Inflammatory bowel disease (IBD) is a debilitating disease of the intestinal tract, plagued by chronic inflammation of the gut. A dysregulated gut immune response causes the persistent inflamed condition leading to erosion of the protective intestinal barrier.
Existing studies have shown that a series of molecular events called the NF-κB (NF-kappa B) canonical pathway plays a crucial role in triggering the immune response in the gut. Usually, the pathway controls the activity of a set of proteins called NF-κB transcription factors to regulate the inflammation process while fighting pathogens in the gut. However, in IBD, the regulation is skewed, leading to the hyperactivation of inflammatory proteins. The molecular details of what provokes this runaway process has remained elusive.
Now, a collaborative study by a team of researchers from the National Institute of Immunology, New Delhi, Indian Institute of Technology Delhi, New Delhi, All India Institute of Medical Sciences, New Delhi, and Agency for Science, Technology and Research (A*STAR), Singapore, has deciphered a mechanism underlying the aberrant inflammation in Inflammatory Bowel Disease. The study, led by Soumen Basak, Staff Scientist VI, National Institute of Immunology, revealed unwarranted crosstalk of the canonical pathway with another seemingly harmless pathway called the noncanonical NF-κB pathway.
NF-κB is a family of transcription factors – proteins that, once activated and inside the nucleus, can bind to specific DNA regions and help transcribe (or decipher) the target genes. This subsequently generates specific proteins for physiological responses. NF-κB factors respond to a variety of signals and activate the expression of immune response genes. Their nuclear activation is controlled by two signalling pathways: canonical and noncanonical, both regulating a myriad of physiological functions.
When pathogens invade, the canonical NF-κB pathway is activated and leads to the generation of an immune response. The canonical NF-κB pathway triggers the release of cytokines and mediates a controlled inflammatory process. Once the pathogens are flushed out, the triggers subside, and the canonical pathway is inhibited.
However, when the canonical pathway is hyperactivated, as seen in IBD, along with flushing out the pathogens, the over-abundant cytokines also destroy the mucosal barrier of the intestine. This exposes the intestinal epithelial tissue (the outermost layer of cells inside the intestine), causing uncontrolled inflammation and tissue damage at the affected site.
NF-κB factors also respond to signals via the noncanonical pathway, which regulates the development of immune organs and tissues. “The noncanonical pathway has different molecules that monitor other bodily functions, such as generating primary cells that form organs,” says Basak.
Recent studies have shown that the two pathways are intertwined. However, whether the noncanonical NF-κB pathway has a role in IBD was not known. Noticing this, the team hypothesised that the noncanonical pathways could be playing an indirect role in regulating the inflammatory NF-κB activity and, in turn, modulating the inflammatory response in the gut.
The team conducted molecular analyses on colon biopsy samples of IBD patients and compared them with non-IBD control group samples. In the IBD patient samples, they found increased processing of a key protein of the noncanonical pathway, called p100.
p100 is encoded by a gene named Nfkb2. p100 is processed to generate another protein p52 during noncanonical signalling and p52 is responsible for inducing the target genes in the noncanonical NF-κB pathway. “We noticed up to 53% of IBD patients displayed elevated p100 processing to form p52,” says Basak. So, the question now was whether the elevated noncanonical signalling enhanced the canonical pathway’s activity in intestinal inflammation.
To find out, the researchers experimented on mice with chemically-induced intestinal inflammation as seen in IBD. First, they genetically engineered mice such that they lacked the Nfkb2 gene in the intestinal epithelial cells and treated them with a chemical reagent known to induce IBD-like conditions via the canonical pathway. Compared to mice with an intact Nfkb2 gene, the mice that lacked it showed significantly reduced canonical pathway activation subsequent to the chemical treatment, indicating its dependence on the Nfkb2 gene. “We performed gene expression analyses which revealed that noncanonical NF-κB signalling events were amplifying inflammatory gene expressions in the intestinal epithelial cells,” says Meenakshi Chawla, first author of the study. This showed that the enhanced activity of the noncanonical pathway was causing unwarranted communication between the two pathways. This caused the heightened inflammation in gut epithelial cells leading to the chronically inflamed condition.
Ellora Sen, Scientist at the National Brain Research Centre, Manesar who was not involved in the research, says, “This study highlights the previously unknown crosstalk between the canonical and noncanonical NF-κB pathway that drives the hyper-inflammatory response in IBD. Thus, targeting Nfkb2 may hold promise as a therapeutic option, not only in IBD but several other inflammatory diseases.”