Direct reprogramming makes use of transcription factors (TFs) that induce the identity of specific cell types. These TFs often act in a context dependent manner, and are restricted in most cell types by inhibitory mechanisms that maintain cell fates (Graf & Enver, 2009; Zhau & Melton, 2008). In order to identify these barriers, we study the model organism Caenorhabditis elegans (C. elegans) using the zinc-finger TF CHE-1 that is required to induce the glutamatergic ASE neuron fate. Upon ectopic expression of CHE-1 and removal of barrier genes by RNAi, induction of the ASE neuronal fate marker
gcy-5prom::gfp can be seen in a variety of cell types including germ cells3, the epidermis and the intestine. We identified a candidate barrier gene for reprogramming germ cells into neurons (we call this the GeCo phenotype), which is the NAD+-dependent mitochondrial isocitrate dehydrogenase
idha-1. We observed that upon RNAi knockdown of
idha-1 and ectopic expression of CHE-1, cells in the germline acquire neuron-like morphology and express a number of neuronal fate markers. Furthermore, upon ectopic expression of the Pitx family of homeodomain-containing TF UNC-30, which specifies the fate of GABAergic motor neurons, germ cells express a reporter for the GABAergic neuron fate upon knockdown of
idha-1. Recent studies on mitochondria in the context of reprogramming and iPSCs, show that mitochondrial dynamics change during the process of differentiation and cells show distinct mitochondrial and metabolic signatures during proliferation and differentiation (Bukowiecki et al. 2014). This suggests that disturbing mitochondrial function may feed back to chromatin thereby altering gene expression and allowing reprogramming. However, this process seems to be more specific since the depletion of most other genes that are required for mitochondrial function do not result in the GeCo phenotype. Interestingly, the
idha-1 depletion-mediated reprogramming of germ cells to neurons is partially repressed in animals that lack the hypoxia-induced factor HIF-1. HIF-1 has been implicated in regulating iPSC reprogramming, a process that is triggered by changes in the level of the metabolite alpha-ketoglutarate (DeBerardinis et al., 2008; Xu et al., 2010). Importantly, alpha-ketoglutarate levels are regulated by isocitrate dehydrogenases such as IDHA-1 (Zeng et al., 2014; Zhang et al., 2015). Moreover, alpha-ketoglutarate is known to act as a co-factor of histone demethylases (Tsukada et al., 2006) and we are currently studying the underlying signaling mechanisms as well as the chromatin regulation involved during this cellular conversion.