Cellular interactions are key to many events in vascular biology. Cell surface adhesion molecules mediate many of the interactions between leukocytes, platelets and the vessel wall. In response to inflammatory stimuli, leukocytes and platelets in the adjacent vasculature initially roll on the blood vessel wall, then stick, and finally transmigrate to the site of insult. The initial rolling event involves a class of adhesion proteins termed selectins (P-, E-, and L-selectin) which mediate the interaction between leukocytes, platelets and endothelial cells by their recognition of specific carbohydrate counter-structures, including sialyl-Lewis x. The primary sequence/motif structure of each of the selectins is similar. Each contains a N-terminal, 118-amino acid calcium-dependent lectin domain, an EGF motif, a variable number of tandem repetitive motifs related to motifs found in complement regulatory domains, a transmembrane domain and a short cytoplasmic tail.
P-selectin is a 140-kDa integral granule membrane glycoprotein localized to the α-granules of platelets and the Weibel-Palade bodies of endothelial cells and is rapidly expressed on both cell types on cell activation. This suggests that endothelial P-selectin is a critical molecule mediating initial adhesion events in acute inflammation, a view recently supported by a number of in vivo inflammatory models including neutrophil-dependent acute lung injury (Mulligan et al. (1992) J. Clin. Invest. 90, 1600), endotoxin-induced neutropenia (Coughlan et al. (1994) J. Exp. Med. 179, 329), reperfusion injury (Asako et al. (1994) J. Clin. Invest. 93, 1508) and histamine-induced leukocyte rolling in post capillary venules (Weyrich et al. (1993) J. Clin. Invest. 91, 2620). P-selectin binds to 10,000-20,000 copies of a single class of binding sites on neutrophils and HL60 cells.
Sako et al. ((1993) Cell 75, 1179) have cloned a ligand for P-selectin, termed P-selectin glycoprotein ligand-1 (PSGL-1) found on the surface of leukocytes (see also copending application Ser. No. 08/316,305). PSGL-1 is a 220 kDa, disulfide-linked homodimeric sialomucin which, when expressed by recombinant methodology with the appropriate fucosyltransferase, binds P-selectin, E-selectin and L-selectin in a similar calcium-dependent manner to the PSGL-1 on neutrophils. PSGL-1 has a signal peptide sequence of 17 amino acids followed by a 24-amino acid PACE cleaved propeptide sequence. The mature N-terminus of PSGL-1 contains an unusual stretch of twenty amino acids which is rich in negatively-charged aspartate and glutamate residues and which contains three tyrosine residues which meet the consensus sequence for 0-sulfation by a golgi sulfotransferase. At least one of these tyrosine residues is sulfated as evaluated by site-directed mutagenesis (Sako et al.).
In addition to binding P-selectin, PSGL-1 also binds L- and E-selectin. In contrast to P-selectin, however, the requirements for E-selectin recognition are much less rigid. (Spertinit et al., J. Cell. Biol. 135:523 (1996)). E-selectin binds a wide variety of sialomucin structures if they co-express the sialyl-Lewis x structure. L-selectin binds to a number of different counter-receptors, GLYCAM-1, MadCAM-1 and CD34, which like PSGL-1, are also sialomucins. A major question currently unresolved is what determines selectin specificity in the recognition of specific counter-receptor structures. P-, E- and L-selectin are 60-70% homologous in their N-terminal, 118-amino acid lectin motifs and each similarly recognizes the sialyl-Lewis x and sialyl-Lewis a carbohydrate structures. Further, binding of P-selectin to its receptor on neutrophils is four to five orders of magnitude more avid than the binding of sialyl-Lewis x. While differences in specificity and avidity may in part be accounted for by either the presentation of multiple sialyl-Lewis carbohydrate structures on the receptor mucin core or by subtle differences in carbohydrate structure, it is probable that the protein component of the sialomucin also determines selectin interaction.
Although the inflammatory response mediated by the P-selectin/PSGL-1 interaction is a part of the body's normal defense system, inappropriate inflammatory responses can also result in the development of various inflammatory disease states. It would, therefore, be desirable to provide agents for interfering with or blocking the selectin/PSGL-1 interaction in order to treat inflammatory disease.
GPIbα is a component of the glycoprotein (GP) Ib-IX complex found on the surface of platelets and serving as a receptor for von Willibrand factor (vWF). The interaction of the GP Ib-IX complex with vWF mediates attachment of platelets to the blood vessel wall at the site of injury. It has also can cause aggregation of platelets in high shear conditions and enable platelet activation at low concentrations of thrombin.
Mocarhagin, a protease found in the venom of cobras (including the Mozambiquan spitting cobra, Naja mossambica mossambica, a.k.a. Naja mocambique mocambique), has been found to cleave PSGL-1, resulting in disruption of P- and L-selectin mediated cell adhesion. Preparations of mocarhagin have been reported and demonstrated to serve this purpose. See, U.S. Pat. No. 5,659,018; DeLuca et al., J. Biol. Chem. 270: 26734 (1995); Ward et al., Biochem. 35: 4929 (1996). (Spertini et al.)
In addition, it also has been reported that Mocarhagin is capable of cleaving GPIbα at a position proximal to sulfated tyrosine residues within the critical vWF binding domain and disrupting the binding activity of GPIbα: DeLuca et al., J. Biol. Chem. 270: 26734 (1995); Dong et al., Biochemistry, 33: 13946 (1994).
It is therefore anticipated that an agent that can disrupt this interaction may have therapeutic application in a variety of thrombotic disorders such as restenosis and DVT.
However, applicants have discovered that the preparations described in these documents is only partially purified. Since it is necessary for mocarhagin proteins to be provided in highly purified form for such proteins to be used for therapeutic purposes, it would be desirable to provide highly purified preparations of mocarhagin proteins.
It would also be desirable to identify and isolate polynucleotides encoding mocarhagin proteins in order to produce such proteins by recombinant methods.