Expression of myelin basic protein isoforms in nonglial cells

Brain organoids engineered to give rise to glia and neural networks after 90 days in culture exhibit human-specific proteoforms

Human brain organoids are emerging as translationally relevant models for the study of human brain health and disease. However, it remains to be shown whether human-specific protein processing is conserved in human brain organoids. Herein, we demonstrate that cell fate and composition of unguided brain organoids are dictated by culture conditions during embryoid body formation, and that culture conditions at this stage can be optimized to result in the presence of glia-associated proteins and neural network activity as early as three-months in vitro. Under these optimized conditions, unguided brain organoids generated from induced pluripotent stem cells (iPSCs) derived from male-female siblings are similar in growth rate, size, and total protein content, and exhibit minimal batch-to-batch variability in cell composition and metabolism. A comparison of neuronal, microglial, and macroglial (astrocyte and oligodendrocyte) markers reveals that profiles in these brain organoids are more similar to autopsied human cortical and cerebellar profiles than to those in mouse cortical samples, providing the first demonstration that human-specific protein processing is largely conserved in unguided brain organoids. Thus, our organoid protocol provides four major cell types that appear to process proteins in a manner very similar to the human brain, and they do so in half the time required by other protocols. This unique copy of the human brain and basic characteristics lay the foundation for future studies aiming to investigate human brain-specific protein patterning (e.g., isoforms, splice variants) as well as modulate glial and neuronal processes in an in situ-like environment.

White matter damage as a consequence of vascular dysfunction in a spontaneous mouse model of chronic mild chronic hypoperfusion with eNOS deficiency

Vascular cognitive impairment and dementia (VCID) is the second most common form of dementia after Alzheimer's disease (AD). Currently, the mechanistic insights into the evolution and progression of VCID remain elusive. White matter change represents an invariant feature. Compelling clinical neuroimaging and pathological evidence suggest a link between white matter changes and neurodegeneration. Our prior study detected hypoperfused lesions in mice with partial deficiency of endothelial nitric oxide (eNOS) at very young age, precisely matching to those hypoperfused areas identified in preclinical AD patients. White matter tracts are particularly susceptible to the vascular damage induced by chronic hypoperfusion. Using immunohistochemistry, we detected severe demyelination in the middle-aged eNOS-deficient mice. The demyelinated areas were confined to cortical and subcortical areas including the corpus callosum and hippocampus. The intensity of demyelination correlated with behavioral deficits of gait and associative recognition memory performances. By Evans blue angiography, we detected blood-brain barrier (BBB) leakage as another early pathological change affecting frontal and parietal cortex in eNOS-deficient mice. Sodium nitrate fortified drinking water provided to young and middle-aged eNOS-deficient mice completely prevented non-perfusion, BBB leakage, and white matter pathology, indicating that impaired endothelium-derived NO signaling may have caused these pathological events. Furthermore, genome-wide transcriptomic analysis revealed altered gene clusters most related to mitochondrial respiratory pathways selectively in the white matter of young eNOS-deficient mice. Using eNOS-deficient mice, we identified BBB breakdown and hypoperfusion as the two earliest pathological events, resulting from insufficient vascular NO signaling. We speculate that the compromised BBB and mild chronic hypoperfusion trigger vascular damage, along with oxidative stress and astrogliosis, accounting for the white matter pathological changes in the eNOS-deficient mouse model. We conclude that eNOS-deficient mice represent an ideal spontaneous evolving model for studying the earliest events leading to white matter changes, which will be instrumental to future therapeutic testing of drug candidates and for targeting novel/specific vascular mechanisms contributing to VCID and AD.

Selective and compartmentalized myelin expression of HspB5

In the present study, we reveal myelin-specific expression and targeting of mRNA and biochemical pools of HspB5 in the mouse CNS. Our observations are based on in situ hybridization, electron microscopy and co-localization with 2',3'-Cyclic-Nucleotide 3'-Phosphodiesterase (CNPase), reinforcing this myelin-selective expression. HspB5 mRNA might be targeted to these structures based on its presence in discrete clusters resembling RNA granules and the presence of a putative RNA transport signal. Further, sub-cellular fractionation of myelin membranes reveals a distinct sub-compartment-specific association and detergent solubility of HspB5. This is akin to other abundant myelin proteins and is consistent with HspB5's association with cytoskeletal/membrane assemblies. Oligodendrocytes have a pivotal role in supporting axonal function via generating and segregating the ensheathing myelin. This specialization places extreme structural and metabolic demands on this glial cell type. Our observations place HspB5 in oligodendrocytes which may require selective and specific chaperone capabilities to maintain normal function and neuronal support.

SncRNA715 Inhibits Schwann Cell Myelin Basic Protein Synthesis

Myelin basic proteins (MBP) are major constituents of the myelin sheath in the central nervous system (CNS) and the peripheral nervous system (PNS). In the CNS Mbp translation occurs locally at the axon-glial contact site in a neuronal activity-dependent manner. Recently we identified the small non-coding RNA 715 (sncRNA715) as a key inhibitor of Mbp translation during transport in oligodendrocytes. Mbp mRNA localization in Schwann cells has been observed, but has not been investigated in much detail. Here we could confirm translational repression of Mbp mRNA in Schwann cells. We show that sncRNA715 is expressed and its levels correlate inversely with MBP in cultured Schwann cells and in the sciatic nerve in vivo. Furthermore we could reduce MBP protein levels in cultured Schwann cells by increasing the levels of the inhibitory sncRNA715. Our findings suggest similarities in sncRNA715-mediated translational repression of Mbp mRNA in oligodendrocytes and Schwann cells.

Myelin basic protein cleaves cell adhesion molecule L1 and promotes neuritogenesis and cell survival

The cell adhesion molecule L1 is a Lewis(x)-carrying glycoprotein that plays important roles in the developing and adult nervous system. Here we show that myelin basic protein (MBP) binds to L1 in a Lewis(x)-dependent manner. Furthermore, we demonstrate that MBP is released by murine cerebellar neurons as a sumoylated dynamin-containing protein upon L1 stimulation and that this MBP cleaves L1 as a serine protease in the L1 extracellular domain at Arg(687) yielding a transmembrane fragment that promotes neurite outgrowth and neuronal survival in cell culture. L1-induced neurite outgrowth and neuronal survival are reduced in MBP-deficient cerebellar neurons and in wild-type cerebellar neurons in the presence of an MBP antibody or L1 peptide containing the MBP cleavage site. Genetic ablation of MBP in shiverer mice and mutagenesis of the proteolytically active site in MBP or of the MBP cleavage site within L1 as well as serine protease inhibitors and an L1 peptide containing the MBP cleavage site abolish generation of the L1 fragment. Our findings provide evidence for novel functions of MBP in the nervous system.

Making myelin basic protein -from mRNA transport to localized translation

In the central nervous system (CNS) of most vertebrates, oligodendrocytes enwrap neuronal axons with extensions of their plasma membrane to form the myelin sheath. Several proteins are characteristically found in myelin of which myelin basic protein (MBP) is the second most abundant one after proteolipid protein. The lack of functional MBP in rodents results in a severe hypomyelinated phenotype in the CNS demonstrating its importance for myelin synthesis. Mbp mRNA is transported from the nucleus to the plasma membrane and is translated locally at the axon-glial contact site. Axonal properties such as diameter or electrical activity influence the degree of myelination. As oligodendrocytes can myelinate many axonal segments with varying properties, localized MBP translation represents an important part of a rapid and axon-tailored synthesis machinery. MBP's ability to compact cellular membranes may be problematic for the integrity of intracellular membranous organelles and can also explain why MBP is transported in oligodendrocytes in the form of an mRNA rather than as a protein. Here we review the recent findings regarding intracellular transport and signaling mechanisms leading to localized translation of Mbp mRNA in oligodendrocytes. More detailed insights into the MBP synthesis pathway are important for a better understanding of the myelination process and may foster the development of remyelination therapies for demyelinating diseases.

Myelin management by the 18.5-kDa and 21.5-kDa classic myelin basic protein isoforms

The classic myelin basic protein (MBP) splice isoforms range in nominal molecular mass from 14 to 21.5 kDa, and arise from the gene in the oligodendrocyte lineage (Golli) in maturing oligodendrocytes. The 18.5-kDa isoform that predominates in adult myelin adheres the cytosolic surfaces of oligodendrocyte membranes together, and forms a two-dimensional molecular sieve restricting protein diffusion into compact myelin. However, this protein has additional roles including cytoskeletal assembly and membrane extension, binding to SH3-domains, participation in Fyn-mediated signaling pathways, sequestration of phosphoinositides, and maintenance of calcium homeostasis. Of the diverse post-translational modifications of this isoform, phosphorylation is the most dynamic, and modulates 18.5-kDa MBP's protein-membrane and protein-protein interactions, indicative of a rich repertoire of functions. In developing and mature myelin, phosphorylation can result in microdomain or even nuclear targeting of the protein, supporting the conclusion that 18.5-kDa MBP has significant roles beyond membrane adhesion. The full-length, early-developmental 21.5-kDa splice isoform is predominantly karyophilic due to a non-traditional P-Y nuclear localization signal, with effects such as promotion of oligodendrocyte proliferation. We discuss in vitro and recent in vivo evidence for multifunctionality of these classic basic proteins of myelin, and argue for a systematic evaluation of the temporal and spatial distributions of these protein isoforms, and their modified variants, during oligodendrocyte differentiation.

Nucleus-localized 21.5-kDa myelin basic protein promotes oligodendrocyte proliferation and enhances neurite outgrowth in coculture, unlike the plasma membrane-associated 18.5-kDa isoform

The classic myelin basic protein (MBP) family of central nervous system (CNS) myelin arises from transcription start site 3 of the Golli (gene of oligodendrocyte lineage) complex and comprises splice isoforms ranging in nominal molecular mass from 14 kDa to (full-length) 21.5 kDa. We have determined here a number of distinct functional differences between the major 18.5-kDa and minor 21.5-kDa isoforms of classic MBP with respect to oligodendrocyte (OLG) proliferation. We have found that, in contrast to 18.5-kDa MBP, 21.5-kDa MBP increases proliferation of early developmental immortalized N19-OLGs by elevating the levels of phosphorylated ERK1/2 and Akt1 kinases and of ribosomal protein S6. Coculture of N2a neuronal cells with N19-OLGs transfected with the 21.5-kDa isoform (or conditioned medium from), but not the 18.5-kDa isoform, caused the N2a cells to have increased neurite outgrowth and process branching complexity. These roles were dependent on subcellular localization of 21.5-kDa MBP to the nucleus and on the exon II-encoded segment, suggesting that the nuclear localization of early minor isoforms of MBP may play a crucial role in regulating and/or initiating myelin and neuronal development in the mammalian CNS.

Sexual dimorphism in the white matter of rodents

Sexual dimorphism of astrocytes and neurons is well documented in many brain and spinal cord structures. Sexual dimorphism of oligodendrocytes (Olgs) and myelin has received less attention. We recently showed that density of Olgs in corpus callosum, fornix, and spinal cord of wild-type male rodents is more densely packed than in females; myelin proteins and myelin gene expression are likewise greater in males than in female rodents. However, glial cell proliferation and cell death were two times greater in female corpus callosum. Endogenous sex hormones, specifically lack of androgens, produce an Olg female phenotype in castrated male mouse. In vitro studies using Olgs culture also showed differences between males and females Olg survival and signaling pathways in response to sexual hormones. Sexual dimorphism of white matter tracts and glia in rodents indicates the necessity for controlling gender in the experimental studies of neurodegenerative disorders. Most importantly, our studies suggest that hormones may contribute to sexual dimorphism observed in certain human diseases including multiple sclerosis.

Post-transcriptional regulation of myelin formation

Myelin is a specialized structure of the nervous system that both enhances electrical conductance and protects neurons from degeneration. In the central nervous system, extensively polarized oligodendrocytes form myelin by wrapping cellular processes in a spiral pattern around neuronal axons. Myelin formation requires the oligodendrocyte to regulate gene expression in response to changes in its extracellular environment. Because these changes occur at a distance from the cell body, post-transcriptional control of gene expression allows the cell to fine-tune its response. Here, we review the RNA-binding proteins that control myelin formation in the brain, highlighting the molecular mechanisms by which they control gene expression and drawing parallels from studies in other cell types.

Adeno-associated virus-mediated L1 expression promotes functional recovery after spinal cord injury

Paucity of permissive molecules and abundance of inhibitory molecules in the injured spinal cord of adult mammals prevent axons from successful regeneration and, thus, contribute to the failure of functional recovery. Using an adeno-associated viral (AAV) vector, we expressed the regeneration-promoting cell adhesion molecule L1 in both neurons and glia in the lesioned spinal cord of adult mice. Exogenous L1, detectable already 1 week after thoracic spinal cord compression and immediate vector injection, was expressed at high levels up to 5 weeks, the longest time-period studied. Dissemination of L1-transduced cells throughout the spinal cord was wide, spanning over more than 10 mm rostral and 10 mm caudal to the lesion scar. L1 was not detectable in the fibronectin-positive lesion core. L1 overexpression led to improved stepping abilities and muscle coordination during ground locomotion over a 5-week observation period. Superior functional improvement was associated with enhanced reinnervation of the lumbar spinal cord by 5-HT axons. Corticospinal tract axons did not regrow beyond the lesion scar but extended distally into closer proximity to the injury site in AAV-L1-treated compared with control mice. The expression of the neurite outgrowth-inhibitory chondroitin sulphate proteoglycan NG2 was decreased in AAV-L1-treated spinal cords, along with reduction of the reactive astroglial marker GFAP. In vitro experiments confirmed that L1 inhibits astrocyte proliferation, migration, process extension and GFAP expression. Analyses of intracellular signalling indicated that exogenous L1 activates diverse cascades in neurons and glia. Thus, AAV-mediated L1 overexpression appears to be a potent means to favourably modify the local environment in the injured spinal cord and promote regeneration. Our study demonstrates a clinically feasible approach of promising potential.

Restoration of FcRγ/Fyn signaling repairs central nervous system demyelination

Disruption of myelin causes severe neurological diseases. An understanding of the mechanisms that control myelination and remyelination is needed to develop therapeutic strategies for demyelinating diseases such as multiple sclerosis (MS). Our previous finding indicating the critical involvement of the gamma chain of immunogloblin Fc receptors (FcRgamma) and Fyn signaling in oligodendrocyte differentiaion and myelination demands a fundamental revision of the strategies used for MS therapy, because antigen-antibody complexes in MS patients may induce the direct dysregulation of myelination process as well as the inflammatory destruction of myelin sheath. Here we show that the FcRgamma/Fyn signaling cascade is critically involved in cuprizone-induced demyelination/remyelination, with no lymphocytic response. The levels of phosphorylated myelin basic proteins (p-MBPs), especially the 21.5-kDa isoform, but not the levels of total MBPs, decreased markedly during demyelination induced by aging, cuprizone treatment, and double knockout of FcRgamma/Fyn genes. We also showed that the recovery from demyelination in cuprizone-treated and aged mice is achieved after administration of the herbal medicine Ninjin'yoeito, an effective therapy targeting the FcRgamma/Fyn-Rho (Rac1)-MAPK (P38 MAPK)-p-MBPs signaling cascade. These results suggest that the restoration of FcRgamma/Fyn signaling represents a new approach for the treatment of demyelinating diseases.

Proliferation and death of oligodendrocytes and myelin proteins are differentially regulated in male and female rodents

Sexual dimorphism of neurons and astrocytes has been demonstrated in different centers of the brain, but sexual dimorphism of oligodendrocytes and myelin has not been examined. We show, using immunocytochemistry and in situ hybridization, that the density of oligodendrocytes in corpus callosum, fornix, and spinal cord is 20-40% greater in males compared with females. These differences are present in young and aged rodents and are independent of strain and species. Proteolipid protein and carbonic anhydrase-II transcripts, measured by real-time PCR, are approximately two to three times greater in males. Myelin basic protein and 2', 3'-cyclic nucleotide 3'-phosphodiesterase, measured by Western blots, are 20-160% greater in males compared with females. Surprisingly, both generation of new glia and apoptosis of glia, including oligodendrocytes, are approximately two times greater in female corpus callosum. These results indicate that the lifespan of oligodendrocytes is shorter in females than in males. Castration of males produces a female phenotype characterized by fewer oligodendrocytes and increased generation of new glia. These findings indicate that exogenous androgens differentially affect the lifespan of male and female oligodendrocytes, and they can override the endogenous production of neurosteroids. The data imply that turnover of myelin is greater in females than in males. Mu-calpain, a protease upregulated in degeneration of myelin, is dramatically increased at both transcriptional and translational levels in females compared with males. These morphological, molecular, and biochemical data show surprisingly large differences in turnover of oligodendrocytes and myelin between sexes. We discuss the potential significance of these differences to multiple sclerosis, a sexually dimorphic disease, whose progression is altered by exogenous hormones.

T-cell properties determine disease site, clinical presentation, and cellular pathology of experimental autoimmune encephalomyelitis

Two distinct clinical phenotypes of experimental autoimmune encephalomyelitis are observed in BALB interferon-gamma knockout mice immunized with encephalitogenic peptides of myelin basic protein. Conventional disease, characterized by ascending weakness and paralysis, occurs with greater frequency after immunizing with a peptide comprising residues 59 to 76. Axial-rotatory disease, characterized by uncontrolled axial rotation, occurs with greater frequency in mice immunized with a peptide corresponding to exon 2 of the full length 21.5-kd protein. The two clinical phenotypes are histologically distinguishable. Conventional disease is characterized by inflammation and demyelination primarily in spinal cord, whereas axial-rotatory disease involves inflammation and demyelination of lateral medullary areas of brain. Both types have infiltrates in which neutrophils are a predominating component. By isolating T cells and transferring disease to naive recipients, we show here that the type of disease is determined entirely by the inducing T cell. Furthermore, studies using CXCR2 knockout recipients, unable to recruit neutrophils to inflammatory sites, show that although neutrophils are critical for some of these T cells to effect disease, there are also interferon-gamma-deficient T cells that induce disease in the absence of both interferon-gamma and neutrophils. These results highlight the multiplicity of T-cell-initiated effector pathways available for inflammation and demyelination.

Staufen recruitment into stress granules does not affect early mRNA transport in oligodendrocytes

Staufen is a conserved double-stranded RNA-binding protein required for mRNA localization in Drosophila oocytes and embryos. The mammalian homologues Staufen 1 and Staufen 2 have been implicated in dendritic RNA targeting in neurons. Here we show that in rodent oligodendrocytes, these two proteins are present in two independent sets of RNA granules located at the distal myelinating processes. A third kind of RNA granules lacks Staufen and contains major myelin mRNAs. Myelin Staufen granules associate with microfilaments and microtubules, and their subcellular distribution is affected by polysome-disrupting drugs. Under oxidative stress, both Staufen 1 and Staufen 2 are recruited into stress granules (SGs), which are stress-induced organelles containing transiently silenced messengers. Staufen SGs contain the poly(A)-binding protein (PABP), the RNA-binding proteins HuR and TIAR, and small but not large ribosomal subunits. Staufen recruitment into perinuclear SGs is paralleled by a similar change in the overall localization of polyadenylated RNA. Under the same conditions, the distribution of recently transcribed and exported mRNAs is not affected. Our results indicate that Staufen 1 and Staufen 2 are novel and ubiquitous SG components and suggest that Staufen RNPs are involved in repositioning of most polysomal mRNAs, but not of recently synthesized transcripts, during the stress response.

Differential effects of apotransferrin on two populations of oligodendroglial cells

In the central nervous system (CNS), apotransferrin (aTf) is produced by oligodendroglial cells (OLGcs), and aTf is essential for cell survival. We previously demonstrated that a single intracranial injection of aTf in 3-day-old rats accelerates differentiation of OLGc and that aTf acts at early stages of development on certain populations of OLGcs, promoting accelerated maturation, with no effect on late markers of cell differentiation. The objective of the present study was to analyze OLGc maturation at two different stages of rat development, 4 and 10 days of age, in OLGcs isolated from the brain after intracranial injection of aTf at 3 days of age, and to explore the in vitro effect of aTf added to cultures of OLGc isolated from aTf-injected and control brains. The maturational cell stages were identified by immunocytochemistry with different OLGc markers and by analysis of their morphological complexity. The OLGcs isolated from 4- and 10-day-old animals intracranially injected with aTf were more differentiated than control cells. Treatment with aTf of the cultures of OLGcs that were isolated from 4-day-old saline-injected control animals induced their differentiation, while a similar treatment of the cultures of OLGcs that were isolated from 10-day-old animals did not induce further maturation of the cells. The results presented in the present report demonstrate that the in vivo effects of aTf on OLGc maturation can be reproduced in cultures and that the effects of aTf occur early in development during a narrow, transient "temporal window" within which OLGcs are sensitive to its action.

Increased A beta peptides and reduced cholesterol and myelin proteins characterize white matter degeneration in Alzheimer's disease

Relative to the gray matter, there is a paucity of information regarding white matter biochemical alterations and their contribution to Alzheimer's disease (AD). Biochemical analyses of AD white matter combining size-exclusion, normal phase, and gas chromatography, immunoassays, and Western blotting revealed increased quantities of Abeta40 and Abeta42 in AD white matter accompanied by significant decreases in the amounts of myelin basic protein, myelin proteolipid protein, and 2',3'-cyclic nucleotide 3'-phosphodiesterase. In addition, the AD white matter cholesterol levels were significantly decreased while total fatty acid content was increased. In some instances, these white matter biochemical alterations were correlated with patient apolipoprotein E genotype, Braak stage, and gender. Our observations suggest that extensive white matter axonal demyelination underlies Alzheimer's pathology, resulting in loss of capacitance and serious disturbances in nerve conduction, severely damaging brain function. These white matter alterations undoubtedly contribute to AD pathogenesis and may represent the combined effects of neuronal degeneration, microgliosis, oligodendrocyte injury, microcirculatory disease, and interstitial fluid stasis. To accurately assess the success of future therapeutic interventions, it is necessary to have a complete appreciation of the full scope and extent of AD pathology.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Membrane traffic in myelinating oligodendrocytes

In the central nervous system (CNS), the myelin sheath is synthesised by oligodendrocytes as a specialised subdomain of an extended plasma membrane, reminiscent of the segregated membrane domains of polarised cells. Myelination takes place within a relatively short period of time and oligodendrocytes must have adapted membrane sorting and transport mechanisms to achieve such a high rate of myelin synthesis and to maintain the unique organisation of the myelin membrane. In adult life, maintenance of the functional myelin sheath requires a carefully orchestrated balance of myelin synthesis and turnover. Imbalance in these processes may cause dys- or demyelination and disease. This review summarises what is currently known about myelin protein trafficking and mistrafficking in oligodendrocytes. We also present data demonstrating distinct transport pathways for myelin structural proteins and the expression of SNARE proteins in differentiating oligodendrocytes. Myelinating glial cells may well serve as a model system for studying general aspects of membrane trafficking and organisation of membrane domains.

Morphometric analysis of the myelin-associated oligodendrocytic basic protein-deficient mouse reveals a possible role for myelin-associated oligodendrocytic basic protein in regulating axonal diameter

Myelin-associated oligodendrocytic basic protein is a member of the proteins constituting the central nervous system myelin. By morphometric analysis, we demonstrated that axons of myelin-associated oligodendrocytic basic protein-deficient mice had larger diameters and more myelin lamellae as compared to those of wild-type mice at the same age. It is known that the number of myelin lamellae increases linearly with axonal diameter, and that the rate of radial axonal growth is the factor controlling the rate of myelin formation. In line with these observations, we found that the regression line for axonal diameter and the number of myelin lamellae in myelin-associated oligodendrocytic basic protein-deficient mice appeared to be identical to that in wild-type mice, indicating that the increase in the number of myelin lamellae was the result of the increase in axonal diameter. Furthermore, we generated myelin basic protein/myelin-associated oligodendrocytic basic protein-double-deficient mice through mating myelin-associated oligodendrocytic basic protein-deficient mice with shiverer mice, an autosomal recessive mutant characterized by a lack of all isoforms of myelin basic protein. With these knock-out mice, we showed that axons of the double-deficient mice had larger diameters and smaller form factor, an index of the deformation of the fiber contour, in ensheathed fibers than those of shiverer mice, although there was no difference in axonal diameter of unmyelinated fibers between them. Taken together, myelin-associated oligodendrocytic basic protein seemed to play a role in controlling axonal diameter and in keeping axons round.

Fluorescent myelin proteins provide new tools to study the myelination process

We present here a new approach which permits us to follow myelin proteins within living, actively myelinating cells. We have developed probes to study the spatial and temporal incorporation of proteins into the myelin sheath by expressing myelin proteins fused to the green fluorescent protein (GFP). GFP from the jellyfish Aequorea victoria and its derivatives, e.g., blue fluorescent protein (BFP) were used as molecular reporters to monitor the intracellular distribution of myelin proteins. Fusion proteins (14 kD myelin basic protein [MBP]-GFP, 21 kD MBP-GFP) were expressed in primary Schwann cells (SCs) and their distribution was monitored by confocal microscopy. The autofluorescent chimeric proteins were readily visualized and their subcellular localization was unaffected by the GFP reporter. However, because of the length of culturing time necessary to establish permanent cell lines, we found that it was not possible to obtain MBP-GFP stable SCs that also were capable of myelinating neuronal axons. We therefore devised a way of introducing vectors under conditions where cells are dividing in response to endogenous stimuli, and therefore are still capable of myelinating. We designed a protocol in which SCs cocultured with dorsal root ganglion (DRG) neurons are transfected while they are actively dividing. SCs transfected in this way exhibit a good level of protein expression and retain their myelinating phenotype. The fusion protein expression lasts long enough to observe "green myelin. " These fluorescently tagged myelin proteins will allow high-resolution examination of the protein and membrane traffic in normal myelinating cells.

Myelin basic protein gene dosage effects in the PNS

Myelin basic protein (MBP) plays an essential adhesive role in the formation of compact myelin in the central nervous system (CNS), but not in the peripheral nervous system (PNS). Morphologic data suggest that MBP controls the number of cytoplasmic channels or Schmidt-Lanterman incisures (SLI) present in PNS myelin. The levels of connexin-32 (Cx32) and myelin-associated glycoprotein (MAG), two components of the incisures, are inversely proportional to the levels of MBP in sciatic nerves of mice affected by the shiverer (shi) mutation, while protein zero (P0) and peripheral membrane protein 22 (PMP22), two structural components of compact myelin, remain constant. The levels of P0, PMP22, Cx32, and MAG mRNA do not vary in relationship to the levels of MBP. This indicates that MBP exerts its effect on Cx32 and MAG at a posttranscriptional level and suggests a new function for MBP in regulating gene expression in the PNS.

Retrovirus mediated gene transfer of the self antigen MBP into the bone marrow of mice alters resistance to experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is a prototypic model of organ specific autoimmunity. MHC class II restricted T-cells directed against myelin basic protein (MBP) have been shown to cause EAE in susceptible strains of mice. We have asked whether the introduction of a gene encoding an autoantigen (MBP) into the hematopoetic stem cells of mice would result in tolerance to that protein. We have introduced cDNA encoding the 21.5 kDa isoform of MBP into the hematopoetic stem cells of B10.PL (73NS), SJL, and B10 mice by retrovirus-mediated gene transfer. Our experiments show expression of proviral MBP in peripheral blood and thymus following transplantation of genetically modified stem cells. Such expression does not result in deletion of MBP-specific T cells or tolerance to MBP, nor does it alter susceptibility to MBP-induced EAE in susceptible strains B10.PL and SJL. However, retrovirus-mediated gene transfer resulted in resistant B10 mice developing mild EAE. This report demonstrates that autoreactive MBP-specific T cells can be selected in the presence of endogenous antigen or an MBP-encoding retrovirus.

Adhesion of acidic lipid vesicles by 21.5 kDa (recombinant) and 18.5 kDa isoforms of myelin basic protein

Myelin basic protein (MBP) is thought to be responsible for adhesion of the intracellular surfaces of compact myelin to give the major dense line. The 17 and 21.5 kDa isoforms containing exon II have been reported by others to localize to the cytoplasm and nucleus of murine oligodendrocytes and HeLa cells while the 14 and 18.5 kDa isoforms lacking exon II are confined to the plasma membrane. However, we show that the exon II(-) 18.5 kDa form and a recombinant exon II(+) 21.5 kDa isoform both caused similar aggregation of acidic lipid vesicles, indicating that they should have similar abilities to bind to the intracellular lipid surface of the plasma membrane and to cause adhesion of those surfaces to each other. The circular dichroism spectra of the two isoforms indicated that both had a similar secondary structure. Thus, both isoforms should be able to bind to and cause adhesion of the cytosolic surfaces of compact myelin. The fact that they do not could be due to differences in post-translational modification in vivo, trafficking through the cell and/or subcellular location of synthesis, but it is not due to differences in their lipid binding.

Phosphorylated human keratinocyte ornithine decarboxylase is preferentially associated with insoluble cellular proteins

Ornithine decarboxylase (ODC), the first enzyme in polyamine biosynthesis, is highly regulated by many trophic stimuli, and changes in its levels and organization correlate with cytoskeletal changes in normal human epidermal keratinocytes (NHEK). NHEK ODC exhibits a filamentous perinuclear/nuclear localization that becomes more diffuse under conditions that alter actin architecture. We have thus asked whether ODC colocalizes with a component of the NHEK cytoskeleton. Confocal immunofluorescence showed that ODC distribution in NHEK was primarily perinuclear; upon disruption of the actin cytoskeleton with cytochalasin D, ODC distribution was diffuse. The ODC distribution in untreated NHEK overlapped with that of keratin in the perinuclear but not cytoplasmic area; after treatment with cytochalasin D, overlap between staining for ODC and for keratin was extensive. No significant overlap with actin and minimal overlap with tubulin filament systems were observed. Subcellular fractionation by sequential homogenizations and centrifugations of NHEK lysates or detergent and salt extractions of NHEK in situ revealed that ODC protein and activity were detectable in both soluble and insoluble fractions, with mechanical disruption causing additional solubilization of ODC activity (three- to sevenfold above controls). Fractionation and ODC immunoprecipitation from [(32)P]orthophosphate-labeled NHEK lysates showed that a phosphorylated form of ODC was present in the insoluble fractions. Taken together, these data suggest that two pools of ODC exist in NHEK. The first is the previously described soluble pool, and the second is enriched in phospho-ODC and associated with insoluble cellular material that by immunohistochemistry appears to be organized in conjunction with the keratin cytoskeleton.

Subcellular fractionation and association with the cytoskeleton of messengers encoding myelin proteins

The targeting of polypeptides to restricted cytoplasmic domains by means of mRNA sorting is a widespread phenomena utilized by many cell types. In the central nervous system, in situ hybridization analysis has shown previously that the mRNAs encoding several myelin-specific proteins are specifically located within the myelinating processes of oligodendrocytes. Here, by means of biochemical and subcellular fractionation methods, we show that a myelin fraction is selectively enriched in those mRNAs. The four major myelin basic protein (MBP) mRNAs that arise by alternative splicing of exons II and VI of the MBP gene are concentrated in this subcellular fraction. Furthermore, an interaction of MBP and MOBP 81A mRNAs with the cytoskeleton was observed. This interaction might serve to mediate the anchoring of these messengers after translocation to the subcellular site of translation.

Peripheral myelin protein 22 and protein zero: a novel association in peripheral nervous system myelin

Mutations found in the two major glycosylated transmembrane proteins of the PNS myelin, the peripheral myelin protein zero (P0) and peripheral myelin protein 22 (PMP22), have been independently associated with the most common hereditary demyelinating peripheral neuropathies. Genotype-phenotype correlations in humans and transgenic animals have provided functional evidence that P0 and PMP22 are involved in formation and maintenance of compact myelin. Here, we demonstrate for the first time that P0 and PMP22 proteins form complexes in the myelin membrane, as shown by coimmunoprecipitation experiments, and that glycosylation is not involved in mediating these interactions. Complex formation was also detected when the two proteins were coexpressed in heterologous cells. In transfected cells, P0 and PMP22 are recruited and colocalize at the apposed plasma membranes of expressors as shown by confocal microscopy. These findings provide a new basis for a better understanding of myelin assembly and of the pathomechanisms involved in demyelinating peripheral neuropathies. Furthermore, these results propose a possible explanation why alterations in either of these molecules are sufficient to destabilize the myelin structure and cause a similar disease phenotype.

Peripheral myelin protein 22 and protein zero: a novel association in peripheral nervous system myelin

Mutations found in the two major glycosylated transmembrane proteins of the PNS myelin, the peripheral myelin protein zero (P0) and peripheral myelin protein 22 (PMP22), have been independently associated with the most common hereditary demyelinating peripheral neuropathies. Genotype-phenotype correlations in humans and transgenic animals have provided functional evidence that P0 and PMP22 are involved in formation and maintenance of compact myelin. Here, we demonstrate for the first time that P0 and PMP22 proteins form complexes in the myelin membrane, as shown by coimmunoprecipitation experiments, and that glycosylation is not involved in mediating these interactions. Complex formation was also detected when the two proteins were coexpressed in heterologous cells. In transfected cells, P0 and PMP22 are recruited and colocalize at the apposed plasma membranes of expressors as shown by confocal microscopy. These findings provide a new basis for a better understanding of myelin assembly and of the pathomechanisms involved in demyelinating peripheral neuropathies. Furthermore, these results propose a possible explanation why alterations in either of these molecules are sufficient to destabilize the myelin structure and cause a similar disease phenotype.

Protein zero, a myelin IgCAM, induces physiologically operative tight junctions in nonadhesive carcinoma cells

In the peripheral nervous system, protein zero (P0), a homophilic immunoglubulin cell adhesion molecule, mediates adhesion of Schwann cell membranes as they enwrap axons and generate compact myelin. Although P0 is naturally only expressed in peripheral myelin, it can behave as a vigorous adhesion molecule in a variety of cell types (Filbin et al. [1990] Nature 344:871-872; Schneider-Schaulies et al. [1990] J Neurosci Res 27:286-297; Doyle et al. [1995] J Cell Biol 131:465-482) and can thus be characterized as an obligatory adhesion molecule. Previously, we showed that when HeLa, a cervical carcinoma cell line devoid of epithelial junctions, is forced to express P0, strong cell-cell adhesion is induced, proteins associated with junctional elements are upregulated, and ultrastructurally tight junctions, adherens junctions, and desmosomes become apparent (Doyle et al., 1995). In this report, we assessed whether the tight junctions were physiologically operative in P0 HeLa expressors. Consistent with the presence of operative tight junctions, we found that P0 expressors in monolayers maintained endogenous proteins in their apical and basolateral plasma membrane subdomains. Furthermore, these cells generated a higher transepithelial resistance than did control HeLa cells, which is indicative of the formation of an effective intercellular permeability barrier.

Cytoskeletal and nuclear localization of myelin oligodendrocytic basic protein isoforms

The recently described single copy myelin-associated oligodendrocytic basic protein (Mobp) gene is expressed exclusively in the central nervous system (CNS). The gene encodes a family of small highly basic polypeptides with predicted amino acid lengths of 69, 71, 81, 99 and 170, all of which share a 68 residue amino terminal. Here we report on the subcellular distribution of two of these polypeptides termed MOBP81 and MOBP170 in transiently transfected Cos7 cells using an antibody raised against a region common to all isoforms of MOBP. Additionally, we describe MOBP trafficking in cultured mouse spinal cord oligodendrocytes. Immunostaining for MOBP81 is intense in the perinuclear region and extends throughout the cytoplasm colocalizing with the microtubular cytoskeletal network. Consistent with this we demonstrate that MOBP partitions with the cytoskeletal fraction prepared from myelin. In contrast, although MOBP170 is present in the cytoplasm it does not colocalize with the cytoskeleton and displays a greater variation in distribution. In the majority of transfectants immunostaining is present throughout the karyoplasm but with increased intensity around the nucleolus. Within mouse primary oligodendrocytes endogenous MOBP is present in the cell body and processes colocalizing with the microtubular network. Immunoreactivity is not detectable in the nucleus in these mature oligodendrocytes. These significant differences in MOBP81 and MOBP170 protein kinesis coupled to different expression profiles of their respective message populations may be indicative of both myelin structural and cellular/regulatory functions, respectively, for these polypeptides.

Overexpression of a minor component of myelin basic protein isoform (17.2 kDa) can restore myelinogenesis in transgenic shiverer mice

Shiverer (shi) mice, which are neurologically mutant, lack a large portion of the gene for the myelin basic proteins (MBPs), have virtually no myelin in their central nervous system (CNS), and shiver, undergo seizures, and die early. At least five types of MBPs (21.5, 18.5, 17.3, 17.2 and 14.0 kDa) are known to be generated through alternative splicing from a single MBP gene. We have produced transgenic shi mice carrying a cDNA encoding mouse 14-kDa MBP isoform, the most abundant form of MBPs, under control of a mouse MBP gene promoter, and showed that expression of the 14-kDa MBP can restore CNS myelination. To test whether the 17.2-kDa MBP isoform, one of the minor components of MBPs, can also elicit myelination in homozygous shi mutants, we produced seven independent transgenic shi mice carrying cDNA encoding the mouse 17.2-kDa MBP isoform, and the transcription of which was driven by a mouse MBP gene promoter. The axons in the cerebellum of one transgenic line, which exhibited the highest expression of transgene-derived mRNA ( approximately 50% of the level of total MBP mRNA in the normal mouse brain), were myelinated. This mouse exhibited nearly normal behavior. These findings indicate that the 17.2-kDa MBP isoform, even when the only 17.2-kDa MBP isoform is present, has the ability to elicit CNS myelination in transgenic shi mice. This transgenic strategy will be useful for elucidating the role of each type of MBP isoform in CNS myelinogenesis.

CNP overexpression induces aberrant oligodendrocyte membranes and inhibits MBP accumulation and myelin compaction

2',3'-Cyclic nucleotide 3'-phosphodiesterase (CNP) is highly enriched in myelin-forming cells where it is concentrated at the cytoplasmic side of all surface membranes except those of compact myelin. Previous studies have provided evidence that CNP is functionally involved in migration or expansion of membranes during myelination. This hypothesis is supported, in part, by the production of aberrant myelin membranes in transgenic mice that have a 6-fold increase in CNP expression. In addition, many myelin lamellae in these CNP-overexpressing mice lacked major dense lines (MDLs). The purpose of the present study was to determine if CNP overexpression altered: (1) oligodendrocyte and myelin membrane production during early stages of myelination, and (2) the ultrastructural distribution of CNP and myelin basic protein (MBP) in myelin membranes. We identified aberrant membrane expanses that extended from premyelinating oligodendrocyte processes, the periaxonal membrane, and the contact point between oligodendrocyte processes and myelin internodes. Myelin membranes without MDLs were deficient in MBP and enriched in CNP. These data support a functional role for CNP during oligodendrocyte membrane expansion and indicate, for the first time, that CNP may help target MBP to compact myelin.

Nuclear transport of myelin basic protein

The multiple myelin basic protein (MBP) isoforms expressed by myelinating cells are now known to have different expression patterns. The relative abundance of the isoforms containing exon II is greater early in myelinogenesis, whereas in compact myelin the isoforms lacking this exon are more abundant. Further, the individual MBPs exhibit different intracellular localizations, indicating that the isoforms may not be functionally equivalent in myelinating cells. The major MBPs (14 kD and 18.5 kD) have strong affinity for membranes, while on the other hand, the less abundant isoforms (17 kD and 21.5 kD) localize to the nucleus of young oligodendrocytes, suggesting a regulatory role in the myelination program. The same intracellular distribution patterns have been observed when the MBPs are expressed in Hela cells and in shiverer oligodendrocytes. Thus, the intracellular fate of these proteins seems to be generally directed through alternative expression of exon II. Furthermore, the extent of MBPexII entry into the nucleus was found to be directly related to the growth state of host cells. In this paper, we demonstrate that nuclear proteins constitutively expressed by Hela cells also exhibit an apparently growth-related nucleo-cytoplasmic distribution revealing that MBPexII exhibits the same behavior as bona fide nuclear proteins. Also, to further characterize MBP nuclear transport, we explored various parameters of the translocation of MBP into the nucleus using an in vitro system. This experimental paradigm permits the uncoupling of synthesis and translocation events; thus, the transport of MBP into cell nuclei can be studied as a function of time. We also evaluated how changes in temperature as well as energy depletion affect the in vitro nuclear transport of MBP.

Ins and outs of peripheral myelin protein-22: mapping transmembrane topology and intracellular sorting

Molecular genetic studies have shown that the peripheral myelin protein 22 (PMP22) is a key gene in hereditary peripheral neuropathies and appears to be essential for the formation and maintenance of myelin in the PNS. Based on the amino acid sequence the predicted structure of PMP22 protein contains two major distinct hydrophilic regions and four transmembrane domains. To analyze the cellular localization and membrane topology of PMP22 we inserted an octapeptide tag-sequence at the amino or at the carboxyl terminus of the PMP22 open reading frame and generated different chimeric constructs which were expressed in HeLa cells. The expression of the tagged PMP22 protein and its orientation with respect to the plasma membrane were analyzed using antibodies raised against specific PMP22 epitopes and the tag sequence. Combined indirect, double-immunofluorescence labeling and confocal microscopy showed that PMP22 is synthesized in the rough endoplasmic reticulum of transfected cells and passes through the Golgi apparatus to the cell surface. We determined the transmembrane organization of PMP22 providing the first experimental evidence that confirms the cytoplasmic disposition of its N and C termini and the extracellular localization of the two hydrophilic domains containing amino acids 28-40 and 118-131. This study provides the basis for further analysis aimed to identify functional domains of wild-type PMP22 and the cellular sorting of mutant forms of PMP22.

The active transport of myelin basic protein into the nucleus suggests a regulatory role in myelination

The myelin basic proteins (MBPs) are a set of membrane proteins that function to adhere the cytoplasmic leaflets of the myelin bilayer. During oligodendrocyte maturation prior to compact myelin formation, however, certain MBPs have been observed within the cell body and nucleus. We explored the parameters of the translocation of the exon II-containing MBPs (MBPexII) from the site of synthesis in the cell cytoplasm into the nucleus and in some experiments used GFP as a molecular reporter to monitor the intracellular distribution of MBP-GFP fusion proteins in living cells. We show here that the transport of MBPexII into cell nuclei is an active process, which is temperature and energy dependent, and may be regulated by phosphorylation state. Further, MBPexII can direct the entry of macromolecular complexes into cell nuclei, revealing that the exon II peptide segment may provide a nuclear localization signal (NLS), perhaps a novel one, or may induce a conformational change in the full-length protein that exposes a cryptic NLS. The MBPexII are thus very unusual in that they are plasma membrane proteins that are also targeted to the nucleus. In oligodendrocytes and Schwann cells, where the MBPs are naturally expressed, it is likely that karyophilic MBPs subserve a regulatory function in implementing the myelination program.

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.

Protein zero, a nervous system adhesion molecule, triggers epithelial reversion in host carcinoma cells

Protein zero (P(o)) is the immunoglobulin gene superfamily glycoprotein that mediates the self-adhesion of the Schwann cell plasma membrane that yields compact myelin. HeLa is a poorly differentiated carcinoma cell line that has lost characteristic morphological features of the cervical epithelium from which it originated. Normally, HeLa cells are not self-adherent. However, when P(o) is artificially expressed in this line, cells rapidly aggregate, and P(o) concentrates specifically at cell-cell contact sites. Rows of desmosomes are generated at these interfaces, the plasma membrane localization of cingulin and ZO-1, proteins that have been shown to be associated with tight junctions, is substantially increased, and cytokeratins coalesce into a cohesive intracellular network. Immunofluorescence patterns for the adherens junction proteins N-cadherin, alpha-catenin, and vinculin, and the desmosomal polypeptides desmoplakin, desmocollin, and desmoglein, are also markedly enhanced at the cell surface. Our data demonstrate that obligatory cell-cell adhesion, which in this case is initially brought about by the homophilic association of P(o) molecules across the intercellular cleft, triggers pronounced augmentation of the normally sluggish or sub-basal cell adhesion program in HeLa cells, culminating in suppression of the transformed state and reversion of the monolayer to an epithelioid phenotype. Furthermore, this response is apparently accompanied by an increase in mRNA and protein levels for desmoplakin and N-cadherin which are normally associated with epithelial junctions. Our conclusions are supported by analyses of ten proteins we examined immunochemically (P(o), cingulin, ZO-1, desmoplakin, desmoglein, desmocollin, N-cadherin, alpha-catenin, vinculin, and cytokeratin-18), and by quantitative polymerase chain reactions to measure relative amounts of desmoplakin and N-cadherin mRNAs. P(o) has no known signaling properties; the dramatic phenotypic changes we observed are highly likely to have developed in direct response to P(o)-induced cell adhesion. More generally, the ability of this "foreign" membrane adhesion protein to stimulate desmosome and adherens junction formation by augmenting well-studied cadherin-based adhesion mechanisms raises the possibility that perhaps any bona fide cell adhesion molecule, when functionally expressed, can engage common intracellular pathways and trigger reversion of a carcinoma to an epithelial-like phenotype.

Myelin basic protein mRNA localization and polypeptide targeting

Myelin basic proteins (MBPs), the major peripheral membrane proteins of central nervous system (CNS) myelin, are encoded by mRNAs that are selectively segregated to the myelinating processes of oligodendrocytes. In order to test whether the intracellular mechanisms responsible for MBP mRNA translocation are oligodendrocyte-specific, or alternatively, are present in other cell types and may therefore be more general, we have studied the localization of the 14 kD MBP mRNA and its encoded polypeptide (MBP14) in transiently transfected HeLa cells (a cervical carcinoma cell line) and in the rat pheochromocytoma cell line PC12. Unlike the situation in oligodendrocytes in situ, where MBP mRNAs are translocated and become "centrifugally" distributed, in both of the non-glial cells MBP mRNA was primarily detected in the perinuclear region. The MBP14 polypeptide was found associated with intracellular membranes, and not exclusively with the plasma membrane. Our results indicate that the inability of HeLa and PC12 cells to correctly target MBP mRNAs to the cell periphery leads to a failure to incorporate MBP polypeptides directly into the plasma membrane. Further, the data lend credence to the concept that MBP mRNA segregation appears to be a specific feature of myelin-forming cells which is required for the precise delivery of the encoded polypeptides to the forming myelin membrane.

A sequence compilation and comparison of exons that are alternatively spliced in neurons

Alternative splicing is an important regulatory mechanism to create protein diversity. In order to elucidate possible regulatory elements common to neuron specific exons, we created and statistically analysed a database of exons that are alternatively spliced in neurons. The splice site comparison of alternatively and constitutively spliced exons reveals that some, but not all alternatively spliced exons have splice sites deviating from the consensus sequence, implying diverse patterns of regulation. The deviation from the consensus is most evident at the -3 position of the 3' splice site and the +4 and -3 position of the 5' splice site. The nucleotide composition of alternatively and constitutively spliced exons is different, with alternatively spliced exons being more AU rich. We performed overlapping k-tuple analysis to identify common motifs. We found that alternatively and constitutively spliced exons differ in the frequency of several trinucleotides that cannot be explained by the amino acid composition and may be important for splicing regulation.

Intracellular transport and sorting of the oligodendrocyte transmembrane proteolipid protein

Delineating the properties and functions of the major central nervous system myelin proteins has been the focus of intensive research for decades. For PLP, this task has been confounded by its unusual properties, the complexity of the cellular membrane in which it resides, and the absence of a functional assay for the protein. The development of new experimental paradigms in which to study PLP may shed fresh light on the properties and functions of this intrinsic membrane protein. In the present communication we have used indirect, double label, immunofluorescence, and confocal microscopy to examine the distribution of PLP in Cos-7 cells transfected with an expression vector bearing the human PLP cDNA. Our results show that PLP is synthesized in the rough endoplasmic reticulum of transfected cells and passes through the Golgi apparatus to the cell surface. These results are consistent with previous studies showing PLP reaches the cell surface by transport through the secretory pathway. Levels of PLP at the cell surface are modest, most likely because protein deposited in this compartment can be endocytosed and subsequently transported to perinuclear lysosomes. Similar results are reported in the companion communication by Sinoway et al. (J Neurosci Res, 37:551-562, 1994). Using transfected HeLa cells they show that DM20 alone and PLP coexpressed with DM20 assume appropriate conformations for transport to the cell surface. The presence of PLP in subcellular compartments beyond the endoplasmic reticulum in Cos-7 cells indicates that the protein achieves a conformation appropriate for transport in the absence of other oligodendrocyte-specific factors; however, accumulation of large amounts of PLP in the cytoplasmic membrane compartment may require interactions with such glial-specific factors. Thus, the transfection paradigm described herein should prove a useful tool for investigating the folding and sorting of wild type and mutant forms of PLP as well as its membrane topology and posttranslational processing.

Isoprenylated proteins in myelin

Incubation of rat brainstem slices with [3H]-mevalonate ([3H]MVA) in the presence of lovastatin resulted in the incorporation of label into three groups of myelin-associated proteins with molecular masses of 47, 21-27, and 8 kDa, as revealed on sodium dodecyl sulfate-polyacrylamide rod gel electrophoresis. Although the gel patterns of [3H]MVA-derived prenylated proteins were similar, the relative level of 3H incorporated into each protein species differed between myelin and the brainstem homogenate. Immunoprecipitation studies identified the 47-kDa prenylated protein as a 2'-3'-cyclic nucleotide phosphodiesterase, whereas the 8-kDa protein proved to be the gamma subunit of membrane-associated guanine nucleotide regulatory protein. The 3H-labeled 21-27-kDa group in myelin corresponds to the molecular mass of the extensive Ras-like family of monomeric GTP-binding proteins known to be prenylated in other tissues. Increase in lovastatin concentration resulted in reduced levels of [3H]MVA-labeled species in myelin and concomitantly increased levels in the cytosol. A cold MVA chase restored to normality the appearance of [3H]MVA-labeled proteins in myelin. Furthermore, a high lovastatin concentration in the brainstem slice incubation mixture altered the appearance of newly synthesized nonprenylated myelin proteins, including proteolipid protein and the 17-kDa subspecies of myelin basic protein. Because other myelin proteins were unaffected by the high lovastatin concentration, restricting the availability of MVA in myelin-forming cells may selectively alter processes required for myelinogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Proteolipid protein interactions in transfectants: implications for myelin assembly

The proteolipid proteins (PLP and DM20) are major constituents of CNS myelin, but how they are delivered to and organized within the oligodendrocyte plasma membrane is incompletely understood. We have expressed both PLP and DM20 singly or together in a host cell line, HeLa. In either DM20 or PLP transfectants, at early time points (24 hours), the expressed proteins are found within intracellular compartments. In DM20 transfectants, the protein is delivered to the plasma membrane by 48 hours. In HeLa cells, PLP remains intracellular when expressed in the absence of DM20; only when it is coexpressed with DM20 is it transported to the plasma membrane. In cotransfectants, PLP can also be localized to organelles involved in both the protein biosynthetic and the endocytic pathways. Since, in HeLa cells at least, the delivery of PLP to the plasma membrane is facilitated by the coexpression of DM20, we suggest that the two proteins interact intracellularly to form a complex. In some PLP/DM20 cotransfectants, the proteolipids are concentrated in regions of cell-cell contact. The regional accumulation of these proteins at cell-cell interfaces is highly reminiscent of the behavior in transfected cells of another myelin protein, P0, and certain cadherin polypeptides, both of which have readily demonstrable membrane adhesive properties. Our data suggests that at certain stoichiometric ratios, proteolipids can become stabilized at cell surfaces to form adhesive bonds.

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

The segregation of proteins to specific cellular membranes is recognized as a common phenomenon. In oligodendrocytes of the central nervous system, localization of certain proteins to select regions of the plasma membrane gives rise to the myelin membrane. Whilst the fundamental structure and composition of myelin is well understood, less is known of the mechanisms by which the constituent proteins are specifically recruited to those regions of plasma membrane that are forming myelin. The two principal proteins of myelin, the myelin basic protein and proteolipid protein, differ greatly in character and sites of synthesis. The message for myelin basic protein is selectively translocated to the ends of the cell processes, where it is translated on free ribosomes and is incorporated directly into the membrane. Proteolipid protein synthesized at the rough endoplasmic reticulum, processed through the Golgi apparatus, and presumably transported via vesicles to the myelin membrane. This review examines the mechanisms by which these two proteins are targeted to the myelin membrane.

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.