cells Review Myelin Fat Facts: An Overview of Lipids and Fatty Acid Metabolism Yannick Poitelon , Ashley M. Kopec and Sophie Belin * Department of Neuroscience and Experimental Therapeutics, Albany Medical College, Albany, NY 12208, USA; [email protected] (Y.P.); [email protected] (A.M.K.) * Correspondence: [email protected] Received: 5 March 2020; Accepted: 25 March 2020; Published: 27 March 2020 Abstract: Myelin is critical for the proper function of the nervous system and one of the most complex cell–cell interactions of the body. Myelination allows for the rapid conduction of action potentials along axonal fibers and provides physical and trophic support to neurons. Myelin contains a high content of lipids, and the formation of the myelin sheath requires high levels of fatty acid and lipid synthesis, together with uptake of extracellular fatty acids. Recent studies have further advanced our understanding of the metabolism and functions of myelin fatty acids and lipids. In this review, we present an overview of the basic biology of myelin lipids and recent insights on the regulation of fatty acid metabolism and functions in myelinating cells. In addition, this review may serve to provide a foundation for future research characterizing the role of fatty acids and lipids in myelin biology and metabolic disorders affecting the central and peripheral nervous system. Keywords: Schwann cell; oligodendrocyte; myelin; lipid; fatty acid 1. Introduction Myelin is a specialized multilamellar membrane consisting of 40 or more tightly wrapped lipid bilayers [1]. The deposition of compact myelin in a spiraling pattern around an axon generates two morphological features that can be observed by electron microscopy, (1) the major dense line (MDL), which is the tight apposition of the cytoplasmic leaflets, and (2) the intraperiod line (IPL), which is the apposition of the extracellular leaflets (Figure1). Myelin is made by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS). CNS and PNS myelin differ in several important ways. One oligodendrocyte forms myelin sheath segments for several neurons, whereas a single Schwann cell myelinates one segment for a single neuron. During the development of the vertebrate nervous system, myelination starts around birth first in the PNS, then the spinal cord and finally in the brain. Although most myelination will be completed in the PNS shortly after birth (within two years after birth in humans; within four weeks after birth in rodents), myelination in the CNS is an ongoing process that continues throughout adulthood. During active CNS myelination in rodents, the myelin sheath expands at a rate 5 103 – 5 104 µm2/cell/day (15 × × to 150 times faster than a normal cell body membrane extension). In addition, one oligodendrocyte or one Schwann cell supports a surface area of myelin up to 2 mm2 or 20 mm2 respectively, which can represent 2000 times the cell surface area of an epithelial cell [2,3]. Although the turnover of myelin lipids in humans is uncharacterized, studies in mice showed that lipids within the myelin sheath are continuously remodeled, and lipid turnover rates are differently regulated through life [4]. Thus, biosynthesis, storage, and cellular trafficking of myelin lipids are essential to the assembly and maintenance of myelin in the nervous system through life span. Recent reviews have provided updates on the genes, proteins, and molecular signals controlling myelination both in the PNS and CNS [5–7]. Herein, we focus on the most recent developments regarding myelin lipids and fatty acids. Cells 2020, 9, 812; doi:10.3390/cells9040812 www.mdpi.com/journal/cells Cells 2020, 9, x FOR PEER REVIEW 2 of 17 Cells 2020updates, 9, 812 on the genes, proteins, and molecular signals controlling myelination both in the PNS and 2 of 17 CNS [5–7]. Herein, we focus on the most recent developments regarding myelin lipids and fatty acids. Figure 1.FigureThe 1. structure The structure of peripheral of peripheral nervous nervous systemsystem myelin myelin sheath. sheath. Schematic Schematic representation representation of a (i) of a (i) myelinated axon, (ii) myelin sheath, (iii) bilayer membrane, and (iv) major lipids classes. Myelin is myelinated axon, (ii) myelin sheath, (iii) bilayer membrane, and (iv) major lipids classes. Myelin is formed by apposition of the external surfaces and internal surfaces of the myelin bilayer that formedconstitute by apposition the intraperiodic of the external line (IPL) surfaces and the and major internal dense line surfaces (MDL), of respectively the myelin (ii, bilayer iii). The thatmyelin constitute the intraperiodicbilayer has linean asymmetric (IPL) and thelipid major composition dense line(iii, (MDL),iv). Myelin respectively protein are ( iialso, iii ).asymmetrically The myelin bilayer has an asymmetricdistributed with lipid for example composition PLP and ( iiiP0, iniv the). IP MyelinL and MBP protein and PMP2 are also at the asymmetrically MDL (iii). Cholesterol distributed with for(Chl.), example Galactosylceramide PLP and P0 in(Galc, the cyan), IPL andPlasmalogen MBP and (PE, PMP2 yellow), at Phosphatidylcholine the MDL (iii). Cholesterol (PC, dark (Chl.), Galactosylceramideblue), Sphingomyelin (Galc, (SM, cyan), light Plasmalogenblue) and other (PE,phospholipids yellow), (PL, Phosphatidylcholine red). P0, PMP2 proteins (PC, and dark the blue), enrichment of sphingomyelin in the myelin are specific to PNS myelin. Sphingomyelin (SM, light blue) and other phospholipids (PL, red). P0, PMP2 proteins and the enrichment2. Myelin of Lipids sphingomyelin—Role of Lipids in the in myelin Myelin are specific to PNS myelin. 2. Myelin Lipids—RoleThe myelin sheath of is Lipids characterized in Myelin by a high proportion of lipids (70%–85%) and consequently a low proportion of proteins (15%–30%). In contrast, most biological membranes have approximatively Theequivalent myelin ratio sheath of proteins is characterized to lipids (50% by lipid a high/50%proportion protein) [8]. T ofhe lipidshigh lipid/protein (70%–85%) ratio and in consequentlymyelin a low proportioncontributes ofto proteinsthe close (15%–30%).packing and tight In contrast, organization most of biological the myelin membranes sheath through have non approximatively-covalent equivalentinteractions ratio of between proteins lipids to and lipids myelin (50% proteins lipid/ [9]50%. In protein)addition, the [8]. enrichment The high in lipid specific/protein classes ratioof lipids in myelin contributesis also to required the close for the packing long-term and maintenance tight organization of myelin [10] of. The the three myelin major sheathclasses of through membrane non-covalent lipids are cholesterol, phospholipids (e.g., plasmalogen, lecithin, sphingomyelin) and glycolipids (e.g., interactions between lipids and myelin proteins [9]. In addition, the enrichment in specific classes of galactosylceramide). The lipid composition of myelin sheath is distinctive, made of high amounts of lipids ischolesterol also required and forenriched the long-term in glycolipid, maintenance in a ratio ofof myelin 40%:40%:20% [10]. The(cholesterol, three major phospholipid classes of, and membrane lipids areglycolipid, cholesterol, respectively) phospholipids compared to (e.g., most plasmalogen, biological membranes lecithin, (25%:65%:10% sphingomyelin)) [11]. and glycolipids (e.g., galactosylceramide).Myelin is not The a simple lipid homogeneous composition layer of myelin of proteins sheath and lipids. is distinctive, It also contains made discrete of high and amounts of cholesteroldynamic andlipid enricheddomains in intheglycolipid, external leaflet in of a its ratio plasma of membrane 40%:40%:20% called lipid (cholesterol, rafts [12,13 phospholipid,]. Lipid rafts are characterized by the concentration of selected membrane lipids such as cholesterols, and glycolipid, respectively) compared to most biological membranes (25%:65%:10%) [11]. galactosylceramide, and low levels of phosphatidylcholine. These lipids have roles in myelin Myelinformation is not and a stabilization simple homogeneous [14,15] by includ layering ofand proteins excluding and spec lipids.ific proteins It also into contains particular discrete and dynamiccompartments lipid domains and orchestrating in the external membrane leaflet protein of its trafficking plasma membrane[16,17] and calledsubsequent lipid signal rafts [12,13]. Lipid raftstransduction are characterized [18] (see [16,19 by] for the review). concentration In CNS and of PNS selected myelin, membrane lipid rafts have lipids been such suggested as cholesterols, to galactosylceramide,promote cellular and adhesion low levels by facilitating of phosphatidylcholine. the localization of Thesemembrane lipids and have transmembrane roles in myelin proteins formation and stabilizationmediating axon [14,-15glia] byrecognition including [14,20 and–24 excluding]. specific proteins into particular compartments and orchestrating membrane protein trafficking [16,17] and subsequent signal transduction [18] (see [16,19] for review). In CNS and PNS myelin, lipid rafts have been suggested to promote cellular adhesion by facilitating the localization of membrane and transmembrane proteins mediating axon-glia recognition [14,20–24]. While there are no lipids that are specific to the myelin [25], the three most abundant lipids are (i) cholesterol, (ii) galactosylceramide, and (iii) plasmalogen. Together, these three lipids comprise 65% of the total myelin lipids. Lipid composition