Caveolae in smooth muscles: nanocontacts

Abstract

Smooth muscle cell (SMC) caveolae have been investigated by quantitative and qualitative analysis of transmission electron microscopy (TEM) images of rat stomach, bladder and myometrium, guinea pig taenia coli, human ileum, and rat aortic SMCs. Ultrathin (below 30 nm) serial sections were used for examination of caveolar morphology and their connections with SMC organelles. Average caveolar diameter was smaller in vascular SMCs (70 nm, n=50) than in visceral SMCs (77 nm, n=100), but with the same morphology. Most of the caveolae, featured as flask-shaped plasma membrane (PM) invaginations, opened to the extracellular space through a 20 nm stoma (21, 3nm) having a 7 nm thick diaphragm. A small percentage of caveolae (3%), gathered as grape-like clusters, did not open directly to the extracellular space, but to irregular PM pockets having a 20-30 nm opening to the extracellular space. In visceral SMCs, caveolae were disposed in 4 - 6 rows, parallel to myofilaments, whilst aortic SMCs caveolae were arranged as clusters. This caveolar organization in rows or clusters minimizes the occupied volume, providing more space for the contractile machinery. The morphometric analysis of relative volumes (% of cell volume) showed that caveolae were more conspicuous in visceral than in vascular SMCs (myometrium - 2.40%; bladder - 3.66%, stomach - 2.61%, aorta - 1.43%). We also observed a higher number of caveolae per length unit of cell membrane in most visceral SMCs compared to vascular SMCs (myometrium - 1.06/μm, bladder - 0.74/μm, aorta - 0.57/μm, stomach - 0.48/μm). Caveolae increase the cellular perimeter up to 15% and enlarge the surface area of the plasma membrane about 80% in SMCs. Three-dimensional reconstructions (15μ 3 ) showed that most caveolae, in both visceral and vascular SMCs, have nanocontacts with SR (87%), or with mitochondria (10%), and only 3%, apparently, have no contact with these organelles. Usually, 15 nm wide junctional spaces exist between caveolae and SR, some of them with nanostructural links between each other or with mitochondria: direct contacts (space < 2 nm or none ) and molecular links, so called ‘feet’ (about 12 nm electron dense structures between organellar membranes). Direct contacts possibly allow molecular translocation between the two membranes. Electron-dense ‘feet’-like structures suggest a molecular link between these organelles responsible for intracellular Ca 2+ homeostasis ( excitation-contraction coupling or pharmaco-mechan-ical coupling ). Close appositions (∼15 nm) have also been observed between caveolae and perinuclear SR cisterna, suggesting that caveolae might be directly implicated in excitation-transcription coupling.

Calcium Pumps in Health and Disease

“…One interesting property of the PMCA pump that is related to the multiplicity of its isoforms is its inhomoge- neous distribution in the plasma membrane: in a number of cell types, the pump has been shown to be associated with the caveolae (96), a functionally convenient location, as caveolae, at least in some cell types (e.g., intestinal smooth muscle), are in close contact with the ER (239) and would thus facilitate the extrusion of Ca 2ϩ liberated by it. In addition to caveolae, the cholesterol-rich lipid rafts, which are emerging as important domains in cell signaling, also appear to be loci for specific association with the PMCA.…”

Molecular assembly and dynamics of fluorescent protein-tagged single KCa1.1 channel in expression system and vascular smooth muscle cells

“…Caveolae are a type of lipid raft structure and are observed as ⍀-shaped invaginations on the plasma membrane (40). In VSMCs and endothelial cells, BK channels distribute with high density in caveolae, which contain abundant cholesterol and caveolin proteins and act effectively against agonists and specific stimuli (1,3,49).…”

“…These Ca 2ϩ transients originating from the ryanodine receptor (RyR) on the superficial sarcoplasmic reticulum (SR) activate BK channels located in plasmalemma near the Ca 2ϩ release site (14). Caveolae, ⍀-shaped invaginations on the membrane surface (50ϳ100 nm in diameter), may be essential for the compartmentalization of signaling molecules during physiological events (40). It has been suggested that caveolae are junctional structures of plasmalemma specific to Ca 2ϩ sparks and hotspot sites (27).…”

Regulation of Cellular Communication by Signaling Microdomains in the Blood Vessel Wall

“…The plasma membrane is composed of a variety of lipids, and it is now well understood that specialized lipid regions, especially those enriched with cholesterol, can harbor proteins together to create a signaling platform at the plasma membrane. Perhaps the most well-known of these specialized plasma membrane regions are lipid rafts and caveolin 1 (Cav1)-enriched caveolae, which are known to concentrate membrane receptors, transporters, and other signaling proteins (for review, see Popescu et al, 2006). The Regulation of Cellular Communication in Blood Vessel Wall association of endothelial nitric-oxide synthase (eNOS) with Cav1 can be spatially enriched in the vicinity of a number of cofactors and substrates (Mineo and Shaul, 2012).…”