The study centers on a mitochondrial surveillance pathway known as the Ethanol and Stress Response Element, or ESRE, pathway. First identified more than two decades ago as an eleven-nucleotide DNA motif, TCTGCGTCTCT, in genes induced by ethanol exposure, the ESRE motif later turned up in genes activated during infection with the opportunistic pathogen Pseudomonas aeruginosa and by acute iron starvation. Subsequent work from the same laboratory showed that the pathway responds to reactive oxygen species, or ROS, generated by disruptions of the mitochondrial electron transport chain. Unlike the better-known mitochondrial unfolded protein response, which senses misfolded proteins inside mitochondria through the transcription factor ATFS-1, or the PINK-1/Parkin pathway, which licenses damaged mitochondria for autophagic destruction, ESRE appears exquisitely sensitive to redox perturbation. What remained unclear was which molecular players actually assemble and drive this signal.

To find them, the researchers started from a chemical genetics angle. In earlier high-throughput screens, their team had identified small molecules that improve survival of the roundworm Caenorhabditis elegans during Liquid Killing, a P. aeruginosa pathogenesis model in which the bacterium poisons worms in liquid culture. One of the most protective compounds, LK56, had been shown to depend on MDT-15 for its activity, but its mechanism was unknown. Bioinformatic analysis of the genes induced by LK56, using tools that scan promoter sequences for overrepresented motifs, revealed that more than thirty percent of the upregulated genes carried the ESRE motif upstream of their coding regions. The same analysis flagged lipid metabolism, detoxification, and pathogen response as the dominant functional categories, hinting that fat handling and mitochondrial surveillance were connected long before any direct experiments began.

The first direct test was straightforward but decisive. The team used a reporter strain in which green fluorescent protein is driven by three tandem copies of the ESRE motif, then activated the pathway with rotenone, a compound that inhibits mitochondrial Complex I and reliably triggers ESRE signaling. When the researchers knocked down mdt-15 by RNA interference, the fluorescent signal was essentially abolished, even under maximal rotenone stress. MDT-15, an evolutionarily conserved subunit of the Mediator complex, is not a DNA-binding transcription factor itself; instead, it acts as a co-activator that helps DNA-binding factors turn genes on. Its partners include the nuclear hormone receptor NHR-49, the heat shock factor HSF-1, and the SREBP-like protein SBP-1, and among its established targets are the delta-9 fatty acid desaturases FAT-5, FAT-6, and FAT-7. These enzymes are the workhorses of monounsaturated fat production: FAT-5 converts palmitic acid into palmitoleic acid, while FAT-6 and FAT-7 convert saturated stearic acid into oleic acid, a major membrane lipid in the worm.

Given that connection, the investigators silenced each desaturase in turn and found that loss of fat-5, fat-6, or fat-7 each blunted ESRE activation, with genetic mutants in fat-6 and fat-7 showing significantly reduced expression of native ESRE genes measured by quantitative PCR. The team then asked whether the box C/D snoRNPs, which they had previously implicated as a molecular switch governing ESRE and the unfolded protein response while repressing the PMK-1/p38 innate immune pathway, also controlled fatty acid enzymes. Using fluorescent protein reporters for FAT-6 and FAT-7, they showed that both proteins accumulated strongly during P. aeruginosa liquid pathogenesis and during chemical mitochondrial stress, and that this accumulation required both MDT-15 and the snoRNP core protein FIB-1. Strikingly, the regulation ran one way: knocking down MDT-15 reduced FIB-1 protein levels and the expression of native box C/D snoRNP genes, whereas depleting FIB-1 left MDT-15 untouched. MDT-15, in other words, sits at the top of the hierarchy, commanding the very RNA-editing complexes that in turn modulate the fat enzymes it induces.

The search for host defense implications followed logically from earlier observations that the ESRE pathway is activated during liquid-based P. aeruginosa infection. Worms reared on mdt-15 RNAi or fat-6 RNAi became markedly more susceptible to the pathogen, but they remained perfectly healthy when the same liquid-killing protocol was run with harmless E. coli substituted for P. aeruginosa, ruling out nonspecific sickness. The pattern was also selective for the type of infection: only mdt-15 knockdown sensitized worms to agar-based P. aeruginosa infection, a model that relies more heavily on the p38 MAPK pathway, which MDT-15 independently supports. The result ties lipid-dependent ESRE surveillance directly to resistance against an acute bacterial threat, extending the growing recognition that mitochondria sit at the crossroads of innate immunity.

Perhaps the most unexpected finding concerned mitophagy, the selective autophagic disposal of damaged mitochondria. Using a PINK-1::GFP reporter, the researchers observed that knocking down mdt-15 or fib-1 caused PINK-1, the kinase that initiates the mitophagy cascade, to accumulate, and that this accumulation translated into genuine autophagic flux: a marker of autophagosome formation, LGG-1::GFP, formed more puncta when fib-1 was depleted, and that effect disappeared when pink-1 was also silenced. Because MDT-15 and FIB-1 are both required for full ESRE activation, the simplest interpretation is that a functional ESRE pathway acts as a brake on mitophagy, either by improving mitochondrial function enough that clearance becomes unnecessary, or by actively restraining the PINK-1 licensing step. The authors are careful to note that their reporter measurements cannot yet distinguish between increased mitochondrial damage, PINK-1 stabilization, and increased mitophagic flux, so whether ESRE directly represses mitophagy remains an open question. Still, the genetic interplay had functional consequences: in a pink-1 mutant background, the heightened mortality caused by mdt-15 loss during P. aeruginosa infection was no longer amplified, suggesting that MDT-15 and PINK-1 operate in an interconnected protective network.

The team then turned to supplementing the worms with fatty acids downstream of FAT-6 and FAT-7: oleic acid, linoleic acid, gamma-linolenic acid, and arachidonic acid. The expectation had been that supplying the missing lipid product might rescue ESRE activation when its upstream enzymes were silenced. Instead, supplementation boosted ESRE activation in every genetic background tested, including wild-type worms, desaturase knockdowns, and snoRNP-depleted animals, implying that fatty acids act through a parallel mechanism rather than simply replenishing a missing product. Arachidonic acid also increased expression of native ESRE genes, confirming biological relevance. Yet the effect was conditional: fatty acids alone, in the absence of mitochondrial stress, activated nothing. They amplified only when rotenone was present. Even more selectively, supplementation did not boost the mitochondrial unfolded protein response and in some cases dampened it, arguing that the enhancement is specific to ESRE and mitophagy rather than a general stress amplifier.

Why would unsaturated fats sharpen a redox-sensitive pathway but leave a proteostasis-sensing one untouched? The authors propose that the answer lies in what each pathway measures. Long-chain fatty acids can impair mitochondrial redox activity and raise steady-state ROS production, and polyunsaturated species embedded in membrane phospholipids are prime targets for peroxidation, which itself generates further ROS and membrane damage. ESRE, tuned to ROS, would amplify; the unfolded protein response, tuned to protein misfolding, would not. Supporting this model, the antioxidant ascorbate abolished rotenone-induced ESRE activation regardless of fatty acid supplementation, while basal ROS measurements showed that the fatty acids alone did not create oxidative stress. The findings also raise intriguing questions about ferroptosis, the iron-dependent cell death program driven by lipid peroxidation: the authors speculate that ESRE might either restrain ferroptosis by restoring homeostasis in time, or actively propagate a ferroptotic signal to contain failed mitochondria. And because a human box C/D snoRNA, SNORD88C, is already known to boost translation of the desaturase SCD1 by methylating a conserved site in 28S ribosomal RNA, the worm findings may well extend to human biology, linking rRNA editing, lipid balance, mitochondrial health, and aging in a single regulatory axis.

Subject of Research: Regulation of mitochondrial surveillance, mitophagy, and host defense by MDT-15, box C/D snoRNPs, and fatty acid metabolism

Article Title: Box C/D snoRNPs and MDT-15/MED15 regulate mitochondrial surveillance and mitophagy via fatty acid metabolism

Article References: Armendariz, L., Chan, A., Tjahjono, E., Wang, M., Acevedo, Y., & Kirienko, N. V. (2026). Box C/D snoRNPs and MDT-15/MED15 regulate mitochondrial surveillance and mitophagy via fatty acid metabolism. GeroScience. https://doi.org/10.1007/s11357-026-02541-z

Image Credits: AI Generated

DOI: 10.1007/s11357-026-02541-z

Keywords: mitochondrial surveillance, ESRE pathway, MDT-15/MED15, box C/D snoRNPs, fatty acid metabolism, mitophagy, PINK-1, oleic acid, Pseudomonas aeruginosa, C. elegans, UPRmt, host defense

A research team at Sun Yat-sen University in Guangzhou now reports a solution that could reshape how lung immunology is done. In a study published in Advanced Biotechnology, the researchers describe an immortalized mouse alveolar macrophage cell line, created not by introducing cancer-causing oncogenes but by switching on telomerase, the enzyme that naturally maintains the protective caps at the ends of chromosomes. The resulting cells, named iAMs, divide indefinitely yet, according to the team’s detailed molecular and functional profiling, retain an unmistakably primary-like identity.

The motivation is easy to appreciate. Harvesting primary alveolar macrophages requires flushing the airways of experimental animals, a procedure that yields only 200,000 to 500,000 cells per mouse. That quantity is quickly consumed by basic characterization experiments, leaving little material for the high-throughput screens, genetic manipulations, and replication studies on which modern biomedical science depends. Worse still, the cells rapidly lose their defining traits once removed from the alveolar environment, senescing quickly in culture dishes.

Existing cell lines have not filled the gap convincingly. The AMJ2-C8 and AMJ2-C11 lines, created decades ago by v-raf and v-myc oncogene transduction, respond abnormally to bacterial products such as lipopolysaccharide and muramyl dipeptide and fail to secrete adequate amounts of the inflammatory cytokines IL-1 and TNF. The widely used MH-S line, immortalized with SV40 T antigen, lacks Siglec-F, one of the most recognizable surface markers of the mouse alveolar macrophage, and does not express the receptor CD116, leaving it deaf to GM-CSF, a growth factor central to alveolar macrophage biology, and blunted in its responses to bacterial stimulation.

The Sun Yat-sen team chose a gentler strategy. Human telomerase reverse transcriptase, or hTERT, has previously been used to immortalize a range of primary cell types without dragging them toward a cancer-like state, because it extends replicative lifespan rather than rewiring growth-control pathways. The researchers packaged hTERT into a lentiviral vector and delivered it to primary alveolar macrophages isolated from C57BL/6 mice by bronchoalveolar lavage. After selection with puromycin and isolation of single-cell clones, four stable lines emerged, all confirmed free of mycoplasma contamination and indistinguishable from one another in morphology, marker expression, and growth. Short tandem repeat profiling verified that the final line traced back to the original primary cells, and all subsequent work used cells between passages 1 and 12.

The proliferative leap is dramatic. Where primary alveolar macrophages stagnate, the iAMs double roughly every 12 hours, nearly all of them staining positive for the proliferation marker Ki67 and incorporating the DNA precursor EdU at rates far above those of their primary counterparts. Crucially, this vigor did not come at the cost of appearance. Under bright-field and transmission electron microscopy, the cells resemble freshly isolated alveolar macrophages, with similar Giemsa and Oil Red O staining patterns. Scanning electron microscopy revealed one subtle difference: the immortalized cells keep the spherical shape of their primary ancestors but extend fewer pseudopodia, the membrane protrusions macrophages use to crawl and engulf.

Transcriptionally, the resemblance holds up under rigorous scrutiny. Bulk RNA sequencing compared iAMs with primary alveolar macrophages, with AM-like cells differentiated from bone marrow precursors using GM-CSF, TGF-beta and rosiglitazone, and with bone marrow-derived macrophages, the workhorse surrogate in many labs. Principal component analysis placed the iAMs closest to primary alveolar macrophages, and pairwise correlation and hierarchical clustering confirmed the same picture. Out of the whole transcriptome, differential expression analysis flagged 2,953 genes changing by at least twofold between iAMs and primary cells, concentrated in immune signaling and metabolic pathways. Functional module scoring showed the immortalized cells mirroring primary cells across lysosome, phagocytosis, oxidative phosphorylation, chemotaxis and M2 polarization programs, with antigen presentation scoring somewhat higher in the immortalized line.

Surface phenotype and function tracked the transcriptome. Flow cytometry confirmed the canonical alveolar macrophage signature of intermediate F4/80, low CD11b, high Siglec-F and high CD11c, and quantitative PCR validated expression of AM-specific, alternative activation and self-renewal genes at levels comparable to primary cells. In functional assays, the iAMs actually outperformed primary cells in some respects: they engulfed fluorescently labeled E. coli more avidly than either primary alveolar macrophages or AM-like cells, and they migrated more efficiently toward Pseudomonas aeruginosa in Transwell chemotaxis chambers. When stimulated with lipopolysaccharide, lipoteichoic acid, or heat-inactivated Pseudomonas, they mounted inflammatory cytokine responses, including Il1a, Il1b, Il6, Ccl2 and Tnfa, closely matching those of primary cells.

The most demanding test came in living animals. The team depleted resident alveolar macrophages in mice with clodronate liposomes, achieving roughly 95.7 percent depletion, and then transferred iAMs directly into the airways before challenging the animals with Pseudomonas aeruginosa. The transferred cells engrafted stably in the alveolar space and, tracked with the fluorescent dye PKH26, were still detectable 14 days later, by which point they accounted for more than half of the CD11c-positive population. Mice lacking alveolar macrophages developed severe lung injury, with blood-tinged and turbid lavage fluid, elevated protein levels, massive neutrophil infiltration and high bacterial burdens. Reconstitution with iAMs significantly eased every one of these pathological features, restoring bacterial clearance and restraining neutrophil recruitment, evidence that the immortalized cells can genuinely stand in for their natural counterparts in pulmonary host defense.

The authors are candid about the model’s limits. Immortalization and long-term culture can introduce genetic or epigenetic drift and clonal selection, and the enhanced antigen-presenting capacity and reduced pseudopod formation show the cells are not perfect copies of freshly isolated macrophages. The team recommends working within the validated passage range and monitoring key markers and functions, with critical findings confirmed in primary cells when maximum fidelity is required. Even with those caveats, the resource fills a conspicuous void. Bone marrow-derived macrophages and monocyte-derived macrophages differ fundamentally from tissue-resident macrophages in developmental origin, transcriptional wiring and functional repertoire, and oncogene-transformed lines carry distortions of their own. A telomerase-immortalized line that preserves the alveolar macrophage identity opens the door to high-throughput drug screening, CRISPR-based mechanistic dissection and reproducible functional assays that were previously impractical, and it promises to accelerate research on lung immunology, host defense and pulmonary homeostasis for years to come.

Subject of Research: Generation and characterization of an hTERT-immortalized mouse alveolar macrophage cell line

Article Title: hTERT-immortalized mouse alveolar macrophages retain primary-like transcriptional and functional programs

Article References: hTERT-immortalized mouse alveolar macrophages retain primary-like transcriptional and functional programs. (n.d.). https://doi.org/10.1007/s44307-026-00130-x

Image Credits: AI Generated

DOI: 10.1007/s44307-026-00130-x

Keywords: alveolar macrophages, hTERT, cell immortalization, telomerase, lung immunology, phagocytosis, Pseudomonas aeruginosa, RNA sequencing, host defense, macrophage cell line, pulmonary homeostasis, GM-CSF