Annexin A5 stimulates autophagy and inhibits endocytosis

Abstract

SummaryMacroautophagy is a major lysosomal catabolic process activated particularly under starvation in eukaryotic cells. A new organelle, the autophagosome, engulfs cytoplasmic substrates, which are degraded after fusion with endosomes and/or lysosomes. During a shotgun proteome analysis of purified lysosomal membranes from mouse fibroblasts, a Ca 2+ -dependent phospholipid-binding protein, annexin A5, was found to increase on lysosomal membranes under starvation. This suggests a role for this protein, an abundant annexin with a still unknown intracellular function, in starvation-induced lysosomal degradation. Transient overexpression and silencing experiments showed that annexin A5 increased lysosomal protein degradation, and colocalisation experiments, based on GFP sensitivity to lysosomal acidic pH, indicated that this was mainly the result of inducing autophagosome-lysosome fusion. Annexin A5 also inhibited the endocytosis of a fluid-phase marker and cholera toxin, but not receptor-mediated endocytosis. Therefore, we propose a double and opposite role of annexin A5 in regulating the endocytic and autophagic pathways and the fusion of autophagosomes with lysosomes and endosomes.

Annexins – insights from knockout mice

“…Inside most cells, upon Ca 2+ elevation, AnxA5 translocates to the plasma membrane and nucleus. Other locations, including the Golgi, endoplasmic reticulum, late endosomes/ lysosomes, phagosomes, and mitochondria, have also been reported (Dubois et al, 1998;Gerke et al, 2005;Ghislat et al, 2012). At these cellular sites, AnxA5 has been associated with membrane trafficking and organization, Ca 2+ signaling, regulation of ion channels and Ca 2+ -influx as well as cell cycle regulation and apoptosis Wang et al, 2003;Gerke et al, 2005;Monastyrskaya et al, 2007;Faria et al, 2011).…”

Regulation of Lysosomal Degradation by CA2+And CA2+-Binding Proteins

“…In fact, in the same proteomic analysis, we found that, like annexin A5, its levels on lysosomal membranes increased upon starvation 15 . Also, a different group showed that annexin A1 promotes autophagic degradation and suggested a role of annexin A1 in increasing the formation of amphisomes 18 .…”

“…For detection of endocytosis of cholera toxin B or epidermal growth factor (EGF), cells were treated with Alexa-Fluor-488-cholera-toxin-subunit-B (1.5 mg/ml) or with Alexa-Fluor-488-EGF (2 mg/ml) for 5,15,30,60 or 90 min at 37°C.…”

“…Using a proteomic approach, we recently identified annexin A5 as a regulator of autophagosome maturation, especially in the starvation response where it localizes to lysosomal membranes in a Ca ++ -dependent way 15 . Under starvation annexin A5 translocates from the Golgi complex to lysosomes and, to a lesser extent, to late endosomes.…”

“…Copine 1, another protein whose levels also increased on lysosomal membranes under high proteolysis conditions 15 , shares with the annexin family of proteins the property of binding to phospholipid membranes in a Ca ++ -dependent manner 30 . In addition, its presence in autophagosomal, phagosomal and lysosomal delimiting membranes has been previously reported 31,32 .…”