In vivo functional brain mapping in a conditional mouse model of human tauopathy (taup301l) reveals reduced neural activity in memory formation structures
© Perez et al.; licensee BioMed Central Ltd. 2013
Received: 18 September 2012
Accepted: 18 January 2013
Published: 4 February 2013
Tauopathies are characterized by intracellular deposition of the microtubule-associated protein tau as filamentous aggregates. The rTg4510 mouse conditionally expresses mutant human tau protein in various forebrain areas under the Tet-off expression system. Mice develop neurofibrillary tangles, with significant neuronal loss and cognitive deficits by 6 months of age. Previous behavioral and biochemical work has linked the expression and aggregates of mutant tau to functional impairments. The present work used manganese-enhanced magnetic resonance imaging (MEMRI) to investigate basal levels of brain activity in the rTg4510 and control mice.
Our results show an unmistakable curtailment of neural activity in the amygdala and hippocampus, two regions known for their role in memory formation, but not the cortex, cerebellum, striatum and hypothalamus in tau expressing mice.
Behavioral impairments associated with changes in activity in these areas may correspond to age progressive mutant tauP301L-induced neurodegeneration.
KeywordsTauopathy Neurodegenerative disease Alzheimer’s disease rTg4510 Manganese enhanced MRI
Accumulation of intracellular neurofibrillary tangles (NFTs) consisting of microtubule-associated protein tau is a major hallmark of Alzheimer’s disease (AD) and related neurodegenerative diseases regarded as ‘tauopathies’ [1–3]. Findings of tau mutations in subjects affected by frontotemporal dementia and mutant tau expression systems, which lead to tau-positive inclusions, neuron loss and behavioral abnormalities in various animal models, have established a role of this protein in neurodegeneration [4–11].
The rTg4510 mouse line was specifically developed to model aspects of human tauopathy, with overexpression of human tau containing the P301L mutation that is associated with frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17-Tau). Expression of human tau in the rTg4510 mouse is controlled by the tetracycline transactivator (tTA) transgene under the Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα) promoter. This leads to selective tau expression from an otherwise transcriptionally inactive tau transgene . Mice develop robust intracellular deposition of tau protein in cortico-limbic areas, which is physiologically relevant to AD and other tauopathies, and they also show age-related forebrain atrophy.
The main pathological features of rTg4510 mice have been obtained as a result of postmortem analysis of brain tissue, except recent work employing multiphoton microscopical analysis [13, 14]. The advent of rodent magnetic resonance imaging methods can fill the need for investigating in vivo correlates of early signs of disease. Recent developments in the animal imaging have led to the use of the manganese ion (Mn2+) as a neuronal contrast agent that provides a useful tool for functional brain mapping at good spatial resolution [15, 16]. MEMRI thus provides a noninvasive approach for mapping neural activity in transgenic mice. Mn2+ is highly paramagnetic and enhances signal intensity in T1 weighted brain images [17, 18]. It enters neurons largely through voltage dependent calcium channels, and possibly also via vesicular reuptake mechanisms, which correlates well with changes in synaptic firing of neuronal populations [19–21]. Other MEMRI studies in rodents have shown Mn2+ signal intensity in various ROI, including the hippocampus, under basal conditions [17, 18]. Here, we performed in vivo brain MEMRI of rTg4510 mice and non-transgenic (nonTg) littermates. Mutant rTg4510 mice sustain significant tau-associated hippocampal atrophy at 6 months of age and above and this was anticipated to result in less Mn2+ signal within this region in basal un-stimulated conditions. We observed clear differences in basal neural activity between mutant tau expressing mice and controls, with the magnitude of activity being lower in rTg4510 mice. Our study extends previous applications of MEMRI in the study of axonal transport rates in transgenic mice by showing reduced neural activity in two structures involved in learning and memory [22–24].
rTg4510 mice develop NFTs and show significant loss of hippocampal neurons at around 5–6 months. Neuronal loss may be severe (~80%) by 8 months . These factors are likely to contribute to impairments observed in spatial navigation memory previously reported using the water maze test , a behavior that is associated with neuronal activity in the dorsal hippocampus. The present study used in vivo MEMRI to map brain activation in control and rTg4510 mice under baseline, non-stimulated conditions. We observed significantly lower signal intensity, which may reflect underlying reductions in neuronal firing in rTg4510 versus control littermates. This effect appeared selective for memory formation structures, such as the hippocampus and amygdala, and was not observed in other ROI such as the septum, cerebellum, striatum, midbrain and brainstem. The hippocampus and amygdala play important roles in learning and memory, with the hippocampal structures contributing significantly to declarative memory while the amygdala contributes to implicit or emotional memory formation. While there is growing evidence that progressive reductions in hippocampal activity lead to generalized memory impairments, the implications for the amygdala are new for the rTg4510 mouse. The amygdala, particularly the basolateral regions, develop NFT neuropathology, however, a mechanism in rTg4510 mice remains to be thoroughly investigated. Dickey et al. (2009) immunohistochemically stained for tau and found no staining in thalamus. Indirect effects of tau pathology (or tau expression) in other regions, such as the neocortex, may have contributed to the differences observed here. In addition, we observed increases in signal intensity (activity) in the striatum, an effect that might reflect some changes in disinhibitory process arising from cortical inputs. However, despite the lack of any evidence of neuronal loss in the striatum, this region does accumulate NFTs beginning around 5.5 months .
Progression of NFT pathology in rTg4510 mice has been considerably explored by several groups. Immunohistochemical analysis revealed that rTg4510 mice develop pre-tangle pathology at 2.5 M and fully formed NFTs at 4 M in the cortex and at 5.5 M in the hippocampus [12, 28]. NFT pathology in the hippocampal formation occurs in a distinctly staged sub regional pattern beginning from CA1 to CA3 and dentate gyrus . Biochemical analysis of the rTg4510 hippocampus confirmed that the progression of NFT formation was drastically increased from 4 months to 6 months of age [27, 29–31]. In agreement with these neuropathological observations, our in vivo MRI analysis from rTg4510 mice (5.5-6.3 M) showed significant reduction of both neural activity and volume in the hippocampus. We observed significant reduction of signal intensity in the dentate gyrus of rTg4510 mice. Because CA1 pyramidal neurons were significantly decreased at this age, other sub regions of the hippocampus may receive less signaling associated with neural activity. Similar to our present observations, Yang et al. (2011) showed increased reactive astrocytic gliosis and enlarged microglia in CA1 from 8 M male rTg4510. One interesting possibility may be the sequestering of Mn2+ in microglial cells occurs in rTg4510 CA1. This would either increase CA1 signal or pull away signal from the DG. An in-depth neurochemical investigation is warranted. An important follow up will be to control the expression of transgenic tau and the progression of NFT formation and/or neuronal loss through the administration of doxycycline and examine whether mice show unbalanced neural activity across hippocampal sub regions at different stages of disease progression.
A series of experiments using MEMRI have been carried out to examine axonal transport rates along the olfactory and optic tracts in transgenic mice for amyloid precursor protein (APP) expression. Using a fast temporal series of T1 weighted image acquisitions it was shown that there is an age progressive decline in olfactory tract axonal transport rates of Mn2+ that worsened after plaque formation. In a subsequent study it was demonstrated that loss of APP in knockout mice resulted in reduced axonal transport rates for Mn2+, which was recovered by over-expressing human wildtype tau . Similar deficits in axonal transport rates have been reported in regions of the visual system and hippocampal formation of APP knockout mice  and in the olfactory system of triple transgenic mice expressing APP and human presenilin . The above references illustrate the versatility of the MEMRI method. Localized in vivo treatment can be used to track not only global brain activity as in the present work, but also transport rates on selective regions of the CNS . Our present results are consistent with recent work published by Kimura et al. (2007) showing lower activity in hippocampal and surrounding parahippocampal areas of aged mice with hyperphosphorylated tau protein. Levels of signal intensity in the parahippocampal areas highly correlated with performance on a spatial learning task . In the latter study Mn2+ was provided to animals only 4 hours prior to imaging session and thus provides evidence of significant brain uptake of Mn2+ across the blood brain barrier of mice. This is supported by in vitro studies showing rapid uptake of Mn2+ in brain, which appears to be aided by both active and passive transport processes .
To our knowledge, this is the first MEMRI study examining neural activation in the rTg4510 mouse. Previous work on this transgenic mouse has focused on chemical and volumetric differences compared to controls. Both male and female mutant tau-expressing mice show reduced total brain volume which is largely associated with lower cortical and hippocampal volume . We observed this feature as well and also noted the previously reported widening of ventricles . This is consistent with postmortem tissue analysis in these same animals [12, 26]. Proton MR spectroscopy showed greater myoinositol concentrations in the hippocampus and thalamus of rTg4510 mice, and this is consistent with increased gliosis . Interestingly, N-acetylaspartate, which is often taken as a marker of neuronal function, was not different between controls and rTg4510 mice. We observe here that in vivo neural activity is reduced during baseline conditions in these animals. The previously reported lower number of neurons , which may lead to lower hippocampal volume , also may have accounted for the generally lower signal intensity in this region in rTg4510 mice. However, cortical neural activity was not different between control mice and rTg4510 mice despite the reduced volume in the latter. This could signify that the lower neural activity is due to reduced functional activity in memory structures. We also found that the reduction in hippocampal signal intensity is not uniform through the subanatomical layers of this paleocortical region. rTg4510 mice show a somewhat modest, but significant increase in signal intensity in the CA1 region. It is tempting to speculate on what such an effect may be associated with. One possibility could be due to compensatory changes in neuronal activity across the dorsal hippocampal areas; however, this would potentially result in maintenance of higher levels of performance on spatial navigation and memory tasks. Past data do not support such a contention at this time. One alternative might be linked to gliosis, which would be consistent with past work by Yang et al. (2012). Microglial cells migrating to this region might sequester Mn2+ to such an extent that they result in increased signal intensity. This would not be due to increased neuronal activity, but rather it would be in line with the pathogenicity observed with tau over-expression at this age of the mice. This is somewhat supported by the literature. Quinolic acid lesions of neural tissue result in greater levels of Mn2+ accumulation than in controls, thus supporting internalization of Mn2+ in both neurons and glial cells . In conclusion, we find that mice over expressing human mutant tau(P301L) show lower brain activity in selected regions involved in memory formation. The present novel findings are consistent with previous behavioral and biochemical studies in the rTg4510 mouse. Moreover, the methods used provide a novel approach to explore the specific relation between genetically-driven expression of pathogenic tau and functional changes over neurodegenerative disease progression.
The parental P301L tau responder line, parental tTA activator line, and the resultant F1 rTg4510 mice and littermates were generated and maintained as previously described . Mice were maintained on a standard diet lacking doxycycline to ensure that transgenic tau was expressed throughout the lifetime of the experimental animals (weights were 25-40 g at the time of imaging). All mice were kept in standard size mouse cages (29 x 18 x 13 cm; up to 5 per same sex groups) at 20-26°C on a daily 12 hour light–dark cycle (light on during 0700 – 1900) with ad libitum access to food and water. Mice kept under these conditions were handled twice per week. Animals were generated and cared for in accordance with the guidelines published in the Guide for the Care and Use of Laboratory Animals (8th Edition, 2011) and in adherence to the National Institutes of Health and the American Association for Laboratory Animal Science guidelines. All procedures involving live mice received prior approval from the Institutional Animal Care and Use Committee of the University of Florida.
Manganese chloride pretreatment
Manganese (II) chloride tetrahydrate was purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) and was dissolved in ddH2O and sterile filtered prior to administration. We first carried out pilot experiments to examine the optimal MnCl2 dose (20 and 70 mg/kg) that produced T1 weighted signal enhancement in the absence of motor impairments. These experiments were carried out on a separate cohort of animals at 5.3-6.3 months of age (M) and resulted in selection of the 70 mg/kg MnCl2 solution. This dose produced results similar to those reported in previous work . In order to map basal brain activity in rTg4150 mice (n=5; 5.5-6.3 M) and control nonTg littermates (n=5; 5.5-6.3 M), MnCl2 was administered at a dose of 70 mg/kg, i.p. 24 hrs prior to MRI scanning. Animals were handled every two weeks and acclimatized to the investigator providing the injections. After injections, mice were returned to their home cage and imaged the following day.
Magnetic resonance imaging
Images of anesthetized mice were collected on an actively shielded 4.7-Tesla Magnex Scientific MR scanner controlled by Agilent Technologies VnmrJ 3.1 console software. A 38-mm quadrature transmit/receive radiofrequency (RF) coil tuned to 200 MHz was used (Insight NeuroImaging Systems, LLC, Leominster, MA). Anesthesia was initially induced under 2.0-2.5% isoflurane (0.1 mL/min) delivered in 100% oxygen for 30–60 seconds and levels were then maintained between 1.0-1.25% throughout the entire setup and imaging session. Mice were placed prone on a custom-made plastic bed with a respiratory pad placed underneath the abdomen. They were covered with disposable paper towel to aid in preserving body warmth. Body temperatures were maintained using a warm air recirculation system that received feedback from a fiber optic thermocouple microprobe (SA Instruments, Inc., New York). Respiratory rates were monitored continuously and maintained between 20–30 beats per minute by adjusting isoflurane levels between 1–1.25%. Incisors were secured on the front end of the plastic bed to minimize motion. The front half of the bed was aligned and clamped inside the quad RF coil and placed inside the isocenter of the scanner. Images were acquired using a T1 -weighted 3D gradient echo sequence with the following parameters: repetition time (TR) = 35 ms, echo time (TE) = 12 ms, flip angle α = 50°, data matrix 256 X 1282 (along read X phase X slice directions), size 30 mm X 20 mm2 (resolution 200 X 156 μm2 along the same read X phase X slice directions). Total scan time per mouse was 57 minutes.
Data processing and statistical analysis
A pre-set threshold of Z ≥ 1 was selected based on a priori observation of individual datasets and a close inspection of their intensity distribution histograms. This corresponds to a threshold of 1 standard deviation above the mean. All voxels with z score values below this threshold were set to zero. The intensity histograms in Figure 1B show mean voxel counts for the entire brain of 5 rTg4510 mice and 5 control mice included in the study. Rescaling of intensity values based on each subjects’ signal intensity variance allowed consistent sampling of brain voxels showing increase signal across each subject. The voxels exceeding the threshold value of z =1 were considered in our statistical analysis as having higher signal intensities (exceeding 1 standard deviation above the mean signal intensity across the whole brain). The segmented ROIs in the digital atlas of the mouse brain included the neocortex, striatum, globus pallidus, olfactory bulb, hypothalamus, septum/basalforebrain, midbrain, brainstem, thalamus, hippocampus, amygdala and cerebellum. Upon analyzing all regions, the hippocampus showed major differences between rTg4510 and control mice. However, the digital atlas does not segment sub regions of the hippocampus. Therefore, a separate analysis was also carried out to examine in detail the volume of the hippocampus, which has previously been reported to be smaller in rTg4510 . For this analysis, dorsal hippocampal sub regions CA1 and DG were manually traced. Mean normalized signal intensity values for each ROI were compared using an unpaired two-tailed t-test (homoscedastic variances, α ≤ 0.05).
Formalin fixed brains were paraffin embedded and cut into sagittal (5 μm) sections. Standard immunohistochemical procedure was implemented with the Dako Universal Autostainer (Dako, Carpinteria, CA). Primary antibodies used CP13 (phosphorylated tau at Ser202, provided by Dr. P.Davies, Albert Einstein College of Medicine) and ionized calcium-binding adaptor molecule 1 (Iba-1, Wako Chemicals USA, Richmond, VA).
Region of interest
Conditional transgenic mouse for human mutant tau expression
Calcium/calmodulin kinase IIα
Manganese enhanced magnetic resonance imaging
Magnetic resonance imaging.
MF is supported by NIH grant DA019946 and a seed grant from the McKnight Brain Institute. NS is supported by NIH grant NS067127 and Thomas H. Maren Junior Investigator Fund from University of Florida. NS and JL are supported by the Center for Translational Research in Neurodegenerative Disease and the Department of Neuroscience at University of Florida. The authors thank Dr. Stephen J. Blackband for providing the mouse brain atlas. MRI data were obtained at the Advanced Magnetic Resonance Imaging and Spectroscopy (AMRIS) facility in the McKnight Brain Institute of the University of Florida.
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