Dissection of the phenotype and genotype of the X-linked myelin mutants

Claudin Proteins And Neuronal Function

The identification and characterization of the claudin family of tight junction (TJ) proteins in the late 1990s ushered in a new era for research into the molecular and cellular biology of intercellular junctions. Since that time, TJs have been studied in the contexts of many diseases including deafness, male infertility, cancer, bacterial invasion and liver and kidney disorders. In this review, we consider the role of claudins in the nervous system focusing on the mechanisms by which TJs in glial cells are involved in neuronal function. Electrophysiological evidence suggests that claudins may operate in the central nervous system (CNS) in a manner similar to polarized epithelia. We also evaluate hypotheses that TJs are the gatekeepers of an immune-privileged myelin compartment and that TJs emerged during evolution to form major adhesive forces within the myelin sheath. Finally, we consider the implications of CNS myelin TJs in the contexts of behavioral disorders (schizophrenia) and demyelinating/hypomyelinating diseases (multiple sclerosis and the leukodystrophies), and explore evidence of a possible mechanism governing affective disorder symptoms in patients with white matter abnormalities.

CHOP and the endoplasmic reticulum stress response in myelinating glia

The unfolded protein response (UPR) comprises kinase signaling and transcription factor activation cascades delineated over the past 20 years. Most studies conclude that this stress response is adaptive but, nevertheless, includes maladaptive programs involving CHOP expression that drives cell-autonomous apoptosis. Herein, we highlight several studies of UPR diseases involving myelinating glia of the central and peripheral nervous systems that do not support a primary role for CHOP in apoptosis. In oligodendrocytes, CHOP expression apparently protects against death whereas in Schwann cells, CHOP promotes demyelination in the absence of cell death. Together, these studies demonstrate that CHOP should be viewed more broadly as a cell-specific and context-specific mediator of adaptive or maladaptive responses to stress rather than a proapoptotic transcription factor.

The unfolded protein response modulates disease severity in Pelizaeus-Merzbacher disease

The unfolded protein response (UPR) is a eukaryotic signaling pathway linking protein flux through the endoplasmic reticulum to transcription and translational repression. Herein, we demonstrate UPR activation in the leukodystrophy Pelizaeus-Merzbacher disease (PMD) as well as in three mouse models of this disease and transfected fibroblasts expressing mutant protein. The CHOP protein, widely known as a proapoptotic transcription factor, modulates pathogenesis in the mouse models of PMD; however, this protein exhibits antiapoptotic activity. Together, these data show that the UPR has the potential to modulate disease severity in many cells expressing mutant secretory pathway proteins. Thus, PMD represents the first member of a novel class of disparate degenerative diseases for which UPR activation and signaling is the common pathogenic mechanism.

Myelin biogenesis: vesicle transport in oligodendrocytes

Intracellular trafficking of membranes plays an essential role in the biogenesis and maintenance of myelin. The requisite proteins and lipids are transported from their sites of synthesis to myelin via vesicles. Vesicle transport is tightly coordinated with synthesis of lipids and proteins. To maintain the structural and functional organization of oligodendrocytes it is essential synchronize the various pathways of vesicle transport and to coordinate vesicle transport with reorganization of cytoskeleton. The systems that regulate the targeting of protein to myelin by vesicle transport are now being described. Here we review the current knowledge of these systems including those involved in (a) protein folding, (b) protein sorting and formation of carrier vesicles, (c) vesicle transport along elements of the cytoskeleton, and (d) vesicle targeting/fusion.

Locomotor analysis of the taiep rat

Locomotor activity (tremor, ataxia, immobility, epilepsy, and paralysis) in the taiep rat, which suffers from a myelin deficient disorder, has not been previously documented. This study used walking track analysis of footprints to analyze locomotor activity in the taiep rat in comparison to normal, age-matched controls. The results confirmed differences between normal and taiep rats in terms of stride length, step length, and stride width. In addition, we found significant interactions between age and condition for stride and step length. The results suggest that locomotor analysis is a sensitive indicator of myelin deficiency. The results are discussed in terms of the underlying myelin deficiency and possible treatment regimens.

The evolution of lipophilin genes from invertebrates to tetrapods: DM-20 cannot replace proteolipid protein in CNS myelin

The proteolipid protein (PLP) gene encodes two myelin-specific protein isoforms, DM-20 and PLP, which are members of the highly conserved lipophilin family of transmembrane proteins. While the functions of this family are poorly understood, the fact that null mutations of the PLP gene cause leukodystrophy in man is testament to the importance of DM-20 and PLP in normal CNS function. PLP differs from DM-20 by the presence of a 35 amino acid domain exposed to the cytoplasm, which is not encoded by other lipophilin genes and appears to have arisen in amphibians approximately 300 million years before present. However, the lipophilin gene family can be traced back at least 550 million years and is represented in Drosophila and silkworms. Thus, from an evolutionary perspective PLP can reasonably be anticipated to perform functions in CNS myelin that cannot be accomplished by other lipophilins. Herein we use a novel knock-in strategy to generate mice expressing wild-type levels of a Plp gene that has been modified to encode only DM-20. Although DM-20 is incorporated into functional compact myelin sheaths in young animals, our data show that the 35 amino acid PLP-specific peptide is required to engender the normal myelin period and to confer long-term stability on this multilamellar membrane.

Intracellular transport of the DM-20 bearing shaking pup (shp) mutation and its possible phenotypic consequences

Paralytic tremor (pt) in rabbits and shaking pup (shp) in dogs are allelic dysmyelinated mutants of the proteolipid protein (Plp) gene. Both mutations affect the same amino acid, histidine36, which is replaced by glutamine in pt and by proline in shp. Phenotypic expression of these two mutations is very different. Paralytic tremor presents a much milder form of dysmyelination than shaking pup. The number of oligodendrocytes in the mutant rabbit is normal, while in the dog, the oligodendrocyte number is reduced due to early death or incomplete maturation. We have previously reported an abnormal intracellular transport of the PLPpt, whereas DM-20pt was normally transported to the cell membrane. In the present study, we show that the transport of the two isoforms containing the shp mutation is impaired in transfected Cos-7 cells. Cotransfecting cells with different ratios and combinations of mutated PLP and DM-20 cDNAs, we demonstrated that DM-20pt, but not DM-20shp, facilitates intracellular trafficking and integration into the plasma membrane of either of the two mutated PLPs. The phenotypic difference between these two allelic mutations can result from differences in DM-20 protein trafficking and sorting. These results show that the loss of function of PLP is not position-dependent but depends on the nature of the mutation.

Transplantation of CG4 oligodendrocyte progenitor cells in the myelin-deficient rat brain results in myelination of axons and enhanced oligodendroglial markers

Transplantation of oligodendrocyte (Ol) progenitor cells into the central nervous system is a promising approach for the treatment of myelin disorders. This approach requires providing adequate numbers of healthy cells with myelinating potential. We recently showed the successful transplantation of Ol progenitors into the myelin-deficient (md) rat brain. In the present work, CG4 cells, a cell line with properties of Ol progenitors, were labeled with fast blue and grafted into P3-P5 pups born to carrier mothers. Examination of host brains 2 weeks posttransplant indicated that CG4 cells display a much more extensive migration capacity than their wild-type counterparts. These cells synthesized myelin components. In addition, ultrastructural analysis showed myelin formation along axons of md hosts in various brain regions, including corpus callosum, cerebellum, and brainstem. Furthermore, in situ hybridization studies performed on sagittal sections revealed extensive expression of transferrin-mRNA within the md host parenchyma. The high survival and functional features displayed by CG4 cells after transplantation, together with their striking wide distribution within the host parenchyma, as assessed by the presence of myelinated fibers in mutant hosts, emphasizes the importance of using highly motile and proliferative Ol progenitor cells. Strategies to improve the condition and life span of md rat pups are currently under investigation.

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.

Developmental expression of the murine Mobp gene

In this report we describe the developmental expression of the murine (Mobp) gene encoding myelin-associated oligodendrocytic basic protein. We have characterized three Mobp cDNA clones which have been used as probes. Murine Mobp splice variant-1 (mmsv-1), a portion of 3' untranslated region (UTR), is homologous to 3' UTR sequences found in the rat Mobp splice variants rOP1, Mobp81-A and Mobp-99. The mmsv-2 sequence, encoding 81 amino acids, closely resembles the rat Mobp81-A splice variant. The mmsv-3 cDNA, encoding 170 amino acids corresponding closely to the rat rOPRP1 splice variant, detects a single mRNA species present in low levels from E12 onward, suggesting this MOBP may have a function alternative or additional to involvement in myelin formation. The mmsv-1 probe detects an mRNA species abundantly expressed in the postnatal central nervous system (CNS) but barely detectable at E18. This mRNA is located initially in the cell bodies of oligodendrocytes, moving distally into their processes as myelination proceeds. The most abundant mmsv(s) in the adult CNS are present at detectable levels after expression of the myelin basic protein (Mbp) gene and marginally after or coincident with the proteolipid protein (Plp) gene. The level of the abundant, late-expressed mRNA correlates closely with the capacity to form myelin and the maturity of oligodendrocytes, as shown in two hypomyelinated mutants, rumpshaker and jimpy, which represent mildly and severely affected phenotypes, respectively.

Conservation of topology, but not conformation, of the proteolipid proteins of the myelin sheath

The proteolipid protein gene products DM-20 and PLP are adhesive intrinsic membrane proteins that make up >/=50% of the protein in myelin and serve to stabilize compact myelin sheaths at the extracellular surfaces of apposed membrane lamellae. To identify which domains of DM-20 and PLP are positioned topologically in the extracellular space to participate in adhesion, we engineered N-glycosylation consensus sites into the hydrophilic segments and determined the extent of glycosylation. In addition, we assessed the presence of two translocation stop-transfer signals and, finally, mapped the extracellular and cytoplasmic dispositions of four antibody epitopes. We find that the topologies of DM-20 and PLP are identical, with both proteins possessing four transmembrane domains and N and C termini exposed to the cytoplasm. Consistent with this notion, DM-20 and PLP contain within their N- and C-terminal halves independent stop-transfer signals for insertion into the bilayer of the rough endoplasmic reticulum during de novo synthesis. Surprisingly, the conformation (as opposed to topology) of DM-20 and PLP may differ, which has been inferred from the divergent effects that many missense mutations have on the intracellular trafficking of these two isoforms. The 35 amino acid cytoplasmic peptide in PLP, which distinguishes this protein from DM-20, imparts a sensitivity to mutations in extracellular domains. This peptide may normally function during myelinogenesis to detect conformational changes originating across the bilayer from extracellular PLP interactions in trans and trigger intracellular events such as membrane compaction in the cytoplasmic compartment.

Evidence that some oligodendrocyte progenitors in the developing optic pathway express the plp gene

DM-20, a product of the proteolipid protein (plp) gene, has been demonstrated in the spinal cord of the mouse embryo as early as embryonic day 12 (E12) in certain cells, some of which are identifiable as oligodendrocyte progenitors. The present work uses optic pathways of rat and mouse as well-characterized systems for the study of gliogenesis. plp gene expression was monitored with a combination of reverse transcriptase polymerase chain reaction, in situ hybridization, and immunostaining with antibodies to different PLP peptide sequences, combined with O-2A lineage markers. In tissue sections, hybridizing cells were detected initially in the proximal optic tracts between E18 and birth and thereafter progressively in the chiasm and optic nerves. Small unbranched cells expressing DM-20 but not myelin basic protein (MBP) and probably representing progenitors were detectable by immunostaining in similar locations. With increasing postnatal ages, cells representing maturing oligodendrocytes which co-label for PLP and MBP are present in the optic pathways. In vitro analysis of freshly dissociated cells from premyelinated optic nerve demonstrated that the plp gene is expressed in some O-2A progenitor cells as well as mature oligodendrocytes. We also present evidence that increase in expression of the plp gene along the O-2A lineage differentiation is not progressive but that downregulation at the proligodendroblast (O4+/O1-) stage probably occurs. We suggest that progenitors express the dm-20 isoform while oligodendrocytes express predominantly the plp isoform. Not all progenitors express the plp gene at the times studied, indicating that the presence of DM-20 is either transitory in individual cells or that only a sub-population is involved. The function of DM-20 at this early stage of the oligodendrocyte lineage has yet to be determined.

Myelin mutants: model systems for the study of normal and abnormal myelination

Spontaneous mutations that perturb myelination occur in a range of species including man, and together with engineered mutations have been used to study disease, normal myelination and axon/glial inter-relationships. Only a minority of the currently defined mutations have an apparently simple pathogenesis due to lack of a functional protein. Mutations in the myelin basic protein gene lead to a lack of protein, resulting in changes in the structure of myelin, which can be rescued by transgenic complementation. The pathogenesis of autosomal dominant and X-linked mutations affecting either oligodendrocytes or Schwann cells is more complex. Point mutations may act in a dominant negative manner and gene dosage is clearly linked to phenotypic change. Mutations in regulatory genes, such as those encoding transcription factors, can also disturb myelination by selected cell types. Other less-well studied and unexpected consequences of myelin mutations, such as seizures in mutations affecting genes expressed in Schwann cells and axonal changes associated with dysmyelination, are also considered. With the major developments in gene mapping and cloning it is now relevant to study mutations in a variety of species with the real prospect of defining their molecular basis. Examples are given of unusual, but potentially useful, uncharacterized mutations in dog and bovine.

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

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

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.

Expression of myelin-specific proteins during development of normal and hypomyelinated Paralytic tremor mutant rabbits. I. Studies on the brain homogenates

The paralytic tremor (pt) rabbit is an X-linked recessive mutant characterized by hypomyelination of the CNS. The onset of myelin mutants' neurological symptoms typically occurs about the tenth postnatal day. A partial recovery is often observed; thus, the life-span of affected animals is almost normal and they can breed successfully. Mutants presenting this phenotype were chosen for our study. Because proteins can serve as excellent markers for the myelin formation process, we examined the developmental expression of several important myelin proteins (PLP/DM-20, MBP, CNP, MAG, and MOG) in both pt mutant and control rabbit brain homogenates. Expression of the investigated proteins occurs in rabbits as follows: CNP and MAG are already present at the early postnatal stage; PLP/DM-20 and MBP appear about the 10th postnatal day; MOG, expressed last, has been detected on the 28th postnatal day. Whereas the MBP, CNP, MAG, and MOG content is only slightly reduced in mature pt mutant brain homogenates (80-90% of control values), the amount of PLP corresponds to approximately 30-40% of that present in controls. Expression of all of the examined proteins is substantially retarded in maturing brains, which leads to the conclusion that besides severe PLP deficiency, retardation of oligodendrocyte maturation is another probable feature of pt mutation.

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.

The proteolipid protein gene

Proteolipid protein (PLP) is the major myelin protein of the CNS and is believed to have a structural role in maintaining the intraperiod line of compact myelin. An isoform, DM-20, produced by alternative splicing of exon 3B is expressed earlier than PLP in the CNS and may be involved in glial cell development. DM-20 is also present in myelin-forming and non-myelin-forming Schwann cells, olfactory nerve ensheathing cells, some glial cell lines and cardiac myocytes. Molecular studies suggest the existence of a PLP gene family with sequence similarities between molecules of different species. Such studies also lend credence to the suggestion that PLP and/or DM-20 may function as a membrane pore. Mutations in the PLP gene occur in several animal species and cause severe pleiotropic effects on myelination. In man this presents as Pelizaeus-Merzbacher disease (PMD). The phenotype of such mutants is characterized by dysmyelination with myelin of abnormal periodicity, paucity of mature oligodendrocytes and astrocytosis. Duplication of the PLP gene in transgenic animals or in one form of PMD also results in dysmyelination. X-linked spastic paraplegia (SPG2) is allelic to PMD and is associated with PLP mutations in which the levels of the DM-20 isoform are probably relatively normal. The effects of PLP gene dosage on CNS myelination can be compared in many ways to the variety of phenotypes in the PNS in hereditary neuropathies of the Charcot-Marie-Tooth type in which the peripheral myelin-22 gene is mutated.

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).

Mechanisms of myelin basic protein and proteolipid protein targeting in oligodendrocytes (review)

The segregation of proteins to specific cellular membranes is recognized as a common phenomenon. In oligodendrocytes of the central nervous system, localization of certain proteins to select regions of the plasma membrane gives rise to the myelin membrane. Whilst the fundamental structure and composition of myelin is well understood, less is known of the mechanisms by which the constituent proteins are specifically recruited to those regions of plasma membrane that are forming myelin. The two principal proteins of myelin, the myelin basic protein and proteolipid protein, differ greatly in character and sites of synthesis. The message for myelin basic protein is selectively translocated to the ends of the cell processes, where it is translated on free ribosomes and is incorporated directly into the membrane. Proteolipid protein synthesized at the rough endoplasmic reticulum, processed through the Golgi apparatus, and presumably transported via vesicles to the myelin membrane. This review examines the mechanisms by which these two proteins are targeted to the myelin membrane.

Paralytic tremor (pt) rabbit: a sex-linked mutation affecting proteolipid protein-gene expression

Paralytic tremor (pt) is a neurological sex-linked recessive mutation in rabbits which is characterized by a coarse body tremor and limb paresis. Morphological studies showed that this mutation affects CNS myelination. Although the number of oligodendrocytes is not reduced, myelination is slower, irregular and defective. We have made a biochemical and molecular analysis of 4-wk-old mutant and normal rabbits. The amount of myelin in the mutant represents only approximately 25% of the normal level. Radioimmunoassay for myelin basic protein showed a reduction to approximately 40% in pt whole-brain homogenate but the difference was not significant in purified myelin. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of brain homogenates followed by immunoblotting showed that all major myelin proteins are affected by the pt mutation, although to different degrees. While most of the myelin proteins are reduced to approximately 60-80% of the normal level, an important reduction to approximately 30%, was measured for the proteolipid protein (PLP). In purified myelin, the difference in PLP concentration was significant while the other specific proteins were less affected. A similar reduction in myelin-protein gene expression was detected at the mRNA level. Sex-linked transmission, low concentrations of PLP and its specific mRNA in the CNS indicate that the pt mutation primarily affects the expression of the Plp gene.

Oligodendrocyte development and differentiation in the rumpshaker mutation

The jimpy rumpshaker (jprsh) mutation is an amino acid substitution in exon 4 (Ile186-->Thr) of the proteolipid protein (PLP) gene on the X chromosome. Affected mice show moderate hypomyelination of the central nervous system (CNS) with increased numbers of oligodendrocytes in the white matter of the spinal cord, a feature distinguishing them from other PLP mutations such as jp, in which premature cell death occurs with reduced numbers of oligodendrocytes. Myelin sheaths of jprsh immunostain for myelin basic protein (MBP) and DM-20, but very few contain PLP. This study examines the differentiation of oligodendrocytes cultured from the spinal cords of young mutant and wild type mice using various surface and cytoplasmic antigenic markers to define the stage of development. The majority of oligodendrocytes from mutant mice progress normally to express MBP; approximately 30%, relative to wild type, contain DM-20 at the in vivo age of 16 days, but very few immunostain for PLP or the O10 and O11 markers. The morphology of mutant cells in respect to membrane sheets and processes appears similar to normal. The jprsh oligodendrocyte is, therefore, characterized by a failure to express the markers indicative of the most mature cell; however, it is probably able to achieve a normal period of survival. These data, taken in conjunction with previous results, suggest that the PLP gene has at least two functions; one, probably involving PLP, is concerned with a structural role in normal myelin compaction; the other, perhaps related to DM-20 (or another lower molecular weight proteolipid), is essential for cell survival. The mutation in jprsh at residue 186 suggests that this region, which is common to PLP and DM-20, is not critical for this latter function.

The molecular genetics of myelination: an update

Recent molecular genetic studies have provided new insights into the structure and function of 2 of the major integral membrane proteins of myelin--the proteolipid protein (PLP) and protein zero (P0)--and have uncovered a third such protein--PMP22/gas3. The rumpshaker mouse has been shown to carry a point mutation in the PLP gene that uncouples a deleterious effect on CNS myelin assembly, which these mice exhibit, from oligodendrocyte degeneration and cell death, which they do not. The developmental importance of the P0 protein in PNS myelination has been dramatically demonstrated by the analysis of loss-of-function mutations engineered through the expression of antisense RNA and through the insertional inactivation of the P0 gene by homologous recombination in embryonic stem cells and the generation of P0-deficient mice. The cloned promoter of the P0 gene has been shown to drive quantitative, Schwann cell-specific expression of heterologous genes in transgenic mice. The PMP22/gas3 gene, previously cloned from fibroblast cell lines, has been found to encode an axonally regulated Schwann cell protein that is assembled into PNS myelin. Importantly, this gene appears to be the target of mutations that result in the Trembler alleles in mice, and in Charcot-Marie-Tooth disease Type 1a, the most common inherited peripheral neuropathy in humans.

The taiep rat: a myelin mutant with an associated oligodendrocyte microtubular defect

This report describes a new inherited disorder of myelination in the rat, named taiep, in which failure of normal myelination of the CNS and subsequent demyelination result in a progressive neurological disturbance. At two months of age, myelin is present throughout the spinal cord, but is immature in the fasciculus gracilis and corticospinal tracts despite the presence of abundant oligodendrocytes. By 12 months, myelin has largely been lost in these spinal cord tracts and also in more rostral parts of the CNS, such as the cerebellum and optic nerves. Other funiculi of the spinal cord show a more diffuse lack of myelin. Oligodendrocytes develop a unique cellular abnormality, most obviously in older rats, which is characterized by the accumulation of microtubules throughout their cytoplasm. As the mutant rats age, there is a continued protracted breakdown of myelin throughout the CNS, with evidence suggesting either persistent hypomyelination or attempts at remyelination of affected axons. It is proposed that the microtubular defect in oligodendrocytes results in a disruption of the normal myelination process in certain areas of the CNS of this mutant, and eventually leads to failure of maintenance of the myelin sheath.

Developmental expression of major myelin protein genes in the CNS of X-linked hypomyelinating mutant rumpshaker

Rumpshaker (rsh) is an X-linked mutation causing hypomyelination of the CNS of mice and has recently been identified as an allele of jimpy (jp). The mutation (known as jprsh) differs in several respects from other X-linked myelin mutants, including jp, in that mice have normal longevity, oligodendrocyte numbers are not decreased, and cell death is not a feature. Myelin sheaths are deficient in immunostainable PLP protein. The present study examines the developmental expression of the major myelin protein genes and translatability of PLP and MBP mRNA. Differences between the spinal cord and brain of mutants are evident in that mRNA levels are more markedly decreased in the brain. Protein levels are severely reduced in both locations and to a proportionately greater extent than the mRNA, particularly in the spinal cord where PLP RNA and protein are approximately 80% and 10-20%, respectively, of age-matched wild type mice. DM-20 protein, the other major product of the PLP gene, is disproportionately expressed in rumpshaker as is a 10 kDa proteolipid. In vitro translation studies indicate a marked decrease in PLP translation products from mutant RNA. There is no deficiency in the number of PLP mRNA-expressing oligodendrocytes although the abundance per cell is reduced. The data suggest that the phenotypic effects of the mutation may be associated with reduced translation of major myelin proteins, in particular PLP and its incorporation into compact myelin. However, the mutation is compatible with survival of oligodendrocytes and their differentiation to the stage of expressing PLP/DM-20 mRNA.

Rumpshaker: an X-linked mutation causing hypomyelination: developmental differences in myelination and glial cells between the optic nerve and spinal cord

The X-linked mutation rumpshaker (rsh), which is probably an allele of jimpy (jp), causes hypomyelination in the CNS of mice. This study examines the developmental expression of the morphology, glial cells, and immunostaining of myelin proteins in the optic nerve and spinal cord. The optic nerve contains varying numbers of amyelinated and myelinated fibres. The majority of such sheaths are of normal thickness whereas in the spinal cord most axons are associated with a disproportionately thin sheath which changes little in thickness during development. In the optic nerve glial cell numbers are elevated in mutants during early and peak myelination but then fall slightly below normal in adults. In contrast, the number of glial cells is consistently elevated after 16 days of age in the spinal cord. The majority of the alterations to total glial cells are due to corresponding changes in the oligodendrocyte population. Immunostaining intensity is somewhat reduced for myelin basic protein (MBP) and the C-terminal common to proteolipid protein (PLP) and DM-20 and profoundly decreased for the PLP-specific peptide. Glial fibrillary acidic protein (GFAP) is increased in rsh. It is probable that some of the variation in myelination between optic nerve and cord in rsh is related to the difference in axon diameter in the two locations, as there are adequate numbers of oligodendrocytes at the time of myelination. However, the effect of the mutation on cell development in the brain and the spinal cord may be different. The immunostaining indicates a marked deficiency in PLP in myelin but suggests that DM-20 levels may be relatively normal. rsh shows several major differences from jp and other X-linked myelin mutants, particularly in relation to oligodendrocyte numbers, and will be useful to elucidate the role of the PLP gene in influencing oligodendrocyte differentiation and survival.