PSF Suppresses Tau Exon 10 Inclusion by Interacting with a Stem-Loop Structure Downstream of Exon 10

Abstract

Microtubule binding protein Tau has been implicated in a wide range of neurodegenerative disorders collectively classified as tauopathies. Exon 10 of the human tau gene, which codes for a microtubule binding repeat region, is alternatively spliced to form Tau protein isoforms containing either four or three microtubule binding repeats, Tau4R and Tau3R, respectively. The levels of different Tau splicing isoforms are fine-tuned by alternative splicing with the ratio of Tau4R/Tau3R maintained approximately at one in adult neurons. Mutations that disrupt tau exon 10 splicing regulation cause an imbalance of different tau splicing isoforms and have been associated with tauopathy. To search for factors interacting with tau pre-messenger RNA (pre-mRNA) and regulating tau exon 10 alternative splicing, we performed a yeast RNA–protein interaction screen and identified polypyrimidine tract binding protein associated splicing factor (PSF) as a candidate tau exon 10 splicing regulator. UV crosslinking experiments show that PSF binds to the stem-loop structure at the 5′ splice site downstream of tau exon 10. This PSF-interacting RNA element is distinct from known PSF binding sites previously identified in other genes. Overexpression of PSF promotes tau exon 10 exclusion, whereas down-regulation of the endogenous PSF facilitates exon 10 inclusion. Immunostaining shows that PSF is expressed in the human brain regions affected by tauopathy. Our data reveal a new player in tau exon 10 alternative splicing regulation and uncover a previously unknown mechanism of PSF in regulating tau pre-mRNA splicing.

ALS-Associated FUS Mutations Result in Compromised FUS Alternative Splicing and Autoregulation

“…Activation or repression of cassette exon splicing can be dependent on RNA binding positions, as suggested in Nova [36] and FOX2 [61] CLIP-seq data, which showed that Nova and FOX2 binding proximal to 3′ splice sites repressed cassette exons, while conversely binding proximal to 5′ splice sites promoted cassette exons. However, splicing factors such as hnRNP A1 and PSF binding to introns proximal to 5′ splice sites can also repress cassette exons [62], [63]; which suggests that activation or repression of cassette exon splicing is likely more complex. We experimentally demonstrated in this paper that FUS is a repressor of its own exon 7.…”

PSF: nuclear busy‐body or nuclear facilitator?

“…PSF has recently been shown to influence alternative splicing of both the CD45 (cluster of differentiation 45) and Tau genes through direct interaction with specific RNA sequences. In the Tau gene, PSF interacts with a stem–loop structure at the exon–intron boundary downstream of exon 10 to repress inclusion of this exon in the final mRNA . Similarly, PSF represses inclusion of exon 4 of the human CD45 gene by binding to a pyrimidine‐rich region within this exon …”

PSF contacts exon 7 of SMN2 pre-mRNA to promote exon 7 inclusion

“…In other cases, PSF is recruited to the exon splicing silencer complex to inhibit exon inclusion, indicating that PSF forms a large RNA-protein complex. PSF can also regulate alternative splicing through direct phosphorylation by GSK3 [34, 35, 46]. Whereas various RNA structures were recognized by PSF for its activity, we have shown in this report that PSF contacts purine rich sequence on exon 7 of SMN genes to promote exon 7 inclusion of SMN2 pre-mRNA.…”

“…Since the PTB-PSF complex is required early in spliceosome formation and is essential in catalytic step II [30, 32, 33], PSF has been suggested to be important in regulation of pre-mRNA splicing. For example, PSF is known to regulate alternative splicing of the tau and CD45 pre-mRNA [34, 35]. PSF contacts purine rich sequence in the 3’ site of U5 snRNA [36] and is able to mediate transcriptional activator-dependent stimulation of pre-mRNA processing.…”