Ischemic stroke is a leading cause of death and long term disability in the United States. Ischemic stroke results in death of neurons in the affected area with limited capacity for regeneration in the adult brain. Recent advances in stem cell techniques provide the possibility of ameliorating ischemic damage by replacing lost neurons with transplanted neuronal precursor cells that can terminally differentiate into mature neurons and integrate with the host circuitry. The focus of many transplantation studies currently centers on neuronal differentiation and transplantation of ES or iPS cells into the ischemic brain. More recently, studies demonstrate the panneuronal transcription factor NeuroD1 (ND1) can reprogram astrocytes directly into neurons, a process called direct reprogramming. Lentiviral vector delivery of ND1 to astrocytes results in permanently reprogrammed neurons without the need for maintained ectopic expression of the introduced transcription factor.
Astrocytes primarily provide support to surrounding neurons but also proliferate reactively in response to pathologies including ischemic stroke. Reactive astrocytes proliferate and hypertrophy in response to ischemic stroke and form a border around the site of injury forming a glial scar. Without intervention, there are an abundance of these reactive astrocytes in the peri-infarct region around the injury. Intra-lineage direct reprogramming provides an endogenous source of new neurons from existing proliferative astrocytes and has immense potential to reduce the burden of stroke.
Compared to traditional stem cell transplantation approaches, converted neurons derived from endogenous astrocytes will have the advantage of already being “settled” in a microenvironment that is more conducive to synaptogenesis and survival. Effective reprogramming of astrocytes to neurons further acts to “melt” the glial scar which normally exerts an inhibitory barrier for synaptogenesis and axonogenesis. Finally, new neurons are autologous and post-mitotic eliminating risk associated with rejection or tumor formation. Direct reprogramming of astrocytes has not yet been explored as a therapeutic tool in a model of ischemic stroke. The following work aims to use this novel approach to directly reprogram proliferative astrocytes into neurons in vivo following ischemic stroke and to augment activity dependent repair using whisker stimulation resulting in enhanced functional recovery in mice. We hypothesize direct conversion of astrocytes to neurons in the peri-infarct area will improve functional recovery in a mouse model of ischemic stroke that can be enhanced with whisker stimulation. The following study utilizes a novel approach to cell replacement in a model of ischemic brain injury and evaluates the efficacy of intra-lineage direct reprogramming of astrocytes into neurons in vivo. Direct reprogramming with activity dependent repair may improve functional recovery and reduce the morbidity of this common but devastating disease.
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