The muscle pattern of the Drosophila abdomen depends on a subdivision of the anterior compartment of each segment

Activin is a neural inducer of a male-specific muscle in Drosophila

Drosophila melanogaster has a pair of male-specific muscles called the muscle of Lawrence (MOL) in abdominal segment 5 (A5) of adult flies. The MOL is produced only when its innervating motoneuron expresses FruitlessM (FruM) neural masculinizing proteins. We show that MOL induction is hampered by: (1) silencing electrical activities in the motoneuron, (2) blocking vesicular release from the motoneuron, and (3) knocking down Activin ß (Actß) in the motoneuron or knocking down Actß signaling pathway components in the myoblasts. Our timelapse live imaging of the developing neuromuscular system reveals that, upon contact with the presumptive MOL, the motoneuronal axon retracts concomitant with the progression of MOL degeneration resulting from neural silencing. We conclude that MOL formation depends on the bidirectional trophic interactions between pre- and postsynaptic cells, with motoneuron-derived Actß playing an inducing role in MOL formation.

Mechanical tension and spontaneous muscle twitching precede the formation of cross-striated muscle in vivo

Muscle forces are produced by repeated stereotypical actomyosin units called sarcomeres. Sarcomeres are chained into linear myofibrils spanning the entire muscle fiber. In mammalian body muscles, myofibrils are aligned laterally, resulting in their typical cross-striated morphology. Despite this detailed textbook knowledge about the adult muscle structure, it is still unclear how cross-striated myofibrils are built in vivo. Here, we investigate the morphogenesis of Drosophila abdominal muscles and establish them as an in vivo model for cross-striated muscle development. By performing live imaging, we find that long immature myofibrils lacking a periodic actomyosin pattern are built simultaneously in the entire muscle fiber and then align laterally to give mature cross-striated myofibrils. Interestingly, laser micro-lesion experiments demonstrate that mechanical tension precedes the formation of the immature myofibrils. Moreover, these immature myofibrils do generate spontaneous Ca²⁺-dependent contractions in vivo, which, when chemically blocked, result in cross-striation defects. Taken together, these results suggest a myofibrillogenesis model in which mechanical tension and spontaneous muscle twitching synchronize the simultaneous self-organization of different sarcomeric protein complexes to build highly regular cross-striated myofibrils spanning the length of large muscle fibers.

Summary: In Drosophila, immature myofibrils are built simultaneously across an entire muscle fiber, and then self-organize in a manner dependent on spontaneous contractions and mechanical tension.

Amontillado is required for Drosophila Slit processing and for tendon-mediated muscle patterning

Slit cleavage into N-terminal and C-terminal polypeptides is essential for restricting the range of Slit activity. Although the Slit cleavage site has been characterized previously and is evolutionally conserved, the identity of the protease that cleaves Slit remains elusive. Our previous analysis indicated that Slit cleavage is essential to immobilize the active Slit-N at the tendon cell surfaces, mediating the arrest of muscle elongation. In an attempt to identify the protease required for Slit cleavage we performed an RNAi-based assay in the ectoderm and followed the process of elongation of the lateral transverse muscles toward tendon cells. The screen led to the identification of the Drosophila homolog of pheromone convertase 2 (PC2), Amontillado (Amon), as an essential protease for Slit cleavage. Further analysis indicated that Slit mobility on SDS polyacrylamide gel electrophoresis (SDS-PAGE) is slightly up-shifted in amon mutants, and its conventional cleavage into the Slit-N and Slit-C polypeptides is attenuated. Consistent with the requirement for amon to promote Slit cleavage and membrane immobilization of Slit-N, the muscle phenotype of amon mutant embryos was rescued by co-expressing a membrane-bound form of full-length Slit lacking the cleavage site and knocked into the slit locus. The identification of a novel protease component essential for Slit processing may represent an additional regulatory step in the Slit signaling pathway.

Summary: The Drosophila homolog of pheromone convertase 2 (PC2), amontillado (Amon), is shown to contribute to Slit processing and further cleavage into an N-Slit, essential for Slit activity in directing muscle patterning.

Insulin signaling controls neurotransmission via the 4eBP-dependent modification of the exocytotic machinery

Altered insulin signaling has been linked to widespread nervous system dysfunction including cognitive dysfunction, neuropathy and susceptibility to neurodegenerative disease. However, knowledge of the cellular mechanisms underlying the effects of insulin on neuronal function is incomplete. Here, we show that cell autonomous insulin signaling within the Drosophila CM9 motor neuron regulates the release of neurotransmitter via alteration of the synaptic vesicle fusion machinery. This effect of insulin utilizes the FOXO-dependent regulation of the thor gene, which encodes the Drosophila homologue of the eif-4e binding protein (4eBP). A critical target of this regulatory mechanism is Complexin, a synaptic protein known to regulate synaptic vesicle exocytosis. We find that the amounts of Complexin protein observed at the synapse is regulated by insulin and genetic manipulations of Complexin levels support the model that increased synaptic Complexin reduces neurotransmission in response to insulin signaling.

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

Muscle differentiation requires the assembly of high-order structures called myofibrils, composed of sarcomeres. Even though the molecular organization of sarcomeres is well known, the mechanisms underlying myofibrillogenesis are poorly understood. It has been proposed that integrin-dependent adhesion nucleates myofibrils at the periphery of the muscle cell to sustain sarcomere assembly. Here, we report a role for the gene perdido (perd, also known as kon-tiki, a transmembrane chondroitin proteoglycan) in myofibrillogenesis. Expression of perd RNAi in muscles, prior to adult myogenesis, can induce misorientation and detachment of Drosophila adult abdominal muscles. In comparison to controls, perd-depleted muscles contain fewer myofibrils, which are localized at the cell periphery. These myofibrils are detached from each other and display a defective sarcomeric structure. Our results demonstrate that the extracellular matrix receptor Perd has a specific role in the assembly of myofibrils and in sarcomeric organization. We suggest that Perd acts downstream or in parallel to integrins to enable the connection of nascent myofibrils to the Z-bands. Our work identifies the Drosophila adult abdominal muscles as a model to investigate in vivo the mechanisms behind myofibrillogenesis.

Methods for studying planar cell polarity

Planar cell polarity (PCP) is the polarity of epithelial cells in the plane orthogonal to the apical-basal axis, and is controlled by a partially defined signaling system. PCP related signaling also plays roles in cell migration, tissue re-organization and stem cell differentiation during embryonic development, and later, in regeneration and repair. Aberrant signaling has been linked to a broad range of pathophysiologies including cancer, developmental defects, and neurological disorders. The deepest mechanistic insights have come from studies of PCP in Drosophila. In this chapter we review tools and methods to study PCP signaling in Drosophila epithelia, where it was found to involve asymmetric protein localization that is coordinated between adjacent cells. Such signaling has been most extensively studied in wing, eye, and abdomen, but also in other tissues such as leg and notum. In the adult fly, PCP is manifested in the coordinated direction of hairs and bristles, as well as the organization of ommatidia in the eye. The polarity of these structures is preceded by asymmetric localization of PCP signaling proteins at the apical junctions of epithelial cells. Based on genetic and molecular criteria, the proteins that govern PCP can be divided into distinct modules, including the core module, the Fat/Dachsous/Four-jointed (Fat/Ds/Fj) module (often referred to as the 'global' module) as well as tissue specific effector modules. Different tissues and tissue regions differ in their sensitivity to disturbances in the various modules of the PCP signaling system, leading to controversies about the interactions among the modules, and emphasizing the value of studying PCP in multiple contexts. Here, we review methods including those generally applicable, as well as some that are selectively useful for analyses of PCP in eye (including eye discs), wing (including wing discs), pupal and adult abdomen, and the cuticle of larvae and embryos.

Plasticity of both planar cell polarity and cell identity during the development of Drosophila

Drosophila has helped us understand the genetic mechanisms of pattern formation. Particularly useful have been those organs in which different cell identities and polarities are displayed cell by cell in the cuticle and epidermis (Lawrence, 1992; Bejsovec and Wieschaus, 1993; Freeman, 1997). Here we use the pattern of larval denticles and muscle attachments and ask how this pattern is maintained and renewed over the larval moult cycles. During larval growth each epidermal cell increases manyfold in size but neither divides nor dies. We follow individuals from moult to moult, tracking marked cells and find that, as cells are repositioned and alter their neighbours, their identities change to compensate and the pattern is conserved. Single cells adopting a new fate may even acquire a new polarity: an identified cell that makes a forward-pointing denticle in the first larval stage may make a backward-pointing denticle in the second and third larval stages. DOI: http://dx.doi.org/10.7554/eLife.01569.001.

Ancestral patterning of tergite formation in a centipede suggests derived mode of trunk segmentation in trilobites

Trilobites have a rich and abundant fossil record, but little is known about the intrinsic mechanisms that orchestrate their body organization. To date, there is disagreement regarding the correspondence, or lack thereof, of the segmental units that constitute the trilobite trunk and their associated exoskeletal elements. The phylogenetic position of trilobites within total-group Euarthropoda, however, allows inferences about the underlying organization in these extinct taxa to be made, as some of the fundamental genetic processes for constructing the trunk segments are remarkably conserved among living arthropods. One example is the expression of the segment polarity gene engrailed, which at embryonic and early postembryonic stages is expressed in extant panarthropods (i.e. tardigrades, onychophorans, euarthropods) as transverse stripes that define the posteriormost region of each trunk segment. Due to its conservative morphology and allegedly primitive trunk tagmosis, we have utilized the centipede Strigamia maritima to study the correspondence between the expression of engrailed during late embryonic to postembryonic stages, and the development of the dorsal exoskeletal plates (i.e. tergites). The results corroborate the close correlation between the formation of the tergite borders and the dorsal expression of engrailed, and suggest that this association represents a symplesiomorphy within Euarthropoda. This correspondence between the genetic and phenetic levels enables making accurate inferences about the dorsoventral expression domains of engrailed in the trunk of exceptionally preserved trilobites and their close relatives, and is suggestive of the widespread occurrence of a distinct type of genetic segmental mismatch in these extinct arthropods. The metameric organization of the digestive tract in trilobites provides further support to this new interpretation. The wider evolutionary implications of these findings suggest the presence of a derived morphogenetic patterning mechanism responsible for the reiterated occurrence of different types of trunk dorsoventral segmental mismatch in several phylogenetically distant, extinct and extant, arthropod groups.

Loss of Drosophila A-type lamin C initially causes tendon abnormality including disintegration of cytoskeleton and nuclear lamina in muscular defects

Lamins are the major components of nuclear envelope architecture, being required for both the structural and informational roles of the nuclei. Mutations of lamins cause a spectrum of diseases in humans, including muscular dystrophy. We report here that the loss of the A-type lamin gene, lamin C in Drosophila resulted in pupal metamorphic lethality caused by tendon defects, matching the characteristics of human A-type lamin revealed by Emery-Dreifuss muscular dystrophy (EDMD). In tendon cells lacking lamin C activity, overall cell morphology was affected and organization of the spectraplakin family cytoskeletal protein Shortstop which is prominently expressed in tendon cells gradually disintegrated, notably around the nucleus and in a manner correlating well with the degradation of musculature. Furthermore, lamin C null mutants were efficiently rescued by restoring lamin C expression to shortstop-expressing cells, which include tendon cells but exclude skeletal muscle cells. Thus the critical function of A-type lamin C proteins in Drosophila musculature is to maintain proper function and morphology of tendon cells.