Ascaris suum Informs Extrasynaptic Volume Transmission in Nematodes.

Louise E Atkinson,Elke Vandewyer,Nikki J Marks,Qingxiao Liang,Isabel Beets,Allister Irvine,Aaron G Maule,Bruce A Rosa,Fiona Mckay,Makedonka Mitreva,Lingjun Li,Yang Liu,Angela Mousley,Charles Viau,Zihui Li

doi:10.1021/acschemneuro.1c00281

Abstract

Neural circuit synaptic connectivities (the connectome) provide the anatomical foundation for our understanding of nematode nervous system function. However, other nonsynaptic routes of communication are known in invertebrates including extrasynaptic volume transmission (EVT), which enables short- and/or long-range communication in the absence of synaptic connections. Although EVT has been highlighted as a facet of Caenorhabditis elegans neurosignaling, no experimental evidence identifies body cavity fluid (pseudocoelomic fluid; PCF) as a vehicle for either neuropeptide or biogenic amine transmission. In the parasitic nematode Ascaris suum, FMRFamide-like peptides encoded on flp-18 potently stimulate female reproductive organs but are expressed in cells that are anatomically distant from the reproductive organ, with no known synaptic connections to this tissue. Here we investigate nonsynaptic neuropeptide signaling in nematodes mediated by the body cavity fluid. Our data show that (i) A. suum PCF (As-PCF) contains a catalog of neuropeptides including FMRFamide-like peptides and neuropeptide-like proteins, (ii) the A. suum FMRFamide-like peptide As-FLP-18A dominates the As-PCF peptidome, (iii) As-PCF potently modulates nematode reproductive muscle function ex vivo, mirroring the effects of synthetic FLP-18 peptides, (iv) As-PCF activates the C. elegans FLP-18 receptors NPR-4 and -5, (v) As-PCF alters C. elegans behavior, and (vi) FLP-18 and FLP-18 receptors display pan-phylum distribution in nematodes. This study provides the first direct experimental evidence to support an extrasynaptic volume route for neuropeptide transmission in nematodes. These data indicate nonsynaptic signaling within the nematode functional connectome and are particularly pertinent to receptor deorphanization approaches underpinning drug discovery programs for nematode pathogens.

Highlights

Our current understanding of nematode neuronal circuitry is based on comprehensive Caenorhabditis elegans synaptic connectome data.[1−3] The C. elegans blueprint underpins fundamental parasitic nematode neurobiology driving anatomical and functional connectomics studies in model parasites such as Ascaris suum, in which the simple neuronal architecture described in C. elegans appears to be highly conserved; see ref 4
This, in conjunction with the experimental tractability of Ascaris where tissues and organs can be readily dissected for physiology studies, offers a unique opportunity to investigate extrasynaptic volume transmission (EVT) in nematodes
We show that A. suum PCF (As-PCF) contains a complex array of neuropeptides that modulate nematode behavior

Summary

Introduction

Our current understanding of nematode neuronal circuitry is based on comprehensive Caenorhabditis elegans synaptic connectome data.[1−3] The C. elegans blueprint underpins fundamental parasitic nematode neurobiology driving anatomical and functional connectomics studies in model parasites such as Ascaris suum, in which the simple neuronal architecture described in C. elegans appears to be highly conserved; see ref 4. While not directly experimentally demonstrated in nematodes to date, the existence and significance of additional forms of nonsynaptic neuronal communication have been widely recognized among invertebrates.[5,6] For example, in crustaceans, extrasynaptic volume transmission (EVT) operates beyond the synaptic connectome, mediating long-range hormonal communication in the absence of neuronal synapses.[7,8] Notably, EVT has been implicated in nematode neuronal signaling including in C. elegans.[9−14] the C. elegans wired connectome does not always support receptor−ligand interactions that have been functionally linked; there are examples of signaling pathways where receptors are not located cosynaptically with neurons in both monoamine and neuropeptide systems,[15−18] supporting a role for EVT in neurotransmission. Neuropeptides are well-known neuromodulators of a wide array of essential neuronal functions in nematodes, including locomotion, reproduction, and feeding, highlighting the importance of neuropeptidergic signaling to nematode biology.[20,21] Despite this, neuropeptide signaling remains unexploited underscoring its appeal as a novel control target

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