Caveolae Vesicles Research Articles

Transcellular transport of CCL2 across brain microvascular endothelial cells

The means by which the chemokine CCL2 produced in the brain parenchyma can recruit leukocytes lying behind the highly impervious endothelium of the blood-brain barrier (BBB) has remained a paradox. As other chemokines have been evidenced to stimulate their own synthesis and release by peripheral microvascular endothelial cells, and/or undergo transcytosis in the abluminal-to-luminal direction, we determined whether CCL2 experiences similar fates across brain microvascular endothelial cells (BMEC). Using cultured BMEC as a paradigm of the BBB, it was observed that exogenous unlabeled CCL2 actually depressed the release of endogenous CCL2, and further caused diminished CCL2 mRNA levels in these cells. On the other hand, exogenous (125)I-labeled CCL2 exhibited transport across BMEC in a manner that was sensitive to temperature, competition by excess unlabeled CCL2 but not unlabeled CCL3, knockdown of caveolin-1/caveolae, and elimination of the cognate CCL2 receptor CCR2. These results implied a facet of CCL2 transport by a transcellular mechanism partly involving binding of CCL2 to CCR2, and subsequent transfer to caveolae vesicles for transcytosis. This notion was supported by double-label immuno-electronmicroscopy, which revealed co-localization of caveolin-1 with exogenous CCL2, during this chemokine's transit across BMEC. Collectively, these findings provide a rationale by which CCL2, deposited on the abluminal side of the brain microvasculature during inflammatory episodes, can be relayed across the BBB to foster leukocyte recruitment.

Sodium pumps: ouabain, ion transport, and signaling in hypertension

TO THE EDITOR: A recent Editorial Focus in this journal (7) addresses the newly recognized role of the sodium pump as both an ion transport protein and a hormone receptor with its own signaling cascade. The relative importance of the cardiotonic steroids (CTS) as endogenous signaling hormones versus their ability to inhibit the sodium pump is of great interest. Indeed, the article noted that “although all digitalislike compounds inhibit Na-K-ATPase-mediated transport, they differ considerably in terms of their hypertensive effects.” The author concludes that “these findings do not support the prevailing hypothesis that the pathophysiological effect of digitalis results from inhibition of . . . transport activity.” This implies that the hypertensive effect of CTS may be due to the action of these agents as regulators of signal transduction, entirely unrelated “to its (Na-K-ATPase’s) role as an ion pump.” The discoveries related to Src kinase-mediated signal transduction by Na-K-ATPase add a new dimension to the view that the endogenous cardiotonic steroids comprise a novel hormone system. Nevertheless, the view that CTS induced hypertension is unrelated to inhibition of ion transport is inconsistent with published reports. Regrettably, the Editorial Focus does not cite key articles that support the conclusion that CTS hypertension is secondary to inhibition of sodium pumps. It is true that the tendency of certain CTS to induce hypertension does not correlate with their relative potency as Na-KATPase inhibitors (8, 9). The most obvious comparison is that ouabain induces hypertension while digoxin (1, 9) and a digitoxigenin derivative, PST-2238 (4), are antihypertensive. This need not imply that ouabain’s effect on blood pressure is unrelated to inhibition of Na transport. Instead, these data are consistent with the interpretation that some CTS have mixed agonist/antagonist properties. Viability of the hypothesis that enhancement of cytoplasmic Ca 2 responses by ouabain is related to inhibition of Na-KATPase relies on the secondary effect of Na on Na/Ca exchange. There is now direct evidence that elevation of blood pressure is a consequence of reduced transport activity of the ouabain-sensitive Na-K-ATPase 2-isoform (2, 3, 10) and the resultant Ca 2 gain mediated by Na/Ca exchange (3, 5, 10). Moreover, reduced 2 expression leads to murine hypertension (10). The vascular myogenic effect of that model is prevented by the Na/Ca exchange inhibitor SEA0400 (10). A key role for Na/Ca exchange is revealed by the observation that SEA0400 lowers blood pressure in ouabain-induced hypertension and other forms of salt-dependent hypertension, but not salt-independent hypertension (5). These findings demonstrate that ouabain’s hypertensive effect depends on its inhibition of the Na-K-ATPase and Ca 2 gain via Na/Ca exchange. Thus Src signaling, alone, appears to be insufficient to raise blood pressure in ouabain-induced hypertension although an accessory role is not excluded. Comparison of the effect of reduced expression of the 2 Na-K-ATPase (6) with that of low-dose ouabain on cell signaling may be a useful way to determine the roles of altered Na and Ca 2 in mediating the effects of ouabain on Src kinase signaling.

Participation of caveolae in β1 integrin-mediated mechanotransduction

We previously reported that caveolin-1 is a key component in a β1 integrin-dependent mechanotransduction pathway suggesting that caveolae organelles and integrins are functionally linked in their mechanotransduction properties. Here, we exposed BAEC monolayers to shear stress then isolated caveolae vesicles form the plasma membrane. While little β1 integrin was detected in caveolae derived from cells kept in static culture, shear stress induced β1 integrin transposition to the caveolae. To evaluate the significance of shear-induced β1 integrin localization to caveolae, cells were pretreated with cholesterol sequestering compounds or caveolin-1 siRNA to disrupt caveolae structural domains. Cholesterol depletion attenuated integrin-dependent caveolin-1 phosphorylation, Src activation and Csk association with β1 integrin. Reduction of both caveolin-1 protein and membrane cholesterol inhibited downstream shear-induced, integrin-dependent phosphorylation of myosin light chain. Taken together with our previous findings, the data supports the concept that β1 integrin-mediated mechanotransduction is mediated by caveolae domains.

Caveolin-1 in extracellular matrix vesicles secreted from osteoblasts

Caveolin-1 is an essential and signature protein of caveolae, which are small invaginations of the plasma membrane enriched in cholesterol and sphingolipids. Although high levels of expression of caveolin-1 have been demonstrated in osteoblasts as well as endothelial cells, fibroblasts, and muscular cells, the role of caveolin-1 in osteoblasts has not been clarified. Here, we show that caveolin-1 is secreted from osteoblasts in the form of matrix vesicles; extracellular vesicles released from the plasma membrane of osteoblasts. In this study, caveolae and matrix vesicles were similarly enriched in cholesterol and sphingomyelin in fractions isolated from mineralizing MC3T3-E1 cells. Interestingly, in the MC3T3-E1 cells caveolin-1 was enriched in the matrix vesicle fraction as well as the caveolar membrane fraction, and the amount of caveolin-1 in the matrix vesicle fraction increased as differentiation progressed. Localization of caveolin-1 in matrix vesicles was also confirmed in murine tibia. Furthermore, overexpression of caveolin-1 enhanced matrix calcification in MC3T3-E1 cells, whereas knockdown of caveolin-1 diminished it. These results suggest that secreted caveolin-1 as a component of matrix vesicles may play an important role in osteoblast calcification.

Participation of caveolae in beta‐1 integrin‐mediated mechanotransduction

Experimental evidence suggests that caveolae and integrins function as primary mechanosensors. We recently reported that shear stress-induced activation of β1 integrin resulted in Src-family kinase dependent phosphorylation of caveolin-1, a process that was necessary for recruitement of signaling molecules to β1 integrins sites and propagation of a mechanotransduction response. Based on these findings, we propose that caveolae and integrins are functionally linked in their mechanotransduction properties. Here, we exposed BAEC monolayers to shear stress then isolated caveolae vesicles form the plasma membrane. While little β1 integrin was present in caveolae derived from cells kept in static culture, shear stress induced β1 integrin transposition to the caveolae. Csk was detected in caveolae derived from non-shear cells and additional Csk was recruited to caveolae after shear. To evaluate the significance of shear-induced β1 integrin localization to caveolae, cells were pretreated with methyl-β-cyclodextrin (CD) to disrupt rafts/caveolae. CD attenuated integrin-dependent caveolin-1 phosphorylation, Csk association with β1 integrin and myosin light chain phosphorylation induced by shear. Taken together with our previous findings, the data supports the concept that β1 integrin-mediated mechanotransduction is mediated by raft/caveolae domains. Supported by NIH HL66301.

Peculiarities of Host Cholesterol Transport to the Unique Intracellular Vacuole Containing Toxoplasma

The intracellular protozoan Toxoplasma gondii is auxotrophic for low-density lipoprotein (LDL)-derived cholesterol (C). We previously showed that T. gondii scavenges this essential lipid from host endolysosomal compartments and that C delivery to the parasitophorous vacuole (PV) does not require transit through host Golgi or endoplasmic reticulum. In this study, we explore the itinerary of C from the host endolysosomes to the PV. Labeled C incorporated into LDL is rapidly detected in intravacuolar parasites and partially esterified by the parasites. In contrast to diverse mammalian organelles, the post-endolysosomal transfer of C to the PV does not involve the host plasma membrane as an intermediate. Nevertheless, the PV membrane is accessible to extracellular sterol acceptors, suggesting C trafficking from intracellular parasites to host plasma membrane. C movement to the PV requires temperatures permissive for vesicular transport, metabolic energy and functional microtubules. Host caveolae vesicles and the sterol carrier protein-2 do not participate in this process. Proteolytic treatment of purified PV or free parasites abolishes C acquisition by the parasites. Altogether, these results support a vesicular transport system from host endolysosomes to the PV, and a requirement for PV membrane and parasite plasma membrane proteins in C delivery to T. gondii.

Caveolar endocytosis and virus entry

The endocytic function of caveolae has been controversial for a long time. However, a real-time-imaging analysis of Simian virus 40 (SV40) 's entry in cells has indicated the existence of caveolar endocytosis during virus entry. The caveolae engulfed SV40 virions begin budding from plasma membrane depending on dynamin. SV40 enclosed in caveolae vesicles move to the caveosome, then to the endoplasmic reticulum. In addition, it was demonstrated that human coronavirus-229E enters the cell through caveolae. This review examines the involvement of caveolae in endocytosis used by the viral entry system.

N-terminal processing and modifications of caveolin-1 in caveolae from human adipocytes

Caveolin, the principal structural protein of caveolae membrane domains, has a cytosol-exposed N-terminal part that was cleaved off by trypsin treatment of caveolae vesicles isolated from primary human adipocytes. Sequencing of the released tryptic peptides by nanospray quadrupole time-of-flight mass spectrometry revealed that both caveolin-1α and caveolin-1β were processed by excision of the starting methionines. The N-terminus of the mature caveolin-1α was acetylated, while caveolin-1β was found in acetylated as well as in non-acetylated forms. Fractional phosphorylation of serine-36 in the mature caveolin-1α and of the homologous serine-5 in caveolin-1β was identified. This is the first experimental evidence for in vivo phosphorylation of caveolin-1 at the consensus site for phosphorylation by protein kinase C. The phosphorylation was found in both the acetylated and non-acetylated variants of caveolin-1β. This variability in modifications is consistent with critical involvement of the N-terminal domain of caveolin in the regulation of caveolae.

Caveolae: an alternative membrane transport compartment.

Caveolae are omega-shaped invaginations of the plasma membrane with a diameter of 50-100 nm. Caveolae invaginations can detach from the plasma membrane to form discrete functional caveolae vesicles within the cell cytoplasm. Caveolae are most prominent in adipocytes, fibroblasts, muscle cells (skeletal, smooth and cardiac), capillary endothelium and type I pneumocytes, although other cell types also display these structures but at a lower numerical density. The key structural and functional protein for caveolae is caveolin. At the plasma membrane caveolae serve to compartmentalize and integrate a wide range of signal transduction processes. Caveolae also serve transport functions including that of the vesicular internalisation of small molecules by the process of potocytosis, and the endocytic and transcytotic movements of macromolecules. Opportunities exist for basic and applied investigators working within the pharmaceutical sciences to exploit caveolae membrane interactions with the aim to develop novel cellular or transcellular drug delivery strategies.

Testosterone signaling through internalizable surface receptors in androgen receptor-free macrophages.

Testosterone acts on cells through intracellular transcription-regulating androgen receptors (ARs). Here, we show that mouse IC-21 macrophages lack the classical AR yet exhibit specific nongenomic responses to testosterone. These manifest themselves as testosterone-induced rapid increase in intracellular free [Ca(2+)], which is due to release of Ca(2+) from intracellular Ca(2+) stores. This Ca(2+) mobilization is also inducible by plasma membrane-impermeable testosterone-BSA. It is not affected by the AR blockers cyproterone and flutamide, whereas it is completely inhibited by the phospholipase C inhibitor U-73122 and pertussis toxin. Binding sites for testosterone are detectable on the surface of intact IC-21 cells, which become selectively internalized independent on caveolae and clathrin-coated vesicles upon agonist stimulation. Internalization is dependent on temperature, ATP, cytoskeletal elements, phospholipase C, and G-proteins. Collectively, our data provide evidence for the existence of G-protein-coupled, agonist-sequestrable receptors for testosterone in plasma membranes, which initiate a transcription-independent signaling pathway of testosterone.

Caveolin forms a hetero-oligomeric protein complex that interacts with an apical GPI-linked protein: implications for the biogenesis of caveolae.

Glycosyl-phosphatidylinositol (GPI)-linked proteins are transported to the apical surface of epithelial cells where they undergo cholesterol-dependent clustering in membrane micro-invaginations, termed caveolae or plasmalemmal vesicles. However, the sorting machinery responsible for this caveolar-clustering mechanism remains unknown. Using transfected MDCK cells as a model system, we have identified a complex of cell surface molecules (80, 50, 40, 22-24, and 14 kD) that interact in a pH- and cholesterol-dependent fashion with an apical recombinant GPI-linked protein. A major component of this hetero-oligomeric protein complex is caveolin, a type II transmembrane protein. As this hetero-oligomeric caveolin complex is detectable almost immediately after caveolin synthesis, our results suggest that caveolae may assemble intracellularly during transport to the cell surface. As such, our studies have implications for understanding both the intracellular biogenesis of caveolae and their subsequent interactions with GPI-linked proteins in epithelia and other cell types.