Muscular tissues of the squid Doryteuthis pealeii express identical myosin heavy chain isoforms: an alternative mechanism for tuning contractile speed

Abstract

SUMMARYThe speed of muscle contraction is largely controlled at the sarcomere level by the ATPase activity of the motor protein myosin. Differences in amino acid sequence in catalytically important regions of myosin yield different myosin isoforms with varying ATPase activities and resulting differences in cross-bridge cycling rates and interfilamentary sliding velocities. Modulation of whole-muscle performance by changes in myosin isoform ATPase activity is regarded as a universal mechanism to tune contractile properties, especially in vertebrate muscles. Invertebrates such as squid, however, may exhibit an alternative mechanism to tune contractile properties that is based on differences in muscle ultrastructure, including variable myofilament and sarcomere lengths. To determine definitively whether contractile properties of squid muscles are regulated via different myosin isoforms (i.e. different ATPase activities), the nucleotide and amino acid sequences of the myosin heavy chain from the squid Doryteuthis pealeii were determined from the mantle, arm, tentacle, fin and funnel retractor musculature. We identified three myosin heavy chain isoforms in squid muscular tissues, with differences arising at surface loop 1 and the carboxy terminus. All three isoforms were detected in all five tissues studied. These results suggest that the muscular tissues of D. pealeii express identical myosin isoforms, and it is likely that differences in muscle ultrastructure, not myosin ATPase activity, represent the most important mechanism for tuning contractile speeds. Supplementary material available online at

Tuning of shortening speed in coleoid cephalopod muscle: no evidence for tissue‐specific muscle myosin heavy chain isoforms

“…In conclusion, we have shown that S. officinalis and O. bimaculoides , like the squid D. pealeii (Shaffer & Kier ), do not appear to express tissue‐specific myosin heavy chain isoforms. Although additional work is needed, these results suggest that coleoid cephalopods in general may use ultrastructural differences, primarily variation in thick filament length, to tune contractile velocity.…”

“…), so a functional difference, if any, would not affect function in a tissue‐specific manner. A final possibility is that the squid also expresses more than three myosin heavy chain isoforms, but that they were missed in the previous study by Shaffer & Kier (). While possible, we think that it is unlikely due to the large number of samples prepared and sequenced over the course of the study.…”

“…While it is widely accepted that animals express tissue‐specific myosin heavy chain isoforms to tune muscle contractile velocities (Barany ), squids appear to be an exception. Shaffer & Kier () showed that the muscular tissues of the squid Doryteuthis pealeii do not express tissue‐specific myosin heavy chain isoforms. The results from the current study are consistent with this finding and provide preliminary support for the hypothesis that cephalopod mollusks in general use ultrastructural differences rather than differences in myosin heavy chain isoforms to tune contractile velocity.…”

“…The protocols for RACE were based on those described previously (Scotto-Lavino et al 2006a,b). For additional details, see Shaffer & Kier (2012).…”

The length-force behavior and operating length range of squid muscle varies as a function of position in the mantle wall

“…Squid modulate isometric force and shortening velocity by varying thick filament length, not by expressing different isoforms of myosin heavy chain (Kier and Schachat, 1992;Kier and Schachat, 2008;Thompson et al, 2010b;Shaffer and Kier, 2012). Although other myofilament lattice proteins have the potential to affect the length-force relationship as well (e.g.…”