Family 13: Protein Homeostasis and Degradation
Family overview. Cells maintain protein quality through the heat shock response, the ubiquitin-proteasome system (UPS), and autophagy/mitophagy. When these fail, misfolded or damaged proteins accumulate, disrupting cellular function and triggering danger-response inflammation.
Concrete mechanisms and ME/CFS evidence:
Heat shock protein (HSP) dysregulation. Abnormal HSP70 and HSP90 expression in ME/CFS samples; HSPs normally prevent protein misfolding, support immune cell function, and facilitate recovery from stress (Nijs et al. 2012). Hochecker et al. (2025) demonstrated that HSP70 (HSPA5) can be acutely upregulated (48.33% increase) in ME/CFS PBMCs via whole-body hyperthermia, and that this induction correlates with mitochondrial respiratory improvement — suggesting that HSP deficiency is dynamically modifiable rather than a fixed deficit (Hochecker et al. 2025). Thermal challenge may additionally recruit TRPV1-mediated autonomic pathways (Section Post-Exercise Plasma LPA Panel as PEM Subtyping Biomarker), providing a secondary, non-mitochondrial mechanism by which controlled hyperthermia could serve as a diagnostic stress test for autonomic recovery capacity (Nelson et al. 2021).
Autophagy block — ATG13 elevation. ATG13 (autophagy-initiating protein) is significantly elevated in ME/CFS patient serum, indicating autophagy has been initiated but aborted. Phosphorylated ATG13 escaping into serum signals failed autophagic flux. This serum ATG13 stimulates ROS and nitric oxide production in microglial cells via RAGE receptor, directly linking failed autophagy to neuroinflammation (Gottschalk et al. 2022).
Impaired mitophagy. Defective clearance of damaged mitochondria allows dysfunctional mitochondria to accumulate in cells, worsening energy production (Family 1) and promoting inflammatory signalling.
Ubiquitin-proteasome system (UPS). UPS impairment is theoretically plausible given the protein quality control demands of chronic immune activation and oxidative stress; direct proteasome activity measurements in ME/CFS are absent from the literature.
Extracellular protein aggregation. Fibrinaloid microclots (Family 6) represent documented extracellular protein aggregation. Whether intracellular protein aggregation occurs in ME/CFS neurons or immune cells — as in long COVID preliminary data — is unstudied.
Endoplasmic reticulum stress and the unfolded protein response (UPR). The UPR’s three sensors — PERK, IRE1α, ATF6 — detect misfolded proteins in the ER and initiate adaptive or apoptotic programs. When chronically activated, each sensor drives inflammation: IRE1α-XBP1 induces IL-6 and TNF-α; PERK-eIF2α activates NF-κB via IκB degradation; ATF6 upregulates complement components (Kawano et al. 2023). Kawano et al. (2023) demonstrated that ER proteostasis dysfunction in peripheral tissues generates systemic signals regulating sleep, linking ER stress directly to the sleep disruption that characterizes ME/CFS (Kawano et al. 2023). Chronic UPR activation would provide a feed-forward mechanism linking any protein-folding disturbance (from oxidative stress, viral infection, or mitochondrial dysfunction) to sustained inflammatory output — without ongoing antigenic stimulation. No study has measured UPR activation (phospho-PERK, spliced XBP1, nuclear ATF6) in ME/CFS tissues.
Evidence status: Emerging (ATG13/autophagy block is a key discovery; UPS and UPR/ER stress uncharacterised in ME/CFS).
Does proteostasis failure in ME/CFS extend beyond extracellular fibrin microclots to intracellular protein aggregation in neurons, NK cells, or endothelial cells? Direct assessment using amyloid detection assays and cryo-EM on patient-derived cells would determine whether protein aggregation is a primary or secondary phenomenon.
Additionally: Is the UPR chronically activated in ME/CFS tissues? Measurement of phospho-PERK, spliced XBP1, ATF6 nuclear translocation, and downstream CHOP in patient-derived PBMCs and muscle biopsies would indicate whether ER stress contributes to the inflammatory state.