Intersectin, a multiple Eps15 homology and Src homology 3 (SH3) domain-containing protein, is a component of the endocytic machinery in neurons and nonneuronal cells. However, its role in endocytosis via caveolae in endothelial cells (ECs) is unclear. We demonstrate herein by coimmunoprecipitation, velocity sedimentation on glycerol gradients, and cross-linking that intersectin is present in ECs in a membrane-associated protein complex containing dynamin and SNAP-23. Electron microscopy (EM) immunogold labeling studies indicated that intersectin associated preferentially with the caveolar necks, and it remained associated with caveolae after their fission from the plasmalemma. A cell-free system depleted of intersectin failed to support caveolae fission from the plasma membrane. A biotin assay used to quantify caveolae internalization and extensive EM morphological analysis of ECs overexpressing wt-intersectin indicated a wide range of morphological changes (i.e., large caveolae clusters marginated at cell periphery and pleiomorphic caveolar necks) as well as impaired caveolae internalization. Biochemical evaluation of caveolae-mediated uptake by ELISA showed a 68.4% inhibition by reference to control. We also showed that intersectin interaction with dynamin was important in regulating the fission and internalization of caveolae. Taken together, the results indicate the crucial role of intersectin in the mechanism of caveolae fission in endothelial cells.

Predescu SA, Predescu DN, Malik AB. Molecular determinants of endothelial transcytosis and their role in endothelial permeability.

Basal vascular endothelial permeability is normally kept low in part by the restrictiveness of interendothelial junctions (IEJs). We investigated the possible role of nitric oxide (NO) in controlling IEJ integrity and thereby regulating basal vascular permeability. We determined the permeability of continuous endothelia in multiple murine vascular beds, including lung vasculature, of wild-type mice, endothelial nitric oxide synthase (eNOS) null mice, and mice treated with NOS inhibitor N-nitro-L-arginine methyl ester (L-NAME). Light and electron microscopic studies revealed that L-NAME treatment resulted in IEJs opening within a few minutes with a widespread response within 30 min. We observed a 35% increase in transendothelial transport of albumin, using as tracer dinitrophenylated albumin in mouse lungs and other organs studied. To rule out the involvement of blood cells in the mechanism of increased endothelial permeability, vascular beds were flushed free of blood, treated with L-NAME, and perfused with the tracer. The open IEJs observed in these studies indicated a direct role for NO in preserving the normal structure of endothelial junctions. We also used the electron-opaque tracer lanthanum chloride to assess vascular permeability. Lanthanum chloride was presented by perfusion to various vascular beds of mice lacking NO. Open IEJs were seen only in capillary and venular endothelial segments of mice lacking NO, and there was a concomitant increase in vascular permeability to the tracer. Together, these data demonstrate that constitutive eNOS-derived NO is a crucial determinant of IEJ integrity and thus serves to maintain the low basal permeability of continuous endothelia.

Caveolin-1, the principal integral membrane protein of caveolae, has been implicated in regulating the structural integrity of caveolae, vesicular trafficking, and signal transduction. Although the functions of caveolin-1 are beginning to be explored in caveolin-1-/- mice, these results are confounded by unknown compensatory mechanisms and the development of pulmonary hypertension, cardiomyopathy, and lung fibrosis. To address the role of caveolin-1 in regulating lung vascular permeability, in the present study we used small interfering RNA (siRNA) to knock down caveolin-1 expression in mouse lung endothelia in vivo. Intravenous injection of siRNA against caveolin-1 mRNA incorporated in liposomes selectively reduced the expression of caveolin-1 by approximately 90% within 96 h of injection compared with wild-type mice. We observed the concomitant disappearance of caveolae in lung vessel endothelia and dilated interendothelial junctions (IEJs) as well as increased lung vascular permeability to albumin via IEJs. The reduced caveolin-1 expression also resulted in increased plasma nitric oxide concentration. The nitric oxide synthase inhibitor L-NAME, in part, blocked the increased vascular albumin permeability. These morphological and functional effects of caveolin-1 knockdown were reversible within 168 h after siRNA injection, corresponding to the restoration of caveolin-1 expression. Thus our results demonstrate the essential requirement of caveolin-1 in mediating the formation of caveolae in endothelial cells in vivo and in negatively regulating IEJ permeability.

We investigated the location and the structural identity of the small pore system, postulated by the pore theory of capillary permeability, using a murine heart perfusion system and small protein molecules as preferential probes for the small pores. Dinitrophenylated proteins were perfused in situ in the absence and in the presence of N-ethylmaleimide (NEM), a reagent known to interfere with membrane fusion of vesicular carriers with their target membranes. The exit pathways of the tracers from vascular lumina to the interstitia were followed by immunoelectron microscopy and by tissue fractionation biochemistry to quantitate their transport and to estimate the extent of transport inhibition by NEM. After 5 min of perfusion, all tracers used were found essentially restricted to plasmalemmal vesicles (PVs) within the endothelium and NEM inhibited their transport by 80-85%. The transport of [14C]inulin and [14C]sucrose, assumed to follow the paracellular pathway, was marginally affected by NEM. These findings indicate that PVs function as structural equivalents of the small pore system for molecules >2 nm in diameter.

In a murine heart perfusion system, we were able to "turn off" the transport of derivatized albumin [dinitrophenylated albumin (DNP-albumin)] from the perfusate to the tissue, by preperfusing the system with 1 mM N-ethylmale~imide (NEM) for 5 min at 37C, followed by a 5-min perfusion of DNP-albumin in the presence of NEM. Using a postembedding immunocychemica procedure, we showed that (t0 a 30-sec to 1-min treatment of heart vasculature with 1 mM NEM reduces the traniendothelal transport of DNP albumin and nearly stops it after 5 min, and (is) DNP-albumin is detected exclusively in plasmalemmal vesicles (PVs) PVs are organized in sessile dendritic structures with no continuity across the endothelium and only a few free vesicles scattered in the cytoplasm (6, 7). More recently PVs (in fibroblasts) have been ascribed to an entirely different function, called "potocytosis" (8,9), assumed to involve uptake of substrate followed by digestion and transport to the cytosol. Explicitly or implicitly, the conclusions reached in refs. 6 and 8 are not compatible with PV transcytosis in vascular endothelia.Transcytosis implies repeated PV fission from one domain of the plasmalemma coupled with repeated PV fusion to the opposite domain, in a process generally comparable to that at work in transport by vesicular carriers along the intracellular exocytic and endocytic pathways. Work on reconstituted vesicular carrier transport in cell-free systems (10, 11) has identified an N-ethylmaleimide (NEM)-sensitive factor (NSF) as an essential component of a "membrane fusion machine" required for vesicular transport along the pathways mentioned above (12,13). Although the PVs belong to a different type of vesicular carriers than those so far studied (14, 15), we surmised that their fusion to the target membrane may depend on NSF (or related proteins) and, hence, be sensitive to NEM. To test this assumption, we have studied quantitatively the transcytosis of DNP-albumin perfused through the microvasculature of murine heart in the presence or absence of NEM in the perfusate. The results presented in this paper indicate that NEM severely inhibits transcytosis of DNP-albumin by PVs. MATERIALS AND METHODS Animals. The studies were performed on 15-to 20-g BALB/c male mice kept under standard housing conditions and anesthetized intraperitoneally as described (16).Antibodies and Reagents. Rabbit anti-DNP and the horseradish peroxidase (HRP)-conjugated anti-DNP antibodies were from Dako; reporter antibodies-i.e., goat anti-rabbit IgG, alkaline phosphatase (AP)-conjugated goat anti-rabbit IgG, and HRP-conjugated goat anti-rabbit IgG-were from Cappel and Kirkegaard and Perry Laboratories. All other chemicals were of reagent grade.Preparation ofProbes. DNP-albumin prepared as described (3) was separated from free DNP by gel filtration over PD-10 columns from Pharmacia. The reporter antibody, 9-nm-goldtagged goat anti-rabbit IgG, was prepared and stabilized as described (17), stored at 40C in phosphate-buffered saline (PBS) containing 2...

We determined the organization of target (t) SNARE proteins on the basolateral endothelial plasma membrane (PM) and their role in the mechanism of caveolar fusion. Studies were performed in a cellfree system involving endothelial PM sheets and isolated biotinlabeled caveolae. We monitored the fusion of caveolae with the PM by the detection of biotin-streptavidin complexes using correlative high resolution fluorescence microscopy and gold labeling electron microscopy on ultrathin sections of PM sheets. Imaging of PM sheets demonstrated and biochemical findings showed that the t-SNARE proteins present in endothelial cells (SNAP-23 and syntaxin-4) formed cholesterol-dependent clusters in discrete areas of the PM. Upon fusion of caveolae with the target PM, 50% of the caveolae co-localized with the t-SNARE clusters, indicating that these caveolae were at the peak of the fusion reaction. Fluorescent streptavidin staining of PM sheets correlated with the ultrastructure in the same area. These findings demonstrate that t-SNARE clusters in the endothelial target PM serve as the fusion sites for caveolae during exocytosis. The endothelial cells (ECs)3 lining the blood vessel walls are differentiated to mediate the rapid exchange of substances between the plasma and interstitial fluid. The process involving the fission of caveolae from the apical plasma membrane (PM) and fusion with the basolateral PM is termed transcytosis. Caveolae "pinch off " from the apical PM through a process requiring the recruitment of the GTPase dynamin to the caveolar necks as regulated by another protein intersectin (1, 2). Upon fission, caveolae form vesicular carriers that shuttle through the cytosol and deliver their cargo to the subendothelium by fusion with the basolateral PM (3). The basis of caveolar fusion may involve the same machinery as other vesicular carriers, i.e. the SNARE proteins of the syntaxin, synaptobrevin/cellubrevin, and SNAP-23/25 families (4 -7). However, the organization of SNARE proteins and their function in the fusion of caveolae in ECs are incompletely understood. It is known that syntaxin, cellubrevin, SNAP-23, and the cytosolic factors N-ethylmaleimide-sensitive factor (NSF) and ␣-and ␥-SNAP are key components of the endothelial multimolecular transcytotic complex, the assembly of which depends on the membrane fusion ATPase, NSF, which can be inhibited by alkylation of NSF by N-ethylmaleimide (7). Studies have also shown that N-ethylmaleimide interferes with transcytosis in ECs (4, 8). Based on the SNARE hypothesis, membrane fusion occurs when SNARE proteins on opposing membranes form four helix bundles, bringing the membranes in close apposition, thus providing the driving force necessary for fusion (9, 10). Ultrastructural studies have shown several states of association between caveolae and their target PM, varying in proximity, stability, and readiness for fusion (11). However, the pre-fusion states of caveolae have not been experimentally clarified, and the sequence of events responsible for caveolar fusion...

We have demonstrated that the plasmalemmal vesicles (caveolae) of the continuous microvascular endothelium function as transcytotic vesicular carriers for protein molecules Ͼ20 Å and that transcytosis is an N-ethylmaleimide-sensitive factor (NSF)-dependent, N-ethylmaleimide-sensitive process. We have further investigated NSF interactions with endothelial proteins to find out 1) whether a complete set of fusion and targeting proteins is present in the endothelium; 2) whether they are organized in multimolecular complexes as in neurons; and 3) whether the endothelial multimolecular complexes differ from their neuronal counterparts, because of their specialized role in transcytosis. To generate the complexes, we have used myc-NSF, cultured pulmonary endothelial cells, and rat lung cytosol and membrane preparations; to detect them we have applied coimmunoprecipitation with myc antibodies; and to characterize them we have used velocity sedimentation and cross-linking procedures. We have found that both cytosolic and membrane fractions contain complexes that comprise beside soluble NSF attachment proteins and SNAREs (soluble NSF attachment protein receptor), rab 5, dynamin, caveolin, and lipids. By immunogold labeling and negative staining we have detected in these complexes, myc-NSF, syntaxin, dynamin, caveolin, and endogenous NSF. Similar complexes are formed by endogenous NSF. The results indicate that complexes with a distinct protein-lipid composition exist and suggest that they participate in targeting, fusion, and fission of caveolae with the endothelial plasmalemma.