Blood–brain barrier P-glycoprotein function decreases in specific brain regions with aging: A possible role in progressive neurodegeneration

Abstract

Cerebrovascular P-glycoprotein (P-gp) acts at the blood–brain barrier (BBB) as an active cell membrane efflux pump for several endogenous and exogenous compounds. Age-associated decline in P-gp function could facilitate the accumulation of toxic substances in the brain, thus increasing the risk of neurodegenerative pathology with aging. We hypothesised a regionally reduced BBB P-gp function in older healthy subjects.

We studied cerebrovascular P-gp function using [¹¹C]-verapamil positron emission tomography (PET) in seventeen healthy volunteers with age 18–86. Logan analysis was used to calculate the distribution volume (DV) of [¹¹C]-verapamil in the brain. Statistical Parametric Mapping was used to study specific regional differences between the older compared with the younger adults.

Older subjects showed significantly decreased P-gp function in internal capsule and corona radiata white matter and in orbitofrontal regions.

Decreased BBB P-gp function in those regions could thus explain part of the vulnerability of the aging brain to white matter degeneration. Moreover, decreased BBB P-gp function with aging could be a mechanism by which age acts as the main risk factor for the development of neurodegenerative disease.

Introduction

Advancing age regulates the onset of many neurodegenerative diseases, such as Parkinson's disease (PD) and Alzheimer's disease (AD). Immigrant surveys attempting to differentiate between genetic and environmental causes favour an environmental cause in the aetiology of PD (Schoenberg et al., 1988), and epidemiological studies suggest that chronic exposure to pesticides may be a causative factor (Di Monte, 2003). The contention put forward in this study is that the link between environment, genetic factors and age affecting the brain can possibly be found at the level of the blood–brain barrier (BBB) and its protective function.

The BBB limits entrance of proteins and polar compounds into brain parenchyma. Passive transport across the BBB depends highly on the physico-chemical features of the compound. Importantly, the concept of the BBB has expanded to accommodate newly recognised functions: proteins that are involved in cellular efflux prevent accumulation in the brain of various potentially toxic compounds. They include P-glycoprotein (P-gp) and multidrug resistance associated protein (MRP) (Higgins, 1992). P-gp is expressed in high concentrations at the luminal side of the BBB endothelium (Demeule et al., 2001), and functions as active efflux pump by extruding a substrate from the brain to the blood, which is important for maintaining loco-regional homeostasis in the brain.

P-gp was shown to play an important role in protecting the brain from accumulation of potentially toxic substances. In P-gp knockout mice, higher brain uptake of the P-gp substrate ivermectine was seen, causing severe neurotoxicity and death (Schinkel et al., 1994). Environmental toxins have important effects on central nervous system (CNS) degenerative changes. Exposure to environmental toxic substances combined with aging could increase the risk for developing PD. It has indeed been demonstrated that the aging nigrostriatal dopamine pathway has increased sensitivity to pesticides (Thiruchelvam et al., 2000).

Dysfunctional P-gp in the BBB has already been suggested to play a role in the development of neurodegenerative diseases, such as PD (Drozdzik et al., 2003, Kortekaas et al., 2005) and AD (Vogelgesang et al., 2001). Genetic variations in the MDR1 gene, associated with decreased P-gp function at the BBB (Hoffmeyer et al., 2000), has been associated with higher risk of PD in interaction with exposure to neurotoxic substances (Drozdzik et al., 2003, Furuno et al., 2002). Decreased P-gp expression was also found in patients with Creutzfeldt–Jakob Disease (CJD), which was suggested to facilitate the accumulation of the pathogenic prion PrP^Sc (Vogelgesang et al., 2006a). In AD, a compensatory mechanism with up-regulation of P-gp expression in the early pathogenesis was suggested, while at later disease stages P-gp expression in the capillaries was lost (Vogelgesang et al., 2001). It was shown that β-amyloid (Aβ) is transported by P-gp in vitro (Vogelgesang et al., 2006b), and in elderly non-demented humans, BBB P-gp expression was found to be inversely correlated with the number of Aβ plaques (Vogelgesang et al., 2002). Furthermore, deficiency of the BBB in white matter areas was suggested to play a role in white matter lesions (Ueno et al., 2000), the incidence of which also increases with aging.

It can thus be hypothesised that P-gp forms a compensatory mechanism to increase clearance of harmful proteins that play a role in neurodegenerative diseases. Decreased P-gp function in the BBB, possibly with a genetic predisposition, may increase the risk of neurological diseases as a result of increased exposure to exogenous and/or endogenous toxic substances. If this hypothesis is true, it can give insight in a mechanism by which age is the main risk factor for neurodegenerative diseases.

Relatively few papers have been published so far on the impact of advancing age on P-gp activity and expression in different tissues, and more studies are warranted to characterise the impact of age on P-gp activity at crucial P-gp locations such as the BBB (Mangoni, 2007). Effect of age on BBB P-gp expression has first been studied in rodents. In rats and mice, decreased BBB P-gp expression was found with senescence (Matsuoka et al., 1999, Rosati et al., 2003, Tsai et al., 2002). It is now also possible to measure functional P-gp in vivo in humans using [¹¹C]-verapamil and positron emission tomography (PET) (Hendrikse et al., 2001, Hendrikse et al., 1999). Verapamil is a substrate for P-gp and is hence actively transported out of the brain by P-gp. The volume of distribution (DV) of [¹¹C]-verapamil in the brain inversely reflects P-gp function in the BBB. Recently, a pilot study of P-gp function in vivo in the human BBB with [¹¹C]-(R)-verapamil PET found decreased P-gp activity as measured in overall brain of five older healthy subjects compared to a younger group (Toornvliet et al., 2006).

The present study is the first to assess regionally accentuated loss of BBB P-gp function in relation to age. We hypothesised decreased BBB P-gp function with aging with possible accentuated decline in white matter regions. We again studied in vivo P-gp function using [¹¹C]-verapamil PET in seventeen healthy volunteers with age 18–86. The C3435T polymorphism of the MDR1 gene, associated with diminished P-gp function, was also assessed.