Steroid hormone regulation of innate immunity in Drosophila melanogaster

Abstract

Endocrine signaling networks control diverse biological processes and life history traits across metazoans. In both invertebrate and vertebrate taxa, steroid hormones regulate immune system function in response to intrinsic and environmental stimuli, such as microbial infection. The mechanisms of this endocrine-immune regulation are complex and constitute an ongoing research endeavor facilitated by genetically tractable animal models. The 20-hydroxyecdysone (20E) is the major steroid hormone in arthropods, primarily studied for its essential role in mediating developmental transitions and metamorphosis; 20E also modulates innate immunity in a variety of insect taxa. This review provides an overview of our current understanding of 20E-mediated innate immune responses. The prevalence of correlations between 20E-driven developmental transitions and innate immune activation are summarized across a range of holometabolous insects. Subsequent discussion focuses on studies conducted using the extensive genetic resources available in Drosophila that have begun to reveal the mechanisms underlying 20E regulation of immunity in the contexts of both development and bacterial infection. Lastly, I propose directions for future research into 20E regulation of immunity that will advance our knowledge of how interactive endocrine networks coordinate animals' physiological responses to environmental microbes.

Fig 1. 20E signaling activity during Drosophila development coincides with up-regulation of immune genes.

(A) Schematic illustrating the temporal dynamics of 20E titer and immune gene induction during insect pupariation. Distinct peaks in hemolymph 20E levels drive developmental transitions including the onset of metamorphosis in Drosophila and other holometabolous insects [20,30,31]. A dramatic, microbe-independent up-regulation of immune gene transcripts, including AMPs, also occurs at pupariation [40,45]. (B) 20E activates a hierarchical, temporally controlled gene regulatory network. Circulating 20E binds to and activates the heterodimeric NHR complex consisting of EcR and Usp. 20E-EcR/Usp directly activates transcription of “early” response genes encoding additional NHRs and transcription factors, including Broad (Br), Eip74EF (E74), and Eip75B (E75). These NHRs up-regulate expression of “late” response genes, which code for proteins that actuate a variety of cellular and physiological processes [20,22,81]. 20E chemical structure depiction from PubChem [151]. AMP, antimicrobial peptide; EcR, ecdysone receptor; NHR, nuclear hormone receptor; Usp, ultraspiracle; 20E, 20-hydroxyecdysone.

Fig 2. Proposed transcriptional regulatory mechanisms through which 20E signaling can promote immune activation and AMP expression.

The 20E-EcR/Usp complex induces expression of early genes encoding additional transcription factors including Br, Eip78C, Hr3, Eip93F, and others, each of which is required for 20E-dependent immune gene induction [40,41,89]. (A) 20E-activated transcription factors such as Br and other NHRs may directly regulate and increase the expression of the PGRP-LC receptor, which facilitates IMD signaling in response to DAP-type peptidoglycan (PGN) derived from gram-negative bacteria. IMD signaling downstream of up-regulated PGRP-LC results in Relish (Rel)-dependent transcriptional induction of IMD-regulated AMPs [41,87,89]. (B) 20E-activated NHRs may bind to cis regulatory sequences in the promoters of particular AMPs and thereby directly induce their up-regulation in response to as yet unknown intrinsic (developmental) or external (microbial) signals [40,41,46]. Direct regulation of immune genes including PGRP-LC and various AMPs by 20E-regulated transcription factors warrants further investigation. Detailed mechanistic studies have shown that Br regulates expression of the miRNAs let-7 and miR-34 [121,124]. Br promotes expression of let-7, which directly targets Dpt transcripts [124]. miR-34 activates immunity in part by targeting transcripts of the 20E-induced negative IMD regulator Eip75B [121]. 20E signaling via Br suppresses miR-34 expression, derepressing Eip75B which diminishes IMD activity through unclear mechanisms that might involve PGRP-LC suppression [41,121]. (C) 20E could potentiate induction of Toll-responsive AMPs by driving expression of genes coding for Toll pathway components, such as the activating receptor PGRP-SA [55] or fungal-responsive proteins like GNBP1 and GNBP3. Analogous to the PGRP-LC-dependent mechanism (A), this could facilitate AMP up-regulation via Dorsal-related immunity factor (Dif)-Dorsal (Dl) activation downstream of canonical Toll signaling [40]. This putative mechanism and the cellular/physiological contexts in which it might occur have not been thoroughly investigated in Drosophila. AMP, antimicrobial peptide; EcR, ecdysone receptor; NHR, nuclear hormone receptor; PGRP, peptidoglycan recognition protein; Usp, ultraspiracle; 20E, 20-hydroxyecdysone.