Cortical atrophy in experimental autoimmune encephalomyelitis: In vivo imaging

Abstract

There are strong correlations between cortical atrophy observed by MRI and clinical disability and disease duration in multiple sclerosis (MS). The objective of this study was to evaluate the progression of cortical atrophy over time in vivo in experimental autoimmune encephalomyelitis (EAE), the most commonly used animal model for MS. Volumetric changes in brains of EAE mice and matched healthy controls were quantified by collecting high-resolution T2-weighted magnetic resonance images in vivo and labeling anatomical structures on the images. In vivo scanning permitted us to evaluate brain structure volumes in individual animals over time and we observed that though brain atrophy progressed differently in each individual animal, all mice with EAE demonstrated significant atrophy in whole brain, cerebral cortex, and whole cerebellum compared to normal controls. Furthermore, we found a strong correlation between cerebellar atrophy and cumulative disease score in mice with EAE. Ex vivo MRI showed a significant decrease in brain and cerebellar volume and a trend that did not reach significance in cerebral cortex volume in mice with EAE compared to controls. Cross modality correlations revealed a significant association between neuronal loss on neuropathology and in vivo atrophy of the cerebral cortex by neuroimaging. These results demonstrate that longitudinal in vivo imaging is more sensitive to changes that occur in neurodegenerative disease models than cross-sectional ex vivo imaging. This is the first report of progressive cortical atrophy in vivo in a mouse model of MS.

Introduction

Multiple sclerosis (MS) is an inflammatory, demyelinating disease of the central nervous system (CNS). Magnetic resonance imaging (MRI) techniques have been developed in order to visualize disease activity in MS primarily in the form of the number and volume of lesions in the CNS (Zivadinov and Bakshi, 2004b). However, even though lesion volume has been shown to correlate with clinical measures of disease progression, it is clear that lesions can represent both reversible and irreversible effects of the disease (Bermel and Bakshi, 2006). A growing body of literature has documented the neurodegenerative aspect of MS (Peterson and Fujinami, 2007, Trapp et al., 1999) and shown gray matter atrophy to be a better indicator of irreversible clinical disease progression and cognitive impairment (Amato et al., 2008, Bermel and Bakshi, 2006, Miller et al., 2002, Zivadinov and Bakshi, 2004b). In fact, measurement of whole brain atrophy has been correlated strongly with disability (Rudick et al., 1999, Zivadinov and Bakshi, 2004a) and in one study involving an eight-year observation period brain atrophy was found to have the most robust correlation with disability out of several MRI measures (Fisher et al., 2002). Additionally, anatomically localized atrophy has been shown to correlate strongly to disability, specifically cerebellar cortex atrophy correlated with cerebellar disability (Calabrese et al., 2010).

Mouse disease models play a critical role in the understanding of disease mechanisms, progression and drug efficacy testing because of similarities in disease shared between mouse and human and the feasibility of genetic manipulation in this species. Brain morphology is often an important biomarker to detect and monitor neurological disease in mouse models; including the atrophy of specific brain regions. Histology has been widely used for the characterization of phenotypes and the evaluation of therapeutic interventions. However, in recent years in vivo magnetic resonance imaging (MRI) in mice has become a promising and widely available technique to examine brain morphology (Bock et al., 2006, Borg and Chereul, 2008, Delatour et al., 2006). MRI has permitted the repeated measure of neuroanatomical structures to evaluate the progression of atrophy in these structures in vivo (Lau et al., 2008, Zhang et al., 2010). It has high anatomical accuracy without the tissue deformation associated with the embedding, sectioning and staining procedures used in histology. Also, its quantitative three-dimensional (3D) format makes it much more accurate at volume measurement than conventional histological methods (Badea et al., 2007, Jacobs et al., 1999, Kovacevic et al., 2005, Ma et al., 2005, MacKenzie-Graham et al., 2009).

MRI has been used extensively in the study of the most commonly used animal model of MS, experimental autoimmune encephalomyelitis (EAE). A great deal of work has focused primarily on white matter lesions and their visualization (Ahrens et al., 1998, Levy et al., 2010, Morrissey et al., 1996, Pirko et al., 2008). However, recent work has reinforced the “clinico-radiological paradox” describing a discrepancy between lesion burden visualized by MRI and the extent of clinical disability (Wuerfel et al., 2007). Measures of axonal damage in the spinal cord using diffusion tensor imaging (DTI) have shown greater correlation with clinical scores (Budde et al., 2008), but do not assess injury to the gray matter in EAE.

In previous studies we have demonstrated ex vivo gray matter atrophy in the cerebellar cortex of mice with EAE (MacKenzie-Graham et al., 2006, MacKenzie-Graham et al., 2009). In this study we proposed to analyze gray matter atrophy in vivo and to examine the neuropathological underpinnings of that atrophy. Despite the generally lower spatial resolution of in vivo compared to ex vivo MRI (as a result of shorter acquisition times), we hypothesized that in vivo MRI might nevertheless be more sensitive for detection of atrophy, particularly in the cerebral cortex. This greater sensitivity would be due to maintenance of the in vivo morphology of the brain, with full ventricles and anatomical structures adjacent to the ventricles retaining their in vivo configuration, as opposed to displacement into the collapsed ventricles that occurs in ex vivo MRI. Thus, in order to detect progression of gray matter atrophy in the cerebral cortex of mice with EAE we induced disease in female C57BL/6 mice with myelin oligodendrocyte glycoprotein (MOG) and imaged the mice before disease induction and during the course of disease.