Distinct isoforms of the PI3K catalytic subunit have specialized functions in the brain, but their role in cognition is unknown. Here, we show that the catalytic subunit p110β plays an important role in prefrontal cortex (PFC)-dependent cognitive defects in mouse models of Fragile X syndrome (FXS), an inherited intellectual disability. FXS is caused by loss of function of the fragile X mental retardation protein (FMRP), which binds and translationally represses mRNAs. PFC-selective knockdown of p110β, an FMRP target that is translationally upregulated in FXS, reverses deficits in higher cognition in Fmr1 knockout mice. Genetic full-body reduction of p110β in Fmr1 knockout mice normalizes excessive PI3K activity, restores stimulus-induced protein synthesis, and corrects increased dendritic spine density and behavior. Notably, adult-onset PFC-selective Fmr1 knockdown mice show impaired cognition, which is rescued by simultaneous p110β knockdown. Our results suggest that FMRP-mediated control of p110β is crucial for neuronal protein synthesis and cognition.
Fragile X syndrome (FXS), the most common monogenic cause of inherited intellectual disability and autism, is caused by the silencing of the FMR1 gene, leading to the loss of fragile X mental retardation protein (FMRP), a synaptically expressed RNA-binding protein regulating translation. The Fmr1 knockout model recapitulates the main traits of the disease. Uncontrolled activity of metabotropic glutamate receptor 5 (mGluR5) and mammalian target of rapamycin (mTOR) signaling seem crucial in the pathology of this disease. The endocannabinoid system (ECS) is a key modulator of synaptic plasticity, cognitive performance, anxiety, nociception and seizure susceptibility, all of which are affected in FXS. The cannabinoid receptors CB1 (CB1R) and CB2 (CB2R) are activated by phospholipid-derived endocannabinoids, and CB1R-driven long-term regulation of synaptic strength, as a consequence of mGluR5 activation, is altered in several brain areas of Fmr1 knockout mice. We found that CB1R blockade in male Fmr1 knockout (Fmr1(-/y)) mice through pharmacological and genetic approaches normalized cognitive impairment, nociceptive desensitization, susceptibility to audiogenic seizures, overactivated mTOR signaling and altered spine morphology, whereas pharmacological blockade of CB2R normalized anxiolytic-like behavior. Some of these traits were also reversed by pharmacological inhibition of mTOR or mGluR5. Thus, blockade of ECS is a potential therapeutic approach to normalize specific alterations in FXS.
The role of the fragile X mental retardation protein (FMRP) is well established in brain, where its absence leads to the fragile X syndrome (FXS). FMRP is almost ubiquitously expressed, suggesting that, in addition to its effects in brain, it may have fundamental roles in other organs. There is evidence that FMRP expression can be linked to cancer. FMR1 mRNA, encoding FMRP, is overexpressed in hepatocellular carcinoma cells. A decreased risk of cancer has been reported in patients with FXS while a patient-case with FXS showed an unusual decrease of tumour brain invasiveness. However, a role for FMRP in regulating cancer biology, if any, remains unknown. We show here that FMRP and FMR1 mRNA levels correlate with prognostic indicators of aggressive breast cancer, lung metastases probability and triple negative breast cancer (TNBC). We establish that FMRP overexpression in murine breast primary tumours enhances lung metastasis while its reduction has the opposite effect regulating cell spreading and invasion. FMRP binds mRNAs involved in epithelial mesenchymal transition (EMT) and invasion including E-cadherin and Vimentin mRNAs, hallmarks of EMT and cancer progression.
Fragile X syndrome, a common cause of intellectual disability and autism, is thought to occur due to abnormal regulation of neuronal protein synthesis. A study by Osterweil et al. (2013), in this issue, demonstrates that the HMG-CoA reductase inhibitor lovastatin can normalize protein synthesis and also reduce audiogenic seizures in Fmr1 knockout mice.
Fragile X-associated tremor/ataxia syndrome (FXTAS) results from a “premutation” size 55-200 CGG repeat expansion in the fragile X mental retardation 1 (FMR1) gene. Core motor features include cerebellar gait ataxia and kinetic tremor, resulting in progressive mobility disability. There are no published studies characterizing balance deficits in FMR1 premutation carriers with and without FXTAS using a battery of quantitative measures to test the sensory integration underlying postural control, automatic postural reflexes, and dynamic postural stability limits. Computerized dynamic posturography (CDP) and two performance-based balance measures were administered in 44 premutation carriers, 21 with FXTAS and 23 without FXTAS, and 42 healthy controls to compare balance and functional mobility between these groups. Relationships between FMR1 molecular variables, age, and sex and CDP scores were explored. FXTAS subjects demonstrated significantly lower scores on the sensory organization test (with greatest reductions in the vestibular control of balance), longer response latencies to balance perturbations, and reduced stability limits compared to controls. Premutation carriers without FXTAS also demonstrated significantly delayed response latencies and disrupted sensory weighting for balance control. Advancing age, male sex, increased CGG repeat size, and reduced X activation of the normal allele in premutation carrier women predicted balance dysfunction. These postural control deficits in carriers with and without FXTAS implicate dysfunctional cerebellar neural networks and may provide valuable outcome markers for tailored rehabilitative interventions. Our findings suggest that CDP may provide sensitive measures for early detection of postural control impairments in at-risk carriers and better characterize balance dysfunction and progression in FXTAS.
Fragile X syndrome (FXS), caused by the loss of functional FMRP, is a leading cause of autism. Neurons lacking FMRP show aberrant mRNA translation and intracellular signalling. Here, we identify that, in Fmr1 knockout neurons, type 1 adenylyl cyclase (Adcy1) mRNA translation is enhanced, leading to excessive production of ADCY1 protein and insensitivity to neuronal stimulation. Genetic reduction of Adcy1 normalizes the aberrant ERK1/2- and PI3K-mediated signalling, attenuates excessive protein synthesis and corrects dendritic spine abnormality in Fmr1 knockout mice. Genetic reduction of Adcy1 also ameliorates autism-related symptoms including repetitive behaviour, defective social interaction and audiogenic seizures. Moreover, peripheral administration of NB001, an experimental compound that preferentially suppresses ADCY1 activity over other ADCY subtypes, attenuates the behavioural abnormalities in Fmr1 knockout mice. These results demonstrate a connection between the elevated Adcy1 translation and abnormal ERK1/2 signalling and behavioural symptoms in FXS.
Fragile X syndrome (FXS) is the leading monogenic cause of autism spectrum disorders (ASD). Trinucleotide repeat expansions in FMR1 abolish FMRP expression, leading to hyperactivation of ERK and mTOR signaling upstream of mRNA translation. Here we show that metformin, the most widely used drug for type 2 diabetes, rescues core phenotypes in Fmr1(-/y) mice and selectively normalizes ERK signaling, eIF4E phosphorylation and the expression of MMP-9. Thus, metformin is a potential FXS therapeutic.
A leading theory holds that neurodevelopmental brain disorders arise from imbalances in excitatory and inhibitory (E/I) brain circuitry. However, it is unclear whether this one-dimensional model is rich enough to capture the multiple neural circuit alterations underlying brain disorders. Here we combined computational simulations with analysis of in vivo 2-photon Ca(2+) imaging data from somatosensory cortex of Fmr1 knock-out (KO) mice, a model of Fragile-X Syndrome, to test the E/I imbalance theory. We found that: 1) The E/I imbalance model cannot account for joint alterations in the observed neural firing rates and correlations; 2) Neural circuit function is vastly more sensitive to changes in some cellular components over others; 3) The direction of circuit alterations in Fmr1 KO mice changes across development. These findings suggest that the basic E/I imbalance model should be updated to higher-dimensional models that can better capture the multidimensional computational functions of neural circuits.
Fragile X mental retardation protein (FMRP) and its autosomal paralog FXR2P are selective neuronal RNA-binding proteins, and mice that lack either protein exhibit cognitive deficits. Although double-mutant mice display more severe learning deficits than single mutants, the molecular mechanism behind this remains unknown. In the present study, we discovered that FXR2P (also known as FXR2) is important for neuronal dendritic development. FMRP and FXR2P additively promote the maturation of new neurons by regulating a common target, the AMPA receptor GluA1, but they do so via distinct mechanisms: FXR2P binds and stabilizes GluA1 mRNA and enhances subsequent protein expression, whereas FMRP promotes GluA1 membrane delivery. Our findings unveil important roles for FXR2P and GluA1 in neuronal development, uncover a regulatory mechanism of GluA1, and reveal a functional convergence between fragile X proteins in neuronal development.
Recent advances in techniques for manipulating genomes have allowed the generation of transgenic animals other than mice. These new models enable cross-mammalian comparison of neurological disease from core cellular pathophysiology to circuit and behavioural endophenotypes. Moreover they will enable us to directly test whether common cellular dysfunction or behavioural outcomes of a genetic mutation are more conserved across species. Using a new rat model of Fragile X Syndrome (FXS), we report that Fmr1 knockout (KO) rats exhibit elevated basal protein synthesis and an increase in mGluR-dependent long-term depression (LTD) in CA1 of the hippocampus that is independent of new protein synthesis. These defects in plasticity are accompanied by an increase in dendritic spine density selectively in apical dendrites and subtle changes in dendritic spine morphology of CA1 pyramidal neurons. Behaviourally, Fmr1 KO rats show deficits in hippocampal-dependent, but not hippocampal-independent, forms of associative recognition memory indicating that the loss of FMRP causes defects in episodic-like memory. In contrast to previous reports from mice, Fmr1 KO rats show no deficits in spatial reference memory reversal learning. One-trial spatial learning in a delayed matching to place water maze task was also not affected by the loss of FMRP in rats. This is the first evidence for conservation across mammalian species of cellular and physiological hippocampal phenotypes associated with the loss of FMRP. Furthermore, while key cellular phenotypes are conserved they manifest in distinct behavioral dysfunction. Finally, our data reveal novel information about the selective role of FMRP in hippocampus-dependent associative memory.