Molecular determinants involved in activation of caspase 7

Inflammatory Caspases: Toward a Unified Model for Caspase Activation by Inflammasomes

Inflammasomes are inflammatory signaling complexes that provide molecular platforms to activate the protease function of inflammatory caspases. Caspases-1, -4, -5, and -11 are inflammatory caspases activated by inflammasomes to drive lytic cell death and inflammatory mediator production, thereby activating host-protective and pathological immune responses. Here, we comprehensively review the mechanisms that govern the activity of inflammatory caspases. We discuss inflammatory caspase activation and deactivation mechanisms, alongside the physiological importance of caspase activity kinetics. We also examine mechanisms of caspase substrate selection and how inflammasome and cell identities influence caspase activity and resultant inflammatory and pyroptotic cellular programs. Understanding how inflammatory caspases are regulated may offer new strategies for treating infection and inflammasome-driven disease. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Caspase-9 Activation of Procaspase-3 but Not Procaspase-6 Is Based on the Local Context of Cleavage Site Motifs and on Sequence

Studying the interactions between a protease and its protein substrates at a molecular level is crucial for identifying the factors facilitating selection of particular proteolytic substrates and not others. These selection criteria include both the sequence and the local context of the substrate cleavage site where the active site of the protease initially binds and then performs proteolytic cleavage. Caspase-9, an initiator of the intrinsic apoptotic pathway, mediates activation of executioner procaspase-3 by cleavage of the intersubunit linker (ISL) at site ¹⁷²IETD↓S. Although procaspase-6, another executioner, possesses two ISL cleavage sites (site 1, ¹⁷⁶DVVD↓N; site 2, ¹⁹⁰TEVD↓A), neither is directly cut by caspase-9. Thus, caspase-9 directly activates procaspase-3 but not procaspase-6. To elucidate this selectivity of caspase-9, we engineered constructs of procaspase-3 (e.g., swapping the ISL site, ¹⁷²IETD↓S, with DVVDN and TEVDA) and procaspase-6 (e.g., swapping site 1, ¹⁷⁶DVVD↓N, and site 2, ¹⁹⁰TEVD↓A, with IETDS). Using the substrate digestion data of these constructs, we show here that the P4-P1' sequence of procaspase-6 ISL site 1 (DVVDN) can be accessed but not cleaved by caspase-9. We also found that caspase-9 can recognize the P4-P1' sequence of procaspase-6 ISL site 2 (TEVDA); however, the local context of this cleavage site is the critical factor that prevents proteolytic cleavage. Overall, our data have demonstrated that both the sequence and the local context of the ISL cleavage sites play a vital role in preventing the activation of procaspase-6 directly by caspase-9.

Pyroptotic and non-pyroptotic effector functions of caspase-11

Innate immune cells, epithelial cells, and many other cell types are capable of detecting infection or tissue injury, thus mounting regulated immune response. Inflammasomes are highly sophisticated and effective orchestrators of innate immunity. These oligomerized multiprotein complexes are at the center of various innate immune pathways, including modulation of the cytoskeleton, production and maturation of cytokines, and control of bacterial growth and cell death. Inflammasome assembly often results in caspase‐1 activation, which is an inflammatory caspase that is involved in pyroptotic cell death and release of inflammatory cytokines in response to pathogen patterns and endogenous danger stimuli. However, the nature of stimuli and inflammasome components are diverse. Caspase‐1 activation mediated release of mature IL‐1β and IL‐18 in response to canonical stimuli initiated by NOD‐like receptor (NLR), and apoptosis‐associated speck‐like protein containing a caspase recruitment domain (ASC). On the other hand, caspase‐11 delineates a non‐canonical inflammasome that promotes pyroptotic cell death and non‐pyroptotic functions in response to non‐canonical stimuli. Caspase‐11 in mice and its homologues in humans (caspase‐4/5) belong to caspase‐1 family of cysteine proteases, and play a role in inflammation. Knockout mice provided new genetic tools to study inflammatory caspases and revealed the role of caspase‐11 in mediating septic shock in response to lethal doses of lipopolysaccharide (LPS). Recognition of LPS mediates caspase‐11 activation, which promotes a myriad of downstream effects that include pyroptotic and non‐pyroptotic effector functions. Therefore, the physiological functions of caspase‐11 are much broader than its previously established roles in apoptosis and cytokine maturation. Inflammation induced by exogenous or endogenous agents can be detrimental and, if excessive, can result in organ and tissue damage. Consequently, the existence of sophisticated mechanisms that tightly regulate the specificity and sensitivity of inflammasome pathways provides a fine‐tuning balance between adequate immune response and minimal tissue damage. In this review, we summarize effector functions of caspase‐11.

Dimerization and auto-processing induce caspase-11 protease activation within the non-canonical inflammasome

This study provides a detailed molecular mechanism for caspase-11 activation within the non-canonical inflammasome, giving new insight into host defence against cytosolic bacterial infection.

Caspase-11 is a cytosolic sensor and protease that drives innate immune responses to the bacterial cell wall component, LPS. Caspase-11 provides defence against cytosolic Gram-negative bacteria; however, excessive caspase-11 responses contribute to murine endotoxic shock. Upon sensing LPS, caspase-11 assembles a higher order structure called the non-canonical inflammasome that enables the activation of caspase-11 protease function, leading to gasdermin D cleavage and cell death. The mechanism by which caspase-11 acquires protease function is, however, poorly defined. Here, we show that caspase-11 dimerization is necessary and sufficient for eliciting basal caspase-11 protease function, such as the ability to auto-cleave. We further show that during non-canonical inflammasome signalling, caspase-11 self-cleaves at site (D285) within the linker connecting the large and small enzymatic subunits. Self-cleavage at the D285 site is required to generate the fully active caspase-11 protease (proposed here to be p32/p10) that mediates gasdermin D cleavage, macrophage death, and NLRP3-dependent IL-1β production. This study provides a detailed molecular mechanism by which LPS induces caspase-11–driven inflammation and cell death to provide host defence against cytosolic bacterial infection.

Investigation of the neuroprotective effects of Lycium barbarum water extract in apoptotic cells and Alzheimer's disease mice

Alzheimer's disease (AD) affects people worldwide and is caused by chronic and progressive damage to the central nervous system. Lycium barbarum (LB), a renowned functional food and medicinal plant in Southeast Asia, may possess protective effects against nerve injury. The present study aimed to investigate the neuroprotective effects of LB water extract in a differentiated (D)PC12 cellular apoptosis model induced by L-glutamic acid (L-Glu), and a mouse model of AD, induced by the combination of AlCl₃ and D-galactose. LB markedly increased DPC12 cell survival against L-Glu induced damage by increasing cell viability, reducing the apoptosis rate and G1 phase arrest, suppressing intracellular reactive oxygen species accumulation, blocking Ca²⁺ overload and preventing mitochondrial membrane potential depolarization. LB additionally normalized the expression levels of apoptosis regulator Bcl-2, apoptosis regulator BAX, and cleaved caspase-3, −8 and −9 in L-Glu exposed cells. In the AD mouse model, LB increased the amount of horizontal and vertical movement in the autonomic activity test, improved endurance time in the rotarod test and decreased escape latency time in the Morris water maze test. Additionally, the levels of acetylcholine and choline acetyltransferase were significantly increased in the serum and hypothalamus in the LB-treated AD mice. These data suggested that LB may exert neuroprotective effects and may aid in preventing neurodegenerative disease.

Modulation of procaspase-7 self-activation by PEST amino acid residues of the N-terminal prodomain and intersubunit linker

Procaspase-7 zymogen polypeptide is composed of a short prodomain, a large subunit (p20), and a small subunit (p10) connected to an intersubunit linker. Caspase-7 is activated by an initiator caspase-8 and -9, or by autocatalysis after specific cleavage at IQAD¹⁹⁸↓S located at the intersubunit linker. Previously, we identified that PEST regions made of amino acid residues Pro (P), Glu (E), Asp (D), Ser (S), Thr (T), Asn (N), and Gln (Q) are conserved flanking amino acid residues in the cleavage sites within a prodomain and intersubunit linker of all caspase family members. Here we tested the impact of alanine substitution of PEST amino acid residues on procaspase-7 proteolytic self-activation directly in Escherichia coli. The p20 and p10 subunit cleavage were significantly delayed in double caspase-7 mutants in the prodomain (N18A/P26A) and intersubunit linker (S199A/P201A), compared with the wild-type caspase-7. The S199A/P201A mutants effectively inhibited the p10 small subunit cleavage. However, the mutations did not change the kinetic parameters (k_cat/K_M) and optimal tetrapeptide specificity (DEVD) of the purified mutant enzymes. The results suggest a role of PEST-amino acid residues in the molecular mechanism for prodomain and intersubunit cleavage and caspase-7 self-activation.

Calf Spleen Extractive Injection (CSEI), a small peptides enriched extraction, induces human hepatocellular carcinoma cell apoptosis via ROS/MAPKs dependent mitochondrial pathway

Calf Spleen Extractive Injection (CSEI), a small peptides enriched extraction, performs immunomodulatory activity on cancer patients suffering from radiotherapy or chemotherapy. The present study aims to investigate the anti-hepatocellular carcinoma effects of CSEI in cells and tumor-xenografted mouse models. In HepG2 and SMMC-7721 cells, CSEI reduced cell viability, enhanced apoptosis rate, caused reactive oxygen species (ROS) accumulation, inhibited migration ability, and induced caspases cascade and mitochondrial membrane potential dissipation. CSEI significantly inhibited HepG2-xenografted tumor growth in nude mice. In cell and animal experiments, CSEI increased the activations of pro-apoptotic proteins including caspase 8, caspase 9 and caspase 3; meanwhile, it suppressed the expressions of anti-apoptotic protein B-cell lymphoma 2 (Bcl-2) and anti-oxidation proteins, such as nuclear factor-erythroid 2 related factor 2 (Nrf2) and catalase (CAT). The enhanced phosphorylation of P38 and c-JunN-terminalkinase (JNK), and decreased phosphorylation of extra cellular signal-regulated protein kinase (ERKs) were observed in CSEI-treated cells and tumor tissues. CSEI-induced cell viability reduction was significantly attenuated by N-Acetyl-l-cysteine (a ROS inhibitor) pretreatment. All data demonstrated that the upregulated oxidative stress status and the altered mitogen-activated protein kinases (MAPKs) phosphorylation contributed to CSEI-driven mitochondrial dysfunction. Taken together, CSEI exactly induced apoptosis in human hepatocellular carcinoma cells via ROS/MAPKs dependent mitochondrial pathway.

Cordycepin, a Natural Antineoplastic Agent, Induces Apoptosis of Breast Cancer Cells via Caspase-dependent Pathways

Cordycepin, a major compound separated from Cordyceps sinensis, is known as a potential novel candidate for cancer therapy. Breast cancer, the most typical cancer diagnosed among women, remains a global health problem. In this study, the anti-breast cancer property of cordycepin and its underlying mechanisms was investigated. The direct effects of cordycepin on breast cancer cells both in in vitro and in vivo experiments were evaluated. Cordycepin exerted cytotoxicity in MCF-7 and MDA-MB-231 cells confirmed by reduced cell viability, inhibition of cell proliferation, enhanced lactate dehydrogenase release and reactive oxygen species accumulation, induced mitochondrial dysfunction and nuclear apoptosis in human breast cancer cells. Cordycepin increased the activation of pro-apoptotic proteins, including caspase-8, caspase-9, caspase-3 and Bax, and suppressed the expression of the anti-apoptotic protein, B-cell lymphoma 2 (Bcl-2). The inhibition on MCF-7-xenografted tumor growth in nude mice further confirmed cordycepin's anti-breast cancer effect. These aforementioned results reveal that cordycepin induces apoptosis in human breast cancer cells via caspase-dependent pathways. The data shed light on the possibility of cordycepin being a safe agent for breast cancer treatment.

General in vitro caspase assay procedures

One of the most valuable tools that have been developed for the study of apoptosis is the availability of recombinant active caspases. The determination of caspase substrate preference, the design of sensitive substrates and potent inhibitors, the resolution of caspase structures, the elucidation of their activation mechanisms, and the identification of their substrates were made possible by the availability of sufficient amounts of enzymatically pure caspases. The current chapter describes at length the expression, purification, and basic enzymatic characterization of apoptotic caspases.

Lengthening the intersubunit linker of procaspase 3 leads to constitutive activation

The conformational ensemble of procaspase 3, the primary executioner in apoptosis, contains two major forms, inactive and active, with the inactive state favored in the native ensemble. A region of the protein known as the intersubunit linker (IL) is cleaved during maturation, resulting in movement of the IL out of the dimer interface and subsequent active site formation (activation-by-cleavage mechanism). We examined two models for the role of the IL in maintaining the inactive conformer, an IL-extension model versus a hydrophobic cluster model, and we show that increasing the length of the IL by introducing 3-5 alanines results in constitutively active procaspases. Active site labeling and subsequent analyses by mass spectrometry show that the full-length zymogen is enzymatically active. We also show that minor populations of alternately cleaved procaspase result from processing at D169 when the normal cleavage site, D175, is unavailable. Importantly, the alternately cleaved proteins have little to no activity, but increased flexibility of the linker increases the exposure of D169. The data show that releasing the strain of the short IL, in and of itself, is not sufficient to populate the active conformer of the native ensemble. The IL must also allow for interactions that stabilize the active site, possibly from a combination of optimal length, flexibility in the IL, and specific contacts between the IL and interface. The results provide further evidence that substantial energy is required to shift the protein to the active conformer. As a result, the activation-by-cleavage mechanism dominates in the cell.

Structural asymmetry of procaspase-7 bound to a specific inhibitor

Caspase-7 is expressed as a proenzyme and is activated by initiator caspases upon the transmission of cell-death signals. Despite extensive structural and biochemical analyses, many questions regarding the mechanism of caspase-7 activation remain unanswered. Caspase-7 is auto-activated during overexpression in Escherichia coli, even in the absence of initiator caspases, indicating that procaspase-7 has intrinsic catalytic activity. When variants of procaspase-7 with altered L2 loops were prepared, a variant with six inserted amino acids showed meaningful catalytic activity which was inhibited by Ac-DEVD-CHO. The kinetic constants of the procaspase-7 variant were determined and its three-dimensional structure was solved with and without bound inhibitor. The homodimeric procaspase-7 bound to the inhibitor revealed an asymmetry. One monomer formed a complete active site bound to the inhibitor in collaboration with the L2 loop from the other monomer, whereas the other monomer had an incomplete active site despite the bound inhibitor. Consequently, the two L2 loops in homodimeric procaspase-7 served as inherent L2 and L2' loops forming one complete active site. These data represent the first three-dimensional structure of a procaspase-7 variant bound to a specific inhibitor, Ac-DEVD-CHO, and provide insight into the folding mechanism during caspase-7 activation and the basal activity level of procaspase-7.

A designed redox-controlled caspase

Caspases are a powerful class of cysteine proteases. Introduction of activated caspases in healthy or cancerous cells results in induction of apoptotic cell death. In this study, we have designed and characterized a version of caspase-7 that can be inactivated under oxidizing extracellular conditions and then reactivated under reducing intracellular conditions. This version of caspase-7 is allosterically inactivated when two of the substrate-binding loops are locked together via an engineered disulfide. When this disulfide is reduced, the protein regains its full function. The inactive loop-locked version of caspase-7 can be readily observed by immunoblotting and mass spectrometry. The reduced and reactivated form of the enzyme observed crystallographically is the first caspase-7 structure in which the substrate-binding groove is properly ordered even in the absence of an active-site ligand. In the reactivated structure, the catalytic-dyad cysteine-histidine are positioned 3.5 Å apart in an orientation that is capable of supporting catalysis. This redox-controlled version of caspase-7 is particularly well suited for targeted cell death in concert with redox-triggered delivery vehicles.

Cysteine protease attribute of eukaryotic ribosomal protein S4

BACKGROUND

Ribosomal proteins often carry out extraribosomal functions. The protein S4 from the smaller subunit of Escherichia coli, for instance, regulates self synthesis and acts as a transcription factor. In humans, S4 might be involved in Turner syndrome. Recent studies also associate many ribosomal proteins with malignancy, and cell death and survival. The list of extraribosomal functions of ribosomal proteins thus continues to grow.

METHODS

Enzymatic action of recombinant wheat S4 on fluorogenic peptide substrates Ac-XEXD↓-AFC (N-acetyl-residue-Glu-residue-Asp-7-amino-4-trifluoromethylcoumarin) and Z-FR↓-AMC (N-CBZ-Phe-Arg-aminomethylcoumarin) as well as proteins has been examined under a variety of solution conditions.

RESULTS

Eukaryotic ribosomal protein S4 is an endoprotease exhibiting all characteristics of cysteine proteases. The K(m) value for the cleavage of Z-FR↓-AMC by a cysteine mutant (C41F) is about 70-fold higher relative to that for the wild-type protein under identical conditions, implying that S4 is indeed a cysteine protease. Interestingly, activity responses of the S4 protein and caspases toward environmental parameters, including pH, temperature, ionic strength, and Mg(2+) and Zn(2+) concentrations, are quite similar. Respective kinetic constants for their cleavage action on Ac-LEHD↓-AFC are also similar. However, S4 cannot be a caspase, because unlike the latter it also hydrolyzes the cathepsin substrate Z-FR↓-AMC.

GENERAL SIGNIFICANCE

The eukaryotic S4 is a generic cysteine protease capable of hydrolyzing a broad spectrum of synthetic substrates and proteins. The enzyme attribute of eukaryotic ribosomal protein S4 is a new phenomenon. Its possible involvement in cell growth and proliferations are presented in the light of known extraribosomal roles of ribosomal proteins.

FLIP(L) induces caspase 8 activity in the absence of interdomain caspase 8 cleavage and alters substrate specificity

Caspase 8 is an initiator caspase that is activated by death receptors to initiate the extrinsic pathway of apoptosis. Caspase 8 activation involves dimerization and subsequent interdomain autoprocessing of caspase 8 zymogens, and recently published work has established that elimination of the autoprocessing site of caspase 8 abrogates its pro-apoptotic function while leaving its proliferative function intact. The observation that the developmental abnormalities of caspase 8-deficient mice are shared by mice lacking the dimerization adapter FADD (Fas-associated death domain) or the caspase paralogue FLIP(L) [FLICE (FADD-like interleukin 1β-converting enzyme)-inhibitory protein, long form] has led to the hypothesis that FADD-dependent formation of heterodimers between caspase 8 and FLIP(L) could mediate the developmental role of caspase 8. In the present study, using an inducible dimerization system we demonstrate that cleavage of the catalytic domain of caspase 8 is crucial for its activity in the context of activation by homodimerization. However, we find that use of FLIP(L) as a partner for caspase 8 in dimerization-induced activation rescues the requirement for intersubunit linker proteolysis in both protomers. Moreover, before processing, caspase 8 in complex with FLIP(L) does not generate a fully active enzyme, but an attenuated species able to process only selected natural substrates. Based on these results we propose a mechanism of caspase 8 activation by dimerization in the presence of FLIP(L), as well as a mechanism of caspase 8 functional divergence in apoptotic and non-apoptotic pathways.