Single‐Cell Transcriptome Analysis of the Drosophila Salivary Gland to Identify Genes Involved in Secretory Granule Maturation

Abstract

Regulated secretion is an essential process by which proteins, stored in membranous secretory granules, are released into the extracellular environment in response to either a hormonal or physiological stimulus. Defects in this process, as well as in the synthesis and maturation of secreted proteins, can contribute to numerous diseases, including cystic fibrosis and inflammatory bowel disease. Exocrine cells, in particular, are often responsible for the synthesis and secretion of large, highly-glycosylated proteins, such as mucins. In Drosophila salivary glands, mucins are produced, packaged, and released in a highly-organized fashion in response to developmentally-regulated pulses of the hormone, 20-hydroxyecdysone. Mucin-containing secretory granules initially bud from the trans-Golgi network and undergo a series of maturation steps until every secretory cell within the gland is filled with mature granules. To understand this highly orchestrated process and identify the factors responsible for mucin biosynthesis and secretory granule maturation, we performed single-cell RNA sequencing of Drosophila salivary glands. Through Seurat and Monocle analyses, we are able to identify and characterize secretory cells at each stage of the maturation process, as well as other cell populations associated with or integral to the salivary glands. This study will not only allow us to gain a better understanding of how highly-glycosylated proteins such as mucins are synthesized, packaged and secreted, but will also provide insight into salivary gland maturation and function.