Neuronal Trans-differentiation by Transcription Factors Ascl1 and Nurr1: Induction of a Dopaminergic Neurotransmitter Phenotype in Cortical GABAergic Neurons

Neurons with a desired neurotransmitter phenotype can be differentiated from induced pluripotent stem cells or from somatic cells only through tedious protocols with relatively low yield. Readily available cortical neurons isolated from embryonic rat brain, which have already undergone a complete neuronal differentiation process, might serve as alternative template source. These cultures consist of 85% glutamatergic and 15% GABAergic neurons, and we attempted to trans-differentiate them into dopaminergic neurons. Transcription factors Nurr1, Lmx1A and Pitx3, essential determinants of a dopaminergic cell fate during CNS development, were not sufficient to induce tyrosine hydroxylase expression in a significant number of cells. Combining Nurr1 with the generic neuronal differentiator and re-programming factor Ascl1, however, resulted in generation of neurons which express dopaminergic markers TH, AADC, VMAT2 and DAT. Only neurons of GABAergic phenotype could be trans-differentiated towards a dopaminergic neurotransmitter phenotype, while for glutamatergic neurons, this process proved to be neurotoxic. Intriguingly, GABAergic neurons isolated from embryonal midbrain could not be trans-differentiated into dopaminergic neurons by Ascl1 and Nurr1. Thus, in principle, post-mitotic embryonal neurons can serve as templates for neurons with a desired neurotransmitter phenotype. However, neurotransmitter phenotype plasticity critically depends on the differentiation history of the template neurons, which can result in relatively low yields of dopaminergic neurons.