InteractiveFly: GeneBrief

obstructor-A: Biological Overview | References


Gene name - obstructor-A

Synonyms -

Cytological map position - 19C1-19C1

Function - extracellular scaffold protein

Keywords - scaffold-like protein - binds chitin - recruits proteins for chitin-matrix growth - modulates localization of proteins at the ECM - controls tracheal tube diameter - affects cuticle stiffness during wound repair - promotes morphogenesis and axonal growth in the prothoracic gland - regulates molting

Symbol - obst-A

FlyBase ID: FBgn0031097

Genetic map position - chrX:20,239,855-20,242,539

Classification - Chitin binding Peritrophin-A domain

Cellular location - secreted



NCBI links: EntrezGene, Nucleotide, Protein
BIOLOGICAL OVERVIEW

Identification of signals for systemic adaption of hormonal regulation would help to understand the crosstalk between cells and environmental cues contributing to growth, metabolic homeostasis and development. Physiological states are controlled by precise pulsatile hormonal release, including endocrine steroids in human and ecdysteroids in insects. This study shows in Drosophila that regulation of genes that control biosynthesis and signaling of the steroid hormone ecdysone, a central regulator of developmental progress, depends on the extracellular matrix protein Obstructor-A (Obst-A). Ecdysone is produced by the prothoracic gland (PG), where sensory neurons projecting axons from the brain integrate stimuli for endocrine control. By defining the extracellular surface, Obst-A promotes morphogenesis and axonal growth in the PG. This process requires Obst-A-matrix reorganization by Clathrin/Wurst-mediated endocytosis. Wurst (Wus) is a transmembrane protein essential for clathrin-mediated endocytosis. These data identifies the extracellular matrix as essential for endocrine ring gland function, which coordinates physiology, axon morphogenesis, and developmental programs. As Obst-A and Wurst homologs are found among all arthropods, it is proposed that this mechanism is evolutionary conserved (Pesch, 2018).

Steroid hormones are small, lipophilic compounds that can pass through cell membranes and constitute important regulators of growth, metabolism, and reproduction. To coordinate steroid production neuronal and endocrine systems need to integrate external and internal cues in the vertebrate hypothalamic-pituitary system. Furthermore, reciprocal interactions between nerves and glands maintain homeostasis and allow for responses to environmental stimuli. As a response to stimulatory and inhibitory signals coming from the hypothalamus, pituitary gland cells synthesize and secrete a variety of specific pituitary trophic hormones, such as ACTH (adrenocorticotropic hormone), Thyroid stimulating hormone (TSH), and Follicle stimulating hormone (FSH), in a pulsatile and episodic manner. Any malfunctions in the interactions between nerves and the pituitary gland can lead to multiple endocrine disorders, neurological manifestations and has substantial impact on metabolism, sexual maturation, reproduction, blood pressure and other vital physical functions (Pesch, 2018).

In Drosophila distinct neurosecretory cells from the brain stimulate hormone responses in the endocrine ring gland. Most prominent Prothoracicotropic hormone (PTTH) expressing neurosecretory cells project axons from the brain to directly stimulate ecdysone steroid production. In addition, other neuroendocrine centers in the brain send axons through the nervus corporis cardiaci (Ncc) NccI and NccII towards the ring gland. The ring gland itself is formed by the two prothoracic gland (PG, ventrally) lobes, corpora allata (CA, dorsally), and corpora cardiaca (CC, laterally). The CC cells affect metabolism by secreting Adipokinetic Hormone (AKH) for manipulating sugar levels in the hemolymph, as well as glycemic factors and heart rate accelerating peptides/hormones. Drosophila that lack the CA, which is the source of JH, pupariated at smaller size due to reduced larval growth rates and were lethal at pupal stages. Most of the ring gland volume is taken up by the PGs, whose cells grow in size, while CC cells remain small. PG cells produce and secrete the steroid hormone ecdysone. Thus, insect development and metamorphosis is coordinately controlled by the sesquiterpenoid juvenile hormones (JH) and the molting hormone 20-hydroxyecdysone (20E) (Pesch, 2018).

Ecdysone biosynthesis relies on dietary cholesterol which is processed by a number of enzymes. Npc1a protein, named after the Niemann-Pick type C disease, is required for providing cholesterol as substrate for ecdysone biosynthesis. In the PGs sterols are then converted into Ecdysone by subsequent modifications regulated by a number of ecdysteroidogenic enzymes encoded by neverland and the Halloween genes such as spook, shroud and phantom. Secreted from PGs, Ecdysone is released into the circulatory system and converted into the more active 20 hydroxyecdysone (20E) by the P450 (Halloween) enzyme Shade. 20E finally binds and controls the activity of the nuclear ecdysone receptor complex to initiate transcription of many target genes that precisely regulate larval molting and metamorphosis. The activated EcR/Usp receptor complex binds genomic response elements to initiate transcription of early response target genes, of which E74A is a prominent representative. The Cyp18a1 enzyme is involved in the termination of ecdysone pulses by feedback mechanism. Finally, ecdysis-triggering hormones (ETH) trigger centrally patterned ecdysis and accompanied behaviors by induction of the eclosion hormone (EH) and its release from neurosecretory cells (Pesch, 2018).

Obstructor (Obst)-A belongs to the Drosophila obstructor multigene family which is well-conserved among arthropods. The Obst-A protein is characterized by three Chitin-binding domains type 2 (CBDs), a specific domain composition that has been identified even in nematodes Obst-A is expressed in ectodermal epithelia and secreted towards the extracellular space where it is located at the chitinous matrix. Acting as a scaffold-like protein Obst-A binds chitin and recruits other proteins and enzymes for chitin-matrix growth. Obst-A modulates localization of proteins and enzymes at the extracellular matrix that in turn control physical and chemical properties. Thereby Obst-A affects cuticle stiffness during wound repair and integrity against numerous stresses. This study reports surprising evidence that Obst-A is needed for proper ring gland morphogenesis. Immunofluorescent studies show that Obst-A defines the extracellular matrix (ECM) of PG cells. Mutant studies further confirm that the extracellular Obst-A-matrix is required for upregulation of RNA levels of ecdysteroidogenic enzyme genes for the PG in the onset of ecdysis. Genetic studies indicate further that normal axon growth at the PG surface depends on the Obst-A defined PG matrix. This study shows that axons in obst-A null mutants and PG specific knockdown embryos are prevented from normal growth at the PG. This would be consistent with the fact that not only chemical signals but also substrate properties, such as stiffness can determine axon growth. Finally, extracellular Obst-A localization and its functional consequences for PG cells depends on its internalization via Wurst/Clathrin-mediated endocytosis. Since both Obst-A and Wurst (orthologous to vertebrate DNAJC22) proteins are largely conserved, their roles in steroid control is potentially relevant to all arthropods (Pesch, 2018).

Loss of obst-A and the PG-specific obst-A knockdown caused a range of phenotypes characteristic of ecdysone deficiency mutants. A PG specific requirement of Obst-A was further confirmed by the fact that CA (Aug21-Gal4) and CC (akh-Gal4 driven) specific obst-A knockdown did not result in ring gland defects, retained cuticles or larval lethality. Mutant studies show the requirement of Obst-A at the PG cell surface for axon formation and the pulsatile up-regulation of genes involved in ecdysone machinery. Thus, this work demonstrates the importance of the PG specific extracellular matrix for ring gland morphology and physiology during late embryonic and early larval development (Pesch, 2018).

A recent study provided evidence that the circadian clock is a key driver of steroid hormone production. Steroidogenic genes, appeared selectively expressed at night and day in the third instar ring gland. It was shown that Halloween gene expression was dependent on Timeless, which couples the circadian machinery directly to steroid synthesis. Interestingly and comparable with Halloween genes, obst-A expression is under control of circadian rhythm depending on timeless and period in the PG cells (Pesch, 2018).

obst-A mutant analysis shows that most genes representative of ecdysteroidogenesis are prevented from regular pulse-like up-regulation prior to larval ecdysis. This includes even the primary ecdysone-inducible transcription factor E74A. The findings are consistent with the observation of elevated npc1a but reduced cyp18a expression, suggesting that animals could try to raise ecdysone levels in obst-A mutants at the end of first instar development trying to restore the ability to molt. In the same context, application of the active 20E to first instar larvae had beneficial effect on survival when obst-A knocked down or completely absent, indicating that ecdysteroid production was prevented in the mutants. Although ecdysteroid application was beneficial, it did not rescue lethality to adulthood as was found for genes exclusively involved in ecdysone production. Thus, on one hand partial rescue proves ecdysteroid deficiency caused by knockdown or loss of obst-A in the PG, on the other hand it shows that larvae suffer from severe defects in the tracheal and epidermal cuticles. Obst-A is not maternally contributed (Petkau, 2012) and ring gland Obst-A expression starts at late embryonic stage 16, indicating that Ecdysone signaling in earlier embryos cannot be affected by Obst-A. By contrast, at molt from first to second instar obst-A mutants most likely become arrested when 20E levels rise. This would be consistent with data showing reduced gene expression of ecdysteroidogenic factors in obst-A mutants (Pesch, 2018).

At the cell surface Obst-A provides the capacity for binding chitin and associated proteins that modulate the chitin-matrix. Obst-A controls the proper localization of chitin-deacetylases Serpentine (Serp) and Vermiform (Verm) and the chitin protector Knickkopf (Knk). The failure in forming a normal chitin-matrix in obst-A mutants disturbs tracheal and epidermal cuticle integrity and stiffness. Larvae lacking obst-A display wrinkled trachea and epidermis, as well as deformed body shape (Pesch, 2015; Petkau, 2012; Tiklova, 2013). However, lethality at larval transition, growth arrest, and retained cuticles found in obst-A mutant is typical for defects in the ecdysone pathway. Consistent with this, a restricted Obst-A requirement in the cuticle would not explain why gene expression of serp, verm and knk failed to be up-regulated in obst-A mutant larvae in the onset of ecdysis (Pesch, 2015). In addition, none of these gene products were detected in the ring gland. This altogether provides evidence for a PG specific but chitin-independent function of the Obst-A at the ring gland. Despite its tracheal expression Serp is secreted by the fat body, transported via hemolymph and taken up by tracheal cells. Thus, levels of free Serp molecules in the hemolymph could reflect the current status of chitin-matrix maturation and accompanied cuticle formation. Whether Obst-A binds Serp also at the PG surface was not addressed, but it would be a potential mechanisms to precisely sense the current status of developmental progress (Pesch, 2018).

The ECM provides a scaffold for cellular support and mediates many processes including signaling during morphogenesis and tissue homeostasis. Tissue stiffness enhances matrix-directed differentiation for example through nuclear Lamin-A to enhance tissue specific differentiation. Local tissue stiffness is critically involved in instructing neuronal growth, and softening of tissue leads to aberrant axon growth. Axons in softened brains dispersed from their normal trajectory showing reduced directionality, while axons grown on stiff substrates were longer than those on soft substrates (Koser, 2016). These studies did not address the influences of Obst-A on PG matrix stiffness. But in analogy to its role in the cuticle (Petkau, 2012), where it modulates the chitin-matrix-properties, Obst-A may also influence extracellular matrix properties at PG cell surfaces in late embryos at a time when axons need to grow at the surfaces of the forming ring gland. It is speculated that the lack of Obst-A may alter ECM composition, leading to local changes in matrix properties at the PG surfaces contributing to normal axon growth at the ring gland. This is consistent to the findings that inhibition of endocytosis led to phenotypes that were similar to PG specific obst-A knockdown, including larval lethality and lack of Fas2 axon-like structures at the PG. Thus, depending on axonal growth progress at the PG surface, local ECM properties underlie dynamic changes via endocytosis of factors, such as Obst-A, that may contribute to matrix stiffness. Since a direct affect was excluded by a general Obst-A overexpression in the PG cells, this model would explain the observed changes in the regular pattern of Fas2 axon-like structures in obst-A mutants, PG specific obst-A knockdown, and PG specific endocytic mutants (Pesch, 2018).

This study provides evidence that genetic regulation of ecdysone production in PG cells is under control of a specific ECM. A chitin-binding protein, Obst-A, defines the surface of PG cells, thereby controlling larval survival, molting and growth. In addition to its role in the cuticle of epithelial organs, Obst-A supports function of PGs. By modulating the cell matrix Obst-A essentially contributes to axonal growth at the PG. Importantly, local matrix properties depend on Obst-A internalization by Wurst/Clathrin dependent endocytosis. Collectively, Obst-A provides a new link between the endocrine system, nervous system, and developmental growth control in insects and, due to evolutionary conservation of the obstructor gene family, potentially also in other arthropod species (Pesch, 2018).

rebuff regulates apical luminal matrix to control tube size in Drosophila trachea

The Drosophila embryonic tracheal network is an excellent model to study tube size. The chitin-based apical luminal matrix and cell polarity are well known to regulate tube size in Drosophila trachea. Defects in luminal matrix and cell polarity lead to tube overexpansion. This study addressed the novel function of the rebuff (reb) gene, which encodes an evolutionarily conserved Smad-like protein. In reb mutants, tracheal tubes are moderately over-elongated. Despite the establishment of normal cell polarity, significantly reduced apical luminal matrix was observed in reb mutants. Among various luminal components, luminal Obstructor-A (ObstA) is drastically reduced. Interestingly, ObstA is localized in vesicle-like structures that are apically concentrated in reb mutants. To investigate the possibility that reb is involved in the endocytosis of ObstA, the co-localization of ObstA and endocytic markers was examined in reb mutants. It was observed that ObstA is localized in late endosomes and recycling endosomes. This suggests that in reb mutant trachea, endocytosed ObstA is degraded or recycled back to the apical region. However, ObstA vesicles are retained in the apical region and are failed to be secreted to the lumen. Taken together, these results suggest one function of reb is regulating the endocytosis of luminal matrix components (Chandran, 2018).

Obstructor-A organizes matrix assembly at the apical cell surface to promote enzymatic cuticle maturation in Drosophila

Assembly and maturation of the apical extracellular matrix (aECM) is crucial for protecting organisms, but underlying molecular mechanisms remain poorly understood. Epidermal cells secrete proteins and enzymes that assemble at the apical cell surface to provide epithelial integrity and stability during developmental growth and upon tissue damage. This study analyzed molecular mechanisms of aECM assembly and identified the conserved chitin-binding protein Obstructor (Obst)-A as an essential regulator. In Drosophila Obst-A was shown to be required to coordinate protein- and chitin-matrix packaging at the apical cell surface during development. Secreted by epidermal cells, the Obst-A protein was specifically enriched in the apical assembly zone where matrix components are packaged into their highly ordered architecture. In obst-A null mutant larvae, the assembly zone was strongly diminished resulting in severe disturbance of matrix-scaffold organization and impaired aECM integrity. Furthermore, enzymes that support aECM stability were mislocalized. As a biological consequence, cuticle architecture, integrity and function were disturbed in obst-A mutants finally resulting in immediate lethality upon wounding. These studies identify a new core-organizing center, the assembly zone that controls aECM assembly at the apical cell surface. They propose a genetically conserved molecular mechanism by which Obst-A forms a matrix-scaffold to coordinate trafficking and localization of proteins and enzymes in the newly deposited aECM. This mechanism is essential for maturation and stabilization of the aECM in a growing and remodeling epithelial tissue as an outermost barrier (Pesch, 2015).

Molting, wounding, and other types of cuticle disruption require complex actions of enzymes and scaffold proteins that form new and remodel existing cuticles throughout development. To some extent, chitin synthases and chitinolytic enzymes have been studied in the past, but little is known about underlying mechanisms that control the assembly, stability, and integrity of newly synthesized cuticles. The assembly zone is the first apical extracellular area where all cuticle components are deposited and the chitin matrix is packaged into the highly ordered procuticle. The assembly zone is a permeable matrix for components that process into the cuticle. Indeed, cuticle proteins and putative enzymes were identified to be part of the assembly zone; however, they have not been molecularly characterized (Pesch, 2015).

This study demonstrates that Obst-A plays a key role in organizing the chitin matrix. Normal chitin levels in obst-A null mutants exclude a role in chitin synthesis. Given that Obst-A is required for exoskeletal function at the epidermis, this study investigated the extracellular region where Obst-A organizes the chitin matrix. Indeed, confocal, ultrastructure, and mutant analyses identify Obst-A as a chitin-binding protein essential for assembly zone formation. Moreover, Obst-A and its partner proteins are required for larval cuticle stability. In addition, the observations about obst-A expression pattern are in line with recent data showing that the loss of obst-A results in severe molting defects and lethality shortly after ecdysis to second instar larval stage. Collectively, Obst-A is an essential regulator at the apical cell surfaces coordinating chitin matrix formation and thereby promoting epidermal cuticle integrity (Pesch, 2015).

Cuticle defects in obst-A mutants could implicate impaired protection of newly synthesized cuticle. In the beetle Tribolium castaneum, the Knk protein is required for protection of newly synthesized chitin matrix. In the embryonic tracheal matrix, Obst-A maintains extracellular Knk localization, which prevents premature degradation of the cuticle during tracheal tube size control. A few hours later at the end of embryogenesis, the dispensable intraluminal chitin matrix becomes cleared by clathrin-mediated endocytosis, resulting in a rather thin apical chitin-rich cuticle. This study found that epidermal extracellular Knk co-localization with chitin was not affected in obst-A mutants, and conversely weak apical Obst-A enrichment was found in knk knockdown larvae. However, knk gene and protein levels appeared reduced in the obst-A mutants at a time when wild type larvae start to molt. This suggests that chitin matrix protection must be affected. The current findings may not exclude other potential protective proteins, and yet uncharacterized Drosophila Knk-like proteins could depend on Obst-A. However, the wrinkled cuticle specifically observed in obst-A null and transheterozygous obst-A;knk mutants and the localization of Obst-A and Knk proteins in the outer cuticle led to the speculative hypothesis that they act in cuticle stability of the newly synthesized and packaged chitin matrix. In summary, these findings might further suggest that aspects of obst-A mutant cuticle phenotypes could be the result of mistimed or ectopic degradation of chitin in the exoskeleton (Pesch, 2015).

The procuticle is capable of extending throughout larval development, whereas its integrity remains stable. Previous data about Obst-A binding with chitin and ultrastructure observations suggest that Obst-A could play a role in coordinating chitin scaffold formation. This would be consistent with gene expression data, protein localization, and genetic studies that propose a genetic link of obst-A with serp and verm in organizing maturation of the epidermal cuticle throughout larval development. The data further suggest that Obst-A proteins may recruit chitin fibrils at the apical cell surface to organize their packaging and maturation into a more compact procuticle. In addition, it was observed Obst-A scattering along the stratified chitin lamellae in third instar larval cuticle. Therefore, it is also possible that Obst-A is involved in providing stability of the outer epidermal cuticle (Pesch, 2015).

The zona pellucida protein Piopio has been shown to provide a structural network in trachea that may link the apical epithelial cell surface with the overlaying aECM at the epidermis. In addition, Alas (δ-aminolevulinate synthase), expressed in the hepatocyte-like oenocytes, is involved in formation of a dityrosine network at the apical cell surface to resist hydrostatic pressure of the hemolymph and to prevent dehydration. Local detachment between cell surface and cuticle observed in piopio and alas mutants was also identified in obst-A null mutants. Interestingly, cuticle detachment was found in obst-A mutants, serp knockdown larvae and in the transheterozygous obst-A serp,verm mutants. Because of defective cuticle structure in obst-A mutants, it is speculated that cuticle formation at the apical cell surface, likely within the assembly zone, affects essential aspects of cell adhesion to the aECM (Pesch, 2015).

These data provide evidence that Obst-A is required for the assembly zone formation. Whether Obst-A may act in the elongation or ordering of chitin filaments or whether it prevents premature chitin fibril assembly remains elusive. However, the data propose that Obst-A provides a well structured chitin scaffold for deacetylation enzymes to mature and improve aECM stability and integrity. Furthermore, genetic studies suggest that Obst-A is linked to Knk-mediated protection of the newly synthesized cuticle (see Obst-A organizes the aECM at the apical cell surface). All insects establish compact and structured exoskeletal cuticles at their outermost body parts. The current data argue that structural similarities of the body wall cuticle and molecular conservation of involved proteins point toward a highly conserved mechanism of procuticle formation among chitinous invertebrates (Pesch, 2015).

Control of airway tube diameter and integrity by secreted chitin-binding proteins in Drosophila

The transporting function of many branched tubular networks like lungs and circulatory system depend on the sizes and shapes of their branches. Understanding the mechanisms of tube size control during organ development may offer new insights into a variety of human pathologies associated with stenoses or cystic dilations in tubular organs. This study presents the first secreted luminal proteins involved in tube diametric expansion in the Drosophila airways. obst-A and gasp are conserved among insect species and encode secreted proteins with chitin binding domains. The widely used tracheal marker 2A12, recognizes the Gasp protein. Analysis of obst-A and gasp single mutants and obst-A; gasp double mutant shows that both genes are primarily required for airway tube dilation. Similarly, Obst-A and Gasp control epidermal cuticle integrity and larval growth. The assembly of the apical chitinous matrix of the airway tubes is defective in gasp and obst-A mutants. The defects become exaggerated in double mutants indicating that the genes have partially redundant functions in chitin structure modification. The phenotypes in luminal chitin assembly in the airway tubes are accompanied by a corresponding reduction in tube diameter in the mutants. Conversely, overexpression of Obst-A and Gasp causes irregular tube expansion and interferes with tube maturation. These results suggest that the luminal levels of matrix binding proteins determine the extent of diametric growth. It is proposed that Obst-A and Gasp organize luminal matrix assembly, which in turn controls the apical shapes of adjacent cells during tube diameter expansion (Tiklova, 2013).

Obstructor-A is required for epithelial extracellular matrix dynamics, exoskeleton function, and tubulogenesis

The epidermis and internal tubular organs, such as gut and lungs, are exposed to a hostile environment. They form an extracellular matrix to provide epithelial integrity and to prevent contact with pathogens and toxins. In arthropods, the cuticle protects, shapes, and enables the functioning of organs. During development, cuticle matrix is shielded from premature degradation; however, underlying molecular mechanisms are poorly understood. Previous work has identified the conserved obstructor multigene-family, which encodes chitin-binding proteins. This study shows that Obstructor-A is required for extracellular matrix dynamics in cuticle forming organs. Loss of obstructor-A causes severe defects during cuticle molting, wound protection, tube expansion and larval growth control. Obstructor-A interacts and forms a core complex with the polysaccharide chitin, the cuticle modifier Knickkopf and the chitin deacetylase Serpentine. Knickkopf protects chitin from chitinase-dependent degradation and deacetylase enzymes ensure extracellular matrix maturation. This study provides evidence that Obstructor-A is required to control the presence of Knickkopf and Serpentine in the extracellular matrix. A model is proposed suggesting that Obstructor-A coordinates the core complex for extracellular matrix protection from premature degradation. This mechanism enables exoskeletal molting, tube expansion, and epithelial integrity. The evolutionary conservation suggests a common role of Obstructor-A and homologs in coordinating extracellular matrix protection in epithelial tissues of chitinous invertebrates (Petkau, 2012).

This study has analyzed the molecular process of chitin ECM dynamics in Drosophila. These studies demonstrate the specific requirement of Obst-A for chitin ECM protection. Obst-A binds and co-localizes with chitin in cuticle forming organs. Additionally, Obst-A interacts with Serp and Knk forming a core complex with chitin. Serp preserves the cuticle structure and Knk organizes chitin and protects it from chitinases. Mutant studies demonstrate that Obst-A is required to control Serp and Knk presence in the cuticle. The premature reduction of Serp and Knk in obst-A mutants was independent from chitin. Genetic studies further show that Obst-A and Serp may interact to control the Knk localization for chitin protection. It is proposed that Obst-A is required to form a core complex in the chitin ECM that coordinates Knk presence for protecting chitin from premature degradation. Consistent with previous observations, these findings suggest that Obst-A coordinated chitin ECM protection is required for tracheal tube expansion and presumably for cuticle function during development (Petkau, 2012).

ECM is involved in organizing proper tube expansion which is vital for transport of liquids and gases across the body. The organization of distinct branches and the tracheal epithelium are determinants of tube size. Furthermore, apical secretion of chitin matrix components into the forming lumina is involved in uniform tracheal tube expansion. Chitin modifications and filament structure are involved in tube length control. These studies demonstrate that chitin ECM protection organized by Obst-A at stage 16 is required to coordinate tube size. The lack of Obst-A resulted in tracheal overexpansion. Obst-A is expressed in tracheal cells and secreted towards the developing tube lumina where it co-localizes and interacts with the ECM core components chitin, Serp, and Knk. Additionally, obst-A-null mutants studies showed that apical matrix components started to degrade irregularly from tracheal tube lumina. These findings suggest that Obst-A is required to prevent a premature degradation of the luminal ECM thereby providing a matrix for determining final tube size at late embryogenesis (Petkau, 2012).

The cuticle is one of the most abundant ECM structures in nature. It is common among arthropods and serves as an exoskeleton throughout their lifetime that accommodates growth. It further provides a protective layer against mechanical, physical and chemical stress. The chitin ECM lines epithelial cells of the epidermis, spiracles, trachea, and parts of the digestive tract. The dynamic organization of ECM during cuticle molting in these organs is poorly understood. These studies demonstrate the importance of Obst-A for molting. Obst-A co-localized with core ECM components of epidermis, head skeleton, trachea, foregut, and posterior spiracles. Loss of obst-A resulted in severe cuticle integrity, structure, and molting defects in these organs. Additionally, obst-A mutants showed larval growth arrest and lethality. The impaired ECM protection in obst-A mutants resulted in a premature degradation of the luminal matrix of trachea. It is assume that Obst-A mediated ECM protection could also affect Obst-A expressing, cuticle forming organs, such as the epidermal exoskeleton. However, it remains elusive whether the defective ECM protection causes cuticle molting defects observed in the obst-A mutants. It is interesting to note that Serp and Knk are evolutionarily conserved. Importantly, Obst-A homologs were identified in arthropods and nematodes. This suggests that a similar mechanism may exist to control chitin ECM degradation among chitinous invertebrates (Petkau, 2012).


Functions of Obstructor family members in Drosophila

Mechanical control of whole body shape by a single cuticular protein Obstructor-E in Drosophila melanogaster

Body shapes are much more variable than body plans. One way to alter body shapes independently of body plans would be to mechanically deform bodies. To what extent body shapes are regulated physically, or molecules involved in physical control of morphogenesis, remain elusive. During fly metamorphosis, the cuticle (exoskeleton) covering the larval body contracts longitudinally and expands laterally to become the ellipsoidal pupal case (puparium). This study shows that Drosophila melanogaster Obstructor-E (Obst-E) is a protein constituent of the larval cuticle that confers the oriented contractility/expandability. Obst-E is a member of the obstructor multigene family encoding putative cuticular proteins with three type 2 chitin-binding domains (CBDs). In the absence of obst-E function, the larval cuticle fails to undergo metamorphic shape change and finally becomes a twiggy puparium. Results are presented indicating that Obst-E regulates the arrangement of chitin, a long-chain polysaccharide and a central component of the insect cuticle, and directs the formation of supracellular ridges on the larval cuticle. It was further shown that Obst-E is locally required for the oriented shape change of the cuticle during metamorphosis, which is associated with changes in the morphology of those ridges. Thus, Obst-E dramatically affects the body shape in a direct, physical manner by controlling the mechanical property of the exoskeleton (Tajiri, 2017).


REFERENCES

Search PubMed for articles about Drosophila Obstructor-A

Chandran, R. R., Scholl, A., Yang, Y. and Jiang, L. (2018). rebuff regulates apical luminal matrix to control tube size in Drosophila trachea. Biol Open 7(9). PubMed ID: 30185423

Koser, D. E., Thompson, A. J., Foster, S. K., Dwivedy, A., Pillai, E. K., Sheridan, G. K., Svoboda, H., Viana, M., Costa, L. D., Guck, J., Holt, C. E. and Franze, K. (2016). Mechanosensing is critical for axon growth in the developing brain. Nat Neurosci 19(12): 1592-1598. PubMed ID: 27643431

Pesch, Y. Y., Riedel, D. and Behr, M. (2015). Obstructor A organizes matrix assembly at the apical cell surface to promote enzymatic cuticle maturation in Drosophila. J Biol Chem 290(16): 10071-10082. PubMed ID: 25737451

Pesch, Y. Y., Hesse, R., Ali, T. and Behr, M. (2018). A cell surface protein controls endocrine ring gland morphogenesis and steroid production. Dev Biol 445(1):16-28. PubMed ID: 30367846

Petkau, G., Wingen, C., Jussen, L. C., Radtke, T. and Behr, M. (2012). Obstructor-A is required for epithelial extracellular matrix dynamics, exoskeleton function, and tubulogenesis. J Biol Chem 287(25): 21396-21405. PubMed ID: 22544743

Tajiri, R., Ogawa, N., Fujiwara, H. and Kojima, T. (2017). Mechanical control of whole body shape by a single cuticular protein Obstructor-E in Drosophila melanogaster. PLoS Genet 13(1): e1006548. PubMed ID: 28076349

Tiklova, K., Tsarouhas, V. and Samakovlis, C. (2013). Control of airway tube diameter and integrity by secreted chitin-binding proteins in Drosophila. PLoS One 8(6): e67415. PubMed ID: 23826295


Biological Overview

date revised: 21 January 2019

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