Human ApoE Isoforms Differentially Modulate Glucose and Amyloid Metabolic Pathways in Female Brain: Evidence of the Mechanism of Neuroprotection by ApoE2 and Implications for Alzheimer's Disease Prevention and Early Intervention

Abstract

Three major genetic isoforms of apolipoprotein E (ApoE), ApoE2, ApoE3, and ApoE4, exist in humans and lead to differences in susceptibility to Alzheimer's disease (AD). This study investigated the impact of human ApoE isoforms on brain metabolic pathways involved in glucose utilization and amyloid-β (Aβ) degradation, two major areas that are significantly perturbed in preclinical AD. Hippocampal RNA samples from middle-aged female mice with targeted human ApoE2, ApoE3, and ApoE4 gene replacement were comparatively analyzed with a qRT-PCR custom array for the expression of 85 genes involved in insulin/insulin-like growth factor (Igf) signaling. Consistent with its protective role against AD, ApoE2 brain exhibited the most metabolically robust profile among the three ApoE genotypes. When compared to ApoE2 brain, both ApoE3 and ApoE4 brains exhibited markedly reduced levels of Igf1, insulin receptor substrates (Irs), and facilitated glucose transporter 4 (Glut4), indicating reduced glucose uptake. Additionally, ApoE4 brain exhibited significantly decreased Pparg and insulin-degrading enzyme (Ide), indicating further compromised glucose metabolism and Aβ dysregulation associated with ApoE4. Protein analysis showed significantly decreased Igf1, Irs, and Glut4 in ApoE3 brain, and Igf1, Irs, Glut4, Pparg, and Ide in ApoE4 brain compared to ApoE2 brain. These data provide the first documented evidence that human ApoE isoforms differentially affect brain insulin/Igf signaling and downstream glucose and amyloid metabolic pathways, illustrating a potential mechanism for their differential risk in AD. A therapeutic strategy that enhances brain insulin/Igf1 signaling activity to a more robust ApoE2-like phenotype favoring both energy production and amyloid homeostasis holds promise for AD prevention and early intervention.

Keywords: Alzheimer’s disease; apolipoprotein E2; apolipoprotein E3; apolipoprotein E4; early intervention; glucose metabolism; glucose transporter 4; insulin-degrading enzyme; insulin-like growth factor 1; peroxisome proliferator-activated receptor gamma.

Fig. 1

Gene expression profiles in the brains of middle-aged female mice carrying human ApoE2, ApoE3, or ApoE4 gene. Hippocampal RNA samples were analyzed with a qRT-PCR gene array for the expression of a focused panel of genes involved in insulin/Igf signaling. Red indicates high expression, green indicates low expression. Control Group = ApoE2 brain; Group 1 = ApoE3 brain; Group 2 = ApoE4 brain; N = 5 per ApoE genotype group. A list of all genes profiled is provided in Supplemental Table 1.

Fig. 2

The heat map (a) includes genes that significantly differed between any two of the three ApoE genotypes. The volcano plots show fold changes (X-axis) and P-values (Y-axis) between (b) ApoE3 brain versus ApoE2 brain and (c) ApoE4 brain versus ApoE2 brain; each dot represents a gene; red indicates greater expression; green indicates lower expression; dots that fall above the horizontal blue line indicate significantly differed genes (p<0.05). Highlighted significantly altered expression - Igf1: insulin-like growth factor 1; Irs1/2: insulin receptor substrate 1/2; Slc2a4: solute carrier family 2 facilitated glucose transporter member 4 (Glut4); Pparg: peroxisome proliferator-activated receptor gamma; Ide: insulin-degrading enzyme. N = 5 per ApoE genotype group.

Fig. 3

When compared to ApoE2 brain, ApoE3 and ApoE4 brain exhibit both similar and distinct expression profiles of genes involved in insulin/Igf signaling. Major genes that showed a significantly lower expression in both ApoE3 and ApoE4 brain than ApoE2 brain include Igf1, Irs1, and Glut4, involved in glucose metabolism. Ide, a key player involved in the degradation of Aβ monomers in the brain, and Pparg, involved in insulin sensitivity and mitochondrial biogenesis, were significantly low-expressed in only ApoE4 but not ApoE3 brain when compared to ApoE2 brain.

Fig. 4

ApoE isoforms differentially modulate the expression, at both the gene and protein level, of major players involved in Igf1 signaling and glucose and Aβ metabolism including Igf1, Irs1, Glut4, Pparg, and Ide. Specifically, when compared to ApoE2 brain, both ApoE3 and ApoE4 brain exhibited a significantly lower expression of Igf1, Irs1 and Glut4. Moreover, ApoE4 brain was associated with a significantly lower expression of Pparg than ApoE2 brain and significantly lower expression of Ide than both ApoE2 and ApoE3 brain, and there was not a significant difference between ApoE2 and ApoE3 brain. N = 5 per ApoE genotype group; *p<0.05, **p<0.01, and ***p<0.001.

Fig. 5

Our hypotheses and conclusion: 1) When compared to the ApoE3 and ApoE4 brain, the anti-AD ApoE2 brain is associated with a more robust Igf1 signaling and mechanisms involved in glucose uptake, glucose metabolism, and Aβ degradation, which could serve as a molecular basis underlying its protective role against the development of AD. 2) When compared to the ApoE2 and ApoE3 brain, the pro-AD ApoE4 brain is associated with a weaker Igf1 signaling and mechanisms involved in glucose uptake, glucose metabolism, and Aβ degradation, which could mechanistically contribute to its detrimental role in the etiology of AD. Therefore, targeting Igf1/Irs/Glut4/Pparg-related energy metabolism and the downstream energy production as well as Ide and associated Aβ degradation could serve as a vital strategy in order to transform a pro-AD ApoE4 brain to the anti-AD ApoE2 phenotype, and, as a result, increase the defense ability of an ApoE4 brain against the development of AD.