Myelin damage and repair in pathologic CNS: challenges and prospects

Abstract

Injury to the central nervous system (CNS) results in oligodendrocyte cell death and progressive demyelination. Demyelinated axons undergo considerable physiological changes and molecular reorganizations that collectively result in axonal dysfunction, degeneration and loss of sensory and motor functions. Endogenous adult oligodendrocyte precursor cells and neural stem/progenitor cells contribute to the replacement of oligodendrocytes, however, the extent and quality of endogenous remyelination is suboptimal. Emerging evidence indicates that optimal remyelination is restricted by multiple factors including (i) low levels of factors that promote oligodendrogenesis; (ii) cell death among newly generated oligodendrocytes, (iii) inhibitory factors in the post-injury milieu that impede remyelination, and (iv) deficient expression of key growth factors essential for proper re-construction of a highly organized myelin sheath. Considering these challenges, over the past several years, a number of cell-based strategies have been developed to optimize remyelination therapeutically. Outcomes of these basic and preclinical discoveries are promising and signify the importance of remyelination as a mechanism for improving functions in CNS injuries. In this review, we provide an overview on: (1) the precise organization of myelinated axons and the reciprocal axo-myelin interactions that warrant properly balanced physiological activities within the CNS; (2) underlying cause of demyelination and the structural and functional consequences of demyelination in axons following injury and disease; (3) the endogenous mechanisms of oligodendrocyte replacement; (4) the modulatory role of reactive astrocytes and inflammatory cells in remyelination; and (5) the current status of cell-based therapies for promoting remyelination. Careful elucidation of the cellular and molecular mechanisms of demyelination in the pathologic CNS is a key to better understanding the impact of remyelination for CNS repair.

Keywords: astrocytes; cell therapy; demyelination; neural stem cells; oligodendrocyte precursor cells; oligodendrocytes; remyelination; spinal cord injury.

FIGURE 1

Structural and molecular organization of myelinated axons in normal and demyelinating conditions. (A) Schematic diagram shows structure and molecular configuration of a myelinated axon at the node of Ranvier, paranodal and juxtaparanodal regions. Nav 1.6 and Kv7 (KCQN) are located in the nodal region and are essential for formation and propagation of action potential. Na⁺/Ca²⁺ exchanger (NCX) is also located in nodal area and exchanges intracellular sodium with extracellular Ca²⁺ in an ATP dependent manner. Ion channels are precisely localized to specific domains of axons through their contact with adhesion molecules such as neurofascin (NF)-186. These adhesion molecules aid in stabilizing ion channels by connecting them with extracellular matrix (ECM) and glial cell processes surrounding the nodal region. Paranodal junctions are the region where myelin loops are tethered to axonal membranes. Contactin and contactin associated protein (Caspr) play key roles in formation of paranodal loops through their interaction with neurofascin (NF)-155 and other adhesion molecules from myelinating glia. Juxtaparanode contains voltage gated Kv⁺ channels (Kv1.1 and 1.2) that are essential for restoring resting membrane potential. Kv channels allow for potassium to exit the axons quickly following depolarization. Caspr2/TAG-1 adhesion complex stabilizes these Kv1.1 and Kv1.2 channels in axonal membrane. Stationary mitochondria (brown) are mainly located in juxtaparanodal and internodal regions where Na/K ATPases are abundant to provide energy for ion homeostasis. There is another mitochondrial population called motile mitochondria (green) which can translocate in both retrograde and anterograde directions along the axon. These mitochondria are being produced in the cell body and can stop in stationary sites. They are important for the turnover and redistribution of mitochondria along the axons and during changes in energy demand. (B) Following demyelination, due to the disruption of paranodal myelin loops, all ion channels, pumps and exchangers become dispersed along the axon and sodium influx increases through Nav1.6 channels. Expression of Nav1.6, Kv1.1, and Kv1.2 channels increases significantly following demyelination. Sodium overload causes axonal calcium to reach toxic levels as it is being exchanged with sodium through NCXs by an energy dependent process. Following demyelination, speed of mitochondrial transportation and size of stationary mitochondria significantly increase to compensate for the increased energy demand. Despite robust increase in mitochondrial content, demyelinated axons are unable to maintain a balance between their energy production and expenditure that results in axonal degeneration eventually.

FIGURE 2

Transplanted adult NPCs (aNPCs) promote the aggregation of K⁺ channels and the formation of nodes of Ranvier in the spinal cord axons of shi/shi mice. Confocal immunostaining of Kv1.2 subunits (red) and pan-Na⁺ hannels (blue) in the spinal cord of wild-type mice (A–C), control shi/shi mice (G–I), and transplanted shi/shi mice (M–P) is depicted. Kv1.2 subunits were clearly localized to the juxtaparanodal regions of wild-type spinal cord axons (A–C), confirmed with nodal pan-Na⁺ immunostaining. In shi/shi mice, Kv1.2 immunostaining was abnormally distributed along the axonal internodes (G–I), but Na⁺ clusters were observed as aberrant nodal aggregates. Six weeks after aNPCs transplantation, spinal cord segments of shi/shi mice showed restoration of Kv1.2 subunit clusters (M–P). YFP-positive processes of transplanted aNPCs were observed in close association with axons containing restored K⁺ channels aggregates (P). Nodal localization of Na⁺ channels was further confirmed using Nav1.6 (red) immunostaining in wild-type (D–F), control shi/shi (J–L), and transplanted shi/shi (Q–T). Caspr (blue) immunostaining was used to identify the paranodal area. A 3D reconstruction clearly shows a node of Ranvier that is bordered by an MBP-expressing NPC derived oligodendrocyte. Note that the processes of YFP-labeled oligodendrocytes avoid the nodal region (U). From Eftekharpour et al. (2007).