The main focus in this department is to elucidate the molecular mechanisms of viral diseases including human immunodeficiency virus (HIV) infection. The following projects are currently underway.
(1) Anti-retroviral factors
HIV does not establish a productive infection in any other monkey except for the chimpanzee; this is thought to be due to inhibitors in simian lymphocytes that act at the early stage (reverse transcription) of viral infection. To date, TRIM5α and cyclophilin A have been identified as such restriction factors. We had shown that differences in the amino acid sequences in the C-terminal domain of TRIM5α of different monkey species affect the species-specific restriction of retrovirus infection (Fig.1, left). We also found that sequence variations in the N-terminal half of the viral capsid protein (Fig. 1, right) determine viral sensitivity to TRIM5α-mediated restriction, which indicates that there is an interaction between TRIM5α and the virus capsid. In addition, we showed that HIV-2 replication levels in infected individuals are associated with capsid variations, and we suggested that viral sequence analysis can predict AIDS progression. Furthermore, we have succeeded in improving the simian-tropic HIV-1 virus and the methods of monkey genome analysis. These new developments greatly facilitate the generation of an HIV-1 animal model, which would be a highly useful tool in research aiming to understand AIDS pathogenesis and to develop an effective vaccine. We are also seeking to identify the binding surface between the viral capsid protein and TRIM5α, as this may be useful for the development of new anti-retroviral drugs.
 |
||
| Structure of C-terminal SPRY domain of TRIM5α.The amino acids that are important for viral restriction are located in the surface of SPRY domain. V1 and V2 denote the regions that vary between the different monkey species. | The 3D structure model of viral capsid protein. A single amino acid change from P to A or Q radically affected the cofiguration of the loop. | |
| Fig. 1 Structural models of TRIM5α (left) and the viral capsid (right). | ||
(2) Host factors that participate in HIV pathogenesis and anti-retroviral drug side-effects.
As an animal model of AIDS has not yet been established, we are utilizing epidemiological procedures to understand the mechanisms of AIDS pathogenesis. There are cases who are not infected despite repeated exposure to HIV. There are also HIV-positive patients who do not develop symptoms of AIDS despite not receiving any anti-retroviral treatment. These cases are suspected to bear a resistance-inducing factor (RIF) against HIV. To characterize these RIFs, we have compared the genome sequences of the cases described above to those of HIV-infected patients and uninfected individuals. We found (a) the deletion mutant CCR5-893 (-), which fails to produce a co-receptor that is needed for HIV entry, (b) a polymorphism in the promoter of the chemokine RANTES, and (c) a polymorphism in the promoter of IL4, which regulates the expression of the co-receptor. We then demonstrated that these mutations affect susceptibility to HIV infection and the rate with which the disease progresses to AIDS.
At present, in collaboration with Thai groups, we are also focusing on the relationship between human genomic variation and anti-retroviral therapy side-effects, with the aim of establishing “tailor-made therapies” that will improve the quality of life of HIV-infected patients.
(3) Molecular mechanisms of HIV particle formation.
The HIV genomic RNA always forms dimers in the mature virion. It was suggested previously that the presence of the dimerized genome in the virion is advantageous for survival, as it provides an extra template that can be used when one RNA molecule is damaged; it may also endow the progeny with genetic variety. We were able to identify the minimal HIV genome region that is suffcient for genome dimerization. Our data suggest that RNA dimerization is part of RNA packaging. We also found that HIV genome dimerization affects the early stage of HIV replication after its entry into cells.
Fig. 2. HIV-1 genome RNA dimeriation.