Meeting Abstract
We are interested in the contribution of giant sarcomere associated proteins (gSAPs, e.g. titin) to history dependent modulation of force production and have developed an in vivo model with which to transiently knock down expression of these proteins. The sallimus gene of fruit fly transcribes several gSAPs that contain repeated PEVK and Ig domains and appear to act similarly to titin in chordate muscle. Several aspects of the larval fruit fly preparation make it especially useful for further exploration of gSAP physiology: muscle length change is relatively large, genetic tools are well established, muscle orientation and geometry are repeated across multiple segments, and motor behavior is simple. Our model utilizes RNAi, under the regulation of a temperature sensitive gene, to reduce transcription of sls. Expression of gSAPs decreases with increased exposure to a restrictive temperature. The molecular and physiologic efficacy of this paradigm will be addressed and early results are given in the context of the hypothesis that gSAPs dampen sarcomere length changes and promote maintenance of force produced after actomyosin activation. Force hysteresis in this preparation is well described and not of synaptic origin; wild type animals maintain force that is greater than predicted when an initial activation precedes the contraction being examined. We tested our hypothesis using isometric conditions, giving an initial burst of actomyosin activation and modulating activation frequency thereafter, and force ergometer / mechanical approaches. Animals with reduced gSAP expression exhibited reduced hysteresis but maintained fundamental parameters of contraction; peak force and time constants were not significantly different from controls. These findings support the hypothesis that gSAPs can act as tethers in arthropod muscle.
