The protein in question is connexin 26, or Cx26, encoded by the gene GJB2. In healthy tissue, six connexin subunits assemble into hemichannels that dock with neighboring cells to form gap junctions, pores through which small molecules and ions pass freely. Cx26 was long considered a tumor suppressor, yet accumulating evidence has linked its overexpression to metastasis and poor prognosis in several cancers. Earlier work had noted that Cx26 accumulates in the cytoplasm of colorectal cancer cells rather than sitting on the plasma membrane, but whether this mislocalization was merely a passive hallmark of aggressiveness or an active driver of it remained unresolved.

To answer that question, the team led by researchers at Nanjing University turned to a uniquely informative pair of cell lines: SW480, isolated from a primary colon adenocarcinoma, and SW620, derived a year later from a lymph node metastasis in the same patient. The metastatic SW620 cells expressed far more Cx26 than their primary counterparts, yet their gap junction function was weaker, not stronger. Surface-level Cx26 was nearly absent in SW620 cells. Instead, fractionation and immunofluorescence experiments revealed that the protein had accumulated in an unexpected destination: the mitochondria. When the researchers induced epithelial-mesenchymal transition, or EMT, in SW480 cells with TGF-β, Cx26 levels rose and the protein shifted toward mitochondria in step with the cells’ acquisition of invasive traits.

The pattern held beyond the laboratory dish. In a human colorectal cancer tissue microarray, multiplex immunofluorescence showed that Cx26 overlapped strongly with the mitochondrial marker TOM20 in metastatic lesions of the liver, lung, and ovary, while primary tumors retained substantial membrane-localized Cx26 with little mitochondrial colocalization. A parallel comparison of the mouse melanoma sublines B16F1 and B16F10—which share a genetic background but differ sharply in metastatic capacity—reproduced the same relationship: the highly metastatic B16F10 cells carried less membrane Cx26 and more mitochondrial Cx26. Mitochondrial translocation of Cx26, the authors conclude, is a defining feature of the aggressive, metastatic phenotype.

What steers the protein to mitochondria? Signal peptide predictions pointed to the first forty amino acids of Cx26, but deleting them changed nothing. The answer emerged from a co-immunoprecipitation screen coupled to mass spectrometry, which identified PIMT—protein L-isoaspartyl/D-aspartyl O-methyltransferase, encoded by PCMT1—among 118 candidate interacting proteins. PIMT is a conserved repair enzyme that methylates isoaspartyl residues created by the spontaneous deamidation of asparagine, a chemical decay process that kinks protein backbones and undermines stability. In the metastatic SW620 cells, the interaction between PIMT and Cx26 was far stronger than in SW480 cells, and silencing PIMT pushed Cx26 back toward the cell membrane, restored gap junction communication, and drained the protein out of mitochondria.

Mass spectrometry then pinpointed the chemistry: PIMT methylates Cx26 at asparagine 176, a residue within the extracellular loop that normally helps dock hemichannels between adjacent cells. Mutating that residue to alanine shortened the protein’s half-life, reduced its mitochondrial abundance, and increased its cytoplasmic pool. PIMT overexpression, by contrast, protected wild-type Cx26 from degradation while leaving the N176A mutant untouched. The team showed that Cx26 is degraded by the ubiquitin-proteasome system, and that PIMT suppresses Cx26 ubiquitination through its enzymatic activity. In short, PIMT acts as a chaperone of sorts—repairing Cx26’s conformation, stabilizing it, and licensing its journey to the mitochondrial compartment.

Inside the mitochondria, Cx26 proved functionally potent. Overexpression of Cx26 increased mitochondrial calcium uptake, measured with a genetically encoded calcium sensor, while the N176A mutation blunted that uptake. Electron microscopy and co-immunoprecipitation revealed that Cx26 strengthens contacts between the endoplasmic reticulum and mitochondria by bridging the calcium release channel IP3R with the mitochondrial channel VDAC1. The consequences for tumor behavior were dramatic: Cx26 overexpression accelerated EMT in SW480 cells, enlarged xenograft tumors, and multiplied pulmonary metastatic burden in mouse models, while Cx26 knockdown in SW620 and B16F10 cells did the opposite. Blocking mitochondrial calcium uptake with ruthenium red or the MCU inhibitor MCU-i4 reversed the EMT phenotype, establishing calcium flux as the mechanistic link between mitochondrial Cx26 and metastatic capacity.

The therapeutic twist came from an unbiased screen. Querying the Genomics of Drug Sensitivity in Cancer databases across 23 colorectal cancer cell lines, the researchers found that higher GJB2 expression generally predicted resistance to standard chemotherapeutics, including 5-fluorouracil and oxaliplatin. Bortezomib, a 26S proteasome inhibitor used as first-line therapy for multiple myeloma and mantle cell lymphoma, broke the pattern: cells with high GJB2 expression were more sensitive, showing lower IC50 values. In direct comparisons, bortezomib killed metastatic SW620 cells more effectively than primary SW480 cells, while the conventional colorectal cancer drugs showed no such preference. Cx26 overexpression sensitized SW480 cells to the drug, confirming the connection.

Curiously, the mechanism had little to do with the proteasome. Cellular thermal shift assays, molecular docking, differential scanning fluorimetry, and surface plasmon resonance all converged on an unexpected target: PIMT itself. Bortezomib bound PIMT directly, raised its melting temperature by 5.6 degrees Celsius, and enhanced its methyltransferase activity in vitro. Treatment with the drug strengthened the PIMT-Cx26 interaction, drove more Cx26 into mitochondria, and triggered a catastrophic surge of intracellular calcium. That calcium flood generated reactive oxygen species and activated ERK signaling, producing PARP cleavage and LC3-II accumulation—hallmarks of apoptosis and autophagy. Scavenging reactive oxygen species, blocking mitochondrial calcium uptake, inhibiting ERK with PD0325901, or knocking down Cx26 or PIMT all blunted bortezomib’s killing of SW620 cells, mapping the pathway end to end.

The study reframes Cx26 mislocalization from a passive biomarker into an organelle-specific engine of malignancy, and it challenges the assumption that solid tumors are inherently resistant to proteasome inhibitors. Bortezomib, in this context, would not work as a proteasome poison but as a molecular lever that amplifies the very PIMT-Cx26 axis on which metastatic cells have come to depend, converting their addiction into a death signal. The authors caution that mitochondrial Cx26 localization must be validated as a survival predictor in larger clinical cohorts, and that direct biochemical confirmation of the N176 modification with purified substrates is still needed. Even so, the work offers a concrete strategy for a disease state with few options: identify tumors addicted to mitochondrial Cx26, and turn that dependency against them with a drug already sitting on the pharmacy shelf.

Subject of Research: PIMT-mediated mitochondrial translocation of connexin 26 in metastatic colorectal cancer and its therapeutic targeting by bortezomib

Article Title: Mitochondrial Localization of Cx26 Promotes Colorectal Cancer Malignancy and Exposes a Fatal Vulnerability to Drug Targeting

Article References: Wang, J., Chen, L., Qin, J., Wang, X., Ye, S., An, N., Bai, M., Shen, Y., Wu, X., & Xu, Q. (2026). Mitochondrial Localization of Cx26 Promotes Colorectal Cancer Malignancy and Exposes a Fatal Vulnerability to Drug Targeting. Advanced Science, Article e78117. https://doi.org/10.1002/advs.78117

Image Credits: AI Generated

DOI: 10.1002/advs.78117

Keywords: colorectal cancer, connexin 26, Cx26, PIMT, mitochondria, metastasis, EMT, bortezomib, calcium signaling, gap junctions, drug sensitivity, Advanced Science

The story begins with drying. Most crop species produce what botanists call orthodox seeds, which acquire desiccation tolerance during the late stages of maturation. As the seed matures, it loses up to 95 percent of its water content, and its cytoplasm transitions from a fluid to a glassy state. In this vitrified matrix, molecular mobility and relaxation rates drop dramatically, metabolic activity grinds to a near halt, and the seed enters a state of quiescence that can persist for years, decades, or even centuries. This programmed drying is an evolutionary masterpiece, but it comes at a cost. The removal of hydration shells compromises the hydrogen bonding network that holds proteins in their proper shapes, causing partial unfolding and the abnormal exposure of hydrophobic regions that promotes aggregation. Membranes suffer too: as bound water diminishes, lipid bilayers shift from a liquid-crystalline phase to a rigid gel phase, setting the stage for leakage upon rehydration.

The chemistry of drying is even more insidious at the molecular level. Molecular oxygen interacts with intracellular transition metals such as ferrous iron and cuprous copper through Fenton and Haber-Weiss reactions, generating hydroxyl radicals that oxidize DNA, proteins, and lipids. Drying stress can also initiate Maillard reactions, in which sugars cross-link with proteins and impede the mobilization of storage reserves during germination. Paradoxically, while desiccation slows the kinetics of DNA damage by nearly shutting down metabolism, it simultaneously disables the very repair machinery that would normally fix such lesions. Genome damage therefore accumulates silently over prolonged dry storage, building what the review’s authors describe as a molecular scar load within the genome. Studies in pear have shown that even moderate drying alters DNA methylation patterns, and prolonged storage induces further epigenetic drift, adding another layer of risk to genomic stability.

Seed aging, once underway, is commonly described as irreversible, cumulative, and inexorable. Its pace is governed by an interplay of genetics and environment. Storage temperature and moisture content are critical: below roughly 20 degrees Celsius and at moisture levels of 0.1 grams of water per gram of dry weight or less, the glassy cytoplasm impedes molecular diffusion and slows the biochemical reactions of deterioration. Higher humidity raises seed moisture, reactivates respiration, and exacerbates the generation of reactive oxygen species. Even in thoroughly dry seeds, lipids remain susceptible to oxidation and serve as the predominant source of free radicals. The resulting redox imbalance is particularly damaging because it inactivates or misfolds DNA repair proteins while simultaneously causing strand breaks and base modifications. Non-enzymatic modifications compound the problem: in mung bean seeds, Amadori products rise during early aging, while Maillard products accumulate throughout storage, and both correlate with declining vigor.

Against this assault, seeds deploy a sophisticated antioxidant arsenal, and the review highlights glutathione as its centerpiece. This low-molecular-weight thiol is the most abundant water-soluble antioxidant in orthodox seeds, neutralizing reactive oxygen species either by direct scavenging or by donating electrons to detoxifying enzymes. As seeds age, the ratio of oxidized to reduced glutathione rises, marking a shift toward a more oxidizing cellular environment and a corresponding decline in viability. Indeed, the redox potential of the glutathione half-cell has been proposed as a quantitative metric of seed viability. Arabidopsis ecotypes with elevated glutathione levels show enhanced longevity, and genotypes with impaired tocopherol biosynthesis exhibit diminished lifespan, underscoring the importance of lipid-soluble antioxidants as well. Genome-wide association studies and reverse genetics in Arabidopsis have identified genes such as DHAR1, which regenerates ascorbate during hydrogen peroxide detoxification, and PSAD1, a photosystem I subunit, as contributors to seed longevity. Intriguingly, loss of the enzyme FAHD1 shifts seeds into a more reducing state and increases their resistance to aging.

Reactive oxygen species are not simply enemies. During early imbibition, antioxidant protein activity drops as a consequence of protein carbonylation, producing a transient rise in ROS that acts as a signaling cue to initiate germination. Germination is governed by an oxidative window in which ROS levels must remain within an optimal range to release dormancy and orchestrate signaling pathways. ROS also cleave cell wall polysaccharides, loosening the wall to permit rapid water uptake. The lesson from the review is that redox control in seeds is a balancing act: too much oxidation destroys viability, but a carefully calibrated pulse of oxidation is part of the ignition sequence for life.

When water finally returns, the seed faces its most critical test. Rehydration reanimates metabolism but simultaneously triggers a surge of ROS and mechanical stress at the chromatin level, compounding the lesions accumulated during storage. Base oxidation is the most common DNA lesion in aging seeds, with guanine especially vulnerable because of its low redox potential; oxidation produces the mutagenic 8-oxoguanine, which mispairs with adenine. Base excision repair, initiated by DNA glycosylases that excise damaged bases, is the primary defense, and elevated expression of 8-oxoG DNA glycosylase enzymes during imbibition in Medicago truncatula highlights its importance. Nucleotide excision repair handles bulky lesions, and mutations in the XPB helicase of this pathway compromise germination under oxidative stress. Double-strand breaks, the most catastrophic form of genome damage, are repaired through non-homologous end joining and homologous recombination. Mutations in DNA ligase 4 and DNA ligase 6, which mediate the canonical and alternative end-joining pathways, render seeds exceptionally vulnerable to accelerated aging, and a longevity quantitative trait locus in Arabidopsis maps to the LIG4 gene. The checkpoint kinase ATM further links damage sensing to germination control, while rad51 mutants in maize show delayed germination and increased seedling mortality after irradiation.

Proteins, too, require active repair, and here the review spotlights two unsung heroes: methionine sulfoxide reductase and protein L-isoaspartyl methyltransferase. Oxidation converts methionine residues to methionine sulfoxide, a modification that disrupts protein structure and serves as a biomarker of aging across organisms. MSR enzymes reverse this damage reversibly, and higher natural MSR activity correlates with increased seed longevity in Medicago and alfalfa. In rice, the MSR isoform MSRB5 physically associates with ascorbate peroxidase and PIMT themselves, both of which are vulnerable to methionine oxidation, restoring their function and maintaining seed vigor. PIMT, meanwhile, repairs isoaspartyl residues that arise from spontaneous deamidation of asparagine and glutamine, which destabilize storage proteins and enzymes. Overexpression of chickpea PIMT genes extends Arabidopsis seed longevity, and PIMT repairs the enolase 2 protein in rice, safeguarding its function through maturation and storage.

The deepest insight of the review is that these systems do not operate in isolation but form a functional synergy. Proteins needed at germination come from three sources: the stored proteome that survived drying, stored mRNAs translated upon imbibition, and new transcripts. The translational machinery is the Achilles’ heel of seed longevity, and PIMT targets include DEAD-box RNA helicases whose activity is lost upon isoaspartyl formation but restored by repair. PIMT also repairs the ABI3, ABI4, and ABI5 transcription factors that orchestrate desiccation tolerance, the antioxidant enzymes superoxide dismutase and catalase, and the heat-shock transcription factor OsHSFC1b, whose function is disrupted by isoaspartyl modification. Protective proteins complete the network: late embryogenesis abundant proteins, of which Arabidopsis possesses 51 genes across eight families, act as molecular chaperones that stabilize proteins and membranes, buffer iron ions that catalyze ROS production, and contribute to vitrification alongside sugars. In Medicago, accumulation of four LEA polypeptides correlated with a 30-fold enhancement in longevity. Small heat-shock proteins, developmentally induced by the seed-specific transcription factor HSFA9 under ABI3 control, prevent aggregation without requiring ATP, and expression of rice HSP18.2 enhances vigor by mitigating ROS accumulation. Intrinsically disordered LEA proteins even refold into amphipathic helices upon drying and bind DNA and RNA, protecting the genetic material itself.

The practical implications are substantial. Understanding these molecular timekeepers enables the development of biomarkers for seed storage stability, guides the selection of genotypes with superior repair capacity for breeding, and informs storage and priming protocols that sustain viability. As climate change intensifies pressure on global agriculture and gene banks worldwide safeguard plant genetic resources as an insurance policy against biodiversity loss, deciphering how seeds preserve their genomes and proteomes through decades of quiescence offers a roadmap for engineering crops whose seeds remain vigorous longer. The alliance of redox control, repair, and protection, refined over hundreds of millions of years of plant evolution, may prove to be one of the most valuable toolkits that nature has left for humanity to borrow.

Subject of Research: Molecular mechanisms of seed longevity involving redox homeostasis, DNA and protein repair, and protective proteins

Article Title: Molecular timekeepers: the curious alliance of redox, repair, and protective proteins in preserving seed longevity

Article References: Das, A., & Majee, M. (2026). Molecular timekeepers: the curious alliance of redox, repair, and protective proteins in preserving seed longevity. Crop Health, 4(1), Article 3. https://doi.org/10.1007/s44297-026-00064-9

Image Credits: AI Generated

DOI: 10.1007/s44297-026-00064-9

Keywords: seed longevity, redox homeostasis, DNA repair, reactive oxygen species, LEA proteins, heat-shock proteins, PIMT, methionine sulfoxide reductase, glutathione, desiccation tolerance, seed aging, crop improvement