Within the past thirty years, the discovery and characterization of the microtubule-associated motor proteins, kinesins and cytoplasmic dynein, has radically expanded our understanding of intracellular trafficking and motile phenomena. Nevertheless, the mechanisms by which eukaryotic cells integrate motor functionality and cargo interactions over multiple subcellular domains in a spatiotemporally controlled way remain largely mysterious.
During transport within the neuronal axon, dynein and the kinesins run in opposite directions along uniformly polarized microtubule tracks, so that each motor must switch between active transport and being, itself, a cargo in order to be properly positioned and carry out its function. The axon thus represents a model system in which to study the regulatory mechanisms governing intracellular transport, especially under conditions when it must be modulated in response to changing environmental cues, such as during axon outgrowth and development.
Recently, the localization of certain messenger RNAs and their local translation to yield protein has emerged as a critical process for the development of axons and other neuronal compartments. I observed that transcripts encoding the dynein cofactors Lis1 and dynactin are among those localized to axons, so I hypothesized that stimulus-dependent changes in axonal transport may occur via local synthesis of dynein cofactors. In these studies, I have shown that different conditions of nerve growth factor signaling on developing axons trigger acute changes in the transport of various axonal cargoes, contemporaneous with rapid translational activation and production of Lis1 and dynactin’s main subunit, p150Glued, within the axons themselves. Differential synthesis of these cofactors in axons was confirmed to be required for the observed stimulus-dependent transport changes, which were completely prevented by axon-specific pharmacologic inhibition of protein synthesis or RNA interference targeted against Lis1 and p150Glued. In fact, Lis1 was, in an apparent paradox, locally synthesized in response to both nerve growth factor stimulation and withdrawal. I demonstrated that this is due to the fact that Lis1 is produced from a heterogeneous population of localized transcripts, differentiated chiefly by whether they interact with the RNA-binding protein APC. Preventing the binding of APC to Lis1 transcripts thus inhibited axonal synthesis of Lis1 and its resultant transport effects under conditions of nerve growth factor stimulation, while having no bearing on the similar phenomena seen during nerve growth factor withdrawal. This demonstrates that association with RNA-binding proteins can functionally distinguish sub-populations of localized messenger RNAs, which, in turn, provides a foundation for mechanistically understanding how localized protein synthesis is coupled to specific stimuli.
Axonally synthesized Lis1 also was shown to have a particular role in mediating transport of a retrograde death signal originating in nerve growth factor-deprived axons, as neurons exhibited greatly reduced cell death when axonal synthesis of Lis1 was blocked. Through the application of pharmacologic agents inhibiting different steps in the propagation of this pro-apoptotic signal, I established that the signal depends upon effective endocytosis and the activity of glycogen synthase kinase 3β. It is therefore likely that the retrogradely transported signaling cargo in question is a glycogen synthase kinase 3β-containing endosome or multivesicular body—a type of large cargo consistent with Lis1’s known role in adapting the dynein motor for high-load transport. Preliminary results further indicate that axons exposed to another type of degenerative stress, in the form of toxic amyloid-β oligomers, may also employ local synthesis of Lis1 as a means of regulating transport and survival signaling.
These findings establish a previously undescribed mechanism of regulating dynein activity and cargo interactions through local synthesis of its cofactors, allowing for rapid responses to environmental cues and stimuli that are especially relevant during the development of the nervous system. In addition to illustrating a regulatory principle that may be generally applicable to subcellular compartments throughout polarized cells, these studies provide new insights into intracellular transport disruptions that occur in lissencephaly, neurodegeneration, and other human disease states.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/D8S46S27 |
| Date | January 2016 |
| Creators | Villarin, Joseph Manuel |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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