Many activated growth factor receptors traffic through multivesicular endosomes1
. This raises the question of whether other signalling pathways might sequester cytosolic proteins as a means of regulating signalling.
One example may be NF-κB signalling, which is a key pathway in innate immunity. HRS is required for NF-κB signalling, reminiscent of its role in WNT signalling. An RNA interference (RNAi) screen also revealed a positive role for MVB formation in D. melanogaster
NF-κB (known as Toll) signalling42
. RNAi-mediated depletion of Myopic or HRS, two critical components of the ESCRT-0 complex, prevented the degradation of the D. melanogaster
Inhibitor of NF-κB (IκB) homologue Cactus, which normally inhibits the translocation of Toll into the nucleus. These findings show that endocytic trafficking and MVB formation are required to activate, rather than to downregulate, the Toll signalling pathway, and it is possible that this requires sequestration of Cactus or another negative regulator in MVBs.
Sequestration of SRC may also be relevant for signalling downstream of the β2 adrenergic GPCR. Receptor activation by its ligand triggers the relocalization of SRC Tyr kinase into cytoplasmic vesicle-like structures43
. The adaptor protein β-arrestin mediates binding of SRC to the receptor, targeting the complex for endocytosis. In addition, phosphorylation of dynamin Tyr residues mediated by SRC is essential for its endocytosis44
. Thus, SRC activity is required for the endocytosis of its associated receptor, which may lead to its sequestration in MVBs and the depletion of its cytoplasmic activity. Similarly to SRC, GSK3 is also required for its own endocytosis, as it must phosphorylate the cytoplasmic tail of the LRP6 receptor for WNT receptor complexes to be assembled and internalized13,14
. Intriguingly, the β2 adrenergic receptor requires HRS for its resensitization at the membrane, suggesting that this GPCR is recycled back to the plasma membrane45
. Whether SRC does indeed accumulate in MVBs, and what cytosolic targets this may afford protection for, remains to be seen.
The JAK–STAT pathway may also require endosomal trafficking. There have been mixed reports as to whether inhibition of HRS in D. melanogaster
the nuclear accumulation of the transcription factor STAT. Further studies will be needed to assess the possible role of MVB sequestration in this pathway.
A positive function of MVBs has been documented for Notch signalling48
. Endocytosis of Notch upon ligand binding leads to its relocalization to a multivesicular endosomal compartment. Degradation of the Notch extracellular domain by lysosomal enzymes facilitates cleavage by the intra-membrane protease presenilin, releasing the Notch intracellular domain (NICD) into the cytoplasm; the NICD then translocates into the nucleus, where it helps to initiate transcription49
. Two possibilities have been proposed for Delta–Notch interactions in multivesicular endosomes50
. Notch might remain in the outer endosomal membrane bound to Delta located in ILVs; Delta binding would release the NICD into the cytosol for its subsequent translocation into the nucleus. Alternatively, the Notch receptor could be incorporated in ILVs with Delta remaining on the outer endosomal membrane; in this model, after digestion by lysosomal enzymes and presenilins, the still-intact NICD would be delivered to the cytosol by back-fusion of ILVs to the outer endosomal membrane50
In summary, the ESCRT machinery and MVBs seem to have two principal roles in growth factor signalling. First, as has long been known, endosomal trafficking to lysosomes negatively regulates signalling downstream of many growth factors by degrading their receptors. Second, ESCRT-dependent removal of cytosolic inhibitory components, such as GSK3 in the WNT pathway and IκB in the NF-κB pathway, is starting to emerge as a positive function of multivesicular endosomes in signalling.