In order for organs and tissues to grow, or regenerate after injury, certain proteins must enter the cell nucleus and activate genes needed for cell proliferation. One of these proteins, the yes-associated protein (YAP), plays an important role in regulating this growth process, and enters the nucleus in response to specific mechanical signals generated by trauma or disease. Previously, the details of YAP’s nuclear entry process were unknown, but now a team at the Centro Nacional de Investigaciones Cardiovasculares (National Centre for Cardiovascular Research), or CNIC, have identified the shuttle protein that enables nuclear import of YAP. This discovery presents a new potential drug target for diseases affected by uncontrolled import of YAP, such as cancer and fibrosis.
The protein importin-7 (Imp7) was identified as the nuclear transport receptor that drives YAP’s entry into the nucleus, in a process highly regulated by mechanical forces within the body. The association between Imp7 and YAP was tested through various assays including proximity ligation assays, co-immunoprecipitation assays and an experiment based on an FG hydrogel designed to mimic nuclear pore perm-selectivity. In the latter experiment, YAP stayed outside the hydrogel in the absence of any nuclear import receptors, but strongly accumulated in the gel particles with the addition of Imp7. Additionally, adding different nuclear transport receptors, such as Imp5 and transportin-1 (Trn1), did not induce entry of YAP into the gel. Lastly, a cell-based nuclear import assay showed that Imp7 enabled accumulation of YAP in the nucleus.
The researchers further found that YAP acts as the dominant cargo of Imp7, further regulating nuclear shuttle activity and restricting nuclear import of other proteins that might be carried by Imp7, like Smad3 and Erk2. To determine whether Imp7 could be a potential drug target to block uncontrolled nuclear import of YAP, which leads to tissue overgrowth in cancer and other diseases, the team reduced expression of Imp7 in the organs of Drosophila fruit flies, and found that this prevented YAP nuclear import and excess organ growth when YAP was overexpressed. This research was published in Nature Communications.
“This finding has major clinical potential because the progression of several diseases, including atherosclerosis, cancer, and fibrosis, is in part driven by uncontrolled and deleterious nuclear import of YAP,” said first author María García. “Blocking the binding of YAP to importin-7 would thus provide a way to prevent inappropriate YAP nuclear entry and subsequent disease.”
Photo: Importin-7 is required for YAP accumulation in the nucleus. (A) Human cells stained for YAP (green), showing nuclear localization in control cells. (B) Localization outside the nucleus in cells lacking the nuclear shuttle protein importin-7. Cell nuclei are stained blue in the lower panels. Credit: CNIC