Spatial heterogeneity of the axolemma of non-myelinated fibers in the optic disc of the adult rat. Freeze-fracture observations

Building sensory axons: Delivery and distribution of NaV1.7 channels and effects of inflammatory mediators

Sodium channel Na_V1.7 controls firing of nociceptors, and its role in human pain has been validated by genetic and functional studies. However, little is known about Na_V1.7 trafficking or membrane distribution along sensory axons, which can be a meter or more in length. We show here with single-molecule resolution the first live visualization of Na_V1.7 channels in dorsal root ganglia neurons, including long-distance microtubule-dependent vesicular transport in Rab6A-containing vesicles. We demonstrate nanoclusters that contain a median of 12.5 channels at the plasma membrane on axon termini. We also demonstrate that inflammatory mediators trigger an increase in the number of Na_V1.7-carrying vesicles per axon, a threefold increase in the median number of Na_V1.7 channels per vesicle and a ~50% increase in forward velocity. This remarkable enhancement of Na_V1.7 vesicular trafficking and surface delivery under conditions that mimic a disease state provides new insights into the contribution of Na_V1.7 to inflammatory pain.

Regional node-like membrane specializations in non-myelinated axons of rat retinal nerve fiber layer

The axons in the nerve fiber layer (NFL) of the adult rat retina were examined by transmission electron microscopy. NFL axons range in size from 0.12 to about 2.0 microm, with a peak at 0.3-0.4 microm. In addition to conventional small mitochondria in the NFL axons contain some large ones, which are similar to astrocytic gliosomes. Two types of regional axon membrane specialization are found in the NFL. One of these represents portions of the initial axon segments of retinal ganglion cells. Apart from features typical for initial axon segments in general, a corona of lamelliform, villous or blunt glial processes is always present. The glial processes originate from MUller cells. The other regional axon membrane specialization consists of patches of an electron-dense subaxolemmal undercoating with associated tufts of Miller cell processes. These patches cover a varying but always limited proportion of the axon circumference and their longitudinal extent varies between 0.5 and 5.0 microm. They are clearly distinct from the initial axon segment and from the initial heminode in the optic nerve. Similar undercoated patches in the optic disc axons are apposed by astrocytic processes. It is concluded that rat NFL axons represent an example of central non-myelinated axons with distinct regional membrane specializations, which have some structural characteristics in common with nodes of Ranvier.

Regional membrane heterogeneity in premyelinated CNS axons: factors influencing the binding of sterol-specific probes

Binding of the sterol-specific probe filipin to developing optic nerve axonal membrane is spatially heterogeneous prior to association of glial cells with the axons. Experiments were performed using different sterol binding probes (filipin, tomatin, and saponin), at different temperatures (4 degrees C, 23 degrees C, and 37 degrees C), after incubation in different ionic conditions (10 mM Ca2+, 10 mM EGTA, and 20 mM Mg2+), to examine factors that may be responsible for this membrane heterogeneity in rat optic nerve. The patchy pattern of filipin binding is apparent with each sterol-specific probe, even prior to glial ensheathment, and is retained when membrane fluidity is increased at higher temperatures. Increased Ca2+ concentration increased membrane stability, and increased Mg2+ reduced the patchiness of filipin binding. After tannic acid staining, regions of the cytoskeleton are seen associated with the membrane via filaments extending from microtubules to the membrane, preferentially in regions where filipin interaction with the membrane is inhibited. The non-uniform interaction of filipin with the axolemma suggests an underlying heterogeneity in the sterol composition and stability of the membrane. Heterogeneity of premyelinated axonal membrane may provide an important formative influence in the differentiation of axons to their mature morphology and function.

Filipin-cholesterol binding in CNS axons prior to myelination: evidence for microheterogeneity in premyelinated axolemma

The distribution of cholesterol in axonal membrane of developing rat optic nerves prior to myelination was studied by freeze-fracture cytochemistry. Binding of the cholesterol-specific probe, filipin, to the axolemma of premyelinated axons was heterogeneous; this suggests the presence of microdomains of axolemma with different membrane composition and/or cytoskeletal/extracellular matrix association. Although the reasons for this binding pattern have not yet been determined, heterogeneity occurs prior to association of glia with the axon, and may reflect regional differences in lipid/sterol composition of the axonal membrane bilayer, or distribution of membrane-associated cytoskeleton. The distribution of intramembranous particles was not obviously associated with the pattern of filipin binding in early developing axons, however, as might have been expected from the attending differences in fluidity of the membrane microdomains. Microheterogeneity in axonal membranes of developing axons could have an influence on several membrane properties, and may be associated with processes important for growth and differentiation of axons.

Axo-glial relations in the retina-optic nerve junction of the adult rat: freeze-fracture observations on axon membrane structure

The axolemmal ultrastructure of nerve fibres within the retina-optic nerve junction (ROJ) from adult rats was examined by freeze-fracture electron microscopy. In the juxtaocular (proximal) region of the ROJ, all fibres are unmyelinated. The axons generally have a membrane ultrastructure similar to that of retinal nerve fibre layer axons, with a high density of intramembranous particles (IMPs) on the P-fracture face and a low density of IMPs on the E-face. However, along some axons in this region of the ROJ, localized aggregations of E-face IMPs are observed. At levels of the ROJ closer to the optic nerve proper, the unmyelinated fibres enter a transition zone in which the axons acquire myelin sheaths. By the distal boundary of the transitional zone (optic nerve proper), virtually all fibres are myelinated. Within the transitional zone, conventional axo-glial associations and axolemmal ultrastructure is present at nodes of Ranvier. In addition, atypical axo-glial relationships and atypical nodal segments are observed in this region. At some nodes, an isolated oligodendroglial process, the axolemma usually displays a paranodal-like ultrastructure. Finger-like oligodendroglial processes were also observed in association with non-nodal unmyelinated axon membrane. At these sites of association, the axon membrane tends to be indented and may have a paranodal-like morphology. Nodal axolemma may exhibit several atypical forms in the transition zone. At some nodes, the nodal axolemma has a low density of E-face particles. Also, nodes of extended linear length (approximately 2 micron) exhibit a lower-than-normal density of P-face IMPs. At heminodes, the axolemma immediately adjacent to the terminal loops lacks the usual nodal characteristics of high IMP density and high percentage of large particles. The results show that aberrant axo-glial associations accompanied by unusual ultrastructural characteristics of the axolemma are present in the ROJ of normal adult rats.

Axo-glial relations in the retina-optic nerve junction of the adult rat: electron-microscopic observations

The retina-optic nerve junction (ROJ) was examined by electron microscopy in adult rats, with particular emphasis on the unmyelinated-myelinated nerve fibre transition. Both single sections and serial sections were used. The non-retinal part of the ROJ is covered by an extensively folded glia limitans, facing the choroidea, sclera and pia mater. The blood vessels within the ROJ follow a transverse course and are surrounded by unusually wide perivascular spaces with a glia limitans-like outer delimitation. The endothelial cells exhibit numerous pinocytotic vesicles on their abluminal aspect. In the unmyelinated part of the ROJ the axons are embedded in an extensive meshwork of fibrous astrocytic processes. Some unmyelinated axons exhibit patches of axolemmal undercoating with externally associated astrocytic processes. Typical oligodendrocytes are not found, but a few small dark glial cells of unknown identity can be observed. Atypical ensheathment and myelination of axons at this level by ectopic Schwann cells occurred in one case. In the transition segment of the ROJ a pattern similar to that along dysmyelinated axons is observed, including aberrant axo-glial contacts, unusually thin and short myelin sheaths, intercalated unmyelinated segments, distorted myelin termination regions, bizarre paranodes and myelin termination regions without associated nodally differentiated axolemma. Neither sheath length nor number of myelin lamellae is related to axon diameter in the transition region. Axon diameter tends to be somewhat larger at myelinated than unmyelinated levels of the same axon. We suggest that the unusual axo-glial relations in this region are due to a deficient proliferation and differentiation of oligodendroglial cells, and that the pattern of glial ensheathment in the ROJ might be a consequence of the locally deficient blood-brain barrier.

Membrane specialization and axo-glial association in the rat retinal nerve fibre layer: freeze-fracture observations

The ultrastructure of non-myelinated ganglion cell axolemma within the retinal nerve fibre layer of adult rats was examined by thin section and freeze-fracture electron microscopy. Most of the axolemma within the nerve fibre layer does not exhibit any membrane specializations; intramembranous particles are partitioned with a density of approximately 1750 microns-2 on the P-fracture face and approximately 225 microns-2 on the E-face of the non-specialized axolemma. The nerve fibres also exhibit specialized foci of axolemma, at which the axons are abutted by the tips of blunt, radially oriented processes from Müller cells. At such sites of axo-glial association, an electron-dense undercoating is present beneath the axon membrane. Freeze-fracture analysis revealed a substantial increase in the density of E-face particles (greater than 500 microns-2) at sites of association between the tips of blunt glial processes and the axon. These findings demonstrate that non-myelinated axolemma of the retinal nerve fibre layer can exhibit spatial heterogeneity, with patches of node-like membrane at regions of specialized association with glial cell processes. On the basis of their morphological similarity to nodes of Ranvier, we suggest that these specialized axon regions represent foci of inward ionic current.

Freeze-fracture ultrastructure of developing and adult non-myelinated ganglion cell axolemma in the retinal nerve fibre layer

The ultrastructure of non-myelinated ganglion cell axolemma in the retinal nerve fibre layer from developing and adult rats was examined by freeze-fracture electron microscopy. The axolemma of fibres from neonatal (2-8 days) rats had a moderate density (510-556/micrometers 2) of intramembranous particles (IMPs) on the P-fracture face, while there was a low density (101-146/micrometers 2) of particles on the E-fracture face. Particle density on the P-face increased with development, such that by 28 days the density of IMPs was 1281/micrometers 2. Adult fibres had a high (1741/micrometers 2) density of particles on the P-face. On the E-face, the density of IMPs did not change substantially throughout development, and remained less than 225/micrometers 2 at all ages. Mean particle diameters were compiled for P- and E-fracture faces at the various developmental ages and were greatest in fibres from adult animals. P-face particle density of non-myelinated axons in the retinal nerve fibre layer changed at approximately the time that myelination occurred in distal (optic nerve) segments of these axons. The alteration in membrane structure during development of non-myelinated axons in the retinal nerve fibre layer suggests that conduction properties may also change with development.