UCSF

Work in the lab of Graeme Davis has shown that genetic disruption of the presynaptic Spectrin-Ankyrin skeleton results in catastrophic degeneration of the presynaptic motoneuron terminal at the Drosophila neuromuscular junction (NMJ). My dissertation work has focused on identifying genes that protect against presynaptic neuromuscular degeneration following disruption of this presynaptic Spectrin-Ankyrin skeleton. The following chapters identify and describe presynaptic neuroprotective signaling, which can significantly slow the progression of NMJ degeneration and dysfunction.

In Chapter 2, I demonstrate that NMJ disassembly following loss of α-Spectrin can be suppressed by a WldS (Wallerian degeneration slow) transgene, providing evidence for a Wallerian-type degenerative mechanism. I subsequently describe a second neuroprotective signaling system, and show that enhanced MAPK-JNK-Fos signaling suppresses NMJ disassembly. Importantly, Fos over-expression does not restore α-Spectrin protein levels at the NMJ, nor does it rescue defects in the underlying microtubule cytoskeleton, demonstrating that Fos has a neuroprotective activity that is separable from cytoskeletal disruption. This signaling pathway is activated following an acute cytoskeletal disruption, suggesting an endogenous role during neurological stress, and includes delayed, negative feedback via the JNK phosphatase puckered. This negative feedback of JNK-Fos allows NMJ disassembly in the presence of persistent cytoskeletal stress. I present a model in which signaling via JNK-Fos functions as a stress response that is transiently activated following cytoskeletal disruption to enhance NMJ stability, and is then shut off through puckered to allow NMJ disassembly during persistent cytoskeletal disruption.

In Chapter 3, I present evidence that the transcription factor Fos may also be required for maintaining baseline stability, as well as overall motoneuron health and function. Neuronal expression of a dominant-negative Fos construct results in significant degeneration of the NMJ terminal, as well as motor dysfunction and premature death. Interestingly, this impairment of NMJ stability occurs without any obvious preceding disruption of the underlying synaptic microtubule cytoskeleton, further confirming the placement of this stabilizing Fos activity downstream of the microtubule cytoskeleton. Lastly, in Chapter 4, I present evidence that this neuroprotective pathway may also play a role in regulating axonal transport and subsequent signaling within the axonal compartment.