Biogenesis and maintenance of cytoplasmic domains in myelin of the central nervous system

Abstract

Myelin is a multi-lamellar membrane structure, produced by oligodendrocytes which are special glial cells, that myelinate axons in the central nervous system (CNS) (Aggarwal, Yurlova, & Simons, 2011; Vassall, Bamm, & Harauz, 2015). The main role of these tightly-packed and stable structures is to electrically insulate the axon. During the biogenesis of myelin, two processes have to be coordinated. At first, the incorporation of myelin adjacent to the axon at the innermost tongue is accompanied by the lateral expansion of newly formed layers. At the same time, a complex system of cytoplasmic channels (CPCs) is formed, enabling membrane trafficking from the cell body to the leading edge in thin-caliber-axons of the immature optic nerve (Snaidero et al., 2014). These channels are known in the peripheral nervous system (PNS) as Schmidt-Lanterman Incisures, but have not been yet established in the CNS (Gould, Byrd, & Barbarese, 1995; Small, Ghabriel, & Allt, 1987). The development of an improved protocol for high-pressure freezing (HPF), allowed us to better preserve the native myelin ultrastructure close to its native state. Using HPF and freeze-substitution for transmission electron microscopy (TEM), we were able to visualize a system of cytoplasmic (myelinic) channels within myelin surrounding large-caliber axons in the CNS for the first time. In line with their presence in developing myelin lamellae, here, we present how a system of interconnected CPCs is organized in mature myelin of axons with different calibers. Beside the morphological analysis of these channels by TEM, we combined different in vivo and in vitro approaches to describe the biogenesis, molecular structures, and possible roles of CPCs. We elucidated a mechanism that regulates the formation and determines the molecular organization and their involved key components. In this study, we identified 2’,3’-cyclic-nucleotide 3’-phosphodiesterase (CNP) as an essential determinant in generating and maintaining cytoplasmic domains within compact myelin sheaths. Our observations provide evidence that the protein-protein interaction of CNP and filamentous actin (F-actin) results in the formation of a stable structure that helps to keep opposing myelin leaflets separated. The close interaction of CNP and F-actin prevents membrane compaction that is exercised by the classic myelin basic protein (MBP).