Posts tagged zebrafish
Articles and news from the latest research reports.
How Different Nerve Cells Develop in the Eye
Neurobiologists from Heidelberg University’s Centre for Organismal Studies (COS) have gained new insights into how different types of nerve cells are formed in the developing animal. Through specialised microscopes, they were able to follow the development of the neural retina in the eye of living zebrafish embryos. Using high-resolution three-dimensional time-lapse images the researchers simultaneously observed the division of retinal nerve cells and changes in gene expression. This enabled them to gain insights into the way in which the two processes are linked during eye development and how the number and proportion of different cell types are regulated.
A central question in developmental and regenerative neurobiology concerns the growth processes in animal organisms: How does a growing animal control the generation of the right number of each type and subtype of nerve cell in the brain and what is the relationship between these cells? The retina consists of many different kinds of nerve cells, which are well characterised and common to all vertebrates. Thus, the retina is a particularly good model for studying neuronal development. The researchers studied such retinal developmental processes in living organisms using zebrafish embryos, which are completely transparent and grow rapidly outside their mother.
All retinal cells, which are either excitatory or inhibitory, arise from a relatively small number of apparently homogeneous progenitor cells. These progenitors are able to generate all the different retinal cell types. “It is a challenge to understand how each progenitor cell contributes to the correct number and subtype of nerve cells that compose the final retinal network. Our work contributes to the understanding of how different genes orchestrate neuronal diversity along a progenitor cell lineage, that is the number of cell divisions and types of neurons generated”, says Heidelberg researcher Dr. Lucia Poggi.
To tackle this challenge, Dr. Poggi’s team used different lines of transgenic zebrafish, in which fluorescent reporter proteins highlight the expression of different genes in dividing cells. Working in close cooperation with Dr. Patricia Jusuf of the Australian Regenerative Medicine Institute at Monash University, the researchers found that some particular kinds of excitatory and inhibitory nerve cells tend to be lineally related, i.e. they derive from a common progenitor cell. For the first time, 4D recordings permitted an in vivo analysis of how the generation of particular inhibitory cells is regulated through coordination of cell division mode and gene expression within individual retinal progenitors of excitatory nerve cells.
This study has established a model of how cell lineage influences neuronal subtype specification and neuronal circuitry formation in the native environment of the vertebrate brain. The results were published in the Journal of Neuroscience.
(Source: uni-heidelberg.de)
See-through ‘MitoFish’ opens a new window on brain diseases
Scientists have demonstrated a new way to investigate mechanisms at work in Alzheimer’s and other neurodegenerative diseases, which also could prove useful in the search for effective drugs. For new insights, they turned to the zebrafish, which is transparent in the early stages of its life. The researchers developed a transgenic variety, the “MitoFish,” that enables them to see – within individual neurons of living animals – how brain diseases disturb the transport of mitochondria, the power plants of the cell.
Neurodegenerative diseases such as Alzheimer’s, Parkinson’s, ALS (amyotrophic lateral sclerosis), and MS (multiple sclerosis) are quite different in their effects on patients’ cognitive and motor functions, behavior, and prognosis. Yet on the level of individual neurons, common mechanisms can be observed that either cause or accompany nerve degeneration in a number of different diseases. One of these is a disturbance in the transport of mitochondria, organelles that play several vital roles in the life of a cell — above all, delivering energy where it is needed. And in a neuron, an extremely power-hungry cell, that means moving mitochondria all the way down its longest extension, the axon. Studying mitochondria transport in other animal models of neurodegenerative disease, particularly in mice, has been revealing. But the MitoFish model opens up new possibilities.
The new model was jointly developed in the labs of Prof. Thomas Misgeld of the Technische Universität München (TUM) and Dr. Bettina Schmid, a senior scientist of the German Center for Neurodegenerative Diseases (DZNE) based at the institute of LMU Prof. Christian Haass. “This collaboration has provided a system,” Misgeld says, “with which we can try to understand the traffic rules or the life cycle of a given organelle, in this case mitochondria, in the context of a nerve cell that’s existing in its physiological environment, where it is developing and changing. Most of these things we don’t understand well enough to model them in another setting, so we have the organism do it for us.”
The MitoFish is both readily manipulated, enabling researchers to pose specific questions, and literally transparent — allowing non-invasive in vivo observation of changes relevant to disease processes. It is possible to image a whole, living neuron over time and to follow the movements of mitochondria within it. “The zebrafish is an established genetic model,” Schmid explains, “which means you can bring foreign genes or certain proteins into a fish to test hypotheses about basic biology, disease mechanisms, or potential therapies. And because the early embryo is transparent, you can label specific nerve cells with a fluorescent protein and then look at them in an intact, living animal.”
A Blind Circadian Clock in Cavefish Reveals that Opsins Mediate Peripheral Clock Photoreception
The circadian clock is synchronized with the day-night cycle primarily by light. Fish represent fascinating models for deciphering the light input pathway to the vertebrate clock since fish cell clocks are regulated by direct light exposure. Here we have performed a comparative, functional analysis of the circadian clock involving the zebrafish that is normally exposed to the day-night cycle and a cavefish species that has evolved in perpetual darkness. Our results reveal that the cavefish retains a food-entrainable clock that oscillates with an infradian period. Importantly, however, this clock is not regulated by light. This comparative study pinpoints the two extra-retinal photoreceptors Melanopsin (Opn4m2) and TMT-opsin as essential upstream elements of the peripheral clock light input pathway.
Inflammation for Regeneration
The secret to zebrafish’s remarkable capacity for repairing their brains is inflammation, according to a report published online in Science. Neural stem cells in the fish’s brains express a receptor for inflammatory signaling molecules, which prompt the cells to multiply and develop into new neurons.
“This is a very interesting paper,” said Guo-li Ming, a professor of neurology and neuroscience at The Johns Hopkins University in Baltimore, who was not involved in the study. “It is well known that fish have this ability to self-repair, and this paper provides a mechanism,” she said.
Zebrafish, like many other vertebrates, are able to regenerate a variety of body tissues, including their brains. In fact, said Michael Brand, a professor of developmental genetics at the Technische Universität in Dresden, Germany, “mammals are the ones that seem to have lost this ability—they are kind of the odd ones out.” Given the therapeutic potential of neuron regeneration for patients with brain or spinal injuries, “we’d like to figure out if we can somehow reactivate this potential in humans,” Brand said.
With an incredible diversity of cell types, the central nervous system (CNS), comprising the brain, spinal cord and retina, can be considered to be the most complex organ in the body.
Professor Bill Harris, an experimental biologist and Head of the Department of Physiology, Development and Neuroscience, is fascinated by how this complex and sophisticated system is built out of a collection of undifferentiated cells.
By putting an advanced technology to novel use, he has been able to observe for the first time the entire process of retinal development at the cellular level in zebrafish embryos. This has achieved a long-sought goal in developmental neurobiology: a complete analysis of the building of a vertebrate CNS structure in vivo.
The zebrafish is a major player in the study of vertebrate biology and human disease. Its transparent, externally fertilized eggs, short reproductive cycle and fast growth mean that its embryonic development can be studied closely while the animal is alive, and the fish is a useful model for studying gene behaviour and function.
Now, researchers led by Stephen Ekker, a molecular biologist at the Mayo Clinic in Rochester, Minnesota, have for the first time made custom changes to parts of the zebrafish (Danio rerio) genome, using artificial enzymes to cut portions of DNA out of targeted positions in a gene sequence, and replace them with synthetic DNA.
Light on the Brain
By Sabrina Richards | September 20, 2012
Researchers find that photoreceptors expressed in zebrafish hypothalamus contribute to light-dependent behavior.
Juvenile zebrafish.
Zebrafish larvae without eyes or pineal glands can still respond to light using photopigments located deep within their brains. Published in Current Biology, the findings are the first to link opsins, photoreceptors located in the hypothalamus and other brain areas, to increased swimming in response to darkness, a behavior researchers hypothesize may help the fish move toward better-lit environments.
“[It’s a] strong demonstration that opsin-dependent photoreceptors in deep brain areas affect behaviors,” said Samer Hattar, who studies light reception in mammals at Johns Hopkins University but did not participate in the research.
Photoreceptors in eyes enable vision, and photoreceptors in the pineal gland, a small endocrine gland located in the center of the vertebrate brain, regulate circadian rhythms. But photoreceptors are also found in other brain areas of both invertebrates and vertebrate lineages. The function of these extraocular photoreceptors has been best studied in birds, where they regulate seasonal reproduction, explained Harold Burgess, a behavioral neurogeneticist at the Eunice Kennedy Shriver National Institute for Child Health and Human Development.
Many opsins have been reported in the brains of tiny and transparent larval zebrafish, raising the possibility that light could be stimulating the photoreceptors even deep in the brain. To test for behaviors that may be regulated by deep brain photoreceptors, Burgess and his colleagues in Wolfgang Driever’s lab at the University of Freiburg removed the eyes of zebrafish larvae, and compared their behavior to larvae that retained their eyes. Although most light-dependent behavior required eyes, the eyeless larvae did respond when the lights were turned off, increasing their activity for a several minutes, though to a somewhat lesser extent than control larvae. But the fact that they responded at all suggests that non-retinal photoreceptors contributed to the behavior.
To confirm the role of the deep brain photoreceptors, the researchers also tested eyeless larvae that had been genetically modified to block expression of photoreceptors in the pineal gland. This fish still showed this jump in activity for several minutes after entering darkness.
Two different types of opsins—melanopsin and multiple tissue opsin—are expressed in the same type of neuron in zebrafish hypothalamus. Burgess and his colleagues looked at zebrafish missing the transcription factor Orthopedia, which is unique to these neurons, and found that the darkness-induced activity boost is nearly absent in these fish. To further narrow the search for the responsible photoreceptors, the researchers overexpressed melanopsin in hypothalamus neurons that co-express Orthopedia and melanopsin, and found that it increased the sensitivity of eyeless zebrafish to reductions in light. The results point to both melanopsin and Orthopedia as key players in modulating this behavior and pinpoint the location to neurons that coexpress these factors in the zebrafish hypothalamus.
Interestingly, the hypothalamus is one of the oldest parts of the vertebrate brain, said Detlev Arendt, a developmental biologist at the European Molecular Biology Laboratory in Heidelberg. “It’s very possible that this is one of the oldest functions”—one that evolved in “non-visual organisms” that had no eyes but still needed to sense light.
Although not as directed and efficient as eye-dependent behaviors that help fish swim toward light, Burgess speculates that deep brain opsins can still benefit zebrafish larvae. “You could imagine situation where it can’t see light, if a leaf falls on it and it doesn’t know where to swim. I think this behavior puts it in a hyperactive state where it swims wildly for several minutes until it reaches enough light for eyes to take over,” he explained, noting that such behavior is common in invertebrates.
It remains to be seen whether these deep brain opsins regulate other behaviors, perhaps in similar fashion to seasonal hormonal regulation in birds, but Hattar believes it is likely. “It’s beyond reasonable doubt there are many functions for these deep brain photoreceptors.”
(Source: the-scientist.com)
Zebrafish Reveal Promising Process for Healing Spinal Cord Injury
ScienceDaily (July 6, 2012) — Yona Goldshmit, Ph.D., is a former physical therapist who worked in rehabilitation centers with spinal cord injury patients for many years before deciding to switch her focus to the underlying science.
"After a few years in the clinic, I realized that we don’t really know what’s going on," she said.
Now a scientist working with Peter Currie, Ph.D., at Monash University in Australia, Dr. Goldshmit is studying the mechanisms of spinal cord repair in zebrafish, which, unlike humans and other mammals, can regenerate their spinal cord following injury. On June 23 at the 2012 International Zebrafish Development and Genetics Conference in Madison, Wisconsin, she described a protein that may be a key difference between regeneration in fish and mammals.
One of the major barriers to spinal regeneration in mammals is a natural protective mechanism, which incongruously results in an unfortunate side effect. After a spinal injury, nervous system cells called glia are activated and flood the area to seal the wound to protect the brain and spinal cord. In doing so, however, the glia create scar tissue that acts as a physical and chemical barrier, which prevents new nerves from growing through the injury site.
One striking difference between the glial cells in mammals and fish is the resulting shape: mammalian glia take on highly branched, star-like arrangements that appear to intertwine into dense tissue. Fish glia cells, by contrast, adopt a simple elongated shape — called bipolar morphology — that bridges the injury site and appears to help new nerve cells grow through the damaged area to heal the spinal cord.
"Zebrafish don’t have so much inflammation and the injury is not so severe as in mammals, so we can actually see the pro-regenerative effects that can happen," Dr. Goldshmit explained.
Studies in mice have found that mammalian glia can take up the same elongated shape, but in response to the environment around the injury they instead mature into scar tissue that does not allow nerve regrowth.
Dr. Goldshmit and her colleagues have focused on a family of molecules called fibroblast growth factors (Fgf), which have shown some evidence of improving recovery in mice and humans with spinal cord damage. The Monash University group found that Fgf activity around the damage site promotes the bipolar glial shape and encourages nerve regeneration in zebrafish.
Preliminary results in mice show that Fgf injections near a spinal injury increase both the number of glia cells at the site and the elongated morphology. Their evidence suggests that Fgfs may work to create an environment more supportive of regeneration in mammals as well and could be a valuable therapeutic target.
Spinal injury patients usually have few options, Dr. Goldshmit emphasized, and development of new, biologically-based approaches will be critical.
"This is a nice example of how we can use the zebrafish model," she said. "When we learn from the zebrafish what to look at, we can find things that give us hope for finding therapeutic approaches for spinal cord injury in humans."
Source: Science Daily