Autophagy, in its textbook form, is a beautifully choreographed process. When a cell is starved or stressed, a cascade of autophagy-related proteins, the ATG family, assembles a double-membraned vesicle called an autophagosome around cytoplasmic cargo. The hallmark protein ATG8, known as LC3 in mammals, decorates the growing membrane and helps tether and engulf the cargo. The completed vesicle then fuses with lysosomes, where acidic hydrolases dismantle the contents into raw materials the cell can reuse. Giardia, however, lacks most ATG genes, including ATG8 itself, which led many researchers to conclude that this parasite, with its famously simplified cellular architecture, simply does not perform autophagy. The new work, led by Angélica Hollunder Klippel and Alexander R. Paredez and their colleagues, overturns that assumption by revealing a degradative compartment system that behaves remarkably like autophagy despite lacking its familiar molecular cast.

The team’s central discovery is that GlRac marks and regulates a population of double-membraned compartments that arise under precisely the conditions that trigger autophagy in other organisms. When Giardia cells are deprived of nutrients or induced to encyst, the developmental program by which the parasite transforms into its hardy cyst form, GlRac-positive compartments accumulate in the cytoplasm. When amino acids are restored to the medium, these structures rapidly disappear. Notably, glucose replenishment has no such effect, indicating that the pathway is tuned specifically to amino acid availability rather than to general energy status. That kind of nutrient-specific regulation is a signature feature of canonical autophagy, and finding it in an organism without ATG8 suggests that the logic of the pathway has been conserved even as its molecular components have been replaced beyond recognition.

The regulatory circuitry extends one step further upstream to Giardia’s Target of Rapamycin, or GTOR, the parasite’s counterpart of the TOR kinase that serves as the master nutrient sensor in eukaryotes from yeast to humans. The researchers found that GTOR protein levels drop during nutrient depletion, mirroring the rise in GlRac-positive compartments. When they knocked down GTOR, compartment abundance increased, even in the absence of starvation. This positions GTOR as a negative regulator of the pathway: as long as nutrients are plentiful and GTOR is active, compartment formation is suppressed, but when GTOR signaling wanes, the degradative compartments bloom. The parallel with canonical autophagy is striking, because TOR inhibition is the canonical trigger for autophagy induction across the tree of life. Giardia appears to have preserved this regulatory architecture while swapping out the execution machinery downstream.

Live-cell time-lapse microscopy allowed the team to watch the compartments form in real time, and the footage revealed a morphological sequence familiar to anyone who has studied autophagosome biogenesis. The structures first appear as linear profiles, then curve into cup-shaped intermediates resembling the phagophores, or isolation membranes, of classical autophagy, before sealing into spheres. Upon nutrient replenishment, the mature compartments are cleared from the cell. This progression from flat membrane to curved cup to closed vesicle is precisely how autophagosomes are thought to grow and complete in ATG8-bearing organisms, making it all the more remarkable that Giardia accomplishes the same geometric feat without the membrane-deformation proteins usually credited with driving it.

To test whether any remnant of the canonical machinery might be involved, the researchers examined nine putative ATG orthologs encoded in the Giardia genome. None of them localized to the GlRac-positive compartments with anything approaching GlRac’s specificity, supporting the conclusion that the pathway is genuinely ATG8-independent and highly divergent. Yet the compartments themselves display a suite of features that are unmistakably autophagy-like. They are bounded by double membranes, the defining ultrastructural trait of autophagosomes. They recruit actin, they undergo acidification, and they harbor cysteine protease activity, the enzymatic workhorse of intracellular degradation. In other words, the output of the pathway looks like autophagy even though the input machinery does not.

Pharmacological experiments reinforced the idea that these compartments are sites of continuous degradative turnover rather than static storage structures. When the team blocked cysteine proteases with the inhibitor E-64d, or when they prevented vacuolar acidification by targeting the V-ATPase proton pump with concanamycin A, the compartments accumulated in the cells. Both outcomes are consistent with a system in which cargo is constantly being delivered, degraded, and cleared, and in which disrupting any stage of that pipeline causes intermediates to pile up. The same logic underlies classic autophagy assays in model organisms, where lysosomal inhibitors cause autophagosomes to accumulate because the downstream disposal step has been jammed.

GlRac itself proved to be a bidirectional control switch for the pathway. When the researchers engineered a constitutively active version of the GTPase, compartment abundance and size both increased, indicating that active GlRac drives biogenesis. Conversely, knocking down GlRac reduced the number of compartments, showing that the protein is required for their formation in the first place. This places GlRac, the sole Rho family GTPase in Giardia, in a role reminiscent of the small GTPases that regulate membrane trafficking and autophagy in other eukaryotes, but here concentrated into a single multifunctional protein in an organism with an unusually stripped-down signaling repertoire. The finding also hints that Rho GTPase involvement in autophagy-like membrane remodeling may be more ancient than the ATG8-centric version of the pathway familiar from model systems.

The work carries a potential translational payoff. Quinacrine, an FDA-approved drug long used against Giardia infections, accumulates in acidic organelles, and the researchers found that it perturbs the GlRac-positive compartments, consistent with reports that quinacrine disrupts autophagy in other eukaryotes. If the autophagy-like pathway contributes to parasite fitness, as the authors suggest, then the compartments may represent a previously unrecognized vulnerability that existing drugs already exploit, and one that new therapeutics could target more precisely. More broadly, the study is a reminder that evolutionary simplification can be deceptive. Giardia has not lost the function of autophagy; it has rebuilt it from unfamiliar parts, with a lone Rho GTPase standing in for an entire protein family. For evolutionary cell biologists, the parasite now offers a living experiment in how a fundamental eukaryotic process can be re-engineered, and for parasitologists, it opens a fresh front in the search for drugs against one of the world’s most common causes of diarrheal disease.

Subject of Research: An ATG8-independent autophagy-like pathway regulated by the Rho GTPase GlRac in the parasite Giardia lamblia

Article Title: Gl Rac regulates an ATG8-independent autophagy-like pathway in Giardia lamblia

Article References: Klippel, A. H., Newman-Boulle, C., Reed, G., Dule, D., Kedia, S., Kelty, M. T., Chan, W. P., Shih, H.-W., & Paredez, A. R. (2026). GlRac regulates an ATG8-independent autophagy-like pathway in Giardia lamblia. PLOS Pathogens, 22(10), e1014624. https://doi.org/10.1371/journal.ppat.1014624

Image Credits: AI Generated

DOI: 10.1371/journal.ppat.1014624

Keywords: Giardia lamblia, autophagy, ATG8-independent pathway, GlRac, Rho GTPase, GTOR, nutrient sensing, encystation, double-membrane compartments, cysteine proteases, quinacrine, PLOS Pathogens

The research, led by Ana Cristina Reguera and Rosalía Rodríguez-Rodríguez at the Universitat Internacional de Catalunya, together with colleagues in Spain, Austria, and the United Kingdom, focuses on an unusual phospholipid known as bis(monoacylglycero)phosphate, or BMP. Unlike most cellular lipids, BMP is not scattered across membranes; it is almost exclusively found in late endosomes, the acidic compartments where cellular cargo is sorted, degraded, and recycled. BMP is required for the formation of intraluminal vesicles, the small bubbles that carry cargo into the interior of these compartments, and it plays a central role in cholesterol handling and lysosomal function. Without adequate BMP, the late endosomal system essentially loses its operating fluid.

What has puzzled researchers is how neurons maintain the right amount of this specialized lipid. BMP is chemically unusual, resistant to many common phospholipases, but it does have one known executioner: an enzyme called ABHD6, alpha-beta hydrolase domain containing 6. ABHD6 can hydrolyze BMP, breaking it down. The new work shows that the activity of this hydrolase is not constant; it is tuned by the metabolic state of the cell, and the tuning knob is CPT1C, a brain-specific member of the carnitine palmitoyltransferase family that sits in the endoplasmic reticulum and is known to respond to nutrient and energy cues.

CPT1C is an intriguing protein in its own right. Unlike its relatives CPT1A and CPT1B, which drive fatty acid import into mitochondria, CPT1C has negligible transferase activity. Instead, it behaves as a lipid and energy sensor, accumulating in neurons during fasting and low-energy states and helping neurons adapt their metabolism accordingly. The new study reveals an unexpected downstream target for this sensor: by restraining ABHD6, CPT1C protects the BMP pool that late endosomes depend on. In other words, a protein long studied for its role in energy sensing turns out to be a critical regulator of the lipid composition of the degradative compartments themselves.

The experimental logic of the study is straightforward and compelling. When the researchers removed CPT1C from neurons, ABHD6 activity rose, intracellular BMP levels fell, and the endolysosomal system began to falter. The consequences were measurable at several points along the pathway. Endosomal maturation was impaired, meaning early endosomes failed to progress properly into functional late compartments. The release of extracellular vesicles, including exosomes, which bud from late endosomes and depend on intraluminal vesicle formation, was reduced. Lysosomes lost their normal acidity, and their proteolytic activity, the enzymatic digestion of cargo, declined. Each of these defects is consistent with a shortage of the BMP that late endosomes and lysosomes need to build and maintain their internal architecture.

Crucially, the team showed that the problem was not a failure to synthesize BMP but an acceleration of its destruction. When they acutely inhibited ABHD6 in CPT1C-deficient neurons, BMP levels recovered, and the endolysosomal defects were rescued. This pharmacological rescue is one of the strongest signals in the paper, because it establishes a clear causal chain: CPT1C loss elevates ABHD6 activity, elevated ABHD6 depletes BMP, and BMP depletion undermines endolysosomal function. Blocking the hydrolase reverses the cascade at its source.

The nutrient dimension of the story emerged when the researchers exposed cells and animals to lipid overload. In wild-type neurons and in the hypothalamus of mice fed excess fat, ABHD6 activity increased and BMP levels dropped, mirroring the phenotype seen in CPT1C-deficient cells. But there was a telling difference: CPT1C-deficient neurons and mice already displayed constitutively elevated ABHD6 activity, and when faced with additional lipid excess, they failed to adapt further. Their regulatory range was exhausted. This suggests that CPT1C does not merely set a baseline for BMP homeostasis; it provides the adaptive capacity that neurons need when the lipid environment changes, as it does in high-fat diets and obesity.

Why does this matter for whole-body metabolism? The hypothalamus is the command center that integrates hormonal and nutrient signals to regulate food intake, energy expenditure, and glucose homeostasis. If lipid overload degrades the endolysosomal health of hypothalamic neurons, it could compromise the very cells responsible for sensing metabolic state and orchestrating the response. Defective lysosomal acidification and proteolysis would impair the degradation of proteins and organelles, including through autophagy, while reduced exosome release could alter intercellular communication within hypothalamic circuits. The study does not claim to have demonstrated these downstream physiological effects, but it establishes the molecular lesion, BMP depletion through unrestrained ABHD6, that could plausibly feed into hypothalamic dysfunction in metabolic disease.

The findings also connect to a broader and rapidly growing literature on lysosomal lipids in neurobiology. BMP has attracted intense interest because it is the receptor for a family of DNAJ proteins that stimulate lysosomal activity, and because altered BMP metabolism has been implicated in neurodegenerative conditions in which lysosomal failure is a hallmark. Neurons are particularly vulnerable to lysosomal dysfunction because they are long-lived and cannot dilute accumulated damage by cell division. A pathway in which a nutrient sensor controls the abundance of the signature lipid of the degradative pathway therefore offers a direct mechanistic bridge between diet, neuronal metabolism, and cellular housekeeping.

Therapeutically, the study points to ABHD6 as a druggable node. Pharmacological ABHD6 inhibition restored BMP levels in CPT1C-deficient neurons, raising the possibility that modulating this hydrolase could protect endolysosomal function in settings of chronic lipid excess. ABHD6 inhibitors have already been explored in other contexts, including endocannabinoid signaling, which gives medicinal chemists a starting point. The authors caution, implicitly, that much remains to be worked out: how CPT1C physically restrains ABHD6, whether the axis operates in other neuronal populations, and whether restoring BMP in vivo can correct hypothalamic dysfunction in obesity models. But the identification of a nutrient-sensitive CPT1C-ABHD6 axis governing BMP homeostasis gives researchers a concrete molecular handle on a process that had been almost entirely opaque. It reframes CPT1C not just as a metabolic sensor at the endoplasmic reticulum but as a custodian of the neuronal recycling system, and it suggests that the road from a fatty diet to faulty cellular waste disposal in the brain may run through a single, targetable enzyme.

Subject of Research: Nutrient-sensitive regulation of BMP lipid homeostasis and endolysosomal function in hypothalamic neurons via the CPT1C-ABHD6 axis

Article Title: A nutrient-sensitive CPT1C-ABHD6 axis regulates BMP homeostasis and endolysosomal function in hypothalamic neurons

Article References: Reguera, A. C., Miralpeix, C., Bernard, M., Zagmutt, S., Bolaños-Hurtado, M., Palacín, C., Fuentes, A., Rodríguez-García, M., Fosch, A., Garcia-Chica, J., Barnadas-Rodríguez, R., Fawzy, N., Zimmermann, R., Eden, E., Casals, N., & Rodríguez-Rodríguez, R. (2026). A nutrient-sensitive CPT1C-ABHD6 axis regulates BMP homeostasis and endolysosomal function in hypothalamic neurons. Cellular and Molecular Life Sciences. https://doi.org/10.1007/s00018-026-06451-3

Image Credits: AI Generated

DOI: 10.1007/s00018-026-06451-3

Keywords: CPT1C, ABHD6, BMP, endolysosomal system, hypothalamus, neurons, lipid metabolism, lysosomes, nutrient sensing, exosomes, obesity, endosomes

The finding matters because mTORC1, the mechanistic target of rapamycin complex 1, sits at the very hub of how cells interpret the nutritional environment. When amino acids are abundant, mTORC1 switches on growth programs; when nutrients are scarce, it falls silent and cells shift toward maintenance and repair. Decades of work, from yeast to mice, have established that dampening this pathway, whether by drugs such as rapamycin or by dietary restriction, reliably extends lifespan. Yet mTORC1 is not a single lever but a control panel, and different outputs of the pathway can be engaged or disengaged independently. Understanding which specific downstream branch actually drives aging, rather than growth, has been a stubborn problem in the field.

One of mTORC1’s most enigmatic outputs is the preferential translation of a special class of messenger RNAs known as TOP mRNAs, so named for the 5′ terminal oligopyrimidine motif that caps them. These transcripts overwhelmingly encode ribosomal proteins, the structural components of the protein-synthesizing machinery itself. Because TOP mRNAs are extraordinarily abundant, they impose a heavy translational burden on the cell, and their regulation is notoriously resistant to rapamycin, the canonical mTORC1 inhibitor. In other words, even when rapamycin shuts down most mTORC1 outputs, TOP mRNA translation marches largely on. How this branch is physiologically tuned, and whether it connects to lifespan at all, was unknown.

The new study began with a systems-level observation. By comparing transcriptomic datasets from flies subjected to different diets, ages, and genetic perturbations, the researchers noticed that Lsp2 expression tracked nutrient status with striking fidelity. Feeding flies yeast, a rich source of essential amino acids, potently induced Lsp2, and subsequent dissection showed that this induction ran through mTORC1 itself. Essential amino acids, particularly the branched-chain amino acids leucine, isoleucine and valine, drove Lsp2 expression in the fly’s fat body, an organ that combines the functions of liver and adipose tissue. But the induction was not automatic: it was gated by additional layers of nutrient and hormonal signaling, including insulin signaling, ecdysone, and stress-responsive factors such as FoxO and ATF4, which suppressed the response. Lsp2, in effect, acts as an integration point that only fires when multiple independent signals agree that conditions are truly plentiful.

Then came the surprise. Rather than being a passive storage protein, Lsp2 feeds back into the mTORC1 pathway as an activator, sustaining the phosphorylation of the translational repressor 4E-BP even under conditions where other mTORC1 outputs have been curtailed. 4E-BP is a well-studied brake on translation initiation: when unphosphorylated, it binds the cap-binding protein eIF4E and blocks the assembly of translation initiation complexes. mTORC1 phosphorylation disables 4E-BP, freeing the translational machinery. The team showed that Lsp2 specifically promotes the phosphorylation of 4E-BP in mature adult flies, thereby keeping TOP mRNA translation running at full tilt, while leaving earlier-life mTORC1 activity essentially untouched. This adult-specific, rapamycin-resistant mode of regulation suggested that Lsp2 occupies a unique position in the aging-relevant arm of the pathway.

The functional consequences were dramatic. Flies engineered to lack Lsp2, either through a transposon-derived mutant allele or through adult-specific RNA interference, lived substantially longer than controls across multiple diets and genetic backgrounds. Crucially, the long-lived mutants did not show the trade-offs that often accompany lifespan extension. Egg laying, climbing ability, spontaneous locomotion, and resistance to starvation, oxidative stress, and bacterial infection were all preserved. Developmental timing was normal. The authors even found that Lsp2 acts as an adipokine that limits the release of insulin-like peptide 2 from the brain, yet systemic insulin signaling was not detectably altered. The lifespan effect appeared to be channeled through a specific molecular route rather than a general slowing of metabolism.

That route was revealed by translatomic profiling, a suite of techniques including polysome profiling and ribosome-associated sequencing that measure which mRNAs are actually being translated, not merely transcribed. In Lsp2-deficient flies, the translation of TOP mRNAs, above all the mRNAs encoding cytoplasmic ribosomal proteins, dropped selectively, while the bulk of the transcriptome was barely affected. Polysome profiles revealed a shift in the balance of ribosomal subunits, with an increase in free 60S subunits and a decrease in 40S subunits, consistent with impaired production of the small-subunit components encoded by TOP mRNAs. Genetic epistasis nailed down the mediator: when the gene encoding 4E-BP, called Thor, was deleted, the lifespan extension and the translational changes of Lsp2 loss were both abolished. Lsp2 therefore extends lifespan by relieving an Lsp2-dependent, rapamycin-resistant drive on 4E-BP phosphorylation, which in turn restrains TOP mRNA translation.

The study also resolves a long-standing question about TOP motifs themselves. In mammals, 5′ TOP motifs are the textbook hallmark of ribosomal protein mRNAs, but whether fruit flies possessed functional TOP motifs had been uncertain. By mining a large collection of CAGE-seq datasets, which precisely map transcription start sites across seven species, the researchers showed that TOP motifs are present in nearly all Drosophila ribosomal protein mRNAs and that they co-emerged evolutionarily with 4E-BP itself. This co-evolution makes elegant mechanistic sense: a translational control element is only meaningful in an organism that also carries the repressor that reads it. The conservation of this regulatory logic across hundreds of millions of years underscores how central the governance of ribosome biogenesis is to life history.

The broader implications reach well beyond flies. Rapamycin extends lifespan across species, yet its clinical translation has been complicated by side effects stemming from broad pathway suppression. The Lsp2 study suggests a cleaner target: if a specific, drug-resistant branch of mTORC1 signaling, channeled through 4E-BP and TOP mRNA translation, is what actually drives aging, then interventions aimed selectively at that branch might capture longevity benefits while sparing the pathway’s necessary growth and immune functions. Notably, the researchers found that long-lived Lsp2 mutants were still further responsive to rapamycin, implying that the two interventions act through partly distinct mechanisms and could in principle be combined. Whether mammals harbor an equivalent adipose-derived amplifier of mTORC1 output is now the obvious question, and the answer could reshape how the field thinks about the connection between diet, protein synthesis, and the biology of growing old.

Subject of Research: The role of the Lsp2 protein in linking mTORC1 signaling, TOP mRNA translation, and lifespan regulation in Drosophila.

Article Title: Lsp2 links mTORC1 to TOP mRNA translation and lifespan in Drosophila

Article References: Wang, J., Cai, Z., Gu, J., Xiong, S., Yi, J., Yang, M., Chang, K., Ning, X., Wen, Y., Yan, Y., Lu, J., Wang, Y., & Zhai, Z. (2026). Lsp2 links mTORC1 to TOP mRNA translation and lifespan in Drosophila. Nature. https://doi.org/10.1038/s41586-026-11029-x

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11029-x

Keywords: Lsp2, mTORC1, TOP mRNAs, 4E-BP, Drosophila, lifespan, aging, translation, ribosomal proteins, nutrient sensing, rapamycin, fat body