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Keywords:

  • caveolae;
  • caveosomes;
  • endocytosis;
  • GPI-anchored proteins;
  • glycosphingolipids;
  • lipid rafts;
  • Simian Virus 40

Abstract

  1. Top of page
  2. Abstract
  3. Caveolae
  4. Caveolar Entry of Simian Virus 40
  5. Caveolar Endocytosis of Other Ligands and Membrane Constituents
  6. How Does It Work?
  7. Perspectives
  8. Acknowledgments
  9. References

Caveolae are flask-shaped invaginations present in the plasma membrane of many cell types. They have long been implicated in endocytosis, transcytosis, and cell signaling. Recent work has confirmed that caveolae are directly involved in the internalization of membrane components (glycosphingolipids and glycosylphosphatidylinositol-anchored proteins), extracellular ligands (folic acid, albumin, autocrine motility factor), bacterial toxins (cholera toxin, tetanus toxin), and several nonenveloped viruses (Simian virus 40, Polyoma virus). Unlike clathrin-mediated endocytosis, internalization through caveolae is a triggered event that involves complex signaling. The mechanism of internalization and the subsequent intracellular pathways that the internalized substances take are starting to emerge.

While clathrin-mediated endocytosis constitutes the main pathway for internalization of extracellular ligands and plasma membrane components in most cell types, it has been recognized for some time that alternative, parallel uptake mechanisms also exist. These ‘clathrin-independent pathways’ have been more difficult to study; hence, detailed information is still scarce [for recent reviews see (1–3)]. In this review, we focus on one of these alternatives: endocytosis via caveolae. Our own interest in this field was evoked by the observation that certain non-enveloped viruses such as Simian Virus 40 (SV40) use caveolae-mediated endocytosis to enter cells (4–6). Similar uptake processes are likely to be involved in many interesting endogenous processes including cholesterol homeostasis, recycling of glycosylphosphatidylinositol (GPI)-anchored proteins, glycosphingolipid transport, and transcytosis of serum components (7–12).

Caveolae

  1. Top of page
  2. Abstract
  3. Caveolae
  4. Caveolar Entry of Simian Virus 40
  5. Caveolar Endocytosis of Other Ligands and Membrane Constituents
  6. How Does It Work?
  7. Perspectives
  8. Acknowledgments
  9. References

Caveolae were first identified in the 1950s by Palade (13) and Yamada (14) due to their characteristic morphology observed by electron microscopy of thin sections. They typically appear as rounded plasma membrane invaginations of 50–80 nm in diameter. Their composition, appearance and function are cell-type dependent. In endothelial cells, caveolae can be more constricted at the mouth, or they may contain a diaphragm that restricts diffusion (15,16). In muscle cells, caveolae are often observed in the form of composite clusters or linear rows of multiple flask-shaped units involved in the formation of T-tubules (17,18). In epithelial tissue culture cells, caveolae are open to the extracellular medium, do not contain a diaphragm, appear as single indentations or grape-like structures, and are on average a bit smaller (19,20). Recent video microscopy and fluorescence recovery after photobleaching (FRAP) analysis has shown that in these cells caveolae are stationary and held in place by the cortical actin cytoskeleton underlying the plasma membrane (21,22). As discussed below, only upon specific signals do they detach from the membrane as an endocytic vesicle.

Although caveolae do not show an electron-dense layer on their cytosolic surface in thin-section electron micrographs, they do have a protein ‘coat’ composed primarily of a protein called caveolin-1 (or caveolin-3 in muscle cells) (20). Caveolins are integral membrane proteins of 21 kDa. They have an unusual topology in that the cytosolic N- and C-terminal domains are cytosolic connected by a hydrophobic sequence that is buried in the membrane but does not span the bilayer (23,24). Caveolins are palmitoylated in the C-terminal segment (25), they can be phosphorylated on tyrosine residues (26), they bind cholesterol (27), and they form dimers and higher oligomers (24). On the cytosolic surface they can be visualized by electron microscopy as part of parallel, shallow ridges in replicas obtained after shadowing (20,28).

Caveolins are essential for the formation and stability of caveolae: in their absence no caveolae are seen, and, when expressed in cells that lack caveolae, they induce caveolar formation (29). In addition to the plasma membrane, caveolins are present in the trans-Golgi network (TGN) (23,30) and in a newly discovered organelle called the ‘caveosome’ (21), which will be discussed below. Upon extraction or oxidation of plasma membrane cholesterol, caveolins re-localize to intracellular structures that can be endosomes, the Golgi complex, or the ER (18,31). When caveolins are over-expressed or retained in the ER, they can also localize to intracellular lipid droplets (32–34). Immunofluorescence with conformation-specific anti-caveolin antibodies suggests that the conformation of caveolin-1 on the plasma membrane and in caveosomes is similar, but different from that in the Golgi complex (21,23).

During endocytosis of caveolae, caveolin-1 moves along with the vesicles into the cytosol with no visible remnant left in the plasma membrane (6, 21, 35–37). Whether caveolin has a direct role in the endocytic process is not clear. On the one hand, it has been shown that N-terminally truncated or N-terminally GFP-tagged caveolin constructs serve as dominant negative inhibitors of caveolar endocytosis during SV40 entry (21,38). On the other hand, recent data indicate that the presence of caveolin-1 can actually slow down the endocytic process via caveolae (39,40). This implies that the role of caveolin may be to stabilize caveolae and thus to counteract an underlying raft-dependent endocytic process. The finding that certain ligands internalize via a lipid raft-dependent but clathrin-independent mechanism in cells that lack caveolae, has led to the postulation that lipid rafts can internalize independently of caveolae (41). In this context, it is significant that caveolin knockout mice survive surprisingly well, although their cells do not have detectable caveolae (42–44).

In addition to caveolins, caveolae are known to contain dynamin (45,46), a GTPase involved in the formation of clathrin-coated vesicles (47). This molecule has been localized to the neck of flask-shaped caveolar indentations (45,46), and is therefore most likely involved in pinching off the caveolar vesicle in a way similar to its role in coated vesicle fission (48,49). Recent video imaging of the dynamics of GFP-tagged dynamin2 during SV40 entry showed that it is only transiently recruited to virus-loaded caveolae (50). It appears as small spots on the site of caveolae with a residence time of about 8 s. Thus, dynamin is not a permanent component of caveolae but rather belongs to a group of factors recruited in response to specific signals. Also, caveolae contain the molecular machinery for vesicle docking and fusion (51).

Caveolae are, moreover, rich in GPI-anchored proteins and several receptor and nonreceptor protein tyrosine kinases (52,53). Since these kinases are involved in signal transduction events, caveolae are thought to constitute especially active sites for signal transmission (9,54,55). Furthermore, some of the kinases might be involved in regulating the internalization of caveolae, as will be discussed below.

The lipid composition of caveolae corresponds to that of lipid rafts, i.e. caveolae are rich in cholesterol and sphingolipids [extensively reviewed in (54,56)]. These are, in fact, essential for the formation and stability of caveolae. If cholesterol is removed from the plasma membrane, caveolae disappear (20). Consistent with their high content of raft lipids, caveolae resist solubilization by nonionic detergents at 4 °C. One may thus define caveolae as caveolin-containing plasma membrane invaginations rich in raft lipids.

Caveolar Entry of Simian Virus 40

  1. Top of page
  2. Abstract
  3. Caveolae
  4. Caveolar Entry of Simian Virus 40
  5. Caveolar Endocytosis of Other Ligands and Membrane Constituents
  6. How Does It Work?
  7. Perspectives
  8. Acknowledgments
  9. References

We will first concentrate on one of the most extensively studied ligands for caveolar endocytosis, SV40, which uses this pathway for infectious entry into the cell (4–6, 21, 38, 50, 57–67). The entry process of this non-enveloped DNA virus, analyzed in several laboratories including ours, is emerging as a useful paradigm in the field.

SV40 has several advantages as a model ligand. The particle itself is well characterized in terms of composition and structure. The X-ray structure of this non-enveloped DNA virus has been determined at a resolution of 3.1Å and shows an icosahedral shell with 360 subunits of the major coat protein VP1 (62,66). The diameter of the particle is 50 nm. Although replication of the virus is restricted to monkey cells, the entry process and initial infection can be followed in most cell types.

Uptake of SV40 occurs specifically by the caveolar pathway. Even when cells are incubated with amounts of virus particles by far exceeding receptor saturation, less than 5% of internalized particles is found to pass through clathrin-coated pits and vesicles (4,21,57,61). When clathrin-dependent endocytosis was inhibited by expressing a dominant negative mutant of the EGF receptor pathway substrate 15 (Eps15), SV40 endocytosis and infection were not affected (50). In contrast, when dominant negative mutants of caveolin were expressed, internalization or infection was not observed (21,38).

The interaction of the incoming virus particles with cells has been studied by morphological techniques (electron microscopy and light microscopy), by biochemical techniques (quantitative endocytosis assays) and by virological methods (infection assays). Most of the studies have been performed in tissue culture cells from green African monkeys. A variety of inhibitors have been tested, as well as expression of dominant negative mutant proteins. Extensive use has recently been made of video microscopy in live cells with fluorescently labeled virus particles and GFP-labeled cellular proteins. This has made it possible to follow single virus particles during their journey into the cell. The stepwise process of SV40 entry is today known in some detail.

Sequestration and internalization (Figure 1)

image

Figure 1. Initial stages of SV40 internalization via caveolae. After binding to the membrane, virus particles are mobile until trapped in caveolae, which are linked to the actin cytoskeleton (step 1). In the caveolae, SV40 particles trigger a signal transduction cascade that leads to local protein tyrosine phosphorylation and depolymerization of the cortical actin cytoskeleton (step 2). Actin monomers are recruited to the virus-loaded caveolae and an actin patch is formed (step 3). Concomitantly, Dynamin is recruited to the virus-loaded caveolae and a burst of actin polymerization occurs on the actin patch (step 4). Virus-loaded caveolae vesicles are now released from the membrane and can move into the cytosol (step 5). After internalization, the cortical actin cytoskeleton returns to its normal pattern (step 6).

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After binding to the plasma membrane via major histocompatibility (MHC) class I antigens (64), virus particles diffuse laterally along the membrane, until trapped in caveolae (21). Virus-containing caveolae are slightly smaller in diameter than the virus-free caveolae (6). The reason may be the tight interaction between the virus and the caveolar membrane; in electron micrographs it almost looks like the viruses would be ‘budding’ into the cell.

Virus particles stay in the caveolae for 20 min or more, after which the caveolae pinch off and move as caveolin-coated endocytic vesicles into the cytoplasm. Caveolae devoid of virus particles do not internalize (21). Interestingly, virus uptake does not involve endocytosis of MHC class I antigens (68). This implies the presence of a secondary receptor that mediates the tight binding of the virus to the caveolar membrane, and possibly triggers the signal needed to induce the endocytic process (see below).

During the lag period preceding internalization, a complex series of virus-induced events takes place. The arrival of the SV40 particle in the caveolae triggers phosphorylation of tyrosine residues in proteins associated with the caveolae (50). However, the kinases and substrates involved remain to be identified.

One effect of the activated signal transduction cascade is the disassembly of nearby actin stress fibers. Subsequently, actin is recruited to the virus-loaded caveolae as a small actin patch, followed by bursts of actin polymerization resulting in the transient appearance of actin ‘tails’ (average length about 1.5 μm) emanating from the virus-loaded caveolae. Dynamin is also recruited to the sites of virus internalization but, as already mentioned, it stays there only for a short period of time (50). Another effect is the up-regulation of the primary response genes c-myc, c-jun and c-sis (69).

Studies with inhibitors and dominant negative mutants show that the association of SV40 with caveolae, the tyrosine phosphorylation, the recruitment of actin, the formation of actin tails, and the association of dynamin are all necessary for efficient closure of caveolar vesicles and ultimately for infection of the cell (50,67). Most of the changes seem to be transient; once the virus particles are internalized, the phosphotyrosines disappear, and the actin cytoskeleton returns to the normal pattern.

Transport to caveosomes

The caveolar vesicles have a diameter of 60–70 nm and contain single virus particles surrounded by a tight-fitting membrane (4). The local depolymerization of cortical actin seems to be necessary for their passage deeper into the cytosol (50) and the actin tail is not needed once the vesicle is released.

These primary endocytic vesicles transfer their viral cargo, presumably by membrane fusion, to larger, more complex tubular membrane organelles that we have termed ‘caveosomes’ (21) (Figure 2). Since the uptake of virus particles is relatively slow (half-time 90 min and maximal uptake after 3 h), accumulation of virus particles in caveosomes becomes visible after about 40–60 min and continues for the following 3 h.

image

Figure 2. Subcellular distribution and morphology of caveosomes in green African monkey kidney epithelial cells.Left: Laser scanning confocal micrograph (intracellular plane) of a CV−1 cell expressing caveolin-1-GFP (cav1-GFP, green) and Texas Red-labeled Simian Virus 40 (TRX-SV40, red). Virus particles have been allowed to accumulate in caveosomes for 3 h. Most caveosomes are now filled with virus particles (yellow), but some are still empty (green). Scale-bar, 10 μm Middle: Thin-section electron micrograph of an intracellular region of a CV−1 cell having internalized virus particles for 2 h. Caveosomes appear as heterogeneous organelles containing tightly packed SV40 particles. Vacuolar endosomes do not accumulate virus particles. (CCV: clathrin-coated vesicle, CV: caveolar vesicle). Scale-bar, 500 nm. Right: Higher-magnification thin-section electron micrograph shows the heterogeneous morphology of caveosomes, with virus particles appearing as ‘peas in a pod’. Scale-bar, 100 nm.

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Caveosomes are pre-existing, caveolin-1-containing membrane organelles distributed throughout the cytoplasm. The pH in caveosomes is neutral, and the membrane is rich in cholesterol and glycosphingolipids ((21,70) (L. Pelkmans, C. Buser, A. Helenius, unpublished results). Caveosomes do not accumulate ligands endocytosed via clathrin-coated pits, or components such as transferrin that cycle between endosomes and the plasma membrane. Nor do they accumulate detectable amounts of fluid phase markers such as FITC-dextran or horseradish peroxidase, even when these are added to cells together with SV40 (21,61). Antibodies against markers of endosomes, lysosomes, TGN, Golgi complex, or the ER do not stain the caveosome. Ultra-structural analysis shows tubulovesicular structures, heterogeneous in size and shape, with the virus particles usually present in narrow tubules like peas in a pod (21,61).

Molecular sorting and transport to the ER

During the accumulation of virus particles, the caveosomes become increasingly dynamic. Longer tubular extensions filled with virus particles but devoid of caveolin-1 emerge and detach from the caveosomes leaving caveolin-1 behind (21). These vesicles are transported along microtubules to perinuclear membrane organelles identified as the smooth ER. The receiving compartment can grow in size when more virus particles are added, forming an anastomosing, tubular expansion of the ER (57,61). Here, the majority of virus particles remain undegraded for up to 16 h or longer. How the viral genome is transported from the ER to the nucleus is unclear, but it seems to passage through the cytosol and the nuclear pore complex (71).

In summary, the entry pathway of SV40 has revealed several key features that seem to define the caveolar endocytic pathway and make it distinct from clathrin-mediated endocytosis. First, SV40 uptake through caveolae is ligand triggered. Triggering involves one or more tyrosine kinases that initiate a phosphorylation-dependent signaling cascade. Second, the pathway taken by the virus inside the cell bypasses the organelles involved in clathrin-coated vesicle endocytosis. Moreover, along the route, the pH is maintained at a neutral level and no degradative end-station is reached. Instead, the cargo is delivered to the ER. Thus, this pathway provides a direct route from the plasma membrane to the ER.

Remarkable also is the role of the cytoskeleton. Actin plays a central role in the formation of the caveolar vesicles, and in the initial transfer of the newly formed vesicles through the cortex of the plasma membrane. The ability to locally de-polymerize, and also the ability to polymerize actin are important. This is in contrast to the internalization of clathrin-coated pits, which appears to be enhanced by, but is not strictly dependent on, a functional actin cytoskeleton (72,73).

After internalization, the microtubule cytoskeleton takes over. Initial transport to caveosomes does not seem to be dependent on microtubules, but video microscopy shows that early vesicles can move along them. Later, microtubules play an essential role in the sorting of SV40 from caveosomes and transport to the smooth ER.

Caveolar Endocytosis of Other Ligands and Membrane Constituents

  1. Top of page
  2. Abstract
  3. Caveolae
  4. Caveolar Entry of Simian Virus 40
  5. Caveolar Endocytosis of Other Ligands and Membrane Constituents
  6. How Does It Work?
  7. Perspectives
  8. Acknowledgments
  9. References

Other ligands or membrane constituents that can be internalized via caveolae are cholera toxin (19,35), folic acid (74,75), serum albumin (76), autocrine motility factor (AMF) (77), alkaline phosphatase (35), GPI-anchored green fluorescent protein (GPI-GFP) (78), and the bodipy-labeled glycosphingolipid Lactosyl Ceramide (LacCer) (70). (See Table 1 for more details.) Certain FimH-expressing bacteriaare also internalized in a caveolae-dependent manner in mast cells (79).

Table 1. : Materials endocytosed via caveolae or lipid rafts
LigandReceptor orInductionAffinityReferences
or consitituentmembrane moiety Caveolae/ Lipid raftsClathrin 
  1. TM, transmembrane insertion; ND, not determined; E. coli, Escherichia coli.

Ligands
Folic acidFolic acid receptorYes+ ++(74,75,96)
 (GPI anchor)
Albumingp60 (TM),Yes+ ++(39,76)
 others
Autocrine motility factor (AMF)AMF receptor (TM)ND+ ++(77,97)
Interleukin-2 (IL2)IL2 receptor (TM)ND+ +(41)
Membrane constituents
Alkaline phosphataseGPI-anchorYes+ +(35)
GPI-GFPGPI-anchorND+ +(78)
Lactosyl ceramideGlycosphingolipidYes+ +(70)
Toxins
Cholera toxinGM1 gangliosidesND+ ++(19,35,94,95)
Tetanus toxin GPI-anchoredND+ +(19,98,99)
  molecules
Viruses
Simian virus 40MHC1 (TM), NDYes+ + (4–6, 64)
Polyoma virus Sialic acid, GM1ND+ + (80, 100, 101)
  gangliosides
Echovirus 1α2β1-IntegrinND+ND (82)
Respiratory syncytial virusNDND+ND (83)
Bacteria (caveolae-mediated phagocytosis)
FimH expressing E. coliCD48 (GPI anchor)Yes+ + (79)

SV40 is not the only virus that enters via caveolae. The closely related polyoma virus was recently found to enter mouse cells by the caveolar route and to have a similar dissociating effect on actin stress fibers (80). Interestingly, polyoma virus appears to specifically bind to branched sialic acid groups present in glycoproteins with O-linked carbohydrates and in GM1 gangliosides (81). Echovirus 1, a member of the picorna virus family, which binds to α2β1-integrin, also internalizes via caveolae (82). In addition, respiratory syncytial virus has also been reported to associate with caveolae (83). Finally, there is evidence that HIV-1 uses caveolae for transcytosis across endothelia (84). It is not unlikely that the caveolar pathway is important for a variety of viruses for which the entry mechanism has remained obscure.

How Does It Work?

  1. Top of page
  2. Abstract
  3. Caveolae
  4. Caveolar Entry of Simian Virus 40
  5. Caveolar Endocytosis of Other Ligands and Membrane Constituents
  6. How Does It Work?
  7. Perspectives
  8. Acknowledgments
  9. References

The systems studied so far provide a rather heterogeneous picture of caveolar endocytosis, and it is not easy to define a common denominator. However, caveolae seem to be used by cells to internalize membrane components that are enriched in rafts, such as cholesterol, glycosphingolipids, GPI-anchored proteins, and any ligands that bind to them. However, to be internalized, affinity for rafts or raft components is, in general, not sufficient; some form of induction is in addition is needed.

One way to induce caveolar uptake is by cross-linking caveolar components. This has been shown for several GPI-anchored proteins (35,85). Many of the ligands in Table 1 are multivalent and therefore capable of cross-linking their receptors. Such clustering ability may explain why multivalent ligands such as virus particles and bacteria are sequestered into caveolae. They may also induce the formation of new caveolae (86,87).

In several cases internalization of caveolae is induced upon activation of a phosphorylation cascade (9,39,50). It is also known that phosphatase inhibitors can induce internalization of caveolae (22,35). How extracellular ligands activate phosphorylation is unclear. One way to transmit a signal is by cross-linking transmembrane tyrosine kinase receptors, of which several are enriched in caveolae. Another, more recently postulated mechanism suggests that clustering of GPI-anchored proteins or glycosphingolipids in the extracellular leaflet may stabilize lipid rafts, which in turn may lead to the recruitment of proteins with high affinity for rafts, such as caveolin-1 and Src family protein tyrosine kinases, on the cytosolic leaflet (54,87).

Direct signaling via raft-enriched lipids such as ceramide or phosphatidyl inositols could also be involved. For instance, internalization of folic acid via caveolae is regulated by the serine/threonine kinase Protein Kinase Cα (PKCα) (9), which is activated by diacylglycerol, a hydrolysis product of phosphatidyl inositol 4,5-bisphosphate (PIP2). It has also been suggested that PKCα regulates the internalization or receptor recycling of SV40, since the phorbol ester PMA, which mimics diacylglycerol, reduces SV40 uptake (5).

The actin cytoskeleton seems to play an essential role in the internalization of caveolae. First of all, local recruitment of an actin patch to caveolae, similar to that observed in clustered lipid rafts (88), is necessary and might function as a scaffold for the build-up of the internalization machinery (50). Temporary depolymerization of the cortical actin cytoskeleton is necessary to allow proper closure of caveolae and transit to the cytosol (22,50) (P. Verkade and K. Simons, personal communication). The actin patches are likely formed by actin–protein or actin–lipid interactions. At least during SV40 entry, the patches are formed in conditions where actin depolymerization is temporarily favored. The mechanism of actin recruitment is not known, but it may involve enrichment of PIP2 on the cytosolic leaflet of caveolae, which can recruit the necessary machinery to locally polymerize actin (89,90). It likely also involves several SH2 and SH3 adapter proteins that link the actin cytoskeleton to tyrosine phosphorylation. Furthermore, filamin, an actin-binding protein that binds caveolin-1, might play a role (104).

Dynamin is also involved in the internalization of caveolae. Over-expression of a mutant dynamin, defective in GTP hydrolysis, inhibits release of caveolae from plasma membranes in an in vitro assay (45). EM images showed that caveolae have extended necks in the presence of this mutant (45,46). Functional data come from the observation that this dynamin mutant inhibits uptake of cholera toxin and muscarinic cholinergic receptors via caveolae and uptake of interleukin-2 (IL2) via lipid rafts (41,45,46,91). It now appears that, as a result of the ligand-induced kinase activity, dynamin is temporarily recruited to the internalization site to perform its function in the fission of caveolar vesicles.

Once internalized, caveolar vesicles seem to follow an intracellular route distinct from the classical endocytic pathway (Figure 3). They enter a caveolin-1-rich sorting compartment, the caveosome, which, as already discussed, is distinct from endosomes. From caveosomes, the internalized substances are distributed to the ER, to the Golgi complex, and possibly to other compartments. The pathway is best analyzed for SV40, GPI-GFP, and LacCer (21,70,78). Available information regarding internalization of cholera and tetanus toxins, albumin, folic acid, and FimH expressing Escherichia coli is consistent with this picture (70,75,78,9,92,93). The issue is somewhat complicated, however, by the observation that some of the ligands (cholera toxin, albumin and the folic acid receptor) are also internalized via clathrin-coated pits, and can therefore be found in endosomes as well (94–96).

image

Figure 3. Proposed pathway for intracellular trafficking of ligands and constituents internalized via caveolae or lipid rafts. Depicted is a model for internalization via caveolae and lipid rafts. After internalization, caveolae- or lipid raft-derived vesicles travel to caveosomes, which are distinct from endosomes in content and pH. In caveosomes, internalized ligands or membrane constituents could reside, be sorted to the Golgi complex, or to the endoplasmic reticulum (ER). Membrane constituents that are sorted to the Golgi complex are GPI-GFP and Lactosyl Ceramide (LacCer). Ligands that are sorted to the ER are Simian Virus 40 (SV40) and autocrine motility factor (AMF). Whether ligands or constituents can cycle from caveosomes directly back to the plasma membrane has not yet been studied. Caveolin-1 partly resides in caveosomes. Examples of ligands internalized via clathrin-coated pits that travel to endosomes are also depicted. From endosomes, the envelope of Semliki Forest Virus (SFV) and low-density lipoprotein (LDL) are sorted to lysosomes, Shiga toxin is sorted to the Golgi complex and transferrin (Tfn) is recycled back to the plasma membrane (1,102,103). Whether ligands or membrane constituents can travel between endosomes and caveosomes has not been studied.

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Perspectives

  1. Top of page
  2. Abstract
  3. Caveolae
  4. Caveolar Entry of Simian Virus 40
  5. Caveolar Endocytosis of Other Ligands and Membrane Constituents
  6. How Does It Work?
  7. Perspectives
  8. Acknowledgments
  9. References

Endocytosis through caveolae provides a true alternative to the clathrin-mediated pathway. Being ligand-triggered, it provides an intrinsically more selective way for uptake of specified substances, and it allows tighter control by cellular regulation. It can be used to route incoming ligands and membrane components to organelles that are not easily accessed by other endocytic mechanisms such as the ER. This advantage could, for example, be important in the homeostasis of cholesterol for which most of the regulatory sensors reside in the ER (12). A direct connection between the plasma membrane and the ER could also be important in other processes such as the immune response and in the regulation of secretion. Moreover, the pathway bypasses low pH and digestive compartments, which may be important for some of the internalized ligands.

The analysis of caveolar endocytosis is still in its early stages. Many conceptual and mechanistic issues remain to be solved. It is important to address the differences and similarities between caveolae and other non-clathrin-mediated uptake systems. Is caveolar uptake just a special form of raft-mediated endocytosis? The mild phenotype of knockout mice suggests that, while required for formation of caveolae, caveolin-1 may not be essential for endocytosis nor for crucial signaling functions. The molecular mechanisms of endocytosis and the connection with the cytoskeleton also need to be addressed in detail. Here, complicated signaling and regulatory cascades are involved, of which we now only see the tip of the iceberg.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Caveolae
  4. Caveolar Entry of Simian Virus 40
  5. Caveolar Endocytosis of Other Ligands and Membrane Constituents
  6. How Does It Work?
  7. Perspectives
  8. Acknowledgments
  9. References

The authors would like to thank all members of the laboratory for helpful suggestions and valuable comments. Work is supported by grants from the Swiss National Science Foundation and the Swiss Federal Institute of Technology.

References

  1. Top of page
  2. Abstract
  3. Caveolae
  4. Caveolar Entry of Simian Virus 40
  5. Caveolar Endocytosis of Other Ligands and Membrane Constituents
  6. How Does It Work?
  7. Perspectives
  8. Acknowledgments
  9. References
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