The Human Immunodeficiency Virus Coat Protein gp120 Promotes Forward Trafficking and Surface Clustering of NMDA Receptors in Membrane Microdomains

Abstract

Infection by the Human immunodeficiency virus (HIV) can result in debilitating neurological syndromes collectively known as HIV associated neurocognitive disorders (HAND). While the HIV coat protein gp120 has been identified as a potent neurotoxin that enhances NMDA receptor function, the exact mechanisms for effect are not known. Here we provide evidence that gp120 activates two separate signaling pathways that converge to enhance NMDA-evoked calcium flux by clustering NMDA receptors in modified membrane microdomains. HIV gp120 enlarged, and stabilized the structure of lipid rafts on neuronal dendrites by mechanisms that involved a redox-regulated translocation of a sphingomyelin hydrolase (neutral sphingomyelinase-2; nSMase2) to the plasma membrane. A concurrent pathway was activated that enhanced the forward traffic of NMDA receptors by promoting a PKA-dependent phopshorylation of the NR1 C-terminal serine 897 (that masks an ER retention signal), followed by a PKC-dependent phosphorylation of serine 896 (important for surface expression). NMDA receptors were preferentially targeted to synapses, and clustered in modified membrane microdomains. In these conditions, NMDA receptors were unable to laterally disperse, and did not internalize, even in response to strong agonist induction. Focal NMDA-evoked calcium bursts were enhanced three-fold in these regions. Inhibiting membrane modification or NR1 phosphorylation prevented gp120 from enhancing the surface localization and clustering of NMDA receptors, while disrupting the structure of membrane microdomains restored the ability of NMDA receptors to disperse and internalize following gp120. These findings demonstrate that gp120 contributes to synaptic dysfunction in the setting of HIV-infection by interfering with the traffic of NMDA receptors.

Dendritic Spine Injury Induced by the 8-Hydroxy Metabolite of Efavirenz

“…Hippocampal neuronal cultures were prepared from embryonic day 18 Sprague-Dawley rats by using methods that have been described previously (Wheeler et al, 2009;Xu et al, 2011). In brief, hippocampi were isolated and trypsinized, and cells were dissociated by tituration in a calcium-and magnesium-free Hanks' balanced salt solution.…”

HIV-1 gp120Bal down-Regulates Phosphorylated NMDA Receptor Subunit 1 in Cortical Neurons via Activation of Glutamate and Chemokine Receptors

“…Phosphorylation is a key mechanism that regulates the localization and channel function of NMDARs (Scott et al , 2003). Previous studies indicated that phosphorylation of NR1 at Ser897 and Ser896 contributes to HIV gp120 IIIB-induced NMDAR trafficking and surface expression in cultured hippocampal neurons (Xu et al , 2011). We wanted to determine in this study if gp120Bal acts similarly in the regulation of NR1 phosphorylation.…”

“…In this context, it is interesting to note that gp120 IIIB enhances the phosphorylation of the NR1 at both Ser896 and Ser897. It also promotes NMDAR trafficking and surface expression (Xu et al , 2011). We currently do not know how the different types of gp120 cause apparently opposite effects on NR1 phosphorylation.…”

“…Synapse loss also seems to occur in the gp120 transgenic mice (Toggas et al , 1994). In addition to synaptic degeneration, HIV-1 proteins may also interfere with synaptic activity (Haughey and Mattson, 2002; Kasyanov et al , 2006; Krogh et al , 2014; Nath et al , 1996; Tovar et al , 2013; Xu et al , 2011). For instance, gp120 was shown to activate N-methyl D-aspartate receptor (NMDAR), and this could induce synaptic damage (Gemignani et al , 2000; Kaul and Lipton, 1999; Lipton, 1992; Pattarini et al , 1998; Toggas et al , 1996; Viviani et al , 2006; Yang et al , 2013).…”

“…Interestingly, T-cell-tropic (T-tropic) gp120 IIIB promoted trafficking and surface clustering of NMDA receptors in membrane microdomains in cultured hippocampal neurons. The gp120 IIIB-regulated NMDAR trafficking was associated with the phosphorylation of the NMDAR subunit NR1 (also known as the GluN1 subunit) at its C-terminal serine 896 (Ser896) and serine 897 (Ser897) (Xu et al , 2011). It has been shown that phosphorylation at these residues are important posttranslational modifications for NMDAR to be transported from the Golgi complex and thus is a critical step in the process of NMDAR surface expression, which is required for it to become functional at synapses (Scott et al , 2003; Scott et al , 2001).…”