Microtubule associated protein (MAP1B) is present in cultured oligodendrocytes and co-localizes with tubulin

Microtubule organization and dynamics in oligodendrocytes, astrocytes, and microglia

Though much is known about microtubule organization and microtubule-based transport in neurons, the development and function of microtubules in glia are more enigmatic. In this review, we provide an overview of the literature on microtubules in ramified brain cells, including oligodendrocytes, astrocytes, and microglia. We focus on normal cell biology-how structure relates to function in these cells. In oligodendrocytes, microtubules are important for extension of processes that contact axons and for elongating the myelin sheath. Recent studies demonstrate that new microtubules can form outside of the oligodendrocyte cell body off of Golgi outpost organelles. In astrocytes and microglia, changes in cell shape and ramification can be influenced by neighboring cells and the extracellular milieu. Finally, we highlight key papers implicating glial microtubule defects in neurological injury and disease and discuss how microtubules may contribute to invasiveness in gliomas. Thus, future research on the mechanisms underlying microtubule organization in normal glial cell function may yield valuable insights on neurological disease pathology.

Neurofilament-tubulin binding site peptide NFL-TBS.40-63 increases the differentiation of oligodendrocytes in vitro and partially prevents them from lysophosphatidyl choline toxiciy

During multiple sclerosis (MS), the main axon cystoskeleton proteins, neurofilaments (NF), are altered, and their release into the cerebrospinal fluid correlates with disease severity. The role of NF in the extraaxonal location is unknown. Therefore, we tested whether synthetic peptides corresponding to the tubulin-binding site (TBS) sequence identified on light NF chain (NFL-TBS.40-63) and keratin (KER-TBS.1-24), which could be released during MS, modulate remyelination in vitro. Biotinylated NFL-TBS.40-63, NFL-Scramble2, and KER-TBS.1-54 (1-100 μM, 24 hr) were added to rat oligodendrocyte (OL) and astrocyte (AS) cultures, grown in chemically defined medium. Proliferation and differentiation were characterized by using specific antibodies (A2B5, CNP, MBP, GFAP) and compared with untreated cultures. Lysophosphatidyl choline (LPC; 2 × 10(-5) M) was used to induce OL death and to test the effects of TBS peptides under these conditions. NFL-TBS.40-63 significantly increased OL differentiation and maturation, with more CNP(+) and MBP(+) cells characterized by numerous ramified processes, along with myelin balls. When OL were challenged with LPC, concomitant treatment with NFL-TBS.40-63 rescued more than 50% of OL compared with cultures treated with LPC only. Proliferation of OL progenitors was not affected, nor were AS proliferation and differentiation. NFL-TBS.40-63 peptide induces specific effects in vitro, increasing OL differentiation and maturation without altering AS fate. In addition, it partially protects OL from demyelinating injury. Thus release of NFL-TBS.40-63 caused by axonal damage in vivo could improve repair through increased OL differentiation, which is a prerequisite for remyelination.

Cellular and molecular characterization of multipolar Map5-expressing cells: a subset of newly generated, stage-specific parenchymal cells in the mammalian central nervous system

Although extremely interesting in adult neuro-glio-genesis and promising as an endogenous source for repair, parenchymal progenitors remain largely obscure in their identity and physiology, due to a scarce availability of stage-specific markers. What appears difficult is the distinction between real cell populations and various differentiation stages of the same population. Here we focused on a subset of multipolar, polydendrocyte-like cells (mMap5 cells) expressing the microtubule associated protein 5 (Map5), which is known to be present in most neurons. We characterized the morphology, phenotype, regional distribution, proliferative dynamics, and stage-specific marker expression of these cells in the rabbit and mouse CNS, also assessing their existence in other mammalian species. mMap5 cells were never found to co-express the Ng2 antigen. They appear to be a population of glial cells sharing features but also differences with Ng2+progenitor cells. We show that mMap5 cells are newly generated, postmitotic parenchymal elements of the oligodendroglial lineage, thus being a stage-specific population of polydendrocytes. Finally, we report that the number of mMap5 cells, although reduced within the brain of adult/old animals, can increase in neurodegenerative and traumatic conditions.

The light chains of microtubule-associated proteins MAP1A and MAP1B interact with α1-syntrophin in the central and peripheral nervous system

Microtubule-associated proteins of the MAP1 family (MAP1A, MAP1B, and MAP1S) share, among other features, a highly conserved COOH-terminal domain approximately 125 amino acids in length. We conducted a yeast 2-hybrid screen to search for proteins interacting with this domain and identified α1-syntrophin, a member of a multigene family of adapter proteins involved in signal transduction. We further demonstrate that the interaction between the conserved COOH-terminal 125-amino acid domain (which is located in the light chains of MAP1A, MAP1B, and MAP1S) and α1-syntrophin is direct and occurs through the pleckstrin homology domain 2 (PH2) and the postsynaptic density protein 95/disk large/zonula occludens-1 protein homology domain (PDZ) of α1-syntrophin. We confirmed the interaction of MAP1B and α1-syntrophin by co-localization of the two proteins in transfected cells and by co-immunoprecipitation experiments from mouse brain. In addition, we show that MAP1B and α1-syntrophin partially co-localize in Schwann cells of the murine sciatic nerve during postnatal development and in the adult. However, intracellular localization of α1-syntrophin and other Schwann cell proteins such as ezrin and dystrophin-related protein 2 (DRP2) and the localization of the axonal node of Ranvier-associated protein Caspr1/paranodin were not affected in MAP1B null mice. Our findings add to a growing body of evidence that classical MAPs are likely to be involved in signal transduction not only by directly modulating microtubule function, but also through their interaction with signal transduction proteins.

Association of microtubule-associated protein (MAP1B) with growing axons in cultured hippocampal neurons

Microtubule-associated protein 1B (MAP1B) is a major constituent of the neuronal cytoskeleton early in development. This protein is present in embryonic brain and is composed of two isoforms that are the result of differential phosphorylation. We examined the distribution of MAP1B during the differentiation of cultured hippocampal neurons and compared it to that of MAP2 and tubulin. We demonstrated by immunofluorescent doublestaining that MAP1B and MAP2 are colocalized in cell bodies and the minor processes of hippocampal neurons during the early stages of development, before the establishment of neuronal polarity. Later, when neurons acquire axonal and dendritic characteristics, MAP1B is sorted into growing axons, including the growth cone, whereas MAP2 is restricted to dendrites and cell bodies. Unlike tubulin, the localization of MAP1B in growing axons is not uniform. Rather, the protein is found concentrated in the distal portion. During later stages of development, the neurons extend a network of fasciculating axonal and dendritic neurites in which the segregation of MAP1B and MAP2 is maintained. However, the staining of MAP1B in mature neuronal cultures decreases in a pattern that resembles the decline of this protein during brain development. These results support the association of MAP1B with growing axons and its correct developmental regulation in the hippocampal culture system.

Adult neurogenesis in mammals--a theme with many variations

Investigations of adult neurogenesis in recent years have revealed numerous differences among mammalian species, reflecting the remarkable diversity in brain anatomy and function of mammals. As a mechanism of brain plasticity, adult neurogenesis might also differ due to behavioural specialization or adaptation to specific ecological niches. Because most research has focused on rodents and only limited data are available on other mammalian orders, it is hotly debated whether, in some species, adult neurogenesis also takes place outside of the well-characterized subventricular zone of the lateral ventricle and subgranular zone of the dentate gyrus. In particular, evidence for the functional integration of new neurons born in 'non-neurogenic' zones is controversial. Considering the promise of adult neurogenesis for regenerative medicine, we posit that differences in the extent, regional occurrence and completion of adult neurogenesis need to be considered from a species-specific perspective. In this review, we provide examples underscoring that the mechanisms of adult neurogenesis cannot simply be generalized to all mammalian species. Despite numerous similarities, there are distinct differences, notably in neuronal maturation, survival and functional integration in existing synaptic circuits, as well as in the nature and localization of neural precursor cells. We also propose a more appropriate use of terminology to better describe these differences and their relevance for brain plasticity under physiological and pathophysiological conditions. In conclusion, we emphasize the need for further analysis of adult neurogenesis in diverse mammalian species to fully grasp the spectrum of variation of this adaptative mechanism in the adult CNS.

Role of the oligodendroglial cytoskeleton in differentiation and myelination

Oligodendrocytes, the myelin-forming cells of the central nervous system, are in culture characterized by an elaborate process network, terminating in flat membranous sheets that are rich in myelin-specific proteins and lipids, and spirally wrap axons forming a compact insulating layer in vivo. By analogy with other cell types, maintenance and stability of these processes, as well as the formation of the myelin sheath, likely rely on a pronounced cytoskeleton consisting of microtubules and microfilaments. While the specialized process of wrapping and compaction forming the myelin sheath is not well understood, considerably more is known about how cytoskeletal organization is mediated by extracellular and intracellular signals and other interaction partners during oligodendrocyte differentiation and myelination. Here, we review the current state of knowledge on the role of the oligodendrocyte cytoskeleton in differentiation with an emphasis on signal transduction mechanisms and will attempt to draw out implications for its significance in myelination.

Human myelin proteome and comparative analysis with mouse myelin

Myelin, formed by oligodendrocytes (OLs) in the CNS, is critical for axonal functions, and its damage leads to debilitating neurological disorders such as multiple sclerosis. Understanding the molecular mechanisms of myelination and the pathogenesis of human myelin disease has been limited partly by the relative lack of identification and functional characterization of the repertoire of human myelin proteins. Here, we present a large-scale analysis of the myelin proteome, using the shotgun approach of 1-dimensional PAGE and liquid chromatography/tandem MS. Three hundred eight proteins were commonly identified from human and mouse myelin fractions. Comparative microarray analysis of human white and gray matter showed that transcripts of several of these were elevated in OL-rich white matter compared with gray matter, providing confidence in their detection in myelin. Comparison with other databases showed that 111 of the identified proteins/transcripts also were expressed in OLs, rather than in astrocytes or neurons. Comparison with 4 previous myelin proteomes further confirmed more than 50% of the identified proteins and revealed the presence of 163 additional proteins. A select group of identified proteins also were verified by immunoblotting. We classified the identified proteins into biological subgroups and discussed their relevance in myelin biogenesis and maintenance. Taken together, the study provides insights into the complexity of this metabolically active membrane and creates a valuable resource for future in-depth study of specific proteins in myelin with relevance to human demyelinating diseases.

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.

Essential function, sophisticated regulation and pathological impact of the selective RNA-binding protein QKI in CNS myelin development

The selective RNA-binding protein QKI play a key role in advancing oligodendrocyte-dependent myelination, which is essential for the function and development of the CNS. The emerging evidence that QKI abnormalities are associated with schizophrenia and may underlie myelin impairment in this devastating disease has greatly increased interest in understanding the function of QKI. Despite the discovery of the biochemical basis for QKI-RNA interaction, a comprehensive model is currently missing regarding how QKI regulates its mRNA ligands to promote normal myelinogenesis and how deficiency of the QKI pathway is involved in the pathogenesis of human diseases that affect CNS myelin. In this review, we will focus on the role of QKI in regulating distinct mRNA targets at critical developmental steps to promote oligodendrocyte differentiation and myelin formation. In addition, we will discuss molecular mechanisms that control QKI expression and activity during normal myelinogenesis as well as the pathological impact of QKI deficiency in dysmyelination mutant animals and in human myelin disorders.

Truncated Tau with the Fyn-binding domain and without the microtubule-binding domain hinders the myelinating capacity of an oligodendrocyte cell line

The mechanisms underlying developmental myelination have therapeutic potential following CNS injury and degeneration. We report that transplanted central glial (CG)-4 cells had a diminished myelinating capacity in myelin-deficient (md) rats when cells express a mutated form of Tau (Tau [688]), which binds Fyn but not the microtubules. In the brain of the md rats, Tau [688]-transfected CG-4 cells displayed a decrease in cellular process outgrowth and myelination; in the spinal cord the extent of myelination rostral and caudal to the injection site was decreased. In contrast, control Tau [605]-transfected CG-4 cells formed long cellular processes and substantial areas of myelin both in the brain and spinal cord. In culture, Tau [688]-transfected CG-4 cells displayed a decrease in cellular process outgrowth, and Fyn localized largely in the cell body, not the processes. Thus, Tau in oligodendrocytes plays a key role in myelination, and a functional Tau-Fyn interaction might have therapeutic potential during demyelination and myelin repair following CNS injury and degeneration.

Inhibition of extension outgrowth in differentiating rat C6 glioma cells by chlorpyrifos and chlorpyrifos oxon: effects on microtubule proteins

The aim of this work was to assess the toxic effects of the phosphorothionate insecticide chlorpyrifos (CPF) and its major in vivo metabolite chlorpyrifos oxon (CPO) on differentiating rat C6 glioma cells. At sublethal concentrations (1-10 microM), both compounds were able to inhibit the development of extensions from C6 cells induced to differentiate by sodium butyrate. Western blot analysis of C6 cell lysates revealed that 4 h exposure to CPF was associated with decreased levels of the cytoskeletal protein MAP1B compared to controls, whereas the levels of the cytoskeletal proteins tubulin and MAP2c were not significantly affected. Western blot analysis of extracts of cells treated with CPO showed a significant, concentration-dependent decrease in the levels of tubulin after 24 h. MAP-1B levels were also significantly decreased. The above changes were not temporally related to acetylcholinesterase (AChE) inhibition. These results suggest that both CPF and CPO can exert toxic effects directly on glial cell differentiation and that the latter compound has a potent effect on the microtubule network.

The cytoskeleton in oligodendrocytes. Microtubule dynamics in health and disease

Oligodendrocytes have a complex cytoarchitecture and are characterized by an elaborate network of microtubules. They provide the tracks for organelle trafficking and the intracellular translocation of myelin-specific gene products. The integrity of the cytoskeleton is an essential determinant of the function and survival of oligodendrocytes. Microtubule growth and stability are regulated by microtubule-associated proteins. Oligodendrocytes contain a number of microtubule-associated proteins, including the tau proteins, which are developmentally regulated and especially prominent in the branching points of the cellular processes. Process outgrowth is regulated by the interaction of Fyn kinase with the cytoskeleton and by microtubule-severing proteins, such as stathmin. Alterations or disruption of the cytoskeleton and abundant abnormal aggregates of cytoskeletal proteins often accompany neurodegenerative diseases, and inclusion bodies, resembling protein aggregates found in neurons, are prominent in oligodendroglial lesions in white matter pathology. This review emphasizes the role of the cytoskeleton, particularly of microtubules and their associated proteins, in oligodendrocytes during developmental processes. Furthermore, recent data on protein aggregate formation in oligodendroglial cells, which might occur during aging and disease processes, are summarized.

The selective RNA-binding protein quaking I (QKI) is necessary and sufficient for promoting oligodendroglia differentiation

Quaking I (QKI) is a selective RNA-binding protein essential for myelination of the central nervous system. Three QKI isoforms with distinct C termini and subcellular localization, namely QKI-5, QKI-6, and QKI-7, are expressed in oligodendroglia progenitor cells (OPCs) prior to the initiation of myelin formation and implicated in promoting oligodendrocyte lineage development. However, the functional requirement for each QKI isoform and the mechanisms by which QKI isoforms govern OPC development still remain elusive. We report here that exogenous expression of each QKI isoform is sufficient to enhance differentiation of OPCs with different efficiency, which is abolished by a point mutation that abrogates the RNA binding activity of QKI. Reciprocally, small interfering RNA-mediated QKI knockdown blocks OPC differentiation, which can be partly rescued by QKI-5 and QKI-6 but not by QKI-7, indicating the differential requirement of QKI isoform function in advancing OPC differentiation. Furthermore, we found that abrogation of OPC differentiation, as a result of QKI deficiency, is not due to altered proliferation capacity or cell cycle progression. These results indicate that QKI isoforms are necessary and sufficient for promoting OPC development, which must involve direct influence of QKI on differentiation/maturation of OPCs independent of cell cycle exit, likely via regulating the expression of the target mRNAs of QKI that support OPC differentiation.

Microtubule-associated protein 1B, a growth-associated and phosphorylated scaffold protein

Microtubule-associated protein 1B, MAP1B, is one of the major growth associated and cytoskeletal proteins in neuronal and glial cells. It is present as a full length protein or may be fragmented into a heavy chain and a light chain. It is essential to stabilize microtubules during the elongation of dendrites and neurites and is involved in the dynamics of morphological structures such as microtubules, microfilaments and growth cones. MAP1B function is modulated by phosphorylation and influences microtubule stability, microfilaments and growth cone motility. Considering its large size, several interactions with a variety of other proteins have been reported and there is increasing evidence that MAP1B plays a crucial role in the stability of the cytoskeleton and may have other cellular functions. Here we review molecular and functional aspects of this protein, evoke its role as a scaffold protein and have a look at several pathologies where the protein may be involved.

QKI binds MAP1B mRNA and enhances MAP1B expression during oligodendrocyte development

Microtubule-associated protein 1B (MAP1B) is essential for neural development. Besides the abundant expression in neurons, MAP1B recently was found in myelinating oligodendroglia. Moreover, MAP1B deficiency causes delayed myelin development, suggesting the functional importance of MAP1B in oligodendroglia. However, molecular mechanisms that control MAP1B expression in oligodendroglia remain elusive. We report here that MAP1B mRNA is markedly up-regulated in the oligodendroglia cell line CG4 upon induced differentiation, leading to elevated MAP1B protein production. A coordinated regulation of homeoprotein transcription factors was observed during CG4 cell differentiation, which recapitulates the regulation in neurons that promotes MAP1B transcription. Hence, transcriptional regulation of MAP1B appears to be a common mechanism in both neurons and oligodendroglia. In addition, we found posttranscriptional regulation of MAP1B mRNA by the selective RNA-binding protein QKI in oligodendroglia. The 3'UTR of MAP1B mRNA interacts with QKI, and oligodendroglia-specific QKI-deficiency in the quakingviable mutant mice resulted in reduced MAP1B mRNA expression. Moreover, RNAi-mediated QKI-knockdown caused destabilization of the MAP1B mRNA in CG4 cells. Furthermore, forced expression of exogenous QKI was sufficient for promoting MAP1B expression. Because QKI is absent in neurons, QKI-dependent stabilization of MAP1B mRNA provides a novel mechanism for advancing MAP1B expression specifically in oligodendroglia during brain development.

Mice deficient in microtubule-associated protein MAP1B show a distinct behavioral phenotype and altered retina function

We investigated mice deficient for the microtubule-associated protein MAP1B, a cytoskeletal element highly expressed in the developing nervous system, for altered performance in behavior, learning, and memory. Using the multiple T-maze, the open field and the Morris water maze we found that mice homozygous for a deletion of the MAP1B gene demonstrate impaired locomotor activity most likely correlated to a lack of physical endurance in general. In contrast, there were no significant differences in cognitive function and memory retention. In addition, we performed electroretinography and observed a reduction of the a-wave amplitude in response to single flash, white light stimulation. Taken together, these data provide further evidence for an important role of MAP1B in synaptic neurotransmission.

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.

Aberrant neuronal-glial differentiation in Taylor-type focal cortical dysplasia (type IIA/B)

Focal cortical dysplasia (FCD) type IIA/B (Taylor type) is a malformation of cortical development characterized by laminar disorganization and dysplastic neurons. FCD IIA and FCD IIB denote subtypes in which balloon cells are absent or present, respectively. The etiology of FCD IIA/B is unknown, but previous studies suggest that its pathogenesis may involve aberrant, mixed neuronal-glial differentiation. To investigate whether aberrant differentiation is a consistent phenotype in FCD IIA/B, we studied a panel of neuronal and glial marker antigens in a series of 15 FCD IIB cases, and 2 FCD IIA cases. Double-labeling immunofluorescence and confocal imaging revealed that different combinations of neuronal and glial antigens were co-expressed by individual cells in all cases of FCD IIA/B, but not in control cases of epilepsy due to other causes. Co-expression of neuronal and glial markers was most common in balloon cells, but was also observed in dysplastic neurons. The relative expression of neuronal and glial antigens varied over a broad range. Microtubule-associated protein 1B, an immature neuronal marker, was more frequently co-expressed with glial antigens than were mature neuronal markers, such as neuronal nuclear antigen. Our results indicate that aberrant neuronal-glial differentiation is a consistent and robust phenotype in FCD IIA/B, and support the hypothesis that developmental defects of neuronal and glial fate specification play an important role in its pathogenesis.

Expression of MAP1B protein and its phosphorylated form MAP1B-P in the CNS of a continuously growing fish, the rainbow trout

Microtubule-associated protein-1B (MAP1B), and particularly its phosphorylated isoform MAP1B-P, play an important role in axonal outgrowth during development of the mammalian nervous system and have also been shown to be associated with axonal plasticity in the adult. Here, we used antibodies and mRNA probes directed against mammalian MAP1B to extend our analysis to fish species, trout (Oncorhynchus mykiss), at different stages of development. The specificity of the cross-reaction of our anti-total-MAP1B/MAP1B-P antibodies was confirmed by Western blotting. Trout MAP1B-like proteins exhibited about the same apparent molecular weight (320 kDa) as rat-MAP1B. Immunohistochemistry and in situ hybridization analysis performed on hindbrain and spinal cord revealed the presence of MAP1B in neurons and some glial subpopulations. Primary sensory neurons and motoneurons maintain high levels of MAP1B expression from early stages throughout adulthood, as has been shown for mammals. Unlike mammals, however, MAP1B and axon-specific MAP1B-P continue to be strongly expressed by hindbrain neurons projecting into spinal cord, with the important exception of Mauthner cells. MAP1B/MAP1B-P immunostaining were also detected elsewhere within the brain, including axons of the retino-tectal projection. This obvious difference between adult fish and mammals is likely to reflect the capacity of fish for continued growth and regeneration. Our results suggest that MAP1B/MAP1B-P expression is generally maintained in neurons known to regenerate after axotomy. The regenerative potential of the adult nervous system may in fact depend on continued expression of neuron-intrinsic growth related proteins, a feature of MAP1B that appears phylogenetically conserved.

Microtubule-associated protein 1B function during normal development, regeneration, and pathological conditions in the nervous system

Microtubule-associated protein 1B is the first MAP to be expressed during the development of the nervous system. Several different approaches have revealed that MAP1B function is associated with microtubule and actin microfilament polymerization and dynamics. In recent years, the generation of molecular models to inactivate MAP1B function in invertebrates and mammals has sparked some controversy about the real role of MAP1B. Despite discrepancies between some studies, it is clear that MAP1B plays a principal role in the development of the nervous system. In this article, we summarize the evidence for MAP1B function in a wide variety of cellular processes implicated in the proper construction of the nervous system. We also discuss the role of MAP1B in pathological processes.

Microfilament and microtubule organization and dynamics in process extension by central glia-4 oligodendrocytes: evidence for a microtubule organizing center

Microfilaments in freshly adhering CG-4 cells and differentiated CG-4 oligodendrocytes are concentrated at the tips and edges of rapidly forming processes while microtubules are concentrated in new processes and extend from a concentrated spot of alpha-tubulin staining in the cell body to the cell periphery. In motile bipolar CG-4 cells, microfilaments are heavily concentrated at the flattened end of one process and along the rim of processes and the cell body: microtubules are concentrated along main processes and splay out into process tips and the cell body. In differentiated CG-4 oligodendrocytes, microfilaments are concentrated at the many process tips, in filopodia and in fine processes, but are not obvious in main processes where separate bundles of microtubules, which diverge at process branch points, are concentrated. gamma-tubulin, involved in microtubule nucleation, is concentrated at a small discrete area in the cell body, indicative of a microtubule organizing center. Polymerization of both actin and tubulin is required for initial process elaboration. Depolymerization of microtubules, but not of microfilaments, causes complete retraction of bipolar CG-4 cell processes. This process retraction does not occur if microfilaments are depolymerized first, indicating that process extension/retraction in motile bipolar CG-4 cells may occur by a balance of motor protein-driven forces as suggested for growth cone motility. Cytoskeleton organization in CG-4 cells is very similar to that reported for oligodendrocytes. CG-4 cells are thus a useful model for investigating the signals and mechanisms regulating oligodendrocyte process dynamics.

Localization of the neuronal class III beta-tubulin in oligodendrogliomas: comparison with Ki-67 proliferative index and 1p/19q status

The class III beta-tubulin isotype (betaIII) is widely regarded as a neuronal marker in development and neoplasia. Whereas the expression of betaIII in neuronal/neuroblastic tumors is differentiation-dependent, the aberrant expression of this cytoskeletal protein in astrocytomas is associated with an ascending gradient of malignancy. To test the generality of this observation we have compared the immunoreactivity (IR) profiles of the betaIII isotype with the Ki-67 nuclear antigen proliferative index in 41 archival, surgically excised oligodendrogliomas (32 classical [WHO grade II] and 9 anaplastic [WHO grade III]). Seventeen of 41 tumors were examined by quantitative microsatellite analysis for loss of 1p and/or 19q. Minimal deletion regions were defined on 1p (D1S468, D1S214) and 19q (D19S408, D19S867). Three of 10 classical oligodendrogliomas had combined 1p/19q loss, while 2 exhibited loss of either 1p or 19q. Three of 7 anaplastic tumors had combined 1p/19q loss. BetaIII IR was present in all tumors, but was significantly greater in the anaplastic (median labeling index [MLI] 61%, interquartile range [IQR] 55%-64%) as compared with the classical variants (MLI, 19%, IQR, 11-36%) (p < 0.0001). A highly significant relationship was found to exist between betaIII and Ki-67 LIs (betaIII, p < 0.0001 and Ki-67, p < 0.0001. r = 0.809). BetaIII localization delineated hitherto understated unipolar or bipolar tumor phenotypes with growth cones and leading cell processes resembling migrating oligodendrocyte progenitor cells. Codistribution of betaIII and GFAP IR was present in "gliofibrillary" tumor areas. Synaptophysin IR was detected in rare tumor cells (mean LI, 0.7%), and only in 4/41 samples (10%), denoting a lack of relationship between betaIII and synaptophysin expression. No significant differences in betaIII LIs were observed in tumors with 1p and/or 19q loss as compared to those with 1p/19q intact status. Increased betaIII IR in oligodendrogliomas is associated with an ascending degree of malignancy and thus is a potentially useful tumor marker. However, the significance of high betaIII LIs in low-grade oligodendrogliomas with respect to prognostic and predictive value requires further evaluation. Class III beta-tubulin expression in oligodendrogliomas should not be construed as a priori evidence of divergent neuronal differentiation.

Distribution of the phosphorylated form of microtubule associated protein 1B in the fish visual system during optic nerve regeneration

Microtubule associated proteins are a heterogeneous group of proteins that have been implicated in regulating microtubule stability. They play an important role in the organisation of the neuronal cytoskeleton during neurite outgrowth, plasticity and regeneration. The fish visual system presents a considerable degree of plasticity. Thus, the retina grows continually throughout life and the optic nerve regenerates after crush. In the present study, we compared the distribution of the microtubule associated protein 1B in its phosphorylated form (MAP1B-phos) in the normal adult fish visual system with that observed during optic nerve regeneration after adult optic nerve crush using a specific monoclonal antibody mAb-150. Expression of MAP1B-phos was observed in some ganglion cell somata and in developing, growing axons within the control optic nerve. Few immunoreactive terminals were seen in the control optic tectum. After optic nerve crush, we found additional MAP1B-phos expression in regenerating axons throughout the visual system. Our results demonstrate that MAP1B-phos is present in growing and regenerating axons of fish retinal ganglion cells, which suggests that the phosphorylated form of MAP1B may play an important role in developmental and regeneration processes within the fish central nervous system.

Neu differentiation factor regulates tau protein and mRNA in cultured neonatal oligodendrocytes

Axonal signals activate myelinogenesis via regulation of the extent to which oligodendrocyte (OLG) processes wrap around the axon. The cytoskeleton in OLG processes is actively involved in myelination and is a putative target for axonal regulation of myelination. The axon-associated neuregulins may regulate the cytoskeleton extensions in OLG processes. Here, we report that the neuregulin neu differentiation factor (NDF) increases the expression of tau mRNA and tau protein in OLGs. Treatment of neonatal OLGs with alpha-NDF or beta-NDF resulted in dramatic increases in the length of OLG processes, which appeared either as singular unbranched extensions or as a network of extensively branched processes. By immunoblot analysis with tau-1 mAb, which recognizes the dephosphorylated form of the tau proteins, neonatal OLGs treated with alpha-NDF or beta-NDF, had an increase in tau protein levels. The increase of tau levels in beta-NDF-treated cells is much greater than the twofold increase present in alpha-NDF-treated cells. By immunoblot analysis with the phosphorylation-insensitive tau-5 mAb, beta-NDF-treated cells had a twofold increase in tau. Immunoblot analysis suggest that alpha-NDF and beta-NDF promote a twofold increase in the tau protein levels in OLG, with the beta-factor also promoting a tau dephosphorylation. Using promoters spanning the amino-terminal region of tau, we found that OLGs treated with alpha-NDF or beta-NDF contained approximately twofold more tau mRNA than untreated cells. However, there was no qualitative difference between control and NDF-treated cells in the pattern of tau mRNA isoforms expressed. A model is proposed in which the axonal NDF-induced regulation of tau expression in OLGs may be part of the mechanism by which the axon regulates myelination.

Organization and functional roles of the cytoskeleton in oligodendrocytes

Mature oligodendrocytes are characterized by their numerous cytoplasmic extensions and flat membranous sheets. These sheets contain an extensive cytoskeletal network of microtubules (MTs) that maintain the cellular morphology, are specifically important for cellular sorting, and provide the rails for organelle trafficking. Mitochondria are localized in the primary and secondary processes and follow the tracks of the MTs in the cytoplasmic extensions. Oligodendrocytes express microtubule associated proteins (MAPs), specifically MAP2 and tau, which might be involved in the regulation and stabilization of the dynamic MT network in the myelin-containing cellular processes. Tau and MAP2 heterogeneity increases during oligodendroglia maturation, and in mature oligodendrocytes tau mRNA with four MT binding domains are more prominent than in progenitor cells. Filamentous cell inclusions are a unifying mechanism underlying a variety of late-onset neurodegenerative disorders and have mainly been viewed as neuron-specific. Recent evidence indicated that glial changes occur in CNS degenerative diseases and seem to be a more common feature than previously thought. Glial fibrillary tangles (GFTs) in oligodendrocytes were observed in familial multiple system tauopathy, and glial cytoplasmic inclusions (GCIs) and oligodendroglia degeneration are the histological hallmark of multiple system atrophy (MSA). GCIs are associated with MTs and contain stress proteins and MAPs. Thus, neurons and glial cells share common cytoskeletal pathologies. During health and disease, MAPs might be important regulators of the structural stability and plasticity of the oligodendroglia cytoskeleton.

Intracellular distribution of myelin protein gene products is altered in oligodendrocytes of the taiep rat

Hypomyelination and subsequent demyelination of the taiep rat CNS are thought to result from the abnormal accumulation of microtubules (MTs) in oligodendrocytes that disrupts intracellular transport of components needed to form and maintain the myelin sheath. In this study, myelin gene expression was evaluated in mutant and age-matched controls to determine if MT abnormalities affect the distribution of myelin proteins and their mRNAs. Immunohistochemical analysis of taiep brains and spinal cords revealed a gradual decrease in levels of several myelin proteins including myelin basic protein (MBP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), and 2',3'-cyclic nucleotide 3'-phosphodiesterase. Accompanying early declines in MAG and PLP, accumulations of immunoreactive products were detected within oligodendrocytes, consistent with a defect in protein trafficking. Northern blot analysis indicated that diminishing protein levels could not be attributed to changes in transcriptional activity, except for MBP of which mRNA levels decreased with age. Cellular localization of MBP mRNA by in situ hybridization further revealed that transcripts were concentrated within oligodendrocyte cell bodies instead of uniformly distributed throughout processes. These results demonstrate that changes in expression and intracellular localization of myelin gene products are concurrent with increases in MT mass in taiep oligodendrocytes and support our hypothesis that cytoskeletal defects prevent the normal transport of elements required for the formation and maintenance of the myelin sheath.

Glial cells as targets and producers of neurotrophins

Glial cells fulfill important tasks within the neural network of the central and peripheral nervous systems. The synthesis and secretion of various polypeptidic factors (cytokines) and a number of receptors, with which glial cells are equipped, allow them to communicate with their environment. Evidence has accumulated during recent years that neurotrophins play an important role not only for neurons but also for glial cells. This brief update of some morphological, immunocytochemical, and biochemical characteristics of glial cell lineages conveys our present knowledge about glial cells as targets and producers of neurotrophins under normal and pathological conditions. The chapter discusses the presence of neurotrophin receptors on glial cells, glial cells as producers of neurotrophins, signaling pathways downstream Trk and p75NTR, and the significance of neurotrophins and their receptors for glial cells during development, in cell death and survival, and in neurological disorders.

The cytoskeletal components of the myelin fraction are affected by a single intracranial injection of apotransferrin in young rats

We have previously shown that in rat pups intracranially injected with a single dose of apotransferrin (aTf), there is an early oligodendroglial cell OLGc differentiation. The expression of the mRNAs of myelin basic proteins and of 2',3' cyclic nucleotide 3'-phosphodiesterase and the amount of the corresponding proteins, as well as myelin glycolipids and phospholipids, were significantly increased in these animals at 10 and 17 days of age. Microtubules and myelin basic proteins appear to be closely associated in OLGc and it has been shown that the mRNAs of myelin basic proteins are concentrated in the OLGc processes. The aim of this work was to clarify if the accelerated myelination produced by aTf could be linked to changes in certain cytoskeletal elements present in the myelin fraction such as tubulin, actin, and different microtubule-associated proteins (MAPs). A significant increase in the expression of the mRNA of tubulin and actin was observed in the brain of the aTf-treated animals. Several MAPs, particularly MAP 1B and stable tubule only peptide as well as actin and tubulin, were markedly increased in the Triton X-100 insoluble pellet obtained from the myelin fraction of these animals. The changes that we have previously described in the myelin of aTf intracranially injected rats, could be the consequence of its action on the cytoskeletal network of the OLGc. An enlargement of this structure would result in a more efficient and faster movement of the different components that are normally transported to the myelin by the cytoskeleton of this cell.

The oligodendroglia cytoskeleton in health and disease

Oligodendrocytes have a high rate of synthetic activity and produce vast amounts of myelin. The membrane production requires specific sorting and transport processes and structural support. In culture, oligodendrocytes extend flat membranous sheets containing an extensive cytoskeletal network of microtubules (MTs) and microfilaments (MFs). The microtubules participate in the elaboration and stabilization of the myelin-containing cellular processes and have an impact not only on the complex oligodendroglia architecture but also influence their functions. They participate in intracellular sorting processes and the translocation of myelin basic protein (MBP) mRNAs to the forming myelin sheath. The two major groups of neuronal microtubule-associated proteins (MAPs), MAP2 and tau are expressed in oligodendrocytes and might be involved in the regulation of MT stability and organization. Myelin-specific proteins, such as MBP and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), interact with the cytoskeleton. Glial changes occur in a variety of neurodegenerative diseases, and glial fibrillary tangles and glial cytoplasmic inclusions (GCls), containing abnormal microtubular structures which stain positively for stress proteins and microtubule-associated proteins, are found in oligodendrocytes of the affected brains. The role of MTs and their associated proteins in oligodendrocytes during normal development and pathological situations is specifically emphasized in this review.

Developmental regulation of alternatively spliced isoforms of mRNA encoding MAP2 and tau in rat brain oligodendrocytes during culture maturation

Oligodendrocytes are responsible for the formation and maintenance of the myelin sheaths in the central nervous system (CNS), and microtubules essentially participate in the elaboration and stabilization of myelin-containing cellular processes. We have shown before that the two major groups of neuronal microtubule-associated proteins (MAPs), MAP2 and tau, are expressed in the myelin forming cells of the CNS (Mueller et al. [1997] Cell Tissue Res. 288:239-249). Here we demonstrate for the first time that during culture maturation, changes in mRNA splicing and a shift from immature to mature MAP2 and tau mRNAs occur in oligodendrocytes. Similarly to neurons, a developmental shift from MAP2 isoforms with 3 microtubule (MT)-binding domains (3R) to the isoforms with 4 MT-binding domains (4R) is observable. MAP2c constitutes the major MAP2 isoform in oligodendrocytes. They contain tau mRNA splice products with both 3 and 4 MT-binding repeats (3R, 4R) with no amino terminal insert or with exon 2, and do not express isoforms containing exon 3. The shortest form tau 1 (3R; no inserts) representing the immature tau isoform is most prominently expressed in early progenitor cells and gradually decreases during culture maturation, while tau 5 (4R; with exon 2) appears later during in vitro differentiation. The product corresponding to tau 2 (3R; with exon 2) and tau 4 (4R; no inserts) remains approximately at the same level. Hence, the occurrence of MAPs in oligodendrocytes is developmentally regulated. While in progenitor cells, 3R- and 4R-MAP2c are expressed at approximately the same level, in mature oligodendrocytes after 12 days in vitro, the ratio of 4R- to 3R-MAP2c is nearly 2. In contrast, the ratio of 4R- to 3R-tau in progenitor cells is 1:3 and shifts to 1:1 after 12 days in culture.

Induction of microtubule-associated protein 1B expression in Schwann cells during nerve regeneration

Microtubule-associated protein 1B (MAP1B) is expressed at high levels during development of the nervous system and is localized primarily in neurons while specific phosphorylated isoforms of MAP1B are localized exclusively in growing axons. The levels of MAP1B are down regulated in most regions of the adult CNS, but remain high in neurons and axons of the PNS. This study demonstrates that the expression of MAP1B is induced in adult Schwann cells following sciatic nerve lesion and regeneration. High levels of both mRNA and the MAP1B protein were detected in Schwann cells associated with the axotomized distal stump. Expression of MAP1B was also observed in cultured primary Schwann cells from neonatal rats. The properties of the MAP1B protein in cultured Schwann cells were further characterized by Western blot analysis using specific antibodies that recognize the N-terminal, middle and C-terminal domains of MAP1B. All of these antibodies detected a protein of 320-340 kDa demonstrating that MAP1B expressed by Schwann cells is very similar, or identical, to MAP1B expressed by neurons. The phosphorylation of MAP1B in Schwann cells was also studied using monoclonal antibodies (mAb) that recognize specific phosphorylation epitopes. The results indicated that the expression of MAP1B in Schwann cells exhibited a differential phosphorylation state that was recognized by mAb 1B6 but not by other mAbs, including 1B-P, 150 and RT97, that recognize phosphorylated MAP1B in growing axons. We therefore conclude that MAP1B is expressed in Schwann cells during both development and axonal regeneration, suggesting that the developmental pattern of MAP1B in these cells is recapitulated in adult Schwann cells during the early stages of regeneration and remyelination of injured peripheral axons. The presence of MAP1B in Schwann cells may support morphological changes of these cells, particularly the formation of processes prior to their differentiation into myelin forming Schwann cells.

Class III beta-tubulin isotype (beta III) in the adrenal medulla: III. Differential expression of neuronal and glial antigens identifies two distinct populations of neuronal and glial-like (sustentacular) cells in the PC12 rat pheochromocytoma cell line maintained in a Gelfoam matrix system

BACKGROUND

The rat PC12 pheochromocytoma cell line provides an established system for the study of neuronal differentiation. To our knowledge, glial differentiation has not been reported in this cell line.

METHODS

We have studied, by immunohistochemistry and immunoblotting, the presence of neuronal cytoskeletal antigens [class III beta-tubulin isotype (beta III), microtubule associated proteins MAP2, MAP1B and tau, and different neurofilament (NF) protein components], and synaptophysin in comparison with the glial fibrillary acidic protein (GFAP) and S-100 protein in the PC12 cell line. In three different experiments, PC12 cells were maintained in a three-dimensional gelatin foam (Gelfoam) matrix system for up to 34 days with and without treatment with 1 mM dibutyryl cyclic (dc)AMP. Immunohistochemistry was performed on explants ranging from 2 to 32 days-in vitro, which were fixed in either Bouin's solution, 70% ethanol, or 10% neutral-buffered formalin and embedded in paraffin. Immunoblotting was performed on Gelfoam explants with a panel of antibodies against all aforementioned neuronal and glial markers. Additional immunoblot experiments using anti-GFAP and anti-beta III monoclonal antibodies in cell suspensions and homogenates from PC12 monolayer cultures were carried out to compare growth conditions in relation to the expression of these proteins.

RESULTS

Beta III and MAP2 were demonstrated by immunohistochemistry and immunoblotting of PC12 explants maintained for up to 32 days in Gelfoam matrices with and without treatment with dcAMP. Intense filamentous and granular beta III staining of PC12 cells was observed in dcAMP-treated cultures concomitant with neuronal morphologic alterations (neuritogenesis and ganglionic phenotype). In untreated cultures, beta III staining was present in less differentiated cells, as well in cells undergoing neuritic development. The neuronal phenotype of PC12 cells was confirmed by staining for MAP2, tau, and NF proteins, as well as for synaptophysin. The presence of beta III, MAP2, MAP1B, tau, and NF proteins was confirmed by immunoblotting. Clusters of GFAP-positive and S-100 protein-positive spindle cells, phenotypically distinct from the chromaffin-like or neuronal cells, were demonstrated in Gelfoam explants at 5-30 days in vitro. In 30-day-old cultures treated with dcAMP, there was strong filamentous GFAP and diffuse S-100 protein staining in an increased number of sustentacular-like PC12 cells. GFAP staining was corroborated by immunoblotting of explants maintained under identical conditions in vitro. In contrast, immunoblots performed on homogenates from PC12 suspension and monolayer cultures were GFAP-negative.

CONCLUSIONS

Neuronal and glial-like, presumed sustentacular, phenotypes were demonstrated in PC12 cells grown in Gelfoam matrices with and without treatment with dcAMP for up to 34 days. To our knowledge, the occurrence of glial differentiation in the PC12 line is a hitherto unreported finding. Adult rat medullary sustentacular cells are known to express S-100 and GFA proteins (Suzuki and Kachi, Kaibogaku Zasshi-Anat 70(2): 130-139, 1995), and the organ culture system employed in our study may well have favored this direction of differentiation.

Delayed development of nervous system in mice homozygous for disrupted microtubule-associated protein 1B (MAP1B) gene

Microtubule-associated protein 1B (MAP1B), one of the microtubule-associated proteins (MAPs), is a major component of the neuronal cytoskeleton. It is expressed at high levels in immature neurons during growth of their axons, which indicates that it plays a crucial role in neuronal morphogenesis and neurite extension. To better define the role of MAP1B in vivo, we have used gene targeting to disrupt the murine MAP1B gene. Heterozygotes of our MAP1B disruption exhibit no overt abnormalities in their development and behavior, while homozygotes showed a slightly decreased brain weight and delayed nervous system development. Our data indicate that while MAP1B is not essential for survival, it is essential for normal time course development of the murine nervous system. These conclusions are very different from those of a previous MAP1B gene-targeting study (Edelmann, W., M. Zervas, P. Costello, L. Roback, I. Fischer, A. Hammarback, N. Cowan, P. Davis, B. Wainer, and R. Kucherlapati. 1996. Proc. Natl. Acad. Sci. USA. 93: 1270-1275). In this previous effort, homozygotes died before reaching 8-d embryos, while heterozygotes showed severely abnormal phenotypes in their nervous systems. Because the gene targeting event in these mice produced a gene encoding a 571-amino acid truncated product of MAP1B, it seems likely that the phenotypes seen arise from the truncated MAP1B product acting in a dominant-negative fashion, rather than a loss of MAP1B function.

Microtubule-associated proteins (MAPs) in the peripheral nervous system during development and regeneration

In this article, we have described the structure and distribution of the various variants of the microtubule-associated proteins (MAPs), tau, MAP2, MAP1A, and MAP1B, that are expressed in the dorsal root ganglion (DRG) and spinal cord during development and regeneration. We have summarized the data on their gene structure and compared the sequence of the major transcripts encoding these MAPs that are expressed in the brain, the spinal cord, and the DRG. Finally, we have surveyed the studies that used a variety of experimental approaches (e.g., antisense inhibition, transgenic knockouts, and expression in neuronal and nonneuronal cells) to understand the functional significance of MAPs heterogeneity and differences observed between the central nervous system (CNS) and the peripheral nervous system (PNS) both during development and regeneration.

Role of vimentin in early stages of neuritogenesis in cultured hippocampal neurons

Vimentin is expressed initially by nearly all neuronal precursors in vivo, and is replaced by neurofilaments shortly after the immature neurons become post-mitotic. Moreover, both vimentin and neurofilaments can be detected transiently within the same neurite, leaving open the possibility that vimentin may play a role in the early stages of neuritogenesis. In the present study, cultured hippocampal neurons, which transiently express vimentin in culture, were treated with sense- and antisense-oriented deoxyoligonucleotides encoding regions of the vimentin sequence that overlap the translation initiation codon. Antisense oligonucleotide treatment reduced vimentin-immunoreactivity to background levels. Moreover, while 90-100% of cultured hippocampal neurons elaborated neurites within the first 24 hr following plating, only 24-30% did so in the presence of vimentin antisense oligonucleotides. Inhibition of neurite outgrowth was reversible following removal of antisense oligonucleotide. These findings substantiate earlier studies in neuroblastoma cells, indicating a possible role for vimentin in the initiation of neurite outgrowth.

Microtubule-associated proteins in developing oligodendrocytes: transient expression of a MAP2c isoform in oligodendrocyte precursors

The morphological differentiation of oligodendrocytes is characterized by the formation of multiple, microtubule-rich processes which endow these cells with the ability to myelinate many axons simultaneously. Since microtubule-associated proteins (MAPs) strongly influence the structure and function of microtubules, we have investigated their expression in cultured differentiating oligodendrocytes in order to gain insights into MAP function during process formation and stabilization. MAP1B has been compared with two other structural MAPs: MAP4, which is an ubiquitously expressed protein, and MAP2, which hitherto was thought to be confined to neurons and reactive astrocytes. Immunofluorescence microscopy showed that the colocalization of MAP4 with microtubules in oligodendrocyte processes is not as extensive as found previously for MAP1B (Vouyiouklis and Brophy: J Neurosci Res 35:257-267, 1993). Nevertheless, like MAP1B, the expression of MAP4 increases during oligodendrocyte differentiation. In contrast, the expression of MAP2 is transiently elevated in preoligodendrocytes but declines precipitously at the onset of terminal differentiation. Cells of the oligodendrocyte lineage exclusively express a novel isoform of MAP2c which is primarily localized in the cell bodies of preoligodendrocytes. This suggests that MAP2c assists in the initiation of process extension rather than in the stabilization of microtubules in the cytoplasm-filled membranous extensions of mature cells. MAP-tau was not expressed at any developmental stage by oligodendrocytes. The distinct subcellular localizations and patterns of developmental expression of MAP1B, MAP4, and MAP2c suggest that these MAPs have different roles in the regulation of the microtubule network during the differentiation of myelin-forming oligodendrocytes.

Myelin mutants: new models and new observations

The myelin mutants have been extensively used as tools to study the complex process of myelination in the central and peripheral nervous system. A multidisciplinary approach to the study of these models ultimately allows a correlation to be made between phenotype and genotype. This correlation may then lead to the formation of new hypotheses about the functions of the products of genes involved in myelination. This review presents a number of new myelin mutants which have recently been described. The species involved include mouse, rat, rabbit, hamster, and dog models. The genetic defect has not been elucidated in all of these animals, but most have been characterized clinically and pathologically, and, in some cases, biochemically. In addition, a better known myelin mutant, the trembler mouse, is discussed. Recent molecular findings have brought this fascinating mutant to the forefront of the field of peripheral nervous system research. The range of abnormalities in the mutants described in this review includes defects in specific myelin proteins, suspected abnormalities in membrane formation, and apparent defects of the oligodendrocyte cytoskeleton. These findings underscore the complexity of the myelination process and highlight the numerous ways in which it can be disrupted.

Expression and distribution of phosphorylated MAP1B in growing axons of cultured hippocampal neurons

Microtubule associated proteins (MAPs) interact with tubulin to modulate neurite stability and growth during development. The phosphorylated form of one of these MAPs, MAP1B (MAP1B-P) is hypothesized to be of particular importance for the regulation of neurite outgrowth. To investigate the mechanisms by which MAP1B and MAP1B-P contribute to this regulation, we used a new antibody against an isoform of MAP1B-P to determine its pattern of expression during neuronal development in vitro. We examined cultured hippocampal neurons because these provide a well-established system to evaluate the development of axons and dendrites. MAP1B, MAP1B-P and MAP2 colocalized to the cell bodies and minor processes during the first 24 hours of culture, but MAP1B-P also extended well into the growth cones. As neurite outgrowth and differentiation proceeded, MAP1B and MAP1B-P became localized to the cell bodies and axons, and MAP2 to the cell bodies and dendrites. After 3 days, MAP1B-P declined in the cell body and was segregated to the distal axon; MAP1B remained in the cell body, but was also concentrated in the distal axon. Over 5-9 days in culture, MAP1B-P levels decreased and became undetectable; MAP1B levels decreased later (19-23 days). MAP2 levels, however, remained high through the entire culture period in cell bodies and dendrites. These results are consistent with the hypothesis that MAP1B-P plays an important role in the initiation and elongation of axons by regulating the dynamics of microtubules near the growth cone: MAP1B-P expression is greatest during the period of active neurite extension, is particularly prominent in growth cones where axon outgrowth is most active, and decreases along with the decline in active axon extension.

Identification of novel mRNAs expressed in oligodendrocytes

To identify new proteins, which are expressed in oligodendrocytes and which may have a functional role in myelination, a rat oligodendrocyte cDNA library was screened using differential and subtractive screening techniques. Ten clones that have elevated levels of expression in brain were isolated. Two of these clones were characterized further and one clone, pC26.H2, was found to be closely related to mouse stearoyl-CoA desaturase 2 (SCD2), which catalyzes the synthesis of unsaturated fatty acid. From Northern blot and in situ hybridization studies, SCD2 mRNA was expressed primarily in brain with lower levels found in lung and spleen. In brain sections, SCD2 mRNA was found primarily in oligodendrocytes, although mRNA was detected at a low level in neurons, in particular in Purkinje cells in the cerebellum. Northern blot analysis of the other clone, p973.HB, indicated that it was expressed more selectively in brain. In mixed glial cultures oligodendrocytes were the only cells that expressed this mRNA, whereas in brain, neurons expressed this mRNA at a higher level than in oligodendrocytes. This clone is being actively pursued because of its unique expression exclusively in oligodendrocytes and neurons.

Microtubule-associated protein 1B (MAP1B) is present in glial cells phosphorylated different than in neurones

A panel of four anti-MAP1B antibodies have been used to study the presence and post-translational modification of MAP1B in primary cultures of glial cells. Two antibodies (150 and 125) recognize phosphorylated epitopes whereas the other two (531 and 842) recognize non-phosphorylated phosphorylatable epitopes on the MAP1B molecule. Immunofluorescence and Western blot analysis with antibodies 531 and 842 revealed the presence of small amounts of MAP1B-like immunoreactivity in type 1 astrocytes and a greater content in more differentiated glial cells found in long-term cultures. By immunofluorescence, these latter cells gave positive immunostaining with antibody 125, which recognizes a phosphorylated epitope phosphorylated by casein kinase II. Antibody 150, which reacts to a phosphorylated epitope on the MAP1B molecule, did not show any detectable immunoreactivity in glial cells cultures, either by immunofluorescence or Western blot. All four antibodies recognized hippocampal neurones in culture, with especially intense immunostaining in cell bodies and axons, and reacted strongly with protein present in hippocampal neurones extracts showing an electrophoretic mobility similar to that of brain MAP1B. In mixed optic nerve glial cell cultures, anti-galactocerebroside (GalC) positive cells gave also positive staining with antibodies 531 and 125. We propose that MAP1B is present in cultures of glial cells in moderate amounts and with a phosphorylation state different than in neurones. Thus, less differentiated glial cells, such as type 1 astrocytes, have a small amount of MAP1B, mainly in a non-phosphorylated form, which is spread diffusely in the cytoplasm and probably does not interact with microtubules. More differentiated glial cells, such as oligodendrocytes, show a greater content in MAP1B which, at least in part, is phosphorylated by a casein kinase II-like activity.

The distribution of myelin basic protein mRNAs within myelinating oligodendrocytes

The nervous system contains oligodendrocytes with processes that are greatly extended in space. It is now clear that there are numerous complex, poorly understood mechanisms by which polypeptides are synthesized and delivered to their sites of function in these cells. One mechanism is by the active positioning of mRNAs encoding certain proteins to restricted intracellular subdomains. Perhaps the best studied example of this in the vertebrate CNS is the translocation of myelin basic protein mRNAs to the forming myelin sheath, where the newly synthesized polypeptides, which avidly associate with membranes, can be directly incorporated into the myelin membrane. Evidence for this conclusion is presented here in the context of related work on the general phenomenon of mRNA translocation that is under analysis in other systems.

A phosphorylation epitope on MAP 1B that is transiently expressed in growing axons in the developing rat nervous system

We have isolated a monoclonal antibody (150) that recognizes a phosphorylation epitope on the microtubule-associated protein (MAP) 1B. Immunoblot analysis of the developing rat central nervous system shows that monoclonal antibody 150 is directed against a protein of approximately 325 kDa (MAP 1B) that copolymerizes with microtubules through successive cycles of temperature-dependent assembly and disassembly. Furthermore, immunoprecipitated MAP 1B contains the epitope recognized by monoclonal antibody 150. Removal of phosphate from blotted proteins using alkaline phosphatase abolishes the binding of monoclonal antibody 150 to MAP 1B, indicating that the epitope is phosphorylated. In the developing rat nervous system, immunohistochemistry with monoclonal antibody 150 shows that the phosphorylation epitope on MAP 1B is transiently expressed in growing axons but not in dendrites. For instance, in the neonatal rat cerebellum, the parallel fibres of granule cells are stained only during elongation and not after synaptogenesis. The monoclonal antibody 150 epitope is also transiently expressed in radial glial fibres and in certain cell nuclei. All immunostaining of sections with monoclonal antibody 150 was completely abolished by alkaline phosphatase treatment. These observations and previous ones made by us in cell culture (Mansfield et al., J. Neurocytol., 20, 654-666, 1991) suggest that the phosphorylation epitope on MAP 1B recognized by monoclonal antibody 150, which has not been previously detected in vivo, may be important in axonogenesis.

Microtubule-associated protein MAP1B expression precedes the morphological differentiation of oligodendrocytes

The microtubule-associated protein MAP1B is believed to play an important role in the outgrowth of neurites from neurons (Tucker and Matus, Dev Biol 130: 423-434, 1988). We have investigated the possibility that MAP1B might participate in the formation of processes in cultured oligodendrocytes by an analysis of the expression of MAP1B during oligodendrocyte progenitor development. The appearance of the antigens recognized by the monoclonal antibodies A2B5, O4, and O1 which define distinct stages in the maturation of progenitors, was compared with the developmental expression of MAP1B. MAP1B is first detectable in O4+ preoligodendrocytes prior to the acquisition of galactocerebroside and immediately before they develop the complex process-bearing morphology characteristic of terminally differentiated myelin-forming oligodendrocytes in the CNS. In contrast, astrocytes have negligible amounts of MAP1B. These results demonstrate that the expression of MAP1B precedes the development of the mature oligodendrocyte phenotype and suggest that interactions between microtubules and MAP1B might have a role in the formation and stabilization of myelin-forming processes.

Novel oligodendrocyte transmembrane signaling systems. Investigations utilizing antibodies as ligands

Antibodies are increasingly being used as tools to study the function of cell surface markers. Several types of responses may occur upon the selective binding of an antibody to an epitope on a receptor. Antibody binding may trigger signals that are normally transduced by endogenous ligands. Moreover, antibody binding may activate normal signals in a manner that disrupts a sequence of events that coordinates either differentiation, mitogenesis, or morphogenesis. Alternately, it is possible that binding elicits either a modified signal or no signal. This article focuses on the cascade of events that occur following specific antibody binding to myelin markers expressed by cultured murine oligodendrocytes. Binding of specific antibodies to the oligodendrocyte membrane surface markers myelin/oligodendrocyte glycoprotein (MOG), myelin/oligodendrocyte specific protein (MOSP), galactocerebroside (GalC), and sulfatide on cultured murine oligodendrocytes results in different effects with regard to phospholipid turnover, Ca2+ influxes, and antibody:marker distribution. The consequence of each antibody-elicited cascade of events appears to be the regulation of the cytoskeleton within the oligodendroglial membrane sheets. The antibody binding studies described in this article demonstrate that these myelin surface markers are capable of transducing signals. Since endogenous ligands for these myelin markers have yet to be identified, it is not known if these signals are normally transduced or are a modification of normally transduced signals.

The taiep rat: a myelin mutant with an associated oligodendrocyte microtubular defect

This report describes a new inherited disorder of myelination in the rat, named taiep, in which failure of normal myelination of the CNS and subsequent demyelination result in a progressive neurological disturbance. At two months of age, myelin is present throughout the spinal cord, but is immature in the fasciculus gracilis and corticospinal tracts despite the presence of abundant oligodendrocytes. By 12 months, myelin has largely been lost in these spinal cord tracts and also in more rostral parts of the CNS, such as the cerebellum and optic nerves. Other funiculi of the spinal cord show a more diffuse lack of myelin. Oligodendrocytes develop a unique cellular abnormality, most obviously in older rats, which is characterized by the accumulation of microtubules throughout their cytoplasm. As the mutant rats age, there is a continued protracted breakdown of myelin throughout the CNS, with evidence suggesting either persistent hypomyelination or attempts at remyelination of affected axons. It is proposed that the microtubular defect in oligodendrocytes results in a disruption of the normal myelination process in certain areas of the CNS of this mutant, and eventually leads to failure of maintenance of the myelin sheath.

Immunohistochemical expression of microtubule-associated protein 5 (MAP5) in glial cells in multiple system atrophy

An immunohistochemical study focusing on glial cells was performed using monoclonal antibodies against microtubule-associated proteins (MAP1, MAP2 and MAP5), transferrin, leukocyte common antigen (LCA) and glial fibrillary acidic protein (GFAP) in 5 cases of multiple system atrophy (MSA) exhibiting olivopontocerebellar atrophy and striatonigral degeneration. An antibody to MAP5, a fetal antigen in developing brain, was strongly demonstrated in the glial cytoplasmic inclusions (GCIs) which have recently drawn a great deal of attention and were observed in all 5 cases of MSA. Moreover, MAP5-positive glial cells (MAP5-Gs) were present in significantly higher number than in the controls in various regions where GCIs were found, predominantly in putamen, substantia nigra, cerebellar white matter and internal capsule. LCA and transferrin, markers of microglia and oligodendroglia, respectively, were immunohistochemically detected in some MAP5-Gs. GFAP, on the other hand, was not expressed in MAP5-Gs at all. These findings suggest that MAP5-Gs consist of reactive microglia and oligodendroglia. Our study is the first to demonstrate immunohistochemical detection of MAP5 in glial pathological changes in MSA.