A specific group of transmembrane receptors, including the β1-adrenergic receptor (β1-AR), is internalized through a non-clathrin pathway known as Fast Endophilin Mediated Endocytosis (FEME). A key question is: how does the endocytic machinery assemble and how is it modulated by activated receptors during FEME. Here we show that endophilin, a major regulator of FEME, undergoes a phase transition into liquid-like condensates, which facilitates the formation of multi-protein assemblies by enabling the phase partitioning of endophilin binding proteins. The phase transition can be triggered by specific multivalent binding partners of endophilin in the FEME pathway such as the third intracellular loop (TIL) of the β1-AR, and the C-terminal domain of lamellipodin (LPD). Other endocytic accessory proteins can either partition into, or target interfacial regions of, these condensate droplets, and LPD also phase separates with the actin polymerase VASP. On the membrane, TIL promotes protein clustering in the presence of endophilin and LPD C-terminal domain. Our results demonstrate how the multivalent interactions between endophilin, LPD, and TIL regulate protein assembly formation on the membrane, providing mechanistic insights into the priming and initiation steps of FEME.

Liquid-liquid phase separation has recently emerged as an important fundamental organizational phenomenon in biological settings. Most studies of biological phase separation have focused on droplets that “condense” from solution above a critical concentration, forming so-called “membraneless organelles” suspended in solution. However, membranes are ubiquitous throughout cells, and many biomolecular condensates interact with membrane surfaces. Such membrane-associated phase-separated systems range from clusters of integral or peripheral membrane proteins in the plane of the membrane to free, spherical droplets wetting membrane surfaces to droplets containing small lipid vesicles. In this review, we consider phase-separated liquids that interact with membrane surfaces and we discuss the consequences of those interactions. The physical properties of distinct liquid phases in contact with bilayers can reshape the membrane, and liquid-liquid phase separation can construct membrane-associated protein structures, modulate their function, and organize collections of lipid vesicles dynamically. We summarize the common phenomena that arise in these systems of liquid phases and membranes.

Endophilin plays key roles during endocytosis of cellular receptors, including generating membrane curvature to drive internalization. Electrostatic interactions between endophilin’s BAR domain and anionic membrane lipids have been considered the major driving force in curvature generation. However, the SH3 domain of endophilin also interacts with the proline-rich third intracellular loop (TIL) of various G-protein coupled receptors (GPCRs), and it is unclear whether this interaction has a direct role in generating membrane curvature during endocytosis. To examine this, we designed model membranes with a membrane density of 1400 receptors per µm2 represented by a covalently conjugated TIL region from β1-adrenergic receptor. We observed that TIL recruits endophilin to membranes composed of 95 mol% of zwitterionic lipids via the SH3 domain. More importantly, endophilin recruited via TIL tubulates vesicles and gets sorted onto highly curved membrane tubules. These observations indicate that the cellular membrane bending and curvature sensing activities of endophilin can be facilitated through detection of the TIL of activated GPCRs in addition to binding to anionic lipids. Furthermore, we show that TIL electrostatically interacts with membranes composed of anionic lipids. Therefore, anionic lipids can modulate TIL/SH3 domain binding. Overall, our findings imply that an interplay between TIL, charged membrane lipids, BAR domain, and SH3 domain could exist in the biological system and that these components may act in coordination to regulate the internalization of cellular receptors.

Endocytosis of transmembrane receptors initiates via molecular interactions between the activated receptor and the endocytic machinery. A specific group of receptors, including the β1-adrenergic receptor (β1-AR), is internalized through a non-clathrin pathway known as Fast Endophilin Mediated Endocytosis (FEME). A key question is: how does the endocytic machinery assemble and how is it modulated by activated receptors during FEME. Here we show that endophilin, a major regulator of FEME, undergoes a phase transition into liquid-like condensates, which facilitates the formation of multi-protein assemblies by enabling the phase partitioning of endophilin binding proteins. The phase transition can be triggered by specific multivalent binding partners of endophilin in the FEME pathway such as the third intracellular loop (TIL) of the β1-AR, and the proline-rich-motifs of lamellipodin (LPD-PRMs). Other endocytic accessory proteins can either partition into, or target interfacial regions of, these condensate droplets. On the membrane, TIL promotes protein clustering in the presence of endophilin and LPD-PRMs. Our results demonstrate how the multivalent interactions between endophilin, LPD-PRMs and TIL regulate protein assembly formation on the membrane, providing mechanistic insights into the priming and initiation steps of FEME.

Background Peptide receptor radionuclide therapy (PRRT) is effective for treating midgut neuroendocrine tumors (NETs); however, incorporation of PRRT into routine practice in the U.S. is not well studied. Herein we analyze the first year of PRRT implementation to determine tolerance of PRRT and factors that increase risk of PRRT discontinuation. Materials and Methods Medical records were reviewed and data were abstracted on all patients with NETs scheduled for PRRT during the first year of PRRT implementation at a U.S. NET referral center (August 2018 through July 2019). Logistic regression was used to identify factors associated with PRRT discontinuation. Results Fifty‐five patients (56% male) were scheduled for PRRT over the study period. The most common primary NET location was small bowel (47%), followed by pancreas (26%), and 84% of the NETs were World Health Organization grade 1 or 2. The cohort was heavily pretreated with somatostatin analog (SSA) therapy (98%), non‐SSA systemic therapy (64%), primary tumor resection (73%), and liver‐directed therapy (55%). At the time of analysis, 52 patients completed at least one PRRT treatment. Toxicities including bone marrow suppression and liver function test (LFT) abnormalities were comparable to prior publications. Eleven patients (21%) prematurely discontinued PRRT because of toxicity or an adverse event. Pretreatment LFT abnormality was associated with increased risk of PRRT cancellation (odds ratio: 12; 95% confidence interval: 2.59–55.54; p < .001). Conclusion PRRT can be administered to a diverse NET population at a U.S. NET referral center. Baseline liver function test abnormality increases the likelihood of PRRT discontinuation. Implications for Practice Peptide receptor radionuclide therapy (PRRT) can be successfully implemented at a U.S. neuroendocrine tumor (NET) referral center in a NET population that is diverse in tumor location, grade, and prior treatment history. Toxicity and adverse effects of PRRT are comparable to prior reports; however, 21% of individuals prematurely discontinued PRRT. Patients with baseline liver function test abnormalities were more likely to discontinue PRRT than patients with normal liver function tests, which should be taken into consideration when selecting treatment options for NETs.