Uncoupling of hypomyelination and glial cell death by a mutation in the proteolipid protein gene.

Proteolipid protein (PLP; M(r) 30,000) is a highly conserved major polytopic membrane protein in myelin but its cellular function remains obscure. Neurological mutant mice can often provide model systems for human genetic disorders. Mutations of the X-chromosome-linked PLP gene are lethal, identified first in the jimpy mouse and subsequently in patients with Pelizaeus-Merzbacher disease. The unexplained phenotype of these mutations includes degeneration and premature cell death of oligodendrocytes with associated hypomyelination. Here we show that a new mouse mutant rumpshaker is defined by the amino-acid substitution Ile-to-Thr at residue 186 in a membrane-embedded domain of PLP. Surprisingly, rumpshaker mice, although myelin-deficient, have normal longevity and a full complement of morphologically normal oligodendrocytes. Hypomyelination can thus be genetically separated from the PLP-dependent oligodendrocyte degeneration. We suggest that PLP has a vital function in glial cell development, distinct from its later role in myelin assembly, and that this dichotomy of action may explain the clinical spectrum of Pelizaeus-Merzbacher disease.     

Sodium channel density in hypomyelinated brain increased by myelin basic protein gene deletion.

Trophic control over the expression and membrane distribution of voltage-dependent ion channels is one of the principal organizing events underlying the maturation of excitable cells. The myelin sheath is a major structural determinant of regional ion channel topography in central axons, but the exact molecular signals that mediate local interactions between the oligodendrocyte and axolemma are not known. We have found that large caliber fibre pathways in the brain of the mutant mouse shiverer (shi, gene on chromosome 18), whose developmental fate of myelination is averted by deletion of five exons in the myelin basic protein gene, have a striking excess of sodium channels. As cytoplasmic membranes of shiverer oligodendroglia still adhere to axons, the evidence indicates that myelin basic protein or a myelin basic protein-dependent glial transmembrane signal associated with compact myelin formation, rather than a simple glial-axon contact inhibition or an intrinsic genetic program of neuronal differentiation, could be critical in downregulating sodium channel density in axons. Here we use the shiverer mutant to show that mature central nervous system projection neurons with large caliber unmyelinated fibres sustain functional excitability by increasing sodium channel density. This axon plasticity, triggered by the absence of a single glial protein, contributes to the unexpectedly mild degree of neurological impairment in the mutant brain without myelin, and may be a potentially inducible mechanism determining the recovery of function from dysmyelinating disease.     

Shiverer peripheral myelin contains P2

Myelin-deficient mutant mice, such as shiverer, can provide information about the normal mechanisms involved in myelination. The shiverer mouse carries a recessive, autosomal mutation resulting in an extreme deficiency in central myelin, and the small amount of myelin present is poorly compacted; the peripheral myelin, however, appears essentially normal. As the amount of myelin basic protein (P1) in both central and peripheral nervous system myelin is extremely low in shiverer, it is possible that P1 is essential for the normal formation and compaction of central myelin, but not of peripheral myelin. Some other protein would then be responsible for the formation of compact peripheral myelin in shiverer. Peripheral myelin contains another basic protein, designated P2, which could be a possible candidate for this role. Kirschner and Ganser, however, using SDS-polyacrylamide gel electrophoresis, reported that P2, as well as P1, is absent from shiverer sciatic nerve. This is an important observation if correct, because it not only excludes the possibility that P2 is required for compaction but also makes it less likely that the deficiency in P1 is the primary defect in shiverer. As P2 in rat and mouse has frequently been confused with another small basic protein (related to P1) in SDS-polyacrylamide gels, it seemed worthwhile to reassess this aspect of the Kirschner and Ganser observations. Immunohistochemistry and immunoblotting have been used here to show unambiguously that P2 is present in shiverer peripheral myelin.     

Increased levels of myelin basic protein transcripts in virus-induced demyelination

In multiple sclerosis, a demyelinating disease of young adults, there is a paucity of myelin repair in the central nervous system (CNS) which is necessary for the restoration of fast saltatory conduction in axons. Consequently, this relapsing disease often causes marked disability. In similar diseases of small rodents, however, remyelination can be quite extensive, as in the demyelinating disease caused by the A59 strain of mouse hepatitis virus (MHV-A59), a coronavirus of mice. To investigate when and where oligodendrocytes are first triggered to repair CNS myelin in such disease, we have used a complementary DNA probe specific for one major myelin protein gene, myelin basic protein (MBP), which hybridizes with the four forms of MBP messenger RNA in rodents. Using Northern blot and in situ hybridization techniques, we previously found that MBP mRNA is first detected at about 5 days after birth, peaks at 18 days and progressively decreases to 25% of the peak levels in the adult. We now report that in spinal cord sections of adult animals with active demyelination and inflammatory cells, in situ hybridization reveals a dramatic increase in probe binding to MBP-specific mRNA at 2-3 weeks after virus inoculation and before remyelination can be detected by morphological methods. This increase of MBP-specific mRNA is found at the edge of the demyelinating area and extends into surrounding areas of normal-appearing white matter. Thus, in situ hybridization with myelin-specific probes appears to be a useful method for detecting the timing, intensity and location of myelin protein gene reactivation preceding remyelination. This method could be used to elucidate whether such a reactivation occurs in multiple sclerosis brain tissue. Our results suggest that in mice, glial cells react to a demyelinating process with widespread MBP mRNA synthesis which may be triggered by a diffusible factor released in the demyelinated areas.     

Spreading of T-cell autoimmunity to cryptic determinants of an autoantigen.

Immunization with myelin basic protein (MBP) induces experimental allergic encephalomyelitis (EAE), a prototype of CD4+ T-cell mediated autoimmune disease. In rodents, MBP-reactive T-cell clones are specific for a single, dominant determinant on MBP and use a highly restricted number of T-cell receptor genes. Accordingly, EAE has been prevented by various receptor-specific treatments, suggesting similar strategies may be useful for therapy of human autoimmune disease. Here we report that in (SJL x B10.PL)F1 mice, immune dominance of a single determinant, MBP:Ac1-11, is confined to the inductive phase of EAE. In mice with chronic EAE, several additional determinants of MBP in peptides 35-47, 81-100 and 121-140 recall proliferative responses. Most importantly, reactivity to the latter determinants was also detected after induction of EAE with MBP peptide Ac1-11 alone; this demonstrates priming by endogenous MBP determinants. Thus, determinants of MBP that are cryptic after primary immunization can become immunogenic in the course of EAE. Diversification of the autoreactive T-cell repertoire due to 'determinant spreading' has major implications for the pathogenesis of, and the therapeutic approach to, T-cell driven autoimmune disease.     

Haemophilia B caused by a point mutation in a donor splice junction of the human factor IX gene

Haemophilia B (Christmas disease) is an inherited, recessive, sex-linked, haemorrhagic condition caused by a defect in the intrinsic clotting factor IX. This disease occurs in males at a frequency of approximately 1 in 30,000. Patients differ in the severity of their clinical symptoms, and variation in the clotting activity and in the concentration of factor IX antigen in their plasma has been demonstrated. There is probably heterogeneity in the molecular defects of the factor IX gene causing the disease. Here we study a severely affected, antigen-negative patient, and show that the only significant sequence difference from the normal factor IX gene is a point mutation changing the obligatory GT to a TT within the donor splice junction of exon f. We infer that this change is the cause of the disease in this individual. In addition, we have used oligodeoxynucleotide probes specific for this mutation to demonstrate the feasibility of carrier detection and prenatal diagnosis for relatives of the patient.     

Ia-restricted encephalitogenic T lymphocytes mediating EAE lyse autoantigen-presenting astrocytes

T lymphocytes specific for myelin basic protein (MBP) are responsible for the cellular events leading to autoimmune disease within the central (CNS) and peripheral (PNS) nervous systems. Both in actively induced and T-cell transfer versions of experimental autoimmune encephalomyelitis (EAE) and neuritis (EAN), the autoaggressive T cells are activated outside the nervous system and reach their target tissue via the blood circulation. The target specificity of the autoaggressive T cells is impressive; T-cell lines specific for MBP predominantly home to and affect the white matter of the CNS whereas T cells specific for PNS myelin protein P2 exclusively infiltrate peripheral nerves. Having penetrated the tight blood tissue barriers, the lymphocytes seem to interact with local cells expressing the relevant autoantigen in an immunogenic form. Although the exact mechanism of target finding and destruction is unknown, studies from our laboratory have shown that astrocytes, a main component of the normal CNS glia, can actively present antigen to specific T cells. This observation suggests that astrocytes are involved in natural immune reactivity within the CNS, and that they may be involved in pathological aberrations, such as in the development of autoimmune lesions. Having studied astrocyte/T-cell interactions in more detail, we discovered that encephalitogenic T-cell lines recognizing MBP on astrocytes will subsequently proceed to kill the presenting cells. Here we report that astrocyte killing follows the rules governing 'classical' T-cell-mediated cytolysis; it is antigen-specific, restricted by antigens of the major histocompatibility complex (MHC) and apparently contact-dependent. Our data suggest that the nature of the recognized antigenic epitope determines whether or not antigen recognition is followed by killing; moreover, killing of antigen-presenting astrocytes seems to be correlated with the capacity to transfer encephalomyelitis to normal syngeneic rats.     

Trembler mouse carries a point mutation in a myelin gene.

The autosomal dominant trembler mutation (Tr), maps to mouse chromosome 11 (ref. 2) and manifests as a Schwann-cell defect characterized by severe hypomyelination and continuing Schwann-cell proliferation throughout life. Affected animals move clumsily and develop tremor and transient seizures at a young age. We have recently described a potentially growth-regulating myelin protein, peripheral myelin protein-22 (PMP-22; refs 7, 8), which is expressed by Schwann cells and found in peripheral myelin. We now report the assignment of the gene for PMP-22 to mouse chromosome 11. Cloning and sequencing of PMP-22 complementary DNAs from inbred Tr mice reveals a point mutation that substitutes an aspartic acid residue for a glycine in a putative membrane-associated domain of the PMP-22 protein. Our results identify the PMP-22 gene as a likely candidate for the mouse trembler locus and will encourage the search for mutations in the corresponding human gene in pedigrees with hypertrophic neuropathies such as Charcot-Marie-Tooth and Dejerine-Sottas diseases (hereditary motor and sensory neuropathies I and III).     

An exonic mutation in the HuP2 paired domain gene causes Waardenburg's syndrome.

Here we report the identification and characterization of a gene defect causing Waardenburg's syndrome with hearing loss in a large Brazilian family. This demonstrates a mutation causing Waardenburg's syndrome as well as a mutation causing a form of congenital deafness. The mutation was found in the HuP2 gene, a member of the paired domain family of proteins that bind DNA and regulate gene expression. The mutation occurred in 100% of the cases with the disease in this family and was absent in a random sample of 50 unrelated control subjects. Identification of the Waardenburg's syndrome gene and future characterization of its gene product is likely to increase our understanding of the pathogenesis of this disorder and may allow prevention of deafness of this type.     

The mechanosensory protein MEC-6 is a subunit of the C. elegans touch-cell degenerin channel

Mechanosensory transduction in touch receptor neurons is believed to be mediated by DEG/ENaC (degenerin/epithelial Na+ channel) proteins in nematodes and mammals. In the nematode Caenorhabditis elegans, gain-of-function mutations in the degenerin genes mec-4 and mec-10 (denoted mec-4(d) and mec-10(d), respectively) cause degeneration of the touch cells. This phenotype is completely suppressed by mutation in a third gene, mec-6 (refs 3, 4), that is needed for touch sensitivity. This last gene is also required for the function of other degenerins. Here we show that mec-6 encodes a single-pass membrane-spanning protein with limited similarity to paraoxonases, which are implicated in human coronary heart disease. This gene is expressed in muscle cells and in many neurons, including the six touch receptor neurons. MEC-6 increases amiloride-sensitive Na+ currents produced by MEC-4(d)/MEC-10(d) by approximately 30-fold, and functions synergistically with MEC-2 (a stomatin-like protein that regulates MEC-4(d)/MEC-10(d) channel activity) to increase the currents by 200-fold. MEC-6 physically interacts with all three channel proteins. In vivo, MEC-6 co-localizes with MEC-4, and is required for punctate MEC-4 expression along touch-neuron processes. We propose that MEC-6 is a part of the degenerin channel complex that may mediate mechanotransduction in touch cells.     

T-cell epitope of the autoantigen myelin basic protein that induces encephalomyelitis

Chronic relapsing paralysis and demyelination within the central nervous system (CNS), features associated with the human disease multiple sclerosis (MS), develop in mice after injection of murine T-cell clones specific for the autoantigen myelin basic protein (MBP). We examined the fine specificity of three independently derived encephalitogenic T-cell clones using synthetic polypeptides derived from portions of the N-terminal sequence of MBP. These clones appear functionally identical; they all respond to an epitope in the N-terminal nine amino acid residues in association with the same class II (I-A) molecules of the major histocompatibility complex (MHC). Both the N-terminal acetyl moiety and the first residue (Ala) are necessary for recognition. Only N-terminal MBP peptides recognized by these clones were found to cause encephalomyelitis (EAE) in vivo. These results show that the N-terminal MBP-specific T lymphocytes that mediate autoimmune encephalomyelitis are a small population with a limited repertoire; they all recognise the same combination of MHC and target.     

Adoptive transfer of myelin basic protein-sensitized T cells produces chronic relapsing demyelinating disease in mice

The autoimmune disease of the central nervous system (CNS), experimental allergic encephalomyelitis (EAE), is induced by challenge of genetically susceptible animals with spinal cord homogenates or myelin basic protein (MBP). Chronic and relapsing forms of the disease have some similarities to human demyelinating disorders, namely, multiple sclerosis, and are of particular interest. EAE can be transferred passively with sensitized lymphoid cells into syngeneic animals but transferred EAE has been believed to have limited relevance to human disease because it is usually monophasic and manifested by minimal demyelination. We report here that a single transfer of MBP-sensitized lymph node cells or T cell, in the absence of a peripheral antigen depot, leads to both acute EAE with significant primary demyelination, and chronic relapsing disease with lesions typical of demyelination over a long period. These findings have major implications for the immunological mechanisms involved in experimental and human demyelinating diseases.     

Dynamin-like protein encoded by the Drosophila shibire gene associated with vesicular traffic.

Temperature-sensitive paralysis is the most striking defect of adult Drosophila carrying the shibire mutation. This is believed to be due to a reversible block of endocytosis, which prevents membrane cycling and thus depletes synaptic vesicles. The shibire mutation also affects many tissues outside the nervous system. We have now mapped and characterized the shibire gene. A 275-kilobase yeast artificial chromosome was subcloned into cosmids, among which the gene was then located by analysing with restriction-fragment length polymorphisms. A 15-kilobase fragment of wild-type DNA rescues the mutant phenotype and the sequence of two mutant alleles show differences with wild type, demonstrating that we have isolated the shibire gene. The gene encodes a protein that is highly similar to rat dynamin, 69% of the amino-acid sequence is identical. Dynamin is a GTP-driven mechanochemical enzyme related to mammalian mx-proteins and to the yeast vps 1 gene product. Because the shibire gene product and dynamin have extensive similarity, we propose that they are cognate homologues. Dynamin causes microtubules to slide along each other in vitro and in extracts it is associated with a distinct, but so far uncharacterized, membrane fraction. In light of the shibire phenotype, we suggest that these proteins provide the motor for vesicular transport during endocytosis.     

Adoptive transfer of EAE-like lesions from rats with coronavirus-induced demyelinating encephalomyelitis

Viruses have been found to induce inflammatory demyelinating lesions in central nervous system (CNS) tissue of both animal and man, either by natural infections or after vaccination. At least two different pathogenic mechanisms have been proposed for these changes, a cytopathic viral infection of oligodendroglia cells with subsequent cell death, and a host immune reaction against virus and brain antigens. We now report the occurrence of cell-mediated immune reactions against basic myelin proteins in the course of coronavirus infections in Lewis rats. Infection of rats with the murine coronavirus JHM leads to demyelinating encephalomyelitis developing several weeks to months postinfection. Lymphocytes from these diseased Lewis rats can be restimulated with basic myelin protein (BMP) and adoptive transfer of these cells leads to lesions resembling those of experimental allergic encephalomyelitis (EAE) in recipients, which can be accompanied by a mild clinical disease. This model demonstrates that a virus infection in CNS tissue is capable of initiating an autoimmune response which may be of pathogenic importance.     

Evolution of increased complexity in a molecular machine

Many cellular processes are carried out by molecular 'machines'-assemblies of multiple differentiated proteins that physically interact to execute biological functions. Despite much speculation, strong evidence of the mechanisms by which these assemblies evolved is lacking. Here we use ancestral gene resurrection and manipulative genetic experiments to determine how the complexity of an essential molecular machine--the hexameric transmembrane ring of the eukaryotic V-ATPase proton pump--increased hundreds of millions of years ago. We show that the ring of Fungi, which is composed of three paralogous proteins, evolved from a more ancient two-paralogue complex because of a gene duplication that was followed by loss in each daughter copy of specific interfaces by which it interacts with other ring proteins. These losses were complementary, so both copies became obligate components with restricted spatial roles in the complex. Reintroducing a single historical mutation from each paralogue lineage into the resurrected ancestral proteins is sufficient to recapitulate their asymmetric degeneration and trigger the requirement for the more elaborate three-component ring. Our experiments show that increased complexity in an essential molecular machine evolved because of simple, high-probability evolutionary processes, without the apparent evolution of novel functions. They point to a plausible mechanism for the evolution of complexity in other multi-paralogue protein complexes.     

Identification of receptor contact site involved in receptor-G protein coupling

The mammalian G proteins transduce information from extracellular signals, including neurotransmitters, hormones and sensory stimuli, into regulation of effector enzymes or ion channels within cells. Triggered by appropriate extracellular signals, receptor proteins specifically activate members of the G protein family by catalysing replacement of GDP by GTP at the guanine nucleotide binding site. Like the receptor proteins, the heterotrimeric G proteins exhibit impressive structural similarities, suggesting that all receptor-G protein interactions use homologous structural elements and a single molecular mechanism. Topologically equivalent portions of each G protein may therefore interact with the appropriate receptor. We recently predicted the secondary structure of a composite G protein alpha-chain and proposed that a predicted amphipathic alpha-helix at the extreme carboxy-terminus of the polypeptide directly contacts receptors. This proposal has now been confirmed by sequencing complementary DNAs of the gene that encodes the alpha-chain (alpha s) of the stimulatory regulator (Gs) of adenylyl cyclase in wild-type cells and in a mutant mouse S49 lymphoma cell line, unc, in which Gs cannot be activated by hormone receptors. The sequences reveal a point mutation in the unc gene that substitutes a proline residue for an arginine near the carboxy-terminus of the alpha s-polypeptide. Expression of recombinant alpha s-unc in genetically alpha s-deficient S49 cells reproduces the unc phenotype.     

伴性基因的选择原理

本文根据基因平衡定律和伴性基因的遗传特点,从理论上讨论了伴性基因的选择原理。主要对显性和隐性伴性基因的完全和部分选择问题进行了分析和讨论,并推导出了各种选择方式下基因频率的计算公式。     

Highly efficient endogenous human gene correction using designed zinc-finger nucleases

Permanent modification of the human genome in vivo is impractical owing to the low frequency of homologous recombination in human cells, a fact that hampers biomedical research and progress towards safe and effective gene therapy. Here we report a general solution using two fundamental biological processes: DNA recognition by C2H2 zinc-finger proteins and homology-directed repair of DNA double-strand breaks. Zinc-finger proteins engineered to recognize a unique chromosomal site can be fused to a nuclease domain, and a double-strand break induced by the resulting zinc-finger nuclease can create specific sequence alterations by stimulating homologous recombination between the chromosome and an extrachromosomal DNA donor. We show that zinc-finger nucleases designed against an X-linked severe combined immune deficiency (SCID) mutation in the IL2Rgamma gene yielded more than 18% gene-modified human cells without selection. Remarkably, about 7% of the cells acquired the desired genetic modification on both X chromosomes, with cell genotype accurately reflected at the messenger RNA and protein levels. We observe comparably high frequencies in human T cells, raising the possibility of strategies based on zinc-finger nucleases for the treatment of disease.     

Maintenance of functional equivalence during paralogous Hox gene evolution

Biological diversity is driven mainly by gene duplication followed by mutation and selection. This divergence in either regulatory or protein-coding sequences can result in quite different biological functions for even closely related genes. This concept is exemplified by the mammalian Hox gene complex, a group of 39 genes which are located on 4 linkage groups, dispersed on 4 chromosomes. The evolution of this complex began with amplification in cis of a primordial Hox gene to produce 13 members, followed by duplications in trans of much of the entire unit. As a consequence, Hox genes that occupy the same relative position along the 5' to 3' chromosomal coordinate (trans-paralogous genes) share more similarity in sequence and expression pattern than do adjacent Hox genes on the same chromosome. Studies in mice indicate that although individual family members may have unique biological roles, they also share overlapping functions with their paralogues. Here we show that the proteins encoded by the paralogous genes, Hoxa3 and Hoxd3, can carry out identical biological functions, and that the different roles attributed to these genes are the result of quantitative modulations in gene expression.     

天人菊matK基因克隆及生物信息学分析

为了完善天人菊的遗传信息,丰富菊科植物分子生物学分析数据.通过PCR扩增、基因克隆获得天人菊matK基因的完整核苷酸序列,采用生物信息学方法分析天人菊matK蛋白的结构及性质,并与其他12种matK氨基酸序列进行对比,构建系统进化树.结果表明,天人菊matK基因全长1296 bp,可编码432个氨基酸,二级结构以α-螺...