Acid phosphatases, classified under the enzyme commission number EC 3.1.3.2, are orthophosphoric monoester phosphohydrolases that hydrolyze phosphate esters and carry out transphosphorylation reactions in acidic environments. In plants, they are indispensable players in phosphate metabolism, signal transduction, nutrient mobilization during germination, and the acquisition of phosphorus when soils are depleted. Beyond plant physiology, these enzymes serve as clinical markers for human diseases, including prostate cancer and infertility, which gives any new insight into their active-site architecture a relevance that stretches far beyond the botanical world. Yet for many plant acid phosphatase isoenzymes, the structural basis of catalysis remains poorly understood, and Erythrina indica, a leguminous tree of medicinal and ecological importance, had never had its seed phosphatases characterized at this level of detail.

The investigative strategy rested on a classic but powerful technique: residue-specific chemical modification. The team treated purified AP-II with three targeted reagents, each designed to attack one class of amino acid. Dicyclohexylcarbodiimide, abbreviated DCHC, was deployed against carboxylate groups; N-bromosuccinimide, or NBS, against tryptophan; and phenylmethylsulfonyl fluoride, PMSF, against serine. The results were unambiguous. Modification with NBS at 5 millimolar concentration produced complete loss of enzymatic activity, PMSF treatment destroyed more than 90 percent of activity, and DCHC treatment caused a roughly 90 percent decline. Because each reagent is selective for a different residue type, the near-total inactivation pointed directly to the participation of tryptophan, serine, and carboxylate in the enzyme’s active site.

Crucially, the researchers then ran substrate protection experiments to confirm that these residues were not merely incidental casualties of the reagents. When 2 millimolar p-nitrophenyl phosphate, the enzyme’s model substrate, was added before the modification reactions, the loss of activity caused by all three modifying agents was significantly prevented. The substrate, in effect, occupied the active site and shielded the vulnerable residues from chemical attack, exactly as would be expected if those residues were genuinely part of the catalytic machinery rather than bystanders elsewhere on the protein. This protective effect provided strong evidence that all three residue classes are directly involved in the catalytic process.

Kinetic analysis of the partially inactivated enzyme added a subtle but important layer of interpretation. When AP-II was partially inactivated with NBS, PMSF, or DCHC, the maximum reaction velocity, Vmax, decreased while the Michaelis constant, Km, remained essentially unchanged. That dissociation is telling: Km reflects substrate binding affinity, while Vmax reflects the rate of catalytic turnover. A drop in Vmax with stable Km means the modified residues contribute primarily to the chemistry of catalysis rather than to substrate recognition. The enzyme could still grab its substrate, but it could no longer efficiently execute the hydrolysis reaction, implicating the three residue classes in the bond-breaking step itself.

The pH dependence of activity supplied independent confirmation of the carboxylate’s role. Measuring kinetic parameters across the pH range from 3 to 6 and plotting log(Vmax/Km) against pH yielded pKa values of 4.6 and 5.25 for AP-II, values squarely consistent with the ionization behavior of a carboxylate group participating in the reaction. Meanwhile, Lineweaver-Burk plots showed that Km for p-nitrophenyl phosphate stayed nearly constant across pH 3.0 to 6.0 while Vmax changed, demonstrating that pH alters the rate of reaction without altering substrate affinity. The stoichiometry of modification told a similar story of economy: although two tryptophan residues were modified by NBS, extrapolating the plot of residual activity against modified residues to zero activity revealed that only a single tryptophan is directly involved in catalysis. Likewise, a log-log plot of inactivation rate against PMSF concentration gave a slope of one, indicating a single serine residue participates, and the same slope-of-one analysis for DCHC indicated that binding of one molecule of reagent suffices to inactivate the enzyme.

Fluorescence spectroscopy illuminated the physical environment of the catalytic tryptophan. Excited at 280 and 295 nanometers, AP-II emitted with a maximum at 313 nanometers, a blue-shifted signature indicating that the fluorescent tryptophans are buried in the hydrophobic interior of the protein rather than exposed to solvent. Quenching experiments with three agents of different charge, neutral acrylamide, positively charged cesium chloride, and negatively charged potassium iodide, revealed an unusual microenvironment. Acrylamide quenched the fluorescence completely, and remarkably, cesium ions achieved full accessibility as well, while iodide quenched far less effectively. The researchers concluded that the tryptophan microenvironment is strongly negatively charged, which explains why the positively charged cesium ion penetrates so readily while the negatively charged iodide is repelled. Stern-Volmer plots for acrylamide were linear, characteristic of purely dynamic quenching, whereas the upward-curving plots for cesium chloride and potassium iodide indicated a sphere-of-action static component or two populations of tryptophan residues quenched at different rates.

Structural studies rounded out the picture. Thermal denaturation caused activity loss accompanied by a red shift of the fluorescence maximum from 313 to 333 nanometers, the classic signature of protein unfolding exposing buried tryptophans to water. Chemical denaturation with guanidine hydrochloride abolished activity at concentrations as low as 0.2 molar, further supporting the involvement of carboxylate residues in catalysis. Denatured enzyme titrated with NBS revealed five total tryptophans in AP-II, of which two are surface-exposed and three are buried in the core. Circular dichroism spectroscopy showed that AP-II is composed of roughly 45 percent alpha-helix, 23 percent beta-sheet, and 31 percent random coil, and a sixth-degree polynomial fit of the far-UV CD spectrum achieved a coefficient of determination of approximately 0.89, which the authors interpreted as indicating greater structural stability of the major form. In silico structure prediction went further, suggesting a homodimeric enzyme of 102 kilodaltons with about 5.2 percent N-linked glycosylation and conserved histidine, tryptophan, aspartate, and tyrosine residues, an architecture typical of large plant acid phosphatases.

The authors are candid about the limits of their approach. The identification of active-site residues rests primarily on chemical modification rather than high-resolution crystallography, the three-dimensional structure is computationally predicted and awaits experimental validation, and the entire characterization was performed with the artificial substrate p-nitrophenyl phosphate, which may not mirror the enzyme’s physiological substrate preferences. Future work, they suggest, should include site-directed mutagenesis of the identified residues and testing against natural substrates. Even so, the study delivers a coherent and technically rigorous account of how three amino acids, a carboxylate acting in acid-base catalysis, a buried tryptophan stabilizing the substrate, and a serine participating in phosphoryl transfer, cooperate to hydrolyze phosphate esters in the acidic interior of a seed. It is a reminder that even a tree growing on a residential street in Pune can harbor molecular machinery of elegant precision, and that careful classical biochemistry, bolstered by modern statistics and computation, can still resolve that machinery one residue at a time.

Subject of Research: Identification of the catalytic carboxylate, tryptophan, and serine residues in acid phosphatase form II from Erythrina indica seeds

Article Title: Identification of catalytic residues of acid phosphatase form II from Erythrina indica seeds

Article References: Uzgare, A. S., & Shaikh, F. M. R. (2026). Identification of catalytic residues of acid phosphatase form II from Erythrina indica seeds. Discover Chemistry, 3(1), Article 488. https://doi.org/10.1007/s44371-026-00948-3

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00948-3

Keywords: acid phosphatase, Erythrina indica, active site, chemical modification, tryptophan, serine, carboxylate, enzyme kinetics, fluorescence spectroscopy, circular dichroism, plant biochemistry, catalytic mechanism

The team, led by Dr. Corinne Benakis, set out to understand how signals originating in the intestine reach the immune cells that help determine stroke outcome. Their starting point was an observation in mouse models of severe stroke: bacteria capable of producing a metabolite called indole became more abundant in the gut following the injury. Indole is a small molecule with a distinctive property that makes it a plausible messenger between microbes and the body — it is produced only in the gut, by bacteria that carry a gene known as tnaA. That gene encodes an enzyme allowing these microbes to convert tryptophan, an amino acid obtained from food, into indole. In other words, a dietary building block is transformed by bacterial metabolism into a signaling molecule that can act on the host immune system.

When the researchers examined what this meant for the brain, the results were striking. In mice, the presence of indole-producing bacteria worsened brain injury after stroke. This established a causal link rather than a mere correlation: the microbial metabolite was not simply a byproduct of the disease state but an active participant in shaping how much damage the stroke inflicted. The discovery adds stroke to a growing list of conditions in which gut-derived molecules have been shown to reach beyond the digestive tract and influence distant organs, often through the immune system.

The next question was mechanistic: how does a molecule made in the gut alter events in the brain? The answer lies with a receptor called the aryl hydrocarbon receptor, or AHR, a sensor found in dendritic cells. Dendritic cells are immune sentinels that patrol the intestinal lining, sampling their surroundings and helping to coordinate immune responses throughout the body. Indole, the team found, acts on AHR in these cells, and that interaction changes their behavior in ways that matter for stroke recovery.

To test this directly, the researchers switched off AHR specifically in dendritic cells. The effect on the mice was protective: without the receptor, they were better shielded against brain injury after stroke. The reason, according to the study, lies in how the cells behave once the indole signal is removed. Dr. Rosa Delgado Jiménez, who shares first authorship of the paper with Alexandria Ruggles and Mujeeb Adedokun, described the change in vivid terms. Dendritic cells, she explained, have long extensions, like arms, that allow them to sense changes in their environment, including metabolites produced by gut bacteria. When AHR was switched off, the behavior of these cells changed completely: they became more potent at migrating from the intestine to the meninges, the membrane surrounding the brain.

That migration proved to be consequential. Once in the meninges, the dendritic cells promoted protective T cells, a class of immune cells capable of dampening inflammation and tissue damage, and thereby helped reduce the injury in the brain. The picture that emerges is a chain of events running from the dinner plate to the brain’s outer membranes: dietary tryptophan is converted by tnaA-carrying bacteria into indole; indole engages AHR in intestinal dendritic cells; that engagement restrains the cells from migrating and from fostering protective T cells; and the resulting immune balance tilts toward greater brain damage after a stroke. Blocking the receptor breaks the chain at its immunological link and tips the balance back toward protection.

Crucially, the researchers found evidence that this pathway may not be a peculiarity of mice. In a cohort of human patients with ischemic stroke, the ISD team observed that a higher abundance of bacterial TnaA — the gene that enables indole production — was associated with poorer functional outcome. The patient samples for this analysis were provided by collaborating teams led by Fernández-Cadenas in Barcelona and Lorenz Hirt in Lausanne. The human data, while correlational, align with the mouse experiments and suggest that the same microbial machinery could be operating in people, influencing how well patients recover from stroke in the real world.

The study also uncovered a connection to stroke risk factors themselves. Bacteria carrying the tnaA gene and capable of producing indole were more abundant in people with conditions known to raise stroke risk, including obesity and type 2 diabetes. This observation hints at a possible mechanism by which metabolic disease could prime the immune system for a worse response to brain injury, though the researchers are careful to note that much remains to be established about whether the full pathway identified in mice also operates in patients.

The complexity of the work made collaboration essential. The study drew on the Transregional Collaborative Research Centre TRR 355, a research network funded by the German Research Foundation (DFG) that focuses on regulatory T cells. Benakis emphasized that the project required expertise a small group could not have covered alone: the TRR 355 network provided financial support and access to the knowledge of group leaders within the consortium. In addition, the team worked with Michael Zimmermann at EMBL and Michael Gigl at TUM to detect tryptophan metabolites and microbial indole, measurements that are technically challenging because of the low concentrations and complex chemistry involved. It was, as Benakis put it, a real team effort.

Beyond its immediate findings, the study opens a question that could reshape how scientists think about stroke prevention: could changing the gut microbiome, or the metabolites it produces, in healthy individuals help prepare the immune system to respond differently if a stroke occurs? Most stroke research and therapy focuses on the brain itself — clot removal, neuroprotection, rehabilitation — while the systemic immune context in which the injury unfolds has received far less attention as a target. If the gut’s contribution proves as influential in humans as it appears to be in mice, interventions ranging from diet to microbial or metabolic therapies could conceivably be used to tilt the immune system toward a protective configuration before a stroke ever happens, particularly in people at elevated risk. As Delgado Jiménez observed, stroke is usually thought of as an injury restricted to the brain, but the findings show that the gut can help shape the immune response to that injury. Benakis noted that the team still needs to determine whether the mechanism identified in mice also occurs in patients, but the work opens new avenues for investigating whether the gut microbiota could eventually be targeted to improve stroke outcome, including through preventive approaches in those most vulnerable.

Subject of Research: How the gut bacterial metabolite indole influences immune responses and brain damage after stroke

Article Title: How gut bacteria can shape the brain’s response to stroke

Article References: How gut bacteria can shape the brain’s response to stroke. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: stroke, gut microbiota, indole, tryptophan, aryl hydrocarbon receptor, dendritic cells, T cells, meninges, ischemic stroke, tnaA, immune response, LMU

Cathodoluminescence, or CL, is a spectroscopic technique in which electrons rather than photons excite a material, prompting it to emit light. The method has long been a workhorse for characterizing inorganic semiconductors, phosphors, and wide-bandgap materials, where defect-related emission and excitation dynamics are well understood. Organic molecules, however, have remained largely terra incognita. Their structural complexity, strong nonradiative relaxation pathways, and susceptibility to irradiation-induced chemical modification have made them difficult subjects. Yet the payoff for studying them is considerable: high-energy electrons generate a broad cascade of secondary electrons that produce excitation and ionization events fundamentally different from those triggered by a laser or a lamp, potentially granting access to electronic states that optical methods simply cannot reach.

The research team, led by Wataru Inami and Yoshimasa Kawata with colleagues including Machi Takahashi, Kentaro Nakajima, Yu Masuda, and Kei Hosomi, chose their molecular targets with care. They compared three classes of biologically relevant compounds: the aromatic amino acids tryptophan, tyrosine, and phenylalanine in both their D- and L-forms; the coenzymes NADH and FAD, central players in cellular metabolism; and the carbohydrates glucose, glycogen, and sodium glucose-6-phosphate. All samples were measured as powders mounted on carbon tape inside a scanning electron microscope, with a parabolic collection mirror gathering the emitted light and feeding it through an optical fiber to a spectrometer equipped with a photomultiplier tube. Measurements were performed in vacuum at an acceleration voltage of 10 kilovolts and a beam current of 1 nanoampere, with the beam raster-scanned over the sample to record spatially integrated spectra.

The results were unambiguous in their hierarchy. Aromatic amino acids outshone coenzymes, which in turn outshone carbohydrates. When intensities were normalized to sodium glucose-6-phosphate, the weakest emitter, L-tryptophan registered a relative signal of roughly 305, while L-tyrosine and D-phenylalanine came in at around 10 to 11, NADH at about 3, FAD near 1, and glucose and glycogen at under 2. In absolute terms, L-tryptophan emitted approximately 30 times more light than L-tyrosine and 27 times more than D-phenylalanine. The likely culprit behind this dramatic difference is the indole ring, the fused double-ring structure unique to tryptophan’s side chain, whose conjugated system of delocalized pi-electrons provides an efficient channel for both electron excitation and radiative relaxation.

But here is where the story takes a turn that should make microscopists sit up: the dominant emission from tryptophan appeared not in the near-ultraviolet, where its famous intrinsic fluorescence lives, but as a broad, asymmetric band centered around 570 nanometers in the visible spectrum. The authors caution that this visible band cannot be straightforwardly assigned to the conventional low-lying excited states of the indole chromophore that govern ordinary photoluminescence. Instead, they suggest it may arise from solid-state or irradiation-modified emissive states, possibly involving molecular packing effects or multiple overlapping transitions. Notably, both D- and L-tryptophan showed comparable near-UV emission around 340 nanometers, yet the visible band was markedly stronger in the L-isomer, evidence that the visible component is not simply a scaled copy of the molecule’s familiar fluorescence.

Time-resolved measurements added another layer of intrigue. Under continuous electron irradiation, the 570-nanometer band of L-tryptophan steadily declined over 30 minutes, suggesting that the visible-emitting states are progressively deactivated or degraded rather than continuously generated. L-tyrosine, by contrast, held remarkably stable, its luminescence barely changing over 35 minutes of bombardment. Then came the surprise: L-phenylalanine did the opposite of fading. Its CL intensity increased with irradiation time, most dramatically between 10 and 20 minutes, while its dominant emission peak shifted from about 500 nanometers to 430 nanometers, a blue shift of roughly 0.40 electron-volts toward higher energy. Something about the electron beam appears to generate or activate additional higher-energy emissive states in phenylalanine, a molecule whose intrinsic fluorescence is normally confined to the near-ultraviolet and far too weak to explain these broad visible bands.

What could account for such divergent behavior among three amino acids that share the same backbone? The authors point to a competition between destructive and constructive processes. Electron irradiation can induce radiolysis and other chemical modifications that degrade existing emissive states, as apparently happens in tryptophan. But it can also create new radiative centers, as the phenylalanine results imply. Previous work has shown that self-assembled phenylalanine exhibits aggregation-induced visible emission, hinting that intermolecular organization and molecular packing, not just molecular composition, shape the CL response. The differing 430-to-500-nanometer intensity ratios between D- and L-phenylalanine reinforce this picture: the visible emission likely involves multiple electron-beam-induced emissive states whose populations depend on solid-state structure and irradiation history. The researchers are careful to note that their measurements alone cannot distinguish among molecular rearrangement, changes in packing, phase transformation, or specific radiolysis products, and that structural characterization before and after irradiation, for example by Raman spectroscopy, would be needed to settle the question.

The coenzymes and carbohydrates, though fainter, told a consistent story. NADH produced a broad band peaking near 510 nanometers but at only about one-sixtieth the intensity of L-tryptophan, while FAD yielded barely detectable signal across the visible range. Glucose and glycogen, despite glycogen’s being a complex branched polymer of glucose units, emitted nearly identical broad spectra centered around 460 nanometers, a neat confirmation that their shared glucose-based composition dictates their spectral character. Sodium glucose-6-phosphate, a central metabolic intermediate, glowed only weakly, with a low signal-to-noise ratio. Across all three classes, the degree of aromatic conjugation emerged as the decisive variable: the more extended and delocalized the pi-electron system, the brighter the electron-beam-induced glow.

The practical implications extend well beyond basic molecular spectroscopy. Electron-beam-induced autofluorescence has already been demonstrated in living cells, and cathodoluminescence has been observed in functional proteins such as enhanced green fluorescent protein and even in biogenic materials like human kidney stones. As label-free electron-beam microscopy techniques mature, understanding which biomolecules light up, at what wavelengths, and how their emission evolves under bombardment becomes essential for interpreting images correctly. A tryptophan-rich region of a protein will not simply mimic its optical fluorescence signature; it will glow green-yellow at 570 nanometers and fade with dose, while a phenylalanine-rich region may brighten and blue-shift as the beam lingers. Irradiation history, in other words, is written into the spectrum itself.

The study also demonstrates that CL measurements can serve as a sensitive readout of irradiation-induced changes in soft organic materials, a capability with potential relevance for radiation chemistry, electron-beam lithography of organic films, and even assessing radiation damage in biological specimens. The authors emphasize that systematic measurements as a function of accelerating voltage and quantified electron dose will be needed to disentangle direct excitation from irradiation-induced modification. For now, the message is clear and a little humbling: the molecules of life respond to electron beams in ways that neither textbooks on fluorescence nor intuition from optical spectroscopy would predict, and each molecule carries its own signature of damage, resilience, and transformation under the electron’s gaze.

Subject of Research: Cathodoluminescence properties and electron-beam irradiation responses of biologically relevant organic molecules

Article Title: Electron-beam-induced responses of biologically relevant organic molecules

Article References: Inami, W., Takahashi, M., Nakajima, K., Masuda, Y., Hosomi, K., & Kawata, Y. (2026). Electron-beam-induced responses of biologically relevant organic molecules. Results in Chemistry, 31, Article 103940. https://doi.org/10.1016/j.rechem.2026.103940

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103940

Keywords: cathodoluminescence, tryptophan, amino acids, electron beam, biomolecules, spectroscopy, radiolysis, NADH, FAD, carbohydrates, fluorescence, scanning electron microscopy

The signal in question is the diffusible signaling factor, or DSF, chemically identified as cis-11-methyl-dodecenoic acid. DSF is the linchpin of quorum sensing in Xcc, the process by which bacterial populations gauge their density and synchronize the production of virulence factors, biofilms, and stress defenses. The molecule is synthesized by the enzyme RpfF through the fatty acid elongation cycle, sensed by the RpfC/RpfG two-component system, and funneled into the global regulator Clp. At late stages of growth, a fatty acyl-CoA ligase called RpfB activates the fatty acid beta-oxidation pathway to degrade DSF, effectively switching the conversation off. Because quorum sensing underpins nearly every aggressive behavior of the pathogen, any manipulation of DSF levels has profound consequences for disease.

To test whether IAA interferes with this system, the team, led by Si-Nan Li and Kai Song of Shanghai Jiao Tong University together with colleagues, grew the wild-type strain XC1 in Xylem-Sucrose medium, a formulation designed to mimic the nutrient environment inside plant xylem. Adding IAA at concentrations of 1 to 100 micromolar left bacterial growth untouched but slashed DSF levels dramatically. At 36 hours post-inoculation, DSF concentrations fell to 11.9, 9.2, and 1.4 percent of control levels at the three doses respectively. The effect held in a mutant that overproduces DSF, confirming that the hormone does not merely slow signal synthesis but actively accelerates its turnover.

Genetic dissection pinpointed RpfB as the indispensable executioner. In a strain lacking rpfB, 100 micromolar IAA failed to reduce DSF at all, while restoring a single chromosomal copy of the gene rescued the hormone’s effect. Intriguingly, IAA did not boost rpfB transcription, nor did it stimulate purified RpfB enzyme activity in vitro. The hormone was instead working through the cell’s physiology, and the next clue came from an unexpected quarter: pH.

Left alone in XYS medium, Xcc acidifies its surroundings, with culture pH dropping from 6.75 to 4.40 over 36 hours, and its cytoplasmic pH declining in parallel. IAA reversed this acidification in a dose-dependent manner. At 100 micromolar, the hormone significantly raised both culture and cytoplasmic pH by 24 hours, an effect sustained through 36 hours. This matters because prior work from the same group had shown that elevated cytoplasmic pH enhances RpfB-dependent DSF degradation. IAA, in other words, was not touching the degradation machinery directly; it was remodeling the intracellular environment so that the machinery works faster.

Transcriptome sequencing revealed how the hormone achieves this remodeling, identifying four IAA-regulated gene clusters. The hormone upregulated the atpABCDEFGH operon, encoding the F0F1 ATP synthase, by 2.6- to 5.2-fold. This bifunctional enzyme can run in reverse under acidic stress, hydrolyzing ATP to pump protons out of the cell and defend cytoplasmic pH. IAA also strongly induced hepABCD, an RND-family efflux pump shown previously to generate a proton motive force that raises cytoplasmic pH and stimulates RpfB activity. Digital PCR confirmed the RNA-seq patterns for representative genes, with hepB rising as much as 9.23-fold and atpB 4.96-fold at the highest IAA dose.

The most striking discovery, however, was a self-reinforcing biosynthetic loop. IAA induced the tryptophan synthase gene cluster trpB-A by 21- to 45-fold, driving production of L-tryptophan, the direct precursor of IAA in Xcc. Strains engineered to overexpress trpB-A accumulated more intracellular tryptophan and more IAA, and showed significantly reduced DSF levels, while deleting trpB in a DSF-overproducing background caused DSF to balloon to roughly 672 percent of control. The regulatory logic was completed by TrpI, a transcriptional activator adjacent to the operon: deletion of trpI abolished IAA’s induction of trpB-A, and electrophoretic mobility shift assays confirmed that TrpI binds the trpB promoter specifically. Yet adding IAA directly to the binding reaction changed nothing, meaning TrpI is essential but is not itself the IAA sensor, and the true receptor remains unidentified.

A fourth cluster added a second, pH-independent route to the same endpoint. IAA repressed iaepABCDE, an RND-family efflux pump the authors named the IAA-associated efflux pump, to as little as 0.06-fold of control levels. Deleting the cluster cut DSF levels by about 62 percent without altering culture or cytoplasmic pH, and the effect vanished in an rpfB mutant, showing that iaepABCDE normally suppresses RpfB-dependent turnover through a mechanism that does not involve proton pumping. Its substrate is still unknown, leaving a tantalizing gap in the pathway.

Critically, the laboratory findings translate to real infection. Using gusA reporter strains, the team detected promoter activity of all four IAA-regulated clusters, trpB-A, atpABCDEFGH, hepRABCD, and iaepABCDE, inside cabbage leaves five days after inoculation, confirming that the pathway operates during actual disease. The authors propose a dual-role model: Xcc exploits IAA to enhance its own virulence traits such as exopolysaccharide production and reactive oxygen species degradation, while the plant simultaneously deploys the hormone to erode quorum sensing and push the pathogen toward a quieter, more persistent lifestyle. The work also revealed synergy between IAA and the defense signal salicylic acid, hinting that multiple hormonal signals co-modulate bacterial behavior in planta. Because the study relied on a defined medium that simplifies the complex host environment, the authors caution that spatiotemporal IAA gradients, immune responses, and microbiome interactions still need validation in living plants, ideally using IAA-homeostasis mutants and real-time DSF reporters. Even so, the identification of a hormone-triggered feedback loop that amplifies its own precursor while dismantling bacterial communication opens a genuinely new front in the arms race between crops and one of their most devastating pathogens, and it suggests that auxin signaling could one day be manipulated to blunt quorum-sensing-dependent diseases across the roughly 400 plant species that Xanthomonas attacks.

Subject of Research: IAA-induced, RpfB-dependent turnover of the DSF quorum sensing signal in the phytopathogen Xanthomonas campestris pv. campestris

Article Title: The phytohormone indole-3-acetic acid induces quorum sensing signal DSF turnover via a positive feedback biosynthetic loop in the phytopathogen Xanthomonas campestris

Article References: Li, S.-N., Zhang, M.-L., Cui, Y., Li, L., Thawai, C., Jiang, L., Tian, D.-L., Gu, Y.-C., He, Y.-W., Zhou, L., & Song, K. (2026). The phytohormone indole-3-acetic acid induces quorum sensing signal DSF turnover via a positive feedback biosynthetic loop in the phytopathogen Xanthomonas campestris. Stress Biology, 6(1), Article 22. https://doi.org/10.1007/s44154-026-00298-1

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00298-1

Keywords: Xanthomonas campestris, indole-3-acetic acid, quorum sensing, DSF, RpfB, tryptophan, cytoplasmic pH, efflux pumps, plant hormone, black rot, plant-pathogen interaction, positive feedback loop

The biological rationale behind the trial was compelling. Many tumors defend themselves not just by hiding from immune cells but by actively poisoning the local environment in which those cells operate. One of the most notorious mechanisms involves indoleamine 2,3-dioxygenase-1, or IDO1, an enzyme that catabolizes the essential amino acid tryptophan into kynurenine. When IDO1 is highly active, tryptophan levels in the tumor microenvironment fall while kynurenine levels rise. This metabolic shift is doubly damaging: T cells deprived of tryptophan become dysfunctional and can even undergo stress responses, while kynurenine itself promotes immunosuppressive signaling. The ratio of kynurenine to tryptophan, often abbreviated Kyn/Trp, therefore serves as a convenient biochemical readout of IDO1 activity and of the metabolic hostility of a tumor’s surroundings.

To neutralize this mechanism, the investigators used epacadostat, known in earlier development as INCB024360, an oral small-molecule inhibitor of IDO1 supplied by Incyte Corporation. Patients received 300 milligrams twice daily starting on day zero. Three weeks later, beginning on day 21, they also received MELITAC 12.1, a multipeptide vaccine developed to stimulate T cells against multiple melanoma antigens simultaneously. The idea was a classic one-two punch: the vaccine would expand armies of T cells primed to recognize melanoma, while the IDO1 inhibitor would strip away the metabolic defenses that tumors use to disable those armies once they arrive. The trial enrolled eleven eligible patients with measurable regional or distant melanoma metastases.

A distinctive strength of the study was its insistence on looking inside the tumor, not just at the bloodstream. Patients underwent tumor biopsies at three time points: before treatment, on day 21, and on day 42. Some patients also underwent resection of all clinically evident disease by day 42, providing additional tissue for analysis. The prespecified primary immunologic endpoint was an increase in CD8-positive tumor-infiltrating lymphocytes, the cytotoxic T cells that can directly kill tumor cells, measured by immunohistochemistry. Circulating T cell responses were tracked using interferon-gamma ELIspot assays, while inhibition of IDO1 was assessed by measuring kynurenine and tryptophan levels in both serum and tumor tissue.

On the safety front, the combination performed respectably. Two of the eleven subjects, about 18 percent, experienced dose-limiting toxicities that were transient, and the regimen was otherwise well tolerated. This matters because combination immunotherapies often stack toxicities on top of one another, and a tolerable pairing preserves options for future development. The biochemical data were likewise encouraging. Epacadostat normalized serum kynurenine-to-tryptophan ratios in 91 percent of subjects, demonstrating that the drug was doing exactly what it was designed to do systemically. Perhaps more importantly, the drug also reduced Kyn/Trp ratios inside the tumors themselves, although pretreatment intratumoral ratios varied widely from patient to patient, hinting that IDO1-driven immunosuppression may be relevant in some tumors but not others.

Here, however, the trial’s central hypothesis ran aground. The prespecified primary endpoint, an increase in total CD8-positive T cell infiltration within tumors, was not met. Blocking IDO1, whether alone or in combination with the vaccine, did not meaningfully increase the number of cytotoxic T cells crowding into the tumor microenvironment. This finding lands as part of a broader reckoning with the IDO1 field. Enthusiasm for IDO1 inhibitors was once enormous, but large randomized trials of epacadostat in combination with checkpoint inhibitors in melanoma failed to improve clinical outcomes, and the present study adds mechanistic texture to that disappointment: the drug reliably lowers kynurenine, yet lowering kynurenine alone does not summon more killer T cells into tumors.

The vaccine component also produced puzzling results. In one subject, a day-42 tumor contained T cells reactive to peptides included in the vaccine, a tantalizing sign that vaccine-primed cells could reach tumor tissue. Unfortunately, no baseline tumor sample was available from that patient, making it impossible to determine whether those cells had been recruited or expanded as a result of treatment. More broadly, circulating T cell responses measured by ELIspot were lower than those seen in prior studies of the vaccine given alone, raising the possibility that the IDO1 inhibitor, or the sequencing of the two treatments, somehow blunted rather than enhanced vaccine-induced immunity. That counterintuitive observation alone justifies further mechanistic study.

Clinically, the trial was too small to draw firm conclusions about efficacy, but the signals were modest at best. Among five patients evaluable for clinical response, the best response was a partial remission in one subject, stable disease in three, and progressive disease in one. The partial response was transient. The authors conclude that these findings do not support routine combination of the MELITAC 12.1 vaccine with epacadostat, an honest negative result that spares future patients from an unpromising regimen and redirects resources toward more rational strategies.

Yet the study is far from a dead end. The observation that pretreatment intratumoral Kyn/Trp ratios varied widely suggests a biomarker-driven path forward: rather than giving IDO1 inhibitors to everyone, clinicians might identify the subset of patients whose tumors are genuinely dependent on IDO1-mediated metabolic suppression and treat only those individuals. The authors explicitly note that there may be value in targeted study of IDO1 inhibitors for tumors with the highest kynurenine-to-tryptophan levels. This kind of sequential tumor biopsy design, sampling the microenvironment before and during therapy, is exactly the sort of translational rigor that turns failed trials into roadmaps.

The trial also stands as a testament to collaborative infrastructure. It was funded by an NIH/NCI grant supporting the Cancer Immunotherapy Trials Network, with additional support from cancer center grants at the Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium and the University of Virginia Cancer Center, along with philanthropic support for vaccine peptide production. Specialized analyses of gene expression, IDO1 content, and tumor metabolites were performed with partners including NanoString Technologies and ImaBiotech. In an era when immunotherapy headlines often celebrate dramatic successes, this study is a reminder that negative results, when paired with deep biological measurement, are among the most instructive outputs of clinical science. Eleven patients, three biopsies each, and a meticulously measured metabolic axis have clarified where one immunotherapeutic combination does not work, and pointed toward where it still might.

Subject of Research: A phase II trial combining an IDO1 inhibitor with a multipeptide vaccine in advanced melanoma

Article Title: A phase II pilot trial of an indoleamine 2,3-dioxygenase-1 (IDO1) inhibitor (INCB024360) plus a multipeptide melanoma vaccine (MELITAC 12.1) in patients with advanced melanoma, with sequential evaluation of the tumor microenvironment

Article References: Slingluff, C. L., Ernstoff, M. S., Lawson, D., Delman, K. A., Hanks, B. A., Gastman, B., Mauldin, I. S., Olson, W. C., Ninmer, E. K., Gradecki, S. E., Smith, K. T., Gru, A., Ramchurren, N., Morishima, C., Fling, S. P., Kaiser, J. C., & Cheever, M. A. (2026). A phase II pilot trial of an indoleamine 2,3-dioxygenase-1 (IDO1) inhibitor (INCB024360) plus a multipeptide melanoma vaccine (MELITAC 12.1) in patients with advanced melanoma, with sequential evaluation of the tumor microenvironment. Cancer Immunology, Immunotherapy. https://doi.org/10.1007/s00262-026-04572-z

Image Credits: AI Generated

DOI: 10.1007/s00262-026-04572-z

Keywords: melanoma, IDO1, epacadostat, cancer vaccine, immunotherapy, tumor microenvironment, kynurenine, tryptophan, CD8 T cells, clinical trial, Cancer Immunotherapy Trials Network, tumor-infiltrating lymphocytes

The scope of the stress problem in aquaculture is enormous. Farmed fish face environmental fluctuations such as temperature swings, poor water quality, and salinity changes, alongside physiological challenges including handling, transportation, and overcrowding. Disease outbreaks and suboptimal feeding practices compound the burden. When fish perceive a threat, their bodies activate the hypothalamus-pituitary-interrenal axis, releasing cortisol and catecholamines—the primary stress hormones in teleosts. While these responses are adaptive in the short term, chronic activation leads to immunosuppression, impaired growth, altered behaviour, and oxidative damage. The metabolic demands of stressed fish rise sharply, dramatically increasing their need for amino acids to sustain cellular functions, support immune responses, and maintain overall health.

Among the most extensively studied functional amino acids is glutamine, a non-essential amino acid that becomes conditionally essential during stress. Glutamine serves as the primary energy source for rapidly dividing cells, particularly those of the immune system and gastrointestinal tract. When fish endure poor water quality or overcrowding, their glutamine reserves can deplete, compromising gut integrity and immune function. Supplementation with L-glutamine supports the growth and repair of enterocytes, the cells lining the intestine, preventing the translocation of pathogens and toxins into the bloodstream. Research cited in the review shows that dietary glutamine enhanced growth performance in Nile tilapia under chronic hyperosmotic stress by boosting antioxidant capacity and improving osmoregulation, although one study found no growth benefit for Atlantic salmon parr under high temperatures, underscoring that responses are species- and context-dependent.

Tryptophan offers perhaps the most striking example of how a single amino acid can reshape behaviour and physiology. As the precursor to serotonin, L-tryptophan influences mood, aggression, and the stress response itself. Serotonin exerts an inhibitory effect on the HPI axis, reducing cortisol release during challenging events such as transport or high stocking density. In Atlantic salmon, dietary supplementation with L-tryptophan or melatonin effectively mitigated stress-induced cortisol elevation, enhanced antioxidant defences, and improved osmoregulatory function following acute handling stress. Tryptophan also feeds the kynurenine pathway, which produces immunosuppressive metabolites that prevent excessive inflammation, and serves as a precursor to melatonin, the neurohormone governing sleep and circadian rhythms essential for recovery.

Arginine operates through an entirely different but equally vital mechanism: the L-arginine-nitric oxide pathway. The enzyme nitric oxide synthase converts arginine into nitric oxide, a signalling molecule that promotes vasodilation, improving blood flow, oxygen delivery, and waste removal in stressed tissues. Nitric oxide also modulates the sensitivity of inter-renal cells to adrenocorticotropic hormone and activates macrophages for pathogen defence. Beyond circulation, arginine supports ammonia detoxification through the urea cycle—critical when stress accelerates protein breakdown—and stimulates the proliferation of lymphocytes and macrophages. Studies in common carp, turbot, and Indian major carps have consistently shown that arginine supplementation improves stress resilience, lowers plasma cortisol, and enhances immune parameters.

The antioxidant dimension of amino acid nutrition centres on the glutathione system, and here cysteine and methionine take centre stage. Glutathione, a tripeptide composed of glutamine, cysteine, and glycine, is one of the most important intracellular antioxidants in fish, neutralizing the reactive oxygen species that accumulate during stress. Methionine, an essential sulfur-containing amino acid, is first converted to S-adenosylmethionine, a universal methyl donor, before feeding cysteine and ultimately glutathione synthesis. N-acetyl cysteine, a supplemental form of cysteine, facilitates de novo glutathione production by gamma-glutamylcysteine synthetase, the rate-limiting enzyme in the pathway, and accelerates glutathione regeneration through glutathione reductase. Histidine adds further antioxidant firepower, with its imidazole group capable of directly scavenging free radicals; histidine-deficient grass carp showed increased red blood cell fragility and reduced hypoxia tolerance.

Other amino acids play more specialized protective roles. Taurine, a sulfur-containing compound not incorporated into proteins, functions as a key osmolyte for marine and brackish water species, regulating ion transport across gill and kidney epithelial cells to maintain cellular osmotic balance during salinity fluctuations. It also stabilizes cell membranes, scavenges reactive oxygen species, and supports cardiac and nervous system function. Proline acts as an osmoprotectant shielding cellular membranes and proteins from damage, while contributing to collagen synthesis for tissue repair. Leucine, a branched-chain amino acid, stimulates the mTOR signalling pathway to preserve muscle protein during catabolic stress, though the review cautions that excessive leucine can suppress feed intake through amino acid sensing mechanisms in the brain. Glycine, the simplest amino acid, acts as an inhibitory neurotransmitter that calms nervous system excitability, reducing anxiety and aggression.

The review also highlights tyrosine, the precursor to the catecholamine stress hormones dopamine, norepinephrine, and epinephrine. Although tyrosine can be synthesized from phenylalanine, endogenous production may become insufficient during rapid growth or physiological stress. Studies in stinging catfish fingerlings identified an optimal phenylalanine-to-tyrosine ratio of roughly 65:35 for growth, antioxidant status, and digestive enzyme activity, while research in mrigal fingerlings established a total aromatic amino acid requirement of 21.5 grams per kilogram of diet, with tyrosine able to replace about 36 percent of the phenylalanine requirement. Tyrosine additionally supports melanin production for camouflage and cognitive functions aiding adaptation to changing environments.

Beyond individual mechanisms, the review emphasizes how amino acids intersect with energy metabolism and growth under stress. When carbohydrate and lipid reserves are depleted, fish increasingly rely on amino acids as fuel, converting them to glucose through gluconeogenesis or oxidizing them in the tricarboxylic acid cycle to generate ATP. Glutamine feeds the TCA cycle via alpha-ketoglutarate, while branched-chain amino acids are catabolized in skeletal muscle to produce acetyl-CoA and succinyl-CoA. This metabolic flexibility allows fish to maintain energy homeostasis, but only if dietary amino acid supplies are adequate. The authors argue that strategically timed supplementation—initiated before or during anticipated stress events—can prevent the muscle wasting, immunosuppression, and growth impairment that characterize chronic stress.

The practical implications extend well beyond fish welfare. As aquaculture shifts toward plant-based proteins for sustainability reasons, diets increasingly lack certain essential amino acids and contain anti-nutritional factors that stress the gut mucosa. Supplementing limiting amino acids such as lysine, methionine, and threonine restores amino acid balance, improves feed conversion ratios, and allows feed manufacturers to reduce fishmeal use, easing pressure on marine ecosystems. Improved nitrogen utilization also lowers nitrogenous waste output, a critical concern in intensive systems. Although crystalline amino acids carry a cost premium, the review concludes that their benefits—better stress tolerance, reduced mortality, and enhanced productivity—make them not merely a nutritional necessity but a genuine sustainability tool. Advances in microbial fermentation are further improving the affordability and environmental footprint of amino acid production, positioning these remarkable molecules at the heart of the future of responsible fish farming.

Subject of Research: The role of dietary amino acids in mitigating stress in aquaculture species

Article Title: Amino acids as functional nutrients in stress mitigation of aquatic species: mechanisms and applications in aquaculture

Article References: Chuphal, N., Malik, M. A., Kishore, P. S., & Mohanta, K. N. (2025). Amino acids as functional nutrients in stress mitigation of aquatic species: mechanisms and applications in aquaculture. Blue Biotechnology, 2(1), Article 20. https://doi.org/10.1186/s44315-025-00040-y

Image Credits: AI Generated

DOI: 10.1186/s44315-025-00040-y

Keywords: amino acids, aquaculture, fish stress, glutamine, tryptophan, arginine, taurine, glutathione, cortisol, osmoregulation, fish welfare, sustainable aquafeeds