In silico Cell Therapy Model Restores Failing Human Myocyte Electrophysiology and Calcium Cycling in Fibrotic Myocardium

Abstract

Myocardial delivery of human c-kit+ cardiac interstitial cells (hCICs) and human mesenchymal stem cells (hMSCs), an emerging approach for treating the failing heart, has been limited by an incomplete understanding of the effects on host myocardium. This computational study aims to model hCIC and hMSC effects on electrophysiology and calcium cycling of healthy and diseased human cardiomyocytes (hCM), and reveals a possible cardiotherapeutic benefit independent of putative regeneration processes. First, we developed an original hCIC mathematical model with an electrical profile comprised of distinct experimentally identified ion currents. Next, we verified the model by confirming it is representative of published experiments on hCIC whole-cell electrophysiology and on hCIC co-cultures with rodent cardiomyocytes. We then used our model to compare electrophysiological effects of hCICs to other non-excitable cells, as well as clinically relevant hCIC-hMSC combination therapies and fused hCIC-hMSC CardioChimeras. Simulation of direct coupling of hCICs to healthy or failing hCMs through gap junctions led to greater increases in calcium cycling with lesser reductions in action potential duration (APD) compared with hMSCs. Combined coupling of hCICs and hMSCs to healthy or diseased hCMs led to intermediate effects on electrophysiology and calcium cycling compared to individually coupled hCICs or hMSCs. Fused hCIC-hMSC CardioChimeras decreased healthy and diseased hCM APD and calcium transient amplitude compared to individual or combined cell treatments. Finally, to provide a theoretical basis for optimizing cell-based therapies, we randomized populations of 2,500 models incorporating variable hMSC and hCIC interventions and simulated their effects on restoring diseased cardiomyocyte electrophysiology and calcium handling. The permutation simulation predicted the ability to correct abnormal properties of heart failure hCMs in fibrotic, but not non-fibrotic, myocardium. This permutation experiment also predicted paracrine signaling to be a necessary and sufficient mechanism for this correction, counteracting the fibrotic effects while also restoring arrhythmia-related metrics such as upstroke velocity and resting membrane potential. Altogether, our in silico findings suggest anti-fibrotic effects of paracrine signaling are critical to abrogating pathological cardiomyocyte electrophysiology and calcium cycling in fibrotic heart failure, and support further investigation of delivering an optimized cellular secretome as a potential strategy for improving heart failure therapy.

Computational design of custom therapeutic cells to correct failing human cardiomyocytes

“…To model non-fibrotic HF cardiomyocytes, several parameters were modified as detailed elsewhere by Mora et al ( Mora et al, 2018 ) to reflect the underlying pathophysiology of HF with reduced ejection fraction. HF in fibrotic myocardium was modeled as coupling of an individual HF with reduced ejection fraction cardiomyocyte to 5 fibroblasts with a gap junctional conductance of 1 nS, as detailed in our previous work ( Phillips et al, 2021 ) (adapted from Mora et al ( Mora et al, 2018 )). Pacing frequency was set at 1 Hz, and all simulations were run for 500 beats to achieve a steady state solution, in accordance with our previous work ( Phillips et al, 2021 ).…”

“…Ionic currents from hCIC channels—including a sodium current, inward rectifying potassium current, large-conductance calcium-activated potassium current, and transient outward current—were modeled as described in our recent work ( Phillips et al, 2021 ). Ionic currents from hMSC channels—including a transient outward current, sodium current, L-type calcium current, calcium-activated potassium current, and delayed rectifier potassium current—were modeled as described in our previous work ( Mayourian et al, 2016 ).…”

“…The capacitance of all custom cells was set to 40pF, to reflect the approximate capacitance of hMSCs ( Mayourian et al, 2016 ) and hCICs ( Phillips et al, 2021 ), and to reflect their smaller size compared to hCMs ( O’Hara et al, 2011 ). During simulations, intracellular ion concentrations of all custom cells were assumed to be held constant.…”

“…A human cardiomyocyte (non-fibrotic HF, fibrotic HF, or healthy) was coupled to custom cells using the following pair of equations, similar to our previous work ( Phillips et al, 2021 ) and as described elsewhere ( Xie et al, 2009 ): …”

“…Our lab has recently created a computational pipeline to investigate the heterocellular coupling effects of hMSCs ( Mayourian et al, 2016 ; Mayourian et al, 2017 ; Mayourian et al, 2018a ; Mayourian et al, 2018b ), hCICs ( Phillips et al, 2021 ), and “enhanced” CardioChimeras ( Phillips et al, 2021 ) on healthy and failing cardiomyocytes. In our series of works, we: 1) developed hMSC ( Mayourian et al, 2016 ) and hCIC ( Phillips et al, 2021 ) models that were validated by previously published experimental data ( Li et al, 2005 ; Zhang et al, 2014 ); 2) helped unravel relative paracrine versus heterocellular coupling effects of delivered therapeutic cells on human cardiac contractility and arrhythmogenicity ( Mayourian et al, 2017 ); and 3) demonstrated that heterocellular coupling of standard hMSCs and hCICs are insufficient to correct failing human cardiomyocytes ( Phillips et al, 2021 ).…”