Novel imaging strategies illuminate selective trafficking pathways in neurons

Abstract

The fundamental unit of the nervous system is the neuron. Neurons develop many anatomically and functionally distinct domains to fulfill their role in electrochemical signaling. These domains require different complements of membrane proteins. To accurately localize to their respective domains, membrane proteins undergo three trafficking steps: selective sorting, selective transport, and selective fusion. Little is known about the mechanisms that dictate neuronal protein trafficking, and many contradictory models have been proposed. This dissertation engages with two fundamental questions: what is the predominant trafficking pathway of axonal membrane proteins, and what are the mechanisms that explain kinesin-based selective transport. Chapter 2 develops a novel strategy to selectively label vesicles in different axonal trafficking pathways. It reveals that multiple pathways contribute simultaneously, although most axonal membrane proteins are directly delivered to the axon. It also identifies a novel degradative pathway by which wayward axonal proteins are targeted to lysosomes. Chapter 3 reviews the explanatory power of different kinesin transport models by comparing findings produced from two kinesin-based labeling strategies: truncated motor accumulation, and kinesin tail vesicle labeling. This allows a systematic comparison of the transport preferences of kinesin motor domains to the transport behavior of the vesicles each kinesin binds. For axon-selective transport it concludes that all available data are consistent with the smart motor model. Dendrite-selective transport cannot be explained by the smart motor model alone and may instead involve a variation of cargo steering via on-vesicle regulation. These findings underscore the importance of new technologies in driving our understanding of selective protein trafficking in neurons. They attempt to reconcile longstanding discrepancies within the field by evaluating multiple contradictory models simultaneously, and in doing so provide new insights into the complexities of neuronal trafficking machinery.