Differential requirements for Myocyte Enhancer Factor-2 during adult myogenesis in Drosophila

Single-nuclei multiome ATAC and RNA sequencing reveals the molecular basis of thermal plasticity in Drosophila melanogaster embryos

Embryogenesis is remarkably robust to temperature variability, yet there is limited understanding of the homeostatic mechanisms that offset thermal effects during early development. Here, we measured the thermal acclimation response of upper thermal limits and profiled chromatin state and the transcriptome of D. melanogaster embryos (Bownes Stage 11) using single-nuclei multiome ATAC and RNA sequencing. We report that thermal acclimation, while preserving a common set of primordial cell types, rapidly shifted the upper thermal limit. Cool-acclimated embryos showed a homeostatic response characterized by increased chromatin accessibility at transcription factor binding motifs for the transcriptional activator Zelda, along with enhanced activity of gene regulatory networks in the primordial cell types including the foregut and hindgut, mesoderm, and peripheral nervous system. In addition, cool-acclimated embryos had higher expression of genes encoding ribosomal proteins and enzymes involved in oxidative phosphorylation. Despite the hypothesis that differential heat tolerance might be explained by differential expression of molecular chaperones, we did not observe widespread differences in the chromatin accessibility or expression of heat shock genes. Overall, our results suggest that environmental robustness to temperature during embryogenesis necessitates homeostatic gene expression responses that regulate the speed of development, potentially imposing metabolic costs that constrain upper thermal limits.

Enhanced expression of the myogenic factor Myocyte enhancer factor-2 in imaginal disc myoblasts activates a partial, but incomplete, muscle development program

The Myocyte enhancer factor-2 (MEF2) transcription factor plays a vital role in orchestrating muscle differentiation. While MEF2 cannot effectively induce myogenesis in naïve cells, it can potently accelerate myogenesis in mesodermal cells. This includes in Drosophila melanogaster imaginal disc myoblasts, where triggering premature muscle gene expression in these adult muscle progenitors has become a paradigm for understanding the regulation of the myogenic program. Here, we investigated the global consequences of MEF2 overexpression in the imaginal wing disc myoblasts, by combining RNA-sequencing with RT-qPCR and immunofluorescence. We observed the formation of sarcomere-like structures that contained both muscle and cytoplasmic myosin, and significant upregulation of muscle gene expression, especially genes essential for myofibril formation and function. These transcripts were functional since numerous myofibrillar proteins were detected in discs using immunofluorescence. Interestingly, muscle genes whose expression is restricted to the adult stages were not activated in these adult myoblasts. These studies confirm a broad activation of the myogenic program in response to MEF2 expression and suggest that additional regulatory factors are required for promoting the adult muscle-specific program. Our findings contribute to understanding the regulatory mechanisms governing muscle development and highlight the multifaceted role of MEF2 in orchestrating this intricate process.

The Hox protein Antennapedia orchestrates Drosophila adult flight muscle development

Muscle development and diversity require a large number of spatially and temporally regulated events controlled by transcription factors (TFs). Drosophila has long stood as a model to study myogenesis due to the highly conserved key TFs involved at all stages of muscle development. While many studies focused on the diversification of Drosophila larval musculature, how distinct adult muscle types are generated is much less characterized. Here, we identify an essential regulator of Drosophila thoracic flight muscle development, the Hox TF Antennapedia (Antp). Correcting a long-standing belief that flight muscle development occurs without the input of Hox TFs, we show that Antp intervenes at several stages of flight muscle development, from the establishment of the progenitor pool in the embryo to myoblast differentiation in the early pupa. Furthermore, the precisely regulated clearance of Hox in the developing flight muscle fibers is required to allow for fibrillar muscle fate diversification, setting these muscles apart from all other adult tubular muscle types.

Apolipoprotein E Induces Lipid Accumulation Through Dgat2 That Is Prevented with Time-Restricted Feeding in Drosophila

Background: Apolipoprotein E (ApoE) is the leading genetic risk factor for late-onset Alzheimer's disease (AD), which is the leading cause of dementia worldwide. Most people have two ApoE-ε3 (ApoE3) alleles, while ApoE-ε2 (ApoE2) is protective from AD, and ApoE-ε4 (ApoE4) confers AD risk. How these alleles modulate AD risk is not clearly defined, and ApoE's role in lipid metabolism is also not fully known. Lipid droplets increase in AD. However, how ApoE contributes to lipid accumulation in the brain remains unknown. Methods: Here, we use Drosophila to study the effects of ApoE alleles on lipid accumulation in the brain and muscle in a cell-autonomous and non-cell-autonomous manner. Results: We report that pan-neuronal expression of each ApoE allele induces lipid accumulation specifically in the brain, but not in the muscle. However, this was not the case when expressed with muscle-specific drivers. ApoE2- and ApoE3-induced lipid accumulation is dependent on the expression of Dgat2, a key regulator of triacylglycerol production, while ApoE4 still induces lipid accumulation even with knock-down of Dgat2. Additionally, we find that implementation of time-restricted feeding (TRF), a dietary intervention in which food access only occurs in the active period (day), prevents ApoE-induced lipid accumulation in the brain of flies and modulates lipid metabolism genes. Conclusions: Altogether, our results demonstrate that ApoE induces lipid accumulation in the brain, that ApoE4 is unique in causing lipid accumulation independent of Dgat2, and that TRF prevents ApoE-induced lipid accumulation. These results support the idea that lipid metabolism is critical in AD, and that TRF could be a promising therapeutic approach to prevent ApoE-associated dysfunction in lipid metabolism.

Mechanoresponsive regulation of myogenesis by the force-sensing transcriptional regulator Tono

Muscle morphogenesis is a multi-step program, starting with myoblast fusion, followed by myotube-tendon attachment and sarcomere assembly, with subsequent sarcomere maturation, mitochondrial amplification, and specialization. The correct chronological order of these steps requires precise control of the transcriptional regulators and their effectors. How this regulation is achieved during muscle development is not well understood. In a genome-wide RNAi screen in Drosophila, we identified the BTB-zinc-finger protein Tono (CG32121) as a muscle-specific transcriptional regulator. tono mutant flight muscles display severe deficits in mitochondria and sarcomere maturation, resulting in uncontrolled contractile forces causing muscle rupture and degeneration during development. Tono protein is expressed during sarcomere maturation and localizes in distinct condensates in flight muscle nuclei. Interestingly, internal pressure exerted by the maturing sarcomeres deforms the muscle nuclei into elongated shapes and changes the Tono condensates, suggesting that Tono senses the mechanical status of the muscle cells. Indeed, external mechanical pressure on the muscles triggers rapid liquid-liquid phase separation of Tono utilizing its BTB domain. Thus, we propose that Tono senses high mechanical pressure to adapt muscle transcription, specifically at the sarcomere maturation stages. Consistently, tono mutant muscles display specific defects in a transcriptional switch that represses early muscle differentiation genes and boosts late ones. We hypothesize that a similar mechano-responsive regulation mechanism may control the activity of related BTB-zinc-finger proteins that, if mutated, can result in uncontrolled force production in human muscle.

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.

Drosophila neuronal Glucose-6-Phosphatase is a modulator of neuropeptide release that regulates muscle glycogen stores via FMRFamide signaling

Neuropeptides (NPs) and their cognate receptors are critical effectors of diverse physiological processes and behaviors. We recently reported of a noncanonical function of the Drosophila Glucose-6-Phosphatase (G6P) gene in a subset of neurosecretory cells in the central nervous system that governs systemic glucose homeostasis in food-deprived flies. Here, we show that G6P-expressing neurons define six groups of NP-secreting cells, four in the brain and two in the thoracic ganglion. Using the glucose homeostasis phenotype as a screening tool, we find that neurons located in the thoracic ganglion expressing FMRFamide NPs (FMRFaG6P neurons) are necessary and sufficient to maintain systemic glucose homeostasis in starved flies. We further show that G6P is essential in FMRFaG6P neurons for attaining a prominent Golgi apparatus and secreting NPs efficiently. Finally, we establish that G6P-dependent FMRFa signaling is essential for the build-up of glycogen stores in the jump muscle which expresses the receptor for FMRFamides. We propose a general model in which the main role of G6P is to counteract glycolysis in peptidergic neurons for the purpose of optimizing the intracellular environment best suited for the expansion of the Golgi apparatus, boosting release of NPs and enhancing signaling to respective target tissues expressing cognate receptors.

Aging-affiliated post-translational modifications of skeletal muscle myosin affect biochemical properties, myofibril structure, muscle function, and proteostasis

The molecular motor myosin is post-translationally modified in its globular head, its S2 hinge, and its thick filament domain during human skeletal muscle aging. To determine the importance of such modifications, we performed an integrative analysis of transgenic Drosophila melanogaster expressing myosin containing post-translational modification mimic mutations. We determined effects on muscle function, myofibril structure, and myosin biochemistry. Modifications in the homozygous state decreased jump muscle function by a third at 3 weeks of age and reduced indirect flight muscle function to negligible levels in young flies, with severe effects on flight muscle myofibril assembly and/or maintenance. Expression of mimic mutations in the heterozygous state or in a wild-type background yielded significant, but less severe, age-dependent effects upon flight muscle structure and function. Modification of the residue in the globular head disabled ATPase activity and in vitro actin filament motility, whereas the S2 hinge mutation reduced actin-activated ATPase activity by 30%. The rod modification diminished filament formation in vitro. The latter mutation also reduced proteostasis, as demonstrated by enhanced accumulation of polyubiquitinated proteins. Overall, we find that mutation of amino acids at sites that are chemically modified during human skeletal muscle aging can disrupt myosin ATPase, myosin filament formation, and/or proteostasis, providing a mechanistic basis for the observed muscle defects. We conclude that age-specific post-translational modifications present in human skeletal muscle are likely to act in a dominant fashion to affect muscle structure and function and may therefore be implicated in degeneration and dysfunction associated with sarcopenia.

Drosophila Neuronal Glucose 6 Phosphatase is a Modulator of Neuropeptide Release that Regulates Muscle Glycogen Stores via FMRFamide Signaling

Neuropeptides (NPs) and their cognate receptors are critical effectors of diverse physiological processes and behaviors. We recently reported of a non-canonical function of the Drosophila Glucose-6-Phosphatase ( G6P ) gene in a subset of neurosecretory cells in the CNS that governs systemic glucose homeostasis in food deprived flies. Here, we show that G6P expressing neurons define 6 groups of neuropeptide secreting cells, 4 in the brain and 2 in the thoracic ganglion. Using the glucose homeostasis phenotype as a screening tool, we find that neurons located in the thoracic ganglion expressing FMRFamide neuropeptides ( FMRFa G6P neurons) are necessary and sufficient to maintain systemic glucose homeostasis in starved flies. We further show that G6P is essential in FMRFa G6P neurons for attaining a prominent Golgi apparatus and secreting neuropeptides efficiently. Finally, we establish that G6P dependent FMRFa signaling is essential for the build-up of glycogen stores in the jump muscle which expresses the receptor for FMRFamides. We propose a general model in which the main role of G6P is to counteract glycolysis in peptidergic neurons for the purpose of optimizing the intracellular environment best suited for the expansion of the Golgi apparatus, boosting release of neuropeptides and enhancing signaling to respective target tissues expressing cognate receptors.

SIGNIFICANCE STATEMENT

Glucose-6-phosphtase (G6P) is a critical enzyme in sugar synthesis and catalyzes the final step in glucose production. In Drosophila - and insects in general - where trehalose is the circulating sugar and Trehalose phosphate synthase, and not G6P, is used for sugar production, G6P has adopted a novel and unique role in peptidergic neurons in the CNS. Interestingly, flies lacking G6P show diminished Neuropeptide secretions and have a smaller Golgi apparatus in peptidergic neurons. It is hypothesized that the role of G6P is to counteract glycolysis, thereby creating a cellular environment that is more amenable to efficient neuropeptide secretion.

Bruno 1/CELF regulates splicing and cytoskeleton dynamics to ensure correct sarcomere assembly in Drosophila flight muscles

Muscles undergo developmental transitions in gene expression and alternative splicing that are necessary to refine sarcomere structure and contractility. CUG-BP and ETR-3-like (CELF) family RNA-binding proteins are important regulators of RNA processing during myogenesis that are misregulated in diseases such as Myotonic Dystrophy Type I (DM1). Here, we report a conserved function for Bruno 1 (Bru1, Arrest), a CELF1/2 family homolog in Drosophila, during early muscle myogenesis. Loss of Bru1 in flight muscles results in disorganization of the actin cytoskeleton leading to aberrant myofiber compaction and defects in pre-myofibril formation. Temporally restricted rescue and RNAi knockdown demonstrate that early cytoskeletal defects interfere with subsequent steps in sarcomere growth and maturation. Early defects are distinct from a later requirement for bru1 to regulate sarcomere assembly dynamics during myofiber maturation. We identify an imbalance in growth in sarcomere length and width during later stages of development as the mechanism driving abnormal radial growth, myofibril fusion, and the formation of hollow myofibrils in bru1 mutant muscle. Molecularly, we characterize a genome-wide transition from immature to mature sarcomere gene isoform expression in flight muscle development that is blocked in bru1 mutants. We further demonstrate that temporally restricted Bru1 rescue can partially alleviate hypercontraction in late pupal and adult stages, but it cannot restore myofiber function or correct structural deficits. Our results reveal the conserved nature of CELF function in regulating cytoskeletal dynamics in muscle development and demonstrate that defective RNA processing due to misexpression of CELF proteins causes wide-reaching structural defects and progressive malfunction of affected muscles that cannot be rescued by late-stage gene replacement.

Twist is required for muscle development of the adult legs in Henosepilachna vigintioctopunctata

Although muscle development has been widely studied in Drosophila melanogaster, it was a great challenge to apply to developmental processes of other insect muscles. This study was focused on the functional characterization of a basic helix-loop-helix transcription factor gene twist in an herbivorous ladybird Henosepilachna vigintioctopunctata. Its transcript (Hvtwist) levels were detected in all developmental stages. RNA interference (RNAi)-aided knockdown of Hvtwist at the penultimate larval instar stage impaired pupation, and caused a deformed adult in the legs. The tarsi were malformed and did not support the bodies in an upright position. The climbing ability was impaired. Moreover, around 50% of the impaired adults had a malformed elytrum. In addition, they consumed less foliage and did not lay eggs. A hematoxylin-eosin staining of the leg demonstrated that the tibial extensor (TE) and the tibial flexor (TF) muscles were originated from the femurs while levator and depressor muscles of the tarsus (TL and TD) were located in the tibia in the control adults, in which tarsal segments were devoid of muscles. RNAi treatment specific to Hvtwist expression markedly impaired TE and TF muscles in the femurs, and prevented the development of TL and TD muscles in the tibia. Therefore, our findings demonstrate Twist plays a vital role in the myogenesis in H. vigintioctopunctata adult legs.

Polyglutamine disease in peripheral tissues

This year is a milestone anniversary of the discovery that Huntington's disease is caused by the presence of expanded polyglutamine repeats in the huntingtin gene leading to the formation of huntingtin aggregates. 30 years have elapsed and there is still no cure and the only FDA-approved treatment to alleviate the debilitating locomotor impairments presents several adverse effects. It has long been neglected that the huntingtin gene is almost ubiquitously expressed in many tissues outside of the nervous system. Growing evidence indicates that these peripheral tissues can contribute to the symptoms of the disease. New findings in Drosophila have shown that the selective expression of mutant huntingtin in muscle or fat is sufficient to cause detrimental effects in the absence of any neurodegeneration. In addition, it was discovered that a completely different tissue distribution of Htt aggregates in Drosophila muscles is responsible for a drastic aggravation of the detrimental effects. This review examines the peripheral tissues that express huntingtin with an added focus on the nature and distribution of the aggregates, if any.

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.

Blood meal digestion and changes in lipid reserves are associated with the post-ecdysis development of the flight muscle and ovary in young adults of Rhodnius prolixus

Rhodnius prolixus is a hemimetabolous, hematophagous insect, and both nymphs and adults feed exclusively on blood. The blood feeding triggers the molting process and, after five nymphal instar stages, the insect reaches the winged adult form. After the final ecdysis, the young adult still has a lot of blood in the midgut and, thus, we have investigated the changes in protein and lipid contents that are observed in the insect organs as digestion continues after molting. Total midgut protein content decreased during the days after the ecdysis, and digestion was finished fifteen days later. Simultaneously, proteins and triacylglycerols present in the fat body were mobilized, and their contents decreased, whereas they increased in both the ovary and the flight muscle. In order to evaluate the activity of de novo lipogenesis of each organ, the fat body, ovary and flight muscle were incubated in the presence of radiolabeled acetate, and the fat body showed the highest efficiency rate (around 47%) to convert the taken up acetate into lipids. The levels of de novo lipid synthesis in the flight muscle and ovary were very low. When ³H-palmitate was injected into the young females, its incorporation by the flight muscle was higher than by the ovary or the fat body. In the flight muscle, the ³H-palmitate was similarly distributed amongst triacylglycerols, phospholipids, diacylglycerols and free fatty acids, while in the ovary and fat body it was mostly found in triacylglycerols and phospholipids. The flight muscle was not fully developed after the molt, and at day two no lipid droplets were observed. At day five, very small lipid droplets were present, and they increased in size up to day fifteen. The diameter of the muscle fibers also increased from day two to fifteen, as well as the internuclear distance, indicating that muscle hypertrophy occurred along these days. The lipid droplets from the fat body showed a different pattern, and their diameter decreased after day two, but started to increase again at day ten. The data presented herein describes the development of the flight muscle after the final ecdysis, and modifications that occur regarding lipid stores. We show that, after molting, substrates that are present in the midgut and fat body are mobilized and directed to the ovary and flight muscle, for the adults of R. prolixus to be ready to feed and reproduce.

Gene Structure, Expression and Function Analysis of MEF2 in the Pacific White Shrimp Litopenaeus vannamei

The Pacific white shrimp Litopenaeus vannamei is the most economically important crustacean in the world. The growth and development of shrimp muscle has always been the focus of attention. Myocyte Enhancer Factor 2 (MEF2), a member of MADS transcription factor, has an essential influence on various growth and development programs, including myogenesis. In this study, based on the genome and transcriptome data of L. vannamei, the gene structure and expression profiles of MEF2 were characterized. We found that the LvMEF2 was widely expressed in various tissues, mainly in the Oka organ, brain, intestine, heart, and muscle. Moreover, LvMEF2 has a large number of splice variants, and the main forms are the mutually exclusive exon and alternative 5′ splice site. The expression profiles of the LvMEF2 splice variants varied under different conditions. Interestingly, some splice variants have tissue or developmental expression specificity. After RNA interference into LvMEF2, the increment in the body length and weight decreased significantly and even caused death, suggesting that LvMEF2 can affect the growth and survival of L. vannamei. Transcriptome analysis showed that after LvMEF2 was knocked down, the protein synthesis and immune-related pathways were affected, and the associated muscle protein synthesis decreased, indicating that LvMEF2 affected muscle formation and the immune system. The results provide an important basis for future studies of the MEF2 gene and the mechanism of muscle growth and development in shrimp.

Reversine ameliorates hallmarks of cellular senescence in human skeletal myoblasts via reactivation of autophagy

Cellular senescence leads to the depletion of myogenic progenitors and decreased regenerative capacity. We show that the small molecule 2,6-disubstituted purine, reversine, can improve some well-known hallmarks of cellular aging in senescent myoblast cells. Reversine reactivated autophagy and insulin signaling pathway via upregulation of Adenosine Monophosphate-activated protein kinase (AMPK) and Akt2, restoring insulin sensitivity and glucose uptake in senescent cells. Reversine also restored the loss of connectivity of glycolysis to the TCA cycle, thus restoring dysfunctional mitochondria and the impaired myogenic differentiation potential of senescent myoblasts. Altogether, our data suggest that cellular senescence can be reversed by treatment with a single small molecule without employing genetic reprogramming technologies.

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.

Time-restricted feeding promotes muscle function through purine cycle and AMPK signaling in Drosophila obesity models

Obesity caused by genetic and environmental factors can lead to compromised skeletal muscle function. Time-restricted feeding (TRF) has been shown to prevent muscle function decline from obesogenic challenges; however, its mechanism remains unclear. Here we demonstrate that TRF upregulates genes involved in glycine production (Sardh and CG5955) and utilization (Gnmt), while Dgat2, involved in triglyceride synthesis is downregulated in Drosophila models of diet- and genetic-induced obesity. Muscle-specific knockdown of Gnmt, Sardh, and CG5955 lead to muscle dysfunction, ectopic lipid accumulation, and loss of TRF-mediated benefits, while knockdown of Dgat2 retains muscle function during aging and reduces ectopic lipid accumulation. Further analyses demonstrate that TRF upregulates the purine cycle in a diet-induced obesity model and AMPK signaling-associated pathways in a genetic-induced obesity model. Overall, our data suggest that TRF improves muscle function through modulations of common and distinct pathways under different obesogenic challenges and provides potential targets for obesity treatments.

Differentiation and Maturation of Muscle and Fat Cells in Cultivated Seafood: Lessons from Developmental Biology

Cultivated meat, also known as cultured or cell-based meat, is meat produced directly from cultured animal cells rather than from a whole animal. Cultivated meat and seafood have been proposed as a means of mitigating the substantial harms associated with current production methods, including damage to the environment, antibiotic resistance, food security challenges, poor animal welfare, and-in the case of seafood-overfishing and ecological damage associated with fishing and aquaculture. Because biomedical tissue engineering research, from which cultivated meat draws a great deal of inspiration, has thus far been conducted almost exclusively in mammals, cultivated seafood suffers from a lack of established protocols for producing complex tissues in vitro. At the same time, fish such as the zebrafish Danio rerio have been widely used as model organisms in developmental biology. Therefore, many of the mechanisms and signaling pathways involved in the formation of muscle, fat, and other relevant tissue are relatively well understood for this species. The same processes are understood to a lesser degree in aquatic invertebrates. This review discusses the differentiation and maturation of meat-relevant cell types in aquatic species and makes recommendations for future research aimed at recapitulating these processes to produce cultivated fish and shellfish.

Regulation of the evolutionarily conserved muscle myofibrillar matrix by cell type dependent and independent mechanisms

Skeletal muscles play a central role in human movement through forces transmitted by contraction of the sarcomere. We recently showed that mammalian sarcomeres are connected through frequent branches forming a singular, mesh-like myofibrillar matrix. However, the extent to which myofibrillar connectivity is evolutionarily conserved as well as mechanisms which regulate the specific architecture of sarcomere branching remain unclear. Here, we demonstrate the presence of a myofibrillar matrix in the tubular, but not indirect flight (IF) muscles within Drosophila melanogaster. Moreover, we find that loss of transcription factor H15 increases sarcomere branching frequency in the tubular jump muscles, and we show that sarcomere branching can be turned on in IF muscles by salm-mediated conversion to tubular muscles. Finally, we demonstrate that neurochondrin misexpression results in myofibrillar connectivity in IF muscles without conversion to tubular muscles. These data indicate an evolutionarily conserved myofibrillar matrix regulated by both cell-type dependent and independent mechanisms.

Rbfox1 is required for myofibril development and maintaining fiber type-specific isoform expression in Drosophila muscles

Protein isoform transitions confer muscle fibers with distinct properties and are regulated by differential transcription and alternative splicing. RNA-binding Fox protein 1 (Rbfox1) can affect both transcript levels and splicing, and is known to contribute to normal muscle development and physiology in vertebrates, although the detailed mechanisms remain obscure. In this study, we report that Rbfox1 contributes to the generation of adult muscle diversity in Drosophila Rbfox1 is differentially expressed among muscle fiber types, and RNAi knockdown causes a hypercontraction phenotype that leads to behavioral and eclosion defects. Misregulation of fiber type-specific gene and splice isoform expression, notably loss of an indirect flight muscle-specific isoform of Troponin-I that is critical for regulating myosin activity, leads to structural defects. We further show that Rbfox1 directly binds the 3'-UTR of target transcripts, regulates the expression level of myogenic transcription factors myocyte enhancer factor 2 and Salm, and both modulates expression of and genetically interacts with the CELF family RNA-binding protein Bruno1 (Bru1). Rbfox1 and Bru1 co-regulate fiber type-specific alternative splicing of structural genes, indicating that regulatory interactions between FOX and CELF family RNA-binding proteins are conserved in fly muscle. Rbfox1 thus affects muscle development by regulating fiber type-specific splicing and expression dynamics of identity genes and structural proteins.

Muscle Regeneration of the Tongue: A review of current clinical and regenerative research strategies

Various abnormalities of the tongue, including cancers, commonly require surgical removal to sequester growth and metastasis. However, even minor resections can affect functional outcomes such as speech and swallowing, thereby reducing quality of life. Surgical resections alone create volumetric muscle loss whereby muscle tissue cannot self-regenerate within the tongue. In these cases, the tongue is reconstructed typically in the form of autologous skin flaps. However, flap reconstruction has many limitations and unfortunately is the primary option for oral and reconstructive surgeons to treat tongue defects. The alternative, but yet undeveloped strategy for tongue reconstruction is regenerative medicine, which widely focuses on building new organs with stem cells. Regenerative medicine has successfully treated many tissues, but research has inadequately addressed the tongue as a vital organ in need of tissue engineering. In this review, we address the current standard for tongue reconstruction, the cellular mechanisms of muscle cell development, and the stem cell studies that have attempted muscle engineering within the tongue. Until now, no review has focused on engineering the tongue with regenerative medicine, which could guide innovative strategies for tongue reconstruction.

In Silico and In Vivo Analysis of Amino Acid Substitutions That Cause Laminopathies

Mutations in the LMNA gene cause diseases called laminopathies. LMNA encodes lamins A and C, intermediate filaments with multiple roles at the nuclear envelope. LMNA mutations are frequently single base changes that cause diverse disease phenotypes affecting muscles, nerves, and fat. Disease-associated amino acid substitutions were mapped in silico onto three-dimensional structures of lamin A/C, revealing no apparent genotype–phenotype connections. In silico analyses revealed that seven of nine predicted partner protein binding pockets in the Ig-like fold domain correspond to sites of disease-associated amino acid substitutions. Different amino acid substitutions at the same position within lamin A/C cause distinct diseases, raising the question of whether the nature of the amino acid replacement or genetic background differences contribute to disease phenotypes. Substitutions at R249 in the rod domain cause muscular dystrophies with varying severity. To address this variability, we modeled R249Q and R249W in Drosophila Lamin C, an orthologue of LMNA. Larval body wall muscles expressing mutant Lamin C caused abnormal nuclear morphology and premature death. When expressed in indirect flight muscles, R249W caused a greater number of adults with wing posturing defects than R249Q, consistent with observations that R249W and R249Q cause distinct muscular dystrophies, with R249W more severe. In this case, the nature of the amino acid replacement appears to dictate muscle disease severity. Together, our findings illustrate the utility of Drosophila for predicting muscle disease severity and pathogenicity of variants of unknown significance.

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.

A nutrient-responsive hormonal circuit mediates an inter-tissue program regulating metabolic homeostasis in adult Drosophila

Animals maintain metabolic homeostasis by modulating the activity of specialized organs that adjust internal metabolism to external conditions. However, the hormonal signals coordinating these functions are incompletely characterized. Here we show that six neurosecretory cells in the Drosophila central nervous system respond to circulating nutrient levels by releasing Capa hormones, homologs of mammalian neuromedin U, which activate the Capa receptor (CapaR) in peripheral tissues to control energy homeostasis. Loss of Capa/CapaR signaling causes intestinal hypomotility and impaired nutrient absorption, which gradually deplete internal nutrient stores and reduce organismal lifespan. Conversely, increased Capa/CapaR activity increases fluid and waste excretion. Furthermore, Capa/CapaR inhibits the release of glucagon-like adipokinetic hormone from the corpora cardiaca, which restricts energy mobilization from adipose tissue to avoid harmful hyperglycemia. Our results suggest that the Capa/CapaR circuit occupies a central node in a homeostatic program that facilitates the digestion and absorption of nutrients and regulates systemic energy balance.

Development of the indirect flight muscles of Aedes aegypti, a main arbovirus vector

Background

Flying is an essential function for mosquitoes, required for mating and, in the case of females, to get a blood meal and consequently function as a vector. Flight depends on the action of the indirect flight muscles (IFMs), which power the wings beat. No description of the development of IFMs in mosquitoes, including Aedes aegypti, is available.

Methods

A. aegypti thoraces of larvae 3 and larvae 4 (L3 and L4) instars were analyzed using histochemistry and bright field microscopy. IFM primordia from L3 and L4 and IFMs from pupal and adult stages were dissected and processed to detect F-actin labelling with phalloidin-rhodamine or TRITC, or to immunodetection of myosin and tubulin using specific antibodies, these samples were analyzed by confocal microscopy. Other samples were studied using transmission electron microscopy.

Results

At L3–L4, IFM primordia for dorsal-longitudinal muscles (DLM) and dorsal–ventral muscles (DVM) were identified in the expected locations in the thoracic region: three primordia per hemithorax corresponding to DLM with anterior to posterior orientation were present. Other three primordia per hemithorax, corresponding to DVM, had lateral position and dorsal to ventral orientation. During L3 to L4 myoblast fusion led to syncytial myotubes formation, followed by myotendon junctions (MTJ) creation, myofibrils assembly and sarcomere maturation. The formation of Z-discs and M-line during sarcomere maturation was observed in pupal stage and, the structure reached in teneral insects a classical myosin thick, and actin thin filaments arranged in a hexagonal lattice structure.

Conclusions

A general description of A. aegypti IFM development is presented, from the myoblast fusion at L3 to form myotubes, to sarcomere maturation at adult stage. Several differences during IFM development were observed between A. aegypti (Nematoceran) and Drosophila melanogaster (Brachyceran) and, similitudes with Chironomus sp. were observed as this insect is a Nematoceran, which is taxonomically closer to A. aegypti and share the same number of larval stages.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12861-021-00242-8.

The steroid-hormone ecdysone coordinates parallel pupariation neuromotor and morphogenetic subprograms via epidermis-to-neuron Dilp8-Lgr3 signal induction

Innate behaviors consist of a succession of genetically-hardwired motor and physiological subprograms that can be coupled to drastic morphogenetic changes. How these integrative responses are orchestrated is not completely understood. Here, we provide insight into these mechanisms by studying pupariation, a multi-step innate behavior of Drosophila larvae that is critical for survival during metamorphosis. We find that the steroid-hormone ecdysone triggers parallel pupariation neuromotor and morphogenetic subprograms, which include the induction of the relaxin-peptide hormone, Dilp8, in the epidermis. Dilp8 acts on six Lgr3-positive thoracic interneurons to couple both subprograms in time and to instruct neuromotor subprogram switching during behavior. Our work reveals that interorgan feedback gates progression between subunits of an innate behavior and points to an ancestral neuromodulatory function of relaxin signaling.

Myofibril and mitochondria morphogenesis are coordinated by a mechanical feedback mechanism in muscle

Complex animals build specialised muscles to match specific biomechanical and energetic needs. Hence, composition and architecture of sarcomeres and mitochondria are muscle type specific. However, mechanisms coordinating mitochondria with sarcomere morphogenesis are elusive. Here we use Drosophila muscles to demonstrate that myofibril and mitochondria morphogenesis are intimately linked. In flight muscles, the muscle selector spalt instructs mitochondria to intercalate between myofibrils, which in turn mechanically constrain mitochondria into elongated shapes. Conversely in cross-striated leg muscles, mitochondria networks surround myofibril bundles, contacting myofibrils only with thin extensions. To investigate the mechanism causing these differences, we manipulated mitochondrial dynamics and found that increased mitochondrial fusion during myofibril assembly prevents mitochondrial intercalation in flight muscles. Strikingly, this causes the expression of cross-striated muscle specific sarcomeric proteins. Consequently, flight muscle myofibrils convert towards a partially cross-striated architecture. Together, these data suggest a biomechanical feedback mechanism downstream of spalt synchronizing mitochondria with myofibril morphogenesis.

The Hippo pathway controls myofibril assembly and muscle fiber growth by regulating sarcomeric gene expression

Skeletal muscles are composed of gigantic cells called muscle fibers, packed with force-producing myofibrils. During development, the size of individual muscle fibers must dramatically enlarge to match with skeletal growth. How muscle growth is coordinated with growth of the contractile apparatus is not understood. Here, we use the large Drosophila flight muscles to mechanistically decipher how muscle fiber growth is controlled. We find that regulated activity of core members of the Hippo pathway is required to support flight muscle growth. Interestingly, we identify Dlg5 and Slmap as regulators of the STRIPAK phosphatase, which negatively regulates Hippo to enable post-mitotic muscle growth. Mechanistically, we show that the Hippo pathway controls timing and levels of sarcomeric gene expression during development and thus regulates the key components that physically mediate muscle growth. Since Dlg5, STRIPAK and the Hippo pathway are conserved a similar mechanism may contribute to muscle or cardiomyocyte growth in humans.

The Drosophila CG1674 gene encodes a synaptopodin 2-like related protein that localizes to the Z-disc and is required for normal flight muscle development and function

BACKGROUND

To identify novel myofibrillar components of the Drosophila flight muscles, we carried out a proteomic analysis of chemically demembranated flight muscle myofibrils, and characterized the knockdown phenotype of a novel gene identified in the screen, CG1674.

RESULTS

The CG1674 protein has some similarity to vertebrate synaptopodin 2-like, and when expressed as a FLAG-tagged fusion protein, it was localized during development to the Z-disc and cytoplasm. Knockdown of CG1674 expression affected the function of multiple muscle types, and defective flight in adults was accompanied by large actin-rich structures in the flight muscles that resembled overgrown Z-discs. Localization of CG1674 to the Z-disc depended predominantly upon presence of the Z-disc component alpha-actinin, but also depended upon other Z-disc components, including Mask, Zasp52, and Sals. We also observed re-localization of FLAG-CG1674 to the nucleus in Alpha-actinin and sals knockdown animals.

CONCLUSIONS

These studies identify and characterize a previously unreported myofibrillar component of Drosophila muscle that is necessary for proper myofibril assembly during development.

A Novel Mechanism for Activation of Myosin Regulatory Light Chain by Protein Kinase C-Delta in Drosophila

Myosin is an essential motor protein, which in muscle is comprised of two molecules each of myosin heavy-chain (MHC), the essential or alkali myosin light-chain 1 (MLC1), and the regulatory myosin light-chain 2 (MLC2). It has been shown previously that MLC2 phosphorylation at two canonical serine residues is essential for proper flight muscle function in Drosophila; however, MLC2 is also phosphorylated at additional residues for which the mechanism and functional significance is not known. We found that a hypomorphic allele of Pkcδ causes a flightless phenotype; therefore, we hypothesized that PKCδ phosphorylates MLC2. We rescued flight disability by duplication of the wild-type Pkcδ gene. Moreover, MLC2 is hypophosphorylated in Pkcδ mutant flies, but it is phosphorylated in rescued animals. Myosin isolated from Pkcδ mutant flies shows a reduced actin-activated ATPase activity, and MLC2 in these myosin preparations can be phosphorylated directly by recombinant human PKCδ. The flightless phenotype is characterized by a shortened and disorganized sarcomere phenotype that becomes apparent following eclosion. We conclude that MLC2 is a direct target of phosphorylation by PKCδ, and that this modification is necessary for flight muscle maturation and function.

Diet-MEF2 interactions shape lipid droplet diversification in muscle to influence Drosophila lifespan

The number, size, and composition of lipid droplets can be influenced by dietary changes that shift energy substrate availability. This diversification of lipid droplets can promote metabolic flexibility and shape cellular stress responses in unique tissues with distinctive metabolic roles. Using Drosophila, we uncovered a role for myocyte enhancer factor 2 (MEF2) in modulating diet-dependent lipid droplet diversification within adult striated muscle, impacting mortality rates. Muscle-specific attenuation of MEF2, whose chronic activation maintains glucose and mitochondrial homeostasis, leads to the accumulation of large, cholesterol ester-enriched intramuscular lipid droplets in response to high calorie, carbohydrate-sufficient diets. The diet-dependent accumulation of these lipid droplets also correlates with both enhanced stress protection in muscle and increases in organismal lifespan. Furthermore, MEF2 attenuation releases an antagonistic regulation of cell cycle gene expression programs, and up-regulation of Cyclin E is required for diet- and MEF2-dependent diversification of intramuscular lipid droplets. The integration of MEF2-regulated gene expression networks with dietary responses thus plays a critical role in shaping muscle metabolism and function, further influencing organismal lifespan. Together, these results highlight a potential protective role for intramuscular lipid droplets during dietary adaptation.

Rbf Activates the Myogenic Transcriptional Program to Promote Skeletal Muscle Differentiation

The importance of the retinoblastoma tumor suppressor protein pRB in cell cycle control is well established. However, less is known about its role in differentiation during animal development. Here, we investigated the role of Rbf, the Drosophila pRB homolog, in adult skeletal muscles. We found that the depletion of Rbf severely reduced muscle growth and altered myofibrillogenesis but only minimally affected myoblast proliferation. We identified an Rbf-dependent transcriptional program in late muscle development that is distinct from the canonical role of Rbf in cell cycle control. Unexpectedly, Rbf acts as a transcriptional activator of the myogenic and metabolic genes in the growing muscles. The genomic regions bound by Rbf contained the binding sites of several factors that genetically interacted with Rbf by modulating Rbf-dependent phenotype. Thus, our results reveal a distinctive role for Rbf as a direct activator of the myogenic transcriptional program that drives late muscle differentiation.

Inactivation of the tumor suppressor RB, an obligatory step in most cancers, results in unrestrained cell cycle progression. Zappia et al. show that Rbf, the RB Drosophila ortholog, directly activates the metabolic program that accompanies muscle development. This work expands the understanding of the plethora of Rbf functions.

Bimolecular Fluorescence Complementation (BiFC) for Studying Sarcomeric Protein Interactions in Drosophila

Protein-protein interactions in Drosophila myofibrils are essential for their function and formation. Bimolecular Fluorescence Complementation (BiFC) is an effective method for studying protein interactions and localization. BiFC relies on the reconstitution of a monomeric fluorescent protein from two half-fragments when in proximity. Two proteins tagged with the different half-fragments emit a fluorescent signal when they are in physical contact, thus revealing a protein interaction and its spatial distribution. Because myofibrils are large networks of interconnected proteins, BIFC is an ideal method to study protein-protein interactions in myofibrils. Here we present a protocol for generating transgenic flies compatible with BiFC and a method for analyzing protein-protein interactions based on the fluorescent BiFC signal in myofibrils. Our protocol is applicable to the majority of Drosophila proteins and with few modifications may be used to study any tissue.

Myofibril diameter is set by a finely tuned mechanism of protein oligomerization in Drosophila

Myofibrils are huge cytoskeletal assemblies embedded in the cytosol of muscle cells. They consist of arrays of sarcomeres, the smallest contractile unit of muscles. Within a muscle type, myofibril diameter is highly invariant and contributes to its physiological properties, yet little is known about the underlying mechanisms setting myofibril diameter. Here we show that the PDZ and LIM domain protein Zasp, a structural component of Z-discs, mediates Z-disc and thereby myofibril growth through protein oligomerization. Oligomerization is induced by an interaction of its ZM domain with LIM domains. Oligomerization is terminated upon upregulation of shorter Zasp isoforms which lack LIM domains at later developmental stages. The balance between these two isoforms, which we call growing and blocking isoforms sets the stereotyped diameter of myofibrils. If blocking isoforms dominate, myofibrils become smaller. If growing isoforms dominate, myofibrils and Z-discs enlarge, eventually resulting in large pathological aggregates that disrupt muscle function.

Regulation of fiber-specific actin expression by the Drosophila SRF ortholog Blistered

Serum response factor (SRF) has an established role in controlling actin homeostasis in mammalian cells, yet its role in non-vertebrate muscle development has remained enigmatic. Here, we demonstrate that the single Drosophila SRF ortholog, termed Blistered (Bs), is expressed in all adult muscles, but Bs is required for muscle organization only in the adult indirect flight muscles. Bs is a direct activator of the flight muscle actin gene Act88F, via a conserved promoter-proximal binding site. However, Bs only activates Act88F expression in the context of the flight muscle regulatory program provided by the Pbx and Meis orthologs Extradenticle and Homothorax, and appears to function in a similar manner to mammalian SRF in muscle maturation. These studies place Bs in a regulatory framework where it functions to sustain the flight muscle phenotype in Drosophila Our studies uncover an evolutionarily ancient role for SRF in regulating muscle actin expression, and provide a model for how SRF might function to sustain muscle fate downstream of pioneer factors.

A small proportion of Talin molecules transmit forces at developing muscle attachments in vivo

Cells in developing organisms are subjected to particular mechanical forces that shape tissues and instruct cell fate decisions. How these forces are sensed and transmitted at the molecular level is therefore an important question, one that has mainly been investigated in cultured cells in vitro. Here, we elucidate how mechanical forces are transmitted in an intact organism. We studied Drosophila muscle attachment sites, which experience high mechanical forces during development and require integrin-mediated adhesion for stable attachment to tendons. Therefore, we quantified molecular forces across the essential integrin-binding protein Talin, which links integrin to the actin cytoskeleton. Generating flies expressing 3 Förster resonance energy transfer (FRET)-based Talin tension sensors reporting different force levels between 1 and 11 piconewton (pN) enabled us to quantify physiologically relevant molecular forces. By measuring primary Drosophila muscle cells, we demonstrate that Drosophila Talin experiences mechanical forces in cell culture that are similar to those previously reported for Talin in mammalian cell lines. However, in vivo force measurements at developing flight muscle attachment sites revealed that average forces across Talin are comparatively low and decrease even further while attachments mature and tissue-level tension remains high. Concomitantly, the Talin concentration at attachment sites increases 5-fold as quantified by fluorescence correlation spectroscopy (FCS), suggesting that only a small proportion of Talin molecules are mechanically engaged at any given time. Reducing Talin levels at late stages of muscle development results in muscle–tendon rupture in the adult fly, likely as a result of active muscle contractions. We therefore propose that a large pool of adhesion molecules is required to share high tissue forces. As a result, less than 15% of the molecules experience detectable forces at developing muscle attachment sites at the same time. Our findings define an important new concept of how cells can adapt to changes in tissue mechanics to prevent mechanical failure in vivo.

The protein Talin links the transmembrane cell adhesion molecule integrin to the actin cytoskeleton. Quantitative FRET-based force measurements across Talin in vivo reveal that only few Talin molecules are under force during the development of muscle attachment sites.

Cells in our body are constantly exposed to mechanical forces, which they need to sense and react to. In previous studies, fluorescent force sensors were developed to demonstrate that individual proteins in adhesion structures of a cell experience forces in the piconewton (pN) range. However, these cells were analyzed in isolation in an artificial plastic or glass environment. Here, we explored forces on adhesion proteins in their natural environment within a developing animal and used the muscle–tendon tissue in the fruit fly Drosophila as a model system. We made genetically modified fly lines with force sensors or controls inserted into the gene that produces the essential adhesion protein Talin. Using these force sensor flies, we found that only a small proportion of all the Talin proteins (<15%) present at developing muscle–tendon attachments experience detectable forces at the same time. Nevertheless, a large amount of Talin is accumulated at these attachments during fly development. We found that this large Talin pool is important to prevent rupture of the muscle–tendon connection in adult flies that produce high muscle forces during flight. In conclusion, we demonstrated that a large pool of Talin proteins is required for stable muscle–tendon attachment, likely with the individual Talin molecules dynamically sharing the mechanical load.

The impact of Megf10/Drpr gain-of-function on muscle development in Drosophila

Recessive mutations in multiple epidermal growth factor-like domains 10 (MEGF10) underlie a rare congenital muscle disease known as MEGF10 myopathy. MEGF10 and its Drosophila homolog Draper (Drpr) are transmembrane receptors expressed in muscle and glia. Drpr deficiency is known to result in muscle abnormalities in flies. In the current study, flies that ubiquitously overexpress Drpr, or mouse Megf10, display developmental arrest. The phenotype is reproduced with overexpression in muscle, but not in other tissues, and with overexpression during intermediate stages of myogenesis, but not in myoblasts. We find that tubular muscle subtypes are particularly sensitive to Megf10/Drpr overexpression. Complementary genetic analyses show that Megf10/Drpr and Notch may interact to regulate myogenesis. Our findings provide a basis for investigating MEGF10 in muscle development using Drosophila.

Suppression of myopathic lamin mutations by muscle-specific activation of AMPK and modulation of downstream signaling

Laminopathies are diseases caused by dominant mutations in the human LMNA gene encoding A-type lamins. Lamins are intermediate filaments that line the inner nuclear membrane, provide structural support for the nucleus and regulate gene expression. Drosophila melanogaster models of skeletal muscle laminopathies were developed to investigate the pathological defects caused by mutant lamins and identify potential therapeutic targets. Human disease-causing LMNA mutations were modeled in Drosophila Lamin C (LamC) and expressed in indirect flight muscle (IFM). IFM-specific expression of mutant, but not wild-type LamC, caused held-up wings indicative of myofibrillar defects. Analyses of the muscles revealed cytoplasmic aggregates of nuclear envelope (NE) proteins, nuclear and mitochondrial dysmorphology, myofibrillar disorganization and up-regulation of the autophagy cargo receptor p62. We hypothesized that the cytoplasmic aggregates of NE proteins trigger signaling pathways that alter cellular homeostasis, causing muscle dysfunction. In support of this hypothesis, transcriptomics data from human muscle biopsy tissue revealed misregulation of the AMP-activated protein kinase (AMPK)/4E-binding protein 1 (4E-BP1)/autophagy/proteostatic pathways. Ribosomal protein S6K (S6K) messenger RNA (mRNA) levels were increased and AMPKα and mRNAs encoding downstream targets were decreased in muscles expressing mutant LMNA relative controls. The Drosophila laminopathy models were used to determine if altering the levels of these factors modulated muscle pathology. Muscle-specific over-expression of AMPKα and down-stream targets 4E-BP, Forkhead box transcription factors O (Foxo) and Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α), as well as inhibition of S6K, suppressed the held-up wing phenotype, myofibrillar defects and LamC aggregation. These findings provide novel insights on mutant LMNA-based disease mechanisms and identify potential targets for drug therapy.

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.

Identifying Differential Gene Expression in Wing Polymorphism of Adult Males of the Largest Water Strider: De novo Transcriptome Assembly for Gigantometra gigas (Hemiptera: Gerridae)

Wing polymorphism is common in a wide variety of insect species. However, few studies have reported on adaptations in the wing polymorphism of insects at molecular level, in particular for males. Thus, the adaptive mechanisms need to be explored. The remarkable variability in wing morphs of insects is well represented in the water striders (Hemiptera: Gerridae). Within this family, Gigantometra gigas (China, 1925), the largest water strider known worldwide, displays macropterous and apterous males. In the present study, we used de novo transcriptome assembly to obtain gene expression information and compared body and leg-component lengths of adult males in different wing morphs. The analyses in both gene expression and phenotype levels were used for exploring the adaptive mechanism in wing polymorphism of G. gigas. After checking, a series of highly expressed structural genes were found in macropterous morphs, which were related to the maintenance of flight muscles and the enhancement of flight capacity, whereas in the apterous morphs, the imaginal morphogenesis protein-Late 2 (Imp-L2), which might inhibit wing development and increase the body size of insects, was still highly expressed in the adult stage. Moreover, body and leg-component lengths were significantly larger in apterous than in macropterous morphs. The larger size of the apterous morphs and the differences in highly expressed genes between the two wing morphs consistently demonstrate the adaptive significance of wing polymorphism in G. gigas. These results shed light on the future loss-of-function research of wing polymorphism in G. gigas.

Novel functions for integrin-associated proteins revealed by analysis of myofibril attachment in Drosophila

We use the myotendinous junction of Drosophila flight muscles to explore why many integrin associated proteins (IAPs) are needed and how their function is coordinated. These muscles revealed new functions for IAPs not required for viability: Focal Adhesion Kinase (FAK), RSU1, tensin and vinculin. Genetic interactions demonstrated a balance between positive and negative activities, with vinculin and tensin positively regulating adhesion, while FAK inhibits elevation of integrin activity by tensin, and RSU1 keeps PINCH activity in check. The molecular composition of myofibril termini resolves into 4 distinct layers, one of which is built by a mechanotransduction cascade: vinculin facilitates mechanical opening of filamin, which works with the Arp2/3 activator WASH to build an actin-rich layer positioned between integrins and the first sarcomere. Thus, integration of IAP activity is needed to build the complex architecture of the myotendinous junction, linking the membrane anchor to the sarcomere.

A transcriptomics resource reveals a transcriptional transition during ordered sarcomere morphogenesis in flight muscle

Muscles organise pseudo-crystalline arrays of actin, myosin and titin filaments to build force-producing sarcomeres. To study sarcomerogenesis, we have generated a transcriptomics resource of developing Drosophila flight muscles and identified 40 distinct expression profile clusters. Strikingly, most sarcomeric components group in two clusters, which are strongly induced after all myofibrils have been assembled, indicating a transcriptional transition during myofibrillogenesis. Following myofibril assembly, many short sarcomeres are added to each myofibril. Subsequently, all sarcomeres mature, reaching 1.5 µm diameter and 3.2 µm length and acquiring stretch-sensitivity. The efficient induction of the transcriptional transition during myofibrillogenesis, including the transcriptional boost of sarcomeric components, requires in part the transcriptional regulator Spalt major. As a consequence of Spalt knock-down, sarcomere maturation is defective and fibers fail to gain stretch-sensitivity. Together, this defines an ordered sarcomere morphogenesis process under precise transcriptional control - a concept that may also apply to vertebrate muscle or heart development.

Cell-type specific sequencing of microRNAs from complex animal tissues

MicroRNAs (miRNAs) play an essential role in the post-transcriptional regulation of animal development and physiology. However, in vivo studies aimed at linking miRNA function to the biology of distinct cell types within complex tissues remain challenging, partly because in vivo miRNA-profiling methods lack cellular resolution. We report microRNome by methylation-dependent sequencing (mime-seq), an in vivo enzymatic small-RNA-tagging approach that enables high-throughput sequencing of tissue- and cell-type-specific miRNAs in animals. The method combines cell-type-specific 3'-terminal 2'-O-methylation of animal miRNAs by a genetically encoded, plant-specific methyltransferase (HEN1), with chemoselective small-RNA cloning and high-throughput sequencing. We show that mime-seq uncovers the miRNomes of specific cells within Caenorhabditis elegans and Drosophila at unprecedented specificity and sensitivity, enabling miRNA profiling with single-cell resolution in whole animals. Mime-seq overcomes current challenges in cell-type-specific small-RNA profiling and provides novel entry points for understanding the function of miRNAs in spatially restricted physiological settings.

In Vivo Imaging of Muscle-tendon Morphogenesis in Drosophila Pupae

Muscles together with tendons and the skeleton enable animals including humans to move their body parts. Muscle morphogenesis is highly conserved from animals to humans. Therefore, the powerful Drosophila model system can be used to study concepts of muscle-tendon development that can also be applied to human muscle biology. Here, we describe in detail how morphogenesis of the adult muscle-tendon system can be easily imaged in living, developing Drosophila pupae. Hence, the method allows investigating proteins, cells and tissues in their physiological environment. In addition to a step-by-step protocol with helpful tips, we provide a comprehensive overview of fluorescently tagged marker proteins that are suitable for studying the muscle-tendon system. To highlight the versatile applications of the protocol, we show example movies ranging from visualization of long-term morphogenetic events – occurring on the time scale of hours and days – to visualization of short-term dynamic processes like muscle twitching occurring on time scale of seconds. Taken together, this protocol should enable the reader to design and perform live-imaging experiments for investigating muscle-tendon morphogenesis in the intact organism.

Ral function in muscle is required for flight maintenance in Drosophila

Ral is a small GTPase of the Ras superfamily that is important for a number of cellular functions. Recently, we found that expression of Ral is regulated by store-operated calcium entry (SOCE) in Drosophila neurons. In this study, through genetic and behavioural experiments, we show that Ral function is required in differentiated muscles for flight. Reducing Ral function in muscles, specifically reduced duration of flight bouts but not other motor functions, like climbing. Interestingly, unlike in the nervous system, Ral expression in the muscle is not regulated by SOCE. Moreover, either knockdown or genetic inhibition of SOCE in muscles does not affect flight. These findings demonstrate that a multiplicity of signalling mechanisms very likely regulate Ral expression in different tissues.

Mef2 and the skeletal muscle differentiation program

Mef2 is a conserved and significant transcription factor in the control of muscle gene expression. In cell culture Mef2 synergises with MyoD-family members in the activation of gene expression and in the conversion of fibroblasts into myoblasts. Amongst its in vivo roles, Mef2 is required for both Drosophila muscle development and mammalian muscle regeneration. Mef2 has functions in other cell-types too, but this review focuses on skeletal muscle and surveys key findings on Mef2 from its discovery, shortly after that of MyoD, up to the present day. In particular, in vivo functions, underpinning mechanisms and areas of uncertainty are highlighted. We describe how Mef2 sits at a nexus in the gene expression network that controls the muscle differentiation program, and how Mef2 activity must be regulated in time and space to orchestrate specific outputs within the different aspects of muscle development. A theme that emerges is that there is much to be learnt about the different Mef2 proteins (from different paralogous genes, spliced transcripts and species) and how the activity of these proteins is controlled.

Adult Muscle Formation Requires Drosophila Moleskin for Proliferation of Wing Disc-Associated Muscle Precursors

Adult muscle precursor (AMP) cells located in the notum of the larval wing disc undergo rapid amplification and eventual fusion to generate the Drosophila melanogaster indirect flight muscles (IFMs). Here we find that loss of Moleskin (Msk) function in these wing disc-associated myoblasts reduces the overall AMP pool size, resulting in the absence of IFM formation. This myoblast loss is due to a decrease in the AMP proliferative capacity and is independent of cell death. In contrast, disruption of Msk during pupal myoblast proliferation does not alter the AMP number, suggesting that Msk is specifically required for larval AMP proliferation. It has been previously shown that Wingless (Wg) signaling maintains expression of the Vestigial (Vg) transcription factor in proliferating myoblasts. However, other factors that influence Wg-mediated myoblast proliferation are largely unknown. Here we examine the interactions between Msk and the Wg pathway in regulation of the AMP pool size. We find that a myoblast-specific reduction of Msk results in the absence of Vg expression and a complete loss of the Wg pathway readout β-catenin/Armadillo (Arm). Moreover, msk RNA interference knockdown abolishes expression of the Wg target Ladybird (Lbe) in leg disc myoblasts. Collectively, our results provide strong evidence that Msk acts through the Wg signaling pathway to control myoblast pool size and muscle formation by regulating Arm stability or nuclear transport.

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

Knockout of either of two Drosophila Troponin C genes that are expressed in either the flight muscle or the jump muscle resulted in expansion of transcription of its paralogue into the affected muscle. Although either isoform can support normal jumping, only the flight isoform can support flight.

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.