Viewpoints Autophagy in Myelinating Glia https://doi.org/10.1523/JNEUROSCI.1066-19.2019 Cite as: J. Neurosci 2019; 10.1523/JNEUROSCI.1066-19.2019 Received: 28 May 2019 Revised: 17 October 2019 Accepted: 8 November 2019 This Early Release article has been peer-reviewed and accepted, but has not been through the composition and copyediting processes. The final version may differ slightly in style or formatting and will contain links to any extended data. Alerts: Sign up at www.jneurosci.org/alerts to receive customized email alerts when the fully formatted version of this article is published. Copyright © 2019 the authors 1 Title: Autophagy in Myelinating Glia 2 Abbreviated Title: Autophagy in Myelinating Glia 3 Authors: Jillian Belgrad1, Raffaella De Pace2, and R. Douglas Fields1* 4 Affiliations: 1Section on Nervous System Development and Plasticity, Eunice Kennedy Shriver 5 National Institute of Child Health and Human Development, National Institutes of Health, 6 Bethesda, MD, 20892 7 2Section on Intracellular Protein Trafficking, Eunice Kennedy Shriver National Institute of Child 8 Health and Human Development, National Institutes of Health, Bethesda, MD, 20892 9 10 *Corresponding Author 11 Dr. R. Douglas Fields 12 Chief, Nervous System Development & Plasticity 13 National Institute of Health, NICHD 14 Bldg. 9, Room 1E126 15 9 Memorial Drive 16 Bethesda, MD 20892 17 Office: (301) 480-3209 18 Email: [email protected] 19 20 Keywords: Plasticity, white matter, injury, hypoxia, oligodendrocytes, Schwann cells, 21 autophagic flux 22 23 Number of Pages: 49 24 Number of Figures: 2 25 Number of Tables: 3 26 Number of Words for Abstract: 195 27 Number of Words for Introduction: 350 28 Number of Words for Discussion: 59 1 29 Conflict of Interest: The authors declare no competing financial interests. 30 Acknowledgments: This work is supported by the NIH Intramural Research Grants 31 ZIAHD000713-22 and ZIAHD001607. The authors also thank Dr. Juan S. Bonifacino for his 32 helpful discussion of the manuscript. 33 2 34 Abstract 35 Autophagy is the cellular process involved in transportation and degradation of 36 membrane, proteins, pathogens, and organelles. This fundamental cellular process is vital in 37 development, plasticity, and response to disease and injury. Compared with neurons, little 38 information is available on autophagy in glia, but it is paramount for glia to carry out their 39 critical responses to nervous system disease and injury, including active tissue remodeling and 40 phagocytosis. In myelinating glia, autophagy has expanded roles, particularly in phagocytosis 41 of mature myelin and in generating the vast amounts of membrane proteins and lipids that must 42 be transported to form new myelin. Notably, autophagy plays important roles in removing 43 excess cytoplasm to promote myelin compaction and development of oligodendrocytes, as well 44 as in remyelination by Schwann cells after nerve trauma. This review summarizes the cell 45 biology of autophagy, detailing the major pathways and proteins involved, as well as the roles 46 of autophagy in Schwann cells and oligodendrocytes in development, plasticity, and diseases in 47 which myelin is affected. This includes traumatic brain injury, Alexander’s disease, Alzheimer’s 48 disease, hypoxia, multiple sclerosis, hereditary spastic paraplegia and others. Promising areas for 49 future research are highlighted. 50 51 Introduction 52 Autophagy, derived from the Greek words phagos meaning “eat,” and auto meaning “self,” 53 is an essential and conserved cellular process that drives the capture and recycling of proteins, 54 pathogens, and organelles, allowing their removal from the cytosol and degradation in the 55 lysosome (Levine and Klionsky, 2004; Eskelinen and Saftig, 2009; Dikic and Elazar, 2018). 56 Well characterized in neurons (Lee, 2012; Wong and Holzbaur, 2015; Maday and Holzbaur, 57 2016; Menzies et al., 2017), autophagy in glial cells has been much less studied. This is 3 58 surprising, considering the important involvement of autophagy in development, disease, and 59 response to injury, and the essential functions of glia in these processes. Examples that will be 60 discussed in this review include the new evidence of autophagy during oligodendrocyte 61 development, in peripheral myelin compaction, and in facilitating myelin clearance after nerve 62 injury. The functions of autophagy in myelinating glia (Schwann cells and oligodendrocytes) are 63 even less well known than for astrocytes and microglia, yet the formation of myelin membrane 64 and removal of myelin debris after injury involve robust transport and recycling of proteins, 65 membrane lipids, organelles; cytoskeletal and membrane remodeling; and endo- and exocytosis: 66 processes in which autophagy is critical in other cells. This review summarizes current 67 information on autophagy in myelinating glia in association with a wide range of biological 68 functions and in nervous system disorders, and it highlights promising areas for future research. 69 While other reviews have described autophagy signaling in detail (Dikic and Elazar 2018; 70 Glick et al., 2010), our goal is to synthesize mechanisms of autophagy in myelinating glia in 71 central and peripheral nervous system during lifelong development, plasticity, injury, and 72 disease. We first review the major components and interactions along the autophagy pathways. 73 We then discuss studies that identify and manipulate autophagic processes in Schwann cells and 74 oligodendrocytes across developmental and injury states. In doing so, we hope to underscore the 75 crucial contribution that autophagy plays in myelinating glia and how these new insights may be 76 targeted in therapies for neurological disease and injury. 77 78 Autophagy process and pathways 4 79 At least three forms of autophagy have been identified: (1) chaperone mediated 80 autophagy (Dice, 1990), which targets unfolded cytosolic proteins with chaperone proteins and 81 translocates them through the lysosomal membrane, (2) microautophagy (Marzella et al., 1982), 82 in which the lysosomal membrane undergoes local rearrangements to engulf portions of 83 cytoplasm, and (3) macroautophagy (De Duve and Wattiaux, 1966), which, thorugh double 84 membrane organelles called phagosomes, engulf cellular material that is degraded and recycled. 85 Microautophagy is induced by nitrogen starvation or rapamycin treatment (Li et al., 2012) and 86 can be selective or bulk engulfment (Sahu et al., 2011). Microautophagy happens locally on the 87 surface of lysosomes and only involves a small portion of these organelles. This review ill focus 88 on macroautophagy, henceforth referred to as autophagy. 89 90 Macroautophagy (De Duve and Wattiaux, 1966) can be induced by a number of signals, 91 e.g., hypoxia, nutrient depletion, cellular damage, production of oxygen reactive species. 92 Following induction, the cell starts to protrude double membrane organelles called 93 phagosomes, to surround and engulf the material targeted for degradation. The origin of the cup 94 shaped double membrane is not well understood and is still a matter of intense debate. Some 95 recent in silico work proposes that the double membrane is the result of progressive fusion of 96 vesicles, followed by protein-mediated remodeling (Bahrami et al., 2017). However, 97 macroautophagy does not always involve double membraned structures (Mijaljica and Devenish, 98 2013) . 99 In most cells, basal levels of autophagy help maintain the integrity of intracellular 100 organelles. However, autophagy is strongly induced under starvation (Hosokawa et al., 2009; 101 Kim et al., 2011), hypoxia (Semenza, 2009), aging (Mizushima and Komatsu 2011), cancer 5 102 (Galluzzi et al., 2015), and infection (Gomes and Dikic, 2014), reflecting a critical role of 103 recycling membrane, organelles and macromolecules during these processes. Macroautophagy 104 consist of five main steps: (1) initiation and nucleation, (2) elongation of the nascent double 105 membrane, (3) cargo sequestration, (4) fusion of the mature autophagosome with lysosomes, and 106 (5) recycling of nutrients (Dikic and Elazar, 2018). The autophagy-related (ATG) proteins, 107 several of which have been identified and are encoded by different Atg genes (Dikic and Elazar, 108 2018), are among the most important proteins for this process. The fundamentals of the 109 autophagy pathway are illustrated in Figure 1. 110 Different cellular compartments have been described as the origin for the autophagy 111 membrane: the first organelle proposed as source for autophagosomal membrane is the 112 endoplasmic reticulum (ER) (Axe et al., 2008). More specifically, it has been suggested that the 113 ER–mitochondrion interface domains donate lipids to forming phagosomes (Hailey et al., 2010). 114 In particular, work done in eukaryotic cells shows that the outer membrane of the mitochondria 115 must be physically connected with the ER during starvation in order to share 116 phosphatidylethanolamine (PE) lipids with the nascent autophagosome and promote the 117 lipidation of microtubule-associated protein light chain 3B I(LC3B-I) to LC3B-II (Hailey et al., 118 2010). The plasma membrane has also been hypothesized to contribute to the formation of 119 autophagosomes, involving the heavy chain of clathrin and ATG16L1 protein (Ravikumar et al., 120 2010). The Golgi has also been suggested as a possible independent donor of membrane 121 forming autophagosomes (Ohashi and Munro, 2010; Geng et al., 2010).