Hedgehog Signaling Regulates Neurogenesis in the Larval and Adult Zebrafish Hypothalamus.

Neurogenesis is now known to play a role in adult hypothalamic function, yet the cell-cell mechanisms regulating this neurogenesis remain poorly understood. Here, we show that Hedgehog (Hh)/Gli signaling positively regulates hypothalamic neurogenesis in both larval and adult zebrafish and is necessary and sufficient for normal hypothalamic proliferation rates. Hh-responsive radial glia represent a relatively highly proliferative precursor population that gives rise to dopaminergic, serotonergic, and GABAergic neurons. In situ and transgenic reporter analyses revealed substantial heterogeneity in cell-cell signaling within the hypothalamic niche, with slow cycling Nestin-expressing cells residing among distinct and overlapping populations of Sonic Hh (Shh)-expressing, Hh-responsive, Notch-responsive, and Wnt-responsive radial glia. This work shows for the first time that Hh/Gli signaling is a key component of the complex cell-cell signaling environment that regulates hypothalamic neurogenesis throughout life.

Integrity in nursing students: A concept analysis.

OBJECTIVE: The purpose of this review was to clarify the concept of nursing student integrity. Unlike published reviews that highlight the absence of integrity, the author sought to identify the positive defining characteristics of integrity. DESIGN: Concept analysis. DATA SOURCES: A literature review was conducted using the Cumulative Index for Nursing and Allied Health, Social Science Index, Psyc Info and Medline. Inclusion criteria included peer reviewed articles, written in English, with no limitation on publication date. The search also revealed that a concept analysis of integrity had not been performed. REVIEW METHODS: Walker and Avant's (2005) eight step modification of Wilson's (1963) classic concept analysis procedure was used as the organizing framework to explore how the concept of integrity is defined in the current literature. RESULTS: Nursing student integrity was analyzed using Walker and Avant's method of concept analysis: concept definition, defining attribute, model, borderline, related and contrary cases, antecedents, consequences and empirical referents (Walker and Avant 2005). Defining attributes to nursing student integrity were honesty, ethical behavior and professionalism. Antecedents to integrity included an academic culture of respect, characterized by student-faculty relationships derived from mutual respect, trust and a shared learning goal. CONCLUSIONS: This review identified honesty, ethical behavior and professionalism as the defining attributes of integrity. The importance of faculty as role models of integrity was paramount in building a culture of honesty. Future research should explore faculty perceptions of their professional role as models of integrity, and faculty perceptions of behaviors that promote a culture of respect.

Distinct expression patterns of syndecans in the embryonic zebrafish brain.

Axon pathfinding in the neuroepithelium of embryonic brain is dependent on a variety of short and long range guidance cues. Heparan sulfate proteoglycans such as syndecans act as modulators of these cues and their importance in neural development is highlighted by their phylogenetic conservation. In Drosophilia, a single syndecan is present on the surface of axon growth cones and is required for chemorepulsive signalling during midline crossing. Understanding the role of syndecans in the vertebrate nervous system is challenging given that there are four homologous genes, syndecans 1-4. We show here that syndecans 2-4 are expressed in the zebrafish embryonic brain during the major period of axon growth. These genes show differing expression patterns in the brain which provides putative insights into their functional specificity.

Frizzled-3a and slit2 genetically interact to modulate midline axon crossing in the telencephalon.

The anterior commissure forms the first axon connections between the two sides of the embryonic telencephalon. We investigated the role of the transmembrane receptor Frizzled-3a in the development of this commissure using zebrafish as an experimental model. Knock down of Frizzled-3a resulted in complete loss of the anterior commissure. This defect was accompanied by a loss of the glial bridge, expansion of the slit2 expression domain and perturbation of the midline telencephalic-diencephalic boundary. Blocking Slit2 activity following knock down of Frizzled-3a effectively rescued the anterior commissure defect which suggested that Frizzled-3a was indirectly controlling the growth of axons across the rostral midline. We have shown here that Frizzled-3a is essential for normal development of the commissural plate and that loss-of-function causes Slit2-dependent defects in axon midline crossing in the embryonic vertebrate forebrain. These data supports a model whereby Wnt signaling through Frizzled-3a attenuates expression of Slit2 in the rostral midline of the forebrain. The absence of Slit2 facilitates the formation of a midline bridge of glial cells which is used as a substrate for commissural axons. In the absence of this platform of glia, commissural axons fail to cross the rostral midline of the forebrain.

A dynamic Gli code interprets Hh signals to regulate induction, patterning, and endocrine cell specification in the zebrafish pituitary.

Hedgehog (Hh) signaling is necessary for the induction and functional patterning of the pituitary placode, however the mechanisms by which Hh signals are interpreted by placodal cells are unknown. Here we show distinct temporal requirements for Hh signaling in endocrine cell differentiation and describe a dynamic Gli transcriptional response code that interprets these Hh signals within the developing adenohypophysis. Gli1 is required for the differentiation of selected endocrine cell types and acts as the major activator of Hh-mediated pituitary induction, while Gli2a and Gli2b contribute more minor activator functions. Intriguingly, this Gli response code changes as development proceeds. Gli1 continues to be required for the activation of the Hh response anteriorly in the pars distalis. In contrast, Gli2b is required to repress Hh target gene expression posteriorly in the pars intermedia. Consistent with these changing roles, gli1, gli2a, and gli2b, but not gli3, are expressed in pituitary precursor cells at the anterior neural ridge. Later in development, gli1 expression is maintained throughout the adenohypophysis while gli2a and gli2b expression are restricted to the pars intermedia. Given the link between Hh signaling and pituitary adenomas in humans, our data suggest misregulation of Gli function may contribute to these common pituitary tumors.

BOC, brother of CDO, is a dorsoventral axon-guidance molecule in the embryonic vertebrate brain.

The early axon scaffolding in the embryonic vertebrate brain consists of a series of ventrally projecting axon tracts that grow into a single major longitudinal pathway connected across the midline by commissures. We have investigated the role of Brother of CDO (BOC), an immunoglobulin (Ig) superfamily member distantly related to the Roundabout (Robo) family of axon-guidance receptors, in the development of this embryonic template of axon tracts in the zebrafish brain. A zebrafish homologue of BOC was isolated and shown to be expressed predominantly in the developing neural plate and later in the neural tube and developing brain. Zebrafish boc was initially highly localized to discrete bands in the mid- and hindbrain, but, as the major brain subdivisions emerged, it became more evenly expressed along the rostrocaudal axis, particularly in dorsal regions. The function of zebrafish boc was examined by a loss-of-function approach. Analysis of embryos injected with antisense morpholinos designed against boc revealed highly selective defects in the development of dorsoventrally projecting axon tracts. Loss of boc caused ventrally projecting axons, particularly those arising from the presumptive telencephalon, to follow aberrant trajectories. These data indicate that boc is an axon-guidance molecule playing a fundamental role in pathfinding during the early patterning of the axon scaffold in the embryonic vertebrate brain.

Zebrafish as an experimental model: strategies for developmental and molecular neurobiology studies.

Zebrafish provide a rapid and effective means for assessing gene function in the vertebrate nervous system. By employing gain- and loss-of-function techniques it is possible to obtain insights into the roles of both wild-type and heterologously expressed genes. Such approaches enable rapid progression from gene discovery to gene expression and finally to gene function even when examining development of a tissue as complex as the nervous system. Exploiting the full potential of zebrafish as a bioassay for the nervous system will require, not only an understanding of the molecular and cellular basis of normal zebrafish development, but also an appreciation of comparative processes in other species. When applied to mutant animals, classic morphological approaches and contemporary molecular genetic techniques are providing a wealth of information on the development of the nervous system at the molecular, cell, system and behavioural levels. Zebrafish are now emerging as an important tool, supporting mouse genetical approaches for understanding neural function in vertebrates.

Identifying axon guidance defects in the embryonic zebrafish brain.

The method described here outlines a simple protocol to investigate the in vivo function of axon guidance molecules during the development of the embryonic zebrafish brain. By 24 hours postfertilization, a simple scaffold of axon tracts and commissures can be visualised in the brain using acetylated alpha-tubulin, a panaxonal marker that stains all axons. The highly stereotypical trajectory of axons in the embryonic zebrafish brain provides an ideal system in which to study the molecular mechanisms of axon guidance, as defects in the axon scaffold can be clearly visualised. We describe here our approach to identify defects in the trajectory of axons that establish the initial template of tracts in the embryonic fore- and mid-brain. By combining immunohistochemical techniques and confocal microscopy on dissected wholemounts of embryonic brains we are able to observe at high resolution the complete scaffold of axon tracts. This approach provides a rapid and simple means of assessing axon guidance defects in the developing brain. Given the advantages of the zebrafish as a model system, and the range of molecular perturbation methods now available, this technique provides a valuable tool for assessing the phenotypic effects of gene perturbations in a biologically relevant context.