Lateral mobility of Na,K-ATPase and membrane lipids in renal cells. Importance of cytoskeletal integrity

Size-selective adhesion of calcium oxalate monohydrate crystals to lipid membranes

The retention of calcium oxalate monohydrate (COM) crystals on cell membranes is pivotal in kidney stone formation. However, the mechanisms underlying COM attachment to neutral lipid membranes remain unclear. In this study, we demonstrate that COM exhibits size-selective adhesion to fluid lipid membranes composed of lipids with distinct sizes. Specifically, the (100) facet of COM induces the formation of new domains and establishes strong adhesion in the 18:1 (Δ9-Cis) PC (DOPC) membrane, while the (010) facet induces domains with strong adhesion in the 16:0-14:0 PC membrane. This selectivity is linked to the compatibility of the area per lipid in DOPC with the unit cell area of the (100) facet and the area per lipid in 16:0-14:0 PC with the (010) facet. Our Raman spectroscopic analyses reveal that the lipid acyl chains within these induced domains exhibit a higher degree of ordering compared to the typical fluid state of the membrane. This ordered structural alignment, combined with the lateral size-matching effect, suggests the potential formation of molecular arrays within the lipid bilayer that are in harmony with the lattice dimension of COM. To elucidate the strong adhesion between calcium oxalate and the phospholipid head group in the absence of a direct molecular structural correspondence, we propose that crystal water associated with COM can form hydrogen bonds with the phospholipid head group. Using structure visualization software, we demonstrate the feasibility of such hydrogen bonding networks. The formation of this network could serve to stabilize and enhance the attachment of COM to the lipid membrane. This mediation by water molecules offers a plausible explanation for the pronounced affinity at the interface.

Cell biology and dynamics of Neuronal Na+/K+-ATPase in health and diseases

Neuronal Na⁺/K⁺-ATPase is responsible for the maintenance of ionic gradient across plasma membrane. In doing so, in a healthy brain, Na⁺/K⁺-ATPase activity accounts for nearly half of total brain energy consumption. The α3-subunit containing Na⁺/K⁺-ATPase expression is restricted to neurons. Heterozygous mutations within α3-subunit leads to Rapid-onset Dystonia Parkinsonism, Alternating Hemiplegia of Childhood and other neurological and neuropsychiatric disorders. Additionally, proteins such as α-synuclein, amyloid-β, tau and SOD1 whose aggregation is associated to neurodegenerative diseases directly bind and impair α3-Na⁺/K⁺-ATPase activity. The review will provide a summary of neuronal α3-Na⁺/K⁺-ATPase functional properties, expression pattern, protein-protein interactions at the plasma membrane, biophysical properties (distribution and lateral diffusion). Lastly, the role of α3-Na⁺/K⁺-ATPase in neurological and neurodegenerative disorders will be discussed. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Short- and long-term salinity challenge, osmoregulatory ability, and (Na+, K+)-ATPase kinetics and α-subunit mRNA expression in the gills of the thinstripe hermit crab Clibanarius symmetricus (Anomura, Diogenidae)

The evolutionary history of the Crustacea reveals ample adaptive radiation and the subsequent occupation of many osmotic niches resulting from physiological plasticity in their osmoregulatory mechanisms. We evaluate osmoregulatory ability in the intertidal, thinstripe hermit crab Clibanarius symmetricus after short-term exposure (6 h) or long-term acclimation (10 days) to a wide salinity range, also analyzing kinetic behavior and α-subunit mRNA expression of the gill (Na⁺, K⁺)-ATPase. The crab strongly hyper-regulates its hemolymph at 5 and 15‰S (Salinity, g L^-1) but weakly hyper-regulates up to ≈27‰S. After 6 h exposure to 35‰S and 45‰S, C. symmetricus slightly hypo-regulates its hemolymph, becoming isosmotic after 10 days acclimation to these salinities. (Na⁺, K⁺)-ATPase specific activity decreases with increasing salinity for both exposure periods, reflecting physiological adjustment to isosmoticity. At low salinities, the gill enzyme exhibits a single, low affinity ATP binding site. However, at elevated salinities, a second, high affinity, ATP binding site appears, independently of exposure time. (Na⁺, K⁺)-ATPase α-subunit mRNA expression increases only after 10 days acclimation to 5‰S. Our findings suggest that hemolymph hyper-regulation is effected by alterations in enzyme activity during short-term exposure, but is sustained by increased mRNA expression during long-term acclimation. The decrease in gill (Na⁺, K⁺)-ATPase activity seen as a consequence of increasing salinity appears to underlie biochemical adjustments to hemolymph isosmoticity as hypo-regulatory ability diminishes.

Disruption of Ankyrin B and Caveolin-1 Interaction Sites Alters Na+,K+-ATPase Membrane Diffusion

The Na⁺,K⁺-ATPase is a plasma membrane ion transporter of high physiological importance for ion homeostasis and cellular excitability in electrically active tissues. Mutations in the genes coding for Na⁺,K⁺-ATPase α-subunit isoforms lead to severe human pathologies including Familial Hemiplegic Migraine type 2, Alternating Hemiplegia of Childhood, Rapid-onset Dystonia Parkinsonism, or epilepsy. Many of the reported mutations lead to change- or loss-of-function effects, whereas others do not alter the functional properties, but lead to, e.g., reduced protein stability, reduced protein expression, or defective plasma membrane targeting. Na⁺,K⁺-ATPase frequently assembles with other membrane transporters or cellular matrix proteins in specialized plasma membrane microdomains, but the effects of these interactions on targeting or protein mobility are elusive so far. Mutation of established interaction motifs of the Na⁺,K⁺-ATPase with ankyrin B and caveolin-1 are expected to result in changes in plasma membrane targeting, changes of the localization pattern, and of the diffusion behavior of the enzyme. We studied the consequences of mutations in these binding sites by monitoring diffusion of eGFP-labeled Na⁺,K⁺-ATPase constructs in the plasma membrane of HEK293T cells by fluorescence correlation spectroscopy as well as fluorescence recovery after photobleaching or photoswitching, and observed significant differences compared to the wild-type enzyme, with synergistic effects for combinations of interaction site mutations. These measurements expand the possibilities to study the consequences of Na⁺,K⁺-ATPase mutations and provide information about the interaction of Na⁺,K⁺-ATPase α-isoforms with cellular matrix proteins, the cytoskeleton, or other membrane protein complexes.

Ac2-26 Mimetic Peptide of Annexin A1 Inhibits Local and Systemic Inflammatory Processes Induced by Bothrops moojeni Venom and the Lys-49 Phospholipase A2 in a Rat Model

Annexin A1 (AnxA1) is an endogenous glucocorticoid regulated protein that modulates anti-inflammatory process and its therapeutic potential has recently been recognized in a range of systemic inflammatory disorders. The effect of the N-terminal peptide Ac2-26 of AnxA1 on the toxic activities of Bothrops moojeni crude venom (CV) and its myotoxin II (MjTX-II) were evaluated using a peritonitis rat model. Peritonitis was induced by the intraperitoneal injection of either CV or MjTX-II, a Lys-49 phospholipase A2. Fifteen minutes after the injection, the rats were treated with either Ac2-26 or PBS. Four hours later, the CV and MjTX-II-induced peritonitis were characterized by neutrophilia (in the peritoneal exudate, blood and mesentery) and increased number of mesenteric degranulated mast cells and macrophages. At 24 hours post-injection, the local inflammatory response was attenuated in the CV-induced peritonitis while the MjTX-II group exhibited neutrophilia (peritoneal exudates and blood). Ac2-26 treatment prevented the influx of neutrophils in MjTX-II-induced peritonitis and diminished the proportion of mesenteric degranulated mast cells and macrophages in CV-induced peritonitis. Additionally, CV and MjTX-II promoted increased levels of IL-1β and IL-6 in the peritoneal exudates which were significantly reduced after Ac2-26 treatment. At 4 and 24 hours, the endogenous expression of AnxA1 was upregulated in the mesenteric neutrophils (CV and MjTX-II groups) and mast cells (CV group). In the kidneys, CV and MjTX-II administrations were associated with an increased number of macrophages and morphological alterations in the juxtamedullary nephrons in proximal and distal tubules. Ac2-26 promoted significant recovery of the juxtamedullary structures, decreased the number of macrophages and diminished the AnxA1 in epithelial cells from distal tubules and renal capsules. Our results show that Ac2-26 treatment significantly attenuates local and systemic inflammatory processes and indicate this peptide as a potential target for the development of new therapeutic strategies for the snakebite envenomation treatment.

Adenylyl cyclase AC8 directly controls its micro-environment by recruiting the actin cytoskeleton in a cholesterol-rich milieu

The central and pervasive influence of cAMP on cellular functions underscores the value of stringent control of the organization of adenylyl cyclases (ACs) in the plasma membrane. Biochemical data suggest that ACs reside in membrane rafts and could compartmentalize intermediary scaffolding proteins and associated regulatory elements. However, little is known about the organization or regulation of the dynamic behaviour of ACs in a cellular context. The present study examines these issues, using confocal image analysis of various AC8 constructs, combined with fluorescence recovery after photobleaching and fluorescence correlation spectroscopy. These studies reveal that AC8, through its N-terminus, enhances the cortical actin signal at the plasma membrane; an interaction that was confirmed by GST pull-down and immunoprecipitation experiments. AC8 also associates dynamically with lipid rafts; the direct association of AC8 with sterols was confirmed in Förster resonance energy transfer experiments. Disruption of the actin cytoskeleton and lipid rafts indicates that AC8 tracks along the cytoskeleton in a cholesterol-enriched domain, and the cAMP that it produces contributes to sculpting the actin cytoskeleton. Thus, an adenylyl cyclase is shown not just to act as a scaffold, but also to actively orchestrate its own micro-environment, by associating with the cytoskeleton and controlling the association by producing cAMP, to yield a highly organized signalling hub.

Differential adjustment in gill Na+/K+- and V-ATPase activities and transporter mRNA expression during osmoregulatory acclimation in the cinnamon shrimp Macrobrachium amazonicum (Decapoda, Palaemonidae)

We evaluate osmotic and chloride (Cl(-)) regulatory capability in the diadromous shrimp Macrobrachium amazonicum, and the accompanying alterations in hemolymph osmolality and [Cl(-)], gill Na(+)/K(+)-ATPase activity, and expression of gill Na(+)/K(+)-ATPase α-subunit and V-ATPase B subunit mRNA during salinity (S) acclimation. We also characterize V-ATPase kinetics and the organization of transport-related membrane systems in the gill epithelium. Macrobrachium amazonicum strongly hyper-regulates hemolymph osmolality and [Cl(-)] in freshwater and in salinities up to 25‰ S. During a 10-day acclimation period to 25‰ S, hemolymph became isosmotic and hypo-chloremic after 5 days, [Cl(-)] alone remaining hyporegulated thereafter. Gill Na(+)/K(+)-ATPase α-subunit mRNA expression increased 6.5 times initial values after 1 h, then decreased to 3 to 4 times initial values by 24 h and to 1.5 times initial values after 10 days at 25‰ S. This increased expression was accompanied by a sharp decrease at 5 h then recovery of initial Na(+)/K(+)-ATPase activity within 24 h, declining again after 5 days, which suggests transient Cl(-) secretion. V-ATPase B-subunit mRNA expression increased 1.5-fold within 1 h, then reduced sharply to 0.3 times initial values by 5 h, and remained unchanged for the remainder of the 10-day period. V-ATPase activity dropped sharply and was negligible after a 10-day acclimation period to 21‰ S, revealing a marked downregulation of ion uptake mechanisms. The gill epithelium consists of thick, apical pillar cell flanges, the perikarya of which are coupled to an intralamellar septum. These two cell types respectively exhibit extensive apical evaginations and deep membrane invaginations, both of which are associated with numerous mitochondria, characterizing an ion transporting epithelium. These changes in Na(+)/K(+)- and V-ATPase activities and in mRNA expression during salinity acclimation appear to underpin ion uptake and Cl(-) secretion by the palaemonid shrimp gill.

Pannexin1 and pannexin3 delivery, cell surface dynamics, and cytoskeletal interactions

Pannexins (Panx) are a class of integral membrane proteins that have been proposed to exhibit characteristics similar to those of connexin family members. In this study, we utilized Cx43-positive BICR-M1R(k) cells to stably express Panx1, Panx3, or Panx1-green fluorescent protein (GFP) to assess their trafficking, cell surface dynamics, and interplay with the cytoskeletal network. Expression of a Sar1 dominant negative mutant revealed that endoplasmic reticulum to Golgi transport of Panx1 and Panx3 was mediated via COPII-dependent vesicles. Distinct from Cx43-GFP, fluorescence recovery after photobleaching studies revealed that both Panx1-GFP and Panx3-GFP remained highly mobile at the cell surface. Unlike Cx43, Panx1-GFP exhibited no detectable interrelationship with microtubules. Conversely, cytochalasin B-induced disruption of microfilaments caused a severe loss of cell surface Panx1-GFP, a reduction in the recoverable fraction of Panx1-GFP that remained at the cell surface, and a decrease in Panx1-GFP vesicular transport. Furthermore, co-immunoprecipitation and co-sedimentation assays revealed actin as a novel binding partner of Panx1. Collectively, we conclude that although Panx1 and Panx3 share a common endoplasmic reticulum to Golgi secretory pathway to Cx43, their ultimate cell surface residency appears to be independent of cell contacts and the need for intact microtubules. Importantly, Panx1 has an interaction with actin microfilaments that regulates its cell surface localization and mobility.

Membrane Mobility and Microdomain Association of the Dopamine Transporter Studied with Fluorescence Correlation Spectroscopy and Fluorescence Recovery after Photobleaching

To investigate microdomain association of the dopamine transporter (DAT), we employed FCS (fluorescence correlation spectroscopy) and FRAP (fluorescence recovery after photobleaching). In non-neuronal cells (HEK293), FCS measurements revealed for the YFP-DAT (DAT tagged with yellow fluorescent protein) a diffusion coefficient (D) of approximately 3.6 x 10(-9) cm2/s, consistent with a relatively freely diffusible protein. In neuronally derived cells (N2a), we were unable to perform FCS measurements on plasma membrane-associated protein due to photobleaching, suggesting partial immobilization. This was supported by FRAP measurements that revealed a lower D and a mobile fraction of the YFP-DAT in N2a cells compared to HEK293 cells. Comparison with the EGFP-EGFR (epidermal growth factor receptor) and the EGFP-beta2AR (beta2 adrenergic receptor) demonstrated that this observation was DAT specific. Both the cytoskeleton-disrupting agent cytochalasin D and the cholesterol-depleting agent methyl-beta-cyclodextrin (mbetaCD) increased the lateral mobility of the YFP-DAT but not that of the EGFP-EGFR. The DAT associated in part with membrane raft markers both in the N2a cells and in rat striatal synaptosomes as assessed by sucrose density gradient centrifugation. Raft association was further confirmed in the N2a cells by cholera toxin B patching. It was, moreover, observed that cholesterol depletion, and thereby membrane raft disruption, decreased both the Vmax and KM values for [3H]dopamine uptake without altering DAT surface expression. In summary, we propose that association of the DAT with lipid microdomains in the plasma membrane and/or the cytoskeleton serves to regulate both the lateral mobility of the transporter and its transport capacity.

Paradigm shift of the plasma membrane concept from the two-dimensional continuum fluid to the partitioned fluid: high-speed single-molecule tracking of membrane molecules

Recent advancements in single-molecule tracking methods with nanometer-level precision now allow researchers to observe the movement, recruitment, and activation of single molecules in the plasma membrane in living cells. In particular, on the basis of the observations by high-speed single-particle tracking at a frame rate of 40,000 frames s(1), the partitioning of the fluid plasma membrane into submicron compartments throughout the cell membrane and the hop diffusion of virtually all the molecules have been proposed. This could explain why the diffusion coefficients in the plasma membrane are considerably smaller than those in artificial membranes, and why the diffusion coefficient is reduced upon molecular complex formation (oligomerization-induced trapping). In this review, we first describe the high-speed single-molecule tracking methods, and then we critically review a new model of a partitioned fluid plasma membrane and the involvement of the actin-based membrane-skeleton "fences" and anchored-transmembrane protein "pickets" in the formation of compartment boundaries.

Cellular characterization of adenylate kinase and its isoform: two-photon excitation fluorescence imaging and fluorescence correlation spectroscopy

Adenylate kinase (AK) is a ubiquitous enzyme that regulates the homeostasis of adenine nucleotides in the cell. AK1beta (long form) from murine cells shares the same protein sequence as AK1 (short form) except for the addition of 18 amino acid residues at its N-terminus. It is hypothesized that these residues serve as a signal for protein lipid modification and targeting of the protein to the plasma membrane. To better understand the cellular function of these AK isoforms, we have used several modern fluorescence techniques to characterize these two isoforms of AK enzyme. We fused cytosolic adenylate kinase (AK1) and its isoform (AK1beta) with enhanced green fluorescence protein (EGFP) and expressed the chimera proteins in HeLa cells. Using two-photon excitation scanning fluorescence imaging, we were able to directly visualize the localization of AK1-EGFP and AK1beta-EGFP in live cells. AK1beta-EGFP mainly localized on the plasma membrane, whereas AK1-EGFP distributed throughout the cell except for trace amounts in the nuclear membrane and some vesicles. We performed fluorescence correlation spectroscopy measurements and photon-counting histogram analysis in specific domains of live cells. For AK1-EGFP, we observed only one diffusion component in the cytoplasm. For AK1beta-EGFP, we observed two distinct diffusion components on the plasma membrane. One corresponded to the free diffusing protein, whereas the other represented the membrane-bound AK1beta-EGFP. The diffusion rate of AK1-EGFP was slowed by a factor of 1.8 with respect to that of EGFP, which was 50% more than what we would expect for a free diffusing AK1-EGFP. To rule out the possibility of oligomer formation, we performed photon-counting histogram analysis to direct analyze the brightness difference between AK1-EGFP and EGFP. From our analysis, we concluded that cytoplasmic AK1-EGFP is monomeric. fluorescence correlation spectroscopy proved to be a powerful technique for quantitatively studying the mobility of the target protein in live cells. This technology offers advantages in studying protein interactions and function in the cell.

Regulation of PGE(2) and PGI(2) release from human umbilical vein endothelial cells by actin cytoskeleton

Disruption of microfilaments in human umbilical vein endothelial cells (HUVEC) with cytochalasin D (cytD) or latrunculin A (latA) resulted in a 3.3- to 5.7-fold increase in total synthesis of prostaglandin E(2) (PGE(2)) and a 3.4- to 6.5-fold increase in prostacyclin (PGI(2)) compared with control cells. Disruption of the microtubule network with nocodazole or colchicine increased synthesis of PGE(2) 1.7- to 1.9-fold and PGI(2) 1.9- to 2.0-fold compared with control cells. Interestingly, however, increased release of PGE(2) and PGI(2) from HUVEC into the media occurred only when microfilaments were disrupted. CytD treatment resulted in 6.7-fold more PGE(2) and 3.8-fold more PGI(2) released from HUVEC compared with control cells; latA treatment resulted in 17.7-fold more PGE(2) and 11.2-fold more PGI(2) released compared with control cells. Both increased synthesis and release of prostaglandins in response to all drug treatments were completely inhibited by NS-398, a specific inhibitor of cyclooxygenase-2 (COX-2). Disruption of either microfilaments using cytD or latA or of microtubules using nocodazole or colchicine resulted in a significant increase in COX-2 protein levels, suggesting that the increased synthesis of prostaglandins in response to drug treatments may result from increased activity of COX-2. These results, together with studies demonstrating a vasoprotective role for prostaglandins, suggest that the cytoskeleton plays an important role in maintenance of endothelial barrier function by regulating prostaglandin synthesis and release from HUVEC.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

protomers of Na+,K+-ATPase from microsomes of duck salt gland are mostly monomeric: Formation of higher oligomers does not modify molecular activity

The distance that separates alphabeta protomers of the Na(+), K(+)-ATPase in microsomes and in purified membranes prepared from duck nasal salt glands was estimated by measuring fluorescence resonance energy transfer between anthroylouabain bound to a population of alphabeta protomers and either N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain or 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain bound to the rest. Energy transfer between probes bound in the microsomal preparation was less than in the purified membranes. The efficiency of energy transfer between anthroylouabain and N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain was 29.2% in the microsomes compared with 62.6% in the purified preparation. Similar results were obtained with 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain as acceptor. We calculate that either the protomer bound probes were on the average 13 A farther apart in the microsomes than in the purified membranes, or that 53% of the protomers are monomeric in the microsome preparation. Microsomes prepared in the presence of phalloidin (a toxin that binds to F actin and stabilizes the actin-based cytoskeleton) showed less quench than those prepared in its absence. The data support the hypothesis that protomers are kept apart by their association with the cytoskeleton. The turnover rate while hydrolyzing ATP is the same in the microsomal and purified preparations; higher oligomer formation has no significant effect on the enzyme reaction mechanism.

Alphabeta protomers of Na+,K+-ATPase from microsomes of duck salt gland are mostly monomeric: formation of higher oligomers does not modify molecular activity

The distance that separates alphabeta protomers of the Na(+), K(+)-ATPase in microsomes and in purified membranes prepared from duck nasal salt glands was estimated by measuring fluorescence resonance energy transfer between anthroylouabain bound to a population of alphabeta protomers and either N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain or 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain bound to the rest. Energy transfer between probes bound in the microsomal preparation was less than in the purified membranes. The efficiency of energy transfer between anthroylouabain and N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain was 29.2% in the microsomes compared with 62.6% in the purified preparation. Similar results were obtained with 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain as acceptor. We calculate that either the protomer bound probes were on the average 13 A farther apart in the microsomes than in the purified membranes, or that 53% of the protomers are monomeric in the microsome preparation. Microsomes prepared in the presence of phalloidin (a toxin that binds to F actin and stabilizes the actin-based cytoskeleton) showed less quench than those prepared in its absence. The data support the hypothesis that protomers are kept apart by their association with the cytoskeleton. The turnover rate while hydrolyzing ATP is the same in the microsomal and purified preparations; higher oligomer formation has no significant effect on the enzyme reaction mechanism.

A quantitative immunocytochemical study of Na+,K+-ATPase in rat hepatocytes after STZ-induced diabetes and dietary fish oil supplementation

Because diabetes causes alterations in hepatic membrane fatty acid content, these changes may affect the Na+,K+-ATPase. In this study we documented the effects of streptozotocin (STZ)-induced diabetes on hepatic Na+,K+-ATPase catalytic alpha1-subunit and evaluated whether these changes could be normalized by fish oil supplementation. Two groups of diabetic rats received fish oil or olive oil supplementation. Both groups had a respective control group. We studied the localization of catalytic alpha1-subunit on bile canalicular and basolateral membranes using immunocytochemical methods and confocal laser scanning microscopy, and the Na+, K+-ATPase activity, membrane fluidity, and fatty acid composition on isolated hepatic membranes. A decrease in the alpha1-subunit was observed with diabetes in the bile canalicular membranes, without changes in basolateral membranes. This decrease was partially prevented by dietary fish oil. Diabetes induces significant changes as documented by enzymatic Na+,K+-ATPase activity, membrane fluidity, and fatty acid content, whereas little change in these parameters was observed after a fish oil diet. In conclusion, STZ-induced diabetes appears to modify bile canalicular membrane integrity and dietary fish oil partly prevents the diabetes-induced alterations.

Pseudomonas aeruginosa invasion and cytotoxicity are independent events, both of which involve protein tyrosine kinase activity

Pseudomonas aeruginosa clinical isolates exhibit invasive or cytotoxic phenotypes. Cytotoxic strains acquire some of the characteristics of invasive strains when a regulatory gene, exsA, that controls the expression of several extracellular proteins, is inactivated. exsA mutants are not cytotoxic and can be detected within epithelial cells by gentamicin survival assays. The purpose of this study was to determine whether epithelial cell invasion precedes and/or is essential for cytotoxicity. This was tested by measuring invasion (gentamicin survival) and cytotoxicity (trypan blue staining) of PA103 mutants deficient in specific exsA-regulated proteins and by testing the effect of drugs that inhibit invasion for their effect on cytotoxicity. A transposon mutant in the exsA-regulated extracellular factor exoU was neither cytotoxic nor invasive. Furthermore, several of the drugs that inhibited invasion did not prevent cytotoxicity. These results show that invasion and cytotoxicity are mutually exclusive events, inversely regulated by an exsA-encoded invasion inhibitor(s). Both involve host cell protein tyrosine kinase (PTK) activity, but they differ in that invasion requires Src family tyrosine kinases and calcium-calmodulin activity. PTK inhibitor drugs such as genistein may have therapeutic potential through their ability to block both invasive and cytotoxicity pathways via an action on the host cell.

Effects of integrins on proliferation and apoptosis of renal epithelial cells after acute injury

We sought to determine the importance of integrins for recovery after acute tubular injury and to investigate the signal transduction pathways for integrin effects on cell cycle regulation involving proliferation and apoptosis. Primary cultures of rat renal proximal tubule epithelial cells were exposed to a superoxide-generating system to induce injury in the absence of overt necrosis. Integrin function was antagonized by the integrin recognition sequence tetrapeptide Gly-Arg-Gly-Asp (GRGD) or monoclonal antibody to beta 1-integrin. Injured cells had reduced thymidine uptake compared with normal cells. The presence of GRGD during recovery from injury caused a further 44% reduction in DNA synthesis but did not affect DNA synthesis in normal cells. Injured cells had an increased proportion of apoptosis that was further accentuated by exposed to GRGD during recovery. Integrin antagonism also stimulated apoptosis in uninjured cells. To investigate signal transduction mechanisms for this effect of integrins, inhibitors and activators of protein tyrosine kinase (PTK) and protein kinase C (PKC) were evaluated. Activation of PKC stimulated cellular proliferation, whereas inhibitors of PKC and PTK had no significant effect. Genistein, a PTK inhibitor, induced apoptosis in normal cells, mimicking the effect of integrin inhibition. On the other hand, PMA, an activator of PKC, prevented cells from becoming apoptotic when exposed to injury plus GRGD. The phosphorylation status of intracellular proteins was evaluated by immunoblotting with antiphosphotyrosine antibody. A similar pattern of decreased phosphorylation was observed after either integrin inhibition, injury, both, or PTK inhibition. These findings suggest that kinase cascades are involved in the effects of integrins on renal epithelial cell proliferation and apoptosis. After injury, an interaction between cells and the extracellular matrix is required for cells to proliferate and contribute to repair rather than to enter an apoptotic pathway.

Fatty acid alteration and the lateral diffusion of lipids in the plasma membrane of keratinocytes

The fluorescent probe diI was used to study the lateral mobility of lipids in in vitro strains of living adult human keratinocytes grown in four different media. One medium was essential fatty acid deficient (EFAD) and low in calcium ion, a medium known to yield cells that proliferate rapidly and contain lipid with extremely low levels of essential fatty acids. Two other media were supplemented with essential fatty acids (FAS), media that are known to result in cells that grow more slowly and have normalized fatty acid proportions. A fourth medium consisted of 1 microM all-trans-retinoic acid added to the fatty acid-supplemented medium (FAS-RA), a medium known to produce cells that are highly proliferative, with a growth rate greater than that of the FAS strains and similar to that of the EFAD strains. The keratinocytes grown in these four media were studied using the fluorescence recovery after photobleaching (FRAP) technique to determine the lateral diffusion rate of diI in the plasma membranes. Our results showed a positive correlation between growth rate and diffusion coefficient (D): the diffusion coefficient of diI was higher in the EFAD or FAS-RA cells than in the FAS cells. The measurement of D among the FAS cells fell into two groups. One group was similar to the single group seen in the EFAD cells, but the other group was composed of much lower D values. The other FRAP parameters (mobile fraction and bleach depth) were larger in the "slow" group than in the "fast" group. This trend of negative correlation between these parameters and D was also found within the fast group. These results are interpreted in terms of possible changes in membrane structure or morphology that might be indirectly associated with the fatty acid alterations, including the possible presence of areas in senescing keratinocytes where plasma membranes collapse to form an interacting system of lipid bilayers.

Fluorescence generalized polarization of cell membranes: a two-photon scanning microscopy approach

We use the lipophilic fluorescence probe Laurdan to study cell membranes. The generalized polarization (GP) of Laurdan-labeled cells contains useful information about membrane fluidity and polarity. A high GP is usually associated with low fluidity, low polarity, or high cholesterol content of the membranes, and a low GP is the opposite. We have combined the GP method and two-photon fluorescence microscopy to provide an alternative approach to study cell membranes. Using two-photon excitation in a conventional microscope offers great advantages for studying biological samples. These advantages include efficient background rejection, low photodamage, and improved depth discrimination. We performed GP measurements on mouse fibroblast cells and observed that both intensity and GP images are not spatially uniform. We tested for possible GP artifacts arising from cellular autofluorescence and lifetime quenching, using a procedure for background fluorescence subtraction and by direct lifetime measurements in the microscope. GP measured in a single cell displays a broad distribution, and the GP of 40 different cells grown on the same cover glass is also statistically distributed. The correlations between intensity and GP images were analyzed, and no monotonic dependence between the two was found. By digitally separating high and low GP values, we found that high GP values often associate with the regions of the plasma membrane and low GP values link with the nuclear membranes. Our results also show local GP variations within the plasma and nuclear membranes.

Effects of vasopressin and aldosterone on the lateral mobility of epithelial Na+ channels in A6 renal epithelial cells

We have previously demonstrated that apical Na+ channels in A6 renal epithelial cells are associated with spectrin-based membrane cytoskeleton proteins and that the lateral mobility of these channels, as determined by fluorescence photobleach recovery (FPR) analysis, is severely restricted by this association (Smith et al., 1991. Proc. Natl. Acad. Sci. USA 88:6971-6975). Recent data indicate that the actin component of the cytoskeleton may play a role in modulating Na+ channel activity (Cantiello et al., 1991. Am. J. Physiol. 261:C882-C888); however, it is unknown if the Na+ channel's linkage to the spectrin-based membrane cytoskeleton is also involved in regulating channel activity. In this study, we have used FPR to examine if the linkage of the Na+ channels to the membrane cytoskeleton is a site for modulation of Na+ channel activity in filter grown A6 cells by vasopressin and aldosterone. We hypothesized that if the linkage of the Na+ channels to the membrane cytoskeleton is a site for regulation of Na+ channel activity by vasopressin and aldosterone, then hormone-mediated changes in either the membrane cytoskeleton or the affinity of the Na+ channel for the membrane cytoskeleton, should be reflected in changes in the lateral mobility and/or mobile fraction of Na+ channels on the cell surface. FPR revealed that although the rates of lateral mobility were not affected, there was a twofold increase in mobility fraction (f) of apical Na+ channels in aldosterone-treated (16 hr) monolayers (f = 32.31 +/- 5.42%) when compared to control (unstimulated) (f = 14.2 +/- 0.77%) and vasopressin-treated (20 min) (f = 12.7 +/- 2.4%) monolayers. The twofold increase in mobile fraction of Na+ channels corresponds to the average increase in Na+ transport in response to aldosterone in A6 cells. The aldosterone-induced increase in Na+ transport and mobile fraction can be inhibited by the methylation inhibitor, 3-deazaadenosine, consistent with the hypothesis that a methylation event is involved in aldosterone induced upregulation of Na+ transport. We propose that the membrane cytoskeleton is involved in the aldosterone-mediated activation of epithelial Na+ channels.