Exercised blood plasma promotes hippocampal neurogenesis in the Alzheimer's disease rat brain

BACKGROUND

Exercise training promotes brain plasticity and is associated with protection against cognitive impairment and Alzheimer's disease (AD). These beneficial effects may be partly mediated by blood-borne factors. Here we used an in vitro model of AD to investigate effects of blood plasma from exercise-trained donors on neuronal viability, and an in vivo rat model of AD to test whether such plasma impacts cognitive function, amyloid pathology, and neurogenesis.

METHODS

Mouse hippocampal neuronal cells were exposed to AD-like stress using amyloid-β and treated with plasma collected from human male donors 3 h after a single bout of high-intensity exercise. For in vivo studies, blood was collected from exercise-trained young male Wistar rats (high-intensity intervals 5 days/week for 6 weeks). Transgenic AD rats (McGill-R-Thy1-APP) were injected 5 times/fortnight for 6 weeks at 2 months or 5 months of age with either (a) plasma from the exercise-trained rats, (b) plasma from sedentary rats, or (c) saline. Cognitive function, amyloid plaque pathology, and neurogenesis were assessed. The plasma used for the treatment was analyzed for 23 cytokines.

RESULTS

Plasma from exercised donors enhanced cell viability by 44.1% (p = 0.032) and reduced atrophy by 50.0% (p < 0.001) in amyloid-β-treated cells. In vivo exercised plasma treatment did not alter cognitive function or amyloid plaque pathology but did increase hippocampal neurogenesis by ∼3 fold, regardless of pathological stage, when compared to saline-treated rats. Concentrations of 7 cytokines were significantly reduced in exercised plasma compared to sedentary plasma.

CONCLUSION

Our proof-of-concept study demonstrates that plasma from exercise-trained donors can protect neuronal cells in culture and promote adult hippocampal neurogenesis in the AD rat brain. This effect may be partly due to reduced pro-inflammatory signaling molecules in exercised plasma.

Dissection and culturing of adult lateral entorhinal cortex layer II neurons from APP/PS1 Alzheimer model mice

BACKGROUND

Primary neuronal cultures enable cell-biological studies of Alzheimer's disease (AD), albeit typically non-neuron-specific. The first cortical neurons affected in AD reside in layer II of the lateralmost part of the entorhinal cortex, and they undergo early accumulation of intracellular amyloid-β, form subsequent tau pathology, and start degenerating pre-symptomatically. These vulnerable entorhinal neurons uniquely express the glycoprotein reelin and provide selective inputs to the hippocampal memory system. Gaining a more direct access to study these neurons is therefore highly relevant.

NEW METHOD

We demonstrate a methodological approach for dissection and long-term culturing of adult lateral entorhinal layer II-neurons from AD-model mice.

RESULTS

We maintain adult dissected lateralmost entorhinal layer II-neurons beyond two months in culture. We show that they express neuronal markers, and that they are electrophysiologically active by 15 days in vitro and continuing beyond 2 months.

COMPARISON WITH EXISTING METHODS

Primary neurons are typically harvested from embryonic or early postnatal brains because such neurons are easier to culture compared to adult neurons. Methods to culture adult primary neurons have been reported, however, to our knowledge, culturing of adult entorhinal neuron-type specific primary neurons from AD-model animals have not been reported.

CONCLUSIONS

Our methodological approach offers a window to study initial pathological changes in the AD disease-cascade. This includes the study of proteinopathy, single-neuron changes, and network-level dysfunction.

Intracellular Amyloid-β in the Normal Rat Brain and Human Subjects and Its relevance for Alzheimer's Disease

BACKGROUND

Amyloid-β (Aβ) is a normal product of neuronal activity, including that of the aggregation-prone Aβ42 variant that is thought to cause Alzheimer's disease (AD). Much knowledge about AD comes from studies of transgenic rodents expressing mutated human amyloid-β protein precursor (AβPP) to increase Aβ production or the Aβ42/40 ratio. Yet, little is known about the normal expression of Aβ42 in rodent brains.

OBJECTIVE

To characterize the brain-wide expression of Aβ42 throughout the life span of outbred Wistar rats, and to relate these findings to brains of human subjects without neurological disease.

METHODS

Aβ42 immunolabeling of 12 Wistar rat brains (3-18 months of age) and brain sections from six human subjects aged 20-88 years.

RESULTS

In healthy Wistar rats, we find intracellular Aβ42 (iAβ42) in neurons throughout the brain at all ages, but levels vary greatly between brain regions. The highest levels are in neurons of entorhinal cortex layer II, alongside hippocampal neurons at the CA1/subiculum border. Concerning entorhinal cortex layer II, we find similarly high levels of iAβ42 in the human subjects.

CONCLUSION

Expression of iAβ42 in healthy Wistar rats predominates in the same structures where iAβ accumulates and Aβ plaques initially form in the much used, Wistar based McGill-R-Thy1-APP rat model for AD. The difference between wild-type Wistar rats and these AD model rats, with respect to Aβ42, is therefore quantitative rather that qualitative. This, taken together with our human results, indicate that the McGill rat model in fact models the underlying wild-type neuronal population-specific vulnerability to Aβ42 accumulation.

Lowering levels of reelin in entorhinal cortex layer II-neurons results in lowered levels of intracellular amyloid-β

Projection neurons in the anteriolateral part of entorhinal cortex layer II are the predominant cortical site for hyper-phosphorylation of tau and formation of neurofibrillary tangles in prodromal Alzheimer's disease. A majority of layer II projection neurons in anteriolateral entorhinal cortex are unique among cortical excitatory neurons by expressing the protein reelin. In prodromal Alzheimer's disease, these reelin-expressing neurons are prone to accumulate intracellular amyloid-β, which is mimicked in a rat model that replicates the spatio-temporal cascade of the disease. Two important findings in relation to this are that reelin-signalling downregulates tau phosphorylation, and that oligomeric amyloid-β interferes with reelin-signalling. Taking advantage of this rat model, we used proximity ligation assay to assess whether reelin and intracellular amyloid-β directly interact during early, pre-plaque stages in anteriolateral entorhinal cortex layer II reelin-expressing neurons. We next made a viral vector delivering micro-RNA against reelin, along with a control vector, and infected reelin-expressing anteriolateral entorhinal cortex layer II-neurons to test whether reelin levels affect levels of intracellular amyloid-β and/or amyloid precursor protein. We analysed 25.548 neurons from 24 animals, which results in three important findings. First, in reelin-expressing anteriolateral entorhinal cortex layer II-neurons, reelin and intracellular amyloid-β engage in a direct protein-protein interaction. Second, injecting micro-RNA against reelin lowers reelin levels in these neurons, amounting to an effect size of 1.3-4.5 (Bayesian estimation of Cohen's d effect size, 95% credible interval). This causes a concomitant reduction of intracellular amyloid-β ranging across three levels of aggregation, including a reduction of Aβ42 monomers/dimers amounting to an effect size of 0.5-3.1, a reduction of Aβ prefibrils amounting to an effect size of 1.1-3.5 and a reduction of protofibrils amounting to an effect size of 0.05-2.1. Analysing these data using Bayesian estimation of mutual information furthermore reveals that levels of amyloid-β are dependent on levels of reelin. Third, the reduction of intracellular amyloid-β occurs without any substantial associated changes in levels of amyloid precursor protein. We conclude that reelin and amyloid-β directly interact at the intracellular level in the uniquely reelin-expressing projection neurons in anteriolateral entorhinal cortex layer II, where levels of amyloid-β are dependent on levels of reelin. Since amyloid-β is known to impair reelin-signalling causing upregulated phosphorylation of tau, our findings are likely relevant to the vulnerability for neurofibrillary tangle-formation of this entorhinal neuronal population.

Temporal changes in personal activity intelligence and the risk of incident dementia and dementia related mortality: A prospective cohort study (HUNT)

BACKGROUND

The Personal Activity Intelligence (PAI) translates heart rate during daily activity into a weekly score. Obtaining a weekly PAI score ≥100 is associated with reduced risk of premature morbidity and mortality from cardiovascular diseases. Here, we determined whether changes in PAI score are associated with changes in risk of incident dementia and dementia-related mortality.

METHODS

We conducted a prospective cohort study of 29,826 healthy individuals. Using data from the Trøndelag Health-Study (HUNT), PAI was estimated 10 years apart (HUNT1 1984-86 and HUNT2 1995-97). Adjusted hazard-ratios (aHR) and 95%-confidence intervals (CI) for incidence of and death from dementia were related to changes in PAI using Cox regression analyses.

FINDINGS

During a median follow-up time of 24.5 years (interquartile range [IQR]: 24.1-25.0) for dementia incidence and 23.6 years (IQR: 20.8-24.2) for dementia-related mortality, there were 1998 incident cases and 1033 dementia-related deaths. Individuals who increased their PAI score over time or maintained a high PAI score at both assessments had reduced risk of dementia incidence and dementia-related mortality. Compared with persistently inactive individuals (0 weekly PAI) at both time points, the aHRs for those with a PAI score ≥100 at both occasions were 0.75 (95% CI: 0.58-0.97) for incident dementia, and 0.62 (95% CI: 0.43-0.91) for dementia-related mortality. Using PAI score <100 at both assessments as the reference cohort, those who increased from <100 at HUNT1 to ≥100 at HUNT2 had aHR of 0.83 (95% CI: 0.72-0.96) for incident dementia, and gained 2.8 (95% CI: 1.3-4.2, P<0.0001) dementia-free years. For dementia-related mortality, the corresponding aHR was 0.74 (95% CI: 0.59-0.92) and years of life gained were 2.4 (95% CI: 1.0-3.8, P=0.001).

INTERPRETATION

Maintaining a high weekly PAI score and increases in PAI scores over time were associated with a reduced risk of incident dementia and dementia-related mortality. Our findings extend the scientific evidence regarding the protective role of PA for dementia prevention, and suggest that PAI may be a valuable tool in guiding research-based PA recommendations.

FUNDING

The Norwegian Research Council, the Liaison Committee between the Central Norway Regional Health Authority and Norwegian University of Science and Technology (NTNU), Trondheim, Norway.

Can exercise training teach us how to treat Alzheimer's disease?

Alzheimer's disease (AD) is the most common cause of dementia and there is currently no cure. Novel approaches to treat AD and curb the rapidly increasing worldwide prevalence and costs of dementia are needed. Physical inactivity is a significant modifiable risk factor for AD, estimated to contribute to 12.7% of AD cases worldwide. Exercise interventions in humans and animals have shown beneficial effects of exercise on brain plasticity and cognitive functions. In animal studies, exercise also improved AD pathology. The mechanisms underlying these effects of exercise seem to be associated mainly with exercise performance or cardiorespiratory fitness. In addition, exercise-induced molecules of peripheral origin seem to play an important role. Since exercise affects the whole body, there likely is no single therapeutic target that could mimic all the benefits of exercise. However, systemic strategies may be a viable means to convey broad therapeutic effects in AD patients. Here, we review the potential of physical activity and exercise training in AD prevention and treatment, shining light on recently discovered underlying mechanisms and concluding with a view on future development of exercise-free treatment strategies for AD.

Structural connectivity-based segmentation of the human entorhinal cortex

The medial (MEC) and lateral entorhinal cortex (LEC), widely studied in rodents, are well defined and characterized. In humans, however, the exact locations of their homologues remain uncertain. Previous functional magnetic resonance imaging (fMRI) studies have subdivided the human EC into posteromedial (pmEC) and anterolateral (alEC) parts, but uncertainty remains about the choice of imaging modality and seed regions, in particular in light of a substantial revision of the classical model of EC connectivity based on novel insights from rodent anatomy. Here, we used structural, not functional imaging, namely diffusion tensor imaging (DTI) and probabilistic tractography to segment the human EC based on differential connectivity to other brain regions known to project selectively to MEC or LEC. We defined MEC as more strongly connected with presubiculum and retrosplenial cortex (RSC), and LEC as more strongly connected with distal CA1 and proximal subiculum (dCA1pSub) and lateral orbitofrontal cortex (OFC). Although our DTI segmentation had a larger medial-lateral component than in the previous fMRI studies, our results show that the human MEC and LEC homologues have a border oriented both towards the posterior-anterior and medial-lateral axes, supporting the differentiation between pmEC and alEC.

Neuronal spreading and plaque induction of intracellular Aβ and its disruption of Aβ homeostasis

The amyloid-beta peptide (Aβ) is thought to have prion-like properties promoting its spread throughout the brain in Alzheimer’s disease (AD). However, the cellular mechanism(s) of this spread remains unclear. Here, we show an important role of intracellular Aβ in its prion-like spread. We demonstrate that an intracellular source of Aβ can induce amyloid plaques in vivo via hippocampal injection. We show that hippocampal injection of mouse AD brain homogenate not only induces plaques, but also damages interneurons and affects intracellular Aβ levels in synaptically connected brain areas, paralleling cellular changes seen in AD. Furthermore, in a primary neuron AD model, exposure of picomolar amounts of brain-derived Aβ leads to an apparent redistribution of Aβ from soma to processes and dystrophic neurites. We also observe that such neuritic dystrophies associate with plaque formation in AD-transgenic mice. Finally, using cellular models, we propose a mechanism for how intracellular accumulation of Aβ disturbs homeostatic control of Aβ levels and can contribute to the up to 10,000-fold increase of Aβ in the AD brain. Our data indicate an essential role for intracellular prion-like Aβ and its synaptic spread in the pathogenesis of AD.

Re-emphasizing early Alzheimer's disease pathology starting in select entorhinal neurons, with a special focus on mitophagy

The entorhinal-hippocampal system contains distinct networks subserving declarative memory. This system is selectively vulnerable to changes of ageing and pathological processes. The entorhinal cortex (EC) is a pivotal component of this memory system since it serves as the interface between the neocortex and the hippocampus. EC is heavily affected by the proteinopathies of Alzheimer's disease (AD). These appear in a stereotypical spatiotemporal manner and include increased levels of intracellular amyloid-beta Aβ (iAβ), parenchymal deposition of Aβ plaques, and neurofibrillary tangles (NFTs) containing abnormally processed Tau. Increased levels of iAβ and the formation of NFTs are seen very early on in a population of neurons belonging to EC layer II (EC LII), and recent evidence leads us to believe that this population is made up of highly energy-demanding reelin-positive (RE+) projection neurons. Mitochondria are fundamental to the energy supply, metabolism, and plasticity of neurons. Evidence from AD postmortem brain tissues supports the notion that mitochondrial dysfunction is one of the initial pathological events in AD, and this is likely to take place in the vulnerable RE + EC LII neurons. Here we review and discuss these notions, anchored to the anatomy of AD, and formulate a hypothesis attempting to explain the vulnerability of RE + EC LII neurons to the formation of NFTs. We attempt to link impaired mitochondrial clearance to iAβ and signaling involving both apolipoprotein 4 and reelin, and argue for their relevance to the formation of NFTs specifically in RE + EC LII neurons during the prodromal stages of AD. We believe future studies on these interactions holds promise to advance our understanding of AD etiology and provide new ideas for drug development.

Neuronal chemo-architecture of the entorhinal cortex: A comparative review

The identification of neuronal markers, that is, molecules selectively present in subsets of neurons, contributes to our understanding of brain areas and the networks within them. Specifically, recognizing the distribution of different neuronal markers facilitates the identification of borders between functionally distinct brain areas. Detailed knowledge about the localization and physiological significance of neuronal markers may also provide clues to generate new hypotheses concerning aspects of normal and abnormal brain functioning. Here, we provide a comprehensive review on the distribution within the entorhinal cortex of neuronal markers and the morphology of the neurons they reveal. Emphasis is on the comparative distribution of several markers, with a focus on, but not restricted to rodent, monkey and human data, allowing to infer connectional features, across species, associated with these markers, based on what is revealed by mainly rodent data. The overall conclusion from this review is that there is an emerging pattern in the distribution of neuronal markers in the entorhinal cortex when aligning data along a comparable coordinate system in various species.

Are the neuroprotective effects of exercise training systemically mediated?

To date there is no cure available for dementia, and the field calls for novel therapeutic targets. A rapidly growing body of literature suggests that regular endurance training and high cardiorespiratory fitness attenuate cognitive impairment and reduce dementia risk. Such benefits have recently been linked to systemic neurotrophic factors induced by exercise. These circulating biomolecules may cross the blood-brain barrier and potentially protect against neurodegenerative disorders such as Alzheimer's disease. Identifying exercise-induced systemic neurotrophic factors with beneficial effects on the brain may lead to novel molecular targets for maintaining cognitive function and preventing neurodegeneration. Here we review the recent literature on potential systemic mediators of neuroprotection induced by exercise. We focus on the body of translational research in the field, integrating knowledge from the molecular level, animal models, clinical and epidemiological studies. Taken together, the current literature provides initial evidence that exercise-induced, blood-borne biomolecules, such as BDNF and FNDC5/irisin, may be powerful agents mediating the benefits of exercise on cognitive function and may form the basis for new therapeutic strategies to better prevent and treat dementia.

Reelin-immunoreactive neurons in entorhinal cortex layer II selectively express intracellular amyloid in early Alzheimer's disease

The onset of Alzheimer's disease (AD) is associated with subtle pathological changes including increased intracellular expression of amyloid-β (Aβ). A structure affected particularly early in the course of AD is the entorhinal cortex, where neuronal death in layer II is observed already at initial stages. Neurons in EC-layer II, particularly those that express the protein Reelin, give rise to projections to the hippocampal dentate gyrus and this projection shows severe loss of synaptic contacts during early-stage AD. Given this anatomical specificity, we sought to determine whether increased intracellular expression of Aβ is selectively associated with Reelin-immunoreactive neurons in layer II of the entorhinal cortex. Here we report that in a transgenic rat model, which mimics the onset and distribution of extracellular amyloid deposits seen in human AD subjects, expression of intracellular Aβ in entorhinal layer II selectively occurs in Reelin-immunoreactive neurons during the early, pre-plaque stage. This Reelin-Aβ association is also present in human subjects with AD-related pathological changes, even in early disease stages. These findings strongly indicate that Reelin-immunoreactive neurons in entorhinal layer II play a crucial role during the initial stages of AD, and may therefore lead to refined hypotheses concerning the origin of this devastating condition.

Stereological estimation of neuron number and plaque load in the hippocampal region of a transgenic rat model of Alzheimer's disease

The main hallmarks of Alzheimer's disease (AD) are senile plaques, neurofibrillary tangles and neuronal death. The McGill-R-Thy1-APP rat is one of the few transgenic rat models of AD that displays progressive amyloid pathology. This study aimed to further characterise this rat model, focusing on the pathological changes in the hippocampal formation and the parahippocampal region. These structures, that are important for episodic memory and spatial navigation, are affected in the early stages of the disease. This study used unbiased stereology to investigate possible neuronal loss in the CA1, subiculum and entorhinal cortex of 18-month-old homozygous McGill-R-Thy1-APP rats, and also quantified the plaque load in all the areas of the hippocampal formation and parahippocampal region from 9 to 18 months old. A significant reduction of neurons at 18 months was only seen in the subiculum. The first plaque pathology was seen at 9 months in the subiculum. Although the quantified plaque load was variable between animals, the pattern of spatiotemporal progression was similar for all animals. The spread of plaque pathology mainly affected anatomically connected regions. Overall, the plaque pathology observed in the transgenic rats was similar to the early phases of amyloid beta (Aβ)-deposition described in human patients. The findings here thus indicate that the McGill-R-Thy1-APP rat could be a good model of the Aβ pathology in AD, but less so with respect to neuron loss.