The role of mammalian Brinps in neurobiology and neural development

Background: Bone morphogenetic protein / retinoic acid inducible neural-specific proteins, Brinp1, Brinp2 and Brinp3 are almost exclusively in the mammalian nervous system from mid-embryonic development through to adulthood. Brinps are highly conserved in vertebrates, indicating an important function. Brinps share homology via a MACPF domain with neuronal migration proteins, Astrotactin1 and Astrotactin2. BRINPs and Astrotactins are found at two conserved loci, with a high number of neural genes. The loci have been implicated in several neurodevelopmental disorders. Approach: To investigate the function of Brinps in the developing and adult mammalian brain, Brinp1, Brinp2 or Brinp3 conditional knockout (KO) mice were generated via by Cre-recombinase mediated removal of the 3rd exon of Brinp1, Brinp2 or Brinp3. Histopathological analysis was performed on each of the three KO lines. Cohorts of Brinp1, Brinp2 and Brinp3 KO mice were then tested by behavioral screening, which examined multiple aspects of behavior: motor co-ordination, sensory processing, anxiety response, sociability and memory. To investigate whether the absence of multiple Brinps would result in an enhanced phenotype, a double KO mouse line (Brinp2/3) and a triple KO line were generated (Brinp1/2/3). These lines were characterized in a similar manner to the single KO lines. Further phenotypic analysis was undertaken for the Brinp1 KO mice, with additional behavioral testing and immuno-staining of cell populations in the adult and embryonic (E18.5) Brinp1 KO mouse brains. Results: Adult Brinp1 knockout adult mice exhibited a marked reduction in sociability and altered ultrasonic vocalizations. Brinp1 KO mice also demonstrated impaired short-term memory, hyperactivity and hyper-exploratory behavior in a novel environment. Methylphenidate (Ritalin), a drug commonly used to treat ADHD in humans, did not reduce hyperactivity in the Brinp1 KO mice. Brinp3 KO mice also showed hyper-exploratory behavior, whilst Brinp2 KO mice showed no impairments in behavior. The absence of both Brinp2 and Brinp3 in the double reproduced the Brinp3 KO mice behavioral phenotype. Brinp1 KO mice were found to have a reduced density of pyramidal neurons in layer IV of the somatosensory neocortex. Altered layer IV formation was detected in the E18.5 neocortex by labelling E14.5-born cells with 5-bromo-2'-deoxyuridine (BrdU). An increased density of parvalbumin-expressing interneurons was identified in both the neocortex and hippocampus. An increase in Astrotactin1 and Astrotactin2 mRNA was detected in the embryonic and postnatal brains of Brinp1 KO mice. Conclusions: The altered behavior of Brinp1 KO mice resemble features of a subset of human neurological disorders, namely autism spectrum disorder (ASD) and the hyperactivity aspect of attention deficit hyperactivity disorder (ADHD). Brinp1 influences neuronal distribution within the cortex, indicating a role for Brinp1 in cell migration.