Robert Ho Ph.D.
Associate Professor, Department of Organismal Biology and AnatomyThe Joseph P. Kennedy Intellectual and Developmental Disabilities Research Center
University of Chicago
5801 S Ellis , BSDLC 402
Chicago, IL 60637
773-834-8423 (tel)
773-702-0037 (fax)
[email protected]
Biography/Curriculum Vitae:
Research Interests:
Narrative of Current Research Efforts:
The zebrafish, Danio rerio, is a relatively simple vertebrate whose potential as a model system for developmental studies is only recently being realized. Embryos are easy to obtain in large numbers, develop external to the mother in fresh water and are optically transparent throughout the early stages of development. These features make the zebrafish embryo easily accessible to experimental manipulations such as the microinjection of lineage tracer molecules, cell ablations and cell transplantation. In addition to being an excellent embryological preparation, the zebrafish has an extensive history of genetic analyses and many interesting mutations have already been isolated. The ability to combine a workable genetics with an accessible embryology is perhaps the most advantageous feature of working with the zebrafish and has provided us with many novel insights into vertebrate development.
The theme of the work being performed in the laboratory is to address classical problems of vertebrate embryogenesis using modern techniques in the zebrafish embryo. The general goal is to gain insights into the cellular, molecular and genetic mechanisms leading to the assignment of cell fate and, ultimately, to the formation of a complex vertebrate body plan. We are especially interested in the processes leading to the specification of the embryonic body axes and how the movements of individual cells within the embryo influence/correlate with cell fate decisions.
Maternal Determinants (animal-vegetal axis) The localization of determinant molecules during development is an attractive hypothesis put forth to explain how cell fate is specified in a variety of organisms. The early cleavages of the zebrafish are "meroblastic" that is, the division planes do not extend completely through the embryo. Therefore the early blastomeres, those cells that make up the embryo proper, remain cytoplasmically connected to each other and to the underlying yolk cell, forming a syncytium where mRNAs, proteins and organelles could form gradients within the early embryo. In order to characterize possible maternal molecules that may play a role in the development of the zebrafish embryo, we have isolated a number of maternally supplied mRNAs in the developing oocyte and early embryo. We have concentrated our studies upon those mRNAs which show interesting localization patterns in the early embryo and whose functions appear to mediate early axis formation.
Organizer Specification (dorso-ventral axis) During gastrulation, the functions of a Spemann organizer-type region within the embryo are essential for the specification of the vertebrate body plan, however the organizer is thought to be induced from another signaling center called the Nieuwkoop center. In the zebrafish, the functional equivalent of the amphibian Spemann organizer is the dorsal shield. Most of our organizer studies have focused upon the functions of a paired-like homeobox gene that we have isolated called nieuwkoid. In situ hybridization analysis shows that this transcription factor is expressed immediately following the mid-blastula transition in an extra-embryonic tissue region directly underlying the future dorsal shield of the zebrafish embryo. Mis-expression of nieuwkoid in blastomeres of zebrafish embryos is sufficient for the early induction of ectopic organizer regions and secondary, duplicated axes. We have concluded that the dynamic and restricted expression of the nieuwkoid gene, combined with its potent dorsalizing activity shows that nieuwkoid is an important component in the regionalization of the organizer, possibly characterizing and mediating a Nieuwkoop center-like activity. In addition, we and others have identified a mutation in the nieuwkoid gene and are further characterizing the null phenotype in the mutant called bozozok. We continue to study the nieuwkoid gene in over-expression and subtraction assays for the purpose of finding down-stream elements such as early signaling molecules. In addition, we are interested in isolating factors such as maternally expressed T-box genes that may be upstream and controlling the expression of the nieuwkoid gene.
Hox Genes (antero-posterior axis) The clustered Hox genes as originally described in Drosophila, play a central role in the assignment of different antero-posterior fates along the body axis. We have spent considerable effort in the isolation and characterization of the hox cluster genes of the zebrafish. We have found intriguing differences in the genomic organization and expression patterns of the teleost hox genes relative to the homologous genes of tetrapods. For instance, it is generally accepted that all vertebrates have four clusters of Hox genes. We originally reported the surprising fact that the zebrafish contained several "extra" hox cluster genes not present in tetrapods, both within and outside of the canonical four tetrapod clusters; this finding has led directly to the view that the zebrafish genome has gone through an extra round of duplication relative to tetrapods. It has now been shown that the zebrafish genome contains seven separate hox clusters as opposed to the four clusters found in tetrapods. We are currently considering the implications of this genome-wide duplication upon future developmental, genetic and evolutionary studies, using a combination of over-expression and knock-down studies in mutant lines of zebrafish which exhibit altered antero-posterior and dorso-ventral patterning defects. We are especially intrigued by the evolutionary implications of having a duplicated genome and determining how the zebrafish uses and retains or modifies the functions of these duplicated genes.
Tail Development (posterior body axis) Whereas embryonic development of the head and trunk regions has been carefully described, we believe that some of the confusion surrounding the development of the posterior body has stemmed from difficulties in characterizing the cell fates and monitoring the cell movements within the tail bud region. Because it is possible to follow individually labeled cells within the living, intact organism, the zebrafish embryo is the vertebrate system of choice for fate mapping and cell movement studies. By labeling individual cells and time-lapsing their movements in the growing tail region, we have come to better understand the pattern of cell movements in this region of the vertebrate embryo and have produced the most detailed fate map for the development of the posterior body region in any vertebrate. Recently, several mutations have been isolated that differentially affect the trunk and tail regions of embryonic zebrafish; we are using these developmental mutations as further aids towards the description of posterior body development. We are currently engaged in transplantation and induction experiments in a effort to discover which cells of the tail bud are serving as a "tail-organizer" region and are also investigating the role of a "para"-hox gene, namely the caudal gene, upon hox gene and t-box gene regulation in the posterior body region.
Somite Formation and Segmentation (tissue and organ formation) A general understanding of the cellular events underlying a segmented body pattern is noticeably lacking in vertebrate embryos. We are studying the cellular and molecular cues used by cells in the embryo to form segments. The most obvious manifestation of the segmental nature of vertebrate embryos is the formation of mesodermal somites from which the skeletal muscles and vertebrae of the embryo derive. Less obvious is the organization of neurons in the spinal cord, especially motoneurons, into iterated groups of cells which mirror the segmental pattern of the somites. We are interested in understanding the interaction that occur between the paraxial somitic mesoderm and midline structures, like the nervous system and notochord, to arrive at an organized, metameric body plan. To aid us in these studies we are concentrating on the functions and unusual expression patterns of members of the Delta:Notch signaling pathway. Several members of this pathway exhibit novel "cyclic" patterns of waves of stripes which prefigure the later placement of segmental boundaries.
T-box Genes (tissue and organ formation) The T-box genes are a family of transcription factors related to the brachyury gene, first described in mouse. These genes appear to be playing a role in specifying identity within large regions of the embryonic body plan. We have recently characterized two t-box genes, tbx5 and tbx4, which are expressed in the fore limb and hind limb, respectively, and therefore are the only pair of markers that distinguish between a fore and hind limb bud fate. Conclusions from this work show that the pectoral and pelvic fins of fish are indeed homologous structures to the fore and hind limbs, respectively, of tetrapods, and we are especially interested in continuing comparative studies between tetrapod and teleost limb development. For instance, whereas higher tetrapods species generally develop both the hind limb bud and fore limb bud simultaneously during embryogenesis, in fish species, the development of the fore limb bud during embryogenesis can precede the development of the hind limb bud by many weeks if not months. Also, whereas the placement of the limbs is at precisely fixed positions along the antero-posterior body of tetrapods, in many fish species the placement of the limbs is variable from species to species, with some teleosts even developing the "hind" limbs at a more anterior position than the "fore" limbs. In addition we have recently shown that the limb specific t-box genes are absolutely required for limb bud outgrowth and we continue to investigate the implications of these genes' functions upon developmental and evolutionary issues.
Major Honors and Awards:
Representative Publications:
Ahn, D-g., Kourakis, M.J., Rohde, L. A., Silver, L.M., Ho, R.K.: T-box gene tbx5 is essential for formation of the pectoral limb bud. Nature 417:754-758 (2002)
Oates, A.C., Ho, R.K.: Hairy/E(spl)-Related (HER) genes are central components of the segmentation oscillator and display redundancy with the Delta/Notch signaling pathway in the formation of anterior segmental boundaries in the zebrafish. Development 129:2929-2946 (2002)
Bruce, A.E.E., Oates, A., Prince, V.E., Ho, R.K.: Additional hox clusters in the zebrafish: Divergent expression patterns belie conserved function of duplicate hoxB5 genes. Evolution and Development 3,3:127-144 (2001)
Ahn, D-A.; Ruvinsky, I.; Oates. A.C.; Silver, L.; Ho, R.K.: tbx20, a new vertebrate T-box gene implicated in the development of cranial motoneurons and cardiovascular structures in zebrafish. Mech. Dev. 95:253-258 (2000)
Oates, A.C.; Bruce, A.E.E.; Ho, R.K.: Too much interference: injection of double stranded RNA fails to produce specific effects in the zebrafish embryo. Dev. Bio. 224:20-28 (2000)
Howley, C.; Ho, R.K.: mRNA localization patterns in zebrafish oocytes. Mech. Dev. 92:305-309 (2000)
Ruvinsky, I.; Silver, L.M.; Ho, R.K.: The evolution of paired appendages in vertebrates: t-box genes in zebrafish. Dev. Genes. Evol. 210:82-91 (2000)
Koos, D.S., Ho, R.K.: The nieuwkoid/dharma homeobox gene is essential for bmp2b repression in the zebrafish pregastrula. Dev. Bio. 215:190-207 (1999)
Bally-Cuif, L.; Schatz, W.J.; Ho, R.K.: Characterization of the zebrafish Orb/CPEB-related RNA-binding protein and localization of maternal components in the zebrafish oocyte. Mech. Dev. 77:31-47 (1998)
Prince, V.E.; Price, A.L.; Ho, R.K.: Hox gene expression reveals regionalization along the anteroposterior axis of the zebrafish notochord. Dev. Genes Evol. 208:517-522 (1998)
Koos, D.S.; Ho, R.K.: The nieuwkoid gene characterizes and mediates a Nieuwkoop center-like activity in the zebrafish. Curr. Biol. 8:1199-1206 (1998)
Ruvinsky, I.; Silver, L.M.; Ho, R.K.; Characterization of the zebrafish tbx16 gene and evolution of the vertebrate T-box family. Dev. Genes. Evol. 208/2 (1998)
Prince, V.E.; Joly, L.; Ekker, M.: Ho, R.K.: Zebrafish hox genes: genomic organization and modified colinear expression patterns in the trunk. Development, 125:407-420 (1998)
Prince, V.E; Moens, C.B., Kimmel, C.B.; Ho, R.K.: Zebrafish hox genes: expression in the hindbrain region of wild-type and mutants of the segmentation gene, valentino. Development, 125:393-406 (1998)
Kanki, J.P.; Ho, R.K.: The development of the posterior body in zebrafish. Development 124:881-893 (1997)
Created 10/15/2007 by Danielle Onunkwo
Last modified 11/9/2007 by Miriam Domowicz
