Thick Filament Activation and Post-Tetanic Potentiation Mechanisms Evolved Differently in Invertebrate and Vertebrate Striated Muscle

Abstract

Striated muscle contraction involves sliding of actin thin filaments along myosin thick filaments, controlled by calcium through thin filament activation. In relaxation, the two heads of myosin interact with each other to form the interacting-heads motif, consisting of blocked and free heads helically arranged around the filament backbone. A key question is how both free and blocked heads are activated and released from the backbone to approach actin and produce force. In a proposed mechanosensing mechanism for vertebrate striated muscle, a ∼1.4% backbone elongation leads to thick filament activation. We used time-resolved synchrotron X-ray diffraction to determine whether a similar mechanism is present in tarantula striated muscle. Tetanic isometric contraction produced only very small backbone elongation (∼0.27%), much smaller than in frog, suggesting that a mechanosensing mechanism is not of primary importance in tarantula. Rather, thick filament activation is produced by a phosphorylation mechanism, revealed by the X-ray diffraction patterns and force measurements before and after tetanus, correlated with the levels of myosin regulatory light chain phosphorylation. The results provide insights into the mechanism of force potentiation and post-tetanic potentiation. We also show that tarantula (Aphonopelma hentzi) muscle has an additional thermosensing mechanism that disorders the myosin heads above and below 22.5° C. Together, these findings suggest that -apart from insect flight muscle stretch activation - two other molecular mechanisms for thick filament activation have evolved to disrupt the intermolecular interactions that establish the relaxed helices of free and blocked heads: one in invertebrates (with a rigid paramyosin core), by either RLC phosphorylation (as in arthropods) or Ca2+-binding (as in mollusks, that lack phosphorylation), and another in vertebrates (lacking paramyosin), by mechanosensing. Supported by NIH grants AR067279, GM103622, AR072036 and HL139883.