Urinary ACE Phenotyping as a Research and Diagnostic Tool: Identification of Sex-Dependent ACE Immunoreactivity

BACKGROUND

Angiotensin-converting enzyme (ACE) is highly expressed in renal proximal tubules, but ACE activity/levels in the urine are at least 100-fold lower than in the blood. Decreased proximal tubular ACE has been associated with renal tubular damage in both animal models and clinical studies. Because ACE is shed into urine primarily from proximal tubule epithelial cells, its urinary ACE measurement may be useful as an index of tubular damage.

OBJECTIVE AND METHODOLOGY

We applied our novel approach-ACE phenotyping-to characterize urinary ACE in volunteer subjects. ACE phenotyping includes (1) determination of ACE activity using two substrates (ZPHL and HHL); (2) calculation of the ratio of hydrolysis of the two substrates (ZPHL/HHL ratio); (3) quantification of ACE immunoreactive protein levels; and (4) fine mapping of local ACE conformation with mAbs to ACE.

PRINCIPAL FINDINGS

In normal volunteers, urinary ACE activity was 140-fold less than in corresponding plasma/serum samples and did not differ between males and females. However, urinary ACE immunoreactivity (normalized binding of 25 mAbs to different epitopes) was strongly sex-dependent for the several mAbs tested, an observation likely explained by differences in tissue ACE glycosylation/sialylation between males and females. Urinary ACE phenotyping also allowed the identification of ACE outliers. In addition, daily variability of urinary ACE has potential utility as a feedback marker for dieting individuals pursuing weight loss.

CONCLUSIONS/SIGNIFICANCE

Urinary ACE phenotyping is a promising new approach with potential clinical significance to advance precision medicine screening techniques.

Cryo-EM reveals mechanisms of angiotensin I-converting enzyme allostery and dimerization

Hypertension (high blood pressure) is a major risk factor for cardiovascular disease, which is the leading cause of death worldwide. The somatic isoform of angiotensin I‐converting enzyme (sACE) plays a critical role in blood pressure regulation, and ACE inhibitors are thus widely used to treat hypertension and cardiovascular disease. Our current understanding of sACE structure, dynamics, function, and inhibition has been limited because truncated, minimally glycosylated forms of sACE are typically used for X‐ray crystallography and molecular dynamics simulations. Here, we report the first cryo‐EM structures of full‐length, glycosylated, soluble sACE (sACE^S1211). Both monomeric and dimeric forms of the highly flexible apo enzyme were reconstructed from a single dataset. The N‐ and C‐terminal domains of monomeric sACE^S1211 were resolved at 3.7 and 4.1 Å, respectively, while the interacting N‐terminal domains responsible for dimer formation were resolved at 3.8 Å. Mechanisms are proposed for intradomain hinging, cooperativity, and homodimerization. Furthermore, the observation that both domains were in the open conformation has implications for the design of sACE modulators.

Novel Therapeutic Approaches Targeting the Renin-Angiotensin System and Associated Peptides in Hypertension and Heart Failure

Despite the success of renin-angiotensin system (RAS) blockade by angiotensin-converting enzyme (ACE) inhibitors and angiotensin II type 1 receptor (AT1R) blockers, current therapies for hypertension and related cardiovascular diseases are still inadequate. Identification of additional components of the RAS and associated vasoactive pathways, as well as new structural and functional insights into established targets, have led to novel therapeutic approaches with the potential to provide improved cardiovascular protection and better blood pressure control and/or reduced adverse side effects. The simultaneous modulation of several neurohumoral mediators in key interconnected blood pressure-regulating pathways has been an attractive approach to improve treatment efficacy, and several novel approaches involve combination therapy or dual-acting agents. In addition, increased understanding of the complexity of the RAS has led to novel approaches aimed at upregulating the ACE2/angiotensin-(1-7)/Mas axis to counter-regulate the harmful effects of the ACE/angiotensin II/angiotensin III/AT1R axis. These advances have opened new avenues for the development of novel drugs targeting the RAS to better treat hypertension and heart failure. Here we focus on new therapies in preclinical and early clinical stages of development, including novel small molecule inhibitors and receptor agonists/antagonists, less conventional strategies such as gene therapy to suppress angiotensinogen at the RNA level, recombinant ACE2 protein, and novel bispecific designer peptides.

Interacting cogs in the machinery of the renin angiotensin system

Somatic angiotensin converting enzyme (sACE) is well-known for its role in blood pressure regulation and consequently, ACE inhibitors are widely prescribed for the treatment of hypertension. More than 60 years after the discovery of sACE, however, the molecular details of its substrate hydrolysis and inhibition are still poorly understood. Isothermal titration calorimetry, molecular dynamics simulations and fine epitope mapping suggest that substrate or inhibitor binding triggers a hinging motion between the two subdomains of each domain. Ligand binding to one domain further induces a conformational change in sACE to negatively affect the second domain's function and can also cause dimerization between sACE molecules. This has been linked to an increase in sACE expression via intracellular signalling. Inhibitor-induced dimerization could thus decrease the efficacy of hypertension treatment. At present, the only structural information available for sACE are crystal structures of the truncated domains in the closed conformation due to the presence of ligands. These structures do not provide any information regarding the open active site conformation prior to ligand binding, the relative orientation of the two domains in full-length sACE, or the dimerization interface. To guarantee effective therapeutic intervention, further research is required to investigate the hinging, negative cooperativity and dimerization of sACE. This review describes our current understanding of these interactions and proposes how recent advances in cryo-electron microscopy could enable structural elucidation of their mechanisms.

Advances in Structural Biology of ACE and Development of Domain Selective ACE-inhibitors

BACKGROUND

The Angiotensin-I converting enzyme (ACE) is one of the most important components of the renin-angiotensin-aldosterone system controlling blood pressure and renal functions. Inhibitors of ACE are first line therapeutics used in the treatment of hypertension and related cardiovascular diseases. Somatic ACE consists of two homologous catalytic domains, the C- and N-domains. Recent findings have shown that although both domains are highly homologous in structure, they may have different physiological functions. The C-domain is primarily involved in the control of blood pressure, in contrast to the N-domain that is engaged in the regulation of hematopoietic stem cell proliferation. The currently available ACE inhibitors have some adverse effects that can be attributed to the non-selective inhibition of both domains. In addition, specific Ndomain inhibitors have emerged as potential antifibrotic drugs. Therefore, ACE is still an important drug target for the development of novel domain-selective drugs not only for the cardiovascular system but also for other systems.

OBJECTIVE

Detailed structural information about interactions in the protein-ligand complex is crucial for rational drug design. This review highlights the structural information available from crystallographic data which is essential for the development of domain selective inhibitors of ACE.

METHODS

Over eighty crystal complexes of ACE are placed into the Protein Database. An overview of X-ray ACE complexes with various inhibitors in C- and N-domains and an analysis of their binding mode have given mechanistic explanation of the structural determinants of selective ligand binding. In addition, ACE domain selective inhibitors with dual modes of action in complexes with ACE are also discussed.

CONCLUSION

Selectivity of ACE inhibitors for the N- and C-domain is controlled by subtle differences in the amino-acids forming the active site. Reported studies of crystal complexes of inhibitors in the C- and N-domains revealed that most selective inhibitors interact with non-conserved amino-acids between domains and have distinct interactions with the residues in the S2 and S2' subsites of the ACE catalytic site. Moreover, unusual binding of the second molecule of inhibitors in the binding cavity opens new possibilities of exploiting more distant regions of the catalytic center in structure-based design of novel drugs.

The influence of angiotensin converting enzyme mutations on the kinetics and dynamics of N-domain selective inhibition

Angiotensin-1-converting enzyme (ACE) is a zinc metalloprotease that plays a major role in blood pressure regulation via the renin-angiotensin-aldosterone system. ACE consists of two domains with differences in inhibitor binding affinities despite their 90% active site identity. While the C-domain primarily controls blood pressure, the N-domain is selective for cleavage of the antifibrotic N-acetyl-Ser-Asp-Lys-Pro. Inhibitors, such as 33RE, that selectively bind to the N-domain thus show potential for treating fibrosis without affecting blood pressure. The aim of this study was to elucidate the molecular mechanism of this selectivity. ACE inhibition by 33RE was characterized using a continuous kinetic assay with fluorogenic substrate. The N-domain displayed nanomolar (K_i = 11.21 ± 0.74 nm) and the C-domain micromolar (K_i = 11 278 ± 410 nm) inhibition, thus 1000-fold selectivity. Residues predicted to contribute to selectivity based on the N-domain-33RE co-crystal structure were subsequently mutated to their C-domain counterparts. S₂ subsite mutation with resulting loss of a hydrogen bond drastically decreased the affinity (K_i = 2 794 ± 156 nm), yet did not entirely account for selectivity. Additional substitution of all unique S₂ ' residues, however, completely abolished selectivity (K_i = 10 009 ± 157 nm). Interestingly, these residues do not directly bind 33RE. All mutants were therefore subjected to molecular dynamics simulations in the presence and absence of 33RE. Trajectory analyses highlighted the importance of these S₂ ' residues in formation of a favourable interface between the ACE subdomains and thus a closed, ligand-bound complex. This study provides a molecular basis for the intersubsite synergism governing 33RE's 1000-fold N-selectivity and aids the future design of novel inhibitors for fibrosis treatment.

ENZYMES

Angiotensin converting enzyme (ACE, EC 3.4.15.1).

A modern understanding of the traditional and nontraditional biological functions of angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is a zinc-dependent peptidase responsible for converting angiotensin I into the vasoconstrictor angiotensin II. However, ACE is a relatively nonspecific peptidase that is capable of cleaving a wide range of substrates. Because of this, ACE and its peptide substrates and products affect many physiologic processes, including blood pressure control, hematopoiesis, reproduction, renal development, renal function, and the immune response. The defining feature of ACE is that it is composed of two homologous and independently catalytic domains, the result of an ancient gene duplication, and ACE-like genes are widely distributed in nature. The two ACE catalytic domains contribute to the wide substrate diversity of ACE and, by extension, the physiologic impact of the enzyme. Several studies suggest that the two catalytic domains have different biologic functions. Recently, the X-ray crystal structure of ACE has elucidated some of the structural differences between the two ACE domains. This is important now that ACE domain-specific inhibitors have been synthesized and characterized. Once widely available, these reagents will undoubtedly be powerful tools for probing the physiologic actions of each ACE domain. In turn, this knowledge should allow clinicians to envision new therapies for diseases not currently treated with ACE inhibitors.

Apoplast proteome reveals that extracellular matrix contributes to multistress response in poplar

BACKGROUND

Riverine ecosystems, highly sensitive to climate change and human activities, are characterized by rapid environmental change to fluctuating water levels and siltation, causing stress on their biological components. We have little understanding of mechanisms by which riverine plant species have developed adaptive strategies to cope with stress in dynamic environments while maintaining growth and development.

RESULTS

We report that poplar (Populus spp.) has evolved a systems level "stress proteome" in the leaf-stem-root apoplast continuum to counter biotic and abiotic factors. To obtain apoplast proteins from P. deltoides, we developed pressure-chamber and water-displacement methods for leaves and stems, respectively. Analyses of 303 proteins and corresponding transcripts coupled with controlled experiments and bioinformatics demonstrate that poplar depends on constitutive and inducible factors to deal with water, pathogen, and oxidative stress. However, each apoplast possessed a unique set of proteins, indicating that response to stress is partly compartmentalized. Apoplast proteins that are involved in glycolysis, fermentation, and catabolism of sucrose and starch appear to enable poplar to grow normally under water stress. Pathogenesis-related proteins mediating water and pathogen stress in apoplast were particularly abundant and effective in suppressing growth of the most prevalent poplar pathogen Melampsora. Unexpectedly, we found diverse peroxidases that appear to be involved in stress-induced cell wall modification in apoplast, particularly during the growing season. Poplar developed a robust antioxidative system to buffer oxidation in stem apoplast.

CONCLUSION

These findings suggest that multistress response in the apoplast constitutes an important adaptive trait for poplar to inhabit dynamic environments and is also a potential mechanism in other riverine plant species.

The N domain of human angiotensin-I-converting enzyme: the role of N-glycosylation and the crystal structure in complex with an N domain-specific phosphinic inhibitor, RXP407

Angiotensin-I-converting enzyme (ACE) plays a critical role in the regulation of blood pressure through its central role in the renin-angiotensin and kallikrein-kinin systems. ACE contains two domains, the N and C domains, both of which are heavily glycosylated. Structural studies of ACE have been fraught with severe difficulties because of surface glycosylation of the protein. In order to investigate the role of glycosylation in the N domain and to create suitable forms for crystallization, we have investigated the importance of the 10 potential N-linked glycan sites using enzymatic deglycosylation, limited proteolysis, and mass spectrometry. A number of glycosylation mutants were generated via site-directed mutagenesis, expressed in CHO cells, and analyzed for enzymatic activity and thermal stability. At least eight of 10 of the potential glycan sites are glycosylated; three C-terminal sites were sufficient for expression of active N domain, whereas two N-terminal sites are important for its thermal stability. The minimally glycosylated Ndom389 construct was highly suitable for crystallization studies. The structure in the presence of an N domain-selective phosphinic inhibitor RXP407 was determined to 2.0 Å resolution. The Ndom389 structure revealed a hinge region that may contribute to the breathing motion proposed for substrate binding.

Angiotensin I-converting enzyme Gln1069Arg mutation impairs trafficking to the cell surface resulting in selective denaturation of the C-domain

Background

Angiotensin-converting enzyme (ACE; Kininase II; CD143) hydrolyzes small peptides such as angiotensin I, bradykinin, substance P, LH-RH and several others and thus plays a key role in blood pressure regulation and vascular remodeling. Complete absence of ACE in humans leads to renal tubular dysgenesis (RTD), a severe disorder of renal tubule development characterized by persistent fetal anuria and perinatal death.

Methodology/Principal Findings

Patient with RTD in Lisbon, Portugal, maintained by peritoneal dialysis since birth, was found to have a homozygous substitution of Arg for Glu at position 1069 in the C-terminal domain of ACE (Q1069R) resulting in absence of plasma ACE activity; both parents and a brother who are heterozygous carriers of this mutation had exactly half-normal plasma ACE activity compared to healthy individuals. We hypothesized that the Q1069R substitution impaired ACE trafficking to the cell surface and led to accumulation of catalytically inactive ACE in the cell cytoplasm. CHO cells expressing wild-type (WT) vs. Q1069R-ACE demonstrated the mutant accumulates intracellularly and also that it is significantly degraded by intracellular proteases. Q1069R-ACE retained catalytic and immunological characteristics of WT-ACE N domain whereas it had 10–20% of the nativity of the WT-ACE C domain. A combination of chemical (sodium butyrate) or pharmacological (ACE inhibitor) chaperones with proteasome inhibitors (MG 132 or bortezomib) significantly restored trafficking of Q1069R-ACE to the cell surface and increased ACE activity in the cell culture media 4-fold.

Conclusions/Significance

Homozygous Q1069R substitution results in an ACE trafficking and processing defect which can be rescued, at least in cell culture, by a combination of chaperones and proteasome inhibitors. Further studies are required to determine whether similar treatment of individuals with this ACE mutation would provide therapeutic benefits such as concentration of primary urine.