Assessing Whether the Adult Neocortex Can Incorporate New Projection Neurons

The neocortex is the seat of our highest cognitive functions. Neocortical projection neurons can be lost due to neurodegenerative diseases and once lost they are not normally replaced, leading to permanent functional deficits. There are a number of therapies in development to alleviate the symptoms and retard the onset of neurodegenerative diseases, such as Alzheimer's, in which neocortical projection neurons are lost. However, additional treatment, such as the replacement of cortical projection neurons, will eventually be necessary to return patients to pre-disease states. Developing strategies to replace lost projection neurons is a daunting task because of the complexity and size of the neocortex. Previous attempts at replacing projection neurons in the neocortex using several types of transplanted neural stem or precursor cells have not provided viable strategies. One reason for the limited integration of new neurons is that thus far transplanted cells have remained primarily in a clump at the transplant site. To achieve the goal of functionally integrating new projection neurons throughout broad areas of the neocortex, a novel strategy is required that takes into account cell dispersion. We are developing an approach for introducing new, widely dispersed, projection neurons in the adult neocortex, providing a paradigm for testing whether they can functionally integrate. Our preliminary data suggest that we can achieve the wide dispersion of precursors in the adult neocortex and that new neurons can extend long projections along their normal tracks and to their normal targets. We are using engineering precursors, which have a natural ability to disperse in the adult neocortex, with doxycycline-inducible transcription factors that could, once the precursors have dispersed, allow their reprogramming to a cortical projection neuron fate. We are also assess the ability of transplanted neuronal cells to disperse and integrate in a healthy environment compared with one in which projection neurons are degenerating.