Transgenic tools for Drosophila muscle research

A Candidate RNAi Screen Reveals Diverse RNA-Binding Protein Phenotypes in Drosophila Flight Muscle

The proper regulation of RNA processing is critical for muscle development and the fine-tuning of contractile ability among muscle fiber-types. RNA binding proteins (RBPs) regulate the diverse steps in RNA processing, including alternative splicing, which generates fiber-type specific isoforms of structural proteins that confer contractile sarcomeres with distinct biomechanical properties. Alternative splicing is disrupted in muscle diseases such as myotonic dystrophy and dilated cardiomyopathy and is altered after intense exercise as well as with aging. It is therefore important to understand splicing and RBP function, but currently, only a small fraction of the hundreds of annotated RBPs expressed in muscle have been characterized. Here, we demonstrate the utility of Drosophila as a genetic model system to investigate basic developmental mechanisms of RBP function in myogenesis. We find that RBPs exhibit dynamic temporal and fiber-type specific expression patterns in mRNA-Seq data and display muscle-specific phenotypes. We performed knockdown with 105 RNAi hairpins targeting 35 RBPs and report associated lethality, flight, myofiber and sarcomere defects, including flight muscle phenotypes for Doa, Rm62, mub, mbl, sbr, and clu. Knockdown phenotypes of spliceosome components, as highlighted by phenotypes for A-complex components SF1 and Hrb87F (hnRNPA1), revealed level- and temporal-dependent myofibril defects. We further show that splicing mediated by SF1 and Hrb87F is necessary for Z-disc stability and proper myofibril development, and strong knockdown of either gene results in impaired localization of kettin to the Z-disc. Our results expand the number of RBPs with a described phenotype in muscle and underscore the diversity in myofibril and transcriptomic phenotypes associated with splicing defects. Drosophila is thus a powerful model to gain disease-relevant insight into cellular and molecular phenotypes observed when expression levels of splicing factors, spliceosome components and splicing dynamics are altered.

Contributions of alternative splicing to muscle type development and function

Animals possess a wide variety of muscle types that support different kinds of movements. Different muscles have distinct locations, morphologies and contractile properties, raising the question of how muscle diversity is generated during development. Normal aging processes and muscle disorders differentially affect particular muscle types, thus understanding how muscles normally develop and are maintained provides insight into alterations in disease and senescence. As muscle structure and basic developmental mechanisms are highly conserved, many important insights into disease mechanisms in humans as well as into basic principles of muscle development have come from model organisms such as Drosophila, zebrafish and mouse. While transcriptional regulation has been characterized to play an important role in myogenesis, there is a growing recognition of the contributions of alternative splicing to myogenesis and the refinement of muscle function. Here we review our current understanding of muscle type specific alternative splicing, using examples of isoforms with distinct functions from both vertebrates and Drosophila. Future exploration of the vast potential of alternative splicing to fine-tune muscle development and function will likely uncover novel mechanisms of isoform-specific regulation and a more holistic understanding of muscle development, disease and aging.

Mechanical Regulation of Cardiac Aging in Model Systems

Unlike diet and exercise, which individuals can modulate according to their lifestyle, aging is unavoidable. With normal or healthy aging, the heart undergoes extensive vascular, cellular, and interstitial molecular changes that result in stiffer less compliant hearts that experience a general decline in organ function. Although these molecular changes deemed cardiac remodeling were once thought to be concomitant with advanced cardiovascular disease, they can be found in patients without manifestation of clinical disease. It is now mostly acknowledged that these age-related mechanical changes confer vulnerability of the heart to cardiovascular stresses associated with disease, such as hypertension and atherosclerosis. However, recent studies have aimed at differentiating the initial compensatory changes that occur within the heart with age to maintain contractile function from the maladaptive responses associated with disease. This work has identified new targets to improve cardiac function during aging. Spanning invertebrate to vertebrate models, we use this review to delineate some hallmarks of physiological versus pathological remodeling that occur in the cardiomyocyte and its microenvironment, focusing especially on the mechanical changes that occur within the sarcomere, intercalated disc, costamere, and extracellular matrix.

Muscle-specific overexpression of the catalytic subunit of DNA polymerase γ induces pupal lethality in Drosophila melanogaster

We show the physiological effects and molecular characterization of overexpression of the catalytic core of mitochondrial DNA (mtDNA) polymerase (pol γ-α) in muscle of Drosophila melanogaster. Muscle-specific overexpression of pol γ-α using the UAS/GAL4 (where UAS is upstream activation sequence) system produced more than 90% of lethality at the end of pupal stage at 25°C, and the survivor adult flies showed a significant reduction in life span. The survivor flies displayed a decreased mtDNA level that is accompanied by a corresponding decrease in the levels of the nucleoid-binding protein mitochondrial transcription factor A (mtTFA). Furthermore, an increase in apoptosis is detected in larvae and adults overexpressing pol γ-α. We suggest that the pupal lethality and reduced life span of survivor adult flies are both caused mainly by massive apoptosis of muscle cells induced by mtDNA depletion.

Courtship song analysis of Drosophila muscle mutants

As part of the mating ritual, males of Drosophila species produce species-specific courtship songs through wing vibrations generated by the thoracic musculature. While previous studies have shown that indirect flight muscles (IFM) are neurally activated during courtship song production, the precise role of these muscles in song production has not been investigated. Fortunately, IFM mutants abound in Drosophila melanogaster and studies spanning several decades have shed light on the role of muscle proteins in IFM-powered flight. Analysis of courtship songs in these mutants offers the opportunity to uncover the role of the IFM in a behavior distinct than flight and subject to different evolutionary selection regimes. Here, we describe protocols for the recording and analysis of courtship behavior and mating song of D. melanogaster muscle transgenic and mutant strains. To record faint acoustic signal of courtship songs, an insulated mating compartment was used inside a recording device (INSECTAVOX) equipped with a modified electret microphone, a low-noise power supply, and noise filters. Songs recorded in the INSECTAVOX are digitized using Goldwave, whose several features enable extraction of critical song parameters, including carrier frequencies for pulse song and sine song. We demonstrate the utility of this approach by showing that deletion of the N-terminal region of the myosin regulatory light chain, a mutation known to decrease wing beat frequency and flight power, affects courtship song parameters.

Establishment and characterisation of salmon louse (Lepeophtheirus salmonis (Krøyer 1837)) laboratory strains

The salmon louse (Lepeophtheirus salmonis (Krøyer 1837)) is an ectoparasitic copepod which represents a major pathogen of wild and farmed salmonid fishes in the marine environment. In order to facilitate research on this ecologically and economically important parasite, a hatchery and culturing system permitting the closure of the life-cycle of L. salmonis in the laboratory was developed. Here, the hatchery system, breeding practices, and selected louse strains that have been maintained in culture in the period 2002-2009 are presented. The hatchery and culture protocol gave rise to predictable hatching of larvae and infections of host fish, permitting the cultivation of specific strains of L. salmonis for 22 generations. Both in- and out-bred lice and mutant colour strains have been established, and some of these strains were characterised by microsatellite DNA markers confirming their pedigree. No evidence of inbreeding depression, fitness or morphological changes was observed in any of the strains cultured. It is suggested that the culturing system, and the strains produced represent a significant resource for future research on this parasite.