Plasticity of hippocampal circuitry in Alzheimer's disease

Effects of repetitive paired associative stimulation on brain plasticity and working memory in Alzheimer's disease: a pilot randomized double-blind-controlled trial

DESIGN

Pilot randomized double-blind-controlled trial of repetitive paired associative stimulation (rPAS), a paradigm that combines transcranial magnetic stimulation (TMS) of the dorsolateral prefrontal cortex (DLPFC) with peripheral median nerve stimulation.

OBJECTIVES

To study the impact of rPAS on DLPFC plasticity and working memory performance in Alzheimer's disease (AD).

METHODS

Thirty-two patients with AD (females = 16), mean (SD) age = 76.4 (6.3) years were randomized 1:1 to receive a 2-week (5 days/week) course of active or control rPAS. DLPFC plasticity was assessed using single session PAS combined with electroencephalography (EEG) at baseline and on days 1, 7, and 14 post-rPAS. Working memory and theta-gamma coupling were assessed at the same time points using the N-back task and EEG.

RESULTS

There were no significant differences between the active and control rPAS groups on DLPFC plasticity or working memory performance after the rPAS intervention. There were significant main effects of time on DLPFC plasticity, working memory, and theta-gamma coupling, only for the active rPAS group. Further, on post hoc within-group analyses done to generate hypotheses for future research, as compared to baseline, only the rPAS group improved on post-rPAS day 1 on all three indices. Finally, there was a positive correlation between working memory performance and theta-gamma coupling.

CONCLUSIONS

This study did not show a beneficial effect of rPAS for DLPFC plasticity or working memory in AD. However, post hoc analyses showed promising results favoring rPAS and supporting further research on this topic. (Clinicaltrials.gov-NCT01847586).

The effect of hippocampal radiomic features and functional connectivity on the relationship between hippocampal volume and cognitive function in Alzheimer's disease

Hippocampal volume is associated with cognitive function in Alzheimer's disease (AD). Hippocampal radiomic features and resting-state functional connectivity (rs-FC) are promising biomarkers and correlate with AD pathology. However, few studies have been conducted on how hippocampal biomarkers affect the cognition-structure relationship. Therefore, we aimed to investigate the effects of hippocampal radiomic features and resting-state functional connectivity (rs-FC) on this relationship in AD. We enrolled 70 AD patients and 65 healthy controls (HCs). The FreeSurfer software was used to measure hippocampal volume. We selected hippocampal radiomic features to build a model to distinguish AD patients from HCs and used a seed-based approach to calculate the hippocampal rs-FC. Furthermore, we conducted mediation and moderation analyses to investigate the effect of hippocampal radiomic features and rs-FC on the relationship between hippocampal volume and cognition in AD. The results suggested that hippocampal radiomic features mediated the association between bilateral hippocampal volume and cognition in AD. Additionally, patients with AD showed weaker rs-FC between the bilateral hippocampus and right ventral posterior cingulate cortex and stronger rs-FC between the left hippocampus and left insula than HCs. The rs-FC between the hippocampus and insula moderated the relationship between hippocampal volume and cognition in AD, suggesting that this rs-FC could exacerbate or ameliorate the effects of hippocampal volume on cognition and may be essential in improving cognitive function in AD. Our findings may not only expand existing biological knowledge of the interrelationships among hippocampal biomarkers and cognition but also provide potential targets for treatment strategies for AD.

Lesion-induced sprouting promotes neurophysiological integration of septal and entorhinal inputs to granule cells in the dentate gyrus of rats

Axonal sprouting of dentate gyrus (DG) afferents after entorhinal cortex (EC) lesion is a model preparation to assess lesion-induced functional reorganization in a denervated target structure. Following a unilateral EC lesion, the surviving contralateral entorhinal projection, termed the crossed temporodentate pathway (CTD), and the heterotypic septal input to the DG, the septodentate pathway (SD), undergo extensive axonal sprouting. We explored whether EC lesion alters the capacity of the SD pathway to influence CTD-evoked granule cell excitability in the DG. We recorded extracellular field excitatory postsynaptic potentials (fEPSPs) after CTD stimulation alone and paired SD-CTD stimulation. Male rats were given unilateral EC lesions or sham operations; evoked fEPSPs in the DG were recorded at 4-, 15-, and 90-days post-entorhinal lesion to assess functional reorganization of the CTD and SD pathways. We found significantly increased fEPSP amplitudes in cases with unilateral lesions compared to sham-operates at 15- and 90-days post lesion. Within each time point, paired SD-CTD stimulation resulted in significantly depressed fEPSP amplitudes compared to amplitudes evoked after CTD stimulation alone and this effect was solely seen in cases with EC lesion. In cases where granule cell discharge was observed, SD stimulation increased discharge amplitude elicited by the CTD stimulation at 90-days postlesion. These findings demonstrate that synaptic remodeling following unilateral cortical lesion results in a synergistic interaction between two established hippocampal afferents that is not seen in uninjured brains. This work may be important for models of neurodegenerative disease and neural injury that target these structures and associated hippocampal circuitry.

All the Tau We Cannot See

Alzheimer's disease (AD) was described in 1906 as a dementing disease marked by the presence of two types of fibrillar aggregates in the brain: neurofibrillary tangles and senile plaques. The process of aggregation and formation of the aggregates has been a major focus of investigation ever since the discoveries that the tau protein is the predominant protein in tangles and amyloid β is the predominant protein in plaques. The idea that smaller, oligomeric species of amyloid may also be bioactive has now been clearly established. This review examines the possibility that soluble, nonfibrillar, bioactive forms of tau-the "tau we cannot see"-comprise a dominant driver of neurodegeneration in AD.

Excessive/Aberrant and Maladaptive Synaptic Plasticity: A Hypothesis for the Pathogenesis of Alzheimer’s Disease

The amyloid hypothesis for the pathogenesis of Alzheimer’s disease (AD) is widely accepted. Last year, the US Food and Drug Administration considered amyloid-β peptide (Aβ) as a surrogate biomarker and approved an anti-Aβ antibody, aducanumab, although its effectiveness in slowing the progression of AD is still uncertain. This approval has caused a great deal of controversy. Opinions are divided about whether there is enough evidence to definitely consider Aβ as a causative substance of AD. To develop this discussion constructively and to discover the most suitable therapeutic interventions in the end, an alternative persuasive hypothesis needs to emerge to better explain the facts. In this paper, I propose a hypothesis that excessive/aberrant and maladaptive synaptic plasticity is the pathophysiological basis for AD.

Synaptic plasticity in Alzheimer's disease and healthy aging

The strength and efficiency of synaptic connections are affected by the environment or the experience of the individual. This property, called synaptic plasticity, is directly related to memory and learning processes and has been modeled at the cellular level. These types of cellular memory and learning models include specific stimulation protocols that generate a long-term strengthening of the synapses, called long-term potentiation, or a weakening of the said long-term synapses, called long-term depression. Although, for decades, researchers have believed that the main cause of the cognitive deficit that characterizes Alzheimer's disease (AD) and aging was the loss of neurons, the hypothesis of an imbalance in the cellular and molecular mechanisms of synaptic plasticity underlying this deficit is currently widely accepted. An understanding of the molecular and cellular changes underlying the process of synaptic plasticity during the development of AD and aging will direct future studies to specific targets, resulting in the development of much more efficient and specific therapeutic strategies. In this review, we classify, discuss, and describe the main findings related to changes in the neurophysiological mechanisms of synaptic plasticity in excitatory synapses underlying AD and aging. In addition, we suggest possible mechanisms in which aging can become a high-risk factor for the development of AD and how its development could be prevented or slowed.

A novel tau-based rhesus monkey model of Alzheimer's pathogenesis

INTRODUCTION

Alzheimer's disease (AD) is a devastating condition with no effective treatments, with promising findings in rodents failing to translate into successful therapies for patients.

METHODS

Targeting the vulnerable entorhinal cortex (ERC), rhesus monkeys received two injections of an adeno-associated virus expressing a double tau mutation (AAV-P301L/S320F) in the left hemisphere, and control AAV-green fluorescent protein in the right ERC. Noninjected aged-matched monkeys served as additional controls.

RESULTS

Within 3 months we observed evidence of misfolded tau propagation, similar to what is hypothesized to occur in humans. Viral delivery of human 4R-tau also coaptates monkey 3R-tau via permissive templating. Tau spreading is accompanied by robust neuroinflammatory response driven by TREM2+ microglia, with biomarkers of inflammation and neuronal loss in the cerebrospinal fluid and plasma.

DISCUSSION

These results highlight the initial stages of tau seeding and propagation in a primate model, a more powerful translational approach for the development of new therapies for AD.

Single-cell-resolved measurement of enzyme activity at the tissue level using drop-on-demand microkits

Drop-on-demand microkits with a diameter of ∼20 μm are used to measure the activity of acetylcholinesterase (AChE) in a brain slice with single-cell resolution. The relative standard deviation from 25 cellular regions reached 73.3% exhibiting the difference of enzyme activity in the brain slice. Therefore, this approach utilizing the well-established kits provides an alternative single-cell-resolved strategy for the elucidation of enzymatic heterogeneity at the tissue level.

In vivo exploration of brain phosphorus 31 metabolism in patients with senile dementia of Alzheimer type

In vivo NMR 31p spectroscopy is a non invasive, non ionizing method of exploration of energy and phospholipid metabolism in the brain. This study consisted of comparing 31p spectra in five patients with Senile Dementia of Alzheimer Type (SDAT) with those of four controls of similar ages. Abnormal phosphonionocsters (PME) concentrations, either high or low, were found in the patients, but statistical analysis did not elicit any significant difference relative to controls.

Neuronal diversity and reciprocal connectivity between the vertebrate hippocampus and septum

A hallmark of the nervous system is the precision with which myriad cell types are integrated into functional networks that control complex behaviors. The limbic system governs evolutionarily conserved processes essential for survival. The septum and the hippocampus are central to the limbic system, and control not only emotion-related behaviors but also learning and memory. Here, we provide a developmental and evolutionary perspective of the hippocampus and septum and highlight the neuronal diversity and circuitry that connects these two central components of the limbic system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development Nervous System Development > Vertebrates: General Principles Comparative Development and Evolution > Regulation of Organ Diversity.

Perforant Path Fiber Loss Results in Mnemonic Discrimination Task Deficits in Young Rats

The observation that entorhinal input to the hippocampus declines in old age is well established across human studies and in animal models. This loss of perforant path fibers is exaggerated in individuals with episodic memory deficits and Mild Cognitive Impairment, suggesting that perforant path integrity is associated with progression to Alzheimer's Disease. During normal aging, behaviors that measure the ability of a study participant to discriminate between stimuli that share features is particularly sensitive to perforant fiber loss. Evidence linking perforant path changes to cognitive decline, however, has been largely correlational. Thus, the current study tested the causative role of perforant path fiber loss in behavioral decline by performing a unilateral knife cut to disconnect the entorhinal cortex from the hippocampus in the right hemisphere in young male and female rats. This approach does not completely disconnect the hippocampus from the entorhinal cortex but rather reduces the effective connectivity between these two structures. Male and female rats were then tested on the rodent variant of the mnemonic discrimination task, which is believed to critically rely on perforant path fiber integrity. Right hemisphere perforant path transections produced a significant impairment in the abilities of lesioned animals to discriminate between objects with high levels of feature overlap. This deficit was not observed in the male and female sham groups that received a cut to cortex above the white matter. Together these data support the view that, across species, age-related perforant path fiber loss produces behavioral deficits in the ability to discriminate between stimuli with perceptual overlap.

Basal forebrain septal nuclei are enlarged in healthy subjects prior to the development of Alzheimer's disease

Alzheimer's disease (AD) is known to be associated with loss of cholinergic neurons in the nucleus basalis of Meynert, located in the posterior basal forebrain. Structural changes of septal nuclei, located in the anterior basal forebrain, have not been well studied in AD. Using a validated algorithm, we manually traced septal nuclei on high-resolution coronal magnetic resonance imaging (MRI) in 40 subjects with mild cognitive impairment (MCI) or AD, 89 healthy controls, and 18 subjects who were cognitively normal at the time of MRI but went on to develop AD an average of 2.8 years later. We found that cognitively normal subjects destined to develop AD in the future had enlarged septal nuclei as compared to both healthy controls and patients with current MCI or AD. To our knowledge, this is the first time a brain structure has been found to be enlarged in association with risk of AD. Further research is needed to determine if septal enlargement reflects neuroplastic compensation, amyloid deposition, inflammation, or another process and to determine whether it can serve as an early MRI biomarker of AD.

A second wind for the cholinergic system in Alzheimer's therapy

Notwithstanding tremendous research efforts, the cause of Alzheimer's disease (AD) remains elusive and there is no curative treatment. The cholinergic hypothesis presented 35 years ago was the first major evidence-based hypothesis on the etiology of AD. It proposed that the depletion of brain acetylcholine was a primary cause of cognitive decline in advanced age and AD. It relied on a series of observations obtained in aged animals, elderly, and AD patients that pointed to dysfunctions of cholinergic basal forebrain, similarities between cognitive impairments induced by anticholinergic drugs and those found in advanced age and AD, and beneficial effects of drugs stimulating cholinergic activity. This review revisits these major results to show how this hypothesis provided the drive for the development of anticholinesterase inhibitor-based therapies of AD, the almost exclusively approved treatment in use despite transient and modest efficacy. New ideas for improving cholinergic therapies are also compared and discussed in light of the current revival of the cholinergic hypothesis on the basis of two sets of evidence from new animal models and refined imagery techniques in humans. First, human and animal studies agree in detecting signs of cholinergic dysfunctions much earlier than initially believed. Second, alterations of the cholinergic system are deeply intertwined with its reactive responses, providing the brain with efficient compensatory mechanisms to delay the conversion into AD. Active research in this field should provide new insight into development of multitherapies incorporating cholinergic manipulation, as well as early biomarkers of AD enabling earlier diagnostics. This is of prime importance to counteract a disease that is now recognized to start early in adult life.

APOE-Sensitive Cholinergic Sprouting Compensates for Hippocampal Dysfunctions Due to Reduced Entorhinal Input

Brain mechanisms compensating for cerebral lesions may mitigate the progression of chronic neurodegenerative disorders such as Alzheimer's disease (AD). Mild cognitive impairment (MCI), which often precedes AD, is characterized by neuronal loss in the entorhinal cortex (EC). This loss leads to a hippocampal disconnection syndrome that drives clinical progression. The concomitant sprouting of cholinergic terminals in the hippocampus has been proposed to compensate for reduced EC glutamatergic input. However, in absence of direct experimental evidence, the compensatory nature of the cholinergic sprouting and its putative mechanisms remain elusive. Transgenic mice expressing the human APOE4 allele, the main genetic risk factor for sporadic MCI/AD, display impaired cholinergic sprouting after EC lesion. Using these mice as a tool to manipulate cholinergic sprouting in a disease-relevant way, we showed that this sprouting was necessary and sufficient for the acute compensation of EC lesion-induced spatial memory deficit before a slower glutamatergic reinnervation took place. We also found that partial EC lesion generates abnormal hyperactivity in EC/dentate networks. Dentate hyperactivity was abolished by optogenetic stimulation of cholinergic fibers. Therefore, control of dentate hyperactivity by cholinergic sprouting may be involved in functional compensation after entorhinal lesion. Our results also suggest that dentate hyperactivity in MCI patients may be directly related to EC neuronal loss. Impaired sprouting during the MCI stage may contribute to the faster cognitive decline reported in APOE4 carriers. Beyond the amyloid contribution, the potential role of both cholinergic sprouting and dentate hyperactivity in AD symptomatogenesis should be considered in designing new therapeutic approaches.

SIGNIFICANCE STATEMENT

Currently, curative treatment trials for Alzheimer's disease (AD) have failed. The endogenous ability of the brain to cope with neuronal loss probably represents one of the most promising therapeutic targets, but the underlying mechanisms are still unclear. Here, we show that the mammalian brain is able to manage several deleterious consequences of the loss of entorhinal neurons on hippocampal activity and cognitive performance through a fast cholinergic sprouting followed by a slower glutamatergic reinnervation. The cholinergic sprouting is gender dependent and highly sensitive to the genetic risk factor APOE4 Our findings highlight the specific impact of early loss of entorhinal input on hippocampal hyperactivity and cognitive deficits characterizing early stages of AD, especially in APOE4 carriers.

Hippocampal Proteomic Analysis Reveals Distinct Pathway Deregulation Profiles at Early and Late Stages in a Rat Model of Alzheimer's-Like Amyloid Pathology

The cerebral accumulation and cytotoxicity of amyloid beta (Aβ) is central to Alzheimer's pathogenesis. However, little is known about how the amyloid pathology affects the global expression of brain proteins at different disease stages. In order to identify genotype and time-dependent significant changes in protein expression, we employed quantitative proteomics analysis of hippocampal tissue from the McGill-R-Thy1-APP rat model of Alzheimer-like amyloid pathology. McGill transgenic rats were compared to wild-type rats at early and late pathology stages, i.e., when intraneuronal Aβ amyloid burden is conspicuous and when extracellular amyloid plaques are abundant with more pronounced cognitive deficits. After correction for multiple testing, the expression levels of 64 proteins were found to be considerably different in transgenic versus wild-type rats at the pre-plaque stage (3 months), and 86 proteins in the post-plaque group (12 months), with only 9 differentially regulated proteins common to the 2 time-points. This minimal overlap supports the hypothesis that different molecular pathways are affected in the hippocampus at early and late stages of the amyloid pathology throughout its continuum. At early stages, disturbances in pathways related to cellular responses to stress, protein homeostasis, and neuronal structure are predominant, while disturbances in metabolic energy generation dominate at later stages. These results shed new light on the molecular pathways affected by the early accumulation of Aβ and how the evolving amyloid pathology impacts other complex metabolic pathways.

High plasticity of axonal pathology in Alzheimer's disease mouse models

Axonal dystrophies (AxDs) are swollen and tortuous neuronal processes that are associated with extracellular depositions of amyloid β (Aβ) and have been observed to contribute to synaptic alterations occurring in Alzheimer's disease. Understanding the temporal course of this axonal pathology is of high relevance to comprehend the progression of the disease over time. We performed a long-term in vivo study (up to 210 days of two-photon imaging) with two transgenic mouse models (dE9xGFP-M and APP-PS1xGFP-M). Interestingly, AxDs were formed only in a quarter of GFP-expressing axons near Aβ-plaques, which indicates a selective vulnerability. AxDs, especially those reaching larger sizes, had long lifetimes and appeared as highly plastic structures with large variations in size and shape and axonal sprouting over time. In the case of the APP-PS1 mouse only, the formation of new long axonal segments in dystrophic axons (re-growth phenomenon) was observed. Moreover, new AxDs could appear at the same point of the axon where a previous AxD had been located before disappearance (re-formation phenomenon). In addition, we observed that most AxDs were formed and developed during the imaging period, and numerous AxDs had already disappeared by the end of this time. This work is the first in vivo study analyzing quantitatively the high plasticity of the axonal pathology around Aβ plaques. We hypothesized that a therapeutically early prevention of Aβ plaque formation or their growth might halt disease progression and promote functional axon regeneration and the recovery of neural circuits.

Aberrant Axonal Arborization of PDF Neurons Induced by Aβ42-Mediated JNK Activation Underlies Sleep Disturbance in an Alzheimer's Model

Impaired sleep patterns are common symptoms of Alzheimer's disease (AD). Cellular mechanisms underlying sleep disturbance in AD remain largely unknown. Here, using a Drosophila Aβ42 AD model, we show that Aβ42 markedly decreases sleep in a large population, which is accompanied with postdevelopmental axonal arborization of wake-promoting pigment-dispersing factor (PDF) neurons. The arborization is mediated in part via JNK activation and can be reversed by decreasing JNK signaling activity. Axonal arborization and impaired sleep are correlated in Aβ42 and JNK kinase hemipterous mutant flies. Image reconstruction revealed that these aberrant fibers preferentially project to pars intercerebralis (PI), a fly brain region analogous to the mammalian hypothalamus. Moreover, PDF signaling in PI neurons was found to modulate sleep/wake activities, suggesting that excessive release of PDF by these aberrant fibers may lead to the impaired sleep in Aβ42 flies. Finally, inhibition of JNK activation in Aβ42 flies restores nighttime sleep loss, decreases Aβ42 accumulation, and attenuates neurodegeneration. These data provide a new mechanism by which sleep disturbance could be induced by Aβ42 burden, a key initiator of a complex pathogenic cascade in AD.

Spatiotemporal dynamics of lesion-induced axonal sprouting and its relation to functional architecture of the cerebellum

Neurodegenerative lesions induce sprouting of new collaterals from surviving axons, but the extent to which this form of axonal remodelling alters brain functional structure remains unclear. To understand how collateral sprouting proceeds in the adult brain, we imaged post-lesion sprouting of cerebellar climbing fibres (CFs) in mice using in vivo time-lapse microscopy. Here we show that newly sprouted CF collaterals innervate multiple Purkinje cells (PCs) over several months, with most innervations emerging at 3–4 weeks post lesion. Simultaneous imaging of cerebellar functional structure reveals that surviving CFs similarly innervate functionally relevant and non-relevant PCs, but have more synaptic area on PCs near the collateral origin than on distant PCs. These results suggest that newly sprouted axon collaterals do not preferentially innervate functionally relevant postsynaptic targets. Nonetheless, the spatial gradient of collateral innervation might help to loosely maintain functional synaptic circuits if functionally relevant neurons are clustered in the lesioned area.

Neurodegenerative lesions induce sprouting from surviving axons, but the patterns of re-innervation of these collaterals in relation to existing functional networks remains unclear. Here the authors performed long term in vivo imaging in mice, of sprouts from cerebellar climbing fibers after a lesion, and describe the patterns of connectivity relative to functionally active zones.

Loss of presenilin function is associated with a selective gain of APP function

Presenilin 1 (PS1) is an essential γ-secretase component, the enzyme responsible for amyloid precursor protein (APP) intramembraneous cleavage. Mutations in PS1 lead to dominant-inheritance of early-onset familial Alzheimer’s disease (FAD). Although expression of FAD-linked PS1 mutations enhances toxic Aβ production, the importance of other APP metabolites and γ-secretase substrates in the etiology of the disease has not been confirmed. We report that neurons expressing FAD-linked PS1 variants or functionally deficient PS1 exhibit enhanced axodendritic outgrowth due to increased levels of APP intracellular C-terminal fragment (APP-CTF). APP expression is required for exuberant neurite outgrowth and hippocampal axonal sprouting observed in knock-in mice expressing FAD-linked PS1 mutation. APP-CTF accumulation initiates CREB signaling cascade through an association of APP-CTF with Gαs protein. We demonstrate that pathological PS1 loss-of-function impinges on neurite formation through a selective APP gain-of-function that could impact on axodendritic connectivity and contribute to aberrant axonal sprouting observed in AD patients.

DOI:http://dx.doi.org/10.7554/eLife.15645.001

One of the hallmarks of Alzheimer’s disease is the accumulation within the brain of sticky deposits called plaques. These plaques form from clumps of molecules called amyloid-beta peptide. An enzyme called gamma-secretase generates the amyloid-beta peptide, by cutting it from a membrane-associated protein called APP. This enzyme consists of multiple subunits, and a mutation in one of these – presenilin-1 – causes a particularly severe form of Alzheimer’s disease.

For decades, research into Alzheimer’s disease has focused on the harmful effects of amyloid-beta peptides and plaques. However, Deyts et al. now argue that the protein that gives rise to amyloid-beta peptides has a more direct role in Alzheimer’s disease than previously thought. Specifically, APP may contribute to the harmful effects of the presenilin-1 mutations.

By studying genetically modified mice carrying a human presenilin-1 mutation, Deyts et al. show that some of these animals’ nerve cells grow abnormally. Their cell bodies sprout too many branches, while their nerve fibers – which carry electrical signals away from the cell body – become too long. These abnormalities resemble changes seen in the brain in Alzheimer’s disease. Unexpectedly, however, deleting the gene for APP in the presenilin-1 mutant mice prevents the changes from occurring. This suggests that APP must be present for the presenilin-1 mutation to exert this unwanted effect.

An increase in APP-driven signaling within cells seems to trigger the observed abnormalities in nerve cells. The presenilin-1 mutation modifies how gamma-secretase cuts APP at the cell membrane to produce amyloid-beta peptides. This frees up the APP to instead interact with signaling cascades inside the cell. Given that gamma-secretase is a key therapeutic target in Alzheimer’s disease, further work is needed to explore the implications of these protein interactions for potential treatments.

DOI:http://dx.doi.org/10.7554/eLife.15645.002

Hippocampal plasticity during the progression of Alzheimer's disease

Neuroplasticity involves molecular and structural changes in central nervous system (CNS) throughout life. The concept of neural organization allows for remodeling as a compensatory mechanism to the early pathobiology of Alzheimer's disease (AD) in an attempt to maintain brain function and cognition during the onset of dementia. The hippocampus, a crucial component of the medial temporal lobe memory circuit, is affected early in AD and displays synaptic and intraneuronal molecular remodeling against a pathological background of extracellular amyloid-beta (Aβ) deposition and intracellular neurofibrillary tangle (NFT) formation in the early stages of AD. Here we discuss human clinical pathological findings supporting the concept that the hippocampus is capable of neural plasticity during mild cognitive impairment (MCI), a prodromal stage of AD and early stage AD.

Hippocampal endosomal, lysosomal, and autophagic dysregulation in mild cognitive impairment: correlation with aβ and tau pathology

Endosomal-lysosomal and autophagic dysregulation occurs in the hippocampus in prodromal Alzheimer disease (AD), but its relationship with β-amyloid (Aβ) and tau pathology remains unclear. To investigate this issue, we performed immunoblot analysis of hippocampal homogenates from cases with an antemortem clinical diagnosis of no cognitive impairment, mild cognitive impairment (MCI), and AD. Western blot analysis revealed significant increases in the acid hydrolase cathepsin D and early endosome marker rabaptin5 in the MCI group compared with AD, whereas levels of phosphorylated mammalian target of rapamycin proteins (pmTOR), total mammalian target of rapamycin (mTOR), p62, traf6, and LilrB2 were comparable across clinical groups. Hippocampal Aβ1-40 and Aβ1-42 concentrations and AT8-immunopositive neurofibrillary tangle density were not significantly different across the clinical groups. Greater cathepsin D expression was associated with global cognitive score and episodic memory score but not with mini mental state examination or advanced neuropathology criteria. These results indicate that alterations in hippocampal endosomal-lysosomal proteins in MCI are independent of tau or Aβ pathology.

Impact of continuous versus discontinuous progesterone on estradiol regulation of neuron viability and sprouting after entorhinal cortex lesion in female rats

Because the estrogen-based hormone therapy (HT) in postmenopausal women typically contains a progestogen component, understanding the interactions between estrogens and progestogens is critical for optimizing the potential neural benefits of HT. An important issue in this regard is the use of continuous vs discontinuous hormone treatments. Although sex steroid hormone levels naturally exhibit cyclic fluctuation, many HT formulations include continuous delivery of hormones. Recent findings from our laboratory and others have shown that coadministration of progesterone (P4) can either attenuate or augment beneficial actions of 17β-estradiol (E2) in experimental models depending in part upon the delivery schedule of P4. In this study, we demonstrate that the P4 delivery schedule in combined E2 and P4 treatments alters degenerative and regenerative outcomes of unilateral entorhinal cortex lesion. We assessed how lesion-induced degeneration of layer II neurons in entorhinal cortex layer and deafferentation in dentate gyrus are affected by ovariectomy and treatments with E2 alone or in combination with either continuous or discontinuous P4. Our results demonstrate the combined efficacy of E2 and P4 is dependent on the administration regimen. Importantly, the discontinuous-combined E2+P4 regimen had the greatest neuroprotective efficacy for both end points. These data extend a growing literature that indicates qualitative differences in the neuroprotective effects of E2 as a function of cotreatment with continuous versus discontinuous P4, the understanding of which has important implications for HT in postmenopausal women.

Tau-amyloid interactions in the rTgTauEC model of early Alzheimer's disease suggest amyloid-induced disruption of axonal projections and exacerbated axonal pathology

Early observations of the patterns of neurofibrillary tangles and amyloid plaques in Alzheimer's disease suggested a hierarchical vulnerability of neurons for tangles, and a widespread nonspecific pattern of plaques that nonetheless seemed to correlate with the terminal zone of tangle-bearing neurons in some instances. The first neurofibrillary cortical lesions in Alzheimer's disease occur in the entorhinal cortex, thereby disrupting the origin of the perforant pathway projection to the hippocampus, and amyloid deposits are often found in the molecular layer of the dentate gyrus, which is the terminal zone of the entorhinal cortex. We modeled these anatomical changes in a transgenic mouse model that overexpresses both P301L tau (uniquely in the medial entorhinal cortex) and mutant APP/PS1 (in a widespread distribution) to examine the anatomical consequences of early tangles, plaques, or the combination. We find that tau uniformly occupies the terminal zone of the perforant pathway in tau-expressing mice. By contrast, the addition of amyloid deposits in this area leads to disruption of the perforant pathway terminal zone and apparent aberrant distribution of tau-containing axons. Moreover, human P301L tau-containing axons appear to increase the extent of dystrophic axons around plaques. Thus, the presence of amyloid deposits in the axonal terminal zone of pathological tau-containing neurons profoundly impacts their normal connectivity.

Synaptic alterations in the rTg4510 mouse model of tauopathy

Synapse loss, rather than the hallmark amyloid-β (Aβ) plaques or tau-filled neurofibrillary tangles (NFT), is considered the most predictive pathological feature associated with cognitive status in the Alzheimer's disease (AD) brain. The role of Aβ in synapse loss is well established, but despite data linking tau to synaptic function, the role of tau in synapse loss remains largely undetermined. Here we test the hypothesis that human mutant P301L tau overexpression in a mouse model (rTg4510) will lead to age-dependent synaptic loss and dysfunction. Using array tomography and two methods of quantification (automated, threshold-based counting and a manual stereology-based technique) we demonstrate that overall synapse density is maintained in the neuropil, implicating synapse loss commensurate with the cortical atrophy known to occur in this model. Multiphoton in vivo imaging reveals close to 30% loss of apical dendritic spines of individual pyramidal neurons, suggesting these cells may be particularly vulnerable to tau-induced degeneration. Postmortem, we confirm the presence of tau in dendritic spines of rTg4510-YFP mouse brain by array tomography. These data implicate tau-induced loss of a subset of synapses that may be accompanied by compensatory increases in other synaptic subtypes, thereby preserving overall synapse density. Biochemical fractionation of synaptosomes from rTg4510 brain demonstrates a significant decrease in expression of several synaptic proteins, suggesting a functional deficit of remaining synapses in the rTg4510 brain. Together, these data show morphological and biochemical synaptic consequences in response to tau overexpression in the rTg4510 mouse model.

Reversal of neurofibrillary tangles and tau-associated phenotype in the rTgTauEC model of early Alzheimer's disease

Neurofibrillary tangles (NFTs), a marker of neuronal alterations in Alzheimer's disease (AD) and other tauopathies, are comprised of aggregates of hyperphosphorylated tau protein. We recently studied the formation of NFTs in the entorhinal cortex (EC) and their subsequent propagation through neural circuits in the rTgTauEC mouse model (de Calignon et al., 2012). We now examine the consequences of suppressing transgene expression with doxycycline on the NFT-associated pathological features of neuronal system deafferentation, NFT progression and propagation, and neuronal loss. At 21 months of age we observe that EC axonal lesions are associated with an abnormal sprouting response of acetylcholinesterase (AChE)-positive fibers, a phenotype reminiscent of human AD. At 24 months, NFTs progress, tau inclusions propagate to the dentate gyrus, and neuronal loss is evident. Suppression of the transgene expression from 18 to 24 months led to reversal of AChE sprouting, resolution of Gallyas-positive and Alz50-positive NFTs, and abrogation of progressive neuronal loss. These data suggest that propagation of NFTs, as well as some of the neural system consequences of NFTs, can be reversed in an animal model of NFT-associated toxicity, providing proof in principle that these lesions can be halted, even in established disease.

Reduced plasticity and mild cognitive impairment-like deficits after entorhinal lesions in hAPP/APOE4 mice

Mild cognitive impairment (MCI) is a clinical condition that often precedes Alzheimer disease (AD). Compared with apolipoprotein E-ε3 (APOE3), the apolipoprotein E-ε4 (APOE4) allele is associated with an increased risk of developing MCI and spatial navigation impairments. In MCI, the entorhinal cortex (EC), which is the main innervation source of the dentate gyrus, displays partial neuronal loss. We show that bilateral partial EC lesions lead to marked spatial memory deficits and reduced synaptic density in the dentate gyrus of APOE4 mice compared with APOE3 mice. Genotype and lesion status did not affect the performance in non-navigational tasks. Thus, partial EC lesions in APOE4 mice were sufficient to induce severe spatial memory impairments and synaptic loss in the dentate gyrus. In addition, lesioned APOE4 mice showed no evidence of reactional increase in cholinergic terminals density as opposed to APOE3 mice, suggesting that APOE4 interferes with the ability of the cholinergic system to respond to EC input loss. These findings provide a possible mechanism underlying the aggravating effect of APOE4 on the cognitive outcome of MCI patients.

Functional segmentation of the hippocampus in the healthy human brain and in Alzheimer's disease

In this study we segment the hippocampus according to functional connectivity assessed from resting state functional magnetic resonance images in healthy subjects and in patients with Alzheimer's disease (AD). We recorded the resting FMRI signal from 16 patients and 22 controls. We used seed-based functional correlation analyses to calculate partial correlations of all voxels in the hippocampus relative to characteristic regional signal changes in the thalamus, the prefrontal cortex (PFC) and the posterior cingulate cortex (PCC), while controlling for ventricular CSF and white matter signals. Group comparisons were carried out controlling for age, gender, hippocampal volume and brain volume. The strength of functional connectivity in each region also was correlated with neuropsychological measures. We found that the hippocampus can be segmented into three distinct functional subregions (head, body, and tail), according to the relative connectivity with PFC, PCC and thalamus, respectively. The AD group showed stronger hippocampus-PFC and weaker hippocampus-PCC functional connectivity, the magnitudes of which correlated with MMSE in both cases. The results are consistent with an adaptive role of the PFC in the context of progression of dysfunction in PCC during earlier stages of AD. Extension of our approach could integrate regional volume measures for the hippocampus with their functional connectivity patterns in ways that should increase sensitivity for assessment of AD onset and progression.

Malignant synaptic growth and Alzheimer's disease

In this article, we will describe the malignant synaptic growth hypothesis of Alzheimer's disease. Originally presented in 1994, the hypothesis remains a viable model of the functional and biophysical mechanisms underlying the development and progression of Alzheimer's disease. In this article, we will refresh the model with references to relevant empirical support that has been generated in the intervening two decades since it's original presentation. We will include discussion of its relationship, in terms of points of alignment and points of contention, to other models of Alzheimer's disease, including the cholinergic hypothesis and the tau and β-amyloid models of Alzheimer's disease. Finally, we propose several falsifiable predictions made by the malignant synaptic growth hypothesis and describe the avenues of treatment that hold the greatest promise under this hypothesis.

Cognitive deterioration and associated pathology induced by chronic low-level aluminum ingestion in a translational rat model provides an explanation of Alzheimer's disease, tests for susceptibility and avenues for treatment

A translational aging rat model for chronic aluminum (Al) neurotoxicity mimics human Al exposure by ingesting Al, throughout middle age and old age, in equivalent amounts to those ingested by Americans from their food, water, and Al additives. Most rats that consumed Al in an amount equivalent to the high end of the human total dietary Al range developed severe cognitive deterioration in old age. High-stage Al accumulation occurred in the entorhinal cortical cells of origin for the perforant pathway and hippocampal CA1 cells, resulting in microtubule depletion and dendritic dieback. Analogous pathological change in humans leads to destruction of the perforant pathway and Alzheimer's disease dementia. The hippocampus is thereby isolated from neocortical input and output normally mediated by the entorhinal cortex. Additional evidence is presented that Al is involved in the formation of neurofibrillary tangles, amyloid plaques, granulovacuolar degeneration, and other pathological changes of Alzheimer's disease (AD). The shared characteristics indicate that AD is a human form of chronic Al neurotoxicity. This translational animal model provides fresh strategies for the prevention, diagnosis, and treatment of AD.

Frontiers of spinal cord and spine repair: experimental approaches for repair of spinal cord injury

Regeneration of injured CNS neurons was once thought to be an unachievable goal. Most patients with significant damage to the spinal cord suffer from permanently impaired neurological function. A century of research, however, has led to an understanding of multiple factors that limit CNS regeneration and from this knowledge experimental strategies have emerged for enhancing CNS repair. Some of these approaches have undergone human translation. Nevertheless, translating experimental findings to human trials has been more challenging than anticipated. In this chapter, we will review the current state of knowledge regarding central axonal growth failure after injury, and approaches taken to enhance recovery after SCI.

Abnormal accumulation of autophagic vesicles correlates with axonal and synaptic pathology in young Alzheimer's mice hippocampus

Dystrophic neurites associated with amyloid plaques precede neuronal death and manifest early in Alzheimer's disease (AD). In this work we have characterized the plaque-associated neuritic pathology in the hippocampus of young (4- to 6-month-old) PS1(M146L)/APP(751SL) mice model, as the initial degenerative process underlying functional disturbance prior to neuronal loss. Neuritic plaques accounted for almost all fibrillar deposits and an axonal origin of the dystrophies was demonstrated. The early induction of autophagy pathology was evidenced by increased protein levels of the autophagosome marker LC3 that was localized in the axonal dystrophies, and by electron microscopic identification of numerous autophagic vesicles filling and causing the axonal swellings. Early neuritic cytoskeletal defects determined by the presence of phosphorylated tau (AT8-positive) and actin-cofilin rods along with decreased levels of kinesin-1 and dynein motor proteins could be responsible for this extensive vesicle accumulation within dystrophic neurites. Although microsomal Aβ oligomers were identified, the presence of A11-immunopositive Aβ plaques also suggested a direct role of plaque-associated Aβ oligomers in defective axonal transport and disease progression. Most importantly, presynaptic terminals morphologically disrupted by abnormal autophagic vesicle buildup were identified ultrastructurally and further supported by synaptosome isolation. Finally, these early abnormalities in axonal and presynaptic structures might represent the morphological substrate of hippocampal dysfunction preceding synaptic and neuronal loss and could significantly contribute to AD pathology in the preclinical stages.

Is there a specific pattern of attention deficit in mild cognitive impairment with subcortical vascular features? Evidence from the Attention Network Test

BACKGROUND

Mild cognitive impairment (MCI) is an intermediate state between normal aging and early dementia. Some MCI patients show white matter hyperintensities in magnetic resonance imaging, revealing subcortical vascular damage (SVD). This study aimed to evaluate potential attention deficits not previously described in these patients. Specifically, we evaluated attention network functioning in MCI on the basis of Posner's cognitive neuroscience model, which considers attention as a set of networks: alerting, orienting and executive control.

METHODS

Three groups of participants were tested: 19 MCI patients with SVD (svMCI), 15 MCI patients free from SVD (nvMCI) and 19 healthy controls (HC). We used a task in which the three attention networks and their interactions can be assessed simultaneously, the Attention Network Test (ANT).

RESULTS

The svMCI group showed smaller orienting effect compared with the nvMCI and HC groups. In contrast to the HC and nvMCI groups, svMCI patients did not show improvement in the executive network from the valid visual cue.

CONCLUSIONS

svMCI patients show a deficit in orienting attention networks. This deficit could be related to an effect of SVD on the cholinergic system because acetylcholine is implicated in the modulation of covert orienting responses of attention.

Relationships between hippocampal microstructure, metabolism, and function in early Alzheimer's disease

Abnormal microstructural integrity and glucose metabolism of the hippocampus are common in subjects with Alzheimer's disease (AD) that typically manifest as episodic memory impairment. The above-tissue alterations can be captured in vivo using diffusion tensor imaging (DTI) and positron emission tomography with [18F]fluorodeoxyglucose (FDG-PET). Here, we explored relationships between the above neuroimaging and cognitive markers of early AD-specific hippocampal damage. Twenty patients with early AD (MMSE 25.7 ± 1.7) were studied using DTI and FDG-PET. Episodic memory performance was assessed using the free delayed verbal recall task (DVR). In the between-modality correlation analysis, FDG uptake was strongly associated with diffusivity in the left anterior hippocampus only (r = -0.81, p < 0.05 Bonferroni's corrected for multiple tests). Performance on DVR significantly correlated with left anterior (r = -0.80, p < 0.05) and left mean (r = -0.72, p < 0.05) hippocampal diffusivity, while the correlation with left anterior FDG uptake did not reach statistical significance (r = 0.52, n.s.). DTI-derived diffusivity of the anterior hippocampus might be a sensitive early marker of hippocampal dysfunction as reflected at the synaptic and cognitive levels. This neurobiological distinction of the anterior hippocampus might be related to the disruption of the perforant pathway that is known to occur early in the course of AD.

Progesterone influence on neurite outgrowth involves microglia

Progesterone (P4) antagonizes estradiol (E2) in synaptic remodeling in the hippocampus during the rat estrous cycle. To further understand how P4 modulates synaptic plasticity, we used entorhinal cortex lesions, which induce E2-dependent neurite sprouting in the hippocampus. In young ovariectomized rats, the E2-dependent entorhinal cortex lesion-induced sprouting was attenuated by concurrent treatment with P4 and E2. Microglial activation also showed the E2-P4 antagonism. These findings extend reports on the estrous cycle synaptic remodeling without lesions by showing the P4-E2 antagonism during simultaneous treatment with both E2 and P4. Glial mechanisms were analyzed with the wounding-in-a-dish model of cocultured glia and embryonic d-18 cortical neurons from rat. In cocultures of mixed glia (astrocytes plus 30% microglia), P4 antagonized the E2-dependent neurite outgrowth (number and length) and neuron viability in the presence of E2, as observed in vivo. However, removal of microglia (astrocyte-neuron coculture) abolished the antagonism of E2 by P4 on neuron sprouting. The P4 receptor antagonists ORG-31710 and RU-486 blocked the antagonism of P4 on E2-dependent sprouting. These findings suggest a new role for microglia in P4 antagonism of E2 in neuronal plasticity and show its dependence on progesterone receptors. These findings are also relevant to the inclusion of progestins in hormone therapy, which is controversial in relation to cognitive declines during aging and in Alzheimer's disease.

Spinal cord injury: plasticity, regeneration and the challenge of translational drug development

Over the past three decades, multiple mechanisms limiting central nervous system regeneration have been identified. Here, we address plasticity arising from spared systems as a particularly important and often unrecognized mechanism that potentially contributes to functional recovery in studies of 'regeneration' after spinal cord injury. We then discuss complexities involved in translating findings from animal models to human clinical trials in spinal cord injury; current strategies might be too limited in scope to yield detectable benefits in the complex and variable arena of human injury. Our animal models are imperfect, and the very variability that we attempt to control in the course of conducting rigorous research might, ironically, limit our ability to identify the most promising therapies in the human arena. Therapeutic candidates are most likely to have a detectable effect in human trials if they elicit benefits in severe contusion and larger animal models and pass the test of independent replication.

Multi-modal imaging predicts memory performance in normal aging and cognitive decline

This study (n=161) related morphometric MR imaging, FDG-PET and APOE genotype to memory scores in normal controls (NC), mild cognitive impairment (MCI) and Alzheimer's disease (AD). Stepwise regression analyses focused on morphometric and metabolic characteristics of the episodic memory network: hippocampus, entorhinal, parahippocampal, retrosplenial, posterior cingulate, precuneus, inferior parietal, and lateral orbitofrontal cortices. In NC, hippocampal metabolism predicted learning; entorhinal metabolism predicted recognition; and hippocampal metabolism predicted recall. In MCI, thickness of the entorhinal and precuneus cortices predicted learning, while parahippocampal metabolism predicted recognition. In AD, posterior cingulate cortical thickness predicted learning, while APOE genotype predicted recognition. In the total sample, hippocampal volume and metabolism, cortical thickness of the precuneus, and inferior parietal metabolism predicted learning; hippocampal volume and metabolism, parahippocampal thickness and APOE genotype predicted recognition. Imaging methods appear complementary and differentially sensitive to memory in health and disease. Medial temporal and parietal metabolism and morphometry best explained memory variance. Medial temporal characteristics were related to learning, recall and recognition, while parietal structures only predicted learning.

Hippocampal hyperperfusion in Alzheimer's disease

Many of the regions with the earliest atrophy in Alzheimer's Disease (AD) do not show prominent deficits on functional imaging studies of flow or metabolism. This paradox may provide unique insights into the pathophysiology of AD. We sought to examine the relationship between function and atrophy in AD using MRI blood flow and anatomic imaging. 22 subjects diagnosed with AD, mean Mini Mental State Exam (MMSE) score 22.2, and 16 healthy elderly controls were imaged with a volumetric arterial spin labeling blood flow MRI technique and an anatomical imaging method using the identical spatial resolution, image orientation, and spatial encoding strategy. Cerebral blood flow (CBF) and gray matter (GM) maps derived from the imaging were transformed to a standard anatomical space. GM and CBF maps were tested for significant differences between groups. Additionally, images were tested for regions with significant mismatch of the CBF and GM differences between groups. CBF was significantly lower in the bilateral precuneus, parietal association cortex and the left inferior temporal lobe but was non-significantly increased in the hippocampus and other medial temporal structures. After correction for GM loss, CBF was significantly elevated in the hippocampus and other medial temporal structures. The hippocampus and other regions affected early in AD are characterized by elevated atrophy-corrected perfusion per cm(3) of tissue. This suggests compensatory or pathological elevation of neural activity, inflammation, or elevated production of vasodilators.

Hippocampal hyperperfusion in Alzheimer's disease

Many of the regions with the earliest atrophy in Alzheimer's Disease (AD) do not show prominent deficits on functional imaging studies of flow or metabolism. This paradox may provide unique insights into the pathophysiology of AD. We sought to examine the relationship between function and atrophy in AD using MRI blood flow and anatomic imaging. 22 subjects diagnosed with AD, mean Mini Mental State Exam (MMSE) score 22.2, and 16 healthy elderly controls were imaged with a volumetric arterial spin labeling blood flow MRI technique and an anatomical imaging method using the identical spatial resolution, image orientation, and spatial encoding strategy. Cerebral blood flow (CBF) and gray matter (GM) maps derived from the imaging were transformed to a standard anatomical space. GM and CBF maps were tested for significant differences between groups. Additionally, images were tested for regions with significant mismatch of the CBF and GM differences between groups. CBF was significantly lower in the bilateral precuneus, parietal association cortex and the left inferior temporal lobe but was non-significantly increased in the hippocampus and other medial temporal structures. After correction for GM loss, CBF was significantly elevated in the hippocampus and other medial temporal structures. The hippocampus and other regions affected early in AD are characterized by elevated atrophy-corrected perfusion per cm(3) of tissue. This suggests compensatory or pathological elevation of neural activity, inflammation, or elevated production of vasodilators.

Alzheimer's disease in Down syndrome: neurobiology and risk

Down syndrome (DS) is characterized by increased mortality rates, both during early and later stages of life, and age-specific mortality risk remains higher in adults with DS compared with the overall population of people with mental retardation and with typically developing populations. Causes of increased mortality rates early in life are primarily due to the increased incidence of congenital heart disease and leukemia, while causes of higher mortality rates later in life may be due to a number of factors, two of which are an increased risk for Alzheimer's disease (AD) and an apparent tendency toward premature aging. In this article, we describe the increase in lifespan for people with DS that has occurred over the past 100 years, as well as advances in the understanding of the occurrence of AD in adults with DS. Aspects of the neurobiology of AD, including the role of amyloid, oxidative stress, Cu/ZN dismutase (SOD-1), as well as advances in neuroimaging are presented. The function of risk factors in the observed heterogeneity in the expression of AD dementia in adults with DS, as well as the need for sensitive and specific biomarkers of the clinical and pathological progressing of AD in adults with DS is considered.