While nitric oxide is an important signaling molecule produced almost everywhere within the human body, scientists are still working to fill gaps in our knowledge about how this chemical functions on the eye’s surface in the hope of developing exciting therapies in the future.
A review1 published by Australian scientists in The Ocular Surface this month examined the role of nitric oxide (NO) on the ocular surface, including its benefits and detriments, and proposed suitable techniques to measure the concentrations of this molecule within the eye’s tissues as well as in tears.
The review, which examined over 250 research papers, also looked at the potential for nitric oxide to be used as a biomarker to spot a number of ocular surface diseases as well as its potential use as an antimicrobial agent on the ocular surface.
“[The] revolution in medicine that arose from the discovery of NO as a signaling agent has touched ocular research as it has all other areas of physiology and pathology. NO is truly the double-edged sword, underpinning crucial biological processes but also inducing cellular damage. This remains a field ripe for discovery, and future research in this area may revolutionise the diagnosis and treatment of conditions of the ocular surface,” the researchers said.
Nitric oxide itself is a colourless gas whose molecules consist of a single nitrogen and oxygen atom. It is a free radical, having an unpaired electron, and is highly reactive as a result. This high reactivity gives it some important roles within our bodies as well as elsewhere in the environment.
A double-edged sword
In 1987, nitric oxide was discovered as essential for life. It plays three main roles in the body: widening the blood vessels and reducing blood pressure, acting as a neurotransmitter promoting cognitive skills and modulating pain, and targeting and destroying microbes and cancer cells within the immune system.
In our blood vessels and nervous system, nitric oxide is continuously produced. Because it has a half-life of one second, it disappears fairly rapidly from the body and has to be constantly replaced. Within the immune system however, nitric oxide is only produced in response to inflammation or infection.
Dr Shyam Sunder Tummanapalli of the School of Optometry and Vision Science at the University of New South Wales was co-author on the review and noted that those with low levels of nitric oxide could experience a range of health problems.
“For example, low nitric oxide levels may cause vascular conditions like cardiopulmonary diseases, heart attacks, hypertension, and erectile dysfunction because of low blood circulation. It also plays a major role in bowel movement and can be a major factor in irritable bowel syndrome,” he told Lab Down Under.
“Research has also shown that low levels of nitric oxide cause loss of neurotransmitter function which can lead to cognitive and neurodegenerative disorders, especially dementia and Alzheimer’s disease.”
If the body produces too much NO, other adverse symptoms can also be produced, Dr Tummanapalli said. For instance, chronic inflammation in conditions such a diabetes and neuroinflammation produce about a thousand times more NO than the normal physiological limits, leading to highly negative consequences.
“If this chronic inflammation continues then the overproduction of nitric oxide also continues. That could be cytotoxic because it produces a highly reactive radical called peroxynitrite which acts as a proinflammatory mediator instead of an anti-inflammatory agent. It enhances the inflammation and causes cell death and tissue damage.”
Video: The Science of Nitric Oxide
Pros and cons of nitric oxide in the eyes
Originally focusing his research on diabetes, Dr Tummanapalli and his team found that nerve damage caused by diabetes stemmed from low levels of nitric oxide which caused low blood circulation in the feet and other extremities. Working in the areas of optometry and vision science, the obvious next step was to look at what happened with nitric oxide on the ocular surface.
“When I looked back into the research, I was astonished and surprised that a lot of research had been done on the ocular surface and nitric oxide. It plays a major role on the ocular surface,” he said.
For instance nitric oxide is created by the endothelial cells which make up a thin membrane in our blood vessels, including those found in the cornea (the transparent front part of our eyes). The nitric oxide modulates the eye’s pump mechanism, with its by-products, nitrite and nitrate (NO2 and NO3), found in tears as a result.
“The cornea needs to be transparent all the time, so we need to have a pumping mechanism which pushes the fluid in and out to maintain this transparency. That pump mechanism is really pushed by the nitric oxide and ensures that the cornea maintains its proper thickness, curvature and transparency,” Dr Tummanapalli said.
“We hypothesise that nitrates that are found in tears come from the nitric oxide produced by the endothelial cells of the conjunctival blood vessels which maintain vascular tone and blood flow in the conjunctiva — that’s the white portion of your eye.”
Excessive amounts of NO can also wreak havoc in the eyes. For instance, dry eyes in chronic autoimmune diseases such as Sjogren’s syndrome are thought to be caused by large quantities of this chemical causing cell death in the acinar cells which are responsible for tear production, he told Lab Down Under.
While there were plenty of systemic studies examining the role of nitric oxide in the ocular surface, Dr Tummanapalli said there were very few in vivo and in vitro studies — respectively studies conducted within living things and within a test tube, petri dish, etc. The short half-life of nitric oxide made it difficult to detect and directly measure in the eye and in tears, with researchers opting instead to examine end products such as NO2 and NO3.
Possible therapeutic uses of nitric oxide
The review also shed light on a number of therapeutic uses of NO in the ocular surface. For instance where there is an overproduction of nitric oxide, chemical inhibitors and scavengers could be used to bring nitric oxide levels back down to normal levels and combat adverse symptoms such as dry eyes in the aforementioned Sjogren’s syndrome.
Research also showed that nitric oxide, which is normally produced by bacteria destroying cells called macrophages, could improve the healing of wounds in the cornea, Dr Tummanapalli said.
“In corneal wound healing, macrophages on the cornea or ocular surface produce very low amounts of nitric oxide. So if you exogenously apply nitric oxide donors, this can improve the proliferation of cells and make the corneal epithelial wound healing faster.”
Finally, NO also prevented the build up of biofilms which could be found on contact lenses and contact lens cases, he told Lab Down Under.
“This biofilm is a community of bacteria living together in a protective sheet or a matrix. These bacteria are highly resistant to antibiotics so research has now started looking at nitric oxide to disperse the biofilm.”
Building better eye treatments
Dr Tummanapalli noted that there are still large gaps in our knowledge about the role of NO on the ocular surface and its possible therapeutic uses. Firstly, there is no established method of detecting nitric oxide on the ocular surface, he said.
“Researchers have to look into how to develop a rapid, very effective and robust method to take direct measurements of nitric oxide on the ocular surface.”
Because NO disappears rapidly thanks to its short half-life, researchers also have to develop an effective sample collection method so that measurements could be taken before too much NO was lost to the atmosphere or the process of oxidation.
Thirdly, there is a need to examine the antimicrobial effects of nitric oxide and develop treatments targeted towards eye infections through something like an eye drop which could be locally applied and long-lasting, Dr Tummanapalli said.
“Now researchers are coming up with different silicon nanoparticles which deliver nitric oxide. They’re looking at this now but we’ll have to see what the effects of this are in the future,” he told Lab Down Under.
Researchers involved in this review came from UNSW as well as University of Sydney.
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1 Tummanapalli SS, Kuppusamy R, Yeo JH, Kumar N, New EJ, Willcox MDP. The role of nitric oxide in ocular surface physiology and pathophysiology. The Ocular Surface, Volume 21, July 2021, Pages 37-51.
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