Haematology

Tissue factor (TF) is a transmembrane protein essential for the maintenance of haemeostasis (Mackman and Taubman, 2009). It is present on the surface of vascularised organ cells including the heart, kidneys and intestines (Chu, 2011). TF has both coagulant and signalling activities. Its coagulant activities include activation of platelets thrombin formation and deposition of fibrin. The signalling activities include PAR-2 activation by FVIIa and FXa, as well PAR-1 activation by FXa and thrombin. TF amino acid sequence contains three domains including extracellular, transmembrane and cytoplasmic domains. The extracellular domain consists of N- and C- carbohydrate terminus that have three potential glycosylation sites at Asn11, Asn 124 and Ans137 (Butenas et al, 2009). The N-terminus has a cystine49-cystine57 disulfide bond link the two and the C-terminus has Cystine 186-Cystine 209 (cys186-cys209).

It is suggested that the presence of TF in cells is said to be inactive, i.e. encrypted state, with low procoagulant activity, therefore it requires conversion into a fully active state (Rao et al. 2012), suggesting that TF is present in two different states being low activity state (encrypted) and high activity state (decrypted) (Butenas et al, 2009). In recent years, there has been a debate about the mechanisms of encryption-decryption of TF. Different hypotheses have been suggested to explain the mechanisms of TF decryption which has remained indefinable for two decades (Mackman and Taubman, 2009), this may be because TF has been greatly recognised with important roles in inflammation, cancer and angiogenesis (Ahamed et al, 2006). The mechanism of TF decryption that has received the most consideration over the year consists of a thiol/disulfide switch in TF (Chen and Hogg, 2013). Some of these hypotheses suggested are discussed below.

TF encryption studied on cultured cells seems to mirrors the inactive blood-borne TF which does not activate factor X until decrypted. Four mechanisms have been suggested to keep TF in the inactive form. Inhibition of FX activation can be obtained when TF is found in lipid rafts (Sevinsky et al, 1996). Dimerisation and oligomerisation of TF also prevent FX activation, but in other studies these have not been expressed consistently (Bach, 2006). The formation of coagulation complexes and full activity of TF require exposure to the phosphatidylserine present on the surface of blood and vessel walls as well as a negatively charged phospholipid environment, shown in figure 1. However, the amount of contribution the phospholipid environment has on TF activity was questioned because of evidence that suggests TF is susceptible to post-translational modification in the cys186-cys209 disulfied bond (Hogg, 2003).

Figure 1. Components included in encryption and decryption of TF. TF is maintained in a cryptic state by a neutral phospholipid environment and by factors that keep TF cysteine 186 and 209 bond in the reduced free-thiol form. Thioredoxin (Trx)/thioredoxin reductase (TR)/NADPH system, the thiol alkylators, reduced glutathione (GSH) and nitric oxide (NO) have been involved in keeping TF in a reduced state. TF is activated or decrypted by activation of a purinergic receptor (P2X7) or complement (C5b-7) on myeloid cells and protein disulphide isomerase (PDI) has been involved in both systems. Contact of phosphatidyserine on the cell surface is required for full TF activity (Chen and Hogg, 2013).

(Figure 1. Taken from Chen and Hogg, 2013)

Maximal TF activity or decryption requires a negatively charged phospholipid, shown in Figure 1, (Ohkubo et al, 2010) as well as cys186-cys209 oxidation as evidence for this has been put together using non-physiological TF (Butenas and Krudysz-Amblo, 2012). Various studies imply that TF decryption involves both the redox reaction of cys186 and cys209 residues on cryptic TF and the exposure to phosphatidylserine on the surface of the cell. Both of these seem to participate in TF decryption. However, evidence for cys186-cys209 redox state in vivo is required but not yet available (Chen and Hogg, 2013).

Initial studies such as Ahamed et al (2006), have found that, extracellular cys186-cys209 disulfide bond is necessary for the activation and initiation phase of coagulation through the extrinsic pathway. Their study also reported that coagulant activity of TF is suppressed as extracellular protein disulfide isomerase (PDI) targets and breaks extracellular cys186-cys209 disulfide bond. Their data outlines that TF-VIIa complex can switch from coagulation to cell signalling, this provides a potential solution to how TF-VIIa signalling can occur independently of coagulation activation.