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From 2012.igem.org

Nitric oxide (NO) is produced in different environments, be that within humans or by bacteria in soil. It is a nitrogenous species that is involved in the nitrogen cycle. With further research, development and integration into various systems, the future of medicine and agriculture could involve NO sensors.

Agriculture

NO can be produced by nitrifying bacteria during oxidation of ammonium (1). These gases can be released into the atmosphere. NO is also converted back to atmospheric nitrogen in the process of denitrification. The differences in the bacterial species that compose soil flora produce and reduce nitric oxide lead to different levels of NO in soil. Besides soil flora, soil composition, temperature, water levels, tilling and fertiliser quantities (2) can all affect the levels of NO. The levels of NO can affect agricultural yield. This is particularly significant now as the world population is ever increasing; the higher the agricultural yield the more people than can be supported. An accurate NO sensor can allow a farmer to be able to better utilise their resources and to accurately apply fertiliser, water and needed farming techniques to maximise the crop yield.

Medicine

NO is a very important compound used within the human systems. It regulates multiple biological processes such as vasodilation and innate immunity. NO syntheses include inducible (iNOS), endothelial (eNOS) and neuronal (nNOS) (3). In different systems, NO induces different effects. NO in the blood, causes vasodilation through increase of cGMP, a second messenger which activates many receptors and processes (4). As NO signalling is so widespread within the body there are many applications NO sensors can have in medicine.

Cancer Research

All the isoforms of NO synthases aforementioned are involved in the promotion or inhibition of the tumour cells (3). Research has shown that high levels of NO can be cytotoxic /or cytostatic to tumour cells; however, at the low levels produced by tumour cells, NO signals angiogenic factors such as VEGF leading to angiogenesis and enlargement of the tumour (3). Macrophages, as part of the innate immune system, produce NO to kill off the tumour. Research by Xu et al., shows that NO has greater activity when the tumour is less differentiated and hence a higher grade.

Using NO as a chemoattractant, the low and high levels of NO produced by cancer cells and macrophages, respectively can be perceived by the sensor which can target drugs directly to the tumour. An alternative method is to use oxygen as a chemorepellent so modified cells can specifically target hypoxic cells. Specificity can be increased through the use of targeting using specific peptide sequences which home onto receptors or glycoproteins which are specific to tumour cells such as the NGR peptide (5).

Diagnostics

NO is produced throughout the body. An abnormal level of NO is an indication of diseases and disorders such as hypertension, impotence and obesity. A NO sensor can be augmented into two ways to aid diagnosis and treatment. Building upon the Cambridge 2009 iGEM E.Chromi project, the NO sensor can be incorporated to perceive abnormal levels of NO in the gut flora, which can visually be seen in the form of colours in faecal matter. The colours could correspond to different diseases and disorders. Another use is through the cells being stimulated to create a protein coat for resistance and persistence within the human body. These cells could be mass produced and contain vaccines which can be taken at birth. These cells can be used to target pathogens or other abnormalities that arise in the body.

Understanding Pathogens

Pathogens have many different mechanisms and strategies of invasion and attack. For example some pathogens produce biofilms. To create biofilms, bacteria signal to other bacteria to assess the population size. When numbers are sufficient, they switch on gene expression which leads to aggregation and adhesion. This is an example of quorum sensing. A greater understanding of NO can be applied to research into pathogenic diseases which could lead to advances in the creation of cures and vaccines.

NO sensing

Many NO sensors exist but none that can accurately and specifically measure the levels of NO. To do this a comparator circuit system can be put into place. The circuit could involve multiple sensors which sense different reactive oxygen and nitrogenous species. Within this system, short interfering RNA (siRNA) could be implemented to inhibit the expression certain enzymes and proteins. This would produce an overall output that corresponds only to NO levels. To give a visual quantitative output, the use of bioluminescence can be augmented. The brightness would relate to the amount of NO in cells environment.

Furthermore, as mentioned previously the E.chromi project can be incorporated. In addition to its application in disease communication relative to NO levels, different sensors can be incorporated into the system which accurately measures all nitrogenous compounds. The levels of these can be qualitatively measured through the resulting colour product.