**4.2** *Drosophila* **hormones and neuropeptides**

Apart from using neurons, fly intestine can also communicate with other organs through systemic signals. Intestinal physiology is modulated by both extrinsic hormonal signals (emanating from endocrine glands, neuroendocrine structures, or organs such as the fat body) and by its own peptide hormones, produced by EE cells. In turn, gut-derived signals such as EE cell-derived peptide hormones can have long-range effects on other internal organs. EE cells accounts for 5–10% of midgut epithelial cells in flies compared to 0.4–0.6% in the mammalian small intestine [217–219]. Majority of them express peptide hormones, often more than one and with regional stereotypy [219–222]. The developmental program of EE cells shares similarities with that of neurons, probably reflecting a common phylogenetic origin [223–225]. Consistent with this idea, all known EE peptide hormones (exception insect CCHamides) [226] are also produced by the brain. Acting through these hormones, EE cells may play "neural-like" roles in regulating intestinal physiology and conveying intestinal as well as nutritional state to other cell types or organs. These roles are particularly prominent in the midgut given the relatively sparse innervation of midgut region. *Scute* mutant flies lacks all EE cells and show normal food intake and fertility, but are short-lived and display abnormal intestinal homeostasis [49].

A role for EE cells on muscle peristalsis has been suggested by the finding that ablation of Diuretic hormone 31 (Dh31)-expressing EE cells or Dh31 downregulation both reduce muscle peristalsis in the larval anterior midgut, which may function as a valve to minimize mixing of acidified and non-acidified food in the acidic region of the midgut [227]. Adult EE cells produce Bursicon. Signaling through the Bursicon/DLGR2 receptor in visceral muscle, represses the production of the visceral muscle-derived mitogen Vein and, consequently, ISC proliferation. Another study found in *scute* mutants, depletion on EE cells compromised the nutrientdependent midgut growth that occurs post-eclosion [49] partly by the lack of EE cell-derived Tk, which normally promotes expression of the visceral muscle-derived Ilp3 insulin-like peptide shown to sustain ISC proliferation and nutrient-dependent midgut growth [49, 228]. A recent comparative fly–mouse–human study has pointed to neurotensin-like signaling from EE cells to ECs in flies, with effects on lipid metabolism and AMPK activation. Indeed, expression of mouse neurotensin from *Drosophila* EE cells (and possibly also peripheral sensory neurons) promoted lipid accumulation in both standard and high-fat diets in the midgut, fat body, and oenocytes, and also decreased gut AMPK activation [229]. The effect was dependent

#### *Gut Feeding the Brain:* Drosophila *Gut an Animal Model for Medicine to Understand… DOI: http://dx.doi.org/10.5772/intechopen.96503*

on expression of the Pyrokinin 1 receptor in ECs, but did not seem to be mediated by EE cell-derived Pyrokinin 1, pointing to an involvement of a different ligand [229].

A high-sugar diet leads to increased midgut EE cell number and enhanced production of EE-derived Activin ligand (Activin-β not Daw) [230] suggesting systemic roles for EE-derived peptide hormones. Mirroring the activin-mediated fat-to-gut signaling involved in sucrose repression, midgut-derived Activin-β binds to the TGF-β receptor Baboon in fat cells which, in turn, leads to enhancement of Akh signaling it the fat body and consequent hyperglycemia [230]. CCHamides are insect hormones [231, 232] and their expression is promoted by nutrient availability and sites of expression include the gut EE cells, a subset of central neurons and, possibly, the fat body [226, 233, 234]. Their receptors are expressed in the nervous system including the insulin-producing neurons, and are absent from the gut [226, 234]. Although not strictly gut-derived, a new peptide hormone produced not by EE cells, but by an adjacent secretory gland may have provided the most compelling example to date of gut-to-brain communication. Indeed, Limostatin (Lst) peptide is produced by the *corpus cardiacum*: the Akh-producing gland which, in the adult, is found adjacent to the hypocerebral ganglion on the gastrointestinal tract, at the junction between the esophagus and anterior midgut. Lst is released in response to nutrient restriction and suppresses insulin production by the insulin-producing cells of the brain PI. *Lst* mutant flies accumulate excess fat and display phenotypes associated with insulin excess [235].

In animals with a vascular system, peptides secreted from EE cells can enter the bloodstream and reach tissues at a considerable distance, ranging from other cells in the digestive tract to brain centers regulating appetite [236]. Nutrient availability can also affect the number of EE cells; signaling through the nuclear hormone receptor Hr96, dietary lipids control EE differentiation during the first few days of adult life, providing another way to couple nutrient availability with tissue architecture and physiology [237]. Modulation of intestinal physiology by systemic signals has also been looked into [220, 221]. Control of epithelial turnover by insulin-like peptides or JH (juvenile hormone), and the coupling of dietary availability of sugars with EC digestive enzyme production via the fat body-derived Activin ligand Daw are some examples. The actions of the diuretic peptide Leucokinin (Lk), secreted into the circulation from CNS-derived nerves that terminate at the abdominal wall [14, 238, 239] is an another example. Downregulation of either this peptide or its receptor leads to abnormal excreta and extreme fluid retention that can rupture the abdominal wall [14]. Finally, a link between energy balance, intestinal permeability, and immunity has been suggested by the finding that sNPF is a target of the Crtc/ CREB energy sensing pathway, and functions to maintain epithelial barrier integrity acting through its receptor in ECs [240]. Although the precise source of sNPF remains to be established, tissue-specific genetic and expression data points to roles in neurosecretory cells [240], consistent with roles as a neuroendocrine hormone or in gut-innervating neurons. Gut can also produce long-range signals to affect the physiology of other organs, for example by production of the signaling protein Hh by larval EC. Circulating Hh regulates developmental timing by controlling ecdysteroid production in the prothoracic gland, and is required for mobilization of fat body TAG stores during starvation [241].

Other functions of brain-gut peptides and hormones include detection and utilization of nutrients during hunger, stress or normal conditions. Diuretic hormone 44 (Dh44), a homolog of the mammalian corticotropin –releasing -hormone (CRH) activate by nutritive sugars. Disturbed activity of Dh44 neurons leads to fail to select nutritive sugars [131]. These neurons localized to PI in adult brain,

counterpart of mammalian hypothalamus [242] are filled with neurosecretory cells [131]. Dh44 conveys information from Dh44 neurons to Dh44 receptor R1 neurons in the brain and R2 cells in the gut suggesting requirement for nutrient selection. Artificial activation of these neurons causes rapid PER and it has been suggested that Dh44 is necessary and sufficient for gut motility and excretion in flies [131]. Both Dh44 neurons and the gut-innervating insulin-producing neurons of the PI are innervated by Hugin-producing neurons that suppress food intake and induce locomotion, providing a possible link between food-related behaviors and intestinal physiology [243]. It is seen later that Dh44+ neurons rapidly activate during amino acid feeding and are a direct sensor of dietary amino acids [244].

Fly gut peptide Dromyosuppressin [181] expresses in the number of cells in central nervous system (CNS) in adult stage, extending into the rectum, near the anus; part of the adult gut. Their immunoreactive fibers also project into the crop and show expression of Dromyosuppressin [172] and crop abundantly expresses Dromyosuppressin receptors (Dromyosuppressin receptor I) [220]. The effects of neuropeptide on neural regulation of crop motility and contractions have been shown [245]. Serotonergic neurons have also been shown to regulate insulin producing cells (IPC) located in the PI of the adult brain of the fly [246]. DILP2, 3 and 5 express particularly in midgut [187, 220, 247] and extend their axons to proventriculus, crop and corpora cardiaca [248]. DILP2 is particularly involved in carbohydrate metabolism [249]. Decreased levels of DILP2 affects stored trehalose as well [248]. IPC knockdown flies show increased glycogen storage, high levels of circulating triglycerides and extended lifespan [248].

Mammalian neuropeptide NPY (Neuropeptide – Y- precursor) has an invertebrate homologous peptide called NPF due to characteristic C- terminal F residue [250]. NPF has been shown to co- localize within midgut cells in *Drosophila* and in brain and it plays a role of co- transmitter in many neural circuits [251, 252]. Its receptors are expressed primarily in the malpighian tubules but also hindgut and midgut [253]. There are some regulatory peptides found in both gut and brain. Small neuropeptide-F (sNPF) is found in the neurons in the hypocerebral ganglion innervating the midgut as well. sNPF gene encodes four kinds of sNPFs and is predominantly found in the central nervous system suggesting it might be directly involved in several neural circuits that affect hunger and feeding mechanisms [254]. The receptor for sNPF is further identified and found to be expressed in the crop, Malpighian tubules, hindgut, and the midgut [253]. Over expression or knockdown of sNPF leads to an increase or decrease in adult feeding respectively [255]. sNPF also directly affects and alters DILPs levels in larval and adult IPCs [254]. RFAmides are a class of different neuropeptides, all containing a common C-terminal RFamide sequence [256]. Using antisera to recognize the gene products of the five genes in *Drosophila* genome encoding for RFAmides, endocrine cells in anterior, middle and posterior midgut are labeled [220], axons in the midgut and crop as well as hypocerebral ganglion. RFAmides also play a key role in food intake, sensing and feeding mechanisms pointing toward conservation in these pathways from insects to mammals [257].

Other neuropeptides include Allatostatin characterized into three kinds namely Allatostatin A/B/C [258–260]. The endocrine cells producing Allatostatin are found in the posterior midgut and is innervated by axons from thoracico-abdominal ganglion [220]. Its receptors, DAR1 and DAR2 are predominantly located in the central nervous system and gastrointestinal tract (including crop, midgut and hindgut) respectively [261, 262]. Allatostain is also present throughout midgut. Another major peptide is PDF. It is expressed in central nervous system [263] and its neurons are also found in thoracico-abdominal ganglion [184]. Axons from these neurons innervate midgut and hindgut and in the crop. These neuropeptides are closely associated to circadian rhythm [264] and locomotor activity as well.

*Gut Feeding the Brain:* Drosophila *Gut an Animal Model for Medicine to Understand… DOI: http://dx.doi.org/10.5772/intechopen.96503*
