Science Advances

Postmenopausal osteoporosis is a highly prevalent disease characterized by low bone mass and structural deterioration of bone tissue leading to increased bone fragility and fracture risk. Osteocytes orchestrate local bone (re)modeling and therefore, characterizing their 3D morphological and spatial distribution across distinct (re)modeling microenvironments is essential for understanding the cellular mechanisms underlying divergent tissue-level responses. To address this, we used a 3D-3D multiscale registration framework mapping endpoint osteocyte lacunae onto dynamic, time-lapsed in vivo (re)modeling sites. Ovariectomized 15-week-old mice were subjected to 4 weeks of anabolic parathyroid hormone (PTH) or anti-catabolic bisphosphonate (BIS) treatment as monotreatment or combined with cyclic mechanical loading (ML). Biweekly in vivo micro-computed tomography (micro-CT) to identify (re)modeling sites and endpoint ultra-high resolution (ultra-HR) micro-CT to characterize osteocyte lacunae was performed. Osteocyte lacunae were characterized by lacunar density, volume and sphericity. Our findings showed that physical and pharmacological therapies differentially regulate the osteocyte lacunar network through distinct perilacunar adaptations across functional microenvironments. While BIS stabilizes local lacunar architecture, PTH drives asymmetrical lacunar expansion (increased volume with reduced sphericity) through perilacunar remodeling that supersedes any added ML effects. Crucially, estrogen loss disrupts the natural spatial maturation gradient by inducing rounding of lacunae in newly formed bone. Therapies counteract this disruption, restoring the healthy gradients in lacunar morphometry between spatially distinct microenvironments. Additionally, despite restoring tissue-level bone volume, physical and pharmacological osteoporosis therapies do not fully recover persistent lacunar density deficits. The overall lacunar morphological spatial signatures confirm active osteocytic tuning to endocrine, physical and matrix cues.

Mechanical stability critically influences bone fracture healing, yet how the early mechanical environment directs the transition from inflammation to regeneration remains unclear. Using a murine femoral osteotomy model, we compared rigid, semirigid, and dynamically adjusted semirigid-to-rigid fixation. Semirigid fixation delayed healing on day 21, whereas a strategy of early compliance followed by increased stiffness after 7 days restored bridging and improved bone regeneration beyond constant rigid fixation, pointing to a critical early mechanosensitive window after fracture. Single-cell RNA sequencing across the first week, comparing semirigid with rigid fixation, showed that fixation stiffness altered intercellular signaling within one day of injury. Signaling among myeloid populations was more broadly increased under semirigid fixation, with monocytes the single exception, and signaling associated with resolution of inflammation was reduced. From day 5, periosteal and skeletal progenitor populations increasingly directed cartilage-associated matrix and growth factor signaling toward macrophages and chondrocytes, accompanied by increased Sox9 regulon activity and increased COL10+ hypertrophic cartilage matrix by day 7. Reduced fixation stiffness therefore promotes a chondrogenic trajectory of repair, but increasing stiffness is required to redirect this response toward bone formation. Piezo1 was expressed across the responding populations and shifted between compartments as healing progressed. PIEZO1 activation with Yoda1 under rigid fixation enhanced bone formation but provided no additional benefit under semirigid or dynamically adjusted fixation, whereas inhibition of mechanosensitive signaling with GsMTx4 impaired repair across all conditions. Together, these findings establish the early mechanical environment as a determinant of fracture-healing trajectory, shaping the transition from inflammation to regeneration, and show that successful repair depends not on maximal stability but on when stiffness is applied. Increasing either early mechanical stimulation or cellular mechanosensitivity enhanced regeneration, supporting mechanosensitive signaling as a key component of the early healing response.

Marine heatwaves cause massive, but often patchy coral bleaching. Why neighboring colonies often respond differently remains unresolved. Here, we show that interactions between metabolic demand and flow-mediated resupply generate heterogeneous oxygen microenvironments inside branching corals under heat stress. During the 2023 and 2024 Red Sea heatwaves, high-frequency in situ measurements revealed hypoxic to near-anoxic interbranch conditions despite well-oxygenated surrounding seawater. In 2023, neighboring colonies developed divergent oxygen regimes preceding paling and bleaching; in 2024, stronger and earlier thermal stress was associated with recurrent nighttime near-anoxia before tissue disintegration and mortality. A physics-based three-dimensional fluid-metabolism model reproduced these patterns. Oxygen depletion and coral impacts emerged when metabolic oxygen demand exceeded flow-mediated resupply. Organism-scale variability in oxygen regimes may explain divergent coral fate under shared heatwave exposure.

The Ocean sustains planetary health through climate regulation, biodiversity, and essential ecosystem services. Plankton underpins these processes by driving primary production, sustaining food webs, and mediating carbon sequestration. Yet the spatial organization of plankton-based ecosystem services remains poorly understood. The High Seas Treaty and the 30% Ocean protection target by 2030 create an urgent need for science-based guidance for global ocean governance. Here, we aim to identify key planktonic areas associated with functional diversity, net primary productivity, and carbon export globally. We used machine learning to integrate omics-derived plankton functions and satellite observations with diversity metric and ecosystem services. Leveraging metagenomic data, we identified molecular functions strongly linked to ecosystem functioning and project their global distributions across seasons. We reveal putative hotspots of functional diversity, net primary productivity, and carbon export. Our analysis pinpoints areas where all three indicators converge synchronously, predominantly along oceanographic structures such as polar fronts and principal current systems. By relying on satellite data as core input, our approach captures the temporal variability of these hotspots, providing a global, dynamic protocol for projections of plankton-based ecosystem services. These findings establish a foundation for developing integrative conservation tools based on ecosystem functioning and inform adaptive management under climate change.

Coral reefs support an extraordinary diversity and biomass of fishes. However, whether species coexist through fine-scale niche partitioning, and whether this impacts ecosystem functioning, remain unresolved. We combined DNA metabarcoding and stable isotope analysis to reconstruct a high-resolution food web and quantify the trophic niches of 2,060 reef fishes across 261 species. We observed extreme trophic partitioning: species pairs diverged by a median of 93% (DNA metabarcoding) and 86% (stable isotopes), consistently overlapping far less than expected by chance. Further, communities with greater trophic complementarity supported higher standing-stock biomass, particularly at high species richness. Our results confirm limiting similarity and trophic complementarity as the driving forces of coexistence and ecosystem functioning in the ocean's most diverse ecological community.

The ability to identify dangerous temperatures is critical, but the neural basis for cold sensation remains enigmatic. By combining in vivo functional imaging with tissue-specific spatiotemporal cooling, we unveil myelinated high threshold-mechanonociceptors (HTMRs) as the neural origin of tooth cold sensitivity. Critically, this neural encoding is driven by threshold noxious cold temperature, not cooling rate and remains independent of the cool receptor TRPM8 or pain transducer Nav1.8. Furthermore, while Bmpr1b-Cre captures the intradental HTMRs, it further reveals myelinated HTMRs mediate trigeminal cold encoding in soft tissues. Finally, we determine that inflammation potentiates intradental HTMRs, providing functional basis for clinical hypersensitivity in pulpitis. Collectively, our results reveal a new neural mediator of cold sensation and provide insight into painful transformation of cold sensory encoding in inflammation.

As the most recently evolved neuroanatomical trait of our species, our globular head shape ('globularity') constitutes a promising point of entry to probe possible associations between brain morphology and cognitive traits that may be distinctive of Homo sapiens. To shed additional light on the underlying biology of globularity and its cognitive consequences, we capitalize on data pairing genetic variation to brain shape differences from magnetic resonance imaging (MRI) scans of 34,595 adults of European ancestry, and embed genome-wide associations of globularity within genetic variation underpinning up to 75 brain-related traits. Through extensive hypergeometric tests, we observe significant associations between globularity and white matter traits at the gene level, but stronger associations with cerebellar volume, ventricular size, and the fornix at the genomic variant level. Combining conjunctional false discovery rate (FDR) with genetic correlations, we notice homogeneously concordant relations between globularity and the volume of the brain ventricles, and homogeneously discordant relations with glymphatic function. We also unravel more selective, spatially organized relations between globularity and cerebellar volume, white matter connectivity and sulcal width. Spatial permutation tests reveal that sulcal width associations are systematically distributed along a sensorimotor-association gradient of cortical organization. Additionally, correlations of brain- and lifespan-wise gene expression patterns linked to globularity and the brain ventricles highlight recurrent associations with the pentose phosphate metabolic pathway. Finally, deploying an open repository of genetic variant age estimations and an extensive catalog of single nucleotide changes distinguishing hominins, we uncover a coincidence between the establishment of globularity and the time of emergence of modifications in the cerebrospinal fluid proteome. Overall, we provide a finer-grained genetic architecture for globularity, with novel biological associations to traits across evolutionary and developmental timescales, leading us to draw implications for the early development of cognitive networks but also for liabilities to neurodegeneration late in life.

Cilia are highly conserved organelles that use hundreds of unique proteins to drive extracellular motility and serve as signaling hubs for eukaryotic cells. The Ciliogenesis and Planar Polarity Effector (CPLANE) protein complex controls basal body docking during early ciliogenesis and recruits the intraflagellar transport (IFT) proteins to the base of growing cilia. Leveraging recent advantages in deep learning neural networks and high-performance computing, we predict the structure of JBTS17, the largest CPLANE subunit, and the tertiary structure of the complete CPLANE complex, revealing direct contacts between JBTS17 and three other CPLANE members. We then used this model for an in silico protein-protein interaction screen that identified several novel JBTS17 interactors. Experiments in Xenopus confirmed a functional link in vivo between Jbts17 and the Spinocerebellar ataxia-associated protein TTBK2. The data provide new insights into mechanisms of CPLANE function during ciliogenesis and demonstrate the power of modeling protein complexes for the discovery of physical and functional protein interactions in cell homeostasis and disease.

Fibrous dysplasia (FD) arises from activating GNAS mutations that increase cAMP signaling in skeletal stem/progenitor cells (SSPCs), but the underlying mechanisms and whether the resulting fate imbalance can be reversed remain unclear. Using controlled GNAS induction with transcriptomic, pharmacologic, and cell-fate analyses, we identify a PKA-dependent checkpoint governing SSPC lineage commitment. GNASR201C redirects SSPCs toward osteoclastogenesis while suppressing osteoblast maturation; genetic or pharmacologic PKA inhibition reverses both effects, revealing this fate decision is dynamically reversible. Mechanistically, PKA phosphorylates RUNX2, the master regulator of osteogenesis, enforcing its cytoplasmic retention and blocking osteogenic transcription; a non-phosphorylatable RUNX2 mutant restores nuclear localization and osteoblast differentiation, defining RUNX2 phosphorylation as a disease-initiating molecular switch. Critically, PKA inhibition in vivo reduces fibrosis and osteoclast burden in established bone lesions, demonstrating reversibility even in advanced FD. These findings define a druggable GNAS-PKA-RUNX2 checkpoint and identify PKA inhibition as a strategy to reverse existing disease in FD.

Intervertebral disc (IVD) degeneration, a major cause of low back pain, is driven by the disruption of nucleus pulposus (NP) tissue homeostasis. The NP arises from embryonic notochord cells that differentiate and adapt to the hypoxic, matrix-rich environment of the developing disc. Recapitulating the developmental trajectory from notochordal progenitors to NP-like cells in vitro remains challenging, as most protocols bypass early specification steps and fail to capture this understudied lineage. Here, using a three-dimensional (3D) system, we reconstruct this developmental path from human pluripotent stem cells into mesendoderm progenitor cells (MEPCs), notochord-like cells (NLCs), and NP-like cells, guided by single-cell transcriptomics and spatial profiling. We identify MEPCs as a distinct progenitor population with a unique gene signature and conserved regulatory networks (GSC, KLF8, LHX1, MIXL1) shared with early organizer and mesendodermal cells. Self-organization in MEPCs-derived aggregates induced spontaneous expression of NOTO, a key transcription factor orchestrating notochordal identity, and enhanced commitment to notochordal fate. Long-term culture in a medium containing TGF-{beta}1 and GDF-5 promoted maturation into SOX9+, KRT8+, CD109+ NP-like cells exhibiting robust matrix deposition and prominent LAMP1-positive intracytoplasmic vacuoles. This scalable, physiologically relevant model enables fundamental studies of human disc biology and early degenerative processes, while advancing regenerative strategies.

The basal body (BB) is essential for axoneme assembly and ciliary function, and its defects are associated with ciliogenesis disorders and ciliopathies (1,2). However, the molecular basis of the BB's region-specific transitions along the proximal-distal axis, and the mechanism by which ciliary central pair (CP) microtubules are nucleated during ciliogenesis, have remained largely unknown. Here, we resolve the subnanometer architecture of native BBs in mammalian multiciliated cells using cryo-electron tomography and subtomogram averaging, and identify 11 ciliary-base microtubule-associated proteins (CMAPs), including 5 previously uncharacterized core components. These CMAPs localize specifically along ciliary base that facilitate the triplet-to-doublet microtubule transition. We further identify luminal gamma-tubulin ring complexes (gamma-TuRCs) anchored to the microtubule wall via the augmin complex, and propose that they may serve as candidate nucleation sites for CP microtubules biogenesis. Together, these findings define a molecular blueprint of the BB and provide a structural framework for understanding how regional architectural specialization and luminal nucleation collectively drive ciliary assembly and how their dysfunction causes ciliopathies.

Spatial transcriptomics has transformed molecular characterization of the human brain, but most studies profile individual two-dimensional tissue sections and therefore cannot capture molecular organization across 3D neuroanatomical axes. Here, we developed a spatial genomics framework to reconstruct the 3D cellular architecture of the human nucleus accumbens (NAc) by densely sampling and aligning serial sections across its anterior-posterior (AP) extent. Combining single-cell Xenium profiling with near-transcriptome-wide VisiumHD, we mapped cell type composition and topography across AP, mediolateral and dorsoventral axes. Neuronal populations varied in relative abundance across the AP axis while maintaining characteristic spatial topographies. These differences were particularly pronounced among D1-islands, a topographically organized structure within the NAc in which transcriptionally distinct subpopulations of DRD1-expressing medium spiny neurons (MSNs) aggregate into discrete cellular formations. D1-islands comprised two molecularly distinct spatial domains with conserved counterparts across species. Both domains varied in relative abundance across the AP axis, but demonstrated distinct 3D distribution; one was present throughout the NAc, whereas the other, corresponding to islands of Calleja (ICj), was more restricted to intermediate-to-posterior levels. Local spatial analyses revealed region- and AP-dependent cellular relationships, including segregation of the two island populations despite their anatomical proximity. Together, these findings establish a framework for three dimensional spatial genomics of human brain tissue and reveal molecular and cellular organization not captured by individual tissue sections.

Urbanization is an accelerating evolutionary force, yet the patterns and drivers of urban gene flow remain largely obscured. This leaves a critical gap in testing the interplay between demographic and genetic models of urban connectivity. Specifically: the urban fragmentation model, where structural barriers restrict gene flow and drive genetic drift; the urban facilitation model, where permeable matrices enhance functional connectivity; and the urban sink hypothesis, a demographic model where high mortality creates unsustainable population sinks. We used high-resolution RAD-capture data to conduct landscape genomics and gene flow analyses to resolve these models and investigate contemporary evolution in urban coyotes (Canis latrans) across the greater Los Angeles, CA, USA metropolitan area. By incorporating fine-scale environmental, social-ecological, and historical variables of the built environment, we rigorously assessed how urban heterogeneity shapes intra-city evolutionary dynamics. We found weak population structure, heavily influenced by family groups. There was extensive landscape permeability and gene flow across the study area including across four major highways. We also identified heavily asymmetric gene flow from exurban source populations into the urban core, supporting the urban sink paradigm. However, rather than acting as a genetic dead-end, the urban matrix showed striking functional heterogeneity: one urban core functioned as a 'genetic sponge,' absorbing unique transient alleles from multiple surrounding sources, and both urban cores operated as permeable 'leaky sinks,' exporting critical gene flow to other populations. Consequently, urban demographic sinks can simultaneously act as vital genetic bridges. Our findings reject a pure urban fragmentation model while demonstrating that demographic sink dynamics and genetic facilitation are not mutually exclusive. This demonstrates that high-turnover urban sinks can buffer regional biodiversity against genetic erosion, highlighting the need to prioritize matrix permeability and inter-patch connectivity in urban conservation strategies.

Animal societies are organized across scales, from moment-to-moment actions of individuals to dynamic group structure. A central challenge in sociobiology is to understand how behaviors of individuals shape group structure. Here, using machine vision to map recurrent fine-scale behavioral modules in the eusocial naked mole-rat (Heterocephalus glaber), we show that spontaneous behavior in isolation predicts group-level social organization, including colony membership and individual identity. We determined dominance relationships and found highly linear hierarchies, with ranks stable over months to years. Remarkably, fine-scale behavioral profiles in isolation correctly predicted pairwise dominance relationships, allowing reconstruction of rank order within each colony. Models trained within one colony predicted rank poorly in others, suggesting that naked mole-rats express social position through behavioral patterns specific to each colony. Together, our results connect fine-scale behavior to social structure maintained over years and show that, even when an animal is alone, its behavior carries signatures of its identity, colony, and place within that society.

Aberrant phase transitions of -synuclein (-Syn) condensates, the pathological hallmark of Parkinson disease (PD) and related synucleinopathies, are difficult to dissect in living cells due to limited tools for spatiotemporal control of intracellular phase behavior. Here, we use an optogenetic-based light-inducible protein aggregation (LIPA) system to drive -Syn liquid-liquid phase separation (LLPS) in mammalian HEK-293T cells and human iPSC-derived neurons (iNeurons). Blue-light stimulation of LIPA--Syn induces dynamic, liquid like biomolecular condensates that initially remain fully reversible, but gradually undergo condensate maturation into an irreversible, solid like state upon prolonged stimulation, consistent with a liquid to solid phase transition. This transition is accompanied by a robust accumulation of phosphatase-resistant Ser129-phosphorylated -synuclein (pS129), a hallmark biochemical feature of Lewy pathology in PD and related synucleinopathies. Mechanistically, PLK2-mediated phosphorylation at Ser129 promotes condensate stabilization and irreversibility, identifying the PLK2-pS129 axis as a key regulator of the conversion of dynamic -Syn condensates into persistent assemblies. Importantly, transient liquid-like -Syn condensates are efficiently cleared, primarily through the proteasomal degradation pathway. In contrast, mature solid-like condensates become refractory to degradation, resulting in proteostasis impairment and progressive intracellular -Syn accumulation. A major finding of this study is the identification of a strong link between -Syn condensate maturation and lipid droplet (LD) biology. The formation of irreversible -Syn condensates is associated with a marked increase in both LD abundance and size. Moreover, mature LIPA--Syn condensates exhibit extensive interactions with LDs, disrupting lipid homeostasis and increasing cellular vulnerability. These findings suggest that LDs are not merely passive bystanders but active participants in the pathological maturation of -Syn condensates. Collectively, our results provide mechanistic insight into the molecular and cellular processes that govern the transition of -Syn condensates from reversible liquid assemblies to pathogenic solid-like aggregates. They identify Ser129 phosphorylation and lipid droplet interactions as critical determinants of condensate fate and establish aberrant condensate maturation as a potential source of neurotoxic -Syn species that contribute to the pathogenesis of PD and other synucleinopathies.

Environmental exposures shape disease risk, but the generalizability of circulating protein-based readouts of exposures across diverse populations remains unknown. Using plasma proteomic profiles of 5,416 proteins in 10,043 participants from the All of Us Research Program, we evaluated associations with and composite scores of 100+ exposures, spanning adiposity, air pollution, ambient temperature, and substance use. We find that proteomic exposure signatures are largely consistent across populations and socioeconomic strata, with <1% of proteins showing significant cross-population heterogeneity, which included BMI as an exception, with leptin being the most heterogeneous association. Population-specific leptin trajectories adiposity, together with residual leptin-type 2 diabetes associations after BMI adjustment, revealed sex-specific metabolic risk not captured by BMI alone. Aggregated protein scores were largely exposure-specific, but substance-use scores showed cross-prediction and SES scores were broadly associated. PM2.5 protein scores captured within-region exposure variation and outperformed geographic and socioeconomic correlates, but spatial confounders remain embedded within protein-environment associations. Scores trained in UK Biobank transferred to All of Us for biologically proximal exposures, supporting cross-biobank portability. These findings establish plasma proteomics as a scalable framework for exposure measurement across diverse populations, while underscoring that responsible applications require careful consideration of biological context, social patterning, and cohort structure.

Phytoplankton drive ~46% of global primary production, yet whether their protein repertoires encode ocean conditions - and whether reference databases capture the environmentally responsive fraction - is unknown. We used a protein language model to classify 447.7 million proteins from 2,357 samples and analyzed domain profiles for 231.7 million algal proteins across 2,044 samples. Under spatial block cross-validation, environment predicted individual domain abundances at R2 up to 0.59, while domain profiles predicted sea surface temperature at R2 = 0.38. The strongest coupling lay beyond annotated sequence space: 33,950 families clustered from 201 million Pfam-dark proteins coupled to environment 2.29-fold more strongly than Pfam domains. Cross-taxonomic selection, protein language modeling, and AlphaFold 3 predictions of 138 well-folded, InterPro-unannotated representatives support the Pfam-dark proteome as a biologically structured evolutionary compartment. Thus, the proteins most tightly coupled to ocean state are those least represented in reference databases.

Histone H3 Gln 5 serotonylation (H3Q5ser) is a recently identified epigenetic modification in brain that modulates reader interactions with adjacent H3 Lys 4 trimethylation (H3K4me3) to promote transcriptional permissiveness. However, whether H3K4me3Q5ser and its associated binding proteins regulate neurodevelopmental gene expression programs remains unknown. Here, we identified the catalytically inactive Lysine methyltransferase 2E (KMT2E) as a reader of combinatorial H3K4me3Q5ser. KMT2E preferentially binds H3K4me3Q5ser over H3K4me3 alone, and enriches at broad chromatin domains marking actively transcribed neurodevelopmental loci. Notably, heterozygous variants in KMT2E have been implicated in ODLURO syndrome, a recently characterized neurodevelopmental disorder (NDD). To identify the molecular mechanisms underlying ODLURO syndrome, we generated a Kmt2e transgenic mouse model that reproduces behavioral, physiological, and cellular endophenotypes associated with this and other NDDs. Furthermore, we observed that KMT2E mediates these effects by recruiting the NCoR/HDAC3 repressor complex to H3K4me3Q5ser-marked loci to restrict spreading of co-localized H3 Lys 9 acetylation (H3K9ac). Inhibition of aberrant H3K9ac spreading was sufficient to rescue transcriptional dysregulation in Kmt2e haploinsufficient neurons. These findings thus establish KMT2E as a critical reader of H3 serotonylation during neurodevelopment and provide mechanistic insights into the pathogenesis of ODLURO syndrome.

The habenula (Hb) is a small epithalamic midline structure critical for reward processing and mood regulation that is implicated in many complex brain disorders, including depression and addiction. The molecular anatomy of the Hb has been well-characterized in model organisms, but little is known about its organization in the human brain, including the transcriptomic and epigenetic signatures of spatially-organized lateral (LHb) and medial (MHb) cell types. Here, we generated a high resolution spatiomolecular map of the human Hb using both multiomic single nucleus sequencing (snMultiome) and spatially-resolved transcriptomics. We measured gene expression and chromatin accessibility across 4 MHb and 5 LHb neuronal cell types as defined by transcriptomic conservation with rodents and zebrafish. We identified cell type-specific molecular specializations related to GABAergic and glutamatergic signaling as well as regulatory relationships between open chromatin regions, transcription factors, and target genes associated with depression and addiction. High resolution spatial transcriptomics identified the anatomical location of heterogeneous LHb subpopulations, including two putative inhibitory populations in the LHb. We further characterized cell-type relationships between Hb neurons and glial subtypes and predicted ligand-receptor interactions between LHb and MHb neurons. Finally, we delineated Hb cell types and spatial domains enriched in genes associated with genetic risk for psychiatric disorders, identifying the strongest enrichment in the MHb. Collectively, the generated data and analyses resolve the spatiomolecular organization of the human Hb and identify molecular-genetic associations with neuropsychiatric disease.

Myelin membranes are tightly connected to the axon, forming an electrical route for rapid saltatory conduction of action potentials from node to node. Beneath the myelin, adjacent to the node, lie voltage-gated potassium (K+) channels, whose fundamental role in conduction remains poorly understood. Constrained by biological data, we developed a finite element model of a node of Ranvier encompassing an adaxonal membrane separating oligodendrocyte cytoplasm from the periaxonal space, and solved the full coupled set of physical equations governing electrodiffusion and voltage profiles at the nanoscale. Remarkably, we find that nodal action potentials invert at the inner adaxonal myelin membrane. The hyperpolarization-driven opening of myelinic Kir4.1 channels produces an instantaneous, powerful K+ prebuffering, clamping the periaxonal K+ concentration and enabling high-frequency action potential generation. Together, these findings indicate that the periaxon maintains an electrical feedback loop with the node, and suggest a key evolutionary advantage of juxtaparanodal K+ channel positioning.