Xenopus tropicalis oocytes as an advantageous model system for the study of intracellular Ca²⁺ signalling

1. The purpose of this study was to compare oocytes from the pipid frogs Xenopus tropicalis and Xenopus laevis, with respect to their utility for studying Ca²⁺ signalling mechanisms and for expression of heterologous proteins. 2. We show that X. tropicalis oocytes possess an intracellular Ca²⁺ store that is mobilized by inositol (1,4,5) trisphosphate (IP₃). Ca²⁺ signalling is activated by endogenous lysophosphatidic acid receptors and cytosolic Ca²⁺ activates a plasma membrane chloride conductance. The spatiotemporal organization of cytosolic Ca²⁺ signals, from the microscopic architecture of elementary Ca²⁺ 'puffs' to the macroscopic patterns of Ca²⁺ spiking are closely similar to the local and global patterns of Ca²⁺ release previously characterized in oocytes from X. laevis. 3. By injecting X. tropicalis oocytes with cDNA encoding an ER-targeted fluorescent protein construct, we demonstrate the capacity of the X. tropicalis oocyte to readily express heterologous proteins. The organization of ER is polarized across the oocyte, with the IP₃-releaseable store targeted within an ∼8 μm wide band that circumscribes the cell. 4. We conclude that the X. tropicalis oocyte shares many of the characteristics that have made oocytes of X. laevis a favoured system for studying Ca²⁺ signalling mechanisms. Moreover, X. tropicalis oocytes display further practical advantages in terms of imaging depth, Ca²⁺ signal magnitude and electrical properties. These further enhance the appeal of X. tropicalis as an experimental system, in addition to its greater amenability to transgenic approaches.