Author response: Belly roll, a GPI-anchored Ly6 protein, regulates Drosophila melanogaster escape behaviors by modulating the excitability of nociceptive peptidergic interneurons
Full text Figures and data Side by side Abstract Editor's evaluation Introduction Results Discussion Materials and methods Appendix 1 Data availability References Decision letter Author response Article and author information Metrics Abstract Appropriate modulation of escape behaviors in response to potentially damaging stimuli is essential for survival. Although nociceptive circuitry has been studied, it is poorly understood how genetic contexts affect relevant escape responses. Using an unbiased genome-wide association analysis, we identified an Ly6/α-neurotoxin family protein, Belly roll (Bero), which negatively regulates Drosophila nociceptive escape behavior. We show that Bero is expressed in abdominal leucokinin-producing neurons (ABLK neurons) and bero knockdown in ABLK neurons resulted in enhanced escape behavior. Furthermore, we demonstrated that ABLK neurons responded to activation of nociceptors and initiated the behavior. Notably, bero knockdown reduced persistent neuronal activity and increased evoked nociceptive responses in ABLK neurons. Our findings reveal that Bero modulates an escape response by regulating distinct neuronal activities in ABLK neurons. Editor's evaluation This convincing and fundamental study uses unbiased genome-wide association analysis to identify a gene, called Bero, encoding Ly6/α-neurotoxin family protein, that affects the way larval Drosophila respond to nociceptive stimuli. This discovery is followed up by the identification of neurons in which Bero function is relevant for the modulation of nociceptive behaviour, namely the abdominal leucokinin-producing neurons. These neurons are activated by nociceptive sensory neurons and can initiate escape behavior. In these neurons Bero modulates both persistent and evoked activities. This elegant work will be of interest to neurobiologists working on genes, neural circuits, and behaviour. https://doi.org/10.7554/eLife.83856.sa0 Decision letter Reviews on Sciety eLife's review process Introduction Appropriate escape behavior in response to threatening stimuli is essential for organismal survival (Branco and Redgrave, 2020; Burrell, 2017; Chin and Tracey, 2017; Im and Galko, 2012; Peirs and Seal, 2016). In both vertebrates and invertebrates, the modulation of escape behavior depends on environmental contexts as well as intensity and modality of stimuli, which are sensed and processed in a multilayered neuronal network, subject to genetic variation, that integrates external and internal sensory stimuli (Burnett et al., 2016; Jennings et al., 2014; Mu et al., 2012; Ohyama et al., 2015). Within the network, peptidergic neuromodulation plays a critical role in neuronal information processing (Nässel and Winther, 2010; Taghert and Nitabach, 2012). When Drosophila melanogaster larvae encounter noxious stimuli, such as harsh mechanical stimulations intrinsic to attacks by parasitoid wasps, they show stereotypic escape behaviors composed of an initial abrupt bending and subsequent corkscrew-like rolling (Hwang et al., 2007; Onodera et al., 2017; Tracey et al., 2003). In larvae, the noxious stimuli are mainly sensed by class IV dendritic arborization neurons (Class IV neurons) underneath the body wall (Tracey et al., 2003). In addition, a type of tracheal dendrite neuron (v’td2 neurons) and class III dendritic arborization neurons (Class III neurons) are known to detect noxious light stimuli and noxious cold stimuli, respectively (Imambocus et al., 2022; Turner et al., 2016). Recent studies have identified several important downstream components of the nociceptive neural circuitry (Ohyama et al., 2015; Takagi et al., 2017; Yoshino et al., 2017). In parallel, several genes and neurotransmitters have been characterized as context-dependent modulators of the behaviors (Burgos et al., 2018; Dason et al., 2020; Hu et al., 2017; Kaneko et al., 2017). While most of these studies identified the nociceptive neural circuitry by connectomic approaches, we used a genetic strategy to provide additional insights into the nociceptive modulation. Importantly, we discovered that D. melanogaster larvae displayed an extensive natural variation in nociceptive escape behavior. This broad behavioral variation allowed us to perform an unbiased genome-wide association (GWA) analysis to explore genetic variants affecting the nociceptive escape behaviors. In this study, we have demonstrated that an Ly6/α-neurotoxin family protein, Belly roll (Bero), modulates the neuronal excitability of peptidergic interneurons that initiates and facilitates Drosophila nociceptive escape behaviors. We first identified the bero gene through GWA analysis and validated its role in regulating nociception using pan-neuronal bero knockdown and null mutant animals. By searching for bero-expressing neurons, we identified abdominal leucokinin-producing neurons (ABLK neurons) that have been reported to be involved in controlling larval escape behaviors (Hu et al., 2020; Imambocus et al., 2022). We then found that bero knockdown in ABLK neurons resulted in enhanced escape behaviors, similar to bero mutants. Furthermore, we demonstrated that ABLK neurons responded to the activation of nociceptors and initiated nociceptive escape behavior. Notably, bero knockdown in ABLK neurons resulted in reduced persistent activities and increased evoked nociceptive responses in ABLK neurons, implying an essential role of bero in regulating neuronal activities. Based on our results regarding the ABLK neurons, we propose that these neurons may integrate external and internal sensory stimuli, thereby orchestrating nociceptive escape behaviors. Results Natural variation in nociceptive responses in wild-type strains of the Drosophila melanogaster Genetic Reference Panel Upon noxious thermal stimulation, Drosophila larvae show stereotypical nociceptive responses: bending and rolling behavior (Figure 1A; Tracey et al., 2003). Using the Heat Probe Assay (HPA; Tracey et al., 2003), we noticed that there is a large difference in the latency of rolling behavior (rolling latency) between the two commonly used strains, w1118 and Canton-S (Figure 1B). In order to evaluate natural variation in nociceptive responses, we took advantage of the Drosophila melanogaster Genetic Reference Panel (DGRP; Huang et al., 2014; Mackay et al., 2012), a collection of wild-caught Drosophila lines. We quantified the rolling behavior in 38 representative inbred wild-type strains of DGRP and observed a considerable variation in the behavior: the strain-specific median rolling latency ranged from 2.83 s to 10 s and the occurrence rates of rolling behavior (rolling probability) within 10 s ranged from around 30% to almost 100% among these strains (Figure 1C and D). We categorized the rolling probability into three response classes (rolling probability in 2, 5, and 10 s) based on previous studies (Onodera et al., 2017; Terada et al., 2016; Tracey et al., 2003) and found that the three response classes are highly correlated with one another (Figure 1—figure supplement 1A). To identify genetic variants associated with the behavioral variation, we performed multiple trials of GWA mapping analyses using the publicly available web-based analysis tool, DGRP 2.0 (Huang et al., 2014), with four statistical metrics that characterize larval rolling behavior and are mutually dependent to some extent (Figure 1E, Figure 1—source data 1, ‘Materials and methods’). We then identified in total 37 single-nucleotide polymorphisms (SNPs), two deletions, and three insertions in or near 36 candidate genes (Figure 1—source data 2), at a threshold p-value of 1.0 × 10–5, a commonly used threshold p-value in previous studies with DGRP (Figure 1F, Figure 1—figure supplement 1B; Mackay and Huang, 2018). Figure 1 with 1 supplement see all Download asset Open asset Natural diversity in nociceptive rolling escape behavior in wild-type strains of Drosophila melanogaster. (A) A schematic representation of the Heat Probe Assay (see ‘Materials and methods’ for details). (B) The latency of rolling escape behavior (rolling latency) is different between two wild-type strains, w1118 and Canton-S (CS) (46°C, [w1118] n = 40, [CS] n = 38; 48°C, [w1118] n = 32, [CS] n = 33; 50°C, [w1118] n = 33, [CS] n = 33; ***p<0.001, Wilcoxon rank-sum test). Violin plots provide a kernel density estimate of the data, where the middle circle shows the median, a boxplot shape indicates 25th and 75th percentiles, and whiskers to the left and right of the box indicate the 90th and 10th percentiles, respectively, in this and the following figures. Each data point represents an individual larva. See source data tables for detailed genotypes in this and the following figures. (C) The rolling latency of 38 representative Drosophila melanogaster Genetic Reference Panel (DGRP) lines in ascending order (n = 30 or 31 larvae/line). Boxplots indicate the median and 25th and 75th percentiles, and whiskers to the left and right of the box indicate the 90th and 10th percentiles, respectively, in this and the following figures. (D) The rolling probability of 38 representative DGRP lines in ascending order (n = 30 or 31 larvae/line). The stacked bar chart indicates the rolling probability within 2, 5, and 10 s, respectively. (E) A diagram showing experimental procedures for the genome-wide association (GWA) analysis. (F) GWA analysis for median rolling latency of the rolling escape behavior. The p-values (−log10 transformed) are shown on the y axis. The gray dotted line marks the nominal p-value threshold (1.0 × 10–5). Each data point corresponds to an individual genetic variant (single-nucleotide polymorphism, deletion, or insertion). Data points are arranged by relative chromosome (genomic) position, the color code indicates the respective chromosome to which they belong. See also Figure 1—source data 1 and Figure 1—figure supplement 1A. Figure 1—source data 1 Summary table of genotypes, statistical testing, and graph data for Figure 1. https://cdn.elifesciences.org/articles/83856/elife-83856-fig1-data1-v1.zip Download elife-83856-fig1-data1-v1.zip Figure 1—source data 2 Genome-wide association results for rolling behavior. https://cdn.elifesciences.org/articles/83856/elife-83856-fig1-data2-v1.zip Download elife-83856-fig1-data2-v1.zip Identification of CG9336/belly roll (bero) through an RNA interference knockdown screen of candidate genes To explore impacts of these candidate genes on nociceptive responses, we carried out a secondary functional screen by pan-neuronal RNA interference (RNAi)-mediated gene knockdown. By evaluating 17 RNAi strains that were currently available, we found that pan-neuronal knockdown of the CG9336 gene resulted in significant enhancement of the rolling probability (Figure 2A). To further test whether the phenotype was specifically caused by suppression of CG9336, we generated two distinct short-hairpin RNAs (shRNAs) against CG9336 (shRNA#1 and #2; Figure 2B; ‘Materials and methods’), validated the efficiency of shRNA#2 (Figure 2—figure supplement 1A and B; ‘Materials and methods’), and found that both shRNAs caused a comparable enhancement (Figure 2E, Figure 2—figure supplement 1C). We also performed three sets of effector-control tests for UAS-bero shRNA#2 with different genomic backgrounds. In each group, both the UAS-bero shRNA#2 control animal and the corresponding controls exhibited comparable rolling behavior, which ensures that any observed phenotypes were not due to leaky expression from the UAS transgene (Figure 2E). We then designated the gene as belly roll (bero), after a high-jump style in athletics. bero encodes a member of the Ly6/α-neurotoxin protein superfamily, which is characterized by a three-finger structure, including various membrane-tethered and secreted polypeptides (Loughner et al., 2016). Using several bioinformatics tools, we predicted a signal peptide sequence at the amino terminus, two possible glycosylation sites, a GPI-anchorage site, a putative transmembrane region at the carboxyl terminus, and five putative disulfide bridges, which are typical structural features of the Ly6/α-neurotoxin protein superfamily (Figure 2C, Figure 2—figure supplement 1D; Almagro Armenteros et al., 2019; Baek et al., 2021; Eisenhaber et al., 1999; Gupta and Brunak, 2001; Krogh et al., 2001). A three-dimensional model of Bero was generated by protein structure prediction using AlphaFold2 (Jumper et al., 2021; Figure 2D). Figure 2 with 2 supplements see all Download asset Open asset bero negatively regulates nociceptive rolling escape behavior. (A) A secondary functional screen by pan-neuronal RNA interference. The rolling latency of each UAS-RNAi line was measured. Pan-neuronal knockdown of CG9336/bero reduces rolling latency (Heat Probe Assay, n = 30 or 31 larvae/genotype; ****p<0.0001, Wilcoxon rank-sum test). (B) A schematic representation of bero-associated SNP (2L_20859945_SNP; minor allele, cytosine; major allele, thymine; minor allele frequency = 0.4167). (C) A schematic representation of unprocessed and mature Bero protein. SP, signal peptide; TM, transmembrane region. Orange lines mark the predicted disulfide bonds. Yellow lines mark the predicted N-glycosylation sites. Pink lines mark the predicted GPI-modification site. See also Figure 2—figure supplement 1D. (D) A three-dimensional protein structure prediction of unprocessed Bero protein by AlphaFold2. Red region, signal peptide; blue region, transmembrane region; orange sticks, predicted disulfide bonds; yellow region, predicted N-glycosylation sites; pink region, predicted GPI-modification site. (E) Rolling latency of nSyb>attP2 control (n = 63), pan-neuronal bero knockdown animals (nSyb>bero RNAishRNA#2, n = 65), and the corresponding three sets of effector-controls (w1118 background: attP2 control, n = 64, bero RNAishRNA#2, n = 60; Canton-S background: attP2 control, n = 60, bero RNAishRNA#2, n = 60, y w background: attP2 control, n = 55, bero RNAishRNA#2, n = 55). **p<0.01; n.s., nonsignificant; Wilcoxon rank-sum test. The genetic backgrounds are controlled (see source data tables for detailed genotypes in this and the following figures). (F) Rolling latency of Canton-S control (n = 38), bero heterozygous (beroKO/+, n = 38), and homozygous (beroKO/beroKO, n = 37) mutant animals. **p<0.01, ***p<0.001, Wilcoxon rank-sum test. The bero KO strain had been outcrossed to Canton-S for 11 generations. Figure 2—source data 1 Summary table of genotypes, statistical testing, and graph data for Figure 2. https://cdn.elifesciences.org/articles/83856/elife-83856-fig2-data1-v1.zip Download elife-83856-fig2-data1-v1.zip The bero-associated SNP, 2L_20859945_SNP, is located in the second intron of CG9336 isoform A (FBtr0081427) and comprises two nucleotide variations in the DGRP core 40 lines: a minor allele, cytosine, and a major allele, thymine (Figure 2B). We found that most of the 38 DGRP core lines with the bero minor allele (C) showed higher responsiveness than those with the major allele (T) upon noxious thermal stimulations (Figure 2—figure supplement 2A). Based on these observations and our bero knockdown results, we speculated that animals with the bero minor allele (C) might have lower expression levels of bero compared with those possessing the major allele (T). Thus, using reverse transcriptase PCR, we compared the expression levels of bero in the larval central nervous system (CNS) among four DGPR strains that showed extremely quick or slow behavioral responses. We found that animals with the minor allele have lower expression levels of bero (Figure 2—figure supplement 2B). Using online sequence analysis based on JASPAR 2022 (Castro-Mondragon et al., 2022), we predicted that the neighboring nucleotide sequences of the bero-associated SNP might be a target site of certain transcription factors that control the expression level of bero in CNS (Supplementary file 2). Furthermore, we generated a bero null mutant line using CRISPR-mediated mutagenesis, which deleted the entire protein-coding sequence (Figure 2—figure supplement 1E; ‘Materials and methods’), and observed in HPA experiments that both bero heterozygous (beroKO/+) and homozygous (beroKO/beroKO) mutant animals displayed an enhanced rolling escape behavior (Figure 2F). Taken together, these results strongly suggest that bero expression in neurons negatively regulates nociceptive escape behavior. bero is expressed in several subgroups of peptidergic neurons in the larval CNS We next sought to find which neurons express bero in the larval CNS. To do this, we examined the overlap between bero-expressing neurons labeled in a protein-trap strain bero-YFP (CPTI-001654; Lowe et al., 2014), in which the endogenous Bero protein is fused with a yellow fluorescent protein variant mVenus, and candidate neurons labeled by a CD4:tdTomato fusion protein driven by distinct GAL4-drivers. We found no bero-YFP expression in the Class IV neurons (nociceptors) or in the downstream neurons that have been reported to be required for the nociceptive escape behavior (Figure 3—figure supplement 1A; Ohyama et al., 2015). Consistent with this, bero knockdown specifically in the corresponding neurons did not alter the rolling behavior (Figure 3—figure supplement 1B). Instead, we observed bero-YFP expression in several groups of peptidergic neurons specified by the bHLH transcription factor dimmed (dimm), including insulin-producing cells (IPCs) and a subset of Eclosion hormone-producing neurons (EH neurons) in the larval brain, and abdominal leucokinin-producing neurons (ABLK neurons) in the lateral region of abdominal ganglions in the larval ventral nerve cord (VNC; Figure 3A and B). In support of our observations, a recent single-cell transcriptomic atlas showed that bero/CG9336 is selectively enriched in peptidergic neurons (Corrales et al., 2022). Bero is also expressed in certain non-neuronal tissues, such as midline glia (Figure 3A). However, we did not study Bero function in the glia in this study. Figure 3 with 1 supplement see all Download asset Open asset Expression of bero in ABLK neurons plays an essential role in the negative regulation of nociceptive behavior. (A) Confocal image stacks (maximum projection) showing fluorescence of the endogenous Bero reporter, Bero-YFP (green), and the colocalized peptidergic neurons with GAL4, UAS-CD4-tdTomato (magenta) in third-instar larvae. Scale bars, 50 μm. (B) A schematic representation of bero-expressing neurons. IPC, insulin-producing cells; EH, eclosion hormone-producing neurons; ABLK, abdominal leucokinin-producing neurons; SEG, the subesophageal ganglion; VNC, the ventral nerve cord. (C) Confocal image stack showing endogenous Bero expression (labeled by Bero-YFP, yellow), LK neurons (Lk-GAL4.TH, UAS-CD4-tdTomato, magenta), and leucokinin (labeled by anti-Lk, cyan) in a third-instar larva. Bottom and left panels show an XZ and YZ cross-section of the ABLK somatic region (locations of the cross-sections are indicated by horizontal and vertical gray lines in the primary image), respectively. Middle panels show magnified views of the boxed regions: Individual optical sections depicting ABLK neurons (ABLK neurons are indicated by A1–A7 label, and an image of A7 with enhanced intensity is showed), Right: image stack depicting SELK neurons (indicated by arrowheads), and image stack depicting ABLK neurites (an image with enhanced intensity is showed). Scale bars: 50 μm; 10 μm or 20 μm for the magnified view. (D) Confocal image stack showing the dendrite (labeled by ABLK >DenMark, magenta) and axon terminal markers (labeled by ABLK >syt:eGFP, green) in ABLK neurons in a third-instar larva. Scale bars: 50 μm; 20 μm for the magnified view. (E) Rolling latency of LK-specific bero knockdown larvae (Lk>bero RNAishRNA #2, n = 73) and control (Lk>attP2, n = 75). Rolling latency of Eh-specific bero knockdown larvae (Eh>bero RNAishRNA #2, n = 38) and control (Eh>attP2, n = 41). Rolling latency of Ilp2-specific bero knockdown larvae (Ilp2>bero RNAishRNA #2, n = 40) and control (Ilp2>attP2, n = 39). ***p<0.001; n.s., nonsignificant; Wilcoxon rank-sum test. (F) Rolling latency of ABLK-specific bero overexpression larvae (ABLK>Bero:FLAG, n = 72), control (ABLK>myr:GFP, n = 73), and the corresponding effector-controls (y v background: myr:GFP control, n = 60, Bero:FLAG, n = 61). **p<0.01; n.s., nonsignificant; Wilcoxon rank-sum test. Figure 3—source data 1 Summary table of genotypes, statistical testing, and graph data for Figure 3. https://cdn.elifesciences.org/articles/83856/elife-83856-fig3-data1-v1.zip Download elife-83856-fig3-data1-v1.zip bero expression in ABLK neurons negatively regulates nociceptive rolling escape behavior To identify the particular subgroups of neurons that require bero expression for negative regulation of nociceptive escape behavior, we examined rolling escape behavior in larvae with cell type-specific RNAi knockdowns of bero. We found that knockdown of bero using Lk-GAL4, a leucokinin-producing neuron-specific driver, caused enhanced rolling escape behavior (Figure 3E). In contrast, knockdown of bero using either Eh-GAL4, a neuron-specific driver, or an driver, did not in any in rolling escape behavior (Figure 3E). LK neurons can be into four distinct subgroups in the larval LK neurons lateral LK neurons subesophageal LK neurons and abdominal LK neurons et al., bero is expressed in all of ABLK neurons and is to not expressed in any LK neurons (Figure 3A and Consistent with this, the of bero-YFP labeled neurons were for (Figure of individual optical sections showed that Bero on both the and internal of ABLK neurons (Figure We expressed the dendrite and axon terminal namely and in ABLK neurons (Figure that Bero both and of the neurons. To the role of bero expression in ABLK neurons in rolling behavior, we to ABLK-specific bero overexpression using an et al., 2010; 2016; Figure 3—figure supplement 1C). We observed that bero overexpression rolling behavior (Figure that bero expression in ABLK neurons plays an essential role in the negative regulation of nociceptive behavior. In contrast, in the effector-control both the control animal and the corresponding controls showed comparable levels of rolling behavior (Figure we our analysis on the of ABLK neurons and the functional of bero in these neurons. bero persistent activities and evoked nociceptive responses in ABLK neurons We next whether ABLK neurons respond to of To test this, we expressed a blue variant of et al., in and a fluorescence et al., in LK neurons. We responses in ABLK neurons in and observed that cell and of ABLK neurons displayed persistent of (Figure and Figure supplement which was with a previous study et al., Notably, activation of the nociceptors evoked evoked neuronal activities in ABLK neurons, that these neurons respond to nociceptive (Figure and Hu et al., We used larvae on a with no additional all as controls activation in the A was also in the control larvae on a with no additional the the of the blue light (Figure This indicates that the be to the persistent in ABLK neurons. Figure with 2 supplements see all Download asset Open asset bero is for the of persistent activities and suppression of evoked nociceptive activity in ABLK neurons. (A) results of responses showing the persistent signal of ABLK neurons in control and bero knockdown larvae bero RNAishRNA (B) A of the of persistent activities of ABLK neurons in control (n = neurons from three and bero knockdown larvae (n = neurons from three ****p<0.0001, Wilcoxon rank-sum test. (C) of the nociceptive responses of ABLK neurons in control and bero knockdown larvae bero RNAishRNA after activation of Class IV neurons Scale bars, 10 μm. (D) for responses see ‘Materials and methods’ for of ABLK neurons in control n = neurons from three and bero knockdown larvae bero RNAishRNA #2, n = neurons from three upon activation of Class IV neurons light is indicated by at and for blue and gray indicate (E) for responses of ABLK neurons in control n = neurons from three and bero knockdown larvae bero RNAishRNA #2, n = neurons from three upon activation of Class IV neurons. (F) of responses of ABLK neurons in control n = neurons from three n = neurons from three and bero knockdown larvae n = neurons from three n = neurons from three upon activation of Class IV neurons n.s., nonsignificant; test followed by test. Figure data 1 Summary table of genotypes, statistical testing, and graph data for Figure Download To further the of bero in ABLK neurons, we whether knockdown of bero persistent activities nociceptive responses of the neurons. We found that bero knockdown almost persistent activities (Figure and it increased the of evoked nociceptive responses (Figure and that bero knockdown enhanced rolling escape behavior in the HPA experiments (Figure we speculated that the evoked nociceptive responses in ABLK neurons are important for rolling escape behavior. the phenotype of the bero knockdown on persistent activities in ABLK neurons, we performed a behavioral analysis of and found that bero knockdown did not affect larval (Figure supplement persistent in ABLK neurons of bero knockdown animals to be enhanced compared to controls (Figure Figure supplement 1B). We that the used for in nociceptive Furthermore, ABLK neurons with bero knockdown displayed increased to these nociceptive than which may provide a possible the neural activity may not Thus, we compared activity the we have that bero plays a critical role in not a higher level of persistent also the evoked nociceptive responses of the ABLK neurons, thereby the nociceptive escape behavior (Figure of evoked activity in ABLK neurons is for the control of rolling escape behavior To test whether of ABLK activity to escape behaviors, we activated the neurons using a et al., We first activated neurons and found that light a rolling behavior that was observed in control larvae (Figure and B). We then the to selectively express in ABLK or SELK neurons (Figure activation of ABLK neurons almost resulted in not rolling bending behavior, which is as a initial of the entire sequence of escape (Figure Figure supplement 1A and B). In contrast, activation of SELK neurons rolling behavior in all larvae. Figure with 1 supplement see all
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