Fat Body Hormone SPARC Drives Insulin Release and Body Growth in Fly Larvae

How does an animal know when to grow? For a young insect, the answer depends heavily on food. A growing fruit fly larva constantly assesses the quality and quantity of its nutrients, and it translates that information into a body-wide growth program. The central player in this translation is the fat body, an organ that combines the functions of the mammalian liver and adipose tissue. It stores energy, senses nutrition, and broadcasts chemical messages that tell distant tissues whether conditions are favorable for expansion. Now, a team of researchers at Julius-Maximilians-Universität Würzburg, working with collaborators, has identified a key messenger in this communication network: a secreted protein called SPARC, which turns out to act as an evolutionarily conserved trigger for insulin release. The discovery, published in BMC Biology, reveals a new layer of control over systemic growth and offers an unexpected parallel to hormone signaling in humans.

The study began with a deceptively simple question: which genes, when silenced in the fat body, change the growth of organs elsewhere in the animal? Answering it required a screening system that could measure growth in living larvae rather than relying on laborious dissection and imaging. The team engineered a reporter line, dubbed GQ, in which the fat body drives genetic knockdowns while the growth response is read out in peripheral tissues, specifically the imaginal discs, the larval structures that will eventually form adult wings, legs, and eyes. These discs carry a firefly luciferase reporter whose activity scales with tissue growth, while a Renilla luciferase reporter provides an internal reference. By homogenizing individual larvae at a standardized developmental time point, 115 hours after egg deposition, and running dual luciferase assays, the researchers could quantify the growth consequences of each fat-body-specific gene knockdown in a single, rapid measurement.

Using RNA interference lines from the Vienna Drosophila Resource Center, the team systematically knocked down secreted and secretory-pathway genes in the fat body and scored the resulting effects on imaginal disc growth. The primary screen flagged dozens of candidate genes, and a rigorous secondary screen retested each hit with multiple independent RNAi transgenes, including lines from the Bloomington stock center and the National Institute of Genetics in Japan. Only candidates whose effects could be reproduced with independent reagents were considered genuine. Out of the retested genes, 32 were confirmed with high confidence, three showed moderate effects, and 13 failed validation, a sobering reminder of why independent confirmation is essential in genetic screens. Among the confirmed hits, one protein stood out for both the strength of its effect and the elegance of its mechanism: SPARC, short for Secreted Protein Acidic and Rich in Cysteine, also known as BM-40.

SPARC is a matricellular protein, meaning it is secreted into the extracellular environment but does not serve as a structural component of the matrix itself. Instead, matricellular proteins modulate cell-matrix interactions and cell signaling. In mammals, SPARC has been implicated in tissue remodeling, wound repair, fibrosis, and cancer biology, but its role as an endocrine growth regulator had not been established. The new work shows that when SPARC production is reduced in the larval fat body, the entire animal grows less. Adult flies with fat-body SPARC knockdown emerge smaller, with reduced body weight and smaller wings, effects that were quantified by measuring adult dry weight and wing area across multiple independent experiments and temperatures. The consistency of the size reduction across different driver lines and RNAi constructs pointed to a genuine developmental function rather than a technical artifact.

The critical question was how a protein secreted from the fat body could slow the growth of tissues it never touches directly. The answer lies in a small cluster of neurosecretory cells in the larval brain called the insulin-producing cells, or IPCs. These cells are the functional counterparts of human pancreatic beta cells: they release Drosophila insulin-like peptides, particularly Ilp2, into the circulation, and circulating insulin activates the insulin signaling pathway in target tissues, driving cell growth and proliferation. When the researchers examined the IPCs of larvae in which the fat body had been depleted of SPARC, they found that Ilp2 accumulated inside the cells instead of being released. In other words, the peptide was being made but retained, a classic secretory defect. Fat-body SPARC, it appears, is required for the proper endocrine release of insulin from the brain, acting as an incretin-like signal that promotes insulin secretion in response to nutritional conditions.

Several control experiments strengthened this interpretation. A short, one-day depletion of SPARC did not grossly disrupt larval feeding behavior or fat body integrity, arguing against the possibility that the growth defects were secondary to starvation or tissue damage. Larvae with SPARC knockdown still ingested food normally, as shown by feeding assays with dye-labeled yeast paste, and their fat body architecture, visualized with nuclear and membrane stains, remained intact. The team also showed that SPARC is indeed secreted into the hemolymph, the insect circulatory fluid, where it can be detected by western blotting, and that its expression responds to nutritional state, with starvation reducing SPARC mRNA levels in the fat body. These observations fit a model in which the fat body, upon sensing adequate nutrition, secretes SPARC into the circulation, and SPARC in turn stimulates the IPCs to release Ilp2, thereby licensing systemic growth.

Perhaps the most striking result of the study is its evolutionary punchline. When the researchers supplied synthetic human SPARC to fly larvae lacking their own SPARC, the human protein could substitute for the fly version, restoring insulin release and growth. This cross-species rescue demonstrates that the biochemical function of SPARC in this signaling axis has been conserved over hundreds of millions of years of evolution separating insects and vertebrates. It also raises an obvious and tantalizing question for mammalian biology: could SPARC play a comparable role in human insulin regulation? Mammalian incretins, such as GLP-1, are gut-derived hormones that potentiate insulin secretion from pancreatic beta cells, and they form the basis of a major class of diabetes drugs. A fat-derived, SPARC-dependent pathway that promotes insulin release would represent a fundamentally new branch of this regulatory logic, though the authors are careful to frame the human implications as a hypothesis for future work rather than a demonstrated mechanism.

The technical achievement behind the discovery deserves attention in its own right. Traditional genetic screens for growth regulators require laborious measurements of organ size, often by microscopy, which limits throughput and introduces variability. By coupling a tissue-specific knockdown system to a luciferase-based growth readout, the Würzburg team created a pipeline in which a single larva yields a quantitative growth measurement in a plate reader. The dual-reporter design, with firefly luciferase reporting imaginal disc growth and Renilla luciferase normalizing for larval size and handling differences, allowed hundreds of genotypes to be assayed individually. The supplementary data accompanying the paper document the full trajectory of the screen, from the primary results across the tested lines to the validation tables listing every independent transgene and its effect, providing an unusually transparent record of how the conclusions were reached.

Beyond SPARC itself, the study delivers a conceptual message about how growth is coordinated across an animal. The fat body is not merely a passive energy depot; it is an endocrine gland whose secreted products reach into the brain and tune the output of the insulin-producing cells. This architecture mirrors the inter-organ communication circuits of mammals, in which the liver, adipose tissue, gut, and pancreas exchange hormonal signals to match growth and metabolism to nutrient availability. Disruptions in such circuits are central to metabolic disease, and understanding their components in a genetically tractable organism like Drosophila provides a framework for dissecting the corresponding human pathways. The identification of SPARC as an incretin-like, insulin-promoting signal adds a new name to the roster of conserved growth regulators and suggests that the extracellular matrix, long viewed as structural scaffolding, participates directly in endocrine control.

Future experiments will need to determine exactly how SPARC acts on the IPCs, whether it binds a specific receptor on these cells, modulates their secretory machinery, or acts through intermediate signals in the hemolymph. It will also be important to establish whether the SPARC-dependent pathway responds to specific nutrients and how it integrates with other known fat-body-derived signals that influence insulin release and growth. For now, the study stands as a vivid example of modern genetics at work: a systematic in vivo screen, a rigorous validation pipeline, and a mechanistic story that connects a secreted matrix protein in the fat body to insulin granules in the brain and, ultimately, to the size of the adult fly. In doing so, it transforms our picture of how a growing animal counts its calories and converts them into body mass, and it plants a flag for SPARC in a territory where few would have thought to look.

Subject of Research: Fat body-derived SPARC signaling in the regulation of insulin release and systemic larval growth in Drosophila

Article Title: Fat body secreted SPARC promotes Ilp2 release and systemic growth in Drosophila larvae

Article References: Fat body secreted SPARC promotes Ilp2 release and systemic growth in Drosophila larvae. (n.d.). https://doi.org/10.1186/s12915-026-02761-x

Image Credits: AI Generated

DOI: 10.1186/s12915-026-02761-x

Keywords: Drosophila, SPARC, insulin signaling, fat body, Ilp2, incretin, larval growth, inter-organ communication, genetic screen, nutrient signaling, insulin-producing cells, BMC Biology