Projects
The figure above shows a tiny region of a muscle. The purple squares are the sarcomeres, a typical muscle cell contains thousands of sarcomeres, and a fascinating thing about them is that they all have the same size. We want to know how nature preserves sarcomere with such precision.
Our projects have to do with the regulation of sarcomere shape. One project looks specifically at how sarcomeres keep their shape despite actively contracting at all times. We think muscles use elastic proteins that serve as molecular springs that hold the sarcomere together. Another project looks at how sarcomeres assemble as the muscles differentiate and grow. Sarcomeres first assemble at the cytoplasm as tiny protein condensates that grow to their final size by recruiting cytoplasmic proteins. We want to know how the muscle dictates sarcomere growth rate and duration.
Project 1 - The proteins that hold the sarcomere together
In every contraction cycle, myosin filaments pull actin filaments toward the middle of the sarcomere. Because actin filaments are anchored to the Z-disc, myosin forces result in the shortening of the sarcomere. The sarcomere relies on elastic molecules to resist myosin pulling forces from extracting actin filaments away from the Z-disc and to hold the sarcomeres together during each contraction cycle. The giant protein titin stretches from the Z-disc to the myosin filaments and serves as an elastic element that holds the sarcomere together. The actin-binding protein filamin also provides sarcomere cohesion and in filamin mutants, the Z-disc brake. In flies, filamin contains 2 actin-binding domains and 22 Ig domains. The region between Ig domains 14 and 19 is called the mechanosensory region. Normally, it adopts a closed conformation in which Ig domains 14-19 coil around each other, upon pulling forces, it adopts a linear or open conformation. We hypothesize that filamin uses this conformational switch to provide elastic protection to the Z-disc and resist contractile forces.
Project 2 - The metabolic regulation of sarcomere growth
The Zasp/Enigma proteins and actinin are at the core of Z-disc assembly and growth. They are required for the first steps of Z-disc formation and they localize to nascent Z-discs. Actinin cross-links actin filaments from opposite sides of the Z-discs and Zasp is a scaffold protein that coordinates protein recruitment. To discover novel Z-disc proteins involved in sarcomere growth, we did a genetic screen using a combination of bioinformatic and genetic approaches. We uncovered a few very promising candidates, which we plan to further investigate. One is the oxoglutarate dehydrogenase enzyme, a member of the Krebs cycle. Without this enzyme, the Z-discs assemble but do not grow. Because oxoglutarate dehydrogenase is critical to metabolic regulation, we are also interested in other related enzymes.