Tau is a microtubule-associated protein that becomes dysregulated in a group of neurodegenerative diseases called tauopathies. Differential tau isoforms, expression levels, promoters, and disruption of endogenous genes in transgenic mouse models of tauopathy make it difficult to draw definitive conclusions about the biological role of tau in these models. We addressed this shortcoming by characterizing the molecular and cognitive phenotypes associated with the pathogenic P301L tau mutation (rT2 mice) in relation to a genetically matched transgenic mouse overexpressing nonmutant (NM) 4-repeat (4R) human tau (rT1 mice). Both male and female mice were included in this study. Unexpectedly, we found that 4R NM human tau (hTau) exhibited abnormal dynamics in young mice that were lost with the P301L mutation, including elevated protein stability and hyperphosphorylation, which were associated with cognitive impairment in 5-month-old rT1 mice. Hyperphosphorylation of NM hTau was observed as early as 4 weeks of age, and transgene suppression for the first 4 or 12 weeks of life prevented abnormal molecular and cognitive phenotypes in rT1, demonstrating that NM hTau pathogenicity is specific to postnatal development. We also show that NM hTau exhibits stronger binding to microtubules than P301L hTau, and is associated with mitochondrial abnormalities. Overall, our genetically matched mice have revealed that 4R NM hTau overexpression is pathogenic in a manner distinct from classical aging-related tauopathy, underlining the importance of assaying the effects of transgenic disease-related proteins at appropriate stages in life.

Abnormal intraneuronal accumulation of the presynaptic protein α-synuclein (α-syn) is implicated in the etiology of dementia with Lewy bodies (DLB) and Parkinson's disease with dementia (PDD). Recent work revealed that mice expressing human α-syn with the alanine-53-threonine (A53T) mutation have a similar phenotype to the human condition, exhibiting long-term potentiation deficits, learning and memory deficits, and inhibitory hippocampal remodeling, all of which were reversed by genetic ablation of microtubule-associated protein tau. Significantly, memory deficits were associated with histological signs of network hyperactivity/seizures. Electrophysiological abnormalities are often seen in parkinsonian dementias. Baseline electroencephalogram (EEG) slowing is used as a supportive diagnostic feature in DLB and PDD, and patients with these diseases may exhibit indicators of broad network dysfunction such as sleep dysregulation, myoclonus, and seizures. Given the translational significance, we examined whether human A53T α-syn expressing mice exhibit endogenous-tau-dependent EEG abnormalities, as measured with epidural electrodes over the frontal and parietal cortices. Using template-based waveform sorting, we determined that A53T mice have significantly high numbers of epileptiform events as early as 3-4 months of age and throughout life, and this effect is markedly attenuated in the absence of tau. Epileptic myoclonus occurred in half of A53T mice and was markedly reduced by tau ablation. In spectral analysis, tau ablation partially reduced EEG slowing in 6-7 month transgenic mice. We found abnormal sleeping patterns in transgenic mice that were more pronounced in older groups, but did not find evidence that this was influenced by tau genotype. Together, these data support the notion that tau facilitates A53T α-syn-induced hyperexcitability that both precedes and coincides with associated synaptic, cognitive, and behavioral effects. Tau also contributes to some aspects of EEG slowing in A53T mice. Importantly, our work supports tau-based approaches as an effective early intervention in α-synucleinopathies to treat aberrant network activity.

Synaptic and cognitive deficits mediated by a severe reduction in excitatory neurotransmission caused by a disproportionate accumulation of the neuronal protein tau in dendritic spines is a fundamental mechanism that has been found repeatedly in models of tauopathies, including Alzheimer’s disease, Lewy body dementia, frontotemporal dementia, and traumatic brain injury. Synapses thus damaged may contribute to dementia, among the most feared cause of debilitation in the elderly, and currently there are no treatments to repair them. Caspase-2 (Casp2) is an essential component of this pathological cascade. Although it is believed that Casp2 exerts its effects by hydrolyzing tau at aspartate-314, forming Δtau314, it is also possible that a noncatalytic mechanism is involved because catalytically dead Casp2 is biologically active in at least one relevant cellular pathway, that is, autophagy. To decipher whether the pathological effects of Casp2 on synaptic function are due to its catalytic or noncatalytic properties, we discovered and characterized a new Casp2 inhibitor, compound 1 [pK i (Casp2) = 8.12], which is 123-fold selective versus Casp3 and >2000-fold selective versus Casp1, Casp6, Casp7, and Casp9. In an in vitro assay based on Casp2-mediated cleavage of tau, compound 1 blocked the production of Δtau314. Importantly, compound 1 prevented tau from accumulating excessively in dendritic spines and rescued excitatory neurotransmission in cultured primary rat hippocampal neurons expressing the P301S tau variant linked to FTDP-17, a familial tauopathy. These results support the further development of small-molecule Casp2 inhibitors to treat synaptic deficits in tauopathies.

BackgroundPatients with Alzheimer's disease (AD) are at risk for seizures and accelerated cognitive decline. Mechanisms of seizures and related synaptic dysfunction in AD are active areas of investigation. Alterations in ion channels have been identified in transgenic mouse models of AD, but levels of voltage‐gated potassium channels (VGKCs) have not been fully explored. In particular, little is known about Kv1.1 channels in AD. Mutations in Kv1.1 or autoantibodies against Kv1.1 cause neuronal overexcitation in several human diseases, including episodic ataxia type 1, epilepsy, myokymia, and limbic encephalitis, indicating that Kv1.1 is critical for regulating neuronal excitability.MethodTo explore VGKCs in AD, we used hAPP‐J20 mice ages 4‐6 months and determined mRNA and protein levels of four highly expressed VGKCs ‐ Kv1.1, Kv1.2, Kv1.4, and Kv4.2 ‐ in dentate gyrus, entorhinal cortex, motor cortex, and somatosensory cortex (SSC).ResultUsing RT‐qPCR, we identified reductions in Kv1.1 mRNA in the SSC of hAPP‐J20 mice. Western blotting revealed reductions in the Kv1.1 channel in the SSC of hAPP‐J20 mice and human parietal cortex in mild and moderate stages of AD. Immunolabeling revealed that the reductions in Kv1.1 channels in the SSC occurred in pyramidal cells and GABAergic interneurons. Kv1.1 levels in the SSC were lower in hAPP‐J20 mice, which overexpress mutant hAPP and have high Aβ(1‐42) levels, than in the wild‐type hAPP line I5, suggesting some dependence on Aβ(1‐42) levels. Levetiracetam treatment (75 mg/kg/day) administered for 28 days did not affect Kv1.1 levels in hAPP‐J20 mice. To assess Kv1.1 depletion in an AD model, we crossed Kv1.1 heterozygous mice (Kv1.1 +/‐) with hAPP‐J9 mice, which have lower expression of mutant hAPP than J20 mice. We performed behavioral tests on mice ages 4‐9 months. While Kv1.1 +/‐ mice and hAPP‐J9 mice had normal survival, the double mutant (Kv1.1 +/‐ hAPP‐J9) mice had premature mortality and impairments in elevated plus maze and light‐dark box tests (n=16‐25 mice/genotype) of anxiety.ConclusionThese data indicate that elevated Aβ(1‐42) levels may act synergistically with Kv1.1 depletion to exacerbate cognitive deterioration in AD. Ongoing investigation will further characterize the relationships between Kv1.1 loss and AD‐related hyperexcitability.

BackgroundAbnormal α‐synuclein (αS) plays an important role in the pathology of Parkinson’s disease dementia (PDD) and dementia with Lewy bodies (DLB). New evidence shows network hyperexcitability can accelerate synaptic and cognitive deficits in PDD and DLB. While 90% of epileptiform activity is detected during sleep in Alzheimer’s disease (AD), epileptiform activity during sleep in PDD or DLB is poorly understood. There are currently no known treatments to delay PDD or DLB progression. Although antiseizure drugs (ASDs) have been used to improve memory deficits in AD, this treatment approach has not been investigated in α‐synucleinopathies.MethodsTo explore the effect of ASDs in attenuating αS‐dependent seizure activity, we used transgenic mice expressing the A53T mutant human α‐synuclein (TgA53T) as a model of motor and cognitive decline in PDD and DLB. We have begun screening FDA‐approved ASDs using cortical electroencephalography with electromyography to examine epileptic events 24 hours before and after intraperitoneal injection. The two ASDs exhibiting the strongest efficacy in reducing epileptic activity across sleep‐wake states will be tested for effects on cognitive function and synaptic plasticity following chronic administration. We are also quantifying pathological high frequency network oscillations (HFOs), biomarkers of epileptogenesis.Results3‐4‐month‐old TgA53T mice experienced epileptiform events but had normal long‐term potentiation (LTP) and memory, indicating that epileptic activity onset precedes αS‐induced synaptic and cognitive deficits. Starting at 6 months of age, TgA53T hippocampal slices show the absence of LTP, with reduced c‐fos and calbindin and increased neuropeptide Y indicating alterations in hippocampal inhibitory circuitry. In behavioral testing, TgA53T mice exhibit impairment in fear conditioning, Y‐maze spatial recognition, and Barnes maze spatial learning and memory with age. Preliminary studies indicate lacosamide and brivaracetam as the most effective of ASDs tested in reducing epileptic myoclonus and interictal epileptiform events. We also found more HFOs in TgA53T mice and TgA53T mice with tau ablation.ConclusionSuppression of epileptic activity by ASDs would support ASDs as a novel treatment for reducing or preventing cognitive dysfunction in DLB and PDD. Furthermore, ASD‐induced reversal of synaptic and cognitive deficits would provide evidence that epileptic activity plays a causal role in αS‐dependent pathology.