Myelin basic protein gene and the function of antisense RNA in its repression in myelin-deficient mutant mouse

Gene regulation and genetics in neurochemistry, past to future

Ask any neuroscientist to name the most profound discoveries in the field in the past 60 years, and at or near the top of the list will be a phenomenon or technique related to genes and their expression. Indeed, our understanding of genetics and gene regulation has ushered in whole new systems of knowledge and new empirical approaches, many of which could not have even been imagined prior to the molecular biology boon of recent decades. Neurochemistry, in the classic sense, intersects with these concepts in the manifestation of neuropeptides, obviously dependent upon the central dogma (the established rules by which DNA sequence is eventually converted into protein primary structure) not only for their conformation but also for their levels and locales of expression. But, expanding these considerations to non-peptide neurotransmitters illustrates how gene regulatory events impact neurochemistry in a much broader sense, extending beyond the neurochemicals that translate electrical signals into chemical ones in the synapse, to also include every aspect of neural development, structure, function, and pathology. From the beginning, the mutability - yet relative stability - of genes and their expression patterns were recognized as potential substrates for some of the most intriguing phenomena in neurobiology - those instances of plasticity required for learning and memory. Near-heretical speculation was offered in the idea that perhaps the very sequence of the genome was altered to encode memories. A fascinating component of the intervening progress includes evidence that the central dogma is not nearly as rigid and consistent as we once thought. And this mutability extends to the potential to manipulate that code for both experimental and clinical purposes. Astonishing progress has been made in the molecular biology of neurochemistry during the 60 years since this journal debuted. Many of the gains in conceptual understanding have been driven by methodological progress, from automated high-throughput sequencing instruments to recombinant-DNA vectors that can convey color-coded genetic modifications in the chromosomes of live adult animals. This review covers the highlights of these advances, both theoretical and technological, along with a brief window into the promising science ahead. This article is part of the 60th Anniversary special issue.

[Letter to the Editor] Accelerated RNA-RNA hybridization by concentrated guanidinium thiocyanate solution in single-step RNA isolation

Address correspondence to Mart Speek, Department of Gene Technology, Akadeemia tee 15, Room 129, Tallinn University of Technology, Tallinn 19086, Estonia. E-mail: mart.speek@ttu.ee.

A genome-wide investigation of expression characteristics of natural antisense transcripts in liver and muscle samples of pigs

Natural antisense transcripts are endogenous transcripts that are complementary to the sense-strand of DNA. These transcripts have been identified in various eukaryotic species and are involved in a broad range of regulatory events and biological processes. However, their general biological functions, expression characteristics and regulatory mechanisms are still unclear. In this study, 497 liver and 586 muscle samples were harvested from a White Duroc×Erhualian F₂ resource population. The expression profiles of sense and antisense transcripts were determined by tag-based RNA sequencing. We identified 33.7% and 20.4% of transcripts having both sense and antisense expression, and 12.5% and 6.1% of transcripts only expressing antisense transcripts in liver and muscle, respectively. More than 32.2% of imprinting or predicted imprinting genes in the geneimprint database were detected with both sense and antisense expression. The correlations between sense and antisense expression in sense-antisense pairs were diverse in both liver and muscle, showing positive, negative or absent correlation. Antisense expression increases gene expression variability. More interestingly, compared to eQTL mapping of sense transcripts in which more than one eQTL was mapped for a transcript, only one eQTL was identified for each antisense transcript, and the percentage of cis-eQTL in antisense eQTL was higher than that in sense eQTL. This suggests that the expressions of antisense transcripts tend to be cis-regulated by a single genomic locus. To our knowledge, this study is the first systematical investigation of antisense transcription in pigs. The findings improve our understanding of the complexity of porcine transcriptome.

Non-coding RNAs with essential roles in neurodegenerative disorders

The importance of various classes of regulatory non-protein-coding RNA molecules (ncRNAs) in the normal functioning of the CNS is becoming increasingly evident. ncRNAs are involved in neuronal cell specification and patterning during development, but also in higher cognitive processes, such as structural plasticity and memory formation in the adult brain. We discuss advances in understanding of the function of ncRNAs in the CNS, with a focus on the potential involvement of specific species, such as microRNAs, endogenous small interfering RNAs, long intergenic non-coding RNAs, and natural antisense transcripts, in various neurodegenerative disorders. This emerging field is anticipated to profoundly affect clinical research, diagnosis, and therapy in neurology.

Novel noncoding antisense RNA transcribed from human anti-NOS2A locus is differentially regulated during neuronal differentiation of embryonic stem cells

Here, we report on the discovery of a locus in the human genome, which evolved by gene duplication followed by an internal DNA inversion. This locus exhibits high sequence similarity to the gene for the inducible isoform of NOS protein (NOS2A) and is transcribed into a noncoding RNA containing a region of significant antisense homology with the NOS2A mRNA. We show that this antisense transcript (anti-NOS2A RNA) is expressed in different types of brain tumors, including meningiomas and glioblastomas. More importantly, we demonstrate that the expression profiles of the anti-NOS2A RNA and the NOS2A mRNA exhibit concurrent reciprocal changes in undifferentiated human embryonic stem cells (hESCs) and in hESCs induced to differentiate into neurogenic precursors such as neurospheres. As NOS2A has a role in neurogenesis, our results suggest that the anti-NOS2A RNA is involved in the regulation of neuronal differentiation of hESCs through the modulation of NOS2A gene expression.

Trans-natural antisense transcripts including noncoding RNAs in 10 species: implications for expression regulation

Natural antisense transcripts are at least partially complementary to their sense transcripts. Cis-Sense/Antisense pairs (cis-SAs) have been extensively characterized and known to play diverse regulatory roles, whereas trans-Sense/Antisense pairs (trans-SAs) in animals are poorly studied. We identified long trans-SAs in human and nine other animals, using ESTs to increase coverage significantly over previous studies. The percentage of transcriptional units (TUs) involved in trans-SAs among all TUs was as high as 4.13%. Particularly 2896 human TUs (or 2.89% of all human TUs) were involved in 3327 trans-SAs. Sequence complementarities over multiple segments with predicted RNA hybridization indicated that some trans-SAs might have sophisticated RNA-RNA pairing patterns. One-fourth of human trans-SAs involved noncoding TUs, suggesting that many noncoding RNAs may function by a trans-acting antisense mechanism. TUs in trans-SAs were statistically significantly enriched in nucleic acid binding, ion/protein binding and transport and signal transduction functions and pathways; a significant number of human trans-SAs showed concordant or reciprocal expression pattern; a significant number of human trans-SAs were conserved in mouse. This evidence suggests important regulatory functions of trans-SAs. In 30 cases, trans-SAs were related to cis-SAs through paralogues, suggesting a possible mechanism for the origin of trans-SAs. All trans-SAs are available at http://trans.cbi.pku.edu.cn/.

RNA silencing in mammalian oocytes and early embryos

RNA silencing is a common term for homology-dependent silencing phenomena found in the majority of eukaryotic species. RNA silencing pathways share several conserved components. The common denominator of these pathways is the presence of specific, short (21-25 nt) RNA molecules generated from different double-stranded RNA substrates by a specific RNase III activity. Short RNA molecules serve as a template for sequence-specific effects including transcriptional silencing, mRNA degradation, and inhibition of translation. This review will discuss possible roles of RNA silencing pathways in mouse oocytes and early embryos as well as the use of RNA silencing for experimental inhibition of gene expression in this model system.

Noncoding RNAs and RNA Editing in Brain Development, Functional Diversification, and Neurological Disease

The progressive maturation and functional plasticity of the nervous system in health and disease involve a dynamic interplay between the transcriptome and the environment. There is a growing awareness that the previously unexplored molecular and functional interface mediating these complex gene-environmental interactions, particularly in brain, may encompass a sophisticated RNA regulatory network involving the twin processes of RNA editing and multifaceted actions of numerous subclasses of non-protein-coding RNAs. The mature nervous system encompasses a wide range of cell types and interconnections. Long-term changes in the strength of synaptic connections are thought to underlie memory retrieval, formation, stabilization, and effector functions. The evolving nervous system involves numerous developmental transitions, such as neurulation, neural tube patterning, neural stem cell expansion and maintenance, lineage elaboration, differentiation, axonal path finding, and synaptogenesis. Although the molecular bases for these processes are largely unknown, RNA-based epigenetic mechanisms appear to be essential for orchestrating these precise and versatile biological phenomena and in defining the etiology of a spectrum of neurological diseases. The concerted modulation of RNA editing and the selective expression of non-protein-coding RNAs during seminal as well as continuous state transitions may comprise the plastic molecular code needed to couple the intrinsic malleability of neural network connections to evolving environmental influences to establish diverse forms of short- and long-term memory, context-specific behavioral responses, and sophisticated cognitive capacities.

Golli-MBP copy number analysis by FISH, QMPSF and MAPH in 195 patients with hypomyelinating leukodystrophies

The inherited disorders of CNS myelin formation represent a heterogeneous group of leukodystrophies. The proteolipoprotein (PLP1) gene has been implicated in two X-linked forms, Pelizaeus-Merzbacher disease (PMD) and spastic paraplegia type 2, and the gap junction protein alpha12 (GJA12) gene in a recessive form of PMD. The myelin basic protein (MBP) gene, which encodes the second most abundant CNS myelin protein after PLP1, presents rearrangements in hypomyelinating murine mutants and is always included in the minimal region deleted in 18q- patients with an abnormal hypomyelination pattern on cerebral MRI. In this study, we looked at the genomic copy number at the Golli-MBP locus in 195 patients with cerebral MRI suggesting a myelin defect, who do not have PLP1 mutation. Although preliminary results obtained by FISH suggested the duplication of Golli-MBP in 3 out of 10 patients, no abnormal gene quantification was found using Quantitative Multiplex PCR of Short Fluorescent fragments (QMPSF), Multiplex Amplifiable Probe Hybridization (MAPH), or another FISH protocol using directly-labelled probes. Pitfalls and interest in these different techniques to detect duplication events are emphasised. Finally, the study of this large cohort of patients suggests that Golli-MBP deletion or duplication is rarely involved in inherited defects of myelin formation.

The pathobiology of myelin mutants reveal novel biological functions of the MBP and PLP genes

Substantial biological data indicate that the myelin basic protein (MBP) and myelin proteolipid protein (PLP/DM20) genes produce products with functions beyond that of serving as myelin structural proteins. Much of this evidence comes from studies on naturally-occurring and man-made mutations of these genes in mice and other species. This review focuses upon recent evidence showing the existence of other products of these genes that may account for some of these other functions, and recent studies providing evidence for alternative biological functions of PLP/DM20. The MBP and PLP/DM20 genes each encode the classic MBP and PLP isoforms, as well as a second family of proteins that are not involved in myelin structure. The biological roles of these other products of the genes are becoming clarified. The non-classic MBP gene products appear to be components of transcriptional complexes in the nucleus, and they also may be involved in signaling pathways in T-cells and in neural cells. The non-classic PLP/DM20 gene products appear to be components of intracellular transport vesicles in oligodendrocytes. There is evidence for other functions of the classic PLP/DM20 proteins, including a role in neural cell death mechanisms, autocrine and paracrine regulation of oligodendrocytes and neurons, intracellular transport and oligodendrocyte migration.

Extensive adenosine-to-inosine editing detected in Alu repeats of antisense RNAs reveals scarcity of sense-antisense duplex formation

One type of RNA editing converts adenosine residues to inosine in double-stranded regions. Recent transcriptome analysis has revealed that numerous Alu repeats, present within introns and untranslated regions of human transcripts, are subject to this A-->I RNA editing. Furthermore, it revealed global transcription of antisense RNAs. Here, we demonstrate that antisense RNAs are also edited extensively but only in their Alu repeat sequences, and editing does not extend to the surrounding sequence. Our findings imply that sense and antisense RNAs form two separate intramolecular double-stranded RNAs consisting of inversely oriented Alu repeats, but rarely form intermolecular duplexes.

Natural antisense RNAs in the nervous system

Natural antisense RNAs are endogenous molecules that are complementary to RNA transcripts of already established function. They were discovered first in prokaryotes in which they are now recognised as an important component of molecular mechanisms involved in the regulation of gene expression. Recently, through the cumulative efforts of molecular biologists and bioinformaticians, natural antisense RNAs have been demonstrated in significant numbers in eukaryotic systems also. Probably the most exciting outcome of these studies is that natural antisense RNAs are particularly prevalent in the nervous system. Here we discuss the major known types of natural antisense RNAs in eukaryotic systems and focus on their potential roles in the regulation of gene expression in the brain.

Targeted expression of anti-apoptotic protein p35 in oligodendrocytes reduces delayed demyelination and functional impairment after spinal cord injury

Functional impairment after spinal cord injury (SCI) is attributed to neuronal cell necrosis death and axonotmesis, with further worsening caused by the accompanying apoptosis of myelin-forming oligodendrocytes (OLGs). However, it is unclear as to how much OLG apoptosis contributes to functional impairment. To address this issue, we used transgenic mice characterized by the targeted expression of p35, a broad-spectrum caspase inhibitor, in OLGs using the cre/loxP system (referred to as cre/p35 transgenic mice). In this study, we examined the motor function and histopathologic changes after a contusive thoracic spinal cord injury in the cre/p35 transgenic mice. A larger number of OLGs and a lesser extent of demyelination were observed after SCI in the cre/p35 transgenic mice than in the control cre mice, which did not carry the p35 transgene. Furthermore, the motor function of the hindlimbs recovered to a significantly better degree in the cre/p35 transgenic mice than in the control cre mice. Thus, the inhibition of OLG apoptosis decreased the extent of functional impairment after SCI. These findings suggest that the inhibition of OLG apoptosis may be a potential treatment for SCI.

Diversity of antisense regulation in eukaryotes: multiple mechanisms, emerging patterns

High-throughput analysis of RNA molecules in multicellular eukaryotes has revealed an abundance of complementary antisense RNAs that are transcribed from separate or overlapping genes. In mammals these include many novel non-coding RNAs of unknown function. This unexpected complexity of the mammalian transcriptome suggests that expression of many genes is regulated post-transcriptionally by mechanisms mediated by RNA-RNA base pairing. The recent discovery of the widespread expression of microRNAs in animals and plants provides a prototypic example of such regulation in eukaryotes. However, there are likely to be numerous other types of antisense regulation in eukaryotes, many as yet uncharacterized.

Transgenic RNAi in mouse oocytes: a simple and fast approach to study gene function

Double-strand RNA (dsRNA)-mediated posttranscriptional gene silencing, also known as RNA interference (RNAi), is a powerful tool to inhibit gene expression in several experimental model systems, including Arabidopsis, Caenorhabditis, and Drosophila. We previously described that the microinjection of Mos dsRNA into fully grown mouse oocytes results in the specific degradation of Mos mRNA in a time- and concentration-dependent manner. We report here a transgenic RNAi approach that is suitable to study gene function during mouse oocyte development and differentiation. The oocyte-specific Zp3 promoter was used to drive the expression of a long hairpin dsRNA ( approximately 500 bp) targeting Mos mRNA. Transgenic founder animals appeared healthy, but while males were fertile, females were not, in accordance with the known Mos null phenotype. The amount of Mos mRNA in the transgenic F(1) females was reduced by >90%, whereas there was no decrease in the nontargeted tissue plasminogen activator (Plat) mRNA. Moreover, the maturation-associated increase in mitogen-activated protein (MAP) kinase activity was not observed, and the metaphase II eggs underwent spontaneous parthenogenetic activation, thus recapitulating the Mos null phenotype. This approach provides a powerful method to study the functions of any oocyte-synthesized gene during oocyte development and early embryogenesis.

Mutations in the rat myelin basic protein gene are associated with specific alterations in other myelin gene expression

The Long Evans shaker (les) rat is a myelin basic protein (MBP) mutant that exhibits severe central nervous system (CNS) dysmyelination. We used a combination of immunohistochemistry, immunoblot and Northern blot analyses to determine the effect of MBP deficits on the expression of other CNS myelin genes in this mutant. Immunohistochemistry revealed a marked reduction in all major myelin proteins and differences in their intracellular distribution. Immunoblots confirmed the decreased expression of these proteins and indicated that relative levels of proteolipid protein (PLP) and DM20 were altered in this mutant. Quantitation of mRNA levels indicated that decreases in PLP and DM20 were a result of changes in mRNA levels but detected no change in other myelin gene transcripts.

Natural antisense (rTSalpha) RNA induces site-specific cleavage of thymidylate synthase mRNA

The 3' untranslated region (UTR) of rTSalpha RNA is complementary (i.e., antisense) to human thymidylate synthase (TS) RNA. When HEp2 cells (human epidermoid carcinoma) progressed from late-log to plateau phase growth, ribonuclease protection assay (RPA) revealed an inverse correlation between the levels of rTSalpha RNA and TS mRNA, suggesting a possible effect of rTSalpha RNA on TS mRNA levels. HEp2 cells expressing a Tet-On transactivator were transiently co-transfected with pHook-1 and a construct containing rTSalpha (protein and antisense RNA), rTSalphaDelta3' (rTSalpha protein only), rTSalpha-3' (antisense RNA-luciferase) or luciferase. Transfected cells were selected and evaluated for the effects of induced transgene expression on TS mRNA. Induced expression of transfected rTSalpha or rTSalpha-3', but not rTSalphaDelta3' or luciferase, resulted in decreased TS mRNA levels as measured by RPA. These results demonstrated that the antisense region of rTSalpha RNA is necessary and sufficient for this down-regulation of TS mRNA. RPA for TS mRNA also showed the enhanced appearance of two partial-length protected fragments in rTSalpha or rTSalpha-3' transfected cells. RPA stringency evaluations and primer extension assays indicated that TS mRNA is cleaved in vivo in a site-specific manner. These data demonstrate that rTS gene expression likely plays a role in down-regulating TS through a natural RNA-based antisense mechanism.

The KLHL1-antisense transcript ( KLHL1AS) is evolutionarily conserved

Spinocerebellar ataxia type 8 (SCA8) is caused by a CTG expansion in an untranslated, endogenous antisense RNA that overlaps the Kelch-like 1 ( KLHL1) gene. The normal function of this transcript is currently unknown. We have now identified the promoter region for the KLHL1-antisense ( KLHL1AS) RNA and report that a Klhl1as transcript is present in the mouse as well. Human and mouse KLHL1AS are transcribed from homologous promoter regions in the first intron of KLHL1 and extend through the transcription and translation start sites as well as the first splice donor sequence of KLHL1. We found that the mouse Klhl1as RNA is not spliced and terminates in a polyadenylation site in the Klhl1 promoter region, whereas both the present and previous work show that human KLHL1AS is highly variably spliced into processed transcripts that contain up to six exons. Mouse Klhl1as transcript was detected in RNA isolated from the cerebellum and from total adult brain and total fetal tissue, and at a low level in testis and ovary. Similarly, human KLHL1AS is expressed in various brain tissues, including the cerebellum, the tissue most affected by SCA8, and was detected at low levels in testis and kidney. The evolutionary conservation of this antisense/sense transcriptional organization strongly indicates that KLHL1AS transcripts play a significant biological role in both human and mouse, presumably as a regulator of KLHL1 expression.

Lipodystrophy in the fld mouse results from mutation of a new gene encoding a nuclear protein, lipin

Mice carrying mutations in the fatty liver dystrophy (fld) gene have features of human lipodystrophy, a genetically heterogeneous group of disorders characterized by loss of body fat, fatty liver, hypertriglyceridemia and insulin resistance. Through positional cloning, we have isolated the gene responsible and characterized two independent mutant alleles, fld and fld(2J). The gene (Lpin1) encodes a novel nuclear protein which we have named lipin. Consistent with the observed reduction of adipose tissue mass in fld and fld(2J)mice, wild-type Lpin1 mRNA is expressed at high levels in adipose tissue and is induced during differentiation of 3T3-L1 pre-adipocytes. Our results indicate that lipin is required for normal adipose tissue development, and provide a candidate gene for human lipodystrophy. Lipin defines a novel family of nuclear proteins containing at least three members in mammalian species, and homologs in distantly related organisms from human to yeast.

Targeted expression of baculovirus p35 caspase inhibitor in oligodendrocytes protects mice against autoimmune-mediated demyelination

The mechanisms underlying oligodendrocyte (OLG) loss and the precise roles played by OLG death in human demyelinating diseases such as multiple sclerosis (MS), and in the rodent model of MS, experimental autoimmune encephalomyelitis (EAE), remain to be elucidated. To clarify the involvement of OLG death in EAE, we have generated transgenic mice that express the baculovirus anti-apoptotic protein p35 in OLGs through the Cre-loxP system. OLGs from cre/p35 transgenic mice were resistant to tumor necrosis factor-alpha-, anti-Fas antibody- and interferon-gamma-induced cell death. cre/p35 transgenic mice were resistant to EAE induction by immunization with the myelin oligodendrocyte glycoprotein. The numbers of infiltrating T cells and macrophages/microglia in the EAE lesions were significantly reduced, as were the numbers of apoptotic OLGs expressing the activated form of caspase-3. Thus, inhibition of apoptosis in OLGs by p35 expression alleviated the severity of the neurological manifestations observed in autoimmune demyelinating diseases.

Insertion of a retrotransposon in Mbp disrupts mRNA splicing and myelination in a new mutant rat

Our understanding of myelination has been greatly enhanced via the study of spontaneous mutants that harbor a defect in a gene encoding one of the major myelin proteins (myelin mutants). In this study, we describe a unique genetic defect in a new myelin mutant called the Long Evans shaker (les) rat that causes severe dysmyelination of the CNS. Myelin deficits result from disruption of the myelin basic protein (Mbp) gene caused by the insertion of an endogenous retrotransposon [early transposons (ETn) element] into a noncoding region (intron 3) of the gene. The ETn element alters the normal splicing dynamics of MBP mRNA, leading to a dramatic reduction in the levels of full-length isoforms (<5% of normal) and the appearance of improperly spliced, chimeric transcripts. Although these aberrant transcripts contain proximal coding regions of the MBP gene (exons 1-3), they are unable to encode functional proteins required to maintain the structural integrity of the myelin sheath. These chimeric transcripts seem capable, however, of producing the necessary signal to initiate and coordinate myelin gene expression because normal numbers of mature oligodendrocytes synthesizing abundant levels of other myelin proteins are present in the mutant CNS. The les rat is thus an excellent model to study alternative functions of MBP beyond its well characterized role in myelin compaction.

EVI2B, a gene lying in an intron of the neurofibromatosis type 1 (NF1) gene, is as the NF1 gene involved in differentiation of melanocytes and keratinocytes and is overexpressed in cells derived from NF1 neurofibromas

The EVI2B gene is one of three genes embedded in intron 27b of the neurofibromatosis type 1 (NF1; M. Recklinghausen) gene, which are transcribed in the direction opposite that of the NF1 gene. The function of EVI2B and its relation to NF1 symptoms is unknown. Here, the amounts of NF1 and EVI2B mRNA were investigated in detail in cells involved in NF1 manifestations as café-au-lait macules and neurofibromas. These investigations showed that aside from the NF1 gene, EVI2B is involved in melanocyte and keratinocyte differentiation. Whereas in NF1 melanocytes from café-au-lait macules, EVI2B expression was not altered, in fibroblast-like cells derived from neurofibromas, an increased level of EVI2B mRNA was found. We investigated whether this increase was attributable to an influence of NF1 gene expression on the expression of the EVI2B gene, as suggested by the fact that the EVI2B primary transcript is antisense to the NF1 primary transcript. Investigations of cells derived from patients with different amounts of NF1 pre-mRNA showed no correlation between the amount of NF1 pre-mRNA and the increased level of EVI2B mRNA.

Antisense RNA: function and fate of duplex RNA in cells of higher eukaryotes

There is ample evidence that cells of higher eukaryotes express double-stranded RNA molecules (dsRNAs) either naturally or as the result of viral infection or aberrant, bidirectional transcriptional readthrough. These duplex molecules can exist in either the cytoplasmic or nuclear compartments. Cells have evolved distinct ways of responding to dsRNAs, depending on the nature and location of the duplexes. Since dsRNA molecules are not thought to exist naturally within the cytoplasm, dsRNA in this compartment is most often associated with viral infections. Cells have evolved defensive strategies against such molecules, primarily involving the interferon response pathway. Nuclear dsRNA, however, does not induce interferons and may play an important posttranscriptional regulatory role. Nuclear dsRNA appears to be the substrate for enzymes which deaminate adenosine residues to inosine residues within the polynucleotide structure, resulting in partial or full unwinding. Extensively modified RNAs are either rapidly degraded or retained within the nucleus, whereas transcripts with few modifications may be transported to the cytoplasm, where they serve to produce altered proteins. This review summarizes our current knowledge about the function and fate of dsRNA in cells of higher eukaryotes and its potential manipulation as a research and therapeutic tool.

Do natural antisense transcripts make sense in eukaryotes?

The existence of naturally occurring antisense RNAs has been illustrated, in eukaryotes, by an increasing number of reports. The following review presents the major findings in this field, with a special focus on the regulation of gene expression exerted by endogenous complementary transcripts. A large variety of eukaryotic organisms, contains antisense transcripts. Moreover, the great diversity of genetic loci encoding overlapping sense and antisense RNAs suggests that such transcripts may be involved in numerous biological functions, such as control of development, adaptative response. viral infection. The regulation of gene expression by endogenous antisense RNAs seems of general importance in eukaryotes as already established in prokaryotes: it is likely to be involved in the control of various biological functions and to play a role in the development of pathological situations. Several experimental evidences for coupled, balanced or unbalanced expression of sense and antisense RNAs suggest that antisense transcripts may govern the expression of their sense counterparts. Furthermore, documented examples indicate that this control may be exerted at many levels of gene expression (transcription, maturation, transport, stability and translation). This review also addresses the underlying molecular mechanisms of antisense regulation and presents the current mechanistic hypotheses.

Sense and antisense transcripts of the developmentally regulated murine hsp70.2 gene are expressed in distinct and only partially overlapping areas in the adult brain

We have examined the spatial pattern of expression of a member of the hsp70 gene family, hsp70.2, in the mouse central nervous system. Surprisingly, RNA blot analysis and in situ hybridization revealed abundant expression of an 'antisense' hsp70.2 transcript in several areas of adult mouse brain. Two different transcripts recognized by sense and antisense riboprobes for the hsp70.2 gene were expressed in distinct and only partially overlapping neuronal populations. RNA blot analysis revealed low levels of the 2.7 kb transcript of hsp70.2 in several areas of the brain, with highest signal in the hippocampus. Abundant expression of a slightly larger (approximately 2.8 kb) 'antisense' transcript was detected in several brain regions, notably in the brainstem, cerebellum, mesencephalic tectum, thalamus, cortex, and hippocampus. In situ hybridization revealed that the sense and antisense transcripts were both predominantly neuronal and localized to the same cell types in the granular layer of the cerebellum, trapezoid nucleus of the superior olivary complex, locus coeruleus and hippocampus. The hsp70.2 antisense transcripts were particularly abundant in the frontal cortex, dentate gyrus, subthalamic nucleus, zona incerta, superior and inferior colliculi, central gray, brainstem, and cerebellar Purkinje cells. Our findings have revealed a distinct cellular and spatial localization of both sense and antisense transcripts, demonstrating a new level of complexity in the function of the heat shock genes.

Glial cell degeneration and hypomyelination caused by overexpression of myelin proteolipid protein gene

Myelin proteolipid protein (PLP), the major myelin protein in the CNS, has been thought to function in myelin assembly. Thus, mutations within the gene coding for PLP (Plp) cause hypomyelination, such as the jimpy phenotype in mice and Pelizaeus-Merzbacher disease in humans. However, these mutants often exhibit premature death of oligodendrocytes, which form CNS myelin. To elucidate the functional roles of Plp gene products in the maturation and/or survival of oligodendrocytes, we produced transgenic mice overexpressing the Plp gene by introducing extra wild-type mouse Plp genes. Surprisingly, transgenic mice bearing 4 more Plp genes exhibited dysmyelination in the CNS, whereas those with 2 more Plp genes showed normal myelination at an early age (3 weeks after birth), but later developed demyelination. Overexpression of the Plp gene resulted in arrested maturation of oligodendrocytes, and the severity of arrest was dependent on the extent of overexpression. Overexpression also led to oligodendrocyte cell death, apparently caused by abnormal swelling of the Golgi apparatus. Thus, tight regulation of Plp gene expression is necessary for normal oligodendrocyte differentiation and survival, and its overexpression can be the cause of both dys- and demyelination.

The genetics of myelin

Myelin formation and maintenance requires complex interactions between neurons and glia, and between the integral protein and lipid components of the myelin sheath. Many of the underlying mechanisms may be examined by studying the perturbations caused by spontaneous and targeted mutations in myelin protein genes. This review summarizes the progress in our understanding of these mutations with an emphasis on integrating the recent advances in the genetics of myelin into a more generalized view of myelin organization and function.

Constitutive activation of Src family kinases in mouse embryos that lack Csk

Csk is a novel cytoplasmic protein-tyrosine kinase that has been shown to inactivate members of the Src family of protein-tyrosine kinases in vitro. To examine the function of Csk in vivo, Csk-deficient mouse embryos were generated by gene targeting in embryonic stem cells. These embryos were developmentally arrested at the 10 to 12 somite stage and exhibited growth retardation and necrosis in the neural tissues. The kinase activity of p60c-src, p59fyn, and p53/56lyn in these embryos was greatly enhanced as an apparent consequence of enhanced specific activity. The increase in kinase activity was associated with an increase in tyrosine phosphorylation of several proteins, especially those around 85 and 120 kd. Thus, these results suggest that Csk indeed acts as an indispensable negative regulator of Src family kinases in vivo.

Mechanism of action of antisense RNA. Sometime inhibition of transcription, processing, transport, or translation

Anyone considering the use of AS RNA, generated endogenously, to inhibit gene expression should plan to generate several independent transfectants with nonoverlapping sequences; strategies that maximize both the transcription rate and the stability of the AS RNA are obviously desirable. Reasons why different results are obtained in different systems or with different constructs likely include the specific nucleotide sequence under investigation, the location of the AS gene in the nucleus relative to the endogenous gene, and the rate-limiting step in the expression of the target gene. Splicing may not be necessary, but an efficient polyadenylation signal likely is. Employment of a ribozyme-mediated strategy, discussed by various investigators in this volume, may be beneficial. There is no reason at present to conclude that any gene, however abundant its transcript might be, is inherently recalcitrant to AS-mediated down-regulation of expression.

Molecular genetic analysis of the mldr mouse: a spontaneous revertant at the mld locus containing a recombinant myelin basic protein gene

The mld mutation is a complex genetic lesion affecting the myelin basic protein (MBP) locus in the mouse. The mutation consists of a variety of DNA rearrangements including: tandem duplication of the MBP structural gene, partial inversion of the 3' end of the upstream gene copy, duplication of a region flanking the rearrangement junction in the upstream copy and insertion between the two gene copies of a segment of extraneous DNA not associated with the wild-type MBP locus. The net result of the mutation is a dysfunctional MBP locus. Homozygous mld/mld mice produce very little MBP and consequently very little myelin. They exhibit a clinical phenotype characteristic of hypomyelination (shaking, convulsions). We have discovered a revertant mld mouse which does not exhibit clinical symptoms of hypomyelination. Genetic analysis indicates that the reversion is allelic to mld. We have designated the revertant locus mldr. Restriction analysis of mldr genomic DNA indicates that there is a single intact MBP gene. Analysis of various junction regions using the polymerase chain reaction indicates that the single MBP gene in mldr is derived by recombination from the 5' end of the upstream gene and the 3' end of the downstream gene. Studies on MBP expression in mldr mice indicate that the developmental regulation, level of expression and pattern of post-transcriptional processing of MBP gene products in mldr are similar to wild type. These results indicate that the recombinant MBP gene in mldr is fully functional. From this we infer that the MBP-deficient phenotype of the original mld mutant is attributable to the complex rearrangements in the upstream gene copy which render the locus dysfunctional.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of the new isoforms of mouse myelin basic protein: the existence of exon 5a

Myelin basic protein (MBP) is a major constituent in the myelin of the CNS. In mice, five forms of MBPs (14 kDa, two types of 17 kDa, 18.5 kDa, and 21.5 kDa) encoded by separate mRNAs have been identified based on cDNA cloning studies. These mRNAs are considered to be produced by alternative splicing from a single gene composed of seven exons. Here we report the existence of two novel MBP mRNAs encoding 19.7-kDa and 21-kDa MBPs identified by cDNA cloning using the polymerase chain reaction. Both of these MBPs contain a sequence of a previously unidentified exon of 66 nucleotides, which was mapped to be just 5' of exon 5 in the MBP gene. MBP mRNAs containing this novel exon (exon 5a) belong to a minor population in the whole brain and PNS and are somewhat enriched in the spinal cord. Exon 5a encodes a very hydrophobic segment rich in valine residues, which presumably forms a beta-pleated sheet.