Calcium Handling Hearing New Things About Calcium

Elizabeth M. Adler

Science's STKE, AAAS, Washington, DC 20005, USA

Our ability to sense gravity--and our sense of balance--depends on otoconia, particles in the inner ear that consist of a proteinaceous core coated with CaCo₃ crystals. Otoconial development depends on otopetrin 1 (Otop1), a member of a protein family defined by a highly conserved multitransmembrane domain of unknown function. Its predicted structure, together with its extracellular location, suggested to Hughes et al. that Otop1 was likely associated with extruded membrane vesicles (globular substance vesicles) that were previously shown to respond to adenosine triphosphate (ATP) with an increase in intravesicular Ca²⁺, leading them to investigate its effects on the Ca²⁺ response to purinergic stimulation. When overexpressed in COS7 cells, Otop1 abolished an early peak in intracellular Ca²⁺ concentration elicited in wild-type cells by extracellular ATP or uridine triphosphate (UTP). ATP and UTP both activate P2Y purinergic receptors, which signal through the G_q family of heterotrimeric guanine nucleotide-binding proteins. Signaling through other G_q­-coupled receptors was maintained in the presence of Otop1, suggesting that an Otop1-mediated decrease in endoplasmic reticulum Ca²⁺ stores (determined in experiments with thapsigargin) was not sufficient to abolish all rapid G_q-dependent Ca²⁺ signals and that P2Y inhibition occurred upstream of G_q. ATP also elicited a slower sustained phase of increased intracellular Ca²⁺ (Ca²⁺ plateau). Cells overexpressing Otop1 exhibited a Ca²⁺ plateau even after treatment with thapsigargin; this plateau was abolished by chelating extracellular Ca²⁺ (unlike the P2Y-mediated rapid Ca²⁺ peak in wild-type cells). Moreover, overexpression of Otop1 in rat osteosarcoma cells that do not independently respond to purinergic stimuli enabled an ATP-dependent increase in intracellular Ca²⁺. The effects of Otop1 on the Ca²⁺ response to ATP were reversibly inhibited by suramin (previously shown to inhibit the ATP-dependent Ca²⁺ response of globular substance vesicles). The authors conclude that Otop1 has marked effects on the Ca²⁺ response to ATP that could be critical to its role in otoconia formation.

I. Hughes, M. Saito, P. H. Schlesinger, D. M. Ornitz, Otopetrin 1 activation by purinergic nucleotides regulates intracellular calcium. Proc. Natl. Acad. Sci. U.S.A. 104, 12023-12028 (2007). [Abstract] [Full Text]