Intermediate filament-like protein syncoilin in normal and myopathic striated muscle

Abstract

The intermediate filament-like protein syncoilin is a member of the dystrophin protein complex, and links the complex to the cytoskeleton through binding α-dystrobrevin and desmin in muscle. Here, we identify further sites of syncoilin location in normal muscle: at the perinuclear space, myotendinous junction, and enrichment in the sarcolemma and sarcoplasm of oxidative muscle fibers in mice. To understand the importance of the dystrophin protein complex-syncoilin-cytoskeletal link and its implication to disease, we analyzed syncoilin in mice null for α-dystrobrevin (adbn−/−) and desmin (des−/−). Syncoilin was upregulated in dystrophic muscles of adbn−/− mice, without alteration in its subcellular location. In des−/− mice, syncoilin was severely reduced in skeletal muscle; lost from sarcomeric Z-lines and neuromuscular junctions, and redistributed from the sub-sarcolemmal cytoskeleton to the cytoplasm. The data show that absence of α-dystrobrevin or desmin leads to dynamic changes in syncoilin that may compensate for, or participate in, different muscle myopathies.

Introduction

Duchenne muscular dystrophy (DMD) is caused by mutations of the dystrophin gene that lead to its absence at the sarcolemma, resulting in exhaustive cycles of muscle degeneration and regeneration [1]. Dystrophin interacts with a number of proteins to form the dystrophin protein complex, which can be divided into three subcomplexes: the dystroglycan complex, the sarcoglycan complex, and the cytoplasmic complex consisting of syntrophins and α-dystrobrevin. Many members of this complex are mutated in muscle disorders [1]. The dystrophin protein complex provides mechanical stability to the muscle plasma membrane by linking the intracellular actin cytoskeleton with the extracellular matrix. The dystrophin protein complex is also likely to be involved in intracellular signaling, the loss of which contributes to the pathology of muscular dystrophy [2]. α-Dystrobrevin is one of several components of the dystrophin protein complex that plays a signaling role in muscle [2], [3]. The importance of α-dystrobrevin to muscle integrity is evident from the null mutant mouse (adbn−/−), which develops muscular dystrophy without disruption of the dystrophin protein complex [3]. Abnormalities are observed at the neuromuscular junction, including irregular distribution of acetylcholine receptors and a reduction in junctional folds [3], [4], but the causal mechanism underlying the muscular dystrophy is unknown.

We, and others, have examined the function of α-dystrobrevin in muscle and have identified several binding partners which include dysbindin, syncoilin, synemin/desmuslin and DAMAGE, although the exact function of these proteins is unknown [5], [6]. We are particularly interested in syncoilin, since it is an intermediate filament-like protein and intermediate filament proteins are known to be important in human disease [7]. Intermediate filament expression patterns are varied and they are considered to play a role in structural support, cell migration and cell signaling. Syncoilin is a 64-kDa protein, containing the coiled-coil domains typical of intermediate filament proteins, but it is unique in having a novel N-terminal head domain with no similarity to any known protein [8]. In addition, unlike most intermediate filaments, syncoilin is unable to form either homofilaments or heterofilaments with other intermediate filaments [9]. Syncoilin is predominantly expressed in skeletal and cardiac muscle [8] and in the former it is concentrated at the sarcolemma, Z-lines and neuromuscular junctions. Previously, we suggested that syncoilin is enriched in the slow oxidative type I muscle fibers [8], although this has not been clarified in detail.

We have also shown that syncoilin co-localizes and associates with desmin in muscle [9]. Desmin is the main intermediate filament protein in mature striated muscle and co-localizes with syncoilin at the plasma membrane, Z-lines, and neuromuscular junctions [9]. Desmin is known to be enriched at the myotendinous junction and perinuclear space [10], [11], but these regions have not been examined for syncoilin. Desmin is postulated to maintain the integrity of muscle fibers by linking myofibrils to the sarcolemma. Thus, syncoilin provides an important link between the dystrophin protein complex via α-dystrobrevin to the desmin intermediate filament cytoskeletal network, presumably contributing to the stability of contracting muscle fibers under stress. Mice deficient for desmin (des−/−) are viable, but develop a myopathy in skeletal, cardiac and smooth muscle [12], [13]. Muscle architecture is disrupted, which is manifested by loss of myofibril alignment and attachment to the sarcolemma with deformed and disorganized mitochondria. The pathology is pronounced in muscles that are continually used, such as the soleus, diaphragm and heart [12], [13].

In this work, we further characterize syncoilin in normal muscle by immunochemical methods. First, we establish the muscle fiber dependence of syncoilin localization. The myotendinous junction and perinuclear space are identified as new regions where syncoilin is enriched. Second, we describe the effect of α-dystrobrevin and desmin knock-out upon the distribution of syncoilin in skeletal muscle. We show that syncoilin expression is altered in the absence of either α-dystrobrevin or desmin, suggesting that it compensates or contributes to the myopathies in these mutant mice.