Occluding-like junctions at mesaxons of central myelin in Anolis carolinensis are not 'tight'. A freeze-fracture-protein tracer analysis

Claudin-11 Tight Junctions in Myelin Are a Barrier to Diffusion and Lack Strong Adhesive Properties

The radial component is a network of interlamellar tight junctions (TJs) unique to central nervous system myelin. Ablation of claudin-11, a TJ protein, results in the absence of the radial component and compromises the passive electrical properties of myelin. Although TJs are known to regulate paracellular diffusion, this barrier function has not been directly demonstrated for the radial component, and some evidence suggests that the radial component may also mediate adhesion between myelin membranes. To investigate the physical properties of claudin-11 TJs, we compared fresh, unfixed Claudin 11-null and control nerves using x-ray and neutron diffraction. In Claudin 11-null tissue, we detected no changes in myelin structure, stability, or membrane interactions, which argues against the notion that myelin TJs exhibit significant adhesive properties. Moreover, our osmotic stressing and D2O-H2O exchange experiments demonstrate that myelin lacking claudin-11 is more permeable to water and small osmolytes. Thus, our data indicate that the radial component serves primarily as a diffusion barrier and elucidate the mechanism by which TJs govern myelin function.

Blood barriers of the insect

The blood-brain barrier (BBB) ensures brain function in vertebrates and insects by maintaining ionic integrity of the neuronal bathing fluid. Without this barrier, paralysis and death ensue. The structural analogs of the BBB are occlusive (pleated-sheet) septate and tight junctions between perineurial cells, glia and perineurial cells, and possibly between glia. Immature Diptera have such septate junctions (without tight junctions) while both junctional types are found in the imago. Genetic and molecular biology of these junctions are discussed, namely tight (occludin) and pleated-sheet septate (neurexin IV). A temporal succession of blood barriers form in immature Diptera. The first barrier forms in the peripheral nervous system where pleated-sheet septate junctions bond cells of the nascent (embryonic) chordotonal organs in early neurogenesis. At the end of embryonic life, the central nervous system is fully vested with a blood-brain barrier. A blood-eye barrier arises in early pupal life. Future prospects in blood-barrier research are discussed.

Radial component of CNS myelin: junctional subunit structure and supramolecular assembly

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 approximately 40 A diameter) at the extracellular apposition which is linked by approximately 15 A diameter filaments extending through the bilayer to approximately 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.

New X-ray spacings from central myelinated tissue

New X-ray reflections have been detected from myelinated tissue of the mammalian C.N.S. Diffraction patterns from unfixed optic nerves of mouse, rat, guinea-pig and rabbit, and unfixed corpus callosum of calf were recorded during X-ray exposures of 1-6 days. The equatorial pattern was typical of lamellar myelin of the C.N.S.; however, the meridional pattern showed new features: a strong, sharp intensity maximum at 30.4 +/- 0.4 A (N = 11) spacing, and a weak, diffuse ring centered at 11.0 +/- 0.5 A (N = 5) spacing. The 30.4 A reflection was neither strictly arced like the equatorial reflections from lamellar myelin nor fully horizontal. Since the 30.4 A meridional reflection was not observed in patterns from myelinated nerve of the P.N.S., we suggest that this diffraction comes from the radial component, or interlamellar tight junctions, which is unique to mammalian C.N.S. myelinated tissue. The diffuse ring at 11.0 A probably comes from myelin protein.

Opening of the blood-brain barrier in Anolis carolinensis. A high voltage electron microscope protein tracer study

The tight junctions between endothelial cells of capillaries in the forebrain of Anolis carolinensis are a common component of the structural basis for the blood-brain barrier in this reptile. The complexity of these junctions, which is apparent in platinum replicas of freeze-fractured brain capillaries, is unchanged by treatments designed to render the blood-brain barrier of these lizards leaky to horseradish peroxidase. An alternative route for extravasation of horseradish peroxidase, following injection of chameleons with 2.7 mg of D-glucose to render their brain capillaries leaky, is a system of transient cytoplasmic vesicles and vesiculo-tubular channels whose lumina may be open to the luminal or abluminal surface (or both) of the capillary endothelial cell. High voltage electron microscopy (HVEM) of 0.25 and 0.5 micron thick plastic sections of experimental brain capillary endothelium confirmed the existence of vesiculo-tubular conduits. These channels display a sigmoid morphology and are situated in the cytoplasm at angles oblique to the luminal and abluminal surfaces of the endothelium. Occasionally, the channels spanned the entire endothelial wall of the capillary, and in such cases, appeared to connect the lumen with the brain extracellular compartment. HVEM images (including stereo pairs) of the vesiculo-tubular channels show them to have a scalloped, irregular profile consistent with their proposed formation by fusion of pinocytotic vesicles. Also, HVEM examinations of experimental capillaries from peroxidase-treated lizards reveal massive quantities of dense reaction product in cytoplasmic vesicles and vesiculo-tubular membrane compartments of the endothelium, and the complex pleomorphism exhibited by these structures. Observations made in the present study suggest that as a consequence of severe hyperglycaemia, transendothelial channels form in the brain capillaries by fusion of pinocytotic vesicles generated by accelerated pinocytosis at the luminal surface of the endothelium, and subsequently serve as open routes for massive floods of tracer into the central nervous system.

The gastric mucosal barrier: structure of intercellular junctions in the dog

The canine gastric mucosa consists of two regions, the surface mucous cells and gland area cells including parietal, chief, and mucous-containing cells. We have used quantitative freeze-fracture methods in conjunction with thin-section extracellular tracers to document and correlate tight junction morphology with epithelial permeability. The number of strands in the tight junction complexes of the surface cells and gland cells is the same, but differences in strand arrangement exist. The surface cells have an interwoven tight junction configuration which is impermeable to extracellular tracers. The gland cell junctions are regularly arranged and often permeable to extracellular lanthanum. The possibility that the observed difference in permeability between the tight junctions of the surface mucous cells and those of the gland cells is related to their structural configuration is discussed.

The periaxonal space in an experimental model of neuropathy: the mutant Syrian hamster with hindleg paralysis

The periaxonal space of peripheral myelinated axons was studied in the mutant Syrian hamster with hindleg paralysis, an experimental model of neuropathy. Despite pronounced alterations of the axon and the surrounding sheath, sometimes leading to demyelination, the periaxonal space showed remarkable resistance to change in most instances. When the space was widened as the result of the infiltration of extracellular fluid, the axon was found at the periphery of the enlarged inner perimeter of the sheath. Even under these extreme conditions the axon maintained close to normal distance from the inner collar of cytoplasm. The significance of these findings with regard to both the normal anatomical relationship within the sheath and to the mechanisms of demyelination are discussed.

Interlamellar tight junctions of central myelin. I. Developmental mechanisms during myelogenesis

The process of myelination in the central nervous system (CNS) of the rat (optic nerve) was studied with the freeze-fracturing technique and ultrathin sectioning to obtain information on the developmental mechanisms of interlamellar tight junctions. Using a tilting cartridge for analysis of thin sections, it could be demonstrated that during the initial phase of wrapping a tight junction formation develops between the joining tips of the oligodendrocytic process. In tannic acid-stained samples these junctions appear as typical quintuple-layered membrane fusions, while in potassium permanganate-stained material membrane thickenings between the apposing glial tips are prevalent. The latter configuration represents the characteristic feature of the so-called radial component of central myelin. Using the freeze-fracturing technique, a biphasic mode of the myelinic tight junction assembly was detected. It is suggested that tight junctions represent a prerequisite of the myelination process.

Interlamellar tight junctions of central myelin. II. A freeze fracture and cytochemical study on their arrangement and composition

The interlamellar tight junctions (ITJ) of central myelin (white matter from the parietal lobe and the medulla oblongata of the rat) were analyzed electron microscopically, making use of a wide range of different preparatory techniques. Freeze-fracture observations indicate that the ITJ are composed of rows of particulate subunits in glutaraldehyde-fixed or formaldehyde-fixed material, and in the unfixed state. The particulate subunits of the ITJ are preferentially associated with the protoplasmic (P) face in the aldehyde-fixed state, and no shift in the binding characteristics of the particles was observed after omission of aldehyde fixation. Tracer studies in conjunction with the dissociated appearance of the junctional globules suggest that the ITJ represent a leaky type of zonula occludens. It is assumed that the ITJ particles represent an "integral-type protein" that preferentially serves as a mechanical device maintaining the structural integrity of the central myelin sheath. By means of cytochemical experiments, the proteinaceous character of the ITJ subunits is established. An attempt is made, based on results from lipid extraction and protein digestion, to define certain cytochemical parameters of the ITJ proteins and to compare them with the current collection of chemically identified proteins of central myelin.