Michiyo Iba

Synthetic Tau Fibrils Mediate Transmission of Neurofibrillary Tangles in a Transgenic Mouse Model of Alzheimer's-Like Tauopathy

Tauopathies, including Alzheimers disease (AD) and frontotemporal lobar degeneration with tau pathologies, are neurodegenerative diseases characterized by neurofibrillary tangles (NFTs) comprising filamentous tau protein. Although emerging evidence suggests that tau pathology may be transmitted, we demonstrate here that synthetic tau fibrils are sufficient to transmit tau inclusions in a mouse model. Specifically, intracerebral inoculation of young PS19 mice overexpressing mutant human tau (P301S) with synthetic preformed fibrils (pffs) assembled from recombinant full-length tau or truncated tau containing four microtubule binding repeats resulted in rapid induction of NFT-like inclusions that propagated from injected sites to connected brain regions in a time-dependent manner. Interestingly, injection of tau pffs into either hippocampus or striatum together with overlaying cortex gave rise to distinct pattern of spreading. Moreover, unlike tau pathology that spontaneously develops in old PS19 mice, the pff-induced tau inclusions more closely resembled AD NFTs because they were Thioflavin S positive, acetylated, and more resistant to proteinase K digestion. Together, our study demonstrates that synthetic tau pffs alone are capable of inducing authentic NFT-like tau aggregates and initiating spreading of tau pathology in a tauopathy mouse model.

The Microtubule-Stabilizing Agent, Epothilone D, Reduces Axonal Dysfunction, Neurotoxicity, Cognitive Deficits, and Alzheimer-Like Pathology in an Interventional Study with Aged Tau Transgenic Mice

Neurodegenerative tauopathies, such as Alzheimers disease (AD), are characterized by insoluble deposits of hyperphosphorylated tau protein within brain neurons. Increased phosphorylation and decreased solubility has been proposed to diminish normal tau stabilization of microtubules (MTs), thereby leading to neuronal dysfunction. Earlier studies have provided evidence that small molecule MT-stabilizing drugs that are used in the treatment of cancer may have utility in the treatment of tauopathies. However, it has not been established whether treatment with a small molecule MT-stabilizing compound will provide benefit in a transgenic model with pre-existing tau pathology, as would be seen in human patients with clinical symptoms. Accordingly, we describe here an interventional study of the brain-penetrant MT-stabilizing agent, epothilone D (EpoD), in aged PS19 mice with existing tau pathology and related behavioral deficits. EpoD treatment reduced axonal dystrophy and increased axonal MT density in the aged PS19 mice, which led to improved fast axonal transport and cognitive performance. Moreover, the EpoD-treated PS19 mice had less forebrain tau pathology and increased hippocampal neuronal integrity, with no dose-limiting side effects. These data reveal that brain-penetrant MT-stabilizing drugs hold promise for the treatment of AD and related tauopathies, and that EpoD could be a candidate for clinical testing.

Epothilone D Improves Microtubule Density, Axonal Integrity and Cognition in a Transgenic Mouse Model of Tauopathy

Neurons in the brains of those with Alzheimers disease (AD) and many frontotemporal dementias (FTDs) contain neurofibrillary tangles comprised of hyperphosphorylated tau protein. Tau normally stabilizes microtubules (MTs), and tau misfolding could lead to a loss of this function with consequent MT destabilization and neuronal dysfunction. Accordingly, a possible therapeutic strategy for AD and related “tauopathies” is treatment with a MT-stabilizing anti-cancer drug such as paclitaxel. However, paclitaxel and related taxanes have poor blood–brain barrier permeability and thus are unsuitable for diseases of the brain. We demonstrate here that the MT-stabilizing agent, epothilone D (EpoD), is brain-penetrant and we subsequently evaluated whether EpoD can compensate for tau loss-of-function in PS19 tau transgenic mice that develop forebrain tau inclusions, axonal degeneration and MT deficits. Treatment of 3-month-old male PS19 mice with low doses of EpoD once weekly for a 3 month period significantly improved CNS MT density and axonal integrity without inducing notable side-effects. Moreover, EpoD treatment reduced cognitive deficits that were observed in the PS19 mice. These results suggest that certain brain-penetrant MT-stabilizing agents might provide a viable therapeutic strategy for the treatment of AD and FTDs.

α-Synuclein Immunotherapy Blocks Uptake and Templated Propagation of Misfolded α-Synuclein and Neurodegeneration

SUMMARY Accumulation of misfolded alpha-synuclein (α-syn) into Lewy bodies (LBs) and Lewy neurites (LNs) is a major hallmark of Parkinson’s disease (PD) and dementia with LBs (DLB). Recent studies showed that synthetic preformed fibrils (pffs) recruit endogenous α-syn and induce LB/LN pathology in vitro and in vivo, thereby implicating propagation and cell-to-cell transmission of pathological α-syn as mechanisms for the progressive spread of LBs/LNs. Here, we demonstrate that α-syn monoclonal antibodies (mAbs) reduce α-syn pff-induced LB/LN formation and rescue synapse/neuron loss in primary neuronal cultures by preventing both pff uptake and subsequent cell-to-cell transmission of pathology. Moreover, intraperitoneal (i.p.) administration of mAb specific for misfolded α-syn into nontransgenic mice injected intrastriatally with α-syn pffs reduces LB/LN pathology, ameliorates substantia nigra dopaminergic neuron loss, and improves motor impairments. We conclude that α-syn antibodies could exert therapeutic effects in PD/DLB by blocking entry of pathological α-syn and/or its propagation in neurons.

Chronic Stress Exacerbates Tau Pathology, Neurodegeneration, and Cognitive Performance through a Corticotropin-Releasing Factor Receptor-Dependent Mechanism in a Transgenic Mouse Model of Tauopathy

Because overactivation of the hypothalamic–pituitary–adrenal (HPA) axis occurs in Alzheimers disease (AD), dysregulation of stress neuromediators may play a mechanistic role in the pathophysiology of AD. However, the effects of stress on tau phosphorylation are poorly understood, and the relationship between corticosterone and corticotropin-releasing factor (CRF) on both β-amyloid (Aβ) and tau pathology remain unclear. Therefore, we first established a model of chronic stress, which exacerbates Aβ accumulation in Tg2576 mice and then extended this stress paradigm to a tau transgenic mouse model with the P301S mutation (PS19) that displays tau hyperphosphorylation, insoluble tau inclusions and neurodegeneration. We show for the first time that both Tg2576 and PS19 mice demonstrate a heightened HPA stress profile in the unstressed state. In Tg2576 mice, 1 month of restraint/isolation (RI) stress increased Aβ levels, suppressed microglial activation, and worsened spatial and fear memory compared with nonstressed mice. In PS19 mice, RI stress promoted tau hyperphosphorylation, insoluble tau aggregation, neurodegeneration, and fear–memory impairments. These effects were not mimicked by chronic corticosterone administration but were prevented by pre-stress administration of a CRF receptor type 1 (CRF1) antagonist. The role for a CRF1-dependent mechanism was further supported by the finding that mice overexpressing CRF had increased hyperphosphorylated tau compared with wild-type littermates. Together, these results implicate HPA dysregulation in AD neuropathogenesis and suggest that prolonged stress may increase Aβ and tau hyperphosphorylation. These studies also implicate CRF in AD pathophysiology and suggest that pharmacological manipulation of this neuropeptide may be a potential therapeutic strategy for AD.

Aβ Accelerates the Spatiotemporal Progression of Tau Pathology and Augments Tau Amyloidosis in an Alzheimer Mouse Model

Senile plaques formed by β-amyloid peptides (Aβ) and neurofibrillary tangles (NFTs) formed by hyperphosphorylated tau, a microtubule-associated protein, are the hallmark lesions of Alzheimers disease (AD) in addition to loss of neurons. While several transgenic (Tg) mouse models have recapitulated aspects of AD-like Aβ and tau pathologies, a spatiotemporal mapping paradigm for progressive NFT accumulation is urgently needed to stage disease progression in AD mouse models. Braak and co-workers developed an effective and widely used NFT staging paradigm for human AD brains. The creation of a Braak-like spatiotemporal staging scheme for tau pathology in mouse models would facilitate mechanistic studies of AD-like tau pathology. Such a scheme would also enhance the reproducibility of preclinical AD therapeutic studies. Thus, we developed a novel murine model of Aβ and tau pathologies and devised a spatiotemporal scheme to stage the emergence and accumulation of NFTs with advancing age. Notably, the development of NFTs followed a spatiotemporal Braak-like pattern similar to that observed in authentic AD. More significantly, the presence of Aβ accelerated NFT formation and enhanced tau amyloidosis; however, tau pathology did not have the same effect on Aβ pathology. This novel NFT staging scheme provides new insights into the mechanisms of tau pathobiology, and we speculate that this scheme will prove useful for other basic and translational studies of AD mouse models.

P301S Mutant Human Tau Transgenic Mice Manifest Early Symptoms of Human Tauopathies with Dementia and Altered Sensorimotor Gating

Tauopathies are neurodegenerative disorders characterized by the accumulation of abnormal tau protein leading to cognitive and/or motor dysfunction. To understand the relationship between tau pathology and behavioral impairments, we comprehensively assessed behavioral abnormalities in a mouse tauopathy model expressing the human P301S mutant tau protein in the early stage of disease to detect its initial neurological manifestations. Behavioral abnormalities, shown by open field test, elevated plus-maze test, hot plate test, Y-maze test, Barnes maze test, Morris water maze test, and/or contextual fear conditioning test, recapitulated the neurological deficits of human tauopathies with dementia. Furthermore, we discovered that prepulse inhibition (PPI), a marker of sensorimotor gating, was enhanced in these animals concomitantly with initial neuropathological changes in associated brain regions. This finding provides evidence that our tauopathy mouse model displays neurofunctional abnormalities in prodromal stages of disease, since enhancement of PPI is characteristic of amnestic mild cognitive impairment, a transitional stage between normal aging and dementia such as Alzheimers disease (AD), in contrast with attenuated PPI in AD patients. Therefore, assessment of sensorimotor gating could be used to detect the earliest manifestations of tauopathies exemplified by prodromal AD, in which abnormal tau protein may play critical roles in the onset of neuronal dysfunctions.

In vivo measurement of glutamate loss is associated with synapse loss in a mouse model of tauopathy

Glutamate is the primary excitatory neurotransmitter in the brain, and is implicated in neurodegenerative diseases such as Alzheimers disease (AD) and several other tauopathies. The current method for measuring glutamate in vivo is proton magnetic resonance spectroscopy ((1)H MRS), although it has poor spatial resolution and weak sensitivity to glutamate changes. In this study, we sought to measure the effect of tau pathology on glutamate levels throughout the brain of a mouse model of tauopathy using a novel magnetic resonance imaging (MRI) technique. We employed glutamate chemical exchange saturation transfer (GluCEST) imaging, which has been previously validated as a complimentary method for measuring glutamate levels with several important advantages over conventional (1)H MRS. We hypothesized that the regional changes in glutamate levels would correlate with histological measurements of pathology including pathological tau, synapse and neuron loss. Imaging and spectroscopy were carried out on tau transgenic mice with the P301S mutation (PS19, n=9) and their wild-type littermates (WT, n=8), followed by immunohistochemistry of their brain tissue. GluCEST imaging resolution allowed for sub-hippocampal analysis of glutamate. Glutamate was significantly decreased by 29% in the CA sub-region of the PS19 hippocampus, and by 15% in the thalamus, where synapse loss was also measured. Glutamate levels and synapse density remained high in the dentate gyrus sub-region of the hippocampus, where neurogenesis is known to occur. The further development of GluCEST imaging for preclinical applications will be valuable, as therapies are being tested in mouse models of tauopathy.

Development of Multidimensional Representations of Task Phases in the Lateral Prefrontal Cortex

The temporal structuring of multiple events is essential for the purposeful regulation of behavior. We investigated the role of the lateral prefrontal cortex (LPFC) in transforming external signals of multiple sensory modalities into information suitable for monitoring successive events across behavioral phases until an intended action is prompted and then initiated. We trained monkeys to receive a succession of 1 s visual, auditory, or tactile sensory signals separated by variable intervals and to then release a key as soon as the fourth signal appeared. Thus, the animals had to monitor and update information about the progress of the task upon receiving each signal preceding the key release in response to the fourth signal. We found that the initial, short-latency responses of LPFC neurons reflected primarily the sensory modality, rather than the phase or progress of the task. However, a task phase-selective response developed within 500 ms of signal reception, and information about the task phase was maintained throughout the presentation of successive cues. The task phase-selective activity was updated with the appearance of each cue until the planned action was initiated. The phase-selective activity of individual neurons reflected not merely a particular phase of the task but also multiple successive phases. Furthermore, we found combined representations of task phase and sensory modality in the activity of individual LPFC neurons. These properties suggest how information representing multiple phases of behavioral events develops in the LPFC to provide a basis for the temporal regulation of behavior.

Longitudinal imaging reveals subhippocampal dynamics in glutamate levels associated with histopathologic events in a mouse model of tauopathy and healthy mice

Tauopathies are neurodegenerative disorders characterized by abnormal intracellular aggregates of tau protein, and include Alzheimers disease, corticobasal degeneration, frontotemporal dementia, and traumatic brain injury. Glutamate metabolism is altered in neurodegenerative disorders manifesting in higher or lower concentrations of glutamate, its transporters or receptors. Previously, glutamate chemical exchange saturation transfer (GluCEST) magnetic resonance imaging (MRI) demonstrated that glutamate levels are reduced in regions of synapse loss in the hippocampus of a mouse model of late‐stage tauopathy. We performed a longitudinal GluCEST imaging experiment paired with a cross‐sectional study of histologic markers of tauopathy to determine whether (1) early GluCEST changes are associated with synapse loss before volume loss occurs in the hippocampus, and whether (2) subhippocampal dynamics in GluCEST are associated with histopathologic events related to glutamate alterations in tauopathy. Live imaging of the hippocampus in three serial slices was performed without exogenous contrast agents, and subregions were segmented based on a k‐means cluster model. Subregions of the hippocampus were analyzed (cornu ammonis CA1, CA3, dentate gyrus DG, and ventricle) in order to associate local MRI‐observable changes in glutamate with histological measures of glial cell proliferation (GFAP), synapse density (synaptophysin, VGlut1) and glutamate receptor (NMDA–NR1) levels. Early differences in GluCEST between healthy and tauopathy mice were measured in the CA1 and DG subregions (30% reduction, P ≤ 0.001). Synapse density was also significantly reduced in every subregion of the hippocampus in tauopathy mice by 6 months. Volume was not significantly reduced in any subregion until 13 months. Further, a gradient in glutamate levels was observed in vivo along hippocampal axes that became polarized as tauopathy progressed. Dynamics in hippocampal glutamate levels throughout lifetime were most closely correlated with combined changes in synaptophysin and GFAP, indicating that GluCEST imaging may be a surrogate marker of glutamate concentration in glial cells and at the synaptic level.