Redox state is a central modulator of the balance between self-renewal and differentiation in a dividing glial precursor cell

Chemically diverse toxicants converge on Fyn and c-Cbl to disrupt precursor cell function

Identification of common mechanistic principles that shed light on the action of the many chemically diverse toxicants to which we are exposed is of central importance in understanding how toxicants disrupt normal cellular function and in developing more effective means of protecting against such effects. Of particular importance is identifying mechanisms operative at environmentally relevant toxicant exposure levels. Chemically diverse toxicants exhibit striking convergence, at environmentally relevant exposure levels, on pathway-specific disruption of receptor tyrosine kinase (RTK) signaling required for cell division in central nervous system (CNS) progenitor cells. Relatively small toxicant-induced increases in oxidative status are associated with Fyn kinase activation, leading to secondary activation of the c-Cbl ubiquitin ligase. Fyn/c-Cbl pathway activation by these pro-oxidative changes causes specific reductions, in vitro and in vivo, in levels of the c-Cbl target platelet-derived growth factor receptor-α and other c-Cbl targets, but not of the TrkC RTK (which is not a c-Cbl target). Sequential Fyn and c-Cbl activation, with consequent pathway-specific suppression of RTK signaling, is induced by levels of methylmercury and lead that affect large segments of the population, as well as by paraquat, an organic herbicide. Our results identify a novel regulatory pathway of oxidant-mediated Fyn/c-Cbl activation as a shared mechanism of action of chemically diverse toxicants at environmentally relevant levels, and as a means by which increased oxidative status may disrupt mitogenic signaling. These results provide one of a small number of general mechanistic principles in toxicology, and the only such principle integrating toxicology, precursor cell biology, redox biology, and signaling pathway analysis in a predictive framework of broad potential relevance to the understanding of pro-oxidant–mediated disruption of normal development.

Chemically different toxins (lead, methylmercury, and paraquat) each cause the intracellular environment to become more oxidized, and thereby activate a common pathway that suppresses signaling from growth factor receptors that may be associated with developmental impairments.

Discovering general principles underlying the effects of toxicant exposure on biological systems is one of the central challenges of toxicological research. We have discovered a previously unrecognized regulatory pathway on which chemically diverse toxicants converge, at environmentally relevant exposure levels, to disrupt the function of progenitor cells of the developing central nervous system. We found that the ability of low levels of methylmercury, lead, and paraquat to make progenitor cells more oxidized causes activation of an enzyme called Fyn kinase. Activated Fyn then activates another enzyme (c-Cbl) that modifies specific proteins—receptors that are required for cell division and survival—to initiate the proteins' degradation. By enhancing degradation of these receptors, their downstream signaling functions are repressed. Analysis of developmental exposure to methylmercury provided evidence that this same pathway is activated in vivo by environmentally relevant toxicant levels. The remarkable sensitivity of progenitor cells to low levels of toxicant exposure, and the discovery of the redox/Fyn/c-Cbl pathway as a mechanism by which small increases in oxidative status can markedly alter cell function, provide a novel and specific means by which exposure to chemically diverse toxicants might perturb normal development. In addition, the principles revealed in our studies appear likely to have broad applicability in understanding the regulation of cell function by alterations in redox balance, regardless of how they might be generated.

Coenzyme Q(10)--its role as a prooxidant in the formation of superoxide anion/hydrogen peroxide and the regulation of the metabolome

Coenzyme Q10 plays a central role in cellular bioenergy generation and its regulation. Closed membrane systems generate a proton motive force to create transient localized bio-capacitors; the captured energy is used for the synthesis of mitochondrial ATP but also for many other processes, such as metabolite translocations, nerve conduction and a host of other bioenergy requiring processes. Coenzyme Q10 plays a key role in many of these sub-cellular membrane energy generating systems. Integral to this phenomenon is the prooxidant role of coenzyme Q10 in generating the major superoxide anion/hydrogen peroxide second messenger system. This messenger system, largely but not exclusively, arises from coenzyme Q10 semiquinone function; it contributes to the regulation of sub-cellular redox potential levels; transcription/gene expression control; is essential for modulated protein turnover and activation; mediates hormone and growth factor extracellular signaling. The regulated prooxidant formation of the superoxide anion/H2O2 second messenger system is essential for the normal physiological function of the metabolome. The normally functioning metabolome is the expression of a finely tuned dynamic equilibrium comprised of thousands of anabolic and catabolic reactions and all cellular signaling systems must be finely regulated. There is still much to be learnt about the up/down regulation of the H2O2 messenger system. The concept that superoxide anion/H2O2 cause random macromolecular damage is rebutted. The administration of antioxidants to quench the inferred toxicity of these compounds as a therapy for age associated diseases is unsupported by extant mammalian clinical trials and should be subject to serious re-evaluation. The role of ascorbic acid as a beneficial hydrogen peroxide prodrug is discussed.

Radiation Response of Neural Precursor Cells

Considerable data are now available regarding the radiation responsiveness of neural precursor cells that exist in the neurogenic regions of the mammalian forebrain. These cells and their progeny are extremely sensitive to irradiation, undergoing apoptosis after clinically relevant doses that do not result in overt tissue injury. In addition, there is compelling evidence that radiation significantly affects the whole process of neurogenesis and that the sensitivity depends, at least in part, on alterations in the microenvironment within which the precursor cells exist. Although provocative data exist suggesting that inflammation, oxidative stress, or morphologic relations influence neurogenesis, the precise mechanisms involved remain obscure and need to be investigated. Additionally, it is important to try to understand what these findings may mean in the context of radiation paradigms associated with the treatment of intracranial disease. Understanding how neural precursor cells respond to noxious stimuli is likely to lead to new therapeutic approaches that should restore neurogenesis and perhaps improve cognitive performance.

The essential requirement for superoxide radical and nitric oxide formation for normal physiological function and healthy aging

Contrary to the dogma that superoxide anion and hydrogen peroxide formation are highly deleterious to cell function and healthy aging, we suggest this premise is flawed. Superoxide anion and hydrogen peroxide formation are essential to normal cellular function; they constitute a second messenger system absolutely required for the regulation of the metabolome. Embraced within this regulation is the modulation of cellular redox poise, bioenergy output, gene expression and cell differentiation. A key component in the overall process is coenzyme Q10 whose prooxidant function through the formation of superoxide anion and hydrogen peroxide is a major factor in the overall processes. The free radical gas, nitric oxide (similarly to superoxide anion), functions in the regulation of a wide range of cell systems. As part of the normal physiological process, superoxide anion and NO function separately and interactively as second messengers. Superoxide anion and nitric oxide play an intrinsic role in the regulated ordered turnover of proteins, rather than randomly cause protein damage and their inactivation. The proposition that metabolic free radical formation is unequivocally deleterious to cell function is rebutted; their toxicity as primary effectors in the aging process has been overemphasized. The concept that a dietary supplement of high concentrations of small-molecule antioxidants is a prophylactic/amelioration therapy for the aging process and age-associated diseases is questioned as to its clinical validity.

Reactive oxygen species modulate the differentiation of neurons in clonal cortical cultures

Reactive oxygen species (ROS) are important regulators of intracellular signaling. We examined the expression of ROS during rat brain development and explored their role in differentiation using cortical cultures. High levels of ROS were found in newborn neurons. Neurons produced ROS, not connected with cell death, throughout embryogenesis and postnatal stages. By P20, ROS-producing cells were found only in neurogenic regions. Cells with low levels of ROS, isolated from E15 brains by FACS, differentiated into neurons, oligodendrocytes, and astrocytes in clonal cultures. Neurons produced high ROS early in culture and later differentiated into two types: large pyramidal-like neurons that fired no or only a single action potential and smaller neurons that expressed nuclear calretinin and fired repeated action potentials. Antioxidant treatment did not alter neuron number but increased the ratio of small to large neurons. These findings suggest that modulation of ROS levels influences multiple aspects of neuronal differentiation.

Cellular redox regulation and prooxidant signaling systems: a new perspective on the free radical theory of aging

The overarching role of coenzyme Q(10) in gene regulation, bioenergy formation, cellular redox poise regulation, and hydrogen peroxide formation is presented. Coenzyme Q(10) has a central role acting as a prooxidant in the generation of H(2)O(2). Contrary to the dogma that superoxide and H(2)O(2) formation are highly deleterious to cell survival this premise is rejected. Data are discussed that continuous superoxide and hydrogen peroxide formation are essential for normal cell function and that they play a major role in subcellular redox state modulation. It is the prooxidant activity of the so-called antioxidants that may be responsible for previously claimed benefits for high doses of oxido-reduction nutritional supplements such as alpha lipoic acid and coenzyme Q(10). Oxygen-free radical formation is essential for the biological function and is not a direct causation of the mammalian aging process; aging is a multisystem stochastic process.

Effect of reduced culture temperature on antioxidant defences of mesenchymal stem cells

Mesenchymal stem cells (MSC) promise to be valuable therapeutic tools but, due to their low numbers, require considerable in vitro expansion before use. This leads to in vitro aging, the accumulation of intracellular oxidative damage, and subsequently a decreased potential for proliferation and differentiation. Optimised culture conditions might help to reduce oxidative damage in MSC in vitro, and therefore, as reduced temperature is known to reduce oxidative stress in other somatic cells, we have investigated the effect of reduced temperature on rat MSC viability, differentiation, and oxidative damage. Temperature reduction did not affect MSC viability but increased differentiation and reduced apoptosis. Oxidative-damage-related indices were improved; reactive oxide species, nitric oxide, thiobarbituric acid reactive substances, carbonyl, and lipofuscin levels were reduced and glutathione peroxidase and superoxide dimutase levels increased. Levels of antiapoptotic heat shock proteins (HSP-27, -70, and -90) were raised and levels of the proapoptotic HSP-60 reduced. These data demonstrate that culturing MSC at reduced temperature decreases the accumulation of oxidative damage and therefore would probably improve long-term viability and successful engraftment of MSC used for tissue engineering or cell therapeutic purposes.

Increased "vigilance" of antioxidant mechanisms in neural stem cells potentiates their capability to resist oxidative stress

Although the potential value of transplanted and endogenous neural stem cells (NSCs) for the treatment of the impaired central nervous system (CNS) has widely been accepted, almost nothing is known about their sensitivity to the hostile microenvironment in comparison to surrounding, more mature cell populations. Since many neuropathological insults are accompanied by oxidative stress, this report compared the alertness of antioxidant defense mechanisms and cell survival in NSCs and postmitotic neural cells (PNCs). Both primary and immortalized cells were analyzed. At steady state, NSCs distinguished themselves in their basal mitochondrial metabolism from PNCs by their lower reactive oxygen species (ROS) levels and higher expression of the key antioxidant enzymes uncoupling protein 2 (UCP2) and glutathione peroxidase (GPx). Following exposure to the mitochondrial toxin 3-nitropropionic acid, PNC cultures were marked by rapidly decreasing mitochondrial activity and increasing ROS content, both entailing complete cell loss. NSCs, in contrast, reacted by fast upregulation of UCP2, GPx, and superoxide dismutase 2 and successfully recovered from an initial deterioration. This recovery could be abolished by specific antioxidant inhibition. Similar differences between NSCs and PNCs regarding redox control efficiency were detected in both primary and immortalized cells. Our first in vivo data from the subventricular stem cell niche of the adult mouse forebrain corroborated the above observations and revealed strong baseline expression of UCP2 and GPx in the resident, proliferating NSCs. Thus, an increased "vigilance" of antioxidant mechanisms might represent an innate characteristic of NSCs, which not only defines their cell fate, but also helps them to encounter oxidative stress in diseased CNS.

Effect of developmental exposure to chlorpyrifos on the expression of neurotrophin growth factors and cell-specific markers in neonatal rat brain

Chlorpyrifos (CPS), a known neurotoxicant, is a widely used agricultural organophosphorus insecticide. The effects of postnatal exposure to CPS on the expression of mRNA for two factors critical to brain development, nerve growth factor (NGF) and reelin, were investigated in the forebrain of rats. In addition, the expression of mRNA for the muscarinic acetylcholine receptor (mAChR) M(1) subtype and cell-specific markers for developing neurons (beta-III tubulin), astrocytes (glial fibrillary acidic protein, GFAP), and oligodendrocytes (myelin-associated glycoprotein, MAG) was also investigated. Oral administration of CPS (1.5 or 3.0 mg/kg) or the corn oil vehicle was performed daily from postnatal days (PNDs) 1 through 6. No signs of overt toxicity or of cholinergic hyperstimulation were observed after CPS administration. Body weight was significantly different from controls on PND7 in both males and females exposed to 3.0 mg/kg CPS. Quantitative PCR was performed on the forebrain. The expression of NGF, reelin, and M(1) mAChR mRNA was significantly reduced with both dosages of CPS in both sexes. beta-III Tubulin mRNA expression remained unchanged after exposure, whereas MAG mRNA expression was significantly decreased with both dosages of CPS in both sexes, suggesting effects on the developing oligodendrocytes. In contrast, GFAP mRNA levels were significantly increased with both dosages of CPS in both sexes, suggesting increased astrocyte reactivity. Our findings indicate that dosages of CPS which cause significant cholinesterase inhibition but do not exert overt toxicity can adversely affect the expression levels of critical genes involved in brain development during the early postnatal period in the rat.

A role for mitochondria in the establishment and maintenance of the maize root quiescent center

Mitochondria in the oxidizing environment of the maize (Zea mays) root quiescent center (QC) are altered in function, but otherwise structurally normal. Compared to mitochondria in the adjacent, rapidly dividing cells of the proximal root tissues, mitochondria in the QC show marked reductions in the activities of tricarboxylic acid cycle enzymes. Pyruvate dehydrogenase activity was not detected in the QC. Use of several mitochondrial membrane potential (DeltaPsi(m)) sensing probes indicated a depolarization of the mitochondrial membrane in the QC, which suggests a reduction in the capacity of QC mitochondria to generate ATP and NADH. We postulate that modifications of mitochondrial function are central to the establishment and maintenance of the QC.

Metabolic changes in mesenchymal stem cells in osteogenic medium measured by autofluorescence spectroscopy

The purpose of this study was to measure metabolic changes in mesenchymal stem cells (MSCs) placed in osteogenic medium by autofluorescence spectroscopy. MSCs were plated in stem cell-supporting or osteogenic medium and imaged. Shift from the basic growth environment to the inductive osteogenic environment was confirmed by reverse transcription-polymerase chain reaction. Reduced pyridine nucleotides were detected by exciting near 366 nm and measuring fluorescence at 450 nm, and oxidized flavoproteins were detected by exciting at 460 nm and measuring fluorescence at 540 nm. The ratio of these fluorescence measurements, reduction-oxidation (redox) fluorometry, is a noninvasive measure of the cellular metabolic state. The detected pyridine nucleotide to flavoprotein ratio decreased upon transitioning from the stem cell to the differentiated state, as well as with increasing cell density and cell-cell contact. MSC metabolism increased upon placement in differentiating medium and with increasing cell density and contact. Redox fluorometry is a feasible, noninvasive technique for distinguishing MSCs from further differentiated cells.

Ovotransferrin is a redox-dependent autoprocessing protein incorporating four consensus self-cleaving motifs flanking the two kringles

Embryos of avian eggs and mammals are highly sensitive to oxidative stress and hence maintaining a steady reducing environment during the embryonic development is known to confer protection. Although information is completely lacking, proteins of avian egg albumin which have been suggested to play various biological functions, are the major targets for such reducing state during embryogenesis. In this study, we found that ovotransferrin (OTf), the second major protein in egg albumin, undergoes autocleavage at distinct sites upon reduction with thiol-reducing agent or thioredoxin-reducing system. Mass spectral and microsequencing analysis indicated that OTf is able to cleave itself through the unique chemical reactivity of four tripeptides motifs, HTT (residues 209-211), HST (residues 542-544) and two CHT (residues 115-117 and 454-456). Intriguingly, these self-cleavage sites were uniquely located upstream and downstream of the two disulfide kringle domains (residues 115-211 and 454-544) of OTf. These reduction-scissile sequences, His/Cys-X-Thr, are evolutionary conserved self-cleavage motifs found in several autoprocessing proteins including hedgehog proteins. Interestingly, reduction of other two members of transferrin family induced autocleavage patterns, similar to that of OTf, in bovine lactoferrin (bLf) while human lactoferrin (hLf) showed much less self-cleaving activity. This finding is the first to describe that transferrins are a new subset in the class of proteins able to carry out autoprocessing, providing insight into this unusual biochemical process that appears to be a molecular switch involved in triggering a yet unidentified function(s) of OTf as well as bLf.

Redox regulation of precursor cell function: insights and paradoxes

Studies on oligodendrocytes, the myelin-forming cells of the central nervous system, and on the progenitor cells from which they are derived, have provided several novel insights into the role of intracellular redox state in cell function. This review discusses our findings indicating that intracellular redox state is utilized by the organism as a means of regulating the balance between progenitor cell division and differentiation. This regulation is achieved in part through cell-intrinsic differences that modify the response of cells to extracellular signaling molecules, such that cells that are slightly more reduced are more responsive to inducers of cell survival and division and less responsive to inducers of differentiation or cell death. Cells that are slightly more oxidized, in contrast, show a greater response to inducers of differentiation or cell death, but less response to inducers of proliferation or survival. Regulation is also achieved by the ability of exogenous signaling molecules to modify intracellular redox state in a highly predictable manner, such that signaling molecules that promote self-renewal make progenitor cells more reduced and those that promote differentiation make cells more oxidized. In both cases, the redox changes induced by exposure to exogenous signaling molecules are a necessary component of their mode of action. Paradoxically, the results obtained through studies on the oligodendrocyte lineage are precisely the opposite of what might be predicted from a large number of studies demonstrating the ability of reactive oxidative species to enhance the effects of signaling through receptor tyrosine kinase receptors and to promote cell proliferation. Taken in sum, available data demonstrate clearly the existence of two distinct programs of cellular responses to changes in oxidative status. In one of these, becoming even slightly more oxidized is sufficient to inhibit proliferation and induce differentiation. In the second program, similar changes enhance proliferation. It is not yet clear how cells can interpret putatively identical signals in such opposite manners, but it does already seem clear that resolving this paradox will provide insights of considerable relevance to the understanding of normal development, tissue repair, and tumorigenesis.

Efficient Production of Reactive Oxygen Species in Neural Precursor Cells after Exposure to 250 MeV Protons

The space radiation environment is composed of highly energetic ions, dominated by protons, that pose a range of potential health risks to astronauts. Traversals of these particles through certain tissues may compromise the viability and/or function of sensitive cells, including neural precursors found within the dentate subgranular zone of the hippocampus. Irradiation has been shown to deplete these cells in vivo, and reductions of these critical cells are believed to impair neurogenesis and cognition. To more fully understand the mechanisms underlying the behavior of these precursor cells after irradiation, we have developed an in vitro neural precursor cell system and used it to assess acute (0-48 h) changes in ROS and mitochondrial end points after exposure to Bragg-peak protons of 250 MeV. Relative ROS levels were increased at nearly all doses (1-10 Gy) and postirradiation times (6-24 h) compared to unirradiated controls. The increase in ROS after proton irradiation was more rapid than that observed with X rays and showed a well-defined dose response at 6 and 24 h, increasing approximately 10% and 3% per gray, respectively. However, by 48 h postirradiation, ROS levels fell below controls and coincided with minor reductions in mitochondrial content. Use of the antioxidant alpha-lipoic acid (before or after irradiation) was shown to eliminate the radiation-induced rise in ROS levels. Our results corroborate earlier studies using X rays and provide further evidence that elevated ROS are integral to the radioresponse of neural precursor cells.

Alterations of the glutathione redox state improve apical meristem structure and somatic embryo quality in white spruce (Picea glauca)

In white spruce, an improvement of somatic embryo number and quality can be achieved through experimental manipulations of the endogenous levels of reduced (GSH) and oxidized (GSSG) glutathione. An optimal protocol for embryo production included an initial application of GSH in the maturation medium, followed by replacement with GSSG during the remaining maturation period. Under these conditions, the overall embryo population more than doubled, and the percentage of fully developed embryos increased from 22% to almost 70%. These embryos showed improved post-embryonic growth and conversion frequency. Structural studies revealed remarkable differences between embryo types, especially in storage product deposition pattern and organization of the shoot apical meristem (SAM). Compared with their control counterparts, glutathione-treated embryos accumulated a larger amount of starch during the early stages of development, and more protein and lipid bodies during the second half of development. Differences were also noted in the organization of SAMs. Shoot meristems of control embryos were poorly organized and were characterized by the presence of intercellular spaces, which caused separation of the subapical cells. Glutathione-treated embryos had well-organized meristems composed of tightly packed cells which lack large vacuoles. The improved organization of the shoot apical meristems in treated embryos was ascribed to a lower production of ethylene. Differences in meristem structure between control and treated embryos were also related to the localization pattern of HBK1, a shoot apical meristem 'molecular marker' gene with preferential expression to the meristematic cells of the shoot pole. Expression of this gene, which was localized to the apical cells in control embryos, was extended to the subapical cells of treated embryos. Overall, it appears that meristem integrity and embryo quality are under the direct control of the glutathione redox state.

Accelerated growth and prolonged lifespan of adipose tissue-derived human mesenchymal stem cells in a medium using reduced calcium and antioxidants

Human mesenchymal stem cells (MSCs) have been isolated from bone marrow and other adult tissues and are potentially useful for tissue engineering. Adipose tissue has several clear advantages as a starting material for harvesting stem cells, as it is abundant and relatively easy to procure. However, existing methods to expand adipose-derived MSCs are less than optimal. Here we describe a new cell culture method that accelerates greatly the growth rate and prolongs the lifespan of adipose MSCs. This was accomplished by using a growth medium with low calcium and supplemented with N-acetyl-L-cysteine and L-ascorbic acid-2-phosphate. Cells produced early in these cultures displayed characteristics similar to those previously reported for multipotential stem cells, including a high frequency of anchorage- independent growth in soft agar, lack of gap junctional intercellular communication in a cell type with serpiginous morphology, and the expression of Oct-4. Furthermore, these cells could readily be induced to differentiate into adipocytes, osteoblasts, and chondrocytes. Thus, modification of growth medium by reduction of calcium and addition of antioxidants greatly enhanced the growth rate and extended the lifespan of adipose-derived multipotential human MSCs.

Temporal changes of multiple redox couples from proliferation to growth arrest in IEC-6 intestinal epithelial cells

Changes in intracellular redox couples and redox reactive molecules have been implicated in the regulation of a variety of cellular processes, including cell proliferation and growth arrest by contact inhibition. However, the magnitude, direction, and temporal relationship of redox changes to cellular responses are incompletely defined. The present work sought to characterize redox and metabolic changes associated with proliferative stages to contact inhibition of growth in rat IEC-6 intestinal epithelial cells. From the first day of culture until 1 day before confluence, an increase in GSH concentrations and a significant reduction in the redox potential of the GSSG/2GSH couple were observed. These changes were accompanied by a decrease in relative reactive oxygen species (ROS) and nitric oxide (NO) concentrations and oxidation of the redox potential of the NADP(+)/reduced NADP and NAD(+)/NADH couples. Postconfluent cells exhibited a significant decrease in GSH concentrations and a significant oxidation of the GSSG/2GSH couple. When cell proliferation decreased, relative ROS concentrations increased (P < 0.01), whereas NO concentrations remained unchanged, and the NAD(+)/NADH couple became more reduced. Together, these data indicate that the redox potential of distinct couples varies differentially in both magnitude and direction during successive stages of IEC-6 growth. This finding points out the difficulty of defining intracellular redox status at particular stages of cell growth by examining only one redox species. In addition, the data provide a numerical framework for future research of regulatory mechanisms governed by distinct intracellular redox couples.

Newborn neurons acquire high levels of reactive oxygen species and increased mitochondrial proteins upon differentiation from progenitors

A population of embryonic rat cortical cells cultured in the presence of FGF2 and having neuronal morphology expressed higher levels of reactive oxygen species (ROS) than did progenitor cells, astrocytes, and several cell lines of neuronal and non-neuronal origin. ROS were assessed using 5-(and-6)-chlormethyl-2',7'-dichlorodihydrofluorescein diacetate, acetyl ester (CM-H2DCF-DA), and high levels persisted in the presence of antioxidants or lowered levels of ambient oxygen. Greater than 95% of high ROS-producing cells, isolated by fluorescence-activated cell sorting, expressed the neuronal marker beta III tubulin. These cells did not incorporate BrdU or express nestin, unlike low ROS-producing cells, 99% of which exhibited both of these characteristics. Upon growth factor removal, low ROS-expressing cells differentiated into neurons and astrocytes and these neurons expressed high levels of ROS, indicating that ROS accumulation accompanies the differentiation of progenitors into neurons. ROS levels were decreased by added superoxide dismutase and catalase, suggesting that both superoxide and hydrogen peroxide contribute to the ROS signal. High ROS-expressing cells also contained higher levels of several mitochondrial respiratory chain components. Although ROS have been associated with conditions that lead to cell death, our results and recent studies on the role of ROS as regulators of signal pathways are consistent with the possibility that ROS play a role in the development of the neuronal phenotype. Moreover, the differential production of ROS provides a useful method to isolate from mixed populations cells that are highly enriched for either progenitor cells or neurons.

The effects of stress and aging on glutathione metabolism

Glutathione plays a critical role in many biological processes both directly as a co-factor in enzymatic reactions and indirectly as the major thiol-disulfide redox buffer in mammalian cells. Glutathione also provides a critical defense system for the protection of cells from many forms of stress. However, mild stress generally increases glutathione levels, often but not exclusively through effects on glutamate cysteine ligase, the rate-limiting enzyme for glutathione biosynthesis. This upregulation in glutathione provides protection from more severe stress and may be a critical feature of preconditioning and tolerance. In contrast, during aging, glutathione levels appear to decline in a number of tissues, thereby putting cells at increased risk of succumbing to stress. The evidence for such a decline is strongest in the brain where glutathione loss is implicated in both Parkinson's disease and in neuronal injury following stroke.

Intracellular redox state: towards quantitative description

Redox state is a widely used term for the description of redox phenomena in biological systems. The regulating mechanisms responsible for maintaining the redox state are not yet fully known. But it was shown that changes in the redox state might lead to a cascade of intracellular events, beneficial or deleterious to the cell. There are several methods for the description of the intracellular redox state. These methods are based on using measured intracellular concentrations of reduced and oxidized glutathione in the Nernst equation. However, glutathione is not always a basic redox component in biological fluids, organelles, cells or tissues. As a result, changes in the intracellular redox state are not always accompanied by considerable changes of glutathione concentration. In this work it was proposed to use the concept of effective reduction potential for the quantitative characteristic of the intracellular redox state. The effective reduction potential was substantiated on the basis of a thermodynamic description. A new equation for the calculation of the effective reduction potential was derived. This equation summarizes the contribution of different oxidizing and reducing agents in the formation of an effective redox potential. The theoretical estimation of the effective reduction potential values for the different biological fluids and cells was carried out with the use of a method developed.

Cell-density-dependent regulation of neural precursor cell function

Stress-induced reductions of neural precursor cells from the subgranular zone of the hippocampal dentate gyrus have been linked to impaired neurogenesis and cognitive dysfunction. Given the importance of redox state in regulating multiple damage-responsive pathways in the CNS, we hypothesize that oxidative stress plays a major role in affecting neurogenesis and subsequent cognitive function after cell injury/depletion. Using an in vitro system, we showed that the level of reactive oxygen species (ROS), which depend critically on changes in cell density, were significantly higher in neural precursor cells when compared with other primary and transformed cell lines. ROS were significantly elevated ( approximately 4-fold) under low- (<1 x 10(4) cells per cm(2)) versus high-density (>1 x 10(5) cells per cm(2)) conditions. Higher ROS levels found at lower cell densities were associated with elevated proliferation and increased metabolic activity. These ROS were likely a result of altered mitochondrial function that ultimately compromised the growth rate of cells. At high cell densities, intracellular ROS and oxidative damage were reduced in concert with an increased expression of mitochondrial superoxide dismutase 2. Our finding that DNA-damage-induced depletion of neural precursor cells in the subgranular zone of mice also led to increased ROS and altered proliferation validated our in vitro system. Increased ROS and proliferation associated with the reduction of precursor cell numbers both in vitro and in vivo could be reversed with the antioxidant alpha-lipoic acid. These data showed that neural precursor cells were predisposed to microenvironmental cues that regulate redox-sensitive pathways to control cellular proliferation after CNS damage.

Redox-mediated enrichment of self-renewing adult human pancreatic cells that possess endocrine differentiation potential

OBJECTIVES

The limited availability of transplantable human islets has stimulated the development of methods needed to isolate adult pancreatic stem/progenitor cells capable of self-renewal and endocrine differentiation. The objective of this study was to determine whether modulation of intracellular redox state with N-acetyl-L-cysteine (NAC) would allow for the propagation of pancreatic stem/progenitor cells from adult human pancreatic tissue.

METHODS

Cells were propagated from human pancreatic tissue using a serum-free, low-calcium medium supplemented with NAC and tested for their ability to differentiate when cultured under different growth conditions.

RESULTS

Human pancreatic cell (HPC) cultures coexpressed alpha-amylase, albumin, vimentin, and nestin. The HPC cultures, however, did not express other genes associated with differentiated pancreatic exocrine, duct, or endocrine cells. A number of transcription factors involved in endocrine cell development including Beta 2, Islet-1, Nkx6.1, Pax4, and Pax6 were expressed at variable levels in HPC cultures. In contrast, pancreatic duodenal homeobox factor 1 (Pdx-1) expression was extremely low and at times undetectable. Overexpression of Pdx-1 in HPC cultures stimulated somatostatin, glucagon, and carbonic anhydrase expression but had no effect on insulin gene expression. HPC cultures could form 3-dimensional islet-like cell aggregates, and this was associated with expression of somatostatin and glucagon but not insulin. Cultivation of HPCs in a differentiation medium supplemented with nicotinamide, exendin-4, and/or LY294002, an inhibitor of phosphatidylinositol-3 kinase, stimulated expression of insulin mRNA and protein.

CONCLUSION

These data support the use of intracellular redox modulation for the enrichment of pancreatic stem/progenitor cells capable of self-renewal and endocrine differentiation.

Expression of thyroid hormone receptor isoforms in the oligodendrocyte lineage

Thyroid hormone (T3) regulates brain development and function and in particular ensures normal myelination. Animal models and in vitro systems have been employed to demonstrate the effects of T3, which acts via nuclear hormone receptors. T3 receptors (TRs) are transcription factors that activate or suppress target gene expression, such as myelin basic protein (MBP), in a hormone-dependent or -independent fashion. Two distinct genes, TR alpha and TR beta, encode several receptor isoforms with specific functions. This overview summarizes current knowledge on the cellular expression and the role of these isoforms and also examines the action of T3 on oligodendrocyte lineage cell types at defined developmental stages. Re-expression of TRs and also that of other transcription factors in oligodendrocytes may constitute some of the metabolic changes required for succesfull remyelination in the adult central nervous system after demyelinating lesions.

A combined in vitro/bioinformatic investigation of redox regulatory mechanisms governing cell cycle progression

The intracellular reduction-oxidation (redox) environment influences cell cycle progression; however, underlying mechanisms are poorly understood. To examine potential mechanisms, the intracellular redox environment was characterized per cell cycle phase in Chinese hamster ovary fibroblasts via flow cytometry by measuring reduced glutathione (GSH), reactive oxygen species (ROS), and DNA content with monochlorobimane, 2',7'-dichlorohydrofluorescein diacetate (H2DCFDA), and DRAQ5, respectively. GSH content was significantly greater in G2/M compared with G1 phase cells, whereas GSH was intermediate in S phase cells. ROS content was similar among phases. Together, these data demonstrate that G2/M cells are more reduced than G1 cells. Conventional approaches to define regulatory mechanisms are subjective in nature and focus on single proteins/pathways. Proteome databases provide a means to overcome these inherent limitations. Therefore, a novel bioinformatic approach was developed to exhaustively identify putative redox-regulated cell cycle proteins containing redox-sensitive protein motifs. Using the InterPro (http://www.ebi.ac.uk/interpro/) database, we categorized 536 redox-sensitive motifs as: 1) active/functional-site cysteines, 2) electron transport, 3) heme, 4) iron binding, 5) zinc binding, 6) metal binding (non-Fe/Zn), and 7) disulfides. Comparing this list with 1,634 cell cycle-associated proteins from Swiss-Prot and SpTrEMBL (http://us.expasy.org/sprot/) revealed 92 candidate proteins. Three-fourths (69 of 92) of the candidate proteins function in the central cell cycle processes of transcription, nucleotide metabolism, (de)phosphorylation, and (de)ubiquitinylation. The majority of oxidant-sensitive candidate proteins (68.9%) function during G2/M phase. As the G2/M phase is more reduced than the G1 phase, oxidant-sensitive proteins may be temporally regulated by oscillation of the intracellular redox environment. Combined with evidence of intracellular redox compartmentalization, we propose a spatiotemporal mechanism that functionally links an oscillating intracellular redox environment with cell cycle progression.

Ischemic preconditioning attenuates ischemia-reperfusion-induced mucosal apoptosis by inhibiting the mitochondria-dependent pathway in rat small intestine

Ischemic preconditioning provides a way of protecting organs from damage inflicted with prolonged ischemia-reperfusion. In this study, we investigated the mechanism of ischemic preconditioning involved in inhibition of prolonged ischemia-reperfusion-induced mucosal apoptosis in rat small intestine. Ischemic preconditioning was triggered by a transient occlusion of the superior mesenteric artery followed by reperfusion. Ischemia-reperfusion was induced by 60-min occlusion of the superior mesenteric artery followed by 60-min reperfusion in the small intestine. Ischemia-reperfusion alone induced mucosal apoptosis and mitochondrial respiratory dysfunction via promoted reactive oxygen species generation, reduced mitochondrial glutathione oxidation, increased mitochondrial lipid peroxidation, reduced mitochondrial membrane potential, and enhanced release of cytochrome c from mitochondria to activate caspase-9 and caspase-6 in the small intestine. Pretreatment with 20-min ischemia followed by 5-min reperfusion significantly inhibited the prolonged ischemia-reperfusion-induced mucosal apoptosis by 30%. Ischemic preconditioning ameliorated mitochondrial respiratory dysfunction by 50%, reduced reactive oxygen species generation by 38%, and suppressed mitochondrial lipid peroxidation by 36%, resulting in improvement of the mitochondrial membrane potential and prevention of cytochrome c release as well as caspase-6 activation. Results suggest that ischemic preconditioning attenuated ischemia-reperfusion-induced mucosal apoptosis partly by inhibiting the reactive oxygen species-mediated mitochondria-dependent pathway in the rat small intestine.

Getting a GR(i)P on oligodendrocyte development

One of the most extensively studied of mammalian cells is the oligodendrocyte, the myelin-forming cell of the central nervous system. The ancestry and development of this cell have been studied with every approach utilized by developmental biologists. Such detailed efforts have the potential of providing paradigms of relevance to those interested in analyzing the ancestry and development of any cell type. One of the striking features of studies on the development of oligodendrocytes is that different analytical approaches have led to strikingly different theoretical views regarding the ancestry of these cells. On one extreme is the hypothesis that the steps leading to the generation of oligodendrocytes begin with the generation of a glial-restricted precursor (GRP) cell from neuroepithelial stem cells. GRP cells are thought to be capable of giving rise to all glial cells (including oligodendrocytes and multiple astrocyte populations), but not to neurons, a process that appears to require progression through further stages of greater lineage restriction. On the other extreme is the hypothesis that oligodendrocytes are derived from a precursor cell that generates only motor neurons and oligodendrocytes, with astrocytes being generated through a separate lineage. In this review, we critically consider the various contributions to understanding the ancestry of oligodendrocytes, with particular attention to the respective merits of the GRP cell vs. the motor neuron-oligodendrocyte precursor (MNOP) cell hypothesis. We draw the conclusion that, at present, the strengths of the GRP cell hypothesis outweigh those of the MNOP hypothesis and other hypotheses suggesting oligodendrocytes are developmentally more related to motor neurons than to astrocytes. Moreover, it is clear from existing data that, following the period of motor neuron generation, the major glial precursor cell in the embryonic spinal cord is the GRP cell, and that multiple previous studies on the earliest stages of oligodendrocyte generation in the developing spinal cord have been focused on a differentiation stage of GRP cells.

Radiation Response of Neural Precursor Cells: Linking Cellular Sensitivity to Cell Cycle Checkpoints, Apoptosis and Oxidative Stress

Therapeutic irradiation of the brain can cause a progressive cognitive dysfunction that may involve defects in neurogenesis. In an effort to understand the mechanisms underlying radiation-induced stem cell dysfunction, neural precursor cells isolated from the adult rat hippocampus were analyzed for acute (0-24 h) and chronic (3-33 days) changes in apoptosis and reactive oxygen species (ROS) after exposure to X rays. Irradiated neural precursor cells exhibited an acute dose-dependent apoptosis accompanied by an increase in ROS that persisted over a 3-4-week period. The radiation effects included the activation of cell cycle checkpoints that were associated with increased Trp53 phosphorylation and Trp53 and p21 (Cdkn1a) protein levels. In vivo, neural precursor cells within the hippocampal dentate subgranular zone exhibited significant sensitivity to radiation. Proliferating precursor cells and their progeny (i.e. immature neurons) exhibited dose-dependent reductions in cell number. These reductions were less severe in Trp53-null mice, possibly due to the disruption of apoptosis. These data suggest that the apoptotic and ROS responses may be tied to Trp53-dependent regulation of cell cycle control and stress-activated pathways. The temporal coincidence between in vitro and in vivo measurements of apoptosis suggests that oxidative stress may provide a mechanistic explanation for radiation-induced inhibition of neurogenesis in the development of cognitive impairment.

Toward cell replacement therapy: promises and caveats

Studies in animal models have suggested a role for stem cells in repair and regeneration of the nervous system. Human equivalents of stem and precursor cells have been isolated and their efficacy is being evaluated in rodent and primate models. Difficulties exist in translating results of these preclinical models to therapy in humans. Evolutionary differences among rodents, primates, and humans; fundamental differences in the anatomy and physiology; differences in immune responses in xenotransplant models; the paucity of good transplant models of chronic disease; and allelic variability in the cells themselves make any study evaluating the efficacy of cells in transplant models difficult to interpret. As no better alternatives to testing in animals exist, we suggest that at this early stage a considered step-by-step approach to testing and comparison of different transplant strategies in isolation will prepare us better for clinical trials than simple evaluation of functional outcomes in various models of disease. We emphasize that we do not recommend delaying or abandoning clinical trials; rather, we suggest that one anticipate failures and design experiments and data collection such that we learn from these failures to ensure future success in as rapid a time frame as possible.

Superoxide signalling required for multicellular development of Dictyostelium

Reactive oxygen species are known to have a signalling role in many organisms. In bacteria and yeast various response systems have evolved to combat oxidative stress which are triggered by reactive oxygen species. Mammals and plants are known to actively generate reactive oxygen species such as superoxide during signalling responses to a variety of extracellular factors. We report here the generation of superoxide as a signalling molecule in early development of Dictyostelium discoideum. Dictyostelium grows as single amoebae but, on starvation, the single cells aggregate to form a multicellular organism. Superoxide is generated in response to a secreted factor during the transition to the multicellular phase of development. Scavenging superoxide, either pharmacologically or by overexpressing the enzyme superoxide dismutase, inhibits the formation of the aggregate. This report of the use of superoxide as a signalling molecule in a lower eukaryote as it switches to a multicellular phase suggests that this signalling mechanism arose early in the evolution of multicellular organisms, perhaps as a necessary consequence of the need to diversify the number and type of signalling pathways available to facilitate intercellular communication.

Hydrogen sulfide induces serum-independent cell cycle entry in nontransformed rat intestinal epithelial cells

Hydrogen sulfide (H2S), produced by commensal sulfate-reducing bacteria, is an environmental insult that potentially contributes to chronic intestinal epithelial disorders. We tested the hypothesis that exposure of nontransformed intestinal epithelial cells (IEC-18) to the reducing agent sodium hydrogen sulfide (NaHS) activates molecular pathways that underlie epithelial hyperplasia, a phenotype common to both ulcerative colitis (UC) and colorectal cancer. Exposure of IEC-18 cells to NaHS rapidly increased the NADPH/NADP ratio, reduced the intracellular redox environment, and inhibited mitochondrial respiratory activity. The addition of 0.2-5 mM NaHS for 4 h increased the IEC-18 proliferative cell fraction (P<0.05), as evidenced by analysis of the cell cycle and proliferating cell nuclear antigen expression, while apoptosis occurred only at the highest concentration of NaHS. Thirty minutes of NaHS exposure increased (P<0.05) c-Jun mRNA concentrations, consistent with the observed activation of mitogen activated protein kinases (MAPK). Microarray analysis confirmed an increase (P<0.05) in MAPK-mediated proliferative activity, likely reflecting the reduced redox environment of NaHS-treated cells. These data identify functional pathways by which H2S may initiate epithelial dysregulation and thereby contribute to UC or colorectal cancer. Thus, it becomes crucial to understand how genetic background may affect epithelial responsiveness to this bacterial-derived environmental insult.

Redox state as a central modulator of precursor cell function

In our attempts to understand how the balance between self-renewal and differentiation is regulated in dividing precursor cells, we have discovered that intracellular redox state appears to be a critical modulator of this balance in oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells. The intracellular redox state of freshly isolated progenitor cells allows prospective isolation of cells with different self-renewal characteristics, which can be further modulated in opposite directions by prooxidants and antioxidants. Redox state is itself modulated by cell-extrinsic signaling molecules that alter the balance between self-renewal and differentiation: growth factors that promote self-renewal cause progenitors to become more reduced, while exposure to signaling molecules that promote differentiation causes progenitors to become more oxidized. Moreover, pharmacological antagonists of the redox effects of these cell-extrinsic signaling molecules antagonize their effects on self-renewal and differentiation, further suggesting that cell-extrinsic signaling molecules that modulate this balance converge on redox modulation as a critical component of their effector mechanism. A further example of the potential relevance of intracellular redox state to development processes emerges from our attempts to understand why different central nervous system (CNS) regions exhibit different temporal patterns of oligodendrocyte generation and myelinogenesis. Characterization of O-2A progenitor cells (O-2A/OPCs) isolated from different regions indicates that these developmental patterns are consistent with properties of the specific O-2A/OPCs resident in each region. Marked differences were seen in self-renewal and differentiation characteristics of O-2A/OPCs isolated from cortex, optic nerve, and optic chiasm. In conditions where optic nerve-derived O-2A/OPCs generated oligodendrocytes within 2 days, oligodendrocytes arose from chiasm-derived cells after 5 days and from cortical O-2A/OPCs after only 7-10 days. These differences, which appear to be cell intrinsic, were manifested both in reduced percentages of clones producing oligodendrocytes and in a lesser representation of oligodendrocytes in individual clones. In addition, responsiveness of optic nerve-, chiasm-, and cortex-derived O-2A/OPCs to thyroid hormone (TH) and ciliary neurotrophic factor (CNTF), well-characterized inducers of oligodendrocyte generation, was inversely related to the extent of self-renewal observed in basal division conditions. These results demonstrate hitherto unrecognized complexities among the precursor cells thought to be the immediate ancestors of oligodendrocytes and suggest that the properties of these different populations may contribute to the diverse time courses of myelination in different CNS regions. Strikingly, O-2A/OPCs isolated from cortex and analyzed immediately upon isolation were more reduced in their redox state than were optic nerve-derived cells, precisely as would be predicted from our analysis of the role of redox state in modulating the balance between self-renewal and differentiation. Chiasm-derived cells, which exhibited self-renewal properties intermediate between cortex- and optic nerve-derived cells, were more reduced than optic nerve cells but more oxidized that cortical O-2A/OPCs.

Calcium-permeable AMPA/kainate receptors mediate toxicity and preconditioning by oxygen-glucose deprivation in oligodendrocyte precursors

Hypoxic-ischemic brain injury in premature infants results in cerebral white matter lesions with prominent oligodendroglial injury and loss, a disorder termed periventricular leukomalacia (PVL). We have previously shown that glutamate receptors mediate hypoxic-ischemic injury to oligodendroglial precursor cells (OPCs) in a model of PVL in the developing rodent brain. We used primary OPC cultures to examine the mechanism of cellular toxicity induced by oxygen-glucose deprivation (OGD) to simulate brain ischemia. OPCs were more sensitive to OGD-induced toxicity than mature oligodendrocytes, and OPC toxicity was attenuated by nonselective [2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[f]quinoxaline (NBQX), 6-cyano-7-nitroquinoxaline-2,3-dione], alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring (GYKI 52466), kainate-preferring (gamma-d-glutamylaminomethanesulfonic acid), or Ca2+-permeable AMPA/kainate receptor antagonists (joro spider toxin, JSTx) administered either during or after OGD. Furthermore, NBQX or JSTx blocked OGD-induced Ca2+ influx. Relevant to recurrent hypoxic-ischemic insults in developing white matter, we examined the effects of sublethal OGD preconditioning. A prior exposure of OPCs to sublethal OGD resulted in enhanced vulnerability to subsequent excitotoxic or OGD-induced injury associated with an increased Ca2+ influx. AMPA/kainate receptor blockade with NBQX or JSTx either during or after sublethal OGD prevented its priming effect. Furthermore, OGD preconditioning resulted in a down-regulation of the AMPA receptor subunit GluR2 on cell surface that increased Ca2+ permeability of the receptors. Overall, these data suggest that aberrantly enhanced activation of Ca2+-permeable AMPA/kainate receptors may be a major mechanism in acute and repeated hypoxic-ischemic injury to OPCs in disorders of developing cerebral white matter, such as PVL.

Transcription factor expression and cellular redox in immature oligodendrocyte cell death: effect of Bcl-2

Multiple sclerosis (MS) is characterized by the progressive damage or loss of oligodendrocytes. In an effort to better understand the causes of oligodendrocyte destruction in MS plaques, we treated immature oligodendrocytes with glucose oxidase, ceramide, or brefeldin A. These treatments model the different mechanisms by which oligodendrocytes are thought to die. We report that the AP-1 and Egr-1 transcription factors are induced within an hour of treatment. Of the AP-1 proteins studied, c-Jun was expressed at the highest level, followed by JunD, c-Fos, and Fra-2, although different treatments induced slightly different levels of expression. Bcl-2 overexpression protects against all treatments, to differing degrees. Although Bcl-2 did not have a dramatic effect on AP-1 or Egr-1 induction within the first 3 h, it caused a lowering of steady-state redox levels with a concomitant increase in cellular glutathione. We propose that the lowering of cellular redox and the upregulation of glutathione are responsible in part for the protective properties of Bcl-2.

Quiescent center formation in maize roots is associated with an auxin-regulated oxidizing environment

Embedded within the meristem of all Angiosperm roots is a population of slowly dividing cells designated the quiescent center (QC). In maize roots the QC can constitute upwards of 800-1200 cells, most of which spend an extended period of time (180-200 hours) in the G(1) phase of the cell cycle. How the QC forms and is maintained is not known. Here we report that cells of the QC are characterized by their highly oxidized status. Glutathione and ascorbic acid occur predominately in the oxidized forms in the QC. This is contrasted with the status of these redox intermediates in adjacent, rapidly dividing cells in the root meristem, in which the reduced forms of these two species are favored. Using a redox sensitive fluorescent dye we were able to visualize an overall oxidizing environment in the QC, and we also made comparisons with the adjacent, rapidly dividing cells in the root meristem. Altering the distribution of auxin and the location of the auxin maximum in the root tip activates the QC, and cells leave G(1) and enter mitosis. Commencement of relatively more rapid cell division in the QC is preceded by changes in the overall redox status of the QC, which becomes less oxidizing. We discuss how the position of the auxin maximum may influence the redox status of the QC and thereby modulate the cell cycle.

The cortical ancestry of oligodendrocytes: common principles and novel features

Studies on the development of cortical oligodendrocytes indicate that although general principles that apply to other parts of the CNS are applicable, there are important differences that appear to be critical to the analysis of this lineage in the cortex. Herein, we review previous studies demonstrating that oligodendrocyte-type-2 astrocyte progenitor cells (or oligodendrocyte precursor cells; aka O-2A/OPCs) of the developing postnatal cortex exhibit a striking cell-intrinsic bias towards undergoing prolonged self-renewal in the relative absence of oligodendrocyte generation [Power et al., Dev Biol 2002;245:362-375]. This phenotype is quite distinct from that observed in comparable cells isolated from the optic tract. This predilection for self-renewal is associated with a lessened response to inducers of oligodendrocyte generation and of possible mechanistic importance in regards to these other properties. We also review studies on stem/progenitor cells isolated from the embryonic cortex that are able to generate oligodendrocytes. As for the studies on O-2A/OPCs, important differences also distinguish these early cells from those studied in other CNS regions in their response to signaling molecules and expression of the Dlx family of transcriptional regulators [He et al., J Neurosci 2001;21:8854-8862; Yung et al., Proc Natl Acad Sci USA 2002;99:16273-16278]. We also present new data on clonal analysis of A2B5+ precursor cells isolated from the E13.5 cortex, demonstrating that this tissue appears to contain a cell similar in properties to the tripotential glial-restricted precursor cell that has been isolated from embryonic spinal cord [Rao et al., Proc Natl Acad Sci USA 1998;95:3996-4001]. Moreover, the A2B5+ precursor cells isolated from embryonic cortex are much more heterogeneous than is seen in the spinal cord at this age, even to the point of including an A2B5/PSA-NCAM double-positive cell that can generate neurons.

Oligodendroglia in developmental neurotoxicity

The developing nervous system has been long recognized as a primary target for a variety of toxicants. To date, most efforts to understand the impact of neurotoxic agents on the brain have focused primarily on neurons and to a lesser degree astroglia as cellular targets. The role of oligodendroglia, the myelin-forming cells in the central nervous system (CNS), in developmental neurotoxicity has been emphasized only in recent years. Oligodendrocytes originate from migratory, mitotic progenitors that mature progressively into postmitotic myelinating cells. During differentiation, oligodendroglial lineage cells pass through a series of distinct phenotypic stages that are characterized by different proliferative capacities and migratory abilities, as well as dramatic changes in morphology with sequential expression of unique developmental markers. In recent years, it has become appreciated that oligodendrocyte lineage cells have important functions other than those related to myelin formation and maintenance, including participation in neuronal survival and development, as well as neurotransmission and synaptic function. Substantial knowledge has accumulated on the control of oligodendroglial survival, migration, proliferation, and differentiation, as well as the cellular and molecular events involved in oligodendroglial development and myelin formation. Recently, studies have been initiated to address the role of oligodendrocyte lineage cells in neurotoxic processes. This article examines recent progress in oligodendroglial biology, focuses attention on the characteristic features of the oligodendrocyte developmental lineage as a model system for neurotoxicological studies, and explores the role of oligodendrocyte lineage cells in developmental neurotoxicity. The potential role of oligodendroglia in environmental lead neurotoxicity is presented to exemplify this thesis.

Aging-related changes in the thiol/disulfide redox state: implications for the use of thiol antioxidants

Genetic and biochemical studies suggest that free radical-derived reactive oxygen species play a key role in a common mechanism of aging in many or all animal species. This led to the hypothesis that the quality of life in old age may be improved by pharmacological or dietary thiol antioxidants. This review describes important details about how the organism deals with its own thiol antioxidants. Aging was found to be associated with an oxidative shift in the thiol/disulfide redox state (REDST) of the intracellular glutathione pool and of the plasma cyst(e)ine and albumin pools. There is also a decrease in plasma thiol (mainly cysteine) concentration. The oxidative shift in intracellular REDST was found to be typically associated with cellular dysfunctions. Studies in humans related to plasma REDST revealed correlations with aging-related pathophysiological processes, suggesting that oxidative changes in REDST play a key role in processes and diseases which limit the human life span. The age-related shift in plasma REDST is mediated, at least partly, by the decreasing capacity to remove dietary cysteine from the oxidative environment of the blood. Thiol antioxidants were found to ameliorate various aging-related processes but obviously ought to be used with caution in consideration of the oxidative environment of the blood.

Regulation of oligodendrocyte development in the vertebrate CNS

The vertebrate central nervous system (CNS) contains two major classes of macroglial cells, oligodendrocytes and astrocytes. Oligodendrocytes are responsible for the formation of myelin in the central nervous system, while the functions of astrocytes are more diverse and less well established. Recent studies have provided new insights into when, where and how these different classes of cell arise during CNS development. The founder cells of the oligodendrocyte lineage initially arise in distinct regions of the ventricular zone during early development as the result of local signals including sonic hedgehog. In the spinal cord, oligodendrocyte precursors appear to share a developmental lineage with motor neurons, although they may also develop from restricted glial precursors. Immature oligodendrocyte precursors are highly migratory. They migrate from their site of origin to developing white matter tracts using a variety of guidance cues including diffusible chemorepellents. The majority of oligodendrocyte precursor proliferation occurs in developing white matter as a result of the local expression of mitogenic signals. Oligodendrocyte precursor cell proliferation is regulated by a number of distinct growth factors that act at distinct stages in the lineage and whose activity is modulated by synergy with other molecules including chemokines. The final matching of oligodendrocyte and axon number is accomplished through a combination of local regulation of cell proliferation, differentiation and cell death. Not all oligodendrocyte precursors differentiate during development, and the adult CNS contains a significant population of precursors. Understanding the regulation of oligodendrogenesis will facilitate the use of these endogenous precursors to enhance repair in a variety of pathological conditions.

Integrins engage mitochondrial function for signal transduction by a mechanism dependent on Rho GTPases

We show here the transient activation of the small GTPase Rac, followed by a rise in reactive oxygen species (ROS), as necessary early steps in a signal transduction cascade that lead to NFkappaB activation and collagenase-1 (CL-1)/matrix metalloproteinase-1 production after integrin-mediated cell shape changes. We show evidence indicating that this constitutes a new mechanism for ROS production mediated by small GTPases. Activated RhoA also induced ROS production and up-regulated CL-1 expression. A Rac mutant (L37) that prevents reorganization of the actin cytoskeleton prevented integrin-induced CL-1 expression, whereas mutations that abrogate Rac binding to the neutrophil NADPH membrane oxidase in vitro (H26 and N130) did not. Instead, ROS were produced by integrin-induced changes in mitochondrial function, which were inhibited by Bcl-2 and involved transient membrane potential loss. The cells showing this transient decrease in mitochondrial membrane potential were already committed to CL-1 expression. These results unveil a new molecular mechanism of signal transduction triggered by integrin engagement where a global mitochondrial metabolic response leads to gene expression rather than apoptosis.

Apoptosis of oligodendrocytes in secondary cultures from neonatal rat brains

The plaques in multiple sclerosis (MS) autopsy tissue contain tumor necrosis factor-alpha (TNF-alpha) at high concentrations. Moreover, microglia are able to convert L-tryptophan to quinolinic acid. Thus, TNF-alpha and quinolinic acid are endogenous compounds which may compromise oligodendrocytes during inflammatory demyelination. It is also known that cellular functions depend on adequate concentrations of glutathione (GSH). As some apoptotic oligodendrocytes have been observed in MS plaques, it was therefore logical to determine whether oligodendrocyte apoptosis would occur in response to TNF-alpha, quinolinic acid or GSH depletion. Oligodendrocytes were treated in vitro with TNF-alpha, quinolinic acid and the GSH-depleting agent, buthionine sulfoximine (BSO), respectively, and the numbers of intact and apoptotic cells were counted. TNF-alpha reduced the numbers of mature oligodendrocytes, but not immature oligodendrocytes, without producing apoptosis. Quinolinic acid and BSO each caused oligodendrocyte loss via apoptosis, and GSH ethyl ester partly protected the cells against BSO. The data suggest that oligodendrocytes undergo apoptosis under adverse conditions that result from an endogenous toxicant or depletion of GSH.

Oligodendrocyte precursor cells from different brain regions express divergent properties consistent with the differing time courses of myelination in these regions

Different CNS regions exhibit different temporal patterns of oligodendrocyte generation and myelinogenesis. Characterization of oligodendrocyte-type-2 astrocyte progenitor cells (here abbreviated as O-2A/OPCs) isolated from different regions indicates these developmental patterns are consistent with properties of the specific O-2A/OPCs resident in each region. Marked differences were seen in self-renewal and differentiation characteristics of O-2A/OPCs isolated from cortex, optic nerve and optic chiasm. In conditions where optic nerve-derived O-2A/OPCs generated oligodendrocytes within 2 days, oligodendrocytes arose from chiasm-derived cells after 5 days and from cortical O-2A/OPCs only after 7-10 days. These differences, which appear to be cell-intrinsic (and may be related to intracellular redox state), were manifested both in reduced percentages of clones producing oligodendrocytes and in a lesser representation of oligodendrocytes in individual clones. In addition, responsiveness of optic nerve-, chiasm- and cortex-derived O-2A/OPCs to thyroid hormone (TH) and ciliary neurotrophic factor (CNTF), well-characterized inducers of oligodendrocyte generation, was inversely related to the extent of self-renewal observed in basal division conditions. Our results demonstrate hitherto unrecognized complexities among the precursor cells thought to be the immediate ancestors of oligodendrocytes, and suggest that the properties of these different populations may contribute to the diverse time courses of myelination in different CNS regions.

Human aging and global function of coenzyme Q10

In this paper, we review two parts of our recent work on human skeletal muscle. The first part mainly describes changes occurring during aging, whereas the second part discusses the functions of coenzyme Q10 (CoQ10), particularly in relation to the aging process. During the lifetime of an individual, mtDNA undergoes a variety of mutation events and rearrangements. These mutations and their consequent bioenergenic decline, together with nuclear DNA damage, contribute to the reduced function of cells and organs, especially in postmitotic tissues. In skeletal muscle, this functional decline can be observed by means of changes with age in fiber type profile and the reduction in the number and size of the muscle fibers. In addition to the functions of coenzyme Q10 as an electron carrier in the respiratory chain and as an antioxidant, CoQ10 has been shown to regulate global gene expression in skeletal muscle. We hypothesize that this regulation is achieved via superoxide formation with H2O2 as a second messenger to the nucleus.

The tripotential glial-restricted precursor (GRP) cell and glial development in the spinal cord: generation of bipotential oligodendrocyte-type-2 astrocyte progenitor cells and dorsal-ventral differences in GRP cell function

We have found that the tripotential glial-restricted precursor (GRP) cell of the embryonic rat spinal cord can give rise in vitro to bipotential cells that express defining characteristics of oligodendrocyte-type-2 astrocyte progenitor cells (O2A/OPCs). Generation of O2A/OPCs is regulated by environmental signals and is promoted by platelet-derived growth factor (PDGF), thyroid hormone (TH) and astrocyte-conditioned medium. In contrast to multiple observations indicating that oligodendrocyte precursor cells in the embryonic day 14 (E14) spinal cord are ventrally restricted, GRP cells are already present in both the dorsal and ventral spinal cord at E13.5. Ventral-derived GRP cells, however, were more likely to generate O2A/OPCs and/or oligodendrocytes than were their dorsal counterparts when exposed to TH, PDGF, or even bone morphogenetic protein-4. The simplest explanation of our results is that oligodendrocyte generation occurs as a result of generation of GRP cells from totipotent neuroepithelial stem cells, of O2A/OPCs from GRP cells and, finally, of oligodendrocytes from O2A/OPCs. In this respect, the responsiveness of GRP cells to modulators of this process may represent a central control point in the initiation of this critical developmental sequence. Our findings provide an integration between the earliest known glial precursors and the well-studied O2A/OPCs while opening up new questions concerning the intricate spatial and temporal regulation of precursor cell differentiation in the CNS.

Redox control of the transsulfuration and glutathione biosynthesis pathways

Intracellular reduction-oxidation status is increasingly recognized as a primary regulator of cellular growth and development. The relative reduction-oxidation state of the cell depends primarily on the precise balance between concentrations of reactive oxygen species and the cysteine-dependent antioxidant thiol buffers glutathione and thioredoxin, which by preferentially reacting with reactive oxygen species, protect other intracellular molecules from oxidative damage. The transsulfuration pathway constitutes the major route of cysteine biosynthesis, and may thus be central in controlling the intracellular reduction-oxidation state and the balance between self-renewal and differentiation programs. This review discusses new findings on reciprocal reduction-oxidation modulation of enzymes involved in the transsulfuration and glutathione biosynthesis pathways, as well as studies elucidating the impact of sulfur amino acid availability on these pathways.

Free radicals in the physiological control of cell function

At high concentrations, free radicals and radical-derived, nonradical reactive species are hazardous for living organisms and damage all major cellular constituents. At moderate concentrations, however, nitric oxide (NO), superoxide anion, and related reactive oxygen species (ROS) play an important role as regulatory mediators in signaling processes. Many of the ROS-mediated responses actually protect the cells against oxidative stress and reestablish "redox homeostasis." Higher organisms, however, have evolved the use of NO and ROS also as signaling molecules for other physiological functions. These include regulation of vascular tone, monitoring of oxygen tension in the control of ventilation and erythropoietin production, and signal transduction from membrane receptors in various physiological processes. NO and ROS are typically generated in these cases by tightly regulated enzymes such as NO synthase (NOS) and NAD(P)H oxidase isoforms, respectively. In a given signaling protein, oxidative attack induces either a loss of function, a gain of function, or a switch to a different function. Excessive amounts of ROS may arise either from excessive stimulation of NAD(P)H oxidases or from less well-regulated sources such as the mitochondrial electron-transport chain. In mitochondria, ROS are generated as undesirable side products of the oxidative energy metabolism. An excessive and/or sustained increase in ROS production has been implicated in the pathogenesis of cancer, diabetes mellitus, atherosclerosis, neurodegenerative diseases, rheumatoid arthritis, ischemia/reperfusion injury, obstructive sleep apnea, and other diseases. In addition, free radicals have been implicated in the mechanism of senescence. That the process of aging may result, at least in part, from radical-mediated oxidative damage was proposed more than 40 years ago by Harman (J Gerontol 11: 298-300, 1956). There is growing evidence that aging involves, in addition, progressive changes in free radical-mediated regulatory processes that result in altered gene expression.

The tripotential glial-restricted precursor (GRP) cell and glial development in the spinal cord: generation of bipotential oligodendrocyte-type-2 astrocyte progenitor cells and dorsal-ventral differences in GRP cell function

We have found that the tripotential glial-restricted precursor (GRP) cell of the embryonic rat spinal cord can give rise in vitro to bipotential cells that express defining characteristics of oligodendrocyte-type-2 astrocyte progenitor cells (O2A/OPCs). Generation of O2A/OPCs is regulated by environmental signals and is promoted by platelet-derived growth factor (PDGF), thyroid hormone (TH) and astrocyte-conditioned medium. In contrast to multiple observations indicating that oligodendrocyte precursor cells in the embryonic day 14 (E14) spinal cord are ventrally restricted, GRP cells are already present in both the dorsal and ventral spinal cord at E13.5. Ventral-derived GRP cells, however, were more likely to generate O2A/OPCs and/or oligodendrocytes than were their dorsal counterparts when exposed to TH, PDGF, or even bone morphogenetic protein-4. The simplest explanation of our results is that oligodendrocyte generation occurs as a result of generation of GRP cells from totipotent neuroepithelial stem cells, of O2A/OPCs from GRP cells and, finally, of oligodendrocytes from O2A/OPCs. In this respect, the responsiveness of GRP cells to modulators of this process may represent a central control point in the initiation of this critical developmental sequence. Our findings provide an integration between the earliest known glial precursors and the well-studied O2A/OPCs while opening up new questions concerning the intricate spatial and temporal regulation of precursor cell differentiation in the CNS.

Identification of a discrete population of human bone marrow-derived mesenchymal cells exhibiting properties of uncommitted progenitors

Human bone marrow-derived mesenchymal progenitor cells (MPC) after ex vivo expansion give rise to a heterogeneous mixture of cells with distinct proliferative potential at various stages of differentiation. Here we show that when proliferative MPC were forced to metabolic death by exposure to 5-fluorouracil, the remaining subset (5-20%) contains a population of quiescent, uncommitted, and undifferentiated mesenchymal cells. The isolated cells self-renew and generate precursors committed at least to the adipogenic and osteogenic lineages. Taken together, these results demonstrate that within ex vivo-expanded bone marrow-derived MPC, there exist a discrete population of mesenchymal cells with properties of uncommitted progenitors. Because these cells are capable of engraftment into bone marrow, spleen, bone, and skeletal muscle after intravenous infusion and can be efficiently transduced with adenoviral vectors, they may represent an interesting option for cellular and gene therapies for a wide range of disorders of mesenchymal tissues.

Antioxidants N-acetylcysteine (NAC) and 2-mercaptoethanol (2-ME) affect the survival and differentiative potential of cholinergic precursors from the embryonic septal nuclei and basal forebrain: involvement of ras signaling

We investigated the effects of antioxidants N-acetylcysteine (NAC) and 2-mercaptoethanol (2-ME) on the expression of choline acetyltransferase (ChAT) in cultured cholinergic precursors from the embryonic rat septal nuclei and basal forebrain. Carboxy-dichlorofluorescein fluorescence confirmed that 2-ME inhibited intracellular oxidation. Low micromolar concentrations of 2-ME produce as much as a 12-fold increase in ChAT; this is enhanced further by inclusion of nerve growth factor (NGF). NAC effects are biphasic: 0.15 mM produces profound increases in ChAT while 1.5 mM has no effect. Immature (E16) cultures respond with increases in ChAT while more highly differentiated cultures (E18) do not. Labeling of single precursors with a lacZ-expressing retrovirus reveals that the increase in ChAT is due primarily to an increased number and size of clones, not an increase in cholinergic neurons per clone, suggesting an effect on precursor survival. Inhibition of ras farnesylation inhibits both 2-ME and NAC induction of ChAT suggesting a ras-mediated pathway. Inclusion of the MEK inhibitor PD98059 does not affect low doses of NAC, but at doses of NAC that fail to increase ChAT activity, inhibition of the pathway actually raises ChAT. Immunocytochemical investigation of the cultures indicates that cells exposed to low doses of NAC develop healthy neuronal arbors in the apparent absence of glial support. At higher concentrations of NAC, neurons were found in association with astrocytes, making contact via elaborate varicose fibers. Treatment of the cultures with PD98059 to inhibit MEK returned cultures to a 'low-dose' phenotype. These data suggest that redox status of basal forebrain precursors affect both their survival and differentiative potential.

Reprogramming of the macrophage transcriptome in response to interferon-gamma and Mycobacterium tuberculosis: signaling roles of nitric oxide synthase-2 and phagocyte oxidase

Macrophage activation determines the outcome of infection by Mycobacterium tuberculosis (Mtb). Interferon-gamma (IFN-gamma) activates macrophages by driving Janus tyrosine kinase (JAK)/signal transducer and activator of transcription-dependent induction of transcription and PKR-dependent suppression of translation. Microarray-based experiments reported here enlarge this picture. Exposure to IFN-gamma and/or Mtb led to altered expression of 25% of the monitored genome in macrophages. The number of genes suppressed by IFN-gamma exceeded the number of genes induced, and much of the suppression was transcriptional. Five times as many genes related to immunity and inflammation were induced than suppressed. Mtb mimicked or synergized with IFN-gamma more than antagonized its actions. Phagocytosis of nonviable Mtb or polystyrene beads affected many genes, but the transcriptional signature of macrophages infected with viable Mtb was distinct. Studies involving macrophages deficient in inducible nitric oxide synthase and/or phagocyte oxidase revealed that these two antimicrobial enzymes help orchestrate the profound transcriptional remodeling that underlies macrophage activation.

Induction of mild intracellular redox imbalance inhibits proliferation of CaCo-2 cells

Intracellular redox status plays a critical role in cell function, such as proliferation. Oxidative stress, which elicits redox imbalance, also affects cell growth. Therefore, it is often difficult to distinguish the effects of redox imbalance from those of oxidative stress. The objective of this study was to determine the role of redox imbalance independent of reactive oxygen species (ROS) production, in proliferation of human colonic CaCo-2 cells. Low concentrations of diamide plus 1,3-bis(2 chloroethyl)-1-nitrosourea (BCNU) increased intracellular GSSG and decreased GSH and the GSH:GSSG ratio. These changes occurred within 30 min, which preceded a decrease in thymidine incorporation at 6 and 24 h. ROS formation was not detected under these conditions. This suppression of cell proliferative activity was attenuated by N-acetyl cysteine, in parallel with restoration of the intracellular GSH redox status. dl-buthionine-[S, R]-sulfoximine (BSO) decreased intracellular GSH level, but did not change the GSH:GSSG ratio. BSO alone had no effect on cell proliferation, but its presence exaggerated the suppressive effect of diamide plus BCNU. Flow cytometric analysis showed that cells were arrested at G1-to-S transition and G2/M phase. Collectively, this study shows that mild intracellular redox imbalance inhibited cell proliferation independent of ROS generation. Moreover, cells with compromised cellular GSH were susceptible to redox imbalance-induced inhibition of proliferation.