Piers Ruddle

Specific Loss of Brain ABCA1 Increases Brain Cholesterol Uptake and Influences Neuronal Structure and Function

The expression of the cholesterol transporter ATP-binding cassette transporter A1 (ABCA1) in the brain and its role in the lipidation of apolipoproteins indicate that ABCA1 may play a critical role in brain cholesterol metabolism. To investigate the role of ABCA1 in brain cholesterol homeostasis and trafficking, we characterized mice that specifically lacked ABCA1 in the CNS, generated using the Cre/loxP recombination system. These mice showed reduced plasma high-density lipoprotein (HDL) cholesterol levels associated with decreased brain cholesterol content and enhanced brain uptake of esterified cholesterol from plasma HDL. Increased levels of HDL receptor SR-BI in brain capillaries and apolipoprotein A-I in brain and CSF of mutant mice were evident. Cholesterol homeostasis changes were mirrored by disturbances in motor activity and sensorimotor function. Changes in synaptic ultrastructure including reduced synapse and synaptic vesicle numbers were observed. These data show that ABCA1 is a key regulator of brain cholesterol metabolism and that disturbances in cholesterol transport in the CNS are associated with structural and functional deficits in neurons. Moreover, our findings also demonstrate that specific changes in brain cholesterol metabolism can lead to alterations in cholesterol uptake from plasma to brain.

Cholesterol efflux via ATP-binding cassette transporter A1 (ABCA1) and cholesterol uptake via the LDL receptor influences cholesterol-induced impairment of beta cell function in mice

Aims/hypothesisCellular cholesterol accumulation is an emerging mechanism for beta cell dysfunction in type 2 diabetes. Absence of the cholesterol transporter ATP-binding cassette transporter A1 (ABCA1) results in increased islet cholesterol and impaired insulin secretion, indicating that impaired cholesterol efflux leads to beta cell dysfunction. In this study, we aimed to determine the role of the LDL receptor (LDLr) in islet cholesterol uptake and to assess the contributions of cholesterol uptake compared with efflux to islet cholesterol levels.MethodsIslet cholesterol and beta cell function were assessed in mice lacking LDLr (Ldlr−/−), or apolipoprotein E (Apoe−/−), as well as in mice with beta-cell-specific deficiency of Abca1 crossed to Ldlr−/− mice.ResultsHypercholesterolaemia resulted in increased islet cholesterol levels and decreased beta cell function in Apoe−/− mice but not in Ldlr−/− mice, suggesting that the LDL receptor is required for cholesterol uptake leading to cholesterol-induced beta cell dysfunction. Interestingly, when wild-type islets with functional LDL receptors were transplanted into diabetic, hypercholesterolaemic mice, islet graft function was normal compared with Ldlr−/− islets, suggesting that compensatory mechanisms can maintain islet cholesterol homeostasis in a hypercholesterolaemic environment. Indeed, transplanted wild-type islets had increased Abca1 expression. However, lack of the Ldlr did not protect Abca1−/− mice from islet cholesterol accumulation, suggesting that cholesterol efflux is the critical regulator of cholesterol levels in islets.Conclusions/interpretationOur data indicate that islet cholesterol levels and beta cell function are strongly influenced by LDLr-mediated uptake of cholesterol into beta cells. Cholesterol efflux mediated by ABCA1, however, can compensate in hypercholesterolaemia to regulate islet cholesterol levels in vivo.

Despite antiatherogenic metabolic characteristics, SCD1-deficient mice have increased inflammation and atherosclerosis

Objective—Absence of stearoyl-CoA desaturase-1 (SCD1) in mice reduces plasma triglycerides and provides protection from obesity and insulin resistance, which would be predicted to be associated with reduced susceptibility to atherosclerosis. The aim of this study was to determine the effect of SCD1 deficiency on atherosclerosis. Methods and Results—Despite an antiatherogenic metabolic profile, SCD1 deficiency increases atherosclerosis in hyperlipidemic low-density lipoprotein receptor (LDLR)-deficient mice challenged with a Western diet. Lesion area at the aortic root is significantly increased in males and females in two models of SCD1 deficiency. Inflammatory changes are evident in the skin of these mice, including increased intercellular adhesion molecule (ICAM)-1 and ulcerative dermatitis. Increases in ICAM-1 and interleukin-6 are also evident in plasma of SCD1-deficient mice. HDL particles demonstrate changes associated with inflammation, including decreased plasma apoA-II and apoA-I and paraoxonase-1 and increased plasma serum amyloid A. Lipopolysaccharide-induced inflammatory response and cholesterol efflux are not altered in SCD1-deficient macrophages. In addition, when SCD1 deficiency is limited to bone marrow–derived cells, lesion size is not altered in LDLR-deficient mice. Conclusions—These studies reinforce the crucial role of chronic inflammation in promoting atherosclerosis, even in the presence of antiatherogenic biochemical and metabolic characteristics.

Absence of stearoyl-CoA desaturase-1 ameliorates features of the metabolic syndrome in LDLR-deficient mice

A combination of the interrelated metabolic risk factors obesity, insulin resistance, dyslipidemia, and hypertension, often described as the “metabolic syndrome,” is known to increase the risk of developing cardiovascular disease and diabetes. Stearoyl-coenzyme A desaturase (SCD) activity has been implicated in the metabolic syndrome, but detailed studies of the beneficial metabolic effects of SCD deficiency have been limited. Here, we show that absence of the Scd1 gene product reduces plasma triglycerides and reduces weight gain in severely hyperlipidemic low density lipoprotein receptor (LDLR)-deficient mice challenged with a Western diet. Absence of SCD1 also increases insulin sensitivity, as measured by intraperitoneal glucose and insulin tolerance testing. SCD1 deficiency dramatically reduces hepatic lipid accumulation while causing more modest reductions in plasma apolipoproteins, suggesting that in conditions of sustained hyperlipidemia, SCD1 functions primarily to mediate lipid stores. In addition, absence of SCD1 partially ameliorates the undesirable hypertriglyceridemic effect of antiatherogenic liver X receptor agonists. Our results demonstrate that constitutive reduction of SCD activity improves the metabolic phenotype of LDLR-deficient mice on a Western diet.

ABCA1 in adipocytes regulates adipose tissue lipid content, glucose tolerance, and insulin sensitivity

Adipose tissue contains one of the largest reservoirs of cholesterol in the body. Adipocyte dysfunction in obesity is associated with intracellular cholesterol accumulation, and alterations in cholesterol homeostasis have been shown to alter glucose metabolism in cultured adipocytes. ABCA1 plays a major role in cholesterol efflux, suggesting a role for ABCA1 in maintaining cholesterol homeostasis in the adipocyte. However, the impact of adipocyte ABCA1 on adipose tissue function and glucose metabolism is unknown. Our aim was to determine the impact of adipocyte ABCA1 on adipocyte lipid metabolism, body weight, and glucose metabolism in vivo. To address this, we used mice lacking ABCA1 specifically in adipocytes (ABCA1−ad/−ad). When fed a high-fat, high-cholesterol diet, ABCA1−ad/−ad mice showed increased cholesterol and triglyceride stores in adipose tissue, developed enlarged fat pads, and had increased body weight. Associated with these phenotypic changes, we observed significant changes in the expression of genes involved in cholesterol and glucose homeostasis, including ldlr, abcg1, glut-4, adiponectin, and leptin. ABCA1−ad/−ad mice also demonstrated impaired glucose tolerance, lower insulin sensitivity, and decreased insulin secretion. We conclude that ABCA1 in adipocytes influences adipocyte lipid metabolism, body weight, and whole-body glucose homeostasis.

Loss of Cyp8b1 Improves Glucose Homeostasis by Increasing GLP-1

Besides their role in facilitating lipid absorption, bile acids are increasingly being recognized as signaling molecules that activate cell-signaling receptors. Targeted disruption of the sterol 12α-hydroxylase gene (Cyp8b1) results in complete absence of cholic acid (CA) and its derivatives. Here we investigate the effect of Cyp8b1 deletion on glucose homeostasis. Absence of Cyp8b1 results in improved glucose tolerance, insulin sensitivity, and β-cell function, mediated by absence of CA in Cyp8b1−/− mice. In addition, we show that reduced intestinal fat absorption in the absence of biliary CA leads to increased free fatty acids reaching the ileal L cells. This correlates with increased secretion of the incretin hormone GLP-1. GLP-1, in turn, increases the biosynthesis and secretion of insulin from β-cells, leading to the improved glucose tolerance observed in the Cyp8b1−/− mice. Thus, our data elucidate the importance of Cyp8b1 inhibition on the regulation of glucose metabolism.

ABCA1 influences neuroinflammation and neuronal death

ATP-binding cassette transporter A1 (ABCA1) mediates cellular cholesterol efflux in the brain and influences whole brain cholesterol homeostasis. Activation of liver X receptors (LXRs), transcription factors that increase the expression of cholesterol transport genes including ABCA1, reduces neuroinflammation and pathology in neurodegenerative animal models suggesting that in addition to its involvement in cholesterol transport, ABCA1 may play a role in modulating the inflammatory response in the brain. We investigated the cell-type specific role of ABCA1 in neuroinflammation in vivo using mice specifically lacking brain ABCA1 (ABCA1(-B/-B)) as well as mice lacking neuronal (ABCA1(-N/-N)) and astrocytic (ABCA1(-Ast/-Ast)) ABCA1. ABCA1(-B/-B) mice exhibit cortical astrogliosis, increased inflammatory gene expression as well as activation of mitogen-activated protein kinases (MAPKs) following acute lipopolysaccharide (LPS) administration. Microglia cultured from ABCA1(-B/-B) mice exhibit augmented LPS-induced secretion of tumor necrosis factor α (TNFα) and decreased phagocytic activity, indicating an increase in a pro-inflammatory response. ABCA1(-N/-N) mice develop astrogliosis but show no change in inflammatory gene expression. Intriguingly, ABCA1(-Ast/-Ast) mice show neither astrogliosis nor elevated expression of inflammatory markers. Cortical apolipoprotein E (apoE) levels are reduced in ABCA1(-Ast/-Ast) but not in ABCA1(-N/-N) mice, providing in vivo evidence for the specific role of astrocyte ABCA1 in regulating brain apoE levels. Interestingly, cortical neuronal death is increased in 17month-old ABCA1(-B/-B) mice but not in ABCA1(-N/-N) or ABCA1(-Ast/-Ast) mice. Our findings suggest that coordinated ABCA1 activity across neurons and glial cells influences neuroinflammation and neurodegeneration.

Hepatic ABCA1 Expression Improves β-Cell Function and Glucose Tolerance

Low HDL is a risk factor for the development of type 2 diabetes. Hepatic ABCA1 is the rate-limiting protein in HDL biogenesis, and mice lacking hepatic ABCA1 (ABCA1-l/-l) have very low plasma HDL concentrations. To investigate the role of hepatic ABCA1 in glucose tolerance and β-cell function, we used ABCA1-l/-l mice, which showed impaired glucose tolerance without changes in insulin sensitivity. Insulin secretion was reduced following glucose gavage. Ex vivo, glucose stimulated insulin secretion from β-cells from wild-type (WT) and ABCA1-l/-l mice was similar. Insulin secretion was, however, reduced upon addition of ABCA1-l/-l serum to the medium compared with WT serum, whereas islets lacking β-cell ABCA1 were not affected differently by ABCA1-l/-l or WT serum. After high-fat feeding, WT and ABCA1-l/-l mice showed no difference in glucose tolerance or insulin secretion, and serum from ABCA1-l/-l and WT mice fed a high-fat diet did not affect insulin secretion differently. We conclude that hepatic ABCA1 improves glucose tolerance by improving β-cell function through both HDL production and interaction with β-cell ABCA1. The beneficial effect of hepatic ABCA1 is decreased under metabolic stress. Increasing hepatic ABCA1 may represent a novel therapeutic strategy for improving glucose homeostasis in diabetes.

Hepatic Abca1 Expression Improves Β-Cell Function And Glucose Tolerance Hepatic ABCA1 improves β-cell function

Low HDL is a risk factor for the development of type 2 diabetes. Hepatic ABCA1 is the rate-limiting protein in HDL biogenesis, and mice lacking hepatic ABCA1 (ABCA1-l/-l) have very low plasma HDL concentrations. To investigate the role of hepatic ABCA1 in glucose tolerance and β-cell function, we used ABCA1-l/-l mice, which showed impaired glucose tolerance without changes in insulin sensitivity. Insulin secretion was reduced following glucose gavage. Ex vivo, glucose stimulated insulin secretion from β-cells from wild-type (WT) and ABCA1-l/-l mice was similar. Insulin secretion was, however, reduced upon addition of ABCA1-l/-l serum to the medium compared with WT serum, whereas islets lacking β-cell ABCA1 were not affected differently by ABCA1-l/-l or WT serum. After high-fat feeding, WT and ABCA1-l/-l mice showed no difference in glucose tolerance or insulin secretion, and serum from ABCA1-l/-l and WT mice fed a high-fat diet did not affect insulin secretion differently. We conclude that hepatic ABCA1 improves glucose tolerance by improving β-cell function through both HDL production and interaction with β-cell ABCA1. The beneficial effect of hepatic ABCA1 is decreased under metabolic stress. Increasing hepatic ABCA1 may represent a novel therapeutic strategy for improving glucose homeostasis in diabetes.

Antisense oligonucleotide-mediated inhibition of miR-33 in cultured neurons, astrocytes and microglia: Effects on ABCA1 expression, APOE lipidation, cellular cholesterol and beta-amyloid neurotoxicity

Background: DSP-8658, a novel selective peroxisome proliferator-activated receptor (PPAR)a/gmodulator, is a Phase I clinical stage drug candidate for the treatment of Alzheimer’s disease (AD). We have previously reported that DSP-8658 enhances the microglial uptake of amyloid b-peptide(Ab), which is a proposed trigger of the onset of AD, and also improves cognitive function in APP/PS1 transgenic mice. The present study describes the acute or subacute symptomatic effects of DSP-8658 on cognitive function and the results in 104-week rat carcinogenicity study.Methods: To investigate procognitive effects of DSP-8658 at 1-30 mg/kg, p.o., we performed a mouse object recognition test, a mouse Y-maze test and a rat passive avoidance test. To clarify the safety profile of DSP-8658 more, the 104-week carcinogenicity study of rats was conducted. Results: DSP8658 (30mg/kg) significantly prevented the object memory loss in aged mice. DSP-8658 (3 mg/kg) also improved the spatial working memory impairment caused by LPS-induced neuroinflammation in mice. In the rat passive avoidance test, DSP-8658 (10mg/kg) significantly ameliorated scopolamine-induced memory impairment. On the other hand, Wy14643 (PPARa agonist) and Rosiglitazone (PPARg agonist) also showed procognitive effects, but those maximum effects were less than that of DSP8658. These results indicate that DSP-8658 has a great potential to restore the memory deficits caused by aging, cholinergic dysfunction and inflammation, which are the important characteristic features of AD pathology. The potent efficacies of DSP-8658 on procognitive function, which is superior to those of the single agonists of PPAR subtypes, may be mediated by its dual agonistic activities on both PPARa and g. In the 104-week rat carcinogenicity study, DSP-8658 induced tumors only at the highest doses producing AUC 10-fold therapeutic exposures with the maximum recommended human dose, and had enough cardiovascular safety margin, indicating that DSP-8658 exhibits good safety profiles as a PPARmodulator.Conclusions: Based on the nonclinical pharmacological and safety data, DSP-8658 has a highly promising therapeutic profile as a drug for both symptomatic and disease-modifying treatment of AD. DSP-8658 is expected to be efficacious for AD in the following clinical trials.