Restriction of Lentiviruses by SAMHD1
Final Report Abstract
The HIV1 restriction factor SAMHD1 blocks viral replication in non-dividing cells such as macrophages, dendritic cells and resting T cells by lowering the cellular dNTP pool thus starving HIV1 at reverse transcription. SAMHD1 has no antiviral activity in dividing cells such as activated T cells. The virion packaged viral accessory protein Vpx which is encoded by HIV2 and some SIVs, but not by HIV1, degrades SAMHD1 upon infection allowing for reverse transcription in the infected cell. During the funding period of this fellowship I investigated the regulation of SAMHD1 activity in dividing versus non-dividing cells; the Vpx-dependent and independent ubiquitination of SAMHD1; and I identified cellular proteins interacting with SAMHD1. By mass spectrometry analysis of dividing and differentiated THP1 cells I explored that residue T592 in SAMHD1 is predominantly phosphorylated in dividing cells. While I explored the consequences of this post-translational modification, three other groups reported the same finding and showed that phosphorylation at T592 results in the loss of SAMHD1 antiviral function. I could reproduce these results in my studies. Phosphorylation of SAMHD1 at T592 was observed in activated CD4+ T cells, but not in resting CD4+ T cells. Interestingly I found that Vpx barely increased HIV1 infection of resting T cells, even though analysis of the newly synthesized viral DNA in the resting T cells showed that Vpx provided a dramatic increase in the amount of viral DNA. This suggests a second block to infection of resting CD4+ T cells, which I could identify to be at the level of integration. I am currently expanding my studies to HIV2 because in contrast to HIV1 this virus encodes a Vpx gene and would face the second block to infection in resting T cells. Although HIV1 replication is already blocked at the level of reverse transcription, understanding the molecular mechanism of the block to integration might allow to generate new lentiviral transduction vectors for resting CD4+ T cell.
