Characterization of mouse angiogenin-related protein: implications for functional studies on angiogenin

Human and mouse angiogenins: Emerging insights and potential opportunities

Angiogenin, a well-known angiogenic factor, is crucial to the angiogenesis in gastrointestinal tumors. Human angiogenin has only one gene, whereas the murine angiogenin family has extended to incorporate six genes. Evolutionary studies have suggested functional variations among murine angiogenin paralogs, even though the three-dimensional structures of angiogenin proteins are remarkably similar. In addition to angiogenesis, the ubiquitous pattern of angiogenin expression suggests a variety of functions, such as tumorigenesis, neuroprotective, antimicrobial activity, and innate immunity. Here, we comprehensively reviewed studies on the structures and functions of human and mouse angiogenins. Understanding the structure and function of angiogenins from a broader perspective could facilitate future research related to development of novel therapeutics on its biological processes, especially in gastrointestinal cancers.

SLFN2 protection of tRNAs from stress-induced cleavage is essential for T cell-mediated immunity

Reactive oxygen species (ROS) increase in activated T cells because of metabolic activity induced to support T cell proliferation and differentiation. We show that these ROS trigger an oxidative stress response that leads to translation repression. This response is countered by Schlafen 2 (SLFN2), which directly binds transfer RNAs (tRNAs) to protect them from cleavage by the ribonuclease angiogenin. T cell-specific SLFN2 deficiency results in the accumulation of tRNA fragments, which inhibit translation and promote stress-granule formation. Interleukin-2 receptor β (IL-2Rβ) and IL-2Rγ fail to be translationally up-regulated after T cell receptor stimulation, rendering SLFN2-deficient T cells insensitive to interleukin-2's mitogenic effects. SLFN2 confers resistance against the ROS-mediated translation-inhibitory effects of oxidative stress normally induced by T cell activation, permitting the robust protein synthesis necessary for T cell expansion and immunity.

Attenuated portal hypertension in germ-free mice: Function of bacterial flora on the development of mesenteric lymphatic and blood vessels

Intestinal bacterial flora may induce splanchnic hemodynamic and histological alterations that are associated with portal hypertension (PH). We hypothesized that experimental PH would be attenuated in the complete absence of intestinal bacteria. We induced prehepatic PH by partial portal vein ligation (PPVL) in germ-free (GF) or mice colonized with altered Schaedler's flora (ASF). After 2 or 7 days, we performed hemodynamic measurements, including portal pressure (PP) and portosystemic shunts (PSS), and collected tissues for histomorphology, microbiology, and gene expression studies. Mice colonized with intestinal microbiota presented significantly higher PP levels after PPVL, compared to GF, mice. Presence of bacterial flora was also associated with significantly increased PSS and spleen weight. However, there were no hemodynamic differences between sham-operated mice in the presence or absence of intestinal flora. Bacterial translocation to the spleen was demonstrated 2 days, but not 7 days, after PPVL. Intestinal lymphatic and blood vessels were more abundant in colonized and in portal hypertensive mice, as compared to GF and sham-operated mice. Expression of the intestinal antimicrobial peptide, angiogenin-4, was suppressed in GF mice, but increased significantly after PPVL, whereas other angiogenic factors remained unchanged. Moreover, colonization of GF mice with ASF 2 days after PPVL led to a significant increase in intestinal blood vessels, compared to controls. The relative increase in PP after PPVL in ASF and specific pathogen-free mice was not significantly different.

CONCLUSION

In the complete absence of gut microbial flora PP is normal, but experimental PH is significantly attenuated. Intestinal mucosal lymphatic and blood vessels induced by bacterial colonization may contribute to development of PH.

Crystal structures of murine angiogenin-2 and -3-probing 'structure--function' relationships amongst angiogenin homologues

Angiogenin (Ang) is a potent inducer of neovascularization. Point mutations in human Ang have been linked to cancer progression and two neurodegenerative diseases: amyotrophic lateral sclerosis and Parkinson's disease. Intensive structural and functional analyses of Ang have been paramount in assigning functions to this novel homologue of bovine pancreatic RNase A. However, inhibitor-binding studies with crystalline Ang (for designing potential anti-cancer drugs) have been hampered as a result of the inaccessibility of the active site. Experiments with the murine homologues of Ang have not only overcome the obvious practical limitations encountered when studying the role of a human protein in healthy individuals, but also the crystal structures of murine angiogenins (mAng and mAng-4) have revealed themselves to have greater potential for the visualization of small-molecule inhibitor binding at the active site. In the present study, we report the crystal structures of two more murine Ang paralogues, mAng-2 and mAng-3, at 1.6 and 1.8 Å resolution, respectively. These constitute the first crystal structures of an Ang with a zinc ion bound at the active site and provide some insight into the possible mode of inhibition of the ribonucleolytic activity of the enzyme by these divalent cations. Both structures show that the residues forming the putative P₁, B₁ and B₂ subsites occupy positions similar to their counterparts in human Ang and are likely to have conserved roles. However, a less obtrusive conformation of the C-terminal segment in mAng-3 and the presence of a sulfate ion in the B₁ subsite of mAng-2 suggest that these proteins have the potential to be used for inhibitor-binding studies. We also discuss the biological relevance of the structural similarities and differences between the different Ang homologues.

Mutational dynamics of murine angiogenin duplicates

Background

Angiogenin (Ang) is a protein involved in angiogenesis by inducing the formation of blood vessels. The biomedical importance of this protein has come from findings linking mutations in Ang to cancer progression and neurodegenerative diseases. These findings highlight the evolutionary constrain on Ang amino acid sequence. However, previous studies comparing human Angiogenin with homologs from other phylogenetically related organisms have led to the conclusion that Ang presents a striking variability. Whether this variability has an adaptive value per se remains elusive. Understanding why many functional Ang paralogs have been preserved in mouse and rat and identifying functional divergence mutations at these copies may explain the relationship between mutations and function. In spite of the importance of testing this hypothesis from the evolutionarily and biomedical perspectives, this remains yet unaccomplished. Here we test the main mutational dynamics driving the evolution and function of Ang paralogs in mammals.

Results

We analysed the phylogenetic asymmetries between the different Ang gene copies in mouse and rat in the context of vertebrate Ang phylogeny. This analysis shows strong evidence in support of accelerated evolution in some Ang murine copies (mAng). This acceleration is not due to non-functionalisation because constraints on amino acid replacements remain strong. We identify many of the amino acid sites involved in signal localization and nucleotide binding by Ang to have evolved under diversifying selection. Compensatory effects of many of the mutations at these paralogs and their key structural location in or nearby important functional regions support a possible functional shift (functional divergence) in many Ang copies. Similarities between 3D-structural models for mAng copies suggest that their divergence is mainly functional.

Conclusions

We identify the main evolutionary dynamics shaping the variability of Angiogenin in vertebrates and highlight the plasticity of this protein after gene duplication. Our results suggest functional divergence among mAng paralogs. This puts forward mAng as a good system candidate for testing functional plasticity of such an important protein while stresses caution when using mouse as a model to infer the consequences of mutations in the single Ang copy of humans.

A new role for angiogenin in neurite growth and pathfinding: implications for amyotrophic lateral sclerosis

Mutations in human angiogenin (hANG), an angiogenic member of the RNase A superfamily, have been recently reported in patients with amyotrophic lateral sclerosis (ALS), a progressive late-onset neurodegenerative disorder. However, very little is known about the expression and subcellular distribution of ANG in the nervous system or its role in differentiation. Here we report that mouse angiogenin-1 (mAng-1) is strongly expressed in the developing nervous system during mouse embryogenesis and neuroectodermal differentiation of pluripotent P19 embryonal carcinoma cells. mAng1 is strongly expressed in motor neurons (MNs) in the spinal cord and dorsal root ganglia as well as in post-mitotic MNs derived from P19 cells. We also show for the first time that ANG expression is in the growth cones and neurites. NCI 65828, an inhibitor of the ribonucleolytic activity of hANG, affected pathfinding by P19-derived neurons but not neuronal differentiation. Our findings clearly show that ANG plays an important role in neurite pathfinding and this has implications for ALS.

Structure of murine angiogenin: features of the substrate- and cell-binding regions and prospects for inhibitor-binding studies

Angiogenin is an unusual member of the pancreatic ribonuclease superfamily that induces blood-vessel formation and is a promising anticancer target. The three-dimensional structure of murine angiogenin (mAng) has been determined by X-ray crystallography. Two structures are presented: one is a complex with sulfate ions (1.5 Angstroms resolution) and the other a complex with phosphate ions (1.6 Angstroms resolution). Residues forming the putative B(1), P(1) and B(2) subsites occupy positions similar to their hAng counterparts and are likely to play similar roles. The anions occupy the P(1) subsite, sulfate binding conventionally and phosphate adopting two orientations, one of which is novel. The B(1) subsite is obstructed by Glu116 and Phe119, with the latter assuming a less invasive position than its hAng counterpart. Hydrophobic interactions between the C-terminal segment and the main body of the protein are more extensive than in hAng and may underly the lower enzymatic activity of the murine protein. Elsewhere, the structure of the H3-B2 loop supports the view that hAng Asn61 interacts directly with cell-surface molecules and does not merely stabilize adjacent regions of the hAng structure. mAng crystals appear to offer small-molecule inhibitors a clear route to the active site and may even withstand a reorientation of the C-terminal segment that provides access to the cryptic B(1) subsite. These features represent considerable advantages over crystalline hAng and bAng.

Regeneration in MRL mice: further genetic loci controlling the ear hole closure trait using MRL and M.m. Castaneus mice

The MRL mouse has been shown to display an epimorphic regenerative response after ear hole punching leading to complete closure within 30 days and cartilage regrowth. The regenerative capacity of the MRL has also been seen after a severe cryoinjury to the heart leads to complete healing without scarring and functional myocardium. The wound healing ear hole closure response that occurs in MRL mice has been shown to be genetically controlled. We have previously identified 11 quantitative trait loci (QTL) that govern healing in an intercross of (MRL x C57BL/6 J) mice. However, it is desirable to use another poorly healing mouse strain to elucidate the full range of genetic factors that affect this important process. In the current study, we have used an inbred subspecies of the mouse, M. castaneus, and have confirmed a number of loci identified previously. In addition, we report three new healing QTL. Furthermore, in this strain combination, we note a strong sexual dimorphism also observed in the MRL x C57BL/6 cross, both in the healing trait and in the QTL that control it.

Pseudogenization of the tumor-growth promoter angiogenin in a leaf-eating monkey

Physiological functions of human genes may be studied by gene-knockout experiments in model organisms such as the mouse. This strategy relies on the existence of one-to-one gene orthology between the human and mouse. When lineage-specific gene duplication occurs and paralogous genes share a certain degree of functional redundancy, knockout mice may not provide accurate functional information on human genes. Angiogenin is a small protein that stimulates blood-vessel growth and promotes tumor development. Humans and related primates only have one angiogenin gene, while mice have three paralogous genes. This makes it difficult to generate angiogenin-knockout mice and even more difficult to interpret the genotype-phenotype relation from such animals should they be generated. We here show that in the douc langur (Pygathrix nemaeus), an Asian leaf-eating colobine monkey, the single-copy angiogenin gene has a one-nucleotide deletion in the sixth codon of the mature peptide, generating a premature stop codon. This nucleotide deletion is found in five unrelated individuals sequenced, and therefore is likely to have been fixed in the species. Five colobine species that are closely related to the douc langur have intact angiogenin genes, suggesting that the pseudogenization event was recent and unique to the douc langur lineage. This natural knockout experiment suggests that primate angiogenin is dispensable even in the wild. Further physiological studies of douc langurs may offer additional information on the role of this cancer-related gene in normal physiology of primates, including humans. Our findings also provide a strong case for the importance of evolutionary analysis in biomedical studies of gene functions.

Angiogenins: a new class of microbicidal proteins involved in innate immunity

Although angiogenins have been implicated in tumor-associated angiogenesis, their normal physiologic function remains unclear. We show that a previously uncharacterized angiogenin, Ang4, is produced by mouse Paneth cells, is secreted into the gut lumen and has bactericidal activity against intestinal microbes. Ang4 expression is induced by Bacteroides thetaiotaomicron, a predominant member of the gut microflora, revealing a mechanism whereby intestinal commensal bacteria influence gut microbial ecology and shape innate immunity. Furthermore, mouse Ang1 and human angiogenin, circulating proteins induced during inflammation, exhibit microbicidal activity against systemic bacterial and fungal pathogens, suggesting that they contribute to systemic responses to infection. These results establish angiogenins as a family of endogenous antimicrobial proteins.

Diversifying selection of the tumor-growth promoter angiogenin in primate evolution

Diversifying selection drives the rapid differentiation of gene sequences and is one of the main forces behind adaptive evolution. Most genes known to be shaped by diversifying selection are those involved in host-pathogen or male-female interactions characterized as molecular "arms races." Here we report the unexpected detection of diversifying selection in the evolution of a tumor-growth promoter, angiogenin (ANG). A comparison among 11 primate species demonstrates that ANG has a significantly higher rate of nucleotide substitution at nonsynonymous sites than at synonymous sites, a hallmark of positive selection acting at the molecular level. Furthermore, we observed significant charge diversity at the molecular surface, suggesting the presence of selective pressures in the microenvironment of ANG, including its binding molecules. A population survey of ANG in chimpanzees, however, reveals no polymorphism, which may have resulted from a recent selective sweep of a charge-altering substitution in chimpanzee evolution. Functional assays of recombinant ANGs from the human and owl monkey indicate that primate ANGs retain angiogenic activity despite rapid evolution. Our study, together with findings of similar selection in the primate breast cancer suppressor gene, BRCA1, reveals an intriguing phenomenon of unusual selective pressures on, and adaptive evolution of, cancer-related genes in primate evolution.

Serum angiogenin concentrations in young patients with diabetes mellitus

BACKGROUND

Angiogenin serum levels were measured in a large group of type 1 diabetic young patients, looking at whether increased Angiogenin concentrations are associated with long-term glycemic control and microvascular complications.

MATERIALS AND METHODS

Four groups of patients were compared to 223 age- and sex- matched healthy controls: 196 type 1 diabetic patients (age range 3-24 years, onset of diabetes before the age of 12 years; duration of disease longer than 2 years), without microvascular complications were divided into three groups on the basis of age (group 1, n = 37, age < 6 years; group 2, n = 71, age 6-12 years; group 3, n = 88, age > 12 years); 53 adolescents and young adults (age 16.1-29.7 years) with diabetic microvascular complications (background, preproliferative or proliferative retinopathy, albumin excretion rate 20-200 microg min-1) (group 4).

RESULTS

Angiogenin serum levels were significantly increased in diabetic pre-school and pre-pubertal children, and particularly elevated in pubertal subjects compared with age- and sex-matched controls. Adolescents and young adults with microvascular complications had very high angiogenin concentrations. One-year mean HbA1c values were correlated with angiogenin levels (r = 0.389; p < 0.01). In poorly controlled diabetics (HbA1c > 10%), long-term (2 years) improvement of glycemic control determined a significant reduction of angiogenin concentrations in both pre-school and pre-pubertal children as well as in pubertal youngsters.

CONCLUSIONS

Angiogenin serum concentrations are increased in diabetic children even before puberty. Severity of microvascular complications is associated with markedly increased angiogenin serum levels. Long-term tight glycemic control determines a consistent reduction of angiogenin concentrations.

High-level expression of three members of the murine angiogenin family in Escherichia coli and purification of the recombinant proteins

Angiogenin (Ang) is a small basic protein which belongs to the pancreatic ribonuclease superfamily. It potently induces the formation of new blood vessels and has emerged as a promising anticancer target. Mice possess genes encoding one ortholog (mAng) and three homologs of Ang, designated angiogenin-related protein (mAngrp), angiogenin-3 (mAng-3), and angiogenin-4 (mAng-4). Structural and functional study of these homologs has been hampered by the low yield of protein from the existing heterologous expression system. In the experiments described, we used a pET expression vector to express these proteins in the cytoplasm of Escherichia coli BL21-CodonPlus(DE3)-RIL cells, whereupon substantial amounts of each accumulated in the form of insoluble aggregates. The proteins were renatured using an arginine-assisted procedure and subsequently purified by cation-exchange chromatography and reversed-phase HPLC; each purified protein was shown to be enzymatically active toward tRNA. The yields of pure mAngrp and mAng-3 were 7.6 and 12 mg/liter culture, respectively, representing substantial increases over previously reported experiments. This is also the first report of the expression and purification of mAng-4, obtained here in a yield of 30 mg/liter culture. The ready availability of milligram quantities of these proteins will enable further functional studies and high-resolution structural analyses to be conducted.

Molecular analysis of commensal host-microbial relationships in the intestine

Human beings contain complex societies of indigenous microbes, yet little is known about how resident bacteria shape our physiology. We colonized germ-free mice with Bacteroides thetaiotaomicron, a prominent component of the normal mouse and human intestinal microflora. Global intestinal transcriptional responses to colonization were observed with DNA microarrays, and the cellular origins of selected responses were established by laser-capture microdissection. The results reveal that this commensal bacterium modulates expression of genes involved in several important intestinal functions, including nutrient absorption, mucosal barrier fortification, xenobiotic metabolism, angiogenesis, and postnatal intestinal maturation. These findings provide perspectives about the essential nature of the interactions between resident microorganisms and their hosts.

Refined crystal structures of native human angiogenin and two active site variants: implications for the unique functional properties of an enzyme involved in neovascularisation during tumour growth

Human angiogenin (Ang), an unusual member of the pancreatic RNase superfamily, is a potent inducer of angiogenesis in vivo. Its ribonucleolytic activity is weak (10(4) to 10(6)-fold lower than that of bovine RNase A), but nonetheless seems to be essential for biological function. Ang has been implicated in the establishment of a wide range of human tumours and has therefore emerged as an important target for the design of new anti-cancer compounds. We report high-resolution crystal structures for native Ang in two different forms (Pyr1 at 1.8 A and Met-1 at 2.0 A resolution) and for two active-site variants, K40Q and H13A, at 2.0 A resolution. The native structures, together with earlier mutational and biochemical data, provide a basis for understanding the unique functional properties of this molecule. The major structural features that underlie the weakness of angiogenin's RNase activity include: (i) the obstruction of the pyrimidine-binding site by Gln117; (ii) the existence of a hydrogen bond between Thr44 and Thr80 that further suppresses the effectiveness of the pyrimidine site; (iii) the absence of a counterpart for the His119-Asp121 hydrogen bond that potentiates catalysis in RNase A (the corresponding aspartate in Ang, Asp116, has been recruited to stabilise the blockage of the pyrimidine site); and (iv) the absence of any precise structural counterparts for two important purine-binding residues of RNase A. Analysis of the native structures has revealed details of the cell-binding region and nuclear localisation signal of Ang that are critical for angiogenicity. The cell-binding site differs dramatically from the corresponding regions of RNase A and two other homologues, eosinophil-derived neurotoxin and onconase, all of which lack angiogenic activity. Determination of the structures of the catalytically inactive variants K40Q and H13A has now allowed a rigorous assessment of the relationship between the ribonucleolytic and biological activities of Ang. No significant change outside the enzymatic active site was observed in K40Q, establishing that the loss of angiogenic activity for this derivative is directly attributable to disruption of the catalytic apparatus. The H13A structure shows some changes beyond the ribonucleolytic site, but sites involved in cell-binding and nuclear translocation are essentially unaffected by the amino acid replacement.

mAngiogenin-3, a target gene of oncoprotein E2a-Pbx1, encodes a new angiogenic member of the angiogenin family

Angiogenins are proteins in the pancreatic ribonuclease superfamily that utilize their ribonuclease activity to induce formation of new blood vessels. Recently we identified a new member of the angiogenin gene family, mouse angiogenin-3, by virtue of its transcriptional activation in NIH3T3 fibroblasts coincident with transformation by the chimeric leukemia oncogene, E2a-Pbx1. Here we have isolated the cDNA encoding mouse angiogenin-3 and used it to produce the protein in E. coli. We demonstrate that mouse angiogenin-3 is a ribonuclease whose activity and specificity towards tRNA and dinucleotide substrates differ from those of mouse angiogenin or of mouse angiogenin-related protein, a non-angiogenic factor. Mouse angiogenin-3 induced angiogenesis in both the chicken embryo chorioallantoic membrane assay and the rat cremaster muscle. Electron microscopy revealed that endothelial cells within vessels induced by both mouse angiogenin-3 and mouse angiogenin contain fenestrations similar to those observed in endothelial cells from neovasculature induced by vascular endothelial growth factor and basic fibroblast growth factor. Mouse angiogenin-3 also induced other molecular events typical of rapidly proliferating endothelial cells, such as increases in rough endoplasmic reticulum, polysomes, and mitochondria.

Serum angiogenin levels in children and adolescents with insulin-dependent diabetes mellitus

Microangiopathy, one of the most important complications of diabetes mellitus in humans, is associated with increased angiogenic response and proliferative lesions in various organs. Angiogenin, a polypeptide with a molecular size of 14 kD, is a potent inducer of vascular growth. This study aimed at investigating whether serum angiogenin levels are elevated in children and adolescents (youngsters) with insulin-dependent diabetes mellitus and whether angiogenin levels are affected by duration and metabolic control of the disease. It is assumed that angiogenin levels reflect the increased angiogenesis associated with microangiopathy, whether clinically evident or not. Forty diabetic youngsters were compared with 30 healthy control subjects (mean age +/- SD, 14.3 +/- 3.6 y and 13.8 +/- 3.6 y, respectively). The patients' disease duration and glycosylated Hb were (mean +/- SD) 6.2 +/- 3.8 y and 9.6 +/- 1.8%, respectively. Angiogenin (ng/mL) was measured in serum samples by an enzyme immunoassay and was found to be significantly higher (mean +/- SE) in patients (353.3 +/- 20.0) than in control subjects (244.7 +/- 9.6) (p = 0.0002). Levels did not vary with age, but were significantly higher in females compared with male subjects (p = 0.01). In the diabetic youngsters no significant differences were noticed with respect to duration or metabolic control of the disease. In conclusion, serum angiogenin levels were found to be increased among diabetic youngsters, irrespective of the duration and metabolic control of the disease, as well as in female subjects, with or without diabetes.

An angiogenic protein from bovine serum and milk--purification and primary structure of angiogenin-2

Bovine serum and milk contain a basic angiogenic protein that binds tightly to placental ribonuclease inhibitor. It was purified from both sources by ion-exchange and reversed-phase chromatographies. Its amino acid sequence revealed that it is a member of the ribonuclease superfamily. It contains 123 amino acids in a single polypeptide chain, is cross-linked by three disulfide bonds, is glycosylated at Asn33, and is 57% identical to bovine angiogenin. The amino-terminal and carboxyl-terminal residues are pyroglutamic acid and proline, respectively. The protein has ribonucleolytic activity that is similar to, but somewhat lower than, that of bovine angiogenin, i.e. very low relative to RNase. It is angiogenically potent on chicken chorioallantoic membrane, but less so than angiogenin. The sequence and activities demonstrate that this protein is a second, distinct, member of the angiogenin sub-family of pancreatic ribonucleases, and is referred to as angiogenin-2.

Changes of angiogenin serum concentrations in the perinatal period

The polypeptide angiogenin, a normal constituent of human plasma, might be involved in endothelium homeostasis, angiogenesis, and neovascularization accompanying various diseases. This study aimed at determining angiogenin serum concentrations in the perinatal period of healthy newborns and at forming a baseline for this protein, which in the future may serve as a diagnostic index in developmental errors of the placenta and/or newborn. One milliliter of blood was drawn on d 1 and 4 of life from 30 healthy full-term neonates, and angiogenin serum concentrations were measured by an enzyme immunoassay using a commercially available kit. In 10 cases angiogenin serum concentrations were also measured in the maternal serum before delivery and in the umbilical vein serum. Angiogenin serum concentrations (microgram/L) were significantly higher in maternal serum (225.7 +/- 49.6) compared with umbilical vein serum (119.0 +/- 34.2) (p < 0.0002), as well as that compared with day 1 (166.4 +/- 44.9) (p < 0.01) but not to d 4 neonatal serum (240.8 +/- 52.6). Angiogenin serum concentrations showed a statistically significant increase from d 1 to 4 (p < 10(-7)), as well as from umbilical cord serum to d 1 neonatal serum (p < 0.0002). A statistically significant correlation existed between values in umbilical cord serum and d 1 neonatal serum (r = 0.84, n = 10, p < 0.002) and between those in d 1 and 4 neonatal serum (r = 0.37, n = 30, p < 0.04). Sex, birth weight, or mode of delivery did not influence angiogenin serum concentrations. We conclude that a rapid increase of angiogenin serum concentrations to maternal levels takes place during the first four postnatal days in healthy full-term neonates.