Caveolae

Caveolae are invaginations of the plasma membrane. They contain the protein caveolin and are rich in certain phospholipids. Similar to coated pits, they bud off internally forming endocytic vesicles. Caveolae play an important role in the internalization of certain cell surface receptors. 

Fyn is a nonreceptor tyrosine kinase related to Src that is frequently found in cell junctions. Die protein is N-myristoylated and palmitoylated and thereby becomes associated with caveolae-like membrane microdomains. Fyn can interact with a variety of other signaling molecules and control a diversity of biological processes such as T cell receptor signaling, regulation of brain function, and adhesion mediated signaling. 

Similar to nNOS, Ca2+-activated calmodulin is important for the regulation of eNOS activity. However, several other proteins interact with eNOS and regulate its activity. Heat shock protein 90 (hsp90) is found associated with eNOS and probably acts as an allosteric modulator that activates the enzyme. Caveolin-1 binds eNOS and directs it to caveolae. Caveolin-1 is viewed as an inhibitor of eNOS activity, which is being replaced by CaM upon activation of endothelial cells [2]. 

Pelkmans L, Fava E, Grabner H, Hannus M, Habermann B, Krausz E, Zerial M (2005) Genomewide analysis of human kinases in clathrin- and caveolae/raft-mediated endocytosis. Nature 436 78-86... 

Lipid Rafts Caveolae Are Special Features of Some Membranes... 

While the fluid mosaic model of membrane stmcture has stood up well to detailed scrutiny, additional features of membrane structure and function are constantly emerging. Two structures of particular current interest, located in surface membranes, are tipid rafts and caveolae. The former are dynamic areas of the exo-plasmic leaflet of the lipid bilayer enriched in cholesterol and sphingolipids they are involved in signal transduction and possibly other processes. Caveolae may derive from lipid rafts. Many if not all of them contain the protein caveolin-1, which may be involved in their formation from rafts. Caveolae are observable by electron microscopy as flask-shaped indentations of the cell membrane. Proteins detected in caveolae include various components of the signal-transduction system (eg, the insutin receptor and some G proteins), the folate receptor, and endothetial nitric oxide synthase (eNOS). Caveolae and lipid rafts are active areas of research, and ideas concerning them and their possible roles in various diseases are rapidly evolving. 

Marx J Caveolae a once-elusive structure gets some respect. Science 2001 294 1862. 

New information has been added in appropriate chapters on hpid rafts and caveolae, aquaporins, connexins, disorders due to mutations in genes encoding proteins involved in intracellular membrane transport, absorption of iron, and conformational diseases and pharmacogenomics. 

Uittenbogaard, A, Everson, WV, Matveev, SV, and Smart, EJ, 2002. Cholesteryl ester is transported from caveolae to internal membranes as part of a caveolin-annexin II lipid-protein complex. J Biol Chem 277,4925—4-931. 

LDH-FITC is well overlapped with red fluorescent clathrin-TR, but not with caveolin-1-TR (Figure 13.10). This is dear evidence that clathrin-mediated endocytosis is the prindpal mechanism for the cellular internalization of LDH particles. Caveolae-mediated endocytosis, if any, seems not to be responsible for LDH uptake. 

Rejman J, Oberle V, Zuhom IS, Hoekstra D (2004) Size-dependent internalization of particles via the pathways of clathrin- and caveolae-mediated endocytosis. Biochem J 377 159-169... 

Rejman J, Bragonzi A, Conese M (2005) Role of clathrin- and caveolae-mediated endocytosis in gene transfer mediated by lipo- and polyplexes. Mol Ther 12 468 174... 

Although apoE HDL particles are formed by astrocytes in vitro, the brain contents of apoE knockout (-/-) were not found to differ in lipid content in comparison to those obtained from normal animals [14]. A probable explanation is that newly synthesized cholesterol can be transported from astrocyte ER to plasma membrane via an alternative route that employs caveolae to form apoAl-HDL [15]. 

Liu, L., Mohammadi, K., Aynafshar, B. et al. Role of caveolae in the signal transducing function of cardiac Na+/K+-ATPase. Am. J. Physiol. Cell Physiol. 284 C1550-C1560, 2003. 

Other pinocytotic pathways also exist that do not depend on either caveolae or clathrin, although these are not as well defined [55]. Specific receptors continue to be internalized in the absence of clathrin or caveolin and these pathways can be monitored by following glycosyl phos-phatidylinositol (GPI (-anchored proteins. Nonclathrin, noncaveolin pathways may also be responsible for the reuptake of membrane in neuroendocrine cells after stimulated secretion. Some, but not all, of these pathways appear to require dynamin. 

There is growing evidence that the membrane receptors for estrogens are very important in tissues as the vascular endothelium (Chambliss et al. 2002 Hodgin et al. 2002 Mendelsohn 2002 White 2002). In the endothelial cells ERs appear to be located in specific zones of the membranes called caveolas, but not in the greater part of the membrane. Such receptors mediate rapid responses to... 

Kim HP, Lee JY, Jeong JK, Bae SW, Lee HK, Jo I (1999) Nongenomic stimulation of nitric oxide release by estrogen is mediated by estrogen receptor alpha localized in caveolae. Biochem Biophys Res Commun 263(l) 257-262... 

Taggart MJ 2001 Smooth muscle excitation-contraction coupling a role for caveolae and caveolins News Physiol Sci 16 61-65... 

FIG. 4. Ultrastructure of vascular smooth muscle of the rabbit inferior vena cava revealed with electron microscopy. Serial cross-sections of VSMCs are shown in series 1 (panel A—D) and series 2 (panel E—G). Series 1 illustrates the close spatial apposition between the superficial SR sheet and the PM with the apices of the caveolae perforating through the superficial SR sheets to come into contact with the bulk cytoplasm. The membranes of the PM (dotted line) and the SR (solid line) in panel A-D are outlined to the right of the respective panels. The close apposition between the superficial SR sheet, the PM and the neck region of the caveolae creates a narrow and expansive restricted space. Series 2 illustrates the perpendicular sheets of SR, which appear to arise from the superficial SR sheets. Mitochondria also come into close contact with the perpendicular SR sheets. Panel H contains a stylized illustration of the close association between the superficial SR sheet, which is continuous with the perpendicular sheet, the perforating caveolae (C), the PM and a mitochondrion (M). Panel I shows calyculin-A mediated dissociation of the superficial SR sheets from the PM (see arrows). The black scale bar indicated represents 200 nm of distance. 

Isenberg I would like to address the question of the diffusional barrier once more. There is one argument that this barrier is flat sheet. On the other hand, Casey van Breemen has shown that in the region of the caveolae, there are interruptions of... 

Hlaustein The question is whether those holes are holes in the barrier or not. If you look at the reticulum where the caveolae stick up, this is not a hole in the barrier because the diffusion is restricted within the barrier itself. These holes are places where the plasma membrane at the top of the caveolae can actually communicate with bulk cytoplasm. This is a different region than that between the plasma membrane and the reticulum. 

Somljo It is within the measurement errors. There is a fenestration of the SR sheet, and sticking out come the caveolae. No one has really measured accurately this distance, or the distance between the caveolae and SR on top. The surface coupling space is pretty consistent. With regard to what is different in smooth muscle, if you are talking about the SR at the junction having Ca-ATPase or not, we don t know. What we do know from freeze-fracture studies of striated muscle is that the Ca-ATPase does not seem to be at the SR terminal cisternae. We don t know the answer in smooth muscle, but if there is Ca-ATPase at the junctional surface itself, this is different from what one sees in striated muscle. 

Brading How close would the caveolae membrane get to the SR where it butts through ... 

Brading So if the TRP channels were in the caveolae they would be close to the SR. 

Bolton-. The SR normally underlies the rows of caveolae. Between those are the dense bars where the contractile proteins are attached. It seems likely that the caveolae contain receptors and channels, and that the SR occurs under the voltage-dependent channels. 

FIG. 5. View of the SR network in a longitudinally oriented rabbit portal vein smooth muscle cell stained with osmium ferricyanide. Note the close apposition of the SR to the plasma membrane (small arrows) and caveolae, as well as its relationship to mitochondria (M) in some instances the SR completely surrounds mitochondria. 

Bolton I want to ask your opinion on a quantitative point. How far are the caveolae and their associated SR from the plasma membrane My point is, if it is a small distance, let s say some 0.2 /tm, which is just below the resolution of the confocal microscope so that the caveolae, their SR and the plasma membrane cannot be separately resolved, then the plasma membrane ought to stain strongly with DiOCg or labelled ryanodine. Is this the case ... 

Somlyo Dil18 stains the whole plasma membrane and we can t tell the difference between the caveolae and the non-caveolar membrane. We also have a pretty good idea that when we see dense bodies this pretty much excludes SR and caveolae. At this resolution we would not want to distinguish caveolae. The best lateral resolution a confocal will give is about 0.2 fim, and by the time you get close to... 

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