Spontaneous activity of the mitochondrial apoptotic apparatus controls mitochondrial antiviral signaling protein (MAVS) and cyclic GMP-AMP synthase (cGAS) aggregation and activity

Introduction

MAVS is the adaptor protein of cytosolic helicases. Upon activation, MAVS monomers aggregate to serve as a platform for recruitment of effector proteins, leading to IRF3 and NF-κB activation. cGAS recognizes dsDNA and synthesizes cGAMP, leading to an IRF3 and NF-kB response. cGAS also aggregates when activated. Spontaneous sub-lethal activation of the mitochondrial apoptotic apparatus has been observed in proliferating epithelial cells, generating low-level caspase activity. Caspases play various roles in inflammation. One of the anti-inflammatory functions of caspases is the proteolytic degradation of signaling proteins during apoptosis. MAVS, cGAS and IRF3 are substrates of caspase-3 and their cleavage limits the IFN response. We propose a novel role of sub-lethal activity of the mitochondrial apoptotic apparatus that tunes MAVS/cGAS activity.

Goals

To provide an exact measurement of steady-state sub-lethal activity of cell death caspases and to understand how the mitochondrial apoptotic apparatus controls MAVS/cGAS signaling.

Materials & Methods

We analyzed MAVS aggregation and activity in cells deficient in mitochondrial apoptosis apparatus components. To detect MAVS aggregation, we performed SDD-AGE. To answer the question of MAVS-sensitivity to low-level caspase-3 activity, we exposed mitochondria to recombinant caspase-3. We also assessed IFN type I response and IL-6 secretion.

Results

Spontaneous activity and substrate cleavage by caspase-9 and -3 but not -7 were observed in non-apoptotic HeLa cells. Loss of caspase-activity in the mitochondrial apoptosis pathway increased MAVS aggregation at steady-state and upon experimental stimulation of MAVS. We observed that MAVS complexes have a greater sensitivity to low-level caspase-3 activity than monomers. In the absence of caspase activity, MAVS-signals were enhanced. cGAS activity was also regulated by steady-state caspase-activity.

Summary

Spontaneous activation of the mitochondrial apoptotic apparatus generates low-level caspase-3 activity that specifically cleaves MAVS- and probably cGAS-complexes but not the monomers. We propose that this is a way to tune their basal activity.