The invention provides compositions and in vivo, ex vivo and in vitro methods for trans-differentiation of or re-programming mammalian cells to functional neurons. In particular, the invention provides methods for engineering non-neuronal cells into neurons, including fully functional human neuronal cells, and methods for engineering non-neuronal cells into neurons, e.g., fully functional human neuronal cells, in the brain to treat a neurodegenerative disease. In alternative embodiments, the invention provides compositions comprising re-differentiated or re-programmed mammalian cells, such as human cells, of the invention. The invention also provides compositions and methods for direct reprogramming of cells to a second phenotype or differentiated phenotype, such as a neuron, including a fully functional human neuronal cell. The invention also provides formulations, products of manufacture, implants, artificial organs or tissues, or kits, comprising a trans-differentiated or re-programmed cell of the invention, e.g., a fully functional human neuronal cell.
