Local Biosynthetic Trafficking Of Synaptic Proteins In Neuronal Dendrites

Neurons face the challenge of regulating the abundance, distribution and repertoire of integral membrane proteins on the surface of their immense, architecturally complex dendritic arbors. While the endoplasmic reticulum (ER) supports local translation of secretory cargo in all dendrites, most dendrites lack the Golgi apparatus (GA), an essential organelle for conventional secretory trafficking. Thus, whether secretory cargo is locally trafficked in dendrites through a non-canonical pathway remains a fundamental question. We have defined the trafficking itinerary for key synaptic molecules in dendrites. Following ER exit, the AMPA-type glutamate receptor GluA1 and neuroligin 1 undergo spatially restricted entry into the dendritic secretory pathway and accumulate in recycling endosomes (REs) located in dendrites and spines prior to reaching the plasma membrane. Surprisingly, surface delivery of GluA1 occurred even when GA function was disrupted. Thus, in addition to their canonical role in protein recycling, REs are critical mediators of forward secretory trafficking in neuronal dendrites and spines through a specialized GA-independent trafficking network. While the SNARE machinery that supports biosynthetic trafficking from the GA to the plasma membrane in neurons is not known, the SNAREs that mediate RE exocytosis have been partially defined. Surprisingly, we found that constitutive trafficking of GluA1 through REs does not depend on VAMP2, an R-SNARE with a well-defined role in RE exocytosis. Instead, the clostridial-neurotoxin insensitive SNARE VAMP7 defined a pool of GluA1-containing transport vesicles and was required for their delivery to the plasma membrane. Synaptic stimulation accelerated the delivery of GluA1 from this pool. Interestingly, while this activity-regulated delivery required VAMP2, inhibition of VAMP7 had no effect on activity-induced exocytosis. Thus, VAMP2 and VAMP7 play complementary roles in activity-induced and constitutive delivery of new synaptic proteins. Overall we have identified a novel biosynthetic pathway that involves GA-independent transfer of cargoes to the dendritic RE compartment. Subsequent exocytosis of biosynthetic REs is constitutively maintained by VAMP7, but can be promoted by synaptic activity in a VAMP2-dependent manner. These results provide crucial insight into membrane trafficking processes that could support experience-dependent learning by rapidly delivering locally synthesized proteins to synaptic locations. Ongoing efforts are focused on the development of novel optical approaches to control secretory trafficking that will ultimately expand our capability to dissect the spatial trafficking of cargoes within the dendrite.