Diversification of Endosomes in the Nervous System: NEEP21 and TrkA Trafficking

Neurons project all over the body to maintain homeostasis and respond to external cues. This results in an intricate network of neurons stretching throughout the body with neurons as long as one meter in humans. Given the polarized state of neurons and the long distance over which proteins must be distributed, neurons have adapted special mechanisms to deal with these challenges. One way neurons can deal with the heightened burden of protein trafficking across long distances is through utilizing endosomal transport. To further interrogate the role of endosomes in the neuronal system, I focused on two proteins: neuron enriched endosomal protein of 21 kDa (NEEP21), and the receptor tyrosine kinase TrkA. NEEP21 is expressed almost exclusively in neurons, and has been implicated in trafficking of multiple receptors in cultured primary neurons. However, its expression pattern in the brain over development was unknown. Here, I found that NEEP21 is developmentally and spatially regulated in the brain, maintaining high expression in adulthood only in Purkinje neurons, hippocampal pyramidal neurons, and Layer V cortical neurons. Interestingly, I also found that it was expressed highly in the superior cervical ganglia (SCG), and co-localized with TrkA in culture. TrkA is a receptor tyrosine kinase critical for the formation and maintenance of the sympathetic nervous system. However, loss of NEEP21 in the knockout mouse did not have any effect on the trafficking or signaling of TrkA. To more closely probe the endosomal proteins responsible for the transport and signaling of TrkA, I performed a detailed analysis of the endosomal proteins associated with the TrkA signaling endosome in the axon and in dendrites, as well as describe the endosomal proteins involved with the novel TrkA trafficking event, retrograde transcytosis. I found that in all aspects of TrkA signaling, TrkA was spread among multiple different endosomal compartments and underwent dynamic movements in the dendrites. In my work here, I describe two detailed studies aimed at more thoroughly understanding how the neuronal endosomal system is adapted to the large size and complex morphology by investigating the possible roles of NEEP21 and identifying somatodendritic retrograde transcytosis as a mechanism for NGF-TrkA signaling endosome diversification in the peripheral nervous system.