Social approach in genetically engineered mouse lines relevant to autism

Abstract

Profound impairment in social interaction is a core symptom of autism, a severe neurodevelopmental disorder. Deficits can include a lack of interest in social contact and low levels of approach and proximity to other children. In this study, a three-chambered choice task was used to evaluate sociability and social novelty preference in five lines of mice with mutations in genes implicated in autism spectrum disorders. Fmr1(tm1Cgr/Y)(Fmr1(-/y)) mice represent a model for fragile X, a mental retardation syndrome that is partially comorbid with autism. We tested Fmr1(-/y)mice on two genetic backgrounds, C57BL/6J and FVB/N-129/OlaHsd (FVB/129). Targeted disruption of Fmr1 resulted in low sociability on one measure, but only when the mutation was expressed on FVB/129. Autism has been associated with altered serotonin levels and polymorphisms in SLC6A4 (SERT), the serotonin transporter gene. Male mice with targeted disruption of Slc6a4 displayed significantly less sociability than wild-type controls. Mice with conditional overexpression of Igf-1 (insulin-like growth factor-1) offered a model for brain overgrowth associated with autism. Igf-1 transgenic mice engaged in levels of social approach similar to wild-type controls. Targeted disruption in other genes of interest, En2 (engrailed-2) and Dhcr7, was carried on genetic backgrounds that showed low levels of exploration in the choice task, precluding meaningful interpretations of social behavior scores. Overall, results show that loss of Fmr1 or Slc6a4 gene function can lead to deficits in sociability. Findings from the fragile X model suggest that the FVB/129 background confers enhanced susceptibility to consequences of Fmr1 mutation on social approach.

Figure 1. Open field locomotion in Fmr1 mouse lines from Cohort 2

Significant increases in distance traveled were seen in the Fmr1-null mice on an FVB/129, but not a C57BL/6J, background. Activity was assessed by a 1-hour trial in an open field chamber. Data shown are mean ± SEM. *p<0.05.

Figure 2. Time spent in each side during the test for sociability in (a) Cohort 1 and (b) Cohort 2 of the Fmr1 mouse lines, and (c) numbers of entries during the test

Fmr1-null mice on the FVB/129 background, from both cohorts, did not have a significant preference for proximity to stranger 1. The loss of Fmr1 did not have significant effects on number of entries in either background strain. Side choice for Cohort 1 included an empty side (without any wire cage); for Cohort 2, an empty cage side. Data shown are mean + SEM. * p<0.05, within-group comparison, stranger 1 side different from empty (Cohort 1) or empty cage (Cohort 2) side. # p<0.05, comparison with same measure in ^+/y mice with C57BL/6J background.

Figure 3. (a) Time spent sniffing each cage during the test for sociability and (b) time spent in each side during the test for social novelty preference in Fmr1 mouse lines from Cohort 2

All groups had a significant preference for the wire cage containing an unfamiliar mouse, stranger 1, in comparison to an empty cage (a), and a significant preference for proximity to the more-novel stranger 2 (b). Data shown are mean + SEM. * p<0.05, within-group comparison, stranger 1 side different from opposite side.

Figure 4. Time spent in each side by Slc6a4 mice during the tests for (a) sociability and (b) preference for social novelty

Neither male nor female Slc6a4-null mice, or female heterozygous mice, had a significant preference for proximity to stranger 1. Data shown are mean + SEM for each group. * p<0.05, within-group comparison, stranger 1 side different from empty cage side (a) or stranger 2 side (b). # p<0.05, comparison with same measure in both ^+/+ and ^+/− male mice.

Figure 5. Time spent sniffing each cage by Slc6a4 mice during the tests for (a) sociability and (b) preference for social novelty

During the test for sociability, all groups had a significant preference for the cage containing stranger 1. Data shown are mean + SEM for each group. * p<0.05, within-group comparison, stranger 1 side different from empty cage side (a) or stranger 2 side (b).

Figure 6. Time spent in each side by Igf-1 mice during the tests for (a) sociability and (b) preference for social novelty

All groups had a significant preference for proximity to stranger 1 in the test for sociability. Data shown are mean + SEM for each group. * p<0.05, within-group comparison, stranger 1 side different from empty cage side. # p<0.05, comparison with same measure in ^+/+ female mice.