Loss of the starvation-induced gene Rack1 leads to glycogen deficiency and impaired autophagic responses in Drosophila

Abstract

Autophagy delivers cytoplasmic material for lysosomal degradation in eukaryotic cells. Starvation induces high levels of autophagy to promote survival in the lack of nutrients. We compared genome-wide transcriptional profiles of fed and starved control, autophagy-deficient Atg7 and Atg1 null mutant Drosophila larvae to search for novel regulators of autophagy. Genes involved in catabolic processes including autophagy were transcriptionally upregulated in all cases. We also detected repression of genes involved in DNA replication in autophagy mutants compared with control animals. The expression of Rack1 (receptor of activated protein kinase C 1) increased 4.1- to 5.5-fold during nutrient deprivation in all three genotypes. The scaffold protein Rack1 plays a role in a wide range of processes including translation, cell adhesion and migration, cell survival and cancer. Loss of Rack1 led to attenuated autophagic response to starvation, and glycogen stores were decreased 11.8-fold in Rack1 mutant cells. Endogenous Rack1 partially colocalized with GFP-Atg8a and early autophagic structures on the ultrastructural level, suggesting its involvement in autophagosome formation. Endogenous Rack1 also showed a high degree of colocalization with glycogen particles in the larval fat body, and with Shaggy, the Drosophila homolog of glycogen synthase kinase 3B (GSK-3B). Our results, for the first time, demonstrated the fundamental role of Rack1 in autophagy and glycogen synthesis.

Application and interpretation of current autophagy inhibitors and activators

“…Drosophila Rack1, a receptor of activated protein kinase C 1, increases 4.1-to 5.5-fold during nutrient deprivation in all three genotypes. Recently, it was reported that loss of Rack1 led to an attenuated autophagic response to starvation [22] . This starvationinduced protein potentially acts as a scaffold protein during autophagosome formation.…”

Receptor for Activated C Kinase 1 (RACK1) Promotes Dishevelled Protein Degradation via Autophagy and Antagonizes Wnt Signaling

“…RACK1 Interacts with LC3-It was reported that loss of RACK1 resulted in attenuated autophagy and glycogen store in Drosophila, and RACK1 was partially colocalized with the Drosophila LC3 homolog Atg8 (27). We observed that RACK1 interacted with LC3 with endogenous or exogenously FIGURE 3.…”

RACK1 Is an Interaction Partner of ATG5 and a Novel Regulator of Autophagy

“…In fact, involvement of RACK1 was suggested in Drosophila studies where loss of RACK1 was associated with attenuated autophagy responses and glycogen storage defects (14). While our manuscript was in preparation, Zhao et al (15) showed that RACK1 was required for hepatocyte autophagy and lipophagy.…”

“…Recent studies showed that WD domain-containing proteins, such as ATG16L1, ALFY, and WIPI1, play significant roles in different stages of the autophagy pathway (12)(13)(14)(15)(16). In this study, we describe RACK1 (receptor-activated C-kinase 1, GNB2L1), a WD40 repeat protein with a seven-bladed ␤-propeller structure, as an interaction partner of ATG5 and a novel autophagy protein.…”