Camilla Nilsberth

The 'Arctic' APP mutation (E693G) causes Alzheimer's disease by enhanced Aβ protofibril formation

Several pathogenic Alzheimers disease (AD) mutations have been described, all of which cause increased amyloid β-protein (Aβ) levels. Here we present studies of a pathogenic amyloid precursor protein (APP) mutation, located within the Aβ sequence at codon 693 (E693G), that causes AD in a Swedish family. Carriers of this Arctic mutation showed decreased Aβ42 and Aβ40 levels in plasma. Additionally, low levels of Aβ42 were detected in conditioned media from cells transfected with APPE693G. Fibrillization studies demonstrated no difference in fibrillization rate, but Aβ with the Arctic mutation formed protofibrils at a much higher rate and in larger quantities than wild-type (wt) Aβ. The finding of increased protofibril formation and decreased Aβ plasma levels in the Arctic AD may reflect an alternative pathogenic mechanism for AD involving rapid Aβ protofibril formation leading to accelerated buildup of insoluble Aβ intra- and/or extracellularly.

The Arctic mutation interferes with processing of the amyloid precursor protein.

The Arctic amyloid precursor protein (APP) Alzheimer mutation, is located inside the &bgr;-amyloid (A&bgr;) domain. Here, hybrid APP mutants containing both the Swedish and the Arctic APP mutations were investigated. ELISA measurements of cell media showed decreased levels of both A&bgr;40 and A&bgr;42. Similar results were obtained for the Dutch and Italian mutations, whereas the Flemish mutation displayed increased amounts of A&bgr;40 and A&bgr;42. Immunoprecipitation studies revealed increased A&bgr;40/p340 and A&bgr;42/p342 ratios for the Arctic mutation. These results were further verified by quantification revealing decreased levels of &agr;APPs accompanied by increased &bgr;APPs levels in the media. Thus, the pathogenic effects of the Arctic mutation may not only be due to the changed properties of the peptide but also altered processing of Arctic APP.

Alzheimer's disease presenilin-1 exon 9 deletion and L250S mutations sensitize SH-SY5Y neuroblastoma cells to hyperosmotic stress-induced apoptosis.

Mutations in the presenilin-1 (PS1) and presenilin-2 (PS2) genes account for the majority of early-onset familial Alzheimers disease cases. Recent studies suggest that presenilin gene mutations predispose cells to apoptosis by mechanisms involving altered calcium homeostasis and oxidative damage. In the present study, we determined whether PS1 mutations also sensitize cells to hyperosmotic stress-induced apoptosis. For this, we established SH-SY5Y neuroblastoma cell lines stably transfected with wild-type PS1 or either the PS1 exon 9 deletion (deltaE9) or PS1 L250S mutants. Cultured cells were exposed to an overnight (17 h) serum deprivation, followed by a 30 min treatment with either 20 mM glucose, 10 nM insulin-like growth factor-1 or 20 mM glucose + 10 nM insulin-like growth factor-1. Cells were then cultured for a further 3, 6 or 24 h and stained for apoptotic condensed nuclei using propidium iodide. Confirmation that cells were undergoing an active apoptotic process was achieved by labelling of DNA strand breaks using the terminal dUTP nick end labelling (TUNEL) technique. We also determined cell viability using 3-(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction. Propidium iodide staining revealed that all cell lines and controls showed an increased number of apoptotic cells appearing with condensed nuclei at 24 h compared with 6 h and 3 h. High glucose-induced hyperosmotic stress resulted in significantly more apoptotic cells in the PS1 deltaE9 and PS1 L250S mutation cell lines at 24 h, compared with the wild-type PS1 lines (P < 0.001, ANOVA for both comparisons). Mean values (+/-S.D.) for the percentage number of apoptotic cells at 24 h following high glucose treatment were 16.1 +/- 3.5%, 26.7 +/- 5.5% and 31.0 +/- 5.7% for the wild-type PS1, PS1 deltaE9 and PS1 L250S lines, respectively. The pro-apoptotic effects of high glucose treatment were reversed by 10 nM insulin-like growth factor-1, although to a lesser extent in the mutation cell lines (5.8 +/- 2.4%, 15.2 +/- 7.3% and 13.2 +/- 2.0% for the wild-type PS1, PS1 deltaE9 (P < 0.01 for comparison with wild-type PS1) and PS1 L250S (P < 0.01 for comparison with wild-type PS1) transfected lines, respectively. TUNEL labelling of cells at 24 h following treatment gave essentially the same results pattern as obtained using propidium iodide. The percentage number of apoptotic cells with DNA strand breaks (means +/- S.D.) following high glucose treatment was 15.4 +/- 2.6% for the wild-type PS1, 26.8 +/- 3.2% for the PS1 deltaE9 (P < 0.001 for comparison with wild-type PS1) and 29.7 +/- 6.1% for the PS1 L250S transfected lines (P < 0.001 for comparison with wild-type PS1). The PS1 deltaE9 and PS1 L250S transfected lines also showed a higher number of apoptotic cells with DNA strand breaks at 24 h following high glucose plus insulin-like growth factor-1 treatment (11.4 +/- 2.0% and 14.3 +/- 2.8%, respectively), compared with values for the wild-type PS1 lines (8.5 +/- 2.4%). These differences were significant (P < 0.01) for the comparison of wild-type PS1 and PS1 L250S, but not PS1 deltaE9 lines. The mutation-related increases in number of apoptotic cells at 24 h following high glucose treatment were not accompanied by significant differences in cell viability at this time-point. Our results indicate that PS1 mutations predispose to hyperosmotic stress-induced apoptosis and that the anti-apoptotic effects of insulin-like growth factor-1 are compromised by these mutations. Perturbations of insulin-like growth factor-1 signalling may be involved in PS1 mutation-related apoptotic neuronal cell death in Alzheimers disease.

Plasma Levels of Aβ42 and Aβ40 in Alzheimer Patients during Treatment with the Acetylcholinesterase Inhibitor Tacrine

Deregulation of amyloid precursor protein (APP) processing with increased production of amyloid β-peptide (Aβ) is considered to be a key pathogenic event in Alzheimer’s disease (AD). It has been suggested that stimulation of the muscarinic M1 receptor subtype affects APP processing and leads to a change in Aβ concentration. To test the hypothesis that treatment with a cholinesterase inhibitor could change the levels of Aβ in plasma, we measured Aβ42 and Aβ40 plasma levels in AD subjects before tacrine treatment and at weeks 2 and 6 of treatment. Treatment with tacrine had no statistically significant effect on plasma Aβ42 and Aβ40 either at 2 weeks or at 6 weeks of administration compared to baseline levels. Plasma Aβ42 and Aβ40 levels showed large subject-to-subject variation but small variation within the same patient over the 3-sample interval. After 2 weeks of treatment with tacrine, there was a strong negative correlation between tacrine concentration and levels of Aβ42 (r = –0.64; p = 0.01) and Aβ40 (r = –0.55; p = 0.04). However, after 6 weeks there was no correlation between plasma concentrations of tacrine and Aβ42 (r = 0.33; p = 0.34) or Aβ40 (r = –0.22; p = 0.54) levels in plasma. After 2 weeks of treatment with an acetylcholinesterase inhibitor, we found a correlation between higher drug concentrations and lower β-amyloid levels. This might indicate an effect on APP metabolism with an increased α-cleavage. But after 6 weeks of drug treatment, there was no obvious drug effect on β-amyloid concentrations. This finding may indicate that compensatory mechanisms have started at 6 weeks and that no long-term effect on key pathological features in AD is to be expected by an inhibition of acetylcholinesterase.

Changes in APP, PS1 and other Factors Related to Alzheimer's Disease Pathophysiology after Trimethyltin-Induced Brain Lesion in the Rat

Trimethyltin (TMT) chloride induces limbic system neurodegeneration, resulting in behavioral alterations including cognitive deficits. Different factors related to Alzheimer’s disease (AD) were studied after TMT lesion in Sprague-Dawley rats. The expression of amyloid precursor protein (APP) containing 695 amino acids (APP695), APP containing the Kuniz protease inhibitor domain (APP-KPI), presenilin 1 (PS1), c-fos and IL-1β was investigated at different timepoints after a single TMT injection (7 mg/kg i.p.) using in situ hybridization and immunohistochemistry.After the TMT treatment, extensive degeneration of pyramidal neurons was observed in the CA3 region of the hippocampus, concomitant with neurodegeneration in the outer layer of the CA1 region and layer II of enterhinal and piriform cortex. The affected regions showed abundant condensed eosinophilic and TUNEL-positive neuronal cells, that were apparent at day 4 after TMT, increasing to day 7 and subsequently disappearing. In the affected regions the levels of APP695 mRNA gradually declined with time after the TMT injection. While there was no apparent alteration in the overall expression of APP-KPI or PS1 mRNA, detailed analysis of the CA3c region showed that the mRNA expression shifted from neurons to glial cells. Three days after TMT, neurons in the piriform cortex, the CA3 region and DG expressed high levels of c-fos mRNA that slowly declined to become normalized when analyzed at day 28. At day 7 after TMT a few distinct IL-1β mRNA expressing glial cells were observed in the CA3c region.Thus, TMT exposure leads to alterations in the expression of APP, APP-KPI, PS1, c-fos and IL-1β in the limbic system. These findings suggest that TMT lesions, not only share certain key features of AD symptomatology and regional neurodegeneration, but also induce effects on important factors related to the pathophysiology of AD.

The Arctic Alzheimer mutation enhances sensitivity to toxic stress in human neuroblastoma cells.

The E693G (Arctic) mutation of the amyloid precursor protein was recently found to lead to early-onset Alzheimers disease in a Swedish family. In the present study, we report that the Arctic mutation decreases cell viability in human neuroblastoma cells. The cell viability, as measured by the MTT assay and propidium iodide staining, was further compromised following exposure to calcium ionophore A23187, microtubule-binding colchicine or oxidative stress inducer hydrogen peroxide. The manner of cell death was found to be apoptotic. During apoptosis, cells with the Arctic mutation also decreased their secretion of beta-secretase cleaved amyloid precursor protein. The enhanced sensitivity to toxic stress in cells with the Arctic mutation most likely contributes to the pathogenic pathway leading to Alzheimers disease.

Expression of presenilin 1 mRNA in rat peripheral organs and brain.

At least 50 different mutations in the presenilin 1 gene have been shown to cause early onset familial Alzheimers disease. Although presenilin 1 has an obvious role in the pathogenesis of Alzheimers disease, its function is still unknown. In the present study, the occurrence and distribution of presenilin 1 mRNA was examined in rat peripheral organs as well as in the brain by in situ hybridization histochemistry, using a radiolabelled oligonucleotide probe. In comparison to the brain, a high presenilin 1 mRNA expression was found in the testis, kidney, spleen, adrenal gland and thymus. It was also observed in skeletal muscle, liver, small intestine and lung, whereas no presenilin 1 could be detected in the heart, spinal cord and pancreas. Since presenilin 1 mRNA was found to be abundant in peripheral tissues which apparently are not affected in Alzheimers disease, additional functions of presenilin 1 are suggested, unrelated to its role in the pathological processes of the disease.