Malate initiates a proton-sensing pathway essential for pH regulation of inflammation, 2024, Chen et al.

Malate initiates a proton-sensing pathway essential for pH regulation of inflammation

Yu-jia-nan Chen, Rong-chen Shi, Yuan-cai Xiang, Li Fan, Hong Tang, Gang He, Mei Zhou, Xin-zhe Feng, Jin-dong Tan, Pan Huang, Xiao Ye, Kun Zhao, Wen-yu Fu, Liu-li Li, Xu-ting Bian, Huan Chen, Feng Wang, Teng Wang, Chen-ke Zhang, Bing-hua Zhou, Wan Chen, Tao-tao Liang, Jing-tong Lv, Xia Kang, …Kang-lai Tang

Abstract

Metabolites can double as a signaling modality that initiates physiological adaptations. Metabolism, a chemical language encoding biological information, has been recognized as a powerful principle directing inflammatory responses. Cytosolic pH is a regulator of inflammatory response in macrophages.

Here, we found that L-malate exerts anti-inflammatory effect via BiP-IRF2BP2 signaling, which is a sensor of cytosolic pH in macrophages. First, L-malate, a TCA intermediate upregulated in pro-inflammatory macrophages, was identified as a potent anti-inflammatory metabolite through initial screening. Subsequent screening with DARTS and MS led to the isolation of L-malate-BiP binding.

Further screening through protein‒protein interaction microarrays identified a L-malate-restrained coupling of BiP with IRF2BP2, a known anti-inflammatory protein. Interestingly, pH reduction, which promotes carboxyl protonation of L-malate, facilitates L-malate and carboxylate analogues such as succinate to bind BiP, and disrupt BiP-IRF2BP2 interaction in a carboxyl-dependent manner.

Both L-malate and acidification inhibit BiP-IRF2BP2 interaction, and protect IRF2BP2 from BiP-driven degradation in macrophages. Furthermore, both in vitro and in vivo, BiP-IRF2BP2 signal is required for effects of both L-malate and pH on inflammatory responses.

These findings reveal a previously unrecognized, proton/carboxylate dual sensing pathway wherein pH and L-malate regulate inflammatory responses, indicating the role of certain carboxylate metabolites as adaptors in the proton biosensing by interactions between macromolecules.

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Explain like I'm brain-foggy:

Malate is a TCA cycle metabolite critical for ATP production through oxidative phosphorylation. Its role in cellular metabolism is well characterized.

However, this paper shows that it also exerts an immunomodulatory effect on macrophages that is independent from its role in cellular metabolism.

This happens through L-malate binding to BiP. IRF2BP2 is a protein that normally interrupts interferon production.

Under homeostatic stress conditions, BiP is a protein that binds to and inhibits IRF2BP2, allowing interferon production to ramp up. However, L-malate can bind to BiP and prevent it from binding to IRF2BP2.

pH changes in the cell can make L-malate (and other TCA cycle metabolites) even more effective at this.

The cumulative effect is that L-malate blocks the thing that normally blocks the thing that blocks interferon production...which is all to say that L-malate --> decreased interferon production (where the --> is a little Rube Goldberg machine of its own)

 

Some interesting tidbits on temporal dynamics:

 

It depends on where exactly the malate-aspartate shuttle is being inhibited (if it is at all). If it's being inhibited upstream at succinate dehydrogenase (a target of itaconate), then you would have increased succinate but less fumarate and malate. If it's at another point (e.g. GOT2), then you'd have a situation where malate crosses into the mitochondria once, but can never get back out (after converting to other forms), so it would need to be constantly replenished by another source. In both cases, you're probably ending up with less or "normal" (if adequately replenished by some backup mechanism) cytosolic malate levels.

The findings of paulendat's paper seemed to suggest that, if itaconate is involved, the primary way it leads to increased interferon production is via PRDX5. This might just be another potential way in which itaconate would result in increased interferon production, though it would be dependent on ER stress.

Interestingly, the text noted that succinate and fumarate were also capable of binding to BiP, but I don't think it did any quantification on whether there might be differences between the metabolites.

My intuition tells me that L-malate binding would only affect BiP detection if the antibody for BiP directly recognized the binding site with L-malate or any of those other metabolites. In practice I've never come across a situation where that happened (though I'm hardly a western blot expert). Without knowing the exact site of antibody binding on BiP, I can't be sure though.

 

Would ingested malic acid (apples, sour candies) affect immune activity via this mechanism? I'm intolerant of malic acid, I assume due to my ME.

 

That’s fascinating—have you happened to see my posts about a malic acid supplement? It was a bit of a miraculous supplement for my ME.

However, only certain sources of malic acid gave me a benefit (pure malic acid or sumac, not apples/apple juice/other fruits). I assumed that something else usually present in fruits (my guess is certain sugars, but I’m not sure. Malonic acid, which has different properties, is another option) counteracted the benefit for me.

What do you mean specifically by intolerance, and which sources of malic acid have you noticed this effect from?