Rod Domain Research Articles

Tailoring talin function

Different configurations of the integrin-talin-vinculin complex may enable talin to sense the different forces in developing tissues.

Ameliorating pathogenesis by removing an exon containing a missense mutation: a potential exon-skipping therapy for laminopathies.

Exon skipping, as a therapy to restore a reading frame or switch protein isoforms, is under clinical trial. We hypothesised that removing an in-frame exon containing a mutation could also improve pathogenic phenotypes. Our model is laminopathies: incurable tissue-specific degenerative diseases associated with LMNA mutations. LMNA encodes A-type lamins, that together with B-type lamins, form the nuclear lamina. Lamins contain an alpha-helical central rod domain composed of multiple heptad repeats. Eliminating LMNA exon 3 or 5 removes six heptad repeats, so shortens, but should not otherwise significantly alter, the alpha-helix. Human Lamin A or Lamin C with a deletion corresponding to amino acids encoded by exon 5 (Lamin A/C-Δ5) localised normally in murine lmna-null cells, rescuing both nuclear shape and endogenous Lamin B1/emerin distribution. However, Lamin A carrying pathogenic mutations in exon 3 or 5, or Lamin A/C-Δ3, did not. Furthermore, Lamin A/C-Δ5 was not deleterious to wild-type cells, unlike the other Lamin A mutants including Lamin A/C-Δ3. Thus Lamin A/C-Δ5 function as effectively as wild-type Lamin A/C and better than mutant versions. Antisense oligonucleotides skipped LMNA exon 5 in human cells, demonstrating the possibility of treating certain laminopathies with this approach. This proof-of-concept is the first to report the therapeutic potential of exon skipping for diseases arising from missense mutations.

Blocking integrin inactivation as an anti-angiogenic therapy.

During angiogenesis, endothelial cell migration is coordinated by integrin‐mediated contact with the extra‐cellular matrix (ECM), coupled with receptor tyrosine kinase signalling to regulate dynamic cytoskeletal and plasma membrane reorganization. A recent paper by Vitorino et al (2015) defined a new MAP4K4–moesin–talin–β1‐integrin pathway that could be therapeutically exploited to suppress pathologic angiogenesis.

Characteristics of an R-phycoerythrin with two γ subunits prepared from red macroalga Polysiphonia urceolata.

An R-phycoerythrin (R-PE) was isolated by gel filtrations on Sepharose CL-4B and Sephadex G-150 from the phycobiliprotein extract of the marine red macroalga Polysiphonia urceolata Grev and further purified by ion exchange chromatography on DEAE-Sepharose Fast Flow. The purified R-PE showed three absorption peaks at 498 nm, 538 nm, 566 nm and one fluorescent emission maximum at 577 nm. Although the R-PE showed a single band on the examination by native PAGE, it exhibited two very close bands at pH about 4.7 in native isoelectric focusing (IEF). Polypeptide analysis of the R-PE demonstrated that it contained four chromophore-carrying subunits, α18.2, β20.6, γ31.6 (γ'), γ34.6 (γ), and no colorless polypeptide; its subunit composition was 6α18.2:6β20.6:1 γ31.6:2γ34.6. The α and β subunits were distributed within a acidic pH range from 5.0 to 6.0 in denaturing IEF and the γ subunits were in a basic pH range from 7.6 to 8.1. These results reveal that the prepared R-PE may exist in two hexamers of γ (αβ)3 γ (αβ)3γ' and γ (αβ)3 γ'(αβ)3 γ and that the R-PE participate in the rod domain assembly of P. urceolata phycobilisomes by stacking each of its trimer (αβ)3 face-to-face with the aid of one γ subunit (γ or γ').

Keratin 1 Plays a Critical Role in Golgi Localization of Core 2 N-Acetylglucosaminyltransferase M via Interaction with Its Cytoplasmic Tail

Core 2 N-acetylglucosaminyltransferase 2/M (C2GnT-M) synthesizes all three β6GlcNAc branch structures found in secreted mucins. Loss of C2GnT-M leads to development of colitis and colon cancer. Recently we have shown that C2GnT-M targets the Golgi at the Giantin site and is recycled by binding to non-muscle myosin IIA, a motor protein, via the cytoplasmic tail (CT). But how this enzyme is retained in the Golgi is not known. Proteomics analysis identifies keratin type II cytoskeletal 1 (KRT1) as a protein pulled down with anti-c-Myc antibody or C2GnT-M CT from the lysate of Panc1 cells expressing bC2GnT-M tagged with c-Myc. Yeast two-hybrid analysis shows that the rod domain of KRT1 interacts directly with the WKR(6) motif in the C2GnT-M CT. Knockdown of KRT1 does not affect Golgi morphology but increases the interaction of C2GnT-M with non-muscle myosin IIA and its transportation to the endoplasmic reticulum, ubiquitination, and degradation. During Golgi recovery after brefeldin A treatment, C2GnT-M forms a complex with Giantin before KRT1, demonstrating CT-mediated sequential events of Golgi targeting and retention of C2GnT-M. In HeLa cells transiently expressing C2GnT-M-GFP, knockdown of KRT1 does not affect Golgi morphology but leaves C2GnT-M outside of the Golgi, resulting in the formation of sialyl-T antigen. Interaction of C2GnT-M and KRT1 was also detected in the goblet cells of human colon epithelial tissue and primary culture of colonic epithelial cells. The results indicate that glycosylation and thus the function of glycoconjugates can be regulated by a protein that helps retain a glycosyltransferase in the Golgi.

Novel linkage of LMNA Single Nucleotide Polymorphism with Dilated Cardiomyopathy in an Indian case study

BackgroundDilated Cardiomyopathy (DCM) is one of the most commonly encountered heart diseases reported globally. It is characterized by enlarged ventricles with impaired systolic and diastolic functions. Mutations in LMNA gene are one of the causative factors to precipitate the disease. However, association of SNPs of LMNA with DCM in particular has not been well documented. MethodHere we present a limited and restricted case study of patients from south eastern part of India afflicted with idiopathic DCM and conduction defects. By using next generation sequencing we have sequenced the exons of LMNA gene from genomic DNA isolated from patients. ResultWe have identified the linkage of 8 different LMNA SNPs with idiopathic DCM viz. rs121117552, rs538089, rs505058, rs4641, rs646840, rs534807, rs80356803 and rs7339. These SNPs are scattered throughout the gene with prevalence for the region encoding the central rod domain of lamin A/C. ConclusionMost of these SNPs in LMNA were previously reported to be involved in various disorders other than DCM. We conclude that, variation in LMNA is one of the major underlying genetic causes for the pathogenesis of DCM, as observed in few Indian populations.

Flexibility in the N-terminal actin-binding domain: clues from in silico mutations and molecular dynamics.

Dystrophin is a long, rod-shaped cytoskeleton protein implicated in muscular dystrophy (MDys). Utrophin is the closest autosomal homolog of dystrophin. Both proteins have N-terminal actin-binding domain (N-ABD), a central rod domain and C-terminal region. N-ABD, composed of two calponin homology (CH) subdomains joined by a helical linker, harbors a few disease causing missense mutations. Although the two proteins share considerable homology (>72%) in N-ABD, recent structural and biochemical studies have shown that there are significant differences (including stability, mode of actin-binding) and their functions are not completely interchangeable. In this investigation, we have used extensive molecular dynamics simulations to understand the differences and the similarities of these two proteins, along with another actin-binding protein, fimbrin. In silico mutations were performed to identify two key residues that might be responsible for the dynamical difference between the molecules. Simulation points to the inherent flexibility of the linker region, which adapts different conformations in the wild type dystrophin. Mutations T220V and G130D in dystrophin constrain the flexibility of the central helical region, while in the two known disease-causing mutants, K18N and L54R, the helicity of the region is compromised. Phylogenetic tree and sequence analysis revealed that dystrophin and utrophin genes have probably originated from the same ancestor. The investigation would provide insight into the functional diversity of two closely related proteins and fimbrin, and contribute to our understanding of the mechanism of MDys.

Structural insight into the Phosphoinositide-Regulated Cellular Dynamics of Alpha-Actinin

Alpha-actinin2 is a fundamental component of striated muscle sarcomeres. It localizes to the Z-disk, the macromolecular assembly that connects two adjacent sarcomeres where it provides mechanical stability during contraction but also plays a pivotal role during Z-disk assembly. A regulatory role by phosphatidylinositol-bis-phosphate (PiP2) has been proposed for all 4 alpha-actinin isoforms but not validated at structural level. The X-ray structure of full-length alpha-actinin2 suggests the existence of two different conformational states. In the ground state, alpha-actinin exists in an intramolecular autoinhibited conformation, where the C-terminal EF-hands bind a titin-like pseudoligand at the connection between rod and actin-binding domains. PiP2 acts as a potential trigger for the transition to an open state capable of titin-binding. We hypothesize this mechanism plays a key role during Z-disk maturation, with the open-state allowing interaction with key binding partners. Super-resolution and correlative electron microscopy of mutant-transfected Neonatal Rat Cardiomyocytes (NRC) shed new light on the internal organization of the Z-disk and the regulatory mechanism of alpha-actinin ligand binding. Combining fluorescence confocal microscopy and live-cell imaging in NRC, we identified a crucial role for PiP2, which regulates the intracellular dynamics of alpha-actinin as seen in Fluorescence Recovery After Photobleaching experiments with a PiP2-binding deficient mutant. A second alpha-actinin mutant, in constitutively open-conformation, impaired overall sarcomere assembly by reduced intracellular dynamics, suggesting that PiP2 might not be limited to a positive regulation, but is also essential for the dynamic equilibrium of the protein.

The filler-rubber interface in styrene butadiene nanocomposites with anisotropic silica particles: morphology and dynamic properties.

Silica-styrene butadiene rubber (SBR) nanocomposites were prepared by using shape-controlled spherical and rod-like silica nanoparticles (NPs) with different aspect ratios (AR = 1-5), obtained by a sol-gel route assisted by a structure directing agent. The nanocomposites were used as models to study the influence of the particle shape on the formation of nanoscale immobilized rubber at the silica-rubber interface and its effect on the dynamic-mechanical behavior. TEM and AFM tapping mode analyses of nanocomposites demonstrated that the silica particles are surrounded by a rubber layer immobilized at the particle surface. The spherical filler showed small contact zones between neighboring particles in contact with thin rubber layers, while anisotropic particles (AR > 2) formed domains of rods preferentially aligned along the main axis. A detailed analysis of the polymer chain mobility by different time domain nuclear magnetic resonance (TD-NMR) techniques evidenced a population of rigid rubber chains surrounding particles, whose amount increases with the particle anisotropy, even in the absence of significant differences in terms of chemical crosslinking. Dynamic measurements demonstrate that rod-like particles induce stronger reinforcement of rubber, increasing with the AR. This was related to the self-alignment of the anisotropic silica particles in domains able to immobilize rubber.

Telomere Repeat-Binding Factor 2 Is Responsible for the Telomere Attachment to the Nuclear Membrane.

Telomeres are nucleoprotein structures that specify ends of eukaryotic chromosomes. They enable complete DNA replication, protect chromosomes from end-to-end fusions, and help organize chromatin structure. These functions are mediated by special telomeric proteins. TRF2 (telomeric repeat-binding factor 2) is an essential component of shelterin, a telomere-binding protein complex. TRF2 induces formation of a special structure of telomeric DNA, counteracts activation of double-strand break response pathway and ataxia telangiectasia mutated kinase pathway at telomeres. Some line of evidence implicates TRF2 in interactions with the nuclear envelope (NE). TRF2 is tightly bound to the nuclear membrane in frog oocytes nucleus, and it was found colocalized with NE or its remnants in mouse cells. Computer analysis of TRF2 amino acid sequence has shown that TRF2 possesses motifs, which resemble rod domain characteristic of intermediate filament proteins. These observations suggest that TRF2 is a good candidate for the attachment of telomeres to the NE in somatic cells.

Mapping the accessibility of the disulfide crosslink network in the wool fiber cortex.

The disulfide bond network within the cortex of mammalian hair has a critical influence on the physical and mechanical characteristics of the fiber. The location, pattern, and accessibility of free and crosslinked cysteines underpin the properties of this network, but have been very difficult to map and understand, because traditional protein extraction techniques require the disruption of these disulfide bonds. Cysteine accessibility in both trichocyte keratins and keratin associated proteins (KAPs) of wool was investigated using staged labeling, where reductants and chaotropic agents were used to expose cysteines in a stepwise fashion according to their accessibility. Cysteines thus exposed were labeled with distinguishable alkylation agents. Proteomic profiling was used to map peptide modifications and thereby explore the role of KAPs in crosslinking keratins. Labeled cysteines from KAPs were detected when wool was extracted with reductant only. Among them were sequences from the end domains of KAPs, indicating that those cysteines were easily accessible in the fiber and could be involved in forming interdisulfide linkages with keratins or with other KAPs. Some of the identified peptides were from the rod domains of Types I and II keratins, with their cysteines positioned on the exposed surface of the α-helix. Peptides were also identified from keratin head and tail domains, demonstrating that they are not buried within the filament structure and, hence, have a possible role in forming disulfide linkages. From this study, a deeper understanding of the accessibility and potential reactivity of cysteine residues in the wool fiber cortex was obtained.

Molecular Alterations in Bone Marrow Mesenchymal Stroma Cells of AML Patients

It is increasingly recognized that the tumor microenvironment plays a pivotal role in cancer initiation and progression. In mouse models it was shown that a genetically altered bone marrow (BM) micro milieu was sufficient to induce leukemia (Raaijmakers, Nature 2010); however, the pathogenic role and contribution of the BM stroma in leukemia initiation and during disease progression warrants further investigation. To address this, we have performed gene expression, methylation, RNAseq, whole exome sequencing (WES) in BM mesenchymal stroma cells (BM-MSC) and leukemic cells from AML patients (pts) to unravel underlying molecular alterations.We collected BM hematopoietic cells (BM-HC) as well as plastic-adherent BM-MSC from aspirates from AML pts and healthy donors (HD). BM-MSC were expanded to passage 4 and defined as CD73+/CD105+/CD271+/low/CD45-/CD33-. We investigated gene expression profiles (Affymetrix) of BM-MSC from newly diagnosed AML pts (n=20) and compared these to BM-MSC from HD (n=4). BM-MSC from AML pts displayed an altered expression signature with 191and 175genesbeingsignificantly 2-fold over- and under-expressed. KEGG analysis of differentially expressed genes in BM-MSC from AML pts exhibited enrichment for TGF-ß signalling, whereas downregulated genes were enriched for cytokine receptor interactions. Several of these candidates were validated in a larger set of BM-MSC samples by RT-PCR. One putative stroma-leukemia interaction molecule, lumican (LUM) was highly overexpressed in BM-MSC (n=60) from AML pts compared to HD (n=5; p value =0.019) indicating that LUM may affect the BM niche in AML.To explore the altered expression pattern in AML BM-MSC compared to HD BM-MSC, global methylation analyses (Illumina Infinium HumanMethylation 450 bead chip arrays) were performed in 5 AML pts where we had collected BM-HC and BM-MSC at 3 sequential time points [initial diagnosis (ID), remission (CR), relapse (REL); n=30] as well as in BM-HC and BM-MSC from HDs (n=6). A significantly different methylation profile was evident comparing AML BM-HC to the corresponding AML BM-MSC samples, the latter showing a homogenous pattern during the course of disease. When AML BM-MSC were compared to a set of HD BM-MSC, we identified 2416 differentially methylated CpG sites (p value <0.01) indicating that an epigenetic deregulation contributes to the altered gene expression profile observed in AML BM-MSC.These 30 AML BM-MSC/BM-HC samples were subsequently analyzed by WES to unravel genetic alterations in the compartments of the mesenchymal and hematopoietic cell fractions. In WES (HiSeq2000, 100bp paired-end), we obtained an average of 100 reads for the target region; more than 90% of the exome target region was covered at least 30-fold. When the AML BM-HC CR sample was used as germline control, a median of 3 SNVs were detected in AML BM-MSC samples. The only BM-MSC-specific alteration present in one AML patient at all time points (ID, CR, REL) was a mutation in the plectin gene (PLEC). This mutation in the ROD domain of this cytoskeletal linker protein is located in the hot spot for mutations described in epidermolysis bullosa. This mutation was validated by Sanger sequencing, however in a larger cohort of 50 AML ID BM-MSC, no additional PLEC mutation at the same position was found. The set of AML BM-MSC (n=15) samples further allowed us to identify lesions (SNVs, Indels) in the corresponding BM-HC (n=15). When we used the AML BM-MSC as germline control we identified in total 43 lesions in the AML BM-HC fractions, which were not found when the corresponding BM-HC CR sample was used as germline control. This unraveled pre-leukemic lesions present in the AML BM-HC at remission: importantly using this approach, lesions in ASXL1 (Y591*) and DNMT3A (R882H), and in another patient a DNMT3A (M880V) mutation were revealed.In conclusion, the altered gene expression profile and methylation signature of AML BM-MSC provide novel insights into the pathogenic role of the leukemic BM microenvironment. Genetic alterations explored by WES revealed only very few genetic hits that will require further functional exploration. However, the low number of genetic alterations suggests that the transcriptional and epigenetic alterations are directed by extrinsic factors. At the same time, AML BM-MSC provides a non-hematopoietic derived germline control that allows to unravel pre-leukemic lesions in BM-HC. DisclosuresNo relevant conflicts of interest to declare.

Conformational activation of talin by RIAM triggers integrin-mediated cell adhesion.

The membrane localization and activation of cytoskeletal protein talin are key steps to initiate integrin transmembrane receptors activation, which mediates many cellular adhesive responses such as cell migration, spreading, and proliferation. RIAM, a membrane anchor and small GTPase RAP1 effector, is known to bind to the C-terminal rod domain of talin (talin-R) and promote localizations of talin to membrane. Through systematic mapping analysis, we find that RIAM also binds to the N-terminal head of talin (talin-H), a crucial domain involved in binding and activating integrins. We show that the RIAM binding to talin-H sterically occludes the binding of a talin-R domain that otherwise masks the integrin binding site on talin-H. We further provide functional evidence that such RIAM-mediated steric unmasking of talin triggers integrin activation. Our findings thus uncover a novel role for RIAM in conformational regulation of talin during integrin activation and cell adhesion.

Characterization of unfolding mechanism of human lamin A Ig fold by single-molecule force spectroscopy-implications in EDMD.

A- and B-type lamins are intermediate filament proteins constituting the nuclear lamina underneath the nuclear envelope thereby conferring proper shape and mechanical rigidity to the nucleus. Lamin proteins are also shown to be related diversely to basic nuclear processes. More than 400 mutations in human lamin A protein alone have been reported to produce at least 11 different disease conditions jointly termed as laminopathies. These mutations in lamin A are scattered throughout its helical rod domain, as well as the C-terminal domain containing the conserved Ig-fold region. The commonality of phenotypes in all these diseases is characterized by misshapen nuclei of the affected tissues which might stem from altered rigidity of the supporting lamina hence lamins. Here we have focused on autosomal dominant Emery-Dreifuss Muscular Dystrophy, one such laminopathy where R453W is the causative mutation located in the Ig domain of lamin A. We have investigated by single-molecule force spectroscopy how a stretching mechanical perturbation senses the destabilizing effect of the mutation in the lamin A Ig domain and compared the mechanoelastic properties of the mutant R453W with that of the wild-type in conjunction with steered molecular dynamics. Furthermore, we have shown the interaction of Ig domain with emerin, another key player and interacting partner in the pathogenesis of EDMD, is disrupted in the R453W mutant. This altered mechanoresistance of Ig domain itself and consequent uncoupling of lamin A-emerin interaction might underlie the altered mechanotransduction properties of EDMD affected nuclei.

Carboxyl-terminal-dependent recruitment of nonmuscle myosin II to megakaryocyte contractile ring during polyploidization

AbstractEndomitosis is a unique megakaryocyte (MK) differentiation process that is the consequence of a late cytokinesis failure associated with a contractile ring defect. Evidence from in vitro studies has revealed the distinct roles of 2 nonmuscle myosin IIs (NMIIs) on MK endomitosis: only NMII-B (MYH10), but not NMII-A (MYH9), is localized in the MK contractile ring and implicated in mitosis/endomitosis transition. Here, we studied 2 transgenic mouse models in which nonmuscle myosin heavy chain (NMHC) II-A was genetically replaced either by II-B or by a chimeric NMHCII that combined the head domain of II-A with the rod and tail domains of II-B. This study provides in vivo evidence on the specific role of NMII-B on MK polyploidization. It demonstrates that the carboxyl-terminal domain of the heavy chains determines myosin II localization to the MK contractile ring and is responsible for the specific role of NMII-B in MK polyploidization.

The Talin Dimer Structure Orientation Is Mechanically Regulated

Formation of a stable cell-substrate contact can be regulated by mechanical force, especially at the focal adhesion. Individual proteins that make up the focal adhesions, such as talin, can exhibit mechanosensing. We previously described one mode of talin mechanosensing in which the vinculin-binding site of talin is exposed after force-induced stretch of a single talin rod domain. Here, we describe a second mode of talin mechanosensing in which the talin dimer itself can adopt different orientations in response to mechanical stimulation. Using molecular dynamics models, we demonstrate that the C-terminus region of the talin dimer is flexible mainly at the linker between the dimerization helices and the nearby actin-binding helical bundle. Our molecular dynamics simulations reveal two possible orientations of the talin dimer at its C-terminus. The extracellular matrix (ECM)-bound integrins cross-linked by talin can be forced apart leading to an elongated orientation of the talin dimer, and the ECM-bound integrins can be forced together by the ECM producing a collapsed orientation of the talin dimer. Formation of the elongated orientation is shown to be more favorable. Switching between the two talin dimer orientations constitutes a mode of mechanosensing.

G.P.87: Prospects for single antisense oligonucleotide-induced multiple exon skipping for rare non-hotspot mutations in Duchenne muscular dystrophy

Antisense oligonucleotides (AONs) for Duchenne muscular dystrophy (DMD) are designed to induce skipping of a single exon during pre-mRNA splicing, thus restoring the transcript’s open reading frame and consequently synthesis of the deficient dystrophin protein. Although this approach is mutation-dependent, the majority of mutations cluster in a hotspot region from exon 45 to 53, allowing application of one AON to a subpopulation of patients with grouped mutations. Some mutations however are ultra-rare with only few patients worldwide, and require the skipping of an exon outside the hotspot region. Combining these ultra-rare mutations by applying multiple exon skipping is therefore attractive. The feasibility of multiple exon skipping using a cocktail of AONs has been demonstrated, but a drawback is the increased complexity of the development pathway and manufacturing costs. We therefore studied the possibility of designing a single AON capable of inducing simultaneous skipping of multiple exons. The repetitive nature of the central rod domain in dystrophin implies the occurrence of homologous stretches within the transcript. Indeed, in the exon 10 to 40 region several exon pairs were identified sharing such a homologous stretch, which we explored as a potential dual target for a single AON. In-frame exon combinations that would mimic a Becker-like mutation associated with a relatively mild phenotype and that would apply to a considerable patient subpopulation (up to ∼15%) were selected. In vitro proof-of-concept was obtained for healthy donor as well as for DMD patient-derived muscle cells and resulted in novel dystrophin expression. Results demonstrating the applicability in the mdx mouse model in vitro and in vivo will be presented. The data support further (pre) clinical development of such multiple exon skipping AONs, not only for more rare mutations that are not targeted by current single skip programs but also for mutations in the hotspot region.

G.P.104: Oral small molecule compounds that promote the skipping of exon 51 in the DMD gene

Duchenne muscular dystrophy (DMD) is a severe progressive X-linked genetic disorder that affects 1 in 3500 males. It is a monogenic disease caused by mutations in the gene encoding the dystrophin protein. Approximately two thirds of the mutations are deletions in the open reading frame that result in the production of C-terminus-truncated non-functional dystrophin protein. One potential therapeutic approach to address a subset of the reading frame-altering deletions, located in the central rod domain, is to cause skipping of an exon downstream of the deletion thereby restoring the reading frame. The resulting protein, containing an internal deletion, would be partially functional but expected to lead to a milder, Becker muscular dystrophy-like phenotype. The largest fraction (∼13%) of all DMD patients have the above described mutations that can be addressed by skipping exon 51. We are pursuing an innovative approach to discover and develop orally bioavailable tissue-penetrable small molecules that induce skipping of DMD exon 51. Using PTC’s proprietary drug discovery platform technology referred to as GEMS AS (Gene Expression Modulation by Small-molecules – Alternative Splicing™) we have designed and conducted a high throughput screen of our compound library. We identified several structurally distinct classes of small molecules that promote the skipping of DMD exon 51. These scaffolds are undergoing structural optimization to improve biological, pharmacological and pharmaceutical properties with the ultimate goal of identifying a Drug Candidate for preclinical and clinical development.

G.P.147: Two novel epsilon isoforms of nesprin-2, a protein linked to Emery–Dreifuss muscular dystrophy

Nesprin proteins have a central rod domain of spectrin repeats and are intracellular linkers and scaffolds. Full length or "giant" nesprin-1 and nesprin-2 have N-terminal calponin homology domains that bind the actin cytoskeleton and C-terminal transmembrane KASH domains which anchor the nesprins to the outer nuclear membrane. A number of short isoforms of nesprin-1 and nesprin-2 are produced by internal promotion and by alternative splicing. Mutations in nesprin-1 and nesprin-2 can cause Emery-Dreifuss muscular dystrophy and dilated cardiomyopathy. Nesprin-1-alpha-2 is found almost exclusively in skeletal muscle and heart and all the known mutations in nesprins that are associated with EDMD or DCM lie within the alpha isoform sequence. Two novel "epsilon" isoforms of nesprin-2 were predicted by bioinformatics and we have demonstrated the existence of corresponding proteins for the first time. Because they are similar to nesprin-1 alpha in size and structure, we evaluated the significance of the epsilon isoforms in 20 human tissues and 7 human cell lines using qPCR. N2-epsilon-1 was expressed only in ovary and early embryonic cells (Ntera-2 and ESC), while N2-epsilon-2 was expressed in several mature tissues, including cardiac, but not skeletal, muscle. Western blotting confirmed the presence of epsilon-1 protein in Ntera-2 and ESC and epsilon-2 protein in heart and brain. Total nesprin transcript in ESC was mainly nesprin-2-giant (77%) and nesprin-2-epsilon-1 (21%). PCR indicated that most of the nesprin-2 mRNA in ESC lacked the KASH domain, whereas heart and skeletal muscle appeared to contain high levels of nesprin-2 KASH. Immunofluorescence microscopy with a monoclonal antibody against nesprin-2 showed nuclear rim staining in heart and skeletal muscle sections, but in KASH-less ESC there was an intense nucleoplasmic speckle-like distribution. Supported by British Heart Foundation (PG/11/71/29091).

Aggregate-prone desmin mutations impair mitochondrial calcium uptake in primary myotubes

Desmin, being a major intermediate filament of mature muscle cell, interacts with mitochondria within the cell and participates in mitochondria proper localization. The goal of the present study was to assess the effect of aggregate-prone and non-aggregate-prone desmin mutations on mitochondrial calcium uptake. Primary murine satellite cells were transduced with lentiviruses carrying desmin in wild type or mutant form, and were induced to differentiate into myotubes. Four mutations resulting in different degree of desmin aggregates formation were analyzed. Tail domain mutation Asp399Tyr has the mildest impact on desmin filament polymerization, rod domain mutation Ala357Pro causes formation of large aggregates composed of filamentous material, and Leu345Pro and Leu370Pro are considered to be the most severest in their impact on desmin polymerization and structure. For mitochondrial calcium measurement cells were loaded with rhod 2-AM. We found that aggregate-prone mutations significantly decreased [Ca2+]mit, whereas non-aggregate-prone mutations did not decrease [Ca2+]mit. Moreover aggregate-prone desmin mutations resulted in increased resting cytosolic [Ca2+]. However this increase was not accompanied by any alterations in sarcoplasmic reticulum calcium release. We suggest that the observed decline in [Ca2+]mit was due to desmin aggregate accumulation resulting in the loss of desmin mitochondria interactions.