Biology subjects

Georgiou, J.

Publications and source records attributed to Georgiou, J..

11 recordsLinked to original sources

bioRxiv2026

Characterization of the Grin1Q536R/+ mouse: a preclinical model for GRIN1-related neurodevelopmental disorder

N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors playing critical roles in brain development, synaptic plasticity, and cognition. GRIN1-Related Neurodevelopmental Disorder (GRIN1-NDD) is a rare genetic condition caused by pathogenic variations in the GRIN1 gene, which encodes the obligatory subunit of NMDARs. The spectrum of GRIN1 clinical symptoms is hypothesized to result from the functional consequences that different missense variants have on NMDARs. To investigate the disease mechanism in vivo, we generated a novel heterozygous Grin1Q536R/+ knock-in mouse model that carries the identical variation as an adolescent male patient. We describe the clinical presentation of this patient and conduct comprehensive molecular, morphological, electrophysiological, and behavioural characterization in the juvenile, adult, and aging mice. Compared to wildtype littermates, Grin1Q536R/+ mice displayed reduced whole-cell NMDA-induced currents in cortical pyramidal neurons, and reduced NMDAR-mediated synaptic transmission, decreased long-term potentiation, but intact depotentiation at the hippocampal CA1 synapses. Morphological change was observed in the dentate gyrus region of the Grin1Q536R/+ mice. Behavioral phenotyping revealed age- and sex-dependent differences from controls, including hyperlocomotion, reduced muscle strength, and spatial learning deficits. These phenotypes are in line with the clinical manifestations and the relative disease severity of the male patient. The age-dependent phenotypic shift in Grin1Q536R/+ mice highlights the model's value for investigating GRIN1-NDD disease progression and informing longitudinal monitoring as well as potential therapeutic adjustments with age. Taken together, our findings establish a novel and robust in vivo model for studying NMDAR mechanisms and disease pathology in GRIN1-NDD, while supporting the preclinical development of new therapeutic strategies.

neuroscience↗

bioRxiv2026

Spacing theta-burst stimulation enhances synaptic potentiation in the vulnerable prefrontal cortex

Neuromodulation with intermittent theta-burst stimulation (iTBS) is a clinical treatment for major depression. One postulated mechanism of iTBS is to strengthen the synaptic connections which activate the prefrontal cortex to regulate mood. With ex vivo electrical stimulation and neuronal calcium imaging, we demonstrate that a common, clinical iTBS pattern (600-stimuli, ~3-minutes) reliably strengthens the synaptic recruitment of the adult mouse prefrontal cortex. This synaptic potentiation, however, becomes less reliable in the depressive-like mouse model of prolonged social isolation. To better understand this change, we examine the impact of social isolation on the complex pattern of calcium elevation during clinical iTBS. In social isolation, the calcium peaks rise more prominently, but the induction peak no longer predicts the synaptic potentiation outcome. To better regulate calcium dynamics during induction, we test a paradigm with fewer iTBS episodes, separated by longer intervals. This spaced iTBS (90-stimuli, ~10-minutes) in the adult prefrontal cortex limits calcium elevation during induction and yields reliable long-term potentiation (LTP) following either juvenile- or adult-onset social isolation. This research illustrates new strategies to interrogate and to enhance synaptic plasticity in the vulnerable prefrontal cortex.

neuroscience↗

bioRxiv2026

Emergence of microglia structural and functional heterogeneity between hippocampal subregions during development into early adulthood

Phagocytosis is a key function performed by microglia to maintain tissue homeostasis. The degree of microglial phagocytic activity differs across ages and between gross anatomical brain regions, dictated by the local environment. Here, we asked whether microglial phagocytic phenotype exhibits subregion-scale tuning by circuit-specific features within a brain region. To address this, we took advantage of the highly stereotyped architecture of the hippocampus. We examined three adjacent synaptic subregions, the CA1 stratum radiatum (SR), stratum lacunosum moleculare (SLM) and dentate gyrus molecular layer (DGML). These three subregions provide an ideal system for examining local microglia heterogeneity, as each subregion contains distinct neuropil features, creating three adjacent unique micro-environments to which the microglia are exposed. We measured the phagocytic activity and morphological properties of over 1,000 individual microglia at two developmental points, mid-postnatal (P16) and early adulthood (P60) in the CA1 SR, SLM and DGML. We found that microglial phenotype diversified with development into early adulthood. At the mid-postnatal age, phagocytic activity and morphology were homogeneous across subregions. Conversely, in young adulthood, microglia in the CA1 SR and DGML exhibited a reduction in phagocytic activity, while microglia in the CA1 SLM maintained a highly phagocytic phenotype reminiscent of an immature-like state. These findings uncover a fine-scale tuning of microglia activity that emerges during maturation and is dictated at the sub-region level of the hippocampus, uncovering a distinct population of microglia in the CA1 SLM that exhibit a persistent immature phenotype under physiological conditions. Understanding the target(s) of this phagocytosis and consequences for CA1 SLM function will provide new insight into the role of local tuning of microglia properties for circuit-specific needs in both health and disease.

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bioRxiv2025

Activation of CP-AMPARs is required for homosynaptic and heterosynaptic structural LTP in the hippocampus

Long-term potentiation (LTP) involves alterations in synaptic structure that are believed to underlie the persistent increase in synaptic efficacy. Here we compared structural LTP (sLTP) in EGFP-labelled spines with functional LTP, using field potential recording, at CA3-CA1 synapses in mouse hippocampal slices for [~] 2 h following theta-burst stimulation (TBS). Activity-dependent labelling with FM4-64 allowed us to compare activated (FM+) and non-activated synapses and thereby compare homo- and hetero-synaptic sLTP. In addition, we related spine volume changes according to the probability of release, P(r), of activated synapses. At homosynaptic sites there was the expected NMDA receptor (NMDAR)-dependent potentiation of spine volume that persisted throughout the recording period. We found that this sLTP also required the synaptic activation of CP-AMPARs. There was also sLTP at heterosynaptic sites that, surprisingly, developed more quickly than the associated homosynaptic sLTP. This heterosynaptic sLTP was also dependent on the synaptic activation of both NMDARs and CP-AMPARs. Additionally, we observed a trans- and hetero-synaptic interaction, whereby the heterosynaptic spines grew according to the P(r) of the neighbouring active (homosynaptic) synapse. These observations have therefore advanced our understanding of sLTP in several ways; the first demonstration of the absolute requirement for the synaptic activation of CP-AMPARs for sLTP; the magnitude of heterosynaptic sLTP relative to homosynaptic sLTP and the hitherto unexpected combination of trans- and hetero-synaptic interactions.

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bioRxiv2025

Tau deletion results in sex-dependent modulation of synaptic weakening in rat hippocampus

Tangles, a defining characteristic of Alzheimers disease (AD), are composed principally of hyperphosphorylated and misfolded tau species. However, dysregulation of tau precedes tangle formation by many years and is associated with cognitive decline, the best functional correlate of which is synaptic weakening and eventual synaptic loss. Although tau is present at synapses, its normal synaptic function is poorly understood - information vital to the rational development of tau-modifying therapies. Since rats have a greater cognitive repertoire than mice and display more robust signs of tau pathology in AD models, we developed a tau knockout (Mapt-/-) rat to investigate the impact of tau elimination on synaptic function. We observed that long-term depression (LTD), a form of synaptic weakening, is enhanced in the hippocampus of male, but not female, rats. This enhanced LTD was dependent on the synaptic activation of group I metabotropic glutamate receptors and involved altered synaptic actin polymerization. These studies therefore provide mechanistic insights into how tau modulates synaptic function via regulation of the actin cytoskeleton. Our findings are relevant to the understanding of the physiological roles of synaptic tau, the functional consequences of tau-lowering therapies, and the influence of sex in AD susceptibility.

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bioRxiv2024

Activation of group I mGluRs is required for heterosynaptic priming of long-term potentiation in mouse hippocampus

Metaplasticity involves changes in the state of neurons or synapses that influence their ability to generate synaptic plasticity. One form of heterosynaptic metaplasticity, known as synaptic tagging and capture (STC), has been intensively studied but the underlying mechanisms are not fully understood. In experiments using hippocampal slices prepared from C57BL/6J mice, we have examined the role of group I metabotropic glutamate receptors (mGluRs) in STC. We used a version of STC where a strong theta-burst stimulus (TBS), delivered to one set of Schaffer collateral-commissural pathway inputs to CA1, preceded a weak TBS delivered to a second independent set of inputs. We observed that, firstly, dual inhibition of mGluR1 and mGluR5, using YM 298198 and MTEP respectively, did not affect a form of protein synthesis-independent LTP (LTP1), but substantially inhibited a form of protein synthesis-dependent LTP (LTP2). Secondly, these inhibitors prevented the small heterosynaptic potentiation, which is often associated with LTP2. Thirdly, STC was abolished when these antagonists were applied either during the strong (priming) TBS or during the subsequent weak TBS at the independent pathway. It is proposed that the activation of group I mGluRs serves as a trigger for local protein synthesis both during the strong and weak TBS and, as such, are an integral part of the STC process. STC is involved in associative learning and memory, a cognitive function that is disrupted in many brain disorders including Alzheimers disease.

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bioRxiv2024

Grin1Y647S/+ Mice: A Preclinical Model of GRIN1-Related Neurodevelopmental Disorder

GRIN1-related neurodevelopmental disorder (GRIN1-NDD) is characterized by clinically significant variation in the GRIN1 gene, which encodes the obligatory GluN1 subunit of N-methyl-D-aspartate receptors (NMDARs). The identified p.Tyr647Ser (Y647S) variant is carried by a 34-year-old female with seizures and intellectual disability. This study builds upon initial in vitro investigations of the functional impacts of this variant in the SYTANLAAF domain of the GluN1 M3 helix and examines its in vivo consequences in a mouse model. To investigate in vitro functional impacts of NMDARs containing GluN1-Y647S variant subunits, GluN1-Y647S was co-expressed with wildtype GluN2A or GluN2B subunits in Xenopus laevis oocytes and HEK cells. Grin1Y647S/+ mice were created by CRISPR-Cas9 endonuclease-mediated transgenesis and the molecular, electrophysiological, and behavioural consequences of the variant were examined. Additionally, de-identified patient data were collected to examine the representative nature of Grin1Y647S/+ mice in modelling specific aspects of patient symptomology. In vitro, NMDARs containing GluN1-Y647S showed altered sensitivity to endogenous agonists and negative allosteric modulators, and reduced cell surface trafficking. Ex vivo, Grin1Y647S/+ mice displayed a reduction in whole brain GluN1 levels and a deficiency in NMDAR-mediated synaptic transmission in the hippocampus. Behaviourally, Grin1Y647S/+ mice exhibited altered vocalizations, muscle strength, sociability, and problem-solving, as well as spontaneous convulsions that were ameliorated with supplementation of {beta}-hydroxybutyrate (BHB), an endogenously produced ketone body. The Y647S variant confers a complex in vivo phenotype, which reflects largely diminished properties of NMDAR function. As a result, Grin1Y647S/+ mice display atypical behaviour in domains relevant to the clinical characteristics of GRIN1-NDD and the individual carrying the variant, which allowed for the identification of BHB supplementation as a potential anti-convulsant treatment. Ultimately, the characterization of Grin1Y647S/+ mice accomplished in the present work, expands our understanding of the mechanisms underlying GRIN1-NDD and provides a foundation for the continued development of novel therapeutics.

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bioRxiv2022

Amyloid-β1-42 oligomers enhance mGlu5R-dependent synaptic weakening via NMDAR activation and complement C3aR/C5aR signaling

Synaptic dysfunction, weakening, and loss of synapses are well-correlated with the pathology of Alzheimers disease (AD). Oligomeric amyloid beta (oA{beta}) is considered a major synaptotoxic trigger for AD. Recent studies have implicated hyperactivation of the complement cascade as the driving force for loss of synapses caused by oA{beta}. However, the initial synaptic cues that trigger pathological complement activity remain elusive. Here, we examined a form of synaptic long-term depression (LTD) mediated by metabotropic glutamate receptors (mGluR) which is disrupted in rodent models of AD. Exogenous application of oA{beta} (1-42) to mouse hippocampal slices enhanced the magnitude of mGlu subtype 5 receptor (mGlu5R)-dependent LTD. We found that the enhanced synaptic weakening occurred via both NMDARs and complement C3aR/C5aR signaling. Our findings reveal a mechanistic interaction between mGlu5R, NMDARs, and the complement cascade in synaptic weakening induced by oA{beta}, which could represent an early trigger of synaptic loss and degeneration in AD.

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bioRxiv2020

Further evidence that CP-AMPARs are critically involved in synaptic tag and capture at hippocampal CA1 synapses

The synaptic tag and capture (STC) hypothesis provides an important theoretical basis for understanding the synaptic basis of associative learning. We recently provided pharmacological evidence that calcium-permeable AMPA receptors (CP-AMPARs) are a crucial component of this process. Here we have investigated two predictions that arise on the basis of CP-AMPARs serving as a trigger of the STC effect. Firstly, we compared the effects of the order in which we delivered a strong theta burst stimulation (TBS) protocol (75 pulses) and a weak TBS protocol (15 pulses) to two independent inputs. We only observed a significant STC effect when the strong preceded the weak TBS. Second, we found that pausing stimulation following either the sTBS or the wTBS for [~]20 min largely eliminates the STC effect. These observations are exactly as predicted for a process that is triggered by the synaptic insertion of CP-AMPARs and provide a framework for establishing the underlying molecular mechanism.

neuroscience↗

bioRxiv2020

PKA drives an increase in AMPA receptor unitary conductance during LTP in the hippocampus

Long-term potentiation (LTP) at hippocampal CA1 synapses can be expressed by an increase either in the number (N) of AMPA (-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors or in their single channel conductance ({gamma}). Here we have established how these distinct synaptic processes contribute to the expression of LTP in hippocampal slices obtained from young adult rodents. LTP induced by compressed theta burst stimulation (TBS), with a 10 s inter-episode interval, involved purely an increase in N (LTPN). In contrast, either a spaced TBS, with a 10 min inter-episode interval, or a single TBS, delivered when PKA was activated, resulted in LTP that was associated with a transient increase in {gamma} (LTP{gamma}). This {gamma} increase was due to the insertion of calcium-permeable (CP)-AMPA receptors. Activation of CaMKII was necessary and sufficient for LTPN whilst PKA was additionally required for LTP{gamma}. Thus, two mechanistically distinct forms of LTP co-exist at these synapses.

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bioRxiv2020

Complement C3-dependent glutamatergic synapse elimination in the developing hippocampus is region- and synapse-specific

Withdrawal statementThe authors have withdrawn their manuscript owing to the results of this pre-print being based on experiments performed prior to COVID-19 shutdowns, which required a near complete culling of the mouse colony. Upon re-initiating the colony, the phenotype outlined in the pre-print could no longer be observed and as such we have decided to withdraw the pre-print. Therefore, the authors do not wish this work to be cited as reference for the project. If you have any questions, please contact the corresponding author.

neuroscience↗