Comparison of neurolin (ALCAM) and neurolin-like cell adhesion molecule (NLCAM) expression in zebrafish

Retinal regeneration is facilitated by the presence of surviving neurons

Teleost fish regenerate their retinas after damage, in contrast to mammals. In zebrafish subjected to an extensive ouabain-induced lesion that destroys all neurons and spares Müller glia, functional recovery and restoration of normal optic nerve head (ONH) diameter take place at 100 days postinjury. Subsequently, regenerated retinas overproduce cells in the retinal ganglion cell (RGC) layer, and the ONH becomes enlarged. Here, we test the hypothesis that a selective injury, which spares photoreceptors and Müller glia, results in faster functional recovery and fewer long-term histological abnormalities. Following this selective retinal damage, recovery of visual function required 60 days, consistent with this hypothesis. In contrast to extensively damaged retinas, selectively damaged retinas showed fewer histological errors and did not overproduce neurons. Extensively damaged retinas had RGC axons that were delayed in pathfinding to the ONH, and showed misrouted axons within the ONH, suggesting that delayed functional recovery following an extensive lesion is related to defects in RGC axons exiting the eye and/or reaching their central targets. The atoh7, fgf8a, Sonic hedgehog (shha), and netrin-1 genes were differentially expressed, and the distribution of hedgehog protein was disrupted after extensive damage as compared with selective damage. Confirming a role for Shh signaling in supporting rapid regeneration, shha(t4) +/- zebrafish showed delayed functional recovery after selective damage. We suggest that surviving retinal neurons provide structural/molecular information to regenerating neurons, and that this patterning mechanism regulates factors such as Shh. These factors in turn control neuronal number, retinal lamination, and RGC axon pathfinding during retinal regeneration.

Nlcam modulates midline convergence during anterior neural plate morphogenesis

During development, different cell types must undergo distinct morphogenetic programs so that tissues develop the right dimensions in the appropriate place. In early eye morphogenesis, retinal progenitor cells (RPCs) move first towards the midline, before turning around to migrate out into the evaginating optic vesicles. Neighbouring forebrain cells, however, converge rapidly and then remain at the midline. These differential behaviours are regulated by the transcription factor Rx3. Here, we identify a downstream target of Rx3, the Ig-domain protein Nlcam, and characterise its role in regulating cell migration during the initial phase of optic vesicle morphogenesis. Through sophisticated live imaging and comprehensive cell tracking experiments in zebrafish, we show that ectopic expression of Nlcam in RPCs, as is observed in Rx3 mutants, causes enhanced convergence of these cells. Expression levels of Nlcam therefore regulate the migratory properties of RPCs. Our results provide evidence that the two phases of optic vesicle morphogenesis: slowed convergence and outward-directed migration, are under different genetic control. We propose that Nlcam forms part of the guidance machinery directing rapid midline migration of forebrain precursors, where it is normally expressed, and that its ectopic expression upon loss of Rx3 imparts these migratory characteristics upon RPCs.

Myogenesis and molecules - insights from zebrafish Danio rerio

Myogenesis is a fundamental process governing the formation of muscle in multicellular organisms. Recent studies in zebrafish Danio rerio have described the molecular events occurring during embryonic morphogenesis and have thus greatly clarified this process, helping to distinguish between the events that give rise to fast v. slow muscle. Coupled with the well-known Hedgehog signalling cascade and a wide variety of cellular processes during early development, the continual research on D. rerio slow muscle precursors has provided novel insights into their cellular behaviours in this organism. Similarly, analyses on fast muscle precursors have provided knowledge of the behaviour of a sub-set of epitheloid cells residing in the anterior domain of somites. Additionally, the findings by various groups on the roles of several molecules in somitic myogenesis have been clarified in the past year. In this study, the authors briefly review the current trends in the field of research of D. rerio trunk myogenesis.

A HaemAtlas: characterizing gene expression in differentiated human blood cells

Hematopoiesis is a carefully controlled process that is regulated by complex networks of transcription factors that are, in part, controlled by signals resulting from ligand binding to cell-surface receptors. To further understand hematopoiesis, we have compared gene expression profiles of human erythroblasts, megakaryocytes, B cells, cytotoxic and helper T cells, natural killer cells, granulocytes, and monocytes using whole genome microarrays. A bioinformatics analysis of these data was performed focusing on transcription factors, immunoglobulin superfamily members, and lineage-specific transcripts. We observed that the numbers of lineage-specific genes varies by 2 orders of magnitude, ranging from 5 for cytotoxic T cells to 878 for granulocytes. In addition, we have identified novel coexpression patterns for key transcription factors involved in hematopoiesis (eg, GATA3-GFI1 and GATA2-KLF1). This study represents the most comprehensive analysis of gene expression in hematopoietic cells to date and has identified genes that play key roles in lineage commitment and cell function. The data, which are freely accessible, will be invaluable for future studies on hematopoiesis and the role of specific genes and will also aid the understanding of the recent genome-wide association studies.

Asymmetric involution of the myocardial field drives heart tube formation in zebrafish

Many vertebrate organs are derived from monolayered epithelia that undergo morphogenesis to acquire their shape. Whereas asymmetric left/right gene expression within the zebrafish heart field has been well documented, little is known about the tissue movements and cellular changes underlying early cardiac morphogenesis. Here, we demonstrate that asymmetric involution of the myocardium of the right-posterior heart field generates the ventral floor, whereas the noninvoluting left heart field gives rise to the dorsal roof of the primary heart tube. During heart tube formation, asymmetric left/right gene expression within the myocardium correlates with asymmetric tissue morphogenesis. Disruption of left/right gene expression causes randomized myocardial tissue involution. Time-lapse analysis combined with genetic analyses reveals that motility of the myocardial epithelium is a tissue migration process. Our results demonstrate that asymmetric morphogenetic movements of the 2 bilateral myocardial cell populations generate different dorsoventral regions of the zebrafish heart tube. Failure to generate a heart tube does not affect the acquisition of atrial versus ventricular cardiac cell shapes. Therefore, establishment of basic cardiac cell shapes precedes cardiac function. Together, these results provide the framework for the integration of single cell behaviors during the formation of the vertebrate primary heart tube.

Dystrobrevin and dystrophin family gene expression in zebrafish

Dystrophin/dystrobrevin superfamily proteins play structural and signalling roles at the plasma membrane of many cell types. Defects in them or the associated multiprotein complex cause a range of neuromuscular disorders. Members of the dystrophin branch of the family form heterodimers with members of the dystrobrevin branch, mediated by their coiled-coil domains. To determine which combinations of these proteins might interact during embryonic development, we set out to characterise the gene expression pattern of dystrophin and dystrobrevin family members in zebrafish. gamma-dystrobrevin (dtng), a novel dystrobrevin recently identified in fish, is the predominant form of dystrobrevin in embryonic development. Dtng and dmd (dystrophin) have similar spatial and temporal expression patterns in muscle, where transcripts are localized to the ends of differentiated fibres at the somite borders. Dtng is expressed in the notochord while dmd is expressed in the chordo-neural hinge and then in floor plate and hypochord. In addition, dtng is dynamically expressed in rhombomeres 2 and 4-6 of the hindbrain and in the ventral midbrain. alpha-dystrobrevin (dtna) is expressed widely in the brain with particularly strong expression in the hypothalamus and the telencephalon; drp2 is also expressed widely in the brain. Utrophin expression is found in early pronephros and lateral line development and utrophin and dystrophin are both expressed later in the gut. beta-dystrobrevin (dtnb) is expressed in the pronephric duct and widely at low levels. In summary, we find clear instances of co-expression of dystrophin and dystrobrevin family members in muscle, brain and pronephric duct development and many examples of strong and specific expression of members of one family but not the other, an intriguing finding given the presumed heterodimeric state of these molecules.

Vestigial-like-2b (VITO-1b) and Tead-3a (Tef-5a) expression in zebrafish skeletal muscle, brain and notochord

The vestigial gene has been shown to control skeletal muscle formation in Drosophila and the related Vestigial-like 2 (Vgl-2) protein plays a similar role in mice. Vgl-family proteins are thought to regulate tissue-specific gene expression by binding to members of the broadly expressed Scalloped/Tef/TEAD transcription factor family. Zebrafish have at least four Vgl genes, including two Vgl-2s, and at least three TEAD genes, including two Tead3s. We describe the cloning and expression of one member from each family in the zebrafish. A novel gene, vgl-2b, with closest homology to mouse and human vgl-2, is expressed transiently in nascent notochord and in muscle fibres as they undergo terminal differentiation during somitogenesis. Muscle cells also express a TEAD-3 homologue, a possible partner of Vgl-2b, during myoblast differentiation and early fibre assembly. Tead-3a is also expressed in rhombomeres, eye and epiphysis regions.

A conserved molecular pathway mediates myoblast fusion in insects and vertebrates

Skeletal muscles arise by fusion of precursor cells, myoblasts, into multinucleated fibers. In vertebrates, mechanisms controlling this essential step in myogenesis remain poorly understood. Here we provide evidence that Kirrel, a homolog of receptor proteins that organize myoblast fusion in Drosophila melanogaster, is necessary for muscle precursor fusion in zebrafish. Within developing somites, Kirrel expression localized to membranes of fusion-competent myoblasts of the fast-twitch lineage. Unlike wild-type myoblasts that form spatially arrayed syncytial (multinucleated) fast myofibers, those deficient in Kirrel showed a significant reduction in fusion capacity. Inhibition of Rac, a GTPase and the most downstream intracellular transducer of the fusion signal in D. melanogaster, also compromised fast-muscle precursor fusion in zebrafish. However, unlike in D. melanogaster, constitutive Rac activation in zebrafish led to hyperfused giant syncytia, highlighting an entirely new function for this protein in zebrafish for gating the number and polarity of fusion events. These findings uncover a substantial degree of evolutionary conservation in the genetic regulation of myoblast fusion.

ALCAM (CD166) Is a Surface Marker for Early Murine Cardiomyocytes

ALCAM (activated leukocyte cell adhesion molecule, CD166) belongs to the immunoglobulin superfamily and is involved in axon guidance, hematopoiesis, immune response and tumor metastasis. During embryogenesis, mRNA encoding ALCAM was expressed in the cardiac crescent and the neural groove at embryonic day (E) 7.75 and predominately in the tubular heart at E8.5. A newly generated monoclonal antibody against the ALCAM molecule (ALC-48) exclusively stained cardiomyocytes at E8.25-10.5. However, ALCAM expression was lost by cardiomyocytes by E12.5 and its expression shifts to a variety of organs during later stages. ALCAM was found to be a prominent surface marker for cardiomyocytes in early embryonic hearts. The transient expression of ALCAM during early developmental stages marks specific developmental stages in cardiomyocyte differentiation.