cGMP-Dependent Protein Kinase Inhibition Extends the Upper Temperature Limit of Stimulus-Evoked Calcium Responses in Motoneuronal Boutons of Drosophila melanogaster Larvae

While the mammalian brain functions within a very narrow range of oxygen concentrations
and temperatures, the fruit fly, Drosophila melanogaster, has employed strategies to deal
with a much wider range of acute environmental stressors. The foraging (for) gene encodes
the cGMP-dependent protein kinase (PKG), has been shown to regulate thermotolerance
in many stress-adapted species, including Drosophila, and could be a potential therapeutic
target in the treatment of hyperthermia in mammals. Whereas previous thermotolerance
studies have looked at the effects of PKG variation on Drosophila behavior or excitatory
postsynaptic potentials at the neuromuscular junction (NMJ), little is known about PKG
effects on presynaptic mechanisms. In this study, we characterize presynaptic calcium
([Ca^2+]i) dynamics at the Drosophila larval NMJ to determine the effects of high temperature
stress on synaptic transmission. We investigated the neuroprotective role of PKG modulation
both genetically using RNA interference (RNAi), and pharmacologically, to
determine if and how PKG affects presynaptic [Ca^2+]i dynamics during hyperthermia. We
found that PKG activity modulates presynaptic neuronal Ca^2+ responses during acute
hyperthermia, where PKG activation makes neurons more sensitive to temperatureinduced
failure of Ca^2+ flux and PKG inhibition confers thermotolerance and maintains normal
Ca^2+ dynamics under the same conditions. Targeted motoneuronal knockdown of PKG
using RNAi demonstrated that decreased PKG expression was sufficient to confer thermoprotection.
These results demonstrate that the PKG pathway regulates presynaptic motoneuronal
Ca^2+ signaling to influence thermotolerance of presynaptic function during acute
hyperthermia.