Apocrine Secretion in Drosophila Salivary Glands: Subcellular Origin, Dynamics, and Identification of Secretory Proteins

Abstract

In contrast to the well defined mechanism of merocrine exocytosis, the mechanism of apocrine secretion, which was first described over 180 years ago, remains relatively uncharacterized. We identified apocrine secretory activity in the late prepupal salivary glands of Drosophila melanogaster just prior to the execution of programmed cell death (PCD). The excellent genetic tools available in Drosophila provide an opportunity to dissect for the first time the molecular and mechanistic aspects of this process. A prerequisite for such an analysis is to have pivotal immunohistochemical, ultrastructural, biochemical and proteomic data that fully characterize the process. Here we present data showing that the Drosophila salivary glands release all kinds of cellular proteins by an apocrine mechanism including cytoskeletal, cytosolic, mitochondrial, nuclear and nucleolar components. Surprisingly, the apocrine release of these proteins displays a temporal pattern with the sequential release of some proteins (e.g. transcription factor BR-C, tumor suppressor p127, cytoskeletal β-tubulin, non-muscle myosin) earlier than others (e.g. filamentous actin, nuclear lamin, mitochondrial pyruvate dehydrogenase). Although the apocrine release of proteins takes place just prior to the execution of an apoptotic program, the nuclear DNA is never released. Western blotting indicates that the secreted proteins remain undegraded in the lumen. Following apocrine secretion, the salivary gland cells remain quite vital, as they retain highly active transcriptional and protein synthetic activity.

Xanthine oxidoreductase mediates membrane docking of milk‐fat droplets but is not essential for apocrine lipid secretion

“…Farkas et al . () suggest that Drosophila salivary glands could be a useful model system to study apocrine secretion, especially for understanding the sorting of cellular components for secretion and retention within the cell. However, the eventual cell death of this epithelium after major apocrine secretory processes makes it substantively different from the lactating mammary cell, which must continue apocrine secretion for days, weeks or even months.…”

Respiratory metabolism of salivary glands during the late larval and prepupal development of Drosophila melanogaster

“…We showed that at 1-6/7 h APF, the glands undergo intense vacuolation associated with complex endosomal trafficking, and then, at about 8-10 h APF, they display apocrine secretion (Farkaš et al, 2014(Farkaš et al, , 2015. In order to provide a physiological explanation for the relationship of these new cytological observations to the previously well-characterized chromosomal puffing patterns, we measured the respiratory metabolism of the Drosophila SG during the nonfeeding prepupal period.…”

“…During the next set of cytological stages characterized by neutralization and disappearance of the vacuoles, apocrine secretion and PCD ( Fig. 2; Farkaš et al, 2014Farkaš et al, , 2015, the regular metabolic decline, initiated already in the wandering larval stage and interrupted temporarily during pupariation, continues until the complete dissolution of the SG.…”

“…This has been shown by following the molecular changes in the pools of proteins separable by SDS-PAGE and by autoradiography (Tissiéres et al, 1974;Lewis et al, 1975;Mitchell et al, 1977;Zhimulev et al, 1981;Farkaš et al, 2014). The patterns of gene and protein expression in the SG change hourly until the programmed death of the glands (Tissiéres et al, 1974;Lewis et al, 1975;Zhimulev et al, 1981;Farkaš et al, 2014). From the described course of postendocytotic changes in respiratory metabolism and RNA synthesis, we conclude that the SG of the nonfeeding prepupal stages have only limited resources for the constitutive transcription and translation of housekeeping gene products, which had been actively expressed during the earlier feeding stages.…”

Vacuole dynamics in the salivary glands ofDrosophila melanogasterduring prepupal development

“…Very recently, we have described massive apocrine secretion in 8–10 h prepupal salivary glands of Drosophila (Farkaš et al . ). Here, we have used genetic tools and confocal microscopy to more precisely characterize the range of cellular processes that occur between 0 and 7/8 h of prepupal development.…”