Involvement of TRPV4 channels in Aβ40-induced hippocampal cell death and astrocytic Ca2+ signalling
Neurotoxicology. 2014 Mar;41:64-72.
Ji-Zhong Bai* and Janusz Lipski
Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Private bag 92-019, Auckland, New Zealand
Previous studies suggested that amyloid β (Aβ)-induced disruption of astrocytic Ca2+ signallingand oxidative stressplay a major role in the progression towards neuronal and glial death in Alzheimer’s disease. We have recently demonstrated that Ca2+-permeable TRPV4 channels are highly expressed in rat hippocampal astrocytes and are involved in oxidative stress-induced cell damage. The aim of this study was to test the hypothesis that TRPV4 channels also contribute to hippocampal damage evoked by Aβ. Synthetic Aβ40 evoked cell death in hippocampal slice cultures in a concentration (0-20 mM) and time (12-48 hr) dependent manner, after cultures were preconditioned with sublethal concentration of buthionine sulfoximine (BSO, 1.5 µM) which enhanced endogenous ROS production. As demonstrated by propidium iodide fluorescence, damage was observed in the granule cell layer of the dentate gyrus and to a smaller degree in pyramidal neurons of the CA1-CA3 region, as well as in glia cells mainly at the edge of the slice. Immunocytochemistry revealed an altered pattern of TRPV4 and GFAP protein expression, andreactive astrogliosis surrounding pyramidal CA1-CA3 neurons. Neuronal and astrocytic damage was attenuated by the antioxidant Trolox, TRPV4 channel blockers Gd3+ and ruthenium red (RR), and a specific inhibitor of the redox and Ca2+-sensitive phospholipase A2 enzyme (MAFP). In disassociated co-cultures of hippocampal neurons and astrocytes without BSO preconditioning, Aβ40 evoked pronounced neuronal damage,enhanced the expression of TRPV4 and GFAP proteins (indicative of reactive astrogliosis), and increased intracellular free Ca2+ concentration in astrocytes. The latter effect was attenuated by RR and in Ca2+-free media. These data show that Ab40 can activate astrocytic TRPV4 channels in the hippocampus, leading to neuronal and astrocytic damage in a Ca2+ and oxidative stress-dependent manner.
Amyloid deposition and neuron death are the pathologic hallmarks of Alzheimer’s disease (AD). Accumulation of toxic amyloid β (Aβ) species in the brain is widely accepted as the primary factor in AD pathogenesis. While previous research on the action of Aβ has predominantly focused on neurons, glial cells (mainly astrocytes, the most abundant group of brain cells) have been largely ignored due to their spongiform appearance as ‘passive bystanders’. There is strong evidence that astrocytes supply neurons with metabolic substrates and the precursors of antioxidant glutathione (GSH), as well as play an active role in other aspects of neuronal function. Although reactive astrocytes are known to surround amyloid plaques indicating their role in the deposition and clearance of Aβand activation of the inflammatory response, a role of dysfunctional astrocytes in the pathogenesis of AD remains unanswered.
Previous studies suggested that astrocytes can be the primary target of Aβ, and that Aβ-induced changes in astrocytic [Ca2+]i, ROS generation and GSH depletion play a role in the progression towards neuronal death (e.g., 1).However,the cellular and molecular mechanisms of these effects remain to be determined. We have recently demonstrated that Ca2+-permeable TRPV4 channels are highly expressed in rat hippocampal astrocytes and are involved in oxidative stress-induced cell damage (2). The aim of the current study was to examine the potential role of these channels in Aβ-induced damage of the hippocampus, a brain region highly vulnerable in AD.
Our results show thatin hippocampal slice cultures obtained from rats, Ab40 evokes oxidative stress-dependent and region-specific cell damage associated with a change of TRPV4 immunoreactivity. Aβ40 mainly damaged neurons in the dentate gyrus and the CA1 pyramidal region, similarly to what has been reportedin the hippocampus in AD human brains. Under our experimental conditions, oxidative preconditioning of slice cultures with low concentrations of BSO was essential for Ab40 cytotoxicity.This is consistent with the notion that oxidative stress in brain tissue increases with age, representing a profound risk factor for AD. Most interestingly, immunocytochemistry revealed an altered pattern of TRPV4 and GFAP protein expression, andreactive astrogliosis surrounding pyramidal CA1-CA3 neurons. Cell damage could be prevented by the antioxidant Trolox, non-selective TRPV4 channel blockers, and inhibition of redox and Ca2+-sensitive PLA2.