Gray matter covariance networks in the mouse brain

The presence of networks of correlation between gray matter volumes of brain regions - as measured across subjects in a group of individuals - has been consistently described in several human studies, an approach termed structural covariance MRI (scMRI). Complementary to prevalent brain connectivity modalities like functional and diffusion-weighted imaging, this approach can provide valuable insight into the mutual influence of regional trophic and plastic processes occurring between brain regions. Previous investigations highlighted coordinated growth of these regions within specific structural networks in healthy populations and described their derangement in pathological states. However, a number of fundamental questions about the origin and significance of these couplings remains open and the mechanisms behind the formation of scMRI networks are still poorly understood. To investigate whether analogous scMRI networks are present in lower mammal species amenable to genetic and experimental manipulation such as the laboratory mouse, I coupled high resolution morpho-anatomical MRI with network-based approaches on a large cohort of genetically-homogeneous wild-type mice (C57Bl6/J). To this purpose, I first developed a semi-automated pipeline enabling reliable Voxel Based Morphometry (VBM) of gray matter volumes in the mouse. To validate this approach and its ability to detect plastic changes in brain structures, I applied it to a cohort of aged mice treated with omega-3 polyunsaturated fatty acids (n3-PUFA). This study revealed that treatment with n3PUFA, but not isocaloric olive oil preserved gray matter volume of the hippocampus and frontal cortices, an effect coincident with amelioration of hippocampal-based spatial memory functions. I next employed VBM to investigate scMRI networks in inbred mice using a seed-based approach. In striking resemblance with human findings, I observed the presence of homotopic (i.e. bilateral) architecture in several scMRI cortical and subcortical networks, a finding corroborated by Independent Component Analyses. Subcortical structures also showed highly symmetric inter-hemispheric correlations, with evidence of distributed antero-posterior networks in diencephalic regions of the thalamus and hypothalamus. Hierarchical cluster analysis revealed six identifiable clusters of cortical and sub-cortical regions corresponding to previously described neuroanatomical systems. This work documents for the first time the presence of homotopic cortical and subcortical scMRI networks in the mouse brain, and is poised to pave the way to translational use of this species to investigate the elusive biological and neuroanatomical underpinnings of scMRI network development and its derangement in neuropathological states.