Bruno-3 regulates sarcomere components expression and contributes to muscle phenotypes of Myotonic dystrophy type 1

Abstract

Steinert disease, or myotonic dystrophy type 1 (DM1), is a multisystemic disorder caused by toxic noncoding CUG repeat transcripts, leading to altered levels of two RNA binding factors, MBNL1 and CELF1. The contribution of CELF1 to DM1 phenotypes is controversial. Here, we show that the Drosophila CELF1 family member, Bru-3, contributes to pathogenic muscle defects observed in a Drosophila model of DM1. Bru-3 displays predominantly cytoplasmic expression in muscles and its muscle-specific overexpression causes a range of phenotypes also observed in the fly DM1 model, including affected motility, fiber splitting, reduced myofiber length and altered myoblast fusion. Interestingly, comparative genome-wide transcriptomic analyses revealed that Bru-3 negatively regulates levels of mRNAs encoding a set of sarcomere components, including Actn transcripts. Conversely, it acts as a positive regulator of Actn translation. As CELF1 displays predominantly cytoplasmic expression in differentiating C2C12 myotubes and binds to Actn mRNA, we hypothesize that it might exert analogous functions in vertebrate muscles. Altogether, we propose that cytoplasmic Bru-3 contributes to DM1 pathogenesis in a Drosophila model by regulating sarcomeric transcripts and protein levels.

Identification of a pathogenic mutation in ATP2A1 via in silico analysis of exome data for cryptic aberrant splice sites

“…Type II fibers predominate in mouse diaphragm, and SERCA1-null mice have been observed to die from respiratory failure shortly after birth (Pan et al, 2003). In Drosophila, previous studies have shown that missplicing of SERCA leads to hypercontraction in flies (a phenotype which is correlated with myotonia) (Picchio et al, 2018). Fly SERCA had also been demonstrated to play an important role in mediating calcium storage-regulation of muscle action potential (Sanyal et al, 2005), and cardiac function (Abraham & Wolf, 2013;Sanyal, Jennings, Dowse, & Ramaswami, 2006).…”

Dissecting Pathogenetic Mechanisms and Therapeutic Strategies in Drosophila Models of Myotonic Dystrophy Type 1

“…MBNL1 sequestration and CELF1 accumulation were the first and now the most thoroughly studied mechanisms of DM1 pathogenesis, but the specific contribution of CELF1 to DM1-associated phenotypes has not yet been entirely elucidated. Recent work by our group [59] using Drosophila as a model revealed new functions of the fly CELF1 counterpart, Bruno-3 (Bru-3) and its contribution to the DM1 phenotype. Our data demonstrate that the increased level of Bru-3 in muscles contribute to impaired larva motility and muscle morphology defects in DM1.…”

“…Genome-wide transcriptional profiling of Bru-3 overexpressing larvae has led to identification of a set of 11 genes encoding conserved sarcomeric components whose expression is down-regulated (Figure 1). These include α-Actinin ( Actn ), Myosin heavy and light chains ( Mhc , Mlc1 and Mlc2 ), Tropomyosin 1 and 2 ( Tm1 and Tm2 ), Troponin I ( wupA ) and C47D ( TpnC47D ), bent , Paramyosin ( Prm ), Zasp52 and Unc-89 [59]. Interestingly, as demonstrated for Actn , the down-regulation of sarcomeric genes involves the cytoplasmic Bru-3 function and its potential role in co-translational mRNA decay [59].…”

“…Another recent study reporting transcriptional profiling of the cardiac DM1 models [59] revealed that the inhibition of mbl or overexpression of Bru-3 specifically in the fly heart, that mimic the pathogenic MBNL1/CELF1 misbalance in DM1 patient, induce asynchronous heartbeat and cardiac arrhythmia (Figure 1). The data analysis revealed that the cardiac disorders observed in these DM1 models are the result of deregulation of genes involved in cellular calcium level and cardiac conduction.…”

Straightjacket/α2δ3deregulation is associated with cardiac conduction defects in Myotonic Dystrophy type 1

“…The heart of the fruit fly, even if simple in structure, nevertheless displays just like the human heart, pacemaker-regulated rhythmic beating involving functions of conserved ion channels (Ocorr et al, 2007;Taghli-Lamallem et al, 2016). We simulated pathogenic MBNL1/CELF1 misbalance specifically in the fly heart by attenuating the Drosophila MBNL1 ortholog Muscleblind (Mbl) and by overexpressing the CELF1 counterpart Bruno-3 (Bru-3) (Picchio et al, 2018). This led to asynchronous heartbeat (anterior and posterior heart segments beating at a different rate) that in Drosophila results from partial conduction block (Birse et al, 2010).…”