Microglia, amyloid, and cognition in Alzheimer's disease: An [11C](R)PK11195-PET and [11C]PIB-PET study

Abstract

[11C](R)PK11195-PET is a marker of activated microglia while [11C]PIB-PET detects raised amyloid load. Here we studied in vivo the distributions of amyloid load and microglial activation in Alzheimer's disease (AD) and their relationship with cognitive status. Thirteen AD subjects had [11C](R)PK11195-PET and [11C]PIB-PET scans. Ten healthy controls had [11C](R)PK11195-PET and 14 controls had [11C]PIB-PET scans. Region-of-interest analysis of [11C](R)PK11195-PET detected significant 20–35% increases in microglial activation in frontal, temporal, parietal, occipital and cingulate cortices (p < 0.05) of the AD subjects. [11C]PIB-PET revealed significant two-fold increases in amyloid load in these same cortical areas (p < 0.0001) and SPM (statistical parametric mapping) analysis confirmed the localisation of these increases to association areas. MMSE scores in AD subjects correlated with levels of cortical microglial activation but not with amyloid load. The inverse correlation between MMSE and microglial activation is compatible with a role of microglia in neuronal damage.

Introduction

The pathological hallmark of Alzheimer's disease (AD) is the presence of extracellular amyloid plaques and intraneuronal neurofibrillary tangles (NFTs). The exact cause of the neurofibrillary tangle formation associated with neuronal degeneration and synaptic loss in AD remains uncertain. The amyloid cascade hypothesis suggests formation of β-amyloid fibrils (Aβ) is directly responsible for triggering tau phosphorylation and neurofibrillary tangle formation leading to neuronal death and dementia (Hardy and Allsop, 1991, Selkoe, 1991). A modified amyloid cascade/neuroinflamation hypothesis suggests that Aβ formation activates microglial cells which in turn release potentially neurotoxic substances, such as nitric oxide, pro-inflammatory cytokines, complement proteins, and other inflammatory mediators, resulting in tau phosphorylation and neurodegenerative changes (Akiyama et al., 2000, Eikelenboom et al., 2002, McGeer and McGeer, 2001). It has been shown in vitro that Aβ fibrils result in microglial activation (Giulian et al., 1996, McDonald et al., 1997, Meda et al., 1995).

Plaque associated microglia display dilated intracellular channels of smooth endoplasmic reticulum containing amyloid fibers, (Wisniewski et al., 1989, Wisniewski et al., 1992) suggesting a role of microglia in clearing beta amyloid. Microglia may normally be responsible for amyloid phagocytosis in health (Wisniewski et al., 1991). In AD it has been argued that the microglia clustered around amyloid deposits have become dysfunctional and incapable of removing amyloid (Rogers et al., 2002).

The peripheral benzodiazepine binding receptor (PBBR) is a nuclear encoded mitochondrial protein that is abundant in peripheral organs, particularly in adrenal glands, kidney, heart and lungs as well as haematogenous cells but present in the normal CNS only at low levels (Parola et al., 1993). Normal brain shows detectable levels only in the choroid plexus, ependyma, olfactory bulbs, endothelial cells and in smooth muscle of the tunica media of intra and extra parenchymal arteries (Beurdeley-Thomas et al., 2000). Normally microglia are in a resting state in brain. AD is associated with activation of microglia and the mitochondria of these cells express increased PBBRs (Kreutzberg, 1996). [11C](R)PK11195 [1-(2-chlorophenyl)-N-methyl-N-(1-methyl propyl)-3-isoquinoline carboxamide] is a specific ligand for the PBBR (Shah et al., 1994). In normal brain tissue specific binding of [11C](R)PK11195 is low as microglia are in a resting state. Autoradiography combined with immunocytochemical studies has shown that brain binding of [11C](R)PK11195 closely follows the distribution of microglia activated by brain injury in conditions such as multiple sclerosis (Banati et al., 2000). A previous PET study from our group reported that increased [11C](R)PK11195 binding could be detected in entorhinal, temporoparietal and posterior cingulate cortex in established AD (Cagnin et al., 2001a).

[11C]PIB (N-methyl-[11-C]2-(4′-methyl amino phenyl )-6-hydroxy benzothiazole), binds to fibrillar β amyloid in both neuritic and non-neuritic plaques in the cortex and striatum of AD patients, but not to amorphous β amyloid deposits; the latter can be found in the cerebellum (Klunk et al., 2003). Studies have reported two-fold increases in cortical uptake of [11C]PIB in AD subjects compared with healthy controls (Archer et al., 2006, Edison et al., 2007, Engler et al., 2006, Fagan et al., 2006, Klunk et al., 2004, Mintun, 2005, Verhoeff et al., 2004).

To date the relationship between microglial activation and amyloid deposition in vivo has not been investigated simultaneously. In this study, for the first time we have examined the relative distributions of raised amyloid load and microglial activation in a group of AD subjects using [11C]PIB-PET and [11C](R)PK11195-PET.