Molecular mechanisms of myelin dysfunctions in the nervous system
Creators: name: Kim, Jihyun
Publisher: name: No Publisher Supplied
Year: 2017

description: Myelin is generated by Schwann cells (SCs) in the peripheral nervous system (PNS) and by oligodendrocytes (OLs) in the central nervous system (CNS). Myelin not only provides physical and structural support to the axon, it also provides trophic and metabolic supports. In addition, myelin is important for rapid nerve conduction, which is essential for proper communication within a neuronal circuit. Therefore, damage to myelin or myelin loss disrupts axonal integrity and impairs neuronal functions. Elucidating molecular mechanisms underlying myelin defects under pathological conditions will be important in gaining insights into developing a strategy for preventing myelin loss and improving myelin repair. In this study, we investigated molecular mechanisms of aberrant myelination and myelin loss associated with Charcot Marie Tooth disease (CMT) in the PNS and traumatic brain injury (TBI) in the CNS.    In Chapter 1, we investigated the impact of CMT4B1-associated MTMR2 loss in SCs. CMT4B1 is a genetically inherited disorder of the PNS that is caused by loss of MTMR2 gene function. In CMT4B1 patients, myelin outfolding and demyelination are observed resulting in decreased nerve conduction, muscle weakness, atrophy, and sensory deficits. Previously, it has been shown that SC-specific deletion of MTMR2 in mice results in reduced nerve conduction and myelin abnormalities similar to defects observed in CMT4B1patients. However, the mechanism(s) by which loss of MTMR2 function leads to the myelin abnormalities are not fully understood. To elucidate the underlying mechanisms, we generated MTMR2 knockdown SCs and analyzed the effect of MTMR2 loss on intracellular signaling pathways that are essential for SC myelination. Since MTMR2 is a phosphoinositide 3-phosphatase that regulates the PI(3,5)P2 metabolism, it is possible that abnormal regulation of the PI(3,5)P2 level in MTMR2 KD SCs may be associated with the aberrant SC functions. Recently, PI(3,5)P2 has been shown to serve as a platform for mTORC1 signaling on lysosomal membrane. Since mTORC1 has an important role in SC myelination, we monitored whether MTMR2 loss affects the mTORC1 signaling pathway in SCs. Here, we report an aberrant increase in mTORC1 activity in MTMR2 KD SCs. The mTORC1 activation is also associated with inhibition of autophagy and transcription activity of TFEB, a regulator of lysosomal biogenesis and function.   Myelin repair or promoting remyelination in the PNS is important for improving neuronal function in patients with peripheral myelin dysfunctions. In Chapter 2, we elucidated the promyelination function of recombinant TIMP-3 in SCs. TIMP-3 is a member of the tissue inhibitor of metalloproteinase family proteins and one of the targets includes ADAM17. Endogenous ADAM17 in the PNS negatively regulate axonal Nrg1 type III signaling that is essential for SC myelination. Here, we report that recombinant TIMP-3 enhances myelin formation by SCs. The TIMP-3 function is associated with an increase in axonal Nrg1 signaling and laminin deposition during the early stages of myelin formation.   In Chapter 3, we investigated molecular mechanisms underlying myelin dysfunction in the CNS associated with TBI. Myelin loss following TBI contributes to axonal degeneration, neuronal death and in long-term, neuronal dysfunction in the patients. Recent studies provide evidence that primary myelin loss contributes to the myelinated axon pathology following TBI. The myelin loss appears to occur without OL death, indicating that demyelination results from a mechanism that actively destroys myelin in viable OLs. Therefore, understanding the mechanisms of OL response to injury may provide insights into preventing demyelination and/or to protecting proper axon- myelin units following TBI. To this end, we investigated the direct impact of mechanical injury on OLs. We developed an OL monoculture system established on a deformable silicone membrane that can be rapidly stretched by a computer-controlled air pulse, which mimics diffused mechanical injury in the brain following TBI. Our data show that stretch injury induces activation of the Erk1/2 pathway in OLs, which leads to myelin protein loss. Furthermore, the Erk1/2 activation was induced by intracellular calcium increase. Inhibition of Erk1/2 or chelating intracellular calcium prevents myelin protein loss after stretch injury. Furthermore, TBI in vivo results in rapid Erk1/2 activation in white matter OLs accompanied by losing the mature OL phenotype.    By studying the molecular mechanisms responsible for myelin malformation or myelin loss in demyelinating diseases, we provide evidences of signaling pathways or signaling molecules that could be potential therapeutic targets for preventing myelin dysfunctions.