Mural cell dysfunction leads to altered cerebrovascular tau uptake following repetitive head trauma.

A pathological characteristic of repetitive traumatic brain injury (TBI) is the deposition of hyperphosphorylated and aggregated tau species in the brain and increased levels of extracellular monomeric tau are believed to play a role in the pathogenesis of neurodegenerative tauopathies. The pathways by which extracellular tau is eliminated from the brain, however, remains elusive. The purpose of this study was to examine tau uptake by cerebrovascular cells and the effect of TBI on these processes. We found monomeric tau interacts with brain vascular mural cells (pericytes and smooth muscle cells) to a greater extent than other cerebrovascular cells, indicating mural cells may contribute to the elimination of extracellular tau, as previously described for other solutes such as beta-amyloid. Consistent with other neurodegenerative disorders, we observed a progressive decline in cerebrovascular mural cell markers up to 12 months post-injury in a mouse model of repetitive mild TBI (r-mTBI) and human TBI brain specimens, when compared to control. These changes appear to reflect mural cell degeneration and not cellular loss as no difference in the mural cell population was observed between r-mTBI and r-sham animals as determined through flow cytometry. Moreover, freshly isolated r-mTBI cerebrovessels showed reduced tau uptake at 6 and 12 months post-injury compared to r-sham animals, which may be the result of diminished cerebrovascular endocytosis, as caveolin-1 levels were significantly decreased in mouse r-mTBI and human TBI cerebrovessels compared to their respective controls. Further emphasizing the interaction between mural cells and tau, similar reductions in mural cell markers, tau uptake, and caveolin-1 were observed in cerebrovessels from transgenic mural cell-depleted animals. In conclusion, our studies indicate repeated injuries to the brain causes chronic mural cell degeneration, reducing the caveolar-mediated uptake of tau by these cells. Alterations in tau uptake by vascular mural cells may contribute to tau deposition in the brain following head trauma and could represent a novel therapeutic target for TBI or other neurodegenerative disorders.

Apolipoprotein E isoforms differentially regulate matrix metallopeptidase 9 function in Alzheimer's disease.

Apolipoprotein E (APOE) has been shown to influence amyloid-ß (Aß) clearance from the brain in an isoform-specific manner. Our prior work showed that Aß transit across the blood-brain-barrier was reduced by apoE4, compared to other apoE isoforms, due to elevated lipoprotein receptor shedding in brain endothelia. Recently, we demonstrated that matrix metallopeptidase 9 (MMP-9) induces lipoprotein receptor proteolysis in an apoE isoform-dependent manner, which impacts Aß elimination from the brain. The current studies interrogated the relationship between apoE and MMP-9 and found that apoE impacted proMMP-9 cellular secretion from brain endothelia (apoE2 < apoE3 = apoE4). In a cell-free assay, apoE dose-dependently reduced MMP-9 activity, with apoE4 showing a significantly weaker ability to inhibit MMP-9 function than apoE2 or apoE3. Finally, we observed elevated MMP-9 expression and activity in the cerebrovasculature of both human and animal AD brain specimens with an APOE4 genotype. Collectively, these findings suggest a role for apoE in regulating MMP-9 disposition and may describe the effect of apoE4 on Aß pathology in the AD brain.

Multiscale causal networks identify VGF as a key regulator of Alzheimer's disease.

Though discovered over 100 years ago, the molecular foundation of sporadic Alzheimer's disease (AD) remains elusive. To better characterize the complex nature of AD, we constructed multiscale causal networks on a large human AD multi-omics dataset, integrating clinical features of AD, DNA variation, and gene- and protein-expression. These probabilistic causal models enabled detection, prioritization and replication of high-confidence master regulators of AD-associated networks, including the top predicted regulator, VGF. Overexpression of neuropeptide precursor VGF in 5xFAD mice partially rescued beta-amyloid-mediated memory impairment and neuropathology. Molecular validation of network predictions downstream of VGF was also achieved in this AD model, with significant enrichment for homologous genes identified as differentially expressed in 5xFAD brains overexpressing VGF. Our findings support a causal role for VGF in protecting against AD pathogenesis and progression.

Influence of Matrix Metallopeptidase 9 on Beta-Amyloid Elimination Across the Blood-Brain Barrier.

Lipoprotein receptor transport across the blood-brain barrier (BBB) mediates beta-amyloid (Aß) accumulation in the brain and may be a contributing factor in Alzheimer's disease (AD) pathogenesis. Lipoprotein receptors are susceptible to proteolytic shedding at the cell surface, which precludes the endocytic transport of ligands. A ligand that closely interacts with the lipoprotein receptors is apolipoprotein E (apoE), which exists as three isoforms (apoE2, apoE3, apoE4). Our prior work showed an inverse relationship between lipoprotein receptor shedding and Aß transport across the BBB, which was apoE-isoform dependent. To interrogate this further, the current studies investigated an enzyme implicated in lipoprotein receptor shedding, matrix metalloproteinase 9 (MMP9). Treatment with MMP9 dose-dependently elevated lipoprotein receptor shedding in brain endothelial cells and freshly isolated mouse cerebrovessels. Furthermore, treatment with a MMP9 inhibitor (SB-3CT) mitigated Aß-induced lipoprotein receptor shedding in brain endothelial cells and the brains of apoE4 animals. In terms of BBB transit, SB-3CT treatment increased the transport of Aß across an in vitro model of the BBB. In vivo, administration of SB-3CT to apoE4 animals significantly enhanced Aß clearance from the brain to the periphery following intracranial administration of Aß. The current studies show that MMP9 impacts lipoprotein receptor shedding and Aß transit across the BBB, in an apoE  isoform-specific manner. In total, MMP9 inhibition can facilitate Aß clearance across the BBB, which could be an effective approach to lowering Aß levels in the brain and mitigating the AD phenotype, particularly in subjects carrying the apoE4 allele.

Apolipoprotein E-mediated Modulation of ADAM10 in Alzheimer's Disease.

BACKGROUND: The APOE4 allele is the strongest genetic risk factor for Alzheimer's disease (AD). It has been associated with an accumulation of amyloid-ß (Aß) in the brain, which is produced through the sequential cleavage of the amyloid-ß precursor protein (AßPP) by ß - and γ-secretases. Alternatively, AßPP is also cleaved by α -secretases such as A Disintegrin and Metalloproteinase Domain-containing Protein 10 (ADAM10). OBJECTIVE: While several studies have investigated the impact of apoE on ß- and γ-secretase, interactions between apoE and α-secretases have not been fully examined. We investigated the effect of each apoE isoform on ADAM10 in vitro and in human cortex samples. METHOD: ADAM10 activity and kinetics was assessed in cell-free assays and the biological activity of ADAM10 further investigated in 7WCHO cells over-expressing wild type AßPP through ELISA. Finally, ADAM10 expression and activity was observed in the soluble fraction of both control and Alzheimer's Disease human cortex samples through ELISA. RESULTS: In a cell free assay, ADAM10 activity was found to be significantly lower in apoE4 samples compared to apoE2. 7WCHO cells over expressing wild type AßPP exposed to apoE4 demonstrated reduced formation of sAßPPα compared to other apoE isoforms. We also identified APOE and AD dependent changes in ADAM10 activity and expression in the soluble brain fraction of human brain cortex. CONCLUSION: Overall, our data demonstrates an apoE isoform-dependent effect on ADAM10 function and AßPP processing which may describe the elevated amyloid levels in the brains of AD subjects carrying the APOE4 allele.

APOE ε4 specific imbalance of arachidonic acid and docosahexaenoic acid in serum phospholipids identifies individuals with preclinical Mild Cognitive Impairment/Alzheimer's Disease.

This study was designed to explore the influence of apolipoprotein E (APOE) on blood phospholipids (PL) in predicting preclinical Alzheimer's disease (AD). Lipidomic analyses were also performed on blood from an AD mouse model expressing human APOE isoforms (EFAD) and five AD mutations and from 195 cognitively normal participants, 23 of who converted to mild cognitive impairment (MCI)/AD within 3 years. APOE ε4-carriers converting to MCI/AD had high arachidonic acid (AA)/docosahexaenoic acid (DHA) ratios in PL compared to cognitively normal ε4 and non-ε4 carriers. Arachidonic acid and DHA containing PL species, ε4-status and Aß42/Aß40 ratios provided 91% accuracy in detecting MCI/AD. Fish oil/omega-3 fatty acid consumption was associated with lower AA/DHA ratios even among ε4 carriers. High plasma AA/DHA ratios were observed in E4FAD compared to EFAD mice with other isoforms. In particular, alterations in plasma AA and DHA containing PL species were also observed in the brains of E4FAD mice compared to E3FAD mice. Despite the small sample size and a short follow-up, these results suggest that blood PL could potentially serve as biomarkers of preclinical MCI/AD.

Apolipoprotein E isoform-specific effects on lipoprotein receptor processing.

Recent findings indicate an isoform-specific role for apolipoprotein E (apoE) in the elimination of beta-amyloid (Aß) from the brain. ApoE is closely associated with various lipoprotein receptors, which contribute to Aß brain removal via metabolic clearance or transit across the blood­brain barrier (BBB). These receptors are subject to ectodomain shedding at the cell surface, which alters endocytic transport and mitigates Aß elimination. To further understand the manner in which apoE influences Aß brain clearance, these studies investigated the effect of apoE on lipoprotein receptor shedding. Consistent with prior reports, we observed an increased shedding of the low-density lipoprotein receptor (LDLR) and the LDLR-related protein 1 (LRP1) following Aß exposure in human brain endothelial cells. When Aß was co-treated with each apoE isoform, there was a reduction in Aß-induced shedding with apoE2 and apoE3, while lipoprotein receptor shedding in the presence of apoE4 remained increased. Likewise, intracranial administration of Aß to apoE-targeted replacement mice (expressing the human apoE isoforms) resulted in an isoform-dependent effect on lipoprotein receptor shedding in the brain (apoE4 > apoE3 > apoE2). Moreover, these results show a strong inverse correlation with our prior work in apoE transgenic mice in which apoE4 animals showed reduced Aß clearance across the BBB compared to apoE3 animals. Based on these results, apoE4 appears less efficient than other apoE isoforms in regulating lipoprotein receptor shedding, which may explain the differential effects of these isoforms in removing Aß from the brain.