Three-dimensional fine structure of cytoskeletal-membrane interactions at nodes of Ranvier

Abstract

Cytoskeleton-membrane-extracellular matrix interactions at the node of Ranvier were examined in both central and peripheral axons by combining three different methods for tissue preparation with three different electron microscopic techniques for imaging supramolecular structure. Conventional and three-dimensional high voltage electron microscopy of thin and semithick sections of tissues stained en bloc with ferric chloride revealed the presence of transcellular structures across the nodal gap traversing the paranodal glial-axonal junction. These structures penetrate both axonal and glial membranes and are further traced to the cortical axoplasm. This observation was verified by an examination of similar regions in rapidly-frozen freeze-substituted fresh axons. The filamentous nature of these structures, their focal attachment to the external true surface of the nodal and paranodal axolemma and their association with membrane particles were visualized in deep etch rotary-shadow replicas. At the node, both extracellular gap-crossing filaments and membrane-cytoskeletal linkers in the nodal axoplasm are joined to one of the prominent membrane particles of the nodal axolemma. At the paranodal axo-glial junction, the anchoring site of these membrane-cytoskeleton linkers are found on the linear arrays of 16 nm particles. Thus, cytoplasmic filaments and extracellular filaments or bridge structures are involved in the membrane-cytoskeletal interaction at the node and paranode. Some of these membrane particles are known to play a role in ionic conductances known to occur at this site. An additional role in cell adhesion or maintenance of the membrane specialization of this functionally important site of axolemma is now indicated.

Electron tomographic analysis of cytoskeletal cross-bridges in the paranodal region of the node of Ranvier in peripheral nerves

“…This group discovered that efficient association of neurofascin 155 with lipid raft domains is required for the maintenance of the paranodal axo-glial junction and without this junction, demyelination occurs in MS. The PJB complex was seen previously in images of deep-etched rotary-shadowed preparations and described as 16-nm "pegs" that line up in ordered arrays (Ellisman, 1979;Ichimura and Ellisman, 1991;Wiley and Ellisman, 1980;Wiley-Livingston and Ellisman, 1981). In our reconstructions, the PJBs were only occasionally observed as discrete pegs, but more often formed a continuous rod of density.…”

“…The glial margin wraps around the paranodal axolemma in a helical fashion. Both intracellular and intercellular bridging or junctional molecules are abundant in the paranodal and juxtaparanodal regions (Ichimura and Ellisman, 1991). Macromolecular junctional complexes, form between the axolemma to the spiral paranodal loops.…”

“…Microtubules are the "highways" of transport for materials moving rapidly within the axons of neurons (Li et al, 2004) and have a diameter of about 24 nm; they vary in length from several hundred nanometers to hundreds of micrometers in axons. Microfilaments, composed of F-actin, and found in the cytoplasm of the paranodal loops (Ichimura and Ellisman, 1991;Scherer, 1996;Trapp et al, 1989) measure about 7 nm in diameter and are likely also involved with transport. In addition, all three filament types seem to function as mechanical stabilizers of the cytoplasm to resist mechanical stress (Fuchs and Cleveland, 1998;Garcia and Cleveland, 2001).…”

“…Between the loops of the paranode and the axolemma in the extracellular matrix are linear arrays of heterogeneously sized particles of 8-16 nm in diameter that bridge this space (Ellisman, 1979;Wiley and Ellisman, 1980;Wiley-Livingston and Ellisman, 1981); we name these paranodal junctional bridges (PJB). They have also been called "septate junctions" or "septate-like junctions" (Ichimura and Ellisman, 1991;Sosinsky et al, 2005;Spiegel and Peles, 2002). These structures, which appear continuous, are comprised of heterologous macromolecules and appear to insert into both the glial and axonal plasma membranes.…”

Abnormal myelination in the spinal cord of PTPα-knockout mice

“…Well-defined, electron-dense, ladder-like partitions or septa, also called transverse bands, can be visualized at regular intervals in the narrow cleft between paranodal loops and the axolemma. 1,2 This structure also serves as a partial diffusion barrier to the periaxonal space to prevent the random lateral diffusion of membrane components. 3 Increasing evidence demonstrate that axoglial interactions at the paranodes regulate myelination.…”