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Immediately upon synthesis nascent transcripts associate with proteins to form large ribonucleoprotein (RNP) complexes, the protein content of which evolves as pre-mRNA is processed into mRNA and nuclear mRNA is exported to the cytoplasm (for recent reviews, see Dreyfuss et al. 2002; Reed and Hurt 2002). After being released from the gene templates, which are distributed throughout the nucleoplasm, messenger ribonucleoprotein particles (mRNPs) must reach the nuclear pore complexes in order to be transported to the cytoplasm - but what drives this intranuclear movement remains unknown. [ABSTRACT FROM AUTHOR]

Recent advances in mRNA visualization technology have placed biomedical imaging at an interface between molecular biology and cellular biology. It is now increasingly possible to study in vivo gene expression at the transcript level. Analyses of mRNA expression, movement, interactions, and localization will enhance understanding of cellular responses to various conditions and will complement studies of protein expression. [ABSTRACT FROM AUTHOR]

The expression of protein-coding genes in eukaryotic cells is a multi-step process that starts with the synthesis of a pre-messenger RNA (pre-mRNA) in the cell nucleus and finishes with the synthesis of a protein in the cytoplasm. In the nucleus, the pre-mRNA must undergo several processing reactions to become a mature mRNA that is subsequently exported to the cytoplasm. Many different enzymatic activities participate in the gene expression pathway, and the molecules or multi-molecular complexes responsible for these activities have been identified and characterized in considerable detail at the biochemical level (e.g., Ban et al. 2000; Shatkin and Manley 2000; Cramer et al. 2001; Will and Lührmann 2001a). Indeed, our knowledge about the gene expression processes is such that many of the steps of the gene expression pathway can be reproduced individually in vitro. However, nucleocytoplasmic transport can only be envisaged in relation to the cellular architecture, and for this reason the understanding of the transport mechanisms relies mostly on in situ studies. The larval salivary glands of the dipteran Chironomus tentans constitute a valuable model system for in situ studies of gene expression, as they offer the possibility of directly visualizing the synthesis, processing and export of a specific transcript: the Balbiani ring (BR) pre-mRNA. The BR genes and their expression in the salivary gland cells have been reviewed in detail previously (e.g., Mehlin and Daneholt 1993; Wieslander 1994; Daneholt 2001). In this chapter, we will describe the BR system, we will discuss the contribution of the BR system to our understanding of mRNA trafficking, and we will discuss the possible implication of nonchromatin nuclear structures in mRNA biogenesis. [ABSTRACT FROM AUTHOR]

The nucleolus is a subnuclear organelle (a membraneless nuclear compartment) in which, besides some other functions (Olson et al. 2002; Bernardi and Pandolfi 2003), the biogenesis of ribosomes takes place (e.g., Lewis et al. 1966; Busch and Smetana 1970; Goessens 1984; Hadjiolov 1985; Fakan 1986; Risueno and Medina 1986; Ra ka et al. 1990; Scheer and Benavente 1990; Hernandez- Verdun 1991; Thiry et al. 1991; Wachtler and Stahl 1993; Shaw and Jordan 1995; Carmo-Fonseca et al. 1996; Reeder 1999; Scheer and Hock 1999; Warner 1999; Olson et al. 2000; Fatica and Tollervey 2002). It is characterized by a high concentration of RNA (Brachet 1940; Caspersson and Schultz 1940) and thus is the most conspicuous nuclear structure seen through light and electron microscopy (LM and EM). Indeed, in metabolically active animal and plant somatic cells, the nucleolus harbors many tens, sometimes hundreds, of active ribosomal genes that are usually characterized by high levels of transcription accounting for more than 50% of the total cellular RNA production. At the same time, however, a significant fraction of nucleolar ribosomal genes, possibly in the order of magnitude of 50-95% (including the situation in yeast), are transcriptionally silent and likely exhibit the nucleosomal chromatin arrangement (Scheer 1978; Prior et al. 1983; Conconi et al. 1989; Gonzales- Melendi et al. 2001; Santoro and Grummt 2001; Sandmeier et al. 2002; Santoro et al. 2002). [ABSTRACT FROM AUTHOR]

Eukaryotic cells contain many types of ribonucleoproteins (RNPs) which are complexes formed by RNA molecules associated with proteins. The majority of these RNPs are in the nucleus and can be classified in two groups, the small nuclear RNPs (snRNPs) that function in the maturation of messenger RNAs, and the small nucleolar RNPs that reside in the cell nucleolus and are required for maturation of ribosomal RNAs. Although both types of RNPs function in two different fundamental processes, progress made during the last few years shows that their assembly requires similar protein components. This review focuses on the factors and mechanisms governing snRNPs and snoRNPs biogenesis as well as on the mechanisms implicated in the sorting of these RNPs to their intracellular destinations. [ABSTRACT FROM AUTHOR]

A variety of subnuclear structures, including nucleoli and Cajal bodies, are involved in the normal synthesis and processing of various types of gene transcripts as well as the formation and remodeling of their associated ribonucleoprotein (RNP)-RNA complexes. In contrast to this normal synthesis and assembly process, the expression of some mutant genes results in the formation of aberrant intranuclear inclusions that may interfere with nuclear function. These genes contain repetitive DNA sequence motifs, microsatellite tri- to penta-nucleotide repeats that contract and expand in length due to errors in DNA replication, recombination and repair. When microsatellite expansions occur within the protein coding region a toxic gene product may be synthesized. Recent studies suggest that some of these mutant proteins interfere with the function of factors important for transcriptional regulation. Surprisingly, a group of autosomal dominant neuromuscular diseases, the myotonic dystrophies, are associated with expansions in untranslated regions, and expression of these unstable microsatellites results in the accumulation of mutant allele transcripts together with specific proteins in nuclear foci. Are these RNA foci pathogenic or protective and do they provide evidence for the existence of yet another subnuclear structure in wild-type cells? Here, we review unstable microsatellites and their effects on nuclear structure and function with particular emphasis on the hypothesis that the transcription of microsatellite expansions results in the production of toxic RNA. [ABSTRACT FROM AUTHOR]

The cell nucleus, described by Franz Bauer in 1802, was one of the first intracellular structures to be identified by early microscopists. The nucleus is surrounded by a double membrane, of which the outer membrane is continuous with the endoplasmic reticulum, and this serves to separate the sites of gene transcription from the sites of mRNA translation in the cytoplasm. Transport of molecules between the nuclear and cytoplasmic compartments of the cell occurs via specific protein complexes located in the nuclear envelope, termed nuclear pore complexes (NPCs, see Fig. 1). [ABSTRACT FROM AUTHOR]