Elsevier

Peptides

Volume 136, February 2021, 170466
Peptides

The neuropeptide SMYamide, a SIFamide paralog, is expressed by salivary gland innervating neurons in the American cockroach and likely functions as a hormone

https://doi.org/10.1016/j.peptides.2020.170466Get rights and content

Highlights

  • SIFamide and SMYamide are differentially expressed in the American cockroach.

  • a bilateral SMYaminergic neuron innervates the salivary gland in this species.

  • SMYamide likely has a hormonal function.

Abstract

The SMYamide genes are paralogs of the SIFamide genes and code for neuropeptides that are structurally similar to SIFamide. In the American cockroach, Periplanea americana, the SMYamide gene is specifically expressed in the SN2 neurons that innervate the salivary glands and are known to produce action potentials during feeding. The SN2 axon terminals surround rather than directly innervate the salivary gland acini. Therefore one may expect that on activation of these neurons significant amounts of SMYamide will be released into the hemolymph, thus suggesting that SMYamide may also have a hormonal function. In the Periplaneta genome there are two putative SIFamide receptors and these are both expressed not only in the central nervous system and the salivary gland, but also in the gonads and other peripheral tissues. This reinforces the hypothesis that SMYamide also has an endocrine function in this species.

Introduction

SIFamide is an arthropod neuropeptide that was initially identified from a flesh fly by its ability to stimulate oviduct contractions in a locust. Antiserum raised to this neuropeptide showed it to be present in four large cells in the pars intermedia, a brain nucleus containing various types of neuroendocrine cells that project to the corpus cardiacum where hormones are released into the hemolymph. However, no SIFamide immunoreactivity was found in the corpus cardiacum [1]. We isolated and sequenced the peptide from the fruit fly Drosophila melanogaster. As in the fleshfly only four large cells in the brain were found to produce the peptide as shown by immunohistology, in situ hybridisation and transgene expression [2]. In the larva these neurons each project a major axon to the ventral neuromeres, but during metamorphosis they develop extensive arborizations within the entire central nervous system. Nevertheless, there is no anatomical evidence that any such axons could release the peptide into the hemolymph.

Using transgenesis we expressed in the Drosophila SIFamide neurons either an apoptotic protein to kill them or RNAi to eliminate the peptide. Those experiments revealed that the four neurons and the peptide they produce are essential for the correct execution of male sexual behavior; in the absence of SIFamide males court males as intensely as females [2]. In Drosophila the fruitless gene produces male and female specific transcription factors that allow for the correct development of male and female brain architecture and subsequent sexual behavior. The effects of SIFamide on male sexual behavior is at least in part through neurons expressing fruitless [3].

More recent studies have started to look at other species, such as the blood feeding bug Rhodnius prolixus, the locust Schistocerca gregaria as well as several cockroaches [[4], [5], [6]]. In all these species homologs of the four neuroendocrine cells in the pars intermedia are the major SIFamide immunoreactive neurons. In cockroaches and Schistocerca there are other additional SIFamide immunoreactive neurons, but only in the cockroach Rhyparobia maderae and Rhodnius have SIFamide immunoreactive projections been found in the corpora cardiaca [4,6], suggesting an endocrine function for SIFamide in those species.

The deorphanization of the G-protein coupled receptor (GPCR) encoded by Drosophila gene CG10823 as the SIFamide receptor [7] allowed the identification of SIFamide receptor homologs from other species, such as one from the tick Dermacentor variabilis that is expressed in the male reproductive system [8].

These initial data might suggest an important and perhaps preponderant role in reproduction and sexual behavior, but SIFamide and its receptor have now been been shown to be important for the correct execution of other behaviors as well, such as sleep in Drosophila and aggression in a decapod crustacean [[9], [10], [11], [12]], while in the tick Ixodes scapularis SIFamide innervates the salivary gland and in Rhodnius it is released during feeding and increases meal size as well as the rate of heart beat [4,13].

In some insect species the SIFamide gene has a paralog. IMFamide is a SIFamide paralog that was initially described from the silkworm Bombyx mori [14], but seems to be generally present in Lepidoptera. An independent SIFamide gene duplication occurred in the termite Zootermopsis nevadensis and the locust Locusta migratoria where the paralog peptide is SMYamide [15]. Whereas the primary sequence of IMFamide shows significant differences from SIFamide, SIFamide and SMYamide peptides appear more similar. The recently published genome sequence for the American cockroach Periplaneta americana [16] also reveals both a SIFamide and a SMYamide gene [17]. I here report that in the latter species SMYamide is specifically expressed in two neurons innervating the salivary glands and that it seems plausible that SMYamide also has a hormonal function.

Section snippets

Animals

Periplaneta americana are from a small colony that I maintain on mouse chow and water and that was intially established from animals obtained from Professor Peter Kloppenburg (University of Cologne, Germany). Only adults were used here.

RNA extraction and cDNA synthesis

Total RNA was isolated from specific tissues using a kit from Macherey-Nagel (Hoerdt, France). Moloney murine leukemia virus reverse transcriptase (New England Biolabs, Evry, France) and random hexamer primers were used to transcribe 1 μg RNA in a 20 μl reaction

SMYamide distribution and sequences

Previously Bläser and Predel [22] reported that in transcriptome SRAs SMYamide precursors were only found in Polyneoptera insect orders and they suggested that the SIFamide gene duplication may have occurred within the Plecoptera. This is confirmed by analysis of insect genomes; they only yielded SMYamide genes in Orthoptera, Embioptera, Phasmatodea, Mantodea and Blattodea, but not in genomes from Plecoptera or Polyneopteran insect orders that branched off even earlier. The structure of the

Discussion

This study reveals that two previously identified neurons in the subesophageal ganglion that innervate the salivary gland are immunoreactive with antiserum to SIFamide. In situ hybridization experiment show that these neurons express the SMYamide gene. Publicly available expression data from the brain and subesophageal ganglion support this conclusion. Results also show that the SIFamide receptor is expressed in tissues other than the brain and the salivary glands.

It is likely that the SIFamide

Credit author statement

Jan A. Veenstra conceptualized and executed the experiments, wrote the manuscript and prepared the figures.

Acknowledgements

I thank two anonymous reviewers for their helpful comments on the manuscript. Institutional funding was from the CNRS.

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