The cleavage of APP, APP overexpression, and the consequences of APP mutations have been well studied in the brain and muscle for AD pathogenesis and inclusion body myositis 
, respectively. Researchers have proposed roles for APP in cell adhesion, neuron migration and synaptogenesis 
. However, it is important to note that APP is not only expressed in the brain and muscle, but in adipose tissue (and other tissues) as well 
. For several years, there has been a renewed focus on defining the role of APP beyond its association with extracellular plaques and intracellular inclusions, with the hope that understanding the effects of APP in peripheral tissues would potentially provide more information about the link between APP and AD risk. Our results demonstrate novel effects of mutated APP on adipose tissue: severe hypoleptinemia, decreased adiposity, and reduced adipocyte size (). These effects were reversed under obesogenic conditions, but this reversal was accompanied by mutant APP-associated increases in insulin resistance, increased circulating triglycerides, and increased macrophage infiltration of visceral adipose tissue ().
Overview of mutant APP effects on adipose biology.
It has previously been shown that APP expression is upregulated in adipose tissue in response to obesity 
and APP knock-out mice have reduced body weight 
. Recent studies have identified that conditions which are associated with reduced adiposity are also associated with reduced amyloid pathology in mouse models expressing APP mutations 
. Such studies provide correlative support for changes in adiposity modulating specific aspects of amyloid pathogenesis. The current study builds upon these findings by raising the potential for mutations in APP in humans to promote alterations in adipose tissue. These changes in adipose tissue could induce insulin resistance and endocrine alterations which also contribute to adipose-mediated changes in amyloid pathology. Our studies reveal decreased adiposity in a mutant APP mouse model of CAA. However, this novel phenotype was only observed under non-obesogenic conditions, while CAA mice given an obesity-inducing diet exhibited similar adiposity as wild-type mice under the same conditions. Taken together, these data link mutant APP expression to the regulation of adiposity, potentially impairing adipogenesis under non-obesogenic conditions. While the basis for mutant APP to decrease adiposity is not known, it is important to note the significantly reduced adipocyte size in control diet-fed mice. These data suggest that mutant APP is not decreasing adiposity via decreasing the number of adipocytes, but rather primarily through decreased size and potentially decreased differentiation of adipocytes. In order to evaluate changes in adipocyte differentiation due to mutant APP, we evaluated PPAR and C/EBPα mRNA expression in visceral and subcutaneous fat depots. A significant decrease in PPARγ and C/EBPα mRNA was determined due to genotype in subcutaneous fat ( +3B) while protein analysis revealed slight increases in these proteins for CAA-CD mice. Coupled with our data on mutant APP altering adipose size, it is likely changes are occurring at the level of adipocyte differentiation. One possibility for altered adipose size in the current study is mutant APP-associated changes in lipase expression. However, it is important to point out that these effects may be adipose depot-specific. Of further note is the lack of adipocyte changes following mutant APP expression in vitro
(using the 3T3-L1 model), with mutant APP expression induced after differentiation had occurred. Taken together, these data suggest that mutant APP may preferentially mediate its effects in pre-adipocytes, although more work needs to be conducted to experimentally define mutant APP effects on pre-adipocytes.
In addition to the observed changes in adiposity for mice expressing mutant APP, there was also a striking level of hypoleptinemia observed in mutant APP expressing mice. This decrease in leptin was so severe in the present study that it resulted in nearly undetectable levels of leptin in the serum (ELISA analysis). We also detected reduced levels of leptin in subcutaneous and visceral fat depots and 3T3-L1 preadipocytes expressing mutated APP. To our knowledge, these are the first studies to demonstrate such severe hypoleptinemia occurring as a result of mutant APP expression. Furthermore, this level of hypoleptinemia cannot be explained based on the levels of reduced adiposity in CAA mice fed the control diet. Previous studies have reported decreased plasma leptin in the Tg2576 mouse 
, in which APP expression is driven by a hamster prion promoter, resulting in a primary localization of mutant APP in the brain 
. Additionally, impairments in circulating leptin have been proposed to play a role in the regulation of AD pathogenesis. For example, administration of leptin decreases AD-like pathology in transgenic mouse models of AD pathology (TgCRND8 and Tg2576) 
, with increased AD risk in human patients reported for subjects with low levels of circulating leptin 
. CAA mice fed the HF diet exhibited a reversal of hypoleptinemia, exhibiting comparable levels of leptin to C-HF mice. The ability of CAA-HF leptin levels to reach C-HF leptin levels provides evidence that the systems responsible for regulating leptin are intact in this mouse model. The basis for leptin downregulation under CD conditions, as well as the apparent selectivity of mutant APP to modulate leptin over other adipokines, are unknown and must be further explored. However, it is well known that leptin expression is dependent on adipocyte size, as shown in rodent and human studies 
While the high level of lipids in adipose tissue made Aβ measurements impossible, recent studies suggest that APP processing in adipose tissue may be important to the regulation of adiposity. For example, a recent study has shown that transgenic mice lacking BACE1 (β-site amyloid precursor protein-cleaving enzyme 1), an essential enzyme for APP processing, are leaner than wild-type controls and have less adipose tissue 
. Coupled with the findings in APP outlined above, it is clear that BACE1 and APP are both involved in regulating adipogenesis and adipocyte homeostasis. The present study suggests that the ultimate impact of changes in APP or BACE1 expression, or the ultimate impact of mutations in these genes, is dictated by caloric intake and energy expenditure. In conditions which are obesogenic (excess calories, reduced energy expenditure), and that have altered expression of APP or APP processing components, there is likely a promotion of adiposity and exacerbation of obesity-induced metabolic dysfunction as the result of impaired adipose function. Conversely, under non-obesogenic conditions and altered APP expression, these same genes result in impaired endocrine homeostasis due to impaired adipose function. It is well known that BACE and APP expression are highly related to the genesis of Aβ: whereby when BACE is moderately overexpressed, APP is processed to generate more Aβ 
and when BACE is genetically ablated, Aβ is not produced 
. Interestingly, the leptin promoter contains numerous regulatory elements 
that are also involved in the expression and processing of APP, suggesting complex interactions between multiple signaling pathways and the effects of mutant APP on adipose tissue function. For example, Sp1 regulates BACE1 expression 
and HIF-1 regulates APP and BACE1 expression 
. These regulatory elements are also known to be modulated in response to diet 
, providing further evidence for a connection between APP expression and adiposity.
While CAA-HF mice were able to reverse certain phenotypes found in the CAA-CD mice, they also presented evidence for an exacerbation of obesity-induced metabolic dysfunction including insulin resistance, elevated triglycerides, and macrophage infiltration of adipose tissue. The factors leading to insulin resistance are multifactorial, but numerous studies have linked the development of insulin resistance to increased levels of inflammation 
. Obesity is often accompanied by insulin resistance and inflammatory visceral fat tissue 
. However, it is intriguing that the CAA-HF mice revealed this phenotype whereas the C-HF mice did not, suggesting an increased susceptibility for the CAA genotype. With the epidemic proportion of obese individuals in the United States, and soon to be worldwide 
, we hypothesize this phenotype to be more common among individuals with genetic mutations of APP. This supports the current findings that Alzheimer’s disease is correlated with obesity and type II diabetes 
In conclusion, novel metabolic abnormalities have been demonstrated in this study for the CAA mouse under two different dietary conditions. This data has implications for the prevalence and risk of metabolic disturbances in human subjects and presents new consequences for APP mutations beyond Alzheimer’s disease risk. Numerous rodent studies have also demonstrated increased amyloid plaque deposition, cognitive impairment, and increased APP expression due to consumption of a high fat diet or under the conditions of diet-induced obesity (DIO) 
. Altogether, this data provides further evidence for a link between adiposity and APP biology.