Protocol for affinity purification-mass spectrometry interactome profiling in larvae of Drosophila melanogaster

Many techniques exist for the identification of protein interaction networks. We present a protocol that relies on an affinity purification-mass spectrometry (AP-MS) approach to detect proteins that co-purify with a tagged bait of interest from Drosophila melanogaster larval muscles using the GAL4/upstream activating sequence (UAS) expression system. We also describe steps for the isolation and identification of protein complexes, followed by streamlined bioinformatics analysis for rapid and reproducible results. This protocol can be extended to investigate protein interactions in other tissues. For complete details on the use and execution of this protocol, please refer to Guo et al.1.

Cell fusion: Inter-organ tissue communication promotes a union between myoblasts

Repeated rounds of fusion between apposing myoblasts allow muscles to become multinucleated. New research finds that myoblasts undergoing fusion in the Drosophila embryo respond to hormone signaling from a nearby tissue, resulting in the activation of a myoblast-specific gene necessary for the fusion process.

Identification of CryAB as a target of NUAK kinase activity in Drosophila muscle tissue

Phosphorylation reactions performed by protein kinases are one of the most studied post-translational modifications within cells. Much is understood about conserved residues within protein kinase domains that perform catalysis of the phosphotransfer reaction, yet the identity of the target substrates and downstream biological effects vary widely among cells, tissues, and organisms. Here, we characterize key residues essential for NUAK kinase activity in Drosophila melanogaster myogenesis and homeostasis. Creation of a NUAK kinase-dead mutation using Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 results in lethality at the embryo to larval transition, while loss of NUAK catalytic function later in development produces aggregation of the chaperone protein αB-crystallin/CryAB in muscle tissue. Yeast 2-hybrid assays demonstrate a physical interaction between NUAK and CryAB. We further show that a phospho-mimetic version of NUAK promotes the phosphorylation of CryAB and this post-translational modification occurs at 2 previously unidentified phosphosites that are conserved in the primary sequence of human CryAB. Mutation of these serine residues in D. melanogaster NUAK abolishes CryAB phosphorylation, thus, proving their necessity at the biochemical level. These studies together highlight the importance of kinase activity regulation and provide a platform to further explore muscle tissue proteostasis.

Orchestration of autophagosome fusion by STRIPAK complex components in muscle tissue

Autophagy is a central process responsible for the disposal of normal as well as damaged cellular proteins and organelles. Proper regulation of multiple steps - including initiation and the fusion between autophagosomes and lysosomes - is essential for the completion of cargo disposal. While the function of many proteins that mediate canonical autophagy has been characterized, the identification of new autophagy regulators may shed light on differences between tissues and/or responses to cellular stresses. In this punctum, we discuss our recent findings about how the Striatin-Interacting Phosphatase and Kinase (STRIPAK)-NUAK-Starvin (Stv) complex coordinately regulates autophagy in the muscle tissue of Drosophila melanogaster.

A conserved STRIPAK complex is required for autophagy in muscle tissue

Autophagy is important for cellular homeostasis and to prevent the abnormal accumulation of proteins. While many proteins that comprise the canonical autophagy pathway have been characterized, the identification of new regulators may help understand tissue and/or stress-specific responses. Using an in-silico approach, we identified Striatin interacting protein (Strip), MOB kinase activator 4, and fibroblast growth factor receptor 1 oncogene partner 2 as conserved mediators of muscle tissue maintenance. We performed affinity purification-mass spectrometry (AP-MS) experiments with Drosophila melanogaster Strip as a bait protein and copurified additional Striatin-interacting phosphatase and kinase (STRIPAK) complex members from larval muscle tissue. NUAK family kinase 1 (NUAK) and Starvin (Stv) also emerged as Strip-binding proteins and these physical interactions were verified in vivo using proximity ligation assays. To understand the functional significance of the STRIPAK-NUAK-Stv complex, we employed a sensitized genetic assay combined with RNA interference (RNAi) to demonstrate that both NUAK and stv function in the same biological process with genes that encode for STRIPAK complex proteins. RNAi-directed knockdown of Strip in muscle tissue led to the accumulation of ubiquitinated cargo, p62, and Autophagy-related 8a, consistent with a block in autophagy. Indeed, autophagic flux was decreased in Strip RNAi muscles, while lysosome biogenesis and activity were unaffected. Our results support a model whereby the STRIPAK-NUAK-Stv complex coordinately regulates autophagy in muscle tissue.

Independent pathways control muscle tissue size and sarcomere remodeling

Cell growth and proliferation must be balanced during development to attain a final adult size with the appropriate proportions of internal organs to maximize fitness and reproduction. While multiple signaling pathways coordinate Drosophila development, it is unclear how multi-organ communication within and between tissues converge to regulate systemic growth. One such growth pathway, mediated by insulin-like peptides that bind to and activate the insulin receptor in multiple target tissues, is a primary mediator of organismal size. Here we uncover a signaling role for the NUAK serine/threonine kinase in muscle tissue that impinges upon insulin pathway activity to limit overall body size, including a reduction in the growth of individual organs. In skeletal muscle tissue, manipulation of NUAK or insulin pathway components influences sarcomere number concomitant with modulation of thin and thick filament lengths, possibly by modulating the localization of Lasp, a nebulin repeat protein known to set thin filament length. This mode of sarcomere remodeling does not occur in other mutants that also exhibit smaller muscles, suggesting that a sensing mechanism exists in muscle tissue to regulate sarcomere growth that is independent of tissue size control.

Gene expression profiling of NUAK kinase overexpression in Drosophila larval muscle development

Signal transduction pathways mediated by kinases control diverse biological outputs at the level of cells and tissues to regulate a diverse array of biological and developmental events. To gain insight into how muscle expression of the evolutionarily conserved NUAK kinase regulates the transcriptional landscape during Drosophila melanogaster development, we performed high-throughput sequencing of RNA from either whole larvae or dissected muscle fillets at the end of larval development. Raw data was generated using the Illumina HiSeq 4000 platform. After trimming and mapping to the Drosophila reference genome, differential gene expression and GO enrichment analysis were completed. Raw data are deposited in the NCBI Gene Expression Ominbus (GEO) repository under GEO accession GSE204894.

Complex protein interactions mediate Drosophila Lar function in muscle tissue

The type IIa family of receptor protein tyrosine phosphatases (RPTPs), including Lar, RPTPσ and RPTPδ, are well-studied in coordinating actin cytoskeletal rearrangements during axon guidance and synaptogenesis. To determine whether this regulation is conserved in other tissues, interdisciplinary approaches were utilized to study Lar-RPTPs in the Drosophila musculature. Here we find that the single fly ortholog, Drosophila Lar (Dlar), is localized to the muscle costamere and that a decrease in Dlar causes aberrant sarcomeric patterning, deficits in larval locomotion, and integrin mislocalization. Sequence analysis uncovered an evolutionarily conserved Lys-Gly-Asp (KGD) signature in the extracellular region of Dlar. Since this tripeptide sequence is similar to the integrin-binding Arg-Gly-Asp (RGD) motif, we tested the hypothesis that Dlar directly interacts with integrin proteins. However, structural analyses of the fibronectin type III domains of Dlar and two vertebrate orthologs that include this conserved motif indicate that this KGD tripeptide is not accessible and thus unlikely to mediate physical interactions with integrins. These results, together with the proteomics identification of basement membrane (BM) proteins as potential ligands for type IIa RPTPs, suggest a complex network of protein interactions in the extracellular space that may mediate Lar function and/or signaling in muscle tissue.

An Investigation of the Altered Textural Property in Woody Breast Myopathy Using an Integrative Omics Approach

Woody breast (WB) is a myopathy observed in broiler Pectoralis major (PM) characterized by its tough and rubbery texture with greater level of calcium content. The objective of this study was to investigate the functionality/integrity of WB sarcoplasmic reticulum (SR), which may contribute to the elevated calcium content observed in WB and other factors that may influence WB texture. Fourteen Ross line broiler PM [7 severe WB and 7 normal (N)] were selected, packaged, and frozen at −20°C at 8 h postmortem from a commercial processing plant. Samples were used to measure pH, sarcomere length, proteolysis, calpain activity, collagenase activity, collagen content, collagen crosslinks density, and connective tissue peak transitional temperature. Exudate was also collected from each sample to evaluate free calcium concentration. The SR fraction of the samples was separated and utilized for proteomic and lipidomic analysis. The WB PM had a higher pH, shorter sarcomeres, lower % of intact troponin-T, more autolyzed μ/m calpain, more activated collagenase, greater collagen content, greater mature collagen crosslinks density, and higher connective tissue peak transitional temperature than the N PM (p ≤ 0.05). Exudate from WB PM had higher levels of free calcium than those from N PM (p < 0.05). Proteomics data revealed an upregulation of calcium transport proteins and a downregulation of proteins responsible for calcium release (p < 0.05) in WB SR. Interestingly, there was an upregulation of phospholipase A2 (PLA2), and cholinesterase exhibited a 7.6-fold increase in WB SR (p < 0.01). Lipidomics data revealed WB SR had less relative % of phosphatidylcholine (PC) and more lysophosphatidylcholine (LPC; p < 0.05). The results indicated that upregulation of calcium transport proteins and downregulation of calcium-release proteins in WB SR may be the muscle’s attempt to regulate this proposed excessive signaling of calcium release due to multiple factors, such as upregulation of PLA2 resulting in PC hydrolysis and presence of cholinesterase inhibitors in the system prolonging action potential. In addition, the textural abnormality of WB may be the combined effects of shorter sarcomere length and more collagen with greater crosslink density being deposited in the broiler PM.

Integration of proteomic and genetic approaches to assess developmental muscle atrophy

Muscle atrophy, or a decline in muscle protein mass, is a significant problem in the aging population and in numerous disease states. Unraveling molecular signals that trigger and promote atrophy may lead to a better understanding of treatment options; however, there is no single cause of atrophy identified to date. To gain insight into this problem, we chose to investigate changes in protein profiles during muscle atrophy in Manduca sexta and Drosophila melanogaster. The use of insect models provides an interesting parallel to probe atrophic mechanisms as these organisms undergo a normal developmental atrophy process during the pupal transition stage. Leveraging the inherent advantages of each model organism, we first defined protein signature changes during M. sexta intersegmental muscle (ISM) atrophy and then used genetic approaches to confirm their functional importance in the D. melanogaster dorsal internal oblique muscles (DIOMs). Our data reveal an upregulation of proteasome and peptidase components and a general downregulation of proteins that regulate actin filament formation. Surprisingly, thick filament proteins that comprise the A-band are increased in abundance, providing support for the ordered destruction of myofibrillar components during developmental atrophy. We also uncovered the actin filament regulator ciboulot (Cib) as a novel regulator of muscle atrophy. These insights provide a framework towards a better understanding of global changes that occur during atrophy and may eventually lead to therapeutic targets.

TRIM32: A Multifunctional Protein Involved in Muscle Homeostasis, Glucose Metabolism, and Tumorigenesis

Human tripartite motif family of proteins 32 (TRIM32) is a ubiquitous multifunctional protein that has demonstrated roles in differentiation, muscle physiology and regeneration, and tumor suppression. Mutations in TRIM32 result in two clinically diverse diseases. A mutation in the B-box domain gives rise to Bardet–Biedl syndrome (BBS), a disease whose clinical presentation shares no muscle pathology, while mutations in the NHL (NCL-1, HT2A, LIN-41) repeats of TRIM32 causes limb-girdle muscular dystrophy type 2H (LGMD2H). TRIM32 also functions as a tumor suppressor, but paradoxically is overexpressed in certain types of cancer. Recent evidence supports a role for TRIM32 in glycolytic-mediated cell growth, thus providing a possible mechanism for TRIM32 in the accumulation of cellular biomass during regeneration and tumorigenesis, including in vitro and in vivo approaches, to understand the broad spectrum of TRIM32 functions. A special emphasis is placed on the utility of the Drosophila model, a unique system to study glycolysis and anabolic pathways that contribute to the growth and homeostasis of both normal and tumor tissues.

Costameric integrin and sarcoglycan protein levels are altered in a Drosophila model for Limb-girdle muscular dystrophy type 2H

Mutations in two different domains of the ubiquitously expressed TRIM32 protein give rise to two clinically separate diseases, one of which is Limb-girdle muscular dystrophy type 2H (LGMD2H). Uncovering the muscle-specific role of TRIM32 in LGMD2H pathogenesis has proven difficult, as neurogenic phenotypes, independent of LGMD2H pathology, are present in TRIM32 KO mice. We previously established a platform to study LGMD2H pathogenesis using Drosophila melanogaster as a model. Here we show that LGMD2H disease-causing mutations in the NHL domain are molecularly and structurally conserved between fly and human TRIM32. Furthermore, transgenic expression of a subset of myopathic alleles (R394H, D487N, and 520fs) induce myofibril abnormalities, altered nuclear morphology, and reduced TRIM32 protein levels, mimicking phenotypes in patients afflicted with LGMD2H. Intriguingly, we also report for the first time that the protein levels of βPS integrin and sarcoglycan δ, both core components of costameres, are elevated in TRIM32 disease-causing alleles. Similarly, murine myoblasts overexpressing a catalytically inactive TRIM32 mutant aberrantly accumulate α- and β-dystroglycan and α-sarcoglycan. We speculate that the stoichiometric loss of costamere components disrupts costamere complexes to promote muscle degeneration.

Insect Cuticular Chitin Contributes to Form and Function

Chitin contributes to the rigidity of the insect cuticle and serves as an attachment matrix for other cuticular proteins. Deficiency of chitin results in abnormal embryos, cuticular structural defects and growth arrest. When chitin is not turned over during molting, the developing insect is trapped inside the old cuticle. Partial deacetylation of cuticular chitin is also required for proper laminar organization of the cuticle and vertical pore canals, molting, and locomotion. Thus, chitin and its modifications strongly influence the structure of the exoskeleton as well as the physiological functions of the insect. Internal tendons and specialized epithelial cells called "tendon cells" that arise from the outer layer of epidermal cells provide attachment sites at both ends of adult limb muscles. Membrane processes emanating from both tendon and muscle cells interdigitate extensively to strengthen the attachment of muscles to the extracellular matrix (ECM). Protein ligands that bind to membrane-bound integrin complexes further enhance the adhesion between muscles and tendons. Tendon cells contain F-actin fiber arrays that contribute to their rigidity. In the cytoplasm of muscle cells, proteins such as talin and other proteins provide attachment sites for cytoskeletal actin, thereby increasing integrin binding and activation to mechanically couple the ECM with actin in muscle cells. Mutations in integrins and their ligands, as well as depletion of chitin deacetylases, result in defective locomotion and muscle detachment from the ECM. Thus, chitin in the cuticle and chitin deacetylases strongly influence the shape and functions of the exoskeleton as well as locomotion of insects.

Drosophila NUAK functions with Starvin/BAG3 in autophagic protein turnover

The inability to remove protein aggregates in post-mitotic cells such as muscles or neurons is a cellular hallmark of aging cells and is a key factor in the initiation and progression of protein misfolding diseases. While protein aggregate disorders share common features, the molecular level events that culminate in abnormal protein accumulation cannot be explained by a single mechanism. Here we show that loss of the serine/threonine kinase NUAK causes cellular degeneration resulting from the incomplete clearance of protein aggregates in Drosophila larval muscles. In NUAK mutant muscles, regions that lack the myofibrillar proteins F-actin and Myosin heavy chain (MHC) instead contain damaged organelles and the accumulation of select proteins, including Filamin (Fil) and CryAB. NUAK biochemically and genetically interacts with Drosophila Starvin (Stv), the ortholog of mammalian Bcl-2-associated athanogene 3 (BAG3). Consistent with a known role for the co-chaperone BAG3 and the Heat shock cognate 71 kDa (HSC70)/HSPA8 ATPase in the autophagic clearance of proteins, RNA interference (RNAi) of Drosophila Stv, Hsc70-4, or autophagy-related 8a (Atg8a) all exhibit muscle degeneration and muscle contraction defects that phenocopy NUAK mutants. We further demonstrate that Fil is a target of NUAK kinase activity and abnormally accumulates upon loss of the BAG3-Hsc70-4 complex. In addition, Ubiquitin (Ub), ref(2)p/p62, and Atg8a are increased in regions of protein aggregation, consistent with a block in autophagy upon loss of NUAK. Collectively, our results establish a novel role for NUAK with the Stv-Hsc70-4 complex in the autophagic clearance of proteins that may eventually lead to treatment options for protein aggregate diseases.

Drosophila TRIM32 cooperates with glycolytic enzymes to promote cell growth

Cell growth and/or proliferation may require the reprogramming of metabolic pathways, whereby a switch from oxidative to glycolytic metabolism diverts glycolytic intermediates towards anabolic pathways. Herein, we identify a novel role for TRIM32 in the maintenance of glycolytic flux mediated by biochemical interactions with the glycolytic enzymes Aldolase and Phosphoglycerate mutase. Loss of Drosophila TRIM32, encoded by thin (tn), shows reduced levels of glycolytic intermediates and amino acids. This altered metabolic profile correlates with a reduction in the size of glycolytic larval muscle and brain tissue. Consistent with a role for metabolic intermediates in glycolysis-driven biomass production, dietary amino acid supplementation in tn mutants improves muscle mass. Remarkably, TRIM32 is also required for ectopic growth - loss of TRIM32 in a wing disc-associated tumor model reduces glycolytic metabolism and restricts growth. Overall, our results reveal a novel role for TRIM32 for controlling glycolysis in the context of both normal development and tumor growth.

A tissue communication network coordinating innate immune response during muscle stress

Complex tissue communication networks function throughout an organism's lifespan to maintain tissue homeostasis. Using the genetic model Drosophila melanogaster, we have defined a network of immune responses that are activated following the induction of muscle stresses, including hypercontraction, detachment and oxidative stress. Of these stressors, loss of the genes that cause muscle detachment produced the strongest levels of JAK-STAT activation. In one of these mutants, fondue (fon), we also observe hemocyte recruitment and the accumulation of melanin at muscle attachment sites (MASs), indicating a broad involvement of innate immune responses upon muscle detachment. Loss of fon results in pathogen-independent Toll signaling in the fat body and increased expression of the Toll-dependent antimicrobial peptide Drosomycin. Interestingly, genetic interactions between fon and various Toll pathway components enhance muscle detachment. Finally, we show that JAK-STAT and Toll signaling are capable of reciprocal activation in larval tissues. We propose a model of tissue communication for the integration of immune responses at the local and systemic level in response to altered muscle physiology.

Thin is required for cell death in the Drosophila abdominal muscles by targeting DIAP1

In holometabolous insects, developmentally controlled programmed cell death (PCD) is a conserved process that destroys a subset of larval tissues for the eventual creation of new adult structures. This process of histolysis is relatively well studied in salivary gland and midgut tissues, while knowledge concerning larval muscle destruction is limited. Here, we have examined the histolysis of a group of Drosophila larval abdominal muscles called the dorsal external oblique muscles (DEOMs). Previous studies have defined apoptosis as the primary mediator of DEOM breakdown, whose timing is controlled by ecdysone signaling. However, very little is known about other factors that contribute to DEOM destruction. In this paper, we examine the role of thin (tn), which encodes for the Drosophila homolog of mammalian TRIM32, in the regulation of DEOM histolysis. We find that loss of Tn blocks DEOM degradation independent of ecdysone signaling. Instead, tn genetically functions in a pathway with the death-associated inhibitor of apoptosis (DIAP1), Dronc, and death-associated APAF1-related killer (Dark) to regulate apoptosis. Importantly, blocking Tn results in the absence of active Caspase-3 immunostaining, upregulation of DIAP1 protein levels, and inhibition of Dronc activation. DIAP1 and Dronc mRNA levels are not altered in tn mutants, showing that Tn acts post-transcriptionally on DIAP1 to regulate apoptosis. Herein, we also find that the RING domain of Tn is required for DEOM histolysis as loss of this domain results in higher DIAP1 levels. Together, our results suggest that the direct control of DIAP1 levels, likely through the E3 ubiquitin ligase activity of Tn, provides a mechanism to regulate caspase activity and to facilitate muscle cell death.

Optimization of wrMTrck to monitor Drosophila larval locomotor activity

An efficient and low-cost method of examining larval movement in Drosophila melanogaster is needed to study how mutations and/or alterations in the muscular, neural, and olfactory systems affect locomotor behavior. Here, we describe the implementation of wrMTrck, a freely available ImageJ plugin originally developed for examining multiple behavioral parameters in the nematode C. elegans. Our optimized method is rapid, reproducible and does not require automated microscope setups or the purchase of proprietary software. To demonstrate the utility of this method, we analyzed the velocity and crawling paths of two Drosophila mutants that affect muscle structure and/or function. Additionally, we show that this approach is useful for tracking the behavior of adult insects, including Tribolium castaneum and Drosophila melanogaster.

Drosophila clueless is involved in Parkin-dependent mitophagy by promoting VCP-mediated Marf degradation

PINK1/Parkin-mediated mitochondrial quality control (MQC) requires valosin-containing protein (VCP)-dependent Mitofusin/Marf degradation to prevent damaged organelles from fusing with the healthy mitochondrial pool, facilitating mitochondrial clearance by autophagy. Drosophila clueless (clu) was found to interact genetically with PINK1 and parkin to regulate mitochondrial clustering in germ cells. However, whether Clu acts in MQC has not been investigated. Here, we show that overexpression of Drosophila Clu complements PINK1, but not parkin, mutant muscles. Loss of clu leads to the recruitment of Parkin, VCP/p97, p62/Ref(2)P and Atg8a to depolarized swollen mitochondria. However, clearance of damaged mitochondria is impeded. This paradox is resolved by the findings that excessive mitochondrial fission or inhibition of fusion alleviates mitochondrial defects and impaired mitophagy caused by clu depletion. Furthermore, Clu is upstream of and binds to VCP in vivo and promotes VCP-dependent Marf degradation in vitro Marf accumulates in whole muscle lysates of clu-deficient flies and is destabilized upon Clu overexpression. Thus, Clu is essential for mitochondrial homeostasis and functions in concert with Parkin and VCP for Marf degradation to promote damaged mitochondrial clearance.

Analysis of mitochondrial structure and function in the Drosophila larval musculature

Mitochondria are dynamic organelles that change their architecture in normal physiological conditions. Mutations in genes that control mitochondrial fission or fusion, such as dynamin-related protein (Drp1), Mitofusins 1 (Mfn1) and 2 (Mfn2), and Optic atrophy 1 (Opa1), result in neuropathies or neurodegenerative diseases. It is increasingly clear that altered mitochondrial dynamics also underlie the pathology of other degenerative diseases, including Parkinson's disease (PD). Thus, understanding mitochondrial distribution, shape, and dynamics in all cell types is a prerequisite for developing and defining treatment regimens that may differentially affect tissues. The majority of Drosophila genes implicated in mitochondrial dynamics have been studied in the adult indirect flight muscle (IFM). Here, we discuss the utility of Drosophila third instar larvae (L3) as an alternative model to analyze and quantify mitochondrial behaviors. Advantages include large muscle cell size, a stereotyped arrangement of mitochondria that is conserved in mammalian muscles, and the ability to analyze muscle-specific gene function in mutants that are lethal prior to adult stages. In particular, we highlight methods for sample preparation and analysis of mitochondrial morphological features.

Loss of a Clueless-dGRASP complex results in ER stress and blocks Integrin exit from the perinuclear endoplasmic reticulum in Drosophila larval muscle

Drosophila Clueless (Clu) and its conserved orthologs are known for their role in the prevention of mitochondrial clustering. Here, we uncover a new role for Clu in the delivery of integrin subunits in muscle tissue. In clu mutants, αPS2 integrin, but not βPS integrin, abnormally accumulates in a perinuclear endoplasmic reticulum (ER) subdomain, a site that mirrors the endogenous localization of Clu. Loss of components essential for mitochondrial distribution do not phenocopy the clu mutant αPS2 phenotype. Conversely, RNAi knockdown of the DrosophilaGolgi reassembly and stacking protein GRASP55/65 (dGRASP) recapitulates clu defects, including the abnormal accumulation of αPS2 and larval locomotor activity. Both Clu and dGRASP proteins physically interact and loss of Clu displaces dGRASP from ER exit sites, suggesting that Clu cooperates with dGRASP for the exit of αPS2 from a perinuclear subdomain in the ER. We also found that Clu and dGRASP loss of function leads to ER stress and that the stability of the ER exit site protein Sec16 is severely compromised in the clu mutants, thus explaining the ER accumulation of αPS2. Remarkably, exposure of clu RNAi larvae to chemical chaperones restores both αPS2 delivery and functional ER exit sites. We propose that Clu together with dGRASP prevents ER stress and therefore maintains Sec16 stability essential for the functional organization of perinuclear early secretory pathway. This, in turn, is essential for integrin subunit αPS2 ER exit in Drosophila larval myofibers.

Drosophila importin-7 functions upstream of the Elmo signaling module to mediate the formation and stability of muscle attachments

Establishment and maintenance of stable muscle attachments is essential for coordinated body movement. Studies in Drosophila have pioneered a molecular understanding of the morphological events in the conserved process of muscle attachment formation, including myofiber migration, muscle-tendon signaling, and stable junctional adhesion between muscle cells and their corresponding target insertion sites. In both Drosophila and vertebrate models, integrin complexes play a key role in the biogenesis and stability of muscle attachments through the interactions of integrins with extracellular matrix (ECM) ligands. We show that Drosophila importin-7 (Dim7) is an upstream regulator of the conserved Elmo-Mbc→Rac signaling pathway in the formation of embryonic muscle attachment sites (MASs). Dim7 is encoded by the moleskin (msk) locus and was identified as an Elmo-interacting protein. Both Dim7 and Elmo localize to the ends of myofibers coincident with the timing of muscle-tendon attachment in late myogenesis. Phenotypic analysis of elmo mutants reveal muscle attachment defects similar to those previously described for integrin mutants. Furthermore, Elmo and Dim7 interact both biochemically and genetically in the developing musculature. The muscle detachment phenotype resulting from mutations in the msk locus can be rescued by components in the Elmo signaling pathway, including the Elmo-Mbc complex, an activated Elmo variant, or a constitutively active form of Rac. In larval muscles, the localization of Dim7 and activated Elmo to the sites of muscle attachment is attenuated upon RNAi knockdown of integrin heterodimer complex components. Our results show that integrins function as upstream signals to mediate Dim7-Elmo enrichment to the MASs.

Genetic interaction screens identify a role for hedgehog signaling in Drosophila border cell migration

BACKGROUND

Cell motility is essential for embryonic development and physiological processes such as the immune response, but also contributes to pathological conditions such as tumor progression and inflammation. However, our understanding of the mechanisms underlying migratory processes is incomplete. Drosophila border cells provide a powerful genetic model to identify the roles of genes that contribute to cell migration.

RESULTS

Members of the Hedgehog signaling pathway were uncovered in two independent screens for interactions with the small GTPase Rac and the polarity protein Par-1 in border cell migration. Consistent with a role in migration, multiple Hh signaling components were enriched in the migratory border cells. Interference with Hh signaling by several different methods resulted in incomplete cell migration. Moreover, the polarized distribution of E-Cadherin and a marker of tyrosine kinase activity were altered when Hh signaling was disrupted. Conservation of Hh-Rac and Hh-Par-1 signaling was illustrated in the wing, in which Hh-dependent phenotypes were enhanced by loss of Rac or par-1.

CONCLUSIONS

We identified a pathway by which Hh signaling connects to Rac and Par-1 in cell migration. These results further highlight the importance of modifier screens in the identification of new genes that function in developmental pathways.

"Importin" signaling roles for import proteins: the function of Drosophila importin-7 (DIM-7) in muscle-tendon signaling

The formation of a mature myotendinous junction (MTJ) between a muscle and its site of attachment is a highly regulated process that involves myofiber migration, cell-cell signaling, and culminates with the stable adhesion between the adjacent muscle-tendon cells. Improper establishment or maintenance of muscle-tendon attachment sites results in a decrease in force generation during muscle contraction and progressive muscular dystrophies in vertebrate models. Many studies have demonstrated the important role of the integrins and integrin-associated proteins in the formation and maintenance of the MTJ. We recently demonstrated that moleskin (msk), the gene that encodes for Drosophila importin-7 (DIM-7), is required for the proper formation of muscle-tendon adhesion sites in the developing embryo. Further studies demonstrated an enrichment of DIM-7 to the ends of muscles where the muscles attach to their target tendon cells. Genetic analysis supports a model whereby msk is required in the muscle and signals via the secreted epidermal growth factor receptor (Egfr) ligand Vein to regulate tendon cell maturation. These data demonstrate a novel role for the canonical nuclear import protein DIM-7 in establishment of the MTJ.

The DOCK protein sponge binds to ELMO and functions in Drosophila embryonic CNS development

Cell morphogenesis, which requires rearrangement of the actin cytoskeleton, is essential to coordinate the development of tissues such as the musculature and nervous system during normal embryonic development. One class of signaling proteins that regulate actin cytoskeletal rearrangement is the evolutionarily conserved CDM (C. elegansCed-5, human DOCK180, DrosophilaMyoblast city, or Mbc) family of proteins, which function as unconventional guanine nucleotide exchange factors for the small GTPase Rac. This CDM-Rac protein complex is sufficient for Rac activation, but is enhanced upon the association of CDM proteins with the ELMO/Ced-12 family of proteins. We identified and characterized the role of Drosophila Sponge (Spg), the vertebrate DOCK3/DOCK4 counterpart as an ELMO-interacting protein. Our analysis shows Spg mRNA and protein is expressed in the visceral musculature and developing nervous system, suggesting a role for Spg in later embryogenesis. As maternal null mutants of spg die early in development, we utilized genetic interaction analysis to uncover the role of Spg in central nervous system (CNS) development. Consistent with its role in ELMO-dependent pathways, we found genetic interactions with spg and elmo mutants exhibited aberrant axonal defects. In addition, our data suggests Ncad may be responsible for recruiting Spg to the membrane, possibly in CNS development. Our findings not only characterize the role of a new DOCK family member, but help to further understand the role of signaling downstream of N-cadherin in neuronal development.

Drosophila ELMO/CED-12 interacts with Myoblast city to direct myoblast fusion and ommatidial organization

Members of the CDM (CED-5, Dock180, Myoblast city) superfamily of guanine nucleotide exchange factors function in diverse processes that include cell migration and myoblast fusion. Previous studies have shown that the SH3, DHR1 and DHR2 domains of Myoblast city (MBC) are essential for it to direct myoblast fusion in the Drosophila embryo, while the conserved DCrk-binding proline rich region is expendable. Herein, we describe the isolation of Drosophila ELMO/CED-12, an approximately 82 kDa protein with a pleckstrin homology (PH) and proline-rich domain, by interaction with the MBC SH3 domain. Mass spectrometry confirms the presence of an MBC/ELMO complex within the embryonic musculature at the time of myoblast fusion and embryos maternally and/or zygotically mutant for elmo exhibit defects in myoblast fusion. Overexpression of MBC and ELMO in the embryonic mesoderm causes defects in myoblast fusion reminiscent of those seen with constitutively-activated Rac1, supporting the previous finding that both the absence of and an excess of Rac activity are deleterious to myoblast fusion. Overexpression of MBC and ELMO/CED-12 in the eye causes perturbations in ommatidial organization that are suppressed by mutations in Rac1 and Rac2, demonstrating genetically that MBC and ELMO/CED-12 cooperate to activate these small GTPases in Drosophila.

Myoblast fusion in Drosophila

Myogenic differentiation in Drosophila melanogaster, as in many other organisms, involves the generation of multinucleate muscle fibers through the fusion of myoblasts. Prior to fusion, the myoblasts become specified as one of two distinct cell types. They then become competent to fuse and express genes associated with cell recognition and adhesion. Initially, cell-type- specific adhesion molecules mediate recognition and fusion between these two distinct populations of myoblasts. Intracellular proteins that are essential for the fusion process are then recruited to points of cell-cell contact at the membrane, where the cell surface molecules have become localized. Many of these cytosolic proteins contribute to reorganization of the cytoskeleton through activation of small guanosine triphosphatases and recruitment of actin nucleating proteins. Following the initial fusion event, the ultimate size of the syncytia is achieved through multiple rounds of fusion between the developing syncytia and mononucleate myoblasts. Ultrastructural changes associated with cell fusion include recruitment of electron-dense vesicles to points of cell-cell contact, resolution of these vesicles into fusion plaques, fusion pore formation, and membrane vesiculation. This chapter reviews our current understanding of the genes, pathways, and ultrastructural events associated with fusion in the Drosophila embryo, giving rise to multinucleate syncytia that will be used throughout larval life.

The CDM superfamily protein MBC directs myoblast fusion through a mechanism that requires phosphatidylinositol 3,4,5-triphosphate binding but is independent of direct interaction with DCrk

Myoblast city (mbc), a member of the CDM superfamily, is essential in the Drosophila melanogaster embryo for fusion of myoblasts into multinucleate fibers. Using germ line clones in which both maternal and zygotic contributions were eliminated and rescue of the zygotic loss-of-function phenotype, we established that mbc is required in the fusion-competent subset of myoblasts. Along with its close orthologs Dock180 and CED-5, MBC has an SH3 domain at its N terminus, conserved internal domains termed DHR1 and DHR2 (or "Docker"), and C-terminal proline-rich domains that associate with the adapter protein DCrk. The importance of these domains has been evaluated by the ability of MBC mutations and deletions to rescue the mbc loss-of-function muscle phenotype. We demonstrate that the SH3 and Docker domains are essential. Moreover, ethyl methanesulfonate-induced mutations that change amino acids within the MBC Docker domain to residues that are conserved in other CDM family members nevertheless eliminate MBC function in the embryo, which suggests that these sites may mediate interactions specific to Drosophila MBC. A functional requirement for the conserved DHR1 domain, which binds to phosphatidylinositol 3,4,5-triphosphate, implicates phosphoinositide signaling in myoblast fusion. Finally, the proline-rich C-terminal sites mediate strong interactions with DCrk, as expected. These sites are not required for MBC to rescue the muscle loss-of-function phenotype, however, which suggests that MBC's role in myoblast fusion can be carried out independently of direct DCrk binding.

Requirement for JAK/STAT signaling throughout border cell migration in Drosophila

The evolutionarily conserved JAK/STAT signaling pathway is essential for the proliferation, survival and differentiation of many cells including cancer cells. Recent studies have implicated this transcriptional pathway in the process of cell migration in humans, mice, Drosophila and Dictyostelium. In the Drosophila ovary, JAK/STAT signaling is necessary and sufficient for the specification and migration of a group of cells called the border cells; however, it is not clear to what extent the requirement for cell fate is distinct from that for cell migration. We found that STAT protein is enriched in the migrating border cells throughout their migration and is an indicator of cells with highest JAK/STAT activity. In addition, stat(ts) mutants exhibited border cell migration defects after just 30 minutes at the non-permissive temperature, prior to any detectable change in the expression of cell fate markers. At later times, cell fate changes became evident, indicating that border cell fate is labile. JAK/STAT signaling was also required for organization of the border cell cluster. Finally, we show that both the accumulation of STAT protein and nuclear accumulation are positively regulated by JAK/STAT activity. The activity of the pathway is negatively regulated by overexpression of a SOCS protein and by blocking endocytosis. Together, our findings suggest that the requirement for STAT in border cells extends beyond the initial specification and delamination of cells from the epithelium.

A Role for Drosophila IAP1-Mediated Caspase Inhibition in Rac-Dependent Cell Migration

Border cell migration in the Drosophila ovary is a relatively simple and genetically tractable model for studying the conversion of epithelial cells to migratory cells. Like many cell migrations, border cell migration is inhibited by a dominant-negative form of the GTPase Rac. To identify new genes that function in Rac-dependent cell motility, we screened for genes that when overexpressed suppressed the migration defect caused by dominant-negative Rac. Overexpression of the Drosophila inhibitor of apoptosis 1 (DIAP1), which is encoded by the thread (th) gene, suppressed the migration defect. Moreover, loss-of-function mutations in th caused migration defects but, surprisingly, did not cause apoptosis. Mutations affecting the Dark protein, an activator of the upstream caspase Dronc, also rescued RacN17 migration defects. These results indicate an apoptosis-independent role for DIAP1-mediated Dronc inhibition in Rac-mediated cell motility.

Myosin VI is required for E-cadherin-mediated border cell migration

Myosin VI (MyoVI) is a pointed-end-directed, actin-based motor protein, and mutations in the gene result in disorganization of hair cell stereocilia and cause deafness in mice. MyoVI also localizes to the leading edges of growth-factor-stimulated fibroblast cells and has been suggested to be involved in cell motility. There has been no direct test of this hypothesis, however. Drosophila melanogaster MyoVI is expressed in a small group of migratory follicle cells, known as border cells. Here we show that depletion of MyoVI specifically from border cells severely inhibited their migration. Similar to MyoVI, E-cadherin is required for border cell migration. We found that E-cadherin and Armadillo (Arm, Drosophila beta-catenin) protein levels were specifically reduced in cells lacking MyoVI, whereas other proteins were not. In addition, MyoVI protein levels were reduced in cells lacking DE-cadherin or Arm. MyoVI and Arm co-immunoprecipitated from ovarian protein extracts. These data suggest that MyoVI is required for border cell migration where it stabilizes E-cadherin and Arm. Mutations in MyoVIIA, another unconventional myosin protein, also lead to deafness, and MyoVIIA interacts with E-cadherin through a membrane protein called vezatin. Multiple biochemical mechanisms may exist, therefore, for cadherins to associate with diverse unconventional myosins that are required for normal stereocilium formation or maintenance.