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

Expression of caspase 3 in ovarian follicle cells of the lizard Podarcis sicula

In this study, our aim was to determine whether caspase 3 plays a role, during previtellogenesis, in the ovarian follicular epithelium of the lizard Podarcis sicula. We investigated the presence and localization of proform and active caspase 3 by enzyme assay, Western blotting and immunocytochemistry. In parallel, a fragment of caspase 3 was cloned for the first time in this species, sequenced and used for in situ hybridization to localize messengers and analysed by a phylogenetic survey to shed light on its homology with reptilian caspases. Results demonstrated that: (1) the follicle cells expressed a caspase of the 3/7 group and the mRNA for caspase 3 was transcribed in the stem phase and was completely translated during cell differentiation; (2) the proform protein was stored during the differentiated (nurse) stage and activated at the end of previtellogenesis provoking the degeneration of cells; (3) the predicted protein sequence, although partial, had a strong similarity with the known reptilian caspases 3. The epithelial cells of the ovarian follicle, therefore, do not employ caspase 3 during the nurse stage but, instead, prepare for apoptosis long before the process actually begins. The relevance of this strategy is discussed.

Intracellular Protein Shuttling: A Mechanism Relevant for Myelin Repair in Multiple Sclerosis?

A prominent feature of demyelinating diseases such as multiple sclerosis (MS) is the degeneration and loss of previously established functional myelin sheaths, which results in impaired signal propagation and axonal damage. However, at least in early disease stages, partial replacement of lost oligodendrocytes and thus remyelination occur as a result of resident oligodendroglial precursor cell (OPC) activation. These cells represent a widespread cell population within the adult central nervous system (CNS) that can differentiate into functional myelinating glial cells to restore axonal functions. Nevertheless, the spontaneous remyelination capacity in the adult CNS is inefficient because OPCs often fail to generate new oligodendrocytes due to the lack of stimulatory cues and the presence of inhibitory factors. Recent studies have provided evidence that regulated intracellular protein shuttling is functionally involved in oligodendroglial differentiation and remyelination activities. In this review we shed light on the role of the subcellular localization of differentiation-associated factors within oligodendroglial cells and show that regulation of intracellular localization of regulatory factors represents a crucial process to modulate oligodendroglial maturation and myelin repair in the CNS.

Cyclin dependent kinase 5 is required for the normal development of oligodendrocytes and myelin formation

The development of oligodendrocytes, the myelinating cells of the vertebrate CNS, is regulated by a cohort of growth factors and transcription factors. Less is known about the signaling pathways that integrate extracellular signals with intracellular transcriptional regulators to control oligodendrocyte development. Cyclin dependent kinase 5 (Cdk5) and its co-activators play critical roles in the regulation of neuronal differentiation, cortical lamination, neuronal cell migration and axon outgrowth. Here we demonstrate a previously unrecognized function of Cdk5 in regulating oligodendrocyte maturation and myelination. During late embryonic development Cdk5 null animals displayed a reduction in the number of MBP+ cells in the spinal cord, but no difference in the number of OPCs. To determine whether the reduction of oligodendrocytes reflected a cell-intrinsic loss of Cdk5, it was selectively deleted from Olig1+ oligodendrocyte lineage cells. In Olig1(Cre/+); Cdk5(fl/fl) conditional mutants, reduced levels of expression of MBP and PLP mRNA were observed throughout the CNS and ultrastructural analyses demonstrated a significant reduction in the proportion of myelinated axons in the optic nerve and spinal cord. Pharmacological inhibition or RNAi knockdown of Cdk5 in vitro resulted in the reduction in oligodendrocyte maturation, but had no effect on OPC cell proliferation. Conversely, over-expression of Cdk5 promoted oligodendrocyte maturation and enhanced process outgrowth. Consistent with this data, Cdk5(-/-) oligodendrocytes developed significantly fewer primary processes and branches than control cells. Together, these findings suggest that Cdk5 function as a signaling integrator to regulate oligodendrocyte maturation and myelination.

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.

Mast cell adhesion induces cytoskeletal modifications and programmed cell death in oligodendrocytes

In this paper we show that rat peritoneal mast cells (RPMC) adhere to rat oligodendrocytes (ODC) in culture and switch on a bi-directional signal affecting both adhering cell and its target. Following heterotypic interaction, RPMC release granule content and ODC show morphological changes and enter the apoptotic programme. Altogether, these findings indicate that the interaction of MC with ODC could play a role in the mechanism of CNS damage induced by the inflammatory reaction.

The N-glycan profile of mouse myelin, a specialized central nervous system membrane

Understanding the rich complement of sugar chains found in cellular membranes is impeded by the complexity of cell types and membrane diversity. To overcome this, we have analyzed the N-linked sugar chain composition of the glycoproteins of CNS myelin, an elaboration of the plasma membranes of oligodendrocytes (OLs) that result in a multilamellar wrapping of neuronal axons, facilitating nerve conduction with dramatic savings of space and energy. Due to an usually high lipid to protein ratio, myelin can be separated readily from other heavier membranes on sucrose gradients and further fractionated into subdomains related to myelin structure and function, including compact myelin and myelin-associated axolemmal membrane (Menon et al. 2003). We analyzed these fractions for N-linked sugar chains, using 2D HPLC following hydrazinolysis and pyridylamination. Our results indicate that compared with total brain homogenate, the amount of N-glycans is 1.3-fold higher in the myelin-associated axolemmal membranes, but it is 0.5-fold less in CM. M5 [Manalpha1-3((Manalpha1-3)(Manalpha1-6)Manalpha1-6)Manbeta1-4GlcNAcbeta1-4GlcNAc] is the most abundant sugar chain in total brain homogenate, compact myelin, and myelin-associated axolemma, constituting approximately 20% of sugar chains. Although the types of sugar chains are similar among the fractions, their expression levels vary significantly. In addition to high mannose type oligosaccharides, the core fucosylated, biantennary N-glycans with bisecting N-acetylglucosamine (GlcNAc) residue, A2G1(3)FB [Galbeta1-4GlcNAcbeta1-2Manalpha1-3(GlcNAcbeta1-2Manalpha1-6)(GlcNAcbeta1-4)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6)GlcNAc], A2G1(6)FB [GlcNAcbeta1-2Manalpha1-3(Galbeta1-4GlcNAcbeta1-2Manalpha1-6)(GlcNAcbeta1-4)Manbeta1-4GlcNAcbeta1-4 (Fucalpha1-6)GlcNAc] and BA-1 [Manalpha1-3(GlcNAcbeta1-2Manalpha1-6)(GlcNAcbeta1-4)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6)GlcNAc], and A1(6)G0F [Manalpha1-3(GlcNAcbeta1-2Manalpha1-6)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6) GlcNAc] are also present in relatively large proportions in compact myelin. We suggest that these differences may be related to myelin-axolemmal function.

Myelin-associated oligodendrocytic basic protein: a family of abundant CNS myelin proteins in search of a function

The myelin-associated oligodendrocytic basic protein (MOBP) family constitutes the third most abundant protein in CNS myelin. The mouse Mobp gene comprises eight exons. Mobp pre-mRNA processing gives rise to at least seven Mobp splice variants which are expressed solely in the oligodendrocyte. The predicted proteins all, with one exception, share a 68 residue amino terminus, encoded by exon 3. The carboxyl termini differ in length, giving rise to the diverse array of the protein isoforms. Like myelin basic protein, MOBP is present in the major dense line of CNS myelin suggesting a role in the compaction or stabilization of myelin. However, Mobp homozygous null mice display no overt clinical phenotype and no defect in the process of myelination. MOBP can induce experimental allergic encephalomyelitis in mice and has been proposed to have a role in the pathogenesis of multiple sclerosis. Despite 10 years of rigorous study, the normal physiological function of MOBP remains unknown.

Potential structural role of non-coding and coding RNAs in the organization of the cytoskeleton at the vegetal cortex of Xenopus oocytes

The localization of RNA within a cell or embryo is crucial for proper cellular function or development. There is evidence that the cytoskeleton and RNA may function in the anchoring of localized RNAs at the vegetal cortex of Xenopus laevis oocytes. We found that the organization of the cytokeratin filaments but not the actin cytoskeleton depends on the presence of intact VegT mRNA and a noncoding RNA, Xlsirts. Destruction of either of these transcripts results in disruption of the cytokeratin cytoskeleton in a transcript-specific manner and interferes with proper formation of the germinal granules and subsequent development of the germline. Analysis of the distribution of endogenous VegT and Xlsirts in live oocytes using molecular beacons showed that these RNAs are integrated into the cytokeratin cytoskeleton. These results demonstrate a novel structural role of coding and noncoding RNAs in the organization of the vegetal cortex of Xenopus oocytes.

The microtubule-associated protein tumor overexpressed gene binds to the RNA trafficking protein heterogeneous nuclear ribonucleoprotein A2

In neural cells, such as oligodendrocytes and neurons, transport of certain RNAs along microtubules is mediated by the cis-acting heterogeneous nuclear ribonucleoprotein A2 response element (A2RE) trafficking element and the cognate trans-acting heterogeneous nuclear ribonucleoprotein (hnRNP) A2 trafficking factor. Using a yeast two-hybrid screen, we have identified a microtubule-associated protein, tumor overexpressed gene (TOG)2, as an hnRNP A2 binding partner. The C-terminal third of TOG2 is sufficient for hnRNP A2 binding. TOG2, the large protein isoform of TOG, is the only isoform detected in oligodendrocytes in culture. TOG coimmunoprecipitates with hnRNP A2 present in the cytoskeleton (CSK) fraction of neural cells, and both coprecipitate with microtubule stabilized pellets. Staining with anti-TOG reveals puncta that are localized in proximity to microtubules, often at the plus ends. TOG is colocalized with hnRNP A2 and A2RE-mRNA in trafficking granules that remain associated with CSK-insoluble tissue. These data suggest that TOG mediates the association of hnRNP A2-positive granules with microtubules during transport and/or localization.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.

Identification of a distinct class of cytoskeleton-associated mRNAs using microarray technology

BACKGROUND

Interactions between mRNA and the cytoskeleton are critical for the localization of a number of transcripts in eukaryotic somatic cells. To characterize additional transcripts that may be subject to this form of regulation, we developed a two-step approach that utilizes biochemical fractionation of cells to isolate transcripts from different subcellular compartments followed by microarray analysis to examine and compare these subpopulations of transcripts in a massively-parallel manner.

RESULTS

Using this approach, mRNA was extracted from the cytoskeleton-rich and the cytosolic fractions of the promyelocytic HL-60 cell line. We identify a subset of 22 transcripts that are significantly enriched in the cytoskeleton-associated population. The majority of these encode structural proteins and/or proteins known to interact with elements of the cytoskeleton. Localization required an intact actin cytoskeleton and was largely conserved upon differentiation of precursor HL-60 cells to a macrophage-like phenotype.

CONCLUSIONS

We conclude that the association of transcripts with the actin cytoskeleton in somatic cells may be a critical post-transcriptional regulatory event that controls a larger class of genes than has previously been recognized.

Organization and functional roles of the cytoskeleton in oligodendrocytes

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

Targeting of mRNAs within the glial cell cytoplasm: how to hide the message along the journey

The subcellular targeting of mRNAs encoding myelin proteins to the oligodendrocyte processes is an accepted fact in myelin formation. How these messengers are kept silent during their movement to the subcellular domain where they are turned on remains a mystery. This review focuses on aspects of mRNA targeting and speculates on possible molecular mechanisms for the translational control of myelin-located mRNAs.

Analogous structural motifs in myelin basic protein and in MARCKS

Myelin basic protein (MBP) and myristoylated alanine-rich C-kinase substrate (MARCKS) are similar in terms of having extended conformations regulated by their environment (i.e., solubilised or lipid-associated), N-terminal modifications, a dual nature of interactions with lipids, binding to actin and Ca2+-calmodulin, and being substrates for different kinds of protein kinases. The further sequence similarities of segments of MBP with lipid effector regions of MARCKS, and numerous reports in the literature, support the thesis that some developmental isoform of MBP functions in signal transduction.