Lithium Sensitivity of Store Operated Ca2+ Entry and Survival of Fibroblasts Isolated from Chorea-Acanthocytosis Patients.

BACKGROUND: The widely expressed protein chorein fosters activation of the phosphoinositide 3 kinase (PI3K) pathway thus supporting cell survival. Loss of function mutations of the chorein encoding gene VPS13A (vacuolar protein sorting-associated protein 13A) causes chorea-acanthocytosis (ChAc), a neurodegenerative disorder paralleled by deformations of erythrocytes. In mice, genetic knockout of chorein leads to enhanced neuronal apoptosis. PI3K dependent signalling upregulates Orai1, a pore forming channel protein accomplishing store operated Ca2+ entry (SOCE). Increased Orai1 expression and SOCE have been shown to confer survival of tumor cells. SOCE could be up-regulated by lithium. The present study explored, whether SOCE and/or apoptosis are altered in ChAc fibroblasts and could be modified by lithium treatment. METHODS: Fibroblasts were isolated from ChAc patients and age-matched healthy volunteers. Cytosolic Ca2+ activity ([Ca2+]i) was estimated from Fura-2-fluorescence, SOCE from increase of [Ca2+]i following Ca2+ re-addition after Ca2+-store depletion with sarcoendoplasmatic Ca2+-ATPase (SERCA) inhibitor thapsigargin (1 µM), and apoptosis from annexin-V/propidium iodide staining quantified in flow cytometry. RESULTS: SOCE was significantly smaller in ChAc fibroblasts than in control fibroblasts. Lithium (2 mM, 24 hours) significantly increased and Orai1 blocker 2-Aminoethoxydiphenyl Borate (2-APB, 50 µM, 24 hours) significantly decreased SOCE. Annexin-V-binding and propidium iodide staining were significantly higher in ChAc fibroblasts than in control fibroblasts. In ChAc fibroblasts annexin-V-binding and propidium iodide staining were significantly decreased by lithium treatment, significantly increased by 2-APB and virtually lithium insensitive in the presence of 2-APB. CONCLUSIONS: In ChAc fibroblasts, downregulation of SOCE contributes to enhanced susceptibility to apoptosis. Both, decreased SOCE and enhanced apoptosis of ChAc fibroblasts can be reversed by lithium treatment.

Up-Regulation of the Excitatory Amino Acid Transporters EAAT1 and EAAT2 by Mammalian Target of Rapamycin.

BACKGROUND: The excitatory amino-acid transporters EAAT1 and EAAT2 clear glutamate from the synaptic cleft and thus terminate neuronal excitation. The carriers are subject to regulation by various kinases. The EAAT3 isoform is regulated by mammalian target of rapamycin (mTOR). The present study thus explored whether mTOR influences transport by EAAT1 and/or EAAT2. METHODS: cRNA encoding wild type EAAT1 (SLC1A3) or EAAT2 (SLC1A2) was injected into Xenopus oocytes without or with additional injection of cRNA encoding mTOR. Dual electrode voltage clamp was performed in order to determine electrogenic glutamate transport (IEAAT). EAAT2 protein abundance was determined utilizing chemiluminescence. RESULTS: Appreciable IEAAT was observed in EAAT1 or EAAT2 expressing but not in water injected oocytes. IEAAT was significantly increased by coexpression of mTOR. Coexpression of mTOR increased significantly the maximal IEAAT in EAAT1 or EAAT2 expressing oocytes, without significantly modifying affinity of the carriers. Moreover, coexpression of mTOR increased significantly EAAT2 protein abundance in the cell membrane. CONCLUSIONS: The kinase mTOR up-regulates the excitatory amino acid transporters EAAT1 and EAAT2.

Down-Regulation of the Na+,Cl- Coupled Creatine Transporter CreaT (SLC6A8) by Glycogen Synthase Kinase GSK3ß.

BACKGROUND: The Na+,Cl- coupled creatine transporter CreaT (SLC6A8) is expressed in a variety of tissues including the brain. Genetic defects of CreaT lead to mental retardation with seizures. The present study explored the regulation of CreaT by the ubiquitously expressed glycogen synthase kinase GSK3ß, which contributes to the regulation of neuroexcitation. GSK3ß is phosphorylated and thus inhibited by PKB/Akt. Moreover, GSK3ß is inhibited by the antidepressant lithium. The present study thus further tested for the effects of PKB/Akt and of lithium. METHODS: CreaT was expressed in Xenopus laevis oocytes with or without wild-type GSK3ß or inactive K85RGSK3ß. CreaT and GSK3ß were further expressed without and with additional expression of wild type PKB/Akt. Creatine transport in those oocytes was quantified utilizing dual electrode voltage clamp. RESULTS: Electrogenic creatine transport was observed in CreaT expressing oocytes but not in water-injected oocytes. In CreaT expressing oocytes, co-expression of GSK3ß but not of K85RGSK3ß, resulted in a significant decrease of creatine induced current. Kinetic analysis revealed that GSK3ß significantly decreased the maximal creatine transport rate. Exposure of CreaT and GSK3ß expressing oocytes for 24 hours to Lithium was followed by a significant increase of the creatine induced current. The effect of GSK3ß on CreaT was abolished by co-expression of PKB/Akt. CONCLUSION: GSK3ß down-regulates the creatine transporter CreaT, an effect reversed by treatment with the antidepressant Lithium and by co-expression of PKB/Akt.

Lithium Sensitive ORAI1 Expression, Store Operated Ca2+ Entry and Suicidal Death of Neurons in Chorea-Acanthocytosis.

Chorea-Acanthocytosis (ChAc), a neurodegenerative disorder, results from loss-of-function-mutations of chorein-encoding gene VPS13A. In tumour cells chorein up-regulates ORAI1, a Ca2+-channel accomplishing store operated Ca2+-entry (SOCE) upon stimulation by STIM1. Furthermore SOCE could be up-regulated by lithium. The present study explored whether SOCE impacts on neuron apoptosis. Cortical neurons were differentiated from induced pluripotent stem cells generated from fibroblasts of ChAc patients and healthy volunteers. ORAI1 and STIM1 transcript levels and protein abundance were estimated from qRT-PCR and Western blotting, respectively, cytosolic Ca2+-activity ([Ca2+]i) from Fura-2-fluorescence, as well as apoptosis from annexin-V-binding and propidium-iodide uptake determined by flow cytometry. As a result, ORAI1 and STIM1 transcript levels and protein abundance and SOCE were significantly smaller and the percentage apoptotic cells significantly higher in ChAc neurons than in control neurons. Lithium treatment (2 mM, 24 hours) increased significantly ORAI1 and STIM1 transcript levels and protein abundance, an effect reversed by inhibition of Serum & Glucocorticoid inducible Kinase 1. ORAI1 blocker 2-APB (50 µM, 24 hours) significantly decreased SOCE, markedly increased apoptosis and abrogated the anti-apoptotic effect of lithium. In conclusion, enhanced neuronal apoptosis in ChAc at least partially results from decreased ORAI1 expression and SOCE, which could be reversed by lithium treatment.