The BPAG1 locus: Alternative splicing produces multiple isoforms with distinct cytoskeletal linker domains, including predominant isoforms in neurons and muscles

BPAG1a and b associate with EB1 and EB3 and modulate vesicular transport, Golgi apparatus structure, and cell migration in C2.7 myoblasts

BPAG1a and BPAG1b (BPAG1a/b) constitute two major isoforms encoded by the dystonin (Dst) gene and show homology with MACF1a and MACF1b. These proteins are members of the plakin family, giant multi-modular proteins able to connect the intermediate filament, microtubule and microfilament cytoskeletal networks with each other and to distinct cell membrane sites. They also serve as scaffolds for signaling proteins that modulate cytoskeletal dynamics. To gain better insights into the functions of BPAG1a/b, we further characterized their C-terminal region important for their interaction with microtubules and assessed the role of these isoforms in the cytoskeletal organization of C2.7 myoblast cells. Our results show that alternative splicing does not only occur at the 5′ end of Dst and Macf1 pre-mRNAs, as previously reported, but also at their 3′ end, resulting in expression of additional four mRNA variants of BPAG1 and MACF1. These isoform-specific C-tails were able to bundle microtubules and bound to both EB1 and EB3, two microtubule plus end proteins. In the C2.7 cell line, knockdown of BPAG1a/b had no major effect on the organization of the microtubule and microfilament networks, but negatively affected endocytosis and maintenance of the Golgi apparatus structure, which became dispersed. Finally, knockdown of BPAG1a/b caused a specific decrease in the directness of cell migration, but did not impair initial cell adhesion. These data provide novel insights into the complexity of alternative splicing of Dst pre-mRNAs and into the role of BPAG1a/b in vesicular transport, Golgi apparatus structure as well as in migration in C2.7 myoblasts.

Focal Contact and Hemidesmosomal Proteins in Keratinocyte Migration and Wound Repair

Significance: During wound healing of the skin, keratinocytes should move over while still adhering to their underlying matrix. Thus, mechanistic insights into the wound-healing process require an understanding of the forms and functions of keratinocyte matrix adhesions, specifically focal contacts and hemidesmosomes, and their components. Recent Advances: Although the structure and composition of focal contacts and hemidesmosomes are relatively well defined, the functions of their components are only now being delineated using mouse genetic models and knockdown approaches in cell culture systems. Remarkably, both focal contact and hemidesmosomal proteins appear involved in determining the speed and directional migration of epidermal cells by modulating several signal transduction pathways. Critical Issues: Although many publications are centered on focal contacts, their existence in tissues such as the skin is controversial. Nonetheless, focal contact proteins are central to mechanisms that regulate skin cell motility. Conversely, hemidesmosomes have been identified in intact skin but whether hemidesmosomal components play a positive regulatory function in keratinocyte motility remains debated in the field. Future Directions: Defective wound healing is a developing problem in the aged, hospitalized and diabetic populations. Hence, deriving new insights into the molecular roles of matrix adhesion proteins in wound healing is a prerequisite to the development of novel therapeutics to enhance tissue repair and regeneration.

Plakins, a versatile family of cytolinkers: roles in skin integrity and in human diseases

The plakin family consists of giant proteins involved in the cross-linking and organization of the cytoskeleton and adhesion complexes. They further modulate several fundamental biological processes, such as cell adhesion, migration, and polarization or signaling pathways. Inherited and acquired defects of plakins in humans and in animal models potentially lead to dramatic manifestations in the skin, striated muscles, and/or nervous system. These observations unequivocally demonstrate the key role of plakins in the maintenance of tissue integrity. Here we review the characteristics of the mammalian plakin members BPAG1 (bullous pemphigoid antigen 1), desmoplakin, plectin, envoplakin, epiplakin, MACF1 (microtubule-actin cross-linking factor 1), and periplakin, highlighting their role in skin homeostasis and diseases.

BPAG1-e restricts keratinocyte migration through control of adhesion stability

Bullous pemphigoid antigen 1 (BPAG1-e, also known as BP230) is a member of the plakin family of hemidesmosome cytoskeletal linker proteins that is encoded by an isoform of the dystonin (DST) gene. Recently, we reported two unrelated families with homozygous nonsense mutations in this DST isoform that led to ultrastructural loss of hemidesmosomal inner plaques and clinical features of trauma-induced skin fragility. We now demonstrate that keratinocytes isolated from these individuals have significant defects in adhesion, as well as increased cell spreading and migration. These mutant keratinocytes also display reduced levels of β4 integrins at the cell surface but increased total protein levels of keratin-14 and β1 integrins. These alterations in cell behavior and protein expression were not seen in control keratinocytes in which BPAG1-e expression had been silenced by stable expression of short hairpin RNA to target DST. The failure of knockdown approaches to recapitulate the changes in morphology, adhesion, and migration seen in patient cells therefore suggests such approaches are not appropriate to study loss of this protein in vivo. The contrasting findings in keratinocytes harboring naturally occurring mutations, however, demonstrate a previously unappreciated key role for BPAG1-e in regulating keratinocyte adhesion and migration and suggest a requirement for this protein in controlling functional switching between integrin types in epithelial cells.

Using fly genetics to dissect the cytoskeletal machinery of neurons during axonal growth and maintenance

The extension of long slender axons is a key process of neuronal circuit formation, both during brain development and regeneration. For this, growth cones at the tips of axons are guided towards their correct target cells by signals. Growth cone behaviour downstream of these signals is implemented by their actin and microtubule cytoskeleton. In the first part of this Commentary, we discuss the fundamental roles of the cytoskeleton during axon growth. We present the various classes of actin- and microtubule-binding proteins that regulate the cytoskeleton, and highlight the important gaps in our understanding of how these proteins functionally integrate into the complex machinery that implements growth cone behaviour. Deciphering such machinery requires multidisciplinary approaches, including genetics and the use of simple model organisms. In the second part of this Commentary, we discuss how the application of combinatorial genetics in the versatile genetic model organism Drosophila melanogaster has started to contribute to the understanding of actin and microtubule regulation during axon growth. Using the example of dystonin-linked neuron degeneration, we explain how knowledge acquired by studying axonal growth in flies can also deliver new understanding in other aspects of neuron biology, such as axon maintenance in higher animals and humans.

Immunopathological characteristics of patients with bullous pemphigoid and neurological disease

BACKGROUND

The relationship between bullous pemphigoid (BP) and neurological disease has been the subject of numerous recent studies and BP antigens and their isoforms have been identified in the central nervous system (CNS). Whilst epidemiological data support this association, little is known about the pathomechanism behind this link and the immunological characteristics of patients with BP and neurological disease, other than multiple sclerosis (MS), has not been studied.

OBJECTIVE

We aimed to compare the cutaneous immune response in BP patients with and without neurological disease, to investigate whether or not there is a distinctive immunopathological profile in patients with concomitant BP and neurological disease.

METHODS

Seventy-two patients with BP were included and divided into two groups; those with neurological disease (BP+N, n = 43) and those without (BP-N, n = 29). Patients in BP+N group had a confirmed neurological disease by a hospital physician, neurologist or psychiatrist with positive neurological imaging where appropriate, or a Karnofsky score of 50 or less due to mental impairment. All sera were analysed with indirect immunofluorescence (IIF) using serial dilutions up to 1:120000, immunoblotting (IB) and Enzyme-linked immunosorbent assay (ELISA) for BP180 and BP230.

RESULTS

Median antibody titres by IIF were 1:1600 vs. 1:800 for BP-N and BP+N, respectively, although the difference did not reach statistic significance (P = 0.93, Mann-Whitney U-test). ELISA values for both BP180 and BP230 did not differ significantly between the two groups. Similarly, autoantibodies to specific antigens as identified by ELISA and IB were not related to the presence of neurological disease.

CONCLUSION

The results of this study indicate that patients with BP and neurological disease exhibit an immune response to both BP180 and BP230, thus the link between the CNS and the skin is not dependent on a specific antigen, but possibly both antigens or their isoforms may be exposed following a neurological insult, and play a role in generation of an immune response.

Herpesvirus tegument protein pUL37 interacts with dystonin/BPAG1 to promote capsid transport on microtubules during egress

Herpes simplex virus 1 (HSV-1) is a neurotropic virus that travels long distances through cells using the microtubule network. Its 125-nm-diameter capsid is a large cargo which efficiently recruits molecular motors for movement. Upon entry, capsids reach the centrosome by minus-end-directed transport. From there, they are believed to reach the nucleus by plus-end-directed transport. Plus-end-directed transport is also important during egress, when capsids leave the nucleus to reach the site of envelopment in the cytoplasm. Although capsid interactions with dynein and kinesins have been described in vitro, the actual composition of the cellular machinery recruited by herpesviruses for capsid transport in infected cells remains unknown. Here, we identify the spectraplakin protein, dystonin/BPAG1, an important cytoskeleton cross-linker involved in microtubule-based transport, as a binding partner of the HSV-1 protein pUL37, which has been implicated in capsid transport. Viral replication is delayed in dystonin-depleted cells, and, using video microscopy of living infected cells, we show that dystonin depletion strongly inhibits capsid movement in the cytoplasm during egress. This study provides new insights into the cellular requirements for HSV-1 capsid transport and identifies dystonin as a nonmotor protein part of the transport machinery.

Spectraplakins: master orchestrators of cytoskeletal dynamics

The dynamics of different cytoskeletal networks are coordinated to bring about many fundamental cellular processes, from neuronal pathfinding to cell division. Increasing evidence points to the importance of spectraplakins in integrating cytoskeletal networks. Spectraplakins are evolutionarily conserved giant cytoskeletal cross-linkers, which belong to the spectrin superfamily. Their genes consist of multiple promoters and many exons, yielding a vast array of differential splice forms with distinct functions. Spectraplakins are also unique in their ability to associate with all three elements of the cytoskeleton: F-actin, microtubules, and intermediate filaments. Recent studies have begun to unveil their role in a wide range of processes, from cell migration to tissue integrity.

Untethering the nuclear envelope and cytoskeleton: biologically distinct dystonias arising from a common cellular dysfunction

Most cases of early onset DYT1 dystonia in humans are caused by a GAG deletion in the TOR1A gene leading to loss of a glutamic acid (ΔE) in the torsinA protein, which underlies a movement disorder associated with neuronal dysfunction without apparent neurodegeneration. Mutation/deletion of the gene (Dst) encoding dystonin in mice results in a dystonic movement disorder termed dystonia musculorum, which resembles aspects of dystonia in humans. While torsinA and dystonin proteins do not share modular domain architecture, they participate in a similar function by modulating a structural link between the nuclear envelope and the cytoskeleton in neuronal cells. We suggest that through a shared interaction with the nuclear envelope protein nesprin-3α, torsinA and the neuronal dystonin-a2 isoform comprise a bridge complex between the outer nuclear membrane and the cytoskeleton, which is critical for some aspects of neuronal development and function. Elucidation of the overlapping roles of torsinA and dystonin-a2 in nuclear/endoplasmic reticulum dynamics should provide insights into the cellular mechanisms underlying the dystonic phenotype.

Clinical features and practical diagnosis of bullous pemphigoid

Bullous pemphigoid (BP) represents the most common autoimmune subepidermal blistering disease. BP typically affects the elderly and is associated with significant morbidity. It has usually a chronic course with spontaneous exacerbations. The cutaneous manifestations of BP can be extremely protean. While diagnosis of BP in the bullous stage is straightforward, in the non-bullous stage or in atypical variants of BP signs and symptoms are frequently non-specific with eg, only itchy excoriated, eczematous, papular and/or urticarial lesions that may persist for several weeks or months. Diagnosis of BP critically relies on immunopathologic examinations including direct immunofluorescence microscopy and detection of serum autoantibodies by indirect immunofluorescence microscopy or BP180-ELISA.

Leucine-rich repeat kinase 2 and alternative splicing in Parkinson's disease

Mutations of the leucine-rich repeat kinase 2 (LRRK2) gene are the most common genetic cause of Parkinson's disease (PD) and are associated with pleiomorphic neuropathology. We hypothesize that LRRK2 mediates its pathogenic effect through alternative splicing of neurodegeneration genes. Methods used in this study included western blotting analysis of subcellular protein fractions, exon-array analysis of RNA from cultured neuroblastoma cells transfected with LRRK2 expression vectors, and reverse-transcription polymerase chain reaction (RT-PCR) of RNA from cultured cells and postmortem tissue. Overexpression of the LRRK2 G2019S mutant resulted in a significant (2.6-fold; P = 0.020) decrease in nuclear transactive response DNA-binding protein 43 levels. Exon-array analyses revealed that wild-type LRRK2 had a significant effect on the expression of genes with nuclear (P < 10(-22) ) and cell-cycle functions (P < 10(-15) ). We replicated changes in gene expression in 30% of selected genes by quantitative RT-PCR. Overexpression of LRRK2 resulted in the altered splicing of two genes associated with PD, with an increased inclusion of exon 10 of microtubule-associated protein tau (1.7-fold; P = 0.001) and exon 5 of the alpha-synuclein (SNCA) gene (1.6-fold; P =0.005). Moreover, overexpression of LRRK2 (G2019S) and two mutant genes associated with neurodegeneration, TARDBP (M337V) and FUS (R521H), were associated with decreased inclusion out of the dystonin (DST) 1e precursor exons in SK-N-MC cells. Altered splicing of SNCA (1.9-fold; P < 0.001) and DST genes (log(2) 2.3-fold; P = 0.005) was observed in a cohort of PD, compared with neurologically healthy, brains. This suggests that aberrant RNA metabolism is an important contributor to idiopathic PD.

Autoimmune bullous diseases associations

The presence of one autoimmune disorder helps lead to the discovery of other autoimmune conditions. It is thought that diseases in which autoimmunity is a feature tend to be associated together more often than one can ascribe to chance. A variety of diseases have been implicated in the onset of intraepidermal and subepidermal autoimmune diseases. The presence of one autoimmune disease should alert the physician to watch for a second immunologic disorder. A list of autoimmune bullous diseases associations includes autoimmune bullous diseases, pemphigus, pemphigoid, epidermolysis bullosa acquisita, dermatitis herpetiformis (Duhring), linear immunoglobulin A disease, and multiple autoimmune syndrome.

Microtubule stability, Golgi organization, and transport flux require dystonin-a2-MAP1B interaction

Loss of interaction between the dystonin-a2 isoform and the microtubule-associated protein MAP1B induces microtubule instability and trafficking defects that may underlie certain neuropathies.

Loss of function of dystonin cytoskeletal linker proteins causes neurodegeneration in dystonia musculorum (dt) mutant mice. Although much investigation has focused on understanding dt pathology, the diverse cellular functions of dystonin isoforms remain poorly characterized. In this paper, we highlight novel functions of the dystonin-a2 isoform in mediating microtubule (MT) stability, Golgi organization, and flux through the secretory pathway. Using dystonin mutant mice combined with isoform-specific loss-of-function analysis, we found dystonin-a2 bound to MT-associated protein 1B (MAP1B) in the centrosomal region, where it maintained MT acetylation. In dt neurons, absence of the MAP1B–dystonin-a2 interaction resulted in altered MAP1B perikaryal localization, leading to MT deacetylation and instability. Deacetylated MT accumulation resulted in Golgi fragmentation and prevented anterograde trafficking via motor proteins. Maintenance of MT acetylation through trichostatin A administration or MAP1B overexpression mitigated the observed defect. These cellular aberrations are apparent in prephenotype dorsal root ganglia and primary sensory neurons from dt mice, suggesting they are causal in the disorder.

Neuronal dystonin isoform 2 is a mediator of endoplasmic reticulum structure and function

Dystonin/Bpag1 is a cytoskeletal linker protein whose loss of function in dystonia musculorum (dt) mice results in hereditary sensory neuropathy. Although loss of expression of neuronal dystonin isoforms (dystonin-a1/dystonin-a2) is sufficient to cause dt pathogenesis, the diverging function of each isoform and what pathological mechanisms are activated upon their loss remains unclear. Here we show that dt(27) mice manifest ultrastructural defects at the endoplasmic reticulum (ER) in sensory neurons corresponding to in vivo induction of ER stress proteins. ER stress subsequently leads to sensory neurodegeneration through induction of a proapoptotic caspase cascade. dt sensory neurons display neurodegenerative pathologies, including Ca(2+) dyshomeostasis, unfolded protein response (UPR) induction, caspase activation, and apoptosis. Isoform-specific loss-of-function analysis attributes these neurodegenerative pathologies to specific loss of dystonin-a2. Inhibition of either UPR or caspase signaling promotes the viability of cells deficient in dystonin. This study provides insight into the mechanism of dt neuropathology and proposes a role for dystonin-a2 as a mediator of normal ER structure and function.

Bullous pemphigoid: from the clinic to the bench

Bullous pemphigoid (BP) constitutes the most frequent autoimmune subepidermal blistering disease. It is associated with autoantibodies directed against the BP antigens 180 (BP180, BPAG2) and BP230 (BPAG1-e). The pathogenicity of anti-BP180 antibodies has been convincingly demonstrated in animal models. The clinical features of BP are extremely polymorphous. The diagnosis of BP critically relies on immunopathologic findings. The recent development of novel enzyme-linked immunosorbent assays has allowed the detection of circulating autoantibodies with relatively high sensitivity and specificity. Although potent topical steroids have emerged in the past decade as first-line treatment of BP, management of the disease may be challenging.

Cardiac autoantibodies from patients affected by a new variant of endemic pemphigus foliaceus in Colombia, South America

Several patients affected by a new variant of endemic pemphigus foliaceus in El Bagre, Colombia (El Bagre-EPF) have experienced a sudden death syndrome, including persons below the age of 50. El Bagre-EPF patients share several autoantigens with paraneoplastic pemphigus patients, such as reactivity to plakins. Further, paraneoplastic pemphigus patients have autoantibodies to the heart. Therefore, we tested 15 El Bagre-EPF patients and 15 controls from the endemic area for autoreactivity to heart tissue using direct and indirect immunofluorescence, confocal microscopy, immunohistochemistry, immunoblotting, and immunoelectron microscopy utilizing heart extracts as antigens. We found that 7 of 15 El Bagre patients exhibited a polyclonal immune response to several cell junctions of the heart, often colocalizing with known markers. These colocalizing markers included those for the area composita of the heart, such as anti-desmoplakins I and II; markers for gap junctions, such as connexin 43; markers for tight junctions, such as ezrin and junctional adhesion molecule A; and adherens junctions, such pan-cadherin. We also detected colocalization of the patient antibodies within blood vessels, Purkinje fibers, and cardiac sarcomeres. We conclude that El Bagre-EPF patients display autoreactivity to multiple cardiac epitopes, that this disease may resemble what is found in patients with rheumatic carditis, and further, that the cardiac pathophysiology of this disorder warrants further evaluation.

MAP1B and clathrin are novel interacting partners of the giant cyto-linker dystonin

Dystonin is a large multidomain cytoskeletal-associated protein that plays an essential role in the nervous system. Loss of dystonin results in neuromuscular dysfunction and early death in a mouse mutant called dystonia musculorum. Conserved among related proteins, the plakin domain is a defining feature of all major dystonin isoforms, yet its interactions have not been explored in detail. The purpose of the present study was to identify novel interacting partners of the plakin domain of the neuronal isoform of dystonin (dystonin-a). Newly identified interacting proteins discovered through a pull-down assay were validated using coimmunoprecipitation, coimmunofluorescence, and proximity ligation assays. Microtubule associated protein 1B (MAP1B), a microtubule stabilizing protein, and clathrin heavy chain, the major component of the clathrin triskelion, were identified as interaction partners for dystonin-a. Increased levels of phosphorylated MAP1B suggest a misregulation of MAP1B and a potentially novel component of the dt pathology. This work will further facilitate our understanding of how cytoskeletal proteins can affect and regulate neurodegenerative disorders.

Autoimmune bullous diseases in Austria

Autoimmune bullous diseases (AIBD) are chronic disorders associated with significant morbidity and even mortality, for which the 19 dermatologic departments in Austria apply standard modalities to provide state-of-the-art diagnosis and treatment. Most of the affected individuals are initially treated on an inpatient basis, with follow-up done in specialized outpatient clinics or in private practices. A well-established system of care for AIBD patients is thus available nationwide. Considering the significant morbidity and mortality but also rareness of AIBD, national and international standardization of AIBD administration in registries is a major requirement of further improvement in patient care.

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.

Motor unit abnormalities in Dystonia musculorum mice

Dystonia musculorum (dt) is a mouse inherited sensory neuropathy caused by mutations in the dystonin gene. While the primary pathology lies in the sensory neurons of dt mice, the overt movement disorder suggests motor neurons may also be affected. Here, we report on the contribution of motor neurons to the pathology in dt(27J) mice. Phenotypic dt(27J) mice display reduced alpha motor neuron cell number and eccentric alpha motor nuclei in the ventral horn of the lumbar L1 spinal cord region. A dramatic reduction in the total number of motor axons in the ventral root of postnatal day 15 dt(27J) mice was also evident. Moreover, analysis of the trigeminal nerve of the brainstem showed a 2.4 fold increase in number of degenerating neurons coupled with a decrease in motor neuron number relative to wild type. Aberrant phosphorylation of neurofilaments in the perikaryon region and axonal swellings within the pre-synaptic terminal region of motor neurons were observed. Furthermore, neuromuscular junction staining of dt(27J) mouse extensor digitorum longus and tibialis anterior muscle fibers showed immature endplates and a significant decrease in axon branching compared to wild type littermates. Muscle atrophy was also observed in dt(27J) muscle. Ultrastructure analysis revealed amyelinated motor axons in the ventral root of the spinal nerve, suggesting a possible defect in Schwann cells. Finally, behavioral analysis identified defective motor function in dt(27J) mice. This study reveals neuromuscular defects that likely contribute to the dt(27J) pathology and identifies a critical role for dystonin outside of sensory neurons.

Coordinated interactions between actin and microtubules through crosslinkers in neurite retraction induced by lysophosphatidic acid

Neurite development requires rearrangement of cytoskeletal elements, which are mechanically and functionally integrated with each other. Although the process of how an extracellular signal induces rearrangement of a single element has been closely examined, the mechanisms by which the signal regulates cytoskeletal integration during cell shape changes are poorly understood. We previously reported that lysophosphatidic acid (LPA) induces actin polymerization-dependent microtubule (MT) rearrangement, leading to neurite retraction in cultured neurons. Here we examined whether the crosslinker proteins were involved in LPA-induced neurite retraction using immortalized mouse neuroblast TR cells. When the MT-binding domains of MACF (MT actin-crosslinking factor) were exogenously expressed in TR cells, MTs were found to be stabilized and become resistant to exposure to LPA. On the other hand, expression of MT-associated protein 2c showed no effect on LPA-induced neurite retraction. These findings suggest that MACF is involved in actin-dependent MT rearrangement during LPA-induced neurite retraction.

Nervous-tissue-specific elimination of microtubule-actin crosslinking factor 1a results in multiple developmental defects in the mouse brain

The microtubule-actin crosslinking factor 1 (MACF1) is a ubiquitous cytoskeletal linker protein with multiple spliced isoforms expressed in different tissues. The MACF1a isoform contains microtubule and actin-binding regions and is expressed at high levels in the nervous system. Macf1-/- mice are early embryonic lethal and hence the role of MACF1 in the nervous system could not be determined. We have specifically knocked out MACF1a in the developing mouse nervous system using Cre/loxP technology. Mutant mice died within 24-36h after birth of apparent respiratory distress. Their brains displayed a disorganized cerebral cortex with a mixed layer structure, heterotopia in the pyramidal layer of the hippocampus, disorganized thalamocortical and corticofugal fibers, and aplastic anterior and hippocampal commissures. Embryonic neurons showed a defect in traversing the cortical plate. Our data suggest a critical role for MACF1 in neuronal migration that is dependent on its ability to interact with both microfilaments and microtubules.

Hearts of dystonia musculorum mice display normal morphological and histological features but show signs of cardiac stress

Dystonin is a giant cytoskeletal protein belonging to the plakin protein family and is believed to crosslink the major filament systems in contractile cells. Previous work has demonstrated skeletal muscle defects in dystonin-deficient dystonia musculorum (dt) mice. In this study, we show that the dystonin muscle isoform is localized at the Z-disc, the H zone, the sarcolemma and intercalated discs in cardiac tissue. Based on this localization pattern, we tested whether dystonin-deficiency leads to structural defects in cardiac muscle. Desmin intermediate filament, microfilament, and microtubule subcellular organization appeared normal in dt hearts. Nevertheless, increased transcript levels of atrial natriuretic factor (ANF, 66%) β-myosin heavy chain (beta-MHC, 95%) and decreased levels of sarcoplasmic reticulum calcium pump isoform 2A (SERCA2a, 26%), all signs of cardiac muscle stress, were noted in dt hearts. Hearts from two-week old dt mice were assessed for the presence of morphological and histological alterations. Heart to body weight ratios as well as left ventricular wall thickness and left chamber volume measurements were similar between dt and wild-type control mice. Hearts from dt mice also displayed no signs of fibrosis or calcification. Taken together, our data provide new insights into the intricate structure of the sarcomere by situating dystonin in cardiac muscle fibers and suggest that dystonin does not significantly influence the structural organization of cardiac muscle fibers during early postnatal development.

Role of dermal-epidermal basement membrane zone in skin, cancer, and developmental disorders

The dermal-epidermal basement membrane zone is an important epithelial and stromal interface, consisting of an intricately organized collection of intracellular, transmembrane, and extracellular matrix proteins. The basement membrane zone has several main functions including acting as a permeability barrier, forming an adhesive interface between epithelial cells and the underlying matrix, and controlling cellular organization and differentiation. This article identifies key molecular players of the dermal-epidermal membrane zone, and highlights recent research studies that have identified structural and functional roles of these components in the context of various blistering, neoplastic, and developmental syndromes.

Loss of dystrophin and the microtubule-binding protein ELP-1 causes progressive paralysis and death of adult C. elegans

EMAP-like proteins (ELPs) are conserved microtubule-binding proteins that function during cell division and in the behavior of post-mitotic cells. In Caenorhabditis elegans, ELP-1 is broadly expressed in many cells and tissues including the touch receptor neurons and body wall muscle. Within muscle, ELP-1 is associated with a microtubule network that is closely opposed to the integrin-based adhesion sites called dense bodies. To examine ELP-1 function, we utilized an elp-1 RNA interference assay and screened for synthetic interactions with mutated adhesion site proteins. We reveal a synthetic lethal relationship between ELP-1 and the dystrophin-like protein, DYS-1. Reduction of ELP-1 in a dystrophin [dys-1(cx18)] mutant results in adult animals with motility defects, splayed and hypercontracted muscle with altered cholinergic signaling. Worms fill with vesicles, become flaccid, and die. We conclude that ELP-1 is a genetic modifier of a C. elegans model of muscular dystrophy.

Mouse ACF7 and drosophila short stop modulate filopodia formation and microtubule organisation during neuronal growth

Spectraplakins are large actin-microtubule linker molecules implicated in various processes, including gastrulation, wound healing, skin blistering and neuronal degeneration. Expression data for the mammalian spectraplakin ACF7 and genetic analyses of the Drosophila spectraplakin Short stop (Shot) suggest an important role during neurogenesis. Using three parallel neuronal culture systems we demonstrate that, like Shot, ACF7 is essential for axon extension and describe, for the first time, their subcellular functions during axonal growth. Firstly, both ACF7 and Shot regulate the organisation of neuronal microtubules, a role dependent on both the F-actin- and microtubule-binding domains. This role in microtubule organisation is probably the key mechanism underlying the roles of Shot and ACF7 in growth cone advance. Secondly, we found a novel role for ACF7 and Shot in regulating the actin cytoskeleton through their ability to control the formation of filopodia. This function in F-actin regulation requires EF-hand motifs and interaction with the translational regulator Krasavietz/eIF5C, indicating that the underlying mechanisms are completely different from those used to control microtubules. Our data provide the basis for the first mechanistic explanation for the role of Shot and ACF7 in the developing nervous system and demonstrate their ability to coordinate the organisation of both actin and microtubule networks during axonal growth.

Dystonin/Bpag1 is a necessary endoplasmic reticulum/nuclear envelope protein in sensory neurons

Dystonin/Bpag1 proteins are cytoskeletal linkers whose loss of function in mice results in a hereditary sensory neuropathy with a progressive loss of limb coordination starting in the second week of life. These mice, named dystonia musculorum (dt), succumb to the disease and die of unknown causes prior to sexual maturity. Previous evidence indicated that cytoskeletal defects in the axon are a primary cause of dt neurodegeneration. However, more recent data suggests that other factors may be equally important contributors to the disease process. In the present study, we demonstrate perikaryal defects in dorsal root ganglion (DRG) neurons at stages preceding the onset of loss of limb coordination in dt mice. Abnormalities include alterations in endoplasmic reticulum (ER) chaperone protein expression, indicative of an ER stress response. Dystonin in sensory neurons localized in association with the ER and nuclear envelope (NE). A fusion protein ofthe dystonin-a2 isoform, which harbors an N-terminal transmembrane domain, associated with and reorganized the ER in cell culture. This isoform also interacts with the NE protein nesprin-3alpha, but not nesprin-3beta. Defects in dt mice, as demonstrated here, may ultimately result in pathogenesis involving ER dysfunction and contribute significantly to the dt phenotype.

Abnormal cellular translocation of alpha-internexin in spinal motor neurons of Dystonia musculorum mice

Dystonia musculorum (dt) is a mutant mouse with hereditary neuropathy where the dysfunction is mainly found in the dorsal root ganglia (DRG) neurons but not in the spinal motor neurons. However, the accumulation of intermediate filament (IF) proteins in the swelling axons of spinal motor neurons could be found in dt/dt mice. In order to understand the pathological role of neuronal IFs in the swelling axons of spinal motor neurons from dt/dt mice, we extensively examined the distribution of neuronal IF proteins. By immunofluorescence staining, our results indicated that alpha-internexin was a major component in the swelling axon and showed abnormal translocation in the nuclei of spinal motor neurons in dt/dt mice. This abnormal translocation of alpha-internexin in the nuclei of spinal motor neurons was also confirmed by Western blotting and immunoelectron microscopy. Instead of the 10-nm filamentous structure, a diffuse immunopositive pattern of alpha-internexin was observed in the nucleus of spinal motor neurons in dt/dt mutants. We further examined the cell death of spinal motor neurons by TUNEL assay, and no TUNEL-positive cells could be identified from spinal motor neurons in dt/dt mice. From these observations we suggest that abnormal accumulation of neuronal IFs in the swelling axons and abnormal translocation of alpha-internexin in the nuclei of the spinal motor neurons from dt/dt mice may not directly cause cell death of the spinal motor neurons.

Dystonin/Bpag1--a link to what?

The dystonin/Bpag1 cytoskeletal interacting proteins play important roles in maintaining cytoarchitecture integrity in skin and in the neuromuscular system. The most profound phenotype observed in the dystonin mutant dystonia musculorum (dt) mice is a severe movement disorder, attributed in large part to sensory neuron degeneration. The molecular basis for this phenotype is currently not clear, despite several studies indicating possible causes for the pathology in dt mice. Complicating the picture of what essential dystonin functions are lost in dt mice is the fact that our understanding of the very nature of what dystonin is has evolved greatly over the past decade. Elucidating the roles of dystonin most relevant to neuronal function and survival should help to shed light on some of the common mechanisms underlying neurodegeneration.

Muscular integrity--a matter of interlinking distinct structures via plectin

Myocytes are characterized by the presence of highly specialized cytoskeletal structures that are part of regularly spaced functional units distributed over long distances. In this chapter we discuss previously published evidence as well as novel data showing that the proper positioning and architecture of Z-disks and of sarcolemma-associated costameric structures are largely dependent on the cytolinker protein plectin and its associated intermediate filament (desmin) cytoskeleton. Deficiency in either plectin or desmin lead to muscular dystrophies of similar pathology. However, while in the absence of plectin, desmin networks collapse and form aggregates, when desmin is missing, plectin retains its typical localization. This suggests that plectin recruits and anchors desmin filaments to both Z-disks and costameres and thus is a key element for maintaining and reinforcing myocyte cytoarchitecture. We hypothesize that as an essential link of the Z-disk-costamere axis, plectin is likely to play also a crucial role in myofiber signaling.

Plakins in development and disease

Plakins are large multi-domain molecules that have various functions to link cytoskeletal elements together and to connect them to junctional complexes. Plakins were first identified in epithelial cells where they were found to connect the intermediate filaments to desmosomes and hemidesmosomes [Ruhrberg, C., and Watt, F.M. (1997). The plakin family: versatile organizers of cytoskeletal architecture. Curr Opin Genet Dev 7, 392-397.]. They were subsequently found to be important for the integrity of muscle cells. Most recently, they have been found in the nervous system, where their functions appear to be more complex, including cross-linking of microtubules (MTs) and actin filaments [Leung, C.L., Zheng, M., Prater, S.M., and Liem, R.K. (2001). The BPAG1 locus: Alternative splicing produces multiple isoforms with distinct cytoskeletal linker domains, including predominant isoforms in neurons and muscles. J Cell Biol 154, 691-697., Leung, C.L., Sun, D., Zheng, M., Knowles, D.R., and Liem, R.K. (1999). Microtubule actin cross-linking factor (MACF): a hybrid of dystonin and dystrophin that can interact with the actin and microtubule cytoskeletons. J Cell Biol 147, 1275-1286.]. These plakins have also indicated their relationship to the spectrin superfamily of proteins and the plakins appear to be evolutionarily related to the spectrins, but have diverged to perform different specialized functions. In invertebrates, a single plakin is present in both Drosophila melanogaster and Caenorhabditis elegans, which resemble the more complex plakins found in mammals [Roper, K., Gregory, S.L., and Brown, N.H. (2002). The 'spectraplakins': cytoskeletal giants with characteristics of both spectrin and plakin families. J Cell Sci 115, 4215-4225.]. In contrast, there are seven plakins found in mammals and most of them have alternatively spliced forms leading to a very complex group of proteins with potential tissue specific functions [Jefferson, J.J., Leung, C.L., and Liem, R.K. (2004). Plakins: goliaths that link cell junctions and the cytoskeleton. Nat Rev Mol Cell Biol 5, 542-553.]. In this review, we will first describe the plakins, desmoplakin, plectin, envoplakin and periplakin and then describe two other mammalian plakins, Bullous pemphigoid antigen 1 (BPAG1) and microtubule actin cross-linking factor 1 (MACF1), that are expressed in multiple isoforms in different tissues. We will also describe the relationship of these two proteins to the invertebrate plakins, shortstop (shot) in Drosophila and VAB-10 in C. elegans. Finally, we will describe an unusual mammalian plakin, called epiplakin.