Cognitive Profiles on the Severe Impairment Battery Are Similar in Alzheimer Disease and Down Syndrome With Dementia

The Role of Oxidative Stress in Trisomy 21 Phenotype

Extensive research has been conducted to gain a deeper understanding of the deregulated metabolic pathways in the development of trisomy 21 (T21) or Down syndrome. This research has shed light on the hypothesis that oxidative stress plays a significant role in the manifestation of the T21 phenotype. Although in vivo studies have shown promising results in mitigating the detrimental effects of oxidative stress, there is currently a lack of introduced antioxidant treatment options targeting cognitive impairments associated with T21. To address this gap, a comprehensive literature review was conducted to provide an updated overview of the involvement of oxidative stress in T21. The review aimed to summarize the insights into the pathogenesis of the Down syndrome phenotype and present the findings of recent innovative research that focuses on improving cognitive function in T21 through various antioxidant interventions. By examining the existing literature, this research seeks to provide a holistic understanding of the role oxidative stress plays in the development of T21 and to explore novel approaches that target multiple aspects of antioxidant intervention to improve cognitive function in individuals with Down syndrome. The guides -base systematic review process (Hutton et al. 2015).

Basal forebrain cholinergic neurons are vulnerable in a mouse model of Down syndrome and their molecular fingerprint is rescued by maternal choline supplementation

Basal forebrain cholinergic neuron (BFCN) degeneration is a hallmark of Down syndrome (DS) and Alzheimer's disease (AD). Current therapeutics in these disorders have been unsuccessful in slowing disease progression, likely due to poorly understood complex pathological interactions and dysregulated pathways. The Ts65Dn trisomic mouse model recapitulates both cognitive and morphological deficits of DS and AD, including BFCN degeneration and has shown lifelong behavioral changes due to maternal choline supplementation (MCS). To test the impact of MCS on trisomic BFCNs, we performed laser capture microdissection to individually isolate choline acetyltransferase-immunopositive neurons in Ts65Dn and disomic littermates, in conjunction with MCS at the onset of BFCN degeneration. We utilized single population RNA sequencing (RNA-seq) to interrogate transcriptomic changes within medial septal nucleus (MSN) BFCNs. Leveraging multiple bioinformatic analysis programs on differentially expressed genes (DEGs) by genotype and diet, we identified key canonical pathways and altered physiological functions within Ts65Dn MSN BFCNs, which were attenuated by MCS in trisomic offspring, including the cholinergic, glutamatergic and GABAergic pathways. We linked differential gene expression bioinformatically to multiple neurological functions, including motor dysfunction/movement disorder, early onset neurological disease, ataxia and cognitive impairment via Ingenuity Pathway Analysis. DEGs within these identified pathways may underlie aberrant behavior in the DS mice, with MCS attenuating the underlying gene expression changes. We propose MCS ameliorates aberrant BFCN gene expression within the septohippocampal circuit of trisomic mice through normalization of principally the cholinergic, glutamatergic, and GABAergic signaling pathways, resulting in attenuation of underlying neurological disease functions.

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.

Adiponectin Modulation by Genotype and Maternal Choline Supplementation in a Mouse Model of Down Syndrome and Alzheimer's Disease

Down syndrome (DS) is a genetic disorder caused by the triplication of human chromosome 21, which results in neurological and physiological pathologies. These deficits increase during aging and are exacerbated by cognitive decline and increase of Alzheimer's disease (AD) neuropathology. A nontoxic, noninvasive treatment, maternal choline supplementation (MCS) attenuates cognitive decline in mouse models of DS and AD. To evaluate potential underlying mechanisms, laser capture microdissection of individual neuronal populations of MCS offspring was performed, followed by RNA sequencing and bioinformatic inquiry. Results at ~6 months of age (MO) revealed DS mice (the well-established Ts65Dn model) have significant dysregulation of select genes within the Type 2 Diabetes Mellitus (T2DM) signaling pathway relative to normal disomic (2N) littermates. Accordingly, we interrogated key T2DM protein hormones by ELISA assay in addition to gene and encoded protein levels in the brain. We found dysregulation of adiponectin (APN) protein levels in the frontal cortex of ~6 MO trisomic mice, which was attenuated by MCS. APN receptors also displayed expression level changes in response to MCS. APN is a potential biomarker for AD pathology and may be relevant in DS. We posit that changes in APN signaling may be an early marker of cognitive decline and neurodegeneration.

Increased excitatory to inhibitory synaptic ratio in parietal cortex samples from individuals with Alzheimer's disease

Synaptic disturbances in excitatory to inhibitory (E/I) balance in forebrain circuits are thought to contribute to the progression of Alzheimer's disease (AD) and dementia, although direct evidence for such imbalance in humans is lacking. We assessed anatomical and electrophysiological synaptic E/I ratios in post-mortem parietal cortex samples from middle-aged individuals with AD (early-onset) or Down syndrome (DS) by fluorescence deconvolution tomography and microtransplantation of synaptic membranes. Both approaches revealed significantly elevated E/I ratios for AD, but not DS, versus controls. Gene expression studies in an independent AD cohort also demonstrated elevated E/I ratios in individuals with AD as compared to controls. These findings provide evidence of a marked pro-excitatory perturbation of synaptic E/I balance in AD parietal cortex, a region within the default mode network that is overly active in the disorder, and support the hypothesis that E/I imbalances disrupt cognition-related shifts in cortical activity which contribute to the intellectual decline in AD.

TTC3-Mediated Protein Quality Control, A Potential Mechanism for Cognitive Impairment

The tetrapeptide repeat domain 3 (TTC3) gene falls within Down's syndrome (DS) critical region. Cognitive impairment is a common phenotype of DS and Alzheimer’s disease (AD), and overexpression of TTC3 can accelerate cognitive decline, but the specific mechanism is unknown. The TTC3-mediated protein quality control (PQC) mechanism, similar to the PQC system, is divided into three parts: it acts as a cochaperone to assist proteins in folding correctly; it acts as an E3 ubiquitin ligase (E3s) involved in protein degradation processes through the ubiquitin–proteasome system (UPS); and it may also eventually cause autophagy by affecting mitochondrial function. Thus, this article reviews the research progress on the structure, function, and metabolism of TTC3, including the recent research progress on TTC3 in DS and AD; the role of TTC3 in cognitive impairment through PQC in combination with the abovementioned attributes of TTC3; and the potential targets of TTC3 in the treatment of such diseases.

Longitudinal assessment of dementia measures in Down syndrome

INTRODUCTION

Early detection of dementia symptoms is critical in Down syndrome (DS) but complicated by clinical assessment barriers. The current study aimed to characterize cognitive and behavioral impairment using longitudinal trajectories comparing several measures of cognitive and behavioral functioning.

METHODS

Measures included global cognitive status (Severe Impairment Battery [SIB]), motor praxis (Brief Praxis Test [BPT]), and clinical dementia informant ratings (Dementia Questionnaire for People with Learning Disabilities [DLD]). One-year reliability was assessed using a two-way mixed effect, consistency, single measurement intraclass correlation among non-demented participants. Longitudinal assessment of SIB, BPT, and DLD was completed using linear mixed effect models.

RESULTS

One-year reliability (n = 52; 21 male) was moderate for DLD (0.69 to 0.75) and good for SIB (0.87) and BPT (0.80). Longitudinal analysis (n = 72) revealed significant age by diagnosis interactions for SIB (F(2, 115.02) = 6.06, P = .003), BPT (F(2, 85.59) = 4.56, P = .013), and DLD (F(2, 103.56) = 4.48, P = .014). SIB progression (PR) had a faster decline in performance versus no-dementia (ND) (t(159) = -2.87; P = .013). Dementia had a faster decline in BPT performance versus ND (t(112) = -2.46; P = .041). PR showed quickly progressing scores compared to ND (t(128) = -2.86; P = .014).

DISCUSSION

Current measures demonstrated moderate to good reliability. Longitudinal analysis revealed that SIB, BPT, and DLD changed with age depending on diagnostic progression; no change rates were dependent on baseline cognition, indicating usefulness across a variety of severity levels in DS.

Synaptic actin stabilization protein loss in Down syndrome and Alzheimer disease

Reduced spine densities and age-dependent accumulation of amyloid β and tau pathology are shared features of Down syndrome (DS) and Alzheimer's disease (AD). Both spine morphology and the synaptic plasticity that supports learning depend upon the actin cytoskeleton, suggesting that disturbances in actin regulatory signaling might underlie spine defects in both disorders. The present study evaluated the synaptic levels of two proteins that promote filamentous actin stabilization, the Rho GTPase effector p21-activated kinase 3 (PAK3) and Arp2, in DS vs. AD. Fluorescent deconvolution tomography was used to determine postsynaptic PAK3 and Arp2 levels for large numbers of excitatory synapses in the parietal cortex of individuals with DS plus AD pathology (DS + AD) or AD alone relative to age-matched controls. Though numbers of excitatory synapses were not different between groups, synaptic PAK3 levels were greatly reduced in DS + AD and AD individuals vs. controls. Synaptic Arp2 levels also were reduced in both disorders, but to a greater degree in AD. Western blotting detected reduced Arp2 levels in the AD group, but there was no correlation with phosphorylated tau levels suggesting that the Arp2 loss does not contribute to mechanisms that drive tau pathology progression. Overall, the results demonstrate marked synaptic disturbances in two actin regulatory proteins in adult DS and AD brains, with greater effects in individuals with AD alone. As both PAK and the Arp2/3 complex play roles in the actin stabilization that supports synaptic plasticity, reductions in these proteins at synapses may be early events in spine dysfunction that contribute to cognitive impairment in these disorders.

Dementia in Down syndrome: unique insights for Alzheimer disease research

Virtually all adults with Down syndrome (DS) show the neuropathological changes of Alzheimer disease (AD) by the age of 40 years. This association is partially due to overexpression of amyloid precursor protein, encoded by APP, as a result of the location of this gene on chromosome 21. Amyloid-β accumulates in the brain across the lifespan of people with DS, which provides a unique opportunity to understand the temporal progression of AD and the epigenetic factors that contribute to the age of dementia onset. This age dependency in the development of AD in DS can inform research into the presentation of AD in the general population, in whom a longitudinal perspective of the disease is not often available. Comparison of the risk profiles, biomarker profiles and genetic profiles of adults with DS with those of individuals with AD in the general population can help to determine common and distinct pathways as well as mechanisms underlying increased risk of dementia. This Review evaluates the similarities and differences between the pathological cascades and genetics underpinning DS and AD with the aim of providing a platform for common exploration of these disorders.

Cerebral vasoreactivity and intima-media thickness in Down syndrome: A case-control study

Subjects with Down Syndrome (DS) have high prevalence of cerebral vascular amyloidosis, cognitive decline and dementia. In Alzheimer Disease, impaired vasoreactivity has been reported as the results of vascular amyloid deposition. Aim of our study was to verify presence of impaired cerebral vasoreactivity and to study carotid intima media-thickness (IMT) by carotid and transcranial ultrasound. We studied 25 DS and compared them with 25 age- and sex-matched normal controls. Vasomotor reactivity was evaluated by means of breath-holding index (BHI) test. There was no difference in IMT, both considering the two side separately (left: 0.70±0.10 vs 0.69±0.12mm, p=0.6) (right: 0.67±0.13 vs 0.68±0.10mm, p=0.5), and considering the sum of both sides (1.38±0.22 vs 1.38±0.23mm, p=1). There was a significant difference in peak systolic velocities (PSV) (139.75±27.67 vs. 123.89±25.73cm/s, p=0.04) and in pulsatility index (PI) (0.95±0.14 vs. 0.86±0.12, p=0.02). BHI was significantly lower in DS than in controls (1.15±0.38 vs 1.88±0.72, p<0.001). In conclusion, subjects with DS have increased PSV and PI, and show a reduction of BHI, expression of impaired vasomotor reserve, possibly due to micro-vascular impairment. Larger study with longitudinal design is needed to verify our data.

Clinical aspects and biomarkers of Alzheimer's disease in Down syndrome

Alzheimer's disease (AD) may affect in excess of 90% of individuals with Down syndrome (DS) after age 60, due to duplication of the APP gene in trisomy of chromosome 21, with neuropathology that is comparable to Sporadic AD and Familial AD (FAD). Previous literature suggested some unique features in clinical presentation of dementia in DS (DSd), which might be due to diagnostic difficulties, or represent a real difference compared to SAD or FAD. We review current knowledge on clinical diagnosis and presentation of dementia in DS in comparison with FAD due to APP mutations and APP duplication. We suggest that the clinical presentation in DS (prominent memory decline and behavioral symptoms, and early development of myoclonus and seizures) are similar to the clinical features associated with APP mutations that is known to have an increased Aβ42/ Aβ40 ratio, and highlight the relative lack of vascular complications associated with cerebral amyloid angiopathy in DS in comparison with those rare individuals with FAD due to duplication APP. We consider the biomarker evidence associated with DS and DSd with reference to Aβ peptide levels and oxidative stress, and suggest future directions for research to explore the potential mechanisms associated with the clinical presentation of DSd.

Cognitive decline and dementia in Down syndrome

PURPOSE OF REVIEW

Alzheimer's disease is most likely universal in older individuals with Down syndrome, due to having three copies of the amyloid precursor protein gene, resulting in amyloid-beta plaque deposition. Down syndrome is an important population in which to consider clinical trials of treatments to prevent or delay the development of dementia. However, assessment of subtler cognitive changes is challenging due to the presence of intellectual disability.

RECENT FINDINGS

Recent research confirmed that older adults with Down syndrome often present with cognitive decline: more than 80% may experience dementia by age 65 years. Efforts have been made to improve and validate neuropsychological assessment and to describe the relationship with comorbidities such as epilepsy and haemorrhagic stroke. There have also been advances in biomarkers such as neuroimaging using amyloid PET.

SUMMARY

Clinical trials of treatments, particularly in the presymptomatic phase of Alzheimer's disease, are important to consider in individuals with Down syndrome given their high dementia burden, and may also serve as proof of concept for other forms of Alzheimer's disease. However, further work is required to improve outcome measures and better understand the biomarkers of progression of disorder and their relationship with symptom development during the presymptomatic period.