Endocytic sorting of signaling receptors between recycling and degradative pathways is a key cellular process controlling the surface complement of receptors and, accordingly, the cell’s ability to respond to specific extracellular stimuli. The beta-2 adrenergic receptor (β2AR) is a prototypical seven-transmembrane signaling receptor that recycles rapidly and efficiently to the plasma membrane after ligand-induced endocytosis. β2AR recycling is dependent on the receptor’s C-terminal PDZ ligand and Rab41,2. This active sorting process is required for functional resensitization of β2AR-mediated signaling3,4. Here we show that sequence-directed sorting occurs at the level of entry into retromer tubules and that retromer tubules are associated with Rab4. Further, we show that sorting nexin 27 (SNX27) serves as an essential adapter protein linking β2ARs to the retromer tubule. SNX27 does not appear to directly interact with the retromer core complex, but does interact with the retromer associated Wiskott-Aldrich Syndrome Protein and SCAR Homolog (WASH) complex. The present results identify a role for retromer in endocytic trafficking of signaling receptors, in regulating a receptor-linked signaling pathway, and in mediating direct endosome-to-plasma membrane traffic.

The functional consequences of signaling receptor endocytosis are determined by the endosomal sorting of receptors between degradation and recycling pathways. How receptors recycle efficiently, in a sequence-dependent manner that is distinct from bulk membrane recycling, is not known. Here, in live cells, we visualize the sorting of a prototypical sequence-dependent recycling receptor, the beta-2 adrenergic receptor, from bulk recycling proteins and the degrading delta-opioid receptor. Our results reveal a remarkable diversity in recycling routes at the level of individual endosomes, and indicate that sequence-dependent recycling is an active process mediated by distinct endosomal sub-domains distinct from those mediating bulk recycling. We identify a specialized subset of tubular microdomains on endosomes, stabilized by a highly localized but dynamic actin machinery, that mediate this sorting, and provide evidence that these actin-stabilized domains provide the physical basis for a two-step kinetic and affinity-based model for protein sorting into the sequence-dependent recycling pathway.

G protein–coupled receptors rely on the PDZ domain of SNX27 for endosomal recycling.

Several symptoms associated with chronic pain, including fatigue and depression, are characterized by reduced motivation to initiate or complete goal-directed tasks. However, it is unknown whether maladaptive modifications in neural circuits that regulate motivation occur during chronic pain. Here, we demonstrate that the decreased motivation elicited in mice by two different models of chronic pain requires a galanin receptor 1–triggered depression of excitatory synaptic transmission in indirect pathway nucleus accumbens medium spiny neurons. These results demonstrate a previously unknown pathological adaption in a key node of motivational neural circuitry that is required for one of the major sequela of chronic pain states and syndromes.

A fundamental and still largely unresolved question is how neurons achieve rapid delivery of selected signaling receptors throughout the elaborate dendritic arbor. Here we show that this requires a conserved sorting machinery called retromer. Retromer-associated endosomes are distributed within dendrites in ~2 μm intervals and supply frequent membrane fusion events into the dendritic shaft domain immediately adjacent to (<300 nm from) the donor endosome and typically without full endosome discharge. Retromer-associated endosomes contain β-adrenergic receptors as well as ionotropic glutamate receptors, and retromer knockdown reduces extrasynaptic insertion of adrenergic receptors as well as functional expression of AMPA and NMDA receptors at synapses. We propose that retromer supports a broadly distributed network of plasma membrane delivery to dendrites, organized in micron-scale axial territories to render essentially all regions of the postsynaptic surface within rapid diffusion distance of a local exocytic event.