Golgi-associated retrograde protein (GARP) complex-dependent endosomes to trans Golgi network retrograde trafficking is controlled by Rab4b

BACKGROUND

The trafficking of cargoes from endosomes to the trans-Golgi network requires numerous sequential and coordinated steps. Cargoes are sorted into endosomal-derived carriers that are transported, tethered, and fused to the trans-Golgi network. The tethering step requires several complexes, including the Golgi-associated retrograde protein complex, whose localization at the trans-Golgi network is determined by the activity of small GTPases of the Arl and Rab family. However, how the Golgi-associated retrograde protein complex recognizes the endosome-derived carriers that will fuse with the trans-Golgi network is still unknown.

METHODS

We studied the retrograde trafficking to the trans-Golgi network by using fluorescent cargoes in cells overexpressing Rab4b or after Rab4b knocked-down by small interfering RNA in combination with the downregulation of subunits of the Golgi-associated retrograde protein complex. We used immunofluorescence and image processing (Super Resolution Radial Fluctuation and 3D reconstruction) as well as biochemical approaches to characterize the consequences of these interventions on cargo carriers trafficking.

RESULTS

We reported that the VPS52 subunit of the Golgi-associated retrograde protein complex is an effector of Rab4b. We found that overexpression of wild type or active Rab4b increased early endosomal to trans-Golgi network retrograde trafficking of the cation-independent mannose-6-phosphate receptor in a Golgi-associated retrograde protein complex-dependent manner. Conversely, overexpression of an inactive Rab4b or Rab4b knockdown attenuated this trafficking. In the absence of Rab4b, the internalized cation-independent mannose 6 phosphate receptor did not have access to VPS52-labeled structures that look like endosomal subdomains and/or endosome-derived carriers, and whose subcellular distribution is Rab4b-independent. Consequently, the cation-independent mannose-6-phosphate receptor was blocked in early endosomes and no longer had access to the trans-Golgi network.

CONCLUSION

Our results support that Rab4b, by controlling the sorting of the cation-independent mannose-6-phosphate receptor towards VPS52 microdomains, confers a directional specificity for cargo carriers en route to the trans-Golgi network. Given the importance of the endocytic recycling in cell homeostasis, disruption of the Rab4b/Golgi-associated retrograde protein complex-dependent step could have serious consequences in pathologies.

The function of Golgi apparatus in LRRK2-associated Parkinson's disease

Parkinson's disease (PD) is a chronic neurodegenerative disease associated with the intracellular organelles. Leucine-rich repeat kinase 2 (LRRK2) is a large multi-structural domain protein, and mutation in LRRK2 is associated with PD. LRRK2 regulates intracellular vesicle transport and function of organelles, including Golgi and lysosome. LRRK2 phosphorylates a group of Rab GTPases, including Rab29, Rab8, and Rab10. Rab29 acts in a common pathway with LRRK2. Rab29 has been shown to recruit LRRK2 to the Golgi complex (GC) to stimulate LRRK2 activity and alter the Golgi apparatus (GA). Interaction between LRRK2 and Vacuolar protein sorting protein 52 (VPS52), a subunit of the Golgi-associated retrograde protein (GARP) complex, mediates the function of intracellular soma trans-Golgi network (TGN) transport. VPS52 also interacts with Rab29. Knockdown of VPS52 leads to the loss of LRRK2/Rab29 transported to the TGN. Rab29, LRRK2, and VPS52 work together to regulate functions of the GA, which is associated with PD. We highlight recent advances in the roles of LRRK2, Rabs, VPS52, and other molecules, such as Cyclin-dependent kinase 5 (CDK5) and protein kinase C (PKC) in the GA, and discuss their possible association with the pathological mechanisms of PD.

The Parkinson's Disease Protein LRRK2 Interacts with the GARP Complex to Promote Retrograde Transport to the trans-Golgi Network

Mutations in Leucine-rich repeat kinase 2 (LRRK2) cause Parkinson's disease (PD). However, the precise function of LRRK2 remains unclear. We report an interaction between LRRK2 and VPS52, a subunit of the Golgi-associated retrograde protein (GARP) complex that identifies a function of LRRK2 in regulating membrane fusion at the trans-Golgi network (TGN). At the TGN, LRRK2 further interacts with the Golgi SNAREs VAMP4 and Syntaxin-6 and acts as a scaffolding platform that stabilizes the GARP-SNAREs complex formation. Therefore, LRRK2 influences both retrograde and post-Golgi trafficking pathways in a manner dependent on its GTP binding and kinase activity. This action is exaggerated by mutations associated with Parkinson's disease and can be blocked by kinase inhibitors. Disruption of GARP sensitizes dopamine neurons to mutant LRRK2 toxicity in C. elegans, showing that these pathways are interlinked in vivo and suggesting a link in PD.