People with spinal cord injuries suffer from many complications in addition to paralysis and numbness. Some of these problems are caused by a lack of the neurotransmitter GABA in the injured spinal cord. Now research in mice is showing that human embryonic stem cells differentiated into medial ganglionic eminence (MGE)-like cells, which produce GABA, may help alleviate two of the most severe side effects–chronic neuropathic pain and bladder dysfunction. The results appear September 22 in Cell Stem Cell.

This study, a collaboration between senior authors Arnold Kriegstein, Director of the Developmental and Stem Cell Biology Program at the University of California, San Francisco (UCSF), and Linda Noble-Haeusslein, a professor in the Departments of Neurological Surgery and Physical Therapy and Rehabilitation Science at UCSF, addressed both neuropathic pain and bladder dysfunction, which are at least in part attributed to overactive spinal cord circuits. “We reasoned if we could take inhibitory neurons and directly place them into the spinal cord in the regions that are overactive, they might integrate into those circuits and suppress the activity,” says Kriegstein.

GABA (gamma-Aminobutyric acid) is an inhibitory neurotransmitter that is found throughout the central nervous system. It plays an important role in reducing the excitability of neurons by binding to receptors that act on synapses. In the study, the investigators used GABAergic inhibitory neuron precursors called MGE-like cells that were derived from human embryonic stem cells.

The neural precursor cells were placed into the spinal cords of mice two weeks after injury had been induced, where they could differentiate into GABA-producing neuron subtypes and form synaptic connections.

“Rather than implanting these cells into the site of injury, at the mid-thoracic level, we injected them in the lumbosacral region, where the circuits are known to be overactive,” says Thomas Fandel, a research specialist at UCSF and the study’s co-first author. “Six months later we could see broad dispersion of the cells in that area. They were integrated into the spinal cord.”

The researchers used several measures to determine whether the stem cells were effective in alleviating neuropathic pain and bladder dysfunction at six months. To assess for bladder control, the mice were placed in cages with filter paper that showed where the mice had urinated. The treated mice had fewer, larger spots, indicating less leakage. Bladder function was also assessed by measuring bladder volume and tension, which confirmed the improved voiding ability of animals receiving transplants.

By six months after transplant, animals exhibited significantly reduced pain sensitivities. Grooming and scratching behaviors were also evaluated, as decreased GABA in the spinal cord can similarly cause pathological itch (pruritus). The researchers found that mice receiving the stem cell transplants showed decreased overgrooming compared to controls.

“The fact that these cells were implanted in the spinal cords two weeks after injury is also important to note,” says Alpa Trivedi, a researcher at UCSF and co-first author of the study. “Many of the current Phase I trials for spinal cord injury are run in the acute phase, which is right after injury. But the vast majority of people with spinal cord injuries are the chronic patient population, and a treatment that might work for them would capture a larger number of patients who are really in need of better treatments.”

Current treatments for neuropathic pain in people with spinal cord injuries most often involve opioids and other pain medications, as well as certain antidepressants, which have many side effects and tend to have limited efficacy. Treatments for bladder dysfunction are often anticholinergics, but these drugs have side effects like dizziness and dry mouth. Botulinum toxin (Botox®) may help with bladder spasms, but the benefits tend to be transient. “The current approaches for treatment are not very effective and clearly more options are needed,” Dr. Fandel says. “Our hope is that this treatment would last a long time, or maybe even be permanent.”

Additional research is needed before the stem cells can be tested in patients, but the researchers have already taken the first step by using human-derived stem cells rather than mouse stem cells in their animal models. “Chronic pain and bladder dysfunction remain significant quality-of-life issues for many people with spinal cord injuries. Inhibitory cell-based neuro-therapy is a new approach and has shown promise to date in early animal studies, warranting further development,” says Cory Nicholas, a co-first author.

Autism spectrum disorders (ASDs) are characterized by impaired social interactions and repetitive behaviors, often accompanied by abnormal reactions to sensory stimuli. ASD is generally thought to be caused by deficits in brain development, but a study in mice, published June 9 in Cell, now suggests that at least some aspects of the disorder–including how touch is perceived, anxiety, and social abnormalities–are linked to defects in another area of the nervous system, the peripheral nerves found throughout the limbs, digits, and other parts of the body that communicate sensory information to the brain.

“An underlying assumption has been that ASD is solely a disease of the brain, but we’ve found that may not always be the case,” says senior author David Ginty, a Professor of Neurobiology at Harvard Medical School and a Howard Hughes Medical Institute Investigator. “Advances in mouse genetics have made it possible for us to study genes linked to ASD by altering them only in certain types of nerve cells and studying the effects.”

In the new study, the researchers examined the effects of gene mutations known to be associated with ASD in humans. In particular, they focused on Mecp2, which causes Rett syndrome, a disorder that is often associated with ASD, and Gabrb3, which also is implicated in ASD. They looked at two other genes connected to ASD-like behaviors as well.

These genes are believed to be essential for the normal function of nerve cells, and previous studies have linked these mutations to problems with synaptic function–how neurons communicate with each other.

“Although we know about several genes associated with ASD, a challenge and a major goal has been to find where in the nervous system the problems occur,” Ginty says. “By engineering mice that have these mutations only in their peripheral sensory neurons, which detect light touch stimuli acting on the skin, we’ve shown that mutations there are both necessary and sufficient for creating mice with an abnormal hypersensitivity to touch.”

The investigators measured how the mice reacted to touch stimuli, such as a light puff of air on their backs, and tested whether they could discriminate between objects with different textures. Mice with ASD gene mutations in only their sensory neurons exhibited heightened sensitivity to touch stimuli and were unable to discriminate between textures. The transmission of neural impulses between the touch-sensitive neurons in the skin and the spinal cord neurons that relay touch signals to the brain was also abnormal. Together, these results show that mice with ASD-associated gene mutations have deficits in tactile perception.

The investigators next examined anxiety and social interactions in the mice using established tests looking at how much mice avoided being out in the open and how much they interacted with mice they’d never seen before. Surprisingly, the animals with ASD gene mutations only in peripheral sensory neurons showed heightened anxiety and interacted less with other mice. “How closely these behaviors mimic anxiety seen in ASD in humans is up for debate,” Ginty says, “but in our field, these are well-established measures of what we consider to be anxiety-like behavior and social interaction deficits.”

“A key aspect of this work is that we’ve shown that a tactile, somatosensory dysfunction contributes to behavioral deficits, something that hasn’t been seen before,” Ginty says. “In this case, that deficit is anxiety and problems with social interactions.” How problems with processing the sense of touch lead to anxiety and social problems isn’t clear at this point, however.

“Based on our findings, we think mice with these ASD-associated gene mutations have a major defect in the ‘volume switch’ in their peripheral sensory neurons,” says first author Lauren Orefice, a postdoctoral fellow in Ginty’s lab. Essentially, she says, the volume is turned up all the way in these neurons, leading the animals to feel touch at an exaggerated, heightened level.

“We think it works the same way in humans with ASD,” Ginty adds.

“The sense of touch is important for mediating our interactions with the environment, and for how we navigate the world around us,” Orefice says. “An abnormal sense of touch is only one aspect of ASD, and while we don’t claim this explains all the pathologies seen in people, defects in touch processing may help to explain some of the behaviors observed in patients with ASD.”

The investigators are now looking for approaches that might turn the “volume” back down to normal levels in the peripheral sensory neurons, including both genetic and pharmaceutical approaches.

Cocaine is one of the most addictive substances known to man, and for good reason: By acting on levels of the “feel-good” chemical dopamine, it produces a tremendous sensation of euphoria.

Now the laboratory of Rockefeller University Professor and Nobel Laureate Paul Greengard has shown for the first time in mice how a protein called WAVE1 regulates the brain’s response to cocaine. Their discovery, which was published recently in the journal Proceedings of the National Academy of Sciences, offers fundamental insights into the brain’s inner workings—and could lead to better interventions for treating addiction to cocaine and other drugs.

Cocaine and the brain

Researchers have long used cocaine as a model to study how certain messages are transmitted in the brain. And Greengard’s group, which investigates the molecular basis of communication between nerve cells in the brains of mammals, has studied WAVE1, a protein involved in cell signaling, for more than a decade. But their PNAS study reveals something new about the way in which WAVE1 and dopamine interact.

“We knew about the connection between WAVE1 and dopamine many years ago, but until now no one knew the mechanism of how cocaine stimulates WAVE1 and how WAVE1 regulates cocaine’s actions,” says Yong Kim, a Research Assistant Professor in Greengard’s lab and the senior author of the new study.

No WAVE1, no reward

In the new work, the team observed that WAVE1 became active in the brain of mice exposed to cocaine, and that this cocaine effect on WAVE1 could be prevented by blocking dopamine receptors. The research also provides new clues about how WAVE1 influences changes in the brain’s synapses— the junctions between nerves through which impulses pass—in response to cocaine exposure.

Specifically, the investigators looked at changes in an area of the brain called the nucleus accumbens, a key component of the neural reward system that is known to play a critical part in addiction—and in which dopamine is heavily involved. When these synapses form, they allow the signals from dopamine and another neurotransmitter called glutamate to be transmitted.

To investigate the interaction between WAVE1 and dopamine more specifically, the team looked at mice that had WAVE1 selectively removed in nerve cells. These nerve cells also contained one of the subtypes of dopamine receptor (called D1). They found a significant decrease in the preference for cocaine in these mice, compared with those producing normal WAVE1 levels.  This suggested that the dopamine signals were not being transmitted.

However, this effect was not seen when WAVE1 was removed from nerve cells containing a different dopamine receptor subtype (called D2). Those results suggest previously unknown details about how cocaine works.

Addiction intervention

“It’s well known that cocaine increases the signaling of dopamine in the brain,” Kim says. “Understanding more about the mechanism of cocaine action is providing new insight into the neurobiology of addiction. Our eventual goal is to use these findings to find a way to develop a drug to treat addiction.”

However, Kim says there are limitations to the current work, largely because the mice were injected with cocaine by the researchers. Future studies will need a system in which the mice can self-administer the cocaine by pushing a lever and injecting themselves, a model that more closely mimics human addiction behavior.