bioRxiv Subject Collection: Neuroscience's Journal
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Wednesday, August 14th, 2024
| Time |
Event |
| 8:34a |
Inference technique for the synaptic conductances in rhythmically active networks and application to respiratory central pattern generation circuits
Unraveling synaptic interactions between excitatory and inhibitory interneurons within rhythmic neural circuits, such as central pattern generation (CPG) circuits for rhythmic motor behaviors, is critical for deciphering circuit interactions and functional architecture, which is a major problem for understanding how neural circuits operate. Here we present a general method for extracting and separating patterns of inhibitory and excitatory synaptic conductances at high temporal resolution from single neuronal intracellular recordings in rhythmically active networks. These post-synaptic conductances reflect the combined synaptic inputs from the key interacting neuronal populations and can reveal the functional connectome of the active circuits. To illustrate the applicability of our analytic technique, we employ our method to infer the synaptic conductance profiles in identified rhythmically active interneurons within key microcircuits of the mammalian (mature rat) brainstem respiratory CPG and provide a perspective on how our approach can resolve the functional interactions and circuit organization of these interneuron populations. We demonstrate the versatility of our approach, which can be applied to any other rhythmic circuits where conditions allow for neuronal intracellular recordings. | | 9:47a |
Cognitive dysfunction following brain trauma results from sex-specific reactivation of the developmental pruning processes
Cognitive losses resulting from severe brain trauma have long been associated with the focal region of tissue damage, leading to devastating functional impairment. For decades, researchers have focused on the sequelae of cellular alterations that exist within the perilesional tissues; however, few clinical trials have been successful. Here, we employed a mouse brain injury model that resulted in expansive synaptic damage to regions outside the focal injury. Our findings demonstrate that synaptic damage results from the prolonged increase in D-serine release from activated microglia and astrocytes, which leads to hyperactivation of perisynaptic NMDARs, tagging of damaged synapses by complement components, and the reactivation of developmental pruning processes. We show that this mechanistic pathway is reversible at several stages within a prolonged and progressive period of synaptic loss. Importantly, these key factors are present in acutely injured brain tissue acquired from patients with brain injury, which supports a therapeutic neuroprotective strategy. | | 9:47a |
Human-specific features of cerebellar astrocytes and Purkinje cells: an anatomical comparison with mice and macaques
Little is known about the morphological diversity and distribution of cerebellar astrocytes in the human brain and how or if these features differ from those of cerebellar astrocytes in species used to model human illnesses. To address this, we performed a comparative post-mortem examination of cerebellar astrocytes and Purkinje cells (PCs) in healthy humans, macaques, and mice using microscopy-based techniques. Visualizing with canonical astrocyte markers glial fibrillary acidic protein (GFAP) and aldehyde dehydrogenase-1 family member L1 (ALDH1L1), we mapped astrocytes within a complete cerebellar hemisphere. Astrocytes were observed to be differentially distributed across the cerebellar layers, displayed overall increases in area coverages with evolution, and showed features uniquely hominoid. Stereological quantifications in 3 functionally distinct cerebellar lobules demonstrated opposing trends for the canonical astrocyte markers across species with ALDH1L1+ astrocytes increasing with evolution and GFAP+ astrocytes decreasing. PC analyses revealed that while humans have the lowest PC densities, their cell body sizes were the largest with more ALDH1L1 immunoreactive astrocytes surrounding. Notably, the cognitive lobule crus I displayed the highest ratio of Bergmann glia to PC in all species. These findings align with the growing literature for astrocyte and PC heterogeneity and suggest cerebellar astrocyte and PC divergence both within and across species, possibly indicative of a role for these cells in higher-order cerebellar processing.
O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=178 SRC="FIGDIR/small/607849v1_ufig1.gif" ALT="Figure 1">
View larger version (51K):
org.highwire.dtl.DTLVardef@1bfbf0borg.highwire.dtl.DTLVardef@1977861org.highwire.dtl.DTLVardef@16067aborg.highwire.dtl.DTLVardef@361683_HPS_FORMAT_FIGEXP M_FIG C_FIG Main PointsO_LIALDH1L1 and GFAP immunoreactive astrocytes are differentially distributed across cerebellar layers and lobules.
C_LIO_LIOpposing trends across species with ALDH1L1+ astrocyte densities increasing and GFAP+ astrocyte densities decreasing with larger cerebella.
C_LIO_LIWith evolution, Purkinje cell densities decrease while their cell bodies and the number of Bergmann glia surrounding increase.
C_LI | | 9:47a |
Population coding of predator imminence in the hypothalamus
Hypothalamic VMHdmSF1 neurons are activated by predator cues and are necessary and sufficient for instinctive defensive responses. However, such data do not distinguish which features of a predator encounter are encoded by VMHdmSF1 neural activity. To address this issue, we imaged VMHdmSF1 neurons at single-cell resolution in freely behaving mice exposed to a natural predator in varying contexts. Our results reveal that VMHdmSF1 neurons do not represent different defensive behaviors, but rather encode predator identity and multiple predator-evoked internal states, including threat-evoked fear/anxiety; neophobia or arousal; predator imminence; and safety. Notably, threat and safety are encoded bi-directionally by anti-correlated subpopulations. Finally, individual differences in predator defensiveness are correlated with differences in VMHdmSF1 response dynamics. Thus, different threat-related internal state variables are encoded by distinct neuronal subpopulations within a genetically defined, anatomically restricted hypothalamic cell class.
HighlightsO_LIDistinct subsets of VMHdmSF1 neurons encode multiple predator-evoked internal states.
C_LIO_LIAnti-correlated subsets encode safety vs. threat in a bi-directional manner
C_LIO_LIA population code for predator imminence is identified using a novel assay
C_LIO_LIVMHdmSF1 dynamics correlate with individual variation in predator defensiveness.
C_LI | | 9:47a |
Lower Aperiodic Activity is Associated with Reduced Verbal Fluency Performance Across Adulthood
Age-related cognitive decline associations with human electroencephalography (EEG) have previously focused on periodic activity. However, EEG is primarily made up of non-oscillatory aperiodic activity, which can be characterised with an exponent and offset value. In a secondary analysis of a cohort of 111 healthy participants aged 17 to 71 years, we examined the associations of the aperiodic exponent and offset with a battery of cognitive tests assessing processing speed and response inhibition, working memory, verbal learning and memory, psychomotor speed, and verbal fluency. Using Principal Component Analysis and K-Means Clustering, we identified clusters of electrodes that exhibited similar aperiodic exponent and offset activity during resting-state eyes-closed EEG. Robust linear models were then used to model how aperiodic activity interacted with age and their associations with performance during each cognitive test. Exponent by age and offset by age interactions were identified for the verbal fluency model where flatter exponents and smaller offsets were associated with poorer performance in adults as early as 30 years of age. Steeper exponents and greater offsets become increasingly related to verbal fluency performance and executive functioning in adulthood. | | 9:47a |
A bone-derived protein primes rapid visual escape via GPR37 receptor in a subpopulation of VTA GABAergic neurons
Rapid escape against visual threats is critical for survival. Whether it requires a permissive mechanism is unknown. Here, we show that osteocalcin (OCN), a protein produced by bone and persisted in the brain, primes rapid visual escape response by increasing excitability of VTA GABAergic neuron subpopulation via OCN-GPR37-cAMP-THIK-1 (K2P13.1) pathway. Knock-out of OCN or its receptor GPR37, and conditional knock-out of GPR37 in VTA GABAergic or glutamatergic neurons caused delayed escape. Reconstituting OCN-GPR37 signaling specifically in VTA was sufficient to restore normal response. Single-cell transcriptomics combined with electrophysiology showed that OCN decreases potassium currents in a subpopulation of VTA GABAergic neurons via GPR37-induced cAMP reduction and subsequent THIK-1 suppression. This elevation of excitability in VTA neuron subpopulation can be recapitulated by HM4Di, an inhibitory chemogenetic GPCR commonly used to suppress neuronal activity. Our study demonstrated that visual behavior requires a bone-derived protein that tunes electrophysiology of central nervous system neurons. | | 9:47a |
A cleaved cytosolic FOXG1 promotes excitatory neurogenesis by modulation of mitochondrial translation - a new therapeutic target for brain disorders.
Modulation of mitochondrial function is at the core of cell fate decisions and tissue homeostasis, yet the mechanisms that govern their activity are not understood. Here, we provide evidence that mitochondrial activity is controlled in a tissue-specific manner through a non-canonical cytoplasmic function of the transcription factor FOXG1. Using zebrafish and human models of the neurodevelopmental disorder, FOXG1 Syndrome, we found that FOXG1 mutations inducing a premature stop codon unexpectedly lead to the production of a short C-terminal peptide. The expression of this truncated protein is responsible for an excess of excitatory neurons and a structural, functional, and translational mitochondrial phenotype in mutants. We demonstrate that this activity is a gain of function, normally carried out by a cleavage product in wildtype. Both peptides promote the translation of mitochondrially-encoded transcripts, are preferentially transported to the mitochondria, and interact with mito-ribosomal proteins. These findings unveil a mechanism that integrates cell fate decisions with metabolic output. Adjusting the dosage of the mutant peptide rescues aspects of FOXG1 Syndrome, offering a new therapeutic avenue for the treatment of disorders involving mitochondrial dysfunctions. | | 9:47a |
A multi-model approach defines function altering MECP2 missense variants identified in individuals with autism spectrum disorder
MECP2 is commonly mutated in Rett syndrome, where MECP2s function as a DNA cytosine methylation reader is believed critical. MECP2 variants are also catalogued in individuals with autism spectrum disorder (ASD), including nine missense variants with no known clinical significance. To assess these nine as risk alleles for ASD, we developed MECP2 variant function assays using yeast, Drosophila and human cell lines. We calibrated these assays with known reference pathogenic and benign variants. Our data predict that four ASD variants are loss of function (LoF) and five are functional. Protein destabilization or nuclear delocalization offers insight into the altered function of a number of these variants. Notably, yeast and Drosophila lack DNA methylation, yet all Rett reference pathogenic and ASD variants in the methyl DNA binding domain that we analyzed proved to be LoF, suggesting a clinically-relevant role for non-methyl DNA-binding by MECP2. | | 9:47a |
A novel functional coordination in UNC-13 regulates neurotransmitter release
Munc13 plays a crucial role in short-term synaptic plasticity by regulating synaptic vesicle (SV) exocytosis and neurotransmitter release at the presynaptic terminals. However, the intricate mechanisms governing these processes have remained elusive due to the presence of multiple functional domains within Munc13, each playing distinct roles in neurotransmitter release. Here we report a coordinated mechanism in the C. elegans Munc13 homolog UNC-13 that controls the functional switch of UNC-13 during synaptic transmission. Mutations disrupting the interactions of C1 and C2B with diacylglycerol (DAG) and phosphatidylinositol 4,5-bisphosphate (PIP2) on the plasma membrane induced the gain-of-function state of UNC-13L, the long UNC-13 isoform, resulting in enhanced SV release. Concurrent mutations in both domains counteracted this enhancement, highlighting the functional interdependence of C1 and C2B. Intriguingly, the individual C1 and C2B domains exhibited significantly stronger facilitation of SV release compared to the presence of both domains, supporting a mutual inhibition of C1 and C2B under basal conditions. Moreover, the N-terminal C2A and X domains exhibited opposite regulation on the functional switch of UNC-13L. Furthermore, we identified the polybasic motif in the C2B domain that facilitates SV release. Finally, we found that disruption of C1 and C2B membrane interaction in UNC-13S, the short isoform, leads to functional switch between gain-of-function and loss-of-function. Collectively, our findings provide a novel mechanism for SV exocytosis wherein UNC-13 undergoes functional switches through the coordination of its major domains, thereby regulating synaptic transmission and short-term synaptic plasticity. | | 11:46p |
Spatially organized striatal neuromodulator release encodes trajectory errors
Goal-directed navigation requires animals to continuously evaluate their current direction and speed of travel relative to landmarks to discern whether they are approaching or deviating from their goal. Striatal dopamine and acetylcholine are powerful modulators of goal-directed behavior, but it is unclear whether and how neuromodulator dynamics at landmarks incorporate relative motion for effective behavioral guidance. Using optical measurements in mice, we demonstrate that cue-evoked striatal dopamine release encodes bi-directional 'trajectory errors' reflecting relationships between ongoing speed and direction of locomotion and visual flow relative to optimal goal trajectories. Striatum-wide micro-fiber array recordings resolved an anatomical gradient of trajectory error signaling across the anterior-posterior axis, distinct from trajectory error independent cue signals. Dynamic regression modeling revealed that positive and negative trajectory error encoding emerges early and late respectively during learning and over different time courses in the medial and lateral striatum, enabling region specific contributions to learning. Striatal acetylcholine release also encodes trajectory errors, but encoding is more spatially restricted, opposite polarity, and delayed relative to dopamine, supporting distinct roles in modulating striatal output and behavior. Dopamine trajectory error signaling and task performance were reproduced in a reinforcement learning model incorporating a conjunctive state space representation, suggesting a potential neural substrate for trajectory error generation. Our results establish region specific neuromodulator signals positioned to guide the speed and direction of locomotion to reach goals based on environmental landmarks during navigation. | | 11:46p |
Opto-CLIP reveals dynamic FMRP regulation of mRNAs upon CA1 neuronal activation
Neuronal diversity and function are intricately linked to the dynamic regulation of RNA metabolism, including splicing, localization, and translation. Electrophysiologic studies of synaptic plasticity, models for learning and memory, are disrupted in Fragile X Syndrome (FXS). FXS is characterized by the loss of FMRP, an RNA-binding protein (RBP) known to bind and suppress translation of specific neuronal RNAs. Since molecular studies have demonstrated that synaptic plasticity in CA1 excitatory hippocampal neurons is protein-synthesis dependent, together these observations have suggested a potential role for FMRP in synaptic plasticity in FXS. To explore this model, we developed a new experimental platform, Opto-CLIP, to integrate optogenetics with cell-type specific FMRP CLIP and RiboTag in CA1 hippocampal neurons, allowing investigation of FMRP- regulated dynamics after neuronal activation. We tracked changes in FMRP binding and ribosome- associated RNA profiles 30 minutes after neuronal activation. Our findings reveal a significant reduction in FMRP-RNA binding to transcripts encoding nuclear proteins, suggesting FMRP translational inhibition may be de-repressed to allow rapid translational responses required for neuronal homeostasis. In contrast, FMRP binding to transcripts encoding synaptic targets were generally stable after activation, but all categories of targets demonstrated variability in FMRP translational control. Opto-CLIP revealed differential regulation of subsets of transcripts within CA1 neurons rapidly after depolarization, and offers promise as a generally useful platform to uncover mechanisms of RBP-mediated RNA regulation in the context of synaptic plasticity. |
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