Evidence of widespread cerebral microglial activation in amyotrophic lateral sclerosis: an [11C](R)-PK11195 positron emission tomography study

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Abstract

Microglial activation is implicated in the pathogenesis of ALS and can be detected in animal models of the disease that demonstrate increased survival when treated with anti-inflammatory drugs. PK11195 is a ligand for the “peripheral benzodiazepine binding site” expressed by activated microglia. Ten ALS patients and 14 healthy controls underwent [11C](R)-PK11195 PET of the brain. Volumes of interest were defined to obtain [11C](R)-PK11195 regional binding potential values for motor and “extra-motor” regions. Significantly increased binding was found in motor cortex (P = 0.003), pons (P = 0.004), dorsolateral prefrontal cortex (P = 0.010) and thalamus (P = 0.005) in the ALS patients, with significant correlation between binding in the motor cortex and the burden of upper motor neuron signs clinically (r = 0.73, P = 0.009). These findings indicate that cerebral microglial activation can be detected in vivo during the evolution of ALS, and support the previous observations that cerebral pathology is widespread. They also argue for the development of therapeutic strategies aimed at inflammatory pathways.

Introduction

ALS is a progressive neurodegenerative disease characterised by degeneration of corticospinal, brain stem and spinal cord motor neurons (Rowland and Shneider, 2001). Median survival (in a clinic setting) is 3–4 years from symptom onset (Turner et al., 2002), but can be more than 10 years for a small proportion of patients (Turner et al., 2003), reflecting the heterogeneous nature of the disease (Shinsuke et al, 2003). The cause of ALS is unknown with the exception of rare familial forms, particularly those associated with mutations of the superoxide dismutase-1 gene Al-Chalabi and Leigh, 2000, Cleveland and Rothstein, 2001. There is no specific diagnostic test or reliable biological marker to assess disease progression, nor are there any reliable techniques for assessing the presence or extent of cortical pathology. Transcranial magnetic stimulation Pohl et al., 2001, Schulte-Mattler et al., 1999, MRI Chan et al., 1999, Ellis et al., 2001 and PET Abrahams et al., 1996, Lloyd et al., 2000, Turner and Leigh, 2000 have all shown potential in this respect.

The ligand PK11195 (1-[2-chlorophenyl]-N-methyl-N-[1-methyl-propyl]-3-isoquinolone carboxamide) binds specifically to the “peripheral benzodiazepine binding site” (PBBS). The PBBS is expressed by mitochondria in cells of the mononuclear phagocyte lineage and within the central nervous system is highly expressed by activated, though not resting, microglia—the brain's intrinsic population of tissue macrophages. Based on the molecular specificity of radioligand–receptor interaction, PET allows the quantitative in vivo functional assessment of specific cellular pathway and their involvement in disease. In combination with volumetric MRI to provide detailed structural information, the entantiomeric PET ligand [11C](R)-PK11195 has been used to measure microglial activation in acute and chronic inflammatory, and non-inflammatory, brain disease (Banati, 2002a).

Microglial cell activation has been implicated in the pathogenesis of several neurodegenerative disorders McGeer and McGeer, 1998, McGeer et al., 1993. There is evidence that inflammatory mechanisms, in which microglial cells may play a central role, are important mediators of cell death or survival specifically in ALS Kriz et al., 2002, McGeer and McGeer, 2002. Moreover, drugs aimed at inflammatory pathways have beneficial effects on survival of transgenic mouse models of ALS Drachman et al., 2002, Zhu et al., 2002. There are also several emerging hypotheses concerning selective motoneuronal cell death, in which microglia may have the central role Ciesielski-Treska et al., 2001, Raoul et al., 2002, Tortarolo et al., 2003.

In this in vivo neuropathological study, [11C](R)-PK11195 PET was used to assess the presence and spatial distribution pattern of microglial activation in the brain in ALS.

Section snippets

Participants

Sporadic ALS patients fulfilling the categories of “probable” or “definite” ALS according to revised El Escorial criteria (Brooks et al., 1998) were invited to take part in the study. Ten ALS patients (4 female, 6 male; mean age 50 years, range 27–63, SD 13) underwent imaging. Eight of the patients had limb-onset disease, two bulbar-onset, and seven of the 10 patients had bulbar signs at the time of the scan. Subjects were scored at the time of the scan using the revised ALS functional rating

Results

[11C](R)-PK11195 BP values for all subjects are summarised in Table 1 (the occasional negative mean BP values represent variation near to zero). Variable binding of [11C](R)-PK11195 was seen in all regions in both patients and controls with overlap of the ranges of the two groups (Fig. 2). Despite this, significantly increased mean binding of [11C](R)-PK11195 was demonstrable in the region of the motor cortex (P = 0.003), pons (P = 0.004) (Fig. 3b), frontal lobe region (DLPFC) (P = 0.010) (Fig.

Discussion

This [11C](R)-PK11195 PET study provides the first in vivo evidence of diffusely increased microglial activation in both motor and “extra-motor” cerebral regions in a group of ALS patients during the evolution of the disease. The sample is small and the conclusions must therefore be approached cautiously. Although there was considerable individual variability in the ALS BP values for [11C](R)-PK11195, these correlated significantly with clinical UMN “burden”, particularly in the region of the

Conclusions

This in vivo PET study demonstrates what appears to be a more diffuse microglial activation compared to the greater, more focal binding seen in [11C](R)-PK11195 PET studies of other cerebral diseases. Nonetheless, these findings represent the first evidence that microglial activation is a feature—and not only an end-stage feature—of ALS. This has important implications for our understanding of pathogenic mechanisms, in many of which, a clear potential role for activated microglia is emerging.

Acknowledgements

MRT is supported by a Wellcome Trust Clinical Research Fellowship. The King's MND Care and Research Centre (MRT, CES, PNL) receives support from the Motor Neurone Disease Association (UK). DJB receives support from the Medical Research Council. RBB has received support from the Max-Planck-Institute of Neurobiology (Martinsried, Germany) and the Deutsche Forschungsgemeinschaft grant: “The mitochondrial benzodiazepine receptor as indicator of early CNS pathology, clinical application in PET”; the

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