The first strand cDNA was used as the template for real-time quantitative PCR analysis. KO ACM-treated neurons. gene. Recent studies suggest a role of astrocytes in neuronal development. However, the mechanisms involved in the regulation process of astrocytes from FXS remain unclear. In this study, we found that astrocytes derived from a Fragile X model, the knockout (KO) mouse which lacks FMRP expression, inhibited the proper elaboration of dendritic processes of neurons KO mice. However, the levels of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), glial cell-derived neurotrophic factor (GDNF), and ciliary neurotrophic factor (CNTF) were normal. FMRP has multiple RNACbinding motifs and is involved in translational regulation. RNACbinding protein immunoprecipitation (RIP) showed the mRNA interacted with FMRP in WT astrocytes. Addition of high concentrations of exogenous NT-3 to culture medium reduced the dendrites of neurons and synaptic protein levels, whereas these measures were ameliorated by neutralizing antibody to NT-3 or knockdown of NT-3 expression in KO astrocytes through short hairpin RNAs (shRNAs). Prefrontal cortex microinjection of WT astrocytes or NT-3 shRNA infected KO astrocytes rescued the deficit of trace fear memory in KO mice, concomitantly decreased the NT-3 levels in the prefrontal cortex. This study indicates that excessive NT-3 from astrocytes contributes to the abnormal neuronal dendritic development and that astrocytes could be a potential therapeutic target for FXS. Author Summary Fragile X syndrome is a form of inherited mental retardation in humans that results from expansion of a CGG repeat in the gene. Recent studies suggest that astrocytes play a role in neuronal growth. In this study, we find that astrocytes derived from a Fragile X model, the knockout (KO) mouse, inhibit the proper elaboration of dendritic processes of neurons KO mice. Blockage of NT-3 by neutralizing antibodies and knockdown of NT-3 by using short IL10A hairpin RNAs (shRNAs) in KO astrocytes can rescue the neuronal dendritic development. experiments show that prefrontal cortex microinjection of WT astrocytes or NT-3 shRNACinfected KO astrocytes rescues the deficit of trace fear memory in KO mice. This study provides the evidence that a lack of FMRP leads to an overexpression of NT-3, which reduces dendritic growth in neurons. Introduction Fragile X syndrome is a form of inherited mental retardation in humans that results from expansion of a CGG repeat in the gene on the X chromosome , . This syndrome is characterized by low intelligence quotient, attention deficits, and anxiety C. As an mRNA binding protein, FMRP is associated with polyribosomes and involved in the translational efficiency and/or trafficking of certain mRNAs . FMRP is widely expressed in the brain , , and its absence is expected to disrupt the synthesis and/or the subcellular localization of several proteins, which is important in long-term synaptic plasticity , . knockout (KO) mice serve as a model to study fragile X mental retardation , . Our previous study has identified FMRP as a key messenger for dopamine modulation in the forebrain and provided insights on the cellular and molecular mechanisms underlying FXS . However, the cellular pathophysiology of FXS is still under discussion. Emerging evidence suggests that glia may LTX-401 also be involved in the development of FXS. For example, astrocytes, the major glia of the central nervous system (CNS), have been shown to regulate the stability, dynamics, and maturation of dendritic spines ,  and take part in the regulation of synaptic plasticity and synaptic transmission , . FMRP is expressed in the astrocyte lineage during development , and a lack of FMRP in astrocytes affects LTX-401 the dendritic morphology of neurons . Evidence is steadily implicating astrocytes in synaptic maturation and elimination, suggesting that FMRP may be essential to the role of astrocytes in synaptogenesis during development . LTX-401 However, the underlying mechanisms of astrocytes in regulating neuronal dendritic development in FXS are still unclear. In this study, we found that a lack of FMRP leads to an overexpression of neurotrophin-3 (NT-3) and LTX-401 this in turn reduces dendritic growth in neurons. Therefore, excessive NT3 from astrocytes contributed to the dendritic LTX-401 developmental disorder of KO mice. The present study.