Tobias Hartmann

Simvastatin strongly reduces levels of Alzheimer's disease β-amyloid peptides Aβ42 and Aβ40 in vitro and in vivo

Recent epidemiological studies show a strong reduction in the incidence of Alzheimers disease in patients treated with cholesterol-lowering statins. Moreover, elevated Aβ42 levels and the ɛ4 allele of the lipid-carrier apolipoprotein E are regarded as risk factors for sporadic and familial Alzheimers disease. Here we demonstrate that the widely used cholesterol-lowering drugs simvastatin and lovastatin reduce intracellular and extracellular levels of Aβ42 and Aβ40 peptides in primary cultures of hippocampal neurons and mixed cortical neurons. Likewise, guinea pigs treated with high doses of simvastatin showed a strong and reversible reduction of cerebral Aβ42 and Aβ40 levels in the cerebrospinal fluid and brain homogenate. These results suggest that lipids are playing an important role in the development of Alzheimers disease. Lowered levels of Aβ42 may provide the mechanism for the observed reduced incidence of dementia in statin-treated patients and may open up avenues for therapeutic interventions.

Treatment with simvastatin in normocholesterolemic patients with Alzheimer's disease: A 26-week randomized, placebo-controlled, double-blind trial

In a randomized, placebo‐controlled, double‐blind study, we investigated whether statins alter cholesterol metabolites and reduce Aβ levels in the cerebrospinal fluid of 44 patients with Alzheimers disease. Individuals were given up to 80mg simvastatin daily or placebo for 26 weeks. Overall, simvastatin did not significantly alter cerebrospinal fluid levels of Aβ40 and Aβ42. In post hoc analysis, simvastatin significantly decreased Aβ40 levels in the cerebrospinal fluid of patients with mild Alzheimers disease. The reduction of Aβ40 correlated with the reduction of 24S‐hydroxycholesterol. These changes were not observed in more severely affected patients.

Regulation of cholesterol and sphingomyelin metabolism by amyloid-beta and presenilin

Amyloid beta peptide (Aβ) has a key role in the pathological process of Alzheimers disease (AD), but the physiological function of Aβ and of the amyloid precursor protein (APP) is unknown. Recently, it was shown that APP processing is sensitive to cholesterol and other lipids. Hydroxymethylglutaryl-CoA reductase (HMGR) and sphingomyelinases (SMases) are the main enzymes that regulate cholesterol biosynthesis and sphingomyelin (SM) levels, respectively. We show that control of cholesterol and SM metabolism involves APP processing. Aβ42 directly activates neutral SMase and downregulates SM levels, whereas Aβ40 reduces cholesterol de novo synthesis by inhibition of HMGR activity. This process strictly depends on γ-secretase activity. In line with altered Aβ40/42 generation, pathological presenilin mutations result in increased cholesterol and decreased SM levels. Our results demonstrate a biological function for APP processing and also a functional basis for the link that has been observed between lipids and Alzheimers disease (AD).

The LPS receptor (CD14) links innate immunity with Alzheimer's disease

To rapidly respond to invading microorganisms, humans call on their innate immune system. This occurs by microbe‐detecting receptors, such as CD14, that activate immune cells to eliminate the pathogens. Here, we link the lipopolysaccharide receptor CD14 with Alzheimers disease, a severe neurodegenerative disease resulting in dementia. We demonstrate that this key innate immunity receptor interacts with fibrils of Alzheimer amyloid peptide. Neutralization with antibodies against CD14 and genetic deficiency for this receptor significantly reduced amyloid peptide induced microglial activation and microglial toxicity. The observation of strongly enhanced microglial expression of the LPS receptor in brains of animal models of Alzheimers disease indicates a clinical relevance of these findings. These data suggest that CD14 may significantly contribute to the overall neuroinflammatory response to amyloid peptide, highlighting the possibility that the enormous progress currently being made in the field of innate immunity could be extended to research on Alzheimers disease.

Impact of different saturated fatty acid, polyunsaturated fatty acid and cholesterol containing diets on beta-amyloid accumulation in APP/PS1 transgenic mice.

The present study assessed the influence of dietary lipids on accumulation of amyloid beta-peptide (Abeta) in the brain. Seven experimental diets with varying n-6/n-3-ratio, saturated and polyunsaturated fatty acid and cholesterol contents were fed to transgenic APPswe/PS1dE9 mice for 3-4 months beginning at a young adult age (6 months). Hippocampal Abeta levels were determined with ELISA and plaque load by using immunocytochemistry. A typical Western diet with 40% saturated fatty acids and 1% of cholesterol increased, while diets supplemented with docosahexaenoic acid (DHA) decreased Abeta levels compared to regular (soy oil based) diet. DHA diet also decreased the number of activated microglia in hippocampus and increased exploratory activity of transgenic mice, but did not improve their spatial learning in the water maze. The favorable effect of DHA on Abeta production was verified in two different cell lines. Regulation of dietary lipid intake may offer a new tool to reduce the risk of Alzheimers disease at the population level.

Cerebrospinal fluid Aβ42 is increased early in sporadic Alzheimer's disease and declines with disease progression

All mutations known to cause familial Alzheimers disease (AD) act by increasing the levels of soluble β‐amyloid peptide (Aβ), especially the longer form, Aβ42. However, in vivo elevation of soluble Aβ in sporadic AD has so far not been shown. In the present study, we used enzyme‐linked immunosorbent assays specific for Aβ42 and Aβ40 to investigate cerebrospinal fluid from sporadic AD at different stages of disease severity, to clarify the roles of Aβ42 and Aβ40 during disease progression. We also evaluated three other groups—one group of patients with mild cognitive impairment who were at risk of developing dementia, a cognitively intact, nondemented reference group diagnosed with depression, and a perfectly healthy control group. We found that Aβ42 is strongly elevated in early and mid stages of AD, and thereafter it declines with disease progression. On the contrary, Aβ40 levels were decreased in early and mid stages of AD. The group of cognitively impaired patients and the depression reference group had significantly higher levels of Aβ42 than the healthy control group, implying that Aβ42 is increased not only in AD, but in other central nervous system conditions as well. Our data also point out the importance of having thoroughly examined control material. The initial increase and subsequent decrease of Aβ42 adds a new biochemical tool to follow the progression of AD and might be important in the monitoring of therapeutics. Ann Neurol 1999;45:504–511

TNFα plus IFNγ induce the production of Alzheimer β-amyloid peptides and decrease the secretion of APPs

The appearance of inflammatory markers associated with amyloid plaques indicates a state of chronic inflammation in Alzheimers disease (AD). Multiple epidemiological studies also suggest that patients taking anti‐inflammatory drugs have a decreased risk of developing AD. Here we present evidence that inflammatory cytokines can alter the metabolism of the β‐am‐yloid precursor protein (βAPP). We show that the combination of tumor necrosis factor α and interferon γ triggers the production of β‐amyloid peptides and inhibits the secretion of soluble APPs by human neuronal and extraneuronal cells. The results demonstrate a new mechanism by which inflammatory components can exacerbate the fundamental pathology in AD.—Blasko, I., Marx, F., Steiner, E., Hartmann, T., Grubeck‐Loebenstein, B. TNFα plus IFNγ induce the production of Alzheimer β‐amyloid peptides and decrease the secretion of APPs. FASEB J. 13, 63–68 (1999)

Key factors in Alzheimer's disease: β-Amyloid precursor protein processing, metabolism and intraneuronal transport

During the last years it has become evident that the β‐amyloid (Aβ) component of senile plaques may be the key molecule in the pathology of Alzheimers disease (AD). The source and place of the neurotoxic action of Aβ, however, is still a matter of controversy. The precursor of the β‐amyloid peptide is the predominantly neuronal β‐amyloid precursor protein. We, and others, hypothesize that intraneuronal misregulation of APP leads to an accumulation of Aβ peptides in intracellular compartments. This accumulation impairs APP trafficking, which starts a cascade of pathological changes and causes the pyramidal neurons to degenerate. Enhanced Aβ secretion as a function of stressed neurons and remnants of degenerated neurons provide seeds for extracellular Aβ aggregates, which induce secondary degenerative events involving neighboring cells such as neurons, astroglia and macrophages/microglia.

A worldwide multicentre comparison of assays for cerebrospinal fluid biomarkers in Alzheimer's disease

Background Different cerebrospinal fluid (CSF) amyloid-beta 1–42 (Aβ 1–42), total Tau (Tau) and Tau phosphorylated at threonine 181 (P-Tau) levels are reported, but currently there is a lack of quality control programmes. The aim of this study was to compare the measurements of these CSF biomarkers, between and within centres. Methods Three CSF-pool samples were distributed to 13 laboratories in 2004 and the same samples were again distributed to 18 laboratories in 2008. In 2004 six laboratories measured Aβ 1–42, Tau and P-Tau and seven laboratories measured one or two of these marker(s) by enzyme-linked immunosorbent assays (ELISAs). In 2008, 12 laboratories measured all three markers, three laboratories measured one or two marker(s) by ELISAs and three laboratories measured the markers by Luminex. Results In 2004, the ELISA intercentre coefficients of variance (interCV) were 31%, 21% and 13% for Aβ 1–42, Tau and P-Tau, respectively. These were 37%, 16% and 15%, respectively, in 2008. When we restricted the analysis to the Innotest® (N = 13) for Aβ 1–42, lower interCV were calculated (22%). The centres that participated in both years (N = 9) showed interCVs of 21%, 15% and 9% and intra-centre coefficients (intraCV) of variance of 25%,18% and 7% in 2008. Conclusions The highest variability was found for Aβ 1–42. The variabilities for Tau and P-Tau were lower in both years. The centres that participated in both years showed a high intraCV comparable to their interCV, indicating that there is not only a high variation between but also within centres. Besides a uniform standardization of (pre)analytical procedures, the same assay should be used to decrease the inter/intracentre variation.

TLR2 Is a Primary Receptor for Alzheimer’s Amyloid β Peptide To Trigger Neuroinflammatory Activation

Microglia activated by extracellularly deposited amyloid β peptide (Aβ) act as a two-edged sword in Alzheimer’s disease pathogenesis: on the one hand, they damage neurons by releasing neurotoxic proinflammatory mediators (M1 activation); on the other hand, they protect neurons by triggering anti-inflammatory/neurotrophic M2 activation and by clearing Aβ via phagocytosis. TLRs are associated with Aβ-induced microglial inflammatory activation and Aβ internalization, but the mechanisms remain unclear. In this study, we used real-time surface plasmon resonance spectroscopy and conventional biochemical pull-down assays to demonstrate a direct interaction between TLR2 and the aggregated 42-aa form of human Aβ (Aβ42). TLR2 deficiency reduced Aβ42-triggered inflammatory activation but enhanced Aβ phagocytosis in cultured microglia and macrophages. By expressing TLR2 in HEK293 cells that do not endogenously express TLR2, we observed that TLR2 expression enabled HEK293 cells to respond to Aβ42. Through site-directed mutagenesis of tlr2 gene, we identified the amino acids EKKA (741–744) as a critical cytoplasmic domain for transduction of inflammatory signals. By coexpressing TLR1 or TLR6 in TLR2-transgenic HEK293 cells or silencing tlrs genes in RAW264.7 macrophages, we observed that TLR2-mediated Aβ42-triggered inflammatory activation was enhanced by TLR1 and suppressed by TLR6. Using bone marrow chimeric Alzheimer’s amyloid precursor transgenic mice, we observed that TLR2 deficiency in microglia shifts M1- to M2-inflammatory activation in vivo, which was associated with improved neuronal function. Our study demonstrated that TLR2 is a primary receptor for Aβ to trigger neuroinflammatory activation and suggested that inhibition of TLR2 in microglia could be beneficial in Alzheimer’s disease pathogenesis.