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

Abstract

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.