Search

Mutations in nuclear envelope proteins (NEPs) cause devastating genetic diseases, known as envelopathies, which primarily affect the heart and skeletal muscle. A mutation in the NEP LEMD2 causes severe cardiomyopathy in humans. However, the roles of LEMD2 in the heart and the pathological mechanisms responsible for its association with cardiac disease are unknown. As a consequence, there is no specific therapy for this cardiomyopathy. To explore the basis of this pathology, we generated mice carrying the human c.T38>G Lemd2 mutation. These mice represent a new preclinical model that phenocopied the human disease and developed severe dilated cardiomyopathy with extensive cardiac fibrosis leading to premature death. At the cellular level, LEMD2 mutant cardiomyocytes exhibited disorganization of the transcriptionally silent heterochromatin associated with the nuclear envelope. Moreover, mice with cardiac-specific deletion of LEMD2 also died shortly after birth due to heart abnormalities. Both LEMD2 mutant models displayed aberrant cardiac gene expression and extensive DNA damage linked to p53 activation. Importantly, cardiomyocyte-specific Lemd2 gene therapy via adeno-associated virus significantly rescued cardiac function in the c.T38>G mice. Together, our results reveal the essentiality of the nuclear envelope protein LEMD2 for genome stability and cardiac function and unveil its mechanistic association with human disease.

Lamins and transmembrane proteins within the nuclear envelope regulate nuclear structure and chromatin organization. Nuclear envelope transmembrane protein 39 (Net39) is a muscle nuclear envelope protein whose functions in vivo have not been explored. We show that mice lacking Net39 succumb to severe myopathy and juvenile lethality, with concomitant disruption in nuclear integrity, chromatin accessibility, gene expression, and metabolism. These abnormalities resemble those of Emery–Dreifuss muscular dystrophy (EDMD), caused by mutations in A-type lamins (LMNA) and other genes, like Emerin (EMD). We observe that Net39 is downregulated in EDMD patients, implicating Net39 in the pathogenesis of this disorder. Our findings highlight the role of Net39 at the nuclear envelope in maintaining muscle chromatin organization, gene expression and function, and its potential contribution to the molecular etiology of EDMD., The nuclear envelope tethers chromatin to the nuclear periphery to control genome architecture. Here, the authors show that Net39 preserves the integrity and gene expression of muscle nuclei in mice, and it may contribute to the pathogenesis of Emery–Dreifuss muscular dystrophy.

Micromanaging muscle cell fusion Adult skeletal muscles are characterized by long, multinucleated cells called myofibers. Myofibers form when muscle precursor cells, or myoblasts, differentiate and fuse together during embryogenesis. The fusion process is not fully understood. Studying cell culture and mouse models, Bi et al. identified an 84–amino acid peptide that promotes myoblast fusion. This small peptide, called Myomixer, physically interacts with and stimulates the activity of a fusogenic membrane protein called Myomaker. Notably, the Myomaker-Myomixer pair can also promote the fusion of nonmuscle cells, such as fibroblasts. Science , this issue p. 323

The formation of new myofibers in vertebrates occurs by myoblast fusion and requires fusogenic activity of the muscle-specific membrane protein myomaker. Here, using in silico (BLAST) genome analyses, we show that the myomaker gene from trout includes 14 minisatellites, indicating that it has an unusual structure compared with those of other animal species. We found that the trout myomaker gene encodes a 434 –amino acid (aa) protein, in accordance with its apparent molecular mass (40 kDa) observed by immunoblotting. The first half of the trout myomaker protein (1–220 aa) is similar to the 221-aa mouse myomaker protein, whereas the second half (222–234 aa) does not correspond to any known motifs and arises from two protein extensions. The first extension (70 aa) apparently appeared with the radiation of the bony fish clade Euteleostei, whereas the second extension (up to 236 aa) is restricted to the superorder Protacanthopterygii (containing salmonids and pike) and corresponds to the insertion of minisatellites having a length of 30 nucleotides. According to gene expression analyses, trout myomaker expression is consistently associated with the formation of new myofibers during embryonic development, postlarval growth, and muscle regeneration. Using cell-mixing experiments, we observed that trout myomaker has retained the ability to drive the fusion of mouse fibroblasts with C2C12 myoblasts. Our work reveals that trout myomaker has fusogenic function despite containing two protein extensions., SCOPUS: ar.j, DecretOANoAutActif, info:eu-repo/semantics/published

Multinucleate cellular syncytial formation is a hallmark of skeletal muscle differentiation. Myomaker, encoded by Mymk (Tmem8c), is a well-conserved plasma membrane protein required for myoblast fusion to form multinucleated myotubes in mouse, chick, and zebrafish. Here, we report that autosomal recessive mutations in MYMK (OMIM 615345) cause Carey-Fineman-Ziter syndrome in humans (CFZS; OMIM 254940) by reducing but not eliminating MYMK function. We characterize MYMK-CFZS as a congenital myopathy with marked facial weakness and additional clinical and pathologic features that distinguish it from other congenital neuromuscular syndromes. We show that a heterologous cell fusion assay in vitro and allelic complementation experiments in mymk knockdown and mymkinsT/insT zebrafish in vivo can differentiate between MYMK wild type, hypomorphic and null alleles. Collectively, these data establish that MYMK activity is necessary for normal muscle development and maintenance in humans, and expand the spectrum of congenital myopathies to include cell-cell fusion deficits., During embryogenesis, the cytoplasmic protein Myomarker (MYMK) mediates muscle fibre formation by fusion of myoblasts. Here, the authors identify autosomal recessive mutations in MYMK that cause Carey-Fineman-Ziter syndrome in humans, and model the disease variants in zebrafish.

MicroRNAs (miRNAs) perform critical functions in normal physiology and disease by associating with Argonaute proteins and downregulating partially complementary messenger RNAs (mRNAs). Here we use clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9) genome-wide loss-of-function screening coupled with a fluorescent reporter of miRNA activity in human cells to identify new regulators of the miRNA pathway. By using iterative rounds of screening, we reveal a novel mechanism whereby target engagement by Argonaute 2 (AGO2) triggers its hierarchical, multi-site phosphorylation by CSNK1A1 on a set of highly conserved residues (S824-S834), followed by rapid dephosphorylation by the ANKRD52-PPP6C phosphatase complex. Although genetic and biochemical studies demonstrate that AGO2 phosphorylation on these residues inhibits target mRNA binding, inactivation of this phosphorylation cycle globally impairs miRNA-mediated silencing. Analysis of the transcriptome-wide binding profile of non-phosphorylatable AGO2 reveals a pronounced expansion of the target repertoire bound at steady-state, effectively reducing the active pool of AGO2 on a per-target basis. These findings support a model in which an AGO2 phosphorylation cycle stimulated by target engagement regulates miRNA:target interactions to maintain the global efficiency of miRNA-mediated silencing.