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Deciphering Fragile X Syndrome: FMRP Regulation at Synapses

Explore the intricate relationship between FMRP, BC1 RNA, and mRNA translation in Fragile X Syndrome. Discover how dysregulation affects synaptic plasticity and dendritic spine morphology in FRAXA patients and FMR1 KO mice.

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Deciphering Fragile X Syndrome: FMRP Regulation at Synapses

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  1. The Fragile X Protein FMRP Associates with BC1 RNA and Regulates the Translation of Specific mRNAs at SynapsesFrancesca Zalfa et al. (2003) Cell Jeremy Logue

  2. Dendritic Spines Nimchinsky, E. et al (2002) Annual Review of Physiology

  3. Spine Shape and Plasticity Nimchinsky, E. et al (2002) Annual Review of Physiology

  4. Acitivity Dependent Metamorphosis of Spines Heike Hering and Morgan Sheng (2001) Nature Reviews Neuroscience

  5. Fragile X Syndrome • Fragile X accounts for about one-half of cases of X-linked • mental retardation and is the most second most common • cause of mental impairment after trisomy 21. • Caused by a (CGG)n expansion in the FMR1 gene promoter. • Normal individuals harbor 45 +/- 25 repeats, more than 200 • produces fragile X. Repeats induce hypermethylation of the • promoter, silencing the gene. • FMR1 encodes fragile X mental retardation protein (FMRP), • an RNA binding protein expressed in the brain. • FRAXA patients and FMR1 KO mice exhibit abnormal • dendritic spines.

  6. Fragile X Spine Morphology • Fragile X patient spines • Normal Irwin, S. et al (2000) Cerebral Cortex

  7. Macromolecular Structure of the Synapse Heike Hering and Morgan Sheng (2001) Nature Reviews Neuroscience

  8. FMRP Shuttling Claudia Bagni and William T. Greenough (2005) Nature Reviews Neuroscience

  9. FMRP and Synapse Pruning Claudia Bagni and William T. Greenough (2005) Nature Reviews Neuroscience

  10. FMRP Granules Antar, L et al (2004) J. Neurosci

  11. mGluR Activation Regulates FMRP Localization Antar, L et al (2004) J. Neurosci

  12. FMRP at Synapses Claudia Bagni and William T. Greenough (2005) Nature Reviews Neuroscience

  13. BC1/BC200 RNA • BC1 is a 200-nucleotide-long RNA pol III product. • BC1 RNA is a non-translatable psuedogene. • BC1 is highly expressed in the brain. • Composed of three domains; • 5’ portion homologous to the Alu Lm • a central adenosine rich region • (3) terminal 43-nt non-repetitive domain • Believed that BC1 was retropositionally generated and • exapted into a function regulating synaptic plasticity.

  14. Polysome/mRNP Distrubution of mRNAs Sucrose gradient centrifugation of extracts and RT-PCR for mRNAs. FMR1 KO exhibits shift to polysome fraction (Note: this includes BC1).

  15. Polysome/mRNP Distribution of mRNAs 22% 13% 17% 38% Polysome/mRNP analysis

  16. Protein Levels for FMR1 KO and WT

  17. Polysome/mRNP Distribution of FMRP Protein Ribo protein (control) FMRP co-sediments with monomeric 80S ribosomes and with mRNPs in total brain and in synaptoneurosomes.

  18. mRNAs Associated with FMRP Complex IP salt dependent Human qRT-PCR Input KO KO KO KO wt wt wt wt

  19. FMRP Binds Directly to BC1 RNA EMSA using in vitro transcribed BC1 RNA. 750 mM NaCl used. Concentrations? 25% of RNA bound. Authors argue that other factors are required for high-affinity binding.

  20. Homology Between BC1 RNA and Regulated mRNAs

  21. BC1 RNA Interacts with FMRP-Targeted mRNAs 21mer DNA oligonucleotide toward region of complementarity to MAP1B. BC1 RNA hybiridization to mRNAs is required for FMRP complex formation.

  22. BC1 RNA Interacts with FMRP-Targeted mRNAs Total brain RNA mixed with biotin labeled BC1 RNA. Input Pull Down (-) BC1 Input Pull Down (-) BC1 Input Pull Down (-) BC1 Input Pull Down (-) BC1 Input Pull Down (-) BC1 Input Pull Down (-) BC1 BC1 RNA and FMRP targeted mRNAs interact in the absence of protein.

  23. Conclusions • Translation of key mRNAs is upregulated in • FMR1 KO mice. • Translational repression is stronger at synapses. • BC1 RNA determines specificity of mRNAs to be • regulated by FMRP. • FMRP and BC1 RNA associate directly. • The association between BC1 RNA and FMRP-regulated mRNAs occurs in • the absence of proteins. • FMRP’s role in translational repression at synapses likely underlies • abnormalities in dedritic spine morphology in FRAXA patients and in FMR1 • KO mice. • Mechanism of translation repression remains unkown.

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