6-OHDA-induced dopaminergic neurodegeneration in <i>Caenorhabditis elegans</i> is promoted by the engulfment pathway and inhibited by the transthyretin-related protein TTR-33

<div><p>Oxidative stress is linked to many pathological conditions including the loss of dopaminergic neurons in Parkinson’s disease. The vast majority of disease cases appear to be caused by a combination of genetic mutations and environmental factors. We screened for genes protecting <i>Caenorhabditis elegans</i> dopaminergic neurons from oxidative stress induced by the neurotoxin 6-hydroxydopamine (6-OHDA) and identified the <u>t</u>rans<u>t</u>hyretin-<u>r</u>elated gene <i>ttr-33</i>. The only described <i>C</i>. <i>elegans</i> transthyretin-related protein to date, TTR-52, has been shown to mediate corpse engulfment as well as axon repair. We demonstrate that TTR-52 and TTR-33 have distinct roles. TTR-33 is likely produced in the posterior arcade cells in the head of <i>C</i>. <i>elegans</i> larvae and is predicted to be a secreted protein. TTR-33 protects <i>C</i>. <i>elegans</i> from oxidative stress induced by paraquat or H₂O₂ at an organismal level. The increased oxidative stress sensitivity of <i>ttr-33</i> mutants is alleviated by mutations affecting the KGB-1 MAPK kinase pathway, whereas it is enhanced by mutation of the JNK-1 MAPK kinase. Finally, we provide genetic evidence that the <i>C</i>. <i>elegans</i> cell corpse engulfment pathway is required for the degeneration of dopaminergic neurons after exposure to 6-OHDA. In summary, we describe a new neuroprotective mechanism and demonstrate that TTR-33 normally functions to protect dopaminergic neurons from oxidative stress-induced degeneration, potentially by acting as a secreted sensor or scavenger of oxidative stress.</p></div

The neurotoxic MEC-4(d) DEG/ENac sodium channel conducts calcium: implications for necrosis initiation. Nat Neurosci

Rises in intracellular Ca have crucial roles in promoting apoptosis 1 and necrosis 2 , contributing to the debilitating neuronal damage that accompanies ischemia, injury and neurodegenerative disease 3 . The ER can be the source of a life-threatening increase in Ca 2+ in these conditions Hyperactivation of ion channels initiates excitotoxicity and underlies several inherited neurodegenerative disorders across species In vivo, mec-4(d)-induced neuronal death requires the function of ER Ca 2+ -storing proteins calreticulin and calnexin as well as ER Ca 2+ release channels 5 , suggesting that the ER regulates an essential cytoplasmic concentration of Ca 2+ required for progression through necrosis. When the intracellular Ca 2+ concentration rises to a neurotoxic threshold, specific Ca 2+ -activated calpain proteases function to dismantle the neuron 23 . Ion channel hyperactivation, the requirement for a rise in intracellular Ca 2+ , and calpain activation are common features of mammalian necrosis, indicating that, similar to apoptosis 24 , necrotic neuronal death occurs by an evolutionarily conserved mechanism 8 . A key issue in the nematode necrosis model, which is relevant to problems in mammalian traumatic neuronal injury and channelinduced death, is how an increase in Na + influx might provoke release of Ca 2+ from the ER, the paradox being that Ca 2+ release from the ER is usually induced by signals that are not dependent on Na + , namely, inositol (1,4,5)-trisphosphate (IP3) or Ca 2+ itself RESULTS VGCCs are not required for mec-4(d)-induced necrosis We tested the hypothesis that hyperactivation of the MEC-4(d) channel induces secondary Ca 2+ influx through plasma membrane sources. In response to gentle touch, mec-4(+) touch receptor neurons generate Ca 2+ transients that depend on MEC-4 as well as on the EGL-19 α and UNC-36 α-2/δ L-type VGCC subunits 13 . To test whether Ca 2+ influx through these or other VGCC subunits are needed for necrosi