Loss of Fragile X mental retardation protein (FMRP) leads to Fragile X syndrome (FXS), the most common form of inherited intellectual disability and autism. Although the functions of FMRP and its homologues FXR1P and FXR2P are well studied in the somatodendritic domain, recent evidence suggests that this family of RNA binding proteins also plays a role in the axonal and presynaptic compartments. Fragile X granules (FXGs) are morphologically- and genetically-defined structures containing Fragile X proteins that are expressed axonally and presynaptically in a subset of circuits. To further understand the role of presynaptic Fragile X proteins in the brain we have systematically mapped the FXG distribution in the mouse central nervous system. This analysis revealed both the circuits and the neuronal types that express FXGs. FXGs are enriched in circuits that mediate sensory processing and motor planning - functions that are particularly perturbed in FXS patients. Analysis of FXG expression in the hippocampus suggests that CA3 pyramidal neurons utilize presynaptic Fragile X proteins to modulate recurrent but not feedforward processing. Neuron-specific FMRP mutants revealed a requirement for neuronal FMRP in the regulation of FXGs. Finally, conditional FMRP ablation demonstrated that FXGs are expressed in axons of thalamic relay nuclei that innervate cortex, but not in axons of thalamic reticular nuclei, striatal nuclei, or cortical neurons that innervate thalamus. Together, these findings support the proposal that dysregulation of axonal and presynaptic Fragile X proteins contribute to the neurological symptoms of FXS.
Local axonal protein synthesis plays a crucial role in the formation and function of neuronal circuits. Understanding the role of this mechanism in specific circuits requires identifying the protein composition and mRNA cargos of the ribonucleoprotein particles (RNPs) that form the substrate for axonal translation. FXGs (Fragile X granules) are axonal RNPs present in a stereotyped subset of mature axons in the intact brain that contain one or more of the Fragile X related (FXR) proteins
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