Fig. 2

The developmental strategies to generate a biological pacemaker. A Different systems to generate biological PCs. Functional re-engineering induces the expression of specific ion channels in VMs to generate ion currents to generate electrical firing. Other than this, cell morphology, structure, and functions remain the same as VMs. The ion channel genes could be expressed in non-cardiomyocyte cells (e.g., MSCs) to generate ion currents to induce pacemaker activity. However, spontaneous action potentials from engineered non-cardiomyocyte cells must be coupled with the nearby VMs. This explains why the efficacy of the cell-gene hybrid approach is worse than those of engineered VMs. Direct reprogramming changes VMs to PCs with holistic changes of morphology, structure, function, and transcriptions. The reprogramming could be reached by the re-expression of transcription factors (e.g., Tbx18) or biomaterials (e.g., silk fibroin). The last is the strategy of cell therapy. Human IPS-PCs could be derived from the subtype differentiation of human IPS cells, and implanted into the heart directly. B Biological products for the therapy. The virus (adeno or adeno-associated) or non-viral vectors (lipid nanoparticles for mRNA, DNA, and minicircle DNA) could express the candidate genes for VMs for functional re-engineering or direct reprogramming. The biomaterial per se could be applied as the biological product to induce reprogramming in the heart. For cell therapy, immunocompatible human IPS-PCs could be implanted into the heart with different designs, including PCs alone, cell mixture (e.g., with MSCs), or engineered PC tissues. C A minimally invasive procedure should be used to deliver the biological products into the heart from preclinical large animal models to humans. We illustrate that the vectors could be delivered by transvenous catheter into the specific area of the ventricular septum, His bundle, to generate a biological pacemaker. D The biological pacemaker could be used to construct organoids (self-organized three-dimensional SANs) or engineered SAN-like tissues. These could be applied as the in-vitro model for screening drugs or exploring pathogenesis, and replace the animals for the preclinical studies. HiPSC, human induced pluripotent stem cell; AVN, atrioventricular node