Journal of Neurocytotogy 19, 187-199 (1990) Radial component of CNS myelin: junctional subunit structure and supramolecular assembly B. KOSARAS and D. A. KIRSCHNER* Neurology Research, Children's Hospital, Boston, MA 02115, Department of Neurology, Harvard Medical School, Boston, MA 02115, USA received 27 July 1989; revised 18 September 1989; accepted 25 September 1989 Summary The radial component is a structural specialization within CNS myelin that is believed to stabilize the apposition of membranes in the internode. Previous observations on thin sections and freeze-fracture replicas show that this junctional complex consists of linear, particulate strands that run parallel to the nerve fibre axis and radially through the myelin sheath, but details on its molecular organization are lacking. The objective of our current study was to gain further insight into its arrangement and composition by examining its fine-structure and incidence in: myelin with known deficits in protein composition (e.g., shiverer, transgenic shiverer, myelin deficient and jimpy mutant mice); isolated CNS myelin, which has been shown by X-ray diffraction to be more stable than intact CNS myelin; and human white matter, in which this junctional complex has not yet been described. Our results confirm the localization and general appearance of the radial component as previously reported. In addition, we found that: (1) the radial component occurs abundantly in human CNS myelin where it has a complex subunit structure; (2) the constituent junctional unit of this structure is organized as a pair of globular domains (each -40 • diameter) at the extracellular apposition which is linked by -15 A diameter filaments extending through the bilayer to ~25 A globular domains in the adjacent cytoplasmic apposition; (3) the radial component is present with apparently normal structure in the sparse, compact myelin of murine mutants containing either different amounts of MBP or no PLP which indicates that neither of these proteins is necessary for junctional integrity; (4) the radial component is present in purified CNS myelin membranes which may account for the stability of these membranes; and (5) the radial component is structurally resistant to Triton, which suggests a method for its further biochemical characterization. Finally, from an analysis of images from tilted transverse and longitudinal sections, we have reconstructed a model of its three-dimensional, supramolecular organization Introduction The complex morphology of the nerve myelin sheath is stabilized by a variety of inter- and intracellular adhesion mechanisms. In the paranodal region nu- merous rows of glial-axonal junctions bridge between the terminal loops of the ensheathing cell and the underlying axonal membrane; and a tight junction links adjacent terminal loops to one another, separat- ing the extracellular compartment within myelin from the intercellular space (Raine, 1984). In the internodal region the periaxonal space between the ensheathing cell and axon is stabilized by the myelin-associated glycoprotein (Trapp et al., 1984; Poltorak et aI., 1987). The most abundant adhesion, however, occurs in the compact myelin where a combination of non-specific forces and specific, short-range interactions organize and stabilize the packing of membranes at their cytoplasmic and extracellular appositions (Inouye & * To whom correspondence should be addressed. Kirschner, 1988a, b). The aforementioned mechan- isms of attaching membranes to one another occur in both CNS and PNS myelins; however, in CNS myelin there is in addition a unique network called the radial component (also termed interlamellar or intramyelinic tight junctions). Consisting of nearly parallel strands of junctions arranged both through the stack Of membranes and in the planes of the membranes parallel to the fibre axis, the radial component occurs mostly in the region between the inner and outer tongue processes. Detailed information about its fine structure and macromolecular organization is lacking. In cross-sections stained with permanganate the junc- tions are revealed as conspicuous, recurrent enhance- ments of electron density at the intraperiod line (Peters, 1964). Freeze-fracture replicas reveal strands of 90-120 • diameter particles running parallel to the 0300M864/90 $03.00 4-.12 9 1990 Chapman and Hall Ltd.