Construction of a Global Pain Systems Network Highlights Phospholipid Signaling as a Regulator of Heat Nociception

Abstract

The ability to perceive noxious stimuli is critical for an animal's survival in the face of environmental danger, and thus pain perception is likely to be under stringent evolutionary pressure. Using a neuronal-specific RNAi knock-down strategy in adult Drosophila, we recently completed a genome-wide functional annotation of heat nociception that allowed us to identify α2δ3 as a novel pain gene. Here we report construction of an evolutionary-conserved, system-level, global molecular pain network map. Our systems map is markedly enriched for multiple genes associated with human pain and predicts a plethora of novel candidate pain pathways. One central node of this pain network is phospholipid signaling, which has been implicated before in pain processing. To further investigate the role of phospholipid signaling in mammalian heat pain perception, we analysed the phenotype of PIP5Kα and PI3Kγ mutant mice. Intriguingly, both of these mice exhibit pronounced hypersensitivity to noxious heat and capsaicin-induced pain, which directly mapped through PI3Kγ kinase-dead knock-in mice to PI3Kγ lipid kinase activity. Using single primary sensory neuron recording, PI3Kγ function was mechanistically linked to a negative regulation of TRPV1 channel transduction. Our data provide a systems map for heat nociception and reinforces the extraordinary conservation of molecular mechanisms of nociception across different species.

Phosphoinositide regulation of TRPV1 revisited

“…In the following years several papers reported a seemingly contradictory result: PI(4,5)P 2 in excised inside-out patches activated, rather than inhibited TRPV1 [49, 64, 109], raising doubts about PI(4,5)P 2 being inhibitory. Several other reports however were compatible with the existence of a partial inhibitory effect of PI(4,5)P 2 , present in intact cells [45, 64, 75, 83]. Most of the data in the literature had been consistent with a general dependence of TRPV1 activity on phosphoinositides, most likely via direct interactions of these lipids with the channel, and the possible presence of an indirect partial inhibition by PI(4,5)P 2 [29, 95].…”

“…Neely et al [75] found that PIP5K1α −/− mice had increased sensitivity to radiant heat and to capsaicin in behavioral assays. This finding is compatible with PI(4,5)P 2 exerting a negative effect.…”

Lipid kinases as therapeutic targets for chronic pain

“…The only GPCR-coupled PI3K, PI3Kγ, is expressed in nociceptive neurons and has been implicated in morphine-induced peripheral analgesia and tolerance [16; 42]. Surprisingly, PI3Kγ knockout ( Pik3cg −/− ) mice exhibit enhanced responses to heat and capsaicin, suggesting that PI3Kγ acts as a negative regulator of thermal and TRPV1 responses [60]. Interestingly, antagonism of this isozyme with a specific inhibitor in the periphery (via intraplantar injections) was shown to be anti-allodynic in a carrageenan-induced allodynia model [45].…”

“…A recent functional genomics study identified phospholipid signaling and lipid kinases as key regulators of heat nociception in flies [60]. It was also found that PIP5KIα knockout ( Pip5k1a −/− ) mice displayed hypersensitivity to noxious heat and capsaicin but the precise underlying mechanism is unknown.…”

RNAi screening in Drosophila cells and in vivo

“…At least one group has performed a small-scale in vivo double-knockdown screen [50]. In addition, a number of full-genome studies have been reported [33,51–54]. Not all assay are dependent on transgenic RNAi; a large-scale screen in which dsRNA was injected into embryos, followed by time-lapse confocal microscopy, has also been reported [55].…”