With $95,000 in grants from FRAXA Research Foundation from 2001-2006, Dr. Justin Fallon and his team at Brown University studied systematic mapping of fragile X granules in developing mouse brains, revealing a potential role for presynaptic FMRP in sensorimotor functions.
Sandra Won, PhD
FRAXA Postdoctoral Fellow (2001)
Graduate Student (2004-2005)
Results published in J Comp Neurol.
Akins MR, Leblanc HF, Stackpole EE, Chyung E, Fallon JR, 4/23/2012
Loss of fragile X mental retardation protein (FMRP) leads to fragile X syndrome (FXS), the most common form of inherited intellectual disability and autism. Although the functions of FMRP and its homologues FXR1P and FXR2P are well studied in the somatodendritic domain, recent evidence suggests that this family of RNA binding proteins also plays a role in the axonal and presynaptic compartments. Fragile X granules (FXGs) are morphologically- and genetically-defined structures containing fragile X proteins that are expressed axonally and presynaptically in a subset of circuits. To further understand the role of presynaptic fragile X proteins in the brain we have systematically mapped the FXG distribution in the mouse central nervous system. This analysis revealed both the circuits and the neuronal types that express FXGs. FXGs are enriched in circuits that mediate sensory processing and motor planning – functions that are particularly perturbed in FXS patients. Analysis of FXG expression in the hippocampus suggests that CA3 pyramidal neurons utilize presynaptic fragile X proteins to modulate recurrent but not feedforward processing. Neuron-specific FMRP mutants revealed a requirement for neuronal FMRP in the regulation of FXGs. Finally, conditional FMRP ablation demonstrated that FXGs are expressed in axons of thalamic relay nuclei that innervate cortex, but not in axons of thalamic reticular nuclei, striatal nuclei, or cortical neurons that innervate thalamus. Together, these findings support the proposal that dysregulation of axonal and presynaptic fragile X proteins contribute to the neurological symptoms of FXS.
Project Plan by Anne Booker, 2/1/2005
Our laboratory is interested in the molecular mechanisms that regulate Fmr1 expression. Recently, we have developed a new model to study Fmr1 transcription in the central nervous system (CNS). We have shown that olfactory experience (smelling) bi-directionally regulates Fmr1 gene expression. Moreover, this regulation is highly dependent on developmental stage. In preliminary experiments we have observed that DNA methylation patterns in the Fmr1 promoter are correlated with the developmental changes in gene expression. We will investigate the transcriptional regulation of fmr1 using a combined molecular, cellular and biochemical approach in both in vitro and in vivo systems.
The main goal of this study is to discover the specific processes that are responsible for the bi-directional regulation of the fmr1 gene. There are several ways in which genes can be regulated. One aim is to identify transcription factors-proteins that bind to specific regions of DNA and promote the synthesis of messenger RNA. Second, recent studies have shown that epigenetic modifications — such as the methylation of a gene and histone acetylation — can also be influenced by both development and activity. Our preliminary data suggest that the fmr1 gene is regulated throughout development and following activity by these epigenetic modifications. We hypothesize that some transcription factors and epigenetic modifications are particularly important in early development, while others play a prominent role later in life.
Results from these studies will help us understand the normal function and regulation of Fmr1 in early childhood and in the adult. These insights could aid in the design of therapeutic strategies to treat FXS that are tailored to particular developmental stages.
Transcriptional Regulation of the Fragile X Gene (fmr1) in the Olfactory Bulb
by Justin Fallon, 7/1/2001
Fragile X syndrome is caused by the absence of the FMR1 gene’s protein product, FMRP. However, little is known about the normal function and regulation of FMRP or how its loss leads to cognitive impairment. We do know that the translation of RNAs into proteins at synapses (the junctions between nerve cells) is essential for learning and memory. A growing body of evidence suggests a role for FMRP in RNA binding, transport, and/or translation. Intriguingly, FMR1 messenger RNA is present at the synapses and its translation can be stimulated by neurotransmitters. The close relationship between FMRP protein and message and RNA metabolism at synapses provides a pathway to link FMRP function at the molecular level to its role in higher functions in the brain. Therefore, an understanding of the translational regulation of FMRP is necessary for understanding the molecular mechanisms leading to fragile X mental retardation.
We are investigating the molecular mechanisms of activity-induced fragile X protein synthesis using a combined molecular, cellular and biochemical approach in cultured neurons and in mice. Of special interest is the potential role of a particular process, recently identified in our laboratory, by which synaptic mRNAs are translated into proteins. The mRNAs encoding FMRP and a related protein, FXR2P, contain unique tags indicating that they may be regulated by this process. The overall goal of our studies is to understand the role FMRP plays in translating other proteins and thereby strengthening and/or weakening synapses and, ultimately, enabling learning and memory. Such information could contribute to designing strategies and treatments for overcoming the loss of FMRP in fragile X syndrome.