HITS-CLIP in various brain areas reveals new targets and new modalities of RNA binding by fragile X mental retardation protein

Abstract

Fragile X syndrome (FXS), the most common form of inherited intellectual disability, is due to the functional deficiency of the fragile X mental retardation protein (FMRP), an RNA-binding protein involved in translational regulation of many messenger RNAs, playing key roles in synaptic morphology and plasticity. To date, no effective treatment for FXS is available. We searched for FMRP targets by HITS-CLIP during early development of multiple mouse brain regions (hippocampus, cortex and cerebellum) at a time of brain development when FMRP is most highly expressed and synaptogenesis reaches a peak. We identified the largest dataset of mRNA targets of FMRP available in brain and we defined their cellular origin. We confirmed the G-quadruplex containing structure as an enriched motif in FMRP RNA targets. In addition to four less represented motifs, our study points out that, in the brain, CTGKA is the prominent motif bound by FMRP, which recognizes it when not engaged in Watson–Crick pairing. All of these motifs negatively modulated the expression level of a reporter protein. While the repertoire of FMRP RNA targets in cerebellum is quite divergent, the ones of cortex and hippocampus are vastly overlapping. In these two brain regions, the Phosphodiesterase 2a (Pde2a) mRNA is a prominent target of FMRP, which modulates its translation and intracellular transport. This enzyme regulates the homeostasis of cAMP and cGMP and represents a novel and attractive therapeutic target to treat FXS.

Carbamazepine restores neuronal signaling, protein synthesis and cognitive function in a mouse model of fragile X syndrome

“…5b). This is consistent with that PDE2A may be affected only in juvenile but not adult FXS mice 16,31 . In contrast to the effects of carbamazepine, BAY607550, which has inhibitory activity against PDE2 32 , did not show significant effect on protein translation (Fig.…”

“…Further, whether alteration of ADCY and PDE in specific brain region, sub-cellular domain and developmental stage should also be considered. FMRP binds Adcy1 and Pde2a mRNA and the protein levels of ADCY1 and PDE2A are elevated in Fmr1 KO mice 8-10, 16 . One complication is that, while elevated ADCY1 is detected in adult FXS mouse samples (Fig.…”

“…ADCY and PDE have counteracting enzymatic action on cAMP metabolism and may have different effects on cAMP-regulated signaling molecule such as ERK1/2 11, 30 . With regard to the consideration of targeting ADCY1-ERK1/2 for potential therapeutic strategy, one complication is that PDE2A was also identified as an FMRP target 16 . We found that PDE2A is also primarily expressed in neurons but not glial cells (Fig.…”

“…Notably, it is reported that FMRP also regulates the translation of certain PDE subtype (e.g. PDE2A) and the PDE2A protein level is elevated in Fmr1 KO mice 16 . Thus, it is intriguing and seemingly possible that FMRP deficiency could concurrently affect signaling molecules with counteracting enzymatic activity.…”

Identification of FMRP target genes expressed in corticogenesis: implication for common phenotypes among neurodevelopmental disorders

“…We also observed strong expression of FMRP in the cortical plate consisting with immature neurons ( Figure 1A-C), which has not been reported in the above-mentioned previous studies (Saffary and Xie, 2011;La Fata et al, 2014;Pilaz et al, 2016). This may be attributed to difference in antibodies used, although it is not surprising that FMRP is indeed expressed in neurons of adult mice (Darnell et al, 2011;Maurin et al, 2018).…”