Mechanisms of EndocytosisAn Overview

Routes of Endocytosis

Endocytosis in Depth

Signal Pathway

Dynaminy

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Dynamin is a GTPase responsible for endocytosis in the eukaryotic cell. Dynamins are principally involved in the scission of newly formed vesicles from the membrane of one cellular compartment and their targeting to, and fusion with, another compartment, both at the cell surface (particularly caveolae internalization) as well as at the Golgi apparatus. Dynamin has been extensively studied within clathrin-coated vesicle budding from the cell membrane. As a vesicle invaginates, dynamin forms a spiral around the neck of the vesicle. Once the spiral is in place, it extends lengthwise and constricts through GTP hydrolysis. This lengthening and tightening of the coil around the vesicle neck causes it to break and results in the pinching off of the vesicle from the parent membrane. An example of a vesicle is a clathrin-coated pit.

GTPGTP-induced Constriction by Dynamin

Three-dimensional maps of dynamin in the non-constricted (left) and constricted (right) states (Chen et al, 2004). Dynamin undergoes a large conformational change upon GTP hydrolysis which leads to a significant constriction of underlying lipid bilayer.

CaveolaeCaveolae-dependent Endocytosis

Models for regulation of caveolar budding

Nichols, B. J Cell Sci 2003;116:4707-4714

CaveolinCaveolin-mediated Internalization

Caveosomesy

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It is now clear that uptake by such a clathrin-independent mechanism can lead to delivery to caveolin-1-containing endosomes, termed caveosomes. Endocytosis to caveosomes is blocked by overexpression of a dynamin 2 K44A mutant and by cholesterol depletion, although neither of these treatments is likely to be highly specific. Caveosomes are separate from the early and recycling endosomes fed by uptake from clathrin-coated pits and were first characterized as intermediates in the trafficking of SV40 virus from the cell surface to the endoplasmic reticulum (ER). They are also likely to function constitutively in non-infected cells during transport of sphingolipids and GPI-linked proteins from the plasma membrane to the Golgi apparatus. Several more recent papers have focused on the role of trafficking via caveolin-positive endosomes without using the term caveosome. For simplicity, I will refer to all caveolin-1-containing endosomes as caveosomes.

Endocytosis of lipid-anchored proteins lipid-

CLathrin-Independent Carriers (CLICs) CLathrinand GPI-Enriched Endocytic PICompartments (GEECs)

This CLIC/GEEC pathway relies upon cellular signaling and activation through small G proteins, but mechanistic insight into the biogenesis of its tubular and tubulovesicular carriers is lacking. Rho-GAP-domain-containing protein GRAF1 marks, and is indispensable for, a major Clathrin-independent endocytic pathway. This pathway is characterized by its ability to internalize bacterial exotoxins, GPI-linked proteins, and extracellular fluid. GRAF1 localizes to PtdIns(4,5)P2-enriched, tubular, and punctate lipid structures via Nterminal BAR and PH domains. These membrane carriers are relatively devoid of caveolin1 and flotillin1 but are associated with activity of the small G protein Cdc42.

ClathrinClathrin-independent carriers (CLIC)y

Clathrin-independent carriers (CLICs) are prevalent tubulovesicular membranes responsible for non-clathrin mediated endocytic events. They appear to endocytose material into GEECs. Collectively, CLICs and GEECs comprise the CLIC/GEEC endocytic pathway, which is regulated by GRAF1.

GPI-Enriched Endocytic Compartments PI(GEECs)y

GEECs (or GPI-AP-enriched early endosomal compartments) are the endosomal compartment[clarification needed] associated with the CLIC/GEEC endocytic pathway, a highly prevalent clathrin-independent endocytic pathway regulated by GRAF1.

IL2RB, Arf6, and Flotillin-dependent Flotillin-

IL2RB Receptory

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This beta subunit is involved in receptor mediated endocytosis and transduces the mitogenic signals of IL2. IL2R exists in 3 different forms: a high affinity dimer, an intermediate affinity monomer (beta subunit), and a low affinity monomer (alpha subunit). The high and intermediate affinity forms also associate with a gamma subunit. Interacts with SHB upon interleukin stimulation. Interacts with HTLV-1 accessory protein p12I. Protein type: Receptor, cytokine Cellular Component: integral to plasma membrane; plasma membrane; external side of plasma membrane Molecular Function: protein binding; interleukin-2 receptor activity; receptor activity; interleukin-2 binding Biological Process: cytokine and chemokine mediated signaling pathway; interspecies interaction between organisms; protein complex assembly; signal transduction; positive regulation of survival gene product activity

Arf6y

ADP-ribosylation factor 6 (ARF6) is a member of the ADP ribosylation factor family of GTP-binding proteins. ARF6 has a variety of cellular functions that are frequently involved in trafficking of biological membranes and transmembrane protein cargo. ARF6 has specifically been implicated in endocytosis of plasma membrane proteins and also, to a lesser extent, plasma membrane protein recycling.

Flotilliny

Flotillin-1 is a protein that in humans is encoded by the FLOT1 gene. Caveolae are small domains on the inner cell membrane involved in vesicular trafficking and signal transduction. FLOT1 encodes a caveolae-associated, integral membrane protein. The function of flotillin 1 has not been determined. Flotillin-2 is a protein in humans encoded by the FLOT2 gene.

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Phagocytosis - Processy 1. 2. 3.

Phagocytosis in three steps: Unbound phagocyte surface receptors do not trigger phagocytosis. Binding of receptors causes them to cluster. Phagocytosis is triggered and the particle is taken up by the phagocyte.

Macropinocytosis Brief Overview

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Macropinocytosis is a form of endocytosis that accompanies cell surface ruffling. It is distinct in many ways from the better characterized micropinocytosis, which includes clathrin-coated vesicle endocytosis and small uncoated vesicles. Because macropinosomes are relatively large, they provide an efficient route for non-selective endocytosis of solute macromolecules. This route may facilitate MHC-class-II-restricted antigen presentation by dendritic cells. Because the ruffling that leads to macropinocytosis is regulated, it has been exploited by some pathogenic bacteria as a novel route for entry into cells.

Nature Review Endocytosis & Cancery

Hallmark of malignant cellsDefective vesicular trafficking of growth factor receptors Unbalanced recycling of integrin- and cadherin-based adhesion complexes

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Oncogenic alterations in endocytosisAltered ubiquitylation (Cbl and Nedd4 ubiquitin ligases) Altered cytoskeletal interactions Alterations to Rab family members

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ProposalPharmaceutical interception of the propensity of tumour cells to derail their signalling and their adhesion receptors may constitute a novel target for cancer therapy.

Aberrant Endocytosis of Transmembrane Proteins Contributes to Malignant Transformation

Aberrant Endocytosis of Transmembrane Proteins Contributes to Malignant Transformation (cont.)

Endocytic Mechanisms Underlying Dissolution of Cell-Cell Contacts Celland Loss of Tumor Cell Polarity

Mosesson, Mills, and Yarden. Nature Reviews: Derailed endocytosis an emerging feature of cancer.

Endocytosis and Cancer Polo, Pece, Pece, and Fiore (2004)y 1. 2. 3.

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Subversion of endocytic control is predicted to play a causative role in hyperproliferative conditions - cancer". Sustained signaling via interference of Cbl function by molecular circuitries involving: Src, Cdc42, or Spry2 Possibility that clathrin-mediated internalization of TGF-beta receptor would enhance signaling (Di Guglielmo, et al.) Loss of tumor-suppressor function of E-cadherin (transmembrane adhesion molecule that down-regulates the beta-catenin pathway and participates in epithelial tissue homeostasis by establishing homophilic interactions at sites of cell-to-cell contacts) Over-expression of Hakai (E3 ligase) causes perturbation of E-cadherinbased adherens junctions and enhances growth-factor-induced signaling leading to cell scattering Deregulation of Notch signaling The paper proposes that harnessing the endocytic machinery would prove to be a viable and promising cancer therapeutic strategy. It must be noted that the paper specifically focused on the basis of endocytic role in signal attenuation.