Cell blebbing is a key feature in apoptosis. Because blebbing dynamically alters cell volume and regulatory volume changes have been linked to chloride (Cl) channels, we evaluated an association between blebbing and Cl channels activity. We used scanning electron microscopy, confocal laser microscopy, and cell sorting to quantify cell volume and blebbing and whole-cell recording to characterize Cl^- currents. We found that blockade of Cl channel activity as well as inhibition of adenylyl cyclase or protein kinase A (PKA) activity suppressed ammonia-induced blebbing in the microglia cell line, BV-2. In further experiments, we elucidated the common mechanism of Cl channel activity and cyclic adenosine 3’,5’-monophosphate (cAMP)-dependent pathway on cell blebbing. These experiments indicated that perfusion of cells with cAMP or the catalytic subunit of PKA activated a Cl^- current under normotonic conditions. The pharmacological profile (sensitivity to 5-nitro-2-(3-phenylpropylamino)benzoic acid [NPPB], flufenamic acid, and [(dihydroindenyl)oxy]alkanoic acid [DIOA]), outward rectification, and kinetic of the current were identical to the swelling-activated Cl channel. Superfusion of cells with ammonia elicited an outwardly rectifying current sensitive to Cl channel blockers. We propose that ammonia induces a PKA-dependent phosphorylation of Cl channels. Localized influx of Cl^- is followed by influx of water, required for bleb expansion.

Although ammonia is a well-known neuropathogenic factor, the cellular mechanisms of ammonia toxicity are less characterized. Up to now, the main focus of ammonia toxicity has been on astrocytes and neurons. Despite the significance of microglia in neurodegenerative diseases, little is known about their responsiveness to ammonia. In the present study, we found that ammonia triggered mitosis at concentrations between 30 microm and 3.0 mm but apoptosis at concentrations >or= 1.0 mm in the murine microglial cell line BV-2. Most apoptotic cells showed an accumulation of condensed chromatin at the nuclear envelope, blebbing of the plasma membrane, formation of apoptotic bodies and an increase in caspase 3/7 activity. Blockade of caspase 3/7 activity by Ac-DEVD-CHO suppressed ammonia-induced apoptosis. Surprisingly, some BV-2 cells exposed to ammonia displayed clear signs of mitotic catastrophe, a type of cell death occurring during mitosis. In a further series of experiments, we found that cyclic adenosine 3',5'-monophosphate (cAMP) mediated the apoptogenic effects of ammonia, because (i) ammonia dose-dependently elevated the intracellular cAMP level, (ii) blockade of the adenylyl cyclase by SQ-22536 suppressed ammonia-induced apoptosis, (iii) inhibition of phosphodiesterases (PDEs) by the nonselective PDE inhibitor IBMX, or by the PDE4-selective inhibitor rolipram, increased the relative number of apoptotic cells, and (iv) the cAMP analogues 8-bromoadenosine cAMP and Sp-cAMP mimicked the effect of ammonia and triggered apoptosis. Taken together, our results indicate that distinct concentrations of ammonia trigger opposite signalling pathways in microglial cells.

Under physiological conditions, astrocytes take up L-glutamate from the synaptic gap, metabolize it to L-glutamine and return it to neurons, where L-glutamine is metabolized to L-glutamate and stored in neurotransmitter vesicles. However, under pathological conditions, such as hepatic failure, L-glutamine and ammonium are elevated globally in the brain. The Trojan horse hypothesis of L-glutamine toxicity assumes that intramitochondrial hydrolysis of L-glutamine enhances ammonium locally and leads to mitochondrial dysfunction. In the present study, we show that exposure of murine primary microglia as well as of the microglial cell-line BV-2 to L-glutamine promotes chromatin condensation and formation of crescent-like structures in the nucleus. Furthermore, L-glutamine induced an increase in annexin-V labelling, cell shrinkage (apoptotic volume decrease), cell fragmentation and formation of apoptotic bodies. Inhibition of the phosphate-activated glutaminase with 6-diazo-5-oxo-L-norleucine suppressed chromatin condensation and annexin-V labelling in L-glutamine-exposed cells. In addition, inhibition of the glutamine synthetase with L-methionine sulfoximine suppressed chromatin condensation and annexin-V labelling in ammonium-exposed cells. L-glutamine and ammonium enhanced production of reactive oxygen species, as detected with CM-H(2)DCFDA. Apoptosis, induced by L-glutamine, was inhibited either by the radical scavenger alpha-tocopherol or by the nitric oxide synthase blocker N (G)-methyl-L-arginine. Cyclosporin A, a ligand of the permeability transition pore complex component cyclophilin D, prevented L-glutamine-triggered apoptosis. Furthermore, blockade of caspase-9 activity with Z-LEHD-FMK prevented L-glutamine-triggered apoptosis. Taken together, our results indicate that hydrolysis of l-glutamine and, accordingly, accumulation of ammonium in mitochondria induce the intrinsic pathway of apoptosis, characterized by mitochondrial dysfunction and activation of caspase-9, which activates caspase-3.

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