Effects of Ca(II) ions on Mn(II) dynamics in chick glia and rat astrocytes: Potential regulation of glutamine synthetase

Abstract

Previous studies have demonstrated that in glia and astrocytes Mn(II) is distributed with ca. 30-40% in the cytoplasm, 60-70% in mitochondria. Ca(II) ions were observed to alter both the flux rates and distribution of Mn(II) ions in primary cultures of chick glia and rat astrocytes. External (influxing) Ca(II) ions had the greatest effect on Mn(II) uptake and efflux, compared to internal (effluxing) or internal-external equilibrated Ca(II) ions. External (influxing) Ca(II) ions inhibited the net rate and extent of Mn(II) uptake but enhanced Mn(II) efflux from mitochondria. These observations differ from Ca(II)-Mn(II) effects previously reported with "brain" (neuronal) mitochondria. Overall, increased cytoplasmic Ca(II) acts to block Mn(II) uptake and enhance Mn(II) release by mitochondria, which serve to increase the cytoplasmic concentration of free Mn(II). A hypothesis is presented involving external L-glutamate acting through membrane receptors to mobilize cell Ca(II), which in turn causes mitochondrial Mn(II) to be released. Because the concentration of free cytoplasmic Mn(II) is poised near the Kd for Mn(II) with glutamine synthetase, a slight increase in cytoplasmic Mn(II) will directly enhance the activity of glutamine synthetase, which catalyzes removal of neurotoxic glutamate and ammonia.

Regulation of astrocyte glutamine synthetase in epilepsy

“…There is evidence that brain GS is a Mn++ metalloprotein (Denman and Wedler, 1984; Wedler et al, 1982; Wedler and Ley, 1994; Wedler et al, 1994). Consistent with this conclusion are the observations that, regardless of whether the transferase activities or the biosynthetic activities of sheep brain GS were measured, the apparent K m for Mn ++ (∼20 µM) was approximately two orders of magnitude lower than the corresponding apparent K m for Mg(II) (1.7–3.5 mM) (Wedler and Toms, 1986).…”

“…Consistent with this conclusion are the observations that, regardless of whether the transferase activities or the biosynthetic activities of sheep brain GS were measured, the apparent K m for Mn ++ (∼20 µM) was approximately two orders of magnitude lower than the corresponding apparent K m for Mg(II) (1.7–3.5 mM) (Wedler and Toms, 1986). In conjunction with the differential roles of divalent cations in activating brain GS (Meister, 1985), the finding that brain GS is a Mn metalloprotein, having a high affinity for Mn ++ , strongly suggests that intracellular levels of Mn ++ in astrocytes constitute one critical factor in regulating brain GS activity and flux (Wedler et al, 1994), because brain GS is largely astroglial in localization (Martinez-Hernandez et al, 1977). Brain levels of manganese become elevated in several neurodegenerative (e.g., Alzheimer’s disease and Parkinson’s disease) and metabolic (e.g., hepatic encephalopathy) diseases and in manganese toxicity (Lai et al, 1999; Lai et al, 2000).…”

“…Brain levels of manganese become elevated in several neurodegenerative (e.g., Alzheimer’s disease and Parkinson’s disease) and metabolic (e.g., hepatic encephalopathy) diseases and in manganese toxicity (Lai et al, 1999; Lai et al, 2000). Of relevance to the regulation of brain GS is the finding that significant levels of the elevated Mn are likely detected in astrocytes, because these cells are known to accumulate Mn avidly (Lai et al, 1999; Lai et al, 2000; Wedler et al, 1994). Consequently, the elevation of astrocytic Mn levels may exert modulatory effects on GS activity and flux and ultimately on glutamate-glutamine and glutamate-GABA-glutamine cycling in those disease states (Chowdhury et al, 2007; Jain et al, 2011; Lai et al, 1999; Lai et al, 2000; Sibson et al, 2001; Wedler et al, 1994).…”

Structure–function relationships of glutamine synthetases

“…Wedler and coworkers proposed that GS II is a Mn(II) containing enzyme [85], whereas Ginsburg and coworkers found that GS binds Mg(II) instead [74,76]. Wedler and Ley reported later [93], that the concentration of free cytoplasmic Mn(II) in chicken brain cells is near the K d for the GS^Mn(II) complex [87,93]. In the presence of mM Mg(II) and WM Mn(II), 20^30% of GS subunits were trapped with bound Mn(II) [93].…”

The effects of exposure to the environmental neurotoxicant manganese on α-synuclein and its cell-to-cell transmission via exosomes

“…This increase in Ca 2+ blocks Mn 2+ uptake, prompting a release of mitochondrial Mn 2+ into the cytosol. Finally, high levels of cytosolic Mn 2+ in astrocytes activates glutamine synthetase, which removes excess glutamate (Wedler et al, 1994). However, excessive extracellular Mn 2+ can disrupt intracellular Ca 2+ signaling in astrocytes by competitively occupying Ca 2+ binding sites, thus interfering with mitochondrial Ca 2+ homeostasis (Farina et al, 2013).…”