Different organizational states of fodrin in cultured MDCK cells are induced by treatment with low pH, calmodulin antagonist TFP, and tumor promoter PMA

Activation of endothelial cell kinin receptors leads to intracellular calcium increases and filamin translocation: regulation by protein kinase C

Membrane-associated cytoskeletal proteins provide support for endothelial cell (EC) junctional cell adhesion molecules. Nonmuscle filamin is a dimeric actin cross-linking protein that interacts with F-actin and membrane glycoproteins. Both bradykinin and des-Arg9-bradykinin cause filamin redistribution from the plasma membrane to the cytosol of confluent EC. Kinin-induced filamin translocation parallels the dynamics of intracellular Ca2+ increases. Pretreatment with kinin receptor antagonists blocks the Ca2+ response as well as filamin translocation induced by kinins. Protein kinase C activation prior to kinin stimulation attenuates intracellular Ca2+ increases and filamin translocation. BAPTA, a cell-permeable Ca2+ chelator, attenuates bradykinin-induced intracellular Ca2+ increases and filamin translocation. This study demonstrates that bovine pulmonary artery ECs express both kinin B1 and B2 receptors, and that activation of either receptor leads to intracellular Ca2+ increases. This Ca2+ signalling, which is downregulated by protein kinase C activation, is essential for kinin-induced filamin translocation.

Regulation of the disassembly/assembly of the membrane skeleton in Madin-Darby canine kidney cells

The effects of pH, temperature, block of energy production, calcium/calmodulin, protein phosphorylation, and cytoskeleton-disrupting agents (cytochalasin D, nocodazole) on the integrity of the membrane skeleton were studied in polarized MDCK cells. The intracellular distributions of alpha-fodrin, actin, and ankyrin were monitored by immunofluorescence microscopy. The membrane skeleton, once assembled, seemed to be quite stable; the only factors releasing alpha-fodrin from the lateral walls were the acidification of the cytoplasm and the depletion of extracellular calcium ions. Upon cellular acidification, some actin was also released from its normal location along the lateral walls and was seen in colocalization with alpha-fodrin in the cytoplasm, whereas ankyrin remained associated with the lateral walls. No accumulation of plasma membrane lipids was observed in the cytoplasm of acidified cells, as visualized by TMA-DPH. These results suggest that the linkages between the fodrin-actin complex and its membrane association sites are broken upon acidification. The pH-induced change in alpha-fodrin localization was reversible upon restoring the normal pH. Reassembly of the membrane skeleton, however, required temperatures above +20 degrees C, normal energy production, proper cell-cell contacts, and polymerized actin. Release of alpha-fodrin from the lateral walls to the cytoplasm was also observed upon depletion of extracellular calcium ions. This change was accompanied by the disruption of cell-cell contacts, supporting the role of proper cell-cell contacts in the maintenance of the membrane skeleton polarity. These results suggest that local alterations of the cytoplasmic pH and calcium ion concentration may be important in regulating the integrity of the membrane skeleton.

Redistribution of fodrin in an in vitro wound healing model of the corneal epithelium

We previously observed the redistribution of a membrane skeletal protein, fodrin, after wounding in the corneal epithelium in vivo. In this study, we made an in vitro wound healing model using cultured corneal epithelial cells to investigate the redistribution mechanism of fodrin in the corneal epithelial cells. The distributional change of fodrin from the plasmalemma to the cytoplasm was observed soon after wounding by indirect immunofluorescence microscopy and laser scanning confocal microscopy. A similar change was caused by treating intact cells with phorbol-12-myristate-13-acetate (PMA), but not with calcium ionophore, A23187. The redistribution occurred even in cells pretreated with 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetomethyl ester (BAPTA-AM) before wounding. The redistribution caused by wounding or by PMA was inhibited by pretreating the cells with protein kinase C inhibitors, H-7 or calphostin C. Moreover, the reagents were found to slow down the migration of corneal epithelial cells after wounding. These results suggest that the redistribution of fodrin in the wounded corneal epithelium is caused through the activation of protein kinase C and might be related to the ensuing cell migration.

The polarity of the membrane skeleton in retinal pigment epithelial cells of developing chicken embryos and in primary culture

We studied the morphogenesis and the membrane skeleton in the retinal pigment epithelium during chicken embryogenesis and in culture, by using immunofluorescence and electron microscopy. During embryogenesis two distinct membrane skeletal structures were formed, an apical and a basolateral one. The former was seen in the apical surface already in the 10-day-old embryos. It was comprised of ankyrin and alpha-fodrin and showed a codistribution with Na+,K(+)-ATPase and an as yet uncharacterized cadherin-like molecule. The basolateral membrane skeleton was seen in the lateral walls already in the 10-day-old embryos, and later, between the 13th and 17th embryonic days, it also appeared at the basal membrane, coincidentally with the formation of the basal infoldings. It consisted of ankyrin and alpha-fodrin, but did not codistribute with any of the integral membrane proteins studied (Na+,K(+)-ATPase and cadherins). In culture, the retinal pigment epithelial cells retained their polarized morphology. Compared with the situation in vivo, however, there was a distinct translocation of the membrane skeletal components fodrin and ankyrin from the apical surface to the lateral walls, accompanied by a similar redistribution of Na+,K(+)-ATPase and the cadherin-like molecule. The results suggest that (1) there is, in the retinal pigment epithelium, an apical Na+,K(+)-ATPase-membrane skeleton structure stabilized by contacts between the retinal pigment epithelium and the neural retina, possibly mediated by a cadherin-like molecule, and that (2) there is another fodrin/ankyrin-based membrane skeleton in the basolateral walls that is important for the maintenance of the extensive folding of these surface areas.

Phorbol myristate acetate-mediated stimulation of transcytosis and apical recycling in MDCK cells

We observed that phorbol myristate acetate (PMA) stimulates transcytosis of the polymeric immunoglobulin receptor (pIgR) in MDCK cells. Apical release of pre-endocytosed ligand (dimeric IgA) bound to the pIgR can be stimulated twofold within 7 min of addition of PMA while recycling of the ligand from the basal surface is not affected. In addition, apical surface delivery of pIgR and cleavage of its ectodomain to secretory component (SC) is also stimulated by PMA. The recycling of apically internalized ligand back to the apical surface is similarly stimulated. These results suggest that the stimulation of apical delivery is from an apical recycling compartment. The effect of PMA suggests that protein kinase C (PKC) is involved in the regulation of pIgR trafficking in MDCK cells. To test this we down regulated PKC activity by pre-treating cells with PMA for 16 h and observed that transcytosis could no longer be stimulated by PMA. Western blots show that the PKC isozymes alpha and to a lesser extent epsilon, are depleted from MDCK cells which have been pre-treated with PMA for 16 h and that treatment of MDCK cells with PMA for 5 min causes a dramatic translocation of the PKC alpha isozyme and a partial translocation of the epsilon isozyme from the cytosol to the membrane fraction of cell homogenates. This translocation suggests that the alpha and/or epsilon isozymes may be involved in PMA mediated stimulation of transcytosis. A mutant pIgR in which serines 664 and 726, the major sites of phosphorylation, are replaced by alanine is stimulated to transcytose by PMA, suggesting that phosphorylation of pIgR at these sites is not required for the effect of PMA. These results suggest that PMA-mediated stimulation of pIgR transcytosis may involve the activation of PKC alpha and/or epsilon, and that this stimulation occurs independently of the major phosphorylation sites on the pIgR. Finally, PMA stimulates transcytosis of basolaterally internalized transferrin, suggesting that PMA acts to generally stimulate delivery of endocytosed proteins to the apical surface.

Induction of cell fusion in cultured fibroblasts and epithelial cells by microinjection of EGTA, GTP gamma S and antifodrin antibodies

CaCl2, EGTA, GTP gamma S and anti-alpha-fodrin antibodies were injected into fibroblast-like IMR-33 cells and Madin-Darby bovine kidney (MDBK) epithelial cells cultured both in the presence and absence of cycloheximide and fetal calf serum. EGTA, GTP gamma S antifodrin antibody induced fusion of MDBK cells within one hour after injection. The cells formed polykaryons with up to 15 nuclei, reaching an average fusion index of 20%. IMR-33 cells fused at a slower kinetics and only upon injection of GTP gamma S or antifodrin antibodies. No fusions were seen in serum-deprived, quiescent cells. On the other hand, cycloheximide treatment did not prevent the fusions. The results show that cells can be induced to fuse by using agents that interfere with the regulation of the G-proteins, intracellular calcium level or membrane skeleton. We suggest that the putative fusogens are resident proteins of the plasma membrane which become exposed upon destabilization of the membrane skeleton.

The effects of PMA and TFP and alterations in intracellular pH and calcium concentration on the membrane associations of phospholipid-binding proteins fodrin, protein kinase C and annexin II in cultured MDCK cells

Annexin II, alpha-fodrin and protein kinase C (PKC) are associated with the cytoplasmic surface of the plasma membranes. When assayed with liposomes, they show affinity for acidic phospholipids and bind calcium ions. They also respond to or participate in cell signal transduction by altered membrane binding properties. In the present work we have studied the properties of these proteins in epithelial MDCK cells in response to elevated intracellular calcium ion concentration, lowered pH, treatment with tumor promoter phorbol myristoyl acetate (PMA) and calmodulin inhibitor trifluoperazine (TFP). In untreated polarized MDCK cells annexin II was seen both along the lateral walls and membranes of intracellular vesicles, fodrin was located along the lateral walls, whereas PKC was seen in the cytoplasm. There was no observable translocation of these proteins upon elevation of the intracellular calcium concentration using a calcium ionophore A23187. On the other hand, treatment with TFP led to a release of annexin II from the plasma membranes which was accompanied by a transient peak in the intracellular calcium. Treatment with PMA led to a loss of the cubic form of the cells, a slight elevation in the intracellular calcium concentration and a drop in the intracellular pH. Simultaneously fodrin was released from the lateral walls, but still remained insoluble in Triton X-100, PKC became associated with the intracellular membranes and fibers, whereas annexin II remained along the lateral walls. These changes could be prevented by clamping the intracellular pH neutral during PMA treatment. On the other hand, lowering of intracellular pH below 6.5 with the nigericin treatment led to a similar translocation of fodrin and PKC as PMA. This suggests that the protein redistribution is caused by cytoplasmic acidification and is due to an increased hydrophobicity and enhanced protonation of lipids and proteins. In contrast, no changes were seen in the annexin II distribution in response to altered pH. Hence, its release by TFP is presumably due to changes in the cationic properties of the inner phase of the plasma membrane. Thus, proteins which show similar binding properties with liposomes show different characteristics in their association with the intracellular membranes.