The GABA A α5-selective Modulator, RO4938581, Rescues Protein Anomalies in the Ts65Dn Mouse Model of Down Syndrome

Brain circuit pathology in Down syndrome: from neurons to neural networks

Down syndrome (DS), a genetic pathology caused by triplication of chromosome 21, is characterized by brain hypotrophy and impairment of cognition starting from infancy. While studies in mouse models of DS have elucidated the major neuroanatomical and neurochemical defects of DS, comparatively fewer investigations have focused on the electrophysiology of the DS brain. Electrical activity is at the basis of brain functioning. Therefore, knowledge of the way in which brain circuits operate in DS is fundamental to understand the causes of behavioral impairment and devise targeted interventions. This review summarizes the state of the art regarding the electrical properties of the DS brain, starting from individual neurons and culminating in signal processing in whole neuronal networks. The reported evidence derives from mouse models of DS and from brain tissues and neurons derived from individuals with DS. EEG data recorded in individuals with DS are also provided as a key tool to understand the impact of brain circuit alterations on global brain activity.

Profiling Basal Forebrain Cholinergic Neurons Reveals a Molecular Basis for Vulnerability Within the Ts65Dn Model of Down Syndrome and Alzheimer's Disease

Basal forebrain cholinergic neuron (BFCN) degeneration is a hallmark of Down syndrome (DS) and Alzheimer's disease (AD). Current therapeutics have been unsuccessful in slowing disease progression, likely due to complex pathological interactions and dysregulated pathways that are poorly understood. The Ts65Dn trisomic mouse model recapitulates both cognitive and morphological deficits of DS and AD, including BFCN degeneration. We utilized Ts65Dn mice to understand mechanisms underlying BFCN degeneration to identify novel targets for therapeutic intervention. We performed high-throughput, single population RNA sequencing (RNA-seq) to interrogate transcriptomic changes within medial septal nucleus (MSN) BFCNs, using laser capture microdissection to individually isolate ~500 choline acetyltransferase-immunopositive neurons in Ts65Dn and normal disomic (2N) mice at 6 months of age (MO). Ts65Dn mice had unique MSN BFCN transcriptomic profiles at ~6 MO clearly differentiating them from 2N mice. Leveraging Ingenuity Pathway Analysis and KEGG analysis, we linked differentially expressed gene (DEG) changes within MSN BFCNs to several canonical pathways and aberrant physiological functions. The dysregulated transcriptomic profile of trisomic BFCNs provides key information underscoring selective vulnerability within the septohippocampal circuit. We propose both expected and novel therapeutic targets for DS and AD, including specific DEGs within cholinergic, glutamatergic, GABAergic, and neurotrophin pathways, as well as select targets for repairing oxidative phosphorylation status in neurons. We demonstrate and validate this interrogative quantitative bioinformatic analysis of a key dysregulated neuronal population linking single population transcript changes to an established pathological hallmark associated with cognitive decline for therapeutic development in human DS and AD.

Long-lasting correction of in vivo LTP and cognitive deficits of mice modelling Down syndrome with an α5-selective GABAA inverse agonist

Background and Purpose

Excessive GABAergic inhibition contributes to cognitive dysfunctions in Down syndrome (DS). Selective negative allosteric modulators (NAMs) of α5‐containing GABA_A receptors such as the α5 inverse agonist (α5IA) restore learning and memory deficits in Ts65Dn mice, a model of DS. In this study we have assessed the long‐lasting effects of α5IA on in vivo LTP and behaviour in Ts65Dn mice.

Experimental Approach

We made in vivo LTP recordings for six consecutive days in freely moving Ts65Dn mice and their wild‐type littermates, treated with vehicle or α5IA. In parallel, Ts65Dn mice were assessed by various learning and memory tests (Y maze, Morris water maze, or the novel object recognition) for up to 7 days, following one single injection of α5IA or vehicle.

Key Results

LTP was not evoked in vivo in Ts65Dn mice at hippocampal CA3‐CA1 synapses. However, this deficit was sustainably reversed for at least six consecutive days following a single injection of α5IA. This long‐lasting effect of α5IA was also observed when assessing working and long‐term memory deficits in Ts65Dn mice.

Conclusion and Implications

We show for the first time in vivo LTP deficits in Ts65Dn mice. These deficits were restored for at least 6 days following acute treatment with α5IA and might be the substrate for the long‐lasting pharmacological effects of α5IA on spatial working and long‐term recognition and spatial memory tasks. Our results demonstrate the relevance of negative allosteric modulators of α5‐containing GABA_A receptors to the treatment of cognitive deficits associated with DS.

Using mouse models to understand Alzheimer's disease mechanisms in the context of trisomy of chromosome 21

People who have Down syndrome are at significantly elevated risk of developing early onset Alzheimer's disease that causes dementia (AD-DS). Here we review recent progress in modeling the development of AD-DS in mouse models. These studies provide insight into mechanisms underlying Alzheimer's disease and generate new clinical research questions. In addition, they suggest potential new targets for disease prevention therapies.

Enhanced Dendritic Inhibition and Impaired NMDAR Activation in a Mouse Model of Down Syndrome

Down syndrome (DS) or Trisomy 21 is a developmental disorder leading to cognitive deficits, including disruption of hippocampus-dependent learning and memory. Enhanced inhibition has been suggested to underlie these deficits in DS based on studies using the Ts65Dn mouse model. Here we show that, in this mouse model, GABAergic synaptic inhibition onto dendrites of hippocampal pyramidal cells is increased. By contrast, somatic inhibition was not altered. In addition, synaptic NMDAR currents were reduced. Furthermore, dendritic inhibition was mediated via nonlinear α5-subunit containing GABA_ARs that closely matched the kinetics and voltage dependence of NMDARs. Thus, enhanced dendritic inhibition and reduced NMDA currents strongly decreased burst-induced NMDAR-mediated depolarization and impaired LTP induction. Finally, selective reduction of α5-GABA_AR-mediated inhibition rescued both burst-induced synaptic NMDAR activation and synaptic plasticity. These results demonstrate that reduced synaptic NMDAR activation and synaptic plasticity in the Ts65Dn mouse model of DS can be corrected by specifically targeting nonlinear dendritic inhibition.SIGNIFICANCE STATEMENT Mild to moderate intellectual disability is a prominent feature of Down syndrome. Previous studies in mouse models suggest that increased synaptic inhibition is a main factor for decreased synaptic plasticity, the cellular phenomenon underlying memory. The present study shows that increased inhibition specifically onto dendrites together with reduced NMDAR content in excitatory synapses may be the cause. Reducing a slow nonlinear component that is specific to dendritic inhibitory inputs and mediated by α5 subunit-containing GABA_A receptors rescues both NMDAR activation and synaptic plasticity.

In silico identification of critical proteins associated with learning process and immune system for Down syndrome

Understanding expression levels of proteins and their interactions is a key factor to diagnose and explain the Down syndrome which can be considered as the most prevalent reason of intellectual disability in human beings. In the previous studies, the expression levels of 77 proteins obtained from normal genotype control mice and from trisomic Ts65Dn mice have been analyzed after training in contextual fear conditioning with and without injection of the memantine drug using statistical methods and machine learning techniques. Recent studies have also pointed out that there may be a linkage between the Down syndrome and the immune system. Thus, the research presented in this paper aim at in silico identification of proteins which are significant to the learning process and the immune system and to derive the most accurate model for classification of mice. In this paper, the features are selected by implementing forward feature selection method after preprocessing step of the dataset. Later, deep neural network, gradient boosting tree, support vector machine and random forest classification methods are implemented to identify the accuracy. It is observed that the selected feature subsets not only yield higher accuracy classification results but also are composed of protein responses which are important for the learning and memory process and the immune system.