High-performance serial block-face SEM of nonconductive biological samples enabled by focal gas injection-based charge compensation.

A longstanding limitation of imaging with serial block-face scanning electron microscopy is specimen surface charging. This charging is largely due to the difficulties in making biological specimens and the resins in which they are embedded sufficiently conductive. Local accumulation of charge on the specimen surface can result in poor image quality and distortions. Even minor charging can lead to misalignments between sequential images of the block-face due to image jitter. Typically, variable-pressure SEM is used to reduce specimen charging, but this results in a significant reduction to spatial resolution, signal-to-noise ratio and overall image quality. Here we show the development and application of a simple system that effectively mitigates specimen charging by using focal gas injection of nitrogen over the sample block-face during imaging. A standard gas injection valve is paired with a precisely positioned but retractable application nozzle, which is mechanically coupled to the reciprocating action of the serial block-face ultramicrotome. This system enables the application of nitrogen gas precisely over the block-face during imaging while allowing the specimen chamber to be maintained under high vacuum to maximise achievable SEM image resolution. The action of the ultramicrotome drives the nozzle retraction, automatically moving it away from the specimen area during the cutting cycle of the knife. The device described was added to a Gatan 3View system with minimal modifications, allowing high-resolution block-face imaging of even the most charge prone of epoxy-embedded biological samples.

Automated microscopy system for mosaic acquisition and processing.

An automatic mosaic acquisition and processing system for a multiphoton microscope is described for imaging large expanses of biological specimens at or near the resolution limit of light microscopy. In a mosaic, a larger image is created from a series of smaller images individually acquired systematically across a specimen. Mosaics allow wide-field views of biological specimens to be acquired without sacrificing resolution, providing detailed views of biological specimens within context. The system is composed of a fast-scanning, multiphoton, confocal microscope fitted with a motorized, high-precision stage and custom-developed software programs for automatic image acquisition, image normalization, image alignment and stitching. Our current capabilities allow us to acquire data sets comprised of thousands to tens of thousands of individual images per mosaic. The large number of individual images involved in creating a single mosaic necessitated software development to automate both the mosaic acquisition and processing steps. In this report, we describe the methods and challenges involved in the routine creation of very large scale mosaics from brain tissue labelled with multiple fluorescent probes.

PKA, PKC, and AKAP localization in and around the neuromuscular junction.

BACKGROUND: One mechanism that directs the action of the second messengers, cAMP and diacylglycerol, is the compartmentalization of protein kinase A (PKA) and protein kinase C (PKC). A-kinase anchoring proteins (AKAPs) can recruit both enzymes to specific subcellular locations via interactions with the various isoforms of each family of kinases. We found previously that a new class of AKAPs, dual-specific AKAPs, denoted D-AKAP1 and D-AKAP2, bind to RIalpha in addition to the RII subunits. RESULTS: Immunohistochemistry and confocal microscopy were used here to determine that D-AKAP1 colocalizes with RIalpha at the postsynaptic membrane of the vertebrate neuromuscular junction (NMJ) and the adjacent muscle, but not in the presynaptic region. The labeling pattern for RIalpha and D-AKAP1 overlapped with mitochondrial staining in the muscle fibers, consistent with our previous work showing D-AKAP1 association with mitochondria in cultured cells. The immunoreactivity of D-AKAP2 was distinct from that of D-AKAP1. We also report here that even though the PKA type II subunits (RIIalpha and RIIbeta) are localized at the NMJ, their patterns are distinctive and differ from the other R and D-AKAP patterns examined. PKCbeta appeared to colocalize with the AKAP, gravin, at the postsynaptic membrane. CONCLUSIONS: The kinases and AKAPs investigated have distinct patterns of colocalization, which suggest a complex arrangement of signaling micro-environments. Because the labeling patterns for RIalpha and D-AKAP 1 are similar in the muscle fibers and at the postsynaptic membrane, it may be that this AKAP anchors RIalpha in these regions. Likewise, gravin may be an anchor of PKCbeta at the NMJ.

Phalloidin-eosin followed by photo-oxidation: a novel method for localizing F-actin at the light and electron microscopic levels.

We describe a novel high-resolution method to detect F-actin at the light and electron microscopic levels through the use of the actin-binding protein phalloidin conjugated to the fluorophore eosin, followed by photo-oxidation of diaminobenzidine. This method possesses several key advantages over antibody-based labeling and structural methods. First, phalloidin binding to F-actin can tolerate relatively high concentrations of glutaraldehyde (up to 1%) in the primary fixative, resulting in good ultrastructural preservation. Second, because both eosin and phalloidin are relatively small molecules, considerable penetration of reagents into aldehyde-fixed tissue was obtained without any permeabilization steps, allowing 3D reconstructions at the electron microscopic level. By employing a secondary fixation with tannic acid combined with low pH osmication, conditions known to stabilize actin filaments during preparation for electron microscopy, we were able to visualize individual actin filaments in some structures. Finally, we show that fluorescent phalloidin can be directly injected into neurons to label actin-rich structures such as dendritic spines. These results suggest that the fluorescent phalloidin is an excellent tool for the study of actin networks at high resolution.

Selective localization of high concentrations of F-actin in subpopulations of dendritic spines in rat central nervous system: a three-dimensional electron microscopic study.

Dendritic spines differ considerably in their size, shape, and internal organization between brain regions. We examined the actin cytoskeleton in dendritic spines in hippocampus (areas CA1, CA3, and dentate gyrus), neostriatum, and cerebellum at both light and electron microscopic levels by using a novel high-resolution photoconversion method based in the high affinity of phalloidin for filamentous (F)-actin. In all brain regions, labeling was strongest in the heads of dendritic spines, diminishing in the spine neck. The number of labeled spines varied by region. Compared with the cerebellar molecular layer and area CA3, where nearly every dendritic spine was labeled, less than half the spines were labeled in CA1, dentate gyrus, and neostriatum. Serial section reconstructions of spines in these areas indicated that phalloidin labeling was restricted to the largest and most morphologically diverse dendritic spines. The resolution of the photoconversion technique allowed us to examine the localization and organization of actin filaments in the spine. The most intense staining for actin was found in the postsynaptic density and associated with the spines internal membrane system. In mushroom-shaped spines, F-actin staining was particularly strong between the lamellae of the spine apparatus. Three-dimensional reconstruction of labeled spines by using electron tomography showed that the labeled dense material was in continuity with the postsynaptic density. These results highlight differences in the actin cytoskeleton between different spine populations and provide novel information on the organization of the actin cytoskeleton in vivo.

A specific activation of the mitogen-activated protein kinase kinase 1 (MEK1) is required for Golgi fragmentation during mitosis.

Incubation of permeabilized cells with mitotic extracts results in extensive fragmentation of the pericentriolarly organized stacks of cisternae. The fragmented Golgi membranes are subsequently dispersed from the pericentriolar region. We have shown previously that this process requires the cytosolic protein mitogen-activated protein kinase kinase 1 (MEK1). Extracellular signal-regulated kinase (ERK) 1 and ERK2, the known downstream targets of MEK1, are not required for this fragmentation (Acharya et al. 1998). We now provide evidence that MEK1 is specifically phosphorylated during mitosis. The mitotically phosphorylated MEK1, upon partial proteolysis with trypsin, generates a different peptide population compared with interphase MEK1. MEK1 cleaved with the lethal factor of the anthrax toxin can still be activated by its upstream mitotic kinases, and this form is fully active in the Golgi fragmentation process. We believe that the mitotic phosphorylation induces a change in the conformation of MEK1 and that this form of MEK1 recognizes Golgi membranes as a target compartment. Immunoelectron microscopy analysis reveals that treatment of permeabilized normal rat kidney (NRK) cells with mitotic extracts, treated with or without lethal factor, converts stacks of pericentriolar Golgi membranes into smaller fragments composed predominantly of tubuloreticular elements. These fragments are similar in distribution, morphology, and size to the fragments observed in the prometaphase/metaphase stage of the cell cycle in vivo.

Localization of a passively transferred human recombinant monoclonal antibody to herpes simplex virus glycoprotein D to infected nerve fibers and sensory neurons in vivo.

A human recombinant monoclonal antibody to herpes simplex virus (HSV) glycoprotein D labeled with the fluorescent dye Cy5 was administered to mice infected in the cornea with HSV type 1 (HSV-1). The distribution of such antibody in the corneas and trigeminal ganglia of the mice was then investigated by confocal microscopy. The antibody was detected on HSV-infected nerve fibers in the cornea--identified by colocalization with HSV antigens and the neuritic markers neurofilament, GAP-43, synapsin-1, and CNPase--and on the perikarya of sensory neurons in the HSV-1-infected neurons in ipsilateral trigeminal ganglia. Antibodies have been shown to be effective against many neurotropic viruses, often in the absence of obvious cell damage. Observations from experimental HSV infections suggest that antibodies could act in part by interfering with virus expression in the ganglia and/or with axonal spread. The present results provide morphological evidence of the localization of antiviral antibodies at anatomical sites relevant to such putative antibody-mediated protective actions and suggest that viral glycoproteins are accessible to antibodies on infected nerve fibers and sensory neurons.

Identification and localization of Ca(2+)-activated K+ channels in rat sciatic nerve.

To understand the physiology of Schwann cells and myelinated nerve, we have been engaged in identifying K+ channels in sciatic nerve and determining their subcellular localization. In the present study, we examined the slo family of Ca(2+)-activated K+ channels, a class of channel that had not previously been identified in myelinated nerve. We have determined that these channels are indeed expressed in peripheral nerve, and have cloned rat homologues of slo that are more than 95% identical to the murine slo. We found that sciatic nerve RNA contained numerous alternatively spliced variants of the slo homologue, as has been seen in other tissues. We raised a polyclonal antibody against a peptide from the carboxyl terminal of the channels. Immunocytochemistry revealed that the channel proteins are in Schwann cells and are associated with canaliculi that run along the outer surface of the cells. They are also relatively concentrated near the node of Ranvier in the Schwann cell outer membrane. This staining pattern is quite similar to what we previously reported for the voltage-dependent K+ channel Kv 1.5. We did not observe staining of axons or connective tissue in the nerve and so it seems likely that most or all of the splicing variants are located in the Schwann cells. The localization of these channels also suggests that they may participate in maintaining the resting potential of the Schwann cells during K+ buffering.

NH2-Terminal targeting motifs direct dual specificity A-kinase-anchoring protein 1 (D-AKAP1) to either mitochondria or endoplasmic reticulum.

Subcellular localization directed by specific targeting motifs is an emerging theme for regulating signal transduction pathways. For cAMP-dependent protein kinase (PKA), this is achieved primarily by its association with A-kinase-anchoring proteins (AKAPs). Dual specificity AKAP1, (D-AKAP1) binds to both type I and type II regulatory subunits and has two NH2-terminal (N0 and N1) and two COOH-terminal (C1 and C2) splice variants (. J. Biol. Chem. 272:8057). Here we report that the splice variants of D-AKAP1 are expressed in a tissue-specific manner with the NH2-terminal motifs serving as switches to localize D-AKAP1 at different sites. Northern blots showed that the N1 splice is expressed primarily in liver, while the C1 splice is predominant in testis. The C2 splice shows a general expression pattern. Microinjecting expression constructs of D-AKAP1(N0) epitope-tagged at either the NH2 or the COOH terminus showed their localization to the mitochondria based on immunocytochemistry. Deletion of N0(1-30) abolished mitochondrial targeting while N0(1-30)-GFP localized to mitochondria. Residues 1-30 of N0 are therefore necessary and sufficient for mitochondria targeting. Addition of the 33 residues of N1 targets D-AKAP1 to the ER and residues 1-63 fused to GFP are necessary and sufficient for ER targeting. Residues 14-33 of N1 are especially important for targeting to ER; however, residues 1-33 alone fused to GFP gave a diffuse distribution. N1(14-33) thus serves two functions: (a) it suppresses the mitochondrial-targeting motif located within residues 1-30 of N0 and (b) it exposes an ER-targeting motif that is at least partially contained within the N0(1-30) motif. This represents the first example of a differentially targeted AKAP and adds an additional level of complexity to the PKA signaling network.

The distribution of ganglioside-like moieties in peripheral nerves.

GM1 ganglioside has been implicated as a target of immune attack in some diseases of the peripheral nervous system. Anti-GM1 ganglioside antibodies are associated with certain acquired immune-mediated neuropathies. It is not clear how anti-GM1 antibodies cause nerve dysfunction and injury; however, sodium and/or potassium ion channel dysfunction at the node of Ranvier has been implicated. To gain insight into the pathogenesis of these neuropathies, we examined the distribution of GM1 ganglioside and Gal(beta1-3)GalNAc moieties in nerve fibres and their relationship to voltage-gated sodium and potassium (Kv1.1, 1.5) channels at the nodes of Ranvier in peripheral nerves from human, rat and dystrophic mice. Gal(beta1-3)GalNAc moieties were localized via the binding of cholera toxin and peanut agglutinin. As a control for the specificity of these findings, we compared the distribution of GM1 moieties to that of the ganglioside GT1b. Our study provides definitive evidence for the presence of Gal(beta1-3)GalNAc bearing moieties on the axolemmal surface of mature myelinated fibres and on Schwann cells. Gal(beta1-3)GalNAc binding sites did not have an obligatory co-localization with voltage-gated sodium channels or the potassium ion channels Kv1.1 and Kv1.5 and are thus not likely carried by these ion channels. In contrast with Gal(beta1-3)GalNAc, GT1b-like moieties are restricted to the axolemma.

The perinucleolar compartment and transcription.

The perinucleolar compartment (PNC) is a unique nuclear structure localized at the periphery of the nucleolus. Several small RNAs transcribed by RNA polymerase III and two hnRNP proteins have been localized in the PNC (Ghetti, A., S. Piñol-Roma, W.M. Michael, C. Morandi, and G. Dreyfuss. 1992. Nucleic Acids Res. 20:3671-3678; Matera, A.G., M.R. Frey, K. Margelot, and S.L. Wolin. 1995. J. Cell Biol. 129:1181- 1193; Timchenko, L.T., J.W. Miller, N.A. Timchenko, D.R. DeVore, K.V. Datar, L. Lin, R. Roberts, C.T. Caskey, and M.S. Swanson. 1996. Nucleic Acids Res. 24: 4407-4414; Huang, S., T. Deerinck, M.H. Ellisman, and D.L. Spector. 1997. J. Cell Biol. 137:965-974). In this report, we show that the PNC incorporates Br-UTP and FITC-conjugated CTP within 5 min of pulse labeling. Selective inhibition of RNA polymerase I does not appreciably affect the nucleotide incorporation in the PNC. Inhibition of all RNA polymerases by actinomycin D blocks the incorporation completely, suggesting that Br-UTP incorporation in the PNC is due to transcription by RNA polymerases II and/or III. Treatment of cells with an RNA polymerase II and III inhibitor induces a significant reorganization of the PNC. In addition, double labeling experiments showed that poly(A) RNA and some of the factors required for pre-mRNA processing were localized in the PNC in addition to being distributed in their previously characterized nucleoplasmic domains. Fluorescence recovery after photobleaching (FRAP) analysis revealed a rapid turnover of polypyrimidine tract binding protein within the PNC, demonstrating the dynamic nature of the structure. Together, these findings suggest that the PNC is a functional compartment involved in RNA metabolism in the cell nucleus.

Electron tomography of mitochondria from brown adipocytes reveals crista junctions.

Electron microscope tomography was used to examine the membrane topology of brown adipose tissue (BAT) mitochondria prepared by cryofixation or chemical fixation techniques. These mitochondria contain an uncoupling protein which results in the conversion of energy from electron transport into heat. The three-dimensional reconstructions of BAT mitochondria provided a view of the inner mitochondrial membrane different in important features from descriptions found in the literature. The work reported here provides new insight into BAT mitochondria architecture by identifying crista junctions, including multiple junctions connecting a crista to the same side of the inner boundary membrane, in a class of mitochondria that have no tubular cristae, but only lamellar cristae. Crista junctions were defined previously as the tubular membranes of relatively uniform diameter that connect a crista membrane with the inner boundary membrane. We have also found that the cristae architecture of cryofixed mitochondria, including crista junctions, is similar to that found in chemically fixed mitochondria, suggesting that this architecture is not a fixation artifact. The stacks of lamellar cristae extended through more of the BAT mitochondrial volume than did the cristae we observed in neuronal mitochondria. Hence, the inner membrane surface area was larger in the former. In chemically fixed mitochondria, contact sites were easily visualized because the outer and inner boundary membranes were separated by an 8 nm space. However, in cryofixed mitochondria almost all the outer membrane was observed to be in close contact with the inner boundary membrane.

Myosin I is associated with zymogen granule membranes in the rat pancreatic acinar cell.

BACKGROUND & AIMS: The mechanisms whereby intracellular messengers mediate zymogen granule transport and exocytosis in the pancreatic acinar cell are not well defined. Electron microscopy has shown a periluminal network of actin in the acinar cell, suggesting a role for actin and myosin in the transport process. The possible involvement of two types of myosin in the secretory process was investigated, and their distribution in acinar cells was determined. METHODS: Antibodies specific to myosin I or to myosin II were used for immunocytochemistry and Western blot analysis. Ultrastructural studies were also performed. RESULTS: Western blot analysis showed that myosin I and myosin II were present in total pancreatic homogenate but that only myosin I was present on isolated zymogen granules and their membranes. By immunocytochemistry, myosin I was shown in the apical aspect of acinar cells colocalized with glycoprotein 2, a marker for zymogen granules, and actin. By immunocytochemistry, myosin I was also localized on isolated zymogen granules. CONCLUSIONS: The immunolocalization of myosin I to zymogen granule membranes and its close association with periluminal actin suggest that myosin I plays a direct role in the process of transport and exocytosis of zymogen granules in the pancreatic acinar cell.

Clustering of voltage-sensitive sodium channels on axons is independent of direct Schwann cell contact in the dystrophic mouse.

The distribution of voltage-sensitive sodium channels on axons in the dorsal and ventral spinal roots of the dystrophic mouse 129/ReJ-Lama2dy was determined via immunocytochemistry. In these nerves there are regions in which Schwann cells fail to proliferate and myelinate axons in a normal manner, leaving bundles of closely packed large-diameter amyelinated axons. We have identified discrete and focal concentrations of sodium channel immunoreactivity on these axons by both confocal immunofluorescence and immunoelectron microscopy, using a peptide-derived polyclonal antibody. In addition, simultaneous labeling with an antibody recognizing neuronal-specific ankyrinG revealed a distinct colocalization with the sodium channels on both normal and amyelinated axons. The presence of patches of sodium channels along with their anchoring protein on amyelinated axons in the absence of intervening Schwann cells demonstrates that axons can form and maintain independently these initial aggregations. This confirms that direct contact between Schwann cell and axon is not required for the formation of sodium channel patches of nodal dimensions and density. Furthermore, this strongly suggests that local transfer of sodium channels from Schwann cells to axons is not required for this process.

The dynamic organization of the perinucleolar compartment in the cell nucleus.

The perinucleolar compartment (PNC) is a unique nuclear structure preferentially localized at the periphery of the nucleolus. Several small RNAs transcribed by RNA polymerase III (e.g., the Y RNAs, MRP RNA, and RNase P H1 RNA) and the polypyrimidine tract binding protein (PTB; hnRNP I) have thus far been identified in the PNC (Ghetti, A., S. PinolRoma, W.M. Michael, C. Morandi, and G. Dreyfuss. 1992. Nucleic Acids Res. 20:3671-3678; Matera, A.G., M.R. Frey, K. Margelot, and S.L. Wolin. 1995. J. Cell Biol. 129:1181-1193; Lee, B., A.G. Matera, D.C. Ward, and J. Craft. 1996. Proc. Natl. Acad. Sci. USA. 93: 11471-11476). In this report, we have further characterized this structure in both fixed and living cells. Detection of the PNC in a large number of human cancer and normal cells showed that PNCs are much more prevalent in cancer cells. Analysis through the cell cycle using immunolabeling with a monoclonal antibody, SH54, specifically recognizing PTB, demonstrated that the PNC dissociates at the beginning of mitosis and reforms at late telophase in the daughter nuclei. To visualize the PNC in living cells, a fusion protein between PTB and green fluorescent protein (GFP) was generated. Time lapse studies revealed that the size and shape of the PNC is dynamic over time. In addition, electron microscopic examination in optimally fixed cells revealed that the PNC is composed of multiple strands, each measuring approximately 80-180 nm diam. Some of the strands are in direct contact with the surface of the nucleolus. Furthermore, analysis of the sequence requirement for targeting PTB to the PNC using a series of deletion mutants of the GFP-PTB fusion protein showed that at least three RRMs at either the COOH or NH2 terminus are required for the fusion protein to be targeted to the PNC. This finding suggests that RNA binding may be necessary for PTB to be localized in the PNC.

Visualization of the peroxisomal compartment in living mammalian cells: dynamic behavior and association with microtubules.

Peroxisomes in living CV1 cells were visualized by targeting the green fluorescent protein (GFP) to this subcellular compartment through the addition of a COOH-terminal peroxisomal targeting signal 1 (GFP-PTS1). The organelle dynamics were examined and analyzed using time-lapse confocal laser scanning microscopy. Two types of movement could be distinguished: a relatively slow, random, vibration-like movement displayed by the majority (approximately 95%) of the peroxisomes, and a saltatory, fast directional movement displayed by a small subset (approximately 5%) of the peroxisomes. In the latter instance, peak velocities up to 0.75 micron/s and sustained directional velocities up to 0.45 micron/s over 11.5 microns were recorded. Only the directional type of motion appeared to be energy dependent, whereas the vibrational movement continued even after the cells were depleted of energy. Treatment of cells, transiently expressing GFP-PTS1, with microtubule-destabilizing agents such as nocodazole, vinblastine, and demecolcine clearly altered peroxisome morphology and subcellular distribution and blocked the directional movement. In contrast, the microtubule-stabilizing compound paclitaxel, or the microfilament-destabilizing drugs cytochalasin B or D, did not exert these effects. High resolution confocal analysis of cells expressing GFP-PTS1 and stained with anti-tubulin antibodies revealed that many peroxisomes were associated with microtubules. The GFP-PTS1-labeled peroxisomes were found to distribute themselves in a stochastic, rather than ordered, manner to daughter cells at the time of mitosis.

Differential distribution and subcellular localization of ryanodine receptor isoforms in the chicken cerebellum during development.

The distribution of ryanodine receptor (RyR) isoforms was examined using isoform-specific monoclonal antibodies in the developing chicken brain, from E18 through adulthood, using light and electron microscopic immunocytochemistry. Monoclonal antibody 110F is specific for the alpha-skeletal muscle form of RyR, while monoclonal antibody 110E recognizes both the beta-skeletal muscle and cardiac isoforms, but does not distinguish between the two. Significant differences in the distribution of the alpha- and beta/cardiac forms were observed. Labeling for the alpha-form was restricted to cerebellar Purkinje neurons while the beta/cardiac form was observed in neurons throughout the brain. A major finding was the presence of labeling for the beta/cardiac in presynaptic terminals of the parallel fibers in the molecular layer and the mossy fiber terminals in the granular layer glomeruli in late development and during adulthood. Labeling for the beta/cardiac, but not the alpha-form, underwent a major redistribution in the cerebellum during the course of development. At 1 day of age, beta/cardiac labeling was present mainly in Purkinje neurons. From 1 day to 4 weeks, immunolabeling for the beta/cardiac form gradually disappeared from Purkinje neurons, but increased in granule cells. Within the molecular layer, the labeling pattern changed from being primarily within Purkinje dendrites to a more diffuse pattern. Electron microscopic examination of the cerebellar molecular layer of 2-week-old chicks revealed that beta/cardiac-labeling was mainly present in the axons and presynaptic processes of the parallel fibers. No developmental changes were observed in other brain regions. This study represents the first demonstration of ryanodine receptor immunoreactivity in presynaptic boutons and suggests that the ryanodine receptor may modulate neurotransmitter release through local regulation of intracellular calcium in the parallel fiber synapse.

Translocation of RNA granules in living neurons.

Sorting of RNAs to specific subcellular loci occurs in diverse settings from fly oocytes to mammalian neurons. Using the membrane-permeable nucleic acid stain SYTO 14, we directly visualized the translocation of endogenous RNA in living cells. Labeled RNA was distributed nonrandomly as discrete granules in neuronal processes. The labeled granules colocalized with poly(A+) mRNA, with the 60S ribosomal subunit, and with elongation factor 1alpha, suggesting that granules represent a translational unit. A subset of labeled granules colocalized with beta-actin mRNA. Correlative light and electron microscopy indicated that the fluorescent granules corresponded to clusters of ribosomes at the ultrastructural level. Poststaining of sections with heavy metals confirmed the presence of ribosomes within these granules. In living neurons, a subpopulation of RNA granules was motile during the observation period. They moved at an average rate of 0.1 microm/sec. In young cultures their movements were exclusively anterograde, but after 7 d in culture, one-half of the motile granules moved in the retrograde direction. Granules in neurites were delocalized after treatment with microtubule-disrupting drugs. These results raise the possibility of a cellular trafficking system for the targeting of RNA in neurons.

Clusters of axonal Na+ channels adjacent to remyelinating Schwann cells.

Rat sciatic nerve fibres were demyelinated by injection of lysolecithin and examined at several stages as Schwann cells proliferated, adhered, and initiated remyelination. Immunoperoxidase EM has been used to follow the clustering of Na+ channels that represents an early step in the formation of new nodes of Ranvier. At the peak of demyelination, 1 week post-injection, only isolated sites, suggestive of the original nodes, were labelled. As Schwann cells adhered and extended processes along the axons, regions of axonal Na+ channel immunoreactivity were often found just beyond their leading edges. These channel aggregates were associated only with the axolemma and Na+ channels were not detected on glial membranes. Sites with more than one cluster in close proximity and broadly labelled aggregates between Schwann cells suggested that new nodes of Ranvier formed as neighbouring Na+ channel groups merged. Schwann cells thus seem to play a major role in ion channel distributions in the axolemma. In all of these stages Na+ channel label was found primarily just outside the region of close contact between axon and Schwann cell. This suggests that Schwann cell adherence acts in part to exclude Na+ channels, or that diffusible substances are involved and can act some distance from regions of direct contact.

Inwardly rectifying K+ channels that may participate in K+ buffering are localized in microvilli of Schwann cells.

The presence of K+ channels on the Schwann cell plasma membrane suggests that Schwann cells may participate actively during action potential propagation in the peripheral nervous system. One such role for Schwann cells may be to maintain a constant extracellular concentration of K+ in the face of K+ efflux from a repolarizing axon. This buffering is likely to involve the influx of K+ through inward rectifying K+ channels. The molecular cloning of these genes allowed us to examine their expression and localization in Schwann cells in detail. In this study, we demonstrate the expression of two inward rectifying K+ channels, IRK1 and IRK3, in adult rat sciatic nerve. Immunocytochemistry using a polyclonal antibody against these proteins showed that the channels were highly localized at nodes in sciatic nerve. By immunoelectron microscopy, the nodal staining was shown to be concentrated in the microvilli of Schwann cells (also called nodal processes). The large surface area of the microvilli and their presence in the nodal space suggest involvement with ionic buffering. Thus, IRK1 and IRK3 are well suited to K+ buffering by virtue of both their biophysical properties and their localization. The restricted distribution of the inward rectifying K+ channels also provides an example of the highly regulated localization of ion channels to their specialized membrane domains. In the Schwann cell, where the nodal processes are a minute fraction of the total cell membrane, a potent mechanism must be present to concentrate the channels in this structure.