Scientists have developed a compound that successfully promotes rebuilding of the protective sheath around nerve cells that is damaged in conditions such as multiple sclerosis.
In a study published today in the journal Glia, scientists described successfully testing the compound in mice.
Researchers at Oregon Health & Science University have already started to apply the compound on a rare population of macaque monkeys at the Oregon National Primate Research Center at OHSU who develop a disease that is similar to MS in humans.
“I think we’ll know in about a year if this is the exact right drug to try in human clinical trials,” said senior author Larry Sherman, Ph.D., an OHSU professor in the Division of Neuroscience at the primate center.
“If it’s not, we know from the mouse studies that this approach can work.
The question is, can this drug be adapted to bigger human brains?”
The discovery culminates more than a decade of research following a 2005 breakthrough by Sherman’s lab.
In that study, scientists discovered that a molecule called hyaluronic acid, or HA, accumulates in the brains of patients with MS. Further, the scientists linked this accumulation of HA to the failure of cells called oligodendrocytes to mature. Oligodendrocytes generate myelin.
Myelin, in turn, forms a protective sheath covering each nerve cell’s axon—the threadlike portion of a cell that transmits electrical signals between cells.
Damage to myelin is associated with MS, stroke, brain injuries, and certain forms of dementia such as Alzheimer’s disease.
In addition, delay in myelination can affect infants born prematurely, leading to brain damage or cerebral palsy.
Subsequent studies led by the Sherman lab showed that HA is broken down into small fragments in multiple sclerosis lesions by enzymes called hyaluronidases.
In collaboration with Stephen Back, M.D., Ph.D., a professor of pediatrics in the OHSU School of Medicine, Sherman discovered that the fragments of HA generated by hyaluronidases send a signal to immature oligodendrocytes not to turn on their myelin genes.
That led researchers to explore how they might block hyaluronidase activity and promote remyelination.
For the past decade, an international team of researchers led by OHSU has been working to develop a compound that neutralizes the hyaluronidase in the brains of patients with MS and other neurodegenerative diseases, thereby reviving the ability of progenitor cells to mature into myelin-producing oligodendrocytes.
The study published today describes a modified flavonoid – a class of chemicals found in fruits and vegetables – that does just that.
The compound, called S3, reverses the effect of HA in constraining the growth of oligodendrocytes and promotes functional remyelination in mice.
Lead author Weiping Su, Ph.D., senior scientist in the Sherman lab, dedicated years of intensive research to make the discovery.
“It’s not only showing that the myelin is coming back, but it’s causing the axons to fire at a much higher speed,” Sherman said. “That’s exactly what you want functionally.”
The next phase of research involves testing, and potentially refining, the compound in macaque monkeys who carry a naturally occurring version of MS called Japanese macaque encephalomyelitis.
The condition, which causes clinical symptoms similar to multiple sclerosis in people, is the only spontaneously occurring MS-like disease in nonhuman primates in the world.
More information: Glia, DOI: 10.1002/glia.23715
Provided by Oregon Health & Science University
Myelination of CNS axons by oligodendrocytes facilitates the rapid transmission of neural impulses and global synchronization of neuronal circuit activity. In a number of disorders of the CNS, myelin and oligodendrocytes are targets of pathology, where demyelination impairs normal conduction properties, producing functional disability. Eventually, the chronic demyelination of axons results in neurodegeneration and progressive accumulation of disability, which can be prevented with efficient remyelination (Duncan et al., 2009).
Remyelination represents an endogenous regenerative program that becomes activated following demyelination. Here, a uniformly distributed quiescent population of lineage-restricted oligodendrocyte progenitor cells (OPCs) are mobilized, whereupon they migrate to the area of demyelination, locally proliferate and differentiate, and remyelinate axons (Plemel et al., 2017).
This multistep process is both negatively and positively modulated by a number of intrinsic and extrinsic cues.
Although the spontaneous regeneration of myelin does occur, the process is commonly incomplete, typically attributed to an accumulation of inhibitors during demyelination and a paucity of positive myelination cues in the pathological adult CNS. Thus, strategies to enhance remyelination are required to significantly improve outcomes in a number of CNS disorders.
In the demyelinating disorder multiple sclerosis (MS), physical activity and engagement in activities of daily living have been implicated as partial determinants of disease severity. In pediatric patients, physical inactivity is associated with an increased lesion burden (Grover et al., 2015).
In a model of MS, experimental autoimmune encephalomyelitis, exercise delays disease onset and reduces peak clinical disability (Bernardes et al., 2013, Le Page et al., 1994). It has generally been presumed that the effect is due to anti-inflammatory immunomodulation or growth factor upregulation by exercise. Considering the responsiveness of CNS stem cells to physical activity (Jensen and Yong, 2016), including the induction of neurogenesis by exercise (van Praag et al., 1999), we hypothesized that activity could enhance remyelination and contribute to the reduction in disability by exercise in MS.
Here, a focal lesion is induced in the ventrolateral white matter of the murine spinal cord that displays a temporally consistent evolution wherein demyelination occurs in the first 3 days post lesion (dpl), OPC proliferation between 3 and 7 dpl, and oligodendrocyte differentiation between 5 and 14 dpl, concluding with remyelination between 10 and 28 dpl (Figures S1A and S1B).
To administer exercise we used a voluntary running task in which mice were given unrestricted access to a monitored running wheel.
Animals were allowed access immediately following lysolecithin injection; animals spontaneously exercised and reached a consistent velocity following recovery from the surgical procedure.
This activity level is maintained for the majority of their nocturnal active period (Figure S1D). Control animals were housed in an identical setting with a locked wheel to serve as a control for the environmental enrichment.
Exercise Enhances Oligodendrogenesis following Demyelination
To evaluate the ability of physical activity to promote remyelination, we first examined the effect of activity on the generation of new oligodendrocytes.
Here, we immunohistochemically labeled oligodendrocyte lineage cells with the pan-oligodendrocyte lineage marker Olig2 and segregated this population into OPCs using PDGFRα expression and oligodendrocytes using CC1 expression at 3, 7, and 14 dpl (Figure 1A).
Quantification of the density of oligodendrocytes lineage cells over the evolution of the lesion (Figure 1B) revealed an increase in the number of cells at 7 and 14 dpl (32% and 30%, respectively) as a result of exercise.
Separating this population into oligodendrocytes and OPCs (Figures 1C and 1D) demonstrated that this global increase was driven by both oligodendrocytes and OPCs at 7 dpl and exclusively by oligodendrocytes at 14 dpl (34% increase).
Furthermore, when we examined the ratio of mature oligodendrocytes to oligodendrocyte lineage cells, at no point did we see a change as a result of exercise, indicating that exercise does not alter the kinetics of differentiation and predominantly acts through increasing the number of cells within the lesion.
Thus, exercise increases the generation of new oligodendrocytes within demyelinated lesions.
We next sought to identify which step in the processes of oligodendrogenesis was altered by exercise to increase the number of mature oligodendrocytes within the lesion. As the major increase in oligodendrocyte lineage cells occurs between 3 and 7 dpl (the proliferative phase), and because we did not detect a reduction in OPCs (indicating enhanced differentiation), we hypothesized that exercise enhances OPCs proliferation and directly examined this on the basis of entry into the cell cycle (Ki67+; Figure 1E).
We found an increase from 41% to 60% at 7 dpl as a result of exercise (Figure 1G). Together, these data suggest that exercise prolongs the proliferative phase of OPCs within demyelinated lesions, allowing the production of a greater number of oligodendrocytes.
Exercise Enhances Remyelination
We examined the impact of physical activity on remyelination at 14 dpl by first visualizing myelin through immunohistochemical labeling of myelin basic protein (MBP). Quantification of the fluorescence intensity of lesion-associated MBP revealed a 48% increase in MBP as a result of exercise (Figures 2A and 2B). This MBP was observed as ring-like structures surrounding SMI312-positive axons, indicating that this signal is derived from remyelination and not increased myelin debris.
To further confirm remyelination, we performed electron microscopy (Figure 2C). Here, we observed a 2.11-fold increase in remyelinated axon density (Figure 2D) and a decrease in average g ratio (a metric describing myelin thickness equal to inner axon diameter divided by outer myelin diameter; a g ratio of 1 indicates demyelination) from 0.88 to 0.81 (Figure 2E), with significantly different linear regression lines (p < 0.0001). Considering that the average g ratio of a developmentally myelinated axon within the spinal cord is between 0.6 and 0.7, the change from 0.88 to 0.81 by exercise amounts to a ∼20% increase in thickness.
Furthermore, although 51% of axons remained demyelinated in control animals, only 13% of axons remained demyelinated in exercising animals. Importantly, we show that the density of axons between exercising and control mice is unchanged before the onset of remyelination (7 dpl; Figures S2A and S2B); thus, exercise improves the rate of remyelination rather than preventing axonal loss during demyelination inflicted by lysolecithin. These findings provide clear evidence of a substantial positive effect of exercise on a number of aspects of remyelination.
The authors used irregularly spaced running wheels as a motor skill learning task and demonstrated the production of new oligodendrocytes during the learning period. However, oligodendrogenesis occurred only after the first exposure to the running wheel; a subsequent course of exercise had no effect on oligodendrogenesis.
Thus, to ensure that the effects of exercise on remyelination are due to the exercise itself and not the novel stimulus of the running wheel, we allowed mice to exercise for 7 days followed by a 7 day break (preconditioning) prior to demyelination and exercise, as done before (Figures S2C–S2F).
We found that preconditioning produced no decline in the efficacy of exercise, with a 37% increase in the number of oligodendrocyte lineage cells at 14 dpl, mirroring the results shown in Figure 1. We conclude that exercise promotes remyelination independent of a novel motor task.