As the myelin sheath is pretty central to MS, I thought I’d write something about it and the CNS.
The neuron cell body (soma) acts as a receptive area for synaptic inputs from other cells. To better receive contacts, neurons can add dendritic processes off its body with various branching covered in spines to enlarge its receptive area. Receptors on these surfaces receive information by reacting to the transmitters released by axon terminals of other neurons.
The axon comes out of the soma near the axon hillock, which holds cytoskeletal elements and organelles ready to send into the axon. The axon initial segment connects off this area before the myelin sheath begins and is where the action potential is initiated. Nerve endings gather here to transfer the signal into the axon. The synapse is a specialized part of this nerve ending plasma membrane that can form functional contact with other cells.
Neurons only have one axon that mostly contains microtubules, neurofilaments, and mitochondria.
This is where the myelin sheath works its magic. It creates super compact multilayer insulating segments ‘internodes’ on the axon and helps create the unmyelinated nodes of Ranvier (NR) between the internodes. NRs are located about midway from each other on the axon and are the only spots where a myelinated axon can contact the extracellular environment. They recharge neuron impulses, to make sure signals spread along the entire axon length. The term saltatory conduction comes from the Latin word ‘saltus’, meaning to leap like the electrical impulse seems to do from NR to NR. This process uses voltage-gated Na+ and K+ channel clusters that come together at the NR, and open and close by changing the NR membrane potential. Putting them here restricts action potential regeneration to the NR, and the high membrane resistance and low capacitance generated by the myelin sheath promotes rapid current flow along the myelinated axon section to the next node, greatly increasing conduction velocities. The axon exposed at the NR actually isn’t bare, as it’s often the site of axon branching, synaptic contacts, or covered with glial processes. The saltatory conduction process greatly increases conduction velocity relative to axonal size. Forming these clusters entails many different types of specialized molecules. In development, clustering of voltage-gated sodium and potassium channels to these domains coincides with myelination. The myelin sheath is locked to the axon on both sides of the node to set the size of the NR and any alterations can affect conduction speed. OLs contact axons at the paranode (via NF155-Caspr/Contactin1) and are crucial to organize, cluster, and maintain sodium channels (primarily Nav1.6) at the nodes, and Kv1 potassium channels at the juxtaparanodes.
The four myelinated axon region subdomains include the internode (compacted myelin),
the paranodes (where outer myelin loops contact the axon), the nodes of Ranvier (∼1 μm gap between myelin internodes), and the juxtaparanode (interface between paranode and compact myelin, rich in potassium channels).
The myelin sheath can have as many as 100 layers or more tightly wound on top of each other around the axon. The number of myelin layers is set by the sheath thickness, which depends on the axon diameter: the larger it is, the thicker the myelin. Its relative thickness is measured by the ratio between the inner and outer diameter, ‘the g-ratio’. The thinner the sheath, the closer the g-value is to 1. The optimal g-ratio depends on optimizing conduction speeds versus minimizing conduction delays. Optimal g-ratio in the CNS is found to be ∼0.77 and deviations from this can affect neural development and neurologic disease. However, the g-ratio changes in different brain regions, with more myelin needed in highly interconnected areas than in peripheral connections.
The myelin sheath, like all cell membranes, has three main components – water, lipids, and proteins, but their ratio in myelin isn’t the same as in typical cell membranes. The dry myelin sheath is high in lipids (70-85%) with low protein (15-30%), while the typical cell membrane has an even ratio (50/50%). High myelin lipid proportions makes it less permeable to ions for better electrical insulation and affects rigidity and membrane deformation.
Myelin is highly susceptible to composition changes, and even small changes in the ratio of its components can break it down.
CNS myelin is 33-55% water, the lowest water content of any morphological compartment. Near the polar phospholipid headgroups, water molecules have an electrostatic orienting effect and form bonds with the hydrophilic lipid and myelin protein groups. Myelin prevents water diffusion toward the axon which adds to anisotropy that reflects an increase in myelination.
Lipids differ from other major biological macromolecules as they self-assemble due to the hydrophobic effect into macromolecular aggregates, like lipid bilayers, the basic structure of all cell membranes. Lipids are the main constituents of membranes, but myelin isn’t like usual cell membranes in higher lipid proportions, and in ratios of the major lipid components. In the myelin sheath, major lipid proportions are 40% cholesterol, 40% phospholipids, and 20% glycolipids, while in most biological membranes, it’s closer to 25%:65%:10%. Cholesterol and glycolipids are higher due to myelin’s unique multilayer compact structure. Slight changes in myelin lipid composition can affect intermembrane adhesion which can destroy myelin structures.
Lipids aren’t directly genetically encoded, but are synthesized by genetically-encoded enzymes. Thus, myelinogenesis is a strictly regulated process with coordinated gene expressions coding for enzymes involved in myelin lipid and myelin protein synthesis.
Lipid molecules spontaneous self-organize into a lipid bilayer in water due to their hydrophobic properties. When lipids disperse in water, their hydrophobic tails cause water molecules to form quasi-regular ‘clathrate cages’ around these hydrophobic parts. Depending on the phospholipid head group, 6 or more water molecules surround a lipid molecule. When lipid molecules come together, water molecules lose their cage structure and form disordered clusters, increasing total system entropy that makes lipid molecule monolayer self-organization a thermodynamically favorable process. The free energy decreases more if two lipid monolayers pack tail-to-tail for a better arrangement with minimal water contact, a phospholipid bilayer, the basic structure of biomembranes.
Cholesterol has both polar (hydrophilic) and nonpolar (hydrophobic) regions. It has a polar head with only one hydroxyl group and four rings and a hydrophobic hydrocarbon tail that can readily insert into hydrophobic cell membranes interiors. The four fused hydrocarbon rings have an almost flat rigid structure, and their contact with other lipids and proteins in the membrane creates higher packing density. Cholesterol helps reduce water, gases, and small neutral molecule penetration through the membrane. Cholesterol content is key for membrane structural organization and permeability. High cholesterol content (30-50%) ensures high membrane hydrophobicity, increases membrane packing, and helps set membrane fluidity. Remyelination is more efficient with increased cholesterol synthesis.
Membrane phospholipids classes, sphingomyelins and phosphatidylcholines, are +50% of membrane phospholipids. Long phospholipid hydrophobic tail lengths increase interaction between tails, promote tight packing, decrease lipid association fluidity, and make a more nonpermeable barrier.
Two abundant glycolipids in myelin membranes are galactocerebroside (GalC) and galactosulfatide (sGalC). Glycolipid long alkyl chains closely align and can form up to 8 intermolecular hydrogen bonds. They interact with phospholipids and cholesterol to form dense packing in the myelin membrane bilayer. Phospholipids and glycolipids asymmetrically arrange on the membrane, with phospholipids higher on the inner lipid bilayer sheet and glycolipids on the outer sheet. The network of hydrogen contacts among lipids helps form micro lipid rafts, a kind of liquid crystal structure. These densely packed regions decrease motion for a more rigid and resist fluid/solid phase transition. Deficient glycolipids impairs lipid bilayer packing, increases membrane permeability, and breaks down myelinated axon conduction. Glycolipids increase 2x in myelin vs typical biomembranes.
CNS myelin is relatively low in protein quantity, but it’s highly diverse (223 proteins). They have a variety of sequences, functions, and structures, but all are small, no more than 30 KDa, with long half-lives, and are multifunctional. Many myelin proteins are intrinsically disordered (IDP) or have intrinsically disordered regions (IDR). Absent a fixed, ordered 3-D structure in their part or the whole is due to a relatively small proportion of hydrophobic amino acids and more disorder-promoting amino acids. Thus, high conformational flexibility to make variable structures depending on neighboring contacts. Upon binding with other molecules in myelin, IDRs undergo a disorder-to-order transition ‘coupled folding’ and binding. IDRs in myelin proteins are key to form multilayer myelin membranes.
PLP is the most abundant CNS myelin protein, 38% of total myelin protein mass. PLP1 gene encodes human PLP and expresses in OLs, astrocytes, and even in some NPCs. A high PLP level in myelin is needed for myelin integrity. PLP helps form a compact multilayer membrane structure by bringing myelin membranes closer together. Reducing it by 50% causes axonal pathology and mutations in PLP1 gene cause hypomyelination.
A small PLP chain fragment (residues 45–53), peptide (KLIETYFSK), that only covers 3% of the PLP molecule, forms a complex with HLA class I histocompatibility molecule HLA-A*0301 and may act in autoimmunity. MS patients elevate T-cell and antibody responses to PLP.
MBP is the second most abundant myelin protein in the CNS: ~30% of dry protein CNS myelin mass. MBP also interacts with other proteins and helps transmit extracellular signals to the cytoskeleton and tight junctions. MBP is the ‘executive’ myelin membrane molecule due to its critical role in compact myelin sheath formation.
In mammals, the MBP gene has 7 exons and differential primary mRNA splicing leads to different protein isoforms. ‘Classic myelin isoforms’ vary in molecular mass from 14 to 21.5 kDa. The 18.5-kDa isoform is the most abundant in CNS myelin. MBP isoforms undergo a key post-translational modification by citrullinating arginyl residues. This creates the eight charged 18.5 kDa isomers (C1-C8). Mostly unmodified C1 isomer has highest charge (+19 at pH 7), while mostly modified C8 has smallest charge (+13 at pH 7) due to deimination (citrullination) of 6 arginine residues into uncharged citrulline. The irreversible citrullination reaction reduces MBP positive surface charges, which weakens its interactions with negatively charged lipids.
MS T cells preferentially respond to citrullinated MBP, inducing and perpetuating MS.
C1, C2, and C3 stabilize myelin, while C8 helps it develop. C1 has low hydrophobic content, ~25%. It localizes to cytoplasmic myelin membrane parts. MBP brings together two apposing negatively charged cytoplasmic myelin leaflets that form the major dense line in the myelin sheath. Maximum adhesion force and minimum cytoplasmic spacing occurs when each negative lipid in the membrane binds to a positively charged lysine or arginine group on MBP. Excess MBP forms a weak gel between myelin surfaces, while excess negative charges cause the water gap to electrostatically swell. Excess or deficient MBP causes violent bilayers to repel each other and may lead to demyelination.
Myelin leaflet lipid composition majorly impacts its interactions with MBP.
MBP also interacts with a large group of proteins related to protein expression and may play a regulatory role in myelinogenesis.
The CNS (brain, spinal cord, olfactory and optic nerves), is myelinated by oligodendrocytes (OLs). OPCs differentiate to form mature OLs that extend their processes to wrap nearby axons with concentric membrane layers. Depending on the CNS locale, each OL myelinates 20- 60 axon segments, but only one segment per axon. An OL usually only needs ~5 h to generate all its myelin, which includes synthesizing all the necessary proteins and lipids. The leading edge of the developing myelin sheath circles repetitively around the axon, staying close to the axon to pass under previous myelin wraps each turn. Outer wraps extend laterally, with the terminal edges attaching to the axon in loops that form the paranode. Over time the cytoplasm is excluded from most myelin regions, producing compact myelin.
OLs are located in both gray and white matter, but are predominant in WM. In GM, they usually locate near neurons and capillaries.
The OL forming a myelin internode (i.e. the myelin between two nodes) is seldom seen adjacent to the myelin-wrapped process, as thin cytoplasmic bridges connect the OL cell body to the myelin. This is good, as the one drawback of myelinating the axon is that it blocks access for it to take up nutrients and other metabolites directly from the extracellular space. Non-compact myelin remains in the paranodal loops and innermost myelin layer next to the axon for cytoplasmic channels to pass through the myelin sheath to connect to the OL. These are more prominent in development, but stay in to transfer organelles and molecules to support the myelin sheath and axon in adults. OL-specific protein 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNP) is essential to preserve cytoplasmic spaces between inner non-compact myelin leaflets. MBP also retains some less dense citrullinated forms for these channels. OLs express monocarboxylate transporter (MCT)-1 and can transfer glycolysis products like lactate to axons, where it’s converted to ATP. MCT-1 expresses in the myelin sheath and along the adaxonal surface to directly supply myelinated axons with energy. OLs secrete exosomes to support neuronal health and buffer potassium by expressing Kir4.1. OL glucose transporter (GLUT)-1 expression is regulated by NMDA receptor activity. OLs don’t store glycogen, so OL glucose uptake and glycolysis product supply to axons may have to match their activity levels, supporting axons during heightened metabolic load.
Extracellular vesicles (EVs) are lipid bilayer-bound structures that carry metabolites, proteins, lipids, mRNAs, and miRNAs. Based on their size and how they are released, they are either exosomes [30–100 nm, released from multivesicular bodies (MVBs)] or microvesicles (100–1,000 nm, released by plasma membrane budding). Once secreted, EVs can be scooped up by other cells to use. OL MVBs are located in non-compact myelin regions and at the adaxonal loop. After secretion, OL EVs are taken up by neurons, and promote their survival, at least in culture. EVs released along the periaxonal spaces are preferentially taken up by the myelinated axon, for a relatively targeted and activity-regulated transfer. OL-derived exosomes contain chaperone proteins and enzymes to protect against OS, but individual components are unknown.
Astrocytes in a normal CNS maintain glutamate, water, and extracellular potassium homeostasis. They functionally connect to adjacent astrocytes and OLs by gap junctions, forming large syncytium-like glial networks that contain hundreds of cells.
OLs use heterotypic gap junctions to siphon K+ away from the inner myelin layer and to astrocytes. Gap junctions also likely help traffic glucose between astrocytes and OLs. Cx47 is needed to fully connect OLs to astrocytes. Gap junctions may link glial networks and distribute metabolites ultimately shuttled to axons via OLs. Astrocytes and OLs may also act together to regulate glutamate breakdown and redistribute its metabolites, as OL subsets in the spinal cord and midbrain express glutamine synthetase.
That, my friends is one of the biggest problems in MS, because reactive astrocytes can’t resume homeostatic functions and OLs, axons, and neurons depend on their support.
*Organization of Cell Types (Section 1, Chapter 8) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy – The University of Texas Medical School at Houston
10:1041961.doi:10.3389/fchem.2022.1041961
*Kister A and Kister I (2023) Overview of Myelin, major myelin lipids, and myelin-associated proteins. Front. Chem. 10:1041961.doi:10.3389/fchem.2022.1041961
*Duncan GJ, Simkins TJ and Emery B (2021) Neuron-Oligodendrocyte Interactions in the Structure and Integrity of Axons. Front. Cell Dev. Biol. 9:653101. doi: 10.3389/fcell.2021.653101