Cloning and expression of the proteolipid protein DM20 cDNA from the brain of the rainbow trout, Oncorhynchus mykiss

A quantitative real-time RT-PCR assay for European eel tyrosine hydroxylase

Dopamine (DA) plays a key inhibitory role in pubertal development of the European eel, but how DAergic neuronal activity is regulated is not known in this species. In order to investigate the regulation of DA inhibition at the molecular level, we developed a quantitative real-time RT-PCR (qrtRT-PCR) assay, using the Light Cycler system, for the expression of eel tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis. Two different reference genes were compared: the previously cloned eel cytochrome b, and eel acidic ribosomal phosphoprotein P0, the latter of which we cloned and partly sequenced. To further validate the assay, different methods of total RNA extraction were tested and compared. When applied to cDNA extracted from dissected brains of juvenile eels, the expression of TH was highest in the olfactory bulb, followed by the telencephalon including preoptic area, and the di-/mesencephalic areas excluding the optic lobes. TH expression in the optic lobes and in the medulla oblongata was low, whereas no expression could be detected in corpus cerebellum. This distribution pattern is in agreement with earlier studies on TH in the eel using immunohistochemistry, RT-PCR, and Northern blotting. The developed qrtRT-PCR assay provides a new tool for understanding the mechanisms regulating central DA inhibition of puberty in juvenile eels.

The 36K protein of zebrafish CNS myelin is a short-chain dehydrogenase

Previous studies identified homologues to mammalian myelin genes expressed in the teleost central nervous system (CNS), including myelin basic protein (MBP), protein zero (P0), and a member of the proteolipid protein family, DM20. In addition, an uncharacterized 36-kDa (36K) protein is a major component of teleost myelin, but is not a major component of myelin in other species. In the present study, we sought to better understand myelin proteins and myelination in one teleost, zebrafish, by molecular characterization of the zebrafish 36K protein. Purified zebrafish CNS myelin was isolated and the amino acid sequences of peptides present in the 36-kDa band were determined by mass spectrometry. These sequences matched a previously uncharacterized EST in The Institute for Genome Research (TIGR) zebrafish database that is related to the short-chain dehydrogenase/reductase (SDR) protein family. In vitro expression of the zebrafish 36K cDNA in Neuro 2a cells resulted in a protein product that was recognized by a 36K polyclonal antibody. The zebrafish 36K mRNA and protein expression patterns were determined and correlated to other known myelin gene expression profiles. In addition, we determined by in situ hybridization that a human 36K homologue (FLJ13639) is expressed in oligodendrocytes and neurons in the adult human cortex. This study identified a major myelin protein in zebrafish, 36K, as a member of the SDR superfamily; an expression pattern similar to other myelin genes was demonstrated.

Characterization of myelination in the developing zebrafish

Myelination, the process by which glial cells ensheath and electrically insulate axons, has been investigated intensely. Nevertheless, knowledge of how myelination is regulated or how myelinating cells communicate with neurons is still incomplete. As a prelude to genetic analyses of these processes, we have identified zebrafish orthologues of genes encoding major myelin proteins and have characterized myelination in the larval zebrafish. Expression of genes corresponding to proteolipid protein (PLP/DM20), myelin protein zero (P0), and myelin basic protein (MBP) is detected at 2 days postfertilization (dpf), first in the ventral hindbrain, close to the midline. During the next 8 days, expression spreads rostrally to the midbrain and optic nerve, and caudally to the spinal cord. DM20 is expressed in the CNS only, while MBP transcripts are detected both in the CNS and in Schwann cells of the lateral line, cranial nerves, and spinal motor nerves. Unlike its closest homologue, trout IP1, zebrafish P0 transcripts were restricted to the CNS. Ultrastructurally, the expression of myelin genes correlated well with myelination, although myelination showed a temporal lag. Myelinated axons were first detected at 4 dpf in the ventral hindbrain, where they were loosely wrapped by processes of glia cells. By 7 dpf, bundles of heavily myelinated axons were observed in the same region. Axons in the lateral line and optic nerves were also surrounded by compact myelin. Conservation in gene expression patterns and the early appearance of myelinated axons, support using the zebrafish to dissect the process of myelination by a genetic approach.

Molecular synapomorphies resolve evolutionary relationships of extant jawed vertebrates

The evolutionary relationships of gnathostomes (jawed vertebrates), which comprise chondrichthyans (cartilaginous fishes), lobe-finned fishes (coelacanths and lungfishes), tetrapods, and actinopterygians (ray-finned fishes), have been debated for almost a century. Phylogenetic analyses based on fossils, morphology, and molecular sequences have generated different models of relationships that remain unresolved. We identified 13 derived shared molecular markers (synapomorphies) that define clades in the vertebrate lineage and used them to resolve the phylogenetic relationships of extant jawed vertebrates. Our markers include the presence or absence of insertions and deletions in coding sequences, nuclear introns, and alternatively spliced transcripts. The synapomorphies identified by us are congruent with each other and give rise to a single phylogenetic tree. This tree confirms that chondrichthyans are basal to all living gnathostomes, that lungfishes (Dipnoi) are the closest living relatives of tetrapods, and that bichirs (Cladistia) are the living members of the most ancient family of ray-finned fishes. Our study also provides molecular evidence to support the monophyly of living tetrapods and teleosts.

A proteolipid protein-specific pre-mRNA (Ppm-1) contains intron 3 and is up-regulated during myelination in the CNS

Alternative splicing of the precursor for messenger RNA (pre-mRNA) is a common process utilised by higher eukaryotes to modulate gene expression. A single primary transcript may generate several proteins with distinct functions, expressed in tissue-specific, developmental patterns. This article describes an oligodendrocyte-specific pre-mRNA product of proteolipid protein gene (P/p) transcription, which is the precursor for P/p but not Dm20 mRNA in the CNS. This P/p-specific pre-mRNA (Ppm-1) includes the intact intron 3 of the P/p gene. It is first expressed during active myelination, and it localises to the nucleus of oligodendrocytes, in both normal and jimpy (jp) murine CNS. In addition to mouse, Ppm-1 is found also in rat and dog, but not toad or trout. Our work suggests that alternative splicing of the P/p gene primary transcript follows a branching pattern, resulting in the presence of at least one P/p isoform-specific pre-mRNA molecule, Ppm-1. Therefore, Dm20 mRNA may be the product of a divergent set of pre-mRNA splicing events.

Modification of Schwann cell phenotype with Plp transgenes: evidence that the PLP and DM20 isoproteins are targeted to different cellular domains

The X-linked proteolipid protein (Plp) gene encodes PLP, the major protein of central nervous system myelin, and its alternative RNA splice product, termed DM20. Schwann cells also express the Plp gene but, in contrast to oligodendrocytes, neither protein is incorporated into peripheral myelin. In the present study, we use different transgenes encoding PLP and DM20 to modify the expression of these proteins in myelin-forming Schwann cells of wild-type and jimpy mice. Increasing the level of PLP, either singly or in combination with DM20, leads to the incorporation of PLP into the compacted myelin sheath; however, DM20 always remains restricted to cytoplasmic regions of the Schwann cell. The insertion of PLP into the membrane does not appear to depend on a cooperativity of the two isoproteins. The presence of PLP does not visibly alter the ultrastructure and periodicity of peripheral nervous system (PNS) myelin. The results indicate that the absence of PLP in the peripheral myelin of normal animals most probably reflects the very low amounts of this isoprotein synthesised by Schwann cells. The preferential incorporation of PLP, as opposed to DM20, in peripheral myelin may indicate that a myelin targeting signal is present in the PLP-specific region of the molecule.