I am investigating the protein/nucleic acid interactions and signal transduction pathways involved in two neurodegenerative disorders, Alzheimer’s disease (AD) and Fragile X syndrome (FXS).  My current research project examines the translational regulation of amyloid precursor protein (APP) by fragile X mental retardation protein (FMRP).  APP is the parent molecule that is cleaved by secretases to produce beta-amyloid, which is the causative agent in senile plaque formation in AD, while FMRP is an RNA binding protein that is lacking in individuals with FXS.  We have discovered that APP levels are increased in a mouse model of Fragile X syndrome (FXS). 

FXS is the most common form of inherited mental retardation with a frequency of 1 in 4,000 males and 1 in 8,000 females.  The disorder results from a mutation in a single gene, fmr-1, located on the X chromosome and the symptoms include moderate to severe mental retardation (overall IQ<70), autistic behavior, macroorchidism and facial abnormalities.  FXS is a trinucleotide repeat disorder resulting in blocked expression of the fragile X mental retardation protein (FMRP).  The normal population has 5-50 CGG trinucleotide repeats in the 5’-UTR of fmr-1 gene (chromosome Xq27.3) while FXS patients have >200 copies.  The CGG repeats are associated with hyper-methylation, chromatin condensation and subsequent transcriptional silencing of the fmr-1 promoter.

FXS occurs because the mutated fmr-1 gene does not produce enough FMRP, which is needed for normal dendritic spine development.  FMRP is a multi-functional mRNA binding protein involved in the transport, localization and translational regulation of mRNA ligands.  The protein has 2 hnRNP K homology domains and 1 RGG box as well as nuclear localization and export signals.  The RGG box of FMRP mediates interactions with G quartet sequences in mRNA ligands.  In neurons, FMRP is located in polysomes and in nontranslating ribonucleoprotein (RNP) particles of dendrites.  The RNPs contain the FMRP homologs FXR1 and 2, the guide RNA BC1 and other proteins and mRNAs involved in synaptic plasticity.  Hundreds of mRNA ligands have been identified with many found at synapses and having the potential to influence synapse formation and synaptic plasticity.  It has been proposed that FMRP is an “immediate early protein” at the synapse orchestrating synaptic development and plasticity.

We are investigating the translational regulation of synaptic density proteins by FMRP in a mouse model of FXS.  In our PLoS paper (2007), we demonstrate that FMRP binds to the coding region of APP mRNA at a guanine-rich region adjacent to a G-quartet–like sequence.  Stimulation of cortical synaptoneurosomes or primary neuronal cells with the group 1 metabotropic glutamate receptor (mGluR) agonist DHPG increases APP translation in wild type but not fmr-1 knockout samples.  APP mRNA co-immunoprecipitates with FMRP in resting synaptoneurosomes, but the interaction is lost shortly after DHPG treatment.  Soluble beta-amyloid1-40 and beta-amyloid1-42 levels are significantly higher in two strains of fmr-1 knockout mice compared to wild type controls.  These findings incriminate FMRP in translational repression of APP mRNA at the synapse and suggest that group 1 mGluR activation over-rides tonic inhibition. 

APP mRNA is highly expressed in neurons and dendrites with over-expression implicated in Alzheimer’s disease (AD).  AD is a progressive form of dementia characterized histologically by beta-amyloid plaques, neurofibrillary tangles and neuronal cell death.  Patients suffer memory loss, impaired judgment, cognitive dysfunction, the inability to perform everyday tasks, and behavioral problems.  beta-amyloid is processed from the APP parent molecule.  The normal physiological function of APP is not well defined, but it is expressed at synapses where it promotes synapse differentiation during development.  APP exhibits increased expression during neuronal differentiation with maximal levels at synaptic connection completion.  Of note, the role of synaptic dysfunction has received increasing attention as a primary upstream lesion in early AD.  Thus, APP facilitates synapse formation in the developing brain, while beta-amyloid accumulation results in synaptic loss and impaired neurotransmission.  Our data indicates that postsynaptic FMRP binds to and regulates the translation of APP mRNA through group 1 mGluR activation and suggests a possible link between two neurological disorders, Alzheimer’s disease (AD) and FXS.  Roles for APP in FXS, and FMRP in AD, have not been previously investigated.