A homozygous nonsense mutation in the PLEC1 gene in patients with epidermolysis bullosa simplex with muscular dystrophy

Different effects of bariatric surgery on epigenetic plasticity in skeletal muscle of individuals with and without type 2 diabetes

AIM

Bariatric surgery is highly effective for the treatment of obesity in individuals without (OB1) and in those with type 2 diabetes (T2D2). However, whether bariatric surgery triggers similar or distinct molecular changes in OB and T2D remains unknown. Given that individuals with type 2 diabetes often exhibit more severe metabolic deterioration, we hypothesized that bariatric surgery induces distinct molecular adaptations in skeletal muscle, the major site of glucose uptake, of OB and T2D after surgery-induced weight loss.

METHODS

All participants (OB, n = 13; T2D, n = 13) underwent detailed anthropometry before and one year after the surgery. Skeletal muscle biopsies were isolated at both time points and subjected to transcriptome and methylome analyses using a comprehensive bioinformatic pipeline.

RESULTS

Before surgery, T2D had higher fasting glucose and insulin levels but lower whole-body insulin sensitivity, only glycemia remained higher in T2D than in OB after surgery. Surgery-mediated weight loss affected different subsets of genes with 2,013 differentially expressed in OB and 959 in T2D. In OB differentially expressed genes were involved in insulin, PPAR signaling and oxidative phosphorylation pathways, whereas ribosome and splicesome in T2D. LASSO regression analysis revealed distinct candidate genes correlated with improvement of phenotypic traits in OB and T2D. Compared to OB, DNA methylation was less affected in T2D in response to bariatric surgery. This may be due to increased global hydroxymethylation accompanied by decreased expression of one of the type 2 diabetes risk gene, TET2, encoding a demethylation enzyme in T2D.

CONCLUSION

OB and T2D exhibit differential skeletal muscle transcriptome responses to bariatric surgery, presumably resulting from perturbed epigenetic flexibility.

Plectin Deficiency in Fibroblasts Deranges Intermediate Filament and Organelle Morphology, Migration, and Adhesion

Plectin, a highly versatile and multifunctional cytolinker, has been implicated in several multisystemic disorders. Most sequence variations in the human plectin gene (PLEC) cause epidermolysis bullosa simplex with muscular dystrophy (EBS-MD), an autosomal recessive skin-blistering disorder associated with progressive muscle weakness. In this study, we performed a comprehensive cell biological analysis of dermal fibroblasts from three different patients with EBS-MD, where PLEC expression analyses revealed preserved mRNA levels in all cases, whereas full-length plectin protein content was significantly reduced or completely absent. Downstream effects of pathogenic PLEC sequence alterations included massive bundling of vimentin intermediate filament networks, including the occurrence of ring-like nuclei-encasing filament bundles, elongated mitochondrial networks, and abnormal nuclear morphologies. We found that essential fibroblast functions such as wound healing, migration, or orientation upon cyclic stretch were significantly impaired in the cells of patients with EBS-MD. Finally, EBS-MD fibroblasts displayed reduced adhesion capacities, which could be attributed to smaller focal adhesion contacts. Our study not only emphasizes plectin's functional role in human skin fibroblasts, it also provides further insights into the understanding of EBS-MD-associated disease mechanisms.

Mutation update: The spectra of PLEC sequence variants and related plectinopathies

Plectin, encoded by PLEC, is a cytoskeletal linker of intermediate filaments expressed in many cell types. Plectin consists of three main domains that determine its functionality: the N-terminal domain, the Rod domain, and the C-terminal domain. Molecular defects of PLEC correlating with the functional aspects lead to a group of rare heritable disorders, plectinopathies. These multisystem disorders include an autosomal dominant form of epidermolysis bullosa simplex (EBS-Ogna), limb-girdle muscular dystrophy (LGMD), aplasia cutis congenita (ACC), and an autosomal recessive form of EBS, which may associate with muscular dystrophy (EBS-MD), pyloric atresia (EBS-PA), and/or congenital myasthenic syndrome (EBS-MyS). In this study, genotyping of over 600 Iranian patients with epidermolysis bullosa by next-generation sequencing identified 15 patients with disease-causing PLEC variants. This mutation update analyzes the clinical spectrum of PLEC in our cohort and in the literature and demonstrates the relationship between PLEC genotype and phenotypic manifestations. This study has integrated our seven novel PLEC variants and phenotypic findings with previously published data totaling 116 variants to provide the most complete overview of pathogenic PLEC variants and related disorders.

Muscle-Related Plectinopathies

Plectin is a giant cytoskeletal crosslinker and intermediate filament stabilizing protein. Mutations in the human plectin gene (PLEC) cause several rare diseases that are grouped under the term plectinopathies. The most common disorder is autosomal recessive disease epidermolysis bullosa simplex with muscular dystrophy (EBS-MD), which is characterized by skin blistering and progressive muscle weakness. Besides EBS-MD, PLEC mutations lead to EBS with nail dystrophy, EBS-MD with a myasthenic syndrome, EBS with pyloric atresia, limb-girdle muscular dystrophy type R17, or EBS-Ogna. In this review, we focus on the clinical and pathological manifestations caused by PLEC mutations on skeletal and cardiac muscle. Skeletal muscle biopsies from EBS-MD patients and plectin-deficient mice revealed severe dystrophic features with variation in fiber size, degenerative myofibrillar changes, mitochondrial alterations, and pathological desmin-positive protein aggregates. Ultrastructurally, PLEC mutations lead to a disorganization of myofibrils and sarcomeres, Z- and I-band alterations, autophagic vacuoles and cytoplasmic bodies, and misplaced and degenerating mitochondria. We also summarize a variety of genetically manipulated mouse and cell models, which are either plectin-deficient or that specifically lack a skeletal muscle-expressed plectin isoform. These models are powerful tools to study functional and molecular consequences of PLEC defects and their downstream effects on the skeletal muscle organization.

Advances in gene editing strategies for epidermolysis bullosa

Epidermolysis bullosa represents a monogenetic disease comprising a variety of heterogeneous mutations in at least 16 genes encoding structural proteins crucial for skin integrity. Due to well-defined mutations but still lacking causal treatment options for the disease, epidermolysis bullosa represents an ideal candidate for gene therapeutic interventions. Recent developments and improvements in the genome editing field have paved the way for the translation of various gene repair strategies into the clinic. With the ability to accurately predict and monitor targeting events within the human genome, the translation might soon be possible. Here, we describe current advancements in the genome editing field for epidermolysis bullosa, along with a discussion of aspects and strategies for precise and personalized gene editing-based medicine, in order to develop efficient and safe ex vivo as well as in vivo genome editing therapies for epidermolysis bullosa patients in the future.

Diverse dystonin gene mutations cause distinct patterns of Dst isoform deficiency and phenotypic heterogeneity in Dystonia musculorum mice

Loss-of-function mutations in dystonin (DST) can cause hereditary sensory and autonomic neuropathy type 6 (HSAN-VI) or epidermolysis bullosa simplex (EBS). Recently, DST-related diseases were recognized to be more complex than previously thought because a patient exhibited both neurological and skin manifestations, whereas others display only one or the other. A single DST locus produces at least three major DST isoforms: DST-a (neuronal isoform), DST-b (muscular isoform) and DST-e (epithelial isoform). Dystonia musculorum (dt) mice, which have mutations in Dst, were originally identified as spontaneous mutants displaying neurological phenotypes. To reveal the mechanisms underlying the phenotypic heterogeneity of DST-related diseases, we investigated two mutant strains with different mutations: a spontaneous Dst mutant (Dst^dt-23Rbrc mice) and a gene-trap mutant (Dst^Gt mice). The Dst^dt-23Rbrc allele possesses a nonsense mutation in an exon shared by all Dst isoforms. The Dst^Gt allele is predicted to inactivate Dst-a and Dst-b isoforms but not Dst-e. There was a decrease in the levels of Dst-a mRNA in the neural tissue of both Dst^dt-23Rbrc and Dst^Gt homozygotes. Loss of sensory and autonomic nerve ends in the skin was observed in both Dst^dt-23Rbrc and Dst^Gt mice at postnatal stages. In contrast, Dst-e mRNA expression was reduced in the skin of Dst^dt-23Rbrc mice but not in Dst^Gt mice. Expression levels of Dst proteins in neural and cutaneous tissues correlated with Dst mRNAs. Because Dst-e encodes a structural protein in hemidesmosomes (HDs), we performed transmission electron microscopy. Lack of inner plaques and loss of keratin filament invasions underneath the HDs were observed in the basal keratinocytes of Dst^dt-23Rbrc mice but not in those of Dst^Gt mice; thus, the distinct phenotype of the skin of Dst^dt-23Rbrc mice could be because of failure of Dst-e expression. These results indicate that distinct mutations within the Dst locus can cause different loss-of-function patterns among Dst isoforms, which accounts for the heterogeneous neural and skin phenotypes in dt mice and DST-related diseases.

Summary: This paper describes the mechanism by which diverse dystonin gene mutations result in phenotypic heterogeneity in neural and cutaneous tissues of Dystonia musculorum mice.

Atypical Familial Amyotrophic Lateral Sclerosis with Slowly Progressing Lower Extremities-predominant Late-onset Muscular Weakness and Atrophy

Objective Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease characterized by the progressive loss of the upper and lower motor neurons that progresses to paralysis of almost all skeletal muscles of the extremities, bulbar, and respiratory system. Although most ALS cases are sporadic, about 10% are dominantly inherited. We herein report an atypical phenotype of familial ALS (fALS). To elucidate the phenotype-genotype correlation of this atypical phenotype of fALS, clinical and genetic investigations were performed. Methods and Patients Five sibling patients (three men, two women) from a Japanese family and one healthy sibling (a woman) were clinically interviewed and examined. Genetic analyses, including genome-wide linkage analyses and whole-exome sequencing, were performed using genomic DNA extracted from the peripheral blood samples of these siblings. Results The clinical features of fALS are characterized by slow progression (mean duration of the disease±standard deviation [SD]: 19.6±3.9 years) and lower extremities-predominant late-onset muscular weakness (mean onset of muscular weakness±SD: 52.8±2.6 years). Genetic analyses revealed novel heterozygous missense mutations of c.2668C>T, p.R890C in the PLEC gene and c.421G>C, p.V141L in the ST3GAL6 gene in all affected siblings. Conclusion A new atypical fALS family with a benign clinical course is herein reported. We identified two candidate gene mutations of PLEC and ST3GAL6 linked to this phenotype.

Epidermolysis bullosa simplex with muscular dystrophy. Review of the literature and a case report

BACKGROUND

Epidermolysis bullosa simplex associated with muscular dystrophy is a genetic skin disease caused by plectin deficiency. A case of a 19-year-old Czech patient affected with this disease and a review all previously published clinical cases are presented.

MAIN OBSERVATIONS

In our patient, skin signs of the disease developed after birth. Bilateral ptosis at the age of 8 years was considered as the first specific symptom of muscular dystrophy. Since then, severe scoliosis, urological and psychiatric complication have quickly developed. The signs of plectin deficiency were found by histopathological studies, electron microscopy and antigen mapping of the skin and muscular samples. Two autosomal recessive mutations in the plectin gene leading to premature termination codon were disclosed by mutation analysis. By review of all published clinical cases, 49 patients with this disease were found. 54 different mutations in the plectin gene were published, p.(Arg2319*) in exon 31 being the most frequently found. Median age of muscular dystrophy development was 9.5 years. Hoarseness and respiratory complications were the most often complications beside skin involvement.

CONCLUSION

Epidermolysis bullosa simplex with muscular dystrophy was diagnosed based on clinical, histopathological (skin and muscle biopsy) and mutation analysis of the plectin gene. Overview of the genetic and clinical characteristic of this disease could be presented by review of all previously published clinical cases.

Plectin-related skin diseases

Plectin has been characterized as a linker protein that is expressed in many cell types and is distinctive in various isoforms in the N-terminus and around the rod domain due to complicated alternative splicing of PLEC, the gene encoding plectin. Plectin deficiency causes autosomal recessive epidermolysis bullosa simplex (EBS) with involvement of the skin and other organs, such as muscle and gastrointestinal tract, depending on the expression pattern of the defective protein. In addition, a point mutation in the rod domain of plectin leads to autosomal dominant EBS, called as EBS-Ogna. Plectin can be targeted by circulating autoantibodies in subepidermal autoimmune blistering diseases. This review summarizes plectin-related skin diseases, from congenital to autoimmune disorders.

Plectin-intermediate filament partnership in skin, skeletal muscle, and peripheral nerve

Plectin is a large, 500-kDa, intermediate filament (IF)-associated protein. It acts as a cytoskeletal crosslinker and signaling scaffold, affecting mechanical as well as dynamic properties of the cytoskeleton. As a member of the plakin family of cytolinker proteins, plectin has a multidomain structure that is responsible for its vast binding portfolio. It not only binds to all types of IFs, actin filaments and microtubules, but also to transmembrane receptors, proteins of the subplasma membrane protein skeleton, components of the nuclear envelope, and several kinases with known roles in migration, proliferation, and energy metabolism of cells. Due to alternative splicing, plectin is expressed as various isoforms with differing N-terminal heads that dictate their differential subcellular targeting. Through specific interactions with other proteins at their target sites and their ability to bind to all types of IFs, plectin molecules provide strategically located IF anchorage sites within the cytoplasm of cells. In this review, we will present an overview of the structural features and functional properties of plectin and discuss recent progress in defining the role of its isoforms in stress-prone tissues and the implicated diseases, with focus on skin, skeletal muscle, and Schwann cells of peripheral nerve.

The many faces of plectin and plectinopathies: pathology and mechanisms

Plectin, a giant multifunctional cytolinker protein, plays a crucial role in stabilizing and orchestrating intermediate filament networks in cells. Mutations in the human plectin gene result in multiple diseases manifesting with muscular dystrophy, skin blistering, and signs of neuropathy. The most common disease caused by plectin deficiency is epidermolysis bullosa simplex (EBS)-MD, a rare autosomal-recessive skin blistering disorder with late-onset muscular dystrophy. EBS-MD patients and plectin-deficient mice display pathologic desmin-positive protein aggregates, degenerated myofibrils, and mitochondrial abnormalities, the hallmarks of myofibrillar myopathies. In addition to EBS-MD, plectin mutations have been shown to cause EBS-MD with a myasthenic syndrome, limb-girdle muscular dystrophy type 2Q, EBS with pyloric atresia, and EBS-Ogna. This review focuses on clinical and pathological manifestations of these plectinopathies. It addresses especially plectin's role in skeletal muscle, where a loss of muscle fiber integrity and profound changes of myofiber cytoarchitecture are observed in its absence. Furthermore, the highly complex genetic and molecular structure of plectin is discussed; a high number of differentially spliced exons give rise to a variety of different isoforms, which fulfill distinct functions in different cell types and tissues. Plectin's abilities to act as a dynamic organizer of intermediate filament networks and to interact with a multitude of different interaction partners are the basis for its function as a scaffolding platform for proteins involved in signaling. Finally, the article addresses a series of genetically manipulated mouse lines that were generated to serve as powerful models to study functional and molecular consequences of plectin gene defects.

Epidermolysis bullosa simplex associated with muscular dystrophy and cardiac involvement

We report a new clinical variant of epidermolysis bullosa simplex with muscular dystrophy (EBS-MD) that was associated with cardiac involvement. A 33-year-old patient had atrial fibrillation, pericardial effusion, and hypokinetic left ventricular cardiac walls. The muscle biopsy material revealed diffuse endomysial fibrosis and small atrophic muscle fibers with rounded contours. A positive desmin expression with abnormal localization in the subsarcolemmal groups was observed. We concluded that patients with EBS-MD should be investigated carefully when there are associated cardiac findings.

Molecular organization of the basement membrane zone

The dermal-epidermal basement membrane is a complex assembly of proteins that provide adhesion and regulate many important processes such as development, wound healing, and cancer progression. This contribution focuses on the structure and function of individual components of the basement membrane, how they assemble together, and how they participate in human tissues and diseases, with an emphasis on skin involvement. Understanding the composition and structure of the basement membrane provides insight into the pathophysiology of inherited blistering disorders, such as epidermolysis bullosa, and acquired bullous diseases, such as the pemphigoid group of autoimmune diseases and epidermolysis bullosa acquisita.

Myasthenic syndrome caused by plectinopathy

BACKGROUND

Plectin crosslinks intermediate filaments to their targets in different tissues. Defects in plectin cause epidermolysis bullosa simplex (EBS), muscular dystrophy (MD), and sometimes pyloric atresia. Association of EBS with a myasthenic syndrome (MyS) was documented in a single patient in 1999.

OBJECTIVES

To analyze the clinical, structural, and genetic aspects of a second and fatal case of EBS associated with a MyS and search for the genetic basis of the disease in a previously reported patient with EBS-MD-MyS.

METHODS

Clinical observations; histochemical, immunocytochemical, and electron microscopy studies of skeletal muscle and neuromuscular junction; and mutation analysis.

RESULTS

An African American man had EBS since early infancy, and progressive muscle weakness, hyperCKemia, and myasthenic symptoms refractory to therapy since age 3 years. Eventually he became motionless and died at age 42 years. At age 15 years, he had a marked EMG decrement, and a reduced miniature endplate potential amplitude. The myopathy was associated with dislocated muscle fiber organelles, structurally abnormal nuclei, focal plasmalemmal defects, and focal calcium ingress into muscle fibers. The neuromuscular junctions showed destruction of the junctional folds, and remodeling. Mutation analysis demonstrated a known p.Arg2319X and a novel c.12043dupG mutation in PLEC1. The EBS-MD-MyS patient reported in 1999 also carried c.12043dupG and a novel p.Gln2057X mutation. The novel mutations were absent in 200 Caucasian and 100 African American subjects.

CONCLUSIONS

The MyS in plectinopathy is attributed to destruction of the junctional folds and the myopathy to defective anchoring of muscle fiber organelles and defects in sarcolemmal integrity.

Plectin expression patterns determine two distinct subtypes of epidermolysis bullosa simplex

Plectin is a cytoskeletal linker protein that has a dumbbell-like structure with a long central rod and N- and C-terminal globular domains. Mutations in the gene encoding plectin (PLEC1) cause two distinct autosomal recessive subtypes of epidermolysis bullosa (EB): EB simplex with muscular dystrophy (EBS-MD), and EB simplex with pyloric atresia (EBS-PA). Here, we demonstrate that normal human fibroblasts express two different plectin isoforms including full-length and rodless forms of plectin. We performed detailed analysis of plectin expression patterns in six EBS-MD and three EBS-PA patients. In EBS-PA, expression of all plectin domains was found to be markedly attenuated or completely lost; in EBS-MD, the expression of the N- and C-terminal domains of plectin remained detectable, although the expression of rod domains was absent or markedly reduced. Our data suggest that loss of the full-length plectin isoform with residual expression of the rodless plectin isoform leads to EBS-MD, and that complete loss or marked attenuation of full-length and rodless plectin expression underlies the more severe EBS-PA phenotype. These results also clearly account for the majority of EBS-MD PLEC1 mutation restriction within the large exon 31 that encodes the plectin rod domain, whereas EBS-PA PLEC1 mutations are generally outside exon 31.

Plectin gene defects lead to various forms of epidermolysis bullosa simplex

Plectin is an important organizer of the keratin filament cytoskeleton in basal keratinocytes. It is essential for anchoring these filaments to the extracellular matrix via hemidesmosomal integrins. Loss of plectin or incorrect function of the protein due to mutations in its gene can lead to various forms of the skin blistering disease, epidermolysis bullosa simplex. Severity and subtype of the disease is dependent on the specific mutation and can be associated with (late-onset) muscular dystrophy or pyloric atresia. Mouse models mimicking the human phenotypes allow detailed study of plectin function.

Extracutaneous manifestations and complications of inherited epidermolysis bullosa: part II. Other organs

It is well known, primarily via case reports and limited case series, that nonepithelial tissues may become injured in patients with epidermolysis bullosa. Only recently, however, have there been data generated from large, well characterized cohorts. Our objective is to provide dermatologists with a comprehensive review of each of these major extracutaneous complications, with a summary of the pertinent literature and evidence-based recommendations for surveillance, evaluation, and management. Some epidermolysis bullosa subtypes are at risk for severe injury of the bone marrow, musculoskeletal system, heart, kidney, and teeth, and for the development of squamous cell carcinoma, basal cell carcinoma, or malignant melanoma. If untreated, significant morbidity or mortality may result.

Obscurin-like 1, OBSL1, is a novel cytoskeletal protein related to obscurin

Cytoskeletal adaptor proteins serve vital functions in linking the internal cytoskeleton of cells to the cell membrane, particularly at sites of cell-cell and cell-matrix interactions. The importance of these adaptors to the structural integrity of the cell is evident from the number of clinical disease states attributable to defects in these networks. In the heart, defects in the cytoskeletal support system that surrounds and supports the myofibril result in dilated cardiomyopathy and congestive heart failure. In this study, we report the cloning and characterization of a novel cytoskeletal adaptor, obscurin-like 1 (OBSL1), which is closely related to obscurin, a giant structural protein required for sarcomere assembly. Multiple isoforms arise from alternative splicing, ranging in predicted molecular mass from 130 to 230 kDa. OBSL1 is located on human chromosome 2q35 within 100 kb of SPEG, another gene related to obscurin. It is expressed in a broad range of tissues and localizes to the intercalated discs, to the perinuclear region, and overlying the Z lines and M bands of adult rat cardiac myocytes. Further characterization of this novel cytoskeletal linker will have important implications for understanding the physical interactions that stabilize and support cell-matrix, cell-cell, and intracellular cytoskeletal connections.

Progress in epidermolysis bullosa: the phenotypic spectrum of plectin mutations

Plectin, a large multidomain adhesive protein with versatile binding functions, is expressed in a number of tissues and cell types. In the skin, plectin is a critical component of hemidesmosomes, interacting with keratin intermediate filaments and beta4 integrin. Mutations in the plectin gene (PLEC1) result in fragility of skin, demonstrating blister formation at the level of hemidesmosomes. These blistering disorders belong to the spectrum of epidermolysis bullosa (EB) phenotypes, and three distinct variants because of plectin mutations have been identified. First, EB with muscular dystrophy, an autosomal recessive syndrome, is frequently caused by premature termination codon-causing mutations leading to the absence of plectin both in the skin and in the muscle. Second, a heterozygous missense mutation (R2110W) in PLEC1 has been documented in patients with EB simplex of the Ogna type, a rare autosomal dominant disorder. Finally, recent studies have disclosed plectin mutations in patients with EB with pyloric atresia, an autosomal recessive syndrome, frequently with lethal consequences. Collectively, these observations attest to the phenotypic spectrum of plectin mutations, and provide the basis for accurate genetic counselling with prognostic implications, as well as for prenatal diagnosis in families at the risk of recurrence of the disease.

Epidermolysis bullosa simplex associated with pyloric atresia is a novel clinical subtype caused by mutations in the plectin gene (PLEC1)

Epidermolysis bullosa (EB) is an inherited mechano-bullous disorder of the skin, and is divided into three major categories: EB simplex (EBS), dystrophic EB, and junctional EB (JEB). Mutations in the plectin gene (PLEC1) cause EBS associated with muscular dystrophy, whereas JEB associated with pyloric atresia (PA) results from mutations in the alpha6 and beta4 integrin genes. In this study, we examined three EB patients associated with PA from two distinct families. Electron microscopy detected blister formation within the basal keratinocytes leading to the diagnosis of EBS. Surprisingly, immunohistochemical studies using monoclonal antibodies to a range of basement membrane proteins showed that the expression of plectin was absent or markedly attenuated. Sequence analysis demonstrated four novel PLEC1 mutations. One proband was a compound heterozygote for a nonsense mutation of Q305X and a splice-site mutation of 1344G-->A. An exon-trapping experiment suggested that the splice-site mutation induced aberrant splicing of the gene. The second proband harbored a heterozygous maternal nonsense mutation, Q2538X and homozygous nonsense mutations R1189X. Analysis of the intragenic polymorphisms of PLEC1 suggested that R1189X mutations were due to paternal segmental uniparental isodisomy. These results indicate that PLEC1 is a possible causative gene in this clinical subtype, EBS associated with PA. Furthermore, two patients out of our three cases died in infancy. In terms of clinical prognosis, this novel subtype is the lethal variant in the EBS category.

The keratins and their disorders

Diseases caused by mutations in gene encoding keratin intermediate filaments (IF) are characterized by a loss of structural integrity in the cells expressing those keratins in vivo. This is manifested as cell fragility, compensatory epidermal hyperkeratosis, and keratin filament aggregation in some affected tissues. Keratin disorders are a novel molecular category including quite different phenotypes such as epidermolysis bullosa simplex (EBS), bullous congenital ichthyosiform erthroderma (BCIE), pachyonychia congenital (PC), steatocystoma multiplex, ichthyosis bullosa of Siemens (IBS), and white sponge nevus (WSN) of the orogenital mucosa.

Defining the properties of the nonhelical tail domain in type II keratin 5: insight from a bullous disease-causing mutation

Inherited mutations in the intermediate filament (IF) proteins keratin 5 (K5) or keratin 14 (K14) cause epidermolysis bullosa simplex (EBS), in which basal layer keratinocytes rupture upon trauma to the epidermis. Most mutations are missense alleles affecting amino acids located in the central alpha-helical rod domain of K5 and K14. Here, we study the properties of an unusual EBS-causing mutation in which a nucleotide deletion (1649delG) alters the last 41 amino acids and adds 35 residues to the C terminus of K5. Relative to wild type, filaments coassembled in vitro from purified K5-1649delG and K14 proteins are shorter and exhibit weak viscoelastic properties when placed under strain. Loss of the C-terminal 41 residues contributes to these alterations. When transfected in cultured epithelial cells, K5-1649delG incorporates into preexisting keratin IFs and also forms multiple small aggregates that often colocalize with hsp70 in the cytoplasm. Aggregation is purely a function of the K5-1649delG tail domain; in contrast, the cloned 109 residue-long tail domain from wild type K5 is distributed throughout the cytoplasm and colocalizes partly with keratin IFs. These data provide a mechanistic basis for the cell fragility seen in individuals bearing the K5-1649delG allele, and point to the role of the C-terminal 41 residues in determining K5's assembly properties.

Plectin deficient epidermolysis bullosa simplex with 27-year-history of muscular dystrophy

BACKGROUND

Epidermolysis bullosa simplex associated with muscular dystrophy is caused by plectin deficiency.

OBJECTIVE

To report clinical, immunohistochemical, ultrastructural and molecular features of a 52-year-old Japanese patient affected with this disease, whose muscular disease had been followed-up for 27 years.

METHODS

We performed histopathological study, immunofluorescence, electron microscopic study and mutation detection analysis for plectin.

RESULTS

The patient developed blisters and erosions followed by nail deformity on the traumatized regions from birth. The skin lesions were continuously developed to date. The histopathological study showed subepidermal blister. Electron microscopic study showed blister formation inside the basal cells at the level just above the attachment plaque of hemidesmosome. Immunofluorescence showed complete loss of staining to plectin. The mutation analysis using protein truncation test and DNA sequencing revealed a C-to-T transition at nucleotide position 7006 of the plectin cDNA sequence, which lead a novel homozygous nonsense mutation (R2319X).

CONCLUSION

From the above results, the diagnosis of epidermolysis bullosa simplex associated with muscular dystrophy was made. Slight muscular dystrophy was noticed at the age of 25 years. The muscular dystrophy gradually progressed and she could not walk at the age of 46 years. However, she can still breathe and swallow by herself. This is the patient of this disease with the longest follow-up, and may indicate the slow progress of muscular condition of this disease.

Progress in epidermolysis bullosa: Genetic classification and clinical implications

Epidermolysis bullosa (EB), a heterogenous group of genodermatoses, is characterized by fragility and blistering of the skin associated with extracutaneous manifestations. Based on clinical severity, constellation of the phenotypic manifestations, and the level of tissue separation within the cutaneous basement membrane zone (BMZ), EB has been divided into distinct subcategories. Traditionally, these include the simplex, junctional, and dystrophic forms of EB, and recently attention has been drawn to hemidesmosomal variants demonstrating tissue separation at the level of the hemidesmosomes. Specific mutations in ten distinct genes expressed within the cutaneous BMZ have been delineated in >500 families with different variants of EB. The types of mutations, their positions along the affected genes, and their consequences at the mRNA and protein levels provide explanation for the phenotypic variability and genetic heterogeneity of this group of genodermatoses. Elucidation of mutations in different forms of EB has direct translational applications for improved diagnosis and molecularly based classification with prognostic implications as well as for genetic counseling and DNA-based prenatal testing in families with EB.

Severe mucous membrane involvement in epidermolysis bullosa simplex with muscular dystrophy due to a novel plectin gene mutation

Epidermolysis bullosa simplex with muscular dystrophy (OMIM 226670) is an autosomal recessive disorder caused by mutations of the human plectin gene on chromosome 8q24. Here, we report a 3-year-old girl, offspring of a consanguineous Lebanese family, who presented with skin blistering and recurrent episodes of severe respiratory distress necessitating tracheotomy at the age of 2 years. Repeated examination did not provide any evidence of muscle involvement. Indirect immunofluorescence analysis of a diagnostic skin biopsy with four different domain specific plectin antibodies showed a complete absence of plectin staining. Mutation analysis revealed a novel homozygous single guanine insertion mutation (5588insG/5588insG) residing in the N-terminal part of exon 31 of the plectin gene.

CONCLUSION

The complete lack of protein expression, which may be attributed to a nonsense-mediated plectin mRNA decay, is likely to cause muscular dystrophy and other multisystem involvement later in life.

Hemidesmosomes: molecular organization and their importance for cell adhesion and disease

In the skin, basal epithelial cells constantly divide to renew the epidermis. The newly formed epithelial cells then differentiate in a process called keratinization, ultimately leading to the death of these cells and a pile-up of cell material containing vast amounts of keratins. The basal keratinocytes in skin are attached to their underlying basement membrane via specialized adhesion complexes termed hemidesmosomes (HDs). These complexes ascertain stable adhesion of the epidermis to the dermis, and mutations in components of these complexes often result in tissue fragility and blistering of the skin. In this review, we will describe the various hemidesmosomal proteins in detail as well as, briefly, the protein families to which they belong. Specifically, we will report the protein-protein interactions involved in the assembly of hemidesmosomes and their molecular organization. Some signaling pathways involving primarily the alpha6beta4 integrin will be discussed, since they appear to profoundly modulate the assembly and function of hemidesmosomes. Furthermore, the importance of these hemidesmosomal components for the maintenance of tissue homeostasis and their involvement in various clinical disorders will be emphasized. Finally, we will present a model for the assembly of HDs, based on our present knowledge.

Identification of a Lethal Form of Epidermolysis Bullosa Simplex Associated with a Homozygous Genetic Mutation in Plectin

Genetic mutations in plectin, a cytoskeleton linker protein expressed in a large variety of tissues including skin, muscle, and nerves, cause epidermolysis bullosa simplex with muscular dystrophy, a recessive inherited disease characterized by blistering of the skin and late onset of muscular dystrophy, and Ogna epidermolysis bullosa simplex, a rare dominant inherited form of epidermolysis bullosa simplex with no muscular involvement. Here we report a novel homozygous genetic mutation (2727del14) in the plectin gene (PLEC1) associated with a lethal form of recessive inherited epidermolysis bullosa in a consanguineous family with three affected offspring. This new clinical variant of epidermolysis bullosa is characterized by general skin blistering, aplasia cutis of the limbs, developmental complications, and rapid demise after birth. Mutation 2727del14 is the first genetic defect described in PLEC1 that disrupts the plakin domain of plectin. The severe phenotype of the patients may be linked to the role of the N-terminal domain in the function of plectin and develops the understanding of the genotype-phenotype correlations in the genodermatoses affecting the dermal-epidermal junction.

Rapid Decay of α6 Integrin Caused by a Mis-Sense Mutation in the Propeller Domain Results in Severe Junctional Epidermolysis Bullosa with Pyloric Atresia

Genetic mutations in alpha6beta4 integrin cause junctional epidermolysis bullosa with pyloric atresia, a genodermatosis characterized by blistering of the skin and pyloric occlusion. The lethal form of junctional epidermolysis bullosa with pyloric atresia has been mainly associated with the presence of premature termination codons in the mRNA encoding either the alpha6 or beta4 subunit causing rapid decay of the mutated transcript and absence of alpha6beta4 integrin. In this study, we disclose the genetic background of lethal junctional epidermolysis bullosa with pyloric atresia in a patient presenting absent expression of alpha6 integrin despite normal steady-state level of the alpha6beta4 mRNA. Screening for mutation in the alpha6 gene detected a homozygous base pair substitution (286 C-to-T), which results in the substitution of a serine with a leucine residue (S47L). The amino acid substitution S47L localizes in the first beta-strand of the seven-bladed beta-propeller structure of the extracellular head of alpha6 integrin, and triggers a rapid proteolysis of the aberrant polypeptides involving the lysosomal degradation pathway. This study provides new insight into the pathogenic effect of a mis-sense mutation affecting a functional domain of a protein, and identifies a critical peptide sequence of the beta-propeller domain conserved among the alpha integrin cell receptors.

Expression of plectin in muscle fibers with cytoarchitectural abnormalities

Plectin is a protein belonging to the cytoskeletal anchoring system, concentrated at sites of mechanical stress in different cell types. In normal skeletal muscle, plectin is located at level of Z-discs, sarcolemma, post-synaptic membrane, and intermyofibrillar network. We investigated plectin immunocytochemistry in lobulated fibers, fibers with tubular aggregates, target fibers, central core disease and centronuclear myopathy. Thirty to forty percent of lobulated fibers had patchy increase of plectin immunoreactivity at sarcolemmal level with focal subsarcolemmal increases. Tubular aggregates revealed a low binding for plectin. Ten percent of central cores exhibited faint focal increase of plectin immunoreactivity. Target formations had a normal plectin pattern. In centronuclear myopathy, plectin immunoreactivity was increased around the centrally located nuclei in 8-12% of the fibers, at the sarcolemma of 50% of type 2 fibers, and at the membrane of small vacuoles located peripherally around the central nuclei. We postulate that plectin may play a role in the subsarcolemmal aggregation of mitochondria in the lobulated fibers, and in the central position of nuclei as well as in shape formation, positioning and moving of the vacuoles in centronuclear myopathy.

Prenatal diagnosis for epidermolysis bullosa: a study of 144 consecutive pregnancies at risk

Epidermolysis bullosa (EB) is a group of inherited disorders characterized by increased skin fragility, resulting in blisters and erosions after minor trauma. Mutations in 10 structural genes expressed in the cutaneous basement membrane zone have been reported. The DebRA Molecular Diagnostics Laboratory at Jefferson Medical College has performed 144 DNA-based prenatal diagnoses since 1993 in families at risk for recurrence of the most severe forms of EB, including the recessive dystrophic EB (RDEB), junctional EB (JEB), EB with pyloric atresia (EB-PA), and EB simplex (EBS). A mutation-detection strategy using either conformation-sensitive gel electrophoresis (CSGE) or denaturing high-performance liquid chromatography (dHPLC) scanning analysis, followed by nucleotide sequencing, was applied to most cases with DEB and to all JEB, EB-PA, and EBS families. For some RDEB families, linkage analysis was performed, either alone when the inheritance pattern was clear or in combination with one mutation. Among the 144 prenatal diagnoses, 63 were for RDEB, 69 for JEB, 6 for EB-PA, and 6 for EBS. Twenty-eight normal, 73 heterozygous carrier, and 28 affected RDEB, JEB, and EB-PA pregnancies were reported in these recessively inherited diseases. Two affected and four normal pregnancies were predicted in dominantly inherited EBS. Among the 144 pregnancies, 9 were terminated without confirmation, 13 cases were lost to follow-up, and 6 pregnancies are ongoing. There were 6 families with inconclusive results due either to recombination events between flanking markers, absence of informative markers for one allele, or lack of sample from the previously affected child. There were three discordant results, one that was explained by maternal contamination of the chorionic villus sample and two that were unresolved. Overall, the availability, relative ease, and over 98% success rate make molecular DNA-based prenatal diagnosis a viable option for EB families at risk.

Acute effects of desmin mutations on cytoskeletal and cellular integrity in cardiac myocytes

Mutations in desmin have been associated with a subset of human myopathies. Symptoms typically appear in the second to third decades of life, but in the most severe cases can manifest themselves earlier. How desmin mutations lead to aberrant muscle function, however, remains poorly defined. We created a series of four mutations in rat desmin and tested their in vitro filament assembly properties. RDM-G, a chimera between desmin and green fluorescent protein, formed protofilament-like structures in vitro. RDM-1 and RDM-2 blocked in vitro assembly at the unit-length filament stage, while RDM-3 had more subtle effects on assembly. When expressed in cultured rat neonatal cardiac myocytes via adenovirus infection, these mutant proteins disrupted the endogenous desmin filament to an extent that correlated with their defects in in vitro assembly properties. Disruption of the desmin network by RDM-1 was also associated with disruption of plectin, myosin, and alpha-actinin organization in a significant percentage of infected cells. In contrast, expression of RDM-2, which is similar to previously characterized human mutant desmins, took longer to disrupt desmin and plectin organization and had no significant effect on myosin or alpha-actinin organization over the 5-day time course of our studies. RDM-3 had the mildest effect on in vitro assembly and no discernable effect on either desmin, plectin, myosin, or alpha-actinin organization in vivo. These results indicate that mutations in desmin have both direct and indirect effects on the cytoarchitecture of cardiac myocytes.

Plectin tethers desmin intermediate filaments onto subsarcolemmal dense plaques containing dystrophin and vinculin

Plectin is a versatile cytoskeletal linker protein that preferentially localizes at interfaces between intermediate filaments and the plasma membrane in muscle, epithelial cells, and other tissues. Its deficiency causes muscular dystrophy with epidermolysis bullosa simplex. To better understand the functional roles of plectin beneath the sarcolemma of skeletal muscles and to gain some insights into the underlying mechanism of plectin-deficient muscular dystrophy, we studied in vivo structural and molecular relationships of plectin to subsarcolemmal cytoskeletal components, such as desmin, dystrophin, and vinculin, in rat skeletal muscles. Immunogold electron microscopy revealed that plectin fine threads tethered desmin intermediate filaments onto subsarcolemmal dense plaques overlying Z-lines and I-bands. These dense plaques were found to contain dystrophin and vinculin, and thus may be the structural basis of costameres. The in vivo association of plectin with desmin, (meta-)vinculin, dystrophin, and actin was demonstrated by immunoprecipitation experiments. Treatment of plectin immunoprecipitates with gelsolin reduced actin, dystrophin, and (meta-)vinculin but not desmin, implicating that subsarcolemmal actin could partly mediate the interaction between plectin and dystrophin or (meta-)vinculin. Altogether, our data suggest that plectin, along with desmin intermediate filaments, might serve a vital structural role in the stabilization of the subsarcolemmal cytoskeleton.

Congenital myopathy, recurrent secretory diarrhea, bullous eruption of skin, microcephaly, and deafness: a new genetic syndrome?

We describe three siblings with congenital myopathy, bullous eruption of the skin, secretory diarrhea, apparent zinc deficiency, failure to thrive, deafness, and microcephaly. The parents are not consanguineous and there are no other affected relatives. This new syndrome, which follows an apparent autosomal recessive pattern, appears to be distinct from known syndromes of secretory diarrhea, myopathy, deafness, microcephaly, and zinc deficiency.

Epidermolysis bullosa: new and emerging trends

Epidermolysis bullosa is a family of inherited blistering skin disorders characterized by blister formation in response to mechanical trauma. Major types of epidermolysis bullosa include epidermolysis bullosa simplex, hemidesmosomal epidermolysis bullosa, junctional epidermolysis bullosa, and dystrophic epidermolysis bullosa. Current treatment for epidermolysis bullosa consists of supportive care for skin and other organ systems and entails a combination of wound management, infection support for chronic wounds, surgical management as needed, nutritional support, and preventative screening for squamous cell carcinoma in recessive dystrophic epidermolysis bullosa. The regimen must be tailored specifically to the severity and extent of skin and systemic involvement in each case. Recent studies have identified specific protein and genetic abnormalities for most epidermolysis bullosa subtypes. These new advancements in the understanding of molecular pathophysiology have provided much of the basis for current efforts to develop effective gene and protein therapy for epidermolysis bullosa.

Differentiation potential of primary myogenic cells derived from skeletal muscle of dystonia musculorum mice

The dystonia musculorum (dt) mouse has a mutation in the gene encoding the cytoskeletal crosslinker protein bullous pemphigoid antigen 1 (Bpag1). These mice have perturbations in the cytoarchitecture of skeletal muscle. Bpag1 has been hypothesized to be involved in the maintenance rather than the establishment of the muscle cell architecture given that cytoskeletal disruptions are observed in the muscle tissue of post-natal dt mice. Not known is whether Bpag1-deficiency affects the proliferative and differentiation potential of myogenic cells. In the present investigation, we show that the growth rate of cultured primary myogenic cells derived from dt mice, as assessed by BrdU incorporation, is similar to that of myogenic cells derived from wild-type littermates. The myogenic differentiation potential of dt versus wild-type cells was monitored by examining the expression of myosin heavy chain by immunofluorescence, and by analyzing the expression profiles of myogenic regulatory factors and myogenic differentiation markers by RT-PCR. In all instances, both dt and wild-type myogenic cells displayed a similar differentiation profile. Furthermore, the absence of any observable differences in the proliferation and differentiation rates of dt and wild-type cells was not due to an overexpression of plectin, another crosslinker protein, in dt cells. Together, these findings demonstrate that the early phases of myogenic differentiation occur independently of Bpag1.

A site-specific plectin mutation causes dominant epidermolysis bullosa simplex Ogna: two identical de novo mutations

Plectin is one of the largest and most versatile cytolinker proteins known. In basal keratinocytes it links the intermediate filament network to cell membrane-associated hemidesmosomes. Several mutations in its gene have been identified that lead to the recessive disease epidermolysis bullosa with muscular dystrophy. We report here a mutation that leads to a dominant form of the disease, epidermolysis bullosa simplex Ogna. We found that the epidermolysis bullosa simplex Ogna phenotype is due to a site-specific missense mutation within plectin's rod domain. Further, we show that epidermolysis bullosa simplex Ogna is not restricted to a single Norwegian kindred as previously believed. A German family with the phenotypic hallmarks of epidermolysis bullosa simplex Ogna was found to carry an identical de novo mutation. These two mutations arose about 200 y apart in time. Consistent with the absence of muscular symptoms in these patients, muscle biopsies from several epidermolysis bullosa simplex Ogna members of the Norwegian kindred showed normal staining patterns using antibodies to plectin. Skin changes in epidermolysis bullosa simplex Ogna patients are documented on the ultrastructural level.

Cutting edge: integration of human T lymphocyte cytoskeleton by the cytolinker plectin

Chemokine-induced polarization of lymphocytes involves the rapid collapse of vimentin intermediate filaments (IFs) into an aggregate within the uropod. Little is known about the interactions of lymphocyte vimentin with other cytoskeletal elements. We demonstrate that human peripheral blood T lymphocytes express plectin, an IF-binding, cytoskeletal cross-linking protein. Plectin associates with a complex of structural proteins including vimentin, actin, fodrin, moesin, and lamin B in resting peripheral blood T lymphocytes. During chemokine-induced polarization, plectin redistributes to the uropod associated with vimentin and fodrin; their spatial distribution indicates that this vimentin-plectin-fodrin complex provides a continuous linkage from the nucleus (lamin B) to the cortical cytoskeleton. Overexpression of the plectin IF-binding domain in the T cell line Jurkat induces the perinuclear aggregation of vimentin IFs. Plectin is therefore likely to serve as an important organizer of the lymphocyte cytoskeleton and may regulate changes of lymphocyte cytoarchitecture during polarization and extravasation.

Intermediate filament-related myopathies

The dynamic and critical role of intermediate filaments in muscle is highlighted by myopathies characterized by aberrant accumulation of intermediate filaments. In some affected patients, mutations in genes encoding intermediate filaments that are expressed in muscle have been confirmed. The importance of intermediate filaments in muscle is further strengthened by murine models in which genetically designed intermediate filament mutations are expressed, leading to progressive skeletal or cardioskeletal myopathy in affected mice. In this article the intermediate filaments expressed in muscle are reviewed, and the clinical and pathologic features of myopathies known to relate to intermediate filaments are described. With the increasing awareness of intermediate filaments in muscle and the rapid advances in genetic investigation, it is likely that the list of intermediate filament-related myopathies will expand.

Cadherins and catenins, Wnts and SOXs: embryonic patterning in Xenopus

Wnt signaling plays a critical role in a wide range of developmental and oncogenic processes. Altered gene regulation by the canonical Wnt signaling pathway involves the cytoplasmic stabilization of beta-catenin, a protein critical to the assembly of cadherin-based cell-cell adherence junctions. In addition to binding to cadherins, beta-catenin also interacts with transcription factors of the TCF-subfamily of HMG box proteins and regulates their activity. The Xenopus embryo has proven to be a particularly powerful experimental system in which to study the role of Wnt signaling components in development and differentiation. We review this literature, focusing on the role of Wnt signaling and interacting components in establishing patterns within the early embryo.

Desmin-related myopathies in mice and man

Desmin, the main intermediate filament (IF) protein in skeletal and heart muscle cells, is of great importance as a part of the cytoskeleton. The IFs surround and interlink myofibrils, and connect the peripheral myofibrils with the sarcolemma. In myotendinous junctions and neuromuscular junctions of skeletal muscle fibres, desmin is enriched. In the heart, desmin is increased at intercalated discs, the attachment between cardiomyocytes, and it is the main component in Purkinje fibres of the conduction system. Desmin is the first muscle-specific protein to appear during myogenesis. Nevertheless, lack of desmin, as shown from experiments with desmin knockout (K/O) mice, does not influence myogenesis or myofibrillogenesis. However, the desmin knock-out mice postnatally develop a cardiomyopathy and a muscle dystrophy in highly used skeletal muscles. In other skeletal muscles the organization of myofibrils is remarkably unaffected. Thus, the main consequence of the lack of desmin is that the muscle fibres become more susceptible to damage. The loss of membrane integrity leads to a dystrophic process, with degeneration and fibrosis. In the heart cardiac failure develops, whereas in affected skeletal muscles regenerative attempts are seen. In humans, accumulations of desmin have been a hallmark for presumptive desmin myopathies. Recent investigations have shown that some families with such a myopathy have a defect in the gene coding for alphaB-crystallin, whereas others have mutations in the desmin gene. Typical features of these patients are cardiac affections and muscle weakness. Thus, mutations in the desmin gene is pathogenic for a distinct type of muscle disorder.

Cultured keratinocytes from plectin/HD1-deficient epidermolysis bullosa simplex showed altered ability of adhesion to the matrix

Epidermolysis bullosa simplex associated with late onset of muscular dystrophy has been found to show defective expression of plectin, an intracytoplasmic protein in hemidesmosomes. In this report, we examined ability of cell-to-matrix attachment of cultured keratinocytes derived from a case with this disease by various cell biological methods, and compared it to that of normal keratinocytes. In cell adhesion assay, the patient keratinocytes showed more prominent short-time cell adhesion than normal keratinocytes. In contrast, the patient keratinocytes could be detached much easier than normal keratinocytes in cell detachment assay by treatment with dispase. In phagokinetic track assay, no apparent difference of cell migration was observed between the patient and normal keratinocytes. These results indicate that plectin-deficiency may up-regulate short-term cell contact and reduce stable cell-matrix adhesion at the epidermal basement membrane zone.

Laminins and human disease

The laminin protein family has diverse tissue expression patterns and is involved in the pathology of a number of organs, including skin, muscle, and nerve. In the skin, laminins 5 and 6 contribute to dermal-epidermal cohesion, and mutations in the constituent chains result in the blistering phenotype observed in patients with junctional epidermolysis bullosa (JEB). Allelic heterogeneity is observed in patients with JEB: mutations that results in premature stop codons produce a more severe phenotype than do missense mutations. Gene therapy approaches are currently being studied in the treatment of this disease. A blistering phenotype is also observed in patients with acquired cicatricial pemphigoid (CP). Autoantibodies targeted against laminins 5 and 6 destabilize epithelial adhesion and are pathogenic. In muscle cells, laminin alpha 2 is a component of the bridge that links the actin cytoskeleton to the extracellular matrix. In patients with laminin alpha 2 mutations, the bridge is disrupted and mature muscle cells apoptose. Congenital muscular dystrophy (CMD) results. The role of laminin in diseases of the nervous system is less well defined, but the extracellular protein has been shown to serve an important role in peripheral nerve regeneration. The adhesive molecule influences neurite outgrowth, neural differentiation, and synapse formation. The broad spatial distribution of laminin gene products suggests that laminin may be involved in a number of diseases for which pathogenic mechanisms are still being unraveled.

Progress in desmin-related myopathies

Desmin-related myopathies are sporadic and familial neuromuscular conditions of considerable clinical heterogeneity uniformly marked by the pathologic accretion of desmin, often in a filamentous fashion. A large variety of other proteins, some of them cytoskeletal, also accrue. Morphologically, two types may be distinguished, one characterized by inclusions such as cytoplasmic and spheroid bodies or desmin-dystrophin plaques and another marked by granulofilamentous material. The genetic spectrum of desmin-related myopathies is quite diverse in that missense mutations and deletions in the desmin gene and a missense mutation in the alpha-B crystallin gene have been detected and several genes on other chromosomes have been mapped; the encoded protein products of these genes, however, are unknown. Accumulation of desmin and other proteins appears to be due to impaired nonlysosomal proteolysis. Mutant desmin that appears to be hyperphosphorylated seems to act as a seed protein for filament aggregation, inducing formation of inclusions and granulofilamentous material in these conditions. This condition is part of the group of disorders known as "surplus protein myopathies."

Association of plectin with Z-discs is a prerequisite for the formation of the intermyofibrillar desmin cytoskeleton

Plectin is a high-molecular mass protein (approximately 500 kd) that binds actin, intermediate filaments, and microtubules. Mutations of the plectin gene cause a generalized blistering skin disorder and muscular dystrophy. In adult muscle, plectin is colocalized with desmin at structures forming the intermyofibrillar scaffold and beneath the plasma membrane. To study the involvement of plectin in myofibrillogenesis, we analyzed the spatial and temporal expression patterns of plectin in cultured differentiating human skeletal muscle cells and its relationship to desmin intermediate filaments during this process. Northern and Western blot analyses demonstrated that at least two different plectin isoforms are expressed at all developmental stages from proliferating myoblasts to mature myotubes. Using immunocytochemistry, we show that the localization of plectin dramatically changes from a network-like distribution into a cross-striated distribution during maturation of myocytes. Double immunofluorescence experiments revealed that desmin and plectin are colocalized in premyofibrillar stages and in mature myotubes. Interestingly, plectin was often found to localize to the periphery of Z-discs during the actual alignment of neighboring myofibrils, and an obvious cross-striated plectin staining pattern was observed before desmin was localized in the Z-disc region. We conclude that the association of plectin with Z-discs is an early event in the lateral alignment of myofibrils that precedes the formation of the intermyofibrillar desmin cytoskeleton.