The Role of Astrocyte BMP Signaling in Fragile X Syndrome

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Project Summary/Abstract This project addresses the role of astrocyte bone morphogenetic protein (BMP) signaling in the pathogenesis of Fragile X Syndrome (FXS). FXS is the most common inherited form of intellectual disability (ID) and autism spectrum disorder (ASD). FXS is caused by trinucleotide repeat expansion in the FMR1 gene promoter leading to transcriptional silencing, and FXS is most often modeled with the Fmr1 knockout (KO) mouse. Although research has implicated several pathways mediating the effects of the Fmr1 loss of function, most targeted treatments have failed in clinical trials, and FXS is predominantly treated by symptom only. The majority of research in FXS has focused on intrinsic changes within neurons. However, emerging research in FXS implicates astrocytes, specifically through astrocyte-secreted factors. Wild-type (WT) neurons cultured with astrocytes or astrocyte-conditioned media (ACM) from Fmr1 KO mice exhibit stunted neurite outgrowth and recapitulate the immature dendritic spine phenotype observed in vivo in Fmr1 KO mice and human FXS patients, providing direct evidence for a causal role of astrocyte-secreted factors in FXS. Preliminary data profiling FXS astrocyte transcription and protein secretion identified four proteins both overexpressed in mRNA and oversecreted, one of which is BMP6. Furthermore, activation of BMP signaling in WT astrocytes generates over a third of the protein secretion changes of FXS astrocytes, while abrogation of BMP signaling in FXS astrocytes abolishes neurite outgrowth deficits. The goal of this proposal is to test the hypothesis that BMP signaling in astrocytes is upstream of neurodevelopmental FXS deficits in vivo and to identify the astrocyte-secreted proteins that mediate this effect. A combined genetic and viral approach to selective knock out Bmpr2 or Smad4 in astrocytes will be used to assess whether downregulation of astrocyte BMP signaling can rescue in vivo FXS abnormalities in dendritic spines, plasticity, and behavior. An astrocyte-specific in vivo proteomic approach combined with characterization of specific proteins in vitro will identify proteins downstream of BMP signaling responsible for FXS deficits. These experiments will determine if BMP signaling in astrocytes mediates FXS deficits in vivo and elucidate mechanisms by which it occurs, thereby providing new insight into a previously underappreciated aspect of FXS pathophysiology. The proposed research will take place in the Allen Laboratory at the Salk Institute for Biological Studies, a collaborative research environment that provides access to all necessary equipment and training. Through theoretical and practical training in molecular neurobiology, collaboration with supporters of diverse research and clinical backgrounds, and a research team committed to mentorship, the proposed research training plan will enable rigorous instruction in research and lay the foundation for a future career as an independent physician-scientist.