Caspase involvement in autophagy

Caspases are a family of cysteine proteases widely known as the principal mediators of the apoptotic cell death response, but considerably less so as the contributors to the regulation of pathways outside cellular demise. In regards to autophagy, the modulatory roles of caspases have only recently begun to be adequately described. In contrast to apoptosis, autophagy promotes cell survival by providing energy and nutrients through the lysosomal degradation of cytoplasmic constituents. Under basal conditions autophagy and apoptosis cross-regulate each other through an elaborate network of interconnections which also includes the interplay between autophagyrelated proteins (ATGs) and caspases. In this review we focus on the effects of this crosstalk at the cellular level, as we aim to concentrate the main observations from research conducted so far on the fine-tuning of autophagy by caspases. Several members of this protease-family have been found to directly interact with key ATGs involved in different tiers across the autophagic cascade. Therefore, we firstly outline the core mechanism of macroautophagy in brief. In an effort to emphasize the importance of the intricate cross-regulation of ATGs and caspases, we also present examples drawn from Drosophila and plant models regarding the contribution of autophagy to apoptotic cell death during normal development

Caspase cleavage of the Golgi stacking factor GRASP65 is required for Fas/CD95-mediated apoptosis

GRASP65 (Golgi reassembly and stacking protein of 65 KDa) is a cis-Golgi protein with roles in Golgi structure, membrane trafficking and cell signalling. It is cleaved by caspase-3 early in apoptosis, promoting Golgi fragmentation. We now show that cleavage is needed for Fas-mediated apoptosis: expression of caspase-resistant GRASP65 protects cells, whereas expression of membrane proximal caspase-cleaved GRASP65 fragments dramatically sensitises cells. GRASP65 coordinates passage through the Golgi apparatus of proteins containing C-terminal hydrophobic motifs, via its tandem PDZ type ‘GRASP' domains. Fas/CD95 contains a C-terminal leucine–valine pairing so its trafficking might be coordinated by GRASP65. Mutagenesis of the Fas/CD95 LV motif reduces the number of cells with Golgi-associated Fas/CD95, and generates a receptor that is more effective at inducing apoptosis; however, siRNA-mediated silencing or expression of mutant GRASP65 constructs do not alter the steady state distribution of Fas/CD95. We also find no evidence for a GRASP65–Fas/CD95 interaction at the molecular level. Instead, we find that the C-terminal fragments of GRASP65 produced following caspase cleavage are targeted to mitochondria, and ectopic expression of these sensitises HeLa cells to Fas ligand. Our data suggest that GRASP65 cleavage promotes Fas/CD95-mediated apoptosis via release of C-terminal fragments that act at the mitochondria, and we identify Bcl-XL as a candidate apoptotic binding partner for GRASP65

Atg4D at the interface between autophagy and apoptosis

The Atg4 family of endopeptidases regulates autophagosome biogenesis by priming newly synthesised Atg8 to enable covalent attachment of phosphatidylethanolamine, and by delipidating Atg8 at the lysosomal fusion step. Control of Atg4 activity is therefore crucial, although little is known about how these molecules are regulated in living cells. We have found that one human Atg4 family member (Atg4D) is cleaved at DEVD63K by caspase-3 during apoptosis. Importantly, our studies suggest that native Atg4D is enzymatically inactive, but gains GABARAP-L1 priming/delipidation activity following caspase cleavage. Caspase-cleaved Atg4D is also highly cytotoxic; however, toxicity is not due to enhanced autophagy, but is mediated by a putative C-terminal BH3 domain, and is associated with transient recruitment of Atg4D to mitochondria.The Atg4 family of endopeptidases regulates autophagosome biogenesis by priming newly synthesised Atg8 to enable covalent attachment of phosphatidylethanolamine, and by delipidating Atg8 at the lysosomal fusion step. Control of Atg4 activity is therefore crucial, although little is known about how these molecules are regulated in living cells. We have found that one human Atg4 family member (Atg4D) is cleaved at DEVD63K by caspase-3 during apoptosis. Importantly, our studies suggest that native Atg4D is enzymatically inactive, but gains GABARAP-L1 priming/delipidation activity following caspase cleavage. Caspase-cleaved Atg4D is also highly cytotoxic; however, toxicity is not due to enhanced autophagy, but is mediated by a putative C-terminal BH3 domain, and is associated with transient recruitment of Atg4D to mitochondria

A cryptic mitochondrial targeting motif in Atg4D links caspase cleavage with mitochondrial import and oxidative stress

The Atg4 cysteine proteases play crucial roles in the processing of Atg8 proteins during autophagy, but their regulation during cellular stress and differentiation remains poorly understood. We have found that two Atg4 family members—Atg4C and Atg4D—contain cryptic mitochondrial targeting sequences immediately downstream of their canonical (DEVD) caspase cleavage sites. Consequently, caspase-cleaved Atg4D (ΔN63 Atg4D) localizes to the mitochondrial matrix when expressed in mammalian cells, where it undergoes further processing to a ~42 kDa mitochondrial form. Interestingly, caspase cleavage is not needed for Atg4D mitochondrial import, because ~42 kDa mitochondrial Atg4D is observed in cells treated with caspase inhibitors and in cells expressing caspase-resistant Atg4D (DEVA(63)). Using HeLa cell lines stably expressing ΔN63 Atg4D, we showed that mitochondrial Atg4D sensitizes cells to cell death in the presence of the mitochondrial uncoupler, CCCP, and that mitochondrial cristae are less extensive in these cells. We further showed that the organization of mitochondrial cristae is altered during the mitochondrial clearance phase in differentiating primary human erythroblasts stably expressing ΔN63 Atg4D, and that these cells have elevated levels of mitochondrial reactive oxygen species (ROS) during late stages of erythropoiesis. Together these data suggest that the import of Atg4D during cellular stress and differentiation may play important roles in the regulation of mitochondrial physiology, ROS, mitophagy and cell viability