Introduction

Intracellular Ca2+ signals provide astrocytes with a specific form of excitability that enables them to regulate synaptic transmission. In this study, we demonstrate that NAADP-AM, a membrane-permeant analogue of the new second messenger nicotinic acid-adenine dinucleotide phosphate (NAADP), mobilizes Ca2+ in astrocytes and that the response is blocked by Ned-19, an antagonist of NAADP signalling. We also show that NAADP receptors are expressed in lysosome-related acidic vesicles. Pharmacological disruption of either NAADP or lysosomal signalling reduced Ca2+ responses induced by ATP and endothelin-1, but not by bradykinin. Furthermore, ATP increased endogenous NAADP levels. Overall, our data provide evidence for NAADP being an intracellular messenger for agonist-mediated calcium signalling in astrocytes

NAADP mediates ATP-induced Ca2+ signals in astrocytes

Intracellular Ca2+ signals provide astrocytes with a specific form of excitability that enables them to regulate synaptic transmission. In this study, we demonstrate that NAADP-AM, a membrane-permeant analogue of the new second messenger nicotinic acid-adenine dinucleotide phosphate (NAADP), mobilizes Ca2+ in astrocytes and that the response is blocked by Ned-19, an antagonist of NAADP signalling. We also show that NAADP receptors are expressed in lysosome-related acidic vesicles. Pharmacological disruption of either NAADP or lysosomal signalling reduced Ca2+ responses induced by ATP and endothelin-1, but not by bradykinin. Furthermore, ATP increased endogenous NAADP levels. Overall, our data provide evidence for NAADP being an intracellular messenger for agonist-mediated calcium signalling in astrocytes

JNK and Ceramide Kinase Govern the Biogenesis of Lipid Droplets through Activation of Group IVA Phospholipase A2*

The biogenesis of lipid droplets (LD) induced by serum depends on group IVA phospholipase A2 (cPLA2α). This work dissects the pathway leading to cPLA2α activation and LD biogenesis. Both processes were Ca2+-independent, as they took place after pharmacological blockade of Ca2+ transients elicited by serum or chelation with 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid tetrakis(acetoxymethyl ester). The single mutation D43N in cPLA2α, which abrogates its Ca2+ binding capacity and translocation to membranes, did not affect enzyme activation and formation of LD. In contrast, the mutation S505A did not affect membrane relocation of the enzyme in response to Ca2+ but prevented its phosphorylation, activation, and the appearance of LD. Expression of specific activators of different mitogen-activated protein kinases showed that phosphorylation of cPLA2α at Ser-505 is due to JNK. This was confirmed by pharmacological inhibition and expression of a dominant-negative form of the upstream activator MEKK1. LD biogenesis was accompanied by increased synthesis of ceramide 1-phosphate. Overexpression of its synthesizing enzyme ceramide kinase increased phosphorylation of cPLA2α at Ser-505 and formation of LD, and its down-regulation blocked the phosphorylation of cPLA2α and LD biogenesis. These results demonstrate that LD biogenesis induced by serum is regulated by JNK and ceramide kinase