Minocycline corrects early, pre-plaque neuroinflammation and inhibits BACE-1 in a transgenic model of Alzheimer's disease-like amyloid pathology

Background

A growing body of evidence indicates that inflammation is one of the earliest neuropathological events in Alzheimer's disease. Accordingly, we have recently shown the occurrence of an early, pro-inflammatory reaction in the hippocampus of young, three-month-old transgenic McGill-Thy1-APP mice in the absence of amyloid plaques but associated with intracellular accumulation of amyloid beta petide oligomers. The role of such a pro-inflammatory process in the progression of the pathology remained to be elucidated.

Methods and results

To clarify this we administered minocycline, a tetracyclic derivative with anti-inflammatory and neuroprotective properties, to young, pre-plaque McGill-Thy1-APP mice for one month. The treatment ended at the age of three months, when the mice were still devoid of plaques. Minocycline treatment corrected the up-regulation of inducible nitric oxide synthase and cyclooxygenase-2 observed in young transgenic placebo mice. Furthermore, the down-regulation of inflammatory markers correlated with a reduction in amyloid precursor protein levels and amyloid precursor protein-related products. Beta-site amyloid precursor protein cleaving enzyme 1 activity and levels were found to be up-regulated in transgenic placebo mice, while minocycline treatment restored these levels to normality. The anti-inflammatory and beta-secretase 1 effects could be partly explained by the inhibition of the nuclear factor kappa B pathway.

Conclusions

Our study suggests that the pharmacological modulation of neuroinflammation might represent a promising approach for preventing or delaying the development of Alzheimer's disease neuropathology at its initial, pre-clinical stages. The results open new vistas to the interplay between inflammation and amyloid pathology.

Transgenic mice as a model of pre-clinical Alzheimer's disease

At diagnosis, Alzheimer's disease (AD) brains are extensively burdened with plaques and tangles and display a degree of synaptic failure most likely beyond therapeutic treatment. It is therefore crucial to identify early pathological events in the progression of the disease. While it is not currently feasible to identify and study early, pre-clinical stages of AD, transgenic (Tg) models offer a valuable tool in this regard. Here we investigated cognitive, structural and biochemical CNS alterations occurring in our newly developed McGill-Thyl-APP Tg mice (over-expressing the human amyloid precursor protein with the Swedish and Indiana mutations) prior to extracellular plaque deposition. Pre-plaque, 3-month old Tg mice already displayed cognitive deficits concomitant with reorganization of cortical cholinergic pre-synaptic terminals. Conformational specific antibodies revealed the early appearance of intracellular amyloid β (Aβ)-oligomers and fibrillar oligomers in pyramidal neurons of cerebral cortex and hippocampus. At the same age, the cortical levels of insulin degrading enzyme -a well established Aβ-peptidase, were found to be significantly down-regulated. Our results suggest that, in the McGill-Thy1-APP Tg model, functional, structural and biochemical alterations are already present in the CNS at early, pre-plaque stages of the pathology. Accumulation of intraneuronal neurotoxic Aβ-oligomers (possibly caused by a failure in the clearance machinery) is likely to be the culprit of such early, pre-plaque pathology. Similar neuronal alterations might occur prior to clinical diagnosis in AD, during a yet undefined 'latent' stage. A better understanding of such pre-clinical AD might yield novel therapeutic targets and or diagnostic tools.

Engagement of the PFC in consolidation and recall of recent spatial memory

The standard model of system consolidation proposes that memories are initially hippocampus dependent and become hippocampus independent over time. Previous studies have demonstrated the involvement of the medial prefrontal cortex (mPFC) in the retrieval of remote memories. The transformations required to make a memory undergo system's consolidation are thought to require synaptic plasticity. In this study, we investigated the participation of the mitogen-activated protein kinase (MAPK)/ERK pathway in acquisition, memory consolidation, and recent memory recall of the Morris water maze (MWM) task using a 1-d training protocol. To this end, bilateral injections of the MEK inhibitor U0126 into the rat mPFC were performed. The injection of the MEK inhibitor in the mPFC did not affect the acquisition of the MWM. However, MEK inhibitor resulted in impairments on recent memory retrieval either when applied at the end of the learning phase (memory consolidation) or prior to the retention test. The results strongly support the concept that recently acquired and consolidated spatial memories require the mPFC, and that local activation of the MAPK/ERK pathway in the mPFC is necessary for the consolidation and recall of recent memories.

A novel transgenic rat model with a full Alzheimer's-like amyloid pathology displays pre-plaque intracellular amyloid-beta-associated cognitive impairment

Alzheimer's disease (AD) is a neurodegenerative pathology in which amyloid-beta (Abeta) peptide accumulates in different brain areas leading to deposition of plaques and a progressive decline of cognitive functions. After a decade in which a number of transgenic (Tg) mouse models mimicking AD-like amyloid-deposition pathology have been successfully generated, few rat models have been reported that develop intracellular and extracellular Abeta accumulation, together with impairment of cognition. The generation of a Tg rat reproducing the full AD-like amyloid pathology has been elusive. Here we describe the generation and characterization of a new transgenic rat line, coded McGill-R-Thy1-APP, developed to express the human amyloid-beta precursor protein (AbetaPP) carrying both the Swedish and Indiana mutations under the control of the murine Thy1.2 promoter. The selected mono-transgenic line displays an extended phase of intraneuronal Abeta accumulation, already apparent at 1 week after birth, which is widespread throughout different cortical areas and the hippocampus (CA1, CA2, CA3, and dentate gyrus). Homozygous Tg animals eventually produce extracellular Abeta deposits and, by 6 months of age, dense, thioflavine S-positive, amyloid plaques are detected, associated with glial activation and surrounding dystrophic neurites. The cognitive functions in transgenic McGill-R-Thy1-APP rats, as assessed using the Morris water maze task, were found already altered as early as at 3 months of age, when no CNS plaques are yet present. The spatial cognitive impairment becomes more prominent in older animals (13 months), where the behavioral performance of Tg rats positively correlates with the levels of soluble Abeta (trimers) measured in the cortex.

Early-stage inflammation and experimental therapy in transgenic models of the Alzheimer-like amyloid pathology

BACKGROUND

Intracellular accumulation of beta-amyloid (Abeta) is one of the early features in the neuropathology of Alzheimer's disease (AD) and Down's syndrome. This can be reproduced in cell and transgenic animal models of the AD-like amyloid pathology. In a transgenic rat model, our lab has previously shown that the intracellular accumulation of Abeta is sufficient to provoke cognitive impairments and biochemical alterations in the cerebral cortex and hippocampus in the absence of amyloid plaques.

OBJECTIVE

To investigate an early, pre-plaque inflammatory process in AD-like transgenic models and establish whether the neurotoxic effects of Abeta oligomers and proinflammatory responses can be arrested with minocycline.

METHODS

For these studies, we used naïve mice and transgenic animal models of the AD-like amyloid pathology and applied neurochemical, immunohistochemical and behavioral experimental approaches.

RESULTS

In the early stages of the AD-like amyloid pathology, intracellular Abeta oligomers accumulate within neurons of the cerebral cortex and hippocampus. Coincidental with this, behavioral impairments occur prior to the appearance of amyloid plaques, together with an upregulation of MHC-II, i-NOS and COX-2, well-known proinflammatory markers. Treatment with minocycline corrected behavioral impairments, lowered inflammatory markers and levels of Abeta trimers.

CONCLUSION

A pharmacological approach targeting the early neuroinflammatory effects of Abeta might be a promising strategy to prevent or delay the onset of AD.

Changes with aging in the dopaminergic and noradrenergic innervation of rat neocortex

In normal aging, the mammalian cortex undergoes significant remodeling. Although neuromodulation by dopamine and noradrenaline in the cortex is known to be important for proper cognitive function, little is known on how cortical noradrenergic and dopaminergic presynaptic boutons are affected in normal aging. Using rats we investigated whether these two neurotransmitter systems undergo structural reorganization in aging, and if these changes correlated with cognitive loss. Young and aged rats were tested for cognitive performance using the Morris water maze. Following the behavioral characterization, the animals were sacrificed and the cortical tissue was processed for immunofluorescence using antibodies directed against tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) to detect and discriminate noradrenergic and dopaminergic varicosities. We observed a significant increase in dopaminergic varicosities in lamina V of the anterior cingulate cortex (ACC) of aged cognitively unimpaired rats when compared to young and aged-impaired animals. In laminae II and III of the ACC, we observed a significant decrease of dopaminergic varicosities in aged-impaired animals when compared to young or aged cognitively unimpaired animals. Changes in noradrenergic varicosities never reached statistical significance in any group or brain region. The data suggests that the remodeling of mesocortical dopaminergic fibers may participate in age-associated cognitive decline.

Increased matrix metalloproteinase 9 activity in mild cognitive impairment

Nerve growth factor (NGF)-dependent cholinergic basal forebrain neurons degenerate during the progression of Alzheimer disease (AD). Elevated proNGF and reduced levels of the TrkA high-affinity NGF receptor occur in prodromal and advanced stages of AD. We recently described a protease cascade responsible for the conversion of proNGF to mature NGF (mNGF) in which matrix metalloproteinase 9 (MMP-9) degrades mNGF in the extracellular space. To determine whether this proteolytic cascade is altered during the progression of AD, we examined human frontal and parietal cortex tissues from aged subjects with a clinical diagnosis of AD, mild cognitive impairment, or no cognitive impairment. The analysis demonstrated greater MMP-9 activity in both AD and mild cognitive impairment compared with no cognitive impairment brain samples (p < 0.01), which supports the notion that a metabolic failure in the NGF-maturation/degradation pathway may be associated with an exacerbated degradation of mNGF in the cerebral cortex in early AD. Moreover, there were inverse correlations between Global Cognitive Score and Mini-Mental State Examination score and MMP-9 activity. These findings suggest that a reduction in mNGF as a consequence of MMP-9-mediated degradation may in part underlie the pathogenesis of cognitive deficits in mild cognitive impairment and AD.

Localization of substance P in neuronal pathways

The main neuronal systems containing substance P are summarized on the basis of immunohistochemical evidence. The substance P striatonigral projection is one of the most conspicuous of these. Electron microscopic studies using the peroxidase-antiperoxidase technique reveal some heterogeneity in the substance P-immunostained material in the substantia nigra. Immunoreactivity for the peptide is found in terminals establishing both symmetrical and asymmetrical synapses with substantia nigra dendrites. Substance P immunoreactivity in the substantia gelatinosa of the trigeminal nerve and in the skin of the trigeminal territory was found to be depleted after sensory denervation. Electron microscopy showed that in this area of the rat brain substance P-immunoreactive elements are largely associated with dendrites and establish asymmetrical axo-dendritic synapses. Substance P-immunoreactive terminals synapsing with presynaptic dendrites were also observed (i.e. dendrites that themselves are presynaptic to other dendrites). The origin of substance P-containing fibres in the prevertebral ganglia has been investigated in the guinea-pig by combining surgical procedures and immunohistochemistry. Only procedures which disconnected dorsal root ganglia from prevertebral ganglia depleted substance P immunofluorescence in the latter. This substance P-immunoreactive material disappeared after administration of capsaicin. Electron microscopic studies in prevertebral ganglia show that substance P-immunoreactive varicosities establish axodendritic contacts with the sympathetic neurons. These observations provide strong evidence for direct synaptic sensory-autonomic interactions in the prevertebral ganglia involving substance P-containing collaterals of peripheral sensory nerve fibres.

ADAM-10 over-expression increases cortical synaptogenesis

Cortical cholinergic, glutamatergic and GABAergic terminals become upregulated during early stages of the transgenic amyloid pathology. Abundant evidence suggests that sAPP alpha, the product of the non-amyloidogenic alpha-secretase pathway, is neurotrophic both in vitro and when exogenously applied in vivo. The disintegrin metalloprotease ADAM-10 has been shown to have alpha-secretase activity in vivo. To determine whether sAPP alpha has an endogenous biological influence on cortical presynaptic boutons in vivo, we quantified cortical cholinergic, glutamatergic and GABAergic presynaptic bouton densities in either ADAM-10 moderate expressing (ADAM-10 mo) transgenic mice, which moderately overexpress ADAM-10, or age-matched non-transgenic controls. Both early and late ontogenic time points were investigated. ADAM-10 mo transgenic mice display significantly elevated cortical cholinergic, glutamatergic and GABAergic presynaptic bouton densities at the early time point (8 months). Only the cholinergic presynaptic bouton density remains significantly elevated in late-staged ADAM-10 mo transgenic animals (18 months). To confirm that the observed elevations were due to increased levels of endogenous murine sAPP alpha, exogenous human sAPP alpha was infused into the cortex of non-transgenic control animals for 1 week. Exogenous infusion of sAPP alpha led to significant elevations in the cholinergic, glutamatergic and GABAergic cortical presynaptic bouton populations. These results are the first to demonstrate an in vivo influence of ADAM-10 on neurotransmitter-specific cortical synaptic plasticity and further confirm the neurotrophic influence of sAPP alpha on cortical synaptogenesis.

Impact of intracellular beta-amyloid in transgenic animals and cell models

The present commentary based on cell and animal models of intracellular beta-amyloid (iAbeta) expression indicates that low levels of microscopically undetectable iAbeta could have a physiological role in the modulation of the cyclic AMP response element (CRE)-dependent gene expression and, as a consequence, a positive influence on synaptic plasticity (the 'good' Abeta?). On the other hand, high levels of iAbeta resembling the pathological and microscopically visible accumulation of this amyloid peptide, akin to that observed in Down syndrome and Alzheimer's disease, disrupt CRE-regulated gene expression, therefore compromising the protein synthesis-dependent component of long-term potentiation (the 'bad' Abeta?). Moreover, intracellular pathology would be independent and additive to the toxic effects of the extracellular Abeta burden.

Cognitive impairment and transmitter-specific pre- and postsynaptic changes in the rat cerebral cortex during ageing

Recent studies suggest that age-related cognitive decline is correlated with an excitatory-inhibitory imbalance in synaptic discharges on pyramidal neurons. This study focuses on whether ageing and cognitive status correlates with relative numbers of excitatory and inhibitory presynaptic boutons. We investigated the density of excitatory and inhibitory presynaptic inputs across several areas of the rat cerebral cortex in young and aged male Fischer 344 rats. Aged animals were segregated into aged cognitively impaired (AI) and aged cognitively unimpaired (AU) groups using the Morris water maze. We applied immunohistochemistry to reveal the majority of excitatory and inhibitory presynaptic boutons captured with confocal microscopy and quantitative image analysis. A gradual decline in the density of excitatory and inhibitory presynaptic boutons occurred from young to AU to AI animals; however, the ratios of excitatory to inhibitory presynaptic bouton densities were not significantly altered. We further investigated the density of receptor scaffolding proteins representing key excitatory and inhibitory receptor postsynaptic sites, using antibodies against specific markers of excitatory and inhibitory postsynaptic densities, respectively. Significant changes in the ratios of excitatory to inhibitory postsynaptic densities were observed only in AI compared to young rats.

NGF-cholinergic dependency in brain aging, MCI and Alzheimer's disease

Forebrain cholinergic neurons are highly dependent on nerve growth factor (NGF) for phenotype maintenance. We have established that in addition to "target-derived" NGF neurotrophic stimulation, cholinergic neurons also respond dose-dependently, to intra-parenchymal NGF administration in the somato-dendritic region of the nucleus Basalis, thus illustrating the potential of alternative reparative therapies which would by-pass the undesirable effects of diffuse neurotrophin application. Moreover, our lab has also observed that the steady-state number of cortical cholinergic synapses is dependent on continuous NGF supply, as anti-NGF monoclonal antibodies and TrkA receptor antagonists deplete pre-existing cholinergic bouton numbers. Furthermore, the application of either NGF or TrkA NGF-mimetic agonists successfully rescues the age-dependent loss of cortical cholinergic boutons in aged-impaired rats. The vulnerability of the cortical cholinergic system has also been demonstrated in transgenic animal models of the Alzheimer's disease (AD) amyloid pathology. It is of interest to note however, that an up-regulation of cholinergic presynaptic boutons has been observed in certain transgenic mouse models prior to plaque formation. This observation is similar to the visibly increased immunoreactivity of cortical and hippocampal choline acetyltransferase (ChAT) fibers in patients with Mild Cognitive Impairment (MCI). A series of ex-vivo experiments conducted by our group have demonstrated that contrary to popular belief, proNGF, as opposed to mature NGF, is released from the cerebral cortex in an activity-dependent manner. In addition, proNGF appears to be released with a series of pro-enzymes and enzymes, which are involved in its subsequent maturation to NGF and degradation in the extracellular space. Given that proNGF is known to be upregulated in AD patients a dysregulation in the maturation or degradation of mature NGF might explain the preferential vulnerability of the cholinergic system in the AD pathology.

Paradoxical upregulation of glutamatergic presynaptic boutons during mild cognitive impairment

Synaptic integrity is now recognized as a central component of Alzheimer's disease. Surprisingly, however, the structural status of glutamatergic synapses in Alzheimer's disease is unclear, despite the fact that glutamate is the major excitatory transmitter of the CNS and has key roles in excitotoxicity and long-term potentiation. The identification of specific markers of glutamatergic neurons now allows an assessment of the structural involvement of the glutamatergic system across progressive stages of the Alzheimer's pathology, an opportunity not afforded by previously used neurochemical approaches. Glutamatergic presynaptic bouton density and dystrophic neurite abundance were quantified in midfrontal gyrus brain tissue from subjects with no cognitive impairment, mild cognitive impairment, or mild- or severe-stage Alzheimer's disease. Our study demonstrates a striking pathology-dependent pattern of glutamatergic synaptic remodeling with disease progression. Subjects with mild cognitive impairment display a paradoxical elevation in glutamatergic presynaptic bouton density, a situation akin to that observed in the cholinergic system, which then depletes and drops with disease progression. This pattern of synaptic remodeling mirrors our previous findings in transgenic animal models and is of major relevance to current transmitter-based therapeutics.

High intracellular concentrations of amyloid-beta block nuclear translocation of phosphorylated CREB

The beta-amyloid peptide (Abeta) is considered responsible for the pathogenesis of Alzheimer's disease. Despite the magnitude of reports describing a neurotoxic role of extracellular Abeta, the role for intracellular Abeta (iAbeta) has not been elucidated. We previously demonstrated that in rat pheochromocytoma cells expression of moderate levels of Abeta results in the up-regulation of phospho-extracellular signal-regulated kinases (ERK1)/2 along with an elevation of cyclic AMP-response element (CRE)-regulated gene expression; however, the effect of high intracellular levels of Abeta were not examined. Towards this goal we generated constructs that endogenously produce different expression levels of iAbeta in a human cell line. We show a bimodal response to Abeta in a neural human cell line. A moderate increase of endogenous Abeta up-regulates certain cyclic AMP-response element-binding protein (CREB) responsive genes such as presenilin 1, presenilin 2, brain-derived neurotrophic factor, and mRNA and protein levels by CREB activation and Synapsin 1 nuclear translocation. On the other hand, high-loads of iAbeta resulted in sustained hyper-phosphorylation of CREB that did not translocate to the nucleus and did not stimulate activation of CRE-regulated gene expression. Our study suggests that variations in levels of iAbeta could influence signaling mechanisms that lead to phosphorylation of CREB, its nuclear translocation and CRE-regulated genes involved in production of Abeta and synaptic plasticity in opposite directions.

The failure in NGF maturation and its increased degradation as the probable cause for the vulnerability of cholinergic neurons in Alzheimer's disease

This short review discusses the arguments to consider the dismetabolism of the pathway responsible for both the maturation and degradation of NGF as the culprit of vulnerability of the forebrain cholinergic system to the Alzheimer's disease neuropathology. This summary includes information regarding a novel metabolic cascade converting Pro-NGF to mature NGF in the extracellular space and its ultimate degradation by a metalloprotease. It also describes how this pathway is altered in Alzheimer's disease with the consequential CNS accumulation of proNGF and impairment in the formation of NGF along with increased degradation of this key trophic factor. This metabolic scenario in Alzheimer's disease should result in the failure of NGF trophic support to forebrain cholinergic neurons and thus explaining the vulnerability of these neurons in this neurodegenerative condition.

Altered synaptic function in Alzheimer's disease

Alzheimer's disease is the leading cause of dementia in the elderly, presenting itself clinically by progressive loss of memory and learning. Since synaptic density correlates more closely with cognitive impairment than any other pathological lesion observable in the disease pathology, an increased understanding of the mechanisms behind synaptic disconnection is of vital importance. Our lab investigated the neurotransmitter-specific status of distinct cortical presynaptic bouton populations in various transgenic mouse models of the Alzheimer's-like amyloid pathology in order to assess their involvement throughout the progression of the pathology. These studies have revealed that the amyloid pathology appears to progress in a neurotransmitter-specific manner where the cholinergic terminals appear most vulnerable, followed by the glutamatergic terminals and finally by the somewhat more resilient GABAergic terminals. This review will discuss additional studies which also provide evidence of a neurotransmitter-specific pathology as well as comment on the potential explanations for the observed vulnerabilities, touching upon metabolic demand, trophic support and receptor mediated activation.