The microtubule-associated protein tau is expressed throughout the nervous system, most highly in neurons but also in glial cells. Its functions in adult and aging mammals remain to be defined. Previous studies in mouse models found either protective or detrimental effects of genetic tau ablation. Though tau ablation prevented synaptic, network, and cognitive dysfunctions in several models of Alzheimer's disease and made mice more resistant to epileptic seizures, a recent study described a parkinsonian phenotype in aging Tau knockout mice. Here we tested cognition and motor functions in Tau(+/+), Tau(+/-), and Tau(-/-) mice at approximately 1 and 2 years of age. Tau ablation did not impair cognition and caused only minor motor deficits that were much more subtle than those associated with the aging process. Tau ablation caused a mild increase in body weight, which correlated with and might have contributed to some of the motor deficits. However, tau ablation did not cause significant dopaminergic impairments, and dopamine treatment did not improve the motor deficits, suggesting that they do not reflect extrapyramidal dysfunction.

In light of the rising prevalence of Alzheimer's disease (AD), new strategies to prevent, halt, and reverse this condition are needed urgently. Perturbations of brain network activity are observed in AD patients and in conditions that increase the risk of developing AD, suggesting that aberrant network activity might contribute to AD-related cognitive decline. Human amyloid precursor protein (hAPP) transgenic mice simulate key aspects of AD, including pathologically elevated levels of amyloid-β peptides in brain, aberrant neural network activity, remodeling of hippocampal circuits, synaptic deficits, and behavioral abnormalities. Whether these alterations are linked in a causal chain remains unknown. To explore whether hAPP/amyloid-β-induced aberrant network activity contributes to synaptic and cognitive deficits, we treated hAPP mice with different antiepileptic drugs. Among the drugs tested, only levetiracetam (LEV) effectively reduced abnormal spike activity detected by electroencephalography. Chronic treatment with LEV also reversed hippocampal remodeling, behavioral abnormalities, synaptic dysfunction, and deficits in learning and memory in hAPP mice. Our findings support the hypothesis that aberrant network activity contributes causally to synaptic and cognitive deficits in hAPP mice. LEV might also help ameliorate related abnormalities in people who have or are at risk for AD.

Hyperacetylation of tau has been implicated in neurodegeneration and cognitive decline in tauopathy brains. The nicotinamide adenosine dinucleotide-dependent class-III protein deacetylase SIRT1 is one of the major enzymes involved in removal of acetyl groups from tau in vitro However, whether SIRT1 regulates acetylation of pathogenic tau and ameliorates tau-mediated pathogenesis remains unclear. Here, we report deacetylating activity of SIRT1 for acetylated Lys174 (K174) of tau in tauP301S transgenic mice with a brain-specific SIRT1 deletion. We show that SIRT1 deficiency leads to exacerbation of premature mortality, synapse loss, and behavioral disinhibition in tauP301S transgenic mice of both sexes. By contrast, SIRT1 overexpression by stereotaxic delivery of adeno-associated virus that encodes SIRT1 into the hippocampus reduces acetylated K174 tau. Furthermore, SIRT1 overexpression significantly attenuates the spread of tau pathology into anatomically connected brain regions of tauP301S transgenic mice of both sexes. These findings suggest the functional importance of SIRT1 in regulating pathogenic tau acetylation and in suppressing the spread of tau pathology in vivoSIGNIFICANCE STATEMENT In neurodegenerative disorders with inclusions of microtubule-associated protein tau, aberrant lysine acetylation of tau plays critical roles in promoting tau accumulation and toxicity. Identifying strategies to deacetylate tau could interfere with disease progression; however, little is known about how pathogenic tau is deacetylated in vivo Here we show that the protein deacetylase SIRT1 reduces tau acetylation in a mouse model of neurodegeneration. SIRT1 deficiency in the brain aggravates synapse loss and behavioral disinhibition, and SIRT1 overexpression ameliorates propagation of tau pathology.

In light of the rising prevalence of Alzheimer's disease (AD), new strategies to prevent, halt, and reverse this condition are needed urgently. Perturbations of brain network activity are observed in AD patients and in conditions that increase the risk of developing AD, suggesting that aberrant network activity might contribute to AD-related cognitive decline. Human amyloid precursor protein (hAPP) transgenic mice simulate key aspects of AD, including pathologically elevated levels of amyloid-β peptides in brain, aberrant neural network activity, remodeling of hippocampal circuits, synaptic deficits, and behavioral abnormalities. Whether these alterations are linked in a causal chain remains unknown. To explore whether hAPP/amyloid-β-induced aberrant network activity contributes to synaptic and cognitive deficits, we treated hAPP mice with different antiepileptic drugs. Among the drugs tested, only levetiracetam (LEV) effectively reduced abnormal spike activity detected by electroencephalography. Chronic treatment with LEV also reversed hippocampal remodeling, behavioral abnormalities, synaptic dysfunction, and deficits in learning and memory in hAPP mice. Our findings support the hypothesis that aberrant network activity contributes causally to synaptic and cognitive deficits in hAPP mice. LEV might also help ameliorate related abnormalities in people who have or are at risk for AD.

Maintaining DNA integrity is vital for all cells and organisms. Defective DNA repair may contribute to neurological disorders, including Alzheimer's disease (AD). We found reduced levels of BRCA1, but not of other DNA repair factors, in the brains of AD patients and human amyloid precursor protein (hAPP) transgenic mice. Amyloid-β oligomers reduced BRCA1 levels in primary neuronal cultures. In wild-type mice, knocking down neuronal BRCA1 in the dentate gyrus caused increased DNA double-strand breaks, neuronal shrinkage, synaptic plasticity impairments, and learning and memory deficits, but not apoptosis. Low levels of hAPP/Amyloid-β overexpression exacerbated these effects. Physiological neuronal activation increased BRCA1 levels, whereas stimulating predominantly extrasynaptic N-methyl-D-aspartate receptors promoted the proteasomal degradation of BRCA1. We conclude that BRCA1 is regulated by neuronal activity, protects the neuronal genome, and critically supports neuronal integrity and cognitive functions. Pathological accumulation of Aβ depletes neuronal BRCA1, which may contribute to cognitive deficits in AD.

Astrocytes express a variety of G protein-coupled receptors and might influence cognitive functions, such as learning and memory. However, the roles of astrocytic Gs-coupled receptors in cognitive function are not known. We found that humans with Alzheimer's disease (AD) had increased levels of the Gs-coupled adenosine receptor A2A in astrocytes. Conditional genetic removal of these receptors enhanced long-term memory in young and aging mice and increased the levels of Arc (also known as Arg3.1), an immediate-early gene that is required for long-term memory. Chemogenetic activation of astrocytic Gs-coupled signaling reduced long-term memory in mice without affecting learning. Like humans with AD, aging mice expressing human amyloid precursor protein (hAPP) showed increased levels of astrocytic A2A receptors. Conditional genetic removal of these receptors enhanced memory in aging hAPP mice. Together, these findings establish a regulatory role for astrocytic Gs-coupled receptors in memory and suggest that AD-linked increases in astrocytic A2A receptor levels contribute to memory loss.

BACKGROUND: Although extensive research has demonstrated the importance of excitatory granule neurons in the dentate gyrus of the hippocampus in normal learning and memory and in the pathogenesis of amnesia in Alzheimers disease (AD), the role of hilar GABAergic inhibitory interneurons, which control the granule neuron activity, remains unclear. METHODOLOGY AND PRINCIPAL FINDINGS: We explored the function of hilar GABAergic interneurons in spatial learning and memory by inhibiting their activity through Cre-dependent viral expression of enhanced halorhodopsin (eNpHR3.0)--a light-driven chloride pump. Hilar GABAergic interneuron-specific expression of eNpHR3.0 was achieved by bilaterally injecting adeno-associated virus containing a double-floxed inverted open-reading frame encoding eNpHR3.0 into the hilus of the dentate gyrus of mice expressing Cre recombinase under the control of an enhancer specific for GABAergic interneurons. In vitro and in vivo illumination with a yellow laser elicited inhibition of hilar GABAergic interneurons and consequent activation of dentate granule neurons, without affecting pyramidal neurons in the CA3 and CA1 regions of the hippocampus. We found that optogenetic inhibition of hilar GABAergic interneuron activity impaired spatial learning and memory retrieval, without affecting memory retention, as determined in the Morris water maze test. Importantly, optogenetic inhibition of hilar GABAergic interneuron activity did not alter short-term working memory, motor coordination, or exploratory activity. CONCLUSIONS AND SIGNIFICANCE: Our findings establish a critical role for hilar GABAergic interneuron activity in controlling spatial learning and memory retrieval and provide evidence for the potential contribution of GABAergic interneuron impairment to the pathogenesis of amnesia in AD.

Aging is the predominant risk factor for neurodegenerative diseases. One key phenotype as the brain ages is an aberrant innate immune response characterized by proinflammation. However, the molecular mechanisms underlying aging-associated proinflammation are poorly defined. Whether chronic inflammation plays a causal role in cognitive decline in aging and neurodegeneration has not been established. Here we report a mechanistic link between chronic inflammation and aging microglia and a causal role of aging microglia in neurodegenerative cognitive deficits. We showed that SIRT1 is reduced with the aging of microglia and that microglial SIRT1 deficiency has a causative role in aging- or tau-mediated memory deficits via IL-1β upregulation in mice. Interestingly, the selective activation of IL-1β transcription by SIRT1 deficiency is likely mediated through hypomethylating the specific CpG sites on IL-1β proximal promoter. In humans, hypomethylation of IL-1β is strongly associated with chronological age and with elevated IL-1β transcription. Our findings reveal a novel epigenetic mechanism in aging microglia that contributes to cognitive deficits in aging and neurodegenerative diseases.

A major sex difference in Alzheimer's disease (AD) is that men with the disease die earlier than do women. In aging and preclinical AD, men also show more cognitive deficits. Here, we show that the X chromosome affects AD-related vulnerability in mice expressing the human amyloid precursor protein (hAPP), a model of AD. XY-hAPP mice genetically modified to develop testicles or ovaries showed worse mortality and deficits than did XX-hAPP mice with either gonad, indicating a sex chromosome effect. To dissect whether the absence of a second X chromosome or the presence of a Y chromosome conferred a disadvantage on male mice, we varied sex chromosome dosage. With or without a Y chromosome, hAPP mice with one X chromosome showed worse mortality and deficits than did those with two X chromosomes. Thus, adding a second X chromosome conferred resilience to XY males and XO females. In addition, the Y chromosome, its sex-determining region Y gene (Sry), or testicular development modified mortality in hAPP mice with one X chromosome such that XY males with testicles survived longer than did XY or XO females with ovaries. Furthermore, a second X chromosome conferred resilience potentially through the candidate gene Kdm6a, which does not undergo X-linked inactivation. In humans, genetic variation in KDM6A was linked to higher brain expression and associated with less cognitive decline in aging and preclinical AD, suggesting its relevance to human brain health. Our study suggests a potential role for sex chromosomes in modulating disease vulnerability related to AD.