Low-dose RGFP966 Molecule Aids Myelin Formation in CMT1A Mice
A low dose of a molecule known as RGFP966 boosted myelin production and improved electrical impulse conduction in nerves of a mouse model of Charcot-Marie-Tooth disease type 1 subtype A (CMT1A), a study found.
RGFP966 is a selective inhibitor (blocker) of histone deacetylase 3 (HDAC3), a protein that stops myelination, which is the process by which sheaths of myelin, a fatty substance, form around nerves to protect them and allow them to send messages (electrical impulses) quickly and efficiently.
Researchers also observed that low-dose RGFP966 increased the speed with which electrical impulses were being sent along the nerves of CMT1A mice.
However, when given at a higher dose, RGFP966 decreased the ability of CMT1A mice to run and hold onto a metal grid, indicating worse motor performance. High-dose RGFP966 also resulted in an accumulation of macrophages — a type of immune cell involved in inflammation — within the peripheral nerves that reside outside the brain and spinal cord.
“Correct dosing of HDAC3 inhibitors is of crucial importance if translated to a clinical setting for demyelinating forms of CMT or other neurological disorders,” the researchers wrote, noting that “HDAC3 plays a multifaceted role in myelination, in muscle physiology, and in the neuroimmune response.”
The study, “HDAC3 inhibition stimulates myelination in a CMT1A mouse model,” was published in Molecular Neurobiology by a team of researchers in Belgium.
HDAC3 works as a switch for signaling pathways that keep myelination from occurring. A signaling pathway is a series of chemical reactions whereby signals are passed on from molecule to molecule to control certain functions in cells or tissues.
Countering HDAC3 is expected to prevent myelination from being stopped. The researchers wondered if RGFP966 could protect against the loss of myelin and serve as a therapeutic option for CMT1A.
They used mice, called C3-PMP22, which have been engineered to carry the human PMP22 gene. This gene provides instructions for making a protein of the same name that is part of myelin. In people, carrying one extra copy of PMP22 is enough to cause CMT1A. Myelination is delayed in these mice.
The researchers found that CMT1A mice actually carried five copies of human PMP22, which is more than the three to four reported. When they looked at the signaling pathways involved in myelination, they found that they were indeed tuned down, which translated into delayed myelination at an early age. CMT1A mice also manifested CMT-like symptoms, such as limb clasping.
The researchers also looked at the sciatic nerve, a long nerve that runs from the lower back down through the leg, to get a more detailed understanding of how the poorly myelinated nerves worked. They found that electrical impulses from the sciatic nerve of CMT1A mice were impaired compared to healthy (wild-type) mice.
Next, the scientists investigated the effects of three weeks of treatment with RGFP966 at either a low dose (5 mg/kg of body weight) or a high dose (10 mg/kg of body weight), starting on the sixth day of life. During the first week of treatment, RGFP966 was administered subcutaneously (under the skin) once daily, and thereafter intraperitoneally (into the peritoneal cavity) every other day for two weeks.
Compared to untreated CMT1A, treatment with RGFP966 increased myelin production. It also increased the g-ratio, which is a ratio of the inner versus outer diameter of a myelin sheath. A higher g-ratio means the myelin sheath is thicker, which is expected to speed up electrical impulses.
In line with this finding, treatment with RGFP966 increased the speed with which electrical impulses were being sent along the nerves of CMT1A mice.
The researchers then watched for changes in the ability of the mice to run on a rotating rod (rotarod). There were no differences in the amount of time wild-type mice remained on the rotarod, whether they had received treatment with RGFP966 or had not been treated. However, CMT1A mice treated with high-dose RGFP966 fell off the rotarod earlier than did wild-type or untreated CMT1A mice on a placebo.
Similar observations were made on a test measuring grip strength, although wild-type mice receiving the 5 mg/kg dose had an increased overall grip strength compared to untreated wild-type mice.
In addition, high-dose RGFP966 resulted in a high number of macrophages within peripheral nerves of CMT1A mice, but not wild-type mice. According to the research team, improvements seen in tissue- and nerve-based observations indicate that this may be beneficial, but can also have a “minor adverse effect in the nerve which is overcompensated by the positive effect of HDAC3i [HDAC3 blockage] in the nerve.”
“HDAC3 does not only play a role in regulating myelination but is also important in the neuroimmune modulation,” the researchers concluded, adding that “correct dosing is of crucial importance … when reaching a clinical setting for neurological or other disorders.”
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