Contribution of F‐Actin to Barrier Properties of the Blood‐Joint Pathway

Abstract

Objectives: Because fibroblast filamentous actin (F-actin) influences cutaneous interstitial matrix swelling pressure (5), we investigated whether F-actin in fibroblast-derived synoviocytes influences the hydraulic permeability of the transsynovial interstitial pathway. The study also tested whether F-actin in fenestrated synovial endothelium contributes to the blood-joint barrier in vivo. Methods: The clearance of Evans blue-albumin (EVA) from plasma into the knee joint cavity was determined in joint infused with F-actin disrupting cytochalasin D (1-200 M), latrunculin B (100 M) or vehicle in anesthetized rabbits. The hydraulic permeability of the lining was determined as the slope relating net trans-synovial flow Q s to intra-articular pressure P j . Synovium was examined histologically after i.v. Monastral blue to assess endothelial leakiness. Results: EVA permeation in vivo was increased up to 25-fold by cytochalasin (p ‫ס‬ 0.0002, ANOVA), with an EC 50 of 23 M (95% confidence limits 13-43 M). Washout quickly reversed the increase. Latrunculin had a similar effect. F-actin disruption switched Q s from drainage (control) to filtration into the cavity at low P j in vivo and raised the conductance dQ s /dP j by 2.13 (p ‫ס‬ 0.001, ANOVA). Circulatory arrest abolished these effects. Monastral blue revealed numerous endothelial leaks. Conclusions: F-actin is crucial to the barrier function of fenestrated endothelium in situ. No significant effect of synoviocyte F-actin on matrix permeability was detected.

Ste20-Related Proline/Alanine-Rich Kinase (SPAK) Regulated Transcriptionally by Hyperosmolarity Is Involved in Intestinal Barrier Function

“…However, in agreement with previous reports, in the present study hyperosmolarity also activated the Erk1/2 pathway (Figure S1) [75], [78], [79], [80], which is involved in the mechanism by which permeability is decreased [81]. Furthermore, hyperosmolarity leads to redistribution and polymerization of cortical F-actin, which is involved in the downregulation of cellular permeability [82], [83], [84]. Hyperosmolarity can induce activation of focal adhesion kinase and redistribution of focal adhesion protein, which can lead to increased barrier function [85], [86].…”

Regulation of hyaluronan secretion into rabbit synovial joints in vivo by protein kinase C

“…PMA might, in principle, indirectly stimulate hyaluronan secretion by triggering an inflammatory synovitis. We tested this in a joint using the inflammation‐assay method of Poli et al . (2001, 2002).…”

“…The joints were then left for 6 h to secrete fresh hyaluronan. Higher drug concentrations were injected in vivo (see later) than is customary in cell culture because small, rapidly diffusible solutes are cleared rapidly by the synovial microcirculation (clearance ≈55 μl min −1 ; Poli et al . 2002).…”

“…Subsidiary experiments were carried out to test whether PMA causes significant synovial inflammation. Inflammation was assessed as increased fluid filtration into the joint and increased permeation of plasma albumin labelled with Evans blue into the joint cavity, as described by Poli et al . (2001, 2002).…”

Hyperosmolarity enhances the lung capillary barrier

“…The association of endothelial barrier deterioration with actin depolymerization (35,36) has established the view that cortical actin filaments stabilize endothelial barrier function. This view was challenged by recent studies involving the dominant negative and constitutively active forms of the small GTPase Rac1, which decrease and enhance cortical actin, respectively.…”