Identification and expression of proteases C. sonorensis and C. imicola important for African horsesickness virus replication

Abstract

African horsesickness (AHS) is one of the most deadly diseases of horses, with a mortality rate of over 90% in horses that have not been exposed to any African horsesickness virus (AHSV) serotype previously (Howell, 1960; Darpel et al., 2011). The Orbiviruses, African horsesickness virus (AHSV) and Bluetongue virus (BTV), are primarily transmitted to their mammalian hosts through certain haematophagous midge vectors (Culicoides spp.) (Erasmus, 1973). The selective cleavage of BTV and AHSV VP2 by trypsin-like serine proteases (Marchi et al., 1995) resulted in the generation of subsequent infectious sub-viral particles (ISVP) (Marchi et al., 1995; van Dijk & Huismans, 1982). It is believed that this cleavage affects the ability of the virus to infect cells of the mammalian and vector host (Darpel et al., 2011). Darpel et al (2011) identified a trypsinlike serine protease in the saliva of Culicoides sonorensis (C. sonorensis), which also cleaves the serotype determinant viral protein 2 (VP2) of BTV. And, a similar cleavage pattern was also observed by van Dijk & Huismans (1982) and Marchi et al (1995) with the use of trypsin and chymotrypsin. Manole et al (2012) recently determined the structure of a naturally occurring African horsesickness virus serotype 7 (AHSV7) strain with a truncated VP2. Upon further investigation, this strain was also shown to be more infective than the AHSV4 HS32/62 strain, since it outgrew AHSV4 in culture (Manole et al., 2012). Therefore, through proteolytic cleavage of these viral particles, the ability of the adult Culicoides to transmit the virus might be significantly increased (Dimmock, 1982; Darpel et al., 2011). Based on these findings, it is important to investigate the factors that influence the capability of arthropod-borne viruses to infect their insect vectors, mammalian hosts and their known reservoirs. In this study, we postulated that one of the vectors for AHSV, Culicoides imicola (C. imicola), has a protease similar to the 29 kDa C. sonorensis trypsin-like serine protease identified by Darpel et al (2011). Proteins in the total homogenate of C. imicola were separated on SDS-PAGE and yielded several protein bands, one of which also had a molecular mass of around 29 kDa. Furthermore, proteolytic activity was observed on a gelatin-based sodium dodecyl sulfate polyacryamide gel electrophoresis (SDS-PAGE) gel. The activity of the protein of interest was also confirmed to be a trypsin-like serine protease with the use of class-specific protease inhibitors. A recombinant trypsin-like serine protease of C. sonorensis was generated using the pColdIII bacterial expression vector. The expressed protein was partially purified with nickel ion affinity chromatography. Zymography also confirmed proteolytic activity. With the use of the protease substrates containing fluorescent tags and class specific protease inhibitors, the expressed protein was classified as a serine protease. It was also proposed that incubation of purified AHSV4 with the recombinant protease would result in the cleavage of AHSV4 VP2, resulting in similar VP2 digestion patterns as observed in BTV by Darpel et al (2011) or the truncated VP2 of AHSV7 by Manole et al (2012). BHK-21 cell cultured AHSV4 was partially purified through Caesium chloride gradient ultracentrifugation after which the virus was incubated with the recombinant protease. Since not enough virus sample was obtained, the outcome of VP2 digestion was undetermined. In the last part of this study, it was postulated that C. imicola and C. sonorensis have the same trypsin-like serine protease responsible for the cleavage of VP2 based on the protease activity visualised in the whole midge homogenate. Since the genome of C. imicola is not yet sequenced, the sequence of this likely protease is still unknown. Therefore, we attempted to identify this C. imicola protease through polymerase chain reaction (PCR) amplification. Total isolated ribonucleic acid (RNA) of C. imicola was used to synthesize complementary deoxyribonucleic acid (cDNA). The cDNA was subjected to PCR using C. sonorensis trypsin-like serine protease-based primers. An 830 bp DNA fragment was amplified. However, sequence alignment and the basic local alignment software tool (BLAST), revealed that DNA did not encode with any other known proteins or proteases. From the literature it seems that there is a correlation between the proteases in the vector and the mammalian species that succumb to AHS (Darpel et al., 2011, Wilson et al., 2009, Marchi et al., 1995). Based on the work performed in the study, a proteolytically active protein similar to the 29 kDa protein of C. sonorensis is present in C. imicola. The 29 kDa protease of C. sonorensis can also be expressed in bacteria which could aid in future investigations on how proteolytic viral modifications affect infectivity between different host species.