Increased Human Wildtype Tau Attenuates Axonal Transport Deficits Caused by Loss of APP in Mouse Models

Manganese-Enhanced Magnetic Resonance Imaging: Application in Central Nervous System Diseases

Manganese-enhanced magnetic resonance imaging (MEMRI) relies on the strong paramagnetism of Mn2+. Mn2+ is a calcium ion analog and can enter excitable cells through voltage-gated calcium channels. Mn2+ can be transported along the axons of neurons via microtubule-based fast axonal transport. Based on these properties, MEMRI is used to describe neuroanatomical structures, monitor neural activity, and evaluate axonal transport rates. The application of MEMRI in preclinical animal models of central nervous system (CNS) diseases can provide more information for the study of disease mechanisms. In this article, we provide a brief review of MEMRI use in CNS diseases ranging from neurodegenerative diseases to brain injury and spinal cord injury.

Manganese Enhanced MRI for Use in Studying Neurodegenerative Diseases

MRI has been extensively used in neurodegenerative disorders, such as Alzheimer’s disease (AD), frontal-temporal dementia (FTD), mild cognitive impairment (MCI), Parkinson’s disease (PD), Huntington’s disease (HD) and amyotrophic lateral sclerosis (ALS). MRI is important for monitoring the neurodegenerative components in other diseases such as epilepsy, stroke and multiple sclerosis (MS). Manganese enhanced MRI (MEMRI) has been used in many preclinical studies to image anatomy and cytoarchitecture, to obtain functional information in areas of the brain and to study neuronal connections. This is due to Mn²⁺ ability to enter excitable cells through voltage gated calcium channels and be actively transported in an anterograde manner along axons and across synapses. The broad range of information obtained from MEMRI has led to the use of Mn²⁺ in many animal models of neurodegeneration which has supplied important insight into brain degeneration in preclinical studies. Here we provide a brief review of MEMRI use in neurodegenerative diseases and in diseases with neurodegenerative components in animal studies and discuss the potential translation of MEMRI to clinical use in the future.

Manganese-Enhanced Magnetic Resonance Imaging: Overview and Central Nervous System Applications With a Focus on Neurodegeneration

Manganese-enhanced magnetic resonance imaging (MEMRI) rose to prominence in the 1990s as a sensitive approach to high contrast imaging. Following the discovery of manganese conductance through calcium-permeable channels, MEMRI applications expanded to include functional imaging in the central nervous system (CNS) and other body systems. MEMRI has since been employed in the investigation of physiology in many animal models and in humans. Here, we review historical perspectives that follow the evolution of applied MRI research into MEMRI with particular focus on its potential toxicity. Furthermore, we discuss the more current in vivo investigative uses of MEMRI in CNS investigations and the brief but decorated clinical usage of chelated manganese compound mangafodipir in humans.

In Vivo Evaluation of Neuronal Transport in Murine Models of Neurodegeneration Using Manganese-Enhanced MRI

Manganese-enhanced MRI (MRI) is a technique that allows for a noninvasive in vivo estimation of neuronal transport. It relies on the physicochemical properties of manganese, which is both a calcium analogue being transported along neurons by active transport, and a paramagnetic compound that can be detected on conventional T1-weighted images. Here, we report a multi-session MEMRI protocol that helps establish time-dependent curves relating to neuronal transport along the olfactory tract over several days. The characterization of these curves via unbiased fitting enables us to infer objectively a set of three parameters (the rate of manganese transport from the maximum slope, the peak intensity, and the time to peak intensity). These parameters, measured previously in wild type mice during normal aging, have served as a baseline to demonstrate their significant sensitivity to pathogenic processes associated with Tau pathology. Importantly, the evaluation of these three parameters and their use as indicators can be extended to monitor any normal and pathogenic processes where neuronal transport is altered. This approach can be applied to characterize and quantify the effect of any neurological disease conditions on neuronal transport in animal models, together with the efficacy of potential therapies.

Preclinical Magnetic Resonance Imaging and Spectroscopy Studies of Memory, Aging, and Cognitive Decline

Neuroimaging provides for non-invasive evaluation of brain structure and activity and has been employed to suggest possible mechanisms for cognitive aging in humans. However, these imaging procedures have limits in terms of defining cellular and molecular mechanisms. In contrast, investigations of cognitive aging in animal models have mostly utilized techniques that have offered insight on synaptic, cellular, genetic, and epigenetic mechanisms affecting memory. Studies employing magnetic resonance imaging and spectroscopy (MRI and MRS, respectively) in animal models have emerged as an integrative set of techniques bridging localized cellular/molecular phenomenon and broader in vivo neural network alterations. MRI methods are remarkably suited to longitudinal tracking of cognitive function over extended periods permitting examination of the trajectory of structural or activity related changes. Combined with molecular and electrophysiological tools to selectively drive activity within specific brain regions, recent studies have begun to unlock the meaning of fMRI signals in terms of the role of neural plasticity and types of neural activity that generate the signals. The techniques provide a unique opportunity to causally determine how memory-relevant synaptic activity is processed and how memories may be distributed or reconsolidated over time. The present review summarizes research employing animal MRI and MRS in the study of brain function, structure, and biochemistry, with a particular focus on age-related cognitive decline.

Pharmocologic treatment with histone deacetylase 6 inhibitor (ACY-738) recovers Alzheimer's disease phenotype in amyloid precursor protein/presenilin 1 (APP/PS1) mice

Introduction

Current therapy for Alzheimer's disease (AD) focuses on delaying progression, illustrating the need for more effective therapeutic targets. Histone deacetylase 6 (HDAC6) modulates tubulin acetylation and has been implicated as an attractive target. HDAC6 is also elevated in postmortem tissue samples from patients. However, HDAC6 inhibitors have had limited success preclinically due to low blood-brain barrier penetration.

Method

We investigated a specific, potent HDAC6 inhibitor (ACY-738) in a mouse model of AD. We determined the effects of ACY-738 treatment on axonal transport, behavior, and pathology in amyloid precursor protein/presenilin 1 mice.

Results

We demonstrated improvements in in vivo axonal transport in two treatment groups as a result of ACY-738 brain levels. We also demonstrated recovery of short-term learning and memory deficits, hyperactivity, and modifications of tau and tubulin.

Discussion

Our findings implicate specific, targeted HDAC6 inhibitors as potential therapeutics and demonstrate that further investigations are warranted into effects of HDAC6 inhibitors in AD.

Global axonal transport rates are unaltered in htau mice in vivo

Microtubule-based axonal transport is believed to become globally disrupted in Alzheimer's disease in part due to alterations of tau expression or phosphorylation. We previously showed that axonal transport rates along retinal ganglion axons are unaffected by deletion of normal mouse tau or by overexpression of wild-type human tau. Here, we report that htau mice expressing 3-fold higher levels of human tau in the absence of mouse tau also display normal fast and slow transport kinetics despite the presence of abnormally hyperphosphorylated tau in some neurons. In addition, markers of slow transport (neurofilament light subunit) and fast transport (snap25) exhibit normal distributions along optic axons of these mice. These studies demonstrate that human tau overexpression, even when associated with a limited degree of tau pathology, does not necessarily impair general axonal transport function in vivo.

Age-related decline in white matter integrity in a mouse model of tauopathy: an in vivo diffusion tensor magnetic resonance imaging study

Elevated expression of human hyperphosphorylated tau is associated with neuronal loss and white matter (WM) pathology in Alzheimer's disease (AD) and related neurodegenerative disorders. Using in vivo diffusion tensor magnetic resonance imaging (DT-MRI) at 11.1 Tesla we measured age-related alterations in WM diffusion anisotropy indices in a mouse model of human tauopathy (rTg4510) and nontransgenic (nonTg) control mice at the age of 2.5, 4.5, and 8 months. Similar to previous DT-MRI studies in AD subjects, 8-month-old rTg4510 mice showed lower fractional anisotropy (FA) values in WM structures than nonTg. The low WM FA in rTg4510 mice was observed in the genu and splenium of the corpus callosum, anterior commissure, fimbria, and internal capsule and was associated with a higher radial diffusivity than nonTg. Interestingly, rTg4510 mice showed lower estimates for the mode of anisotropy than controls at 2.5 months suggesting that changes in this diffusivity metric are detectable at an early stage preceding severe tauopathy. Immunogold electron microscopy partly supports our diffusion tensor imaging findings. At the age of 4 months, rTg4510 mice show axonal tau inclusions and unmyelinated processes. At later ages (12 months and 14 months) we observed inclusions in myelin sheath, axons, and unmyelinated processes, and a "disorganized" pattern of myelinated fiber arrangement with enlarged inter-axonal spaces in rTg4510 but not in nonTg mice. Our data support a role for the progression of tau pathology in reduced WM integrity measured by DT-MRI. Further in vivo DT-MRI studies in the rTg4510 mouse should help better discern the detailed mechanisms of reduced FA and anisotropy mode, and the specific role of tau during neurodegeneration.

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.

The airbag problem-a potential culprit for bench-to-bedside translational efforts: relevance for Alzheimer's disease

For the last 20 years, the "amyloid cascade hypothesis" has dominated research aimed at understanding, preventing, and curing Alzheimer's disease (AD). During that time researchers have acquired an enormous amount of data and have been successful, more than 300 times, in curing the disease in animal model systems by treatments aimed at clearing amyloid deposits. However, to date similar strategies have not been successful in human AD patients. Hence, before rushing into further clinical trials with compounds that aim at lowering amyloid-beta (Aβ) levels in increasingly younger people, it would be of highest priority to re-assess the initial assumption that accumulation of Aβ in the brain is the primary pathological event driving AD. Here we question this assumption by highlighting experimental evidence in support of the alternative hypothesis suggesting that APP and Aβ are part of a neuronal stress/injury system, which is up-regulated to counteract inflammation/oxidative stress-associated neurodegeneration that could be triggered by a brain injury, chronic infections, or a systemic disease. In AD, this protective program may be overridden by genetic and other risk factors, or its maintenance may become dysregulated during aging. Here, we provide a hypothetical example of a hypothesis-driven correlation between car accidents and airbag release in analogy to the evolution of the amyloid focus and as a way to offer a potential explanation for the failure of the AD field to translate the success of amyloid-related therapeutic strategies in experimental models to the clinic.

Non-invasive, in vivo monitoring of neuronal transport impairment in a mouse model of tauopathy using MEMRI

The impairment of axonal transport by overexpression or hyperphosphorylation of tau is well documented for in vitro conditions; however, only a few studies on this phenomenon have been conducted in vivo, using invasive procedures, and with contradictory results. Here we used the non-invasive, Manganese-Enhanced Magnetic Resonance Imaging technique (MEMRI), to study for the first time a pure model of tauopathy, the JNPL3 transgenic mouse line, which overexpresses a mutated (P301L) form of the human tau protein. We show progressive impairment in neuronal transport as tauopathy advances. These findings are further supported by a significant correlation between the severity of the impairment in neuronal transport assessed by MEMRI, and the degree of abnormal tau assessed by histology. Unlike conventional techniques that focus on axonal transport measurement, MEMRI can provide a global analysis of neuronal transport, i.e. from dendrites to axons and at the macroscopic scale of fiber tracts. Neuronal transport impairment has been shown to be a key pathogenic process in Alzheimer's disease and numerous other neurodegenerative disorders. Hence, MEMRI provides a promising set of functional biomarkers to be used during preclinical trials to facilitate the selection of new drugs aimed at restoring neuronal transport in neurodegenerative diseases.

Impact of repeated topical-loaded manganese-enhanced MRI on the mouse visual system

PURPOSE

Optic nerve degeneration in diseases such as glaucoma and multiple sclerosis evolves in months to years. The use of Mn(2+)-Enhanced Magnetic Resonance Imaging (MEMRI) in a time-course study may provide new insights into the disease progression. Previously, we demonstrated the feasibility of using a topical administration for Mn(2+) delivery to the visual system. This study is to evaluate the impact of biweekly or monthly repeated Mn(2+) topical administration and the pH levels of the Mn(2+) solutions for MEMRI on the mouse visual pathway.

METHODS

Using groups of mice, the MEMRI with an acidic or pH neutralized 1 M MnCl(2) solution was performed. To evaluate the feasibility of repeated MEMRIs, topical-loaded MEMRI was conducted biweekly seven times or monthly three times. The enhancement of MEMRI in the visual system was quantified. After repeated MEMRIs, the corneas were examined by optical coherence tomography. The retinal ganglion cells (RGCs) and optic nerves were examined by histology.

RESULTS

All mice exhibited consistent enhancements along the visual system following repeated MEMRIs. The acidic Mn(2+) solution induced a greater MEMRI enhancement as compared with a neutral pH Mn(2+) solution. Significant 20% RGC loss was found after three biweekly Mn(2+) inductions, but no RGC loss was found after three monthly Mn(2+) treatments. The corneal thickness was found increased after seven biweekly topical-loaded MEMRI.

CONCLUSIONS

Acidic Mn(2+) solutions enhanced the uptake of Mn(2+) observed on the MEMRI. Increasing the time intervals of repeated Mn(2+) topical administration reduced the adverse effects caused by MEMRI.

Axonal transport and neurodegenerative disease: can we see the elephant?

Although it is well established that axonal transport defects are part of the initiation or progression of some neurodegenerative diseases, the precise role of these defects in disease development is poorly understood. Thus, in this article, rather than enumerate the already well-reviewed evidence that there are transport deficits in disease, I will focus on a discussion of two crucial and unanswered questions about the possible role of axonal transport defects in HD and AD. (1) Are alterations in axonal transport caused by changes in the normal function of proteins mutated or altered in HD and AD and/or do such alterations in transport occur as a result of the formation of toxic aggregates of peptides or proteins? (2) Do alterations in axonal transport contribute to the causes of HD and AD or are they early, or late, secondary consequences of other cellular defects caused by disease-induction?

Deficits in axonal transport in hippocampal-based circuitry and the visual pathway in APP knock-out animals witnessed by manganese enhanced MRI

Mounting evidence implicates axonal transport defects, typified by the presence of axonal varicosities with aberrant accumulations of cargo, as an early event in Alzheimer's disease (AD) pathogenesis. Work identifying amyloid precursor protein (APP) as a vesicular motor receptor for anterograde axonal transport further implicates axonal transport in AD. Manganese-enhanced MRI (MEMRI) detects axonal transport dynamics in preclinical studies. Here we pursue an understanding of the role of APP in axonal transport in the central nervous system by applying MEMRI to hippocampal circuitry and to the visual pathway in living mice homozygous for either wild type or a deletion in the APP gene (n=12 for each genotype). Following intra-ocular or stereotaxic hippocampal injection, we performed time-lapse MRI to detect Mn(2+) transport. Three dimensional whole brain datasets were compared on a voxel-wise basis using within-group pair-wise analysis. Quantification of transport to structures connected to injection sites via axonal fiber tracts was also performed. Histology confirmed consistent placement of hippocampal injections and no observable difference in glial-response to the injections. APP-/- mice had significantly reduced transport from the hippocampus to the septal nuclei and amygdala after 7h and reduced transport to the contralateral hippocampus after 25 h; axonal transport deficits in the APP-/- animals were also identified in the visual pathway. These data support a system-wide role for APP in axonal transport within the central nervous system and demonstrate the power of MEMRI for assessing neuronal circuitry involved in memory and learning.