A novel transporter, Pfcrt, confers antimalarial drug resistance

The elucidation of the molecular details of drug resistance phenomena is a very active area of research that crosses many disciplinary boundaries. Drug resistance is due to altered drug-target interaction, and/or dysregulated signaling related to cell growth and death. Since many drugs need to rapidly diffuse into and within cells in order to find their targets, and since transmembrane ion transport is an important facet of cellular signaling, it is not surprising that membrane transport phenomena have been implicated in the evolution of drug resistance in tumor cells, bacteria, and intracellular parasites such as Plasmodium falciparum, the causative agent of the most lethal form of human malaria. The most infamous membrane transport protein involved in drug resistance is "MDR protein" or "P-glycoprotein" (Pgp),1 which was found to be overexpressed in drug-resistant tumor cells over 15 years ago, and which is representative of the ATP-binding cassette (ABC) superfamily that also includes the important cystic fibrosis transmembrane conductance regulator (CFTR) and sulfonyl urea receptor (SUR) ion channels. Availability of mouse and human Pgp cDNA rather quickly led to the identification of homologues in many species, including P. falciparum, and these were de facto assumed to be the ultimate determinants of drug resistance in these systems as well. However, research over the past 10 years has taught us that this assumption likely is wrong and that the situation is more complex. We now know that human Pgp plays a relatively minor role in clinically relevant tumor drug resistance, and that an integral membrane protein with no homology to the ABC superfamily, Pfcrt, ultimately confers chloroquine resistance in P. falciparum. Thus, the general hypothesis that membrane transport and membrane transport proteins are important in drug resistance phenomena remains correct, but at a genetic, biochemical, and physiological level we have recently witnessed a few very interesting surprises.

Evidence for altered ion transport in Saccharomyces cerevisiae overexpressing human MDR 1 protein

Recently [Hoffman, M. M., and Roepe, P. D. (1997) Biochemistry 36, 11153-11168] we presented evidence for a novel Na+- and Cl--dependent H+ transport process in LR73/hu MDR 1 CHO transfectants that likely explains pHi, volume, and membrane potential changes in eukaryotic cells overexpressing the hu MDR 1 protein. To further explore this process, we have overexpressed human MDR 1 protein in yeast strain 9.3 following a combination of approaches used previously [Kuchler, K., and Thorner, J. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 2302-2306; Ruetz, S., et al. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11588-11592]. Thus, a truncated hu MDR 1 cDNA was cloned behind a tandem array of sterile 6 (Ste6) and alchohol dehydrogenase (Adh) promoters to create the yeast expression vector pFF1. Valinomycin resistance of intact cells and Western blot analysis with purified yeast plasma membranes confirmed the overexpression of full length, functional, and properly localized hu MDR 1 protein in independently isolated 9.3/pFF1 colonies. Interestingly, relative valinomycin resistance and growth of the 9.3/hu MDR 1 strains are found to strongly depend on the ionic composition of the growth medium. Atomic absorption reveals significant differences in intracellular K+ for 9.3/hu MDR 1 versus control yeast. Transport assays using [3H]tetraphenylphosphonium ([3H]TPP+) reveal perturbations in membrane potential for 9.3/hu MDR 1 yeast that are stimulated by KCl and alkaline pHex. ATPase activity of purified plasma membrane fractions from yeast strains and LR73/hu MDR 1 CHO transfectants constructed previously [Hoffman, M. M., et al. (1996) J. Gen. Physiol. 108, 295-313] was compared. MDR 1 ATPase activity exhibits a higher pH optimum and different salt dependencies, relative to yeast H+ ATPase. Inside-out plasma membrane vesicles (ISOV) fabricated from 9.3/hu MDR 1 and control strains were analyzed for formation of H+ gradients +/- verapamil. Similar pharmacologic profiles are found for verapamil stimulation of MDR 1 ATPase activity and H+ pumping in 9.3/hu MDR 1 ISOV. In sum, these experiments strongly support the notion that hu MDR 1 catalyzes H+ transport in some fashion and lowers membrane potential in yeast when K+ contributes strongly to that potential. In the accompanying paper [Santai, C. T., Fritz, F., and Roepe, P. D. (1999) Biochemistry 38, XXXX-XXXX] the effects of ion gradients on H+ transport by hu MDR 1 are examined.

Transient neurophysiological changes in CA3 neurons and dentate granule cells after severe forebrain ischemia in vivo

Transient neurophysiological changes in CA3 neurons and dentate granule cells after severe forebrain ischemia in vivo. J. Neurophysiol. 80: 2860-2869, 1998. The spontaneous activities, evoked synaptic responses, and membrane properties of CA3 pyramidal neurons and dentate granule cells in rat hippocampus were compared before ischemia and </=7 days after reperfusion with intracellular recording and staining techniques in vivo. A four-vessel occlusion method was used to induce approximately 14 min of ischemic depolarization. No significant change in spontaneous firing rate was observed in both cell types after reperfusion. The amplitude and slope of excitatory postsynaptic potentials (EPSPs) in CA3 neurons decreased to 50% of control values during the first 12 h reperfusion and returned to preischemic levels 24 h after reperfusion. The amplitude and slope of EPSPs in granule cells slightly decreased 24-36 h after reperfusion. The amplitude of inhibitory postsynaptic potentials in CA3 neurons transiently increased 24 h after reperfusion, whereas that in granule cells showed a transient decrease 24-36 h after reperfusion. The duration of spike width of CA3 and granule cells became longer than that of control values during the first 12 h reperfusion. The spike threshold of both cell types significantly increased 24-36 h after reperfusion, whereas the frequency of repetitive firing evoked by depolarizing current pulse was decreased during this period. No significant change in rheobase and input resistance was observed in CA3 neurons. A transient increase in rheobase and a transient decrease in input resistance were detected in granule cells 24-36 h after reperfusion. The amplitude of fast afterhyperpolarization in both cell types increased for 2 days after ischemia and returned to normal values 7 days after reperfusion. The results from this study indicate that the neuronal excitability and synaptic transmission in CA3 and granule cells are transiently suppressed after severe forebrain ischemia. The depression of synaptic transmission and neuronal excitability may provide protection for neurons after ischemic insult.

Indoor emissions from conversion varnishes

Conversion varnishes are two-component, acid-catalyzed varnishes that are commonly used to finish cabinets. They are valued for their water and stain resistance, as well as their appearance. They have been found, however, to contribute to indoor emissions of organic compounds. For this project, three commercially available conversion varnish systems were selected. A U.S. Environmental Protection Agency (EPA) Method 24 analysis was performed to determine total volatile content, and a sodium sulfite titration method was used to determine uncombined (free) formaldehyde content of the varnish components. The resin component was also analyzed by gas chromatography/mass spectroscopy (GC/MS) (EPA Method 311 with an MS detector) to identify individual organic compounds. Dynamic small chamber tests were then performed to identify and quantify emissions following application to coupons of typical kitchen cabinet wood substrates, during both curing and aging. Because conversion varnishes cure by chemical reaction, the compounds emitted during curing and aging are not necessarily the same as those in the formulation. Results of small chamber tests showed that the amount of formaldehyde emitted from these coatings was 2.3-8.1 times the amount of free formaldehyde applied in the coatings. A long-term test showed a formaldehyde emission rate of 0.17 mg/m2/hr after 115 days.

Electrophysiological changes of CA3 neurons and dentate granule cells following transient forebrain ischemia

The electrophysiological responses of CA3 pyramidal neurons and dentate granule (DG) cells in rat hippocampus were studied after transient forebrain ischemia using intracellular recording and staining techniques in vivo. Approximately 5 min of ischemic depolarization was induced using 4-vessel occlusion method. The spike threshold and rheobase of CA3 neurons remained unchanged up to 12 h following reperfusion. No significant change in spike threshold was observed in DG cells but the rheobase transiently increased 6-9 h after ischemia. The input resistance and time constant of CA3 neurons increased 0-3 h after ischemia and returned to control ranges at later time periods. The spontaneous firing rate in CA3 neurons transiently decreased shortly following reperfusion, while that of DG cells progressively decreased after ischemia. In CA3 neurons, the amplitude and slope of excitatory postsynaptic potentials (EPSPs) transiently decreased 0-3 h after reperfusion, and the stimulus intensity threshold for EPSPs transiently increased at the same time. No significant changes in amplitude and slope of EPSPs were observed in DG cells, but the stimulus intensity threshold for EPSPs slightly increased shortly after reperfusion. The present study demonstrates that the excitability of CA3 pyramidal neurons and DG cells after 5 min ischemic depolarization is about the same as control levels, whereas the synaptic transmission to these cells was transiently suppressed after the ischemic insult. These results suggest that synaptic transmission is more sensitive to ischemia than membrane properties, and the depression of synaptic transmission may be a protective mechanism against ischemic insults.

Digit removal leads to discrepancies between the structural and functional organization of the forepaw barrel subfield in layer IV of rat primary somatosensory cortex

The physiological representation of the forepaw in rat primary somatosensory cortex (SI) is topographically organized. This representation is associated with the unique arrangement of barrels in layer IV of the forepaw barrel subfield (FBS) in SI and provides an example of a relationship between cortical structure and function. It has been reported that removal of peripheral afferent input to the FBS prior to postnatal day 5 or 6 results in a disorganized FBS, while deafferentation at later times produces little or no alteration of the FBS. Therefore, restricted deafferentations of individual digits in adult rats should result in little, if any, disruption of the FBS, while at the same time eliminating afferent input to the FBS from a localized region of the periphery. This manipulation is likely to create a mismatch between structure and function and offer insight into what barrels actually represent in the adult deafferent. In the present study, we amputated digit three (D3) in eight adult rats, allowed a 1-month survival time, physiologically mapped the representation of D2, D4, and the stump, and compared this physiological map to the underlying barrels in the FBS. Our results showed that FBS barrels formerly associated with the representation of D3 were now associated with the representation of surrounding digits D2 and D4, as well as the remaining stump. By superimposing the morphological and physiological map upon one another, it was clear that the D2 and D4 representations expanded into the former D3 barrel territory and septae between the barrels. The reorganized physiological map was somatotopically organized, even though the general configuration of the morphological map remained unaltered, as visualized with cytochrome oxidase staining. These results suggest that in the deafferent, neurons within FBS barrels previously associated with the representation of punctate regions of skin become associated with neighboring regions of skin. A morphological substrate to account for this cortical reorganization is described.

Cell and molecular analysis of the developing and adult mouse subventricular zone of the cerebral hemispheres

The subventricular zone (SVZ) of the lateral ventricle remains mitotically active in the adult mammalian central nervous system (CNS). Recent studies have suggested that this region may contain neuronal precursors (neural stem cells) in adult rodents. A variety of neuronal and glial markers as well as three extracellular matrix (ECM) markers were examined with the hope of understanding factors that may affect the growth and migration of neurons from this region throughout development and in the adult. This study has characterized the subventricular zone of late embryonic, postnatal, and adult mice using several neuronal markers [TuJ1, nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), neuron-specific enolase (NSE)], glial markers [RC-2, vimentin, glial fibrillary acidic protein (GFAP), galactocerebroside (Gal-C)], ECM markers [tenascin-C (TN-C), chondroitin sulfate, a chondroitin sulfate proteoglycan termed dermatan sulfate-dependent proteoglycan-1 (DSD-1-PG)], stem-cell marker (nestin), and proliferation-specific marker [bromodeoxyuridine (BrdU)]. TuJ1+ and nestin+ cells (neurons and stem cells, respectively) persist in the region into adulthood, although the numbers of these cells become more sparse as the animal develops, and they appear to be immature compared to the cells in surrounding forebrain structures (e.g., not expressing NSE and having few, if any, processes). Likewise, NADPH-d+ cells are found in and around the SVZ during early postnatal development but become more sparse in the proliferative zone through maturity, and, by adulthood, only a few labeled cells can be found at the border between the SVZ and surrounding forebrain structures (e.g., the striatum), and even smaller numbers of positive cells can be found within the adult SVZ proper. BrdU labeling also seems to decrease significantly after the first postnatal week, but it still persists in the SVZ of adult animals. The disappearance of RC-2+ (radial) glia during postnatal development and the persistence of glial-derived ECM molecules such as tenascin and chondroitin sulfate proteoglycans (as well as other "boundary" molecules) in the adult SVZ may be associated with a persistence of immaturity, cell death, and a lack of cell emigration from the SVZ in the adult.

A new anti-inflammatory test, utilizing the chorio-allantoic membrane of the chick embryo

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