GABAergic hyperinnervation of dentate granule cells in the Ts65Dn mouse model of down syndrome: Exploring the role of App

It has been suggested that increased GABAergic innervation in the hippocampus plays a significant role in cognitive dysfunction in Down syndrome (DS). Bolstering this notion, are studies linking hyper-innervation of the dentate gyrus (DG) by GABAergic terminals to failure in LTP induction in the Ts65Dn mouse model of DS. Here, we used extensive morphometrical methods to assess the status of GABAergic interneurons in the DG of young and old Ts65Dn mice and their 2N controls. We detected an age-dependent increase in GABAergic innervation of dentate granule cells (DGCs) in Ts65Dn mice. The primary source of GABAergic terminals to DGCs somata is basket cells (BCs). For this reason, we assessed the status of these cells and found a significant increase in the number of BCs in Ts65Dn mice compared with controls. Then we aimed to identify the gene/s whose overexpression could be linked to increased number of BCs in Ts65Dn and found that deleting the third copy of App gene in Ts65Dn mice led to normalization of the number of BCs in these mice. Our data suggest that App overexpression plays a major role in the pathophysiology of GABAergic hyperinnervation of the DG in Ts65Dn mice. © 2016 Wiley Periodicals, Inc.

Physical exercise induces structural alterations in the hippocampal astrocytes: exploring the role of BDNF-TrkB signaling

While it has been known that physical activity can improve cognitive function and protect against neurodegeneration, the underlying mechanisms for these protective effects are yet to be fully elucidated. There is a large body of evidence indicating that physical exercise improves neurogenesis and maintenance of neurons. Yet, its possible effects on glial cells remain poorly understood. Here, we tested whether physical exercise in mice alters the expression of trophic factor-related genes and the status of astrocytes in the dentate gyrus of the hippocampus. In addition to a significant increase in Bdnf mRNA and protein levels, we found that 4 weeks of treadmill and running wheel exercise in mice, led to (1) a significant increase in synaptic load in the dentate gyrus, (2) alterations in astrocytic morphology, and (3) orientation of astrocytic projections towards dentate granule cells. Importantly, these changes were possibly linked to increased TrkB receptor levels in astrocytes. Our study suggests that astrocytes actively respond and could indeed mediate the positive effects of physical exercise on the central nervous system and potentially counter degenerative processes during aging and neurodegenerative disorders.

Noradrenergic System in Down Syndrome and Alzheimer's Disease A Target for Therapy

Locus coeruleus (LC) neurons in the brainstem send extensive noradrenergic (NE)-ergic terminals to the majority of brain regions, particularly those involved in cognitive function. Both Alzheimer's disease (AD) and Down syndrome (DS) are characterized by similar pathology including significant LC degeneration and dysfunction of the NE-ergic system. Extensive loss of NE-ergic terminals has been linked to alterations in brain regions vital for cognition, mood, and executive function. While the mechanisms by which NE-ergic abnormalities contribute to cognitive dysfunction are not fully understood, emergent evidence suggests that rescue of NE-ergic system can attenuate neuropathology and cognitive decline in both AD and DS. Therapeutic strategies to enhance NE neurotransmission have undergone limited testing. Among those deployed to date are NE reuptake inhibitors, presynaptic α-adrenergic receptor antagonists, NE prodrugs, and β-adrenergic agonists. Here we examine alterations in the NE-ergic system in AD and DS and suggest that NE-ergic system rescue is a plausible treatment strategy for targeting cognitive decline in both disorders.

Increased incidence of intermittent hypoxemia in the Ts65Dn mouse model of Down syndrome

In addition to nervous system, cardiovascular and respiratory systems are primarily affected in Down syndrome (DS). The Ts65Dn mouse model is widely used to recapitulate cognitive dysfunction in DS. While these mice consistently show failure in learning and memory along with functional and structural abnormalities in the hippocampal region, the underlying mechanisms behind cognitive dysfunction remain to be fully elucidated. Convergent evidence implicates chronic episodes of hypoxemia in cognitive dysfunction in people with DS. Using an infra-red detection system to assess oxygen saturation in free-moving mice, we assessed arterial blood oxygenation in both adolescent and adult Ts65Dn mice and found a significant increase in the incidence of hypoxemia in both groups. Notably, the severity of hypoxemia increased during the dark cycle, suggesting a link between hypoxemia and increased motor activity. Postmortem analysis showed significant increase in the expression of mitochondrial Cox4i2, the terminal enzyme of the mitochondrial respiratory chain and oxygen response element. Altogether these data suggest early and chronic occurrence of hypoxemia in the Ts65Dn mouse model of DS, which can contribute to cognitive dysfunction in these mice.