Alternative splicing regulation by Muscleblind proteins: from development to disease

Abstract

Regulated use of exons in pre-mRNAs, a process known as alternative splicing, strongly contributes to proteome diversity. Alternative splicing is finely regulated by factors that bind specific sequences within the precursor mRNAs. Members of the Muscleblind (Mbl) family of splicing factors control critical exon use changes during the development of specific tissues, particularly heart and skeletal muscle. Muscleblind homologs are only found in metazoans from Nematoda to mammals. Splicing targets and recognition mechanisms are also conserved through evolution. In this recognition, Muscleblind CCCH-type zinc finger domains bind to intronic motifs in pre-mRNA targets in which the protein can either activate or repress splicing of nearby exons, depending on the localization of the binding motifs relative to the regulated alternative exon. In humans, the Muscleblind-like 1 (MBNL1) proteins play a critical role in hereditary diseases caused by microsatellite expansions, particularly myotonic dystrophy type 1 (DM1), in which depletion of MBNL1 activity through sequestration explains most misregulated alternative splicing events, at least in murine models. Because of the involvement of these proteins in human diseases, further understanding of the molecular mechanisms by which MBNL1 regulates splicing will help design therapies to revert pathological splicing alterations. Here we summarize the most relevant findings on this family of proteins in recent years, focusing on recently described functional motifs, transcriptional regulation of Muscleblind, regulatory activity on splicing, and involvement in human diseases.

MBNL proteins and their target RNAs, interaction and splicing regulation

“…This pattern of splicing regulation is akin to other factors, such as neurooncological ventral antigen (NOVA), FOX or heterogeneous nuclear ribonucleoprotein L [hnRNP L, (90)]. Such configuration indicates that repression of exon inclusion is a consequence of MBNLs directly blocking the exonic enhancers, 3′ splice-sites, polypyrimidine-rich tracts or intronic branch site elements, whereas activation might be caused by MBNLs competition with splicing silencers, activation of intronic splicing enhancers or enhanced recognition of 5′ splice-site by spliceosome (7,9,54,91). Other positional splicing activities of MBNL1 also occur [Figures 5 and 6A, (3,7,10,13,23,92)].…”

“…Increased concentrations of MBNL1 and 2 during differentiation result in at least two key developmental transitions (9–11). The first promotes differentiation of embryonic stem cells and the second induces a shift from a fetal to adult splice pattern of target mRNAs (Figure 2C).…”

Combinatorial Mutagenesis of MBNL1 Zinc Fingers Elucidates Distinct Classes of Regulatory Events

“…Muscleblind-like 1, or MBNL1, is an alternative splicing factor that has been the focus of intense study over the last decade due to its involvement in myotonic dystrophy (DM) pathogenesis (10,23). DM is a debilitating, multisystemic disease that is caused by the expansion of certain noncoding, CTG-and CCTG-containing repeats within the genome (3,26,30).…”

Transcriptional regulation and alternative splicing cooperate in muscle fiber-type specification in flies and mammals

“…While strong evidence supports the involvement of splicing factors such as Muscleblind-like [19] and Rbfox [58] in muscular dystrophies, the relationship between RNA-binding motif protein 20 (RBM20) and titin in heart disease is perhaps the most provocative link to-date between alternative splicing and muscle disease. As discussed above, titin is the elastic component of the myofibril, maintaining the precise structural arrangement of thick and thin filaments and generating passive muscle stiffness.…”