Molecular characterisation of the smooth endoplasmic reticulum Ca2+-ATPase ofPorcellio scaberand its expression in sternal epithelia during the moult cycle (original) (raw)

Abstract

Recent investigations on specialized epithelia suggest a role of the smooth endoplasmic reticulum Ca 2+-ATPase (SERCA) in epithelial Ca 2+-transport (Franklin et al., 2001; Hagedorn and Ziegler, 2002). These epithelia are involved in quick mineralisation processes in which epithelial Ca 2+-transport predominantly follows a transcellular route (transport through the cells) (Hubbard, 2000; Roer, 1980; Wheatly, 1997; Ziegler, 2002), rather than a paracellular pathway in which Ca 2+ would move extracellularly across the apical cell contacts and between the epithelial cells along its electrochemical gradient (Bronner, 1991). In the transcellular pathway, Ca 2+ enters the cells passively across the plasma membrane at one side of the cells and is actively extruded on the other side (Ahearn and Franco, 1993; Ahearn and Zhuang, 1996; Roer, 1980). The most critical step in transcellular epithelial Ca 2+-transport, however, is the Ca 2+ transport within the cells, through the cytoplasm from one side to the plasma membrane on the opposite side. Sustained elevated concentrations of Ca 2+ within the cytoplasm would interfere with the multiple regulatory functions of cytosolic free Ca 2+ signals. In addition, such elevated concentrations of ionized calcium can lead to cell damage and even cell death (Berridge, 1993). How such a toxic rise is prevented during epithelial Ca 2+-transport is still unknown. Simkiss (1996) proposed a model in which organelles, e.g. the smooth endoplasmic reticulum (SER), function as a transient calcium store allowing vectorial bulk flow through epithelial cells without toxic effects. Within cells the SER generally functions as source and sink for cytosolic Ca 2+ signals, with the inositol 1,4,5 trisphosphate (IP3)-receptors and/or ryanodine-receptors releasing Ca 2+ and the SERCA pumping Ca 2+ back in a regulated fashion (Hussain and Inesi, 1999). Previous investigations on the anterior sternal epithelium (ASE) of Porcellio scaber (Hagedorn and Ziegler, 2002; Ziegler, 2002) suggest that the SER contributes to transcellular calcium transport. Like most crustaceans, P. scaber has a calcified cuticle, which is moulted regularly to allow for growth of the animal. During premoult the ASE transports Ca 2+ , originating from the posterior cuticle to form large CaCO3 deposits located within the ecdysial gap of the first four anterior sternites (Messner, 1965

Loading Preview

Sorry, preview is currently unavailable. You can download the paper by clicking the button above.