Outstations of the Golgi complex are present in the processes of cultured rat oligodendrocytes

Golgi Outposts Nucleate Microtubules in Cells with Specialized Shapes

Classically, animal cells nucleate or form new microtubules off the perinuclear centrosome. In recent years, the Golgi outpost has emerged as a satellite organelle that can function as an acentrosomal microtubule-organizing center (MTOC), nucleating new microtubules at distances far from the nucleus or cell body. Golgi outposts can nucleate new microtubules in specialized cells with unique cytoarchitectures, including Drosophila neurons, mouse muscle cells, and rodent oligodendrocytes. This review compares and contrasts topics of functional relevance, including Golgi outpost heterogeneity, formation and transport, as well as regulation of microtubule polarity and branching. Golgi outposts have also been implicated in the pathology of diseases including muscular dystrophy, and neurodegenerative diseases, such as Parkinson's disease (PD). Since Golgi outposts are relatively understudied, many outstanding questions regarding their function and roles in disease remain.

The Golgi Outpost Protein TPPP Nucleates Microtubules and Is Critical for Myelination

Oligodendrocytes extend elaborate microtubule arbors that contact up to 50 axon segments per cell, then spiral around myelin sheaths, penetrating from outer to inner layers. However, how they establish this complex cytoarchitecture is unclear. Here, we show that oligodendrocytes contain Golgi outposts, an organelle that can function as an acentrosomal microtubule-organizing center (MTOC). We identify a specific marker for Golgi outposts-TPPP (tubulin polymerization promoting protein)-that we use to purify this organelle and characterize its proteome. In in vitro cell-free assays, recombinant TPPP nucleates microtubules. Primary oligodendrocytes from Tppp knockout (KO) mice have aberrant microtubule branching, mixed microtubule polarity, and shorter myelin sheaths when cultured on 3-dimensional (3D) microfibers. Tppp KO mice exhibit hypomyelination with shorter, thinner myelin sheaths and motor coordination deficits. Together, our data demonstrate that microtubule nucleation outside the cell body at Golgi outposts by TPPP is critical for elongation of the myelin sheath.

A Microtubule-Myelination Connection

Microtubules are critical for the extension of oligodendrocyte processes and myelin deposition, yet our knowledge of their microtubule biogenesis is limited. In this issue of Cell, Fu et al. (2019) identify an oligodendrocyte-enriched microtubule regulator that promotes microtubule growth from Golgi outposts and controls myelin sheath elongation, linking microtubule cytoarchitecture and myelination in the CNS.

NDE1 positively regulates oligodendrocyte morphological differentiation

Oligodendrocytes, the myelin-forming cells in the central nervous system (CNS), undergo morphological differentiation characterized by elaborated branched processes to enwrap neuronal axons. However, the basic molecular mechanisms underlying oligodendrocyte morphogenesis remain unknown. Herein, we describe the essential roles of Nuclear Distribution E Homolog 1 (NDE1), a dynein cofactor, in oligodendrocyte morphological differentiation. In the mouse corpus callosum, Nde1 mRNA expression was detected in oligodendrocyte lineage cells at the postnatal stage. In vitro analysis revealed that downregulation of NDE1 by siRNA impaired the outgrowth and extensive branching of oligodendrocyte processes and led to a decrease in the expression of myelin-related markers, namely, CNPase and MBP. In myelinating co-cultures with dorsal root ganglion (DRG) neurons, NDE1-knockdown oligodendrocyte precursor cells (OPCs) failed to develop into MBP-positive oligodendrocytes with multiple processes contacting DRG axons. Immunoprecipitation studies showed that NDE1 interacts with the dynein intermediate chain (DIC) in oligodendrocytes, and an overexpressed DIC-binding region of NDE1 exerted effects on oligodendrocyte morphogenesis that were similar to those following NDE1 knockdown. Furthermore, NDE1-knockdown-impaired oligodendrocyte process formation was rescued by siRNA-resistant wild-type NDE1 but not by DIC-binding region-deficient NDE1 overexpression. These results suggest that NDE1 plays a crucial role in oligodendrocyte morphological differentiation via interaction with dynein.

Differential regulation of sphingomyelin synthesis and catabolism in oligodendrocytes and neurons

Neurons (both primary cultures of 3-day rat hippocampal neurons and embryonic chick neurons) rapidly converted exogenous NBD-sphingomyelin (SM) to NBD-Cer but only slowly converted NBD-Cer to NBD-SM. This was confirmed by demonstrating low in vitro sphingomyelin synthase (SMS) and high sphingomyelinase (SMase) activity in neurons. Similar results were observed in a human neuroblastoma cell line (LA-N-5). In contrast, primary cultures of 3-day-old rat oligodendrocytes only slowly converted NBD-SM to NBD-Cer but rapidly converted NBD-Cer to NBD-SM. This difference was confirmed by high in vitro SMS and low SMase activity in neonatal rat oligodendrocytes. Similar results were observed in a human oligodendroglioma cell line. Mass-Spectrometric analyses confirmed that neurons had a low SM/Cer ratio of (1.5 : 1) whereas oligodendroglia had a high SM/Cer ratio (9 : 1). Differences were also confirmed by [(3)H]palmitate-labeling of ceramide, which was higher in neurons compared with oligodendrocytes. Stable transfection of human oligodendroglioma cells with neutral SMase, which enhanced the conversion of NBD-SM to NBD-Cer and increased cell death, whereas transfection with SMS1 or SMS2 enhanced conversion of NBD-Cer to NBD-SM and was somewhat protective against cell death. Thus, SMS rather than SMases may be more important for sphingolipid homeostasis in oligodendrocytes, whereas the reverse may be true for neurons.

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.

Endoplasmic reticulum of animal cells and its organization into structural and functional domains

The endoplasmic reticulum (ER) in animal cells is an extensive, morphologically continuous network of membrane tubules and flattened cisternae. The ER is a multifunctional organelle; the synthesis of membrane lipids, membrane and secretory proteins, and the regulation of intracellular calcium are prominent among its array of functions. Many of these functions are not homogeneously distributed throughout the ER but rather are confined to distinct ER subregions or domains. This review describes the structural and functional organization of the ER and highlights the dynamic properties of the ER network and the mechanisms that support the positioning of ER membranes within the cell. Furthermore, we outline processes involved in the establishment and maintenance of an anisotropic distribution of ER-resident proteins and, thus, in the organization of the ER into functionally and morphologically different subregions.

The golgi apparatus is present in perisynaptic, subependymal and perivascular processes of astrocytes and in processes of retinal Müller glia

The Golgi apparatus (GA) of innervated rat and chicken skeletal muscle is present in a typical perinuclear location, and in subsynaptic areas where it disperses after denervation. It was suggested that the subsynaptic segments of the GA are linked with functions involved in the maturation and targeting of synaptic proteins. Similarly, the GA of rat myocardium is found in a perinuclear location and between myofibrils, adjacent to the T system of tubules. These findings raise the question whether the GA of polarized cells is present in a typical perinuclear location, for the performance of general "housekeeping" functions, and in distal areas, for the mediation of specialized functions. Astrocytes may contain GA within their long cytoplasmic processes which are difficult to identify in thin sections. To ensure the astrocytic origin of GA in otherwise unidentifiable small processes, we used transgenic mice expressing the rat MG160 medial Golgi sialoglycoprotein only in the GA of astrocytes, and visualized the GA with monoclonal antibody 10A8 (mAb10A8) which reacts only with rat MG160. Thus, we identified cisternae of the GA in distal perisynaptic and subependymal processes, in perivascular foot plates of cerebral astrocytes, and in processes of the Müller glia in the retina. A similar strategy may be adopted in future investigations aiming at the detection of elements of the GA in distal processes of neurons and oligodendrocytes. The functional implications of GA in perisynaptic astrocytic processes and other processes are unknown. However, the isolation and molecular characterization of the perisynaptic subset of astrocytic Golgi may be feasible, since others have purified the astrocytic glutamate transporter 1 (GLT1) from crude synaptosomal fractions in which astrocytic processes are probably unavoidable contaminants.

Protein kinase C prevents oligodendrocyte differentiation: modulation of actin cytoskeleton and cognate polarized membrane traffic

In a previous study, we showed that activation of protein kinase C (PKC) prevents oligodendrocyte differentiation at the pro-oligodendrocyte stage. The present study was undertaken to identify downstream targets of PKC action in oligodendrocyte progenitor cells. Activation of PKC induced the predominant phosphorylation of an 80-kD protein, identified as myristoylated alanine-rich C-kinase substrate (MARCKS). Upon phosphorylation, MARCKS is translocated from the plasma membrane to the cytosol. Furthermore, PKC activation perturbed the organization of the actin cytoskeleton, causing a redistribution of actin filaments to the submembranous or cortical actin cytoskeleton. As a consequence, transport of a protein traffic marker, the vesicular stomatitis virus glycoprotein, from the trans-Golgi network to the plasma membrane becomes perturbed. The effect of disruption of the actin filament network by cytochalasin D perfectly matched the effect of PKC. These data thus favor the existence of a causal relationship between actin rearrangement and docking and/or fusion of proteins to the plasma membrane. Interestingly, neither in control cells nor in PKC-activated cells did another protein traffic marker, influenza hemagglutinin (HA), reach the cell surface. However, an eminent and specific accumulation of HA just underneath the plasma membrane became apparent upon PKC activation. Yet, this effect could not be simulated by cytochalasin D treatment. Therefore, these observations imply that although MARCKS represents a prominent PKC target site in regulating differentiation, another target involves the differential control of cognate polarized trafficking pathways, which are apparently operating in oligodendrocyte progenitor cells.

Internalization and sorting of plasma membrane sphingolipid analogues in differentiating oligodendrocytes

We studied the formation of early endosomes in differentiating oligodendrocytes and type-2 astrocytes, which are derived from common precursor cells in rat neonates, using fluorescent analogues of lactosylceramide (LacCer) and sulfatide labeled with 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene++ +-3-pentanoic acid (BODIPY FL C5). These sphingolipid analogues exhibit a concentration-dependent shift in their fluorescence emission maximum from green to red wavelengths that can be used to estimate the relative concentration of an analogue in the intracellular membranes of living cells by quantitative fluorescence microscopy. When oligodendrocytes at various stages of differentiation were incubated with 1 microM BODIPY-sphingolipid at 10 degrees C and washed, yellow/green plasma membrane fluorescence was observed. Quantitative studies confirmed that the amount of BODIPY-LacCer or -sulfatide incorporated into the plasma membrane of a given cell type was identical. When these cells were subsequently warmed to 37 degrees C for 2-10 min to allow internalization to occur, the BODIPY-sphingolipid analogues were distributed in a punctate pattern throughout the cytoplasm. Within individual cells labeled with BODIPY-sulfatide, some endosomes exhibited green fluorescence, whereas others emitted red/orange fluorescence. In contrast, when BODIPY-LacCer was used, only green endosomes were observed. Although this phenomenon could be observed at earlier stages of differentiation, it was most obvious in mature oligodendrocytes, where quantitative measurements of the red/green ratio of individual endosomes suggested about a threefold difference between the concentration of the LacCer and sulfatide analogues in endosomes. These results suggest that "lipid sorting" takes place during endocytosis in mature oligodendrocytes, resulting in selective exclusion of certain lipid species during the internalization process. This sorting event may result in the net addition of lipids to the differentiated oligodendrocyte plasma membrane.

An apical-type trafficking pathway is present in cultured oligodendrocytes but the sphingolipid-enriched myelin membrane is the target of a basolateral-type pathway

Myelin sheets originate from distinct areas at the oligodendrocyte (OLG) plasma membrane and, as opposed to the latter, myelin membranes are relatively enriched in glycosphingolipids and cholesterol. The OLG plasma membrane can therefore be considered to consist of different membrane domains, as in polarized cells; the myelin sheet is reminiscent of an apical membrane domain and the OLG plasma membrane resembles the basolateral membrane. To reveal the potentially polarized membrane nature of OLG, the trafficking and sorting of two typical markers for apical and basolateral membranes, the viral proteins influenza virus-hemagglutinin (HA) and vesicular stomatitis virus-G protein (VSVG), respectively, were examined. We demonstrate that in OLG, HA and VSVG are differently sorted, which presumably occurs upon their trafficking through the Golgi. HA can be recovered in a Triton X-100-insoluble fraction, indicating an apical raft type of trafficking, whereas VSVG was only present in a Triton X-100-soluble fraction, consistent with its basolateral sorting. Hence, both an apical and a basolateral sorting mechanism appear to operate in OLG. Surprisingly, however, VSVG was found within the myelin sheets surrounding the cells, whereas HA was excluded from this domain. Therefore, despite its raft-like transport, HA does not reach a membrane that shows features typical of an apical membrane. This finding indicates either the uniqueness of the myelin membrane or the requirement of additional regulatory factors, absent in OLG, for apical delivery. These remarkable results emphasize that polarity and regulation of membrane transport in cultured OLG display features that are quite different from those in polarized cells.

Transport of proteolipid protein to the plasma membrane does not depend on glycosphingolipid cotransport in oligodendrocyte cultures

The possibility that transport of proteolipid protein (PLP) from its site of synthesis to the plasma membrane is dependent on cotransport with (sulfo)galacto-cerebrosides was investigated in primary cultured oligodendrocytes and Chinese hamster ovary (CHO) cells expressing PLP. Sulfation was inhibited by growing oligodendrocytes in the presence of a competitive inhibitor of this process, sodium chlorate. Under these circumstances, sulfatide synthesis was inhibited by 85%. Nevertheless, PLP was still delivered to the plasma membrane in quantitative amounts. Furthermore, when PLP was expressed in CHO cells, which normally synthesize very low amounts of galactosyl ceramide (GalCer) and no sulfatide, PLP was transported to the plasma membrane. Moreover, in CHO cells coexpressing PLP and ceramide galactosyl transferase, PLP cell surface labeling was unaltered. Noting that it has been demonstrated that proteins destined for the apical surface of epithelial cells colocalize with glycolipid-enriched microdomains, we isolated detergent-insoluble membrane complexes from cultured oligodendrocytes. We found, however, that most of the PLP is present in the detergent-soluble fraction and, furthermore, that PLP could not be chased into or out of the insoluble fraction. Taken together, these data make it very likely that in oligodendrocytes PLP transport takes place irrespective of the presence of glycosphingolipids GalCer and sulfatide.

Regulation of oligodendrocyte differentiation: protein kinase C activation prevents differentiation of O2A progenitor cells toward oligodendrocytes

Oligodendrocytes differentiate on a specific schedule in vivo in order to myelinate axons at the precise time and at the appropriate position. The current study was undertaken to obtain further insight as to how this timed appearance is regulated intracellularly. We observed that exposure of O2A progenitor cells in culture to phorbol 12-myristate 13-acetate (PMA; an activator of protein kinase C, PKC) inhibited their differentiation to oligodendrocytes by suppressing the expression of specific myelin markers at the O4-stage. To positively identify a role of PKC per se in differentiation, the use of a minimal medium with low serum content turned out to be essential. This was demonstrated by showing that the inhibitory effect of PMA on oligodendrocyte differentiation could be completely abolished by a combined action of insulin, triiodothyronine (T3), hydrocortisone and other components of a chemically defined medium (CDM). Furthermore, the PMA-mediated inhibition of oligodendrocyte differentiation could be partially restored by activation of the cAMP signal transduction pathway. The results indicate that PKC plays a crucial role in the differentiation of O2A progenitor cells toward oligodendrocytes: PKC activation prevents differentiation of O2A progenitor cells, whereas differentiation toward oligodendrocytes is dependent on other signaling compounds which may counteract the PKC signal transduction route.

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.

Differential and cell development-dependent localization of myelin mRNAs in oligodendrocytes

In oligodendrocytes (OLG), the mRNAs for the various myelin proteins localize to different intracellular sites. Whereas the confinement of myelin basic protein (MBP) mRNA to the processes of the cell has been well established, we demonstrate that most other myelin mRNA species are mainly present in the perinuclear region. Using in situ hybridization of cultured rat OLG we found that mRNAs are localized to at least three different locations: 1) to the perinuclear region [myelin-associated glycoprotein (MAG) mRNA]; 2) mainly to the processes (the mRNA for the 14-kDa isoform of MBP); and 3) to both the perinuclear region and the primary processes [2',3'-cyclic nucleotide phosphodiesterase (CNPase) and proteolipid protein (PLP) mRNAs]. Thus, depending on their primary structure, the mRNA species in OLG either remain near the nucleus or localize to primary or secondary processes before their translation. The myelin mRNA localization correlates well with that of the proteins encoded in them, as demonstrated by immunocytochemistry. Since different isoforms of MBP have different locations in transfected HeLa cells (Staugaitis et al.: J Cell Biol 110:1719-1727, 1990), we also have investigated the localization of the various mRNAs in OLG, using exon 2-minus and exon 2-specific probes in situ hybridization. The exon 2-minus MBP mRNAs are transported far into the processes, whereas exon 2-specific mRNA was only detected in the cell body. This suggests that sorting and trafficking of MBP mRNA are regulated by the presence or absence of the exon 2 sequence. Furthermore, during maturation of OLG, exon 2-plus mRNAs disappear, whereas exon 2-minus mRNAs increase. The developmentally regulated expression of exon 2-plus transcripts suggest a role of their protein products in differentiation rather than in myelination.

Central glial and neuronal populations display differential sensitivity to ceramide-dependent cell death

Ceramide is a lipid second messenger implicated in the mechanism of apoptotic cell death. The effect of the cell-permeable ceramide analogue C2 has been tested on primary cortical cultures of neurons, astrocytes, and oligodendrocytes as well as on the bipotential glial precursor cell line CG-4. After 24 hr of treatment, C2 ceramide induced a dose-dependent cell death in primary oligodendrocytes and precursor cells, with a maximum effect at 10 microM. Commitment of oligodendrocytes to cell death occurred within the first 6 hr of treatment. Ultramicroscopic analysis of primary oligodendrocytes exposed to C2 ceramide for 3.5 hr revealed extensive membrane blebbing in the absence of nuclear condensation. In contrast, similar treatment of primary neuronal or astrocytic cortical cultures had no effect on cell survival. Neurons and astrocytes were resistant to 10 microM C2 ceramide. Furthermore, bipotential progenitors that were differentiated toward astrocytes also became resistant to ceramide treatment as they acquired a mature astrocytic phenotype. These experiments suggest that cell type specific factors are required for ceramide-mediated cell death in the nervous system.

Nerve-dependent plasticity of the Golgi complex in skeletal muscle fibres: compartmentalization within the subneural sarcoplasm

Several recent reports have highlighted the plasticity of the Golgi apparatus during myogenesis, yet the organization of this specialized organelle in innervated skeletal muscle fibres remains poorly understood. Using four bona fide anti-Golgi antibodies, directed against a 210 kDa protein, a 160 kDa sialoglycoprotein, the small GTP-binding protein rab6p, and TGN38, the localization of which covers the various compartments of the Golgi complex, we show by immunofluorescence microscopy that the Golgi complex undergoes considerable reorganization in the course of myogenic differentiation and motor endplate formation in the rat. Unlike the typical perinuclear distribution of the Golgi stacks associated with every nucleus in myotubes, a striking subneural compartmentalization is observed in adult innervated myofibres. In short-term denervated adult muscle fibres, we noticed the presence of the perinuclear Golgi apparatus in extrajunctional regions, a pattern reminiscent of that of developing myotubes. At variance with anti-Golgi antibodies, antibodies to the rough endoplasmic reticulum label structures dispersed throughout the entire sarcoplasm, hence suggesting that it is not the entire membrane/secretory protein synthesis machinery which is compartmentalized, but only the Golgi apparatus. Also, an unexpected lack of immunoreactivity with the TGN38 and alpha-mannosidase II antibodies points to biochemical differentiation of the subneural Golgi apparatus at the adult motor endplate. These new data extend our previous observations on the compartmentalization of the Golgi apparatus in the postsynaptic sarcoplasm of chick muscle fibres, and further illustrate the plasticity of the Golgi apparatus in muscle cells. The specialization of the Golgi apparatus within the subneural compartment provides this particular region with a compartmentalized secretory pathway, and these observations highlight the notion that the level of differentiation of this domain is not only maintained via transcriptional regulation but also by post-translational control mechanisms.