Mechanisms of myelin basic protein and proteolipid protein targeting in oligodendrocytes (review)

Oligodendroglial Energy Metabolism and (re)Myelination

Central nervous system (CNS) myelin has a crucial role in accelerating the propagation of action potentials and providing trophic support to the axons. Defective myelination and lack of myelin regeneration following demyelination can both lead to axonal pathology and neurodegeneration. Energy deficit has been evoked as an important contributor to various CNS disorders, including multiple sclerosis (MS). Thus, dysregulation of energy homeostasis in oligodendroglia may be an important contributor to myelin dysfunction and lack of repair observed in the disease. This article will focus on energy metabolism pathways in oligodendroglial cells and highlight differences dependent on the maturation stage of the cell. In addition, it will emphasize that the use of alternative energy sources by oligodendroglia may be required to save glucose for functions that cannot be fulfilled by other metabolites, thus ensuring sufficient energy input for both myelin synthesis and trophic support to the axons. Finally, it will point out that neuropathological findings in a subtype of MS lesions likely reflect defective oligodendroglial energy homeostasis in the disease.

Yet more evidence that myelin protons can be directly imaged with UTE sequences on a clinical 3T scanner: Bicomponent T2* analysis of native and deuterated ovine brain specimens

PURPOSE

UTE sequences with a minimal nominal TE of 8 µs have shown promise for direct imaging of myelin protons (T2 , < 1 ms). However, there is still debate about the efficiency of 2D slice-selective UTE sequences in exciting myelin protons because the half excitation pulses used in these sequences have a relatively long duration (e.g., 0.3-0.6 ms). Here, we compared UTE and inversion-recovery (IR) UTE sequences used with either hard or half excitation pulses (durations 32 µs or 472 µs, respectively) for imaging myelin in native and deuterated ovine brain at 3T.

METHODS

Freshly frozen ovine brains were dissected into ∼2 mm-thick pure white matter and ∼3 to 8 mm-thick cerebral hemisphere specimens, which were imaged before and/or after different immersion time in deuterium oxide.

RESULTS

Bicomponent T2* analysis of UTE signals obtained with hard excitation pulses detected an ultrashort T2 component (STC) fraction (fS ) of 0% to 10% in native specimens, and up to ∼86% in heavily deuterated specimens. fS values were significantly affected by the TIs used in IR-UTE sequences with either hard or half excitation pulses in native specimens but not in heavily deuterated specimens. The STC T2* was in the range of 150 to 400 µs in all UTE and IR-UTE measurements obtained with either hard or half excitation pulses.

CONCLUSION

Our results further support myelin protons as the major source of the ultrashort T2* signals seen on IR-UTE images and demonstrate the potential of IR-UTE sequences with half excitation pulses for directly imaging myelin using clinical scanners. Magn Reson Med 80:538-547, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

Signs of Myelin Impairment in Cerebrospinal Fluid After Osmotic Opening of the Blood-Brain Barrier in Rats

A number of clinical neurological pathologies are associated with increased permeability of the blood brain barrier (BBB). Induced changes of the homeostatic mechanisms in the brain microenvironment lead among others to cellular changes in the CNS. The question was whether some of these changes can be induced by osmotic opening of BBB in an in vivo experiment and whether they can be detected in cerebrospinal fluid (CSF). CSF was taken via the suboccipital puncture from 10 healthy rats and six rats after the osmotic opening of the BBB. In all 16 animals, concentration of myelin basic protein (MBP ng/ml), Neuron-specific enolase (NSE ng/ml) and Tau-protein (Tau pg/ml) were determined in CSF by ELISA. Values in both groups were statistically evaluated. Significant difference between the control and experimental group was revealed only for the concentration of myelin basic protein (p<0.01). The presented results indicate that osmotic opening of the BBB in vivo experiment without the presence of other pathological conditions of the brain leads to a damage of myelin, without impairment of neurons or their axons.

Effects of the jimpy mutation on mouse retinal structure and function

The Jimpy mutant mouse has a point mutation in the proteolipid protein gene (plp1). The resulting misfolding of the protein leads to oligodendrocyte death, myelin destruction, and failure to produce adequately myelinated axons in the central nervous system (CNS). It is not known how the absence of normal myelination during development influences neural function. We characterized the Jimpy mouse retina to find out whether lack of myelination in the optic nerve during development has an effect on normal functioning and morphology of the retina. Optokinetic reflex measurements showed that Jimpy mice had, in general, a functional visual system. Both PLP1 antibody staining and reverse transcriptase-polymerase chain reaction for plp1 mRNA showed that plp1 is not expressed in the wild-type retina. However, in the optic nerve, plp1 is normally expressed, and consequently, in Jimpy mutant mice, myelination of axons in the optic nerve was mostly absent. Nevertheless, neither axon count nor axon ultrastructure in the optic nerve was affected. Physiological recordings of ganglion cell activity using microelectrode arrays revealed a decrease of stimulus-evoked activity at mesopic light levels. Morphological analysis of the retina did not show any significant differences in the gross morphology, such as thickness of retinal layers or cell number in the inner and outer nuclear layer. The cell bodies in the inner nuclear layer, however, were larger in the peripheral retina of Jimpy mutant mice. Antibody labeling against cell type-specific markers showed that the number of rod bipolar and horizontal cells was increased in Jimpy mice. In conclusion, whereas the Jimpy mutation has dramatic effects on the myelination of retinal ganglion cell axons, it has moderate effects on retinal morphology and function.

Effect of 2-fluoropalmitate, cerulenin and tunicamycin on the palmitoylation and intracellular translocation of myelin proteolipid protein

We have investigated the effect of documented protein palmitoylation inhibitors on the fatty acylation and intracellular transport of myelin proteolipid protein (PLP). To this end, brain slices from 20-day-old rats were incubated with either [3H]palmitate or [3H]leucine in the presence or absence of various concentrations of 2-fluoropalmitate (FP), cerulenin (CER), or tunicamycin (TM). FP (> or = 10 microM) decreased the cellular uptake of [3H]palmitate and consequently reduced the labeling of palmitoyl-CoA, glycerolipids and PLP. CER (> or = 1 mM) reduced the palmitoylation of PLP with a concomitant decline in protein thiols. Consistent with being a fatty acyl-CoA analogue, TM (> or = 200 microM) diminished the palmitoylation of PLP and lipids while increasing the amount of [3H]palmitoyl-CoA. Although both CER and TM decreased protein palmitoylation, only the latter affected the appearance of newly synthesized PLP into myelin. Because TM, but not CER, also reduced the formation of lipids, it is concluded that palmitoylation is not required for intracellular transport. Finally, comparison of the effect of TM in brain slices and in a cell-free system suggests that palmitoylation of PLP in whole cells may be an enzymatic process.

Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) can now be defined as an X-linked recessive leukodystrophy that is caused by a mutation in the proteolipid protein (PLP) gene on chromosome Xq22. The most common mutation is gene duplication followed in frequency by missense mutations, insertions, and deletions. The clinical spectrum ranges from severe neonatal cases to relatively benign adult forms and X-linked recessive spastic paraplegia type 2. The lack of PLP is accompanied by deficits in the other myelin proteins of the central nervous system, including myelin basic protein, myelin-associated glycoprotein, and cyclic nucleotide phosphodiesterase. Surprisingly, the total absence of PLP due to gene deletion or a null allele causes a relatively benign form of PMD. Abnormal PLP is thought to impair protein trafficking and to induce apoptosis in oligodendroglia. Immunocytochemistry with specific antibodies reveals the PLP deficiency and insufficient generation of myelin sheaths with the remaining proteins. Both excessive biosynthesis of PLP, as in gene duplications, or conformational change of the protein, as in missense mutations, are detrimental to myelination. Several naturally occurring and transgenic animal models with PLP gene mutations or deletions have contributed to our understanding of dysmyelination in PMD and the general knowledge of myelination and myelin repair.

A new algorithm for analysis of oligonucleotide arrays: application to expression profiling in mouse brain regions

Oligonucleotide arrays are a powerful technology for measuring the expression of thousands of genes simultaneously. Improvements in the sensitivity and precision of the measurements, which often pose a challenge to users, would assist the practical application of the technology. Here, we describe a new analysis algorithm for assessing changes in gene expression using oligonucleotide arrays. Changes in expression are detected in terms of the statistical significance (S-score) of change, which combines signals detected by multiple probe pairs according to an error model characteristic of oligonucleotide arrays. We show that the S-score is sensitive and reliable, enabling us to obtain more consistent results than with existing methods. Cluster analysis of S-score data of four brain regions exhibits patterns that are more distinctive because of improved data quality. In our case study of two mouse brain regions, over 200 genes were identified to have detectable changes between the ventral tegmental area and the prefrontal cortex. The genes with the most distinctive changes are found to be related to myelin or neurofilament synthesis, calcium signaling, and transcription factors. Many of these findings are in agreement with previous studies, using other techniques, such as in situ hybridization. Overall, our findings suggest that this new algorithm may have broad applicability for improving the analysis of oligonucleotide array data.

Endoplasmic reticulum stress in PLP-overexpressing transgenic rats: gray matter oligodendrocytes are more vulnerable than white matter oligodendrocytes

Studies dealing with transport of proteins from the oligodendrocyte cell body to the myelin sheath reveal the presence of different transport pathways. Proteolipid protein (PLP) is synthesized at the rough endoplasmic reticulum (ER) and then processed through the Golgi apparatus and transported to the myelin membranes. Myelin basic protein (MBP) on the other hand is synthesized locally at the ends of cell processes where its messenger RNA is translated on free ribosomes. Here we show that in rats that overexpress PLP, impairment of PLP transport from the cell body to the processes interferes with the translocation of other membrane proteins such as myelin-associated glycoprotein (MAG) and myelin oligodendrocyte glycoprotein (MOG), but not with peripherally translated MBP. In addition, it also impedes the transport of non-myelin proteins, for example the amyloid precursor protein (APP). At the ultrastructural level, the ER of these metabolically disturbed oligodendrocytes revealed extreme swelling of the cisternae, and immunohistochemistry revealed intense expression of the ER chaperone molecule BiP/GRP78 and ER folding enzyme protein disulfide isomerase (PDI). These features suggest that these oligodendrocytes, which were found exclusively in gray matter areas of the spinal cord, started an unfolded protein response while suffering from ER stress. Some of these disturbed oligodendrocytes were seen to undergo programmed cell death. These results indicate that gray matter oligodendrocyte differ from white matter oligodendrocytes in their capacity to stabilize metabolic disturbances by an unfolded protein response.

Normal CNS myelination in transgenic mice overexpressing MHC class I H-2L(d) in oligodendrocytes

In demyelinating diseases, such as multiple sclerosis, an upregulation of MHC class I expression is thought to contribute to oligodendrocyte/myelin damage. In order to investigate potential physiological consequences of upregulated MHC class I expression in oligodendrocytes, we generated transgenic mice that overexpress H-2L(d) under the control of the proteolipid protein (PLP) promoter (PLP-L(d) mice). We focused our studies on the MHC class I molecule H-2L(d), because of its unique intracellular transport characteristics. In the CNS of PLP-L(d) mice, H-2L(d) was expressed by oligodendrocytes. Furthermore, H-2L(d) protein was transported to and expressed on the surface of oligodendrocytes. Most importantly, this upregulation of MHC class I expression in the CNS of PLP-L(d) mice did not by itself result in a de- or dysmyelinating phenotype. These transgenic mice are likely to provide a unique and novel tool for the analysis of potential roles of MHC class I-mediated mechanisms in demyelinating pathologies.

Messenger RNAs located in myelin sheath assembly sites

The targeting of mRNAs to specific subcellular locations is believed to facilitate the rapid and selective incorporation of their protein products into complexes that may include membrane organelles. In oligodendrocytes, mRNAs that encode myelin basic protein (MBP) and select myelin-associated oligodendrocytic basic proteins (MOBPs) locate in myelin sheath assembly sites (MSAS). To identify additional mRNAs located in MSAS, we used a combination of subcellular fractionation and suppression subtractive hybridization. More than 50% of the 1,080 cDNAs that were analyzed were derived from MBP or MOBP mRNAs, confirming that the method selected mRNAs enriched in MSAS. Of 90 other cDNAs identified, most represent one or more mRNAs enriched in rat brain myelin. Five cDNAs, which encode known proteins, were characterized for mRNA size(s), enrichment in myelin, and tissue and developmental expression patterns. Two of these, peptidylarginine deiminase and ferritin heavy chain, have recognized roles in myelination. The corresponding mRNAs were of different sizes than the previously identified mRNA, and they had tissue and development expression patterns that were indistinguishable from those of MBP mRNA. Three other cDNAs recognize mRNAs whose proteins (SH3p13, KIF1A, and dynein light intermediate chain) are involved in membrane biogenesis. Although enriched in myelin, the tissue and developmental distribution patterns of these mRNAs differed from those of MBP mRNA. Six other cDNAs, which did not share significant sequence homology to known mRNAs, were also examined. The corresponding mRNAs were highly enriched in myelin, and four had tissue and developmental distribution patterns indistinguishable from those of MBP mRNA. These studies demonstrate that MSAS contain a diverse population of mRNAs, whose locally synthesized proteins are placed to contribute to myelin sheath assembly and maintenance. Characterization of these mRNAs and proteins will help provide a comprehensive picture of myelin sheath assembly.

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.

On the biogenesis of the myelin sheath: cognate polarized trafficking pathways in oligodendrocytes

Oligodendrocytes, the myelinating cells of the central nervous system, are capable of transporting vast quantities of proteins and of lipids, in particular galactosphingolipids, to the myelin sheath. The sheath is continuous with the plasma membrane of the oligodendrocyte, but the composition of both membrane domains differs substantially. Given its high glycosphingolipid and cholesterol content the myelin sheath bears similarity to the lipid composition of the apical domain of a polarized cell. The question thus arises whether myelin components, like typical apical membrane proteins are transported by an apical-like trafficking mechanism to the sheath, involving a 'raft'-mediated mechanism. Indeed, the evidence indicates the presence of cognate apical and basolateral pathways in oligodendrocytes. However, all major myelin proteins do not participate in this pathway, and remarkably apical-like trafficking seems to be restricted to the oligodendrocyte cell body. In this review, we summarize the evidence on the existence of different trafficking pathways in the oligodendrocyte, and discuss possible mechanisms separating the oligodendrocyte's membrane domains.

Microglial reactivity correlates to the density and the myelination of the anterogradely degenerating axons and terminals following perforant path denervation of the mouse fascia dentata

Transection of the entorhino-dentate perforant path is a well known model for lesion-induced axonal sprouting and glial reactions in the rat. In this study, we have characterized the microglial reaction in the dentate molecular layer of the SJL/J and C57Bl/6 mouse. The morphological transformation of the microglial cells and their densitometrically measured Mac-1 immunoreactivity were correlated with the density of silver-impregnated axonal and terminal degeneration and the myelination of the degenerating medial and lateral perforant pathways. Anterograde axonal and terminal degeneration leads to: (i) altered myelin basic protein immunoreactivity with the appearance of discrete myelin deposits preferentially in the denervated medial and significantly less so in the lateral perforant path zone from day 2 after lesioning; (ii) an increase in number and Mac-1 immunoreactivity of morphologically-changed microglial cells in the denervated perforant path zones with more pronounced morphological transformation of microglia in the medial than in the lateral perforant path zones at day 2 but not day 5 after lesioning; and (iii) a linear correlation between the density of microglial Mac-1 reactivity and axonal degeneration in the medial but not in the lateral perforant path zone at two days postlesion, and a linear correlation in both zones at five days postlesion. We propose that the differentiated microglial response is due to the different densities of axonal and terminal degeneration, as observed in the individual cases. The finding of a potentiated or accelerated microglial activation in the medial as compared to the lateral perforant path zone suggests different kinetics of microglial activation in areas with degenerating myelinated and unmyelinated fibers.

Human proteolipid protein (PLP) mediates winding and adhesion of phospholipid membranes but prevents their fusion

Proteolipid protein (PLP or lipophilin) is a highly conserved, strongly hydrophobic, integral membrane protein, and is the major protein component of central nervous system myelin. Although PLP has been implicated in many functions, its in vivo role is still uncertain. Here, we report the investigation of PLP's putative adhesive function using purified PLP and reconstituted phospholipid vesicles made of either 100% phosphatidylcholine (PC), or a mixture of 92% PC and 8% phosphatidylserine (PS), by weight. PLP-induced changes in the phospholipid bilayer surfaces were directly examined by transmission electron microscopy. We found that upon the introduction of PLP, larger lipid vesicles became smaller and unilamellar. At the PLP:lipid molar ratio of 1:20, vesicle membranes rolled onto themselves forming 'croissant'-like structures that subsequently adhered to each other. The phenomena of PLP-induced bilayer rolling and adhesion were dependent on the concentration of PLP and the period of incubation, but were independent of the presence of calcium and types of phospholipids (PC or PC:PS). Furthermore, the presence of PLP in the lipid bilayers prevented the fusion of membranes. These findings show that PLP can induce membrane 'winding' while preventing the fusion of adjacent lipid bilayers. Hence, our data provide direct evidence for PLP's suspected function of membrane adhesion, and also suggest that PLP could potentially play a role in the formation of the myelin sheath.

A quantitative study of the progress of myelination in the rat central nervous system, using the immunohistochemical method for proteolipid protein

The temporal changes in intensity of myelination of the nervous pathways in 0 to 42-day-old Wistar rats were quantitatively analyzed by immunohistochemistry with anti-proteolipid protein and compared with that obtained by immunohistochemistry with anti-myelin basic protein. Immunohistochemistry was performed on paraffin-embedded tissue according to the standard ABC technique. Intensity of myelination was examined by an image analyzing system. We analyzed nine nervous pathways: corpus callosum, optic tract, internal capsule, spinal tract of the trigeminal nerve, inferior cerebellar peduncle, cerebellar white matter, pyramidal tract, medial longitudinal fasciculus, and cuneate fasciculus. The presence of immunoreactive fibers for proteolipid protein (PLP) in the spinal tract of the trigeminal nerve, medial longitudinal fasciculus and cuneate fasciculus was noted on postnatal day 0. Those of the corpus callosum, inferior cerebellar peduncle, cerebellar white matter, pyramidal tract and internal capsule were noted on day 7, and that of optic tract on day 14. The time required to reach the intensity of myelination of day 42 was day 14 for the cuneate fasciculus, day 21 for the spinal tract of the trigeminal nerve, inferior cerebellar peduncle and medial longitudinal fasciculus, day 28 for the optic and pyramidal tracts, day 35 for the corpus callosum and day 42 for the internal capsule and cerebellar white matter. The appearance of immunoreactive fibers for PLP was usually earlier than that for myelin basic protein (MBP) and the pattern of difference between PLP and MBP can be classified into three groups: (1) their time of appearance and progress are almost the same, as in the optic tract; (2) the appearance and progress of PLP occurs earlier than those of MBP, as in the pyramidal tract; (3) the appearance of PLP occurs earlier than that of MBP, but their progress is the same. Our findings revealed that the time of appearance and progress of myelination as measured by PLP are different among the nervous pathways, and that there is also a difference between PLP and MBP. This difference between PLP and MBP may indicate a functional difference between them.

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.

Targeting of mRNAs into Neuronal and Glial Processes: Intracellular and Extracellular Influences

Neurons and macroglia share the common, polarizing, morphological feature of multiple processes extending from a cell body, thereby defining two cellular domains. Frequently, specialized cellular activities occur within these processes, such as the dendrites of neurons and the myelin sheath of oligodendrocytes, which serve to define some of the functions of the cell. As a consequence, molecules involved in carrying out these functions need to be targeted to these domains, and mechanisms must exist for selecting and delivering these molecules to their appropriate locations. One mechanism that is emerging as increasingly important in targeting proteins to distal processes of neural cells is the translocation of the mRNAs encoding those proteins. In this review, we present many examples of such translocated mRNAs in neurons, astrocytes, and oligodendrocytes. There is a growing consensus that four major steps occur in mRNA targeting after transcription and exit of these molecules from the nucleus. These include 1) the assembly of mRNA into an RNA-protein granule, presumably around some translocation signal within the mRNA; 2) transport of the mRNA granule complex to distal sites via the cytoskeleton; 3) anchoring of the granule at the targeting site; and 4) translation of the localized mRNA to generate protein products in situ. It has become increasingly apparent that mRNA translocation is an active process, although many of the components of the translocation apparatus remain to be identified. Recent evidence also indicates that a number of factors can regulate the transport of mRNAs from within and without the cell. These include cell-cell contact, differentiation state, electrical activity, and trophic factors, which seem to exert their influence through signal transduction mechanisms that are only beginning to be defined. NEUROSCIENTIST 4:77-87, 1998

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.

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.

Membrane adhesion and other functions for the myelin basic proteins

The myelin basic proteins are a set of peripheral membrane polypeptides which play an essential role in myelination. Their most well-documented property is the unique ability to 'seal' the cytoplasmic aspects of the myelin membrane, but this is probably not the only function for these highly charged molecules. Despite extensive homology, the individual myelin basic proteins (MBPs) exhibit different expression patterns and biochemical properties, and so it is now believed that the various isoforms are not functionally equivalent in myelinating cells. We now think that while the major MBPs are intracellular adhesion molecules, some of the quantitatively less abundant isoforms that are expressed very early in development may have regulatory effects on the myelination program.