A role for diacylglycerol in annexin A7-mediated fusion of lung lamellar bodies

Abstract

Lung surfactant secretion in alveolar type II cells occurs following lamellar body fusion with plasma membrane. Annexin A7 is a Ca2+-dependent membrane-binding protein that is postulated to promote membrane fusion during exocytosis in some cell types including type II cells. Since annexin A7 preferably binds to lamellar body membranes, we postulated that specific lipids could modify the mode of annexin A7 interaction with membranes and its membrane fusion activity. Initial studies with phospholipid vesicles containing phosphatidylserine and other lipids showed that certain lipids affected protein interaction with vesicle membranes as determined by change in protein tryptophan fluorescence, protein interaction with trans membranes, and by protein sensitivity to limited proteolysis. The presence of signaling lipids, diacylglycerol or phosphatidylinositol-4,5-bisphosphate, as minor components also modified the lipid vesicle effect on these characteristics and membrane fusion activity of annexin A7. In vitro incubation of lamellar bodies with diacylglycerol or phosphatidylinositol-4,5-bisphosphate caused their enrichment with either lipid, and increased the annexin A7 and Ca2+-mediated fusion of lamellar bodies. Treatment of isolated lung lamellar bodies with phosphatidylinositol- or phosphatidylcholine phospholipase C to increase diacylglycerol, without or with preincubation with phosphatidylinositol-4,5-bisphosphate, augmented the fusion activity of annexin A7. Thus, increased diacylglycerol in lamellar bodies following cell stimulation with secretagogues may enhance membrane fusion activity of annexin A7.

Secretagogues of lung surfactant increase annexin A7 localization with ABCA3 in alveolar type II cells

“…Our previous studies have demonstrated that molecular organization of A7 can be modified by membrane lipid composition and that PIP 2 , which is mostly present in the plasma membrane, has a dramatic influence on annexin A7 properties [6]. Similarly, membrane insertion of synaptotagmin, which acts as a calcium sensor in SNARE-mediated release of synaptic vesicles, is facilitated by PIP 2 in the membranes [36].…”

“…Similarly, membrane insertion of synaptotagmin, which acts as a calcium sensor in SNARE-mediated release of synaptic vesicles, is facilitated by PIP 2 in the membranes [36]. Although this study did not investigate A7 co-localization with PIP 2 , but A7 interaction with PIP 2 and consequent changes in protein conformation [6] would suggest that A7 possibly binds to PIP 2 -rich domains in the plasma membrane. Previous studies have demonstrated activation of exocytic sites by microdomains of PIP 2 and syntaxin [37].…”

“…Previous studies have demonstrated activation of exocytic sites by microdomains of PIP 2 and syntaxin [37]. In case of A7 also, phospholipase C-mediated hydrolysis of PIP 2 could increase diacylglycerol content in the membranes that may facilitate the membrane-fusion function of A7 [6]. …”

“…We have previously shown that one of the annexin proteins, annexin A7 (A7), can facilitate membrane fusion between lamellar bodies and plasma membrane in vitro[4] and that it can promote surfactant secretion in semi-permeable alveolar type II cells [5]. More recently, our in vitro studies suggested that diacylglycerol could regulate A7 function during membrane fusion, since lamellar body enrichment with diacylglycerol increased the A7-mediated membrane fusion activity [6]. …”

Defining the structural characteristics of annexin V binding to a mimetic apoptotic membrane

“…They have structural and functional features of both soluble and peripheral membrane proteins and are widely distributed in a variety of tissues and species (Gerke et al 2005;Gerke and Moss 2002;Rescher and Gerke 2004). Annexins have been identified on the basis of activity that includes phospholipase A2 inhibition (Croxtall et al 1995;De Coupade et al 2000;Kim et al 1994;Lim and Pervaiz 2007), inhibition of blood coagulation (Rand et al 2012;Reutelingsperger and Van Heerde 1997;Thiagarajan and Tait 1990;Wahezi et al 2013), and mediation of membrane fusion (Chander et al 2007(Chander et al , 2013Chattopadhyay et al 2003;Creutz 1992). This activity is always a result of Ca 2+ -dependent interaction of annexins with phospholipids.…”