Poster No. 134 Zebrafish as a tool to study cardiovascular effects caused by fibrillin impairment

Introduction

Marfan syndrome (MFS) is the most common type of fibrillinopathy with a high predisposition to develop TAAD. A thorough understanding of the underlying mechanisms is still lacking, indicating a particular need for more flexible in vivo models to address this knowledge gap.

Objectives

We aimed to generate a relevant zebrafish model to gain insight into the molecular mechanisms relating fibrillin defects to the cardiovascular system.

Methods

The CRISPR/Cas9 system was used to systematically target the three different fibrillin genes (fbn1, fbn2a and fbn2b) in Tg(kdrl:GFP) reporter zebrafish. Time-lapse fluorescent microscopy was used to evaluate the cardiovascular phenotype.

Results

zebrafish lacking fbn1 and/or fbn2a do not show any cardiovascular phenotype during early-stage development. On the other hand, approximately 50% of homozygous fbn2b mutant (fbn2b-/-) zebrafish embryo's show a severe phenotype characterized by endocardial detachment, leading to vascular embolism and premature mortality at 7–9 dpf. Interestingly, the remaining fbn2b-/- zebrafish survive until adulthood, but during larval stages already develop a dilation of the bulbus arteriosus. The caudal vein of all fbn2b-/- embryos also develops abnormally as a cavernous structure lacking vessel integrity. This phenotype is resolved in embryos retaining normal blood flow and aggravated upon pharmacological inhibition of blood flow during development.

Conclusion

These data indicate that fbn2b-/- zebrafish model recapitulates cardiovascular complications, and can be considered as a relevant model to study the mechanisms underlying MFS pathogenesis. Our preliminary data suggest that there is an interplay between fibrillin deficiency and biomechanical signaling in the regulation of cardiovascular development.

Characterizing the cardiovascular phenotype of a new zebrafish model of Marfan syndrome

Background

Marfan syndrome (MFS) is a rare disease caused by a defect in the fibrillin-1 gene (FBN1), with potentially severe cardiovascular manifestations. MFS patients are particularly susceptible to a progressive aortic dilation leading to potential dissection and wall rupture. No causal treatment for the disease is available and current medical treatment is aimed at slowing aortic disease progression to minimize severe complications. When indicated, surgical repair of the aortic defect is performed. Although these strategies have clearly led to improved survival, some patients still present with fatal complications.

Purpose

To generate a new flexible zebrafish model of MFS to gain a better understanding of the underlying pathophysiological mechanisms and to find new treatment options.

Methods

We used the CRISPR/Cas9 method to disrupt the 3 fibrillin genes in zebrafish (fbn1, fbn2a, and fbn2b). The Tg(kdrl:EGFP) reporter was used to visualize cardiovascular structure by fluorescent microscopy up to 8 days post fertilization (dpf). A subset of embryos was treated with the myosin inhibitor 2,3-butanedione monoxime (BDM).

Results

We found that zebrafish lacking fbn1 and/or fbn2a do not show any detectable phenotype during development. No evidence of induction of genetic compensation was found in these mutant lines.

Zebrafish deficient in fbn2b however do show strong phenotypes, including fully penetrant finfold atrophy (Fig. 1A-B and E, arrowhead). On average 50% of homozygous fbn2b mutant (fbn2b−/−) zebrafish embryos show endocardial detachment (Fig. 1C-D; a:atrium, v:ventricle), leading to vascular embolism, pericardial edema (Fig. 1B, arrow), loss of blood flow, and ultimately death at 7–9 dpf. Interestingly, fbn2b−/− without endocardial detachment survive normally, but develop a dilated bulbus arteriosus phenotype during larval stages (Fig. 1F-G, arrow; 1H: diameter during minimal and maximal distension, *: P<0.05 and ***: P<0.001 by Sidak's post-test after Two-Way ANOVA). This anatomical structure is strongly related to the aortic root in humans, which is the predominant location of aortic dilation in MFS.

All fbn2b−/− embryos show abnormal early development of the caudal vein as a cavernous structure lacking vessel integrity (Fig. 2, arrowheads). This phenotype resolves in embryos retaining normal blood flow. We found that fbn2b−/− embryos raised in BDM to inhibit blood flow show a more severe caudal vein phenotype than wild-type (WT) controls (Fig. 2, yellow line: severe vascular dilation).

Conclusion

Loss of fbn2b, but not the other fibrillin genes, in zebrafish results in cardiovascular manifestations overlapping with MFS. These data indicate that fbn2b−/− zebrafish can be a relevant model to explore the mechanisms leading from fibrillin deficiency to the cardiovascular symptoms observed in MFS. Our preliminary results suggest that there is an interplay between fibrillin deficiency and biomechanical signaling.

Funding Acknowledgement

Type of funding sources: Foundation. Main funding source(s): Fund Baillet Latour Grant for Medical Research Figure 1. Phenotype of fbn2b−/− larvaeFigure 2. Effect of blood flow in fbn2b−/−

Spontaneous Right Ventricular Pseudoaneurysms and Increased Arrhythmogenicity in a Mouse Model of Marfan Syndrome

Patients with Marfan syndrome (MFS), a connective tissue disorder caused by pathogenic variants in the gene encoding the extracellular matrix protein fibrillin-1, have an increased prevalence of primary cardiomyopathy, arrhythmias, and sudden cardiac death. We have performed an in-depth in vivo and ex vivo study of the cardiac phenotype of Fbn1mgR/mgR mice, an established mouse model of MFS with a severely reduced expression of fibrillin-1. Using ultrasound measurements, we confirmed the presence of aortic dilatation and observed cardiac diastolic dysfunction in male Fbn1mgR/mgR mice. Upon post-mortem examination, we discovered that the mutant mice consistently presented myocardial lesions at the level of the right ventricular free wall, which we characterized as spontaneous pseudoaneurysms. Histological investigation demonstrated a decrease in myocardial compaction in the MFS mouse model. Furthermore, continuous 24 h electrocardiographic analysis showed a decreased heart rate variability and an increased prevalence of extrasystolic arrhythmic events in Fbn1mgR/mgR mice compared to wild-type littermates. Taken together, in this paper we document a previously unreported cardiac phenotype in the Fbn1mgR/mgR MFS mouse model and provide a detailed characterization of the cardiac dysfunction and rhythm disorders which are caused by fibrillin-1 deficiency. These findings highlight the wide spectrum of cardiac manifestations of MFS, which might have implications for patient care.

Zebrafish type I collagen mutants faithfully recapitulate human type I collagenopathies

The type I collagenopathies are a group of heterogeneous connective tissue disorders, that are caused by mutations in the genes encoding type I collagen and include specific forms of osteogenesis imperfecta (OI) and the Ehlers-Danlos syndrome (EDS). These disorders present with a broad disease spectrum and large clinical variability of which the underlying genetic basis is still poorly understood. In this study, we systematically analyzed skeletal phenotypes in a large set of zebrafish, with diverse mutations in the genes encoding type I collagen, representing different genetic forms of human OI, and a zebrafish model resembling human EDS, which harbors a number of soft connective tissues defects, typical of EDS. Furthermore, we provide insight into how zebrafish and human type I collagen are compositionally and functionally related, which is relevant in the interpretation of human type I collagen-related disease models. Our studies reveal a high degree of intergenotype variability in phenotypic expressivity that closely correlates with associated OI severity. Furthermore, we demonstrate the potential for select mutations to give rise to phenotypic variability, mirroring the clinical variability associated with human disease pathology. Therefore, our work suggests the future potential for zebrafish to aid in identifying unknown genetic modifiers and mechanisms underlying the phenotypic variability in OI and related disorders. This will improve diagnostic strategies and enable the discovery of new targetable pathways for pharmacological intervention.

Accurate quantification of homologous recombination in zebrafish: brca2 deficiency as a paradigm

Homologous Recombination (HR) repair is essential for repairing DNA double strand breaks (DSB) in dividing cells and preventing tumorigenesis. BRCA2 plays an important role in HR by recruiting the DNA recombinase RAD51 to the DSB. Despite being a popular model organism in genetic and cancer research, knowledge on the conservation of the HR pathway and function of zebrafish Brca2 is limited. To evaluate this, we developed a Rad51 foci assay in zebrafish embryos. We identified the zebrafish embryonic intestinal tissue as an ideal target for Rad51 immunostaining. After inducing DSB through irradiation, Rad51 foci were present in irradiated embryos but not in unirradiated controls. We present a method for accurate quantification of HR. Both morpholino-induced knockdown and knockout of Brca2 lead to almost complete absence of Rad51 foci in irradiated embryos. These findings indicate conserved function of Brca2 in zebrafish. Interestingly, a statistically significant decrease in Rad51 foci was observed in Brca2 heterozygous carriers compared to wild types, indicative of haploinsufficiency, a hypothesised cause of some tumours in patients with a germline BRCA2 mutation. In conclusion, we demonstrated the suitability of zebrafish as an excellent in vivo model system for studying the HR pathway and its functionality.

ZEB2-transgene expression in the epidermis compromises the integrity of the epidermal barrier through the repression of different tight junction proteins

Epithelial homeostasis within the epidermis is maintained by means of multiple cell-cell adhesion complexes such as adherens junctions, tight junctions, gap junctions, and desmosomes. These complexes co-operate in the formation and the regulation of the epidermal barrier. Disruption of the epidermal barrier through the deregulation of the above complexes is the cause behind a number of skin disorders such as psoriasis, dermatitis, keratosis, and others. During epithelial-to-mesenchymal transition (EMT), epithelial cells lose their adhesive capacities and gain mesenchymal properties. ZEB transcription factors are key inducers of EMT. In order to gain a better understanding of the functional role of ZEB2 in epidermal homeostasis, we generated a mouse model with conditional overexpression of Zeb2 in the epidermis. Our analysis revealed that Zeb2 expression in the epidermis leads to hyperproliferation due to the combined downregulation of different tight junction proteins compromising the epidermal barrier. Using two epidermis-specific in vivo models and in vitro promoter assays, we identified occludin as a new Zeb2 target gene. Immunohistological analysis performed on human skin biopsies covering various pathogeneses revealed ZEB2 expression in the epidermis of pemphigus vulgaris. Collectively, our data support the notion for a potential role of ZEB2 in intracellular signaling of this disease.

SIP1/ZEB2 induces EMT by repressing genes of different epithelial cell-cell junctions

SIP1/ZEB2 is a member of the deltaEF-1 family of two-handed zinc finger nuclear factors. The expression of these transcription factors is associated with epithelial mesenchymal transitions (EMT) during development. SIP1 is also expressed in some breast cancer cell lines and was detected in intestinal gastric carcinomas, where its expression is inversely correlated with that of E-cadherin. Here, we show that expression of SIP1 in human epithelial cells results in a clear morphological change from an epithelial to a mesenchymal phenotype. Induction of this epithelial dedifferentiation was accompanied by repression of several cell junctional proteins, with concomitant repression of their mRNA levels. Besides E-cadherin, other genes coding for crucial proteins of tight junctions, desmosomes and gap junctions were found to be transcriptionally regulated by the transcriptional repressor SIP1. Moreover, study of the promoter regions of selected genes by luciferase reporter assays and chromatin immunoprecipitation shows that repression is directly mediated by SIP1. These data indicate that, during epithelial dedifferentiation, SIP1 represses in a coordinated manner the transcription of genes coding for junctional proteins contributing to the dedifferentiated state; this repression occurs by a general mechanism mediated by Smad Interacting Protein 1 (SIP1)-binding sites.

The two-handed E box binding zinc finger protein SIP1 downregulates E-cadherin and induces invasion

Transcriptional downregulation of E-cadherin appears to be an important event in the progression of various epithelial tumors. SIP1 (ZEB-2) is a Smad-interacting, multi-zinc finger protein that shows specific DNA binding activity. Here, we report that expression of wild-type but not of mutated SIP1 downregulates mammalian E-cadherin transcription via binding to both conserved E2 boxes of the minimal E-cadherin promoter. SIP1 and Snail bind to partly overlapping promoter sequences and showed similar silencing effects. SIP1 can be induced by TGF-beta treatment and shows high expression in several E-cadherin-negative human carcinoma cell lines. Conditional expression of SIP1 in E-cadherin-positive MDCK cells abrogates E-cadherin-mediated intercellular adhesion and simultaneously induces invasion. SIP1 therefore appears to be a promoter of invasion in malignant epithelial tumors.

Mechanisms of downregulation of transfected E-cadherin cDNA during formation of invasive tumors in syngeneic mice

Loss of E-cadherin expression has been observed both in experimental tumors and in human cancers and is related to invasiveness and poor differentiation. The E-cadherin-negative mouse mesenchymal tumor cell line MO4 was transfected with several plasmids expressing mouse E-cadherin cDNA. These plasmids differed from each other by the extent of E-cadherin-specific 3' untranslated region (UTR) sequences and by the use of different constitutive promoters. Transfectants were isolated that expressed functional E-cadherin in a homogeneous way. In syngeneic mice, such MO4-Ecad transfectants invariably produced malignant fibrosarcoma-like tumors, which were completely E-cadherin-negative at the protein level. Northern blotting revealed that E-cadherin mRNA expression was downregulated in some but not all MO4-Ecad tumors. Downregulation was caused by mRNA instability triggered by particular 3' UTR sequences. This in vivo downregulation of E-cadherin in malignant MO4-Ecad tumors turned out to be reversible and is likely to be mediated by host factors to be further identified.

E-cadherin and metastasin (mts-1/S100A4) expression levels are inversely regulated in two tumor cell families

Metastasin is putatively associated with cytoskeletal proteins and may influence cell motility, although its exact physiological role is not known. Because E-cadherin is an invasion suppressor molecule, and metastasin a metastasis-inducing molecule, we wondered which molecule was playing a dominant role in the balance that finally leads to noninvasiveness or invasiveness. The expression levels of E-cadherin and metastasin were monitored in two mouse tumor cell families and were found to be inversely regulated. Moreover, overexpression obtained via transfection of plasmids coding for either one of these two molecules abrogated expression of the other molecule as investigated via Northern and Western blotting experiments. Invasiveness was accordingly influenced. Expression levels of alpha- and beta-catenins were not influenced by up-regulated metastasin, but their intracellular distribution was disturbed. The present results suggest that metastasin-induced invasiveness of several malignant tumor cells is at least partially caused by down-regulation of E-cadherin.