**4.6. Cholesterol-25-hydroxylase**

interfering with one or more of the following processes: 1) HIV-1 uncoating; 2) nuclear import

Fricke and colleagues [169] suggested a model in which MxB binds to the HIV-1 core in the cytoplasm of the cell and prevents the uncoating process of HIV-1 through stabilization of incoming viral capsides. In addition, they demonstrated that MxB requires capsid binding and

More recently, Matreyek et al. [170] observed that MxB restricts HIV-1 after DNA synthesis at

HIV-1 RNA is reverse transcribed into double stranded linear DNA and carries a fraction of the viron CA protein [171, 172]. HIV-1 CA protein is known to play a central role in mediating physical interactions with several host proteins involved in the post-entry step of infection. Some identified residues of CA involved in binding to cyclophilin A (CypA), TRIM5α, TNP03, CPSF6, NUP153 and NUP358/RanBP2 are also critical for the sensitivity of HIV-1 to the antiviral action of MxB. Results obtained by Liu and colleagues indicate that both silencing of CypA expression or disruption of the CA-CypA interaction by addition of cyclosporine A abrogated the antiviral activity of MxB, thus CypA binding to the HIV-1 CA appears to be required for MXB restriction. Furthermore, results obtained by diverse groups indicate that

The viral integrase (IN) protein processes the long terminal repeat (LTR) ends of the viral DNA to yield the integration-competent PIC, which subsequently transports the viral DNA into the nucleus for IN-mediated integration [173]. Matreyek and collaborators [170] found evidence for an additional block in the formation of 2-LTR circular viral DNA (that are only present in the nucleus, and thus have been utilized as a marker of nuclear entry of viral DNA [174]). In contrast, results obtained by Liu and collaborators [167] showed that MxB reduces the levels of integrated HIV-1 DNA, though it does not affect the amount of 2-LTR circles. They con‐ cluded that MxB impairs the integration step and spares the nuclear entry of viral DNA.

Apparently, MxB antiviral activity is independent of its GTPase active site residues or stalk domain Loop4 (both previously shown to be necessary for MxA function) that confer func‐ tional oligomerization to related dynamin family proteins [166, 168]. There are two locations in MxB that exhibit the greatest sequence dissimilarity with MxA. The first one is Loop4 that is not critical for MxB antiviral activity but is important for the MxA inhibition of Influenza A and Thogotovirus infection [170, 175]. The other part of MxB with greatest dissimilarity to MxA is the N-terminal region. The specific particular functions conferred by this region are

In a global perspective, the post-entry step of HIV-1 replication cycle appears to be quite vulnerable to the actions of IFN-inducible restriction factors: TRIM5α, APOBEC3 proteins, SAMHD1 and, more recently, MxB use distinct mechanisms to prevent integration of this pathogenic virus in host genome. Certainly it will continue to be of interest to the scientific community the study of restriction factors of viral infection by antiviral host factors due to its impact in many areas. These findings raises hope as a potential clinical and epidemiological

particularly important for MxB activity and consequent HIV-1 restriction [170].

relevant approach which could be exploited to control HIV infections and AIDS.

of the HIV-1 PIC; or 3) nuclear maturation of the PIC.

86 Trends in Basic and Therapeutic Options in HIV Infection - Towards a Functional Cure

steps that are coincident with PIC nuclear import and integration.

CA mutations counteracted MxB restriction [165-168, 170].

oligomerization for effective restriction.

Recently, a new antiviral IFN-induced protein (cholesterol 25-hydroxilase; CH25H) was identified as being able to block the fusion between viral envelope and target cell membrane. It exhibits a broadly antiviral activity against several enveloped virus including HIV, Ebola virus (Zaire strain), vesicular stomatitis virus, herpes simplex virus I, Rift Valley fever virus, Nipah virus, Influenza A (H1N1) virus and varicella zoster virus [176, 177]. It also revealed antiviral effect against poliovirus [178], a non-enveloped virus. The IFN-induced cholester‐ ol-25-hydroxylase (*Ch25h*) gene encodes an endoplasmic-reticulum-associated enzyme (CH25H) that mediates the oxidation of cholesterol, by the addition of an extra hydroxyl group at position 25, converting it to 25-hydroxycholesterol (25HC). 25HC belongs to a large class of endogenous cholesterol derivatives named oxysterols. In addition to their involvement in basic metabolic processes, e.g. bile acids production in the liver [179], oxysterols also play a key role in several signaling pathways that influence the activation of macrophages, T-cells and B-cells, and thus the regulation of inflammatory response [177, 180-188].

Although several antiviral mechanisms have been suggested for CH25H and 25HC, they seem to inhibited HIV-1 replication by blocking the virus-cell fusion step [176]. One possible mechanism underlying this effect is the induction of cellular membrane changes affecting the topology and permissiveness for fusion of host cell membrane. There is extensive evidence that the lipid composition of target cell membrane influences HIV-1 fusion and entry. In fact, though the fusion event is triggered by HIV envelope glycoproteins, lipids also play a key role in virus-cell membrane fusion by themselves, directly affecting the viral receptor accessibility and distribution in lipid rafts domains of the plasma membrane, or the membrane fluidity and curvature [189]. The modifications in cellular membrane architecture induced by 25HC (considerably more hydrophilic than cholesterol [190]) would be of outstanding importance in the complex protein-lipid interplay required for successful virus-cell fusion events [176].
