Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

Highlights

• Neuropeptides regulate almost all aspects of daily life.

• Drosophila has emerged as an excellent model in neuropeptide research.

• Drosophila neuropeptides regulate physiology and a broad set of behaviors.

Abstract

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

Introduction

Animal survival and reproduction depend on the ability to display flexible physiology and behavior that enables adaptations to multiple environmental challenges. The nervous and endocrine systems are key organizers of these adjustments and ensure coordination between the external and the internal milieu, as well as optimal nutrient and energy balance and successful reproduction (Anderson, 2016; Owusu-Ansah and Perrimon, 2014, Owusu-Ansah and Perrimon, 2015; Yapici et al., 2014). These systems utilize signals with different spatial precision and temporal scales of action (Kim et al., 2017; Marder, 2012; Nässel, 2009, Nässel, 2018; Nusbaum et al., 2017; Taghert and Nitabach, 2012; van den Pol, 2012). Thus, synaptic transmission is usually fast (milliseconds), spatially precise (synaptic junctions) and utilizes small molecule neurotransmitters or electric conduction via gap junctions. At a slower timescale (seconds to hours), signaling is performed by various types of neuromodulators, neurohormones or hormones. These are commonly released at sites distant from bona fide synapses or receptor sites. This type of signaling utilizes monoamines, neuropeptides, peptide hormones, steroids, fatty acids and other molecules (as well as nitric oxide). The precision of the slower and more diffuse signaling is determined by the expression of appropriate receptors in target cells, rather than spatial proximity. This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known to play roles in almost all aspects of daily life as well as in developmental processes.

Neuropeptides and peptide hormones are interesting because they can act over a wide range or temporal and spatial scales (Kim et al., 2017; Nusbaum et al., 2017; van den Pol, 2012). In many cases, neuropeptides are considered as neuronal co-transmitters, acting along with small molecule fast transmitters at synaptic sites (Hökfelt et al., 1987; Kim et al., 2017; Nässel, 2018; Vaaga et al., 2014). However, in invertebrates, neuropeptides are usually thought of as neuromodulators and only a few studies have actually revealed co-transmitter functions [see Nässel, 2018; Nusbaum et al., 2017]. For some neuropeptides, one and the same molecule can play roles both as co-transmitter, neuromodulator and neurohormone. One example of this in Drosophila is tachykinin-type peptides (TKs), which are produced by interneurons, neuroendocrine cells of the central nervous system (CNS) and endocrine cells of the intestine (Ignell et al., 2009; Kahsai et al., 2010a; Siviter et al., 2000; Winther et al., 2003). Thus, TKs are part of several different neuronal circuits, where they are sometimes co-expressed with GABA or other neuropeptides (Glantz et al., 2000; Ignell et al., 2009; Im et al., 2015), and also seem to act via endocrine and paracrine signaling (Johard et al., 2003; Kahsai et al., 2010a; Song et al., 2014). A more general example is cholecystokinin (CCK), which in mammals is produced by gut endocrine cells and brain neurons, and plays multiple roles in stomach acid secretion, gall bladder contractions, pancreatic enzyme secretion, intestinal motility, regulation of satiety and multiple functions as a neuromodulator/co-transmitter in the central and peripheral nervous system (see Rehfeld, 2017). Insect sulfakinins (SKs) are orthologs of CCK, which also regulate satiety and food ingestion, aggression, hyperactivity and gut function [see Downer et al., 2007; Nässel and Williams, 2014; Söderberg et al., 2012; Wei et al., 2000; Williams et al., 2014; Yu et al., 2013; Zels et al., 2015]. These are only two examples of multifunctional peptides, and we will highlight further cases where peptidergic signaling orchestrates simple or complex behaviors, and regulates physiological homeostasis and other aspects of the daily life.

Neuropeptides and peptide hormones are crucial in regulation of a rich variety of developmental, physiological and behavioral functions throughout the life cycle of animals. Signaling with neuropeptides is very complex, which is underpinned by the large number of genes encoding peptide precursors (prepropeptides) and receptors in a given species (Hauser et al., 2006; Hewes and Taghert, 2001; Jekely, 2013; Mirabeau and Joly, 2013; Nässel and Winther, 2010; Semmens et al., 2016; Veenstra, 2014, Veenstra, 2016a). In invertebrates, at least 50 different neuropeptide genes have been identified in each species, and each of these display unique expression patterns in cells and tissues (Husson et al., 2007; Marder, 2012; Nässel and Winther, 2010; Park et al., 2008; Roller et al., 2008; Santos et al., 2007a; Taghert and Nitabach, 2012). Much of this complexity was unveiled fairly recently by whole genome and transcriptome sequencing of quite a few model and non-model organisms [see e. g. Veenstra, 2016a, Mirabeau and Joly, 2013, Jekely, 2013, Hauser et al., 2010, Caers et al., 2012, Elphick et al., 2018, Koziol, 2018, Varoqueaux et al., 2018a)]. At first, the massive amount of new information mined from sequence databases was daunting, but with the rapid development of powerful genetic and imaging tools much experimental progress has been made to increase the understanding of the multiple functions of neuropeptides, especially in the worm Caenorhabditis elegans [see Husson et al., 2007, de Bono and Maricq, 2005, Kaletsky and Murphy, 2010, Bendena et al., 2008, Bendena et al., 2012, Chalasani et al., 2010, Geary and Kubiak, 2005, Herrero et al., 2015, Janssen et al., 2008, Janssen et al., 2009, Lindemans et al., 2009a] and the fly Drosophila melanogaster (as will be discussed in this review) and more recently, in the marine annelid Platynereis dumerilii (Bauknecht and Jekely, 2015, Conzelmann et al., 2013a, Conzelmann et al., 2013b, Shahidi et al., 2015, Williams et al., 2015). It is generally accepted that many of the genes encoding neuropeptides and peptide receptors in insects and other invertebrates are ancestrally related to those found in mammals (Mirabeau and Joly, 2013, Jekely, 2013, Elphick et al., 2018, Bauknecht and Jekely, 2015, Zandawala et al., 2017), and recent work has suggested that in quite a few cases functions of peptidergic signaling are also partly conserved over evolution (Im et al., 2015, Nässel and Williams, 2014, Lindemans et al., 2009b, Schlegel et al., 2016, Terhzaz et al., 2012, Van Sinay et al., 2017). Thus, invertebrates with their less complex neuronal and endocrine systems are being explored as models of neuropeptide signaling in a variety of functions [reviewed in Owusu-Ansah and Perrimon, 2014, Owusu-Ansah and Perrimon, 2015, Marder, 2012, Taghert and Nitabach, 2012, Kim et al., 2017, Nässel, 2018, Nusbaum et al., 2017, Veenstra, 2016a, Veenstra, 2014, Caers et al., 2012, Varoqueaux et al., 2018a, Schoofs et al., 2017, Padmanabha and Baker, 2014, Simpson, 2009, Christie et al., 2008, Veenstra, 2015, Senatore et al., 2017, Varoqueaux et al., 2018b, Semmens and Elphick, 2017].

Although neuropeptides in Drosophila and other invertebrates have been reviewed over the years, developments in the field have been rapid and extensive and it is felt that there is a need for a comprehensive update on what we know about signaling with this ubiquitous and complex group of molecules. In this review, we summarize recent advances in neuropeptide biology and highlight neuropeptide and receptor evolution, the anatomy of peptide signaling systems, as well as the large progress in understanding neuropeptide function in different aspects of the life cycle of Drosophila. We also discuss Drosophila models of peptidergic signaling where the advantages with a small, short-lived, and less complex organism, combined with genetic tractability are promising. Such studies have employed Drosophila to model certain diseases, as well as metabolic regulation, sleep, aggression, reproduction, learning, stress responses, aging and lifespan amongst others. Also, investigations on the role of insulin/IGF-like peptides and associated mechanisms in development, growth, longevity, metabolism, stress responses and reproduction have been numerous in the last 10 years and generated novel insights. Although this review deals primarily with Drosophila neuropeptide signaling, it does so in the perspective of findings in other organisms at different levels of organization. For more detailed information and overviews on neuropeptide signaling in insect species other than Drosophila, as well as other invertebrates, the reader is referred to some fairly recent reviews (Marder, 2012, Taghert and Nitabach, 2012, Kim et al., 2017, Nusbaum et al., 2017, Mirabeau and Joly, 2013, Jekely, 2013, Hauser et al., 2006, Roller et al., 2008, Husson et al., 2007, Hauser et al., 2010, Caers et al., 2012, Elphick et al., 2018, Koziol, 2018, Bendena et al., 2012, Bauknecht and Jekely, 2015, Zandawala et al., 2017, Schoofs et al., 2017, Varoqueaux et al., 2018b, Nässel, 2002, Nässel and Homberg, 2006, Ons, 2016, Audsley and Weaver, 2009, Audsley and Down, 2015, Vanden Broeck, 2001a, Coast et al., 2002, Dircksen et al., 2011, Hauser et al., 2008, Kiss and Pirger, 2006, Nusbaum and Blitz, 2012, Orchard et al., 2001, Predel and Neupert, 2007, Riehle et al., 2002, Spit et al., 2012, Takahashi et al., 2008, Veenstra, 2010a, Veenstra et al., 2012, Lizbinski and Dacks, 2017, Vanden Broeck, 2001b, Jekely et al., 2018).

Of all the neuropeptides known in Drosophila, there are several that have received substantial attention in the last few years, while others have been largely neglected since their discovery. We hope to stimulate interest in investigating these neglected neuropeptides and in providing more complete functional characterization of the better-known peptides, where a lot is still to be learned. Furthermore there is a need to understand the extent to which specific neuropeptides serve multiple disparate functions as local co-transmitters or as part of systems that orchestrate global unified functions. A largely neglected aspect of neuropeptide research in Drosophila is the role of these molecules in co-transmission in concert with other neuropeptides or small molecule neurotransmitters |see Nässel, 2018]. Another gap in our knowledge is the cellular distribution of neuropeptide receptors and a correlation between peptide release sites and target receptors within the CNS. Finally, there is a need to better understand the evolution of peptide signaling systems and to what extent the functional roles of neuropeptides are ancestrally related. Note that Section 5, which provides an update on the biology and functions of all the known Drosophila neuropeptides, is presented in a tabular form in Supplementary materials (Supplementary Material Appendix 1).