Yazi D. Ke

Dendritic Function of Tau Mediates Amyloid-β Toxicity in Alzheimer's Disease Mouse Models

Alzheimers disease (AD) is characterized by amyloid-beta (Abeta) and tau deposition in brain. It has emerged that Abeta toxicity is tau dependent, although mechanistically this link remains unclear. Here, we show that tau, known as axonal protein, has a dendritic function in postsynaptic targeting of the Src kinase Fyn, a substrate of which is the NMDA receptor (NR). Missorting of tau in transgenic mice expressing truncated tau (Deltatau) and absence of tau in tau(-/-) mice both disrupt postsynaptic targeting of Fyn. This uncouples NR-mediated excitotoxicity and hence mitigates Abeta toxicity. Deltatau expression and tau deficiency prevent memory deficits and improve survival in Abeta-forming APP23 mice, a model of AD. These deficits are also fully rescued with a peptide that uncouples the Fyn-mediated interaction of NR and PSD-95 in vivo. Our findings suggest that this dendritic role of tau confers Abeta toxicity at the postsynapse with direct implications for pathogenesis and treatment of AD.

Sodium selenate mitigates tau pathology, neurodegeneration, and functional deficits in Alzheimer's disease models.

Alzheimers disease (AD) brains are characterized by amyloid-β-containing plaques and hyperphosphorylated tau-containing neurofibrillary tangles (NFTs); however, in frontotemporal dementia, the tau pathology manifests in the absence of overt amyloid-β plaques. Therapeutic strategies so far have primarily been targeting amyloid-β, although those targeting tau are only slowly beginning to emerge. Here, we identify sodium selenate as a compound that reduces tau phosphorylation both in vitro and in vivo. Importantly, chronic oral treatment of two independent tau transgenic mouse strains with NFT pathology, P301L mutant pR5 and K369I mutant K3 mice, reduces tau hyperphosphorylation and completely abrogates NFT formation. Furthermore, treatment improves contextual memory and motor performance, and prevents neurodegeneration. As hyperphosphorylation of tau precedes NFT formation, the effect of selenate on tau phosphorylation was assessed in more detail, a process regulated by both kinases and phosphatases. A major phosphatase implicated in tau dephosphorylation is the serine/threonine-specific protein phosphatase 2A (PP2A) that is reduced in both levels and activity in the AD brain. We found that selenate stabilizes PP2A-tau complexes. Moreover, there was an absence of therapeutic effects in sodium selenate-treated tau transgenic mice that coexpress a dominant-negative mutant form of PP2A, suggesting a mediating role for PP2A. Taken together, sodium selenate mitigates tau pathology in several AD models, making it a promising lead compound for tau-targeted treatments of AD and related dementias.

Parkinsonism and impaired axonal transport in a mouse model of frontotemporal dementia

Frontotemporal dementia (FTD) is characterized by cognitive and behavioral changes and, in a significant subset of patients, Parkinsonism. Histopathologically, FTD frequently presents with tau-containing lesions, which in familial cases result from mutations in the MAPT gene encoding tau. Here we present a novel transgenic mouse strain (K3) that expresses human tau carrying the FTD mutation K369I. K3 mice develop a progressive histopathology that is reminiscent of that in human FTD with the K369I mutation. In addition, K3 mice show early-onset memory impairment and amyotrophy in the absence of overt neurodegeneration. Different from our previously generated tau transgenic strains, the K3 mice express the transgene in the substantia nigra (SN) and show an early-onset motor phenotype that reproduces Parkinsonism with tremor, bradykinesia, abnormal gait, and postural instability. Interestingly, motor performance of young, but not old, K3 mice improves upon L-dopa treatment, which bears similarities to Parkinsonism in FTD. The early-onset symptoms in the K3 mice are mechanistically related to selectively impaired anterograde axonal transport of distinct cargos, which precedes the loss of dopaminergic SN neurons that occurs in aged mice. The impaired axonal transport in SN neurons affects, among others, vesicles containing the dopamine-synthesizing enzyme tyrosine hydroxylase. Distinct modes of transport are also impaired in sciatic nerves, which may explain amyotrophy. Together, the K3 mice are a unique model of FTD-associated Parkinsonism, with pathomechanistic implications for the human pathologic process.

Neuronal MicroRNA Deregulation in Response to Alzheimer's Disease Amyloid-β

Normal brain development and function depends on microRNA (miRNA) networks to fine tune the balance between the transcriptome and proteome of the cell. These small non-coding RNA regulators are highly enriched in brain where they play key roles in neuronal development, plasticity and disease. In neurodegenerative disorders such as Alzheimers disease (AD), brain miRNA profiles are altered; thus miRNA dysfunction could be both a cause and a consequence of disease. Our study dissects the complexity of human AD pathology, and addresses the hypothesis that amyloid-β (Aβ) itself, a known causative factor of AD, causes neuronal miRNA deregulation, which could contribute to the pathomechanisms of AD. We used sensitive TaqMan low density miRNA arrays (TLDA) on murine primary hippocampal cultures to show that about half of all miRNAs tested were down-regulated in response to Aβ peptides. Time-course assays of neuronal Aβ treatments show that Aβ is in fact a powerful regulator of miRNA levels as the response of certain mature miRNAs is extremely rapid. Bioinformatic analysis predicts that the deregulated miRNAs are likely to affect target genes present in prominent neuronal pathways known to be disrupted in AD. Remarkably, we also found that the miRNA deregulation in hippocampal cultures was paralleled in vivo by a deregulation in the hippocampus of Aβ42-depositing APP23 mice, at the onset of Aβ plaque formation. In addition, the miRNA deregulation in hippocampal cultures and APP23 hippocampus overlaps with those obtained in human AD studies. Taken together, our findings suggest that neuronal miRNA deregulation in response to an insult by Aβ may be an important factor contributing to the cascade of events leading to AD.

Experimental diabetes mellitus exacerbates tau pathology in a transgenic mouse model of Alzheimer's disease.

Diabetes mellitus (DM) is characterized by hyperglycemia caused by a lack of insulin, insulin resistance, or both. There is increasing evidence that insulin also plays a role in Alzheimers disease (AD) as it is involved in the metabolism of β-amyloid (Aβ) and tau, two proteins that form Aβ plaques and neurofibrillary tangles (NFTs), respectively, the hallmark lesions in AD. Here, we examined the effects of experimental DM on a pre-existing tau pathology in the pR5 transgenic mouse strain that is characterized by NFTs. pR5 mice express P301L mutant human tau that is associated with dementia. Experimental DM was induced by administration of streptozotocin (STZ), which causes insulin deficiency. We determined phosphorylation of tau, using immunohistochemistry and Western blotting. Solubility of tau was determined upon extraction with sarkosyl and formic acid, and Gallyas silver staining was employed to reveal NFTs. Insulin depletion by STZ administration in six months-old non-transgenic mice causes increased tau phosphorylation, without its deposition or NFT formation. In contrast, in pR5 mice this results in massive deposition of hyperphosphorylated, insoluble tau. Furthermore, they develop a pronounced tau-histopathology, including NFTs at this early age, while the pathology in sham-treated pR5 mice is moderate. Whereas experimental DM did not result in deposition of hyperphosphorylated tau in non-transgenic mice, a predisposition to develop a tau pathology in young pR5 mice was both sufficient and necessary to exacerbate tau deposition and NFT formation. Hence, DM can accelerate onset and increase severity of disease in individuals with a predisposition to developing tau pathology.

Primary support cultures of hippocampal and substantia nigra neurons

Primary cultures of rat and murine hippocampal neurons are widely used to reveal cellular mechanisms in neurobiology. Their use is limited, as culturing at low density is often not possible or is dependent on sophisticated methods. Here we present a novel method for culturing embryonic (E16.5) murine hippocampal neurons, using a spatially separated ring of cortical neurons for neurotrophic support. This method allows long-term cultures at a very low cell density, and therefore, the study of single embryo preparations and isolated neurons. This method has been adopted for neurons from the substantia nigra (E16.5), with support from a ring of striatal neurons.

A decade of tau transgenic animal models and beyond

The first tau transgenic mouse model was established more than a decade ago. Since then, much has been learned about the role of tau in Alzheimer’s disease and related disorders. Animal models, both in vertebrates and invertebrates, were significantly improved and refined as a result of the identification of pathogenic mutations in Tau in human cases of frontotemporal dementia. They have been instrumental for dissecting the cross‐talk between tau and the second hallmark lesion of Alzheimer’s disease, the Aβ peptide‐containing amyloid plaque. We discuss how the tau models have been used to unravel the pathophysiology of Alzheimer’s disease, to search for disease modifiers and to develop novel treatment strategies. While tau has received less attention than Aβ, it is rapidly acquiring a more prominent position and the emerging view is one of a synergistic action of Aβ and tau in Alzheimer’s disease. Moreover, the existence of a number of neurodegenerative diseases with tau pathology in the absence of extracellular deposits underscores the relevance of research on tau.

Tau-Targeted Immunization Impedes Progression of Neurofibrillary Histopathology in Aged P301L Tau Transgenic Mice

In Alzheimers disease (AD) brains, the microtubule-associated protein tau and amyloid-β (Aβ) deposit as intracellular neurofibrillary tangles (NFTs) and extracellular plaques, respectively. Tau deposits are furthermore found in a significant number of frontotemporal dementia cases. These diseases are characterized by progressive neurodegeneration, the loss of intellectual capabilities and behavioral changes. Unfortunately, the currently available therapies are limited to symptomatic relief. While active immunization against Aβ has shown efficacy in both various AD mouse models and patients with AD, immunization against pathogenic tau has only recently been shown to prevent pathology in young tau transgenic mice. However, if translated to humans, diagnosis and treatment would be routinely done when symptoms are overt, meaning that the histopathological changes have already progressed. Therefore, we used active immunization to target pathogenic tau in 4, 8, and 18 months-old P301L tau transgenic pR5 mice that have an onset of NFT pathology at 6 months of age. In all age groups, NFT pathology was significantly reduced in treated compared to control pR5 mice. Similarly, phosphorylation of tau at pathological sites was reduced. In addition, increased astrocytosis was found in the oldest treated group. Taken together, our data suggests that tau-targeted immunization slows the progression of NFT pathology in mice, with practical implications for human patients.

Phosphorylated Tau Interacts with c-Jun N-terminal Kinase-interacting Protein 1 (JIP1) in Alzheimer Disease

In Alzheimer disease (AD) and frontotemporal dementia the microtubule-associated protein Tau becomes progressively hyperphosphorylated, eventually forming aggregates. However, how Tau dysfunction is associated with functional impairment is only partly understood, especially at early stages when Tau is mislocalized but has not yet formed aggregates. Impaired axonal transport has been proposed as a potential pathomechanism, based on cellular Tau models and Tau transgenic mice. We recently reported K369I mutant Tau transgenic K3 mice with axonal transport defects that suggested a cargo-selective impairment of kinesin-driven anterograde transport by Tau. Here, we show that kinesin motor complex formation is disturbed in the K3 mice. We show that under pathological conditions hyperphosphorylated Tau interacts with c-Jun N-terminal kinase- interacting protein 1 (JIP1), which is associated with the kinesin motor protein complex. As a result, transport of JIP1 into the axon is impaired, causing JIP1 to accumulate in the cell body. Because we found trapping of JIP1 and a pathological Tau/JIP1 interaction also in AD brain, this may have pathomechanistic implications in diseases with a Tau pathology. This is supported by JIP1 sequestration in the cell body of Tau-transfected primary neuronal cultures. The pathological Tau/JIP1 interaction requires phosphorylation of Tau, and Tau competes with the physiological binding of JIP1 to kinesin light chain. Because JIP1 is involved in regulating cargo binding to kinesin motors, our findings may, at least in part, explain how hyperphosphorylated Tau mediates impaired axonal transport in AD and frontotemporal dementia.

Site-specific phosphorylation of tau inhibits amyloid-β toxicity in Alzheimer’s mice

Tau phosphorylation—not all bad Alzheimers disease presents with amyloid-β (Aβ) plaques and tau tangles. The prevailing idea in the field is that Aβ induces phosphorylation of tau, which in turn mediates neuronal dysfunction. Working in Alzheimers disease mouse models, Ittner et al. found evidence for a protective role of tau in early Alzheimers disease. This protection involves specific tau phosphorylation at threonine 205 at the postsynapse. A protective role of phosphorylated tau in disease challenges the dogma that tau phosphorylation only mediates toxic processes. Science, this issue p. 904 Phosphorylation of tau at a specific site mitigates, rather than enhances, symptoms in a mouse model of Alzheimer’s disease. Amyloid-β (Aβ) toxicity in Alzheimer’s disease (AD) is considered to be mediated by phosphorylated tau protein. In contrast, we found that, at least in early disease, site-specific phosphorylation of tau inhibited Aβ toxicity. This specific tau phosphorylation was mediated by the neuronal p38 mitogen-activated protein kinase p38γ and interfered with postsynaptic excitotoxic signaling complexes engaged by Aβ. Accordingly, depletion of p38γ exacerbated neuronal circuit aberrations, cognitive deficits, and premature lethality in a mouse model of AD, whereas increasing the activity of p38γ abolished these deficits. Furthermore, mimicking site-specific tau phosphorylation alleviated Aβ-induced neuronal death and offered protection from excitotoxicity. Our work provides insights into postsynaptic processes in AD pathogenesis and challenges a purely pathogenic role of tau phosphorylation in neuronal toxicity.