The Golgi ribbon in mammalian cells negatively regulates autophagy by modulating mTOR activity

Abstract

In vertebrates, individual Golgi stacks are joined into a compact ribbon structure; however, the relevance of a ribbon structure has been elusive. Here, we exploit the finding that the membrane tether of the -Golgi network, GCC88 (encoded by), regulates the balance between Golgi mini-stacks and the Golgi ribbon. Loss of Golgi ribbons in stable cells overexpressing GCC88 resulted in compromised mechanistic target of rapamycin (mTOR) signaling and a dramatic increase in LC3-II-positive autophagosomes, whereas RNAi-mediated depletion of GCC88 restored the Golgi ribbon and reduced autophagy. mTOR was absent from dispersed Golgi mini-stacks whereas recruitment of mTOR to lysosomes was unaffected. We show that the Golgi ribbon is a site for localization and activation of mTOR, a process dependent on the ribbon structure. We demonstrate a strict temporal sequence of fragmentation of Golgi ribbon, loss of Golgi mTOR and subsequent increased autophagy. Golgi ribbon fragmentation has been reported in various neurodegenerative diseases and we demonstrate the potential relevance of our findings in neuronal cells using a model of neurodegeneration. Overall, this study highlights a role for the Golgi ribbon in pathways central to cellular homeostasis.This article has an associated First Person interview with the first author of the paper.

Form and function of the Golgi apparatus: scaffolds, cytoskeleton and signalling

“…HeLa cell clones expressing twice the level of endogenous GCC88 showed an absence of the typical Golgi ribbon, rather Golgi mini-stacks were scattered throughout the cytoplasm, as defined by super-resolution SIM light microscopy and EM tomography [54]. Depletion of GCC88 by siRNA in these clones restored the Golgi ribbon, with the length of the Golgi cisternae even longer than those of the parental cells [54]. Treatment with actin depolymerising drugs resulted in a reversal of the phenotype mediated by the overexpression of GCC88 [42].…”

“…A modest overexpression of GCC88 results in disruption of the Golgi ribbon and dispersal of intact Golgi mini-stacks throughout the cytoplasm, a finding which provided a strategy to explore the function of the Golgi ribbon. HeLa cell clones expressing twice the level of endogenous GCC88 showed an absence of the typical Golgi ribbon, rather Golgi mini-stacks were scattered throughout the cytoplasm, as defined by super-resolution SIM light microscopy and EM tomography [54]. Depletion of GCC88 by siRNA in these clones restored the Golgi ribbon, with the length of the Golgi cisternae even longer than those of the parental cells [54].…”

“…Increased autophagy was not due to either ER stress or nutrient deprivation. Notably, fragmentation of the Golgi ribbon by a variety of other treatments also results in an autophagic response .…”

Golgi Structure and Function in Health, Stress, and Diseases

“…On the other hand, another TGN golgin GCC88 is reported to play a role in TGN organization and ribbon-linking. Overexpression of GCC88 causes a loss of the compact Golgi ribbon and dispersal of mini-stacks throughout the cytoplasm, while knockdown of GCC88 results in a longer ribbon structure (Gosavi et al 2018). A recent report suggests that GCC88-induces Golgi ribbon dispersal via actin and non-muscle myosin IIA.…”

“…In addition to Golgi fragmentation, GCC88 overexpression also induces autophagy via reducing the activity of mTOR. A considerable pool of mTOR is localized and activated on the Golgi, which is dependent on the ribbon structure for recruitment but independent of lysosomal mTOR activation (Gosavi et al 2018).…”