Mark Bear, PhD, Principal Investigator
Miquel Bosch, PhD, Postdoctoral Fellow (2010)
Asha Bhakar, PhD, Postdoctoral Fellow (2009-10)
Dilja Krueger, PhD, Postdoctoral Fellow (2008-10)
Emily Osterweil, PhD, Postdoctoral Fellow (2006-8)
Massachusetts Institute of Technology (MIT)

by Miquel Bosch and Mark F. Bear, 5/2010

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.

by Asha Bhakar, PhD, Postdoctoral Fellow, 5/2009

The majority of Fragile X individuals suffer from mental disabilities, suggesting that cognitive impairment is a central problem in Fragile X Syndrome (FXS). The mechanistic basis for cognitive impairment, however, remains unclear, and currently there is no known cure. Hope for a treatment is plausible though, since patient neurons do not appear damaged or destroyed. In fact, the neuronal circuitry that is largely formed before birth is grossly normal in Fragile X patients, and it is the fine synaptic connections, the structural sites for neurotransmission between neurons, that appear disrupted in FXS. These synaptic connections, and their modification by experience, are thought to underlie the fundamental mechanisms of cognitive functioning. Given that synaptic connections are highly plastic after birth and remain so in Fragile X, we believe that drugs that can correct the disease will work by influencing the components that regulate synaptic connectivity. In this research project, we will begin to test this hypothesis by developing a high-throughput assay that specifically studies the synaptic connectivity defects underlying FXS and is suitable for drug discovery.

One type of high-throughput assay, High-Content Screening (HCS), is capable of reporting on subtle features of cultured neurons including changes in synaptic connectivity. In this research project, we will use HCS to develop an assay sensitive to the effect of the FXS genotype (validation step 1), and then to test the ability of an mGluR5 antagonist in restoring the effects of the FXS genotype to normal in this assay (validation step 2). Once validated, this HCS assay will be used to screen existing Food and Drug Administration (FDA)-approved compounds for effectiveness in Fragile X.

Results from this project would establish the first cell-based morphological and functional tool directly relevant for treating the neuronal deficits underlying cognitive impairment in FXS and would bridge the gap between basic synaptic/cellular biology and preclinical animal model testing. Moreover, since several collections of FDA approved drugs have proven to be rich sources of undiscovered bioactivity and therapeutic potential, drugs discovered to treat mental impairments in this HCS assay may have immediate application to treatment of FXS and may also help treat autism

by Mark Bear, 6/2003

We hypothesize that Fragile X syndrome is a consequence of exaggerated responses to synaptic activation of the group 1 mGluRs that are coupled to local protein synthesis. One consequence of this defect is that some AMPA receptors are pulled away from the surface of the neuron, leaving fewer AMPA receptors at the cell surface to perform their normal function. This hypothesis fits neatly with the studies of Dr. Berry-Kravis and Dr. Lauterborn, because Ampakine drugs work by enhancing the function of the fewer AMPA receptors still left. Another consequence of excessive mGluR function is initiation of epileptiform activity, which may explain why many children with Fragile X have seizures.

The goal of this project is to determine if the malfunction in the mGluR pathway causes the delayed development of synapses, using the Fragile X mouse model. If so, we will investigate whether mGluR antagonists, like MPEP, will correct this delayed development. Thus the overall goal of this study is to further investigate mGluR antagonists as potential treatments for Fragile X.