Zinc Inhibits Astrocyte Glutamate Uptake by Activation of Poly(ADP-ribose) Polymerase-1

Sang Won Suh,Conrad C Alano,Koji Aoyama,Aaron M Hamby,Raymond A Swanson,Christopher M Anderson

doi:10.2119/2007-00043.suh

Abstract

Several processes by which astrocytes protect neurons during ischemia are now well established. However, less is known about how neurons themselves may influence these processes. Neurons release zinc (Zn2+) from presynaptic terminals during ischemia, seizure, head trauma, and hypoglycemia, and modulate postsynaptic neuronal function. Peak extracellular zinc may reach concentrations as high as 400 microM. Excessive levels of free, ionic zinc can initiate DNA damage and the subsequent activation of poly(ADP-ribose) polymerase 1 (PARP-1), which in turn lead to NAD+ and ATP depletion when DNA damage is extensive. In this study, cultured cortical astrocytes were used to explore the effects of zinc on astrocyte glutamate uptake, an energy-dependent process that is critical for neuron survival. Astrocytes incubated with 100 or 400 microM of zinc for 30 min showed significant decreases in ATP levels and glutamate uptake capacity. These changes were prevented by the PARP inhibitors benzamide or DPQ (3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-isoquinolinone) or PARP-1 gene deletion (PARP-1 KO). These findings suggest that release of Zn2+ from neurons during brain insults could induce PARP-1 activation in astrocytes, leading to impaired glutamate uptake and exacerbation of neuronal injury.

Highlights

Astrocytes perform several functions that are essential for normal neuron activity, including glutamate uptake, K+ and H+ buffering, water transport, and metabolite exchange [1,2]
Astrocytes incubated with Zn2+ (100–400 μM) for 30 min exhibited delayed and dose-dependent reductions in glutamate uptake that became more pronounced with progressive time after zinc exposure and washout (Figure 1)
Zinc-induced glutamate uptake inhibition was attenuated in astrocytes prepared from poly(ADP-ribose) polymerase 1 (PARP-1)–/– mice, further suggesting that the effect of Zn2+ on glutamate uptake is mediated by PARP-1 activation (Figure 2C)

Summary

Introduction

Astrocytes perform several functions that are essential for normal neuron activity, including glutamate uptake, K+ and H+ buffering, water transport, and metabolite exchange [1,2]. These astrocyte functions can influence neuronal survival during ischemia and other brain insults [2]. These transporters move glutamate into astrocytes against a steep concentration gradient by coupling glutamate translocation to the transmembrane Na+, K+, and voltage gradients These gradients are in turn maintained by membrane Na+/K+ ATPase activity, such that glutamate uptake is ATP dependent

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