TXN2

TXN2 is the mitochondrial thioredoxin, a small redox protein that maintains mitochondrial redox homeostasis by serving as the electron donor for the mitochondrial peroxiredoxin PRDX3/SP-22, with electrons flowing from NADPH through mitochondrial thioredoxin reductase to TXN2 and onward to PRDX3 to reduce H2O2 and organic hydroperoxides (PMID:9363753, PMID:32866605). This redox-buffering activity is essential for mitochondrial function: partial loss in mice impairs electron transport chain activity and ATP production while raising mitochondrial ROS and oxidative damage to DNA, lipids, and proteins, and sensitizing cells to ROS-induced apoptosis (PMID:18164269), and complete human loss-of-function abolishes the protein, dysregulates ROS and oxidative phosphorylation in patient fibroblasts, and is rescued by TXN2 re-expression, establishing a causal requirement for mitochondrial redox homeostasis and cell viability (PMID:26626369). By controlling mitochondrial ROS, TXN2 acts upstream of redox-sensitive signaling: it limits NF-κB activation and pro-inflammatory cytokine induction (PMID:38246558), and through ROS/NF-κB it restrains BACE1 transcription and amyloid-β production (PMID:32920833). TXN2 expression is itself transcriptionally regulated by multiple inputs, including AMPK in hypothalamic glucose-sensing neurons (PMID:32839348), KAT2A-dependent H3K36 acetylation at its promoter (PMID:38417317), and post-transcriptional silencing by miR-27a/b (PMID:28356525). A human TXN2 promoter polymorphism that reduces transcriptional activity is associated with spina bifida risk, linking TXN2 dosage to neural tube closure (PMID:19165900).

1. 1997 High

   Established the core enzymatic role of mitochondrial thioredoxin by defining it as the electron donor that couples NADPH oxidation to peroxiredoxin-mediated peroxide detoxification.

   Evidence Protein purification and in vitro reconstitution of NADPH/thioredoxin reductase/mt-Trx/SP-22 with peroxidase activity assays, bovine adrenal cortex

   PMID:9363753

   Open questions at the time

   • Reconstitution used bovine proteins; full electron-transfer kinetics and structural basis not resolved
   • Did not address in vivo physiological requirement
2. 2007 High

   Showed in vivo that TXN2 dosage is rate-limiting for mitochondrial bioenergetics and oxidative-damage protection, moving the protein from a biochemical activity to a physiological mitochondrial guardian.

   Evidence Txn2 heterozygous knockout mouse with mitochondrial ATP/ETC assays, ROS and oxidative damage markers, apoptosis sensitivity

   PMID:18164269

   Open questions at the time

   • Haploinsufficiency only; complete knockout phenotype not examined here
   • Mechanism linking ROS to ETC decline not dissected
3. 2015 High

   Demonstrated that TXN2 is causally required in humans, linking loss-of-function to mitochondrial redox failure and OXPHOS dysfunction with rescue by re-expression.

   Evidence Exome sequencing of a patient, patient fibroblast ROS/OXPHOS assays, TXN2 reconstitution rescue, antioxidant supplementation

   PMID:26626369

   Open questions at the time

   • Single patient; genotype-phenotype spectrum unknown
   • Neurodegeneration mechanism downstream of redox failure not detailed
4. 2017 Medium

   Identified post-transcriptional control of TXN2 by miR-27a/b and linked TXN2 levels to cell-cycle progression and viral replication.

   Evidence miRNA mimic/inhibitor, 3' UTR reporter assay, TXN2 siRNA, flow cytometry cell cycle, adenoviral genome quantification

   PMID:28356525

   Open questions at the time

   • Link between TXN2 redox function and G1 arrest mechanistically unspecified
   • Single lab
5. 2020 High

   Placed TXN2 within redox-sensitive signaling and tissue physiology: as an AMPK target controlling hypothalamic glucose-sensing neuron activity, and as a suppressor of BACE1/Aβ via ROS/NF-κB.

   Evidence AMPK conditional KO with Txn2 re-expression rescue and GI neuron electrophysiology (mouse); TXN2 KD/OE in SH-SY5Y/HEK-APP with BACE1, Aβ, and p65 epistasis assays

   PMID:32839348 PMID:32920833

   Open questions at the time

   • Direct transcriptional mechanism downstream of AMPK not shown
   • NF-κB/BACE1 axis evidence is Medium-confidence and single-lab
   • Whether TXN2 acts on other ROS-sensitive signaling nodes unknown
6. 2024 Medium

   Generalized the TXN2→mitochondrial ROS→NF-κB inflammatory axis and defined epigenetic control of TXN2 expression.

   Evidence TXN2 KD/OE in bovine adipocytes with IκBα/phospho-NF-κB, cytokine qPCR, and NAC rescue; ChIP-qPCR of KAT2A-dependent H3K36ac at TXN2 promoter in SH-SY5Y with KAT2A OE rescue

   PMID:38246558 PMID:38417317

   Open questions at the time

   • Both single-lab; direct KAT2A occupancy at TXN2 promoter not shown by independent methods
   • Cross-species generality of NF-κB suppression untested in human cells

• How TXN2 redox status is sensed and integrated by NF-κB and other downstream signaling, and the structural basis of TXN2–PRDX3 electron transfer, remain open.
• No structural model of the human TXN2–PRDX3 complex
• Molecular link between TXN2 oxidation state and NF-κB activation not defined
• Full substrate/interactome of TXN2 beyond PRDX3 uncharacterized