Larry H. Bernstein, MD, FCAP, Curator
http://pharmaceuticalintelligence.com/6/7/2014/Immune activation, immunity, antibacterial activity
This segment is an update on activation of innate immunity, which has had a great amount of basic science resurgence in the last several decades. It also addresses the issue of antibiotic resistance, which shall be covered more fully in later segments. Antimicrobial resistance is a growing threat, and a challenge to the pharmaceutical industry. Moreover, worldwide travel increases the possibility of transfer of strains of virus and microbiota to distant communities.
8-OH-dG: A novel immune activator.
Innate immunity against viral or pathogenic infection involves sensing of non-self-molecules, otherwise known at pathogen-associated molecular patterns (PAMPs). This same sensing mechanism can be applied to damaged self-molecules, which are called damage-associated molecular patterns (DAMPs). One type of molecular pattern, for both groups, is cytosolic or extracellular DNA. However, there is not an extensive amount of research showing specifically what type of DAMP DNA molecule is best at activating this immune sensing response. A recent study investigated the mechanism behind how oxidized DNA from UV damage activates an immune sensing response.
A group of researchers found that, compared to a variety of types of cellular damage, damage from UV irradiation created a strong immune response (type I IFN response), seen across different types of immune regulatory cells. This was compared with freeze/thaw, physical damage and nutritional deprivation, each of which did not produce a noticeable immune response. Additionally, this immune response was seen when DNA was exposed to UV-A and UV-B (the type of radiation produced by our sun) and UV-C radiation.
DNA can be damaged by UV light directly, or through reactive oxygen species (ROS) caused by UV light. A well-known mark of DNA damaged by ROS is the oxidation of guanine to create 8-hydroxyguanine (8-OH-dG). These researchers saw an increase in 8-OH-dG dependant on the level of UV dose, and this also correlated with an increase in immune response; showing that DNA damage created by UV light in the form of 8-OH-dG is sufficient to activate an immune response. This study shows that 8-OH-dG can be classified at a DAMP.
Next, this group wanted to place a mechanism to these observations. They found that the ability of oxidation-damaged DNA to activate an immune response was dependant on cGAS and STING. Free DNA in the cytosol binds cGAS, a cGAMP synthase. This action produces a messenger molecule which proceeds to bind to and activate STING, an endoplasmic reticulum protein. STING activation will ultimately stimulate a type I IFN response.
When a cell’s own DNA is damaged, the cell’s machinery does all it can to repair it. This sometimes involves erasing, or degrading, the DNA that has been damaged. The enzyme, TREX1 exonuclease, has this job in a cell. However, this group found that when DNA was modified with an 8-OH-dG, it was resistant to this degradation by TREX1. This implies that the observed increase in immune response due to the presence of 8-OH-dG occurred because of an accumulation of damaged DNA, because it was not being degraded by TREX1 and could therefore sufficiently activate cGAS and STING.
This type of study has important implications for autoimmune diseases like lupus erythematosus (LE), which is characterized by its abnormally high number of autoantibodies against DNA. It is possible that this uncontrollable immune response is activated by oxidation-damaged DNA. Studies in this area, therefore, hold great importance.
– See more at: http://www.stressmarq.com/Blog/November-2013/8-OH-dG-A-novel-immune-activator.aspx#sthash.CvSdK0H1.dpuf
Oxidative Damage of DNA Confers Resistance to Cytosolic Nuclease TREX1 Degradation and Potentiates STING-Dependent Immune Sensing
Nadine Gehrke, Christina Mertens, Thomas Zillinger, Jörg Wenzel,…,Winfried Barchet
DOI: http://dx.doi.org/10.1016/j.immuni.2013.08.004
Highlights
- •UV or ROS damage potentiates immunorecognition of DNA via cGAS and STING
- •The oxidation product 8-OHG in DNA is sufficient for enhanced immunorecognition
- •Oxidized self-DNA acts as a DAMP and induces skin lesions in lupus-prone mice
- •Oxidized DNA is resistant to cytosolic nuclease TREX1-mediated degradation
Summary
Immune sensing of DNA is critical for antiviral immunity but can also trigger autoimmune diseases such as lupus erythematosus (LE). Here we have provided evidence for the involvement of a damage-associated DNA modification in the detection of cytosolic DNA. The oxidized base 8-hydroxyguanosine (8-OHG), a marker of oxidative damage in DNA, potentiated cytosolic immune recognition by decreasing its susceptibility to 3′ repair exonuclease 1 (TREX1)-mediated degradation. Oxidizative modifications arose physiologically in pathogen DNA during lysosomal reactive oxygen species (ROS) exposure, as well as in neutrophil extracellular trap (NET) DNA during the oxidative burst. 8-OHG was also abundant in UV-exposed skin lesions of LE patients and colocalized with type I interferon (IFN). Injection of oxidized DNA in the skin of lupus-prone mice induced lesions that closely matched respective lesions in patients. Thus, oxidized DNA represents a prototypic damage-associated molecular pattern (DAMP) with important implications for infection, sterile inflammation, and autoimmunity.
ribonuclease TREX1 and immunity
Immunity 19 Sep 2013;39(3), p482–495,
New Weapon in Fight Against ‘Superbugs’
Some harmful bacteria are increasingly resistant to treatment with antibiotics. A discovery might be able to help the antibiotics treat the disease.
By ANN LUKITS June 30, 2014 8:47 p.m. ET
Some harmful bacteria are increasingly resistant to treatment with antibiotics. This common fungus found in soil might be able to help the antibiotics combat diseases. Corbis
A soil sample from a national park in eastern Canada has produced a compound that appears to reverse antibiotic resistance in dangerous bacteria.
Scientists at McMaster University in Ontario discovered that the compound almost instantly turned off a gene in several harmful bacteria that makes them highly resistant to treatment with a class of antibiotics used to fight so-called superbug infections. The compound, called aspergillomarasmine A, or AMA, was extracted from a common fungus found in soil and mold.
Antibiotic resistance is a growing public-health threat. Common germs such asEscherichia coli, or E. coli, are becoming harder to treat because they increasingly don’t respond to antibiotics. Some two million people in the U.S. are infected each year by antibiotic-resistant bacteria and 23,000 die as a result, according to the Centers for Disease Control and Prevention. The World Health Organization has called antibiotic resistance a threat to global public health.
The Canadian team was able to disarm a gene—New Delhi Metallo-beta-Lactamase-1, or NDM-1—that has become “public enemy No. 1” since its discovery in 2009, says Gerard Wright, director of McMaster’s Michael G. DeGroote Institute for Infectious Disease Research and lead researcher on the study. The report appears on the cover of this week’s issue of the journal Nature.
“Discovery of a fungus capable of rendering these multidrug-resistant organisms incapable of further infection is huge,” says Irena Kenneley, a microbiologist and infectious disease specialist at Frances Payne Bolton School of Nursing at Cleveland’s Case Western Reserve University. “The availability of more treatment options will ultimately save many more lives,” says Dr. Kenneley, who wasn’t involved in the McMaster research.
The McMaster team plans further experiments to determine the safety and effective dosage of AMA. It could take as long as a decade to complete clinical trials on people with superbug infections, Dr. Wright says.
The researchers found that AMA, extracted from a strain of Aspergillus versicolor and combined with a carbapenem antibiotic, inactivated the NDM-1 gene in three drug-resistant superbugs—Enterobacteriaceae, a group of bacteria that includes E. coli;Acenitobacter, which can cause pneumonia and blood infections; and Pseudomonas, which often infect patients in hospitals and nursing homes. The NDM-1 gene encodes an enzyme that helps bacteria become resistant to antibiotics and that requires zinc to survive. AMA works by removing zinc from the enzyme, freeing the antibiotic to do its job, Dr. Wright says. Although AMA was only tested on carbapenem-resistant bacteria, he expects the compound would have a similar effect when combined with other antibiotics.
AMA was first identified in the 1960s in connection with leaf wilt in plants and later investigated as a potential drug for treating high blood pressure. The compound turned up in Dr. Wright’s lab a few years ago during a random screening of organisms derived from 10,000 soil samples stored at McMaster. The sample that produced AMA was collected by one of Dr. Wright’s graduate students during a visit to a Nova Scotia park. It was the only sample of 500 tested that inhibited NDM-1 in cell cultures.
“It was a lucky hit,” says Dr. Wright. “It tells us that going back to those environmental organisms, where we got antibiotics in the first place, is a really good idea.”
The McMaster team developed a purified form of AMA for experiments on mice injected with a lethal form of drug-resistant pneumonia. Treatment with either AMA or a carbapenem antibiotic alone proved ineffective. But combining the substances resulted in more than 95% of the mice still being alive after five days. The combination was also tested on 229 cell cultures from human patients infected with resistant superbugs. The treatment resensitized 88% of the samples to carbapenem.
Still, bacteria could someday find a way to outwit AMA. “I can’t imagine anything we could make where resistance would never be an issue,” he says. “At the end of the day, this is evolution and you can’t fight evolution.”
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