Measurement of altered APP isoform expression in adipose tissue of diet-induced obese mice by absolute quantitative real-time PCR

Abstract

Obesity is associated with increased risk of Alzheimer's disease. Previous studies have demonstrated that amyloid-beta precursor protein (APP) is expressed in subcutaneous adipose tissue (SAT), upregulated with obesity, and correlates with insulin resistance and adipose tissue inflammation. APP is alternatively spliced into several isoforms, which may be indicative of the pathogenesis of APP-related diseases, but the accurate quantification has been difficult to standardize and reproduce. In light of this, we developed isoform-specific absolute cDNA standards for absolute quantitative real-time PCR (AQ-PCR), and measured transcript copy numbers for three major APP isoforms (APP770, APP751, and APP695), in SAT from C57BL/6 mice fed either a normal or high-fat diet. Expression of all three major APP isoforms was increased in diet-induced obese mice. Transcript copy numbers of APP770 and APP695 correlated with plasma insulin and CCL2 gene expression. The ratios of APP770 and APP751 to APP695 gradually decreased with aging, and correlated with plasma glucose levels. In addition, APP770 was significantly decreased in thiazolidinedione-treated mice. We describe quantification of APP isoform transcripts by AQ-PCR, which allows for direct comparison of gene copy number across isoforms, between experiments, and across studies conducted by independent research groups, which relative quantitative PCR does not allow. Our results suggest a possible role of differential expression of APP isoforms in the development of obesity-related insulin resistance and adipose tissue inflammation. In addition, it is important to determine if altered ratios of APP isoforms in SAT contribute to higher circulating Aβ peptides and increased risk of abnormalities in obesity.

Keywords: Alternative splicing; absolute quantitative real-time PCR; adipose tissue; amyloid-beta precursor protein (APP); obesity.

Figure 1.

Schematic diagram showing the positions of primers (solid arrows) for standard curves samples, and primers (dashed arrows) and probes (dotted bars) for AQ-PCR for each APP isoform and GAPDH. (A) APP770, (B) APP751, (C) APP695, (D) GAPDH.

Figure 2.

Transcript copy number of APP770 (A), APP751 (B), and APP695 (C) per 1000 mRNA transcript copies of GAPDH in SAT of C57BL/6 mice fed either a ND or HFD until 16, 26, 36, 47, or 77 weeks of age. *p < .05, **p < .01, ***p < .001 between mice on ND and HFD.

Figure 3.

Transcript copy number of APP770 and APP695 (per 1000 mRNA transcript of GAPDH) in SAT correlated with non-fasting plasma insulin levels (A and C) and log expression level of CCL2 in SAT (B and D) of C57BL/6 mice fed either a ND or HFD until the age of 16, 26, 36, 47, or 77 weeks.

Figure 4.

Correlation of non-fasting plasma glucose levels with ratios of transcript copy number of APP770 (A) and APP-KPI (B) to that of APP695 in SAT of C57BL/6 mice fed either a ND or HFD until 16, 26, 36, 47, or 77 weeks of age.

Figure 5.

Transcript copy number of APP isoforms (per 1000 transcript copies of GAPDH) in SAT from TZD treated mice (closed bars) and control mice treated with vehicle only (open bars). **p < .01.