The E3 ubiquitin ligase Slimb/ß-TrCP is required for normal copper homeostasis in Drosophila.

The Drosophila Slimb (Slmb) gene encodes a Skp1-Cul1-F-box (SCP) E3 ubiquitin ligase orthologous to the human ß-TrCP/BTRC protein. Slmb and/or BTRC play regulatory roles in numerous biological processes by ubiquitinating several substrate proteins which are then targeted for proteasomal degradation. Here, we demonstrate an additional role for Slmb in maintaining cellular copper homeostasis. In the thorax, midgut and eye, Slmb knockdown causes copper deficiency phenotypes which can be rescued by increasing cellular copper levels via decreased efflux or increased uptake. Furthermore, Slmb knockdown results in decreased levels of the copper transporters Ctr1A and ATP7, indicating Slmb is required to regulate copper homeostasis. We also present evidence that the transcription factor Cap-n-Collar (Nrf2 in mammals), a known substrate of Slmb/BTRC, mediates Slmb's regulatory effect on Ctr1A in a post-transcriptional manner.

Vacuolar-type H(+)-ATPase subunits and the neurogenic protein big brain are required for optimal copper and zinc uptake.

Copper and zinc homeostasis in polarized epithelial cells requires the correct localization and regulation of membrane-bound transport proteins at the apical and basolateral cell membranes. We have identified a subunit of the vacuolar-type H(+)-ATPase (V-ATPase) complex, vhaPPA1-2, and the Drosophila aquaporin homolog big brain (bib), as being required for the correct localization of the copper uptake transporters Ctr1A and Ctr1B and the zinc uptake protein dZip89B and hence necessary for optimal copper and zinc accumulation in vivo. Knockdown of vhaPPA1-2 or bib resulted in cuticle hypo-pigmentation phenotypes typical of copper deficiency in the fly and induction of midgut Ctr1B expression, a known response to low cellular copper levels. Furthermore, midgut-specific knockdown of bib increased tolerance to elevated dietary zinc levels. Ctr1A, Ctr1B and dZip89B are normally localized to the apical plasma membrane. Upon knockdown of vhaPPA1-2 or bib, this localization was strongly disrupted as was that of the generic plasma membrane marker CD8-GFP, indicating that these two genes are not acting specifically on metal ion homeostasis but rather are necessary for general apical membrane protein localization in polarized epithelial cells. These results suggest that metal ion transport is particularly sensitive to disturbances in cellular protein localization processes.

Tissue-specific interplay between copper uptake and efflux in Drosophila.

The vinegar fly Drosophila melanogaster is proving to be an excellent system to study the in vivo regulation of the essential metal copper. The Ctr1A/B and DmATP7 copper transport proteins have well-established roles in Drosophila copper uptake and efflux, respectively. Both Ctr1A and DmATP7 are essential genes, whereas Ctr1B mutants are viable but die in excess or depleted copper conditions. Less is known about the tissue-specific requirements for these three genes and how they interact to maintain copper homeostasis in different cell types. Here, we use targeted overexpression and suppression of each gene to examine these questions in vivo. We find that in the epidermal cells that form the adult thoracic and abdominal cuticle, Ctr1A plays a major role in copper uptake, whereas Ctr1B plays only a minor supporting role and DmATP7, as previously shown, is essential for transfer of copper to the trans-Golgi network. We also find that the copper chaperone dSco1 appears necessary for supplying the mitochondria with copper in these tissues. In contrast, in the developing Drosophila eye, DmATP7 appears to be non-essential unless copper levels in these cells are artificially elevated. Again, Ctr1A is the main copper uptake gene in the eye, but when ectopically expressed, Ctr1B has greater phenotypic effects than Ctr1A. Furthermore, Ctr1A and Ctr1B show a dramatic synergistic interaction, indicating their relationship is more complicated than a simply additive one and that they may in fact act cooperatively for optimal copper import.