Cognitive and pharmacological insights from the Ts65Dn mouse model of Down syndrome

Pleiotropic effects of trisomy and pharmacologic modulation on structural, functional, molecular, and genetic systems in a Down syndrome mouse model

Down syndrome (DS) is characterized by skeletal and brain structural malformations, cognitive impairment, altered hippocampal metabolite concentration and gene expression imbalance. These alterations were usually investigated separately, and the potential rescuing effects of green tea extracts enriched in epigallocatechin-3-gallate (GTE-EGCG) provided disparate results due to different experimental conditions. We overcame these limitations by conducting the first longitudinal controlled experiment evaluating genotype and GTE-EGCG prenatal chronic treatment effects before and after treatment discontinuation. Our findings revealed that the Ts65Dn mouse model reflected the pleiotropic nature of DS, exhibiting brachycephalic skull, ventriculomegaly, neurodevelopmental delay, hyperactivity, and impaired memory robustness with altered hippocampal metabolite concentration and gene expression. GTE-EGCG treatment modulated most systems simultaneously but did not rescue DS phenotypes. On the contrary, the treatment exacerbated trisomic phenotypes including body weight, tibia microarchitecture, neurodevelopment, adult cognition, and metabolite concentration, not supporting the therapeutic use of GTE-EGCG as a prenatal chronic treatment. Our results highlight the importance of longitudinal experiments assessing the co-modulation of multiple systems throughout development when characterizing preclinical models in complex disorders and evaluating the pleiotropic effects and general safety of pharmacological treatments.

Large-scale organoid study suggests effects of trisomy 21 on early fetal neurodevelopment are more subtle than variability between isogenic lines and experiments

This study examines cortical organoids generated from a panel of isogenic trisomic and disomic iPSC lines (subclones) as a model of early fetal brain development in Down syndrome (DS). An initial experiment comparing organoids from one trisomic and one disomic line showed many genome-wide transcriptomic differences and modest differences in cell-type proportions, suggesting there may be a neurodevelopmental phenotype that is due to trisomy of chr21. To better control for multiple sources of variation, we undertook a highly robust study of ∼1,200 organoids using an expanded panel of six all-isogenic lines, three disomic, and three trisomic. The power of this experimental design was indicated by strong detection of the ∼1.5-fold difference in chr21 genes. However, the numerous expression differences in non-chr21 genes seen in the smaller experiment fell away, and the differences in cell-type representation between lines did not correlate with trisomy 21. Results suggest that the initial smaller experiment picked up differences between small organoid samples and individual isogenic lines, which "averaged out" in the larger panel of isogenic lines. Our results indicate that even when organoid and batch variability are better controlled for, variation between isogenic cell lines (even subclones) may obscure, or be conflated with, subtle neurodevelopmental phenotypes that may be present in ∼2nd trimester DS brain development. Interestingly, despite this variability between organoid batches and lines, and the "fetal stage" of these organoids, an increase in secreted Aβ40 peptide levels-an Alzheimer-related cellular phenotype-was more strongly associated with trisomy 21 status than were neurodevelopmental shifts in cell-type composition.

Enhanced GIRK2 channel signaling in Down syndrome: A feasible role in the development of abnormal nascent neural circuits

The most distinctive feature of Down syndrome (DS) is moderate to severe cognitive impairment. Genetic, molecular, and neuronal mechanisms of this complex DS phenotype are currently under intensive investigation. It is becoming increasingly clear that the abnormalities arise from a combination of initial changes caused by triplication of genes on human chromosome 21 (HSA21) and later compensatory adaptations affecting multiple brain systems. Consequently, relatively mild initial cognitive deficits become pronounced with age. This pattern of changes suggests that one approach to improving cognitive function in DS is to target the earliest critical changes, the prevention of which can change the ‘trajectory’ of the brain development and reduce the destructive effects of the secondary alterations. Here, we review the experimental data on the role of KCNJ6 in DS-specific brain abnormalities, focusing on a putative role of this gene in the development of abnormal neural circuits in the hippocampus of genetic mouse models of DS. It is suggested that the prevention of these early abnormalities with pharmacological or genetic means can ameliorate cognitive impairment in DS.

GIRK2 Channels in Down Syndrome and Alzheimer's Disease

Cognitive impairment in Down syndrome (DS) results from the abnormal expression of hundreds of genes. However, the impact of KCNJ6, a gene located in the middle of the 'Down syndrome critical region' of chromosome 21, seems to stand out. KCNJ6 encodes GIRK2 (KIR3.2) subunits of G protein-gated inwardly rectifying potassium channels, which serve as effectors for GABAB, m2, 5HT1A, A1, and many other postsynaptic metabotropic receptors. GIRK2 subunits are heavily expressed in neocortex, cerebellum, and hippocampus. By controlling resting membrane potential and neuronal excitability, GIRK2 channels may thus affect both synaptic plasticity and stability of neural circuits in the brain regions important for learning and memory. Here, we discuss recent experimental data regarding the role of KCNJ6/GIRK2 in neuronal abnormalities and cognitive impairment in models of DS and Alzheimer's disease (AD). The results compellingly show that signaling through GIRK2 channels is abnormally enhanced in mouse genetic models of Down syndrome and that partial suppression of GIRK2 channels with pharmacological or genetic means can restore synaptic plasticity and improve impaired cognitive functions. On the other hand, signaling through GIRK2 channels is downregulated in AD models, such as models of early amyloidopathy. In these models, reduced GIRK2 channel signaling promotes neuronal hyperactivity, causing excitatory-inhibitory imbalance and neuronal death. Accordingly, activation of GABAB/GIRK2 signaling by GIRK channel activators or GABAB receptor agonists may reduce Aβ-induced hyperactivity and subsequent neuronal death, thereby exerting a neuroprotective effect in models of AD.

Oxidative Phosphorylation Is Dysregulated Within the Basocortical Circuit in a 6-month old Mouse Model of Down Syndrome and Alzheimer's Disease

Down syndrome (DS) is the primary genetic cause of intellectual disability (ID), which is due to the triplication of human chromosome 21 (HSA21). In addition to ID, HSA21 trisomy results in a number of neurological and physiological pathologies in individuals with DS, including progressive cognitive dysfunction and learning and memory deficits which worsen with age. Further exacerbating neurological dysfunction associated with DS is the concomitant basal forebrain cholinergic neuron (BFCN) degeneration and onset of Alzheimer's disease (AD) pathology in early mid-life. Recent single population RNA sequencing (RNA-seq) analysis in the Ts65Dn mouse model of DS, specifically the medial septal cholinergic neurons of the basal forebrain (BF), revealed the mitochondrial oxidative phosphorylation pathway was significantly impacted, with a large subset of genes within this pathway being downregulated. We further queried oxidative phosphorylation pathway dysregulation in Ts65Dn mice by examining genes and encoded proteins within brain regions comprising the basocortical system at the start of BFCN degeneration (6 months of age). In select Ts65Dn mice we demonstrate significant deficits in gene and/or encoded protein levels of Complex I-V of the mitochondrial oxidative phosphorylation pathway in the BF. In the frontal cortex (Fr Ctx) these complexes had concomitant alterations in select gene expression but not of the proteins queried from Complex I-V, suggesting that defects at this time point in the BF are more severe and occur prior to cortical dysfunction within the basocortical circuit. We propose dysregulation within mitochondrial oxidative phosphorylation complexes is an early marker of cognitive decline onset and specifically linked to BFCN degeneration that may propagate pathology throughout cortical memory and executive function circuits in DS and AD.

Detoxification Improves Multidomain Cognitive Dysfunction in High-Dose Benzodiazepine Abusers

Purpose

High-dose benzodiazepines (BZDs) abuse has been documented to cause multidomain cognitive dysfunction. We explored whether cognitive abnormalities to high-dose BZD abuse might be reversed by detoxification with slow subcutaneous infusion of flumazenil.

Methods

We recruited 96 patients consecutively admitted to the Department of Internal Medicine, Addiction Medicine Unit, Verona University Hospital, Italy for detoxification from high-dose BZD dependence. After selection for inclusion and exclusion criteria, 50 patients (23 men, 27 women; age 42.7 ± 10.3 years) were included. They underwent a comprehensive neuropsychological battery to explore verbal memory, visuospatial memory, working memory, attention, and executive functions 28–30 days prior to admission for detoxification (T0) and at the end of detoxification, i.e., 7 days after admission (T1). A group of 50 healthy adults (24 men, 26 women; mean age 44.5 ± 12.8 years) matched for age, sex, and education served as controls.

Results

At T0, patients scored significantly worse than healthy controls in all the neuropsychological tests. Depression and anxiety scores were associated with impaired verbal memory at T0 in patients. T1–T0 comparison showed improved performances in all neuropsychological tests after the end of detoxification in patients.

Conclusion

We confirmed that all neuropsychological domains were significantly and profoundly impaired by high-dose BZD abuse and documented that cognitive abnormalities improved after detoxification with slow subcutaneous infusion of flumazenil.

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.

Hydroxyurea Improves Spatial Memory and Cognitive Plasticity in Mice and Has a Mild Effect on These Parameters in a Down Syndrome Mouse Model

Down syndrome (DS), a genetic disorder caused by partial or complete triplication of chromosome 21, is the most common genetic cause of intellectual disability. DS mouse models and cell lines display defects in cellular adaptive stress responses including autophagy, unfolded protein response, and mitochondrial bioenergetics. We tested the ability of hydroxyurea (HU), an FDA-approved pharmacological agent that activates adaptive cellular stress response pathways, to improve the cognitive function of Ts65Dn mice. The chronic HU treatment started at a stage when early mild cognitive deficits are present in this model (∼3 months of age) and continued until a stage of advanced cognitive deficits in untreated mice (∼5–6 months of age). The HU effects on cognitive performance were analyzed using a battery of water maze tasks designed to detect changes in different types of memory with sensitivity wide enough to detect deficits as well as improvements in spatial memory. The most common characteristic of cognitive deficits observed in trisomic mice at 5–6 months of age was their inability to rapidly acquire new information for long-term storage, a feature akin to episodic-like memory. On the background of severe cognitive impairments in untreated trisomic mice, HU-treatment produced mild but significant benefits in Ts65Dn by improving memory acquisition and short-term retention of spatial information. In control mice, HU treatment facilitated memory retention in constant (reference memory) as well as time-variant conditions (episodic-like memory) implicating a robust nootropic effect. This was the first proof-of-concept study of HU treatment in a DS model, and indicates that further studies are warranted to assess a window to optimize timing and dosage of the treatment in this pre-clinical phase. Findings of this study indicate that HU has potential for improving memory retention and cognitive flexibility that can be harnessed for the amelioration of cognitive deficits in normal aging and in cognitive decline (dementia) related to DS and other neurodegenerative diseases.

Proteomic analysis of six- and twelve-month hippocampus and cerebellum in a murine Down syndrome model

This study was designed to investigate the brain proteome of the Ts65Dn mouse model of Down syndrome. We profiled the cerebellum and hippocampus proteomes of 6- and 12-month-old trisomic and disomic mice by difference gel electrophoresis. We quantified levels of 2082 protein spots and identified 272 (170 unique UniProt accessions) by mass spectrometry. Four identified proteins are encoded by genes trisomic in the Ts65Dn mouse. Three of these (CRYZL11, EZR, and SOD1) were elevated with p-value <0.05, and 2 proteins encoded by disomic genes (MAPRE3 and PHB) were reduced. Intergel comparisons based on age (6 vs. 12 months) and brain region (cerebellum vs. hippocampus) revealed numerous differences. Specifically, 132 identified proteins were different between age groups, and 141 identified proteins were different between the 2 brain regions. Our results suggest that compensatory mechanisms exist, which ameliorate the effect of trisomy in the Ts65Dn mice. Differences observed during aging may play a role in the accelerated deterioration of learning and memory seen in Ts65Dn mice.

Disinhibition, an emerging pharmacology of learning and memory

Learning and memory are dependent on interactive excitatory and inhibitory mechanisms. In this review, we discuss a mechanism called disinhibition, which is the release of an inhibitory constraint that effectively results in an increased activity in the target neurons (for example, principal or projection neurons). We focus on discussing the role of disinhibition in learning and memory at a basic level and in disease models with cognitive deficits and highlight a strategy to reverse cognitive deficits caused by excess inhibition, through disinhibition of α5-containing GABA _A receptors mediating tonic inhibition in the hippocampus, based on subtype-selective negative allosteric modulators as a novel class of drugs.

Maternal Choline Supplementation: A Potential Prenatal Treatment for Down Syndrome and Alzheimer's Disease

Although Down syndrome (DS) can be diagnosed prenatally, currently there are no effective treatments to lessen the intellectual disability (ID) which is a hallmark of this disorder. Furthermore, starting as early as the third decade of life, DS individuals exhibit the neuropathological hallmarks of Alzheimer's disease (AD) with subsequent dementia, adding substantial emotional and financial burden to their families and society at large. A potential therapeutic strategy emerging from the study of trisomic mouse models of DS is to supplement the maternal diet with additional choline during pregnancy and lactation. Studies demonstrate that maternal choline supplementation (MCS) markedly improves spatial cognition and attentional function, as well as normalizes adult hippocampal neurogenesis and offers protection to basal forebrain cholinergic neurons (BFCNs) in the Ts65Dn mouse model of DS. These effects on neurogenesis and BFCNs correlate significantly with spatial cognition, suggesting functional relationships. In this review, we highlight some of these provocative findings, which suggest that supplementing the maternal diet with additional choline may serve as an effective and safe prenatal strategy for improving cognitive, affective, and neural functioning in DS. In light of growing evidence that all pregnancies would benefit from increased maternal choline intake, this type of recommendation could be given to all pregnant women, thereby providing a very early intervention for individuals with DS, and include babies born to mothers unaware that they are carrying a fetus with DS.

Synaptic Vesicle Recycling Is Unaffected in the Ts65Dn Mouse Model of Down Syndrome

Down syndrome (DS) is the most common genetic cause of intellectual disability, and arises from trisomy of human chromosome 21. Accumulating evidence from studies of both DS patient tissue and mouse models has suggested that synaptic dysfunction is a key factor in the disorder. The presence of several genes within the DS trisomy that are either directly or indirectly linked to synaptic vesicle (SV) endocytosis suggested that presynaptic dysfunction could underlie some of these synaptic defects. Therefore we determined whether SV recycling was altered in neurons from the Ts65Dn mouse, the best characterised model of DS to date. We found that SV exocytosis, the size of the SV recycling pool, clathrin-mediated endocytosis, activity-dependent bulk endocytosis and SV generation from bulk endosomes were all unaffected by the presence of the Ts65Dn trisomy. These results were obtained using battery of complementary assays employing genetically-encoded fluorescent reporters of SV cargo trafficking, and fluorescent and morphological assays of fluid-phase uptake in primary neuronal culture. The absence of presynaptic dysfunction in central nerve terminals of the Ts65Dn mouse suggests that future research should focus on the established alterations in excitatory / inhibitory balance as a potential route for future pharmacotherapy.

The legacy of the benzodiazepine receptor: from flumazenil to enhancing cognition in Down syndrome and social interaction in autism

The study of the psychopharmacology of benzodiazepines continues to provide new insights into diverse brain functions related to vigilance, anxiety, mood, epileptiform activity, schizophrenia, cognitive performance, and autism-related social behavior. In this endeavor, the discovery of the benzodiazepine receptor was a key event, as it supplied the primary benzodiazepine drug-target site, provided the molecular link to the allosteric modulation of GABAA receptors and, following the recognition of GABAA receptor subtypes, furnished the platform for future, more selective drug actions. This review has two parts. In a retrospective first part, it acknowledges the contributions to the field made by my collaborators over the years, initially at Hoffmann-La Roche in Basle and later, in academia, at the University and the ETH of Zurich. In the second part, the new frontier of GABA pharmacology, targeting GABAA receptor subtypes, is reviewed with special focus on nonsedative anxiolytics, antidepressants, analgesics, as well as enhancers of cognition in Down syndrome and attenuators of symptoms of autism spectrum disorders. It is encouraging that a clinical trial has been initiated with a partial inverse agonist acting on α5 GABAA receptors in an attempt to alleviate the cognitive deficits in Down syndrome.

Nest building is impaired in the Ts65Dn mouse model of Down syndrome and rescued by blocking 5HT2a receptors

Down syndrome (DS) has an incidence of about 1/700 births, and is therefore the most common cause of cognitive and behavioral impairments in children. Recent studies on mouse models of DS indicate that a number of pharmacotherapies could be beneficial for restoring cognitive abilities in individuals with DS. Attention deficits that are present in DS account in part for learning and memory deficiencies yet have been scarcely studied in corresponding models. Investigations of this relevant group of behaviors is more difficult in mouse models because of the difficulty in homologizing mouse and human behaviors and because standard laboratory environments do not always elicit behaviors of interest. Here we characterize nest building as a goal-directed behavior that is seriously impaired in young Ts65Dn mice, a genetic model of DS. We believe this impairment may reflect in part attention deficits, and we investigate the physiological, genetic, and pharmacological factors influencing its expression. Nesting behavior in young Ts65Dn mice was severely impaired when the animals were placed in a novel environment. But this context-dependent impairment was transient and reversible. The genetic determinants of this deficiency are restricted to a ∼100 gene segment on the murine chromosome 16. Nest building behavior is a highly integrated phenotypic trait that relies in part on limbic circuitry and on the frontal cortex in relation to cognitive and attention processes. We show that both serotonin content and 5HT2a receptors are increased in the frontal cortex of Ts65Dn mice and that pharmacological blockage of 5HT2a receptors in Ts65Dn mice rescues their context dependent nest building impairment. We propose that the nest-building trait could represent a marker of attention related deficits in DS models and could be of value in designing pharmacotherapies for this specific aspect of DS. 5HT2a modulation may improve goal-directed behavior in DS.

Hedgehog agonist therapy corrects structural and cognitive deficits in a Down syndrome mouse model

Down syndrome (DS) is among the most frequent genetic causes of intellectual disability, and ameliorating this deficit is a major goal in support of people with trisomy 21. The Ts65Dn mouse recapitulates some major brain structural and behavioral phenotypes of DS, including reduced size and cellularity of the cerebellum and learning deficits associated with the hippocampus. We show that a single treatment of newborn mice with the Sonic hedgehog pathway agonist SAG 1.1 (SAG) results in normal cerebellar morphology in adults. Further, SAG treatment at birth rescued phenotypes associated with hippocampal deficits that occur in untreated adult Ts65Dn mice. This treatment resulted in behavioral improvements and normalized performance in the Morris water maze task for learning and memory. SAG treatment also produced physiological effects and partially rescued both N-methyl-d-aspartate (NMDA) receptor-dependent synaptic plasticity and NMDA/AMPA receptor ratio, physiological measures associated with memory. These outcomes confirm an important role for the hedgehog pathway in cerebellar development and raise the possibility for its direct influence in hippocampal function. The positive results from this approach suggest a possible direction for therapeutic intervention to improve cognitive function for this population.

A 'cure' for Down syndrome: what do parents want?

Recent advancements in molecular genetics raise the possibility that therapeutics or a 'cure' for Down syndrome (DS) may become available. However, there are no data regarding how parents of children with DS perceive the possibility of mitigating specific manifestations such as the intellectual disability (ID) associated with DS, or curing the condition entirely. To explore these issues, we distributed a questionnaire to members of the Lower Mainland Down Syndrome Society in British Columbia, Canada. Questionnaires were completed by 101 parents (response rate=41%). A majority (61%) viewed the possibility of reversing ID in DS positively, but only 41% said that they would 'cure' their child of DS if it were possible. Twenty-seven percent of respondents said they would not 'cure' their child, and 32% were unsure if they would 'cure' their child. The most commonly cited motivation for opting for a 'cure' was to increase their child's independence. However, parental attitudes' towards a 'cure' for DS were complex, affected by ethical issues, perceived societal values, and pragmatic factors such as the age of the individual and long-term care-giving burden. These findings could be used by healthcare professionals supporting families who include a member with DS and to direct future research.

Development of DANDYs, new 3,5-diaryl-7-azaindoles demonstrating potent DYRK1A kinase inhibitory activity

A series of 3,5-diaryl-1H-pyrrolo[2,3-b]pyridines were synthesized and evaluated for inhibition of DYRKIA kinase in vitro. Derivatives having hydroxy groups on the aryl moieties (2c, 2j-l) demonstrated high inhibitory potencies with Kis in the low nanomolar range. Their methoxy analogues were up to 100 times less active. Docking studies at the ATP binding site suggested that these compounds bind tightly to this site via a network of multiple H-bonds with the peptide backbone. None of the active compounds were cytotoxic to KB cells at 10(-6) M. Kinase profiling revealed that compound 2j showed 2-fold selectivity for DYRK1A with respect to DYRK2 and DYRK3.

Aging and intellectual disability: insights from mouse models of Down syndrome

Down syndrome (DS) is one of many causes of intellectual disability (ID), others including but not limited to, fetal alcohol syndrome, Fragile X syndrome, Rett syndrome, Williams syndrome, hypoxia, and infection. Down syndrome is characterized by a number of neurobiological problems resulting in learning and memory deficits and early onset Alzheimer's disease. The cognitive impairment in people with DS is virtually universal but varies considerably with respect to expressivity and severity. Significant advances in medical treatment and social inclusion have increased longevity in people with DS resulting in an increased aging population, thus highlighting the significance of early onset of dementia and the importance of identifying pharmacotherapies to treat DS-associated health complications in adults. Given its prevalence and established mouse models, this review will focus on ID in the DS population; specifically, the superimposed effect of aging on the complications already manifest in DS adults and the cognitive insights gained from studies on mouse models of DS.