MANTENIMIENTO PERIODICO Y PEQUEÑAS REPARACIONES
MANTENER ALEJADA DEL ALCANCE DE LOS NIÑOS
It is necessary for all living organisms to be able to sense and respond to different environmental cues and changing organismal demands. These processes are highly dependent on complex inter-organ communication programs. Understanding how tissues communicate with each other is necessary to understand human physiology and pathology. Due to their simpler physiology and genetic amenability,
Drosophila has pioneered inter-organ communication studies.
1.5.1 Gut derived factors regulating metabolism
The gut is the first organ sensing nutrients and many gut-derived secreted factors are known to act on distant tissues to regulate metabolism.
It has been shown that Hedgehog (Hh) is increased within the larval gut upon starvation, it is secreted into the hemolymph and binds its receptor Patch on the
fat body to mobilise lipid stores under starvation condition (Rodenfels et al., 2014). Furthermore, circulating Hh also regulated Ecdysone levels through Patch binding on the prothoracic gland, thus regulating pupariation (Rodenfels et al., 2014).
Recent work on the adult Drosophila midgut has shown that ISC proliferation impacts brain derived Insulin signals and therefore has major effects on metabolism. Activation of the Hippo pathway, by overexpressing an activated form of yorkie in stem progenitor cells (ISCs and EBs) induced ISC proliferation, which was shown to induce ImpL2 production within the gut (Kwon et al., 2015). Imaginal morphogenesis protein Late 2 (ImpL2), the homolog to the mammalian IGFBP7, belongs to the immunoglobulin-superfamily and is a secreted Insulin/IGF antagonist, therefore leading to reduced nutrient uptake from the circulation, which consequently also impacts nutrient storage and induces tissue wasting (Kwon et al., 2015). In agreement with reduced Insulin signalling, animals bearing
yorki-driven hyperproliferative midguts were hyperglycemic and showed reduced
lipid and glycogen levels, which was independent from feeding behaviour (Kwon et al., 2015). Furthermore, intestinal hyperproliferation led to ovary and muscle wasting, which could be rescued by the introduction of a mutant allele for impl2 (Kwon et al., 2015).
Those relatively recent studies provided a great foundation and opened up the field of endocrine regulation of metabolism in Drosophila research, but yet many more investigations are needed to fully understand and uncover the endocrine system in the fruit fly.
1.5.2 Non-gut derived factors regulating metabolism
The Drosophila fat body and oenocytes, the homolog to the mammalian adipose tissue and liver, are the primary sites for energy storage and release, which needs to be carefully regulated by hormones. In Drosophila, many fat body derived peptides, for example Unpaired 2 (Upd2), Dawdle, ImpL2 and Dilp6 have been identified.
In mammals, Leptin, the satiety hormone has been identified as a hormone responding to Insulin levels. It was shown that adipose derived Leptin binds to its
receptor in neuroendocrine organs to regulate metabolism (Ahima et al., 1996; Tartaglia et al., 1995). Interestingly, there is no Drosophila protein, which has recognisable sequence similarity with mammalian Leptin. But recently, in
Drosophila larvae it was shown, that Unpaired 2 (Upd2) acts in a similar fashion
as human Leptin (Rajan and Perrimon, 2012). Fat body derived Upd2 can bind to its receptor Domeless (Dome, JAK/Stat receptor) on GABA+ive neurons to mediate
Dilp secretion from IPCs (Rajan and Perrimon, 2012).
Drosophila Dawdle is a TGF-β/ Activin-like ligand, which is produced by the fat
body in response to dietary sugars (Chng et al., 2014). Dawdle was shown to act on midgut enterocytes through Baboon/Punt receptors to supress digestive enzymes, thus working as a negative feedback loop (Chng et al., 2014). This sugar sensing mechanism has been found to be specific to nutritious sugars and dependent on Smad2 activation, but independent to Insulin- or AKH- (Glucagon) signalling (Chng et al., 2014). Interestingly, it has also been shown that muscle derived dawdle is a Foxo target and therefore being controlled by Insulin (Bai et al., 2013). Finally, dawdle, and its receptor baboon were expressed throughout various larval tissues (Ghosh and O'Connor, 2014). dawdle mutant larvae showed increased levels of Dilp2 within IPCs, thus suggesting that wild type Dawdle protein promotes Insulin secretion (Ghosh and O'Connor, 2014). Furthermore, dawdle mutant larvae displayed higher TAG, glycogen and glucose levels compared to control or heterozygous dawdle mutants (Ghosh and O'Connor, 2014). Those studies demonstrate that TGFβ/ Activin-like signalling is a regulator of metabolism in Drosophila.
ImpL2, the homolog to the mammalian IGFBP7 has been found to act as an inhibitor of Insulin signalling by binding extracellular Dilp2 (Honegger et al., 2008). ImpL2 in the fat body was increased upon 24h starvation, suggesting that ImpL2 is important for regulating the starvation response of the animal (Honegger et al., 2008). Consistently, mutant larvae for impl2 showed decreased survival when fed with 1% sugar or PBS only, compared to fully fed animals (Honegger et al., 2008). Another inhibitor of circulating Insulin is secreted decoy of insulin receptor (Sdr), therefore leading to the inhibition of growth (Okamoto et al., 2013). Sdr is expressed by the CNS and is also necessary for adequate response to starvation (Okamoto et al., 2013). Interestingly, Sdr and Impl2 act independently from each
other and they bind circulating Insulins (Dilp1-7 tested) with different affinities (Okamoto et al., 2013). Furthermore, Impl2 was also found as a secreted factor from the muscles as a protective mechanism upon mitochondrial stress (Owusu- Ansah et al., 2013).
Dilp6, which displays structural similarity to the Insulin-like growth factor (IGF) was highly increased in non-feeding stages of the larval fat body (Okamoto et al., 2009). dilp6 expression in the fat body is essential to achieve normal overall animal size, lipid metabolism and survival upon starvation (Chatterjee et al., 2014; Okamoto et al., 2009; Slaidina et al., 2009).
It has recently been discovered that terminal tracheal branches, akin to mammalian vasculature, play an important role in nutrient sensing and systemic metabolism in Drosophila (Linneweber et al., 2014) Nutrients were sensed by enteric neurons producing Insulin-like peptide 7 (Dilp7) and Pigment Dispersing Factor (PDF). These neuropeptides bind to Insulin and PDF receptors within gut- associated trachea, which increased or decreased their branching in conditions of abundant or poor nutrients, respectively (Linneweber et al., 2014). Reducing terminal gut-tracheal branching throughout animal development by inhibition of Insulin or PDF signalling led to the reduction of organismal TAG levels in larvae and adult flies (Linneweber et al., 2014).
The endocrine system in Drosophila also consists of endocrine glands, called corpora allata (CA) and corpora cardiaca (CC), which produce key factors to maintain metabolic homeostasis. Those factors are Adipokinetic Hormone (AKH), Limostatin and Juvenile Hormone (JH).
AKH is a Glucogon-like peptide and therefore works as an opposing factor to Insulins. AKH is produced and released by the CC (Galikova et al., 2015; Kim and Rulifson, 2004) and binds to its receptor AKHR in various tissues to increase the release of stored nutrients (Galikova et al., 2015; Gronke et al., 2007; Kim and Rulifson, 2004; Lee and Park, 2004). AKH ablation in Drosophila larvae led to reduction of circulating sugar levels (Kim and Rulifson, 2004). akh and akhr mutants also showed higher lipid content compared to control flies, which was responsible for increased starvation resistance (Galikova et al., 2015). Furthermore adult flies carrying a mutation for akh or akhr showed reduced
circulating sugar levels, which wasn’t due to an upregulation in stored glucose (Galikova et al., 2015). Interestingly, AKH ablated flies didn’t show the starvation induced hyperactivity as seen in control flies (Lee and Park, 2004). This shows that adequate AKH signalling is essential in Drosophila.
Limostatin (Lst), another CC produced peptide, is secreted in response to nutrient deprivation to reduce Insulin secretion by binding to its receptor LstR on IPCs and therefore acts as a Decretin (Alfa et al., 2015). Limostatin signalling was found to be ortholog to mammalian Neuromedin U/ Neuromedin U receptor signalling (Alfa et al., 2015). lst expression was upregulated upon starvation and reduced again specifically after re-feeding with carbohydrates, but not proteins (Alfa et al., 2015). lst mutants showed decreased glucose levels and increased dilp2 transcription, as well as increased circulating Dilp2 protein, increased stored lipid content and reduced lifespan (Alfa et al., 2015). lst mutant phenotypes were rescued upon blockage of Dilp2 secretion or when lst was overexpressed in CC cells in a lst mutant background (Alfa et al., 2015). The differential regulation of Lst by nutrients represents an excellent paradigm to understand nutrient sensing mechanism.
The corpora allata produces Juvenile Hormone, which regulates larval growth and adult reproduction through its receptor Germ cell-expressed (Gce)/ Methopren- tolerant (Met) (Jindra et al., 2015; Mirth et al., 2014; Reiff et al., 2015). Ablation of JH producing CA cells led to smaller larvae due to Foxo dependent reduction in growth rate (Mirth et al., 2014). Recently, it was found that JH is important for proliferation, growth and metabolic status of the midgut of mated females in preparation for reproduction (Reiff et al., 2015). Virgin flies displayed smaller and less proliferative midguts as well as lower lipid content compared to mated females (Reiff et al., 2015). Interestingly, this lack of growth and proliferation could be rescued by feeding virgin flies with JH supplemented food (Reiff et al., 2015). Knockdown of the Juvenile hormone receptor met or gce specifically in stem/ progenitor cells or enterocytes resulted in flies with reduced fecundity, showing that JH acts on the midgut to increase its size, which leads to increased lipid metabolism for functional fecundity (Reiff et al., 2015).
All above mentioned hormones and signalling pathways uncovered and characterised show the great advantage of using Drosophila as a model organism
to study complex inter-organ communication leading to the regulation of local and systemic metabolic homeostasis through highly conserved molecular mechanism.