Lessons from a tarantula: new insights into myosin interacting-heads motif evolution and its implications on disease

Abstract

Tarantula's leg muscle thick filament is the ideal model for the study of the structure and function of skeletal muscle thick filaments. Its analysis has given rise to a series of structural and functional studies, leading, among other things, to the discovery of the myosin interacting-heads motif (IHM). Further electron microscopy (EM) studies have shown the presence of IHM in frozen-hydrated and negatively stained thick filaments of striated, cardiac, and smooth muscle of bilaterians, most showing the IHM parallel to the filament axis. EM studies on negatively stained heavy meromyosin of different species have shown the presence of IHM on sponges, animals that lack muscle, extending the presence of IHM to metazoans. The IHM evolved about 800 MY ago in the ancestor of Metazoa, and independently with functional differences in the lineage leading to the slime mold Dictyostelium discoideum (Mycetozoa). This motif conveys important functional advantages, such as Ca²⁺ regulation and ATP energy-saving mechanisms. Recent interest has focused on human IHM structure in order to understand the structural basis underlying various conditions and situations of scientific and medical interest: the hypertrophic and dilated cardiomyopathies, overfeeding control, aging and hormone deprival muscle weakness, drug design for schistosomiasis control, and conditioning exercise physiology for the training of power athletes.

Keywords: Hypertrophic cardiomyopathy; Muscle disease; Muscle evolution; Myosin filaments; Myosin interacting-heads motif; Tarantula.

Fig. 1

3D reconstructions from electron micrographs of negatively stained or frozen-hydrated thick filaments for smooth, skeletal, and cardiac muscles of different invertebrate and vertebrate animals. According to the presence of interacting-heads motif (IHM) on the 3D reconstructions for each species, the IHM seem to be present in the bilaterians metazoans. These thick filaments exhibits three, four, or seven helices of IHMs. All these 3D reconstructions show densities corresponding to IHMs (red circles) approximately parallel to the filament axis except the IHM for indirect flight muscle (IFM) of the insect Lethocerus, which is approximately perpendicular. Bare zone location at the top. Zebrafish 3D reconstruction reproduced with permission from González-Solá et al. (2014) and the Biophysical Journal. Images of metazoans from Wikipedia (S. mansoni: David Williams, Illinois State University, Scallop: Manfred Heyde, Lethocerus: Richard Orr, H. sapiens: Michelangelo Buonarroti, The Creation of Adam c. 1512)

Fig. 2

Averaged images of heavy meromyosins (HMMs) from different species. The IHM is present in the studied metazoans irrespective of the muscle type (smooth, skeletal, and cardiac muscle), nonmuscle cells, or its absence, as in sponges. The IHM presence was not observed in the yeast Schizosaccharomyces pombe (Ascomycota, Fungi) and the Acanthamoeba castellanii (Discosea, Amoebozoa) (images shown are not averaged), while a differently folded tail with extra tail–IHM interactions was observed in Dictyostelium discoideum (Mycetozoa, Amoebozoa). The presence of the IHM in sponges suggests that the IHM is present in the metazoans. The drawings at the bottom show the folded inhibited form of the myosin heads (blocked head green, free head blue) or not (black heads) and the different ways the tail pack on the two folded myosin heads. The span across species of the three MHCII types (striated-like, smooth-like, and nonmuscle-like) is shown. Electron micrographs of turkey HMM reproduced with permission from Burgess et al. (2007) and the Journal of Molecular Biology. Images of organisms from Wikipedia (sponge: Nick Hobgood, Anemone: Attrattorestrano at Italian Wikipedia, Drosophila: André Karwath, S. Pombe: David O. Morgan, A. castellanii: Lorenzo-Morales et al. 2015)

Fig. 3

Consensus phylogenetic relationships of Unikonta, i.e., Metazoa, Fungi, Amoebozoa, and some related unicellular eukaryotes, for which the IHM structure has been studied directly via electron microscopy (EM) and small-angle X-ray scattering (SAXS) (like the squid Loligo pealeii). Phylogenetic relations and node ages are based on recent studies (Parfrey et al. ; Cavalier-Smith et al. ; Erwin ; Telford et al. 2015). Solid blue lines: myosin II establishing the IHM structure (dashed blue lines represent unstudied metazoans presumably with IHM); cyan line: myosin II with different tail folding exhibiting extra tail–IHM interactions (as found in D. discoideum); red lines: no IHM detected; black lines: unstudied taxa. The emergence of MHCII, RLC, ELC, MLCK and MCLP mypt genes (Steinmetz et al. 2012) are shown with horizontal arrows (cf. Mycetozoa (Griffith et al. see text).

Fig. 4

Assessment of the effects of hypertrophic (HCM) and dilated (DCM) variants according to their localization on the same positions on the blocked (BH) and free (FH) heads but exposed to different environments. Mapping of pathogenic and likely pathogenic HCM (a) and DCM (b) variants on the homologous human myosin β-cardiac IHM PDB 5TBY, based on the tarantula PDB 3JBH quasi-atomic model. Each variant appears as a pair, one located on or associated with the blocked head (olive) and one on the free head (green). Essential light chain ELC (BH, brown; FH, purple), regulatory light chain RLC (BH, dark blue; FH, light blue). a HCM variants that alter residues involved in IHM interactions (73/135 variants, 54%) are represented by colored balls: priming, green (“f” and “g”, see interaction colors code on c top panel); anchoring, orange (“i” and “j”); stabilizing, magenta (“a”, “d”, and “e”); scaffolding, white (ELC–MHC and RLC–MHC); RLC–RLC interface, yellow. Variants that do not alter residues involved in IHM interactions are shown in gray. b DCM variants (7/27; 26%) colored as described above. c The molecular pathogenesis of HCM and DCM assessed in the context of the IHM paradigm. Myosin interactions involved in IHM assembly and myosin motor domain (MD) functions that are altered by variants are depicted. Top panel: Relaxed healthy cardiac muscle contains myosin heads populations in the super-relaxed (SRX) state (left) with lowest ATP consumption and a disordered relaxed (DRX) state (right) with swaying free heads that generate force with higher ATP consumption (see Alamo et al. 2016). The population of cardiac myosins in SRX is more stable than in skeletal muscle (Hooijman et al. 2011), which supports physiologic contraction and relaxation, energy conservation, and normal cardiac morphology. Middle panel: HCM myosin variants (colored stars) both alter residues involved in MD functions (causing increased biophysical power, Tyska et al. 2000) and destabilize IHM interactions (particularly those with altered electrostatic charge). Reduced populations of myosins in the SRX state and increased populations of myosins in DRX as well as enhanced MD properties will result in increased contractility, decreased relaxation, and increased ATP consumption, the three major phenotypes observed in HCM hearts. Compensatory signals may promote ventricular hypertrophy. Bottom panel: MYH7 DCM variants (colored stars) have modest effects on IHM interactions but substantially reduce MD functions, particularly nucleotide binding, resulting in reduced ATP consumption and sarcomere power (Schmitt et al. 2006), with minimal impact on relaxation and overall diminished contractility. Compensatory signals result in ventricular dilatation to maintain circulatory demands in DCM hearts. Reproduced with permission from Alamo et al. (2017b) and eLife