An IAP retrotransposon in the mouse ADAMTS13 gene creates ADAMTS13 variant proteins that are less effective in cleaving von Willebrand factor multimers

Mechanisms of ADAMTS13 regulation

Recombinant ADAMTS13 is currently undergoing clinical trials as a treatment for hereditary thrombotic thrombocytopenic purpura, a lethal microvascular condition resulting from ADAMTS13 deficiency. Preclinical studies have also demonstrated its efficacy in treating arterial thrombosis and inflammation without causing bleeding, suggesting that recombinant ADAMTS13 may have broad applicability as an antithrombotic agent. Despite this progress, we currently do not understand the mechanisms that regulate ADAMTS13 activity in vivo. ADAMTS13 evades canonical means of protease regulation because it is secreted as an active enzyme and has a long half-life in circulation, suggesting that it is not inhibited by natural protease inhibitors. Although shear can spatially and temporally activate von Willebrand factor to capture circulating platelets, it is also required for cleavage by ADAMTS13. Therefore, spatial and temporal regulation of ADAMTS13 activity may be required to stabilize von Willebrand factor-platelet strings at sites of vascular injury. This review outlines potential mechanisms that regulate ADAMTS13 in vivo including shear-dependency, local inactivation, and biochemical and structural regulation of substrate binding. Recently published structural data of ADAMTS13 is discussed, which may help to generate novel hypotheses for future research.

The conserved DNMT1-dependent methylation regions in human cells are vulnerable to neurotoxicant rotenone exposure

BACKGROUND

Allele-specific DNA methylation (ASM) describes genomic loci that maintain CpG methylation at only one inherited allele rather than having coordinated methylation across both alleles. The most prominent of these regions are germline ASMs (gASMs) that control the expression of imprinted genes in a parent of origin-dependent manner and are associated with disease. However, our recent report reveals numerous ASMs at non-imprinted genes. These non-germline ASMs are dependent on DNA methyltransferase 1 (DNMT1) and strikingly show the feature of random, switchable monoallelic methylation patterns in the mouse genome. The significance of these ASMs to human health has not been explored. Due to their shared allelicity with gASMs, herein, we propose that non-traditional ASMs are sensitive to exposures in association with human disease.

RESULTS

We first explore their conservancy in the human genome. Our data show that our putative non-germline ASMs were in conserved regions of the human genome and located adjacent to genes vital for neuronal development and maturation. We next tested the hypothesized vulnerability of these regions by exposing human embryonic kidney cell HEK293 with the neurotoxicant rotenone for 24 h. Indeed,14 genes adjacent to our identified regions were differentially expressed from RNA-sequencing. We analyzed the base-resolution methylation patterns of the predicted non-germline ASMs at two neurological genes, HCN2 and NEFM, with potential to increase the risk of neurodegeneration. Both regions were significantly hypomethylated in response to rotenone.

CONCLUSIONS

Our data indicate that non-germline ASMs seem conserved between mouse and human genomes, overlap important regulatory factor binding motifs, and regulate the expression of genes vital to neuronal function. These results support the notion that ASMs are sensitive to environmental factors such as rotenone and may alter the risk of neurological disease later in life by disrupting neuronal development.

Mouse germ line mutations due to retrotransposon insertions

Transposable element (TE) insertions are responsible for a significant fraction of spontaneous germ line mutations reported in inbred mouse strains. This major contribution of TEs to the mutational landscape in mouse contrasts with the situation in human, where their relative contribution as germ line insertional mutagens is much lower. In this focussed review, we provide comprehensive lists of TE-induced mouse mutations, discuss the different TE types involved in these insertional mutations and elaborate on particularly interesting cases. We also discuss differences and similarities between the mutational role of TEs in mice and humans.

Phylogenetic and functional analysis of ADAMTS13 identifies highly conserved domains essential for allosteric regulation

The metalloprotease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats member 13) prevents microvascular thrombosis by cleaving von Willebrand factor (VWF) within platelet-rich thrombi, and cleavage depends on allosteric activation of ADAMTS13 by the substrate VWF. Human ADAMTS13 has a short propeptide, metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains (proximal domains), followed by 7 T and 2 CUB (complement components C1r and C1s, sea urchin protein Uegf, and bone morphogenetic protein-1) domains (distal domains). Distal domains inhibit the catalytic proximal domains; binding of distal T8-CUB domains to the VWF D4 domain relieves autoinhibition and promotes cleavage of the nearby VWF A2 domain. However, the role of specific ADAMTS13 distal domains in this allosteric mechanism is not established. Assays of plasma ADAMTS13 from 20 placental mammals, birds, and amphibians show that allosteric regulation is broadly conserved, and phylogenetic analysis of 264 vertebrates shows the long propeptide, T3, T4, T6, and T6a domains have been deleted several times in placental mammals, birds, and fish. Notably, pigeon ADAMTS13 has only 3 distal T domains but was activated normally by human VWF D4 and cleaved VWF multimers, preferentially under fluid shear stress. Human ADAMTS13 constructed to resemble pigeon ADAMTS13 retained normal allosteric regulation and shear-dependent cleavage of VWF. Thus, the T3-T6 domains of human ADAMTS13 are dispensable. Conversely, deletion of T7 or T8 abolished allosteric activation. For most species, some sequence changes in the VWF substrate can markedly increase the rate of cleavage, suggesting that ADAMTS13 and VWF have not evolved to be optimal enzyme-substrate pairs. These properties may reflect evolutionary pressure to balance the risk for VWF-dependent bleeding and thrombosis.

A novel role for von Willebrand factor in the pathogenesis of experimental cerebral malaria

Plasmodium falciparum malaria infection is associated with an early marked increase in plasma von Willebrand factor (VWF) levels, together with a pathological accumulation of hyperreactive ultra-large VWF (UL-VWF) multimers. Given the established critical role of platelets in malaria pathogenesis, these increases in plasma VWF raise the intriguing possibility that VWF may play a direct role in modulating malaria pathogenesis. To address this hypothesis, we used an established murine model of experimental cerebral malaria (ECM), in which wild-type (WT) C57BL/6J mice were infected with Plasmodium berghei ANKA. In keeping with findings in children with P falciparum malaria, acute endothelial cell activation was an early and consistent feature in the murine model of cerebral malaria (CM), resulting in significantly increased plasma VWF levels. Despite the fact that murine plasma ADAMTS13 levels were not significantly reduced, pathological UL-VWF multimers were also observed in murine plasma following P berghei infection. To determine whether VWF plays a role in modulating the pathogenesis of CM in vivo, we further investigated P berghei infection in VWF(-/-) C57BL/6J mice. Clinical ECM progression was delayed, and overall survival was significantly prolonged in VWF(-/-) mice compared with WT controls. Despite this protection against ECM, no significant differences in platelet counts or blood parasitemia levels were observed between VWF(-/-) and WT mice. Interestingly, however, the degree of ECM-associated enhanced blood-brain barrier permeability was significantly attenuated in VWF(-/-) mice compared with WT controls. Given the significant morbidity and mortality associated with CM, these novel data may have direct translational significance.

Probing ADAMTS13 substrate specificity using phage display

Von Willebrand factor (VWF) is a large, multimeric protein that regulates hemostasis by tethering platelets to the subendothelial matrix at sites of vascular damage. The procoagulant activity of plasma VWF correlates with the length of VWF multimers, which is proteolytically controlled by the metalloprotease ADAMTS13. To probe ADAMTS13 substrate specificity, we created phage display libraries containing randomly mutated residues of a minimal ADAMTS13 substrate fragment of VWF, termed VWF73. The libraries were screened for phage particles displaying VWF73 mutant peptides that were resistant to proteolysis by ADAMTS13. These peptides exhibited the greatest mutation frequency near the ADAMTS13 scissile residues. Kinetic assays using mutant and wild-type substrates demonstrated excellent agreement between rates of cleavage for mutant phage particles and the corresponding mutant peptides. Cleavage resistance of selected mutations was tested in vivo using hydrodynamic injection of corresponding full-length expression plasmids into VWF-deficient mice. These studies confirmed the resistance to cleavage resulting from select amino acid substitutions and uncovered evidence of alternate cleavage sites and recognition by other proteases in the circulation of ADAMTS13 deficient mice. Taken together, these studies demonstrate the key role of specific amino acids residues including P3-P2’ and P11’, for substrate specificity and emphasize the importance in flowing blood of other ADAMTS13–VWF exosite interactions outside of VWF73.

Nxf1 natural variant E610G is a semi-dominant suppressor of IAP-induced RNA processing defects

Endogenous retroviruses and retrotransposons contribute functional genetic variation in animal genomes. In mice, Intracisternal A Particles (IAPs) are a frequent source of both new mutations and polymorphism across laboratory strains. Intronic IAPs can induce alternative RNA processing choices, including alternative splicing. We previously showed IAP I∆1 subfamily insertional mutations are suppressed by a wild-derived allele of the major mRNA export factor, Nxf1. Here we show that a wider diversity of IAP insertions present in the mouse reference sequence induce insertion-dependent alternative processing that is suppressed by Nxf1CAST alleles. These insertions typically show more modest gene expression changes than de novo mutations, suggesting selection or attenuation. Genome-wide splicing-sensitive microarrays and gene-focused assays confirm specificity of Nxf1 genetic modifier activity for IAP insertion alleles. Strikingly, CRISPR/Cas9-mediated genome editing demonstrates that a single amino acid substitution in Nxf1, E610G, is sufficient to recreate a quantitative genetic modifier in a co-isogenic background.

Allosteric activation of ADAMTS13 by von Willebrand factor

The metalloprotease ADAMTS13 cleaves von Willebrand factor (VWF) within endovascular platelet aggregates, and ADAMTS13 deficiency causes fatal microvascular thrombosis. The proximal metalloprotease (M), disintegrin-like (D), thrombospondin-1 (T), Cys-rich (C), and spacer (S) domains of ADAMTS13 recognize a cryptic site in VWF that is exposed by tensile force. Another seven T and two complement C1r/C1s, sea urchin epidermal growth factor, and bone morphogenetic protein (CUB) domains of uncertain function are C-terminal to the MDTCS domains. We find that the distal T8-CUB2 domains markedly inhibit substrate cleavage, and binding of VWF or monoclonal antibodies to distal ADAMTS13 domains relieves this autoinhibition. Small angle X-ray scattering data indicate that distal T-CUB domains interact with proximal MDTCS domains. Thus, ADAMTS13 is regulated by substrate-induced allosteric activation, which may optimize VWF cleavage under fluid shear stress in vivo. Distal domains of other ADAMTS proteases may have similar allosteric properties.

Crystal structure and enzymatic activity of an ADAMTS-13 mutant with the East Asian-specific P475S polymorphism

BACKGROUND

An East Asian-specific P475S polymorphism in the gene encoding ADAMTS-13 causes an approximately 16% reduction in plasma ADAMTS-13 activity.

OBJECTIVES

To demonstrate the impact of this dysfunctional polymorphism by characterizing the structure and activity of the P475S mutant protein.

METHODS

We determined the crystal structure of the P475S mutant of ADAMTS-13-DTCS (DTCS-P475S, residues 287-685) and compared it with the wild-type structure. We determined the enzymatic parameters of ADAMTS-13-MDTCS (residues 75-685) and MDTCS-P475S, and further examined the effects of denaturants and reaction temperature on their activity. We also examined the cleavage of shear-treated von Willebrand factor (VWF) by MDTCS-P475S.

RESULTS

MDTCS-P475S showed a reaction rate similar to that of wild-type MDTCS, but showed two-fold lower affinity for the peptidyl substrate, indicating that the Pro475-containing V-loop (residues 474-481) in the CA domain is a substrate-binding exosite. Structural analysis showed that the conformation of the V-loop was significantly different in DTCS-P475S and the wild type, where no obvious interactions of Ser475 with other residues were observed. This explains the higher susceptibility of the enzymatic activity of MDTCS-P475S to reaction environments such as denaturants and high temperature. MDTCS-P475S can moderately cleave shear-treated VWF.

CONCLUSIONS

We have provided structural evidence that the P475S polymorphism in ADAMTS-13 leads to increased local structural instability, resulting in lowered affinity for the substrate without changing the reaction rate. The moderate activity of ADAMTS-13-P475S for shear-treated VWF is sufficient to prevent thrombotic thrombocytopenic purpura (TTP) onset.

Global cerebral ischemia induces increased expression of multiple retrotransposons

Retrotransposons (RTs) account for ~45% of the mammalian genome. They are capable of inserting into new genomic locations, which can result in deleterious outcomes. We examined the response of nine RTs to global cerebral ischemia (GCI) and explored the DNA methylation status of the two significantly altered RTs. Seven of the nine RTs were significantly increased at 24h post-insult in ischemic hippocampus. GCI also led to a significant decrease in the DNA methylation status of intracisternal A-particle (IAP) RT, but had no marked effect upon DNA methylation of long interspersed nucleotide element 1 (L1) RT. In summary, GCI produced marked increases in RT RNA expression and had a differential effect on the DNA methylation status of two RTs in vulnerable hippocampal neurons destined to die. These data suggest that RTs may play an active role in ischemic brain pathology and that these endogenous mutagens and their regulatory elements could be targeted as potential therapeutic targets in this devastating condition.

Mouse endogenous retroviruses can trigger premature transcriptional termination at a distance

Endogenous retrotransposons have caused extensive genomic variation within mammalian species, but the functional implications of such mobilization are mostly unknown. We mapped thousands of endogenous retrovirus (ERV) germline integrants in highly divergent, previously unsequenced mouse lineages, facilitating a comparison of gene expression in the presence or absence of local insertions. Polymorphic ERVs occur relatively infrequently in gene introns and are particularly depleted from genes involved in embryogenesis or that are highly expressed in embryonic stem cells. Their genomic distribution implies ongoing negative selection due to deleterious effects on gene expression and function. A polymorphic, intronic ERV at Slc15a2 triggers up to 49-fold increases in premature transcriptional termination and up to 39-fold reductions in full-length transcripts in adult mouse tissues, thereby disrupting protein expression and functional activity. Prematurely truncated transcripts also occur at Polr1a, Spon1, and up to ∼5% of other genes when intronic ERV polymorphisms are present. Analysis of expression quantitative trait loci (eQTLs) in recombinant BxD mouse strains demonstrated very strong genetic associations between the polymorphic ERV in cis and disrupted transcript levels. Premature polyadenylation is triggered at genomic distances up to >12.5 kb upstream of the ERV, both in cis and between alleles. The parent of origin of the ERV is associated with variable expression of nonterminated transcripts and differential DNA methylation at its 5'-long terminal repeat. This study defines an unexpectedly strong functional impact of ERVs in disrupting gene transcription at a distance and demonstrates that ongoing retrotransposition can contribute significantly to natural phenotypic diversity.

Unraveling the scissile bond: how ADAMTS13 recognizes and cleaves von Willebrand factor

von Willebrand factor (VWF) is a large adhesive glycoprotein with established functions in hemostasis. It serves as a carrier for factor VIII and acts as a vascular damage sensor by attracting platelets to sites of vessel injury. VWF size is important for this latter function, with larger multimers being more hemostatically active. Functional imbalance in multimer size can variously cause microvascular thrombosis or bleeding. The regulation of VWF multimeric size and platelet-tethering function is carried out by ADAMTS13, a plasma metalloprotease that is constitutively active. Unusually, protease activity of ADAMTS13 is controlled not by natural inhibitors but by conformational changes in its substrate, which are induced when VWF is subject to elevated rheologic shear forces. This transforms VWF from a globular to an elongated protein. This conformational transformation unfolds the VWF A2 domain and reveals cryptic exosites as well as the scissile bond. To enable VWF proteolysis, ADAMTS13 makes multiple interactions that bring the protease to the substrate and position it to engage with the cleavage site as this becomes exposed by shear. This article reviews recent literature on the interaction between these 2 multidomain proteins and provides a summary model to explain proteolytic regulation of VWF by ADAMTS13.

Role of zinc finger transcription factor zfp69 in body fat storage and diabetes susceptibility of mice

Type 2 diabetes is a polygenic disease resulting from a combination of different disease alleles reflecting obesity, insulin resistance, and hyperglycemia. Using a positional cloning strategy with different inbred strains of mice, we mapped a disease locus for obesity-associated diabetes on chromosome 4. We analyzed all genes in this region and identified distinct differences in the expression levels of the transcription factor Zfp69. The expression of this gene mediated diabetes progression in a leptin-deficient congenic mouse line. The animals developed a disease pattern of hyperglycemia, reduced gonadal fat mass, and increased plasma and liver triglycerides, resembling a potential defect in triglyceride storage . In order to elucidate the impact of the human ortholog of Zfp69 in the development of type 2 diabetes, we tested its mRNA expression in human white adipose tissue. Consistent with the mouse data, mRNA-expression was significantly higher in diabetic subjects than in unaffected controls.

Escherichia coli-derived von Willebrand factor-A2 domain fluorescence/Förster resonance energy transfer proteins that quantify ADAMTS13 activity

The cleavage of the A2 domain of von Willebrand factor (VWF) by the metalloprotease ADAMTS13 regulates VWF size and platelet thrombosis rates. Reduction or inhibition of this enzyme activity leads to thrombotic thrombocytopenic purpura (TTP). We generated a set of novel molecules called VWF-A2 FRET (fluorescence/Förster resonance energy transfer) proteins, where variants of yellow fluorescent protein (Venus) and cyan fluorescent protein (Cerulean) flank either the entire VWF-A2 domain (175 amino acids) or truncated fragments (141, 113, and 77 amino acids) of this domain. These proteins were expressed in Escherichia coli in soluble form, and they exhibited FRET properties. Results show that the introduction of Venus/Cerulean itself did not alter the ability of VWF-A2 to undergo ADAMTS13-mediated cleavage. The smallest FRET protein, XS-VWF, detected plasma ADAMTS13 activity down to 10% of normal levels. Tests of acquired and inherited TTP could be completed within 30 min. VWF-A2 conformation changed progressively, and not abruptly, on increasing urea concentrations. Although proteins with 77 and 113 VWF-A2 residues were cleaved in the absence of denaturant, 4M urea was required for the efficient cleavage of larger constructs. Overall, VWF-A2 FRET proteins can be applied both for the rapid diagnosis of plasma ADAMTS13 activity and as a tool to study VWF-A2 conformation dynamics.

The distal carboxyterminal domains of murine ADAMTS13 influence proteolysis of platelet-decorated VWF strings in vivo

BACKGROUND

The multidomain metalloprotease ADAMTS13 regulates the size of von Willebrand factor (VWF) multimers upon their release from endothelial cells. How the different domains in ADAMTS13 control VWF proteolysis in vivo remains largely unidentified.

METHODS

Seven C-terminally truncated murine ADAMTS13 (mADAMTS13) mutants were constructed and characterized in vitro. Their ability to cleave VWF strings in vivo was studied in the ADAMTS13(-/-) mouse.

RESULTS

Murine MDTCS (devoid of T2-8 and CUB domains) retained full enzyme activity in vitro towards FRETS-VWF73 and the C-terminal T6-8 (del(T6-CUB)) and CUB domains (delCUB) are dispensable under these assay conditions. In addition, mADAMTS13 fragments without the spacer domain (MDT and M) had reduced catalytic efficiencies. Our results hence indicate that similar domains in murine and human ADAMTS13 are required for activity in vitro, supporting the use of mouse models to study ADAMTS13 function in vivo. Interestingly, using intravital microscopy we show that removal of the CUB domains abolishes proteolysis of platelet-decorated VWF strings in vivo. In addition, whereas MDTCS is fully active in vivo, partial (del(T6-CUB)) or complete (delCUB) addition of the T2-8 domains gradually attenuates its activity.

CONCLUSIONS

Our data demonstrate that the ADAMTS13 CUB and T2-8 domains influence proteolysis of platelet-decorated VWF strings in vivo.

Pathophysiology of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a disorder with characteristic von Willebrand factor (VWF)-rich microthrombi affecting the arterioles and capillaries of multiple organs. The disorder frequently leads to early death unless the patients are treated with plasma exchange or infusion. Studies in the last decade have provided ample evidence to support that TTP is caused by deficiency of a plasma metalloprotease, ADAMTS13. When exposed to high shear stress in the microcirculation, VWF and platelets are prone to form aggregates. This propensity of VWF and platelet to form microvascular thrombosis is mitigated by ADAMTS13, which cleaves VWF before it is activated by shear stress to cause platelet aggregation in the circulation. Deficiency of ADAMTS13, due to autoimmune inhibitors in patients with acquired TTP and mutations of the ADAMTS13 gene in hereditary cases, leads to VWF-platelet aggregation and microvascular thrombosis of TTP. In this review, we discuss the current knowledge on the pathogenesis, diagnosis and management of TTP, address the ongoing controversies, and indicate the directions of future investigations.

An insertion of intracisternal A-particle retrotransposon in a novel member of the phosphoglycerate mutase family in the lew allele of mutant mice

Intracisternal A-particle retrotransposons (IAPs) are known, moveable, retrovirus-like elements and are defective in envelope protein synthesis in the mouse genome. Insertion of IAP elements can either interupt or enhance gene function or expression. Using a mouse model called lethal wasting (lew), we recently identified the insertion of an IAP sequence in a gene, 9630033F20Rik, that contains domains involved in glycolysis. The expression pattern of the 9630033F20Rik gene between various normal and diseased tissues was determined by semi-quantitative RT-PCR. The effect of the insertion mutation in 9630033F20Rik on glycolysis in heart, muscle, and brain tissues was further investigated using oligonuleotide microarray analysis. Results indicated that the expression of 9630033F20Rik is ubiquitous and its signal is relatively higher in heart and brain tissues. The insertion caused the deletion of exon 5 and decreased expression of this gene in all the tissues studied in the lew mice. Changes in the expression levels of glycolytic genes mainly occured in muscle tissue, raising a possibility that 9630033F20Rik may function as one of the transcriptional regulators of glycolytic genes in skeletal muscle. However, considering the fact that a single nucleotide mutation in vesicle-associated membrane protein 1 (VAMP1) has been reported as the causal gene for the lew mouse, how much of an impact the IAP insertion in the lew mouse phenotype has on glycolytic genes compared to the effect from the VAMP1 mutation responsible for the lew mouse phenotype should be further investigated.

The function of ADAMTS13 in thrombogenesis in vivo: insights from mutant mice

Recently, two independent groups have established ADAMTS13-deficient mice using gene-targeting techniques. In humans, genetic or acquired deficiency in ADAMTS13 leads to a potentially fatal syndrome, thrombotic thrombocytopenic purpura (TTP). Surprisingly, ADAMTS13-deficient mice are viable with no apparent signs of TTP. However, these mouse models indicate that ADAMTS13 down-regulates platelet adhesion and aggregation in vivo, and ADAMTS13 deficiency can provide enhanced thrombus formation at the site of vascular lesions. In addition, ADAMTS13 by cleaving hyperactive ultra-large von Willebrand factor multimers not only down-regulates thrombosis but also inflammation. ADAMTS13-congenic mice that carry a truncated form of ADAMTS13 lacking the C-terminal domains have also been developed. Phenotypes of the congenic mice indicate the physiological significance of the C-terminal domains of ADAMTS13 in down-regulating thrombus growth. The studies mentioned here in different mouse models uncover the in vivo function of ADAMTS13 and strengthened the understanding of the mechanism of systemic disease TTP.

Multi-step binding of ADAMTS-13 to von Willebrand factor

BACKGROUND

ADAMTS-13 proteolytic activity is controlled by the conformation of its substrate, von Willebrand factor (VWF), and changes in the secondary structure of VWF are essential for efficient cleavage. Substrate recognition is mediated through several non-catalytic domains in ADAMTS-13 distant from the active site.

OBJECTIVES

We hypothesized that not all binding sites for ADAMTS-13 in VWF are cryptic and analyzed binding of native VWF to ADAMTS-13.

METHODS

Immunoprecipiation of VWF-ADAMTS-13 complexes using anti-VWF antibodies and magnetic beads was used. Binding was assessed by Western blotting and immunosorbent assays.

RESULTS

Co-immunoprecipitation demonstrated that ADAMTS-13 binds to native multimeric VWF (K(d) of 79 +/- 11 nmol L(-1)) with no measurable proteolysis. Upon shear-induced unfolding of VWF, binding increased 3-fold and VWF was cleaved. Binding to native VWF was saturable, time dependent, reversible and did not vary with ionic strength (I of 50-200). Moreover, results with ADAMTS-13 deletion mutants indicated that binding to native VWF is mediated through domains distal to the ADAMTS-13 spacer, probably thrombospondin-1 repeats. Interestingly, this interaction occurs in normal human plasma with an ADAMTS-13 to VWF stoichiometry of 0.0040 +/- 0.0004 (mean +/- SEM, n = 10).

CONCLUSIONS

ADAMTS-13 binds to circulating VWF and may therefore be incorporated into a platelet-rich thrombus, where it can immediately cleave VWF that is unfolded by fluid shear stress.

A novel active endogenous retrovirus family contributes to genome variability in rat inbred strains

Endogenous retroviruses (ERVs) contribute to a range of germline, as well as somatic mutations in mammals. However, autonomous retrotransposition of potentially active elements has not been demonstrated in the rat genome. We cloned an insertion that disrupted the normal splicing of the Cntrob gene that was subsequently identified as a nonautonomous, novel endogenous retrovirus of the RnERV-K8e family. The RnERV-K8e family is closely related to the recently reported MmERV-K10c elements, but differs from the autonomous mouse MusD or IAP families. In addition, we identified a novel, unexpectedly close relative of RnERV-K8e in the mouse, suggesting ERV-K cross-species transmission between mice and rats. We cloned a potentially autonomous RnERV-K8e element identified by in silico analysis and, using an in vitro retrotransposition assay, demonstrated that it is capable of retrotransposition. This particular element (named Rat-rho, pronounced "retro") encodes a retroviral envelope gene (env); however, env is not required for de novo retrotransposition events. Significant levels of RnERV-K8e-associated genetic polymorphisms were detected among inbred rat strains, suggesting ongoing retrotransposition in the rat genome. This study identifies an ERV-K-type family in rats that shows obvious signs of recent activity. Ongoing retrotranspositional activity may significantly add to genomic variability among inbred rat strains.

A novel binding site for ADAMTS13 constitutively exposed on the surface of globular VWF

ADAMTS13 metalloprotease regulates the multimeric size of von Willebrand factor (VWF) by cleaving the Tyr1605-Met1606 bond in the VWF A2 domain. The mechanisms of VWF recognition by ADAMTS13 have yet to be fully resolved. Most studies have focused on the role of exosites within the VWF A2 domain, involved in interaction with the ADAMTS13 spacer domain. In the present study, we expressed different C-terminal domain VWF fragments and evaluated their binding to ADAMTS13 and its truncated mutants, MDTCS and del(TSP5-CUB). Using plate binding assay and surface plasmon resonance, we identified a novel ADAMTS13 binding site (K(D) approximately 86 nM) in the region of VWF spanning residues 1874 to 2813, which includes the VWF D4 domain and that interacts with the C-terminal domains of ADAMTS13. We show that the interaction occurs even when VWF is in static conditions, assumed to be globular and where the VWF A2 domain is hidden. We demonstrate that C-terminal VWF fragments, as well as an antibody specifically directed toward the VWF D4 domain, inhibit VWF proteolysis by ADAMTS13 under shear conditions. We propose that this novel VWF C-terminal binding site may participate as the initial step of a multistep interaction ultimately leading to proteolysis of VWF by ADAMTS13.

Positional cloning of zinc finger domain transcription factor Zfp69, a candidate gene for obesity-associated diabetes contributed by mouse locus Nidd/SJL

Polygenic type 2 diabetes in mouse models is associated with obesity and results from a combination of adipogenic and diabetogenic alleles. Here we report the identification of a candidate gene for the diabetogenic effect of a QTL (Nidd/SJL, Nidd1) contributed by the SJL, NON, and NZB strains in outcross populations with New Zealand Obese (NZO) mice. A critical interval of distal chromosome 4 (2.1 Mbp) conferring the diabetic phenotype was identified by interval-specific congenic introgression of SJL into diabetes-resistant C57BL/6J, and subsequent reporter cross with NZO. Analysis of the 10 genes in the critical interval by sequencing, qRT–PCR, and RACE–PCR revealed a striking allelic variance of Zfp69 encoding zinc finger domain transcription factor 69. In NZO and C57BL/6J, a retrotransposon (IAPLTR1a) in intron 3 disrupted the gene by formation of a truncated mRNA that lacked the coding sequence for the KRAB (Krüppel-associated box) and Znf-C2H2 domains of Zfp69, whereas the diabetogenic SJL, NON, and NZB alleles generated a normal mRNA. When combined with the B6.V-Lep^ob background, the diabetogenic Zfp69^SJL allele produced hyperglycaemia, reduced gonadal fat, and increased plasma and liver triglycerides. mRNA levels of the human orthologue of Zfp69, ZNF642, were significantly increased in adipose tissue from patients with type 2 diabetes. We conclude that Zfp69 is the most likely candidate for the diabetogenic effect of Nidd/SJL, and that retrotransposon IAPLTR1a contributes substantially to the genetic heterogeneity of mouse strains. Expression of the transcription factor in adipose tissue may play a role in the pathogenesis of type 2 diabetes.

Type 2 diabetes in humans as well as in obese mice is caused by a combination of adipogenic and diabetogenic gene variants. We have identified a gene that appears to be involved in the pathogenesis of hyperglycaemia in obese mice: in some mouse strains, the gene Zfp69 is disrupted by a retroviral transposon (IAPLTR1a), which generates a truncated mRNA. Disruption of the gene was associated with a reduced susceptibility for diabetes, whereas the normal allele enhanced hyperglycaemia in obese mice. Zfp69 encodes a transcription factor which appears to interfere with lipid storage in adipose tissue, and thereby enhances lipid deposition in liver. In humans with type 2 diabetes, mRNA levels of the human orthologue of Zfp69 (ZNF642) were increased in adipose tissue. Thus, the transcription factor ZFP69/ZNF642 may be involved in the pathogenesis of obesity-associated diabetes.

Species-dependent variability of ADAMTS13-mediated proteolysis of human recombinant von Willebrand factor

BACKGROUND

von Willebrand factor (VWF) is composed of a series of multimers, the sizes of which are regulated by the plasma metalloprotease ADAMTS13.

OBJECTIVE

Proteolysis of human recombinant VWF (rVWF) by ADAMTS13 present in the plasma of different species typically used as preclinical animal models was investigated to evaluate the efficacy and safety of rVWF.

METHODS

Degradation of rVWF was studied in vitro under moderate denaturing conditions and was monitored by multimer analysis, residual collagen binding, and immunoblot analysis. In vivo cleavage was determined by administration of rVWF to cynomolgus monkeys, rabbits and VWF-deficient mice and subsequent analysis of plasma samples by immunoblot. Plasma ADAMTS13 levels were determined with a synthetic human VWF peptide (FRETS-VWF73).

RESULTS

From the animals tested, only rabbit plasma was as efficient as human plasma in proteolysing rVWF in vitro. Mouse plasma virtually failed to cleave rVWF. Administration of human rVWF resulted in ADAMTS13-specific cleavage products in rabbits and, to a lesser extent, in cynomolgus monkeys at various doses of rVWF. Virtually no cleavage occurred in mice. ADAMTS13 activity levels in rabbit and monkey plasma were similar to those in human plasma and were not significantly altered upon infusion of rVWF up to very high doses, indicating that rVWF did not lead to an exhaustion of endogenous ADAMTS13 in both species.

CONCLUSIONS

The differences in susceptibility to cleavage of rVWF by different species need to be considered when interpreting the physiology of human rVWF from results of tests in animal models.

von Willebrand factor is a major determinant of ADAMTS-13 decrease during mouse sepsis induced by cecum ligation and puncture

SUMMARY BACKGROUND

During sepsis, von Willebrand factor (VWF) is abundantly secreted; the main mechanism regulating its size involves specific proteolysis by the metalloprotease ADAMTS-13.

OBJECTIVES

To determine whether ADAMTS-13 consumption due to its binding to, and/or cleavage, of VWF contributes to its decrease during sepsis and whether abrogating or enhancing ADAMTS-13 activity influences sepsis outcome.

METHODS

ADAMTS-13 activity was evaluated in a model of sepsis induced by cecum ligature and puncture (CLP) in wild-type and Vwf(-/-) mice. Sepsis outcome was studied in those mice and in Adamts-13(-/-) mice. Finally, survival was studied in wild-type mice injected hydrodynamically with the human ADAMTS-13 gene.

RESULTS

In wild-type mice, CLP-induced sepsis elicited a significant ADAMTS-13 decrease, and a strong negative correlation existed between VWF and ADAMTS-13. In Vwf(-/-) mice, CLP also induced severe sepsis, but ADAMTS-13 was not significantly diminished. Notably, Vwf(-/-) mice lived significantly longer than wild-type mice. In contrast, Adamts-13(-/-) mice and wild-type mice were comparable with regard to thrombocytopenia, VWF concentrations, absence of thrombi, and survival. Hydrodynamic hADAMTS-13 gene transfer with the pLIVE expression vector resulted in high and stable ADAMTS13 activity in CLP mice; however, no impact on survival was observed.

CONCLUSIONS

VWF secretion is a major determinant of ADAMTS-13 decrease in the CLP model, and plays an important role in sepsis-induced mortality, but the complete absence of its regulating protease, ADAMTS-13, had no detectable impact in this sepsis model. Furthermore, increasing ADAMTS-13 activity had no impact on survival.

Molecular basis of ADAMTS13 dysfunction in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a thrombotic microangiopathic disorder characterized by thrombocytopenia, hemolytic anemia, neurological and renal manifestations, and fever. It is associated with dysfunctional von Willebrand factor (VWF) proteolysis and the occurrence of VWF- and platelet-rich thrombi in the microcirculation of multiple organs, including the kidneys. Von Willebrand factor is a large glycoprotein that circulates in plasma as a series of multimers, and it plays a major role in primary hemostasis by inducing the formation of platelet plugs at sites of vascular injury and high-shear stress. Its activity is dependent on the sizes of the multimers, with ultra-large (UL) VWF multimers being biologically very potent. The ULVWF multimers are rapidly degraded upon their secretion from endothelial cells in normal individuals but not in the circulation of TTP patients, causing the formation of disseminated thrombi in the latter. The defective breakdown of VWF is attributed to a severely deficient activity of the VWF-cleaving protease ADAMTS13, a plasma metalloprotease synthesized in the liver, kidneys, and endothelium. This protease rapidly degrades VWF-platelet strings under flow by proteolytic cleavage of the VWF subunit, thereby regulating the size of the platelet thrombus. Congenital TTP occurs due to ADAMTS13 mutations, with the usual debut occurring during the first years of life, while acquired TTP is associated with auto-antibodies against ADAMTS13.

Thrombotic thrombocytopenic purpura related to severe ADAMTS13 deficiency in children

Thrombotic thrombocytopenic purpura (TTP) related to a severely deficient activity of the von Willebrand factor cleaving protease, ADAMTS (A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats) 13, is a life-threatening event, the onset of which may occur as early as childhood. TTP is either inherited (Upshaw-Schulman syndrome) via ADAMTS13 gene mutations (neonatal onset) or acquired via anti-ADAMTS13 autoantibodies (childhood onset). TTP is due to platelet- and von-Willebrand-factor-rich thrombi of the microvasculature, inducing mechanical hemolytic anemia, consumption thrombocytopenia, and multivisceral ischemia. Clinical course consists of relapsing acute events triggered mostly by infections, associated icterus and hyperbilirubinemia, severe hemolytic anemia with schistocytosis and a negative Coombs test, severe thrombocytopenia, and sometimes symptoms related to visceral ischemia (renal failure, central nervous system vascular events, other organ failure). The recently available ADAMTS13 laboratory investigation combining measurement of ADAMTS13 activity in plasma, search for an ADAMTS13 circulating inhibitor, and anti-ADAMTS13 IgG and ADAMTS13 gene sequencing is a crucial addition to TTP diagnosis. Plasma exchanges are first-line treatment of acquired TTP, combined with steroids and immunosuppressive drugs. Curative treatment of acute events in Upshaw-Schulman syndrome relies on plasma infusions (provider of active ADAMTS13). Guidelines for preventive treatment of relapses are not clearly established but should associate plasmatherapy and caution to triggers of relapses. Therapeutic perspectives are focused on the development of concentrated plasma-derived ADAMTS13 or recombinant ADAMTS13.

The distal carboxyl-terminal domains of ADAMTS13 are required for regulation of in vivo thrombus formation

ADAMTS13 is a multidomain protease that limits platelet thrombogenesis through the cleavage of von Willebrand factor (VWF). We previously identified 2 types of mouse Adamts13 gene: the 129/Sv-strain Adamts13 gene encodes the long-form ADAMTS13 having the same domains as human ADAMTS13, whereas the C57BL/6-strain Adamts13 gene encodes the short-form ADAMTS13 lacking the distal C-terminal domains. To assess the physiologic significance of the distal C-terminal domains of ADAMTS13, we generated and analyzed 129/Sv-genetic background congenic mice (Adamts13(S/S)) that carry the short-form ADAMTS13. Similar to wild-type 129/Sv mice (Adamts13(L/L)), Adamts13(S/S) did not have ultralarge VWF multimers in plasma, in contrast to 129/Sv-genetic background ADAMTS13-deficient mice (Adamts13(-/-)). However, in vitro thrombogenesis under flow at a shear rate of 5000 s(-1) was accelerated in Adamts13(S/S) compared with Adamts13(L/L). Both in vivo thrombus formation in ferric chloride-injured arterioles and thrombocytopenia induced by collagen plus epinephrine challenge were more dramatic in Adamts13(S/S) than in Adamts13(L/L) but less than in Adamts13(-/-). These results suggested that the C-terminally truncated ADAMTS13 exhibited decreased activity in the cleavage of VWF under high shear rate. Role of the C-terminal domains may become increasingly important under prothrombotic conditions.

N-Glycans of ADAMTS13 modulate its secretion and von Willebrand factor cleaving activity

Severe deficiency of ADAMTS13, a plasma metalloprotease, leads to thrombotic thrombocytopenic purpura. ADAMTS13 contains 10 putative N-glycosylation sites in or near its metalloprotease sequence, spacer region, thrombospondin type 1 repeat no. 4 (TSR no. 4), and CUB domains. Tunicamycin treatment markedly decreased the secretion of ADAMTS13 into the culture medium of transfected cells. Nevertheless, the protease was efficiently secreted from N-acetylglucosaminyltransferase I-deficient Lec1 Chinese hamster ovary cells, indicating that N-glycosylation in the endoplasmic reticulum, but not the conversion of oligomannose to complex N-glycans in the Golgi complex, is important for secretion. However, ADAMTS13 with oligomannose N-glycans cleaved its substrate, von Willebrand factor (VWF) multimers, less effectively, with a higher K(m) but similar k(cat) value. In mutagenesis analysis, decreased secretion and VWF cleaving activity was observed with the N146Q and N828Q mutants, while decreased secretion only was observed with the N552Q mutant of ADAMTS13. Enzymatic removal of N-glycans from ADAMTS13 did not affect its VWF cleaving activity. Thus, N-glycosylation is necessary for efficient secretion of ADAMTS13, while conversion of the N-glycans from oligomannose to complex type in the Golgi complex enhances the proteolytic activity of the protease toward VWF multimers. After its secretion, ADAMTS13 does not require N-glycans for its VWF cleaving activity.

Extensive contacts between ADAMTS13 exosites and von Willebrand factor domain A2 contribute to substrate specificity

The metalloprotease ADAMTS13 efficiently cleaves only the Tyr(1605)-Met(1606) bond in the central A2 domain of multimeric von Willebrand factor (VWF), even though VWF constitutes only 0.02% of plasma proteins. This remarkable specificity depends in part on binding of the noncatalytic ADAMTS13 spacer domain to the C-terminal alpha-helix of VWF domain A2. By kinetic analysis of recombinant ADAMTS13 constructs, we show that the first thrombospondin-1, Cys-rich, and spacer domains of ADAMTS13 interact with segments of VWF domain A2 between Gln(1624) and Arg(1668), and together these exosite interactions increase the rate of substrate cleavage by at least approximately 300-fold. Internal deletion of Gln(1624)-Arg(1641) minimally affected the rate of cleavage, indicating that ADAMTS13 does not require a specific distance between the scissile bond and auxiliary substrate binding sites. Smaller deletions of the P2-P9 or the P4'-P18' residues on either side of the Tyr(1605)-Met(1606) bond abolished cleavage, indicating that the metalloprotease domain interacts with additional residues flanking the cleavage site. Thus, specific recognition of VWF depends on cooperative, modular contacts between several ADAMTS13 domains and discrete segments of VWF domain A2.

The kidney in thrombotic thrombocytopenic purpura

The kidney is commonly affected in thrombotic thrombocytopenic purpura (TTP), a multi-system disorder with microvascular thrombosis of the capillaries and arterioles. Nevertheless, due to difference in its diagnostic criteria, the frequency and severity of renal dysfunction in TTP remains controversial. Recent studies indicate that the severe deficiency of a von Willebrand factor (VWF) cleaving protease, ADAMTS13, is the main cause of platelet thrombosis in TTP; it is now possible to define TTP at the molecular level. Among patients with acquired TTP due to inhibitory antibodies of ADAMTS13, renal dysfunction is usually mild; oliguria, fluid overload, hypertension, and need of dialysis support are infrequent. When any of these complications occur, one must re-examine the diagnosis of TTP and consider the possibility that the patient has another cause for these complications. In a patient with hereditary TTP, acute renal failure may ensue. However, the renal dysfunction is mostly reversible if the patients are promptly treated with plasma to replenish ADAMTS13. Patients with TTP, particularly of the hereditary type, may develop chronic renal failure. This complication may be a consequence of repeated insults by overt or subclinical microvascular thrombosis to the kidney, or it may have a separate cause. Therapy of hereditary TTP should aim not only to prevent acute exacerbations but also to minimize the risk of irreversible renal injury.