Steroid Receptor Isoform Expression in Drosophila Nociceptor Neurons Is Required for Normal Dendritic Arbor and Sensitivity

Abstract

Steroid hormones organize many aspects of development, including that of the nervous system. Steroids also play neuromodulatory and other activational roles, including regulation of sensitivity to painful stimuli in mammals. In Drosophila, ecdysteroids are the only steroid hormones, and therefore the fly represents a simplified model system in which to explore mechanisms of steroid neuromodulation of nociception. In this report, we present evidence that ecdysteroids, acting through two isoforms of their nuclear ecdysone receptor (EcR), modulate sensitivity to noxious thermal and mechanical stimuli in the fly larva. We show that EcRA and EcRB1 are expressed by third instar larvae in the primary nociceptor neurons, known as the class IV multidendritic neurons. Suppression of EcRA by RNA interference in these cells leads to hyposensitivity to noxious stimulation. Suppression of EcRB1 leads to reduction of dendritic branching and length of nociceptor neurons. We show that specific isoforms of the ecdysone receptor play critical cell autonomous roles in modulating the sensitivity of nociceptor neurons and may indicate human orthologs that represent targets for novel analgesic drugs.

Growth control through regulation of insulin-signaling by nutrition-activated steroid hormone

“…All of these nuclear receptors could be efficiently genetically depleted in the IPCs with exception of Usp, for which knockdown was incomplete. The latter is consistent with an earlier report that Usp expression is difficult to reduce in the CNS ( Figure 6, A-D; (49)).…”

Steroid hormone signaling activates thermal nociception during Drosophila peripheral nervous system development

“…To investigate EcR dynamics during the developmental period with different levels of thermal nociception, we examined the localization of EcR in 2 nd and 3 rd instar larval C4da neurons with an antibody which recognizes a common domain of all EcR isoforms. Consistent with previous findings (Kuo et al 2005;Ou et al 2008;Kirilly et al 2009;McParland et al 2015), we observed nuclear localization of EcR in wandering 3 rd instar larvae (Figure 2A,B). Earlier in larval development, EcR was evenly distributed between the nucleus and cytoplasm in the 2 nd instar (2 hr and 8 hr After L2 Ecdysis) and at the beginning of the 3 rd instar, 2 hr After L3 Ecdysis (AL3E).…”

“…These isoforms are identical in the DNA binding and ligand binding domains but differ in the Activation Function 1 (AF1) domain (Figure 2 figure supplement 1A). C4da neurons express isoforms EcR-A and EcR-B1 at embryonic stages, during the 3 rd instar, and dynamically during pupal development (Kuo et al 2005;Ou et al 2008;Kirilly et al 2009;McParland et al 2015). EcR-B1 regulates dendrite growth (Ou et al, 2008) and pruning (Kuo et al, 2005, Kirilly et al, 2009.…”

“…EcR-B1 regulates dendrite growth (Ou et al, 2008) and pruning (Kuo et al, 2005, Kirilly et al, 2009. EcR-A is required for thermal nociception in the 3 rd instar, as revealed by the greater reduction of nociception caused by RNAi knockdown of EcR-A as compared to RNAi knockdown of EcR-B1 (McParland et al 2015).…”

“…Expression of EcR-A RNAi or EcR-B1-dominant negative mutant in the 3 rd instar larvae reduces the number of dendrite tips (Ou et al 2008;McParland et al 2015). Insensitivity to nociception is often associated with a reduction in dendrite structure, while hypersensitivity to nociception can be associated with both increased or decreased dendrite coverage (Honjo et al 2016).…”