NALP expression in Paneth cells provides a novel track in IBD signaling

The complex role of inflammasomes in the pathogenesis of Inflammatory Bowel Diseases - lessons learned from experimental models

Inflammasomes are a large family of multiprotein complexes recognizing pathogen-associated molecular pattern molecules (PAMPs) and damage-associated molecular patterns (DAMPs). This leads to caspase-1 activation, promoting the secretion of mature IL-1β, IL-18 and under certain conditions even induce pyroptosis. Inflammatory Bowel Diseases (IBD) is associated with alterations in microbiota composition, inappropriate immune responses and genetic predisposition associated to bacterial sensing and autophagy. Besides their acknowledged role in mounting microbial induced host responses, a crucial role in maintenance of intestinal homeostasis was revealed in inflammasome deficient mice. Further, abnormal activation of these functions appears to contribute to the pathology of intestinal inflammation including IBD and colitis-associated cancer. Herein, the current literature implicating the inflammasomes, microbiota and IBD is comprehensively reviewed.

Inflammatory bowel disease: mechanisms, redox considerations, and therapeutic targets

Oxidative stress is thought to play a key role in the development of intestinal damage in inflammatory bowel disease (IBD), because of its primary involvement in intestinal cells' aberrant immune and inflammatory responses to dietary antigens and to the commensal bacteria. During the active disease phase, activated leukocytes generate not only a wide spectrum of pro-inflammatory cytokines, but also excess oxidative reactions, which markedly alter the redox equilibrium within the gut mucosa, and maintain inflammation by inducing redox-sensitive signaling pathways and transcription factors. Moreover, several inflammatory molecules generate further oxidation products, leading to a self-sustaining and auto-amplifying vicious circle, which eventually impairs the gut barrier. The current treatment of IBD consists of long-term conventional anti-inflammatory therapy and often leads to drug refractoriness or intolerance, limiting patients' quality of life. Immune modulators or anti-tumor necrosis factor α antibodies have recently been used, but all carry the risk of significant side effects and a poor treatment response. Recent developments in molecular medicine point to the possibility of treating the oxidative stress associated with IBD, by designing a proper supplementation of specific lipids to induce local production of anti-inflammatory derivatives, as well as by developing biological therapies that target selective molecules (i.e., nuclear factor-κB, NADPH oxidase, prohibitins, or inflammasomes) involved in redox signaling. The clinical significance of oxidative stress in IBD is now becoming clear, and may soon lead to important new therapeutic options to lessen intestinal damage in this disease.

Paneth cells: maestros of the small intestinal crypts

Paneth cells are highly specialized epithelial cells of the small intestine, where they coordinate many physiological functions. First identified more than a century ago on the basis of their readily discernible secretory granules by routine histology, these cells are located at the base of the crypts of Lieberkühn, tiny invaginations that line the mucosal surface all along the small intestine. Investigations over the past several decades determined that these cells synthesize and secrete substantial quantities of antimicrobial peptides and proteins. More recent studies have determined that these antimicrobial molecules are key mediators of host-microbe interactions, including homeostatic balance with colonizing microbiota and innate immune protection from enteric pathogens. Perhaps more intriguing, Paneth cells secrete factors that help sustain and modulate the epithelial stem and progenitor cells that cohabitate in the crypts and rejuvenate the small intestinal epithelium. Dysfunction of Paneth cell biology contributes to the pathogenesis of chronic inflammatory bowel disease.

Nucleotide-binding oligomerization domain-like receptors and inflammasomes in the pathogenesis of non-microbial inflammation and diseases

The nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) or nucleotide-binding domain leucine-rich repeat-containing family of genes plays an important role in the development of innate immune responses. Some family members are known to form multiprotein complexes known as inflammasomes that regulate the processing and secretion of proinflammatory mediators, such as interleukin-1β and interleukin-18. Activity of the inflammasome is triggered not only by microbial infection, but also by a wide range of both exogenous and endogenous noninfectious stimuli. Consequently, the dysregulation of inflammasome activity is associated with numerous proinflammatory, non-microbial human diseases. The discovery of NLRP3 gene mutations in autoinflammatory diseases such as Muckle-Wells syndrome has led to the association of NLRs in the pathogenesis of many non-microbial diseases that include arthritis, neurodegenerative disorders, metabolic disorders (obesity and diabetes), cardiovascular disease (atherosclerosis, myocardial infarction), inflammatory bowel disease, kidney disease and hypersensitivity dermatitis. A number of NLRs are also associated with human disease in the absence of inflammasome activity, suggesting additional roles for NLRs in the regulation of inflammation and disease. This review serves to provide a summary of NLR-associated diseases and, where possible, the mechanisms behind the associations.