Attention-Deficit/Hyperactivity Disorder (ADHD) is a highly prevalent condition that affects up to 7% of all children and for which there is currently no cure and little understanding of the underlying cellular and molecular mechanisms. Despite its high heritability, there are no single gene mutations that explain a significant percentage of ADHD cases. Rather, it has been suggested that environmental factors may strongly influence the manifestation of this disorder. Among several known environmental risk factors, allergies have long been observed to occur in striking comorbidity with ADHD. However, and despite the abundance of data linking allergies with ADHD, there are still few studies addressing the causal link between these conditions, mainly due to the lack of animal models that would allow for mechanistic insight into the disorder. While working to address this issue, we have observed that IL-4 – a cytokine that mediates allergic reactions – plays an important role during the critical period of postnatal remodeling of the cerebellum. This brain region, which is largely affected in ADHD patients, is unique in the sense that it undergoes a profound reorganization during the postnatal period which, in mice, corresponds to the second postnatal week. In fact, an entire layer of cerebellar neurons – the external granule cell layer - is eliminated during the same time window, when mammals are learning to interact with the environment and refining motor skills. According to our results, to help in this process, cerebellar microglia retain an “activated” phenotype that helps prune excessive neurons, thus contributing to cerebellar circuit maturation. However, for this to occur, IL-4 levels need to be tightly regulated, rising progressively - after the first two postnatal weeks - to help dampen microglia from an “activated” to a “surveillance” phenotype. Our recent work has led us to propose that increases in the levels of IL-4, which may occur in allergic conditions, perturb the physiological activity of microglia by attenuating its ability to prune excessive neurons during the critical period of cerebellar remodeling. The effects of IL-4 deregulation may persistently damage the cerebellar circuit, leading to the motor disturbances observed in ADHD. Thus, our observations have led us to propose IL-4-injected animals as a new model to study this disorder. We believe this work will help clarify a novel mechanism of microglia-neuronal interplay that is crucial for the physiological remodeling of the cerebellar circuit, while shedding light on the emerging relationship between allergies and ADHD. Additionally, if our hypothesis is correct, we will be able to put forth a novel animal model for ADHD, which will undoubtedly help overcome one of the major limitations in this field, and to establish a clear path of research for the early detection, prevention and treatment of this condition.
To validate this new animal model and gain further insight into the IL-4-dependent mechanistic link between allergies and ADHD, our main goals for this project are: (i) to perform an in-depth characterization of cerebellar activity, employing electrophysiological tools and resting state functional MRI (ii) to thoroughly characterize the behavioral alterations observed in IL-4-treated mice, (iii) to conduct a genetic dissection of the effects of IL-4 signaling specifically onto microglia versus its direct effects on neurons and (iv) to test the circulating cytokine and IgE profile of ADHD patients and age-matched controls, in order to achieve the first comprehensive allergic profile of a Portuguese ADHD cohort.