Another link between phospholipid transmembrane migration and ABC transporter gene family, inferred from a rare inherited disorder of phosphatidylserine externalization

The ABCs of Candida albicans Multidrug Transporter Cdr1

In the light of multidrug resistance (MDR) among pathogenic microbes and cancer cells, membrane transporters have gained profound clinical significance. Chemotherapeutic failure, by far, has been attributed mainly to the robust and diverse array of these proteins, which are omnipresent in every stratum of the living world. Candida albicans, one of the major fungal pathogens affecting immunocompromised patients, also develops MDR during the course of chemotherapy. The pivotal membrane transporters that C. albicans has exploited as one of the strategies to develop MDR belongs to either the ATP binding cassette (ABC) or the major facilitator superfamily (MFS) class of proteins. The ABC transporter Candida drug resistance 1 protein (Cdr1p) is a major player among these transporters that enables the pathogen to outplay the battery of antifungals encountered by it. The promiscuous Cdr1 protein fulfills the quintessential need of a model to study molecular mechanisms of multidrug transporter regulation and structure-function analyses of asymmetric ABC transporters. In this review, we cover the highlights of two decades of research on Cdr1p that has provided a platform to study its structure-function relationships and regulatory circuitry for a better understanding of MDR not only in yeast but also in other organisms.

Cellular mechanisms underlying the formation of circulating microparticles

Microparticles (MPs) derived from platelets, monocytes, endothelial cells, red blood cells, and granulocytes may be detected in low concentrations in normal plasma and at increased levels in atherothrombotic cardiovascular diseases. The elucidation of the cellular mechanisms underlying the generation of circulating MPs is crucial for improving our understanding of their pathophysiological role in health and disease. The flopping of phosphatidylserine (PS) to the outer leaflet of the plasma membrane is the key event that will ultimately lead to the shedding of procoagulant MPs from activated or apoptotic cells. Research over the last few years has revealed important roles for calcium-, mitochondrial-, and caspase-dependent mechanisms leading to PS exposure. The study of Scott cells has unraveled different molecular mechanisms that may contribute to fine-tuning of PS exposure and MP release in response to a variety of specific stimuli. The pharmacological modulation of MP release may have a substantial therapeutic impact in the management of atherothrombotic vascular disorders. Because PS exposure is a key feature in pathological processes different from hemostasis and thrombosis, the most important obstacle in the field of MP-modulating drugs seems to be carefully targeting MP release to relevant cell types at an optimal level, so as to achieve a beneficial action and limit possible adverse effects.

[Congenital thrombocytopathies]

Inherited thrombocytopathies are much less frequent in comparison to acquired platelet function disorders. However, congenital disorders can lead to severe bleeding tendency and are often not diagnosed. They are induced by different platelet defects based on disorders of platelet adhesion, receptors, secretion and signal transduction. In some cases they are associated with thrombocytopenia, giant platelets and various comorbidities. This article gives an overview regarding diverse defects, their diagnosis and treatment options.

Catfish egg lectin causes rapid activation of multidrug resistance 1 P-glycoprotein as a lipid translocase

Rhamnose-binding lectin from catfish (Silurus asotus) eggs (SAL) has the ability to induce externalization of phosphatidylserine (PS), followed by cell shrinkage in globotriaosylceramide (Gb3)-expressing Burkitt's lymphoma Raji cells. Because phospholipid scramblase and aminophospholipid translocase did not participate in SAL-induced PS externalization, we examined the relationship of ATP-binding cassette (ABC) transporters, such as multidrug resistance (MDR) 1 P-glycoprotein (MDR1 P-gp) and MDR-associated protein 1 (MRP1), for translocation of PS. Since cyclosporin A (MDR1 P-gp inhibitor) but not MK571 (MRP1 inhibitor) inhibited SAL-induced PS externalization, it was suggested that MDR1 P-gp is involved in this phenomenon. On the other hand, SAL activated both of the ABC transporters for efflux of rhodamine123 (MDR1 P-gp substrate, Rho123) and 5-carboxyfluorescein diacetate (MRP1 substrate, 5-CFDA) in Raji cells. In contrast, SAL did not activate these two transporters in Gb3-negative cell lines, such as K562 and doxorubicin-resistant K562 cells, involving not only PS externalization but also efflux of Rho123 or 5-CFDA. Since Gb3 and both transporters in Raji cells are located in the glycosphingolipid-enriched microdomain (GEM), it is suggested that the binding of SAL to Gb3 localized in the GEM specifically induces MDR1 P-gp activation in Raji cells.

A novel missense mutation in ABCA1 results in altered protein trafficking and reduced phosphatidylserine translocation in a patient with Scott syndrome

Scott syndrome (SS) is a bleeding disorder characterized by a failure to expose phosphatidylserine (PS) to the outer leaflet of the platelet plasma membrane. Because the adenosine triphosphate (ATP)–binding cassette transporter A1 (ABCA1) is implicated in the exofacial translocation of PS, we assessed its role in the pathophysiology of a patient with SS. Substantially reduced levels of ABCA1 mRNA were found in the patient's leukocytes, compared with controls. The SS patient was heterozygous for a novel missense mutation c.6064G>A (ABCA1 R1925Q), absent from unaffected family members and controls. Both mutant and wild-type alleles were reduced in mRNA expression, and no causative mutation for this phenomenon was identified in the ABCA1 gene or its proximal promoter, suggesting a putative second mutation in a trans-acting regulatory gene may also be involved in the disorder in this patient. In vitro expression studies showed impaired trafficking of ABCA1 R1925Q to the plasma membrane. Overexpression of wild-type ABCA1 in SS lymphocytes complemented the Ca2+-dependent PS exposure at the cell surface. These data identify a mutation in ABCA1 that contributes to the defective PS translocation phenotype in our patient with SS.

ABC transporters as phenotypic markers and functional regulators of stem cells

Characterization of molecules with tightly controlled expression patterns during differentiation represents an approach to understanding regulation of hematopoietic stem cell commitment. The multidrug resistance-1 (MDR1) gene product, P-glycoprotein, and the breast cancer resistance protein (BCRP) are expressed differentially during hematopoiesis, with the highest levels in primitive bone marrow stem cell populations that are CD34(low) and CD34(-), respectively. Roles for ATP-binding cassette (ABC) transporter superfamily members in conferring drug resistance have been extensively described. However, recent hematopoietic overexpression studies have begun to reveal previously unknown roles for ABC transporter function in normal and malignant hematopoiesis. Expression of MDR1 and BCRP transporters in the myeloid lineage has been reported in blasts from acute myeloid leukemia, but very low to undetectable in normal myelomonocytic cells. Retroviral-mediated dysregulated expression of the MDR1 transporter resulted in increased hematopoietic repopulating activity and myeloproliferative disease in mice. A distinct functional role for the BCRP transporter as a negative regulator of hematopoietic repopulating activity has recently been demonstrated using the same approach. Additionally, the presence of BCRP expression specifically on hematopoietic side-population stem cells and neural stem/progenitors, makes BCRP an attractive candidate marker for isolation of stem cells with the ability to respond to diverse environmental cues. Regulation of stem cell biology by ABC transporters has emerged as an important new field of investigation. In light of these findings, it will be critical to further characterize this family of proteins in hematopoietic lineage-restricted stem cells and in pluripotent stem cells capable of crossing lineage barriers.

Apoptotic cell removal

Ingestion by professional or amateur phagocytes is the fate of most cells that undergo apoptosis. Studies in both Caenorhabditis elegans and mammals are now converging to reveal some of the key mechanisms and consequences of this removal process. At least seven corpse removal genes in nematodes have mammalian equivalents, and represent elements of signaling pathways involved in uptake. In mammals, a wide variety of apoptotic cell recognition receptors has been implicated and appears to be divided into two categories, involved in tethering the apoptotic cell or triggering an uptake mechanism related to macropinocytosis. Apoptotic cell removal is normally efficient and non-inflammatory. By contrast, the process may become subverted by parasites to yield a more favorable growth environment, or in other cases lead to fibrosis. Removal may also clinch the apoptotic process itself in cells not yet completely committed to death.

Microvillar cell surface as a natural defense system against xenobiotics: a new interpretation of multidrug resistance

The phenomenon of multidrug resistance (MDR) is reinterpreted on the basis of the recently proposed concept of microvillar signaling. According to this notion, substrate and ion fluxes across the surface of differentiated cells occur via transporters and ion channels that reside in membrane domains at the tips of microvilli (MV). The flux rates are regulated by the actin-based cytoskeletal core structure of MV, acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. The expression of this diffusion barrier system is a novel aspect of cell differentiation and represents a functional component of the natural defense system of epithelial cells against environmental hazardous ions and lipophilic compounds. Because of the specific organization of epithelial Ca(2+) signaling and the secretion, lipophilic compounds associated with the plasma membrane are transferred from the basal to the apical cell surface by a lipid flow mechanism. Drug release from the apical pole occurs by either direct secretion from the cell surface or metabolization by the microvillar cytochrome P-450 system and efflux of the metabolites and conjugation products through the large multifunctional anion channels localized in apical MV. The natural microvillar defense system also provides a mechanistic basis of acquired MDR in tumor cells. The microvillar surface organization is lost in rapidly growing cells such as tumor or embryonic cells but is restored during exposure of tumor cells to cytotoxins by induction of a prolonged G(0)/G(1) resting phase.

Lipid translocation across the plasma membrane of mammalian cells

The plasma membrane, which forms the physical barrier between the intra- and extracellular milieu, plays a pivotal role in the communication of cells with their environment. Exchanging metabolites, transferring signals and providing a platform for the assembly of multi-protein complexes are a few of the major functions of the plasma membrane, each of which requires participation of specific membrane proteins and/or lipids. It is therefore not surprising that the two leaflets of the membrane bilayer each have their specific lipid composition. Although membrane lipid asymmetry has been known for many years, the mechanisms for maintaining or regulating the transbilayer lipid distribution are still not completely understood. Three major players have been presented over the past years: (1) an inward-directed pump specific for phosphatidylserine and phosphatidylethanolamine, known as aminophospholipid translocase; (2) an outward-directed pump referred to as 'floppase' with little selectivity for the polar headgroup of the phospholipid, but whose actual participation in transport of endogenous lipids has not been well established; and (3) a lipid scramblase, which facilitates bi-directional migration across the bilayer of all phospholipid classes, independent of the polar headgroup. Whereas a concerted action of aminophospholipid translocase and floppase could, in principle, account for the maintenance of lipid asymmetry in quiescent cells, activation of the scramblase and concomitant inhibition of the aminophospholipid translocase causes a collapse of lipid asymmetry, manifested by exposure of phosphatidylserine on the cell surface. In this article, each of these transporters will be discussed, and their physiological importance will be illustrated by the Scott syndrome, a bleeding disorder caused by impaired lipid scrambling. Finally, phosphatidylserine exposure during apoptosis will be briefly discussed in relation to inhibition of translocase and simultaneous activation of scramblase.

Mammalian Bax triggers apoptotic changes in yeast

Apoptosis is co-regulated by the conserved family of Bcl-2-related proteins, which includes both its agonists (Bax) and antagonists (Bcl-X(L)). A mutant strain of the yeast Saccharomyces cerevisiae has been shown to express all morphological signs of apoptosis. Overexpression of Bax is lethal in S. cerevisiae, whereas simultaneous overexpression of Bcl-X(L) rescues the cells. We report that overexpression of mammalian Bax in a S. cerevisiae wild type strain triggers morphological changes similar to those of apoptotic metazoan cells: the loss of asymmetric distribution of plasma membrane phosphatidylserine, plasma membrane blebbing, chromatin condensation and margination, and DNA fragmentation. Simultaneous overexpression of Bcl-X(L) prevents these changes. We demonstrate that Bax triggers phenotypic alterations in yeast strongly resembling those it causes in metazoan apoptotic cells.

Expression of Proteins Controlling Transbilayer Movement of Plasma Membrane Phospholipids in the B Lymphocytes From a Patient With Scott Syndrome

Scott syndrome is a rare inherited bleeding disorder in which platelets and other blood cells fail to promote normal assembly of the membrane-stabilized proteases of the plasma coagulation system. The defect in Scott blood cells is known to reflect inability to mobilize phosphatidylserine from inner plasma membrane leaflet to the cell surface in response to an elevation of Ca2+ at the endofacial surface. To gain insight into the molecular basis of this membrane defect, we examined the expression in Scott cells of plasma membrane proteins that have been implicated to participate in the accelerated transbilayer movement of plasma membrane PL. By both reverse transcriptase-polymerase chain reaction (RT-PCR) and functional assay, the level of expression of the multidrug resistance (MDR)1 and MDR3 P-glycoproteins in immortalized B-lymphoblast cell lines from the patient with Scott syndrome were indistinguishable from matched cell lines derived from normal controls. Whereas the plasma membrane of Scott cells are insensitive to activation of the plasma membrane PL scramblase pathway, it had been shown that PL scramblase protein isolated from detergent-solubilized Scott erythrocytes exhibits normal function when incorporated into proteoliposomes (Stout JG, Basse F, Luhm RA, Weiss HJ, Wiedmer T, Sims PJ: J Clin Invest 99:2232, 1997). Consistent with this finding in Scott erythrocytes, we found that Scott lymphoblasts expressed normal levels of PL scramblase mRNA and protein, and that the deduced sequence of PL scramblase in Scott cells is identical to that of normal controls. These data suggest that the defect in Scott syndrome is related either to aberrant posttranslational processing of the PL scramblase polypeptide or to a defect or deficiency in an unknown cofactor that is required for normal expression of plasma membrane PL scramblase function in situ, or alternatively, reflects the presence of a detergent-dissociable inhibitor of this pathway.

© 1998 by The American Society of Hematology.

Expression of Proteins Controlling Transbilayer Movement of Plasma Membrane Phospholipids in the B Lymphocytes From a Patient With Scott Syndrome

Scott syndrome is a rare inherited bleeding disorder in which platelets and other blood cells fail to promote normal assembly of the membrane-stabilized proteases of the plasma coagulation system. The defect in Scott blood cells is known to reflect inability to mobilize phosphatidylserine from inner plasma membrane leaflet to the cell surface in response to an elevation of Ca2+ at the endofacial surface. To gain insight into the molecular basis of this membrane defect, we examined the expression in Scott cells of plasma membrane proteins that have been implicated to participate in the accelerated transbilayer movement of plasma membrane PL. By both reverse transcriptase-polymerase chain reaction (RT-PCR) and functional assay, the level of expression of the multidrug resistance (MDR)1 and MDR3 P-glycoproteins in immortalized B-lymphoblast cell lines from the patient with Scott syndrome were indistinguishable from matched cell lines derived from normal controls. Whereas the plasma membrane of Scott cells are insensitive to activation of the plasma membrane PL scramblase pathway, it had been shown that PL scramblase protein isolated from detergent-solubilized Scott erythrocytes exhibits normal function when incorporated into proteoliposomes (Stout JG, Basse F, Luhm RA, Weiss HJ, Wiedmer T, Sims PJ: J Clin Invest 99:2232, 1997). Consistent with this finding in Scott erythrocytes, we found that Scott lymphoblasts expressed normal levels of PL scramblase mRNA and protein, and that the deduced sequence of PL scramblase in Scott cells is identical to that of normal controls. These data suggest that the defect in Scott syndrome is related either to aberrant posttranslational processing of the PL scramblase polypeptide or to a defect or deficiency in an unknown cofactor that is required for normal expression of plasma membrane PL scramblase function in situ, or alternatively, reflects the presence of a detergent-dissociable inhibitor of this pathway.

© 1998 by The American Society of Hematology.