Acid-dependent dismutation of nitrogen oxides may be a critical source of nitric oxide in human macrophages

Signaling and stress: The redox landscape in NOS2 biology

Nitric oxide (NO) has a highly diverse range of biological functions from physiological signaling and maintenance of homeostasis to serving as an effector molecule in the immune system. However, deleterious as well as beneficial roles of NO have been reported. Many of the dichotomous effects of NO and derivative reactive nitrogen species (RNS) can be explained by invoking precise interactions with different targets as a result of concentration and temporal constraints. Endogenous concentrations of NO span five orders of magnitude, with levels near the high picomolar range typically occurring in short bursts as compared to sustained production of low micromolar levels of NO during immune response. This article provides an overview of the redox landscape as it relates to increasing NO concentrations, which incrementally govern physiological signaling, nitrosative signaling and nitrosative stress-related signaling. Physiological signaling by NO primarily occurs upon interaction with the heme protein soluble guanylyl cyclase. As NO concentrations rise, interactions with nonheme iron complexes as well as indirect modification of thiols can stimulate additional signaling processes. At the highest levels of NO, production of a broader range of RNS, which subsequently interact with more diverse targets, can lead to chemical stress. However, even under such conditions, there is evidence that stress-related signaling mechanisms are triggered to protect cells or even resolve the stress. This review therefore also addresses the fundamental reactions and kinetics that initiate signaling through NO-dependent pathways, including processes that lead to interconversion of RNS and interactions with molecular targets.

Conservation of the sterol regulatory element-binding protein pathway and its pathobiological importance in Cryptococcus neoformans

The mammalian sterol regulatory element-binding protein (SREBP) homolog, Sre1, is important for adaptation and growth of Cryptococcus neoformans in the mouse brain, where oxygen concentration and nutritional conditions are suboptimal for fungal growth. The extent of conservation of the SREBP pathway in C. neoformans or in any other fungi, however, has not been investigated. We generated mutants susceptible to low oxygen and identified six genes that play a role in the SREBP pathway. Three of these genes (SFB2, KAP123, and GSK3) are not known to be involved in the SREBP pathway in other fungi. Furthermore, we show that C. neoformans contains an additional gene, DAM1, which functions in the SREBP pathway but is yet to be described. Mutants associated with the steps prior to formation of the nuclear Sre1 form dramatically reduced accumulation of the nuclear form under low-oxygen conditions. Concurrently, two mutant strains, scp1Delta and stp1Delta, and the previously isolated sre1Delta strain showed reduction in ergosterol levels, hypersensitivity to several chemical agents, including azole antifungals, CoCl(2), and compounds producing reactive oxygen or nitrogen species, and most importantly, reduced virulence in mice. Mutants affecting genes involved in later steps of the Sre1 pathway, such as those required for import and phosphorylation of proteins in the nucleus, showed less compelling phenotypes. These findings suggest that the SREBP pathway is highly conserved in C. neoformans and it serves as an important link between sterol biosynthesis, oxygen sensing, CoCl(2) sensitivity, and virulence in C. neoformans.

Importance of mitochondria in survival of Cryptococcus neoformans under low oxygen conditions and tolerance to cobalt chloride

Cryptococcus neoformans is an environmental fungal pathogen that requires atmospheric levels of oxygen for optimal growth. For the fungus to be able to establish an infection, it must adapt to the low oxygen concentrations in the host environment compared to its natural habitat. In order to investigate the oxygen sensing mechanism in C. neoformans, we screened T-DNA insertional mutants for hypoxia-mimetic cobalt chloride (CoCl₂)-sensitive mutants. All the CoCl₂-sensitive mutants had a growth defect under low oxygen conditions at 37°C. The majority of mutants are compromised in their mitochondrial function, which is reflected by their reduced rate of respiration. Some of the mutants are also defective in mitochondrial membrane permeability, suggesting the importance of an intact respiratory system for survival under both high concentrations of CoCl₂ as well as low oxygen conditions. In addition, the mutants tend to accumulate intracellular reactive oxygen species (ROS), and all mutants show sensitivity to various ROS generating chemicals. Gene expression analysis revealed the involvement of several pathways in response to cobalt chloride. Our findings indicate cobalt chloride sensitivity and/or sensitivity to low oxygen conditions are linked to mitochondrial function, sterol and iron homeostasis, ubiquitination, and the ability of cells to respond to ROS. These findings imply that multiple pathways are involved in oxygen sensing in C. neoformans.

Cryptococcus neoformans is an obligate aerobic fungus that requires atmospheric levels of oxygen (21%) for optimal growth. However, the fungus is able to cause life-threatening brain infections in humans, where the oxygen tension is significantly lower than 21%. To understand the pathobiology of Cryptococcus neoformans, it is important to explore the molecular mechanisms adopted by the fungus to survive under low oxygen conditions. By using cobalt chloride, a hypoxia-mimicking agent, we isolated a number of mutants that are unable to grow in the presence of 0.7 mM CoCl₂ as well as at low oxygen conditions. In this study, we show that mitochondria play an important role for C. neoformans to survive in low oxygen conditions. We demonstrate that mutants harboring mutations in the genes related to mitochondrial functions have mitochondrial membrane permeability defect and lowered respiration rate and are more sensitive to stress generating chemicals, in addition to their inability to survive at low oxygen conditions. Finally, we also show that when wild-type cells are exposed to hypoxia-mimicking cobalt chloride, expression of genes involved in respiration and iron and sterol homeostasis, as well as ubiquitination, changes significantly.

Reactive nitrogen and oxygen species in airway inflammation

The free radical nitric oxide (NO) is an important mediator of many biological processes. Interestingly, the molecule appears to be a two-edged sword. Apart from NO having a function as a paracrine messenger, NO-derived oxidants are important weapons against invading pathogens. The role of NO in the airways is similarly ambiguous. Besides the task as a bronchodilator, NO and its derivatives play a role in the pathophysiology of asthma via their putative damaging effects on the airways. This deleterious effect can be increased by a nitrosative response to respiratory tract infections, since both the infectious agent and the host may suffer from the consequent nitrosative stress. Interestingly, respiratory infections can also compromise the beneficial (bronchodilator) effects of NO. This paper gives an overview on NO and its derivatives in the pathophysiology of airway inflammation.