Tau overexpression impacts a neuroinflammation gene expression network perturbed in Alzheimer's disease.

Filamentous inclusions of the microtubule-associated protein, tau, define a variety of neurodegenerative diseases known as tauopathies, including Alzheimer's disease (AD). To better understand the role of tau-mediated effects on pathophysiology and global central nervous system function, we extensively characterized gene expression, pathology and behavior of the rTg4510 mouse model, which overexpresses a mutant form of human tau that causes Frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). We found that the most predominantly altered gene expression pathways in rTg4510 mice were in inflammatory processes. These results closely matched the causal immune function and microglial gene-regulatory network recently identified in AD. We identified additional gene expression changes by laser microdissecting specific regions of the hippocampus, which highlighted alterations in neuronal network activity. Expression of inflammatory genes and markers of neuronal activity changed as a function of age in rTg4510 mice and coincided with behavioral deficits. Inflammatory changes were tau-dependent, as they were reversed by suppression of the tau transgene. Our results suggest that the alterations in microglial phenotypes that appear to contribute to the pathogenesis of Alzheimer's disease may be driven by tau dysfunction, in addition to the direct effects of beta-amyloid

Passive immunization with phospho-tau antibodies reduces tau pathology and functional deficits in two distinct mouse tauopathy models.

In Alzheimer's disease (AD), an extensive accumulation of extracellular amyloid plaques and intraneuronal tau tangles, along with neuronal loss, is evident in distinct brain regions. Staging of tau pathology by postmortem analysis of AD subjects suggests a sequence of initiation and subsequent spread of neurofibrillary tau tangles along defined brain anatomical pathways. Further, the severity of cognitive deficits correlates with the degree and extent of tau pathology. In this study, we demonstrate that phospho-tau (p-tau) antibodies, PHF6 and PHF13, can prevent the induction of tau pathology in primary neuron cultures. The impact of passive immunotherapy on the formation and spread of tau pathology, as well as functional deficits, was subsequently evaluated with these antibodies in two distinct transgenic mouse tauopathy models. The rTg4510 transgenic mouse is characterized by inducible over-expression of P301L mutant tau, and exhibits robust age-dependent brain tau pathology. Systemic treatment with PHF6 and PHF13 from 3 to 6 months of age led to a significant decline in brain and CSF p-tau levels. In a second model, injection of preformed tau fibrils (PFFs) comprised of recombinant tau protein encompassing the microtubule-repeat domains into the cortex and hippocampus of young P301S mutant tau over-expressing mice (PS19) led to robust tau pathology on the ipsilateral side with evidence of spread to distant sites, including the contralateral hippocampus and bilateral entorhinal cortex 4 weeks post-injection. Systemic treatment with PHF13 led to a significant decline in the spread of tau pathology in this model. The reduction in tau species after p-tau antibody treatment was associated with an improvement in novel-object recognition memory test in both models. These studies provide evidence supporting the use of tau immunotherapy as a potential treatment option for AD and other tauopathies

Heirarchical clustering of age-dependent gene expression changes in rTg4510 mice.

<p>Heirarchical clustering of the 165 probe sets that showed age-dependent changes in rTg4510 mice (A) using all ages and genotyopes analysed, or (B) only 6.1 month old animals, illustrates probe sets that were either downregulated or upregulated with age in rTg4510 animals. Standardization was achieved by shifting the expression values to a mean of zero and scaling to a standard deviation of one.</p

Spatial working memory and object recognition memory in rTg4510 mice.

<p>(A) At 2 and 4 months of age, rTg4510 displayed cognitive deficits in spatial working memory compared to DN (1-way ANOVA followed by Tukey’s post-hoc ***p<0.001; student’s t-test *p<0.05). By 6 months of age, spatial working memory was confounded by stereotypic behavior observed in rTg4510 mice. n = 10–12/group. (B) At 2 months of age, all 3 genotypes displayed a preference for the novel object over the familiar object (2-way ANOVA, Bonferroni’s post-hoc **p<0.01, ***p<0.001). By 4 months of age, there was no significant difference in the amount of time rTg4510 mice explored the novel object compared to the familiar object, and the percent of time that rTg4510 explored the novel object was significantly less than the time spent by DN animals (2-way ANOVA, Bonferroni’s post-hoc *p<0.05, ***p<0.001). By 6 months of age, both rTg4510 and tTA animals explored the novel object less than DN animals (2-way ANOVA, Bonferroni’s post-hoc *p<0.05, ***p<0.001). n = 12–15/group. Error bars indicate SEM.</p

Inflammatory genes upregulated in the rTg4510 frontal cortex.

<p><i>C4b</i>, <i>Gfap</i> and <i>Spp1</i> mRNA expression levels were higher in rTg4510 animals compared to all other genotypes in the frontal cortex, as determined by qRT-PCR. mRNA expression levels are all normalized to tTA. **p<0.01, ***p<0.001 compared to tTA using the Dunnett multiple comparison test. Error bars indicate SEM.</p

Overview of gene expression changes identified in microdissected hippocampal subfields.

<p>(A) Venn diagram of number of differentially expressed genes in the CA1, CA3 and DG microdissected samples. Most of the gene expression changes were observed in the CA1 region, with only modest overlap with the other two regions. (B) Venn diagram of the number of differentially expressed genes in the hippocampal subfields with the whole hippocampus.</p

Upregulation of the complement system in rTg4510 mice.

<p>The complement cascade, shown here adapted from the IPA library, is the most significant pathway overrepresented by genes with age-dependent changes in expression in rTg4510 hippocampus. All gene expression changes in this pathway were increases in expression. The degree of shading in red corresponds to the amount of upregulation in rTg4510 hippocampus compared to tTA at 6.1 months of age, with the numbers indicating the fold-change.</p

Suppression of tau expression leads to reversal of inflammation.

<p>(A) Administration of doxycycline (Dox) to rTg4510 mice resulted in a decrease in total human tau (HT7) in the cortex and hippocampus. The most dramatic reduction was observed in cortical layer IV (arrowhead). Doxycycline administration also resulted in a decrease of Iba-1 and GFAP staining in the cortex. Scale bar, 200 µm. (B) Doxycycline resulted in a decrease in human tau and <i>Gfap</i> gene expression measured by qRT-PCR after 1 week of treatment. Further reductions in <i>Gfap</i> gene expression were observed with progressively longer treatments prior to 5.3 months of age. (C) Quantitation of GFAP levels by ELISA reveal a 3-fold reduction in rTg4510 mice with 6 weeks of doxycycline treatment, but no change in tTA mice. (D) Plasma levels of the monocyte marker, CD40, were higher in rTg4510 animals than tTA controls, and were suppressed to tTA levels after 6 weeks of doxycycline administration. **p<0.01, ***p<0.001 using the Dunnett multiple comparison test. Error bars indicate SEM.</p