Aminopeptidase-A. I. CDNA cloning and expression and localization in rat tissues

Bioinformatic characterization of ENPEP, the gene encoding a potential cofactor for SARS-CoV-2 infection

Research on SARS-CoV-2, the viral pathogen that causes COVID-19, has identified angiotensin converting enzyme 2 (ACE2) as the primary viral receptor. Several genes that encode viral cofactors, such as TMPRSS2, NRP1, CTSL, and possibly KIM1, have since been discovered. Glutamyl aminopeptidase (APA), encoded by the gene ENPEP, is another cofactor candidate due to similarities in its biological role and high correlation with ACE2 and other human coronavirus receptors, such as aminopeptidase N (APN) and dipeptidyl peptidase 4 (DPP4). Recent studies have proposed a role for ENPEP as a viral receptor in humans, and ENPEP and ACE2 are both closely involved in the renin-angiotensin-aldosterone system proposed to play an important role in SARS-CoV-2 pathophysiology. We performed bioinformatic analyses using publicly available bulk (>17,000 samples from 49 distinct tissues) and single-cell (>2.5 million cells) RNA-Seq gene expression datasets to evaluate the expression and function of the ENPEP gene. We also investigated age- and sex-related changes in ENPEP expression. Overall, expression of ENPEP was highest in the small intestine enterocyte brush border and the kidney cortex. ENPEP is widely expressed in a subset of vascular smooth muscle cells (likely pericytes) in systemic vasculature, the heart, and the brain. ENPEP is expressed at low levels in the lower respiratory epithelium. In the lung, ENPEP is most highly expressed in para-alveolar fibroblasts. Single-cell data revealed ENPEP expression in a substantial fraction of ependymal cells, a finding not reported before in humans. Age increases ENPEP expression in skeletal muscle and the prostate, while decreasing it in the heart and aorta. Angiogenesis was found to be a central biological function associated with the ENPEP gene. Tissue-specific roles, such as protein digestion and fat metabolism, were also identified in the intestine. In the liver, the gene is linked to the complement system, a connection that has not yet been thoroughly investigated. Expression of ENPEP and ACE2 is strongly correlated in the small intestine and renal cortex. Both overall and in blood vessels, ENPEP and ACE2 have a stronger correlation than many other genes associated with SARS-CoV-2, such as TMPRSS2, CTSL, and NRP1. Possible interaction between glutamyl aminopeptidase and SARS-CoV-2 should be investigated experimentally.

Alterations in Gene Expression of Renin-Angiotensin System Components and Related Proteins in Colorectal Cancer

MATERIALS AND METHODS

Quantitative expression of the RNA of these 17 genes in normal and cancerous tissues obtained using chip arrays from the public functional genomics data repository, Gene Expression Omnibus (GEO) application, was compared statistically.

RESULTS

Expression of four genes, AGT (angiotensinogen), ENPEP (aminopeptidase A) MME (neprilysin), and PREP (prolyl endopeptidase), was significantly upregulated in CRC specimens. Expression of REN (renin), THOP (thimet oligopeptidase), NLN (neurolysin), PRCP (prolyl carboxypeptidase), ANPEP (aminopeptidase N), and MAS1 (Mas receptor) was downregulated in CRC specimens.

CONCLUSIONS

Presuming gene expression parallel protein expression, these results suggest that increased production of the angiotensinogen precursor of angiotensin (ANG) peptides, with the reduction of the enzymes that metabolize it to ANG II, can lead to accumulation of angiotensinogen in CRC tissues. Downregulation of THOP, NLN, PRCP, and MAS1 gene expression, whose proteins contribute to the ACE2/ANG 1-7/Mas axis, suggests that reduced activity of this RAS branch could be permissive for oncogenicity. Components of the RAS may be potential therapeutic targets for treatment of CRC.

Knockout of aminopeptidase A in mice causes functional alterations and morphological glomerular basement membrane changes in the kidneys

Aminopeptidase A is one of the most potent enzymes within the renin-angiotensin system in terms of angiotensin II degradation. Here, we examined whether there is a kidney phenotype and any compensatory changes in other renin angiotensin system enzymes involved in the metabolism of angiotensin II associated with aminopeptidase A deficiency. Kidneys harvested from aminopeptidase A knockout mice were examined by light and electron microscopy, immunohistochemistry and immunofluorescence. Kidney angiotensin II levels and the ability of renin angiotensin system enzymes in the glomerulus to degrade angiotensin II ex vivo, their activities, protein and mRNA levels in kidney lysates were evaluated. Knockout mice had increased blood pressure and mild glomerular mesangial expansion without significant albuminuria. By electron microscopy, knockout mice exhibited a mild increase of the mesangial matrix, moderate thickening of the glomerular basement membrane but a striking appearance of knob-like structures. These knobs were seen in both male and female mice and persisted after the treatment of hypertension. In isolated glomeruli from knockout mice, the level of angiotensin II was more than three-fold higher as compared to wild type control mice. In kidney lysates from knockout mice angiotensin converting enzyme activity, protein and mRNA levels were markedly decreased possibly as a compensatory mechanism to reduce angiotensin II formation. Thus, our findings support a role for aminopeptidase A in the maintenance of glomerular structure and intra-kidney homeostasis of angiotensin peptides.

Targeting Brain Aminopeptidase A: A New Strategy for the Treatment of Hypertension and Heart Failure

The pathophysiology of heart failure (HF) and hypertension are thought to involve brain renin-angiotensin system (RAS) hyperactivity. Angiotensin III, a key effector peptide in the brain RAS, provides tonic stimulatory control over blood pressure (BP) in hypertensive rats. Aminopeptidase A (APA), the enzyme responsible for generating brain angiotensin III, constitutes a potential therapeutic target for hypertension treatment. We focus here on studies of RB150/firibastat, the first prodrug of the specific and selective APA inhibitor EC33 able to cross the blood-brain barrier. We consider its development from therapeutic target discovery to clinical trials of the prodrug. After oral administration, firibastat crosses the gastrointestinal and blood-brain barriers. On arrival in the brain, it is cleaved to generate EC33, which inhibits brain APA activity, lowering BP in various experimental models of hypertension. Firibastat was clinically and biologically well tolerated, even at high doses, in phase I trials conducted in healthy human subjects. It was then shown to decrease BP effectively in patients of various ethnic origins with hypertension in phase II trials. Brain RAS hyperactivity leads to excessive sympathetic activity, which can contribute to HF after myocardial infarction (MI). Chronic treatment with oral firibastat (4 or 8 weeks after MI) has been shown to normalize brain APA activity in mice. This effect is accompanied by a normalization of brain RAS and sympathetic activities, reducing cardiac fibrosis and hypertrophy and preventing cardiac dysfunction. Firibastat may therefore represent a novel therapeutic advance in the clinical management of patients with hypertension and potentially with HF after MI.

Deficiency of the Angiotensinase Aminopeptidase A Increases Susceptibility to Glomerular Injury

Aminopeptidase A (APA) is expressed in glomerular podocytes and tubular epithelia and metabolizes angiotensin II (AngII), a peptide known to promote glomerulosclerosis. In this study, we tested whether APA expression changes in response to progressive nephron loss or whether APA exerts a protective role against glomerular damage and during AngII-mediated hypertensive kidney injury. At advanced stages of FSGS, fawn-hooded hypertensive rat kidneys exhibited distinctly increased APA staining in areas of intact glomerular capillary loops. Moreover, BALB/c APA-knockout (KO) mice injected with a nephrotoxic serum showed persistent glomerular hyalinosis and albuminuria 96 hours after injection, whereas wild-type controls achieved virtually full recovery. We then tested the effect of 4-week infusion of AngII (400 ng/kg per minute) in APA-KO and wild-type mice. Although we observed no significant difference in achieved systolic BP, AngII-treated APA-KO mice developed a significant rise in albuminuria not observed in AngII-treated wild-type mice along with increased segmental and global sclerosis and/or collapse of juxtamedullary glomeruli, microcystic tubular dilation, and tubulointerstitial fibrosis. In parallel, AngII treatment significantly increased the kidney AngII content and attenuated the expression of podocyte nephrin in APA-KO mice but not in wild-type controls. These data show that deficiency of APA increases susceptibility to glomerular injury in BALB/c mice. The augmented AngII-mediated kidney injury observed in association with increased intrarenal AngII accumulation in the absence of APA suggests a protective metabolizing role of APA in AngII-mediated glomerular diseases.

The role of the brain renin-angiotensin system in hypertension: implications for new treatment

Hypertension affects 26% of adults and is in constant progress related to increased incidence of obesity and diabetes. One-third of hypertensive patients may be successfully treated with one antihypertensive agent, one-third may require two agents and in the remaining patients will need three agents for effective blood pressure (BP) control. The development of new classes of antihypertensive agents with different mechanisms of action therefore remains an important goal. Brain renin-angiotensin system (RAS) hyperactivity has been implicated in hypertension development and maintenance in several types of experimental and genetic hypertension animal models. Among the main bioactive peptides of the brain RAS, angiotensin (Ang) II and Ang III have similar affinities for type 1 (AT1) and type 2 (AT2) Ang II receptors. Following intracerebroventricular (i.c.v.) injection, Ang II and Ang III similarly increase arginine-vasopressin (AVP) release and BP. Blocking the brain RAS may be advantageous as it simultaneously (1) decreases sympathetic tone and consequently vascular resistance, (2) decreases AVP release, reducing blood volume and vascular resistance and (3) blocks angiotensin-induced baroreflex inhibition, decreasing both vascular resistance and cardiac output. However, as Ang II is converted to Ang III in vivo, the exact nature of the active peptide is not precisely determined. We summarize here the main findings identifying AngIII as one of the major effector peptides of the brain RAS in the control of AVP release and BP. Brain AngIII exerts a tonic stimulatory effect on BP in hypertensive rats, identifying brain aminopeptidase A (APA), the enzyme generating brain Ang III, as a potentially candidate target for hypertension treatment. This has led to the development of potent orally active APA inhibitors, such as RB150--the prototype of a new class of centrally acting antihypertensive agents.

Jacques Benoit lecture: the neuroendocrine view of the angiotensin and apelin systems

The hyperactivity of the brain renin-angiotensin system (RAS) has been implicated in the development and maintenance of hypertension in several types of experimental and genetic hypertension animal models. Among the main bioactive peptides of the brain RAS, angiotensin (Ang) II and Ang III display the same affinity for type 1 and type 2 Ang II receptors. Both peptides, injected intracerebroventricularly, similarly increase arginine vasopressin (AVP) release and blood pressure (BP); however, because Ang II is converted in vivo to Ang III, the identity of the true effector is unknown. We review new insights into the predominant role of brain Ang III in the control of BP, underlining the fact that brain aminopeptidase A (APA), the enzyme generating brain Ang III, may therefore be an interesting candidate target for the treatment of hypertension. This justifies the development of potent systemically active APA inhibitors, such as RB150, as prototypes of a new class of antihypertensive agents for the treatment of certain forms of hypertension. We also searched for a putative angiotensin receptor subtype specific for Ang III and isolated a seven transmembrane-domain G protein-coupled receptor corresponding to the receptor for apelin, a newly-discovered peptide isolated from bovine stomach. Apelin and its receptor are expressed in magnocellular vasopressinergic neurones in the hypothalamus. The central injection of apelin in lactating rats decreases the phasic electrical activity of vasopressinergic neurones and the systemic secretion of AVP, inducing water diuresis. Apelin is therefore a natural inhibitor of the antidiuretic effect of AVP. In addition, systemic administration of apelin decreases BP, improves cardiac contractility and reduces cardiac loading. The development of nonpeptide agonists of the apelin receptor may provide new therapeutic tools for treating water retention, hyponatraemia and cardiovascular diseases. Angiotensins and apelin thus exert opposing but complementary effects, and are thereby determinant for the maintenance of body fluid homeostasis and cardiovascular functions.

Aminopeptidase A inhibitors as centrally acting antihypertensive agents

Among the main bioactive peptides of the brain renin-angiotensin system, angiotensin (Ang) II and AngIII exhibit the same affinity for the type 1 and type 2 Ang receptors. Both peptides, injected intracerebroventricularly, cause similar increase in blood pressure (BP). Because AngII is converted in vivo to AngIII, the identity of the true effector is unknown. This review summarized recent insights into the predominant role of brain AngIII in the central control of BP underlining the fact that brain aminopeptidase A (APA), the enzyme forming central AngIII, could constitute a putative central therapeutic target for the treatment of hypertension. This led to the development of potent, systematically active APA inhibitors, such as RB150, as a prototype of a new class of centrally acting antihypertensive agents for the treatment of certain forms of hypertension.

Characterization and cDNA cloning of aminopeptidase A from the venom of Gloydius blomhoffi brevicaudus

The aminopeptidase activities of snake venoms from Gloydius blomhoffi brevicaudus, Gloydius halys blomhoffii, Trimeresurus flavoviridis, Bothrops jararaca and Crotalus atrox were investigated. Aminopeptidase A (APA), aminopeptidase B and aminopeptidase N activities were present in all snake venoms. The strongest APA activity was found in venom from G. blomhoffi brevicaudus. The susceptibility to metallopeptidase inhibitors and the pH optimum of the partially purified enzyme from G. blomhoffi brevicaudus venom were similar to those of known APAs from mammals. A G. blomhoffi brevicaudus venom gland cDNA library was screened to isolate cDNA clones using probes based on highly conserved amino acid sequences in known APAs. Molecular cloning of APA from G. blomhoffi brevicaudus venom predicted that it was a type II integral membrane protein containing 958 amino acid residues with 17 potential N-linked glycosylation sites. It possessed a His-Glu-Xaa-Xaa-His-(Xaa)(18)-Glu zinc binding motif that allowed the classification of this protein as a member of the M1 family of zinc-metallopeptidases, or gluzincins. The deduced amino acid sequence shows approximately 60% sequence identity to mammalian APA sequences. This is the first study to report the primary structure of APA from a reptile.

Cellular signaling, AGE accumulation and gene expression in hepatocytes of lean aging rats fed ad libitum or food-restricted

The effects of food restriction on liver glucagon and vasopressin V1a receptors, on AGE accumulation and on gene expression were investigated in 10- and 30-month-old WAG/Rij female rats fed ad libitum or chronically food-restricted by 30%. The age-related increase in glucagon and vasopressin V1a receptor density, as well as the rise in glucagon-induced cAMP generation was prevented by the restriction. AGE accumulation, characteristic of the aging process, was normalized in food-restricted animals. Gene expression determined with rat Atlas cDNA Expression Arrays containing 1176 cDNA indicates that a few genes exhibited a greater than twofold change in mRNA ratios with age. Most down-regulated genes were related to oxidative metabolism of lipids, and most of the up-regulated genes were concerned with the cell cycle and transcription factors. Chronic food restriction partially prevents these changes in gene expression and induces up- and down-regulation of several mRNAs which are not modified with age in ad libitum fed rats.

Study of asparagine 353 in aminopeptidase A: characterization of a novel motif (GXMEN) implicated in exopeptidase specificity of monozinc aminopeptidases

Aminopeptidase A (EC 3.4.11.7, APA) is a 160 kDa membrane-bound zinc enzyme that contains the HEXXH consensus sequence found in members of the zinc metalloprotease family, the zincins. In addition, the monozinc aminopeptidases are characterized by another conserved motif, GXMEN, the glutamate residue of which has been shown to be implicated in the exopeptidase specificity of aminopeptidase A [Vazeux G. (1998) Biochem. J. 334, 407-413]. In carboxypeptidase A (EC 3.4.17.1, CPA), the exopeptidase specificity is conferred by an arginine residue (Arg-145) and an asparagine residue (Asn-144). Thus, we hypothesized that Asn-353 of the GXMEN motif in APA plays a similar role to Asn-144 in CPA and contributes to the exopeptidase specificity of APA. We investigated the functional role of Asn-353 in APA by substituting this residue with a glutamine (Gln-353), an alanine (Ala-353) or an aspartate (Asp-353) residue by site-directed mutagenesis. Expression of wild-type and mutated APAs revealed that Gln-353 and Ala-353 are similarly routed and glycosylated to the wild-type APA, whereas Asp-353 is trapped intracellularly and partially glycosylated. Kinetic studies, using alpha-L-glutamyl-beta-naphthylamide (GluNA) as a substrate showed that the K(m) values of the mutants Gln-353 and Ala-353 were increased 11- and 8-fold, respectively, whereas the k(cat) values were decreased (2-fold) resulting in a 24- and 14-fold reduction in cleavage efficiency. When alpha-L-aspartyl-beta-naphthylamide or angiotensin II were used as substrates, the mutations had a greater effect on k(cat), leading to a similar decrease in cleavage efficiencies as that observed with GluNA. We then measured the inhibitory potencies of several classes of inhibitors, glutamate thiol, glutamine thiol and two isomers (L- or D-) of glutamate phosphonate to explore the functional role of Asn-353. The data indicate that Asn-353 is critical for the integrity and catalytic activity of APA. This residue is involved in substrate binding via interactions with the free N-terminal part and with the P1 carboxylate side chain of the substrate. In conclusion, Asn-353 of the GXMEN motif, together with Glu-352, contributes to the exopeptidase specificity of APA and plays an equivalent role to Asn-144 in CPA.

Molecular and cellular biology of small-bowel mucosa

Study of the molecular and cellular biology of the small-intestinal mucosa is providing insights into the remarkable properties of this unique tissue. With its structured pattern of cell proliferation, differentiation, and apoptosis, and its ability to adapt following exposure to luminal nutrients or injury from surgery or pathogens, it functions in a regulated but responsive manner. We review recent publications on factors affecting development, gene expression, cell turnover, and adaptation.

Aminopeptidase-A. II. Genomic cloning and characterization of the rat promoter

Aminopeptidase-A (APA) has a widespread tissue distribution consistent with a role in the metabolism of circulating or locally produced ANG II or CCK-8. APA is also highly expressed in pre-B lymphocytes, but its role in lymphoid cell development is unknown. To begin to understand the basis for cell-specific regulation of APA expression, we sought to clone and characterize the rat gene promoter. Screening of a rat genomic library with a partial rat APA cDNA resulted in isolation of a 12-kb clone found to contain the first exon and >3 kb of 5'-flanking sequence. Primer extension of rat kidney mRNA indicated that the major transcription start site was 312 bp upstream of the translation start codon and 22 bp downstream from a TATA box. Constructs containing portions of the 5'-flanking region placed upstream of a chloramphenicol acetyltransferase reporter gene indicated that expression was cell specific and that high activity could be obtained with constructs containing as little as 110 bp of 5'-flanking region sequence. We further identified an upstream regulatory element between -1063 and -348 that suppressed transcription in a cell-specific manner. This element (termed upstream suppressor of APA, or USA) also suppressed transcription of a heterologous promoter. These results indicate that the organization and regulation of the rat APA is not consistent with it being a housekeeping gene and further suggest that rat APA gene transcription might be regulated through the presence of a novel strong upstream suppressor element.