RNA-binding Protein Muscleblind-like 3 (MBNL3) Disrupts Myocyte Enhancer Factor 2 (Mef2) β-Exon Splicing

Abstract

Mammalian MBNL (muscleblind-like) proteins are regulators of alternative splicing and have been implicated in myotonic dystrophy, the most common form of adult onset muscular dystrophy. MBNL3 functions as an inhibitor of muscle differentiation and is expressed in proliferating muscle precursor cells but not in differentiated skeletal muscle. Here we demonstrate that MBNL3 regulates the splicing pattern of the muscle transcription factor myocyte enhancer factor 2 (Mef2) by promoting exclusion of the alternatively spliced ␤-exon. Expression of the transcriptionally more active (؉)␤ isoform of Mef2D was sufficient to overcome the inhibitory effects of MBNL3 on muscle differentiation. These data suggest that MBNL3 antagonizes muscle differentiation by disrupting Mef2 ␤-exon splicing. MBNL3 regulates Mef2D splicing by directly binding to intron 7 downstream of the alternatively spliced exon in the pre-mRNA. The RNA binding activity of MBNL3 requires the CX 7 CX 4 -6 CX 3 H zinc finger domains. Using a cell culture model of myotonic dystrophy and myotonic dystrophy patient tissue, we have evidence that expression of CUG expanded RNAs can lead to an increase in MBNL3 expression and a decrease in Mef2D ␤-exon splicing. These studies suggest that elevating MBNL3 activity in myogenic cells could lead to muscle degeneration disorders such as myotonic dystrophy.

Disrupted prenatal RNA processing and myogenesis in congenital myotonic dystrophy

“…5F). These results suggested that MBNL3 represses adult pattern RNA isoforms, as has been proposed previously (Lee et al 2010). In agreement, Mbnl3 knockout myoblasts also displayed early changes in mRNA abundance compared with wild-type controls (Supplemental Fig.…”

Myotonic Dystrophy Type 1: Focus on the RNA Pathology and Therapy

“…Using a DM1 cell culture model and DM1 patient tissue, they provided evidence that expression of CUG expanded RNAs can lead to an increase in MBNL3 expression accompanied by a decrease in Mef2D β-exon splicing (Lee et al, 2010). These studies suggest that an increase in MBNL3 activity may play a role in the skeletal muscle degeneration experienced by DM1 patients (Lee et al, 2010).…”

“…MBNL3 antagonizes muscle differentiation by disrupting the expression of (+)β isoform of Mef2D which is transcriptionally more active (Lee et al, 2010). Using a DM1 cell culture model and DM1 patient tissue, they provided evidence that expression of CUG expanded RNAs can lead to an increase in MBNL3 expression accompanied by a decrease in Mef2D β-exon splicing (Lee et al, 2010).…”

“…MBNL3 regulates the splicing pattern of the muscle transcription factor myocyte enhancer factor 2 (Mef2) (Lee et al, 2010). MBNL3 antagonizes muscle differentiation by disrupting the expression of (+)β isoform of Mef2D which is transcriptionally more active (Lee et al, 2010).…”

Monoclonal Antibodies Against Muscleblind-like 3, a Protein with Punctate Nuclear Localization

“…The subnuclear localization pattern of MBNL3 in interchromatin regions supports our discovery that MBNL3 is a regulator of pre-mRNA splicing in myogenic cells. (17) Discussion Mammalian MBNL proteins have been implicated in muscle differentiation based on their homology to muscleblind, a Drosophila protein required for terminal muscle differentiation in flies. (18) The existence of three MBNL genes in vertebrates as opposed to a single gene in Drosophila implies that there are functional differences between the three mammalian MBNL proteins MBNL1, MBNL2, and MBNL3.…”