Functional regeneration after nervous system injury requires transected axons to reconnect with their original target tissue. Axonal fusion, a spontaneous regenerative mechanism identified in several species, provides an efficient means of achieving target reconnection as a regrowing axon is able to contact and fuse with its own separated axon fragment, thereby re-establishing the original axonal tract. Here we report a molecular characterization of this process in Caenorhabditis elegans, revealing dynamic changes in the subcellular localization of the EFF-1 fusogen after axotomy, and establishing phosphatidylserine (PS) and the PS receptor (PSR-1) as critical components for axonal fusion. PSR-1 functions cell-autonomously in the regrowing neuron and, instead of acting in its canonical signalling pathway, acts in a parallel phagocytic pathway that includes the transthyretin protein TTR-52, as well as CED-7, NRF-5 and CED-6 (refs 9, 10, 11, 12). We show that TTR-52 binds to PS exposed on the injured axon, and can restore fusion several hours after injury. We propose that PS functions as a 'save-me' signal for the distal fragment, allowing conserved apoptotic cell clearance molecules to function in re-establishing axonal integrity during regeneration of the nervous system.

Chromosome fragmentation is a hallmark of apoptosis, conserved in diverse organisms. In mammals, caspases activate apoptotic chromosome fragmentation by cleaving and inactivating an apoptotic nuclease inhibitor. We report that inactivation of the C. elegans dcr-1 gene, which encodes the Dicer ribonuclease important for processing of small RNAs, compromises apoptosis and blocks apoptotic chromosome fragmentation. DCR-1 was cleaved by the CED-3 caspase to generate a C-terminal fragment with deoxyribonuclease activity, which produced 3’ hydroxyl DNA breaks on chromosomes and promoted apoptosis. Thus caspase-mediated activation of apoptotic DNA degradation is conserved. DCR-1 functions in fragmenting chromosomal DNA during apoptosis, in addition to processing of small RNAs, and undergoes a protease-mediated conversion from a ribonuclease to a deoxyribonuclease.

Mitochondria are inherited maternally in most animals, but the mechanisms of selective paternal mitochondrial elimination (PME) are unknown. While examining fertilization in C. elegans, we observe that paternal mitochondria rapidly lose their inner membrane integrity. CPS-6, a mitochondrial endonuclease G, serves as a paternal mitochondrial factor that is critical for PME. The CPS-6 endonuclease relocates from the intermembrane space of paternal mitochondria to the matrix following fertilization to degrade mitochondrial DNA. It acts with maternal autophagy and proteasome machineries to promote PME. Loss of cps-6 delays breakdown of mitochondrial inner membranes, autophagosome enclosure of paternal mitochondria, and PME. Delayed removal of paternal mitochondria causes increased embryonic lethality, demonstrating that PME is important for normal animal development. Thus, CPS-6 functions as a paternal mitochondrial degradation factor during animal development.

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Summary Non-steroidal anti-inflammatory drugs (NSAIDs) are among the most commonly used drugs in the world. While the role of NSAIDs as cyclooxygenase (COX) inhibitors is well established, other targets may contribute to anti-inflammation. Here we report caspases as a new pharmacological target for NSAID-family drugs such as ibuprofen, naproxen, and ketorolac at physiologic concentrations both in vitro and in vivo. We characterize caspase activity both in vitro and in cell culture, and combine computational modeling and biophysical analysis to determine the mechanism of action. We observe that inhibition of caspase catalysis reduces cell death and the generation of pro-inflammatory cytokines. Further, NSAID inhibition of caspases is COX-independent, representing a new anti-inflammatory mechanism. This finding expands upon existing NSAID anti-inflammatory behaviors, with implications for patient safety and next-generation drug design.