Interaction of angiotensin-converting enzyme (ACE) with membrane-bound carboxypeptidase M (CPM) - a new function of ACE

Aβ degradation or cerebral perfusion? Divergent effects of multifunctional enzymes

There is increasing evidence that deficient clearance of β-amyloid (Aβ) contributes to its accumulation in late-onset Alzheimer disease (AD). Several Aβ-degrading enzymes, including neprilysin (NEP), endothelin-converting enzyme (ECE), and angiotensin-converting enzyme (ACE) reduce Aβ levels and protect against cognitive impairment in mouse models of AD. In post-mortem human brain tissue we have found that the activity of these Aβ-degrading enzymes rise with age and increases still further in AD, perhaps as a physiological response that helps to minimize the build-up of Aβ. ECE-1/-2 and ACE are also rate-limiting enzymes in the production of endothelin-1 (ET-1) and angiotensin II (Ang II), two potent vasoconstrictors, increases in the levels of which are likely to contribute to reduced blood flow in AD. This review considers the possible interdependence between Aβ-degrading enzymes, ischemia and Aβ in AD: ischemia has been shown to increase Aβ production both in vitro and in vivo, whereas increased Aβ probably enhances ischemia by vasoconstriction, mediated at least in part by increased ECE and ACE activity. In contrast, NEP activity may help to maintain cerebral perfusion, by reducing the accumulation of Aβ in cerebral blood vessels and lessening its toxicity to vascular smooth muscle cells. In assessing the role of Aβ-degrading proteases in the pathogenesis of AD and, particularly, their potential as therapeutic agents, it is important to bear in mind the multifunctional nature of these enzymes and to consider their effects on other substrates and pathways.

C-terminal clipping of chemokine CCL1/I-309 enhances CCR8-mediated intracellular calcium release and anti-apoptotic activity

Carboxypeptidase M (CPM) targets the basic amino acids arginine and lysine present at the C-terminus of peptides or proteins. CPM is thought to be involved in inflammatory processes. This is corroborated by CPM-mediated trimming and modulation of inflammatory factors, and expression of the protease in inflammatory environments. Since the function of CPM in and beyond inflammation remains mainly undefined, the identification of natural substrates can aid in discovering the (patho)physiological role of CPM. CCL1/I-309, with its three C-terminal basic amino acids, forms a potential natural substrate for CPM. CCL1 plays a role not only in inflammation but also in apoptosis, angiogenesis and tumor biology. Enzymatic processing differently impacts the biological activity of chemokines thereby contributing to the complex regulation of the chemokine system. The aim of the present study was to investigate whether (i) CCL1/I-309 is prone to trimming by CPM, and (ii) the biological activity of CCL1 is altered after C-terminal proteolytic processing. CCL1 was identified as a novel substrate for CPM in vitro using mass spectrometry. C-terminal clipping of CCL1 augmented intracellular calcium release mediated by CCR8 but reduced the binding of CCL1 to CCR8. In line with the higher intracellular calcium release, a pronounced increase of the anti-apoptotic activity of CCL1 was observed in the BW5147 cellular model. CCR8 signaling, CCR8 binding and anti-apoptotic activity were unaffected when CPM was exposed to the carboxypeptidase inhibitor DL-2-mercaptomethyl-3-guanidino-ethylthiopropanoic acid. The results of this study suggest that CPM is a likely candidate for the regulation of biological processes relying on the CCL1-CCR8 system.

Identification and disruption of sperm-specific angiotensin converting enzyme-3 (ACE3) in mouse

BACKGROUND

IZUMO1 is the only sperm protein which is proven to be essential for sperm-egg fusion. However, the IZUMO1 is a structurally simple protein with single Ig domain and seems not to include either a "fusogenic peptide" or a fusion machinery domain. This led us to assume the existence of an IZUMO1-interacting protein(s) which makes a functional fusion machine interacting with IZUMO1.

METHODOLOGY/PRINCIPAL FINDINGS

We produced a transgenic mouse line which expresses His-tagged IZUMO1 in the Izumo1(-/-) genetic background. After solubilization of sperm membranes, we purified His-tagged IZUMO1 using anti-His affinity chromatography and found a protein that interacts with IZUMO1. After being separated on SDS-PAGE gel, the IZUMO1-interacting protein was subjected to LC-MS/MS analysis and from the partial fragments, we identified the protein as ACE3. We raised the antibody against ACE3 and found that ACE3 is localized on the acrosomal cap area as in the case of IZUMO1. However, ACE3 disappeared from sperm after acrosome reaction while IZUMO1 remained on sperm. In order to investigate the role of ACE3 in vivo, we generated Ace3-deficient mice by homologous recombination and examined the fertilizing ability of the males. Unexpectedly, the male mice showed no defect in fertilizing ability in in vivo or in an in vitro fertilization system.

CONCLUSIONS/SIGNIFICANCE

We identified an IZUMO1-interacting protein in sperm, which we identified as testis specific ACE homologue ACE3. We produced an Ace3 disrupted mouse line, and found the localization of IZUMO1 spread in a little wider area on sperm, but the elimination of ACE3 did not result in a loss of sperm fertilizing ability, differing from the case of ACE disruption.

Fine epitope mapping of monoclonal antibodies 9B9 and 3G8 to the N domain of angiotensin-converting enzyme (CD143) defines a region involved in regulating angiotensin-converting enzyme dimerization and shedding

A panel of monoclonal antibodies (mAbs) raised against both the N and C domains of angiotensin-I-converting enzyme (ACE, peptidyl dipeptidase, EC 3.4.15.2) have been extensively mapped and have facilitated the study of various aspects of ACE structure and biology. In this study, we characterize two mAbs, 9B9 and 3G8, that recognize the N domain of ACE and that influence shedding and dimerization. Fine epitope mapping was performed, which mapped the epitopes for these mAbs to the N terminal region of the N domain where they overlap to a large extent, despite having different effects on ACE processing. The mAb 3G8 epitope appears to be shielded by the C domain and to be carbohydrate dependent as binding increased significantly as a result of underglycosylation, whereas these factors did not influence mAb 9B9 recognition. Three mutations within the overlapping region of these two epitopes, Q18H, L19E, and Q22A, which decreased mAb 3G8 binding to the soluble N domain, were introduced into full-length somatic ACE (sACE) to determine their influence on ACE expression and processing. Increased ACE expression, cell surface expression, and basal shedding were observed with all three mutations. Furthermore, cross-linking and western blotting of Chinese hamster ovary (CHO) cell lysates detected two distinct ACE dimers, a native and cross-linked dimer. Increasing amounts of the cross-linked dimer were observed for the mutant sACEQ22A, further implicating the overlapping region of the mAb 9B9 and 3G8 epitopes in ACE processing.