RESTRICTION OF APOPTOSIS IN MATURE NEURONS BY MULTIPLE REDUNDANT BRAKES

Apoptotic cell death is a key part of normal nervous system development, as a process that allows for the controlled removal of developing neurons that fail to properly integrate into the nervous system. However, once the nervous system is established, the production of new neurons is halted in most areas of the nervous system, meaning that neurons remaining after the establishment of the nervous system must last for the lifetime of the organism. Aberrant or accidental activation of the apoptotic pathway would be deleterious for this long-term neuronal survival, and neurons have been found to restrict the apoptotic pathway as they mature. However, while the resistance of mature neurons to apoptosis is well documented, the precise molecular mechanisms underlying this resistance have remained unclear. Recent findings from our lab found that one way neurons can restrict Bax activation is by upregulating the members of the miR-29 family of microRNAs, which can inhibit Bax activation by blocking the upregulation of multiple redundant members of the BH3-only family of proteins. While overexpression of miR-29 has been found to be sufficient to render young neurons resistant to apoptosis, it was unclear whether the miR-29 family of miRNAs was the only brake restricting Bax activation in mature neurons, or if other brakes on Bax activation exist. The work presented in this dissertation addresses this question by examining the status of the apoptotic pathway in neurons from mice in which all three members of the miR-29 family have been deleted. I found that in neurons lacking miR-29 expression, the apoptotic pathway remains restricted. I identified miR-24 as a microRNA that is also upregulated with neuronal maturation, which can act redundantly with miR-29 by repressing a similar repertoire or BH3-only domain genes. Additonally, I found that mature neurons are more resistant to Bax activation induced by direct injection of BH3-only peptides, and that mature neurons may also block apoptosis by restricting the phosphorylation and activation of the pro-apoptotic transcription factor c-jun. My findings also suggest that mature neurons are able to restrict the apoptotic pathway in the soma while maintaining a permissive environment in their axons.Doctor of Philosoph