Testis- and developmental stage-specific expression of hnRNP A2/B1 splicing isoforms, B0a/b

Effect of prenatal perfluoroheptanoic acid exposure on spermatogenesis in offspring mice

BACKGROUND

Perfluoroheptanoic acid (PFHpA), a persistent organic pollutant widespread in the environment, is suspected as an environmental endocrine disruptor for its disturbance effect on hormone homeostasis and reproductive development. Whereas the effect of intrauterine PFHpA exposure during gestation on spermatogenesis of male offspring mice is still unknown.

OBJECTIVE

This study aimed to explore the effect of prenatal PFHpA exposure on the reproductive development of male offspring mice and the role of N⁶-methyladenosine (m⁶A) during the process.

METHODS

Fifty-six C57BL/6 pregnant mice were randomly divided into 4 groups. During the gestation period, the pregnant mice were exposed to 0, 0.0015, 0.015, and 0.15 mg/kg bw/d PFHpA from gestational day 1 (GD1) to GD16 by oral gavage. The male offspring mice were sacrificed by spinal dislocation at 7 weeks old. The body weight, testicular weight, and brain weight were weighed, and the intra-testicular testosterone was detected. The sperm qualities were analyzed with computer-aided sperm analysis (CASA). The testicular tissues were taken to analyze the pathological changes and examine the global m⁶A RNA methylation levels. Quantitative real-time PCR (qRT-PCR) was adopted to figure out the mRNA expression levels of m⁶A-related enzymes in testicular tissues of different PFHpA treated groups. Methylated RNA immunoprecipitation sequencing (MeRIP-seq) was applied to further explore the m⁶A RNA methylation at a whole-genome scale.

RESULTS

Compared with the control group, no significant differences were observed in body weight, testicular weight, testicular coefficient, and the visceral-brain ratio of testicular tissue in the PFHpA treated groups. And no significant change was observed in intra-testicular testosterone among the four groups. CASA results showed a decrease of sperm count, sperm concentration, and total cell count, as well as an increase of sperm progressive cells' head area after prenatal PFHpA exposure (P < 0.05). Hematoxylin and eosin staining of pathological sections showed seminiferous tubules morphological change, disorder arrangement of seminiferous epithelium, and reduction of spermatogenic cells in the PFHpA treated groups. PFHpA significantly decreased global levels of m⁶A RNA methylation in testicular tissue (P < 0.05). Besides, qRT-PCR results showed significant alteration of the mRNA expression levels of seven m⁶A-related enzymes (Mettl3, Mettl5, Mettl14, Pcif1, Wtap, Hnrnpa2b1, and Hnrnpc) in the PFHpA treated groups (P < 0.05). MeRIP-seq results showed a correlation between prenatal PFHpA exposure and activation and binding of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Cnga3 and Mpzl3 showed differential expression in the enrichment subcategories or pathways.

CONCLUSIONS

Exposure to PFHpA during the gestation period would adversely affect the development of seminiferous tubules and testicular m⁶A RNA methylation in offspring mice, which subsequently interferes with spermatogenesis and leads to reproductive toxicity.

Uncoupling transcription and translation through miRNA-dependent poly(A) length control in haploid male germ cells

As one of the post-transcriptional regulatory mechanisms, uncoupling of transcription and translation plays an essential role in development and adulthood physiology. However, it remains elusive how thousands of mRNAs get translationally silenced while stability is maintained for hours or even days before translation. In addition to oocytes and neurons, developing spermatids display significant uncoupling of transcription and translation for delayed translation. Therefore, spermiogenesis represents an excellent in vivo model for investigating the mechanism underlying uncoupled transcription and translation. Through full-length poly(A) deep sequencing, we discovered dynamic changes in poly(A) length through deadenylation and re-polyadenylation. Deadenylation appeared to be mediated by microRNAs (miRNAs), and transcripts with shorter poly(A) tails tend to be sequestered into ribonucleoprotein (RNP) granules for translational repression and stabilization. In contrast, re-polyadenylation might allow for translocation of the translationally repressed transcripts from RNP granules to polysomes. Overall, our data suggest that miRNA-dependent poly(A) length control represents a previously unreported mechanism underlying uncoupled translation and transcription in haploid male mouse germ cells.

Key proteins of proteome underlying sperm malformation of rats exposed to low fenvalerate doses are highly related to P53

Fenvalerate (Fen) is an endocrine disruptor, capable of interfering with the activity of estrogen and androgen. Our objective was to explore the molecular mechanisms of Fen on sperm in vivo. Adult male Sprague-Dawley rats were orally exposed to 0, 0.00625, 0.125, 2.5, 30 mg/kg/day Fen for 8 weeks. Sperm morphology, differential proteomics of sperm and testes, bioinformatic analysis, western blotting (WB), and RT-PCR were used to explore the mechanism of Fen on sperm. Data showed that low Fen doses significantly induced sperm malformations. In sperm proteomics, 47 differentially expressed (DE) proteins were enriched in biological processes (BPs) related to energy metabolism, response to estrogen, spermatogenesis; and enriched in cellular components (CCs) relating to energy-metabolism, sperm fibrous sheath and their outer dense fibers. In testicular proteomics, 56 DE proteins were highly associated with mRNA splicing, energy metabolism; and enriched in CCs relating to vesicles, myelin sheath, microtubules, mitochondria. WB showed that the expression of selected proteins was identical to their tendency in 2D gels. Literature indicates that key DE proteins in proteomic profiles (such as Trap1, Hnrnpa2b1, Hnrnpk, Hspa8, and Gapdh) are involved in P53-related processes or morphogenesis or spermatogenesis. Also, P53 mRNA and protein levels were significantly increased by Fen; bioinformatic re-analysis showed that 88.5% DE proteins and P53 formed a complex interacting network, and the key DE proteins were coenriched with P53-related BPs. Results indicate that key DE proteins of proteome underlying sperm malformations of rats exposed to low Fen doses are highly related to P53.

RNA N6-methyladenosine modification, spermatogenesis, and human male infertility

RNA N6-methyladenosine (m6A) modification is one of the main forms of posttranscriptional modification, and its dysregulation is involved in a series of pathological processes. RNA m6A regulators, which mediate dynamic RNA m6A modification, are expressed in almost all types of testicular cells, including spermatogenetic cells and somatic cells. Cumulative studies have found that knockout of RNA m6A regulators in the testis leads to abnormal metabolism of the target mRNAs, which eventually causes spermatogenetic disorders and infertility. To date, a role for dysregulated RNA m6A modification in human male infertility remains elusive; however, dysregulated expression of RNA m6A regulators in abnormal human semen samples, including oligospermia, asthenozoospermia and azoospermia, has been found. Therefore, we speculate that abnormal RNA m6A methylation may be an important mechanism of male infertility. In this review, we summarize the recent findings regarding the spatiotemporal expression of RNA m6A regulators in the testes, mechanisms of RNA m6A modification in spermatogenesis and the relation between dysregulated RNA m6A regulators and human male infertility. In addition, we also discuss future directions in studying the molecular mechanism of male infertility and exploring their clinical applications from the viewpoint of RNA m6A modification.

Unraveling transcriptome dynamics in human spermatogenesis

Spermatogenesis is a dynamic developmental process that includes stem cell proliferation and differentiation, meiotic cell divisions and extreme chromatin condensation. Although studied in mice, the molecular control of human spermatogenesis is largely unknown. Here, we developed a protocol that enables next-generation sequencing of RNA obtained from pools of 500 individually laser-capture microdissected cells of specific germ cell subtypes from fixed human testis samples. Transcriptomic analyses of these successive germ cell subtypes reveals dynamic transcription of over 4000 genes during human spermatogenesis. At the same time, many of the genes encoding for well-established meiotic and post-meiotic proteins are already present in the pre-meiotic phase. Furthermore, we found significant cell type-specific expression of post-transcriptional regulators, including expression of 110 RNA-binding proteins and 137 long non-coding RNAs, most of them previously not linked to spermatogenesis. Together, these data suggest that the transcriptome of precursor cells already contains the genes necessary for cellular differentiation and that timely translation controlled by post-transcriptional regulators is crucial for normal development. These established transcriptomes provide a reference catalog for further detailed studies on human spermatogenesis and spermatogenic failure.

Highlighted Article: Using laser capture microscopy, a comprehensive transcriptomic dataset of well-defined and distinct germ cell subtypes based on morphology and localization in the human testis is generated.

Novel role of hnRNP-A2/B1 in modulating aryl hydrocarbon receptor ligand sensitivity

The aryl hydrocarbon receptor (AHR) is responsible for susceptibility to its ligand-dependent responses. However, the effect of non-AHR factors is less clear. To explore the non-AHR factors, we used two mouse strains with different AHR genetic variants, namely C3H/lpr and MRL/lpr strains with Ala and Val as the 375th amino acid residue, respectively. To assess the contribution of AHR alone, COS-7 cells transiently expressing AHR from each strain were treated with 6-formylindolo[3,2-b]carbazole (FICZ) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and xenobiotic-responsive element (XRE)-driven reporter gene activities were measured. FICZ-EC50 values for the C3H/lpr and MRL/lpr AHR-mediated transactivation were 0.023 and 0.046 nM, respectively, indicating a similar susceptibility in both AHR genotypes. In contrast, C3H/lpr AHR was fourfold more sensitive to TCDD than MRL/lpr AHR. By a pull-down assay using a XRE-containing PCR product as bait and the hepatic nuclear extracts of both FICZ-treated mouse strains, we identified two interacting proteins as heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNP-A2) and its splicing variant (hnRNP-A2b). Immunoprecipitation assays demonstrated the AHR interaction with hnRNP-A2/B1. When hnRNP-A2 was co-expressed with the MRL/lpr or C3H/lpr AHR in COS-7, FICZ treatment decreased EC50 to about threefold in both AHR genotypes, compared with EC50 in AHR alone. Similarly, hnRNP-A2b co-expression also lowered the FICZ-EC50 values. In TCDD-treated COS-7, responses depended on the AHR genotype; while no change in TCDD-EC50 was observed for C3H/lpr AHR when hnRNP-A2 was co-expressed, the value was reduced to nearly tenfold for MRL/lpr AHR. Co-transfection with hnRNP-A2b attenuated the AHR sensitivity to TCDD. In conclusion, the hnRNP-A2/B1 interacting with AHR may be a modulator of the AHR ligand sensitivity.

Linking spermatid ribonucleic acid (RNA) binding protein and retrogene diversity to reproductive success

Spermiogenesis is a postmeiotic process that drives development of round spermatids into fully elongated spermatozoa. Spermatid elongation is largely controlled post-transcriptionally after global silencing of mRNA synthesis from the haploid genome. Here, rats that differentially express EGFP from a lentiviral transgene during early and late steps of spermiogenesis were used to flow sort fractions of round and elongating spermatids. Mass-spectral analysis of 2D gel protein spots enriched >3-fold in each fraction revealed a heterogeneous RNA binding proteome (hnRNPA2/b1, hnRNPA3, hnRPDL, hnRNPK, hnRNPL, hnRNPM, PABPC1, PABPC4, PCBP1, PCBP3, PTBP2, PSIP1, RGSL1, RUVBL2, SARNP2, TDRD6, TDRD7) abundantly expressed in round spermatids prior to their elongation. Notably, each protein within this ontology cluster regulates alternative splicing, sub-cellular transport, degradation and/or translational repression of mRNAs. In contrast, elongating spermatid fractions were enriched with glycolytic enzymes, redox enzymes and protein synthesis factors. Retrogene-encoded proteins were over-represented among the most abundant elongating spermatid factors identified. Consistent with these biochemical activities, plus corresponding histological profiles, the identified RNA processing factors are predicted to collectively drive post-transcriptional expression of an alternative exome that fuels finishing steps of sperm maturation and fitness.

Functional implications of the emergence of alternative splicing in hnRNP A/B transcripts

The heterogeneous nuclear ribonucleoproteins (hnRNPs) A/B are a family of RNA-binding proteins that participate in various aspects of nucleic acid metabolism, including mRNA trafficking, telomere maintenance, and splicing. They are both regulators and targets of alternative splicing, and the patterns of alternative splicing of their transcripts have diverged between paralogs and between orthologs in different species. Surprisingly, the extent of this splicing variation and its implications for post-transcriptional regulation have remained largely unexplored. Here, we conducted a detailed analysis of hnRNP A/B sequences and expression patterns across six vertebrates. Alternative exons emerged via the introduction of new splice sites, changes in the strengths of existing splice sites, and the accumulation of auxiliary splicing regulatory motifs. Observed isoform expression patterns could be attributed to the frequency and strength of cis-elements. We found a trend toward increased splicing variation in mammals and identified novel alternatively spliced isoforms in human and chicken. Pulldown and translational assays demonstrated that the inclusion of alternative exons altered the affinity of hnRNP A/B proteins for their cognate nucleic acids and modified protein expression levels. As the hnRNPs A/B regulate several key steps in mRNA processing, the involvement of diverse hnRNP isoforms in multiple cellular contexts and species implies concomitant differences in the transcriptional output of these systems. We conclude that the emergence of alternative splicing in the hnRNPs A/B has contributed to the diversification of their roles in the regulation of alternative splicing and has thus added an unexpected layer of regulatory complexity to transcription in vertebrates.

Exposure of male rats to cyclophosphamide alters the chromatin structure and basic proteome in spermatozoa

BACKGROUND

The formation of mature sperm involves the expression of numerous proteins during spermiogenesis and the replacement of histones with protamines to package the genome. Exposure to cyclophosphamide (CPA), an anticancer alkylating agent, during spermiogenesis may disrupt chromatin condensation with adverse consequences to the offspring.

METHODS

Adult male rats were given CPA in one of two schedules: (i) subchronic, 4 days - day 1 (100 mg kg(-1)) and days 2-4 (50 mg kg(-1) per day) or (ii) chronic - daily (6.0 mg kg(-1) per day). Animals were euthanized on days 14, 21 or 28.

RESULTS

The effects of CPA on epididymal sperm chromatin structure were germ-cell-phase specific; mid-spermiogenic spermatids were most sensitive. The acridine orange DNA denaturation assay showed significant increases in susceptibility to denaturation (P < 0.01). Chromatin packaging assessment revealed 1,4-dithiothreitol-dependent chromomycin A3 DNA binding and less condensed, protamine-deficient sperm; the total thiol (P < 0.001) and protamine contents (P < 0.01), measured using monobromobimane and the HUP1N protamine 1 antibody, respectively, were reduced. The sperm basic proteome was also altered; proteins that were identified are involved in events during spermiogenesis and fertilization.

CONCLUSIONS

Paternal exposure to CPA alters sperm chromatin structure, as well as the composition of sperm head basic proteins. We speculate that these changes underlie effects on fertilization and embryo development.

Aberrant Expression of the Autoantigen Heterogeneous Nuclear Ribonucleoprotein-A2 (RA33) and Spontaneous Formation of Rheumatoid Arthritis-Associated Anti-RA33 Autoantibodies in TNF-α Transgenic Mice

Human TNF-alpha transgenic (hTNFtg) mice develop erosive arthritis closely resembling rheumatoid arthritis (RA). To investigate mechanisms leading to pathological autoimmune reactions in RA, we examined hTNFtg animals for the presence of RA-associated autoantibodies including Abs to citrullinated epitopes (anti-cyclic citrullinated peptide), heterogeneous nuclear ribonucleoprotein (hnRNP)-A2 (anti-RA33), and heat shock proteins (hsp) (anti-hsp). Although IgM anti-hsp Abs were detected in 40% of hTNFtg and control mice, IgG anti-hsp Abs were rarely seen, and anti-cyclic citrullinated peptide Abs were not seen at all. In contrast, >50% of hTNFtg mice showed IgG anti-RA33 autoantibodies, which became detectable shortly after the onset of arthritis. These Abs were predominantly directed to a short epitope, which was identical with an epitope previously described in MRL/lpr mice. Incidence of anti-RA33 was significantly decreased in mice treated with the osteoclast inhibitor osteoprotegerin and also in c-fos-deficient mice lacking osteoclasts. Pronounced expression of hnRNP-A2 and a smaller splice variant was seen in joints of hTNFtg mice, whereas expression was low in control animals. Although the closely related hnRNP-A1 was also overexpressed, autoantibodies to this protein were infrequently detected. Because expression of hnRNP-A2 in thymus, spleen, brain, and lung was similar in hTNFtg and control mice, aberrant expression appeared to be restricted to the inflamed joint. Finally, immunization of hTNFtg mice with recombinant hnRNP-A2 or a peptide harboring the major B cell epitope aggravated arthritis. These findings suggest that overproduction of TNF-alpha leads to aberrant expression of hnRNP-A2 in the rheumatoid joint and subsequently to autoimmune reactions, which may enhance the inflammatory and destructive process.

Up-regulation of the ubiquitous alternative splicing factor Tra2β causes inclusion of a germ cell-specific exon

We have discovered a new exon of the homeodomain-interacting kinase HipK3 that incorporates a premature stop codon and is included only in the human testis. To investigate this, we tested the effects of transfecting cells with green fluorescent protein fusions of RNA-binding proteins implicated in spermatogenesis using a novel assay based on multi-fraction fluorescence-activated cell sorting (MF-FACS). This allows the effect of a controlled titration of any splicing factor on the splicing of endogenous genes to be studied in vivo. We found that Tra2beta recapitulates testis-specific splicing of endogenous HipK3 in a concentration-dependent manner and binds specifically to a long purine-rich sequence in the novel exon. This sequence was also specifically bound by hnRNP A1, hnRNP H, ASF/SF2 and SRp40, but not by 9G8. Consistent with these observations, in vitro studies showed that this sequence shifts splicing to a downstream 5' splice site within a heterologous pre-mRNA substrate in the presence of Tra2beta, ASF/SF2 and SRp40, whereas hnRNP A1 specifically inhibits this choice. By mutating the purine-rich sequence in the context of the HipK3 gene, we also show that it is the major determinant of Tra2beta- and hnRNP A1-mediated regulation. Tra2 is essential for sex determination and spermatogenesis in flies, and Tra2beta protein was most highly expressed in testis out of six mouse tissues, whereas hnRNP A1 is down-regulated during germ cell development. Therefore, our data imply an evolutionarily conserved role for Tra2 proteins in spermatogenesis and suggest that an elevated concentration of Tra2beta may convert it into a tissue-specific splicing factor.

Angiomyofibroblastoma of the vulva: report of a case with immunohistochemical and molecular analysis

A 43-year-old woman presented with a mass in the subcutaneous tissue of the right labium majus. A lipoma or Bartholin gland cyst was suspected and excision of the lesion was performed. The lesion was well circumscribed, and histological examination revealed a typical angiomyofibroblastoma. The lesion was composed of alternating hypocellular edematous and hypercellular areas with abundant vessels, and plump tumor cells were loosely dispersed or aggregated mainly around the vessels. Tumor cells were immunoreactive for vimentin and desmin but negative for muscle actins. Ultrastructurally, the tumor cells contained a moderate amount of rough endoplasmic reticulum and abundant intermediate filaments, and had primitive junctions. Pinocytotic vesicles or basal lamina were not evident. Immunohistochemical studies also revealed that the tumor cells expressed basic fibroblast-growth factor, vascular-endothelial-growth factor, and stem-cell factor, factors that may contribute to the rich vascularity and mast cells within the tumor. Reverse transcription-polymerase chain reaction detected high mobility group I-C (HMGI-C) transcripts in the tumor tissue. Because the expression of HMGI-C is regulated by developmental and differentiation processes and is not found in adult normal tissues, HMGI-C may be involved in the tumorigenesis of angiomyofibroblastoma.

Characterization of the mouse hnRNP A2/B1/B0 gene and identification of processed pseudogenes

The mouse hnRNP A2/B1/B0 gene has been cloned using a PCR-based strategy and sequenced. Analysis of this sequence showed that the gene organization closely follows that of the human orthologue with 12 exons and 11 introns. The hnRNP A2/B1/B0 gene gives rise to four splice variants through alternative splicing of exons 2 and 9. RT-PCR assays indicated that all splice variants were expressed in mouse brain, skin, and stomach tissues of varying ages, although their ratios to one another varied with age and tissue type. We also identified a small subset of all polyadenylated splice variants that included intron 11, which shows 94% sequence identity between human and mouse. Several processed pseudogenes were identified in the mouse genome. A search of the mouse genome databases located five pseudogenes, four of which are presumed to be non-functional because of the presence of premature stop codons, large deletions or rearrangements within the coding region. The fifth, which possesses putative promoter elements and has a coding sequence identical to that of the hnRNP A2 mRNA variant, may be functional.

Heterogeneous nuclear ribonucleoprotein A3, a novel RNA trafficking response element-binding protein

The cis-acting response element, A2RE, which is sufficient for cytoplasmic mRNA trafficking in oligodendrocytes, binds a small group of rat brain proteins. Predominant among these is heterogeneous nuclear ribonucleoprotein (hnRNP) A2, a trans-acting factor for cytoplasmic trafficking of RNAs bearing A2RE-like sequences. We have now identified the other A2RE-binding proteins as hnRNP A1/A1(B), hnRNP B1, and four isoforms of hnRNP A3. The rat and human hnRNP A3 cDNAs have been sequenced, revealing the existence of alternatively spliced mRNAs. In Western blotting, 38-, 39-, 41-, and 41.5-kDa components were all recognized by antibodies against a peptide in the glycine-rich region of hnRNP A3, but only the 41- and 41.5-kDa bands bound antibodies to a 15-residue N-terminal peptide encoded by an alternatively spliced part of exon 1. The identities of these four proteins were verified by Edman sequencing and mass spectral analysis of tryptic fragments generated from electrophoretically separated bands. Sequence-specific binding of bacterially expressed hnRNP A3 to A2RE has been demonstrated by biosensor and UV cross-linking electrophoretic mobility shift assays. Mutational analysis and confocal microscopy data support the hypothesis that the hnRNP A3 isoforms have a role in cytoplasmic trafficking of RNA.

Base excision repair is limited by different proteins in male germ cell nuclear extracts prepared from young and old mice

The combined observations of elevated DNA repair gene expression, high uracil-DNA glycosylase-initiated base excision repair, and a low spontaneous mutant frequency for a lacI transgene in spermatogenic cells from young mice suggest that base excision repair activity is high in spermatogenic cell types. Notably, the spontaneous mutant frequency of the lacI transgene is greater in spermatogenic cells obtained from old mice, suggesting that germ line DNA repair activity may decline with age. A paternal age effect in spermatogenic cells is recognized for the human population as well. To determine if male germ cell base excision repair activity changes with age, uracil-DNA glycosylase-initiated base excision repair activity was measured in mixed germ cell (i.e., all spermatogenic cell types in adult testis) nuclear extracts prepared from young, middle-aged, and old mice. Base excision repair activity was also assessed in nuclear extracts from premeiotic, meiotic, and postmeiotic spermatogenic cell types obtained from young mice. Mixed germ cell nuclear extracts exhibited an age-related decrease in base excision repair activity that was restored by addition of apurinic/apyrimidinic (AP) endonuclease. Uracil-DNA glycosylase and DNA ligase were determined to be limiting in mixed germ cell nuclear extracts prepared from young animals. Base excision repair activity was only modestly elevated in pachytene spermatocytes and round spermatids relative to other spermatogenic cells. Thus, germ line short-patch base excision repair activity appears to be relatively constant throughout spermatogenesis in young animals, limited by uracil-DNA glycosylase and DNA ligase in young animals, and limited by AP endonuclease in old animals.

Interaction of hnRNP A2/B1 isoforms with telomeric ssDNA and the in vitro function

Overexpression of heterogeneous nuclear ribonucleoprotein (hnRNP) A2/B1, especially of B1 has been reported as a useful marker to detect cancers in early stage, although the biological reason is not clear. A2/B1 proteins were previously reported to bind telomeric DNA repeats. Alternative splicing of A2/B1 gene produces abundant A2, less abundant B1, and testis-specific minor isoforms B0a and B0b. In this study, B1 and B0b that have the N-terminal 12 amino acid insertion were suggested to have higher affinities to telomeric single-stranded DNA (ssDNA) than A2 and B0a. Kinetic analyses using purified B1 and B0b indicated that they interact dynamically with a single array of telomeric repeats. Furthermore, functional assays demonstrated that B1 and B0b bind with telomeric repeats in a tandem fashion and protect them from a nuclease and promote telomerase activity. A2/B1 proteins, especially B1 and B0b, may function as telomeric ssDNA-binding proteins in cancer and reproductive cells.