Connecting the Dots - Part 3: When Oxygen Can't Be Used — Linking WASF3 Complex IV Inhibition to the Post-Exertional P5C Surge

In Part 1 and Part 2 of this series, we looked at how the new PNAS study from Paul Hwang’s team (Syed et al., 2026) expands our understanding of WASF3 from skeletal muscle into the brain, and how the endoplasmic reticulum (ER) and mitochondria-associated membranes (MAMs) act as a central hub for cellular stress in ME/CFS.

The core takeaway from both the muscle and brain studies is striking: when cells experience ER stress, WASF3 is upregulated, enters the mitochondria, and disrupts respiratory complex IV (cytochrome c oxidase). Complex IV is the final step of the mitochondrial electron transport chain—the exact molecular machinery where oxygen is actually consumed to generate energy. When WASF3 interferes with complex IV, oxygen consumption drops, oxidative phosphorylation stalls, and the cell is forced toward glycolysis and a "hypometabolic" state.

This brings me back to a blog post I wrote back in May 2022 "Efter motion har ME patienter forhøjet niveau af pyrroline-5-carboxylate"  ("After exercise, ME patients have elevated levels of pyrroline-5-carboxylate") about an intriguing metabolic finding in ME/CFS patients after two consecutive days of exercise. Looking at that older data through the lens of WASF3 suddenly makes the biochemical puzzle pieces snap into place.

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Revisiting the 2022 Data: The Post-Exertional Surge in P5C and Proline

In 2022, Maureen Hanson’s group (Germain et al., 2022) published a plasma metabolomics study examining female ME/CFS patients and healthy controls before and after maximal cardiopulmonary exercise testing on two consecutive days:

• D1PRE (Day 1, before exercise)
• D1POST (Day 1, after exercise)
• D2PRE (Day 2, before exercise)
• D2POST (Day 2, after exercise)

One of the standout metabolites was pyrroline-5-carboxylate (P5C), an intermediate in the interconversion of the amino acids proline, glutamate, and ornithine. Notably, elevated P5C had also been reported earlier in ME/CFS by Naviaux et al. (2016). In Germain et al.’s two-day exercise study, P5C didn’t just stay elevated—it escalated dramatically with exertion (shown as Fold Change, FC, compared to controls):

• D1PRE: FC = 1.18 (p = 0.10)
• D1POST: FC = 1.36 (p = 0.009)
• D2PRE: FC = 1.20 (p = 0.13)
• D2POST: FC = 1.72 (p = 0.0000122)

By the end of the second day of exercise (D2POST), proline itself also became significantly elevated in ME/CFS patients compared to controls:

• D1PRE: FC = 1.03 (p = 0.76)
• D1POST: FC = 1.04 (p = 0.88)
• D2PRE: FC = 1.15 (p = 0.13)
• D2POST: FC = 1.18 (p = 0.03)

At the time, the big question was: Why do P5C and proline spike specifically when ME/CFS patients are pushed physically, and what does this have to do with oxygen?

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1. WASF3 Creates Intracellular "Pseudohypoxia"

To understand why P5C rises, we have to look at what happens when oxygen cannot be used. Under true hypoxia (oxygen deprivation), P5C and proline act as a specialized redox couple that helps the cell manage its NADP+/NADPH and NAD+/NADH balance (Chalecka et al., 2021).

In ME/CFS, oxygen levels in the blood are typically normal. However, if WASF3 is disrupting complex IV inside the mitochondria, the cell cannot utilize the oxygen delivered to it. Biochemically, this creates functional hypoxia (or pseudohypoxia). At rest (D1PRE), the cell may barely cope, which could explain why P5C is only slightly—and not yet statistically significantly—elevated before exercise.

The moment physical exertion begins, energy demand skyrockets. Because complex IV is throttled by WASF3, electrons back up in the electron transport chain, causing reduced cofactors NADH and NADPH to accumulate.

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2. The P5C/Proline Cycle as a Metabolic Emergency Valve

When the electron transport chain is jammed, the cell desperately needs ways to offload excess electrons and regenerate NADP+ and NAD+ so that backup energy pathways—like glycolysis and the pentose phosphate pathway—don't grind to a halt.

This is where the P5C–proline cycle steps in as a biochemical emergency valve:

• Offloading electrons: Converting P5C into proline (via the enzyme PYCR) consumes NADPH and NADH and regenerates NADP+ and NAD+. Under hypoxic or pseudohypoxic stress, pushing glutamate toward P5C and proline helps keep cellular redox balance afloat.
• A traffic jam in the mitochondria: Under healthy conditions, proline can be oxidized back into P5C by proline dehydrogenase (PRODH), feeding electrons directly into the mitochondrial electron transport chain. But if WASF3 has already created a roadblock at complex IV, those electrons have nowhere to go. The cycle becomes backed up, causing both the intermediate (P5C) and the end product (proline) to accumulate and spill out into the bloodstream—peaking after the second day of exertion (D2POST).

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3. P5CS: A Mitochondrial Stress Sensor Working Overtime?

There is another fascinating layer to this story. P5C is synthesized from glutamate by the mitochondrial enzyme pyrroline-5-carboxylate synthase (P5CS).

Research has shown that P5CS is far more than a simple metabolic assembly-line worker—it acts as a cellular stress sensor (Pan et al., 2021). When cells experience starvation or oxidative stress, P5CS physically reorganizes inside the mitochondria and plays an active role in maintaining the structural organization of the mitochondrial respiratory complexes.

Think about what this means in the context of WASF3:

1. ER stress induces WASF3, which destabilizes respiratory complex IV and generates oxidative stress.
2. P5CS senses this exact combination of cellular energy crisis and oxidative stress, likely ramping up its activity in an attempt to stabilize mitochondrial respiration and supply P5C for redox buffering.
3. Yet because WASF3 continues to block complex IV, this compensatory response cannot fully restore oxidative ATP production—leaving behind a massive, measurable surge of P5C and proline in the blood after exertion.

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Connecting the Dots

When we place the 2022 metabolomics data alongside the 2023 and 2026 WASF3 studies, the post-exertional rise in P5C and proline no longer looks like a random metabolic quirk. Instead, it looks like a downstream biochemical footprint of an obstructed electron transport chain:

• WASF3 blocks complex IV, preventing mitochondria from using oxygen properly during exertion.
• Pseudohypoxia and redox stress build up rapidly, especially across two consecutive days of exercise.
• P5CS and the P5C/proline redox couple kick into overdrive to buffer NADP+/NADPH ratios and compensate for impaired mitochondrial respiration, resulting in the sharp D1POST and D2POST spikes observed in ME/CFS patients.

Figure 1. Proposed mechanistic model linking WASF3-induced complex IV inhibition to post-exertional P5C and proline accumulation in ME/CFS.

It is remarkable how older metabolomics findings continue to gain new mechanistic meaning as upstream regulators like ER stress, MAMs, and WASF3 come into focus.

A Note on Data vs. Hypothesis
The post-exertional elevation of P5C and proline in ME/CFS patients (Germain et al., 2022) and WASF3-mediated inhibition of complex IV (Wang et al., 2023; Syed et al., 2026) are both established findings from peer-reviewed studies. However, the mechanistic link proposed here—that WASF3-driven pseudohypoxia directly triggers the P5C/proline redox shunt during exertion—is a theoretical synthesis awaiting direct experimental validation.

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References

1. Syed et al. Stress-inducible WASF3 regulates brain metabolism and cognitive function. PNAS. 2026. https://pubmed.ncbi.nlm.nih.gov/42945241/
2. Wang PY, et al. WASF3 disrupts mitochondrial respiration and may mediate exercise intolerance in myalgic encephalomyelitis/chronic fatigue syndrome. PNAS. 2023;120(34):e2302738120.
3. Germain A, et al. Plasma metabolomics reveals disrupted response and recovery following maximal exercise in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. JCI Insight. 2022;7(9):e157621. jci.org
4. Naviaux RK, et al. Metabolic features of chronic fatigue syndrome. PNAS. 2016;113(37):E5472-E5480.
5. Chalecka M, et al. P5C as an Interface of Proline Interconvertible Amino Acids and Its Role in Regulation of Cell Survival and Apoptosis. Int J Mol Sci. 2021;22(21):11763. https://www.mdpi.com/1422-0067/22/21/11763
6. Pan X, et al. Pyrroline-5-carboxylate synthase senses cellular stress and modulates metabolism by regulating mitochondrial respiration. Cell Death Differ. 2021;28(1):391-405. https://www.nature.com/articles/s41418-020-0601-5

Connecting the Dots - Part 2: Revisiting the PACS2 Master Switch and the MAM Interface Across Tissues in ME/CFS

In my previous post, we looked at how the new WASF3 brain study (Syed et al., 2026) creates a fascinating bridge to historical ME/CFS transcriptomic data, pointing toward a cellular energy crisis driven by the IER3 (IEX-1) mitochondrial brake. In Syed et al.’s proposed pathway, cellular stress triggers—such as viral infection, DNA damage, or endoplasmic reticulum (ER) stress—activate two parallel, interconnected branches:

• Branch A (Direct Bioenergetic Suppression): Stress upregulates WASF3, which directly impairs mitochondrial respiration and fatty acid oxidation.
• Branch B (The p53/IER3 Signaling Cascade): Stress—amplified by WASF3—activates p38 and p53, driving up IER3 (IEX-1), reducing ATPIF1, and ultimately impairing cellular and cognitive function.

To understand where and why this two-pronged breakdown occurs, we have to look closer at the physical contact sites where the ER and mitochondria communicate.

This brings me back to a blog post I wrote in October 2018 ("PACS2 and ME"). Back then, we explored how the ER and mitochondria form a dynamic tubular network connected by mitochondria-associated ER membranes (MAMs). These specialized contact sites govern lipid exchange, calcium Ca 2+ signaling, bioenergetics, and apoptosis. At the time, MAM dysfunction was already recognized in neurodegenerative conditions like Alzheimer’s, ALS, and Parkinson’s disease. Viewed alongside the new WASF3 findings, it becomes compelling to hypothesize that ME/CFS shares a similar structural vulnerability at the MAM interface.

The Dynamic Switch in the Middle of the Bridge

As reviewed in my 2018 post via Liu and Zhu (2017), specific protein tethers physically anchor the ER to mitochondria. One of the master regulators of this bridge is phosphofurin acidic cluster sorting protein 2 (PACS2), which modulates the crucial Fis1–Bap31 tether.

Beyond simple structural tethering, PACS2 acts as a multifunctional metabolic switch:

• It coordinates membrane trafficking, MAM-localized Ca2+ flux, and lipid metabolism.
• Crucially, it directly regulates p53-dependent stress signaling and cell-cycle/apoptotic decisions in response to metabolic cues.

When we overlay my 2018 PACS2 post onto Syed et al.’s two-branch model, PACS2 sits right at the crossroads between WASF3 (Branch A) and p53/IER3 (Branch B):

• Stabilizing or Destabilizing the Bridge: WASF3 is induced by ER stress and acts directly at the mitochondrial interface to suppress respiration and fatty acid oxidation. Because PACS2 maintains the physical integrity and Ca2+/lipid homeostasis of the ER–mitochondria bridge, any dysregulation of PACS2 leaves the mitochondria structurally and metabolically exposed.
• Gating the p53–IER3 Cascade: In Syed et al.’s pathway, WASF3 feeds into p38/p53 activation, pushing IER3 (IEX-1) upward and ATPIF1 downward. Since PACS2 is a known regulator of p53 stress responses, an impaired PACS2 switch removes a critical checkpoint on this exact cascade.

A Cross-Tissue Hypothesis: From Immune Cells to Muscle and Brain

A key nuance when connecting these studies is paying attention to which tissues were examined:

• WASF3 elevation and mitochondrial disruption have now been demonstrated in skeletal muscle (Wang et al., 2023) and brain tissue (Syed et al., 2026).
• PACS2 dysregulation comes from immune cell data: in 2018, I highlighted a finding by de Vega et al. (2018) showing that the PACS2 gene (specifically in the TSS1500 promoter region) is differentially methylated in peripheral immune cells (PBMCs) from ME/CFS patient subtypes.

Because DNA methylation acts as the cell’s long-term volume dial, altered methylation of PACS2 suggests an epigenetic scar that impairs how cells regulate this master MAM switch. While we do not yet know whether PACS2 methylation is similarly altered in muscle or brain—or whether PACS2 dysfunction is primarily a systemic immune feature—combining these tissue findings suggests two plausible, potentially reinforcing scenarios across the body:

1. Viral Hijacking and Epigenetic Priming: Viruses routinely target PACS2 and the ER–mitochondria interface to manipulate host bioenergetics and evade immunity. An acute viral trigger—the classic onset event in ME/CFS—could leave a lasting epigenetic mark on PACS2, lowering the threshold for ER stress and WASF3 accumulation during subsequent physical or cognitive exertion.
2. Chronic WASF3 Stress Driving Epigenetic Drift: Conversely, persistent ER stress and WASF3-driven mitochondrial dysfunction alter cellular redox state and key metabolites (such as S-adenosylmethionine and alpha -ketoglutarate) required by DNA-modifying enzymes. Over time, chronic bioenergetic stress across tissues could itself promote compensatory epigenetic silencing of PACS2 in an attempt to dampen MAM-mediated apoptosis.

Connecting the Dots

(Note: While WASF3-driven pathway disruption in muscle/brain and differential PACS2 methylation in ME/CFS immune cells are published empirical findings, linking them into a unified cross-tissue MAM model remains a working hypothesis without experimental validation.)

Taken together, the 2018 PACS2 data and the 2023–2026 WASF3 studies point toward a shared anatomical weak spot inside the cell: the ER–mitochondria interface. Whether initiated by viral hijacking of PACS2 or sustained ER stress driving WASF3, a compromised MAM bridge provides a coherent framework for how a single cellular stress pathway can manifest as bioenergetic failure across immune cells, skeletal muscle, and the brain.

Figure 1. Proposed Cross-Tissue Model of the WASF3–PACS2 Axis at the MAM Interface. Building on the two-branch cellular stress pathway described by Syed et al. (2026)—where stress triggers upregulate WASF3 (suppressing mitochondrial respiration and fatty acid oxidation) alongside p38/p53 and IER3/IEX-1 signaling—this hypothesis positions PACS2 as a central gatekeeper at mitochondria-associated ER membranes (MAMs). Epigenetic alterations in PACS2 (reported in ME/CFS immune cells by de Vega et al., 2018) and WASF3 accumulation (observed in skeletal muscle and brain) may mutually reinforce MAM instability, driving bioenergetic decline across multiple tissues.

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References

1. Liu Y, Zhu X. Endoplasmic reticulum-mitochondria tethering in neurodegenerative diseases. Transl Neurodegener. 2017;6:21. [PMC5567882]
2. Thomas G, Aslan JE, Thomas L, Shinde P, Shinde U, Simmen T. Caught in the act - protein adaptation and the expanding roles of the PACS proteins in tissue homeostasis and disease. J Cell Sci. 2017;130(11):1865-1876. [PMC5482974]
3. de Vega WC, Herrera S, Vernon SD, McGowan PO. Integration of DNA methylation & health scores identifies subtypes in myalgic encephalomyelitis/chronic fatigue syndrome. Epigenomics. 2018;10(5):539-557.
4. Wang PY, et al. WASF3 disrupts mitochondrial respiration and may mediate exercise intolerance in myalgic encephalomyelitis/chronic fatigue syndrome. PNAS. 2023;120(34):e2302738120.
5. Syed et al. Stress-inducible WASF3 regulates brain metabolism and cognitive function. PNAS. 2026.