Apolipoprotein E alters metabolism of AbetaPP in cells engaged in beta-amyloidosis

Association of Apolipoprotein E4-related Microvascular Disease in the Alzheimer's Disease Hippocampal CA1 Stratum Radiatum

Current data suggest a hypothesis of vascular pathogenesis for the development and progression of Alzheimer's disease (AD). To investigate this, we studied the association of apolipoprotein E4 (APOE4) gene on microvessels in human autopsy-confirmed AD with and without APOE4, compared with age/sex-matched control (AC) hippocampal CA1 stratum radiatum. AD arterioles (without APOE4 gene) had mild oxidative stress and loss of vascular endothelial growth factor (VEGF) and endothelial cell density, reflecting aging progression. In AD + APOE4, an increase in strong oxidative DNA damage marker 8-hydroxy-2'-deoxyguanosine (8-OHdG), VEGF, and endothelial cell density were associated with increased diameter of arterioles and perivascular space dilation. In cultured human brain microvascular cells (HBMECs), treatment of ApoE4 protein plus amyloid-β (Aβ) oligomers increased superoxide production and the apoptotic marker cleaved caspase 3, sustained hypoxia inducible factor-1α (HIF-1α) stability that was associated with an increase in MnSOD, VEGF, and cell density. This cell over-proliferation was inhibited with the antioxidants N-acetyl cysteine and MnTMPyP, the HIF-1α inhibitor echinomycin, the VEGFR-2 receptor blocker SU1498, the protein kinase C (PKC) ε knock-down (KD) and the extracellular signal-regulated kinase 1/2 (ERK) inhibitor FR180204. The PKCε KD and echinomycin decreased VEGF and/or ERK. In conclusion, AD capillaries and arterioles in hippocampal CA1 stratum radiatum of non-APOE4 carriers are related with aging, while those in APOE4 carriers with AD are related with pathogenesis of cerebrovascular disease.

The neuropathology of stillbirth - correlation with apolipoprotein genotype in a Scottish population based study

BACKGROUND

The neuropathology of stillbirths has been widely studied but rarely on a population basis. Whether foetal apolipoprotein E (APOE) genotype exerts any influence has been little investigated, despite well known effects in adult brains.

AIMS

To establish the neuropathology of a population cohort of stillbirths and compare with the APOE genotype.

STUDY DESIGN AND SUBJECTS

The brains of 191 stillbirths (≥24weeks of gestation) were recruited from a Scottish population cohort and grouped by clinical history. APOE genotype was available for 97%.

RESULTS AND CONCLUSIONS

One or more neuropathological features, most appearing relatively recent, were found in 54% of 157 antepartum singletons, 44% of 9 abruption-associated stillbirths, 85% of 13 in multiple pregnancies but in only 19% of 12 intrapartum stillbirths. White matter injury (WMI) occurred in 36% of preterm and 21% mature stillbirths. Fresh petechial haemorrhages were common in all groups (29%) but germinal matrix haemorrhage (GMH) (7%) and periventricular leucomalacia (1%) were confined to preterm. GMH was significantly associated with WMI (p=0.003). Placental inflammation was common in intrapartum stillbirths (50%), compared with antepartum (15%), multiple pregnancy (23%) and abruption (0%). β-Amyloid precursor protein (βAPP) positive axons (36% stillbirths overall) correlated closely with WMI (p<0.0001), justifying future routine inclusion in foetal neuropathological investigation. This study highlights the paucity of brain damage in intrapartum stillbirths. While APOE2 was significantly overrepresented in stillbirths, there was no correlation between APOE genotype and neuropathological findings. We conclude that APOE does not influence neuropathological outcomes in stillbirths.

Murine cerebrovascular cells as a cell culture model for cerebral amyloid angiopathy: isolation of smooth muscle and endothelial cells from mouse brain

The use of murine cerebrovascular endothelial and smooth muscle cells has not been widely employed as a cell culture model for the investigation of cellular mechanisms involved in cerebral amyloid angiopathy (CAA). Difficulties in isolation and propagation of murine cerebrovascular cells and insufficient yields for molecular and cell culture studies have deterred investigators from using mice as a source for cerebrovascular cells in culture. Instead, cerebrovascular cells from larger mammals are preferred and several methods describing the isolation of endothelial and smooth muscle cells from human, canine, rat, and guinea pig have been published. In recent years, several transgenic mouse lines showing CAA pathology have been established; consequently murine cerebrovascular cells derived from these animals can serve as a key cellular model to study CAA. Here, we describe a procedure for isolating murine microvessels that yields healthy smooth muscle and endothelial cell populations and produce sufficient material for experimental purposes. Murine smooth muscle cells isolated using this protocol exhibit the classic "hill and valley" morphology and are immunoreactive for the smooth muscle cell marker α-actin. Endothelial cells display a "cobblestone" pattern phenotype and show the characteristic immunostaining for the von Willebrand factor and the factor VIII-related antigen. In addition, we describe methods designed to preserve these cells by storage in liquid nitrogen and reestablishing viable cell cultures. Finally, we compare our methods with protocols designed to isolate and maintain human cerebrovascular cell cultures.

Human apolipoprotein E4 targeted replacement mice show increased prevalence of intracerebral hemorrhage associated with vascular amyloid deposition

Previous studies show that APOE *4 carriers are at increased risk for ischemic stroke and intracerebral hemorrhage (ICH). The APOE *4 gene is also linked to increased incidence of cerebral amyloid angiopathy. It has been suggested that apolipoprotein E4 expression leads to increased vascular amyloid deposition, which may explain the increased incidence of ICH in APOE *4 carriers. Here we show a significant increase in ICH in apoE4 targeted replacement mice compared with apoE3 mice. In all, 89% of the vessels in the apoE4 mice that showed evidence for hemorrhage contained fibrillar amyloid beta based on thioflavine-S staining. Aged apoE4 mice contained predominantly vascular amyloid deposits in the frontal cortex and hippocampus, but also showed evidence for parenchymal amyloid deposits. Most of the parenchymal amyloid appeared diffuse in nature; however, a small fraction was thioflavine-S positive, indicating presence of fibrillar amyloid. Electron microscopy further revealed evidence for fibrillar deposits in the vessel walls of apoE4 mice, but not apoE3 mice. The apoE4 targeted replacement mice do not harbor any mutation in the amyloid precursor protein gene and, therefore, are similar to the majority of humans susceptible to cerebral amyloid angiopathy and ICH, where the APOE genetic polymorphism is the only known genetic risk factor.

Abeta42 neurotoxicity in primary co-cultures: effect of apoE isoform and Abeta conformation

Autosomal dominant mutations that increase amyloid-beta(1-42) (Abeta42) cause familial Alzheimer's disease (AD), and the most common genetic risk factor for AD is the presence of the epsilon4 allele of apolipoprotein E (apoE). Previously, we characterized stable preparations of Abeta42 oligomers and fibrils and reported that oligomers induced a 10-fold greater increase in neurotoxicity than fibrils in Neuro-2A cells. To determine the effects of apoE genotype on Abeta42 oligomer- and fibril-induced neurotoxicity in vitro, we co-cultured wild type (WT) neurons with glia from WT, apoE-knockout (apoE-KO), and human apoE2-, E3-, and E4-targeted replacement (TR) mice. Dose-dependent neurotoxicity was induced by oligomeric Abeta42 with a ranking order of apoE4-TR>KO=apoE2-TR=apoE3-TR>WT. Neurotoxicity induced by staurosporine or glutamate were not affected by apoE genotype, indicating specificity for oligomeric Abeta42-induced neurotoxicity. These in vitro data demonstrate a gain of negative function for apoE4, synergistic with oligomeric Abeta42, in mediating neurotoxicity.

Induction of vascular amyloidosis-β by oxidative stress depends on APOE genotype

The reduced antioxidant defense in apolipoprotein E epsilon4/epsilon4 carriers may contribute to beta-amyloidosis. Previously we found that Fe(2+)-induced oxidative stress caused greater protein oxidation in epsilon4/epsilon4 than in epsilon3/epsilon3 human brain vascular smooth muscle cells. Moreover, Fe(2+) induced lysosomal accumulation of endogenous Abeta and APOE in cultured cells, and Abeta deposition in vascular tunica media in organotypic cultures of brain vessels. Here we demonstrated that Fe(2+) enhanced an uptake of exogenous Abeta 1-40 and its deposition together with APOE in lysosomes in myocytes. Abeta deposits were associated with lipid-peroxidation and protein ubiquitination, and were more abundant and stable in epsilon4/epsilon4 than in epsilon3/epsilon3 cells. In organotypic cultures of brain vessels Fe(2+) induced deposition of non-fibrillar and fibrillar Abeta 1-40 in vascular tunica media. We hypothesize that locally increased concentrations of iron induce accumulation of exogenous and endogenous Abeta in SMCs, triggering beta-amyloid angiopathy. The greater susceptibility of epsilon4 carriers to Fe(2+) ions may result in an increased risk of beta-amyloidosis.

Extracellular Deposits of Aβ Produced in Cultures of Alzheimer Disease Brain Vascular Smooth Muscle Cells

Alzheimer disease (AD) and Down syndrome (DS) brains contain deposits of amyloid-beta peptide that are located extracellularly in the neuropil and in blood vessels walls. A small fraction of brain Abeta is detected intracellularly in neurons, smooth muscle cells, and microglia. The roles of these extracellular and intracellular pools of Abeta in pathogenesis of AD-type dementia are controversial. Cell culture models of vascular amyloidosis-beta revealed intracellular, but not extracellular deposition of Abeta. Here we demonstrate for the first time, formation of extracellular deposits of Abeta in primary cultures of vascular smooth muscle cells isolated from AD cases with cerebrovascular amyloid angiopathy. Extracellular Abeta deposition required the use of cultures that produced high quantities of Abeta, which contained at least 50% of cells forming intracellular Abeta deposits, and providing extracellular matrix proteins. During 12 days of culture in this system, we observed accumulation of nonfibrillar, granular deposits in extracellular matrix, similar to early stages of vascular amyloidogenesis in vivo. This is a valuable system to study the effects of various potential amyloidogenic factors on formation of extracellular Abeta deposits.

The effect of oxidative stress on amyloid precursor protein processing in cells engaged in beta-amyloidosis is related to apolipoprotein E genotype

The reduced antioxidative defense in allele epsilon4 carriers is suggested to contribute to beta-amyloidosis in Alzheimer's disease and Down's syndrome. We studied the effect of oxidative stress on accumulation of amyloid-beta peptide (Abeta) in vascular smooth muscle cells (SMCs) that are engaged in production of amyloid-beta in vivo. Previously, we found that oxidative stress caused by ferrous ions induced accumulation of Abeta-apolipoprotein E deposits in lysosomes and was associated with a greater oxidative protein damage in epsilon4 carriers. Here, we demonstrate that ferrous ions induce formation of Abeta deposits also in vascular tunica media in organotypic cultures of whole brain vessels, suggesting the role of oxidative stress in development of vascular beta-amyloidosis. Cellular accumulation of Abeta in SMCs treated with ferrous ions was associated with a greater accumulation of C-terminal amyloid precursor protein (APP) fragments in epsilon4/epsilon4 than in epsilon3/epsilon3 myocytes and reduced the amount of soluble APPalpha in epsilon3/epsilon3, but not epsilon4/epsilon4, cultures. Antioxidant vitamin E prevented these effects, and, when applied alone, diminished the amount of APP C-terminal fragments and increased the amount of secreted APP in epsilon3/epsilon3, but not epsilon4/epsilon4, cells. C-terminal APP-immunoreactive material was accumulated in lysosomes partly with Abeta- and N-terminal APP immunoreactivities. These results suggest that the increased accumulation of APP and its fragments in lysosomes may yield additional amounts of cellular Abeta, particularly in epsilon4 carriers. We hypothesize that the altered processing of APP in SMCs locally exposed to oxidative stress facilitates cellular deposition of Abeta and contribute to the increased risk of development of beta-amyloidosis in epsilon4/epsilon4 carriers.

Lysosomal deposition of Abeta in cultures of brain vascular smooth muscle cells is enhanced by iron

Recently, we found that brain vascular smooth muscle cells from Tg2576 mice over-expressed the APP transgene in culture, secreted amyloid-beta peptide (Abeta) and accumulated Abeta intracellularly. Now we detected this intracellular Abeta inside lysosomes, which were also rich in C-terminal domain of APP, but not in endoplasmic reticulum, Golgi apparatus, or trans-Golgi network. Treatment of cultures with ferrous ions (50-150 microM) increased the proportion of muscle cells with Abeta immunoreactive granules and the amounts of intracellular Abeta1-40 and Abeta1-42 in a dose-dependent manner. This increase of intracellular Abeta1-40 by iron was inhibited by alpha-tocopherol, but not by a water-soluble antioxidant melatonin. The increase of intracellular Abeta1-42 by iron was not inhibited by alpha-tocopherol or melatonin. Cell treatment with iron did not alter the lysosomal localization of Abeta immunoreactivity. Cell treatment with iron (II and III), copper (II), zinc (II) and aluminum (III) increased cellular levels of carbonyls. However, the effect of zinc on Abeta accumulation in cultures was weak, and there were no effects of copper and aluminum. The data suggest that iron may be the factor that triggers vascular amyloidosis. Lysosomal accumulation of APP and Abeta initiates deposition of amyloid in blood vessels in Tg2576 mice.

The effect of oxidative stress on accumulation of apolipoprotein E3 and E4 in a cell culture model of beta-amyloid angiopathy (CAA)

Apolipoprotein E (apoE) is a multifunctional molecule that is active during brain development, maintenance, and injury. Allele epsilon 4 of apoE is recognized as a risk factor for beta-amyloidosis, but the responsible mechanisms are not clear. Recently, we showed that vascular smooth muscle cells (SMCs) from epsilon 4/ epsilon 4 carriers are the most susceptible to oxidative protein damage that was associated with the appearance of apoE-Abeta-immunoreactive granules in cells. Here, we demonstrate that apoE4 is more readily accumulated in SMCs treated with ferrous ions than is apoE3. ApoE accumulated in lysosomes in the form of monomers, dimers, apoE-containing complexes, and apoE fragments. ApoE4 and apoE4-containing complexes persisted in SMCs longer than apoE3 and its complexes. Both isoforms of apoE stimulated formation of apoE-Abeta deposits and increased immobilization of iron in cultures treated with ferrous ions. The accumulation of apoE-Abeta deposits in lysosomes was associated with the appearance of lipid peroxidation products such as malondialdehyde and 4-hydroxynonenal-2-nonenal. The higher cellular accumulation of apoE4 than apoE3 in SMCs exposed to oxidative stress may facilitate development of beta-amyloid angiopathy that is more frequent in epsilon 4/ epsilon 4 carriers.

Secretion and accumulation of Abeta by brain vascular smooth muscle cells from AbetaPP-Swedish transgenic mice

Alzheimer amyloid-beta is deposited in the neuropil and in brain blood vessels in transgenic Tg2576 mice that overexpress human amyloid-beta precursor protein (AbetaPP) containing the Swedish mutation (AbetaPP-Swe). Because the AbetaPP transgene in Tg2576 mice is placed behind the PrP promoter, all amyloid-beta, including vascular amyloid, is considered to be of neuronal origin. We studied the expression of the transgenic AbetaPP in smooth muscle cells cultured from brain blood vessels from Tg2576 mice. We found that brain vascular smooth muscle cells overexpressed human AbetaPP-Swe approximately 4 times the physiological levels of mouse AbetaPP. The cultured cells secreted abundant Abeta1-40 and Abeta1-42 and formed intracellular Abeta-immunoreactive granules. The percentage of cells containing intracellular Abeta and the amount of intracellular Abeta were significantly higher in cultures obtained from 14-month-old than from 4-month-old mice, as tested on first or second passages. During cell senescence in culture, intracellular accumulation of Abeta and C-terminal fragments of AbetaPP increased in cells derived from both 4- and 14-month-old mice. Vascular muscle cells from Tg2576 mice appear to be a valuable model of the intracellular accumulation of Abeta. We suggest that vascular muscle cells may be involved in the production of cerebrovascular amyloid in Tg2576 mice.

TGFbeta1 enhances formation of cellular Abeta/apoE deposits in vascular myocytes

Brain injury increases the risk of Alzheimer's disease (AD) through unknown mechanisms. We studied deposition of amyloid-beta protein (Abeta) in cells exposed to transforming growth factor beta1 (TGFbeta1), a cytokine that regulates cell metabolism during brain injury, and apolipoproteinE (apoE), the major lipid transporter in the brain. The studies were conducted by using brain vascular smooth muscle cells that are engaged in beta-amyloidosis in vivo and produce Abeta in cell culture. We found that cell treatment with TGFbeta1 together with apoE4 strongly increased the amount of cellular Abeta. The intracellular Abeta co-localized with apoE but not with TGFbeta, similarly as in vascular beta-amyloid. Some cellular Abeta/apoE deposits increased in size and persisted in culture even after the TGFbeta1 and apoE4 were removed. The appearance of cellular deposits of Abeta was associated with increased production of the amyloid-beta precursor protein and cellular retention of its mature form. The results suggest that the concomitant presence of apoE and TGFbeta1 can trigger vascular beta-amyloidosis by inducing intracellular formation of stable Abeta/apoE deposits.

Declining expression of neprilysin in Alzheimer disease vasculature: possible involvement in cerebral amyloid angiopathy

Molecular, genetic, and pharmacological studies have shown that neprilysin (also called NEP) catabolizes amyloid beta peptides (A beta) in healthy conditions. However, in Alzheimer disease (AD), A beta accumulates forming senile plaques in brain parenchyma and amyloid deposition around blood vessels. In this study, we tested at cellular level the relationship between neprilysin and A beta in human healthy and AD brain. Our results provided evidence for declining levels of neprilysin in AD brains as compared to healthy controls in parallel with increasing deposition of A beta. In hippocampus of AD individuals we observed a significant down-regulation of neprilysin expression in pyramidal neurons, consistent with the possibility that neprilysin controls the level of A beta accumulation and plaque formation in this area. In the cortex and leptomeninges, neprilysin was expressed in the smooth muscle cells of blood vessels. In sections from AD patients we observed a clear inverse relationship between neprilysin and A beta peptide levels in the vasculature, implicating neprilysin in cerebral amyloid angiopathy.

Oxidative protein damage in cells engaged in beta-amyloidosis is related to apoE genotype

The epsilon4 allele of apolipoprotein E (apoE) is a risk factor for Alzheimer's disease. The reduced antioxidant defense in epsilon4 carriers is suggested to contribute to beta-amyloidosis. We found that oxidative stress induced by treatment with Fe2+ ions raised more protein carbonyls in vascular smooth muscle cells isolated from human brains with apoE genotype epsilon4/epsilon4 than with 3epsilon/epsilon3 and epsilon3/epsilon4. Antioxidant vitamin E prevented formation of carbonyls but not in cells with genotype epsilon4/epsilon4. Treatment with Fe2+ ions induced cellular accumulation of amyloid-beta protein (Abeta)-immunoreactive material that co-localized with heme oxygenase, a marker of oxidative stress, and apoE. We hypothesize that the damage caused by oxidation in epsilon4/epsilon4 carriers facilitates development of beta-amyloidosis.

Deposition of Alzheimer's vascular amyloid-beta is associated with decreased expression of brain L-3-hydroxyacyl-coenzyme A dehydrogenase (ERAB)

L-3-hydroxyacyl-coenzyme A dehydrogenase type II (HADH) was described as an endoplasmic reticulum amyloid beta-peptide-binding protein (ERAB), which enhances Abeta toxicity, and accumulates in neurons in Alzheimer's disease (AD). Hence, HADH/ERAB was suggested to mediate the amyloid-induced neurodegeneration. We estimated the in vivo interactions of HADH and Abeta in an immunocytochemical study of ten Alzheimer's disease and seven normal brains using five monoclonal HADH-specific antibodies. We found no HADH in amyloid plaques or vascular amyloid. The neuronal expression of HADH was not correlated with the severity of amyloid load in neuropil. HADH was expressed in vascular smooth muscle cells in young and old controls and in amyloid-free blood vessels in AD cases, but little or no HADH was in smooth muscle cells in arteries with amyloid deposits. The putative intracellular interaction between HADH and Abeta in amyloid-producing cells was further studied in vascular smooth muscle cells isolated from brain blood vessels with amyloid-beta angiopathy - the cells that were shown previously to accumulate Abeta intracellularly ['Research advances in Alzheimer's disease and related disorders' (1995) 747; Brain Res. 676 (1995) 225; Neurosci. Lett. 183 (1995) 120]. HADH had a mitochondrial localization and did not co-localize with an endoplasmic reticulum marker. Cells that accumulated Abeta were those with low expression of HADH and the proteins did not co-localize. Explanation of the association between low levels of HADH and deposition of Abeta by brain smooth muscle cells requires further studies.

Role of perivascular cells and myocytes in vascular amyloidosis

Amyloidogenic processing of amyloid-beta precursor protein (APP) by cells of the brain is the major pathologic component of Alzheimer's disease. Amyloid-beta (A beta) is of heterogeneous origin. Perivascular cells of monocyte-macrophage-microglial cell lineage produce fibrillar A beta in the wall of capillaries, whereas parenchymal microglial cells produce fibrillar A beta in the parenchyma of gray matter. Fibrillar A beta deposition by perivascular cells lead to endothelial cell degeneration and death, obliteration of affected capillaries, and reduction of the length of the vascular network. These changes cause local ischemia with neuronal degeneration and death. Smooth muscle cells are the source of A beta in the tunica media of parenchymal and leptomeningeal arteries and veins. Fibrillar A beta in the tunica media of leptomeningeal and parenchymal vessels causes degeneration and necrosis of smooth muscle cells and leads to multiple cortical hemorrhages. Smooth muscle cells isolated from blood vessels with amyloid deposits secrete A beta and accumulate nonfibrillar A beta intracellularly. The amyloidogenic processing of APP can be enhanced by apolipoprotein E, reduced by transthyretin, and modulated by several cytokines.