Regulatable transgenic mouse models of Alzheimer disease: onset, reversibility and spreading of Tau pathology

In vivo axonal transport deficits in a mouse model of fronto-temporal dementia

Background

Axonal transport is vital for neurons and deficits in this process have been previously reported in a few mouse models of Alzheimer's disease prior to the appearance of plaques and tangles. However, it remains to be determined whether axonal transport is defective prior to the onset of neurodegeneration. The rTg4510 mouse, a fronto-temporal dementia and parkinsonism-17 (FTDP-17) tauopathy model, over-express tau-P301L mutation found in familial forms of FTDP-17, in the forebrain driven by the calcium–calmodulin kinase II promoter. This mouse model exhibits tau pathology, neurodegeneration in the forebrain, and associated behavioral deficits beginning at 4–5 months of age.

Animal model

rTg4510 transgenic mice were used in these studies. Mice were given 2 μL of MnCl₂ in each nostril 1 h prior to Magnetic Resonance Imaging (MRI). Following MnCl₂ nasal lavage, mice were imaged using Manganese enhanced Magnetic Resonance Imaging (MEMRI) Protocol with TE = 8.5 ms, TR = 504 ms, FOV = 3.0 cm, matrix size = 128 × 128 × 128, number of cycles = 15 with each cycle taking approximately 2 min, 9 s, and 24 ms using Paravision software (BrukerBioSpin, Billerica, MA). During imaging, body temperature was maintained at 37.0 °C using an animal heating system (SA Instruments, Stony Brook, NY).

Data analysis

Resulting images were analyzed using Paravision software. Regions of interest (ROI) within the olfactory neuronal layer (ONL) and the water phantom consisting of one pixel (ONL) and 9 pixels (water) were selected and copied across each of the 15 cycles. Signal intensities (SI) of ONL and water phantom ROIs were measured. SI values obtained for ONL were then normalized the water phantom SI values. The correlation between normalized signal intensity in the ONL and time were assessed using Prism (GraphPad Software, San Diego, CA).

Results

Using the MEMRI technique on 1.5, 3, 5, and 10-month old rTg4510 mice and littermate controls, we found significant axonal transport deficits present in the rTg4510 mice beginning at 3 months of age in an age-dependent manner. Using linear regression analysis, we measured rates of axonal transport at 1.5, 3, 5, and 10 months of age in rTg4510 and WT mice. Axonal transport rates were observed in rTg4510 mice at 48% of WT levels at 3 months, 40% of WT levels at 5 months, and 30% of WT levels at 10 months of age. In order to determine the point at which tau appears in the cortex, we probed for phosphorylated tau levels, and found that pSer262 is present at 3 months of age, not earlier at 1.5 months of age, but observed no pathological tau species until 6 months of age, months after the onset of the transport deficits. In addition, we saw localization of tau in the ONL at 6 months of age.

Discussion

In our study, we identified the presence of age-dependent axonal transport deficits beginning at 3 months of age in rTg4510 mice. We correlated these deficits at 3 months to the presence of hyperphosphorylated tau in the brain and the presence within the olfactory epithelium. We observed tau pathology not only in the soma of these neurons but also within the axons and processes of these neurons. Our characterization of axonal transport in this tauopathy model provides a functional time point that can be used for future therapeutic interventions.

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We used MEMRI to define axonal transport rate changes in the rTg4510 mouse.

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We observed significant hyperphosphorylated tau starting at 3 months of age.

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We found an age-dependent decline in axonal transport rates.

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Declines in axonal transport correlated with increases in hyperphosphorylated tau.

cAMP-PKA phosphorylation of tau confers risk for degeneration in aging association cortex

The pattern of neurodegeneration in Alzheimer's disease (AD) is very distinctive: neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau selectively affect pyramidal neurons of the aging association cortex that interconnect extensively through glutamate synapses on dendritic spines. In contrast, primary sensory cortices have few NFTs, even in late-stage disease. Understanding this selective vulnerability, and why advancing age is such a high risk factor for the degenerative process, may help to reveal disease etiology and provide targets for intervention. Our study has revealed age-related increase in cAMP-dependent protein kinase (PKA) phosphorylation of tau at serine 214 (pS214-tau) in monkey dorsolateral prefrontal association cortex (dlPFC), which specifically targets spine synapses and the Ca(2+)-storing spine apparatus. This increase is mirrored by loss of phosphodiesterase 4A from the spine apparatus, consistent with increase in cAMP-Ca(2+) signaling in aging spines. Phosphorylated tau was not detected in primary visual cortex, similar to the pattern observed in AD. We also report electron microscopic evidence of previously unidentified vesicular trafficking of phosphorylated tau in normal association cortex--in axons in young dlPFC vs. in spines in aged dlPFC--consistent with the transneuronal lesion spread reported in genetic rodent models. pS214-Tau was not observed in normal aged mice, suggesting that it arises with the evolutionary expansion of corticocortical connections in primates, crossing the threshold into NFTs and degeneration in humans. Thus, the cAMP-Ca(2+) signaling mechanisms, needed for flexibly modulating network strength in young association cortex, confer vulnerability to degeneration when dysregulated with advancing age.

Rescue of tau-induced synaptic transmission pathology by paclitaxel

Behavioral and electrophysiological studies of Alzheimer’s disease (AD) and other tauopathies have revealed that the onset of cognitive decline correlates better with synaptic dysfunctions than with hallmark pathologies such as extracellular amyloid-β plaques, intracellular hyperphosphorylated tau or neuronal loss. Recent experiments have also demonstrated that anti-cancer microtubule (MT)-stabilizing drugs can rescue tau-induced behavioral decline and hallmark neuron pathologies. Nevertheless, the mechanisms underlying tau-induced synaptic dysfunction as well as those involved in the rescue of cognitive decline by MTs-stabilizing drugs remain unclear. Here we began to study these mechanisms using the glutaminergic sensory-motoneuron synapse derived from Aplysia ganglia, electrophysiological methods, the expression of mutant-human tau (mt-htau) either pre or postsynaptically and the antimitotic drug paclitaxel. Expression of mt-htau in the presynaptic neurons led to reduced excitatory postsynaptic potential (EPSP) amplitude generated by rested synapses within 3 days of mt-htau expression, and to deeper levels of homosynaptic depression. mt-htau-induced synaptic weakening correlated with reduced releasable presynaptic vesicle pools as revealed by the induction of asynchronous neurotransmitter release by hypertonic sucrose solution. Paclitaxel totally rescued tau-induced synaptic weakening by maintaining the availability of the presynaptic vesicle stores. Postsynaptic expression of mt-htau did not impair the above described synaptic-transmission parameters for up to 5 days. Along with earlier confocal microscope observations from our laboratory, these findings suggest that tau-induced synaptic dysfunction is the outcome of impaired axoplasmic transport and the ensuing reduction in the releasable presynaptic vesicle stores rather than the direct effects of mt-htau or paclitaxel on the synaptic release mechanisms.

New perspectives on the role of tau in Alzheimer's disease. Implications for therapy

Alzheimer's disease (AD) and related dementias constitute a major public health issue due to an increasingly aged population as a consequence of generally improved medical care and demographic changes. Current drug treatment of AD, the most prevalent dementia, with cholinesterase inhibitors or NMDA antagonists have demonstrated very modest, symptomatic efficacy, leaving an unmet medical need for new, more effective therapies. While drug development efforts in the last two decades have primarily focused on the amyloid cascade hypothesis, so far with disappointing results, tau-based strategies have received little attention until recently despite that the presence of extensive tau pathology is central to the disease. The discovery of mutations within the tau gene that cause fronto-temporal dementia demonstrated that tau dysfunction, in the absence of amyloid pathology, was sufficient to cause neuronal loss and clinical dementia. Abnormal levels and hyperphosphorylation of tau protein have been reported to be the underlying cause of a group of neurodegenerative disorders collectively known as 'tauopathies'. The detrimental consequence is the loss of affinity between this protein and the microtubules, increased production of fibrillary aggregates and the accumulation of insoluble intracellular neurofibrillary tangles. However, it has become clear in recent years that tau is not only a microtubule interacting protein, but rather has additional roles in cellular processes. This review focuses on emerging therapeutic strategies aimed at treating the underlying causes of the tau pathology in tauopathies and AD, including some novel approaches on the verge of providing new treatment paradigms within the coming years.

Specific serotonergic denervation affects tau pathology and cognition without altering senile plaques deposition in APP/PS1 mice

Senile plaques and neurofibrillary tangles are major neuropathological features of Alzheimer's Disease (AD), however neuronal loss is the alteration that best correlates with cognitive impairment in AD patients. Underlying neurotoxic mechanisms are not completely understood although specific neurotransmission deficiencies have been observed in AD patients and, in animal models, cholinergic and noradrenergic denervation may increase amyloid-beta deposition and tau phosphorylation in denervated areas. On the other hand brainstem neurodegeneration has been suggested as an initial event in AD, and serotonergic dysfunction, as well as reductions in raphe neurones density, have been reported in AD patients. In this study we addressed whether specific serotonergic denervation, by administering 5,7-dihydroxitriptamine (5,7-DHT) in the raphe nuclei, could also worsen central pathology in APPswe/PS1dE9 mice or interfere with learning and memory activities. In our hands specific serotonergic denervation increased tau phosphorylation in denervated cortex, without affecting amyloid-beta (Aβ) pathology. We also observed that APPswe/PS1dE9 mice lesioned with 5,7-DHT were impaired in the Morris water maze test, supporting a synergistic effect of the serotonergic denervation and the presence of APP/PS1 transgenes on learning and memory impairment. Altogether our data suggest that serotonergic denervation may interfere with some pathological aspects observed in AD, including tau phosphorylation or cognitive impairment, without affecting Aβ pathology, supporting a differential role of specific neurotransmitter systems in AD.

Alzheimer disease therapy--moving from amyloid-β to tau

Disease-modifying treatments for Alzheimer disease (AD) have focused mainly on reducing levels of amyloid-β (Aβ) in the brain. Some compounds have achieved this goal, but none has produced clinically meaningful results. Several methodological issues relating to clinical trials of these agents might explain this failure; an additional consideration is that the amyloid cascade hypothesis--which places amyloid plaques at the heart of AD pathogenesis--does not fully integrate a large body of data relevant to the emergence of clinical AD. Importantly, amyloid deposition is not strongly correlated with cognition in multivariate analyses, unlike hyperphosphorylated tau, neurofibrillary tangles, and synaptic and neuronal loss, which are closely associated with memory deficits. Targeting tau pathology, therefore, might be more clinically effective than Aβ-directed therapies. Furthermore, numerous immunization studies in animal models indicate that reduction of intracellular levels of tau and phosphorylated tau is possible, and is associated with improved cognitive performance. Several tau-related vaccines are in advanced preclinical stages and will soon enter clinical trials. In this article, we present a critical analysis of the failure of Aβ-directed therapies, discuss limitations of the amyloid cascade hypothesis, and suggest the potential value of tau-targeted therapy for AD.

What Renders TAU Toxic

TAU is a microtubule-associated protein that under pathological conditions such as Alzheimer's disease (AD) forms insoluble, filamentous aggregates. When 20 years after TAU's discovery the first TAU transgenic mouse models were established, one declared goal that was achieved was the modeling of authentic TAU aggregate formation in the form of neurofibrillary tangles. However, as we review here, it has become increasingly clear that TAU causes damage much before these filamentous aggregates develop. In fact, because TAU is a scaffolding protein, increased levels and an altered subcellular localization (due to an increased insolubility and impaired clearance) result in the interaction of TAU with cellular proteins with which it would otherwise either not interact or do so to a lesser degree, thereby impairing their physiological functions. We specifically discuss the non-axonal localization of TAU, the role phosphorylation has in TAU toxicity and how TAU impairs mitochondrial functions. A major emphasis is on what we have learned from the four available TAU knock-out models in mice, and the knock-out of the TAU/MAP2 homolog PTL-1 in worms. It has been proposed that in human pathological conditions such as AD, a rare toxic TAU species exists which needs to be specifically removed to abrogate TAU's toxicity and restore neuronal functions. However, what is toxic in one context may not be in another, and simply reducing, but not fully abolishing TAU levels may be sufficient to abrogate TAU toxicity.