Kendal Broadie

Kendal Broadie, Ph.D.
Principal Investigator
Cheryl Gatto, Ph.D.
FRAXA Fellow (2008-9)
Vanderbilt University

FRAXA Awards:
  $40,000 in 2009
  $40,000 in 2008
  $40,000 in 2007
  $40,000 in 2006
  $35,000 in 2000

Cheryl Gatto

To develop any treatment for Fragile X Syndrome (FraX), it is critical to understand the developmental progression of the disease. It remains poorly understood whether FraX is primarily a ‘developmental disease’, reflecting a transient, age-dependent requirement for the Fragile X Mental Retardation Protein (FMRP) in brain development, a ‘plasticity disease’, reflecting a maintained, constant requirement for FMRP in the mature brain, or some two-phase combination of FMRP requirements giving rise to different FraX symptoms. In other words, do we need FMRP early in life, as a fetus, young child or budding adolescent, to allow normal brain development and to establish a platform for mature brain functions, or is FMRP more important later, and throughout life, to allow brain circuitry and communication to be adjusted as necessary? This knowledge is absolutely vital for the design and implementation of any effective FraX interventions, including therapeutic exploitation of the ‘mGluR (metabotropic glutamate receptor) theory of FraX’. We must know when to treat and why.

In the coming year, we will use our well-established Drosophila FraX disease model to dissect the spatial and temporal requirements for FMRP in behavior regulation, brain structure and synaptic function. FMRP expression will be conditionally driven in the brain using the inducible, transgenic system, called the Gene-Switch (GS) method, in animals otherwise completely lacking FMRP. This approach allows FMRP to be turned on throughout the brain, or within specific targeted brain regions, during defined windows of time. This will enable the definition of the critical periods requiring FMRP, and thus therapeutically targetable signaling events, including mGluR signaling. The findings from these studies will direct the timing of drug trials and subsequent treatments to enhance efficacy and resolution of FraX disease symptoms. If we know when to treat and why, we will be able to improve intervention outcomes.

Over the past 4 years, we have developed a genetic model of Fragile X Syndrome in that best-characterized genetic workhorse system; the fruitfly Drosophila. 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 suggests that FMRP may be regulated downstream of signaling via metabotropic glutamate receptors (mGluRs). The mGluR theory suggests that FMRP functions to limit neuronal activity in response to these receptors.

We are testing the mGluR theory 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 pathways. This research may lead to 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

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.