20996
Disrupted Short- and Long-Range Neural Connectivity in a Mouse Model of Autism
Autism spectrum disorder (ASD) is characterized by deficits in socio-emotional functions and language development, as well as repetitive and/or restrictive behaviours. The behavioural domains that define ASD together with the vast heterogeneity of the clinical symptoms suggest that these deficits likely involve widely distributed neural systems. Accordingly, it has been proposed that ASD may represent a condition of disrupted connectivity. Neuroimaging studies have shown diffuse impairment in brain networks of ASD patients, including both focal deficits in specific brain regions and aberrant long-range connections. However, their cellular substrates and molecular underpinnings remain poorly understood.
Objectives:
We investigated the development of neural circuitry in the BTBR mouse model of autism, which displays all of the three defining behavioural features of ASD. First, we examined eye-specific segregation in the lateral geniculate nucleus (LGN), which is a model system to study how precise patterning of synaptic connections form and refine during development. Second, we investigated the development of dendritic arbors and neuronal densities of hippocampal pyramidal neurons, which integrate spatial, contextual, and emotional information and transmit it to various regions throughout the brain.
Methods:
We labelled retinal afferents from both eyes with an anterograde tracer conjugated with different fluorophores, and compared eye-specific segregation in the LGN between the BTBR and B6 animals, a strain commonly used as a control for the BTBR mice in ASD studies. Next, we used Golgi staining to reveal dendritic structure, and Nissl staining to measure neuronal density of hippocampal CA1 pyramidal neurons. Western blots were used to quantify relative expression levels of proteins known to regulate dendritic structure.
Results:
We found that in neonatal animals, the total area of dorsal LGN occupied by retinal afferents was more rounded in shape, and significantly smaller in size in the BTBR compared to B6 mice. Notably, the degree of overlap between the ipsi- and contralateral afferents was significantly greater in the BTBR mice. Moreover, these abnormalities continued into adulthood in the BTBR animals, suggesting persistent deficits rather than delayed maturation of axonal refinement. Further, we found that both total length and branching of the dendritic arbor were significantly greater in neonatal BTBR compared to B6 animals, while no differences were found in the density of CA1 pyramidal neurons or the thickness of the neuronal layer. A significant difference in the protein levels of ERK pathway, but not BDNF or S6 kinase signaling, was found between the two strains, indicating that altered ERK signaling may be involved in the dendritic changes observed in the BTBR model.
Conclusions:
Circuit formation and refinement were disrupted in the BTBR model of ASD, including both precise patterning of synaptic connections by long-range axonal projections and the development of dendritic arbor. Together, these results indicate disrupted connectivity at both local and long-distance levels in the BTBR model of autism, suggesting that such abnormalities could contribute to the overall impaired connectivity observed in ASD, and may ultimately contribute to the behavioural symptoms.