eLife

Fossil central nervous systems (CNS) have illuminated several facets of total-group euarthropod evolution, including their phylogenetic affinities, the origin of the head, and their ancestral neuroanatomical organization. However, the CNS remains unknown in trilobites, the most diverse and earliest group of Palaeozoic euarthropods, but can potentially clarify fundamental questions that remain disputed despite thousands of described species and countless studies spanning over two centuries, such as their affinities relative to modern representatives. Here, we describe the CNS of the cheirurid Anacheirurus adserai from the Lower Ordovician Fezouata Shale. Anacheirurus features a brain with well-developed proto, deuto- and tritocerebral neuromeres, an anteriomedian protocerebral protrusion, a central complex, an esophageal foramen, and a ganglionated ventral nerve cord. The CNS is replicated in framboidal iron that differs in texture and composition from the exoskeleton and rock matrix. The overall organization of Anacheirurus's CNS most closely resembles that of Cambrian stem-group mandibulates, and critically shares the presence of a central complex with a protocerebral bridge and lateral accessory lobes with extant pancrustaceans. Our findings provide new morphological support for the hypothesis that Trilobita belongs to the mandibulate stem lineage and bring new insights into the ancestral morphology and evolutionary history of the euarthropod central complex. More broadly, our findings offer direct anatomical evidence of the key sensorimotor system innovations that originated during the Cambrian Information Revolution.

KCNQ2 (Kv7.2) subunits contribute to the channels that carry the neuronal M-current, and variants in KCNQ2 cause epilepsies that range from self-limited neonatal seizures to developmental and epileptic encephalopathy (DEE). Structures of KCNQ2 bound to PIP2 and cannabidiol show a widened activation gate, yet it has remained unclear which point of the activated pore limits K+ flux and how DEE variants at that point act. We simulated the KCNQ2 pore domain (PDB 8J01) in a POPC bilayer under an applied field of about 565 mV, restraining only the terminal residues of S5 and S6 so that G310 and S314 were free. The activated pore conducted K+ at 4.3 pS (95% CI 2.5-6.9; 17 net filter crossings in 1120 ns over four replicas), while the closed-state pore (PDB 7CR3) conducted none in 560 ns. Conduction proceeded by knock-on through a filter that was doubly occupied in 84-92% of frames. Along S6 the narrowest position of the activated pore was not the canonical gate residue S314 but the glycine of the Pro-Ala-Gly motif, G310: activation widens S314 by 0.37 nm but G310 by only 0.07 nm, and restraining G310 to its closed-state width in one replica abolished gate crossings. The DEE variant G310S reduced conduction about six-fold (G310S/wild-type ratio 0.16, 95% CI 0.02-0.66; p = 0.003). Its serine hydroxyls project into the constriction and halve the water column in the G310 plane (8.7 to 4.7 waters), an effect reproduced in CHARMM36m (9.9 to 4.3). The first coordination shell of a K+ ion at the constriction was, however, conserved (6.3 vs 6.5 oxygens), because the serine hydroxyls replace the waters they displace. The hydroxyls did not retain the ion: contacts lasted tens of picoseconds, and ions were instead turned back at the serine ring, fewer reaching the gate plane (45 vs 79 excursions per microsecond) and fewer of those crossing it (5% vs 23%). In mixed tetramers carrying one or two variant subunits the effect on the water column was additive, each subunit removing about a quarter of it (f = 0.22 and 0.58 of the full effect). These results identify G310 as the flux-limiting constriction of the activated KCNQ2 pore in our simulations and point to a permeation defect as one component of G310S loss of function.

Rod photoreceptors are optimized to detect single photons, but their performance near absolute visual threshold is limited by intrinsic phototransduction noise. The two major components are discrete single-photon-like events and continuous fluctuations in outer-segment current. Although spontaneous rhodopsin activation is established as the source of discrete noise, the molecular origin of continuous noise remains controversial, and how these distinct noise sources influence signal processing at the first visual synapse is not fully understood. To address these questions, we combined genetic manipulation of rhodopsin, phosphodiesterase 6 (PDE6), and glutamic acid rich protein 2 (GARP2) with single-cell recordings from rods and rod-bipolar cells (RBCs) and pharmacological manipulation of mGluR6 signaling. Continuous noise was unaffected by reduced rhodopsin concentration but was strongly influenced by PDE6 expression and regulation, supporting basal PDE6 activity as a major source of continuous noise. Reducing either discrete or continuous noise increased signal-to-noise ratio (SNR) in rods, but these changes were differentially processed at the rod-to-RBC synapse. In WT retina, the nonlinear transformation increased SNR, whereas elevated continuous noise limited this increase. Conversely, reduced continuous noise increased rod SNR, which was maintained in RBCs and associated with a lower detection threshold. Pharmacological reduction of the mGluR6-dependent nonlinearity altered these genotype-dependent differences, indicating that nonlinear filtering is well matched to normal signal and noise distributions but does not fully compensate when presynaptic noise is experimentally altered. Together, our study identifies spontaneous PDE6 activity as a major source of continuous noise and demonstrates that continuous noise interacts with nonlinear processing at the first visual synapse to determine the fidelity and detection threshold of dim-light signals.

Many falls and balance disorders in older adults originate in the otolith organs, the gravity sensors of the inner ear. These sensors maintain upright posture through otoconia, calcium carbonate crystals that mass-load the sensory maculae. Otoconia dislodgement causes the most common form of vertigo, and their age-related loss reduces gravity sensation and undermines balance. Yet the cellular mechanisms of otoconia formation and maintenance -and how they fail in disease- remain unknown. Using mineral-preserving histology, crystal-sensitive imaging, volume electron microscopy, and immunolabeling in human and animal otolith organs, we found otoconia biogenesis-related proteins and early crystallization at the pole opposing the macula, the roof domain. We discovered filigree extracellular matrix scaffolds bridging roof and macula, loaded with nascent otoconia, suggesting scaffold-guided transport across the organ. Single-cell transcriptomics nominated a specialized roof mesenchyme, signaling to the roof epithelium, as a driver of otoconia and scaffold production. In guinea pigs with endolymphatic hydrops, fluid expansion of the organs ruptured the otoconia-trafficking scaffolds as roof and macula separated, followed by a decline in macular otoconial mass. We propose a new disease model for common vertigo and balance disorders in which disruption of the otoconia-generating and -trafficking epithelial-mesenchymal roof niche leads to displacement and depletion of otoconia.

Many G protein-coupled receptors (GPCRs) are capable of activating multiple heterotrimeric G protein subtypes. The angiotensin II type 1 receptor (AT1R) couples to both Gq and Gi family members, and the two families mediate distinct aspects of the effects of angiotensin II (AngII) on blood pressure and fluid balance. Previous work has pointed to fundamental differences in the strength and ligand-dependence of Gq versus Gi activation by AT1R. Here we resolve key aspects of the mechanisms underlying these subtype-specific patterns of G protein coupling to AT1R. Using cellular biosensors, we show that AT1R activates Gq more efficaciously than Gi. Leveraging mini-G proteins, engineered versions of G subunits that diverge only in the C-terminal 5 helix, we identify four residues dispersed throughout the 5 helix that are sufficient to account for these differences. We employ radioligand binding to demonstrate that subtype-specific differences originate at the level of allosteric coupling between ligand and transducer binding, and we use Systematic Protein Affinity Strength Modulation biosensors to show that Gq-specific residues in the 5 helix increase affinity for AngII-activated AT1R. We provide evidence that the 5 helix residue with the greatest effect on coupling makes interactions with AT1R intracellular loop 3 that were not resolved in a previously reported AT1R-Gq cryo-electron microscopy structure. These findings enhance our understanding of how AT1R differentially activates Gq and Gi and may ultimately guide pharmacological approaches to modulate certain G proteins downstream of AT1R and other GPCRs.

Cell type designations label uniquely-identifiable groups of related cells that typically function in similar ways. Cell types divide the Drosophila melanogaster central nervous system into functional units, but remain challenging to target and identify. Previously we generated a driver line and image resource for targeting many neuron types (Meissner et al., 2025). Here we improve on identifying the targeted neurons by automated searching against electron microscopy datasets and manual scoring of search results to add 6,368 annotations across 1,772 driver lines in the collection. These annotations provide a potential training resource for future search algorithms. We then estimate the number of cell types and thus the fraction of the central nervous system labeled by the collection as a whole.

PhenoAge, a blood-based biological aging clock, is associated with cancer outcomes, a finding taken as evidence that biological aging shapes prognosis. In 3,067 patients undergoing primary tumor resection, PhenoAge acceleration rose with tumor stage. One or two of its eight blood components, differing by cancer type, carried the prognostic signal, and replacing its general-population weights with equal weights did not weaken it. In cancer, PhenoAge may reflect disease burden rather than biological aging.

At physiological levels glucocorticoid (GC) signalling within the osteoblast lineage has been proposed to be required for skeletal homeostasis, whereas supraphysiological activation supresses bone formation and causes bone loss. While these observations suggest that GC exert dose- and context-dependent effects, the molecular mechanisms underlying these opposing actions remain unresolved. Here we systematically dissected the receptor-, lineage-, and concentration-dependent effects of GC signalling on human bone marrow stromal cell (BMSC) differentiation using integrated transcriptional, epigenomic, and functional analyses. Our findings suggest that GC opposing effects are not simply due to a GR dose-response but instead reflect distinct context- and concentration-dependent modes of action. During early BMSC differentiation, the cellular context induced GR-dependent licensing of lineage-specific gene and enhancer programs. The adverse effects of high-dose GC developed over time and could not be predicted from the early transcriptional response. We found that high-dose GC caused osteoblast maturation arrest, accompanied with a cellular stress response sustained by myeloid zinc finger 1 (MZF1) also identified as an endogenous inhibitor of osteoblast differentiation.

Intrinsically light sensitive retinal ganglion cells through melanopsin photoreception (mRGCs) are important for fundamental regulatory brain processes such as circadian rhythms, mood, acute sleep, and pupillary constriction. While previous research in animal models indicates temporal differences in maturation of mRGC mediated projections, corresponding developmental patterns have not been studied in human infants. To elucidate the roles of mRGCs in early development of regulation of pupillary constriction and sleep/wake patterns, we here explore the maturation of mRGC mediated photoentrainment in human infants aged 1 to 12 months, using non-invasive eye tracking pupillometry and sleep diaries. Although the mRGCs relay rod/cone activation in a range of light intensities, it is possible to separate rod/cone/mRGC activation due to the photoreceptors' color sensitivity and temporal responsiveness. We therefore used sustained red and blue light stimulation after partial dark adaptation to characterize developmental changes in wavelength-dependent mRGC-relayed pupillary responses. Infants' pupil responses indicate a prolonged post-natal mRGC maturation to the shell of the olivary pretectal nucleus during the first year. This response was separate from infants' sleep/wake patterns, which parallels findings from cellular research on murines: the mRGCs have separate pathways for photoentrainment of pupillary constriction and sleep/wake patterns, which develop with different temporal dynamics. Together, these findings provide the first characterization of developmental changes in mRGC-relayed pupillary responses in human infants, and an initial step towards bridging human mRGC network development with murine and human adult mRGC research.

Predators routinely fail to capture prey despite being larger, faster, and more powerful. Half a century ago, theory predicted that prey could evade biomechanically superior predators using brinkmanship, delaying evasive maneuvers until the last possible instant, but this hypothesis has never been tested in the wild. Here, using more than 2,000 hours of high-speed stereo-camera recordings of predator and prey coral reef fishes, we show that prey successfully evade 97% of attacks. However, contrary to the standard explanation that prey evade through superior maneuverability, we find that prey evade by exploiting their predator's sensory-motor delay - the interval between a stimulus reaching the predator and the onset of the predator's motor response. Although predators partially compensate for this by forecasting prey position during interception steering, prey consistently time evasive maneuvers within a brief window just tens of milliseconds before collision, within which no viable predator trajectory can intercept them. Prey do not, however, turn at the time that maximizes miss distance, but slightly later, a bias that makes escape robust to variability in their own response timing. These findings reveal how temporal delays in perception and action place fundamental limits on the outcome of predator-prey interactions, and show that prey can exploit delays in a way that approaches the theoretical limit of escape performance.

Innate sensorimotor policies enable rapid and reliable behavior but can become ineffective when environments change. How these policies interact with feedback-driven processes to support behavioral flexibility remains unclear. Using closed-loop virtual reality in larval zebrafish, we reversed the visual consequences of self-generated turns. Larvae initially followed their innate mapping and then rapidly switched to corrective turns in the opposite direction. Purkinje cell ablation abolished this correction while preserving innate optomotor responses. Imaging revealed outcome-invariant sensorimotor representations in granule cells and an outcome-dependent modulation of inferior olive neurons and Purkinje cells. Correction weakened as the optic-flow angle increased, despite persistent error-related representations. The innate behavioral repertoire also shifted as the optic-flow angle increased, making corrective turns less likely. An actor-critic model initialized with the measured behavioral repertoire reproduced the limits of correction. Together, these results show that adaptive behavior emerges from the interaction between an innate feed-forward policy and an olivo-cerebellar instructive signal, revealing innate behavioral structure as a fundamental constraint on cerebellar adaptation.

Innate host defense depends on transcriptional programs, which support survival through two strategies that can vary independently. Resistance lowers pathogen burden, while resilience limits the damage a given burden causes. In Caenorhabditis elegans infected with Staphylococcus aureus, the highly conserved transcription factor HLH-30/TFEB drives most of the host response, which mediates infection resilience. We previously showed that EGL-30/Gq, PLC-1/PLC{varepsilon}, and DKF-1/PRKD activate HLH-30/TFEB during infection. The same phospholipase C and protein kinase D step activates TFEB in mouse macrophages infected with Salmonella or S. aureus. However, the input that drives the EGL-30/Gq, PLC-1/PLC{varepsilon}, and DKF-1/PRKD module to activate HLH-30/TFEB during infection remained unknown. Here we report that acetylcholine, acting on muscarinic receptors, is one such input. Atropine and scopolamine, two muscarinic antagonists in clinical use, as well as muscarinic receptor gene silencing blunted HLH-30/TFEB activation by infection. Moreover, a mutation of unc-17/SLC18A3 that lowers acetylcholine release from neurons impaired HLH-30/TFEB activation and induction of its target genes, while pathogen burden remained high. In uninfected animals, muscarinic agonist arecoline was sufficient to activate HLH-30/TFEB, in a muscarinic receptor-dependent manner. Nicotine, in contrast, had no effect. The agonist also lengthened survival of infection without lowering pathogen burden, in an HLH-30/TFEB-dependent manner. Silencing egl-30/GNAQ, plc-1/PLCE1, or dkf-1/PRKD1-3 cut activation by the agonist just as it cut activation by infection, showing that both stimuli require the same module. In mammals the link documented so far between acetylcholine and TFEB runs through nicotinic receptors, so whether the muscarinic route is conserved remains open. Thus, we favor a model in which infection drives neurons to release acetylcholine onto the intestinal epithelium, where muscarinic receptors acting through EGL-30/Gq, PLC-1/PLC{varepsilon}, and DKF-1/PRKD activate HLH-30/TFEB and strengthen infection resilience.

Antibiotic heteroresistance, a trait in which a drug-susceptible population harbors a rare resistant subpopulation, has been presented as an unstable intermediate state on the way to classic stable antibiotic resistance. However, the timescales and pathways under which this could occur has not been examined. Extending a recombination-based model of copy- heteroresistance to include compensatory mutations, with rates and costs from experimental estimates, we simulated populations across thousands of generations of growth with {beta}-lactam exposure. In the first days a founder population survives by one of two paths, either amplifying the resistance gene to a high copy-number or gaining compensatory mutations and staying at low copy-number. However, both paths converge over weeks to months on the same low-copy compensated state, which would be indistinguishable from classical resistance. Importantly, the ending state would provide no information as to the route taken: an isolate sequenced afterward carries no sign of the intermediate amplified state. With the parameters estimated and across a wide range of parameters, we find that amplification-mediated heteroresistance can be a transient stepping stone across lethal selection that leaves no sequence trace in surviving isolates.

The anterolateral system (ALS) originates from neurons in various regions of the spinal cord and conveys somatosensory information related to pain, skin temperature and itch to several brain targets. One component of the ALS is the spinotectal tract, which projects to the superior colliculus (SC) and is thought to underlie directed movements of the head towards a painful stimulus. Using anterograde and retrograde tracing, we demonstrate that the input from the spinal enlargements to the SC originates predominantly from neurons in the lateral spinal nucleus (LSN), many of which express calbindin. This input, which mainly targets the caudal and lateral parts of the SC, is somatotopically organised, and is associated with the clusters of Pitx2-expressing neurons that have been identified in previous studies. Since activation of Pitx2 neurons drives directed head movements, input from LSN neurons to the Pitx2 cells is likely to be a critical component of orientation and attention towards a noxious stimuli. We also provide evidence that spinotectal LSN cells receive excitatory synaptic input from neurons in superficial dorsal horn that express substance P or the gastrin-releasing peptide receptor (GRPR). Since GRPR neurons are strongly implicated in itch, our findings raise the possibility that the SC has a role in coordinating movements in response to pruritic stimuli.

Cognitive stability, or task focus, enables goal-directed behavior through representational dynamics: by prioritizing task-relevant representations over competing task-irrelevant ones. Beyond corrective reactive mechanisms operating after conflict arises, stability can be deployed predictively through the statistical learning of conflict regularities, either proactively, guided by global conflict expectations, or through a predictive reactive mode guided by item-specific ones. Yet, the representational mechanisms enabling such predictive adjustments remain underspecified. We addressed this by re-analyzing EEG data from a spatial Stroop task concurrently manipulating list-wide (LWPC) and item-specific (ISPC) proportion congruency. Moving beyond existing analytical approaches, we developed a framework combining trial-by-trial Bayesian conflict-expectancy modeling with a novel spatio-temporo-spectral searchlight Representational Similarity Analysis, and leveraged directional representational modeling complemented by ridge-regression decoding. We first identified the necessary computational substrate for stability predictive reconfigurations: the spatio-temporo-spectral patterns encoding task (Target, Distractor) and conflict-expectancy (LWPC, ISPC) representations, the latter representations encoded already in the pre-stimulus windows. Crucially, conflict expectations modulated Target and Distractor encoding strength with distinct spatio-temporal and spectral dynamics, revealing a functional dissociation. LWPC-induced proactive stability follows a "Target-first" policy, primarily strengthening the Target early post-stimulus via Alpha and lower-Beta patterns, weakening Distractor encoding only later on; conversely, ISPC-induced predictive reactive stability follows a "Distractor-first" policy, rapidly weakening Distractor encoding via response-locked posterior Beta patterns, strengthening the Target only downstream. Our study refines theoretical cognitive control models, incorporating a mechanistic explanation for predictive stability, while also establishing a multidimensional methodological template for interrogating the underlying substrates of adaptive behavior and beyond.

The B cell receptor (BCR) drives the differentiation of naive B cells into activated plasma cells that produce antibodies. The BCR consists of a plasma membrane-bound surface immunoglobulin (sIg) that binds antigens, and two coreceptor subunits CD79A and CD79B. The intracellular domains of CD79A and CD79B initiate immune signaling and endocytosis of the receptor in healthy B cells. In diffuse large B cell lymphoma (DLBCL), an aggressive form of human blood cancer, the activated B cell like (ABC) subtype exhibits constitutive signaling that drives survival and proliferation. Here, in a model of human ABC DLBCL, we determine the localization of BCR components relative to plasma membrane structures using correlative super-resolution light and platinum replica transmission electron microscopy. We find that spontaneous clusters of the surface Immunoglobulin common to ABC lymphoma localize to smooth raised membrane domains. These structures are involved in the endocytosis of large receptor clusters. Surprisingly, activated phosphorylated CD79A (pCD79A) shows little colocalization with clathrin on or off smooth raised membranes. Furthermore, pCD79A spatially segregates away from both the slg and CD79B subunits of the BCR complex. A similar distribution was found for downstream phosphorylated Src family kinases. We propose that a disengagement of pCD79A away from sIg and CD79B allows the receptor to evade down-regulation through endocytosis and maintains signaling at the plasma membrane. This mechanism could drive aberrant signaling and the proliferation of lymphomas leading to disease.

Lipid transfer proteins (LTPs) mediate non-vesicular lipid transport at membrane contact sites, yet how transient membrane tethering contributes to lipid transfer remains poorly understood. Here, we use the single-domain yeast phosphatidylserine transporter Osh6, a member of the oxysterol-binding protein-related protein family, to investigate tethering-dependent lipid transport. We show that Osh6 can access two transport modes depending on membrane composition: conventional shuttling between membranes and a tethering-dependent mode in which the opposing membrane facilitates cargo extraction from the donor membrane. Combining lipid transport and membrane-tethering measurements with kinetic analysis, lipid partitioning, and in vivo complementation, we provide evidence for a transient membrane-bridged intermediate during lipid transfer. Productive phosphatidylinositol 4-phosphate (PI4P) extraction destabilizes this bridged state, allowing cargo-loaded Osh6 to disengage and continue the transport cycle. We term this mechanism kiss-and-ride, in which transient membrane contact facilitates cargo extraction ("kiss") before the cargo-loaded protein disengages ("ride"). Consistent with this model, PI4P transport is preserved in a lidless Osh6 mutant with impaired membrane dissociation, whereas phosphatidylserine transport is strongly reduced, revealing distinct mechanistic requirements for the two cargoes. Together, our findings reveal a tethering-dependent transport mode in a cup-like LTP and suggest that membrane contact sites can actively shape the mechanism of lipid transport rather than merely bringing donor and acceptor membranes into proximity.

Body-brain communication is crucial for optimal neuronal performance across the lifespan, yet whether peripheral organs regulate spinal cord motoneuron (MN) function through circulating molecules remains unknown. The expression of hormone receptors by spinal MNs raises the possibility that circulating cues allow the spinal cord to sense the body's energetic state and sustain motor activity. Here, we show that the bone-derived hormones osteocalcin (OCN) signals on spinal MNs via its receptor GPR158. GPR158 is selectively enriched in spinal cholinergic MNs, and its loss, globally or specifically in spinal MNs, or the one of OCN, impairs locomotor function and reduces choline acetyltransferase (ChAT) levels. Mechanistically, OCN signaling through GPR158 fine-tunes autophagy, mitophagy and mitochondrial activity in spinal MNs. Restoring any of these processes, by pharmacologically inducing mitophagy or chemogenetically activating mitochondria, is sufficient to rescue the locomotor deficits of Gpr158-deficient mice. Finally, OCN restoration in aged mice reverses age-related locomotor decline through a mechanism that requires MN autophagy and mitophagy. This study reveals that through OCN, the skeleton influences motor functions by fine-tuning motoneuronal mitochondria, and suggests that age-related motor decline is, at least in part, reversible.

Phagocytosis and killing of bacteria by neutrophils is a key driver of human immunity against invading infections. IgG antibodies targeting the bacterial surface can potently enhance phagocytic uptake by engaging Fc gamma receptors (Fc{gamma}Rs) on the neutrophil membrane. In addition, IgG-mediated complement activation triggers surface deposition of complement opsonins that engage complement receptors (CRs) to further enhance target cell clearance. In this paper, we reveal that complement deposition prevents engagement of Fc{gamma}Rs to the Fc-domain of surface-bound IgG molecules. When complement is present, we observe that blockade of Fc{gamma}Rs does not influence phagocytosis of both Gram-positive and -negative bacteria (using Staphylococcus aureus and Klebsiella pneumoniae as model organisms). Complement-mediated hindrance of Fc{gamma}R engagement was observed in different model conditions, for example using either pre-existing polyclonal antibodies in human serum or recombinant monoclonal antibodies (mAbs) targeting different bacterial antigens. Using Fc engineering, we found that inhibition of Fc{gamma}R binding is dependent on the ability of IgGs to form IgG hexamers. Mechanistically, we show that complement proteins, in particular C1q, directly prevents the interaction between IgG-Fc and Fc{gamma}R by steric hindrance. Together, these insights help us define the respective roles of Fc{gamma}R and CR in neutrophil phagocytosis, while also providing important fundamental insights for therapeutic antibody development.

The citric acid cycle (TCA cycle) is the common terminal pathway for the oxidation of all nutrients. Citrate oxidation to oxaloacetate produces CO2, and citrate synthase (CS) uses nutrient-derived acetyl groups to regenerate citrate and fuel cycle turning. However, the essentiality of cycle fueling and turning in vivo remains unclear. Here we use hematopoiesis, the most proliferative system in the body, as a model to show that, contrary to common assumptions, TCA cycle turning is dispensable for respiration, survival, and proliferation of stem and progenitor cells in vivo, and its loss promotes stem cell function. Hematopoietic-specific Cs deletion in adult mice blocked citrate cycling without reducing the frequency of hematopoietic stem (HSC) and progenitor cells. HSCs and progenitor cells adapted to TCA cycle loss by markedly increasing nutrient consumption and biosynthesis. Disruption of cycle turning increased HSC regeneration, myeloid progenitor proliferation, and myelopoiesis in vivo. HSCs without a turning TCA cycle outcompeted wild-type HSCs within the same environment. The effect of CS deletion on HSC function was not phenocopied by genetic ablation of cytosolic citrate use and was rescued by ablation of glutamine use in biosynthesis. Therefore, TCA cycle turning restrains nutrient uptake, biosynthesis, cell proliferation, and stem cell function. These results suggest an explanation for the reduction in cycle activity observed in many normal proliferating cells and cancer cells.