A ‘Trp’ to the land of Kyns
by Felicita Fedelis Jusof
Despite being implicated in immune responses of a number of normal body functions as well as disease processes, the kynurenine (Kyn) pathway is known amongst researchers only if one has worked on it, or topics related to it. Worse still, if one has not done any immunology-related research, one may never hear of it although its implication in medicine is transdisciplinary in nature, extending beyond a single disease. The Kyn pathway is essentially a metabolic pathway involving the amino acid, tryptophan (Trp), which is an essential building block for proteins (see Figure 1). Through this metabolic pathway, more than 95% of Trp is catabolized .
This pathway is a curious one as it has the capacity to elicit immune responses that may both protect or harm the body depending on the extent of the activation of the pathway as well as the cells it is activated in. One of the earliest documented roles of the Kyn pathway was in bacterial infections . Its role in other infectious diseases such as HIV/AIDS and hepatitis, as well as malaria, has since been established [3-5]. The role of the pathway in the pathogenesis of dengue infections has also been suggested . Other pathological conditions in which the Kyn pathway has been implicated include neurological disorders such as Alzheimer’s, AIDS-related dementia and schizophrenia [7-8], as well as autoimmune disorders such as cancer, allergy, arthritis and asthma in which it has been gaining traction [9-13].
The Kyn pathway also contributes to the immunotolerance observed in pregnancy, where the activation of this pathway suppresses the mother’s local immunity in the placenta, preventing the mother’s immunity from rejecting her foetus, which would otherwise be recognised as a foreign body . This discovery was monumental in contributing to our understanding of how the pathway modulates immunity. A similar mechanism of immunotolerance has been documented in cases of transplantation, where induction of Trp-catabolic enzyme rendered protection from organ rejection in cases of liver transplantation . The ability of this pathway to induce immunosuppression also contributes to a poorer outcome in several types of cancers as it allows the tumour to persist in the host [16-18].
One of the first mechanisms proposed through which the Kyn pathway exerts its immunological response involves the manipulation of Trp availability to cells and pathogens in the microenvironment. As Trp is a food source for pathogens in infections, the activation of this pathway during microbial infections strategically depletes Trp, effectively starving and eventually eliminating the pathogens . The alternative mechanism through which the Kyn pathway exerts an immunomodulatory effect is either through the suppression of immune cells known as T cells or through the generation of metabolites that can act on neurons [19-21]. These metabolites may either be neuroprotective or neurotoxic.
“Trp is a food source for pathogens in infections, the activation of this pathway … depletes Trp, effectively starving and eventually eliminating the pathogens”
The first step in this pathway is the conversion of Trp to kynurenine (Kyn). This step of the pathway can be catalysed by three enzymes, tryptophan 2,3 dioxygenase (TDO), indoleamine 2,3 dioxygenase 1 (IDO1) and the most recently discovered IDO2. While the three enzymes catalyse the same reaction, their distribution and role in tissues differ. The longest known enzyme of the three, TDO is expressed constitutively in high amounts in the mammalian liver and its best-understood role is in the regulation of dietary Trp . However, as it also is expressed constitutively albeit at low levels in other tissues such as epididymis, testis, pancreas and heart, an alternative role for TDO in these tissues has been considered [23,24]. IDO1, on the other hand, which is expressed constitutively in relatively low levels in the epididymis, intestine and placenta, is induced in various cells and tissue types in the presence of inflammatory stimuli, namely interferon gamma (IFNᵧ), TNF and LPS [14, 25-27].
The least understood of the three is the isozyme of IDO1, IDO2. Based on the most recent phylogenetic study, it is shown that IDO1 is more similar to the ancestral IDO gene and that IDO2 arose from duplication of the ancestral gene that occurred in early vertebrate evolution with IDO1 being eventually lost in a number of the lower vertebrates . Despite both IDO homologues in mammals possessing striking genomic structural similarities and Trp-catabolic properties, the enzymatic activity of IDO2 is relatively low and its substrate specificity differs from IDO1, leading to speculations that it may possess functions aside from its ability to metabolise Trp. The expression of IDO2 protein has been confirmed only in the mammalian liver while its presence in other tissues are ambiguous, as its RNA has been detected in kidney, brain, colon as well as in epididymis [29,30]. IFNγ-inducible IDO2 was detected in antigen-presenting dendritic cells, macrophages, astrocytes and mesenchymal stem cells [31-33]. In terms of clinical significance, IDO2 has been implicated in some forms of cancer [32-34]. However, more recently, it was also reported to be involved in the pathogenesis of rheumatoid arthritis and contact hypersensitivity [35,36].
The generation of neuroactive metabolites through the Kyn pathway are known to contribute to the progression of some of the diseases it is implicated in. These metabolites include 3-hydroxykynurenine, kynurenic acid, quinolinic acid and picolinic acid. Of these, kynurenic acid (KA) has been reported to be neuroprotective [37,38], whereas downstream metabolites of the pathway, 3-hydroxykynurenine, quinolinic acid (QA) and picolinic acid (PA) have been shown to exert neurotoxic effects [19,39].
Fascinatingly, not all cell types express enzymes downstream in the pathway. In cells where enzymes downstream in the pathway are absent or present in very minute levels, the Kyn pathway often does not metabolise substrates beyond kynurenine. For example, in endothelial cells, Trp is catabolised to kynurenic acid constitutively while generating both kynurenine and kynurenic acid when primed with IFNᵧ [40,41]. However, endothelial cells do not possess the capacity to synthesise kynurenine metabolites downstream in the pathway, such as quinolinic or picolinic acid, as the downstream enzymes of the pathway are either inactive, absent or present in levels too low to exert an effect. Human foetal brain cultures and lung cells also exhibited similar features [42,42].. On the other hand, macrophages, monocytes, microglia and liver cells were reported to have a high level of quinolinic acid, indicating the capacity of these cells to metabolise substrates downstream of the pathway [40, 42,43].
How is this pathway relevant to the Malaysian context, one may ask.
The involvement of this pathway in immune responses of various diseases reflects the potential of this pathway to help us better understand more than just one immune- and inflammation-related disease. The major health burden of Malaysia according to the National Strategic Plan for Non-communicable Disease, Ministry of Health Malaysia, is non-communicable diseases which include cardiovascular-related illness, diabetes and cancer . In addition to these non-communicable diseases, infectious diseases, namely dengue and HIV/AIDS, rounds up the eight leading disease burdens of Malaysia. Strikingly, the Kyn pathway has been implicated in all of the above mentioned diseases. It has been suggested that the balance between the neuroprotective and neurotoxic metabolites generated by the pathway could be used for early diagnosis, prognosis and intervention in diseases in which Kyn pathway is involved. It would be interesting to investigate if the balance between the neuroprotective and neurotoxic metabolites influences the progression of these diseases and whether inhibiting the generation of the neurotoxic metabolites, or administering neuroprotective metabolites could be a possible approach to intervention. The most exciting thing about deepening our understanding of this pathway is the potential to apply the knowledge in more than just one disease.
The remarkable thing about research is that the solutions are never quite straightforward and often enough, the direction a study takes can be unpredictable and surprising. The best of research projects begins with a very simple yet logical and exciting idea which, with further persistent probing, yields valuable information for our understanding of the human body and disease processes.
Perhaps this is the vision that the understanding of kynurenine pathway and the key players in it calls us to. That one day, a better understanding of this pathway may help us with the diagnosis and treatment of more than one physiological or pathological condition.
About the Author:
FELICITA FEDELIS JUSOF considers herself a newbie in the academic world. Having completed her doctoral degree in the University of Sydney in the year 2015, she returned to serve the very people who funded her doctoral degree overseas (Malaysian taxpayers) through her service to the University of Malaya. Although she loves the Big Bang Theory, she sees herself as an ordinary person who enjoys normal things like The Voice, amazing Asian food and Ed Sheeran. Despite this normalcy, she can sometimes be accused of being an adrenaline junkie who enjoys bungee jumping, bridge climbing and slingshot rides to name a few.
This article first appeared in the Scientific Malaysian Magazine Issue 12. Check out other articles in Issue 12 by downloading the PDF version for free here: Scientific Malaysian Magazine Issue 12 (PDF version)
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