Lipoprotein lipase mutations and Alzheimer's disease

Evaluation of altered cell-cell communication between glia and neurons in the hippocampus of 3xTg-AD mice at two time points

Alzheimer's disease (AD) is the most common form of dementia and is characterized by progressive memory loss and cognitive decline, affecting behavior, speech, and motor abilities. The neuropathology of AD includes the formation of extracellular amyloid-β plaque and intracellular neurofibrillary tangles of phosphorylated tau, along with neuronal loss. While neuronal loss is an AD hallmark, cell-cell communication between neuronal and non-neuronal cell populations maintains neuronal health and brain homeostasis. To study changes in cellcell communication during disease progression, we performed snRNA-sequencing of the hippocampus from female 3xTg-AD and wild-type littermates at 6 and 12 months. We inferred differential cell-cell communication between 3xTg-AD and wild-type mice across time points and between senders (astrocytes, microglia, oligodendrocytes, and OPCs) and receivers (excitatory and inhibitory neurons) of interest. We also assessed the downstream effects of altered glia-neuron communication using pseudobulk differential gene expression, functional enrichment, and gene regulatory analyses. We found that glia-neuron communication is increasingly dysregulated in 12-month 3xTg-AD mice. We also identified 23 AD-associated ligand-receptor pairs that are upregulated in the 12-month-old 3xTg-AD hippocampus. Our results suggest increased AD association of interactions originating from microglia. Signaling mediators were not significantly differentially expressed but showed altered gene regulation and TF activity. Our findings indicate that altered glia-neuron communication is increasingly dysregulated and affects the gene regulatory mechanisms in neurons of 12-month-old 3xTg-AD mice.

Myelin repair in Alzheimer's disease: a review of biological pathways and potential therapeutics

This literature review investigates the significant overlap between myelin-repair signaling pathways and pathways known to contribute to hallmark pathologies of Alzheimer's disease (AD). We discuss previously investigated therapeutic targets of amyloid, tau, and ApoE, as well as other potential therapeutic targets that have been empirically shown to contribute to both remyelination and progression of AD. Current evidence shows that there are multiple AD-relevant pathways which overlap significantly with remyelination and myelin repair through the encouragement of oligodendrocyte proliferation, maturation, and myelin production. There is a present need for a single, cohesive model of myelin homeostasis in AD. While determining a causative pathway is beyond the scope of this review, it may be possible to investigate the pathological overlap of myelin repair and AD through therapeutic approaches.

Identification of a pleiotropic effect of ADIPOQ on cardiac dysfunction and Alzheimer's disease based on genetic evidence and health care records

Observations of comorbidity in heart diseases, including cardiac dysfunction (CD) are increasing, including and cognitive impairment, such as Alzheimer's disease and dementia (AD/D). This comorbidity might be due to a pleiotropic effect of genetic variants shared between CD and AD/D. Here, we validated comorbidity of CD and AD/D based on diagnostic records from millions of patients in Korea and the University of California, San Francisco Medical Center (odds ratio 11.5 [8.5-15.5, 95% Confidence Interval (CI)]). By integrating a comprehensive human disease-SNP association database (VARIMED, VARiants Informing MEDicine) and whole-exome sequencing of 50 brains from individuals with and without Alzheimer's disease (AD), we identified missense variants in coding regions including APOB, a known risk factor for CD and AD/D, which potentially have a pleiotropic role in both diseases. Of the identified variants, site-directed mutation of ADIPOQ (268 G > A; Gly90Ser) in neurons produced abnormal aggregation of tau proteins (p = 0.02), suggesting a functional impact for AD/D. The association of CD and ADIPOQ variants was confirmed based on domain deletion in cardiac cells. Using the UK Biobank including data from over 500000 individuals, we examined a pleiotropic effect of the ADIPOQ variant by comparing CD- and AD/D-associated phenotypic evidence, including cardiac hypertrophy and cognitive degeneration. These results indicate that convergence of health care records and genetic evidences may help to dissect the molecular underpinnings of heart disease and associated cognitive impairment, and could potentially serve a prognostic function. Validation of disease-disease associations through health care records and genomic evidence can determine whether health conditions share risk factors based on pleiotropy.

Decreased Plasma Level of Lipoprotein Lipase Predicted Verbal Disfluency in Chinese Type 2 Diabetes Mellitus Patients with Early Cognitive Deficits

BACKGROUND

Lipoprotein Lipase (LPL) is the rate-limiting enzyme catalyzing the hydrolysis of triglycerides and contributes to the amyloid-β formation, which shows promise as a pathological factor of cognitive decline in Type 2 Diabetes Mellitus (T2DM). This study aimed to investigate the pathogenetic roles of LPL and rs328 polymorphism in Mild Cognitive Impairment (MCI) in patients with T2DM.

METHODS

Chinese patients with T2DM were recruited and divided into two groups based on the Montreal Cognitive Assessment score. Demographic data were collected, LPL was measured and neuropsychological test results were examined.

RESULTS

Seventy-nine patients with diabetes and MCI had significantly decreased plasma LPL levels (p = 0.007) when compared with health-cognition controls (n = 91). Correlation analysis revealed that LPL was positively correlated with clock drawing test (r = 0.158, p = 0.043) and logical memory test (r = 0.162, p = 0.037), while lipoprotein a (r = -0.214, p = 0.006) was inversely associated with LPL. Logistic regression analysis further demonstrated that LPL concentration was an independent factor for diabetic MCI (p = 0.036). No significant differences were observed in the distributions of rs328 variants between patients with MCI and the controls. Moreover, no remarkable association was found among plasma LPL levels, cognitive performances, and lipid levels between the genotypic subgroups. The trail making test A was increased in the GC group when compared with the CC genotype in the control group.

CONCLUSION

Decreased plasma level of LPL could probably predict early cognitive deficits, especially verbal disfluency.

Eckel R. Genetic Variants of Lipoprotein Lipase and Regulatory Factors Associated with Alzheimer's Disease Risk

Lipoprotein lipase (LPL) is a key enzyme in lipid and lipoprotein metabolism. The canonical role of LPL involves the hydrolysis of triglyceride-rich lipoproteins for the provision of FFAs to metabolic tissues. However, LPL may also contribute to lipoprotein uptake by acting as a molecular bridge between lipoproteins and cell surface receptors. Recent studies have shown that LPL is abundantly expressed in the brain and predominantly expressed in the macrophages and microglia of the human and murine brain. Moreover, recent findings suggest that LPL plays a direct role in microglial function, metabolism, and phagocytosis of extracellular factors such as amyloid- beta (Aβ). Although the precise function of LPL in the brain remains to be determined, several studies have implicated LPL variants in Alzheimer's disease (AD) risk. For example, while mutations shown to have a deleterious effect on LPL function and expression (e.g., N291S, HindIII, and PvuII) have been associated with increased AD risk, a mutation associated with increased bridging function (S447X) may be protective against AD. Recent studies have also shown that genetic variants in endogenous LPL activators (ApoC-II) and inhibitors (ApoC-III) can increase and decrease AD risk, respectively, consistent with the notion that LPL may play a protective role in AD pathogenesis. Here, we review recent advances in our understanding of LPL structure and function, which largely point to a protective role of functional LPL in AD neuropathogenesis.

Human iPSC-Derived Hippocampal Spheroids: An Innovative Tool for Stratifying Alzheimer Disease Patient-Specific Cellular Phenotypes and Developing Therapies

The hippocampus is important for memory formation and is severely affected in the brain with Alzheimer disease (AD). Our understanding of early pathogenic processes occurring in hippocampi in AD is limited due to tissue unavailability. Here, we report a chemical approach to rapidly generate free-floating hippocampal spheroids (HSs), from human induced pluripotent stem cells. When used to model AD, both APP and atypical PS1 variant HSs displayed increased Aβ42/Aβ40 peptide ratios and decreased synaptic protein levels, which are common features of AD. However, the two variants differed in tau hyperphosphorylation, protein aggregation, and protein network alterations. NeuroD1-mediated gene therapy in HSs-derived progenitors resulted in modulation of expression of numerous genes, including those involved in synaptic transmission. Thus, HSs can be harnessed to unravel the mechanisms underlying early pathogenic changes in the hippocampi of AD patients, and provide a robust platform for the development of therapeutic strategies targeting early stage AD.

Involvement of Lipids in Alzheimer's Disease Pathology and Potential Therapies

Lipids constitute the bulk of the dry mass of the brain and have been associated with healthy function as well as the most common pathological conditions of the brain. Demographic factors, genetics, and lifestyles are the major factors that influence lipid metabolism and are also the key components of lipid disruption in Alzheimer's disease (AD). Additionally, the most common genetic risk factor of AD, APOE ϵ4 genotype, is involved in lipid transport and metabolism. We propose that lipids are at the center of Alzheimer's disease pathology based on their involvement in the blood-brain barrier function, amyloid precursor protein (APP) processing, myelination, membrane remodeling, receptor signaling, inflammation, oxidation, and energy balance. Under healthy conditions, lipid homeostasis bestows a balanced cellular environment that enables the proper functioning of brain cells. However, under pathological conditions, dyshomeostasis of brain lipid composition can result in disturbed BBB, abnormal processing of APP, dysfunction in endocytosis/exocytosis/autophagocytosis, altered myelination, disturbed signaling, unbalanced energy metabolism, and enhanced inflammation. These lipid disturbances may contribute to abnormalities in brain function that are the hallmark of AD. The wide variance of lipid disturbances associated with brain function suggest that AD pathology may present as a complex interaction between several metabolic pathways that are augmented by risk factors such as age, genetics, and lifestyles. Herewith, we examine factors that influence brain lipid composition, review the association of lipids with all known facets of AD pathology, and offer pointers for potential therapies that target lipid pathways.

A Unique Microglia Type Associated with Restricting Development of Alzheimer's Disease

Alzheimer's disease (AD) is a detrimental neurodegenerative disease with no effective treatments. Due to cellular heterogeneity, defining the roles of immune cell subsets in AD onset and progression has been challenging. Using transcriptional single-cell sorting, we comprehensively map all immune populations in wild-type and AD-transgenic (Tg-AD) mouse brains. We describe a novel microglia type associated with neurodegenerative diseases (DAM) and identify markers, spatial localization, and pathways associated with these cells. Immunohistochemical staining of mice and human brain slices shows DAM with intracellular/phagocytic Aβ particles. Single-cell analysis of DAM in Tg-AD and triggering receptor expressed on myeloid cells 2 (Trem2)^-/- Tg-AD reveals that the DAM program is activated in a two-step process. Activation is initiated in a Trem2-independent manner that involves downregulation of microglia checkpoints, followed by activation of a Trem2-dependent program. This unique microglia-type has the potential to restrict neurodegeneration, which may have important implications for future treatment of AD and other neurodegenerative diseases. VIDEO ABSTRACT.

Serum cholesterol and variant in cholesterol-related gene CETP predict white matter microstructure

Several common genetic variants influence cholesterol levels, which play a key role in overall health. Myelin synthesis and maintenance are highly sensitive to cholesterol concentrations, and abnormal cholesterol levels increase the risk for various brain diseases, including Alzheimer's disease. We report significant associations between higher serum cholesterol (CHOL) and high-density lipoprotein levels and higher fractional anisotropy in 403 young adults (23.8 ± 2.4 years) scanned with diffusion imaging and anatomic magnetic resonance imaging at 4 Tesla. By fitting a multi-locus genetic model within white matter areas associated with CHOL, we found that a set of 18 cholesterol-related, single-nucleotide polymorphisms implicated in Alzheimer's disease risk predicted fractional anisotropy. We focused on the single-nucleotide polymorphism with the largest individual effects, CETP (rs5882), and found that increased G-allele dosage was associated with higher fractional anisotropy and lower radial and mean diffusivities in voxel-wise analyses of the whole brain. A follow-up analysis detected white matter associations with rs5882 in the opposite direction in 78 older individuals (74.3 ± 7.3 years). Cholesterol levels may influence white matter integrity, and cholesterol-related genes may exert age-dependent effects on the brain.

Prioritization of candidate disease genes by enlarging the seed set and fusing information of the network topology and gene expression

The identification of disease genes is very important not only to provide greater understanding of gene function and cellular mechanisms which drive human disease, but also to enhance human disease diagnosis and treatment. Recently, high-throughput techniques have been applied to detect dozens or even hundreds of candidate genes. However, experimental approaches to validate the many candidates are usually time-consuming, tedious and expensive, and sometimes lack reproducibility. Therefore, numerous theoretical and computational methods (e.g. network-based approaches) have been developed to prioritize candidate disease genes. Many network-based approaches implicitly utilize the observation that genes causing the same or similar diseases tend to correlate with each other in gene-protein relationship networks. Of these network approaches, the random walk with restart algorithm (RWR) is considered to be a state-of-the-art approach. To further improve the performance of RWR, we propose a novel method named ESFSC to identify disease-related genes, by enlarging the seed set according to the centrality of disease genes in a network and fusing information of the protein-protein interaction (PPI) network topological similarity and the gene expression correlation. The ESFSC algorithm restarts at all of the nodes in the seed set consisting of the known disease genes and their k-nearest neighbor nodes, then walks in the global network separately guided by the similarity transition matrix constructed with PPI network topological similarity properties and the correlational transition matrix constructed with the gene expression profiles. As a result, all the genes in the network are ranked by weighted fusing the above results of the RWR guided by two types of transition matrices. Comprehensive simulation results of the 10 diseases with 97 known disease genes collected from the Online Mendelian Inheritance in Man (OMIM) database show that ESFSC outperforms existing methods for prioritizing candidate disease genes. The top prediction results of Alzheimer's disease are consistent with previous literature reports.

Meta-analyses of 8 polymorphisms associated with the risk of the Alzheimer's disease

Aims

The aim of this study was to evaluate the combined contribution of 8 polymorphisms to the risk of Alzheimer's disease (AD).

Methods

Through a comprehensive literature search for genetic variants involved in the AD association study, we harvested a total of 6 genes (8 polymorphisms) for the current meta-analyses. These genes consisted of A2M (5bp I/D and V1000I), ABCA2 (rs908832), CHAT (1882G >A, 2384G >A), COMT (Val158Met), HTR6 (267C >T) and LPL (Ser447Ter).

Results

A total of 33 studies among 9,453 cases and 10,833 controls were retrieved for the meta-analyses of 8 genetic variants. It was showed that A2M V1000I (odd ratio (OR) = 1.26, 95% confidence interval (CI) = 1.07–1.49, P = 0.007), rs908832 allele of ABCA2 (OR = 1.55, 95% CI = 1.12–2.16, P = 0.009), 2384G >A of CHAT (OR = 1.22, 95% CI = 1.00–1.49, P = 0.05) and Ser447Ter of LPL in the Northern-American population (OR = 0.56, 95% CI = 0.35–0.91, P = 0.02) were significantly associated with the risk of AD. No association was found between the rest of the 5 polymorphisms and the risk of AD.

Conclusion

Our results showed that A2M V1000I polymorphism in German, Korean, Chinese, Spanish, Italian and Polish populations, rs90883 of ABCA2 gene in French, American, Swiss, Greek and Japanese populations, 2384G >A of CHAT gene in British and Korean populations and LPL Ser447Ter in the Northern-American population were associated with the risk of AD.

Activated cyclin-dependent kinase 5 promotes microglial phagocytosis of fibrillar β-amyloid by up-regulating lipoprotein lipase expression

Amyloid plaques are crucial for the pathogenesis of Alzheimer disease (AD). Phagocytosis of fibrillar β-amyloid (Aβ) by activated microglia is essential for Aβ clearance in Alzheimer disease. However, the mechanism underlying Aβ clearance in the microglia remains unclear. In this study, we performed stable isotope labeling of amino acids in cultured cells for quantitative proteomics analysis to determine the changes in protein expression in BV2 microglia treated with or without Aβ. Among 2742 proteins identified, six were significantly up-regulated and seven were down-regulated by Aβ treatment. Bioinformatic analysis revealed strong over-representation of membrane proteins, including lipoprotein lipase (LPL), among proteins regulated by the Aβ stimulus. We verified that LPL expression increased at both mRNA and protein levels in response to Aβ treatment in BV2 microglia and primary microglial cells. Silencing of LPL reduced microglial phagocytosis of Aβ, but did not affect degradation of internalized Aβ. Importantly, we found that enhanced cyclin-dependent kinase 5 (CDK5) activity by increasing p35-to-p25 conversion contributed to LPL up-regulation and promoted Aβ phagocytosis in microglia, whereas inhibition of CDK5 reduced LPL expression and Aβ internalization. Furthermore, Aβ plaques was increased with reducing p25 and LPL level in APP/PS1 mouse brains, suggesting that CDK5/p25 signaling plays a crucial role in microglial phagocytosis of Aβ. In summary, our findings reveal a potential role of the CDK5/p25-LPL signaling pathway in Aβ phagocytosis by microglia and provide a new insight into the molecular pathogenesis of Alzheimer disease.

Lipoprotein lipase is a novel amyloid beta (Abeta)-binding protein that promotes glycosaminoglycan-dependent cellular uptake of Abeta in astrocytes

Lipoprotein lipase (LPL) is a member of a lipase family known to hydrolyze triglyceride molecules in plasma lipoprotein particles. LPL also plays a role in the binding of lipoprotein particles to cell-surface molecules, including sulfated glycosaminoglycans (GAGs). LPL is predominantly expressed in adipose and muscle but is also highly expressed in the brain where its specific roles are unknown. It has been shown that LPL is colocalized with senile plaques in Alzheimer disease (AD) brains, and its mutations are associated with the severity of AD pathophysiological features. In this study, we identified a novel function of LPL; that is, LPL binds to amyloid β protein (Aβ) and promotes cell-surface association and uptake of Aβ in mouse primary astrocytes. The internalized Aβ was degraded within 12 h, mainly in a lysosomal pathway. We also found that sulfated GAGs were involved in the LPL-mediated cellular uptake of Aβ. Apolipoprotein E was dispensable in the LPL-mediated uptake of Aβ. Our findings indicate that LPL is a novel Aβ-binding protein promoting cellular uptake and subsequent degradation of Aβ.

Cholesterol metabolism and transport in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder, affecting millions of people worldwide. Apart from age, the major risk factor identified so far for the sporadic form of AD is possession of the epsilon4 allele of apolipoprotein E (APOE), which is also a risk factor for coronary artery disease (CAD). Other apolipoproteins known to play an important role in CAD such as apolipoprotein B are now gaining attention for their role in AD as well. AD and CAD share other risk factors, such as altered cholesterol levels, particularly high levels of low density lipoproteins together with low levels of high density lipoproteins. Statins--drugs that have been used to lower cholesterol levels in CAD, have been shown to protect against AD, although the protective mechanism(s) involved are still under debate. Enzymatic production of the beta amyloid peptide, the peptide thought to play a major role in AD pathogenesis, is affected by membrane cholesterol levels. In addition, polymorphisms in several proteins and enzymes involved in cholesterol and lipoprotein transport and metabolism have been linked to risk of AD. Taken together, these findings provide strong evidence that changes in cholesterol metabolism are intimately involved in AD pathogenic processes. This paper reviews cholesterol metabolism and transport, as well as those aspects of cholesterol metabolism that have been linked with AD.

Chromosome 8p as a potential hub for developmental neuropsychiatric disorders: implications for schizophrenia, autism and cancer

Defects in genetic and developmental processes are thought to contribute susceptibility to autism and schizophrenia. Presumably, owing to etiological complexity identifying susceptibility genes and abnormalities in the development has been difficult. However, the importance of genes within chromosomal 8p region for neuropsychiatric disorders and cancer is well established. There are 484 annotated genes located on 8p; many are most likely oncogenes and tumor-suppressor genes. Molecular genetics and developmental studies have identified 21 genes in this region (ADRA1A, ARHGEF10, CHRNA2, CHRNA6, CHRNB3, DKK4, DPYSL2, EGR3, FGF17, FGF20, FGFR1, FZD3, LDL, NAT2, NEF3, NRG1, PCM1, PLAT, PPP3CC, SFRP1 and VMAT1/SLC18A1) that are most likely to contribute to neuropsychiatric disorders (schizophrenia, autism, bipolar disorder and depression), neurodegenerative disorders (Parkinson's and Alzheimer's disease) and cancer. Furthermore, at least seven nonprotein-coding RNAs (microRNAs) are located at 8p. Structural variants on 8p, such as copy number variants, microdeletions or microduplications, might also contribute to autism, schizophrenia and other human diseases including cancer. In this review, we consider the current state of evidence from cytogenetic, linkage, association, gene expression and endophenotyping studies for the role of these 8p genes in neuropsychiatric disease. We also describe how a mutation in an 8p gene (Fgf17) results in a mouse with deficits in specific components of social behavior and a reduction in its dorsomedial prefrontal cortex. We finish by discussing the biological connections of 8p with respect to neuropsychiatric disorders and cancer, despite the shortcomings of this evidence.

Impact of lipoprotein lipase gene polymorphisms on ulcerative colitis

AIM: To examine the influence of lipoprotein lipase (LPL) gene polymorphism in ulcerative colitis (UC) patients.

METHODS: Peripheral blood was obtained from 131 patients with UC and 106 healthy controls for DNA extraction. We determined LPL gene polymorphisms affecting the enzyme at Ser447stop, as well as HindIII and PvuII polymorphisms using PCR techniques. PCR products were characterized by PCR-RFLP and direct sequencing. Polymorphisms were examined for association with clinical features in UC patients. Genotype frequencies for LPL polymorphisms were also compared between UC patients and controls.

RESULTS: In patients with onset at age 20 years or younger, C/G and G/G genotypes for Ser447stop polymorphism were more prevalent than C/C genotype (OR = 3.13, 95% CI = 0.95-10.33). Patients with H^+/- or H^-/- genotype for HindIII polymorphism also were more numerous than those with H^+/+ genotype (OR = 2.51, 95% CI = 0.85-7.45). In the group with H^+/+ genotype for HindIII polymorphism, more patients had serum triglyceride concentrations over 150 mg/dL than patients with H^+/- or H^-/- genotype (P < 0.01, OR = 6.46, 95% CI = 1.39-30.12). Hypertriglycemia was also more prevalent in patients with P^+/+ genotypes for PvuII polymorphism (P < 0.05, OR = 3.0, 95% CI = 1.06-8.50). Genotype frequency for LPL polymorphism did not differ significantly between UC patients and controls.

CONCLUSION: Ser447stop and HindIII LPL polymorphisms may influence age of onset of UC, while HindIII and PvuII polymorphisms influence serum triglyceride in UC patients.

Re-assessing the relationship between cholesterol, statins and Alzheimer's disease

This communication integrates the purported role of cholesterol and statins in Alzheimer's disease (AD) with recent data. Meta-analysis of association studies relevant to AD indicates that apolipoprotein (apo)E4 is the only cholesterol-related polymorphism that shows clear association with AD. This suggests that the effect of apoE4 on the pathophysiology of AD occurs via a mechanism that is not directly related to cholesterol, such as fibrillization of Abeta. Despite the lack of genetic association, cholesterol and statins clearly modulate amyloid precursor protein (APP) processing in cell culture and animal models. Statins appear to act by a pleiotropic mechanism, involving both cholesterol (via lipid rafts) and isoprenylation. The pleiotropic mechanism of statin action clarifies conflicting data from clinical studies, where statins exert an action on Abeta and AD that might be dose dependent because of actions on both cholesterol and isoprenylation. Reduced isoprenylation can also inhibit inflammation. Our own studies of brains from Alzheimer subjects +/- statins indicate that statins inhibit inflammation in humans but might not reduce cerebral Abeta load. These results suggest that the primary action of statins in humans with AD might be to reduce inflammation rather than decrease Abeta load.

A polymorphism in lipoprotein lipase affects the severity of Alzheimer's disease pathophysiology

Emerging evidences indicate a role for lipoprotein lipase (LPL) in degenerative states. Genetic variations in the LPL gene were previously associated to lipid imbalance and coronary artery disease (CAD) risk and severity, a condition that shares pathological features with common Alzheimer's disease (AD). To evaluate whether these genetic variations associate with the risk and pathophysiology of common AD, autopsy-confirmed patients (242 controls, 153 AD) were genotyped for a PvuII single nucleotide polymorphism (SNP; rs285; referred to as the P+ allele) of LPL. Brain LPL mRNA levels, cholesterol levels, amyloid concentration, senile plaques and neurofibrillary tangles density counts were measured and contrasted with specific LPL genotypes. When adjusted for age and sex, homozygosity for the P+ allele resulted in an odds ratio of 2.3 for the risk of developing AD. More importantly, we report that the presence of the P+ allele of LPL significantly affects its mRNA expression level (n = 51; P = 0.026), brain tissue cholesterol levels (n = 55; P = 0.0013), neurofibrillary tangles (n = 52; P = 0.025) and senile plaque (n = 52; P = 0.022) densities. These results indicate that a common polymorphism in the lipoprotein lipase gene modulates the risk level for sporadic AD in the eastern Canadian population but more importantly, indirectly modulates the pathophysiology of the brain in autopsy-confirmed cases.

Apolipoprotein E, cholesterol metabolism, diabetes, and the convergence of risk factors for Alzheimer's disease and cardiovascular disease

High fat diets and sedentary lifestyles are becoming major concerns for Western countries. They have led to a growing incidence of obesity, dyslipidemia, high blood pressure, and a condition known as the insulin-resistance syndrome or metabolic syndrome. These health conditions are well known to develop along with, or be precursors to atherosclerosis, cardiovascular disease, and diabetes. Recent studies have found that most of these disorders can also be linked to an increased risk of Alzheimer's disease (AD). To complicate matters, possession of one or more apolipoprotein E epsilon4 (APOE epsilon4) alleles further increases the risk or severity of many of these conditions, including AD. ApoE has roles in cholesterol metabolism and Abeta clearance, both of which are thought to be significant in AD pathogenesis. The apparent inadequacies of ApoE epsilon4 in these roles may explain the increased risk of AD in subjects carrying one or more APOE epsilon4 alleles. This review describes some of the physiological and biochemical changes that the above conditions cause, and how they are related to the risk of AD. A diversity of topics is covered, including cholesterol metabolism, glucose regulation, diabetes, insulin, ApoE function, amyloid precursor protein metabolism, and in particular their relevance to AD. It can be seen that abnormal lipid, cholesterol and glucose metabolism are consistently indicated as central in the pathophysiology, and possibly the pathogenesis of AD. As diagnosis of mild cognitive impairment and early AD are becoming more reliable, and as evidence is accumulating that health conditions such as diabetes, obesity, and coronary artery disease are risk factors for AD, appropriate changes to diets and lifestyles will likely reduce AD risk, and also improve the prognosis for people already suffering from such conditions.

Lipoprotein lipase S447X: a naturally occurring gain-of-function mutation

Lipoprotein lipase (LPL) hydrolyzes triglycerides in the circulation and promotes the hepatic uptake of remnant lipoproteins. Since the gene was cloned in 1989, more than 100 LPL gene mutations have been identified, the majority of which cause loss of enzymatic function. In contrast to this, the naturally occurring LPL(S447X) variant is associated with increased lipolytic function and an anti-atherogenic lipid profile and can therefore be regarded as a gain-of-function mutation. This notion combined with the facts that 20% of the general population carries this prematurely truncated LPL and that it may protect against cardiovascular disease has led to extensive clinical and basic research into this frequent LPL mutant. It is only until recently that we begin to understand the molecular mechanisms that underlie the beneficial effects associated with LPL(S447X). This review summarizes the current literature on this interesting LPL variant.

Cholesteryl ester transfer protein TaqI B and lipoprotein lipase Ser447Ter gene polymorphisms are not associated with ischaemic stroke in Greek patients

Cholesteryl ester transfer protein (CETP) and lipoprotein lipase (LPL) are both key players in plasma lipoprotein homeostasis and, as such, genetically induced alterations in their respective activities may affect susceptibility to cerebrovascular diseases. In this study, we examined the distribution of two common polymorphisms, namely CETP TaqI B and LPL Ser447Ter in a cohort of Greek clinically diagnosed late-onset ischaemic stroke patients (n = 98) and an ethnicity-, age- and sex-matched control group with no manifestations of vascular disease (n = 100). Our study revealed no statistically significant differences with respect to the distribution of either polymorphism, examined separately or in combination, between the two groups.

Gene expression profiles in microdissected neurons from human hippocampal subregions

Pyramidal neurons in hippocampal subregions are selectively vulnerable in certain disease states. To investigate, we tested the hypothesis that selective vulnerability in human hippocampus is related to regional differences in neuronal cell death and cell receptor gene expression in CA1 vs. CA3 subregions. We used laser capture microdissection to remove approximately 600 CA1 and 600 CA3 pyramidal neurons each from five fresh-frozen normal post-mortem brains, extracted total RNA and double-amplified mRNA. This was reverse transcribed and labeled for hybridization onto human cDNA array chips containing probes to 10,174 genes and unknown ESTs. RNA from additional microdissections was pooled for replicate hybridizations and quantitative RT-PCR validation. Gene expression differences were few (< 1%). We found 43 enriched genes in CA1 neuronal samples that included peripheral benzodiazipine receptor-associated protein, nicotinic cholinergic receptor, two chemokine receptors (CCR1 and CCR5) and several transcriptional factors. We found 17 enriched genes in the CA3 neuronal samples that included fibroblast growth factor receptor and prostaglandin-endoperoxide synthase 1. We found no differential gene expression for 23 calcium channel proteins; nine transporter proteins; 55 cell death and apoptotic regulator proteins; and an additional 497 cell receptors, including 24 glutamate receptors. Quantitative RT-PCR of four differentially expressed genes confirmed the microarray data. The results confirm the ability to examine gene expression profiles in microdissected neurons from human autopsy brain. They show only minor gene expression differences between two distinct neuronal populations in the hippocampus and suggest that selective hippocampal vulnerability is due to factors other than intrinsic differential expression in glutamate receptors and cell death genes.

Lipoprotein lipase and its role in regulation of plasma lipoproteins and cardiac risk

For over 50 years, biologists and clinicians have studied lipoprotein lipase (LPL) and learned about its structure, function, cellular production, physiology, and human genetics. LPL is the principal enzyme that removes triglyceride from the bloodstream. It also determines plasma levels of high-density lipoprotein. Surprisingly, within the past several years, a number of new and unexpected proteins have been discovered that regulate the actions of LPL. These include the very low-density lipoprotein receptor, angiopoetin-like protein 3, and apolipoprotein A-V. In addition, mouse genetic studies have confirmed tissue culture findings of nonenzymatic roles of LPL both in lipid metabolism and atherogenesis. These basic observations are now being related to new information on human genetic polymorphism in this gene that is likely to affect clinical evaluation of lipoprotein disorders and cardiac risk.

The H+ allele of the lipoprotein lipase (LPL) HindIII intronic polymorphism and the risk for sporadic late-onset Alzheimer’s disease

A sample of 243 Italian patients affected by the sporadic late-onset form of Alzheimer's disease (AD) was studied for the HindIII intronic polymorphism of the lipoprotein lipase (LPL) gene and compared with a sample of 148 healthy subjects. Since this polymorphism has been reported to be associated with CAD and because the two pathologies share common aspects, we decided to study it in AD too. We found a difference in the allele distribution, in that the H+ allele was more frequent in patients (0.782) than in controls (0.720); this difference was not quite significant (P = 0.059). The odds ratio from the logistic regression analysis for the H+ carrying genotypes was 2.7 (95% CI = 1.01-7.21; P = 0.048). When the separate genotypes H+H+ and H+H- were entered into the analysis, only H+H+ was found to significantly increase the risk with respect to H-H- (P = 0.029). This means that carrying this allele significantly increases the risk of developing AD, and the risk is mostly associated with the H+H+ genotype.

Requirement for enzymatically active lipoprotein lipase in neuronal differentiation: a site-directed mutagenesis study

Lipoprotein lipase (LPL) is well known for its role in the catabolism of plasma triglyceride (Tg)-rich lipoproteins, such as very low density lipoproteins (VLDL) and chylomicrons. The action of LPL on Tg-rich lipoproteins provides free fatty acids to skeletal muscle and adipose tissues, the main sites of LPL synthesis. Several studies have demonstrated that LPL is widely expressed in the parenchyma of brain tissues. We have recently shown that LPL expression is essential for promoting VLDL-stimulated differentiation of Neuro-2A cells. In the present study, we have generated stably transfected Neuro-2A cell lines expressing either wild-type LPL or various LPL mutants, including three enzymatically inactive variants (Asp156Asn, Gly188Glu and Pro207Leu), an enzymatically defective variant (Asn291Ser) and a variant known to express increased LPL activity (Ser447Ter). In Neuro-2A cells expressing enzymatically inactive LPL variants, VLDL-stimulated differentiation and neurite extension were not observed. However, in Neuro-2A cells expressing partially active or overactive LPL variants, VLDL added to the cultured medium was able to induce the phenotypic differentiation similar to that observed in Neuro-2A cells expressing wild-type LPL. In summary, these data show that the availability of fatty acids, resulting from the catabolism of VLDL by LPL, is required to promote the phenotypical differentiation of neuroblastoma cells. These findings may have significant relevance to lipoprotein metabolism in the brain as well as to the maturation and regeneration of nervous tissues in carriers of mutant LPL.

Lipoprotein lipase and endothelial lipase expression in mouse brain: regional distribution and selective induction following kainic acid-induced lesion and focal cerebral ischemia

Lipoprotein and endothelial lipases are members of the triglyceride lipase gene family. These genes are expressed in the brain, where the encoded proteins are fulfilling functions that have yet to be elucidated. In this study, we examined the distribution of their respective mRNAs in the C57BL/6 mouse brain by in situ hybridization. In control mice, we observed widespread expression of lipoprotein lipase (LPL) mRNA mainly in pyramidal cells of the hippocampus (CA1, CA2 and CA3 areas), in the striatum and in several cortical areas. Endothelial lipase (EL) mRNA expression was restricted to CA3 pyramidal cells of the hippocampus, to ependymal cells in the ventral part of the third ventricle and to some cortical cell layers. To gain insight into the role played by lipases in the brain, neurodegeneration was induced by intraperitoneal injection of kainic acid (KA) or by occlusion of the middle cerebral artery (MCA). Upon injection of KA, a rapid increase in EL mRNA expression was observed in the piriform cortex, hippocampus, thalamus and neocortex. However, the levels of LPL mRNA were unaffected by KA injection. Remarkably, after focal cerebral ischemia, the expression of EL was unaffected whereas a dramatic increase in LPL expression was observed in neocortical areas of the lesioned side of the brain. These results show that LPL and EL transcripts are selectively upregulated in function of the type of brain injury. LPL and EL could thus fulfill a function in the pathophysiological response of the brain to injury.

Lipoprotein lipase affects the survival and differentiation of neural cells exposed to very low density lipoprotein

Lipoprotein lipase (LPL) is a key enzyme involved in the metabolism of lipoproteins, providing tissues like adipose tissue or skeletal muscle with fatty acids. LPL is also expressed in the brain, fulfilling yet unknown functions. Using a neuroblastoma cell line transfected with a NEO- or a LPL-expression vector, we have developed a model to study the function of LPL in neurons exposed to native or copper-oxidized lipoproteins. The addition to the culture media of VLDL with 10 microm copper sulfate led to a significant reduction in the viability of NEO transfectants whereas LPL-transfectants were protected from this injury. In the presence of VLDL and CuSO(4), LPL transfectants were even able to display significant neurite extension. This neuritogenic effect was also observed in LPL transfectants exposed to native lipoproteins. However, addition of VLDL particles oxidized with CuSO(4) prior to their addition to the culture media resulted in neurotoxic effects on LPL transfectants. These findings suggest that the presence of LPL in cultured neuronal cells modulates the physiological response of neurons following exposure to native or oxidized lipoproteins. LPL could thus play a key role in the differentiation of Neuro-2A cells and in the pathophysiological effects of oxidative stress in several neurodegenerative disorders.

Lipoprotein lipase: genetics, lipid uptake, and regulation

Lipoprotein lipase (LPL) regulates the plasma levels of triglyceride and HDL. Three aspects are reviewed. 1) Clinical implications of human LPL gene variations: common mutations and their effects on plasma lipids and coronary heart disease are discussed. 2) LPL actions in the nervous system, liver, and heart: the discussion focuses on LPL and tissue lipid uptake. 3) LPL gene regulation: the LPL promoter and its regulatory elements are described.

Lack of association of two lipoprotein lipase polymorphisms with Alzheimer's disease

A recent genetic study demonstrated associations between an altered risk of Alzheimer's disease (AD) and two polymorphisms in the lipoprotein lipase (LPL) gene, Asn291Ser and Ser447Ter. LPL immunostains senile plaques, and is a ligand of the low-density lipoprotein receptor-related protein (LRP), a major apolipoprotein E (apoE) receptor. LPL increases the cellular uptake of apoE via LRP, and polymorphisms in LPL alter its ability to mediate apoE-LRP interactions, with potential implications for AD pathogenesis. Here, we tested the genetic association of LPL with AD in a case-control study. For the Asn291Ser polymorphism, we analyzed 277 individuals (141 AD, 136 control) and found no significant difference in allele frequencies between the AD and control groups. For the Ser447Ter polymorphism, we analyzed 187 individuals (108 AD, 79 control) and again found no significant difference in allele frequencies between the AD and control groups. Thus, our study does not support associations between AD and two common polymorphisms in LPL.

Candidate gene association studies in sporadic Alzheimer's disease

The genetics of Alzheimer's disease (AD) is complex. Three genes (amyloid precursor protein, presenilin 1 and presenilin 2) have been described in the relatively rare, early-onset, autosomal dominant familial form of AD. In the common, non-familial (sporadic) late-onset AD, the major known genetic risk factor is the epsilon4 allele of the apolipoprotein E (APOE) gene. However, at least half of the people who develop AD do not carry this allele, and not all people who do carry this allele develop AD even if they live to an old age. Therefore, approximately 30 other candidate genes involving a protein in a critical pathway in the pathogenesis of disease (principally interaction with amyloid-beta, oxidative stress and inflammation/apoptosis) have been considered as risk factors for sporadic AD. Then these genes have been sequenced in search of genetic variability or polymorphisms, and each putative polymorphism has been reported to alter the risk of AD either directly or by an interaction with the APOE epsilon4 allele. However, positive-association studies with these candidate genes have not been consistently confirmed.

No association between the lipoprotein lipase S447X polymorphism and Alzheimer's disease

Results from various genetic association studies of the lipoprotein lipase (LPL) S447X polymorphism and Alzheimer's disease (AD), range from a statistically significant negative association of clinically examined patients to a non-significant but consistent trend for under-representation of the X447 allele in neuropathologically confirmed subjects. In this report we have compared the distribution of the above polymorphism in an independent group of clinically diagnosed AD patients, including a subgroup where the disease was pathologically confirmed, and a spousal control group. No statistically significant differences emerged from this comparison. We conclude that LPL cannot be a major factor in pathogenesis of AD.

Association of lipoprotein lipase Ser447Ter polymorphism with brain infarction: a population-based neuropathological study

BACKGROUND

Variants of the lipoprotein lipase (LPL) gene have been shown to influence serum lipid levels, risk of coronary heart disease and, as found recently, risk of clinical ischaemic cerebrovascular disease. Here we tested for an association between brain infarction and two common polymorphisms of the LPL gene, Ser447Ter and Asn291 Ser.

METHOD

To avoid ascertainment and selection bias involved in many association studies, we compared the distribution of these polymorphisms in neuropathologically verified patients (n = 119) vs controls (n = 133) derived from a prospective, population-based study (the Vantaa 85+ study).

RESULTS

The LPL Ter447 variant was negatively associated with neuropathologically verified brain infarcts (P = 0.006), and even more strongly with small brain infarcts (P = 0.004). In addition, we found that the Ter447 variant was associated with higher serum HDL chblesterol (P = 0.004) and lower triglyceride levels (P= 0.003), and that it was negatively associated with pathologically verified severe coronary artery disease (P=0.001) in the Vantaa 85+ study sample. The Asn291Ser polymorphism was not significantly associated with brain infarction.

CONCLUSION

The Ter447 variant of LPL is associated with decreased risk of brain infarction and coronary artery disease in our very elderly population.

Cardiovascular risk factors and Alzheimer's disease: a genetic association study in a population aged 85 or over

Increasing evidence suggests a relation between vascular disorders and late-onset Alzheimer's disease (AD). We performed an association analysis of low-density lipoprotein receptor-related protein (LRP), lipoprotein lipase (LPL), and angiotensin converting enzyme (ACE) genes, known to be involved in vascular disorders, and AD. Genotyping was carried out in 113 patients with clinically defined Alzheimer's disease (NINCDS-ADRDA criteria) and 203 non-demented controls in a prospective, population-based study of people aged 85 years or over (Vantaa 85+ Study). Corresponding analysis was performed on 121 neuropathologically verified AD patients (CERAD criteria) and 75 controls derived from the same study population. We did not find significant associations between the polymorphisms studied and AD. However, analysis of the LPL polymorphism showed a weak trend (uncorrected P-value 0.095) towards protection against neuropathologically defined AD. Our study is based on very elderly Finns. Therefore, further studies are warranted in other populations.

Hunting for disease genes in multi-functional diseases

Disease genes may be identified through functional, positional, and candidate gene approaches. Although extensive and often labor-intensive studies such as family linkage analysis, functional investigation of gene products and genome database searches are usually involved, thousands of human disease genes, especially for monogenic diseases with Mendelian transmission, have been identified. However, in diseases caused by more than one gene, or by a combination of genetic and environmental factors, identification of the genes is even more difficult. Common examples include atherosclerosis, cancer, Alzheimer's disease, asthma, diabetes, glaucoma, and age-related macular degeneration. There have been conflicting reports on the roles of associated genes. Even with population-based case-control studies and new statistical methods such as the sib-ship disequilibrium test and the discordant alleles test, there is no agreement on whether alpha2-macroglobulin (A2M) is a gene for Alzheimer's disease. Another example is the inconsistent association between age-related macular degeneration and ATP-binding cassette transporter (ABCR). Ethnic variation causes further complications. In our investigation of LDL-receptor variants in familial hypercholesterolemia, and the trabecular meshwork inducible glucocorticoid response protein, or myocillin (TIGR-MYOC) mutation pattern in primary open angle glaucoma, we did find dissimilar results in Chinese compared to Caucasians. New information from the Human Genome Project and advancements in technologies will aid the search for and confirm identification of disease genes despite such challenges.

Roles for lipoprotein lipase in Alzheimer's disease: an association study

Lipoprotein lipase (LPL) assists lipid transport by transferring lipids between lipoprotein particles and cells. LPL binds apolipoprotein E (apoE) lipoprotein particles and a major apoE receptor, low density lipoprotein receptor related protein (LRP). Because apoE and LRP polymorphisms alter Alzheimer's disease (AD) risk, and LPL itself is found in AD amyloid plaques, we examined whether LPL variants also affect AD risk. In case-control studies in the United States and Canada, the frequencies of two LPL alleles known to affect LPL enzymatic activity were measured in Caucasian AD or elderly normal (N) subjects. Pathologically confirmed subjects in both studies exhibited similar trends toward fewer 447Ter and more 291Ser alleles in AD. Combining results from both countries gave allele frequencies for 447Ter of 13.7% (26/190) in N and 9.4% (80/852) in AD (P = 0.10), and for 291Ser of 0.0% (0/184) in N and 1. 3% (8/636) in AD (P = 0.21). The trend appeared even greater for homozygous 447Ter subjects: 4.2% (4/95) of N vs. 1.4% (6/426) of AD (P = 0.09). These trends are consistent with a putative protective effect of 447Ter and causative effect of 291Ser on AD. Furthermore, brains of AD patients with 447Ter showed trends toward fewer plaques, tangles, and glia, and more neurons and cortical thickness than AD patients without 447Ter. Hippocampal plaques were significantly reduced. LPL might affect hippocampal function and thus dementia via its role as supplier of membrane components or antioxidants to neurons. Alternatively, LPL may play a part in plaque formation through its interaction with apoE and LRP.

The role of cerebral ischemia in Alzheimer's disease

The Alzheimer type of dementia and stroke are known to increase at comparable rates with age. Recent advances suggest that vascular risk factors linked to cerebrovascular disease and stroke in the elderly significantly increase the risk of developing Alzheimer's disease (AD). These include atherosclerosis, atrial fibrillation, coronary artery disease, hypertension, and diabetes mellitus. Moreover, review of various autopsy series shows that 60-90% of AD cases exhibit variable cerebrovascular pathology. Although some vascular lesions such as cerebral amyloid angiopathy, endothelial degeneration, and periventricular white matter lesions are evident in most cases of AD, a third will exhibit cerebral infarction. Despite the interpretation of pathological evidence, longitudinal clinical studies suggest that the co-existence of stroke and AD occurs more than by chance alone. Strokes known to occur in patients with Alzheimer syndrome and most frequently in the oldest old substantially worsen cognitive decline and outcome, implicating some interaction between the disorders. Nevertheless, the nature of a true relationship between the two disorders seems little explored. What predisposes to strokes in underlying cognitive decline or AD? Is it possible that cerebral ischemia is a causal factor for AD? I examined several vascular factors and the vascular pathophysiology implicated in stroke and AD, and propose that cerebral ischemia or oligemia may promote Alzheimer type of changes in the aging brain. Irrespective of the ultimate pathogenetic mechanism, these approaches implicate that management of peripheral vascular disease is important in the treatment or prevention of Alzheimer's disease or mixed dementia.

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.

Common mutations of the lipoprotein lipase gene and their clinical significance

The accumulation of triglyceride-rich lipoproteins is an independent factor for an increased risk for premature arteriosclerosis. Common mutations in the lipoprotein lipase (LPL) gene are at least in part inherited susceptibility factors involved in the age- and sex-dependent phenotypic expression of hypertriglyceridemia. It can be estimated that about 20% of patients with hypertriglyceridemia are carriers of common LPL gene mutations (Asp9Asn, Asn291Ser, Trp86Arg, Gly188Glu, Pro207Leu, Asp250Asn) associated with the HLP. Genotyping of these LPL gene mutations is recommended especially in patients with high risk for premature arteriosclerosis. A comparably high number of individuals are carriers of common mutations (Ser447X) or silent mutations (Thr361) associated with low favorable lipids.