Beyond tie-ing up endothelial adhesion: new insights into the action of angiopoietin-1 in regulation of microvessel permeability.

The growth factor angiopoietin-1 (Ang1) has been identified as the primary activating ligand for Tie2, a tyrosine kinase receptor highly expressed in vascular endothelial cells. Genetic studies using targeted mutations in mice have demonstrated that Tie2 activation by Ang1 is crucial for angiogenesis, vascular remodelling, and vascular maturation. However, the angiogenic functions of Ang1 are distinct from those of vascular endothelial growth factor (VEGF). Transgenic mice studies have revealed that blood vessels induced by VEGF overexpression are leaky, whereas blood vessels induced by Ang1 overexpression are not only non-leaky, but also resistant to vascular leakage during inflammation. Previous studies using inflammatory mediator-stimulated animals or cultured endothelial cells have identified the permeability protective effects of Ang1 to be mediated through inhibition of endothelial gap formation and strengthening of endothelial adhesion. In a new study featured in this issue of Cardiovascular Research, Salmon et al. report the effect of Ang1 on the endothelial glycocalyx and propose a novel mechanism for the permeability protective action of Ang1. Using individually perfused mesenteric microvessels with the Landis-Michel technique, Salmon et al. quantified the direct effect of Ang1 on water and albumin permeability in intact microvessels by measuring permeability coefficients, hydraulic conductivity (Lp), and the albumin reflection coefficient (s). Permeability in whole animal or whole vascular bed studies is usually determined by measuring the extravasation of Evans blue-labelled albumin. These measurements can be affected by changes in microvessel permeability as well as by variations of flow dynamics and surface area. Permeability studies in individually perfused microvessels allow precise measurements of perfusion pressure and surface area, thus providing a more direct assessment of Ang1 effects on the permeability properties of the vascular walls. Salmon et al. also assess Ang1-induced permeability changes in glomerular capillaries using an oncopressive technique. They report that Ang1 increased the reflection coefficient in frog mesenteric microvessels and reduced baseline permeability in both mesenteric and glomerular capillaries. Taken together, these results suggest that Ang1 affects both the continuous endothelia of mesenteric vessels as well as the fenestrated endothelia of glomerular capillaries. Most importantly, Salmon et al. report that Ang1-induced permeability protection is associated with the restoration of the endothelial glycocalyx, a mechanism independent of the previously established role of Ang1 in strengthening endothelial adhesion in inflammatory agent-stimulated vessels. An earlier study by Adamson implicated glycocalyx function as being critical in maintaining vascular permeability, as brief perfusion with pronase, a proteolytic enzyme that degrades the endothelial glycocalyx, moderately increased Lp, and was accompanied by structural changes in the glycocalyx with intact intercellular clefts. Salmon et al. show through electron microscopy that Ang1 perfusion for 30 min increased glycocalyx depth with an associated reduction of baseline Lp. More importantly, Ang1 both prevented and reversed Lp increases induced by pronase. The studies by both Adamson and Salmon et al. support the fibre matrix hypothesis that the endothelial glycocalyx contributes to the hydraulic resistance and molecular selectivity of the microvessel walls. An interesting observation from Salmon et al. is that pronase treatment induces only a moderate change in glycocalyx depth, whereas the main change is the separation of glycocalyx from plasma membrane. Currently, glycocalyx function is mainly understood to be correlated with depth. However, Salmon et al. find that, although Ang1 prevented and reversed the increase in Lp due to glycocalyx degradation, glycocalyx depth was not fully preserved or restored by Ang1 perfusion. It appears that Ang1 perfusion brought the increased Lp back to baseline by reassembling the glycocalyx, enabling the separated The opinions expressed in this article are not necessarily those of the Editors of the Cardiovascular Research or of the European Society of Cardiology. * Corresponding author. Tel: þ1 304 293 1515; fax: þ1 304 293 3850. E-mail address: [email protected]