Pathophysiology of protein aggregation and extended phenotyping in filaminopathy

Abstract

Mutations in FLNC cause two distinct types of myopathy. Disease associated with mutations in filamin C rod domain leading to expression of a toxic protein presents with progressive proximal muscle weakness and shows focal destructive lesions of polymorphous aggregates containing desmin, myotilin and other proteins in the affected myofibres; these features correspond to the profile of myofibrillar myopathy. The second variant associated with mutations in the actin-binding domain of filamin C is characterized by weakness of distal muscles and morphologically by non-specific myopathic features. A frameshift mutation in the filamin C rod domain causing haploinsufficiency was also found responsible for distal myopathy with some myofibrillar changes but no protein aggregation typical of myofibrillar myopathies. Controversial data accumulating in the literature require re-evaluation and comparative analysis of phenotypes associated with the position of the FLNC mutation and investigation of the underlying disease mechanisms. This is relevant and necessary for the refinement of diagnostic criteria and developing therapeutic approaches. We identified a p.W2710X mutation in families originating from ethnically diverse populations and re-evaluated a family with a p.V930_T933del mutation. Analysis of the expanded database allows us to refine clinical and myopathological characteristics of myofibrillar myopathy caused by mutations in the rod domain of filamin C. Biophysical and biochemical studies indicate that certain pathogenic mutations in FLNC cause protein misfolding, which triggers aggregation of the mutant filamin C protein and subsequently involves several other proteins. Immunofluorescence analyses using markers for the ubiquitin-proteasome system and autophagy reveal that the affected muscle fibres react to protein aggregate formation with a highly increased expression of chaperones and proteins involved in proteasomal protein degradation and autophagy. However, there is a noticeably diminished efficiency of both the ubiquitin-proteasome system and autophagy that impairs the muscle capacity to prevent the formation or mediate the degradation of aggregates. Transfection studies of cultured muscle cells imitate events observed in the patient's affected muscle and therefore provide a helpful model for testing future therapeutic strategies.

Chemical chaperone ameliorates pathological protein aggregation in plectin-deficient muscle

“…In this context, the increased expression of chaperones, as observed in plectin-deficient muscle, presumably is a reaction of the cell to facilitate folding of damaged proteins or degradation of aggregated proteins. Similar to what we observed for plectin-related MFM, a predominant localization of chaperones (and of other proteins involved in protein degradation pathways) within protein aggregates has been described for other MFMs, such as filaminopathies (22), myotilinopathies, and desminopathies (23). Most strikingly, highly increased protein levels of HSP27 as well as a tendency for increased αB-crystallin became apparent even during differentiation of plectin-deficient myotubes, indicating that chaperones represented the first line of defense against desmin network aggregation.…”

Differential proteomic analysis of abnormal intramyoplasmic aggregates in desminopathy

“…Dysfunctions in protein quality control and protein degradation via the ubiquitin-proteasome system (UPS) and the autophagic-lysosomal pathway seem to play an essential pathogenic role in desminopathy (see [4] for review) and other MFM subtypes [12,36]. Immunolocalization studies in desminopathy, myotilinopathy and filaminopathy demonstrated that abnormal muscle fibers show a markedly increased immunoreactivity for a number of UPS and autophagy proteins [36–38].…”

A change of heart: oxidative stress in governing muscle function?

“…In the heart, α-B-crystallin and HSP27 are induced during ischemic injury, heat stress, or end-stage failure (Martin et al 1997;Benjamin and McMillan 1998;Knowlton et al 1998;Yoshida et al 1999;Dohke et al 2006). Small HSPs are induced in both myopathic skeletal (Kley et al 2012) and normal muscles after intense exercise (Paulsen et al 2009) and during aging (Doran et al 2007). HSP27 has been shown to preserve cytoskeletal architecture (Mounier and Arrigo 2002) and to play roles in the resistance to oxidative (Huot et al 1996;Dalle-Donne et al 2001), thermal and ischemic stress (Vander Heide 2002;Hollander et al 2004), and to participate in the regulation of cellular redox states (Arrigo 2001) which in turn prevent apoptosis (Mehlen et al 1996) and correlate with hypoxia (Martin et al 1997).…”