Studies of endosomal-recycling pathways in the distal axon

Cecilia Y Martinez, Purdue University

Abstract

Neurons rely on endocytic pathways for nutrient and trophic factor uptake, retrieval of membrane proteins destined for degradation and sampling the local environment for guidance cues. Although the distribution of the endosomal-lysosomal pathway has been established in the distal axon, it remains unclear how endocytic-recycling pathways are functionally organized. The spatial organization of the endosomal-recycling pathways will influence how rapidly axonally derived cargos are endocytically processed. Thus, it is critical to understand if endocytic recycling compartments are stationed along the axon or are positioned in growth cones and/or branch points. To address this issue, investigations into the functional organization of the endosomal recycling compartment in the distal axon of DRG neurons were conducted. Rab11a, a recycling endosome associated protein, was localized to the distal axon with a uniform punctuate distribution. The exogenous expression of Rab11a in the distal axon of neurons transiently transfected with pEGFP::Rab11a or stably transfected with a virus, RCASeGFP::Rab11a was equivalent in localization to endogenous Rab11a. The identity of Rab11a labeled vesicles in the axon was confirmed by the partial colocalization with a lipid analog, Fast DiI, a lipid analog that was previously demonstrated to label recycling endosome compartments. Further characterization of Rab11a vesicle motility behavior in the axon suggests that long range retrograde axonal transport is MT-dependent while local transport is mediated by both actin and MT filaments. While Rab11a vesicles were highly motile in axon and branch points, the majority of Rab11a vesicles in growth cones were stationary. These data suggest that recycling compartments may be anchored in the growth cone to facilitate the immediate progression of axonally derived endosomes through distal endocytic pathways. Because neurons of the peripheral nervous system are dependent upon retrograde transport of NGF for survival, the endocytic processing of NGF through the recycling compartment was also investigated. Although dual label studies involving Rab11a and NGF were not conclusive, this study provides a stepping stone for future experiments in this area.

Degree

Ph.D.

Advisors

Hollenbeck, Purdue University.

Subject Area

Neurology

COinS