Obesity, metabolic syndrome (MetS) and type 2 diabetes (T2D) are associated with decreased cognit... more Obesity, metabolic syndrome (MetS) and type 2 diabetes (T2D) are associated with decreased cognitive function. While weight loss and T2D remission result in improvements in metabolism and vascular function, it is less clear if these benefits extend to cognitive performance. Here, we highlight the malleable nature of MetS-associated cognitive dysfunction using a mouse model of high fat diet (HFD)-induced MetS. While learning and memory was generally unaffected in mice with type 1 diabetes (T1D), multiple cognitive impairments were associated with MetS, including deficits in novel object recognition, cued fear memory, and spatial learning and memory. However, a brief reduction in dietary fat content in chronic HFD-fed mice led to a complete rescue of cognitive function. Cerebral blood volume (CBV), a measure of vascular perfusion, was decreased during MetS, was associated with long term memory, and recovered following the intervention. Finally, repeated infusion of plasma collected from age-matched, low fat diet-fed mice improved memory in HFD mice, and was associated with a distinct metabolic profile. Thus, the cognitive dysfunction accompanying MetS appears to be amenable to treatment, related to cerebrovascular function, and mitigated by systemic factors.
Human apolipoprotein E (apoE) exists in three isoforms: apoE2, apoE3 and apoE4. APOE ε4 is a majo... more Human apolipoprotein E (apoE) exists in three isoforms: apoE2, apoE3 and apoE4. APOE ε4 is a major genetic risk factor for cardiovascular disease (CVD) and Alzheimer's disease (AD). ApoE mediates cholesterol metabolism by binding various receptors. The low-density lipoprotein receptor (LDLR) has a high affinity for apoE, and is the only member of its receptor family to demonstrate an apoE isoform specific binding affinity (E4 N E3N N E2). Evidence suggests that a functional interaction between apoE and LDLR influences the risk of CVD and AD. We hypothesize that the differential cognitive effects of the apoE isoforms are a direct result of their varying interactions with LDLR. To test this hypothesis, we have employed transgenic mice that express human apoE2, apoE3, or apoE4, and either human LDLR (hLDLR) or no LDLR (LDLR −/−). Our results show that plasma and brain apoE levels, cortical cholesterol, and spatial memory are all regulated by isoform-dependent interactions between apoE and LDLR. Conversely, both anxiety-like behavior and cued associative memory are strongly influenced by APOE genotype, but these processes appear to occur via an LDLR-independent mechanism. Both the lack of LDLR and the interaction between E4 and the LDLR were associated with significant impairments in the retention of long term spatial memory. Finally, levels of hippocampal apoE correlate with long term spatial memory retention in mice with human LDLR. In summary, we demonstrate that the apoE-LDLR interaction affects regional brain apoE levels, brain cholesterol, and cognitive function in an apoE isoform-dependent manner.
Oxidative stress (OS) and reactive oxygen species (ROS) play a modulatory role in synaptic plasti... more Oxidative stress (OS) and reactive oxygen species (ROS) play a modulatory role in synaptic plasticity and signaling pathways. Mitochondria (MT), a major source of ROS because of their involvement in energy metabolism, are important for brain function. MT-generated ROS are proposed to be responsible for a significant proportion of OS and are associated with developmental abnormalities and aspects of cellular aging. The role of ROS and MT function in cognition of healthy individuals is relatively understudied. In this study, we characterized behavioral and cognitive performance of 5-to 6-month-old mice over-expressing mitochondrial catalase (MCAT). MCAT mice showed enhancements in hippocam-pus-dependent spatial learning and memory in the water maze and contextual fear conditioning, and reduced measures of anxiety in the elevated zero maze. Catalase activity was elevated in MCAT mice in all brain regions examined. Measures of oxidative stress (glutathione, protein carbonyl content, lipid peroxidation, and 8-hydroxyguanine) did not significantly differ between the groups. The lack of differences in these markers of oxidative stress suggests that the differences observed in this study may be due to altered redox signaling. Catalase over-expression might be sufficient to enhance cognition and reduce measures of anxiety even in the absence of alteration in levels of OS.
Relatively little is known about early irradiation effects on hippocampal function in wild-type m... more Relatively little is known about early irradiation effects on hippocampal function in wild-type mice. In this study, the effects of 56Fe irradiation on hippocampal function were assessed starting 2 weeks after whole-body irradiation. Compared to sham irradiation, radiation impaired novel object recognition in female and male C57BL/6J wild-type mice. There were no effects of irradiation on contextual fear conditioning or spatial memory retention in the water maze. It is possible that oxidative damage might contribute to radiation-induced cognitive changes. Therefore, hippocampal and cortical levels of 3-nitrotyrosine (3NT) and lipid peroxidation, measures of oxidative damage were assessed. There were no effects of irradiation on these measures of oxidative damage. As 56Fe irradiation can increase reactive oxygen species (ROS) levels, which may contribute to the impairments in novel object recognition, the effects of the antioxidant alpha-lipoic acid (ALA) on cognition following sham irradiation and irradiation were also assessed. ALA did not prevent radiation-induced impairments in novel object recognition and impaired spatial memory retention of sham-irradiated and irradiated mice in the probe trial after the first day of hidden platform training in the water maze. Thus, the novel object recognition test is particularly sensitive to detect early cognitive effects of 56Fe irradiation through a mechanism unlikely involving ROS or oxidative damage.
Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, Jan 10, 2015
Apolipoprotein E (apoE) is an essential component of lipoprotein particles in both the brain and ... more Apolipoprotein E (apoE) is an essential component of lipoprotein particles in both the brain and periphery, and exists in three isoforms in the human population: E2, E3, and E4. ApoE has numerous, well established roles in neurobiology. Most notably, E4 is associated with earlier onset and increased risk of Alzheimer's disease (AD). While possession of E2 is protective in the context of AD, E2 appears to confer an increased incidence and severity of posttraumatic stress disorder (PTSD). However, the biological processes underlying this link remain unclear. In this study, we began to elucidate these associations by examining the effects of apoE on PTSD severity in combat veterans, and on PTSD-like behavior in mice with human apoE. In a group of 92 veterans with PTSD, we observed significantly higher Clinician Administered PTSD Scale (CAPS) and PTSD checklist (PCL) scores in E2+ individuals, as well as alterations in salivary cortisol levels. Furthermore, we measured behavioral an...
Impaired adipogenesis renders an adipose tissue unable to expand, leading to lipotoxicity and con... more Impaired adipogenesis renders an adipose tissue unable to expand, leading to lipotoxicity and conditions such as diabetes and cardiovascular disease. While factors important for adipogenesis have been studied extensively, those that set the limits of adipose tissue expansion remain undetermined. Feeding a Western-type diet to apolipoprotein E2 knock-in mice, a model of metabolic syndrome, produced 3 groups of equally obese mice: mice with normal glucose tolerance, hyperinsulinemic yet glucose-tolerant mice, and prediabetic mice with impaired glucose tolerance and reduced circulating insulin. Using proteomics, we compared subcutaneous adipose tissues from mice in these groups and found that the expression of PTRF (polymerase I and transcript release factor) associated selectively with their glucose tolerance status. Lentiviral and pharmacologically overexpressed PTRF, whose function is critical for caveola formation, compromised adipocyte differentiation of cultured 3T3-L1cells. In human adipose tissue, PTRF mRNA levels positively correlated with markers of lipolysis and cellular senescence. Furthermore, a negative relationship between telomere length and PTRF mRNA levels was observed in human subcutaneous fat. PTRF is associated with limited adipose tissue expansion underpinning the key role of caveolae in adipocyte regulation. Furthermore, PTRF may be a suitable adipocyte marker for predicting pathological obesity and inform clinical management.
Human apolipoprotein E (apoE) exists in three isoforms: apoE2, apoE3 and apoE4. APOE ε4 is a majo... more Human apolipoprotein E (apoE) exists in three isoforms: apoE2, apoE3 and apoE4. APOE ε4 is a major genetic risk factor for cardiovascular disease (CVD) and Alzheimer's disease (AD). ApoE mediates cholesterol metabolism by binding various receptors. The low-density lipoprotein receptor (LDLR) has a high affinity for apoE, and is the only member of its receptor family to demonstrate an apoE isoform specific binding affinity (E4>E3>E2). Evidence suggests that a functional interaction between apoE and LDLR influences the risk of CVD and AD. We hypothesize that the differential cognitive effects of the apoE isoforms are a direct result of their varying interactions with LDLR. To test this hypothesis, we have employed transgenic mice that express human apoE2, apoE3, or apoE4, and either human LDLR (hLDLR) or no LDLR (LDLR(-/-)). Our results show that plasma and brain apoE levels, cortical cholesterol, and spatial memory are all regulated by isoform-dependent interactions between apoE and LDLR. Conversely, both anxiety-like behavior and cued associative memory are strongly influenced by APOE genotype, but these processes appear to occur via an LDLR-independent mechanism. Both the lack of LDLR and the interaction between E4 and the LDLR were associated with significant impairments in the retention of long term spatial memory. Finally, levels of hippocampal apoE correlate with long term spatial memory retention in mice with human LDLR. In summary, we demonstrate that the apoE-LDLR interaction affects regional brain apoE levels, brain cholesterol, and cognitive function in an apoE isoform-dependent manner.
Diabetes is a major risk factor for cardiovascular disease. To examine how diabetes interacts wit... more Diabetes is a major risk factor for cardiovascular disease. To examine how diabetes interacts with a mildly compromised lipid metabolism, we introduced the diabetogenic Ins2(C96Y/+) (Akita) mutation into mice expressing human apoE4 (E4) combined with either an overexpressing human LDL receptor gene (hLDLR) or the wild-type mouse gene. The hLDLR allele caused 2-fold reductions in plasma HDL-cholesterol, plasma apoA1, and hepatic triglyceride secretion. Diabetes increased plasma total cholesterol 1.3-fold and increased apoB48 secretion 3-fold, while reducing triglyceride secretion 2-fold. Consequently, diabetic E4 mice with hLDLR secrete increased numbers of small, cholesterol-enriched, apoB48-containing VLDL, although they have near normal plasma cholesterol (<120 mg/dl). Small foam cell lesions were present in the aortic roots of all diabetic E4 mice with hLDLR that we analyzed at six months of age. None were present in nondiabetic mice or in diabetic mice without hLDLR. Aortic expression of genes affecting leukocyte recruitment and adhesion was enhanced by diabetes. ApoA1 levels, but not diabetes, were strongly correlated with the ability of plasma to efflux cholesterol from macrophages. We conclude that the diabetes-induced proinflammatory changes in the vasculature and the hLDLR-mediated cholesterol accumulation in macrophages synergistically trigger atherosclerosis in mice with human apoE4, although neither alone is sufficient.
We investigated the differential roles of apolipoprotein E (apoE) isoforms in modulating diabetic... more We investigated the differential roles of apolipoprotein E (apoE) isoforms in modulating diabetic dyslipidemia-a potential cause of the increased cardiovascular disease risk of patients with diabetes. Diabetes was induced using streptozotocin (STZ) in human apoE3 (E3) or human apoE4 (E4) mice deficient in the LDL receptor (LDLR(-/-)). Diabetic E3LDLR(-/-) and E4LDLR(-/-) mice have indistinguishable levels of plasma glucose and insulin. Despite this, diabetes increased VLDL triglycerides and LDL cholesterol in E4LDLR(-/-) mice twice as much as in E3LDLR(-/-) mice. Diabetic E4LDLR(-/-) mice had similar lipoprotein fractional catabolic rates compared with diabetic E3LDLR(-/-) mice but had larger hepatic fat stores and increased VLDL secretion. Diabetic E4LDLR(-/-) mice demonstrated a decreased reliance on lipid as an energy source based on indirect calorimetry. Lower phosphorylated acetyl-CoA carboxylase content and higher gene expression of fatty acid synthase in the liver indicated reduced fatty acid oxidation and increased fatty acid synthesis. E4LDLR(-/-) primary hepatocytes cultured in high glucose accumulated more intracellular lipid than E3LDLR(-/-) hepatocytes concomitant with a 60% reduction in fatty acid oxidation. Finally, the exaggerated dyslipidemia in diabetic E4LDLR(-/-) mice was accompanied by a dramatic increase in atherosclerosis. ApoE4 causes severe dyslipidemia and atherosclerosis independent of its interaction with LDLR in a model of STZ-induced diabetes. ApoE4-expressing livers have reduced fatty acid oxidation, which contributes to the accumulation of tissue and plasma lipids.
Arteriosclerosis Thrombosis and Vascular Biology, 2008
To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, ... more To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, on the production of LDL and development of atherosclerosis in vivo. Ldlr(-/-)Apobec1(-/-) mice lacking the LDLR and apoB editing enzyme accumulated LDL in plasma and developed severe atherosclerosis when they had wild-type apoB100. In marked contrast, in Ldlr(-/-)Apobec1(-/-) mice carrying the Apob100-beta mutation, in the 2 putative LDLR-binding domains of apoB prevented both LDL accumulation and atherosclerosis. Intestinal absorption of lipids and triglyceride secretion from the liver were not affected. However, the VLDL particles with apoB100-beta were larger in volume by about 70%, and carried approximately four times as much apoE per particle. ApoB100-beta synthesis rate in the primary hepatocytes was normal, but its intracellular degradation was enhanced. Additionally, mutant apoB100 VLDL cleared from the circulation more quickly in vivo through apoE-LRP-mediated mechanism than VLDL with wild-type apoB100. In contrast, uptake of the 2 VLDL by macrophages were not different. While conformational change to apoB100 during conversion of VLDL to LDL exposes LDLR binding domains and facilitates LDLR-mediated lipoprotein clearance, it may also inhibit LRP-mediated VLDL uptake and contribute to LDL accumulation in familial hypercholesterolemia.
To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, ... more To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, on the production of LDL and development of atherosclerosis in vivo. Ldlr(-/-)Apobec1(-/-) mice lacking the LDLR and apoB editing enzyme accumulated LDL in plasma and developed severe atherosclerosis when they had wild-type apoB100. In marked contrast, in Ldlr(-/-)Apobec1(-/-) mice carrying the Apob100-beta mutation, in the 2 putative LDLR-binding domains of apoB prevented both LDL accumulation and atherosclerosis. Intestinal absorption of lipids and triglyceride secretion from the liver were not affected. However, the VLDL particles with apoB100-beta were larger in volume by about 70%, and carried approximately four times as much apoE per particle. ApoB100-beta synthesis rate in the primary hepatocytes was normal, but its intracellular degradation was enhanced. Additionally, mutant apoB100 VLDL cleared from the circulation more quickly in vivo through apoE-LRP-mediated mechanism than VLDL with wild-type apoB100. In contrast, uptake of the 2 VLDL by macrophages were not different. While conformational change to apoB100 during conversion of VLDL to LDL exposes LDLR binding domains and facilitates LDLR-mediated lipoprotein clearance, it may also inhibit LRP-mediated VLDL uptake and contribute to LDL accumulation in familial hypercholesterolemia.
Apolipoprotein E (apoE), a key protein in lipid metabolism, is highly expressed in adipose tissue... more Apolipoprotein E (apoE), a key protein in lipid metabolism, is highly expressed in adipose tissues. Studies have shown that human APOE*4 is associated with a lower body mass index but with a greater risk of coronary heart disease compared with other APOE alleles. To define the isoform-specific role of apoE in regulating the expandability and functionality of adipose tissues, we investigated the effects of diet-induced obesity in mice whose endogenous Apoe gene has been replaced by either the human APOE*3 or APOE*4 allele. After 8 weeks on a Western-type high-fat diet, male APOE4 mice displayed impaired tolerance to glucose and fat overload compared with APOE3 mice. Subcutaneous fat tissues in APOE4 and APOE3 mice after high fat feeding were not different. In contrast, although epididymal fat tissues in APOE4 mice gained 30% less weight during the high fat feeding than in APOE3 mice, they showed impaired insulin-stimulated glucose uptake ex vivo. Epididymal APOE4 adipocytes were larger in size than APOE3 adipocytes, and expressed reduced levels of mRNA for peroxisome proliferator-activated receptor gamma2 and adiponectin, important markers of adipocyte functionality. Adenoviral expression of apoE3 in apoE-null culture adipocytes induced adiponectin mRNA in a dose-dependent manner, but the induction was significantly blunted in cells overexpressing apoE4. However, in contrast to the apoE3-expressing cells, Glut1, but not Glut4, expression levels were positively correlated with increased apoE4 mRNA, suggesting that apoE4 expression in adipocyte interferes in insulin-sensing pathways. Dysfunctional epididymal adipose tissues contribute to the accelerated impairment of glucose tolerance in APOE4 mice fed a Western-type diet. Our results underscore the importance of functionality of individual fat depots rather than total fat mass as a determinant for metabolic disturbance during diet-induced obesity.
Obesity, metabolic syndrome (MetS) and type 2 diabetes (T2D) are associated with decreased cognit... more Obesity, metabolic syndrome (MetS) and type 2 diabetes (T2D) are associated with decreased cognitive function. While weight loss and T2D remission result in improvements in metabolism and vascular function, it is less clear if these benefits extend to cognitive performance. Here, we highlight the malleable nature of MetS-associated cognitive dysfunction using a mouse model of high fat diet (HFD)-induced MetS. While learning and memory was generally unaffected in mice with type 1 diabetes (T1D), multiple cognitive impairments were associated with MetS, including deficits in novel object recognition, cued fear memory, and spatial learning and memory. However, a brief reduction in dietary fat content in chronic HFD-fed mice led to a complete rescue of cognitive function. Cerebral blood volume (CBV), a measure of vascular perfusion, was decreased during MetS, was associated with long term memory, and recovered following the intervention. Finally, repeated infusion of plasma collected from age-matched, low fat diet-fed mice improved memory in HFD mice, and was associated with a distinct metabolic profile. Thus, the cognitive dysfunction accompanying MetS appears to be amenable to treatment, related to cerebrovascular function, and mitigated by systemic factors.
Human apolipoprotein E (apoE) exists in three isoforms: apoE2, apoE3 and apoE4. APOE ε4 is a majo... more Human apolipoprotein E (apoE) exists in three isoforms: apoE2, apoE3 and apoE4. APOE ε4 is a major genetic risk factor for cardiovascular disease (CVD) and Alzheimer's disease (AD). ApoE mediates cholesterol metabolism by binding various receptors. The low-density lipoprotein receptor (LDLR) has a high affinity for apoE, and is the only member of its receptor family to demonstrate an apoE isoform specific binding affinity (E4 N E3N N E2). Evidence suggests that a functional interaction between apoE and LDLR influences the risk of CVD and AD. We hypothesize that the differential cognitive effects of the apoE isoforms are a direct result of their varying interactions with LDLR. To test this hypothesis, we have employed transgenic mice that express human apoE2, apoE3, or apoE4, and either human LDLR (hLDLR) or no LDLR (LDLR −/−). Our results show that plasma and brain apoE levels, cortical cholesterol, and spatial memory are all regulated by isoform-dependent interactions between apoE and LDLR. Conversely, both anxiety-like behavior and cued associative memory are strongly influenced by APOE genotype, but these processes appear to occur via an LDLR-independent mechanism. Both the lack of LDLR and the interaction between E4 and the LDLR were associated with significant impairments in the retention of long term spatial memory. Finally, levels of hippocampal apoE correlate with long term spatial memory retention in mice with human LDLR. In summary, we demonstrate that the apoE-LDLR interaction affects regional brain apoE levels, brain cholesterol, and cognitive function in an apoE isoform-dependent manner.
Oxidative stress (OS) and reactive oxygen species (ROS) play a modulatory role in synaptic plasti... more Oxidative stress (OS) and reactive oxygen species (ROS) play a modulatory role in synaptic plasticity and signaling pathways. Mitochondria (MT), a major source of ROS because of their involvement in energy metabolism, are important for brain function. MT-generated ROS are proposed to be responsible for a significant proportion of OS and are associated with developmental abnormalities and aspects of cellular aging. The role of ROS and MT function in cognition of healthy individuals is relatively understudied. In this study, we characterized behavioral and cognitive performance of 5-to 6-month-old mice over-expressing mitochondrial catalase (MCAT). MCAT mice showed enhancements in hippocam-pus-dependent spatial learning and memory in the water maze and contextual fear conditioning, and reduced measures of anxiety in the elevated zero maze. Catalase activity was elevated in MCAT mice in all brain regions examined. Measures of oxidative stress (glutathione, protein carbonyl content, lipid peroxidation, and 8-hydroxyguanine) did not significantly differ between the groups. The lack of differences in these markers of oxidative stress suggests that the differences observed in this study may be due to altered redox signaling. Catalase over-expression might be sufficient to enhance cognition and reduce measures of anxiety even in the absence of alteration in levels of OS.
Relatively little is known about early irradiation effects on hippocampal function in wild-type m... more Relatively little is known about early irradiation effects on hippocampal function in wild-type mice. In this study, the effects of 56Fe irradiation on hippocampal function were assessed starting 2 weeks after whole-body irradiation. Compared to sham irradiation, radiation impaired novel object recognition in female and male C57BL/6J wild-type mice. There were no effects of irradiation on contextual fear conditioning or spatial memory retention in the water maze. It is possible that oxidative damage might contribute to radiation-induced cognitive changes. Therefore, hippocampal and cortical levels of 3-nitrotyrosine (3NT) and lipid peroxidation, measures of oxidative damage were assessed. There were no effects of irradiation on these measures of oxidative damage. As 56Fe irradiation can increase reactive oxygen species (ROS) levels, which may contribute to the impairments in novel object recognition, the effects of the antioxidant alpha-lipoic acid (ALA) on cognition following sham irradiation and irradiation were also assessed. ALA did not prevent radiation-induced impairments in novel object recognition and impaired spatial memory retention of sham-irradiated and irradiated mice in the probe trial after the first day of hidden platform training in the water maze. Thus, the novel object recognition test is particularly sensitive to detect early cognitive effects of 56Fe irradiation through a mechanism unlikely involving ROS or oxidative damage.
Neuropsychopharmacology : official publication of the American College of Neuropsychopharmacology, Jan 10, 2015
Apolipoprotein E (apoE) is an essential component of lipoprotein particles in both the brain and ... more Apolipoprotein E (apoE) is an essential component of lipoprotein particles in both the brain and periphery, and exists in three isoforms in the human population: E2, E3, and E4. ApoE has numerous, well established roles in neurobiology. Most notably, E4 is associated with earlier onset and increased risk of Alzheimer's disease (AD). While possession of E2 is protective in the context of AD, E2 appears to confer an increased incidence and severity of posttraumatic stress disorder (PTSD). However, the biological processes underlying this link remain unclear. In this study, we began to elucidate these associations by examining the effects of apoE on PTSD severity in combat veterans, and on PTSD-like behavior in mice with human apoE. In a group of 92 veterans with PTSD, we observed significantly higher Clinician Administered PTSD Scale (CAPS) and PTSD checklist (PCL) scores in E2+ individuals, as well as alterations in salivary cortisol levels. Furthermore, we measured behavioral an...
Impaired adipogenesis renders an adipose tissue unable to expand, leading to lipotoxicity and con... more Impaired adipogenesis renders an adipose tissue unable to expand, leading to lipotoxicity and conditions such as diabetes and cardiovascular disease. While factors important for adipogenesis have been studied extensively, those that set the limits of adipose tissue expansion remain undetermined. Feeding a Western-type diet to apolipoprotein E2 knock-in mice, a model of metabolic syndrome, produced 3 groups of equally obese mice: mice with normal glucose tolerance, hyperinsulinemic yet glucose-tolerant mice, and prediabetic mice with impaired glucose tolerance and reduced circulating insulin. Using proteomics, we compared subcutaneous adipose tissues from mice in these groups and found that the expression of PTRF (polymerase I and transcript release factor) associated selectively with their glucose tolerance status. Lentiviral and pharmacologically overexpressed PTRF, whose function is critical for caveola formation, compromised adipocyte differentiation of cultured 3T3-L1cells. In human adipose tissue, PTRF mRNA levels positively correlated with markers of lipolysis and cellular senescence. Furthermore, a negative relationship between telomere length and PTRF mRNA levels was observed in human subcutaneous fat. PTRF is associated with limited adipose tissue expansion underpinning the key role of caveolae in adipocyte regulation. Furthermore, PTRF may be a suitable adipocyte marker for predicting pathological obesity and inform clinical management.
Human apolipoprotein E (apoE) exists in three isoforms: apoE2, apoE3 and apoE4. APOE ε4 is a majo... more Human apolipoprotein E (apoE) exists in three isoforms: apoE2, apoE3 and apoE4. APOE ε4 is a major genetic risk factor for cardiovascular disease (CVD) and Alzheimer's disease (AD). ApoE mediates cholesterol metabolism by binding various receptors. The low-density lipoprotein receptor (LDLR) has a high affinity for apoE, and is the only member of its receptor family to demonstrate an apoE isoform specific binding affinity (E4>E3>E2). Evidence suggests that a functional interaction between apoE and LDLR influences the risk of CVD and AD. We hypothesize that the differential cognitive effects of the apoE isoforms are a direct result of their varying interactions with LDLR. To test this hypothesis, we have employed transgenic mice that express human apoE2, apoE3, or apoE4, and either human LDLR (hLDLR) or no LDLR (LDLR(-/-)). Our results show that plasma and brain apoE levels, cortical cholesterol, and spatial memory are all regulated by isoform-dependent interactions between apoE and LDLR. Conversely, both anxiety-like behavior and cued associative memory are strongly influenced by APOE genotype, but these processes appear to occur via an LDLR-independent mechanism. Both the lack of LDLR and the interaction between E4 and the LDLR were associated with significant impairments in the retention of long term spatial memory. Finally, levels of hippocampal apoE correlate with long term spatial memory retention in mice with human LDLR. In summary, we demonstrate that the apoE-LDLR interaction affects regional brain apoE levels, brain cholesterol, and cognitive function in an apoE isoform-dependent manner.
Diabetes is a major risk factor for cardiovascular disease. To examine how diabetes interacts wit... more Diabetes is a major risk factor for cardiovascular disease. To examine how diabetes interacts with a mildly compromised lipid metabolism, we introduced the diabetogenic Ins2(C96Y/+) (Akita) mutation into mice expressing human apoE4 (E4) combined with either an overexpressing human LDL receptor gene (hLDLR) or the wild-type mouse gene. The hLDLR allele caused 2-fold reductions in plasma HDL-cholesterol, plasma apoA1, and hepatic triglyceride secretion. Diabetes increased plasma total cholesterol 1.3-fold and increased apoB48 secretion 3-fold, while reducing triglyceride secretion 2-fold. Consequently, diabetic E4 mice with hLDLR secrete increased numbers of small, cholesterol-enriched, apoB48-containing VLDL, although they have near normal plasma cholesterol (<120 mg/dl). Small foam cell lesions were present in the aortic roots of all diabetic E4 mice with hLDLR that we analyzed at six months of age. None were present in nondiabetic mice or in diabetic mice without hLDLR. Aortic expression of genes affecting leukocyte recruitment and adhesion was enhanced by diabetes. ApoA1 levels, but not diabetes, were strongly correlated with the ability of plasma to efflux cholesterol from macrophages. We conclude that the diabetes-induced proinflammatory changes in the vasculature and the hLDLR-mediated cholesterol accumulation in macrophages synergistically trigger atherosclerosis in mice with human apoE4, although neither alone is sufficient.
We investigated the differential roles of apolipoprotein E (apoE) isoforms in modulating diabetic... more We investigated the differential roles of apolipoprotein E (apoE) isoforms in modulating diabetic dyslipidemia-a potential cause of the increased cardiovascular disease risk of patients with diabetes. Diabetes was induced using streptozotocin (STZ) in human apoE3 (E3) or human apoE4 (E4) mice deficient in the LDL receptor (LDLR(-/-)). Diabetic E3LDLR(-/-) and E4LDLR(-/-) mice have indistinguishable levels of plasma glucose and insulin. Despite this, diabetes increased VLDL triglycerides and LDL cholesterol in E4LDLR(-/-) mice twice as much as in E3LDLR(-/-) mice. Diabetic E4LDLR(-/-) mice had similar lipoprotein fractional catabolic rates compared with diabetic E3LDLR(-/-) mice but had larger hepatic fat stores and increased VLDL secretion. Diabetic E4LDLR(-/-) mice demonstrated a decreased reliance on lipid as an energy source based on indirect calorimetry. Lower phosphorylated acetyl-CoA carboxylase content and higher gene expression of fatty acid synthase in the liver indicated reduced fatty acid oxidation and increased fatty acid synthesis. E4LDLR(-/-) primary hepatocytes cultured in high glucose accumulated more intracellular lipid than E3LDLR(-/-) hepatocytes concomitant with a 60% reduction in fatty acid oxidation. Finally, the exaggerated dyslipidemia in diabetic E4LDLR(-/-) mice was accompanied by a dramatic increase in atherosclerosis. ApoE4 causes severe dyslipidemia and atherosclerosis independent of its interaction with LDLR in a model of STZ-induced diabetes. ApoE4-expressing livers have reduced fatty acid oxidation, which contributes to the accumulation of tissue and plasma lipids.
Arteriosclerosis Thrombosis and Vascular Biology, 2008
To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, ... more To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, on the production of LDL and development of atherosclerosis in vivo. Ldlr(-/-)Apobec1(-/-) mice lacking the LDLR and apoB editing enzyme accumulated LDL in plasma and developed severe atherosclerosis when they had wild-type apoB100. In marked contrast, in Ldlr(-/-)Apobec1(-/-) mice carrying the Apob100-beta mutation, in the 2 putative LDLR-binding domains of apoB prevented both LDL accumulation and atherosclerosis. Intestinal absorption of lipids and triglyceride secretion from the liver were not affected. However, the VLDL particles with apoB100-beta were larger in volume by about 70%, and carried approximately four times as much apoE per particle. ApoB100-beta synthesis rate in the primary hepatocytes was normal, but its intracellular degradation was enhanced. Additionally, mutant apoB100 VLDL cleared from the circulation more quickly in vivo through apoE-LRP-mediated mechanism than VLDL with wild-type apoB100. In contrast, uptake of the 2 VLDL by macrophages were not different. While conformational change to apoB100 during conversion of VLDL to LDL exposes LDLR binding domains and facilitates LDLR-mediated lipoprotein clearance, it may also inhibit LRP-mediated VLDL uptake and contribute to LDL accumulation in familial hypercholesterolemia.
To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, ... more To examine the effects of apoB100 structure, specifically a mutation in the LDLr binding region, on the production of LDL and development of atherosclerosis in vivo. Ldlr(-/-)Apobec1(-/-) mice lacking the LDLR and apoB editing enzyme accumulated LDL in plasma and developed severe atherosclerosis when they had wild-type apoB100. In marked contrast, in Ldlr(-/-)Apobec1(-/-) mice carrying the Apob100-beta mutation, in the 2 putative LDLR-binding domains of apoB prevented both LDL accumulation and atherosclerosis. Intestinal absorption of lipids and triglyceride secretion from the liver were not affected. However, the VLDL particles with apoB100-beta were larger in volume by about 70%, and carried approximately four times as much apoE per particle. ApoB100-beta synthesis rate in the primary hepatocytes was normal, but its intracellular degradation was enhanced. Additionally, mutant apoB100 VLDL cleared from the circulation more quickly in vivo through apoE-LRP-mediated mechanism than VLDL with wild-type apoB100. In contrast, uptake of the 2 VLDL by macrophages were not different. While conformational change to apoB100 during conversion of VLDL to LDL exposes LDLR binding domains and facilitates LDLR-mediated lipoprotein clearance, it may also inhibit LRP-mediated VLDL uptake and contribute to LDL accumulation in familial hypercholesterolemia.
Apolipoprotein E (apoE), a key protein in lipid metabolism, is highly expressed in adipose tissue... more Apolipoprotein E (apoE), a key protein in lipid metabolism, is highly expressed in adipose tissues. Studies have shown that human APOE*4 is associated with a lower body mass index but with a greater risk of coronary heart disease compared with other APOE alleles. To define the isoform-specific role of apoE in regulating the expandability and functionality of adipose tissues, we investigated the effects of diet-induced obesity in mice whose endogenous Apoe gene has been replaced by either the human APOE*3 or APOE*4 allele. After 8 weeks on a Western-type high-fat diet, male APOE4 mice displayed impaired tolerance to glucose and fat overload compared with APOE3 mice. Subcutaneous fat tissues in APOE4 and APOE3 mice after high fat feeding were not different. In contrast, although epididymal fat tissues in APOE4 mice gained 30% less weight during the high fat feeding than in APOE3 mice, they showed impaired insulin-stimulated glucose uptake ex vivo. Epididymal APOE4 adipocytes were larger in size than APOE3 adipocytes, and expressed reduced levels of mRNA for peroxisome proliferator-activated receptor gamma2 and adiponectin, important markers of adipocyte functionality. Adenoviral expression of apoE3 in apoE-null culture adipocytes induced adiponectin mRNA in a dose-dependent manner, but the induction was significantly blunted in cells overexpressing apoE4. However, in contrast to the apoE3-expressing cells, Glut1, but not Glut4, expression levels were positively correlated with increased apoE4 mRNA, suggesting that apoE4 expression in adipocyte interferes in insulin-sensing pathways. Dysfunctional epididymal adipose tissues contribute to the accelerated impairment of glucose tolerance in APOE4 mice fed a Western-type diet. Our results underscore the importance of functionality of individual fat depots rather than total fat mass as a determinant for metabolic disturbance during diet-induced obesity.
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Papers by Lance Johnson