Linton Mark Traub (1962-2020)

Meir Aridor and David Owen discuss the life and achievements of Linton Traub, who passed away on October 19, 2020.

Endocytic Adaptor Proteins in Health and Disease: Lessons from Model Organisms and Human Mutations

Cells need to exchange material and information with their environment. This is largely achieved via cell-surface receptors which mediate processes ranging from nutrient uptake to signaling responses. Consequently, their surface levels have to be dynamically controlled. Endocytosis constitutes a powerful mechanism to regulate the surface proteome and to recycle vesicular transmembrane proteins that strand at the plasma membrane after exocytosis. For efficient internalization, the cargo proteins need to be linked to the endocytic machinery via adaptor proteins such as the heterotetrameric endocytic adaptor complex AP-2 and a variety of mostly monomeric endocytic adaptors. In line with the importance of endocytosis for nutrient uptake, cell signaling and neurotransmission, animal models and human mutations have revealed that defects in these adaptors are associated with several diseases ranging from metabolic disorders to encephalopathies. This review will discuss the physiological functions of the so far known adaptor proteins and will provide a comprehensive overview of their links to human diseases.

Melatonin promotes human oocyte maturation and early embryo development by enhancing clathrin-mediated endocytosis

Embryo development potential and reproductive clinical outcomes are all deeply rooted in oocyte maturation. Melatonin has been reported to promote oocyte maturation as an antioxidant in nonprimate species. Its antioxidative functions also help reduce plasma membrane rigidity, which facilitates clathrin-mediated endocytosis (CME). Whether melatonin has effects on human oocyte maturation by regulating CME is worthy of exploration. In this study, we found that the optimal melatonin concentration for human oocyte maturation was 10^-11  M, and the maturation rate of this group was 71.9% (P = .03). The metaphase II (MII) stage oocytes obtained by in vitro maturation with 10^-11  M melatonin had a significantly higher fertilization rate (81.4% vs 61.4%, respectively, P = .017) and blastocyst rate (32.2% vs 15.8%, respectively, P = .039) compared to controls. During maturation, antioxidative melatonin greatly enhanced CME and decreased intra-oocyte cAMP level. The former was evidenced by the increasing numbers of coated pits and vesicles, and the upregulated expression of two major CME markers-clathrin and adaptor protein-2 (AP2). CME inhibitor dynasore increased intra-oocyte cAMP level and blocked oocyte maturation, and melatonin could partly rescue oocyte maturation and significantly elevate the expression of clathrin and AP2 in the presence of dynasore. Therefore, we conclude that melatonin could promote human oocyte maturation and early embryo development through enhancing CME.

Molecular and functional characterization of the American cockroach, Periplaneta americana, Rab5: the first exopterygotan low molecular weight ovarian GTPase during oogenesis

The small Rab GTPases are key regulators of membrane vesicle trafficking. Ovaries of Periplaneta americana (Linnaeus) (Blattodea: Blattidae) have small molecular weight GTP/ATP-binding proteins during early and late vitellogenic periods of oogenesis. However, the identification and characterization of the detected proteins have not been yet reported. Herein, we cloned a cDNA encoding Rab5 from the American cockroach, P. americana, ovaries (PamRab5). It comprises 796 bp, encoding a protein of 213 amino acid residues with a predicted molecular weight of 23.5 kDa. PamRab5 exists as a single-copy gene in the P. americana genome, as revealed by Southern blot analysis. An approximate 2.6 kb ovarian mRNA was transcribed especially at high levels in the previtellogenic ovaries, detected by Northern blot analysis. The muscle and head tissues also showed high levels of PamRab5 transcript. PamRab5 protein was localized, via immunofluorescence labeling, to germline-derived cells of the oocytes, very early during oocyte differentiation. Immunoblotting detected a ∼25 kDa signal as a membrane-associated form revealed after application of detergent in the extraction buffer, and 23 kDa as a cytosolic form consistent with the predicted molecular weight from amino acid sequence in different tissues including ovary, muscles and head. The PamRab5 during late vitellogenic periods is required to regulate the endocytotic machinery during oogenesis in this cockroach. This is the first report on Rab5 from a hemimetabolan, and presents an inaugural step in probing the molecular premises of insect oocyte endocytotic trafficking important for oogenesis and embryonic development.

Targeting receptor-mediated endocytotic pathways with nanoparticles: rationale and advances

Targeting of drugs and their carrier systems by using receptor-mediated endocytotic pathways was in its nascent stages 25 years ago. In the intervening years, an explosion of knowledge focused on design and synthesis of nanoparticulate delivery systems as well as elucidation of the cellular complexity of what was previously-termed receptor-mediated endocytosis has now created a situation when it has become possible to design and test the feasibility of delivery of highly specific nanoparticle drug carriers to specific cells and tissue. This review outlines the mechanisms governing the major modes of receptor-mediated endocytosis used in drug delivery and highlights recent approaches using these as targets for in vivo drug delivery of nanoparticles. The review also discusses some of the inherent complexity associated with the simple shift from a ligand-drug conjugate versus a ligand-nanoparticle conjugate, in terms of ligand valency and its relationship to the mode of receptor-mediated internalization.

High-affinity Dkk1 receptor Kremen1 is internalized by clathrin-mediated endocytosis

Kremens are high-affinity receptors for Dickkopf 1 (Dkk1) and regulate the Wnt/β-catenin signaling pathway by down-regulating the low-density lipoprotein receptor-related protein 6 (LRP6). Dkk1 competes with Wnt for binding to LRP6; binding of Dkk1 inhibits canonical signaling through formation of a ternary complex with Kremen. The majority of down-regulated clathrin-mediated endocytic receptors contain short conserved regions that recognize tyrosine or dileucine sorting motifs. In this study, we found that Kremen1 is internalized from the cell surface in a clathrin-dependent manner. Kremen1 contains an atypical dileucine motif with the sequence DXXXLV. Mutation of LV to AA in this motif blocked Kremen1 internalization; as reported previously for other proteins, the aspartic acid residue in Kremen1 is not critical. Inhibition of expression of the adaptor protein 2 (AP-2) or inhibition of clathrin by pitstop 2 also blocked Kremen1 internalization. The novel amino acid sequence identified in Kremen1 is similar to the motif previously identified in hydra, yeast, and other organisms known to signal from the trans-Golgi network to the endosomal compartment.

The apoptotic engulfment protein Ced-6 participates in clathrin-mediated yolk uptake in Drosophila egg chambers

During oogenesis in Drosophila, the phagocytic engulfment protein Ced-6 recognizes the atypical endocytic sorting signal within the vitellogenin receptor Yolkless. Because Ced-6 displays all of the features of an authentic clathrin adaptor, an unrecognized clathrin dependence for Ced-6/Gulp operation during phagocytosis is possible.

Clathrin-mediated endocytosis and phagocytosis are both selective surface internalization processes but have little known mechanistic similarity or interdependence. Here we show that the phosphotyrosine-binding (PTB) domain protein Ced-6, a well-established phagocytosis component that operates as a transducer of so-called “eat-me” signals during engulfment of apoptotic cells and microorganisms, is expressed in the female Drosophila germline and that Ced-6 expression correlates with ovarian follicle development. Ced-6 exhibits all the known biochemical properties of a clathrin-associated sorting protein, yet ced-6–null flies are semifertile despite massive accumulation of soluble yolk precursors in the hemolymph. This is because redundant sorting signals within the cytosolic domain of the Drosophila vitellogenin receptor Yolkless, a low density lipoprotein receptor superfamily member, occur; a functional atypical dileucine signal binds to the endocytic AP-2 clathrin adaptor directly. Nonetheless, the Ced-6 PTB domain specifically recognizes the noncanonical Yolkless FXNPXA sorting sequence and in HeLa cells promotes the rapid, clathrin-dependent uptake of a Yolkless chimera lacking the distal dileucine signal. Ced-6 thus operates in vivo as a clathrin adaptor. Because the human Ced-6 orthologue GULP similarly binds to clathrin machinery, localizes to cell surface clathrin-coated structures, and is enriched in placental clathrin-coated vesicles, new possibilities for Ced-6/Gulp operation during phagocytosis must be considered.

Dab2 regulates clathrin assembly and cell spreading

The recruitment of clathrin to the membrane and its assembly into coated pits results from its interaction with endocytic adaptors and other regulatory proteins in the context of a specific lipid microenvironment. Dab2 (disabled 2) is a mitotic phosphoprotein and a monomeric adaptor for clathrin-mediated endocytosis. In the present study, we employed GFP (green fluorescent protein) fusion constructs of different isoforms and mutants of rat Dab2 and characterized their effect on the size, distribution and dynamics of clathrin assemblies. Enhanced levels of expression of the p82 isoform of Dab2 in COS7 cells induced enlarged clathrin assemblies at the plasma membrane. p82-clathrin assemblies, which concentrate additional endocytic proteins, such as AP2 (adaptor protein 2) and epsin, are dynamic structures in which both p82 and clathrin exchange actively between the membrane-bound and cytosolic sub-populations. The ability of p82 to induce enlarged clathrin assemblies is dependent on the presence of a functional PTB domain (phosphotyrosine-binding domain), on binding to clathrin and phospholipids, and on a newly identified and evolutionarily conserved poly-lysine stretch which precedes the PTB domain. These same molecular features are required for Dab2 to enhance the spreading of COS7 cells on fibronectin. The ability of the p82 isoform of Dab2 to enhance cell spreading was confirmed in both HeLa cells and HBL cells (human breast epithelial cells). COS7 cells expressing GFP-p82 and plated on to fibronectin concentrate the beta1 integrin into clathrin-p82 assemblies. Furthermore, during cell spreading, p82-clathrin assemblies concentrate at the site of the initial cell-matrix contact and are absent from regions of intense membrane ruffling. We propose a role for Dab2 and clathrin in integrin-mediated cell spreading.