Cornelius Y Taabazuing, Ph.D.

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Voorhees Presidential Assistant Professor
Department: Biochemistry and Biophysics

Contact information
904-905 Stellar Chance
422 Curie Boulevard
Philadelphia, PA 19104
Office: 617 755 4838
B.S. (Biochemistry and Molecular Biology)
University of Massachusetts Amherst, MA, 2009.
Ph.D. (Biological Chemistry)
University of Massachusetts Amherst, MA, 2015.
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Description of Research Expertise

The immune system plays essential roles in fighting off infectious diseases and cancer, but its over-activation can lead to inflammation and autoimmune disorders. Consequently, therapeutic modulation of the immune system can have a wide impact on many human diseases. Caspase-1 is a cysteine protease that plays a critical role in activating the immune system. Briefly, in response to intracellular host-derived or pathogen-derived danger signals, germline-encoded pattern-recognition receptors (PRRs) sense the danger, bind to the adapter protein ASC, and recruit the inflammatory pro-caspase-1 zymogen into large multiprotein complexes known as inflammasomes1. The oligomerization of pro-caspase-1 at inflammasomes leads to caspase-1 auto-proteolytic processing and activation. Active caspase-1 then cleaves and activates cytokines, including IL-1β and IL-18, as ¬¬¬¬well as the pore forming protein gasdermin D (GSDMD), which induces an immunostimulatory cell death called pyroptosis. Despite the critical role of caspase-1 in immune activation, the molecular basis of its regulation and activation remains incompletely understood.

The discovery of pyroptosis and apoptosis crosstalk: When I started my postdoc, the Bachovchin lab was investigating a small molecule called Val-boroPro (VbP) that induces potent anti-cancer immune responses in mice through an unknown mechanism. They had just discovered that VbP inhibits the serine peptidases DPP8 and DPP9 (DPP8/9) to induce caspase-1-dependent pyroptosis. However, the mechanistic basis of DPP8/9 inhibitor-induced pyroptosis was completely unknown. Curiously, the recently identified caspase-1 substrate that induces pyroptosis, GSDMD, appeared to be dispensable for cell death, suggesting additional pyroptotic substrates existed. I first sought to understand how VbP induced death in the absence of GSDMD. My research revealed that caspase-1 activates caspases-3/7 and induces apoptosis in GSDMD knockout cells, demonstrating that GSDMD is the only pyroptotic substrate. Conversely, during apoptosis, caspases-3/7 specifically block pyroptosis by cleaving GSDMD at a distinct site, abolishing its pyroptotic function. This previously unrecognized crosstalk indicates that once a cell has received a pyroptotic signal, it is committed to undergoing death either via an immunostimulatory manner (pyroptosis) or immunologically silent manner (apoptosis). However, if a cell receives an apoptotic signal, it will die while actively inhibiting immune stimulation. This previously unrecognized bidirectional crosstalk between apoptotic and pyroptotic signaling networks illuminate the complex
interplay between cell death pathways and has opened novel avenues of research for further exploration of how different cell death pathways may coregulate each other.

Discovery of the PRRs that mediate DPP8/9 inhibitor-induced pyroptosis and their molecular mechanism of activation: We hypothesized that VbP triggered an uncharacterized PRR to adapt to caspase-1 and activate the protease. In collaboration with grad students in the lab, we identified the key PRRs that mediate VbP-induced pyroptosis in mice, NLRP1B, and in humans, NLRP1 and CARD8. Although NLRP1 was the first PRR discovered to form an inflammasome, an activating stimulus had remained elusive until we discovered that VbP activates the NLRP1/CARD8 inflammasomes. Notably, DPP8/9 inhibitors are the only known stimuli that activate all the NLRP1/CARD8 inflammasomes. It should be noted that mice do not have CARD8, and CARD8 had not previously been known to function as an inflammasome. Thus, my research identified a novel inflammasome that controls innate immune activation in humans. NLRP1/CARD8 are distinct from other inflammasome forming PRRs in that they harbor ZU5 (found in ZO-1 and UNC5) domains and undergo auto-proteolysis at the C-terminal end of their ZU5 domains to generate two noncovalently associated polypeptides. It had been well-established that auto-proteolysis is required for activation, but the molecular mechanism had remained unknown for over a decade. We discovered that proteasomal degradation of the N-terminus releases the C-terminus to activate caspase-1 and induce pyroptosis. Collectively, this work identified CARD8 as a new inflammasome that activates caspase-1 and established ‘functional degradation’ of the inhibitory N-terminus as the molecular mechanism of NLRP1/CARD8 activation. This suggests that N-terminal degradation may regulate the activation mechanism of other ZU5 domain containing proteins. I intend to pursue this line of inquiry at UPenn and determine if N-terminal degradation is the molecular mechanism of activation of two other ZU5 domain containing proteins called PIDD and UNC5CL, which are implicated in cell death and the regulation of immunity.

Discovery that caspase-1 interdomain linker cleavage is required for pyroptosis, and that CARD8 is ASC-independent and NLRP1 is ASC-dependent: All well-characterized inflammasomes can utilize the adaptor protein ASC to form a caspase-1 signaling platform, but not all inflammasomes require ASC. ASC is comprised of a pyrin (PYD) domain, followed by a caspase activation and recruitment domain (CARD). The role of ASC is to recruit caspase-1 to the inflammasome via homotypic CARD-CARD interactions to induce caspase-1 oligomerization and auto-proteolysis. In the absence of the adaptor protein ASC, caspase-1 processing is generally not observed, but cell death still occurs. Consistent with the notion that the unprocessed pro-caspase-1 zymogen can induce pyroptosis, mouse pro-caspase-1 harboring mutations at all auto-processing sites to generate a constitutive pro-caspase-1 zymogen was reported to still induce cell death, leading to the conclusion that ASC is required for caspase-1 processing, but processing is dispensable for caspase-1 activity and cell death. Although caspase-1 processing is thought to be important for activity, processing has not been studied extensively. To test the hypothesis that the pro-caspase-1 zymogen is active and can induce pyroptosis, I constructed cleavage defective mutants of human pro-caspase-1 and discovered that caspase-1 interdomain linker processing is required for NLRP1/CARD8 mediated pyroptosis. Surprisingly, cleavage defective mutants of mouse pro-caspase-1 resulted in spontaneous cell death. Importantly, this death was non-pyroptotic cell death, and interdomain linker processing was also critical for mouse caspase-1 activation and induction of pyroptosis. Thus, the previously reported uncleavable mouse pro-casapase-1 zymogen was a dysregulated enzyme that caused non-pyroptotic cell death. Notably, despite their C-termini being remarkably similar, we discovered that NLRP1 requires the ASC adaptor to bridge caspase-1 and CARD8 does not, establishing CARD8 as the first inflammasome that is completely ASC-independent. Collectively, this demonstrates that the ASC adaptor protein is not required for caspase-1 processing and subsequent activation, but caspase-1 processing is absolutely required for the induction pyroptosis, redefining the current paradigm of caspase-1 activation. Interestingly, some inflammasomes such as the NLRC4 inflammasome, can utilize ASC to form a signaling platform, but ASC is not required for its activation. It currently remains unknown why, how, and when these inflammasomes choose to go through either the ASC-dependent or ASC-independent pathway. Part of my research at UPenn will focus on trying to understand the molecular basis of ASC-dependent and independent inflammasome activation in response to diverse stimuli.

In summary, my postdoctoral studies have led to many significant findings. First, we revealed fundamental bi-directional crosstalk between cell death pathways that was previously unknown. This finding will guide researchers studying cell death pathways as well as clinicians that utilize cell death inducing therapeutics for the treatment of human diseases. Second, we have identified the elusive trigger of the NLRP1/CARD8 inflammasomes: the cellular consequence of DPP8/9 inhibition. Notably, until very recently, the only known stimuli that activate NLRP1/CARD8 were DPP8/9 inhibitors, providing key tools to study how these inflammasomes regulate immunity. As mutations in CARD8 and NLRP1 are associated with inflammatory diseases, understanding their molecular mechanism of activation will also contribute to therapeutic development. Third, we discovered that N-terminal degradation of the auto-proteolyzed polypeptide is the mechanism of inflammasome activation, and this mechanism holds true regardless of the stimuli. This opens an avenue for new research to identify the E3 ligases that may potentially be important new targets for modulating the innate immune system. Fourth, although we have known about caspase-1 processing for the past 20 years, the details of how these processing events regulate its pyroptotic function have not been studied. As a result of my work, we now have a detailed understanding of the auto-proteolytic events that regulate caspase-1 activation and the induction of pyroptosis, forcing reconsideration of the current paradigm of caspase-1 activation. Lastly, my work also offers a mechanistic explanation for the potent anti-cancer properties of VbP. Indeed, we discovered that in vivo, DPP8/9 inhibitors caused immunostimulatory cell death of both immortalized and primary AML cells, leading to reduced tumor burden and significantly increased overall survival. This work has prompted the initiation of new clinical trials to test VbP in combination with other immune-oncology agents, specifically, checkpoint inhibitors. Overall, I have uncovered key mechanistic details of how the innate immune system is regulated that will facilitate the development of novel or complementary therapeutic strategies for autoimmune disorders, immuno-evasive pathogens, and cancer.

Currently I have developed expertise in understanding molecular mechanisms of inflammatory cell death. I plan to continue this line of research at UPenn. A major focus of the lab will be on uncovering the mechanistic basis of immunogenic cell death, and how cell death interfaces with microenvironment to induce immune activation. My expertise in cell death pathways will serve me well as well as the broader UPenn community of researchers interested in cancer immunotherapy. Specifically, projects in the lab will be focused on 1) understanding the molecular regulation and activation of ZU5 domain containing proteins, 2) determining the molecular events that dictate why the NLRC4 inflammasome functions to mediate ASC-dependent and independent cell death, and 3) uncovering the mechanistic basis of immunogenic cell death. These areas of research will synergize with the research efforts currently underway at UPenn.

Selected Publications

Chui AJ, Griswold AR, Taabazuing CY, Orth EL, Gai K, Rao SD, Ball DP, Hsiao JC, Bachovchin DA.: Activation of the CARD8 Inflammasome Requires a Disordered Region. Cell Rep 33(2): 108264, Oct 2020.

Taabazuing CY, Griswold AR, Bachovchin DA.: The NLRP1 and CARD8 inflammasomes. Immunol Rev 297: 13-25, September 2020.

Ball DP, Taabazuing CY, Griswold AR, Orth EL, Rao SD, Kotliar IB, Vostal LE, Johnson DC, Bachovchin DA.: Caspase-1 interdomain linker cleavage is required for pyroptosis. Life Sci Alliance 3(3): e202000664, Feb 2020.

Griswold AR, Ball DP, Bhattacharjee A, Chui AJ, Rao SD, Taabazuing CY, Bachovchin DA.: DPP9's Enzymatic Activity and Not Its Binding to CARD8 Inhibits Inflammasome Activation. ACS Chem Biol 14(11): 2424-2429, Nov 2019.

Chui AJ, Okondo MC, Rao SD, Gai K, Griswold AR, Johnson DC, Ball DP, Taabazuing CY, Orth EL, Vittimberga BA, Bachovchin DA.: N-terminal degradation activates the NLRP1B inflammasome. Science 364(6435): 82-85, Apr 2019.

Johnson DC, Taabazuing CY, Okondo MC, Chui AJ, Rao SD, Brown FC, Reed C, Peguero E, de Stanchina E, Kentsis A, Bachovchin DA.: DPP8/DPP9 inhibitor-induced pyroptosis for treatment of acute myeloid leukemia. Nat Med 24: 1151-1156, August 2018.

Okondo MC, Rao SD, Taabazuing CY, Chui AJ, Poplawski SE, Johnson DC, Bachovchin DA.: Inhibition of Dpp8/9 Activates the Nlrp1b Inflammasome. Cell Chem Biol 25(3): 262-267, Mar 2018.

Taabazuing CY, Okondo MC, Bachovchin DA.: Pyroptosis and Apoptosis Pathways Engage in Bidirectional Crosstalk in Monocytes and Macrophages. Cell Chem Biol 24(4): 507-514, Apr 2017.

Hangasky JA, Taabazuing CY, Martin CB, Eron SJ, Knapp MJ.: The facial triad in the α-ketoglutarate dependent oxygenase FIH: A role for sterics in linking substrate binding to O(2) activation. J Inorg Biochem 166: 26-33, January 2017.

Cornelius Y Taabazuing , Justin Fermann , Scott Garman , Michael J Knapp : Substrate promotes productive gas binding in the αKG-dependent oxygenase FIH. Biochemistry 55(2): 277–286, January 2016.

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Last updated: 04/20/2022
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