Most secreted proteins pass through the endoplasmic reticulum (ER), where protein folding, glycosylation, and quality control occurs. Hisatsune et al. found that the ER chaperone ERp44 also regulated blood pressure by limiting the amount of a leucine aminopeptidase called ERAP1 (also known as ERAAP, A-LAP, PILS-AP, and ARTS-1) that was secreted into the circulation. ERAP1 cleaves angiotensin II (ang II), a peptide that increases blood pressure. Although most ERp44-knockout mice died shortly after birth, a few survived to adulthood. At birth, the knockout mice had decreased urine volume, and the surviving adults had altered kidney morphology similar to that seen in angiotensin-deficient mice and lower blood pressure than control littermates. The knockout mice had less circulating ang II and increased leucine aminopeptidase activity, and plasma from the knockout mice degraded recombinant ang II faster than that from control mice. Mass spectrometry analysis of proteins that coimmunoprecipitated with ERp44 identified ERAP1 as the enzyme likely responsible for the increased ang II metabolism. Indeed, the knockout mice had increased circulating amounts of ERAP1 and undetectable amounts of ERAP1 in the liver. Antibody-mediated depletion of ERAP1 from knockout mouse serum stabilized ang II. In response to interferon-γ (IFN-γ), a pro-inflammatory cytokine that increases ERAP1 production, mouse embryo fibroblasts (MEFs) from ERp44-knockout mice exhibited a greater increase in leucine aminopeptidase activity in the culture medium than control MEFs. This increase was blocked by expression of ERp44 in the knockout cells. Analysis of overexpressed ERAP1 and ERp44 with mutations in specific cysteine residues in HeLa cells indicated that ERp44 Cys29 formed a covalent disulfide bond with Cys487 of ERAP1 and that overexpression of ERp44 that could make this disulfide bond with overexpressed ERAP1 reduced the amount of leucine aminopeptidase activity in the culture medium and increased the amount of ERAP1 associated with cell lysates, indicating retention in the cells rather than secretion. Consistent with these in vitro results, septic ERp44-knockout mice exhibited a greater drop in blood pressure and increased leucine aminopeptidase activity in the circulation. Septic control mice exhibited increased abundance of ERp44 and other ER chaperones and ERAP and increased formation of the disulfide-bridged ERp44-ERAP1 complex in multiple tissues. Thus, ERp44 functions as a redox-regulated trap to maintain blood pressure during periods of systemic inflammation.
C. Hisatsune, E. Ebisui, M. Usui, N. Ogawa, A. Suzuki, N. Mataga, H. Takahashi-Iwanaga, K. Mikoshiba, ERp44 exerts redox-dependent control of blood pressure at the ER. Mol. Cell 58, 1015–1027 (2015). [PubMed]