SNT: a unifying toolbox for quantification of neuronal anatomy

Regenerative failure of sympathetic axons contributes to deficits in functional recovery after nerve injury

Renewed scientific interest in sympathetic modulation of muscle and neuromuscular junctions has spurred a flurry of new discoveries with major implications for motor diseases. However, the role sympathetic axons play in the persistent dysfunction that occurs after nerve injuries remains to be explored. Peripheral nerve injuries are common and lead to motor, sensory, and autonomic deficits that result in lifelong disabilities. Given the importance of sympathetic signaling in muscle metabolic health and maintaining bodily homeostasis, it is imperative to understand the regenerative capacity of sympathetic axons after injury. Therefore, we tested sympathetic axon regeneration and functional reinnervation of skin and muscle, both acute and long-term, using a battery of anatomical, pharmacological, chemogenetic, cell culture, analytical chemistry, and electrophysiological techniques. We employed several established growth-enhancing interventions, including electrical stimulation and conditioning lesion, as well as an innovative tool called bioluminescent optogenetics. Our results indicate that sympathetic regeneration is not enhanced by any of these treatments and may even be detrimental to sympathetic regeneration. Despite the complete return of motor reinnervation after sciatic nerve injury, gastrocnemius muscle atrophy and deficits in muscle cellular energy charge, as measured by relative ATP, ADP, and AMP concentrations, persisted long after injury, even with electrical stimulation. We suggest that these long-term deficits in muscle energy charge and atrophy are related to the deficiency in sympathetic axon regeneration. New studies are needed to better understand the mechanisms underlying sympathetic regeneration to develop therapeutics that can enhance the regeneration of all axon types.

Spatial and Temporal Single-Cell Profiling of RNA Compartmentalization in Neurons with Nanotweezers

Emerging techniques for mapping mRNAs within the subcellular compartments of live cells hold great promise for advancing our understanding of the spatial distribution of transcripts and enabling the study of single-cell dynamics in health and disease. This is particularly critical for polarized cells, such as neurons, where mRNA compartmentalization is essential for regulating gene expression, and defects in these localization mechanisms are linked to numerous neurological disorders. However, many subcellular analysis techniques require a compromise between subcellular precision, live-cell measurements, and nondestructive access to single cells in their native microenvironment. To overcome these challenges, we employ a single-cell technology that we have recently developed, the nanotweezer, which features a nanoscale footprint (∼100 nm), avoids cytoplasmic fluid aspiration, and enables rapid RNA isolation from living cells with minimal invasiveness. Using this tool, we investigate single-cell mRNA compartmentalization in the soma and dendrites of hippocampal neurons at different stages of neuronal development. By combining precise targeting with sequential sampling, we track changes in mRNA abundance at dendritic spine regions of the same neuron, both before and after stimulation. This minimally invasive approach enables time-resolved, subcellular gene expression profiling of the same single cell. This could provide critical insights into polarized cells and advance our understanding of biological processes and complex diseases.

Evolution of temperature preference in flies of the genus Drosophila

The preference for a particular thermal range is a key determinant of the distribution of animal species. However, we know little on how temperature preference behaviour evolves during the colonization of new environments. Here we show that at least two distinct neurobiological mechanisms drive the evolution of temperature preference in flies of the genus Drosophila. Fly species from mild climates (D. melanogaster and D. persimilis) avoid both innocuous and noxious heat, and we show that the thermal activation threshold of the molecular heat receptor Gr28b.d precisely matches species-specific thresholds of behavioural heat avoidance. We find that desert-dwelling D. mojavensis are instead actively attracted to innocuous heat. Notably, heat attraction is also mediated by Gr28b.d (and by the antennal neurons that express it) and matches its threshold of heat activation. Rather, the switch in valence from heat aversion to attraction correlates with specific changes in thermosensory input to the lateral horn, the main target of central thermosensory pathways and a region of the fly brain implicated in the processing of innate valence1-5. Together, our results demonstrate that, in Drosophila, the adaptation to different thermal niches involves changes in thermal preference behaviour, and that this can be accomplished using distinct neurobiological solutions, ranging from shifts in the activation threshold of peripheral thermosensory receptor proteins to a substantial change in the way temperature valence is processed in the brain.

N-cadherin facilitates trigeminal sensory neuron outgrowth and target tissue innervation

The trigeminal ganglion emerges from the condensation of two distinct precursor cell populations, cranial placodes and neural crest. While its dual cellular origin is well understood, the molecules underlying its formation remain relatively obscure. Trigeminal ganglion assembly is mediated, in part, by neural cadherin (N-cadherin), which is initially expressed by placodal neurons and is required for their proper coalescence with neural crest cells. Axon outgrowth first occurs from placodal neurons, but as gangliogenesis proceeds, neural crest cells also differentiate into N-cadherin-expressing neurons, and both extend axons toward targets. However, the role of N-cadherin in axon outgrowth and target innervation has not been explored. Our data show that N-cadherin knockdown in chick trigeminal placode cells decreases trigeminal ganglion size, nerve growth and target innervation in vivo, and reduces neurite complexity of neural crest-derived neurons in vitro. Furthermore, blocking N-cadherin-mediated adhesion prevents axon extension in most placodal neurons in vitro. Collectively, these findings reveal cell- and non-cell autonomous functions for N-cadherin, highlighting its crucial role in mediating reciprocal interactions between neural crest- and placode-derived neurons throughout trigeminal ganglion development.

Molecular and Morphological Circuitry of the Octopus Sucker Ganglion

The octopus sucker is a profoundly complex sensorimotor structure. Each of the hundreds of suckers that line the octopus arm can move independently or in concert with one another. These suckers also contain an intricate sensory epithelium, enriched with chemotactile receptors. Much of the massive nervous system embedded in the octopus arm mediates control of the suckers. Each arm houses a large axial nerve cord (ANC), which features local enlargements corresponding to each sucker. There is also a sucker ganglion, a peripheral nervous element, situated in the stalk of every sucker. The structure and function of the sucker ganglion remain obscure. We examined the cellular organization and molecular composition of the sucker ganglion in Octopus bimaculoides. The sucker ganglion has an ellipsoid shape and features an unusual organization: the neuropil of the ganglion is distributed as a cap aborally (away from the sucker) and a small pocket orally (toward the sucker), with neuronal cell bodies concentrated in the space between. Using in situ hybridization, we detected positive expression of sensory (PIEZO) and motor (LHX3 and MNX) neuron markers in the sucker ganglion cell bodies. Nerve fibers spread out from the sucker ganglion, targeting the surrounding sucker musculature and the oral roots extending to the ANC. Our results indicate that the sucker ganglion is composed of both sensory and motor elements and suggest that this ganglion is not a simple relay for the ANC, but facilitates local reflexes for each sucker.

Olfactory projection neuron rewiring in the brain of an ecological specialist

Animal behaviors can differ greatly between closely related species. These behavioral changes are frequently linked to sensory system modifications, but central brain cell-type alterations might also be involved. Here, we develop advanced genetic tools to compare homologous central neurons in Drosophila sechellia, an ecological specialist, with the generalist Drosophila melanogaster. Through systematic morphological analysis of olfactory projection neurons (PNs), we reveal that the global anatomy of these second-order neurons is conserved. However, high-resolution, quantitative comparisons identify a striking case of convergent rewiring of PNs in two olfactory pathways critical for D. sechellia's host location. Calcium imaging and labeling of pre-synaptic sites in these evolved D. sechellia PNs indicate that species-specific connections with third-order partners are formed. This work demonstrates that peripheral sensory evolution is accompanied by selective wiring changes in the central brain to facilitate ecological specialization and paves the way to compare other cell types throughout the nervous system.

Pumilio differentially binds to mRNA 3' UTR isoforms to regulate localization of synaptic proteins

In neuronal cells, the regulation of RNA is crucial for the spatiotemporal control of gene expression, but how the correct localization, levels, and function of synaptic proteins are achieved is not well understood. In this study, we globally investigate the role of alternative 3' UTRs in regulating RNA localization in the synaptic regions of the Drosophila brain. We identify direct mRNA targets of the translational repressor Pumilio, finding that mRNAs bound by Pumilio encode proteins enriched in synaptosomes. Pumilio differentially binds to RNA isoforms of the same gene, favoring long, neuronal 3' UTRs. These longer 3' UTRs tend to remain in the neuronal soma, whereas shorter UTR isoforms localize to the synapse. In cultured pumilio mutant neurons, axon outgrowth defects are accompanied by mRNA isoform mislocalization, and proteins encoded by these Pumilio target mRNAs display excessive abundance at synaptic boutons. Our study identifies an important mechanism for the spatiotemporal regulation of protein function in neurons.

Piezo2 in sensory neurons regulates systemic and adipose tissue metabolism

Systemic metabolism ensures energy homeostasis through inter-organ crosstalk regulating thermogenic adipose tissue. Unlike the well-described inductive role of the sympathetic system, the inhibitory signal ensuring energy preservation remains poorly understood. Here, we show that, via the mechanosensor Piezo2, sensory neurons regulate morphological and physiological properties of brown and beige fat and prevent systemic hypermetabolism. Targeting runt-related transcription factor 3 (Runx3)/parvalbumin (PV) sensory neurons in independent genetic mouse models resulted in a systemic metabolic phenotype characterized by reduced body fat and increased insulin sensitivity and glucose tolerance. Deletion of Piezo2 in PV sensory neurons reproduced the phenotype, protected against high-fat-diet-induced obesity, and caused adipose tissue browning and beiging, likely driven by elevated norepinephrine levels. Finding that brown and beige fat are innervated by Runx3/PV sensory neurons expressing Piezo2 suggests a model in which mechanical signals, sensed by Piezo2 in sensory neurons, protect energy storage and prevent a systemic hypermetabolic phenotype.

Comparative characterization of human accelerated regions in neurons

Human accelerated regions (HARs) are conserved genomic loci that have experienced rapid nucleotide substitutions following the divergence from chimpanzees1,2. HARs are enriched in candidate regulatory regions near neurodevelopmental genes, suggesting their roles in gene regulation3. However, their target genes and functional contributions to human brain development remain largely uncharacterized. Here we elucidate the cis-regulatory functions of HARs in human and chimpanzee induced pluripotent stem (iPS) cell-induced excitatory neurons. Using genomic4 and chromatin looping information, we prioritized 20 HARs and their chimpanzee orthologues for functional characterization via single-cell CRISPR interference, and demonstrated their species-specific gene regulatory functions. Our findings reveal diverse functional outcomes of HAR-mediated cis-regulation in human neurons, including attenuated NPAS3 expression by altering the binding affinities of multiple transcription factors in HAR202 and maintaining iPS cell pluripotency and neuronal differentiation capacities through the upregulation of PUM2 by 2xHAR.319. Finally, we used prime editing to demonstrate differential enhancer activity caused by several HAR26;2xHAR.178 variants. In particular, we link one variant in HAR26;2xHAR.178 to elevated SOCS2 expression and increased neurite outgrowth in human neurons. Thus, our study sheds new light on the endogenous gene regulatory functions of HARs and their potential contribution to human brain evolution.

The PVD neuron has male-specific structure and mating function in Caenorhabditis elegans

Neurons display unique shapes and establish intricate networks, which may differ between sexes. In complex organisms, studying sex differences in structure and function of individual neurons is difficult. The nematode Caenorhabditis elegans hermaphrodites and males present an exceptional model for studying neuronal morphogenesis in a simple, sexually dimorphic system. We focus on the polymodal sensory bilateral neuron pair PVD, which forms a complex but stereotypic dendritic tree composed of multiple subunits that resemble candelabra. PVD is well studied in hermaphrodites, but not in males. We show here that during larval development, male PVD extends a similar architecture to the hermaphrodite utilizing the sexually shared Menorin patterning mechanism. In early adulthood, however, male PVD develops a unique extension into the copulatory tail structure. Alongside established tail ray neurons RnA and RnB, we show PVD is a third, previously unrecognized, neuron within the tail rays. Unlike RnA and RnB, PVD extends anterogradely, branches and turns within the ray hypodermis, and is nonciliated. This PVD sexually dimorphic arborization is absent in mutant backgrounds which perturb the Menorin guidance complex. SAX-7/L1CAM, a hypodermal component of this complex, shows a male-specific expression pattern which precedes PVD extension, and its presence allows PVD to enter the tail rays. Further, our results reveal that genetically altered arborization or ablation of the PVD results in male mating behavioral defects, particularly as males turn around the hermaphrodite. These results uncover an adult-stage sexual dimorphism of dendritic branching and a function for PVD in male sexual behavior.

Supra-second tracking and live-cell karyotyping reveal principles of mitotic chromosome dynamics

Mitotic chromosome dynamics are essential for the three-dimensional organization of the genome during the cell cycle, but the spatiotemporal characteristics of this process remain unclear due to methodological challenges. While Hi-C methods capture interchromosomal contacts, they lack single-cell temporal dynamics, whereas microscopy struggles with bleaching and phototoxicity. Here, to overcome these limitations, we introduce Facilitated Segmentation and Tracking of Chromosomes in Mitosis Pipeline (FAST CHIMP), pairing time-lapse super-resolution microscopy with deep learning. FAST CHIMP tracked all human chromosomes with 8-s resolution from prophase to telophase, identified 15 out of 23 homologue pairs in single cells and compared chromosomal positioning between mother and daughter cells. It revealed a centrosome-motion-dependent flow that governs the mapping between chromosome locations at prophase and their metaphase plate position. In addition, FAST CHIMP measured supra-second dynamics of intra- and interchromosomal contacts. This tool adds a dynamic dimension to the study of chromatin behaviour in live cells, promising advances beyond the scope of existing methods.

Condensates of synaptic vesicles and synapsin are molecular beacons for actin sequestering and polymerization

Neuronal communication relies on precisely maintained synaptic vesicle (SV) clusters, which assemble via liquid-liquid phase separation (LLPS). This process requires synapsins, the major synaptic phosphoproteins, which are known to bind actin. The reorganization of SVs, synapsins and actin is a hallmark of synaptic activity, but their interplay is still unclear. Here, we combined the reconstitution approaches, expansion microscopy, super-resolution imaging and cryo-electron tomography to dissect the roles of synapsin-SV condensates in the organization of the presynaptic actin cytoskeleton. Our data indicate that LLPS of synapsin initiates actin polymerization, allowing for SV:synapsin:actin assemblies to facilitate the mesoscale organization of SV clusters along axons mimicking the native presynaptic organization in both lamprey and mammalian synapses. Understanding the relationship between the actin network and synapsin-SVs condensates is an essential building block on a roadmap to unravel how coordinated neurotransmission along the axon enables circuit function and behavior.

Absence of GluN2A in hippocampal CA1 neurons leads to altered dendritic structure and reduced frequency of miniature excitatory synaptic events

GluN2A is a NMDA receptor subunit postulated as important for learning and memory. In humans, heterozygous loss of function variants in the gene encoding it (GRIN2A) increase the risk of epilepsy, intellectual disability and schizophrenia. Haploinsufficient mouse models show electrophysiological abnormalities and thus to improve and widen understanding of the pathogenesis of GRIN2A-associated disorders in humans, this study aimed to assess the impact of Grin2a absence and haploinsufficiency on core neuronal and synaptic properties in genetically modified rats. Electrophysiological whole-cell current- and voltage-clamp recordings were made from CA1 pyramidal neurons in acute hippocampal slices from wild-type and Grin2a heterozygous (Grin2a+/- ) and homozygous (Grin2a-/- ) knock out rats aged postnatal day 27-34. While reduced levels or absence of GluN2A did not affect neuronal excitability or intrinsic membrane properties in both Grin2a+/- and Grin2a-/- rats, we found a reduced frequency of miniature excitatory post synaptic currents and a reduced density of proximal dendrites suggestive of a reduced number of excitatory synapses. Recordings from CA1 neurons in slices prepared from Grin2a+/- and Grin2a-/- rats revealed there was a reduced ratio of the current mediated by NMDA receptors compared to AMPA receptors, while in Grin2a-/- recordings, there was a slowing of the decay time-constant of the NMDA receptor-mediated excitatory postsynaptic currents. Moreover, neither summation of sub-threshold excitatory postsynaptic potentials nor summation of supra-threshold excitatory postsynaptic potentials to initiate action potential firing in CA1 pyramidal neurons indicated any dependence on GluN2A. We conclude that reduced levels of GluN2A alters the kinetics of NMDA receptor-mediated synaptic events and dendritic structure of CA1 neurons, but do not affect several other core neuronal functions. These relatively subtle changes are consistent with the largely intact neural functioning of the majority of humans carrying GRIN2A loss of function variants. Further research could explore whether the changes in synaptic properties we observed contribute to alterations in higher level circuit dynamics and computation, which may manifest as disorders of cognition and excitability in humans.

Biological effects of rapid short pulses of focused ultrasound for drug delivery to the brain

Focused ultrasound in combination with intravenously injected microbubbles offers a non-invasive and localised method to deliver drugs across the blood-brain barrier, enabling targeted treatment of brain disorders. Recently, we have shown that applying sequences of Rapid Short-Pulses (RaSP; 5 μs pulses emitted at 1.25 kHz grouped into 10 ms bursts) of ultrasound can deliver drugs with an improved efficacy and safety profile compared with traditionally-used longer pulses (> 10 ms). In this study, we examined the extent to which RaSP sequences allowed the extravasation of endogenous blood proteins, including albumin and immunoglobulin, as well as T cells, into the brain parenchyma. We also investigated the effect of RaSP ultrasound treatments on synaptic connectivity, and the distribution and excretion of fluorescently-labelled 3 kDa dextran delivered to the brain with RaSP. The left hippocampus of mice was sonicated with either a RaSP sequence (5 μs at 1.25 kHz in groups of 10 ms at 0.5 Hz) or a long pulse sequence (10 ms at 0.5 Hz), at 0.35, 0.53 and 0.71 MPa with a 1-MHz center frequency. Significantly less albumin was detected in RaSP-treated brains immediately after treatment and was cleared within 10 min compared to those treated with long pulses, while immunoglobulin was hardly detected in RaSP-treated brains at 0, 10 or 20 min after treatment. No T cells were detected in RaSP-treated brains at 0.35, 0.53 or 0.71 MPa after 0 or 2 h. In long pulse samples, however, T cells did extravasate when using the two higher acoustic pressures, 0.53 and 0.71 MPa, immediately after treatment. Quantification of dendritic spine area revealed no differences between RaSP-treated hippocampi compared to untreated contralateral hippocampi and control mice following three weekly ultrasound treatments. Finally, fluorescently-labelled dextran increasingly moved towards blood vessels and away from the parenchyma once delivered to the brain with both RaSP and long pulse sequences. Uptake of dextran within cells decreased over time with both sequences, and long pulses lead to a larger number of vessels with dextran uptake. This study highlights that RaSP ultrasound sequences can deliver molecules across the blood-brain barrier with minimal extravasation of endogenous proteins and no T cell infiltration, while preserving dendritic spine integrity, thus offering an improved safety profile.

Correlative light and electron microscopy reveals the fine circuit structure underlying evidence accumulation in larval zebrafish

Accumulating information is a critical component of most circuit computations in the brain across species, yet its precise implementation at the synaptic level remains poorly understood. Dissecting such neural circuits in vertebrates requires precise knowledge of functional neural properties and the ability to directly correlate neural dynamics with the underlying wiring diagram in the same animal. Here we combine functional calcium imaging with ultrastructural circuit reconstruction, using a visual motion accumulation paradigm in larval zebrafish. Using connectomic analyses of functionally identified cells and computational modeling, we show that bilateral inhibition, disinhibition, and recurrent connectivity are prominent motifs for sensory accumulation within the anterior hindbrain. We also demonstrate that similar insights about the structure-function relationship within this circuit can be obtained through complementary methods involving cell-specific morphological labeling via photo-conversion of functionally identified neuronal response types. We used our unique ground truth datasets to train and test a novel classifier algorithm, allowing us to assign functional labels to neurons from morphological libraries where functional information is lacking. The resulting feature-rich library of neuronal identities and connectomes enabled us to constrain a biophysically realistic network model of the anterior hindbrain that can reproduce observed neuronal dynamics and make testable predictions for future experiments. Our work exemplifies the power of hypothesis-driven electron microscopy paired with functional recordings to gain mechanistic insights into signal processing and provides a framework for dissecting neural computations across vertebrates.

Microtubule-dependent cell polarity regulates skin-resident macrophage phagocytosis and directed cell migration

Immune cells rapidly respond to tissue damage through dynamic properties of the cytoskeleton. How microtubules control immune cell functions during injury responses remains poorly understood. Within skin, tissue-resident macrophages known as Langerhans cells use dynamic dendrites to surveil the epidermis for damage and migrate through a densely packed epithelium to wounds. Here, we use Langerhans cells within the adult zebrafish epidermis as a model to investigate roles for microtubules in immune cell tissue surveillance, phagocytosis, and directed migration. We describe microtubule organization within Langerhans cells, and show that depolymerizing the microtubule cytoskeleton alters dendrite morphology, debris engulfment, and migration efficiency. We find that the microtubule organizing center positions adjacent to engulfed debris and that its position correlates with navigational pathfinding during directed cell migration. Stabilizing microtubules prevents Langerhans cell motility during directed cell migration by impairing navigation around cellular obstacles. Collectively, our work demonstrates requirements for microtubules in the dynamic actions of tissue-resident macrophages during epithelial surveillance and wound repair.

Modeling and correction of protein conformational disease in iPSC-derived neurons through personalized base editing

Altered protein conformation can cause incurable neurodegenerative disorders. Mutations in SERPINI1, the gene encoding neuroserpin, can alter protein conformation resulting in cytotoxic aggregation leading to neuronal death. Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is a rare autosomal dominant progressive myoclonic epilepsy that progresses to dementia and premature death. We developed HEK293T and induced pluripotent stem cell (iPSC) models of FENIB, harboring a patient-specific pathogenic SERPINI1 variant or stably overexpressing mutant neuroserpin fused to GFP (MUT NS-GFP). Here, we utilized a personalized adenine base editor (ABE)-mediated approach to correct the pathogenic variant efficiently and precisely to restore neuronal dendritic morphology. ABE-treated MUT NS-GFP cells demonstrated reduced inclusion size and number. Using an inducible MUT NS-GFP neuron system, we identified early prevention of toxic protein expression allowed aggregate clearance, while late prevention halted further aggregation. To address several challenges for clinical applications of gene correction, we developed a neuron-specific engineered virus-like particle to optimize neuronal ABE delivery, resulting in higher correction efficiency. Our findings provide a targeted strategy that may treat FENIB and potentially other neurodegenerative diseases due to altered protein conformation such as Alzheimer's and Huntington's diseases.

Downregulation of Pten Improves Huntington's Disease Phenotype by Reducing Htt Aggregates and Cell Death

Huntington's disease (HD) is a dominantly inherited neurodegenerative disorder that stems from the expansion of CAG repeats within the coding region of Huntingtin (HTT) gene. Currently, there exists no effective therapeutic intervention that can prevent the progression of the disease. Our study aims to identify a novel genetic modifier with therapeutic potential. We employ transgenic flies containing HTT.ex1.Q93 and mRFP-HTT.588.Q138 constructs, which encode mutant pathogenic Huntingtin (Htt) proteins featuring 93 and 138 polyglutamine (Q) repeats respectively. The resultant mutant proteins cause the loss of photoreceptor neurons in the eye and a progressive loss of neuronal tissues in the brain and motor neurons in Drosophila. Several findings have demonstrated the association of HD with growth factor signaling defects. Phosphatase and tensin homolog (Pten) have been implicated in the negative regulation of the Insulin signaling/receptor tyrosine signaling pathway which regulates the growth and survival of cells. In the present study, we downregulated Pten and found a significant improvement in morphological phenotypes in the eye, brain, and motor neurons. These findings were further correlated with the enhancement of the functional vision and climbing ability of the flies. We also found the reduction in both Htt aggregate and caspase levels which are involved in the apoptotic pathway. In alignment with the genetic modulation of Pten, we elucidated the protective role of Pten inhibition through the utilization of VO-OHpic. VO-OHpic improved the climbing ability of flies and reduced the poly(Q) aggregates and apoptosis levels. A similar reduction in Htt aggregates was observed in the mouse neuronal inducible HD cell line model. Our study illustrates that Pten inhibition is a potential therapeutic approach for HD.

Sympathetic neurons can modify the intrinsic structural and functional properties of human pluripotent stem cell-derived cardiomyocytes

The sympathetic nervous system densely innervates all cardiac chambers and is a key player in cardiac control, yet this relationship has scarcely been investigated using a stem cell-based model. This study investigates the effects that sympathetic neurons (SNs) have on human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) in vitro, and whether they induce any degree of functional or structural maturity in these conventionally immature cells. SNs were isolated from neonatal rat pups, and cocultured with hPSC-CMs for up to 15 days. Structural changes in hPSC-CMs were analysed by microscopy techniques. Fluorescence resonance energy transfer was used to measure second messenger molecule cAMP production and β-adrenergic receptor (βAR) response. Contractile and Ca2+ transient activity was measured using CytoCypher. These cocultures promoted hPSC-CM structural elongation and increased sarcomere organization. Furthermore, the βAR response of cocultured hiPSC-CMs was larger, indicated by increased cAMP production upon neuronal nicotinic stimulation. Faster contraction and ratiometric Ca2+ transient peak height and kinetic parameters strongly indicate increased chronotropic response in coculture. Coculture with SNs also elicited an increase in action potential amplitude and depolarization velocity, further confirming that SNs contribute to hiPSC-CM functional maturation. Overall, we have found that SNs modulate hPSC-CMs in vitro, inducing a more mature functional response. As an in vitro tool, these cocultures could serve as a model of sympathoadrenergic disease, enabling new discovery avenues. KEY POINTS: The sympathetic nervous system controls the involuntary 'fight-or-flight' response, with the heart being one of key target organs. In certain neuro-cardiac diseases, the input from the sympathetic nervous system is hyperregulated, and can lead to increased speed or force of the heart's contraction. Human induced pluripotent stem cells (hiPSCs) represent a rapidly evolving field which allow us to create a cell of interest and model its structural and functional activity in a dish. Here we have created hiPSC-derived cardiomyocytes (hiPSC-CMs) and cocultured them with sympathetic neurons (SNs). We found that SNs are able to modulate structure of the hiPSC-CMs by reducing their circularity and increasing sarcomeric organization, and can significantly increase the speed of contraction and Ca2+ handling. Together, our data provide a platform to investigate the neuro-cardiac relationship in vitro, which could be used for patient-specific disease modelling in future.

Cardiac sympathetic neurons are additional cells affected in genetically determined arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (AC) is a familial cardiac disease, mainly caused by mutations in desmosomal genes, which accounts for most cases of stress-related arrhythmic sudden death, in young and athletes. AC hearts display fibro-fatty lesions that generate the arrhythmic substrate and cause contractile dysfunction. A correlation between physical/emotional stresses and arrhythmias supports the involvement of sympathetic neurons (SNs) in the disease, but this has not been confirmed previously. Here, we combined molecular, in vitro and ex vivo analyses to determine the role of AC-linked DSG2 downregulation on SN biology and assess cardiac sympathetic innervation in desmoglein-2 mutant (Dsg2mut/mut) mice. Molecular assays showed that SNs express DSG2, implying that DSG2-mutation carriers would harbour the mutant protein in SNs. Confocal immunofluorescence of heart sections and 3-D reconstruction of SN network in clarified heart blocks revealed significant changes in the physiologialc SN topology, with massive hyperinnervation of the intact subepicardial layers and heterogeneous distribution of neurons in fibrotic areas. Cardiac SNs isolated from Dsg2mut/mut neonatal mice, prior to the establishment of cardiac innervation, show alterations in axonal sprouting, process development and distribution of varicosities. Consistently, virus-assisted DSG2 downregulation replicated, in PC12-derived SNs, the phenotypic alterations displayed by Dsg2mut/mut primary neurons, corroborating that AC-linked Dsg2 variants may affect SNs. Our results reveal that altered sympathetic innervation is an unrecognized feature of AC hearts, which may result from the combination of cell-autonomous and context-dependent factors implicated in myocardial remodelling. Our results favour the concept that AC is a disease of multiple cell types also hitting cardiac SNs. KEY POINTS: Arrhythmogenic cardiomyopathy is a genetically determined cardiac disease, which accounts for most cases of stress-related arrhythmic sudden death. Arrhythmogenic cardiomyopathy linked to mutations in desmoglein-2 (DSG2) is frequent and leads to a left-dominant form of the disease. Arrhythmogenic cardiomyopathy has been approached thus far as a disease of cardiomyocytes, but we here unveil that DSG2 is expressed, in addition to cardiomyocytes, by cardiac and extracardiac sympathetic neurons, although not organized into desmosomes. AC-linked DSG2 downregulation primarily affect sympathetic neurons, resulting in the significant increase in cardiac innervation density, accompanied by alterations in sympathetic neuron distribution. Our data supports the notion that AC develops with the contribution of several 'desmosomal protein-carrying' cell types and systems.

Comparative specialization of intrinsic cardiac neurons in humans, mice and pigs

Intrinsic cardiac neurons (ICNs) play a crucial role in the proper functioning of the heart; yet a paucity of data pertaining to human ICNs exist. We took a multidisciplinary approach to complete a detailed cellular comparison of the structure and function of ICNs from mice, pigs and humans. Immunohistochemistry of whole and sectioned ganglia, transmission electron microscopy, intracellular microelectrode recording and dye filling for quantitative morphometry were used to define the neurophysiology, histochemistry and ultrastructure of these neurons across species. The densely packed, smaller ICNs of mouse lacked dendrites, formed axosomatic connections and had high synaptic efficacy constituting an obligatory synapse. At pig ICNs, a convergence of subthreshold cholinergic inputs onto extensive dendritic arbors supported greater summation and integration of synaptic input. Human ICNs were tonically firing, with synaptic stimulation evoking large suprathreshold EPSPs like mouse, and subthreshold potentials like pig. Ultrastructural examination of synaptic terminals revealed conserved architecture, yet small clear vesicles were larger in pigs and humans. The presence and localization of ganglionic neuropeptides was distinct, with abundant vasoactive intestinal polypeptide observed in human but not pig or mouse ganglia, and little substance P or calcitonin gene-related peptide in pig ganglia. Action potential waveforms were similar, but human ICNs had larger after-hyperpolarizations. Intrinsic excitability differed; 95% of human neurons were tonic, all pig neurons were phasic, and both phasic and tonic phenotypes were observed in mouse. In combination, this publicly accessible, multimodal atlas of ICNs from mice, pigs and humans identifies similarities and differences in the evolution of ICNs. KEY POINTS: Intrinsic cardiac neurons (ICNs) are essential to the regulation of cardiac function. We investigated the neurochemistry, morphology, ultrastructure, membrane physiology and synaptic transmission of ICNs from donated human hearts in parallel with identical studies of ICNs from mice and pigs to create a publicly accessible cellular atlas detailing the structure and function of these neurons across species. In addition to presenting foundational data on human ICNs, this comparative study identifies both conserved and derived attributes of these neurons within mammals. The findings have significant implications for understanding the regulation of cardiac autonomic function in humans and may greatly influence strategies for neuromodulation in conditions such as atrial fibrillation and heart failure.

Muscle fibroblasts and stem cells stimulate motor neurons in an age and exercise-dependent manner

Exercise preserves neuromuscular function in aging through unknown mechanisms. Skeletal muscle fibroblasts (FIB) and stem cells (MuSC) are abundant in skeletal muscle and reside close to neuromuscular junctions, but their relative roles in motor neuron maintenance remain undescribed. Using direct cocultures of embryonic rat motor neurons with either human MuSC or FIB, RNA sequencing revealed profound differential regulation of the motor neuron transcriptome, with FIB generally favoring neuron growth and cell migration and MuSC favoring production of ribosomes and translational machinery. Conditioned medium from FIB was superior to MuSC in preserving motor neurons and increasing their maturity. Lastly, we established the importance of donor age and exercise status and found an age-related distortion of motor neuron and muscle cell interaction that was fully mitigated by lifelong physical activity. In conclusion, we show that human muscle FIB and MuSC synergistically stimulate the growth and viability of motor neurons, which is further amplified by regular exercise.

CCN1 derived from vascular endothelial cells impairs cognitive function in Alzheimer's disease model mice

Vascular endothelial cell-expressing molecules regulate neuronal function. Although cerebrovascular dysregulation is a hallmark of Alzheimer's disease (AD), the effect of changes in molecular expression on neuronal function in vascular endothelial cells during disease progression is not clear. In this study, we demonstrated that the cellular communication network factor 1 (CCN1), which is highly expressed in vascular endothelial cells during the chronic stage of AD in mice, is involved in the impairment of cognitive function. Vascular endothelial cells isolated from the brains of AppNL-G-F mice show differential expression of genes, including CCN1. CCN1 treatment decreased the synaptic number in cultured hippocampal cells, with changes in the expression of genes associated with morphological changes. In vivo, AppNL-G-F mice with CCN1 silencing in vascular endothelial cells demonstrated high spine density and improved spatial learning. No significant change was observed in the number of microglia/macrophages, astrocytes, and amyloid-beta (Aβ) accumulation in the hippocampus of the mice. These results suggest that CCN1 is a key factor modulating neurological dysfunction through neurovascular interactions.

A Critical Role for the Fascin Family of Actin Bundling Proteins in Axon Development, Brain Wiring and Function

Actin-based cell motility drives many neurodevelopmental events including guided axonal growth. Fascin is a major family of F-actin bundling proteins, but its role in axon development and brain wiring is unknown. Here, we report that fascin is required for axon development, brain wiring and function. We show that fascin is enriched in the motile filopodia of axonal growth cones and its inhibition impairs axonal extension and branching of hippocampal neurons in culture. We next provide evidence that fascin is essential for axon development and brain wiring using Drosophila melanogaster as an in vivo model. Drosophila express a single ortholog of mammalian fascin called Singed (SN), which is highly expressed in the mushroom body (MB) of the central nervous system. We observe that loss of SN results in drastic MB disruption, highlighted by α- and β-lobe defects that are consistent with altered axonal guidance. SN-null flies also exhibit defective sensorimotor behaviors as assessed by the negative geotaxis assay. MB-specific expression of SN in SN-null flies rescues MB structure and sensorimotor deficits, indicating that SN functions autonomously in MB neurons. Together, our data from primary neuronal culture and in vivo models highlight a critical role for fascin in brain development and function.

Significance statement

Fascin is a major family of actin-binding proteins (ABPs) that crosslink and bundle actin filaments to underline membrane protrusions essential for cell motility and neuronal development.Inhibition of fascin in cultured cells causes it to dissipate from F-actin-rich membrane protrusions and become cytosolic, reduces filopodia formation, and decreases axon outgrowth and branching.In fruit flies, loss of the fascin orthologue, Singed, causes defects in brain development, specifically in mushroom body (MB) lobes, and leads to behavioral impairments which can be rescued upon MB-specific expression of Singed.

Subthreshold electric fields bidirectionally modulate neurotransmitter release through axon polarization

Subthreshold electric fields modulate brain activity and demonstrate potential in several therapeutic applications. Electric fields are known to generate heterogenous membrane polarization within neurons due to their complex morphologies. While the effects of somatic and dendritic polarization in postsynaptic neurons have been characterized, the functional consequences of axonal polarization on neurotransmitter release from the presynapse are unknown. Here, we combined noninvasive optogenetic indicators of voltage, calcium and neurotransmitter release to study the subcellular response within single neurons to subthreshold electric fields. We first captured the detailed spatiotemporal polarization profile produced by uniform electric fields within individual neurons. Small polarization of presynaptic boutons produces rapid and powerful modulation of neurotransmitter release, with the direction - facilitation or inhibition - depending on the direction of polarization. We determined that subthreshold electric fields drive this effect by rapidly altering the number of synaptic vesicles participating in neurotransmission, producing effects which resemble short-term plasticity akin to presynaptic homeostatic plasticity. These results provide key insights into the mechanisms of subthreshold electric fields at the cellular level.

Abstract Figure

Cerebellar output neurons can impair non-motor behaviors by altering development of extracerebellar connectivity

The capacity of the brain to compensate for insults during development depends on the type of cell loss, whereas the consequences of genetic mutations in the same neurons are difficult to predict. We reveal powerful compensation from outside the mouse cerebellum when the excitatory cerebellar output neurons are ablated embryonically and demonstrate that the main requirement for these neurons is for motor coordination and not basic learning and social behaviors. In contrast, loss of the homeobox transcription factors Engrailed1/2 (EN1/2) in the cerebellar excitatory lineage leads to additional deficits in adult learning and spatial working memory, despite half of the excitatory output neurons being intact. Diffusion MRI indicates increased thalamo-cortico-striatal connectivity in En1/2 mutants, showing that the remaining excitatory neurons lacking En1/2 exert adverse effects on extracerebellar circuits regulating motor learning and select non-motor behaviors. Thus, an absence of cerebellar output neurons is less disruptive than having cerebellar genetic mutations.

A dendritic guidance receptor functions in both ligand dependent and independent modes

The formation of an appropriately shaped dendritic arbor is critical for a neuron to receive information. Dendritic morphogenesis is a dynamic process involving growth, branching, and retraction. How the growth and stabilization of dendrites are coordinated at the molecular level remains a key question in developmental neurobiology. The highly arborized and stereotyped dendritic arbors of the Caenorhabditis elegans PVD neuron are shaped by the transmembrane DMA-1 receptor through its interaction with a tripartite ligand complex consisting of SAX-7, MNR-1, and LECT-2. However, receptor null mutants exhibit strongly reduced dendrite outgrowth, whereas ligand null mutants show disordered branch patterns, suggesting a ligand-independent function of the receptor. To test this idea, we identified point mutations in dma-1 that disrupt receptor-ligand binding and introduced corresponding mutations into the endogenous gene. We show that the ligand-free receptor is sufficient to drive robust, disordered dendritic branch formation but results in a complete loss of arbor shape. This disordered outgrowth program utilizes similar downstream effectors as the stereotyped outgrowth program, further arguing that ligand binding is not necessary for outgrowth. Finally, we demonstrate that ligand binding is required to maintain higher-order dendrites after development is complete. Taken together, our findings support a surprising model in which ligand-free and ligand-bound DMA-1 receptors have distinct functions: the ligand-free receptor promotes stochastic outgrowth and branching, whereas the ligand-bound receptor guides stereotyped dendrite morphology by stabilizing arbors at target locations.

Bioelectric stimulation outperforms brain derived neurotrophic factor in promoting neuronal maturation

Neuronal differentiation and maturation are crucial for developing research models and therapeutic applications. Brain-derived neurotrophic factor (BDNF) is a widely used biochemical stimulus for promoting neuronal maturation. However, the broad effects of biochemical stimuli on multiple cellular functions limit their applicability in both in vitro models and clinical settings. Electrical stimulation (ES) offers a promising physical method to control cell fate and function, but it is hampered by lack of standard and optimised protocols. In this study, we demonstrate that ES outperforms BDNF in promoting neuronal maturation in human neuroblastoma SH-SY5Y. Additionally, we address the question regarding which ES parameters regulate biological responses. The neuronal differentiation and maturation of SH-SY5Y cells were tested under several pulsed ES regimes. We identified accumulated charge and effective electric field time as novel criteria for determining optimal ES regimes. ES parameters were obtained using electrochemical characterisation and equivalent circuit modelling. Our findings show that neuronal maturation in SH-SY5Y cells correlates with the amount of accumulated charge during ES. Higher charge accumulation (~ 50 mC/h) significantly promotes extensive neurite outgrowth and ramification, and enhances the expression of synaptophysin, yielding effects exceeding those of BDNF. In contrast, fewer charge injection to the culture (~ 0.1 mC/h) minimally induces maturation but significantly increases cell proliferation. Moreover, ES altered the concentration and protein cargo of secreted extracellular vesicles (EV). ES with large enough accumulated charge significantly enriched EV proteome associated with neural development and function. These results demonstrate that each ES regime induces distinct cellular responses. Increased accumulated charge facilitates the development of complex neuronal morphologies and axonal ramification, outperforming exogenous neurotrophic factors. Controlled ES methods are immediately applicable in creating mature neuronal cultures in vitro with minimal chemical intervention.

Expression of Osteopontin in M2 and M4 Intrinsically Photosensitive Retinal Ganglion Cells in the Mouse Retina

Purpose

Melanopsin-expressing intrinsically photosensitive (ip) retinal ganglion cells (RGCs) can be divided into six different subtypes (M1 - M6). Yet, specific markers exist for only some of these subtypes that could be employed to study the function of individual subtypes. Osteopontin (Spp1) marks αRGC, suggesting that, across ipRGCs, it would only mark the M4-ipRGC subtype (synonymous to ON-sustained αRGCs). Recent evidence suggests that osteopontin expression could spread to other ipRGC subtypes. Therefore, this study aims to characterize the expression pattern of osteopontin across ipRGC subtypes in mice.

Methods

Single-cell RNA (scRNA-seq) sequencing data from murine RGCs were analyzed to identify expression patterns of Spp1 across ipRGCs. Immunohistochemistry (IHC) was performed on retinal cryosections and flatmounts from C57BL/6J mice to characterize the localization of osteopontin across ipRGCs. Neurite tracing was employed to study dendritic morphology and identify individual ipRGC subtypes.

Results

scRNA-seq analysis revealed Spp1 expression in two distinct clusters of ipRGCs. IHC confirmed osteopontin colocalization with neurofilament heavy chain, an established marker for αRGCs, including M4-ipRGCs. Spp1 immunoreactivity was moreover identified in one additional group of ipRGCs. By dendritic morphology and stratification, those cells were clearly identified as M2-ipRGCs.

Conclusions

Our findings demonstrate that osteopontin is expressed in both M2- and M4-ipRGCs, challenging the notion of osteopontin as a marker exclusively for αRGCs. IHC double-labeling for osteopontin and melanopsin provides a novel method to identify and differentiate M2 ipRGCs from other subtypes. This will support the study of ipRGC physiology in a subtype -specific manner and may, for instance, foster research in the field of optic nerve injury.

RhoA regulates oligodendrocyte differentiation and myelination by orchestrating cortical and membrane tension

Timely differentiation and myelin formation by oligodendrocytes are essential for the physiological functioning of the central nervous system (CNS). While the Rho GTPase RhoA has been hinted as a negative regulator of myelin sheath formation, the precise in vivo mechanisms have remained elusive. Here we show that RhoA controls the timing and progression of myelination by oligodendrocytes through a fine-tuned balance between cortical tension, membrane tension and cell shape. Using a conditional mouse model, we observe that Rhoa ablation results in the acceleration of myelination driven by hastened differentiation and facilitated through membrane expansion induced by changes in MLCII activity and in F-actin redistribution and turnover within the cell. These findings reveal RhoA as a central molecular integrator of alterations in actin cytoskeleton, actomyosin contractility and membrane tension underlying precise morphogenesis of oligodendrocytes and normal myelination of the CNS.

Schizophrenia-related Xpo7 haploinsufficiency leads to behavioral and nuclear transport pathologies

Recent genetic studies by the Schizophrenia Exome Sequencing Meta-Analysis (SCHEMA) consortium have identified that protein-truncating variants of exportin 7 (XPO7) can increase the risk of schizophrenia (odds ratio, 28.1). Here we show that mice with Xpo7 haploinsufficiency (Xpo7+/- mice) present with cognitive and social behavioral impairments. Through proteome analysis using immunoprecipitation and frontal cortex nuclear isolation of Xpo7+/- mice, we identify 45 molecules interacting with Xpo7, including CutC, Rbfox3, and Gria3. Through single-nucleus RNA sequencing of the frontal cortex and striatum of Xpo7+/- mice differentiating between the onset and progressive stages, we also identify 284 gene expression changes that correlate with these stages. These genes encompass high-odds risk genes of schizophrenia identified by SCHEMA, including Gria3, Grin2A, Herc1, and Trio. Furthermore, our approach reveals 15 gene expression changes in the frontal cortex that correlate with the progressive stages. Our findings indicate the importance of investigating whether the interactions among the high-risk genes identified by SCHEMA contribute to a common schizophrenia pathology and underscore the significance of stage-dependent analysis.

A cell type-specific mechanism driving the rapid antidepressant effects of transcranial magnetic stimulation

Repetitive transcranial magnetic stimulation (rTMS) is an emerging treatment for brain disorders, but its therapeutic mechanism is unknown. We developed a novel mouse model of rTMS with superior clinical face validity and investigated the neural mechanism by which accelerated intermittent theta burst stimulation (aiTBS) - the first rapid-acting rTMS antidepressant protocol - reversed chronic stress-induced behavioral deficits. Using fiber photometry, we showed that aiTBS drives distinct patterns of neural activity in intratelencephalic (IT) and pyramidal tract (PT) projecting neurons in dorsomedial prefrontal cortex (dmPFC). However, only IT neurons exhibited persistently increased activity during both aiTBS and subsequent depression-related behaviors. Similarly, aiTBS reversed stress-related loss of dendritic spines on IT, but not PT neurons, further demonstrating cell type-specific effects of stimulation. Finally, chemogenetic inhibition of dmPFC IT neurons during rTMS blocked the antidepressant-like behavioral effects of aiTBS. Thus, we demonstrate a prefrontal mechanism linking rapid aiTBS-driven therapeutic effects to cell type-specific circuit plasticity.

Multifactorial approach is needed to unravel the maturation phases of human neurons derived from induced pluripotent stem cells

Neurons derived from induced pluripotent stem cells (h-iPSC-Ns) provide an invaluable model for studying the physiological aspects of human neuronal development under healthy and pathological conditions. However, multiple studies have demonstrated that h-iPSC-Ns exhibit a high degree of functional and epigenetic diversity. Due to the imprecise characterization and significant variation among the currently available maturation protocols, it is essential to establish a set of criteria to standardize models and accurately characterize and define the developmental properties of human neurons derived from iPSCs. In this study, we conducted comprehensive cellular and network level analysis of the functional development of human neurons, generated from iPSCs obtained from healthy young female peripheral blood mononuclear cells by BDNF and GDNF treatment. We provide a thorough description of the maturation process of h-iPSC-Ns over a 10-week in vitro period using conventional whole-cell patch clamp and dynamic clamp techniques, alongside with morphometry and immunocytochemistry. Additionally, we utilized calcium imaging to monitor the progression of synaptic activity and network communication. At the single cell level, human neurons exhibited gradually decreasing membrane resistance in parallel with improved excitability. By the fifth week of maturation, firing profiles were consistent with those of mature regular firing type of neurons. At the network level, fast glutamatergic and depolarizing GABAergic synaptic connections were abundant together with synchronized network activity from the sixth week of maturation. Alterations in the expression of GABAA receptor subunits were also observed during the process of maturation. The sequence of differentiation events was consistent, providing a robust temporal framework to execute experiments at defined stages of neuronal maturation as well as to use a specific set of experiments to assess a culture's maturation. The uncovered progression of differentiation events provides a powerful tool to aid the planning and designing of targeted experiments during defined stages of neuronal maturation.

Primary Neuronal Culture and Transient Transfection

Primary neuronal culture and transient transfection offer a pair of crucial tools for neuroscience research, providing a controlled environment to study the behavior, function, and interactions of neurons in vitro. These cultures can be used to investigate fundamental aspects of neuronal development and plasticity, as well as disease mechanisms. There are numerous methods of transient transfection, such as electroporation, calcium phosphate precipitation, or cationic lipid transfection. In this protocol, we used electroporation for neurons immediately before plating and cationic lipid transfection for neurons that have been cultured for a few days in vitro. In our experience, the transfection efficiency of electroporation can be as high as 30%, and cationic lipid transfection has an efficiency of 1%-2%. While cationic lipid transfection has much lower efficiency than electroporation, it does offer the advantage of a higher expression level. Therefore, these transfection methods are suitable for different stages of neurons and different expression requirements. Key features • Culture of primary neurons from the CNS. • Electroporation for freshly isolated neurons in suspension. • Cationic lipid transfection for adherent neurons.

Regenerative failure of sympathetic axons contributes to deficits in functional recovery after nerve injury

Renewed scientific interest in sympathetic modulation of muscle and neuromuscular junctions has spurred a flurry of new discoveries with major implications for motor diseases. However, the role sympathetic axons play in the persistent dysfunction that occurs after nerve injuries remains to be explored. Peripheral nerve injuries are common and lead to motor, sensory, and autonomic deficits that result in lifelong disabilities. Given the importance of sympathetic signaling in muscle metabolic health and maintaining bodily homeostasis, it is imperative to understand the regenerative capacity of sympathetic axons after injury. Therefore, we tested sympathetic axon regeneration and functional reinnervation of skin and muscle, both acute and long-term, using a battery of anatomical, pharmacological, chemogenetic, cell culture, analytical chemistry, and electrophysiological techniques. We employed several established growth-enhancing interventions, including electrical stimulation and conditioning lesion, as well as an innovative tool called bioluminescent optogenetics. Our results indicate that sympathetic regeneration is not enhanced by any of these treatments and may even be detrimental to sympathetic regeneration. Despite the complete return of motor reinnervation after sciatic nerve injury, gastrocnemius muscle atrophy and deficits in muscle cellular energy charge, as measured by relative ATP, ADP, and AMP concentrations, persisted long after injury, even with electrical stimulation. We suggest that these long-term deficits in muscle energy charge and atrophy are related to the deficiency in sympathetic axon regeneration. New studies are needed to better understand the mechanisms underlying sympathetic regeneration to develop therapeutics that can enhance the regeneration of all axon types.

Neuronal segmentation in cephalopod arms

Prehensile arms are among the most remarkable features of the octopus, but little is known about the neural circuitry controlling arm movements. Here, we report on the cellular and molecular organization of the arm nervous system, focusing on its massive axial nerve cords (ANCs). We found that the ANC is segmented. In transverse cross sections, the ANC cell body layer wraps around the neuropil with no apparent segregation of sensory and motor neurons. In longitudinal sections, however, ANC neurons form segments, setting up a modular organization to the adjoining neuropil. ANC nerves exit in the septa between segments, and for each sucker, form a spatial topographic map ("suckerotopy"). A strong link between ANC segmentation and flexible sucker-laden arms is confirmed by comparative study of squid arms and tentacles. These ANC modules offer a template for modeling the motor control of soft tissues and provide a compelling example of nervous system segmentation in molluscs.

Neuro-Mesenchymal Interaction Mediated by a β2-Adrenergic Nerve Growth Factor Feedforward Loop Promotes Colorectal Cancer Progression

SIGNIFICANCE

Our work demonstrates that the bidirectional interplay between sympathetic nerves and NGF-expressing CAFs drives colorectal tumorigenesis. This study also offers novel mechanistic insights into catecholamine action in colorectal cancer. Inhibiting the neuro-mesenchymal interaction by TRK blockade could be a potential strategy for treating colorectal cancer.

ATG8 delipidation is not universally critical for autophagy in plants

Intracellular recycling via autophagy is governed by post-translational modifications of the autophagy-related (ATG) proteins. One notable example is ATG4-dependent delipidation of ATG8, a process that plays critical but distinct roles in autophagosome formation in yeast and mammals. Here, we aim to elucidate the specific contribution of this process to autophagosome formation in species representative of evolutionarily distant green plant lineages: unicellular green alga Chlamydomonas reinhardtii, with a relatively simple set of ATG genes, and a vascular plant Arabidopsis thaliana, harboring expanded ATG gene families. Remarkably, the more complex autophagy machinery of Arabidopsis renders ATG8 delipidation entirely dispensable for the maturation of autophagosomes, autophagic flux, and related stress tolerance; whereas autophagy in Chlamydomonas strictly depends on the ATG4-mediated delipidation of ATG8. Importantly, we also demonstrate the distinct impact of different Arabidopsis ATG8 orthologs on autophagosome formation, especially prevalent under nitrogen depletion, providing new insight into potential drivers behind the expansion of the ATG8 family in higher plants. Our findings underscore the evolutionary diversification of the molecular mechanism governing the maturation of autophagosomes in eukaryotic lineages and highlight how this conserved pathway is tailored to diverse organisms.

INSIHGT: an accessible multi-scale, multi-modal 3D spatial biology platform

Biological systems are complex, encompassing intertwined spatial, molecular and functional features. However, methodological constraints limit the completeness of information that can be extracted. Here, we report the development of INSIHGT, a non-destructive, accessible three-dimensional (3D) spatial biology method utilizing superchaotropes and host-guest chemistry to achieve homogeneous, deep penetration of macromolecular probes up to centimeter scales, providing reliable semi-quantitative signals throughout the tissue volume. Diverse antigens, mRNAs, neurotransmitters, and post-translational modifications are well-preserved and simultaneously visualized. INSIHGT also allows multi-round, highly multiplexed 3D molecular probing and is compatible with downstream traditional histology and nucleic acid sequencing. With INSIHGT, we map undescribed podocyte-to-parietal epithelial cell microfilaments in mouse glomeruli and neurofilament-intensive inclusion bodies in the human cerebellum, and identify NPY-proximal cell types defined by spatial morpho-proteomics in mouse hypothalamus. We anticipate that INSIHGT can form the foundations for 3D spatial multi-omics technology development and holistic systems biology studies.

Transport properties of canonical PIN-FORMED proteins from Arabidopsis and the role of the loop domain in auxin transport

The phytohormone auxin is polarly transported in plants by PIN-FORMED (PIN) transporters and controls virtually all growth and developmental processes. Canonical PINs possess a long, largely disordered cytosolic loop. Auxin transport by canonical PINs is activated by loop phosphorylation by certain kinases. The structure of the PIN transmembrane domains was recently determined, their transport properties remained poorly characterized, and the role of the loop in the transport process was unclear. Here, we determined the quantitative kinetic parameters of auxin transport mediated by Arabidopsis PINs to mathematically model auxin distribution in roots and to test these predictions in vivo. Using chimeras between transmembrane and loop domains of different PINs, we demonstrate a strong correlation between transport parameters and physiological output, indicating that the loop domain is not only required to activate PIN-mediated auxin transport, but it has an additional role in the transport process by a currently unknown mechanism.

Modeling and Correction of Protein Conformational Disease in iPSC-derived Neurons through Personalized Base Editing

Altered protein conformation can cause incurable neurodegenerative disorders. Mutations in SERPINI1 , the gene encoding neuroserpin, can alter protein conformation resulting in cytotoxic aggregation leading to neuronal death. Familial encephalopathy with neuroserpin inclusion bodies (FENIB) is a rare autosomal dominant progressive myoclonic epilepsy that progresses to dementia and premature death. We developed HEK293T and induced pluripotent stem cell (iPSC) models of FENIB, harboring a patient-specific pathogenic SERPINI1 variant or stably overexpressing mutant neuroserpin fused to GFP (MUT NS-GFP). Here, we utilized a personalized adenine base editor (ABE)-mediated approach to correct the pathogenic variant efficiently and precisely to restore neuronal dendritic morphology. ABE-treated MUT NS-GFP cells demonstrated reduced inclusion size and number. Using an inducible MUT NS-GFP neuron system, we identified early prevention of toxic protein expression allowed aggregate clearance, while late prevention halted further aggregation. To address several challenges for clinical applications of gene correction, we developed a neuron-specific engineered virus-like particle to optimize neuronal ABE delivery, resulting in higher correction efficiency. Our findings provide a targeted strategy which may treat FENIB and potentially other neurodegenerative diseases due to altered protein conformation such as Alzheimer's and Huntington's diseases.

Human iPSC-derived myelinating organoids and globoid cells to study Krabbe disease

Krabbe disease (Kd) is a lysosomal storage disorder (LSD) caused by the deficiency of the lysosomal galactosylceramidase (GALC) which cleaves the myelin enriched lipid galactosylceramide (GalCer). Accumulated GalCer is catabolized into the cytotoxic lipid psychosine that causes myelinating cells death and demyelination which recruits microglia/macrophages that fail to digest myelin debris and become globoid cells. Here, to understand the pathological mechanisms of Kd, we used induced pluripotent stem cells (iPSCs) from Kd patients to produce myelinating organoids and microglia. We show that Kd organoids have no obvious defects in neurogenesis, astrogenesis, and oligodendrogenesis but manifest early myelination defects. Specifically, Kd organoids showed shorter but a similar number of myelin internodes than Controls at the peak of myelination and a reduced number and shorter internodes at a later time point. Interestingly, myelin is affected in the absence of autophagy and mTOR pathway dysregulation, suggesting lack of lysosomal dysfunction which makes this organoid model a very valuable tool to study the early events that drive demyelination in Kd. Kd iPSC-derived microglia show a marginal rate of globoid cell formation under normal culture conditions that is drastically increased upon GalCer feeding. Under normal culture conditions, Kd microglia show a minor LAMP1 content decrease and a slight increase in the autophagy protein LC3B. Upon GalCer feeding, Kd cells show accumulation of autophagy proteins and strong LAMP1 reduction that at a later time point are reverted showing the compensatory capabilities of globoid cells. Altogether, this supports the value of our cultures as tools to study the mechanisms that drive globoid cell formation and the compensatory mechanism in play to overcome GalCer accumulation in Kd.

KATP channel mutation disrupts hippocampal network activity and nocturnal gamma shifts

ATP-sensitive potassium (KATP) channels couple cell metabolism to cellular electrical activity. Humans affected by severe activating mutations in KATP channels suffer from developmental delay, epilepsy and neonatal diabetes (DEND syndrome). While the aetiology of diabetes in DEND syndrome is well understood, the pathophysiology of the neurological symptoms remains unclear. We hypothesized that impaired activity of parvalbumin-positive interneurons (PV-INs) may result in seizures and cognitive problems. We found, by performing electrophysiological experiments, that expressing the DEND mutation Kir6.2-V59M selectively in mouse PV-INs reduced intrinsic gamma frequency preference and short-term depression as well as disturbed cognition-associated gamma oscillations and hippocampal sharp waves. Furthermore, the risk of seizures was increased and the day-night shift in gamma activity disrupted. Blocking KATP channels with tolbutamide partially rescued the network oscillations. The non-reversible part may, to some extent, result from observed altered PV-IN dendritic branching and PV-IN arrangement within CA1. In summary, PV-INs play a key role in DEND syndrome, and this provides a framework for establishing treatment options.

Characterization of the Retinal Circulation of the Mouse

Purpose

Mice are highly used in retinal research because, like humans, mice have vascularized retinas and choroidal circulation. Although the retinal circulation has been well-characterized in development, its stability during adulthood is less understood. To examine this network, we quantified several key metrics of the trilaminar vasculature.

Methods

We used mice (n = 15) with transgenic fluorescent NG2-DsRed (JX: #00824), a vascular-associated label in the retina. One eye per mouse was imaged using confocal microscopy (Nikon A1 Ti2 Eclipse) and traced with ImageJ SNT tools. Using an adaptive optics scanning light ophthalmoscope, additional mice (n = 3) were imaged at single-cell resolution within the living eye to measure the same vasculature.

Results

Across mice, we found a stable retinal circulation that formed and maintained a trilaminar stratification throughout early adulthood at all eccentricities. Bridging these layers, microvessels had five distinct anatomical branching patterns. The superficial, intermediate, and deep plexuses increased in density with depth: 16.14 ± 3.61 mm/mm2, 22.14 ± 6.86 mm/mm2, and 31.01 ± 6.24 mm/mm2, respectively. This patterning was not impacted by eccentricity or age (13-61 weeks). Similar metrics were achieved using adaptive optics scanning light ophthalmoscope in vivo with the same analysis pipeline.

Conclusions

The mouse retinal vasculature was stable up to 50 weeks of age, providing a robust and extensive baseline dataset with which models of retinal vascular and neural disease may be compared. Vessels connecting the laminae were more complex than previously reported and represented a uniquely vulnerable population due to their relatively low density.

Host circuit engagement of human cortical organoids transplanted in rodents

Human neural organoids represent promising models for studying neural function; however, organoids grown in vitro lack certain microenvironments and sensory inputs that are thought to be essential for maturation. The transplantation of patient-derived neural organoids into animal hosts helps overcome some of these limitations and offers an approach for neural organoid maturation and circuit integration. Here, we describe a method for transplanting human stem cell-derived cortical organoids (hCOs) into the somatosensory cortex of newborn rats. The differentiation of human induced pluripotent stem cells into hCOs occurs over 30-60 days, and the transplantation procedure itself requires ~0.5-1 hours per animal. The use of neonatal hosts provides a developmentally appropriate stage for circuit integration and allows the generation and experimental manipulation of a unit of human neural tissue within the cortex of a living animal host. After transplantation, animals can be maintained for hundreds of days, and transplanted hCO growth can be monitored by using brain magnetic resonance imaging. We describe the assessment of human neural circuit function in vivo by monitoring genetically encoded calcium responses and extracellular activity. To demonstrate human neuron-host functional integration, we also describe a procedure for engaging host neural circuits and for modulating animal behavior by using an optogenetic behavioral training paradigm. The transplanted human neurons can then undergo ex vivo characterization across modalities including dendritic morphology reconstruction, single-nucleus transcriptomics, optogenetic manipulation and electrophysiology. This approach may enable the discovery of cellular phenotypes from patient-derived cells and uncover mechanisms that contribute to human brain evolution from previously inaccessible developmental stages.

Serotonin Inhibition of Claustrum Projection Neurons: Ionic Mechanism, Receptor Subtypes and Consequences for Claustrum Computation

The claustrum is a small but densely interconnected brain structure that is innervated by axons containing serotonin (5-HT), a neuromodulator that has been implicated in control of sleep and in the actions of psychedelic drugs. However, little is known about how 5-HT influences the claustrum. We have combined whole-cell patch-clamp measurements of ionic currents, flash photolysis, and receptor pharmacology to characterize the 5-HT responses of individual claustral projection neurons (PNs) in mouse brain slices. Serotonin application elicited a long-lasting outward current in claustral PNs. This current was due to an increase in membrane permeability to K+ ions and was mediated mainly by the type 1A 5-HT receptor (5-HTR-1A). The 5-HT-induced K+ current hyperpolarized, and thereby inhibited, the PNs by reducing action potential firing. Focal uncaging of 5-HT revealed that inhibitory 5-HTR-1As were located at both the soma and dendrites of PNs. We conclude that 5-HT creates a net inhibition in the claustrum, an action that should decrease claustrum sensitivity to excitatory input from other brain areas and thereby contribute to 5-HT action in the brain.

Adaptive protein synthesis in genetic models of copper deficiency and childhood neurodegeneration

Rare inherited diseases caused by mutations in the copper transporters SLC31A1 (CTR1) or ATP7A induce copper deficiency in the brain, causing seizures and neurodegeneration in infancy through poorly understood mechanisms. Here, we used multiple model systems to characterize the molecular mechanisms by which neuronal cells respond to copper deficiency. Targeted deletion of CTR1 in neuroblastoma cells produced copper deficiency that was associated with a metabolic shift favoring glycolysis over oxidative phosphorylation. Proteomic and transcriptomic analysis of CTR1 KO cells revealed simultaneous upregulation of mTORC1 and S6K signaling and reduced PERK signaling. Patterns of gene and protein expression and pharmacogenomics show increased activation of the mTORC1-S6K pathway as a pro-survival mechanism, ultimately resulting in increased protein synthesis. Spatial transcriptomic profiling of Atp7a flx/Y :: Vil1 Cre/+ mice identified upregulated protein synthesis machinery and mTORC1-S6K pathway genes in copper-deficient Purkinje neurons in the cerebellum. Genetic epistasis experiments in Drosophila demonstrated that copper deficiency dendritic phenotypes in class IV neurons are partially rescued by increased S6k expression or 4E-BP1 (Thor) RNAi, while epidermis phenotypes are exacerbated by Akt, S6k, or raptor RNAi. Overall, we demonstrate that increased mTORC1-S6K pathway activation and protein synthesis is an adaptive mechanism by which neuronal cells respond to copper deficiency.

Oligodendrocytes use postsynaptic proteins to coordinate myelin formation on axons of distinct neurotransmitter classes

Axon myelination can tune neuronal circuits through placement and modulation of different patterns of myelin sheaths on distinct types of axons. How myelin formation is coordinated on distinct axon classes remains largely unknown. Recent work indicates neuronal activity and vesicle release promote myelin formation, and myelin-producing oligodendrocytes express canonical postsynaptic factors that potentially facilitate oligodendrocyte-axon interaction for myelin ensheathment. Here, we examined whether the inhibitory postsynaptic scaffold protein Gephyrin (Gphn) mediates selective myelination of specific axon classes in the larval zebrafish. Consistent with this possibility, Gphn was enriched in myelin on GABAergic and glycinergic axons. Strikingly, in gphnb deficient larvae, myelin sheaths were longer specifically on GABAergic axons, and the frequency of myelin placement shifted toward glutamatergic axons at the expense of GABAergic axons. Collectively, our results indicate that oligodendrocytes use postsynaptic machinery to coordinate myelin formation in an axon identity-dependent manner.

Neuronal fate resulting from indirect neurogenesis in the mouse neocortex

Excitatory cortical neurons originate from cortical radial glial cells (RGCs). Initially, these neurons were thought to derive directly from RGCs (direct neurogenesis) and be distributed in an inside-out fashion. However, the discovery of indirect neurogenesis, whereby intermediate neuronal progenitors (INPs) generate neurons, challenged this view. To investigate the integration of neurons via these two modes, we developed a method to identify INP progeny and analyze their fate using transgenic mice expressing tamoxifen-inducible Cre recombinase under the neurogenin-2 promoter, alongside thymidine analog incorporation. Their fate was further analyzed using mosaic analysis with double markers in mice. Indirect neurogenesis was prominent during early neurogenesis, generating neuron types that would emerge slightly later than those produced via direct neurogenesis. Despite the timing difference, both neurogenic modes produced fundamentally similar neuron types, as evidenced by marker expression and cortical-depth location. Furthermore, INPs generated pairs of similar phenotype neurons. These findings suggest that indirect neurogenesis, like direct neurogenesis, generates neuron types in a temporally ordered sequence and increases the number of similar neuron types, particularly in deep layers. Thus, both neurogenic modes cooperatively generate a diverse array of neuron types in a similar order, and their progeny populate together to form a coherent cortical structure.

Length-sensitive partitioning of Caenorhabditis elegans meiotic chromosomes responds to proximity and number of crossover sites

Sensing and control of size are critical for cellular function and survival. A striking example of size sensing occurs during meiosis in the nematode Caenorhabditis elegans. C. elegans chromosomes compare the lengths of the two chromosome "arms" demarcated by the position of their single off-center crossover, and they differentially modify these arms to ensure that sister chromatid cohesion is lost specifically on the shorter arm in the first meiotic division, while the longer arm maintains cohesion until the second division. While many of the downstream steps leading to cohesion loss have been characterized, the length-sensing process itself remains poorly understood. Here, we have used cytological visualization of short and long chromosome arms, combined with quantitative microscopy, live imaging, and simulations, to investigate the principles underlying length-sensitive chromosome partitioning. By quantitatively analyzing short-arm designation patterns on fusion chromosomes carrying multiple crossovers, we develop a model in which a short-arm-determining factor originates at crossover designation sites, diffuses within the synaptonemal complex, and accumulates within crossover-bounded chromosome segments. We demonstrate experimental support for a critical assumption of this model: that crossovers act as boundaries to diffusion within the synaptonemal complex. Further, we develop a discrete simulation based on our results that recapitulates a wide variety of observed partitioning outcomes in both wild-type and previously reported mutants. Our results suggest that the concentration of a diffusible factor is used as a proxy for chromosome length, enabling the correct designation of short and long arms and proper segregation of chromosomes.