FGF signaling promotes myoblast proliferation through activation of wingless signaling.

Indirect flight muscles (IFMs) are the largest muscles in Drosophila and are made up of hundreds of myonuclei. The generation of these giant muscles requires a large pool of wing disc associated adult muscle precursors (AMPs), however the factors that control proliferation to form this myoblast pool are incompletely known. Here, we examine the role of fibroblast growth factor (FGF) signaling in the proliferation of wing disc associated myoblasts. We find that the components of FGF signaling are expressed in myoblasts and surrounding epithelial cells of the wing disc. Next, we show that attenuation of FGF signaling results in a diminished myoblast pool. This reduction in the pool size is due to decreased myoblast proliferation. By contrast, activating the FGF signaling pathway increases the myoblast pool size and restores the proliferative capacity of FGF knockdown flies. Finally, our results demonstrate that the FGF receptor Heartless acts through up-regulating ß-catenin/Armadillo signaling to promote myoblast proliferation. Our studies identify a novel role for FGF signaling during IFM formation and uncover the mechanism through which FGF coordinates with Wingless signaling to promote myoblast proliferation.

The Drosophila Z-disc protein Z(210) is an adult muscle isoform of Zasp52, which is required for normal myofibril organization in indirect flight muscles.

The Z-disc is a critical anchoring point for thin filaments as they slide during muscle contraction. Therefore, identifying components of the Z-disc is critical for fully comprehending how myofibrils assemble and function. In the adult Drosophila musculature, the fibrillar indirect flight muscles accumulate a >200 kDa Z-disc protein termed Z(210), the identity of which has to date been unknown. Here, we use mass spectrometry and gene specific knockdown studies, to identify Z(210) as an adult isoform of the Z-disc protein Zasp52. The Zasp52 primary transcript is extensively alternatively spliced, and we describe its splicing pattern in the flight muscles, identifying a new Zasp52 isoform, which is the one recognized by the Z(210) antibody. We also demonstrate that Zasp52 is required for the association of α-actinin with the flight muscle Z-disc, and for normal sarcomere structure. These studies expand our knowledge of Zasp isoforms and their functions in muscle. Given the role of Zasp proteins in mammalian muscle development and disease, our results have relevance to mammalian muscle biology.

Muscle degeneration in aging Drosophila flies: the role of mechanical stress.

Muscle wasting is a universal hallmark of aging which is displayed by a wide range of organisms, although the causes and mechanisms of this phenomenon are not fully understood. We used Drosophila to characterize the phenomenon of spontaneous muscle fiber degeneration (SMFD) during aging. We found that SMFD occurs across diverse types of somatic muscles, progresses with chronological age, and positively correlates with functional muscle decline. Data from vital dyes and morphological markers imply that degenerative fibers most likely die by necrosis. Mechanistically, SMFD is driven by the damage resulting from muscle contractions, and the nervous system may play a significant role in this process. Our quantitative model of SMFD assessment can be useful in identifying and validating novel genetic factors that influence aging-related muscle wasting.

Quantitative model of aging-related muscle degeneration: a Drosophila study.

Changes in the composition and functionality of somatic muscles is a universal hallmark of aging that is displayed by a wide range of species. In humans, complications arising from muscle decline due to sarcopenia aggravate morbidity and mortality rates. The genetics of aging-related deterioration of muscle tissue is not well understood, which prompted us to characterize aging-related muscle degeneration in Drosophila melanogaster (fruit fly), a leading model organism in experimental genetics. Adult flies demonstrate spontaneous degeneration of muscle fibers in all types of somatic muscles, which correlates with functional, chronological, and populational aging. Morphological data imply that individual muscle fibers die by necrosis. Using quantitative analysis, we demonstrate that muscle degeneration in aging flies has a genetic component. Chronic neuronal overstimulation of muscles promotes fiber degeneration rates, suggesting a role for the nervous system in muscle aging. From the other hand, muscles decoupled from neuronal stimulation retain a basal level of spontaneous degeneration, suggesting the presence of intrinsic factors. Based on our characterization, Drosophila can be adopted for systematic screening and validation of genetic factors linked to aging-related muscle loss.

Phosphorylation of the Myogenic Factor Myocyte Enhancer Factor-2 Impacts Myogenesis In Vivo.

Activity of the myogenic regulatory protein myocyte enhancer factor-2 (MEF2) is modulated by post-translational modification. We investigated the in vivo phosphorylation of Drosophila MEF2, and identified serine 98 (S98) as a phosphorylated residue. Phospho-mimetic (S98E) and phospho-null (S98A) isoforms of MEF2 did not differ from wild-type in their activity in vitro, so we used CRISPR/Cas9 to generate an S98A allele of the endogenous gene. In mutant larvae we observed phenotypes characteristic of reduced MEF2 function, including reduced body wall muscle size and reduced expression of myofibrillar protein genes; conversely,S98A homozygotes showed enhanced MEF2 function through muscle differentiation within the adult myoblasts associated with the wing imaginal disc. In adults, S98A homozygotes were viable with normal mobility, yet showed patterning defects in muscles that were enhanced when the S98A allele was combined with a Mef2 null allele. Overall our data indicate that blocking MEF2 S98 phosphorylation in myoblasts enhances its myogenic capability, whereas blocking S98 phosphorylation in differentiating muscles attenuates MEF2 function. Our studies are among the first to assess the functional significance of MEF2 phosphorylation sites in the intact animal, and suggest that the same modification can have profoundly different effects upon MEF2 function depending upon the developmental context.

Myogenesis in Drosophila melanogaster: Dissection of Distinct Muscle Types for Molecular Analysis.

Drosophila is a useful model organism for studying the molecular signatures that define specific muscle types during myogenesis. It possesses significant genetic conservation with humans for muscle disease causing genes and a lack of redundancy that simplifies functional analysis. Traditional molecular methods can be utilized to understand muscle developmental processes such as Western blots, in situ hybridizations, RT-PCR and RNAseq, to name a few. However, one challenge for these molecular methods is the ability to dissect different muscle types. In this protocol we describe some useful techniques for extracting muscles from the pupal and adult stages of development using flight and jump muscles as an example.

Functional redundancy and nonredundancy between two Troponin C isoforms in Drosophila adult muscles.

We investigated the functional overlap of two muscle Troponin C (TpnC) genes that are expressed in the adult fruit fly, Drosophila melanogaster: TpnC4 is predominantly expressed in the indirect flight muscles (IFMs), whereas TpnC41C is the main isoform in the tergal depressor of the trochanter muscle (TDT; jump muscle). Using CRISPR/Cas9, we created a transgenic line with a homozygous deletion of TpnC41C and compared its phenotype to a line lacking functional TpnC4 We found that the removal of either of these genes leads to expression of the other isoform in both muscle types. The switching between isoforms occurs at the transcriptional level and involves minimal enhancers located upstream of the transcription start points of each gene. Functionally, the two TpnC isoforms were not equal. Although ectopic TpnC4 in TDT muscles was able to maintain jumping ability, TpnC41C in IFMs could not effectively support flying. Simultaneous functional disruption of both TpnC genes resulted in jump-defective and flightless phenotypes of the survivors, as well as abnormal sarcomere organization. These results indicated that TpnC is required for myofibril assembly, and that there is functional specialization among TpnC isoforms in Drosophila.

An undergraduate laboratory class using CRISPR/Cas9 technology to mutate drosophila genes.

CRISPR/Cas9 genome editing technology is used in the manipulation of genome sequences and gene expression. Because of the ease and rapidity with which genes can be mutated using CRISPR/Cas9, we sought to determine if a single-semester undergraduate class could be successfully taught, wherein students isolate mutants for specific genes using CRISPR/Cas9. Six students were each assigned a single Drosophila gene, for which no mutants currently exist. Each student designed and created plasmids to encode single guide RNAs that target their selected gene; injected the plasmids into Cas9-expressing embryos, in order to delete the selected gene; carried out a three-generation cross to test for germline transmission of a mutated allele and generate a stable stock of the mutant; and characterized the mutant alleles by PCR and sequencing. Three genes out of six were successfully mutated. Pre- and post- survey evaluations of the students in the class revealed that student attitudes towards their research competencies increased, although the changes were not statistically significant. We conclude that it is feasible to develop a laboratory genome editing class, to provide effective laboratory training to undergraduate students, and to generate mutant lines for use by the broader scientific community. © 2016 by The International Union of Biochemistry and Molecular Biology, 44:263-275, 2016.

Expression of the Troponin C at 41C Gene in Adult Drosophila Tubular Muscles Depends upon Both Positive and Negative Regulatory Inputs.

Most animals express multiple isoforms of structural muscle proteins to produce tissues with different physiological properties. In Drosophila, the adult muscles include tubular-type muscles and the fibrillar indirect flight muscles. Regulatory processes specifying tubular muscle fate remain incompletely understood, therefore we chose to analyze the transcriptional regulation of TpnC41C, a Troponin C gene expressed in the tubular jump muscles, but not in the fibrillar flight muscles. We identified a 300-bp promoter fragment of TpnC41C sufficient for the fiber-specific reporter expression. Through an analysis of this regulatory element, we identified two sites necessary for the activation of the enhancer. Mutations in each of these sites resulted in 70% reduction of enhancer activity. One site was characterized as a binding site for Myocyte Enhancer Factor-2. In addition, we identified a repressive element that prevents activation of the enhancer in other muscle fiber types. Mutation of this site increased jump muscle-specific expression of the reporter, but more importantly reporter expression expanded into the indirect flight muscles. Our findings demonstrate that expression of the TpnC41C gene in jump muscles requires integration of multiple positive and negative transcriptional inputs. Identification of the transcriptional regulators binding the cis-elements that we identified will reveal the regulatory pathways controlling muscle fiber differentiation.

Genetic Evidence for the Role of the Vacuole in Supplying Secretory Organelles with Ca2+ in Hansenula polymorpha.

Processes taking place in the secretory organelles require Ca2+ and Mn2+, which in yeast are supplied by the Pmr1 ion pump. Here we observed that in the yeast Hansenula polymorpha Ca2+ deficiency in the secretory pathway caused by Pmr1 inactivation is exacerbated by (i) the ret1-27 mutation affecting COPI-mediated vesicular transport, (ii) inactivation of the vacuolar Ca2+ ATPase Pmc1 and (iii) inactivation of Vps35, which is a component of the retromer complex responsible for protein transport between the vacuole and secretory organelles. The ret1-27 mutation also exerted phenotypes indicating alterations in transport between the vacuole and secretory organelles. These data indicate that ret1-27, pmc1 and vps35 affect a previously unknown Pmr1-independent route of the Ca2+ delivery to the secretory pathway. We also observed that the vacuolar protein carboxypeptidase Y receives additional modifications of its glycoside chains if it escapes the Vps10-dependent sorting to the vacuole.

Analysis of skeletal muscle development in Drosophila.

The Drosophila system has been invaluable in providing important insights into mesoderm specification, muscle specification, myoblast fusion, muscle differentiation, and myofibril assembly. Here, we present a series of Drosophila protocols that enable the researcher to visualize muscle precursors and differentiated muscles, at all stages of development. In doing so, we also highlight the variety of techniques that are used to create these findings. These protocols are directly used for the Drosophila system, and are provided with explanatory detail to enable the researcher to apply them to other systems.

Extradenticle and homothorax control adult muscle fiber identity in Drosophila.

Here we identify a key role for the homeodomain proteins Extradenticle (Exd) and Homothorax (Hth) in the specification of muscle fiber fate in Drosophila. exd and hth are expressed in the fibrillar indirect flight muscles but not in tubular jump muscles, and manipulating exd or hth expression converts one muscle type into the other. In the flight muscles, exd and hth are genetically upstream of another muscle identity gene, salm, and are direct transcriptional regulators of the signature flight muscle structural gene, Actin88F. Exd and Hth also impact muscle identity in other somatic muscles of the body by cooperating with Hox factors. Because mammalian orthologs of exd and hth also contribute to muscle gene regulation, our studies suggest that an evolutionarily conserved genetic pathway determines muscle fiber differentiation.

Cloning, characterization, and heat stress-induced redistribution of a protein homologous to human hsp27 in the zebrafish Danio rerio.

Hsp27 is a small heat shock protein (shsp) regulating stress tolerance and increasingly thought to play roles in tissue homeostasis and differentiation. The zebrafish Danio rerio is an important model for the study of developmental processes, but little is known regarding shsps in this animal. Here, we report the sequence, expression, regulation, and function of a zebrafish protein (zfHsp27) homologous to human Hsp27. zfHsp27 contains three conserved phosphorylatable serines and a cysteine important for regulation of apoptosis, but it lacks much of a C-terminal tail domain and shows low homology in two putative actin interacting domains that are features of mammalian Hsp27. zfHsp27 mRNA is most abundant in adult skeletal muscle and heart and is upregulated during early embryogenesis. zfHsp27 expressed in mammalian fibroblasts was phosphorylated in response to heat stress and anisomycin, and this phosphorylation was prevented by treatment with SB202190, an inhibitor of p38 MAPK. Expression of zfHsp27 and human Hsp27 in mammalian fibroblasts promoted a similar degree of tolerance to heat stress. zfHsp27 fusion proteins entered the nucleus and associated with the cytoskeleton of heat stressed cells in vitro and in zebrafish embryos. These results reveal conservation in regulation and function of mammalian and teleost Hsp27 proteins and define zebrafish as a new model for the study of Hsp27 function.

C-terminal truncation of alpha-COP affects functioning of secretory organelles and calcium homeostasis in Hansenula polymorpha.

In eukaryotic cells, COPI vesicles retrieve resident proteins to the endoplasmic reticulum and mediate intra-Golgi transport. Here, we studied the Hansenula polymorpha homologue of the Saccharomyces cerevisiae RET1 gene, encoding alpha-COP, a subunit of the COPI protein complex. H. polymorpha ret1 mutants, which expressed truncated alpha-COP lacking more than 300 C-terminal amino acids, manifested an enhanced ability to secrete human urokinase-type plasminogen activator (uPA) and an inability to grow with a shortage of Ca2+ ions, whereas a lack of alpha-COP expression was lethal. The alpha-COP defect also caused alteration of intracellular transport of the glycosylphosphatidylinositol-anchored protein Gas1p, secretion of abnormal uPA forms, and reductions in the levels of Pmr1p, a Golgi Ca2+-ATPase. Overexpression of Pmr1p suppressed some ret1 mutant phenotypes, namely, Ca2+ dependence and enhanced uPA secretion. The role of COPI-dependent vesicular transport in cellular Ca2+ homeostasis is discussed.