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Targeting the endocannabinoid system in the treatment of fragile X syndrome

Abstract

Fragile X syndrome (FXS), the most common monogenic cause of inherited intellectual disability and autism1, is caused by the silencing of the FMR1 gene, leading to the loss of fragile X mental retardation protein (FMRP)2, a synaptically expressed RNA-binding protein regulating translation3. The Fmr1 knockout model recapitulates the main traits of the disease4. Uncontrolled activity of metabotropic glutamate receptor 5 (mGluR5)5,6 and mammalian target of rapamycin (mTOR) signaling7,8,9 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 susceptibility10, 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 activation11, is altered in several brain areas of Fmr1 knockout mice12,13,14. 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.

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Figure 1: Pharmacological modulation of object-recognition memory impairment in Fmr1−/y mice.
Figure 2: Cellular and molecular effects of pharmacological treatment in adult Fmr1−/y mice.
Figure 3: Prevention of other FXS-related phenotypes by pharmacological treatment.
Figure 4: Genetic attenuation of CB1R corrects behavioral deficits and mTOR overactivation in Fmr1−/y mice.

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Acknowledgements

We thank C. Fernández-Avilés, D. Real, M. Linares and H. Gómez for expert technical assistance and O.J. Manzoni for helpful comments. Fmr1 knockout mice in the C57BL/6J background were kindly provided by D. Nelson at Baylor College of Medicine. A.B.-G. is the recipient of a predoctoral fellowship (Ministerio de Educación y Cultura). S.M. is the recipient of a Ramón y Cajal contract (Ministerio de Educación y Cultura). This study was supported by grants from Fundació La Marató de TV3 (#090910 to A.O.), Grants from the Ministerio de Ciencia e Innovación (#SAF2009-07309 to A.O., #SAF2011-29864 to R.M. and SAF2010-21547 to C.M.), CureFXS E-Rare EU/FIS PS09102673 to M.D., Instituto de Salud Carlos III (RD06/0001/0001 to R.M.), PLAN E (Plan Español para el Estímulo de la Economía y el Empleo), Generalitat de Catalunya (SGR-2009-00731 to R.M. and SGR-2009-00718 to R.d.l.T.) and ICREA (Institució Catalana de Recerca i Estudis Avançats) Academia to R.M.

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Contributions

A.B.-G. participated in experimental design, conducted biochemical and behavioral experiments, and wrote the manuscript. M.G.-G. conducted biochemical and behavioral experiments. T.G. performed dendritic spine morphology analysis. C.A.-P. performed stereotaxic surgeries, and M.D. provided Fmr1 knockout and WT mice, and discussed the experiments. A.P. and R.d.l.T. measured endocannabinoid levels. S.M. designed and conducted electrophysiological experiments, and wrote the manuscript. A.P.-S. and S.M. were in charge of electrophysiological equipment. C.M. funded the project. R.M. funded the project and wrote the manuscript. A.O. conceptualized, participated in experimental design, supervised, funded the project and wrote the manuscript.

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Correspondence to Andrés Ozaita.

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The authors declare no competing financial interests.

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Busquets-Garcia, A., Gomis-González, M., Guegan, T. et al. Targeting the endocannabinoid system in the treatment of fragile X syndrome. Nat Med 19, 603–607 (2013). https://doi.org/10.1038/nm.3127

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