Axonal damage and astrocytosis are biological correlates of grey matter network integrity loss: a cohort study in autosomal dominant Alzheimer disease.

Brain development and maturation leads to grey matter networks that can be measured using magnetic resonance imaging. Network integrity is an indicator of information processing capacity which declines in neurodegenerative disorders such as Alzheimer disease (AD). The biological mechanisms causing this loss of network integrity remain unknown. Cerebrospinal fluid (CSF) protein biomarkers are available for studying diverse pathological mechanisms in humans and can provide insight into decline. We investigated the relationships between 10 CSF proteins and network integrity in mutation carriers (N=219) and noncarriers (N=136) of the Dominantly Inherited Alzheimer Network Observational study. Abnormalities in Aß, Tau, synaptic (SNAP-25, neurogranin) and neuronal calcium-sensor protein (VILIP-1) preceded grey matter network disruptions by several years, while inflammation related (YKL-40) and axonal injury (NfL) abnormalities co-occurred and correlated with network integrity. This suggests that axonal loss and inflammation play a role in structural grey matter network changes. Key points: Abnormal levels of fluid markers for neuronal damage and inflammatory processes in CSF are associated with grey matter network disruptions.The strongest association was with NfL, suggesting that axonal loss may contribute to disrupted network organization as observed in AD.Tracking biomarker trajectories over the disease course, changes in CSF biomarkers generally precede changes in brain networks by several years.

Tau positron emission tomography imaging in C9orf72 repeat expansion carriers.

BACKGROUND AND PURPOSE: AV-1451 (18 F-AV-1451, flortaucipir) positron emission tomography was performed in C9orf72 expansion carriers to assess tau accumulation and disease manifestation. METHODS: Nine clinically characterized C9orf72 expansion carriers and 18 age- and gender- matched cognitively normal individuals were psychometrically evaluated and underwent tau positron emission tomography imaging. The regional AV-1451 standard uptake value ratios from multiple brain regions were analyzed. Spearman correlation was performed to relate the AV-1451 standard uptake value ratio to clinical, psychometric and cerebrospinal fluid measures. RESULTS: C9orf72 expansion carriers had increased AV-1451 binding in the entorhinal cortex compared to controls. Primary age-related tauopathy was observed postmortem in one patient. AV-1451 uptake did not correlate with clinical severity, disease duration, psychometric performance or cerebrospinal fluid markers. CONCLUSION: C9orf72 expansion carriers exhibited increased AV-1451 uptake in entorhinal cortex compared to cognitively normal controls, suggesting a propensity for primary age-related tauopathy. However, AV-1451 accumulation was not associated with psychometric performance in our cohort.

Preclinical trials in autosomal dominant AD: implementation of the DIAN-TU trial.

The Dominantly Inherited Alzheimer's Network Trials Unit (DIAN-TU) was formed to direct the design and management of interventional therapeutic trials of international DIAN and autosomal dominant Alzheimer's disease (ADAD) participants. The goal of the DIAN-TU is to implement safe trials that have the highest likelihood of success while advancing scientific understanding of these diseases and clinical effects of proposed therapies. The DIAN-TU has launched a trial design that leverages the existing infrastructure of the ongoing DIAN observational study, takes advantage of a variety of drug targets, incorporates the latest results of biomarker and cognitive data collected during the observational study, and implements biomarkers measuring Alzheimer's disease (AD) biological processes to improve the efficiency of trial design. The DIAN-TU trial design is unique due to the sophisticated design of multiple drugs, multiple pharmaceutical partners, academics servings as sponsor, geographic distribution of a rare population and intensive safety and biomarker assessments. The implementation of the operational aspects such as home health research delivery, safety magnetic resonance imagings (MRIs) at remote locations, monitoring clinical and cognitive measures, and regulatory management involving multiple pharmaceutical sponsors of the complex DIAN-TU trial are described.

Improving CSF biomarker accuracy in predicting prevalent and incident Alzheimer disease.

OBJECTIVE: To investigate factors, including cognitive and brain reserve, which may independently predict prevalent and incident dementia of the Alzheimer type (DAT) and to determine whether inclusion of identified factors increases the predictive accuracy of the CSF biomarkers Aß(42), tau, ptau(181), tau/Aß(42), and ptau(181)/Aß(42). METHODS: Logistic regression identified variables that predicted prevalent DAT when considered together with each CSF biomarker in a cross-sectional sample of 201 participants with normal cognition and 46 with DAT. The area under the receiver operating characteristic curve (AUC) from the resulting model was compared with the AUC generated using the biomarker alone. In a second sample with normal cognition at baseline and longitudinal data available (n = 213), Cox proportional hazards models identified variables that predicted incident DAT together with each biomarker, and the models' concordance probability estimate (CPE), which was compared to the CPE generated using the biomarker alone. RESULTS: APOE genotype including an ε4 allele, male gender, and smaller normalized whole brain volumes (nWBV) were cross-sectionally associated with DAT when considered together with every biomarker. In the longitudinal sample (mean follow-up = 3.2 years), 14 participants (6.6%) developed DAT. Older age predicted a faster time to DAT in every model, and greater education predicted a slower time in 4 of 5 models. Inclusion of ancillary variables resulted in better cross-sectional prediction of DAT for all biomarkers (p < 0.0021), and better longitudinal prediction for 4 of 5 biomarkers (p < 0.0022). CONCLUSIONS: The predictive accuracy of CSF biomarkers is improved by including age, education, and nWBV in analyses.

Cognitively unimpaired HIV-positive subjects do not have increased 11C-PiB: a case-control study.

OBJECTIVES: Diagnostic challenges exist for differentiating HIV dementia from Alzheimer disease (AD) in older HIV-infected (HIV+) individuals. Similar abnormalities in brain amyloid-beta42 (Alphabeta42) metabolism may be involved in HIV-associated neuropathology and AD. We evaluated the amyloid-binding agent (11)C-Pittsburgh compound B ((11)C-PiB), a biomarker for Alphabeta42 deposition, in cognitively unimpaired HIV+ (n = 10) participants and matched community controls without dementia (n = 20). METHODS: In this case-control study, all participants had an (11)C-PiB scan within 2 years of concomitant CSF studies and neuropsychometric testing. Statistical differences between HIV+ and community controls for demographic and clinical values were assessed by chi(2) tests. Participants were further divided into either low (<500 pg/mL) or normal (>or=500 pg/mL) CSF Alphabeta42 groups with Student t tests performed to determine if regional differences in fibrillar amyloid plaque deposition varied with CSF Alphabeta42. RESULTS: Regardless of CSF Alphabeta42 level, none of the HIV+ participants had fibrillar amyloid plaques as assessed by increased (11)C-PiB mean cortical binding potential (MCBP) or binding potential within 4 cortical regions. In contrast, some community controls with low CSF Alphabeta42 (<500 pg/mL) had high (11)C-PiB MCBP with elevated binding potentials (>0.18 arbitrary units) within cortical regions. CONCLUSIONS: Cognitively unimpaired HIV+ participants, even with low CSF Alphabeta42 (<500 pg/mL), do not have (11)C-PiB parameters suggesting brain fibrillar amyloid deposition. The dissimilarity between unimpaired HIV+ and preclinical AD may reflect differences in Abeta42 production and/or formation of diffuse plaques. Future longitudinal studies of HIV+ participants with low CSF Abeta42 and normal (11)C-PiB are required.

CSF biomarkers of Alzheimer disease in HIV-associated neurologic disease.

BACKGROUND: HIV-associated neurologic disorders (HAND) continue to develop in many patients with HIV. CSF amyloid measurements in HAND have been reported to be similar to those in dementia of the Alzheimer type (DAT). Confirmatory evaluation of this finding in carefully evaluated subjects is needed. METHODS: CSF specimens were obtained from subjects clinically categorized with normal cognition from the general population, HIV+ subjects with normal cognition, HIV+ subjects with impaired cognition, or presumed HIV- subjects with mild DAT. CSF measurements of beta-amyloid((1-42)) (Abeta42), beta-amyloid((1-40)) (Abeta40), total tau (t-tau), and phosphorylated tau (p-tau181) were performed. RESULTS: CSF Abeta42 measured in 49 HAND subjects had a median level of 501 pg/mL, which was lower than that of 50 controls of similar age who had median of 686 pg/mL (p < 0.0001) or 21 HIV+ subjects without cognitive impairment who had median of 716 pg/mL (p < 0.003). HAND subjects had similar CSF Abeta42 to 68 subjects with mild DAT. There was no difference of CSF Abeta40 between the groups. Tau and p-tau181 was elevated in DAT, but slightly lower than control in both HIV+ groups. CONCLUSIONS: beta-Amyloid((1-42)) (Abeta42) measurements in CSF of cognitively impaired patients with HIV are similar to those in patients with mild dementia of the Alzheimer type (DAT). Normal or slightly depressed CSF tau and p-tau181 measurements distinguish these patients with HIV-associated neurologic disorders (HAND) from patients with DAT. Further evaluation of amyloid metabolism in patients with HIV cognitive disorder is needed to understand the implications of depressed CSF Abeta42 in the setting of HAND.

Neuroprotective natural antibodies to assemblies of amyloidogenic peptides decrease with normal aging and advancing Alzheimer's disease.

A number of distinct beta-amyloid (Abeta) variants or multimers have been implicated in Alzheimer's disease (AD), and antibodies recognizing such peptides are in clinical trials. Humans have natural Abeta-specific antibodies, but their diversity, abundance, and function in the general population remain largely unknown. Here, we demonstrate with peptide microarrays the presence of natural antibodies against known toxic Abeta and amyloidogenic non-Abeta species in plasma samples and cerebrospinal fluid of AD patients and healthy controls aged 21-89 years. Antibody reactivity was most prominent against oligomeric assemblies of Abeta and pyroglutamate or oxidized residues, and IgGs specific for oligomeric preparations of Abeta1-42 in particular declined with age and advancing AD. Most individuals showed unexpected antibody reactivities against peptides unique to autosomal dominant forms of dementia (mutant Abeta, ABri, ADan) and IgGs isolated from plasma of AD patients or healthy controls protected primary neurons from Abeta toxicity. Aged vervets showed similar patterns of plasma IgG antibodies against amyloid peptides, and after immunization with Abeta the monkeys developed high titers not only against Abeta peptides but also against ABri and ADan peptides. Our findings support the concept of conformation-specific, cross-reactive antibodies that may protect against amyloidogenic toxic peptides. If a therapeutic benefit of Abeta antibodies can be confirmed in AD patients, stimulating the production of such neuroprotective antibodies or passively administering them to the elderly population may provide a preventive measure toward AD.

Purification and characterization of astrocyte-secreted apolipoprotein E and J-containing lipoproteins from wild-type and human apoE transgenic mice.

The varepsilon4 allele of apolipoprotein E (apoE) is a genetic risk factor for Alzheimer's disease (AD). In order to gain a better understanding of the molecular mechanisms by which apoE and possibly other apolipoproteins produced in the central nervous system (CNS) influence AD pathogenesis, we have purified and characterized the two most abundant apolipoproteins produced in the CNS, apoE and apoJ. We purified apoE and apoJ from primary cultures of mouse astrocytes, which were derived from transgenic mice expressing human apoE isoforms in the absence of mouse apoE. Utilizing antibody affinity columns, we were able to purify both human apoE3 and apoE4, as well as mouse apoJ-containing lipoproteins. Astrocyte-secreted human apoE was present in high density-like lipoproteins of three predominant sizes ranging from 8 to 15 nm in diameter. Mouse apoJ was in particles between 10 and 17 nm in diameter with a peak size range of approximately 11 nm. ApoE and apoJ were in distinct lipoproteins. Utilization of quick-freeze, deep-etch electron microscopy revealed the apoE particles were discs while the apoJ particles were smaller and more irregular in appearance. The lipid composition of apoE particles was very different from those containing apoJ. ApoE-particles contained a similar mass of apoE and lipid, with cholesterol and phospholipid being about equal in mass per particle. ApoJ-particles were relatively lipid poor (three parts protein, one part lipid), with phospholipids being much more abundant than cholesterol. Detailed characterization of phospholipid composition by electrospray ionization mass spectrometry analysis revealed ethanolamine glycerophospholipids to be the most abundant phospholipid present in both apoE and apoJ particles. Analysis of cerebrospinal fluid from apoE3 and apoE4 transgenic mice revealed that human and mouse apoE were in particles the same size as those secreted by astrocytes. Further use of physiological preparations of CNS-derived lipoproteins may allow for a detailed understanding of the role of these molecules in the normal brain and in diseases such as AD.

No effect of apolipoprotein E on neuronal cell death due to excitotoxic and apoptotic agents in vitro and neonatal hypoxic ischaemia in vivo.

The epsilon4 allele of apolipoprotein E (apoE) is a genetic risk factor for Alzheimer's disease. Studies also suggest that the epsilon4 allele may be a risk factor for poor outcome following head trauma, brain haemorrhage and ischaemia. The mechanism by which the presence of an apoE epsilon4 allele and certain brain injuries act to predispose to Alzheimer's disease and poor outcome following brain injury is unknown. We questioned whether poor outcome after brain injury was due to direct modification by apoE protein and its gene variants of susceptibility to glutamate-mediated excitotoxic injury and apoptosis, mechanisms of cell death which occur following ischaemia and trauma. We investigated the effect of the presence or absence of endogenous murine apoE protein and different apoE isoforms in modification of the survival of murine embryonic cortical neurons exposed to the glutamate agonist, N-methyl-D-aspartic acid (NMDA) or apoptotic insult by staurosporine, and on the amount of brain injury sustained following a hypoxic-ischaemic insult in vivo to the brain of neonatal mice transgenically expressing human apoE epsilon3 or epsilon4. Our data provide evidence that apoE does not appear to alter neuronal viability following diverse types of acute neuronal insult, e.g. hypoxic-ischaemic or acute exposure to injurious agents in the models we have examined. This suggests that if apoE does modify the extent of brain damage and recovery after injury, it seems unlikely to be a result of direct or indirect modulation of excitotoxic or apoptotic cell death.

Astrocyte lipoproteins, effects of apoE on neuronal function, and role of apoE in amyloid-beta deposition in vivo.

The genetic association between the E4 isoform of apolipoprotein E (apoE) and increased risk for Alzheimer's disease (AD) has prompted interest in the neurobiology of apoE and the possible relationship between lipoprotein metabolism in the brain and neurodegenerative disease. ApoE, a product of astrocytes, is abundant in brain and in cerebrospinal fluid (CSF) where it is found in lipoproteins the size of large plasma high-density lipoproteins (HDL). Cultured astrocytes also secrete apoE/HDL, although the lipid and apoprotein composition of these nascent particles differs from that found in CSF, suggesting possible functional differences. In vitro studies have demonstrated isoform-specific effects of apoE on neurite outgrowth, neuronal plasticity, neurotoxicity, lipid peroxidation, oxidative injury, binding to cytoskeletal proteins, and interactions with amyloid-beta (Abeta), a primary component of senile plaques in AD. A number of these proposed functions have also been assessed in apoE -/- mice and transgenic mice expressing human apoE3 or apoE4. Importantly, analysis of transgenic mice overexpressing a mutant form of the human amyloid precursor protein (APP(V717F)) in the presence of mouse apoE, no apoE, or human apoE3 or E4 has demonstrated a critical and isoform-specific role for apoE in neuritic plaque formation, a pathologic hallmark of AD. Together, these data have provided important clues as to possible mechanism(s) by which apoE genotype modifies AD risk.

Differences in the Abeta40/Abeta42 ratio associated with cerebrospinal fluid lipoproteins as a function of apolipoprotein E genotype.

The epsilon4 allele of apolipoprotein E (ApoE) is a risk factor for Alzheimer's disease (AD). ApoE, which is important for lipid metabolism, is also a major constituent of cerebrospinal fluid (CSF) lipoproteins (LPs). Although ApoE in the CSF is derived from the central nervous system, the relation between LP metabolism in plasma and CSF is not clear. Soluble amyloid-beta (Abeta) protein may normally be associated with CSF LPs. It is converted in AD to a fibrillar form in brain parenchyma. ApoE and CSF LPs may regulate this process. The purpose of this study was to characterize CSF LPs from healthy, cognitively normal, fasted, elderly individuals at different risk for AD based on ApoE genotype. Lipid composition of CSF LPs did not differ with ApoE genotype. Interestingly, plasma and CSF high-density lipoprotein (HDL) cholesterol and apolipoprotein AI (ApoAI) levels were correlated. Importantly, as assessed by size-exclusion chromatography, Abeta in CSF coeluted in fractions containing LPs and was influenced by ApoE genotype: E4-positive subjects displayed significant elevations in Abeta40/Abeta42 ratios. These results suggest that plasma ApoAI/HDL levels can influence CSF ApoAI/HDL levels and that interactions between Abeta and central nervous system LPs may reflect changes in brain Abeta metabolism before the onset of clinical disease.

Apolipoprotein E facilitates neuritic and cerebrovascular plaque formation in an Alzheimer's disease model.

The epsilon4 allele of apolipoprotein E (ApoE) is an important genetic risk factor for Alzheimer's disease (AD). Increasing evidence suggests that this association may be linked to the ability of ApoE to interact with the amyloid-beta (Abeta) peptide and influence its concentration and structure. To determine the effect of ApoE on Abeta and other AD pathology in vivo, we used APPsw transgenic mice and ApoE knockout (-/-) mice to generate APPsw animals that carried two (ApoE +/+), one (ApoE +/-), or no copies (ApoE -/-) of the normal mouse ApoE gene. At 12 months of age, Abeta deposition was present in the cortex and hippocampus and was also prominent within leptomeningeal and cortical blood vessels of all APPsw ApoE +/+ mice. Importantly, although Abeta deposition still occurred in APPsw ApoE -/- mice, no fibrillar Abeta deposits were detected in the brain parenchyma or cerebrovasculature. There was also no neuritic degeneration associated with Abeta deposition in the absence of ApoE. These data demonstrate that ApoE facilitates the formation of both neuritic and cerebrovascular plaques, which are pathological hallmarks of AD and cerebral amyloid angiopathy.

Lipoproteins in the central nervous system.

Although the synthesis and metabolism of plasma lipoproteins are well characterized, little is known about lipid delivery and clearance within the central nervous system (CNS). Our work has focused on characterizing the lipoprotein particles present in the cerebrospinal fluid (CSF) and the nascent particles secreted by astrocytes. In addition to carrying lipids, we have found that beta-amyloid (A beta) associates with lipoproteins, including the discoidal particles secreted by cultured astrocytes and the spherical lipoproteins found in CSF. We believe that association with lipoproteins provides a means of transport and clearance for A beta. This process may be further influenced by an interaction between A beta and apoprotein E (apoE), the primary protein component of CNS lipoproteins. Specifically, we have investigated the formation and physiologic relevance of a SDS-stable complex between apoE and A beta. In biochemical assays, native apoE2 and E3 (associated with lipid particles) form an SDS-stable complex with A beta that is 20-fold more abundant than the apoE4:A beta complex. In cell culture, native apoE3 but not E4 prevents A beta-induced neurotoxicity by a mechanism dependent on cell surface apoE receptors. In addition, apoE and the inhibition of apoE receptors prevent A beta-induced astrocyte activation. Therefore, we hypothesize that the protection from A beta-induced neurotoxicity afforded by apoE3 may result from clearance of the peptide by SDS-stable apoE3:A beta complex formation and uptake by apoE receptors.

Apolipoprotein E isoform-dependent amyloid deposition and neuritic degeneration in a mouse model of Alzheimer's disease.

Apolipoprotein E (apoE) alleles determine the age-adjusted relative risk (epsilon4 > epsilon3) for Alzheimer's disease (AD). ApoE may affect AD pathogenesis by promoting deposition of the amyloid-beta (Abeta) peptide and its conversion to a fibrillar form. To determine the effect of apoE on Abeta deposition and AD pathology, we compared APP(V717F) transgenic (TG) mice expressing mouse, human, or no apoE (apoE(-/-)). A severe, plaque-associated neuritic dystrophy developed in APP(V717F) TG mice expressing mouse or human apoE. Though significant levels of Abeta deposition also occurred in APP(V717F) TG, apoE(-/-) mice, neuritic degeneration was virtually absent. Expression of apoE3 and apoE4 in APP(V717F) TG, apoE(-/-) mice resulted in fibrillar Abeta deposits and neuritic plaques by 15 months of age and substantially (>10-fold) more fibrillar deposits were observed in apoE4-expressing APP(V717F) TG mice. Our data demonstrate a critical and isoform-specific role for apoE in neuritic plaque formation, a pathological hallmark of AD.

Unique lipoproteins secreted by primary astrocytes from wild type, apoE (-/-), and human apoE transgenic mice.

Composition of central nervous system lipoproteins affects the metabolism of lipoprotein constituents within the brain. The epsilon4 allele of apolipoprotein E (apoE) is a risk factor for Alzheimer's disease via an unknown mechanism(s). As glia are the primary central nervous system cell type that synthesize apoE, we characterized lipoproteins secreted by astrocytes from wild type (WT), apoE (-/-), and apoE transgenic mice expressing human apoE3 or apoE4 in a mouse apoE (-/-) background. Nondenaturing size exclusion chromatography demonstrates that WT, apoE3, and apoE4 astrocytes secrete particles the size of plasma high density lipoprotein (HDL) composed of phospholipid, free cholesterol, and protein, primarily apoE and apoJ. However, the lipid:apoE ratio of particles containing human apoE is significantly lower than WT. ApoE localizes across HDL-like particle sizes. ApoJ localizes to the smallest HDL-like particles. ApoE (-/-) astrocytes secrete little phospholipid or free cholesterol despite comparable apoJ expression, suggesting that apoE is required for normal secretion of astrocyte lipoproteins. Further, particles were not detected in apoE (-/-) samples by electron microscopy. Nondenaturing immunoprecipitation experiments indicate that apoE and apoJ reside predominantly on distinct particles. These studies suggest that apoE expression influences the unique structure of astrocyte lipoproteins, a process further modified by apoE species.

Expression of human apolipoprotein E reduces amyloid-beta deposition in a mouse model of Alzheimer's disease.

The epsilon4 allele of apolipoprotein E (apo E) is associated with an increased risk for developing Alzheimer's disease (AD). This may be due to interactions between apo E and the amyloid-beta protein (Abeta). To assess the effects of human apo E isoforms on Abeta deposition in vivo, we bred apo E3 and apo E4 hemizygous (+/-) transgenic mice expressing apo E by astrocytes to mice homozygous (+/+) for a mutant amyloid precursor protein (APPV717F) transgene that develop age-dependent AD neuropathology. All mice were on a mouse apo E null (-/-) background. By nine months of age, APPV717F+/-, apo E-/- mice had developed Abeta deposition, and, as reported previously, the quantity of Abeta deposits was significantly less than that seen in APPV717F+/- mice expressing mouse apo E. In contrast to effects of mouse apo E, similar levels of human apo E3 and apo E4 markedly suppressed early Abeta deposition at nine months of age in APPV717F+/- transgenic mice, even when compared with mice lacking apo E. These findings suggest that human apo E isoforms decrease Abeta aggregation or increase Abeta clearance relative to an environment in which mouse apo E or no apo E is present. The results may have important implications for understanding mechanisms underlying the link between apo E and AD.

Evidence for normal aging of the septo-hippocampal cholinergic system in apoE (-/-) mice but impaired clearance of axonal degeneration products following injury.

The association of the epsilon4 allele of apoE with increased risk for Alzheimer's disease (AD) and with poor clinical outcome after certain acute brain injuries has sparked interest in the neurobiology of apoE. ApoE (-/-) mice provide a tool to investigate the role of apoE in the nervous system in vivo. Since integrity of the basal forebrain cholinergic system is severely compromised in AD, with severity of dysfunction correlating with apoE4 gene dosage, the present study tested the hypothesis that apoE is required to maintain the normal integrity of basal forebrain cholinergic neurons (BFCNs). Histological and biochemical analyses of the septo-hippocampal cholinergic system were performed in apoE (-/-) mice during aging and following injury. Using unbiased quantitative methods, there was little or no evidence for defects in the septo-hippocampal cholinergic system, as assessed by p75(NTR)-immunoreactive neuron number and size in the medial septum, cholinergic fiber density in the hippocampus, and choline acetyltransferase activity in the hippocampus, cortex, and striatum in aged apoE (-/-) mice (up to 24 months of age) as compared to age-matched wild-type mice of the same strain. In addition, cholinergic neuronal survival and size following fimbria-fornix transection in apoE (-/-) mice did not differ from controls. However, following entorhinal cortex lesion, there was persistence of degeneration products in the deafferented hippocampus in apoE (-/-) mice. These data suggest that although apoE is not required for the maintenance of BFCNs in vivo, it may play a role in the clearance of cholesterol-laden neurodegeneration products following brain injury.

Glial fibrillary acidic protein-apolipoprotein E (apoE) transgenic mice: astrocyte-specific expression and differing biological effects of astrocyte-secreted apoE3 and apoE4 lipoproteins.

The epsilon4 allele of apolipoprotein E (apoE) is associated with increased risk for Alzheimer's disease (AD) and poor outcome after brain injury. In the CNS, apoE is expressed by glia, predominantly astrocytes. To define the potential biological functions of different human apoE isoforms produced within the brain, transgenic mice were generated in which human apoE3 and apoE4 expression is under control of the astrocyte-specific glial fibrillary acidic protein (GFAP) promoter. These animals were then bred back to apoE knock-out mice. Human apoE protein is found within astrocytes and the neuropil throughout development and into the adult period, as assessed by immunocytochemistry and immunoblot analysis in several GFAP-apoE3 and E4 lines. Cultured astrocytes from these mice secrete apoE3 and apoE4 in lipoproteins that are high-density lipoprotein-like in size. When primary hippocampal neurons are grown in the presence of astrocyte monolayers derived from these transgenic mice, there is significantly greater neurite outgrowth from neurons grown in the presence of apoE3-secreting astrocytes compared with apoE4-secreting or apoE knock-out astrocytes. These effects are not dependent on direct astrocyte-neuron contact and appear to require the low-density lipoprotein receptor-related protein. These data suggest that astrocyte-secreted, apoE3-containing lipoproteins have different biological effects than apoE4-containing lipoproteins. In addition to providing information regarding the role of astrocyte-secreted apoE lipoproteins in the normal brain, these animals will also be useful in models of both AD and CNS injury.

Potential role of apoE in structural plasticity in the nervous system; implications for disorders of the central nervous system.

Apolipoprotein E (apoE) is a well characterized 299 amino acid protein that participates in the regulation of plasma cholesterol and lipid metabolism. In humans, apoE has three major protein isoforms: E2 (cys(112), cys(158)); E3 (cys(112), arg(158)); and E4 (arg(112), arg(158)) that are encoded for by a single gene on chromosome 19. Genetic studies have shown that apoE4 is a risk factor for Alzheimer's disease (AD) as well as for poor outcome following certain injuries to the central nervous system (CNS). These genetic data, as well as other data reviewed herein, suggest that apoE may play an important role in the nervous system under certain conditions. This review focuses on studies demonstrating that apoE can modulate neuronal structure and the potential implication of these findings for its role following CNS injury, in AD, and in other neurodegenerative diseases.

Differential dependency of cutaneous mechanoreceptors on neurotrophins, trk receptors, and P75 LNGFR.

The impact of null mutations of the genes for the NGF family of neurotrophins and their receptors was examined among the wide variety of medium to large caliber myelinated mechanoreceptors which have a highly specific predictable organization in the mystacial pad of mice. Immunofluorescence with anti-protein gene product 9.5, anti-200-kDa neurofilament protein (RT97), and anti-calcitonin gene-related product was used to label innervation in mystacial pads from mice with homozygous null mutations for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), the three tyrosine kinase receptors (trkA, trkB, trkC), and the low-affinity nerve growth factor receptor p75. Specimens were sacrificed at birth and at 1, 2, and 4 weeks for each type of mutation as well as at 11 weeks and 1 year for p75 and trkC mutations, respectively. Our results demonstrate several major concepts about the role of neurotrophins in the development of cutaneous mechanoreceptors that are supplied by medium to large caliber myelinated afferents. First, each of the high-affinity tyrosine kinase receptors, trkA, trkB, and trkC, as well as the low-affinity p75 receptor has an impact on at least one type of mechanoreceptor. Second, consistent with the various affinities for particular trk receptors, the elimination of NGF, BDNF, and NT-3 has an impact comparable to or more complex than the absence of their most specific high-affinity receptors: trkA, trkB, and trkC, respectively. These complexities include potential NT-3 signaling through trkA and trkB to support some neuronal survival. Third, most types of afferents are dependent on a different combination of neurotrophins and receptors for their survival: reticular and transverse lanceolate afferents are dependent upon NT-3, NGF, and trkA; Ruffini afferents upon BDNF and trkB; longitudinal lanceolate afferents upon NGF, trkA, BDNF, and trkB; and Merkel afferents on NGF, trkA, NT-3, trkC, and p75. NT-4 has no obvious detrimental impact on the mechanoreceptor development in the presence of BDNF. Fourth, NT-4 and BDNF signaling through trkB may suppress Merkel innervation and NT-3 signaling through trkC may suppress Ruffini innervation. Finally, regardless of the neurotrophin/receptor dependency for afferent survival and neurite outgrowth, NT-3 has an impact on the formation of all the sensory endings. In the context of these findings, indications of competitive and suppressive interactions that appear to regulate the balance of innervation density among the various sets of innervation were evident.