Oxysterol-binding protein related-proteins (ORPs) 5 and 8 regulate calcium signaling at specific cell compartments.

Oxysterol-binding protein related-protein 5 and 8 (ORP5/8) localize to the membrane contact sites (MCS) of the endoplasmic reticulum (ER) and the mitochondria, as well as to the ER-plasma membrane (PM) MCS. The MCS are emerging as important regulators of cell signaling events, including calcium (Ca) signaling. ORP5/8 have been shown to interact with phosphatidylinositol-4,5-bisphosphate (PIP) in the PM, and to modulate mitochondrial respiration and morphology. PIP is the direct precursor of inositol trisphosphate (IP), a key second messenger responsible for Ca-release from the intracellular Ca stores. Further, mitochondrial respiration is linked to Ca transfer from the ER to the mitochondria. Hence, we asked whether ORP5/8 would affect Ca signaling in these cell compartments, and employed genetically engineered aequorin Ca probes to investigate the effect of ORP5/8 in the regulation of mitochondrial and caveolar Ca. Our results show that ORP5/8 overexpression leads to increased mitochondrial matrix Ca as well as to increased Ca concentration at the caveolar subdomains of the PM during histamine stimulation, while having no effect on the cytoplasmic Ca. Also, we found that ORP5/8 overexpression increases cell proliferation. Our results show that ORP5/8 regulate Ca signaling at specific MCS foci. These local ORP5/8-mediated Ca signaling events are likely to play roles in processes such as mitochondrial respiration and cell proliferation.