Excision of the first intron from the gonadotropin-releasing hormone (GnRH) transcript serves as a key regulatory step for GnRH biosynthesis

Intron retention as an alternative splice variant of the cattle ANGPTL6 gene

Angiopoietin-like protein 6, which is encoded by ANGPTL6 gene (also known as angiopoietin growth factor, AGF), has been extensively characterized with regard to its proposed functions as angiogenesis and energy metabolism. The present results showed the occurrence of alternative splicing by intron retention (IR) event in the bovine ANGPTL6 gene (bANGPTL6). By means of RT-PCR, TA clone and sequencing, we have shown that the bANGPTL6 gene has a splice variant generated by the retention of its partial intron 3. The computational analysis of the bANGPTL6 genomic sequence showed that its intron 3 has a high percentage of GC (62.31%) and a length of 199 nt, characteristics that have been associated with an IR event. The IR event does not interfere with the coding region as the bANGPTL6 prepropeptide is entirely coded in the third exon. Additionally, both the intronless (namely, bANGPTL6α) and intron-retaining (namely, bANGPTL6β) ANGPTL6 transcripts are constitutively co-expressed in the bovine liver. Further, the relative expression level of different variants in liver was tested by both semi-RT-PCR and RT-qPCR methods. The results suggested bANGPTL6β are significantly higher than bANGPTL6α. Overall, our findings will be helpful for studies on the molecular mechanism of IR events and the functions of ANGPTL6 gene. Specially, bANGPTL6β gene probably contributes to a new target for treatment of obesity and obesity-related diseases.

Alternative splicing of GnRH2 and GnRH2-associated peptide plays roles in gonadal differentiation of the rice field eel, Monopterus albus

The rice field eel, Monopterus albus, is a protogynous hermaphrodite fish, in which the gonads are initially female ovaries which then transform into male testes. The exact mechanisms governing sex reversal in the rice field eel are unknown. In this study, a novel alternative splicing variant of GnRH2 (GnRH2-SV), retaining the second intron, was discovered in the gonad of the rice field eel. Compared to GnRH2, GnRH2-SV may give rise to a novel truncated GnRH2-associated peptide (New GAP2). The normal transcript of GnRH2 was primarily expressed in the brain, and could also be detected in the liver, spleen, ovary, and testis. However, GnRH2-SV was only expressed in the ovary and testis. During sex reversal, GnRH2 expression levels increased significantly at late stages; however, expression levels of GnRH2-SV were lower in ovary than in ovotestis and testis. We also examined the effect of three peptides (GnRHa, GAP2, and New GAP2) on gonadal sex differentiation during the third stage of ovarian development of the rice field eel. Compared to the control group, the expression of amh increased significantly following incubation with each of the three peptides. However, only New GAP2 stimulated the expression of sox9a1 mRNA in vitro. After intraperitoneal injection of GAP2, the expression of amh, foxl2, and cyp19a1a increased significantly after 12 h; the concentration of serum 11-KT was also significantly increased at the 12 h time point. Treatment with New GAP2 significantly increased the expression of amh, dmrt1a, and sox9a1, and also increased the concentration of serum 11-KT. After treated with GnRHa, the expression of amh, dmrt1a, sox9a1, cyp19a1a, and foxl2 increased significantly, as did the level of serum E2. These results indicated that both GAP2 and New GAP2 play a crucial role in inducing expression changes of sex-differentiation related genes, and may be involved in the gonadal development and sex reversal in the rice field eel.

Developing a set of strong intronic promoters for robust metabolic engineering in oleaginous Rhodotorula (Rhodosporidium) yeast species

Background

Red yeast species in the Rhodotorula/Rhodosporidium genus are outstanding producers of triacylglyceride and cell biomass. Metabolic engineering is expected to further enhance the productivity and versatility of these hosts for the production of biobased chemicals and fuels. Promoters with strong activity during oil-accumulation stage are critical tools for metabolic engineering of these oleaginous yeasts.

Results

The upstream DNA sequences of 6 genes involved in lipid biosynthesis or accumulation in Rhodotorula toruloides were studied by luciferase reporter assay. The promoter of perilipin/lipid droplet protein 1 gene (LDP1) displayed much stronger activity (4–11 folds) than that of glyceraldehyde-3-phosphate dehydrogenase gene (GPD1), one of the strongest promoters known in yeasts. Depending on the stage of cultivation, promoter of acetyl-CoA carboxylase gene (ACC1) and fatty acid synthase β subunit gene (FAS1) exhibited intermediate strength, displaying 50–160 and 20–90% levels of GPD1 promoter, respectively. Interestingly, introns significantly modulated promoter strength at high frequency. The incorporation of intron 1 and 2 of LDP1 (LDP1in promoter) enhanced its promoter activity by 1.6–3.0 folds. Similarly, the strength of ACC1 promoter was enhanced by 1.5–3.2 folds if containing intron 1. The intron 1 sequences of ACL1 and FAS1 also played significant regulatory roles. When driven by the intronic promoters of ACC1 and LDP1 (ACC1in and LDP1in promoter, respectively), the reporter gene expression were up-regulated by nitrogen starvation, independent of de novo oil biosynthesis and accumulation. As a proof of principle, overexpression of the endogenous acyl-CoA-dependent diacylglycerol acyltransferase 1 gene (DGA1) by LDP1in promoter was significantly more efficient than GPD1 promoter in enhancing lipid accumulation.

Conclusion

Intronic sequences play an important role in regulating gene expression in R. toruloides. Three intronic promoters, LDP1in, ACC1in and FAS1in, are excellent promoters for metabolic engineering in the oleaginous and carotenogenic yeast, R. toruloides.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0600-x) contains supplementary material, which is available to authorized users.

The Methylcytosine Dioxygenase Ten-Eleven Translocase-2 (tet2) Enables Elevated GnRH Gene Expression and Maintenance of Male Reproductive Function

Reproduction depends on the establishment and maintenance of elevated GnRH neurosecretion. The elevation of primate GnRH release is accompanied by epigenetic changes. Specifically, cytosine residues within the GnRH gene promoter are actively demethylated, whereas GnRH mRNA levels and peptide release rise. Whether active DNA demethylation has an impact on GnRH neuron development and consequently reproductive function remains unknown. In this study, we investigated whether ten-eleven translocation (tet) enzymes, which initiate the process of active DNA demethylation, influence neuronal function and reproduction. We found that tet2 expression increases with age in the developing mouse preoptic area-hypothalamus and is substantially higher in a mature (GT1-7) than an immature (GN11) GnRH cell line. GnRH mRNA levels and mean GnRH peptide release elevated after overexpression of tet2 in GN11 cells, whereas CRISPR/cas9-mediated knockdown of tet2 in GT1-7 cells led to a significant decline in GnRH expression. Manipulations of tet2 expression altered tet2 genome binding and histone 3 lysine 4 trimethylation abundance at the GnRH promoter. Mice with selective disruption of tet2 in GnRH neurons (GnRH-specific tet2 knockout mice) exhibited no sign of altered pubertal timing in either sex, although plasma LH levels were significantly lower, and fecundity was altered specifically in adult male GnRH-specific tet2 knockout animals, indicating that tet2 may participate in the maintenance GnRH neuronal function. Exposure to bisphenol A, an environmental contaminant that alters GnRH neuron activity, caused a shift in tet2 subcellular localization and a decrease in histone 3 lysine 4 trimethylation abundance at the GnRH promoter. Finally, evaluation of tet2 protein interactions in GT1-7 cells suggests that the influence of tet2 on neuronal function are not limited to nuclear mechanisms but could depend on mitochondrial function, and RNA metabolism. Together, these studies implicate tet2 in the maintenance of GnRH neuronal function and neuroendocrine control of male reproduction.

Class-C SOX transcription factors control GnRH gene expression via the intronic transcriptional enhancer

GnRH is a pivotal hypothalamic neurohormone governing reproduction and sexual development. Because transcriptional regulation is crucial for the spatial and temporal expression of the GnRH gene, a region approximately 3.0 kb upstream of the mammalian GnRH promoter has been extensive studied. In the present study, we demonstrate a transcription-enhancer located in the first intron (intron A) region of the GnRH gene. This transcriptional enhancer harbors putative sex-determining region Y-related high-mobility-group box (SOX) family transcription factor-binding sites, which are well conserved across many mammalian species. The class-C SOX member proteins (SOX-C) (SOX4 and SOX11) specifically augment this transcriptional activation by binding to these SOX-binding sites. In accordance, SOX11 is highly enriched in immortalized GnRH-producing GT1-1 cells, and suppression of its expression significantly decreases GnRH gene expression as well as GnRH secretion. Chromatin immunoprecipitation shows that endogenous SOX-C factors recognize and bind to the intronic enhancer in GT1-1 cells and the hypothalamus. Accompanying immunohistochemical analysis demonstrates that SOX4 or SOX11 are highly expressed in the majority of hypothalamic GnRH neurons in adult mice. Taken together, these findings demonstrate that SOX-C transcription factors function as important transcriptional regulators of cell type-specific GnRH gene expression by acting on the intronic transcriptional enhancer.

Nova-1 mediates glucocorticoid-induced inhibition of pre-mRNA splicing of gonadotropin-releasing hormone transcripts

Glucocorticoid (GC) is known to affect the reproductive system by suppressing the gonadotropin-releasing hormone (GnRH) gene expression in the hypothalamus. However, the mechanism of this effect is poorly understood. We show here that the GC-induced reduction of GnRH mRNA is due to attenuation of a post-transcriptional process i.e. splicing of intron A. Treatment of dexamethasone (DEX), a synthetic GC, lowered GnRH mRNA transcripts and was accompanied by reduced excision of the first intron (intron A) from the GnRH pre-mRNA both in vitro and in vivo. While seeking to identify the splicing factors involved in GC-inhibited GnRH pre-mRNA splicing, we found that DEX down-regulated neuro-oncological ventral antigen-1 (Nova-1) mRNA and protein and that knockdown of Nova-1 reduced intron A excision from GnRH pre-mRNA. Nova-1 overexpression reversed the DEX-induced reduction of intron A excision. Nova-1 appears to promote intron A excision by binding to the distal region of exon 1 of the GnRH pre-mRNA. Taken together, our findings indicate that the intron A excision by Nova-1 is a target of GC for down-regulation of GnRH gene expression, and more importantly, we characterized Nova-1, a brain-enriched splicing regulator responsible for GnRH pre-mRNA splicing.

Oxytocin and vasopressin gene expression and RNA splicing patterns in the rat supraoptic nucleus

In this study, we test the hypothesis that there are differential splicing patterns between the expressed oxytocin (OT) and vasopressin (VP) genes in the rat supraoptic nucleus (SON). We quantify the low abundance, intron-containing heteronuclear RNAs (hnRNAs) and the higher abundance mRNAs in the SON using two-step, quantitative SYBR Green real-time reverse transcription (RT)-PCR and external standard curves constructed using synthetic 90 nt sense-strand oligonucleotides. The levels of OT and VP mRNA in the SON were found to be similar, approximately 10(8) copies/SON pair, whereas the copy numbers of VP hnRNAs containing intron 1 or 2 and the OT hnRNA containing intron 1 are much lower, i.e., approximately 10(2)-10(3) copies/rat SON pair. However, the estimated copy number of the intron 2-containing OT hnRNA is much larger, approximately 10(6) copies/SON pair. The relative distributions of all the OT and VP RNA species were invariant and independent of the physiological status of the rats (e.g., osmotically stimulated or lactating rats). Using intron-specific riboprobes against hnRNAs, we demonstrate by fluorescence in situ hybridization strong signals of OT hnRNA containing intron 2 predominantly in the cytoplasm, in contrast to the localization of the VP hnRNA found only in the nuclei. Taken together, these data support the view that the splicing patterns between OT and VP gene transcripts are different and show that there is a selective cytoplasmic retention of OT intron 2.

Restricted expression of Epstein-Barr virus latent genes in murine B cells derived from embryonic stem cells

Background

Several human malignancies are associated with Epstein-Barr virus (EBV) and more than 95% of the adult human population carries this virus lifelong. EBV efficiently infects human B cells and persists in this cellular compartment latently. EBV-infected B cells become activated and growth transformed, express a characteristic set of viral latent genes, and acquire the status of proliferating lymphoblastoid cell lines in vitro. Because EBV infects only primate cells, it has not been possible to establish a model of infection in immunocompetent rodents. Such a model would be most desirable in order to study EBV's pathogenesis and latency in a suitable and amenable host.

Methodology/Principal Findings

We stably introduced recombinant EBV genomes into mouse embryonic stem cells and induced their differentiation to B cells in vitro to develop the desired model. In vitro differentiated murine B cells maintained the EBV genomes but expression of viral genes was restricted to the latent membrane proteins (LMPs). In contrast to human B cells, EBV's nuclear antigens (EBNAs) were not expressed detectably and growth transformed murine B cells did not arise in vitro. Aberrant splicing and premature termination of EBNA mRNAs most likely prevented the expression of EBNA genes required for B-cell transformation.

Conclusions/Significance

Our findings indicate that fundamental differences in gene regulation between mouse and man might block the route towards a tractable murine model for EBV.

Generation of transgenic rats expressing green fluorescent protein in S-100beta-producing pituitary folliculo-stellate cells and brain astrocytes

Folliculo-stellate (FS) cells are known to act as sustentacular cells or scavenger cells in the anterior lobe. However, the precise function and origin of FS cells are still under discussion. Like brain astrocytes, FS cells contain S-100beta protein, and FS cells can be detected immunocytochemically using antibodies for S-100beta protein after fixation; however, living FS cells can not be detected. The generation of transgenic rats expressing green fluorescent protein (GFP) under the control of S-100beta protein gene promoter may allow the detection of living FS cells, which may be an excellent tool for the study of FS cells. With the aim of generation of transgenic rats, we analyzed the promoter activity of the S-100beta gene and found that intron 1 is important for cell-specific expression of the S-100beta gene. Therefore, we obtained a DNA construct containing GFP gene under a part of the S-100 promoter with intron 1. We transfected the construct into rat embryos and succeeded in generating transgenic rats. The transgenic rats expressed GFP in FS cells specifically in the anterior lobe. GFP is also expressed in other known S-100beta-expressing cells, i.e. brain astrocytes, adipocytes, and chondrocytes. We believe that the newly generated transgenic rats will provide a new approach for the study of FS cells and other S-100beta protein-producing cells.

Some thoughts about translational regulation: Forward and backward glances

This review discusses the need to re-examine some popular but unproven ideas about regulation of translation in eukaryotes. Translational control is invoked often on superficial grounds, such as a discrepancy between mRNA and protein levels which could be explained instead by rapid turnover of the protein. It is essential to verify that there is translational control (i.e., essential to rule out alternative mechanisms) before asking how translation is regulated. Many of the postulated control mechanisms are dubious. It is easy to create artifactual regulation (a slight increase or decrease in translation) by over-expressing recombinant RNA-binding proteins. The internal-initiation hypothesis is the source of other misunderstandings. Recent claims about the involvement of internal ribosome entry sequences (IRESs) in cancer and other diseases are discussed. The scanning model for initiation provides a more credible framework for understanding many aspects of translation, including ways to restrict the production of potent regulatory proteins which would be harmful if over-expressed. The rare production in eukaryotes of dicistronic mRNAs (e.g., from retrotransposons) raises questions about how the 3' cistron gets translated. Some proposed mechanisms are discussed, but the available evidence does not allow resolution of the issue.

BMAL1 shuttling controls transactivation and degradation of the CLOCK/BMAL1 heterodimer

CLOCK and BMAL1 are bHLH-PAS-containing transcription factors that bind to E-box elements and are indispensable for expression of core circadian clock components such as the Per and Cry genes. A key step in expression is the heterodimerization of CLOCK and BMAL1 and their accumulation in the nucleus with an approximately 24-h periodicity. We show here that nucleocytoplasmic shuttling of BMAL1 is essential for transactivation and for degradation of the CLOCK/BMAL1 heterodimer. Using serial deletions and point mutants, we identified a functional nuclear localization signal and Crm1-dependent nuclear export signals in BMAL1. Transient-transfection experiments revealed that heterodimerization of CLOCK and BMAL1 accelerates their turnover, as well as E-box-dependent clock gene transcription. Moreover, in embryonic mouse fibroblasts, robust transcription of Per2 is tightly associated with massive degradation of the CLOCK/BMAL1 heterodimer. CRY proteins suppressed this process during the transcription-negative phase and led to nuclear accumulation of the CLOCK/BMAL1 heterodimer. Thus, these findings suggest that the decrease of BMAL1 abundance during the circadian cycle reflects robust transcriptional activation of clock genes rather than inhibition of BMAL1 synthesis.

Intron retention as an alternative splice variant of the rat urocortin 1 gene

Urocortin 1, highly conserved metazoan gene of the corticotropin-releasing hormone family, is a simple gene structured in two exons and the corresponding intron. The urocortin 1 prepropeptide is entirely coded in the second exon. Preliminary non-isotopic in situ hybridization experiments with an oligonucleotide complementary to an intron sequence of the urocortin 1 gene showed a significant cytoplasmic-like staining, suggesting the occurrence of an intron-retained urocortin 1 transcript. This observation prompted us to study whether the urocortin 1 gene presents alternative splicing by intron retention event. Confocal fluorescent in situ hybridization for urocortin 1 RNA and the use of the specific DNA dye TOPRO-3 allowed us to show significant expression of the intron-retained urocortin 1 transcript that did not colocalize with TOPRO-3 staining indicating a cytoplasmic localization for the intron-retained urocortin 1 transcript. The natural occurrence of a polyadenylated intron-retained urocortin 1 RNA was further documented by reverse transcriptase polymerase chain reaction (PCR), primed with oligo(dT), of total RNA extracted from three brain regions, a midbrain region containing the Edinger-Westphal nucleus, cerebellum and prefrontal cortex. In the three brain regions studied, it was possible to amplify both intron-less as well as intron-retained urocortin 1 transcripts. The use of PCR primers that simultaneously amplify both urocortin 1 transcripts allowed us to show that the expression of both urocortin 1 transcripts differs among the brain regions analyzed, suggesting a tissue specific regulation of this alternative splicing. In silico analysis of the five known mammalian urocortin 1 genomic sequences showed high conservation of the urocortin 1 intron sequence. Further studies should investigate the regulation of this intron retention event and its consequence for the functionality of the urocortin 1 gene.

Molecular and evolutionary characterization of the GnRH-II gene in the chicken: Distinctive genomic organization, expression pattern, and precursor sequence

Of all the structural variants of GnRH (gonadotropin-releasing hormone), GnRH-II has been found to be universally present in and uniquely conserved among jawed vertebrates without any sequence substitutions. Our previous study found that the GnRH-II precursor sequences have become divergent in the lineage of eutherian mammals, based on a comparison between reptilian and mammalian GnRH-II. To elucidate the molecular evolution of GnRH-II throughout amniotes, we have performed the first identification of the avian GnRH-II cDNA/gene from the chicken, the species used for the initial discovery of GnRH-II peptide. Gene arrangement around the GnRH-II in the chicken was similar to that in mammals; however, a gene MRPS26 was partly overlapped with the downstream part of the GnRH-II in the chicken. It was identified that the GnRH-II/MRPS26 locus generated at least five distinct types of transcripts with different expression patterns and three of them may produce functional GnRH-II decapeptide. Sequence comparison revealed that the prepro-GnRH-II polypeptide of the chicken was substantially different from those of other species regarding the length and similarity. The present results strongly indicated that considerable variations were generated in the precursor sequence of the evolutionarily conserved GnRH-II during amniote evolution. It was also suggested that the sequence divergence seen in the chicken may have occurred independently of that in the mammalian lineage.

Cooperative actions of Tra2alpha with 9G8 and SRp30c in the RNA splicing of the gonadotropin-releasing hormone gene transcript

In earlier studies, we demonstrated that excision of the first intron (intron A) from the gonadotropin-releasing hormone (GnRH) transcript is highly cell type- and developmental stage-specific. The removal of GnRH intron A requires exonic splicing enhancers on exons 3 and 4 (ESE3 and ESE4, respectively). Tra2alpha,a serine/arginine-rich (SR)-like protein, specifically binds to ESE4, although it requires additional nuclear co-factors for efficient removal of this intron. In the present study, we demonstrate the cooperative action of multiple SR proteins in the regulation of GnRH pre-mRNA splicing. SRp30c specifically binds to both ESE3 and ESE4, whereas 9G8 binds to an element in exon 3 and strongly enhances the excision of GnRH intron A in the presence of minimal amount of other nuclear components. Interestingly, Tra2alpha can interact with either 9G8 or SRp30c, whereas no interaction between 9G8 and SRp30c is observed. Tra2alpha has an additive effect on the RNA binding of these proteins. Overexpression or knock-down of these three proteins in cultured cells further suggests their essential role in intron A excision activities, and their presence in GnRH neurons of the mouse preoptic area further strengthens this possibility. Together, these results indicate that interaction of Tra2alpha with 9G8 and SRp30c appears to be crucial for ESE-dependent GnRH pre-mRNA splicing, allowing efficient generation of mature mRNA in GnRH-producing cells.

Synthesis and secretion of GnRH

Comprehensive studies have provided a clear understanding of the effects of gonadal steroids on the secretion of gonadotropin releasing hormone (GnRH), but some inconsistent results exist with regard to effects on synthesis. It is clear that regulation of both synthesis and the secretion of GnRH are effected by neurotransmitter systems in the brain. Thus, steroid regulation of GnRH synthesis and secretion can be direct, but the predominant effects are transmitted through steroid-responsive neuronal systems in various parts of the brain. There is also emerging evidence of direct effects on GnRH cells. Overriding effects on synthesis and secretion of GnRH can be observed during aging, in undernutrition and under stressful situations; these involve various neuronal systems, which may have serial or parallel effects on GnRH cells. The effect of aging is accompanied by changes in GnRH synthesis, but comprehensive studies of synthesis during undernutrition and stress are less well documented. Altered GnRH and gonadotropin secretion that occurs in seasonal breeding animals and during the pubertal transition is not generally accompanied by changes in GnRH synthesis. Secretion of GnRH from the brain is a reflection of the inherent function of GnRH cells and the inputs that integrate all of the central regulatory elements. Ultimately, the pattern of secretion dictates the reproductive status of the organism. In order to fully understand the central mechanisms that control reproduction, more extensive studies are required on the neuronal circuitry that provides input to GnRH cells.

Regulation of translation via mRNA structure in prokaryotes and eukaryotes

The mechanism of initiation of translation differs between prokaryotes and eukaryotes, and the strategies used for regulation differ accordingly. Translation in prokaryotes is usually regulated by blocking access to the initiation site. This is accomplished via base-paired structures (within the mRNA itself, or between the mRNA and a small trans-acting RNA) or via mRNA-binding proteins. Classic examples of each mechanism are described. The polycistronic structure of mRNAs is an important aspect of translational control in prokaryotes, but polycistronic mRNAs are not usable (and usually not produced) in eukaryotes. Four structural elements in eukaryotic mRNAs are important for regulating translation: (i) the m7G cap; (ii) sequences flanking the AUG start codon; (iii) the position of the AUG codon relative to the 5' end of the mRNA; and (iv) secondary structure within the mRNA leader sequence. The scanning model provides a framework for understanding these effects. The scanning mechanism also explains how small open reading frames near the 5' end of the mRNA can down-regulate translation. This constraint is sometimes abrogated by changing the structure of the mRNA, sometimes with clinical consequences. Examples are described. Some mistaken ideas about regulation of translation that have found their way into textbooks are pointed out and corrected.

Differential adaptive responses to chronic stress of maternally stressed male mice offspring

It is well established that stress in early life can alter the activity of the hypothalamus-pituitary-adrenal (HPA) axis, but most studies to date have focused on HPA reactivity in response to a single acute stress. The present study addressed whether stress in pregnant mice could influence the adaptive responses of their offspring to chronic stress. Male offspring were exclusively used in this study. Elevated plus maze tests revealed that 14 d of repeated restraint stress (6 h per day; from postnatal d 50-63) significantly increased anxiety-like behavior in maternally stressed mice. NBI 27914, a CRH receptor antagonist, completely eliminated anxiety-related behaviors in a dose-dependent manner, indicating an involvement of a hyperactive CRH system. In accordance with increased anxiety, CRH contents in the hypothalamus and amygdala were significantly higher in these mice. Despite an increased basal activity of the CRH-ACTH system, the combination of chronic prenatal and postnatal stress resulted in a significant reduction of basal plasma corticosterone level, presumably because of a defect in adrenal function. Along with alterations in hypothalamic and hippocampal corticosteroid receptors, it was also demonstrated that a dysfunction in negative feedback inhibition of the HPA axis could be deteriorated by chronic stress in maternally stressed male mice. Taken together, these results indicate that exposure to maternal stress in the womb can affect an animal's coping capacity to chronic postnatal stress.

GnRH pre-mRNA splicing: solving the mystery of a nature's knockout, hpg mouse

The hypogonadal (hpg) mouse represents a unique animal model for hypogonadism. In this mutant the truncation of the gene encoding gonadotropin-releasing hormone (GnRH) leads to drastically lowered gonadotropin levels and prepubertal gonads. The deletional mutation encompasses only the distal half of the gene leaving the region encoding GnRH decapeptide intact. The partially deleted gene is transcriptionally active, but translationally inactive. Even though several aspects have been considered to account for the phenomenon, there is no satisfactory explanation so far. Recent reports showed that excision of the GnRH first intron is delicately regulated in a cell type- and developmental stage-specific manner mediated by putative-specific splicing factors acting on cis-acting elements located in exon 3 and 4, and is significantly decreased in hpg mouse whose exonic splicing enhancers are absent. Furthermore, the suppressing effect of intron A retention on the translational activity of downstream open reading frame was reported, giving an insight into the understanding the mystery of hpg mice.

LCN6, a novel human epididymal lipocalin

BACKGROUND

The lipocalin (LCN) family of structurally conserved hydrophobic ligand binding proteins is represented in all major taxonomic groups from prokaryotes to primates. The importance of lipocalins in reproduction and the similarity to known epididymal lipocalins prompted us to characterize the novel human epididymal LCN6.

METHODS AND RESULTS

LCN6 cDNA was identified by database analysis in a comprehensive human library sequencing program. Macaca mulatta (rhesus monkey) cDNA was obtained from an epididymis cDNA library and is 93% homologous to the human. The gene is located on chromosome 9q34 adjacent LCN8 and LCN5. LCN6 amino acid sequence is most closely related to LCN5, but the LCN6 beta-barrel structure is best modeled on mouse major urinary protein 1, a pheromone binding protein. Northern blot analysis of RNAs isolated from 25 human tissues revealed predominant expression of a 1.0 kb mRNA in the epididymis. No other transcript was detected except for weak expression of a larger hybridizing mRNA in urinary bladder. Northern hybridization analysis of LCN6 mRNA expression in sham-operated, castrated and testosterone replaced rhesus monkeys suggests mRNA levels are little affected 6 days after castration. Immunohistochemical staining revealed that LCN6 protein is abundant in the caput epithelium and lumen. Immunofluorescent staining of human spermatozoa shows LCN6 located on the head and tail of spermatozoa with the highest concentration of LCN6 on the post-acrosomal region of the head, where it appeared aggregated into large patches.

CONCLUSIONS

LCN6 is a novel lipocalin closely related to Lcn5 and Lcn8 and these three genes are likely products of gene duplication events that predate rodent-primate divergence. Predominant expression in the epididymis and location on sperm surface are consistent with a role for LCN6 in male fertility.