Thomas M. Butler, PhD

Contact Dr. Butler

1020 Locust Street
445 JAH
Philadelphia, PA 19107

(215) 503-6583

Expertise & Research Interests

The research in my laboratory is directed towards understanding the mechanisms that regulate tonic force maintenance in smooth muscle. This force maintenance is associated with little energy usage by the muscle and a very slow myosin crossbridge cycling rate. In vertebrate smooth muscle this is called the "latch" state, and the force output persists even though phosphorylation of the regulatory myosin light chain (which regulates actin-activated myosin ATPase activity) is very low. Many years of investigation by many laboratories have failed to elucidate the mechanism responsible for latch, so we have focused our research efforts on the mechanism of catch force maintenance which is the prototype of the latch state. We felt that this approach offered the best potential of obtaining a handle on how highly economical force is maintained and eventually released during relaxation in smooth muscles. Catch smooth muscle from invertebrates shows an extreme version of latch with little or no myosin crossbridge cycling associated with force maintenance. We have found that the phosphorylation state of the mini-titin, twitchin, located on the thick filament, is a major site of regulation of tonic force maintenance. Catch force is associated with the dephosphorylation of twitchin, and release of catch, or relaxation, is associated with its protein kinase A mediated phosphorylation. We have also characterized distinct mechanical states and their regulation by twitchin phosphorylation, and determined the sequence of Mytilus twitchin. The catch muscle is an ideal experimental model because of the ease (a) by which distinct mechanical states (calcium-driven cycling, non-cycling or catch crossbridges, relaxation) can be produced in intact and permeabilized muscles, and (b) of controlling the regulatory mechanisms, i.e., the state of twitchin phosphorylation and calcium. Conserved portions of proteins such as titin, projectin, C-protein and caldesmon, implicated in the regulatory processes of contraction are similar to portions of twitchin, suggesting its role in regulatory processes in general. The specific experiments underway include the following: 1) determination of the role, if any, of twitchin phosphorylation-mediated changes in ADP binding to and release from myosin in controlling the mechanical states of catch muscle; 2) determination of the mechanism by which twitchin regulates catch through measurement of the effects of recombinant portions of the molecule on mechanical parameters in permeabilized muscle; and 3) determination of the thick and thin filament binding characteristics of twitchin and recombinant portions of twitchin.