Autism Spectrum Disorders (ASD) are a group of complex developmental conditions that affect roughly one percent of the total population, and for which there is no cure. Since the diagnostic criteria are currently based on behavioral symptoms, such as difficulty with social interaction and communication, as well as repetitive and stereotypic behaviors, the development of mouse models that recapitulate these behavioral features is crucial to study the molecular bases underlying ASD etiology.
Serotonin (5-HT) plays a key role during neurodevelopment. In mice, increasing or decreasing 5-HT availability during development alters emotional and sensory neuronal circuits maturation, and has long-term effects on anxiety and sociability in adulthood. Accordingly, hyperserotonemia is observed in 25% of patients with ASD and was one of the first biomarkers identified for ASD. Nevertheless, the mechanisms underlying the implication of 5-HT in the etiology of neurodevelopmental disorders remain partly elusive due to the potential action of 5-HT on different cell types. In our study, recently published in Molecular Psychiatry, we tested the hypothesis that impaired 5-HT sensing specifically in microglia during a postnatal window between birth and P30 is sufficient to alter the refinement of postnatal circuits, thereby inducing abnormal behaviors relevant to ASD. To this aim, we used a conditional knock-out murine model in which the Htr2b gene (which encodes for 5-HT2B receptor, the main 5-HT sensor in microglia) was deleted only in microglia since birth or during adulthood.
To assess long-lasting consequences of interrupting 5-HT2B receptor-mediated signaling in microglia, we performed a battery of tests to profile the behavior of adult conditional knockout mice in three domains commonly altered in neurodevelopmental disorders: behavior in a novel environment, social skills, and flexibility.
Following neonatal Htr2b gene deletion in microglia, adult mice were significantly more active when exposed to a novel environment, such as an open-field area or an unfamiliar cylinder, suggesting an increased propensity to repetitive stereotypic behavior under unknown and stressful conditions. Furthermore, prevention of 5-HT signaling in microglia triggers social deficits: during the juvenile period, conditional knockout mice were less interested in social familiar stimuli – their mother - than control, wild-type mice. The social deficit seen in the juvenile mice toward their mother persisted in adulthood toward unfamiliar mice. We further explored the social skills of male mice by investigating their social flexibility. To this aim, we recorded the spontaneous social behavior in home cage of male mice housed with a familiar cage mate, during two consecutive days, using the change of litter bedding to challenge the social hierarchy. As expected, wild-type mice adapted their behavior in agreement to the challenge in social hierarchy, showing, on day two, less social investigation and more aggressive behavior than on day one when social hierarchies were defined. By contrast, conditional knockout mice spent less time interacting with the conspecific and showed an abnormal aggressive behavior overall, independently of the environmental challenge. These results confirm a strong social impairment and reveal traits of social inflexibility in mice invalidated for the serotonin receptor in microglia. To understand if the impairment in flexibility could be generalized beyond the social domain, we also performed a reversal learning test, demonstrating that early invalidation of 5-HT2B receptors in microglia decreases not only social but also cognitive flexibility.
Importantly, such behavioral alterations were not seen when microglial 5-HT2B receptors ablation was made at adulthood. This made us wonder whether these behavioral deficits were linked to early microglial dysfunction in the absence of 5-HT signaling. During the first weeks of postnatal development, microglia undergo morphological and functional maturation, with modifications in their phagocytic activity. A precise timing of these changes is necessary for microglia to properly regulate brain circuits formation through the induction of dendritic filopodia formation, engulfment of presynaptic elements, or promotion of spine maturation. Even if the crucial role of microglia in brain wiring refinement is well established, the determinants behind such processes are largely unknown. Interestingly, we showed that abrogating the serotonergic control of microglia neonatally impacts the phagolysosomal compartment of these cells and their proximity to synapses and perturbs neuronal circuits maturation in several brain regions.
Overall, our data point to 5-HT as a global modulator of microglia during the critical time-window of postnatal development, which is needed for the establishment of specific behavioral domains. Furthermore, this link between 5-HT and microglia may explain the association between serotonergic dysfunctions and behavioral traits, like impaired sociability and inadaptability to novelty, which are prominent in several neurodevelopment disorders, notably ASD. Given the number of exogenous factors known to change 5-HT levels during postnatal development, such as physical abuse, maternal separation and maternal inflammation, stress, antidepressants, and their correlation to neurodevelopmental disorders, our findings further emphasize the need of optimal brain levels of 5-HT for proper brain development and function throughout life. Besides, as the serotonergic system is targeted by an array of pharmacological compounds, this newly identified pathway of microglia regulation might open new therapeutic avenues for early diagnosis and intervention in neurodevelopmental disorders.
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