Muscleblind, BSF and TBPH are mislocalized in the muscle sarcomere of aDrosophilamyotonic dystrophy model

Abstract

SUMMARYMyotonic dystrophy type 1 (DM1) is a genetic disease caused by the pathological expansion of a CTG trinucleotide repeat in the 3′ UTR of the DMPK gene. In the DMPK transcripts, the CUG expansions sequester RNA-binding proteins into nuclear foci, including transcription factors and alternative splicing regulators such as MBNL1. MBNL1 sequestration has been associated with key features of DM1. However, the basis behind a number of molecular and histological alterations in DM1 remain unclear. To help identify new pathogenic components of the disease, we carried out a genetic screen using a Drosophila model of DM1 that expresses 480 interrupted CTG repeats, i(CTG)480, and a collection of 1215 transgenic RNA interference (RNAi) fly lines. Of the 34 modifiers identified, two RNA-binding proteins, TBPH (homolog of human TAR DNA-binding protein 43 or TDP-43) and BSF (Bicoid stability factor; homolog of human LRPPRC), were of particular interest. These factors modified i(CTG)480 phenotypes in the fly eye and wing, and TBPH silencing also suppressed CTG-induced defects in the flight muscles. In Drosophila flight muscle, TBPH, BSF and the fly ortholog of MBNL1, Muscleblind (Mbl), were detected in sarcomeric bands. Expression of i(CTG)480 resulted in changes in the sarcomeric patterns of these proteins, which could be restored by coexpression with human MBNL1. Epistasis studies showed that Mbl silencing was sufficient to induce a subcellular redistribution of TBPH and BSF proteins in the muscle, which mimicked the effect of i(CTG)480 expression. These results provide the first description of TBPH and BSF as targets of Mbl-mediated CTG toxicity, and they suggest an important role of these proteins in DM1 muscle pathology.

Derepressing muscleblind expression by miRNA sponges ameliorates myotonic dystrophy-like phenotypes in Drosophila

“…To further analyze the effect of dme-miR-277 or dme-miR-304 silencing we stained Muscleblind distribution in longitudinal sections of indirect flight muscles (IFMs). We had previously shown that endogenous Muscleblind protein is localized mainly in sarcomeric Z and H bands of muscle42. Consistently, we detected Muscleblind proteins in the bands of muscle sarcomeres in control flies that express the scramble-SP construct (Fig.…”

“…The recombinant line Mhc-Gal4 UAS-i(CTG)480 was generated in ref. 42. UAS-mblC flies58 and UAS-IR-mbl flies42 were previously reported.…”

Emerging Roles for hnRNPs in post-transcriptional regulation: what can we learn from flies?

“…Interestingly, the expansion of CTG trinucleotide repeats in the 3′ UTR of the DMPK gene (responsible for myotonic dystrophy type 1 (DM1)) could sequester TBPH and other related proteins into nuclear foci, thus depriving cell of these vital protein functions (Llamusi et al 2013) (Fig. 4d), with a mechanism remembering the previously described interaction between Hrb87F and the CGG trinucleotide repeat at the 5′ untranslated region of FMR1 gene (Fig.…”

“…Furthermore, an interaction between TBPH and the dystrophia myotonica-protein kinase (DMPK) gene has also been recently identified (Llamusi et al 2013). Interestingly, the expansion of CTG trinucleotide repeats in the 3′ UTR of the DMPK gene (responsible for myotonic dystrophy type 1 (DM1)) could sequester TBPH and other related proteins into nuclear foci, thus depriving cell of these vital protein functions (Llamusi et al 2013) (Fig.…”

Roles for RNA-binding proteins in development and disease

“…As regulators of alternative splicing both CELF and MBNL proteins typically bind to the introns downstream or upstream of an alternative exon to promote inclusion or exclusion, respectively (Kalsotra et al, 2008; Wang et al, 2012). In the cytoplasm, MBNL affects mRNA localization or stability by binding the 3’ UTR of target mRNAs (Llamusi et al, 2013; Masuda et al, 2012). …”