Microglia are the resident immune cells in the central nervous system and key players against pathogens and injury. However, persistent microglial activation often exacerbates pathological damage and has been implicated in many neurological diseases. Despite their pivotal physiological and pathophysiological roles, how the survival and death of activated microglia is regulated remains poorly understood. We report here that microglia activated through
Toll-like receptors (TLRs) undergo RIP1/RIP3-dependent programmed
necrosis (necroptosis) when exposed to the pan
caspase inhibitor
zVAD-fmk. Although
zVAD-fmk and the
caspase-8 inhibitor
IETD-fmk had no effect on unstimulated primary microglia, they markedly sensitized microglia to TLR1/2,3,4,7/8
ligands or TNF treatment, triggering programmed
necrosis that was completely blocked by R1P1
kinase inhibitor
necrostatin-1. Interestingly, necroptosis induced by TLR
ligands and zVAD was restricted to microglial cells and was not observed in astrocytes, neurons or oligodendrocytes even though they are known to express certain TLRs. Deletion of genes encoding TNF or
TNFR1 failed to prevent
lipopolysaccharide- and
poly(I:C)-induced microglial necroptosis, unveiling a TNF-independent programmed
necrosis pathway in TLR3- and TLR4-activated microglia. Microglia from mice lacking functional TRIF were fully protected against TLR3/4 activation and
zVAD-fmk-induced
necrosis, and genetic deletion of rip3 also prevented microglia necroptosis. Activation of
c-jun N-terminal kinase and generation of specific
reactive oxygen species were downstream signaling events required for microglial cell death execution. Taken together, this study reveals a robust RIP3-dependent necroptosis signaling pathway in TLR-activated microglia upon
caspase blockade and suggests that TLR signaling and programmed cell death pathways are closely linked in microglia, which could contribute to neuropathology and
neuroinflammation when dysregulated.