Augmentations vs. Restoration: A computational study of the effects of bacterial sodium channels on cardiac conduction.
Cardiac arrhythmias, including ventricular tachycardia, ventricular fibrillation, and atrial fibrillation, are associated with ectopic triggers such as those resulting from afterdepolarizations and structural changes within the cardiac changes. While ectopic triggers can be dealt with via radio frequency ablation, structural causes of arrhythmia, such as microscale source-load mismatches, do not have available treatments. Augmentation of cardiomyocytes with exogenous sodium channels such as Nav1.4 or prokaryotic voltage-gated sodium channels, or BacNavs, have shown promise for potentially alleviating these arrhythmias. However, due to size constraints, only the BacNavs are available for the highest efficiency viral vectors for stable transduction. Limitations in the ability to test these channels in adult mammalian cardiac tissue, particularly tissue with source-load mismatches, have led to a lack of understanding about BacNav’s therapeutic value. This dissertation aims to build models of engineered BacNavs and compare their impact in simulated diseased and healthy cardiac tissue with increases of the endogenous Nav1.5 current to probe mechanisms for therapy.Patch clamp data was analyzed to derive steady-state values and kinetics for the activation and inactivation gating of the BacNavs using techniques dating back to Hodgkin and Huxley’s squid axon model. Models using a cubic activation function and only a single slow inactivation channel were best able to replicate the data, including action potential traces and restitution curves for both action potential duration and conduction velocity. The single slow inactivation channel matches what has been observed in crystallography studies of other BacNav channels. Including the derived BacNav model into membrane models for guinea pig and human ventricular myocytes revealed general trends of action potential duration reduction, action potential amplitude increase, and increases in conduction velocity and upstroke velocity. The action potential duration and amplitude trends were more significant for BacNav than Nav1.5, but the endogenous channel was superior for conduction velocity increase. These effects existed despite different responses in relative and absolute current densities between the two membrane models. Despite evidence that late sodium current can lead to afterdepolarizations, BacNav did not increase susceptibility to them in vulnerable midmyocardial cells except at extremely high current densities. Finally, reductions in action potential duration removed alternans present in the restitution curves for single cells. To study how BacNav affected arrhythmias, BacNav was incorporated into one-dimensional cables and two-dimensional tissues with source-load mismatches present, premature stimuli that could induce unidirectional block or channelopathies such as mutations leading to Brugada syndrome. BacNavs outperformed the endogenous channel in source-load mismatches due to increased action potential amplitude and slower inactivation kinetics. These conclusions were stable to spatial heterogeneity in the treatment. It was also able to rescue Brugada syndrome in a dose-dependent manner and narrow the vulnerable window to unidirectional block for one-dimensional cables. In two dimensions, Nav1.5 had a smaller window to spiral wave induction but experienced wave breaks and multiple wavelets, whereas rotors with BacNav-treated cells were stable. These findings help generate hypotheses to be tested experimentally and further refine the model. Further studies may uncover engineering principles for designing optimal sodium channels for specific pathologies.
bacterial sodium channels
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