Proteasomes in distal myopathy with rimmed vacuoles

Unfolded protein response and activated degradative pathways regulation in GNE myopathy

Although intracellular beta amyloid (Aβ) accumulation is known as an early upstream event in the degenerative course of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) myopathy, the process by which Aβdeposits initiate various degradative pathways, and their relationship have not been fully clarified. We studied the possible secondary responses after amyloid beta precursor protein (AβPP) deposition including unfolded protein response (UPR), ubiquitin proteasome system (UPS) activation and its correlation with autophagy system. Eight GNE myopathy patients and five individuals with normal muscle morphology were included in this study. We performed immunofluorescence and immunoblotting to investigate the expression of AβPP, phosphorylated tau (p-tau) and endoplasmic reticulum molecular chaperones. Proteasome activities were measured by cleavage of fluorogenic substrates. The expression of proteasome subunits and linkers between proteasomal and autophagy systems were also evaluated by immunoblotting and relative quantitative real-time RT-PCR. Four molecular chaperones, glucose-regulated protein 94 (GRP94), glucose-regulated protein 78 (GRP78), calreticulin and calnexin and valosin containing protein (VCP) were highly expressed in GNE myopathy. 20S proteasome subunits, three main proteasome proteolytic activities, and the factors linking UPS and autophagy system were also increased. Our study suggests that AβPP deposition results in endoplasmic reticulum stress (ERS) and highly expressed VCP deliver unfolded proteins from endoplasmic reticulum to proteosomal system which is activated in endoplasmic reticulum associated degradation (ERAD) in GNE myopathy. Excessive ubiquitinated unfolded proteins are exported by proteins that connect UPS and autophagy to autophagy system, which is activated as an alternative pathway for degradation.

Expression of the inclusion body myopathy 3 mutation in Drosophila depresses myosin function and stability and recapitulates muscle inclusions and weakness

A Drosophila model of myosin-based inclusion body myopathy type 3 is presented. Muscle function, ATPase activity, and actin sliding velocity were dramatically reduced. The mutant myosin is prone to aggregate, likely accounting for the observed cytoplasmic inclusions and disorganized muscle filaments reminiscent of the human disease.

Hereditary myosin myopathies are characterized by variable clinical features. Inclusion body myopathy 3 (IBM-3) is an autosomal dominant disease associated with a missense mutation (E706K) in the myosin heavy chain IIa gene. Adult patients experience progressive muscle weakness. Biopsies reveal dystrophic changes, rimmed vacuoles with cytoplasmic inclusions, and focal disorganization of myofilaments. We constructed a transgene encoding E706K myosin and expressed it in Drosophila (E701K) indirect flight and jump muscles to establish a novel homozygous organism with homogeneous populations of fast IBM-3 myosin and muscle fibers. Flight and jump abilities were severely reduced in homozygotes. ATPase and actin sliding velocity of the mutant myosin were depressed >80% compared with wild-type myosin. Light scattering experiments and electron microscopy revealed that mutant myosin heads bear a dramatic propensity to collapse and aggregate. Thus E706K (E701K) myosin appears far more labile than wild-type myosin. Furthermore, mutant fly fibers exhibit ultrastructural hallmarks seen in patients, including cytoplasmic inclusions containing aberrant proteinaceous structures and disorganized muscle filaments. Our Drosophila model reveals the unambiguous consequences of the IBM-3 lesion on fast muscle myosin and fibers. The abnormalities observed in myosin function and muscle ultrastructure likely contribute to muscle weakness observed in our flies and patients.

Prophylactic treatment with sialic acid metabolites precludes the development of the myopathic phenotype in the DMRV-hIBM mouse model

Distal myopathy with rimmed vacuoles (DMRV)-hereditary inclusion body myopathy (hIBM) is an adult-onset, moderately progressive autosomal recessive myopathy; eventually, affected individuals become wheelchair bound1. It is characterized clinically by skeletal muscle atrophy and weakness, and pathologically by rimmed vacuoles, which are actually accumulations of autophagic vacuoles2, 3, 4, scattered angular fibers and intracellular accumulation of amyloid and other proteins5. To date, no therapy is available for this debilitating myopathy, primarily because the disease pathomechanism has been enigmatic. It is known that the disease gene underlying DMRV-hIBM is GNE, encoding glucosamine (UDP-N-acetyl)-2-epimerase and N-acetylmannosamine kinase6, 7, 8--two essential enzymes in sialic acid biosynthesis9. It is still unclear, however, whether decreased sialic acid production causes muscle degeneration, as GNE has been proposed to have roles other than for sialic acid biosynthesis10, 11, 12. By showing that muscle atrophy and weakness are completely prevented in a mouse model of DMRV-hIBM after treatment with sialic acid metabolites orally, we provide evidence that hyposialylation is indeed one of the key factors in the pathomechanism of DMRV-hIBM. These results support the notion that DMRV-hIBM can potentially be treated simply by giving sialic acids, a strategy that could be applied in clinical trials in the near future.

[Distal myopathy due to mutations of GNE gene: clinical spectrum and diagnosis]

Distal myopathies are rare muscular disorders clinically characterized by a predominantly distal muscular involvement. Among recessive forms, the myopathy resulting from mutations in the UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase (GNE) gene, often designated as Nonaka myopathy, primarily affect young adults and are characterized by muscle wasting and weakness predominating on the anterior compartment of the leg, a remarkable quadriceps sparing and a frequent evolution towards ambulation loss after a few years. Finding rimmed vacuoles on muscle biopsy is a further argument for the diagnosis. However, the presentation and course may vary and we describe four patients who illustrate the clinical spectrum of the disease: the first patient had a classical form with progressive weakness over several years, the second one a rapidly progressive myopathy leading to ambulation loss within three years from onset, the third one a very slow course with no ambulation loss after several decades, and the last one a progressive form with misleading neurogenic features on the EMG. One of our four patients harbored a homozygous mutation, and three others were compound heterozygous, two of them displaying an original mutation: one had a c.2036 T>G (p.Val679Gly) substitution, the c.829 C>T (p.Arg277Cys) substitution.

A Gne knockout mouse expressing human V572L mutation develops features similar to distal myopathy with rimmed vacuoles or hereditary inclusion body myopathy

Distal myopathy with rimmed vacuoles (DMRV) or hereditary inclusion myopathy (h-IBM) is an early adult-onset distal myopathy caused by mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene which encodes for a bifunctional enzyme involved in sialic acid biosynthesis. It is pathologically characterized by the presence of rimmed vacuoles especially in atrophic fibers, which also occasionally contain congophilic materials that are immunoreactive to beta-amyloid, lysosomal proteins, ubiquitin and tau proteins. To elucidate the pathomechanism of this myopathy and to explore the treatment options, we generated a mouse model of DMRV/h-IBM. We knocked out the Gne gene in the mouse, but this resulted in embryonic lethality. We therefore generated a transgenic mouse that expressed the human GNEV572L mutation, which is the most prevalent among Japanese DMRV patients, and crossed this with Gne((+/-)) mouse to obtain Gne((-/-))hGNEV572L-Tg. Interestingly, these mice exhibit marked hyposialylation in serum, muscle and other organs. Reduction in motor performance in these mice can only be seen from 30 weeks of age. A compelling finding is the development of beta-amyloid deposition in myofibers by 32 weeks, which clearly precedes rimmed vacuole formation at 42 weeks. These results show that the Gne((-/-)) hGNEV572L-Tg mouse mimics the clinical, histopathological and biochemical features of DMRV/h-IBM, making it useful for understanding the pathomechanism of this myopathy and for employing different strategies for therapy. Our findings underscore the notion that hyposialylation plays an important role in the pathomechanism of DMRV/h-IBM.

Pathways of myocyte death: implications for development of clinical laboratory biomarkers

The recognition that cardiac myocytes die by multiple mechanisms and thus substantially affect ventricular remodeling in diseased human hearts supports the concept of ongoing myocyte death in the progression of heart failure and constitutes the basis of this review. In addition, based on the pathophysiology of myocardial cell deaths, the present study emphasizes that currently methodologies, although with some inherent limitations, are available to recognize and measure quantitatively the contribution of myocyte cell death to the progression of the pathologic state of the heart. Our own studies show that application of such methodologies including modern microscopy techniques and the use of different molecular and immunohistochemical markers may generate the consensus that myocyte cell death is a quantifiable parameter in the normal and pathological human heart. The present study also demonstrates that myocyte cell death, apoptotic, oncotic or autophagic in nature, has to be regarded as an additional critical variable of the multifactorial events implicated in the alterations of cardiac anatomy and myocardial structure of the diseased human heart.

Distal myopathy with rimmed vacuoles and hereditary inclusion body myopathy

Distal myopathy with rimmed vacuoles (DMRV) and hereditary inclusion body myopathy (hIBM) share similar clinical features, including onset in young adulthood with preferential involvement of the anterior compartment of the lower legs and sparing of the quadriceps femoris muscles. The most significant muscle pathology is the presence of rimmed vacuoles, which appear to play a major role in muscle atrophy and weakness. After the discovery of the gene locus in both DMRV and hIBM on chromosome 9 and mutations in the gene encoding the enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), it became clear that they are allelic disorders. From gene analysis, it is evident that these diseases are not restricted to people of Japanese and Jewish ancestry, but that they are widely distributed throughout all ethnic groups. Although defective glycosylation to a muscle fiber has been suggested, the mechanism by which myofibrillar degeneration is followed by rimmed vacuole formation remains to be clarified.

Proteasomal expression, induction of immunoproteasome subunits, and local MHC class I presentation in myofibrillar myopathy and inclusion body myositis

Inclusion body myositis (IBM) and myofibrillar myopathy (MM) are diseases characterized by the abnormal accumulation of proteins in muscle fibers, including desmin, alphaB-crystallin, gelsolin, actin, kinases, and phospho-tau, along with ubiquitin in muscle fibers, suggesting abnormal protein degradation as a possible cause of the surplus myopathy. Since the ubiquitin-proteasome system plays a crucial role in non-lysosomal protein degradation, the present study has examined by immunohistochemistry the expression of components of the catalytic core of 20S proteasomes and its regulators: 19S and PA28alpha/beta, and the expression of immunoproteasome subunits LMP2, LMP7, and MECL1 in 8 patients with MM and 10 patients with IBM. The patients with MM were from 6 unrelated families, 2 sporadic cases, I with autosomal recessive and 5 with autosomal dominant inheritance. One sporadic patient had a de novo R406W mutation in the desmin gene, and 1 patient with autosomal dominant MM had a single amino acid deletion at position 366 in the desmin gene. Increased immunoreactivity to 20S, 19S, and PA28alpha/beta colocalizing abnormal protein deposits, as revealed in consecutive serial sections, was seen in all cases with MM and IBM. In all cases, the subunits of the immunoproteasome LMP2, LMP7, and MECL1 colocalized with proteasomal immunoreactivity and abnormal protein accumulation. Immunohistochemistry revealed focal MHC class I immunoreactivity in the cytoplasmic membrane of muscle fibers in IBM and in association with protein aggregates in IBM, and to a lesser degree, in MM. The present findings provide a link between abnormal protein accumulation and altered proteasomal expression in IBM and MM.

Inclusion body myositis: genetic factors, aberrant protein expression, and autoimmunity

Sporadic inclusion body myositis (s-IBM) is an inflammatory myopathy mainly affecting elderly individuals. It has a chronic progressive course leading to severe disability. Immunosuppressive treatment is in most instances ineffective. S-IBM is morphologically characterized by mononuclear cell infiltrates and vacuolated muscle fibers with pathologic accumulation of a large number of different proteins. Recent research has focused on the expression of various factors that may contribute to the inflammatory reaction and the typical inclusions. This review summarizes the new information on genetic factors, abnormal protein expression and inflammation, which provides a basis for linking the different typical morphologic features of s-IBM to a cascade of pathogenic events.

Autosomal dominant myopathy: missense mutation (Glu-706 --> Lys) in the myosin heavy chain IIa gene

We here report on a human myopathy associated with a mutation in a fast myosin heavy chain (MyHC) gene, and also the genetic defect in a hereditary inclusion body myopathy. The disorder has previously been described in a family with an "autosomal dominant myopathy, with joint contractures, ophthalmoplegia, and rimmed vacuoles." Linkage analysis and radiation hybrid mapping showed that the gene locus (Human Genome Map locus name: IBM3) is situated in a 2-Mb region of chromosome 17p13, where also a cluster of MyHC genes is located. These include the genes encoding embryonic, IIa, IIx/d, IIb, perinatal, and extraocular MyHCs. Morphological analysis of muscle biopsies from patients from the family indicated to us that the type 2A fibers frequently were abnormal, whereas other fiber types appeared normal. This observation prompted us to investigate the MyHC-IIa gene, since MyHC-IIa is the major isoform in type 2A fibers. The complete genomic sequence for this gene was deduced by using an "in silico" strategy. The gene, found to consist of 38 exons, was subjected to a complete mutation scan in patients and controls. We identified a missense mutation, Glu-706 --> Lys, which is located in a highly conserved region of the motor domain, the so-called SH1 helix region. By conformational changes this region communicates activity at the nucleotide-binding site to the neck region, resulting in the lever arm swing. The mutation in this region is likely to result in a dysfunctional myosin, compatible with the disorder in the family.

Distal myopathies

Among various previously described distal myopathies, several diseases have now been established as clinically and genetically distinct entities. The most representative diseases are dominantly inherited Welander distal myopathy and tibial muscular dystrophy, and the recessively inherited distal myopathy with rimmed vacuoles and distal muscular dystrophy (Miyoshi myopathy). Since the discovery of the gene loci for several distal myopathies, several diseases previously categorized as different disorders have now proven to be the same or allelic disorders (e.g. distal myopathy with rimmed vacuoles and hereditary inclusion body myopathy, Miyoshi myopathy and limb-girdle muscular dystrophy with gene locus at 2p13). Except for Miyoshi myopathy, which has the typical findings of muscular dystrophy, most of the distal myopathies share the common pathologic features of myopathic changes with rimmed vacuoles. The pathologic changes are somewhat similar to those seen in chronic muscular dystrophy, but necrotic and regenerative processes are less prominent and creatine kinase levels are either normal or only mildly elevated. Further study is necessary to determine why rimmed vacuoles are so common in the distal myopathies, and what role they play in the pathogenesis of muscle fibre atrophy and loss, predominantly in the distal portions of the extremities.