Acquisition of dietary copper: a role for anion transporters in intestinal apical copper uptake

Effects of visceral inputs on the processing of labyrinthine signals by the inferior and caudal medial vestibular nuclei: ramifications for the production of motion sickness

Neurons located in the caudal aspect of the vestibular nucleus complex have been shown to receive visceral inputs and project to brainstem regions that participate in generating emesis, such as nucleus tractus solitarius and the "vomiting region" in the lateral tegmental field (LTF). Consequently, it has been hypothesized that neurons in the caudal vestibular nuclei participate in triggering motion sickness and that visceral inputs to the vestibular nucleus complex can affect motion sickness susceptibility. To obtain supporting evidence for this hypothesis, we determined the effects of intragastric infusion of copper sulfate (CuSO4) on responses of neurons in the inferior and caudal medial vestibular nuclei to rotations in vertical planes. CuSO4 readily elicits nausea and emesis by activating gastrointestinal (GI) afferents. Infusion of CuSO4 produced a >30 % change in spontaneous firing rate of approximately one-third of neurons in the caudal aspect of the vestibular nucleus complex. These changes in firing rate developed over several minutes, presumably in tandem with the emetic response. The gains of responses to vertical vestibular stimulation of a larger fraction (approximately two-thirds) of caudal vestibular nucleus neurons were altered over 30 % by administration of CuSO4. The response gains of some units went up, and others went down, and there was no significant relationship with concurrent spontaneous firing rate change. These findings support the notion that the effects of visceral inputs on motion sickness susceptibility are mediated in part through the caudal vestibular nuclei. However, our previous studies showed that infusion of CuSO4 produced larger changes in response to vestibular stimulation of LTF neurons, as well as parabrachial nucleus neurons that are believed to participate in generating nausea. Thus, integrative effects of GI inputs on the processing of labyrinthine inputs must occur at brain sites that participate in eliciting motion sickness in addition to the caudal vestibular nuclei. It seems likely that the occurrence of motion sickness requires converging inputs to brain areas that generate nausea and vomiting from a variety of regions that process vestibular signals.

Rate and regulation of copper transport by human copper transporter 1 (hCTR1)

Human copper transporter 1 (hCTR1) is a homotrimer of a 190-amino acid monomer having three transmembrane domains believed to form a pore for copper permeation through the plasma membrane. The hCTR1-mediated copper transport mechanism is not well understood, nor has any measurement been made of the rate at which copper ions are transported by hCTR1. In this study, we estimated the rate of copper transport by the hCTR1 trimer in cultured cells using (64)Cu uptake assays and quantification of plasma membrane hCTR1. For endogenous hCTR1, we estimated a turnover number of about 10 ions/trimer/s. When overexpressed in HEK293 cells, a second transmembrane domain mutant of hCTR1 (H139R) had a 3-fold higher Km value and a 4-fold higher turnover number than WT. Truncations of the intracellular C-terminal tail and an AAA substitution of the putative metal-binding HCH C-terminal tripeptide (thought to be required for transport) also exhibited elevated transport rates and Km values when compared with WT hCTR1. Unlike WT hCTR1, H139R and the C-terminal mutants did not undergo regulatory endocytosis in elevated copper. hCTR1 mutants combining methionine substitutions that block transport (M150L,M154L) on the extracellular side of the pore and the high transport H139R or AAA intracellular side mutations exhibited the blocked transport of M150L,M154L, confirming that Cu(+) first interacts with the methionines during permeation. Our results show that hCTR1 elements on the intracellular side of the hCTR1 pore, including the carboxyl tail, are not essential for permeation, but serve to regulate the rate of copper entry.

A re-evaluation of the role of hCTR1, the human high-affinity copper transporter, in platinum-drug entry into human cells

Cisplatin (cDDP) is an anticancer drug used in a number of malignancies, including testicular, ovarian, cervical, bladder, lung, head, and neck cancers. Its use is limited by the development of resistance, often rationalized via effects on cellular uptake. It has been claimed that human copper transporter 1 (hCTR1), the human high-affinity copper transporter, is the major entry pathway for cDDP and related drugs via a mechanism that mimics copper. This is an unexpected property of hCTR1, a highly selective copper (I) transporter. We compared the uptake rates of copper with cDDP (and several analogs) into human embryonic kidney 293 cells overexpressing wild-type or mutant hCTR1, mouse embryonic fibroblasts that do or do not express CTR1, and human ovarian tumor cells that are sensitive or resistant to cDDP. We have also compared the effects of extracellular copper, which causes regulatory endocytosis of hCTR1, to those of cDDP. We confirm the correlation between higher hCTR1 levels and higher platinum drug uptake in tumor cells sensitive to the drug. However, we show that hCTR1 is not the major entry route of platinum drugs, and that the copper transporter is not internalized in response to extracellular drug. Our data suggest the major entry pathway for platinum drugs is not saturable at relevant concentrations and not protein-mediated. Clinical trials have been initiated that depend upon regulating membrane levels of hCTR1. If reduced drug uptake is a major factor in resistance, hCTR1 is unlikely to be a productive target in attempts to enhance efficacy, although the proteins involved in copper homeostasis may play a role.

Nutritional deficiencies after bariatric surgery

Lifestyle intervention programmes often produce insufficient weight loss and poor weight loss maintenance. As a result, an increasing number of patients with obesity and related comorbidities undergo bariatric surgery, which includes approaches such as the adjustable gastric band or the 'divided' Roux-en-Y gastric bypass (RYGB). This Review summarizes the current knowledge on nutrient deficiencies that can develop after bariatric surgery and highlights follow-up and treatment options for bariatric surgery patients who develop a micronutrient deficiency. The major macronutrient deficiency after bariatric surgery is protein malnutrition. Deficiencies in micronutrients, which include trace elements, essential minerals, and water-soluble and fat-soluble vitamins, are common before bariatric surgery and often persist postoperatively, despite universal recommendations on multivitamin and mineral supplements. Other disorders, including small intestinal bacterial overgrowth, can promote micronutrient deficiencies, especially in patients with diabetes mellitus. Recognition of the clinical presentations of micronutrient deficiencies is important, both to enable early intervention and to minimize long-term adverse effects. A major clinical concern is the relationship between vitamin D deficiency and the development of metabolic bone diseases, such as osteoporosis or osteomalacia; metabolic bone diseases may explain the increased risk of hip fracture in patients after RYGB. Further studies are required to determine the optimal levels of nutrient supplementation and whether postoperative laboratory monitoring effectively detects nutrient deficiencies. In the absence of such data, clinicians should inquire about and treat symptoms that suggest nutrient deficiencies.

Substrate profile and metal-ion selectivity of human divalent metal-ion transporter-1

Divalent metal-ion transporter-1 (DMT1) is a H(+)-coupled metal-ion transporter that plays essential roles in iron homeostasis. DMT1 exhibits reactivity (based on evoked currents) with a broad range of metal ions; however, direct measurement of transport is lacking for many of its potential substrates. We performed a comprehensive substrate-profile analysis for human DMT1 expressed in RNA-injected Xenopus oocytes by using radiotracer assays and the continuous measurement of transport by fluorescence with the metal-sensitive PhenGreen SK fluorophore. We provide validation for the use of PhenGreen SK fluorescence quenching as a reporter of cellular metal-ion uptake. We determined metal-ion selectivity under fixed conditions using the voltage clamp. Radiotracer and continuous measurement of transport by fluorescence assays revealed that DMT1 mediates the transport of several metal ions that were ranked in selectivity by using the ratio I(max)/K(0.5) (determined from evoked currents at -70 mV): Cd(2+) > Fe(2+) > Co(2+), Mn(2+) ≫ Zn(2+), Ni(2+), VO(2+). DMT1 expression did not stimulate the transport of Cr(2+), Cr(3+), Cu(+), Cu(2+), Fe(3+), Ga(3+), Hg(2+), or VO(+). (55)Fe(2+) transport was competitively inhibited by Co(2+) and Mn(2+). Zn(2+) only weakly inhibited (55)Fe(2+) transport. Our data reveal that DMT1 selects Fe(2+) over its other physiological substrates and provides a basis for predicting the contribution of DMT1 to intestinal, nasal, and pulmonary absorption of metal ions and their cellular uptake in other tissues. Whereas DMT1 is a likely route of entry for the toxic heavy metal cadmium, and may serve the metabolism of cobalt, manganese, and vanadium, we predict that DMT1 should contribute little if at all to the absorption or uptake of zinc. The conclusion in previous reports that copper is a substrate of DMT1 is not supported.

Charting the travels of copper in eukaryotes from yeast to mammals

Throughout evolution, all organisms have harnessed the redox properties of copper (Cu) and iron (Fe) as a cofactor or structural determinant of proteins that perform critical functions in biology. At its most sobering stance to Earth's biome, Cu biochemistry allows photosynthetic organisms to harness solar energy and convert it into the organic energy that sustains the existence of all nonphotosynthetic life forms. The conversion of organic energy, in the form of nutrients that include carbohydrates, amino acids and fatty acids, is subsequently released during cellular respiration, itself a Cu-dependent process, and stored as ATP that is used to drive a myriad of critical biological processes such as enzyme-catalyzed biosynthetic processes, transport of cargo around cells and across membranes, and protein degradation. The life-supporting properties of Cu incur a significant challenge to cells that must not only exquisitely balance intracellular Cu concentrations, but also chaperone this redox-active metal from its point of cellular entry to its ultimate destination so as to avert the potential for inappropriate biochemical interactions or generation of damaging reactive oxidative species (ROS). In this review we chart the travels of Cu from the extracellular milieu of fungal and mammalian cells, its path within the cytosol as inferred by the proteins and ligands that escort and deliver Cu to intracellular organelles and protein targets, and its journey throughout the body of mammals. This article is part of a Special Issue entitled: Cell Biology of Metals.

Integrative responses of neurons in parabrachial nuclei to a nauseogenic gastrointestinal stimulus and vestibular stimulation in vertical planes

The parabrachial and adjacent Kölliker-Fuse (PBN/KF) nuclei play a key role in relaying visceral afferent inputs to the hypothalamus and limbic system and are, thus, believed to participate in generating nausea and affective responses elicited by gastrointestinal (GI) signals. In addition, the PBN/KF region receives inputs from the vestibular system and likely mediates the malaise associated with motion sickness. However, previous studies have not considered whether GI and vestibular inputs converge on the same PBN/KF neurons, and if so, whether the GI signals alter the responses of the cells to body motion. The present study, conducted in decerebrate cats, tested the hypothesis that intragastric injection of copper sulfate, which elicits emesis by irritating the stomach lining, modifies the sensitivity of PBN/KF neurons to vertical plane rotations that activate vestibular receptors. Intragastric copper sulfate produced a 70% median change in the gain of responses to vertical plane rotations of PBN/KF units, whose firing rate was modified by the administration of the compound; the response gains for 16 units increased and those for 17 units decreased. The effects were often dramatic: out of 51 neurons tested, 13 responded to the rotations only after copper sulfate was injected, whereas 10 others responded only before drug delivery. These data show that a subset of PBN/KF neurons, whose activity is altered by a nauseogenic stimulus also respond to body motion and that irritation of the stomach lining can either cause an amplification or reduction in the sensitivity of the units to vestibular inputs. The findings imply that nausea and affective responses to vestibular stimuli may be modified by the presence of emetic signals from the GI system.

Regulation of copper transporters in human cells

Copper is essential for normal growth and development of human organisms. The role of copper as a cofactor of important metabolic enzymes, such as cytochrome c oxidase, superoxide dismutase, lysyl oxidase, dopamine-β-hydroxylase, and many others, has been well established. In recent years, new regulatory roles of copper have emerged. Accumulating evidence points to the involvement of copper in lipid metabolism, antimicrobial defense, neuronal activity, resistance of tumor cells to platinum-based chemotherapeutic drugs, kinase-mediated signal transduction, and other essential cellular processes. For many of these processes, the precise mechanism of copper action remains to be established. Nevertheless, it is increasingly clear that many regulatory and signaling events are associated with changes in the intracellular localization and abundance of copper transporters, as well as distinct compartmentalization of copper itself. In this review, we discuss current data on regulation of the localization and abundance of copper transporters in response to metabolic and signaling events in human cells. Regulation by kinase-mediated phosphorylation will be addressed along with the emerging area of the redox-driven control of copper transport. We highlight mechanistic questions that await further testing.

Role of RhoA and its effectors ROCK and mDia1 in the modulation of deformation-induced FAK, ERK, p38, and MLC motogenic signals in human Caco-2 intestinal epithelial cells

Repetitive deformation enhances intestinal epithelial migration across tissue fibronectin. We evaluated the contribution of RhoA and its effectors Rho-associated kinase (ROK/ROCK) and mammalian diaphanous formins (mDia1) to deformation-induced intestinal epithelial motility across fibronectin and the responsible focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), p38, and myosin light chain (MLC) signaling. We reduced RhoA, ROCK1, ROCK2, and mDia1 by smart-pool double-stranded short-interfering RNAs (siRNA) and pharmacologically inhibited RhoA, ROCK, and FAK in human Caco-2 intestinal epithelial monolayers on fibronectin-coated membranes subjected to 10% repetitive deformation at 10 cycles/min. Migration was measured by wound closure. Stimulation of migration by deformation was prevented by exoenzyme C3, Y27632, or selective RhoA, ROCK1, and ROCK2 or mDia1 siRNAs. RhoA, ROCK inhibition, or RhoA, ROCK1, ROCK2, mDia1, and FAK reduction by siRNA blocked deformation-induced nuclear ERK phosphorylation without preventing ERK phosphorylation in the cytoplasmic protein fraction. Furthermore, RhoA, ROCK inhibition or RhoA, ROCK1, ROCK2, and mDia1 reduction by siRNA also blocked strain-induced FAK-Tyr(925), p38, and MLC phosphorylation. These results suggest that RhoA, ROCK, mDia1, FAK, ERK, p38, and MLC all mediate the stimulation of intestinal epithelial migration by repetitive deformation. This pathway may be an important target for interventions to promote mechanotransduced mucosal healing during inflammation.

Integrative responses of neurons in nucleus tractus solitarius to visceral afferent stimulation and vestibular stimulation in vertical planes

Anatomical studies have demonstrated that the vestibular nuclei project to nucleus tractus solitarius (NTS), but little is known about the effects of vestibular inputs on NTS neuronal activity. Furthermore, lesions of NTS abolish vomiting elicited by a variety of different triggering mechanisms, including vestibular stimulation, suggesting that emetic inputs may converge on the same NTS neurons. As such, an emetic stimulus that activates gastrointestinal (GI) receptors could alter the responses of NTS neurons to vestibular inputs. In the present study, we examined in decerebrate cats the responses of NTS neurons to rotations of the body in vertical planes before and after the intragastric administration of the emetic compound copper sulfate. The activity of more than one-third of NTS neurons was modulated by vertical vestibular stimulation, with most of the responsive cells having their firing rate altered by rotations in the head-up or head-down directions. These responses were aligned with head position in space, as opposed to the velocity of head movements. The activity of NTS neurons with baroreceptor, pulmonary, and GI inputs could be modulated by vertical plane rotations. However, injection of copper sulfate into the stomach did not alter the responses to vestibular stimulation of NTS neurons that received GI inputs, suggesting that the stimuli did not have additive effects. These findings show that the detection and processing of visceral inputs by NTS neurons can be altered in accordance with the direction of ongoing movements.