TPM3 (tropomyosin 3) encodes multiple actin-binding protein isoforms that determine the functional capacity of actin filaments. In skeletal muscle, muscle-specific TPM3 isoform is exclusively expressed in slow muscle fibers, and regulates muscle contraction by controlling the access of myosin to the thin filaments of actin. TPM3 mutations are associated with muscle weakness and selective atrophy of slow muscle fibers (type 1). However, the role(s) of tropomyosin 3 in muscle development is poorly understood which currently impedes the establishment of new concepts in the pathogenesis of TPM3-related myopathy.
We took advantage of TPM3-deficient zebrafish to study muscle development and function at different stages. During embryonic development, loss of tropomyosin 3 resulted in defective myofibril organization (2 days post-fertilization, dpf). The swimming performance was then significantly impaired (5 dpf). From juvenile to adult stage, slow muscle fibers were either severely atrophied or did not form. It resulted in significant reduction of slow muscle fiber content. The cytoskeletal organization was impaired in slow muscle fibers as indicated by sarcomere breakdown, and disruption of tubular and mitochondrial networks. In contrast, the content and size of fast muscle fibers were unaffected. At adult stage, impairment of slow muscle fibers was also associated with reduction in swimming activity, and muscle fatigue. Interestingly, the unaffected fast fibers were not able to compensate for the loss of slow fibers in zebrafish.
In this study, we showed that TPM3 is essential for myofibril organization, as well as slow muscle fiber growth and maintenance. Our study also suggests that zebrafish is a potent complementary model in which to study these new concepts in the context of TPM3-related myopathy.