Isolation and characterization of a cDNA encoding the zebra finch myelin proteolipid protein

Neurogenetics of birdsong

Songbirds are a productive model organism to study the neural basis of auditory-guided vocal motor learning. Like human babies, juvenile songbirds learn many of their vocalizations by imitating an adult conspecific. This process is a product of genetic predispositions and the individual's life experience and has been investigated mainly by neuroanatomical, physiological and behavioral methods. Results have revealed general principles governing vertebrate motor behavior, sensitive periods, sexual dimorphism, social behavior regulation and adult neurogenesis. More recently, the emerging field of birdsong neurogenetics has advanced the way we think about genetic contributions to communication, mechanistically and conceptually.

Leukemia inhibitory factor regulates the timing of oligodendrocyte development and myelination in the postnatal optic nerve

Leukemia inhibitory factor (LIF) promotes the survival of oligodendrocytes both in vitro and in an animal model of multiple sclerosis, but the possible role of LIF signaling in myelination during normal development has not been investigated. We find that LIF(-/-) mice have a pronounced myelination defect in optic nerve at postnatal day 10. Myelin basic protein (MBP)- and proteolipid protein (PLP)-positive myelin was evident throughout the optic nerve in the wild-type mice, but staining was present only at the chiasmal region in LIF(-/-) mice of the same age. Further experiments suggest that the myelination defect was a consequence of a delay in maturation of oligodendrocyte precursor cell (OPC) population. The number of Olig2-positive cells was dramatically decreased in optic nerve of LIF(-/-) mice, and the distribution of Olig2-positive cells was restricted to the chiasmal region of the nerve in a steep gradient toward the retina. Gene expression profiling and cell culture experiments revealed that OPCs from P10 optic nerve of LIF(-/-) mice remained in a highly proliferative immature stage compared with littermate controls. Interestingly, by postnatal day 14, MBP immunostaining in the LIF(-/-) optic nerve was comparable to that of LIF(+/+) mice. These results suggest that, during normal development of mouse optic nerve, there is a defined developmental time window when LIF is required for correct myelination. Myelination seems to recover by postnatal day 14, so LIF is not necessary for the completion of myelination during postnatal development.

Monoclonal antibodies to distinct regions of human myelin proteolipid protein simultaneously recognize central nervous system myelin and neurons of many vertebrate species

Myelin proteolipid protein (PLP), the major protein of mammalian CNS myelin, is a member of the proteolipid gene family (pgf). It is an evolutionarily conserved polytopic integral membrane protein and a potential autoantigen in multiple sclerosis (MS). To analyze antibody recognition of PLP epitopes in situ, monoclonal antibodies (mAbs) specific for different regions of human PLP (50-69, 100-123, 139-151, 178-191, 200-219, 264-276) were generated and used to immunostain CNS tissues of representative vertebrates. mAbs to each region recognized whole human PLP on Western blots; the anti-100-123 mAb did not recognize DM-20, the PLP isoform that lacks residues 116-150. All of the mAbs stained fixed, permeabilized oligodendrocytes and mammalian and avian CNS tissue myelin. Most of the mAbs also stained amphibian, teleost, and elasmobranch CNS myelin despite greater diversity of their pgf myelin protein sequences. Myelin staining was observed when there was at least 40% identity of the mAb epitope and known pgf myelin proteins of the same or related species. The pgf myelin proteins of teleosts and elasmobranchs lack 116-150; the anti-100-123 mAb did not stain their myelin. In addition to myelin, the anti-178-191 mAb stained many neurons in all species; other mAbs stained distinct neuron subpopulations in different species. Neuronal staining was observed when there was at least approximately 30% identity of the PLP mAb epitope and known pgf neuronal proteins of the same or related species. Thus, anti-human PLP epitope mAbs simultaneously recognize CNS myelin and neurons even without extensive sequence identity. Widespread anti-PLP mAb recognition of neurons suggests a novel potential pathophysiologic mechanism in MS patients, i.e., that anti-PLP antibodies associated with demyelination might simultaneously recognize pgf epitopes in neurons, thereby affecting their functions.

Evolution of myelin proteolipid proteins: Gene duplication in teleosts and expression pattern divergence

The coevolution of neurons and their supporting glia to the highly specialized axon-myelin unit included the recruitment of proteolipids as neuronal glycoproteins (DMbeta, DMgamma) or myelin proteins (DMalpha/PLP/DM20). Consistent with a genome duplication at the root of teleosts, we identified three proteolipid pairs in zebrafish, termed DMalpha1 and DMalpha2, DMbeta1 and DMbeta2, DMgamma1 and DMgamma2. The paralogous amino acid sequences diverged remarkably after gene duplication, indicating functional specialization. Each proteolipid has adopted a distinct spatio-temporal expression pattern in neural progenitors, neurons, and in glia. DMalpha2, the closest homolog to mammalian PLP/DM20, is coexpressed with P0 in oligodendrocytes and upregulated after optic nerve lesion. DMgamma2 is expressed in multipotential stem cells, and the other four proteolipids are confined to subsets of CNS neurons. Comparing protein sequences and gene structures from birds, teleosts, one urochordate species, and four invertebrates, we have reconstructed major steps in the evolution of proteolipids.

Gene structure and amino acid sequence of Latimeria chalumnae (coelacanth) myelin DM20: phylogenetic relation of the fish

The structure of Latimeria chalumnae (coelacanth) proteolipid protein/DM20 gene excluding exon 1 was determined, and the amino acid sequence of Latimeria DM20 corresponding to exons 2-7 was deduced. The nucleotide sequence of exon 3 suggests that only DM20 isoform is expressed in Latimeria. The structure of proteolipid protein/DM20 gene is well preserved among human, dog, mouse, and Latimeria. Southern blot analysis indicates that Latimeria DM20 gene is a single-copy gene. When the amino acid sequences of DM20 were compared among various species, Latimeria was more similar to tetrapods than other fishes including lungfish, confirming the previous finding by immunoreactivity (Waehneldt and Malotka 1989 J. Neurochem. 52:1941-1943). However, when phylogenetic trees were constructed from the DM20 sequences, lungfish was clearly the closest to tetrapods. Latimeria was situated outside of lungfish by the maximum likelihood method. The apparent similarity of Latimeria DM20 to tetrapod proteolipid protein/DM20 is explained by the slow amino acid substitution rate of Latimeria DM20.

Embryonic expression of the myelin basic protein gene: identification of a promoter region that targets transgene expression to pioneer neurons

The myelin basic protein (MBP) gene produces two families of structurally related proteins from three different promoters-the golli products, generated from the most upstream promoter, and the MBPs, produced from the two downstream promoters. In this report we describe the expression of golli proteins within some of the earliest neuronal populations of the brain, including Cajal-Retzius cells and preplate neurons of the forebrain, representing a new marker for these cells. To identify elements responsible for neuronal expression of the golli products, we generated transgenic animals from constructs containing different portions of the upstream promoter. A construct containing 1.1 kb immediately upstream of the golli transcription start site targeted expression of beta-galactosidase to preplate neurons and a subset of Cajal-Retzius cells in transgenic mice-the first reported genetic element to target expression to these pioneer cortical populations. Although expression in Cajal-Retzius cells declined with embryonic development, preplate cells continued to express the transgene after arriving at their final destination in the subplate. Interestingly, expression persisted in subplate neurons found within a distinct layer between the white matter and cortical layer VI well into postnatal life. Birth dating studies with bromodeoxyuridine indicated that these neurons were born between E10.5 and E12.5. Thus, the transgene marked subplate neurons from their birth, providing a fate marker for these cells. This work suggests a role for the MBP gene in the early developing brain long before myelination and especially in the pioneer cortical neurons important in the formation of the cortical layers.

A history of proteolipids: a personal memoir

This review is a personal memoir of the history of proteolipids and is limited to aspects of the field with which the author has been involved in one way or another. The discovery of proteolipids was a serendipitous observation made in the course of the study of sulfatides. Initial focus was on the chemical characterization of brain proteolipids, their behavior under different conditions and their identification as the major protein of CNS myelin. The sequence of PLP was obtained using solid phase protein sequencing techniques. This, in turn, made possible a new era in which biochemical, cellular and molecular approaches could be applied to address new questions about PLP. Identification of genetic defects in the PLP molecule and its regulation has contributed to understanding myelin biology. Studies of the encephalitogenic activity of PLP in animal models have influenced the views of inflammatory processes in multiple sclerosis. Despite remarkable progress, much remains to be learned about the structure and function of PLP.

Molecular evolution of myelin proteolipid protein

We show that the major membrane protein of central nervous system myelin, proteolipid protein, evolved much more rapidly than it does now more than 300 million years ago. We reason that myelin proteolipid protein evolved rapidly just after its appearance in vertebrates and that its evolutionary rate then gradually decreased. Comparison of the rates between the synonymous and nonsynonymous nucleotide substitutions for the cDNA suggests the possibility that positive selection operated on myelin proteolipid protein at least when it appeared in vertebrates.

Parallel evolution and coexpression of the proteolipid proteins and protein zero in vertebrate myelin

Vertebrate myelin contains two proteins that mediate compaction: protein zero (P0), an immunoglobulin gene superfamily member, or proteolipid proteins, 4-hydrophobic domain-motif proteins biogenetically unrelated to P0. The prevailing view has been that expression of P0 and proteolipid proteins is mutually exclusive; P0, which mediates myelin compaction in fish, is thought to be completely replaced by the newer proteolipid proteins in the terrestrial vertebrate CNS. However, we now find that proteolipid proteins are actually major myelin constituents in bony fish and amphibia, and so are coexpressed with P0. Clearly, myelin proteolipids are not new additions to the myelin protein repertoire, but instead were ancestral sheath components, expressed approximately 440 million years ago in the first myelinated fish that existed at least approximately 100 million years before the origin of amphibians. In conclusion, P0 and the proteolipid proteins are evolving in parallel in myelinating cells of most vertebrate species.