Striated muscle laminopathies

Comprehensive review on gene mutations contributing to dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is one of the most common primary myocardial diseases. However, to this day, it remains an enigmatic cardiovascular disease (CVD) characterized by ventricular dilatation, which leads to myocardial contractile dysfunction. It is the most common cause of chronic congestive heart failure and the most frequent indication for heart transplantation in young individuals. Genetics and various other factors play significant roles in the progression of dilated cardiomyopathy, and variants in more than 50 genes have been associated with the disease. However, the etiology of a large number of cases remains elusive. Numerous studies have been conducted on the genetic causes of dilated cardiomyopathy. These genetic studies suggest that mutations in genes for fibronectin, cytoskeletal proteins, and myosin in cardiomyocytes play a key role in the development of DCM. In this review, we provide a comprehensive description of the genetic basis, mechanisms, and research advances in genes that have been strongly associated with DCM based on evidence-based medicine. We also emphasize the important role of gene sequencing in therapy for potential early diagnosis and improved clinical management of DCM.

Preclinical Advances of Therapies for Laminopathies

Laminopathies are a group of rare disorders due to mutation in LMNA gene. Depending on the mutation, they may affect striated muscles, adipose tissues, nerves or are multisystemic with various accelerated ageing syndromes. Although the diverse pathomechanisms responsible for laminopathies are not fully understood, several therapeutic approaches have been evaluated in patient cells or animal models, ranging from gene therapies to cell and drug therapies. This review is focused on these therapies with a strong focus on striated muscle laminopathies and premature ageing syndromes.

Skeletal and Cardiac Muscle Disorders Caused by Mutations in Genes Encoding Intermediate Filament Proteins

Intermediate filaments are major components of the cytoskeleton. Desmin and synemin, cytoplasmic intermediate filament proteins and A-type lamins, nuclear intermediate filament proteins, play key roles in skeletal and cardiac muscle. Desmin, encoded by the DES gene (OMIM *125660) and A-type lamins by the LMNA gene (OMIM *150330), have been involved in striated muscle disorders. Diseases include desmin-related myopathy and cardiomyopathy (desminopathy), which can be manifested with dilated, restrictive, hypertrophic, arrhythmogenic, or even left ventricular non-compaction cardiomyopathy, Emery–Dreifuss Muscular Dystrophy (EDMD2 and EDMD3, due to LMNA mutations), LMNA-related congenital Muscular Dystrophy (L-CMD) and LMNA-linked dilated cardiomyopathy with conduction system defects (CMD1A). Recently, mutations in synemin (SYNM gene, OMIM *606087) have been linked to cardiomyopathy. This review will summarize clinical and molecular aspects of desmin-, lamin- and synemin-related striated muscle disorders with focus on LMNA and DES-associated clinical entities and will suggest pathogenetic hypotheses based on the interplay of desmin and lamin A/C. In healthy muscle, such interplay is responsible for the involvement of this network in mechanosignaling, nuclear positioning and mitochondrial homeostasis, while in disease it is disturbed, leading to myocyte death and activation of inflammation and the associated secretome alterations.

Clinical spectrum and genetic variations of LMNA-related muscular dystrophies in a large cohort of Chinese patients

Background

LMNA-related muscular dystrophy is caused by mutations in LMNA gene. We aimed to identify genetic variations and clinical features in a large cohort of Chinese patients with LMNA mutations in an attempt to establish genotype-phenotype correlation.

Methods

The clinical presentations of patients with LMNA-related muscular dystrophy were recorded using retrospective and prospective cohort study. LMNA mutation analysis was performed by Sanger sequencing or next-generation sequencing. Mosaicism was detected by personal genome machine amplicon deep sequencing for mosaicism.

Results

Eighty-four patients were identified to harbour LMNA mutations. Forty-one of those were diagnosed with LMNA-related congenital muscular dystrophy (L-CMD), 32 with Emery-Dreifuss muscular dystrophy (EDMD) and 11 with limb-girdle muscular dystrophy type 1B (LGMD1B). We identified 21 novel and 29 known LMNA mutations. Two frequent mutations were identified: c.745C>T and c.1357C>T. A correlation between the location of mutation and the clinical phenotype was observed: mutations affecting the head and coil 2A domains mainly occurred in L-CMD, while the coil 2B and Ig-like domains mainly related to EDMD and LGMD1B. We found somatic mosaicism in one parent of four probands. Muscle biopsies revealed 11 of 20 biopsied L-CMD exhibited inflammatory changes, and muscle cell ultrastructure showed abnormal nuclear morphology.

Conclusions

Our detailed clinical and genetic analysis of 84 patients with LMNA-related muscular dystrophy expands clinical spectrum and broadens genetic variations caused by LMNA mutations. We identified 21 novel and 29 known LMNA mutations and found two frequent mutations. A correlation between the location of mutation and the clinical severity was observed. Preliminary data suggested that low-dose corticosteroid treatment may be effective.

Lamin A/C Assembly Defects in LMNA-Congenital Muscular Dystrophy Is Responsible for the Increased Severity of the Disease Compared with Emery-Dreifuss Muscular Dystrophy

LMNA encodes for Lamin A/C, type V intermediate filaments that polymerize under the inner nuclear membrane to form the nuclear lamina. A small fraction of Lamin A/C, less polymerized, is also found in the nucleoplasm. Lamin A/C functions include roles in nuclear resistance to mechanical stress and gene regulation. LMNA mutations are responsible for a wide variety of pathologies, including Emery–Dreifuss (EDMD) and LMNA-related congenital muscular dystrophies (L-CMD) without clear genotype–phenotype correlations. Both diseases presented with striated muscle disorders although L-CMD symptoms appear much earlier and are more severe. Seeking for pathomechanical differences to explain the severity of L-CMD mutations, we performed an in silico analysis of the UMD-LMNA database and found that L-CMD mutations mainly affect residues involved in Lamin dimer and tetramer stability. In line with this, we found increased nucleoplasmic Lamin A/C in L-CMD patient fibroblasts and mouse myoblasts compared to the control and EDMD. L-CMD myoblasts show differentiation defects linked to their inability to upregulate muscle specific nuclear envelope (NE) proteins expression. NE proteins were mislocalized, leading to misshapen nuclei. We conclude that these defects are due to both the absence of Lamin A/C from the nuclear lamina and its maintenance in the nucleoplasm of myotubes.

Clinical aspects of Emery-Dreifuss muscular dystrophy

Emery-Dreifuss muscular dystrophy (EDMD), clinically characterized by scapulo-humero-peroneal muscle atrophy and weakness, multi-joint contractures with spine rigidity and cardiomyopathy with conduction defects, is associated with structural/functional defect of genes that encode the proteins of nuclear envelope, including lamin A and several lamin-interacting proteins. This paper presents clinical aspects of EDMD in context to causative genes, genotype-phenotype correlation and its emplacement within phenotypic spectrum of skeletal muscle diseases associated with envelopathies.

Effect of genetic background on the cardiac phenotype in a mouse model of Emery-Dreifuss muscular dystrophy

A-type lamins gene (LMNA) mutations cause an autosomal dominant inherited form of Emery-Dreifuss muscular dystrophy (EDMD). EDMD is characterized by slowly progressive muscle weakness and wasting and dilated cardiomyopathy, often leading to heart failure-related disability. EDMD is highly penetrant with poor prognosis and there is currently no specific therapy available. Clinical variability ranges from early onset with severe presentation in childhood to late onset with slow progression in adulthood. Genetic background is a well-known factor that significantly affects phenotype in several mouse models of human diseases. This phenotypic variability is attributed, at least in part, to genetic modifiers that regulate the disease process. To characterize the phenotype of A-type lamins mutation on different genetic background, we created and phenotyped C57BL/6JRj-Lmna^H222P/H222P mice (C57^Lmnap.H222P) and compared them with the 129S2/SvPasCrl-Lmna^H222P/H222P mice (129^Lmnap.H222P). These mouse strains were compared with their respective control strains at multiple time points between 3 and 10 months of age. Both contractile and electrical cardiac muscle functions, as well as survival were characterized. We found that 129^Lmnap.H222P mice showed significantly reduced body weight and reduced cardiac function earlier than in the C57^Lmnap.H222P mice. We also revealed that only 129^Lmnap.H222P mice developed heart arrhythmias. The 129^Lmnap.H222P model with an earlier onset and more pronounced cardiac phenotype may be more useful for evaluating therapies that target cardiac muscle function, and heart arrhythmias.

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Mouse model of Emery-Dreifuss muscular dystrophy generated on 129S2/svPasCrl genetic background have a greater life expectancy.

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Mouse model of Emery-Dreifuss muscular dystrophy generated on 129S2/svPasCrl genetic background showed exacerbated arrhythmia susceptibility.

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Mouse model of Emery-Dreifuss muscular dystrophy generated on 129S2/svPasCrl genetic background showed more pronounced dilated cardiomyopathy.

The Driving Force: Nuclear Mechanotransduction in Cellular Function, Fate, and Disease

Cellular behavior is continuously affected by microenvironmental forces through the process of mechanotransduction, in which mechanical stimuli are rapidly converted to biochemical responses. Mounting evidence suggests that the nucleus itself is a mechanoresponsive element, reacting to cytoskeletal forces and mediating downstream biochemical responses. The nucleus responds through a host of mechanisms, including partial unfolding, conformational changes, and phosphorylation of nuclear envelope proteins; modulation of nuclear import/export; and altered chromatin organization, resulting in transcriptional changes. It is unclear which of these events present direct mechanotransduction processes and which are downstream of other mechanotransduction pathways. We critically review and discuss the current evidence for nuclear mechanotransduction, particularly in the context of stem cell fate, a largely unexplored topic, and in disease, where an improved understanding of nuclear mechanotransduction is beginning to open new treatment avenues. Finally, we discuss innovative technological developments that will allow outstanding questions in the rapidly growing field of nuclear mechanotransduction to be answered.

The Pathogenesis and Therapies of Striated Muscle Laminopathies

Emery-Dreifuss muscular dystrophy (EDMD) is a genetic condition characterized by early contractures, skeletal muscle weakness, and cardiomyopathy. During the last 20 years, various genetic approaches led to the identification of causal genes of EDMD and related disorders, all encoding nuclear envelope proteins. By their respective localization either at the inner nuclear membrane or the outer nuclear membrane, these proteins interact with each other and establish a connection between the nucleus and the cytoskeleton. Beside this physical link, these proteins are also involved in mechanotransduction, responding to environmental cues, such as increased tension of the cytoskeleton, by the activation or repression of specific sets of genes. This ability of cells to adapt to environmental conditions is altered in EDMD. Increased knowledge on the pathophysiology of EDMD has led to the development of drug or gene therapies that have been tested on mouse models. This review proposed an overview of the functions played by the different proteins involved in EDMD and related disorders and the current therapeutic approaches tested so far.

Modeling Skeletal Muscle Laminopathies Using Human Induced Pluripotent Stem Cells Carrying Pathogenic LMNA Mutations

Laminopathies are a clinically heterogeneous group of disorders caused by mutations in LMNA. The main proteins encoded by LMNA are Lamin A and C, which together with Lamin B1 and B2, form the nuclear lamina: a mesh-like structure located underneath the inner nuclear membrane. Laminopathies show striking tissue specificity, with subtypes affecting striated muscle, peripheral nerve, and adipose tissue, while others cause multisystem disease with accelerated aging. Although several pathogenic mechanisms have been proposed, the exact pathophysiology of laminopathies remains unclear, compounded by the rarity of these disorders and lack of easily accessible cell types to study. To overcome this limitation, we used induced pluripotent stem cells (iPSCs) from patients with skeletal muscle laminopathies such as LMNA-related congenital muscular dystrophy and limb-girdle muscular dystrophy 1B, to model disease phenotypes in vitro. iPSCs can be derived from readily accessible cell types, have unlimited proliferation potential and can be differentiated into cell types that would otherwise be difficult and invasive to obtain. iPSC lines from three skeletal muscle laminopathy patients were differentiated into inducible myogenic cells and myotubes. Disease-associated phenotypes were observed in these cells, including abnormal nuclear shape and mislocalization of nuclear lamina proteins. Nuclear abnormalities were less pronounced in monolayer cultures of terminally differentiated skeletal myotubes than in proliferating myogenic cells. Notably, skeletal myogenic differentiation of LMNA-mutant iPSCs in artificial muscle constructs improved detection of myonuclear abnormalities compared to conventional monolayer cultures across multiple pathogenic genotypes, providing a high-fidelity modeling platform for skeletal muscle laminopathies. Our results lay the foundation for future iPSC-based therapy development and screening platforms for skeletal muscle laminopathies.

Unweaving the role of nuclear Lamins in neural circuit integrity

Lamins are type-V intermediate filament proteins that comprise the nuclear lamina. Although once considered static structural components that provide physical support to the inner nuclear envelope, recent studies are revealing additional functional and regulatory roles for Lamins in chromatin organization, gene regulation, DNA repair, cell division and signal transduction. In this issue of Cell Stress, Oyston et al. (2018) reports the function of Lamin in the maintenance of nervous system integrity and neural circuit function using Drosophila. A number of laminopathies in humans exhibit age-dependent neurological phenotypes, but understanding how defects in specific neural cell types or circuitries contribute to patient phenotypes is very challenging. Drosophila provides a simple yet sophisticated system to begin untangling the vulnerability of diverse neuronal cell types and circuits against cellular stressors induced by defects in nuclear lamina organization.

Increasing autophagy and blocking Nrf2 suppress laminopathy-induced age-dependent cardiac dysfunction and shortened lifespan

Mutations in the human LMNA gene cause a collection of diseases known as laminopathies. These include myocardial diseases that exhibit age-dependent penetrance of dysrhythmias and heart failure. The LMNA gene encodes A-type lamins, intermediate filaments that support nuclear structure and organize the genome. Mechanisms by which mutant lamins cause age-dependent heart defects are not well understood. To address this issue, we modeled human disease-causing mutations in the Drosophila melanogaster Lamin C gene and expressed mutant Lamin C exclusively in the heart. This resulted in progressive cardiac dysfunction, loss of adipose tissue homeostasis, and a shortened adult lifespan. Within cardiac cells, mutant Lamin C aggregated in the cytoplasm, the CncC(Nrf2)/Keap1 redox sensing pathway was activated, mitochondria exhibited abnormal morphology, and the autophagy cargo receptor Ref2(P)/p62 was upregulated. Genetic analyses demonstrated that simultaneous over-expression of the autophagy kinase Atg1 gene and an RNAi against CncC eliminated the cytoplasmic protein aggregates, restored cardiac function, and lengthened lifespan. These data suggest that simultaneously increasing rates of autophagy and blocking the Nrf2/Keap1 pathway are a potential therapeutic strategy for cardiac laminopathies.

"Get the Balance Right": Pathological Significance of Autophagy Perturbation in Neuromuscular Disorders

Recent research has revealed that autophagy, a major catabolic process in cells, is dysregulated in several neuromuscular diseases and contributes to the muscle wasting caused by non-muscle disorders (e.g. cancer cachexia) or during aging (i.e. sarcopenia). From there, the idea arose to interfere with autophagy or manipulate its regulatory signalling to help restore muscle homeostasis and attenuate disease progression. The major difficulty for the development of therapeutic strategies is to restore a balanced autophagic flux, due to the dynamic nature of autophagy. Thus, it is essential to better understand the mechanisms and identify the signalling pathways at play in the control of autophagy in skeletal muscle. A comprehensive analysis of the autophagic flux and of the causes of its dysregulation is required to assess the pathogenic role of autophagy in diseased muscle. Furthermore, it is essential that experiments distinguish between primary dysregulation of autophagy (prior to disease onset) and impairments as a consequence of the pathology. Of note, in most muscle disorders, autophagy perturbation is not caused by genetic modification of an autophagy-related protein, but rather through indirect alteration of regulatory signalling or lysosomal function. In this review, we will present the mechanisms involved in autophagy, and those ensuring its tight regulation in skeletal muscle. We will then discuss as to how autophagy dysregulation contributes to the pathogenesis of neuromuscular disorders and possible ways to interfere with this process to limit disease progression.

Gene Therapy via Trans-Splicing for LMNA-Related Congenital Muscular Dystrophy

We assessed the potential of Lmna-mRNA repair by spliceosome-mediated RNA trans-splicing as a therapeutic approach for LMNA-related congenital muscular dystrophy. This gene therapy strategy leads to reduction of mutated transcript expression for the benefit of corresponding wild-type (WT) transcripts. We developed 5′-RNA pre-trans-splicing molecules containing the first five exons of Lmna and targeting intron 5 of Lmna pre-mRNA. Among nine pre-trans-splicing molecules, differing in the targeted sequence in intron 5 and tested in C2C12 myoblasts, three induced trans-splicing events on endogenous Lmna mRNA and confirmed at protein level. Further analyses performed in primary myotubes derived from an LMNA-related congenital muscular dystrophy (L-CMD) mouse model led to a partial rescue of the mutant phenotype. Finally, we tested this approach in vivo using adeno-associated virus (AAV) delivery in newborn mice and showed that trans-splicing events occurred in WT mice 50 days after AAV delivery, although at a low rate. Altogether, while these results provide the first evidence for reprogramming LMNA mRNA in vitro, strategies to improve the rate of trans-splicing events still need to be developed for efficient application of this therapeutic approach in vivo.

Myofibril contraction and crosslinking drive nuclear movement to the periphery of skeletal muscle

Nuclear movements are important for multiple cellular functions, and are driven by polarized forces generated by motor proteins and the cytoskeleton. During skeletal myofibre formation or regeneration, nuclei move from the centre to the periphery of the myofibre for proper muscle function. Centrally located nuclei are also found in different muscle disorders. Using theoretical and experimental approaches, we demonstrate that nuclear movement to the periphery of myofibres is mediated by centripetal forces around the nucleus. These forces arise from myofibril contraction and crosslinking that 'zip' around the nucleus in combination with tight regulation of nuclear stiffness by lamin A/C. In addition, an Arp2/3 complex containing Arpc5L together with γ-actin is required to organize desmin to crosslink myofibrils for nuclear movement. Our work reveals that centripetal forces exerted by myofibrils squeeze the nucleus to the periphery of myofibres.

Ryanodine receptor modification and regulation by intracellular Ca2+ and Mg2+ in healthy and failing human hearts

RATIONALE

Heart failure is a multimodal disorder, of which disrupted Ca²⁺ homeostasis is a hallmark. Central to Ca²⁺ homeostasis is the major cardiac Ca²⁺ release channel - the ryanodine receptor (RyR2) - whose activity is influenced by associated proteins, covalent modification and by Ca²⁺ and Mg²⁺. That RyR2 is remodelled and its function disturbed in heart failure is well recognized, but poorly understood.

OBJECTIVE

To assess Ca²⁺ and Mg²⁺ regulation of RyR2 from left ventricles of healthy, cystic fibrosis and failing hearts, and to correlate these functional changes with RyR2 modifications and remodelling.

METHODS AND RESULTS

The function of RyR2 from left ventricular samples was assessed using lipid bilayer single-channel measurements, whilst RyR2 modification and protein:protein interactions were determined using Western Blots and co-immunoprecipitation. In all failing hearts there was an increase in RyR2 activity at end-diastolic cytoplasmic Ca²⁺ (100nM), a decreased cytoplasmic [Ca²⁺] required for half maximal activation (K_a) and a decrease in inhibition by cytoplasmic Mg²⁺. This was accompanied by significant hyperphosphorylation of RyR2 S²⁸⁰⁸ and S²⁸¹⁴, reduced free thiol content and a reduced interaction with FKBP12.0 and FKBP12.6. Either dephosphorylation of RyR2 using PP1 or thiol reduction using DTT eliminated any significant difference in the activity of RyR2 from healthy and failing hearts. We also report a subgroup of RyR2 in failing hearts that were not responsive to regulation by intracellular Ca²⁺ or Mg²⁺.

CONCLUSION

Despite different aetiologies, disrupted RyR2 Ca²⁺ sensitivity and biochemical modification of the channel are common constituents of failing heart RyR2 and may underlie the pathological disturbances in intracellular Ca²⁺ signalling.

Emerin self-assembly mechanism: role of the LEM domain

At the nuclear envelope, the inner nuclear membrane protein emerin contributes to the interface between the nucleoskeleton and the chromatin. Emerin is an essential actor of the nuclear response to a mechanical signal. Genetic defects in emerin cause Emery-Dreifuss muscular dystrophy. It was proposed that emerin oligomerization regulates nucleoskeleton binding, and impaired oligomerization contributes to the loss of function of emerin disease-causing mutants. We here report the first structural characterization of emerin oligomers. We identified an N-terminal emerin region from amino acid 1 to amino acid 132 that is necessary and sufficient for formation of long curvilinear filaments. In emerin monomer, this region contains a globular LEM domain and a fragment that is intrinsically disordered. Solid-state nuclear magnetic resonance analysis identifies the LEM β-fragment as part of the oligomeric structural core. However, the LEM domain alone does not self-assemble into filaments. Additional residues forming a β-structure are observed within the filaments that could correspond to the unstructured region in emerin monomer. We show that the delK37 mutation causing muscular dystrophy triggers LEM domain unfolding and increases emerin self-assembly rate. Similarly, inserting a disulfide bridge that stabilizes the LEM folded state impairs emerin N-terminal region self-assembly, whereas reducing this disulfide bridge triggers self-assembly. We conclude that the LEM domain, responsible for binding to the chromatin protein BAF, undergoes a conformational change during self-assembly of emerin N-terminal region. The consequences of these structural rearrangement and self-assembly events on emerin binding properties are discussed.

Diseases of the Nucleoskeleton

The nucleus is separated from the cytosol by the nuclear envelope, which is a double lipid bilayer composed of the outer nuclear membrane and the inner nuclear membrane. The intermediate filament proteins lamin A, lamin B, and lamin C form a network underlying the inner nuclear membrane. This proteinaceous network provides the nucleus with its strength, rigidity, and elasticity. Positioned within the inner nuclear membrane are more than 150 inner nuclear membrane proteins, many of which interact directly with lamins and require lamins for their inner nuclear membrane localization. Inner nuclear membrane proteins and the nuclear lamins define the nuclear lamina. These inner nuclear membrane proteins have tissue-specific expression and diverse functions including regulating cytoskeletal organization, nuclear architecture, cell cycle dynamics, and genomic organization. Loss or mutations in lamins and inner nuclear membrane proteins cause a wide spectrum of diseases. Here, I will review the functions of the well-studied nuclear lamina proteins and the diseases associated with loss or mutations in these proteins. © 2016 American Physiological Society. Compr Physiol 6:1655-1674, 2016.

The effect of the lamin A and its mutants on nuclear structure, cell proliferation, protein stability, and mobility in embryonic cells

LMNA gene encodes for nuclear intermediate filament proteins lamin A/C. Mutations in this gene lead to a spectrum of genetic disorders, collectively referred to as laminopathies. Lamin A/C are widely expressed in most differentiated somatic cells but not in early embryos and some undifferentiated cells. To investigate the role of lamin A/C in cell phenotype maintenance and differentiation, which could be a determinant of the pathogenesis of laminopathies, we examined the role played by exogenous lamin A and its mutants in differentiated cell lines (HeLa, NHDF) and less-differentiated HEK 293 cells. We introduced exogenous wild-type and mutated (H222P, L263P, E358K D446V, and ∆50) lamin A into different cell types and analyzed proteins’ impact on proliferation, protein mobility, and endogenous nuclear envelope protein distribution. The mutants give rise to a broad spectrum of nuclear phenotypes and relocate lamin C. The mutations ∆50 and D446V enhance proliferation in comparison to wild-type lamin A and control cells, but no changes in exogenous protein mobility measured by FRAP were observed. Interestingly, although transcripts for lamins A and C are at similar level in HEK 293 cells, only lamin C protein is detected in western blots. Also, exogenous lamin A and its mutants, when expressed in HEK 293 cells underwent posttranscriptional processing. Overall, our results provide new insight into the maintenance of lamin A in less-differentiated cells. Embryonic cells are very sensitive to lamin A imbalance, and its upregulation disturbs lamin C, which may influence gene expression and many regulatory pathways.

Skeletal Muscle Laminopathies: A Review of Clinical and Molecular Features

LMNA-related disorders are caused by mutations in the LMNA gene, which encodes for the nuclear envelope proteins, lamin A and C, via alternative splicing. Laminopathies are associated with a wide range of disease phenotypes, including neuromuscular, cardiac, metabolic disorders and premature aging syndromes. The most frequent diseases associated with mutations in the LMNA gene are characterized by skeletal and cardiac muscle involvement. This review will focus on genetics and clinical features of laminopathies affecting primarily skeletal muscle. Although only symptomatic treatment is available for these patients, many achievements have been made in clarifying the pathogenesis and improving the management of these diseases.

A Novel Mutation in Nucleoporin 35 Causes Murine Degenerative Colonic Smooth Muscle Myopathy

Chronic intestinal pseudo-obstruction (CIPO) is a rare but life-threatening disease characterized by severe intestinal dysmotility. Histopathologic studies in CIPO patients have identified several different mechanisms that appear to be involved in the dysmotility, including defects in neurons, smooth muscle, or interstitial cells of Cajal. Currently there are few mouse models of the various forms of CIPO. We generated a mouse with a point mutation in the RNA recognition motif of the Nup35 gene, which encodes a component of the nuclear pore complex. Nup35 mutants developed a severe megacolon and exhibited a reduced lifespan. Histopathologic examination revealed a degenerative myopathy that developed after birth and specifically affected smooth muscle in the colon; smooth muscle in the small bowel and the bladder were not affected. Furthermore, no defects were found in enteric neurons or interstitial cells of Cajal. Nup35 mice are likely to be a valuable model for the subtype of CIPO characterized by degenerative myopathy. Our study also raises the possibility that Nup35 polymorphisms could contribute to some cases of CIPO.

The SUN protein UNC-84 is required only in force-bearing cells to maintain nuclear envelope architecture

SUN-KASH bridges that connect the nucleoskeleton to the cytoskeleton are only required to maintain nuclear envelope spacing in cells subjected to increased mechanical forces, such as muscle cells.

The nuclear envelope (NE) consists of two evenly spaced bilayers, the inner and outer nuclear membranes. The Sad1p and UNC-84 (SUN) proteins and Klarsicht, ANC-1, and Syne homology (KASH) proteins that interact to form LINC (linker of nucleoskeleton and cytoskeleton) complexes connecting the nucleoskeleton to the cytoskeleton have been implicated in maintaining NE spacing. Surprisingly, the NE morphology of most Caenorhabditis elegans nuclei was normal in the absence of functional SUN proteins. Distortions of the perinuclear space observed in unc-84 mutant muscle nuclei resembled those previously observed in HeLa cells, suggesting that SUN proteins are required to maintain NE architecture in cells under high mechanical strain. The UNC-84 protein with large deletions in its luminal domain was able to form functional NE bridges but had no observable effect on NE architecture. Therefore, SUN-KASH bridges are only required to maintain NE spacing in cells subjected to increased mechanical forces. Furthermore, SUN proteins do not dictate the width of the NE.

Mouse models of heart failure: cell signaling and cell survival

Heart failure is one of the paramount global causes of morbidity and mortality. Despite this pandemic need, the available clinical counter-measures have not altered substantially in recent decades, most notably in the context of pharmacological interventions. Cell death plays a causal role in heart failure, and its inhibition poses a promising approach that has not been thoroughly explored. In previous approaches to target discovery, clinical failures have reflected a deficiency in mechanistic understanding, and in some instances, failure to systematically translate laboratory findings toward the clinic. Here, we review diverse mouse models of heart failure, with an emphasis on those that identify potential targets for pharmacological inhibition of cell death, and on how their translation into effective therapies might be improved in the future.