Anatomical Phenotyping in the IntegrinB3 Mouse Model Related to Autism

Background: The serotonin system, and genes relevant to it, are emerging as strong autism susceptibility candidates. Integrinβ3 is one of those genes.  Integrinβ3 participates in cell adhesion and cell-surface mediated signaling (http://gene.sfari.org), and has been shown to be associated with autism in a large mutiplex study (Weiss et al. 2006a), Further, Integrinβ3 gene expression has been shown to correlate with another autism susceptibility gene, the serotonin transporter gene (SLC6A4) (Weiss et al 2006b).

Objectives: The purpose of this study was to examine the volume changes in the Integrinβ3 knockout mouse model related to autism with high resolution MRI.

Methods: Specimen Preparation – Sixteen B6/129 fixed mouse brains were examined, 8 wild-type and 8 Integrinβ3 knockout mice.  

MRI Acquisition - A 7.0 Tesla MRI (Varian Inc., Palo Alto, CA) was used to acquire anatomical images of brains within skulls. A T2- weighted, 3D fast spin-echo sequence was used, with a TR of 325 ms, and TEs of 10 ms per echo for 6 echos, field-of-view of 14 × 14 × 25 mm3 and matrix size = 250 x 250 x 450 giving an image with 0.056 mm isotropic voxels. Total imaging time was ~12 h.

Data Analysis – We use image registration to align a neuroanatomical atlas defining 62 brain regions towards each scan.  Volumes of structures for each mouse were calculated in mm³.  Group differences in volume were calculated using t-tests, multiple comparisons controlled using the false discovery rate (q value).

Results: The total brain volume of the Integrinβ3 knockout was found to be 12% less than that of the corresponding wild-type; therefore, the brain regions were calculated as relative volumes (% total brain volume).    The corpus callosum, which is widely implicated in autism as being thinned or smaller in volume, was also found to be decreased in size in the Integrinβ3 mouse model (6%, q=0.01).  Further, in spite of the large relative volume decreases in 14 of the 62 regions, there were a few (6) regions that increased in size, the aymgdala (8%, q=0.01) was one of those regions, which is particularly interesting as it has been related to emotions, memory, and social interaction all of which are relevant to the autism phenotype.  Also we looked specifically at changes in the raphe nuclei, due to their relevance to the serotonin system, and we noticed localized decreases within the nuclei themselves, which may have had some impact in the neurological development.

Conclusions: The Integrinβ3 mouse model used here shows some characteristic anatomical changes that are relevant to autism and the serotonin system in general.  Further, when we compared the changes to 18 other mouse models related to autism, the changes found here correlated best with the BTBR mouse, which has been shown to have altered serotonin transporter functionality (Gould et al. 2011) as well as the Neuroligin R451C which displayed similar white matter changes as the Integrinβ3 mouse.  Interestingly, the changes found in the Integrinβ3 did not correlate well with the SLC6A4 mouse as expected due to there close association.