The Cx43-like connexin protein Cx40.8 is differentially localized during fin ontogeny and fin regeneration

Atrial natriuretic peptide signaling co-regulates lipid metabolism and ventricular conduction system gene expression in the embryonic heart

It has been shown that atrial natriuretic peptide (ANP) and its high affinity receptor (NPRA) are involved in the formation of ventricular conduction system (VCS). Inherited genetic variants in fatty acid oxidation (FAO) genes are known to cause conduction abnormalities in newborn children. Although the effect of ANP on energy metabolism in noncardiac cell types is well documented, the role of lipid metabolism in VCS cell differentiation via ANP/NPRA signaling is not known. In this study, histological sections and primary cultures obtained from E11.5 mouse ventricles were analyzed to determine the role of metabolic adaptations in VCS cell fate determination and maturation. Exogenous treatment of E11.5 ventricular cells with ANP revealed a significant increase in lipid droplet accumulation, FAO and higher expression of VCS marker Cx40. Using specific inhibitors, we further identified PPARγ and FAO as critical downstream regulators of ANP-mediated regulation of metabolism and VCS formation.

Transgenic fluorescent zebrafish lines that have revolutionized biomedical research

Since its debut in the biomedical research fields in 1981, zebrafish have been used as a vertebrate model organism in more than 40,000 biomedical research studies. Especially useful are zebrafish lines expressing fluorescent proteins in a molecule, intracellular organelle, cell or tissue specific manner because they allow the visualization and tracking of molecules, intracellular organelles, cells or tissues of interest in real time and in vivo. In this review, we summarize representative transgenic fluorescent zebrafish lines that have revolutionized biomedical research on signal transduction, the craniofacial skeletal system, the hematopoietic system, the nervous system, the urogenital system, the digestive system and intracellular organelles.

Impaired Cx43 gap junction endocytosis causes morphological and functional defects in zebrafish

Gap junctions mediate direct cell-to-cell communication by forming channels that physically couple cells, thereby linking their cytoplasm, permitting the exchange of molecules, ions, and electrical impulses. Gap junctions are assembled from connexin (Cx) proteins, with connexin 43 (Cx43) being the most ubiquitously expressed and best studied. While the molecular events that dictate the Cx43 life cycle have largely been characterized, the unusually short half-life of connexins of only 1-5 hours, resulting in constant endocytosis and biosynthetic replacement of gap junction channels has remained puzzling. The Cx43 C-terminal (CT) domain serves as the regulatory hub of the protein affecting all aspects of gap junction function. Here, deletion within the Cx43 CT (amino acids 256-289), a region known to encode key residues regulating gap junction turnover is employed to examine the effects of dysregulated Cx43 gap junction endocytosis using cultured cells (Cx43^∆256-289) and a zebrafish model (cx43^lh10). We report that this CT deletion causes defective gap junction endocytosis as well as increased gap junction intercellular communication (GJIC). Increased Cx43 protein content in cx43^lh10 zebrafish, specifically in the cardiac tissue, larger gap junction plaques and longer Cx43 protein half-lives coincide with severely impaired development. Our findings demonstrate for the first time that Cx43 gap junction endocytosis is an essential aspect of gap junction function and when impaired, gives rise to significant physiological problems as revealed here for cardiovascular development and function. [Media: see text] [Media: see text] [Media: see text] [Media: see text] [Media: see text].

In vivo Functional Genomics for Undiagnosed Patients: The Impact of Small GTPases Signaling Dysregulation at Pan-Embryo Developmental Scale

While individually rare, disorders affecting development collectively represent a substantial clinical, psychological, and socioeconomic burden to patients, families, and society. Insights into the molecular mechanisms underlying these disorders are required to speed up diagnosis, improve counseling, and optimize management toward targeted therapies. Genome sequencing is now unveiling previously unexplored genetic variations in undiagnosed patients, which require functional validation and mechanistic understanding, particularly when dealing with novel nosologic entities. Functional perturbations of key regulators acting on signals' intersections of evolutionarily conserved pathways in these pathological conditions hinder the fine balance between various developmental inputs governing morphogenesis and homeostasis. However, the distinct mechanisms by which these hubs orchestrate pathways to ensure the developmental coordinates are poorly understood. Integrative functional genomics implementing quantitative in vivo models of embryogenesis with subcellular precision in whole organisms contribute to answering these questions. Here, we review the current knowledge on genes and mechanisms critically involved in developmental syndromes and pediatric cancers, revealed by genomic sequencing and in vivo models such as insects, worms and fish. We focus on the monomeric GTPases of the RAS superfamily and their influence on crucial developmental signals and processes. We next discuss the effectiveness of exponentially growing functional assays employing tractable models to identify regulatory crossroads. Unprecedented sophistications are now possible in zebrafish, i.e., genome editing with single-nucleotide precision, nanoimaging, highly resolved recording of multiple small molecules activity, and simultaneous monitoring of brain circuits and complex behavioral response. These assets permit accurate real-time reporting of dynamic small GTPases-controlled processes in entire organisms, owning the potential to tackle rare disease mechanisms.

Zebrafish as an animal model for biomedical research

Zebrafish have several advantages compared to other vertebrate models used in modeling human diseases, particularly for large-scale genetic mutant and therapeutic compound screenings, and other biomedical research applications. With the impactful developments of CRISPR and next-generation sequencing technology, disease modeling in zebrafish is accelerating the understanding of the molecular mechanisms of human genetic diseases. These efforts are fundamental for the future of precision medicine because they provide new diagnostic and therapeutic solutions. This review focuses on zebrafish disease models for biomedical research, mainly in developmental disorders, mental disorders, and metabolic diseases.

Role of the Connexin C-terminus in skin pattern formation of Zebrafish

Background

Zebrafish display a striped skin pattern on their body; two types of connexins, namely, Connexin39.4 (Cx39.4) and Connexin41.8 (Cx41.8), are involved in stripe pattern formation. Herein, we investigated the role of the C-terminal (CT) domains of Cx39.4 and Cx41.8 in vivo and in vitro.

Methods

To investigate the role of CT domains in vivo, we established transgenic zebrafish lines expressing the CT-domain-modified connexin series in pigmented cells and observed skin patterns in fish. To investigate the role of the CT domains in vitro, we expressed the CT-domain modified connexin series in Neuro-2a (N2a) cells and calculated the plaque formation frequency.

Results

The overexpression of Cx39.4 lacking a CT domain produced skin patterns similar to that produced by full-length Cx39.4 in the cx39.4 ^-/- mutant and in cx39.4 and cx41.8 double-knockout mutant zebrafish. Fluorescence-protein-fused CT-domain-modified Cx39.4 formed gap junction plaques between N2a cells. The overexpression of CT-truncated Cx41.8 rescued the mutant phenotype in the cx41.8 ^-/- mutant but did not function in the double knockout zebrafish. Fluorescence-protein-fused CT-truncated Cx41.8 hardly formed plaques between N2a cells without Cx39.4 but formed gap junction plaques when co-expressed with Cx39.4.

Conclusions

The CT domain of Cx39.4 is not required for protein function, at least in the pigment cells of zebrafish. However, the need for the CT domain of Cx41.8 depends on Cx39.4 expression.

General significance

These results provide evidence for the interactions between Cx39.4 and Cx41.8 in pigment cells of zebrafish and suggest that at least one connexin must have a CT domain.

Connexin Communication Compartments and Wound Repair in Epithelial Tissue

Epithelial tissues line the lumen of tracts and ducts connecting to the external environment. They are critical in forming an interface between the internal and external environment and, following assault from environmental factors and pathogens, they must rapidly repair to maintain cellular homeostasis. These tissue networks, that range from a single cell layer, such as in airway epithelium, to highly stratified and differentiated epithelial surfaces, such as the epidermis, are held together by a junctional nexus of proteins including adherens, tight and gap junctions, often forming unique and localised communication compartments activated for localised tissue repair. This review focuses on the dynamic changes that occur in connexins, the constituent proteins of the intercellular gap junction channel, during wound-healing processes and in localised inflammation, with an emphasis on the lung and skin. Current developments in targeting connexins as corrective therapies to improve wound closure and resolve localised inflammation are also discussed. Finally, we consider the emergence of the zebrafish as a concerted whole-animal model to study, visualise and track the events of wound repair and regeneration in real-time living model systems.

Gap Junction in the Teleost Fish Lineage: Duplicated Connexins May Contribute to Skin Pattern Formation and Body Shape Determination

Gap junctions are intercellular channels that allow passage of ions and small molecules between adjacent cells. Gap junctions in vertebrates are composed of connexons, which are an assembly of six proteins, connexins. Docking of two connexons on the opposite cell surfaces forms a gap junction between the cytoplasm of two neighboring cells. Connexins compose a family of structurally related four-pass transmembrane proteins. In mammals, there are ~20 connexins, each of which contributes to unique permeability of gap junctions, and mutations of some connexin-encoding genes are associated with human diseases. Zebrafish has been predicted to contain 39 connexin-encoding genes; the high number can be attributed to gene duplication during fish evolution, which resulted in diversified functions of gap junctions in teleosts. The determination of body shapes and skin patterns in animal species is an intriguing question. Mathematical models suggest principle mechanisms explaining the diversification of animal morphology. Recent studies have revealed the involvement of gap junctions in fish morphological diversity, including skin pattern formation and body shape determination. This review focuses on connexins in teleosts, which are integrated in the mathematical models explaining morphological diversity of animal skin patterns and body shapes.

trappc11 is required for protein glycosylation in zebrafish and humans

Activation of the unfolded protein response (UPR) can be either adaptive or pathological. We term the pathological UPR that causes fatty liver disease a "stressed UPR." Here we investigate the mechanism of stressed UPR activation in zebrafish bearing a mutation in thetrappc11gene, which encodes a component of the transport protein particle (TRAPP) complex.trappc11mutants are characterized by secretory pathway defects, reflecting disruption of the TRAPP complex. In addition, we uncover a defect in protein glycosylation intrappc11mutants that is associated with reduced levels of lipid-linked oligosaccharides (LLOs) and compensatory up-regulation of genes in the terpenoid biosynthetic pathway that produces the LLO anchor dolichol. Treating wild-type larvae with terpenoid or LLO synthesis inhibitors phenocopies the stressed UPR seen intrappc11mutants and is synthetically lethal withtrappc11mutation. We propose that reduced LLO level causing hypoglycosylation is a mechanism of stressed UPR induction intrappc11mutants. Of importance, in human cells, depletion of TRAPPC11, but not other TRAPP components, causes protein hypoglycosylation, and lipid droplets accumulate in fibroblasts from patients with theTRAPPC11mutation. These data point to a previously unanticipated and conserved role for TRAPPC11 in LLO biosynthesis and protein glycosylation in addition to its established function in vesicle trafficking.

Esco2 regulates cx43 expression during skeletal regeneration in the zebrafish fin

BACKGROUND

Roberts syndrome (RBS) is a rare genetic disorder characterized by craniofacial abnormalities, limb malformation, and often severe mental retardation. RBS arises from mutations in ESCO2 that encodes an acetyltransferase and modifies the cohesin subunit SMC3. Mutations in SCC2/NIPBL (encodes a cohesin loader), SMC3 or other cohesin genes (SMC1, RAD21/MCD1) give rise to a related developmental malady termed Cornelia de Lange syndrome (CdLS). RBS and CdLS exhibit overlapping phenotypes, but RBS is thought to arise through mitotic failure and limited progenitor cell proliferation while CdLS arises through transcriptional dysregulation. Here, we use the zebrafish regenerating fin model to test the mechanism through which RBS-type phenotypes arise.

RESULTS

esco2 is up-regulated during fin regeneration and specifically within the blastema. esco2 knockdown adversely affects both tissue and bone growth in regenerating fins-consistent with a role in skeletal morphogenesis. esco2-knockdown significantly diminishes cx43/gja1 expression which encodes the gap junction connexin subunit required for cell-cell communication. cx43 mutations cause the short fin (sof(b123) ) phenotype in zebrafish and oculodentodigital dysplasia (ODDD) in humans. Importantly, miR-133-dependent cx43 overexpression rescues esco2-dependent growth defects.

CONCLUSIONS

These results conceptually link ODDD to cohesinopathies and provide evidence that ESCO2 may play a transcriptional role critical for human development.

In vivo cell biology in zebrafish - providing insights into vertebrate development and disease

Over the past decades, studies using zebrafish have significantly advanced our understanding of the cellular basis for development and human diseases. Zebrafish have rapidly developing transparent embryos that allow comprehensive imaging of embryogenesis combined with powerful genetic approaches. However, forward genetic screens in zebrafish have generated unanticipated findings that are mirrored by human genetic studies: disruption of genes implicated in basic cellular processes, such as protein secretion or cytoskeletal dynamics, causes discrete developmental or disease phenotypes. This is surprising because many processes that were assumed to be fundamental to the function and survival of all cell types appear instead to be regulated by cell-specific mechanisms. Such discoveries are facilitated by experiments in whole animals, where zebrafish provides an ideal model for visualization and manipulation of organelles and cellular processes in a live vertebrate. Here, we review well-characterized mutants and newly developed tools that underscore this notion. We focus on the secretory pathway and microtubule-based trafficking as illustrative examples of how studying cell biology in vivo using zebrafish has broadened our understanding of the role fundamental cellular processes play in embryogenesis and disease.