Intracellular calcium release: A conductor of compartmentalized dendritic plasticity.

Dendrites endow neurons with multiple compartments within their elaborate morphologies. In a recent study published in the journal Science, O'Hare et al. (2022) used elegant techniques to show that augmenting the intracellular calcium released by the endoplasmic reticulum caused behaviorally relevant plasticity to occur in spatially distinct dendritic compartments.

Cellular bases of olfactory circuit assembly revealed by systematic time-lapse imaging.

Neural circuit assembly features simultaneous targeting of numerous neuronal processes from constituent neuron types, yet the dynamics is poorly understood. Here, we use the Drosophila olfactory circuit to investigate dynamic cellular processes by which olfactory receptor neurons (ORNs) target axons precisely to specific glomeruli in the ipsi- and contralateral antennal lobes. Time-lapse imaging of individual axons from 30 ORN types revealed a rich diversity in extension speed, innervation timing, and ipsilateral branch locations and identified that ipsilateral targeting occurs via stabilization of transient interstitial branches. Fast imaging using adaptive optics-corrected lattice light-sheet microscopy showed that upon approaching target, many ORN types exhibiting "exploring branches" consisted of parallel microtubule-based terminal branches emanating from an F-actin-rich hub. Antennal nerve ablations uncovered essential roles for bilateral axons in contralateral target selection and for ORN axons to facilitate dendritic refinement of postsynaptic partner neurons. Altogether, these observations provide cellular bases for wiring specificity establishment.

RTN4/NoGo-receptor binding to BAI adhesion-GPCRs regulates neuronal development.

RTN4-binding proteins were widely studied as "NoGo" receptors, but their physiological interactors and roles remain elusive. Similarly, BAI adhesion-GPCRs were associated with numerous activities, but their ligands and functions remain unclear. Using unbiased approaches, we observed an unexpected convergence: RTN4 receptors are high-affinity ligands for BAI adhesion-GPCRs. A single thrombospondin type 1-repeat (TSR) domain of BAIs binds to the leucine-rich repeat domain of all three RTN4-receptor isoforms with nanomolar affinity. In the 1.65 Å crystal structure of the BAI1/RTN4-receptor complex, C-mannosylation of tryptophan and O-fucosylation of threonine in the BAI TSR-domains creates a RTN4-receptor/BAI interface shaped by unusual glycoconjugates that enables high-affinity interactions. In human neurons, RTN4 receptors regulate dendritic arborization, axonal elongation, and synapse formation by differential binding to glial versus neuronal BAIs, thereby controlling neural network activity. Thus, BAI binding to RTN4/NoGo receptors represents a receptor-ligand axis that, enabled by rare post-translational modifications, controls development of synaptic circuits.

General Anesthesia Decouples Cortical Pyramidal Neurons.

The mystery of general anesthesia is that it specifically suppresses consciousness by disrupting feedback signaling in the brain, even when feedforward signaling and basic neuronal function are left relatively unchanged. The mechanism for such selectiveness is unknown. Here we show that three different anesthetics have the same disruptive influence on signaling along apical dendrites in cortical layer 5 pyramidal neurons in mice. We found that optogenetic depolarization of the distal apical dendrites caused robust spiking at the cell body under awake conditions that was blocked by anesthesia. Moreover, we found that blocking metabotropic glutamate and cholinergic receptors had the same effect on apical dendrite decoupling as anesthesia or inactivation of the higher-order thalamus. If feedback signaling occurs predominantly through apical dendrites, the cellular mechanism we found would explain not only how anesthesia selectively blocks this signaling but also why conscious perception depends on both cortico-cortical and thalamo-cortical connectivity.

Cell-Surface Proteomic Profiling in the Fly Brain Uncovers Wiring Regulators.

Molecular interactions at the cellular interface mediate organized assembly of single cells into tissues and, thus, govern the development and physiology of multicellular organisms. Here, we developed a cell-type-specific, spatiotemporally resolved approach to profile cell-surface proteomes in intact tissues. Quantitative profiling of cell-surface proteomes of Drosophila olfactory projection neurons (PNs) in pupae and adults revealed global downregulation of wiring molecules and upregulation of synaptic molecules in the transition from developing to mature PNs. A proteome-instructed in vivo screen identified 20 cell-surface molecules regulating neural circuit assembly, many of which belong to evolutionarily conserved protein families not previously linked to neural development. Genetic analysis further revealed that the lipoprotein receptor LRP1 cell-autonomously controls PN dendrite targeting, contributing to the formation of a precise olfactory map. These findings highlight the power of temporally resolved in situ cell-surface proteomic profiling in discovering regulators of brain wiring.

Spatially Stable Mitochondrial Compartments Fuel Local Translation during Plasticity.

Local translation meets protein turnover and plasticity demands at synapses, however, the location of its energy supply is unknown. We found that local translation in neurons is powered by mitochondria and not by glycolysis. Super-resolution microscopy revealed that dendritic mitochondria exist as stable compartments of single or multiple filaments. To test if these mitochondrial compartments can serve as local energy supply for synaptic translation, we stimulated individual synapses to induce morphological plasticity and visualized newly synthesized proteins. Depletion of local mitochondrial compartments abolished both the plasticity and the stimulus-induced synaptic translation. These mitochondrial compartments serve as spatially confined energy reserves, as local depletion of a mitochondrial compartment did not affect synaptic translation at remote spines. The length and stability of dendritic mitochondrial compartments and the spatial functional domain were altered by cytoskeletal disruption. These results indicate that cytoskeletally tethered local energy compartments exist in dendrites to fuel local translation during synaptic plasticity.

Human Cortical Dendrites: Stretched to Perform Better?

Most functional properties of human dendrites have been inferred from data obtained in model organisms. In this issue, Beaulieu-Laroche et al. record directly from human dendrites of cortical neurons and show that the considerably larger human neurons differ from rat neurons in the way they process synaptic signals.

Enhanced Dendritic Compartmentalization in Human Cortical Neurons.

The biophysical features of neurons shape information processing in the brain. Cortical neurons are larger in humans than in other species, but it is unclear how their size affects synaptic integration. Here, we perform direct electrical recordings from human dendrites and report enhanced electrical compartmentalization in layer 5 pyramidal neurons. Compared to rat dendrites, distal human dendrites provide limited excitation to the soma, even in the presence of dendritic spikes. Human somas also exhibit less bursting due to reduced recruitment of dendritic electrogenesis. Finally, we find that decreased ion channel densities result in higher input resistance and underlie the lower coupling of human dendrites. We conclude that the increased length of human neurons alters their input-output properties, which will impact cortical computation. VIDEO ABSTRACT.

A Fine-Scale Functional Logic to Convergence from Retina to Thalamus.

Numerous well-defined classes of retinal ganglion cells innervate the thalamus to guide image-forming vision, yet the rules governing their convergence and divergence remain unknown. Using two-photon calcium imaging in awake mouse thalamus, we observed a functional arrangement of retinal ganglion cell axonal boutons in which coarse-scale retinotopic ordering gives way to fine-scale organization based on shared preferences for other visual features. Specifically, at the ∼6 µm scale, clusters of boutons from different axons often showed similar preferences for either one or multiple features, including axis and direction of motion, spatial frequency, and changes in luminance. Conversely, individual axons could "de-multiplex" information channels by participating in multiple, functionally distinct bouton clusters. Finally, ultrastructural analyses demonstrated that retinal axonal boutons in a local cluster often target the same dendritic domain. These data suggest that functionally specific convergence and divergence of retinal axons may impart diverse, robust, and often novel feature selectivity to visual thalamus.

A Complete Electron Microscopy Volume of the Brain of Adult Drosophila melanogaster.

Drosophila melanogaster has a rich repertoire of innate and learned behaviors. Its 100,000-neuron brain is a large but tractable target for comprehensive neural circuit mapping. Only electron microscopy (EM) enables complete, unbiased mapping of synaptic connectivity; however, the fly brain is too large for conventional EM. We developed a custom high-throughput EM platform and imaged the entire brain of an adult female fly at synaptic resolution. To validate the dataset, we traced brain-spanning circuitry involving the mushroom body (MB), which has been extensively studied for its role in learning. All inputs to Kenyon cells (KCs), the intrinsic neurons of the MB, were mapped, revealing a previously unknown cell type, postsynaptic partners of KC dendrites, and unexpected clustering of olfactory projection neurons. These reconstructions show that this freely available EM volume supports mapping of brain-spanning circuits, which will significantly accelerate Drosophila neuroscience. VIDEO ABSTRACT.

Dendritic Integration of Sensory Evidence in Perceptual Decision-Making.

Perceptual decisions require the accumulation of sensory information to a response criterion. Most accounts of how the brain performs this process of temporal integration have focused on evolving patterns of spiking activity. We report that subthreshold changes in membrane voltage can represent accumulating evidence before a choice. αß core Kenyon cells (αßc KCs) in the mushroom bodies of fruit flies integrate odor-evoked synaptic inputs to action potential threshold at timescales matching the speed of olfactory discrimination. The forkhead box P transcription factor (FoxP) sets neuronal integration and behavioral decision times by controlling the abundance of the voltage-gated potassium channel Shal (KV4) in αßc KC dendrites. αßc KCs thus tailor, through a particular constellation of biophysical properties, the generic process of synaptic integration to the demands of sequential sampling.

Architecture and Dynamics of the Neuronal Secretory Network.

Regulated synthesis and movement of proteins between cellular organelles are central to diverse forms of biological adaptation and plasticity. In neurons, the repertoire of channel, receptor, and adhesion proteins displayed on the cell surface directly impacts cellular development, morphology, excitability, and synapse function. The immensity of the neuronal surface membrane and its division into distinct functional domains present a challenging landscape over which proteins must navigate to reach their appropriate functional domains. This problem becomes more complex considering that neuronal protein synthesis is continuously refined in space and time by neural activity. Here we review our current understanding of how integral membrane and secreted proteins important for neuronal function travel from their sites of synthesis to their functional destinations. We discuss how unique adaptations to the function and distribution of neuronal secretory organelles may facilitate local protein trafficking at remote sites in neuronal dendrites to support diverse forms of synaptic plasticity.

Autophagy in Neurons.

Autophagy is the major cellular pathway to degrade dysfunctional organelles and protein aggregates. Autophagy is particularly important in neurons, which are terminally differentiated cells that must last the lifetime of the organism. There are both constitutive and stress-induced pathways for autophagy in neurons, which catalyze the turnover of aged or damaged mitochondria, endoplasmic reticulum, other cellular organelles, and aggregated proteins. These pathways are required in neurodevelopment as well as in the maintenance of neuronal homeostasis. Here we review the core components of the pathway for autophagosome biogenesis, as well as the cell biology of bulk and selective autophagy in neurons. Finally, we discuss the role of autophagy in neuronal development, homeostasis, and aging and the links between deficits in autophagy and neurodegeneration.

The Cellular and Synaptic Architecture of the Mechanosensory Dorsal Horn.

The deep dorsal horn is a poorly characterized spinal cord region implicated in processing low-threshold mechanoreceptor (LTMR) information. We report an array of mouse genetic tools for defining neuronal components and functions of the dorsal horn LTMR-recipient zone (LTMR-RZ), a role for LTMR-RZ processing in tactile perception, and the basic logic of LTMR-RZ organization. We found an unexpectedly high degree of neuronal diversity in the LTMR-RZ: seven excitatory and four inhibitory subtypes of interneurons exhibiting unique morphological, physiological, and synaptic properties. Remarkably, LTMRs form synapses on between four and 11 LTMR-RZ interneuron subtypes, while each LTMR-RZ interneuron subtype samples inputs from at least one to three LTMR classes, as well as spinal cord interneurons and corticospinal neurons. Thus, the LTMR-RZ is a somatosensory processing region endowed with a neuronal complexity that rivals the retina and functions to pattern the activity of ascending touch pathways that underlie tactile perception.

Classifying Drosophila Olfactory Projection Neuron Subtypes by Single-Cell RNA Sequencing.

The definition of neuronal type and how it relates to the transcriptome are open questions. Drosophila olfactory projection neurons (PNs) are among the best-characterized neuronal types: different PN classes target dendrites to distinct olfactory glomeruli, while PNs of the same class exhibit indistinguishable anatomical and physiological properties. Using single-cell RNA sequencing, we comprehensively characterized the transcriptomes of most PN classes and unequivocally mapped transcriptomes to specific olfactory function for six classes. Transcriptomes of closely related PN classes exhibit the largest differences during circuit assembly but become indistinguishable in adults, suggesting that neuronal subtype diversity peaks during development. Transcription factors and cell-surface molecules are the most differentially expressed genes between classes and are highly informative in encoding cell identity, enabling us to identify a new lineage-specific transcription factor that instructs PN dendrite targeting. These findings establish that neuronal transcriptomic identity corresponds with anatomical and physiological identity defined by connectivity and function.

Real-Time Imaging of Translation on Single mRNA Transcripts in Live Cells.

Translation is under tight spatial and temporal controls to ensure protein production in the right time and place in cells. Methods that allow real-time, high-resolution visualization of translation in live cells are essential for understanding the spatiotemporal dynamics of translation regulation. Based on multivalent fluorescence amplification of the nascent polypeptide signal, we develop a method to image translation on individual mRNA molecules in real time in live cells, allowing direct visualization of translation events at the translation sites. Using this approach, we monitor transient changes of translation dynamics in responses to environmental stresses, capture distinct mobilities of individual polysomes in different subcellular compartments, and detect 3' UTR-dependent local translation and active transport of polysomes in dendrites of primary neurons.

Saturated Reconstruction of a Volume of Neocortex.

We describe automated technologies to probe the structure of neural tissue at nanometer resolution and use them to generate a saturated reconstruction of a sub-volume of mouse neocortex in which all cellular objects (axons, dendrites, and glia) and many sub-cellular components (synapses, synaptic vesicles, spines, spine apparati, postsynaptic densities, and mitochondria) are rendered and itemized in a database. We explore these data to study physical properties of brain tissue. For example, by tracing the trajectories of all excitatory axons and noting their juxtapositions, both synaptic and non-synaptic, with every dendritic spine we refute the idea that physical proximity is sufficient to predict synaptic connectivity (the so-called Peters' rule). This online minable database provides general access to the intrinsic complexity of the neocortex and enables further data-driven inquiries.

Antisense oligonucleotide therapeutic approach for Timothy syndrome.

Timothy syndrome (TS) is a severe, multisystem disorder characterized by autism, epilepsy, long-QT syndrome and other neuropsychiatric conditions1. TS type 1 (TS1) is caused by a gain-of-function variant in the alternatively spliced and developmentally enriched CACNA1C exon 8A, as opposed to its counterpart exon 8. We previously uncovered several phenotypes in neurons derived from patients with TS1, including delayed channel inactivation, prolonged depolarization-induced calcium rise, impaired interneuron migration, activity-dependent dendrite retraction and an unanticipated persistent expression of exon 8A2-6. We reasoned that switching CACNA1C exon utilization from 8A to 8 would represent a potential therapeutic strategy. Here we developed antisense oligonucleotides (ASOs) to effectively decrease the inclusion of exon 8A in human cells both in vitro and, following transplantation, in vivo. We discovered that the ASO-mediated switch from exon 8A to 8 robustly rescued defects in patient-derived cortical organoids and migration in forebrain assembloids. Leveraging a transplantation platform previously developed7, we found that a single intrathecal ASO administration rescued calcium changes and in vivo dendrite retraction of patient neurons, suggesting that suppression of CACNA1C exon 8A expression is a potential treatment for TS1. Broadly, these experiments illustrate how a multilevel, in vivo and in vitro stem cell model-based approach can identify strategies to reverse disease-relevant neural pathophysiology.

Neocortical Layer 1: An Elegant Solution to Top-Down and Bottom-Up Integration.

Many of our daily activities, such as riding a bike to work or reading a book in a noisy cafe, and highly skilled activities, such as a professional playing a tennis match or a violin concerto, depend upon the ability of the brain to quickly make moment-to-moment adjustments to our behavior in response to the results of our actions. Particularly, they depend upon the ability of the neocortex to integrate the information provided by the sensory organs (bottom-up information) with internally generated signals such as expectations or attentional signals (top-down information). This integration occurs in pyramidal cells (PCs) and their long apical dendrite, which branches extensively into a dendritic tuft in layer 1 (L1). The outermost layer of the neocortex, L1 is highly conserved across cortical areas and species. Importantly, L1 is the predominant input layer for top-down information, relayed by a rich, dense mesh of long-range projections that provide signals to the tuft branches of the PCs. Here, we discuss recent progress in our understanding of the composition of L1 and review evidence that L1 processing contributes to functions such as sensory perception, cross-modal integration, controlling states of consciousness, attention, and learning.

A Functional Dissection of the mRNA and Locally Synthesized Protein Population in Neuronal Dendrites and Axons.

Neurons are characterized by a complex morphology that enables the generation of subcellular compartments with unique biochemical and biophysical properties, such as dendrites, axons, and synapses. To sustain these different compartments and carry a wide array of elaborate operations, neurons express a diverse repertoire of gene products. Extensive regulation at both the messenger RNA (mRNA) and protein levels allows for the differentiation of subcellular compartments as well as numerous forms of plasticity in response to variable stimuli. Among the multiple mechanisms that control cellular functions, mRNA translation is manipulated by neurons to regulate where and when a protein emerges. Interestingly, transcriptomic and translatomic profiles of both dendrites and axons have revealed that the mRNA population only partially predicts the local protein population and that this relation significantly varies between different gene groups. Here, we describe the space that local translation occupies within the large molecular and regulatory complexity of neurons, in contrast to other modes of regulation. We then discuss the specialized organization of mRNAs within different neuronal compartments, as revealed by profiles of the local transcriptome. Finally, we discuss the features and functional implications of both locally correlated-and anticorrelated-mRNA-protein relations both under baseline conditions and during synaptic plasticity.