Biomedical subjects

Anand Chandrasekhar

Publications and source records attributed to Anand Chandrasekhar.

6 recordsLinked to original sources

PubMed2005

Neuronal development and migration in zebrafish hindbrain explants.

The zebrafish embryo is an excellent system for studying dynamic processes such as cell migration during vertebrate development. Dynamic analysis of neuronal migration in the zebrafish hindbrain has been hampered by morphogenetic movements in vivo, and by the impermeability of embryos. We have applied a recently reported technique of embryo explant culture to the analysis of neuronal development and migration in the zebrafish hindbrain. We show that hindbrain explants prepared at the somitogenesis stage undergo normal morphogenesis for at least 14 h in culture. Importantly, several aspects of hindbrain development such as patterning, neurogenesis, axon guidance, and neuronal migration are largely unaffected, inspite of increased cell death in explanted tissue. These results suggest that hindbrain explant culture can be employed effectively in zebrafish to analyze neuronal migration and other dynamic processes using pharmacological and imaging techniques.

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PubMed2005

Gli function is essential for motor neuron induction in zebrafish.

The Gli family of zinc-finger transcription factors mediates Hedgehog (Hh) signaling in all vertebrates. However, their roles in ventral neural tube patterning, in particular motor neuron induction, appear to have diverged across species. For instance, cranial motor neurons are essentially lost in zebrafish detour (gli1(-)) mutants, whereas motor neuron development is unaffected in mouse single gli and some double gli knockouts. Interestingly, the expression of some Hh-regulated genes (ptc1, net1a, gli1) is mostly unaffected in the detour mutant hindbrain, suggesting that other Gli transcriptional activators may be involved. To better define the roles of the zebrafish gli genes in motor neuron induction and in Hh-regulated gene expression, we examined these processes in you-too (yot) mutants, which encode dominant repressor forms of Gli2 (Gli2(DR)), and following morpholino-mediated knockdown of gli1, gli2, and gli3 function. Motor neuron induction at all axial levels was reduced in yot (gli2(DR)) mutant embryos. In addition, Hh target gene expression at all axial levels except in rhombomere 4 was also reduced, suggesting an interference with the function of other Glis. Indeed, morpholino-mediated knockdown of Gli2(DR) protein in yot mutants led to a suppression of the defective motor neuron phenotype. However, gli2 knockdown in wild-type embryos generated no discernable motor neuron phenotype, while gli3 knockdown reduced motor neuron induction in the hindbrain and spinal cord. Significantly, gli2 or gli3 knockdown in detour (gli1(-)) mutants revealed roles for Gli2 and Gli3 activator functions in ptc1 expression and spinal motor neuron induction. Similarly, gli1 or gli3 knockdown in yot (gli2(DR)) mutants resulted in severe or complete loss of motor neurons, and of ptc1 and net1a expression, in the hindbrain and spinal cord. In addition, gli1 expression was greatly reduced in yot mutants following gli3, but not gli1, knockdown, suggesting that Gli3 activator function is specifically required for gli1 expression. These observations demonstrate that Gli activator function (encoded by gli1, gli2, and gli3) is essential for motor neuron induction and Hh-regulated gene expression in zebrafish.

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PubMed2004

Turning heads: development of vertebrate branchiomotor neurons.

The cranial motor neurons innervate muscles that control eye, jaw, and facial movements of the vertebrate head and parasympathetic neurons that innervate certain glands and organs. These efferent neurons develop at characteristic locations in the brainstem, and their axons exit the neural tube in well-defined trajectories to innervate target tissues. This review is focused on a subset of cranial motor neurons called the branchiomotor neurons, which innervate muscles derived from the branchial (pharyngeal) arches. First, the organization of the branchiomotor pathways in zebrafish, chick, and mouse embryos will be compared, and the underlying axon guidance mechanisms will be addressed. Next, the molecular mechanisms that generate branchiomotor neurons and specify their identities will be discussed. Finally, the caudally directed or tangential migration of facial branchiomotor neurons will be examined. Given the advances in the characterization and analysis of vertebrate genomes, we can expect rapid progress in elucidating the cellular and molecular mechanisms underlying the development of these vital neuronal networks. Developmental Dynamics 229:143-161, 2004.

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PubMed2003

Neurogenic phenotype of mind bomb mutants leads to severe patterning defects in the zebrafish hindbrain.

Failure of Notch signaling in zebrafish mind bomb (mib) mutants results in a neurogenic phenotype where an overproduction of early differentiating neurons is accompanied by the loss of later-differentiating cell types. We have characterized in detail the hindbrain phenotype of mib mutants. Hindbrain branchiomotor neurons (BMNs) are reduced in number but not missing in mib mutants. In addition, BMN clusters are frequently fused across the midline in mutants. Mosaic analysis indicates that the BMN patterning and fusion defects in the mib hindbrain arise non-cell autonomously. Ventral midline signaling is defective in the mutant hindbrain, in part due to the differentiation of some midline cells into neural cells. Interestingly, while early hindbrain patterning appears normal in mib mutants, subsequent rhombomere-specific gene expression is completely lost. The defects in ventral midline signaling and rhombomere patterning are accompanied by an apparent loss of neuroepithelial cells in the mutant hindbrain. These observations suggest that, by regulating the differentiation of neuroepithelial cells into neurons, Notch signaling preserves a population of non-neuronal cells that are essential for maintaining patterning mechanisms in the developing neural tube.

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PubMed2002

The Zebrafish trilobite gene is essential for tangential migration of branchiomotor neurons.

Newborn neurons migrate extensively in the radial and tangential directions to organize the developing vertebrate nervous system. We show here that mutations in zebrafish trilobite (tri) that affect gastrulation-associated cell movements also eliminate tangential migration of motor neurons in the hindbrain. In the wild-type hindbrain, facial (nVII) and glossopharyngeal (nIX) motor neurons are induced in rhombomeres 4 and 6, respectively, and migrate tangentially into r6 and r7 (nVII) and r7 (nIX). In all three tri alleles examined, although normal numbers of motor neurons are induced, nVII motor neurons are found exclusively in r4, and nIX-like motor neurons are found exclusively in r6. The migration of other neuronal and nonneuronal cell types is unaffected in tri mutants. Rhombomere formation and the development of other hindbrain neurons are also unaffected in tri mutants. Furthermore, tangential neuronal migration occurs normally in the gastrulation mutant knypek, indicating that the trilobite neuron phenotype does not arise nonspecifically from aberrant gastrulation-associated movements. We conclude that trilobite function is specifically required for two types of cell migration that occur at different stages of zebrafish development.

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PubMed2002

Zebrafish trilobite identifies new roles for Strabismus in gastrulation and neuronal movements.

Embryonic morphogenesis is driven by a suite of cell behaviours, including coordinated shape changes, cellular rearrangements and individual cell migrations, whose molecular determinants are largely unknown. In the zebrafish, Dani rerio, trilobite mutant embryos have defects in gastrulation movements and posterior migration of hindbrain neurons. Here, we have used positional cloning to demonstrate that trilobite mutations disrupt the transmembrane protein Strabismus (Stbm)/Van Gogh (Vang), previously associated with planar cell polarity (PCP) in Drosophila melanogaster, and PCP and canonical Wnt/beta-catenin signalling in vertebrates. Our genetic and molecular analyses argue that during gastrulation, trilobite interacts with the PCP pathway without affecting canonical Wnt signalling. Furthermore, trilobite may regulate neuronal migration independently of PCP molecules. We show that trilobite mediates polarization of distinct movement behaviours. During gastrulation convergence and extension movements, trilobite regulates mediolateral cell polarity underlying effective intercalation and directed dorsal migration at increasing velocities. In the hindbrain, trilobite controls effective migration of branchiomotor neurons towards posterior rhombomeres. Mosaic analyses show trilobite functions cell-autonomously and non-autonomously in gastrulae and the hindbrain. We propose Trilobite/Stbm mediates cellular interactions that confer directionality on distinct movements during vertebrate embryogenesis.

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