Apoptotic caspases silence spontaneous innate immune signals by specifically cleaving activated mitochondrial antiviral signalling protein (MAVS)

Abstract

Caspases-9, -3 and -7 are activated in the mitochondrial apoptosis pathway and lead to the apoptotic phenotype. Caspases also function to limit inflammation upon apoptotic mitochondrial permeabilization through degradation of the signalling proteins cGAS, MAVS and IRF3. Cells and mice lacking caspases have higher interferon levels and are resistant to viral infection. We report that in unstimulated, non-apoptotic cells caspase-3 functions to cleave specifically activated MAVS and very likely cGAS. In unstimulated HeLa cells, constitutive caspase-9- and -3-but not 7-dependent proteolytic events were observed. Inhibition of the mitochondrial apoptosis pathway in various healthy cells induced type I interferon (IFN I) through increased cGAS activity in the absence of changes to cGAS levels. We observed enhanced MAVS-dependent signals upon RIG-I-like helicase stimulation in the absence of BAX, caspase-9 or caspase-3 or upon caspase-inhibition. During activation, MAVS forms complexes, and blockade of mitochondrial apoptosis signalling increased complex abundance in unstimulated and stimulated cells. MAVS complexes were more sensitive to caspase-degradation than the monomer, and mutation of caspase-3-cleavage sites in MAVS spontaneously increased complex formation. Inhibition of voltage-dependent anion channel 1 (VDAC1) oligomerization blocked BAX/BAK- and caspase-regulated IFN induction, suggesting a stimulating role of leakage of mitochondrial DNA. We propose that low level, spontaneous activity of the mitochondrial apoptosis pathway, through specific caspase-3-mediated cleavage of only active signaling proteins, counteracts mitochondrial release of nucleic acids to reduce inflammation in the absence of infection. Caspase-3 therefore has a novel function in conformation- and activation-specific cleavage of substrates.