Direct Reprogramming-The Future of Cardiac Regeneration?

Abstract

Today, the only available curative therapy for end stage congestive heart failure (CHF) is heart transplantation. This therapeutic option is strongly limited by declining numbers of available donor hearts and by restricted long-term performance of the transplanted graft. The disastrous prognosis for CHF with its restricted therapeutic options has led scientists to develop different concepts of alternative regenerative treatment strategies including stem cell transplantation or stimulating cell proliferation of different cardiac cell types in situ. However, first clinical trials with overall inconsistent results were not encouraging, particularly in terms of functional outcome. Among other approaches, very promising ongoing pre-clinical research focuses on direct lineage conversion of scar fibroblasts into functional myocardium, termed "direct reprogramming" or "transdifferentiation." This review seeks to summarize strategies for direct cardiac reprogramming including the application of different sets of transcription factors, microRNAs, and small molecules for an efficient generation of cardiomyogenic cells for regenerative purposes.

Keywords: Gata4, Mef2c, Tbx5 (GMT); direct reprogramming; induced cardiomyocytes (iCMs); transdifferentiation.

Figure 1

In vitro approaches for direct reprogramming of fibroblasts into induced cardiomyocytes (iCMs). The CASD lineage conversion method tries to directly convert fibroblasts into iCMs by a transient overexpression of pluripotency factors in combination with lineage specific soluble signals. Other, more direct approaches use transcription factors (TFs), microRNAs, or a combination of both (&) to achieve iCMs. Abbreviations: CASD: Cell-activation and signaling-directed; TF: transcription factor; miR: microRNA; iCM: induced cardiomyocyte; JI1: JAK inhibitor JI1; BMP4: bone morphogenic protein 4; S: SB431542 (ALK4/5/7 inhibitor); C: CHIR99021 (GSK3 inhibitor); P: parnate (LSD1/KDM1 inhibitor); F: forskolin (adenylyl cyclase activator).

Figure 2

Reliability and temporal appearance of cardiomyocyte specific markers during the direct reprogramming process. Different markers for induced cardiomyocytes (iCMs) are depicted with increasing stringency to determine the cardioinducing effect and cellular reprogramming capacity of certain factor combinations.

Figure 3

Remaining challenges of direct reprogramming approaches before a clinical application. Before a translation of direct reprogramming approaches from bench to bedside, several challenges have to be resolved. First of all, issues like the inefficiency of the reprogramming process from fibroblasts (grey cells in front of the arrow) to induced cardiomyocytes (iCMs, red elongated shaped cells behind the arrow), the insufficient maturity of the iCMs, the delivery method, or the understanding of underlying mechanisms have to be addressed. As a next step, especially for evaluating the safety of delivery methods, large animal studies have to be performed. Further effort has to be put into the optimization of the reprogramming technology before a clinical application becomes possible.