Poor production and inefficient γ-carboxylation of synthesized FIX by heterologous expression systems are regarded as major obstacles to the production of large quantities of functional recombinant FIX (Kaufman, Wasley et al. 1986, Baghani and Vatandoost 2016). There are different strategies to increase the expression of functional FIX. Previously, we have exchanged the prepropeptide of human FIX for that of human prothrombin, resulting in an improvement in activity (10 fold) (Khorshidi, Zomorodipour et al. 2015). The prepeptides have different degrees of total hydrophobicity, which may influence protein synthesis and secretion (Bird, Gething et al. 1990, Hatsuzawa, Tagaya et al. 1997, Zhang, Leng et al. 2005, Knappskog, Ravneberg et al. 2007, Khorshidi, Zomorodipour et al. 2015). Since the prepeptide of PT has higher total hydrophobicity than that of the FIX (0.93 vs. 0.79) (Khorshidi, Zomorodipour et al. 2015), it can result in higher production and secretion of FIX, which affects the number of active FIXs. So to eliminate the effect of the prepeptide and study the effect of net hydrophobicity and charge in propeptide γ-CRS, the FIX propeptide was substituted at positions − 12, -13, and − 14 based on the PT propeptide. By increasing both the net charge and hydrophobicity of the γ-CRS in the propeptide region of FIX, this replacement lead to high fully γ-carboxylated material (2.3 fold) and activity (3.4 fold) (Vatandoost and Bos 2019). The effect of hydrophobicity alone by double substituting amino acids at positions E-12Q/H-13P and H-13P/D-14A was also investigated. The net charge was constant in these mutants, but the hydrophobicity increased from − 5.3 in wild FIX to − 3.7 in double E-12Q / H-13P and + 1.6 in double H-13P/ D-14A rFIX mutants, which resulted in 1.4 and 2.9 fold enhanced activity, respectively (Vatandoost and Bos 2019).
Although these results demonstrate the role of net hydrophobicity in γ-carboxylation, the γ-CRS region's charge is also important in the rate of γ-carboxylation. Interestingly, although the PC and FIX γ-CRS sequences display a similar net hydrophobicity, they have a substantial difference in apparent affinity for PC and FIX (230 vs. 33.6 nM), leading to a 7-fold decrease in binding affinity to γ-carboxylase (Camire, Larson et al. 2000, Hallgren, Qian et al. 2006). Hence, hydrophobicity is not the single affecting factor of γ-carboxylase binding. Since a rise in the net charge of the γ-CRS region of PC relative to FIX was observed, the net charge of this region may contribute to γ-carboxylase binding as well. The latter is supported by our previous work, in which substitution of E-12Q with a change of net charge from − 1 to 0 led to 1.2 fold enhanced fully γ-carboxylated material and 1.4 fold activity (Vatandoost and Bos 2019). The second strategy was used to examine and confirm the effect of γ-CRS net charge on γ-carboxylase binding. Specifically, FIX prepropeptide was replaced with that of PC, leading to 1.6 more γ-carboxylation and activity (Table 1). It is speculated that by increasing the FIX γ-CRS region's net charge, the γ-carboxylase turnover and, therefore, γ-carboxylated FIXs may be enhanced. Support for the effect of the net charge of the γ-CRS region on γ-carboxylation comes from several studies. The − 9N to K substitution in FIX, while not affecting the hydrophobicity (from − 5.3 to -5.7), did affect the net charge of the γ-CRS region (from − 1 to 0). This substitution resulted in an increase in apparent affinity from 33 to 370 nM and enhanced γ-carboxylation of FIX (Stanley, Jin et al. 1999). It was also shown that charged side chains at position − 10 as well as the residues − 9 and − 8 in γ-CRS region have an effect on γ-carboxylation (Vermeer 1990). Moreover, charged amino acids that are present close to the predicted α-helical domain in several of the propeptides may affect γ-carboxylation (Czerwiec, Kalume et al. 2006). As such, these examples further point to a role for the charge of the FIX γ-CRS region in the functional interaction with γ-carboxylase, which is essential for an efficient turnover and γ-carboxylation. Since γ-carboxylation is crucial for a normal clotting activity of FIX, an increase in γ-carboxylation leads to more active FIX and a significant increase in activity as observed here (74 vs 45 mU/ml, 1.6-fold).
Furthermore, the FIX recovery assessment following barium citrate adsorption demonstrated that using PC prepropeptide result in 2-fold enhancement in fully γ-carboxylated FIX. In line with the findings of others, it was inferred that factors such as using altered propeptides (Presnell and Stafford 2002, Khorshidi, Zomorodipour et al. 2015), γ-carboxylase (Hallgren, Hommema et al. 2002), and VKOR overexpression (Hallgren, Qian et al. 2006) that can enhance γ-carboxylase turnover and decrease binding affinity may lead to full γ-carboxylation.
Since prepeptide (signal peptide) of PC has the same total hydrophobicity of FIX prepeptide, as anticipated, no difference was observed in total FIX expression (intracellular and conditioned media) when using a PC prepropeptide for the FIX, which is consistent with previous findings (Knappskog, Ravneberg et al. 2007, Tröße, Ravneberg et al. 2007, Tröße, Ravneberg et al. 2007, Vatandoost and Bos 2019). Nonetheless, we observed an improved secretion efficiency for the altered propeptide FIX variant (91% vs. 75%)(Fig. 1B). From this we speculatethat the increased γ-carboxylation leads to higher secretion efficiency and a decrease in trapped FIX in the intracellular space, indicating that secretory machinery is affected by γ-carboxylation. It is in light of other studies that show poor secretion of FIX following γ-carboxylase overexpression (Rehemtulla, Roth et al. 1993, Hallgren, Hommema et al. 2002, Wajih, Hutson et al. 2005) or increased secretion of FIX in overexpressed VKOR cells (Wajih, Hutson et al. 2005, Wajih, Sane et al. 2005, Hallgren, Qian et al. 2006, Pakdaman, Vatandoost et al. 2019), implying that γ-carboxylase acts as an essential factor during protein secretion (McClure, Walls et al. 1992). Moreover, it was shown that γ-carboxylation is part of the protein secretion pathway (Rishavy and Berkner 2012) and was required for both efficient secretion and anticoagulant activity (McClure, Walls et al. 1992). The following hypothesis is also particularly attractive: Glu γ-carboxylation may result in an increasing negative charge on protein substrates, and subsequent interaction with positively charged amino acids could neutralize the resulting charge density or contribute to triggering an active protein release mechanism (Rishavy and Berkner 2012).
Recently, Hao et al. demonstrated the carboxylation efficiency of a reporter-protein as directed by different propeptides (Hao, Jin et al. 2019). Fusion of FIX or PC propeptides to the N-terminus of the chimeric reporter-protein FIXgla-PC indicated that the FIX propeptide is the most efficient propeptide for reporter protein γ-carboxylation, with a 2.5-fold higher level of γ-carboxylation relative to the PC propeptide. Interestingly, these findings are in contrast with earlier observations on the apparent affinity for γ-carboxylase, which led to a 7-fold decrease in binding affinity to γ-carboxylase (Camire, Larson et al. 2000). Since in these studies a chimeric VKD-protein variant was studied in which the FIX GLA domain was fused to mature PC, we speculate that VKD protein domains such as EGF may differentially impact carboxylation in an indirect manner. For instance, the EGF-like region could have subtly altered the global folding of the chimeric protein, leading to a reduction in γ-carboxylation (Yu, Zhang et al. 1994). Furthermore, of note the S2 expression system employed in the current study has been reported to display a higher γ-carboxylation efficiency relative to mammalian cells (Vatandoost, Zomorodipour et al. 2012).
In conclusion, a leader peptide replacement approach is possible to improve the expression and activity of interest protein. Increasing the FIX γ-CRS region's net charge increased the turnover by γ-carboxylase, thereby enhancing the production of fully γ-carboxylated and functional FIX.