Copper (Cu) is essential for multiple cellular functions. Cellular uptake of Cu+ is carried out by the Ctr1 high affinity Cu transporter. The mobilization of endosomal Cu pools is regulated by a protein structurally similar to Ctr1, called Ctr2. It was recently shown that ablation of Ctr2 caused an increase in the concentration of Cu localized to endolysosomes. However, the biological significance of excess endolysosomal Cu accumulation has not been assessed. Here we addressed this issue by investigating the impact of Ctr2 deficiency on mast cells, a cell type unusually rich in endolysosomal organelles (secretory granules). We show that Ctr2−/− mast cells have increased intracellular Cu concentrations and that the absence of Ctr2 results in increased metachromatic staining, the latter indicating an impact of Ctr2 on the storage of proteoglycans in the secretory granules. In agreement with this, the absence of Ctr2 caused a skewed ratio between proteoglycans of heparin and chondroitin sulfate type, with increased amounts of heparin accompanied by a reduction of chondroitin sulfate. Moreover, transmission electron microscopy analysis revealed a higher number of electron dense granules in Ctr2−/− mast cells than in wild-type cells. The increase in granular staining and heparin content is compatible with an impact of Ctr2 on mast cell maturation and, in support of this, the absence of Ctr2 resulted in markedly increased mRNA expression, storage and enzymatic activity of tryptase. Taken together, the present study introduces Ctr2 and Cu as novel actors in the regulation of mast cell maturation and granule homeostasis.
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