Sex differences in the incidence of total callosal agenesis in BALB/cCF mice

[Modern diagnostic of agenesis of the corpus callosum in children]

Observations of the authors regarding main MRI symptoms of agenesis of the corpus callosum (ACC) and literature review on the structure of the corpus callosum in normalcy and pathology are presented. The authors emphasize that some cases of isolated ACC has been found during routine prenatal ultrasound examination. In this regard, prenatal MRI is more effective. In 74% patients with ACC, MRI results are consistent with the results of ultrasound and CT. MRI has advantages in the differentiation of inherited corpus callosum malformation as well as concomitant CNS abnormalities.

Histopathologic characterization of the BTBR mouse model of autistic-like behavior reveals selective changes in neurodevelopmental proteins and adult hippocampal neurogenesis

BACKGROUND

The inbred mouse strain BTBR T+ tf/J (BTBR) exhibits behavioral deficits that mimic the core deficits of autism. Neuroanatomically, the BTBR strain is also characterized by a complete absence of the corpus callosum. The goal of this study was to identify novel molecular and cellular changes in the BTBR mouse, focusing on neuronal, synaptic, glial and plasticity markers in the limbic system as a model for identifying putative molecular and cellular substrates associated with autistic behaviors.

METHODS

Forebrains of 8 to 10-week-old male BTBR and age-matched C57Bl/6J control mice were evaluated by immunohistochemistry using free-floating and paraffin embedded sections. Twenty antibodies directed against antigens specific to neurons, synapses and glia were used. Nissl, Timm and acetylcholinesterase (AchE) stains were performed to assess cytoarchitecture, mossy fibers and cholinergic fiber density, respectively. In the hippocampus, quantitative stereological estimates for the mitotic marker bromodeoxyuridine (BrdU) were performed to determine hippocampal progenitor proliferation, survival and differentiation, and brain-derived neurotrophic factor (BDNF) mRNA was quantified by in situ hybridization. Quantitative image analysis was performed for NG2, doublecortin (DCX), NeuroD, GAD67 and Poly-Sialic Acid Neural Cell Adhesion Molecule (PSA-NCAM).

RESULTS

In midline structures including the region of the absent corpus callosum of BTBR mice, the myelin markers 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and myelin basic protein (MBP) were reduced, and the oligodendrocyte precursor NG2 was increased. MBP and CNPase were expressed in small ectopic white matter bundles within the cingulate cortex. Microglia and astrocytes showed no evidence of gliosis, yet orientations of glial fibers were altered in specific white-matter areas. In the hippocampus, evidence of reduced neurogenesis included significant reductions in the number of doublecortin, PSA-NCAM and NeuroD immunoreactive cells in the subgranular zone of the dentate gyrus, and a marked reduction in the number of 5-bromo-2'-deoxyuridine (BrdU) positive progenitors. Furthermore, a significant and profound reduction in BDNF mRNA was seen in the BTBR dentate gyrus. No significant differences were seen in the expression of AchE, mossy fiber synapses or immunoreactivities of microtubule-associated protein MAP2, parvalbumin and glutamate decarboxylase GAD65 or GAD67 isoforms.

CONCLUSIONS

We documented modest and selective alterations in glia, neurons and synapses in BTBR forebrain, along with reduced neurogenesis in the adult hippocampus. Of all markers examined, the most distinctive changes were seen in the neurodevelopmental proteins NG2, PSA-NCAM, NeuroD and DCX. Our results are consistent with aberrant development of the nervous system in BTBR mice, and may reveal novel substrates to link callosal abnormalities and autistic behaviors. The changes that we observed in the BTBR mice suggest potential novel therapeutic strategies for intervention in autism spectrum disorders.

The Association between handedness, brain asymmetries, and corpus callosum size in chimpanzees (Pan troglodytes)

It has been suggested from studies in human subjects that sex, handedness, and brain asymmetries influence variation in corpus callosum (CC) size and these differences reflect the degree of connectivity between homotopic regions of the left and right cerebral hemispheres. Here we report that handedness is associated with variation in the size of the CC in chimpanzees. We further report that variation in brain asymmetries in a cortical region homologous to Broca's area is associated with the size of the CC but differs for right- and left-handed individuals. Collectively, the results suggest that individual differences in functional and neuroanatomical asymmetries are associated with CC variation not just in humans but also in chimpanzees and therefore may reflect a common neural basis for laterality in these 2 species.

Increased lateralization in rotational side preference in male mice rendered acallosal by prenatal gamma irradiation

In order to test the hypothesis that the ontogenetic development of the corpus callosum is related to the establishment of behavioral laterality, the rotatory behavior in the free-swimming test was studied in male Swiss mice with callosal defects induced by exposure to gamma irradiations at the 16th embryonic day (total dose of 3 Gy). At adulthood, 43 irradiated and 56 non-irradiated mice were submitted to 3 sessions of the free-swimming rotatory test (diameter of the recipient=21 cm; session duration=5 min; inter-test interval=48 h). The number and direction of 30 degrees and 360 degrees turns were recorded. Animals were classified as side-consistent turners (to the right or to the left) when they did not change their preferred side of rotation in all three sessions and in both turning units. In general our results suggested that irradiated animals present more pronounced laterality than non-irradiated ones. In the irradiated group, the percentage of consistent turners was significantly higher than that of non-consistent turners. In the first session, the percentage of animals that presented strong turning preferences in the acallosal group was higher than in the normal group. In first session, the acallosal group presented a higher average number of turns to preferred side than the normal group. Taken together, our results constitute an endorsement to the hypothesis that the normal development of the corpus callosum is related to the establishment of cerebral laterality.

Callosal agenesis affects consistency of laterality in a paw preference task in BALB/cCF mice

We tested the hypothesis that the ontogenetic development of the corpus callosum (CC) affects the consistency of laterality in a paw preference task. Adult male mice (55 normal and 29 acallosal) of the BALB/cCF strain were initially tested (twice; inter-test interval: 72 h) in an unbiased setup in which both forepaws could easily perform a reaching movement. In a subsequent test, animals were placed in a biased setup that favored the use of the non-preferred paw. Acallosal and normal mice were strongly lateralized in the unbiased setup. Additionally, while normal mice did not present a populational bias favoring one of the paws, acallosal mice presented a significant bias favoring the left paw. In the biased setup, left- and right-pawed normal mice were equally consistent (approximately 65% of the animals, in both groups, used the preferred paw of the initial two tests, in spite of the bias). Conversely, while left-pawed acallosal mice were as consistent (65%) as normal mice, only 20% of right-pawed mice were consistent. These results suggest that the development of the CC affects consistency of laterality in a side-dependent manner. These results are discussed considering the role of the CC in the establishment of behavioral lateralization.

Effects of callosal agenesis on rotational side preference of BALB/cCF mice in the free swimming test

In order to test the hypothesis that the ontogenetic development of the corpus callosum is related to the establishment of behavioral laterality, the rotatory behavior in the free swimming test was studied in male mice of the BALB/cCF strain, in which approximately 20% of the animals present total or partial callosal agenesis. All animals were submitted to three sessions of the free-swimming rotatory test in three different sessions (diameter of the recipient = 21 cm; session duration = 5 min; inter-test interval = 48 h). The number and direction of the 30 and 360 degrees turns were recorded. Animals were classified as side-consistent turners (to the right or to the left) when they did not change their preferred side of rotation in all three sessions and in both turning units. In general our results suggested that acallosal animals present more pronounced laterality than normal ones. In the acallosal group, the percentage of consistent turners was significantly higher than that of non-consistent turners. The percentage of animals that presented strong turning preferences in the acallosal group was higher than in the normal group. In first session, the acallosal group presented a higher average number of turns to preferred side than the normal group. Taken together, our results constitute an endorsement to the hypothesis that the normal development of the corpus callosum is related to the establishment of cerebral laterality.