A GFP-tagged muscleblind C protein isoform reporter construct

Drosophila muscleblind (mbl), the ortholog of human Muscleblind-like 1 (MBNL1) gene involved in Myotonic Dystrophy (DM), gives raise to protein isoforms MblA to G. The specific functions and subcellular distribution of isoforms are still largely unknown. To overcome the lack of isoform-specific antibodies we generated transgenic flies that express a GFP:MblC fusion protein under the control of the Gal4/UAS system. The reporter fusion protein was able to functionally complement mbl loss of function mutations, demonstrating activity, and accumulated predominantly in adult muscle nuclei. The fluorescent nature of the reporter makes it appropriate for live imaging detection of MblC protein isoform.

Genetic and chemical modifiers of a CUG toxicity model in Drosophila

Non-coding CUG repeat expansions interfere with the activity of human Muscleblind-like (MBNL) proteins contributing to myotonic dystrophy 1 (DM1). To understand this toxic RNA gain-of-function mechanism we developed a Drosophila model expressing 60 pure and 480 interrupted CUG repeats in the context of a non-translatable RNA. These flies reproduced aspects of the DM1 pathology, most notably nuclear accumulation of CUG transcripts, muscle degeneration, splicing misregulation, and diminished Muscleblind function in vivo. Reduced Muscleblind activity was evident from the sensitivity of CUG-induced phenotypes to a decrease in muscleblind genetic dosage and rescue by MBNL1 expression, and further supported by the co-localization of Muscleblind and CUG repeat RNA in ribonuclear foci. Targeted expression of CUG repeats to the developing eye and brain mushroom bodies was toxic leading to rough eyes and semilethality, respectively. These phenotypes were utilized to identify genetic and chemical modifiers of the CUG-induced toxicity. 15 genetic modifiers of the rough eye phenotype were isolated. These genes identify putative cellular processes unknown to be altered by CUG repeat RNA, and they include mRNA export factor Aly, apoptosis inhibitor Thread, chromatin remodelling factor Nurf-38, and extracellular matrix structural component Viking. Ten chemical compounds suppressed the semilethal phenotype. These compounds significantly improved viability of CUG expressing flies and included non-steroidal anti-inflammatory agents (ketoprofen), muscarinic, cholinergic and histamine receptor inhibitors (orphenadrine), and drugs that can affect sodium and calcium metabolism such as clenbuterol and spironolactone. These findings provide new insights into the DM1 phenotype, and suggest novel candidates for DM1 treatments.

Serpent and a hibris reporter are co-expressed in migrating cells during Drosophila hematopoiesis and Malpighian tubule formation

Motile mesodermal cells contribute several cell types to developing embryos. In Drosophila, blood cell precursors or prohemocytes, are first detected in the procephalic mesoderm by the expression of the GATA transcription factor Serpent. Once specified, a subset of prohemocytes migrate posteriorly to populate most of the embryo and further differentiate as plasmatocytes. Similarly, Drosophila nephrogenesis involves integration of posterior mesodermal cells into the Malpighian tubule primordia where these cells differentiate as stellate cells. Here we investigated the possibility that the immunoglobulin-domain protein Hibris and the GATA factor Serpent were co-expressed in motile mesodermal cells by using the hibris expression reporter P[w(+)]36.1 and antibody staining. We show that P[w(+)]36.1 reproduces the endogenous expression of hibris in several embryonic tissue types and organs, including mesectoderm, early mesoderm, pharyngeal musculature, hindgut, anal plates, posterior spiracles, and antennomaxillary complex. We find that both migrating prohemocytes and posterior mesodermal cells, before their integration into the Malpighian tubule primordia, simultaneously express the hibris reporter and Serpent. We also show that hibris function is not essential for prohemocyte migration out of the procephalic mesoderm NOR maintenance of Serpent expression in prohemocytes.

The Muscleblind family of proteins: an emerging class of regulators of developmentally programmed alternative splicing

Alternative splicing is widely used to generate protein diversity and to control gene expression in many biological processes, including cell fate determination and apoptosis. In this review, we focus on the Muscleblind family of tissue-specific alternative splicing regulators. Muscleblind proteins bind pre-mRNA through an evolutionarily conserved tandem CCCH zinc finger domain. Human Muscleblind homologs MBNL1, MBNL2 and MBNL3 promote inclusion or exclusion of specific exons on different pre-mRNAs by antagonizing the activity of CUG-BP and ETR-3-like factors (CELF proteins) bound to distinct intronic sites. The relative activities of Muscleblind and CELF proteins control a key developmental switch. Defined transcripts follow an embryonic splice pattern when CELF activity predominates, whereas they follow an adult pattern when Muscleblind activity prevails. Human MBNL proteins show functional specializations. While MBNL1 seems to promote muscle differentiation, MBNL3 appears to function in an opposing manner inhibiting expression of muscle differentiation markers. MBNL2, on the other hand, participates in a new RNA-dependent protein localization mechanism involving recruitment of integrin alpha3 protein to focal adhesions. Both muscleblind mutant Drosophila embryos and Mbnl1 knockout mice show muscle abnormalities and altered splicing of specific transcripts. In addition to regulating terminal muscle differentiation through alternative splicing control, results by several groups suggest that Muscleblind participates in the differentiation of photoreceptors, neurons, adipocytes and blood cell types. Misregulation of MBNL activity can lead to human pathologies. Through mechanisms not completely identified yet, expression of transcripts containing large non-coding CUG or CCUG repeat expansions mimics muscleblind loss-of-function phenotypes. Archetypical within this class of disorders are myotonic dystrophies. Our understanding of the biology of Muscleblind proteins has increased dramatically over the last few years, but several key issues remain unsolved. Defining the mechanism of the activity of Muscleblind proteins, their splicing partners, and the functional relevance of its several protein isoforms are just a few examples.

An Interspecific Functional Complementation Test in Drosophila for Introductory Genetics Laboratory Courses

Introductory genetics courses often include evolutionary genetics concepts such as sequence homology and functional conservation. It is usually assumed that two sequences showing homology (i.e., sharing a common ancestral sequence) perform the same molecular function. The correlation, however, does not always hold true, and evidence for functional conservation must come from functional studies. In this study we describe a genetics laboratory class that demonstrates functional conservation between the Drosophila protein Muscleblind (Mbl) and its human ortholog MBNL1. We use the Gal4/UAS system to express MBNL1 in a Drosophila mutant background and measure the in vivo activity of the human protein by rescue of mbl mutant phenotype in embryos. As a control, ubiquitous expression of Drosophila MblC, one of the four protein isoforms encoded by the gene, increased by 71% the viability of mbl mutant embryos and greatly reduced the hypercontracted abdomen of mutant larvae. In a parallel experiment, human MBNL1 provided a robust rescue of the embryonic lethality (78%) and improved abdomen hypercontraction as well. Under both conditions, rescued larvae die as first instars, probably due to overexpression effects, lack of alternative protein isoforms, or incomplete expression in critical tissues such as the nervous system. The use of two constructs in the rescue experiment (UAS-mblC and UAS-MBNL1) and the incomplete rescue prompt several questions for students. The fact that a human protein works in a Drosophila cellular context illustrates the use of an in vivo test to prove functional conservation.