A unique platform for H-Ras signaling involving clathrin-independent endocytosis

Rasosomes spread Ras signals from plasma membrane 'hotspots'

Ras proteins regulate cell growth, differentiation, and apoptosis from various cellular platforms. We have recently identified a novel potential signaling platform, the rasosome, which moves rapidly near the plasma membrane (PM) and in the cytosol, carrying multiple copies of palmitoylated Ras proteins. In the present study we demonstrate that rasosomes are unique entities distinct from PM nanoclusters or from endocytotic compartments. In addition, we examine whether rasosomes can act as regulated Ras signaling platforms. We show that a single rasosome simultaneously carries different types of Ras molecules in their active and inactive state, suggesting that rasosomes can upload and download Ras signals. Total internal reflection fluorescence (TIRF) microscopy combined with fast time-lapse and a new spatial analysis algorithm demonstrate that rasosome movement near the PM is restricted to distinctive areas, rasosomal 'hotspots', localized between actin filament cages. In addition, Ras-binding domain of Raf-1 (RBD) is recruited to Ras in rasosomal hotspots as revealed by bimolecular fluorescence complementation experiments. Interestingly, epidermal growth factor stimulates H/NRas activation on rasosomes and the subsequent recruitment of RBD to rasosomes. Moreover, we show that rasosomes are loaded with Ras downstream effectors and modulators. These findings establish that physiological stimulation originating from PM hotspots is transduced to rasosomes, which appear to serve as robust Ras signaling platforms that spread signals across the cell.

Visualization of Ras-PI3K interaction in the endosome using BiFC

Recent studies indicate the importance of spatiotemporal regulation in the diversity and specificity of intracellular signaling. Here, we show that Ras-PI3K signaling plays an important role in the local regulation of phosphatidylinositol metabolism in the endosome through live-cell imaging by using a bimolecular fluorescence complementation technique, in which molecular interaction is indicated by fluorescence emission. Using several possible combinations of Ras and the Ras-binding domain, we identified an optimal set of probe molecules that yielded the most significant increase in fluorescence intensity between the active and inactive forms of Ras. This combination revealed that, among the Ras effectors tested, phosphatidylinositol 3-kinase (PI3K) was specifically implicated in signaling in the endosome. We also found that full length PI3K was recruited to the endosome in EGF- and Ras-dependent manners, which appears to be essential for the activation of PI3K in this compartment. Taken together, these findings demonstrate that the spatiotemporal regulation of Ras-PI3K signaling may dictate the activation of PI3K and subsequent downstream signaling in the endosome.

Dual acylation is required for trafficking of growth-associated protein-43 (GAP-43) to endosomal recycling compartment via an Arf6-associated endocytic vesicular pathway

GAP-43 (growth-associated protein-43) is a dually palmitoylated protein, at cysteine residues at positions 3 and 4, that mostly localizes in plasma membrane both in neural and non-neural cells. In the present study, we have examined membrane association, subcellular distribution and intracellular trafficking of GAP-43 in CHO (Chinese hamster ovary)-K1 cells. Using biochemical assays and confocal and video microscopy in living cells we demonstrated that GAP-43, at steady state, localizes at the recycling endosome in addition to the cytoplasmic leaflet of the plasma membrane and TGN (trans-Golgi network). Pharmacological inhibition of newly synthesized GAP-43 acylation or double mutation of Cys3 and Cys4 of GAP-43 completely disrupts TGN, plasma membrane and recycling endosome association. A combination of selective photobleaching techniques and time-lapse fluorescence microscopy reveals a dynamic association of GAP-43 with recycling endosomes in equilibrium with the plasma membrane pool. Newly synthesized GAP-43 is found mainly associated with the TGN, but not with the pericentriolar recycling endosome, and traffics to the plasma membrane by a brefeldin A-insensitive pathway. Impairment of plasma membrane fusion and internalization by treatment with tannic acid does affect the trafficking of GAP-43 from plasma membrane to recycling endosomes which reveals a vesicle-mediated retrograde trafficking of GAP-43. Here, we also show that internalization of GAP-43 is regulated by Arf (ADP-ribosylation factor) 6. Taken together, these results demonstrate that dual acylation is required for sorting of peripheral membrane-associated GAP-43 to recycling endosome via an Arf6-associated endocytic vesicular pathway.

Endocytosis and spatial restriction of cell signaling

Endocytosis and recycling are essential components of the wiring enabling cells to perceive extracellular signals and transduce them in a temporally and spatially controlled fashion, directly influencing not only the duration and intensity of the signaling output, but also their correct location. Here, we will discuss key experimental evidence that support how different internalization routes, the generation of diverse endomembrane platforms, and cycles of internalization and recycling ensure polarized compartmentalization of signals, regulating a number of physiological and pathologically-relevant processes in which the resolution of spatial information is vital for their execution.

Virus entry by macropinocytosis

As obligatory intracellular parasites, viruses rely on host-cell functions for most aspects of their replication cycle. This is born out during entry, when most viruses that infect vertebrate and insect cells exploit the endocytic activities of the host cell to move into the cytoplasm. Viruses belonging to vaccinia, adeno, picorna and other virus families have been reported to take advantage of macropinocytosis, an endocytic mechanism normally involved in fluid uptake. The virus particles first activate signalling pathways that trigger actin-mediated membrane ruffling and blebbing. Usually, this is followed by the formation of large vacuoles (macropinosomes) at the plasma membrane, internalization of virus particles and penetration by the viruses or their capsids into the cytosol through the limiting membrane of the macropinosomes. We review the molecular machinery involved in macropinocytosis and describe what is known about its role in virus entry.

Discovery of new cargo proteins that enter cells through clathrin-independent endocytosis

Clathrin-independent endocytosis (CIE) allows internalization of plasma membrane proteins lacking clathrin-targeting sequences, such as the major histocompatibility complex class I protein (MHCI), into cells. After internalization, vesicles containing MHCI fuse with transferrin-containing endosomes generated from clathrin-dependent endocytosis. In HeLa cells, MHCI is subsequently routed to late endosomes or recycled back out to the plasma membrane (PM) in distinctive tubular carriers. Arf6 is associated with endosomal membranes carrying CIE cargo and expression of an active form of Arf6 leads to the generation of vacuolar structures that trap CIE cargo immediately after endocytosis, blocking the convergence with transferrin-containing endosomes. We isolated these trapped vacuolar structures and analyzed their protein composition by mass spectrometry. Here we identify and validate six new endogenous cargo proteins (CD44, CD55, CD98, CD147, Glut1, and ICAM1) that use CIE to enter cells. CD55 and Glut1 appear to closely parallel the trafficking of MHCI, merging with transferrin endosomes before entering the recycling tubules. In contrast, CD44, CD98, and CD147 appear to directly enter the recycling tubules and by-pass the merge with EEA1-positive, transferrin-containing endosomes. This divergent itinerary suggests that sorting may occur along this CIE pathway. Furthermore, the identification of new cargo proteins will assist others studying CIE in different cell types and tissues.

Eps15 homology domain 1-associated tubules contain phosphatidylinositol-4-phosphate and phosphatidylinositol-(4,5)-bisphosphate and are required for efficient recycling

The C-terminal Eps15 homology domain (EHD) 1/receptor-mediated endocytosis-1 protein regulates recycling of proteins and lipids from the recycling compartment to the plasma membrane. Recent studies have provided insight into the mode by which EHD1-associated tubular membranes are generated and the mechanisms by which EHD1 functions. Despite these advances, the physiological function of these striking EHD1-associated tubular membranes remains unknown. Nuclear magnetic resonance spectroscopy demonstrated that the Eps15 homology (EH) domain of EHD1 binds to phosphoinositides, including phosphatidylinositol-4-phosphate. Herein, we identify phosphatidylinositol-4-phosphate as an essential component of EHD1-associated tubules in vivo. Indeed, an EHD1 EH domain mutant (K483E) that associates exclusively with punctate membranes displayed decreased binding to phosphatidylinositol-4-phosphate and other phosphoinositides. Moreover, we provide evidence that although the tubular membranes to which EHD1 associates may be stabilized and/or enhanced by EHD1 expression, these membranes are, at least in part, pre-existing structures. Finally, to underscore the function of EHD1-containing tubules in vivo, we used a small interfering RNA (siRNA)/rescue assay. On transfection, wild-type, tubule-associated, siRNA-resistant EHD1 rescued transferrin and beta1 integrin recycling defects observed in EHD1-depleted cells, whereas expression of the EHD1 K483E mutant did not. We propose that phosphatidylinositol-4-phosphate is an essential component of EHD1-associated tubules that also contain phosphatidylinositol-(4,5)-bisphosphate and that these structures are required for efficient recycling to the plasma membrane.

A phosphoinositide switch controls the maturation and signaling properties of APPL endosomes

The recent identification of several novel endocytic compartments has challenged our current understanding of the topological and functional organization of the endocytic pathway. Using quantitative single vesicle imaging and acute manipulation of phosphoinositides we show that APPL endosomes, which participate in growth factor receptor trafficking and signaling, represent an early endocytic intermediate common to a subset of clathrin derived endocytic vesicles and macropinosomes. Most APPL endosomes are precursors of classical PI3P positive endosomes, and PI3P plays a critical role in promoting this conversion. Depletion of PI3P causes a striking reversion of Rab5 positive endosomes to the APPL stage, and results in enhanced growth factor signaling. These findings reveal a surprising plasticity of the early endocytic pathway. Importantly, PI3P functions as a switch to dynamically regulate maturation and signaling of APPL endosomes.

Kaposi's sarcoma-associated herpesvirus utilizes an actin polymerization-dependent macropinocytic pathway to enter human dermal microvascular endothelial and human umbilical vein endothelial cells

Kaposi's sarcoma-associated herpesvirus (KSHV) utilizes clathrin-mediated endocytosis for its infectious entry into human foreskin fibroblast (HFF) cells (S. M. Akula, P. P. Naranatt, N.-S. Walia, F.-Z. Wang, B. Fegley, and B. Chandran, J. Virol. 77:7978-7990, 2003). Here, we characterized KSHV entry into primary human microvascular dermal endothelial (HMVEC-d) and human umbilical vein endothelial (HUVEC) cells. Similar to the results for HMVEC-d cells, KSHV infection of HUVEC cells also resulted in an initial high level and subsequent decline in the expression of the lytic switch gene, ORF50, while latent gene expression persisted. Internalized virus particles enclosed in irregular vesicles were observed by electron microscopy of infected HMVEC-d cells. At an early time of infection, colocalization of KSHV capsid with envelope was observed by immunofluorescence analysis, thus demonstrating endocytosis of intact enveloped virus particles. Chlorpromazine, an inhibitor of clathrin-mediated endocytosis, and filipin (C(35)H(58)O(11)), a caveolar endocytosis inhibitor, did not have any effect on KSHV binding, entry (DNA internalization), or gene expression in HMVEC-d and HUVEC cells. In contrast to the results for HFF cells, virus entry and gene expression in both types of endothelial cells were significantly blocked by macropinocytosis inhibitors (EIPA [5-N-ethyl-N-isoproamiloride] and rottlerin [C(30)H(28)O(8)]) and by cytochalasin D, which affects actin polymerization. Inhibition of lipid raft blocked viral gene expression in HMVEC-d cells but not in HUVEC or HFF cells. In HMVEC-d and HUVEC cells, KSHV induced the actin polymerization and formation of lamellipodial extensions that are essential for macropinocytosis. Inhibition of macropinocytosis resulted in the distribution of viral capsids at the HMVEC-d cell periphery, and capsids did not associate with microtubules involved in the nuclear delivery of viral DNA. Internalized KSHV in HMVEC-d and HUVEC cells colocalized with the macropinocytosis marker dextran and not with the clathrin pathway marker transferrin or with caveolin. Dynasore, an inhibitor of dynamin, did not block viral entry into endothelial cells but did inhibit entry into HFF cells. KSHV was not associated with the early endosome marker EEA-1 in HMVEC-d cells, but rather with the late endosome marker LAMP1, as well as with Rab34 GTPase that is known to regulate macropinocytosis. Silencing Rab34 with small interfering RNA dramatically inhibited KSHV gene expression. Bafilomycin-mediated disruption of endosomal acidification inhibited viral gene expression. Taken together, these findings suggest that KSHV utilizes the actin polymerization-dependent, dynamin-independent macropinocytic pathway that involves a Rab34 GTPase-dependent late endosome and low-pH environment for its infectious entry into HMVEC-d and HUVEC cells. These studies also demonstrate that KSHV utilizes different modes of endocytic entry in fibroblast and endothelial cells.

Clathrin-independent endocytosis: a unique platform for cell signaling and PM remodeling

There is increasing interest in endocytosis that occurs independently of clathrin coats and the fates of membrane proteins internalized by this mechanism. The appearance of clathrin-independent endocytic and membrane recycling pathways seems to vary with different cell types and cargo molecules. In this review we focus on studies that have been performed using HeLa and COS cells as model systems for understanding this membrane trafficking system. These endosomal membranes contain signaling molecules including H-Ras, Rac1, Arf6 and Rab proteins, and a lipid environment rich in cholesterol and PIP(2) providing a unique platform for cell signaling. Furthermore, activation of some of these signaling molecules (H-Ras, Rac and Arf6) can switch the constitutive form of clathrin-independent endocytosis into a stimulated one, associated with PM ruffling and macropinocytosis.

Shaping cups into phagosomes and macropinosomes

The ingestion of particles or cells by phagocytosis and of fluids by macropinocytosis requires the formation of large endocytic vacuolar compartments inside cells by the organized movements of membranes and the actin cytoskeleton. Fc-receptor-mediated phagocytosis is guided by the zipper-like progression of local, receptor-initiated responses that conform to particle geometry. By contrast, macropinosomes and some phagosomes form with little or no guidance from receptors. The common organizing structure is a cup-shaped invagination of the plasma membrane that becomes the phagosome or macropinosome. Recent studies, focusing on the physical properties of forming cups, indicate that a feedback mechanism regulates the signal transduction of phagocytosis and macropinocytosis.