Phylogenetic and functional analysis of histidine residues essential for pH-dependent multimerization of von Willebrand factor.

von Willebrand factor (VWF) is a multimeric plasma protein that mediates platelet adhesion to sites of vascular injury. The hemostatic function of VWF depends upon the formation of disulfide-linked multimers, which requires the VWF propeptide (D1D2 domains) and adjacent D'D3 domains. VWF multimer assembly occurs in the trans-Golgi at pH ~ 6.2 but not at pH 7.4, which suggests that protonation of one or more His residues (pK(a) ~6.0) mediates the pH dependence of multimerization. Alignment of 30 vertebrate VWF sequences identified 13 highly conserved His residues in the D1D2D'D3 domains, and His-to-Ala mutagenesis identified His³95 and His46° in the D2 domain as critical for VWF multimerization. Replacement of His³95 with Lys or Arg prevented multimer assembly, suggesting that reversible protonation of this His residue is essential. In contrast, replacement of His46° with Lys or Arg preserved normal multimer assembly, whereas Leu, Met, and Gln did not, indicating that the function of His46° depends primarily upon the presence of a positive charge. These results suggest that pH sensing by evolutionarily conserved His residues facilitates the assembly and packaging of VWF multimers upon arrival in the trans-Golgi.

Assembly of Weibel-Palade body-like tubules from N-terminal domains of von Willebrand factor.

Endothelial cells assemble von Willebrand factor (VWF) multimers into ordered tubules within storage organelles called Weibel-Palade bodies, and tubular packing is necessary for the secretion of VWF filaments that can bind connective tissue and recruit platelets to sites of vascular injury. We now have recreated VWF tubule assembly in vitro, starting with only pure VWF propeptide (domains D1D2) and disulfide-linked dimers of adjacent N-terminal D'D3 domains. Assembly requires low pH and calcium ions and is reversed at neutral pH. Quick-freeze deep-etch electron microscopy and three-dimensional reconstruction of negatively stained images show that tubules contain a repeating unit of one D'D3 dimer and two propeptides arranged in a right-handed helix with 4.2 units per turn. The symmetry and location of interdomain contacts suggest that decreasing pH along the secretory pathway coordinates the disulfide-linked assembly of VWF multimers with their tubular packaging.

A Retrospective Chart Review of Chronic Wound Patients Treated with Topical Oxygen Therapy.

Objective: Topical oxygen devices are Food and Drug Administration (FDA) cleared for the following indications for use of various etiologies: skin ulcerations due to diabetes, venous stasis, postsurgical infections and gangrenous lesions, decubitus ulcers; amputations/infected stumps; skin grafts; burns; and frostbite. The goal of this study was to understand the impact of topical oxygen therapy (TOT) on patient outcomes, including amputation and healing rates. Approach: This retrospective chart review included records collected between January 1, 2007, and July 18, 2016, from male and female patients ranging in age from 4 years to 105 years. All wounds were at least 1 cm2 and were treated with at least one separate modality before treatment with TOT and then treated with TOT for a minimum of 2 weeks in compliance with the FDA-approved indications. All records were from wounds that were no longer being treated with TOT. Results: In this study, TOT was associated with an overall rate of 59.4% for a reduction in chronic wound size, while 41.6% of wounds had no healing. The overall amputation rate was 2.4% for wounds in this study. Innovation: To our knowledge, this retrospective chart review represents one of the largest data sets (4,127 total wounds) collected over one of the longest time periods (9.5 years) to evaluate patient outcomes following TOT. Conclusion: This study revealed healing and amputation rates similar to those reported in controlled clinical studies using TOT to treat chronic wounds.

Two Cys residues essential for von Willebrand factor multimer assembly in the Golgi.

Von Willebrand factor (VWF) dimerizes through C-terminal CK domains, and VWF dimers assemble into multimers in the Golgi by forming intersubunit disulfide bonds between D3 domains. This unusual oxidoreductase reaction requires the VWF propeptide (domains D1D2), which acts as an endogenous pH-dependent chaperone. The cysteines involved in multimer assembly were characterized by using a VWF construct that encodes the N-terminal D1D2D'D3 domains. Modification with thiol-specific reagents demonstrated that secreted D'D3 monomer contained reduced Cys, whereas D'D3 dimer and propeptide did not. Reduced Cys in the D'D3 monomer were alkylated with N-ethylmaleimide and analyzed by mass spectrometry. All 52 Cys within the D'D3 region were observed, and only Cys(1099) and Cys(1142) were modified by N-ethylmaleimide. When introduced into the D1D2D'D3 construct, the mutation C1099A or C1142A markedly impaired the formation of D'D3 dimers, and the double mutation prevented dimerization. In full-length VWF, the mutations C1099A and C1099A/C1142A prevented multimer assembly; the mutation C1142A allowed the formation of almost exclusively dimers, with few tetramers and no multimers larger than hexamers. Therefore, Cys(1099) and Cys(1142) are essential for the oxidoreductase mechanism of VWF multimerization. Cys(1142) is reported to form a Cys(1142)-Cys(1142) intersubunit bond, suggesting that Cys(1099) also participates in a Cys(1099)-Cys(1099) disulfide bond between D3 domains. This arrangement of intersubunit disulfide bonds implies that the dimeric N-terminal D'D3 domains of VWF subunits align in a parallel orientation within VWF multimers.

A covalent oxidoreductase intermediate in propeptide-dependent von Willebrand factor multimerization.

The assembly of von Willebrand factor multimers in the Golgi apparatus requires D1D2 domains of the von Willebrand factor propeptide, which may act as an oxidoreductase to promote disulfide bond formation or rearrangement between two D3 domains in the mature subunit. This mechanism predicts that the propeptide should form a transient intrachain disulfide bond with the D3 domain before multimerization. Such an intermediate was detected using truncated subunits that simplify the analysis of the multimerization process. When only the D1D2D'D3 region of von Willebrand factor was expressed in baby hamster kidney cells, the propeptide and D'D3 formed an intrachain disulfide-linked species in the endoplasmic reticulum that could be identified by two-dimensional gel electrophoresis after cleavage with thrombin or furin. This intermediate rearranged in the Golgi to form free propeptide and D'D3 dimers that were secreted. A similar intracellular disulfide-linked species was identified in cells expressing the propeptide and D'D3 as separate proteins and in cells expressing full-length von Willebrand factor. These results support a model in which the propeptide acts as an oxidoreductase to promote von Willebrand factor multimerization in the Golgi apparatus.