Caveolae Act as Membrane Reserves Which Limit Mechanosensitive ICl,swell Channel Activation during Swelling in the Rat Ventricular Myocyte

Abstract

BackgroundMany ion channels are preferentially located in caveolae where compartmentalisation/scaffolding with signal transduction components regulates their activity. Channels that are mechanosensitive may be additionally dependent on caveolar control of the mechanical state of the membrane. Here we test which mechanism underlies caveolar-regulation of the mechanosensitive I Cl,swell channel in the adult cardiac myocyte.Methodology/Principal FindingsRat ventricular myocytes were exposed to solution of 0.02 tonicity (T; until lysis), 0.64T for 10–15 min (swelling), and/or methyl-β-cyclodextrin (MBCD; to disrupt caveolae). MBCD and 0.64T swelling reduced the number of caveolae visualised by electron microscopy by 75 and 50% respectively. MBCD stimulated translocation of caveolin 3 from caveolae-enriched buoyant membrane fractions, but both caveolin 1 and 3 remained in buoyant fractions after swelling. I Cl,swell inhibition in control cells decreased time to half-maximal volume (t 0.5,vol; 0.64T), consistent with a role for I Cl,swell in volume regulation. MBCD-treated cells showed reduced time to lysis (0.02T) and t 0.5,vol (0.64T) compared with controls. The negative inotropic response to swelling (an index of I Cl,swell activation) was enhanced by MBCD.Conclusions/SignificanceThese data show that disrupting caveolae removes essential membrane reserves, which speeds swelling in hyposmotic conditions, and thereby promotes activation of I Cl,swell. They illustrate a general principle whereby caveolae as a membrane reserve limit increases in membrane tension during stretch/swelling thereby restricting mechanosensitive channel activation.

Caveolar domain organization and trafficking is regulated by Abl kinases and mDia1

“…This could be explained by a mechanism in which rosettecaveolae are pinched off from rosettes, or by a flattening of rosette-caveolae as rosettes traffic to the perinuclear area, preventing their identification by morphological criteria; the latter possibility is also consistent with a pinch-off of the whole rosette structure. Caveolar domains respond to tension (Kozera et al, 2009;Sinha et al, 2011), and the increase in caveolar domain complexity observed in suspended cells might therefore reflect an adaptation to sudden loss of membrane tension, reportedly occurring in such conditions (Colbert et al, 2009). In contrast, these increments in caveolar complexity parameters (number of rosettes, rosette-caveolae and rosette size) observed in dRecAA cells upon loss of cell adhesion were not observed in dKOAA cells (Fig.…”

Caveolae – mechanosensitive membrane invaginations linked to actin filaments

“…However, the number of caveolae does not change as a function of sarcomere length in rabbit myocardial cells (Levin and Page, 1980), raising the possibility that caveolae flattening might occur only in vivo in certain tissues. More recent studies have shown that mechanical stretching of cells or their osmotic swelling result in the flattening of about half of the caveolae (Kozera et al, 2009;Gervasio et al, 2011;Sinha et al, 2011). Caveolae flattening is also generated by an excess of stress fibers (Echarri et al, 2012), which results in an increase in membrane tension (Tamura et al, 2010;Echarri et al, 2012).…”

“…5) (Balijepalli and Kamp, 2008;Kozera et al, 2009;Huang et al, 2013). Furthermore, Cav3 deficiency can lead to long QT syndrome, a frequent consequence of defective ion channel functioning (Moss and Kass, 2005).…”

Caveolae at a glance

“…Moreover, Cav1 has been shown to be essential for mechanoactivation of protein kinase B (Akt) and for cell cycle progression under cyclic stretch in vitro and in vivo (Sedding et al, 2005). Recently, Kozera and colleagues demonstrated a direct role for caveolae in mechanosensing and showed that caveolae can act as a membrane reserve and attenuate volume-regulated channels activation by limiting the increase in membrane tension during cellular swelling (Kozera et al, 2009). These reports endorse the hypothesis that caveolae can sense and respond to changes in membrane tension.…”