Calciosome, a sarcoplasmic reticulum-like organelle involved in intracellular Ca2+-handling by non-muscle cells: Studies in human neutrophils and HL-60 cells

Calclosomes are intracellular organelles in HL-60 cells, neutrophils and various other cell types, characterized by their content of a Ca²⁺-binding protein that is biochemically and immunologically similar to calsequestrin (CS) from muscle cells. In subcellular fractionation studies the CS-like protein copurifies with functional markers of the inositol 1,4,5-trisphosphate (IP₃) releasable Ca²⁺-store. These markers (ATP-dependent Ca²⁺-uptake and IP₃-induced Ca²⁺-release) show a subcellular distribution which is clearly distinct from the endoplasmic reticulum and other organelles. In morphological studies, antibodies against rabbit skeletal muscle CS protein specifically stained hitherto unrecognized vesicles with a diameter between 50 and 250 nm. Thus both, biochemical and morphological studies indicate that the calsequestrin containing intracellular Ca²⁺-store, now referred to as the calciosome, is distinct from other known organelles such as endoplasmic reticulum. Calciosomes are likely to play an important role in intracellular Ca²⁺-homeostasis. They are possibly the intracellular target of inositol 1,4,5-trisphosphate and thus the source of Ca²⁺ that is redistributed into the cytosol following surface receptor activation in non-muscle cells.