Insight into Insulin Secretion from Transcriptome and Genetic Analysis of Insulin-Producing Cells of Drosophila

Abstract

Insulin-producing cells (IPCs) in the Drosophila brain produce and release insulin-like peptides (ILPs) to the hemolymph. ILPs are crucial for growth and regulation of metabolic activity in flies, functions analogous to those of mammalian insulin and insulin-like growth factors (IGFs). To identify components functioning in IPCs to control ILP production, we employed genomic and candidate gene approaches. We used laser microdissection and messenger RNA sequencing to characterize the transcriptome of larval IPCs. IPCs highly express many genes homologous to genes active in insulin-producing β-cells of the mammalian pancreas. The genes in common encode ILPs and proteins that control insulin metabolism, storage, secretion, β-cell proliferation, and some not previously linked to insulin production or β-cell function. Among these novelties is unc-104, a kinesin 3 family gene, which is more highly expressed in IPCs compared to most other neurons. Knockdown of unc-104 in IPCs impaired ILP secretion and reduced peripheral insulin signaling. Unc-104 appears to transport ILPs along axons. As a complementary approach, we tested dominant-negative Rab genes to find Rab proteins required in IPCs for ILP production or secretion. Rab1 was identified as crucial for ILP trafficking in IPCs. Inhibition of Rab1 in IPCs increased circulating sugar levels, delayed development, and lowered weight and body size. Immunofluorescence labeling of Rab1 showed its tight association with ILP2 in the Golgi of IPCs. Unc-104 and Rab1 join other proteins required for ILP transport in IPCs.

Synaptic transmission parallels neuromodulation in a central food-intake circuit

“…In accordance with hugin-PC neurons using ACh as neurotransmitter, IPCs were previously shown to express a muscarinic ACh receptor (Cao et al, 2014). Whether additional ACh receptors are expressed is unknown.…”

“…First, by classic synaptic transmission as our data strongly suggest that acetylcholine (ACh) acts as transmitter at these synapses. Accordingly, subsets of mNSCs have been shown to express a muscarinic ACh receptor (Cao et al, 2014). Whether additional ACh receptors are expressed is unknown.…”

The Drosophila ortholog of TMEM18 regulates insulin and glucagon-like signaling

“…Ilps are produced by 14 IPCs with physiological function similar to mammalian pancreatic beta cells (Nassel et al 2013). Moreover, RNAseq experiments from laser-dissected IPCs in Drosophila larvae identified Tmem18 expression in those cells (Cao et al 2014). To limit Tmem18-RNAi specifically to IPCs, we drove expression with an Ilp2 (Ilp2-GAL4) construct.…”

“…Using an RNAi construct with expression driven by the GAL4-UAS system (Brand & Perrimon 1993), we knocked down Tmem18 expression specifically in the IPCs of Drosophila. These cells function physiologically similar to the mammalian pancreas (Cao et al 2014), releasing Drosophila insulinlike proteins (Ilp) (Nassel et al 2013, Cao et al 2014.…”

“…Flies are an excellent model system for studying a variety of molecular pathways, as well as behavior. Specifically for obesity, they have organs that function similar to the human pancreas with corresponding orthologs of insulin and glucagon: insulin-like protein 2 (Ilp2) and adipokinetic hormone (AKH), respectively (Kim & Rulifson 2004, Nassel et al 2013, Cao et al 2014. Moreover, the suite of genetic tools available in Drosophila allows for highly specific experimentation.…”