Heart failure is extremely complex, but at its core, it results in a reduced ability of the heart to fill with blood and then pump it throughout the body. The pump function of the whole heart is driven by the ability of millions of heart cells (myocytes) to contract with every heart beat. Inside each of these cells is a specialized protein lattice called the myofilament, which is what actually causes myocytes, and the heart, to contract. 

The Kirk lab uses sophisticated biophysical assays to study human and animal models of disease to understand the precise molecular mechanisms that cause the myofilament to malfunction. Myofilament function is exquisitely controlled by switches on the proteins that make up the lattice, and we use mass spectrometry to discover which of these have been incorrectly switched on (or off) in disease. Then we determine how to reverse these switches and attempt to restore contractile strength to the heart. 
The Cardiac Myofilament - The Engine of the Heart

    The myofilament is the structure in cardiac myocytes that allows them to contract, providing the pumping function for the heart. The myofilament is organized into individual sarcomeres (a schematic of a single sarcomere is shown below).

Within the sarcomere, there are only a handful of proteins that are directly responsible for generating force. The thick filament (magena in the schematic shown) is composed of myosin, with myosin heads extending towards the actin thin filaments (blue). When the myosin heads attach to their binding spots on the actin thin filament, they can perform a POWERSTROKE which pulls the filaments past each other, generating force. 

    However, when the heart is relaxed and filling with blood (a necessary component of the cardiac cycle, if the heart can’t relax to fill with blood, it doesn’t matter how strong it is), the myosin binding spots on actin are blocked. Specifically, a protein called tropomyosin wraps around actin to block the myosin heads from attaching, while the troponin complex (consisting of troponin I, troponin C, and troponin T) make up the “switch” that moves tropomyosin out of myosin’s binding spot when the heart needs to contract. The signal for troponin to move tropomyosin is a huge increase in intracellular calcium ions that signals the beginning of cardiac contraction.
Cardiac Dyssynchrony - The Heart Out of Beat
    Contraction of the left ventricle is precisely coordinated by the His-Purkinje system, which rapidly conducts electrical excitation to the myocardium. This ensures that shortening throughout the muscle wall occurs synchronously and by a similar magnitude to optimize the pumping efficiency of the heart. Diseases of the conduction system such as a left-bundle branch block (LBBB) lead to a loss of synchrony, and occur in 30-50% of patients with heart failure. As a result, the early-stimulated regions do not generate sufficient pressure to eject blood, but merely stretch the later-activated regions, while the opposite happens later in the cardiac cycle. The “sloshing” of blood within the heart results in a reduction in mechano-energetic performance. In the failing heart, where function is already reduced, dyssynchrony worsens both morbidity and mortality.

    Pioneering studies in the 1990s showed that simultaneous bi-ventricular pacing applied to dyssynchronous hearts improved function and chamber efficiency, while concomitantly lowering morbidity and mortality. This ultimately led to Cardiac Resynchronization Therapy (CRT). To date, CRT remains the singular therapy for heart failure that simultaneously improves both acute and chronic systolic function, increases cardiac work, and yet also prolongs survival.

    Traditionally, CRT has been viewed as a mechanical tuning of the heart. Its relative simplicity and ease of entry into the clinic led to rapid development, testing, and approval – all performed in human subjects. There was very little basic science on CRT reported prior to its clinical adaption. Recently, however, there have been efforts by ourselves and others to “reverse-engineer” CRT, exploring the cellular and molecular mechanisms that are involved. Indeed, dyssynchrony and resynchronization therapy induce a wide range of changes, many unique to both the disease and the treatment. Interestingly, in some instances, CRT does not simply reverse the damage done by dyssynchrony, but acts in entirely novel ways to improve function.

    Our work involves furthering this understanding, with the goal of improving CRT and discovering a way to bring its substantial benefits to the wider heart failure population.

    The myofilament is regulated by protein phosphorylation and other post-translational modifications (PTMs). While antibody-based approaches are useful (and utilized in our lab), they come with a variety of drawbacks including a bias - they can only discover changes that have been looked for. Mass spectrometry provides an un-biased snapshot at a huge number of proteins and PTMs in a sample. Combined with sample fractionation and enrichment, this allows us to observe even very rare events that can have very significant affects in the heart.
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