HIV-1 Nef: a multifaceted modulator of T cell receptor signaling

Abstract

Nef, an accessory protein of the Human Immunodeficiency Virus type 1 (HIV-1), is dispensable for viral replication in cell culture, but promotes virus replication and pathogenesis in the infected host. Acting as protein-interaction adaptor, HIV-1 Nef modulates numerous target cell activities including cell surface receptor expression, cytoskeletal remodeling, vesicular transport, and signal transduction. In infected T-lymphocytes, altering T-cell antigen receptor (TCR) signaling has long been recognized as one key function of the viral protein. However, reported effects of Nef range from inhibition to activation of this cascade. Recent advances in the field begin to explain these seemingly contradictory observations and suggest that Nef alters intracellular trafficking of TCR proximal machinery to disrupt plasma membrane bound TCR signaling while at the same time, the viral protein induces localized signal transduction at the trans-Golgi network. This review summarizes these new findings on how HIV-1 Nef reprograms TCR signalling output from a broad response to selective activation of the RAS-Erk pathway. We also discuss the implications of these alterations in the context of HIV-1 infection and in light of current concepts of TCR signal transduction.

Figure 1

Schematic overview of TCR signaling. Simplified scheme of TCR signal transduction and effector functions. Following TCR ligation with peptide-loaded MHC-class II molecules, F-actin is polymerized at plasma membrane sites of TCR engagement. ITAMs of TCR zeta chains are phosphorylated by the SRC family kinases Lck and Fyn to recruit the downstream kinase ZAP-70. ZAP-70 is activated by phosphorylation, primarily by Lck but also by Fyn, and active ZAP-70 phosphorylates the membrane adaptor LAT at multiple residues. Phospho-LAT serves as scaffold for macromolecular signaling assemblies, via multiple interactions including with PLCγ1, SLP-76 via GADS, and SOS via GRB2. The cytoplasmic adaptor SLP-76 binds to multiple proteins including VAV, NCK, ITK, PLCγ1 and ADAP to regulate signaling pathways necessary for F-actin rearrangement and cell adhesion. PLCγ1 catalyzes formation of IP3 and DAG from PIP2 (not shown). The soluble secondary messenger IP3 binds to IP3R to induce calcium release from intracellular stores and trigger NFAT activation. Active LAT also activates RAS in membrane microdomains via GRB2-SOS which triggers MAPK signaling, culminating in phosphorylation and activation of Erk. Nuclear translocation of activated NFAT/Erk induces gene transcription and IL2 production, hallmarked by T-cell proliferation and differentiation. GRB2: Growth factor receptor-bound protein 2, SOS: son of sevenless, Erk: extracellular-signal-regulated kinase, AP1: Activator protein-1, IP3: Inositol 1,4,5-trisphosphate, CDC42: cell division cycle 42, WAVE2: WASP-family verprolin-homologous protein-2, WASP: Wiskott Aldrich syndrome protein, PAK: p21 activated protein kinase, PM: plasma membrane, NM: nuclear membrane. ‘+p’ and yellow circles with ‘P’ indicate phosphorylation events and phosphorylation, respectively.

Figure 2

Model of the effects of HIV-1 Nef on TCR signaling. Schematic representation of TCR signaling in presence of HIV-1 Nef. Expression of Nef down regulates cell surface exposure of the CD4 receptor. In addition, Nef strongly reduces the availability of Lck, LAT and active RAS at the plasma membrane as these components are retargeted to an intracellular compartment in their active form. This compartment represents the TGN for Lck and RAS and it is assumed here that LAT is recruited to the same compartment. Nef also localizes to these membranes; however, whether its physical presence is required for TCR rewiring remains unclear. Upon TCR stimulation, triggered F-actin remodeling is inhibited by Nef by virtue of its interaction with PAK2 (not shown). The SRC kinase Fyn, whose plasma membrane localization is unaffected by Nef, phosphorylates TCR zeta and ZAP-70, however ZAP-70 mediated phosphorylation of LAT and subsequent SLP-76 microcluster formation is potently disrupted by Nef. Presumably again through the activity of Fyn (indicated by the dotted arrow and question mark), TCR engagement also stimulates Lck-RAS at intracellular membranes, generating MAPK signaling resulting in the activation of Erk. In addition, Nef binds to IP3 receptor to induce calcium release and NFAT activation. By inducing a constitutively active intracellular Lck-RAS signaling module that is partially uncoupled from the plasma membrane, Nef tailors a narrow TCR downstream response that likely optimizes HIV-1 spread in the infected host.