A recipe for remyelination

When Christina Applegate fulfilled her lifelong dream of earning a star on the Hollywood Walk of Fame, she sat barefoot in a chair, bedazzled cane in hand, as her plaque was unveiled. The actress was diagnosed with multiple sclerosis (MS) in the summer of 2021, shutting down production of her hit Netflix series “Dead to Me.” “There was the sense of, ‘Well, let’s get her some medicine so she can get better,'” Applegate told The New York Times. “And there is no better.” 

In people with MS, an autoimmune attack injures myelin, the lipid and protein sheath surrounding the neuron’s axon, causing lesions to develop in the central nervous system. Damage to myelin, which protects the neuron and enables rapid conduction of electrical signals, impairs nerve signaling and eventually leads to degeneration of the underlying neuron structure, resulting in neurological disability. Current treatments for MS include immunomodulatory and anti-inflammatory drugs that manage the destructive immune response, reducing episodes of new or worsening symptoms. But to offer the potential to recover nerve function and prevent further progression of one’s disability, scientists are seeking a new generation of myelin-repairing interventions.

Now, researchers have revealed that molecules ranging from a pregnancy hormone to an over-the-counter antihistamine can promote restoration of the myelin sheath. Using these remyelinating agents, they have demonstrated protection of critical neural components in animal models and developed an imaging-based measure of remyelination in humans. Together, their research advances the development of a long-awaited remyelinating therapy, providing hope for MS patients.  

The protective power of pregnancy

For Rhonda Voskuhl, a neurologist at the University of California, Los Angeles (UCLA), the discovery that a small molecule could have a big impact on MS started with the intriguing observation that pregnancy protects against the disease (1). Women with MS experience a reduction in relapses in the later stages of pregnancy, and undergoing multiple pregnancies associates with decreased disability (2,3). By sifting through the various factors that change over time throughout pregnancy, Voskuhl homed in on estriol, a hormone that peaks during the third trimester. In nonclinical studies, her team found that estriol has anti-inflammatory and neuroprotective effects (4). To validate the estriol dose they planned to administer to humans, Voskuhl used blood from healthy pregnant women, including her own, as a reference. “It just so happens that I was pregnant when we were doing that stuff 20 years ago,” Voskuhl said. “I just wanted to see it myself in our own assays that an eight-milligram dose did induce a level that’s about a mid-pregnancy level.”  

In two Phase 2 clinical trials published in 2002 and 2016, estriol treatment at this dose reduced active inflammatory lesions and relapses in women with MS (5,6). “But we weren't certain what the actual mechanism was,” said Allan MacKenzie-Graham, a neuroscientist at UCLA who works with Voskuhl. “You give the injection, but what's happening inside the cells and inside the brain and inside the spinal cord that's leading to this protection?” 

The team explored this question in a recent study where they administered estriol in a mouse model of MS (7). They observed an increase in myelin in the cerebral cortex, but that didn’t necessarily mean that the hormone restored the myelin sheath. “You can't just give a drug and say that it's remyelinating when you see more myelin there because it can be a decrease in demyelination, [which] is not the same thing as remyelination,” Voskuhl said. Still, the researchers hypothesized that estriol could induce remyelination by binding to estrogen receptor beta (ERβ) on the oligodendrocyte lineage of cells, as newly differentiated oligodendrocytes can regenerate the myelin sheath. 

To distinguish this effect, they turned to a transgenic mouse model that expresses green fluorescent protein (GFP) in newly formed remyelinating oligodendrocytes. They found that treating the mice with estriol enhanced the maturation of oligodendrocyte precursor cells into GFP-expressing oligodendrocytes, confirming the hormone’s ability to promote remyelination. Oligodendrocytes in estriol-treated mice also showed increased expression of two key cholesterol synthesis proteins. Since cholesterol is an important building block for myelin, upregulating cholesterol production supports remyelination. 

The researchers believe that estriol treatment may mimic the process in which the myelin sheath is built in a developing fetus. “It actually makes sense to have a hormone in the mother's circulation during pregnancy, especially the last half, that can support myelination,” Voskuhl said. Therefore, an intuitive strategy to induce remyelination in adults is “to recapitulate that state where your body can actually make myelin,” she said. 

To translate this approach to people with MS, the team confirmed that estriol could promote remyelination in an inflammatory environment. They observed that administering estriol in the mouse model of the disease decreased the activation of microglia, immune cells that secrete proinflammatory factors that can inhibit remyelinating oligodendrocytes and contribute to axon damage. The researchers found that estriol’s effects helped preserve the neuron substructure, thereby reducing atrophy in the cerebral cortex. “The neurons themselves, as opposed to just the myelin coating, are being damaged,” MacKenzie-Graham said. “An important element of multiple sclerosis treatment is to... find neuroprotective treatments that prevent the loss of synapses and axons and neurons because, in a sense, that's irreversible, and it's where the disability comes from.” 

As a naturally occurring hormone that has demonstrated a strong safety profile in women, estriol is an appealing therapeutic for this population of MS patients. “It's important to tailor drugs and treatments for each sex,” Voskuhl said. The researchers hope that administering estriol in combination with existing drugs could move the needle on MS treatment. “To add estriol as a neuroprotective, remyelinating agent with [an] anti-inflammatory, that'd be great,” Voskuhl said. “People need something to not only just decrease relapses, but cause repair.” Next, the team plans to design a trial that evaluates estriol’s ability to induce remyelination in humans. 

An image of myelin

Demonstrating remyelination in clinical settings is a challenge in and of itself. “We can't do away with certain portions or parts of our brains and allow a sample of the tissue to be taken in order for us to study the effects,” said Ari Green, a neurologist at the University of California, San Francisco (UCSF). “The driving force in my own laboratory and career is how do we develop those tools for measuring brain repair in general that can be done in a noninvasive way and in a reproducible, repeatable way?” 

The journey toward developing such tools for remyelination began with a high-throughput screen for compounds that promote restoration of the myelin sheath. Green and his colleagues discovered that clemastine, a widely available antihistamine, enhanced oligodendrocyte differentiation and remyelination via an off-target interaction with muscarinic receptors (8). Following positive results in animal models, the researchers tested the drug as a remyelinating therapy in MS patients in the ReBUILD clinical trial (9). They found that clemastine treatment improved the visual-evoked potential (VEP) score, an electrophysiological measure of the time it takes a signal to travel from the retina to the visual cortex in response to a visual stimulus. Since the anterior visual pathway is a conserved target for demyelination in MS, and restoration of myelin speeds the conduction of electrical signals, the team inferred that the improved VEP score likely reflected remyelination. To find out for sure, they used a transgenic animal model that lacks the ability to form new myelin. Treating the animal with clemastine did not improve its VEP score, indicating that VEP serves as a reliable measure of myelin status (10). 

Having confirmed clemastine’s ability to remyelinate, the researchers recognized a unique opportunity to use the data from the ReBUILD trial to develop an imaging-based measure of remyelination. In doing so, they aimed to document the compound’s structural effects and provide a readily deployable tool for other studies of remyelination. “Why we want an imaging-based method is we'd love for people to be able to do relatively quick, relatively inexpensive clinical trials with potential drugs capable of repair and assess whether or not the drug works,” Green said. 

To develop this imaging-based method, the researchers analyzed the subjects’ MRI data and calculated a parameter called the myelin water fraction (MWF). MWF uses differences in the MRI properties of water that either is trapped in between layers of myelin or freely moving in brain tissue to report the amount of water within myelin relative to the total water content of the brain. With trapped water serving as a proxy for myelin, researchers have proposed that changes in MWF could measure remyelination. 

As part of the ReBUILD trial, Green’s team analyzed MWF in the whole brain and observed that clemastine treatment did not yield an improvement. In considering this result, they realized that “if we use the whole brain, we will quantify myelin water fraction in areas that are rich and poor with myelin, and so there will be a lot of noise,” said Christian Cordano, a neuroscientist at UCSF and collaborator in the project. To make changes in MWF easier to detect, the researchers revisited the data in a recent study, focusing on specific brain regions that are densely myelinated (11). In this way, they found that MWF increased in the myelin-rich corpus callosum when participants were treated with clemastine. The study provides the first direct and biologically validated imaging-based evidence of a remyelinating therapy. 

Interestingly, the team found that the most robust change in MWF took place in the normal-appearing white matter of the corpus callosum rather than in the lesion areas. “We think that’s super important because the field had always earlier presumed that most remyelination would occur within a lesion,” Green said. However, lesions may feature such extensive degeneration of axons that there are few left to remyelinate. Additionally, the activation of astrocyte cells forms scar tissue in lesions that may block remyelinating oligodendrocytes from infiltrating. While remyelination may be limited within the lesion, recent research has demonstrated that significant injury occurs in the normal appearing white matter in MS (12). Green hopes that these findings “may take us beyond the lesion as the principal or maybe the only focus in both repair and understanding the biology of MS,” he said. “The normal looking white matter could be a really important place for us to repair, and it might really bend the arc on preventing disability progression.”  

The normal looking white matter could be a really important place for us to repair, and it might really bend the arc on preventing disability progression. 
- Ari Green, UCSF

While the unexpected activity of an antihistamine spurred important insights into detection methods and locations of remyelination, clemastine may not be the ideal treatment moving forward. Due to its interactions with histamine receptors, the compound causes fatigue, exacerbating an existing symptom of MS. Because of its side effects, the researchers cannot administer clemastine at the dose needed to induce maximum remyelination. To overcome this problem, they are interested in testing a drug that specifically targets muscarinic receptors. 

As scientists evaluate new and improved remyelinating therapies, the team hopes that their imaging-based demonstration of remyelination provides a helpful framework for designing clinical trials. “When you're first trying to establish breakthroughs with new drugs, it's a best educated guess, and now we have data to guide that selection,” Green said. For example, their work sheds light on the MRI sequences, brain regions, patient enrollment size, and study duration that can uncover remyelination. 

Cordano added, “Every step that we do is one step towards our goal, [which] is to make a remyelinating drug something that becomes available for the treatment of patients and that can really change their qualities of life.”

References

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