Prenatal and Postnatal Pharmacotherapy in Down Syndrome: The Search to Prevent or Ameliorate Neurodevelopmental and Neurodegenerative Disorders

Dendritic phenotype and proliferation potency in the hippocampal dentate gyrus of the Ts66Yah model of Down syndrome

The Ts65Dn mouse is the most widely used model of Down syndrome (DS), although, in addition to the triplication of 90 genes homologous to Human Chromosome 21 (Hsa21) genes, it bears the triplication of 46 extra genes. To clarify the latter's impact, the Ts66Yah model has been created from the Ts65Dn mouse by exploiting CRISPR/Cas9 technology for extra gene deletion. It has been found that, similar to the Ts65Dn model, the Ts66Yah model exhibits impairment in hippocampus-dependent learning and memory and age-related hippocampal deterioration, with no increased activity. We examine here the dendritic development of the hippocampal granule neurons and the proliferation potency of granule cell precursors in Ts66Yah mice because these phenotypes are impaired in Ts65Dn mice and individuals with DS starting from early life stages and are thought to underpin cognitive impairment. In Ts66Yah mice aged 15 days and those aged three months, we found no reduction in dendritic arborization, dendritic spine density, proliferation potency, or total number of granule cells, suggesting that other mechanisms may underpin the behavioral impairment found in the Ts66Yah model in adulthood. Thus, the Ts66Yah model is unsuitable to study these neurodevelopmental alterations, although it may be useful to study other DS-related phenotypes.

Intricacies of aging and Down syndrome

Down syndrome is the most frequently occurring genetic condition, with a substantial escalation in risk associated with advanced maternal age. The syndrome is characterized by a diverse range of phenotypes, affecting to some extent all levels of organization, and its progeroid nature - early manifestation of aspects of the senile phenotype. Despite extensive investigations, many aspects and mechanisms of the disease remain unexplored. The current review aims to provide an overview of the main causes and manifestations of Down syndrome, while also examining the phenomenon of accelerated aging and exploring potential therapeutic strategies.

Transcranial Direct Current Stimulation in neurogenetic syndromes: new treatment perspectives for Down syndrome?

This perspective review aims to explore the potential neurobiological mechanisms involved in the application of transcranial Direct Current Stimulation (tDCS) for Down syndrome (DS), the leading cause of genetically-based intellectual disability. The neural mechanisms underlying tDCS interventions in genetic disorders, typically characterized by cognitive deficits, are grounded in the concept of brain plasticity. We initially present the neurobiological and functional effects elicited by tDCS applications in enhancing neuroplasticity and in regulating the excitatory/inhibitory balance, both associated with cognitive improvement in the general population. The review begins with evidence on tDCS applications in five neurogenetic disorders, including Rett, Prader-Willi, Phelan-McDermid, and Neurofibromatosis 1 syndromes, as well as DS. Available evidence supports tDCS as a potential intervention tool and underscores the importance of advancing neurobiological research into the mechanisms of tDCS action in these conditions. We then discuss the potential of tDCS as a promising non-invasive strategy to mitigate deficits in plasticity and promote fine-tuning of the excitatory/inhibitory balance in DS, exploring implications for cognitive treatment perspectives in this population.

Retromer Proteins Reduced in Down Syndrome and the Dp16 Model: Impact of APP Dose and Preclinical Studies of a γ-Secretase Modulator

OBJECTIVE

The retromer complex plays an essential role in intracellular endosomal sorting. Deficits in the retromer complex are linked to enhanced Aβ production. The levels of the components of the retromer complex are reported to be downregulated in Alzheimer disease (AD). Down syndrome (DS) shares neuropathological features with AD. Recent evidence points to dysregulation of the retromer complex in DS. The mechanisms underlying retromer deficits in DS and AD are poorly understood.

METHODS

We measured the levels of retromer components in the frontal cortex of cases of DS-AD (AD in DS) as well as DS; the frontal cortex of a person partially trisomic (PT-DS) for human chromosome 21 (HSA21), whose genome had only the normal 2 copies of the APP gene, was also examined. We also analyzed these proteins in the Dp16 mouse model of DS. To further explore the molecular mechanism for changes in the retromer complex, we treated Dp16 mice with a γ-secretase modulator (GSM; 776890), a treatment that reduces the levels of Aβ42 and Aβ40.

RESULTS

We found VPS26A, VPS26B, and VPS29, but not VPS35, were significantly reduced in both DS and DS-AD, but not in PT-DS. Downregulation of VPS26A, VPS26B, and VPS29 was recapitulated in the brains of old Dp16 mice (at 16 months of age) and required increased App gene dose. Significantly, GSM treatment completely prevented reductions of the retromer complex.

INTERPRETATION

Our studies point to increased APP gene dose as a compromising retromer function in DS and suggest a causal role for Aβ42 and Aβ40. ANN NEUROL 2023;94:245-258.

Reduced synaptic proteins and SNARE complexes in Down syndrome with Alzheimer's disease and the Dp16 mouse Down syndrome model: Impact of APP gene dose

INTRODUCTION

Synaptic failure, a hallmark of Alzheimer's disease (AD), is correlated with reduced levels of synaptic proteins. Though people with Down syndrome (DS) are at markedly increased risk for AD (AD-DS), few studies have addressed synapse dysfunction.

METHODS

Synaptic proteins were measured in the frontal cortex of DS, AD-DS, sporadic AD cases, and controls. The same proteins were examined in the Dp16 model of DS.

RESULTS

A common subset of synaptic proteins were reduced in AD and AD-DS, but not in DS or a case of partial trisomy 21 lacking triplication of APP gene. Pointing to compromised synaptic function, the reductions in AD and AD-DS were correlated with reduced SNARE complexes. In Dp16 mice reductions in syntaxin 1A, SNAP25 and the SNARE complex recapitulated findings in AD-DS; reductions were impacted by both age and increased App gene dose.

DISCUSSION

Synaptic phenotypes shared between AD-DS and AD point to shared pathogenetic mechanisms.

Fatty Acids: A Safe Tool for Improving Neurodevelopmental Alterations in Down Syndrome?

The triplication of chromosome 21 causes Down syndrome (DS), a genetic disorder that is characterized by intellectual disability (ID). The causes of ID start in utero, leading to impairments in neurogenesis, and continue into infancy, leading to impairments in dendritogenesis, spinogenesis, and connectivity. These defects are associated with alterations in mitochondrial and metabolic functions and precocious aging, leading to the early development of Alzheimer’s disease. Intense efforts are currently underway, taking advantage of DS mouse models to discover pharmacotherapies for the neurodevelopmental and cognitive deficits of DS. Many treatments that proved effective in mouse models may raise safety concerns over human use, especially at early life stages. Accumulating evidence shows that fatty acids, which are nutrients present in normal diets, exert numerous positive effects on the brain. Here, we review (i) the knowledge obtained from animal models regarding the effects of fatty acids on the brain, by focusing on alterations that are particularly prominent in DS, and (ii) the progress recently made in a DS mouse model, suggesting that fatty acids may indeed represent a useful treatment for DS. This scenario should prompt the scientific community to further explore the potential benefit of fatty acids for people with DS.

Blood circulating miR-28-5p and let-7d-5p associate with premature ageing in Down Syndrome

Persons with Down Syndrome (DS) undergo a premature ageing with early onset of age-related diseases. The main endpoint of this study was the identification of blood circulating microRNAs (c-miRs) signatures characterizing DS ageing process. A discovery phase based on array was performed in plasma samples obtained from 3 young (31±2 years-old) and 3 elderly DS persons (66±2 years-old). Then, a validation phase was carried out for relevant miRs by RT-qPCR in an enlarged cohort of 43 DS individuals (from 19 up to 68 years-old). A group of 30 non-trisomic subjects, as representative of physiological ageing, was compared. In particular miR-628-5p, miR-152-3p, miR-28-5p, and let-7d-5p showed a lower level in younger DS persons (age ≤ 50 years) respect to the age-matched controls. Among those, miR-28-5p and let-7d-5p were found significantly decreased in physiological ageing (control group with age threshold of 50 years), thus they emerged as possible biomarkers of premature ageing in DS. Moreover, measuring blood levels of beta amyloid peptides, Aβ-42 was assessed at the lowest levels in physiological ageing and correlated with miR-28-5p and let-7d-5p in DS, while Aβ-40 correlated with miR-628-5p in the same cohort. New perspectives in terms of biomarkers are discussed.