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Inflammation or injury to the somatosensory nervous system may result in chronic pain conditions, which affect millions of people and often cause major health problems. Emerging lines of evidence indicate that reactive oxygen species (ROS), such as superoxide anion or hydrogen peroxide, are produced in the nociceptive system during chronic inflammatory and neuropathic pain and act as specific signaling molecules in pain processing. Among potential ROS sources in the somatosensory system are NADPH oxidases, a group of electron-transporting transmembrane enzymes whose sole function seems to be the generation of ROS. Interestingly, the expression and relevant function of the Nox family members Nox1, Nox2, and Nox4 in various cells of the nociceptive system have been demonstrated. Studies using knockout mice or specific knockdown of these isoforms indicate that Nox1, Nox2, and Nox4 specifically contribute to distinct signaling pathways in chronic inflammatory and/or neuropathic pain states. As selective Nox inhibitors are currently being developed and investigated in various physiological and pathophysiological settings, targeting Nox1, Nox2, and/or Nox4 could be a novel strategy for the treatment of chronic pain. Here, we summarize the distinct roles of Nox1, Nox2, and Nox4 in inflammatory and neuropathic processing and discuss the effectiveness of currently available Nox inhibitors in the treatment of chronic pain conditions.
The culture of primary intestinal epithelia cells is not possible in a normal culture system. In 2009 a three-dimensional culture system of intestinal stem cells was established that shows many of the physiological features of the small intestine, such as crypt-villus structure, stem cell niche and all types of differentiated intestinal epithelial cells. These enteroids can be used to analyze biology of intestinal stem cells, gut homeostasis and the development of diseases. They also give the possibility to reduce animal numbers, as enteroids can be cryo-conserved and cultivated for many passages. To investigate the influence of genes such as NADPH oxidases on the gut homeostasis, transgenic approached are the method of choice. The generation of enteroids from knockout mice allows real-time observations of knockout effects. Often conditional knockout or overexpression strategies using inducible Cre recombinase are applied to avoid effects of adaption to the knockout. However, the Cre recombinase has many known caveats from unspecific binding and its endonuclease activity. In this study, we show that although NADPH oxidases are important for in vivo differentiation and proliferation of the intestine, their expression is drastically reduced in the organoid system. Activation of Cre recombinase by 4-hydroxy tamoxifen in freshly isolated enteroids, independently of floxed genes, leads to decreased diameter of organoids. This effect is concentration-dependent and is caused by reduced cell proliferation and induction of apoptosis and DNA damage. In contrast, constitutive expression of Cre has no impact on the enteroids. Therefore, reduction of tamoxifen concentration and treatment duration should be carefully titrated, and appropriate controls are necessary.