RNA silencing
(Right) Frame-by-frame depiction of a microscopy video showing exosome-producing multivesicular bodies (red) transiently interacting with foci of TNRC6 (also called GW182, green). The kymograph is read from left to right and top to bottom. Foci of TNRC6 are labeled with numbers (1-3) and the beginning of their contacts (c) with multivesicular bodies and their eventual disassociation (d) are annotated. (Left) Cells analyzed by video microscopy were flash frozen and analyzed by electron microscopy. Confirming apparent interactions in the videos, multivesicular body-like structures are observed surrounding foci of TNRC6, with TNRC6 on their surfaces (arrows).From Gibbings et al., Nature Struct. and Mol. Biology, 19(5): 517, 2012.
Each neuron is estimated to have, on average, 7000 synapses with other cells. Each synapse must be able to adapt and respond in a manner distinct from its neighboring synapses even though both are governed by the same nucleus and DNA. Post-transcriptional control of mRNA at individual synapses of the type mediated by miRNA enables inter-synaptic distinctions and plasticity. In fact, evidence is accumulating that proteins causing neurodegenerative diseases, such as Huntington’s and Parkinson’s, interact with miRNA complexes and impact miRNA activity, suggesting defects in RNA silencing may be a primary factor in the neuronal dysfunction and degeneration characteristic of these diseases.
Our recent work (Gibbings et al. Nature Structural and Molecular Biology, 19: 517, 2012) demonstrated for the first time that the prion protein (PrP) shares a motif with TNRC6 that allows it to directly bind and traffick Argonaute proteins to permit formation of mature miRNA complexes. PrP is the protein, which in a misfolded form can act as a unique type of protein-only infectious agent, and cause variant Creutzfeldt-Jakob disease, more commonly known as mad cow disease. Fascinatingly, two domains required for PrP to bind Argonaute are tightly linked to severity of pathology in models of mad cow disease. These findings suggest that misregulation of RNA silencing may fundamentally contribute to the pathology of prion diseases.
Intriguingly, exosomes are enriched in PrP, traffick infectious PrP between cells, and appear to accumulate in the plaques associated with pathology of some neurodegenerative diseases. We demonstrated that PrP affects the components of miRNA complexes found in exosomes, suggesting trafficking of miRNA complexes by exosomes in the brain may be relevant to prion diseases.
Future research in the lab will characterize how RNA silencing may be disrupted in prion disease and the role therein of exosomes. More fundamentally, these projects will seek to describe how RNA silencing in neurons is dynamically regulated in response to the highly differentiated and plastic environment of the brain.
in neurons and neurodegenerative disease
