In a recent Cells journal study, researchers are investigating the mechanisms by which the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) induces NLRP3 inflammation conversion leading to pyroptosis. In it, the researchers also discuss the possibility of using NLRP3 and gas dermin D (GSDMD) inhibitors as anti-inflammatory treatments.
Examination: Pyroptosis and its role in SARS-CoV-2 infection. Image credit: Juan Gaertner / Shutterstock.com
What is pyroptosis?
Pyroptosis is a proinflammatory type of cell death associated with gas-intestinal-mediated membrane cavity formation, as well as cell enlargement and lysis. Pyroptosis plays an important role in both infectious and non-infectious diseases, some of which include atherosclerosis, autoimmune diseases, acute injury, adverse pregnancy events, cancer and neuronal diseases.
Several pathogen-associated molecular patterns (PAMPs) and microbial-associated molecular patterns (MAMPs) occur before pyroptosis and thus contribute to the formation of inflammasomes as well as the activation of caspase 1 and 11. NOD, LRR and pyrine Domain-containing protein 3 (NLRP3) inflammasome is one of the most notable upstream events of pyroptosis.
An overview of GSDMD
Upon activation of innate immune receptors by PAMPs, NLRP3 proteins accumulate in primed cells until they eventually undergo conformational changes and oligomerization. These events lead to the recruitment of caspase 1, which subsequently cleaves full-length GSDMD, which is responsible for the release of IL-1 and IL-18 proforms.
Of the five types of gas termines, GSDMD is considered to be the major pyroptosis effector molecule in myeloid cells, including macrophages, monocytes, and dendritic cells. More specifically, GSDMD is primarily responsible for pore formation during pyroptosis, as it is essential for the release of proinflammatory ILs, the flux of ions, and trade in nucleotides.
Gas termine family: tissue-specific signaling. Gasdermin A / B / C / D / E is expressed in different cell types and is activated by different signals, leading to inflammatory or non-inflammatory cell lysis. The upstream events for GSDMA cleavage are not well characterized (represented as ‘?’). GSDMA downregulation in gastric epithelial cells can lead to tumor formation. Proliferating tumor cells are recognized by effector T cells which releases Granzyme A, which cleaves GSDMB in cancer cells, leading to pore formation and tumor cell lysis. Furthermore, GSDMB can also be activated by Caspase 1 or 3 downstream for inflammation transformation or apoptosis, leading to pyroptosis. GSDMC is activated by Caspase 8 downstream of apoptosis, linking apoptosis with pyroptosis. GSDMD is the best characterized GSDM effector molecule, cleaved downstream of inflammasome activation, leading to pyroptosis. Similar to GSDMC, GSDME associates apoptosis with pyroptosis after cleavage of Caspase 3 into apoptotic cells. (Created with BioRender.com on May 7, 2022).
The role of NLRP3 in SARS-CoV-2 infection
Understanding the link between SARS-CoV-2 infection and pyroptosis-mediated inflammation has been crucial since the beginning of the pandemic. In fact, many researchers have been interested in investigating how inflammasomes and pyroptose-mediated inflammatory signatures may offer a therapeutic target for coronavirus disease 2019 (COVID-19).
More in vitro and in vivo studies have revealed that the SARS-CoV-2 tip and coat proteins primer the NLRP3 inflammasome. Furthermore, SARS-CoV-2 infection upregulates the expression of the angiotensin-converting enzyme 2 (ACE-2) receptor, which subsequently initiates nuclear factor κB (NF-κB) -mediated NLRP3 gene transcription and translation.
Previous studies have shown that SARS-CoV open reading frame 3 (ORF3) protein could bind directly to tumor necrosis factor-receptor-associated factor (TRAF) and subsequent active NLRP3. This prompted researchers to also investigate whether the SARS-CoV-2 ORF3 protein showed similar activation of NLRP3, which was eventually confirmed.
In addition to ORF3, SARS-CoV-2 ORF8b interacts directly with the NLRP3 leucine-rich repeat domain (LRR), ultimately leading to pyroptosis. Both SARS-CoV-2 nucleocapsid (N) and non-structural protein 6 (NSP6) proteins have also been identified as pyroptose initiators.
NLRP3 inflammation activation of SARS-CoV-2. The NLRP3 inflammasome can be primed by the spike and sheath of SARS-CoV-2 proteins, and in its opsonized form by activating ACE-2, TLR-2 and the complement receptors, respectively. SARS-CoV-2 infected cells upregulate ACE-2 receptor expression, making these cells more sensitive to SARS-CoV-2 response. Activation of these receptors leads to NFκB-mediated Nlrp3 gene transcription and translation. The NLRP3 inflammasome can be formed in response to ROS generation in SARS-CoV-2 infected cells, via potassium efflux through SARS-CoV-2 Orf3a-generated pores, or via direct interaction with the SARS-CoV-2 proteins orf8a and nucleoprotein. SARS-CoV-2 is capable of NLRP3 inflammation inhibition via NS1 and NS13 proteins, as well as through inhibition of autophagy, a general NLRP3 trigger. Therapeutic potential lies in the inhibition of the effector molecule IL-1β via canakinumab or anakinra, as well as the inhibition of the NLRP3 accessory protein BTK by ibrutinib. (Created with BioRender.com on April 13, 2022).
How NLRP3 predicts COVID-19 symptoms and severity
Some patients infected with SARS-CoV-2 have reported experiencing a wide range of neurological symptoms. A recent study showed that SARS-CoV-2 peak protein interacts directly with the ACE-2 receptor in human microglia, which then leads to the activation of NLRP3 inflammatory isomers. This NLRP3-mediated pyroptosis in microglia may be responsible for the neuroinflammation that has been reported in a subset of COVID-19 patients.
Activation of NLRP3 has also been observed in COVID-19 patients and has been associated with COVID-19 severity. Thus, NLRP3 activation may be a useful predictor of the severity of the disease as well as a possible therapeutic target.
In particular, several biomolecules downstream of NLRP3 have been used as targets for the treatment of COVID-19, especially in patients experiencing severe forms of the disease. Some of these include canakinumab, which is an anti-IL-1β antibody, as well as anakinra, which blocks the IL-1 receptor.
Recently, researchers have identified the potential benefit of ibrutinib, a Bruton tyrosine kinase (BTK) inhibitor, which is an NLRP3-specific regulatory protein. In fact, the use of acalabrutinib, an ibrutinib-like BTK inhibitor, in the treatment of severe COVID-19 patients was found to significantly increase patients’ lung function.
Recent studies have indicated that COVID-19 is an inflammatory condition that can probably be treated by inhibiting pyroptosis. Several agents targeting both upstream and downstream biomolecules of pyroptose have been evaluated for their ability to treat and aid in the diagnosis of COVID-19. Nevertheless, further research is needed to fully assess the usefulness of these agents against COVID-19.
- Bittner, ZA, Schrader, M., George, SE, & Amann, R. (2022). Pyroptosis and its role in SARS-CoV-2 infection. Cells. doi: 10.3390 / cells11101717.