By Hajnalka “Sunny” Nyitrai

Accurate information processing in the brain depends on billions of neurons forming highly complex synaptic networks. Individual neurons communicate with one another at the synaptic contacts encoded with precisely timed chemical signals. Each neuron forms thousands of synapses, all of which require maintenance for proper information transfer. Presynaptic sites are specialized membrane regions that are comprised of many large protein complexes to facilitate accurate signal transmission. However, delivery of presynaptic material over long axonal distances is a daunting challenge for neurons. Several neurodevelopmental and neurodegenerative disorders have been attributed to dysfunctional vesicle trafficking in neurons. Many presynaptic proteins are thought to be synthesized and delivered from the soma via precursor vesicles, the recruitment of which at nerve terminals is crucial for the maintenance of presynaptic function. How do transport vesicles know where to stop along the axon? In most cellular secretory pathways, Golgi-derived cargos are labeled with Rab GTPases that are recognized by tethering complexes at their fusion sites; however, Rab-dependent delivery has not been identified for presynaptic terminals.

In our recent paper (Nyitrai et. al, 2020), we addressed how neurons transport material to the synapse and how they ensure delivery to the appropriate destinations. We have identified a mechanism in which cargo vesicles are marked with Rab6. We have found that Rab6 is used in the Golgi as an address label to mark cargo that is intended for nerve terminals. It then travels with this cargo along axons in its GTP-bound state. We proposed the following mechanism for a Rab6-dependent vesicle transport: When a Rab6-marked cargo passes a pre-synaptic terminal, it is captured by a large presynaptic protein, ELKS1. ELKS1 is anchored at presynaptic sites via its N-terminal sequences and extends its long coiled-coil regions into the cytosol. When travelling Rab6-cargo is in reach, a short C-terminal region recognizes it and binds to cargo-attached Rab6. We conclude that neurons employ Rab-dependent targeting of cargo to deliver Golgi-derived material to presynaptic nerve terminals. Neurons have adapted this capturing mechanisms from pathways for constitutive secretion by spatially separating cargo capture from fusion. This enables regulated secretion after further steps of cargo maturation.

The myriad of protein complexes that are required for regulated vesicle cycling may not all be recruited to synapses by the same means. It is more likely that many mechanisms exist at synapses that recruit cargo by content. The Rab6-ELKS complex could be one such specialized capturing mechanism, which begs the question: What is the content of Rab6-marked transport cargo? At the root of this question lies a long-standing unanswered dilemma that the field has struggled to answer: How well can axonal cargo be defined and categorized by content? The new knowledge that Rab6 labels an axonal transport cargo provides a molecular tool for future studies to assess the composition of axonal vesicle cargo.

Working model of the Rab6-ELKS1 capturing mechanism of axonal transport cargo: Synapse-anchored ELKS1 (in orange) captures axonal Rab6-marked cargo (Rab6 in red).

Hajnalka “Sunny” Nyitrai is a postdoc in the lab of Pascal Kaeser at HMS. She will soon begin a new postdoc appointment in the lab of Joris De Wit at the VIB Center for Brain & Disease Research / KU Leuven, Department of Neurosciences in Belgium.

This story may also be featured in the HMS Neurobiology Department newsletter, The Action Potential.

Learn more in the original paper:
Nyitrai, H., Wang, S., & Kaeser, P. S. (2020). ELKS1 Captures Rab6-Marked Vesicular Cargo in Presynaptic Nerve Terminals. Cell reports, 31(10), 107712. https://doi.org/10.1016/j.celrep.2020.107712