Inflammatory bowel disease is a chronic disorder of the gut. Abdominal pain, rectal bleeding and weight loss affect the overall quality of life of the patients. Studies suggest that it is related to an overactive immune response. But the exact cause of this disorder is still unknown.
What causes the hyper-immune response?
Our intestine is home to trillions of bacteria, mostly beneficial. To overcome any potential threats, the immune surveillance system detects components of the pathogens and activates immune signalling pathways, generating inflammatory responses. Low-grade inflammation is a natural part of the body’s immune response. But inflammation for long durations can pose a problem.
One commonly activated pathway to mount inflammatory responses is a family of transcription factors, proteins that travel to the nucleus and bind to DNA, changing gene expression. When this nuclear factor was first discovered, it was found to bind to kappa light chain enhancers in B-cells. So it was called NF-kappaB.
But direct targeting of the NF-kappaB pathway for treating inflammatory bowel disorders results in undesirable side effects, owing to its involvement in many other physiological processes.
Apart from this NF-kappaB, a similar, yet distinct, ancillary pathway was discovered later. To distinguish the two, it was called the non-canonical NF-kappaB pathway, and the pathway discovered earlier was termed the canonical NF-kappaB pathway. The non-canonical NF-kappaB pathway is now known to promote immune system development.
Could the non-canonical NF-kappaB pathway be a better therapeutic target for inflammatory bowel disorders?
Soumen Basak and his team at the National Institute of Immunology, New Delhi started investigating the possibility. They collaborated with Vineet Ahuja’s group from AIIMS, New Delhi to procure inflammatory bowel disease patient samples.
When they analysed the samples using immunoblotting for proteins actively involved in the ancillary pathway, they noticed an active engagement of this pathway in the samples from the patients.
What is the non-canonical pathway actually doing?
To investigate, the team knocked out the gene involved in activating this pathway in mice. These mice were then subjected to dextran sodium sulphate, a chemical that induces cell apoptosis and inflammation, mimicking human inflammatory bowel diseases. The mice deficient in the non-canonical NF-kappaB signalling were resistant to this induced colon inflammation: they showed reduced colon swelling, diarrhoea and body weight loss. In other words, when the pathway is active, inflammation seems to worsen.
But how?
The team did large-scale ribonucleic acid sequencing of colon cells depleted of the ancillary pathway. The analyses of the sequences showed diminished inflammatory gene expression.
This suggested that the non-canonical pathway could be modulating canonical NF-kappa activity in the gut.
But how?
Meenakshi Chawla, at Soumen’s lab, started cell-culture-based experiments with help from colleagues. To measure the DNA binding ability of the NF-kappaB signalling proteins, she used radiolabeled DNA fragments. When the protein binds to DNA, the mobility of the DNA-protein complex is altered during electrophoresis, and radiolabeled DNA can be used to locate the exact position of the DNA-protein complex.
To identify the bound protein located by this experiment, the team used specific antibodies against canonical NF-kappaB proteins. They found that, in the absence of ancillary signalling, the binding of canonical pathway proteins to DNA is reduced. This, in turn, reduces the expressions of inflammatory genes. Thus, non-canonical NF-kappaB signalling was supplementing canonical NF-kappaB proteins, fuelling inflammation in the gut.
Current therapies for inflammatory bowel diseases involve canonical NF-kappaB inhibition. But that dampens the body’s overall inflammatory response, making individuals more susceptible to other infections. The non-canonical NF-kappaB pathway plays a role in amplifying the action of the canonical pathway. So it may be a better therapeutic target for inflammatory bowel diseases, say the scientists.
Proc. Natl. Acad. Sci. U S A. 22;118(25): e2024828118;
DOI: 10.1073/pnas.2024828118
Reported by Alvina Deka
National Institute of Immunology, New Delhi
*This report was written in an online workshop on science writing organised by NII
For information about an online workshop organised by Current Science, see this link.
STEAMindiaReports 
Leave a comment