With a $160,000 grant from FRAXA Research Foundation from 1999-2000, Drs. Menahem Segal at the Weizmann Institute and Katarina Braun at the Leibnitz Institute for Learning analyzed the fragile X protein’s influence on spine synapses in neurons. Results published.
by Menahem Segal, 1/1/1999
A stunning observation made decades ago is that major deficits in the ability to acquire and store information, as is the case of mental retardation, is associated with minimal, if any, change in the structure of the brain. While scientists are still puzzled by the lack of apparent difference between the brain of a genius and that of a mentally retarded person, exciting information which can explain how an organism becomes mentally retarded begins to emerge. It focuses on the dendritic spine, that part of a nerve cell which is the locus of synaptic interaction, neuronal plasticity and long term memory. The recent advent in molecular cloning helped in isolating the protein FMRP, which is absent in fragile X mentally retarded children, and morphological studies have linked this protein to the dendritic spine. Also, the morphology (shape) of synaptic structures is abnormal in fragile X syndrome. Having access to new and advanced technologies, we have now reached a turning point where, for the first time, it is possible to address these issues in our established models for synaptic plasticity.
The objective of the current proposal is to better understand the role of FMRP in synaptic structure and function in a controlled, in vitro test system involving the tissue-cultured neuron. We propose the hypothesis that mental retardation, typical of patients with the fragile X syndrome, may be manifested by abnormal and malfunctioning synaptic connections. We also propose that expression of FMRP at the synapse could be involved in the synapse maturation process. A continuing role of FMRP expression in adult synaptic plasticity may indicate that synaptic FMRP synthesis is a critical factor in synapse stabilization and elimination, both in developmental brain organization and in adult learning and memory. We will analyze FMRP-mediated functional maturation of spine synapses in two brain regions, the hippocampus and the anterior cingulate cortex. Both regions are known to mature during late postnatal stages and therefore are vulnerable to disruption of normal synaptic selection processes. Both brain regions are part of the limbic system, which mediates emotional responses as well as learning and memory formation, functions that are severely impaired in the fragile X syndrome.
We will study the expression of FMRP in developing cultured hippocampal and cortical neurons and follow the responses of these neurons to experimentally-induced changes in synaptic activity. We will modify the expression of FMRP and study the changes in synaptic properties and integration of the affected neurons. The identification and characterization of the role of FMRP in normal and pathological synaptic plasticity during brain maturation is a prerequisite for future design of clinical preventive or therapeutic strategies for the treatment of mental retardation associated with the fragile X syndrome.
January 2000, by Katarina Braun
We have used tissue-cultured neurons to examine differences in morphology (shape) and synaptic connectivity between wild-type and knockout mice. We have already found that hippocampal neurons taken from knockout mice and grown in culture for three weeks have shorter dendrites and fewer dendritic spines compared to controls. Also, knockout cells tend to develop fewer active synaptic connections, which produce smaller excitatory synaptic currents than controls. These preliminary observations may have important functional implications for the ability of the cells taken from the knockout mice to express long term plasticity, which may underlie the mental retardation seen in patients who lack FMRP.
Results and other research by Dr. Segal can be found here.