Expression of CD59, a regulator of the membrane attack complex of complement, on human skeletal muscle fibers

An up-to-date myopathologic characterisation of facioscapulohumeral muscular dystrophy type 1 muscle biopsies shows sarcolemmal complement membrane attack complex deposits and increased skeletal muscle regeneration

The aim of this study was to identify key routinely used myopathologic biomarkers of FSHD1. Needle muscle biopsies were taken in 34 affected muscles (m. quadriceps femoris (QF), n = 20, m. tibialis anterior (TA), n = 13, m. biceps brachii, n = 1) from 22 patients (age, 53.5 (10) years; M = 12, F = 10). Eleven patients had more than one biopsy (2xQF, n = 1; QF+TA, n = 9; 2xQF+TA, n = 1). Histochemistry, immunoperoxidase, and immunofluorescence stainings were performed and compared to age and muscle type matched muscle specimens of 11 healthy controls. Myopathologic features observed in our FSHD1 cohort were internalized nuclei, type 1 fibre hypertrophy and NADH central clearances/cores. We observed a prominent inflammatory response with MAC deposits, MHC I expression, and muscle regeneration that correlated with the inflammatory score. Our up-to-date characterization of FSHD1 points towards MHC I, MAC, and embryonic Myosin Heavy Chain/muscle regeneration as useful myopathologic readouts of FSHD1.

Complement-mediated synapse loss in Alzheimer's disease: mechanisms and involvement of risk factors

The complement system is increasingly recognized as a key player in the synapse loss and cognitive impairments observed in Alzheimer's disease (AD). In particular, the process of complement-dependent synaptic pruning through phagocytosis is over-activated in AD brains, driving detrimental excessive synapse elimination and contributing to synapse loss, which is the strongest neurobiological correlate of cognitive impairments in AD. Herein we review recent advances in characterizing complement-mediated synapse loss in AD, summarize the underlying mechanisms, and discuss the possible involvement of AD risk factors such as aging and various risk genes. We conclude with an overview of key questions that remain to be addressed.

The emerging role of complement in neuromuscular disorders

The complement cascade is a key arm of the immune system that protects the host from exogenous and endogenous toxic stimuli through its ability to potently regulate inflammation, phagocytosis, and cell lysis. Due to recent clinical trial successes and drug approvals for complement inhibitors, there is a resurgence in targeting complement as a therapeutic approach to prevent ongoing tissue destruction in several diseases. In particular, neuromuscular diseases are undergoing a recent focus, with demonstrated links between complement activation and disease pathology. This review aims to provide a comprehensive overview of complement activation and its role during the initiation and progression of neuromuscular disorders including myasthenia gravis, amyotrophic lateral sclerosis, and Duchenne muscular dystrophy. We will review the preclinical and clinical evidence for complement in these diseases, with an emphasis on the complement-targeting drugs in clinical trials for these indications.

Complement Inhibition for the Treatment of Myasthenia Gravis

Generalized myasthenia gravis (gMG) is a rare autoimmune disorder affecting the neuromuscular junction (NMJ). Approximately 80-90% of patients display antibodies directed against the nicotinic acetylcholine receptor (AChR). A major drive of AChR antibody-positive MG pathology is represented by complement activation. The role of the complement cascade has been largely demonstrated in patients and in MG animal models. Complement activation at the NMJ leads to focal lysis of the post-synaptic membrane, disruption of the characteristic folds, and reduction of AChR. Given that the complement system works as an activation cascade, there are many potential targets that can be considered for therapeutic intervention. Preclinical studies have confirmed the efficacy of complement inhibition in ameliorating MG symptoms. Eculizumab, an antibody directed towards C5, has recently been approved for the treatment of AChR antibody-positive gMG. Other complement inhibitors, targeting C5 as well, are currently under phase III study. Complement inhibitors, however, may present prohibitive costs. Therefore, the identification of a subset of patients more or less prone to respond to such therapies would be beneficial. For such purpose, there is a critical need to identify possible biomarkers predictive of therapeutic response, a field not yet sufficiently explored in MG. This review aims to give an overview of the complement cascade involvement in MG, the evolution of complement-inhibiting therapies and possible biomarkers useful to tailor and monitor complement-directed therapies.

Complement Inhibitor Therapy for Myasthenia Gravis

Complement activation as a driver of pathology in myasthenia gravis (MG) has been appreciated for decades. The terminal complement component [membrane attack complex (MAC)] is found at the neuromuscular junctions of patients with MG. Animals with experimental autoimmune MG are dependent predominantly on an active complement system to develop weakness. Mice deficient in intrinsic complement regulatory proteins demonstrate a significant increase in the destruction of the neuromuscular junction. As subtypes of MG have been better defined, it has been appreciated that acetylcholine receptor antibody-positive disease is driven by complement activation. Preclinical assessments have confirmed that complement inhibition would be a viable therapeutic approach. Eculizumab, an antibody directed toward the C5 component of complement, was demonstrated to be effective in a Phase 3 trial with subsequent approval by the Federal Drug Administration of the United States and other worldwide regulatory agencies for its use in acetylcholine receptor antibody-positive MG. Second- and third-generation complement inhibitors are in development and approaching pivotal efficacy evaluations. This review will summarize the history and present the state of knowledge of this new therapeutic modality.

Acetylcholine receptor antibody-mediated animal models of myasthenia gravis and the role of complement

Because of the failure of many promising therapeutics identified in preclinical evaluation, funding sources have established guidelines for increased rigor in animal evaluations. The myasthenia gravis (MG) community of scientists has developed guidelines for preclinical assessment for potential MG treatments. Here, we provide a focused summary of these recommendations and the role of complement in disease development in experimental models of MG.

Myasthenia gravis: the role of complement at the neuromuscular junction

Generalized myasthenia gravis (gMG) is a rare autoimmune disorder characterized by skeletal muscle weakness caused by disrupted neurotransmission at the neuromuscular junction (NMJ). Approximately 74-88% of patients with gMG have acetylcholine receptor (AChR) autoantibodies. Complement plays an important role in innate and antibody-mediated immunity, and activation and amplification of complement results in the formation of membrane attack complexes (MACs), lipophilic proteins that damage cell membranes. The role of complement in gMG has been demonstrated in animal models and patients. Studies in animals lacking specific complement proteins have confirmed that MAC formation is required to induce experimental autoimmune MG (EAMG) and NMJ damage. Complement inhibition in EAMG models can prevent disease induction and reverse its progression. Patients with anti-AChR⁺ MG have autoantibodies and MACs present at NMJs. Damaged NMJs are associated with more severe disease, fewer AChRs, and MACs in synaptic debris. Current MG therapies do not target complement directly. Eculizumab is a humanized monoclonal antibody that inhibits cleavage of complement protein C5, preventing MAC formation. Eculizumab treatment improved symptoms compared with placebo in a phase II study in patients with refractory gMG. Direct complement inhibition could preserve NMJ physiology and muscle function in patients with anti-AChR⁺ gMG.

Low expression of complement inhibitory protein CD59 contributes to humoral autoimmunity against astrocytes

Neuromyelitis optica spectrum disorder is primarily an anti-aquaporin 4 autoantibody-mediated, central nervous system-restricted channelopathy. Patients frequently develop central nervous system-restricted lesions even though autoantigen aquaporin 4 in neuromyelitis optica spectrum disorder is broadly distributed in the central nervous system and peripheral organs. The cause of such tissue-specific immune response remains largely unknown. We confirmed here that CD59, an inhibitory regulator of the complement membrane attack complex, is expressed and co-localized with aquaporin 4 in peripheral organs but is only minimally expressed in astrocytes in the central nervous system. In addition, we further found that CD59 overexpression in mouse brains decreased demyelination, blocked the loss of astrocytes and aquaporin 4, and inhibited membrane attack complex formation and infiltration of inflammatory cells. Inactivation of CD59 in mouse peripheral aquaporin 4-expressing cells and tissues led to complement-dependent cytotoxicity. In accordance with the mouse data, human samples presented higher expression of CD59 in many aquaporin 4-expressing peripheral tissues but not in astrocytes. Silencing or blocking CD59 in aquaporin 4-expressing human tracheal epithelial and skeletal muscle cells induced membrane attack complex formation and cytotoxicity, which suggests a protective role of CD59 in anti-aquaporin 4 antibodies-mediated complement toxicity. Our findings suggest that low CD59 expression in astrocytes may contribute to central nervous system-restricted lesions in neuromyelitis optica spectrum disorder. Restoring CD59 expression in astrocytes may serve as a novel therapeutic target of neuromyelitis optica spectrum disorder.

The role of complement in experimental autoimmune myasthenia gravis

Complement plays an important role in the pathophysiology of experimental autoimmune myasthenia gravis (EAMG). The deposition of IgG at the neuromuscular junction, followed by the activation and observance of C3 at the site, and finally the insertion of the membrane attack complex results in the destruction of the plasma membrane at the neuromuscular junction. Animal models of complement-deficient components show the importance of the mediated lysis in EAMG. These events have regulators that allow for the limitation in the cascade and the ability of the cell to inhibit complement at many places along the pathway. The complement regulatory proteins have many roles in reducing the activation of the complement cascade and the inflammatory pathways. Mice deficient in complement regulatory proteins, decay accelerating factor, and CD59 demonstrate a significant increase in the destruction at the neuromuscular junction. Inhibition of complement-mediated lysis is an attractive therapeutic in MG.

DAF/CD55 and Protectin/CD59 modulate adaptive immunity and disease outcome in experimental autoimmune myasthenia gravis

The role of regulators of complement activity (RCA) involving CD55 and CD59 in the pathogenesis of experimental autoimmune myasthenia gravis (EAMG) remains unclear. CD55 and CD59 restrict complement activation by inhibiting C3/C5 convertases' activities and membrane attack complex formation, respectively. Actively immunized EAMG mice deficient in either CD55 or CD59 showed significant differences in adaptive immune responses and worsened disease outcome associated with increased levels of serum cytokines, modified production of acetylcholine receptor antibodies, and more complement deposition at the neuromuscular junction. We conclude that modulation of complement activity by RCA represents an alternative in controlling of autoimmune processes in EAMG.

Effect of complement and its regulation on myasthenia gravis pathogenesis

Myasthenia gravis (MG) is primarily caused by antibodies directed towards the skeletal muscle acetylcholine receptor, leading to muscle weakness. Although these antibodies may induce compromise of neuromuscular transmission by blocking acetylcholine receptor function or antigenic modulation, the predominant mechanism of injury to the neuromuscular junction is complement-mediated lysis of the postsynaptic membrane. The vast majority of data to support the role of complement derives from experimentally acquired MG (EAMG). In this article, we review studies that demonstrate the central role of complement in EAMG and MG pathogenesis along with the emerging role of complement in T- and B-cell function, as well as the potential for complement inhibitor-based therapy to treat human MG.

Extraocular muscle susceptibility to myasthenia gravis: unique immunological environment?

Extraocular muscle (EOM) is susceptible to neuromuscular junction disorders, in particular, myasthenia gravis (MG). While EOM physiological characteristics and the ocular motor system requirements contribute to the propensity of ocular motor deficits observed among patients with MG, the authors propose that EOM have immunological features that place the muscles at risk for immune attack. Genomic profiling studies have demonstrated that genes associated with the immune response are differentially expressed in EOM, with particular differences in both classical and alternative complement-mediated immune response pathways. Intrinsic complement regulators are expressed at lower levels at rodent EOM neuromuscular junctions, which would put them at risk for the complement-mediated injury that occurs in MG. In fact, systemic C inhibition in experimental autoimmune MG (EAMG) induced by administration of acetylcholine receptor (AChR) antibodies or immunization with AChR will eliminate complement deposition at junctions of other skeletal muscle, but not EOM. Also, EOM junctions have greater injury in active and passive EAMG by several measures, suggesting that the lack of complement inhibition puts the EOM at risk. Among ocular myasthenia patients, serum AChR antibody levels are low, which would support the concept that EOM junctions are more susceptible to antibody injury than are other junctions. These observations suggest that complement inhibitory therapies may prove to be particularly effective in treatment of ocular myasthenia.

Myasthenia gravis thymus: complement vulnerability of epithelial and myoid cells, complement attack on them, and correlations with autoantibody status

In early-onset myasthenia gravis, the thymus contains lymph node-type infiltrates with frequent acetylcholine receptor (AChR)-specific germinal centers. Our recent evidence/two-step hypothesis implicates hyperplastic medullary thymic epithelial cells (expressing isolated AChR subunits) in provoking infiltration and thymic myoid cells (with intact AChR) in germinal center formation. To test this, we screened for complement attack in a wide range of typical generalized myasthenia patients. Regardless of the exact serology, thymi with sizeable infiltrates unexpectedly showed patchy up-regulation of both C5a receptor and terminal complement regulator CD59 on hyperplastic epithelial cells. These latter also showed deposits of activated C3b complement component, which appeared even heavier on infiltrating B cells, macrophages, and especially follicular dendritic cells. Myoid cells appeared particularly vulnerable to complement; few expressed the early complement regulators CD55, CD46, or CR1, and none were detectably CD59(+). Indeed, when exposed to infiltrates, and especially to germinal centers, myoid cells frequently labeled for C1q, C3b (25 to 48%), or even the terminal C9, with some showing obvious damage. This early/persistent complement attack on both epithelial and myoid cells strongly supports our hypothesis, especially implicating exposed myoid cells in germinal center formation/autoantibody diversification. Remarkably, the similar changes place many apparent AChR-seronegative patients in the same spectrum as the AChR-seropositive patients.

The difficult-to-cultivate coxsackieviruses A can productively multiply in primary culture of mouse skeletal muscle

Coxsackieviruses A (CVA) are associated with several clinical manifestations such as aseptic meningitis and paralytic syndromes in humans. Most CVA are difficult-to-cultivate, which impedes their propagation and isolation from clinical material. Here, we tested the ability of cultivable (CVA-13, CVA-14), and difficult-to-cultivate (CVA-6, CVA-22) strains to infect primary cultures of skeletal muscle cells established from newborn mice. We found that such cultures sustained the multiplication of these CVA, as evidenced by the development of a cytopathic effect, already in the initial preparation or after passaging once. Cultures established for no more than 24h were sensitive to infection whereas older preparations were resistant. Using confocal microscopy after double-immunolabeling of the VP1 capsid protein and the muscle cell marker myosin, we demonstrated that only the myoblasts were infected, resulting in VP1 expression throughout their cytoplasm. Inoculation of infected cultures to suckling mice resulted in paralysis indicating that infection was productive. The nature of candidate receptors for virus entry in such cultures and the influence of cell culture conditions on the expression of these putative receptors are discussed. This work suggests that primary cultures of skeletal muscle cells could be used to propagate and isolate any CVA strain.

Extraocular muscles have fundamentally distinct properties that make them selectively vulnerable to certain disorders

While skeletal muscles generally perform specific limited roles, extraocular muscles (EOMs) have to be responsive over a wider dynamic range. As a result, EOMs have fundamentally distinct structural, functional, biochemical and immunological properties compared to other skeletal muscles. While these properties enable high fatigue resistance and the rapid and precise control of extraocular motility, they might also explain why EOMs are selectively involved in certain disorders, such as chronic progressive external ophthalmoplegia (CPEO), myasthenia gravis and Graves' ophthalmopathy. This review first gives an overview of the novel myofibre classification in EOMs and then focuses on those properties that might explain why ophthalmoplegia should be so prominent in these disorders.

Isoform-specific expression of the Coxsackie and adenovirus receptor (CAR) in neuromuscular junction and cardiac intercalated discs

Background

The Coxsackie and adenovirus receptor (CAR) has a restricted expression pattern in the adult. In skeletal muscle, although CAR is expressed in immature fibers, its transcript levels are barely detectable in mature muscle. This is in contrast to the robust expression observed in the heart. However, both heart and skeletal muscle are susceptible to infection with the Coxsackie B virus which utilizes primarily CAR for cellular internalization. The specific point of viral entry in skeletal and heart muscle remains unknown.

Results

Using antibodies directed against the extracellular and the cytoplasmic domains of CAR, we show CAR in normal human and mouse skeletal muscle to be a novel component of the neuromuscular junction. In cardiac muscle, CAR immunoreactivity is observed at the level of intercalated discs. We demonstrate a single isoform of CAR to be expressed exclusively at the human neuromuscular junction whereas both predominant CAR isoforms are expressed at the intercalated discs of non-diseased human heart.

Conclusion

The localization of CAR to these important junctional complexes suggests that CAR may play both a structural and a regulatory role in skeletal and cardiac muscle, and that these complexes may serve as a point of entry for Coxsackie B virus.

Update on juvenile dermatomyositis: new advances in understanding its etiopathogenesis

PURPOSE OF REVIEW

Juvenile dermatomyositis is the most common of the idiopathic inflammatory myopathies in children. It is considered an autoimmune disease of relatively unknown etiology, although environmental exposures and infectious agents are thought to play a role in disease pathogenesis. More recently, data has become available regarding the molecular genetics of children affected with juvenile dermatomyositis and the impact these genes have on disease expression and clinical course. Additionally, features of the immune response, including specific pathways of the humoral and cellular immune systems, have been further described. This article summarizes the most recent advances in understanding the etiopathogenesis of juvenile dermatomyositis.

RECENT FINDINGS

This article focuses on advances made in understanding the role that complement, soluble adhesion molecules, thrombospondin-1 levels, and genetics play in the evolution of juvenile dermatomyositis. It also describes microarray technology and gene expression profiling as means of identifying those genes overexpressed in affected children and thus likely involved in disease pathogenesis; microarray technology may also be used to distinguish dermatomyositis from the other inflammatory myopathies, as well as from other myopathies.

SUMMARY

In better understanding the pathogenetic mechanisms whereby disease evolves and the means by which genetic profiles influence susceptibility to and expression of disease, immunotherapies to better treat juvenile dermatomyositis may become available in the future.

S-protein is expressed in necrotic fibers in Duchenne muscular dystrophy and polymyositis

Complement regulatory proteins (CD55, CD59) and a fluid-phase complement regulator (S-protein) expression and distribution were studied in Duchenne muscular dystrophy (DMD) and polymyositis by Western blots and immunocytochemistry. In muscle samples from control subjects, no specific signal was detected for CD55 or S-protein, and CD59 was present on the sarcolemma of the muscle fibers. In DMD and polymyositis, Western blots demonstrated a 18-20 kDa band corresponding to CD59, as well as a signal corresponding to S-protein. Immunocytochemistry showed a colocalization between complement membrane attack complex (MAC), a molecule previously demonstrated in necrotic muscle fibers in DMD and polymyositis, and S-protein in necrotic fibers of DMD and polymyositis. Necrotic muscle fibers were more numerous in muscle biopsies of DMD patients with stronger signals for S-protein in Western blots. These results suggest that S-protein is not able to prevent the full assembly of MAC in necrotic fibers of patients with DMD and polymyositis, but might instead inactivate MAC deposits present inside necrotic fibers or participate in the clearance of MAC-attacked muscle fibers.

Complement regulatory protein CD59 involves c-SRC related tyrosine phosphorylation of the creatine transporter in skeletal muscle during sepsis

BACKGROUND

Myocellular creatine (Cr) uptake is predominantly governed by the creatine transporter (CreaT) and plays a pivotal role in skeletal muscle energy metabolism. The CreaT belongs to a neurotransmitter transporter family that is functionally regulated by protein tyrosine kinase induced tyrosine phosphorylation. Recently, complement regulatory protein CD59 has been found not only to protect host tissue from C5b-9 complex attack that occurs in sepsis but also to initiate the activation of Src family kinase and tyrosine phosphorylation of its downstream proteins. The purpose of this study was to determine the association between myocellular free Cr, c-Src related tyrosine phosphorylation of the CreaT, and CD59 during sepsis.

METHODS

Male Sprague-Dawley rats (250 to 300 g) were randomized to undergo cecal ligation and puncture (CLP) or sham operation. Fast-twitch gastrocnemius muscles were harvested 24 hours after operation. Myocellular free Cr levels were measured by high-performance liquid chromatography. Combination of protein immunoprecipitation with Western blotting was used to assess tyrosine phosphorylation status of the CreaT and the association between CD59, c-Src, and CreaT.

RESULTS

Myocellular free Cr levels were 70% greater after CLP. Tyrosine phosphorylation of the CreaT was significantly increased after CLP as compared to sham operation. Tyrosine phosphorylated c-Src (Tyr-416) in the CreaT-c-Src immune complex was 24% higher after CLP. Sepsis also increased protein expression of tyrosine phosphorylated c-Src (Tyr-416) or CreaT in the CD59-c-Src or CD59-CreaT complex by 20% or 30%, respectively.

CONCLUSIONS

During sepsis, an increase in myocellular free Cr levels is associated with enhanced tyrosine phosphorylation of the CreaT, which is likely induced by active c-Src. CD59 is physically associated with both c-Src and CreaT, which suggests that CD59 may participate in the regulation of myocellular Cr metabolism via the CreaT during sepsis.

Extraocular muscle is defined by a fundamentally distinct gene expression profile

Skeletal muscle fibers are defined by patterned covariation of key traits that determine contractile and metabolic characteristics. Although the functional properties of most skeletal muscles result from their proportional content of a few conserved muscle fiber types, some, typically craniofacial, muscles exhibit fiber types that appear to lie outside the common phenotypic range. We analyzed gene expression profiles of three putative muscle classes, limb, masticatory, and extraocular muscle (EOM), in adult mice by high-density oligonucleotide arrays. Pairwise comparisons using conservative acceptance criteria identified expression differences in 287 genes between EOM and limb and/or masticatory muscles. Use of significance analysis of microarrays methodology identified up to 400 genes as having an EOM-specific expression pattern. Genes differentially expressed in EOM reflect key aspects of muscle biology, including transcriptional regulation, sarcomeric organization, excitation-contraction coupling, intermediary metabolism, and immune response. These patterned differences in gene expression define EOM as a distinct muscle class and may explain the unique response of these muscles in neuromuscular diseases.

Massive muscle cell degeneration in the early stage of merosin-deficient congenital muscular dystrophy

Primary merosin-deficient congenital muscular dystrophy (CMD) is a severe form of congenital muscular disorder which is caused by mutations in the laminin alpha2 chain gene (LAMA2). The disease is characterized by marked dystrophic changes in skeletal muscles during early infancy, while little is known about the pathological process of the muscle fiber degeneration. Here, we report the immunohistochemical analysis of skeletal muscle in ten patients with primary merosin-deficient CMD using a panel of molecular markers for skeletal muscle proteins, cellular necrosis, and apoptosis. In the youngest patient (a 52 day old baby), prominent massive muscle cell degeneration occurred in association with the deposition of the C5-9 complement membrane attack complex (MAC). Most of the MAC-positive muscle fibers showed a severely deranged immunoreaction to dystrophin, dystroglycans, and other sarcolemmal proteins. In addition, we found scattered positive signals for apoptosis. Similar but milder changes were also observed in six other patients younger than 1 year. In the patients older than 3 years, muscle fibers positive for MAC and apoptotic signals were barely detectable. These findings imply that massive muscle fiber degeneration occurs in the very early stage of merosin-deficient CMD and may contribute to the severe dystrophic changes in muscle from early infancy.

X-linked vacuolated myopathy: membrane attack complex deposition on the surface membrane of injured muscle fibers is not accompanied by S-protein

We have studied the expression of S-protein on the muscle from patients with X-linked vacuolated myopathy [characterized by the deposition of the complement C5b-9 membrane attack complex (MAC) over abnormal muscle fibers] and controls by immunocytochemistry and immunoblotting. No expression was detected on muscle from controls and patients with X-linked vacuolated myopathy. These findings suggest that S-protein does not render the MAC inactive in X-linked vacuolated myopathy. This situation may be due to the fact that the pathways of MAC activation and the expression of S-protein in X-linked vacuolated myopathy are different from the ones observed in ischemic and/or necrotic, or immune diseases. These results emphasize the role of the membrane complement regulatory proteins (i.e., CD59) in X-linked vacuolated myopathy.