A Spacer Sequence Inserted between the A1 and A2 Domains of Factor VIII Overcomes the Requirement for Proteolytic Cleavage at Arg 372 in the Generation of Cofactor Activity
Authors: Basavaraj Manjunath Goolyam , Krishnaswamy Sriram 
Published: February 2021

Factor VIII (FVIII) with a multi-domain structure (A1-a1-A2-a2-B-a3-A3-C1-C2) is a procofactor and precursor for the anti-hemophilic cofactor protein, FVIIIa. Following the intracellular processing within the B domain, secreted FVIII circulates as a heterodimer with variably sized (90K-200K) heavy chain (A1-a1-A2-a2-B) and an 80K light chain (a3-A3-C1-C2). Proteolytic activation of FVIII by thrombin that yields heterotrimeric FVIIIa (A1-a1/A2-a2/A3-C1-C2), the cofactor for intrinsic tenase, involves cleavage of three peptide bonds between Arg372-Ser373, Arg740-Ser741, and Arg1689-Ser1690. Cleavage at Arg740 removes the B-domain, and cleavage at Arg1689 removes the a3-acidic region and releases FVIII from vWF, its carrier protein, and exposes membrane binding sites within the FVIII light chain. Cleavage at Arg372 separates A1-a1 and A2-a2 domains and is implicated in the cofactor-dependent recognition and enhancement in the rate of factor X (FX) activation by intrinsic tenase. Subsequently, the separated A2-a2 domain dissociates spontaneously from the heterotrimeric FVIIIa resulting in the rapid loss of cofactor activity. We speculated that the requirement for cleavage at Arg372 might be obviated by the insertion of an optimized linker sequence between A1-a1 and A2-a2 domains on an uncleavable Gln372 backbone. To investigate this possibility, we prepared cDNA constructs of B-domain deleted FVIII variants; FVIII wild-type (FVIIIWT), FVIII372Q, and FVIII372Q followed by a rigid (Ala-Pro)5 linker sequence (FVIII372Q-AP5). All three FVIII constructs were stably transfected into BHK cells and high expressing clones were selected by one stage aPTT and western blotting of expression media. Selected stable clones were further expanded to collect 15L of expression media over 5-day period, and recombinant FVIII variants were purified using a three-step chromatographic approach. These FVIII variants were studied using SDS-PAGE, western blotting, aPTT assays, thrombin generation assay (TGA) and purified assays to assess kinetics of FX activation and spontaneous loss of cofactor activity. In contrast to FVIIIWT, FVIII372Q and FVIII372Q-AP5 were completely resistant to cleavage at Gln372 by thrombin, yielding bands corresponding to A1-a1-A2-a2 (90K) and A3-C1-C2 (73K). In one stage aPTT assays, FVIII372Q showed prolonged clotting times with specific activity in the range of 200-400 U/mg, while FVIIIWT and FVIII372Q-AP5 displayed comparable clotting times with specific activities ranging between 8000-10000 U/mg and 4500-5500 U/mg, respectively. In TGA initiated with either 0.1 pM tissue factor or 1 pM factor XIa, both FVIIIWT and FVIII372Q-AP5 displayed similar TGA profiles. In steady state kinetic studies of FX activation using limiting concentrations of factor IXa, saturating concentrations of FVIII variants pretreated with thrombin, membranes and increasing concentrations of FX, the cofactor function of thrombin-cleaved FVIII372Q was severely impaired. However, despite lack of cleavage at Gln372 in FVIII372Q-AP5, catalytic efficiency for FX activation by intrinsic tenase assembled by this variant was comparable to that seen with FVIIIaWT. At the physiological concentration of FX, the initial velocity for Xa formation (v/E) for intrinsic tenase assembled with FVIIIa372Q-AP5 was within a factor of 2 of that observed with FVIIIaWT while the rate observed with FVIIIa372Q was &gt;10-fold lower. Following rapid activation with thrombin, loss of cofactor function was significantly slower for FVIIIa372Q-AP5(t1/2 ~ 10 min) compared to FVIIIaWT (t1/2 ~ 2 min). Our findings indicate that the requirement for cleavage at Arg372 for the development of full FVIIIa cofactor function can be overcome by modulating the A1-A2 connector with an optimized linker sequence. Failure to yield an infinitely stable cofactor in the case of FVIIIa372Q-AP5 suggests that cleavage at Arg372 does not solely explain the spontaneous loss of FVIIIa cofactor function.

Disclosures
Krishnaswamy: Bayer: Research Funding.