Class 4, part 2, Sept-29, Myelination

Transcription

1 1 2 3 Class 4, part 2, Sept-29, Myelination Lecture by Dr. Fournier, Transcribed by Zahra Tabatabaei (Sarah) <sarah_taba@yahoo.com>, Edited by Aki Caramanos Caramanos@gmail.com Table of Contents Table of Contents... 1 Introduction... 3 Title Slide... 3 Factors Affecting Axon conduction speed... 4 Myelinated vs Unmyelinated Axons... 5 Saltatory Conduction... 6 Nodes of Ranvier... 7 Myelination by Schwann cells in the PNS, Oligodendrocytes in the CNS... 9 The Mechanics of Myelin Formation Mechanism of Myelination Myelin Structure Myelin composition Galactolipids in the formation and maintenance of the myelin sheath Myelin Proteins Myelin proteins are different in the CNS and PNS Myelin Basic Protein (MBP)- Mostly in CNS Myelin Shiverer: Mutant Mouse with CNS-Myelin-Basic-Protein Abnormalities Proteolipid protein (PLP)- Largely in CNS Myelin Protein zero (Po) In PNS Myelin Myelin associated Glycoprotein (MAG) Both in CNS and PNS Myelin Molecular architecture of myelin Nodes of Ranvier Question Regarding the Consequences Myelination and Nodal Spacing on Conduction Speed How is appropriate nodal spacing established? Structure of myelinated axons Role of Myelinating Glia in Node Formation Three Accepted Principles Regarding Nodal Formation Formation of axonal membrane domains at nodes and paranodes is initiated by glial cell adhesion molecules (CAMs) Page 1 of 35

2 Myelin and Nerve Regeneration Myelin and nerve regeneration - PNS vs CNS Molecular Determinants Of Axonal Regeneration: Why does axonal regeneration fail in the brain and spinal cord? The molecular inhibitors of the adult CNS glial environment Demyelinating Diseases Demyelinating Diseases: Can Be Either Acquired or Hereditary Multiple Sclerosis: Example of an Acquired Demyelinating Disease Immunopathogenesis of MS Page 2 of 35

3 45 Introduction Title Slide top Page 3 of 35

4 49 50 Factors Affecting Axon conduction speed bottom The speed in which an action potential can propagate down an axon is a direct function of axon diameter. So the fatter an axon is, the faster the conduction is going to be. Squid giant axon: largest known axon and has very fast conduction. However, it has the diameter of a pencil lead and if that was to be in the brain and for every neuron, it would make a very bigger brain! So the solution is to come up with a very thin axon that can fit into a very small space but also enhance the conduction Myelin Myelin is an electrical insulator that sits along the axon only for electrical conduction to only occur in certain areas of the membrane. Saltatory conduction: Action Potential skips over the regions in areas between the unmyelinated spaces. It jumps from one unmyelinated space to the next. Saltatory conduction is 10 times faster than conduction in an unmyelinated axon. Invertebrates, which generally lack myelin, achieve high rates of impulse conduction simply by increasing the radius of their axons. Page 4 of 35

5 66 67 Myelinated vs Unmyelinated Axons top We do have some unmyelinated axons in our nerves system. But many of them are myelinated. The myelination can be seen in periodic wrappings of membrane (depending on how long the axon is). These are perfectly spaced along the axon. The myelinating cell in CNS is Oligodendrocyte and in PNS it is the Schwann cell. Page 5 of 35

6 73 74 Saltatory Conduction bottom [For more details see figure with caption on page 8] When the membrane in the initial segment is depolarized by Sodium influx, because of the myelin insulation of this particular part, the membrane is incapable of depolarizing. So the diffusion of the Sodium gets passed to the next gap through the axon. These gaps are called Nodes of Ranvier. Here then, a depolarization of the membrane can occur and so it will jump to the other nodes of Ranvier and all the way down the axon through Saltatory Conduction. By depolarizing the membrane in some areas instead of all the way, the axonal conduction can be enhanced. The purpose of myelination is to form an insulating membrane that allows electrical conduction to skip in a saltatory fashion from node to node and to enhance this conduction down the axon. Page 6 of 35

7 88 89 Nodes of Ranvier top [For more details see figure with caption on page 8] Nodes of Ranvier They are gaps between sections of myelin. They are booster stations to give strong depolarization. So as an action potential hits a node, the passive diffusion of sodium ions occurs and there will be another action potential which brings the exact same depolarization on the next node. In other words, they retain the action potential of every node as they allow the electrical conductance to ravel down the axon. Keep action potential moving quickly along the axon. Concentrate the voltage-gated Sodium and Potassium ion channels to allow for action potential to propagate at each node. Enhancement of the conduction. Located reproducibly at about every mm along the length of an axon. Page 7 of 35

8 104 Page 8 of 35

9 Myelination by Schwann cells in the PNS, Oligodendrocytes in the CNS bottom [Original figure with caption on next page] Myelination in CNS and PNS are the same in principles. Myelination is PNS is mediated by Schwann cells. There is also a one to one correspondence where only one Schwann cell myelinates one axon. In CNS it is mediated by the complex Oligodendrocytes which due to space issues will myelinate around 50 or more different axons at the same time. Page 9 of 35

10 114 Figure 4-17 The axons of both central and peripheral neurons are insulated by a myelin sheath. A. An axon in the central nervous system receives its myelin sheath from an oligodendrocyte. (Adapted from Bunge 1968.) B. An electron micrograph of a transverse section through an axon (Ax) in the sciatic nerve of a mouse. The spiraling lamellae of the myelin sheath (Ml) start at a structure called the inner mesaxon (IM; circled). The spiraling sheath is still developing and is seen arising from the surface membrane (SM) of the Schwann cell, which is continuous with the outer mesaxon (OM; circled). The Schwann cell cytoplasm (Sc Cyt) is still present, next to the axon; eventually it is squeezed out and the sheath becomes compact. (From Dyck et al ) C. A peripheral nerve fiber is myelinated by a Schwann cell. (Adapted from Williams et al ) Page 10 of 35

11 The Mechanics of Myelin Formation top [Original figure with caption on previous page] How does the myelin wrap around the axon? The Schwann cell starts to fold and two membranes of it will come together in an apposition called Inner Mesaxon. Then one membrane (one leading edge and not the nucleus) tucks underneath the other one and continues to extend on the inner part around the axon to make several layers of myelin. eventually it stops in the Outer Mesaxon. As the myelinating cell wraps around the axon, its membrane keeps condensing into very tight layers by squeezing out the cytoplasm. This goes on until a certain diameter is reached. For animation of myelination go to: Page 11 of 35

12 Mechanism of Myelination bottom An unrolled Schwann cell will look like a flat cell where most of the cytoplasm is squeezed out. There are a few slightly wider cytoplasmic channels that allows the pass of various ions and factors from the nucleus of the Schwann cell to the rest of its body. There are also Adhesive proteins (e.g. P zero in PNS) for cytoplasmic squeezing and pulling the membranes very tight, so that only water and small ions can pass through. Page 12 of 35

13 Myelin Structure top Structure of Myelin An EM of an axon with the many layers of the Schwann cell wraps around it. All the black in the slide is the membrane extension from the Schwann cell. In a cross section of the above, with high magnification, there are alternating black (Major Dense) and light (Interperiod) lines. The Major dense lines are the intracellular space and represent the area where the cytoplasm has been squeezed out. The Interperiod lines are the apposition of extracellular membranes. Page 13 of 35

14 Myelin composition bottom Myelin contains 70-85% lipid and 15-30% protein (i.e., high lipid to protein ratio). Myelin lipids: There is no lipid specific to myelin. Additionally, myelin of the central and peripheral nervous systems, apart from quantitative differences, contain more or less the same lipids. The lipids which are present in myelin are: a) 25-28% cholesterol b) 27-30% galactosphingolipids. c) 40-45% phospholipids (i.e., phosphatidyl ethanolamine and choline) Page 14 of 35

15 Galactolipids in the formation and maintenance of the myelin sheath top The Galactolipids are very critical in terms of keeping the myelin intact. The CGT gene is important information of one particular lipid in myelin in mice. In a wild type versus a knockout mouse for this lipid synthesizing enzyme, we can see that in the wild type the Oligodendrocytes have achieved a nice spacing of the myelin segments along the axons in the CNS while in the mutant mouse the spacing is disrupted. This can lead to severe defects such as tremor and hind limb paralyses (Genotype and phenotype). Page 15 of 35

16 168 Myelin Proteins Myelin proteins are different in the CNS and PNS. bottom a) Major proteins: Myelin basic protein (MBP) - both CNS and PNS myelin. Proteolipid protein (PLP) - specific to CNS myelin. Protein zero (Po)-specific to PNS myelin b) Minor proteins: Myelin associated g1ycoprotein (MAG), P2, CNP (mostly in the CNS), OMgp. Page 16 of 35

17 Myelin Basic Protein (MBP)- Mostly in CNS Myelin top It is located on the cytoplasmic space between the two membranes and so acts to stabilize the major dense line in myelin sheath. It keeps the cytoplasmic faces very tight together. It has been used as an antigen to model MS: Multiple Sclerosis (MS) is a demyelinating disease where there are demyelinated areas in the brain and the spinal cord. It brings a lot of functional defects. In animal studies, it has shown that one s own immune system attacks its own myelin. The injection of Myelin Antigens, including MBP, generates an immune response against it. Doing so will result in an inflammatory disease that is called experimental autoimmune encephalomyelitis (EAE). This is characterized by invasion of immune cells into the nervous system and results in demyelination and chronic paralysis. EAE is believed to be a model of multiple sclerosis (MS) in humans. Page 17 of 35

18 Shiverer: Mutant Mouse with CNS-Myelin-Basic-Protein Abnormalities bottom [Original figure with caption on next page] Shiverer, is a mutant mouse which displays many neurological symptoms that might be similar to MS or other neurological diseases. It shows loss of myelin in the CNS. The PNS myelin is very little affected (perhaps due to some compensatory mechanisms here). This phenotype results from the deletion of a large portion of the MBP gene. The symptoms are tremor, convulsions and finally death. 202 Page 18 of 35

19 203 Figure 4-18 A genetic disorder of myelination in mice (shiverer mutant) can be partially cured by transfection of the normal gene that encodes myelin basic protein. A. Electron micrographs show the state of myelination in the optic nerve of a normal mouse, a shiverer mutant, and a mutant transfected with the gene for myelin basic protein. (From Readhead et al ) B. Myelination is incomplete in the shiverer mutant. As a result, the shiverer mutant exhibits poor posture and weakness. Injection of the wild-type gene into the fertilized egg of the mutant improves myelination. A normal mouse and a transfected shiverer mutant look perky. Page 19 of 35

20 Proteolipid protein (PLP)- Largely in CNS Myelin top Its expression is largely restricted to Oligodendrocytes (CNS). It is a transmembrane protein. It has a small cytoplasmic region as well as an extracellular domain. It makes hemophilic interaction with another PLP molecule in the extracellular region and so it stabilizes the intraperiod line. A mutant mouse named jimpy displays tremors, seizures and CNS demyelination which is characterized by an aberrant, improperly spaced intraperiod line. Page 20 of 35

21 Protein zero (Po) In PNS Myelin bottom It is a member of the immunoglobulin superfamily of cell adhesion molecules. Its expression is restricted to myelin forming Schwann cells only (PNS specific). The extracellular domain of Po which functions as a homophilic cell adhesion molecule promotes the formation of the intraperiod lines whereas the cytoplasmic domain is considered to be involved in making the major dense line of PNS myelin. Page 21 of 35

22 Myelin associated Glycoprotein (MAG) Both in CNS and PNS Myelin top Expressed in both Oligodendrocytes and Schwann cells, particularly in non-compacted regions of myelin sheath. i.e. doesn t have to do with the tight wrapping. It is the mediator for axon-glial adhesion that proceeds myelination. The glial cell initially adheres to the axon before starting to wrap around it, by MAG. The MAG null mutants still generate intact myelin, but they will end up with hypomyelination over time. So it s in vivo role is stabilizing the myelin and keeping it for the long term. Page 22 of 35

23 Molecular architecture of myelin bottom The Glycolipids and Cholesterol are found mainly at the extracellular surfaces in the intraperiod line. PLP passes entirely through the lipid layer and plays a major role in stabilizing the intraperiod line of CNS myelination. By contrast, MBP is an extrinsic protein localized exclusively at the cytoplasmic surface in the major dense line. There is evidence to suggest that MBP forms dimmers and is believed to be the principal protein stabilizing the major dense line. In the PNS, Po protein transverses the bilayer membrane and is believed to stabilize the intraperiod line by homophilic binding to the adjacent layer. Due to positively charged cytoplasmic domain, Po protein also acts in the stabilization of the major dense line in the PNS myelin sheath. MBP also plays a role at the major dense line but it is not as important as in the CNS myelin sheath. P2 protein contributes to the stability of the major dense line. Page 23 of 35

24 251 Nodes of Ranvier Question Regarding the Consequences Myelination and Nodal Spacing on Conduction Speed top myelinated vs non-myelinated? 2X nodes/axon length ---- slower. There should be more depolarization of the membrane.5x nodes/axon length ---- slower. The whole conduction may peter out Sodium diffusion trap ---- no conduction Na2+/K+ pump mutation no conduction. Give explanation of what the Na2+ does and what happens if it is blocked. Page 24 of 35

25 How is appropriate nodal spacing established? bottom Some studies have shown the role of the 1 mm space between the nodes and how it is important to get just the right speed for axon conduction. In the slide, in green, the Na2+ channels are shown to be heavily concentrated at the nodes. There is also other specialization right beside the node; the Paranodal region and further besides them are called the Juxta-Paranodal region where there are K+ channels (red in the slide). Therefore it can be said that the nodes are very well defined molecular structures that are critical to allow for the firing of action potentials. Page 25 of 35

26 Structure of myelinated axons top The schematic of the previous slide. Page 26 of 35

27 Role of Myelinating Glia in Node Formation bottom PNS- contact between axons and Schwann cells is necessary for node formation. CNS some studies suggest that a soluble factor initiates node formation whereas other data implicates contact between Oligodendrocyte and axon as the precipitating event. Page 27 of 35

28 Three Accepted Principles Regarding Nodal Formation top Myelinating Glia initiate clustering of molecules associated with Nodes of Ranvier. So without a Glial cell, there will be no clustering of the Nodal proteins. Intercellular protein-protein interactions, especially involving CAMs, play crucial roles in organizing membrane domains near the Node of Ranvier. A specialized axonal cytoskeleton serves as a scaffold for the maintenance and retention of transmembrane proteins within each membrane domain. Therefore a signal to Na2+ to cluster in the nodal region and some cytoskeletal elements to anchor and hold them in there, so that they don t diffuse away, are needed. Page 28 of 35

29 Formation of axonal membrane domains at nodes and paranodes is initiated by glial cell adhesion molecules (CAMs) bottom The formation of axonal membrane domains at nodes and paranodes is initiated by glial cell adhesion molecules (CAMs) that stabilize the contact between the Glial cell and the axon. Then scaffolding proteins including Ankyrins and Spectrins form a cytoskeleton network underneath and to hold these proteins in place. There are some molecules that are responsible for retaining this micromolecular structure. Page 29 of 35

30 303 Myelin and Nerve Regeneration Myelin and nerve regeneration - PNS vs CNS top Why do the axons of the mature mammalian PNS regenerate after injury while CNS axons do not? Neurons in CNS are incapable of spontaneous regeneration. One molecular contributor to this lack of regeneration is Myelin. An injury to the PNS, leads to an immune response which gets rid of the damaged Schwann cells and Myelin debris. New Schwann cells will then invade. These are very growth permissive and will allow axons to grow along them back to the target (i.e., regeneration). In the CNS, the Myelin is poorly cleared and it remains in the vicinity of the region and it expresses a number of proteins that do have a role in myelin formation, but following the injury they signal to the axon and block its ability to repair. One reason for this could be because there are factors in the Myelin in the CNS that are important for the aberrant plasticity in order to have more cognitive control. Page 30 of 35

31 Molecular Determinants Of Axonal Regeneration: Why does axonal regeneration fail in the brain and spinal cord? bottom An schematic event of the previous slide. After an injury in the CNS, the Myelin debris from the damaged Oligodendrocytes will block the regeneration of these cells. There is also Glial scar components and reexpression of repulsive guidance cue that can signal through the injured axons and block their ability to regenerate themselves. In the PNS, the Myelin gets cleared and the new Schwann cells will come in and make a matrix for these cells to grow over. Page 31 of 35

32 The molecular inhibitors of the adult CNS glial environment top Figure 2 Glial inhibitors and intracellular signalling mechanisms. The molecular inhibitors of the adult CNS glial environment include chondroitin sulphate proteoglycans (CSPGs) associated with reactive astrocytes from the glial scar43, and myelin-associated inhibitors from intact oligodendrocytes and myelin debris, including myelin-associated glycoprotein (MAG)22,23, Nogo-A14 16, oligodendrocyte myelin glycoprotein (OMgp)24, ephrin B3 (REF. 26) and the transmembrane semaphorin 4D (Sema4D)25. Although the topology of Nogo-A remains unclear, both the 66 amino acid loop (Nogo-66) and the amino-terminal domain (amino-nogo) are known to be inhibitory to axon outgrowth14,15,18,19. The neuronal receptors and downstream signalling pathways known to be involved in transducing these inhibitory signals are shown. Among the signalling components that are common to both CSPG and myelin inhibition are the activation of RhoA82 and the rise in intracellular calcium65,89,92,93. Whereas the signals downstream of RhoA that lead to the actin cytoskeleton are well characterized (solid arrows), the relationship between components upstream of RhoA and the role of calcium influx are still ambiguous (dashed arrows). For example, calcium transients might activate protein kinase C (PKC)88,89, which is required for p75 cleavage by -secretase66, or trigger the transactivation of epidermal growth factor receptor (EGFR). Glenn Yiu and Zhigang He NATURE REVIEWS NEUROSCIENCE VOLUME 7 AUGUST There are many different proteins associated with Myelin that have this inhibitory activity for blocking the regeneration (e.g. MAG). Some repulsive guidance cues such as Ephrin family members are also re-expressed for blockage of Myelin and regenerative growth Page 32 of 35

33 351 Demyelinating Diseases Demyelinating Diseases: Can Be Either Acquired or Hereditary bottom Demyelinating Diseases (demyelinating phenotype): Acquired diseases (i.e., multiple sclerosis in which there is also axonal transaction that leads to the blockage of the conduction in the CNS) Hereditary neurodegenerative disorders (i.e., the leukodystrophies). Page 33 of 35

34 Multiple Sclerosis: Example of an Acquired Demyelinating Disease top Page 34 of 35

35 Immunopathogenesis of MS bottom top Page 35 of 35