Fragile X syndrome, the most common inherited form of intellectual disability, is caused by mutation in the FMR1 gene that transcriptionally silences the gene and results in lack of production of the encoded protein, FMRP. A mouse model of the human syndrome (the Fmr1-KO mouse) reproduces the protein (FMRP) deficiency and exhibits abnormalities of synaptic structure and function as well as learning impairments. However, directly relating cellular dysfunction to cognitive impairments is challenging in mouse models. Study of olfactory learning in mice has a number of advantages for this type of work. This research will combine behavioral, electrophysiological, and molecular approaches to elucidate the cellular basis for learning impairment in Fmr1-KO mice.
Experiments in our laboratories have identified several neurobiological abnormalities in Fragile X model mice that are directly related to olfactory learning. First, behavioral studies showed that Fmr1-KO mice learn olfactory discriminations significantly more slowly than wild-type (WT) mice. Second, synapses in primary olfactory cortex (but not hippocampus) of Fmr1-KO mice exhibit a defect in long-term potentiation (LTP), a synaptic plasticity mechanism. Third, synapses in olfactory cortex of Fmr1-KO mice are deficient in NMDA receptors, macromolecular assemblies that are crucial for the induction of LTP. Fourth, Fmr1-KO mice show impaired proliferation and survival of neural progenitor cells and a significant reduction in the population of dentate granule cells in adulthood; this adult neurogenesis is believed to participate in hippocampal-dependent learning and memory.
We hypothesize that FMRP, the protein missing in FXS, participates in two aspects of circuit function that are critical to learning: synaptic plasticity and the generation and survival of new neurons in the adult brain.
Excited by these intriguing observations, we now propose a set of new experiments to determine the cellular mechanisms that are disrupted in mice lacking FMRP and that are critical for learning and memory functions. First, we will use molecular tools to study the cellular basis for neurogenesis deficits in adult Fmr1-KO mice and the signaling pathways by which FMRP regulates neurogenesis in both the olfactory bulb and the hippocampus of the adult brain. In the experiments designed to achieve Specific Aim One, we will determine the effect of FMRP ablation on neural stem and progenitor cell self-renewal, proliferation, migration, and neuronal differentiation in the olfactory bulb and dentate gyrus, determine its time course and cumulative effects, and determine the molecular mechanism underlying FMRP regulation of neurogenesis post-natally. We will test the hypothesis that selective down-regulation of the pro-apoptotic gene, Bax, in neurogenic niches will reverse neurogenic deficits in Fmr1-KO mice.
Second, electrophysiological, biochemical, and pharmacological methods will be used to characterize synaptic dysfunction in the olfactory-hippocampal circuit in Fmr1-KO mice. The experiments of Specific Aim Two are designed to test hypotheses about how FMRP contributes to synaptic function in segments of the olfactory-hippocampal circuit. We hypothesize that absence of FMRP disrupts trafficking of NMDA receptors to synapses, resulting in impairments in NMDA-dependent synaptic plasticity mechanisms such as LTP.
Third, using behavioral analyses, we will attempt to determine the contributions of neurogenic and synaptic dysfunction to learning impairments in Fmr1-KO mice. In studies planned for Specific Aim Three, we will further characterize the nature of this learning impairment as well as test strategies to reverse it. We will compare Fmr1-KO and littermate WT mice for acquisition of olfactory discriminations using a paradigm that does not depend on hippocampal function, to determine if cellular dysfunction within piriform cortex is sufficient to account for learning impairment. We will assess learning in Fmr1-KO mice at different ages at which specific synaptic and neurogenic impairments appear. Finally, we will test the hypothesis that impaired neurogenesis underlies the learning impairment in Fmr1-KO mice by experimentally stimulating neurogenesis in the mice.
In summary, this project will exploit the advantages of the olfactory system to study the cellular basis for learning impairment in a mouse model for fragile X syndrome. This is likely to also have impact on our understanding of mental retardation in general as well as learning disabilities in other autism spectrum disorders. This knowledge will be necessary for the development of rational strategies for prevention and treatment of cognitive impairments from a variety of causes.