UNCOVERING NOVEL MECHANISMS OF NEURONAL AXON PRUNING

Axon pruning is a unique process neurons utilize to selectively degenerate axon branches while keeping the neuronal cell body intact. The mechanisms of axon pruning are best studied in peripheral neurons in a compartmentalized microfluidic model which allows for both spatial separation, and fluidic isolation of distal axons from the cell bodies. This is important as axon pruning is induced by the deprivation of nerve growth factor (NGF) specifically from distal axons. Global NGF deprivation (from both axons and cell bodies) induces apoptosis. Interestingly, the mechanisms of axon pruning have much in common with those of apoptosis. Both the axon pruning and apoptosis pathways result in the activation of the pro-apoptotic protein Bax, and both pathways require caspases, specifically Caspase-9 and Caspase-3. Despite the similarities between these two pathways, there are key differences. For example, axon pruning requires an additional caspase, Caspase-6, and does not require the protein Apaf-1 to activate these caspases. This is striking as the activation of apoptotic caspases is well known to be an Apaf-1 dependent process. Thus, exactly how caspases are activated in an Apaf-1-independent manner during axon pruning is unknown.While investigating potential Apaf-1 independent mechanisms, we considered the possibility that Caspase-6 could be activated by a non-apoptotic protein, Caspase-1. We found that Caspase-1 inhibition or deficiency blocks axon pruning remarkably well in neurons. These results were unexpected as Caspase-1 has been well characterized as an inflammatory protein which functions to mount an immune response to various pathogenic stimuli. In this context Caspase-1 is known to be activated by inflammasomes, protein complexes that sense specific pathogens or pathogenic activity. We examined if inflammasomes were also involved in axon pruning and found that the NLRP1 sensor protein (but not other inflammasome sensors) was essential for axon pruning. Interestingly, neither NLRP1 or Caspase-1 were important for axon degeneration during apoptosis.Our results identify an unexpected function of the NLRP1 inflammasome in physiological axon pruning in neurons. These results also define the overlapping yet markedly distinct mechanisms by which axon degeneration occurs during apoptosis and pruning.