Myelinated neurons have their axons segmentally covered with myelin, a spirally wrapped membrane (lamella). The myelin lamella is formed by glial cells that wrap the axon with protrusions, and then the inner leaflets of the plasma membrane fuse together, with no cytoplasm left. There can be up to 100 turns of myelin wrapped around an axon, and over half of the total diameter of the nerve fibre is made up of myelin lamellae. In CNS, interfascicular oligodendrocytes are the glial cells that form the myelin, each oligodendrocyte myelinating multiple axons and axon segments. In PNS, myelin is formed by glial Schwann cells, one myelin sheath per one Schwann cell. The length of myelin sheaths in PNS is ∼1 mm, with ∼1 µm gaps (nodes of Ranvier) between them, exposing the axon to the extracellular space. The voltage-gated Na+ channels are clustered at the nodes. Myelin between the nodes (internodes) allows the action potential to jump from one node to the next (saltatory conduction), increasing the signal transmission speed up to 200 fold. The nodes of Ranvier are also the sites of presynaptic specializations.
The composition of myelin is variable, especially between CNS and PNS. The water content of myelin is ∼40%. Most of the dry mass is made up of glycolipids, primarily galactocerebroside (galactosyl ceramide), and cholesterol and phospholipids. Myelin proteins consist mainly of proteolipid protein (PLP), myelin basic protein (MBP), and myelin oligodendrocyte glycoprotein (MOG). High lipid content (∼70% of dry weight) limits the diffusion of water molecules and changes proton relaxation properties, which makes MRI sensitive to detecting demyelinating lesions.
Myelin contains also enzymes, including ones with steroid modifying and cholesterol esterifying activity. Phosphatidylcholine can possibly be synthesized within myelin. Carbonic anhydrase in myelin play a role in removing CO2 from axons.
Myelination is regulated by integrins. Oligodendrocytes express α6β1 integrin (laminin receptor), αvβ1, αvβ3, αvβ5, and αvβ8. Schwann cells express α6β1, α6β4, and α7β1 integrins.
Demyelination and remyelination
Genetic abnormalities that affect glial cells or signalling between axons and glial cells can cause defects in axon myelination. Inflammation and metabolic causes can lead to the loss of existing myelin sheaths. Schwann cells can resorpt myelin in peripheral demyelination. Demyelination diseases, such as multiple sclerosis (MS), can lead to various neurological symptoms. Demyelination may also be seen in the brains of subjects with TBI, AD, and some psychiatric disorders, and in spinal cord injuries.
Following demyelination, the otherwise healthy axon can be remyelinated, leading to partial functional recovery. αv-subunit containing integrins in oligodendrocyte progenitor cells may have an important role in remyelination. The myelin sheaths generated during remyelination are typically thinner and shorter than the original.
Myelin contains considerable amounts of Ca2+, especially the myelin in spinal cord. Damage to the myelin may lead to the release of Ca2+, which may trigger necrotic cell death.
In the demyelinated axons, the number of Na+ channels are decreased, and voltage-gated K+ channels are increased and revealed from under the myelin sheaths. MS drug 4AP enhances conduction by blocking K+ channels.
Luxol fast blue is a copper phthalocyanine dye that has been used to stain myelin under light microscopy. The suitability of related compounds for myelin PET imaging has been studied, and led to development of promising stilbene derivatives (Wu et al., 2008). [11C]MeDAS (Myeliviz) has been found to be a sensitive and specific radiopharmaceutical for imaging of myelination in white matter across different brain regions (Wu et al., 2010; de Paula Faria et al., 2014) and in spinal cord (Wu et al., 2013).
Thioflavin-T and its derivatives have affinity toward the multiple β-sheet structures present in fibrillar amyloid-β deposits and MBP in myelin. Since several PET tracers have already been developed for amyloid-β imaging, those could be re-purposed for myelin imaging (Auvity et al., 2020). PET radiopharmaceutical [11C]PIB is one of the thioflavin T derivatives, used extensively in imaging of amyloid plaques in neurological disorders, including Alzheimer's disease, Parkinson's disease, dementia with Lewy bodies, cerebral amyloid angiopathy (CAA), and traumatic brain injury. It has also been shown to be suitable for imaging myelin, and changes in myelin content, especially in multiple sclerosis (Stankoff et al., 2011; Veronese et al., 2015; Bodini et al., 2016). [11C]PIB uptake in cortex of late MS patients is not different from age-matched healthy subjects (Zeydan et al., 2017). However, [11C]PIB uptake is low in the cerebellum, despite of the high myelin density, while [11C]MeDAS distribution correlates well with myelin distribution in both cerebrum and cerebellum (de Paula Faria et al., 2014). Also some other amyloid PET radiopharmaceuticals could be used in myelin imaging: for example [18F]florbetaben and [18F]florbetapir have decreased uptake in the white matter of MS patients (Matias-Guiu et al., 2015; Pietroboni et al., 2019) and in pseudotumoral multiple sclerosis (Matias-Guiu et al., 2017).
Demyelinating lesions are however often very small (∼0.5 mm), which makes lesion detecting difficult using PET tracers that target myelin which is abundant in the surrounding areas. In addition, the high lipid content of myelin leads to high non-specific uptake of most radiopharmaceuticals. (Brugarolas et al., 2018a).
MS drug 4AP binds to the K+-channels which are revealed from under the myelin sheaths after demyelination. The 18F-labelled drug derivative, [18F]3F4AP, is a promising radiotracer for detecting demyelinated brain regions (Brugarolas et al., 2018b).
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Updated at: 2020-02-01
Created at: 2017-11-17
Written by: Vesa Oikonen