Avian sarcoma and leukosis virus-receptor interactions: from classical genetics to novel insights into virus-cell membrane fusion

For over 40 years, avian sarcoma and leukosis virus (ASLV)-receptor interactions have been employed as a useful model system to study the mechanism of retroviral entry into cells. Pioneering studies on this system focused upon the genetic basis of the differential susceptibilities of different lines of chickens to infection by distinct subgroups of ASLV. These studies led to the definition of three distinct autosomal recessive genes that were predicted to encode cellular receptors for different viral subgroups. They also led to the concept of viral interference, i.e. the mechanism by which infection by one virus can render cells resistant to reinfection by other viruses that use the same cellular receptor. Here, we review the contributions that analyses of the ASLV-receptor system have made in unraveling the mechanisms of retroviral entry into cells and focus on key findings such as identification and characterization of the ASLV receptor genes and the subsequent elucidation of an unprecedented mechanism of virus-cell fusion. Since many of the initial findings on this system were published in the early volumes of Virology, this subject is especially well suited to this special anniversary issue of the journal.

Close linkage of genes encoding receptors for subgroups A and C of avian sarcoma/leucosis virus on chicken chromosome 28

Avian sarcoma and leucosis viruses (ASLV) are classified into six major subgroups (A to E and J) according to the properties of the viral envelope proteins and the usage of cellular receptors for virus entry. Subgroup A and B receptors are identified molecularly and their genomic positions TVA and TVB are mapped. The subgroup C receptor is unknown, its genomic locus TVC is reported to be genetically linked to TVA, which resides on chicken chromosome 28. In this study, we used two chicken inbred lines that carry different alleles coding for resistance (TVC(R) and sensitivity (TVC(S)) to infection by subgroup C viruses. A backross population of these lines was tested for susceptibility to subgroup C infection and genotyped for markers from chicken chromosome 28. We confirmed the close linkage between TVA and TVC loci. Further, we have described the position of TVC on chromosome 28 relative to markers from the consensus map of the chicken genome.

CpG island protects Rous sarcoma virus-derived vectors integrated into nonpermissive cells from DNA methylation and transcriptional suppression

CpG islands are important in the protection of adjacent housekeeping genes from de novo DNA methylation and for keeping them in a transcriptionally active state. However, little is known about their capacity to protect heterologous genes and assure position-independent transcription of adjacent transgenes or retroviral vectors. To tackle this question, we have used the mouse aprt CpG island to flank a Rous sarcoma virus (RSV)-derived reporter vector and followed the transcriptional activity of integrated vectors. RSV is an avian retrovirus which does not replicate in mammalian cells because of several blocks at all levels of the replication cycle. Here we show that our RSV-derived reporter proviruses linked to the mouse aprt gene CpG island remain undermethylated and keep their transcriptional activity after stable transfection into both avian and nonpermissive mammalian cells. This effect is most likely caused by the protection from de novo methylation provided by the CpG island and not by enhancement of the promoter strength. Our results are consistent with previous finding of CpG islands in proximity to active but not inactive proviruses and support further investigation of the protection of the gene transfer vectors from DNA methylation.

Retroviruses in foreign species and the problem of provirus silencing

Retroviruses are known to integrate in the host cell genome as proviruses, and therefore they are prone to cell-mediated control at the transcriptional and posttranscriptional levels. This plays an important role especially after retrovirus heterotransmission to foreign species, but also to differentiated cells. In addition to host cell-mediated blocks in provirus expression, also so far undefined host specificities, deciding upon the pathogenic manifestation of retrovirus heterotransmission, are in play. In this respect, we discuss especially the occurrence of wasting disease and immunodeficiency syndrome, which we established also in avian species using avian leukosis virus subgroup C (ALV-C) inoculated in mid-embryogenesis in duck or chicken embryos. The problem of provirus downregulation in foreign species or in differentiated cells has been in the recent years approached experimentally. From a series of observations it became apparent that provirus downregulation is mediated by its methylation, especially in the region of proviral enhancer-promoter located in long terminal repeats (LTR). Several strategies have been devised in order to protect the provirus from methylation using LTR modification and/or introducing in the LTR sequence motifs acting as antimethylation tags. In such a way the expression of retroviruses and vectors in foreign species, as well as in differentiated cells, has been significantly improved. The complexity of the mechanisms involved in provirus downregulation and further possibilities to modulate it are discussed.

Delta-aminolevulinate increases heme saturation and yield of human cystathionine beta-synthase expressed in Escherichia coli

We recently expressed human cystathionine beta-synthase (CBS) in Escherichia coli and purified it to homogeneity. We showed that CBS requires heme in addition to pyridoxal 5'-phosphate for its function. Previously, CBS, only about 20% saturated with heme, was purified from transformed bacteria. In the present study, we supplemented the bacteria with 0.3 mM delta-aminolevulinate (delta ALA), a precursor of heme. While growth of the bacteria did not change, a 50-fold elevation of the heme content per milligram of total protein was observed in the cell extracts of delta ALA-supplemented cells. The increase in heme biosynthesis depended on the overexpression of a heme acceptor--CBS. Our data suggest that bacterial heme synthesis is regulated beyond delta ALA synthase. The delta ALA treatment resulted in 8 times more total CBS activity with a 3.5-fold higher yield of the purified recombinant enzyme, more than 68% saturated with heme. Increased yield, higher specific activity, and improved heme saturation of CBS will facilitate large-scale preparation of the enzyme. This method should be applicable to the overexpression of other recombinant heme proteins in bacteria.