Tissue development in areas such as the nervous system can be likened to the arrangement of various elements in a concert. Genetic control centres play a key role in coordinating which pieces of music are played in which order. These control centres, called master genes or cis regulatory elements, regulate the expression of genes. A team of researchers led by Prof. Dr. Uwe Strähle from the Karlsruhe Institute of Technology (KIT) uses zebrafish to understand the gene networks that control differentiation and nervous system function by unravelling which of the thousands of signalling molecules acts as “conductor” and how “good music” can be played. The researchers are also investigating how these processes are disturbed by environmental toxins.
Thousands of molecules act in concert during the embryonic development of nervous system and other tissues: genetic programmes are initiated in a precisely determined temporal pattern, molecules switch the function of other molecules on or off, tissues communicate with each other, cells develop and differentiate, migrate from one place to another or stop dividing. Over the last few years, Prof. Dr. Uwe Strähle and his team from the Institute of Toxicology and Genetics at the Karlsruhe Institute of Technology (KIT) have been using a systems biology approach to identify the zebrafish genes that regulate these processes. “We are now focusing on the identification of the functional hierarchies between the molecular “teammates” that we believe might be involved in the differentiation and function of the nervous system and musculature, but which could also play a role in the regeneration of adult brain tissue,” said Strähle.
The molecular mechanisms involved in the development of zebrafish organs such as the brain are relatively similar to those of humans. Over the last few years, researchers have discovered signalling molecules in zebrafish that also appear to play a role in human embryonic development. Strähle and his team of researchers decided to use the zebrafish as model system for practical reasons as zebrafish embryos are transparent, which enables researchers to monitor the behaviour of stained genes and molecules live under the microscope. In addition, the short generation time of the fish and the fact that the eggs are easy to cultivate enables high-throughput experiments to be carried out. Over the last few years, another aspect has attracted the researchers’ attention, namely the regenerative ability of zebrafish. While head injuries caused by needles and severed spinal marrow kill a human being, zebrafish are able to regenerate damaged tissue within around three weeks. “Zebrafish are excellently suited to the type of research we are carrying out as they show a remarkable ability to regenerate, even in the adult central nervous system,” said Strähle. “We found out that the fish, unlike human individuals, did not have any scars, which might be one of the reasons why the tissue is able to regenerate so quickly.”
Strähle and his team have characterised stem cell niches in the brains of the fish. A stem cell niche is a microenvironment containing stem cells where new cells are produced in order to close wounds, amongst other things. How are these processes regulated? The researchers from Karlsruhe have screened a large number of genes and have identified around 2,500 regulatory genes, many of which are genetic elements that control entire molecular signalling cascades. They have been able to show how the signalling protein “sonic hedgehog”, which is secreted by specialised cells below the embryonic spinal chord, comes into contact with interneurons in the spinal marrow, where it leads to the activation of NKx genes. The products of these genes are transcription factors, which are differentially expressed and regulate the production of tissue-specific target genes, including those of neurotransmitters, for example.
“The molecular processes that control the development and regeneration of nervous tissue are hierarchically organised reaction cascades; one gene can control several genes, which in turn can control several other genes. All this taken together leads to extremely complex spatial and temporal patterns of molecular activity,” said Strähle. “We know for example that 152 regulatory genes are switched off in the embryonic forebrain; in the adult forebrain, as many as 1000 regulatory genes are silenced. So the question arises as to what their function is and which of the genes regulates what.”
At present, the researchers are taking snapshots of these regulatory processes. They will then go on to elucidate the function of these processes by silencing individual genes in the different cell types of the nervous system and relating potential defects and untypical developments to the function of specific genes. Strähle and his team and their cooperation partners from all over the world are currently working on the establishment of a zebrafish resource centre to store all zebrafish mutants with the aim of providing research groups around the world with access to specific knockout fish. The researchers hope that simple knockout studies will give them insights into the temporal and spatial interaction in the molecular networks during embryogenesis. In addition, Strähle and his team would like to identify and characterise the regulators that control stem cells.
Over the last few years, Strähle and his team have also become involved in a topic that is of interest to all of us. “Day in, day out we are exposed to a steadily increasing number of different chemicals, in our food, in detergents, in the air we breathe, basically everywhere,” said Strähle. What effect do these substances have on the molecular networks in our cells? Or in the embryonic tissue of unborn babies? What impact do they have on embryonic development? Strähle’s group is also interested in finding out how such substances interfere with the molecular pathways in cells. They are using the fish to develop an effective test system to predict the toxic effects (neurotoxic and others) of existing and novel compounds. The researchers believe that the pharmaceutical industry will also benefit from a test system of this kind as it will allow the high-throughput screening of potential drugs targeting defective molecular control mechanisms in cancer and other diseases, thus helping the molecules involved to act in concert once again.
Further information:Prof. Dr. Uwe SträhleKarlsruhe Institute of Technology (KIT)Institute of Toxicology and Genetics, and Universität HeidelbergHermann-von-Helmholtz-Platz 176344 Eggenstein-LeopoldshafenTel.: +49 (0)721/ 608 - 23 291Fax: +49 (0)721/ 608 - 23 354E-mail: uwe.straehle(at)kit.edu