1995 |
Harris, William A |
R01Activity Code Description: To support a discrete, specified, circumscribed project to be performed by the named investigator(s) in an area representing his or her specific interest and competencies. |
Induction and Compentence in the Retina @ University of California San Diego
Lineage studies have shown that retinal cells, as well as many other cells in the vertebrate nervous system, are born pluripotent Culture studies have demonstrated that cellular interactions can drive these pluripotent cells toward particular fates. This is a proposal to investigate the nature of these inductive pathways. For Xenopus photoreceptors, there are two temporally distinct inductive events. The first is necessary to turn on some antigens expressed in both rods and cones, and the second one induces cells to express rod specific markers. Two general schemes of determination are consistent with these separate inductions. In one a single cell is exposed to a series of inductive events that increasingly restrict its fate; in the other, single distinct interactions induce the different cell types. One aim of this proposal is to distinguish between these two possibilities by a using a combination of mixed cell cultures and immunocytochemical tagging. Another main objective of this proposal is to test explicitly whether similar inductive schemes are used for other cell types in the retina. There is an evolutionarily conserved order in retinal histogenesis in vertebrates, and we will investigate the possibilities that this occurs because inductive signals arise in a particular sequence, or because cells change their competence to respond to inductive signals. Changes in competence will be assayed by removing labeled cells from the retina at different stages of development, exposing them to the same inductive cues, and examining the appearance of particular cell types among the labeled cells. The last major goal of this proposal is to examine the role of particular growth factors in retinal development in vivo, by misexpression of dominant-activated and dominant- negative forms of growth factor receptors m vivo, and examining retinal cell development immunohistochemically. The long term objectives of this proposal are to understand the cascade of cell determination in the vertebrate nervous system at a cellular and a molecular level. One has to understand who induces who, what the signals are, and what the receptors are. This proposal paves the way for a better understanding of the order of cellular genesis in the nervous system, in particular a working model for the generation of organized cellular complexity from a specific sequence of inductive events. The retina is a highly accessible and well studied part of the vertebrate central nervous system and will serve well as an experimental system for future work of this sort. The basic understanding of how cells are determined in the retina, and the capability of controlling these inductive events in culture and in vivo, may have implications for retinal regeneration after surgery or injury. This work may also have applications in understanding the embryonic malformations of the retina.
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0.958 |
1995 |
Harris, William A |
R01Activity Code Description: To support a discrete, specified, circumscribed project to be performed by the named investigator(s) in an area representing his or her specific interest and competencies. |
Formation of Neural Pathways in the Embryo @ University of California San Diego
This application seeks to explore the underlying mechanisms of axonal pathfinding and target recognition in the retinal projection of the embryonic Xenopus CNS, a system which is very amenable to such studies because of a recently developed in vivo time-lapse system, modified for the perfusion of drugs. This proposal, a continuation of a long term study of this system, has three major goals. The first is to understand the role of growth cone filopodia in axonal guidance. Retinal axons treated with cytochalasin, an actin depolymerizer, do not pathfind properly in the brain. In this proposal, the filopodial activity of individual axons treated with cytochalasin at various times and doses, will be correlated with their pathfinding behavior. Second, it has become increasingly clear that growth cones in culture can to respond particular cues by growing, stopping, turning or collapsing. Signal transduction cascades of various sorts must be used to translate these signals into growth cone action Which messenger systems are used for which purpose in the developing brain, however, has not yet been approached. One goal of this proposal, therefore is to investigate the role of particular second messenger systems in axon guidance and target recognition in the intact embryonic brain. A screening procedure based on the effects of particular membrane permeant agents has already been used to implicate some of the signaling pathways involved, particularly a G- protein cascade, and to suggest that other pathways, like the calcium pathway may be uninvolved or redundant. In addition, it has been demonstrated that growth filopodia are necessary for pathfinding in the brain, suggesting that this may be the sites of signal transduction. Experiments are proposed to pursue these suggestions with rigorous experiments that distinguish alternate interpretations, and to extend the pathway molecularly. Third, retinal axons dX to the tectum using local positional cues in the neuroepithelium. Evidence suggests that certain positional information genes, such as homeobox genes and paired-box genes, are involved in establishing topographic identity during early vertebrate neurogenesis. Recently, it has been shown that at least some of these genes can directly regulate tahe expression of particular cell adhesion molecules, making it likely that they therefore also affect axonal navigation in the CNS. Advances in gene introduction technology in vivo have made it possible to misexpress genes tin the embryonic Xenopus nervous system. In this proposal, Xenopus homeobox- containing genes, including engrailed, distal-less, Pax-6, and ganglion cells or in the neuroepithelial cells through which the growing retinal axons normally navigate. The misexpressing embyros will be examined for defects in pathfinding and target recognition.
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0.958 |