Two-way traffic in the spinal cord

The progress a baby makes in the first year of life is amazing: a newborn can only wave its arms and legs about randomly, but not so long after the baby can reach out and pick up a crumb from the carpet. What happens in the nervous system that enables this change from random waving to finely coordinated movement? Scientists from the Max Planck Institute of Neurobiology in Martinsried near Munich, working with colleagues from New York and Philadelphia, have described a new type of nerve cell in mice which provides a valuable insight into this developmental phenomenon. During embryonic development, the projections from these cells grow from the spinal cord towards the brain. They may pave the way for other nerve cells which control voluntary movement and which only grow from the brain into the spinal cord after birth.

When we reach out towards an object with our hand or push our foot into a boot, our movements are coordinated and controlled by the brain. For this to be possible there must be a neural pathway for the brain to transmit instructions, for example to the foot; and also in the reverse direction, for stimuli from the surroundings of the foot to be passed back to the brain. Such neural pathways are formed when the projections (axons) grow out from nerve cells during development. Depending on the organism and the body part to be connected, the axons can grow to many centimetres in length. Rüdiger Klein and his team at the Max Planck Institute of Neurobiology investigate how the axons navigate through the body, and which molecules play a part in their pathfinding. In particular, the scientists have been focusing on the signalling molecules known as ephrins and their binding partners, the Eph receptors. Ephrins and Eph receptors are located on the surface of nerve cells, among other places, and help the growing cells find their way and locate their partner cells.

Some time ago, Rüdiger Klein and his team discovered in the mouse that ephrins and Eph receptors play a key role in the development of the neural networks which control our movements. The neurobiologists have been able to demonstrate that the ephrin/Eph system guides nerve cells which, after birth, send their axons from the brain into the spinal cord and direct voluntary movement in the arms and legs. In their investigations into axons which run in the opposite direction, namely from the spinal cord into the brain, the researchers came across a new cell type which also contained Eph receptors. “Just where the ‘descending’ axons were growing, we found the ‘ascending’ axons running in parallel”, says Rüdiger Klein. “That obviously raised the question in our minds as to how this parallel growth is controlled during development.”

Subsequent research by the neurobiologists uncovered something surprising: in contrast with the known cells, the ascending axons of the new cell type did not grow only after birth, but instead already during embryonic development. Moreover, their growth was guided by the same ephrin/Eph signalling system as that involved in the growth of the descending axons. “It would seem that during embryonic development the ascending axons would ‘pre-drill’ a channel for the descending axons which do not grow out until after birth”, explains Rüdiger Klein.

Further investigations into the new, ascending nerve cells have made it clear that they obtain their input from specialised, touch-sensitive cells. A new feedback system could thus be involved here: voluntary movements are refined by signals from touch-sensitive cells, so adapting the intended movement to the environment and your foot slips into the boot. “What we found surprising is the fact that one and the same guidance system directs both the descending and the ascending axons”, says Klein. “This is a wonderful example of how a highly complex nervous system can be built up by making flexible use of individual molecules, and thus a small number of genes.” The next job for the scientists is to find out whether the suspected feedback system actually exists, i.e. whether the ascending and descending cells are connected via synapses. Their aim is to unravel step by step the developmental processes which enable the brain to coordinate sequences of movements.

Researchers create the inner ear from stem cells, opening potential for new treatments

Indiana University scientists have transformed mouse embryonic stem cells into key structures of the inner ear. The discovery provides new insights into the sensory organ’s developmental process and sets the stage for laboratory models of disease, drug discovery and potential treatments for hearing loss and balance disorders.

A research team led by Eri Hashino, Ph.D., Ruth C. Holton Professor of Otolaryngology at Indiana University School of Medicine, reported that by using a three-dimensional cell culture method, they were able to coax stem cells to develop into inner-ear sensory epithelia — containing hair cells, supporting cells and neurons — that detect sound, head movements and gravity. The research was reportedly online Wednesday in the journal Nature.

Previous attempts to “grow” inner-ear hair cells in standard cell culture systems have worked poorly in part because necessary cues to develop hair bundles — a hallmark of sensory hair cells and a structure critically important for detecting auditory or vestibular signals — are lacking in the flat cell-culture dish. But, Dr. Hashino said, the team determined that the cells needed to be suspended as aggregates in a specialized culture medium, which provided an environment more like that found in the body during early development.

The team mimicked the early development process with a precisely timed use of several small molecules that prompted the stem cells to differentiate, from one stage to the next, into precursors of the inner ear. But the three-dimensional suspension also provided important mechanical cues, such as the tension from the pull of cells on each other, said Karl R. Koehler, B.A., the paper’s first author and a graduate student in the medical neuroscience graduate program at the IU School of Medicine.

"The three-dimensional culture allows the cells to self-organize into complex tissues using mechanical cues that are found during embryonic development," Koehler said.

"We were surprised to see that once stem cells are guided to become inner-ear precursors and placed in 3-D culture, these cells behave as if they knew not only how to become different cell types in the inner ear, but also how to self-organize into a pattern remarkably similar to the native inner ear," Dr. Hashino said. "Our initial goal was to make inner-ear precursors in culture, but when we did testing we found thousands of hair cells in a culture dish."

Electrophysiology testing further proved that those hair cells generated from stem cells were functional, and were the type that sense gravity and motion. Moreover, neurons like those that normally link the inner-ear cells to the brain had also developed in the cell culture and were connected to the hair cells.

Additional research is needed to determine how inner-ear cells involved in auditory sensing might be developed, as well as how these processes can be applied to develop human inner-ear cells, the researchers said.

However, the work opens a door to better understanding of the inner-ear development process as well as creation of models for new drug development or cellular therapy to treat inner-ear disorders, they said.

Researchers Find Zinc’s Crucial Pathway to the Brain

A new study helps explain how parts of the brain maintain their delicate balance of zinc, an element required in minute but crucial doses, particularly during embryonic development.

The study, led at the Marine Biological Laboratory (MBL) by Mark Messerli in collaboration with scientists from the University of California, Davis, shows that neural cells require zinc uptake through a membrane transporter referred to as ZIP12. If that route is closed, neuronal sprouting and growth are significantly impaired and is fatal for a developing embryo. Their discovery was published in the Proceedings of the National Academy of Sciences.

“This particular transporter is an essential doorway for many neurons in the central nervous system,” explains Messerli. “You knock out this one gene, this one particular pathway for the uptake of zinc into these cells, and you essentially prevent neuronal outgrowth. That’s lethal to the embryo.”

Previously, scientists thought that zinc could use more than one pathway to enter the cell during early brain development. Some other elements, like calcium, enjoy such luxury of multiple options.

Knocking out ZIP12, affected several critical processes in the brain, the scientists found. For example, frog embryos were unable to develop their neural systems properly. Additionally, neurons had trouble reaching out to connect to other neurons; their extensions were both shorter and fewer in number than normal.

“We were surprised that ZIP12 was required at such an early and critical stage of development,” said Winyoo Chowanadisai, a researcher in nutrition at the University of California at Davis and visiting scientist in the Cellular Dynamics Program at the MBL. Dr. Chowanadisai was the first on the team to realize that ZIP12 is expressed in such abundance in the brain.“This study also reinforces the importance of periconceptional and prenatal nutrition and counseling to promote health during the earliest stages of life.”

ZIP12 is part of a larger family of transporters involved in the movement of metal ions from outside the cell. Other reports showed that simultaneously blocking 3 other transporters in the family – including  ZIP1, 2, and 3 – had no major effects on embryonic development.

Zinc is needed for healthy neural development, helping the brain to learn and remember new information. However, too much zinc can also be problematic.

The research team is investigating the implications of their results on processes like embryonic brain development and wound healing.

“[The result] was not expected,” said Messerli, a physiologist in the MBL’s Bell Center for Regenerative Biology and Tissue Enginering and Cellular Dynamics Program. ““We found that zinc uptake through ZIP12 is a regulatory point for neuronal growth, required for development and possibly required for learning and memory throughout life. We want to elucidate the downstream targets that zinc is affecting. That’s the next exploration.”

Advance in tuberous sclerosis brain science

By manipulating the timing of disease-causing mutations in the brains of developing mice, Brown University researchers have found that early genetic deletions in the thalamus may play an important role in course and severity of the developmental disease tuberous sclerosis complex. Findings appear in the journal Neuron.

Doctors often diagnose tuberous sclerosis complex (TSC) based on the abnormal growths the genetic disease causes in organs around the body. Those overt anatomical structures, however, belie the microscopic and mysterious neurological differences behind the disease’s troublesome behavioral symptoms: autism, intellectual disabilities, and seizures. In a new study in mice, Brown University researchers highlight a role for a brain region called the thalamus and show that the timing of gene mutation during thalamus development makes a huge difference in the severity of the disease.

TSC can arise in humans and mice alike when both alleles (the one from mom and the one from dad) of the TSC1 gene are deleted. One bad gene is often inherited and the other accumulates a mutation some time during embryonic development. This happens to one in 6,000 people.

“We don’t know when during development the mutations are occurring in the patients,” said Elizabeth Normand, a Brown neuroscience graduate student and lead author of the paper in the journal Neuron. “That’s why we chose to look at the timing. It can give us some insight into the role of genes during embryonic development.”

Normand and adviser Mark Zervas, assistant professor of biology, not only wanted to assess the timing but also to probe the role the thalamus might have in contributing to the neurological symptoms of the disease. To do both, their team genetically engineered a clever mouse model in which they could, with a dose of the drug tamoxifen, delete both alleles exclusively in thalamus neurons at the developmental stage of their choosing.

Their interest in the thalamus comes from its role in forging strong but intricate links to the cortex, which is where most other TSC researchers have focused. As for timing, they tested the effect of controlling allele deletions on day 12 of gestation in some mice and day 18 (just before birth) in others. Still other mice were left healthy as experimental controls.

Significant symptoms

Overall, the researchers found they could indeed generate TSC-like behavioral symptoms in the mice, such as seizures, by deleting TSC1 alleles in developing cells of the thalamus. They also found that the timing of the deletion mattered tremendously to the extent of the disease in the brain, the degree of abnormality, and the severity of TSC-like symptoms.

The mice whose alleles were deleted on embryonic day 12 fared much worse behaviorally than the mice whose alleles were deleted on embryonic day 18.

At two months of age, the mice with the embryonic day 12 deletion exhibited excessive self-grooming to the point where they experienced lesions. Among those mice, 10 of 11 experienced seizures at an average rate of more than three per hour.

The mice with the embryonic day 18 deletion, on the other hand, fared better without any over-grooming. By eight months of age, however, four of 17 of the mice did exhibit rare seizures.

These behavioral differences traced to differences in the the way the mice’s brains became wired. A comparison of brain tissue from adult mice — some of which had the early TSC1 deletions and some of which didn’t — revealed differences in the connections between the thalamus and the cortex and in the electrical and physical properties of thalamus cells.

“We’re building off the core idea of the thalamus playing an important role in brain function and showing that if you disrupt the way that the thalamic neurons develop that you can get some of these behavioral consequences such as overgrooming or seizures,” said Zervas, who is affiliated with the Brown Institute for Brain Science.

The extent of mutant neurons was much more severe in the mice with the embryonic day 12 versus day 18 mutations. In embryonic day 12 deleted mice, for example, the deletion disrupted the growth-regulating “mTOR” pathway in 70 percent of neurons versus only 29 percent of neurons in the embryonic day 18 deleted mice. The disruptions occurred in more areas of the thalamus in embryonic day 12 than in day 18 mice as well. The overactivity of mTOR in TSC is what produces the unusual growths around the body, though these new findings indicate additional roles for the mTOR pathway in brain development and function, Zervas said.

In future work, the team plans to study the effects of deleting the TSC1 allele at other days during development as well as to understand whether there is a threshold of mutant neurons with mTOR disruption at which TSC-like symptoms begin to emerge.