Meprin A metalloproteinase and its role in acute kidney injury

Abstract

Meprin A, composed of α- and β-subunits, is a membrane-associated neutral metalloendoprotease that belongs to the astacin family of zinc endopeptidases. It was first discovered as an azocasein and benzoyl-l-tyrosyl-p-aminobenzoic acid hydrolase in the brush-border membranes of proximal tubules and intestines. Meprin isoforms are now found to be widely distributed in various organs (kidney, intestines, leukocytes, skin, bladder, and a variety of cancer cells) and are capable of hydrolyzing and processing a large number of substrates, including extracellular matrix proteins, cytokines, adherens junction proteins, hormones, bioactive peptides, and cell surface proteins. The ability of meprin A to cleave various substrates sheds new light on the functional properties of this enzyme, including matrix remodeling, inflammation, and cell-cell and cell-matrix processes. Following ischemia-reperfusion (IR)- and cisplatin-induced acute kidney injury (AKI), meprin A is redistributed toward the basolateral plasma membrane, and the cleaved form of meprin A is excreted in the urine. These studies suggest that altered localization and shedding of meprin A in places other than the apical membranes may be deleterious in vivo in acute tubular injury. These studies also provide new insight into the importance of a sheddase involved in the release of membrane-associated meprin A under pathological conditions. Meprin A is injurious to the kidney during AKI, as meprin A-knockout mice and meprin inhibition provide protective roles and improve renal function. Meprin A, therefore, plays an important role in AKI and potentially is a unique target for therapeutic intervention during AKI.

Keywords: acute kidney injury; meprin; metalloproteinase; renal proximal tubule.

Fig. 1.

Domain structure and oligomeric forms of meprins. The domain structure of meprins is composed of an N-terminal signal peptide, propeptide, protease motif HExxHxxGxxH, MAM (meprin A5 protein receptor protein tyrosine phosphatase μ), tumor necrosis factor receptor-associated factor (TRAF), after MATH (AM), I (inserted domain), epidermal growth factor (EGF)-like, transmembrane (TM)-spanning, and cytoplasmic domain. Meprin A composed of the α2β2 heterotetramer is the major form present in the brush-border plasma membrane of the mouse kidney. The meprin subunits are bound together by disulfide bridges (horizontal black bars). Meprin A (α2β2 or α3β1) is bound to the brush-border membrane via the meprin β-subunit (membrane-bound meprin A). Meprin α not bound to the β-subunit is secreted as a homo-oligomer (secreted meprin A).

Fig. 2.

A: meprin A distribution in the mouse kidney. Double immunofluorescence staining demonstrating the colocalization of meprin α- and β-subunits of meprin A in kidney brush-border membranes. Double immunofluorescence staining was performed on kidney sections of 8- to 10-wk-old male C57BL/6n mice. Formalin-fixed kidneys sections (5–10 μm) were deparaffinized and stained with primary antibodies against meprin β (polyclonal goat anti-mouse meprin β, R&D Biosystems) and meprin α (polyclonal rabbit anti-human meprin α, Novus Biologicals) followed by Alexa Fluor 594 donkey anti-goat secondary antibody (red, meprin β) and Alexa Fluor 488 donkey anti-rabbit secondary antibody (green, meprin α). Nuclei were stained with 4,6-diamidino-2-phenylindole (DAPI; blue). Epifluorescence pictures were recorded on an Olympus IX51 microscope. The merged picture shows the overlay of signals for both meprin β and α. B: meprin A redistribution in kidneys of mice subjected to ischemia-reperfusion (IR) injury. Double immunofluorescence of meprin A (red) and Na⁺-K⁺-ATPase (green) in kidney sections of control mice (left) and mice subjected to IR (24-h reperfusion after 45 min of ischemia; right) is shown. Formalin-fixed tissue sections were incubated with goat polyclonal anti-meprin-β and rabbit polyclonal anti-Na⁺-K⁺-ATPase antibodies overnight. After a washing with PBS, slices were incubated with secondary antibodies [donkey anti-goat Alexa Fluor 594 (red) and donkey anti-rabbit Alexa Fluor 488 (green)]. Nuclei were stained with DAPI. Epifluorescence pictures were recorded on an Olympus (IX51, IX71, or BX51) or on a Zeiss LSM 510 META confocal microscope.