Increased proliferation of oligodendrocytes in the hypomyelinated mouse mutant-jimpy

Many roles for oligodendrocyte precursor cells in physiology and pathology

Oligodendrocyte precursor cells (OPCs) are a fourth resident glial cell population in the mammalian central nervous system. They are evenly distributed throughout the gray and white matter and continue to proliferate and generate new oligodendrocytes (OLs) throughout life. They were understudied until a few decades ago when immunolabeling for NG2 and platelet-derived growth factor receptor alpha revealed cells that are distinct from mature OLs, astrocytes, neurons, and microglia. In this review, we provide a summary of the known properties of OPCs with some historical background, followed by highlights from recent studies that suggest new roles for OPCs in certain pathological conditions.

Dimethyl fumarate ameliorates myoclonus stemming from protein misfolding in oligodendrocytes

Multiple sclerosis (MS) is considered a primary autoimmune disease; however, this view is increasingly being challenged in basic and clinical science arenas because of the growing body of clinical trials' data showing that exclusion of immune cells from the CNS only modestly slows disease progression to disability. Accordingly, there is significant need for expanding the scope of potential disease mechanisms to understand the etiology of MS. Concomitantly, the use of a broader range of pre-clinical animal models for characterizing existing efficacious clinical treatments may elucidate additional or unexpected mechanisms of action for these drugs that augment insight into MS etiology. Herein, we explore the in vivo mechanism of action of dimethyl fumarate, which has been shown to suppress oxidative stress and immune cell responses in psoriasis and MS. Rather than studying this compound in the context of an experimental autoimmune-induced attack on the CNS, we have used a genetic model of hypomyelination, male rumpshaker (rsh) mice, which exhibit oligodendrocyte metabolic stress and startle-induced subcortical myoclonus during development and into adulthood. We find that myoclonus is reduced 30-50% in treated mutants but we do not detect substantial changes in metabolic or oxidative stress response pathways, cytokine modulation, or myelin thickness (assessed by anova). All procedures involving vertebrate animals in this study were reviewed and approved by the IACUC committee at Wayne State University.

Lineage, fate, and fate potential of NG2-glia

NG2 cells represent a fourth major glial cell population in the mammalian central nervous system (CNS). They arise from discrete germinal zones in mid-gestation embryos and expand to occupy the entire CNS parenchyma. Genetic fate mapping studies have shown that oligodendrocytes and a subpopulation of ventral protoplasmic astrocytes arise from NG2 cells. This review describes recent findings on the fate and fate potential of NG2 cells under physiological and pathological conditions. We discuss age-dependent changes in the fate and fate potential of NG2 cells and possible mechanisms that could be involved in restricting their oligodendrocyte differentiation or fate plasticity. This article is part of a Special Issue entitled SI:NG2-glia(Invited only).

Neurochemistry of Hypomyelination Investigated with MR Spectroscopy

Proton magnetic resonance spectroscopy (MRS) allows the noninvasive exploration of tissue metabolism in vivo, providing neurophysiological and neurochemical information. N-acetylaspartate (NAA) is generally considered to be a marker of neurons and axons, and many neurodegenerative disorders, including demyelinating disorders, exhibit a decrease in total NAA (tNAA). MRS in human hypomyelination disorders, such as Pelizaeus-Merzbacher disease (PMD), is characterized by normal to elevated tNAA, elevated myo-inositol and creatine (Cr), and normal to decreased choline (Cho). MRS in the thalamus of a hypomyelinating mouse model, a myelin synthesis-deficient (msd) mouse, a model of connatal PMD with mutation of the Plp1 gene, revealed increased tNAA and Cr and decreased Cho. That of a shiverer mouse with an autosomal recessive mutation of the Mbp gene showed decreased Cho with normal tNAA and Cr. Accordingly, the reduction of Cho on MRS might be a common marker for hypomyelinating disorders. tNAA concentrations range from normal to increased, probably depending upon the underlying pathology of oligodendrocytes. tNAA may be increased in hypomyelination with a reduced number of mature oligodendrocytes, such as PMD.

Cell-specific loss of kappa-opioid receptors in oligodendrocytes of the dysmyelinating jimpy mouse

Jimpy is a murine mutation in myelin proteolipid protein, leading to premature death of oligodendrocytes and severe central nervous system hypomyelination. Jimpy is a bona fide model of human Pelizaeus-Merzbacher disease. This paper describes a severe reduction in expression of kappa-opioid receptors (KOP) in oligodendrocytes of jimpy mice. A cell-specific reduction of >90% is apparent by 5 days of age. Expression is not reduced in neurons, and mu-opioid receptor expression is normal. Mechanism(s) leading to deficient KOP expression in jimpy mice remain unclear. We speculate that loss of KOP may be related to increased [Ca(2+)](i) and premature death of jimpy oligodendrocytes.

Expression of a myelin proteolipid protein (Plp)-lacZ transgene is reduced in both the CNS and PNS of Plp(jp) mice

Jimpy (Plp(jp)) is an X-linked recessive mutation in mice that causes CNS dysmyelination and early death in affected males. It results from a point mutation in the acceptor splice site of myelin proteolipid protein (Plp) exon 5, producing transcripts that are missing exon 5, with a concomitant shift in the downstream reading frame. Expression of the mutant PLP product in Plp(jp) males leads to hypomyelination and oligodendrocyte death. Expression of our Plp-lacZ fusion gene, PLP(+)Z, in transgenic mice is an excellent readout for endogenous Plp transcriptional activity. The current studies assess expression of the PLP(+)Z transgene in the Plp(jp) background. These studies demonstrate that expression of the transgene is decreased in both the central and peripheral nervous systems of affected Plp(jp) males. Thus, expression of mutated PLP protein downregulates Plp gene activity both in oligodendrocytes, which eventually die, and in Schwann cells, which are apparently unaffected in Plp(jp) mice.

Genetic background determines phenotypic severity of the Plp rumpshaker mutation

The rumpshaker mutation of the proteolipid protein (Plp) gene causes dysmyelination in man and mouse. We show that the phenotype in the mouse depends critically on the genetic background in which the mutation is expressed. On the C3H background there is normal longevity whereas changing to a C57BL/6 strain results in seizures and death at around postnatal day 30. The more severe phenotype is associated with less myelin and reduced levels of major myelin proteins. There are also more apoptotic cells, including oligodendrocytes, increased numbers of proliferating cells, increased numbers of NG2+ oligodendrocyte progenitors and increased microglia compared to the milder phenotype. The number of mature oligodendrocytes is similar to wild-type in both strains of mutant, however, suggesting that increased oligodendrocyte death is matched by increased generation from progenitors. The dichotomy of phenotype probably reflects the influence of modifying loci. The localization of these putative modifying genes and their mode of action remain to be determined.

Oligodendrogenesis is differentially regulated in gray and white matter of jimpy mice

The factors that regulate oligodendrogenesis have been studied extensively in optic nerve, where oligodendrocyte production and myelination quickly follow colonization of the nerve by progenitor cells. In contrast, oligodendrocyte production in the cerebral cortex begins approximately 1 week after progenitor cell colonization and continues for 3-4 weeks. This and other observations raise the possibility that oligodendrogenesis is regulated by different mechanisms in white and gray matter. The present study examined oligodendrocyte production in the developing cerebral cortex of jimpy (jp) and jimpy(msd) (msd) mice, which exhibit hypomyelination and oligodendrocyte death due to mutations in and toxic accumulations of proteolipid protein, the major structural protein of CNS myelin. Proliferation of oligodendrocyte progenitors and production of myelinating oligodendrocytes was reduced in jp cerebral cortex when compared to wild-type (wt) and msd mice. The incidence of oligodendrocyte cell death was similar in jp and msd cortex, but total dying oligodendrocytes were greater in msd. We confirm previous reports of increased oligodendrocyte production in white matter of both jp and msd mice. The jp mutation, therefore, reduces oligodendrocyte production in cerebral cortex but not in white matter. These data provide additional evidence that oligodendrogenesis is differentially regulated in white matter and gray matter and implicate PLP/DM20 as a modulator of these differences.

An immortalized jimpy oligodendrocyte cell line: defects in cell cycle and cAMP pathway

Normal and jimpy oligodendrocytes in secondary cultures were transfected with plasmids containing the SV40 T-antigen gene expressed under the control of the mouse metallothionein-I promoter. Two immortalized stable cell lines, a normal (158N) and jimpy (158JP) cell line, expressed transcripts and proteins of oligodendrocyte markers, including proteolipid protein (PLP), myelin basic protein (MBP), and carbonic anhydrase II (CAII). Galactocerebroside and sulfatide were also detected with immunocytochemistry. Immunoelectron microscopy using gold particles showed that the truncated endogenous jimpy PLP was distributed throughout the cytoplasm and in association with the plasma membrane of cell bodies and processes. The length of the cell cycle in the jimpy oligodendrocytes in the absence of zinc was 31 h, about a 4-h longer cell cycle than the normal line. In the presence of 100 microM zinc, the cell cycle became 3 h shorter for both cell lines, with the jimpy cell cycle duration remaining 4 h longer than the normal line. Interestingly, the jimpy cell line showed a significant deficiency in stimulation via the cAMP pathway. While the level of oligodendrocyte markers (PLP, MBP, and CAII) were significantly increased by dibutyryl cAMP (dbcAMP) treatment in the normal cell line, no changes were observed in the jimpy cell lines. This observation, together with previous results showing jimpy oligodendrocyte's failure to respond to basic fibroblast growth factor (bFGF), suggests a role for PLP in a signal transduction pathway. Jimpy and normal oligodendrocytes transfected with the SV40T antigen gene, driven by the wild-type promoter of mouse metallothionein-I, continue to express properties of oligodendrocytes and therefore provide a powerful model to explore the function of myelin proteins and to dissect the complexity of the jimpy phenotype.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Elevated levels of the chemokine GRO-1 correlate with elevated oligodendrocyte progenitor proliferation in the jimpy mutant

The dysmyelinating mutant jimpy (jp) arises from a point mutation in the mouse gene encoding proteolipid protein and is characterized by severe dysmyelination attributable to oligodendrocyte death. This mutant was used to investigate the regulation of oligodendrocyte progenitor proliferation in the postnatal spinal cord. At postnatal day 18, jp spinal cord contained a three- to eightfold greater number of proliferating oligodendrocyte progenitor cells than did wild-type (wt) spinal cord. Increased proliferation in jp spinal cord was accompanied by a twofold increase in the number of progenitor cells. Semiquantitative reverse transcriptase-PCR revealed no change in the level of mRNA encoding the platelet-derived growth factor A, transforming growth factor-beta, or insulin-like growth factor-I, all of which have been implicated as regulators of proliferation and differentiation of oligodendrocyte progenitor cells. There was, however, a 17-fold increase in the level of mRNA encoding the chemokine GRO-1 and a 5- to 6-fold increase in GRO-1 protein in the jp spinal cord. Double immunofluorescence labeling revealed elevated levels of GRO-1 in reactive astrocytes in jp spinal cord white matter. In vitro studies indicated that extracts from jp spinal cord stimulated oligodendrocyte progenitor proliferation. Furthermore, removal of GRO-1 from jp extracts by immunoprecipitation reduced the proliferation of progenitor cells to a level similar to that achieved by wt extracts. These findings suggest a novel mechanism by which proliferation of oligodendrocyte progenitor cells is regulated in the postnatal spinal cord in response to insult.

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.

Understanding glial abnormalities associated with myelin deficiency in the jimpy mutant mouse

Jimpy is a shortened life-span murine mutant showing recessive sex-linked inheritance. The genetic defect consists of a point mutation in the PLP gene and produces a severe CNS myelin deficiency that is associated with a variety of complex abnormalities affecting all glial populations. The myelin deficiency is primarily due to a failure to produce the normal amount of myelin during development. However, myelin destruction and oligodendrocyte death also account for the drastic myelin deficit observed in jimpy. The oligodendroglial cell line shows complex abnormalities in its differentiation pattern, including the degeneration of oligodendrocytes through an apoptotic mechanism. Oligodendrocytes seem to be the most likely candidate to be primarily altered in a disorder affecting myelination, but disturbances affecting astrocytes and microglia are also remarkable and may have a crucial significance in the development of the jimpy disorder. In fact, the jimpy phenotype may not be attributed to a defect in a single cell but rather to a deficiency in the normal relations between glial cells. Evidences from a variety of sources indicate that the jimpy mutant could be a model for disturbed glial development in the CNS. The accurate knowledge of the significance of PLP and its regulation during development must be of vital importance in order to understand glial abnormalities in jimpy.

Endogenous opioid system in developing normal and jimpy oligodendrocytes: mu and kappa opioid receptors mediate differential mitogenic and growth responses

The early development of both neurons and neuroglia may be modulated by signaling through opioid mediated pathways. Neurons and astroglia not only express specific types of opiate receptors, but also respond functionally to opioids with altered rates of proliferation and growth. The present study was undertaken to determine if opioids also modulate development of the other major CNS macroglial cell, the oligodendrocyte (OL). Using well-characterized polyclonal antibodies specific for delta-, kappa-, and mu-opiate receptors, OLs grown in vitro were shown to express mu-receptors at a very immature stage prior to expression of kappa-receptors. This developmentally regulated sequence differs from the pattern of expression in neurons and astroglia. delta-receptors are apparently absent from cultured OLs. OLs also have physiologic responses to selective mu- and kappa-receptor agonists and antagonists. Exposure of relatively immature O4+ OLs to the mu-receptor agonist PL017 [H-Tyr-Pro-Phe(N-Me)-D-Pro-NH2] resulted in a significant enhancement in the rate of DNA synthesis. This effect, which was not observed in more mature MBP+ OLs, was entirely blocked by the antagonist naloxone. Although the kappa-receptor pathway appeared to be uninvolved in controlling proliferation, the kappa-receptor antagonist nor-binaltorphimine significantly increased the size of myelin-like membranes produced by the cultured OLs. Interestingly, OLs derived from the jimpy mouse, a mutant characterized by an almost complete lack of CNS myelin and premature death of OLs, were found to be deficient in kappa-opiate receptors. Our findings clearly show that OLs not only express specific opiate receptors, but also respond to changes in their level of stimulation in ways that could profoundly impact nervous system morphology and function. If opiate receptors are expressed by OLs in vivo, their pharmacological manipulation might provide a novel pathway for modulating OL and myelin production both during development and in demyelinated conditions.

Induction of metallothionein in astrocytes and microglia in the spinal cord from the myelin-deficient jimpy mouse

Jimpy is a shortened life-span murine mutant whose genetic disorder results in severe pathological alterations in the CNS, including hypomyelination, oligodendrocyte death and strong astroglial and microglial reaction. The knowledge of metallothionein (MT) regulation in the CNS and especially of MT presence in specific glial cell types under pathological conditions is scarce. In the present study, immunocytochemical detection of MT-I + II has been performed in spinal cord sections from 10-12- and 20-22-day-old jimpy and normal animals. The identification of MT-positive glial cells was achieved through double labeling combining MT immunocytochemistry and selective markers for oligodendrocytes, astrocytes and microglia. MT was found in glial cells and was present in the spinal cord of jimpy and normal mice at both ages, but there were remarkable differences in MT expression and in the nature of MT-positive glial cells depending on the type of mouse. The number of MT-positive cells was higher in jimpy than in normal spinal cords. This was apparent in all spinal cord areas, although it was more pronounced in white than in the gray matter and at 20-22 days than at 10-12 days. The mean number of MT-positive glia in the jimpy white matter was 1.9-fold (10-12 days) and 2.4-fold (20-22 days) higher than in the normal one. Astrocytes were the only parenchymal glial cells that were positively identified as MT-producing cells in normal animals. Interestingly, MT in the jimpy spinal cord was localized not only in astrocytes but also in microglial cells. The occurrence of MT induction in relation to reactive astrocytes and microglia, and its role in neuropathological conditions is discussed.

Differentiation and death of premyelinating oligodendrocytes in developing rodent brain

Previous studies have indicated that newly formed oligodendrocytes are dynamic cells whose production, survival, and differentiation depend upon axonal influences. This study has characterized the appearance and fate of newly formed oligodendrocytes in developing rat brain. Oligodendrocytes appear in predictable locations and radially extend DM-20-positive processes that cover 80-microm domains in the cortex and 40-microm domains in the corpus callosum. These premyelinating oligodendrocytes have one of two fates: they myelinate axons or degenerate. Between 7 and 21 d after birth, approximately 20% of premyelinating oligodendrocytes identified in the cerebral cortex were degenerating. Oligodendrocytes that ensheathed axons expressed and selectively targeted proteolipid protein to compact myelin and did not degenerate. These observations support the hypothesis that axonal influences affect oligodendrocyte survival, differentiation, and expression of proteolipid protein gene products.

Temperature-dependent regulation of PLP/DM20 and CNP gene expression in two conditionally-immortalized jimpy oligodendrocyte cell lines

We conditionally immortalized jimpy primary oligodendrocytes (ODCs) with the temperature-sensitive SV40 large T antigen. Two cell lines (clones JP1.1 and JP1.2) were generated that expressed a number of ODC markers. Both jimpy cell lines expressed DM20 mRNAs at the proliferative temperature of 34 degrees C, but not at the "differentiation" temperature of 39 degrees C. Interestingly, at 39 degrees C neither cell line appeared to differentiate further, and neither survived longer than 7 days, in contrast to other ODC cell lines from normal animals that survive many weeks at 39 degrees C. These findings are not consistent with the notion that a PLP/DM20 gene product is the cause of oligodendrocyte cell death in jimpy, since neither jimpy cell line survived at 39 degrees C, and neither line expressed PLP or DM20 proteins. Analysis of the expression of the CNP (2'3' cyclic nucleotide-3'-phosphodiesterase) gene indicated that in both cell lines only one of the two CNP isoforms was expressed at 34 degrees C. Raising the temperature to 39 degrees C caused a greater reduction in the levels of CNP protein than CNP mRNA. Taken together, the DM20 and CNP data suggest that at least some of the decline in myelin/oligodendrocyte components observed in jimpy brains may not be due simply to fewer mature oligodendrocytes, but also to a down regulation of expression of these genes at several levels including transcriptional and post-transcriptional events. Our results provide two cell models for in vitro investigations into the nature of the jimpy mutation at several cellular and molecular levels.

Density-dependent feedback inhibition of oligodendrocyte precursor expansion

The myelin sheath in the vertebrate CNS is formed by oligodendrocytes. The number of oligodendrocytes in a mature axon tract must be sufficient to myelinate all appropriate axons. How the number of oligodendrocytes is matched to axonal requirements and whether such matching involves axon-oligodendrocyte signaling or intrinsic oligodendrocyte self-regulation are not clear. Using a combination of in vitro analyses, we demonstrate that oligodendrocyte precursors closely regulate their numbers through interactions between adjacent precursors. In low-density rat spinal cord cultures, the number of oligodendrocyte lineage cells increases rapidly. The addition of large numbers of oligodendrocyte precursors substantially reduces precursor expansion and results in a normalization of oligodendrocyte lineage cell numbers in the cultures over time. Thus, the number of oligodendrocyte lineage cells that develop appears dependent on the density of oligodendrocyte lineage cells. This normalization of cell number is reflected in assays of clonal potential and proliferation. For example, precursors gave rise to fewer progeny and proliferated less at high density. Reduced precursor expansion at high density was not attributable to the depletion of growth factors. Cocultures of high and low densities did not inhibit precursor expansion in low-density cultures, suggesting the requirement for local cell-cell interactions. The inhibition of precursor expansion was cell-type-specific and dependent on the presence of oligodendrocyte lineage cells. We propose that this density-dependent feedback inhibition of oligodendrocyte precursor expansion may play a primary role in regulating the number of oligodendrocytes in the developing spinal cord.

The microglial reaction in spinal cords of jimpy mice is related to apoptotic oligodendrocytes

Jimpy is a shortened life-span murine mutant whose genetic disorder results in a severe hypomyelination in the central neruons system associated with a variety of glial abnormalities, including oligodendrocyte death. In this study, we report that oligodendrocyte death in jimpy occurs through an apoptotic mechanism, as demonstrated by in situ labeling of nuclear DNA fragmentation. Compared to those of normal littermates, the spinal cords of jimpy mice showed a significantly higher number of apoptotic cells. Our observations also corroborate that specific glial cell death in jimpy is restricted to oligodendrocytes, as evidenced by double labeling for DNA fragmentation and MBP immunocytochemistry. Cells labeled for DNA fragmentation were always negative for astroglial or microglial markers. Apoptotic oligodendrocytes were not aggregated into clusters and were ubiquitously distributed throughout the jimpy spinal cord, although were more numerous in white matter than in gray matter. We found no physical association between astrocytes and dying cells in jimpy. Microglial cells, however, were found closely attached to and even surrounding apoptotic cells. The possible role of microglial cells in relation to apoptotsis is discussed.

X-Linked Developmental Defects of Myelination: From Mouse Mutants to Human Genetic Diseases

Molecular cloning of the major myelin-specific genes and a systematic analysis of mouse mutants have led to the identification of molecular defects in human genetic diseases that affect myelination. In the central nervous system, Pelizaeus-Merzbacher disease (PMD) and X-linked spastic paraplegia (SPG-2) are clinically distinct with respect to the severity of motor dysfunction but involve the same gene for myelin proteolipid protein (PLP). Spontaneous mouse mutants of the PLP gene, such as jimpy and rumpshaker, provide faithful models of these human diseases and allow a detailed analysis of PLP dysfunction. Hypomyelination in jimpy and, presumably, in PMD is largely the result of abnormally increased oligodendrocyte death and a lack of terminal differentiation. In rumpshaker, a model for X-linked spastic paraplegia, myelinating oligodendrocytes appear normal in number but fail to assemble myelin correctly. Recently, PLP-transgenic mice have provided experimental evidence that increasing the normal PLP gene dosage (e.g., by a gene duplication) is by itself sufficient to cause PMD. The latter is strikingly similar to the peripheral neuropathy Charcot-Marie-Tooth disease frequently associated with a duplication of the myelin protein gene PMP-22.

Isolation and characterization of defective jimpy oligodendrocytes in culture

This study characterizes jimpy oligodendrocyte-enriched secondary cultures isolated from 10-12 days in vitro primary glial cell cultures derived from 1-2-day-old jimpy mouse brains. Proliferation of defective oligodendrocytes was carefully investigated with regard to the expression of myelin basic protein and proteolipid protein and their respective mRNAs. Less than 5% of contaminating astrocytes (GFAP+ cells) were usually present. The identity of jimpy oligodendrocytes was confirmed using an antibody directed against a peptide from the wild type proteolipid protein C-terminal sequence for immunocytochemistry and an oligonucleotide complementary to mRNA derived from exon 5 of the proteolipid protein gene for in situ hybridization. Both the antibody and the probe recognize only normal oligondedrocytes while jimpy oligodendrocytes always remain unstained. Proteolipid protein in normal and jimpy oligodendrocytes was detected with antibody recognizing normal and mutated forms. Between 80 and 95% of the cells in normal and jimpy cultures at 2 and 4 days in vitro in secondary cultures express myelin basic protein and proteolipid protein and their respective mRNAs. The percentage of oligodendrocytes (PLP+ or MBP+) in S phase of the cell cycle was 7-10% for both normal and jimpy oligodendrocytes. This contrasts with the in vivo situation where the proliferation rate of oligodendrocytes in jimpy brains is higher than in normal brains. In addition, jimpy oligodendrocytes remain unresponsive to basic fibroblast growth factor treatment while a similar treatment stimulates the proliferation of normal oligodendrocytes.

Myelin mutants: new models and new observations

The myelin mutants have been extensively used as tools to study the complex process of myelination in the central and peripheral nervous system. A multidisciplinary approach to the study of these models ultimately allows a correlation to be made between phenotype and genotype. This correlation may then lead to the formation of new hypotheses about the functions of the products of genes involved in myelination. This review presents a number of new myelin mutants which have recently been described. The species involved include mouse, rat, rabbit, hamster, and dog models. The genetic defect has not been elucidated in all of these animals, but most have been characterized clinically and pathologically, and, in some cases, biochemically. In addition, a better known myelin mutant, the trembler mouse, is discussed. Recent molecular findings have brought this fascinating mutant to the forefront of the field of peripheral nervous system research. The range of abnormalities in the mutants described in this review includes defects in specific myelin proteins, suspected abnormalities in membrane formation, and apparent defects of the oligodendrocyte cytoskeleton. These findings underscore the complexity of the myelination process and highlight the numerous ways in which it can be disrupted.

Microglial cell reaction in the gray and white matter in spinal cords from jimpy mice. An enzyme histochemical study at the light and electron microscope level

Jimpy is a genetic disorder which results in a severe hypomyelination in the central nervous system associated with a variety of astroglial and oligodendroglial abnormalities. In this study, we examined the morphology and distribution of microglial cells in spinal cord sections from jimpy and normal mice at 10-12 and 20-22 days postnatal using a specific microglial marker, the nucleoside diphosphatase staining. Compared to those of normal littermates, the spinal cords of jimpy mice showed an intense microglial cell reaction in white and gray matter, as revealed by quantitative analysis and light and electron microscope study. Microglial reactivity was apparent in all spinal cord areas, although it was more pronounced in white than in gray matter. The mean microglial densities in the jimpy white matter were about threefold (10-12 days) and fivefold (20-22 days) higher than in the normal, whereas in the gray matter, microglial density in jimpy was about 60% higher than in normal at both ages. Morphologically, microglial cells in the normal spinal cord showed a ramified appearance, similar in size and ramification pattern to those reported in other normal CNS areas. In contrast, microglial cells in the jimpy spinal cord showed a reactive morphology, characterized by a shortening and coarsening of their cell processes, swelling of their cell body and accumulation of lipid inclusions. Reactive microglial cells were found in close association with axons and oligodendroglial cells. The possible role of microglial cells in hypomyelination is discussed.

Oligodendrocyte survival and function in the long-lived strain of the myelin deficient rat

This study has examined cellular and molecular aspects of glial cell function in a newly described long-lived myelin deficient rat mutant. In contrast to the shorter-lived mutants which died at 25-30 days, the longer-lived mutant rats lived to 75-80 days of age. Despite living longer, these mutants had a similar frequency of seizures to their younger counterparts. In the spinal cord and optic nerves of the older mutants, myelinated fibres in similar numbers to those seen in the younger myelin deficient rats were present. However, the total glial cell numbers were markedly reduced with few remaining normal appearing oligodendrocytes, and very few microglia compared to the younger mutants. In addition, little or no cell death or division was seen in the longer-lived rats. However, there was some evidence of ongoing myelination and the persistence of immature oligodendrocytes or their progenitors in the older mutant. There was some continued myelin gene expression, although this was at much reduced levels compared to normal, with proteolipid protein and myelin basic protein being most affected. In situ hybridization analysis for proteolipid protein mRNA showed that few proteolipid protein expressing oligodendrocytes remained in the 70-80-day-old mutant. Polymerase chain reaction analysis of exon 3 of the long-lived mutant revealed the same point mutation as described in the younger myelin deficient rat.

Glial transplants: an in vivo analysis of extrinsic and intrinsic determinants of dysmyelination in genetic variants

Myelination in the CNS depends on the ability of oligodendrocytes (Ols) to efficiently colonize the brain, differentiate, and express a precise balance of specific genes necessary for myelin synthesis. Mutations in these genes produce different types of dysmyelination in animal as in human. Defects in the synthesis of myelin constituents usually lead to mild dysmyelinations. IN contrast, mutations affecting the gene encoding the proteolipid, another major protein of myelin, produce various perturbations of Ols biology suggesting a pleiotropic effect of the gene in the development of the CNS. Studies on expansion of cell population and survival have provided contradictory information on the extrinsic and intrinsic action of the gene on Ols biology. On one hand, in vitro studies using conditioned media as in vivo studies on heterozygotes, and transplantations experiments suggest that excess of programmed cell death in these mutants is ruled out by intrinsic factors which could act during embryonic life. On the other hand, attempts to compensate the gene defect by transgenic correction demonstrate a dominant negative effect of the jp mutation on both survival and functional potential of Ols. Finally, total suppression of PLP gene expression has a restricted effect on myelin structure without excess of cell death. These contradictory results are discussed in the perspective of regulation of cell death by competition for growth factors in limiting amount. The proposed model suggests that this contradiction is only apparent, and that excess of cell death in PLP/DM20 mutant is intrinsically determined by diminished competitivity of the mutant Ols for limited amounts of environmental factors.

Programmed cell death in the dysmyelinating mutants

A large number of genetic mutants that are missing a particular myelin protein or that have an aberrant myelin protein composition have been described. These mutations usually cause dysmyelination in the PNS or CNS. Similarly, the nervous system of animals experimentally altered to block synthesis of myelin proteins have recently been generated that show aberrations in the myelin sheath. For both groups of animals, the numbers of myelinating cells remain relatively stable and glial cell death is minimal. The exception is animals with mutations in the proteolipid protein (PLP) gene which exhibit extensive death of oligodendrocytes (OLs). The degree of OL death in the PLP mutants generally correlates with the amount of dysmyelination. Dying OLs in the PLP mutants exhibit the classical features of apoptotic cells. Programmed cell death (PCD) is often, but not necessarily, manifested by cleavage of DNA into abundant oligonucleosomal fragments. Detection of these abundant DNA fragments was examined in normal and jimpy (jp) mice using the TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) method. In normal spinal cord and brain, at least twice as many cells exhibited DNA fragmentation when compared to numbers of pyknotic glia observed microscopically. In jp spinal cord and brain, roughly one-half of cells exhibited DNA fragmentation when compared to numbers of pyknotic glia observed microscopically. PCD of cells in normal development involving DNA fragmentation has been previously described and our results support that conclusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Postmitotic oligodendrocytes generated during postnatal cerebral development are derived from proliferation of immature oligodendrocytes

The phenotype of proliferating glia is examined during postnatal rodent development by combining immunocytochemistry (ICC) with 3H-thymidine autoradiography (ARG) to identify cells in the S phase of the cell cycle. Antibodies (ABs) which are specific for cells in the oligodendrocyte (OL) lineage were utilized, with emphasis placed upon the proliferation of OLs as it remains unclear whether this cell type divides in situ. The results show that proliferating cells stain with ABs which are specific for OLs and myelin glycolipids. The proliferating OLs (oligodendroblasts), although they do not appear to have formed myelin sheaths, have quite elaborate and distinctive morphologies. These oligodendroblasts give rise to very long, thin processes which in turn have additional branches. Their cytoarchitecture corresponds closely to cells described as oligodendroblasts with electron microscopy and whose processes often appear to be in the initial phase of myelination (Skoff et al: J. Comp. Neurol. 169:291-312, 1976a). These proliferating OLs are still quite immature because the expression of myelin specific proteins is only occasionally observed in 3H-thymidine labeled cells. The phenotype of the oligodendroblasts is quite different from that of proliferating astrocytes (astroblasts). As shown in previous studies (Skoff; Dev. Biol. 139:149-163, 1990), the astroblasts, which are identified by the presence of glial fibrillary acidic protein (GFAP), usually have thick, stubby processes, and both their nucleus and cytoplasm are larger and of lighter density than those found in oligodendroblasts. In early myelinating regions of the cerebrum, glycolipid positive cells account for the majority of the 3H-thymidine labeled cells. This data, when combined with the quantification of proliferating astrocytes (ASs) from previous immunocytochemical and electron microscopic studies, indicate that oligodendroblasts and astroblasts constitute the vast majority of the proliferating glia in the brain and in optic nerve at times when ASs and OLs are being generated. In normal postnatal cerebral development, the immature ASs and OLs which proliferate are the direct, immediate precursors for most postmitotic ASs and OLs.

Analysis of myelin proteolipid protein and F0 ATPase subunit 9 in normal and jimpy CNS

Membrane fractions and chloroform-methanol (C-M) extracts of jimpy (jp) and normal CNS at 17-20 days were examined by immunoblot and sequence analysis to determine whether myelin proteolipid protein (FLP) or DM-20 could be detected in jp CNS. No reactivity was detected in jp samples with several PLP antibodies (Abs) except with one Ab to amino acids 109-128 of normal PLP. Proteins in the immunoreactive bands approximately 26 M(r) comigrating with PLP were sequenced for the first 10-12 residues. A sequence corresponding to PLP was found in normal CNS, as expected, but not in the band from jp CNS. Our results provide no evidence for an aberrant form of PLP in jp CNS at 17-20 days. This and other studies suggest that the abnormalities in jp brain are not due to toxicity of the mutant jp PLP/DM-20 proteins. Interestingly, a sequence identical to the amino terminus of the mature proton channel subunit 9 of mitochondrial F0 ATPase was detected in the immunoreactive bands approximately 26 M(r) in both normal and jp samples. This identification was supported by reactivity with an Ab to the F0 subunit and by labeling with dicyclohexylcarbodiimide (DCCD). In contrast to PLP isolated from whole CNS, PLP isolated from myelin was devoid of F0 subunit 9 based on sequence analysis and lack of reactivity with an Ab to the F0 subunit, yet still reacted with DCCD. This finding rules out the possibility that contaminating F0 ATPase gives rise to the DCCD binding exhibited by PLP and confirms the possibility that PLP has proton channel activity, as suggested by Lin and Lees (1,2).

Contribution of transplantations to the understanding of the role of the PLP gene

We present an overview of the results obtained in a cross-transplantation system using respectively controls, jimpy (jp), and shiverer mutant mice as donors and recipients. Homochronic transplantations (O days into O days) demonstrated that jp environment is non-toxic for non-jp cells and that, contrary to in vitro, jp oligodendrocytes phenotype cannot be modified by environmental factors at this age. Transplantations of embryonic fragments into the newborn brain demonstrated that in contrast to oligodendrocyte precursors contained in fragments of newborn tissue, jimpy embryonic stem cells are sensitive to environmental factors able to modulate the proportion of surviving oligodendrocytes. In addition, these series evidenced a disjunction between the surviving and the myelinating capacity of jp cells demonstrating a pleiotropic effect of the jp mutation on oligodendrocyte biology. Results are discussed with regards to the recent molecular biological finding on the role of the DM20/PLP gene.

Transplanted transgenically marked oligodendrocytes survive, migrate and myelinate in the normal mouse brain as they do in the shiverer mouse brain

The dye Hoechst 33342 was combined with an immunodetectable transgene product (chloramphenicol acetyltransferase, CAT) expressed in differentiated oligodendrocytes to trace their fate after transplantation in the normal and the shiverer mouse brain. In the shiverer brain, the technique allowed us to visualize grafted cells inside myelin basic protein-positive myelin patches. Most of these cells were CAT-positive/Hoechst 33342-negative, reinforcing our hypothesis that cell division probably follows migration of grafted oligodendrocytes. Correlation of their morphology and distribution with their location in the host CNS suggested a local effect on the cell division and morphogenesis of the grafted material. When compared with transplantation of fragments of normal newborn donor tissue into the newborn shiverer brain, no difference could be seen between the behaviour of normal and transgenic oligodendrocytes. In the normal brain, transgenic oligodendrocytes survived at least 150 days and successfully myelinated the host axons. The timing of differentiation of grafted cells was similar in both types of recipient brains. Migration occurred rostrally and caudally. Although migrating cells could be observed along the meninges and the blood vessels, migration occurred preferentially along white matter tracts. The extent of migration was influenced by the site of implantation, and grafted cells could be found up to 6 mm from the grafting point. No differences in the timing of differentiation or the pattern or extent of migration could thus be demonstrated when transgenic oligodendrocytes were transplanted in the normal or the shiverer brain. This validates our previous studies using the newborn shiverer mouse as recipient.

Fate of jimpy-type oligodendrocytes in jimpy heterozygote

In the jimpy mutant mouse, as well as in many other animals with mutations in the myelin proteolipid protein (PLP) gene, oligodendrocytes degenerate before their maturation. To analyze whether this degeneration is caused by the loss of function of PLP gene products related to oligodendrocyte maturation/survival acting extrinsically, expression of the PLP gene was investigated in the jimpy heterozygote, in which one-half of the cells are jimpy type and the other half are wild type due to random X-chromosome inactivation. We first showed that jimpy PLP gene expression is normally regulated at the early stages of development in brains of jimpy hemizygotes and heterozygotes, at least to day 2 after birth. However, the great increase in the level of PLP gene transcripts observed in wild-type mouse brain is suppressed in jimpy mouse brain. This increase was also suppressed in the jimpy heterozygote, and by 2 months after birth, very few jimpy-type PLP gene transcripts were detected in heterozygotes. These results indicate that jimpy-type oligodendrocytes cannot survive or are still in the immature stage in the brain of jimpy heterozygotes. Thus, degeneration of jimpy oligodendrocytes is not caused merely by the lack of trophic factors.

Premature arrest of myelin formation in transgenic mice with increased proteolipid protein gene dosage

Proteolipid protein (PLP) is an integral membrane protein of CNS myelin. Mutations of the X chromosome-linked PLP gene cause glial cell death and myelin deficiency in jimpy mice and other neurological mutants. As part of an attempt to rescue these mutants by transgenic complementation, we generated normal mouse lines expressing autosomal copies of the entire wild-type PLP gene. Surprisingly, increase of the PLP gene dosage in nonmutant mice with only 2-fold transcriptional overexpression results in a novel phenotype characterized by severe hypomyelination and astrocytosis, seizures, and premature death. This demonstrates that precise control of the PLP gene is a critical determinant of terminal oligodendrocyte differentiation. Dysmyelination of PLP transgenic mice provides experimental evidence that Pelizaeus-Merzbacher disease, previously associated with a partial duplication of the human X chromosome, can be caused by doubling of the PLP gene dosage.

The pH of jimpy glia is increased: intracellular measurements using fluorescent laser cytometry

The jimpy mutation lies in the gene which codes for myelin proteolipid protein, and the brains and spinal cords of jimpy mice contain little myelin and no measurable proteolipid protein. It has been thought that the mutation affected only the myelin forming oligodendroglial cells, but there is now considerable evidence that astroglia are also a target of the mutation since jimpy astrocytes exhibit a prominent gliosis along with defects in metabolism and proliferation. Because cell proliferation is associated with an increase in intracellular pH, we investigated whether the pH of jimpy glia was abnormal. Using a pH sensitive fluorescent dye and a laser cytometry system we measured the intracellular pH of individual cells in cultures derived from both jimpy and normal brains. The relative pH of flat astrocytes in jimpy cultures was higher than in normal cultures by an average of 0.24 pH units, and these increased values were evident 2-3 days after plating. At this in vitro age the cultures contain only a few oligodendrocytes, none of which express detectable proteolipid protein. The pH of the process-bearing cell population, which contains the oligodendrocytes as well as some astrocytes and presumptive glial precursors, was also increased but not until 7 days in culture. The finding that a mutation in the myelin proteolipid protein gene can alter the normal pH of astrocytes is quite unexpected since, as far as is known, astrocytes do not make proteolipid protein. These results and others discussed in this paper support the hypothesis that either proteolipid protein itself, or some other product of the gene, may have an important role in central nervous system development.

Characterization of a novel peripheral nervous system myelin protein (PMP-22/SR13)

We have recently described a novel cDNA, SR13 (Welcher, A. A., U. Suter, M. De Leon, G. J. Snipes, and E. M. Shooter. 1991. Proc. Natl. Acad. Sci. USA. 88:7195-7199), that is repressed after sciatic nerve crush injury and shows homology to both the growth arrest-specific mRNA, gas3 (Manfioletti, G., M. E. Ruaro, G. Del Sal, L. Philipson, and C. Schneider, 1990. Mol. Cell Biol. 10:2924-2930), and to the myelin protein, PASII (Kitamura, K., M. Suzuki, and K. Uyemura. 1976. Biochim. Biophys. Acta. 455:806-816). In this report, we show that the 22-kD SR13 protein is expressed in the compact portion of essentially all myelinated fibers in the peripheral nervous system. Although SR13 mRNA was found in the central nervous system, no corresponding SR13 protein could be detected by either immunoblot analysis or by immunohistochemistry. Northern and immunoblot analysis of SR13 mRNA and protein expression during development of the peripheral nervous system reveal a pattern similar to other myelin proteins. Furthermore, we demonstrate by in situ mRNA hybridization on tissue sections and on individual nerve fibers that SR13 mRNA is produced predominantly by Schwann cells. We conclude that the SR13 protein is apparently exclusively expressed in the peripheral nervous system where it is a major component of myelin. Thus, we propose the name Peripheral Myelin Protein-22 (PMP-22) for the proteins and cDNA previously designated PASII, SR13, and gas3.

Posttranscriptional regulation of myelin protein gene expression

Regulation of myelin protein gene expression occurs at many different levels including transcription, mRNA translocation, translation, and posttranslational modification of myelin proteins prior to their assembly into the membrane. Translocation of myelin basic protein (MBP) mRNAs into oligodendrocyte processes was observed in vivo and in primary cultures, but no such translocation was observed for the mRNAs encoding the proteolipid protein (PLP) or myelin-associated glycoprotein. More than 99% of the mRNAs encoding 2'3'-cyclic nucleotide phosphodiesterase (CNP) remained associated with cell bodies. In the jimpy mutant, MBP mRNA translocation appeared to be impaired, but translocation occurred normally in quaking brains in vivo. We have found that steroids, such as glucocorticoids, stimulate the translation of MBP and PLP mRNAs in cell-free systems and inhibit the translation of CNP mRNA. This pattern of regulation is consistent with compositional changes noted in myelin during development. We have localized a nine nucleotide segment within the 5'-untranslated region of the MBP mRNA that is involved in the action of steroids on translation of this mRNA. We have also determined that the protein synthetic step modulated by the steroids is chain initiation, enhancing the rate at which new ribosomal subunits bind to the MBP mRNAs.

Oligodendrocytes of the jimpy phenotype can be partially restored by environmental factors in vivo

Cross-transplantations of neural tissue between jimpy (jp) shiverer (shi) and normal mice have been performed under heterochronic conditions. In all series, fragments of E14-E15 embryonic neural tissue from the different donors have been transplanted into newborn host brain in order to study environmental influences by differentiated tissue on transplanted embryonic cell lines. Large patches of proteolipid protein (PLP)-positive myelin have been observed in the jp brain after transplantation of shi or normal embryonic tissue into the newborn jp brain, suggesting that the jp parenchyma did not inhibit the differentiation of other oligodendrocytes (ODCs). Jp embryonic tissue had the same mitotic potential as normal tissue, as demonstrated by the larger size of myelin patches observed when jp embryonic tissue was used instead of newborn jp tissue. By contrast, whatever the conditions, jp myelin patches were always obviously smaller than normal or shi myelin patches, suggesting that the myelinating capacity of jp ODCs was not enhanced by environmental factors. Finally, comparison of the ratio of successful outcomes observed following embryonic vs. newborn jp donor tissue, strongly suggests a partial or total normalization of jp embryonic ODCs survival by a more mature shi environment.

Developmental expression of myelin protein genes in dysmyelinating mutant mice: analysis by nuclear run-off transcription assay, in situ hybridization, and immunohistochemistry

Gene expression for myelin proteolipid protein (PLP) and myelin basic protein (MBP) in the dysmyelinating mutant mice shiverer and jimpy was analyzed by nuclear run-off transcription assay, in situ hybridization, and immunohistochemistry. The level of PLP transcription in shiverer brains was lower than that in controls at postnatal day 18 but relatively higher at later stages. In spite of the considerable amount of hybridization with PLP cDNA, immunoreaction for PLP was greatly reduced in shiverer mice throughout their lives, probably owing to a defect in the assembly of PLP into myelin. Abnormal deposition of PLP in oligodendroglial cell bodies suggested that transport of PLP to myelin is delayed in shiverer brains. The number of oligodendrocytes expressing PLP mRNA was drastically reduced in jimpy mice. MBP mRNA in jimpy mice is localized preferentially in oligodendroglial cell bodies, a result suggesting that oligodendrocytes in jimpy are mostly the immature type. Although transcriptional activity of the MBP gene in jimpy was greatly reduced, a finding reflecting the decrease in the number of mature oligodendrocytes, that of the PLP gene remained high at early stages. The discrepancy of the two gene expressions is discussed relative to the role of PLP transcripts at early stages of myelination.

Division of astroblasts and oligodendroblasts in postnatal rodent brain: evidence for separate astrocyte and oligodendrocyte lineages

What precursor cells are the source of the macroglia generated during postnatal development? In order to answer this question, we studied the expression of glial specific antigens in proliferating neuroglia in postnatal rodent brain and optic nerve. Immunocytochemistry using antibodies to oligodendrocyte (OL) specific markers (sulfatide and galactocerebroside) and an astrocyte (AS) specific marker (glial fibrillary acidic protein) was combined with thymidine autoradiography. During the first week of postnatal development when most ASs are being generated, one third to one half of the proliferating cells in the optic system are positive for glial fibrillary acidic protein after a 1 h injection of thymidine (Skoff, Dev. Biol., 139:149-168, 1990). During the second postnatal week when OLs are being generated, 30 to 100% of the proliferating cells in presumptive white matter tracts are sulfatide positive and at least 10% are galactocerebroside positive. This finding demonstrates that ASs and OLs divide during postnatal development. These results confirm previous electron microscopic autoradiographic studies showing that the vast majority of proliferating cells in postnatal rat optic nerve have the morphologic characteristics of differentiating ASs or OLs (Skoff, J. Comp. Neurol., 169:291-312, 1976). Since proliferating ASs (astroblasts) and OLs (oligodendroblasts) constitute the majority of the dividing cells at the time that ASs and OLs are being generated, these glioblasts must be the major source for the macroglia generated postnatally. The findings strongly suggest that separate lineages exist for ASs and OLs during postnatal development. There is no compelling in vivo evidence for a bipotential progenitor cell that generates the majority of OLs and certain ASs in postnatal rodent brain. There may, of course, be distinct lineages for the subtypes of ASs and possibly even for subtypes of OLs. We review the concepts of commitment and plasticity and apply these terms to glial differentiation. In situ, the presence of oligodendroblasts and astroblasts demonstrates the COMMITMENT of proliferating cells to a specific glial lineage during normal development. Culture conditions may provide an environment that permits proliferating glial cells to vacillate in their selection of a specific lineage. This situation demonstrates developmental PLASTICITY and the ability of glia to adapt to an altered environment. Whether committed glial cells in situ can be induced to switch their lineage when normal CNS conditions are altered is an intriguing question that remains to be answered.

Mutations in the myelin proteolipid protein gene alter oligodendrocyte gene expression in jimpy and jimpymsd mice

The mouse myelin proteolipid protein (PLP) gene has been studied in normal and jimpymsd mice. Potential upstream regulatory regions of the normal gene have been cloned and mapped, but when these regions were studied in jimpymsd mice by Southern blots, no alterations were observed, relative to the normal gene. To assess whether the low ratio of PLP to DM20 proteins in this mutant reflected an altered PLP/DM20 ratio mRNAs, S1 nuclease analyses were undertaken, which demonstrated that at all ages studied in both jimpy and jimpymsd mice, PLP mRNA was elevated above DM20 mRNA. When exon 3 (the site of the alternative splice signal for DM20 mRNA) of the jimpymsd PLP gene was sequenced, no mutation was identified. The transcription of the PLP gene in normal and mutant animals was studied. The transcription rate increases in normal animals with development, and in very young jimpymsd or jimpy mice, the transcription rate of the PLP gene was close to that of age-matched normal animals. However, by 10 days of age, the transcription rate of this gene in both mutants was significantly below that of age-matched controls. The transcription rate of the myelin basic protein (MBP) gene was also reduced, indicating that expression of both genes is affected by this mutation. In contrast, the transcription rate of the glycerol phosphate dehydrogenase (GPDH) gene, an early marker of oligodendrocytes, is equal to or greater than normal in both mutants. We have confirmed an earlier report of a point mutation in exon 6 of the jimpymsd PLP gene, which converts an alanine to a valine.(ABSTRACT TRUNCATED AT 250 WORDS)