Research article
β-Amyloid or its precursor protein is found in epithelial cells of the small intestine and is stimulated by high-fat feeding

https://doi.org/10.1016/j.jnutbio.2006.07.003Get rights and content

Abstract

In Alzheimer's disease (AD), β-amyloid (Aβ) is deposited in extracellular matrices, initiating an inflammatory response and compromising cellular integrity. Epidemiological evidence and studies in animal models provide strong evidence that high-saturated-fat and/or cholesterol-rich diets exacerbate cerebral amyloidosis, although the mechanisms for this are unclear. contains hydrophobic domains and is normally bound to lipid-associated chaperone proteins. In previous studies, we have put forward the notion that Aβ is a regulatory component of postprandial lipoproteins (i.e., chylomicrons) and that aberrations in kinetics may be a contributing risk factor for AD. To explore this further, in this study, we utilized an immunohistochemical approach to determine if Aβ or its precursor protein is expressed in epithelial cells of the small intestine — the site of chylomicron biogenesis. Wild-type mice were fed a low-fat or a high-fat dietary regime and sacrificed, and their small intestines were isolated. We found that, in mice fed low-fat chow, substantial Aβ/precursor protein was found exclusively in absorptive epithelial cells of the small intestine. In contrast, no Aβ/precursor protein was found in epithelial cells when mice were fasted for 65 h. In addition, we found that a high-fat feeding regime strongly stimulates epithelial cell Aβ/precursor protein concentration. Our findings are consistent with the notion that Aβ may serve as a regulatory apolipoprotein of postprandial lipoproteins.

Introduction

β-Amyloid (Aβ) is the predominant protein component of senile plaques found in subjects with Alzheimer's disease (AD) [1]. Current dogma suggests that deposition occurs when synthesis by neuronal cells exceeds the availability of chaperone transporters in the cerebrospinal fluid [2], [3]. However, cerebrospinal fluid is an ultrafiltrate of plasma, raising the possibility that exogenous delivery of Aβ could exacerbate cerebral load [4], [5]. Indeed, soluble forms of Aβ are found in plasma and within the junctions of epithelial cells that form the blood–brain barrier (BBB) [6], and the bidirectional movement of Aβ through the BBB has been described [7], [8].

Sequestration of Aβ by chaperone proteins is pivotal to its continued solubility and underlies its tendency to otherwise cluster into complex oligomers [2], [3], [9]. A number of Aβ transport proteins have been described [10], but common to many of these is their normal coassociation with lipids in vivo. It is conceivable, that the physiological function of Aβ is related to the regulation of lipid metabolism, and consistent with this notion was the finding that Aβ enhanced the uptake of triglyceride-rich lipoproteins (TRLs) by fat-rich tissues, including brain tissues [11].

The kinetics of lipoproteins is dependent on apolipoproteins (apos) that serve as enzyme cofactors, or as ligands for receptors and extracellular matrices. Of particular interest is apoE because the E4 isoform is an established risk factor for AD [12]. ApoE tends to preferentially associate with plasma TRLs derived from the small intestine (chylomicrons) and the liver (very-low-density lipoprotein) [13], [14]. Several lines of evidence suggest that Aβ is involved in the metabolism of dietary fats and that aberrations in postprandial lipemia might be a contributing factor for AD. There is a transient increase in the plasma concentration of amyloid precursor protein (APP), a surrogate marker of Aβ biosynthesis, following the ingestion of dietary fats [15]. Moreover, epidemiological studies have reported a positive association of fat intake with AD prevalence [16], [17]. In animal studies, high-fat feeding induces cerebral amyloidosis, commensurate with dietary-induced hyperlipidemia and raised chylomicron concentration [18], [19], [20].

Aβ can be synthesized by the proteolytic cleavage of APP in the plasma membrane [14]. In addition, there is also significant intracellular abundance of Aβ associated with the rough endoplasmic reticulum (rER) and the Golgi apparatus [21], [22]. The latter observations are consistent with the possibility that Aβ associates with primordial lipoproteins during biosynthesis. Given that lipoprotein synthesis is regulated by lipid–substrate availability, it is conceivable that fat-enriched diets exacerbate cerebral amyloidosis by also stimulating the synthesis and secretion of intestinal-derived chylomicron-Aβ. To explore this further, in this study, we utilized immunohistochemistry (IHC) to explore the putative effects of high-fat feeding on the intestinal expression of Aβ/APP in wild-type (WT) mice fed either a low-fat or a high-fat diet.

Section snippets

Animals

The protocols used in this study were approved by the Curtin University Animal Experimentation and Ethics Committee (reference no. N 55-04). Six-week-old C57BL/6J mice were divided randomly into a low-fat group or a high-fat group. Low-fat mice were given cholesterol-free chow containing 4.0% (wt/wt) total fats (AIM93M; Specialty Feeds, Glen Forrest). Mice on the high-fat regime were given chow containing 1.0% (wt/wt) unsaturated fat, 16.0% total fat, 1% cholesterol and 0.5% cholate (SF00-245;

Body weight and plasma lipids

We found no difference in weight gain during 6 months of (ad libitum) low-fat or high-fat diets; however, mice on high-fat diet were hypercholesterolemic (2.0±0.2 and 6.7±2.0 mmol/L, respectively).

Intestinal expression of Aβ/APP in mice fed low-fat and high-fat diets

In WT mice fed a low-fat and a high-fat diet, a positive Aβ/APP signal was observed throughout the epithelial cells of the intestinal mucosa. The pattern of Aβ/APP immunostaining was uniform throughout the villi and the crypts of Lieberkühn (Fig. 1B, C, E and F). Weak Aβ/APP staining was also present

Discussion

This study utilized an immunohistochemical technique to explore if Aβ/APP is expressed in the absorptive epithelial cells of the small intestine. Aβ-antiserum was specific for Aβ/APP based on the staining of amyloid-rich cerebral plaques in positive control tissues, and this was abolished by competition with exogenous soluble Aβ.

In WT mice, Aβ/APP immunoreactivity was visible in columnar absorptive epithelial cells. The significant abundance of Aβ/APP within the perinuclear region is consistent

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