MOLECULAR MECHANISMS IN THE DEVELOPING AND MATURING BRAIN

Neurons, unlike other cell types, persist throughout the entire lifespan of an organism. Additionally, neurons use multiple anti-apoptotic brakes during different stages of their life cycle in order to maintain their long-term survival. This dissertation investigates the role of two well-known anti-apoptotic genes, Bcl-xL and miR-29, which are believed to serve important functions in preventing cell death during embryonic brain development and during the period of postnatal brain maturation, respectively. Using conditional knockout mouse models of Bcl-xL and miR-29, I found that these genes may have additional non-apoptotic roles at different timepoints during the neuron’s life cycle, challenging the previously accepted roles of these genes. First, I found that Bcl-xL, which is believed to be critical for brain development during the embryonic stages, was expressed at low levels in the rapidly dividing neuronal progenitor cells and was thus dispensable for the survival of these cells during embryonic development. In contrast, the early differentiated postmitotic neurons acutely rely on Bcl-xL for their survival, the lack of which results in severe consequences in mice including microcephaly and neurobehavioral deficits such as hyperactivity, increased risk-taking and self-injurious behaviors. Second, to probe the role of miR-29, a microRNA that is known to be critical for neuronal maturation by preventing apoptosis in neurons, I generated knockout mice that were conditionally deleted for miR-29. We and other labs have predicted that the main role of miR-29 is to prevent apoptosis and that deleting miR-29 could lead to widespread neuronal death. Surprisingly, however, miR-29-deficient mice exhibit no signs of cell death in the brain. In contrast, we identified a novel function of miR-29 in governing DNA methylation, an event that has been primarily studied in the context of cancer and whose roles in the brain are just beginning to be uncovered. I have found that miR-29 has an important epigenetic role in the brain via its ability to target the 3’ untranslated region (3’UTR) of a key DNA methyltransferase known as Dnmt3a. miR-29-deleted brains have increased levels of Dnmt3a, resulting in widespread hypermethylation across the genome. This, in turn, leads to transcriptional repression of multiple neuronal genes in the miR-29 knockout brains, resulting in severe neurobehavioral deficits including hyperactivity, hypersociability, excessive self-grooming and repetitive behaviors. Lastly, using RNAseq and gene association studies, I found that the pathways that are dysregulated in the miR-29-deleted mice are similar to those that are disrupted in the brains of patients with autism spectrum disorder (ASD), suggesting that miR-29 could be tested as a potential therapy in neurodevelopmental disorders in the future.Doctor of Philosoph