Mechanisms of Carrier Formation during Clathrin-Independent Endocytosis

Abstract

Clathrin-independent endocytosis (CIE) mediates the cellular uptake of many extracellular ligands, receptors, and pathogens, including several life-threatening bacterial toxins and viruses. So far, our understanding of CIE carrier formation has lagged behind that of clathrin-coated vesicles. Impediments have been the imprecise definition of some CIE pathways, the lack of specific cargoes being transported and of exclusive cytosolic markers and regulators. Notwithstanding these limitations, three distinct molecular mechanisms by which CIE carriers form can be defined. Cargo capture by cytosolic proteins is the main mechanism used by fast endophilin-mediated endocytosis (FEME) and interleukin 2 receptor (IL-2R) endocytosis. Acute signaling-induced membrane remodeling drives macropinocytosis. Finally, extracellular lipid or cargo clustering by the glycolipid-lectin (GL-Lect) hypothesis mediates the uptake of Shiga and cholera toxins and receptors by the CLIC/GEEC pathway. Here, we review these mechanisms and highlight current gaps in knowledge that will need to be addressed to complete our understanding of CIE.

Clathrin-independent endocytosis: an increasing degree of complexity

“…Also, particles with the plant toxin ricin immobilized on the surface can, in contrast to free ricin, induce a macropinocytotic process (Iversen et al 2012). Moreover, as suggested by Ferreira and Boucrot (2018), results reported for the uptake of IL-2 (interleukin 2) receptor beta subunit might be affected by use of antibodies against this subunit.…”

“…Another challenge it is that one and the same protein can be involved in more than one endocytic pathway, an example being endophilin A2 which plays a role both for clathrin-dependent and clathrin-independent uptake (Boucrot et al 2015; Ferreira and Boucrot 2018; Renard et al 2015; Hohendahl et al 2017). Thus, modifying the level or transfecting with mutants are likely to affect different mechanisms.…”

“…Similarly, Cdc42 is important for both macropinocytosis, FEME as well as for the dynamin-independent CLIC/GEEC (clathrin-independent carrier/glycosylphosphatidylinositol (GPI)-anchored protein-enriched endosomal compartments) pathway (Howes et al 2010a; Sabharanjak et al 2002; Ferreira and Boucrot 2018). However, inhibition of Cdc42 may increase FEME (Boucrot et al 2015).…”

“…This includes Rac1, Pak and phosphatidylinositol 3-kinase (PI3K), so during the years the mechanistic insight has increased, and the requirements for uptake seem to fit with the recently described FEME mechanism (Boucrot et al 2015). However, although IL-2 was reported to be internalized by FEME in T cells (Boucrot et al 2015), it has been suggested that the characteristics found for uptake of IL-2 receptor subunits after crosslinking with antibodies are affected by the antibodies, and that the receptor subunits in spite of many similarities are endocytosed by a different mechanism than FEME (Ferreira and Boucrot 2018). A recent publication reports that endophilin A2 is required for dynamin recruitment during endocytosis of the IL-2 receptor (Bertot et al 2018).…”

“…Perhaps a bit surprising, this Cdc42-dependent form of endocytosis is dynamin independent even though one of the proteins implicated in the process, GRAF1, is able to bind dynamin (Ferreira and Boucrot 2018). However, in this context GRAF1 seems to function as a GAP for Cdc42 (Bendris and Schmid 2017).…”