Identification and characterization of Huntington related pathology: An in vivo DKI imaging study
Highlights
► Novel diffusion kurtosis imaging (DKI) findings in aged transgenic HD rats. ► Identification of sensitive biomarkers to track disease onset and progression. ► Increased diffusion complexity is confirmed by increased fiber dispersion. ► Histological samples from the same specimens confirmed in vivo DKI measurements.
Introduction
Huntington disease (HD) is a neurodegenerative disorder, caused by a CAG trinucleotide repeat expansion in the HD gene. Although considerable progress has been made, no cure exists, and the pathological mechanism by which HD leads to neuronal degeneration remains largely unknown. Due to a lack of early biomarkers, the current standard method for the detection of early onset in HD, is based on clinical evaluation in presymptomatic patients (Bohanna et al., 2008, Zuccato et al., 2010). Given the complex phenotypic heterogeneity of HD, another research challenge is to find highly sensitive screening methods (Esmaeilzadeh et al., 2011). Magnetic Resonance Imaging (MRI) has recently contributed to an improved understanding of striatal atrophy and cortical dysfunction (Paulsen et al., 2004, Rosas et al., 2004, Wolf et al., 2007, Wolf et al., 2009). Moreover, diffusion-based MRI techniques are the only non-invasive imaging techniques that can be applied to study the pathways and connections of not only the human brain, but also of the rodent brain (Douaud et al., 2009, Mascalchi et al., 2004, Reading et al., 2005, Rosas et al., 2006, Seppi et al., 2006, Van Camp et al., 2012). Diffusional kurtosis imaging (DKI) is a very promising diffusion imaging technique (Jensen et al., 2005, Lu et al., 2006), which is expanded towards quantification of non-Gaussian water diffusion. Recent studies have demonstrated that DK measures offer an improved sensitivity in detecting developmental and pathological changes in neuronal tissues, compared to conventional DTI (Cheung et al., 2009, Hui et al., 2008). In addition, directional kurtosis analyses have been formulated to reveal directionally specific information, such as the water diffusional kurtoses along the direction parallel or perpendicular to the principle water diffusion direction (Blockx et al., 2011a, Delgado y Palacios et al., 2011, Hui et al., 2008, Wu and Cheung, 2010). Because kurtosis is a measure of the deviation of the diffusion displacement profile from a Gaussian distribution, DKI analyses quantify the degree of diffusion restriction or tissue complexity.
Transgenic models reflecting elements of human pathology allow the examination of the onset and further progression of neurodegeneration, neuronal dysfunction, and the development and validation of new potential therapies, using translational tools and complementary techniques. The first transgenic rat model of HD (tgHD) was generated by von Hörsten and colleagues in 2003 (von Horsten et al., 2003). These rats display symptoms similar to the late-onset form of HD and develop a progressive phenotype with motor deterioration and emotional, cognitive decline, starting at the age of 7 and 9 months respectively (Kantor et al., 2006, Nguyen et al., 2006, von Horsten et al., 2003), as well as typical histopathological alterations in the form of striatal neuronal nuclear inclusions at the earliest age of 6 months (Bode et al., 2008, Cao et al., 2006, Kantor et al., 2006, Nguyen et al., 2006, Petrasch-Parwez et al., 2007). In an earlier study (Blockx et al., 2011b), we performed a longitudinal multimodal imaging study, combining MR-DTI with μPET imaging on the transgenic rat model of HD, where we aimed to further characterize the phenotype of the tgHD rat model. The outcome of this longitudinal DTI study demonstrated a significant differential aging pattern in the tgHD versus Wt animals, involving the striatum and the white matter structures surrounding it. In a second experiment (Blockx et al., 2011a), we examined the effect of the presence of mutant huntingtin on the postnatal brain development of tgHD rat pups. By using in vivo DKI, temporal age related differences of diffusion parameters were studied. With this DKI study, we demonstrated that there are distinct differences in postnatal brain development of tgHD rat pups, from the age of P15 onwards. These findings prompted us to hypothesize altered diffusion measures in tgHD rat brains, in the same regions as typically affected in HD patients. The aim of the present study was to evaluate whether in vivo DKI measurements provide a more sensitive tool for non-invasive quantification of alterations in brain tissue integrity in tgHD rats. To that end, we first wanted to evaluate this by using aged tgHD rats. We performed hypothesis driven region of interest (ROI) analyses to test whether averaged diffusion parameters were altered in the cerebral cortex, striatum, and associated white matter regions in tgHD rats. Analogous to a study performed in HD patients (Douaud et al., 2009) we extracted the main fiber orientations, to assess possible changes in the orientational organization of the striatum. Finally, to explore possible anatomical substrates underlying diffusion changes observed in tgHD rats, we evaluated astrocyte, oligodendrocyte and microglia cell populations in immuno-histological material from the same brain regions.
Section snippets
Animals
We used 16-month-old male homozygous tgHD rats (n = 7) and wild-type (Wt) (n = 7) littermates. TgHD rats carried a truncated htt cDNA fragment with 51 CAG repeats, under control of the native rat htt promoter (von Horsten et al., 2003). Rats were housed under standard conditions (12-h light/dark cycle) with unrestricted access to food and water. All experimental procedures were performed in accordance with the European guidelines for the care and use of laboratory animals and were approved by the
Volumetric analyses
No significant differences were measured in the average volume of the lateral ventricles (p = 0.21), striatum (p = 0.49), or entire brain (p = 0.72) between the Wt and tgHD group. Thus, no evidence for global or regional atrophy could be measured in tgHD rats (Fig. 2).
DTI–DKI analyses
- Parietal cortex
No significant diffusion changes were found in the parietal cortex of tgHD rats (p > 0.05) (Fig. 3-b).
- (Pre)frontal cortex
Significantly elevated MD, AD and RD values were observed in the (pre)frontal cortex in tgHD animals
Discussion
Since it is established that much cell death has already occurred at the time of diagnosis (Aylward et al., 2004) and that cognitive, sensory, and psychiatric abnormalities often precede motor symptoms in HD (Duff et al., 2007, Paulsen et al., 2006, Solomon et al., 2007), neuroimaging efforts have gained momentum. Diffusion MRI has recently yielded significant insights into brain microstructure in both clinical and preclinical HD (Bohanna et al., 2008, Douaud et al., 2009, Kloppel et al., 2008a
Conclusions
We have shown that DKI provides improved sensitivity and specificity in MR diffusion characterization of neural tissues. It has the potential to expand our understanding of white and gray matter pathology in neurodegenerative diseases. Interestingly, the observations made in gray matter, were comparable to those observed in white matter, which is suggestive for the same pathological process. Although, in complex diseases such as HD, brain regions may experience an unpredictable combination of
Acknowledgments
We thank Yvette C. van Dongen, Anna T. Bore, and Grazyna Babinska for assistance with preparation and processing of histological material, and Alexander P. Osmand for helpful advice.
This study was funded in part by the EC — FP6-project DiMI, LSHB-CT-2005-512146 to AVDL. The study was supported by the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT) SBO/030238, The European Community (RATstream™ STREP, LSHM-CT-2007-037846) to AVDL and SVH, The Research
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