CCM-3 Promotes C. elegans Germline Development by Regulating Vesicle Trafficking Cytokinesis and Polarity

Deficiency of Pdcd10 causes urothelium hypertrophy and vesicle trafficking defects in ureter

The scaffolding protein programmed cell death protein 10 (Pdcd10) has been demonstrated to play a critical role in renal epithelial cell homeostasis and function by maintaining appropriate water reabsorption in collecting ducts. Both ureter and kidney collecting duct systems are derived from the ureter bud during development. Here, we report that cadherin-16 (Cdh16)-cre drives gene recombination with high specificity in the ureter, but not the bladder, urothelium. The consequences of Pdcd10 deletion on the stratified ureter urothelium were investigated using an integrated approach including messenger RNA (mRNA) expression analysis, immunocytochemistry, and high-resolution confocal and electron microscopy. Loss of Pdcd10 in the ureter urothelium resulted in increased expression of uroplakins (Upks) and keratins (Krts), as well as hypertrophy of the ureter urothelium with an associated increase in the number of proliferation marker protein Ki-67 (Ki67)-expressing cells specifically within the basal urothelium layer. Ultrastructural analysis documented significant modification of the intracellular membrane system, including intracellular vesicle genesis and transport along the basal- to umbrella-cell-layer axis. Additionally, Pdcd10 loss resulted in swelling of Golgi compartments, disruption of mitochondrial cristae structure, and increased lysosomal fusion. Lack of Pdcd10 also resulted in decreased fusiform vesicle formation in umbrella cells, increased secretion of exosome vesicles, and alteration in microvillar structure on apical membranes. Our findings indicate that Pdcd10 expression and its influence on homeostasis is associated with modulation of endomembrane trafficking and organelle biogenesis in the ureter urothelium.

Uncoupling cell division and cytokinesis during germline development in metazoans

The canonical eukaryotic cell cycle ends with cytokinesis, which physically divides the mother cell in two and allows the cycle to resume in the newly individualized daughter cells. However, during germline development in nearly all metazoans, dividing germ cells undergo incomplete cytokinesis and germ cells stay connected by intercellular bridges which allow the exchange of cytoplasm and organelles between cells. The near ubiquity of incomplete cytokinesis in animal germ lines suggests that this is an ancient feature that is fundamental for the development and function of this tissue. While cytokinesis has been studied for several decades, the mechanisms that enable regulated incomplete cytokinesis in germ cells are only beginning to emerge. Here we review the current knowledge on the regulation of germ cell intercellular bridge formation, focusing on findings made using mouse, Drosophila melanogaster and Caenorhabditis elegans as experimental systems.

Differential regulation of degradation and immune pathways underlies adaptation of the ectosymbiotic nematode Laxus oneistus to oxic-anoxic interfaces

Eukaryotes may experience oxygen deprivation under both physiological and pathological conditions. Because oxygen shortage leads to a reduction in cellular energy production, all eukaryotes studied so far conserve energy by suppressing their metabolism. However, the molecular physiology of animals that naturally and repeatedly experience anoxia is underexplored. One such animal is the marine nematode Laxus oneistus. It thrives, invariably coated by its sulfur-oxidizing symbiont Candidatus Thiosymbion oneisti, in anoxic sulfidic or hypoxic sand. Here, transcriptomics and proteomics showed that, whether in anoxia or not, L. oneistus mostly expressed genes involved in ubiquitination, energy generation, oxidative stress response, immune response, development, and translation. Importantly, ubiquitination genes were also highly expressed when the nematode was subjected to anoxic sulfidic conditions, together with genes involved in autophagy, detoxification and ribosome biogenesis. We hypothesize that these degradation pathways were induced to recycle damaged cellular components (mitochondria) and misfolded proteins into nutrients. Remarkably, when L. oneistus was subjected to anoxic sulfidic conditions, lectin and mucin genes were also upregulated, potentially to promote the attachment of its thiotrophic symbiont. Furthermore, the nematode appeared to survive oxygen deprivation by using an alternative electron carrier (rhodoquinone) and acceptor (fumarate), to rewire the electron transfer chain. On the other hand, under hypoxia, genes involved in costly processes (e.g., amino acid biosynthesis, development, feeding, mating) were upregulated, together with the worm's Toll-like innate immunity pathway and several immune effectors (e.g., bactericidal/permeability-increasing proteins, fungicides). In conclusion, we hypothesize that, in anoxic sulfidic sand, L. oneistus upregulates degradation processes, rewires the oxidative phosphorylation and reinforces its coat of bacterial sulfur-oxidizers. In upper sand layers, instead, it appears to produce broad-range antimicrobials and to exploit oxygen for biosynthesis and development.

STRIPAK regulation of katanin microtubule severing in the Caenorhabditis elegans embryo

Microtubule severing plays important role in cell structure and cell division. The microtubule severing protein katanin, composed of the MEI-1/MEI-2 subunits in Caenorhabditis elegans, is required for oocyte meiotic spindle formation; however, it must be inactivated for mitosis to proceed as continued katanin expression is lethal. Katanin activity is regulated by 2 ubiquitin-based protein degradation pathways. Another ubiquitin ligase, HECD-1, the homolog of human HECTD1/HECT domain E3 ubiquitin protein ligase 1, regulates katanin activity without affecting katanin levels. In other organisms, HECD-1 is a component of the striatin-interacting kinase phosphatase complex, which affects cell proliferation and a variety of signaling pathways. Here we conducted a systematic screen of how mutations in striatin-interacting kinase phosphatase components affect katanin function in C. elegans. Striatin-interacting kinase phosphatase core components (FARL-11, CASH-1, LET-92, and GCK-1) were katanin inhibitors in mitosis and activators in meiosis, much like HECD-1. By contrast, variable components (SLMP-1, OTUB-2) functioned as activators of katanin activity in mitosis, indicating they may function to alter striatin-interacting kinase phosphatase core function. The core component CCM-3 acted as an inhibitor at both divisions, while other components (MOB-4, C49H3.6) showed weak interactions with katanin mutants. Additional experiments indicate that katanin may be involved with the centralspindlin complex and a tubulin chaperone. HECD-1 shows ubiquitous expression in the cytoplasm throughout meiosis and early development. The differing functions of the different subunits could contribute to the diverse functions of the striatin-interacting kinase phosphatase complex in C. elegans and other organisms.

The exocyst complex regulates C. elegans germline stem cell proliferation by controlling membrane Notch levels

The conserved exocyst complex regulates plasma membrane-directed vesicle fusion in eukaryotes. However, its role in stem cell proliferation has not been reported. Germline stem cell (GSC) proliferation in the nematode Caenorhabditis elegans is regulated by conserved Notch signaling. Here, we reveal that the exocyst complex regulates C. elegans GSC proliferation by modulating Notch signaling cell autonomously. Notch membrane density is asymmetrically maintained on GSCs. Knockdown of exocyst complex subunits or of the exocyst-interacting GTPases Rab5 and Rab11 leads to Notch redistribution from the GSC-niche interface to the cytoplasm, suggesting defects in plasma membrane Notch deposition. The anterior polarity (aPar) protein Par6 is required for GSC proliferation, and for maintaining niche-facing membrane levels of Notch and the exocyst complex. The exocyst complex biochemically interacts with the aPar regulator Par5 (14-3-3ζ) and Notch in C. elegans and human cells. Exocyst components are required for Notch plasma membrane localization and signaling in mammalian cells. Our study uncovers a possibly conserved requirement of the exocyst complex in regulating GSC proliferation and in maintaining optimal membrane Notch levels.

The multiple roles of RAB GTPases in female and male meiosis

BACKGROUND

RAB GTPases constitute the largest family of small GTPases and are found in all eukaryotes. RAB GTPases regulate components of the endomembrane system, the nucleus and the plasma membrane, and are involved in intracellular actin/tubulin-dependent vesicle movement, membrane fusion and cell growth in mitosis.

OBJECTIVE AND RATIONALE

RAB GTPases play multiple critical roles during both female and male meiosis. This review summarizes the progress made in our understanding of the role of RAB GTPases in female and male meiosis in different species. We also discuss the potential relationship between RAB GTPases and oocyte/sperm quality, which may help in understanding the mechanisms underlying oogenesis and spermatogenesis and potential genetic causes of infertility.

SEARCH METHODS

The PubMed database was searched for articles published between 1991 and 2020 using the following terms: 'RAB', 'RAB oocyte', 'RAB sperm' and 'RAB meiosis'.

OUTCOMES

An analysis of 126 relevant articles indicated that RAB GTPases are present in all eukaryotes, and ten subfamilies (almost 70 members) are expressed in human cells. The roles of 25 RAB proteins and orthologues in female meiosis and 12 in male meiosis have been reported. RAB proteins are essential for the accurate continuity of genetic material, successful fertilization and the normal growth of offspring. Distinct and crucial functions of RAB GTPases in meiosis have been reported. In oocytes, RAB GTPases are involved in spindle organization, kinetochore-microtubule attachment, chromosome alignment, actin filament-mediated spindle migration, cytokinesis, cell cycle and oocyte-embryo transition. RAB GTPases function in mitochondrial processes and Golgi-mediated vesicular transport during female meiosis, and are critical for cortical granule transport during fertilization and oocyte-embryo transition. In sperm, RAB GTPases are vital for cytoskeletal organization and successful cytokinesis, and are associated with Golgi-mediated acrosome formation, membrane trafficking and morphological changes of sperm cells, as well as the exocytosis-related acrosome reaction and zona reaction during fertilization.

WIDER IMPLICATIONS

Abnormal expression of RAB GTPases disrupts intracellular systems, which may induce diverse diseases. The roles of RAB proteins in female and male reproductive systems, thus, need to be considered. The mechanisms underlying the function of RAB GTPases and the binding specificity of their effectors during oogenesis, spermatogenesis and fertilization remain to be studied. This review should contribute to our understanding of the molecular mechanisms of oogenesis and spermatogenesis and potential genetic causes of infertility.

Characterization of Strip1 Expression in Mouse Cochlear Hair Cells

Striatin-interacting protein 1 (Strip1) is a core component of the striatin interacting phosphatase and kinase (STRIPAK) complex, which is involved in embryogenesis and development, circadian rhythms, type 2 diabetes, and cancer progression. However, the expression and role of Strip1 in the mammalian cochlea remains unclear. Here we studied the expression and function of Strip1 in the mouse cochlea by using Strip1 knockout mice. We first found that the mRNA and protein expression of Strip1 increases as mice age starting from postnatal day (P) 3 and reaches its highest expression level at P30 and that the expression of Strip1 can be detected by immunofluorescent staining starting from P14 only in cochlear HCs, and not in supporting cells (SCs). Next, we crossed Strip1 heterozygous knockout (Strip +/−) mice to obtain Strip1 homozygous knockout (Strip1−/−) mice for studying the role of Strip1 in cochlear HCs. However, no Strip1−/− mice were obtained and the ratio of Strip +/− to Strip1+/+ mice per litter was about 2:1, which suggested that homozygous Strip1 knockout is embryonic lethal. We measured hearing function and counted the HC number in P30 and P60 Strip +/− mice and found that they had normal hearing ability and HC numbers compared to Strip1+/+ mice. Our study suggested that Strip1 probably play important roles in HC development and maturation, which needs further study in the future.

From Genes and Mechanisms to Molecular-Targeted Therapies: The Long Climb to the Cure of Cerebral Cavernous Malformation (CCM) Disease

Cerebral cavernous malformation (CCM) is a rare cerebrovascular disorder of genetic origin consisting of closely clustered, abnormally dilated and leaky capillaries (CCM lesions), which occur predominantly in the central nervous system. CCM lesions can be single or multiple and may result in severe clinical symptoms, including focal neurological deficits, seizures, and intracerebral hemorrhage. Early human genetic studies demonstrated that CCM disease is linked to three chromosomal loci and can be inherited as autosomal dominant condition with incomplete penetrance and highly variable expressivity, eventually leading to the identification of three disease genes, CCM1/KRIT1, CCM2, and CCM3/PDCD10, which encode for structurally unrelated intracellular proteins that lack catalytic domains. Biochemical, molecular, and cellular studies then showed that these proteins are involved in endothelial cell-cell junction and blood-brain barrier stability maintenance through the regulation of major cellular structures and mechanisms, including endothelial cell-cell and cell-matrix adhesion, actin cytoskeleton dynamics, autophagy, and endothelial-to-mesenchymal transition, suggesting that they act as pleiotropic regulators of cellular homeostasis, and opening novel therapeutic perspectives. Indeed, accumulated evidence in cellular and animal models has eventually revealed that the emerged pleiotropic functions of CCM proteins are mainly due to their ability to modulate redox-sensitive pathways and mechanisms involved in adaptive responses to oxidative stress and inflammation, thus contributing to the preservation of cellular homeostasis and stress defenses.In this introductory review, we present a general overview of 20 years of amazing progress in the identification of genetic culprits and molecular mechanisms underlying CCM disease pathogenesis, and the development of targeted therapeutic strategies.

High Levels of Receptor Tyrosine Kinases in CCM3-Deficient Cells Increase Their Susceptibility to Tyrosine Kinase Inhibition

Cerebral cavernous malformations (CCMs) are vascular malformations that can be the result of the deficiency of one of the CCM genes. Their only present treatment is surgical removal, which is not always possible, and an alternative pharmacological strategy to eliminate them is actively sought. We have studied the effect of the lack of one of the CCM genes, CCM3, in endothelial and non-endothelial cells. By comparing protein expression in control and CCM3-silenced cells, we found that the levels of the Epidermal Growth Factor Receptor (EGFR) are higher in CCM3-deficient cells, which adds to the known upregulation of Vascular Endothelial Growth Factor Receptor 2 (VEGFR2) in these cells. Whereas VEGFR2 is upregulated at the mRNA level, EGFR has a prolonged half-life. Inhibition of EGFR family members in CCM3-deficient cells does not revert the known cellular effects of lack of CCM genes, but it induces significantly more apoptosis in CCM3-deficient cells than in control cells. We propose that the susceptibility to tyrosine kinase inhibitors of CCM3-deficient cells can be harnessed to kill the abnormal cells of these lesions and thus treat CCMs pharmacologically.

Ectopic expression of 35 kDa and knocking down of 78 kDa SG2NAs induce cytoskeletal reorganization, alter membrane sialylation, and modulate the markers of EMT

SG2NA is a protein of the striatin family that organizes STRIPAK complexes. It has splice variants expressing differentially in tissues. Its 78 kDa isoform regulates cell cycle, maintains homeostasis in the endoplasmic reticulum, and prevents oxidative injuries. The 35 kDa variant is devoid of the signature WD-40 repeats in the carboxy terminal, and its function is unknown. We expressed it in NIH 3T3 cells that otherwise express 78 kDa variant only. These cells (35 EE) have altered morphology, faster rate of migration, and enhanced growth as measured by the MTT assay. Similar phenotypes were also seen in cells where the endogenous 78 kDa isoform was downregulated by siRNA (78 KD). Proteomic analyses showed that several cancer-associated proteins are modulated in both 35 EE and 78 KD cells. The 35 EE cells have diffused actin fibers, distinctive ultrastructure, reduced sialylation, and increased expression of MMP2 & 9. The 78 KD cells also had diffused actin fibers and an upregulated expression of MMP2. In both cells, markers epithelial to mesenchymal transition (EMT) viz, E- & N-cadherins, β-catenin, slug, vimentin, and ZO-1 were modulated partially in tune with the EMT process. Since NIH 3T3 cells are mesenchymal, we also expressed 35 kDa SG2NA in MCF-7 cells of epithelial origin. In these cells (MCF-7-35), the actin fibers were also diffused and the modulation of the markers was more in tune with the EMT process. However, unlike in 35 EE cells, in MCF-7-35 cells, membrane sialylation rather increased. We infer that ectopic expression of 35 kDa and downregulation of 78 kDa SG2NAs partially induce transformed phenotypes.

Cells into tubes: Molecular and physical principles underlying lumen formation in tubular organs

Tubular networks, such as the vascular and respiratory systems, transport liquids and gases in multicellular organisms. The basic units of these organs are tubes formed by single or multiple cells enclosing a luminal cavity. The formation and maintenance of correctly sized and shaped lumina are fundamental steps in organogenesis and are essential for organismal homeostasis. Therefore, understanding how cells generate, shape and maintain lumina is crucial for understanding normal organogenesis as well as the basis of pathological conditions. Lumen formation involves polarized membrane trafficking, cytoskeletal dynamics, and the influence of intracellular as well as extracellular mechanical forces, such as cortical tension, luminal pressure or blood flow. Various tissue culture and in vivo model systems, ranging from MDCK cell spheroids to tubular organs in worms, flies, fish, and mice, have provided many insights into the molecular and cellular mechanisms underlying lumenogenesis and revealed key factors that regulate the size and shape of cellular tubes. Moreover, the development of new experimental and imaging approaches enabled quantitative analyses of intracellular dynamics and allowed to assess the roles of cellular and tissue mechanics during tubulogenesis. However, how intracellular processes are coordinated and regulated across scales of biological organization to generate properly sized and shaped tubes is only beginning to be understood. Here, we review recent insights into the molecular, cellular and physical mechanisms underlying lumen formation during organogenesis. We discuss how these mechanisms control lumen formation in various model systems, with a special focus on the morphogenesis of tubular organs in Drosophila.

A Series of Tubes: The C. elegans Excretory Canal Cell as a Model for Tubule Development

Formation and regulation of properly sized epithelial tubes is essential for multicellular life. The excretory canal cell of C. elegans provides a powerful model for investigating the integration of the cytoskeleton, intracellular transport, and organismal physiology to regulate the developmental processes of tube extension, lumen formation, and lumen diameter regulation in a narrow single cell. Multiple studies have provided new understanding of actin and intermediate filament cytoskeletal elements, vesicle transport, and the role of vacuolar ATPase in determining tube size. Most of the genes discovered have clear homologues in humans, with implications for understanding these processes in mammalian tissues such as Schwann cells, renal tubules, and brain vasculature. The results of several new genetic screens are described that provide a host of new targets for future studies in this informative structure.

Novel cytokinetic ring components drive negative feedback in cortical contractility

Actomyosin cortical contractility drives many cell shape changes including cytokinetic furrowing. While positive regulation of contractility is well characterized, counterbalancing negative regulation and mechanical brakes are less well understood. The small GTPase RhoA is a central regulator, activating cortical actomyosin contractility during cytokinesis and other events. Here we report how two novel cytokinetic ring components, GCK-1 (germinal center kinase-1) and CCM-3 (cerebral cavernous malformations-3), participate in a negative feedback loop among RhoA and its cytoskeletal effectors to inhibit contractility. GCK-1 and CCM-3 are recruited by active RhoA and anillin to the cytokinetic ring, where they in turn limit RhoA activity and contractility. This is evidenced by increased RhoA activity, anillin and nonmuscle myosin II in the cytokinetic ring, and faster cytokinetic furrowing, following depletion of GCK-1 or CCM-3. GCK-1 or CCM-3 depletion also reduced RGA-3 levels in pulses and increased baseline RhoA activity and pulsed contractility during zygote polarization. Together, our results suggest that GCK-1 and CCM-3 regulate cortical actomyosin contractility via negative feedback. These findings have implications for the molecular and cellular mechanisms of cerebral cavernous malformation pathologies.

Blocking Signalopathic Events to Treat Cerebral Cavernous Malformations

Cerebral cavernous malformations (CCMs) are pathologies of the brain vasculature characterized by capillary-venous angiomas that result in recurrent cerebral hemorrhages. Familial forms are caused by a clonal loss of any of three CCM genes in endothelial cells, which causes the activation of a novel pathophysiological pathway involving mitogen-activated protein kinase and Krüppel-like transcription factor KLF2/4 signaling. Recent work has shown that cavernomas can undergo strong growth when CCM-deficient endothelial cells recruit wild-type neighbors through the secretion of cytokines. This suggests a treatment strategy based on targeting signalopathic events between CCM-deficient endothelial cells and their environment. Such approaches will have to consider recent evidence implicating 'third hits' from hypoxia-induced angiogenesis signaling or the microbiome in modulating the development of cerebral hemorrhages.

The STRIPAK Complex Regulates Response to Chemotherapy Through p21 and p27

The STRIPAK complex has been linked to a variety of biological processes taking place during embryogenesis and development, but its role in cancer has only just started to be defined. Here, we expand on previous work indicating a role for the scaffolding protein STRIP1 in cancer cell migration and metastasis. We show that cell cycle arrest and decreased proliferation are seen upon loss of STRIP1 in MDA-MB-231 cells due to the induction of cyclin dependent kinase inhibitors, including p21 and p27. We demonstrate that p21 and p27 induction is observed in a subpopulation of cells having low DNA damage response and that the p21^high/γH2AX^low ratio within single cells can be rescued by depleting MST3&4 kinases. While the loss of STRIP1 decreases cell proliferation and tumor growth, cells treated with low dosage of chemotherapeutics in vitro paradoxically escape therapy-induced senescence and begin to proliferate after recovery. This corroborates with already known research on the dual role of p21 and indicates that STRIP1 also plays a contradictory role in breast cancer, suppressing tumor growth, but once treated with chemotherapeutics, allowing for possible recurrence and decreased patient survival.

Effects of programmed cell death protein 10 on fecundity in Schistosoma japonicum

Programmed cell death protein 10 (PCDP10) is widely distributed in animal tissues and exerts extensive biological effects. This study aimed to investigate the effect of Schistosoma japonicum PCDP10 (SjPCDP10) on the fecundity of schistosomes. We performed real-time PCR to assess Sjpcdp10 expression levels at different developmental stages of S. japonicum. Immunoprotection against S. japonicum was assessed in vivo in mice, and Sjpcdp10 expression was inhibited via RNA interference (RNAi) to determine its role in fecundity. Real-time PCR analysis revealed that Sjpcdp10 mRNA was expressed during different developmental stages in S. japonicum, reaching maximum and minimum levels in female worms and lung-stage schistosomula, respectively. Recombinant SjPCDP10 had a molecular weight of approximately 28 kDa, displaying good immunogenicity but poor immunoprotection. SjPCDP10 was primarily localized in the egg, eggshell, epiphragm of adult worms, and especially the vitelline glands of female worms. RNAi-mediated knockdown of Sjpcdp10 by greater than 90%, and the protein expression decreased by 73%, reduced the number of eggs per female worm significantly more than RNAi-mediated knockdown of Egfp (negative control) (P < 0.05). The present results indicate that Sjpcdp10 knockdown affects the fecundity of schistosomes and may play a vital role in oogenesis.

Cerebral Cavernous Malformation Proteins in Barrier Maintenance and Regulation

Cerebral cavernous malformation (CCM) is a disease characterized by mulberry shaped clusters of dilated microvessels, primarily in the central nervous system. Such lesions can cause seizures, headaches, and stroke from brain bleeding. Loss-of-function germline and somatic mutations of a group of genes, called CCM genes, have been attributed to disease pathogenesis. In this review, we discuss the impact of CCM gene encoded proteins on cellular signaling, barrier function of endothelium and epithelium, and their contribution to CCM and potentially other diseases.

BUB-1 targets PP2A:B56 to regulate chromosome congression during meiosis I in C. elegans oocytes

Protein Phosphatase 2A (PP2A) is a heterotrimer composed of scaffolding (A), catalytic (C), and regulatory (B) subunits. PP2A complexes with B56 subunits are targeted by Shugoshin and BUBR1 to protect centromeric cohesion and stabilise kinetochore-microtubule attachments in yeast and mouse meiosis. In Caenorhabditis elegans, the closest BUBR1 orthologue lacks the B56-interaction domain and Shugoshin is not required for meiotic segregation. Therefore, the role of PP2A in C. elegans female meiosis is unknown. We report that PP2A is essential for meiotic spindle assembly and chromosome dynamics during C. elegans female meiosis. BUB-1 is the main chromosome-targeting factor for B56 subunits during prometaphase I. BUB-1 recruits PP2A:B56 to the chromosomes via a newly identified LxxIxE motif in a phosphorylation-dependent manner, and this recruitment is important for proper chromosome congression. Our results highlight a novel mechanism for B56 recruitment, essential for recruiting a pool of PP2A involved in chromosome congression during meiosis I.

Isolation and Purification of Mouse Brain Endothelial Cells to Study Cerebral Cavernous Malformation Disease

We describe a method to purify primary brain microvascular endothelial cells (BMEC) from mice bearing floxed alleles of Krit1 (Krit1fl/fl) or Pdcd10 (Pdcd10fl/fl) and an endothelial-specific tamoxifen-regulated Cre recombinase (Pdgfb-iCreERT2), and used these to delete Krit1 or Pdcd10 genes in a time-controlled manner. These BMEC culture models contain a high degree of purity and have been used to identify the major molecular processes involved in loss of Krit1/Pdcd10-induced altered brain endothelial phenotype and function. In addition, these in vitro models of cerebral cavernous malformations (CCMs) enable molecular, biochemical, and pharmacological studies that have contributed significantly to understand the pathogenesis of CCMs. The findings using this in vitro CCMs model have been validated in mouse CCM models and observed in human CCMs. In this chapter, we summarize procedures for isolation and purification of BMEC from transgenic mice, as well as our experience to genetically inactivate CCM genes in the brain endothelium.

Generation and Analysis of CCM Phenotypes in C. elegans

This chapter presents methods for exploiting the powerful tools available in the nematode worm Caenorhabditis elegans to understand the in vivo functions of cerebral cavernous malformation (CCM) genes and the organization of their associated signaling pathways. Included are methods for assessing phenotypes caused by loss-of-function mutations in the worm CCM genes kri-1 and ccm-3, CRISPR-based gene editing techniques, and protocols for conducting high-throughput forward genetic and small molecule screens.

Interrogating the ccm-3 Gene Network

Cerebral cavernous malformations (CCMs) are neurovascular lesions caused by mutations in one of three genes (CCM1-3). Loss of CCM3 causes the poorest prognosis, and little is known about how it regulates vascular integrity. The C. elegans ccm-3 gene regulates the development of biological tubes that resemble mammalian vasculature, and in a genome-wide reverse genetic screen, we identified more than 500 possible CCM-3 pathway genes. With a phenolog-like approach, we generated a human CCM signaling network and identified 29 genes in common, of which 14 are required for excretory canal extension and membrane integrity, similar to ccm-3. Notably, depletion of the MO25 ortholog mop-25.2 causes severe defects in tube integrity by preventing CCM-3 localization to apical membranes. Furthermore, loss of MO25 phenocopies CCM3 ablation by causing stress fiber formation in endothelial cells. This work deepens our understanding of how CCM3 regulates vascular integrity and may help identify therapeutic targets for treating CCM3 patients.

Distinct cellular roles for PDCD10 define a gut-brain axis in cerebral cavernous malformation

Cerebral cavernous malformation (CCM) is a genetic, cerebrovascular disease. Familial CCM is caused by genetic mutations in KRIT1, CCM2, or PDCD10 Disease onset is earlier and more severe in individuals with PDCD10 mutations. Recent studies have shown that lesions arise from excess mitogen-activated protein kinase kinase kinase 3 (MEKK3) signaling downstream of Toll-like receptor 4 (TLR4) stimulation by lipopolysaccharide derived from the gut microbiome. These findings suggest a gut-brain CCM disease axis but fail to define it or explain the poor prognosis of patients with PDCD10 mutations. Here, we demonstrate that the gut barrier is a primary determinant of CCM disease course, independent of microbiome configuration, that explains the increased severity of CCM disease associated with PDCD10 deficiency. Chemical disruption of the gut barrier with dextran sulfate sodium augments CCM formation in a mouse model, as does genetic loss of Pdcd10, but not Krit1, in gut epithelial cells. Loss of gut epithelial Pdcd10 results in disruption of the colonic mucosal barrier. Accordingly, loss of Mucin-2 or exposure to dietary emulsifiers that reduce the mucus barrier increases CCM burden analogous to loss of Pdcd10 in the gut epithelium. Last, we show that treatment with dexamethasone potently inhibits CCM formation in mice because of the combined effect of action at both brain endothelial cells and gut epithelial cells. These studies define a gut-brain disease axis in an experimental model of CCM in which a single gene is required for two critical components: gut epithelial function and brain endothelial signaling.

Systematic pharmacological screens uncover novel pathways involved in cerebral cavernous malformations

Cerebral cavernous malformations (CCMs) are vascular lesions in the central nervous system causing strokes and seizures which currently can only be treated through neurosurgery. The disease arises through changes in the regulatory networks of endothelial cells that must be comprehensively understood to develop alternative, non-invasive pharmacological therapies. Here, we present the results of several unbiased small-molecule suppression screens in which we applied a total of 5,268 unique substances to CCM mutant worm, zebrafish, mouse, or human endothelial cells. We used a systems biology-based target prediction tool to integrate the results with the whole-transcriptome profile of zebrafish CCM2 mutants, revealing signaling pathways relevant to the disease and potential targets for small-molecule-based therapies. We found indirubin-3-monoxime to alleviate the lesion burden in murine preclinical models of CCM2 and CCM3 and suppress the loss-of-CCM phenotypes in human endothelial cells. Our multi-organism-based approach reveals new components of the CCM regulatory network and foreshadows novel small-molecule-based therapeutic applications for suppressing this devastating disease in patients.

Novel exc Genes Involved in Formation of the Tubular Excretory Canals of Caenorhabditis elegans

Regulation of luminal diameter is critical to the function of small single-celled tubes, of which the seamless tubular excretory canals of Caenorhabditis elegans provide a tractable genetic model. Mutations in several sets of genes exhibit the Exc phenotype, in which canal luminal growth is visibly altered. Here, a focused reverse genomic screen of genes highly expressed in the canals found 18 genes that significantly affect luminal outgrowth or diameter. These genes encode novel proteins as well as highly conserved proteins involved in processes including gene expression, cytoskeletal regulation, and vesicular and transmembrane transport. In addition, two genes act as suppressors on a pathway of conserved genes whose products mediate vesicle movement from early to recycling endosomes. The results provide new tools for understanding the integration of cytoplasmic structure and physiology in forming and maintaining the narrow diameter of single-cell tubules.

Assembly of a heptameric STRIPAK complex is required for coordination of light-dependent multicellular fungal development with secondary metabolism in Aspergillus nidulans

Eukaryotic striatin forms striatin-interacting phosphatase and kinase (STRIPAK) complexes that control many cellular processes including development, cellular transport, signal transduction, stem cell differentiation and cardiac functions. However, detailed knowledge of complex assembly and its roles in stress responses are currently poorly understood. Here, we discovered six striatin (StrA) interacting proteins (Sips), which form a heptameric complex in the filamentous fungus Aspergillus nidulans. The complex consists of the striatin scaffold StrA, the Mob3-type kinase coactivator SipA, the SIKE-like protein SipB, the STRIP1/2 homolog SipC, the SLMAP-related protein SipD and the catalytic and regulatory phosphatase 2A subunits SipE (PpgA), and SipF, respectively. Single and double deletions of the complex components result in loss of multicellular light-dependent fungal development, secondary metabolite production (e.g. mycotoxin Sterigmatocystin) and reduced stress responses. sipA (Mob3) deletion is epistatic to strA deletion by supressing all the defects caused by the lack of striatin. The STRIPAK complex, which is established during vegetative growth and maintained during the early hours of light and dark development, is mainly formed on the nuclear envelope in the presence of the scaffold StrA. The loss of the scaffold revealed three STRIPAK subcomplexes: (I) SipA only interacts with StrA, (II) SipB-SipD is found as a heterodimer, (III) SipC, SipE and SipF exist as a heterotrimeric complex. The STRIPAK complex is required for proper expression of the heterotrimeric VeA-VelB-LaeA complex which coordinates fungal development and secondary metabolism. Furthermore, the STRIPAK complex modulates two important MAPK pathways by promoting phosphorylation of MpkB and restricting nuclear shuttling of MpkC in the absence of stress conditions. SipB in A. nidulans is similar to human suppressor of IKK-ε(SIKE) protein which supresses antiviral responses in mammals, while velvet family proteins show strong similarity to mammalian proinflammatory NF-KB proteins. The presence of these proteins in A. nidulans further strengthens the hypothesis that mammals and fungi use similar proteins for their immune response and secondary metabolite production, respectively.

The multisubunit STRIPAK complex has been studied from yeast to human and plays a range of roles from cell-cycle arrest, fruit body formation to neuronal functions. Molecular assembly of the STRIPAK complex and its roles in stress responses are not well-documented. Fungi, with an estimated 1.5 million members are friends and foes of mankind, acting as pathogens, natural product and enzyme producers. In filamentous fungus Aspergillus nidulans, we found a heptameric STRIPAK core complex made from three subcomplexes, which sits on the nuclear envelope and coordinates signal influx for light-dependent fungal development, secondary metabolism and stress responses. STRIPAK complex controls activities of two major Mitogen Activated Protein Kinase (MAPK) signaling pathways through either promoting their phosphorylation or limiting their nuclear localization under resting conditions. These findings establish a basis for how fungi govern signal influx by using multimeric scaffold protein complexes on the nuclear envelope to control different downstream pathways.

Comprehensive transcriptome analysis of cerebral cavernous malformation across multiple species and genotypes

The purpose of this study was to determine important genes, functions, and networks contributing to the pathobiology of cerebral cavernous malformation (CCM) from transcriptomic analyses across 3 species and 2 disease genotypes. Sequencing of RNA from laser microdissected neurovascular units of 5 human surgically resected CCM lesions, mouse brain microvascular endothelial cells, Caenorhabditis elegans with induced Ccm gene loss, and their respective controls provided differentially expressed genes (DEGs). DEGs from mouse and C. elegans were annotated into human homologous genes. Cross-comparisons of DEGs between species and genotypes, as well as network and gene ontology (GO) enrichment analyses, were performed. Among hundreds of DEGs identified in each model, common genes and 1 GO term (GO:0051656, establishment of organelle localization) were commonly identified across the different species and genotypes. In addition, 24 GO functions were present in 4 of 5 models and were related to cell-to-cell adhesion, neutrophil-mediated immunity, ion transmembrane transporter activity, and responses to oxidative stress. We have provided a comprehensive transcriptome library of CCM disease across species and for the first time to our knowledge in Ccm1/Krit1 versus Ccm3/Pdcd10 genotypes. We have provided examples of how results can be used in hypothesis generation or mechanistic confirmatory studies.

Cold stress response in Arabidopsis thaliana is mediated by GNOM ARF-GEF

Endosomal trafficking plays an important role in regulating plant growth and development both at optimal and stressed conditions. Cold stress response in Arabidopsis root is directly linked to inhibition of the endosomal trafficking of auxin efflux carriers. However, the cellular components that link cold stress and the endosomal trafficking remain elusive. By screening available endosomal trafficking mutants against root growth recovery response under cold stress, we identified GNOM, a SEC7 containing ARF-GEF, as a major modulator of cold response. Contrasting response of partial loss of function mutant gnom^{B4049/emb30-1} and the engineered Brefeldin A (BFA)-resistant GNOM line, both of which contain mutations within SEC7 domain, to cold stress at the whole-plant level highlights the importance of this domain in modulating the cold response pathway of plants. Cold stress selectively and transiently inhibits GNOM expression. The engineered point mutation at 696 amino acid position (Methionine to Leucine) that makes GNOM resistant to BFA in fact results in overexpression of GNOM both at transcriptional and translational levels, and also alters its subcellular localization. Overexpression and altered cellular localization of GNOM were found to be directly linked to conferring striking cold-resistant phenotype in Arabidopsis. Collectively, these results provide a mechanistic link between GNOM, BFA-sensitive GNOM-regulated trafficking and cold stress.

Syncytial germline architecture is actively maintained by contraction of an internal actomyosin corset

Syncytial architecture is an evolutionarily-conserved feature of the germline of many species and plays a crucial role in their fertility. However, the mechanism supporting syncytial organization is largely unknown. Here, we identify a corset-like actomyosin structure within the syncytial germline of Caenorhabditis elegans, surrounding the common rachis. Using laser microsurgery, we demonstrate that actomyosin contractility within this structure generates tension both in the plane of the rachis surface and perpendicular to it, opposing membrane tension. Genetic and pharmacological perturbations, as well as mathematical modeling, reveal a balance of forces within the gonad and show how changing the tension within the actomyosin corset impinges on syncytial germline structure, leading, in extreme cases, to sterility. Thus, our work highlights a unique tissue-level cytoskeletal structure, and explains the critical role of actomyosin contractility in the preservation of a functional germline.

Germline cells in many species are fused to form a syncytium but the mechanics behind the maintenance of these structures are poorly defined. Here, the authors propose an inner contractile actomyosin corset provides a supportive framework to maintain germline architecture in C. elegans.

Recent advances in cerebral cavernous malformation research

Cerebral cavernous malformations (CCM) are manifested by microvascular lesions characterized by leaky endothelial cells with minimal intervening parenchyma predominantly in the central nervous system predisposed to hemorrhagic stroke, resulting in focal neurological defects. Till date, three proteins are implicated in this condition: CCM1 (KRIT1), CCM2 (MGC4607), and CCM3 (PDCD10). These multi-domain proteins form a protein complex via CCM2 that function as a docking site for the CCM signaling complex, which modulates many signaling pathways. Defects in the formation of this signaling complex have been shown to affect a wide range of cellular processes including cell-cell contact stability, vascular angiogenesis, oxidative damage protection and multiple biogenic events. In this review we provide an update on recent advances in structure and function of these CCM proteins, especially focusing on the signaling cascades involved in CCM pathogenesis and the resultant CCM cellular phenotypes in the past decade.

Protein phosphatase 2A is crucial for sarcomere organization in Caenorhabditis elegans striated muscle

Protein phosphatase 2A (PP2A) is a heterotrimer composed of single catalytic and scaffolding subunits and one of several possible regulatory subunits. We identified PPTR-2, a regulatory subunit of PP2A, as a binding partner for the giant muscle protein UNC-89 (obscurin) in Caenorhabditis elegans. PPTR-2 is required for sarcomere organization when its paralogue, PPTR-1, is deficient. PPTR-2 localizes to the sarcomere at dense bodies and M-lines, colocalizing with UNC-89 at M-lines. PP2A components in C. elegans include one catalytic subunit LET-92, one scaffolding subunit (PAA-1), and five regulatory subunits (SUR-6, PPTR-1, PPTR-2, RSA-1, and CASH-1). In adult muscle, loss of function in any of these subunits results in sarcomere disorganization. rsa-1 mutants show an interesting phenotype: one of the two myosin heavy chains, MHC A, localizes as closely spaced double lines rather than single lines. This "double line" phenotype is found in rare missense mutants of the head domain of MHC B myosin, such as unc-54(s74). Analysis of phosphoproteins in the unc-54(s74) mutant revealed two additional phosphoserines in the nonhelical tailpiece of MHC A. Antibodies localize PPTR-1, PAA-1, and SUR-6 to I-bands and RSA-1 to M-lines and I-bands. Therefore, PP2A localizes to sarcomeres and functions in the assembly or maintenance of sarcomeres.

iTAP, a novel iRhom interactor, controls TNF secretion by policing the stability of iRhom/TACE

The apical inflammatory cytokine TNF regulates numerous important biological processes including inflammation and cell death, and drives inflammatory diseases. TNF secretion requires TACE (also called ADAM17), which cleaves TNF from its transmembrane tether. The trafficking of TACE to the cell surface, and stimulation of its proteolytic activity, depends on membrane proteins, called iRhoms. To delineate how the TNF/TACE/iRhom axis is regulated, we performed an immunoprecipitation/mass spectrometry screen to identify iRhom-binding proteins. This identified a novel protein, that we name iTAP (iRhom Tail-Associated Protein) that binds to iRhoms, enhancing the cell surface stability of iRhoms and TACE, preventing their degradation in lysosomes. Depleting iTAP in primary human macrophages profoundly impaired TNF production and tissues from iTAP KO mice exhibit a pronounced depletion in active TACE levels. Our work identifies iTAP as a physiological regulator of TNF signalling and a novel target for the control of inflammation.

Inflammation forms part of the body's defense system against pathogens, but if the system becomes faulty, it can cause problems linked to inflammatory and autoimmune diseases. Immune cells coordinate their activity using specific signaling molecules called cytokines. For example, the cytokine TNF is an important trigger of inflammation and is produced at the surface of immune cells. A specific enzyme called TACE is needed to release TNF, as well as other signaling molecules, including proteins that trigger healing.

Previous work revealed that TACE works with proteins called iRhoms, which regulate its activity and help TACE to reach the surface of the cell to release TNF. To find out how, Oikonomidi et al. screened human cells to see what other proteins interact with iRhoms. The results revealed a new protein named iTAP, which is required to release TNF from the surface of cells. It also protects the TACE-iRhom complex from being destroyed by the cell’s waste disposal system.

When iTAP was experimentally removed in human immune cells, the cells were unable to release TNF. Instead, iRhom and TACE travelled to the cell's garbage system, the lysosome, where the proteins were destroyed. Removing the iTAP gene in mice had the same effect, and the TACE-iRhom complex was no longer found on the surface of the cell, but instead degraded in lysosomes. This suggests that in healthy cells, the iTAP protein prevents the cell from destroying this protein complex.

TNF controls many beneficial processes, including fighting infection and cancer. However, when the immune system releases too many cytokines, it can lead to inflammatory diseases or even cause cancer. Specific drugs that target TNF are not always effective administered on their own, and sometimes, patients stop responding to the drugs. Since the new protein iTAP works as a switch to turn TNF release on or off, it could provide a target for the development of new treatments.

A Sterile 20 Family Kinase and Its Co-factor CCM-3 Regulate Contractile Ring Proteins on Germline Intercellular Bridges

Germ cells in most animals are connected by intercellular bridges, actin-based rings that form stable cytoplasmic connections between cells promoting communication and coordination [1]. Moreover, these connections are required for fertility [1, 2]. Intercellular bridges are proposed to arise from stabilization of the cytokinetic ring during incomplete cytokinesis [1]. Paradoxically, proteins that promote closure of cytokinetic rings are enriched on stably open intercellular bridges [1, 3, 4]. Given this inconsistency, the mechanism of intercellular bridge stabilization is unclear. Here, we used the C. elegans germline as a model for identifying molecular mechanisms regulating intercellular bridges. We report that bridges are actually highly dynamic, changing size at precise times during germ cell development. We focused on the regulation of bridge stability by anillins, key regulators of cytokinetic rings and cytoplasmic bridges [1, 4-7]. We identified GCK-1, a conserved serine/threonine kinase [8], as a putative novel anillin interactor. GCK-1 works together with CCM-3, a known binding partner [9], to promote intercellular bridge stability and limit localization of both canonical anillin and non-muscle myosin II (NMM-II) to intercellular bridges. Additionally, we found that a shorter anillin, known to stabilize bridges [4, 7], also regulates NMM-II levels at bridges. Consistent with these results, negative regulators of NMM-II stabilize intercellular bridges in the Drosophila egg chamber [10, 11]. Together with our findings, this suggests that tuning of myosin levels is a conserved mechanism for the stabilization of intercellular bridges that can occur by diverse molecular mechanisms.