Regulation Of Biosynthetic Cargo Trafficking In Neurons

Neurons are highly polarized cells with extremely long projections: dendrites and axons (together referred to as neurites). The central secretory dogma states that all protein synthesis and early secretory events take place in the cell body of a neuron, the soma. Yet, computational studies fail to reconcile experimental evidence with this notion proposing that there must be local protein synthesis and trafficking routes in distal neurites to support the function of a neuronal cell (reviewed in Chapter 1). Indeed, multiple lines of evidence from transcriptomics and protein translation studies indicate that local protein synthesis takes place independently of the soma. Amongst locally translated proteins, there are numerous secreted and membrane proteins. Yet, it is not clear how these newly synthesized proteins can be secreted locally, mainly, because the secretory pathway is largely understudied, especially in human neurons. Thus, I used human-induced pluripotent stem cells that I differentiated into cortical neurons to study the secretory pathway in distal neurites. In Chapter 2, I show that the secretory events occur in distal neuronal projections independently of the cell body. In fact, I find that nascent proteins are trafficked using an unconventional pathway via recycling endosomes. In addition, I uncover surprising new roles for TFG, a ubiquitously expressed protein and an important player in the early secretory pathway. Missense mutations in TFG have been identified in patients with neurodegenerative disorders though it is not clear how TFG results in disease. In Chapter 2, I show that TFG is not only important in the early secretory events in the conventional ER-to-Golgi trafficking, but it also mediates the unconventional secretory route where newly synthesized proteins exit ER and enter endosomes locally in distal neurites. Curiously, I find that TFG abundantly localizes with endosomes, suggesting that it must have an additional role in the endosomal pathway. Thus, I find that TFG has three distinct roles in neuronal cells: 1) conventional ER-to-Golgi trafficking in the soma, 2) unconventional ER-to-endosome route in distal neurites, 3) regulation of endosomal pathway. Future work, discussed in Chapter 3, will shed light on how TFG regulates the latter and how mutations in TFG lead to neurodegeneration.