Measurements Of Actin Layer Lines In Permeabilized Heart Tissue Reveal New Structural Properties Of The Cardiac Thin Filament

The cardiac thin filament periodicity within the myofilament lattice remain poorly understood mainly due to the challenges of obtaining high order thin filament layer lines (meridional reflections) in heart tissue. Small-angle X-ray diffraction conducted on permeabilized fresh porcine cardiac papillary muscle reveal—for the first time to our knowledge—that cardiac muscle exhibits distinctive thin filament properties compared to skeletal muscle. Furthermore, the use of permeabilized muscle allows us to pinpoint the role of Ca2+ and myosin cross-bridges to thin filament rearrangements during contraction. The results show: 1) the long-pitch helix periodicity of the actin filament intensity decreased due to Ca2+ contribution rather than to rigor cross-bridges; 2) the long-pitch helix periodicity of the actin filament spacing increased during contraction with equal contribution of Ca2+ alone and rigor cross-bridges; and 3) actin first layer reflection was observed under rigor conditions, however, no tropomyosin reflection was detected. These data suggest that both Ca2+ alone and myosin cross-bridges contribute to the thin filament length changes. Furthermore, compared to skeletal muscle, cardiac muscle shows an overall reduction in long-pitch helix periodicity of the actin filament spacing which is similar to the nebulin-knockout soleus muscle (Kiss et al. 2018 PNAS 115:10369) suggesting that while nebulin is important to pre-stretch the skeletal muscle, this mechanism is lacking in the cardiac muscle. In addition, in the nebulin-knockout soleus muscle an impairment in tropomyosin movement was seen during contraction (reduced tropomyosin reflection intensity), similar to what was observed with cardiac muscle here. We speculate that the presence of nebulin in the skeletal muscle facilitates tropomyosin movement and the lack of nebulin in cardiac muscle may provide a structural explanation for why cardiac muscle does not undergo tetanic contractions. NIH-R01HL128683&NIH-P41GM103622