Targeted deletion of the muscular dystrophy gene myotilin does not perturb muscle structure or function in mice.

Myotilin, palladin, and myopalladin form a novel small subfamily of cytoskeletal proteins that contain immunoglobulin-like domains. Myotilin is a thin filament-associated protein localized at the Z-disk of skeletal and cardiac muscle cells. The direct binding to F-actin, efficient cross-linking of actin filaments, and prevention of induced disassembly of filaments are key roles of myotilin that are thought to be involved in structural maintenance and function of the sarcomere. Missense mutations in the myotilin-encoding gene cause dominant limb girdle muscular dystrophy type 1A and spheroid body myopathy and are the molecular defect that can cause myofibrillar myopathy. Here we describe the generation and analysis of mice that lack myotilin, myo(-/-) mice. Surprisingly, myo(-/-) mice maintain normal muscle sarcomeric and sarcolemmal integrity. Also, loss of myotilin does not cause alterations in the heart or other organs of newborn or adult myo(-/-) mice. The mice develop normally and have a normal life span, and their muscle capacity does not significantly differ from wild-type mice even after prolonged physical stress. The results suggest that either myotilin does not participate in muscle development and basal function maintenance or other proteins serve as structural and functional compensatory molecules when myotilin is absent.

Myotilin: a prominent marker of myofibrillar remodelling.

Myofibrillar remodelling with insertion of sarcomeres is a typical feature of biopsies taken from persons suffering of exercise-induced delayed onset muscle soreness. Here we studied the presence of the sarcomeric protein myotilin in eccentric exercise related lesions. Myotilin is a component of sarcomeric Z-discs and it binds several other Z-disc proteins, i.e. alpha-actinin, filamin C, F-actin and FATZ. Myotilin has previously been shown to be present in nemaline rods and central cores and to be mutated in limb girdle muscular dystrophy 1A (LGMD1A) and in a subset of myofibrillar myopathies, indicating an important role in Z-disc maintenance. Our findings on non-diseased muscle affected by eccentric exercise give new information on how myotilin is associated to myofibrillar components upon remodelling. We show that myotilin was present in increased amount in lesions related to Z-disc streaming and events leading to insertion of new sarcomeres in pre-existing myofibrils and can therefore be used as a marker for myofibrillar remodelling. Interestingly, myotilin is preferentially associated with F-actin rather than with the core Z-disc protein alpha-actinin during these events. This suggests that myotilin has a key role in the dynamic molecular events mediating myofibrillar assembly in normal and diseased skeletal muscle.

The proline-rich protein palladin is a binding partner for profilin.

Palladin is an actin-associated protein that has been suggested to play critical roles in establishing cell morphology and maintaining cytoskeletal organization in a wide variety of cell types. Palladin has been shown previously to bind directly to three different actin-binding proteins vasodilator-stimulated phosphoprotein (VASP), alpha-actinin and ezrin, suggesting that it functions as an organizing unit that recruits actin-regulatory proteins to specific subcellular sites. Palladin contains sequences resembling a motif known to bind profilin. Here, we demonstrate that palladin is a binding partner for profilin, interacting with profilin via a poly proline-containing sequence in the amino-terminal half of palladin. Double-label immunofluorescence staining shows that palladin and profilin partially colocalize in actin-rich structures in cultured astrocytes. Our results suggest that palladin may play an important role in recruiting profilin to sites of actin dynamics.

The palladin/myotilin/myopalladin family of actin-associated scaffolds.

The dynamic remodeling of the actin cytoskeleton plays a critical role in cellular morphogenesis and cell motility. Actin-associated scaffolds are key to this process, as they recruit cohorts of actin-binding proteins and associated signaling complexes to subcellular sites where remodeling is required. This review is focused on a recently discovered family of three proteins, myotilin, palladin, and myopalladin, all of which function as scaffolds that regulate actin organization. While myotilin and myopalladin are most abundant in skeletal and cardiac muscle, palladin is ubiquitously expressed in the organs of developing vertebrates. Palladin's function has been investigated primarily in the central nervous system and in tissue culture, where it appears to play a key role in cellular morphogenesis. The three family members each interact with specific molecular partners: all three bind to alpha-actinin; in addition, palladin also binds to vasodilator-stimulated phosphoprotein (VASP) and ezrin, myotilin binds to filamin and actin, and myopalladin also binds to nebulin and cardiac ankyrin repeat protein (CARP). Since mutations in myotilin result in two forms of muscle disease, an essential role for this family member in organizing the skeletal muscle sarcomere is implied.

Different early pathogenesis in myotilinopathy compared to primary desminopathy.

Mutations in the human myotilin gene may cause limb-girdle muscular dystrophy 1A and myofibrillar myopathy. Here, we describe a German patient with the clinically distinct disease phenotype of late adult onset distal anterior leg myopathy caused by a heterozygous S55F myotilin mutation. In addition to a thorough morphological and clinical analysis, we performed for the first time a protein chemical analysis and transient transfections. Morphological analysis revealed an inclusion body myopathy with myotilin- and desmin-positive aggregates. The clinical and pathological phenotype considerably overlaps with late onset distal anterior leg myopathy of the Markesbery-Griggs type. Interestingly, all three analyzed myotilin missense mutations (S55F, S60F and S60C) do not lead to gross changes in the total amount of myotilin or to aberrant posttranslational modifications in diseased muscle, as observed in a number of muscular dystrophies. Transiently transfected wild-type and S55F mutant myotilin similarly colocalised with actin-containing stress fibers in BHK-21 cells. Like the wild-type protein, mutated myotilin did not disrupt the endogenous desmin cytoskeleton or lead to pathological protein aggregation in these cells. This lack of an obvious dominant negative effect sharply contrasts to transfections with, for instance, the disease-causing A357P desmin mutant. In conclusion our data indicate that the disorganization of the extrasarcomeric cytoskeleton and the presence of desmin-positive aggregates are in fact late secondary events in the pathogenesis of primary myotilinopathies, rather than directly related. These findings suggest that unrelated molecular pathways may result in seemingly similar disease phenotypes at late disease stages.

Nrf2/Keap1 Pathway and Expression of Oxidative Stress Lesions 8-hydroxy-2'-deoxyguanosine and Nitrotyrosine in Melanoma.

BACKGROUND/AIM: Increased expression and prognostic significance of major redox regulator nuclear factor erythroid-2-related factor (Nrf2) is recognized in many cancers. Our aim was to investigate the role of oxidative stress markers in melanoma. MATERIALS AND METHODS: We characterized the immunohistochemical expression of Nrf2, kelch-like ECH-associated protein 1 (Keap1), BRAF(V600E), 8-hydroxy-2'-deoxyguanosine (8-OHdG) and nitrotyrosine in 36 nevi, 14 lentigo maligna and 71 malignant melanomas. We measured Nrf2 expression in melanoma cell lines and conducted cytotoxicity assays combining BRAF/NRAS ablation and H2O2treatment. RESULTS: Nuclear Nrf2 expression in melanoma correlated with deeper Breslow (p<0.0005), invasive phenotype (Clark III-V) (p=0.011), nodular growth (p=0.001) and worse melanoma-specific survival (p=0.008). Absence of 8-OHdG in the endothelium was a greater significant predictor of poor prognosis (p=0.024) than ulceration (p=0.17) and had a similar impact on prognosis as Breslow (p=0.024). A decrease of Nrf2 followed the BRAF/NRAS inhibition, but combination of inhibitor with H2O2did not increase cytotoxicity. CONCLUSION: Nrf2 and 8-OHdG influence prognosis in melanoma.

Prospective immunohistochemical analysis of BRAF V600E mutation in melanoma.

The v-raf murine sarcoma viral oncogene homolog B1 (BRAF) V600E mutation is the most common activating genetic alteration of this oncogene and a predictive marker for the therapeutic use of BRAF inhibitors in melanoma. Our aim was to evaluate the performance of BRAF V600E mutation-specific monoclonal antibody (VE1) in a prospective diagnostic setting of melanoma patients (n = 102). All 41 cases (40.2%) that showed a V600E mutation in the cyclic minisequencing analysis of the DNA were also initially scored immunopositive. Two cases that were scored as BRAF V600E mutation positive by immunohistochemistry were negative in the DNA-based mutation analysis and determined to be immunonegative in a repeated staining with more representative specimens. Thus, BRAF V600E mutation detection using immunohistochemistry was 100% sensitive and 96.8% specific, when compared with the analysis of the DNA. None of the BRAF V600K mutations was detected by the VE1 antibody (n = 7). However, the VE1 antibody detected a rare V600E2 mutation. We also studied the role of BRAF V600E mutation in a set of melanoma patients who had been investigated for sentinel node metastasis. Melanoma lymph node metastases were diagnosed in 21.8% (12/55) of the sentinel nodes, and BRAF V600E immunopositivity was detected in 34.5% (19/55) of the cases. BRAF V600E mutation status did not correlate with any clinicopathological parameters. In conclusion, analysis of BRAF V600E mutation in melanoma by immunohistochemistry is a sensitive and specific method, which can be used to identify BRAF inhibitor-sensitive melanoma patients as a first-line method due to its rapid and affordable nature.

Molecular analysis of the interaction between palladin and alpha-actinin.

Palladin is a novel component of stress fiber dense regions. Antisense and transient overexpression studies have indicated an important role for palladin in the regulation of actin cytoskeleton. Palladin colocalizes and coimmunoprecipitates with alpha-actinin, a dense region component, but the molecular details and functional significance of the interaction have not been studied. We show here a direct association between the two proteins and have mapped the binding site within a short sequence of palladin and in the carboxy-terminal calmodulin domain of alpha-actinin. Using transfection-based targeting assays, we show that palladin is involved in targeting of alpha-actinin to specific subcellular foci indicating a functional interplay between the two actin-associated proteins.

Characterization of mouse myotilin and its promoter.

Myotilin is a sarcomeric protein mutated in two forms of muscle disease, limb-girdle muscular dystrophy type 1A and myofibrillar myopathy. Myotilin is expressed late during human myofibrillogenesis and localizes to Z-discs in mature sarcomere. It interacts with alpha-actinin, actin, and filamin C, and has strong F-actin-bundling activity. These features suggest an important role for myotilin in sarcomere organization. In our effort towards the construction of a genetic model for myotilin-related muscle disorders, we have cloned mouse myotilin, including its promoter region, and studied the expression in various tissues. Mouse myotilin is 90% identical with the human orthologue. Northern blot analysis revealed strong mRNA transcripts in skeletal and cardiac muscle, and weak expression in liver and lung tissue. Western blot and RT-PCR analysis showed the presence of one major product in mouse tissues. Analysis of the 5'-flanking region revealed a number of putative regulatory elements that drive expression in differentiating myoblasts. Finally, endogenous myotilin is induced at later stages of Z-disc assembly in C(2)C(12) cells indicating conservation between mouse and human promoter region.

Actin-organising properties of the muscular dystrophy protein myotilin.

Myotilin is a sarcomeric Z-disc protein that binds F-actin directly and bundles actin filaments, although it does not contain a conventional actin-binding domain. Expression of mutant myotilin leads to sarcomeric alterations in the dominantly inherited limb-girdle muscular dystrophy 1A and in myofibrillar myopathy/desmin-related myopathy. Together, with previous in vitro studies, this indicates that myotilin has an important function in the assembly and maintenance of Z-discs. This study characterises further the interaction between myotilin and actin. Functionally important regions in myotilin were identified by actin pull-down and yeast two-hybrid assays and with a novel strategy that combines in vitro DNA transposition-based peptide insertion mutagenesis with phenotype analysis in yeast cells. The shortest fragment to bind actin was the second Ig domain together with a short C-terminal sequence. Concerted action of the first and second Ig domain was, however, necessary for the functional activity of myotilin, as verified by analysis of transposon mutants, actin binding and phenotypic effect in mammalian cells. Furthermore, the Ig domains flanked with N- and C-terminal regions were needed for actin-bundling, indicating that the mere actin-binding sequence was insufficient for the actin-regulating activity. None of the four known disease-associated mutations altered the actin-organising ability. These results, together with previous studies in titin and kettin, identify the Ig domain as an actin-binding unit.

Involvement of palladin and alpha-actinin in targeting of the Abl/Arg kinase adaptor ArgBP2 to the actin cytoskeleton.

Palladin and alpha-actinin are major components of stress fiber dense bodies, cardiomyocyte Z-discs and neuronal synapses. They function as structural molecules and cytoskeletal regulators but also as docking sites to other proteins. Both antisense and transient overexpression experiments have shown that palladin plays an important role in the regulation of actin cytoskeleton. ArgBP2 is a multi-domain scaffolding protein which shares both the tissue distribution and subcellular localization with palladin. ArgBP2 is directly linked to intracellular signaling cascades by its interaction with Abl family kinases, Pyk2 and the ubiquitin ligase Cbl. It has several actin associated binding partners and has been shown to regulate cytoskeletal dynamics. Here, we show by in vivo and in vitro methods that palladin's amino-terminal poly-proline sequences directly interact with the first carboxy-terminal SH3 domain of ArgBP2. We further demonstrate a direct interaction between alpha-actinin and the amino-terminal segment of ArgBP2. Immunoprecipitation and targeting assays suggest that a three-way complex of the proteins occurs in vivo. The interactions provide an explanation to the previously observed Z-disc-specific localization of ArgBP2 and indicate interplay between signaling adaptors and structural proteins of the Z-disc.