Transgenic expression of human S100A12 induces structural airway abnormalities and limited lung inflammation in a mouse model of allergic inflammation

BACKGROUND

The calcium-binding protein S100A12 is highly up-regulated in the serum and sputum of patients with allergic asthma and is suggested to be a biomarker and pathologic mediator of asthma.

OBJECTIVE

To test the role of S100A12 in mediating airway inflammation in a mouse model of allergic lung inflammation.

METHODS

Transgenic (TG) mice that express human S100A12 and wild-type (WT) littermates were sensitized and challenged with ovalbumin (OVA) and assessed for inflammation, lung structure, and function.

RESULTS

Following OVA sensitization and challenge, S100A12 TG mice showed reduced peribronchial and perivascular inflammation, mucus production, and eosinophilia as well as attenuated airway responsiveness to contractile agonist compared with WT sensitized and challenged animals. This is explained, at least in part, by remodelled airways in S100A12 TG mice with thinning of the airway smooth muscle. S100A12 exposure induced Fas expression and activation of caspase 3 in cultured airway smooth muscle cells, suggesting that airway smooth muscle abnormalities observed in S100A12 TG mice may be mediated through myocyte apoptosis.

CONCLUSION AND CLINICAL RELEVANCE

S100A12 is one of the most abundant proteins found in the airways of human asthmatics, and it was postulated that S100A12 could mediate the inflammatory process. Our study shows for the first time that TG expression of S100A12 in the lung of mice does not exacerbate lung inflammation in a model of OVA-induced allergic inflammation. We speculate that the high levels of S100/calgranulins found in bronchoalveolar lavage fluid of asthmatics and of OVA-treated TG S100A12 mice do not significantly mediate pulmonary inflammation.

CD43 interaction with ezrin-radixin-moesin (ERM) proteins regulates T-cell trafficking and CD43 phosphorylation

CD43 interaction with ERM proteins regulates CD43 phosphorylation and T-cell migration. CD43 phosphorylation can also drive CD43 localization in T-cells independently of ERM association.

Cell polarization is a key feature of cell motility, driving cell migration to tissues. CD43 is an abundantly expressed molecule on the T-cell surface that shows distinct localization to the migrating T-cell uropod and the distal pole complex (DPC) opposite the immunological synapse via association with the ezrin-radixin-moesin (ERM) family of actin regulatory proteins. CD43 regulates multiple T-cell functions, including T-cell activation, proliferation, apoptosis, and migration. We recently demonstrated that CD43 regulates T-cell trafficking through a phosphorylation site at Ser-76 (S76) within its cytoplasmic tail. Using a phosphorylation-specific antibody, we now find that CD43 phosphorylation at S76 is enhanced by migration signals. We further show that CD43 phosphorylation and normal T-cell trafficking depend on CD43 association with ERM proteins. Interestingly, mutation of S76 to mimic phosphorylation enhances T-cell migration and CD43 movement to the DPC while blocking ERM association, showing that CD43 movement can occur in the absence of ERM binding. We also find that protein kinase Cθ can phosphorylate CD43. These results show that while CD43 binding to ERM proteins is crucial for S76 phosphorylation, CD43 movement and regulation of T-cell migration can occur through an ERM-independent, phosphorylation–dependent mechanism.

A multidrug ABC transporter with a taste for salt

BACKGROUND

LmrA is a multidrug ATP-binding cassette (ABC) transporter from Lactococcus lactis with no known physiological substrate, which can transport a wide range of chemotherapeutic agents and toxins from the cell. The protein can functionally replace the human homologue ABCB1 (also termed multidrug resistance P-glycoprotein MDR1) in lung fibroblast cells. Even though LmrA mediates ATP-dependent transport, it can use the proton-motive force to transport substrates, such as ethidium bromide, across the membrane by a reversible, H(+)-dependent, secondary-active transport reaction. The mechanism and physiological context of this reaction are not known.

METHODOLOGY/PRINCIPAL FINDINGS

We examined ion transport by LmrA in electrophysiological experiments and in transport studies using radioactive ions and fluorescent ion-selective probes. Here we show that LmrA itself can transport NaCl by a similar secondary-active mechanism as observed for ethidium bromide, by mediating apparent H(+)-Na(+)-Cl(-) symport. Remarkably, LmrA activity significantly enhances survival of high-salt adapted lactococcal cells during ionic downshift.

CONCLUSIONS/SIGNIFICANCE

The observations on H(+)-Na(+)-Cl(-) co-transport substantiate earlier suggestions of H(+)-coupled transport by LmrA, and indicate a novel link between the activity of LmrA and salt stress. Our findings demonstrate the relevance of investigations into the bioenergetics of substrate translocation by ABC transporters for our understanding of fundamental mechanisms in this superfamily. This study represents the first use of electrophysiological techniques to analyze substrate transport by a purified multidrug transporter.

New structure model for the ATP-binding cassette multidrug transporter LmrA

Multidrug resistance of pathogenic microorganisms and mammalian tumors can be associated with the overexpression of multidrug transporters. These integral membrane proteins are capable of extruding a wide range of structurally unrelated compounds from the cell. Among the different classes of multidrug transporters are the ATP binding cassette (ABC) transporters, which are dependent on the binding and hydrolysis of ATP. In the past five years, many researchers have built homology models of ABC extrusion systems using the atomic coordinates of crystallized MsbA, a lipopolysaccharide transporter in Gram-negative bacteria. Likewise, we have previously used the Vibrio cholera MsbA structure as a template in the modeling of the multidrug transporter LmrA from Lactococcus lactis. In view of the recently discovered inaccuracies in the MsbA structure, we have remodelled LmrA using the atomic coordinates of the MsbA homologue Sav1866 from Staphylococcus aureus. To compare and test our MsbA-based and Sav1866-based LmrA models we performed cysteine cross-linking at three key positions in LmrA. The pattern of cross-linking at these positions was consistent with the overall topology of transmembrane helices in Sav1866, suggesting that its crystal structure might be physiologically relevant. We recently identified E314 as a residue important in proton conduction by LmrA. The predicted location of this residue at the interface between the two half-transporters in the Sav1866-based homodimer, within the inner leaflet of the phospholipid bilayer, provides a new structural basis for the role of E314 in LmrA-mediated transport.

New light on multidrug binding by an ATP-binding-cassette transporter

ATP-binding-cassette (ABC) multidrug transporters confer multidrug resistance to pathogenic microorganisms and human tumour cells by mediating the extrusion of structurally unrelated chemotherapeutic drugs from the cell. The molecular basis by which ABC multidrug transporters bind and transport drugs is far from clear. Genetic analyses during the past 14 years reveal that the replacement of many individual amino acids in mammalian multidrug resistance P-glycoproteins can affect cellular resistance to drugs, but these studies have failed to identify specific regions in the primary amino acid sequence that are part of a defined drug-binding pocket. The recent publication of an X-ray crystallographic structure of the bacterial P-glycoprotein homologue MsbA and an MsbA-based homology model of human P-glycoprotein creates an opportunity to compare the original mutagenesis data with the three-dimensional structures of transporters. Our comparisons reveal that mutations that alter specificity are present in three-dimensional 'hotspot' regions in the membrane domains of P-glycoprotein.

Drug-lipid A interactions on the Escherichia coli ABC transporter MsbA

MsbA is an essential ATP-binding cassette half-transporter in the cytoplasmic membrane of the gram-negative Escherichia coli and is required for the export of lipopolysaccharides (LPS) to the outer membrane, most likely by transporting the lipid A core moiety. Consistent with the homology of MsbA to the multidrug transporter LmrA in the gram-positive Lactococcus lactis, our recent work in E. coli suggested that MsbA might interact with multiple drugs. To enable a more detailed analysis of multidrug transport by MsbA in an environment deficient in LPS, we functionally expressed MsbA in L. lactis. MsbA expression conferred an 86-fold increase in resistance to the macrolide erythromycin. A kinetic characterization of MsbA-mediated ethidium and Hoechst 33342 transport revealed apparent single-site kinetics and competitive inhibition of these transport reactions by vinblastine with K(i) values of 16 and 11 microM, respectively. We also detected a simple noncompetitive inhibition of Hoechst 33342 transport by free lipid A with a K(i) of 57 microM, in a similar range as the K(i) for vinblastine, underscoring the relevance of our LPS-less lactococcal model for studies on MsbA-mediated drug transport. These observations demonstrate the ability of heterologously expressed MsbA to interact with free lipid A and multiple drugs in the absence of auxiliary E. coli proteins. Our transport data provide further functional support for direct LPS-MsbA interactions as observed in a recent crystal structure for MsbA from Salmonella enterica serovar Typhimurium (C. L. Reyes and G. Chang, Science 308:1028-1031, 2005).

An ABC transporter with a secondary-active multidrug translocator domain

Multidrug resistance, by which cells become resistant to multiple unrelated pharmaceuticals, is due to the extrusion of drugs from the cell's interior by active transporters such as the human multidrug resistance P-glycoprotein. Two major classes of transporters mediate this extrusion. Primary-active transporters are dependent on ATP hydrolysis, whereas secondary-active transporters are driven by electrochemical ion gradients that exist across the plasma membrane. The ATP-binding cassette (ABC) transporter LmrA is a primary drug transporter in Lactococcus lactis that can functionally substitute for P-glycoprotein in lung fibroblast cells. Here we have engineered a truncated LmrA protein that lacks the ATP-binding domain. Surprisingly, this truncated protein mediates a proton-ethidium symport reaction without the requirement for ATP. In other words, it functions as a secondary-active multidrug uptake system. These findings suggest that the evolutionary precursor of LmrA was a secondary-active substrate translocator that acquired an ATP-binding domain to enable primary-active multidrug efflux in L. lactis.

Reversible transport by the ATP-binding cassette multidrug export pump LmrA: ATP synthesis at the expense of downhill ethidium uptake

The ATP dependence of ATP-binding cassette (ABC) transporters has led to the widespread acceptance that these systems are unidirectional. Interestingly, in the presence of an inwardly directed ethidium concentration gradient in ATP-depleted cells of Lactococcus lactis, the ABC multidrug transporter LmrA mediated the reverse transport (or uptake) of ethidium with an apparent K(t) of 2.0 microm. This uptake reaction was competitively inhibited by the LmrA substrate vinblastine and was significantly reduced by an E314A substitution in the membrane domain of the transporter. Similar to efflux, LmrA-mediated ethidium uptake was inhibited by the E512Q replacement in the Walker B region of the nucleotide-binding domain of the protein, which strongly reduced its drug-stimulated ATPase activity, consistent with published observations for other ABC transporters. The notion that ethidium uptake is coupled to the catalytic cycle in LmrA was further corroborated by studies in LmrA-containing cells and proteoliposomes in which reverse transport of ethidium was associated with the net synthesis of ATP. Taken together, these data demonstrate that the conformational changes required for drug transport by LmrA are (i) not too far from equilibrium under ATP-depleted conditions to be reversed by appropriate changes in ligand concentrations and (ii) not necessarily coupled to ATP hydrolysis, but associated with a reversible catalytic cycle. These findings and their thermodynamic implications shed new light on the mechanism of energy coupling in ABC transporters and have implications for the development of new modulators that could enable reverse transport-associated drug delivery in cells through their ability to uncouple ATP binding/hydrolysis from multidrug efflux.

A new dimer interface for an ABC transporter

The crystallization of MsbA, an ATP-binding cassette (ABC) transporter involved in the transport of Lipid A in Escherichia coli, provided a fascinating glimpse into the high-resolution structure of an ABC transporter at 4.8 A. The E. coli crystal structure of MsbA reveals a dimer. Although the structure of the MsbA monomer is consistent with the biochemistry of ABC transporters, including the human multidrug resistance P-glycoprotein, the interface between the monomers in the MsbA dimer may not reflect the biologically relevant interface. We considered the interface in a two-armed MsbA dimer, named spiral. Our findings indicate that (i) the spiral MsbA dimer may have biological relevance for ABC transporters that interact with lipophilic substrates, and (ii) the dimer interface observed in the crystal structure of E. coli MsbA represents a crystallization artefact. A comparison of the spiral MsbA dimer with the recently published structure of MsbA in Vibrio cholera is also described.