Kendal Broadie

Kendal Broadie, Ph.D. Principal Investigator
Charles Tessier, Ph.D. Postdoctoral Fellow
Vanderbilt University

Over the past 4 years, we have developed an exciting new genetic model of Fragile X Syndrome in that best-characterized genetic workhorse system; the fruitfly Drosophila melanogaster. We previously generated mutant animals lacking or over-expressing the Drosophila Fragile X protein, dFMRP, and demonstrated that mutant animals manifest the characteristic hallmarks of the disease including both neuronal and behavioral defects. An exciting hypothesis developing over the last few years suggests that FMRP may be regulated downstream of signaling via metabotropic glutamate receptors (mGluRs). The mGluR hypothesis suggests that FMRP functions to limit neuronal activity in response to these receptors.

Charles Tessier

We are testing this hypothesis by using animals that lack both dmGluRA and dFMRP. We are employing physiology and molecular tools to understand the roles of each protein in FXS.We are also using drugs that specifically block mGluR signaling to identify the convergence of these two important biological pathways. This research may lead to developing new drugs that specifically target molecules involved in the disease and avoid those which may lead to unwanted side-effects.

Kendal Broadie, Ph.D. Principal Investigator
Yong Zhang, Ph.D. Postdoctoral Fellow
University of Utah

By Yong Zhang and Kendal Broadie, 3/2001

One of the most compelling challenges in fragile X research is to understand how lack of the affected protein (FMRP) gives rise to mental impairment and associated behavioral abnormalities. A potentially fruitful approach is to assay FRAXA gene (FMR1) function within a simple, well-characterized genetic model organism such as the fruitfly, Drosophila melanogaster. Drosophila has a long and distinguished history as a genetic system to assay underlying causes of inherited human genetic diseases. In the last few years, Drosophila has contributed enormously to our understanding of a number of common neurodegenerative diseases including Alzheimer's and Parkinson's disease. We anticipate a similar revolution in our understanding of fragile X through developing a Drosophila model.

Last year, we identified and characterized a Drosophila FMR1 gene homologue (i.e. highly similar gene) which is now named dFMR1. Like its human counterpart, the dFMR1 protein product is highly expressed in most, if not all, nerve cells of the central nervous system, from embryogenesis to adulthood. Like human patients, when dFMR1 is completely removed from the fly genome (i.e. null mutants), the mutant fly is fully viable and morphologically normal, but exhibits uncoordinated movement behaviors. Microscopic assays of these mutants show that neuronal synapses (where a neuron communicates with another neuron or with a muscle cell) develop abnormally, resulting in clear structural defects. When dFMR1 is over-expressed in transgenic flies, making excess protein, an opposite structural change is observed. These results show that the level of dFMR1 protein directly dictates the level of synaptic structural development. Similarly, electrophysiological assays of synaptic function in both mutants and transgenic flies show that neurotransmission is abnormal, in agreement with the severity of the structural defects. These phenotypes, together with complementary human and mouse studies, strongly suggest that fragile X Syndrome may result from synaptic defects.

This year, we will focus on looking for dFMR1 interacting partners by employing powerful genetic interaction screens available only in Drosophila. We will mutate the entire fruit fly genome while screening for genes which can ameliorate fragile X symptoms in flies. Identifying and characterizing genes which interact with dFMR1 will help us understand the mechanism by which the fragile X gene and its protein product perform their normal function - and what goes wrong in the absence of the protein. We intend to use this information to develop treatments for fragile X.

This grant was approved by FRAXA's Directors for a second year of funding, but, happily, it will be funded at a higher level by the NIH/FRAXA joint funding initiative. A small bridge grant was awarded until the NIH funding kicked in.