CD20 expression regulates CD37 levels in B-cell lymphoma - implications for immunotherapies.

Rituximab (RTX) plus chemotherapy (R-CHOP) applied as a first-line therapy for lymphoma leads to a relapse in approximately 40% of the patients. Therefore, novel approaches to treat aggressive lymphomas are being intensively investigated. Several RTX-resistant (RR) cell lines have been established as surrogate models to study resistance to R-CHOP. Our study reveals that RR cells are characterized by a major downregulation of CD37, a molecule currently explored as a target for immunotherapy. Using CD20 knockout (KO) cell lines, we demonstrate that CD20 and CD37 form a complex, and hypothesize that the presence of CD20 stabilizes CD37 in the cell membrane. Consequently, we observe a diminished cytotoxicity of anti-CD37 monoclonal antibody (mAb) in complement-dependent cytotoxicity in both RR and CD20 KO cells that can be partially restored upon lysosome inhibition. On the other hand, the internalization rate of anti-CD37 mAb in CD20 KO cells is increased when compared to controls, suggesting unhampered efficacy of antibody drug conjugates (ADCs). Importantly, even a major downregulation in CD37 levels does not hamper the efficacy of CD37-directed chimeric antigen receptor (CAR) T cells. In summary, we present here a novel mechanism of CD37 regulation with further implications for the use of anti-CD37 immunotherapies.

Thermoprotection by a cell membrane-localized metacaspase in a green alga.

Caspases are restricted to animals, while other organisms, including plants, possess metacaspases (MCAs), a more ancient and broader class of structurally related yet biochemically distinct proteases. Our current understanding of plant MCAs is derived from studies in streptophytes, and mostly in Arabidopsis (Arabidopsis thaliana) with 9 MCAs with partially redundant activities. In contrast to streptophytes, most chlorophytes contain only 1 or 2 uncharacterized MCAs, providing an excellent platform for MCA research. Here we investigated CrMCA-II, the single type-II MCA from the model chlorophyte Chlamydomonas (Chlamydomonas reinhardtii). Surprisingly, unlike other studied MCAs and similar to caspases, CrMCA-II dimerizes both in vitro and in vivo. Furthermore, activation of CrMCA-II in vivo correlated with its dimerization. Most of CrMCA-II in the cell was present as a proenzyme (zymogen) attached to the plasma membrane (PM). Deletion of CrMCA-II by genome editing compromised thermotolerance, leading to increased cell death under heat stress. Adding back either wild-type or catalytically dead CrMCA-II restored thermoprotection, suggesting that its proteolytic activity is dispensable for this effect. Finally, we connected the non-proteolytic role of CrMCA-II in thermotolerance to the ability to modulate PM fluidity. Our study reveals an ancient, MCA-dependent thermotolerance mechanism retained by Chlamydomonas and probably lost during the evolution of multicellularity.

Imaging of proteases using activity-based probes.

Proteases (proteolytic enzymes) are proteins that catalyze one of the most important biochemical reactions, namely the hydrolysis of the peptide bond in peptide and protein substrates. Therefore these molecular biocatalysts participate in virtually all living processes. The proper balance between intact and processed protease substrates enables to maintenance of homeostasis from a single-cell level to the whole living system. However, when the proteolytic activity is altered, this delicate balance is disturbed, which might lead to the development of a plethora of diseases. Given this, monitoring proteolytic activity is indispensable to understanding how proteases operate in disease lesions and how their altered catalytic activity might be harnessed for a better diagnosis and treatment. In this manuscript, we provide a critical review of the recent development of protease chemical probes which are small molecules that detect proteolytic activity by interacting with protease active site, individual proteases as well as complex proteolytic networks.

Selective chemical reagents to investigate the role of caspase 6 in apoptosis in acute leukemia T cells.

Activated effector caspases 3, 6 and 7 are responsible for cleaving a number of target substrates, leading to the ultimate destruction of cells via apoptosis. The functions of caspases 3 and 7 in apoptosis execution have been widely studied over the years with multiple chemical probes for both of these enzymes. In contrast, caspase 6 seems to be largely neglected when compared to the heavily studied caspases 3 and 7. Therefore, the development of new small-molecule reagents for the selective detection and visualization of caspase 6 activity can improve our understanding of molecular circuits of apoptosis and shed new light on how they intertwine with other types of programmed cell death. In this study, we profiled caspase 6 substrate specificity at the P5 position and discovered that, similar to caspase 2, caspase 6 prefers pentapeptide substrates over tetrapeptides. Based on these data, we developed a set of chemical reagents for caspase 6 investigation, including coumarin-based fluorescent substrates, irreversible inhibitors and selective aggregation-induced emission luminogens (AIEgens). We showed that AIEgens are able to distinguish between caspase 3 and caspase 6 in vitro. Finally, we validated the efficiency and selectivity of the synthesized reagents by monitoring lamin A and PARP cleavage via mass cytometry and western blot analysis. We propose that our reagents may provide new research prospects for single-cell monitoring of caspase 6 activity to reveal its function in programmed cell death pathways.

Investigation of osteoclast cathepsin K activity in osteoclastogenesis and bone loss using a set of chemical reagents.

Cathepsin K (CatK) is a lysosomal cysteine protease whose highest expression is found in osteoclasts, which are the cells responsible for bone resorption. Investigations of the functions and physiological relevance of CatK have often relied on antibody-related techniques, which makes studying its activity patterns a challenging task. Hence, we developed a set of chemical tools for the investigation of CatK activity. We show that our probe is a valuable tool for monitoring the proteolytic activation of CatK during osteoclast formation. Moreover, we demonstrate that our inhibitor of CatK impedes osteoclastogenesis and bone resorption and that CatK is stored in its active form in osteoclasts within their lysosomal compartment and mainly in the ruffled borders of osteoclasts. Given that our probe recognizes active CatK within living cells without exhibiting any observed cytotoxicity in the several models tested, we expect that it would be well suited to theranostic applications in CatK-related diseases.

Betulin and Its Derivatives Reduce Inflammation and COX-2 Activity in Macrophages.

Betulin is a heavily studied natural compound for its use as an anticancer or pro-regenerative agent. The structural similarity between betulin to steroids gives rise to the idea that the substance may as well act as an anti-inflammatory drug. This study is the first to describe the anti-inflammatory properties of betulinic acid, betulin, and its derivatives with amino acids 1,4-diaminebutane (Dab), 1,3-diaminepropane (Dap), Ornithine (Orn), and lysine (Lys) on murine macrophages from lymphoma site. The compounds were compared to dexamethasone. To establish the response of the macrophages to the natural compounds, we tested the viability as well as sensitivity to the inflammatory signaling (IFNγR). IL-6 secretory properties and HSP-70 content in the cells were examined. Furthermore, we characterized the effects of compounds on the inhibition of cyclooxygenase-2 (COX-2) activity both in the enzymatic assays and molecular docking studies. Then, the changes in COX-2 expression after betulin treatment were assessed. Betulin and betulinic acid are the low-cytotoxicity compounds with the highest potential to decrease inflammation via reduced IL-6 secretion. To some extent, they induce the reorganization of IFNγR with nearly no effect on COX-2 activity. Conversely, Bet-Orn and Bet-Lys are highly cytotoxic and induce the aggregation of IFNγR. Besides, Bet-Lys reduces the activity of COX-2 to a higher degree than dexamethasone. Bet-Orn is the only one to increase the HSP-70 content in the macrophages. In case of IL-6 reduction, all compounds were more potent than dexamethasone.

Diagnostic and therapeutic potential of protease inhibition.

Proteases are enzymes that hydrolyze peptide bonds in proteins and peptides; thus, they control virtually all biological processes. Our understanding of protease function has advanced considerably from nonselective digestive enzymes to highly specialized molecular scissors that orchestrate complex signaling networks through a limited proteolysis. The catalytic activity of proteases is tightly regulated at several levels, ranging from gene expression through trafficking and maturation to posttranslational modifications. However, when this delicate balance is disturbed, many diseases develop, including cancer, inflammatory disorders, diabetes, and neurodegenerative diseases. This new understanding of the role of proteases in pathologic physiology indicates that these enzymes represent excellent molecular targets for the development of therapeutic inhibitors, as well as for the design of chemical probes to visualize their redundant activity. Recently, numerous platform technologies have been developed to identify and optimize protease substrates and inhibitors, which were further used as lead structures for the development of chemical probes and therapeutic drugs. Due to this considerable success, the clinical potential of proteases in therapeutics and diagnostics is rapidly growing and is still not completely explored. Therefore, small molecules that can selectively target aberrant protease activity are emerging in diseases cells. In this review, we describe modern trends in the design of protease drugs as well as small molecule activity-based probes to visualize selected proteases in clinical settings.

The roles of cellular protease interactions in viral infections and programmed cell death: a lesson learned from the SARS-CoV-2 outbreak and COVID-19 pandemic.

The unprecedented pandemic of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2), which leads to COVID-19, is threatening global health. Over the last 2 years, we have witnessed rapid progress in research focusing on developing new antiviral vaccines and drugs, as well as in academic and clinical efforts to understand the biology and pathology of COVID-19. The roles of proteases among master regulators of SARS-CoV-2 invasion and replication and their pivotal roles in host defence against this pathogen, including programmed cell death, have not been well established. Our understanding of protease function in health and disease has increased considerably over the last two decades, with caspases, matrix metalloproteases, and transmembrane serine proteases representing the most prominent examples. Therefore, during the COVID-19 pandemic, these enzymes have been investigated as potential molecular targets for therapeutic interventions. Proteases that are responsible for SARS-CoV-2 cell entry and replication, such as TMPRSS2, ACE2 or cathepsins, are screened with inhibitor libraries to discover lead structures for further drug design that would prevent virus multiplication. On the other hand, proteases that orchestrate programmed cell death can also be harnessed to enhance the desired demise of infected cells through apoptosis or to attenuate highly inflammatory lytic cell death that leads to undesired cytokine storms, a major hallmark of severe COVID-19. Given the prominent role of proteases in SARS-CoV-2-induced cell death, we discuss the individual roles of these enzymes and their catalytic interactions in the pathology of COVID-19 in this article. We provide a rationale for targeting proteases participating in cell death as potential COVID-19 treatments and identify knowledge gaps that might be investigated to better understand the mechanism underlying SARS-CoV-2-induced cell death.

Resurrection of an ancient inflammatory locus reveals switch to caspase-1 specificity on a caspase-4 scaffold.

Pyroptosis is a mechanism of inflammatory cell death mediated by the activation of the prolytic protein gasdermin D by caspase-1, caspase-4, and caspase-5 in human, and caspase-1 and caspase-11 in mouse. In addition, caspase-1 amplifies inflammation by proteolytic activation of cytokine interleukin-1ß (IL-1ß). Modern mammals of the order Carnivora lack the caspase-1 catalytic domain but express an unusual version of caspase-4 that can activate both gasdermin D and IL-1ß. Seeking to understand the evolutionary origin of this caspase, we utilized the large amount of data available in public databases to perform ancestral sequence reconstruction of an inflammatory caspase of a Carnivora ancestor. We expressed the catalytic domain of this putative ancestor in Escherichia coli, purified it, and compared its substrate specificity on synthetic and protein substrates to extant caspases. We demonstrated that it activates gasdermin D but has reduced ability to activate IL-1ß. Our reconstruction suggests that caspase-1 was lost in a Carnivora ancestor, perhaps upon a selective pressure for which the generation of biologically active IL-1ß by caspase-1 was detrimental. We speculate that later, a Carnivora encountered selective pressures that required the production of IL-1ß, and caspase-4 subsequently gained this activity. This hypothesis would explain why extant Carnivora possess an inflammatory caspase with caspase-1 catalytic function placed on a caspase-4 scaffold.

Engineering caspase 7 as an affinity reagent to capture proteolytic products.

Many proteases recognize their substrates with high specificities, with this in mind, it should theoretically be possible to utilize the substrate binding cleft of a protease as a scaffold to engineer an affinity reagent. In this study, we sought to develop reagents that would differentiate between substrates and products of proteolysis, based on a caspase 7 scaffold. Firstly, we engineered a form of caspase 7 that can undergo conversion to a substrate binding conformation without catalysis. Seeking to generate a product-only trap, we further engineered this construct by incorporating mutations that compensate for the generation of a negative charge in the neo C terminus of a newly generated product. This was accomplished with only three substitutions within the substrate binding cleft. Moreover, the affinity of the product trap for peptides was comparable to the affinity of caspase 7 to parental substrates. Finally, generation of a hybrid fluorescent protein with the product trap provided a reagent that specifically recognized apoptotic cells and highlights the versatility of such an approach in developing affinity and imaging agents for a variety of cysteine and serine proteases.

Multiplexed Probing of Proteolytic Enzymes Using Mass Cytometry-Compatible Activity-Based Probes.

The subset of the proteome that contains enzymes in their catalytically active form can be interrogated by using probes targeted toward individual specific enzymes. A subset of such enzymes are proteases that are frequently studied with activity-based probes, small inhibitors equipped with a detectable tag, commonly a fluorophore. Due to the spectral overlap of these commonly used fluorophores, multiplex analysis becomes limited. To overcome this, we developed a series of protease-selective lanthanide-labeled probes compatible with mass cytometry giving us the ability to monitor the activity of multiple proteases in parallel. Using these probes, we were able to identify the distribution of four proteases with different active site geometries in three cell lines and peripheral blood mononuclear cells. This provides a framework for the use of mass cytometry for multiplexed enzyme activity detection.

Extended subsite profiling of the pyroptosis effector protein gasdermin D reveals a region recognized by inflammatory caspase-11.

Pyroptosis is the caspase-dependent inflammatory cell death mechanism that underpins the innate immune response against pathogens and is dysregulated in inflammatory disorders. Pyroptosis occurs via two pathways: the canonical pathway, signaled by caspase-1, and the noncanonical pathway, regulated by mouse caspase-11 and human caspase-4/5. All inflammatory caspases activate the pyroptosis effector protein gasdermin D, but caspase-1 mostly activates the inflammatory cytokine precursors prointerleukin-18 and prointerleukin-1ß (pro-IL18/pro-IL1ß). Here, in vitro cleavage assays with recombinant proteins confirmed that caspase-11 prefers cleaving gasdermin D over the pro-ILs. However, we found that caspase-11 recognizes protein substrates through a mechanism that is different from that of most caspases. Results of kinetics analysis with synthetic fluorogenic peptides indicated that P1'-P4', the C-terminal gasdermin D region adjacent to the cleavage site, influences gasdermin D recognition by caspase-11. Furthermore, introducing the gasdermin D P1'-P4' region into pro-IL18 enhanced catalysis by caspase-11 to levels comparable with that of gasdermin D cleavage. Pro-IL1ß cleavage was only moderately enhanced by similar substitutions. We conclude that caspase-11 specificity is mediated by the P1'-P4' region in its substrate gasdermin D, and similar experiments confirmed that the substrate specificities of the human orthologs of caspase-11, i.e. caspase-4 and caspase-5, are ruled by the same mechanism. We propose that P1'-P4'-based inhibitors could be exploited to specifically target inflammatory caspases.

Profiling of flaviviral NS2B-NS3 protease specificity provides a structural basis for the development of selective chemical tools that differentiate Dengue from Zika and West Nile viruses.

West Nile virus (WNV) and Dengue virus (DENV) are mosquito-borne pathogenic flaviviruses. The NS2B-NS3 proteases found in these viruses are responsible for polyprotein processing and are therefore considered promising medical targets. Another ortholog of these proteases is found in Zika virus (ZIKV). In this work, we applied a combinatorial chemistry approach - Hybrid Combinatorial Substrate Library (HyCoSuL), to compare the substrate specificity profile at the P4-P1 positions of the NS2B-NS3 proteases found in all three viruses. The obtained data demonstrate that Zika and West Nile virus NS2B-NS3 proteases display highly overlapping substrate specificity in all binding pockets, while the Dengue ortholog has slightly different preferences toward natural and unnatural amino acids at the P2 and P4 positions. We used this information to extract specific peptide sequences recognized by the Dengue NS2B-NS3 protease. Next, we applied this knowledge to design a selective substrate and activity-based probe for the Dengue NS2B-NS3 protease. Our work provides a structural framework for the design of inhibitors, which could be used as a lead structure for drug development efforts.

Protease-activated prodrugs: strategies, challenges, and future directions.

Proteases play critical roles in virtually all biological processes, including proliferation, cell death and survival, protein turnover, and migration. However, when dysregulated, these enzymes contribute to the progression of multiple diseases, with cancer, neurodegenerative disorders, inflammation, and blood disorders being the most prominent examples. For a long time, disease-associated proteases have been used for the activation of various prodrugs due to their well-characterized catalytic activity and ability to selectively cleave only those substrates that strictly correspond with their active site architecture. To date, versatile peptide sequences that are cleaved by proteases in a site-specific manner have been utilized as bioactive linkers for the targeted delivery of multiple types of cargo, including fluorescent dyes, photosensitizers, cytotoxic drugs, antibiotics, and pro-antibodies. This platform is highly adaptive, as multiple protease-labile conjugates have already been developed, some of which are currently in clinical use for cancer treatment. In this review, recent advancements in the development of novel protease-cleavable linkers for selective drug delivery are described. Moreover, the current limitations regarding the selectivity of linkers are discussed, and the future perspectives that rely on the application of unnatural amino acids for the development of highly selective peptide linkers are also presented.

Exploring the prime site in caspases as a novel chemical strategy for understanding the mechanisms of cell death: a proof of concept study on necroptosis in cancer cells.

Caspases participate in regulated cell death mechanisms and are divided into apoptotic and proinflammatory caspases. The main problem in identifying the unique role of a particular caspase in the mechanisms of regulated cell death is their overlapping substrate specificity; caspases recognize and hydrolyze similar peptide substrates. Most studies focus on examining the non-prime sites of the caspases, yet there is a need for novel and more precise chemical tools to identify the molecular participants and mechanisms of programmed cell death pathways. Therefore, we developed an innovative chemical approach that examines the prime area of the caspase active sites. This method permits the agile parallel solid-phase synthesis of caspase inhibitors with a high yield and purity. Using synthesized compounds we have shown the similarities and differences in the prime area of the caspase active site and, as a proof of concept, we demonstrated the exclusive role of caspase-8 in necroptosis.

Legumain is upregulated in acute cardiovascular events and associated with improved outcome - potentially related to anti-inflammatory effects on macrophages.

BACKGROUND AND AIMS: We have previously found increased levels of the cysteine protease legumain in plasma and plaques from patients with carotid atherosclerosis. This study further investigated legumain during acute cardiovascular events. METHODS: Circulating levels of legumain from patients and legumain released from platelets were assessed by enzyme-linked-immunosorbent assay. Quantitative PCR and immunoblotting were used to study expression, while localization was visualized by immunohistochemistry. RESULTS: In the SUMMIT Malmö cohort (n = 339 with or without type 2 diabetes and/or cardiovascular disease [CVD], and 64 healthy controls), the levels of circulating legumain were associated with the presence of CVD in non-diabetics, with no relation to outcome. In symptomatic carotid plaques and in samples from both coronary and intracerebral thrombi obtained during acute cardiovascular events, legumain was co-localized with macrophages in the same regions as platelets. In vitro, legumain was shown to be present in and released from platelets upon activation. In addition, THP-1 macrophages exposed to releasate from activated platelets showed increased legumain expression. Interestingly, primary peripheral blood mononuclear cells stimulated with recombinant legumain promoted anti-inflammatory responses. Finally, in a STEMI population (POSTEMI; n = 272), patients had significantly higher circulating legumain before and immediately after percutaneous coronary intervention compared with healthy controls (n = 67), and high levels were associated with improved outcome. CONCLUSIONS: Our data demonstrate for the first time that legumain is upregulated during acute cardiovascular events and is associated with improved outcome.

Fluorescent probes towards selective cathepsin B detection and visualization in cancer cells and patient samples.

Human cysteine cathepsins constitute an 11-membered family of proteases responsible for degradation of proteins in cellular endosomal-lysosomal compartments as such, they play important roles in antigen processing, cellular stress signaling, autophagy, and senescence. Moreover, for many years these enzymes were also linked to tumor growth, invasion, angiogenesis and metastasis when upregulated. Individual biological roles of each cathepsin are difficult to establish, because of their redundancy and similar substrate specificities. Selective chemical tools that enable imaging of individual cathepsin activities in living cells, tumors, and the tumor microenvironment may provide a better insight into their functions. In this work, we used HyCoSuL technology to profile the substrate specificity of human cathepsin B. The use of unnatural amino acids in the substrate library enabled us to uncover the broad cathepsin B preferences that we utilized to design highly-selective substrates and fluorescent activity-based probes (ABPs). We further demonstrated that Cy5-labeled MP-CB-2 probe can selectively label cathepsin B in eighteen cancer cell lines tested, making this ABP highly suitable for other biological setups. Moreover, using Cy5-labelled MP-CB-2 we were able to demonstrate by fluorescence microscopy that in cancer cells cathepsins B and L share overlapping, but not identical subcellular localization.

Fluorescent activity-based probe for the selective detection of Factor VII activating protease (FSAP) in human plasma.

The zymogen form of circulating Factor VII activating protease (FSAP) is activated by histones that are released as a consequence of tissue damage or excessive inflammation. This is likely to have consequences in a number of disease conditions such as stroke, atherosclerosis, liver fibrosis, thrombosis and cancer. To investigate the existence, as well as the concentration of active FSAP (FSAPa) in complex biological systems an active site probe is needed. We used Hybrid Combinatorial Substrate Library (HyCoSuL) to screen for natural and unnatural amino acids that specifically bind to P4-P2 pockets of FSAPa. This information was used to designing a fluorogenic substrate (Ac-Pro-DTyr-Lys-Arg-ACC) as well as an irreversible, fluorogenic activity-based probe Cy5-6-Ahx-Pro-DTyr-Lys-ArgP(OPh)2. In normal human plasma the probe showed very low non-specific reactivity with some plasma proteins but upon activation of pro-FSAP with histones, strong labelling of FSAPa was observed. This labelling could be inhibited by aprotinin and was not found in the plasma of a subject that was homozygous for a polymorphism, which leads to loss of activity, or in plasma that was depleted of FSAP by antibodies. This 2nd generation substrate exhibited 6-fold higher catalytic efficiency than the 1st generation substrate and a much higher selectivity for FSAPa over other plasma proteases. This substrate and probe can be useful to detect and localize FSAPa in normal and pathological tissue and plasma to gain more insight into its functions.

A New Betulin Derivative Stimulates the Synthesis of Collagen in Human Fibroblasts Stronger than its Precursor.

BACKGROUND/AIM: The exploration of substances that stimulate collagen synthesis and retard the aging process of the skin is an active field of current research. The natural environment and plants used in traditional medicine have been a source of such substances. The aim of this study was to compare the stimulatory effect of betulin (BE), betulinic acid (BA) and the new derivative - betulin ester with diaminobutyl acid (BE-Dab-NH2) on collagen synthesis in human normal fibroblasts. MATERIALS AND METHODS: Primary fibroblast cultures were obtained from the gums of a healthy patient. The effect of the above-mentioned compounds was assessed by Sircol collagen assay, immunocytochemistry, and proliferation test. RESULTS: Fibroblasts cultured in the presence of BE-Dab-NH2 produced 6.85 times more collagen than control cells, 7.85 times more than those cultured in the presence of BA and 6.31 times more than those cultured in the presence of BE. An intense immunocytochemical reaction for collagen type I and III was found in fibroblasts cultured in the presence of BE-Dab-NH2 Conclusion: BE-Dab-NH2 stimulates significantly more collagen synthesis in normal human fibroblasts than its precursor.