miR-285-Yki/Mask double-negative feedback loop mediates blood-brain barrier integrity in Drosophila

Functional consequences of somatic polyploidy in development

Polyploid cells contain multiple genome copies and arise in many animal tissues as a regulated part of development. However, polyploid cells can also arise due to cell division failure, DNA damage or tissue damage. Although polyploidization is crucial for the integrity and function of many tissues, the cellular and tissue-wide consequences of polyploidy can be very diverse. Nonetheless, many polyploid cell types and tissues share a remarkable similarity in function, providing important information about the possible contribution of polyploidy to cell and tissue function. Here, we review studies on polyploid cells in development, underlining parallel functions between different polyploid cell types, as well as differences between developmentally-programmed and stress-induced polyploidy.

Hypoxia-Induced miR-101 Impairs Endothelial Barrier Integrity Through Altering VE-Cadherin and Claudin-5

Stroke is a life-threatening medical condition across the world that adversely affects the integrity of the blood-brain barrier (BBB). The brain microvascular endothelial cells are the important constituent of the BBB. These cells line the blood vessels and form a semipermeable barrier. Disruptions in adherens junction and tight junction proteins of brain microvascular endothelial cells compromise the integrity of BBB. The Vascular Endothelial (VE)-cadherin is an integral adherens junction protein required for the establishment and maintenance of the endothelial barrier integrity. This study aims to investigate the role of miRNA in hypoxia-induced endothelial barrier disruption. In this study, brain endothelial cells were exposed to hypoxic conditions for different time points. Western blotting, overexpression and knockdown of miRNA, real-time PCR, TEER, and sodium fluorescein assay were used to examine the effect of hypoxic conditions on brain endothelial cells. Hypoxic exposure was validated using HIF-1α protein. Exposure to hypoxic conditions resulted to a significant decrease in endothelial barrier resistance and an increase in sodium fluorescein migration across the endothelial barrier. Reduction in endothelial barrier resistance demonstrated compromised barrier integrity, whereas the increase in migration of sodium fluorescein across the barrier indicated the increase in barrier permeability. The present study revealed microRNA-101 decreases the expression of VE-cadherin and claudin-5 in brain endothelial cells exposed to the hypoxic conditions.

Glia as Functional Barriers and Signaling Intermediaries

Glia play a crucial role in providing metabolic support to neurons across different species. To do so, glial cells isolate distinct neuronal compartments from systemic signals and selectively transport specific metabolites and ions to support neuronal development and facilitate neuronal function. Because of their function as barriers, glial cells occupy privileged positions within the nervous system and have also evolved to serve as signaling intermediaries in various contexts. The fruit fly, Drosophila melanogaster, has significantly contributed to our understanding of glial barrier development and function. In this review, we will explore the formation of the glial sheath, blood-brain barrier, and nerve barrier, as well as the significance of glia-extracellular matrix interactions in barrier formation. Additionally, we will delve into the role of glia as signaling intermediaries in regulating nervous system development, function, and response to injury.

Heterochromatic silencing of immune-related genes in glia is required for BBB integrity and normal lifespan in drosophila

Glia and neurons face different challenges in aging and may engage different mechanisms to maintain their morphology and functionality. Here, we report that adult-onset downregulation of a Drosophila gene CG32529/GLAD led to shortened lifespan and age-dependent brain degeneration. This regulation exhibited cell type and subtype-specificity, involving mainly surface glia (comprising the BBB) and cortex glia (wrapping neuronal soma) in flies. In accordance, pan-glial knockdown of GLAD disrupted BBB integrity and the glial meshwork. GLAD expression in fly heads decreased with age, and the RNA-seq analysis revealed that the most affected transcriptional changes by RNAi-GLAD were associated with upregulation of immune-related genes. Furthermore, we conducted a series of lifespan rescue experiments and the results indicated that the profound upregulation of immune and related pathways was not the consequence but cause of the degenerative phenotypes of the RNAi-GLAD flies. Finally, we showed that GLAD encoded a heterochromatin-associating protein that bound to the promoters of an array of immune-related genes and kept them silenced during the cell cycle. Together, our findings demonstrate a previously unappreciated role of heterochromatic gene silencing in repressing immunity in fly glia, which is required for maintaining BBB and brain integrity as well as normal lifespan.

Sertoli cell survival and barrier function are regulated by miR-181c/d-Pafah1b1 axis during mammalian spermatogenesis

Sertoli cells contribute to the formation of the blood-testis barrier (BTB), which is necessary for normal spermatogenesis. Recently, microRNAs (miRNAs) have emerged as posttranscriptional regulatory elements in BTB function during spermatogenesis. Our previous study has shown that miR-181c or miR-181d (miR-181c/d) is highly expressed in testes from boars at 60 days old compared with at 180 days old. Herein, we found that overexpression of miR-181c/d via miR-181c/d mimics in murine Sertoli cells (SCs) or through injecting miR-181c/d-overexpressing lentivirus in murine testes perturbs BTB function by altering BTB-associated protein distribution at the Sertoli cell-cell interface and F-actin organization, but this in vivo perturbation disappears approximately 6 weeks after the final treatment. We also found that miR-181c/d represses Sertoli cell proliferation and promotes its apoptosis. Moreover, miR-181c/d regulates Sertoli cell survival and barrier function by targeting platelet-activating factor acetylhydrolase 1b regulatory subunit 1 (Pafah1b1) gene. Furthermore, miR-181c/d suppresses PAFAH1B1 expression, reduces the complex of PAFAH1B1 with IQ motif-containing GTPase activating protein 1, and inhibits CDC42/PAK1/LIMK1/Cofilin pathway which is required for F-actin stabilization. In total, our results reveal the regulatory axis of miR-181c/d-Pafah1b1 in cell survival and barrier function of Sertoli cells and provide additional insights into miRNA functions in mammalian spermatogenesis.

Non-coding RNAs in the regulation of blood–brain barrier functions in central nervous system disorders

The blood–brain barrier (BBB) is an essential component of the neurovascular unit that controls the exchanges of various biological substances between the blood and the brain. BBB damage is a common feature of different central nervous systems (CNS) disorders and plays a vital role in the pathogenesis of the diseases. Non-coding RNAs (ncRNAs), such as microRNAs (miRNAs), long non-coding RNA (lncRNAs), and circular RNAs (circRNAs), are important regulatory RNA molecules that are involved in almost all cellular processes in normal development and various diseases, including CNS diseases. Cumulative evidences have demonstrated ncRNA regulation of BBB functions in different CNS diseases. In this review, we have summarized the miRNAs, lncRNAs, and circRNAs that can be served as diagnostic and prognostic biomarkers for BBB injuries, and demonstrated the involvement and underlying mechanisms of ncRNAs in modulating BBB structure and function in various CNS diseases, including ischemic stroke, hemorrhagic stroke, traumatic brain injury (TBI), spinal cord injury (SCI), multiple sclerosis (MS), Alzheimer's disease (AD), vascular cognitive impairment and dementia (VCID), brain tumors, brain infections, diabetes, sepsis-associated encephalopathy (SAE), and others. We have also discussed the pharmaceutical drugs that can regulate BBB functions via ncRNAs-related signaling cascades in CNS disorders, along with the challenges, perspective, and therapeutic potential of ncRNA regulation of BBB functions in CNS diseases.

Resources and Methods for the Analysis of MicroRNA Function in Drosophila

Since the widespread discovery of microRNAs (miRNAs) 20 years ago, the Drosophila melanogaster model system has made important contributions to understanding the biology of this class of noncoding RNAs. These contributions are based on the amenability of this model system not only for biochemical analysis but molecular, genetic, and cell biological analyses as well. Nevertheless, while the Drosophila genome is now known to encode 258 miRNA precursors, the function of only a small minority of these have been well characterized. In this review, we summarize the current resources and methods that are available to study miRNA function in Drosophila with a particular focus on the large-scale resources that enable systematic analysis. Application of these methods will accelerate the discovery of ways that miRNAs are embedded into genetic networks that control basic features of metazoan cells.

Zika Virus NS1 Suppresses VE-Cadherin and Claudin-5 via hsa-miR-101-3p in Human Brain Microvascular Endothelial Cells

Zika virus (ZIKV) is a neurotropic virus that causes microcephaly in newborns and Guillain-Barré syndrome (GBS) in adults. ZIKV is known to transmigrate through the blood-brain barrier (BBB) by utilizing different strategies. NS1 is a conserved flavivirus protein, which is secreted extracellularly. ZIKV-NS1 has been shown to target adherens junctions (AJs) and tight junctions (TJs) to disrupt the endothelial barrier integrity. The microRNAs are short non-coding RNAs, which post-transcriptionally regulate the gene expression by binding to 3' UTR of the target gene. In the present study, we studied the ZIKV-NS1-mediated effect through hsa-miR-101-3p on the junctional barrier integrity in human brain microvascular endothelial cells. We exposed hBMVECs and hCMEC/D3 cells with ZIKV-NS1 at different time points (12 h and 24 h) with the doses 500 ng/mL and 1000 ng/mL. The change in the expression of VE-cadherin and claudin-5 was quantified using immunoblotting. The expression of the hsa-miR-101-3p was quantified using qRT-PCR. To prove the targeting of hsa-miR-101-3p to VE-cadherin, we transfected hsa-miR-101-3p mimic, scramble, hsa-miR-101-3p inhibitor, and Cy3 in the ZIKV-NS1-exposed hCMEC/D3 cells. The distribution and expression of the VE-cadherin and claudin-5 were observed using immunofluorescence and immunoblotting. The ZIKV-NS1 compromises the endothelial barrier integrity by disrupting the VE-cadherin and claudin-5 protein expression via hsa-miR-101-3p. The findings of this study suggest that ZIKV-NS1 dysregulates the adherens junction and tight junction proteins through hsa-miR-101-3p, which compromises the barrier integrity of human brain microvascular endothelial cells.

Polyploidy in development and tumor models in Drosophila

Polyploidy, a cell status defined as more than two sets of genomic DNA, is a conserved strategy across species that can increase cell size and biosynthetic production, but the functional aspects of polyploidy are nuanced and vary across cell types. Throughout Drosophila developmental stages (embryo, larva, pupa and adult), polyploid cells are present in numerous organs and help orchestrate development while contributing to normal growth, well-being and homeostasis of the organism. Conversely, increasing evidence has shown that polyploid cells are prevalent in Drosophila tumors and play important roles in tumor growth and invasiveness. Here, we summarize the genes and pathways involved in polyploidy during normal and tumorigenic development, the mechanisms underlying polyploidization, and the functional aspects of polyploidy in development, homeostasis and tumorigenesis in the Drosophila model.

The cAMP effector PKA mediates Moody GPCR signaling in Drosophila blood-brain barrier formation and maturation

The blood-brain barrier (BBB) of Drosophila comprises a thin epithelial layer of subperineural glia (SPG), which ensheath the nerve cord and insulate it against the potassium-rich hemolymph by forming intercellular septate junctions (SJs). Previously, we identified a novel Gi/Go protein-coupled receptor (GPCR), Moody, as a key factor in BBB formation at the embryonic stage. However, the molecular and cellular mechanisms of Moody signaling in BBB formation and maturation remain unclear. Here, we identify cAMP-dependent protein kinase A (PKA) as a crucial antagonistic Moody effector that is required for the formation, as well as for the continued SPG growth and BBB maintenance in the larva and adult stage. We show that PKA is enriched at the basal side of the SPG cell and that this polarized activity of the Moody/PKA pathway finely tunes the enormous cell growth and BBB integrity. Moody/PKA signaling precisely regulates the actomyosin contractility, vesicle trafficking, and the proper SJ organization in a highly coordinated spatiotemporal manner. These effects are mediated in part by PKA's molecular targets MLCK and Rho1. Moreover, 3D reconstruction of SJ ultrastructure demonstrates that the continuity of individual SJ segments, and not their total length, is crucial for generating a proper paracellular seal. Based on these findings, we propose that polarized Moody/PKA signaling plays a central role in controlling the cell growth and maintaining BBB integrity during the continuous morphogenesis of the SPG secondary epithelium, which is critical to maintain tissue size and brain homeostasis during organogenesis.

The Serine Protease Homolog, Scarface, Is Sensitive to Nutrient Availability and Modulates the Development of the Drosophila Blood-Brain Barrier

The adaptable transcriptional response to changes in food availability not only ensures animal survival but also lets embryonic development progress. Interestingly, the CNS is preferentially protected from periods of malnutrition, a phenomenon known as “brain sparing.” However, the mechanisms that mediate this response remain poorly understood. To get a better understanding of this, we used Drosophila melanogaster as a model, analyzing the transcriptional response of neural stem cells (neuroblasts) and glia of the blood–brain barrier (BBB) from larvae of both sexes during nutrient restriction using targeted DamID. We found differentially expressed genes in both neuroblasts and glia of the BBB, although the effect of nutrient deficiency was primarily observed in the BBB. We characterized the function of a nutritional sensitive gene expressed in the BBB, the serine protease homolog, scarface (scaf). Scaf is expressed in subperineurial glia in the BBB in response to nutrition. Tissue-specific knockdown of scaf increases subperineurial glia endoreplication and proliferation of perineurial glia in the blood–brain barrier. Furthermore, neuroblast proliferation is diminished on scaf knockdown in subperineurial glia. Interestingly, reexpression of Scaf in subperineurial glia is able to enhance neuroblast proliferation and brain growth of animals in starvation. Finally, we show that loss of scaf in the blood–brain barrier increases sensitivity to drugs in adulthood, suggesting a physiological impairment. We propose that Scaf integrates the nutrient status to modulate the balance between neurogenesis and growth of the BBB, preserving the proper equilibrium between the size of the barrier and the brain.

SIGNIFICANCE STATEMENT The Drosophila BBB separates the CNS from the open circulatory system. The BBB glia are not only acting as a physical segregation of tissues but participate in the regulation of the metabolism and neurogenesis during development. Here we analyze the transcriptional response of the BBB glia to nutrient deprivation during larval development, a condition in which protective mechanisms are switched on in the brain. Our findings show that the gene scarface reduces growth in the BBB while promoting the proliferation of neural stem, assuring the balanced growth of the larval brain. Thus, Scarface would link animal nutrition with brain development, coordinating neurogenesis with the growth of the BBB.

Targeting microRNAs to Regulate the Integrity of the Blood-Brain Barrier

The blood-brain barrier (BBB) is a highly specialized neurovascular unit that protects the brain from potentially harmful substances. In addition, the BBB also engages in the exchange of essential nutrients between the vasculature and brain parenchyma, which is critical for brain homeostasis. Brain diseases, including neurological disorders and cerebrovascular diseases, are often associated with disrupted BBB integrity, evidenced by increased permeability. Therefore, defining the mechanisms underlying the regulation of BBB integrity is crucial for the development of novel therapeutics targeting brain diseases. MicroRNAs (miRNA), a type of small non-coding RNAs, are emerging as an important regulator of BBB integrity. Here we review recent developments related to the role of miRNAs in regulating BBB integrity.

Identification and profiling of Bactrocera dorsalis microRNAs and their potential roles in regulating the developmental transitions of egg hatching, molting, pupation and adult eclosion

MicroRNAs (miRNAs) are endogenous small noncoding RNAs (18-25 nt) that are involved in many physiological processes including development, cancer, immunity, apoptosis and host-microbe interactions through post-transcriptional regulation of gene expression. In this study, we measured the profile of small RNAs over the developmental transitions of the oriental fruit fly Bactrocera dorsalis from egg hatching, molting, and pupation to adult eclosion. We identified 250 miRNAs, including 83 known and 167 novel miRNAs, and 47 isomiRNAs. In addition, we identified the miRNAs differentially expressed over the developmental transitions. Interestingly, the miR-309 cluster, the miR-2 cluster/family and the let-7 cluster were among these differentially expressed miRNAs, suggesting a role in the regulation of egg hatching, molting and pupation/adult eclosion, respectively. Moreover, a detailed analysis of the temporal expression patterns of 14 highly expressed miRNAs in the pupal stage revealed three types of expression profiles. Furthermore, injection of a miR-100 mimic in the 3rd instar larvae resulted in a significant decrease in pupation and adult eclosion rates, whereas injection of a miR-317 antagomir resulted in a significant decrease in the pupation rate and a decrease in the pupation time, indicating that miR-100 and miR-317 are involved in the process of pupation. Finally, injection of a miR-100/miR-285 mimic or antagomir in pupae resulted in a significant decrease in the eclosion rate and a significant increase in the prevalence of a partial eclosion phenotype, implying the involvement of miR-100 and miR-285 in the process of adult eclosion. This study identified critical miRNAs involved in the transitions of this important holometabolic model and pest insect B. dorsalis from egg hatching to adult eclosion, thus providing a useful resource for exploring the regulatory role of miRNAs during insect post-embryonic development.

Regulatory Mechanisms of Cell Polyploidy in Insects

Polyploidy cells undergo the endocycle to generate DNA amplification without cell division and have important biological functions in growth, development, reproduction, immune response, nutrient support, and conferring resistance to DNA damage in animals. In this paper, we have specially summarized current research progresses in the regulatory mechanisms of cell polyploidy in insects. First, insect hormones including juvenile hormone and 20-hydroxyecdysone regulate the endocycle of variant cells in diverse insect species. Second, cells skip mitotic division in response to developmental programming and conditional stimuli such as wound healing, regeneration, and aging. Third, the reported regulatory pathways of mitotic to endocycle switch (MES), including Notch, Hippo, and JNK signaling pathways, are summarized and constructed into genetic network. Thus, we think that the studies in crosstalk of hormones and their effects on canonical pathways will shed light on the mechanism of cell polyploidy and elucidate the evolutionary adaptions of MES through diverse insect species.

Mask, a component of the Hippo pathway, is required for Drosophila eye morphogenesis

Hippo signaling is an important regulator of tissue size, but it also has a lesser-known role in tissue morphogenesis. Here we use the Drosophila pupal eye to explore the role of the Hippo effector Yki and its cofactor Mask in morphogenesis. We found that Mask is required for the correct distribution and accumulation of adherens junctions and appropriate organization of the cytoskeleton. Accordingly, disrupting mask expression led to severe mis-patterning and similar defects were observed when yki was reduced or in response to ectopic wts. Further, the patterning defects generated by reducing mask expression were modified by Hippo pathway activity. RNA-sequencing revealed a requirement for Mask for appropriate expression of numerous genes during eye morphogenesis. These included genes implicated in cell adhesion and cytoskeletal organization, a comprehensive set of genes that promote cell survival, and numerous signal transduction genes. To validate our transcriptome analyses, we then considered two loci that were modified by Mask activity: FER and Vinc, which have established roles in regulating adhesion. Modulating the expression of either locus modified mask mis-patterning and adhesion phenotypes. Further, expression of FER and Vinc was modified by Yki. It is well-established that the Hippo pathway is responsive to changes in cell adhesion and the cytoskeleton, but our data indicate that Hippo signaling also regulates these structures.

Hippo Signaling in Cancer: Lessons From Drosophila Models

Hippo pathway was initially identified through genetic screens for genes regulating organ size in fruitflies. Recent studies have highlighted the role of Hippo signaling as a key regulator of homeostasis, and in tumorigenesis. Hippo pathway is comprised of genes that act as tumor suppressor genes like hippo (hpo) and warts (wts), and oncogenes like yorkie (yki). YAP and TAZ are two related mammalian homologs of Drosophila Yki that act as effectors of the Hippo pathway. Hippo signaling deficiency can cause YAP- or TAZ-dependent oncogene addiction for cancer cells. YAP and TAZ are often activated in human malignant cancers. These transcriptional regulators may initiate tumorigenic changes in solid tumors by inducing cancer stem cells and proliferation, culminating in metastasis and chemo-resistance. Given the complex mechanisms (e.g., of the cancer microenvironment, and the extrinsic and intrinsic cues) that overpower YAP/TAZ inhibition, the molecular roles of the Hippo pathway in tumor growth and progression remain poorly defined. Here we review recent findings from studies in whole animal model organism like Drosophila on the role of Hippo signaling regarding its connection to inflammation, tumor microenvironment, and other oncogenic signaling in cancer growth and progression.

Differential expression of Öbek controls ploidy in the Drosophila blood-brain barrier

During development, tissue growth is mediated by either cell proliferation or cell growth, coupled with polyploidy. Both strategies are employed by the cell types that make up the Drosophila blood-brain barrier. During larval growth, the perineurial glia proliferate, whereas the subperineurial glia expand enormously and become polyploid. Here, we show that the level of ploidy in the subperineurial glia is controlled by the N-terminal asparagine amidohydrolase homolog Öbek, and high Öbek levels are required to limit replication. In contrast, perineurial glia express moderate levels of Öbek, and increased Öbek expression blocks their proliferation. Interestingly, other dividing cells are not affected by alteration of Öbek expression. In glia, Öbek counteracts fibroblast growth factor and Hippo signaling to differentially affect cell growth and number. We propose a mechanism by which growth signals are integrated differentially in a glia-specific manner through different levels of Öbek protein to adjust cell proliferation versus endoreplication in the blood-brain barrier.

Variant cell cycles regulated by Notch signaling control cell size and ensure a functional blood-brain barrier

Regulation of cell size is crucial in development. In plants and animals two cell cycle variants are employed to generate large cells by increased ploidy: the endocycle and endomitosis. The rationale behind the choice of which of these cycles is implemented is unknown. We show that in the Drosophila nervous system the subperineurial glia (SPG) are unique in using both the endocycle and endomitosis to grow. In the brain, the majority of SPG initially endocycle, then switch to endomitosis during larval development. The Notch signaling pathway and the String Cdc25 phosphatase are crucial for the endocycle versus endomitosis choice, providing the means experimentally to change cells from one to the other. This revealed fundamental insights into the control of cell size and the properties of endomitotic cells. Endomitotic cells attain a higher ploidy and larger size than endocycling cells, and endomitotic SPG are necessary for the blood-brain barrier. Decreased Notch signaling promotes endomitosis even in the ventral nerve cord SPG that normally are mononucleate, but not in the endocycling salivary gland cells, revealing tissue-specific cell cycle responses.

Highlighted Article: In Drosophila brain lobes, Notch and the mitosis-activating phosphatase String regulate the switch of subperineurial glia from endocycle to endomitosis during larval development, with endomitotic cells attaining increased ploidy and size.