The demonstration that mature somatic cells can be converted to new, heterologous cell types, now termed “cellular reprogramming”, led to the 2012 Nobel Prize in Physiology or Medicine, and sparked an explosion of studies investigating the therapeutic potential of such an approach. Direct neuronal reprogramming refers to the conversion of any terminally differentiated cell to an induced (i) Neuron without traversing a pluripotent state. My goal is to examine the therapeutic potential of direct neuronal reprogramming to treat amyotrophic lateral sclerosis (ALS). Specifically, I aim to convert astrocytes in the motor cortex to inhibitory GABAergic neurons in a SOD1G93A mouse model of ALS. The rationale for my approach is that: (1) upper motor neuron pathology in the motor cortex precedes lower motor neuron loss in the brainstem and spinal cord in ALS animal models and patients. Moreover, there is growing support for the idea that interventions targeting the motor cortex can have therapeutic benefits. (2) GABAergic neurons are hypoactive in the SOD1G93A motor cortex, and their activation preserves motor function in SOD1G93A mice. To optimize our approach, we have used our knowledge of embryonic neurogenesis to create phosphosite mutations in Ascl1, a basic-helix-loop-helix bHLH transcription factor that promotes the differentiation of GABAergic neurons. The serine (S) to alanine (A) mutations were made in residues adjacent to prolines (P), creating SA-mutations that keep Ascl1 active even in the presence of inhibitory proline-directed serine-threonine kinases (e.g., ERK, GSK3, CDKs). Using adeno-associated virus (AAV) gene delivery to the motor cortex, I showed that Ascl1 carrying six SA mutations (Ascl1-SA6) is more efficient at inducing neuronal conversion of adult brain astrocytes than native Ascl1. I also have preliminary evidence that Ascl1-SA6 expression in motor cortex astrocytes delays the onset of motor deficits in SOD1G93A mice. I hypothesize that mutated Ascl1 will convert motor cortex astrocytes to GABAergic iNeurons that will integrate into neural circuits to delay ALS progression. I test this hypothesis in three aims:
Aim 1. To confirm glial origins and functional properties of iNeurons in vivo
Aim 2. Assess Ascl1-SA6 reprogramming effects on brain health (cell autonomous & non-cell autonomous).
Aim 3. Assess Ascl1-SA6 reprogramming in a human cerebral organoid (CO) model of ALS.
Impact: Neuronal reprogramming is an innovative way to replace lost neurons. I will uncover new mechanistic insights into how optimized Ascl1 improves brain health in rodent models in vivo and in a human model of ALS.