Mechanistic basis for SNX27-Retromer coupling to ESCPE-1 in promoting endosomal cargo recycling.

Sorting nexin-27 (SNX27)-Retromer is an endosomal sorting complex that orchestrates endosome-to-plasma membrane recycling of hundreds of internalized receptors, channels and transporters, enzymes and adhesion molecules. While SNX27-Retromer is essential for development, subtle functional defects are observed in human disease, most notably neurodegenerative and neurological disorders. Achieving a thorough mechanistic dissection of SNX27-Retromer is central to understanding endosomal sorting in health and disease. Here we combine biochemical, structural and cellular analyses to establish the mechanistic basis through which SNX27-Retromer couples to the membrane tubulating ESCPE-1 complex (Endosomal SNX-BAR sorting complex for promoting exit 1). We show that a conserved surface in the FERM (4.1/ezrin/radixin/moesin) domain of SNX27 directly binds acidic-Asp-Leu-Phe (aDLF) motifs in the disordered amino-termini of the SNX1 and SNX2 subunits of ESCPE-1. This interaction hands-over SNX27-Retromer captured integral membrane proteins into ESCPE-1 tubular profiles to promote their cell surface recycling. Through phylogenetic analysis, we reveal that SNX27:Retromer:ESCPE-1 assembly evolved in a stepwise manner during the early evolution of metazoans, which reflects the increasing complexity of endosomal sorting from the ancestral opisthokont to modern animals.

Opposing functions for retromer and Rab11 in extracellular vesicle traffic at presynaptic terminals

Neuronal extracellular vesicles (EVs) play important roles in intercellular communication and pathogenic protein propagation in neurological disease. However, it remains unclear how cargoes are selectively packaged into neuronal EVs. Here, we show that loss of the endosomal retromer complex leads to accumulation of EV cargoes including amyloid precursor protein (APP), synaptotagmin-4 (Syt4), and neuroglian (Nrg) at Drosophila motor neuron presynaptic terminals, resulting in increased release of these cargoes in EVs. By systematically exploring known retromer-dependent trafficking mechanisms, we show that EV regulation is separable from several previously identified roles of neuronal retromer. Conversely, mutations in rab11 and rab4, regulators of endosome-plasma membrane recycling, cause reduced EV cargo levels, and rab11 suppresses cargo accumulation in retromer mutants. Thus, EV traffic reflects a balance between Rab4/Rab11 recycling and retromer-dependent removal from EV precursor compartments. Our data shed light on previous studies implicating Rab11 and retromer in competing pathways in Alzheimer’s disease, and suggest that misregulated EV traffic may be an underlying defect.

Macropinocytosis-mediated membrane recycling drives neural crest migration by delivering F-actin to the lamellipodium

Individual cell migration requires front-to-back polarity manifested by lamellipodial extension. At present, it remains debated whether and how membrane motility mediates this cell morphological change. To gain insights into these processes, we perform live imaging and molecular perturbation of migrating chick neural crest cells in vivo. Our results reveal an endocytic loop formed by circular membrane flow and anterograde movement of lipid vesicles, resulting in cell polarization and locomotion. Rather than clathrin-mediated endocytosis, macropinosomes encapsulate F-actin in the cell body, forming vesicles that translocate via microtubules to deliver actin to the anterior. In addition to previously proposed local conversion of actin monomers to polymers, we demonstrate a surprising role for shuttling of F-actin across cells for lamellipodial expansion. Thus, the membrane and cytoskeleton act in concert in distinct subcellular compartments to drive forward cell migration.