Identification and characterization of testis specific ornithine decarboxylase antizyme (OAZ-t) gene: expression in haploid germ cells and polyamine-induced frameshifting

The role of polyamine metabolism in remodeling immune responses and blocking therapy within the tumor immune microenvironment

The study of metabolism provides important information for understanding the biological basis of cancer cells and the defects of cancer treatment. Disorders of polyamine metabolism is a common metabolic change in cancer. With the deepening of understanding of polyamine metabolism, including molecular functions and changes in cancer, polyamine metabolism as a new anti-cancer strategy has become the focus of attention. There are many kinds of polyamine biosynthesis inhibitors and transport inhibitors, but not many drugs have been put into clinical application. Recent evidence shows that polyamine metabolism plays essential roles in remodeling the tumor immune microenvironment (TIME), particularly treatment of DFMO, an inhibitor of ODC, alters the immune cell population in the tumor microenvironment. Tumor immunosuppression is a major problem in cancer treatment. More and more studies have shown that the immunosuppressive effect of polyamines can help cancer cells to evade immune surveillance and promote tumor development and progression. Therefore, targeting polyamine metabolic pathways is expected to become a new avenue for immunotherapy for cancer.

Ribosome-associated quality control mediates degradation of the premature translation termination product Orf1p of ODC antizyme mRNA

Decoding of OAZ1 (Ornithine decarboxylase AntiZyme 1) mRNA, which harbours two open reading frames (ORF1 and ORF2) interrupted by a naturally occurring Premature Termination Codon (PTC), produces an 8 kDa truncated polypeptide termed Orf1p, unless the PTC is bypassed by +1 ribosomal frameshifting. In this study, we identified Orf1p as an endogenous ubiquitin-dependent substrate of the 26S proteasome both in yeast and mammalian cells. Surprisingly, we found that the ribosome-associated quality control factor Rqc1 and the ubiquitin ligase Ltn1 are critical for Orf1p degradation. In addition, the cytosolic protein quality control chaperone system Hsp70/Hsp90 and their corresponding co-chaperones Sse1, Fes1, Sti1 and Cpr7 are also required for Orf1p proteolysis. Our study finds that Orf1p, which is naturally synthesized as a result of a premature translation termination event, requires the coordinated role of both ribosome-associated and cytosolic protein quality control factors for its degradation.

Role of Antizyme Inhibitor Proteins in Cancers and Beyond

Polyamines are multivalent organic cations essential for many cellular functions, including cell growth, differentiation, and proliferation. However, elevated polyamine levels are associated with a slew of pathological conditions, including multiple cancers. Intracellular polyamine levels are primarily controlled by the autoregulatory circuit comprising two different protein types, Antizymes (OAZ) and Antizyme Inhibitors (AZIN), which regulate the activity of the polyamine biosynthetic enzyme ornithine decarboxylase (ODC). While OAZ functions to decrease the intracellular polyamine levels by inhibiting ODC activity and exerting a negative control of polyamine uptake, AZIN operates to increase intracellular polyamine levels by binding and sequestering OAZ to relieve ODC inhibition and to increase polyamine uptake. Interestingly, OAZ and AZIN exhibit autoregulatory functions on polyamine independent pathways as well. A growing body of evidence demonstrates the dysregulation of AZIN expression in multiple cancers. Additionally, RNA editing of the Azin1 transcript results in a "gain-of-function" phenotype, which is shown to drive aggressive tumor types. This review will discuss the recent advances in AZIN's role in cancers via aberrant polyamine upregulation and its polyamine-independent protein regulation. This report will also highlight AZIN interaction with proteins outside the polyamine biosynthetic pathway and its potential implication to cancer pathogenesis. Finally, this review will reveal the protein interaction network of AZIN isoforms by analyzing three different interactome databases.

High-throughput interrogation of programmed ribosomal frameshifting in human cells

Programmed ribosomal frameshifting (PRF) is the controlled slippage of the translating ribosome to an alternative frame. This process is widely employed by human viruses such as HIV and SARS coronavirus and is critical for their replication. Here, we developed a high-throughput approach to assess the frameshifting potential of a sequence. We designed and tested >12,000 sequences based on 15 viral and human PRF events, allowing us to systematically dissect the rules governing ribosomal frameshifting and discover novel regulatory inputs based on amino acid properties and tRNA availability. We assessed the natural variation in HIV gag-pol frameshifting rates by testing >500 clinical isolates and identified subtype-specific differences and associations between viral load in patients and the optimality of PRF rates. We devised computational models that accurately predict frameshifting potential and frameshifting rates, including subtle differences between HIV isolates. This approach can contribute to the development of antiviral agents targeting PRF.

New insights of polyamine metabolism in testicular physiology: A role of ornithine decarboxylase antizyme inhibitor 2 (AZIN2) in the modulation of testosterone levels and sperm motility

The specific role of polyamines in the testis physiology is not fully understood. Antizymes (OAZs) and antizyme inhibitors (AZINs) are modulators of ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis and polyamine uptake. Although the three known OAZs are expressed in the testis, only OAZ3 is testis specific and has been proven to have an essential role in male fertility. Regarding the two existing AZINs, AZIN2 is the most abundantly expressed member in this gonad. Whereas previous studies suggested that AZIN2 might participate in mouse spermatogenesis, immunohistological analysis of human testicular sections revealed that AZIN2 is also detected in the steroidogenic Leydig cells but not in the germinal epithelium. In the present study, we found a close ontogenic similarity in the mRNA levels of OAZs and AZINs between mice and rats, but an opposite expression pattern of ODC activity. Further analysis of AZIN2 and OAZ3 in the testis of mice with different alterations in spermatogenesis and fertility, induced either genetically or pharmacologically, corroborated that both AZIN2 and OAZ3 are mainly expressed in the haploid germinal cells. Finally, by using transgenic mice with a truncated Azin2 gene fused to the bacterial lacZ gene, we studied the expression of Azin2 in testes, epididymides and spermatozoa. AZIN2 was detected in spermatids and spermatozoa, as well as in Leydig cells, and in epithelial epidydimal cells. Azin2 knock-out male mice were fertile; however, they showed marked decreases in testicular putrescine and plasma and testicular testosterone levels, and a dramatic reduction in the sperm motility. These results suggest an important role for AZIN2 in testicular cells by modulating polyamine concentrations, testosterone synthesis and sperm function. Overall, our data corroborate the relevance of polyamine regulation in testis functions, where both AZIN2 and OAZ3 play fundamental roles.

The antizyme family for regulating polyamines

The polyamines spermidine, spermine, and their precursor putrescine are organic polycations involved in various cellular processes and are absolutely essential for cellular proliferation. Because of their crucial function in the cell, their intracellular concentration must be maintained at optimal levels. To a large extent, this regulation is achieved through the activity of an autoregulatory loop that involves two proteins, antizyme (Az) and antizyme inhibitor (AzI), that regulate the first enzyme in polyamine biosynthesis, ornithine decarboxylase (ODC), and polyamine uptake activity in response to intracellular polyamine levels. In this Minireview, I will discuss what has been learned about the mechanism of Az expression and its physical interaction with both ODC and AzI in the regulation of polyamines.

Antizyme Inhibitors in Polyamine Metabolism and Beyond: Physiopathological Implications

The intracellular levels of polyamines, cationic molecules involved in a myriad of cellular functions ranging from cellular growth, differentiation and apoptosis, is precisely regulated by antizymes and antizyme inhibitors via the modulation of the polyamine biosynthetic and transport systems. Antizymes, which are mainly activated upon high polyamine levels, inhibit ornithine decarboxylase (ODC), the key enzyme of the polyamine biosynthetic route, and exert a negative control of polyamine intake. Antizyme inhibitors (AZINs), which are proteins highly homologous to ODC, selectively interact with antizymes, preventing their action on ODC and the polyamine transport system. In this review, we will update the recent advances on the structural, cellular and physiological functions of AZINs, with particular emphasis on the action of these proteins in the regulation of polyamine metabolism. In addition, we will describe emerging evidence that suggests that AZINs may also have polyamine-independent effects on cells. Finally, we will discuss how the dysregulation of AZIN activity has been implicated in certain human pathologies such as cancer, fibrosis or neurodegenerative diseases.

Do Gametes Woo? Evidence for Their Nonrandom Union at Fertilization

A fundamental tenet of inheritance in sexually reproducing organisms such as humans and laboratory mice is that gametes combine randomly at fertilization, thereby ensuring a balanced and statistically predictable representation of inherited variants in each generation. This principle is encapsulated in Mendel's First Law. But exceptions are known. With transmission ratio distortion, particular alleles are preferentially transmitted to offspring. Preferential transmission usually occurs in one sex but not both, and is not known to require interactions between gametes at fertilization. A reanalysis of our published work in mice and of data in other published reports revealed instances where any of 12 mutant genes biases fertilization, with either too many or too few heterozygotes and homozygotes, depending on the mutant gene and on dietary conditions. Although such deviations are usually attributed to embryonic lethality of the underrepresented genotypes, the evidence is more consistent with genetically-determined preferences for specific combinations of egg and sperm at fertilization that result in genotype bias without embryo loss. This unexpected discovery of genetically-biased fertilization could yield insights about the molecular and cellular interactions between sperm and egg at fertilization, with implications for our understanding of inheritance, reproduction, population genetics, and medical genetics.

Protein degradation, the main hub in the regulation of cellular polyamines

Ornithine decarboxylase (ODC) is the first and rate-limiting enzyme in the biosynthesis of polyamines, low-molecular-mass aliphatic polycations that are ubiquitously present in all living cells and are essential for fundamental cellular processes. Most cellular polyamines are bound, whereas the free pools, which regulate cellular functions, are subjected to tight regulation. The regulation of the free polyamine pools is manifested by modulation of their synthesis, catabolism, uptake and excretion. A central element that enables this regulation is the rapid degradation of key enzymes and regulators of these processes, particularly that of ODC. ODC degradation is part of an autoregulatory circuit that responds to the intracellular level of the free polyamines. The driving force of this regulatory circuit is a protein termed antizyme (Az). Az stimulates the degradation of ODC and inhibits polyamine uptake. Az acts as a sensor of the free intracellular polyamine pools as it is expressed via a polyamine-stimulated ribosomal frameshifting. Az binds to monomeric ODC subunits to prevent their reassociation into active homodimers and facilitates their ubiquitin-independent degradation by the 26S proteasome. In addition, through a yet unidentified mechanism, Az inhibits polyamine uptake. Interestingly, a protein, termed antizyme inhibitor (AzI) that is highly homologous with ODC, but retains no ornithine decarboxylating activity, seems to regulate cellular polyamines through its ability to negate Az. Overall, the degradation of ODC is a net result of interactions with regulatory proteins and possession of signals that mediate its ubiquitin-independent recognition by the proteasome.

The control of male fertility by spermatid-specific factors: searching for contraceptive targets from spermatozoon's head to tail

Male infertility due to abnormal spermatozoa has been reported in both animals and humans, but its pathogenic causes, including genetic abnormalities, remain largely unknown. On the other hand, contraceptive options for men are limited, and a specific, reversible and safe method of male contraception has been a long-standing quest in medicine. Some progress has recently been made in exploring the effects of spermatid-specifical genetic factors in controlling male fertility. A comprehensive search of PubMed for articles and reviews published in English before July 2016 was carried out using the search terms 'spermiogenesis failure', 'globozoospermia', 'spermatid-specific', 'acrosome', 'infertile', 'manchette', 'sperm connecting piece', 'sperm annulus', 'sperm ADAMs', 'flagellar abnormalities', 'sperm motility loss', 'sperm ion exchanger' and 'contraceptive targets'. Importantly, we have opted to focus on articles regarding spermatid-specific factors. Genetic studies to define the structure and physiology of sperm have shown that spermatozoa appear to be one of the most promising contraceptive targets. Here we summarize how these spermatid-specific factors regulate spermiogenesis and categorize them according to their localization and function from spermatid head to tail (e.g., acrosome, manchette, head-tail conjunction, annulus, principal piece of tail). In addition, we emphatically introduce small-molecule contraceptives, such as BRDT and PPP3CC/PPP3R2, which are currently being developed to target spermatogenic-specific proteins. We suggest that blocking the differentiation of haploid germ cells, which rarely affects early spermatogenic cell types and the testicular microenvironment, is a better choice than spermatogenic-specific proteins. The studies described here provide valuable information regarding the genetic and molecular defects causing male mouse infertility to improve our understanding of the importance of spermatid-specific factors in controlling fertility. Although a male contraceptive 'pill' is still many years away, research into the production of new small-molecule contraceptives targeting spermatid-specific proteins is the right avenue.

TEX13 is a novel male germ cell-specific nuclear protein potentially involved in transcriptional repression

The identification and characterization of male germ cell-specific genes is crucial to understanding the mechanisms of male germ cell development. In this study, we investigated the protein encoded by the novel mouse germ cell-specific gene testis-expressed gene 13 (Tex13). We found that TEX13 expression is testis- and germ cell-specific and is regulated in a stage-specific manner via translational repression. Immunostaining of testicular cells and sperm showed that TEX13 is localized in the nuclei of spermatogenic cells and the redundant nuclear envelope of mature sperm. Remarkably, we found that TEX13 possesses transcriptional repressor activity and that its overexpression in GC-2 cells altered the expression levels of 130 genes. Our results suggest that TEX13 has a potential role in transcriptional regulation during spermatogenesis.

Ribosomal frameshifting and transcriptional slippage: From genetic steganography and cryptography to adventitious use

Genetic decoding is not ‘frozen’ as was earlier thought, but dynamic. One facet of this is frameshifting that often results in synthesis of a C-terminal region encoded by a new frame. Ribosomal frameshifting is utilized for the synthesis of additional products, for regulatory purposes and for translational ‘correction’ of problem or ‘savior’ indels. Utilization for synthesis of additional products occurs prominently in the decoding of mobile chromosomal element and viral genomes. One class of regulatory frameshifting of stable chromosomal genes governs cellular polyamine levels from yeasts to humans. In many cases of productively utilized frameshifting, the proportion of ribosomes that frameshift at a shift-prone site is enhanced by specific nascent peptide or mRNA context features. Such mRNA signals, which can be 5′ or 3′ of the shift site or both, can act by pairing with ribosomal RNA or as stem loops or pseudoknots even with one component being 4 kb 3′ from the shift site. Transcriptional realignment at slippage-prone sequences also generates productively utilized products encoded trans-frame with respect to the genomic sequence. This too can be enhanced by nucleic acid structure. Together with dynamic codon redefinition, frameshifting is one of the forms of recoding that enriches gene expression.

RefSeq curation and annotation of antizyme and antizyme inhibitor genes in vertebrates

Polyamines are ubiquitous cations that are involved in regulating fundamental cellular processes such as cell growth and proliferation; hence, their intracellular concentration is tightly regulated. Antizyme and antizyme inhibitor have a central role in maintaining cellular polyamine levels. Antizyme is unique in that it is expressed via a novel programmed ribosomal frameshifting mechanism. Conventional computational tools are unable to predict a programmed frameshift, resulting in misannotation of antizyme transcripts and proteins on transcript and genomic sequences. Correct annotation of a programmed frameshifting event requires manual evaluation. Our goal was to provide an accurately curated and annotated Reference Sequence (RefSeq) data set of antizyme transcript and protein records across a broad taxonomic scope that would serve as standards for accurate representation of these gene products. As antizyme and antizyme inhibitor proteins are functionally connected, we also curated antizyme inhibitor genes to more fully represent the elegant biology of polyamine regulation. Manual review of genes for three members of the antizyme family and two members of the antizyme inhibitor family in 91 vertebrate organisms resulted in a total of 461 curated RefSeq records.

A Nascent Peptide Signal Responsive to Endogenous Levels of Polyamines Acts to Stimulate Regulatory Frameshifting on Antizyme mRNA

The protein antizyme is a negative regulator of cellular polyamine concentrations from yeast to mammals. Synthesis of functional antizyme requires programmed +1 ribosomal frameshifting at the 3′ end of the first of two partially overlapping ORFs. The frameshift is the sensor and effector in an autoregulatory circuit. Except for Saccharomyces cerevisiae antizyme mRNA, the frameshift site alone only supports low levels of frameshifting. The high levels usually observed depend on the presence of cis-acting stimulatory elements located 5′ and 3′ of the frameshift site. Antizyme genes from different evolutionary branches have evolved different stimulatory elements. Prior and new multiple alignments of fungal antizyme mRNA sequences from the Agaricomycetes class of Basidiomycota show a distinct pattern of conservation 5′ of the frameshift site consistent with a function at the amino acid level. As shown here when tested in Schizosaccharomyces pombe and mammalian HEK293T cells, the 5′ part of this conserved sequence acts at the nascent peptide level to stimulate the frameshifting, without involving stalling detectable by toe-printing. However, the peptide is only part of the signal. The 3′ part of the stimulator functions largely independently and acts at least mostly at the nucleotide level. When polyamine levels were varied, the stimulatory effect was seen to be especially responsive in the endogenous polyamine concentration range, and this effect may be more general. A conserved RNA secondary structure 3′ of the frameshift site has weaker stimulatory and polyamine sensitizing effects on frameshifting.

Diversity and functional convergence of small noncoding RNAs in male germ cell differentiation and fertilization

The small noncoding RNAs (sncRNAs) are considered as post-transcriptional key regulators of male germ cell development. In addition to microRNAs (miRNAs) and PIWI-interacting RNAs (piRNAs), other sncRNAs generated from small nucleolar RNAs (snoRNAs), tRNAs, or rRNAs processing may also play important regulatory roles in spermatogenesis. By next-generation sequencing (NGS), we characterized the sncRNA populations detected at three milestone stages in male germ differentiation: primordial germ cells (PGCs), pubertal spermatogonia cells, and mature spermatozoa. To assess their potential transmission through the spermatozoa during fertilization, the sncRNAs of mouse oocytes and zygotes were also analyzed. Both, microRNAs and snoRNA-derived small RNAs are abundantly expressed in PGCs but transiently replaced by piRNAs in spermatozoa and endo-siRNAs in oocytes and zygotes. Exhaustive analysis of miRNA sequence variants also shows an increment of noncanonical microRNA forms along male germ cell differentiation. RNAs-derived from tRNAs and rRNAs interacting with PIWI proteins are not generated by the ping-pong pathway and could be a source of primary piRNAs. Moreover, our results strongly suggest that the small RNAs-derived from tRNAs and rRNAs are interacting with PIWI proteins, and specifically with MILI. Finally, computational analysis revealed their potential involvement in post-transcriptional regulation of mRNA transcripts suggesting functional convergence among different small RNA classes in germ cells and zygotes.

Ribosomes in the balance: structural equilibrium ensures translational fidelity and proper gene expression

At equilibrium, empty ribosomes freely transit between the rotated and un-rotated states. In the cell, the binding of two translation elongation factors to the same general region of the ribosome stabilizes one state over the other. These stabilized states are resolved by expenditure of energy in the form of GTP hydrolysis. A prior study employing mutants of a late assembling peripheral ribosomal protein suggested that ribosome rotational status determines its affinity for elongation factors, and hence translational fidelity and gene expression. Here, mutants of the early assembling integral ribosomal protein uL2 are used to test the generality of this hypothesis. rRNA structure probing analyses reveal that mutations in the uL2 B7b bridge region shift the equilibrium toward the rotated state, propagating rRNA structural changes to all of the functional centers of ribosome. Structural disequilibrium unbalances ribosome biochemically: rotated ribosomes favor binding of the eEF2 translocase and disfavor that of the elongation ternary complex. This manifests as specific translational fidelity defects, impacting the expression of genes involved in telomere maintenance. A model is presented describing how cyclic intersubunit rotation ensures the unidirectionality of translational elongation, and how perturbation of rotational equilibrium affects specific aspects of translational fidelity and cellular gene expression.

Structural and degradative aspects of ornithine decarboxylase antizyme inhibitor 2

Ornithine decarboxylase (ODC) is the key enzyme in the polyamine biosynthetic pathway. ODC levels are controlled by polyamines through the induction of antizymes (AZs), small proteins that inhibit ODC and target it to proteasomal degradation without ubiquitination. Antizyme inhibitors (AZIN1 and AZIN2) are proteins homologous to ODC that bind to AZs and counteract their negative effect on ODC. Whereas ODC and AZIN1 are well-characterized proteins, little is known on the structure and stability of AZIN2, the lastly discovered member of this regulatory circuit. In this work we first analyzed structural aspects of AZIN2 by combining biochemical and computational approaches. We demonstrated that AZIN2, in contrast to ODC, does not form homodimers, although the predicted tertiary structure of the AZIN2 monomer was similar to that of ODC. Furthermore, we identified conserved residues in the antizyme-binding element, whose substitution drastically affected the capacity of AZIN2 to bind AZ1. On the other hand, we also found that AZIN2 is much more labile than ODC, but it is highly stabilized by its binding to AZs. Interestingly, the administration of the proteasome inhibitor MG132 caused differential effects on the three AZ-binding proteins, having no effect on ODC, preventing the degradation of AZIN1, but unexpectedly increasing the degradation of AZIN2. Inhibitors of the lysosomal function partially prevented the effect of MG132 on AZIN2. These results suggest that the degradation of AZIN2 could be also mediated by an alternative route to that of proteasome. These findings provide new relevant information on this unique regulatory mechanism of polyamine metabolism.

Expression and distribution of genes encoding for polyamine-metabolizing enzymes in the different zones of male and female mouse kidneys

The role of polyamines in renal physiology is only partially understood. Moreover, most of the data on the enzymes of polyamine metabolism come from studies using whole kidneys. The aim of the present study was to analyze the mRNA abundance of the genes implicated in both the polyamine biosynthetic and catabolic pathways in different renal zones of male and female mice, by means of the quantitative reverse transcription-polymerase chain reaction. Our results indicate that there is an uneven distribution of the different mRNAs studied in the five renal zones: superficial cortex, deep cortex, outer stripe of the outer medulla (OS), inner stripe of the outer medulla (IS), and the inner medulla + papilla (IM). The biosynthetic genes, ornithine decarboxylase (ODC) and spermine synthase, were more expressed in the cortex, whereas the mRNAs of the catabolic genes spermine oxidase (SMO) and diamine oxidase were more abundant in IS and IM. The genes involved in the regulation of polyamine synthesis (AZ1, AZ2 and AZIN1) were expressed in all the renal zones, predominantly in the cortex, while AZIN2 gene was more abundant in the OS. ODC, SMO, spermidine synthase and spermidine/spermine acetyl transferase expression was higher in males than in females. In conclusion, the genes encoding for the polyamine metabolism were specifically and quantitatively distributed along the corticopapillary axis of male and female mouse kidneys, suggesting that their physiological role is essential in defined renal zones and/or nephron segments.

Molecular characterization, tissue expression and nucleotide variation of the porcine AZ1 gene

Antizyme 1 (AZ1) is a member of the antizyme family that is involved in many biological processes. As a natural inhibitor, AZ1 controls the normal level of polyamines, which is indispensable to cellular function. Our prior research showed that the expression of the AZ1 gene in Longissimus doris of Landrace pigs was higher (P<0.05) than in Lantang pigs. The AZ1 gene might be involved in the development of muscle and potentially serves as an important target for muscle improvement in pigs. In this study, the molecular characterization, tissue expression, and sequence variation of porcine AZ1 gene were analyzed. A 4082 bp sequence including the 5'-flanking region and gene sequence was obtained through RACE and sequencing. The genomic sequence of AZ1 gene consists of six exons and five introns. The mRNA of AZ1 gene contains three elements: 5'-untranslated regions (UTR, 79 bp), CDS (684 or 207 bp), and 3'-UTR (161 or 423 bp for the two transcripts). Three termination signals of AATAAA were found in the longer 3'-UTR. The mRNA sequence of the AZ1 gene contained two ORFs with a frameshifting site at the 69th codon. The amino acid sequence from the porcine AZ1 gene was similar to other vertebrates and exhibited the highest similarity to cattle. The partial 5'-flanking region was 852 bp with typical cis-regulatory elements such as TATA boxes and CAAT boxes. By DNA pooling and sequencing, nine and 12 single nucleotide polymorphisms (SNPs) were detected in the 5'-flanking region and introns, respectively. Except the SNP at -713, the other eight SNPs were found in putative cis-acting elements and might alter the binding of trans-acting factors. Expression patterns showed that in Lantang pig, the AZ1 gene is differentially expressed in various tissues and displayed higher expression in lung and skeletal muscle.

Polyamines on the reproductive landscape

The polyamines are ubiquitous polycationic compounds. Over the past 40 yr, investigation has shown that some of these, namely spermine, spermidine, and putrescine, are essential to male and female reproductive processes and to embryo/fetal development. Indeed, their absence is characterized by infertility and arrest in embryogenesis. Mammals synthesize polyamines de novo from amino acids or import these compounds from the diet. Information collected recently has shown that polyamines are essential regulators of cell growth and gene expression, and they have been implicated in both mitosis and meiosis. In male reproduction, polyamine expression correlates with stages of spermatogenesis, and polyamines appear to function in promoting sperm motility. There is evidence for polyamine involvement in ovarian follicle development and ovulation in female mammals, and polyamine synthesis is required for steroidogenesis in the ovary. Studies of the embryo indicate a polyamine requirement that can be met from maternal sources before implantation, whereas elimination of polyamine synthesis abrogates embryo development at gastrulation. Polyamines play roles in embryo implantation, in decidualization, and in placental formation and function, and polyamine privation during gestation results in intrauterine growth retardation. Emerging information implicates dietary arginine and dietary polyamines as nutritional regulators of fertility. The mechanisms by which polyamines regulate these multiple and diverse processes are not yet well explored; thus, there is fertile ground for further productive investigation.

Antizyme affects cell proliferation and viability solely through regulating cellular polyamines

Antizymes are key regulators of cellular polyamine metabolism that negatively regulate cell proliferation and are therefore regarded as tumor suppressors. Although the regulation of antizyme (Az) synthesis by polyamines and the ability of Az to regulate cellular polyamine levels suggest the centrality of polyamine metabolism to its antiproliferative function, recent studies have suggested that antizymes might also regulate cell proliferation by targeting to degradation proteins that do not belong to the cellular polyamine metabolic pathway. Using a co-degradation assay, we show here that, although they efficiently stimulated the degradation of ornithine decarboxylase (ODC), Az1 and Az2 did not affect or had a negligible effect on the degradation of cyclin D1, Aurora-A, and a p73 variant lacking the N-terminal transactivation domain whose degradation was reported recently to be stimulated by Az1. Furthermore, we demonstrate that, although Az1 and Az2 could not be constitutively expressed in transfected cells, they could be stably expressed in cells that express trypanosome ODC, a form of ODC that does not bind Az and therefore maintains a constant level of cellular polyamines. Taken together, our results clearly demonstrate that Az1 and Az2 affect cell proliferation and viability solely by modulating cellular polyamine metabolism.

Knockdown of ornithine decarboxylase antizyme 1 causes loss of uptake regulation leading to increased N1, N11-bis(ethyl)norspermine (BENSpm) accumulation and toxicity in NCI H157 lung cancer cells

Ornithine decarboxylase antizyme 1 (AZ1) is a major regulatory protein responsible for the regulation and degradation of ornithine decarboxylase (ODC). To better understand the role of AZ1 in polyamine metabolism and in modulating the response to anticancer polyamine analogues, a small interfering RNA strategy was used to create a series of stable clones in human H157 non-small cell lung cancer cells that expressed less than 5-10% of basal AZ1 levels. Antizyme 1 knockdown clones accumulated greater amounts of the polyamine analogue N (1),N (11)-bis(ethyl)norspermine (BENSpm) and were more sensitive to analogue treatment. The possibility of a loss of polyamine uptake regulation in the knockdown clones was confirmed by polyamine uptake analysis. These results are consistent with the hypothesis that AZ1 knockdown leads to dysregulation of polyamine uptake, resulting in increased analogue accumulation and toxicity. Importantly, there appears to be little difference between AZ1 knockdown cells and cells with normal levels of AZ1 with respect to ODC regulation, suggesting that another regulatory protein, potentially AZ2, compensates for the loss of AZ1. The results of these studies are important for the understanding of both the regulation of polyamine homeostasis and in understanding the factors that regulate tumor cell sensitivity to the anti-tumor polyamine analogues.

Differential expression of ornithine decarboxylase antizyme inhibitors and antizymes in rodent tissues and human cell lines

Ornithine decarboxylase antizyme inhibitors, AZIN1 and AZIN2, are regulators and homologous proteins of ornithine decarboxylase (ODC), the rate limiting enzyme in the biosynthesis of polyamines. In this study, we have examined by means of real-time RT-PCR the relative abundance of mRNA of the three ODC paralogs in different rodent tissues, as well as in several cell lines derived from human tumors. With the exception of mouse and rat testes, ODC mRNA was the most expressed gene in all tissues examined (values higher than 60%). AZIN2 was more expressed than AZIN1 in testis, epididymis, brain, adrenal gland and lung, whereas the opposite was found in liver, kidney, heart, intestine and pancreas, as well as in all the cell lines examined. mRNA abundance of the three antizymes (AZ1, AZ2 and AZ3) that interact with ODC and antizyme inhibitors was also analyzed. AZ1 and AZ2 mRNA were ubiquitously expressed, AZ1 mRNA being more abundant than that of AZ2, although the ratio was dependent on the mouse tissue. In carcinoma-derived cells AZ1 was more expressed than AZ2, whereas in neuroblastoma-derived cells AZ2 mRNA was much more abundant than that of AZ1. AZ3 was expressed exclusively in rodent testes, where it was the most abundant of the three antizymes (~80%). This study is the first comparative-quantitative analysis on the expression of antizymes and antizyme inhibitors in different types of mammalian cells.

Ornithine decarboxylase antizyme Oaz3 modulates protein phosphatase activity

Ornithine decarboxylase antizyme 3 (Oaz3) is expressed in spermatids, makes up the antizyme family of Oaz genes with Oaz1 and Oaz2, and was proposed to encode a 22 kDa antizyme protein involved in polyamine regulation similar to the 22 kDa OAZ1 and OAZ2 proteins. Here we demonstrate however that the major product encoded by Oaz3 is a 12 kDa protein, p12, which lacks the antizyme domain that interacts with ornithine decarboxylase. We show that p12 does not affect ornithine decarboxylase levels, providing an explanation for the surprising observation made in Oaz3 knock-out male mice, which do not display altered testis polyamine metabolism. This suggested a novel activity for Oaz3 p12. Using immuno-electron microscopy we localized p12 to two structures in the mammalian sperm tail, viz. the outer dense fibers and fibrous sheath, as well as to the connecting piece linking head and tail. We identified myosin phosphatase targeting subunit 3 (MYPT3), a regulator of protein phosphatase PP1β, as a major p12-interacting protein, and show that MYPT3 is present in sperm tails and that its ankyrin repeat binds p12. We show that MYPT3 can also bind protein phosphatase PP1γ2, the only protein phosphatase present in sperm tails, and that p12- MYPT3 interaction modulates the activity of both PP1β and PP1γ2. This is, to our knowledge, the first demonstration of a novel activity for an Oaz-encoded protein.

Protocols for studying antizyme expression and function

Antizyme (AZ) is a key molecule in feedback regulation of cellular polyamines. It is induced by polyamines through stimulation of ribosomal frameshifting during its translation. In mammals, AZ is diverged into three paralogs, AZ1-3. Tissue and subcellular distribution are different among the paralogs, as determined by immunochemical methods or expression of fluorescent-tagged proteins. Only AZ2 is known to be phosphorylated. AZ regulates cellular polyamine levels through multiple mechanisms. It binds to ornithine decarboxylase (ODC) to form an inactive complex and to trigger degradation of ODC by 26S proteasomes. The AZ activity to promote ODC degradation can be measured both in vitro and in cells. AZ also inhibits cellular uptake of polyamines. This chapter comprises seven subchapters describing methods for studying expression and function of AZ.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 3: developmental changes in spermatid flagellum and cytoplasmic droplet and interaction of sperm with the zona pellucida and egg plasma membrane

Spermiogenesis constitutes the steps involved in the metamorphosis of spermatids into spermatozoa. It involves modification of several organelles in addition to the formation of several structures including the flagellum and cytoplasmic droplet. The flagellum is composed of a neck region and middle, principal, and end pieces. The axoneme composed of nine outer microtubular doublets circularly arranged to form a cylinder around a central pair of microtubules is present throughout the flagellum. The middle and principal pieces each contain specific components such as the mitochondrial sheath and fibrous sheath, respectively, while outer dense fibers are common to both. A plethora of proteins are constituents of each of these structures, with each playing key roles in functions related to the fertility of spermatozoa. At the end of spermiogenesis, a portion of spermatid cytoplasm remains associated with the released spermatozoa, referred to as the cytoplasmic droplet. The latter has as its main feature Golgi saccules, which appear to modify the plasma membrane of spermatozoa as they move down the epididymal duct and hence may be partly involved in male gamete maturation. The end product of spermatogenesis is highly streamlined and motile spermatozoa having a condensed nucleus equipped with an acrosome. Spermatozoa move through the female reproductive tract and eventually penetrate the zona pellucida and bind to the egg plasma membrane. Many proteins have been implicated in the process of fertilization as well as a plethora of proteins involved in the development of spermatids and sperm, and these are high lighted in this review.

Subcellular localization and phosphorylation of antizyme 2

Antizymes (AZs) are polyamine-induced proteins that negatively regulate cellular polyamine synthesis and uptake. Three antizyme isoforms are conserved among mammals. AZ1 and AZ2 have a broad tissue distribution, while AZ3 is testis specific. Both AZ1 and AZ2 inhibit ornithine decarboxylase (ODC) activity by binding to ODC monomer and target it to the 26S proteasome at least in vivo. Both also inhibit extra-cellular polyamine uptake. Despite their being indistinguishable by these criteria, we show here using enhanced green fluorescent protein (EGFP)-AZ2 fusion protein that in mammalian cells, the subcellular location of AZ2 is mainly in the nucleus, and is different from that of AZ1. The C-terminal part of AZ2 is necessary for the nuclear distribution. Within a few hours, a shift in the distribution of EGFP-AZ2 fusion protein from cytoplasm to the nucleus or from nucleus to cytoplasm is observable in NIH3T3 cells. In addition, we found that in cells a majority of AZ2, but not AZ1, is phosphorylated at Ser-186, likely by protein kinase CK2. There may be a specific function of AZ2 in the nucleus.

Insect ornithine decarboxylase (ODC) complements SPE1 knock-out of yeast Saccharomyces cerevisiae

Ornithine decarboxylase (ODC) is a rate-limiting enzyme in the biosynthesis of polyamines, which are essential for cell growth, differentiation, and proliferation. This report presents the characterization of an ODC-encoding cDNA (SlitODC) isolated from a moth species, the tobacco cutworm, Spodoptera litura (Lepidoptera); its expression in a polyamine-deficient strain of yeast, S. cerevisiae; and the recovery in polyamine levels and proliferation rate with the introduction of the insect enzyme. SlitODC encodes 448 amino acid residues, 4 amino acids longer than B. Mori ODC that has 71% identity, and has a longer C-terminus, consistent with B. mori ODC, than the reported dipteran enzymes. The null mutant yeast strain in the ODC gene, SPE1, showed remarkably depleted polyamine levels; in putrescine, spermidine, and spermine, the levels were > 7, > 1, and > 4%, respectively, of the levels in the wild-type strain. This consequently caused a significant arrest in cell proliferation of > 4% of the wild-type strain in polyaminefree media. The transformed strain, with the substituted SlitODC for the deleted endogenous ODC, grew and proliferated rapidly at even a higher rate than the wild-type strain. Furthermore, its polyamine content was significantly higher than even that in the wild-type strain as well as the spe1-null mutant, particularly with a very continuously enhanced putrescine level, reflecting no inhibition mechanism operating in the putrescine synthesis step by any corresponding insect ODC antizymes to SlitODC in this yeast system.

Developmental alterations in expression and subcellular localization of antizyme and antizyme inhibitor and their functional importance in the murine mammary gland

Ornithine decarboxylase (ODC), antizyme (AZ), and antizyme inhibitor (AIn) play a key role in regulation of intracellular polyamine levels by forming a regulatory circuit through their interactions. To gain insight into their functional importance in cell growth and differentiation, we systematically examined the changes of their expression, cellular polyamine contents, expression of genes related to polyamine metabolism, and beta-casein gene expression during murine mammary gland development. The activity of ODC and AZ1 as well as putrescine level were low in the virgin and involuting stages, but they increased markedly during late pregnancy and early lactation when mammary cells proliferate extensively and begin to augment their differentiated function. The level of spermidine and expression of genes encoding spermidine synthase and AIn increased in a closely parallel manner with that of casein gene expression during pregnancy and lactation. On the other hand, the level of spermidine/spermine N(1)-acetyltransferase (SSAT) mRNA and AZ2 mRNA decreased during those periods. Immunohistochemical analysis showed the translocation of ODC and AIn between the nucleus and cytoplasm and the continuous presence of AZ in the nucleus during gland development. Reduction of AIn by RNA interference inhibited expression of beta-casein gene stimulated by lactogenic hormones in HC11 cells. In contrast, reduction of AZ by AZsiRNA resulted in the small increase of beta-casein gene expression. These results suggested that AIn plays an important role in the mammary gland development by changing its expression, subcellular localization, and functional interplay with AZ.

Antizyme inhibitor 2: molecular, cellular and physiological aspects

Polyamines are small organic polycations essential for cell proliferation and survival. Antizymes (AZs) are small proteins regulated by polyamines that inhibit polyamine biosynthesis and uptake in mammalian cells. In addition, antizyme functions are also regulated by antizyme inhibitors, homologue proteins of ornithine decarboxylase lacking enzymatic activity. There are two antizyme inhibitors (AZIN), known as AZIN1 and AZIN2, that bind to AZs and negate their effects on polyamine metabolism. Here, we review different molecular and cellular properties of the novel AZIN2 with particular emphasis on the role that this protein may have in brain and testis physiology. Whereas AZIN1 is ubiquitously found in mammalian tissues, AZIN2 expression appears to be restricted to brain and testis. In transfected cells, AZIN2 is mainly located in the endoplasmic reticulum-Golgi intermediate compartment and in the cis-Golgi network. AZIN2 is a labile protein that is degraded by the proteasome by a ubiquitin-dependent mechanism. Regarding its physiological role, spatial and temporal analyses of AZIN2 expression in the mouse testis suggest that this protein may have a role in spermiogenesis.

OAZ-t/OAZ3 is essential for rigid connection of sperm tails to heads in mouse

Polyamines are known to play important roles in the proliferation and differentiation of many types of cells. Although considerable amounts of polyamines are synthesized and stored in the testes, their roles remain unknown. Ornithine decarboxylase antizymes (OAZs) control the intracellular concentration of polyamines in a feedback manner. OAZ1 and OAZ2 are expressed ubiquitously, whereas OAZ-t/OAZ3 is expressed specifically in germline cells during spermiogenesis. OAZ-t reportedly binds to ornithine decarboxylase (ODC) and inactivates ODC activity. In a prior study, polyamines were capable of inducing a frameshift at the frameshift sequence of OAZ-t mRNA, resulting in the translation of OAZ-t. To investigate the physiological role of OAZ-t, we generated OAZ-t–disrupted mutant mice. Homozygous OAZ-t mutant males were infertile, although the polyamine concentrations of epididymides and testes were normal in these mice, and females were fertile. Sperm were successfully recovered from the epididymides of the mutant mice, but the heads and tails of the sperm cells were easily separated in culture medium during incubation. Results indicated that OAZ-t is essential for the formation of a rigid junction between the head and tail during spermatogenesis. The detached tails and heads were alive, and most of the headless tails showed straight forward movement. Although the tailless sperm failed to acrosome-react, the heads were capable of fertilizing eggs via intracytoplasmic sperm injection. OAZ-t likely plays a key role in haploid germ cell differentiation via the local concentration of polyamines.

Polyamines are essential for cell proliferation and differentiation, but their role in these processes is unknown. Ornithine decarboxylase antizymes (OAZs) are enzymes that control the concentration of polyamines in cells. To elucidate the role of one of these enzymes, OAZ-t, in the regulation of polyamine concentration during sperm formation, we generated mutant mice in which the OAZ-t gene was disrupted. When we observed sperm from the mice lacking a functional Oaz-t gene, we found that the sperm heads separated easily from the tails, indicating that OAZ-t is essential for the formation of a rigid junction between the head and tail during sperm development. Many of the headless tails could continue swimming, but they were unable to participate in the signaling processes required for successful fertilization. However, tailless heads could produce healthy pups when injected into unfertilized eggs. Such a phenotype has not been previously found. The mutant mice evoked rare cases of infertile human patients whose sperm behaves in a proper fashion. Our study underscores the importance of research into the processes of spermatogenesis and fertilization.

Regulation of cellular polyamine levels and cellular proliferation by antizyme and antizyme inhibitor

Polyamines are small aliphatic polycations present in all living cells. Polyamines are essential for cellular viability and are involved in regulating fundamental cellular processes, most notably cellular growth and proliferation. Being such central regulators of fundamental cellular functions, the intracellular polyamine concentration is tightly regulated at the levels of synthesis, uptake, excretion and catabolism. ODC (ornithine decarboxylase) is the first key enzyme in the polyamine biosynthesis pathway. ODC is characterized by an extremely rapid intracellular turnover rate, a trait that is central to the regulation of cellular polyamine homoeostasis. The degradation rate of ODC is regulated by its end-products, the polyamines, via a unique autoregulatory circuit. At the centre of this circuit is a small protein called Az (antizyme), whose synthesis is stimulated by polyamines. Az inactivates ODC and targets it to ubiquitin-independent degradation by the 26S proteasome. In addition, Az inhibits uptake of polyamines. Az itself is regulated by another ODC-related protein termed AzI (antizyme inhibitor). AzI is highly homologous with ODC, but it lacks ornithine-decarboxylating activity. Its ability to serve as a regulator is based on its high affinity to Az, which is greater than the affinity Az has to ODC. As a result, it interferes with the binding of Az to ODC, thus rescuing ODC from degradation and permitting uptake of polyamines.

High expression of antizyme inhibitor 2, an activator of ornithine decarboxylase in steroidogenic cells of human gonads

High activity of ornithine decarboxylase (ODC), the rate-limiting enzyme of polyamine synthesis, is typically present in rapidly proliferating normal and malignant cells. The mitotically inactive steroidogenic cells in rodent testis and ovaries, however, also display high ODC activity. The activity of ODC in these cells responds to luteinizing hormone, and inhibition of ODC reduces the production of steroid hormones. Polyamines and ODC also control proliferation of germ cells and spermiogenesis. The activity of ODC, especially in proliferating cells, is regulated by antizyme inhibitor (AZIN). This protein displaces ODC from a complex with its inhibitor, antizyme. We have previously identified and cloned a second AZIN, i.e. antizyme inhibitor 2 (AZIN2), which has the highest levels of expression in brain and in testis. In the present study, we have used immunohistochemistry and in situ hybridization to localize the expression of AZIN2 in human gonads. We found a robust expression of AZIN2 in steroidogenic cells: testicular Leydig cells and Leydig cell tumors, in ovarian luteinized cells lining corpus luteum cysts, and in hilus cells. The results suggest that AZIN2 is not primarily involved in regulating the proliferation of the germinal epithelium, indicating a different role for AZIN1 and AZIN2 in the regulation of ODC. The localization of AZIN2 implies possible involvement in the gonadal synthesis and/or release of steroid hormones.

Antizyme and antizyme inhibitor, a regulatory tango

The polyamines are small basic molecules essential for cellular proliferation and viability. An autoregulatory circuit that responds to the intracellular level of polyamines regulates their production. In the center of this circuit is a family of small proteins termed antizymes. Antizymes are themselves regulated at the translational level by the level of polyamines. Antizymes bind ornithine decarboxylase (ODC) subunits and target them to ubiquitin-independent degradation by the 26S proteasome. In addition, antizymes inhibit polyamine transport across the plasma membrane via an as yet unresolved mechanism. Antizymes may also interact with and target degradation of other growth-regulating proteins. An inactive ODC-related protein termed antizyme inhibitor regulates polyamine metabolism by negating antizyme functions. The ability of antizymes to degrade ODC, inhibit polyamine uptake and consequently suppress cellular proliferation suggests that they act as tumor suppressors, while the ability of antizyme inhibitors to negate antizyme function indicates their growth-promoting and oncogenic potential.

Antizyme 3 inhibits polyamine uptake and ornithine decarboxylase (ODC) activity, but does not stimulate ODC degradation

Azs (antizymes) are small polyamine-induced proteins that function as feedback regulators of cellular polyamine homoeostasis. They bind to transient ODC (ornithine decarboxylase) monomeric subunits, resulting in inhibition of ODC activity and targeting ODC to ubiquitin-independent proteasomal degradation. Az3 is a mammalian Az isoform expressed exclusively in testicular germ cells and therefore considered as a potential regulator of polyamines during spermatogenesis. We show here that, unlike Az1 and Az2, which efficiently inhibit ODC activity and stimulate its proteasomal degradation, Az3 poorly inhibits ODC activity and fails to promote ODC degradation. Furthermore, Az3 actually stabilizes ODC, probably by protecting it from the effect of Az1. Its inhibitory effect is revealed only when it is present in excess compared with ODC. All three Azs efficiently inhibit the ubiquitin-dependent degradation of AzI (Az inhibitor) 1 and 2. Az3, similar to Az1 and Az2, efficiently inhibits polyamine uptake. The potential significance of the differential behaviour of Az3 is discussed.

Transcriptomic analysis of polyamine-related genes and polyamine levels in placenta, yolk sac and fetus during the second half of mouse pregnancy

In mammals, polyamines are essential for the maintenance of cell growth. Although early studies reported the highest values of mammalian ornithine decarboxylase (ODC) activity, a key enzyme in polyamine biosynthesis, in rodent placenta, the role of this enzyme in the second half of rodent pregnancy is still controversial. In order to get new insights on polyamine metabolism during this period of pregnancy, we studied polyamine levels, ODC expression and activity and transcript profile of different polyamine-related genes in mouse placenta, fetus and yolk sac. Results indicated that ODC activity and protein levels were higher in placenta than in fetus and yolk sac, especially in the labyrinth, although no correlation between ODC activity and polyamine levels were observed. The half-life of placental ODC ( approximately 190 min) was also higher than the fetal one ( approximately 24 min). Messenger RNAs of all biosynthetic and retroconversion enzymes of polyamine metabolism were present in the three gestational compartments analyzed, as well as those of antizymes 1 and 2 and antizyme inhibitor 1. However, no expression of antizyme 3 and antizyme inhibitor 2 was detected. The catabolic enzyme diamine oxidase was expressed only in the maternal part of placenta but not in the fetal part or in the fetus. The expansion of polyamine pools in the fetus was markedly higher than in placenta, in spite of its lower biosynthetic activity. Our results suggest that the elevated polyamine biosynthetic activity of mouse placenta is required to satisfy the high demand of polyamines required by the growing fetus, during the later period of pregnancy.

Expression of antizyme inhibitor 2 in male haploid germinal cells suggests a role in spermiogenesis

Recently, we have found that the antizyme inhibitor 2, a novel member of the antizyme binding proteins related to polyamine metabolism, was expressed mainly in the adult testes, although its function in testicular physiology is completely unknown. Therefore, in the present work, the spatial and temporal expression of antizyme inhibitor 2, and other genes related to polyamine metabolism were studied in the mouse testis, in an attempt to understand the role of antizyme inhibitor 2 in testicular functions. For that purpose, the temporal expression of different genes, during the first wave of spermatogenesis in postnatal mice, was studied by real-time RT-PCR, and the spatial distribution of transcripts and protein in the adult testis was examined by both RNA in situ hybridization and immunocytochemistry. The results indicated that antizyme inhibitor 2 was specifically expressed in the haploid germinal cells, similarly to antizyme 3, the testis specific antizyme. Conversely, ornithine decarboxylase mRNA was mainly found in the outer part of the seminiferous tubules where spermatogonia and spermatocytes are located. Functional transfection assays and co-immunoprecipitation experiments corroborated that antizyme inhibitor 2 counteracts the negative action of antizyme 3 on polyamine biosynthesis and uptake. All these results indicate that the expression of antizyme inhibitor 2 is postnatally regulated and strongly suggest that antizyme inhibitor 2 may have a role in spermiogenesis.

Antizyme inhibitor 2 (AZIN2/ODCp) stimulates polyamine uptake in mammalian cells

One of the processes that regulate intracellular levels of polyamines in mammalian cells is polyamine uptake. We have measured polyamine uptake in COS7 cells for putrescine, spermidine, and spermine, obtaining K(m) values of 4.5, 1.0, and 0.8 mum, respectively. Treatment of nonconfluent cells with cycloheximide stimulated polyamine uptake and prevented the inhibitory effect found in cells preloaded with polyamines, suggesting the existence of a feedback repression mechanism mediated by antizymes. Transient transfected cells with mutated antizyme forms of AZ1, AZ2, and AZ3, which do not require frameshifting, showed a total blockade of polyamine uptake. Transfection of COS7 cells with mouse or human AZIN2, a novel member of the antizyme inhibitor family, recently characterized by our group, markedly stimulated polyamine uptake and counteracted the action of any of the three antizymes in co-transfected cells. The stimulatory effect of AZIN2 on polyamine uptake was abrogated when the putative antizyme binding sequence, formed by residues 117-140 in AZIN2, was deleted. Real time reverse transcription-PCR analysis of antizyme inhibitor transcripts revealed that in brain and testes AZIN2 is more expressed than AZIN1, especially in the testes where the relative expression was about 25-fold higher. Collectively, our results clearly indicate that AZIN2 affects polyamine homeostasis not only by increasing ornithine decarboxylase activity but also by stimulating polyamine uptake, through negating the inhibitory effect of the antizymes. This finding may have physiological relevance, mostly in testes where AZ3 and AZIN2 are mainly expressed.

Ornithine decarboxylase antizyme finder (OAF): fast and reliable detection of antizymes with frameshifts in mRNAs

BACKGROUND

Ornithine decarboxylase antizymes are proteins which negatively regulate cellular polyamine levels via their affects on polyamine synthesis and cellular uptake. In virtually all organisms from yeast to mammals, antizymes are encoded by two partially overlapping open reading frames (ORFs). A +1 frameshift between frames is required for the synthesis of antizyme. Ribosomes change translation phase at the end of the first ORF in response to stimulatory signals embedded in mRNA. Since standard sequence analysis pipelines are currently unable to recognise sites of programmed ribosomal frameshifting, proper detection of full length antizyme coding sequences (CDS) requires conscientious manual evaluation by a human expert. The rapid growth of sequence information demands less laborious and more cost efficient solutions for this problem. This manuscript describes a rapid and accurate computer tool for antizyme CDS detection that requires minimal human involvement.

RESULTS

We have developed a computer tool, OAF (ODC antizyme finder) for identifying antizyme encoding sequences in spliced or intronless nucleic acid sequenes. OAF utilizes a combination of profile hidden Markov models (HMM) built separately for the products of each open reading frame constituting the entire antizyme coding sequence. Profile HMMs are based on a set of 218 manually assembled antizyme sequences. To distinguish between antizyme paralogs and orthologs from major phyla, antizyme sequences were clustered into twelve groups and specific combinations of profile HMMs were designed for each group. OAF has been tested on the current version of dbEST, where it identified over six thousand Expressed Sequence Tags (EST) sequences encoding antizyme proteins (over two thousand antizyme CDS in these ESTs are non redundant).

CONCLUSION

OAF performs well on raw EST sequences and mRNA sequences derived from genomic annotations. OAF will be used for the future updates of the RECODE database. OAF can also be useful for identifying novel antizyme sequences when run with relaxed parameters. It is anticipated that OAF will be used for EST and genome annotation purposes. OAF outputs sequence annotations in fasta, genbank flat file or XML format. The OAF web interface and the source code are freely available at http://recode.ucc.ie/oaf/ and at a mirror site http://recode.genetics.utah.edu/oaf/.

Polyamines in spermatocytes and residual bodies of rat testis

Immunocytochemistry for polyamines in the rat testis revealed intense staining of small bodies close to the lumen of seminiferous tubules of spermatogenic stage VII and VIII as well as of spermatocytes. Methyl green-pyronin and propidium iodide staining combined with DNase or RNase predigestion showed that the small bodies contained RNA, but not DNA and double fluorescence staining showed that polyamines (PAs) colocalized with RNA in the bodies. Electron microscopy confirmed the absence of nuclei in the bodies and revealed that PA immunoreactivity was associated with ribosomes. These results strongly suggest that the small bodies correspond to the residual bodies, and agree with previous results showing localization of PAs to ribosomes in neurons and gastrointestinal epithelial cells. The accumulation of PAs in residual bodies may reflect a termination of their role in spermiogenesis with respect to protein synthesis.

Ribosomal frameshifting in decoding antizyme mRNAs from yeast and protists to humans: close to 300 cases reveal remarkable diversity despite underlying conservation

The protein antizyme is a negative regulator of intracellular polyamine levels. Ribosomes synthesizing antizyme start in one ORF and at the codon 5′ adjacent to its stop codon, shift +1 to a second and partially overlapping ORF which encodes most of the protein. The ribosomal frameshifting is a sensor and effector of an autoregulatory circuit which is conserved in animals, fungi and protists. Stimulatory signals encoded 5′ and 3′ of the shift site act to program the frameshifting. Despite overall conservation, many individual branches have evolved specific features surrounding the frameshift site. Among these are RNA pseudoknots, RNA stem-loops, conserved primary RNA sequences, nascent peptide sequences and branch-specific ‘shifty’ codons.

Novel antizyme gene in Danio rerio expressed in brain and retina

The synthesis of the protein antizyme requires a +1 ribosomal frameshift event. The frameshifting serves as a regulatory sensor. Antizyme homologs have been identified in diverse organisms ranging from yeast to human and characterized in a disparate subset. Most vertebrates have multiple antizyme paralogs. Here we present identification in the zebrafish Danio rerio of a heretofore unknown member of the antizyme gene family. This novel antizyme does not correspond to any of the known orthologous groups in vertebrates and unlike most other antizymes is preferentially expressed in the retinal ganglion cell layer of the eye. In addition to the retina, it is also expressed in the brain and somites.

Polyamine Analogs with Xylene Rings Induce Antizyme Frameshifting, Reduce ODC Activity, and Deplete Cellular Polyamines

Numerous studies have correlated elevated polyamine levels with abnormal or rapid cell growth. One therapeutic strategy to treat diseases with increased cellular proliferation rates, most obviously cancer, has been to identify compounds which lower cellular polyamine levels. An ideal target for this strategy is the protein antizyme-a negative regulator of polyamine biosynthesis and import, and a positive regulator of polyamine export. In this study, we have optimized two tissue-culture assays in 96-well format, to allow the rapid screening of a 750-member polyamine analog library for compounds which induce antizyme frameshifting and fail to substitute for the natural polyamines in growth. Five analogs (MQTPA1-5) containing xylene (1,4-dimethyl benzene) were found to be equal to or better than spermidine at stimulating antizyme frameshifting and were inefficient at rescuing cell growth following polyamine depletion. These compounds were further characterized for effects on natural polyamine levels and enzymes involved in polyamine metabolism. Finally, direct measurements of antizyme induction in cells treated with two of the lead compounds revealed an 8- to 15-fold increase in antizyme protein over untreated cells. The impact of the xylene moiety and the distance between the positively charged amino groups on antizyme frameshifting and cell growth are discussed.

Mammalian transcription in support of hybrid mRNA and protein synthesis in testis and lung

Post-transcriptional mechanisms including differential splicing expand the protein repertoire beyond that provided by the one gene-one protein model. Trans-splicing has been observed in mammalian systems but is low level (sometimes referred to as noise), and a contribution to hybrid protein expression is unclear. In the study of rat sperm tail proteins a cDNA, called 1038, was isolated representing a hybrid mRNA derived in part from the ornithine decarboxylase antizyme 3 (Oaz3) gene located on rat chromosome 2 fused to sequences encoded by a novel gene on chromosome 4. Cytoplasmic Oaz3 mRNA is completely testis specific. However, in several tissues Oaz3 is transcribed and contributes to hybrid 1038 mRNA synthesis, without concurrent Oaz3 mRNA synthesis. 1038 mRNA directs synthesis of a hybrid 14-kDa protein, part chromosome 2- and part chromosome 4-derived as shown in vitro and in transfected cells. Antisera that recognize a chromosome 4-encoded C-terminal peptide confirm the hybrid character of endogenous 14-kDa protein and its presence in sperm tail structures and 1038-positive tissue. Our data suggest that the testis-specific OAZ3 gene may be an example of a mammalian gene that in several tissues is transcribed to contribute to a hybrid mRNA and protein. This finding expands the repertoire of known mechanisms available to cells to generate proteome diversity.