Defining Post‐transcriptional Regulation of the Copper Transporter ATP7A in Skeletal Muscle Cells

Abstract

Copper is an important trace nutrient that functions as an enzyme cofactor and signaling molecule. Copper is also toxic, so intracellular copper must be tightly regulated by transporters and chaperones. In most cells, copper export is mediated by the trans-Golgi copper transporter, ATP7A, which provides copper to secreted cupro-enzymes. Loss of ATP7A causes Menkes disease, wherein dietary copper absorption is impaired, cellular copper homeostasis is disrupted, and secreted cupro-enzymes are not supplied with copper. One symptom of Menkes disease is muscle weakness, which is largely unexplained. Skeletal muscle is a post-mitotic tissue comprised of multi-nucleated myofibers and can be regenerated by a closely-associated pool of stem cells. Few studies have focused on ATP7A in skeletal muscle. Our previous work showed that total cellular copper, ATP7A levels, and Atp7a mRNA stability increase during muscle cell differentiation. Here, we used an immortalized muscle cell line, C2C12, to demonstrate that Atp7a mRNA is stabilized in higher copper concentrations in undifferentiated muscle cells. We used a candidate-based approach to identify RNA binding proteins that regulate the Atp7a mRNA. Candidates were chosen based on previously described interaction with Atp7a mRNA and other copper homeostasis gene transcripts, their potential ability to bind copper, and their role in muscle development. With these criteria, we identified PTBP1 as a candidate. Using RNA immunoprecipitation, we confirmed that PTBP1 binds Atp7a mRNA in C2C12 cells. We used siRNA to knock down Ptbp1 in C2C12 myoblasts and detected increased Atp7a mRNA and protein levels. Our work further defines the post-transcriptional regulation of Atp7a and identifies PTBP1 as a potential regulator of copper homeostasis.