The conserved transmembrane proteoglycan Perdido/Kon-tiki is essential for myofibrillogenesis and sarcomeric structure in Drosophila

Chondroitin sulfate in invertebrate development

Chondroitin sulfate (CS) is one of the most evolutionarily conserved glycosaminoglycans (GAGs). Although CS's function in skeletal development is well established in vertebrates, CS exists in more primitive animal species with no cartilage or bone, such as C. elegans and Drosophila, indicating that the original role of CS was not in the skeletal system. In this review, we focus on the roles of CS and the mechanisms of action during development of two genetically trackable model organisms, C. elegans and Drosophila.

An engineered in vitro model of the human myotendinous junction

The myotendinous junction (MTJ) is a vulnerable region at the interface of skeletal muscle and tendon that forms an integrated mechanical unit. This study presents a technique for the spatially restrictive co-culture of human embryonic stem cell (hESC)-derived skeletal myocytes and primary tenocytes for two-dimensional modeling of the MTJ. Micropatterned lanes of extracellular matrix and a 2-well culture chamber define the initial regions of occupation. On day 1, both lines occupy less than 20 % of the initially vacant interstitial zone, referred to henceforth as the junction. Myocyte-tenocyte interdigitations are observed by day 7. Immunocytochemistry reveals enhanced organization and alignment of patterned myocyte and tenocyte features, as well as differential expression of multiple MTJ markers. On day 24, electrically stimulated junction myocytes demonstrate negative contractile strains, while positive tensile strains are exhibited by mechanically passive tenocytes at the junction. Unpatterned tenocytes distal to the junction experience significantly decreased strains in comparison to cells at the interface. Unpatterned myocytes have impaired organization and uncoordinated contractile behavior. These findings suggest that this platform is capable of inducing myocyte-tenocyte junction formation and mechanical coupling similar to the native MTJ, showing transduction of force across the cell-cell interface. STATEMENT OF SIGNIFICANCE: The myotendinous junction (MTJ) is an integrated structure that transduces force across the muscle-tendon boundary, making the region vulnerable to strain injury. Despite the clinical relevance, previous in vitro models of the MTJ lack the structure and mechanical accuracy of the native tissue and have difficulty transmitting force across the cell-cell interface. This study demonstrates an in vitro model of the MTJ, using spatially restrictive cues to inform human myocyte-tenocyte interactions and architecture. The model expressed MTJ markers and developed anisotropic myocyte-tenocyte integrations that resemble the native tissue and allow for force transduction from contracting myocytes to passive tenocyte regions. As such, this study presents a system capable of investigating development, injury, and pathology in the human MTJ.

Chondroitin sulfate is required for follicle epithelial integrity and organ shape maintenance in Drosophila

Heparan sulfate (HS) and chondroitin sulfate (CS) are evolutionarily conserved glycosaminoglycans that are found in most animal species, including the genetically tractable model organism Drosophila. In contrast to extensive in vivo studies elucidating co-receptor functions of Drosophila HS proteoglycans (PGs), only a limited number of studies have been conducted for those of CSPGs. To investigate the global function of CS in development, we generated mutants for Chondroitin sulfate synthase (Chsy), which encodes the Drosophila homolog of mammalian chondroitin synthase 1, a crucial CS biosynthetic enzyme. Our characterizations of the Chsy mutants indicated that a fraction survive to adult stage, which allowed us to analyze the morphology of the adult organs. In the ovary, Chsy mutants exhibited altered stiffness of the basement membrane and muscle dysfunction, leading to a gradual degradation of the gross organ structure as mutant animals aged. Our observations show that normal CS function is required for the maintenance of the structural integrity of the ECM and gross organ architecture.

Regulation of morphogen pathways by a Drosophila chondroitin sulfate proteoglycan Windpipe

Morphogens provide quantitative and robust signaling systems to achieve stereotypic patterning and morphogenesis. Heparan sulfate (HS) proteoglycans (HSPGs) are key components of such regulatory feedback networks. In Drosophila, HSPGs serve as co-receptors for a number of morphogens, including Hedgehog (Hh), Wingless (Wg), Decapentaplegic (Dpp) and Unpaired (Upd, or Upd1). Recently, Windpipe (Wdp), a chondroitin sulfate (CS) proteoglycan (CSPG), was found to negatively regulate Upd and Hh signaling. However, the roles of Wdp, and CSPGs in general, in morphogen signaling networks are poorly understood. We found that Wdp is a major CSPG with 4-O-sulfated CS in Drosophila. Overexpression of wdp modulates Dpp and Wg signaling, showing that it is a general regulator of HS-dependent pathways. Although wdp mutant phenotypes are mild in the presence of morphogen signaling buffering systems, this mutant in the absence of Sulf1 or Dally, molecular hubs of the feedback networks, produces high levels of synthetic lethality and various severe morphological phenotypes. Our study indicates a close functional relationship between HS and CS, and identifies the CSPG Wdp as a novel component in morphogen feedback pathways.

Developmental origin of tendon diversity in Drosophila melanogaster

Myogenesis is a developmental process that is largely conserved in both Drosophila and higher organisms. Consequently, the fruit fly is an excellent in vivo model for identifying the genes and mechanisms involved in muscle development. Moreover, there is growing evidence indicating that specific conserved genes and signaling pathways govern the formation of tissues that connect the muscles to the skeleton. In this review, we present an overview of the different stages of tendon development, from the specification of tendon progenitors to the assembly of a stable myotendinous junction across three different myogenic contexts in Drosophila: larval, flight and leg muscle development. We underline the different aspects of tendon cell specification and differentiation in embryo and during metamorphosis that result into tendon morphological and functional diversity.

LanB1 Cooperates With Kon-Tiki During Embryonic Muscle Migration in Drosophila

Muscle development is a multistep process that involves cell specification, myoblast fusion, myotube migration, and attachment to the tendons. In spite of great efforts trying to understand the basis of these events, little is known about the molecular mechanisms underlying myotube migration. Knowledge of the few molecular cues that guide this migration comes mainly from studies in Drosophila. The migratory process of Drosophila embryonic muscles involves a first phase of migration, where muscle progenitors migrate relative to each other, and a second phase, where myotubes migrate searching for their future attachment sites. During this phase, myotubes form extensive filopodia at their ends oriented preferentially toward their attachment sites. This myotube migration and the subsequent muscle attachment establishment are regulated by cell adhesion receptors, such as the conserved proteoglycan Kon-tiki/Perdido. Laminins have been shown to regulate the migratory behavior of many cell populations, but their role in myotube migration remains largely unexplored. Here, we show that laminins, previously implicated in muscle attachment, are indeed required for muscle migration to tendon cells. Furthermore, we find that laminins genetically interact with kon-tiki/perdido to control both myotube migration and attachment. All together, our results uncover a new role for the interaction between laminins and Kon-tiki/Perdido during Drosophila myogenesis. The identification of new players and molecular interactions underlying myotube migration broadens our understanding of muscle development and disease.

Methylmercury myotoxicity targets formation of the myotendinous junction

Methylmercury (MeHg) is a ubiquitous environmental contaminant and developmental toxicant known to cause a variety of persistent motor and cognitive deficits. While previous research has focused predominantly on neurotoxic MeHg effects, emerging evidence points to a myotoxic role whereby MeHg induces defects in muscle development and maintenance. A genome wide association study for developmental sensitivity to MeHg in Drosophila has revealed several conserved muscle morphogenesis candidate genes that function in an array of processes from myoblast migration and fusion to myotendinous junction (MTJ) formation and myofibrillogenesis. Here, we investigated candidates for a role in mediating MeHg disruption of muscle development by evaluating morphological and functional phenotypes of the indirect flight muscles (IFMs) in pupal and adult flies following 0, 5, 10, and 15 μM MeHg exposure via feeding at the larval stage. Developmental MeHg exposure induced a dose-dependent increase in muscle detachments (myospheres) within dorsal bundles of the IFMs, which paralleled reductions eclosion and adult flight behaviors. These effects were selectively phenocopied by altered expression of kon-tiki (kon), a chondroitin sulfate proteoglycan 4/NG2 homologue and a central component of MTJ formation. MeHg elevated kon transcript expression at a crucial window of IFM development and transgene overexpression of kon could also phenocopy myosphere phenotypes and eclosion and flight deficits. Finally, the myosphere phenotype resulting from 10 μM MeHg was partially rescued in a background of reduced kon expression using a targeted RNAi approach. Our findings implicate a component of the MTJ as a MeHg toxicity target which broaden the understanding of how motor deficits can emerge from early life MeHg exposure.

Under construction: The dynamic assembly, maintenance, and degradation of the cardiac sarcomere

The sarcomere is the basic contractile unit of striated muscle and is a highly ordered protein complex with the actin and myosin filaments at its core. Assembling the sarcomere constituents into this organized structure in development, and with muscle growth as new sarcomeres are built, is a complex process coordinated by numerous factors. Once assembled, the sarcomere requires constant maintenance as its continuous contraction is accompanied by elevated mechanical, thermal, and oxidative stress, which predispose proteins to misfolding and toxic aggregation. To prevent protein misfolding and maintain sarcomere integrity, the sarcomere is monitored by an assortment of protein quality control (PQC) mechanisms. The need for effective PQC is heightened in cardiomyocytes which are terminally differentiated and must survive for many years while preserving optimal mechanical output. To prevent toxic protein aggregation, molecular chaperones stabilize denatured sarcomere proteins and promote their refolding. However, when old and misfolded proteins cannot be salvaged by chaperones, they must be recycled via degradation pathways: the calpain and ubiquitin-proteasome systems, which operate under basal conditions, and the stress-responsive autophagy-lysosome pathway. Mutations to and deficiency of the molecular chaperones and associated factors charged with sarcomere maintenance commonly lead to sarcomere structural disarray and the progression of heart disease, highlighting the necessity of effective sarcomere PQC for maintaining cardiac function. This review focuses on the dynamic regulation of assembly and turnover at the sarcomere with an emphasis on the chaperones involved in these processes and describes the alterations to chaperones - through mutations and deficient expression - implicated in disease progression to heart failure.

Drosophila female fertility and juvenile hormone metabolism depends on the type of Wolbachia infection

Maternally inherited intracellular bacteria Wolbachia cause both parasitic and mutualistic effects on their numerous insect hosts, including manipulating the host reproductive system in order to increase the bacteria spreading in a host population, and increasing the host fitness. Here, we demonstrate that the type of Wolbachia infection determines the effect on Drosophila melanogaster egg production as a proxy for fecundity, and metabolism of juvenile hormone (JH), which acts as gonadotropin in adult insects. For this study, we used six D. melanogaster lineages carrying the nuclear background of interbred Bi90 lineage and cytoplasmic backgrounds with or without Wolbachia of different genotype variants. The wMelCS genotype of Wolbachia decreases egg production in infected D. melanogaster females in the beginning of oviposition and increases it later (from the sixth day after eclosion), whereas the wMelPop Wolbachia strain causes the opposite effect, and the wMel, wMel2 and wMel4 genotypes of Wolbachia do not show any effect on these traits compared with uninfected Bi90 D. melanogaster females. The intensity of JH catabolism negatively correlates with the fecundity level in the flies carrying both wMelCS and wMelPop Wolbachia The JH catabolism in females infected with genotypes of the wMel group does not differ from that in uninfected females. The effects of wMelCS and wMelPop infection on egg production can be levelled by the modulation of JH titre (via precocene/JH treatment of the flies). Thus, at least one of the mechanisms promoting the effect of Wolbachia on D. melanogaster female fecundity is mediated by JH.

Structural deciphering of the NG2/CSPG4 proteoglycan multifunctionality

The chondroitin sulfate proteoglycan 4 ( CSPG4) gene encodes a transmembrane proteoglycan (PG) constituting the largest and most structurally complex macromolecule of the human surfaceome. Its transcript shows an extensive evolutionary conservation and, due to the elaborated intracellular processing of the translated protein, it generates an array of glycoforms with the potential to exert variant-specific functions. CSPG4-mediated molecular events are articulated through the interaction with more than 40 putative ligands and the concurrent involvement of the ectodomain and cytoplasmic tail. Alternating inside-out and outside-in signal transductions may thereby be elicited through a tight functional connection of the PG with the cytoskeleton and its regulators. The potential of CSPG4 to influence both types of signaling mechanisms is also asserted by its lateral mobility along the plasma membrane and its intersection with microdomain-restricted internalization and endocytic trafficking. Owing to the multitude of molecular interplays that CSPG4 may engage, and thanks to a differential phosphorylation of its intracellular domain accounted by crosstalking signaling pathways, the PG stands out for its unique capability to affect numerous cellular phenomena, including those purporting pathologic conditions. We discuss here the progresses made in advancing our understanding about the structural-functional bases for the ability of CSPG4 to widely impact on cell behavior, such as to highlight how its multivalency may be exploited to interfere with disease progression.-Tamburini, E., Dallatomasina, A., Quartararo, J., Cortelazzi, B., Mangieri, D., Lazzaretti, M., Perris, R. Structural deciphering of the NG2/CSPG4 proteoglycan multifunctionality.

Nanotopography-responsive myotube alignment and orientation as a sensitive phenotypic biomarker for Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is a fatal genetic disorder currently having no cure. Here we report that culture substrates patterned with nanogrooves and functionalized with Matrigel (or laminin) present an engineered cell microenvironment to allow myotubes derived from non-diseased, less-affected DMD, and severely-affected DMD human induced pluripotent stem cells (hiPSCs) to exhibit prominent differences in alignment and orientation, providing a sensitive phenotypic biomarker to potentially facilitate DMD drug development and early diagnosis. We discovered that myotubes differentiated from myogenic progenitors derived from non-diseased hiPSCs align nearly perpendicular to nanogrooves, a phenomenon not reported previously. We further found that myotubes derived from hiPSCs of a dystrophin-null DMD patient orient randomly, and those from hiPSCs of a patient carrying partially functional dystrophin align approximately 14° off the alignment direction of non-diseased myotubes. Substrates engineered with micron-scale grooves and/or cell adhesion molecules only interacting with integrins all guide parallel myotube alignment to grooves and lose the ability to distinguish different cell types. Disruption of the interaction between the Dystrophin-Associated-Protein-Complex (DAPC) and laminin by heparin or anti-α-dystroglycan antibody IIH6 disenables myotubes to align perpendicular to nanogrooves, suggesting that this phenotype is controlled by the DAPC-mediated cytoskeleton-extracellular matrix linkage.

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.

Tay bridge and extracellular-regulated kinase activity are required for motoneuron function in the Drosophila neural system

Extracellular regulated kinase (Erk) activity is required during neural development for the specification of cell fates in neuroblasts and neuronal lineages, and also regulates several aspects of the activity and survival of mature neurons. The activation of Erk is regulated at multiple levels by kinases and phosphatases that alter its phosphorylation state and by other proteins that regulate its subcellular localization. Here, we find that tay bridge (tay), a negative regulator of Erk in Drosophila imaginal discs, is required in the motoneurons to regulate the number and size of neuromuscular synapses in these cells. The expression of Tay is maximal in motoneurons with low levels of activated ERK, suggesting that Tay modulates the activity of Erk in these cells. We also found that loss of tay expression and increased Erk activity specifically in the motoneurons cause a reversible decrease in walking speed. Impaired motoneurons activity may be caused by alterations in the functionality and number of synaptic boutons developing at the neuromuscular junction in tay mutants.

Mechanical Control of Myotendinous Junction Formation and Tendon Differentiation during Development

The development of the musculoskeletal system is a great model to study the interplay between chemical and mechanical inter-tissue signaling in cell adhesion, tissue morphogenesis and differentiation. In both vertebrates and invertebrates (e.g., Drosophila melanogaster) the formation of muscle-tendon interaction generates mechanical forces which are required for myotendinous junction maturation and tissue differentiation. In addition, these forces must be withstood by muscles and tendons in order to prevent detachment from each other, deformation or even losing their integrity. Extracellular matrix remodeling at the myotendinous junction is key to resist mechanical load generated by muscle contraction. Recent evidences in vertebrates indicate that mechanical forces generated during junction formation regulate chemical signaling leading to extracellular matrix remodeling, however, the mechanotransduction mechanisms associated to this response remains elusive. In addition to extracellular matrix remodeling, the ability of Drosophila tendon-cells to bear mechanical load depends on rearrangement of tendon cell cytoskeleton, thus studying the molecular mechanisms involved in this process is critical to understand the contribution of mechanical forces to the development of the musculoskeletal system. Here, we review recent findings regarding the role of chemical and mechanical signaling in myotendinous junction formation and tendon differentiation, and discuss molecular mechanisms of mechanotransduction that may allow tendon cells to withstand mechanical load during development of the musculoskeletal system.

Kon-tiki/Perdido enhances PS2 integrin adhesion and localizes its ligand, Thrombospondin, in the myotendinous junction

Cell-extracellular matrix adhesion is mediated by cell receptors, mainly integrins and transmembrane proteoglycans, which can functionally interact. How these receptors are regulated and coordinated is largely unknown and key to understand cell adhesion in development. We show that the conserved transmembrane proteoglycan Kon-tiki/Perdido (Kon) interacts with αPS2βPS integrin to mediate muscle-tendon adhesion. Double mutant embryos for kon and inflated show a synergistic increase in muscle detachment. Furthermore, Kon modulates αPS2βPS signaling at the muscle attachment, since P-Fak is reduced in kon mutants. This reduction in integrin signaling can be rescued by the expression of a truncated Kon protein containing the transmembrane and extracellular domains, suggesting that these domains are sufficient to mediate this signaling. We show that these domains are sufficient to properly localize the αPS2βPS ligand, Thrombospondin, to the muscle attachment, and to partially rescue Kon dependent muscle-tendon adhesion. We propose that Kon can engage in a protein complex with αPS2βPS and enhance integrin-mediated signaling and adhesion by recruiting its ligand, which would increase integrin-binding affinity to the extracellular matrix, resulting in the consolidation of the myotendinous junction.

Proteoglycan form and function: A comprehensive nomenclature of proteoglycans

We provide a comprehensive classification of the proteoglycan gene families and respective protein cores. This updated nomenclature is based on three criteria: Cellular and subcellular location, overall gene/protein homology, and the utilization of specific protein modules within their respective protein cores. These three signatures were utilized to design four major classes of proteoglycans with distinct forms and functions: the intracellular, cell-surface, pericellular and extracellular proteoglycans. The proposed nomenclature encompasses forty-three distinct proteoglycan-encoding genes and many alternatively-spliced variants. The biological functions of these four proteoglycan families are critically assessed in development, cancer and angiogenesis, and in various acquired and genetic diseases where their expression is aberrant.

Ectopic expression screen identifies genes affecting Drosophila mesoderm development including the HSPG Trol

Gastrulation of the embryo involves coordinate cell movements likely supported by multiple signaling pathways, adhesion molecules, and extracellular matrix components. Fibroblast growth factors (FGFs) have a major role in Drosophila melanogaster mesoderm migration; however, few other inputs are known and the mechanism supporting cell movement is unclear. To provide insight, we performed an ectopic expression screen to identify secreted or membrane-associated molecules that act to support mesoderm migration. Twenty-four UAS insertions were identified that cause lethality when expressed in either the mesoderm (Twi-Gal4) or the ectoderm (69B-Gal4). The list was narrowed to a subset of 10 genes that were shown to exhibit loss-of-function mutant phenotypes specifically affecting mesoderm migration. These include the FGF ligand Pyramus, α-integrins, E-cadherin, Cueball, EGFR, JAK/STAT signaling components, as well as the heparan sulfate proteoglycan (HSPG) Terribly reduced optic lobes (Trol). Trol encodes the ortholog of mammalian HSPG Perlecan, a demonstrated FGF signaling cofactor. Here, we examine the role of Trol in Drosophila mesoderm migration and compare and contrast its role with that of Syndecan (Sdc), another HSPG previously implicated in this process. Embryos mutant for Trol or Sdc were obtained and analyzed. Our data support the view that both HSPGs function to support FGF-dependent processes in the early embryo as they share phenotypes with FGF mutants: Trol in terms of effects on mesoderm migration and caudal visceral mesoderm (CVM) migration and Sdc in terms of dorsal mesoderm specification. The differential roles uncovered for these two HSPGs suggest that HSPG cofactor choice may modify FGF-signaling outputs.