Lianfeng Wu

Granzyme M Directly Cleaves Inhibitor of Caspase-Activated DNase (CAD) to Unleash CAD Leading to DNA Fragmentation

Granzyme (Gzm)M is constitutively highly expressed in NK cells that may play a critical role in NK cell-mediated cytolysis. However, the function of GzmM has been less defined. Just one report showed GzmM induces a caspase-independent death pathway. In this study, we demonstrate a protein transfection reagent Pro-Ject can efficiently transport GzmM into target cells. GzmM initiates caspase-dependent apoptosis with typical apoptotic nuclear morphology. GzmM induces DNA fragmentation, not DNA nicking. GzmM can directly degrade inhibitor of caspase-activated DNase to release the nuclease activity of caspase-activated DNase for damaging DNA. Furthermore, GzmM cleaves the DNA damage sensor enzyme poly(ADP-ribose) polymerase to prevent cellular DNA repair and force apoptosis.

Cleavage of Survivin by Granzyme M Triggers Degradation of the Survivin-X-linked Inhibitor of Apoptosis Protein (XIAP) Complex to Free Caspase Activity Leading to Cytolysis of Target Tumor Cells

Granzyme M (GzmM) is a chymotrypsin-like serine protease that preferentially cuts its substrates after Met or Leu. GzmM is constitutively expressed in activated innate effector natural killer (NK) cells. GzmM-induced cell death is consistent with the kinetics of cytotoxicity of NK cells. These suggest that GzmM may play an important role in innate immunity. Our previous work demonstrated that GzmM induces caspase-dependent apoptosis. However, it is unknown about how GzmM causes caspase activation. Here, we showed that the inhibitor of the apoptosis gene family member Survivin is a physiological substrate for GzmM. GzmM hydrolyzes Survivin at Leu-138 to remove the last four C-terminal residues. The truncated form (sur-TF) is more rapidly hydrolyzed through proteasome-mediated degradation. In addition, Survivin is in complex with X-linked inhibitor of apoptosis protein (XIAP) to inhibit caspase activation as an endogenous inhibitor. Survivin cleavage by GzmM abolishes the stability of the Survivin-XIAP complex and enhances XIAP hydrolysis, which amplifies caspase-9 and 3 activation of target tumor cells. The noncleavable L138A Survivin overexpression can significantly inhibit GzmM-mediated XIAP degradation, caspase activation, and GzmM- and NK cell-induced cytotoxicity. Moreover, Survivin silencing promotes XIAP degradation and enhances GzmM-induced caspase activation as well as GzmM- and NK cell-induced cytolysis of target tumor cells.

Structural basis for proteolytic specificity of the human apoptosis-inducing granzyme M

Granzyme M (GzmM), a unique serine protease constitutively expressed in NK cells, is important for granule-mediated cytolysis and can induce rapid caspase-dependent apoptosis of tumor cells. However, few substrates of GzmM have been reported to date, and the mechanism by which this enzyme recognizes and hydrolyzes substrates is unknown. To provide structural insights into the proteolytic specificity of human GzmM (hGzmM), crystal structures of wild-type hGzmM, the inactive D86N-GzmM mutant with bound peptide substrate, and the complexes with a catalytic product and with a tetrapeptide chloromethylketone inhibitor were solved to 1.96 Å, 2.30 Å, 2.17 Å and 2.70 Å, respectively. Structure-based mutagenesis revealed that the N terminus and catalytic triad of hGzmM are most essential for proteolytic function. In particular, D86N-GzmM was found to be an ideal inactive enzyme for functional studies. Structural comparisons indicated a large conformational change of the L3 loop upon substrate binding, and suggest this loop mediates the substrate specificity of hGzmM. Based on the complex structure of GzmM with its catalytic product, a tetrapeptide chloromethylketone inhibitor was designed and found to specifically block the catalytic activity of hGzmM.

Identification of SERPINB1 As a Physiological Inhibitor of Human Granzyme H

The granzyme/perforin pathway is a major mechanism for cytotoxic lymphocytes to eliminate virus-infected and tumor cells. The balance between activation and inhibition of the proteolytic cascade must be tightly controlled to avoid self damage. Granzyme H (GzmH) is constitutively expressed in NK cells and induces target cell death; however, how GzmH activity is regulated remains elusive. We reported earlier the crystal structures of inactive D102N-GzmH alone and in complex with its synthetic substrate and inhibitor, as well as defined the mechanisms of substrate recognition and enzymatic activation. In this study, we identified SERPINB1 as a potent intracellular inhibitor for GzmH. Upon cleavage of the reactive center loop at Phe343, SERPINB1 forms an SDS-stable covalent complex with GzmH. SERPINB1 overexpression suppresses GzmH- or LAK cell–mediated cytotoxicity. We determined the crystal structures of active GzmH and SERPINB1 (LM-DD mutant) in the native conformation to 3.0- and 2.9-Å resolution, respectively. Molecular modeling reveals the possible conformational changes in GzmH for the suicide inhibition. Our findings provide new insights into the inhibitory mechanism of SERPINB1 against human GzmH.

Structural insights into the substrate specificity of human granzyme H: the functional roles of a novel RKR motif.

Human granzyme H (GzmH) is constitutively expressed in human NK cells that have important roles in innate immune responses against tumors and viruses. GzmH is a chymotrypsin-like serine protease. Its substrate preference and its mechanism of substrate recognition are poorly understood. To provide structural insights into the substrate recognition mechanisms for GzmH, we solved the crystal structures of a D102N-GzmH mutant alone and in complex with a decapeptide substrate and an inhibitor to 2.2 Å, 2.4 Å, and 2.7 Å, respectively. The Thr189, Gly216, and Gly226 specificity triad in the S1 pocket of GzmH defines its preference for bulky, aromatic residues (Tyr and Phe) at the P1 position. Notably, we discovered that an unusual RKR motif (Arg39-Lys40-Arg41), conserved only in GzmH, helps define the S3′ and S4′ binding regions, indicating the preference for acidic residues at the P3′ and P4′ sites. Disruption of the RKR motif or the acidic P3′ and P4′ residues in the substrate abolished the proteolytic activity of GzmH. We designed a tetrapeptide chloromethylketone inhibitor, Ac-PTSY-chloromethylketone, which can selectively and efficiently block the enzymatic and cytotoxic activity of GzmH, providing a useful tool for further studies on the function of GzmH.