Researchers have uncovered significant insights about the roles of protein disulfide isomerase (PDI) and the P2X7 receptor (P2X7R) during inflammatory responses influenced by lipopolysaccharide (LPS) treatment. This research reveals how PDI integrates signaling pathways from TLR4 and P2X7R and contributes to neuroinflammation through the activation of specific molecular pathways.
P2X7R, known for its nonselective cationic channels, has been implicated as a modulator of inflammatory pathways, particularly in the brain. It especially regulates pro-inflammatory signaling molecules, combining its functions with TLR4, which is activated by LPS—an important factor indicating bacterial infection.
The study conducted by Kim et al. aimed to clarify the relationship between these receptors and PDI, including how they work together to engage the NF-κB pathway, which is central to inflammation. By using wild-type (P2X7R+/+) and knockout (P2X7R−/−) mice, the researchers were able to observe differing responses to LPS treatment.
Initial findings indicated PDI levels rose dramatically following LPS treatment, especially in the P2X7R+/+ strain of mice. This upregulation was found to correlate with microglial activation, and the reduction of PDI expression was noted with P2X7R deletion, pointing to the receptor's role as enhancing neuroinflammation.
Further investigation showed LPS significantly raised PDI levels and activated NF-κB p65, the latter being dependent on the presence of P2X7R. Interestingly, the inhibition of NF-κB via the SN50 inhibitor effectively decreased PDI expression across both mouse strains, emphasizing NF-κB's role as an upstream modulator of PDI.
The role of iNOS was also pivotal, as LPS treatment heightened iNOS expression, indicative of increased nitrosative stress, yet this expression was attenuated when P2X7R was removed. Specifically, the PDI knockdown displayed decreased levels of iNOS and phosphorylated NF-κB, reinforcing the notion of PDI's influential position within this inflammatory signaling.
Investigators also utilized S-nitroso-N-acetyl-DL-penicillamine (SNAP), exploring its effects as a nitric oxide (NO) donor, which resulted in increased levels of S-nitrosylated PDI (SNO-PDI) and P2X7R expression under physiological conditions. This suggests the enhancement of interactions between these pathways when NO is introduced, supporting the hypothesis of complex feedback mechanisms.
While SNO-PDI levels were significantly lower in P2X7R−/− mice at baseline, LPS treatmements elevated these levels again showing dependency upon P2X7R, particularly as SNAP failed to increase SNO-PDI ratios within this strain. The findings strongly suggest PDI acts as a mediator, integrating TLR4 and P2X7R signaling.
Investigators also clarified roles for S-nitrosylation of the p65 subunit of NF-κB during such interactions, noting no alteration to S-nitrosylation levels between strains, indicating PDI's independent regulatory capability over NF-kB under certain physiological circumstances.
Collectively, this study positions PDI as more than just a molecular chaperone, illustrating its function as a key integrator within neuroinflammatory processes. By linking P2X7R to TLR4 and subsequent NF-κB activation, the study suggests novel therapeutic strategies to devastating neurological conditions perpetuated by inflammation.