Acidic Ca2+ stores come to the fore

Changes in the concentration of cytosolic Ca2+ form the basis of a ubiquitous signal transduction pathway. Accumulating evidence implicates acidic organelles in the control of Ca2+ dynamics in organisms across phyla. In this special issue, we discuss Ca2+ signalling by these “acidic Ca2+ stores” which include acidocalcisomes, vacuoles, the endo-lysosomal system, lysosome-related organelles, secretory vesicles and the Golgi complex. Ca2+ release from these morphologically very different organelles is mediated by members of the TRP channel superfamily and two-pore channels. Inositol trisphosphate and ryanodine receptors which are traditionally viewed as endoplasmic reticulum Ca2+ release channels can also mobilize acidic Ca2+ stores. Ca2+ uptake into acidic Ca2+ stores is driven by Ca2+ ATPases and Ca2+/H+ exchangers. In animal cells, the Ca2+-mobilizing messenger NAADP plays a central role in mediating Ca2+ signals from acidic Ca2+ stores through activation of two-pore channels. These signals are important for several physiological processes including muscle contraction and differentiation. Dysfunctional acidic Ca2+ stores have been implicated in diseases such as acute pancreatitis and lysosomal storage disorders. Acidic Ca2+ stores are therefore emerging as essential components of the Ca2+ signalling network and merit extensive further study.