Collectrin, a collecting duct-specific transmembrane glycoprotein, is a novel homolog of ACE2 and is developmentally regulated in embryonic kidneys

The emerging role of angiotensin-converting enzyme-2 in the kidney

PURPOSE OF REVIEW

The intrarenal renin-angiotensin system contributes to the progression of chronic kidney disease. Angiotensin-converting enzyme-2 is a recently identified protein that is highly expressed in the kidney and results in formation of angiotensin-(1-7). This manuscript highlights current information on the localization and function of angiotensin-converting enzyme-2 in the kidney, along with recent studies on the role of the enzyme in hypertension, glomerulosclerosis, and diabetic nephropathy.

RECENT FINDINGS

Angiotensin-converting enzyme-2 is expressed in relative abundance in the proximal tubule and metabolizes angiotensin II to angiotensin-(1-7). Angiotensin-converting enzyme-2 also catalyzes conversion of angiotensin I to angiotensin-(1-9), which is cleaved to angiotensin-(1-7) by angiotensin-converting enzyme. In mice, gene deletion of angiotensin-converting enzyme-2 elevates blood pressure responses to angiotensin II, and increases renal angiotensin II levels. Male angiotensin-converting enzyme-2-deficient mice exhibit accelerated glomerulosclerosis. In early diabetic nephropathy, proximal tubular angiotensin-converting enzyme-2 expression and activity are increased, suggesting that the enzyme may counterregulate the effects of local angiotensin II.

SUMMARY

Angiotensin-converting enzyme-2 appears to be critical in determining the balance between the intrarenal effects of angiotensin II and angiotensin-(1-7). Angiotensin-converting enzyme-2 could thereby represent a target for novel therapeutic approaches in a variety of kidney disorders.

Essential role for collectrin in renal amino acid transport

Angiotensin -converting enzyme 2 (ACE2) is a regulator of the renin angiotensin system involved in acute lung failure, cardiovascular functions and severe acute respiratory syndrome (SARS) infections in mammals. A gene encoding a homologue to ACE2, termed collectrin (Tmem27), has been identified in immediate proximity to the ace2 locus. The in vivo function of collectrin was unclear. Here we report that targeted disruption of collectrin in mice results in a severe defect in renal amino acid uptake owing to downregulation of apical amino acid transporters in the kidney. Collectrin associates with multiple apical transporters and defines a novel group of renal amino acid transporters. Expression of collectrin in Xenopus oocytes and Madin-Darby canine kidney (MDCK) cells enhances amino acid transport by the transporter B(0)AT1. These data identify collectrin as a key regulator of renal amino acid uptake.

Characterization of primary rat proximal tubular cells by gene expression analysis

The kidney plays a major role in excretory and reabsorptive processes. The kidney cortex consists primarily of proximal tubular cells, which are epithelial cells that are often involved in the induction and progression of various kidney diseases. Therefore primary proximal tubular cells are widely used as a renal cell model. To further characterize this kidney in vitro model different time points in culture after isolation of the cells were compared to the cortex in vivo using gene expression analysis based on microarrays. This study revealed that many metabolic pathways and some kidney-specific functions are lacking in the in vitro model. Furthermore genes involved in RNA and protein synthesis, intracellular transport, extracellular matrix and cytoskeletal organization were upregulated in culture compared to in vivo, indicating proliferation of the cells and differentiation into a cell culture phenotype. The data represented here may help to evaluate the in vivo relevance of results obtained with this in vitro model.

Aminoaciduria and altered renal expression of luminal amino acid transporters in mice lacking novel gene collectrin

Defects in renal proximal tubule transport manifest in a number of human diseases. Although variable in clinical presentation, disorders such as Hartnup disease, Dent's disease, and Fanconi syndrome are characterized by wasting of solutes commonly recovered by the proximal tubule. One common feature of these disorders is aminoaciduria. There are distinct classes of amino acid transporters located in the apical and basal membranes of the proximal tubules that reabsorb >95% of filtered amino acids, yet few details are known about their regulation. We present our physiological characterization of a mouse line with targeted deletion of the gene collectrin that is highly expressed in the kidney. Collectrin-deficient mice display a reduced urinary concentrating capacity due to enhanced solute clearance resulting from profound aminoaciduria. The aminoaciduria is generalized, characterized by loss of nearly every amino acid, and results in marked crystalluria. Furthermore, in the kidney, collectrin-deficient mice have decreased plasma membrane populations of amino acid transporter subtypes B(0)AT1, rBAT, and b(0,+)AT, as well as altered cellular distribution of EAAC1. Our data suggest that collectrin is a novel mediator of renal amino acid transport and may provide further insight into the pathogenesis of a number of human disease correlates.

Generation of a large scale repertoire of Expressed Sequence Tags (ESTs) from normalised rainbow trout cDNA libraries

Background

Within the framework of a genomics project on livestock species (AGENAE), we initiated a high-throughput DNA sequencing program of Expressed Sequence Tags (ESTs) in rainbow trout, Oncorhynchus mykiss.

Results

We constructed three cDNA libraries including one highly complex pooled-tissue library. These libraries were normalized and subtracted to reduce clone redundancy. ESTs sequences were produced, and 96 472 ESTs corresponding to high quality sequence reads were released on the international database, currently representing 42.5% of the overall sequence knowledge in this species. All these EST sequences and other publicly available ESTs in rainbow trout have been included on a publicly available Website (SIGENAE) and have been clustered into a total of 52 930 clusters of putative transcripts groups, including 24 616 singletons. 57.1% of these 52 930 clusters are represented by at least one Agenae EST and 14 343 clusters (27.1%) are only composed by Agenae ESTs. Sequence analysis also reveals that normalization and especially subtraction were effective in decreasing redundancy, and that the pooled-tissue library was representative of the initial tissue complexity.

Conclusion

Due to present work on the construction of rainbow trout normalized cDNA libraries and their extensive sequencing, along with other large scale sequencing programs, rainbow trout is now one of the major fish models in term of EST sequences available in a public database, just after Zebrafish, Danio rerio. This information is now used for the selection of a non redundant set of clones for producing DNA micro-arrays in order to examine global gene expression.

Altered blood pressure responses and normal cardiac phenotype in ACE2-null mice

The carboxypeptidase ACE2 is a homologue of angiotensin-converting enzyme (ACE). To clarify the physiological roles of ACE2, we generated mice with targeted disruption of the Ace2 gene. ACE2-deficient mice were viable, fertile, and lacked any gross structural abnormalities. We found normal cardiac dimensions and function in ACE2-deficient animals with mixed or inbred genetic backgrounds. On the C57BL/6 background, ACE2 deficiency was associated with a modest increase in blood pressure, whereas the absence of ACE2 had no effect on baseline blood pressures in 129/SvEv mice. After acute Ang II infusion, plasma concentrations of Ang II increased almost 3-fold higher in ACE2-deficient mice than in controls. In a model of Ang II-dependent hypertension, blood pressures were substantially higher in the ACE2-deficient mice than in WT. Severe hypertension in ACE2-deficient mice was associated with exaggerated accumulation of Ang II in the kidney, as determined by MALDI-TOF mass spectrometry. Although the absence of functional ACE2 causes enhanced susceptibility to Ang II-induced hypertension, we found no evidence for a role of ACE2 in the regulation of cardiac structure or function. Our data suggest that ACE2 is a functional component of the renin-angiotensin system, metabolizing Ang II and thereby contributing to regulation of blood pressure.

Thiazolidinediones ameliorate diabetic nephropathy via cell cycle-dependent mechanisms

Thiazolidinediones are ligands for peroxisome proliferator-activated receptor (PPAR)-gamma, widely used as insulin sensitizer in type 2 diabetic patients and implicated in apoptosis, cell proliferation, and cell cycle regulation. Here, the effect of thiazolidinediones on G1-phase cell cycle arrest, the hallmark in diabetic nephropathy, was investigated. Eight-week-old male Otsuka Long-Evans Tokushima fatty rats were treated with pioglitazone (1 mg x kg body wt(-1) x day(-1)) until 50 weeks of age and compared with insulin treatment. Although similar HbA(1c) levels were observed in both groups, pioglitazone significantly inhibited glomerular hypertrophy and mesangial matrix expansion and reduced urinary albumin excretion compared with the insulin-treated group. In addition, pioglitazone significantly reduced the number of glomerular p27(Kip1)-positive cells. Because prominent expression of PPAR-gamma was observed in podocytes in glomeruli and cultured cells, conditionally immortalized mouse podocyte cells were cultured under 5.5 and 25 mmol/l D-glucose supplemented with pioglitazone. Pioglitazone inhibited cell hypertrophy revealed by [(3)H]thymidine and [(3)H]proline incorporation, and pioglitazone reversed high glucose-induced G1-phase cell cycle arrest, i.e., an increase in G0/G1 phase and decrease in S and G2 phases. Pioglitazone suppressed high glucose-induced phosphorylation of p44/42 mitogen-activated protein kinase and reduced Bcl-2 and p27(Kip1) protein levels. Besides glucose-lowering action, pioglitazone ameliorates diabetic nephropathy via cell cycle-dependent mechanisms.

The HNF-1 target collectrin controls insulin exocytosis by SNARE complex formation

Defective glucose-stimulated insulin secretion is the main cause of hyperglycemia in type 2 diabetes mellitus. Mutations in HNF-1alpha cause a monogenic form of type 2 diabetes, maturity-onset diabetes of the young (MODY), characterized by impaired insulin secretion. Here we report that collectrin, a recently cloned kidney-specific gene of unknown function, is a target of HNF-1alpha in pancreatic beta cells. Expression of collectrin was decreased in the islets of HNF-1alpha (-/-) mice, but was increased in obese hyperglycemic mice. Overexpression of collectrin in rat insulinoma INS-1 cells or in the beta cells of transgenic mice enhanced glucose-stimulated insulin exocytosis, without affecting Ca(2+) influx. Conversely, suppression of collectrin attenuated insulin secretion. Collectrin bound to SNARE complexes by interacting with snapin, a SNAP-25 binding protein, and facilitated SNARE complex formation. Therefore, collectrin is a regulator of SNARE complex function, which thereby controls insulin exocytosis.

Tmem27: a cleaved and shed plasma membrane protein that stimulates pancreatic beta cell proliferation

The signals and molecular mechanisms that regulate the replication of terminally differentiated beta cells are unknown. Here, we report the identification and characterization of transmembrane protein 27 (Tmem27, collectrin) in pancreatic beta cells. Expression of Tmem27 is reduced in Tcf1(-/-) mice and is increased in islets of mouse models with hypertrophy of the endocrine pancreas. Tmem27 forms dimers and its extracellular domain is glycosylated, cleaved and shed from the plasma membrane of beta cells. This cleavage process is beta cell specific and does not occur in other cell types. Overexpression of full-length Tmem27, but not the truncated or soluble protein, leads to increased thymidine incorporation, whereas silencing of Tmem27 using RNAi results in a reduction of cell replication. Furthermore, transgenic mice with increased expression of Tmem27 in pancreatic beta cells exhibit increased beta cell mass. Our results identify a pancreatic beta cell transmembrane protein that regulates cell growth of pancreatic islets.

Identification of adipocyte adhesion molecule (ACAM), a novel CTX gene family, implicated in adipocyte maturation and development of obesity

Few cell adhesion molecules have been reported to be expressed in mature adipocytes, and the significance of cell adhesion process in adipocyte biology is also unknown. In the present study, we identified ACAM (adipocyte adhesion molecule), a novel homologue of the CTX (cortical thymocyte marker in Xenopus) gene family. ACAM cDNA was isolated during PCR-based cDNA subtraction, and its mRNA was shown to be up-regulated in WATs (white adipose tissues) of OLETF (Otsuka Long-Evans Tokushima fatty) rats, an animal model for Type II diabetes and obesity. ACAM, 372 amino acids in total, has a signal peptide, V-type (variable) and C2-type (constant) Ig domains, a single transmembrane segment and a cytoplasmic tail. The amino acid sequence in rat is highly homologous to mouse (94%) and human (87%). ACAM mRNA was predominantly expressed in WATs in OLETF rats, and increased with the development of obesity until 30 weeks of age, which is when the peak of body mass is reached. Western blot analysis revealed that ACAM protein, approx. 45 kDa, was associated with plasma membrane fractions of mature adipocytes isolated from mesenteric and subdermal adipose deposits of OLETF rats. Up-regulation of ACAM mRNAs in obesity was also shown in WATs of genetically obese db/db mice, diet-induced obese ICR mice and human obese subjects. In primary cultured mouse and human adipocytes, ACAM mRNA expression was progressively up-regulated during differentiation. Several stably transfected Chinese-hamster ovary K1 cell lines were established, and the quantification of ACAM mRNA and cell aggregation assay revealed that the degree of homophilic aggregation correlated well with ACAM mRNA expression. In summary, ACAM may be the critical adhesion molecule in adipocyte differentiation and development of obesity.

Distinct roles for ANG II and ANG-(1-7) in the regulation of angiotensin-converting enzyme 2 in rat astrocytes

Angiotensin-converting enzyme 2 (ACE2) is a homolog of ACE that preferentially forms angiotensin-(1-7) [ANG-(1-7)] from angiotensin II (ANG II). Incubation of neonatal rat cerebellar or medullary astrocytes with ANG II reduced ACE2 mRNA by approximately 60%, suggesting transcriptional regulation of the enzyme. In contrast, ANG II had no effect on ACE mRNA in astrocytes isolated from either brain region, demonstrating a differential regulation of the two enzymes by ANG II. The ANG II-mediated reduction in ACE2 mRNA was blocked by the angiotensin type 1 (AT(1)) receptor antagonists losartan or valsartan; the angiotensin type 2 (AT(2)) antagonist PD123319 was ineffective. The reduction in ACE2 mRNA by ANG II also was associated with a 50% decrease in cerebellar and medullary ACE2 protein, which was blocked by losartan. Treatment of medullary astrocytes with ANG-(1-7), the product of ACE2 hydrolysis of ANG II, did not affect ACE2 mRNA; however, ANG-(1-7) prevented the ANG II-mediated reduction in ACE2 mRNA. The addition of [d-Ala(7)]-ANG-(1-7), a selective AT((1-7)) receptor antagonist, blocked the inhibitory actions of ANG-(1-7). These data are the first to demonstrate transcriptional regulation of ACE2 by ANG II and ANG-(1-7). Because ACE2 preferentially converts ANG II to ANG-(1-7), downregulation of the enzyme by ANG II constitutes a novel positive feed-forward system within the brain that may favor ANG II-mediated neural responses. Furthermore, the modulatory role of ANG-(1-7) in the transcriptional regulation of ACE2 by ANG II suggests a complex interplay between these peptides that is mediated by distinct receptor systems.

Visceral adipose tissue-derived serine protease inhibitor: a unique insulin-sensitizing adipocytokine in obesity

There is a rapid global rise in obesity, and the link between obesity and diabetes remains somewhat obscure. We identified an adipocytokine, designated as visceral adipose tissue-derived serpin (vaspin), which is a member of serine protease inhibitor family. Vaspin cDNA was isolated by from visceral white adipose tissues (WATs) of Otsuka Long-Evans Tokushima fatty (OLETF) rat, an animal model of abdominal obesity with type 2 diabetes. Rat, mouse, and human vaspins are made up of 392, 394, and 395 amino acids, respectively; exhibit approximately 40% homology with alpha1-antitrypsin; and are related to serine protease inhibitor family. Vaspin was barely detectable in rats at 6 wk and was highly expressed in adipocytes of visceral WATs at 30 wk, the age when obesity, body weight, and insulin levels peak in OLETF rats. The tissue expression of vaspin and its serum levels decrease with worsening of diabetes and body weight loss at 50 wk. The expression and serum levels were normalized with the treatment of insulin or insulin-sensitizing agent, pioglitazone, in OLETF rats. Administration of vaspin to obese CRL:CD-1 (ICR) (ICR) mice fed with high-fat high-sucrose chow improved glucose tolerance and insulin sensitivity reflected by normalized serum glucose levels. It also led to the reversal of altered expression of genes relevant to insulin resistance, e.g., leptin, resistin, TNFalpha, glucose transporter-4, and adiponectin. In DNA chip analyses, vaspin treatment resulted in the reversal of expression in approximately 50% of the high-fat high-sucrose-induced genes in WATs. These findings indicate that vaspin exerts an insulin-sensitizing effect targeted toward WATs in states of obesity.

ACE2: A novel therapeutic target for cardiovascular diseases

Hypertension afflicts over 65 million Americans and poses an increased risk for cardiovascular morbidity such as stroke, myocardial infarction and end-stage renal disease resulting in significant mortality. Overactivity of the renin-angiotensin system (RAS) has been identified as an important determinant that is implicated in the etiology of these diseases and therefore represents a major target for therapy. In spite of the successes of drugs inhibiting various elements of the RAS, the incidence of hypertension and cardiovascular diseases remain steadily on the rise. This has lead many investigators to seek novel and innovative approaches, taking advantage of new pathways and technologies, for the control and possibly the cure of hypertension and related pathologies. The main objective of this review is to forward the concept that gene therapy and the genetic targeting of the RAS is the future avenue for the successful control and treatment of hypertension and cardiovascular diseases. We will present argument that genetic targeting of angiotensin-converting enzyme 2 (ACE2), a newly discovered member of the RAS, is ideally poised for this purpose. This will be accomplished by discussion of the following: (i) summary of our current understanding of the RAS with a focus on the systemic versus tissue counterparts as they relate to hypertension and other cardiovascular pathologies; (ii) the newly discovered ACE2 enzyme with its physiological and pathophysiological implications; (iii) summary of the current antihypertensive pharmacotherapy and its limitations; (iv) the discovery and design of ACE inhibitors; (v) the emerging concepts for ACE2 drug design; (vi) the current status of genetic targeting of the RAS; (vii) the potential of ACE2 as a therapeutic target for hypertension and cardiovascular disease treatment; and (viii) future perspectives for the treatment of cardiovascular diseases.

Receptor and viral determinants of SARS-coronavirus adaptation to human ACE2

Human angiotensin-converting enzyme 2 (ACE2) is a functional receptor for SARS coronavirus (SARS-CoV). Here we identify the SARS-CoV spike (S)-protein-binding site on ACE2. We also compare S proteins of SARS-CoV isolated during the 2002-2003 SARS outbreak and during the much less severe 2003-2004 outbreak, and from palm civets, a possible source of SARS-CoV found in humans. All three S proteins bound to and utilized palm-civet ACE2 efficiently, but the latter two S proteins utilized human ACE2 markedly less efficiently than did the S protein obtained during the earlier human outbreak. The lower affinity of these S proteins could be complemented by altering specific residues within the S-protein-binding site of human ACE2 to those of civet ACE2, or by altering S-protein residues 479 and 487 to residues conserved during the 2002-2003 outbreak. Collectively, these data describe molecular interactions important to the adaptation of SARS-CoV to human cells, and provide insight into the severity of the 2002-2003 SARS epidemic.

Myocardial infarction increases ACE2 expression in rat and humans

AIMS

Angiotensin converting enzyme (ACE) 2 catalyses the cleavage of angiotensin (Ang) I to Ang 1-9 and of Ang II to Ang 1-7. ACE2 deficiency impairs cardiac contractility and upregulates hypoxia-induced genes, suggesting a link with myocardial ischaemia. We studied the expression of ACE2 after myocardial infarction (MI) in the rat as well as in human failing hearts.

METHODS AND RESULTS

Rats were killed at days 1, 3, and 28 after MI, or treated for 4 weeks with the ACE inhibitor ramipril (1 mg/kg). Cardiac gene and protein expression of ACE and ACE2 were assessed by quantitative real-time reverse transcriptase-polymerase chain reaction and immunohistochemistry/activity assays/in vitro autoradiography, respectively. Both ACE (P = 0.022) and ACE2 (P = 0.015) mRNA increased in the border/infarct area compared with the viable area at day 3 after MI. By day 28, increases in ACE (P = 0.005) and ACE2 (P = 0.006) mRNA were also seen in the viable myocardium of MI rats compared with myocardium of control rats. ACE2 protein localized to macrophages, vascular endothelium, smooth muscle, and myocytes. Ramipril attenuated cardiac hypertrophy and inhibited cardiac ACE. In contrast, ramipril had no effect on cardiac ACE2 mRNA, which remained elevated in all areas of the MI rat heart. Immunoreactivity of both ACE and ACE2 increased in failing human hearts.

CONCLUSION

The increase in ACE2 after MI suggests that it plays an important role in the negative modulation of the renin angiotensin system in the generation and degradation of angiotensin peptides after cardiac injury.

Membrane-associated zinc peptidase families: comparing ACE and ACE2

In contrast to the relatively ubiquitous angiotensin-converting enzyme (ACE), expression of the mammalian ACE homologue, ACE2, was initially described in the heart, kidney and testis. ACE2 is a type I integral membrane protein with its active site domain exposed to the extracellular surface of endothelial cells and the renal tubular epithelium. Here ACE2 is poised to metabolise circulating peptides which may include angiotensin II, a potent vasoconstrictor and the product of angiotensin I cleavage by ACE. To this end, ACE2 may counterbalance the effects of ACE within the renin–angiotensin system (RAS). Indeed, ACE2 has been implicated in the regulation of heart and renal function where it is proposed to control the levels of angiotensin II relative to its hypotensive metabolite, angiotensin-(1–7). The recent solution of the structure of ACE2, and ACE, has provided new insight into the substrate and inhibitor profiles of these two key regulators of the RAS. As the complexity of this crucial pathway is unravelled, there is a growing interest in the therapeutic potential of agents that modulate the activity of ACE2.

Interaction of ACE2 and integrin beta1 in failing human heart

ACE2 purified from failing human heart was found to form a complex with integrin beta1 by immunoprecipitation, Western blotting, activity assay, and ESI tandem mass spectroscopy. The ACE2/integrin complex showed a Km of 6.8 microM and a Vmax of 2.13 pmol/min/microl purified enzyme. Activity was optimal at pH 7.5 with Ang II substrate.

The role of adrenomedullin and receptors in glomerular hyperfiltration in streptozotocin-induced diabetic rats

BACKGROUND

Since adrenomedullin (AM) elicits vasodilatation by binding to specific AM receptors consisted of calcitonin-receptor-like receptor (CRLR)/receptor-activity-modifying protein 2 (RAMP2) or CRLR/receptor-activity-modifying protein 3 (RAMP3) on endothelial cells and stimulating nitric oxide production, AM possibly involves in glomerular capillary dilatation in early phase of diabetic nephropathy.

METHODS

Streptozotocin (STZ)-induced diabetic Sprague-Dawley rats at 4 weeks after the injection were employed for expression studies of AM, RAPM2, and RAMP3. The measurement of AM peptide levels in kidney tissue, plasma, and urine was performed. Human aortic endothelial cells (HAEC) were used to investigate functional link between glucose-induced AM production and nitric oxide release.

RESULTS

STZ rats showed glomerular hypertrophy and increased urinary NO2- and NO3- excretion. By Northern blot analyses, AM and RAPM2 mRNAs significantly increased in the kidneys of STZ rats, while RAMP3 mRNA was not altered. In STZ rats, AM peptide was actively secreted into urine (1280 +/- 360 fmol/day vs. control 110 +/- 36 fmol/day). AM peptide was mainly detected on cortical and medullary collecting duct cells in control rat kidneys and AM peptide and mRNA were up-regulated on afferent arterioles and glomeruli of STZ rats. RAMP2 expression was detected on afferent arterioles and not in glomeruli in control rats and it was up-regulated on glomerular endothelial cells in STZ rats. In HAEC culture, d-glucose stimulated AM and nitric oxide production and they were suppressed by addition of AM antisense oligodeoxynucleotides.

CONCLUSION

Up-regulated expression of AM and RAMP2 in afferent arterioles and glomeruli may be related to selective dilatation of glomerular capillary in acute phase of type 1 diabetes.

ACE2, a new regulator of the renin-angiotensin system

Angiotensin-converting enzyme (ACE) is a zinc metalloproteinase and a key regulator of the renin-angiotensin system (RAS). ACE2 is a newly described enzyme identified in rodents and humans with a more restricted distribution than ACE, and is found mainly in heart and kidney. ACE2 cleaves a single residue from angiotensin I (Ang I) to generate Ang 1-9, and degrades Ang II, the main effector of the RAS, to the vasodilator Ang 1-7. The importance of ACE2 in normal physiology and pathophysiological states is largely unknown. ACE2 might act in a counter-regulatory manner to ACE, modulating the balance between vasoconstrictors and vasodilators within the heart and kidney, and playing a significant role in regulating cardiovascular and renal function.

ACE2 X-ray structures reveal a large hinge-bending motion important for inhibitor binding and catalysis

The angiotensin-converting enzyme (ACE)-related carboxypeptidase, ACE2, is a type I integral membrane protein of 805 amino acids that contains one HEXXH + E zinc-binding consensus sequence. ACE2 has been implicated in the regulation of heart function and also as a functional receptor for the coronavirus that causes the severe acute respiratory syndrome (SARS). To gain further insights into this enzyme, the first crystal structures of the native and inhibitor-bound forms of the ACE2 extracellular domains were solved to 2.2- and 3.0-Å resolution, respectively. Comparison of these structures revealed a large inhibitor-dependent hinge-bending movement of one catalytic subdomain relative to the other (∼16°) that brings important residues into position for catalysis. The potent inhibitor MLN-4760 ((S,S)-2-{1-carboxy-2-[3-(3,5-dichlorobenzyl)-3H-imidazol4-yl]-ethylamino}-4-methylpentanoic acid) makes key binding interactions within the active site and offers insights regarding the action of residues involved in catalysis and substrate specificity. A few active site residue substitutions in ACE2 relative to ACE appear to eliminate the S₂′ substrate-binding subsite and account for the observed reactivity change from the peptidyl dipeptidase activity of ACE to the carboxypeptidase activity of ACE2.

Gene expression in the normal adult human kidney assessed by complementary DNA microarray

The kidney is a highly specialized organ with a complex, stereotyped architecture and a great diversity of functions and cell types. Because the microscopic organization of the nephron, the functional unit of the kidney, has a consistent relationship to the macroscopic anatomy of the kidney, knowledge of the characteristic patterns of gene expression in different compartments of the kidney could provide insight into the functions and functional organization of the normal nephron. We studied gene expression in dissected renal lobes of five adult human kidneys using cDNA microarrays representing approximately 30,000 different human genes. Total RNA was isolated from sections of the inner and outer cortex, inner and outer medulla, papillary tips, and renal pelvis and from glomeruli isolated by sieving. The results revealed unique and highly distinctive patterns of gene expression for glomeruli, cortex, medulla, papillary tips, and pelvic samples. Immunohistochemical staining using selected antisera confirmed differential expression of several cognate proteins and provided histological localization of expression within the nephron. The distinctive patterns of gene expression in discrete portions of the kidney may serve as a resource for further understanding of renal physiology and the molecular and cellular organization of the nephron.

AngRem104, an angiotensin II-induced novel upregulated gene in human mesangial cells, is potentially involved in the regulation of fibronectin expression

Accumulation of extracellular matrix (ECM) in the glomerular mesangium is a common feature of many progressive renal diseases. Angiotensin II (AngII) plays important roles in the proliferation of glomerular mesangial cells (MC) as well as the synthesis of ECM such as fibronectin (FN) and collagens. However, the precise molecular signals responsible for these effects are unknown. To explore possible molecule mechanism of ECM accumulation related to AngII, suppression subtractive hybridization (SSH) was performed to screen and identify upregulated genes induced by AngII in cultured human MC. A novel gene, AngRem104 (GenBank accession number, AF367870), was isolated. The full-length cDNA of AngRem104 is 1690 bp, and it contains a 1041-bp open reading frame (ORF) encoding 347 amino acid residues with a predicted molecular mass of 37.2 kD. AngRem104 widely expressed in human heart, placenta, liver, muscle, kidney, and pancreas. Moreover, AngRem104 was found in human glomeruli and tubule by in situ hybridization. In human MC, the upregulation of AngRem104 induced by AngII was time-dependent, and it was dose-dependently blocked by AngII type 1 receptor antagonist (AT1RA), Losartan. The subcellular localization detected by AngRem104-pEGFP fusion protein revealed that AngRem104 was a nuclear protein. Interestingly, when AngRem104 was overexpressed by transfection of its sense construct, cDNA Microarray showed that two of the ECM-related genes, i.e., human mRNA for FN and integrin-beta-1 (FN receptor), dramatically upregulated their expressions. Furthermore, AngRem104 could regulate the expression of FN induced by AngII, which were detected by RT-PCR and quantitative real-time PCR, when AngRem104 was overexpressed. It is concluded that AngRem104 is a novel human gene potentially involved in the regulation of FN induced by AngII in human MC. These findings may provide new insights into mechanisms of glomerular sclerosis associated with AngII.

A story of two ACEs

According to the World Health Organization predictions cardiovascular diseases will be the leading cause of death by the year 2020. High blood pressure is a major risk factor for myocardial infarction, cerebrovascular disease, and stroke. Modulation of the renin-angiotensin system, particularly inhibition of the angiotensin-converting enzyme (ACE), has become a prime strategy in the treatment of hypertension and heart failure. Recently the gene of a new ACE, termed ACE2, has been characterized. The ACE2 gene maps to defined quantitative trait loci on the X chromosome in three different rat models of hypertension, suggesting ACE2 as a candidate gene for hypertension. In mice the targeted disruption of ACE2 resulted in increased systemic angiotensin II levels, impaired cardiac contractility, and upregulation of hypoxia-induced genes in the heart. Since mice deficient in both ACE2 and ACE show completely normal heart function, it appears that ACE and ACE2 negatively regulate each other. The mechanisms and physiological significance of the interplay between ACE and ACE2 are not yet elucidated, but it may involve several new peptides and peptide systems. In view of drug development the increasing complexity of the renin-angiotensin system offers both challenge and opportunity to develop new and refined treatment strategies against cardiovascular diseases.

Screening and identification of the up-regulated genes in human mesangial cells exposed to angiotensin II

Accumulation of extracellular matrix (ECM) in the glomerular mesangium is a common feature of many progressive renal diseases. Angiotensin II (Ang II) plays a major role in the progression of chronic kidney diseases in part by induction of ECM. However, the precise molecular signals responsible for this effect are unknown. To explore possible molecular mechanisms of ECM production related to Ang II, we screened and identified genes up-regulated by Ang II in cultured human mesangial cells (MC). Detection of up-regulated genes was determined by mRNA populations from human MC with and without Ang II stimulation (10(-6) mol/l, 24 h) by suppression subtractive hybridization. Reverse Northern blot analysis was performed to screen for differentially expressed genes. Full-length cDNAs of three novel genes were isolated by rapid amplification of cDNA ends (RACE)-polymerase chain reaction (PCR). One of these novel genes, AngRem104, was further investigated by Northern blot, Western blot and reverse transcription (RT)-PCR. The bioinformatics analysis implied that AngRem104 coded for a nuclear protein that was widely expressed in various normal human tissues. Moreover, up-regulation of AngRem104 induced by Ang II was time-dependent and was dose-dependently blocked by the Ang II type 1 receptor antagonist, Losartan. Interestingly, we also demonstrated that AngReam104 was associated with increased fibronectin expression. We conclude that AngRem104 is a novel human gene that is related to the expression of fibronectin and that is up-regulated by Ang II in human MC. These findings may lead to new insights into the mechanisms of glomerular sclerosis associated with Ang II.

Perturbation of autocrine/paracrine loops of burst-forming units of erythroid-derived cells in rHuEPO-hyporesponsive hemodialysis patients

BACKGROUND

Quantitative or qualitative abnormalities of erythroid progenitors in patients with chronic renal failure (CRF) could be the major factor for recombinant human erythropoietin (rHuEPO) hyporesponsiveness and severe anemia in hemodialysis (HD) patients receiving rHuEPO therapy.

METHODS

Purified 1 x 10(4) circulating CD34+ cells isolated from rHuEPO-hyporesponsive HD patients (EPO-H; n = 10), rHuEPO-responsive non-HD patients with CRF (EPO-R; n = 8), nonanemic HD patients without rHuEPO therapy (EPO-W/O; n = 10), and healthy volunteer controls (CON; n = 10) were subjected to a methylcellulose culture system supplemented with rHuEPO, recombinant human interleukin-3 (IL-3), recombinant human stem cell factor (SCF), and recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) for 14 days.

RESULTS

The average number of burst-forming units of erythroids (BFU-Es) was significantly less in the EPO-H group compared with the CON and EPO-W/O groups. Furthermore, colony size also was significantly smaller in the EPO-H group. Total RNAs were extracted from approximately 100 colonies/patient and subjected to complementary DNA expression array studies of 268 growth factors, cytokines, chemokines, and their receptors. A characteristic cluster upregulated in the EPO-R and EPO-W/O groups and downregulated in the EPO-H group was identified that contained various cytokines and growth factors, including IL-6, GM-CSF, vascular endothelial growth factor B, IL-9, IL-3, leukemia inhibitory factor, and interferon alpha-2, and such receptors as thrombopoietin receptor, IL-9 receptor, and colony-stimulating factor 1 receptor.

CONCLUSION

These data suggest that the cross-talk network or autocrine/paracrine regulatory loop is critically impaired in BFU-E-derived cells in EPO-H patients, and investigation of these cluster genes would facilitate the development of novel therapeutic strategies for such patients.

Just the beginning: novel functions for angiotensin-converting enzymes

Cardiovascular disease is predicted to be the commonest cause of death worldwide by the year 2020. Diabetes, smoking and hypertension are the main risk factors. The renin-angiotensin system plays a key role in regulating blood pressure and fluid and electrolyte homeostasis in mammals. The discovery of specific drugs that block either the key enzyme of the renin-angiotensin system, angiotensin-converting enzyme (ACE), or the receptor for its main effector angiotensin II, was a major step forward in the treatment of hypertension and heart failure. In recent years, however, the renin-angiotensin system has been shown to be a far more complex system than initially thought. It has become clear that additional peptide mediators are involved. Furthermore, a new ACE, angiotensin-converting enzyme 2 (ACE2), has been discovered which appears to negatively regulate the renin-angiotensin system. In the heart, ACE2 deficiency results in severe impairment of cardiac contractility and upregulation of hypoxia-induced genes. We shall discuss the interplay of the various effector peptides generated by angiotensin-converting enzymes ACE and ACE2, highlighting the role of ACE2 as a negative regulator of the renin-angiotensin system.

Renal interstitial fluid angiotensin I and angiotensin II concentrations during local angiotensin-converting enzyme inhibition

It was recently demonstrated that angiotensin II (AngII) concentrations in the renal interstitial fluid (RIF) of anesthetized rats were in the nanomolar range and were not reduced by intra-arterial infusion of an angiotensin-converting enzyme (ACE) inhibitor (enalaprilat). This study was performed to determine changes in RIF AngI and AngII concentrations during interstitial administration of ACE inhibitors (enalaprilat and perindoprilat). Studies were also performed to determine the effects of enalaprilat on the de novo formation of RIF AngII elicited by interstitial infusion of AngI. Microdialysis probes (cut-off point, 30,000 D) were implanted in the renal cortex of anesthetized rats and were perfused at 2 micro l/min. The effluent dialysate concentrations of AngI and AngII were measured by RIA, and reported values were corrected for the equilibrium rates at this perfusion rate. Basal RIF AngI (0.74 +/- 0.05 nM) and AngII (3.30 +/- 0.17 nM) concentrations were much higher than plasma AngI and AngII concentrations (0.15 +/- 0.01 and 0.14 +/- 0.01 nM, respectively; n = 27). Interstitial infusion of enalaprilat through the microdialysis probe (1 or 10 mM in the perfusate; n = 5 and 8, respectively) significantly increased RIF AngI concentrations but did not significantly alter AngII concentrations. However, perindoprilat (10 mM in the perfusate, n = 7) significantly decreased RIF AngII concentrations by 22 +/- 4% and increased RIF AngI concentrations. Interstitial infusion of AngI (100 nM in the perfusate, n = 7) significantly increased the RIF AngII concentration to 8.26 +/- 0.75 nM, whereas plasma AngI and AngII levels were not affected (0.15 +/- 0.02 and 0.14 +/- 0.02 nM, respectively). Addition of enalaprilat to the perfusate (10 mM) prevented the conversion of exogenously added AngI. These results indicate that addition of AngI in the interstitial compartment leads to low but significant conversion to AngII via ACE activity (blocked by enalaprilat). However, the addition of ACE inhibitors directly into the renal interstitium, via the microdialysis probe, either did not reduce RIF AngII levels or reduced levels by a small fraction of the total basal level, suggesting that much of the RIF AngII is formed at sites not readily accessible to ACE inhibition or is formed via non-ACE-dependent pathways.

The angiotensin-converting enzyme gene family: genomics and pharmacology

Modulation of the renin-angiotensin system (RAS), and particularly inhibition of angiotensin-converting enzyme (ACE), a zinc metallopeptidase, has long been a prime strategy in the treatment of hypertension. However, other angiotensin metabolites are gaining in importance as our understanding of the RAS increases. Recently, genomic approaches have identified the first human homologue of ACE, termed ACEH (or ACE2). ACEH differs in specificity and physiological roles from ACE, which opens a potential new area for discovery biology. The gene that encodes collectrin, a homologue of ACEH, is upregulated in response to renal injury. Collectrin lacks a catalytic domain, which indicates that there is more to ACE-like function than simple peptide hydrolysis.