Exogenous Mesp1 expression induces a cardiac progenitor fate

Objectives: Direct conversion of fibroblasts into cardiomyocyte-like cells was reported by using Gata4, Mef2c and Tbx5 (GMT) as reprogramming transcription factors. Beside us, many other investigators reported the insufficient reprogramming capacity of GMT to induce a complete phenotype conversion into cardiomyocyte like cells.

Mesp1, an early temporarily expressed transcription factor is regulating mesendodermal cell fate during development. Transient expression of Mesp1 in embryonic stem cells (ES) leads to a significantly increased cardiac progenitor number during in-vitro differentiation.

Methods: A murine Nkx 2.5 cardiac enhancer GFP transgenic line was used as reporter line to detect transdifferentiation of somatic cells into cardiac progenitor-like cells. Tail tip fibroblasts as well as satellite cells isolated from the upper limb were used as starting cell material. Transduction of Mesp1 was achieved by using retro - and lentiviral systems. After transduction cells were cultured for additional 14 days. Fluorescence microscopy, FACS analysis, quantitative RT-PCR and gene-expression microarray analysis were performed to evaluate the influence of exogenous Mesp1 overexpression on somatic cells.

Results: Retroviral as well as lentiviral transduction led to a significantly increased expression of Mesp1 (> 80-fold) in somatic cells over untransduced control cells 7 and 14 days after transduction. No activation of the Nkx 2.5 cardiac enhancer GFP reporter transgene could be detected by fluorescence microscopy or FACS analysis after singular transduction of Mesp1. Quantitative RT-PCR revealed a significant induction of the Nkx2.5 expression (12.6-fold after 7 days; 3.5-fold after 14 days). Additionally a significantly increased Troponin T expression (6.1-fold after 7 days; 29.9-fold after 14 days) was detected in Mesp1 transduced fibroblasts over untransduced control cells. Gene-expression microarray analysis showed a similar expression pattern for Mesp1-transduced satellite cells compared to Nkx2.5 cardiac enhancer GFP positive progenitor cells isolated from differentiated Nkx2.5 cardiac enhancer GFP ES cells suggesting a more general induction of a progenitor-like cell expression pattern by Mesp1. So far, no beating cells could be observed after singular transduction of Mesp1.

Conclusion: Mesp1 seems to play an important but not sufficient role for the induction of cardiac progenitor-like cells during direct reprogramming of somatic cells.