Many other questions about vesicular transporter trafficking remain unanswered. Although some motifs have been have tentatively assigned to particular trafficking events, they cannot fully account for all of the known pathways required for sorting to secretory vesicles. Thus, it is likely that additional signals remain to be identified. For example, the potential contribution of ubiquitination to vesicular transporter trafficking is not known. It is also unlikely that all transporter trafficking pathways have been identified. In neuroendocrine cells, trafficking to SLMVs has been suggested to occur in some cases via a late endosome intermediate (100
), but the extent to which neurons use similar, alternative pathways is not known. We note that at least a portion of VGLUT1 would appear to localize to synaptic vesicles in the absence of known endocytosis motifs (112
), supporting the notion that alternative targeting mechanisms are active in at least some neurons.
It is unknown whether vesicular transporters use specific signals for trafficking to the axon versus the somatodendritic compartment. Moreover, there is no information on the potential mechanisms by which vesicular transporters are degraded. This extreme length of many axons highlights the consequences of degrading a transporter at the nerve terminal. To be replaced, it must be synthesized and sorted into a precursor vesicle at a Golgi stack that might be a meter away.
For all of these known and potential trafficking events it remains unclear how variations between neuronal subtypes may influence transporter trafficking. Glutamatergic hippocampal neurons have proven to be a very useful model, but it is possible that variations in neurochemical identity will have dramatic effects on transporter trafficking. This possibility is underscored by the recent finding that loss of dynamin I differentially affects endocytosis at inhibitory versus excitatory synapses (149
). New mammalian model systems representing a variety of different synaptic subtypes are likely to be required to fully understand these effects. Invertebrate systems such as C. elegans
may also be useful in this regard, although the extent to which neuronal trafficking mechanisms are conserved is somewhat controversial.
Surprisingly, the number of vesicular transporters that reside on each secretory vesicle is not known. It has been suggested that as few as one in Drosophila
) or as many as fourteen VGLUT molecules in mammalian preparations (24
) may localize to an individual synaptic vesicle. One of the most important topics for future studies will be determining this number and how it may be regulated. Despite observed variations in neurotransmitter content, it is difficult to imagine that the synapse could tolerate large, random fluctuations in quantal size.
Finally, we believe that it will be critical to determine how changes in vesicular transporter trafficking may affect the nervous system as a whole. In vitro studies have provided tantalizing hints about the potential impact of trafficking on quantal size and synaptic transmission. However, we will not be able to determine how changes in transporter trafficking may affect behavior until we can study trafficking mutants in vivo.