Impacts of endocrine disrupting chemicals on reproduction in wildlife and humans

Endocrine disrupting chemicals (EDCs) are ubiquitous in aquatic and terrestrial environments. The main objective of this review was to summarize the current knowledge of the impacts of EDCs on reproductive success in wildlife and humans. The examples selected often include a retrospective assessment of the knowledge of reproductive impacts over time to discern how the effects of EDCs have changed over the last several decades. Collectively, the evidence summarized here within reinforce the concept that reproduction in wildlife and humans is negatively impacted by anthropogenic chemicals, with several altering endocrine system function. These observations of chemicals interfering with different aspects of the reproductive endocrine axis are particularly pronounced for aquatic species and are often corroborated by laboratory-based experiments (i.e. fish, amphibians, birds). Noteworthy, many of these same indicators are also observed in epidemiological studies in mammalian wildlife and humans. Given the vast array of reproductive strategies used by animals, it is perhaps not surprising that no single disrupted target is predictive of reproductive effects. Nevertheless, there are some general features of the endocrine control of reproduction, and in particular, the critical role that steroid hormones play in these processes that confer a high degree of susceptibility to environmental chemicals. New research is needed on the implications of chemical exposures during development and the potential for long-term reproductive effects. Future emphasis on field-based observations that can form the basis of more deliberate, extensive, and long-term population level studies to monitor contaminant effects, including adverse effects on the endocrine system, are key to addressing these knowledge gaps.

Innovation in regulatory approaches for endocrine disrupting chemicals: The journey to risk assessment modernization in Canada

Globally, regulatory authorities grapple with the challenge of assessing the hazards and risks to human and ecosystem health that may result from exposure to chemicals that disrupt the normal functioning of endocrine systems. Rapidly increasing number of chemicals in commerce, coupled with the reliance on traditional, costly animal experiments for hazard characterization - often with limited sensitivity to many important mechanisms of endocrine disruption -, presents ongoing challenges for chemical regulation. The consequence is a limited number of chemicals for which there is sufficient data to assess if there is endocrine toxicity and hence few chemicals with thorough hazard characterization. To address this challenge, regulatory assessment of endocrine disrupting chemicals (EDCs) is benefiting from a revolution in toxicology that focuses on New Approach Methodologies (NAMs) to more rapidly identify, prioritize, and assess the potential risks from exposure to chemicals using novel, more efficient, and more mechanistically driven methodologies and tools. Incorporated into Integrated Approaches to Testing and Assessment (IATA) and guided by conceptual frameworks such as Adverse Outcome Pathways (AOPs), emerging approaches focus initially on molecular interactions between the test chemical and potentially vulnerable biological systems instead of the need for animal toxicity data. These new toxicity testing methods can be complemented with in silico and computational toxicology approaches, including those that predict chemical kinetics. Coupled with exposure data, these will inform risk-based decision-making approaches. Canada is part of a global network collaborating on building confidence in the use of NAMs for regulatory assessment of EDCs. Herein, we review the current approaches to EDC regulation globally (mainly from the perspective of human health), and provide a perspective on how the advances for regulatory testing and assessment can be applied and discuss the promises and challenges faced in adopting these novel approaches to minimize risks due to EDC exposure in Canada, and our world.

Effects of endocrine disrupting chemicals on gonad development: Mechanistic insights from fish and mammals

Over the past century, evidence has emerged that endocrine disrupting chemicals (EDCs) have an impact on reproductive health. An increased frequency of reproductive disorders has been observed worldwide in both wildlife and humans that is correlated with accidental exposures to EDCs and their increased production. Epidemiological and experimental studies have highlighted the consequences of early exposures and the existence of key windows of sensitivity during development. Such early in life exposures can have an immediate impact on gonadal and reproductive tract development, as well as on long-term reproductive health in both males and females. Traditionally, EDCs were thought to exert their effects by modifying the endocrine pathways controlling reproduction. Advances in knowledge of the mechanisms regulating sex determination, differentiation and gonadal development in fish and rodents have led to a better understanding of the molecular mechanisms underlying the effects of early exposure to EDCs on reproduction. In this manuscript, we review the key developmental stages sensitive to EDCs and the state of knowledge on the mechanisms by which model EDCs affect these processes, based on the roadmap of gonad development specific to fish and mammals.

Hepatic tumours and radiotherapy

We present the update of the recommendations of the French society of oncological radiotherapy on hepatic tumours. Recent technological progress led to develop the concept of focused liver radiation therapy. We must distinguish primary and secondary tumours, as the indications are restricted and must be discussed as an alternative to surgical or medical treatments. The tumour volume, its liver location close to the organs at risk determine the irradiation technique (repositioning method, total dose delivered, dose fractionation regimens). Tumour (and liver) breathing related motions should be taken into account. Strict dosimetric criteria must be observed with particular attention to the dose-volume histograms of non-tumoral liver as well as of the hollow organs, particularly in case of hypofractionated high dose radiotherapy "under stereotaxic conditions". Stereotactic body radiotherapy is being evaluated and is often preferred to radiofrequency for primary or secondary tumours (usually less than 5cm). An adaptation can be proposed, with a conformal fractionated irradiation protocol with or without intensity modulation, for hepatocellular carcinomas larger than 5cm.

[Rib fracture following intra-operative radiotherapy for breast cancer. Case Report and local experience]

Intra-operative radiotherapy for breast cancer has been developed throughout the last two decades. It is already well-established regarding local control and toxicity for intra-operative radiotherapy using electrons as we now have the necessary background knowledge. However, very few data on later toxicity are available for intra-operative radiotherapy using low-energy photons. We report here the case of a 36-year-old woman who experienced rib fracture following intra-operative and external radiotherapy. This patient has been included in the Targit-boost trial. The intra-operative irradiation has been operated with an INTRABEAM device delivering low-energy photons of 50-kV.

[Stereotactic hypofractionated radiation therapy as a bridge to transplantation for hepatocellular carcinoma: Case report of a complete pathological response and review of the literature]

Patients with hepatocellular carcinoma who are on liver transplant waiting list usually require local treatment to limit any risk of tumour growth. Historically percutaneous radiofrequency ablation or transarterial chemoembolization represented the major therapeutic alternatives. Depending on the size, or the topography of the lesion these two techniques may not be feasible. Radiation therapy under stereotactic conditions has recently emerged in the management of localized hepatocellular carcinoma as an alternative to the focused therapies performed to date. We herein report the case of a 43-year-old patient harbouring a complete histological response on explant after liver stereotactic irradiation and discuss its role in the management of hepatocellular carcinoma before liver transplantation.

Investigation of Bubble Instabilities in Film Blowing Process

This paper describes an original on-line video device developed in order to study bubble instabilities occurring in the film blowing process, taking into account their three-dimensional behavior. For a linear low-density polyethylene, two forms of instabilities and combination have been observed: draw resonance and helical instability. These instabilities could be quantitatively described and differences in behavior could be assessed using real objective measurements and criteria. The influence of key processing conditions was investigated and the results showed that the instabilities are enhanced by increasing the draw ratio, blow up ratio and frost line height. These first results are in agreement with the majority of the results reported in the literature, but allow for a more accurate analysis of the phenomena.

Amorphous silicon EPID calibration for dosimetric applications: comparison of a method based on Monte Carlo prediction of response with existing techniques

For EPID dosimetry, the calibration should ensure that all pixels have a similar response to a given irradiation. A calibration method (MC), using an analytical fit of a Monte Carlo simulated flood field EPID image to correct for the flood field image pixel intensity shape, was proposed. It was compared with the standard flood field calibration (FF), with the use of a water slab placed in the beam to flatten the flood field (WS) and with a multiple field calibration where the EPID was irradiated with a fixed 10x10 field for 16 different positions (MF). The EPID was used in its normal configuration (clinical setup) and with an additional 3 mm copper slab (modified setup). Beam asymmetry measured with a diode array was taken into account in MC and WS methods. For both setups, the MC method provided pixel sensitivity values within 3% of those obtained with the MF and WS methods (mean difference<1%, standard deviation<2%). The difference of pixel sensitivity between MC and FF methods was up to 12.2% (clinical setup) and 11.8% (modified setup). MC calibration provided images of open fields (5x5 to 20x20 cm2) and IMRT fields to within 3% of that obtained with WS and MF calibrations while differences with images calibrated with the FF method for fields larger than 10x10 cm2 were up to 8%. MC, WS and MF methods all provided a major improvement on the FF method. Advantages and drawbacks of each method were reviewed.

[New radiotherapy techniques for non-small-cell lung cancer]

Lung cancer is one of the most difficult challenges for radiotherapy. Problems include ballistic targeting compromised by respiratory movements, poor tolerance of neighboring healthy tissues and difficult dosimetry due to the heterogeneous nature of the thoracic tIssues. New perspectives are offered by recent developments allowing a more comprehensive approach to thoracic radiotherapy integrating new advances in imaging techniques, contention, dosimetry, and treatment devices. Two techniques are particularly promising: conformal radiotherapy and respiration-gated radiotherapy. Conformal radiotherapy, a three-dimensional conformal mode of irradiation with or without intensity modulation, is designed to achieve high-precision dose delivery by integrating advanced imaging techniques into the irradiation protocol. These tools are used to optimize irradiation of target Volumes and avoid recurrence while sparing as much as possible healthy tissues. If healthy tissue can be correctly protected, increased doses can be delivered to the target tumor. Respiration-gated techniques offer promising prospects for the treatment of tumors which are displaced by respiratory movements. These techniques allow better adaptation of the irradiation fields to the target tumor and better protection of healthy tissues (lung, heart...). These new approaches are now routine practices in many centers. Early results have been very promising. We describe here the currently available techniques for thoracic radiotherapy.

Contribution of cytosolic cysteine residues to the gating properties of the Kir2.1 inward rectifier

The topological model proposed for the Kir2.1 inward rectifier predicts that seven of the channel 13 cysteine residues are distributed along the N- and C-terminus regions, with some of the residues comprised within highly conserved domains involved in channel gating. To determine if cytosolic cysteine residues contribute to the gating properties of Kir2.1, each of the N- and C-terminus cysteines was mutated into either a polar (S, D, N), an aliphatic (A,V, L), or an aromatic (W) residue. Our patch-clamp measurements show that with the exception of C76 and C311, the mutation of individual cytosolic cysteine to serine (S) did not significantly affect the single-channel conductance nor the channel open probability. However, mutating C76 to a charged or polar residue resulted either in an absence of channel activity or a decrease in open probability. In turn, the mutations C311S (polar), C311R (charged), and to a lesser degree C311A (aliphatic) led to an increase of the channel mean closed time due to the appearance of long closed time intervals (T(c) >or= 500 ms) and to a reduction of the reactivation by ATP of rundown Kir2.1 channels. These changes could be correlated with a weakening of the interaction between Kir2.1 and PIP(2), with C311R and C311S being more potent at modulating the Kir2.1-PIP(2) interaction than C311A. The present work supports, therefore, molecular models whereby the gating properties of Kir2.1 depend on the presence of nonpolar or neutral residues at positions 76 and 311, with C311 modulating the interaction between Kir2.1 and PIP(2).

A specific tryptophan in the I-II linker is a key determinant of beta-subunit binding and modulation in Ca(V)2.3 calcium channels

The ancillary beta subunits modulate the activation and inactivation properties of high-voltage activated (HVA) Ca(2+) channels in an isoform-specific manner. The beta subunits bind to a high-affinity interaction site, alpha-interaction domain (AID), located in the I-II linker of HVA alpha1 subunits. Nine residues in the AID motif are absolutely conserved in all HVA channels (QQxExxLxGYxxWIxxxE), but their contribution to beta-subunit binding and modulation remains to be established in Ca(V)2.3. Mutations of W386 to either A, G, Q, R, E, F, or Y in Ca(V)2.3 disrupted [(35)S]beta3-subunit overlay binding to glutathione S-transferase fusion proteins containing the mutated I-II linker, whereas mutations (single or multiple) of nonconserved residues did not affect the protein-protein interaction with beta3. The tryptophan residue at position 386 appears to be an essential determinant as substitutions with hydrophobic (A and G), hydrophilic (Q, R, and E), or aromatic (F and Y) residues yielded the same results. beta-Subunit modulation of W386 (A, G, Q, R, E, F, and Y) and Y383 (A and S) mutants was investigated after heterologous expression in Xenopus oocytes. All mutant channels expressed large inward Ba(2+) currents with typical current-voltage properties. Nonetheless, the typical hallmarks of beta-subunit modulation, namely the increase in peak currents, the hyperpolarization of peak voltages, and the modulation of the kinetics and voltage dependence of inactivation, were eliminated in all W386 mutants, although they were preserved in part in Y383 (A and S) mutants. Altogether these results suggest that W386 is critical for beta-subunit binding and modulation of HVA Ca(2+) channels.

The effects of melatonin on Ca(2+) homeostasis in endothelial cells

The effect of melatonin on the Ca(2+) signaling process in bovine aortic endothelial cells (BAE) and in primary cultured vascular endothelial cells from normotensive Sprague Dawley (SDR) and genetically hypertensive (SHR) rats was investigated using the Ca(2+) indicator Fura-2. Acute applications of melatonin failed to initiate a Ca(2+) response in the three cell types considered. However, preincubating SHR aortic endothelial cells with exposure to melatonin increased the internal Ca(2+) release triggered by bradykinin (BK) and ATP while stimulating the related agonist-evoked Ca(2+) entry. This effect appeared specific for SHR cells, as a similar incubation period failed to alter the Ca(2+) responses in BAE and SDR cells. Because of the known overproduction of free radicals in SHR cells, the effect of melatonin on Ca(2+) signaling was also tested in SDR and BAE cells exposed to the superoxide anion radical. Melatonin reversed the deleterious action of free radicals on Ca(2+) signaling in both cases, suggesting that its stimulatory effect in SHR was linked to its antioxidative properties. Finally, experiments where melatonin was applied between successive BK stimulation periods showed an enhancement of the agonist-evoked Ca(2+) entry in BAE and SDR cells. This effect appeared to be independent of the production of second messengers as no specific binding sites for melatonin, including MT1, MT2 and MT3 receptors, could be detected in BAE cells. We conclude that melatonin improves Ca(2+) signaling in dysfunctional endothelial cells characterized by an overproduction of free radicals while stimulating the agonist-evoked Ca(2+) entry in normal endothelial cells through a mechanism not related to its antioxidative properties.

Role of aspartate residues in Ca(2+) affinity and permeation of the distal ECaC1

The Ca(2+) affinity and permeation of the epithelial Ca(2+) channel (ECaC1) were investigated after expression in Xenopus oocytes. ECaC1 displayed anomalous mole-fraction effects. Extracellular Ca(2+) and Mg(2+) reversibly inhibited ECaC1 whole cell Li(+) currents: IC(50) = 2.2 +/- 0.4 microM (n = 9) and 235 +/- 35 microM (n = 10), respectively. These values compare well with the Ca(2+) affinity of the L-type voltage-gated Ca(2+) (Ca(V)1.2) channel measured under the same conditions, suggesting that high-affinity Ca(2+) binding is a well-conserved feature of epithelial and voltage-gated Ca(2+) channels. Neutralization of D550 and E535 in the pore region had no significant effect on Ca(2+) and Mg(2+) affinities. In contrast, neutralization of D542 significantly decreased Ca(2+) affinity (IC(50) = 1.1 +/- 0.2 mM, n = 6) and Mg(2+) affinity (IC(50) > 25 +/- 3 mM, n = 4). Despite a 1,000-fold decrease in Ca(2+) affinity in D542N, Ca(2+) permeation properties and the Ca(2+)-to-Ba(2+) conductance ratio remained comparable to values for wild-type ECaC1. Together, our observations suggest that D542 plays a critical role in Ca(2+) affinity but not in Ca(2+) permeation in ECaC1.

State-dependent inhibition of inactivation-deficient Ca(V)1.2 and Ca(V)2.3 channels by mibefradil

The structural determinants of mibefradil inhibition were analyzed using wild-type and inactivation-modified Ca(V)1.2 (alpha1C) and Ca(V)2.3 (alpha1E) channels. Mibefradil inhibition of peak Ba2+ currents was dose- and voltage-dependent. An increase of holding potentials from -80 to -100 mV significantly shifted dose-response curves toward higher mibefradil concentrations, namely from a concentration of 108 +/- 21 microm (n = 7) to 288 +/- 17 microm (n = 3) for inhibition of half of the Ca(V)1.2 currents (IC(50)) and from IC(50) = 8 +/- 2 microm (n = 9) to 33 +/- 7 microm (n = 4) for Ca(V)2.3 currents. In the presence of mibefradil, Ca(V)1.2 and Ca(V)2.3 experienced significant use-dependent inhibition (0.1 to 1 Hz) and slower recovery from inactivation suggesting mibefradil could promote transition(s) to an absorbing inactivated state. In order to investigate the relationship between inactivation and drug sensitivity, mibefradil inhibition was studied in inactivation-altered Ca(V)1.2 and Ca(V)2.3 mutants. Mibefradil significantly delayed the onset of channel recovery from inactivation in CEEE (Repeat I + part of the I-II linker from Ca(V)1.2 in the Ca(V)2.3 host channel), in EC(AID)EEE (part of the I-II linker from Ca(V)1.2 in the Ca(V)2.3 host channel) as well as in Ca(V)1.2 E462R, and Ca(V)2.3 R378E (point mutation in the beta-subunit binding motif) channels. Mibefradil inhibited the faster inactivating chimera EC(IS1-6)EEE with an IC(50) = 7 +/- 1 microm (n = 3), whereas the slower inactivating chimeras EC(AID)EEE and CEEE were, respectively, inhibited with IC(50) = 41 +/- 5 microm (n = 4) and IC(50) = 68 +/- 9 microm (n = 5). Dose-response curves were superimposable for the faster EC(IS1-6)EEE and Ca(V)2.3, whereas intermediate-inactivating channel kinetics (CEEE, Ca(V)1.2 E462R, and Ca(V)1.2 E462K) were inhibited by similar concentrations of mibefradil with IC(50) approximately 55-75 microm. The slower Ca(V)1.2 wild-type and Ca(V)1.2 Q473K channels responded to higher doses of mibefradil with IC(50) approximately 100-120 microm. Mibefradil was also found to significantly speed up the inactivation kinetics of slower channels (Ca(V)1.2, CEEE) with little effect on the inactivation kinetics of faster-inactivating channels (Ca(V)2.3). A open-channel block model for mibefradil interaction with high-voltage-activated Ca2+ channels is discussed and shown to qualitatively account for our observations. Hence, our data agree reasonably well with a "receptor guarded mechanism" where fast inactivation kinetics efficiently trap mibefradil into the channel.

Role of Repeat I in the fast inactivation kinetics of the Ca(V)2.3 channel

The molecular basis for inactivation in Ca(V)2.3 (alpha 1E) channels was studied after expression of alpha 1E/alpha 1C (Ca(V)2.3/Ca(V)1.2) chimeras in Xenopus oocytes. In the presence of 10 mM Ba(2+), the CEEE chimera (Repeat I+part of the I-II linker from Ca(V)1.2) displayed inactivation properties similar to Ca(V)1.2 despite being more than 90% homologous to Ca(V)2.3. The transmembrane segments of Repeat I did not appear to be crucial as inactivation of EC(IS1-6)EEE was not significantly different than Ca(V)2.3. In contrast, EC(AID)EEE, with the beta-subunit binding domain from Ca(V)1.2, tended to behave like Ca(V)1.2 in terms of inactivation kinetics and voltage dependence. A detailed kinetic analysis revealed nonetheless that CEEE and EC(AID)EEE retained the fast inactivation time constant (tau(fast) approximately equal to 20-30 ms) that is a distinctive feature of Ca(V)2.3. Altogether, these data suggest that the region surrounding the AID binding site plays a pivotal albeit not exclusive role in determining the inactivation properties of Ca(V)2.3.

Molecular determinants of inactivation within the I-II linker of alpha1E (CaV2.3) calcium channels

Voltage-dependent inactivation of CaV2.3 channels was investigated using point mutations in the beta-subunit-binding site (AID) of the I-II linker. The quintuple mutant alpha1E N381K + R384L + A385D + D388T + K389Q (NRADK-KLDTQ) inactivated like the wild-type alpha1E. In contrast, mutations of alpha1E at position R378 (position 5 of AID) into negatively charged residues Glu (E) or Asp (D) significantly slowed inactivation kinetics and shifted the voltage dependence of inactivation to more positive voltages. When co-injected with beta3, R378E inactivated with tau(inact) = 538 +/- 54 ms (n = 14) as compared with 74 +/- 4 ms (n = 21) for alpha1E (p < 0.001) with a mid-potential of inactivation E(0.5) = -44 +/- 2 mV (n = 10) for R378E as compared with E(0.5) = -64 +/- 3 mV (n = 9) for alpha1E. A series of mutations at position R378 suggest that positively charged residues could promote voltage-dependent inactivation. R378K behaved like the wild-type alpha1E whereas R378Q displayed intermediate inactivation kinetics. The reverse mutation E462R in the L-type alpha1C (CaV1.2) produced channels with inactivation properties comparable to alpha1E R378E. Hence, position 5 of the AID motif in the I-II linker could play a significant role in the inactivation of Ca(V)1.2 and CaV2.3 channels.

pH and external Ca(2+) regulation of a small conductance Cl(-) channel in kidney distal tubule

A single channel characterization of the Cl(-) channels in distal nephron was undertaken using vesicles prepared from plasma membranes of isolated rabbit distal tubules. The presence in this vesicle preparation of ClC-K type Cl(-) channels was first established by immunodetection using an antibody raised against ClC-K isoforms. A ClC-K1 based functional characterization was next performed by investigating the pH and external Ca(2+) regulation of a small conductance Cl(-) channel which we identified previously by channel incorporation experiments. Acidification of the cis (external) solution from pH 7.4 to 6.5 led to a dose-dependent inhibition of the channel open probability P(O). Similarly, changing the trans pH from 7.4 to 6.8 resulted in a 4-fold decrease of the channel P(O) with no effect on the channel conductance. Channel activity also appeared to be regulated by cis (external) Ca(2+) concentration, with a dose-dependent increase in channel activity as a function of the cis Ca(2+) concentration. It is concluded on the basis of these results that the small conductance Cl(-) channel present in rabbit distal tubules is functionally equivalent to the ClC-K1 channel in the rat. In addition, the present work constitutes the first single channel evidence for a chloride channel regulated by external Ca(2+).

Inhibition of NF-kappa B activation in vitro and in vivo: role of 26S proteasome

It is becoming increasingly apparent that NF-kappa B plays a critical role in regulating the inflammatory response. Data obtained from studies in our laboratories demonstrate that the proteasome plays an important role in the inflammatory cascade by regulating the activation of NF-kappa B. Indeed, the availability of selective and orally active proteasome inhibitors should prove useful in delineating the roles of the proteasome and NF-kappa B in other pathophysiological conditions such as cancer and heart disease.

Role of the proteasome and NF-kappaB in streptococcal cell wall-induced polyarthritis

The transcription factor NF-kappaB activates a number of genes whose protein products are proinflammatory. In quiescent cells, NF-kappaB exists in a latent form and is activated via a signal-dependent proteolytic mechanism in which the inhibitory protein IkappaB is degraded by the ubiquitin-proteasome pathway. Consequently, inhibition of the proteasome suppresses activation of NF-kappaB. This suppression should therefore decrease transcription of many genes encoding proinflammatory proteins and should ultimately have an anti-inflammatory effect. To this end, a series of peptide boronic acid inhibitors of the proteasome, exemplified herein by PS-341, were developed. The proteasome is the large multimeric protease that catalyzes the final proteolytic step of the ubiquitin-proteasome pathway. PS-341, a potent, competitive inhibitor of the proteasome, readily entered cells and inhibited the activation of NF-kappaB and the subsequent transcription of genes that are regulated by NF-kappaB. Significantly, PS-341 displayed similar effects in vivo. Oral administration of PS-341 had anti-inflammatory effects in a model of Streptococcal cell wall-induced polyarthritis and liver inflammation in rats. The attenuation of inflammation in this model was associated with an inhibition of IkappaBalpha degradation and NF-kappaB-dependent gene expression. These experiments clearly demonstrate that the ubiquitin-proteasome pathway and NF-kappaB play important roles in regulating chronic inflammation and that, as predicted, proteasome inhibition has an anti-inflammatory effect.

Mutations in the EF-hand motif impair the inactivation of barium currents of the cardiac alpha1C channel

Calcium-dependent inactivation has been described as a negative feedback mechanism for regulating voltage-dependent calcium influx in cardiac cells. Most recent evidence points to the C-terminus of the alpha1C subunit, with its EF-hand binding motif, as being critical in this process. The EF-hand binding motif is mostly conserved between the C-termini of six of the seven alpha1 subunit Ca2+ channel genes. The role of E1537 in the C-terminus of the alpha1C calcium channel inactivation was investigated here after expression in Xenopus laevis oocytes. Whole-cell currents were measured in the presence of 10 mM Ba2+ or 10 mM Ca2+ after intracellular injection of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Against all expectations, our results showed a significant reduction in the rate of voltage-dependent inactivation as measured in Ba2+ solutions for all E1537 mutants, whereas calcium-dependent inactivation appeared unscathed. Replacing the negatively charged glutamate residue by neutral glutamine, glycine, serine, or alanine significantly reduced the rate of Ba2+-dependent inactivation by 1.5-fold (glutamine) to 3.5-fold (alanine). The overall rate of macroscopic inactivation measured in Ca2+ solutions was also reduced, although a careful examination of the distribution of the fast and slow time constants suggests that only the slow time constant was significantly reduced in the mutant channels. The fast time constant, the hallmark of Ca2+-dependent inactivation, remained remarkably constant among wild-type and mutant channels. Moreover, inactivation of E1537A channels, in both Ca2+ and Ba2+ solutions, appeared to decrease with membrane depolarization, whereas inactivation of wild-type channels became faster with positive voltages. All together, our results showed that E1537 mutations impaired voltage-dependent inactivation and suggest that the proximal part of the C-terminus may play a role in voltage-dependent inactivation in L-type alpha1C channels.

Chicken GABA(A) receptor beta4 subunits form robust homomeric GABA-gated channels in Xenopus oocytes

Chicken GABA(A) receptor beta4L and beta4S subunits were expressed in Xenopus oocytes by cRNA injection. Oocytes expressing either beta4 subunit alone or in combination with the chicken alpha1 subunit were studied using the two-electrode voltage-clamp technique. Both the beta4L and beta4S subunits form homomeric GABA-gated Cl- channels with similar efficiencies. In comparison, oocytes expressing either the chicken alpha1 or beta2S polypeptide show no or barely detectable GABA responses, as reported by others for most single-subunit vertebrate GABA(A) receptors. The GABA-gated currents due to the beta4L-subunit homomer were not affected by the presence of actinomycin D during cRNA expression, indicating that nascent oocyte polypeptides are not required for channel formation. The homomeric beta4L-subunit receptors show high affinity for GABA with an EC50 value of 4.3 +/- 0.4 microM and a Hill coefficient of 1.1 +/- 0.1 (n = 6). In response to GABA application at the EC25 value, currents elicited from the beta4L-subunit receptor are enhanced by 50 microM pentobarbital (110 +/- 10%, n = 3) and 10 microM loreclezole (60 +/- 3%, n = 3), inhibited by 10 microM picrotoxinin (93 +/- 3%, n = 3), but not affected by 1 microM diazepam. These properties are similar to those found for oocytes expressing heteromeric chicken alpha1beta4L and alpha1beta2S receptors. Since the beta subunits of GABA(A) receptors provide essential determinants for receptor assembly and subcellular localization, homomeric beta4-subunit receptors are a useful model system for further study of the structure and function of GABA(A) receptors.

Subunit regulation of the human brain alpha 1E calcium channel

The alpha1 subunit coding for the human brain type E calcium channel (Schneider et al., 1994) was expressed in Xenopus oocytes in the absence, and in combination with auxiliary alpha2delta and beta subunits. alpha1E channels directed with the expression of Ba2+ whole-cell currents that completely inactivated after a 2-sec membrane pulse. Coexpression of alpha1E with alpha2bdelta shifted the peak current by +10 mV but had no significant effect on whole-cell current inactivation. Coexpression of alpha1E with beta2a shifted the peak current relationship by -10 mV, and strongly reduced Ba2+ current inactivation. This slower rate of inactivation explains that a sizable fraction (40 +/- 10%, n = 8) of the Ba2+ current failed to inactivate completely after a 5-sec prepulse. Coinjection with both the cardiac/brain beta2a and the neuronal alpha2bdelta subunits increased by approximately 10-fold whole-cell Ba2+ currents although coinjection with either beta2a or alpha2bdelta alone failed to significantly increase alpha1E peak currents. Coexpression with beta2a and alpha2bdelta yielded Ba2+ currents with inactivation kinetics similar to the beta2a induced currents, indicating that the neuronal alpha2bdelta subunit has little effect on alpha1E inactivation kinetics. The subunit specificity of the changes in current properties were analyzed for all four beta subunit genes. The slower inactivation was unique to alpha1E/beta2a currents. Coexpression with beta1a, beta1b, beta3, and beta4, yielded faster-inactivating Ba2+ currents than currents recorded from the alpha1E subunit alone. Furthermore, alpha1E/alpha2bdelta/beta1a; alpha1E/alpha2bdelta/beta1b; alpha1E/alpha2bdelta/beta3; alpha1E/alpha2bdelta/beta4 channels elicited whole-cell currents with steady-state inactivation curves shifted in the hyperpolarized direction. The beta subunit-induced changes in the properties of alpha1E channel were comparable to modulation effects reported for alpha1C and alpha1A channels with beta3 approximately beta1b > beta1a approximately beta4 >> beta2a inducing fastest to slowest rate of whole-cell inactivation.

Site-specific phosphorylation of IkappaBalpha by a novel ubiquitination-dependent protein kinase activity

Signal-induced activation of the transcription factor NF-kappaB requires specific phosphorylation of the inhibitor IkappaBalpha and its subsequent proteolytic degradation. Phosphorylation of serine residues 32 and 36 targets IkappaBalpha to the ubiquitin (Ub)-proteasome pathway. Here we report the identification of a large, multisubunit kinase (molecular mass approximately 700 kDa) that phosphorylates IkappaBalpha at S32 and S36. Remarkably, the activity of this kinase requires the Ub-activating enzyme (E1), a specific Ub carrier protein (E2) of the Ubc4/Ubc5 family, and Ub. We also show that a ubiquitination event in the kinase complex is a prerequisite for specific phosphorylation of IkappaBalpha. Thus, ubiquitination serves a novel regulatory function that does not involve proteolysis.

Glutamate substitution in repeat IV alters divalent and monovalent cation permeation in the heart Ca2+ channel

In voltage-gated ion channels, residues responsible for ion selectivity were identified in the pore-lining SS1-SS2 segments. Negatively charged glutamate residues (E393, E736, E1145, and E1446) found in each of the four repeats of the alpha 1C subunit were identified as the major determinant of selectivity in Ca2+ channels. Neutralization of glutamate residues by glutamine in repeat I (E393Q), repeat III (E1145Q), and repeat IV (E1446Q) decreased the channel affinity for calcium ions 10-fold from the wild-type channel. In contrast, neutralization of glutamate residues in repeat II failed to significantly alter Ca2+ affinity. Likewise, mutation of neighboring residues in E1149K and D1450N did not affect the channel affinity, further supporting the unique role of glutamate residues E1145 in repeat III and E1446 in repeat IV in determining Ca2+ selectivity. Conservative mutations E1145D and E1446D preserved high-affinity Ca2+ binding, which suggests that the interaction between Ca2+ and the pore ligand sites is predominantly electrostatic and involves charge neutralization. Mutational analysis of E1446 showed additionally that polar residues could achieve higher Ca2+ affinity than small hydrophobic residues could. The role of high-affinity calcium binding sites in channel permeation was investigated at the single-channel level. Neutralization of glutamate residue in repeats I, II, and III did not affect single-channel properties measured with 115 mM BaCl2. However, mutation of the high-affinity binding site E1446 was found to significantly affect the single-channel conductance for Ba2+ and Li+, providing strong evidence that E1446 is located in the narrow region of the channel outer mouth. Side-chain substitutions at 1446 in repeat IV were used to probe the nature of divalent cation-ligand interaction and monovalent cation-ligand interaction in the calcium channel pore. Monovalent permeation was found to be inversely proportional to the volume of the side chain at position 1446, with small neutral residues such as alanine and glycine producing higher Li+ currents than the wild-type channel. This suggests that steric hindrance is a major determinant for monovalent cation conductance. Divalent permeation was more complex. Ba2+ single-channel conductance decreased when small neutral residues such as glycine were replaced by bulkier ones such as glutamine. However, negatively charged amino acids produced single-channel conductance higher than predicted from the size of their side chain. Hence, negatively charged residues at position 1446 in repeat IV are required for divalent cation permeation.

Voltage-dependent inactivation in a cardiac-skeletal chimeric calcium channel

The loci for inactivation in calcium channel proteins are unknown. Mechanisms for inactivation may be distributed across Ca2+ channel subunits and appear to be complex, multiple and interacting. We took advantage of the properties of chimeras, constructed between cardiac (H4) and skeletal muscle (Sk4) calcium channel alpha 1 subunits to study the molecular mechanism of inactivation in L-type calcium channels. Sk1H3, a chimeric construct of these two L-type calcium channels, was expressed in Xenopus oocytes in the absence of auxiliary subunits. Sk1H3 incorporated repeat I from skeletal muscle alpha 1 and repeats II, III, IV from heart alpha 1 subunit. Sk1H3 inactivated faster (tau = 300 ms) and more fully than the wild-type H4 with Ba2+ ions as the charge carrier. Thus, inactivation of Sk1H3 was 90% complete after a 5-s conditioning pulse at +20 mV while inactivation of H4 was only 37% complete. Sk1H3 inactivation also developed at more negative potentials with E0.5 = -15 mV as compared to E0.5 = -5 mV for H4. In the presence of external calcium ions, the extent of inactivation significantly increased from 37 to 83% for H4 while inactivation of Sk1H3 was only slightly increased. Inactivation with Ba2+ as the charge carrier was confirmed at the single- channel level where averaged single-channel ensembles showed a similar rate of inactivation. Collectively, these observations demonstrate that Sk1H3 inactivation appears to have a prominent voltage-dependent component. Whether Sk1H3 inactivation involves interactions within repeat I alone or interactions between repeat I and site(s) located in the three other repeats of the alpha 1 subunit has yet to be determined.

Electrogenic properties of the cloned Na+/glucose cotransporter: I. Voltage-clamp studies

The cloned rabbit intestinal Na+/glucose cotransporter was expressed in Xenopus laevis oocytes. Presteady-state and steady-state currents associated with cotransporter activity were measured with the two-electrode voltage-clamp technique. Steady-state sugar-dependent currents were measured between -150 and +90 mV as a function of external Na+ ([Na]o) and alpha-methyl-D-glucopyranoside concentrations ([alpha MDG]o). K alpha MDG0.5 was found to be dependent upon [Na]o and the membrane potential. At Vm = -50 mV, increasing [Na]o from 10 to 100 mM decreased K alpha MDG0.5 from 1.5 mM to 180 microM. Increasing membrane potential toward negative values decreased K alpha MDG0.5 at nonsaturating [Na]o. For instance, at 10 mM [Na]o, K alpha MDG0.5 decreased from 1.5 mM to 360 microM on increasing the membrane potential from -50 to -150 mV. The i alpha MDGmax was relatively insensitive to [Na]o between 10 and 100 mM and weakly voltage dependent (e-fold increase per 140 mV). KNa0.5 and iNamax were found to be dependent upon membrane potential and [sugar]o. In the presence of 1 mM [alpha MDG]o, KNa0.5 decreased from 50 to 5 mM between 0 and -150 mV and iNamax increased twofold between -30 and -200 mV. The voltage dependence of KNa0.5 is consistent with an effect of potential on Na+ binding (Na(+)-well effect), whereas the voltage dependence of iNamax is compatible with the translocation step being voltage dependent. It is concluded that voltage influences both Na+ binding and translocation. Presteady-state currents were observed for depolarization pulses in the presence of 100 mM [Na]o. The transient current relaxed with a half time of approximately 10 msec, and both the half time and magnitude of the transient varied with the holding potential and the size of depolarization pulse. Presteady-state currents were not observed after the addition of phlorizin or alpha MDG to the external Na+ solution and were not observed for water-injected control oocytes. We conclude that presteady-state currents are due to the activity of the carrier and that they may give a novel insight to the transport mechanism of the Na+/glucose cotransporter.

Electrogenic properties of the cloned Na+/glucose cotransporter: II. A transport model under nonrapid equilibrium conditions

The results of the accompanying electrophysiological study of the cloned Na+/glucose cotransporter from small intestine (Parent, L., Supplisson, S., Loo, D.D.F., Wright, E.M. (1992) J. Mémbrane Biol. 125:49-62) were evaluated in terms of a kinetic model. The steady-state and presteady-state cotransporter properties are described by a 6-state ordered kinetic model ("mirror" symmetry) with a Na+:alpha MDG stoichiometry of 2. Carrier translocation in the membrane as well as Na+ and sugar binding and dissociation are treated as a function of their individual rate constants. Empty carrier translocation and Na+ binding/dissociation are the only steps considered to be voltage dependent. Currents were associated with the translocation of the negatively charged carrier in the membrane. Negative membrane potential facilitates sugar transport. One numerical solution was found for the 14 rate constants that account quantitatively for our experiment observations: i.e., (i) sigmoidal shape of the sugar-specific current-voltage curves (absence of outward currents and inward current saturation at high negative potentials), (ii) Na+ and voltage dependence of Ksugar0.5 and isugarmax, (iii) sugar and voltage dependence of KNa0.5 and iNamax, (iv) presteady-state currents and their dependence on external Na+, alpha MDG and membrane potential, and (v) and carrier Na+ leak current. We conclude that the main voltage effect is on carrier translocation. Na+ ions that migrate from the extracellular medium to their binding sites sense 25 to 35% of the transmembrane voltage, whereas charges associated with the carrier translocation experiences 60 to 75% of the membrane electrical field. Internal Na+ ion binding is not voltage dependent. In our nonrapid equilibrium model, the rate-limiting step for sugar transport is a function of the membrane potential, [Na]o and [alpha MDG]o. At 0 mV and at saturating [Na]o and [alpha MDG]o, the rate-limiting step for sugar transport is the empty carrier translocation (5 sec-1). As the membrane potential is made more negative, the empty carrier translocation gets faster and the internal Na+ dissociation becomes increasingly rate limiting. However, as [Na]o is decreased to less than 10 mM, the rate-limiting step is the external Na+ ions binding in the 0 to -150 mV potential range. At 0 mV, the external Na+ dissociation constant KNa' is 80 mM and decreases to 24 mM at -150 mV. The external sugar dissociation constant KNaS' is estimated to be 200 microM and voltage independent. Finally, the internal leak pathway (CNa2 translocation) is insignificant.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypotonic shock activates a maxi K+ channel in primary cultured proximal tubule cells

The nature and function of the ionic channels at the apical membrane of primary cultured proximal tubule cells (PT) was investigated by use of the extracellular patch-clamp method. Several types of ionic channels were observed, including a calcium-dependent K+ channel of 206 pS in symmetrical 162 mM KCl activated at depolarizing potentials [maxi K+(Ca2+)]. Whole cell experiments were also carried out that clearly indicated that the PT cells respond to a hypotonic shock by activating electroconductive pathways. This response consisted of an initial hyperpolarization (from -47 to -58 mV, SD = 3, n = 4), followed by a strong depolarization (to -23 mV, SD = 4, n = 4). Furthermore, it was found in cell-attached experiments that the maxi K+(Ca2+) channel becomes activated during the hypotonic challenge. The activation process required external Ca2+, although some residual single-channel activity was measured in the absence of extracellular calcium (n = 3). On the basis of these results, it is concluded that the volume regulation process in PT cells in response to a hypotonic shock involves an influx of calcium from the external medium, which in turn triggers the opening of apical maxi K+(Ca2+) channels.