The Structure of the Nucleus

Almost all cells of the human body contain a nucleus, where the genetic material (DNA) is found. The mammalian cell nucleus has a highly ordered structure, containing a number of protein-rich domains in addition to the chromosomes that carry the DNA gene sequences. The complex organization of the nucleus was observed and reported as far back as 1835, when the largest sub-nuclear domain, the nucleolus, was described. Despite this, the detailed organisation of the nucleus and how this affects its function are still far from fully understood. In fact, as increasing numbers of molecular probes become available, so the complexity of nuclear structure appears to expand.

How Genes Work

Essential to the expression, or functioning, of genes are the processes of ‘transcription’ or rewriting of the DNA instructions into a messenger RNA (mRNA) intermediate and ‘translation’ of this temporary RNA message into the protein ‘product’ of the gene. Almost all mammalian genes contain introns, which are sequences present in the DNA template but not represented in the final gene product (protein). These must be removed, or ‘spliced’, from the mRNA message before it can be translated into protein. The accuracy of this mRNA splicing process is essential for correct gene expression. Splicing is carried out by a large, multi-molecular structure known as the spliceosome.

Spinal Muscular Atrophy and snurps

snRNPs (small nuclear ribonucleoproteins, pronounced ‘snurps’) are essential components of the spliceosome and show a complex pattern of distribution within the nucleus, where they localise to a number of different domains including speckles (interchromatin granule clusters) and Cajal bodies. The formation and maturation of snRNPs is a complex process. Early steps in the process occur outside the nucleus in the cytoplasm, and require a protein called Survival of Motor Neurons (SMN). Mutation of the gene coding for this protein is responsible for the inherited neurodegenerative disease, spinal muscular atrophy (SMA). SMN is also found in the nucleus where it concentrates, along with snRNPs, in Cajal bodies. It is not clear how the loss of SMN protein leads to the disease. All cells need to express their genes and, therefore, splice their RNA correctly, but SMA appears to specifically affect one type of cell: the motor neurons.

Our Work

We are studying the maturation of snRNP splicing factors, with a particular emphasis on the dynamics of their localisation within different nuclear structures. Since the first site of accumulation of new snRNPs in the nucleus is Cajal bodies containing the SMN protein, I am also interested in the dynamics of SMN and the protein coilin, which is also found in the Cajal body, but whose function is unknown. A major molecular tool used for our work is the green fluorescent protein (GFP). This is a protein originally found in the jellyfish Aequorea Victoria. Shining blue light on this protein causes it to glow green when viewed using a microscope. Proteins of interest (for example SMN) can be made to glow in the same way by fusing the DNA that codes for them with the DNA of the jellyfish gene. This fused gene can then be put back into mammalian cells and the dynamics of the protein of interest can be watched and recorded in living cells using time-lapse microscopy.

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