Mechanisms of otoconia and otolith development.

BACKGROUND: Otoconia are bio-crystals that couple mechanic forces to the sensory hair cells in the utricle and saccule, a process essential for us to sense linear acceleration and gravity for the purpose of maintaining bodily balance. In fish, structurally similar bio-crystals called otoliths mediate both balance and hearing. Otoconia abnormalities are common and can cause vertigo and imbalance in humans. However, the molecular etiology of these illnesses is unknown, as investigators have only begun to identify genes important for otoconia formation in recent years. RESULTS: To date, in-depth studies of selected mouse otoconial proteins have been performed, and about 75 zebrafish genes have been identified to be important for otolith development. CONCLUSIONS: This review will summarize recent findings as well as compare otoconia and otolith development. It will provide an updated brief review of otoconial proteins along with an overview of the cells and cellular processes involved. While continued efforts are needed to thoroughly understand the molecular mechanisms underlying otoconia and otolith development, it is clear that the process involves a series of temporally and spatially specific events that are tightly coordinated by numerous proteins. Such knowledge will serve as the foundation to uncover the molecular causes of human otoconia-related disorders.

The heparan sulfate editing enzyme Sulf1 plays a novel role in zebrafish VegfA mediated arterial venous identity.

Arterial and venous specification is critical for establishing and maintaining a functioning vascular system, and defects in key arteriovenous signaling pathways including VEGF (vascular endothelial growth factor) lead to congenital arteriopathies. The activities of VEGF, are in part controlled by heparan sulfate (HS) proteoglycans, significant components of the endothelial glycocalyx. The level of 6-O sulfation on HS polysaccharide chains, that mediate the interaction between HS and VEGFA, is edited at the cell surface by the enzyme SULF1. We investigated the role of sulf1 in vascular development. In zebrafish sulf1 is expressed in the head and tail vasculature, corresponding spatially and temporally with vascular development. Targeted knockdown of sulf1 by antisense morpholinos resulted in severe vascular patterning and maturation defects. 93 % of sulf1 morphants show dysmorphogenesis in arterial development leading to occlusion of the distal aorta and lack of axial and cranial circulation. Co-injection of vegfa165 mRNA rescued circulatory defects. While the genes affecting haematopoiesis are unchanged, expression of several arterial markers downstream of VegfA signalling such as notch and ephrinB2 are severely reduced in the dorsal aorta, with a concomitant increase in expression of the venous markers flt4 in the dorsal aorta of the morphants. Furthermore, in vitro, lack of SULF1 expression downregulates VEGFA-mediated arterial marker expression, confirming that Sulf1 mediates arterial specification by regulating VegfA165 activity. This study provides the first in vivo evidence for the integral role of the endothelial glycocalyx in specifying arterial-venous identity, vascular patterning and arterial integrity, and will help to better understand congenital arteriopathies.

Teneurin trans-axonal signaling prunes topographically missorted axons.

Building precise neural circuits necessitates the elimination of axonal projections that have inaccurately formed during development. Although axonal pruning is a selective process, how it is initiated and controlled in vivo remains unclear. Here, we show that trans-axonal signaling mediated by the cell surface molecules Glypican-3, Teneurin-3, and Latrophilin-3 prunes misrouted retinal axons in the visual system. Retinotopic neuron transplantations revealed that pioneer ventral axons that elongate first along the optic tract instruct the pruning of dorsal axons that missort in that region. Glypican-3 and Teneurin-3 are both selectively expressed by ventral retinal ganglion cells and cooperate for correcting missorted dorsal axons. The adhesion G-protein-coupled receptor Latrophilin-3 signals along dorsal axons to initiate the elimination of topographic sorting errors. Altogether, our findings show an essential function for Glypican-3, Teneurin-3, and Latrophilin-3 in topographic tract organization and demonstrate that axonal pruning can be initiated by signaling among axons themselves.

Essential anatomy for core clerkships: A clinical perspective.

Clerkships are defining experiences for medical students in which students integrate basic science knowledge with clinical information as they gain experience in diagnosing and treating patients in a variety of clinical settings. Among the basic sciences, there is broad agreement that anatomy is foundational for medical practice. Unfortunately, there are longstanding concerns that student knowledge of anatomy is below the expectations of clerkship directors and clinical faculty. Most allopathic medical schools require eight "core" clerkships: internal medicine (IM), pediatrics (PD), general surgery (GS), obstetrics and gynecology (OB), psychiatry (PS), family medicine (FM), neurology (NU), and emergency medicine (EM). A targeted needs assessment was conducted to determine the anatomy considered important for each core clerkship based on the perspective of clinicians teaching in those clerkships. A total of 525 clinical faculty were surveyed at 24 United States allopathic medical schools. Participants rated 97 anatomical structure groups across all body regions on a 1-4 Likert-type scale (1 = not important, 4 = essential). Non-parametric ANOVAs determined if differences existed between clerkships. Combining all responses, 91% of anatomical structure groups were classified as essential or more important. Clinicians in FM, EM, and GS rated anatomical structures in most body regions significantly higher than at least one other clerkship (p = 0.006). This study provides an evidence-base of anatomy content that should be considered important for each core clerkship and may assist in the development and/or revision of preclinical curricula to support the clinical training of medical students.

Specific sides to multifaceted glycosaminoglycans are observed in embryonic development.

Ubiquitously found in the extracellular matrix and attached to the surface of most cells, glycosaminoglycans (GAGs) mediate many intercellular interactions. Originally described in 1889 as the primary carbohydrate in cartilage and then in 1916 as a coagulation inhibitor from liver, various GAGs have since been identified as key regulators of normal physiology. GAGs are critical mediators of differentiation, migration, tissue morphogenesis, and organogenesis during embryonic development. While GAGs are simple polysaccharide chains, many GAGs acquire a considerable degree of complexity by extensive modifications involving sulfation and epimerization. Embryos that lack specific GAG modifying enzymes have distinct developmental defects, illuminating the importance of GAG complexity. Revealing how these complex molecules specifically function in the embryo has often required additional approaches, the results of which suggest that GAG modifications might instructively mediate embryonic development.

Massively parallel sequencing identifies the gene Megf8 with ENU-induced mutation causing heterotaxy.

Forward genetic screens with ENU (N-ethyl-N-nitrosourea) mutagenesis can facilitate gene discovery, but mutation identification is often difficult. We present the first study in which an ENU-induced mutation was identified by massively parallel DNA sequencing. This mutation causes heterotaxy and complex congenital heart defects and was mapped to a 2.2-Mb interval on mouse chromosome 7. Massively parallel sequencing of the entire 2.2-Mb interval identified 2 single-base substitutions, one in an intergenic region and a second causing replacement of a highly conserved cysteine with arginine (C193R) in the gene Megf8. Megf8 is evolutionarily conserved from human to fruit fly, and is observed to be ubiquitously expressed. Morpholino knockdown of Megf8 in zebrafish embryos resulted in a high incidence of heterotaxy, indicating a conserved role in laterality specification. Megf8(C193R) mouse mutants show normal breaking of symmetry at the node, but Nodal signaling failed to be propagated to the left lateral plate mesoderm. Videomicroscopy showed nodal cilia motility, which is required for left-right patterning, is unaffected. Although this protein is predicted to have receptor function based on its amino acid sequence, surprisingly confocal imaging showed it is translocated into the nucleus, where it is colocalized with Gfi1b and Baf60C, two proteins involved in chromatin remodeling. Overall, through the recovery of an ENU-induced mutation, we uncovered Megf8 as an essential regulator of left-right patterning.

Ectodermal syndecan-2 mediates left-right axis formation in migrating mesoderm as a cell-nonautonomous Vg1 cofactor.

Heparan sulfate proteoglycans expressed on the Xenopus animal cap ectoderm have been implicated in transmitting left-right information to heart and gut primordia. We report here that syndecan-2 functions in the ectoderm to mediate cardiac and visceral situs, upstream of known asymmetrically expressed genes but independently of its ability to mediate fibronectin fibrillogenesis. Left-right development is dependent on a distinct subset of glycosaminoglycan attachment sites on syndecan-2. A novel in vivo approach with enterokinase demonstrates that syndecan-2 functions in left-right patterning during early gastrulation. We describe a cell-nonautonomous role for ectodermal syndecan-2 in transmitting left-right information to migrating mesoderm. The results further suggest that this function may be related to the transduction of Vg1-related signals.

Expression of Protein-Coding Gene Orthologs in Zebrafish and Mouse Inner Ear Non-sensory Supporting Cells.

Non-mammalian vertebrates, including zebrafish, retain the ability to regenerate hair cells (HCs) due to unknown molecular mechanisms that regulate proliferation and conversion of non-sensory supporting cells (nsSCs) to HCs. This regenerative capacity is not conserved in mammals. Identification of uniquely expressed orthologous genes in zebrafish nsSCs may reveal gene candidates involved in the proliferation and transdifferentiation of zebrafish nsSCs to HCs in the inner ear. A list of orthologous protein-coding genes was generated based on an Ensembl Biomart comparison of the zebrafish and mouse genomes. Our previously published RNA-seq-based transcriptome datasets of isolated inner ear zebrafish nsSCs and HCs, and mouse non-sensory supporting pillar and Deiters' cells, and HCs, were merged to analyze gene expression patterns between the two species. Out of 17,498 total orthologs, 11,752 were expressed in zebrafish nsSCs and over 10,000 orthologs were expressed in mouse pillar and Deiters' cells. Differentially expressed genes common among the zebrafish nsSCs and mouse pillar and Deiters' cells, compared to species-specific HCs, included 306 downregulated and 314 upregulated genes; however, over 1,500 genes were uniquely upregulated in zebrafish nsSCs. Functional analysis of genes uniquely expressed in nsSCs identified several transcription factors associated with cell fate determination, cell differentiation and nervous system development, indicating inherent molecular properties of nsSCs that promote self-renewal and transdifferentiation into new HCs. Our study provides a means of characterizing these orthologous genes, involved in proliferation and transdifferentiation of nsSCs to HCs in zebrafish, which may lead to identification of potential targets for HC regeneration in mammals.

Zebrafish otolith biomineralization requires polyketide synthase.

Deflecting biomineralized crystals attached to vestibular hair cells are necessary for maintaining balance. Zebrafish (Danio rerio) are useful organisms to study these biomineralized crystals called otoliths, as many required genes are homologous to human otoconial development. We sought to identify and characterize the causative gene in a trio of homozygous recessive mutants, no content (nco) and corkscrew (csr), and vanished (vns), which fail to develop otoliths during early ear development. We show that nco, csr, and vns have potentially deleterious mutations in polyketide synthase (pks1), a multi-modular protein that has been previously implicated in biomineralization events in chordates and echinoderms. We found that Otoconin-90 (Oc90) expression within the otocyst is diffuse in nco and csr; therefore, it is not sufficient for otolith biomineralization in zebrafish. Similarly, normal localization of Otogelin, a protein required for otolith tethering in the otolithic membrane, is not sufficient for Oc90 attachment. Furthermore, eNOS signaling and Endothelin-1 signaling were the most up- and down-regulated pathways during otolith agenesis in nco, respectively. Our results demonstrate distinct processes for otolith nucleation and biomineralization in vertebrates and will be a starting point for models that are independent of Oc90-mediated seeding. This study will serve as a basis for investigating the role of eNOS signaling and Endothelin-1 signaling during otolith formation.

RNA-seq transcriptomic analysis of adult zebrafish inner ear hair cells.

Although hair cells are the sensory receptors of the auditory and vestibular systems in the ears of all vertebrates, hair cell properties are different between non-mammalian vertebrates and mammals. To understand the basic biological properties of hair cells from non-mammalian vertebrates, we examined the transcriptome of adult zebrafish auditory and vestibular hair cells. GFP-labeled hair cells were isolated from inner-ear sensory epithelia of a pou4f3 promoter-driven GAP-GFP line of transgenic zebrafish. One thousand hair cells and 1,000 non-sensory surrounding cells (nsSCs) were separately collected for each biological replicate, using the suction pipette technique. RNA sequencing of three biological replicates for the two cell types was performed and analyzed. Comparisons between hair cells and nsSCs allow identification of enriched genes in hair cells, which may underlie hair cell specialization. Our dataset provides an extensive resource for understanding the molecular mechanisms underlying morphology, function, and pathology of adult zebrafish hair cells. It also establishes a framework for future characterization of genes expressed in hair cells and the study of hair cell evolution.

Heparan Sulfate Proteoglycans Regulate Fgf Signaling and Cell Polarity during Collective Cell Migration.

Collective cell migration is a highly regulated morphogenetic movement during embryonic development and cancer invasion that involves the precise orchestration and integration of cell-autonomous mechanisms and environmental signals. Coordinated lateral line primordium migration is controlled by the regulation of chemokine receptors via compartmentalized Wnt/ß-catenin and fibroblast growth factor (Fgf) signaling. Analysis of mutations in two exostosin glycosyltransferase genes (extl3 and ext2) revealed that loss of heparan sulfate (HS) chains results in a failure of collective cell migration due to enhanced Fgf ligand diffusion and loss of Fgf signal transduction. Consequently, Wnt/ß-catenin signaling is activated ectopically, resulting in the subsequent loss of the chemokine receptor cxcr7b. Disruption of HS proteoglycan (HSPG) function induces extensive, random filopodia formation, demonstrating that HSPGs are involved in maintaining cell polarity in collectively migrating cells. The HSPGs themselves are regulated by the Wnt/ß-catenin and Fgf pathways and thus are integral components of the regulatory network that coordinates collective cell migration with organ specification and morphogenesis.

Yes-associated protein 65 (YAP) expands neural progenitors and regulates Pax3 expression in the neural plate border zone.

Yes-associated protein 65 (YAP) contains multiple protein-protein interaction domains and functions as both a transcriptional co-activator and as a scaffolding protein. Mouse embryos lacking YAP did not survive past embryonic day 8.5 and showed signs of defective yolk sac vasculogenesis, chorioallantoic fusion, and anterior-posterior (A-P) axis elongation. Given that the YAP knockout mouse defects might be due in part to nutritional deficiencies, we sought to better characterize a role for YAP during early development using embryos that develop externally. YAP morpholino (MO)-mediated loss-of-function in both frog and fish resulted in incomplete epiboly at gastrulation and impaired axis formation, similar to the mouse phenotype. In frog, germ layer specific genes were expressed, but they were temporally delayed. YAP MO-mediated partial knockdown in frog allowed a shortened axis to form. YAP gain-of-function in Xenopus expanded the progenitor populations in the neural plate (sox2(+)) and neural plate border zone (pax3(+)), while inhibiting the expression of later markers of tissues derived from the neural plate border zone (neural crest, pre-placodal ectoderm, hatching gland), as well as epidermis and somitic muscle. YAP directly regulates pax3 expression via association with TEAD1 (N-TEF) at a highly conserved, previously undescribed, TEAD-binding site within the 5' regulatory region of pax3. Structure/function analyses revealed that the PDZ-binding motif of YAP contributes to the inhibition of epidermal and somitic muscle differentiation, but a complete, intact YAP protein is required for expansion of the neural plate and neural plate border zone progenitor pools. These results provide a thorough analysis of YAP mediated gene expression changes in loss- and gain-of-function experiments. Furthermore, this is the first report to use YAP structure-function analyzes to determine which portion of YAP is involved in specific gene expression changes and the first to show direct in vivo evidence of YAP's role in regulating pax3 neural crest expression.

Heparan sulfate core proteins in cell-cell signaling.

Heparan sulfate (HS) binds numerous extracellular ligands, including cell-cell signaling molecules and their signal-transducing receptors. Ligand binding sites in HS have specific sulfation patterns; and several observations suggest that the HS sulfation pattern is the same for every HS chain that a cell synthesizes, regardless of the core protein to which it is attached. Nonetheless, virtually every Drosophila, zebrafish, Xenopus, and mouse that lacks a specific HS core protein has a mutant phenotype, even though other HS core proteins are expressed in the affected cells. Genetic manipulation of HS core protein genes is beginning to indicate that HS core proteins have functional specificities that are required during distinct stages of development.

PKCgamma regulates syndecan-2 inside-out signaling during xenopus left-right development.

The transmembrane proteoglycan syndecan-2 cell nonautonomously regulates left-right (LR) development in migrating mesoderm by an unknown mechanism, leading to LR asymmetric gene expression and LR orientation of the heart and gut. Here, we demonstrate that protein kinase C gamma (PKCgamma) mediates phosphorylation of the cytoplasmic domain of syndecan-2 in right, but not left, animal cap ectodermal cells. Notably, both phosphorylation states of syndecan-2 are obligatory for normal LR development, with PKCgamma-dependent phosphorylated syndecan-2 in right ectodermal cells and nonphosphorylated syndecan-2 in left cells. The ectodermal cells contact migrating mesodermal cells during early gastrulation, concurrent with the transmission of LR information. This precedes the appearance of monocilia and is one of the earliest steps of LR development. These results demonstrate that PKCgamma regulates the cytoplasmic phosphorylation of syndecan-2 and, consequently, syndecan-2-mediated inside-out signaling to adjacent cells.