Sorting of a nonmuscle tropomyosin to a novel cytoskeletal compartment in skeletal muscle results in muscular dystrophy

Tropomyosin (Tm) is a key component of the actin cytoskeleton and >40 isoforms have been described in mammals. In addition to the isoforms in the sarcomere, we now report the existence of two nonsarcomeric (NS) isoforms in skeletal muscle. These isoforms are excluded from the thin filament of the sarcomere and are localized to a novel Z-line adjacent structure. Immunostained cross sections indicate that one Tm defines a Z-line adjacent structure common to all myofibers, whereas the second Tm defines a spatially distinct structure unique to muscles that undergo chronic or repetitive contractions. When a Tm (Tm3) that is normally absent from muscle was expressed in mice it became associated with the Z-line adjacent structure. These mice display a muscular dystrophy and ragged-red fiber phenotype, suggestive of disruption of the membrane-associated cytoskeletal network. Our findings raise the possibility that mutations in these tropomyosin and these structures may underpin these types of myopathies

Recombinant human tropoelastin : production, properties and interactions

To enable the long-term study of tropoelastin and elastic fibre structure, function and biosynthesis, a recombinant system was developed to provide a source of human tropoelastin

Targeting of a Tropomyosin Isoform to Short Microfilaments Associated with the Golgi Complex

A growing body of evidence suggests that the Golgi complex contains an actin-based filament system. We have previously reported that one or more isoforms from the tropomyosin gene Tm5NM (also known as γ-Tm), but not from either the α- or β-Tm genes, are associated with Golgi-derived vesicles (Heimann et al., (1999). J. Biol. Chem. 274, 10743-10750). We now show that Tm5NM-2 is sorted specifically to the Golgi complex, whereas Tm5NM-1, which differs by a single alternatively spliced internal exon, is incorporated into stress fibers. Tm5NM-2 is localized to the Golgi complex consistently throughout the G1 phase of the cell cycle and it associates with Golgi membranes in a brefeldin A-sensitive and cytochalasin D-resistant manner. An actin antibody, which preferentially reacts with the ends of microfilaments, newly reveals a population of short actin filaments associated with the Golgi complex and particularly with Golgi-derived vesicles. Tm5NM-2 is also found on these short microfilaments. We conclude that an alternative splice choice can restrict the sorting of a tropomyosin isoform to short actin filaments associated with Golgi-derived vesicles. Our evidence points to a role for these Golgi-associated microfilaments in vesicle budding at the level of the Golgi complex

New aspects of tropomyosin-regulated neuritogenesis revealed by the deletion of Tm5NM1 and 2

Previous studies have shown that the overexpression of tropomyosins leads to isoform-specific alterations in the morphology of subcellular compartments in neuronal cells. Here we have examined the role of the most abundant set of isoforms from the ϒ-Tm gene by knocking out the alternatively spliced C-terminal exon 9d. Despite the widespread location of exon 9d-containing isoforms, mice were healthy and viable. Compensation by products containing the C-terminal exon 9c was seen in the adult brain. While neurons from these mice show a mild phenotype at one day in culture, neurons revealed a significant morphological alteration with an increase in the branching of dendrites and axons after four days in culture. Our data suggest that this effect is mediated via altered stability of actin filaments in the growth cones. We conclude that exon 9d-containing isoforms are not essential for survival of neuronal cells and that isoform choice from the g-Tm gene is flexible in the brain. Although functional redundancy does not exist between tropomyosin genes, these results suggest that significant redundancy exists between products from the same gene