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Abbreviations used
LRP

lipoprotein-related protein

NOS

nitric oxide synthase

Neurons exposed to the HIV-1 protein tat develop long-lasting changes in the calcium signals produced by NMDA receptors (NMDAR). NMDA receptors are glutamate and glycine-gated receptors with high calcium permeability. NMDAR-mediated calcium influx is essential to the formation, maturation, and plasticity of central excitatory synapses, and thus to learning and memory (Paoletti et al. 2013). However, NMDAR calcium can also initiate apoptotic cascades, and thus it mediates the excitotoxic properties of glutamate. NMDA receptors are subject to complex regulation by extracellular ligands but are also targets to intracellular signaling cascades. The complex interplay of signals and modulatory mechanisms that converge on NMDAR-calcium and their cellular effects are incompletely understood.

Many of the effects of tat on synaptic density, dendritic morphology, and neuronal death appear to be mediated by NMDARs, however, the complex interactions between tat and NMDA receptors are only beginning to be unraveled. In this issue of Journal of Neurochemistry, Krogh et al. expand our observation window into the effects of tat on neuronal NDMAR-calcium fluxes. They demonstrate that in addition to an acute potentiation of NMDAR-evoked calcium transport which can be observed with 10- or 40-min tat exposure and persists for at least 30 min (Haughey et al. 2001), with longer exposures (24–48 h), neuronal NMDAR calcium responses adapt such that they return to basal levels with 24 h, and continues to drop, falling below basal levels with 48 h exposure (Krogh et al. 2014). Importantly, this adaptation requires that the neuron internalizes tat, and requires the nNOs/sGC/PGK pathway. This novel insight into the dynamic and long-lasting effects of tat on neuronal NMDAR-mediated calcium uptake adds valuable temporal and molecular detail into how neuronal physiology changes upon tat exposure. With this additional knowledge, we can recognize new avenues for deeper insight and also possible targets for therapeutic interventions to alleviate the burden of neurologic disorders in HIV patients (Fig. 1).

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Figure 1. Biphasic effects of tat on neuronal NMDA receptor-mediated calcium In cultured neurons, the NMDA-elicited increase in intracellular calcium surges within the first 8 h of tat treatment but declines with longer exposures returning to basal levels within 24 h. sGC = soluble guanilate cyclase, Tyr = tyrosine, Src kinase, PGK promoter, Gly = glycine.

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A large contingent of HIV-infected patients present neurological symptoms which range in severity from mild impairment to incapacitating loss of cognitive functions and motor deficits. In parallel, brain tissue develops physical changes that range from synaptic loss and dendritic retraction to overt loss of select neuronal populations (Nath and Geiger 1998). Despite evident neurodegeneration, the extent of cognitive decline correlates more closely with dendritic damage and synapse loss, rather than neuronal death, raising the hope that substantial brain function can be recovered even after the onset of neurodegeneration (Ellis et al. 2007).

HIV penetrates the CNS readily but it infects exclusively non-neuronal cells (Watkins et al. 1990), thus its noxious effects on neurons are most likely indirect, through the release of soluble factors (Nath and Geiger 1998). The virally encoded HIV trans-activator protein tat is a potent neurotoxin: when applied to neurons in culture it causes loss of excitatory contacts (Kim et al. 2008; Shin and Thayer 2013) and cell death (Haughey et al. 2001; Eugenin et al. 2007). Tat is a virally encoded regulatory protein that is released from infected cells (Ensoli et al. 1990) and can reach concentrations of 2–40 ng/mL in serum of patients (Xiao et al. 2000). Binding to heparan sulfates further concentrates the toxin on cellular matrices where it binds specifically to the lipoprotein-related protein (LRP) and is efficiently internalized (Liu et al. 2000). Tat neurotoxicity requires LRP-mediated uptake and NMDAR-mediated calcium influx (Haughey et al. 2001; Eugenin et al. 2007).

Acute application of tat onto membrane patches can instantaneously increase NMDAR currents by chelating extracellular zinc, which is a potent NMDAR inhibitor (Song et al. 2003; Chandra et al. 2005). Tat neurotoxicity requires NMDAR activity but also tat internalization by LRP. Exposing cultured neurons to tat (100 nM) for 5 min or 40 min alters them such that long after treatment (> 30 min) they respond to glutamate or NMDA with a calcium influx that is much enlarged relative to the basal level of untreated neurons (Haughey et al. 2001). NMDAR-calcium potentiation corresponds to increased phosphorylation of the GluN2A and GluN2B NMDAR subunits, and requires PKC and threonine kinase activity. In the current report, Krogh and her colleagues replicate this result and identify the tyrosine kinase involved as Src kinase (Krogh et al. 2014). They also investigate how the duration of neuronal exposure to tat affects the NMDAR-calcium potentiation. They observed that when exposed for 2 h or longer to tat (50 ng/mL) neurons responded to NMDA applications with larger and larger intracellular calcium increases. Furthermore, they discovered that the NMDA calcium continued to escalate until it reached almost twice the basal level with 8 h of tat exposure but it began to decline when neurons were exposed to tat longer, such that the NMDAR-mediated calcium influx returned to basal levels with 24-h tat treatment, and fell below basal levels with 48-h treatment. The adaptation required continued LRP activity, and the activity of nitric oxide synthase (NOS), soluble guanilate cyclase (sGC), and c-GMP kinase (PKG). Both NMDAR and NOS activities are required for tat-induced cell death (Eugenin et al. 2007); however, it remains unclear whether these tat-responses, cell death, and adaptation, intersect.

Two additional observations by Krogh and colleagues are notable. First, they argue that the potentiation pathway continues to remain active during adaptation, since LRP or Src inhibitors when applied to fully adapted neurons (> 24-h tat treated) continued to reduce NMDA-calcium below the adapted levels. Second, they argue that the adaptation pathways are tat specific. Although neurons exposed to the pro-inflammatory interleukin IL-1b also produced a marked potentiation of the NMDA-calcium response, this effect persisted even for cells exposed to IL-1b for 24 h and the subsequent adaptation was insensitive to guanilate cyclase inhibitors. Clearly, tat initiates cellular responses that are distinct from other pathways that target NMDAR calcium.

The impact of these new results rests with the temporal and molecular detail they provide but also with the new questions that come to the fore. It will be important to understand what role does the adaptation described in this work play in neuronal physiology? Is it protective? Is it true that neurons that manage to adapt their NMDAR calcium response in this way survive, whereas those that do not succumb to the initial flood produced by NMDAR acute potentiation? Some evidence is consistent with such scenario. Neurons begin to die within several minutes of tat exposure but this degeneration appears to level off within 24 h, with ~80% of neurons becoming apoptotic (Eugenin et al. 2007). Since the attenuation measurements at or after 24 h were done on live neurons, it is important to ask whether preventing adaptation would accelerate cell death, and whether activating the adaptation, through the NO/GMP pathway may save neurons from tat-induced apoptosis. Conversely, the attenuated NMDAR calcium may represent the signal for synaptic pruning and dendritic retraction (Kim et al. 2008). Thus, whether the adaptation of the NMDAR calcium is beneficial or pathological remains to be determined.

Equally important will be to understand the mechanism of NMDAR-calcium adaptation. Is indeed PKG acting directly to phosphorylate NMDAR residues, and if so which ones? This appears to be the case for the NMDAR homologue GluA1, following cocaine administration (Seo et al. 2013). However, whether NMDAR subunits are PKG substrates is unknown. Alternatively, PKG may be acting through an intermediary, a phosphatase or a NMDAR-binding protein. Regardless of the molecules and residues involved, still unknown and important is whether the change in NMDAR calcium occurs through a global downturn of NMDAR current, through allosteric modulation of gating or block, or whether it specifically modulates NMDAR calcium permeability as does Protein Kinase A (PKA). This latter possibility is particularly intriguing because PKA appears to selectively increase NMDAR calcium, with no effect on total current (Skeberdis et al. 2006), and also because precedent exists for opposing regulation by PKA and PKG on L-type calcium channel properties (Vandael et al. 2013). Answers to these questions will add clarity to strategies to enhance or prevent adaptation, as the desired outcome may be.

Acknowledgments and conflict of interest disclosure

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  2. Acknowledgments and conflict of interest disclosure
  3. References

This work was funded by NIH (grant number RO1052669). The authors have no conflict of interest to declare.

References

  1. Top of page
  2. Acknowledgments and conflict of interest disclosure
  3. References
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