Molecular cloning of the myelin basic proteins in the shark, Squalus acanthias, and the ray, Raja erinacia

Myelin in cartilaginous fish

Myelin is probably one of the most fascinating and innovative biological acquisition: a glia plasma membrane tightly wrapped around an axon and insulating it. Chondrichthyans (cartilaginous fishes) form a large group of vertebrates, and they are among oldest extant jawed vertebrate lineage. It has been known from studies 150 years ago, that they are positioned at the root of the successful appearance of compact myelin and main adhesive proteins in vertebrates. More importantly, the ultrastructure of their compact myelin is indistinguishable from the one observed in tetrapods and the first true myelin basic protein (MBP) and myelin protein zero (MPZ) seem to have originated on cartilaginous fish or their ancestors, the placoderms. Thus, the study of their myelin formation would bring new insights in vertebrate׳s myelin evolution. Chondrichthyans central nervous system (CNS) myelin composition is also very similar to peripheral nervous system (PNS) myelin composition. And while they lack true proteolipid protein (PLP) like tetrapods, they express a DM-like protein in their myelin. This article is part of a Special Issue entitled SI: Myelin Evolution.

Structure and expression of myelin basic protein gene products in Xenopus laevis

To study roles of the myelin basic protein (mbp) gene products in nervous system development, cDNA cloning and expression analyses were performed in Xenopus laevis. We cloned cDNAs for XMBP.1 and XMBP.2 encoded by xmbp.1 and xmbp.2 genes, respectively. We also identified xmbp.1 gene transcripts encoding three XGolli (X.laevis gene of the oligodendrocyte lineage) proteins, XBG21.1, XJ37.1, and XTP8.1, which are homologues of mouse BG21, J37, and TP8, respectively. In reverse transcription-polymerase chain reaction (RT-PCR) analyses, the XMBP, XJ37, and XTP8 mRNAs were expressed in brain, ovaries, testes, and/or thymus in frogs and in larvae after hatching. In contrast, the XBG21 mRNA was found fairly ubiquitously in adult tissues, unfertilized eggs and embryos throughout the developmental stages examined. Western blot analyses using three different monoclonal antibodies (mAbs) showed that the central and peripheral myelin contained 20kDa and18.5 kDa XMBP variants. In addition, XMBP was found in thymus by Western blotting and in thymocyte cytoplasm immunocytochemically. However, the XGolli protein, most provably XBG21, was detectable only in testes. The results indicate that the structure of xmbp gene products seems highly conserved among amphibians and mammals, although their expression patterns and thus physiological roles may partially differ. This is the first report that systematically describes the mbp gene products in nonmammalian vertebrates.

Identification and characterization of a MBP isoform specific to hypothalamus in orange-spotted grouper (Epinephelus coioides)

Myelin basic protein (MBP), as a major component of the myelin sheath, has been revealed to play an important role in forming and maintaining myelin structure in vertebrate nervous system. In teleost, hypothalamus is an instinctive brain center and plays significant roles in many physiological functions, such as energy metabolism, growth, reproduction, and stress response. In comparison with other MBP identified in vertebrates, a smallest MBP is cloned and identified from the orange-spotted grouper hypothalamic cDNA plasmid library in this study. RT-PCR analysis and Western blot detection indicate that the EcMBP is specific to hypothalamus, and expresses mainly in the tuberal hypothalamus in adult grouper. Immunofluorescence localization suggests that EcMBP should be expressed by oligodendrocytes, and the expressing cells should be concentrated in hypothalamus and the area surrounding hypothalamus, such as NPOpc, VC, DP, NLTm, and NDLI. The studies on EcMBP expression pattern and developmental behaviour in the brains of grouper embryos and larvae reveal that the EcMBP-expressing cells are only limited in a defined set of cells on the border of hypothalamus, and suggest that the EcMBP-expressing cells might be a subpopulation of oligodendrocyte progenitor cells. This study not only identifies a smallest MBP isoform specific to hypothalamus that can be used as a molecular marker of oligodendrocytes in fish, but also provides new insights for MBP evolution and cellular distribution.

Identification and isolation of a BTB-POZ-containing gene expressed in oocytes and early embryos of the zebrafish Danio rerio

In this report, we describe the cloning of a cDNA from the zebrafish Danio rerio encoding a protein containing a BTB-POZ domain closely resembling the BTBD1 and BTBD2 proteins previously identified in mammals. However, unlike other BTB-POZ-containing genes, expression of this gene in adults is most abundant in oocytes, where the RNA can be detected at all stages of oogenesis examined. The presence of the RNA persists through early cleavage, but is decreased significantly by gastrulation. Although the function of this gene has yet to be determined, its resemblance to the BTB-POZ family of genes coupled with its expression pattern suggests that it may have an important function in oogenesis and (or) early zebrafish development.

Studies of posttranslational modifications in spiny dogfish myelin basic protein

The objective of this investigation was to determine whether nonmammalian myelin basic protein contained charge isomers resulting from extensive posttranslational modifications as seen in mammalian MBP. Four charge isomer components from dogfish MBP have been isolated. These forms arise by phosphorylation and deamidation modifications. Components C1, C2 and C3 have been characterized. We are currently characterizing component C8. Dogfish MBP is less cationic than mammalian MBP and has about 50% lower mobility on a basic pH gel electrophoresis relative to human and to bovine MBP. The mammalian component C1, which is unmodified, is modified in the dogfish by phosphorylation. The reduced electrophoretic mobility is largely attributable to the charge reduction resulting from phosphorylation in serine 72, 83, and 120 or 121 in C1, and C3. In component C2, two or three phosphate groups were distributed among residues 134, 138 and 139. It was found that dogfish amino acid residue 30 was a lysine residue and not a glutamate residue as reported in the literature.

Determination of the sites of posttranslational modifications in the charge isomers of bovine myelin basic protein by capillary electrophoresis-mass spectroscopy

The posttranslational modifications in each of the 18.5 kDa bovine myelin basic protein charge isomers C-1 to C-6 have been determined by the use of capillary electrophoresis-mass spectroscopy. The pattern of modifications is viewed as being unique to each charge isomer and is thought to reflect a specific placement and function for each isomer in the myelin membrane. Several of the sites of posttranslational phosphorylation were found to differ from a number of the reported sites that were phosphorylated in vitro by various kinases. These differences suggest that an extremely cautious approach be taken in identifying in vivo posttranslationally modified amino acid residues from residues that have been modified in vitro by various kinases. We have identified the following posttranslationally phosphorylated and deamidated, modified sites in the bovine MBP components C1-C6. C1 has no modification; C2 represents a deamidation of Gln 146; in C3, Thr 97 and Ser 164 are phosphorylated; in C4, Ser 54, Thr 97, and Ser 160 are phosphorylated; in C5 Ser 7, Ser 54, Thr 97, and Ser 164 are phosphorylated; and in C6, Ser 7, Ser 54, Thr 97, Ser 160, and Ser 164 are phosphorylated.

Neural cells from dogfish embryos express the same subtype-specific antigens as mammalian neural cells in vivo and in vitro

Neural cells are classically identified in vivo and in vitro by a combination of morphological and immunocytochemical criteria. Here, we demonstrate that antibodies used to identify mammalian oligodendrocytes, neurons, and astrocytes recognize these cell types in the developing spiny dogfish central nervous system and in cultures prepared from this tissue. Oligodendrocyte-lineage-specific antibodies O1, O4, and R-mAb labeled cells in the 9 cm dogfish brain stem's medial longitudinal fascicle (MLF) and in areas lateral to it. Process-bearing cells, cultured from the dogfish brain stem, were also labeled with these antibodies. An anti-lamprey neurofilament antibody (LCM), which recognized 60 and 150 kDa proteins in dogfish brain stem homogenates, labeled axons and neurons in the brain stem and axons in the cerebellum of the dogfish embryo. It also labeled cell bodies and/or processes of some cultured cerebellar cells. An anti-bovine glial fibrillary acidic protein antibody, which recognized 42-44 kDa protein(s) in dogfish brain stem homogenates, labeled astrocyte-like processes in the brain stem and cerebellum of the dogfish embryo and numerous large and small flat cells in the cerebellar cultures. These results demonstrate that dogfish oligodendrocytes, neurons, and astrocytes express antigens that are conserved in mammalian neural cells. The ability to culture and identify neural cell types from cartilaginous fish sets the stage for studies to determine if proliferation, migration, and differentiation of these cell types are regulated in a similar fashion to mammalian cells.

Divalent metals of myelin and their differential binding by myelin basic protein of bovine central nervous system

Divalent metal ions are being implicated in the compaction of myelin. Levels of Cd, Co, Cu, Hg, Mn, Pb, Zn, Ca and Mg of isolated myelin of bovine central nervous system (CNS) were measured by flame atomic absorption spectrophotometry. Binding of these metal ions by isolated myelin basic protein (MBP, M(r) 18,500) of bovine CNS was concurrently assessed by centrifugal equilibrium dialysis. Metals were bound in the order of Hg > Cu > Zn > Mg > Cd > Co, exempting Mn, Pb and Ca. The results are indicative of differential metal affinity of MBP which may account for the immobilization or anchoring of MBP in myelin by zinc and other divalent metal cations.

The structural complexities of the myelin basic protein gene from mouse are also present in shark

The Golli-mbp gene complex contains two overlapping transcription units with two distinct promoters, of which the downstream (myelin basic protein [mbp]) promoter is more frequently used. A previous comparison of the downstream promoter sequences from shark and mouse allowed the identification of two DNA sequences called the boxes I and II and the wobble zone. The boxes I and II sequence is a composite cis-acting motif that is thought to be involved in the regulation of the downstream promoter. It contains sequences similar to T-antigen, MyoD/E2A, and glucocorticoid receptor-binding sites. The wobble zone codes for an exon (5a in the nomenclature of Campagnoni et al., 1993) that is included in messenger RNAs transcribed from the upstream promoter. The polypeptides encoded by this exon from shark and mouse are 86 and 84 amino acids long, respectively. These polypeptides are overall 59% identical and include a region (residues 41-75 in shark and 39-73 in mouse) that is 89% identical between the two species. A primary sequence analysis showed that each of these polypeptides contains an N-glycosylation site, phosphorylation sites for Ca2+/calmodulin-dependent protein kinase, protein kinase C and casein kinase II, and partial ATP- and GTP-binding sites. The shark polypeptide also contains a phosphorylation site for proline-directed protein kinase. These observations are consistent with the notion that the intricate structure and regulation of the Golli-mbp gene complex arose during vertebrate evolution within a common ancestor to sharks and mammals.