The E3 SUMO ligase Nse2 regulates sumoylation and nuclear-to-cytoplasmic translocation of skNAC-Smyd1 in myogenesis

SUMOylation regulation of ribosome biogenesis: Emerging roles for USP36

Ribosome biogenesis is essential for cell growth, proliferation, and animal development. Its deregulation leads to various human disorders such as ribosomopathies and cancer. Thus, tight regulation of ribosome biogenesis is crucial for normal cell homeostasis. Emerging evidence suggests that posttranslational modifications such as ubiquitination and SUMOylation play a crucial role in regulating ribosome biogenesis. Our recent studies reveal that USP36, a nucleolar deubiquitinating enzyme (DUB), acts also as a SUMO ligase to regulate nucleolar protein group SUMOylation, thereby being essential for ribosome biogenesis. Here, we provide an overview of the current understanding of the SUMOylation regulation of ribosome biogenesis and discuss the role of USP36 in nucleolar SUMOylation.

Lysine methylation signaling in skeletal muscle biology: from myogenesis to clinical insights

Lysine methylation signaling is well studied for its key roles in the regulation of transcription states through modifications on histone proteins. While histone lysine methylation has been extensively studied, recent discoveries of lysine methylation on thousands of non-histone proteins has broadened our appreciation for this small chemical modification in the regulation of protein function. In this review, we highlight the significance of histone and non-histone lysine methylation signaling in skeletal muscle biology, spanning development, maintenance, regeneration, and disease progression. Furthermore, we discuss potential future implications for its roles in skeletal muscle biology as well as clinical applications for the treatment of skeletal muscle-related diseases.

Sumoylation-deficient phosphoglycerate mutase 2 impairs myogenic differentiation

Phosphoglycerate mutase 2 (PGAM2) is a critical glycolytic enzyme that is highly expressed in skeletal muscle. In humans, naturally occurring mutations in Phosphoglycerate mutase 2 have been etiologically linked to glycogen storage disease X (GSDX). Phosphoglycerate mutase 2 activity is regulated by several posttranslational modifications such as ubiquitination and acetylation. Here, we report that Phosphoglycerate mutase 2 activity is regulated by sumoylation-a covalent conjugation involved in a wide spectrum of cellular events. We found that Phosphoglycerate mutase 2 contains two primary SUMO acceptor sites, lysine (K)49 and K176, and that the mutation of either K to arginine (R) abolished Phosphoglycerate mutase 2 sumoylation. Given that K176 is more highly evolutionarily conserved across paralogs and orthologs than K49 is, we used the CRISPR-mediated homologous recombination technique in myogenic C2C12 cells to generate homozygous K176R knock-in cells (PGAM2^K176R/K176R). Compared with wild-type (WT) C2C12 cells, PGAM2^K176R/K176R C2C12 cells exhibited impaired myogenic differentiation, as indicated by decreased differentiation and fusion indexes. Furthermore, the results of glycolytic and mitochondrial stress assays with the XF96 Extracellular Flux analyzer revealed a reduced proton efflux rate (PER), glycolytic PER (glycoPER), extracellular acidification rate (ECAR), and oxygen consumption rate (OCR) in PGAM2^K176R/K176R C2C12 cells, both at baseline and in response to stress. Impaired mitochondrial function was also observed in PGAM2^K176R/K176R P19 cells, a carcinoma cell line. These findings indicate that the PGAM2-K176R mutation impaired glycolysis and mitochondrial function. Gene ontology term analysis of RNA sequencing data further revealed that several downregulated genes in PGAM2^K176R/K176R C2C12 cells were associated with muscle differentiation/development/contraction programs. Finally, PGAM2 with either of two naturally occurring missense mutations linked to GSDX, E89A (conversion of glutamic acid 89 to alanine) or R90W (conversion of arginine 90 to tryptophan), exhibited reduced Phosphoglycerate mutase 2 sumoylation. Thus, sumoylation is an important mechanism that mediates Phosphoglycerate mutase 2 activity and is potentially implicated in Phosphoglycerate mutase 2 mutation-linked disease in humans.

Nascent polypeptide-Associated Complex and Signal Recognition Particle have cardiac-specific roles in heart development and remodeling

Establishing a catalog of Congenital Heart Disease (CHD) genes and identifying functional networks would improve our understanding of its oligogenic underpinnings. Our studies identified protein biogenesis cofactors Nascent polypeptide-Associated Complex (NAC) and Signal-Recognition-Particle (SRP) as disease candidates and novel regulators of cardiac differentiation and morphogenesis. Knockdown (KD) of the alpha- (Nacα) or beta-subunit (bicaudal, bic) of NAC in the developing Drosophila heart disrupted cardiac developmental remodeling resulting in a fly with no heart. Heart loss was rescued by combined KD of Nacα with the posterior patterning Hox gene Abd-B. Consistent with a central role for this interaction in cardiogenesis, KD of Nacα in cardiac progenitors derived from human iPSCs impaired cardiac differentiation while co-KD with human HOXC12 and HOXD12 rescued this phenotype. Our data suggest that Nacα KD preprograms cardioblasts in the embryo for abortive remodeling later during metamorphosis, as Nacα KD during translation-intensive larval growth or pupal remodeling only causes moderate heart defects. KD of SRP subunits in the developing fly heart produced phenotypes that targeted specific segments and cell types, again suggesting cardiac-specific and spatially regulated activities. Together, we demonstrated directed function for NAC and SRP in heart development, and that regulation of NAC function depends on Hox genes.

The SUMO E3 Ligase MdSIZ1 Sumoylates a Cell Number Regulator MdCNR8 to Control Organ Size

Plant growth and organ size putatively associated with crop yield are regulated by a complex network of genes including ones for controlling cell proliferation. The gene fw2.2 was first identified in tomatoes and reported to govern fruit size variation through controlling cell division. In this study, we isolated a putative ortholog of the tomato fw2.2 gene from apple, Cell Number Regulator 8 (MdCNR8). Our functional analysis showed that MdCNR8 may control fruit size and root growth. MdCNR8 was mediated by the SUMO E3 ligase MdSIZ1, and SUMOylation of MdCNR8 at residue-Lys39 promoted the translocation of MdCNR8 from plasma membrane to the nucleus. The effect of MdCNR8 in inhibiting root elongation could be completely counteracted by the coexpression of MdSIZ1. Moreover, the lower cell proliferation of apple calli due to silencing MdSIZ1 could be rescued by silencing MdCNR8. Collectively, our results showed that the MdSIZ1-mediated SUMOylation is required for the fulfillment of MdCNR8 in regulating cell proliferation to control plant organ size. This regulatory interaction between MdSIZ1 and MdCNR8 will facilitate understanding the mechanism underlying the regulation of organ size.

Smc5/6, an atypical SMC complex with two RING-type subunits

The Smc5/6 complex plays essential roles in chromosome segregation and repair, by promoting disjunction of sister chromatids. The core of the complex is constituted by an heterodimer of Structural Maintenance of Chromosomes (SMC) proteins that use ATP hydrolysis to dynamically associate with and organize chromosomes. In addition, the Smc5/6 complex contains six non-SMC subunits. Remarkably, and differently to other SMC complexes, the Nse1 and Nse2 subunits contain RING-type domains typically found in E3 ligases, pointing to the capacity to regulate other proteins and complexes through ubiquitin-like modifiers. Nse2 codes for a C-terminal SP-RING domain with SUMO ligase activity, assisting Smc5/6 functions in chromosome segregation through sumoylation of several chromosome-associated proteins. Nse1 codes for a C-terminal NH-RING domain and, although it has been proposed to have ubiquitin ligase activity, no Smc5/6-dependent ubiquitylation target has been described to date. Here, we review the function of the two RING domains of the Smc5/6 complex in the broader context of SMC complexes as global chromosome organizers of the genome.

Stability of Smyd1 in endothelial cells is controlled by PML-dependent SUMOylation upon cytokine stimulation

Smyd1 is an epigenetic modulator of gene expression that has been well-characterized in muscle cells. It was recently reported that Smyd1 levels are modulated by inflammatory processes. Since inflammation affects the vascular endothelium, this study aimed to characterize Smyd1 expression in endothelial cells. We detected Smyd1 in human endothelial cells (HUVEC and EA.hy926 cells), where the protein was largely localized in PML nuclear bodies (PML-NBs). By transfection of EA.hy926 cells with expression vectors encoding Smyd1, PML, SUMO1, active or mutant forms of the SUMO protease SuPr1 and/or the SUMO-conjugation enzyme UBC9, as well as Smyd1- or PML-specific siRNAs, in the presence or absence of the translation blocker cycloheximide or the proteasome-inhibitor MG132, and supported by computational modeling, we show that Smyd1 is SUMOylated in a PML-dependent manner and thereby addressed for degradation in proteasomes. Furthermore, transfection with Smyd1-encoding vectors led to PML up-regulation at the mRNA level, while PML transfection lowered Smyd1 protein stability. Incubation of EA.hy926 cells with the pro-inflammatory cytokine TNF-α resulted in a constant increase in Smyd1 mRNA and protein over 24 h, while incubation with IFN-γ induced a transient increase in Smyd1 expression, which peaked at 6 h and decreased to control values within 24 h. The IFN-γ-induced increase in Smyd1 was accompanied by more Smyd1 SUMOylation and more/larger PML-NBs. In conclusion, our data indicate that in endothelial cells, Smyd1 levels are regulated through a negative feedback mechanism based on SUMOylation and PML availability. This molecular control loop is stimulated by various cytokines.

The deubiquitinase USP36 promotes snoRNP group SUMOylation and is essential for ribosome biogenesis

SUMOylation plays a crucial role in regulating diverse cellular processes including ribosome biogenesis. Proteomic analyses and experimental evidence showed that a number of nucleolar proteins involved in ribosome biogenesis are modified by SUMO. However, how these proteins are SUMOylated in cells is less understood. Here, we report that USP36, a nucleolar deubiquitinating enzyme (DUB), promotes nucleolar SUMOylation. Overexpression of USP36 enhances nucleolar SUMOylation, whereas its knockdown or genetic deletion reduces the levels of SUMOylation. USP36 interacts with SUMO2 and Ubc9 and directly mediates SUMOylation in cells and in vitro. We show that USP36 promotes the SUMOylation of the small nucleolar ribonucleoprotein (snoRNP) components Nop58 and Nhp2 in cells and in vitro and their binding to snoRNAs. It also promotes the SUMOylation of snoRNP components Nop56 and DKC1. Functionally, we show that knockdown of USP36 markedly impairs rRNA processing and translation. Thus, USP36 promotes snoRNP group SUMOylation and is critical for ribosome biogenesis and protein translation.

Promyelocytic leukemia protein promotes the phenotypic switch of smooth muscle cells in atherosclerotic plaques of human coronary arteries

Promyelocytic leukemia protein (PML) is a constitutive component of PML nuclear bodies (PML-NBs), which function as stress-regulated SUMOylation factories. Since PML can also act as a regulator of the inflammatory and fibroproliferative responses characteristic of atherosclerosis, we investigated whether PML is implicated in this disease. Immunoblotting, ELISA and immunohistochemistry showed a stronger expression of PML in segments of human atherosclerotic coronary arteries and sections compared with non-atherosclerotic ones. In particular, PML was concentrated in PML-NBs from α-smooth muscle actin (α-SMA)-immunoreactive cells in plaque areas. To identify possible functional consequences of PML-accumulation in this cell type, differentiated human coronary artery smooth muscle cells (dHCASMCs) were transfected with a vector containing the intact PML-gene. These PML-transfected dHCASMCs showed higher levels of small ubiquitin-like modifier (SUMO)-1-dependent SUMOylated proteins, but lower levels of markers for smooth muscle cell (SMC) differentiation and revealed more proliferation and migration activities than dHCASMCs transfected with the vector lacking a specific gene insert or with the vector containing a mutated PML-gene coding for a PML-form without SUMOylation activity. When dHCASMCs were incubated with different cytokines, higher PML-levels were observed only after interferon γ (IFN-γ) stimulation, while the expression of differentiation markers was lower. However, these phenotypic changes were not observed in dHCASMCs treated with small interfering RNA (siRNA) suppressing PML-expression prior to IFN-γ stimulation. Taken together, our results imply that PML is a previously unknown functional factor in the molecular cascades associated with the pathogenesis of atherosclerosis and is positioned in vascular SMCs (VSMCs) between upstream IFN-γ activation and downstream SUMOylation.

The role of SUMOylation during development

During the development of multicellular organisms, transcriptional regulation plays an important role in the control of cell growth, differentiation and morphogenesis. SUMOylation is a reversible post-translational process involved in transcriptional regulation through the modification of transcription factors and through chromatin remodelling (either modifying chromatin remodelers or acting as a ‘molecular glue’ by promoting recruitment of chromatin regulators). SUMO modification results in changes in the activity, stability, interactions or localization of its substrates, which affects cellular processes such as cell cycle progression, DNA maintenance and repair or nucleocytoplasmic transport. This review focuses on the role of SUMO machinery and the modification of target proteins during embryonic development and organogenesis of animals, from invertebrates to mammals.

SUMO system - a key regulator in sarcomere organization

Skeletal muscles constitute roughly 40% of human body mass. Muscles are specialized tissues that generate force to drive movements through ATP-driven cyclic interactions between the protein filaments, namely actin and myosin filaments. The filaments are organized in an intricate structure called the 'sarcomere', which is a fundamental contractile unit of striated skeletal and cardiac muscle, hosting a fine assembly of macromolecular protein complexes. The micrometer-sized sarcomere units are arranged in a reiterated array within myofibrils of muscle cells. The precise spatial organization of sarcomere is tightly controlled by several molecular mechanisms, indispensable for its force-generating function. Disorganized sarcomeres, either due to erroneous molecular signaling or due to mutations in the sarcomeric proteins, lead to human diseases such as cardiomyopathies and muscle atrophic conditions prevalent in cachexia. Protein post-translational modifications (PTMs) of the sarcomeric proteins serve a critical role in sarcomere formation (sarcomerogenesis), as well as in the steady-state maintenance of sarcomeres. PTMs such as phosphorylation, acetylation, ubiquitination, and SUMOylation provide cells with a swift and reversible means to adapt to an altered molecular and therefore cellular environment. Over the past years, SUMOylation has emerged as a crucial modification with implications for different aspects of cell function, including organizing higher-order protein assemblies. In this review, we highlight the fundamentals of the small ubiquitin-like modifiers (SUMO) pathway and its link specifically to the mechanisms of sarcomere assembly. Furthermore, we discuss recent studies connecting the SUMO pathway-modulated protein homeostasis with sarcomere organization and muscle-related pathologies.

siRNA-mediated inhibition of skNAC and Smyd1 expression disrupts myofibril organization: Immunofluorescence and electron microscopy study in C2C12 cells

skNAC (skeletal and heart muscle-specific variant of nascent polypeptide-associated complex) and Smyd1 (SET and MYND domain-containing 1) form a protein dimer which is specific for striated muscle cells. Its function is largely unknown. On the one hand, skNAC-Smyd1 appears to control transcriptional processes in the nucleus, on the other hand, specifically at later stages of myogenic differentiation, both proteins translocate to the sarcoplasm and at least Smyd1 specifically associates with sarcomeric structures and might control myofibrillogenesis and/or sarcomere architecture. Here, using immunofluorescence and electron microscopy, we analyzed sarcomere formation and myofibril organization after siRNA-mediated knockdown of skNAC or Smyd1 expression in murine C2C12 skeletal muscle cells. We found that inhibition of skNAC or Smyd1 expression indeed prevents myofibrillogenesis and sarcomere formation, leading to a disorganized array of myofilaments predominantly within the region immediately beneath the plasma membrane.

The Smyd Family of Methyltransferases: Role in Cardiac and Skeletal Muscle Physiology and Pathology

Protein methylation plays a pivotal role in the regulation of various cellular processes including chromatin remodeling and gene expression. SET and MYND domain-containing proteins (Smyd) are a special class of lysine methyltransferases whose catalytic SET domain is split by an MYND domain. The hallmark feature of this family was thought to be the methylation of histone H3 (on lysine 4). However, several studies suggest that the role of the Smyd family is dynamic, targeting unique histone residues associated with both transcriptional activation and repression. Smyd proteins also methylate several non-histone proteins to regulate various cellular processes. Although we are only beginning to understand their specific molecular functions and role in chromatin remodeling, recent studies have advanced our understanding of this relatively uncharacterized family, highlighting their involvement in development, cell growth and differentiation and during disease in various animal models. This review summarizes our current knowledge of the structure, function and methylation targets of the Smyd family and provides a compilation of data emphasizing their prominent role in cardiac and skeletal muscle physiology and pathology.

Mouse myofibers lacking the SMYD1 methyltransferase are susceptible to atrophy, internalization of nuclei and myofibrillar disarray

The Smyd1 gene encodes a lysine methyltransferase specifically expressed in striated muscle. Because Smyd1-null mouse embryos die from heart malformation prior to formation of skeletal muscle, we developed a Smyd1 conditional-knockout allele to determine the consequence of SMYD1 loss in mammalian skeletal muscle. Ablation of SMYD1 specifically in skeletal myocytes after myofiber differentiation using Myf6(cre) produced a non-degenerative myopathy. Mutant mice exhibited weakness, myofiber hypotrophy, prevalence of oxidative myofibers, reduction in triad numbers, regional myofibrillar disorganization/breakdown and a high percentage of myofibers with centralized nuclei. Notably, we found broad upregulation of muscle development genes in the absence of regenerating or degenerating myofibers. These data suggest that the afflicted fibers are in a continual state of repair in an attempt to restore damaged myofibrils. Disease severity was greater for males than females. Despite equivalent expression in all fiber types, loss of SMYD1 primarily affected fast-twitch muscle, illustrating fiber-type-specific functions for SMYD1. This work illustrates a crucial role for SMYD1 in skeletal muscle physiology and myofibril integrity.

The Arabidopsis SUMO E3 Ligase AtMMS21 Dissociates the E2Fa/DPa Complex in Cell Cycle Regulation

Development requires the proper execution and regulation of the cell cycle via precise, conserved mechanisms. Critically, the E2F/DP complex controls the expression of essential genes during cell cycle transitions. Here, we discovered the molecular function of the Arabidopsis thaliana SUMO E3 ligase METHYL METHANESULFONATE SENSITIVITY GENE21 (AtMMS21) in regulating the cell cycle via the E2Fa/DPa pathway. DPa was identified as an AtMMS21-interacting protein and AtMMS21 competes with E2Fa for interaction with DPa. Moreover, DPa is a substrate for SUMOylation mediated by AtMMS21, and this SUMOylation enhances the dissociation of the E2Fa/DPa complex. AtMMS21 also affects the subcellular localization of E2Fa/DPa. The E2Fa/DPa target genes are upregulated in the root of mms21-1 and mms21-1 mutants showed increased endoreplication. Overexpression of DPa affected the root development of mms21-1, and overexpression of AtMMS21 completely recovered the abnormal phenotypes of 35S:E2Fa-DPa plants. Our results suggest that AtMMS21 dissociates the E2Fa/DPa complex via competition and SUMOylation in the regulation of plant cell cycle.

SUMOylation of KLF4 acts as a switch in transcriptional programs that control VSMC proliferation

The regulation of vascular smooth muscle cell (VSMC) proliferation is an important issue due to its major implications for the prevention of pathological vascular conditions. The objective of this work was to assess the function of small ubiquitin-like modifier (SUMO)ylated Krϋppel-like transcription factor 4 (KLF4) in the regulation of VSMC proliferation in cultured cells and in animal models with balloon injury. We found that under basal conditions, binding of non-SUMOylated KLF4 to p300 activated p21 (p21(WAF1/CIP1))transcription, leading to VSMC growth arrest. PDGF-BB promoted the interaction between Ubc9 and KLF4 and the SUMOylation of KLF4, which in turn recruited transcriptional corepressors to the p21 promoter. The reduction in p21 enhanced VSMC proliferation. Additionally, the SUMOylated KLF4 did not affect the expression of KLF4, thereby forming a positive feedback loop enhancing cell proliferation. These results demonstrated that SUMOylated KLF4 plays an important role in cell proliferation by reversing the transactivation action of KLF4 on p21 induced with PDGF-BB.

SMYD1 and G6PD modulation are critical events for miR-206-mediated differentiation of rhabdomyosarcoma

Rhadomyosarcoma (RMS) is the most common soft tissue sarcoma of childhood. RMS cells resemble fetal myoblasts but are unable to complete myogenic differentiation. In previous work we showed that miR-206, which is low in RMS, when induced in RMS cells promotes the resumption of differentiation by modulating more than 700 genes. To better define the pathways involved in the conversion of RMS cells into their differentiated counterpart, we focused on 2 miR-206 effectors emerged from the microarray analysis, SMYD1 and G6PD. SMYD1, one of the most highly upregulated genes, is a H3K4 histone methyltransferase. Here we show that SMYD1 silencing does not interfere with the proliferative block or with the loss anchorage independence imposed by miR-206, but severely impairs differentiation of ERMS, ARMS, and myogenic cells. Thus SMYD1 is essential for the activation of muscle genes. Conversely, among the downregulated genes, we found G6PD, the enzyme catalyzing the rate-limiting step of the pentose phosphate shunt. In this work, we confirmed that G6PD is a direct target of miR-206. Moreover, we showed that G6PD silencing in ERMS cells impairs proliferation and soft agar growth. However, G6PD overexpression does not interfere with the pro-differentiating effect of miR-206, suggesting that G6PD downmodulation contributes to - but is not an absolute requirement for - the tumor suppressive potential of miR-206. Targeting cancer metabolism may enhance differentiation. However, therapeutic inhibition of G6PD is encumbered by side effects. As an alternative, we used DCA in combination with miR-206 to increase the flux of pyruvate into the mitochondrion by reactivating PDH. DCA enhanced the inhibition of RMS cell growth induced by miR-206, and sustained it upon miR-206 de-induction. Altogether these results link miR-206 to epigenetic and metabolic reprogramming, and suggest that it may be worth combining differentiation-inducing with metabolism-directed approaches.

Defective myogenesis in the absence of the muscle-specific lysine methyltransferase SMYD1

The SMYD (SET and MYND domain) family of lysine methyltransferases harbor a unique structure in which the methyltransferase (SET) domain is intervened by a zinc finger protein-protein interaction MYND domain. SMYD proteins methylate both histone and non-histone substrates and participate in diverse biological processes including transcriptional regulation, DNA repair, proliferation and apoptosis. Smyd1 is unique among the five family members in that it is specifically expressed in striated muscles. Smyd1 is critical for development of the right ventricle in mice. In zebrafish, Smyd1 is necessary for sarcomerogenesis in fast-twitch muscles. Smyd1 is expressed in the skeletal muscle lineage throughout myogenesis and in mature myofibers, shuttling from nucleus to cytosol during myoblast differentiation. Because of this expression pattern, we hypothesized that Smyd1 plays multiple roles at different stages of myogenesis. To determine the role of Smyd1 in mammalian myogenesis, we conditionally eliminated Smyd1 from the skeletal muscle lineage at the myoblast stage using Myf5(cre). Deletion of Smyd1 impaired myoblast differentiation, resulted in fewer myofibers and decreased expression of muscle-specific genes. Muscular defects were temporally restricted to the second wave of myogenesis. Thus, in addition to the previously described functions for Smyd1 in heart development and skeletal muscle sarcomerogenesis, these results point to a novel role for Smyd1 in myoblast differentiation.

skNAC and Smyd1 in transcriptional control

Skeletal and heart muscle-specific variant of the alpha subunit of nascent polypeptide associated complex (skNAC) is exclusively found in striated muscle cells. Its function, however, is largely unknown. Previous reports could demonstrate that skNAC binds to Smyd1 (SET and MYND domain containing protein 1). The facts that (a) SET domains have histone methyltransferase activity, and (b) MYND domains are known recruiters of histone deacetylases (HDACs), implicate the skNAC-Smyd1 complex in transcriptional control. To study potential target genes, we carried out cDNA microarray analysis on differentiating C2C12 myoblasts in which expression of the skNAC gene had been knocked down. We found and confirmed a series of targets, specifically genes encoding regulators of inflammation, cellular metabolism, and cell migration. Mechanistically, as shown by Western blot, ELISA, and ChIP analysis at selected promoter regions, transcriptional control by skNAC-Smyd1 appears to be exerted at least in part by affecting a series of histone modifications, specifically H3K4 di- and trimethylation and potentially also histone acetylation. Taken together, our data suggest that the skNAC-Smyd1 complex is involved in transcriptional regulation both via the control of histone methylation and histone (de)acetylation.

The sarcomeric M-region: a molecular command center for diverse cellular processes

The sarcomeric M-region anchors thick filaments and withstands the mechanical stress of contractions by deformation, thus enabling distribution of physiological forces along the length of thick filaments. While the role of the M-region in supporting myofibrillar structure and contractility is well established, its role in mediating additional cellular processes has only recently started to emerge. As such, M-region is the hub of key protein players contributing to cytoskeletal remodeling, signal transduction, mechanosensing, metabolism, and proteasomal degradation. Mutations in genes encoding M-region related proteins lead to development of severe and lethal cardiac and skeletal myopathies affecting mankind. Herein, we describe the main cellular processes taking place at the M-region, other than thick filament assembly, and discuss human myopathies associated with mutant or truncated M-region proteins.