UCLA

In the mature mammalian central nervous system (CNS), axons do not regenerate after injury. To improve regeneration, past studies have focused on removing the cell extrinsic signals that block regeneration. However, recent studies suggest that the decline of intrinsic growth capacity of neurons might be the major reason why axons do not regenerate in the adult CNS. The Dual Leucine Zipper Kinase (DLK) pathway is a key intrinsic signal for regeneration after axonal injury. Overexpression of dlk leads to robust regeneration while loss of dlk completely abolishes regeneration after injury. In contrast to the extensive studies on DLK and its downstream signaling cascade, upstream signaling pathways that control DLK remain largely unknown. This dissertation aims to identify the upstream regulators of dlk. In chapter two, we showed that the scaffolding proteins X11Lα and X11Lβ are negative regulators of the Drosophila homolog of DLK, Wallenda (Wnd). We found that loss of X11 leads to enhanced axonal regeneration that is dependent on the DLK/Wnd pathway because inhibition of this pathway suppresses the regeneration phenotypes. Our data suggest that X11 normally prevents DLK/Wnd from promoting axonal growth. Upon injury, increase in intracellular Ca2+ triggers degradation of X11 by the Ca2+-dependent protease CalpB. The degradation of X11 in turn activates DLK/Wnd and initiates axonal regeneration. In Chapter three, we identified a small GTPase, rheb, that greatly enhances axonal regeneration and suppresses axonal degeneration in Drosophila when overexpressed. Enhanced axonal regeneration and reduced axonal degeneration caused by rheb overexpression are dependent on the DLK/Wnd pathway. Furthermore, we found that overexpression of rheb leads to an increase of DLK/Wnd protein level. Finally, we found that inhibition of the mTORC1 pathway suppresses rheb-mediated axonal overgrowth. Taken together, our results suggest that rheb regulates Wnd level by increasing protein translation of DLK/Wnd through the mTORC1 pathway. This thesis provides mechanistic insights into DLK/Wnd regulation by X11 and rheb that will help develop therapeutic strategies for regeneration of the injured CNS.