Cellular and Molecular Mechanisms for Acid/Base Sensing and Calcification in Corals

摘要

Coral reefs are highly diverse marine ecosystems that are built upon the calcium carbonate skeletons of coral colonies. Despite their great ecological importance, very little is known about corals at the cellular and molecular level. More specifically, the mechanisms behind acid/base sensing and calcification in corals have yet to be determined. Therefore, for my dissertation research I identified and investigated an enzyme and an ion transporter potentially involved in these processes.;Chapters 1 and 2 of my dissertation focus on the acid/base sensor soluble adenylyl cyclase (sAC) in the corals Pocillopora damicornis and Acropora yongei. sAC is stimulated by HCO3 -- to produce the ubiquitous second messenger, cyclic adenosine monophosphate (cAMP), which may play a role in regulating internal pH through a number of signaling pathways. I have identified multiple sAC transcripts, which undergo alternative splicing, in both species, and have localized sAC protein to all four tissue layers in corals. These findings represent the first molecular evidence for sAC in a coral species.;Chapter 3 of my dissertation focuses on the sodium calcium exchanger (NCX), an ion transporter that typically exports one Ca2+ ion from the cell for every three Na+ ions that it imports. NCX has been implicated in bone calcification in some mammals and birds. I identified a potential NCX protein in Acropora yongei and cloned five distinct splice variants. Immunofluorescence microscopy indicated NCX protein is found in intracellular vesicles within the calicoblastic epithelium, the thin tissue directly involved in calcification of the coral skeleton. This vesicular localization was confirmed by expression of recombinant, fluorescently-tagged coral NCX protein in sea urchin embryos. Based on my findings, I have proposed a new model for coral calcification in which NCX in the membranes of vesicles sequesters Ca2+ ions from the cytoplasm, preventing negative impacts on Ca2+ signaling, and delivers Ca2+ to the site of calcification.;Together, this research helps us better understand the basic cell biology and physiology of marine organisms, improving current models for acid/base sensing and calcification in marine calcifiers such as corals.