Novel subcellular locations and functions for secretory pathway Ca²⁺/Mn²⁺-ATPases

Abstract

Secretory pathway Ca2+/Mn2+-ATPases (SPCAs) are important for maintenance of cellular Ca2+ and Mn2+ homeostasis, and, to date, all SPCAs have been found to localize to the Golgi apparatus. The single Drosophila SPCA gene ( SPoCk) was identified by an in silico screen for novel Ca2+-ATPases. It encoded three SPoCk isoforms with novel, distinct subcellular specificities in the endoplasmic reticulum (ER) and peroxisomes in addition to the Golgi. Furthermore, expression of the peroxisome-associated SPoCk isoform was sexually dimorphic. Overexpression of organelle-specific SPoCk isoforms impacted on cytosolic Ca2+ handling in both cultured Drosophila cells and a transporting epithelium, the Drosophila Malpighian (renal) tubule. Specifically, the ER isoform impacted on inositol ( 1 , 4 , 5 )-trisphosphate-mediated Ca2+ signaling and the Golgi isoform impacted on diuresis, whereas the peroxisome isoform colocalized with Ca2+ “spherites” and impacted on calcium storage and transport. Interfering RNA directed against the common exons of the three SPoCk isoforms resulted in aberrant Ca2+ signaling and abolished neuropeptide-stimulated diuresis by the tubule. SPoCk thus contributed to both of the contrasting requirements for Ca2+ in transporting epithelia: to transport or store Ca2+ in bulk without compromising its use as a signal.

Mislocalization of mitochondria and compromised renal function and oxidative stress resistance inDrosophila SesBmutants

“…Accordingly, the impact of sesB on calcium signaling was determined in intact transgenic tubules using the bioluminescent calcium reporter aequorin (42). The Drosophila neurohormone Capa-1 acts upon the principal cells of tubules via an IP 3 -mediated pathway (38), producing a biphasic rise in cytosolic calcium (21) and eliciting calcium signals in the Golgi, peroxisomes (45), and mitochondria (48) via different calcium channels including TRP channels (10,29). Transgenic fly lines homozygous for UAS-apoaequorin cyto or UAS-apoaequorin mt , and the GAL4 enhancer trap insertion c42, which drives expression in adult tubule principal cells (22,42), were crossed with UASsesB RNAi lines.…”

Increased neuronal activity fragments the Golgi complex

“…Calcium is an important second messenger within cells, where many cellular functions are regulated by the concentration of free cytosolic calcium. Calcium levels are altered by influx from the extracellular space by activation of ion channels or release from intracellular pools in the endoplasmic reticulum, mitochondria, and Golgi complex (38)(39)(40)(41). Because the integrity of the Golgi complex is required for normal calcium signaling and homeostasis (42,43), fragmentation of the Golgi may release calcium from its stores and modify calcium signaling.…”

The Ca2+ Pumps of the Endoplasmic Reticulum and Golgi Apparatus

“…Of these isoforms, only SPoCk-A is targeted to the Golgi apparatus. The subcellular localization of SPoCk-B and SPoCk-C is less clear and unexpected targeting to, respectively, the ER and the peroxisomes was reported (Southall et al 2006). Furthermore, expression of the SPoCk-C variant was shown to be sexually dimorphic (Southall et al 2006).…”

“…Using a genetically transmissible RNAinterference strategy in Drosophila, Southall et al also showed aberrant Ca 2þ signaling combined with defective neuropeptide-stimulated diuresis in the Malpighian tubes of transgenic flies (Southall et al 2006).…”