Direct neural reprogramming in situ: effectiveness of existing approaches and their possible optimizations

Direct neuronal reprogramming (transdifferentiation) in situ of glial cells-astrocytes and microglia-has attracted a substantial interest as a potential approach for the treatment of a wide range of neurodegenerative diseases and injuries of the central nervous system (CNS). The nervous system of higher mammals has a very limited capacity for repair. Disruption of CNS functions due to traumatic injuries or neurodegenerative processes can significantly affect quality of life of patients, leading to mobility- and cognitive impairments, resulting in disability and in some cases death. Restoration of lost neurons in situ, based on direct reprogramming of glial cells without an intermediate stage of pluripotency, appears to be the most attractive approach from the point of view of translational biomedicine. The ability of astroglia to highly proliferate in response to the damage of neural tissue supports the view that these cells, already present at the lesion site and neuronal-like, are good candidates for transdifferentiation into neurons, especially because many independent authors have already shown the possibility of direct neuronal reprogramming of astrocytes in vitro and in vivo. Overexpression of proneuronal transcription factors (TFs), such as NeuroD1-4, NeuroG2, Ascl1, Dlx2, including pioneer factors capable of recognizing target sequences in “closed” chromatin and activating transcription of “silent” genes, has already proven to be a potential therapeutic strategy. Furthermore, blocking the action of PTB and REST TFs via microRNAs, using small molecules or various biomaterials are also utilized for neuronal reprogramming. However, the efficiency of direct in situ reprogramming is limited by a number of factors, including the cell-specificity of the transgene delivery systems, the cell-specificity of promoters in the genetically engineered constructs used for transgene delivery, the brain region in which transdifferentiation occurs, factors affecting the changes of cell metabolism, the influence of microenvironment, etc. Reprogramming in situ, where a large number of cell types are present, requires monitoring and precise phenotypic characterization of subpopulations of cells undergoing transdifferentiation, confirming the fact of reprogramming of astroglia into neurons and their subsequent integration into the CNS. Here, we review and summarize the most effective strategies of neuronal reprogramming, ways to visualize the transdifferentiation process, and also focus on the existing obstacles to effective neuronal conversion and possible approaches to overcome them.