Effects of voluntary physical exercise on adult hippocampal neurogenesis and behavior of Ts65Dn mice, a model of Down syndrome

The Ts65Dn mouse model of Down syndrome shows reduced expression of the Bcl-X(L) antiapoptotic protein in the hippocampus not accompanied by changes in molecular or cellular markers of cell death

The Ts65Dn (TS) mouse, the most widely used model of Down syndrome (DS), has a partial trisomy of a segment of chromosome 16 that is homologous to the distal part of human chromosome 21. This mouse shares many phenotypic characteristics with people with DS including neuromorphological, neurochemical, and cognitive disturbances. Both TS and DS brains show earlier aging and neurodegeneration. Since fibroblast cultures from TS mice and human DS hippocampal regions show increased apoptotic cell death it has been suggested that alterations in cerebral apoptosis might be implicated in the cognitive deficits found in TS mice and in people with DS. In the present study we have evaluated brain expression levels of several proapoptotic and antiapoptotic proteins from the mitochondrial (Bcl-2, Bcl-X(L), Bax and Bad) and the extrinsic (Fas-R and Fas-L) apoptotic pathways as well as the final executioner caspase-3, in the cortex and hippocampus of TS mice. No significant alterations in the expression levels of the proapoptotic Bad and Bax or the antiapoptotic Bcl-2 proteins in the cortex or hippocampus were found in TS mice. However, TS mice showed downregulation of Bcl-X(L) in the hippocampus. In the extrinsic pathway we found unchanged levels of Fas-L in both structures and also in the expression levels of Fas-R in the hippocampus. Although Bcl-X(L) downregulation suggests that the hippocampus of TS mice is less protected against programmed cell death, we did not find any evidence for increased apoptosis in TS mice since neither TUNEL-positive cells nor active caspase-3 expression were found in cortex or hippocampus of TS or CO mice.

Exercise-induced cognitive plasticity, implications for mild cognitive impairment and Alzheimer's disease

Lifestyle factors such as intellectual stimulation, cognitive and social engagement, nutrition, and various types of exercise appear to reduce the risk for common age-associated disorders such as Alzheimer’s disease (AD) and vascular dementia. In fact, many studies have suggested that promoting physical activity can have a protective effect against cognitive deterioration later in life. Slowing or a deterioration of walking speed is associated with a poor performance in tests assessing psychomotor speed and verbal fluency in elderly individuals. Fitness training influences a wide range of cognitive processes, and the largest positive impact observed is for executive (a.k.a. frontal lobe) functions. Studies show that exercise improves additional cognitive functions such as tasks mediated by the hippocampus, and result in major changes in plasticity in the hippocampus. Interestingly, this exercise-induced plasticity is also pronounced in APOE ε4 carriers who express a risk factor for late-onset AD that may modulate the effect of treatments. Based on AD staging by Braak and Braak (1991) and Braak et al. (1993) we propose that the effects of exercise occur in two temporo-spatial continua of events. The “inward” continuum from isocortex (neocortex) to entorhinal cortex/hippocampus for amyloidosis and a reciprocal “outward” continuum for neurofibrillary alterations. The exercise-induced hypertrophy of the hippocampus at the core of these continua is evaluated in terms of potential for prevention to stave off neuronal degeneration. Exercise-induced production of growth factors such as the brain-derived neurotrophic factor (BDNF) has been shown to enhance neurogenesis and to play a key role in positive cognitive effects. Insulin-like growth factor (IGF-1) may mediate the exercise-induced response to exercise on BDNF, neurogenesis, and cognitive performance. It is also postulated to regulate brain amyloid β (Aβ) levels by increased clearance via the choroid plexus. Growth factors, specifically fibroblast growth factor and IGF-1 receptors and/or their downstream signaling pathways may interact with the Klotho gene which functions as an aging suppressor gene. Neurons may not be the only cells affected by exercise. Glia (astrocytes and microglia), neurovascular units and the Fourth Element may also be affected in a differential fashion by the AD process. Analyses of these factors, as suggested by the multi-dimensional matrix approach, are needed to improve our understanding of this complex multi-factorial process, which is increasingly relevant to conquering the escalating and intersecting world-wide epidemics of dementia, diabetes, and sarcopenia that threaten the global healthcare system. Physical activity and interventions aimed at enhancing and/or mimicking the effects of exercise are likely to play a significant role in mitigating these epidemics, together with the embryonic efforts to develop cognitive rehabilitation for neurodegenerative disorders.