Background

The presence of small dense low-density lipoprotein (LDL) is associated with obesity, type II diabetes, and an increased risk for cardiovascular disease. Apolipoprotein C-III (apoC-III) is involved in the formation of small dense LDL, but the exact mechanisms are still not well defined. ApoC-III is a glycosylated apolipoprotein, with 3 major glycoforms: apoC-III₀, apoC-III₁, and apoC-III₂ that contain 0, 1, or 2 molecules of sialic acid, respectively. In our previous work, we reported an association among apoC-III₀ and apoC-III₁, but not apoC-III₂ with fasting plasma triglyceride levels in obesity and type II diabetes.

Objective

The goal of this study was to determine the relationship between changes in the major apoC-III glycoforms and small dense LDL levels after dietary interventions.

Methods

Mass spectrometric immunoassay was performed on fasting plasma samples from 61 subjects who underwent either a high-carbohydrate diet (n = 34) or a weight loss intervention (n = 27).

Results

After both dietary interventions, changes in total apoC-III concentrations were associated with changes in LDL peak particle diameter (r = -0.58, P < .0001). Increases in total apoC-III concentrations after the high-carbohydrate diet were associated with decreases in LDL size (r = -0.53, P = .001), and decreases in apoC-III concentrations after weight loss were associated with increases in LDL peak particle diameter (r = -0.54, P = .004). Changes in concentrations of apoC-III₁ and apoC-III₀, but not apoC-III₂, were associated with changes in LDL peak particle diameter in both the weight loss and high-carbohydrate interventions.

Conclusions

We conclude that apoC-III₀ and apoC-III₁, but not apoC-III₂ are associated with the formation of small dense LDL.

Objective

ApoC-III (apolipoprotein C-III) glycosylation can predict cardiovascular disease risk. Higher abundance of disialylated (apoC-III₂) over monosialylated (apoC-III₁) glycoforms is associated with lower plasma triglyceride levels. Yet, it remains unclear whether apoC-III glycosylation impacts TRL (triglyceride-rich lipoprotein) clearance and whether apoC-III antisense therapy (volanesorsen) affects distribution of apoC-III glycoforms. Approach and Results: To measure the abundance of human apoC-III glycoforms in plasma over time, human TRLs were injected into wild-type mice and mice lacking hepatic TRL clearance receptors, namely HSPGs (heparan sulfate proteoglycans) or both LDLR (low-density lipoprotein receptor) and LRP1 (LDLR-related protein 1). ApoC-III was more rapidly cleared in the absence of HSPG (t_1/2=25.4 minutes) than in wild-type animals (t_1/2=55.1 minutes). In contrast, deficiency of LDLR and LRP1 (t_1/2=56.1 minutes) did not affect clearance of apoC-III. After injection, a significant increase in the relative abundance of apoC-III₂ was observed in HSPG-deficient mice, whereas the opposite was observed in mice lacking LDLR and LRP1. In patients, abundance of plasma apoC-III glycoforms was assessed after placebo or volanesorsen administration. Volanesorsen treatment correlated with a statistically significant 1.4-fold increase in the relative abundance of apoC-III₂ and a 15% decrease in that of apoC-III₁. The decrease in relative apoC-III₁ abundance was strongly correlated with decreased plasma triglyceride levels in patients.

Conclusions

Our results indicate that HSPGs preferentially clear apoC-III₂. In contrast, apoC-III₁ is more effectively cleared by LDLR/LRP1. Clinically, the increase in the apoC-III₂/apoC-III₁ ratio on antisense lowering of apoC-III might reflect faster clearance of apoC-III₁ because this metabolic shift associates with improved triglyceride levels.

Carrying the apolipoprotein E (ApoE) Ɛ4 allele is associated with an increased risk of cerebral amyloidosis and late-onset Alzheimer's disease, but the degree to which apoE glycosylation affects its development is not clear. In a previous pilot study, we identified distinct total and secondary isoform-specific cerebral spinal fluid (CSF) apoE glycosylation profiles, with the E4 isoform having the lowest glycosylation percentage (E2 > E3 > E4). In this work, we extend the analysis to a larger cohort of individuals (n = 106), utilizing matched plasma and CSF samples with clinical measures of AD biomarkers. The results confirm the isoform-specific glycosylation of apoE in CSF, resulting from secondary CSF apoE glycosylation patterns. CSF apoE glycosylation percentages positively correlated with CSF Aβ₄₂ levels (r = 0.53, p < 0.0001). These correlations were not observed for plasma apoE glycosylation. CSF total and secondary apoE glycosylation percentages also correlated with the concentration of CSF small high-density lipoprotein particles (s-HDL-P), which we have previously shown to be correlated with CSF Aβ₄₂ levels and measures of cognitive function. Desialylation of apoE purified from CSF showed reduced Aβ₄₂ degradation in microglia with E4 > E3 and increased binding affinity to heparin. These results indicate that apoE glycosylation has a new and important role in influencing brain Aβ metabolism and can be a potential target of treatment.

Importance

Higher dietary intake of the essential fatty acid docosahexaenoic (DHA) has been associated with better cognitive performance in several epidemiological studies. Animal and in vitro studies also indicate that DHA prevents amyloid deposition in the brain.

Objective

To determine the association between serum DHA levels, cerebral amyloidosis, and the volumes of brain areas affected by Alzheimer disease.

Design, settings, and participants

Cross-sectional analysis of serum DHA levels together with measures of amyloid deposition (Pittsburgh Compound B index), brain volumes, and neuropsychological testing scores from 61 participants in the Aging Brain Study. The study was conducted between June 2008 and May 2013, and the data were analyzed between October 2015 and April 2016. Linear models were adjusted for age, sex, years of education, and apolipoprotein E status.

Main outcomes and measures

Serum DHA levels with cerebral amyloidosis measured using PIB PET.

Results

Samples were available from 61 Aging Brain Study participants (41 women and 20 men) who underwent amyloid PET imaging. The mean (SD) age of the participants was 77 (6) years and ranged from 67 to 88 years. Serum DHA levels (percentage of total fatty acids) were 23% lower in participants with cerebral amyloidosis than those without (0.97 vs 1.25, P = .007) and were inversely correlated with brain amyloid load (r = -0.32, P = .01) independent of age, sex, apolipoprotein E genotype, and years of education. Moreover, greater serum DHA levels were positively associated with brain volume in several subregions affected by AD, in particular the left subiculum (r = 0.38, P = .005) and the left entorhinal volumes (r = 0.51, P = .001). Serum DHA levels were also associated with nonverbal memory scores (r = 0.28, P = .03).

Conclusions and relevance

In this study, serum DHA levels were associated with pathogenesis of cerebral amyloidosis and with preservation of entorhinal and hippocampal volumes. These findings suggest an important role for DHA metabolism in brain amyloid deposition during the preclinical or early symptomatic stages of Alzheimer disease.