Variations in a polymorphic (TG)m sequence near exon 9 of the human CFTR gene have been associated with variable proportions of exon skipping and occurrence of disease. We have recently identified nuclear factor TDP-43 as a novel splicing regulator capable of binding to this element in the CFTR pre-mRNA and inhibiting recognition of the neighboring exon. In this study we report the dissection of the RNA binding properties of TDP-43 and their functional implications in relationship with the splicing process. Our results show that this protein contains two fully functional RNA recognition motif (RRM) domains with distinct RNA/DNA binding characteristics. Interestingly, TDP-43 can bind a minimum number of six UG (or TG) single-stranded dinucleotide stretches, and binding affinity increases with the number of repeats. In particular, the highly conserved Phe residues in the first RRM region play a key role in nucleic acid recognition.We have recently reported the identification of TDP-43 as a splicing regulator that specifically binds the (UG)m-repeated polymorphic region near the 3Ј-splice site of CFTR exon 9 and down-regulates its recognition by the splicing machinery (1). This region, acting in concert with the adjacent (u)n element, is one of the key cis-acting sequences which regulate the proportion of exon 9 skipping in the mature CFTR mRNA transcript (1-3). Considering that exon 9 skipping produces a non-functional CFTR protein (4, 5) the study of the RNA binding properties of TDP-43 is of considerable importance to gain further insight concerning the potential disease-causing consequences of its binding in vivo. Indeed, the clinical relevance of these studies is highlighted by the existence of a clear association between certain (TG)m(T)n alleles with distinct forms of Cystic Fibrosis (1, 6 -9).In addition, the study of (UG)m elements can provide further insight concerning the mRNA splicing process in general because (UG)m sequences have been described to act as splicing regulatory sequences in different genomic contexts. In fact, in addition to the CFTR gene, the presence of simple (UG)mrepeated sequences has been described to influence the splicing process of at least two other genes: the apolipoprotein AII gene (10) and the human cardiac Na ϩ /Ca 2ϩ exchanger (11). In the Apo AII gene the UG tract was shown to be functionally equivalent to a polypyrimidine tract and required for efficient splicing of Apo AII exon 2 (10) while in the human cardiac Na ϩ /Ca 2ϩ exchanger (11) it acts as a strong intronic splicing enhancer situated in intron 2. It should be noted that in contrast with these two genes, the CFTR (UG)m element was found to possess a strong inhibitory effect on CFTR exon 9 splicing, a property that may probably be linked to its peculiar evolutionary history. In fact, sequencing of the mouse CFTR exon 9 genomic region has shown that in the flanking introns, the (TG)m(T)n regulatory elements are absent and that the intron themselves are of very different length when compared with the human introns (...

Alternative splicing of human cystic ®brosis transmembrane conductance regulator (CFTR) exon 9 is regulated by a combination of cis-acting elements distributed through the exon and both¯anking introns (IVS8 and IVS9). Several studies have identi®ed in the IVS8 intron 3¢ splice site a regulatory element that is composed of a polymorphic (TG)m(T)n repeated sequence. At present, no cellular factors have been identi®ed that recognize this element. We have identi®ed TDP-43, a nuclear protein not previously described to bind RNA, as the factor binding speci®c-ally to the (TG)m sequence. Transient TDP-43 overexpression in Hep3B cells results in an increase in exon 9 skipping. This effect is more pronounced with concomitant overexpression of SR proteins. Antisense inhibition of endogenous TDP-43 expression results in increased inclusion of exon 9, providing a new therapeutic target to correct aberrant splicing of exon 9 in CF patients. The clinical and biological relevance of this ®nding in vivo is demonstrated by our characterization of a CF patient carrying a TG10T9(DF508)/ TG13T3(wt) genotype leading to a disease-causing high proportion of exon 9 skipping.

Cellular and plasma fibronectins are heterodimers consisting of similar but not identical polypeptides. The differences between fibronectin subunits are due in part to the variability of internal primary sequences. This results from alternative splicing in at least two regions (ED and IIICS) of the pre‐mRNA. The complete primary structure of human fibronectin, including most of the internal variations, has been determined by sequencing a series of overlapping cDNA clones. In total, they covered 7692 nucleotides and represented the mRNA sequence coding from the amino terminus of the mature protein to the poly(A) tail. The deduced amino acid sequence of fibronectin has been analysed in terms of the arrangement of internal homologies and the different binding domains.

It has been assumed that constitutive and regulated splicing of RNA polymerase II transcripts depends exclusively on signals present in the RNA molecule. Here we show that changes in promoter structure strongly affect splice site selection. We investigated the splicing of the ED I exon, which encodes a facultative type III repeat of fibronectin, whose inclusion is regulated during development and in proliferative processes. We used an alternative splicing assay combined with promoter swapping to demonstrate that the extent of ED I splicing is dependent on the promoter structure from which the transcript originated and that this regulation is independent of the promoter strength. Thus, these results provide the first evidence for coupling between alternative splicing and promoter-specific transcription, which agrees with recent cytological and biochemical evidence of coordination between splicing and transcription.Transcriptional activity by RNA polymerase II (pol II) has been shown to occur in association to 20-50 discrete regions within the cell nucleus, known as nuclear speckles. The fact that these clustered domains not only concentrate Poly(A) ϩ -RNAs, but also small nuclear ribonucleoproteins and the nonsmall nuclear ribonucleoprotein splicing factor SC35, suggests that transcription and splicing might not be independent events, but on the contrary, highly coordinated processes both at the functional and structural levels (1).The study of fibronectin (FN) gene transcription confirmed that pre-mRNAs are constrained from free diffusion. FN transcripts are seen accumulated in elongated tracks, spatially coincident with each allele and colocalizing with speckles. Splicing of FN pre-mRNA appears to occur directly within the track, as evidenced by the fluorescence in situ hybridization detection of intron-containing transcripts in a spatially restricted region proximal to the gene. Each track can be considered as an assembly line where transcription and splicing take place sequentially, exhibiting functional and structural association (2).These observations raise the question whether the intimate spatial association between transcription and splicing also implies influence of one process over the other. Misteli et al. (3) showed that splicing factors are recruited to restricted regions of the nucleus where transcription takes place, thus it becomes important to determine whether cell-specific regulatory transcription factors play a role in splicing. Extending these views we decided to investigate whether modifications in RNA pol II promoter architecture could influence alternative splicing of the ED I exon, which encodes a facultative type III repeat of FN (4-5). Alternative splicing of this exon varies during embryo development and aging, in different adult cell types and in proliferative processes such as healing (6). Inclusion of ED I depends on the presence of a 81-bp sequence, known as ''splicing enhancer'' or SE, located within the central region of the exon (7). The SE markedly stimulates the use...

The extracellular matrix (ECM) is a highly dynamic structure that not only provides a physical framework for cells within connective tissues, but also imparts instructive signals for development, tissue homeostasis and basic cell functions through its composition and ability to exert mechanical forces. The ECM of tissues is composed of, in addition to proteoglycans and hyaluronic acid, a number of proteins, most of which are generated after alternative splicing of their pre-mRNA. However, the precise function of these protein isoforms is still obscure in most cases. Fibronectin (FN), one of the main components of the ECM, is also one of the best-known examples of a family of proteins generated by alternative splicing, having at least 20 different isoforms in humans. Over the last few years, considerable progress on elucidating the functions of the alternatively spliced FN isoforms has been achieved with the essential development of key engineered mouse strains. Here we summarize the phenotypes of the mouse strains having targeted mutations in the FN gene, which may lead to novel insights linking function of alternatively spliced isoforms of fibronectin to human pathologies.