Invited reviewBDNF in fragile X syndrome

•BDNF and FMRP are both implicated in neuronal plasticity and cognitive function.•FMRP and BDNF may regulate each other.•BDNF/TrkB expression and signaling are altered in the developing and adult brain of Fmr1 knockout mice.•BDNF modifies the phenotype of fragile X syndrome.

Fragile X syndrome (FXS) is a monogenic disorder that is caused by the absence of FMR1 protein (FMRP). FXS serves as an excellent model disorder for studies investigating disturbed molecular mechanisms and synapse function underlying cognitive impairment, autism, and behavioral disturbance. Abnormalities in dendritic spines and synaptic transmission in the brain of FXS individuals and mouse models for FXS indicate perturbations in the development, maintenance, and plasticity of neuronal network connectivity. However, numerous alterations are found during the early development in FXS, including abnormal differentiation of neural progenitors and impaired migration of newly born neurons. Several aspects of FMRP function are modulated by brain-derived neurotrophic factor (BDNF) signaling. Here, we review the evidence of the role for BDNF in the developing and adult FXS brain.This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.