The direct and indirect effect of the COVID-19 pandemic on patients with cardiomyopathy

Background

The disease-specific impact of COVID-19 on different cardiac conditions requires further investigation. Whilst direct effects are observed for those infected with SARS-CoV-2, the indirect effects of the pandemic arising from interruption to clinical care may represent a further source of morbidity and mortality.

Purpose

To evaluate the direct and indirect effects of the COVID-19 pandemic on patients with dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM).

Methods

(i) Patients with DCM or HCM previously recruited to a single centre registry were studied using NHS Spine Summary Care Records, hospital records and patient questionnaires. The primary outcome was test-proven COVID-19 infection. Secondary outcomes were the proportion of COVID-19 cases requiring hospitalisation and the proportion of subjects advised to shield. Outcomes were compared with published UK COVID-19 statistics.

(ii) The Heart Hive COVID-19 study is an international online prospective observational cohort study. Subjects registered on an online platform with cardiomyopathy or without heart disease (controls) were invited to enrol. Enrolled subjects complete online surveys, adapted from a WHO Survey tool for behavioural insights on COVID-19.

Results

(i) Of 1236 eligible patients in our registry (703 DCM, 533 HCM), 13 (1.1%) had tested positive for COVID-19 compared to 1.6% in the UK population (p=0.14) up to 2nd November 2020, a higher proportion of whom required hospitalisation compared with the UK population (53.8% vs 16.5%, p=0.002). More patients with cardiomyopathy in our registry were advised to shield than in the UK population (21.9% vs 6.8%, p<0.ehab724.17361, Fig. 1).

(ii) Of 207 subjects enrolled in the Heart Hive COVID-19 study (131 cardiomyopathy, 76 controls), more patients with cardiomyopathy felt their physical health had deteriorated due to the pandemic than controls (32.3% vs 13.2%, p=0.0042) despite only 4.6% reporting COVID-19 symptoms. Of those with cardiomyopathy, 38 (29.0%) reported a rescheduled clinic visit, 21 (16.0%) a cancelled clinic visit, 51 (38.9%) had missed investigations, 10 (7.6%) had cancelled/postponed procedures, 7 (5.3%) had missed medication doses due to the pandemic and 53 (40.4%) felt their health needs could not be met by telemedicine (Fig. 2). The psychological impact of the pandemic was comparable between cardiomyopathy patients and controls.

Conclusions

Patients with cardiomyopathy in our registry were not at a greater risk of testing positive for COVID-19 than the UK population, but a higher proportion of those that tested positive required hospitalisation. Many more patients with cardiomyopathy reported a subjective deterioration in physical health than had experienced COVID-19 symptoms, inferring a significant contribution of the indirect effects of the pandemic. Our findings have implications for both the health needs of these patients and the reorganisation of clinical services to meet these.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): The Wellcome Trust, Medical Research Council (UK) Figure 1Figure 2

In-depth phenotypic characterisation of myocardial fibrosis by cardiovascular magnetic resonance predicts sudden cardiac death in coronary heart disease: a long-term prospective outcome study

Background

Prospective studies harnessing late gadolinium enhancement (LGE) cardiovascular magnetic resonance (CMR) afford the potential to non-invasively characterise the phenotypic substrate for sudden cardiac death (SCD) and simultaneously interrogate its mechanistic drivers.

Purpose

To assess the utility of infarct characterisation by CMR, including scar microstructure analysis, to predict SCD in prospectively investigated patients with coronary heart disease (CHD).

Methods

Patients with stable CHD were prospectively recruited into a registry between August 2009 and January 2016. The primary outcome for this study was SCD or aborted SCD. Patients with a secondary prevention implantable cardioverter defibrillator (ICD) indication were excluded. All patients had CMR with LGE imaging. Infarct quantification (core scar and peri-infarct zone [PIZ]) was performed by an independent level 3 CMR reader. Outcome events were adjudicated by a panel of cardiologists blinded to the CMR data. To investigate fibrosis microstructure, bespoke computational image processing algorithms were applied to the LGE images in order to extract specific morphological and texture related features.

Results

Of 437 patients (mean age 64, mean left ventricular ejection fraction [LVEF] 47%, 91% with LGE) followed for a median of 6.3 years, 49 patients (11.2%) experienced the primary outcome. Patients with higher PIZ mass had an increased risk of the primary outcome (10-year risk 0.7%, 24.0% and 37.8% for patients with PIZ mass <5.66g, 5.66–12.28g and ≥12.29g respectively, P<0.001; figure 1a). On univariable analysis, an increase in PIZ mass and core infarct mass was associated with an increased risk of the primary outcome (per gram: HR 1.12, 95% CI 1.09–1.15, P<0.001 and HR 1.05, 95% CI 1.04–1.06, P<0.001 respectively). PIZ mass and core infarct mass remained independently associated with the primary outcome after adjustment for baseline predictors (per gram: HR 1.10, 95% CI 1.06–1.14, P<0.001 and HR 1.04, 95% CI 1.02–1.06, P<0.001 respectively) and together provided incremental value compared to conventional variables in predicting the endpoint (Harrell's C-statistic 0.76 to 0.82, figure 1b-c). Bespoke analysis of imaging data identified several shape-based scar metrics that associated with the primary outcome (figure 2). These included core infarct transmurality, radiality and interface length (the latter defining the core scar-PIZ boundary length), and the number of PIZ islets.

Conclusions

In this large prospective study of patients with stable CHD, both PIZ mass and core infarct mass independently predicted long-term SCD risk after adjusting for conventional predictors including LVEF. Reassuringly, minimal or absent LGE portended a comparatively low risk of SCD. Analysis of the scar microstructure identified several shape-based features that associated with SCD. These results highlight a potential avenue towards a more personalised approach to ICD implantation decisions.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): National Lung and Heart Institute, Imperial College London

Correction to: Inhibition of cathepsin B by caspase-3 inhibitors blocks programmed cell death in Arabidopsis

We thank D Nicholson for initial advice on caspase activity purification and B Turk for advice on recombinant cathepsin B. We thank N Atanasova for cell death assays. The Bioimaging Facility microscopes used in this study were purchased with grants from BBSRC, Wellcome Trust and the University of Manchester Strategic Fund. Special thanks go to D Knight in the Faculty Biomolecular Analysis facility. We thank P Birch and M Kim for improving the manuscript. The project was partially funded by BBSRC Grants 34/P14516, BB/K009478/1 and China National High-Tech Research and Development Programme(863 programme)NO. 2015AA020903.

A CD4-mimetic compound enhances vaccine efficacy against stringent immunodeficiency virus challenge

The envelope glycoprotein (Env) trimer ((gp120/gp41)₃) mediates human immunodeficiency virus (HIV-1) entry into cells. The “closed,” antibody-resistant Env trimer is driven to more open conformations by binding the host receptor, CD4. Broadly neutralizing antibodies that recognize conserved elements of the closed Env are potentially protective, but are elicited inefficiently. HIV-1 has evolved multiple mechanisms to evade readily elicited antibodies against more open Env conformations. Small-molecule CD4-mimetic compounds (CD4mc) bind the HIV-1 gp120 Env and promote conformational changes similar to those induced by CD4, exposing conserved Env elements to antibodies. Here, we show that a CD4mc synergizes with antibodies elicited by monomeric HIV-1 gp120 to protect monkeys from multiple high-dose intrarectal challenges with a heterologous simian-human immunodeficiency virus (SHIV). The protective immune response persists for at least six months after vaccination. CD4mc should increase the protective efficacy of any HIV-1 Env vaccine that elicits antibodies against CD4-induced conformations of Env.

The HIV Env trimer exhibits a closed confirmation and restricts access to known antibody binding sites. Here the authors show that a small-molecule CD4-mimetic compound binds the HIV Env trimer and enhances antibody-mediated protection in a non-human primate model of infection.

Neutralization Takes Precedence Over IgG or IgA Isotype-related Functions in Mucosal HIV-1 Antibody-mediated Protection

HIV-1 infection occurs primarily through mucosal transmission. Application of biologically relevant mucosal models can advance understanding of the functional properties of antibodies that mediate HIV protection, thereby guiding antibody-based vaccine development. Here, we employed a human ex vivo vaginal HIV-1 infection model and a rhesus macaque in vivo intrarectal SHIV challenge model to probe the protective capacity of monoclonal broadly-neutralizing (bnAb) and non-neutralizing Abs (nnAbs) that were functionally modified by isotype switching. For human vaginal explants, we developed a replication-competent, secreted NanoLuc reporter virus system and showed that CD4 binding site bnAbs b12 IgG1 and CH31 IgG1 and IgA2 isoforms potently blocked HIV-1JR-CSF and HIV-1Bal26 infection. However, IgG1 and IgA nnAbs, either alone or together, did not inhibit infection despite the presence of FcR-expressing effector cells in the tissue. In macaques, the CH31 IgG1 and IgA2 isoforms infused before high-dose SHIV challenge were completely to partially protective, respectively, while nnAbs (CH54 IgG1 and CH38 mIgA2) were non-protective. Importantly, in both mucosal models IgG1 isotype bnAbs were more protective than the IgA2 isotypes, attributable in part to greater neutralization activity of the IgG1 variants. These findings underscore the importance of potent bnAb induction as a primary goal of HIV-1 vaccine development.

Pathogenic infection of Rhesus macaques by an evolving SIV-HIV derived from CCR5-using envelope genes of acute HIV-1 infections

For studies on vaccines and therapies for HIV disease, SIV-HIV chimeric viruses harboring the HIV-1 env gene (SHIVenv) remain the best virus in non-human primate models. However, there are still very few SHIVenv viruses that can cause AIDS in non-CD8-depleted animals. In the present study, a recently created CCR5-using SHIVenv_B3 virus with env gene derived from acute/early HIV-1 infections (AHI) successfully established pathogenic infection in macaques. Through a series of investigations on the evolution, mutational profile, and phenotype of the virus and the resultant humoral immune response in infected rhesus macaques, we found that the E32K mutation in the Env C1 domain was associated with macaque pathogenesis, and that the electrostatic interactions in Env may favor E32K at the gp120 N terminus and "lock" the binding to heptad repeat 1 of gp41 in the trimer and produce a SHIVenv with increased fitness and pathogenesis during macaque infections.

The S-layer proteins of Tannerella forsythia are secreted via a type IX secretion system that is decoupled from protein O-glycosylation

Conserved C-terminal domains (CTD) have been shown to act as a signal for the translocation of certain proteins across the outer membrane of Bacteroidetes via a type IX secretion system (T9SS). The genome sequence of the periodontal pathogen Tannerella forsythia predicts the presence of the components for a T9SS in conjunction with a suite of CTD proteins. T. forsythia is covered with a two-dimensional crystalline surface (S-) layer composed of the glycosylated CTD proteins TfsA and TfsB. To investigate, if T9SS is functional in T. forsythia, T9SS-deficient mutants were generated by targeting either TF0955 (putative C-terminal signal peptidase) or TF2327 (PorK ortholog), and the mutants were analyzed with respect to secretion, assembly and glycosylation of the S-layer proteins as well as proteolytic processing of the CTD and biofilm formation. In either mutant, TfsA and TfsB were incapable of translocation, as evidenced by the absence of the S-layer in transmission electron microscopy of ultrathin-sectioned bacterial cells. Despite being entrapped within the periplasm, mass spectrometry analysis revealed that the S-layer proteins were modified with the complete, mature glycan found on the secreted proteins, indicating that protein translocation and glycosylation are two independent processes. Further, the T9SS mutants showed a denser biofilm with fewer voids compared with the wild-type. This study demonstrates the functionality of T9SS and the requirement of CTD for the outer membrane passage of extracellular proteins in T. forsythia, exemplified by the two S-layer proteins. In addition, T9SS protein translocation is decoupled from O-glycan attachment in T. forsythia.

Breadth of cellular and humoral immune responses elicited in rhesus monkeys by multi-valent mosaic and consensus immunogens

To create an HIV-1 vaccine that generates sufficient breadth of immune recognition to protect against the genetically diverse forms of the circulating virus, we have been exploring vaccines based on consensus and mosaic protein designs. Increasing the valency of a mosaic immunogen cocktail increases epitope coverage but with diminishing returns, as increasingly rare epitopes are incorporated into the mosaic proteins. In this study we compared the immunogenicity of 2-valent and 3-valent HIV-1 envelope mosaic immunogens in rhesus monkeys. Immunizations with the 3-valent mosaic immunogens resulted in a modest increase in the breadth of vaccine-elicited T lymphocyte responses compared to the 2-valent mosaic immunogens. However, the 3-valent mosaic immunogens elicited significantly higher neutralizing responses to Tier 1 viruses than the 2-valent mosaic immunogens. These findings underscore the potential utility of polyvalent mosaic immunogens for eliciting both cellular and humoral immune responses to HIV-1.

Generation of Arabidopsis thaliana plants with complex N-glycans lacking beta1,2-linked xylose and core alpha1,3-linked fucose

The plant glycosyltransferases, beta1,2-xylosyltransferase (XylT) and core alpha1,3-fucosyltransferase (FucT), are responsible for the transfer of beta1,2-linked xylose and core alpha1,3-linked fucose residues to glycoprotein N-glycans. These glycan epitopes are not present in humans and thus may cause immunological responses, which represent a limitation for the therapeutic use of recombinant mammalian glycoproteins produced in transgenic plants. Here we report the genetic modification of the N-glycosylation pathway in Arabidopsis thaliana plants. Knockout plants were generated with complete deficiency of XylT and FucT. These plants lack antigenic protein-bound N-glycans and instead synthesise predominantly structures with two terminal betaN-acetylglucosamine residues (GlcNAc(2)Man(3)GlcNAc(2)).

Tissues of the clawed frog Xenopus laevis contain two closely related forms of UDP-GlcNAc:alpha3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I

UDP-GlcNAc:alpha3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnTI; EC 2.4.1.101) is a medial-Golgi enzyme that is essential for the processing of oligomannose to hybrid and complex N-glycans. On the basis of highly conserved sequences obtained from previously cloned mammalian GnTI genes, cDNAs for two closely related GnTI isoenzymes were isolated from a Xenopus laevis ovary cDNA library. As typical for glycosyltransferases, both proteins exhibit a type II transmembrane protein topology with a short N-terminal cytoplasmic tail (4 amino acids); a transmembrane domain of 22 residues; a stem region with a length of 81 (isoenzyme A) and 77 (isoenzyme B) amino acids, respectively; and a catalytic domain consisting of 341 residues. The two proteins differ not only in length but also at 13 (stem) and 18 (catalytic domain) positions, respectively. The overall identity of the catalytic domains of the X. laevis GnTI isoenzymes with their mammalian and plant orthologues ranges from 30% (Nicotiana tabacum) to 67% (humans). Isoenzymes A and B are encoded by two separate genes that were both found to be expressed in all tissues examined, albeit in varying amounts and ratios. On expression of the cDNAs in the baculovirus/insect cell system, both isoenzymes were found to exhibit enzymatic activity. Isoenzyme B is less efficiently folded in vivo and thus appears of lower physiological relevance than isoenzyme A. However, substitution of threonine at position 223 with alanine was sufficient to confer isoenzyme B with properties similar to those observed for isoenzyme A.

Invasive properties of murine squamous carcinoma cells: secretion of matrix-degrading cathepsins is attributable to a deficiency in the mannose 6-phosphate/insulin-like growth factor II receptor

Penetration of basement membrane layers is a hallmark feature of metastatic tumor cells. The invasive propensity of murine SCC-VII squamous carcinoma cells is in part attributable to the extracellular action of the lysosomal cysteine proteinase cathepsin B. Although most noncancerous cells store this enzyme in the lysosomes, we found that SCC-VII cells release a large fraction (42%) of their newly synthesized procathepsin B into the culture medium. Procathepsins D and L, the precursors of other major lysosomal proteinases, are also secreted in significant amounts (24 and 75%, respectively). In contrast, normal murine 3T3-L1 fibroblasts exocytose only minor amounts of their newly synthesized procathepsins B (10%), D (<1%), and L (16%). Western blotting analysis revealed that SCC-VII cells are deficient in the 300 kDa mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGF2R), a tumor suppressor with a central role in the intracellular transport of lysosomal enzymes. Consistent with the absence of M6P/IGF2R, SCC-VII cells lack dense lysosomes, with the bulk of intracellular acid hydrolases residing in late endosomes/ prelysosomes. On the other hand, the synthesis of the M6P recognition marker on lysosomal enzymes is not impaired in SCC-VII cells, because [33P]Pi is incorporated into the carbohydrate moieties of procathepsins B, D, and L. Furthermore, 69% of the phosphorylated N-linked oligosaccharides synthesized by SCC-VII cells carry phosphomonoester groups and as such constitute high-affinity ligands for M6P receptors. SCC-VII cells express the 46 kDa cation-dependent M6P receptor (MPR46), but intracellular retention of procathepsins B, D, and L is not affected by ammonium chloride and chloroquine, agents known to perturb the M6P receptor system. Our results indicate that failure to express the 300 kDa M6P/IGF2R may enhance the metastatic capacity of tumor cells by inducing the secretion of procathepsins B, D, and L.

Molecular cloning and functional expression of beta1, 2-xylosyltransferase cDNA from Arabidopsis thaliana

The transfer of xylose from UDP-xylose to the core beta-linked mannose of N-linked oligosaccharides by beta1,2-xylosyltransferase (XylT) is a widespread feature of plant glycoproteins which renders them immunogenic and allergenic in man. Here, we report the isolation of the Arabidopsis thaliana XylT gene, which contains two introns and encodes a 60.2 kDa protein with a predicted type II transmembrane protein topology typical for Golgi glycosyltransferases. Upon expression of A. thaliana XylT cDNA in the baculovirus/insect cell system, a recombinant protein was produced that exhibited XylT activity in vitro. Furthermore, the recombinant enzyme displayed XylT activity in vivo in the insect cells, as judged by the acquired cross-reaction of cellular glycoproteins with antibodies against the beta1,2-xylose epitope. The cloned XylT cDNA as well as the recombinant enzyme are essential tools to study the role of beta1,2-xylose in the immunogenicity and allergenicity of plant glycoproteins at the molecular level.

Molecular cloning of cDNA encoding N-acetylglucosaminyltransferase II from Arabidopsis thaliana

N-acetylglucosaminyltransferase II (GnTII, EC 2.4.1.143) is a Golgi enzyme involved in the biosynthesis of glycoprotein-bound N-linked oligosaccharides, catalysing an essential step in the conversion of oligomannose-type to complex N-glycans. GnTII activity has been detected in both animals and plants. However, while cDNAs encoding the enzyme have already been cloned from several mammalian sources no GnTII homologue has been cloned from plants so far. Here we report the molecular cloning of an Arabidopsis thaliana GnTII cDNA with striking homology to its animal counterparts. The predicted domain structure of A. thaliana GnTII indicates a type II transmembrane protein topology as it has been established for the mammalian variants of the enzyme. Upon expression of A. thaliana GnTII cDNA in the baculovirus/insect cell system, a recombinant protein was produced that exhibited GnTII activity.

Modulation of invasive properties of murine squamous carcinoma cells by heterologous expression of cathepsin B and cystatin C

Murine SCC-VII squamous carcinoma cells have the capacity to penetrate reconstituted basement membranes (Matrigel) in vitro. The invasion of Matrigel layers by SCC-VII cells was significantly reduced by E-64, a specific inhibitor of lysosomal cysteine proteinases. The cathepsin-B-selective E-64 derivative, CA-074, inhibited penetration of Matrigel by SCC-VII cells to the same extent, indicating a major role for this particular lysosomal enzyme in extracellular-matrix degradation during squamous-carcinoma-cell invasion. SCC-VII cells were stably transfected with a cDNA encoding human procathepsin B, in an attempt to modulate the invasive properties of the cell line. The transfected cells expressed the heterologous gene, secreted increased amounts of procathepsin B and displayed enhanced invasive potential. In vivo, the activity of cathepsin B is strictly regulated by endogenous inhibitors. SCC-VII cells were therefore also stably transfected with a cDNA encoding human cystatin C, the most potent cysteine-proteinase inhibitor in mammalian tissues. The expression of this transgene resulted in the production of active recombinant cystatin C and a pronounced reduction in Matrigel invasion. These studies demonstrate that the invasive properties of squamous-cell carcinomas can be changed by modulation of the balance between cathepsin B and its endogenous inhibitors, and provide further evidence for the involvement of this lysosomal cysteine proteinase in tumour invasion and metastasis.

Accumulation of sialic acid in endocytic compartments interferes with the formation of mature lysosomes. Impaired proteolytic processing of cathepsin B in fibroblasts of patients with lysosomal sialic acid storage disease

The impact of an altered endocytic environment on the biogenesis of lysosomes was studied in fibroblasts of patients suffering from sialic acid storage disease (SASD). This inherited disorder is characterized by the accumulation of acidic monosaccharides in lysosomal compartments and a concomitant decrease of their buoyant density. We demonstrate that C-terminal trimming of the lysosomal cysteine proteinase cathepsin B is inhibited in SASD fibroblasts. This late event in the biosynthesis of cathepsin B normally takes place in mature lysosomes, suggesting an impaired biogenesis of these organelles in SASD cells. When normal fibroblasts are loaded with sucrose, which inhibits transport from late endosomes to lysosomes, C-terminal cathepsin B processing is prevented to the same extent. Further characterization of the terminal endocytic compartments of SASD cells revealed properties usually associated with late endosomes/prelysosomes. In addition to a decreased buoyant density, SASD "lysosomes" show a reduced acidification capacity and appear smaller than their normal counterparts. We conclude that the accumulation of small non-diffusible compounds within endocytic compartments interferes with the formation of mature lysosomes and that the acidic environment of the latter organelles is a prerequisite for C-terminal processing of lysosomal hydrolases.

Maturation of human procathepsin B. Proenzyme activation and proteolytic processing of the precursor to the mature proteinase, in vitro, are primarily unimolecular processes

Recombinant latent human procathepsin B produced in yeast was purified to near homogeneity. The purified recombinant proenzyme is activated in vitro under acidic conditions resulting in rapid conversion into the mature form of the proteinase. Activation as well as proteolytic maturation of the recombinant cathepsin B precursor were shown to be primarily concentration-independent processes indicating a unimolecular (i.e. intramolecular) mechanism. Only one cleavage site was identified, yielding a mature polypeptide with the same amino-terminal sequence as that found in recombinant active human cathepsin B obtained from yeast culture media. The same peptide bond is cleaved during processing of a nonactivatable mutant of procathepsin B by the purified mature enzyme (i.e. intermolecular processing). Thus, the complete proregion is liberated during procathepsin B processing. This peptide may then act as a reversible inhibitor and stabilizer of the mature proteinase, and it appears likely that cathepsin B-propeptide complexes occur transiently during proteolytic maturation.

Noncovalent complexes between the lysosomal proteinase cathepsin B and its propeptide account for stable, extracellular, high molecular mass forms of the enzyme

Although the lysosomal cysteine proteinase cathepsin B is alkaline pH-labile, active, stable high molecular mass forms have been reported previously from the culture medium of human and murine mammary tumor explants and the sputum of patients with purulent bronchiectasis. A similar, catalytically active, high molecular mass form of recombinant human cathepsin B produced in yeast has now been found to represent a noncovalent complex between the 30-kDa single chain enzyme and its 6-kDa propeptide formed during autocatalytic maturation of the proenzyme (see accompanying article; Mach, L., Mort, J. S., and Glössl, J. (1994) J. Biol. Chem. 269, 13030-13035). Incubation of the complex under acidic conditions resulted in dissociation and degradation of the inhibitory propeptide leading to increased enzymatic activity, as also observed for partially purified cathepsin B isoenzymes from purulent sputum and mammary tumor explant media. The stabilization of the processed proteinase as a noncovalent complex with its proregion provides an important mechanism whereby extracellular cathepsin B can lie dormant until regional acidification mediates its activity.

Activation of procathepsin B in human hepatoma cells: the conversion into the mature enzyme relies on the action of cathepsin B itself

In order to elucidate the processing mechanism of the lysosomal cysteine proteinase, cathepsin B, in mammalian cells, recombinant rat and human cathepsin B precursors were expressed in Saccharomyces cerevisiae. The active-site cysteine residue was changed to serine to prevent autoprocessing. When the purified proenzymes were incubated with the soluble fraction of postnuclear organelles obtained from human hepatoma HepG2 cells, processing to a 33 kDa form corresponding to the mature endogenous single-chain enzyme was observed. Inhibitors of metallo-, serine and aspartic proteinases exerted no significant effect on procathepsin B processing in vitro. However, the processing activity was effectively blocked by cysteine proteinase inhibitors, in particular E-64 and its cathepsin-B-selective derivative CA-074. Processing positions were identified by using anti-peptide antibodies specific for epitopes in the N- and C-terminal cleavage regions. The single-chain form produced in vitro was thus shown to contain an N-terminal extension of at least four residues relative to the mature lysosomal enzyme, as well as a C-terminal extension present in the proenzyme but usually absent in fully processed cathepsin B. On expression of the wild-type proenzyme in yeast, procathepsin B undergoes autoprocessing, yielding a single-chain form of the active enzyme, which contains similar N- and C-terminal extensions. These results indicate that maturation of procathepsin B in vivo in mammalian tissues relies on the proteolytic activity of cathepsin B itself.

Biosynthesis and endocytosis of lysosomal enzymes in human colon carcinoma SW 1116 cells: impaired internalization of plasma membrane-associated cation-independent mannose 6-phosphate receptor

The human colon adenocarcinoma cell lines SW 948, SW 1116, and SW 1222 were tested for their ability to sort and internalize lysosomal enzymes. The biosynthesis of the lysosomal enzymes cathepsin B, arylsulfatase A, and beta-hexosaminidase in these cell lines exhibits no significant differences to that in human fibroblasts. The intracellular targeting of newly synthesized hydrolases to the lysosomes relies in colon carcinoma cells on the mannose 6-phosphate receptor system. Both the cation-independent mannose 6-phosphate receptor (CI-MPR) and the cation-dependent mannose 6-phosphate receptor are expressed in all colon carcinoma cell lines investigated. Endocytosis of lysosomal enzymes via mannose 6-phosphate receptors is reduced in colon carcinoma cells as compared with human fibroblasts. SW 1116 cells were shown to be deficient in receptor-mediated endocytosis of mannose 6-phosphate containing ligands. Ligands of other endocytic receptors as well as the fluid-phase marker horseradish peroxidase were internalized at normal rates. While antibodies against CI-MPR bind to the surface of SW 1116 cells, these antibodies cannot be internalized. These data suggest that the cycling of CI-MPR is specifically impaired in SW 1116 cells.

Rat procathepsin B. Proteolytic processing to the mature form in vitro

Expression of rat procathepsin B in yeast led to the secretion of both the latent and mature forms of the enzyme. Culture in the presence of a cysteine proteinase inhibitor prevented this processing. We have expressed and purified a mutant form of rat procathepsin B whose active-site cysteine residue has been changed to a serine, and which also lacks the glycosylation site in the mature region of the protein. This non-active mutant protein was secreted essentially in an unprocessed form. The purified protein has been incubated with a variety of proteinases, and results indicate that cathepsins D and L, as well as mature cathepsin B itself, can produce a processed (single-chain) form of cathepsin B from this precursor. Amino-terminal sequencing of these processed forms has revealed that they are all elongated by a few residues with respect to the mature form found in vivo. The action of a combination of cathepsin B with dipeptidylpeptidase I produced a single-chain form of cathepsin B with the correct amino terminus. This work has also shown that the processing of procathepsin B to a single-chain form can be an autocatalytic process, in at least an intermolecular manner.

Antibodies to rat procathepsin B recognize the active mature enzyme

Use of mature cathepsin B for immunization invariably yields antisera that react with the denatured protein but not with the native enzyme. This is thought to be due to spontaneous denaturation of the immunizing antigen on introduction into the animal. Recombinant rat procathepsin B has been expressed in yeast as a secreted product. A procathepsin B mutant (Cys29Ser), where autoprocessing is prevented, has been purified and used to raise a rabbit polyclonal antiserum. Both immunodiffusion analysis and an activity depletion assay demonstrated that this antibody recognized native mature cathepsin B. It appears that conformational epitopes existing on the active enzyme are lost on denaturation. The stability of the proenzyme however permits their presentation for antibody generation.

The antigenicity of the carbohydrate moiety of an insect glycoprotein, honey-bee (Apis mellifera) venom phospholipase A2. The role of alpha 1,3-fucosylation of the asparagine-bound N-acetylglucosamine

A rabbit polyclonal antiserum raised against honey-bee (Apis mellifera) venom phospholipase A2 (PLA2) contains antibodies that react exclusively with its glycosylated variants and cross-react with plant glycoproteins. The interaction of anti-(horseradish peroxidase) antiserum with PLA2 suggests the existence of a carbohydrate determinant common to both glycoproteins. E.l.i.s.a. binding and inhibition experiments, employing glycoproteins and glycopeptides of plant and animal origin with known N-glycan structures, in combination with chemical and enzymic deglycosylation, identified alpha 1,3-fucosylation of the asparagine-bound N-acetylglucosamine as the antigenic determinant. This fucose residue is present in the N-glycan of PLA2 and is frequently found in plant glycoproteins, whereas mammalian glycoproteins lack this modification.

Proteolytic processing and glycosylation of cathepsin B. The role of the primary structure of the latent precursor and of the carbohydrate moiety for cell-type-specific molecular forms of the enzyme

The lysosomal cysteine proteinase cathepsin B is synthesized in cultured human hepatoma HepG2 cells as an inactive 44 kDa precursor and subsequently processed to the mature single-chain enzyme with a molecular mass of 33 kDa. Intralysosomal conversion into the two-chain form results in subunits of 27 kDa, 24 kDa (heavy chain) and 5 kDa (light chain). Enzymic deglycosylation reveals that the 27 kDa polypeptide is the glycosylated variant of the carbohydrate-free 24 kDa heavy-chain form. The intracellular transport to the lysosomes is dependent upon mannose 6-phosphate-containing N-linked oligosaccharides. Receptor-mediated endocytosis of human skin-fibroblast-derived procathepsin B by HepG2 cells resulted in processed molecular forms that are not distinguishable from endogenous cathepsin B, thus favouring rather a cell-type-specific processing than structural differences due to the source of the proenzyme. The conversion step of single-chain catehpsin B into the two-chain enzyme is inhibited in vivo by the irreversible cysteine-proteinase inhibitors Z-Phe-Ala-CHN2 and, albeit weaker, Z-Phe-Phe-CHN2. Both substances have no effect on the activation of procathepsin B to the mature enzyme. The carbohydrate moiety of cathepsin B exerts no significant influence on the stability and the enzymatic activity of the enzyme.

Purification and partial characterization of a novel lectin from elder (Sambucus nigra L.) fruit

A previously unknown haemagglutinin, named Sambucus nigra agglutinin-III (SNA-III), has been purified from the fruit of the elder (Sambucus nigra). Whereas elder bark agglutinin I (SNA-I) is highly specific for terminal alpha 2,6-linked sialic acid residues, SNA-III displays a high affinity for oligosaccharides containing exposed N-acetylgalactosamine and galactose residues. Different N-terminal sequences and the amino acid composition distinguish the fruit lectin from elder bark agglutinin II (SNA-II), which shows a similar carbohydrate specificity. The 40-fold higher affinity of SNA-III for asialofetuin than for human asialo-alpha 1-acid glycoprotein and human asialotransferrin respectively suggests a preference for O-linked glycans. SNA-III occurs mainly as a monomeric glycoprotein, but tends to form di- and oligo-meric aggregates. This aggregation seems to mediate the multivalent interaction, leading to agglutination. SDS/PAGE revealed two major polypeptides with apparent molecular masses of 32 and 33 kDa respectively. This heterogeneity is probably a result of proteolysis in the C-terminal region. Binding to concanavalin A and susceptibility to peptide: N-glycosidase F indicated the presence of N-glycosidically linked oligosaccharides.