A weakened recurrent circuit in the hippocampus of Rett syndrome mice disrupts long-term memory representations

Differential effects of left DLPFC anodal and cathodal tDCS interventions on the brain in children with autism: A randomized controlled trial

Background

Autism spectrum disorder (ASD) is a complex and heterogeneous neurodevelopmental disorder with few effective treatment options. In recent years, transcranial direct current stimulation (tDCS) has been applied in interventions for ASD, often targeting the left dorsolateral prefrontal cortex (DLPFC). However, studies investigating anodal and cathodal tDCS interventions have reported differing outcomes. Therefore, this study aimed to compare and analyze the effects of these two stimulations through a randomized controlled trial, utilizing both behavioral assessments and EEG proxy markers capable of characterizing the brain's excitatory-inhibitory balance.

Methods

This study recruited a total of 24 children with ASD (20 males and 4 females; mean age ± SD: 5.5 ± 1.2 years), who were randomly divided into two groups receiving either anodal or cathodal tDCS targeting the DLPFC. The stimulation intensity was set at 1 mA, administered five times per week for a total of 20 sessions. Behavioral intervention outcomes were assessed using the Social Responsiveness Scale (SRS) and the Autism Behavior Checklist (ABC). Additionally, the study evaluated the effects of tDCS on the brain's excitatory-inhibitory balance by analyzing corrected periodic alpha oscillation power and bandwidth, as well as non-periodic exponent and offset derived from EEG data.

Results

Following anodal tDCS intervention, results from the SRS scale indicated a decrease in overall scores, with significant differences observed in social communication and social motivation among children. On the ABC scale, overall scores also decreased, with significant differences noted in sensory behavior, social relating, body and object use, and language and communication skills. Non-periodic exponent and offsets increased following anodal tDCS stimulation, whereas they decreased after cathodal tDCS stimulation. Regarding alpha oscillation power, there was a significant increase following anodal tDCS and a significant decrease following cathodal tDCS. In terms of alpha oscillation bandwidth, there was a reduction following anodal tDCS and an increase following cathodal tDCS. Further correlation analysis revealed that in children who received anodal tDCS intervention, non-periodic exponent showed correlations with behaviors such as social communication.

Conclusion

Our study results demonstrated that anodal and cathodal tDCS targeting the left DLPFC had distinct effects on the behavior and excitatory-inhibitory balance of children with ASD. Anodal tDCS intervention appeared to have a more positive impact compared to cathodal intervention. However, the sample size was small, and we focused solely on the effects of tDCS, with our experimental design perhaps not being able to generalize to all external manipulations of excitability in our study. In future research, we will continue to improve the experiments to address these limitations.

Synaptic disturbance in neurodevelopmental disorders: Perspectives from fragile X and Rett syndromes

Neurodevelopmental disorders (NDDs) are often referred to as "synaptopathies" because many of their behavioral symptoms arise from impaired synaptic development and function. However, the mechanisms that connect synaptic dysfunction to neurological symptoms remain unclear, mainly due to the wide variety of genetic and environmental factors involved in these disorders. Fragile X syndrome and Rett syndrome, two extensively studied monogenic NDDs, provide a unique opportunity to explore these mechanisms at molecular, cellular, and synaptic levels. This review summarizes the current understanding of how synaptic alterations contribute to the neurological symptoms observed in fragile X and Rett syndromes. A comparison of findings from mouse models indicates that an imbalance in local and distal connectivity may serve as a common feature of both disorders.

Engrams across diseases: Different pathologies - unifying mechanisms?

Memories are our reservoir of knowledge and thus, are crucial for guiding decisions and defining our self. The physical correlate of a memory in the brain is termed an engram and since decades helps researchers to elucidate the intricate nature of our imprinted experiences and knowledge. Given the importance that memories have for our lives, their impairment can present a tremendous burden. In this review we aim to discuss engram malfunctioning across diseases, covering dementia-associated pathologies, epilepsy, chronic pain and psychiatric disorders. Current neuroscientific tools allow to witness the emergence and fate of engram cells and enable their manipulation. We further suggest that specific mechanisms of mnemonic malfunction can be derived from engram cell readouts. While depicting the way diseases act on the mnemonic component - specifically, on the cellular engram - we emphasize a differentiation between forms of amnesia and hypermnesia. Finally, we highlight commonalities and distinctions of engram impairments on the cellular level across diseases independent of their pathogenic origins and discuss prospective therapeutic measures.

Acute MeCP2 loss in adult mice reveals transcriptional and chromatin changes that precede neurological dysfunction and inform pathogenesis

Mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene cause Rett syndrome, a severe childhood neurological disorder. MeCP2 is a well-established transcriptional repressor, yet upon its loss, hundreds of genes are dysregulated in both directions. To understand what drives such dysregulation, we deleted Mecp2 in adult mice, circumventing developmental contributions and secondary pathogenesis. We performed time series transcriptional, chromatin, and phenotypic analyses of the hippocampus to determine the immediate consequences of MeCP2 loss and the cascade of pathogenesis. We find that loss of MeCP2 causes immediate and bidirectional progressive dysregulation of the transcriptome. To understand what drives gene downregulation, we profiled genome-wide histone modifications and found that a decrease in histone H3 acetylation (ac) at downregulated genes is among the earliest molecular changes occurring well before any measurable deficiencies in electrophysiology and neurological function. These data reveal a molecular cascade that drives disease independent of any developmental contributions or secondary pathogenesis.

Modeling Cortical Versus Hippocampal Network Dysfunction in a Human Brain Assembloid Model of Epilepsy and Intellectual Disability

Neurodevelopmental disorders often impair multiple cognitive domains. For instance, a genetic epilepsy syndrome might cause seizures due to cortical hyperexcitability and present with memory impairments arising from hippocampal dysfunction. This study examines how a single disorder differentially affects distinct brain regions by using human patient iPSC-derived cortical- and hippocampal-ganglionic eminence assembloids to model Developmental and Epileptic Encephalopathy 13 (DEE-13), a condition arising from gain-of-function mutations in the SCN8A gene. While cortical assembloids showed network hyperexcitability akin to epileptogenic tissue, hippocampal assembloids did not, and instead displayed network dysregulation patterns similar to in vivo hippocampal recordings from epilepsy patients. Predictive computational modeling, immunohistochemistry, and single-nucleus RNA sequencing revealed changes in excitatory and inhibitory neuron organization that were specific to hippocampal assembloids. These findings highlight the unique impacts of a single pathogenic variant across brain regions and establish hippocampal assembloids as a platform for studying neurodevelopmental disorders.

MeCP2 Deficiency Alters the Response Selectivity of Prefrontal Cortical Neurons to Different Social Stimuli

Rett syndrome (RTT), a severe neurodevelopmental disorder caused by mutations in the MeCP2 gene, is characterized by cognitive and social deficits. Previous studies have noted hypoactivity in the medial prefrontal cortex (mPFC) pyramidal neurons of MeCP2-deficient mice (RTT mice) in response to both social and nonsocial stimuli. To further understand the neural mechanisms behind the social deficits of RTT mice, we monitored excitatory pyramidal neurons in the prelimbic region of the mPFC during social interactions in mice. These neurons' activity was closely linked to social preference, especially in wild-type mice. However, RTT mice showed reduced social interest and corresponding hypoactivity in these neurons, indicating that impaired mPFC activity contributes to their social deficits. We identified six mPFC neural ensembles selectively tuned to various stimuli, with RTT mice recruiting fewer neurons to ensembles responsive to social interactions and consistently showing lower stimulus-ON ensemble transient rates. Despite these lower rates, RTT mice exhibited an increase in the percentage of social-ON neurons in later sessions, suggesting a compensatory mechanism for the decreased firing rate. This highlights the limited plasticity in the mPFC caused by MeCP2 deficiency and offers insights into the neural dynamics of social encoding. The presence of multifunctional neurons and those specifically responsive to social or object stimuli in the mPFC emphasizes its crucial role in complex behaviors and cognitive functions, with selective neuron engagement suggesting efficiency in neural activation that optimizes responses to environmental stimuli.

Understanding OLM interneurons: Characterization, circuitry, and significance in memory and navigation

Understanding the intricate mechanisms underlying memory formation and retention relies on unraveling how the hippocampus, a structure fundamental for memory acquisition, is organized. Within the complex hippocampal network, interneurons play a crucial role in orchestrating memory processes. Among these interneurons, Oriens-Lacunosum Moleculare (OLM) cells emerge as key regulators, governing the flow of information to CA1 pyramidal cells. In this review, we explore OLM interneurons in detail, describing their mechanisms and effects on memory processing, particularly in spatial and contextual memory tasks. Our aim is to provide a detailed understanding of how OLM interneurons contribute to the dynamic landscape of memory formation and retrieval.

Use-Dependent, Untapped Dual Kinase Signaling Localized in Brain Learning Circuitry

Imaging brain learning and memory circuit kinase signaling is a monumental challenge. The separation of phases-based activity reporter of kinase (SPARK) biosensors allow circuit-localized studies of multiple interactive kinases in vivo, including protein kinase A (PKA) and extracellular signal-regulated kinase (ERK) signaling. In the precisely-mapped Drosophila brain learning/memory circuit, we find PKA and ERK signaling differentially enriched in distinct Kenyon cell connectivity nodes. We discover that potentiating normal circuit activity induces circuit-localized PKA and ERK signaling, expanding kinase function within new presynaptic and postsynaptic domains. Activity-induced PKA signaling shows extensive overlap with previously selective ERK signaling nodes, while activity-induced ERK signaling arises in new connectivity nodes. We find targeted synaptic transmission blockade in Kenyon cells elevates circuit-localized ERK induction in Kenyon cells with normally high baseline ERK signaling, suggesting lateral and feedback inhibition. We discover overexpression of the pathway-linking Meng-Po (human SBK1) serine/threonine kinase to improve learning acquisition and memory consolidation results in dramatically heightened PKA and ERK signaling in separable Kenyon cell circuit connectivity nodes, revealing both synchronized and untapped signaling potential. Finally, we find that a mechanically-induced epileptic seizure model (easily shocked "bang-sensitive" mutants) has strongly elevated, circuit-localized PKA and ERK signaling. Both sexes were used in all experiments, except for the hemizygous male-only seizure model. Hyperexcitable, learning-enhanced, and epileptic seizure models have comparably elevated interactive kinase signaling, suggesting a common basis of use-dependent induction. We conclude that PKA and ERK signaling modulation is locally coordinated in use-dependent spatial circuit dynamics underlying seizure susceptibility linked to learning/memory potential.

Hippocampal GABAergic interneurons and memory

One of the most captivating questions in neuroscience revolves around the brain's ability to efficiently and durably capture and store information. It must process continuous input from sensory organs while also encoding memories that can persist throughout a lifetime. What are the cellular-, subcellular-, and network-level mechanisms that underlie this remarkable capacity for long-term information storage? Furthermore, what contributions do distinct types of GABAergic interneurons make to this process? As the hippocampus plays a pivotal role in memory, our review focuses on three aspects: (1) delineation of hippocampal interneuron types and their connectivity, (2) interneuron plasticity, and (3) activity patterns of interneurons during memory-related rhythms, including the role of long-range interneurons and disinhibition. We explore how these three elements, together showcasing the remarkable diversity of inhibitory circuits, shape the processing of memories in the hippocampus.

Genetically Defined Subtypes of Somatostatin-Containing Cortical Interneurons

Inhibitory interneurons play a crucial role in proper development and function of the mammalian cerebral cortex. Of the different inhibitory subclasses, dendritic-targeting, somatostatin-containing (SOM) interneurons may be the most diverse. Earlier studies used GFP-expressing and recombinase-expressing mouse lines to characterize genetically defined subtypes of SOM interneurons by morphologic, electrophysiological, and neurochemical properties. More recently, large-scale studies classified SOM interneurons into 13 morpho-electric transcriptomic (MET) types. It remains unclear, however, how these various classification schemes relate to each other, and experimental access to MET types has been limited by the scarcity of specific mouse driver lines. To address these issues, we crossed Flp and Cre driver lines with a dual-color intersectional reporter, allowing experimental access to several combinatorially defined SOM subsets. Brains from adult mice of both sexes were retrogradely dye labeled from the pial surface to identify layer 1-projecting neurons and immunostained against several marker proteins, revealing correlations between genetic label, axonal target, and marker protein expression in the same neurons. Lastly, using whole-cell recordings ex vivo, we analyzed and compared electrophysiological properties between different intersectional subsets. We identified two layer 1-targeting subtypes with nonoverlapping marker protein expression and electrophysiological properties, which, together with a previously characterized layer 4-targeting subtype, account for >50% of all layer 5 SOM cells and >40% of all SOM cells, and appear to map onto 5 of the 13 MET types. Genetic access to these subtypes will allow researchers to determine their synaptic inputs and outputs and uncover their roles in cortical computations and animal behavior.

Oxytocin and serotonin in the modulation of neural function: Neurobiological underpinnings of autism-related behavior

Autism spectrum disorders (ASD) is a group of generalized neurodevelopmental disorders. Its main clinical features are social communication disorder and repetitive stereotyped behavioral interest. The abnormal structure and function of brain network is the basis of social dysfunction and stereotyped performance in patients with autism spectrum disorder. The number of patients diagnosed with ASD has increased year by year, but there is a lack of effective intervention and treatment. Oxytocin has been revealed to effectively improve social cognitive function and significantly improve the social information processing ability, empathy ability and social communication ability of ASD patients. The change of serotonin level also been reported affecting the development of brain and causes ASD-like behavioral abnormalities, such as anxiety, depression like behavior, stereotyped behavior. Present review will focus on the research progress of serotonin and oxytocin in the pathogenesis, brain circuit changes and treatment of autism. Revealing the regulatory effect and neural mechanism of serotonin and oxytocin on patients with ASD is not only conducive to a deeper comprehension of the pathogenesis of ASD, but also has vital clinical significance.

O-LM interneurons: Gatekeepers of pyramidal neuron activity in the hippocampus

A balanced and fine-tuned ratio of neuronal excitation and inhibition is a prerequisite for information processing. In this issue of Neuron, He et al. (2022) reveal a causal link between reduced input to local somatostatin-expressing, MeCP2-negative O-LM interneurons in CA1 and long-term memory impairment in a mouse model of Rett syndrome.