Newborn babies walk the walk

Before you can run, you have to walk, and before you can walk well, you have to walk like a brand-new baby. A new study uncovers the logistics of newborns’ herky-jerky, Frankensteinian stepping action and how this early reflex morphs into refined adult locomotion.

In the study, electrodes on infants’ chubby legs picked up signals from neurons that tell muscles to fire, revealing that three-day old babies tense up many of their leg muscles all at once. Toddlers, preschoolers and adults, by contrast, showed a progressively more sophisticated, selective pattern of neuron activity.

From birth to adulthood, motor neurons in the spine get an overhaul as neurons in different  locations along the spine become specialized for various aspects of walking, such as foot position, balance and direction, Yuri Ivanenko of the Santa Lucia Foundation in Rome and colleagues conclude in the Feb. 13 Journal of Neuroscience.

Researcher Advancing Motor Neuron Studies

Supported by the commitment of the University of Connecticut and the state to stem cell research, a UConn Health Center researcher is advancing the understanding of the devastating inherited condition known as spinal muscular atrophy.

Xue-Jun Li, assistant professor in the Department of Neuroscience, is corresponding author of a paper published in the prestigious journal Cell Research in December 2012 entitled “Recapitulation of spinal motor neuron-specific disease phenotypes in a human cell model of spinal muscular atrophy.” The paper’s other authors are UConn Health Center researcher Zhi-Bo Wang and Xiaoqing Zhang of the Tongji University School of Medicine in Shanghai.

Spinal muscular atrophy (SMA) is a group of inherited diseases that cause muscle damage and debilitation, which progress over time and eventually lead to death. To be affected, a person must inherit the defective gene from both parents. About 1 in 10,000 people have SMA, and most do not survive childhood due to respiratory problems, heart failure and infections.

“There is no effective treatment for spinal muscular atrophy, and one of the roadblocks is not knowing why the spinal motor neuron degenerates,” Li explains. “One of the aspects of our research is to understand how specific types of neurons are specified and degenerated. We are trying to model neurological disorders by using human motor neurons derived from stem cells.”

Establishing human cell models of SMA to mimic motor neuron-specific phenotypes holds the key to understanding this destructive disease, she says. The model described in the journal article provides a unique paradigm for studying how motor neurons degenerate. It also highlights the potential importance of antioxidants for the treatment of SMA.

Understanding how motor neurons are specifically degenerated can lead to effective interventions in the future. “It can help us find some way to rescue the motor neuron degeneration in this disease,” Li points out. “Understanding the role of antioxidants can provide potential clues to finding a treatment.”

New Brain Circuit Sheds Light on Development of Voluntary Movements

All parents know the infant milestones: turning over, learning to crawl, standing, and taking that first unassisted step. Achieving each accomplishment presumably requires the formation of new connections among subsets of the billions of nerve cells in the infant’s brain. But how, when and where those connections form has been a mystery.

Now researchers at Duke Medicine have begun to find answers. In a study reported Jan. 23, 2013, in the scientific journal Neuron, the research team describes the entire network of brain cells that are connected to specific motor neurons controlling whisker muscles in newborn mice.

A better understanding of such motor control circuits could help inform how human brains develop, potentially leading to new ways of restoring movement in people who suffer paralysis from brain injuries, or to the development of better prosthetics for limb replacement.

“Whiskers to mice are like fingers to humans, in that both are moving touch sensors,” said lead investigator Fan Wang, PhD, associate professor of cell biology and member of the Duke Institute for Brain Sciences. “Understanding how the mouse’s brain controls whisker movements may tell us about neural control of finger movements in people.”

Mice are active at night, so they rely heavily on whiskers to detect and discriminate objects in the dark, brushing their whiskers against objects in a rhythmic back-and-forth sweeping pattern referred to as “whisking”. But this whisking behavior does not appear until about two weeks after birth, when young mice start to explore the world outside their nest.

To learn how motor control of whiskers takes place, Wang and postdoctoral fellow Jun Takatoh used a new technique that takes advantage of the rabies virus’ ability to spread through connected nerve cells. A disabled form of the virus used to vaccinate pets was created with the ability to express a fluorescent protein. The researchers were able to trace its path through a network of brain cells directly connected to the motor neurons controlling whisker movement.

“The precision of this mapping method allowed us to ask a key question, namely are parts of the whisker motor control circuitry not yet connected in newborn mice, and are such missing links added later to enable whisking?” Wang said.

By taking a series of pictures in the fluorescently labeled brains during the first two weeks after birth, the research team chronicled the developing circuits before and after mice start whisking.

“When we traced the circuit it was stunning in the sense that we didn’t realize there are so many pools of neurons located throughout the brainstem that are connected to whisker motor neurons,” said Wang. “It’s remarkable that a single motor neuron receives so many inputs, and somehow is able to integrate them.”

At the same time whisking movements emerge, motor neurons receive a new set of inputs from a region of the brainstem called the LPGi. A single LPGi neuron is connected to motor neurons on both sides of the face, putting them in perfect position to synchronize the movements of left and right whiskers.

To learn more about the new circuit formed between LPGi and motor neurons, Wang and Takatoh drew on the expertise of Duke colleague Richard Mooney, PhD, professor of neurobiology, and his student Anders Nelson. Together, the researchers were able to record the labeled neurons and found the LPGi neurons communicate with motor neurons using glutamate, the main neurotransmitter that stimulates the brain. They further discovered that LPGi neurons receive direct inputs from the motor cortex.

“This makes sense because exploratory whisking is a voluntary movement under control of the motor cortex,” Wang said. “Excitatory input is needed for initiating such movements, and LPGi may be critical for relaying signals from the motor cortex to whisker motor neurons.”

The researchers will next explore the connectivity by using genetic, viral and optical tools to see what happens when certain components of the circuits are activated or silenced during various motor tasks.

Researchers turn one form of neuron into another in the brain

A new finding by Harvard stem cell biologists turns one of the basics of neurobiology on its head – demonstrating that it is possible to turn one type of already differentiated neuron into another within the brain.

The discovery by Paola Arlotta and Caroline Rouaux “tells you that maybe the brain is not as immutable as we always thought, because at least during an early window of time one can reprogram the identity of one neuronal class into another,” said Arlotta, an Associate Professor in Harvard’s Department of Stem Cell and Regenerative Biology (SCRB).

The principle of direct lineage reprogramming of differentiated cells within the body was first proven by SCRB co-chair and Harvard Stem Cell Institute (HSCI) co-director Doug Melton and colleagues five years ago, when they reprogrammed exocrine pancreatic cells directly into insulin producing beta cells.

Arlotta and Rouaux now have proven that neurons too can change their mind. The work is being published on-line by the journal Nature Cell Biology.

In their experiments, Arlotta targeted callosal projection neurons, which connect the two hemispheres of the brain, and turned them into neurons similar to corticospinal motor neurons, one of two populations of neurons destroyed in Amyotrophic Lateral Sclerosis (ALS), also known as Lou Gehrig’s disease. To achieve such reprogramming of neuronal identity, the researchers used a transcription factor called Fezf2, which long as been known for playing a central role in the development of corticospinal neurons in the embryo.

What makes the finding even more significant is that the work was done in the brains of living mice, rather than in collections of cells in laboratory dishes. The mice were young, so researchers still do not know if neuronal reprogramming will be possible in older laboratory animals – and humans. If it is possible, this has enormous implications for the treatment of neurodegenerative diseases.

"Neurodegenerative diseases typically effect a specific population of neurons, leaving many others untouched. For example, in ALS it is corticospinal motor neurons in the brain and motor neurons in the spinal cord, among the many neurons of the nervous system, that selectively die," Arlotta said. "What if one could take neurons that are spared in a given disease and turn them directly into the neurons that die off? In ALS, if you could generate even a small percentage of corticospinal motor neurons, it would likely be sufficient to recover basic functioning," she said.

The experiments that led to the new finding began five years ago, when “we wondered: in nature you never seen a neuron change identity; are we just not seeing it, or is this the reality? Can we take one type of neuron and turn it into another?” Arlotta and Rouaux asked themselves.

Over the course of the five years, the researchers analyzed “thousands and thousands of neurons, looking for many molecular markers as well as new connectivity that would indicate that reprogramming was occurring,” Arlotta said. “We could have had this two years ago, but while this was a conceptually very simple set of experiments, it was technically difficult. The work was meant to test important dogmas on the irreversible nature of neurons in vivo. We had to prove, without a shadow of a doubt, that this was happening.”

The work in Arlotta’s lab is focused on the cerebral cortex, but “it opens the door to reprogramming in other areas of the central nervous system,” she said.

Arlotta, an HSCI principal faculty member, is now working with colleague Takao Hensch, of Harvard’s Department of Molecular and Cellular Biology, to explicate the physiology of the reprogrammed neurons, and learn how they communicate within pre-existing neuronal networks.

"My hope is that this will facilitate work in a new field of neurobiology that explores the boundaries and power of neuronal reprogramming to re-engineer circuits relevant to disease," said Paola Arlotta.

(Image courtesy Tulane University)

Molecular ‘Two-Way Radio’ Directs Nerve Cell Branching And Connectivity

Working with fruit flies, Johns Hopkins scientists have decoded the activity of protein signals that let certain nerve cells know when and where to branch so that they reach and connect to their correct muscle targets. The proteins’ mammalian counterparts are known to have signaling roles in immunity, nervous system and heart development, and tumor progression, suggesting broad implications for human disease research. A report of the research was published online Nov. 21 in the journal Neuron.

To control muscle movements, fruit flies, like other animals, have a set of nerve cells called motor neurons that connect muscle fibers to the nerve cord, a structure similar to the spinal cord, which in turn connects to the brain. During embryonic development, the nerve cells send wire-like projections, or axons, from the nerve cord structure out to their targets. Initially, multiple axons travel together in a convoy, but as they move forward, some axons must exit the “highway” at specific points to reach particular targets.

In their experiments, the researchers learned that axons travelling together have proteins on their surfaces that act like two-way radios, allowing the axons to communicate with each other and coordinate their travel patterns, thus ensuring that every muscle fiber gets connected to a nerve cell. “When axons fail to branch, or when they branch too early and too often, fruits flies, and presumably other animals, can be left without crucial muscle-nerve connections,” says Alex Kolodkin, Ph.D., a Howard Hughes investigator and professor of neuroscience at the Institute for Basic Biomedical Sciences at the Johns Hopkins University School of Medicine.

At the center of the communications system, Kolodkin says, is a protein called Sema-1a, already known to reside on the surface of motor neuron axons. If a neighboring axon has a different protein, called PlexA, on its surface, it will be repulsed by Sema-1a and will turn away from the axon bundle. So Sema-1a acts as an instructional signal and PlexA as its receptor. In the fruit fly study, the scientists discovered that Sema-1a can also act as a receptor for PlexA. “We used to think that this pair of surface proteins acted as a one-way radio, with information flowing in a single direction,” says Kolodkin. “What we found is that instructional information flows both ways.”

The Johns Hopkins team identified the “two-way” system by knocking out and otherwise manipulating fruit fly genes and then watching what happened to motor neuron branching. In these experiments, the researchers uncovered still other proteins located within the motor axons that Sema-1a interacts with after receiving a PlexA signal. When the gene for a protein called Pebble was deleted, for example, motor axons bunched together and didn’t branch. When the gene for RhoGAPp190 was deleted, motor axons branched too soon and failed to recognize their target muscles.

Through a series of biochemical tests, Kolodkin’s team found that Pebble and RhoGAPp190 both act on a third protein, Rho1. When Rho1 is activated, it collapses the supporting structures within an axon, making it “limp” and unable to continue toward a target. Sema-1a can bind to Pebble or to RhoGAPp190, and subsequently, these proteins can bind to Rho1. Binding to Pebble activates Rho1, causing axons to branch away from each other. However, binding to RhoGAPp190 shuts down Rho1, causing axons to remain bunched together. Thus, says Kolodkin, balance in the amounts of available Pebble and RhoGAPp190 can determine axon behavior, although what determines this balance is still unknown.

“This signaling is complex and we still don’t understand how it’s all controlled, but we’re one step closer now,” says Kolodkin. He notes that “a relative” of the Sema-1a protein in humans has already been implicated in schizophrenia, although details of this protein’s role in disease remain unclear. “Our experiments affirm how important this protein is to study and understand,” adds Kolodkin.

Scientists Identify Two Genes Essential for Breathing

A team of researchers at the New York University’s Langone Medical Center has discovered that two genes, called Hoxa5 and Hoxc5, play a critical role in establishing the neuronal circuits required for breathing. The discovery could help advance treatments for spinal cord injuries and neurodegenerative diseases.

The three-year study published in the journal Nature Neuroscience identifies a molecular code that distinguishes a group of muscle-controlling nerve cells collectively known as the phrenic motor column (PMC).

“These cells lie about halfway up the back of the neck, just above the fourth cervical vertebra, and are probably the most important motor neurons in your body,” explained senior author Prof Jeremy Dasen of the Howard Hughes Medical Institute.

Harming the part of the spinal cord where the PMC resides can instantly shut down breathing. But relatively little is known about what distinguishes PMC neurons from neighboring neurons, and how PMC neurons develop and wire themselves to the diaphragm in the fetus. The PMC cells relay a constant flow of electrochemical signals down their bundled axons and onto the diaphragm muscles, allowing the lungs to expand and relax in the natural rhythm of breathing.

“We now have a set of molecular markers that distinguish those cells from other populations of motor neurons, so that we can study them in detail and look for ways to selectively enhance their survival,” Prof Dasen said.

Mechanisms Underlying Childhood Neuromuscular Disease Found

A study by scientists from the Motor Neuron Center at Columbia University Medical Center (CUMC) suggests that spinal muscular atrophy (SMA), a genetic neuromuscular disease in infants and children, results primarily from motor circuit dysfunction, not motor neuron or muscle cell dysfunction, as is commonly thought. In a second study, the researchers identified the molecular pathway in SMA that leads to problems with motor function. Findings from the studies, conducted in fruit fly, zebrafish and mouse models of SMA, could lead to therapies for this debilitating and often fatal neuromuscular disease. Both studies were published today in the online edition of the journal Cell (1, 2).

“Scientists call SMA a motor neuron disease, and there is post-mortem evidence that it does cause motor neurons to die,” said Brian McCabe, PhD, assistant professor of pathology and cell biology and of neuroscience in the Motor Neuron Center, who led the first study. “However, it was not clear whether the death of motor neurons is a cause of the disease or an effect. Our findings in the fruit fly SMA model show that the disease originates in other motor circuit neurons, which then causes motor neurons to malfunction.”

In motor circuits, which coordinate muscle movement, specialized sensory neurons called proprioceptive neurons pick up and relay information to the spinal cord and brain about the body’s position in space. The central nervous system then processes and relays the signals, including via interneurons, to motor neurons, which in turn stimulate muscle movement.

“To our knowledge, this is the first clear demonstration in a model organism that defects in the function of a neuronal circuit are the cause of a neurological disease,” added Dr. McCabe.

Amyotrophic lateral sclerosis, also known as Lou Gehrig’s disease or ALS, is a devastating, rapidly advancing disease of the nerve cells in the brain and spinal cord that control voluntary muscle movement. But researchers at NYU School of Medicine have identified a new target for slowing the deterioration of physical function for which the disease is so well known.

In their new study, published August 30, 2012 online ahead of print in Cell Reports, lead investigator Steven J. Burden, PhD, and colleagues show that, by increasing the signaling activity of a protein called muscle skeletal receptor tyrosine-protein kinase (MuSK), they were able to keep nerve cells attached to muscle longer into the progression of the disease in a mouse model of ALS.