Kimberly Huber, PhD, Principal Investigator ((2001 to Present))
Elena Nosyreva, PhD, Postdoctoral Fellow (2006)
Jennifer Ronesi, PhD, Postdoctoral Fellow (2007)
Tong Zang, PhD, Postdoctoral Fellow (2010)
Weirui Guo, PhD, Postdoctoral Fellow ((2012 to Present))

FRAXA Awards:

by Tong Zang, 5/1/2011

Proper synapse maturation and elimination is crucial for the establishment of appropriate neural circuits that underlie sensory processing and cognition. Neuron of Fragile X patients as well as in the mouse model of Fragile X, Fmr1 KO mice, display more dendritic spines, the point of contact for excitatory synapses, as well as long and thin filopodia resembling immature spines. This suggests Fragile X Mental retardation protein (FMRP) has a role in promoting synapse maturation and elimination. Altered regulation of these processes in Fragile X Syndrome likely underlies many of the cognitive deficits associated with Fragile X Syndrome.

There is considerable evidence that synaptic plasticity and structure are altered in Fmr1 KO mice, but the mechanisms by which this occurs are unknown. Previously we demonstrated that acute postsynaptic expression of FMRP in CA1 pyramidal neurons in slice cultures elicits synapse elimination providing a direct cell autonomous role for FMRP in synapse elimination (Pfeiffer et al., 2007). We have gained important mechanistic insight into FMRP induced synapse elimination. The activity-dependent transcription factor, myocyte enhancer factor 2 (MEF2) also causes synapse elimination in neurons. We find that FMRP is required for MEF2 to elicit synapse elimination. Importantly, FMRP functions downstream of MEF2 activation and active MEF2 is required for FMRP to eliminate synapses (Pfeiffer, Zang et al., Neuron, 2010).

Most recently, we have observed developmental differences in the effects of acute FMRP expression on synaptic function. The developmental difference will give us a hint of how the disease occurs and proceeds. We hypothesize that these differences are due to developmental regulation of candidate transcription factors. We anticipate that our results will gain a better understanding of how FMRP regulates synapse development, why synapse density, maturation and connectivity are altered in Fragile X Syndrome and may lead to novel therapeutic strategies for the disease.

by Katie Clapp, 2/1/2011

by Jennifer Ronesi, 9/1/2008

Information storage in the brain is widely attributed to long-lasting activity-dependent changes (potentiation or depression) in the strength of synapses, the connections between neurons. The mGluR theory of Fragile X syndrome (FXS) was motivated by our findings that group 1 mGluR and protein synthesis dependent long-term synaptic depression (mGluR-LTD) is enhanced and no longer requires protein synthesis in the hippocampus of the mouse model of FXS, the Fmr1 knockout (KO) mouse. These results suggested that there is an alteration of group 1 mGluR function in Fmr1 KO mice.

A potential molecular basis for altered mGluR function is suggested by the recent finding that mGluR5 is less associated with the postsynaptic scaffolding protein Homer in Fmr1 KO mice. Homer proteins bind to group 1 mGluRs and to various downstream targets of mGluRs. Due to their distinct dimerization properties, long forms of Homer (short forms of Homer lack a dimerization domain) function as both scaffolds of multi-protein complexes and mediators of mGluR signaling in neurons.

To examine the role of Homer interactions in mGluR-LTD and mGluR signaling to protein synthesis machinery in WT and Fmr1 KO animals, we disrupted mGluR-Homer interactions acutely with a peptide which mimics the C-terminal tail of mGluR5 (mGluR5ct). This peptide was previously shown to disrupt Homer interactions with mGluRs, and we found that introduction into hippocampal slices blocks mGluR-LTD and mGluR-signaling to protein synthesis initiation in wildtype animals. Disruption of mGluR- Homer interactions selectively blocks mGluR activation of the PI3K-Akt-mTOR, but not ERK, pathway and translation of a 5’ terminal oligopyrimidine tract (5’TOP) containing mRNA, Elongation factor 1?. In Fmr1 KO mice, mGluR-LTD is insensitive to disruption of Homer interactions and mGluR activation of PI3K-mTOR is lost.

Our results find specific roles for Homer in mGluR signaling and plasticity and suggest that reduced mGluR-Homer interactions in Fmr1 KO mice lead to a deficit in mGluR stimulation of translation initiation.

What leads to the uncoupling of mGluRs from Homer and translation initiation in Fmr1 KO mice is unknown. Currently, we are examining if increased levels of Homer1a, a dominant negative form of Homer, contribute to the Fragile X phenotype. Our results will provide a better understanding of the role of Homer interactions in mGluR dependent plasticity mechanisms and how these differ in FXS. This knowledge could lead to the development of novel therapeutic strategies for FXS which include the manipulation of Homer isoforms or mGluR interactions with Homers.

by Kimberly Huber, 2/1/2005

It is thought that long-term changes at connections between brain cells (synaptic plasticity) underlie the refinement of neuronal circuitry during development and mediate processes such as learning and memory in the adult. In the past few years we have been studying a form of synaptic weakening, called long-term depression (LTD), in FMR1 knockout mice. We previously demonstrated,with Mark Bear, that LTD induced by metabotropic glutamate receptors (mGluRs) is enhanced in FMR1 knockout mice. Elena Nosyreva demonstrated in normal animals that there is a developmental switch in the cellular and synaptic mechanisms of mGluR-dependent LTD. In other words, different genes and proteins are involved in youth vs. adulthood. We have preliminary data that the GluR-LTD in adult FMR1 knockout mice is similar to the immature form of mGluR-LTD. Dr. Nosyreva characterized the properties ofmGluR-LTD in FMR1 knockout mice to determine if it is indeed the immature form of mGluR-LTD or a mature form of LTD with distinct properties.

by Kimberly Huber, 10/1/2002

This grant was funded by FRAXA with help from The Meadows Foundation.

Our research focuses on how connections between brain cells, called synapses, change in a long term way, termed synaptic plasticity. It is thought that long-term changes at synapses underlie the refinement of neuronal circuitry during development and mediate processes such as learning and memory in the adult. In the past few years we have been studying synaptic plasticity, in a mouse model of mental retardation, specifically fragile X syndrome. The 'fragile X' mouse model was generated by a knockout or removal of the Fragile X mental retardation gene (Fmr1) gene.

To try and understand how synaptic function and plasticity is changed in fragile X syndrome, we studied a form of synaptic weakening, termed long-term depression or LTD, in the fragile X mouse. We have found that LTD is larger in fragile X mice when compared to their normal littermates. This result suggests a potential function for fragile X protein and how abnormal synapse maturation occurs as a consequence of its loss. This result was published this year in Proceedings of the National Academy of Sciences (Huber et al., 2002).

The fact that LTD is larger in fragile X syndrome may provide an avenue which to test therapeutic strategies for treatment of fragile X syndrome. We have done considerable work determining the cellular mechanisms which underlie LTD in rats and know that it requires activation of a subclass of neurotransmitter receptors called metabotropic glutamate receptors or mGluRs. We are currently testing mGluR antagonists with different subtype specificities to find a compound which can reduce LTD in fragile X mice to levels observed in normal mice. We have had success reducing LTD in rats with one mGluR1 antagonist, termed LY367385. We plan to continue testing mGluR1 antagonists in fragile X mice and test them in combination with other mGluR antagonists. In addition, we have demonstrated that LTD occurs in developing synapses in rats and plan to test if LTD is altered in young fragile X mice. This may help to understand why synapses of fragile X syndrome patients and mice have immature synapses.

In the third aim, we have planned experiments which will reveal the cellular mechanisms by which the fragile X protein enhances LTD. This proposed research is expected to lead to therapeutic strategies for the treatment of fragile X syndrome with mGluR antagonists. Furthermore, information about the age-dependent function of fragile X protein could predict the best developmental window for these treatments. We also aim to determine why synaptic plasticity is altered in fragile X syndrome which may lead to hypotheses about the mechanisms of abnormal synapse maturation and cognitive deficits associated with the disease.

Our initial findings that LTD is enhanced in fragile X mice was instrumental in obtaining a McKnight Neuroscience of Brain Disorders award from the McKnight Endowment Fund for Neuroscience. This additional funding has allowed us to expand our efforts to determine the synaptic mechanisms of LTD in cultured neurons, determine how mGluRs contribute to synapse development and develop genetic rescue strategies by transfection of neurons with Fmr1 gene.

The enhancement of LTD in a mental retardation model suggests that pharmacological methods to reduce LTD to normal levels are a potential strategy to treat fragile X syndrome. Results of the proposed research are expected to provide the framework for future clinical trials and facilitate progress towards a treatment of fragile X syndrome.