Organization of microtubules in myelinating Schwann cells

Ribosomal trafficking is reduced in Schwann cells following induction of myelination

Local synthesis of proteins within the Schwann cell periphery is extremely important for efficient process extension and myelination, when cells undergo dramatic changes in polarity and geometry. Still, it is unclear how ribosomal distributions are developed and maintained within Schwann cell projections to sustain local translation. In this multi-disciplinary study, we expressed a plasmid encoding a fluorescently labeled ribosomal subunit (L4-GFP) in cultured primary rat Schwann cells. This enabled the generation of high-resolution, quantitative data on ribosomal distributions and trafficking dynamics within Schwann cells during early stages of myelination, induced by ascorbic acid treatment. Ribosomes were distributed throughout Schwann cell projections, with ~2-3 bright clusters along each projection. Clusters emerged within 1 day of culture and were maintained throughout early stages of myelination. Three days after induction of myelination, net ribosomal movement remained anterograde (directed away from the Schwann cell body), but ribosomal velocity decreased to about half the levels of the untreated group. Statistical and modeling analysis provided additional insight into key factors underlying ribosomal trafficking. Multiple regression analysis indicated that net transport at early time points was dependent on anterograde velocity, but shifted to dependence on anterograde duration at later time points. A simple, data-driven rate kinetics model suggested that the observed decrease in net ribosomal movement was primarily dictated by an increased conversion of anterograde particles to stationary particles, rather than changes in other directional parameters. These results reveal the strength of a combined experimental and theoretical approach in examining protein localization and transport, and provide evidence of an early establishment of ribosomal populations within Schwann cell projections with a reduction in trafficking following initiation of myelination.

Myelination of the nervous system: mechanisms and functions

Myelination of axons in the nervous system of vertebrates enables fast, saltatory impulse propagation, one of the best-understood concepts in neurophysiology. However, it took a long while to recognize the mechanistic complexity both of myelination by oligodendrocytes and Schwann cells and of their cellular interactions. In this review, we highlight recent advances in our understanding of myelin biogenesis, its lifelong plasticity, and the reciprocal interactions of myelinating glia with the axons they ensheath. In the central nervous system, myelination is also stimulated by axonal activity and astrocytes, whereas myelin clearance involves microglia/macrophages. Once myelinated, the long-term integrity of axons depends on glial supply of metabolites and neurotrophic factors. The relevance of this axoglial symbiosis is illustrated in normal brain aging and human myelin diseases, which can be studied in corresponding mouse models. Thus, myelinating cells serve a key role in preserving the connectivity and functions of a healthy nervous system.

Podocytes … What's Under Yours? (Podocytes and Foot Processes and How They Change in Nephropathy)

Most of the described structures of podocytes in health and disease have been inferred from light and electron microscopic studies of rodent models. The variation in filtration barrier features is measured on micrographs, the aim being statistical significance. This is the technical campaign waged against kidney disease but this approach can be misleading. The signaling cascades and connectivity of the podocyte and foot processes (FPs) are inferred from in vitro studies that at best blurr the reality of the in vivo state. This review will outline actin signaling connectivity and the key differences in the structural and functional domains squeezed into the FPs and the relationship of these domains to other parts of the podocyte. It covers the changes in podocytes during nephropathy concentrating on FP and finally proposes an alternative interpretation of FP ultrastructure derived from articles published over the last 60 years.

Biology of Schwann cells

The fundamental roles of Schwann cells during peripheral nerve formation and regeneration have been recognized for more than 100 years, but the cellular and molecular mechanisms that integrate Schwann cell and axonal functions continue to be elucidated. Derived from the embryonic neural crest, Schwann cells differentiate into myelinating cells or bundle multiple unmyelinated axons into Remak fibers. Axons dictate which differentiation path Schwann cells follow, and recent studies have established that axonal neuregulin1 signaling via ErbB2/B3 receptors on Schwann cells is essential for Schwann cell myelination. Extracellular matrix production and interactions mediated by specific integrin and dystroglycan complexes are also critical requisites for Schwann cell-axon interactions. Myelination entails expansion and specialization of the Schwann cell plasma membrane over millimeter distances. Many of the myelin-specific proteins have been identified, and transgenic manipulation of myelin genes have provided novel insights into myelin protein function, including maintenance of axonal integrity and survival. Cellular events that facilitate myelination, including microtubule-based protein and mRNA targeting, and actin based locomotion, have also begun to be understood. Arguably, the most remarkable facet of Schwann cell biology, however, is their vigorous response to axonal damage. Degradation of myelin, dedifferentiation, division, production of axonotrophic factors, and remyelination all underpin the substantial regenerative capacity of the Schwann cells and peripheral nerves. Many of these properties are not shared by CNS fibers, which are myelinated by oligodendrocytes. Dissecting the molecular mechanisms responsible for the complex biology of Schwann cells continues to have practical benefits in identifying novel therapeutic targets not only for Schwann cell-specific diseases but other disorders in which axons degenerate.

MMP2-9 cleavage of dystroglycan alters the size and molecular composition of Schwann cell domains

Myelinating glial cells exhibit a spectacular cytoarchitecture, because they polarize on multiple axes and domains. How this occurs is essentially unknown. The dystroglycan-dystrophin complex is required for the function of myelin-forming Schwann cells. Similar to other tissues, the dystroglycan complex in Schwann cells localizes with different dystrophin family members in specific domains, thus promoting polarization. We show here that cleavage of dystroglycan by matrix metalloproteinases 2 and 9, an event that is considered pathological in most tissues, is finely and dynamically regulated in normal nerves and modulates dystroglycan complex composition and the size of Schwann cell compartments. In contrast, in nerves of Dy(2j/2j) mice, a model of laminin 211 deficiency, metalloproteinases 2 and 9 are increased, causing excessive dystroglycan cleavage and abnormal compartments. Pharmacological inhibition of cleavage rescues the cytoplasmic defects of Dy(2j/2j) Schwann cells. Thus, regulated cleavage may be a general mechanism to regulate protein complex composition in physiological conditions, whereas unregulated processing is pathogenic and a target for treatment in disease.

A laminin-2, dystroglycan, utrophin axis is required for compartmentalization and elongation of myelin segments

Animal and plant cells compartmentalize to perform morphogenetic functions. Compartmentalization of myelin-forming Schwann cells may favor elongation of myelin segments to the size required for efficient conduction of nerve impulses. Compartments in myelinated fibers were described by Ramón y Cajal and depend on periaxin, mutated in the hereditary neuropathy Charcot-Marie-Tooth disease type 4F (Charcot-Marie-Tooth 4F). Lack of periaxin in mice causes loss of compartments, formation of short myelin segments (internodes) and reduced nerve conduction velocity. How compartments are formed and maintained, and their relevance to human neuropathies is largely unknown. Here we show that formation of compartments around myelin is driven by the actin cytoskeleton, and maintained by actin and tubulin fences through linkage to the dystroglycan complex. Compartmentalization and establishment of correct internodal length requires the presence of glycosylated dystroglycan, utrophin and extracellular laminin-2/211. A neuropathic patient with reduced internodal length and nerve conduction velocity because of absence of laminin-2/211 (congenital muscular dystrophy 1A) also shows abnormal compartmentalization. These data link formation of compartments through a laminin2, dystroglycan, utrophin, actin axis to internodal length, and provide a common pathogenetic mechanism for two inherited human neuropathies. Other cell types may exploit dystroglycan complexes in similar fashions to create barriers and compartments.

Antisense suppression of tau in cultured rat oligodendrocytes inhibits process formation

The microtubule-associated protein tau is integral to neuronal process development and has a role in the pathogenesis of several neurodegenerative conditions. We examined possible roles for tau in cultured oligodendrocyte process formation by using antisense oligonucleotide treatment. Inhibition of tau synthesis with single oligonucleotides resulted in decreased tau protein levels and significantly shorter cellular processes. Simultaneous use of two nonoverlapping oligonucleotides caused a major reduction in tau levels and severely inhibited process outgrowth. The timing of oligonucleotide addition to oligodendrocyte cultures was important, with addition of antisense at the time of plating into culture having the most significant effect on morphology through reduction of tau expression.

Regulatory role of cytochrome P450scc and pregnenolone in myelination by rat Schwann cells

To investigate the production of steroid hormones by Schwann cells and to examine the regulation of steroid hormone production during myelination, cultures of rat Schwann cells were differentiated into their myelinating phenotype in the absence of neurons with dibutyryl cAMP (db-cAMP). During this process, the expression of P450scc (involved in steroid biosynthesis) was elevated at both the mRNA and protein levels as evident in RT-PCR, Western blots, and immunostaining. Labeling of the cells with [14C] acetate revealed enhanced production of pregnenolone during differentiation into the myelinating phenotype. Disruption of P450scc's activity with an inhibitor diminished the extent of differentiation into the myelinating phenotype as levels of mRNA and protein expression of myelin protein zero (P0) declined. However, the effect was reversed with the addition of pregnenolone. Furthermore, when the differentiating cultures were treated with pregnenolone, mRNA expression of P0 was upregulated, suggesting the stimulation of the differentiation process. Together, these results provide evidence for Schwann cells as a major producer of steroid hormones and pregnenolone production by P450scc as an important regulatory step during myelination.

P25?/Tubulin polymerization promoting protein expression by myelinating oligodendrocytes of the developing rat brain

P25alpha/tubulin polymerization promoting protein (TPPP) is a brain specific phosphoprotein that displays microtubule bundling activity. In the mature brain, p25alpha/TPPP distributes to oligodendrocytes and choroid plexus epithelium. We mapped the spatial and temporal distribution of p25alpha/TPPP in the developing rat brain. Having localized its expression to neuronal tissue by Western blot analyses, the distribution of p25alpha/TPPP to developing oligodendrocytes was confirmed using a specific antibody. In the pre-natal and post-natal brain, p25alpha/TPPP was localized to the perinuclear cytoplasm of myelinating oligodendrocytes from embryonic (E) day E20 as verified from cellular co-localization with 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP). Oligodendrocyte progenitor cells and pre-myelinating oligodendrocytes identified by the expression of NG2 proteoglycan and CD9, respectively, both failed to contain p25alpha/TPPP. In contrast, P25alpha/TPPP co-localized with beta(IV)-tubulin from post-natal (p) day P10 suggesting that p25alpha/TPPP plays an important role for tubulin-related transport in developing, myelinating oligodendrocytes.

A dual tyrosine-leucine motif mediates myelin protein P0 targeting in MDCK cells

Differential targeting of myelin proteins to multiple, biochemically and functionally distinct Schwann cell plasma membrane domains is essential for myelin formation. In this study, we investigated whether the myelin protein P0 contains targeting signals using Madin-Darby canine kidney (MDCK) cells. By confocal microscopy, P0 was localized to MDCK cell basolateral membranes. C-terminal deletion resulted in apical accumulation, and stepwise deletions defined a 15-mer region that was required for basolateral targeting. Alanine substitutions within this region identified the YAML sequence as a functional tyrosine-based targeting signal, with the ML sequence serving as a secondary leucine-based signal. Replacement of the P0 ectodomain with green fluorescent protein altered the distribution of constructs lacking the YAML signal. Coexpression of the myelin-associated glycoprotein did not alter P0 distribution in MDCK cells. The results indicate that P0 contains a hierarchy of targeting signals, which may contribute to P0 localization in myelinating Schwann cells and the pathogenesis in human disease.

Beta IV tubulin is selectively expressed by oligodendrocytes in the central nervous system

Oligodendrocyte differentiation and myelination involve dramatic changes in cell signaling pathways, gene expression patterns, cell shape, and cytoskeletal organization. In a pilot study investigating CNS angiogenesis, oligodendrocytes were intensely labeled by antisera directed against the C-terminal of Tie-2, a 140-kDa transmembrane receptor for angiopoietin. Immunoprecipitation of rat brain proteins with Tie-2 C-terminal antisera, however, produced a single spot of approximately 55-kDa pI approximately 5 by two-dimensional (2D) electrophoresis, which was identified as beta-tubulin by mass spectrometry. Isotype-specific antibodies for beta(IV) tubulin selectively labeled oligodendrocytes. First detected in premyelinating oligodendrocytes, beta(IV) tubulin was abundant in myelinating oligodendrocyte perinuclear cytoplasm and processes extending to and along developing myelin internodes. Beta(IV) tubulin-positive MTs were diffusely distributed in oligodendrocyte perinuclear cytoplasm and not organized around the centrosome. Beta(IV) tubulin may play a role in establishing the oligodendrocyte MT network, which is essential for the transport of myelin proteins, lipids, and RNA during myelination.

Perisynaptic Schwann cells of the vertebrate motor endplate bear modified cilia

Perisynaptic Schwann cells (PSCs), descendants of the myelinating Schwann cells, cover the axon terminal of the vertebrate motor endplate of the skeletal muscle fiber. PSCs are assumed to support the function of the axon terminal. This function suggests a net material transport in the direction of the axon terminal. Morphologically it is to be expected that these cells have a cytoskeleton aligned to the axon terminal. Investigations clarifying this statement have not yet been undertaken. From previous investigations we know, however, that the PSCs have a microtubule-organizing center, which is a part of this cytoskeleton. The centrioles of the organizing center may also participate in the formation of a modified cilium structure whose function is unknown. In the present investigation, characteristic ultrastructural features of the modified cilium structure and its relationship to the Golgi apparatus and the axon terminal are presented. A function for the modified cilium structure is discussed.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Mechanism of the process formation; podocytes vs. neurons

In this review article we discuss the common mechanism for cellular process formation. Besides the podocyte, the mechanism of process formation, including cytoskeletal organization and signal transduction, etc., has been studied using neurons and glias as model systems. There has been an accumulation of data showing common cell biological features of the podocyte and the neuron: 1) Both cells possess long and short cell processes equipped with highly organized cytoskeletal systems; 2) Both show cytoskeletal segregation; microtubules (MTs) and intermediate filaments (IFs) in podocyte primary processes and in neurites, while actin filaments (AFs) are abundant in podocyte foot processes in neuronal synaptic regions; 3) In both cells, process formation is mechanically dependent on MTs, whose assembly is regulated by various microtubule- associated proteins (MAPs); 4) In both cells, process formation is positively regulated by PP2A, a Ser/Thr protein phosphatase; 5) In both cells, process formation is accelerated by laminin, an extracellular matrix protein. In addition, recent data from our and other laboratories have shown that podocyte processes share many features with neuronal dendrites: 1) Podocyte processes and neuronal dendrites possess MTs with mixed polarity, namely, plus-end-distal and minus-end-distal MTs coexist in these processes; 2) To establish the mixed polarity of MTs, both express CHO1/MKLP1, a kinesin-related motor protein, and when its expression is inhibited formation of both podocyte processes and neuronal dendrites is abolished; 3) The elongation of both podocyte processes and neuronal dendrites is supported by rab8-regulated basolateral-type membrane transport; 4) Both podocyte processes and neuronal dendrites express synaptopodin, an actin-associated protein, in a development-dependent manner; interestingly, in both cells, synaptopodin is localized not in the main shaft of processes but in thin short projections from the main shaft. We propose that the podocyte process and the neuronal dendrite share many features, while the neuronal axon should be thought of as an exceptionally differentiated cellular process.

Ezrin, radixin, and moesin are components of Schwann cell microvilli

Ezrin, radixin, and moesin (ERM proteins), as well as the neurofibromatosis 2 (NF2) tumor suppressor merlin/schwannomin, all belong to the protein 4.1 family, yet only merlin is a tumor suppressor in Schwann cells. To gain insight into the possible functions of ERM proteins in Schwann cells, we examined their localization in peripheral nerve, because we have previously shown that merlin is found in paranodes and in Schmidt-Lanterman incisures. All three ERM proteins were highly expressed in the microvilli of myelinating Schwann cells that surround the nodal axolemma as well as in incisures and cytoplasmic puncta in the vicinity of the node. In all of these locations, ERM proteins were colocalized with actin filaments. In contrast, ERM proteins did not surround nodes in the CNS. The colocalization of ERM proteins with actin indicates that they have functions different from those of merlin in myelinating Schwann cells.

The small myelin-associated glycoprotein binds to tubulin and microtubules

The myelin-associated glycoprotein (MAG) exists as two isoforms, differing only by their respective cytoplasmic domains, that have been suggested to function in the formation and maintenance of myelin. In the present study, a 50 kDa protein binding directly to the small MAG (S-MAG) cytoplasmic domain was detected and identified as tubulin, the core component of the microtubular cytoskeleton. In vitro, the S-MAG cytoplasmic domain slowed the polymerization rate of tubulin and co-purified with assembled microtubules. A significant sequence homology was found between the tau family tubulin-binding repeats and the carboxy-terminus of S-MAG. Our results indicate that S-MAG is the first member of the Ig superfamily that can be classified as a microtubule-associated protein, and place S-MAG in a dynamic structural complex that could participate in linking the axonal surface and the myelinating Schwann cell cytoskeleton.

Nonuniform microtubular polarity established by CHO1/MKLP1 motor protein is necessary for process formation of podocytes

Podocytes are unique cells that are decisively involved in glomerular filtration. They are equipped with a complex process system consisting of major processes and foot processes whose function is insufficiently understood (Mundel, P., and W. Kriz. 1995. Anat. Embryol. 192:385-397). The major processes of podocytes contain a microtubular cytoskeleton. Taking advantage of a recently established cell culture system for podocytes with preserved ability to form processes (Mundel, P., J. Reiser, A. Zúñiga Mejía Borja, H. Pavenstädt, G.R. Davidson, W. Kriz, and R. Zeller. 1997b. Exp. Cell Res. 36:248-258), we studied the functional significance of the microtubular system in major processes. The following data were obtained: (a) Microtubules (MTs) in podocytes show a nonuniform polarity as revealed by hook-decoration. (b) CHO1/ MKLP1, a kinesin-like motor protein, is associated with MTs in podocytes. (c) Treatment of differentiating podocytes with CHO1/MKLP1 antisense oligonucleotides abolished the formation of processes and the nonuniform polarity of MTs. (d) During the recovery from taxol treatment, taxol-stabilized (nocodazole- resistant) MT fragments were distributed in the cell periphery along newly assembled nocodazole-sensitive MTs. A similar distribution pattern of CHO1/MKLP1 was found under these circumstances, indicating its association with MTs. (e) In the recovery phase after complete depolymerization, MTs reassembled exclusively at centrosomes. Taken together, these findings lead to the conclusion that the nonuniform MT polarity in podocytes established by CHO1/MKLP1 is necessary for process formation.

Transient expression of the neurofilament proteins NF-L and NF-M by Schwann cells is regulated by axonal contact

Expression of the genes that encode neurofilament proteins is considered to be confined normally to neurons. However, in demyelinating peripheral nerves Schwann cells upregulate the mRNA for the medium-sized neurofilament protein (NF-M), and cultured Schwann cells of the myelin-forming phenotype can also synthesize and incorporate NF-M protein into their intermediate filament (IF) cytoskeleton. The purpose of this study was to establish how axonal contact might influence glial neurofilament gene expression and regulate the synthesis of neurofilament proteins. We show that the gene encoding NF-M is expressed at early stages of differentiation in myelin-forming Schwann cells in vivo; nevertheless, little NF-M protein can be detected in these cells. The transient induction of NF-M mRNA is also apparent in dedifferentiating Schwann cells during Wallerian degeneration. In these Schwann cells the mRNAs for NF-M and NF-L (the smallest polypeptide), but not NF-H (the largest neurofilament subunit), are coordinately expressed. In contrast to differentiating myelin-forming Schwann cells, the cells of degenerating nerves express both NF-M and NF-L polypeptides. Restoration of axonal contact in the growing nerve stimulates the recapitulation of Schwann cell differentiation including the elevation of NF-M and NF-L mRNA expression. These results demonstrate that the transient induction of neurofilament mRNAs in Schwann cells is a feature of both differentiation and dedifferentiation. However translation of these mRNAs is confined to Schwann cells deprived of axonal contact either by nerve injury or by culture in the absence of axons. These findings suggest that the expression of the NF-M and NF-L polypeptides is an important characteristic of those Schwann cells that will contribute to the repair of damaged peripheral nerves.

Microtubule organization and stability in the oligodendrocyte

The oligodendrocyte is the glial cell responsible for the formation and maintenance of CNS myelin. Because the development of neuronal morphology is known to depend on the presence of highly organized microtubule arrays, it may be hypothesized that the properties of microtubules influence the form and function of oligodendrocytes. The goals of the present study were to define the physical attributes of microtubules in oligodendrocytes maintained in vitro. The results of electron and confocal microscopy indicate that microtubules are present throughout the cell bodies and large and small processes of oligodendrocytes and are rarely associated with discrete microtubule-organizing centers. A modified "hooking" protocol demonstrated that the polarity orientation of microtubules is uniformly plus-end distal in small oligodendrocyte processes, compared with a nonuniform, predominantly plus-end distal orientation in large processes. Oligodendrocytes were exposed to the microtubule-depolymerizing drug nocodazole to examine microtubule stability in these cells. The results suggest that oligodendrocyte microtubules can be resolved into at least three distinct microtubule populations that differ in their kinetics of depolymerization in the presence of nocodazole. These findings suggest that the properties of the oligodendrocyte microtubule array reflect the functions of the different regions of this highly specialized cell.

The transition of microglia to a ramified phenotype is associated with the formation of stable acetylated and detyrosinated microtubules

In situ and in vitro, microglia can have different morphologies, which are thought to reflect distinct physiological activities. Two extreme forms are ameboid and ramified microglia. To study cytoskeletal changes during differentiation, we used defined cell culture systems to yield cultures of ameboid or ramified microglia from mouse brain. With respect to proliferation, secretion, receptor-expression, and phagocytosis, ramified microglia was generally less active. We found that the transition to a ramified phenotype was accompanied by an increase in the relative amount of acetylated and detyrosinated tubulin. Whereas the modified microtubules were restricted to regions close to the microtubule-organizing centers (MTOCs) in ameboid cells, acetylated microtubules were abundant in ramified cells, where they appeared to traverse from one process to another with no apparent anchoring at MTOCs. The increase in acetylated and detyrosinated microtubules was paralleled by an increased stability against nocodazole-induced microtubule disassembly and by a lower rate of change in the length of the processes. Staining of retinal wholemounts confirmed the presence of acetylated microtubules in ramified microglia in situ. We conclude that during the transition of microglia to a ramified phenotype regulated processes exist, which cause a reorganization of microtubules and a change in composition of the microtubule skeleton resulting in a less dynamic and more stable microtubule network. Intracellular factors that are specifically involved in microtubule stabilization in ramified microglia need to be identified in future research and may provide a useful criterion for defining ramified microglia.

Polarity of microtubule assemblies during neuronal cell migration

The active migration of neurons from their sites of origin to their final destinations requires the unidirectional translocation of the nuclei and somatic cytoplasm within the growing leading processes. To explore the cellular machinery underlying this translocation, we determined the polarity of microtubules situated within the leading and trailing processes of migrating cerebellar granule cells in situ. Our analysis reveals that the newly assembled positive ends of the microtubules in the leading process uniformly face the growing tip, while their disintegrating negative ends face the nucleus. In the trailing process, by contrast, microtubule arrays are of mixed polarity. We suggest that the dynamics of slow polymerization in combination with fast disintegration of oriented microtubules create "push" and "pull" forces that contribute to the piston-like saltatory displacement of the nucleus and cytoplasm within the membrane cylinder of the leading process of the migrating neuron.