Developmental regulation of oocyte lipid intake through 'patent' follicular epithelium in Drosophila melanogaster

Bellymount-Pulsed Tracking: A Novel Approach for Real-Time In vivo Imaging of Drosophila Abdominal Tissues

Quantitative live imaging is a valuable tool that offers insights into cellular dynamics. However, many fundamental biological processes are incompatible with current live imaging modalities. Drosophila oogenesis is a well-studied system that has provided molecular insights into a range of cellular and developmental processes. The length of the oogenesis coupled with the requirement for inputs from multiple tissues has made long-term culture challenging. Here, we have developed Bellymount-Pulsed Tracking (Bellymount-PT), which allows continuous, non-invasive live imaging of Drosophila oogenesis inside the female abdomen for up to 16 hours. Bellymount-PT improves upon the existing Bellymount technique by adding pulsed anesthesia with periods of feeding that support the long-term survival of flies during imaging. Using Bellymount-PT we measure key events of oogenesis including egg chamber growth, yolk uptake, and transfer of specific proteins to the oocyte during nurse cell dumping with high spatiotemporal precision within the abdomen of a live female.

Finishing the egg

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Plant secondary metabolites as potential bioinsecticides? Study of the effects of plant-derived volatile organic compounds on the reproduction and behaviour of the pest beetle Tenebrio molitor

Modern agriculture has many environmental consequences, such as soil contamination, accumulation of toxic compounds in the environment or risk of adverse effects on nontarget organisms and for these reasons, scientists are seeking a more environmentally friendly alternative to synthetic insecticides. This study investigated the effects of four plant secondary metabolites classified as volatile organic compounds (VOCs), which have potential as bioinsecticides, (E)-2-decenal, furfural, 2-undecanone and (E,E)-2-4-decadienal, in concentrations 10^-5 and 10^-7 M, on female reproductive processes and larval hatchability of the Tenebrio molitor beetle. Our study indicates proper development of ovaries after application of compounds however the volume of terminal oocytes was significantly reduced, with the strongest effect of (E)- 2-decenal which reduced the volume approximately three times. The relative vitellogenin expression level was reduced, with the strongest effect observed after application of furfural, (E,E)- 2-4-decadienal and (E)- 2-decenal in concentration 10^-7 M, at the same time patency index was significantly reduced up to 2-times after application of furfural at 10^-7 M. What is more important morphological changes translated into physiological ones. The number of laid eggs was affected, with the strongest inhibition after application of furfural (∼43% reduction), (E,E)- 2-4-decadienal (∼33%) and (E)- 2-decenal at concentration 10^-7 M (∼33%). Moreover, we observed up to 13% (in case of 2-undecanone) decrease in larval hatchability. Tested compounds exhibited a repellent effect and caused 60% reduction of insect survivability after (E)- 2-decenal at concentration 10^-5 M. Altogether, VOCs seems like potential bioactive compounds in plant protection.

Actomyosin contractility in olfactory placode neurons opens the skin epithelium to form the zebrafish nostril

Despite their barrier function, epithelia can locally lose their integrity to create physiological openings during morphogenesis. The mechanisms driving the formation of these epithelial breaks are only starting to be investigated. Here, we study the formation of the zebrafish nostril (the olfactory orifice), which opens in the skin epithelium to expose the olfactory neurons to external odorant cues. Combining live imaging, drug treatments, laser ablation, and tissue-specific functional perturbations, we characterize a mechanical interplay between olfactory placode neurons and the skin, which plays a crucial role in the formation of the orifice: the neurons pull on the overlying skin cells in an actomyosin-dependent manner which, in combination with a local reorganization of the skin epithelium, triggers the opening of the orifice. This work identifies an original mechanism to break an epithelial sheet, in which an adjacent group of cells mechanically assists the epithelium to induce its local rupture.

Rational engineering of a synthetic insect-bacterial mutualism

Many insects maintain mutualistic associations with bacterial endosymbionts, but little is known about how they originate in nature. In this study, we describe the establishment and manipulation of a synthetic insect-bacterial symbiosis in a weevil host. Following egg injection, the nascent symbiont colonized many tissues, including prototypical somatic and germinal bacteriomes, yielding maternal transmission over many generations. We then engineered the nascent symbiont to overproduce the aromatic amino acids tyrosine and phenylalanine, which facilitate weevil cuticle strengthening and accelerated larval development, replicating the function of mutualistic symbionts that are widely distributed among weevils and other beetles in nature. Our work provides empirical support for the notion that mutualistic symbioses can be initiated in insects by the acquisition of environmental bacteria. It also shows that certain bacterial genera, including the Sodalis spp. used in our study, are predisposed to develop these associations due to their ability to maintain benign infections and undergo vertical transmission in diverse insect hosts, facilitating the partner-fidelity feedback that is critical for the evolution of obligate mutualism. These experimental advances provide a new platform for laboratory studies focusing on the molecular mechanisms and evolutionary processes underlying insect-bacterial symbiosis.

Juvenile hormone promotes paracellular transport of yolk proteins via remodeling zonula adherens at tricellular junctions in the follicular epithelium

Juvenile hormone (JH) acts as a gonadotrophic hormone stimulating insect vitellogenesis and oogenesis. Paracellular transport of yolk proteins through intercellular channels (patency) in the follicular epithelium is a developmentally regulated and evolutionarily conserved process during vitellogenesis. However, the mechanisms underlying patency opening are poorly understood. Using the migratory locust Locusta migratoria as a model system, we report here that JH-regulated remodeling of zonula adherens (ZA), the belt-like adherens junction maintaining physical linking between follicle cells controlled the opening of patency. JH triggered phosphorylation of Partitioning defective protein 3 (Par3) via a signaling cascade including G protein-coupled receptor (GPCR), small GTPase Cell division cycle 42 (Cdc42) and atypical Protein kinase C (aPKC). Par3 phosphorylation resulted in its disassociation from β-Catenin, the cytoplasmic partner of ZA core component E-Cadherin. Release of Par3 from the β-Catenin/E-Cadherin complex caused ZA disassembly at tricellular contacts, consequently leading to patency enlargement. This study provides new insight into how JH stimulates insect vitellogenesis and egg production via inducing the opening of paracellular route for vitellogenin transport crossing the follicular epithelium barrier.

Vitellogenesis is one of the most emblematic processes in female reproduction of oviparous animals. In many insects, the yolk protein precursor, vitellogenin (Vg) is synthesized in the fat body and transported to oocytes through the intercellular spaces (patency) among follicular cells. Juvenile hormone (JH), the arthropod-specific sesquiterpenoid plays a crucial role in paracellular Vg transport, but the molecular mechanisms of JH-stimulated patency remain elusive. In the present study, we show that JH acts via the GPCR-Cdc42-aPKC signaling cascade that triggers the phosphorylation of Par3, a critical scaffold protein of zonula adherens. JH-dependent Par3 phosphorylation results in its dissociation from the β-Catenin/E-Cadherin complex, consequently leading to patency opening for Vg transport. The findings reveal an important mechanism by which JH induces the remodeling of zonula adherens for the opening of paracellular route for Vg transport crossing the follicular epithelium barrier in the ovary.

DIPA-CRISPR is a simple and accessible method for insect gene editing

Current approaches for insect gene editing require microinjection of materials into early embryos. This severely limits the application of gene editing to a great number of insect species, especially to those whose reproduction systems preclude access to early embryos for injection. To overcome these limitations, we report a simple and accessible method for insect gene editing, termed "direct parental" CRISPR (DIPA-CRISPR). We show that injection of Cas9 ribonucleoproteins (RNPs) into the haemocoel of adult females efficiently introduces heritable mutations in developing oocytes. Importantly, commercially available standard Cas9 protein can be directly used for DIPA-CRISPR, which makes this approach highly practical and feasible. DIPA-CRISPR enables highly efficient gene editing in the cockroaches, on which conventional approaches cannot be applied, and in the model beetle Tribolium castaneum. Due to its simplicity and accessibility, DIPA-CRISPR will greatly extend the application of gene editing technology to a wide variety of insects.

Tricellulin secures the epithelial barrier at tricellular junctions by interacting with actomyosin

The epithelial cell sheet functions as a barrier to prevent invasion of pathogens. It is necessary to eliminate intercellular gaps not only at bicellular junctions, but also at tricellular contacts, where three cells meet, to maintain epithelial barrier function. To that end, tight junctions between adjacent cells must associate as closely as possible, particularly at tricellular contacts. Tricellulin is an integral component of tricellular tight junctions (tTJs), but the molecular mechanism of its contribution to the epithelial barrier function remains unclear. In this study, we revealed that tricellulin contributes to barrier formation by regulating actomyosin organization at tricellular junctions. Furthermore, we identified α-catenin, which is thought to function only at adherens junctions, as a novel binding partner of tricellulin. α-catenin bridges tricellulin attachment to the bicellular actin cables that are anchored end-on at tricellular junctions. Thus, tricellulin mobilizes actomyosin contractility to close the lateral gap between the TJ strands of the three proximate cells that converge on tricellular junctions.

Involvement of Follicular Patency in the Ovarian Developmental Block in Virus-infected, MdSGHV, House Flies, Musca domestica (Diptera: Muscidae)

The Musca domestica salivary gland hypertrophy virus (MdSGHV) is known to have marked effects on the female Musca domestica L. (or common house fly) reproductive system, particularly regarding the size and functionality of the ovaries. Examination of the terminal ovarian follicles can help determine if and how MdSGHV mechanistically causes the block in ovarian development. In this study, terminal ovarian follicle lengths were measured and monitored for patency using Trypan blue dye staining. We examined the effect of MdSGHV infection on female house fly ovarian follicles and attempted to rescue the diminished ovarian follicles in MdSGHV-infected house flies through the application of a hormonal treatment (i.e., methoprene). Comparison of patency in control saline-injected females, virus-injected females with no methoprene application, and virus-injected females with topical methoprene application revealed that none of the virus-infected flies showed an increase in terminal follicular length beyond stage 3 follicles (staging according to Adams 1974). Additionally, none showed evidence of patency. In control, saline-injected females, we found the threshold length of the terminal follicles for the onset of patency to be 600 µm. When examined at 48, 72, and 96 h post-eclosion, average follicle length for infected females seldom reached 250 µm and they also failed to display patency. Thus, the virus is somehow involved in shutting down the mechanism involved in follicular patency. The lack of patency in infected follicles may also be one of the determining factors preventing vertical transmission of the pathogen.

Juvenile hormone signaling promotes ovulation and maintains egg shape by inducing expression of extracellular matrix genes

It is well documented that the juvenile hormone (JH) can function as a gonadotropic hormone that stimulates vitellogenesis by activating the production and uptake of vitellogenin in insects. Here, we describe a phenotype associated with mutations in the Drosophila JH receptor genes, Met and Gce: the accumulation of mature eggs with reduced egg length in the ovary. JH signaling is mainly activated in ovarian muscle cells and induces laminin gene expression in these cells. Meanwhile, JH signaling induces collagen IV gene expression in the adult fat body, from which collagen IV is secreted and deposited onto the ovarian muscles. Laminin locally and collagen IV remotely contribute to the assembly of ovarian muscle extracellular matrix (ECM); moreover, the ECM components are indispensable for ovarian muscle contraction. Furthermore, ovarian muscle contraction externally generates a mechanical force to promote ovulation and maintain egg shape. This work reveals an important mechanism for JH-regulated insect reproduction.