Zebrafish Genetics Laboratory
Zebrafish are small, freshwater fish originating from India that are commonly kept in home aquaria. However, most people are unaware that this fish is also used as a powerful model system for studying how genes control the embryonic development of vertebrates including humans.
Zebrafish have particular characteristics that make them very suitable for genetic research on embryo development. This has led to an explosion of interest in zebrafish research in the last 10 years. Zebrafish embryos develop extremely rapidly - from the one cell stage to hatching of the tiny fish takes only 48 hours. The embryos are also completely transparent allowing us to see every cell in the embryo and to follow its fate. There are techniques that can show where genes are active in particular cells of whole embryos. The relatively large size of zebrafish eggs (compared to mouse eggs for example) makes it very easy to inject DNA to produce transgenic zebrafish. Female zebrafish release large numbers of eggs and this allows us to examine genetic phenomena with great statistical accuracy. Hundreds of mutations have been found that affect embryo development and these are now being investigated intensively in many laboratories around the world. The fact that zebrafish embryos are vertebrates, small, numerous and develop externally is making them increasingly popular with the biomedical industry as a system in which to model human genetic disease and to screen for drugs to treat these diseases.
Our laboratory uses zebrafish embryos to investigate a number of questions involving human disease and brain development in the embryo. Our main interests are in the normal development of the nervous system, the genetics of dementia and in transgenic technology.
Mutations in the two presenilin genes, PSEN1 and PSEN2, cause the majority of inherited Alzheimers disease. These two genes encode two very similar proteins that appear to control a number of different signaling pathways in cells. The best understood function of presenilin proteins is in cleavage of transmembrane proteins within lipid bilayers. This leads to the release of the intracellular domains of signal receptor proteins such as Notch and the Amyloid Precursor Protein. These intracellular domains then enter the nucleus to regulate gene activity. Presenilins also have other activities such as regulation of beta-catenin protein phosphorylation. Despite intensive research, we understand relatively little about the differences in activity between the PSEN1 and PSEN2 proteins. It has also become apparent that many isoforms of these proteins exist that have discrete activities. Zebrafish have two presenilin genes, psen1 and psen2 and we have shown that they appear to function in a similar way to the human presenilin genes. We are currently using the genetically manipulable zebrafish embryo to model aspects of Alzheimer’s Disease in ways not possible using other systems.
The distribution of neurons in the developing spinal cord:
The majority of neuron types in the spinal cord are distributed along it in patterns that appear somewhat “disorganized” (i.e. not regular). However, the generation of these neurons cannot be a completely random process otherwise this complex organ would fail to develop and function correctly. What then controls the generation of irregular distributions of neurons along the spinal cord? We have shown previously that expression of one gene, tbx16 (previously called spadetail), marks a particular type of neuron distributed in an irregular pattern in the spinal cord of developing zebrafish embryos. We have also shown that these neurons migrate after they are formed. We have developed a method to label these neurons and to observe their migration in living embryos. It is very easy to decrease or increase the expression of different genes in zebrafish embryos. Therefore, we can now use these labeled, migrating cells to investigate how genes control the process of neuron migration in living embryos.