Phorbol ester stimulates the phosphorylation of rabbit erythrocyte band 4.1

Emerging metrology for high-throughput nanomaterial genotoxicology

The rapid development of the engineered nanomaterial (ENM) manufacturing industry has accelerated the incorporation of ENMs into a wide variety of consumer products across the globe. Unintentionally or not, some of these ENMs may be introduced into the environment or come into contact with humans or other organisms resulting in unexpected biological effects. It is thus prudent to have rapid and robust analytical metrology in place that can be used to critically assess and/or predict the cytotoxicity, as well as the potential genotoxicity of these ENMs. Many of the traditional genotoxicity test methods [e.g. unscheduled DNA synthesis assay, bacterial reverse mutation (Ames) test, etc.,] for determining the DNA damaging potential of chemical and biological compounds are not suitable for the evaluation of ENMs, due to a variety of methodological issues ranging from potential assay interferences to problems centered on low sample throughput. Recently, a number of sensitive, high-throughput genotoxicity assays/platforms (CometChip assay, flow cytometry/micronucleus assay, flow cytometry/γ-H2AX assay, automated 'Fluorimetric Detection of Alkaline DNA Unwinding' (FADU) assay, ToxTracker reporter assay) have been developed, based on substantial modifications and enhancements of traditional genotoxicity assays. These new assays have been used for the rapid measurement of DNA damage (strand breaks), chromosomal damage (micronuclei) and for detecting upregulated DNA damage signalling pathways resulting from ENM exposures. In this critical review, we describe and discuss the fundamental measurement principles and measurement endpoints of these new assays, as well as the modes of operation, analytical metrics and potential interferences, as applicable to ENM exposures. An unbiased discussion of the major technical advantages and limitations of each assay for evaluating and predicting the genotoxic potential of ENMs is also provided.

DNA Damage Response and DNA Repair in Skeletal Myocytes From a Mouse Model of Spinal Muscular Atrophy

We studied DNA damage response (DDR) and DNA repair capacities of skeletal muscle cells from a mouse model of infantile spinal muscular atrophy (SMA) caused by loss-of-function mutation of survival of motor neuron (Smn). Primary myocyte cultures derived from skeletal muscle satellite cells of neonatal control and mutant SMN mice had similar myotube length, myonuclei, satellite cell marker Pax7 and differentiated myotube marker myosin, and acetylcholine receptor clustering. DNA damage was induced in differentiated skeletal myotubes by γ-irradiation, etoposide, and methyl methanesulfonate (MMS). Unexposed control and SMA myotubes had stable genome integrity. After γ-irradiation and etoposide, myotubes repaired most DNA damage equally. Control and mutant myotubes exposed to MMS exhibited equivalent DNA damage without repair. Control and SMA myotube nuclei contained DDR proteins phospho-p53 and phospho-H2AX foci that, with DNA damage, dispersed and then re-formed similarly after recovery. We conclude that mouse primary satellite cell-derived myotubes effectively respond to and repair DNA strand-breaks, while DNA alkylation repair is underrepresented. Morphological differentiation, genome stability, genome sensor, and DNA strand-break repair potential are preserved in mouse SMA myocytes; thus, reduced SMN does not interfere with myocyte differentiation, genome integrity, and DNA repair, and faulty DNA repair is unlikely pathogenic in SMA.

Worldwide interest in the comet assay: a bibliometric study

The comet assay is a rapid, sensitive and relatively simple method for measuring DNA damage. A bibliometric study was performed to evaluate temporal and geographical trends, research quality and main areas of interest in scientific production in this field. A PubMed search strategy was developed and 7674 citations were retrieved in the period 1990-2013. Notably, the MeSH (Medical Subject Headings) term 'comet assay', officially introduced in 2000, is used by indexers only in two thirds of papers retrieved. Articles on the comet assay were published in 78 countries, spread over the 5 continents. The EU contributed the greatest output, producing >2900 articles with IF (42.0%) and totalling almost 10000 IF points, and was followed by USA. In the new millennium, research with this assay reached a plateau or slow decline in the most industrialised areas (USA, Germany, UK, Italy), while its use has boomed in emerging countries, with increases of 5- to 7-fold in the last 10 years in China, India and Brazil, for instance. This transition resulted in a slow decrease of scientific production quality, as the countries that increased their relative weight typically had lower mIFs. The most common MeSH terms used in papers using the comet assay referred to wide areas of interest, such as DNA damage and repair, cell survival and apoptosis, cancer and oxidative stress, occupational and environmental health. Keywords related to humans, rodents and cell culture were also frequently used. The top journal for the comet assay articles was found to be Mutation Research, followed by Mutagenesis. Most papers using the comet assay as a biomarker were published in genetic and toxicology journals, with a stress on environmental and occupational disciplines.

Micropatterned comet assay enables high throughput and sensitive DNA damage quantification

The single cell gel electrophoresis assay, also known as the comet assay, is a versatile method for measuring many classes of DNA damage, including base damage, abasic sites, single strand breaks and double strand breaks. However, limited throughput and difficulties with reproducibility have limited its utility, particularly for clinical and epidemiological studies. To address these limitations, we created a microarray comet assay. The use of a micrometer scale array of cells increases the number of analysable comets per square centimetre and enables automated imaging and analysis. In addition, the platform is compatible with standard 24- and 96-well plate formats. Here, we have assessed the consistency and sensitivity of the microarray comet assay. We showed that the linear detection range for H2O2-induced DNA damage in human lymphoblastoid cells is between 30 and 100 μM, and that within this range, inter-sample coefficient of variance was between 5 and 10%. Importantly, only 20 comets were required to detect a statistically significant induction of DNA damage for doses within the linear range. We also evaluated sample-to-sample and experiment-to-experiment variation and found that for both conditions, the coefficient of variation was lower than what has been reported for the traditional comet assay. Finally, we also show that the assay can be performed using a 4× objective (rather than the standard 10× objective for the traditional assay). This adjustment combined with the microarray format makes it possible to capture more than 50 analysable comets in a single image, which can then be automatically analysed using in-house software. Overall, throughput is increased more than 100-fold compared to the traditional assay. Together, the results presented here demonstrate key advances in comet assay technology that improve the throughput, sensitivity, and robustness, thus enabling larger scale clinical and epidemiological studies.

Acellular comet assay: a tool for assessing variables influencing the alkaline comet assay

In this study, an acellular modification to the alkaline comet assay to further evaluate key variables within the assay that may influence the outcome of genotoxicity studies is described. This acellular comet assay can detect differences of 0.2 Gy of (60)Co gamma-ray radiation between 0 and 1 Gy and differences of 1 Gy between 0 and 8 Gy; thus, this assay is applicable for a wide range of DNA damage levels. It is also shown that DNA damage from different radiation energies was not significantly different from (60)Co gamma-ray. This assay displayed a statistical increase in DNA damage due to uncontrolled exposure to natural light; however, the slope of the dose-response curve for light-exposed samples was similar to that for samples protected from light. A comparison of the alkaline comet assay with the acellular comet assay allowed for the intrinsic repair capacity of the alkaline comet assay to be quantified.

Introducing a true internal standard for the Comet assay to minimize intra- and inter-experiment variability in measures of DNA damage and repair

The Comet assay (CA) is a sensitive/simple measure of genotoxicity. However, many features of CA contribute variability. To minimize these, we have introduced internal standard materials consisting of 'reference' cells which have their DNA substituted with BrdU. Using a fluorescent anti-BrdU antibody, plus an additional barrier filter, comets derived from these cells could be readily distinguished from the 'test'-cell comets, present in the same gel. In experiments to evaluate the reference cell comets as external and internal standards, the reference and test cells were present in separate gels on the same slide or mixed together in the same gel, respectively, before their co-exposure to X-irradiation. Using the reference cell comets as internal standards led to substantial reductions in the coefficient of variation (CoV) for intra- and inter-experimental measures of comet formation and DNA damage repair; only minor reductions in CoV were noted when the reference and test cell comets were in separate gels. These studies indicate that differences between individual gels appreciably contribute to CA variation. Further studies using the reference cells as internal standards allowed greater significance to be obtained between groups of replicate samples. Ultimately, we anticipate that development will deliver robust quality assurance materials for CA.

Evidence that single-stranded DNA breaks are a normal feature of koala sperm chromatin, while double-stranded DNA breaks are indicative of DNA damage

In this study, we have used single and double comet assays to differentiate between single- and double-stranded DNA damage in an effort to refine the interpretation of DNA damage in mature koala spermatozoa. We have also investigated the likelihood that single-stranded DNA breakage is part of the natural spermiogenic process in koalas, where its function would be the generation of structural bends in the DNA molecule so that appropriate packaging and compaction can occur. Koala spermatozoa were examined using the sperm chromatin dispersion test (SCDt) and comet assays to investigate non-orthodox double-stranded DNA. Comet assays were conducted under 1) neutral conditions; and 2) neutral followed by alkaline conditions (double comet assay); the latter technique enabled simultaneous visualisation of both single-stranded and double-stranded DNA breaks. Following the SCDt, there was a continuum of nuclear morphotypes, ranging from no apparent DNA fragmentation to those with highly dispersed and degraded chromatin. Dispersion morphotypes were mirrored by a similar diversity of comet morphologies that could be further differentiated using the double comet assay. The majority of koala spermatozoa had nuclei with DNA abasic-like residues that produced single-tailed comets following the double comet assay. The ubiquity of these residues suggests that constitutive alkali-labile sites are part of the structural configuration of the koala sperm nucleus. Spermatozoa with ‘true’ DNA fragmentation exhibited a continuum of comet morphologies, ranging from a more severe form of alkaline-susceptible DNA with a diffuse single tail to nuclei that exhibited both single- and double-stranded breaks with two comet tails.

A direct view by immunofluorescent comet assay (IFCA) of DNA damage induced by nicking and cutting enzymes, ionizing (137)Cs radiation, UV-A laser microbeam irradiation and the radiomimetic drug bleomycin

In DNA repair research, DNA damage is induced by different agents, depending on the technical facilities of the investigating researchers. A quantitative comparison of different investigations is therefore often difficult. By using a modified variant of the neutral comet assay, where the histone H1 is detected by immunofluorescence [immunofluorescent comet assay (IFCA)], we achieve previously unprecedented resolution in the detection of fragmented chromatin and show that trillions of ultraviolet A photons (of a few eV), billions of bleomycin (BLM) molecules and thousands of gamma quanta (of 662 keV) generate, in first order, similar damage in the chromatin of HeLa cells. A somewhat more detailed inspection shows that the damage caused by 20 Gy ionizing radiation and by a single laser pulse of 10 microJ are comparable, while the damage caused by 12 microg/ml BLM depends highly on the individual cell. Taken together, this work provides a detailed view of DNA fragmentation induced by different treatments and allows comparing them to some extent, especially with respect to the neutral comet assay.

Bayesian hierarchically weighted finite mixture models for samples of distributions

Finite mixtures of Gaussian distributions are known to provide an accurate approximation to any unknown density. Motivated by DNA repair studies in which data are collected for samples of cells from different individuals, we propose a class of hierarchically weighted finite mixture models. The modeling framework incorporates a collection of k Gaussian basis distributions, with the individual-specific response densities expressed as mixtures of these bases. To allow heterogeneity among individuals and predictor effects, we model the mixture weights, while treating the basis distributions as unknown but common to all distributions. This results in a flexible hierarchical model for samples of distributions. We consider analysis of variance-type structures and a parsimonious latent factor representation, which leads to simplified inferences on non-Gaussian covariance structures. Methods for posterior computation are developed, and the model is used to select genetic predictors of baseline DNA damage, susceptibility to induced damage, and rate of repair.

Assessment of occupational genotoxic risk among Brazilian hairdressers

OBJECTIVES

To evaluate the genotoxic risk to hairdressers exposed daily to chemical substances such as hair dyes, waving and straightening preparations and manicurists' products by the Comet assay test (single-cell gel electrophoresis).

METHODS

The Comet assay was performed on blood samples from 69 female hairdressers (36.4 +/- 10.7 years old) currently employed in 21 different beauty institutes in São Paulo, Brazil, and on 55 female control blood donors (32.6 +/- 10.0 years old) from the São Paulo University Clinical Hospital blood bank. All the control subjects had occupations other than hairdresser. Comet assays were performed by evaluating 100 blood lymphocytes per individual and graded by visual score according to comet tail length.

RESULTS

The hairdressers showed a higher frequency of DNA damage revealed by Comet Score (159.8 +/- 71) when compared to the control group (125.4 +/- 64.1), and the difference was statistically significant by the Student's t-test (P = 0.005). Multiple regression analysis showed that in addition to the hairdressers' profession, tobacco use contributed to the higher frequency of cells with comets (P < 0.05).

CONCLUSIONS

The observed DNA damage could be associated with the hairdressers' occupational environment, where different chemicals are chronically manipulated and inhaled. Considering that this profession in many countries, including Brazil, is not officially regulated, more attention should focus on these professionals not only by legislative bodies but also by multidisciplinary teams able to develop and implement risk prevention and control strategies for chemical, physical and biological agents to which hairdressers are exposed.

Determination of genotoxicity of classical swine fever vaccine in vitro by cytogenetic and comet tests

Chromosome damage in lymphocyte cultures induced by live virus vaccine against classical swine fever (CSF) has been observed in previous studies. In vivo cytogenetic tests were made with several doses of vaccines used in Argentina to control the disease. These studies have shown that genotoxic effects increased with dose. In the present study, two different in vitro assays were performed by recording the frequency of cells with chromosome alterations and by assessing the ability of the vaccine to damage DNA, using the single cell gel microelectrophoretic assay (comet test). Frequencies of cells with chromosomal alterations increased significantly when compared with controls and were dose (microl/ml) dependent: 0 = 1.23, 5 = 2.29, 10 = 5.42 and 20 = 11.71%. In the comet assay the variables measured, tail length (TL) and tail moment (TM), also increased. For control cultures TL was 2.32 microm, whereas with concentrations of 20 and 100 microl/ml TL were 12.47 and 42.3 microm, respectively. TM of control cultures was 0.18, whereas with vaccine concentrations of 20 and 100 microl/ml TM were 5.52 and 24.52, respectively. Comet frequency distributions differed significantly among treatments. These results agree with previous in vivo observations. Regarding CSF pathogeny, our results support a direct effect of CSF vaccinal virus on lymphocyte DNA. Genotoxicity of CSF vaccine was corroborated in vitro at the cytogenetic and molecular levels.

Proapoptotic effect on normal and tumor intestinal cells of cytostatic drugs with enterohepatic organotropism

The proapoptotic effect of cisplatin bile acid derivatives Bamet-R2 [cis-diamminechloro-cholylglycinate-platinum(II)] and Bamet-UD2 [cis-diammine-bisursodeoxycholate-platinum(II)], developed to treat liver and intestinal tumors, was investigated in vitro using human enterohepatic cells HepG2 (hepatoblastoma), LS 174T (colon adenocarcinoma), and its cisplatin-resistant subline LS 174T/R. Uptake by wild-type tumor cells was higher for Bamets than for cisplatin. In LS 174T/R cells, copper transporter-1 was down-regulated and multidrug resistance-associated protein-2 was up-regulated. Consequently, uptake and efflux of cisplatin, but not those of Bamets, were reduced and increased, respectively. The degree of necrosis (lactate dehydrogenase release) induced by these three drugs was small and similar in all cell types. In contrast, proapoptotic effect (caspase-3 activity and DNA fragmentation) was Bamet-UD2 > cisplatin > Bamet-R2 in HepG2 and LS 174T cells, but Bamet-UD2 > Bamet-R2 >> cisplatin in LS 174T/R cells. This effect was consistent with the ability of these compounds to form DNA-adducts (DNA-platination, changes in the DNA melting temperature, and MspI-induced restriction sequence cleavage). Oral administration of Bamet-UD2 to mice induced mild apoptosis in the small intestine (ileum > duodenum), which was not severe enough to modify its structure or function as determined by water absorption and glycocholic acid uptake by in situ perfused ileum. These results indicate that Bamet-UD2 overcomes the resistance to cisplatin when this is due in part to enhanced ability of intestinal tumors to reduce intracellular cisplatin contents. Moreover, its strong proapoptotic versus its weak pronecrotic effect together with its mild effect on normal tissues, including intestinal mucosa, may account for the high antitumor activity of Bamet-UD2 together with its very low toxicity.

Phorbol 12-myristate 13-acetate-stimulated phosphorylation of erythrocyte membrane skeletal proteins is blocked by calpain inhibitors: possible role of protein kinase M

Human erythrocytes contain cytosolic protein kinase C (PKC) which, when activated by phorbol 12-myristate 13-acetate (PMA), induces the phosphorylation of the membrane skeletal proteins band 4.1, band 4.9 and adducin. We found that brief treatments of erythrocytes with PMA resulted in a decrease in cytosolic PKC content and in the transient appearance in the cytosol of a Ca(2+)- and phospholipid-independent 55 kDa fragment of PKC, called PKM. Prolonged treatment with PMA resulted in the complete and irreversible loss of erythrocyte PKC. To investigate the possible role of calpain in this process, the calpain inhibitors leupeptin and E-64 were sealed inside erythrocytes by reversible haemolysis. Both inhibitors prolonged the lifetime of PKC in PMA-treated cells, and leupeptin was shown to block the PMA-stimulated appearance of PKM in the cytosol. Significantly, leupeptin also completely blocked PMA-stimulated phosphorylation of membrane and cytosolic substrates. This effect was mimicked by other calpain inhibitors (MDL-28170 and calpain inhibitor I), but did not occur when other protease inhibitors such as phenylmethanesulphonyl fluoride, pepstatin A or chymostatin were used. In addition, the phosphorylation of exogenous histone sealed inside erythrocytes was also blocked by leupeptin. Immunoblotting showed that leupeptin did not prevent the PMA-induced translocation of PKC to the erythrocyte membrane. Thus inhibition of PKC phosphorylation of membrane skeletal proteins by calpain inhibitors was not due to inhibition of PKC translocation to the membrane. Our results suggest that PMA treatment of erythrocytes results in the translocation of PKC to the plasma membrane, followed by calpain-mediated cleavage of PKC to PKM. This cleavage, or some other leupeptin-inhibitable process, is a necessary step for the phosphorylation of membrane skeletal substrates, suggesting that the short-lived PKM may be responsible for membrane skeletal phosphorylation. Our results suggest a potential mechanism whereby erythrocyte PKC may be subject to continual down-regulation during the lifespan of the erythrocyte due to repeated activation events, possibly related to transient Ca2+ influx. Such down-regulation may play an important role in erythrocyte survival or pathophysiology.

Okadaic acid inhibition of KCl cotransport. Evidence that protein dephosphorylation is necessary for activation of transport by either cell swelling or N-ethylmaleimide

The mechanism of activation of KCl cotransport has been examined in rabbit red blood cells. Previous work has provided evidence that a net dephosphorylation is required for activation of transport by cell swelling. In the present study okadaic acid, an inhibitor of protein phosphatases, was used to test this idea in more detail. We find that okadaic acid strongly inhibits swelling-stimulated KCl cotransport. The IC50 for okadaic acid is approximately 40 nM, consistent with the involvement of type 1 protein phosphatase in transport activation. N-Ethylmaleimide (NEM) is well known to activate KCl cotransport in cells of normal volume. Okadaic acid, added before NEM, inhibits the activation of transport by NEM, indicating that a dephosphorylation is necessary for the NEM effect. Okadaic acid added after NEM inhibits transport only very slightly. After a brief exposure to NEM and rapid removal of unreacted NEM, KCl cotransport activates with a time delay that is similar to that for swelling activation. Okadaic acid causes a slight increase in the delay time. These findings are all consistent with the idea that NEM activates transport not by a direct action on the transport protein but by altering a phosphorylation-dephosphorylation cycle. The simplest hypothesis that is consistent with the data is that both cell swelling and NEM cause inhibition of a protein kinase. Kinase inhibition causes net dephosphorylation of some key substrate (not necessarily the transport protein); dephosphorylation of this substrate, probably by type 1 protein phosphatase, causes transport activation.

Effect of 2,3-diphosphoglycerate on the phosphorylation of protein 4.1 by protein kinase C

We have previously shown that 2,3-diphosphoglycerate (2,3-DPG) inhibits the phosphorylation of erythrocyte membrane cytoskeletal proteins by endogenous casein kinases. Here, we report that 2,3-DPG stimulates the phosphorylation of protein 4.1 by protein kinase C. Studies with red cell membrane preparations showed that while the phosphorylation of most of the membrane proteins by endogenous membrane-bound kinases and purified kinase C was inhibited by 2,3-DPG, the phosphorylation of protein 4.1 was slightly enhanced by the metabolite. The effect of 2,3-DPG was further examined using purified protein 4.1 preparations. Our results indicate that 2,3-DPG stimulates both the rate and the extent of phosphorylation of purified protein 4.1 by kinase C. The amount of phosphate incorporated was found to double to 2 mol of phosphate per mole of protein 4.1 in the presence of 10 mM 2,3-DPG. The increase in phosphorylation was distributed over all phosphorylation sites as revealed by an analysis of the labeling patterns of phosphopeptides resolved by high performance liquid chromatography, but a significantly higher incorporation was detected in two of the phosphopeptides. The stimulatory effect of 2,3-DPG on the phosphorylation of protein 4.1 was observed only with kinase C. Phosphorylation by the cytosolic erythrocyte casein kinase and the cyclic AMP-dependent protein kinase was inhibited by 2,3-DPG. Moreover, the stimulatory effect of 2,3-DPG seemed to be unique to the phosphorylation of protein 4.1 since a similar effect had not been observed with other protein kinase C substrates. Our results suggest that 2,3-DPG may play an important role in the regulation of cytoskeletal interactions.

A protein kinase activity is associated with and specifically phosphorylates the neural cell adhesion molecule L1

The neural cell adhesion molecule L1 is a phosphorylated integral membrane glycoprotein that is recovered from adult mouse brain by immunoaffinity chromatography as a set of polypeptides with apparent molecular masses of 200, 180, 140, 80, and 50 kilodaltons (L1-200, L1-180, L1-140, L1-80, and L1-50, respectively). In the present study, we show that two kinase activities are associated with immunopurified L1: One specifically phosphorylates L1-200 and L1-80 but not L1-180, L1-140, or L1-50. This pattern of phosphorylation corresponds to the one described for L1 after metabolic phosphate incorporation into cultures of cerebellar cells. In both cases, serine is the main amino acid that is labeled by radioactive phosphate. The kinase activity is not activated by Ca2+, calmodulin, phosphatidylserine, diolein, cyclic AMP, or cyclic GMP, a result suggesting that the enzyme is distinct from Ca2+/calmodulin-dependent kinases, from protein kinase C, or from cyclic AMP/cyclic GMP-dependent kinases and may belong to the independent kinase group. The other kinase phosphorylates only casein but not L1, utilizes GTP as well as ATP, and is strongly inhibited by heparin. Because the primary structure of the L1 protein does not contain consensus sequences characteristic for known kinases, we believe that the catalytic activities detectable in immunopurified L1 are due to kinases that are strongly enough associated with L1 to withstand the stringent purification procedures.

Thermal properties of red blood cells infected by malaria parasites

Three membrane thermotropic transitions at 8, 20, and 40 degrees C have been detected in human red blood cells (RBC) by using spin-labeled stearic acids. Red blood cells infected in vitro by Plasmodium falciparum showed the disappearance of the 8 degrees C transition and a lowering of the 40 degrees C transition to 32 degrees C. The disappearance of the 8 degrees C transition was observed in synchronized cultures of P. falciparum trophozoites as well as in mouse RBC infected in vivo by an asynchronous population of P. berghei. Furthermore, erythrocytes infected by P. falciparum showed an increase in the phosphorylation of protein 4.1. This protein was shown previously to be involved in the 8 degrees C transition, (T. Forte, T. L. Leto, M. Minetti, and V. T. Marchesi, Biochemistry 24, 7876-7880 (1985). Our results suggest that the malaria parasite invasion produces a disorganization of the protein 4.1-membrane interaction.

Transmembrane signaling: tumor promoter distribution

Diacylglycerol plays a critical role in transmembrane signaling by activating protein kinase C (PKC). The tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) mimics that action, and in the human erythrocyte, TPA-activated PKC phosphorylates membrane proteins. Although molecular aspects of this process have been investigated, details of the interaction of TPA with plasma membranes remain elusive. Because TPA is hydrophobic, it has been assumed that it associates with the lipid bilayer. However, there is no direct evidence for its transbilayer distribution. Because knowledge of its location would limit molecular models proposed to explain its mode of action, we have used membrane-splitting techniques, based on freeze-fracture of planar cell monolayers, to quantify transmembrane partitioning of [3H]TPA. Under conditions where PKC-mediated phosphorylation was stimulated by [3H]TPA and where more than 90% of the [3H]TPA was associated with the human red cell plasma membrane, two-thirds of the TPA partitioned with the cytoplasmic leaflet after bilayer splitting. This represents the first direct topographic localization of TPA in a biological membrane and supports the hypothesis that the mechanism of TPA activation requires its association with the cytoplasmic leaflet of the bilayer.

Reversible sodium pump defect and swelling in the diabetic rat erythrocyte: effects on filterability and implications for microangiopathy

We have found a defect in the ouabain-sensitive Na+, K+-ATPase (Na+ pump, EC 3.6.1.37) of erythrocytes from streptozocin diabetic rats. This defect was accompanied by an increase in cell volume and osmotic fragility and a decrease in the cytosolic K+/Na+ ratio. There was also a doubling in the time needed for diabetic erythrocytes to pass through 4.7-micron channels in a polycarbonate filter. Our data are consistent with a primary defect in the erythrocyte Na+ pump and secondary changes in cell volume, osmotic fragility, K+/Na+ ratio, and cell filterability. All were reversed or prevented in vivo by insulin or the aldose reductase inhibitor Sorbinil. Protein kinase C agonists (phorbol ester and diacylglycerol) and agonist precursor (myoinositol) reversed the Na+ pump lesion, suggesting that protein kinase C-dependent phosphorylation of the 100-kDa subunit regulates Na+ pump activity and that insulin can influence erythrocyte protein kinase C activity. Ouabain inhibition of the erythrocyte Na+ pump also produced increases in cell size and reductions in rates of filtration. Theoretical treatment of the volume changes also predicts reduction in filterability as a consequence of cell swelling. We suggest that enlarged erythrocytes could play a role in the evolution of the microvascular changes of diabetes mellitus.

The spectrin-actin junction of erythrocyte membrane skeletons

High-resolution electron microscopy of erythrocyte membrane skeletons has provided striking images of a regular lattice-like organization with five or six spectrin molecules attached to short actin filaments to form a sheet of five- and six-sided polygons. Visualization of the membrane skeletons has focused attention on the (spectrin)5,6-actin oligomers, which form the vertices of the polygons, as basic structural units of the lattice. Membrane skeletons and isolated junctional complexes contain four proteins that are stable components of this structure in the following ratios: 1 mol of spectrin dimer, 2-3 mol of actin, 1 mol of protein 4.1 and 0.1-0.5 mol of protein 4.9 (numbers refer to mobility on SDS gels). Additional proteins have been identified that are candidates to interact with the junction, based on in vitro assays, although they have not yet been localized to this structure and include: tropomyosin, tropomyosin-binding protein and adducin. The spectrin-actin complex with its associated proteins has a key structural role in mediating cross-linking of spectrin into the network of the membrane skeleton, and is a potential site for regulation of membrane properties. The purpose of this article is to review properties of known and potential constituent proteins of the spectrin-actin junction, regulation of their interactions, the role of junction proteins in erythrocyte membrane dysfunction, and to consider aspects of assembly of the junctions.

Erythrocyte adducin. Comparison of the alpha- and beta-subunits and multiple-site phosphorylation by protein kinase C and cAMP-dependent protein kinase

Two major substrates for human erythrocyte protein kinase C (PK-C) of Mr 120,000 and 110,000, previously named PKC-1 and PKC-2 [Palfrey, H. C. & Waseem, A. (1985) J. Biol. Chem. 260, 16021-16029] have been found to be identical to CaM-BP 103/97 or 'adducin', recently described by K. Gardner and V. Bennett [(1986) J. Biol. Chem. 261, 1339-1348; (1987) Nature (Lond.) 328, 359-362]. These proteins have been purified from the membrane skeleton by high-salt extraction, ion-exchange and gel filtration chromatography. The two proteins co-fractionate in a ratio of approximately 1:1 under a number of conditions suggesting that they exist as a complex. Physicochemical data indicate that the native adducin complex is probably an asymmetric heterodimer of alpha and beta subunits. Adducin binds to a calmodulin (CaM) affinity matrix in a Ca2+-dependent manner and is specifically eluted with EGTA. Fingerprinting of the iodinated peptides derived from the alpha and beta subunits using three different proteases yields 16-37% overlapping peptides, indicating limited similarity between the two polypeptides. Affinity-purified polyclonal antibodies against each protein show little or no cross-reactivity with the other, indicating that the beta subunit is not derived from the alpha subunit or vice versa. Proteins reactive with both anti-(alpha-adducin) and anti-(beta-adducin) antibodies are found in erythrocytes from rat, rabbit, pig, ferret and duck. Immunoblots of adducin after non-ionic detergent extraction of ghosts reveal that a significant fraction of the protein may associate with non-skeleton membrane components. The phosphorylation of adducin is stimulated by both phorbol esters and cAMP analogues in intact erythrocytes. Fingerprinting suggests that protein kinase C preferentially phosphorylates four distinct sites on the two proteins. Phosphopeptide maps of alpha-adducin are virtually identical to those of beta-adducin after phorbol ester stimulation of intact cells, or after PK-C-catalyzed phosphorylation of the purified protein, indicating strong local similarities in the two proteins. Such maps also suggest that cAMP-dependent protein kinase (cAMP-PK) modifies adducin at some similar and some distinct sites as those modified by PK-C. In vitro phosphorylation of isolated adducin by purified PK-C results in rapid incorporation of phosphate to a final level of approximately 1.5 mol/mol in both alpha and beta subunits.(ABSTRACT TRUNCATED AT 400 WORDS)

Abolition of actin-bundling by phosphorylation of human erythrocyte protein 4.9

Protein 4.9, first identified as a component of the human erythrocyte membrane skeleton, binds to and bundles actin filaments. Protein 4.9 is a substrate for various kinases, including a cyclic AMP(cAMP)-dependent one, in vivo and in vitro. We show here that phosphorylation of protein 4.9 by the catalytic subunit of cAMP-dependent protein kinase reversibly abolishes its actin-bundling activity, but phosphorylation by protein kinase C has no such effect. A quantitative immunoassay showed that human erythrocytes contain 43,000 trimers of protein 4.9 per cell, which is equivalent to one trimer for each actin oligomer in these red blood cells. As analogues of protein 4.9 have been identified together with analogues of other erythroid skeletal proteins in non-erythroid tissues of numerous vertebrates, phosphorylation and dephosphorylation of protein 4.9 may be the basis for a mechanism that regulates actin bundling in many cells.

Topography of protein kinase C substrates analyzed by membrane splitting

We have used the methods of planar cell and membrane monolayer formation and monolayer splitting to study structural details of the transmembrane signaling process mediated by protein kinase C. We analyzed human red cell membrane proteins phosphorylated by phorbol ester activation of protein kinase C. Planar single membrane preparations, extraction procedures, and gel electrophoresis coupled with silver staining and autoradiography confirmed that two bands in the 100 kDa region, and bands 4.1, and 4.9, were peripheral and phosphorylated by treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA). TPA also stimulated minor incorporation of [32 P]Pi into most integral membrane proteins, including band 3, glycophorin A, the band 4.5 region (glucose transporter) and band 7. Planar cell and membrane-splitting methods revealed that neither integral nor peripheral phosphorylated polypeptides were cleaved by freeze fracture, that all phosphorylated peripheral proteins partitioned intact with the cytoplasmic side of the membrane, and that the percentages of [32P]Pi-labeled peripheral proteins were the same in split membrane cytoplasmic leaflets as in intact membranes. As a unique approach to examining protein topographies membrane splitting provides strong evidence that the major phosphorylated products of the polyphosphatidylinositide pathway are topographically associated with the cytoplasmic leaflet of the human erythrocyte plasma membrane. We further conclude that TPA-induced phosphorylation of red cell peripheral proteins does not significantly alter their transbilayer partitioning patterns after membrane splitting.

Stimulation of polyphosphoinositide turnover upon activation of protein kinases in human erythrocytes

Activation of protein kinase C in erythrocytes by 4-beta-phorbol 12-myristate 13-acetate (PMA) resulted in a parallel stimulation (time course and dose response) of the phosphorylation of both membrane proteins (heterodimers of 107 kDa and 97 kDa, protein 4.1 and 4.9, respectively) and of phosphatidylinositol 4-phosphate (PIP) and, to a lesser extent, of phosphatidylinositol 4,5-bisphosphate (PIP2). Evidence that the effect on lipid was mediated by protein kinase C activation and not by a direct action of PMA was provided by (1) the lack of effect of a phorbol ester that did not activate protein kinase C or of PMA addition on isolated membranes from control erythrocytes, (2) the reversal of the effect in the presence of protein kinase C inhibitors (alpha-cobrotoxin, H-7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine) or trifluoperazine). PMA treatment did not change the specific activity of ATP or the content of PIP2, but increased the content of PIP and decreased that of PI, indicating that the phosphorylation or dephosphorylation reactions linking PI and PIP were the target for the action of PMA. PMA treatment had no effect on the Ca2+-dependent PIP/PIP2 phospholipase C activity measured in isolated membranes. Mezerein, another protein kinase activator, had similar effects on both protein and lipid phosphorylation, when added with alpha-cobrotoxin. Activation of protein kinase A by cAMP also produced increases in phosphorylation, although quantitatively different from those induced by protein kinase C, in proteins and PIP. Simultaneous addition of PMA and cAMP at maximal doses resulted in only a partially additive effect on PIP labelling. These results show that inositol lipid turnover can be modulated by a protein kinase C and protein kinase A-dependent process involving the phosphorylation of a common protein. This could be PI kinase or PIP phosphatase or another protein regulating the activity of these enzymes.

Spectrin and related molecules

This review begins with a complete discussion of the erythrocyte spectrin membrane skeleton. Particular attention is given to our current knowledge of the structure of the RBC spectrin molecule, its synthesis, assembly, and turnover, and its interactions with spectrin-binding proteins (ankyrin, protein 4.1, and actin). We then give a historical account of the discovery of nonerythroid spectrin. Since the chicken intestinal form of spectrin (TW260/240) and the brain form of spectrin (fodrin) are the best characterized of the nonerythroid spectrins, we compare these molecules to RBC spectrin. Studies establishing the existence of two brain spectrin isoforms are discussed, including a description of the location of these spectrin isoforms at the light- and electron-microscope level of resolution; a comparison of their structure and interactions with spectrin-binding proteins (ankyrin, actin, synapsin I, amelin, and calmodulin); a description of their expression during brain development; and hypotheses concerning their potential roles in axonal transport and synaptic transmission.

Membrane phospholipid organization in calcium-loaded human erythrocytes

Intracellular Ca2+ levels in human erythrocytes were increased by incubating them with variable concentrations of Ca2+ in the presence of ionophore A23187. Experiments were done to confirm that the Ca2+ loading did induce changes in the cell shape and membrane protein composition. The effect of the increased cytoplasmic Ca2+ levels on the membrane phospholipid organization was analysed using bee venom and pancreatic phospholipases A2, Merocyanine 540 and fluorescamine as the external membrane probes. About 20% phosphatidylethanolamine (PE) and 0% phosphatidylserine (PS) were hydrolysed by the phospholipases in intact control cells, whereas in identical conditions these enzymes readily degraded, 20-30% PE and 7-30% PS, in Ca2+-loaded erythrocytes, depending on the cytoplasmic Ca2+ concentration. Also, Merocyanine 540 failed to stain the fresh or control erythrocytes, but it labeled the cells loaded with Ca2+. Furthermore, fluorescamine labeled approx. 20% PE in fresh or control erythrocytes while in identical conditions, significantly higher amounts of PE were modified in intact Ca2+-loaded cells. These results demonstrate that Ca2+ loading in human erythrocytes leads to loss of the transbilayer phospholipid asymmetry, and suggest that, together with spectrin, polypeptides 2.1 and 4.1 may also play an important role in maintaining the asymmetric distribution of various phospholipids across the erythrocyte membrane bilayer.

The effect of phorbol esters on human erythrocyte morphological discocyte-echinocyte transitions

12-O-Tetradecanoylphorbol-13-acetate (TPA) (100 nM) when incubated with human erythrocytes under conditions of ATP depletion, delayed the onset of the morphological transition from discocytes to echinocytes so that at 2 h, when control incubations were estimated to contain 65% echinocytes, those treated with TPA contained 23% echinocytes. TPA did not alter the subsequent rate of the transition which was complete by 3 h in control cells and 5 h in TPA-treated cells. Addition of 100 nM TPA to ATP-depleted erythrocytes at 2.5 h (greater than 80% echinocytes) for 0.5 h at 37 degrees C resulted in 17% reversal to a discocyte morphology, but as the time of incubation under conditions of ATP depletion was extended, the level of the reversal fell. TPA had no significant effect on the fall in ATP concentrations over the time course of the experiments (5 h). Preincubation of discocytes with TPA for 10 min also prevented, by approx. 50%, the echinocytosis induced by the calcium (0.2 mM) loading of discocytes using 5 microM A23187. TPA was unable to reverse the echinocyte morphology of calcium-loaded cells back to discocytes. The less potent tumour promotor 4-phorbol-12,13-didecanoate had no effect on this discocyte-echinocyte transition. Incubation of discocytes with the diacylglycerol 1-oleoyl-2-acetylglycerol (OAG) (1-10 microM) had complex effects on morphology, and the ATP-induced morphological transition, ranging from stomatocyte formation to echinocyte formation, depending upon the concentration of the agent and the time of incubation.

Lymphoma Thy-1 glycoprotein is linked to the cytoskeleton via a 4.1-like protein

In this study we have found that the phosphoprotein doublet of 68,000 and 65,000 daltons (68/65 kD) in mouse T-lymphoma cells shares several structural and functional similarities with erythrocyte band 4.1. Our evidence for identifying the 68/65-kD doublet as a lymphoma 4.1-like protein is as follows: it displays an immunological cross-reactivity with anti-erythrocyte band 4.1 antibody; it exhibits a Svedberg unit of sedimentation coefficient of 4 S; it is phosphorylated in the presence of phorbol ester (phorbol-12-O-tetradecanoylphorbol-13-acetate) and its phosphorylation requires Ca2+; it is phosphorylated primarily at serine residues; and it can bind directly to fodrin (a spectrin-like actin-binding protein). In addition, this lymphoma 4.1-like protein can be both colocalized and coisolated with the major T-lymphocyte-specific glycoprotein, Thy-1 (gp 25). Therefore, all of these results strongly suggest that the lymphoma 4.1-like protein (68/65-kD doublet) may play a pivotal role in linking the Thy-1 (gp 25) glycoprotein to fodrin which, in turn, binds to the actin filaments that are responsible for recruiting Thy-1 antigens into cap structures.

Limited proteolysis of the erythrocyte membrane skeleton by calcium-dependent proteinases

The action of purified calcium-dependent proteinases on human erythrocyte membrane skeleton proteins has been examined. Preferential cleavage of proteins 4.1 a and b and band 3 and limited cleavage of alpha- and beta-spectrin occur when either calcium-dependent proteinase I or calcium-dependent proteinase II has access to the cytoplasmic side of the ghost membrane skeleton in the presence of calcium. Thus, when these proteinases are incubated with sealed ghosts they do not cleave these proteins. Leupeptin, mersalyl, the specific cellular protein inhibitor of these enzymes, and calcium chelators can inhibit proteolysis of the red cell ghost proteins by Ca2+-dependent proteinases. Each proteinase has also been loaded into erythrocyte ghosts in the absence of calcium at low ionic strength and subsequently trapped inside by resealing the ghosts. The proteinases were activated by incubating these ghosts in the presence of the calcium ionophore A23187 and calcium. Examination of the ghost proteins by electrophoresis demonstrated calcium-dependent proteolysis of Bands 4.1 and 3 and limited cleavage of alpha- and beta-spectrin similar to that observed on proteolysis of the open, leaky ghosts. In the presence of calcium each calcium-dependent proteinase appears to associate with the erythrocyte ghost membrane.

Protein kinase C of human erythrocytes phosphorylates bands 4.1 and 4.9

Addition of 10 nM 12-O-tetradecanoylphorbol 13-acetate (TPA) to intact human erythrocytes results in rapid phosphorylation of two cytoskeletal components, bands 4.1 and 4.9. The synthetic diacylglycerol, 1-oleoyl-2-acetylglycerol, shows a similar effect, while the biologically inactive phorbol ester, 4 alpha-phorbol didecanoate, fails to enhance phosphorylation. That TPA and 1-oleoyl-2-acetylglycerol stimulate this phosphorylation suggests that protein kinase C is being activated. In the presence of TPA, bands 4.1 and 4.9 incorporate 1.5 mol Pi/mol protein and 1.2 mol Pi/mol protein, respectively. The pattern and extent of phosphorylation shows that it is not due to cAMP-dependent protein kinases, which also phosphorylate bands 4.1 and 4.9. Ca2+-phospholipid-dependent protein kinase activity is demonstrable in the soluble fraction of erythrocytes, and has been partially purified (2200-fold) from the hemolysate by affinity chromatography (Uchida and Filburn, 1984. J. Biol. Chem. 259, 12311-12314). The affinity purified erythrocyte kinase has a 42 A Stokes' radius and phosphorylates purified bands 4.1 and 4.9 in vitro in a Ca2+- and phospholipid-dependent manner. These results show that human erythrocytes contain protein kinase C, and that band 4.1 and 4.9 are the major endogenous substrates for this kinase.

Phosphorylation reduces the affinity of protein 4.1 for spectrin

The phosphorylation of protein 4.1 by the membrane kinase and casein kinase A has been investigated. Each of these kinases catalyzed the incorporation of 2 mol of phosphate per mole of protein 4.1. The presence of both kinases in the reaction mixture did not lead to an increase in the incorporation of phosphates into the protein. An analysis of the acid hydrolysis products of the 32P-labeled protein 4.1 indicated that the radioactivities were distributed between phosphothreonine and phosphoserine in a ratio of about 2 to 1. The effects of phosphorylation on the binding of protein 4.1 to spectrin were investigated by using sucrose density gradient centrifugation. The affinity of protein 4.1 for spectrin was reduced about 5-fold, from a KD of 2 X 10(-6) M to a KD of 9.4 X 10(-6) M, by phosphorylation. The phosphorylation of spectrin, on the other hand, appeared to increase slightly its affinity for protein 4.1. The results suggest that phosphorylation may lead to a relaxation of the cytoskeletal network and the formation of a more flexible membrane structure that is important to red cell function.

Phorbol myristate acetate induces endocytosis as well as exocytosis and hydroosmosis in toad urinary bladder

The induction of the hydroosmotic response in the toad urinary bladder is considered to be associated with membrane addition mediated by exocytosis at the affected luminal membrane and reversed by endocytic retrieval at that surface. The permeability, exocytosis and endocytosis are initiated by antidiuretic hormone (ADH) receptor interaction on the basolateral membrane. In other hormone responsive systems, phorbol ester (phorbol myristate acetate, PMA), a tumor promoter, has been implicated in the regulation of various transport processes through the activation of protein kinase C and cytoskeletal protein phosphorylation. We found that addition of 10(-6) M PMA to the mucosa induces an hydroosmotic response which is gradual and which reaches a maximum within 60 min, equal to about 1/3 the maximal ADH response. Morphologically, PMA causes rapid exocytosis of the granules, endocytosis of horseradish peroxidase from the mucosal medium into tubules and multivesicular bodies and elongation of apical microvilli. Controls treated with mucosal 0.1% dimethylsulfoxide (DMSO) or an inactive PMA isomer on the mucosal surface, or PMA on the serosal surface lack the hydroosmotic, exocytic, endocytic and cytoskeletal changes. Addition of serosal ADH to PMA-treated bladders results in a precocious hydroosmotic and exocytic ADH response, but a lowering of the maximal response. Also pretreatment of bladders with PMA prevented the ADH-induced increase in transepithelial potential difference. Thus, apical events mediating the PMA hydroosmotic response are correlated with exo- and endocytosis and elongation of apical microvilli.

The effects of phorbol ester, diacylglycerol, phospholipase C and Ca2+ ionophore on protein phosphorylation in human and sheep erythrocytes

Treatment of human or sheep erythrocytes with PMA (phorbol myristate acetate) enhanced [32P]phosphate labelling of membrane polypeptides of approx. 100, 80 and 46 kDa. The 80 kDa and 46 kDa polypeptides coincided with bands 4.1 and 4.9 respectively on Coomassie-Blue-stained gels. Similar but smaller effects were obtained by treating human cells with 1-oleoyl-2-acetyl-rac-glycerol (OAG), exogenous bacterial phospholipase C or ionophore A23187 + Ca2+, each of which treatments would be expected to raise the concentration of membrane diacylglycerol. In contrast, sheep cells, which do not increase their content of diacylglycerol when treated with phospholipase C or A23187 + Ca2+, only showed enhanced phosphorylation with OAG. Neither human nor sheep cells showed any enhanced [32P]phosphate labelling of phosphoproteins when treated with 1-mono-oleoyl-rac-glycerol. It is concluded that diacylglycerol from a variety of sources can activate erythrocyte protein kinase C, but that the most effective diacylglycerol is that derived from endogenous polyphosphoinositides. In contrast with bacterial phospholipase C and A23187, which stimulate synthesis of phosphatidate by increasing the cell-membrane content of diacylglycerol in human erythrocytes, PMA, OAG or 1-mono-oleoyl-rac-glycerol caused no change in phospholipid metabolism.

A model for intracellular translocation of protein kinase C involving synergism between Ca2+ and phorbol esters

The activation of protein kinase C by diacylglycerol and by tumour promoters has implicated this enzyme in transmembrane signalling and in the regulation of the cell cycle. In vitro studies revealed that catalytic activity requires the presence of calcium and phospholipids with a preference for phosphatidylserine. Diacylglycerol and tumour promoters such as phorbol esters bind to the enzyme, leading to its activation while sharply increasing its affinity for Ca2+ and phospholipid. Addition of diacylglycerol analogues or phorbol esters to intact cells results in the phosphorylation of specific polypeptides. Several cellular processes, including hormone and neurotransmitter release and receptor down-regulation, are modulated by the activation of protein kinase C, while phorbol ester-induced stimulation of the enzyme in whole cells has been associated with its translocation from the cytoplasm to the plasma membrane. Moreover, the use of Ca2+ ionophores has revealed an apparent synergism between Ca2+ mobilization and protein kinase C activation. This synergism has recently also been found to apply to receptor down-regulation (ref. 23 and accompanying paper). Here we describe a reconstitution system in which intracellular translocation of protein kinase C and the synergism between Ca2+ and enzyme activators can be studied. The results suggest a rationale for concomitant Ca2+ mobilization and diacylglycerol formation in response to some hormones, neurotransmitters and growth factors.

Phosphorylation of the glucose transporter in vitro and in vivo by protein kinase C

The Ca2+- and phospholipid-dependent protein kinase (protein kinase C) is present in many mammalian tissues, and its important physiological protein substrates are only now beginning to be identified. A useful advance in identifying these intracellular substrates has been the recognition that the kinase is the receptor for phorbol esters, which stimulate phosphotransferase activity. Phorbol ester-induced changes in protein phosphorylation in intact cells may thus be taken, in part, as a probable indication of protein kinase C activation. The many cellular effects of phorbol esters include the stimulation of glucose uptake, although the response of glucose uptake to phorbol esters appears to be complex, apparently varying in response time and requirement for protein synthesis. Such observations prompted us to explore one possible explanation for the alteration of glucose uptake, namely, phosphorylation of the glucose transporter by protein kinase C. We report here that incubation of purified human erythrocyte glucose transporter with rat brain protein kinase C results in the phosphorylation of a protein of relative molecular mass (Mr) 50,000-60,000 which has subsequently been identified as the glucose transporter by specific immunoprecipitation with a monoclonal antibody. Immunoprecipitation of membrane proteins from 32P-labelled human erythrocytes revealed a phorbol ester-stimulated phosphorylation of the transporter. This covalent modification of the glucose transporter may thus, in part, underlie the ability of phorbol esters and certain hormones to stimulate glucose uptake.