Identifying the role of microglia in mediating ASD-relevant phenotypes in the adult brain

Abstract

The development, maintenance and survival of microglia, the brain’s resident immune cells, requires signalling via the colony stimulating factor 1 receptor (CSF1R). The Fms intronic regulatory element (FIRE), a highly conserved super enhancer found in the second intron of Csf1r, is also required for the development of microglia. Autism spectrum disorder (ASD) is a neurodevelopmental condition characterised by repetitive behaviours and differences in communication. Many studies have indicated that an altered immune response is a risk factor for ASD as many of the high-risk ASD-associated genes are expressed by microglia. In addition, a variety of microglia-deficient rodent models such as Cx3cr1-/- mice display ASD-relevant behaviours such as hyperactivity and excessive grooming. As Cx3cr1 is also expressed by neurones and astrocytes it is unclear whether the ASD phenotypes seen in these mice are microglia specific. Csf1r deficient rodents also lack microglia but have severe growth abnormalities which hinder behavioural studies. Mice with a deletion of the Csf1r enhancer FIRE also lack microglia but retain other macrophage populations and monocytes in the blood and bone marrow. These mice survive into adulthood and are healthy and fertile, providing a novel model to study the role of microglia in adulthood. Although behavioural studies can be performed with mice, rats are better suited for learning based behavioural tasks and are highly social animals. We have created Csf1r ΔFIRE/ΔFIRE rats using CRISPR/Cas9 technology to assess the role of microglia in mediating ASD-relevant phenotypes. Csf1r ΔFIRE/ ΔFIRE rats are healthy, survive to adulthood and have no overt phenotypes. These rats lack microglia yet retain the other brain associated macrophages. Macrophages in the liver, spleen, peritoneum, and lungs are unaffected by the deletion of FIRE, however reductions in macrophage numbers can be seen in the kidney, heart and other tissues. These rats provide us with a model to determine the influence of microglia in ASD-relevant behaviours but also microgliopathies and neurodegenerative diseases.

Adult and aged Csf1r ΔFIRE/ ΔFIRE rats perform comparably to controls in behaviour paradigms that model features of ASD during social, memory and fear-based tasks. Overall, Csf1r ΔFIRE/ ΔFIRE rats with a lack of microglia result in a lack of ASD-relevant behavioural phenotypes but still allow for in depth investigation into the general role of microglia during development.