Research project

I investigated the structural relationship between the T-tubule system and the microtubule cytoskeleton in healthy rabbit right ventricular myocytes. Before I begin to describe what exactly I worked on and what my conclusions were I need to explain a few terms. The rhythmic activation of cardiac myocytes by action potential results in what know as heart beat. T-tubules are invaginations of the muscle cell membrane, called the sarcolemma, and their main function is to propagate the action potential throughout the entire cell to ensure a synchronous contraction of the cell. They do this by being in a close proximity to the calcium ion storage warehouse, the sarcoplasmic reticulum. An incoming action potential opens calcium ion channels on the sarcolemma, resulting in an elevated level of calcium between the T-tubules and the sarcoplasmic reticulum. Through positive feedback, this opens calcium channels on the latter and increases the intracellular calcium concentration enough to activate the contractile machinery of the cell.

A problem arises in heart failure when T-tubules start to lose their regular striated organization and to become displaced from sarcoplasmic reticulum. As a consequence, less calcium ions are released and heart contractility deteriorates. As of now, we still don’t know why T-tubules become misdirected in heart failure. It has been, however, recently showed that T-tubule formation is dependent on the transport of protein and channels essential in T-tubule formation. These proteins are transported in small vesicles by kinesin motor proteins crawling along microtubules from their site of synthesis to the T-tubule system. This is impaired in heart failure when microtubules become denser. We believe that T-tubule misdirection could be related to this densification of microtubule cytoskeleton.

Unfortunately, there were no failing rabbit heart cells available during my bachelor thesis. Therefore, I analysed the structural relationship between the microtubule cytoskeleton and the T-tubule system in healthy cells. Using a state-of-the-art super resolution confocal microscope, I found that T-tubules form transversely across the cell in a striated pattern. I was the first one to quantify the distribution of microtubules and found that they are distributed predominantly longitudinally. Furthermore, microtubule intersections are more likely to occur within the area occupied by T-tubules and they have an approximately perpendicular geometry. We still don’t know, however, whether these play any role in protein transport to T-tubules. It would be therefore very interesting to explore these structures in failing cells and compare them to the healthy ones.