Hydrogen sulfide: An endogenous regulator of the immune system

Hydrogen sulfide (H₂S) is now recognized as an endogenous signaling gasotransmitter in mammals. It is produced by mammalian cells and tissues by various enzymes - predominantly cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3-MST) - but part of the H₂S is produced by the intestinal microbiota (colonic H₂S-producing bacteria). Here we summarize the available information on the production and functional role of H₂S in the various cell types typically associated with innate immunity (neutrophils, macrophages, dendritic cells, natural killer cells, mast cells, basophils, eosinophils) and adaptive immunity (T and B lymphocytes) under normal conditions and as it relates to the development of various inflammatory and immune diseases. Special attention is paid to the physiological and the pathophysiological aspects of the oral cavity and the colon, where the immune cells and the parenchymal cells are exposed to a special "H₂S environment" due to bacterial H₂S production. H₂S has many cellular and molecular targets. Immune cells are "surrounded" by a "cloud" of H₂S, as a result of endogenous H₂S production and exogenous production from the surrounding parenchymal cells, which, in turn, importantly regulates their viability and function. Downregulation of endogenous H₂S producing enzymes in various diseases, or genetic defects in H₂S biosynthetic enzyme systems either lead to the development of spontaneous autoimmune disease or accelerate the onset and worsen the severity of various immune-mediated diseases (e.g. autoimmune rheumatoid arthritis or asthma). Low, regulated amounts of H₂S, when therapeutically delivered by small molecule donors, improve the function of various immune cells, and protect them against dysfunction induced by various noxious stimuli (e.g. reactive oxygen species or oxidized LDL). These effects of H₂S contribute to the maintenance of immune functions, can stimulate antimicrobial defenses and can exert anti-inflammatory therapeutic effects in various diseases.

The PARP inhibitor olaparib exerts beneficial effects in mice subjected to cecal ligature and puncture and in cells subjected to oxidative stress without impairing DNA integrity: A potential opportunity for repurposing a clinically used oncological drug for the experimental therapy of sepsis

Poly(ADP-ribose) polymerase (PARP) is involved in the pathogenesis of cell dysfunction, inflammation and organ failure during septic shock. The goal of the current study was to investigate the efficacy and safety of the clinically approved PARP inhibitor olaparib in experimental models of oxidative stress in vitro and in sepsis in vivo. In mice subjected to cecal ligation and puncture (CLP) organ injury markers, circulating and splenic immune cell distributions, circulating mediators, DNA integrity and survival was measured. In U937 cells subjected to oxidative stress, cellular bioenergetics, viability and DNA integrity were measured. Olaparib was used to inhibit PARP. The results show that in adult male mice subjected to CLP, olaparib (1-10 mg/kg i.p.) improved multiorgan dysfunction. Olaparib treatment reduced the degree of bacterial CFUs. Olaparib attenuated the increases in the levels of several circulating mediators in the plasma. In the spleen, the number of CD4+ and CD8+ lymphocytes were reduced in response to CLP; this reduction was inhibited by olaparib treatment. Treg but not Th17 lymphocytes increased in response to CLP; these cell populations were reduced in sepsis when the animals received olaparib. The Th17/Treg ratio was lower in CLP-olaparib group than in the CLP control group. Analysis of miRNA expression identified a multitude of changes in spleen and circulating white blood cell miRNA levels after CLP; olaparib treatment selectively modulated these responses. Olaparib extended the survival rate of mice subjected to CLP. In contrast to males, in female mice olaparib did not have significant protective effects in CLP. In aged mice olaparib exerted beneficial effects that were less pronounced than the effects obtained in young adult males. In in vitro experiments in U937 cells subjected to oxidative stress, olaparib (1-100 μM) inhibited PARP activity, protected against the loss of cell viability, preserved NAD⁺ levels and improved cellular bioenergetics. In none of the in vivo or in vitro experiments did we observe any adverse effects of olaparib on nuclear or mitochondrial DNA integrity. In conclusion, olaparib improves organ function and extends survival in septic shock. Repurposing and eventual clinical introduction of this clinically approved PARP inhibitor may be warranted for the experimental therapy of septic shock.

Human and Mouse Brown Adipose Tissue Mitochondria Have Comparable UCP1 Function

Brown adipose tissue (BAT) plays an important role in mammalian thermoregulation. The component of BAT mitochondria that permits this function is the inner membrane carrier protein uncoupling protein 1 (UCP1). To the best of our knowledge, no studies have directly quantified UCP1 function in human BAT. Further, whether human and rodent BAT have comparable thermogenic function remains unknown. We employed high-resolution respirometry to determine the respiratory capacity, coupling control, and, most importantly, UCP1 function of human supraclavicular BAT and rodent interscapular BAT. Human BAT was sensitive to the purine nucleotide GDP, providing the first direct evidence that human BAT mitochondria have thermogenically functional UCP1. Further, our data demonstrate that human and rodent BAT have similar UCP1 function per mitochondrion. These data indicate that human and rodent BAT are qualitatively similar in terms of UCP1 function.

Time-Dependent and Organ-Specific Changes in Mitochondrial Function, Mitochondrial DNA Integrity, Oxidative Stress and Mononuclear Cell Infiltration in a Mouse Model of Burn Injury

Severe thermal injury induces a pathophysiological response that affects most of the organs within the body; liver, heart, lung, skeletal muscle among others, with inflammation and hyper-metabolism as a hallmark of the post-burn damage. Oxidative stress has been implicated as a key component in development of inflammatory and metabolic responses induced by burn. The goal of the current study was to evaluate several critical mitochondrial functions in a mouse model of severe burn injury. Mitochondrial bioenergetics, measured by Extracellular Flux Analyzer, showed a time dependent, post-burn decrease in basal respiration and ATP-turnover but enhanced maximal respiratory capacity in mitochondria isolated from the liver and lung of animals subjected to burn injury. Moreover, we detected a tissue-specific degree of DNA damage, particularly of the mitochondrial DNA, with the most profound effect detected in lungs and hearts of mice subjected to burn injury. Increased mitochondrial biogenesis in lung tissue in response to burn injury was also observed. Burn injury also induced time dependent increases in oxidative stress (measured by amount of malondialdehyde) and neutrophil infiltration (measured by myeloperoxidase activity), particularly in lung and heart. Tissue mononuclear cell infiltration was also confirmed by immunohistochemistry. The amount of poly(ADP-ribose) polymers decreased in the liver, but increased in the heart in later time points after burn. All of these biochemical changes were also associated with histological alterations in all three organs studied. Finally, we detected a significant increase in mitochondrial DNA fragments circulating in the blood immediately post-burn. There was no evidence of systemic bacteremia, or the presence of bacterial DNA fragments at any time after burn injury. The majority of the measured parameters demonstrated a sustained elevation even at 20–40 days post injury suggesting a long-lasting effect of thermal injury on organ function. The current data show that there are marked time-dependent and tissue-specific alterations in mitochondrial function induced by thermal injury, and suggest that mitochondria-specific damage is one of the earliest responses to burn injury. Mitochondria may be potential therapeutic targets in the future experimental therapy of burns.

Differential acute and chronic effects of burn trauma on murine skeletal muscle bioenergetics

Altered skeletal muscle mitochondrial function contributes to the pathophysiological stress response to burns. However, the acute and chronic impact of burn trauma on skeletal muscle bioenergetics remains poorly understood. Here, we determined the temporal relationship between burn trauma and mitochondrial function in murine skeletal muscle local to and distal from burn wounds. Male BALB/c mice (8-10 weeks old) were burned by submersion of the dorsum in water (∼ 95 °C) to create a full thickness burn on ∼ 30% of the body. Skeletal muscle was harvested spinotrapezius underneath burn wounds (local) and the quadriceps (distal) of sham and burn treated mice at 3h, 24h, 4d and 10d post-injury. Mitochondrial respiration was determined in permeabilized myofiber bundles by high-resolution respirometry. Caspase 9 and caspase 3 protein concentration were determined by western blot. In muscle local to burn wounds, respiration coupled to ATP production was significantly diminished at 3h and 24h post-injury (P<0.001), as was mitochondrial coupling control (P<0.001). There was a 5- (P<0.05) and 8-fold (P<0.001) increase in respiration in response to cytochrome at 3h and 24h post burn, respectively, indicating damage to the outer mitochondrial membranes. Moreover, we also observed greater active caspase 9 and caspase 3 in muscle local to burn wounds, indicating the induction of apoptosis. Distal muscle mitochondrial function was unaltered by burn trauma until 10d post burn, where both respiratory capacity (P<0.05) and coupling control (P<0.05) were significantly lower than sham. These data highlight a differential response in muscle mitochondrial function to burn trauma, where the timing, degree and mode of dysfunction are dependent on whether the muscle is local or distal to the burn wound.

Browning of Subcutaneous White Adipose Tissue in Humans after Severe Adrenergic Stress

Since the presence of brown adipose tissue (BAT) was confirmed in adult humans, BAT has become a therapeutic target for obesity and insulin resistance. We examined whether human subcutaneous white adipose tissue (sWAT) can adopt a BAT-like phenotype using a clinical model of prolonged and severe adrenergic stress. sWAT samples were collected from severely burned and healthy individuals. A subset of burn victims were prospectively followed during their acute hospitalization. Browning of sWAT was determined by the presence of multilocular adipocytes, uncoupling protein 1 (UCP1), and increased mitochondrial density and respiratory capacity. Multilocular UCP1-positive adipocytes were found in sWAT samples from burn patients. UCP1 mRNA, mitochondrial density, and leak respiratory capacity in sWAT increased after burn trauma. Our data demonstrate that human sWAT can transform from an energy-storing to an energy-dissipating tissue, which opens new research avenues in our quest to prevent and treat obesity and its metabolic complications.

Dendritic cell modification of neutrophil responses to infection after burn injury

Burn patients are highly susceptible to infections due to increased exposure through wounds and impairments in a number of immune functions. Dendritic cells (DCs) are important in activation of numerous immune responses that are essential for the clearance of infections. We have found that prophylactic treatment of burn-injured mice with the DC growth factor FLT3 ligand (FL) significantly increases resistance to burn wound infections in a DC-dependent manner that is correlated closely with enhanced bacterial clearance. However, as DCs are not typically microbicidal, the mechanisms by which DC modulation enhances bacterial clearance are not known. Due to the rapid response of neutrophils to cutaneous wounds, and the reported interactions between DCs and neutrophils, we investigated the role of neutrophils in FL-mediated resistance to burn wound infection. This was examined both in vivo and in vitro through neutrophil depletion, supplementation of neutrophils, and assessment of neutrophil chemotaxis following FL treatment. To test the involvement of DCs, CD11c-diphtheria toxin receptor transgenic mice were used to deplete DCs during FL treatment. Studies revealed that neutrophils do play a critical role in FL-mediated resistance to a burn wound infection. Additionally, treatment with FL after a burn injury enhances neutrophil-mediated control of bacterial spread, neutrophil migratory capacity, and myeloperoxidase production in a DC-dependent manner. The results of this study provide new insight into immunological mechanisms that can offer protection against infection after burn injury.

The role of interferon-γ in the pathogenesis of acute intra-abdominal sepsis

Several studies indicate that IFN-γ facilitates systemic inflammation during endotoxin-induced shock. However, the pathobiology of IFN-γ in clinically relevant models of septic shock, such as CLP, is not well understood. In this study, the role of IFN-γ in the pathogenesis of CLP-induced septic shock was evaluated by examining IFN-γ production at the tissue and cellular levels. The impact of IFN-γ neutralization on systemic inflammation, bacterial clearance, and survival was also determined. Following CLP, concentrations of IFN-γ in plasma and peritoneal lavage fluid were low in comparison with concentrations of IL-6 and MIP-2, as was IFN-γ mRNA expression in liver and spleen. The overall percentage of IFN-γ+ splenocytes was <5% after CLP and not statistically different from control mice. Intracellular IFN-γ was present in a large proportion of peritoneal exudate cells after CLP, primarily in infiltrating myeloid cells and NK cells. i.p. myeloid cell activation was decreased in IFN-γKO mice, and plasma concentrations of IL-6 and MIP-2 were significantly lower in IFN-γKO mice and in mice treated with anti-IFN-γ compared with controls, but bacterial clearance was not affected. IFN-γKO mice were resistant to CLP-induced mortality when treated with systemic antibiotics. However, neutralization of IFN-γ with blocking antibodies did not improve survival significantly. These studies show that IFN-γ facilitates the proinflammatory response during CLP-induced septic shock. However, neutralization of IFN-γ did not improve survival uniformly.

Endogenous Fms-like tyrosine kinase-3 ligand levels are not altered in mice after a severe burn and infection

Background

Fms-like tyrosine kinase-3 ligand (Flt3L) is a hemopoietic cytokine and dendritic cell (DC) growth factor that promotes the proliferation and differentiation of progenitor cells into DCs. We have previously found that treatment of severely burned mice with recombinant Flt3L significantly enhances DC production and bacterial clearance from infected burn wounds, and increases global immune cell activation and survival in response to a burn wound infection. These significant benefits of Flt3L treatment after burn injury have prompted the question of whether or not severe burn injury induces deficits in endogenous Flt3L levels that could affect DCs and subsequent responses to infection.

Results

To address this, male BALB/c mice received a 30% total body surface area scald burn. Blood, spleens, and wound-draining lymph nodes were harvested at various time-points after injury. Some mice received a wound inoculation with P. aeruginosa. Murine Flt3L and G-CSF levels were measured by ELISA. Burn injury had no significant effect on Flt3L levels at any post-burn time-point examined compared to normal Flt3L levels in the sera, spleen, or lymph nodes. Additionally, Flt3L levels in the sera, spleen, and lymph nodes were not significantly altered when wounds were inoculated on the day of burn injury or at post-burn time points examined. Alternatively, levels of G-CSF were increased in response to burn injury and burn wound infection. Additionally, DC numbers and functions were not altered following burn injury alone. There was no significant difference between the number of DCs in the spleens of sham-injured mice and mice at 5 days after burn injury. When naïve T cells from sham-injured mice were co-cultured with DCs from either sham- or burn-injured mice, IFN-γ production was similar, however, IFN-γ levels produced by T cells harvested from burn-injured mice were significantly lower than those produced by T cells from sham mice, regardless of which DC group, sham or burn, was used in the coculture.

Conclusion

These data suggest that the beneficial effects of Flt3L treatments after burn injury are not due to correction of a burn-associated Flt3L deficiency but rather, are likely due to supplementary stimulation of DC production and immune responses to infection.

Prophylactic treatment with fms-like tyrosine kinase-3 ligand after burn injury enhances global immune responses to infection

Severely burned patients are susceptible to infections with opportunistic organisms due to altered immune responses and frequent wound contamination. Immunomodulation to enhance systemic and local responses to wound infections may be protective after burn injury. We previously demonstrated that pretreatments with fms-like tyrosine kinase-3 (Flt3) ligand (Flt3L), a dendritic cell growth factor, increase the resistance of mice to a subsequent burn injury and wound infection by a dendritic cell-dependent mechanism. This study was designed to test the hypothesis that Flt3L administration after burn injury decreases susceptibility to wound infections by enhancing global immune cell activation. Mice were treated with Flt3L after burn injury and examined for survival, wound and systemic bacterial clearance, and immune cell activation after wound inoculation with Pseudomonas aeruginosa. To gain insight into the local effects of Flt3L at the burn wound, localization of Langerhans cells was examined. Mice treated with Flt3L had significantly greater numbers of CD25-expressing T cells and CD69-expressing T and B cells, neutrophils, and macrophages after, but not before, infection. Overall leukocyte apoptosis in response to infection was decreased with Flt3L treatment. Survival and local and systemic bacterial clearance were enhanced by Flt3L. Langerhans cells appeared in the dermis of skin bordering the burn wound, and further increased in response to wound infection. Flt3L augmented the appearance of Langerhans cells in response to both injury and infection. These data suggest that dendritic cell enhancement by Flt3L treatments after burn injury protects against opportunistic infections through promotion of local and systemic immune responses to infection.

Mice depleted of alphabeta but not gammadelta T cells are resistant to mortality caused by cecal ligation and puncture

The present study was undertaken to determine whether the mice depleted of alphabeta or gammadelta T cells show resistance to acute polymicrobial sepsis caused by cecal ligation and puncture (CLP). T-cell receptor beta knockout (betaTCRKO) and T-cell receptor delta knockout (deltaTCRKO) mice were used. An additional group of mice was treated with an antibody against the alphabeta T-cell receptor to induce alphabeta T-cell depletion; a subset of alphabeta T cell-deficient mice was also treated with anti-asialoGM1 to deplete natural killer (NK) cells. The mice underwent CLP and were monitored for survival, temperature, acid-base balance, bacterial counts, and cytokine production. The betaTCRKO mice and the wild-type mice treated with anti-beta T-cell receptor (anti-TCRbeta) antibody showed improved survival after CLP compared with wild-type mice. The treatment of alphabeta T cell-deficient mice with anti-asialoGM1further improved survival after CLP, especially when the mice were treated with imipenem. The improved survival observed in alphabeta T cell-deficient mice was associated with less hypothermia, improved acid-base balance, and decreased production of the proinflammatory cytokines interleukin (IL) 6 and macrophage inflammatory protein (MIP) 2. Compared with wild-type controls, the overall survival was not improved in deltaTCRKO mice. The concentrations of IL-6 and MIP-2 in plasma and cytokine mRNA expression in tissues were not significantly different between wild-type and deltaTCRKO mice. These studies indicate that mice depleted of alphabeta but not of gammadelta T cells are resistant to mortality in an acutely lethal model of CLP. The depletion of NK cells caused further survival benefit in alphabeta T cell-deficient mice. These findings suggest that alphabeta T and NK cells mediate or facilitate CLP-induced inflammatory injury.

Effect of Transforming Growth Factor-β Neutralization on Survival and Bacterial Clearance in a Murine Model of Pseudomonas aeruginosa Burn Wound Infection

Transforming growth factor-beta (TGF-beta), a cytokine with anti-inflammatory properties, may contribute to postburn immunosuppression. This study was designed to determine whether neutralizing TGF-beta in burned mice could improve resistance to infection. C57BL/6J mice received a 35% TBSA flame burn under isoflurane anesthesia. Four days after injury, mice were treated with TGF-beta antibody or nonspecific IgG. On day 5 after burn injury, mice were inoculated with Pseudomonas aeruginosa at the burn wound site or received intraperitoneal injection with P. aeruginosa. Mice treated with anti-TGF-beta exhibited significantly improved survival compared with mice treated with nonspecific IgG after challenge with P. aeruginosa at the burn wound site or after intraperitoneal injection of P. aeruginosa. In mice with burn wound infections, bacterial counts in burn wounds, blood, and lung were decreased in mice treated with anti-TGF-beta compared with mice treated with control IgG. Bacterial counts in lung and blood after intraperitoneal challenge with P. aeruginosa also were significantly lower in burned mice treated with anti-TGF-beta compared with those treated with nonspecific IgG. Our data suggest that neutralization of TGF-beta at 4 days after burn injury in mice improves local and systemic clearance of P. aeruginosa and enhances survival after P. aeruginosa challenge.

Endotoxin priming improves clearance of Pseudomonas aeruginosa in wild-type and interleukin-10 knockout mice

Endotoxin (lipopolysaccharide [LPS]) tolerance is an altered state of immunity caused by prior exposure to LPS, in which production of many cytokines, including gamma interferon (IFN-gamma) and interleukin-12 (IL-12), are reduced but secretion of the anti-inflammatory cytokine IL-10 is increased in response to a subsequent LPS challenge. This pattern of cytokine production is also characteristic of postinflammatory immunosuppression. Therefore, we hypothesized that LPS-primed mice would exhibit an impaired ability to respond to systemic infection with the opportunistic pathogen Pseudomonas aeruginosa. We further hypothesized that depletion of IL-10 would reverse the endotoxin-tolerant state. To test this hypothesis, systemic clearance of Pseudomonas aeruginosa was measured for LPS-primed wild-type and IL-10-deficient mice. LPS-primed wild-type mice exhibited significant suppression of LPS-induced IFN-gamma and IL-12 but increased IL-10 production in blood and spleen compared to levels exhibited by saline-primed wild-type mice. The suppressed production of IFN-gamma and IL-12 caused by LPS priming was ablated in the spleens, but not blood, of IL-10 knockout mice. LPS-primed wild-type mice cleared Pseudomonas aeruginosa from lungs and blood more effectively than saline-primed mice. LPS-primed IL-10-deficient mice were particularly efficient in clearing Pseudomonas aeruginosa after systemic challenge. These studies show that induction of LPS tolerance enhanced systemic clearance of Pseudomonas aeruginosa and that this effect was augmented by neutralization of IL-10.

Diminished bacterial clearance is associated with decreased IL-12 and interferon-gamma production but a sustained proinflammatory response in a murine model of postseptic immunosuppression

After a major illness or injury, immune status in critically ill patients may fluctuate between a marked proinflammatory response and an immunosuppressed state. Postinflammatory immunosuppression can result in increased susceptibility to infection. Alterations of cytokine production, such as suppression of IFNgamma and elevation of the anti-inflammatory cytokine IL-10, are believed to contribute to postinflammatory immunosuppression. We examined antimicrobial immunity in mice that had previously been subjected to a sublethal cecal ligation and puncture (CLP) as a model of major injury. Mice were challenged with Pseudomonas aeruginosa (5 x 10(7) CFU i.v.) on day 5 after CLP or sham surgery. Bacterial clearance in mice after CLP was impaired and associated with decreased production of IFNgamma and increased production of IL-10 in the early response to the Pseudomonas challenge. Pseudomonas-induced production of the IFNgamma-inducing factor IL-12 was also decreased in post-CLP mice. However, splenocytes from post-CLP mice remained responsive to exogenous stimulation with the IFNgamma-inducing cytokines IL-12, IL-15, and IL-18 as well as T-cell receptor activation. Furthermore, production of the proinflammatory cytokines TNF-alpha, IL-1beta, and IL-6 were as high, or higher, in the post-CLP group compared with sham mice after P. aeruginosa challenge. Blockade of IL-10 did not reverse IL-12 and IFNgamma suppression in splenocytes from post-CLP mice. These studies show that suppressed bacterial clearance in post-CLP mice is associated with decreased production of IFNgamma and IL-12 and with increased production of IL-10 and proinflammatory cytokines.

Mechanisms of the inflammatory response

The physiological alterations induced by acute inflammation present significant management challenges for anaesthesiologists. Major surgery, trauma, burns and sepsis all have large inflammatory components. Acute inflammation is characterized by vasodilatation, fluid exudation and neutrophil infiltration. These processes are activated and amplified by a series of intracellular and extracellular factors that tightly co-ordinate the inflammatory process. The innate immune system responds rapidly to infection or injury. Macrophages, natural killer cells, CD8 + T-lymphocytes and neutrophils provide an early response to injurious factors in an effort to contain and eliminate harmful stimuli. The adaptive immune response requires prior exposure to microbial antigens, is mediated primarily by CD4 + T-lymphocytes and serves to further amplify acute inflammation. Although acute inflammation is fundamentally beneficial, severe inflammation can precipitate the systemic inflammatory response syndrome. This syndrome is characterized by hyperinflammation and can cause organ injury, shock and death in its most severe forms. Overall, our understanding of inflammation has increased tremendously during the past 20 years. However, these basic science advances have not yet translated into widespread benefit for patients suffering from trauma, sepsis and systemic inflammation.

Hyperoxia increases AP-1 DNA binding in rat brain

Oxidative stress appears to contribute to neurodegenerative outcomes after ischemia, hypoxia, and hyperoxia. The AP-1 transcription factor is made up of a family of regulatory proteins that can be activated by oxidative stress. In the present study, we examined AP-1 DNA binding activity in terms of specific participating AP-1 proteins in rat brain after hyperoxia. Male Sprague-Dawley rats were exposed to 100% oxygen under isobaric conditions over time. The AP-1 DNA binding activity present in the rat hippocampus and basal forebrain was characterized by electrophoretic mobility shift analysis (EMSA) and the participating AP-1 proteins identified by immunodepletion/supershift and Western blotting analyses. The Fos and Jun proteins were localized by immunohistochemistry to hippocampus. There were significant increases in AP-1 DNA binding in both hippocampus and basal forebrain after hyperoxia. There was also a significant increase in c-Jun protein levels and the proportion of c-Jun present in AP-1 DNA binding complexes in hippocampal nuclei after hyperoxia. These results suggest that AP-1 activation via c-Jun binding to DNA is an important component of brain responses to oxidative stress.

Hyperoxia increases AP-1 DNA binding in rat brain

Oxidative stress appears to contribute to neurodegenerative outcomes after ischemia, hypoxia, and hyperoxia. The AP-1 transcription factor is made up of a family of regulatory proteins that can be activated by oxidative stress. In the present study, we examined AP-1 DNA binding activity in terms of specific participating AP-1 proteins in rat brain after hyperoxia. Male Sprague-Dawley rats were exposed to 100% oxygen under isobaric conditions over time. The AP-1 DNA binding activity present in the rat hippocampus and basal forebrain was characterized by electrophoretic mobility shift analysis (EMSA) and the participating AP-1 proteins identified by immunodepletion/supershift and Western blotting analyses. The Fos and Jun proteins were localized by immunohistochemistry to hippocampus. There were significant increases in AP-1 DNA binding in both hippocampus and basal forebrain after hyperoxia. There was also a significant increase in c-Jun protein levels and the proportion of c-Jun present in AP-1 DNA binding complexes in hippocampal nuclei after hyperoxia. These results suggest that AP-1 activation via c-Jun binding to DNA is an important component of brain responses to oxidative stress.

Attenuated transcriptional responses to oxidative stress in the aged rat brain

The aged nervous system displays impaired cognitive functions, and these impairments are exacerbated in several neurodegenerative diseases. A role for oxidative stress has been suggested for several of these age-associated dysfunctions. In addition, recovery from more acute traumatic insults that also generate oxidative stress is impaired in the aged. Here we examine the response of aged rat hippocampi to normobaric hyperoxia treatments and demonstrate an attenuation in the DNA binding activity of the AP-1 and nuclear factor-kappa B transcription factors, which are important components of stress response signal transduction pathways and can determine shifts in cellular commitments to necrosis, apoptosis, or functional recovery in the central nervous system.

NF-kappaB activity decreases in basal forebrain of young and aged rats after hyperoxia

Hyperoxia is an oxidative stressor that triggers signaling cascades via changes in promoter activation by transcription factors. The transcription factor NF-kappaB has been shown to regulate transcription of many genes that play a role in inflammation and recovery from acute or chronic trauma. Here we describe the effects of hyperoxia on basal levels of NF-kappaB activity in young and aged rat forebrain. The results would suggest that chronic stress may have different effects on NF-kappaB basal activity levels as compared to the effects of an acute stress such as hyperoxia and that there is a diminished response to hyperoxia in the aged basal forebrain.

Nuclear factor kappaB/p49 is a negative regulatory factor in nerve growth factor-induced choline acetyltransferase promoter activity in PC12 cells

Anovel nuclear factor kappaB (NF-kappaB) binding site has been identified within the promoter region of the mouse gene encoding choline acetyltransferase (ChAT), the enzyme that synthesizes acetylcholine and has been implicated in the cognitive deficits associated with aging and Alzheimer's disease. This binding site, which is located within the nerve growth factor (NGF)-responsive enhancer element, was recognized by the NF-kappaB protein p49 but not p65 or p50. p49 from both basal forebrain and PC12 nuclear extracts interacted with this specific sequence in electrophoretic mobility shift assays. Mutation of the NF-kappaB site caused an increase in NGF-induced promoter activation, whereas overexpression of p49 in NGF-differentiated PC12 cells caused a decrease in endogenous ChAT enzyme activity and a decrease in promoter activity that was specifically mediated through this NF-kappaB binding site. Treatment of PC12 cells with NGF resulted in a drastic reduction in nuclear p49 binding to the ChAT NF-kappaB site after 24 h, but nuclear p49 levels were not altered, suggesting that late NGF-mediated events prevent binding of p49 to the ChAT promoter by an unknown mechanism other than nuclear translocation. Decreased ChAT expression and increased NF-kappaB activity in the brain are associated with aging and Alzheimer's disease. These data indicate that p49 is a negative regulator of ChAT expression and suggest a possible mechanism for aging-associated declines in cholinergic function.

NGF-mediated alteration of NF-kappaB binding activity after partial immunolesions to rat cholinergic basal forebrain neurons

There are age-associated cognitive and cholinergic deficits in the neurotrophin-dependent cholinergic basal forebrain neurons (CBFNs). There are also increases in the activity of the transcription factor NF-kappaB in the aged rodent brain that may reflect chronic enhancement of stress response signaling. We used partial immunolesions (PIL) to CBFN to examine the role of endogenous NGF on choline acetyltransferase (ChAT) activity and NGF-mediated NF-kappaB alteration after cholinergic deafferentation. We injected 192 IgG-saporin, an immunotoxin selectively taken up by neurotrophin receptor p75(NTR)-bearing neurons, into lateral ventricles, followed by infusions of anti-NGF to assess NF-kappaB, ChAT and NGF responses to PIL after anti-NGF infusion. Treatment with anti-NGF decreased ChAT activity by 17-34% in the cortex, hippocampus, and olfactory bulb and PIL decreased ChAT activity by 47-73%. Changes in AChE activity levels paralleled those observed for ChAT after PIL. NGF protein levels in the olfactory bulb, but not the cortex or hippocampus, increased significantly after PIL treatment. Infusion of anti-NGF abolished the PIL-induced eight-fold NGF increase in CNS. NF-kappaB binding activity to the IgG-kappaB and ChAT specific NF-kappaB consensus sequences, increased in the cortex but not hippocampus after PIL followed by anti-NGF infusion. It is likely that immunolesion-induced changes in ambient NGF levels may perturb NF-kappaB activity.

Signal transduction in neuronal death

Apoptosis in the nervous system is a necessary event during the development of the nervous system and is also present after genotoxic events, be they chronic as in aging or more acute after trauma and ischemia. Apoptotic events reflect an interplay between intrinsic signaling events that rely on cytokines, neurotransmitters, and growth factors and responses to extrinsic events that increase levels of radical oxygen species. Both intrinsically and extrinsically driven signal-transduction pathways act via transcription factors that regulate the coordinated timely expression of stress-response genes as part of a decision-making process that can commit cells to apoptosis or survival. Here we discuss the role of two transcription factors that participate in apoptosis in the nervous system: the activator protein AP-1 and nuclear factor kappaB.

Responses of young and aged rat CNS to partial cholinergic immunolesions and NGF treatment

The cholinergic neurons of the basal forebrain (CNBF) are the major source of cholinergic innervation of the cortex and hippocampus. In Alzheimer's disease and aged brain, there are severe losses of cholinergic neurons in the nucleus basalis of Meynert, leading to a reduction of cortical cholinergic activity which correlates with the severity of cognitive deficits. While there is evidence that aged central nervous system (CNS) displays impaired stress response signaling, pharmacologic treatments with neurotrophic factors appear to ameliorate these age-associated cholinergic deficits. To mimic these cholinergic deficits in experimental animals and study the acute effects of nerve growth factor (NGF), we induced a partial lesion of CBFNs by the intracerebroventricular (i.c.v.) injection of the cholinergic immunotoxin 192IgG-saporin, in groups of 3- and 30-month-old rats. The lesion was followed 14 days later by i.c.v. administration of NGF, known to restore partial immunolesion-induced cholinergic deficits in rat CNS, and all rats were killed 2 days after the NGF treatment. Here we report the effects of partial immunolesions on the levels of choline acetyltransferase (ChAT) activity and NGF receptor mRNA levels in the basal forebrain of 3- and 30-month-old rats. Because of their presence in the promoters of the NGF, NGF receptors, and ChAT genes, we also measured DNA-binding activity of the transcription factors NFB and AP-1 in the cortex and hippocampus. We discuss these findings in the context of endogenous NGF-mediated signal transduction mechanisms and conclude that we have evidence for age-associated decreases in endogenous NGF responses to partial deafferentation of the basal forebrain cholinergic projections.

Age-associated alterations in hippocampal and basal forebrain nuclear factor kappa B activity

Age-related cognitive deficits are often associated with loss of cholinergic activity within the neurotrophin-dependent cholinergic neurons that project from the basal forebrain to the hippocampus. The cause of reduced cholinergic function is unknown, but alterations in transcription factor-signaling pathways causing altered gene expression may cause decreased specific tissue function, resulting in loss of cholinergic activity. We measured transcription factor Nuclear Factor kappa B by electrophoretic mobility shift assay and Western analysis in young and aged rat brain tissues and report that basal levels of Nuclear Factor kappa B DNA-binding activity increase in the hippocampus and basal forebrain with age to significantly higher levels at 30 months of age. This age-associated increase in binding activity is associated with increased translocation of p65 to the nucleus. These data show an age-associated alteration in Nuclear Factor kappa B signal transduction pathways that may contribute to age-associated decreases in specific tissue function.