SYNAPSE AND DENDRITE DEFICITS INDUCED BY MUTATIONS IN THE X-LINKED INTELLECTUAL DISABILITY GENE IL1RAPL1

Doctorial Thesis of Caterina Montani: Synapse and dendrite deficits induced by mutations in the X-linked intellectual disability gene Il1rapl1 ABSTRACT Mutations and deletions of Interleukin-1 receptor accessory protein like 1 (IL1RAPL1) gene, localized on X chromosome, are strongly associated to intellectual disability (ID) and autism spectrum disorder (ASD) (Carrie et al., 1999; Piton et al., 2008). IL1RAPL1 protein is localized at the postsynaptic compartment of excitatory synapses (Pavlowsky et al.) and plays an important role in synapse formation and stabilization in developing brain, via the interaction with other pre- and post-synaptic proteins (Pavlowsky et al., ; Valnegri et al.). Several studies indicate that ID and ASD are associated with abnormalities of dendrites and dendritic spines morphology (Verpelli et al.), resulting in a defect in neuronal connectivity and thus an altered processing of information at both the cellular and the neural network. Thus, the analysis of IL1RAPL1 function will contribute to identify the molecular and cellular mechanisms that contribute to cognitive brain function. The aim of presented work was to characterize IL1RAPL1 mutants identified in patients with ID and ASD and to perform a behavioral and neuronal morphology analysis on IL1RAPL1 KO mice. In particular, we studied the function of three novel independent mutations of IL1RAPL1 gene in patients presenting mild to moderate ID (Ramos-Brossier et al.): IL1RAPL1∆exon6, IL1RAPL1 C31R and IL1RAPL1 I643V. Through over-expression of the IL1RAPL1 mutants in rat primary cultured neurons, we studied their localization, the ability to recruit presynaptic compartment and to induce changes in neuronal morphology and spine density. To better understand the molecular interaction between IL1RAPL1 and PTPδ, we also investigate, through a cell aggregation assay, the residual ability of IL1RAPL1 mutants to bind PTPδ (Ramos-Brossier et al.). We found that two of the studied mutants lead to a partial loss of function of IL1RAPL1 protein, which is, first, responsible for the cognitive impairments observed in patients and, second, highlights the important function of the extracellular domain for the trans-synaptic PTPδ/IL1RAPL1 interaction in synaptogenesis (this part of the data has already been published (Ramos-Brossier et al.) thus the results and their discussion are not fully reported in this Thesis). Because dendritic abnormalities are one of the most consistent anatomical correlates of ID, we also charactrize the role of wild type and mutants IL1RAPL1 in regulating dendrite morphology using both in vitro neuronal cultures and IL1RAPL1 KO mice. We measured dendritic branching complexity of hippocampal CA1, CA2 and cortex neurons of WT and IL1RAPL1 KO mice. Interestingly we found, associated to hippocampal cognitive impairment, an increased number of dendrite branching in CA1 and CA2 hippocampal neurons but not in cortex of IL1RAPL1 KO mice. In transfected hippocampal neurons the over-expression of full length IL1RAPL1 protein and mutants lacking part of C-terminal domains leads to a simplification of neuronal arborisation. This effect is instead abolished when we overexpressed mutants lacking part of N-terminal domains. Thus our results indicate the importance of IL1RAPL1 extracellular domains not only in synaptogenesis but also in dendrite development. However we also found that for this activity PTPδ interaction is not required suggesting that an unknown IL1RAPL1 binding partner is involved in the effect on dendrite morphology. Understanding how mutations or absence of IL1RAPL1 act on synaptogenesis and dendritic morphology can help to clarify how any changes in IL1RAPL1 function lead to cognitive disorders in humans. In addition, further investigation of the molecular mechanism of IL1RAPL1 would identify potential drug targets to ameliorate cognitive deficits in patients.