Investigators at the Tokyo Metropolitan Institute of Medical Science (TMIMS) have identified the oxidative stress sensor DJ-1 as a previously unknown inflammatory molecule which is released from damaged neurons to activate macrophages in post-stroke neuroinflammation.
The team reported its results in the May 20, 2021, online issue of PLoS Biology.
Neuroinflammation after acute stroke occurs in the absence of invading pathogens. Damage-associated molecular patterns (DAMPs) are considered as sources for the sterile inflammation in the post-ischemic brain. However, the mechanisms underlying the tissue-damage-induced activation of DAMPs have not been sufficiently elucidated.
The DJ-1 gene was originally discovered as an oncogene that shows transforming activity in conjunction with the Ras gene. It was later shown to function as an antioxidant through a variety of mechanisms, including scavenging reactive oxygen species, regulating apoptosis under oxidative stress conditions, or modulating transcription factors like Nrf2. Speaking to BioWorld Science, senior author Takashi Shichita said, "neurons post-ischemic stroke can trigger neuroinflammation via extracellularly released DJ-1 protein, whereas DJ-1 within brain cells has neuroprotective antioxidant effects against Parkinson's disease pathologies."
Shichita has been the project leader of the Stroke Renaissance Project at TMIMS since 2017 where his group studies molecular mechanisms for neural repair in brains damaged by stroke and dementia.
The authors demonstrated that DJ-1 promoted upregulation of inflammatory cytokines in the macrophages by interaction with Toll-like receptors (TLR) on the surface of macrophages. To further confirm DJ-1's proinflammatory potential, Shichita's group showed that its DAMP activity depended on the presence of two key domains of the protein's three-dimensional structure and altering these abolished the proinflammatory effect.
Finally, they showed that DJ-1 was released by dying cells during stroke in a mouse model, and that knocking out DJ-1, or blocking it with an antibody, reduced the damage caused by stroke. Administration of an anti-DJ-1 antibody decreased the expression of the inflammatory cytokine IL-23 in infiltrating immune cells on day 1 after stroke onset, leading to the suppression of inflammation neuroprotection against ischemic brain injury by day 3.
DJ-1 is an atypical member of the peroxiredoxin family proteins that catalyze reactive oxygen species. Peroxiredoxin family proteins ubiquitously exist in not only mammalian cells but also bacteria. Shichita speculated that the peroxiredoxin family proteins may have acquired inflammatogenic activity in the process of evolution. Peroxiredoxin family proteins may trigger sterile inflammation by being mistaken as molecules from pathogens.
Interestingly, the study revealed that the extracellularly released DJ-1 protein from necrotic brain cells triggered cerebral post-ischemic inflammation through the activation of TLR2 and TLR4 in infiltrating immune cells, along with a concomitant increased intracellular expression of DJ-1 within neurons in the ischemic brain.
Those findings suggested that intracellularly accumulated DJ-1 is released into the extracellular space where it then functions as a DAMP.
However, intracellular DJ-1 has been shown to prevent neurodegeneration by reducing oxidative stress within brain cells. Thus, DJ-1 is seen to have opposing functions in the intracellular and extracellular contexts.
A target for stroke, dementia
Inflammation has been generally considered to be detrimental to the functional prognosis of stroke patients. The authors believe that DJ-1 could be a therapeutic target to prevent excessive inflammation and neuronal injury after ischemic stroke.
Since the extracellular release of DJ-1 was usually observed from 12 to 24 hours after the stroke onset, therapeutic strategies targeting extracellularly released DJ-1 could extend the therapeutic time window.
Current therapeutic methods for ischemic stroke, such as rt-PA administration or thrombectomy, need to be started a few hours after stroke onset. Indeed, timely therapeutic intervention is the single most significant predictor of recovery in stroke patients.
"Compounds which inhibit inflammatogenic roles of extracellular DJ-1 but do not disturb neuroprotective antioxidant activity of intracellular DJ-1 may have therapeutic effects for stroke patients," Shishita said.
"What sets DJ-1 apart is that compared to other DAMPs whose expressions were not induced by stresses, DJ-1 is important for triggering neuroinflammation after long-lasting neuronal stresses," he added. "Therefore, extracellularly released DJ-1 may trigger neuroinflammation of chronic neurodegeneration and may be a putative target for therapeutic intervention to prevent the progression of neurodegenerative diseases like Alzheimer's disease."