2015; Yang et al., 2021). In a mouse model of dilated cardiomyopathy caused by the lamin A/C mutation H222P, desmin loses its localization from Z-disks and intercalated disks and forms aggregates (Galata et al., 2018). Interestingly, ameliorating the desmin-associated defects in these cardiomyocytes provided cardioprotection from the H222P lamin A/C mutation (Galata et al., 2018), suggesting that desmin is a promising target for certain laminopathies.

Thus, myocytes, like many other cell types, exhibit tight mechanical coupling from chromatin to ECM. However, the anisotropic subcellular ultrastructure of myocytes and their high degree of force production impose distinct mechanical environments onto myonuclei that may elicit unique and underexplored nuclear mechanosignaling pathways underlying muscle structure and function. In the sections that follow, we discuss specific proteins known to be involved in the maintenance of this process, and the relationship between defects in nuclear mechanosignaling, mechanosensitive gene regulation and protein synthesis, and striated muscle diseases.

The nuclear envelope (NE) and nucleoskeleton provide a connection between the sarcomere, the cytoskeleton, and chromatin in myocytes, which likely facilitates important mechanosensitive gene regulatory programs. In the following subsections, we discuss specific proteins known to be involved in nuclear mechanosignaling or nucleus-related pathologies in striated muscle.

It is clear that nuclear morphology and deformation play an important role in regulating gene expression in many different cell types (Thomas et al., 2002; Kalukula et al., 2022). For example, extracellular-based mechanical tension across the nuclear membrane of aortic valve fibroblasts, mediated by the actin cytoskeleton and LINC complex, facilitates chromatin remodeling that leads to pro-fibrotic myofibroblast persistence (Walker et al., 2021). In striated muscle cells, the large magnitude of myocyte deformations experienced during muscle contraction and relaxation may facilitate processes involving nuclear migration, chromatin organization, and mechanosensitive gene expression. For example, during skeletal muscle contractions, centripetal forces exerted by myofibrils facilitate the migration of the nucleus to the periphery of the myofibers (Roman et al., 2017). Furthermore, myonuclei are readily localized via cytoskeletal protein networks to sites of myocyte damage to accelerate sarcomere repair (Roman et al., 2021). Moreover, physiological myonuclear deformations occur during normal contractions and passive stretches (Kalukula et al., 2022). A recent study (Ghosh et al., 2019) demonstrated that intranuclear deformations during cardiomyocyte contraction depends on the mechanical properties of the extracellular environment, and that areas in the nucleus with denser chromatin exhibited greater strain discrepancies between substrate stiffnesses. This supports the hypothesis that myonuclei transduce the mechanical properties of the extracellular environment into epigenetic control via intranuclear strain distributions that affect chromatin organization and accessibility.

More compliant and fragile nuclei, as well as decoupled nucleus-cytoskeleton connections, can be consequences of nuclear envelope protein mutations associated with various myopathies (Zwerger et al., 2013). In Drosophila larvae with mutated Nesprin, myonuclei deform differently during contractions compared with wild-type flies (Lorber et al., 2020). In an aging context, cardiomyocytes from Drosophila also exhibit decreased Lamin C expression with age, which coincides with decreased nuclear size, increased nuclear stiffness, and altered transcriptional programs (Kirkland et al., 2023). Similarly, aged mice and non-human primates exhibit reduced nuclear size and circularity, with old mice also having reduced Lamin C compared to younger mice (Kirkland et al., 2023). Moreover, mutant lamins in skeletal muscle associated with laminopathies and muscular dystrophy cause nuclear envelope rupture and DNA damage that correlate with disease severity in both mice and humans (Earle et al., 2020). However, microtubule stabilization reinforced myonuclei in these diseased muscle fibers, reducing nuclear damage and rescued myocyte function in viability in cells with mutated lamins (Earle et al., 2020).

Defects or mutations in proteins comprising the nucleoskeleton are not only associated with nuclear envelopathies and laminopathies, but they can also lead to altered nuclear morphology and mechanotransduction (Janin and Gache, 2018; Ross and Stroud, 2021). Mice lacking the nuclear envelope transmembrane protein 39 (Net39) develop skeletal muscle dysfunction and remodeling that resembles Emery Dreifuss muscular dystrophy (Ramirez-Martinez et al., 2021). Moreover, the nuclei in skeletal muscle fibers from Net39-null mice exhibit significant deformations, changes in chromatin accessibility, and altered gene expression, while human muscle biopsies from patients with Emery Dreifuss muscular dystrophy revealed a downregulation of Net39 (Ramirez-Martinez et al., 2021). [See (Bianchi et al., 2018) for a recent review of Emery Dreifuss muscular dystrophy in the context of nuclear mechanosignaling.]

In the heart, a loss of desmin or nesprin in the LINC complex in cardiomyocytes causes the nucleus to collapse in response to neighboring polymerizing microtubules, which can lead to chromatin reorganization, DNA damage, and altered transcriptomic expression (Heffler et al., 2020). Similarly, genetically engineered mice carrying the human p.L13 > R mutation in the nuclear envelope protein LEM domain containing protein 2 (LEMD2) develop severe DCM and cardiac fibrosis, which lead to premature death (Caravia et al., 2022). Homozygous mice for the p.L13 > R LEMD2 variant exhibit disorganized heterochromatin, nuclear envelope deformations, extensive DNA damage, and apoptosis linked to p53 activation (Caravia et al., 2022), suggesting that LEMD2 is essential for genome stability and cardiac function.

Finally, we note that defects in cytoskeletal proteins not directly linked to the nuclear envelope (but associated with other cardiomyopathies) also can cause changes in nuclear structure. For example, a loss of filamin C in adult mouse hearts causes nuclear remodeling and altered gene expression (Zhou et al., 2020; Powers et al., 2022; Powers and McCulloch, 2022), suggesting that cytoskeletal proteins not directly interacting with the nucleoskeleton in myocytes may also influence nuclear structure and mechanosignaling.

In the nucleus, chromatin accessibility and its three-dimensional organization are correlated (Bertero et al., 2019; Bertero and Rosa-Garrido, 2021). As such, mechanical forces and/or defects in the nucleoskeleton structure that affect chromatin organization may be key factors underlying mechanosensitive gene regulation of pathogenic remodeling or dysfunction in myocytes. Lamin has been shown to organize the three-dimensional genome from the nuclear periphery (Zheng et al., 2018). While the conventional view is that chromatin activity is reduced from the nuclear center to periphery, a recent study (Amiad-Pavlov et al., 2021) found that A-type lamin upregulation in instar larva muscle caused chromatin to collapse toward the nuclear center while also reducing overall levels of active chromatin. These findings suggest that: 1) lamina composition regulates the organization of peripheral chromatin, and 2) there may not be a clear spatial distribution of active versus inactive chromatin throughout the nuclear volume. Furthermore, a loss of all isoforms of lamin has been shown to expand or detach specific lamina-associated domains (LADs), and dysregulate the active and repressive chromatin domains among different topologically associated chromatin domains (TADs) (Zheng et al., 2018). Lamin mutations also cause increased Yes-associated protein (YAP) nuclear entry in muscle stem cells (Owens et al., 2020) and alter chromatin interactions and depress alternative fate genes (Shah et al., 2021). With the emerging new findings on lamin A/Polycomb Repressive Complex (PRC) crosstalk (Marullo et al., 2016), it is speculated lamin A can also affect gene expression through histone modifications (Bianchi et al., 2018). PRC proteins are known to form PRC aggregates that form an intranuclear structure that regulates long-range chromatin interactions (Lanzuolo et al., 2007; Bantignies et al., 2011; Cesarini et al., 2015). Polycomb Repressive Complex 2 (PRC2) is responsible for H3K27me3 through the catalytic activity of Ezh2 (Bianchi et al., 2018). It is speculated that, under mechanical stress, lamin A is upregulated, leading to more crosstalk with PCR2, resulting in observed accumulation of H3K27me3 (Bianchi et al., 2018). This hypothesis also explains the higher mechanosignaling sensitivity observed in cells carrying lamin A/C mutation: mutated lamin A/C cannot achieve effective crosstalk with PRC2 which further leads to a loss of repressive activity on mechanosensitive genes.

Mechanical stimulation also affects nuclear translocation of transcription factors. Under mechanical stimulation, myocardin-related transcription factors (MRTFs), YAP, and extracellular signal-regulated kinase (ERK) are translocated into the nucleus in myocytes (Wada et al., 2011; Aragona et al., 2013; Velasquez et al., 2013; Gallo et al., 2019). Two mechanisms of NPC mediated nuclear translocation have been proposed: the first hypothesis is that the NPC might increase the ring structure diameter under the mechanical force because of its connection with SUN2 protein. The enlargement of the NPC tunnel promotes the entry of transcriptional factors. The second hypothesis is that the mechanical stimulation changes the state of transcriptional factors which subsequently increase their binding affinities with nuclear transport receptors (Matsuda and Mofrad, 2022). As cells show reduced mechanosensitivity with dysfunction of NPC, both hypotheses could be correct (Mofrad and Kamm, 2009; Fichtman and Harel, 2014).

Emerin, LAP2, and MAN1 proteins are all referred to as LEM proteins. All of them have a LEM domain where they interact with chromatin indirectly via BAF (Brachner and Foisner, 2011), which helps facilitate the tethering of chromatin around the nuclear periphery. Patients with EDMD show less repressed chromatin and altered chromatin structure in their skeletal muscle (Ognibene et al., 1999; Fidziańska and Hausmanowa-Petrusewicz, 2003; Mewborn et al., 2010). Under force, emerin monomers are phosphorylated, unbind from BAF, and form oligomers which strengthens their interaction with SUN protein and lamin A/C (Guilluy et al., 2014; Fernandez et al., 2022). Through this mechanism, emerin alters the chromatin organization under force. All of the six LAP2 isoforms can bind with BAF and affect chromatin organization (Wilson and Foisner, 2010). Other than its BAF interactions, LAP2 can also affect chromatin organization through interaction with other nuclear proteins. While other isoforms primarily affect chromatin LAD distribution through interacting with B type lamins, LAP2α only binds with lamin A/C (Dechat et al., 2000b; Naetar et al., 2008). Furthermore, LAP2α also interacts with chromatin-binding behavior of the high-mobility group N protein 5 (HMGN5), and downregulation of LAP2α leads to disorganized HMGN5-targeted chromatin regions (Zhang et al., 2013). In addition to interacting with BAF and lamins, MAN1 can also repress TGFβ, activin, and BMP signaling through its interaction with Smad proteins (Pan et al., 2005). However, this interaction is not myocyte or cardiomyocyte specific. In fact, MAN1’s function in mechanosignaling and in myocytes is understudied. It will be interesting to investigate whether MAN1 can interact with Smad-like proteins in myocytes and to subsequently map out the resulting gene regulatory network.

There is a growing body of research that we were not able to include in this review. Nuclear mechanosignaling has been a rapidly developing field of research for decades, and it provides an exciting direction in investigating muscle and heart adaptation, maturation, and disease models. As the majority of the groundwork for understanding the role of nuclear mechanosignaling in cell state and function has been laid in non-muscle cell types, there is now an exciting opportunity to apply this knowledge to investigations into mammalian myocytes. Doing so will reveal novel mechanisms of the mechanical regulation of myocyte structure and function that will be critical in defining disease mechanisms and investigating new therapies for myopathies. Considering the well-established gene editing protocols to edit stem cell-derived myocytes, verifying mechano-sensing protein function and mechanism in human cells is a particularly promising endeavor for future and ongoing research. Moreover, much of our understanding of nucleoskeleton protein function is focused on well-studied proteins like lamins and emerin, while the functions of understudied proteins (such as MAN1) remain far from understood. As such, new research on the variety of other nuclear proteins and their role in regulating nuclear structure, chromatin organization, and nucleoskeletal mechanics will construct a more complete picture of the mechanosignaling processes of healthy and diseased myocytes.

Building upon the rich history of muscle research in which multidisciplinary approaches are embedded, combining multiple existing methodologies and technologies to answer new and exciting questions about myonuclear mechanosignaling will be undoubtedly advantageous. For example, bioinformatics (i.e., quantitative proteomics and transcriptomics) can be applied to future studies to gather a more complete picture of effects of mechanical stimulation and protein mutations in striated muscle mechano-signaling processes. These types of studies will also inform computational investigations into mechanosensitive genetic signaling in myocyte remodeling (Tan et al., 2017; Saucerman et al., 2019; Khalilimeybodi et al., 2023), which will enable a more holistic view of nuclear mechanosensing and myocyte structure/function. Furthermore, studying cells/tissues on a stationary surface/scaffold does not accurately represent the in vivo environment of muscles where the cells are constantly experiencing large forces and deformations. Mechanically stimulating the cells to better mimic the innate physical environment can provide more accurate representations of the mechano-sensing protein functions. Exciting ongoing research in these directions is sure to have a great impact on our understanding of the role of the nucleus in mediating muscle function in health and disease.