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The RNA export pathway is essential for export-competent mRNAs to pass from the nucleus into the cytoplasm, and thus is essential for protein production and normal function of cells.Drosophilawith partial loss of function ofNxt1, a core factor in the pathway, show reduced viability and male and female sterility. The male sterility has previously been shown to be caused by defects in testis-specific gene expression, particularly of genes without introns. Here we describe a specific defect in growth and maintenance of the larval muscles, leading to muscle degeneration inNxt1mutants. RNAseq revealed reduced expression of mRNAs of many genes inNxt1mutant muscles. Despite this, the degeneration was rescued by increased expression of a single gene, the costamere componenttn (abba),in muscles. Genes under-expressed in the mutant typically have long introns, and most normally encode circular RNAs in addition to mRNAs. This is the first report of a specific role for the RNA export pathway geneNxt1in muscle integrity. Our data onNxt1links the mRNA export pathway to a global role in mRNA expression of genes that also produce circular RNAs,in vivo.Author summaryIn eukaryotic cells the DNA encoding instructions for protein synthesis is located in the nucleus, it is transcribed to generate pre-mRNA, which is processed at both ends and spliced to remove internal spacer regions (introns) to generate mRNA. This mRNA is then transported by the mRNA export pathway via nuclear pores to the cytoplasm for protein synthesis. We have previously shown that reduction in activity of a specific protein in the mRNA export pathway, Nxt1, has an additional role in testis-specific transcription. Here we describe a further role for this protein specifically in gene expression, particularly of genes with long introns, and in muscle maintenance.Drosophilalarvae with reduced Nxt1 activity have normal muscle pattern when they are small, but show muscular atrophy and degeneration as they grow, resulting in significant defects in their movement speed. We discovered that expression of many genes is reduced in the mutant larvae, but that restoring the expression of just one of these,abba, theDrosophilahomologue ofTrim32(a human gene involved in muscular dystrophy) is capable of preventing the muscle degeneration.

The RNA export pathway is vital for export-competent mRNA to be exported from the nucleus\ud to the cytoplasm. Without this pathway, RNA is unable to be translated into proteins for their\ud function. A hetero-dimer comprised of Nxt1-Nxf1 is required to bind to the export-competent\ud mRNA for a functional interaction between Nxf1 and the nuclear pore complex. Without Nxt1,\ud Nxf1 interacts less effectively with the complex.\ud In this thesis, the importance of Nxt1 is further examined during metamorphosis and in\ud ovaries. I have shown that a reduction of functional Nxt1 during metamorphosis affects the\ud muscle integrity in third instar larvae. This, in turn, affects the migration of the air bubble,\ud which moves from the posterior to the anterior end, leading to dead pupae with no head\ud eversion. Increasing the expression of a single gene, abba, rescues the muscle phenotype, but\ud not the pupa viability. Further examination of differentially expressed genes revealed that\ud genes with many and long introns were the most affected.\ud In the ovaries, I have shown that Nxt1 is important in the piRNA pathway. Microarray and\ud qRT-PCR have shown that transposons are de-repressed with less Nxt1, leading to sterility\ud and death. Two rounds of small RNA-sequencing were performed, however, discrepancies\ud between the two rounds made it difficult to analyse the data. Either Nxt1 influences the derepression\ud of transposons or Nxt1 has a mild effect on de-repression of transposons and is\ud more involved in the silencing process. Together with our collaborators we also got a first\ud insight into which potential Nxf protein is interacting with Nxt1 in the piRNA pathway.