Abstract
Agenesis of the corpus callosum (AgCC) is a congenital condition associated with wide-ranging emotional and social impairments often overlapping with the diagnostic criteria for autism. Mapping functional connectivity in the acallosal brain can help identify neural correlates of the deficits associated with this condition, and elucidate how congenital white matter alterations shape the topology of large-scale functional networks. By using resting-state BOLD functional magnetic resonance imaging (rsfMRI), here we show that acallosal BTBR T+tpr3tf/J (BTBR) mice, an idiopathic model of autism, exhibit impaired intra-hemispheric connectivity in fronto-cortical, but not in posterior sensory cortical areas. We also document profoundly altered subcortical and intra-hemispheric connectivity networks, with evidence of marked fronto-thalamic and striatal disconnectivity, along with aberrant spatial extension and strength of ipsilateral and local connectivity. Importantly, inter-hemispheric tracing of monosynaptic connections in the primary visual cortex using recombinant rabies virus confirmed the absence of direct homotopic pathways between posterior cortical areas of BTBR mice, suggesting a polysynaptic origin for the synchronous rsfMRI signal observed in these regions. Collectively, the observed long-range connectivity impairments recapitulate hallmark neuroimaging findings in autism, and are consistent with the behavioral phenotype of BTBR mice. In contrast to recent rsfMRI studies in high functioning AgCC individuals, the profound fronto-cortical and subcortical disconnectivity mapped suggest that compensatory mechanism may not necessarily restore the full connectional topology of the brain, resulting in residual connectivity alterations that serve as plausible substrates for the cognitive and emotional deficits often associated with AgCC.
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Acknowledgments
The study was funded by the Istituto Italiano di Tecnologia, and supported by Italian Ministry of Health “Young investigators” GR3-2008 (MLS) and by a grant from the Simons Foundation (SFARI 314688, A.G.). We thank Dr. Angelo Bifone for critically reading the manuscript and Dr. E. Callaway for providing SADΔG-mCherry Rabies Virus and other reagents.
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The authors declare that they have no conflict of interest.
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All in vivo studies were conducted in accordance with the Italian law (DL 116, 1992 Ministero della Sanità, Roma) and the recommendations in the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. Animal research protocols were also reviewed and consented to by the animal care committee of the Istituto Italiano di Tecnologia (permit 2007–2012). All surgical procedures were performed under anesthesia.
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A. Bertero and F. Sforazzini contributed equally this work.
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429_2014_948_MOESM1_ESM.tif
Figure S1 Preserved subcortical inter-hemispheric pathways in BTBR mice. Diffusion tensor tractography of subcortical inter-hemispheric in representative normo-callosal control subjects (B6, top), and in acallosal BTBR mice (bottom). Note the presence of preserved major inter-hemispheric WM projections in the anterior commissure (ac), posterior commissure (pc) and thalamus of B6 and BTBR mice (TIFF 6373 kb)
429_2014_948_MOESM2_ESM.tif
Figure S2 rsfMRI inter-hemispheric connectivity assessment using bilateral volumes of interest. (a) T statistics matrices of inter-hemispheric rsfMRI for all pairs of unilateral brain regions from an atlas-based parcellation of the mouse brain (Sforazzini et al., 2014) in B6 (top right) and BTBR mice (bottom-left), respectively. Positive and negative correlations are depicted in hot and cool colors, respectively. The matrices have been thresholded at pc = 0.05, using Bonferroni correction for multiple comparisons. (c) Matrix depicting the region pairs exhibiting statistically significant (thresholded at pc = 0.05, Bonferroni corrected) inter-strain differences in correlation strength (red indicates connectivity strength in BTBR lower than B6, blue indicates connectivity strength in BTBR greater than B6). [Abbreviations: mPFC, prefrontal cortex; OFC, orbitofrontal cortex; Cg, cingulate cortex; Cg1, cingulate cortex, area 1; Rs, retrosplenial cortex; DG, dentate gyrus of the hippocampus; pDG, posterior dentate gyrus; pHC, posterior hippocampus; Subic, subiculum; vHC, ventral hippocampus; LS, lateral septal nucleus; BF, barrel field; Amyg, amygdala; BNST, bed nuclei of the stria terminalis; Nacc, Nucleus accumbens; Cpu, caudate-putamen; GP, globus pallidus; VTA, ventral tegmental area; Th, thalamus; Hypo, hypothalamus; Fro, frontal cortex; M1, primary motor cortex; M2, secondary motor cortex; S1, primary somatosensory cortex; S2, secondary somatosensory cortex; Aud, auditory cortex; Vis, visual cortex; Pt, parietal association cortex; Tea, temporal association cortex; Ins, insular cortex; Pir, piriform cortex; Rhinal, rhinal cortex; InfCo, inferior colliculus; SupCo, superior colliculus] (TIFF 2139 kb)
429_2014_948_MOESM3_ESM.tif
Figure S3 BTBR show disrupted inter-hemispheric functional connectivity in frontal but not posterior cortical areas. (a) rsfMRI correlation heatmaps (T > 2, pc = 0.001) with respect to unilateral representative right hemisphere seed regions (red labeling) in normo-callosal B6 mice (left), and acallosal BTBR mice (right). Each row reports three representative coronal slices indicating voxel exhibiting significant correlation with the seed region. Slices have been arranged in a caudal-rostral manner (left-to right, and top to bottom). Not the presence of contralateral area of significant correlation in B6 mice for each of the seed, and the lack of contralateral homotopic areas in the most frontal seeds of BTBR mice (i.e. M1, Fro and Ins). [Abbreviations: Au1, primary auditory cortex; Cpu: caudate-putamen; dAc, dorsal anterior cingulate; Fro, frontal association area; Ins, insular cortex; M1, primary motor cortex, S1, somatosensory cortex; V1, primary visual cortex] (TIFF 4227 kb)
429_2014_948_MOESM4_ESM.tif
Figure S4 BTBR show increased local functional connectivity. (a) rsfMRI correlation maps indicating areas of statistically significant increased connectivity in BTBR with respect to control B6 mice (T > 1.6, pc = 0.05) in representative seed regions of the right hemisphere. Seed regions are indicated in yellow, blue indicates greater connectivity in BTBR vs. B6. (b) To quantify the effect, rsfMRI correlation strength with respect to a sphere centered in the seed has been measured (depicted in red in the maps). ** p < 0.01, *** p < 0.001, Student’s t test [Abbreviations: Au1, auditory cortex; dHC, dorsal hippocampus; vHC, ventral hippocampus; Ins, insular cortex; S1, primary somatosensory cortex]. (TIFF 1436 kb)
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Sforazzini, F., Bertero, A., Dodero, L. et al. Altered functional connectivity networks in acallosal and socially impaired BTBR mice. Brain Struct Funct 221, 941–954 (2016). https://doi.org/10.1007/s00429-014-0948-9
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DOI: https://doi.org/10.1007/s00429-014-0948-9