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K A Nave

Publications and source records attributed to K A Nave.

At least 37 records · Page 2Linked to original sources
PubMed1998

Current concepts of PLP and its role in the nervous system.

Proteolipid protein (PLP) and its smaller isoform DM20 constitute the major myelin proteins of the CNS. Mutations of the X-linked Plp gene cause the heterogeneous syndromes of Pelizaeus-Merzbacher disease (PMD) and spastic paraplegia (SPG) in man and similar dysmyelinating disorders in a range of animal species. A variety of mutations including missense mutations, deletions, and duplications are responsible. Missense mutations cause a predicted alteration in primary structure of the encoded protein(s) and are generally associated with early onset of signs and generalised dysmyelination. The severity of the phenotype varies according to the particular codon involved and the influence of uncharacterised modifying genes. There is some evidence that the dysmyelination results from the altered protein acquiring a novel function deleterious to the oligodendrocyte's function. Transgenic mice carrying extra copies of the Plp gene provide a valid model of PMD/SPG due to gene duplication. Depending on the gene dosage, the phenotype can vary from early onset of severe and lethal dysmyelination through to a very late onset of a tract-specific demyelination and axonal degeneration. Mice with a null mutation of the Plp gene assemble and maintain normal amounts of myelin but develop a progressive axonopathy, again demonstrating tract specificity. The results indicate that the functions of PLP are far from clear. There is good evidence that it is involved in the formation of the intraperiod line of myelin, and the results from the knockout and transgenic mice suggest a role in the interaction of oligodendrocyte and axon.

Animals↗
PubMed1998

Late-onset neurodegeneration in mice with increased dosage of the proteolipid protein gene.

Mutations of the proteolipid protein (Plp) gene cause a generalized central nervous system (CNS) myelin deficit in Pelizaeus-Merzbacher disease of man and various tremor syndromes in animal models. X-linked spastic paraplegia is also due to Plp gene mutations but has a different clinical profile and more restricted pathology involving specific tracts and regions. We have shown previously that PLP overexpression in mice homozygous for a Plp transgene results in premature arrest of CNS myelination and premature death. Here, we demonstrate that a low-level increase in Plp gene expression in transgenic mice causes significant axonal degeneration and demyelination with predilection for specific tracts. Following normal motor development, aged mice develop progressive myelin loss, axonal swellings with resultant Wallerian degeneration, and marked vacuolation of the neuropil associated with ataxia, tremor, and seizures. The age of onset and severity of the phenotype is a function of Plp gene dosage. The corticospinal tracts, optic nerve, fasciculus gracilis cerebellum, and brainstem are particularly involved. Although oligodendrocyte cell bodies show little abnormality, their inner adaxonal tongue is often abnormal, suggesting a perturbation of the axon/glial interface that may underlie the axonal changes. We conclude that abnormal expression of an oligodendrocyte-specific gene can cause axonal damage, a finding that is relevant to the pathogenesis of PLP-associated disorders and probably to other myelin-related diseases.

Age of Onset↗
PubMed1998

Neuronal basic helix-loop-helix proteins (NEX, neuroD, NDRF): spatiotemporal expression and targeted disruption of the NEX gene in transgenic mice.

Basic helix-loop-helix (bHLH) genes have emerged as important regulators of neuronal determination and differentiation in vertebrates. Three putative neuronal differentiation factors [NEX for neuronal helix-loop-helix protein-1 (mammalian atonal homolog-2), neuroD (beta-2), and NDRF for neuroD-related factor (neuroD2)] are highly homologous to each other in the bHLH region and comprise a new bHLH subfamily. To study the role of NEX, the first bHLH protein identified in this group, we have disrupted the NEX gene by homologous recombination. NEX-deficient mice have no obvious developmental defect, and CNS neurons appear fully differentiated. To investigate further whether the absence of NEX is compensated for by neuroD and NDRF, we compared the spatiotemporal expression of all three genes. We demonstrate, by in situ hybridization, that the transcription patterns of NEX, neuroD, and NDRF genes are highly overlapping in the developing CNS of normal rats between embryonic day 12 and adult stages but are not strictly identical. The most prominent transcription of each gene marks the dorsal neuroepithelium of the telencephalon in early development and is sustained in the adult neocortex, hippocampus, and cerebellum. In general, neuroD provides the earliest marker of neuronal differentiation in any given region compared with NDRF or NEX. Whereas a few CNS regions are specific for neuroD, no region was detected in which solely NEX or NDRF is expressed. This suggests that the function of the mutant NEX gene in neuronal differentiation is compensated for by neuroD and NDRF and that, in analogy with myogenic bHLH proteins, neuronal differentiation factors are at least in part equivalent in function.

Aging↗
PubMed1998

Mouse models of myelin diseases.

Dys- and demyelination are the common endpoints of several inherited diseases of glial cells, which elaborate myelin and which maintain the myelin sheath very much like an "external" cellular organelle. Whereas some of the genes that are affected by mutations appear to be glial-specific, other genes are expressed in many cell types but their defect is restricted to oligodendrocytes or Schwann cells. Many of the disease genes and their encoded proteins have been studied with the help of mouse models, and a number of different molecular pathomechanisms have emerged which have been summarized in Figure 8. Some of the new concepts in the field, which have been addressed in this review, have only emerged because similar pathomechanisms were discovered for different myelin proteins. Mouse models have clearly helped to address both, the molecular pathology of myelin diseases and the normal function of myelin genes, but as discussed in this review, these questions turned out to be very different. Despite the progress in understanding the role of the abundant myelin proteins, there also remain a number of open questions that concern, among other things, the initial axon-glia recognition, the assembly process of the myelin sheath, and the long-term interaction of axons with their myelinating glia. Finally, animal models of human neurological diseases should not be restricted to the study of pathology, but they should also contribute to the development of experimental treatments. It is encouraging that a few attempts have been made.

Animals↗
PubMed1997

Helix-loop-helix proteins in Schwann cells: a study of regulation and subcellular localization of Ids, REB, and E12/47 during embryonic and postnatal development.

Although basic helix-loop-helix (bHLH) proteins play an important role in transcriptional control in many cell types, the role of HLH proteins in Schwann cells has yet to be assessed. In this study, we have analyzed the expression of the dominant negative HLH genes, Id1 to Id4 and the class A gene REB, during Schwann cell development. We found that mRNA derived from these genes was present in the Schwann cell lineage throughout development including embryonic precursors and mature cells. The mRNA levels were not significantly regulated during development. Nevertheless, by using antibodies against the four different Id proteins, we found clear regulation of some of these genes at the protein level, in particular Id 2, 4, and REB, both in amount and nuclear/cytoplasmic localization. All these proteins are found in the nuclei of Schwann cell precursors but are not seen in nuclei of Schwann cells of newborn nerves. We observed extensive overlap in Id expression, especially in Schwann cell precursors that co-expressed all four Id proteins and REB. We also showed that Id 1 and 2 were up-regulated as Schwann cells progressed through the cell cycle. These data indicate that HLH transcription factors act as regulators of Schwann cell development and point to the existence of as yet unidentified cell type-specific bHLH proteins in these cells.

Aging↗
PubMed1997

Targeted inactivation of the X-linked adrenoleukodystrophy gene in mice.

In its severe form, X-linked adrenoleukodystrophy (ALD) is a lethal neurologic disease of children, characterized by progressive cerebral demyelination and adrenal insufficiency. Associated with a biochemical defect of peroxisomal beta-oxidation, very long-chain fatty acids (VLCFA) build up in tissues that have a high turnover of lipids, such as central nervous system (CNS) white matter, adrenal cortex, and testis. Whether the abnormal accumulation of VLCFA is the underlying cause of demyelination or merely an associated biochemical marker is unknown. ALD is caused by mutations in the gene for a peroxisomal membrane protein (ALDP) that shares structural features with ATP-binding-cassette (ABC) transporters. To analyze the cellular function of ALDP and to obtain an animal model of this debilitating disease, we have generated transgenic mice with a targeted inactivation of the ald gene. Motor functions in ALDP-deficient mice developed at schedule, and unexpectedly, adult animals appeared unaffected by neurologic symptoms up to at least 6 months of age. Biochemical analyses demonstrated impaired beta-oxidation in mutant fibroblasts and abnormal accumulation of VLCFAs in the CNS and kidney. In 6-month-old mutants, adrenal cortex cells displayed a ballooned morphology and needle-like lipid inclusions, also found in testis and ovaries. However, lipid inclusions and demyelinating lesions in the CNS were not a feature. Thus, complete absence of ALDP expression results in a VLCFA storage disease but does not impair CNS function of young adult mice by pathologic and clinical criteria. This suggests that additional genetic or environmental conditions must be fulfilled to model the early-onset and lethality of cerebral ALD in transgenic mice.

ATP Binding Cassette Transporter, Subfamily D, Mem↗
PubMed1997

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.

Animals↗
PubMed1997

Phenotypic severity of murine Plp mutants reflects in vivo and in vitro variations in transport of PLP isoproteins.

Mutations of the major myelin gene, proteolipid protein (Plp), cause Pelizaeus-Merzbacher disease and some forms of spastic paraplegia in man and dysmyelinating phenotypes in animals. The clinical severity is markedly heterogeneous, ranging from relatively mild to severe and fatal. Point mutations, or frame shifts, which are predicted to result in translation of structurally altered proteins account for many of these cases, including 3 of the allelic murine conditions. Plp(jp-rsh), Plp(jp-msd), and Plp(jp) represent an increasing severity of clinical and pathological phenotypes, respectively. In this study we determined whether there was any correlation between the severity of phenotype and the transport of the predicted abnormal protein. We examined the ability of the two products of the Plp gene, PLP and DM20, to insert into the plasma membrane of transfected BHK or COS-7 cells, and into the myelin sheath of oligodendrocytes. With these complementary in vitro and in vivo approaches we find that proteins of Plp(jp-rsh), associated with the mildest phenotype, have a far greater ability to insert into the cell membrane or myelin than those associated with the severe phenotypes. Additionally, altered DM20 is more readily transported to the cell surface and to myelin than the PLP isoprotein. Interestingly, the two clonal cell lines chosen for transient transfection differ in their ability to fold DM20 from Plp(jp-rsh) and Plp(jp-msd) mice correctly, as inferred by staining for the conformation-sensitive O10 epitope. In the case of Plp(jp), which is associated with the most severe phenotype, no PLP or O10 staining is present at the cell surface or in myelin. The perturbation in trafficking observed for altered Plp(jp) PLP and DM20 in oligodendrocytes does not extend to other myelin membrane proteins, such as MAG and MOG, nor to wild type PLP co-expressed in the same cell, all of which are correctly inserted into myelin. As Plp-knockout mice do not have a dysmyelinating phenotype it seems unlikely that absence of PLP and/or DM20 in the membrane is responsible for the pathology. It remains to be determined whether the perturbation in protein trafficking is associated with the dysmyelination, or if the altered product of the mutant alleles acquire a novel function which is deleterious to myelin production by oligodendrocytes.

Animals↗
PubMed1997

SHARPs: mammalian enhancer-of-split- and hairy-related proteins coupled to neuronal stimulation.

In the mammalian central nervous system, a diverse group of basic helix-loop-helix (bHLH) proteins is involved in the determination of progenitor cells and, subsequently, in regulating neuronal differentiation. Here we report the identification of a novel subfamily of bHLH proteins, defined by two mammalian enhancer-of-split- and hairy-related proteins, termed SHARP-1 and SHARP-2. In contrast to known bHLH genes, detectable transcription of SHARP genes begins at the end of embryonic development marking differentiated neurons that have reached a final position, and increases as postnatal development proceeds. In the adult, SHARP genes are expressed in subregions of the CNS that have been associated with adult plasticity. In PC12 cells, a model system to study neurite outgrowth, SHARP genes can be induced by NGF with the kinetics of an immediate-early gene. Similarly, within 1 h after the administration of kainic acid in vivo, SHARP-2 is induced in neurons throughout the rat cerebral cortex. This suggests that neuronal bHLH proteins are also involved in the "adaptive" changes of mature CNS neurons which are coupled to glutamatergic stimulation.

Amino Acid Sequence↗
PubMed1997

Assembly of CNS myelin in the absence of proteolipid protein.

Two proteolipid proteins, PLP and DM20, are the major membrane components of central nervous system (CNS) myelin. Mutations of the X-linked PLP/DM20 gene cause dysmyelination in mouse and man and result in significant mortality. Here we show that mutant mice that lack expression of a targeted PLP gene fail to exhibit the known dysmyelinated phenotype. Unable to encode PLP/DM20 or PLP-related polypeptides, oligodendrocytes are still competent to myelinate CNS axons of all calibers and to assemble compacted myelin sheaths. Ultrastructurally, however, the electron-dense 'intraperiod' lines in myelin remain condensed, correlating with its reduced physical stability. This suggests that after myelin compaction, PLP forms a stabilizing membrane junction, similar to a "zipper." Dysmyelination and oligodendrocyte death emerge as an epiphenomenon of other PLP mutations and have been uncoupled in the PLP null allele from the risk of premature myelin breakdown.

Animals↗
PubMed1997

SHARPs: mammalian enhancer-of-split- and hairy-related proteins coupled to neuronal stimulation.

In the mammalian central nervous system, a diverse group of basic helix-loop-helix (bHLH) proteins is involved in the determination of progenitor cells and, subsequently, in regulating neuronal differentiation. Here we report the identification of a novel subfamily of bHLH proteins, defined by two mammalian enhancer-of-split- and hairy-related proteins, termed SHARP-1 and SHARP-2. In contrast to known bHLH genes, detectable transcription of SHARP genes begins at the end of embryonic development marking differentiated neurons that have reached a final position, and increases as postnatal development proceeds. In the adult, SHARP genes are expressed in subregions of the CNS that have been associated with adult plasticity. In PC12 cells, a model system to study neurite outgrowth, SHARP genes can be induced by NGF with the kinetics of an immediate-early gene. Similarly, within 1 h after the administration of kalnic acid in vivo, SHARP-2 is induced in neurons throughout the rat cerebral cortex. This suggests that neuronal bHLH proteins are also involved in the "adaptive" changes of mature CNS neurons which are coupled to glutamatergic stimulation.

Amino Acid Sequence↗
PubMed1996

Monoclonal antibody O10 defines a conformationally sensitive cell-surface epitope of proteolipid protein (PLP): evidence that PLP misfolding underlies dysmyelination in mutant mice.

Mutations in the gene for proteolipid protein (PLP) have been associated with CNS dysmyelination and abnormal oligodendrocyte death in spontaneous mouse mutants and in Pelizaeus-Merzbacher disease; however, the effect of mutations on PLP structure and function are little understood. We have identified a monoclonal antibody directed against a novel cell surface epitope of PLP, termed O10. By immunofluorescence analysis, COS-7 cells transiently transfected to express PLP (or its isoform DM20) can be stained with antibody O10 and another antibody (A431) directed against the C terminus of PLP/DM20. The subcellular distribution of immunofluorescence labels for the two antibodies is not identical, suggesting that the O10 epitope is acquired post-translationally. When PLP/DM20 from jimpy, jimpymsd, and rumpshaker mutant mice is expressed in COS-7 cells and compared with wild-type PLP/DM20, none of the mutant isoforms displays the O10 epitope, whereas the C-terminal epitope is detected. Because the O10 but not the A431 epitope is also sensitive to SDS and reducing agents, this strongly suggests abnormal protein folding in the PLP mutants. PLP from jimpymsd mice is obviously misfolded, because the amino acid substitution (Ala242 --> Val) is located within a transmembrane domain to which the O10 antibody does not bind. We propose that the O10 epitope emerges as the full length protein reaches a functional tertiary structure and that the absence of this epitope marks a structural defect of PLP that leads to dysmyelination.

Animals↗
PubMed1996

A transgenic rat model of Charcot-Marie-Tooth disease.

Charcot-Marie-Tooth disease (CMT) is the most common inherited neuropathy in humans and has been associated with a partial duplication of chromosome 17 (CMT type 1A). We have generated a transgenic rat model of this disease and provide experimental evidence that CMT1A is caused by increased expression of the gene for peripheral myelin protein-22 (PMP22, gas-3). PMP22-transgenic rats develop gait abnormalities caused by a peripheral hypomyelination, Schwann cell hypertrophy (onion bulb formation), and muscle weakness. Reduced nerve conduction velocities closely resemble recordings in human patients with CMT1A. When bred to homozygosity, transgenic animals completely fail to elaborate myelin. We anticipate that the CMT rat model will facilitate the identification of a cellular disease mechanism and serve in the evaluation of potential treatment strategies.

Animals↗
PubMed1995

Dominant-negative action of the jimpy mutation in mice complemented with an autosomal transgene for myelin proteolipid protein.

Mutations in genes encoding membrane proteins have been associated with cell death of unknown cause from invertebrate development to human degenerative diseases. A point mutation in the gene for myelin proteolipid protein (PLP) underlies oligodendrocyte death and dysmyelination in jimpy mice, an accurate model for Pelizaeus-Merzbacher disease. To distinguish the loss of PLP function from other effects of the misfolded protein, we took advantage of the X chromosomal linkage of the gene and have complemented jimpy with a wild-type PLP transgene. In this artificial heterozygous situation, the jimpy mutation emerged as genetically dominant. At the cellular level oligodendrocytes showed little increase in survival although endogenous PLP gene and autosomal transgene were truly coexpressed. In surviving oligodendrocytes, wild-type PLP was functional and immunodetectable in myelin. Moreover, compacted myelin sheaths regained their normal periodicity. This strongly suggests that, despite the presence of functional wild-type PLP, misfolded jimpy PLP is by itself the primary cause of abnormal oligodendrocyte death.

Animals↗
PubMed1995

Development of lymphoid hyperplasia in transgenic mice expressing the HIV tat gene.

During HIV infection, individuals experience multiorgan disorders such as adenopathy, splenomegaly, and lung and brain diseases. There is an increasing body of evidence that the HIV trans-activating tat gene product possesses multiple activities. First, it can activate several cellular genes; second, in its extracellular soluble form, it plays the role of growth factor in some cells such as Kaposi's sarcoma cells. Thus, we introduced the HIV tat gene, under the control of the cellular proteolipoprotein promoter, into the germline of mice and demonstrate that, when expressed, the tat gene product induces lymphoid hyperplasia in spleen, lymph nodes, and lung, as is observed in AIDS patients, but not in the brain or testes. Our findings indicate that HIV, through some of its genes, directly participates in the pathogenesis of AIDS.

Animals↗
PubMed1995

Transgenic and natural mouse models of proteolipid protein (PLP)-related dysmyelination and demyelination.

The X chromosome-linked PLP/DM-20 gene is the CNS myelin gene most frequently associated with mutations, resulting in dysmyelination in several species including man (Pelizaeus-Merzbacher disease, X-linked Spastic Paraplegia). The pathology of most PLP gene mutations is characterized by hypomyelination, glial cell proliferation, increased numbers of microglia, and premature oligodendrocyte death. In most mutants, residual myelin structures have an abnormal ultrastructure and periodicity. Surprisingly, transgenic mice which carry extra copies of the wild type PLP gene show dysmyelination, demonstrating that the PLP gene is dosage sensitive. Pathological changes of transgenic mice vary from the phenotype of natural mutants. Specifically, many Golgi saccules of oligodendrocytes are vacuolated and the cytoplasm contains autophagic vacuoles hinting at a perturbation in protein trafficking. In fact, upon transgenic overexpression PLP becomes a prominent peripheral myelin protein, whereas in normal Schwann cells PLP is restricted from entering the myelin compartment. Surprisingly, transgenic animals which overexpress PLP/DM-20 at a low level appear normal during early development, but later spontaneously demyelinate. The mechanisms underlying this demyelination phenotype is unknown but an immune-mediated process has been suggested. All attempts to correct the phenotype of natural PLP mutants, such as jimpy mice, with a wild type transgene have had little effects, indicating a dominant-negative effect of the mutant gene product. On the other hand, mice with a targeted disruption of the PLP/DM-20 gene have surprisingly minor clinical signs. This suggests that the lethal phenotype associated with the majority of PLP gene mutations is a complex combination of loss and gain-of-function effects of a mutant myelin protein.

Animals↗