Imaging human cortical responses to intraneural microstimulation using magnetoencephalography

The sensation of touch in the glabrous skin of the human hand is conveyed by thousands of fast-conducting mechanoreceptive afferents, which can be categorised into four distinct types. The spiking properties of these afferents in the periphery in response to varied tactile stimuli are well-characterised, but relatively little is known about the spatiotemporal properties of the neural representations of these different receptor types in the human cortex. Here, we use the novel methodological combination of single-unit intraneural microstimulation (INMS) with magnetoencephalography (MEG) to localise cortical representations of individual touch afferents in humans, by measuring the extracranial magnetic fields from neural currents. We found that by assessing the modulation of the beta (13-30 Hz) rhythm during single-unit INMS, significant changes in oscillatory amplitude occur in the contralateral primary somatosensory cortex within and across a group of fast adapting type I mechanoreceptive afferents, which corresponded well to the induced response from matched vibrotactile stimulation. Combining the spatiotemporal specificity of MEG with the selective single-unit stimulation of INMS enables the interrogation of the central representations of different aspects of tactile afferent signalling within the human cortices. The fundamental finding that single-unit INMS ERD responses are robust and consistent with natural somatosensory stimuli will permit us to more dynamically probe the central nervous system responses in humans, to address questions about the processing of touch from the different classes of mechanoreceptive afferents and the effects of varying the stimulus frequency and patterning.

Differential effects of radiant and mechanically applied thermal stimuli on human C-tactile afferent firing patterns

C-tactile (CT) afferents respond to gentle tactile stimulation, but only a handful of studies in humans and animals have investigated whether their firing is modified by temperature. We describe the effects of radiant thermal stimuli, and of stationary and very slowly moving mechanothermal stimuli, on CT afferent responses. We find that CT afferents are primarily mechanoreceptors, as they fired little during radiant thermal stimuli, but they exhibited different patterns of firing during combined mechano-cool stimulation compared with warming. CTs fired optimally to gentle, very slowly moving, or stationary mechanothermal stimuli delivered at neutral temperature (~32°C, normal skin temperature), but they responded with fewer spikes (median 67% decrease) and at significantly lower rates (47% decrease) during warm (~42°C) tactile stimuli. During cool tactile stimuli (~18°C), their mean instantaneous firing frequency significantly decreased by 35%, but they often fired a barrage of afterdischarge spikes at a low frequency (~5 Hz) that outlasted the mechanical stimulus. These effects were observed under a variety of stimulus conditions, including during stationary and slowly moving touch (0.1 cm/s), and we complemented these tactile approaches using a combined electrical-thermal stimulation experiment where we found a suppression of spiking during warming. Overall, CT afferents are exquisitely sensitive to tactile events, and we show that their firing is modulated with touch temperatures above and below neutral skin temperature. Warm touch consistently decreased their propensity to fire, whereas cool touch produced lower firing rates but afterdischarge spiking. NEW & NOTEWORTHY C-tactile (CT) afferents are thought to underpin pleasant touch, and previous work has shown that they respond optimally to a slow caress delivered at typical (neutral) skin temperature. Here, we show that, although CTs are primarily mechanoreceptive afferents, they are modified by temperature: warm touch decreases their firing, whereas cool touch produces lower firing rates but long-lasting spiking, frequently seen as afterdischarges. This has implications for the encoding of affective sensory events in human skin.

Optimal delineation of single C-tactile and C-nociceptive afferents in humans by latency slowing

Human skin encodes a plethora of touch interactions, and affective tactile information is primarily signaled by slowly conducting C-mechanoreceptive afferents. We show that electrical stimulation of low-threshold C-tactile afferents produces markedly different patterns of activity compared with high-threshold C-mechanoreceptive nociceptors, although the populations overlap in their responses to mechanical stimulation. This fundamental distinction demonstrates a divergence in affective touch signaling from the first stage of sensory processing, having implications for the processing of interpersonal touch.

C-mechanoreceptors in humans comprise a population of unmyelinated afferents exhibiting a wide range of mechanical sensitivities. C-mechanoreceptors are putatively divided into those signaling gentle touch (C-tactile afferents, CTs) and nociception (C-mechanosensitive nociceptors, CMs), giving rise to positive and negative affect, respectively. We sought to distinguish, compare, and contrast the properties of a population of human C-mechanoreceptors to see how fundamental the divisions between these putative subpopulations are. We used microneurography to record from individual afferents in humans and applied electrical and mechanical stimulation to their receptive fields. We show that C-mechanoreceptors can be distinguished unequivocally into two putative populations, comprising CTs and CMs, by electrically evoked spike latency changes (slowing). After both natural mechanical stimulation and repetitive electrical stimulation there was markedly less latency slowing in CTs compared with CMs. Electrical receptive field stimulation, which bypasses the receptor end organ, was most effective in classifying C-mechanoreceptors, as responses to mechanical receptive field stimulation overlapped somewhat, which may lead to misclassification. Furthermore, we report a subclass of low-threshold CM responding to gentle mechanical stimulation and a potential subclass of CT afferent displaying burst firing. We show that substantial differences exist in the mechanisms governing axonal conduction between CTs and CMs. We provide clear electrophysiological “signatures” (extent of latency slowing) that can be used in unequivocally identifying populations of C-mechanoreceptors in single-unit and multiunit microneurography studies and in translational animal research into affective touch. Additionally, these differential mechanisms may be pharmacologically targetable for separate modulation of positive and negative affective touch information.

NEW & NOTEWORTHY Human skin encodes a plethora of touch interactions, and affective tactile information is primarily signaled by slowly conducting C-mechanoreceptive afferents. We show that electrical stimulation of low-threshold C-tactile afferents produces markedly different patterns of activity compared with high-threshold C-mechanoreceptive nociceptors, although the populations overlap in their responses to mechanical stimulation. This fundamental distinction demonstrates a divergence in affective touch signaling from the first stage of sensory processing, having implications for the processing of interpersonal touch.

Oxidative protein folding in the mammalian endoplasmic reticulum

Native disulphide bonds are essential for the structure and function of many membrane and secretory proteins. Disulphide bonds are formed, reduced and isomerized in the endoplasmic reticulum of mammalian cells by a family of oxidoreductases, which includes protein disulphide isomerase (PDI), ERp57, ERp72, P5 and PDIR. This review will discuss how these enzymes are maintained in either an oxidized redox state that allows them to form disulphide bonds in substrate proteins or a reduced form that allows them to perform isomerization and reduction reactions, how these opposing pathways may co-exist within the same compartment and why so many oxidoreductases exist when PDI alone can perform all three of these functions.

Disrupted pulmonary vasculature and decreased vascular endothelial growth factor, Flt-1, and TIE-2 in human infants dying with bronchopulmonary dysplasia

An abnormal pulmonary vasculature may be an important component of bronchopulmonary dysplasia (BPD). We examined human infant lung for the endothelial cell marker PECAM-1 and for angiogenic factors and their receptors. Lung specimens were collected prospectively at approximately 6 h after death. The right middle lobe was inflation fixed and part of the right lower lobe was flash frozen. We compared lungs from infants dying with BPD (n = 5) with lungs from infants dying from nonpulmonary causes (n = 5). The BPD group was significantly more premature and had more days of ventilator and supplemental oxygen support, but died at a postconceptional age similar to control infants. PECAM-1 protein and mRNA were decreased in the BPD group. PECAM-1 immunohistochemistry showed the BPD group had decreased staining intensity and abnormal distribution of alveolar capillaries. The dysmorphic capillaries were frequently in the interior of thickened alveolar septa. The BPD group had decreased vascular endothelial growth factor (VEGF) mRNA and decreased VEGF immunostaining, compared with infants without BPD. Messages for the angiogenic receptors Flt-1 and TIE-2 were decreased in the BPD group. We conclude that infants dying with BPD have abnormal alveolar microvessels and that disordered expression of angiogenic growth factors and their receptors may contribute to these abnormalities.

The cyclin-dependent kinase inhibitor p21 protects the lung from oxidative stress

The lung is a major target tissue for oxidative stress, including hyperoxia used to relieve tissue hypoxia. Unfortunately, severe hyperoxia damages DNA, inhibits proliferation, and kills cells, resulting in morbidity and mortality. Although hyperoxia induces the tumor suppressor p53 and its downstream target, the cyclin-dependent kinase inhibitor p21(Cip1/WAF1/Sdi1) (p21), their role in pulmonary injury remains unknown. Using p53- and p21-deficient mice we demonstrate that hyperoxia induces p21 in the absence of p53, suggesting that previous conclusions that p53 does not modify hyperoxic lung injury cannot be extrapolated to p21. In fact, mean survival of p21-deficient mice decreased by 40% and was associated with terminal deoxyribonucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick-end labeling staining of alveolar debris, indicative of DNA fragmentation and cell death. Ultrastructural analyses revealed that alveolar endothelial and type I epithelial cells died rapidly by necrosis. Although hyperoxia decreased DNA replication in p21-wild-type lungs, it had no effect on replication in p21-deficient lungs. Our findings suggest that p21 protects the lung from oxidative stress, in part, by inhibiting DNA replication and thereby allowing additional time to repair damaged DNA. Our findings have implications for patients suffering from the toxic effects of supplemental oxygen therapies.

Maturation of carbonic anhydrase IV expression in rabbit kidney

Carbonic anhydrase (CA) IV facilitates renal acidification by catalyzing the dehydration of luminal H(2)CO(3). CA IV is expressed in proximal tubules, medullary collecting ducts, and A-intercalated cells of the mature rabbit kidney (Schwartz GJ, Kittelberger AM, Barnhart DA, and Vijayakumar S. Am J Physiol 278: F894-F904, 2000). In view of the maturation of HCO transport in the proximal tubule and collecting duct, the ontogeny of CA IV expression was examined. During the first 2 wk, CA IV mRNA was expressed in maturing cortex and medulla at ~20% of adult levels. The maturational increase was gradual in cortex over 3-5 wk of age but surged in the medulla, so that mRNA levels appeared higher than those in the adult medulla. In situ hybridization showed very little CA IV mRNA at 5 days, with increases in deep cortex and medullary collecting ducts by 21 days. Expression of CA IV protein in the cortex and medulla was minimal at 3 days of age but then apparent in the juxtamedullary region, A-intercalated cells and medullary collecting ducts by 18 days; there was little labeling of the proximal straight tubules of the medullary rays. Thus CA IV expression may be regulated to accommodate the maturational increase in HCO absorption in the proximal tubule. In the medullary collecting duct, there is a more robust maturation of CA IV mRNA and protein, commensurate with the high rate of HCO absorption in the neonatal segment.

Bcl-2 family gene expression during severe hyperoxia induced lung injury

Exposure of the lung to severe hyperoxia induces terminal transferase dUTP end-labeling (TUNEL) indicative of DNA damage or apoptosis and increases expression of the tumor suppressor p53 and of members of the Bcl-2 gene family. Because cell survival and apoptosis are regulated, in part, by the relative abundance of proteins of the Bcl-2 family, we hypothesized that lung cells dying during exposure would show increased expression of pro-apoptotic members, such as Bax, whereas surviving cells would have increased expression of anti-apoptotic members, such as Bcl-X(L). The hypothesis is tested in the current study by determining which Bcl-2 genes are regulated by hyperoxia, with specific focus on correlating expression of Bax and Bcl-X(L) with morphologic evidence of apoptosis or necrosis. Adult mice exposed to greater than 95% oxygen concentrations for 48 to 88 hours had increased whole-lung mRNA levels of Bax and Bcl-X(L), no change in Bak, Bad, or Bcl-2, and decreased levels of Bcl-w and Bfl-1. In situ hybridization revealed that hyperoxia induced Bax and Bcl-X(L) mRNA in uniform and overlapping patterns of expression throughout terminal bronchioles and parenchyma, coinciding with TUNEL staining. Electron microscopy and DNA electrophoresis, however, suggested relatively little classical apoptosis. Unexpectedly, Western analysis demonstrated increased Bcl-X(L), but not Bax, protein in response to hyperoxia. Bax and Bfl-1 were not altered by hyperoxia in p53 null mice; however, oxygen toxicity was not lessened by p53 deficiency. These findings suggest that oxygen-induced lung injury does not depend on the relative expression of these Bcl-2 members.

Expression of vascular endothelial growth factor and Flk-1 in developing and glucocorticoid-treated mouse lung

Although the endothelial cell is the most abundant cell type in the differentiated lung, little is known about regulation of lung developmental vasculogenesis. Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen and angiogenic factor that has putative roles in vascular development. Mitogenic actions of VEGF are mediated by the tyrosine kinase receptor KDR/murine homologue fetal liver kinase Flk-1. HLF (hypoxia-inducible factor-like factor) is a transcription factor that increases VEGF gene transcription. Dexamethasone augments lung maturation in fetal and postnatal animals. However, in vitro studies suggest that dexamethasone blocks induction of VEGF. The objectives for the current study were to measure VEGF mRNA and Flk-1 mRNA in developing mouse lung and to measure the effects of dexamethasone treatment in vivo on VEGF and Flk-1 in newborn mouse lung. Our results show that VEGF and Flk-1 messages increase in parallel during normal lung development (d 13 embryonic to adult) and that the distal epithelium expresses VEGF mRNA at all ages examined. Dexamethasone (0.1-5.0 mg x kg(-1) x d(-1)) treatment of 6-d-old mice resulted in significantly increased VEGF, HLF, and Flk-1 mRNA. Dexamethasone did not affect cell-specific expression of VEGF, VEGF protein, or proportions of VEGF mRNA splice variants. These data suggest that the developing alveolar epithelium has an important role in regulating alveolar capillary development. In addition, unlike effects on cultured cells, dexamethasone, even in relatively high doses, did not adversely affect VEGF expression in vivo. The relatively high levels of VEGF and Flk-1 mRNA in adult lung imply a role for pulmonary VEGF in endothelial cell maintenance or capillary permeability.

p53-independent induction of GADD45 and GADD153 in mouse lungs exposed to hyperoxia

Previous studies have shown that lungs of adult mice exposed to >95% oxygen have increased terminal deoxyribonucleotidyltransferase dUTP nick end-label staining and accumulate p53, the expression of which increases in cells exposed to DNA-damaging agents. The present study was designed to determine whether hyperoxia also increased expression of the growth arrest and DNA damage (GADD) gene 45 and GADD153, which are induced by genotoxic stress through p53-dependent and -independent pathways. GADD proteins have been shown to inhibit proliferation and stimulate DNA repair and/or apoptosis. GADD45 and GADD153 mRNAs were not detected in lungs exposed to room air but were detected after 48 and 72 h of exposure to hyperoxia. In situ hybridization and immunohistochemistry revealed that hyperoxia increased GADD45 and GADD153 expression in the bronchiolar epithelium and GADD45 expression predominantly in alveolar cells that were morphologically consistent with type II cells. Hyperoxia also increased GADD expression in p53-deficient mice. Terminal deoxyribonucleotidyltransferase dUTP nick end-label staining of lung cells from p53 wild-type and p53-null mice exposed to hyperoxia for 48 h revealed that hyperoxia-induced DNA fragmentation was not modified by p53 deficiency. These studies are consistent with the hypothesis that hyperoxia-induced DNA fragmentation is associated with the expression of GADD genes that may participate in DNA repair and/or apoptosis.

Carotenoid composition of marigold (Tagetes erecta) flower extract used as nutritional supplement

Commercially prepared marigold flower (Tagetes erecta) extract was saponified and analyzed for carotenoid composition. HPLC analyses were performed on two normal-phase columns (beta-Cyclobond and silica) and on a C(30) reversed-phase column. The extract contained 93% utilizable pigments (detected at 450 nm), consisting of all-trans and cis isomers of zeaxanthin (5%), all-trans and cis isomers of lutein, and lutein esters (88%). All were identified by chromatographic retention, UV-visible spectra, and positive ion electrospray mass spectrometry in comparison to authentic standards. Contrary to previous findings, insignificant levels (<0.3%) of lutein oxidation products were detected in the saponified extract. This compositional determination is important for the application of marigold extract in nutritional supplements and increases its value as a poultry feed colorant because it contains more biologically useful lutein compounds than previously believed.

Differential expression of VEGF mRNA splice variants in newborn and adult hyperoxic lung injury

Lung development and repair of hyperoxic injury require closely regulated growth and regeneration of alveolar capillaries. Vascular endothelial growth factor (VEGF), a mitogen for endothelial cells, is expressed by alveolar epithelial cells. Alternative splicing of VEGF mRNA results in isoforms of varying mitogenicity and solubility. We examined changes in the proportions of the VEGF splice variant mRNAs in rabbit lung development and in control, oxygen-injured, and recovering newborn and adult rabbit lungs. The proportion of the 189-amino acid VEGF mRNA, which codes for an isoform that binds to the extracellular matrix, increased fivefold during development (from 8% of total VEGF message at 22 days gestation to 40% in 10-day newborn lungs; P < 0.001). During neonatal oxygen injury, its expression declined from 38 to 8% of VEGF message (P < 0.002) and returned to the control value in recovery. A similar pattern was observed in adults. VEGF protein in lung lavage fluid increased slightly during hyperoxia, declined to barely detectable levels at the 50% lethal dose time point, and increased 10-fold (newborn) or up to 40-fold (adult) in recovering animals. We conclude that alternative splicing may have important roles in the regulation of VEGF activity in developing and injured lungs.

Rat primary hepatocytes and H4 hepatoma cells display differential sensitivity to cyclic AMP at the level of expression of tyrosine aminotransferase

We have shown that the sensitivity of isolated hepatocytes and H4 hepatoma cells to cyclic AMP is different. In terms of activation of tyrosine aminotransferase at mRNA and activity level in response to cyclic AMP, isolated hepatocytes are 10-fold more sensitive. Hepatocytes and H4 hepatoma cells show similar sensitivities to cyclic AMP at the level of protein kinase A activation and phosphorylation of cyclic AMP response element binding protein (CREB) and the differential sensitivity must reside at other sites. The consequences of these findings to studies of control phenomena at the transcriptional level is discussed.

Accumulation of p21(Cip1/WAF1) during hyperoxic lung injury in mice

Hyperoxic lung injury results in decreased cell proliferation, DNA damage, and cell death. Because the cyclin-dependent kinase inhibitor p21(Cip1/WAF1) (p21) inhibits cell proliferation in G1/S, enhances DNA repair, and regulates apoptosis in some cells, we hypothesized that the expression of p21 would increase in lungs of C57Bl/6J male mice exposed to and recovered from > 95% oxygen. A low level of p21 messenger RNA (mRNA) expression was detected by Northern blot analysis of room air-exposed lungs. Exposure to hyperoxia resulted in a modest increase in p21 mRNA expression by 24 h, followed by a marked induction by 48 to 72 h. In situ hybridization revealed that p21 mRNA abundance increased in bronchiolar epithelium and in resident alveolar cells, but not in smooth-muscle cells or large airway epithelium. Hyperoxia increased the expression of p21 protein by 24 h and continued to increase at 48 and 72 h. Immunohistochemical staining showed that p21 protein accumulated in the bronchiolar epithelium and in alveolar regions that had increased p21 mRNA expression. In contrast, the expression of the cyclin-dependent kinase inhibitor p27(Kip1) was not altered by hyperoxia. To determine whether p21 expression was altered during the repair process, mice were exposed to hyperoxia for 64 h and allowed to recover for up to 4 d in room air. The abundance of p21 mRNA and protein decreased by 1 to 2 d of recovery and returned to room air-exposed levels by 3 to 4 d of recovery. These findings support the concept that bronchiolar epithelial and alveolar cells damaged by hyperoxia express molecules such as p21, which may participate in regulating cell proliferation, DNA repair, and cell death.

Interleukin-1 expression during hyperoxic lung injury in the mouse

An important component of the pathophysiologic response to hyperoxia (O2) is pulmonary inflammation, although the roles of specific inflammatory mediators during pulmonary O2 toxicity are not completely known. Interleukin-1 (IL-1) is an early inflammatory mediator and is sufficient to elicit many of the responses associated with acute injury. The IL-1 family comprises two bioactive proteins, IL-1alpha and IL-1beta, and their natural antagonist IL-1ra. Here we report studies of IL-1 regulation during hyperoxic lung injury in the adult mouse. When assayed by Northern blot, increases in IL-1beta mRNA were seen after 2 days of hyperoxia. In contrast, IL-1alpha mRNA was barely detectable before 4 days of hyperoxia. To further understand the cellular origin of IL-1beta expression in lungs, in situ hybridization and immunohistochemical analyses were performed. IL-1beta mRNA or protein was not detected in the lungs of unexposed animals. At 3 days, we observed the accumulation of IL-1beta transcripts in pulmonary interstitial macrophages and in a subset of neutrophils, and immunodetectable IL-1beta protein was co-localized in adjacent sections. At 4 days of exposure, IL-1beta transcripts were widespread in lung tissue, but many areas rich in IL-1beta mRNA were devoid of immunodetectable IL-1beta. However, it is not known whether increased synthesis of IL-1beta or the uncoupling of IL-1beta protein and mRNA accumulation has a role in pathophysiology of pulmonary O2 toxicity.

Cell-specific expression of fibronectin and EIIIA and EIIIB splice variants after oxygen injury

Cellular fibronectin (cFN) expression is characteristic of injured tissues. Unlike plasma FN, cFN mRNA often contains the EIIIA or EIIIB domains. We examined the lung cell-specific expression of total cFN mRNA and the EIIIA and EIIIB splice variants in rabbits after acute oxygen injury. By in situ hybridization, control lung had low cFN mRNA. After exposure to > 95% oxygen, mRNAs for total cFN and EIIIA were noted primarily in alveolar macrophages and large-vessel endothelial cells. By 3-5 days recovery, cFN and EIIIA mRNA abundance was increased in alveolar septal cells (i.e., alveolar epithelial, interstitial, or endothelial cells) and in some large-vessel endothelial cells but was low in bronchial epithelial cells. During recovery, EIIIB mRNA was low in alveolar septal cells but was noted mainly in chondrocytes. Immunostaining for EIIIA increased during recovery, paralleling the in situ hybridizations. Because FN may modulate alveolar type II cell phenotype, we investigated type II cell cFN mRNA expression in vivo. During recovery, neither isolated type II cells nor cells with surfactant protein C mRNA in vivo contained FN mRNA. In summary, these data suggest that cFN with the EIIIA domain has a role in alveolar cell recovery from oxygen injury and that type II cells do not express cFN during recovery.

Bixin and norbixin in human plasma: determination and study of the absorption of a single dose of Annatto food color

A procedure was developed for the detection and determination of bixin and norbixin in human plasma by reversed-phase HPLC with a sensitivity limit of 5 micrograms l-1. A group of seven volunteers ingested a single dose of 1 ml of a commercial Annatto Food Color (16 mg of cis-bixin in soybean oil). The presence of bixin (cis and trans) and norbixin (cis and trans) was demonstrated in the plasma at average levels of 11.6, 10.1, 2.8 and 0 micrograms l-1 of bixin and 48, 58, 53 and 29 micrograms l-1 of norbixin after 2, 4, 6 and 8 h, respectively. Considerable individual variations were observed. Complete plasma clearance generally occurred for bixin by 8 h and for norbixin by 24 h after ingestion of cis-bixin.

Hyperoxic injury decreases alveolar epithelial cell expression of vascular endothelial growth factor (VEGF) in neonatal rabbit lung

Normal neonatal lung growth requires a substantial increase in microvascular endothelial cells. Oxygen injury to neonatal lung destroys endothelial cells and alters the normal process of alveolarization, including development of the microvasculature. The mechanisms that regulate lung alveolar capillary growth and development are not known. Vascular endothelial growth factor (VEGF) is a specific mitogen for endothelial cells that is often expressed by epithelial cells in close proximity to capillary beds. VEGF expression is induced by hypoxia and may be inhibited by hyperoxia. We examined the cell-specific expression of VEGF during normal postnatal lung development and the effects of hyperoxic lung injury on VEGF mRNA and protein in vivo. Normal newborn rabbits between 1 day and 5 wk of age had VEGF transcripts located mainly in alveolar epithelial cells, with little or no VEGF mRNA noted in smooth muscle or endothelial cells. A subpopulation of freshly isolated, normal type II cells, but not mesenchymal cells, expressed VEGF mRNA. Newborn rabbits exposed to 100% oxygen for 4 days had no change in VEGF mRNA abundance, transcript location, or immunostaining. Animals exposed to 100% oxygen for an average of 9 days had an 80% decrease in lung VEGF mRNA abundance, decreased alveolar epithelial cell VEGF expression, and decreased VEGF immunostaining. Recovery of VEGF expression to control levels occurred during a 5-day recovery period. We conclude that alveolar epithelial cells in postnatal lung express VEGF, suggesting epithelial regulation of alveolar capillary formation. Furthermore, hyperoxic injury decreases neonatal lung VEGF mRNA and protein, which may be a contributory mechanism of impaired postnatal microvascular development in oxygen injury.

Changes in decorin expression with hyperoxic injury to developing rat lung

Proteoglycans are extracellular matrix components that appear to play important roles in lung development and in the response to injury. Decorin, a small extracellular matrix-associated proteoglycan, is known to be involved in collagen fibrillogenesis and is a likely participant in the pathogenesis of lung injury. We hypothesized that chronic exposure of the developing lung to hyperoxia would result in temporal and spatial changes in decorin expression. To determine the expression of decorin in normal and oxygen-injured lung, newborn rats were exposed to hyperoxia for 6 wk. Decorin mRNA abundance was determined using Northern hybridization analyses, and decorin expression was localized by in situ hybridization and immunohistochemistry. Decorin mRNA expression in type II pneumocytes was studied using reverse transcription-polymerase chain reaction. Oxygen exposure is associated with a 77% reduction in decorin mRNA in whole lung and a decrease in decorin immunoreactivity in connective tissues surrounding large airways and blood vessels, but an increase in decorin mRNA and protein expression at the tips of alveolar septa. Studies using isolated cells indicate that macrophages and polymorphonuclear neutrophils contain decorin core protein but not decorin mRNA. Type II pneumocytes do not contain either decorin mRNA or core protein. These findings demonstrate that hyperoxic lung injury is associated with localized changes in decorin expression, changes that are not reflected in whole lung RNA studies. It is likely that regional changes in lung decorin expression are influenced by factors produced and acting locally, and that such changes may contribute to the morphologic alterations characteristic of oxygen-induced lung injury.

Expression of fibronectin mRNA splice variants by rabbit lung in vivo and by alveolar type II cells in vitro

Fibronectin (FN) is a multidomain glycoprotein with putative functions in tissue development and repair. In repair of alveolar injury, FN may promote the transition of type II epithelial cells to type I epithelial cells. Alternative splicing of FN mRNA, including the EIIIA and EIIIB exons, results in protein isoforms that have cell, tissue, and developmental specificity. The present work found that FN mRNA with the EIIIA exon was in fetal, adult, and oxidant-injured lung. The EIIIB splice variant, however, was restricted to fetal lung and adult lung recovering from oxidant injury. Because alveolar type II cells in vitro express FN, we examined the splice variants in two conditions that induce FN [transforming growth factor-beta 1 (TGF-beta 1) treatment and time in culture]. TGF-beta 1 increased both EIIIA and EIIIB mRNA abundance by 10-fold. Increased EIIIA isoform immunostaining was also noted. Type II cells that spontaneously express FN at 72 h in vitro had increased EIIIA and EIIIB mRNA and increased immunostaining for EIIIA. Nuclear runoff showed induction of FN gene transcription at 72 h in vitro. Together, these data show differential FN splice variant expression in lung, with EIIIB mRNA restricted to fetal and recovering oxidant-injured lung. Furthermore, the transition of type II cells to a type I-like cell is accompanied by increased FN gene transcription and induction of both EIIIA and EIIIB mRNA.

Retinal vascular endothelial growth factor (VEGF) mRNA expression is altered in relation to neovascularization in oxygen induced retinopathy

The temporal and spatial expression of vascular endothelial cell growth factor (VEGF) mRNA was studied in normal developing cat retina, and in oxygen induced retinopathy. Unexposed control and oxygen-exposed animals (80 h of 80% oxygen from day 3, n = 16) were studied at 1, 2, 4, and 6 weeks after birth. India ink injected retinal flat mounts were used to study vessel progression, and in situ hybridizations using retinal cross sections were used to assess VEGF mRNA accumulation. In controls, as the retina matured, VEGF mRNA hybridization was evident in the ganglion cell layer in a scattered line of distinct cells prior to the ingrowth of vessels, involved the most cells in regions just peripheral to invading vessels and persisted in a fewer positive cells, widely spaced in the vascularized retinas of control, six week animals. In the inner nuclear layer, hybridization initially appeared diffusely and later became localized to a narrow portion of that layer and persisted there. In animals with oxygen induced retinopathy, a substantial increase in hybridization was observed in both the ganglion cell and inner nuclear layers of the avascular retina anterior to the advancing neovascularization. VEGF hybridization decreased abruptly to background levels in both layers at the point were neovascularization met avascular retina. By six weeks, when the neovascularization reached the ora, there was a return of VEGF mRNA in the inner nuclear layer which was similar to normal control expression. A low level of unchanging expression was also observed in the retinal pigment epithelium in both groups at all ages. These results indicate that VEGF mRNA abundance is regulated during retinal vascularization and is increased in relation to oxygen induced neovascularization, suggesting that VEGF may play an important role in both normal retinal vessel development and in the pathophysiology of retinopathy of prematurity.

Extracellular glutathione peroxidase in human lung epithelial lining fluid and in lung cells

The epithelial cells of the lower respiratory tract are exposed to high levels of inhaled oxygen and other oxidants. We hypothesized that lung cells would secrete the antioxidant enzyme, extracellular glutathione peroxidase (eGPx), into epithelial lining fluid (ELF). To investigate this hypothesis, we used specific immunoprecipitations of GPx enzymes from ELF, specific immunoprecipitations of 75Se metabolically labeled proteins from lung cells in culture, and in situ hybridization, Northern blot, and reverse transcription-polymerase chain reaction (RT-PCR) analyses. Fifty-seven percent of ELF GPx activity was due to eGPx and 40% was due to cellular GPx (cGPx). Primary bronchial epithelial cells (BEC), primary alveolar macrophages (AM), and two human bronchial epithelial cell lines, BEP2D and A549, synthesized both eGPx and cGPx and secreted eGPx into the medium. Freshly isolated human AM and BEC expressed eGPx mRNA, while freshly isolated rabbit type 2 pneumocytes did not. In lung tissue, eGPx mRNA was found mainly in interstitial cells of tissue surrounding airways. It is concluded that more than half of GPx activity in BAL is due to eGPx, and that BEC, AM, and interstitial cells are potential sources of pulmonary eGPx.

Vascular endothelial growth factor mRNA increases in alveolar epithelial cells during recovery from oxygen injury

Destruction of pulmonary endothelial cells is characteristic of hyperoxic lung injury. During recovery from hyperoxia, pulmonary endothelial cells proliferate to regenerate the vascular endothelium. Vascular endothelial growth factor (VEGF) is a peptide growth factor that is mitogenic specifically for endothelial cells. We hypothesized that VEGF messenger RNA (mRNA) increases during recovery from acute hyperoxic lung injury. Adult rabbits were exposed to 100% oxygen for 64 h and allowed to recover in air for 0, 1, 3, and 5 days. In situ hybridization showed increased VEGF expression in alveolar epithelial cells beginning at 1 day recovery. By 3 days recovery the message was in alveolar epithelial cells throughout the lung. Compared with alveolar epithelial cells, little or no expression was noted in large vessel endothelial cells, airway cells, or smooth muscle cells. Combined in situ hybridization for VEGF and immunostaining for macrophages and other mesenchymal cells found no VEGF message in those cell types. Isolated alveolar macrophages had no detectable VEGF message. Cells expressing VEGF mRNA were enriched in alveolar type II cell preparations from recovering lung. Double in situ hybridization for VEGF and surfactant protein-C (SP-C) showed co-expression in a population of type II cells, but with an inverse relationship: cells with abundant VEGF mRNA did not have abundant SP-C mRNA. Type II cells in vitro expressed VEGF message, but only when the SP-C message abundance was relatively low. We conclude that alveolar type II cells express increased VEGF mRNA during recovery from acute hyperoxia. These findings are consistent with a role for VEGF in regulating microvascular endothelial repair after oxidant injury.

Transforming growth factor-beta 1 modulates type II cell fibronectin and surfactant protein C expression

Extracellular matrix (ECM) deposition by alveolar type II cells is important for repair of a damaged alveolar epithelium. Transforming growth factor-beta (TGF-beta) is abundant in injured lung and has profound effects on ECM production and cell differentiation. We determined the effects of TGF-beta 1 on type II cell expression of fibronectin and surfactant protein C (SP-C) in vitro. TGF-beta 1 increased the proportion of type II cells with detectable mRNA for fibronectin from 9 to 68%, as demonstrated by in situ hybridization, and increased the fibronectin mRNA levels 10-fold. TGF-beta 1-treated cultures had increased immunostaining for fibronectin and increased secretion of metabolically labeled fibronectin. A decreased proportion of type II cells treated with TGF-beta 1 had detectable mRNA for SP-C, and the abundance of this message per cell decreased to 25% of control values. No effects of TGF-beta 1 were noted on the proportion of cells that contained lamellar bodies, which was 87% in both groups. These data indicate that TGF-beta 1 regulates type II cell fibronectin protein an mRNA levels. In addition, the decreased abundance of SP-C mRNA suggests that TGF-beta 1 may also modulate type II cell differentiation in lung injury.

Human kidney proximal tubules are the main source of plasma glutathione peroxidase

The sites of synthesis of extracellular (E) glutathione peroxidase (GPX), a unique selenoglycoprotein present in plasma, are not known. To investigate the possibility that the kidney is the main source for the plasma GPX, we examined GPX activities and selenium concentrations in the plasma of patients with renal failure on dialysis and nephrectomized patients before and after kidney transplantation. Plasma GPX activities in these patients were 42, 22, and 180% of normal EGPX activity, respectively, whereas plasma Se levels were within the normal range. Twenty-four hours after nephrectomy of anesthetized rats, plasma GPX activity was 30.0 +/- 6.4% of the activity at zero time. Northern hybridization analysis of eight human tissues probed with EGPX and cellular glutathione peroxidase (CGPX) cDNA revealed that the ratio of EGPX to CGPX was highest in the kidney. cRNA in situ hybridization studies on kidney slices showed that only proximal tubular epithelial cells and parietal epithelial cells of Bowman's capsule contained EGPX transcripts. Caki-2, a proximal tubular renal carcinoma cell line, makes and actively secretes EGPX. Taken together, these results strongly suggest that kidney proximal tubular cells are the main source for GPX activity in the plasma.

Increased expression of tissue inhibitor of metalloproteinases (TIMP-I) and metallothionein in murine lungs after hyperoxic exposure

Acute exposure to hyperoxia results in well-described pathophysiologic responses in the lungs, beginning with subtle, subcellular changes and ending with severe pulmonary inflammation and edema. The biologic events that underlie or accompany this injury are not well understood. Our previous studies in rabbits have shown that hyperoxia induces large increases in the mRNAs encoding metallothionein (MT) and the tissue inhibitor of metalloproteinases (TIMP-I). Here we report studies of hyperoxic lung injury in two strains of mouse that differ in their relative resistance to O2 toxicity. O2-sensitive (C57BL/6J) mice and O2-resistant (C3H/HeJ) mice were exposed to 100% O2 for up to 96 h. Lung mRNAs were assayed by primer extension and slot blot hybridization. By 72 h of hyperoxia, the sensitive strain showed large increases in MT-I, MT-II, and TIMP-I mRNAs. The resistant strain showed similar changes but with a 24-h delay. In situ hybridization demonstrated that hyperoxic lung injury was accompanied by obvious increases in TIMP-I and MT transcripts in cells surrounding arteries and large airways, where many inflammatory cells were localized. With prolonged exposure, hybridization to MT transcripts had spread throughout lung parenchyma. The two strains showed the same patterns of in situ hybridization for TIMP-I and MT transcripts but, as with the whole lung homogenates, followed a different time course. Corresponding increases in MT protein were shown to occur, using a cadmium binding assay and by immunohistochemistry. The strong spatial correlation between the presence of localized inflammation and increased TIMP-I and MT expression further supports the importance of TIMP-I and MT in acute lung injury.

Increased fibronectin mRNA in alveolar macrophages following in vivo hyperoxia

Oxygen-mediated lung injury can stimulate a fibroproliferative response resulting in the alteration of the pulmonary extracellular matrix and subsequent scarring of parenchymal tissue. Fibronectin (FN), a component of the extracellular matrix, appears in increased quantities in fibrotic lung disease. Alveolar macrophages (AMs) are a potential source of this molecule. Using quantitative in situ hybridization, we demonstrated that AMs from rabbits acutely exposed to 100% oxygen (hyperoxia) for up to 64 h have 20-fold greater levels of FN mRNA relative to cells from control animals. When animals were allowed to recover in room air for up to 72 h after maximal oxygen exposure, AM FN mRNA abundance approached baseline levels. Furthermore, in oxygen-exposed animals, the fraction of lavaged cells expressing FN mRNA was increased 10-fold relative to controls. Although there was marked cell-to-cell variation, we conclude that the AM is a potential source of FN in the events leading to hyperoxia-induced pulmonary fibrosis.

Differential accumulation of surfactant protein A, B, and C mRNAs in two epithelial cell types of hyperoxic lung

The physiologic response of the lung to oxygen toxicity is complex, and similar among all mammals studied. Acute exposure to 100% O2 results in severe decreases in respiratory function and is accompanied by alterations in pulmonary surfactant metabolism, including the regulation of surfactant proteins A, B, and C (SP-A, SP-B, SP-C). Because surfactant proteins and their mRNAs can be expressed in alveolar epithelial type II cells, and nonciliated bronchial epithelial (Clara) cells, we were interested in determining if alterations in the abundance of SP-A, SP-B, and SP-C mRNAs occurred differentially in these two cell types during hyperoxic lung injury. Using quantitative in situ hybridization, we found that hyperoxic lung injury resulted in nearly 20-fold increases in SP-A and SP-B mRNAs in Clara cells, with relatively small (2-fold or less) increases in type II cells. Immunohistochemical analysis suggested a commensurate increase in SP-A protein in Clara cells. SP-C mRNA was only detected in type II cells, and changed little in hyperoxic lung. Because Clara cells are not known to produce surfactant, and appear to lack SP-C mRNA, these observations suggest that increased SP-A and SP-B may serve nonsurfactant functions in hyperoxic lung.

Cell-specific alterations in expression of hyperoxia-induced mRNAs of lung

Hyperoxic lung injury is an unfortunate consequence of ventilatory oxygen therapy that is necessary to sustain life in certain clinical situations. The biochemical events that accompany hyperoxia of the lung, and the molecular mechanisms underlying these events, are incompletely understood. To better understand hyperoxic lung injury, our laboratory has cloned a set of genes corresponding to mRNAs that increase in abundance in the lungs of hyperoxic rabbits. In this report, we focus on three hyperoxia-induced cDNA clones, which encode surfactant apoprotein A (SP-A), the tissue inhibitor of metalloproteinases (TIMP), and metallothionein. In situ hybridizations and RNA dot blots of isolated lung cell populations indicate that the abundance of mRNA encoding all three proteins is increased by hyperoxia in specific cell types. SP-A mRNA increases in type II alveolar epithelial cells and in bronchiolar epithelial cells. TIMP mRNA increases in interstitial fibroblasts, in chondrocytes of the cartilage surrounding airways, and in endothelial cells of a specific subset of vessels, probably venules. Metallothionein transcripts also increase in chondrocytes and pulmonary fibroblasts. A comparison of the increase in these mRNAs during hyperoxic exposure in adults and newborns indicates that adults respond faster and to a greater extent than newborns and suggests that the rate and extent of these increases is correlated with the time course and severity of the injury.

Surfactant apoprotein A (SP-A) is synthesized in airway cells

The pulmonary surfactant apoproteins A, B, and C (SP-A, SP-B, and SP-C, respectively) function in concert with surfactant phospholipids to reduce surface pressure in the alveolus. Surfactant apoproteins also regulate surfactant synthesis, secretion, adsorption, and recycling. SP-A and B have been localized by immunocytochemistry to alveolar epithelial (type II) cells, alveolar macrophages, and nonciliated bronchiolar epithelial (Clara) cells. In contrast, in situ hybridization to SP-A and B mRNA in human lung has shown SP-A and B transcripts in type II cells, but only SP-B message in Clara cells, implying that synthesis of SP-A occurs exclusively in type II cells. In this report, in situ hybridization to SP-A mRNA was performed on adult and developing rabbit lung and on human lung. SP-A transcripts were found in type II cells and bronchiolar epithelium of both species. The distribution of SP-A message-containing cells in the bronchiolar epithelium of rabbits and humans was similar to the distribution of Clara cells in these two species. These data indicate that SP-A is not only synthesized in type II cells but also in Clara cells.