The Nef-infectivity enigma: mechanisms of enhanced lentiviral infection

Abstract

The Nef protein is an essential factor for lentiviral pathogenesis in humans and other simians. Despite a multitude of functions attributed to this protein, the exact role of Nef in disease progression remains unclear. One of its most intriguing functions is the ability of Nef to enhance the infectivity of viral particles. In this review we will discuss current insights in the mechanism of this well-known, yet poorly understood Nef effect. We will elaborate on effects of Nef, on both virion biogenesis and the early stage of the cellular infection, that might be involved in infectivity enhancement. In addition, we provide an overview of different HIV-1 Nef domains important for optimal infectivity and briefly discuss some possible sources of the frequent discrepancies in the field. Hereby we aim to contribute to a better understanding of this highly conserved and therapeutically attractive Nef function.

Fig. (1)

Nef-mediated modifications of the virions during virion biogenesis (at the site of viral assembly). Proposed modifications of nascent virions as a consequence of Nef expression in the virus producing cells are represented. (A) Expression of Nef during viral production results in incorporation of small amounts of Nef into the budding virion. (B) Nef enhances the incorporation of Env glycoproteins into the budding virion, by (left) a CD4-dependent mechanism: Nef downregulates CD4 and prevents sequestration of Env by CD4; by (right) CD4- independent mechanisms: Nef downregulates a yet unknown Env sequestering factor, other than CD4, in a Dyn-2 dependent way, or Nef enhances membrane trafficking of Env by reducing the retention of Env precursors. (C) Nef enhances phophorylation of matrix (MA) proteins, mediated by Nef-associated kinases. (D) Nef regulates the incorporation of yet unknown cellular factors into the budding virion, by (left) binding to cellular factors, resulting in their co-incorporation with Nef in the virion; by (right) regulating surface expression of cellular factors at the site of viral assembly, by affecting their trafficking to the surface membrane or by downregulating them from the cell surface in a Dyn2-dependent way. (E) Nef enhances the incorporation of cholesterol into the budding virion in different ways: (1) alteration of cellular signaling pathways by Nef results in increased expression of the cholesterol biosynthesis genes and thereby increased synthesis of cellular cholesterol; (2) Nef binds to newly produced cholesterol and promotes its transport to the sites of viral assembly; (3) Nef impairs the efflux of cellular cholesterol by stimulating proteosomal degradation of the cholesterol transporter ABCA1.

Fig. (2)

Mechanisms of enhanced infection of the target cell by Nef-modified virions. Proposed steps of the viral infection process that are facilitated due to Nef-mediated modifications of the virion. (A) CD4-dependent virion modifications: higher levels of Env glycoproteins in the virions results in an increase of functional Env-CD4 receptor interactions and thereby increase of the likelihood of productive infection. (B) CD4-independent virion modifications: a yet controversial modification imprinted by Nef during virion production (1) facilitates the dilatation of the fusion pore and consequently the cytoplasmatic delivery of the virion core; (2) facilitates movement of the virion core through the cortical actin barrier; (3) facilitates activation of the reverse transcription complex; (4) protects the virion core against proteosomal degradation.