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Skeletal Muscle
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
A thin filament is about 5–6 nm in diameter, 1 µm long, and consists mainly of a strand of F-actin (F for fibrous), as illustrated in Figure 9.4b. A strand of F-actin is a polymer composed of two twisted rows of 300–400 individual molecules of G-actin (G for globular), each molecule having a diameter of about 5 nm and a molecular weight of about 42 kdaltons. The F-actin strand is held together by a thread of nebulin that extends along the F-actin between the two rows of G-actin molecules. Each G-actin molecule has an active site that can bind to the head of a myosin molecule but is prevented from doing so under resting conditions, when there is no contraction, by tropomyosin molecules that cover the active sites. A tropomyosin molecule is a double strand that joins head-to-tail with other tropomyosin molecules to form a twisted strand over the length of the F-actin. Each tropomyosin molecule covers seven active sites and is bound to a troponin molecule. A troponin molecule is composed of three largely globular subunits: (i) troponin T (tropomyosin-binding troponin) that forms a troponin-tropomyosin complex, (ii) troponin C (Ca2+ binding troponin) that plays a major role in contraction, as explained later, and (iii) troponin I (inhibitory troponin) that is attached to G-actin in the absence of Ca2+ and holds the tropomyosin in a position that blocks myosin from reaching the active sites on G-actin. When Ca2+ bind to troponin C, troponin I detaches from the actin, thereby allowing the tropomyosin to move over the surface of the thin filament.
Striated MusclesSkeletal and Cardiac Muscles
Published in Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal, Principles of Physiology for the Anaesthetist, 2020
Peter Kam, Ian Power, Michael J. Cousins, Philip J. Siddal
Tropomyosin (molecular weight 66,000 Da) consists of two α-helical chains and lies in the groove between the two actin polymers. Tropomyosin covers the myosin-binding site and prevents the interaction of myosin with actin – an effect modulated by troponin.
Muscle Physiology and Electromyography
Published in Verna Wright, Eric L. Radin, Mechanics of Human Joints, 2020
The presence of calcium is required for this process to take place. Recalling Figure 4, the structure of actin is a double helix of G-actin molecules with tropomyosin B in the groove between them. Troponin molecules are arranged along the length of the tropomyosin B. In a resting muscle, tropomyosin B prevents theformation of actomyosin, but free calcium ions cause the troponin to push the tropomyosin B away from the actin, allowing the myosin to bind.
Chemotherapy and targeted treatments of breast sarcoma by histologic subtype
Published in Expert Review of Anticancer Therapy, 2021
Stefania Kokkali, Athina Stravodimou, Jose Duran-Moreno, Nektarios Koufopoulos, Ioannis a Voutsadakis, Antonia Digklia
A promising pathway is the one of Tropomyosin receptor kinases (Trk). They are encoded by the NTRK1, 2 and 3 genes and they are expressed in human neuronal tissue, playing an essential role in nervous system specification and differentiation. The oncogenic Trk gene fusions may induce cell proliferation and engage downstream signaling pathways, such as the MAPK pathway. Trk mutations, although rare, occur in a diverse range of tumors. Rare cases of ETV6-NTRK3 fusions and LMNA-NTRK1 fusions have been seen in infantile fibrosarcoma and low-grade spindle cell sarcoma. Small molecule tyrosine kinase inhibitors larotrectinib and entrectinib have been approved for solid tumors with NTRK fusions and represent an option in sarcoma patients with such fusions [81]. Rare cases of breast carcinomas and breast sarcomas with NTRK fusions have been reported [82,83]. (Table 2 summarizes specific chemotherapy and other systemic therapies according to sub-type of sarcoma).
Tropomyosin autoantibodies associated with checkpoint inhibitor myositis
Published in OncoImmunology, 2020
Pauline Zaenker, David Prentice, Melanie Ziman
Myositis is an autoimmune/antibody-mediated condition and several myositis-related and -associated autoantibodies have been identified to date.4,5 The regionalization of the myositis (paraspinal, ocular and myocardial) in this case is interesting as in a mouse model of muscular dystrophy, deletion of a tropomyosin 3 isoform (Tpm3.1) caused muscle disease in a similar distribution.6 Garaud et al.7 performed microarray antibody analysis in breast cancer, with sera and breast tissue displaying high levels of tumor specific IgA to tumor antigens, including cancer/testis antigen 1B (CTAG1B) and ankyrin repeat domain 30B like protein (ANKRD30BL). These were not related to tumor progression or survival. There was however, a correlation with the development of tertiary lymphoid structures within the tumor suggesting local IgA production. B cell infiltration in tumors is rare but B cell activation does occur in primary, secondary and tertiary lymphoid structures and antibodies may play a role in tumor destruction or progression. Whilst IgA is unable to directly activate the complement pathway, it can do so via the mannose lectin pathway. It is now recognized that monomeric IgA opsonised on cell membranes is able to cause apoptosis and necrosis by binding to the FcαR1 receptor (CD89) on neutrophils.8 The exact mechanism of subsequent tissue damage is under debate and a new novel process called trogoptosis has been suggested.9
Proteomic profiling of giant skeletal muscle proteins
Published in Expert Review of Proteomics, 2019
Sandra Murphy, Paul Dowling, Margit Zweyer, Dieter Swandulla, Kay Ohlendieck
Recent developments in the field of muscle biology that have applied a TDP approach included results published on tropomyosins present in different skeletal muscles from multiple species, including swine, rat and human [43]. This study revealed that tropomyosin isoforms Tpm1.1 and Tpm2.2 are the two major Tpm isoforms in swine and rat skeletal muscles, whereas Tpm1.1, Tpm2.2, and Tpm3.12 are present in human skeletal muscles. This methodology provides an analytical foundation for further studies on these tropomyosin isoforms in muscle-related diseases. A quantitative TDP approach identified significant changes in post-translational modifications of critical myofilament proteins in predominantly fast-twitching skeletal muscles of aging rats, together with age-related alterations in the phosphorylation of isoforms of the Z-disk associated protein Cypher [44].