mtDNA detection by TLR9 was first noted in 2010 2010 by Zhang em et?al /em , who observed that during systemic inflammatory response syndrome (SIRS) mtDNA was released into the blood where it can activate TLR9 on neutrophils 168, 169

mtDNA detection by TLR9 was first noted in 2010 2010 by Zhang em et?al /em , who observed that during systemic inflammatory response syndrome (SIRS) mtDNA was released into the blood where it can activate TLR9 on neutrophils 168, 169. 39 (Fig?2). Of pathophysiological relevance, infection with Herpes simplex virus\1 (HSV\1) or vesicular stomatitis virus (VSV) results in mtDNA stress, TFAM depletion and mtDNA entrance into the cytoplasm. The cytoplasmic mtDNA is then sensed by cGAS, triggering cGAS\STING signalling leading to the up\regulation of a plethora of interferon genes, conferring an anti\viral state on the cell. Importantly, triggers cGAS activation and subsequent IRF3\dependent type I interferon response 50, 51, 52. This was assumed to be solely due to detection of mycobacterium RG108 DNA, but other studies have RG108 identified a role for mitochondrial stress and ensuing release of mtDNA into the cytoplasm 53. This observation is strain\dependent but does propose a role for mitochondrial stress and dynamics on the release from mitochondria in cells infected with discovers a link between IL\1 secretion in infected cells, which can then activate a cGAS\STING\dependent type I interferon response in surrounding bystander cells. Interestingly, IL\1 stimulation of bystander cells increases mitochondrial mass, decreases mitochondrial membrane potential and induces mtDNA release 55. However, mtDNA release is observed in the absence of detectable cytochrome release and cell death, suggesting that this is not the mechanism of mtDNA release, although it does not rule out limited mitochondrial permeabilisation seen by us and others in the context of infection (see below). This is not the first time IL\1R signalling has been implicated in cell\intrinsic defence 56, 57, 58, but it is the first to suggest that mtDNA release plays a key role in the initiation of cGAS\STING signalling in the bystander cells. mtDNA activation of cGAS\STING during cell death During programmed cell death, the pro\apoptotic proteins BAX and BAK permeabilise the mitochondrial outer membrane to allow the passage of pro\apoptotic molecules to move from the inner membrane space into the cytosol, where they can initiate a caspase cascade, resulting in a rapid cell death 59. White and Rongvaux showed that in the absence of apoptotic caspase activation, mtDNA activates cGAS in a promiscuous manner, which leads to mildly elevated IFN\ protein levels in blood, though a level sufficient to induce the expression of interferon\stimulated genes 37, 38 (Fig?3). This suggests that apoptotic caspases play a crucial role in dampening type I interferon responses in dying cells, maintaining the immune\silent nature of apoptosis (Fig?3). Further work has shown that apoptotic caspases directly cleave cGAS, IRF3 and mitochondrial anti\viral signalling protein (MAVS), key proteins required for the production of type I interferon 60, supporting the notion that caspases dampen the immune response during cell death. High\resolution imaging studies have further expanded our understanding of how mtDNA is released from the mitochondria during cell death. We and others recently showed that BAX and BAK can permeabilise the mitochondrial outer membrane, but in the context of caspase inhibition these pores grow dramatically, sufficient to allow inner membrane herniation and extrusion of mtDNA 61, 62, 63 (Fig?3). We found that under caspase\inhibited conditions, mitochondrial permeabilisation leads to Gata3 down\regulation of inhibitor of apoptosis proteins (IAPs), NF\B\inducing kinase (NIK) activation and an NF\B transcriptional program, in addition to mtDNA release\induced cGAS\STING activation 64. The cytokines and chemokines up\regulated via NF\B after mitochondrial permeabilisation can serve to promote macrophage activation 64, 65. This leads to robust anti\tumour effects, highlighting a potential therapeutic role for caspase inhibition in cancer treatment 64. Collectively, these results help to reconcile how predominantly cytosolic cGAS can be activated by mtDNA during cell death. Nevertheless, a number of RG108 unresolved questions remain. Firstly, is inner membrane permeabilisation a regulated process, and if so, how? A rapid inner membrane permeabilisation of sufficient size to allow the passage of small ions is observed minutes after outer membrane permeabilisation 61, but is insufficient to allow mtDNA nucleoid extrusion and is probably only transient,.