Vacuole dynamics in the salivary glands ofDrosophila melanogasterduring prepupal development

Abstract

A central function of the Drosophila salivary glands (SGs), historically known for their polytene chromosomes, is to produce and then release during pupariation the secretory glue used to affix a newly formed puparium to a substrate. This essential event in the life history of Drosophila is regulated by the steroid hormone ecdysone in the late-larval period. Ecdysone triggers a cascade of sequential gene activation that leads to glue secretion and initiates the developmentally-regulated programmed cell death (PCD) of the larval salivary glands, which culminates 16 h after puparium formation (APF). We demonstrate here that, even after the larval salivary glands have completed what is perceived to be one of their major biological functions -glue secretion during pupariation -they remain dynamic and physiologically active up until the execution phase of PCD. We have used specific metabolic inhibitors and genetic tools, including mutations or transgenes for shi, Rab5, Rab11, vha55, vha68-2, vha36-1, syx1A, syx4, and Vps35 to characterize the dramatic series of cellular changes occurring in the SG cells between pupariation and 7-8 h APF. Early in the prepupal period, they are remarkably active in endocytosis, forming acidic vacuoles. Midway through the prepupal period, there is abundant late endosomal trafficking and vacuole growth, which is followed later by vacuole neutralization and disappearance via membrane consolidation. This work provides new insights into the function of Drosophila SGs during the early-to mid-prepupal period.

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

“…Acidification of the vacuolar lumen by proton pumping vATPases was reported earlier as an essential factor for the release of iron (Fe 3+ ) from the dimerized transferrin during endocytosis (El Hage Chahine et al, 2012;Steere et al, 2012;Farkaš et al, 2015). In addition, the temporary enhancement of O 2 consumption after pupariation could be related to a temporary increase in the number of mitochondria in the cytoplasm of the SG cells (Farkaš and Šuťáková, 1998), which occurs over a relatively short time (duration 2-3 h) and is not accompanied by remarkable changes in the mitochondrial morphology.…”

“…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.…”

“…With respect to the temporary enhancement of respiratory metabolism of SG shortly after puparium formation, there seem to be two simultaneously occurring and tightly linked cytological processes: The first of these is the bulk endocytosis and intense vacuolation of the cytoplasm (Farkaš et al, 2015), while the second is the intense acidification of the vacuoles, which is connected with endocytotic import and the release of iron used to enhance of intermediary metabolism. Acidification of the vacuolar lumen by proton pumping vATPases was reported earlier as an essential factor for the release of iron (Fe 3+ ) from the dimerized transferrin during endocytosis (El Hage Chahine et al, 2012;Steere et al, 2012;Farkaš et al, 2015).…”

“…2 and 3) is associated with the termination of the endocytotic phase and the consolidation of vacuoles with endogenous membrane systems, such as the ER and Golgi (Farkaš et al, 2015). This suggests that acidic vacuoles could serve for transport and release of iron that will be used in processes with higher oxygen requirements.…”