Receptor Heterodimerization Modulates Endocytosis through Collaborative and Competitive Mechanisms

Steric pressure between glycosylated transmembrane proteins inhibits internalization by endocytosis

Clathrin-mediated endocytosis is essential for the removal of transmembrane proteins from the plasma membrane in all eukaryotic cells. Many transmembrane proteins are glycosylated. These proteins collectively comprise the glycocalyx, a sugar-rich layer at the cell surface, which is responsible for intercellular adhesion and recognition. Previous work has suggested that glycosylation of transmembrane proteins reduces their removal from the plasma membrane by endocytosis. However, the mechanism responsible for this effect remains unknown. To study the impact of glycosylation on endocytosis, we replaced the ectodomain of the transferrin receptor, a well-studied transmembrane protein that undergoes clathrin-mediated endocytosis, with the ectodomain of MUC1, which is highly glycosylated. When we expressed this transmembrane fusion protein in mammalian epithelial cells, we found that its recruitment to endocytic structures was substantially reduced in comparison to a version of the protein that lacked the MUC1 ectodomain. This reduction could not be explained by a loss of mobility on the cell surface or changes in endocytic dynamics. Instead, we found that the bulky MUC1 ectodomain presented a steric barrier to endocytosis. Specifically, the peptide backbone of the ectodomain and its glycosylation each made steric contributions, which drove comparable reductions in endocytosis. These results suggest that glycosylation constitutes a biophysical signal for retention of transmembrane proteins at the plasma membrane. This mechanism could be modulated in multiple disease states that exploit the glycocalyx, from cancer to atherosclerosis.

Crossroads between membrane trafficking machinery and copper homeostasis in the nerve system

Imbalanced copper homeostasis and perturbation of membrane trafficking are two common symptoms that have been associated with the pathogenesis of neurodegenerative and neurodevelopmental diseases. Accumulating evidence from biophysical, cellular and in vivo studies suggest that membrane trafficking orchestrates both copper homeostasis and neural functions-however, a systematic review of how copper homeostasis and membrane trafficking interplays in neurons remains lacking. Here, we summarize current knowledge of the general trafficking itineraries for copper transporters and highlight several critical membrane trafficking regulators in maintaining copper homeostasis. We discuss how membrane trafficking regulators may alter copper transporter distribution in different membrane compartments to regulate intracellular copper homeostasis. Using Parkinson's disease and MEDNIK as examples, we further elaborate how misregulated trafficking regulators may interplay parallelly or synergistically with copper dyshomeostasis in devastating pathogenesis in neurodegenerative diseases. Finally, we explore multiple unsolved questions and highlight the existing challenges to understand how copper homeostasis is modulated through membrane trafficking.

Weakly Internalized Receptors Use Coated Vesicle Heterogeneity to Evade Competition during Endocytosis

The uptake of receptors by clathrin-mediated endocytosis underlies signaling, nutrient import, and recycling of transmembrane proteins and lipids. In the complex, crowded environment of the plasma membrane, receptors are internalized when they bind to components of the clathrin coat, such as the major adaptor protein, AP2. Receptors with higher affinity for AP2 are known to be more strongly internalized compared to receptors with lower affinity. However, it remains unclear how receptors with different affinities compete for space within crowded endocytic structures. To address this question, we constructed receptors with varying affinities for AP2 and allowed them to compete against one another during internalization. As expected, the internalization of a receptor with high affinity for AP2 was reduced when it was coexpressed with a competing receptor of similar affinity. However, receptors of low affinity for AP2 were surprisingly difficult to displace from endocytic structures, even when expressed alongside receptors with much higher affinity. To understand how these low-affinity receptors are protected from competition, we looked at AP2 heterogeneity across clathrin-coated structures. When we examined structures with lower-than-average AP2 content, we found that they were relatively enriched in cargo of low affinity for AP2 and depleted of cargo with high affinity. These findings suggest that the heterogeneity of adaptor protein content across the population of endocytic structures enables the internalization of diverse receptors. Given the critical role that internalization plays in signaling, this effect may help to prevent strongly internalized receptors from interfering with the cell's ability to process signals from weakly internalized receptors.