Cloning and expression of the M-gene from the human coronavirus NL-63 in different expression systems
Respiratory tract infections are one of the leading causes of morbidity and mortality across the world. This is especially true for infants, young children, the elderly and the immunocompromised. The strain placed on economies and health care systems of all countries by these diseases are phenomenal. Although we are familiar with various agents leading to these kinds of infections (e.g. rhino-, influenza-, parainfluenza, human metapneumo-, respiratory syncytial-, adeno- and coronaviruses), the cause of a substantial portion, (48-70%) of cases remain unidentified (Van der Hoek et al, 2004; Fouchier et al, 2004; File, 2003; Fine et al, 1999; Shay et al, 1999, Henrickson et al 2004; Murray et al 2001). In the past, human coronaviruses have not been known to cause severe disease in humans. For this reason, little research was performed on these viruses, with research focusing on the animal coronaviruses that are of veterinary importance. However, with the outbreak of SARS in 2003, the field of human coronavirus research has received significantly more attention. Also, the subsequent identification of two additional novel human coronaviruses (NL63 and HKU1) has led to an increased awareness of the potential threat of these viruses. With the discovery of these new human coronaviruses, it has become clear that the potential for another outbreak by a yet unknown human coronavirus is a very real possibility. This has made research into the pathogenesis and the role of the various coronavirus genes in the pathogenesis of these viruses of utmost importance. HCoV-NL63 was first discovered in January 2003 in the Netherlands. It causes upper and lower respiratory tract disease in young children, the elderly and immunocompromised individuals. The disease is also associated with croup and has even been implicated as a possible cause of the childhood vascular ailment Kawasaki Disease. HCoV-NL63 is frequently found in combination with other respiratory viruses leading to superinfections. It is still unclear whether HCoV-NL63 is an opportunistic virus or whether it leads the way for co-infection with other respiratory viruses. This particular virus is also the only coronavirus sharing the same cellular receptor as SARS-CoV. The virus is found all over the world and has been identified in countries like Australia (Arden et al, 2005), Japan (Ebihara et al., 2005; Suzuki et al., 2005), Belgium (Moës et al., 2005), Hong Kong (Chiu et al., 2005), Taiwan (Wu et al.,2007) Korea (Choi et al., 2006), Canada (Bastien et al., 2005), France (Vabret etal., 2005), Switzerland (Kaiser et al., 2005; Garbino et al., 2006), Germany (Vander Hoek et al., 2005), Sweden (Koetz et al., 2006) and South Africa (Smuts andHardie, 2006). In this study, the HCoV-NL63 genome was transcribed from RNA to DNA from which the M gene was amplified with various primers designed for use in specific expression systems. The various genes were cloned into the pGEM vector and confirmed by sequencing. The genes were now expressed in cloning vectors suited for each expression system (pFastBac for baculovirus expression, pFlexi for bacterial expression and pCMV for mammalian expression). Clones were sequenced for a second time. The recombinant clone in pFlexi was expressed in KRX cells and a 36hr time course was performed. The recombinant pFastBac clone was used to infect Sf9 insect cells and P1 and P2 viral stocks were obtained. The recombinant pCMV clone was used to transfect Cos1 mammalian cells. The genome was successfully transcribed and the M gene amplified and cloned into pGEM and confirmed by sequencing. Subsequent cloning of the various M genes into pFastBac for baculovirus expression, pFlexi for bacterial expression and pCMV for mammalian expression was achieved and sequencing confirmed the presence of the inserts in frame. pFlexi clones were successfully expressed in bacterial KRX cells with expression of the M protein in the pellet of the lysed bacterial cells. No M protein was seen in the supernatant of the lysed cells. Sf9 insect cells were infected with the recombinant pFastBac clones and P1 and P2 viral stocks were obtained. Protein expression occurs in KRX bacterial cells with optimal expression at approximately 24 hours. The M protein expresses on the cell membrane as can be concluded from the product obtained in the pellet of the lysed bacterial cells. Very little of the expressed protein is present in the plasma of the cell as evidenced by the absence of protein in the supernatant of the lysate.