Raphaëlle Caillierez

Targeting phospho-Ser422 by active Tau Immunotherapy in the THYTau22 mouse model: a suitable therapeutic approach.

Recent data indicate that Tau immunotherapy may be relevant for interfering with neurofibrillary degeneration in Alzheimer disease and related disorders referred to as Tauopathies. The key question for immunotherapy is the choice of the epitope to target. Abnormal phosphorylation is a well-described post-translational modification of Tau proteins and may be a good target. In the present study, we investigated the effects of active immunization against the pathological epitope phospho-Ser422 in the THY-Tau22 transgenic mouse model. Starting from 3-6 months of age, THY-Tau22 mice develop hippocampal neurofibrillary tangle-like inclusions and exhibit phosphorylation of Tau on several AD-relevant Tau epitopes. Three month-old THY-Tau22 mice were immunized with a peptide including the phosphoserine 422 residue while control mice received the adjuvant alone. A specific antibody response against the phospho-Ser422 epitope was observed. We noticed a decrease in insoluble Tau species (AT100- and pS422 immunoreactive) by both biochemical and immunohistochemical means correlated with a significant cognitive improvement using the Y-maze. This Tau immunotherapy may facilitate Tau clearance from the brain toward the periphery since, following immunization, an increase in Tau concentrations was observed in blood. Overall, the present work is, to our knowledge, the first one to demonstrate that active immunotherapy targeting a real pathological epitope such as phospho-Ser422 epitope is efficient. This immunotherapy allows for Tau clearance and improves cognitive deficits promoted by Tau pathology in a well-defined Tau transgenic model.

Tau phosphorylation and sevoflurane anesthesia: an association to postoperative cognitive impairment.

Background: There is a growing interest in the involvement of anesthetic agents in the etiology of postoperative cognitive dysfunction. Recent animal studies suggest that acute anesthesia induces transient hyperphosphorylation of tau, an effect essentially ascribed to hypothermia. The main aim of the present study was to investigate effects, in normothermic conditions, of acute or repeated exposure to sevoflurane, a halogenated anesthetic agent, on hippocampal tau phosphorylation and spatial memory in adult mice. Methods: 5 to 6-month-old C57Bl6/J mice were submitted to acute (1 h) or repeated (five exposures of 1h every month) anesthesia using 1.5 or 2.5% sevoflurane, in normothermic conditions. In the acute protocol, animals were sacrificed 1 and 24 h after exposure. In the chronic protocol, spatial memory was evaluated using the Morris water maze following the fourth exposure, and tau phosphorylation evaluated 1 month following the last exposure using bi- and mono-dimensional electrophoresis. Results: Acute sevoflurane anesthesia in normothermic conditions led to a significant dose-dependent and reversible hippocampal tau phosphorylation, 1 h following the end of exposure (P < 0.001). Conversely, repeated anesthesia led to persistent tau hyperphosphorylation and significant memory impairments, as seen in the retention phase of the Morris water maze in sevoflurane-anesthesized animals. These pathologic features may be related to the activation of both Akt and Erk pathways. Conclusions: The present study demonstrates, in mice, that sevoflurane exposure is associated with increased tau phosphorylation through specific kinases activation and spatial memory deficits. These data support a correlation between exposures to this anesthetic agent and cognitive decline.

Neuron-to-neuron wild-type Tau protein transfer through a trans-synaptic mechanism: relevance to sporadic tauopathies

BackgroundIn sporadic Tauopathies, neurofibrillary degeneration (NFD) is characterised by the intraneuronal aggregation of wild-type Tau proteins. In the human brain, the hierarchical pathways of this neurodegeneration have been well established in Alzheimer’s disease (AD) and other sporadic tauopathies such as argyrophilic grain disorder and progressive supranuclear palsy but the molecular and cellular mechanisms supporting this progression are yet not known. These pathways appear to be associated with the intercellular transmission of pathology, as recently suggested in Tau transgenic mice. However, these conclusions remain ill-defined due to a lack of toxicity data and difficulties associated with the use of mutant Tau.ResultsUsing a lentiviral-mediated rat model of hippocampal NFD, we demonstrated that wild-type human Tau protein is axonally transferred from ventral hippocampus neurons to connected secondary neurons even at distant brain areas such as olfactory and limbic systems indicating a trans-synaptic protein transfer. Using different immunological tools to follow phospho-Tau species, it was clear that Tau pathology generated using mutated Tau remains near the IS whereas it spreads much further using the wild-type one.ConclusionTaken together, these results support a novel mechanism for Tau protein transfer compared to previous reports based on transgenic models with mutant cDNA. It also demonstrates that mutant Tau proteins are not suitable for the development of experimental models helpful to validate therapeutic intervention interfering with Tau spreading.

Neurotoxicity and memory deficits induced by soluble low-molecular-weight amyloid-β1-42 oligomers are revealed in vivo by using a novel animal model.

Neuronal and synaptic degeneration are the best pathological correlates for memory decline in Alzheimers disease (AD). Although the accumulation of soluble low-molecular-weight amyloid-β (Aβ) oligomers has been suggested to trigger neurodegeneration in AD, animal models overexpressing or infused with Aβ lack neuronal loss at the onset of memory deficits. Using a novel in vivo approach, we found that repeated hippocampal injections of small soluble Aβ1–42 oligomers in awake, freely moving mice were able to induce marked neuronal loss, tau hyperphosphorylation, and deficits in hippocampus-dependent memory. The neurotoxicity of small Aβ1–42 species was observed in vivo as well as in vitro in association with increased caspase-3 activity and reduced levels of the NMDA receptor subunit NR2B. We found that the sequestering agent transthyretin is able to bind the toxic Aβ1–42 species and attenuated the loss of neurons and memory deficits. Our novel mouse model provides evidence that small, soluble Aβ1–42 oligomers are able to induce extensive neuronal loss in vivo and initiate a cascade of events that mimic the key neuropathological hallmarks of AD.

Beneficial effects of exercise in a transgenic mouse model of Alzheimer's disease-like Tau pathology.

Tau pathology is encountered in many neurodegenerative disorders known as tauopathies, including Alzheimers disease. Physical activity is a lifestyle factor affecting processes crucial for memory and synaptic plasticity. Whether long-term voluntary exercise has an impact on Tau pathology and its pathophysiological consequences is currently unknown. To address this question, we investigated the effects of long-term voluntary exercise in the THY-Tau22 transgenic model of Alzheimers disease-like Tau pathology, characterized by the progressive development of Tau pathology, cholinergic alterations and subsequent memory impairments. Three-month-old THY-Tau22 mice and wild-type littermates were assigned to standard housing or housing supplemented with a running wheel. After 9 months of exercise, mice were evaluated for memory performance and examined for hippocampal Tau pathology, cholinergic defects, inflammation and genes related to cholesterol metabolism. Exercise prevented memory alterations in THY-Tau22 mice. This was accompanied by a decrease in hippocampal Tau pathology and a prevention of the loss of expression of choline acetyltransferase within the medial septum. Whereas the expression of most cholesterol-related genes remained unchanged in the hippocampus of running THY-Tau22 mice, we observed a significant upregulation in mRNA levels of NPC1 and NPC2, genes involved in cholesterol trafficking from the lysosomes. Our data support the view that long-term voluntary physical exercise is an effective strategy capable of mitigating Tau pathology and its pathophysiological consequences.

A major role for Tau in neuronal DNA and RNA protection in vivo under physiological and hyperthermic conditions

Nucleic acid protection is a substantial challenge for neurons, which are continuously exposed to oxidative stress in the brain. Neurons require powerful mechanisms to protect DNA and RNA integrity and ensure their functionality and longevity. Beside its well known role in microtubule dynamics, we recently discovered that Tau is also a key nuclear player in the protection of neuronal genomic DNA integrity under reactive oxygen species (ROS)-inducing heat stress (HS) conditions in primary neuronal cultures. In this report, we analyzed the capacity of Tau to protect neuronal DNA integrity in vivo in adult mice under physiological and HS conditions. We designed an in vivo mouse model of hyperthermia/HS to induce a transient increase in ROS production in the brain. Comet and Terminal deoxyribonucleotidyltransferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL) assays demonstrated that Tau protected genomic DNA in adult cortical and hippocampal neurons in vivo under physiological conditions in wild-type (WT) and Tau-deficient (KO-Tau) mice. HS increased DNA breaks in KO-Tau neurons. Notably, KO-Tau hippocampal neurons in the CA1 subfield restored DNA integrity after HS more weakly than the dentate gyrus (DG) neurons. The formation of phosphorylated histone H2AX foci, a double-strand break marker, was observed in KO-Tau neurons only after HS, indicating that Tau deletion did not trigger similar DNA damage under physiological or HS conditions. Moreover, genomic DNA and cytoplasmic and nuclear RNA integrity were altered under HS in hippocampal neurons exhibiting Tau deficiency, which suggests that Tau also modulates RNA metabolism. Our results suggest that Tau alterations lead to a loss of its nucleic acid safeguarding functions and participate in the accumulation of DNA and RNA oxidative damage observed in the Alzheimer’s disease (AD) brain.

Detrimental Effects of Diet-Induced Obesity on τ Pathology Are Independent of Insulin Resistance in τ Transgenic Mice

The τ pathology found in Alzheimer disease (AD) is crucial in cognitive decline. Midlife development of obesity, a major risk factor of insulin resistance and type 2 diabetes, increases the risk of dementia and AD later in life. The impact of obesity on AD risk has been suggested to be related to central insulin resistance, secondary to peripheral insulin resistance. The effects of diet-induced obesity (DIO) on τ pathology remain unknown. In this study, we evaluated effects of a high-fat diet, given at an early pathological stage, in the THY-Tau22 transgenic mouse model of progressive AD-like τ pathology. We found that early and progressive obesity potentiated spatial learning deficits as well as hippocampal τ pathology at a later stage. Surprisingly, THY-Tau22 mice did not exhibit peripheral insulin resistance. Further, pathological worsening occurred while hippocampal insulin signaling was upregulated. Together, our data demonstrate that DIO worsens τ phosphorylation and learning abilities in τ transgenic mice independently from peripheral/central insulin resistance.

From tau phosphorylation to tau aggregation: what about neuronal death?

Tau pathology is characterized by intracellular aggregates of abnormally and hyperphosphorylated tau proteins. It is encountered in many neurodegenerative disorders, but also in aging. These neurodegenerative disorders are referred to as tauopathies. Comparative biochemistry of the tau aggregates shows that they differ in both tau isoform phosphorylation and content, which enables a molecular classification of tauopathies. In conditions of dementia, NFD (neurofibrillary degeneration) severity is correlated to cognitive impairment and is often considered as neuronal death. Using tau animal models, analysis of the kinetics of tau phosphorylation, aggregation and neuronal death in parallel to electrophysiological and behavioural parameters indicates a disconnection between cognition deficits and neuronal cell death. Tau phosphorylation and aggregation are early events followed by cognitive impairment. Neuronal death is not observed before the oldest ages. A sequence of events may be the formation of toxic phosphorylated tau species, their aggregation, the formation of neurofibrillary tangles (from pre-tangles to ghost tangles) and finally neuronal cell death. This sequence will last from 15 to 25 years and one can ask whether the aggregation of toxic phosphorylated tau species is a protection against cell death. Apoptosis takes 24 h, but NFD lasts for 24 years to finally kill the neuron or rather to protect it for more than 20 years. Altogether, these data suggest that NFD is a transient state before neuronal death and that therapeutic interventions are possible at that stage.

Ectosomes: a new mechanism for non-exosomal secretion of tau protein.

Tau is a microtubule-associated protein that aggregates in neurodegenerative disorders known as tauopathies. Recently, studies have suggested that Tau may be secreted and play a role in neural network signalling. However, once deregulated, secreted Tau may also participate in the spreading of Tau pathology in hierarchical pathways of neurodegeneration. The mechanisms underlying neuron-to-neuron Tau transfer are still unknown; given the known role of extra-cellular vesicles in cell-to-cell communication, we wondered whether these vesicles could carry secreted Tau. We found, among vesicles, that Tau is predominately secreted in ectosomes, which are plasma membrane-originating vesicles, and when it accumulates, the exosomal pathway is activated.

Hippocampal T cell infiltration promotes neuroinflammation and cognitive decline in a mouse model of tauopathy

Alzheimer’s disease is characterized by the combined presence of amyloid plaques and tau pathology, the latter being correlated with the progression of clinical symptoms. Neuroinflammatory changes are thought to be major contributors to Alzheimer’s disease pathophysiology, even if their precise role still remains largely debated. Notably, to what extent immune responses contribute to cognitive impairments promoted by tau pathology remains poorly understood. To address this question, we took advantage of the THY-Tau22 mouse model that progressively develops hippocampal tau pathology paralleling cognitive deficits and reappraised the interrelationship between tau pathology and brain immune responses. In addition to conventional astroglial and microglial responses, we identified a CD8-positive T cell infiltration in the hippocampus of tau transgenic mice associated with an early chemokine response, notably involving CCL3. Interestingly, CD8-positive lymphocyte infiltration was also observed in the cortex of patients exhibiting frontemporal dementia with P301L tau mutation. To gain insights into the functional involvement of T cell infiltration in the pathophysiological development of tauopathy in THY-Tau22 mice, we chronically depleted T cells using anti-CD3 antibody. Such anti-CD3 treatment prevented hippocampal T cell infiltration in tau transgenic animals and reverted spatial memory deficits, in absence of tau pathology modulation. Altogether, these data support an instrumental role of hippocampal T cell infiltration in tau-driven pathophysiology and cognitive impairments in Alzheimer’s disease and other tauopathies.