Biology subjects

Smart, T. G.

Publications and source records attributed to Smart, T. G..

6 recordsLinked to original sources

bioRxiv2025

Hippocampal circuit-specific enhancement of GABA-inhibition caused by discrete gene regions in a Down syndrome model

Although Down syndrome (DS), trisomy 21, affects [~]6 million people worldwide, the neural circuit mechanisms underlying the neurophenotypes of impaired learning, memory and language are unknown. A prominent candidate mechanism involves dysfunctional GABA-signalling and GABAA receptor ligands have been proposed as therapeutics to reverse the neurophenotypic effects of DS. By investigating GABA neurotransmission in brain regions important for cognition in mouse DS models, we reveal that excessive inhibition is not a ubiquitous feature of DS but instead is brain circuit-specific demonstrating increased phasic and tonic inhibition in the dentate gyrus with no comparative changes to inhibition in CA1 and medial prefrontal cortex. In the dentate, elevated extrasynaptic GABA signalling, and interneuron numbers, likely underpin spike firing defects. We show that increased GABA inhibition is caused by increased dosage of Olig1, Olig2 and Dyrk1a. Overall, DS mice are characterised by circuit-specific dysfunctional inhibition predicted to affect cognition via sparse coding in the hippocampus.

neuroscience↗

bioRxiv2025

Synaptic and intrinsic membrane defects disrupt early neural network dynamics in Down syndrome

Down syndrome, caused by trisomy 21, affects around six million people worldwide and features learning, memory and language deficits. However, the mechanisms underlying trisomy 21 neurophenotypes involving human cortical circuitry are unknown. By characterising developing neural network dynamics and single cell excitability profiles, with synaptic and voltage-dependent ion channel behaviour, using an isogenic induced pluripotent stem cell- derived neuronal model, we show that trisomy 21 impairs the activity and development of cortical circuitry. This is caused by deficient glutamatergic synaptic connectivity and by aberrant intrinsic membrane properties involving K+ and Na+ channels culminating in spike firing defects that weaken neural network activity and disrupt the synchrony of developing neurons. We also identify transiently activated A-type K+ channels, specifically Kv4.3 channels, as a key orchestrator for Down syndrome during neurodevelopment. Overall, these excitability changes will significantly contribute towards the aberrant neurophenotypes observed later on in life.

neuroscience↗

bioRxiv2021

Inhibitory neurosteroid reverses the dendritic spine disorder caused by gain-of-function GABAAR epilepsy variants

GABAA receptors (GABAARs) are key orchestrators of neuronal activity and several GABAAR variants have been linked to genetic neurodevelopmental disorders (NDDs) and epilepsies. Here, we report two variants (Met263Lys, Leu267Ile) in the predominant GABAAR 1 subunit gene (GABRA1) that increase apparent receptor affinity for GABA and confer spontaneous receptor activity. These gain-of-function features are unusual because GABAAR variants are traditionally thought to cause seizures by reducing inhibitory neurotransmission. Both Met263Lys and Leu267Ile increased tonic and spontaneous GABAergic conductances in neurons revealed by competitive inhibition and channel block of GABAARs. Significantly, 1-subunit variant expression in hippocampal neurons also reduced dendritic spine density. Our results indicate that elevated GABAergic signalling can precipitate genetic epilepsies and NDDs. Furthermore, the mechanistic basis may involve the de-compartmentalisation of excitatory inputs due to the removal of dendritic spines. This aberrant structural plasticity can be reversed by the naturally-occurring, therapeutically-tractable, inhibitory neurosteroid, pregnenolone sulphate.

neuroscience↗

bioRxiv2021

AKAP79 enables calcineurin to directly suppress protein kinase A activity

Interplay between the second messengers cAMP and Ca2+ is a hallmark of dynamic cellular processes. A common motif is the opposition of the Ca2+-sensitive phosphatase calcineurin and the major cAMP receptor, protein kinase A (PKA). Calcineurin dephosphorylates sites primed by PKA to bring about changes including synaptic long-term depression (LTD). AKAP79 supports signaling of this type by anchoring PKA and calcineurin in tandem. In this study, we discovered that AKAP79 increases the rate of calcineurin dephosphorylation of type II PKA regulatory subunits by an order of magnitude. Fluorescent PKA activity reporter assays, supported by kinetic modeling, show how AKAP79-enhanced calcineurin activity enables suppression of PKA without altering cAMP levels by increasing PKA catalytic subunit capture rate. Experiments with hippocampal neurons indicate that this mechanism contributes towards LTD. This non- canonical mode of PKA regulation may underlie many other cellular processes.

biochemistry↗

bioRxiv2020

PKA-mediated phosphorylation of Neuroligin-2 regulates its cell surface expression and synaptic stabilisation

The trans-synaptic adhesion molecule Neuroligin-2 (NL2) is essential for the development and function of inhibitory synapses. NL2 recruits the postsynaptic scaffold protein gephyrin, which in turn stabilises GABAA receptors (GABAARs) in the postsynaptic domain. Dynamic regulation of synaptic GABAAR concentration is crucial for inhibitory neurotransmission efficacy. Changes in synaptic levels of NL2 contribute to regulating GABAAR synaptic concentration, however the mechanisms that control NL2 synaptic stabilisation are mostly unknown. Here, by combining biochemistry, imaging, single particle tracking and electrophysiology, we identify a key role for cAMP-dependent protein kinase (PKA) in synaptic stabilisation of NL2. We show that PKA-mediated phosphorylation of NL2 at S714 causes its dispersal from the synapse and reduces NL2 surface levels, leading to a loss of synaptic GABAARs. Conversely, enhanced stability of NL2 at synapses through abolishing phosphorylation leads to increased inhibitory signalling. Thus, PKA plays a key role in regulating NL2 function and synaptic inhibition.

neuroscience↗

bioRxiv2018

Brain neurosteroids are natural anxiolytics targeting α2 subunit γ-aminobutyric acid type-A receptors

Neurosteroids are naturally-occurring molecules in the brain that modulate neurotransmission. They are physiologically important since disrupting their biosynthesis precipitates neurological disorders, such as anxiety and depression. The endogenous neurosteroids, allopregnanolone and tetrahydro-deoxycorticosterone are derived from sex and stress hormones respectively, and exhibit therapeutically-useful anxiolytic, analgesic, sedative, anticonvulsant and antidepressant properties. Their main target is the {gamma}-aminobutyric acid type-A inhibitory neurotransmitter receptor (GABAAR), whose activation they potentiate. However, whether specific GABAAR isoforms and neural circuits differentially mediate endogenous neurosteroid effects is unknown. By creating a knock-in mouse that removes neurosteroid potentiation from 2-GABAAR subunits, we reveal that this isoform is a key target for neurosteroid modulation of phasic and tonic inhibition, and is essential for the anxiolytic role of endogenous neurosteroids, but not for their anti-depressant or analgesic properties. Overall, 2-GABAAR targeting neurosteroids may act as selective anxiolytics for the treatment of anxiety disorders, providing new therapeutic opportunities for drug development.

neuroscience↗