Dynamics and Function of Nuclear Bodies during Embryogenesis

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Dynamics and function of nuclear bodies during embryogenesis Dahyana Arias Escayola, and Karla Neugebauer Biochemistry, Just Accepted Manuscript • DOI: 10.1021/acs.biochem.7b01262 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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Dynamics and function of nuclear bodies during embryogenesis Dahyana Arias Escayola and Karla M. Neugebauer* Molecular Biophysics and Biochemistry, Yale University, New Haven CT [email protected]

ABSTRACT:

Nuclear bodies are RNA-rich membraneless organelles in the cell nucleus that

concentrate specific sets of nuclear proteins and RNA-protein complexes. Nuclear bodies such as the nucleolus, Cajal body (CB), and the histone locus body (HLB) concentrate factors required for nuclear steps of RNA processing. Formation of these nuclear bodies occurs on genomic loci and is frequently associated with active sites of transcription. Whether nuclear body formation is dependent on a particular gene element, an active process such as transcription, or the nascent RNA present at gene loci is a topic of debate. Recently, this question has been addressed through studies in model organisms and their embryos. The switch from maternally provided RNA and protein to zygotic gene products in early embryos has been well characterized in a variety of organisms. This process, termed maternal-to-zygotic transition (MZT), provides an excellent model for studying formation of nuclear bodies before, during and after the transcriptional activation of the zygotic genome. Here, we review findings in embryos that reveal key principles in the study formation and function of nucleoli, CBs, and HLBs. We propose that while

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particular gene elements may contribute to formation of these nuclear bodies, active transcription promotes maturation of nuclear bodies and efficient RNA processing within them INTRODUCTION Membraneless nuclear organelles or “nuclear bodies” include the nucleolus, the Cajal body (CB), the histone locus body (HLB), speckles, paraspeckles, and PML bodies (1). Aside from PML bodies, all nuclear bodies are RNA-rich and contain subsets of nuclear proteins that reflect different functions in the biogenesis of polyadenylated mRNA, replication-dependent histone mRNA and ribosomes. Proteins involved in these processes contain higher than typical tendency to intrinsic disorder, suggesting they may arise through liquid-liquid phase separation (LLPS) and raising the question of how they maintain their unique identities (2,3) . Nuclear bodies have been identified in diverse cells, but their composition has primarily been characterized in aneuploid mammalian tissue culture cells (4). Moreover, only the nucleolus can be biochemically purified, rendering the identification of specific components of other nuclear bodies somewhat haphazard (5). Significant work has addressed the dynamic formation and composition of nucleoli, CBs, and HLBs in model organisms and their embryos. Moreover, critical experiments in these systems have addressed the physiological roles of these nuclear bodies. In this review, we focus on the dynamics and function of nucleoli, CBs, and HLBs. Their overarching cellular roles in embryos reflect the need for the efficient expression and maturation of pre-ribosomal subunits assembled in the nucleolus, spliceosomal small nuclear ribonucleoprotein particles (snRNPs) assembled in the CB and histone mRNAs processed in the HLB (Fig 1, Table 1). We catalog current knowledge of the protein and RNA components of these nuclear bodies in the model organisms studied (Table 2). Because each of these nuclear bodies is associated with distinct genomic sites often present on different chromosomes, nuclear


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bodies appear to organize the genome in the 3-dimensional space of the nucleus (Table 1). We discuss the potential significance of this effect on genome organization in embryos. The key reason to consider nuclear body formation and function in early embryos is that they provide physiologically relevant models for understanding these processes in the context of transcriptional inactivity at fertilization and transcriptional activation – called zygotic genome activation (ZGA) – when demands for pre-mRNA splicing and histone mRNA 3’ end processing are high. Interestingly, the demand for ribosomes is delayed in embryos due to their maternal contribution of ribosomes and the late activation of rDNA genes. The naturally occurring transition from transcriptional silence to activity in early embryos are analogous to transcriptional inactivity in dividing cells in culture (Fig 2). Thus, embryogenesis enables analysis of nuclear bodies in the context of natural transcriptional programs, RNA-protein interactions,

and

gene

positions.


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Figure 1. Nuclear bodies organize chromatin and concentrate components for RNA processing (A) rRNA processing occurs in nucleoli (top) snoRNPs in the nucleolus modify prerRNA (here depicted by a star). The U3 snoRNP binds nascent rRNA and promotes cleavage of nascent pre-rRNA into its 18S, 5.8S, and 28S components. Nucleoli (bottom, blue) form on rDNA repeats and concentrate factors for rRNA processing. (B) snRNP assembly occurs in CBs (top) scaRNAs in the CB modify nascent snRNAs. Assembly of the U4/U6 snRNP occurs in CBs and is depicted here. CBs (bottom, green) form on snRNA gene loci and concentrate factors for snRNP assembly. (C) Histone mRNA 3’ end processing occurs in HLBs (top)The histone cleavage complex (HCC) binds nascent RNA at replication dependent histone genes. U7 snRNA base pairs with the histone downstream element and protein constituents of the snRNP guide


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nascent RNA cleavage in the HLB. HLBs (bottom, purple) form on histone gene clusters and concentrate

factors

for

histone

mRNA

processing.

Table 1. Nuclear body function and genomic location Body

Function

Genomic site

Nucleol us

pre-rRNA cleavage

Nucleolar Organizing Regions (NORs)

rRNA pseudouridylation rRNA 2’ O-methylation pre-ribosomal assembly

Cajal Body

rRNA gene loci

subunit

snRNA gene loci: U1, U2, targeting new snRNAs U11, U4, U5, U7 for export snoRNA gene snRNA loci: U3, U8 pseudouridylation snRNA transcription

snRNA 2’ methylation

O-

snRNP assembly U3/U8 snoRNA 5’ end capping (7-meG) snoRNP assembly Histone Locus Body

histone transcription

gene Replication Dependent histone gene histone mRNA 3’ end loci: HIST1, processing HIST2

NUCLEOLI


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The nucleolus is the site of rDNA transcription, rRNA modification and processing, and preribosomal subunit assembly (Fig 1A and Table 1). In tissue culture cells, formation of a nucleolus is regulated by its epigenetic status; non-coding RNAs transcribed from the intergenic space between rDNA genes regulate methylation of rDNA and nucleolus formation by recruiting the chromatin remodeling complex NoRC and silencing rDNA (6). Recent work on nucleolus formation in embryos has focused on its dependence on rDNA transcription by RNA polymerase I (Pol I). Nucleoli form at rDNA sites called nucleolar organizing regions (NORs). Whether this formation is dependent on the genomic element of rDNA or its active transcription has been a topic of debate. The use of embryos helps answer this question because at fertilization, the embryo’s genome is transcriptionally silent (Fig 2). rDNA transcriptional activation can be identified by the presence of pre-rRNA transcripts in staged embryos. Early work in mouse embryos showed the nucleolus does not assemble until after rRNA transcription is initiated (7). In C. elegans embryos, inhibition of rDNAtranscription abolishes canonical nucleoli formation (8). Additionally, nucleolar size is directly proportional to cell size during development, suggesting a homeostatic relationship between ribosome synthesis and cell growth (9). More recently, the formation of nucleoli has been studied in fruitfly (Drosophila melanogaster) and zebrafish (Danio rerio) embryos, where nucleoli are absent before zygotic genome activation (Fig 2). In early Drosophila embryos, there is a lag in nucleolus formation as a critical concentration of pre-rRNA is reached following rDNA transcription activation (10). In the absence of rDNA repeats, nucleoli do not assemble, but small droplet structures or “assemblies” containing nucleolar proteins are transiently observed. These assemblies are less frequent and smaller than mature nucleoli, and their localization is not specific to a genomic site. While the necessity of a critical concentration of pre-rRNA is consistent with an LLPS model of


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formation, it does not exclude the possibility of active processes driving nuclear body formation. Indeed, recruitment of the nucleolar proteins Ns1 and Mod to Drosophila nucleoli appears to be dependent on active transcription (11). In zebrafish embryos, rDNA transcription by Pol I is activated hours after transcriptional activation by RNA polymerase II (Pol II) has occurred (12,13). Prior to Pol II activation, the small nucleolar ribonucleoprotein particle (snoRNP) component fibrillarin, a common marker of nucleoli, localizes exclusively to CBs. After rDNA transcription, the concentration of fibrillarin decreases in CBs and increases in nucleoli. Taken together, these studies demonstrate that although nucleolus formation occurs at NORs, further formation depends on active rDNA transcription and possibly other active processes.

Figure 2. Relationship between transcriptional activity and nuclear body formation in the early zebrafish embryo and during the cell cycle of tissue culture cells In 24 hours, both HeLa cells and zebrafish embryos undergo a phase of natural transcriptional inactivity (M phase and fertilization), followed by a phase of transcriptional onset (G1 and ZGA). In both cases, transcriptional activation correlates with nuclear body formation. In embryogenesis, the correlated changes in nuclear bodies and transcription occur on a backdrop of rapid cell


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divisions, consistent with the hypothesis that transcription – not the cell cycle per se – is the crucial

factor.

CAJAL BODIES CBs were discovered in vertebrate brains by Ramon y Cajal more than 100 years ago (14,15). They are scaffolded by the intrinsically disordered protein coilin (Table 2), which is required for the concentration of spliceosomal small nuclear RNAs (snRNAs) and snRNP proteins (16). After transcription by Pol II, snRNAs undergo core assembly in the cytoplasm and return to the nucleus for modification and assembly into mature snRNPs (Fig 1B and Table 1)). Immature snRNPs and transient intermediates in the snRNP assembly pathway concentrate in CBs in HeLa cells, providing the first evidence that CBs are the sites of snRNP assembly (17). Specifically, U6 mono-snRNP is targeted to CBs by SART3 (Fig 1B); there, it undergoes secondary structure rearrangements and base-pairing with U4 snRNA, creating the U4/U6 di-snRNP. Next, U4/U6specific proteins join, facilitating U5 snRNP recruitment and formation of the splicing-competent U4/U6•U5 tri-snRNP. Mathematical modeling and fluorescence measurements indicate that assembly in the context of concentrated precursors makes snRNP assembly more efficient (18,19). In addition to snRNP assembly, snRNAs are themselves modified within CBs due to the concentration of snoRNPs containing fibrillarin (a methyltransferase) and dyskerin (a pseudouridylase) are present in CBs to guide RNA modifications of snRNAs (20). These snoRNAs are specifically named scaRNAs, because they localize to the CB and not nucleoli. True nucleolar snoRNAs also traffic through CBs en route to nucleoli, likely to undergo snoRNP assembly steps (21). Although snRNAs are modified in CBs, modification is not dependent on coilin in tissue culture cells (22). A potential explanation arises from the observation that


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scaRNAs concentrate in so-called residual CBs when coilin is absent. Although snRNPs are not concentrated in these residual bodies, snRNA modification may still occur there (22). In agreement with this finding in tissue culture cells, fruit flies lacking CBs still show normal levels of snRNA modifications by scaRNAs (23). Taken together, itseems that snRNA modification can take place independent of snRNPs concentration in CBs, though subtle differences in efficiency cannot be ruled out. Evidence that enhanced snRNP assembly by CBs is important for life comes from experiments in model organisms, which revealed their roles in embryos. Loss of coilin and therefore CBs is lethal for zebrafish embryos. Because viability is rescued through injection of mature human snRNPs, lethality appears to be due to insufficient production of snRNPs needed for splicing zygotic pre-mRNAs expressed in embryos at ZGA (24). These findings are consistent with the coilin knockout phenotype in mice, which is “semi-lethal” and characterized by reduced embryonic viability and fertility of adults (25). Conversely, in Drosophila and Arabidopsis thaliana, homozygous coilin knockouts disperse CBs but are still viable and fertile (26,27). There is no proven explanation for why coilin seems to be essential in vertebrates and inessential in insects and plants. It is tempting to speculate that the cells of fly embryos, which develop as a syncytial blastoderm prior to ZGA, share a common cytoplasm. Thus levels of many nuclear components, like immature snRNPs coming into the nucleus, would be potentially less heterogenous from cell to cell. This system of sharing cellular resources may alleviate the organismal consequences of disadvantages that may follow from environmental conditions or mutation. Clearly, work from the Matera lab on the mouse coilin knockout show the small number of embryos that survive embryogenesis without coilin develop into relatively normal adults. This suggests that COIL-/- mouse embryos are borderline for embryonic gene expression.


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Perhaps coilin -/- flies can better tolerate suboptimal snRNP assembly conditions due to the sharing or buffering of snRNP concentration on a per nucleus basis due to the syncytial blastoderm. To resolve this question, it would be interesting to measure the concentrations of immature and mature snRNPs in WT and mutant zebrafish and fly embryos and compare total numbers to single cell values. Ultimately, species differences may come down to the question of how many cells in an embryo have to die or stop proliferating to cause developmental arrest. Species similarities and differences are also seen in the number of CBs relative to the onset of ZGA. For example, ZGA occurs in the mouse at the two-cell stage, yet CBs are provided by the maternal and paternal pronuclei

(28). Similarly, hundreds of maternally contributed,

extrachromosomal CBs have been observed in amphibians (14). Early transcriptionally inactive zebrafish embryos contain many CBs (29); one cell stage zebrafish embryos can contain up to 30 CBs, a number that is reduced until differentiated cells display only 2 CBs per cell (24). Yet even though CBs are clearly contributed maternally during a period of transcriptional activity, CB number is sensitive to transcription inhibition after ZGA (29). This latter dependence on transcription is similar to tissue culture cells, where CB formation and/or maintenance depends on the cell cycle, overall transcriptional activity, and can even be induced by introducing heterologous active transcription units (21,30–32).

HISTONE LOCUS BODIES HLBs were first classified as a separate nuclear body in Drosophila tissues, where particular makers were show to colocalize with replication dependent (RD) histone gene loci (33). HLB formation may not entirely depend on active processes, as HLBs are able to form in G1 cells when histone transcription is silenced. HLBs contain high concentrations of the factors required


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for histone gene transcription by Pol II and co-transcriptional histone mRNA 3′ end formation (Tables 1 and 2). Mature HLB formation in Drosophila embryos is dependent on a sequence in the H3-H4 promoter (34). Before ZGA, small foci termed “proto-HLBs” form on chromosomes independently of histone gene expression (35). These “proto-HLBs” reflect the recruitment of FLASH (FLICE-associated huge protein) and Mxc, two common HLB markers, at histone gene loci. More recently, CLAMP (chromatin-linked adaptor for male-specific lethal [MSL] proteins), has been identified as a component of proto-HLBs, consistent with the model that DNA binding proteins are the first to be recruited to histone gene loci (36). Proto-HLBs can also form on chromosomes in the absence of histone genes, but are transient in nature and disappear throughout the course of fly embryogenesis (34,37). Mature HLBs do not form until histone gene transcription is activated and the remaining histone mRNA 3’ end processing factors – stem loop binding protein (SLBP) and the U7 snRNP (Fig 1C, Tables 1 and 2)– are recruited (34,35). This suggests that HLB formation is ordered and depends on both a genetic element and transcriptional activity. Recent work in H. pulcherrimus, a sea urchin, showed that interchromosomal interactions of two early histone gene loci are dependent on active transcription of early histone genes(38). There are currently no studies demonstrating interchromosomal interactions in embryonic HLB’s. In fact, histone gene clusters do not colocalize with each other in tissue culture (39–41). In early zebrafish embryos, HLBs form early during ZGA and they become more robust as histone gene transcription increases (29). In Drosophila embryos, HLB formation at sites of active transcription is necessary for efficient histone mRNA biogenesis (42). Failure to recruit sufficient HLB factors results in impaired histone pre-mRNA processing, leading to aberrant polyadenylation of histone mRNAs and transcriptional read through of histone genes.


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SIGNIFICANCE OF NUCLEAR BODY FORMATION AT GENOMIC LOCI Gene expression and subsequent RNA processing is tightly linked to spatial organization in the nucleus (1). We have seen that nuclear bodies form on genomic loci and play an important role in nuclear organization, by concentrating RNA processing factors in each respective organelle. Nucleoli, CBs, and HLBs are separate entities within the nucleus, and assemble on distinct gene loci. The formation of such distinct nuclear bodies that are specifically involved in RNA processing steps taking place on distinct pools of RNA may result from demixing of nuclear body components (8,43–46). The nucleolus assembles at repeated rRNA gene loci and serves as a site of ribosome biogenesis. As rDNA is transcribed, the nucleolus concentrates ribosome maturation factors along NORs (47). Not all rRNA genes are actively transcribed, and rDNA/NOR positioning in nucleoli is dependent upon transcription (48). In C. elegans, RNA plays a modulatory role in nucleolus formation (8), offering an explanation for how enhanced rDNA transcription caused by the ncl-1 mutation leads to larger nucleoli (49) Cajal bodies interact with snRNA and histone loci on separate chromosomes, facilitating intra-chromosomal interactions in these otherwise distant loci (39). Cells lacking CBs do not display this chromosomal organization, and depletion of CBs correlates with lower levels of snRNAs, suggesting potential roles in transcription or stability. Coilin, the CB scaffolding protein, directly interacts with snRNA gene loci and their transcript product (21). Histone genes occur in clusters at which transcription of the different histone genes is tightly regulated (50). These arrays colocalize with HLBs, where efficient histone gene transcription as well as histone pre-mRNA processing occurs. Remarkably, placement of a histone gene transcription unit at an exogenous


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site in the genome leads to formation of a combined CB/HLB at the site in tissue culture cells that fused CBs and HLBs (32). Based on the above findings, three models are currently being considered to explain the formation of nuclear bodies at genomic sites. Two of these follow a “seeding model,” but differ in the nature of the seed. The term “seed” refers to an element that stabilizes the formation of the nuclear body and promotes phase separation. The first of these models suggests that formation is dependent on a particular gene element (DNA) that acts as the seed and recruits nuclear body factors. The second, is that the seed for nuclear bodies is the nascent RNA present at the gene locus. In contrast, the third existing model proposes that an active process (and its associated factors) such as transcription is required for formation. So far, most of the work done in embryos has focused on transcription as the active process in nuclear body formation. However, in tissue culture, post translational modifications (PTMs) of proteins can modulate whether certain proteins form nuclear bodies (51). For example, methylation of coilin regulates whether coilin is in CBs or in residual bodies known as gems (52). While active processes such as PTMs can modulate nuclear body formation, we use transcription as an example of this particular model. In human cells, addition of a non-transcriptionally active mouse NOR is sufficient to recruit human UBF and Pol I transcription machinery, but requires a functional promoter sequence to form a functional nucleolus, suggesting that gene elements are the seed for nucleolar formation (53). Additionally, previous work in Drosophila embryos demonstrated that the H3-H4 promoter sequence is sufficient to provide a scaffold for HLB factors FLASH and Mxc (34). On the other hand, there is strong evidence that nuclear body formation is transcriptionally driven (see above). When transcription is inhibited at the onset of mitosis in somatic cells, the nucleolus disassembles (47). In mammalian cells, recruitment of coilin and snRNAs to CBs is also


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transcription-dependent (30). Reduced transcription of histone genes during cell cycle arrest results in a loss of HLBs (54). Most of these studies have been performed in mammalian tissue culture cells, and the correlation of nuclear body formation to transcription has relied on active tracking of the cell cycle or modulation of transcriptional activity via the use of inhibitors. Prior work on nuclear bodies has used X. laevis oocytes due to their large size and highly abundant nuclear bodies. The germinal vesicle (GV) contains over 1,000 extrachromosomal nucleoli and 50-100 CBs and HLBs, showing that nuclear body association with chromatin is not required for their maintenance (14).This parallels the findings of extrachromosomal, transient, and immature “proto-nuclear bodies”, such as the foci of nucleolar proteins or the proto-HLB in flies (10,26,37). The abundance of nuclear bodies in oocytes and embryos may be to provide embryos with factors that will be required during MZT, when the zygotic genome becomes active for the first time and when cell cycles lack prolonged growth phases for biosynthesis (55). Instead, the embryo relies on maternally provided gene products during the rapid, synchronous cell divisions of cleavage stage. Although nucleoli are abundant in the Xenopus oocyte, they are absent in early embryos until rDNA transcription is activated in both D. melanogaster and D. rerio (29,56). CBs are present in the absence of transcription in early zebrafish embryos and prior to bulk ZGA in fruitflies (26,29) HLBs form at the onset of transcription in both zebrafish and fly embryos (33,37). Nuclear bodies can share components (Table 2), yet which nuclear body a component localizes to is often dependent on an active process. For example, methylation of coilin can determine whether it localizes to CBs or gems (52). During drosophila oogenesis, coilin localizes solely to a single CB, but as HLBs form and CBs disappear, coilin becomes more prominent in the HLB (57). Fibrillarin localizes to CBs in early zebrafish embryos, but is present only in nucleoli once rDNA transcription is activated (29). It appears that components of all three


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nuclear bodies can mix until the transcription of their respective loci triggers maturation and enforces the separate composition of nucleoli, CBs and HLBs. These observations indicate that specificity of demixing is differently balanced due to the seeding process provided by chromatin and

transcription

at

genomic

sites.

Table 2. Nuclear body components identified in oocytes and embryos Protein1

Body

Oocyte

UBF

Nucleolus

frog

NOLC1

Nucleolus

fly

Embryo

fly

(Nopp140) POLR1B

Nucleolus

fly

Nucleostemin

Nucleolus

fly

Fibrillarin

CB,

(rpI135)

frog, fly

fly, fish

fly

Nucleolus SMN

CB

fly

TAF6

CB

frog, newt

PARG

CB

TFIIS

CB

frog, newt

Coilin

CB, HLB

frog, fly, fly newt

SLBP

CB, HLB

frog

fly, fish

Lsm11

CB, HLB

fly

fly, fish

Symplekin

HLB

frog

fly

fly


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NPAT

HLB

fly

CLAMP2

HLB

fly

FLASH

HLB

fly

Lsm10

HLB

fly

SUPT6H

HLB

fly

TBP

HLB

fly

TRF2

HLB

fly

Mute2

HLB

fly

PARP

CB, HLB, Nucleolus

fly

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(mxc)

(Spt6)

1

Human terminology is used for each protein, if the name is different in the corresponding organism, it is shown in parentheses 2

Only found in fly


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Figure 3. Nuclear bodies form to meet demands for RNA processing Extrachromosomal CBs (green) are present and proto-HLBs are bound to the histone gene locus prior to ZGA when transcription is silenced in embryos. Once Pol II transcription is turned on, CBs form on snRNA genes and HLBs (purple) mature on histone loci. Poly-adenylated mRNA, histone mRNA, and mature snRNPs are processed and exported out of the nucleus. Nucleoli (blue) form upon Pol I transcription

activation

and

assemble

pre-ribosomal

subunits.

OUTLOOK Tightly regulated gene expression and RNA processing in the early embryo is essential for survival. We propose that nuclear bodies play an essential role in this regulation by demixing components within the nucleus in order to efficiently process RNA as it is transcribed. rRNA is modified and cleaved in the nucleolus, snRNA is modified in the CB, and 3’ end cleavage of histone mRNA occurs in HLBs. This demixing of components into nuclear bodies is driven by an increasing concentration of RNA as it is transcribed and RNA binding proteins bind and recruit other factors. This demixing is driven by an active process such as transcription, but is also regulated by RNA. Both the transcriptome and RNA binding proteins (RBPs) present in embryos changes drastically during MZT as maternal transcripts are cleared away and ZGA occurs (43–45) (45–47) (46–48) . As chromatin remodeling occurs during MZT, nuclear bodies may help organize nascent transcripts so that they are all in proximity with their partner RBPs or modifiers. In addition, these nuclear bodies impose 3D organization of the genome; whether the clustering of loci in nuclear bodies plays an important role in genome function is unknown. Nucleoli, CBs, and HLBs are therefore easily detected at genomic sites via staining for their components. Given the robust transcription of specific loci at different stages of development, it is possible that additional nuclear bodies exist but have remained undiscovered. For example,


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HnRNP A1, an RBP involved in multiple RNA regulatory events, shifts to the nucleus upon ZGA in zebrafish and aids in processing of mir-430, a microRNA involved in RNA degradation (58). The appearance of a limited number of bright hnRNP A1 loci in embryos at ZGA prompts us to speculate that transcription of the repetitive mir-430 locus may seed a nuclear body dedicated to the processing of pri-mir-430 to pre-mir-430, which would subsequently be exported to the cytoplasm for further processing and function. Given the propensity of transcription and RNA processing factors to intrinsic disorder, we anticipate a regulatory role for LLPS throughout genomes.


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AUTHOR INFORMATION Corresponding Author *333 Cedar St New Haven, CT 06520 [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We thank Valentina Botti for helpful discussions and Vladimir Despic for preparing Figure 2. Due to the brevity of this piece, we apologize to our colleagues whose work was not directly cited in this review. ABBREVIATIONS CB, Cajal body; HLB, histone locus body; MZT, maternal-to-zygotic transition; LLPS, liquidliquid phase separation; snRNP. Spliceosomal small ribonucleoprotein particles; ZGA, zygotic genome

activation;

NOR,

nucleolar

organizing

region;

snoRNP,

small

nucleolar

ribonucleoprotein particle; snRNA, small nuclear RNA; scaRNA, small cajal body specific RNA; Pol II RNA polymerase II; RD, replication dependent; FLASH, FLICE-associated huge protein; CLAMP, chromatin-linked adaptor for male-specific lethal (MSL) proteins; SLBP, stem loop binding protein; PTM, post-translational modifications; GV, germinal vesicle; RBP, RNA binding protein; HCC, histone cleavage complex


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Dynamics and Function of Nuclear Bodies during Embryogenesis

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