Heterogeneity of Ca2+ handling among and within Golgi compartments

The Golgi apparatus (GA) is a dynamic intracellular Ca2+ store endowed with complex Ca2+ homeostatic mechanisms in part distinct from those of the endoplasmic reticulum (ER). We describe the generation of a novel fluorescent Ca2+ probe selectively targeted to the medial GA. We demonstrate that in the medial-GA: i) Ca2+ accumulation takes advantage of two distinct pumps, the sarco-endoplasmic reticulum Ca2+ ATPase and the secretory pathway Ca2+ ATPase1; ii) activation of IP3 or ryanodine receptors causes Ca2+ release, while no functional two-pore channel was found; iii) luminal Ca2+ concentration appears higher than that of the trans-Golgi, but lower than that of the ER, suggesting the existence of a cis- to trans-Golgi Ca2+ concentration gradient. Thus, the GA represents a Ca2+ store of high complexity where, despite the continuous flow of membranes and luminal contents, each sub-compartment maintains its Ca2+ identity with specific Ca2+ homeostatic characteristics. The functional role of such micro-heterogeneity in GA Ca2+ handling is discussed.