Phosphatidic acid induces EHD3-containing membrane tubulation and is required for receptor recycling.

EHD3 is localized on the tubular structures of early endosomes, and it regulates their trafficking pathway. However, the regulatory mechanism of EHD3-containing tubular structures remains poorly understood. An in vitro liposome co-sedimentation assay revealed that EHD3 interacted with phosphatidic acid through its helical domain and this interaction induced liposomal tubulations. Additionally, inhibiting phosphatidic acid synthesis with diacylglycerol kinase inhibitor or lysophosphatidic acid acyltransferase inhibitor significantly reduced the number of EHD3-containing tubules and impaired their trafficking from early endosomes. These results suggest that EHD3 and phosphatidic acid cooperatively regulate membrane deformation and trafficking from early endosomes.

PtdIns4KIIα generates endosomal PtdIns(4)P and is required for receptor sorting at early endosomes.

Phosphatidylinositol 4-kinase IIα (PtdIns4KIIα) localizes to the trans-Golgi network and endosomal compartments and has been implicated in the regulation of endosomal traffic, but the roles of both its enzymatic activity and the site of its action have not been elucidated. This study shows that PtdIns4KIIα is required for production of endosomal phosphatidylinositol 4-phosphate (PtdIns(4)P) on early endosomes and for the sorting of transferrin and epidermal growth factor receptor into recycling and degradative pathways. Depletion of PtdIns4KIIα with small interfering RNA significantly reduced the amount of vesicular PtdIns(4)P on early endosomes but not on Golgi membranes. Cells depleted of PtdIns4KIIα had an impaired ability to sort molecules destined for recycling from early endosomes. We further identify the Eps15 homology domain-containing protein 3 (EHD3) as a possible endosomal effector of PtdIns4KIIα. Tubular endosomes containing EHD3 were shortened and became more vesicular in PtdIns4KIIα-depleted cells. Endosomal PtdIns(4,5)P2 was also significantly reduced in PtdIns4KIIα-depleted cells. These results show that PtdIns4KIIα regulates receptor sorting at early endosomes through a PtdIns(4)P-dependent pathway and contributes substrate for the synthesis of endosomal PtdIns(4,5)P2.

The clathrin assembly protein PICALM is required for erythroid maturation and transferrin internalization in mice.

Phosphatidylinositol binding clathrin assembly protein (PICALM), also known as clathrin assembly lymphoid myeloid leukemia protein (CALM), was originally isolated as part of the fusion gene CALM/AF10, which results from the chromosomal translocation t(10;11)(p13;q14). CALM is sufficient to drive clathrin assembly in vitro on lipid monolayers and regulates clathrin-coated budding and the size and shape of the vesicles at the plasma membrane. However, the physiological role of CALM has yet to be elucidated. Here, the role of CALM in vivo was investigated using CALM-deficient mice. CALM-deficient mice exhibited retarded growth in utero and were dwarfed throughout their shortened life-spans. Moreover, CALM-deficient mice suffered from severe anemia, and the maturation and iron content in erythroid precursors were severely impaired. CALM-deficient erythroid cells and embryonic fibroblasts exhibited impaired clathrin-mediated endocytosis of transferrin. These results indicate that CALM is required for erythroid maturation and transferrin internalization in mice.

Fission of tubular endosomes triggers endosomal acidification and movement.

The early endosome acts as a sorting station for internalized molecules destined for recycling or degradation. While recycled molecules are sorted and delivered to tubular endosomes, residual compartments containing molecules to be degraded undergo "maturation" before final degradation in the lysosome. This maturation involves acidification, microtubule-dependent motility, and perinuclear localization. It is currently unknown how sorting and the processes of maturation cooperate with each other. Here, we show that fission of a tubular endosome triggers the maturation of the residual endosome, leading to degradation. Use of the dynamin inhibitor dynasore to block tubular endosome fission inhibited acidification, endosomal motility along microtubules, perinuclear localization, and degradation. However, tubular endosome fission was not affected by inhibiting endosomal acidification or by depolymerizing the microtubules. These results demonstrate that the fission of recycling tubules is the first important step in endosomal maturation and degradation in the lysosome. We believe this to be the first evidence of a cascade from sorting to degradation.