Redox Signaling in Endosomes Using the Example of EGF Receptors: A Graphical Review

Abstract

Early endosomes represent first-line sorting compartments or even organelles for internalized molecules. They enable the transport of molecules or ligands to other compartments of the cell, such as lysosomes, for degradation or recycle them back to the membrane by various mechanisms. Moreover, early endosomes function as signaling and scaffolding platforms to initiate or prolong distinct signaling pathways. Accordingly, early endosomes have to be recognized as either part of a degradation or recycling pathway. The physical proximity of many ligand-binding receptors with other membrane-bound proteins or complexes such as NADPH oxidases may result in an interaction of second messengers, like reactive oxygen species (ROS) and early endosomes, that promote the correct recognition of individual early endosomes. In fact, redoxosomes comprise an endosomal subsection of signaling endosomes. One example of such potential interaction is epidermal growth factor receptor (EGFR) signaling. Here we summarize recent findings on EGFR signaling as a well-studied example for receptor trafficking and trans-activation and illustrate the interplay between cellular and endosomal ROS.

Keywords: EGFR; early endosomes; reactive oxygen species; redoxosomes; trans-activation.

Figure 1

Overview of major endosomal subtypes and their charactericstic features. Overlapping circles indicate that individual subspecies of endosomes are not always clearly categorizable. Nox = NADPH oxidase; ROS = reactive oxygen species; SARA = SMAD anchor for receptor activation.

Figure 2

Structure of the early endosome. Early endosomes display a dynamic, pleiomorphic appearance with a cis site for incoming cargo and a trans region with tubular and multivesicular structures that differ in their pH according to the destination of cargo, either for recycling or degradation. The most important Rab proteins and exemplarily one of their effectors are displayed. Cargo exits the early endosome for further intracellular trafficking. Early endosomes are connected to both the microtubule organizing center (MTOC) of the cytoskeleton through annexins and to several motor proteins, which allow movement of early endosomes themselves and cargo delivery. The endosomal pH gradient is indicated by color. ACAP2 = ArfGAP with coiled coils, ankyrin repeat, and PH domains 2; APPL = adaptor protein, phosphotyrosine interacting with PH domain, and leucine zipper 1; EEA1 = early endosomal antigen 1; CRIM1 = cysteine-rich motor neuron 1; MICALL = Mical-like protein; OCRL = inositol polyphosphate 5-phosphatase; Rab = Ras-related protein; RUFY1 = RUN and FYVE domain-containing protein 1.

Figure 3

Dynamics and maturation of endosomes. Endocytosis of a dimeric receptor after ligand binding is shown. At early endosomes, the receptor is sorted for recycling or degradation and travels through the endosomal network. Key marker proteins of endosomal compartments are indicated. EEA1 = early endosome antigen 1; Rab = Ras-related protein; Lamp1 = lysosomal associated membrane protein 1.

Figure 4

EGF and MG6 receptor trafficking in detail. Clathrin-mediated endocytosis of EGFR and intracellular trafficking of M6RR are shown. EGFR may be translocated to other cellular compartments like nucleoplasm or mitochondria (not depicted here). No claim to completeness of molecules is shown. (1) Recruitment of major adaptor proteins to EGFR during internalization. The activated receptor dimers are internalized by various pathways depending on ligand type and concentration. The adapter protein Grb2 recruits the ubiquitin ligase Cbl to EGFR, causing mono- or polyubiquitination. Multiple Ub-binding proteins interact with the ubiquitinated EGFR. (2) Association of recycling machinery. Association of Rab4,11,13,35 leads to direct or indirect recycling via the endosomal recycling complex. Deubiquitinating enzymes replenish the free Ub pool. (3) Lysosomal targeting of EGFR. Dephosphorylation of EGF receptor tyrosine kinases by PTPs terminates signaling and promotes the formation of intraluminal vesicles/multivesicular bodies. EGFR interaction with Hrs and recruitment of the ESCRT complex are necessary for the late endosomal–lysosomal route. AMSH = associated molecule with the SH3 domain of STAM; Cbl = Casitas B-lineage lymphoma protein; CIN-85 = Cbl-interacting protein; Eps= epidermal growth factor receptor substrate 15; ESCRT = endosomal sorting complex required for transport; Hrs = hepatocyte growth factor-regulated tyrosine kinase; PTP1B = Phosphotyrosinephosphatase 1 B; SNX = sorting nexin; STAM = signal transducing adaptor molecule Substrate; Tsg101 = tumor susceptibility gene 101; Ub = ubiquitin; WASH = Wiskott–Aldrich syndrome protein and SCAR homolog.

Figure 5

ROS in endosomal sorting and signaling. Redox signaling is initiated by EGFR (1) at the plasma membrane (2) inside of endosomes. Redox-active endosomes (Redoxosomes) harbor Nox enzymes producing ROS downstream of several signaling cascades. Both cellular and endosomal ROS activate c-Src and MMPs followed by shedding of EGFR ligands. This EGFR trans-activation leads as a feed forward mechanism to further ROS creation by Nox. The termination of the ROS signaling by dismutation or other means is not part of the figure. ClC3 provides charge neutralization. ClC3 = chloride channel 3; cSrc = cellular sarcoma kinase; EGFR = epidermal growth factor receptor; IL1R = interleukin 1 receptor; MMP = matrix metalloprotease; Nox = NADPH oxidase; ROS = reactive oxygen species; TNFR = tumor necrosis factor receptor.