Construction of a gammaretrovirus with a novel tropism and wild-type replication kinetics capable of using human APJ as entry receptor

Unique Structure and Distinctive Properties of the Ancient and Ubiquitous Gamma-Type Envelope Glycoprotein

After the onset of the AIDS pandemic, HIV-1 (genus Lentivirus) became the predominant model for studying retrovirus Env glycoproteins and their role in entry. However, HIV Env is an inadequate model for understanding entry of viruses in the Alpharetrovirus, Gammaretrovirus and Deltaretrovirus genera. For example, oncogenic model system viruses such as Rous sarcoma virus (RSV, Alpharetrovirus), murine leukemia virus (MLV, Gammaretrovirus) and human T-cell leukemia viruses (HTLV-I and HTLV-II, Deltaretrovirus) encode Envs that are structurally and functionally distinct from HIV Env. We refer to these as Gamma-type Envs. Gamma-type Envs are probably the most widespread retroviral Envs in nature. They are found in exogenous and endogenous retroviruses representing a broad spectrum of vertebrate hosts including amphibians, birds, reptiles, mammals and fish. In endogenous form, gamma-type Envs have been evolutionarily coopted numerous times, most notably as placental syncytins (e.g., human SYNC1 and SYNC2). Remarkably, gamma-type Envs are also found outside of the Retroviridae. Gp2 proteins of filoviruses (e.g., Ebolavirus) and snake arenaviruses in the genus Reptarenavirus are gamma-type Env homologs, products of ancient recombination events involving viruses of different Baltimore classes. Distinctive hallmarks of gamma-type Envs include a labile disulfide bond linking the surface and transmembrane subunits, a multi-stage attachment and fusion mechanism, a highly conserved (but poorly understood) "immunosuppressive domain", and activation by the viral protease during virion maturation. Here, we synthesize work from diverse retrovirus model systems to illustrate these distinctive properties and to highlight avenues for further exploration of gamma-type Env structure and function.

Directed Molecular Evolution of an Engineered Gammaretroviral Envelope Protein with Dual Receptor Use Shows Stable Maintenance of Both Receptor Specificities

We have previously reported the construction of a murine leukemia virus-based replication-competent gammaretrovirus (SL3-AP) capable of utilizing the human G protein-coupled receptor APJ (hAPJ) as its entry receptor and its natural receptor, the murine Xpr1 receptor, with equal affinities. The apelin receptor has previously been shown to function as a coreceptor for HIV-1, and thus, adaptation of the viral vector to this receptor is of significant interest. Here, we report the molecular evolution of the SL3-AP envelope protein when the virus is cultured in cells harboring either the Xpr1 or the hAPJ receptor. Interestingly, the dual receptor affinity is maintained even after 10 passages in these cells. At the same time, the chimeric viral envelope protein evolves in a distinct pattern in the apelin cassette when passaged on D17 cells expressing hAPJ in three separate molecular evolution studies. This pattern reflects selection for reduced ligand-receptor interaction and is compatible with a model in which SL3-AP has evolved not to activate hAPJ receptor internalization.

IMPORTANCE

Few successful examples of engineered retargeting of a retroviral vector exist. The engineered SL3-AP envelope is capable of utilizing either the murine Xpr1 or the human APJ receptor for entry. In addition, SL3-AP is the first example of an engineered retrovirus retaining its dual tropism after several rounds of passaging on cells expressing only one of its receptors. We demonstrate that the virus evolves toward reduced ligand-receptor affinity, which sheds new light on virus adaptation. We provide indirect evidence that such reduced affinity leads to reduced receptor internalization and propose a novel model in which too rapid receptor internalization may decrease virus entry.

Library screening and receptor-directed targeting of gammaretroviral vectors

Gene- and cell-based therapies hold great potential for the advancement of the personalized medicine movement. Gene therapy vectors have made dramatic leaps forward since their inception. Retroviral-based vectors were the first to gain clinical attention and still offer the best hope for the long-term correction of many disorders. The fear of nonspecific transduction makes targeting a necessary feature for most clinical applications. However, this remains a difficult feature to optimize, with specificity often coming at the expense of efficiency. The aim of this article is to discuss the various methods employed to retarget retroviral entry. Our focus will lie on the modification of gammaretroviral envelope proteins with an in-depth discussion of the creation and screening of envelope libraries.