The impact of TNFalpha on the developing brain

Maternal immune activation (MIA) is known to affect neuronal precursor gene expression, proliferation and differentiation, and has been shown in mice to produce abnormal phenotypes resembling human neurodevelopmental disorders, particularly schizophrenia and autism. The relative contributions of the maternal and foetal immune response is unknown, and the precise role of the microglial cell in normal and pathological brain development has yet to be fully elucidated. To investigate the contribution of TNFalpha, we bred TNFalpha heterozygous C57BL6 mice to generate offspring with 5 different combinations of maternal and foetal TNFalpha gene dosage. These 5 groups were treated with either saline or LPS (10mug/kg) at E13 and killed at E13.5. Normal saline-injected C57BL6 mice were used as controls. Phosphohistone-H3 positive mitotic cells were counted in the embryonic proliferative zones (ventricular and subventricular zones of the dorsal and medial cortex) at E13.5, and microglial number and arborisation was assessed by Iba1 immunohistochemistry. Counts of proliferating cells were corrected for variation in the size of the proliferative zones. In the SVZ of the dorsal cortex, the density of proliferating cells was increased by maternal TNFalpha knockout in a gene dosage-dependent manner. In the VZ of the dorsal cortex, both heterozygous and knockout foetuses showed decreased density of mitotic cells, with no effect of maternal genotype. Immature microglia were found sparsely populating the embryonic brain at E13.5, with both heterozygous and knockout foetuses showing decreased numbers of microglia and an increase in the fraction of non-arborised (rounded) microglia. In the dorsal and medial SVZ, LPS caused a decrease in mitotic cell density. In the dorsal SVZ, density of proliferating cells was much lower in foetuses from KO and Het mothers, while foetal KO status exacerbated the effect of maternal KO status. Together, our results show differential sensitivity to both maternal and foetal TNFalpha status in different proliferative zones of the brain, with pro-proliferative nonlinear effects of foetal TNFalpha status in the dorsal VZ correlating with increased microglia numbers and arborisation and linearly anti-proliferative effects of maternal TNFalpha status in the dorsal SVZ. Thus, our research shows that microglial cell number is dependent on foetal TNFalpha status, but that maternal TNFalpha signalling can directly or indirectly have a profound effect on the development of the foetal brain independently of the ability of the foetus to produce TNFalpha.