RNA sorting in Drosophila oocytes and embryos

In vivo characterization of dynein-driven nanovectors using Drosophila oocytes

Molecular motors transport various cargoes including vesicles, proteins and mRNAs, to distinct intracellular compartments. A significant challenge in the field of nanotechnology is to improve drug nuclear delivery by engineering nanocarriers transported by cytoskeletal motors. However, suitable in vivo models to assay transport and delivery efficiency remain very limited. Here, we develop a fast and genetically tractable assay to test the efficiency and dynamics of fluospheres (FS) using microinjection into Drosophila oocytes coupled with time-lapse microscopy. We designed dynein motor driven FS using a collection of dynein light chain 8 (LC8) peptide binding motifs as molecular linkers and characterized in real time the efficiency of the FS movement according to its linker’s sequence. Results show that the conserved LC8 binding motif allows fast perinuclear nanoparticle's accumulation in a microtubule and dynein dependent mechanism. These data reveal the Drosophila oocyte as a new valuable tool for the design of motor driven nanovectors.

Protein phosphatase 1γ isoforms linked interactions in the brain

Posttranslational protein modifications, in particular reversible protein phosphorylation, are important regulatory mechanisms involved in cellular signaling transduction pathways. Thousands of human proteins are phosphorylatable and the tight regulation of phosphorylation states is crucial for cell maintenance and development. Protein phosphorylation occurs primarily on serine, threonine, and tyrosine residues, through the antagonistic actions of protein kinases and phosphatases. The catalytic subunit of protein phosphatase 1 (PP1), a major Ser/Thr-phosphatase, associates with a large variety of regulatory subunits that define substrate specificity and determine specific cellular pathway responses. PP1 has been shown to bind to different proteins in the brain in order to execute key and differential functions. This work reports the identification of proteins expressed in the human brain that interact with PP1γ1 and PP1γ2 isoforms by the yeast two-hybrid method. An extensive search of PP1-binding motifs was performed for the proteins identified, revealing already known PP1 regulators but also novel interactors. Moreover, our results were integrated with the data of PP1γ interacting proteins from several public web databases, permitting the development of physical maps of the novel interactions. The PP1γ interactome thus obtained allowed for the identification of novel PP1 interacting proteins, supporting novel functions of PP1γ isoforms in the human brain.

Differential localization of mRNAs during early development in the mollusc, Crepidula fornicata

Certain mRNAs have been shown to be segregated in different cells in various metazoan embryos. These events represent aspects of autonomous mechanisms that establish particular embryonic cell fates and axial properties associated with asymmetric cell divisions. The spiralian lophotrochozoans (which include molluscs, annelids, nemerteans, gnathostomulids, dicyemid mesozoans, entoprocts, and platyhelminthes) exhibit a highly conserved pattern of early development that involves stereotypical, asymmetric cell divisions (termed "spiral cleavage"). Recently, it was demonstrated that various mRNAs are dynamically localized to the centrosomes in specific cells during early development in the gastropod mollusc, Ilyanassa obsoleta. During subsequent cell divisions, these messages become segregated in particular daughter cells, and it has been proposed that these events distinguish the developmental potential of these cells within the early embryo of I. obsoleta. The molecular mechanisms underlying these events, however, are still unknown. Here we show for the first time in another spiralian lophotrochozoan (the gastropod Crepidula fornicata) that similar patterns of mRNA localization take place during early development. To characterize the transcriptome of early development, and identify candidate genes for the expression analyses, high-throughput sequencing was carried out, via GS FLX Titanium 454 pyrosequencing. The annotated sequences have been made available as a resource for the scientific community (www.life.illinoi.edu/henry/crepidula_databases.html). Presumably, specific proteins associated with centrosomes may be important for these mRNA localization events. In silico sequence comparisons with known centriolar/centrosomal, ciliary/basal body proteomes shows that a large number of those proteins are represented in the collection of expressed sequence tags of C. fornicata annotated in this study. These data should be useful for future studies of the role of specific mRNAs in controlling cell fate and axial specification in the spiralian Lophotrochozoa, and for dissecting the underlying molecular mechanisms that accomplish these events.

Reaction-diffusion systems in intracellular molecular transport and control

Chemical reactions make cells work only if the participating chemicals are delivered to desired locations in a timely and precise fashion. Most research to date has focused on active-transport mechanisms, although passive diffusion is often equally rapid and energetically less costly. Capitalizing on these advantages, cells have developed sophisticated reaction-diffusion (RD) systems that control a wide range of cellular functions-from chemotaxis and cell division, through signaling cascades and oscillations, to cell motility. These apparently diverse systems share many common features and are "wired" according to "generic" motifs such as nonlinear kinetics, autocatalysis, and feedback loops. Understanding the operation of these complex (bio)chemical systems requires the analysis of pertinent transport-kinetic equations or, at least on a qualitative level, of the characteristic times of the constituent subprocesses. Therefore, in reviewing the manifestations of cellular RD, we also describe basic theory of reaction-diffusion phenomena.

The Opitz syndrome gene product MID1 assembles a microtubule-associated ribonucleoprotein complex

Opitz BBB/G syndrome (OS) is a heterogenous malformation syndrome mainly characterised by hypertelorism and hypospadias. In addition, patients may present with several other defects of the ventral midline such as cleft lip and palate and congenital heart defects. The syndrome-causing gene encodes the X-linked E3 ubiquitin ligase MID1 that mediates ubiquitin-specific modification and degradation of the catalytic subunit of the translation regulator protein phosphatase 2A (PP2A). Here, we show that the MID1 protein also associates with elongation factor 1alpha (EF-1alpha) and several other proteins involved in mRNA transport and translation, including RACK1, Annexin A2, Nucleophosmin and proteins of the small ribosomal subunits. Mutant MID1 proteins as found in OS patients lose the ability to interact with EF-1alpha. The composition of the MID1 protein complex was determined by several independent methods: (1) yeast two-hybrid screening and (2) immunofluorescence, (3) a biochemical approach involving affinity purification of the complex, (4) co-fractionation in a microtubule assembly assay and (5) immunoprecipitation. Moreover, we show that the cytoskeleton-bound MID1/translation factor complex specifically associates with G- and U-rich RNAs and incorporates MID1 mRNA, thus forming a microtubule-associated ribonucleoprotein (RNP) complex. Our data suggest a novel function of the OS gene product in directing translational control to the cytoskeleton. The dysfunction of this mechanism would lead to malfunction of microtubule-associated protein translation and to the development of OS.

The efficiency of Xenopus primordial germ cell migration depends on the germplasm mRNA encoding the PDZ domain protein Grip2

A microarray analysis of vegetal pole sequences in the egg and early Xenopus laevis embryo identified Unigene Xl.14891 as a vegetally localized RNA. Analysis of the Xenopus tropicalis genome showed this Unigene to be localized near the 3' end of the Grip2 (glutamate receptor interacting protein 2) transcription unit. RACE showed that the Unigene represented the 3' UTR of Grip2 mRNA. Grip2 mRNA is present in the mitochondrial cloud of late pre-vitellogenic oocytes and then in the germplasm through oogenesis and early development until tailbud tadpole stages. Interference with Grip2 mRNA translation using two antisense morpholino oligos (MOs) impairs primordial germ cell (PGC) migration to the germinal ridges. Both MOs also inhibit swimming movements of the tailbud tadpole, known to involve glutamate receptors. We conclude that Grip2 has several functions in the embryo, including enabling efficient PGC migration.

Temporal and spatial control of gene expression in early embryos of farm animals

A gradual transition from oocyte-derived mRNA and proteins to full embryonic transcription characterises early embryonic development. Messenger RNAs and proteins of maternal origin are accumulated into the oocyte throughout its growth inthe ovary. Upon fertilisation, sev eral mechanisms ar e activated that controlthe appropriate use of such material and prepare for the synthesis of new products. The present review will describe some of the mechanisms active in early embryos of domestic species. Data will be presented on the control of gene expression by the 3' untranslated regions and their interaction with specialised sequences at the 5' cap end. The process of RNA sorting and localisation, initially described in different cell types and in oocytes of lower species, will also be discussed, particularly in relation to its possible role in regulating early pig development. Finally, specific genes involved in the activation of cattle embryonic transcription will be described. This brief overview will provide some suggestions on how these different mechanisms may be integrated and cooperate to ensure the correct initiation of embryonic development.

Drosophila Dynein light chain (DDLC1) binds to gurken mRNA and is required for its localization

During oogenesis in Drosophila, mRNAs encoding determinants required for the polarization of egg and embryo become localized in the oocyte in a spatially restricted manner. The TGF-alpha like signaling molecule Gurken has a central role in the polarization of both body axes and the corresponding mRNA displays a unique localization pattern, accumulating initially at the posterior and later at the anterior-dorsal of the oocyte. Correct localization of gurken RNA requires a number of cis-acting sequence elements, a complex of trans-acting proteins, of which only several have been identified, and the motor proteins Dynein and Kinesin, traveling along polarized microtubules. Here we report that the cytoplasmic Dynein-light-chain (DDLC1) which is the cargo-binding subunit of the Dynein motor protein, directly bound with high specificity and affinity to a 230-nucleotide region within the 3'UTR of gurken, making it the first Drosophila mRNA-cargo to directly bind to the DLC. Although DDLC1 lacks known RNA-binding motifs, comparison to double-stranded RNA-binding proteins suggested structural resemblance. Phenotypic analysis of ddlc1 mutants supports a role for DDLC1 in gurken RNA localization and anchoring as well as in correct positioning of the oocyte nucleus.

oskar gene expression in the vector mosquitoes, Anopheles gambiae and Aedes aegypti

A disease control strategy based on the introduction into mosquito populations of a gene conferring a pathogen-refractory phenotype is currently under investigation. This population replacement approach requires a drive system that will quickly spread and fix antipathogen effector genes in target populations. Modified transposable elements containing the control sequences of developmentally regulated genes may provide the basis for a gene drive system that regulates gene mobilization in a sex- and stage-restrictive manner. Screening of a Drosophila melanogaster database for genes whose products localize exclusively in the future germ cells during early embryonic development resulted in the identification of several candidate genes. The regulatory sequences of these genes could be used to drive transposition. Mosquito orthologous genes of oskar were identified based on sequence homology and characterized further. The tissue- and sex-specific expression profiles and hybridizations in situ show that oskar orthologous transcripts in Anopheles gambiae and Aedes aegypti accumulate in developing oocytes of adult females and localize to the posterior poles of early embryos. These characteristics potentiate the use of the regulatory sequences of mosquito oskar genes for the control of modified transposable elements.

Characterization of an RNA granule from developing brain

In brain, mRNAs are transported from the cell body to the processes, allowing for local protein translation at sites distant from the nucleus. Using subcellular fractionation, we isolated a fraction from rat embryonic day 18 brains enriched for structures that resemble amorphous collections of ribosomes. This fraction was enriched for the mRNA encoding beta-actin, an mRNA that is transported in dendrites and axons of developing neurons. Abundant protein components of this fraction, determined by tandem mass spectrometry, include ribosomal proteins, RNA-binding proteins, microtubule-associated proteins (including the motor protein dynein), and several proteins described only as potential open reading frames. The conjunction of RNA-binding proteins, transported mRNA, ribosomal machinery, and transporting motor proteins defines these structures as RNA granules. Expression of a subset of the identified proteins in cultured hippocampal neurons confirmed that proteins identified in the proteomics were present in neurites associated with ribosomes and mRNAs. Moreover many of the expressed proteins co-localized together. Time lapse video microscopy indicated that complexes containing one of these proteins, the DEAD box 3 helicase, migrated in dendrites of hippocampal neurons at the same speed as that reported for RNA granules. Although the speed of the granules was unchanged by activity or the neurotrophin brain-derived neurotrophic factor, brain-derived neurotrophic factor, but not activity, increased the proportion of moving granules. These studies define the isolation and composition of RNA granules expressed in developing brain.

Staufen recruitment into stress granules does not affect early mRNA transport in oligodendrocytes

Staufen is a conserved double-stranded RNA-binding protein required for mRNA localization in Drosophila oocytes and embryos. The mammalian homologues Staufen 1 and Staufen 2 have been implicated in dendritic RNA targeting in neurons. Here we show that in rodent oligodendrocytes, these two proteins are present in two independent sets of RNA granules located at the distal myelinating processes. A third kind of RNA granules lacks Staufen and contains major myelin mRNAs. Myelin Staufen granules associate with microfilaments and microtubules, and their subcellular distribution is affected by polysome-disrupting drugs. Under oxidative stress, both Staufen 1 and Staufen 2 are recruited into stress granules (SGs), which are stress-induced organelles containing transiently silenced messengers. Staufen SGs contain the poly(A)-binding protein (PABP), the RNA-binding proteins HuR and TIAR, and small but not large ribosomal subunits. Staufen recruitment into perinuclear SGs is paralleled by a similar change in the overall localization of polyadenylated RNA. Under the same conditions, the distribution of recently transcribed and exported mRNAs is not affected. Our results indicate that Staufen 1 and Staufen 2 are novel and ubiquitous SG components and suggest that Staufen RNPs are involved in repositioning of most polysomal mRNAs, but not of recently synthesized transcripts, during the stress response.

Differential segregation and modification of mRNA during spermiogenesis in Marsilea vestita

We are interested in the mechanisms that underlie cell fate determination in the endosporic male gametophytes of the fern, Marsilea vestita. Synchronous development is initiated by placing dry spores into water and involves the translation of stored mRNAs, with little transcription. Nine division cycles produce 32 spermatids surrounded by 7 sterile cells, and then each spermatid differentiates into a multiciliate gamete. Here, we focus on changes in the distribution of particular proteins, mRNAs, and patterns of polyadenylation as essential prerequisites for cell fate determination and gametogenesis. Earlier, we showed that alpha- and beta-tubulin proteins become concentrated in spermatogenous initials, and that centrin mRNA is translated only in spermatogenous initials. In situ hybridizations reveal that centrin, cyclin B, and beta-tubulin mRNAs are present in both sterile and spermatogenous cells, but that transcripts encoding RNA helicase and PRP-19 (a spliceosome component) become localized in spermatogenous cells. The targeted destruction of these two transcripts by RNAi treatments does not affect the numbers of division cycles, but the gametophytes exhibit anomalous patterns of cytokinesis, and a subsequent failure of spermatid differentiation. Thus, cell fate determination in the gametophyte involves localized translation, and the localization of mRNAs for proteins involved in transcript processing. We found differences in polyadenylation levels in sterile and spermatogenous cells that match the distribution of cytoplasmic poly(A) polymerase (PAP), which, in immunolocalizations, is abundant in spermatogenous cells, but undetectable in sterile cells. The activation of translation in spermatogenous initials, but not in sterile cells, may be under the control of mRNA processing enzymes, which become localized either as proteins or mRNAs in the spermatogenous subdomains before any divisions occur.

Paracentrotus lividus eggs contain different RNAs at the animal and vegetal poles

Paracentrotus lividus eggs were divided by centrifugation into nucleated and anucleated halves. Fertilization and development of the two halves permitted us to establish that nucleated and anucleated fragments correspond, respectively, to the animal and vegetal parts. RNA was extracted from both egg halves and submitted to differential display. Northern blot analysis confirmed their maternal origin and showed that each transcript has a different expression pattern during development. By Northern blot and in situ hybridization experiments we ascertained that Bep2 and PlAn1 are localized in the animal part, whereas 16S rRNA, Plveg1, and L27 in the vegetal part, and that Plun1 is uniformly distributed. Moreover, by treating P. lividus eggs with detergent, in presence or not of drugs such as colchicine and cytochalasin B, we demonstrated the involvement of the cytoskeleton only in localization of Bep2, PlAn1, and Plun1, suggesting that different mechanisms are utilized for animal and vegetal distribution.

Maternal mRNAs of PEM and macho 1, the ascidian muscle determinant, associate and move with a rough endoplasmic reticulum network in the egg cortex

Localization of maternal mRNAs in the egg cortex is an essential feature of polarity in embryos of Drosophila, Xenopus and ascidians. In ascidians, maternal mRNAs such as macho 1, a determinant of primary muscle-cell fate, belong to a class of postplasmic RNAs that are located along the animal-vegetal gradient in the egg cortex. Between fertilization and cleavage, these postplasmic RNAs relocate in two main phases. They further concentrate and segregate in small posterior blastomeres into a cortical structure, the centrosome-attracting body (CAB), which is responsible for unequal cleavages.

By using high-resolution, fluorescent, in situ hybridization in eggs,zygotes and embryos of Halocynthia roretzi, we showed that macho 1 and HrPEM are localized on a reticulated structure situated within 2 μm of the surface of the unfertilized egg, and within 8 μm of the surface the vegetal region and then posterior region of the zygote. By isolating cortices from eggs and zygotes we demonstrated that this reticulated structure is a network of cortical rough endoplasmic reticulum (cER) that is tethered to the plasma membrane. The postplasmic RNAs macho 1 and HrPEM were located on the cER network and could be detached from it. We also show that macho 1 and HrPEM accumulated in the CAB and the cER network. We propose that these postplasmic RNAs relocalized after fertilization by following the microfilament- and microtubule-driven translocations of the cER network to the poles of the zygote. We also suggest that the RNAs segregate and concentrate in posterior blastomeres through compaction of the cER to form the CAB. A multimedia BioClip `Polarity inside the egg cortex' tells the story and can be downloaded at www.bioclips.com/bioclip.html

A molecular mechanism for mRNA trafficking in neuronal dendrites

Specific neuronal mRNAs are localized in dendrites, often concentrated in dendritic spines and spine synapses, where they are translated. The molecular mechanism of localization is mostly unknown. Here we have explored the roles of A2 response element (A2RE), a cis-acting signal for oligodendrocyte RNA trafficking, and its cognate trans-acting factor, heterogeneous nuclear ribonucleoprotein (hnRNP) A2, in neurons. Fluorescently labeled chimeric RNAs containing A2RE were microinjected into hippocampal neurons, and RNA transport followed using confocal laser scanning microscopy. These RNA molecules, but not RNA lacking the A2RE sequence, were transported in granules to the distal neurites. hnRNP A2 protein was implicated as the cognate trans-acting factor: it was colocalized with RNA in cytoplasmic granules, and RNA trafficking in neurites was compromised by A2RE mutations that abrogate hnRNP A2 binding. Coinjection of antibodies to hnRNP A2 halved the number of trafficking cells, and treatment of neurons with antisense oligonucleotides also disrupted A2RE-RNA transport. Colchicine inhibited trafficking, whereas cells treated with cytochalasin were unaffected, implicating involvement of microtubules rather than microfilaments. A2RE-like sequences are found in a subset of dendritically localized mRNAs, which, together with these results, suggests that a molecular mechanism based on this cis-acting sequence may contribute to dendritic RNA localization.

The case for extending storage and secretion functions of human mast cell granules to include synthesis

Ultrastructural studies using standard procedures have for years indicated close associations of ribosomes and secretory granules in human mast cells. These descriptive studies have informed new studies, using established and new ultrastructural methods based on different principles, designed to investigate the possible role of RNA metabolism in secretory granules of human mast cells. In aggregate, these studies indicate human mast cell secretory granule associations with ribosomes, the protein synthetic machine of cells, with ribosomal proteins, with RNA, with poly(A)-positive mRNA and with various long-lived, or short-lived, uridine-rich, and poly(A)-poor RNA species with key roles in RNA processing and splicing. These studies indicate that secretory-storage granules in human mast cells may also be synthetic granules.

Germ line development in the grasshopper Schistocerca gregaria: vasa as a marker

Vasa is a widely conserved germline marker, both in vertebrates and invertebrates. We identify a vasa orthologue, Sgvasa, and use it to study germline development in the grasshopper Schistocerca gregaria, a species in which no germ plasm has been identified. In adults, Sgvasa is specifically expressed in the ovary and testis. It is expressed at high levels during early oogenesis, but no detectable vasa RNA and little Vasa protein are present in mature unlaid eggs. None appears to be localized to any defined region of the egg cortex, suggesting that germline specification may not depend on maternal germ plasm expressing vasa. Vasa protein is expressed in most cleavage energids as they reach the egg surface and persists at high levels in most cells aggregating to form the embryonic primordium. However, after gastrulation, Vasa protein persists only in extraembryonic membranes and in cells at the outer margin of the late heart-stage embryo. In the embryo, it then become restricted to cells at the dorsal margin of the forming abdomen. In older embryos, these Vasa-positive cells move toward the midline; Vasa protein accumulates asymmetrically in their cytoplasm, a pattern closely resembling that of germ cells in late embryonic gonads. Thus, we suggest that the Vasa-stained cells in the abdominal margin are germ cells, as proposed by Nelson (1934), and not cardioblasts, as has been proposed by others.

Differential regulation of maternal and paternal chromosome condensation in mitotic zygotes

A-kinase anchoring protein AKAP95 is implicated in somatic mitotic chromosome condensation by recruiting the condensin complex. Here, we report a differential regulation of condensation of maternal and paternal chromosomes mediated by AKAP95 in mitotic mouse zygotes. AKAP95 is synthesized upon oocyte activation, targeted to the female pronucleus and specifically associates with maternal chromosomes at mitosis. AKAP95 mRNA is highly restricted to the vicinity of the meiotic spindle in metaphase II oocytes. In vivo displacement of endogenous AKAP95 in female pronuclei by microinjection of competitor peptides and rescue experiments show that AKPA95 is required for recruitment of the mCAP-D2 condensin subunit to, and condensation of, maternal chromosomes. In contrast, AKAP95 is dispensable for mCAP-D2 recruitment to,and condensation of, paternal chromosomes. Our results indicate that at first embryonic mitosis, paternal chromosomes target condensins and condense independently of AKAP95, whereas maternal chromosomes require AKAP95 for condensin recruitment and condensation. We propose a concept whereby condensation of chromosomes in gametes, zygotes and somatic cells involves related but distinct mechanisms.

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.

Par-1 regulates stability of the posterior determinant Oskar by phosphorylation

Par-1 kinase is critical for polarization of the Drosophila melanogaster oocyte and the one-cell Caenorhabditis elegans embryo. Although Par-1 localizes specifically to the posterior pole in both cells, neither its targets nor its function at the posterior pole have been elucidated. Here we show that Drosophila Par-1 phosphorylates the posterior determinant Oskar (Osk) and demonstrate genetically that Par-1 is required for accumulation of Osk protein. We show in cell-free extracts that Osk protein is intrinsically unstable and that it is stabilized after phosphorylation by Par-1. Our data indicate that posteriorly localized Par-1 regulates posterior patterning by stabilizing Osk.

The 5'-untranslated region of the ntp303 gene strongly enhances translation during pollen tube growth, but not during pollen maturation

Transcripts of the ntp303 gene accumulate abundantly throughout pollen development, whereas the protein only accumulates to detectable levels after pollen germination. In an attempt to explain the divergence in the accumulation profiles of the mRNA and the protein, we investigated the role of the untranslated regions (UTRs) in enhancing ntp303 translation during the transition from developing to germinating pollen. Luciferase reporter gene fusion constructs containing the ntp303 5'-UTR gave rise to luciferase activity that was up to 60-fold higher during pollen tube growth than that of constructs containing different 5'-UTRs. No apparent differences in the luciferase activity of these constructs were observed during pollen development. The ntp303 5'-UTR-mediated increase in luciferase activity was not significantly influenced by coding region or 3'-UTR sequences. Furthermore, enhanced luciferase activity directed by the ntp303 5'-UTR occurred predominantly at the post-transcriptional level. A series of 5'-UTR deletion constructs was created to identify putative regulatory sequences required for the high level of translation during pollen tube growth. Two predicted stem loop structures (H-I and H-II) caused a complete inhibition of the enhanced translation after their total or partial deletion. A (GAA)(8) repeat within the H-I stem loop structure was demonstrated to be important for the modulation of translation efficiency. The H-II stem loop structure was found to be essential for the determination of mRNA stability.

Translational regulation and RNA localization in Drosophila oocytes and embryos

Translational control is a prevalent means of gene regulation during Drosophila oogenesis and embryogenesis. Multiple maternal mRNAs are localized within the oocyte, and this localization is often coupled to their translational regulation. Subsequently, translational control allows maternally deposited mRNAs to direct the early stages of embryonic development. In this review we outline some general mechanisms of translational regulation and mRNA localization that have been uncovered in various model systems. Then we focus on the posttranscriptional regulation of four maternal transcripts in Drosophila that are localized during oogenesis and are critical for embryonic patterning: bicoid (bcd), nanos (nos), oskar (osk), and gurken (grk). Cis- and trans-acting factors required for the localization and translational control of these mRNAs are discussed along with potential mechanisms for their regulation.

Expression of MSY2 in mouse oocytes and preimplantation embryos

Translational control plays a central role during oocyte maturation and early embryogenesis, as these processes occur in the absence of transcription. MSY2, a member of a multifunctional Y-box protein family, is implicated in repressing the translation of paternal mRNAs. Here, we characterize MSY2 expression in mouse oocytes and preimplantation embryos. Northern blot analysis indicates that MSY2 expression is highly restricted and essentially confined to the oocyte in the female mouse. MSY2 transcript and protein, as assessed by reverse transcription-polymerase chain reaction and immunoblotting, respectively, are expressed in growing oocytes, metaphase II-arrested eggs, and 1-cell embryos, but then are degraded by the late 2-cell stage; no expression is detectable in the blastocysts. During oocyte maturation, MSY2 is phosphorylated and following fertilization it is dephosphorylated. Quantification of the mass amount of MSY2 reveals that it represents 2% of the total protein in the fully grown oocyte, i.e., it is a very abundant protein. Both endogenous MSY2 and MSY2-enhanced green fluorescent protein (EGFP), which is synthesized following microinjection of an mRNA encoding MSY2-EGFP, are primarily localized in the cytoplasm, and about 75% of the MSY2 remains associated with oocyte cytoskeletal preparations. Results of these studies are consistent with the proposal that MSY2 functions by stabilizing and/or repressing the translation of maternal mRNAs.

Elongation factor 1beta is an actin-binding protein

A 17 kDa polypeptide found in association with actin in cellular extracts of Dictyostelium discoideum was identified as a proteolytic fragment of eEF1beta. Antibody elicited against the 17 kDa protein reacted with a single 29 kDa polypeptide in Dictyostelium, indicating that the 17 kDa peptide arises from degradation of a larger precursor. The cDNA isolated from a Dictyostelium library using this antibody as a probe encodes Dictyostelium elongation factor 1beta. Amino acid degradation of the 17 kDa protein fragment confirmed the identity of the protein as eEF1beta. Direct interaction of eEF1beta with actin in vitro was further demonstrated in mixtures of actin with the 17 kDa protein fragment of Dictyostelium eEF1beta, recombinant preparations of Dictyostelium eEF1beta expressed in Escherichia coli, and the intact eEF1betagamma complex purified from wheat germ. Localization of eEF1beta in Dictyostelium by immunofluorescence microscopy reveals both diffuse cytoplasmic staining, and some concentration in the cortical and hyaline cytoplasm. The results support the existence of physical and functional interactions of the translation apparatus with the cytoskeleton, and suggest that eEF1beta may function in a dual role both to promote the elongation phase of protein synthesis, and to interact with cytoplasmic actin.

Barentsz is essential for the posterior localization of oskar mRNA and colocalizes with it to the posterior pole

The localization of Oskar at the posterior pole of the Drosophila oocyte induces the assembly of the pole plasm and therefore defines where the abdomen and germ cells form in the embryo. This localization is achieved by the targeting of oskar mRNA to the posterior and the localized activation of its translation. oskar mRNA seems likely to be actively transported along microtubules, since its localization requires both an intact microtubule cytoskeleton and the plus end-directed motor kinesin I, but nothing is known about how the RNA is coupled to the motor. Here, we describe barentsz, a novel gene required for the localization of oskar mRNA. In contrast to all other mutations that disrupt this process, barentsz-null mutants completely block the posterior localization of oskar mRNA without affecting bicoid and gurken mRNA localization, the organization of the microtubules, or subsequent steps in pole plasm assembly. Surprisingly, most mutant embryos still form an abdomen, indicating that oskar mRNA localization is partially redundant with the translational control. Barentsz protein colocalizes to the posterior with oskar mRNA, and this localization is oskar mRNA dependent. Thus, Barentsz is essential for the posterior localization of oskar mRNA and behaves as a specific component of the oskar RNA transport complex.

In vivo analysis of Drosophila bicoid mRNA localization reveals a novel microtubule-dependent axis specification pathway

Drosophila bicoid mRNA is synthesized in the nurse cells and transported to the oocyte where microtubules and Exuperantia protein mediate localization to the anterior pole. Fluorescent bicoid mRNA injected into the oocyte displays nonpolar microtubule-dependent transport to the closest cortical surface, and the oocyte microtubule cytoskeleton lacks clear axial asymmetry. Nonetheless, bicoid mRNA injected into the nurse cell cytoplasm, withdrawn, and injected into a second oocyte shows microtubule-dependent transport to the anterior cortex. Nurse cells require microtubules and Exuperantia to support anterior transport of bicoid mRNA, and microtubules are required for bicoid mRNA-Exuperantia particle coassembly. We propose that microtubule-dependent Exuperantia-bicoid mRNA complex formation in the nurse cell cytoplasm allows anterior-specific transport on a grossly nonpolar oocyte microtubule network.

Messenger RNA on the move: implications for cell polarity

RNA sorting is an evolutionarily conserved mechanism for establishing asymmetries within a given cell concerning the macromolecular equipment of defined domains. mRNAs as well as non-coding transcripts are delivered to specific subcellular compartments in diverse organisms including developmental systems of Drosophila, Xenopus, ascidiens, zebrafish and echinoderms and in differentiated cells from yeast to mammals. The composition of the RNA localization machinery is complex. Both sequence- or structural motifs within RNA molecules to be transported (cis-acting elements) and various proteins (trans-acting factors) contribute to the localization procedure. Most often these macromolecular ribonucleoprotein complexes exhibit a granular appearance, and granule localization depends on intact microfilaments or microtubules. When delivered to their ultimate destinations mRNAs are anchored to await translational activation at the appropriate timepoint. Beyond doubt, RNA localization plays a pivotal role in embryonic development, where mRNA mislocations cause severe body pattern defects. In terminally differentiated vertebrate cells RNA transport and local on-site translation presumably have an impact on various cellular functions such as cell motility, myelinization of nerve cell axons and nerve cell communications in the central and peripheral nervous system.

Ultrastructural immunogold cytochemistry with autoimmune human sera and an antibody to uridine implicate human mast cell granules in RNA biology

Human mast cells are professional secretory cells that store synthetic products in large granules filling their cytoplasm. Unlike many secretory cells, the principal synthetic organelle, ribosome-rich endoplasmic reticulum, is a minor component of their cytoplasm. Sightings of nonmembrane-bound ribosomes in and near their secretory granules stimulated detailed ultrastructural studies of various RNA species to implicate secretory-storage granules in RNA biology. In the work reported here, postembedding immunogold ultrastructural cytochemistry indicates that human mast cells contain uridine, an integral ingredient of RNA, and ribonucleoproteins, known to associate with small nuclear RNAs important for splicing RNA precursors, several ribonucleoproteins with possible functions in other aspects of RNA biology and ribonucleoproteins known to associate with ribosomes. These findings should catalyse future work toward establishing the full functional repertoire of secretory-storage granules.

Biochemical characterization of a cellular structure retaining vegetally localized RNAs in Xenopus late stage oocytes

Two pathways operate during Xenopus oogenesis to localize a small number of RNAs to the vegetal cortex. Correct localization of these RNAs is essential to normal development as the proteins they encode are involved in specifying cell type and in patterning the early embryo. Binding these RNAs to the vegetal cortex and thus preserving their localized condition is a critical step, although little is known about how this is achieved. In this study, we have used a biochemical approach to examine the anchoring step. Xlsirts, an abundant localized RNA (locRNA), was selectively enriched in a detergent-insoluble fraction (DIF) prepared from oocytes that had completed the RNA localization process. These putative RNA-anchoring complexes were analyzed by density gradient centrifugation and in RNA-protein binding assays. Cortical Xlsirts and other localized RNAs are specifically found in the heavy region of sucrose gradients and in the pellet, quite different from other cellular RNPs. Four proteins were identified by UV-crosslinking that bound the Xlsirts localization signal in the cortex, but not in the soluble fraction. These are likely members of the anchoring complex and appear to include vera, a characterized Vg1 RNA binding protein. Vera was found to co-sediment with other locRNAs found in the vegetal cortex, suggesting that it is a common component of locRNPs. Finally, we found that locRNPs extracted into the soluble fraction had the same buoyant density as typical ooplasmic RNPs. We propose that locRNAs are organized and anchored in the cortex as typical RNPs.

RNA anchoring in the vegetal cortex of the Xenopus oocyte

The body plan of the embryo is established by a polarized source of developmental information in the oocyte. The Xenopus laevis oocyte creates polarity by anchoring mRNAs in the vegetal cortex, including Vg1 and Xwnt-11, which might function in body plan specification, and Xcat-2, which might function in germ cell development. To identify components of the RNA anchoring mechanism, we used the manually isolated vegetal cortex (IVC) to assay loss or change in spatial arrangement of mRNAs caused by disruption of cortical elements. The role of cytoskeleton in mRNA anchoring was tested by treating oocytes with inhibitors that selectively disrupted actin microfilaments and cytokeratin filaments. Treatment of oocytes with cytochalasin B caused clumping of Vg1 and Xwnt-11 as revealed by in situ hybridization of the IVC, but did not cause their release, as confirmed by RT-PCR analysis. These mRNA clumps did not match the distribution of actin microfilament clumps, but were distributed similarly to the remnant cytokeratin filaments. Treatment of oocytes with monoclonal anti-cytokeratin antibody C11 released these mRNAs from the cortex. C11 altered the texture of the cytokeratin network, but did not affect the actin meshwork. These results show that Vg1 and Xwnt-11 are retained by a cytokeratin filament-dependent mechanism, and that organization of the cytokeratin network depend on an intact actin meshwork. Colcemid did not disrupt Vg1 and Xwnt-11 retention in the IVC, so anchoring of these mRNAs are independent of microtubules. Membrane disruption in the IVC by Triton X-100 decreased Vg1 and Xwnt-11. Loss of these mRNAs was due mainly to ribonuclease activity released from membrane components. However, when ribonuclease activity was suppressed under cold temperature, a higher amount of Vg1 and Xwnt-11 was recovered in the supernatant. This result suggested that a fraction of these mRNAs required membranes to be retained in the cortex. By contrast, Xcat-2 mRNA was neither released nor degraded following treatments with cytochalasin B, C11, colcemid and Triton X-100 under cold temperature, so no cortical element could be implicated in its anchoring.

The maternal legacy to the embryo: cytoplasmic components and their effects on early development

RNA molecules and proteins are accumulated in the oocyte cytoplasm during its growth phase and are used to sustain the early phases of embryonic development before embryo DNA transcription begins. This makes the oocyte a very special cell, quite different from somatic cells where RNA and proteins usually undergo a rapid turnover. To enable the storage and timely use of such stored molecules, various mechanisms are effective in the oocyte and are gradually being elucidated. Our understanding of such mechanisms is important for constantly improving therapy for human and animal reproductive disorders as well as for understanding the process of nuclear reprogramming during cloning procedure or stem cell generation. This review focuses on the various aspects of these regulatory processes in an attempt to give an overview of the present knowledge on post-transcriptional and post-translational mechanisms taking place during oocyte maturation and early development. Mechanisms such as cytoplasmic regulation of the poly(A) tail, RNA localization and protein phosphorylation are described in some detail. Because most data are available from lower species these are presented together with appropriate reference to the mammalian oocyte when data are known, or when important differences have been described.

mRNA localization: message on the move

Cytoplasmic messenger RNA localization is a key post-transcriptional mechanism of establishing spatially restricted protein synthesis. The characterization of cis-acting signals within localized mRNAs, and the identification of trans-acting factors that recognize these signals, has opened avenues towards identifying the machinery and mechanisms involved in mRNA transport and localization.

Defining cis-acting elements and trans-acting factors in RNA localization

Research over the last 10 to 15 years has revealed that intracellular RNA localization is a widespread phenomenon found in a large range of different cell types in an equally impressive number of different organisms (Bashirullah et al., 1998; St. Johnston, 1995). Efforts have focused both on the molecular mechanisms involved in localizing RNAs to particular intracellular targets and on the functional importance (to the cell) of placing certain RNAs at particular cellular sites. In many cases, an understanding of the role of RNA localization seems to be predicated on a careful analysis of how a particular RNA achieves its characteristic distribution. A generalized model of RNA localization usually invokes cellular factors recognizing RNA target sequences. This review will focus on several systems in which cis-acting elements and trans-acting factors recognizing these elements are involved in RNA localization: how they have been defined, how they relate to each other, and how they interact and function to help achieve defined intracellular localization. Conservation of both RNA elements and protein factors across species suggests that RNA localization is probably a fundamental cellular process.

VP-RBP, a protein enriched in brain tissue, specifically interacts with the dendritic localizer sequence of rat vasopressin mRNA

The concept of mRNA localization suggests that this process is mediated by sequences residing in the transcript to which proteins specifically bind and ultimately deliver the mRNA along cytoskeletal elements to specific intracellular destinations. The mRNA encoding the vasopressin (VP) precursor protein is localized to the nerve cell processes both in hypothalamic magnocellular neurons and in primary cultured neurons derived from embryonic rat superior cervical ganglia microinjected with a corresponding eukaryotic expression vector. The last 395 nucleotides of the VP mRNA encompassing part of the coding region, as well as the complete 3'-untranslated region, are sufficient to confer dendritic targeting to a normally nonlocalized reporter transcript. Here we report that, by employing in vitro crosslinking analyses with rat brain proteins and radiolabelled VP transcripts, an RNA-binding protein specifically interacts with the dendritic localizer sequence of the VP mRNA. This protein is enriched in nerve cell tissues. Peripheral tissues and various cell lines contain only low amounts of the binding activity. It therefore represents a candidate protein that may be involved in any aspect related to subcellular VP mRNA sorting in nerve cells, including transport and anchoring of the mRNA and possibly its translational control.

Targeting of mRNAs within the glial cell cytoplasm: how to hide the message along the journey

The subcellular targeting of mRNAs encoding myelin proteins to the oligodendrocyte processes is an accepted fact in myelin formation. How these messengers are kept silent during their movement to the subcellular domain where they are turned on remains a mystery. This review focuses on aspects of mRNA targeting and speculates on possible molecular mechanisms for the translational control of myelin-located mRNAs.

In yeast, the 3' untranslated region or the presequence of ATM1 is required for the exclusive localization of its mRNA to the vicinity of mitochondria

We isolated mitochondria from Saccharomyces cerevisiae to selectively study polysomes bound to the mitochondrial surface. The distribution of several mRNAs coding for mitochondrial proteins was examined in free and mitochondrion-bound polysomes. Some mRNAs exclusively localize to mitochondrion-bound polysomes, such as the ones coding for Atm1p, Cox10p, Tim44p, Atp2p, and Cot1p. In contrast, mRNAs encoding Cox6p, Cox5a, Aac1p, and Mir1p are found enriched in free cytoplasmic polysome fractions. Aac1p and Mir1p are transporters that lack cleavable presequences. Sequences required for mRNA asymmetric subcellular distribution were determined by analyzing the localization of reporter mRNAs containing the presequence coding region and/or the 3'-untranslated region (3'UTR) of ATM1, a gene encoding an ABC transporter of the mitochondrial inner membrane. Biochemical analyses of mitochondrion-bound polysomes and direct visualization of RNA localization in living yeast cells allowed us to demonstrate that either the presequence coding region or the 3'UTR of ATM1 is sufficient to allow the reporter mRNA to localize to the vicinity of the mitochondrion, independently of its translation. These data demonstrate that mRNA localization is one of the mechanisms used, in yeast, for segregating mitochondrial proteins.

She2p, a novel RNA-binding protein tethers ASH1 mRNA to the Myo4p myosin motor via She3p

RNA localization is a widespread mechanism to achieve localized protein synthesis. In budding yeast, localization of ASH1 mRNA controls daughter cell-specific accumulation of the transcriptional regulator Ash1p, which determines mating type switching. ASH1 mRNA localization depends on four independently acting sequences ('zipcodes') within the mRNA. In addition, the class V myosin Myo4p and a set of She proteins with as yet unknown function are essential for ASH1 localization. Here we show that She2p is a novel RNA-binding protein that binds specifically to ASH1 mRNA in vivo and to ASH1 RNA zip codes in vitro. She2p can interact with She3 protein via She3p's C-terminus and becomes localized to the daughter cell tip upon ASH1 expression. The N-terminal coiled-coil domain of She3p is required to form an RNA-independent complex with the heavy chain of the myosin motor protein Myo4p. She2p and She3p are the first examples of adapters for tethering a localized mRNA to the motor protein and might serve as prototypes for RNA-motor protein adapters.

A function for kinesin I in the posterior transport of oskar mRNA and Staufen protein

The asymmetric localization of messenger RNA (mRNA) and protein determinants plays an important role in the establishment of complex body plans. In Drosophila oocytes, the anterior localization of bicoid mRNA and the posterior localization of oskar mRNA are key events in establishing the anterior-posterior axis. Although the mechanisms that drive bicoid and oskar localization have been elusive, oocyte microtubules are known to be essential. Here we report that the plus end-directed microtubule motor kinesin I is required for the posterior localization of oskar mRNA and an associated protein, Staufen, but not for the anterior-posterior localization of other asymmetric factors. Thus, a complex containing oskar mRNA and Staufen may be transported along microtubules to the posterior pole by kinesin I.

Germ cells

Recently, cell signaling has been shown to be required for the formation of germ cells in the mouse embryo, direct observation of germ cell migration in living mouse embryos has been achieved, novel genes that control germ-cell migration have been identified in Drosophila, and the roles of many components of germ plasm in several species have become clearer.

Asymmetric segregation of PIE-1 in C. elegans is mediated by two complementary mechanisms that act through separate PIE-1 protein domains

The CCCH finger protein PIE-1 is a regulator of germ cell fate that segregates with the germ lineage in early embryos. At each asymmetric division, PIE-1 is inherited preferentially by the germline daughter and is excluded from the somatic daughter. We show that this asymmetry is regulated at the protein level by two complementary mechanisms. The first acts before cell division to enrich PIE-1 in the cytoplasm destined for the germline daughter. The second acts after cell division to eliminate any PIE-1 left in the somatic daughter. The latter mechanism depends on PIE-1's first CCCH finger (ZF1), which targets PIE-1 for degradation in somatic blastomeres. ZF1s in two other germline proteins, POS-1 and MEX-1, are also degraded in somatic blastomeres, suggesting that localized degradation also acts on these proteins to exclude them from somatic lineages.

Ultrastructural cytochemical, immunocytochemical and in situ hybridization methods with polyuridine probes detect mRNA in human mast cell granules

Mature human mast cells are classical secretory cells that are filled with secretory-storage granules but are poorly endowed with visible free or membrane-bound cytoplasmic ribosomes. We recently reported close associations of ribosomes and various components essential to RNA metabolism in and close to human mast cell granules using multiple ultrastructural imaging methods. In view of these findings and an increased awareness of RNA sorting and localization to specific subcellular sites and organelles, we used human mast cells purified from non-tumour portions of lung samples resected at surgery for carcinoma and ultrastructural methods to investigate this further. Poly(U) probes were used to detect direct en grid binding, and radiolabelled as well as non-radiolabelled poly(U) probes were used in in situ hybridization protocols to detect poly(A)-positive pre-mRNA and mRNA in nuclear, cytoplasmic and granular compartments of mature human mast cells. Negative controls verified specificity of label; expected nuclear and cytoplasmic locations of poly(A)-positive RNA served as positive controls for each sample. These findings lend support to the hypothesis that site-specific synthesis in secretory-storage granules may occur in secretory cells.

Two trans-acting rat-brain proteins, MARTA1 and MARTA2, interact specifically with the dendritic targeting element in MAP2 mRNAs

Different isoforms of the microtubule-associated protein 2 (MAP2) are somatodendritic components of neurons that seem to regulate the stability of the dendritic cytoskeleton. MAP2 localization into dendrites appears to be a complex multicausal mechanism that involves the specific recruitment of MAP2 mRNAs into dendritic compartments. Recently, we have functionally characterized a 640-nucleotide dendritic targeting element (DTE) in the 3' untranslated region (3' UTR) of MAP2 transcripts that mediates extrasomatic mRNA localization in primary neurons (Blichenberg et al. , 1999). In analogy to molecular mechanisms regulating cytoplasmic RNA translocation in other cell systems, we propose that, in vivo, the cis-acting MAP2-DTE interacts with specific protein factors present in neurons. To identify putative trans-acting DTE-binding proteins, we performed in vitro ultraviolet crosslinking assays. Using this experimental system, two 90-kDa and 65-kDa MAP2-RNA trans-acting proteins, MARTA1 and MARTA2, were identified in rat-brain extracts. Both MARTAs bind with high affinity to the MAP2-DTE, but not to other investigated regions of MAP2 transcripts or the somatically restricted alpha-tubulin mRNA. Moreover, MARTA1 and MARTA2 do not bind significantly to other dendritically localized transcripts encoding vasopressin and arg3.1, nor to a dendritic trafficking element from the mRNA encoding the alpha-subunit of the Ca(2+)/calmodulin-dependent protein kinase II. Binding of MARTA1 and MARTA2 to the MAP2-DTE occurs with an affinity in the nanomolar range. Whereas MARTA1 is clearly detectable in crude lysates, cytosolic and ribosomal salt-wash fractions, and in nuclear extracts, MARTA2 is preferentially found in the ribosomal salt-wash preparation. Neither MARTA is restricted to rat brain, and both are present in a number of other rat tissues. Thus, both proteins may be involved in a variety of nuclear and cytoplasmic events that regulate RNA metabolism in different cell types.

Regulation of the vitellogenin receptor during Drosophila melanogaster oogenesis

In many insects, development of the oocyte arrests temporarily just before vitellogenesis, the period when vitellogenins (yolk proteins) accumulate in the oocyte. Following hormonal and environmental cues, development of the oocyte resumes, and endocytosis of vitellogenins begins. An essential component of yolk uptake is the vitellogenin receptor. In this report, we describe the ovarian expression pattern and subcellular localization of the mRNA and protein encoded by the Drosophila melanogaster vitellogenin receptor gene yolkless (yl). yl RNA and protein are both expressed very early during the development of the oocyte, long before vitellogenesis begins. RNA in situ hybridization and lacZ reporter analyses show that yl RNA is synthesized by the germ line nurse cells and then transported to the oocyte. Yl protein is evenly distributed throughout the oocyte during the previtellogenic stages of oogenesis, demonstrating that the failure to take up yolk in these early stage oocyte is not due to the absence of the receptor. The transition to the vitellogenic stages is marked by the accumulation of yolk via clathrin-coated vesicles. After this transition, yolk protein receptor levels increase markedly at the cortex of the egg. Consistent with its role in yolk uptake, immunogold labeling of the receptor reveals Yl in endocytic structures at the cortex of wild-type vitellogenic oocytes. In addition, shortly after the inception of yolk uptake, we find multivesicular bodies where the yolk and receptor are distinctly partitioned. By the end of vitellogenesis, the receptor localizes predominantly to the cortex of the oocyte. However, during oogenesis in yl mutants that express full-length protein yet fail to incorporate yolk proteins, the receptor remains evenly distributed throughout the oocyte.

RNA is closely associated with human mast cell secretory granules, suggesting a role(s) for granules in synthetic processes

The distribution of ribosomes in mature human mast cells, a major granulated secretory cell, does not resemble that in other secretory cells, such as pancreatic acinar cells and plasma cells. By routine ultrastructural analysis, ribosomes in human mast cells are often close to, attached to, or even appear to be within secretory granules. To document better these relationships, we used multiple electron microscopic imaging methods, based on different principles, to define RNA, ribosome, and granule relationships in mature human mast cells. These methods included EDTA regressive staining, RNase digestion, immunogold labeling of ribonucleoproteins or uridine, direct binding or binding after ultrastructural in situ hybridization of various polyuridine probes to polyadenine in mRNA, and ultrastructural autoradiographic localization of [3H]-uridine incorporated into cultured human mast cells. These different labeling methods demonstrated ribosomes, RNA, U1SnRNP (a small nuclear RNP specific for alternative splicing of mRNA), mRNA, and uridine to be associated with secretory granules in human mast cells, implicating granules in a larger synthetic role in mast cell biology.