Bound simian virus 40 translocates to caveolin-enriched membrane domains, and its entry is inhibited by drugs that selectively disrupt caveolae

Molecular characterization of caveolin association with the Golgi complex: identification of a cis-Golgi targeting domain in the caveolin molecule

Caveolins are integral membrane proteins which are a major component of caveolae. In addition, caveolins have been proposed to cycle between intracellular compartments and the cell surface but the exact trafficking route and targeting information in the caveolin molecule have not been defined. We show that antibodies against the caveolin scaffolding domain or against the COOH terminus of caveolin-1 show a striking specificity for the Golgi pool of caveolin and do not recognize surface caveolin by immunofluorescence. To analyze the Golgi targeting of caveolin in more detail, caveolin mutants were expressed in fibroblasts. Specific mutants lacking the NH2 terminus were targeted to the cis Golgi but were not detectable in surface caveolae. Moreover, a 32-amino acid segment of the putative COOH-terminal cytoplasmic domain of caveolin-3 was targeted specifically and exclusively to the Golgi complex and could target a soluble heterologous protein, green fluorescent protein, to this compartment. Palmitoylation-deficient COOH-terminal mutants showed negligible association with the Golgi complex. This study defines unique Golgi targeting information in the caveolin molecule and identifies the cis Golgi complex as an intermediate compartment on the caveolin cycling pathway.

Dominant-negative caveolin inhibits H-Ras function by disrupting cholesterol-rich plasma membrane domains

The plasma membrane pits known as caveolae have been implicated both in cholesterol homeostasis and in signal transduction. CavDGV and CavKSY, two dominant-negative amino-terminal truncation mutants of caveolin, the major structural protein of caveolae, significantly inhibited caveola-mediated SV40 infection, and were assayed for effects on Ras function. We find that CavDGV completely blocked Raf activation mediated by H-Ras, but not that mediated by K-Ras. Strikingly, the inhibitory effect of CavDGV on H-Ras signalling was completely reversed by replenishing cell membranes with cholesterol and was mimicked by cyclodextrin treatment, which depletes membrane cholesterol. These results provide a crucial link between the cholesterol-trafficking role of caveolin and its postulated role in signal transduction through cholesterol-rich surface domains. They also provide direct evidence that H-Ras and K-Ras, which are targeted to the plasma membrane by different carboxy-terminal anchors, operate in functionally distinct microdomains of the plasma membrane.

Chemokine receptor trafficking and viral replication

Chemokines and chemokine receptors have emerged as crucial factors controlling the development and function of leukocytes. Recent studies have indicated that, in addition to these essential roles, both chemokines and chemokine receptors play critical roles in viral infection and replication. Not only are chemokine receptors key components of the receptor/fusion complexes of primate immunodeficiency viruses, but chemokines can also influence virus entry and infection. Many viruses, in particular herpesviruses, encode chemokines and chemokine receptors that influence the replication of both the parent virus and other unrelated viruses. The cell surface expression of the chemokine receptors is regulated through their interaction with membrane trafficking pathways. Ligands induce receptor internalization and downmodulation through endocytosis, and recycling is regulated within endosomes. Part of the mechanism through which chemokines protect cells from HIV infection is through ligand-induced internalization of the specific chemokine receptor co-receptors. In addition, mechanisms may exist to regulate the trafficking of newly synthesized receptors to the cell surface. Here we discuss aspects of the mechanisms through which chemokine receptors interact with membrane-trafficking pathways and the influence of these interactions on viral replication.

Simian virus 40 infection via MHC class I molecules and caveolae

MHC class I molecules are a necessary component of the cell surface receptor for simian virus 40 (SV40). After binding to class I molecules, SV40 enters cells via a unique endocytic pathway that involves caveolae, rather than clathrin-coated pits. This pathway is dependent on a transmembrane signal that SV40 transmits from the cell surface. Furthermore, it delivers SV40 to the endoplasmic reticulum, rather than to the endosomal/lysosomal compartment, which is the usual target for endocytic traffic. The glycosphingolipid and cholesterol-enriched plasma membrane domains that contain caveolae are also enriched for class I molecules, relative to whole plasma membrane. Nevertheless, although class I molecules bind SV40, they do not enter with SV40, nor do they enter spontaneously into uninfected SV40 host cells. Instead, they are shed from the cell surface by the activity of a metalloprotease. These results imply the existence of a putative secondary receptor for SV40 that might mediate SV40 entry. It is not yet clear whether class I molecules are active in transmitting the SV40 signal. Monoclonal antibodies against class I molecules also induce a signal in the SV40 host cells. However, the antibody-induced signal is mediated by mitogen-activated protein kinase (MAP kinase), whereas the SV40 signal is independent of MAP kinase.

Exploitation of major histocompatibility complex class I molecules and caveolae by simian virus 40

Simian virus 40 (SV40), a non-enveloped DNA virus, is transported from the cell surface to the nucleus where virus replication occurs. This pathway of virus uptake involves binding to surface MHC class I molecules, entry via non-coated pits, and subsequent transport to the endoplasmic reticulum (ER). At some stage in this pathway the virus must cross a membrane to reach the cytosol. In the present review, the cellular machinery which the virus has utilized to enter the cell will be examined. In particular, we will consider recent evidence for the involvement of caveolae in the infectious entry step and propose a model involving recruitment of caveolar proteins around the membrane-bound virus. We also speculate that a similar mechanism may have been exploited by bacterial pathogens. The subsequent steps by which SV40 reaches the ER remain unclear but recent evidence suggests that this pathway may be shared with several other proteins that are transported from surface caveolae to the ER.

Detergent-insoluble glycosphingolipid/cholesterol-rich membrane domains, lipid rafts and caveolae (review)

Within the cell membrane glycosphingolipids and cholesterol cluster together in distinct domains or lipid rafts, along with glycosyl-phosphatidylinositol (GPI)-anchored proteins in the outer leaflet and acylated proteins in the inner leaflet of the bilayer. These lipid rafts are characterized by insolubility in detergents such as Triton X-100 at 4 degrees C. Studies on model membrane systems have shown that the clustering of glycosphingolipids and GPI-anchored proteins in lipid rafts is an intrinsic property of the acyl chains of these membrane components, and that detergent extraction does not artefactually induce clustering. Cholesterol is not required for clustering in model membranes but does enhance this process. Single particle tracking, chemical cross-linking, fluorescence resonance energy transfer and immunofluorescence microscopy have been used to directly visualize lipid rafts in membranes. The sizes of the rafts observed in these studies range from 70-370 nm, and depletion of cellular cholesterol levels disrupts the rafts. Caveolae, flask-shaped invaginations of the plasma membrane, that contain the coat protein caveolin, are also enriched in cholesterol and glycosphingolipids. Although caveolae are also insoluble in Triton X-100, more selective isolation procedures indicate that caveolae do not equate with detergent-insoluble lipid rafts. Numerous proteins involved in cell signalling have been identified in caveolae, suggesting that these structures may function as signal transduction centres. Depletion of membrane cholesterol with cholesterol binding drugs or by blocking cellular cholesterol biosynthesis disrupts the formation and function of both lipid rafts and caveolae, indicating that these membrane domains are involved in a range of biological processes.

Visualization of caveolin-1, a caveolar marker protein, in living cells using green fluorescent protein (GFP) chimeras. The subcellular distribution of caveolin-1 is modulated by cell-cell contact

Caveolin-1, a suspected tumor suppressor, is a principal protein component of caveolae in vivo. Recently, we have shown that NIH 3T3 cells harboring anti-sense caveolin-1 exhibit a loss of contact inhibition and anchorage-independent growth. These observations may be related to the ability of caveolin-1 expression to positively regulate contact inhibition. In order to understand the postulated role of caveolin-1 in contact inhibition, it will be necessary to follow the distribution of caveolins in living cells in response to a variety of stimuli, such as cell density. Here, we visualize the distribution of caveolin-1 in living normal NIH 3T3 cells by creating GFP-fusion proteins. In many respects, the behavior of these GFP-caveolin-1 fusion proteins is indistinguishable from endogenous caveolin-1. These GFP-caveolin-1 fusion proteins co-fractionated with endogenous caveolin-1 using an established protocol that separates caveolae-derived membranes from the bulk of cellular membranes and cytosolic proteins, and co-localized with endogenous caveolin-2 in vivo as seen by immunofluorescence microscopy. We show here that as NIH 3T3 cells become confluent, the distribution of GFP-caveolin-1 and endogenous caveolin-1 shifts to areas of cell-cell contact, coincident with contact inhibition. However, unlike endogenous caveolin-1, the levels of GFP-caveolin-1 expression are unaffected by changes in cell density, serum starvation, or growth factor stimulation. These results are consistent with the idea that the levels of endogenous caveolin-1 are modulated by either transcriptional or translational control, and that this modulation is separable from density-dependent regulation of the distribution of caveolin-1. These studies provide a new living-model system for elucidating the dynamic mechanisms underlying the density-dependent regulation of the distribution of caveolin-1 and how this relates to contact inhibition.

Identification of caveolin-1 as a fatty acid binding protein

In an attempt to identify high affinity, fatty acid binding proteins present in 3T3-L1 adipocytes plasma membranes, we labeled proteins in purified plasma membranes with the photoreactive fatty acid analogue, 11-m-diazirinophenoxy[11-3H]undecanoate. A single membrane protein of 22 kDa was covalently labeled after photolysis. This protein fractionated with caveolin-1 containing caveolae and was immunoprecipitated by an anti-caveolin-1 monoclonal antibody. Furthermore, 2D-PAGE analysis revealed that both the alpha and beta isoforms of caveolin-1 could be labeled by the photoreactive fatty acid upon photolysis, indicating that both bind fatty acids. The saturable binding of the photoreactive fatty acid suggests caveolin-1 has a lipid binding site that may either operate during intracellular lipid traffic or regulate caveolin-1 function.

How do animal DNA viruses get to the nucleus?

Genome and pre-genome replication in all animal DNA viruses except poxviruses occurs in the cell nucleus (Table 1). In order to reproduce, an infecting virion enters the cell and traverses through the cytoplasm toward the nucleus. Using the cell's own nuclear import machinery, the viral genome then enters the nucleus through the nuclear pore complex. Targeting of the infecting virion or viral genome to the multiplication site is therefore an essential process in productive viral infection as well as in latent infection and transformation. Yet little is known about how infecting genomes of animal DNA viruses reach the nucleus in order to reproduce. Moreover, this nuclear locus for viral multiplication is remarkable in that the sizes and composition of the infectious particles vary enormously. In this article, we discuss virion structure, life cycle to reproduce infectious particles, viral protein's nuclear import signal, and viral genome nuclear targeting.

Extracellular simian virus 40 transmits a signal that promotes virus enclosure within caveolae

It was reported earlier that entry of simian virus 40 (SV40) into cells is promoted by a signal transmitted by the virus from the cell surface and that SV40 enters cells through caveolae. It is shown here that bound SV40 begins to partition into a caveolae-enriched Triton X-100-insoluble membrane fraction at 30 min postadsorption. Maximal levels of SV40 were seen in that fraction at 1 h. The sterol-binding agent nystatin, which selectively disrupts the cholesterol-enriched caveolae-containing membrane microdomain, selectively blocked the SV40-induced signal. This implies that the SV40 signal is transmitted from that membrane microdomain. The tyrosine kinase inhibitor genistein, which was earlier shown to block the SV40-induced signal and infectious entry, did not block the partitioning of SV40 into the detergent-insoluble membrane fraction. This shows that the signal is not required for the translocation of SV40 to the detergent-insoluble membrane and is consistent with the finding that the signal is likely transmitted from that membrane microdomain. However, electron microscopy of the Triton X-100-insoluble membrane fraction showed that genistein caused SV40 particles to accumulate at the annuli or mouths of the caveolae. In contrast, most SV40 particles were found enclosed within caveolae in parallel samples from untreated control cells. Together, these results imply that SV40 initially binds to flat detergent-soluble membrane. The virus then translocates to a caveolae-containing detergent-insoluble membrane microdomain. From the flat portion of that membrane microdomain the virus induces a signal which promotes its entry into caveolae.

Viral cell entry induced by cross-linked decay-accelerating factor

Decay-accelerating factor (DAF) mediates cellular attachment for many human picornaviruses. In most cases, viral binding to DAF is itself insufficient to permit cell infectivity, with a second, functional internalization receptor being required to facilitate this process. Previously, we postulated that the role of DAF in enterovirus cell infection is as a sequestration receptor, maintaining a reservoir of bound virus in an infectious state, awaiting interaction with functional internalization receptors. Many of these functional receptors possess the capacity to induce relatively rapid changes in capsid conformations, resulting in the formation of altered particles (A-type particles). In this report, we show that antibody-cross-linked DAF, in contrast to endogenous surface-expressed forms, can act as a functional virus receptor to mediate coxsackie A21 virus (CAV21) lytic cell infection. In contrast to the situation with ICAM-1-mediated CAV21 infection, in which high levels of A-type particles are formed, cross-linked DAF-induced CAV21 replication occurs in the absence of detectable A-particle formation.

The caveolae membrane system

The cell biology of caveolae is a rapidly growing area of biomedical research. Caveolae are known primarily for their ability to transport molecules across endothelial cells, but modern cellular techniques have dramatically extended our view of caveolae. They form a unique endocytic and exocytic compartment at the surface of most cells and are capable of importing molecules and delivering them to specific locations within the cell, exporting molecules to extracellular space, and compartmentalizing a variety of signaling activities. They are not simply an endocytic device with a peculiar membrane shape but constitute an entire membrane system with multiple functions essential for the cell. Specific diseases attack this system: Pathogens have been identified that use it as a means of gaining entrance to the cell. Trying to understand the full range of functions of caveolae challenges our basic instincts about the cell.

Mechanisms of enveloped virus entry into animal cells

The ability of viruses to transfer macromolecules between cells makes them attractive starting points for the design of biological delivery vehicles. Virus-based vectors and sub-viral systems are already finding biotechnological and medical applications for gene, peptide, vaccine and drug delivery. Progress has been made in understanding the cellular and molecular mechanisms underlying virus entry, particularly in identifying virus receptors. However, receptor binding is only a first step and we now have to understand how these molecules facilitate entry, how enveloped viruses fuse with cells or non-enveloped viruses penetrate the cell membrane, and what happens following penetration. Only through these detailed analyses will the full potential of viruses as vectors and delivery vehicles be realised. Here we discuss aspects of the entry mechanisms for several well-characterised viral systems. We do not attempt to provide a fully comprehensive review of virus entry but focus primarily on enveloped viruses.

Localization of autocrine motility factor receptor to caveolae and clathrin-independent internalization of its ligand to smooth endoplasmic reticulum

Autocrine motility factor receptor (AMF-R) is a cell surface receptor that is also localized to a smooth subdomain of the endoplasmic reticulum, the AMF-R tubule. By postembedding immunoelectron microscopy, AMF-R concentrates within smooth plasmalemmal vesicles or caveolae in both NIH-3T3 fibroblasts and HeLa cells. By confocal microscopy, cell surface AMF-R labeled by the addition of anti-AMF-R antibody to viable cells at 4 degreesC exhibits partial colocalization with caveolin, confirming the localization of cell surface AMF-R to caveolae. Labeling of cell surface AMF-R by either anti-AMF-R antibody or biotinylated AMF (bAMF) exhibits extensive colocalization and after a pulse of 1-2 h at 37 degreesC, bAMF accumulates in densely labeled perinuclear structures as well as fainter tubular structures that colocalize with AMF-R tubules. After a subsequent 2- to 4-h chase, bAMF is localized predominantly to AMF-R tubules. Cytoplasmic acidification, blocking clathrin-mediated endocytosis, results in the essentially exclusive distribution of internalized bAMF to AMF-R tubules. By confocal microscopy, the tubular structures labeled by internalized bAMF show complete colocalization with AMF-R tubules. bAMF internalized in the presence of a 10-fold excess of unlabeled AMF labels perinuclear punctate structures, which are therefore the product of fluid phase endocytosis, but does not label AMF-R tubules, demonstrating that bAMF targeting to AMF-R tubules occurs via a receptor-mediated pathway. By electron microscopy, bAMF internalized for 10 min is located to cell surface caveolae and after 30 min is present within smooth and rough endoplasmic reticulum tubules. AMF-R is therefore internalized via a receptor-mediated clathrin-independent pathway to smooth ER. The steady state localization of AMF-R to caveolae implicates these cell surface invaginations in AMF-R endocytosis.

Role for beta2-microglobulin in echovirus infection of rhabdomyosarcoma cells

A monoclonal antibody (MAb) that blocks most echoviruses (EVs) from infecting rhabdomyosarcoma (RD) cells has been isolated. By using the CELICS cloning method (T. Ward, P. A. Pipkin, N. A. Clarkson, D. M. Stone, P. D. Minor, and J. W. Almond, EMBO J. 13:5070-5074, 1994), the ligand for this antibody has been identified as beta2-microglobulin (beta2m), the 12-kDa protein that associates with class I heavy chains to form class I HLA complexes. A commercial MAb (MAb 1350) against beta2m was also found to block EV7 infection without affecting binding to its receptor, DAF, or replication of EV7 viral RNA inside cells. Entry of EV7 into cells was reduced by only 30% by antibody and cytochalasin D, an inhibitor of endocytosis mediated by caveolae and clathrin-coated pits, but was not significantly reduced by sodium azide. The block to virus entry by cytochalasin D was additive to the block induced by antibody. We suggest that EV7 rapidly enters into a multicomponent receptor complex prior to entry into cells and that this initial entry event requires beta2m or class I HLA for infection to proceed.

Nuclear import and export of viruses and virus genomes

Many viruses replicate in the nucleus of their animal and plant host cells. Nuclear import, export, and nucleo-cytoplasmic shuttling play a central role in their replication cycle. Although the trafficking of individual virus proteins into and out of the nucleus has been well studied for some virus systems, the nuclear transport of larger entities such as viral genomes and capsids has only recently become a subject of molecular analysis. In this review, the general concepts emerging are discussed and a survey is provided of current information on both plant and animal viruses. Summarizing the main findings in this emerging field, it is evident that most viruses that enter or exit the nucleus take advantage of the cell's nuclear import and export machinery. With a few exceptions, viruses seem to cross the nuclear envelope through the nuclear pore complexes, making use of cellular nuclear import and export signals, receptors, and transport factors. In many cases, they capitalize on subtle control systems such as phosphorylation that regulate traffic of cellular components into and out of the nucleus. The large size of viral capsids and their composition (they contain large RNA and DNA molecules for which there are few precedents in normal nuclear transport) make the processes unique and complicated. Prior capsid disassembly (or deformation) is required before entry of viral genomes and accessory proteins can occur through nuclear pores. Capsids of different virus families display diverse uncoating programs which culminate in genome transfer through the nuclear pores.

Infection of glial cells by the human polyomavirus JC is mediated by an N-linked glycoprotein containing terminal alpha(2-6)-linked sialic acids

The human JC polyomavirus (JCV) is the etiologic agent of the fatal central nervous system (CNS) demyelinating disease progressive multifocal leukoencephalopathy (PML). PML typically occurs in immunosuppressed patients and is the direct result of JCV infection of oligodendrocytes. The initial event in infection of cells by JCV is attachment of the virus to receptors present on the surface of a susceptible cell. Our laboratory has been studying this critical event in the life cycle of JCV, and we have found that JCV binds to a limited number of cell surface receptors on human glial cells that are not shared by the related polyomavirus simian virus 40 (C. K. Liu, A. P. Hope, and W. J. Atwood, J. Neurovirol. 4:49-58, 1998). To further characterize specific JCV receptors on human glial cells, we tested specific neuraminidases, proteases, and phospholipases for the ability to inhibit JCV binding to and infection of glial cells. Several of the enzymes tested were capable of inhibiting virus binding to cells, but only neuraminidase was capable of inhibiting infection. The ability of neuraminidase to inhibit infection correlated with its ability to remove both alpha(2-3)- and alpha(2-6)-linked sialic acids from glial cells. A recombinant neuraminidase that specifically removes the alpha(2-3) linkage of sialic acid had no effect on virus binding or infection. A competition assay between virus and sialic acid-specific lectins that recognize either the alpha(2-3) or the alpha(2-6) linkage revealed that JCV preferentially interacts with alpha(2-6)-linked sialic acids on glial cells. Treatment of glial cells with tunicamycin, but not with benzyl N-acetyl-alpha-D-galactosaminide, inhibited infection by JCV, indicating that the sialylated JCV receptor is an N-linked glycoprotein. As sialic acid containing glycoproteins play a fundamental role in mediating many virus-cell and cell-cell recognition processes, it will be of interest to determine what role these receptors play in the pathogenesis of PML.

A pore-forming toxin interacts with a GPI-anchored protein and causes vacuolation of the endoplasmic reticulum

In this paper, we have investigated the effects of the pore-forming toxin aerolysin, produced by Aeromonas hydrophila, on mammalian cells. Our data indicate that the protoxin binds to an 80-kD glycosyl-phosphatidylinositol (GPI)-anchored protein on BHK cells, and that the bound toxin is associated with specialized plasma membrane domains, described as detergent-insoluble microdomains, or cholesterol-glycolipid "rafts." We show that the protoxin is then processed to its mature form by host cell proteases. We propose that the preferential association of the toxin with rafts, through binding to GPI-anchored proteins, is likely to increase the local toxin concentration and thereby promote oligomerization, a step that it is a prerequisite for channel formation. We show that channel formation does not lead to disruption of the plasma membrane but to the selective permeabilization to small ions such as potassium, which causes plasma membrane depolarization. Next we studied the consequences of channel formation on the organization and dynamics of intracellular membranes. Strikingly, we found that the toxin causes dramatic vacuolation of the ER, but does not affect other intracellular compartments. Concomitantly we find that the COPI coat is released from biosynthetic membranes and that biosynthetic transport of newly synthesized transmembrane G protein of vesicular stomatitis virus is inhibited. Our data indicate that binding of proaerolysin to GPI-anchored proteins and processing of the toxin lead to oligomerization and channel formation in the plasma membrane, which in turn causes selective disorganization of early biosynthetic membrane dynamics.

An Update on Non-clathrin-coated Endocytosis

Recent evidence has proved that in addition to the well-documented clathrin-mediated endocytic route (vesicles of 100-150 nm), at least three distinct non-clathrin-coated endocytic pathways function at the surface of mammalian cells. Endocytosis via these pathways is initiated by caveolae (50-80 nm), macropinosomes (500-2000 nm) and micropinosomes (95-100 nm). The current state of knowledge about these non-clathrin coated endocytic routes is presented and evidence that endocytic routes other than via clathrin-coated vesicles are utilised by viruses is discussed. The recent advances in these areas have provided us with tools to investigate the entry of those viruses which appear to enter cells via endocytosis into non-clathrin-coated vesicles. Data indicate that these four endocytic pathways differ in the absence, presence and/or type of coat on the vesicles, the size of the vesicles, their sensitivity to a variety of inhibitors, and in the ligands endocytosed. A historical perspective of the discovery of these non-clathrin-coated endocytic pathways is provided and recent information is summarised and discussed. The entry of viruses via non-clathrin-coated pits is destined to be an exciting new area of viral-cell entry, as has been indicated recently by the finding that entry of simian virus type 40 into cells occurs via caveolae. Copyright 1997 by John Wiley & Sons, Ltd.

Functional rafts in cell membranes

A new aspect of cell membrane structure is presented, based on the dynamic clustering of sphingolipids and cholesterol to form rafts that move within the fluid bilayer. It is proposed that these rafts function as platforms for the attachment of proteins when membranes are moved around inside the cell and during signal transduction.