Stochastic Gene Expression in a Lentiviral Positive-Feedback Loop: HIV-1 Tat Fluctuations Drive Phenotypic Diversity

Cell. 2005 Jul 29; 122(2): 169-182

Speaker: 鄭伯忻                                                    Time: 2006/5/24 15:00-16:00

Commentator: 王憲威老師                                   Place: room 601

Abstract

Tat is the early gene in HIV. After HIV infect cell, basal level of Tat binds to viral promoter LTR and initiates a positive feedback loop to amplify HIV transcription.  This will result in the death of host cell. Alternatively, HIV can lie in a latent state.  Cells with greater initial burst of Tat production can turn on positive feedback loop to result in infection, while HIV with lower level of Tat production can enter a latent state. In this paper, the authors hypothesized that this phenotypic variation is driven by random fluctuation of Tat production. To test this hypothesis, a lentiviral vector contained LTR-GFP-IRES-Tat (LGIT) was constructed and measuring the fluorescence to represent Tat production. In addition, a feedback loop-disrupted vector, LTR-GFP (LG) served as a control was also constructed. The results showed that LGIT-infected cell exhibited distinct fluorescence expression (Bright, Mid, Dim, and Off), while LG-infected cell only exhibited Off. In LGIT-infected condition, cells sorted from Bright region relaxed slowly to Off. Interestingly, cells sorted from Dim region transitioned to Bright or Off region after three weeks. This phenomenon was defined as a phenotypic bifurcation (PheB). Although these data indicate that low Tat expression generate PheB, authors could not find any correlation in genomic integration pattern or extrinsic cellular factors to generate PheB. A stochastic model was developed and experimental data also confirmed modeling data to describe Tat transactivation and PheB. Therefore, stochastic expression of Tat can play an important role to generate PheB, indicating that cellular determination of Bright or Off depends on stochastic expression of Tat.

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References

1.   Kaern, M., T. C. Elston, W. J. Blake, and J. J. Collins. 2005. Stochasticity in gene expression: from theories to phenotypes. Nat Rev Genet 6:451-64.

2.   Weinberger, L. S., J. C. Burnett, J. E. Toettcher, A. P. Arkin, and D. V. Schaffer. 2005. Stochastic gene expression in a lentiviral positive-feedback loop: HIV-1 Tat fluctuations drive phenotypic diversity. Cell 122:169-82.