Soluble Receptor-Induced Retroviral Infection of Receptor-Deficient Cells

Abstract

Current models of retroviral entry hypothesize that interactions between the host cell receptor(s) and viral envelope protein induce structural changes in the envelope protein that convert it to an active conformation, allowing it to mediate fusion with the membrane. Recent evidence supporting this hypothesis is the demonstration that Tva, the receptor for subgroup A avian sarcoma and leukosis virus (ASLV- Retroviral entry is determined by interactions between the host cell receptor and the envelope glycoproteins that spike the surface of the virus. Viral receptors play critical roles in entry and thus are important determinants of both host range and tissue tropism. The entry process is initiated by binding of the viral envelope protein to the receptor on the cell surface, thereby attaching the virus to the host cell. For retroviruses that enter cells at neutral pH such as subgroup A avian sarcoma and leukosis virus (ASLV-A) and the human and simian immunodeficiency viruses (HIV and SIV, respectively), it has been demonstrated that receptor binding triggers structural alterations in the viral glycoprotein. It is postulated that such conformational changes enable the envelope protein to catalyze the fusion of viral and host membranes necessary for delivery of the viral genome into the cell cytoplasm. Thus, retroviral receptors are believed to participate in both attachment of the virus to the cell and regulation of fusogenic properties of the viral envelope proteins.AStudies of both ASLV-A and its receptor, Tva, have provided evidence supporting this model of receptor-triggered activation during retroviral entry. In vitro analysis indicates that soluble receptor binding can induce temperature-dependent changes in the viral envelope protein EnvA (14,30,34). These changes include exposure of the hydrophobic fusion peptide located within the membrane-associated TM subunit of EnvA as well as changes in the receptor-binding SU subunit (30). In addition, binding of purified, soluble receptor converts a soluble form of EnvA into its membrane-binding form (14,34). Conversion to a membrane-binding state appears to involve cooperative interactions within the oligomeric envelope protein that require binding of multiple receptor molecules (14). Similar molecular and biophysical changes have been detected in the pH-dependent envelope protein of influenza virus, hemagglutinin, under low-pH conditions known to convert this protein into one with a fusion-active conformation (19,43). Thus, it has been postulated that Tva binding triggers the conversion of EnvA to a protein in a fusogenic state, which would mediate the membrane fusion-required viral entry.To further substantiate the ability of receptor to activate the fusogenic potential of EnvA and bypass the requirements of a membrane-associated receptor, we evaluated the capacity of soluble Tva (sTva) to trigger ASLV-A infection of receptordeficient cells. Here we report that Tva can act in solution to mediate infection of mammalian cells devoid of endogenous receptor, suggest...

Expression and Characterization of a Soluble, Active Form of the Jaagsiekte Sheep Retrovirus Receptor, Hyal2

“…sHyal2 does not stimulate JSRV vector transduction of receptor-deficient cells. Others have shown that soluble receptor protein can induce infection of receptor-deficient cells by avian retroviral vectors (13,30), and we tested whether sHyal2 could induce JSRV vector infection of receptor-deficient NIH 3T3 cells in a similar manner. A JSRV-pseudotype vector was incubated with or without sHyal2 at concentrations from 2 to 1,000 nM for 30 min on ice and cells were exposed to the vector in the presence of Polybrene with or without centrifugation as described previously (13).…”

“…Others have shown that soluble receptor protein can induce infection of receptor-deficient cells by avian retroviral vectors (13,30), and we tested whether sHyal2 could induce JSRV vector infection of receptor-deficient NIH 3T3 cells in a similar manner. A JSRV-pseudotype vector was incubated with or without sHyal2 at concentrations from 2 to 1,000 nM for 30 min on ice and cells were exposed to the vector in the presence of Polybrene with or without centrifugation as described previously (13). While the titer of the untreated virus on NIH 3T3 cells expressing human Hyal2 was 7 ϫ10 4 per ml, the titer of treated or untreated vector on receptor-deficient NIH 3T3 cells was Յ50 per ml, indicating that sHyal2 cannot efficiently induce infection of receptor-deficient cells.…”

Premature Activation of the Paramyxovirus Fusion Protein before Target Cell Attachment with Corruption of the Viral Fusion Machinery

“…To elucidate the mechanism by which fusion is inactivated, we drew upon previous work that examined conformational changes of the fusion proteins of influenza (52), Sendai virus (61), and avian sarcoma/leukosis virus (ASLV) (62,63). We developed a biochemical assay to detect a conformational change in F upon activation, observing that the proteaseresistant, native conformation of HPIV3 F transitioned to a protease-sensitive state after activation.…”

Mammalian Models Based on RCAS-TVA Technique

“…Many TVA receptor transgenic models have been generated in mammalian system to accept RCAS vectors, simultaneously new methods are developed. Several studies have indicated that the membrane TVA receptor is not an absolute requirement for virus infection, and RCAS vectors linked with a soluble TVA can be delivered into receptor-deficient cells (Snitkovsky & Young, 1998;Damico & Bates, 2000;Contreras-Alcantara et al, 2006). The viral receptor function of TVA is determined by a 40-residue, cysteine-rich motif called the LDL-A module, which is highly homologous to the human low-density lipoprotein receptor (LDLR) ligand-binding repeats (LDL-A modules).…”