1. Roles for P bodies in polarized cells:
It is widely accepted that neuronal processes ranging from the establishment of long-term memory (LTM) to axon terminal growth requires immediate local protein synthesis in response to a specific local stimulus. Synapitc mRNA translation is controlled by sequence motifs within mRNAs acting in concert with specific RNA binding proteins and noncoding RNAs (ncRNAs). Together, the composition of these ribonucleoprotein (RNP) particles determines whether mRNAs are transported to a specific location (e.g. to the synapse), locally translated, or targeted for storage and/or degradation. While much progress is being made, relatively little is known about the general- and ncRNA-mediated mechanisms underlying local mRNA translation in neurons. Our long-term goal is to elucidate the molecular and cellular mechanisms involved in the storage and translation of synapse-localized mRNAs. New evidence suggests that dendritic RNPs in Drosophila neurons share highly conserved translational repression machinery with cytoplasmic RNA processing bodies (“P bodies”). P bodies have shown to be involved in mRNA decay and both general- and microRNA (miRNA)-mediated translational repression pathways. With this in mind, the underlying objective of this project is to identify and characterize functions for P bodies and P body components in synaptic translational control.
2. Neuronal functions for miRNAs:
New evidence suggests that the microRNA (miRNA) pathway plays an important role in coordinating synaptic processes. miRNAs are small, non-coding, regulatory RNAs implicated in the control of target gene expression at the level of translation or mRNA stability. This is thought to occur via cytoplasmic P bodies. Despite recent advances in this field, there remains a critical need to identify specific miRNAs whose expression is regulated by synaptic activity as well as their key mRNA targets. The goal of this project is to identify novel mechanisms involved in the control of activity-dependent structural plasticity at the glutamatergic larval Drosophila neuromuscular junction (NMJ). This synapse has been extensively characterized as a model system for studying synaptic development, function, and plasticity. We have used several unbiased genomic approaches to identify a subset of neuronal miRNAs whose expression levels are rapidly modulated by spaced synaptic stimulation and are involved in the control of activity-dependent axon terminal growth at the NMJ. We are currently in the process of identifying and functionally characterizing novel bona fide target mRNAs.
3. The molecular pathology of Fragile X Syndrome (FXS):
Defects in neuroplasticity are characteristic features of neural dysfunction. This dysfunction includes conditions ranging from inherited developmental disorders (such as fragile X syndrome or FXS) to age-related memory loss and addiction. FXS is caused by loss of function of the Fragile X Mental Retardation Protein (FMRP). FMRP is an evolutionarily conserved RNA binding protein that has been implicated in the control of synaptic plasticity in the wild-type brain. FMRP is highly expressed throughout the brain where it is predicted to associate with ~ 4% of neuronal mRNAs. While much FMRP is thought to form cytoplasmic RNPs that associate with polyribosomes, a substantial fraction can be found in RNA granules that traffic with repressed mRNAs in axons and dendrites. One abundant population of FMRP-containing neuronal RNPs is highly enriched for conserved components of cytoplasmic P bodies and the miRNA pathway. Within these neuronal granules, FMRP has been shown to act as a translational repressor, presumably through a direct association with structural elements in the untranslated regions (UTRs) of target mRNAs. Recent genetic and biochemical evidence suggests that FMRP may also co-regulate the translation of some neuronal mRNAs through a cooperative interaction with components of the miRNA pathway. However, despite advances in this field, there remains a critical need to further elucidate this mechanism in order to determine how FMRP interacts with P bodies and the miRNA effector complex to mRNA translation. We are currently using genetic, biochemical, and molecular approaches to address these questions in Drosophila as a genetic model system.