The NO cascade, eNOS location, and microvascular permeability

Abstract

The nitric oxide (NO) cascade and endothelial NO synthase (eNOS) are best known for their role in endothelium-mediated relaxation of vascular smooth muscle. Activation of eNOS by certain inflammatory stimuli and enhanced NO release have also been shown to promote increased microvascular permeability. However, it is not entirely clear why activation of eNOS by certain vasodilatory agents, like acetylcholine, does not affect microvascular permeability, whereas activation of eNOS by other inflammatory agents that increase permeability, like platelet-activating factor, does not cause vasodilation. In this review, we discuss the evidence demonstrating the role of eNOS in the elevation of microvascular permeability. We also examine the relative importance of eNOS phosphorylation and localization in its function to promote elevated microvascular permeability as well as emerging topics with regard to eNOS and microvascular permeability regulation.

Figure 1

Simplified scheme of pro-inflammatory signal transduction pathways leading to eNOS activation and endothelial hyperpermeability. Binding of pro-inflammatory agonists to their receptors activates a myriad of intracellular second messengers, leading to release of calcium from intracellular stores, PKC activation, Akt/PKB activation, and ERK-1/2 activation. The overall result is a functional state favouring changes in phosphorylation state of eNOS and elevated production of NO and increased permeability of the microvascular barrier. Several elements of the overall signalling paradigm remain elusive, such as the downstream effectors linking eNOS-derived NO to changes in the cytoskeleton and intercellular junctions that produce the hyperpermeable state, the precise role of the balance between cGMP and cAMP [via phosphodiesterases (PDE)] in permeability regulation. The mechanisms involved in the restoration of basal permeability and/or promotion of enhanced barrier integrity are represented by the Epac/Rap1 pathway.

Figure 2

Putative mechanisms by which eNOS-derived NO causes increased microvascular permeability. A logical sequence supported by experimental data is proposed. (1) During a non-inflammatory state, eNOS is associated with caveolin-1 (depicted as a hairpin) and localized primarily in caveolae at the plasma membrane. (2) Activation of EC by pro-inflammatory signals causes caveolae to pinch off the plasma membrane. Scission of caveolae from the membrane leads to eNOS internalization into the cell. (3) Within the cell, eNOS dissociates from caveolin-1 and may undergo phosphorylation. (3′A) It is also possible that eNOS may dissociate from the caveolar membranes. (4) The combination of activation of eNOS by calcium–calmodulin, dissociation of eNOS from caveolin-1 and eNOS phosphorylation significantly increases the production of NO by eNOS. (5) NO then can bind intracellular target(s) that remain to be identified, which produce signals that (6) alter the junctional proteins between EC and increase microvascular permeability.