Matrix Metalloproteinase Therapeutic Treatments for Fragile X Syndrome

Much research in Fragile X has focused on intracellular signaling pathways — enzyme cascades which translate receptor activation in dendrites into long-lasting effects in neurons. However, it is also know that there are extracellular signaling pathways — enzyme cascades outside the neuron, which remodel the extracellular matrix, the gelatinous bed in which the neurons exist. Intracellular and extracellular signaling pathways both show abnormalities in Fragile X.

More than a decade ago, we developed a Drosophila (fruit fly) genetic model of FXS to identify underlying molecular bases of the disease state and test drug intervention strategies. Using this model, we recently showed that minocycline treatment is an affective means to restore normal synaptic connectivity broadly throughout the nervous system. We showed that minocycline acts to inhibit matrix metalloproteinases (MMPs); extracellular proteases that are known to regulate synapse structure and function. Fortunately, there is only one secreted MMP (1) and one membrane attached MMP (2) in Drosophila, and we have a collection of characterized mutation, over-expression (UAS-WT) and knock-down (RNAi) lines for each MMP to test their requirements.

A current goal of our work is to use these genetic tools to thoroughly characterize MMP roles at synapses and in behavioral outputs. We have found requirements for both secreted MMP-1 and membrane anchored MMP-2 in synaptic structural and functional development. Furthermore, drug tests with MMP inhibitors (MMPIs) show effective restoration of the behavioral learning impairment in our Drosophila FXS disease model. In studying possible downstream reasons, we discovered that synaptic heparan sulfate proteoglycans (HSPGs) are strongly increased in the disease state, and that this causes defects in cell-cell signaling that normally drive synaptic development. We therefore believe that HSPGs represent a potential new FXS drug target.

In this second year of FRAXA funding, we are focused on defining the distinct but also apparently overlapping roles for MMP-1 and MMP-2 in synaptic structural and functional development. In drug studies, we are testing a range of MMPIs in drug treatments to compare effectiveness during development and at maturity, in order to define the contributions of FXS developmental impairments and adult recovery/plasticity. We are expanding our studies to test impacts on long-term memory (LTM), a protein synthesis-dependent behavior directly tied to the immediate FMRP role in translation regulation. Finally, we will test whether correction of HSPG elevation restores normal cell-cell signaling, synaptic structure and function in our Drosophila FXS model.