Calcineurin-ERK Antagonism at the Nuclear Pore Complex Regulates Bi-Directional Nuclear Transport

Calcineurin, the ubiquitously expressed calcium/calmodulin-regulated protein phosphatase, is essential for many cellular processes, including T-cell activation. The dephosphorylation activity of this protein is effectively inhibited by the immunosuppressants cyclosporin A and tacrolimus (FK506). Clinical administration of these immunosuppressants leads to a myriad of adverse effects in non-target tissues, but the underlying causes of these adverse effects are still largely unknown. Recent work offers the foundation for a molecular-scale explanation of these adverse effects by demonstrating that calcineurin directly dephosphorylates multiple proteins in the nuclear pore complex (NPC), and that calcineurin inhibition alters protein transport through the NPC. These findings also suggest that the NPC is a site where calcium signaling takes place. To understand how calcineurin regulates nuclear transport, it is essential to determine whether it acts locally to exert control over nuclear import, nuclear export, or both processes. To pursue this inquiry, I employ NPC-tethered-calcineurin constructs to establish that calcineurin acts locally to regulate transport through the NPC in human cells. Next, I use light-inducible fluorescent reporters for nuclear import and export (LINuS and LEXY, respectively) to show that calcineurin inhibition decreases both import and export rates. These results suggest that calcineurin directly dephosphorylates nuclear pore proteins to promote bidirectional transport. In follow up experiments, I then show that calcineurin opposes ERK, the end effector of the MAPK pathway, to regulate nuclear transport rates. Finally, I demonstrate that calcineurin inhibition also causes importin-β and exportin-1 to mislocalize, thereby revealing a potential mechanism for the promotion of nuclear transport by calcineurin. These findings offer exciting insights into how nuclear transport, a fundamental cellular process, is regulated. Intriguingly, both calcineurin inhibition and nuclear transport defects have been implicated in neurodegenerative phenotypes, suggesting that calcineurin-mediated nuclear transport may be crucial for understanding maladies such as Parkinson’s and Huntington’s disease.