My lab studies brain development in the genetic model system, Drosophila melanogaster. We have a comparative outlook and try to establish similarities and differences in the genetic mechanisms that operate in the developing nervous system in Drosophila and vertebrates. To that end, we compare the patterns in which homologous genes specific for certain brain parts are expressed in fruit flies and vertebrate embryos. We have also begun to study nervous system development in a primitive invertebrate, the flatworm Macrostomum, which has simple and conserved features which are believed to be similar to those of the common ancestor of all bilaterian animals (which include insects and vertebrates). Specific lines of research:
1. Analysis and 3D modeling of the Drosophila brain
A large number of molecular markers for specific brain parts, some of them usable in living animals, have become available. We use these markers to model the structure of the normal brain at a high level of resolution. This work is required in order to embark on a genetic analysis, in which structural defects following the knock-out or ectopic expression of a gene are evaluated. Only detailed knowledge of the normal structure will enable us and others to interpret these defects. Secondly, a three dimensional digital model will serve as an “atlas model‿ in which gene expression patterns will be deposited. We are collaborating with the Berkeley Drosophila Genome project (BDGP) in the analysis and documentation of developmentally regulated genes.
2. The role of cadherin adhesion molecules in brain development
From a structural point of view, brain development consists in the sum total of all of the neurons and glial cells sending out processes and connecting to each other. At the beginning, neurons are undifferentiated “spheres‿ located at a given location, and equipped with a certain set of genes. Both location and genetic equipment then determine how the neuron, or glia, will differentiate. The initial step of differentiation is that cells send out short, unbranched axons. After that, axons branch at specific points. Branches grow in a certain geometry, and establish connections (synapses) with each other. Understanding the geometry and wiring of the brain boils down to the questions: (i) where and when do branches form; (ii) what is the shape of branches; (iii) how do branches connect. In all of these steps, adhesion molecules play an important role. They allow cells to interact, and largely influence the point and shape of branching. We are particularly interested in a class of adhesion molecules, the cadherins, of which about 18 exist in the Drosophila genome. The facts that the Drosophila larval brain is small (around 1000 differentiated neurons) and that numerous powerful tools to address gene function in this organism exist makes this system ideally suited for the genetic study of brain development.
3. The development of the larval visual and neuroendocrine system in Drosophila
The Drosophila larva has a miniature eye, consisting of twelve light sensitive neurons, which target an even smaller number of neurons in the central brain and control larval behavior in a simple but effective manner, in the sense that larvae avoid light and move away from it. The neuroendocrine system is formed by an equally small number of elements, consisting of neurosecretory cells in the brain projecting their axons to an endocrine gland. Along with a group of peripheral neurons associated with the gut, the neuroendocrine system controls feeding behavior and growth of the larva. Despite the enormous difference in size between a fly visual/neuroendocrine systems and their counterparts in vertebrates, these systems function and develop along surprisingly similar lines. For example, the early precursors of these organs obtain similar positions in the early ‘fatemap’ of fly and vertebrate embryos. Furthermore, conserved transcription factors and signaling pathways direct their development. We are studying the steps involved in the development of the visual and neuroendocrine system in the Drosophila embryo, and identify genes and signaling pathways involved in this process.
4. Formation of the Drosophila vascular system and blood
Over recent years our lab collaborates with Dr.U. Banerjee (UCLA, Dept. MCD Biology) in the study of the Drosophila blood system. Our main focus is to identify
the origin of blood and blood vessel cells (which we could show have a common or
igin, just as in vertebrates!), to learn about the transcriptional regulators determining the fate of these cells, and the signaling pathways acting during their development. Insects have a simple, capillary-like ?\200\234heart?\200\235 that produces a directed flow of the body fluid (?\200\234blood?\200\235) through the body. Cellular components of the blood, called hemocytes, fall into only two main classes that correspond to subsets of white blood cells in vertebrates. Both cells of the heart and the
blood are derived from a small domain within the mesoderm (?\200\234cardiogenic mesoderm?\200\235) of the embryo. Our data so far support the idea that numerous similarities
exist between Drosophila and vertebrates in regard to the molecular mechanism that specifies where the cardiogenic mesoderm appears, and how it splits up into
different lineages (heart cells, blood cells and others) that express very different fates.
5. Brain development in Macrostomum, a simple bilaterian animal
Our comparisons of brain development between fruit flies and vertebrates have revealed many stunning similarities, both in the genes controlling aspects of nerve cell fate and differentiation, and in the morphological structures that precede the mature brain in early embryos. This leads us (and, by now, many other researchers in the field) to the conclusion that the last common ancestor of insects
and vertebrates, the so called bilaterian ancestor or ?\200\234Urbilateria?\200\231, must have already possessed these genes, and a nervous system that, even if simple, cont
ained structures that still persist in all of its descendants to the present day. It would be very informative if we could study the brain of the bilaterian ancestor. Since we can?\200\231t do that (ancestors are, by definition, extinct) the next best thing is to look among still living animals for those which may have retain ed many characteristics of the ancestor. In other words: there exist today the
?\200\234highly derived?\200\235 animal groups, like insects or vertebrates, whose body structure has become very elaborate and complicated, next to ?\200\234primitive?\200\235 groups whose body (according to what we know today) has not changed as much. One might call
these groups ?\200\234living fossils?\200\235. Some groups of flatworms, including the genus Macrostomum that we raise in our lab, belong to this latter category. We are stu
dying normal brain development in this system, for which little information existed previously. In addition, we have generated an EST project, isolating several
thousand genes from Macrostomum and assembling them into a web-based database.
Among these genes are many that we know to play a role in brain development in insects and vertebrates. The next step is to analyze the expression pattern of these genes in Macrostomum embryos. Functional studies are also possible, using an approach called ?\200\234RNA interference?\200\235 (RNAi). In this approach, fragments of a d
ouble-stranded RNA of a given gene are generated and injected into embryos, where they interfere with the translation of the corresponding gene.
Neuronal development in Drosophila
Areas of Research
A. Neural development in Drosophila melanogaster.
1. We are focussing on the function of the adhesion molecules Drosophila E-cadherin homolog (DE-cad) and Fas (an Ig-like protein) during neuroblast formation, axonogensis and synapse formation in the embryonic and larval brain. Following the cloning of DE-cad, its phenotypic and expression analysis, we have generated constructs that allow us to overexpress normal and mutant DE-cad forms at specific times and locations during nervous system development.
Tepass, U., Gruszynski-de Feo, E., Haag, T.A., Omatyar, L., TÃ¶rÃ¶k, T., and Hartenstein, V. (1996). shotgun encodes Drosophila E-cadherin and is preferentially required during cell rearrangement in the neuroectoderm and other morphogenetically active epithelia. Genes & Dev. 10, 672-685
Lekven, A., Tepass, U., Keshmeshian, M., Hartenstein, V. (1998) faint sausage encodes a novel member of the Ig superfamily required for cell movement and axonal pathfinding in the Drosophila nervous system. Development 125, 2747-2758
Haag, T., Prtina, N., Lekven, A.C., Hartenstein, V. (1999). Discrete steps in the morphogenesis of the Drosophila heart require faint sausage, shotgun/ DE-cadherin, and laminin A. Dev. Biol. 208, 56-69
2. Of special interest is the question of the dynamic regulation of DE-cad. Based on genetic evidence we hypothesize that the Drosophila EGF-receptor is crucially involved in this regulation. We use a genetic and biochemical approach to investigate this hypothesis
Dumstrei, K., Nassif, C., Abboud, G., Aryai, A., Aryai, AR, Hartenstein, V. (1998) EGFR signaling is required for the differentation and maintenance of neural progenitors along the dorsal midline of the Drosophila embryonic head. Development 125, 3417- 3426
3. Using specific markers, we have initiated a "Drosophila brain mapping" project. The markers are expressed from embryonic stages onward in specif ventral gradient that specifies the different domains within the eye field. Hh is secreted at the lateral boundary of the eye field and may form a gradient that antagonizes the early Dpp gradient. We are reconstructing the details of the fate map of the Drosophila head, and study experimentally the function of the Hh and Dpp gradients in patterning the fatemap.
Rudolph, K., Liaw, G., Daniel, A., Green, P.J., Courey, A.J., Hartenstein, V., Lengyel, J. (1997). Complex regulatory region mediating tailless expression in early embryonic patterning and brain development. Development 124, 4297-4308
Nassif, C., Daniel, A., Lengyel, J.A., and Hartenstein, V. (1998) The role of morphogenetic cell death during embryonic head development of Drosophila Dev. Biol. 197, 170-186
Daniel, A., Dumstrei, K., Lengyel, J., Hartenstein, V. (1999) tailless and atonal control cell fate in the embryonic visual system. Development 126, 2945-2954
Lebestky, T., Chang, T., Hartenstein, V., Banerjee, U. (2000) Specification of Drosophila hematopoietic lineage by conserved transcription factors. Science 288, 146-149
C. Neural development in primitive invertebrates: platyhelminthes
One of the most astounding realizations of modern developmental biology is the high degree to which genes or even complete gene networks are conserved among all animal groups. Examples can be cited for virtually all fundamental developmental steps (e.g., establishment of body axes, gastrulation) and organ systems. In light of these findings the interest in comparative embryology as a basis for discussion of homologies between cells, tissues and organs has increased over the recent years. The fact that animals as divergent as flies and humans express regulatory genes such as eyeless , orthodenticle or the Hox genes implies that the common ancestor had these genes in its genetic repertoire; but what biological function did they serve? It is generally believed that the common ancestor of gastroneuralians ("protostomes") was a simple bilaterian organism with features such as a small acoelomate body, ciliated epidermis with underlying muscle layer, and a single gut opening. Many of these primitive features are conserved among the present day platyhelminths, a taxon that on the basis of morphological evidence has split early from the gastroneuralian (protostomian) line. We have studied the normal development of representative of several flatworm taxa and are now focussing on two species that can be raised in the lab. PCR based cloning of homologs of genes involved in neural development of both Drosophila and vertebrates is under way.
Younossi-Hartenstein, A., Ehlers, U., Hartenstein, V. (2000) Embryonic development of the nervous system of the rhabdocoel flatworm Mesostoma lingua (Abildgaard, 1789). J. Comp. Neur. 416, 461-476
Hartenstein, V., Dwine, K. (2000). A new freshwater dalyellid flatworm, Gieysztoria superba sp. nov. (Dalyellidae: Rhabdocoela) from Southeast Queensland, Australia. Memoirs of the Queensland Museum 45, 381-383
Younossi-Hartenstein, A., Hartenstein, V. (2000a) The embryonic development of the dalyellid flatworm Gieysztoria superba .Int.J.Dev.Biol. (in press)
Younossi-Hartenstein, A., Hartenstein, V. (2000b) The embryonic development of the polyclad flatworm Imgogine mcgrathi Dev. Genes Evol. 210, 383-398
Hartenstein, V., Ehlers, U. (2000) The embryonic development of the rhabdocoel flatworm Mesostoma lingua. Dev. Genes Evol. 210, 399-415
Ramachandra, N.B., Ladurner, P., Jacobs, D. and Hartenstein, V. Neurogenesis in the primitive bilaterian NeochildiaI. Normal development and isolation of genes controlling neural fate. In prep.
A selected list of publications:
Noveen, A., Daniel, A., Hartenstein, V.
The role of eyeless in the embryonic development of the Drosophila mushroom body,
Haag, T., Prtina, N., Lekven, A.C., Hartenstein, V.
Discrete steps in the morphogenesis of the Drosophila heart require faint sausage, shotgun/ DE-cadherin, and laminin A,
Dumstrei, K., Nassif, C., Abboud, G., Aryai, A., Aryai, AR, Hartenstein, V.
EGFR signaling is required for the differentation and maintenance of neural progenitors along the dorsal midline of the Drosophila embryonic head,
Lekven, A., Tepass, U., Keshmeshian, M., Hartenstein, V
faint sausage encodes a novel member of the Ig superfamily required for cell movement and axonal pathfinding in the Drosophila nervous system,
Campos-Ortega, J.A., Hartenstein, V.
The Embryonic Development of Drosophila melanogaster,
Younossi-Hartenstein, A., Nassif, C. and Hartenstein, V.
Early neurogenesis of the Drosophila brain,
J. Comp. Neur,
Tepass, U., Gruszynski-de Feo, E., Haag, T.A., Omaryar, L., Torok, T., and Hartenstein, V.
Shotgun encodes Drosophilia E-Cadherin and is preferentially required during cell rearrangement in the neuroectoderm and other morphogenetically active epithelia,
Genes & Dev,
Hartenstein, V., Lee, A., and Toga, A.W.
Graphic digital database of Drosophila embryogenesis,
Trends in Genetics,