Structural Bases of the Redox-dependent Conformational Switch in the Serpin PAI-2

Analysis of complement deposition and viral RNA in placentas of COVID-19 patients

COVID-19, the disease caused by the novel Coronavirus, SARS-CoV-2, is increasingly being recognized as a systemic thrombotic and microvascular injury syndrome that may have its roots in complement activation. We had the opportunity to study the placental pathology of five full-term births to COVID-19 patients. All five exhibited histology indicative of fetal vascular malperfusion characterized by focal avascular villi and thrombi in larger fetal vessels. Vascular complement deposition in the placentas was not abnormal, and staining for viral RNA and viral spike protein was negative. While all cases resulted in healthy, term deliveries, these findings indicate the systemic nature of COVID-19 infection. The finding of vascular thrombosis without complement deposition may reflect the systemic nature of COVID-19's procoagulant effects unrelated to systemic complement activation.

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This paper explores thrombosis in the placentas COVID-19-positive patients at our hospital

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Potential prothrombotic mechanisms are explored.

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Direct infection of the placentas is ruled out as a cause.

Interaction of the cell adhesion molecule CHL1 with vitronectin, integrins, and the plasminogen activator inhibitor-2 promotes CHL1-induced neurite outgrowth and neuronal migration

The cell adhesion molecule close homolog of L1 (CHL1) plays important functional roles in the developing and adult nervous system. In search of the binding partners that mediate the diverse and sometimes opposing functions of CHL1, the extracellular matrix-associated proteins vitronectin and plasminogen activator inhibitor-2 (PAI-2) were identified as novel CHL1 interaction partners and tested for involvement in CHL1-dependent functions during mouse cerebellar development. CHL1-induced cerebellar neurite outgrowth and cell migration at postnatal days 6-8 were inhibited by a CHL1-derived peptide comprising the integrin binding RGD motif, and by antibodies against vitronectin or several integrins, indicating a vitronectin-dependent integrin-mediated pathway. A PAI-2-derived peptide, or antibodies against PAI-2, urokinase type plasminogen activator (uPA), uPA receptor, and several integrins reduced cell migration. CHL1 colocalized with vitronectin, PAI-2, and several integrins in cerebellar granule cells, suggesting an association among these proteins. Interestingly, at the slightly earlier age of 4-5 d, cerebellar neurons did not depend on CHL1 for neuritogenesis and cell migration. However, differentiation of progenitor cells into neurons at this stage was dependent on homophilic CHL1-CHL1 interactions. These observations indicate that homophilic CHL1 trans-interactions regulate differentiation of neuronal progenitor cells at early postnatal stages, while heterophilic trans-interactions of CHL1 with vitronectin, integrins, and the plasminogen activator system regulate neuritogenesis and neuronal cell migration at a later postnatal stage of cerebellar morphogenesis. Thus, within very narrow time windows in postnatal cerebellar development, distinct types of molecular interactions mediated by CHL1 underlie the diverse functions of this protein.

Redox sensing by proteins: oxidative modifications on cysteines and the consequent events

SIGNIFICANCE

Reactive oxygen species (ROS) are not only essential for the cell's normal functions, but also mediate many pathological effects. When cells experience oxidative stress, proteins are modulated by redox changes and ultimately generate novel signaling patterns. It remains elusive how proteins are modulated, rather than simply damaged, by ROS and then mediate the diverse cellular responses.

RECENT ADVANCES

During the past decade, researchers frequently used "redox sensor" for proteins. However, the term "redox sensing" has not been clearly defined to date. Thiols of cysteines are subjected to oxidative modifications. The conformation changes and the various types of post-translational modifications (PTMs) may result from thiol oxidation of the same protein or other proteins. The molecular effects of redox sensing include changes in protein activity, abundance, localization, and interaction with other biomacromolecules.

CRITICAL ISSUES

We discuss the emerging concept of cysteine-based redox sensing, emphasizing "sensing redox changes by proteins using their thiols." ROS are an input, and the conformation changes and/or the other PTMs after thiol oxidation are the output of redox sensing. Among dozens of redox sensing proteins listed in this article, SENP3 and caspase-9, which have been investigated in our work, are given particular attention. We also introduce the notion of biphasic and compartment-specific redox sensing by nuclear factor kappa B.

FUTURE DIRECTIONS

Understanding chemical modifications and conformational changes following protein redox sensing requires more studies in mass spectrometry and crystallography. Redox-indicative probes in live cells and tissues will help monitor redox-related biological and pathological progresses.

Association of plasminogen activator inhibitor type 2 (PAI-2) with proteasome within endothelial cells activated with inflammatory stimuli

Quiescent endothelial cells contain low concentrations of plasminogen activator inhibitor type 2 (PAI-2). However, its synthesis can be rapidly stimulated by a variety of inflammatory mediators. In this study, we provide evidence that PAI-2 interacts with proteasome and affects its activity in endothelial cells. To ensure that the PAI-2·proteasome complex is formed in vivo, both proteins were coimmunoprecipitated from endothelial cells and identified with specific antibodies. The specificity of this interaction was evidenced after (a) transfection of HeLa cells with pCMV-PAI-2 and coimmunoprecipitation of both proteins with anti-PAI-2 antibodies and (b) silencing of the PAI-2 gene using specific small interfering RNA (siRNA). Subsequently, cellular distribution of the PAI-2·proteasome complexes was established by immunogold staining and electron microscopy analyses. As judged by confocal microscopy, both proteins appeared in a diffuse cytosolic pattern, but they also could be found in a dense perinuclear and nuclear location. PAI-2 was not polyubiquitinated, suggesting that it bound to proteasome not as the substrate but rather as its inhibitor. Consistently, increased PAI-2 expression (a) abrogated degradation of degron analyzed after cotransfection of HeLa cells with pCMV-PAI-2 and pd2EGFP-N1, (b) prevented degradation of p53, as evidenced both by confocal microscopy and Western immunoblotting, and (c) inhibited proteasome cleavage of specific fluorogenic substrate. This suggests that PAI-2, in endothelial cells induced with inflammatory stimuli, can inhibit proteasome and thus tilt the balance favoring proapoptotic signaling.

Human papilloma virus transformed CaSki cells constitutively express high levels of functional SerpinB2

Many malignant tissues, including human papilloma virus (HPV)-associated cancers, express SerpinB2, also known as plasminogen activator inhibitor type-2 (PAI-2). Whether SerpinB2 is expressed by the HPV-transformed cancer cells, and if so, whether SerpinB2 is mutated or behaves aberrantly remains unclear. Here we show that HPV-transformed CaSki cells express high levels of constitutive wild-type SerpinB2, with cellular distribution, glycosylation, secretion, cleavage, induction and urokinase binding similar to that reported for primary cells. Neutralization of secreted SerpinB2 failed to affect CaSki cell migration or growth. Lentivirus-based over-expression of SerpinB2 also had no effect on growth, and we were unable to confirm a role for SerpinB2 in binding or regulating expression of the retinoblastoma protein. CaSki cells thus emerge as a useful tool for studying SerpinB2, with the physiological function of SerpinB2 expression by tumor cells remaining controversial. Using CaSki cells as a source of endogenous SerpinB2, we confirmed that SerpinB2 efficiently binds the proteasomal subunit member β1.

Bomapin is a redox-sensitive nuclear serpin that affects responsiveness of myeloid progenitor cells to growth environment

BACKGROUND

Haematopoiesis is a process of formation of mature blood cells from hematopoietic progenitors in bone marrow. Haematopoietic progenitors are stimulated by growth factors and cytokines to proliferate and differentiate, and they die via apoptosis when these factors are depleted. An aberrant response to growth environment may lead to haematological disorders. Bomapin (serpinb10) is a hematopoietic- and myeloid leukaemia-specific protease inhibitor with unknown function.

RESULTS

We found that the majority of naturally expressed bomapin was located in the nucleus. Both the natural and recombinant bomapin had a disulfide bond which linked the only two bomapin cysteines: one located in the CD-loop and the other near the C-terminus. Computer modelling showed that the cysteines are distant in the reduced bomapin, but can easily be disulfide-linked without distortion of the overall bomapin structure. Low-level ectopic expression of bomapin in bomapin-deficient K562 cells resulted in about 90% increased cell proliferation under normal growth conditions. On the other hand, antisense-downregulation of natural bomapin in U937 cells resulted in a decreased cell proliferation. Bomapin C395S mutant, representing the reduced form of the serpin, had no effect on cell proliferation, suggesting that the disulfide bond-linked conformation of bomapin is biologically important. The bomapin-dependent effect was specific for myeloid cells, since ectopic expression of the serpin in HT1080 cells did not change cell proliferation. In contrast to the survival-promoting activity of bomapin in cells cultured under optimal growth conditions, bomapin enhanced cell apoptosis following growth factor withdrawal.

CONCLUSIONS

We propose that bomapin is a redox-sensitive nuclear serpin that augments proliferation or apoptosis of leukaemia cells, depending on growth factors availability.

A physiological function of inflammation-associated SerpinB2 is regulation of adaptive immunity

SerpinB2 (plasminogen activator inhibitor-2) is widely described as an inhibitor of urokinase plasminogen activator; however, SerpinB2(-/-) mice show no detectable increase in urokinase plasminogen activator activity. In this study, we describe an unexpected immune phenotype in SerpinB2(-/-) mice. After immunization with OVA in CFA, SerpinB2(-/-) mice made approximately 6-fold more IgG2c and generated approximately 2.5-fold more OVA-specific IFN-gamma-secreting T cells than SerpinB2(+/+) littermate controls. In SerpinB2(+/+) mice, high inducible SerpinB2 expression was seen at the injection site and in macrophages low levels in draining lymph nodes and conventional dendritic cells, and no expression was seen in plasmacytoid dendritic, B, T, or NK cells. SerpinB2(-/-) macrophages promoted greater IFN-gamma secretion from wild-type T cells in vivo and in vitro and, when stimulated with anti-CD40/IFN-gamma or cultured with wild-type T cells in vitro, secreted more Th1-promoting cytokines than macrophages from littermate controls. Draining lymph node SerpinB2(-/-) myeloid APCs similarly secreted more Th1-promoting cytokines when cocultured with wild-type T cells. Regulation of Th1 responses thus appears to be a physiological function of inflammation-associated SerpinB2; an observation that may shed light on human inflammatory diseases like pre-eclampsia, lupus, asthma, scleroderma, and periodontitis, which are associated with SerpinB2 polymorphisms or dysregulated SerpinB2 expression.

NETs: a new strategy for using old weapons

As key players in the host innate immune response, neutrophils are recruited to sites of infection and constitute the first line of defense. They employ three strategies to eliminate invading microbes: microbial uptake, the secretion of antimicrobials, and the recently described release of Neutrophil Extracellular Traps (NETs). Composed of decondensed chromatin and antimicrobial proteins, NETs bind and kill a variety of microbes including bacteria, fungi, and parasites. In addition to using a repertoire of known antimicrobials, NETs incorporate histones into the antimicrobial arsenal. Furthermore, NETs may contribute to microbial containment by forming a physical barrier and a scaffold, to enhance antimicrobial synergy while minimizing damage to host tissues. Their role in innate immunity is only now being uncovered.

The CD-loop of PAI-2 (SERPINB2) is redundant in the targeting, inhibition and clearance of cell surface uPA activity

BACKGROUND

Plasminogen activator inhibitor type-2 (PAI-2, SERPINB2) is an irreversible, specific inhibitor of the urokinase plasminogen activator (uPA). Since overexpression of uPA at the surface of cancer cells is linked to malignancy, targeting of uPA by exogenous recombinant PAI-2 has been proposed as the basis of potential cancer therapies. To this end, reproducible yields of high purity protein that maintains this targeting ability is required. Herein we validate the use in vitro of recombinant 6 x His-tagged-PAI-2 lacking the intrahelical loop between C and D alpha-helices (PAI-2 Delta CD-loop) for these purposes.

RESULTS

We show that PAI-2 Delta CD-loop expressed and purified from the pQE9 vector system presents an easier purification target than the previously used pET15b system. Additionally, PAI-2 Delta CD-loop gave both higher yield and purity than wild-type PAI-2 expressed and purified under identical conditions. Importantly, absence of the CD-loop had no impact on the inhibition of both solution phase and cell surface uPA or on the clearance of receptor bound uPA from the cell surface. Furthermore, uPA:PAI-2 Delta CD-loop complexes had similar binding kinetics (KD approximately 5 nM) with the endocytosis receptor Very Low Density Lipoprotein Receptor (VLDLR) to that previously published for uPA:PAI-2 complexes.

CONCLUSION

We demonstrate that the CD-loop is redundant for the purposes of cellular uPA inhibition and cell surface clearance (endocytosis) and is thus suitable for the development of anti-uPA targeted cancer therapeutics.

Identification of regulatory Hck and PAI-2 proteins in the monocyte response to PEG-containing matrices

Mass spectrometry is a powerful proteomic tool enabling researchers to survey the global proteome of a cell. This technique has only recently been employed to investigate cell-material interactions. We had previously identified material scarcity and limited adherent cells as challenges facing mass spectrometric analysis of cell-material interactions. U937 adherent to tissue culture poly(styrene) was used as a model system for identifying proteins expressed by adherent monocytes and analyzed by HPLC coupled offline to MALDI-ToF/ToF (LC-MALDI). We identified 645 proteins from two cation fractions of crude U937 monocyte cell lysate. Forty three proteins of interest from the 645 were chosen based on literature searches for relevance to monocyte-material inflammation and wound healing. Proteins such as 40S ribosomal protein S19 and tyrosyl tRNA synthetase highlight the ability of LC-MALDI to identify proteins relevant to monocyte-material interactions that are currently unexplored. We used PEG-based semi-interpenetrating polymer networks and PEG-only hydrogels to investigate surface dependent effects on the Src family kinase Hck and plasminogen activator inhibitor-2 (PAI-2) using the pyrazolo pyrimidine small molecule inhibitor PP2 and exogenous urokinase plasminogen activator addition, respectively. Hck is well researched in cell adhesion while PAI-2 is virtually unknown in cell-material interactions. U937 on TCPS and PEG-only hydrogels secreted similar levels of inflammatory cytokines and gelatinase MMP-9. MCP-1 secretion from monocytes on PEG-only hydrogels was Hck independent in contrast to Hck-dependent MCP-1 secretion in U937 on TCPS. Overall, U937 adherent to sIPNs secrete low levels of soluble gelatinase MMP-9, IL-1beta, TNF-alpha, IL-6, and MCP-1 independent of Hck and PAI-2. This work demonstrates significant changes in surface dependent expression of proteins from monocytes adherent to PEG-based materials compared to TCPS.

Cytoplasmic and nuclear interaction between Rb family proteins and PAI-2: a physiological crosstalk in human corneal and conjunctival epithelial cells

Extracellular plasminogen activator inhibitor type-2 (PAI-2) is a potent inhibitor of urokinase-type plasminogen activator (u-PA) and also acts as a multifunctional protein. However, the biological activity of intracellular PAI-2, as well as its intracellular targets, until now remain an enigma. Here, we show that pRb2/p130 and Rb1/p105, but not p107, interact with PAI-2 in both the cytoplasm and nucleus of normal primary human corneal and conjunctival epithelial cells. We provided the first in vivo evidence that a specific fragment of the PAI-2 promoter is bound simultaneously by pRb2/ p130, PAI-2, E2F5, histone deacetylase 1 (HDAC1), DNA methyltransferase 1 (DNMT1), and histone methyltransferase (SUV39H1), in normal primary human corneal epithelial cells, and by pRb2/p130, PAI-2, E2F5, HDAC1, and DNMT1, in normal primary human conjunctiva epithelial cells. Our results strongly indicate a physiological interaction between pRb family members and PAI-2, suggesting the hypothesis that pRb2/p130 and PAI-2 may cooperate in modulating PAI-2 gene expression by chromatin remodeling, in normal corneal and conjunctival cells.

Analysis of vertebrate genomes suggests a new model for clade B serpin evolution

BACKGROUND

The human genome contains 13 clade B serpin genes at two loci, 6p25 and 18q21. The three genes at 6p25 all conform to a 7-exon gene structure with conserved intron positioning and phasing, however, at 18q21 there are two 7-exon genes and eight genes with an additional exon yielding an 8-exon structure. Currently, it is not known how these two loci evolved, nor which gene structure arose first--did the 8-exon genes gain an exon, or did the 7-exon genes lose one? Here we use the genomes of diverse vertebrate species to plot the emergence of clade B serpin genes and to identify the point at which the two genomic structures arose.

RESULTS

Analysis of the chicken genome indicated the presence of a single clade B serpin gene locus, containing orthologues of both human loci and both genomic structures. The frog genome and the genomes of three fish species presented progressively simpler loci, although only the 7-exon structure could be identified. The Serpinb12 gene contains seven exons in the frog genome, but eight exons in chickens and humans, indicating that the additional exon evolved in this gene.

CONCLUSION

We propose a new model for clade B serpin evolution from a single 7-exon gene (either Serpinb1 or Serpinb6). An additional exon was gained in the Serpinb12 gene between the tetrapoda and amniota radiations to produce the 8-exon structure. Both structures were then duplicated at a single locus until a chromosomal breakage occurred at some point along the mammalian lineage resulting in the two modern loci.

Antiplasmin

Much of the basic biochemistry of antiplasmin was described more than 20 years ago and yet it remains an enigmatic member of the serine protease inhibitor (serpin) family. It possesses all of the characteristics of other inhibitory serpins but in addition it has unique N- and C-terminal extensions which significantly modify its activities. The N-terminus serves as a substrate for Factor XIIIa leading to crosslinking and incorporation of antiplasmin into a clot as it is formed. Although free antiplasmin is an excellent inhibitor of plasmin, the fibrin bound form of the serpin appears to be the major regulator of clot lysis. The C-terminal portion of antiplasmin is highly conserved between species and contains several charged amino acids including four lysines with one of these at the C-terminus. This portion of the molecule mediates the initial interaction with plasmin and is a key component of antiplasmin's rapid and efficient inhibitory mechanism. Studies of mice with targeted deletion of antiplasmin have confirmed its importance as a major regulator of fibrinolysis and re-emphasized its value as a potential therapeutic target.

The undecided serpin

Plasminogen activator inhibitor type-2 (PAI-2) is a nonconventional serine protease inhibitor (serpin) with unique and tantalizing properties that is generally considered to be an authentic and physiological inhibitor of urokinase. However, the fact that only a small percentage of PAI-2 is secreted has been a long-standing argument for alternative roles for this serpin. Indeed, PAI-2 has been shown to have a number of intracellular roles: it can alter gene expression, influence the rate of cell proliferation and differentiation, and inhibit apoptosis in a manner independent of urokinase inhibition. Despite these recent advances in defining the intracellular function of PAI-2, it still remains one of the most mysterious and enigmatic members of the serpin superfamily.