SPOC1 (PHF13) is required for spermatogonial stem cell differentiation and sustained spermatogenesis

Single-cell analysis identifies critical regulators of spermatogonial development and differentiation in cattle-yak bulls

Spermatogenesis is a continuous process in which functional sperm are produced through a series of mitotic and meiotic divisions and morphological changes in germ cells. The aberrant development and fate transitions of spermatogenic cells cause hybrid sterility in mammals. Cattle-yak, a hybrid animal between taurine cattle (Bos taurus) and yak (Bos grunniens), exhibits male-specific sterility due to spermatogenic failure. In the present study, we performed single-cell RNA sequencing analysis to identify differences in testicular cell composition and the developmental trajectory of spermatogenic cells between yak and cattle-yak. The composition and molecular signatures of spermatogonial subtypes were dramatically different between these 2 animals, and the expression of genes associated with stem cell maintenance, cell differentiation and meiotic entry was altered in cattle-yak, indicating the impairment of undifferentiated spermatogonial fate decisions. Cell communication analysis revealed that signaling within different spermatogenic cell subpopulations was weakened, and progenitor spermatogonia were unable or delayed receiving and sending signals for transformation to the next stage in cattle-yak. Simultaneously, the communication between niche cells and germ cells was also abnormal. Collectively, we obtained the expression profiles of transcriptome signatures of different germ cells and testicular somatic cell populations at the single-cell level and identified critical regulators of spermatogonial differentiation and meiosis in yak and sterile cattle-yak. The findings of this study shed light on the genetic mechanisms that lead to hybrid sterility and speciation in bovid species.

Discovery of a Novel Antiviral Effect of the Restriction Factor SPOC1 against Human Cytomegalovirus

The chromatin-remodeler SPOC1 (PHF13) is a transcriptional co-regulator and has been identified as a restriction factor against various viruses, including human cytomegalovirus (HCMV). For HCMV, SPOC1 was shown to block the onset of immediate-early (IE) gene expression under low multiplicities of infection (MOI). Here, we demonstrate that SPOC1-mediated restriction of IE expression is neutralized by increasing viral titers. Interestingly, our study reveals that SPOC1 exerts an additional antiviral function beyond the IE phase of HCMV replication. Expression of SPOC1 under conditions of high MOI resulted in severely impaired viral DNA replication and viral particle release, which may be attributed to inefficient viral transcription. With the use of click chemistry, the localization of viral DNA was investigated at late time points after infection. Intriguingly, we detected a co-localization of SPOC1, RNA polymerase II S5P and polycomb repressor complex 2 (PRC2) components in close proximity to viral DNA in areas that are hypothesized to harbor viral transcription sites. We further identified the N-terminal domain of SPOC1 to be responsible for interaction with EZH2, a subunit of the PRC2 complex. With this study, we report a novel and potent antiviral function of SPOC1 against HCMV that is efficient even with unrestricted IE gene expression.

MiR-106b-5p Regulates the Reprogramming of Spermatogonial Stem Cells into iPSC (Induced Pluripotent Stem Cell)-Like Cells

Background:

Recent years have brought notable progress in raising the efficiency of the reprogramming technique so that approaches have evolved from known transgenic factors to only a few miRNAs. Nevertheless, there is a poor understanding of both the key factors and biological networks underlying this reprogramming. The present study aimed to investigate the potential of miR-106b-5p in regulating SSCs to iPSC-like cells.

Methods:

We used SSCs because pluripotency is inducible in SSCs under defined culture conditions, and they have a few issues compared to other adult stem cells. As both signaling and post-transcriptional gene controls are critical for pluripotency regulation, we traced the expression of OSKMN. Besides, we considered miR-106b-5p targets using bioinformatic methods.

Results:

Our results showed that transfected SSCs with miR-106b-5p increased the expression of the OSKMN factors, which was significantly more than negative control groups. Moreover, using the functional miRNA enrichment analysis, online tools, and databases, we predicted that miR-106b-5p targeted a signaling pathway gene named MAPK1/ERK2, related to regulating stem cell pluripotency.

Conclusion:

Together, our data suggest that miR-106b-5p regulates the reprogramming of SSCs into iPSC-like cells. Furthermore, noteworthy progress in the in vitro development of SSCs indicates promise reservoirs and opportunities for future clinical trials.

Epigenetic Regulation of Spermatogonial Stem Cell Homeostasis: From DNA Methylation to Histone Modification

Spermatogonial stem cells(SSCs)are the ultimate germline stem cells with the potential of self-renewal and differentiation, and a dynamic balance of SSCs play an essential role in spermatogenesis. During the gene expression process, genomic DNA and nuclear protein, working together, contribute to SSC homeostasis. Recently, emerging studies have shown that epigenome-related molecules such as chromatin modifiers play an important role in SSC homeostasis through regulating target gene expression. Here, we focus on two types of epigenetic events, including DNA methylation and histone modification, and summarize their function in SSC homeostasis. Understanding the molecular mechanism during SSC homeostasis will promote the recognition of epigenetic biomarkers in male infertility, and bring light into therapies of infertile patients.Graphical Abstract.

Tuning the Orchestra: HCMV vs. Innate Immunity

Understanding how the innate immune system keeps human cytomegalovirus (HCMV) in check has recently become a critical issue in light of the global clinical burden of HCMV infection in newborns and immunodeficient patients. Innate immunity constitutes the first line of host defense against HCMV as it involves a complex array of cooperating effectors – e.g., inflammatory cytokines, type I interferon (IFN-I), natural killer (NK) cells, professional antigen-presenting cells (APCs) and phagocytes – all capable of disrupting HCMV replication. These factors are known to trigger a highly efficient adaptive immune response, where cellular restriction factors (RFs) play a major gatekeeping role. Unlike other innate immunity components, RFs are constitutively expressed in many cell types, ready to act before pathogen exposure. Nonetheless, the existence of a positive regulatory feedback loop between RFs and IFNs is clear evidence of an intimate cooperation between intrinsic and innate immunity. In the course of virus-host coevolution, HCMV has, however, learned how to manipulate the functions of multiple cellular players of the host innate immune response to achieve latency and persistence. Thus, HCMV acts like an orchestra conductor able to piece together and rearrange parts of a musical score (i.e., innate immunity) to obtain the best live performance (i.e., viral fitness). It is therefore unquestionable that innovative therapeutic solutions able to prevent HCMV immune evasion in congenitally infected infants and immunocompromised individuals are urgently needed. Here, we provide an up-to-date review of the mechanisms regulating the interplay between HCMV and innate immunity, focusing on the various strategies of immune escape evolved by this virus to gain a fitness advantage.

The Lbc gene promotes differentiation of chicken embryo stem cell into spermatogonial stem cells via the regulation of transcriptional factor Hoxa5

Recently, the surface marker genes of spermatogonial stem cells (SSCs) were increasingly excavated and verified. However, few studies focused on the key genes involved in the regulation of SSCs differentiation. Our laboratory has screened the Lbc gene (GenBank accession number: XM_429585.3), which is specifically expressed on the SSCs. The aim of this study is to investigate the function of Lbc and its regulatory mechanism for SSCs. The indirect immunofluorescence assay (IFA) showed that Lbc was located in both nucleus and cytoplasm. Lbc was also overexpressed and knocked out both in vitro and in vivo to verify its function in SSCs, respectively. As a result, the overexpressed Lbc could promote the formation of spermatogonial stem cells like cells (SSCs-like), while the deficiency of Lbc blocked the formation of SSCs-like. We also identified the core region of Lbc promoter that located into the upstream of the transcription initiation site -247 to -2bp. Moreover, the activity of Lbc promoter could be increased by histone acetylation which is leading to the higher expression of Lbc. When we mutated the transcription factor HOXA5 and SOX10 that bound to the core region of Lbc promoter, HOXA5 could reduce the transcription activity of Lbc whereas the SOX10 was not. Currently, we found Lbc is a new specific marker of SSCs. This gene can be modified by histone acetylated and promote the formation of chicken SSCs via the transcription factor HOXA5. The present research will lay the foundation for further study on the regulatory mechanism of SSCs.

Catch me if you can: the arms race between human cytomegalovirus and the innate immune system

Human cytomegalovirus (HCMV), a common opportunistic pathogen of significant clinical importance, targets immunocompromised individuals of the human population worldwide. The absence of a licensed vaccine and the low efficacy of currently available drugs remain a barrier to combating the global infection. The HCMV's ability to modulate and escape innate immune responses remains a critical step in the ongoing search for potential drug targets. Here, we describe the complex interplay between HCMV and the host immune system, focusing on different evasion strategies that the virus has employed to subvert innate immune responses. We especially highlight the mechanisms and role of host antiviral restriction factors and provide insights into viral modulation of pro-inflammatory NF-κB and interferon signaling pathways.

Chromatin-Remodeling Factor SPOC1 Acts as a Cellular Restriction Factor against Human Cytomegalovirus by Repressing the Major Immediate Early Promoter

The cellular protein SPOC1 (survival time-associated PHD [plant homeodomain] finger protein in ovarian cancer 1) acts as a regulator of chromatin structure and the DNA damage response. It binds H3K4me2/3-containing chromatin and promotes DNA condensation by recruiting corepressors such as KAP-1 and H3K9 methyltransferases. Previous studies identified SPOC1 as a restriction factor against human adenovirus (HAdV) infection that is antagonized by E1B-55K/E4-orf6-dependent proteasomal degradation. Here, we demonstrate that, in contrast to HAdV-infected cells, SPOC1 is transiently upregulated during the early phase of human cytomegalovirus (HCMV) replication. We show that the expression of immediate early protein 1 (IE1) is sufficient and necessary to induce SPOC1. Additionally, we discovered that during later stages of infection, SPOC1 is downregulated in a glycogen synthase kinase 3β (GSK-3β)-dependent manner. We provide evidence that SPOC1 overexpression severely impairs HCMV replication by repressing the initiation of viral immediate early (IE) gene expression. Consistently, we observed that SPOC1-depleted primary human fibroblasts displayed an augmented initiation of viral IE gene expression. This occurs in a multiplicity of infection (MOI)-dependent manner, a defining hallmark of intrinsic immunity. Interestingly, repression requires the presence of high SPOC1 levels at the start of infection, while later upregulation had no negative impact, suggesting distinct temporal roles of SPOC1 during the HCMV replicative cycle. Mechanistically, we observed a highly specific association of SPOC1 with the major immediate early promoter (MIEP), strongly suggesting that SPOC1 inhibits HCMV replication by MIEP binding and the subsequent recruitment of heterochromatin-building factors. Thus, our data add SPOC1 as a novel factor to the endowment of a host cell to restrict cytomegalovirus infections.IMPORTANCE Accumulating evidence indicates that during millennia of coevolution, host cells have developed a sophisticated compilation of cellular factors to restrict cytomegalovirus infections. Defining this equipment is important to understand cellular barriers against viral infection and to develop strategies to utilize these factors for antiviral approaches. So far, constituents of PML nuclear bodies and interferon gamma-inducible protein 16 (IFI16) were known to mediate intrinsic immunity against HCMV. In this study, we identify the chromatin modulator SPOC1 as a novel restriction factor against HCMV. We show that preexisting high SPOC1 protein levels mediate a silencing of HCMV gene expression via a specific association with an important viral cis-regulatory element, the major immediate early promoter. Since SPOC1 expression varies between cell types, this factor may play an important role in tissue-specific defense against HCMV.

Dual role of the chromatin-binding factor PHF13 in the pre- and post-integration phases of HIV-1 replication

Viruses interact with multiple host cell factors. Some of these are required to promote viral propagation, others have roles in inhibiting infection. Here, we delineate the function of the cellular factor PHF13 (or SPOC1), a putative HIV-1 restriction factor. Early in the HIV-1 replication cycle PHF13 increased the number of integrated proviral copies and the number of infected cells. However, after HIV-1 integration, high levels of PHF13 suppressed viral gene expression. The antiviral activity of PHF13 is counteracted by the viral accessory protein Vpr, which mediates PHF13 degradation. Altogether, the transcriptional master regulator and chromatin binding protein PHF13 does not have purely repressive effects on HIV-1 replication, but also promotes viral integration. By the functional characterization of the dual role of PHF13 during the HIV-1 replication cycle, we reveal a surprising and intricate mechanism through which HIV-1 might regulate the switch from integration to viral gene expression. Furthermore, we identify PHF13 as a cellular target specifically degraded by HIV-1 Vpr.

Modes of Interaction of KMT2 Histone H3 Lysine 4 Methyltransferase/COMPASS Complexes with Chromatin

Regulation of gene expression is achieved by sequence-specific transcriptional regulators, which convey the information that is contained in the sequence of DNA into RNA polymerase activity. This is achieved by the recruitment of transcriptional co-factors. One of the consequences of co-factor recruitment is the control of specific properties of nucleosomes, the basic units of chromatin, and their protein components, the core histones. The main principles are to regulate the position and the characteristics of nucleosomes. The latter includes modulating the composition of core histones and their variants that are integrated into nucleosomes, and the post-translational modification of these histones referred to as histone marks. One of these marks is the methylation of lysine 4 of the core histone H3 (H3K4). While mono-methylation of H3K4 (H3K4me1) is located preferentially at active enhancers, tri-methylation (H3K4me3) is a mark found at open and potentially active promoters. Thus, H3K4 methylation is typically associated with gene transcription. The class 2 lysine methyltransferases (KMTs) are the main enzymes that methylate H3K4. KMT2 enzymes function in complexes that contain a necessary core complex composed of WDR5, RBBP5, ASH2L, and DPY30, the so-called WRAD complex. Here we discuss recent findings that try to elucidate the important question of how KMT2 complexes are recruited to specific sites on chromatin. This is embedded into short overviews of the biological functions of KMT2 complexes and the consequences of H3K4 methylation.

Human spermatogonial markers

In this review, we provide an up-to-date compilation of published human spermatogonial markers, with focus on the three nuclear subtypes A_dark, A_pale and B. In addition, we have extended our recently published list of putative spermatogonial markers with protein expression and RNA-sequencing data from the Human Protein Atlas and supported these by literature evidence. Most importantly, we have put substantial effort in acquiring a comprehensive list of new and potentially interesting markers by refiltering the raw data of 15 published germ cell expression datasets (four human, eleven rodent) and subsequent building of intersections to acquire a robust, cross-species set of spermatogonia-enriched or -specific transcripts.

PHF13: A new player involved in RNA polymerase II transcriptional regulation and co-transcriptional splicing

We recently identified PHF13 as an H3K4me2/3 chromatin reader and transcriptional co-regulator. We found that PHF13 interacts with RNAPIIS5P and PRC2 stabilizing their association with active and bivalent promoters. Furthermore, mass spectrometry analysis identified ∼50 spliceosomal proteins in PHF13s interactome. Here, we will discuss the potential role of PHF13 in RNAPII pausing and co-transcriptional splicing.

Imaging of Chromosome Dynamics in Mouse Testis Tissue by Immuno-FISH

The mouse (Mus musculus) represents the central mammalian genetic model system for biomedical and developmental research. Mutant mouse models have provided important insights into chromosome dynamics during the complex meiotic differentiation program that compensates for the genome doubling at fertilization. Homologous chromosomes (homologues) undergo dynamic pairing and recombine during first meiotic prophase before they become partitioned into four haploid sets by two consecutive meiotic divisions that lack an intervening S-phase. Fluorescence in situ hybridization (FISH) has been instrumental in the visualization and imaging of the dynamic reshaping of chromosome territories and mobility during prophase I, in which meiotic telomeres were found to act as pacemakers for the chromosome pairing dance. FISH combined with immunofluorescence (IF) co-staining of nuclear proteins has been instrumental for the visualization and imaging of mammalian meiotic chromosome behavior. This chapter describes FISH and IF methods for the analysis of chromosome dynamics in nuclei of paraffin-embedded mouse testes. The techniques have proven useful for fresh and archived paraffin testis material of several mammalian species.

PHF13 is a molecular reader and transcriptional co-regulator of H3K4me2/3

PHF13 is a chromatin affiliated protein with a functional role in differentiation, cell division, DNA damage response and higher chromatin order. To gain insight into PHF13's ability to modulate these processes, we elucidate the mechanisms targeting PHF13 to chromatin, its genome wide localization and its molecular chromatin context. Size exclusion chromatography, mass spectrometry, X-ray crystallography and ChIP sequencing demonstrate that PHF13 binds chromatin in a multivalent fashion via direct interactions with H3K4me2/3 and DNA, and indirectly via interactions with PRC2 and RNA PolII. Furthermore, PHF13 depletion disrupted the interactions between PRC2, RNA PolII S5P, H3K4me3 and H3K27me3 and resulted in the up and down regulation of genes functionally enriched in transcriptional regulation, DNA binding, cell cycle, differentiation and chromatin organization. Together our findings argue that PHF13 is an H3K4me2/3 molecular reader and transcriptional co-regulator, affording it the ability to impact different chromatin processes.

DOI:http://dx.doi.org/10.7554/eLife.10607.001

In human and other eukaryotic cells, DNA is packaged around proteins called histones to form a structure known as chromatin. Chemical tags added to the histones alter how the DNA is packaged and the activity of the genes encoded by that DNA. For example, many active genes are packaged around histone H3 proteins that have “Lysine 4 tri-methyl” tags attached to them. Another protein that is associated with chromatin is called PHF13 and it has several roles, including repairing damaged DNA. However, it was not known whether PHF13 binds to chromatin via the chemical tags, or in another way.

Ho-Ryun, Xu, Fuchs et al. used several biochemical techniques in mouse and human cells to explore how PHF13 specifically interacts with chromatin. These experiments showed that PHF13 binds specifically to DNA and to two types of methyl tags (lysine 4-tri-methyl or lysine 4-di-methyl). These chemical tags are predominantly found at active promoters as well as at a small subset of less active promoters known as bivalent promoters. PHF13 interacted with other proteins on the chromatin that are known to either drive or repress gene activity and it’s depletion affected the activity of many genes. Whether PHF13 increased or decreased gene activity depended on whether it was bound to active or bivalent promoters. The active promoters targeted by PHF13 had higher numbers of the tri-methyl tags whereas the di-methyl tags were more common on the bivalent promoters.

These findings provide preliminary evidence that a protein binding to different methyl tags in the same place on histone H3 can have opposite effects on gene activity. Ho-Ryun, Xu, Fuchs et al. now intend to find out more about the other proteins that interact with PHF13 on chromatin.

DOI:http://dx.doi.org/10.7554/eLife.10607.002

Histone H3.3 regulates dynamic chromatin states during spermatogenesis

The histone variant H3.3 is involved in diverse biological processes, including development, transcriptional memory and transcriptional reprogramming, as well as diseases, including most notably malignant brain tumors. Recently, we developed a knockout mouse model for the H3f3b gene, one of two genes encoding H3.3. Here, we show that targeted disruption of H3f3b results in a number of phenotypic abnormalities, including a reduction in H3.3 histone levels, leading to male infertility, as well as abnormal sperm and testes morphology. Additionally, null germ cell populations at specific stages in spermatogenesis, in particular spermatocytes and spermatogonia, exhibited increased rates of apoptosis. Disruption of H3f3b also altered histone post-translational modifications and gene expression in the testes, with the most prominent changes occurring at genes involved in spermatogenesis. Finally, H3f3b null testes also exhibited abnormal germ cell chromatin reorganization and reduced protamine incorporation. Taken together, our studies indicate a major role for H3.3 in spermatogenesis through regulation of chromatin dynamics.

SPOC1-mediated antiviral host cell response is antagonized early in human adenovirus type 5 infection

Little is known about immediate phases after viral infection and how an incoming viral genome complex counteracts host cell defenses, before the start of viral gene expression. Adenovirus (Ad) serves as an ideal model, since entry and onset of gene expression are rapid and highly efficient, and mechanisms used 24–48 hours post infection to counteract host antiviral and DNA repair factors (e.g. p53, Mre11, Daxx) are well studied. Here, we identify an even earlier host cell target for Ad, the chromatin-associated factor and epigenetic reader, SPOC1, recently found recruited to double strand breaks, and playing a role in DNA damage response. SPOC1 co-localized with viral replication centers in the host cell nucleus, interacted with Ad DNA, and repressed viral gene expression at the transcriptional level. We discovered that this SPOC1-mediated restriction imposed upon Ad growth is relieved by its functional association with the Ad major core protein pVII that enters with the viral genome, followed by E1B-55K/E4orf6-dependent proteasomal degradation of SPOC1. Mimicking removal of SPOC1 in the cell, knock down of this cellular restriction factor using RNAi techniques resulted in significantly increased Ad replication, including enhanced viral gene expression. However, depletion of SPOC1 also reduced the efficiency of E1B-55K transcriptional repression of cellular promoters, with possible implications for viral transformation. Intriguingly, not exclusive to Ad infection, other human pathogenic viruses (HSV-1, HSV-2, HIV-1, and HCV) also depleted SPOC1 in infected cells. Our findings provide a general model for how pathogenic human viruses antagonize intrinsic SPOC1-mediated antiviral responses in their host cells. A better understanding of viral entry and early restrictive functions in host cells should provide new perspectives for developing antiviral agents and therapies. Conversely, for Ad vectors used in gene therapy, counteracting mechanisms eradicating incoming viral DNA would increase Ad vector efficacy and safety for the patient.

Viruses have acquired functions that target and modulate host cell signaling and diverse regulatory cascades, leading to efficient viral propagation. During the course of productive infection, Ad gene products manipulate destruction pathways to prevent viral clearance or cell death prior to viral genome amplification and release of progeny. Recently, we reported that chromatin formation and cellular SWI/SNF chromatin remodeling processes play a key role in Ad transcriptional regulation. Here, we observe for the first time that SPOC1, identified as a regulator of DNA damage response and chromatin structure, plays an essential role in restricting Ad gene expression and progeny production. This host cell antiviral mechanism is efficiently counteracted by tight association with the major core protein pVII bound to the incoming viral genome. Subsequently, SPOC1 undergoes proteasomal degradation via the Ad E1B-55K/E4orf6-dependent, Cullin-based E3 ubiquitin ligase complex. We also show that other viruses from RNA and DNA families also induce efficient degradation of SPOC1. These analyses of evasion strategies acquired by viruses and other human pathogens should provide important insights into factors manipulating the epigenetic environment to potentially inactivate, or amplify host cell immune responses, since detailed molecular mechanisms and the full repertoire of cellular targets still remain elusive.

SPOC1 modulates DNA repair by regulating key determinants of chromatin compaction and DNA damage response

Survival time-associated plant homeodomain (PHD) finger protein in Ovarian Cancer 1 (SPOC1, also known as PHF13) is known to modulate chromatin structure and is essential for testicular stem-cell differentiation. Here we show that SPOC1 is recruited to DNA double-strand breaks (DSBs) in an ATM-dependent manner. Moreover, SPOC1 localizes at endogenous repair foci, including OPT domains and accumulates at large DSB repair foci characteristic for delayed repair at heterochromatic sites. SPOC1 depletion enhances the kinetics of ionizing radiation-induced foci (IRIF) formation after γ-irradiation (γ-IR), non-homologous end-joining (NHEJ) repair activity, and cellular radioresistance, but impairs homologous recombination (HR) repair. Conversely, SPOC1 overexpression delays IRIF formation and γH2AX expansion, reduces NHEJ repair activity and enhances cellular radiosensitivity. SPOC1 mediates dose-dependent changes in chromatin association of DNA compaction factors KAP-1, HP1-α and H3K9 methyltransferases (KMT) GLP, G9A and SETDB1. In addition, SPOC1 interacts with KAP-1 and H3K9 KMTs, inhibits KAP-1 phosphorylation and enhances H3K9 trimethylation. These findings provide the first evidence for a function of SPOC1 in DNA damage response (DDR) and repair. SPOC1 acts as a modulator of repair kinetics and choice of pathways. This involves its dose-dependent effects on DNA damage sensors, repair mediators and key regulators of chromatin structure.

Differential marker protein expression specifies rarefaction zone-containing human Adark spermatogonia

It is unclear whether the distinct nuclear morphologies of human Adark(Ad) and Apale(Ap) spermatogonia are manifestations of different stages of germ cell development or phases of the mitotic cycle, or whether they may reflect still unknown molecular differences. According to the classical description by Clermont, human dark type A spermatogonium (Ad) may contain one, sometimes two or three nuclear ‘vacuolar spaces’ representing chromatin rarefaction zones. These structures were readily discerned in paraffin sections of human testis tissue during immunohistochemical and immunofluorescence analyses and thus represented robust morphological markers for our study. While a majority of the marker proteins tested did not discriminate between spermatogonia with and without chromatin rarefaction zones, doublesex- and mab-3-related transcription factor (DMRT1), tyrosine kinase receptor c-Kit/CD117 (KIT) and proliferation-associated antigen Ki-67 (KI-67) appeared to be restricted to subtypes which lacked the rarefaction zones. Conversely, exosome component 10 (EXOSC10) was found to accumulate within the rarefaction zones, which points to a possible role of this nuclear domain in RNA processing.