Tractography Study of Brain Asymmetries in a Genetic Mouse Model
Alexandra Petiet1,2, Gonçalo C Vilhais-Neto3,4, Daniel Garcia-Lorenzo1, Stéphane Lehéricy1,2, and Olivier Pourquié3,4,5,6,7

1Center for Neuroimaging Research, Brain and Spine Institute, Paris, France, 2UPMC/Inserm UMRS1127 / CNRS UMR7225, Paris, France, 3Institut de Génétique et de Biologie Moléculaire et Cellulaire, Illkirch, France, 4Stowers Institute for Medical Research, Kansas City, MO, United States, 5Howard Hughes Medical Institute, Kansas City, MO, United States, 6Department of Anatomy and Cell Biology, University of Kansas Medical Center, Kansas City, MO, United States, 7Department of Genetics, Harvard Medical School and Department of Pathology, Brigham and Woman’s Hospital, Boston, MA, United States

Synopsis

While humans show clear preference for right hand usage (90%), normal mice show consistent right or left paw usage (50%). We used a Rere-deficient mouse model (Rere+/om) that shows clear right paw usage preference (80%) compared to wild-type (WT) mice (40%) to evaluate structural connectivity changes in the cortico-spinal tract (CST) using diffusion-based tractography. Our results showed significantly reduced and more asymmetric FA along the CST of the dominant hemisphere in the dextral mutant group compared to the WT group. These results show Rere-dependent structural connectivity changes in the brain that could be clinically relevant to human pathologies.

Purpose

To study brain asymmetries in a Rere (Atrophin2)-deficient mouse model1 using diffusion MRI-based tractography. The Rere gene affects the symmetry of vertebral precursors2. Unlike humans who show clear preference for right hand usage (90%), normal mice show consistent right or left paw usage (50%). The Rere mouse model (Rere+/om mutants, Vilhais-Neto et al., submitted) shows clear right paw usage preference (80%) compared to wild-type mice (WT, 40%) in Mouse Reaching and Grasping (MoRaG) tests. In this study we aimed at evaluating structural connectivity changes in the cortico-spinal tract (CST) of Rere-mutated animals using diffusion-based tractography. We hypothesized that the Rere gene had an effect on the symmetry of the CST in Rere-deficient animals.

Methods

A total of 44 mice (Rere+/om: N=22 and WT: N=22) were imaged after analysis with the MoRaG for paw usage (11-15 months old). All images were acquired on a Bruker Biospec 117/16 USR MRI system (BGA-9S gradients, 750 mT/m, AVIII) running Paravision 5.1. A 72-mm resonator was used for signal emission and a planar surface coil for mouse head was used for signal reception (Bruker Biospin, Ettlingen, Germany). Structural T2-weighted images were acquired for all mice. A multi-slice turbo-RARE sequence was used with TR=6500ms; TE=40ms; matrix=384x384; field-of-view(FOV)=23x23mm2 (60x60μm2 in-plane resolution); 72 slices; slice thickness=0.22mm; number of excitations=4; scan time=2h8min. After first and second order shimming with FASTMAP, respiratory-gated diffusion-weighted images were acquired for all mice. A 3D respiratory-gated EPI sequence was used with TR=1000ms; TE=25ms; matrix=128x96x32; FOV=20x15x16mm3 (resolution=156x156x500μm3); 126 directions; b-value=1000s/mm2; scan time=1h13min. A B0 fieldmap was also acquired and used for distortion correction of the diffusion data. A total of 12 sinistral WT mice (WT L-paw); 8 dextral WT mice (WT R-paw); and 18 dextral Rere+/om mutant mice (Rere+/om R-paw) were included in the analyses. The diffusion acquisitions were preprocessed with FSL software3. We corrected for motion with the Eddycor FSL function and we then used a fieldmap acquisition to correct the EPI geometric distortions. The corrected mean B0 volumes were then registered to the structural T2 volumes. We finally performed the diffusion tensor estimation (with FSL dtifit function) to produce fractional anisotropy (FA) volumes for each mouse. The fiber-tracking analysis was performed with the MRtrix package (http://www.brain.org.au/software/). For the corticospinal tract (CST) reconstruction, six mask ROIs were manually drawn in the left and right motor cortex, internal capsule, and pyramidal tracts of the normalized images, according to the Paxinos and Franklin mouse brain atlas4 and with the MRView viewing tool. The ROIs were then denormalized and the tracts were reconstructed from the native images using spherical deconvolution and probabilistic tractography (Figure 1). Along-fiber FA analysis was performed similarly to the one presented in Colby et al.5. Each fiber was divided into 50 equidistant points; the division was performed using the predefined ROIs to provide good alignment across subjects. Measures were evaluated using a mixed-effects model to take into account local FA and global subject effects. Statistical unpaired t-tests were run for between-group comparisons.

Results

The comparison between all groups (Rere+/om vs WT L-paw; Rere+/om vs WT R-paw; Rere+/om vs WT all; WT R-paw vs WT L-paw) did not show any global differences in FA along the CST. Only fine local differences were found with reduced FA in the dominant tract (left hemisphere) of the Rere+/om as compared with the WT R-paw (FA(WT-Rpaw)>FA(Rere+/om), p<0.00005, Figure 2). The Rere+/om group was found to be locally more asymmetric than both the WT R-paw and L-paw groups (p<0.0005, Figure 3).

Conclusion

The dextral Rere+/om mutant group showed reduced and more asymmetric FA in the dominant CST compared to the matched dextral and sinistral WT groups. These results show Rere-dependent structural connectivity changes in the brain that could be clinically relevant to human pathologies.

Acknowledgements

This work was supported by the Howard Hughes Medical Institute, the Stowers Institute for Medical Research, the European Research Council and the “Investissements d'Avenir”, IHU-A-ICM, Paris Institute of Translational neuroscience ANR-10-IAIHU-06.

References

1. Zoltewicz JS, Stewart NJ, Leung R et al. Atrophin 2 recruits histone deacetylase and is required for the function of multiple signaling centers during mouse embryogenesis. Development 2004;131(1):3-14. 2. Vilhais-Neto GC, Maruhashi M, Smith KT et al. Rere controls retinoic acid signalling and somite bilateral symmetry. Nature 2010; 463(7283):953-7. 3. Smith SM, Jenkinson M, Woolrich MW et al. Advances in functional and structural MR image analysis and implementation as FSL. NeuroImage 2004; 23(S1):208-19. 3. “The mouse brain in stereotaxic coordinates”, Paxinos G and Franklin KB, 2nd Edition, Academic Press 2001. 4. Colby JB, Soderberg L, Lebel C et al. Along-tract statistics allow for enhanced tractography analysis. Neuroimage 2012; 59(4):3227-42.

Figures

Figure 1: Representative left and right CSTs overlaid onto a T2 axial image.

Figure 2: Diagram showing mean FA along the dominant and non-dominant CST from pyramids (left end of curve) to motor cortex (right end). There was reduced FA in Rere+/om R-paw vs WT R-paw in the dominant tract. Mean = green/blue thick lines and SD = grey envelope.

Figure 3: Asymmetry index = (right FA – left FA)/(right FA + left FA) along the CST in Rere+/om R-paw mice (blue lines), L-paw (green lines) and R-paw (red lines) WT groups. Rere+/om R-paw mice showed stronger local asymmetries.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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