|Role of the excessive protein synthesis in the ontogeny of Fragile X Syndrome -- A longitudinal analysis of the structural alterations of dendritic spines
By Miquel Bosch and Mark Bear, 2/26/2011
Brains of patients of Fragile X Syndrome (FXS) and mouse models of the disease show morphological abnormalities in their microarchitecture. Neurons contain an excessive number of dendritic spines with an aberrantly elongated shape. Because dendritic spines receive excitatory synaptic connections, these results suggest altered connectivity in FXS. How does this come about and can it be prevented or reversed?
Another feature of the FXS brain is an increased rate of neuronal protein synthesis that is downstream of signaling via metabotropic glutamate receptor 5 (mGluR5). We hypothesize that increased protein synthesis is pathogenic in FXS, and is responsible for alterations in connectivity and function. If this idea is correct, strategies to reduce protein synthesis could be viable therapeutic approaches.
Our project tests the hypothesis that the excessive rate of protein synthesis is not a consequence, but a primary cause of the structural alterations occurring in FXS. Treatments aimed at blocking mGluR5 signaling might therefore be effective because they eventually reduce the levels of mRNA translation. We propose that the exaggerated protein synthesis might directly alter the plastic properties of synapses and create the superabundance of dendritic spines. This will lead to the functional hyperconnectivity of neuronal networks that eventually generate the cognitive deficits.
In order to test this hypothesis we propose to boost the synthesis of proteins by overexpressing eIF4E, a core member of the translation machinery, and longitudinally analyze whether the development of spines in those neurons reproduce the same phenotype observed in FXS. We will directly visualize the evolution of neuronal morphology over days in an organotypic slice culture using time-lapse two-photon imaging. We will selectively manipulate the genetic backgrounds of these neurons at different time points, either by overexpressing eIF4E or by knocking FMRP down using siRNA. This experimental approach will allow us to neatly observe when (which stage of development), where (which type of spines) and how (by which evolution) dendritic spines exhibit their structural alterations. It will also allow us to test whether a chronic or an acute pharmacological treatment can prevent or rescue the evolution of this abnormal phenotype.
This project will be done in collaboration with Joel Richter, from the Program in Molecular Medicine at the University of Massachusetts Medical School. It represents a new perspective of the disease that can guide the discovery of novel therapeutics, directly designed to act downstream of mGluR5, at the core of the translation machinery.