Contribution of Axon Initial Segment Structure and Channels to Brain Pathology

The dynamic axon initial segment: From neuronal polarity to network homeostasis

The axon initial segment (AIS) is a highly specialized compartment in neurons that resides in between axonal and somatodendritic domains. The localization of the AIS in the proximal part of the axon is essential for its two major functions: generating and modulating action potentials and maintaining neuron polarity. Recent findings revealed that the incredibly stable AIS is generated from highly dynamic components and can undergo extensive structural and functional changes in response to alterations in activity levels. These activity-dependent alterations of AIS structure and function have profound consequences for neuronal functioning, and AIS plasticity has emerged as a key regulator of network homeostasis. This review highlights the functions of the AIS, its architecture, and how its organization and remodeling are influenced by developmental plasticity and both acute and chronic adaptations. It also discusses the mechanisms underlying these processes and explores how dysregulated AIS plasticity may contribute to brain disorders.

Identification of candidate genes involved in Zika virus-induced reversible paralysis of mice

Zika virus (ZIKV) causes a variety of peripheral and central nervous system complications leading to neurological symptoms such as limb weakness. We used a mouse model to identify candidate genes potentially involved in causation or recovery from ZIKV-induced acute flaccid paralysis. Using Zikv and Chat chromogenic and fluorescence in situ RNA hybridization, electron microscopy, immunohistochemistry, and ZIKV RT-qPCR, we determined that some paralyzed mice had infected motor neurons, but motor neurons are not reduced in number and the infection was not present in all paralyzed mice; hence infection of motor neurons were not strongly correlated with paralysis. Consequently, paralysis was probably caused by by-stander effects. To address this, we performed bioinformatics analysis on spinal cord RNA to identify 2058 differentially expressed genes (DEGs) that were altered during paralysis and then normalized after paralysis. Of these "biphasic" DEGs, 951 were up-regulated and 1107 were down-regulated during paralysis, followed by recovery. To refine the search for candidate DEGs we used gene ontology analysis and RT-qPCR to select 3 DEGs that could be involved with the node of Ranvier function and 5 DEGs that could be involved with synaptic function. Among these, SparcL1:Sparc DEG ratios were identified to be inversely correlated with ZIKV-induced paralysis, which is consistent with the known function of SPARC protein to antagonize the synaptogenesis of SPARCL1. Ank3, Sptbn1, and Epb41l3 affecting the structures at and near the nodes of Ranvier were significantly downregulated during ZIKV-induced paralysis. The primary contribution is the identification of 8 candidate genes that may be involved in the causation or recovery of ZIKV-induced paralysis.

Multiomics Reveals Biological Mechanisms Linking Macroscale Structural Covariance Network Dysfunction With Neuropsychiatric Symptoms Across the Alzheimer's Disease Continuum

BACKGROUND

The high heterogeneity of neuropsychiatric symptoms (NPSs) hinders further exploration of their role in neurobiological mechanisms and Alzheimer's disease (AD). We aimed to delineate NPS patterns based on brain macroscale connectomics to understand the biological mechanisms of NPSs on the AD continuum.

METHODS

We constructed regional radiomics similarity networks for 550 participants (AD with NPSs [n = 376], AD without NPSs [n = 111], and normal control participants [n = 63]) from the CIBL (Chinese Imaging, Biomarkers, and Lifestyle) study. We identified regional radiomics similarity network connections associated with NPSs and then clustered distinct subtypes of AD with NPSs. An independent dataset (n = 189) and internal validation were performed to assess the robustness of the NPS subtypes. Subsequent multiomics analysis was performed to assess the distinct clinical phenotype and biological mechanisms in each NPS subtype.

RESULTS

AD patients with NPSs were clustered into severe (n = 187), moderate (n = 87), and mild (n = 102) NPS subtypes, each exhibiting distinct brain network dysfunction patterns. A high level of consistency in clustering NPSs was internally and externally validated. Severe and moderate NPS subtypes were associated with significant cognitive impairment, increased plasma p-tau181 (tau phosphorylated at threonine 181) levels, extensive decreased brain volume and cortical thickness, and accelerated cognitive decline. Gene set enrichment analysis revealed enrichment of differentially expressed genes in ion transport and synaptic transmission with variations for each NPS subtype. Genome-wide association study analysis defined the specific gene loci for each subtype of AD with NPSs (e.g., logical memory), consistent with clinical manifestations and progression patterns.

CONCLUSIONS

This study identified and validated 3 distinct NPS subtypes, underscoring the role of NPSs in neurobiological mechanisms and progression of the AD continuum.

The Inflammation/NF-κB and BDNF/TrkB/CREB Pathways in the Cerebellum Are Implicated in the Changes in Spatial Working Memory After Both Morphine Dependence and Withdrawal in Rat

We aimed to explore the impact of the cerebellum on the decline in spatial working memory following morphine dependence and withdrawal. Two groups of male Wistar rats received intraperitoneal injections of either saline (1 ml/kg) or morphine (10 mg/kg) twice daily for 10 days, serving as the control and dependent groups. Additionally, a withdrawal group underwent a 30-day withdrawal period after the dependence phase. Spatial working memory was assessed using a Y maze test. ELISA and western blot were used to assess protein levels in the cerebellum. On day 1, morphine impaired spatial working memory, deteriorated further after 10 days of morphine use, and nearly returned to its initial level following a 30-day withdrawal period. On day 10, significant increases in TNF-α, IL-1β, and CXCL12 and a notable decrease in IL-10 levels were detected in the morphine-dependent group, which did not completely restore in the withdrawal group. The protein levels of CXCR4, TLR4, P2X7R, and NF-κB sharply increased in the morphine-dependent group. However, these levels almost returned to normal after withdrawal. In the morphine-dependent group, BDNF decreased, while TrkB and CREB1 increased noticeably. Nevertheless, after withdrawal, TrkB and CREB1 but not BDNF levels returned to normal. In the morphine-dependent group, both CACNA1 and KCNMA1 decreased significantly and after withdrawal, only KCNMA1 showed partial restoration, while CACNA1 did not. It can be concluded that inflammation/NF-κB and BDNF/TrkB/CREB pathways play key roles in neural adaptation within the cerebellum, contributing to the decline in spatial working memory after both morphine dependence and withdrawal.

Actin polymerization and longitudinal actin fibers in axon initial segment plasticity

The location of the axon initial segment (AIS) at the junction between the soma and axon of neurons makes it instrumental in maintaining neural polarity and as the site for action potential generation. The AIS is also capable of large-scale relocation in an activity-dependent manner. This represents a form of homeostatic plasticity in which neurons regulate their own excitability by changing the size and/or position of the AIS. While AIS plasticity is important for proper functionality of AIS-containing neurons, the cellular and molecular mechanisms of AIS plasticity are poorly understood. Here, we analyzed changes in the AIS actin cytoskeleton during AIS plasticity using 3D structured illumination microscopy (3D-SIM). We showed that the number of longitudinal actin fibers increased transiently 3 h after plasticity induction. We further showed that actin polymerization, especially formin mediated actin polymerization, is required for AIS plasticity and formation of longitudinal actin fibers. From the formin family of proteins, Daam1 localized to the ends of longitudinal actin fibers. These results indicate that active re-organization of the actin cytoskeleton is required for proper AIS plasticity.

Identification of a novel KCNT2 variant in a family with developmental and epileptic encephalopathies: a case report and literature review

Background: Developmental and epileptic encephalopathies (DEEs) are a group of heterogeneous neurodevelopmental diseases characterized mainly by developmental delay/intellectual disability and early-onset epilepsy. Researchers have identified variations in the KCNT2 gene (OMIM* 610044) as the cause of DEE type 57 (MIM# 617771). Case presentation: We report in this study a 46-year-old woman who presented with early-onset epilepsy, intellectual disability, hypertrichosis, coarse facial features, and short stature. Besides, there were four other affected individuals in her family history, including two elder brothers, a younger brother, and their mother. We collected blood samples from the proband, her two affected brothers, and her clinically normal daughter for genetic analysis. Clinical exome sequencing revealed a novel heterozygous variant in the KCNT2 gene (NM_198503: c.188G>A, p.Arg63His) in the proband and her two affected brothers, while her daughter did not carry this variant. Furthermore, we reviewed all 25 patients identified in the literature with KCNT2 variants and compared their phenotypes. Conclusion: Epilepsy and intellectual disability/developmental delay occur in almost all patients with KCNT2 variants. KCNT2-relevant DEEs partially overlap with the clinical phenotypes of KATP channel diseases, particularly in hypertrichosis and distinctive coarse facial features.

Thalamic Neuron Resilience during Osmotic Demyelination Syndrome (ODS) Is Revealed by Primary Cilium Outgrowth and ADP-ribosylation factor-like protein 13B Labeling in Axon Initial Segment

A murine osmotic demyelinating syndrome (ODS) model was developed through chronic hyponatremia, induced by desmopressin subcutaneous implants, followed by precipitous sodium restoration. The thalamic ventral posterolateral (VPL) and ventral posteromedial (VPM) relay nuclei were the most demyelinated regions where neuroglial damage could be evidenced without immune response. This report showed that following chronic hyponatremia, 12 h and 48 h time lapses after rebalancing osmolarity, amid the ODS-degraded outskirts, some resilient neuronal cell bodies built up primary cilium and axon hillock regions that extended into axon initial segments (AIS) where ADP-ribosylation factor-like protein 13B (ARL13B)-immunolabeled rod-like shape content was revealed. These AIS-labeled shaft lengths appeared proportional with the distance of neuronal cell bodies away from the ODS damaged epicenter and time lapses after correction of hyponatremia. Fine structure examination verified these neuron abundant transcriptions and translation regions marked by the ARL13B labeling associated with cell neurotubules and their complex cytoskeletal macromolecular architecture. This necessitated energetic transport to organize and restore those AIS away from the damaged ODS core demyelinated zone in the murine model. These labeled structures could substantiate how thalamic neuron resilience occurred as possible steps of a healing course out of ODS.