Previous studies have shown that neurodegeneration could be attenuated by NMDA receptor antagonists [9
]. Since IL-1β activation of astrocytes contributes to neurodegeneration during HIV infection, we have examined the effects of ACM recovered from IL-1β-stimulated human astrocytes on NR1a/NR2B receptors expressed on Xenopus
oocytes. Our results showed that pressure ejection of the ACM(+) produced an inward current in NR1a/NR2B-expressing oocytes, but not in non-NR1a/NR2B-expressing oocytes. The ACM(+)-induced inward current was concentration-dependent and blocked by NMDA receptor antagonist APV, but not by AMPA receptor antagonist CNQX. These results suggest that the IL-1β-stimulated astrocytes release soluble NMDAR agonist-like substances, leading to activation of the NR1a/NR2B receptors expressed on Xenopus
oocytes and resultant inward current.
Astrocytes are essential for brain homeostasis and play crucial roles in neuronal function and survival. They are also cellular reservoirs and important participants in the pathogenesis of HAND/HAD [8
]. In the course of HIV-1 brain infection, astrocytes are among the first cells to respond directly to brain injury and participate in disease process largely through their capacity of releasing neurotoxic factors. These astrocyte-derived neurotoxic factors including, but are not limited to, chemokines such as CCL2, cytokines (especially TNF-α), quinolic acid, platelet activating factor, nitrogen oxide and glutamate. These are thought to be important mediators of HIV-1-induced neuronal damage [8
]. Nevertheless, the mechanisms underlying astrocyte/HIV-associated neuronal damage have not been completely elucidated. Previous studies have shown that the detrimental effects of various candidate astrocyte/HIV-associated neurotoxic factors can be blocked in vitro by NMDA receptor antagonists, indicating that activation of NMDA receptors is involved in HIV-1-induced neuronal injury [32
]. These neurotoxic factors may activate NMDA receptors by direct binding or by impairing glutamate uptake functions of astrocytes and thus increase extracellular concentrations of glutamate. Up-to-date, few studies have shown direct activation of NMDA receptors by astrocyte/HIV-associated neurotoxic factors. We found that pressure ejection of IL-1β-stimulated ACM produced an inward current in Xenopus
oocytes expressing NR1a/NR2B NMDA receptors, demonstrating a possible direct action on NMDA receptors by astrocyte/HIV-associated neurotoxic factors.
Numerous studies have shown that the levels of many cytokines increase dramatically following brain insult such as HIV-1 infection. These cytokines may directly interact with neuronal NMDA receptors, or stimulate astrocytes to release soluble factors acting indirectly on neuronal NMDA receptors and resulting in neuronal injury. Among the astrocyte-secreted factors is IL-1β, a cytokine which can recurrently stimulate astrocyte production of more soluble factors including IL-1β. Since IL-1β is a key mediator of inflammation and neuronal death in acute CNS injuries, such as stroke and brain trauma, it has been considered as HAD-relevant pro-inflammatory cytokine and implicated in neurodegenerative diseases such as HAD [35
]. We hypothesize that factors from IL-1β-stimulated astrocytes induces neuronal injury via NMDA receptors. This hypothesis is supported by the experimental results that IL-1β-stimulated ACM activate NMDA receptors expressed on Xenopus
oocytes and induced neuronal injury when added to rat primary human neuronal cultures [35
It is well-known that activation of an NMDA receptor opens a channel permeable to Ca2+
ions and a sustained increase of intracellular Ca2+
has been shown to cause neuronal injury in numerous pathophysiological processes [34
]. As HIV-1-associated neurodegeneration is a chronic disease progression process involving astroglial activation and resultant production of NMDA receptor agonist-like substances, it is most likely that the elevated levels of intracellular Ca2+
, triggered by astrocyte-associated NMDA receptor agonist-like substances, are at least in part involved in the pathogenesis of HIV disease. This postulation is supported by research findings showing that HIV-1-associated neuronal injury can be attenuated or blocked by NMDA receptor antagonists both in vitro and in vivo.
Astrocytes treated with IL-1β induced CCL2, TNF-α, TGF-β2 and MMP observed in the present study suggests that IL-1β induce potential soluble factors secretion in astrocytes. IL-1β-stimulated and/or virus-infected astrocytes release many factors including cytokines (especially TNF-α), quinolinic acid, platelet activating factor, nitrogen oxide glutamate and arachidonic acid [8
]. Upon release, these factors may act alone or in combination on NR1a/NR2B NMDA receptors, producing inward current. It is worth to point out that we did not dissect out the active factor(s) for the activation of NR1a/NR2B receptors observed in this study. The focus of the work, however, was to explore whether the IL-1β-stimulated ACM activates NR1a/NR2B receptors. Nevertheless, our results showed that IL-1β-stimulated ACM directly activate NMDA receptors expressed on Xenopus oocytes
, implicating a potential role for IL-1β-stimulated astrocytes in neuronal injury/dysfunction.
In summary, the present study demonstrated that IL-1β-stimulated astrocytes activate NMDA receptors in vitro. Although the active components remain to be determined, the activation of NR1/NR2B receptors by the IL-1β-stimulated ACM may have implications for the pathogenesis of neurodegenerative disorders including HAND/HAD.