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

2003, The Journal of Experimental Biology

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