Role of nitric oxide in P. aeruginosa keratitis caused by distinct bacterial phenotypes

The role of nitric oxide in ocular surface physiology and pathophysiology

Nitric oxide (NO) has a wide array of biological functions including the regulation of vascular tone, neurotransmission, immunomodulation, stimulation of proinflammatory cytokine expression and antimicrobial action. These functions may depend on the type of isoform that is responsible for the synthesis of NO. NO is found in various ocular tissues playing a pivotal role in physiological mechanisms, namely regulating vascular tone in the uvea, retinal blood circulation, aqueous humor dynamics, neurotransmission and phototransduction in retinal layers. Unregulated production of NO in ocular tissues may result in production of toxic superoxide free radicals that participate in ocular diseases such as endotoxin-induced uveitis, ischemic proliferative retinopathy and neurotoxicity of optic nerve head in glaucoma. However, the role of NO on the ocular surface in mediating physiology and pathophysiological processes is not fully understood. Moreover, methods used to measure levels of NO in the biological samples of the ocular surface are not well established due to its rapid oxidation. The purpose of this review is to highlight the role of NO in the physiology and pathophysiology of ocular surface and propose suitable techniques to measure NO levels in ocular surface tissues and tears. This will improve the understanding of NO's role in ocular surface biology and the development of new NO-based therapies to treat various ocular surface diseases. Further, this review summarizes the biochemistry underpinning NO's antimicrobial action.

Nitric oxide attenuated transforming growth factor-β induced myofibroblast differentiation of human keratocytes

Nitric oxide (NO) has the potential to modulate myofibroblast differentiation. In this study, we investigated the effect of exogenous NO on the myofibroblast differentiation of human keratocytes using sodium nitrite as a NO donor. Myofibroblasts were induced by exposing resting keratocytes to transforming growth factor (TGF)-β1. N-cadherin and α-smooth muscle actin (αSMA) were used as myofibroblast markers. Both resting keratocytes and -stimulated keratocytes were exposed to various concentrations of sodium nitrite (1 μM to 1000 mM) for 24 to 72 h. Exposure to sodium nitrite did not alter keratocytes’ viability up to a 10 mM concentration for 72 h. However, significant cytotoxicity was observed in higher concentrations of sodium nitrite (over 100 mM). The expression of αSMA and N-cadherin was significantly increased in keratocytes by TGF-β1 stimulation after 72 h incubation. The addition of sodium nitrite (1 mM) to TGF-β1-stimulated keratocytes significantly decreased αSMA and N cadherin expression. Smad3 phosphorylation decreased after sodium nitrite (1 mM) exposure in TGF-β1-stimulated keratocytes. The effect of NO was reversed when NO scavenger, 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) was added in the culture medium. Application of sodium nitrite resulted in significant decrease of corneal opacity when measured at 2 weeks after the chemical burn in the mouse. These results verified the potential therapeutic effect of NO to decrease myofibroblast differentiation of human keratocytes and corneal opacity after injury.

Effect of Nitric Oxide on Human Corneal Epithelial Cell Viability and Corneal Wound Healing

Although the wound healing effects of nitric oxide (NO) are known, the mechanism by which NO modulates corneal wound healing remains unclear. In this study, we investigated the effect of exogenous NO donor (NaNO₂) on corneal wound healing. We found that NaNO₂ (0.1 μM to 100 μM) increased human corneal epithelial cell (HCEC) viability and migration. It also modulated the phosphorylation of mitogen-activated protein kinases (MAPKs) in a time- dependent manner in those HCECs. Further, p38 MAPK phosphorylation increased at 6 h and normalized at 24 h, while the phosphorylation of extracellular signal regulated kinase (ERK) was increased both at 6 h and 24 h. Topical treatment with NaNO₂ (10 μM) enhanced corneal epithelial healing and decreased corneal opacity in murine corneal alkali burn model by modulating inflammatory cytokines. Our findings suggest that NO increased HCEC proliferation and migration via time-dependent MAPK activation and eventually enhanced corneal recovery from the alkali burn.