Hypomyelinated mutant mice: description of jpmsd and comparison with jp and qk on their present genetic backgrounds

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.

Minimal role for activating transcription factor 3 in the oligodendrocyte unfolded protein response in vivo

To further our goal of identifying and characterizing the functions of major components of the unfolded protein response (UPR) in oligodendrocytes, the gene encoding the activator of transcription factor 3 protein (ATF3) has been ablated in mice expressing mutant forms of the Proteolipid protein 1 (Plp1) gene and the phenotype of double mutants characterized at several levels. Mature oligodendrocytes in Plp1 mutant mice undergo UPR-induced cell stress, induce ATF3 expression and exhibit a greater propensity to die by apoptosis, which is consistent with pro-death function of ATF3 proposed from in vitro studies. However, we find that the absence of ATF3 has no effect on the levels of apoptosis in Plp1 mutants. Furthermore, we find that oligodendrocyte function appears normal in Atf3(-/-) mice and that motor coordination and neural communication are similarly unaffected. Accordingly, we conclude that ATF3, at best, plays a minor role in UPR signaling and its expression is more likely induced by the UPR as a secondary event in oligodendrocytes that is unrelated to cell death.

Minimal role for caspase 12 in the unfolded protein response in oligodendrocytes in vivo

The unfolded protein response (UPR) is implicated in many neurodegenerative disorders including Alzheimer, Parkinson and prion diseases, and the leukodystrophy, Pelizaeus-Merzbacher disease (PMD). Critical features of degeneration in several of these diseases involve activation of cell death pathways in various neural cell populations, and the initiator caspase 12 has been proposed to play a central role. Accordingly, pharmacological strategies to inhibit caspase 12 activity have received remarkable attention in anticipation of effecting disease amelioration. Our investigation in animal models of PMD demonstrates that caspase 12 is activated following accumulation of mutant proteins in oligodendrocytes; however, eliminating caspase 12 activity does not alter pathophysiology with respect to levels of apoptosis, oligodendrocyte function, disease severity or life span. We conclude that caspase 12 activation by UPR signaling is an epiphenomenon that plays little discernable role in the loss of oligodendrocytes in vivo and may portend the inconsequence of caspase 12 to the pathophysiology of other protein conformational diseases.

Developmental abnormalities of myelin basic protein expression in fyn knock-out brain reveal a role of Fyn in posttranscriptional regulation

Fyn protein-tyrosine kinase (PTK), a member of the Src-PTK family, is essential for myelin development in the central nervous system (CNS). The absence of Fyn activity results in defects in the morphogenesis of oligodendrocyte precursors (OPCs) and CNS hypomyelination. However, molecular mechanisms for Fyn to control CNS myelinogenesis remain elusive. Here we show that Fyn-PTK is significantly up-regulated in early OPC differentiation, concentrated in the compact myelin, and declines during myelin development. Despite the high levels of Fyn-PTK expression during early OPC differentiation, Fyn deficiency does not affect the expression of mRNAs that encode myelin structural proteins, including that for the myelin basic protein (MBP), until postnatal day 13 (P13). However, the accumulation rate of MBP mRNA is significantly attenuated during the most active period of myelinogenesis (P13 and P20). Interestingly, the absence of Fyn causes a preferential reduction of the exon-2 containing MBP mRNA isoforms derived from alternative splicing, providing the first evidence that Fyn is required for posttranscriptional regulation of MBP. Consistent with this idea, Fyn phosphorylates the selective RNA-binding protein QKI, which likely modulates the activity of QKI in binding and stabilizing the MBP mRNA. Furthermore, Fyn deficiency exerts an opposing influence on MBP isoform patterning in comparison to that by QKI deficiency. These observations collectively suggest that Fyn plays critical roles in promoting accelerated MBP expression during myelinogenesis in a MBP isoform-preferential manner, and QKI may act in the same pathway downstream of Fyn for MBP mRNA homeostasis.

The quakingviable mutation affects qkI mRNA expression specifically in myelin-producing cells of the nervous system

The genetic lesion in the quakingviable (qk(v)) mutant mice is a deletion 5' to the qkI gene, resulting in severe hypomyelination. qkI produces several QKI protein isoforms via alternative splicing of the C-terminal coding exons. In the qk(v)/qk(v) brain, immunostaining of QKI proteins is diminished in an isoform-differential manner with undefined mechanisms. We examined the expression of QKI protein isoforms and qkI mRNA isoforms in the qk(v)/qk(v) mutants and the non-phenotypic wt/qk(v) littermates. Our results indicated significant reduction of all qkI mRNA isoforms in the central and peripheral nervous system during active myelination without detectable post-transcriptional abnormalities. In the early stage of myelin development, qkI mRNAs are differentially reduced, which appeared to be responsible for the reduction of the corresponding QKI protein isoforms. The reduced qkI expression was a specific consequence of the qk(v) lesion, not observed in other hypomyelination mutants. Further more, no abnormal qkI expression was found in testis, heart and astroglia of the qk(v)/qk(v) mice, suggesting that the reduction of qkI mRNAs occurred specifically in myelin-producing cells of the nervous system. These observations suggest that diminished qkI expression results from deletion of an enhancer that promotes qkI transcription specifically in myelinating glia during active myelinogenesis.

Abnormal postnatal ontogeny of the locus coeruleus in the epileptic mutant mouse quaking

The tonic-clonic convulsions of the quaking mutant mice have been shown to be associated with the hyperplasia of the nucleus locus coeruleus, the origin of most brain noradrenergic neurons. In the present study, the postnatal ontogeny of the locus coeruleus has been studied by tyrosine hydroxylase immunolabeling in the mutant mice quaking and their controls at postnatal days 1, 30 and 90. In the control mice, the number of immunoreactive neuronal cell bodies increased significantly in the rostral half of the locus coeruleus between birth and postnatal day 30, while it decreased significantly in the caudal half between birth and adulthood. Thus, during postnatal maturation, the distribution of locus coeruleus neurons was shifted in the rostral direction. In the quaking mutant mice, while the increase of immunolabeling between birth and postnatal day 30 was observed in the rostral half of the locus coeruleus, no diminution could be found in the caudal half between birth and adulthood. As a result, the rostral shift of tyrosine hydroxylase immunoreactivity was not observed. Consequently, in adult mice, the caudal part of the mutants locus coeruleus appeared to contain significantly more neurons than the corresponding region in the controls. These results indicate that the hyperplasia of the locus coeruleus of the quaking mice that we had previously reported results from an alteration of the postnatal maturation of this nucleus. This developmental abnormality might be a primary determinant of the inherited epilepsy of the quaking mutant mice.

Rumpshaker behaves like juvenile-lethal Plp mutations when combined with shiverer in double mutant mice

The phenotypes of double mutant mice whose genomes are homozygous for an Mbp (myelin basic protein) mutation and hemizygous for a juvenile-lethal Plp (proteolipid protein) mutation were compared in earlier studies. The results suggested that the shiverer Mpb mutation might have some unexplained ability to partially rescue oligodendrocytes (OLs) from the 'death sentence' that is imposed by the Plp mutations. Conversely, they also indicated that the juvenile-lethal Plp mutations may normalize shiverer OL morphology by reducing the numbers of microprocesses. The Plp mutation rumpshaker produces a mild hypomyelination without reduction in OL numbers and a normal lifespan. This report describes double mutant mice combining two Mbp mutations with rumpshaker, utilizing a common B6C3F1 hybrid-based genetic background. Initial studies on B6C3F1 rumpshaker optic nerve and spinal cord white matter showed unanticipated signs of OL death, with morphologic criteria suggestive of an apoptotic mechanism. In shiverer*rumpshaker double mutant mice, this small class of dying cells could not be identified. White matter morphology was similar to that of mice expressing only the shiverer mutation, except that OL microprocesses were far less abundant. This evidence suggests that, despite their distinctive phenotypic differences, rumpshaker may share more characteristics with the juvenile-lethal Plp mutations than has previously been recognized.

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.

Evidence that CNS hypomyelination does not cause death of jimpy-msd mutant mice

Mice expressing three of the proteolipid protein (Plp) mutations in the mouse (jimpy, jimpy-msd, and jimpy-4J) all have a severe deficiency of CNS myelin and oligodendrocytes (OLs), and die sometime in their 4th postnatal week. The prevailing view has been that the animals' shortened life span and lack of myelin are causally related. Here we describe the survival of jimpy-msd males for as long as postnatal day (P) 210. Although these spontaneously occurring longer-lived jimpy-msd males show a 2- to 8-fold increase in numbers of myelinated axons in many CNS regions, this does not protect them from a later but still premature death. Investigating the cause of premature death may reveal previously undiscovered properties of the myelin genes or the cells that express them, or perhaps additional unsuspected cellular responses that contribute to the disease. This study identifies small accumulations of inflammatory cells in the brain parenchyma of jimpy-msd mice as young as P14 and as old as P60, suggesting that the pathology of the disease produced by at least this Plp mutation may be far more complex than has been previously recognized.

Quaking*shiverer double-mutant mice survive for at least 100 days with no CNS myelin

Mice expressing mutations that produce CNS hypomyelination often die prematurely: the more severe the hypomyelination, the shorter the life span. However, we have previously described jimpy-msd mice that survive twice as long as usual; although they acquire significantly increased amounts of myelin, they still succumb long before their unaffected littermates. This result contradicts any postulated causal relationship between extent of CNS hypomyelination and premature death of the animal. Here we have addressed this question in another way, by using an animal model that does not involve a proteolipid protein (Plp) gene mutation. We demonstrate that quaking*shiverer double-mutant mice can survive for at least 100 days without any CNS myelin whatsoever. Therefore, at least for a mouse, absence of CNS myelin is not lethal per se.

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.

Disrupted proteolipid protein trafficking results in oligodendrocyte apoptosis in an animal model of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a dysmyelinating disease resulting from mutations, deletions, or duplications of the proteolipid protein (PLP) gene. Distinguishing features of PMD include pleiotropy and a range of disease severities among patients. Previously, we demonstrated that, when expressed in transfected fibroblasts, many naturally occurring mutant PLP alleles encode proteins that accumulate in the endoplasmic reticulum and are not transported to the cell surface. In the present communication, we show that oligodendrocytes in an animal model of PMD, the msd mouse, accumulate Plp gene products in the perinuclear region and are unable to transport them to the cell surface. Another important aspect of disease in msd mice is oligodendrocyte cell death, which is increased by two- to threefold. We demonstrate in msd mice that this death occurs by apoptosis and show that at the time oligodendrocytes die, they have differentiated, extended processes that frequently contact axons and are expressing myelin structural proteins. Finally, we define a hypothesis that accounts for pathogenesis in most PMD patients and animal models of this disease and, moreover, can be used to develop potential therapeutic strategies for ameliorating the disease phenotype.

A cellular mechanism governing the severity of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a leukodystrophy linked to the proteolipid protein gene (PLP). We report a cellular basis for the distinction between two disease subtypes, classical and connatal, based on protein trafficking of the two PLP gene products (PLP and DM20). Classical PMD mutations correlate with accumulation of PLP in the ER of transfected COS-7 cells while the cognate DM20 traverses the secretory pathway to the cell surface. On the other hand, connatal PMD mutations lead to the accumulation of both mutant PLP and DM20 proteins in the ER of COS-7 cells with little of either isoform transported to the cell surface. Moreover, we show that transport-competent mutant DM20s facilitate trafficking of cognate PLPs and hence may influence disease severity.

Expression of members of the proteolipid protein gene family in the developing murine central nervous system

Two homologous cDNAs were previously isolated by expression cloning with a monoclonal antibody that recognized a CNS neuronal membrane protein. Both cDNAs, M6a and M6b, bore significant homology with the major myelin proteolipid protein, PLP/DM20. Our initial studies of M6 gene expression in the adult mouse brain showed that M6a was present in neurons, PLP/DM20 in oligodendrocytes, and M6b in both neurons and glia. This led to the recognition of a novel gene family that included the oligodendrocyte-specific PLP/DM20 gene and the neuronal M6 genes. These observations supported the idea that PLP/DM20 may have functions other than myelination. In this report, we describe the spatial and temporal patterns of expression of M6a, M6b, and PLP/DM20 in the developing nervous system. PLP expression was limited to the white matter. M6a appeared in post-mitotic neurons of the brain and spinal cord as early as E10, and later in the hippocampus, cerebral cortex, and the granule cells of the cerebellum. In contrast, M6b was expressed at early embryonic stages in the ventricular zone of the spinal cord, and later during development in both neurons and glia. The early appearance of M6a and M6b mRNAs in the murine CNS suggested that these molecules might play an important role in the development of a variety of neural cell types.

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.

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)

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).

Many naturally occurring mutations of myelin proteolipid protein impair its intracellular transport

The primary structure of the proteolipid protein (PLP) from the central nervous system (CNS) myelin of mammals is highly conserved with only three amino acid differences between the mouse, rat, dog, bovine, and human proteins. Furthermore, within a particular species no polymorphisms in the protein have been identified. Recent interest has focused on the targeting of PLP in oligodendrocytes and the role that mutant forms of this protein play in generating dysmyelinating or hypomyelinating diseases. We previously expressed the human cDNA encoding PLP in transiently transfected Cos-7 cells and characterized the subcellular distribution of the protein in this simple heterologous system. In the current study we have used the same paradigm to examine the effect of five missense mutations in the PLP gene on processing of the encoded protein. The mutations chosen span the carboxy-terminal half of PLP and encompass that part of the protein in which most mutations have been identified. Our results show that transport of all mutations examined was arrested in the secretory pathway at an early stage, causing the mutant proteins to accumulate in the endoplasmic reticulum. Thus, a common mechanism of protein misfolding and failure of PLP to reach the cell surface of oligodendrocytes rather than the inability of the mutant protein to perform some crucial function at the cell surface may be responsible for the diseases caused by many PLP mutations. Our results, together with those of others, prompt us to speculate that the pathobiology observed in PLP mutants may result from oligodendrocyte cell death caused by the accumulation of misfolded protein in the endoplasmic reticulum. This speculation is consistent with the observations that oligodendrocytes bearing misfolded PLP, as in the jimpy mutant, proliferate but die rapidly while oligodendrocytes from PLP deletion survive and produce a myelin-like membrane which lacks PLP.

Spinal dysmyelination in new-born brown Swiss x Braunvieh calves

In four new-born Braunvieh calves suffering from connate recumbency and body tremor, a hitherto not described myelination disorder of the spinal cord was examined. Bilateral symmetric hypo- as well as demyelination in several spinal tracts were the most conspicuous findings, affecting the ascending gracile funiculus, the ascending dorsolateral spinocerebellar tract, and the mainly descending sulcomarginal tract. Deficient myelin production, loss of myelin, consecutive axonal degenerations, and prominent astrogliosis within these tracts were the histological hallmarks of the disease. This possibly inherited primary myelination disorder of the spinal cord differs markedly from known hereditary neurological diseases in Brown Swiss and Braunvieh cattle, respectively, i.e. the weaver-syndrome and the spinal muscular atrophy.

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

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

Spatial distribution of myelin basic protein mRNA and polypeptide in quaking oligodendrocytes in culture

In the CNS, myelin is formed from the expansion of oligodendrocyte processes. In order to study myelin assembly in the hypomyelinating mutant mouse quaking (qk), cultures of oligodendrocytes were established from affected and control animals. The cytoarchitecture of the oligodendrocytes was analyzed by performing morphometric measurements after immunostaining with antitubulin. The results indicate that the gross morphology of the processes is similar in control and mutant cells. The localization of the message for the myelin structural component, myelin basic protein (MBP), was examined by in situ hybridization. In control oligodendrocytes, 80% of MBP mRNA is found in the processes. In contrast, only 23% of MBP mRNA is localized to these structures in the mutant; the majority of MBP mRNA remains in the cell body. The mutant cells are capable of distributing mRNAs to the periphery as shown by the presence of tubulin mRNA in their processes. MBP polypeptide was visualized by immunofluorescence and found in the perikaryon, processes and membranous expansions of the control cells. In the mutant, it is largely confined to the perikaryon, reflecting the distribution of the mRNA. These results suggest that the localization of MBP polypeptide is achieved by restricting the distribution of its mRNA, and that MBP assembly into the myelin membrane occurs in the processes. This step appears to be blocked in qk oligodendrocytes in culture.

Quaking*jimpy double mutant mice: additional evidence for independence of primary deficits in jimpy

Mice which have the genotype qk/qk*Tajp/Y, and therefore simultaneously express both the quaking (qk) and jimpy (jp) mutations, have CNS white matter morphology intermediate between qk and jp with respect to amount of myelin, myelin structure, and oligodendrocyte number. The level of myelin basic protein in the CNS is also intermediate; however, myelin proteolipid protein (PLP) is virtually absent. Thus in the qk/qk*Tajp/Y double mutant mouse the PLP deficit is as severe as in jp alone but the oligodendrocyte survival deficit (reflected in number and myelin production) of jp alone is rendered less severe. The observation that these two cardinal deficits of the jp mutation can be independently altered in double mutant combinations is consistent with our previous suggestion that the PLP genetic locus may encode at least two independently regulated primary gene functions: a structural protein and signal influencing oligodendrocyte behavior.

Postnatal evolution of the gamma-aminobutyric acid/benzodiazepine receptor complex in a model of inherited epilepsy: the quaking mouse

Binding assays of [3H]muscimol and [3H]-flunitrazepam have been performed on brain homogenates of brainstem, cerebellum, and forebrain of genetically epileptic quaking (qk) mutant mice 20, 40, 70, and 90 days old and their corresponding controls of the same strain (C57BL/6J). The endogenous gamma-aminobutyric acid (GABA) content has been determined in various brain regions of 70-day-old qk and control mice. Finally, the behavioral effects of diazepam, of the mixed GABAA/GABAB receptor agonist progabide, and of the selective GABAB receptor agonist baclofen have been assessed in adult qk mutants. Our results strongly suggest a lack of involvement of GABAergic neurotransmission in the inherited epilepsy of the qk mutant mouse.

Myelination in the jimpy mouse in the absence of proteolipid protein

A point mutation in the gene for proteolipid protein (PLP) has been suggested to account for the dysmyelination seen in the jimpy mouse mutant. Despite the absence of PLP, the major integral membrane protein of central nervous system (CNS) myelin, this study shows that there are many scattered myelinated fibers present in the spinal cord of this murine mutant which are immunocytochemically positive for myelin basic protein (MBP), yet negative for PLP. This lack of PLP results in an abnormal compaction of the extracellular leaflets of the myelin sheath and the formation of an abnormal intraperiod line. These results are similar to those seen in another X-linked myelin mutant, the myelin-deficient rat (Duncan et al.: Proc. Natl. Acad. Sci. U.S.A., 84:6287-6291, 1987), and show that a multilamellar membrane can be formed in the absence of its major integral membrane protein.

Quaking shiverer double mutant mice: morphological phenotypes support possible dual actions of the shiverer locus

Mice doubly homozygous for the two different hypomyelination mutations, quaking (qk) and shiverer (shi) or shiverer myelin-deficient (shimld) (abbreviations: qk*shi and qk*shimld), both have much less myelin than either single mutant ancestor, myelin morphology resembling shi or shimld rather than qk, and abundant shi-type oligodendrocytic microprocesses. The qk*shimld double mutant differs from qk*shi only in having small amounts of normal or abnormal major dense line, in keeping with the morphologic difference between the shi and shimld single mutants. By contrast, shi*jp and shimld*jp have clearly different morphological phenotypes; unexpectedly the major dense line is present in the CNS myelin of shi*jp but not shimld*jp. When shi and shimld act alone, their different DNA abnormalities produce similar protein abnormalities. We speculate that the two mutations interact with qk at a different, later step of DNA expression than they interact with jp. In the interaction with qk, the similar proteins produce similar morphologies. In the interaction with jp, the different DNAs are somehow caused to produce protein differences that are reflected in different morphologies. In this study we have observed for the first time a morphological effect of these mutant genes in heterozygous animals. Of particular importance, animals whose genomes combine shi/+ or shimld/+ with qk/qk produce qk-type, compacted myelin but abundant shi-type oligodendrocyte microprocesses. We consider this as evidence that both shi and shimld have two effects: non-production of a normal structural protein, myelin basic protein, and production of an abnormal protein which perturbs the cytogogic function we postulate to be normally exercised by the myelin basic protein gene.

Myelin proteolipid protein gene expression in jimpy and jimpy(msd) mice

Proteolipid protein (PLP) gene expression was studied in the dysmyelinating mouse mutant jimpy(msd) (jpmsd; myelin synthesis deficient) and compared with that in wild-type mice and the allelic mutant, jimpy (jp). Southern analyses of genomic DNA from jpmsd mice revealed no major rearrangements of the PLP gene relative to the wild-type mouse PLP gene. PLP-specific mRNA levels were significantly reduced in these mutant mice, although both the 3.2- and 2.4-kilobase PLP-specific mRNAs were seen. Also, no size differences in either PLP or DM20 mRNAs were found by S1 nuclease assays of brain RNA from either jpmsd or wild-type mice. Both PLP and DM20 protein were detectable at low levels in jpmsd brain homogenates, and these proteins comigrated with PLP and DM20 protein from normal mice. Western analyses showed an altered PLP:DM20 ratio in jpmsd mice relative to wild-type mice; DM20 levels exceeded PLP levels. It is surprising that a similar pattern of expression was seen in normal mice at less than 10 days of age: DM20 protein expression preceding PLP expression. Thus, jpmsd mice are capable of synthesizing normal PLP and DM20 protein; however, the PLP gene defect has affected the normal developmental pattern of expression for these two proteins.

Shiverer jimpy double mutant mice. IV. Five combinations of allelic mutations produce three morphological phenotypes

Mice which simultaneously express mutant genes at both the shi and jp loci (double mutant mice) have phenotypes much more complex than simple addition of individual mutant characteristics. Morphological study of shi jpmsd, shimld jp, and shi/shimld jp, and comparison with previously studied shi jp and shimld jpmsd, shows that 3 classes of central nervous system (CNS) white matter morphology are produced. (1) shi jp shows suppression of most jp characteristics: it is like shi except for more major dense line, and possibly more myelin, than shi alone. No other combination has as much myelin or any major dense line at all. (2) shi jpmsd has qualitative and quantitative characteristics intermediate between the two single mutants. (3) All combinations studied involving shimld have much less myelin than either single mutant. Qualitatively they express most jp locus features but suppress all shi locus features except abnormalities of myelin compaction. The difference between shi and shimld has more influence on the double mutant morphology than the difference between jp and jpmsd. In the 3-mutant combination, shi/shimld jp, the influence of shimld completely overrides that of shi. These morphological phenotypes resist assignment to any hierarchy of normalcy, and their specific features have no simple explanation in presently known molecular biology of the shi and jp locus mutations. They suggest the possibility of multiple copies and multiple primary functions of the messages at these loci.

Expression of myelin basic protein mRNA and polypeptides in mouse oligodendrocytes in culture: differential regulation by genetic and epigenetic factors

We have analyzed the effects of genetic and epigenetic factors on the steady-state levels of myelin basic protein mRNA and polypeptides during development of mouse oligodendrocytes in culture. Oligodendrocytes were characterized by immunofluorescent staining with antibodies for the following markers: galactocerebroside, myelin basic protein, proteolipid protein, myelin-associated glycoprotein and 2',3'-cyclic nucleotide phosphohydrolase. Oligodendrocytes expressing one or more of these markers first appeared at 3 days in culture and increased to a maximum of 1.5 X 10(5) per brain around 6 days, after which the number remained constant up to 31 days. In medium containing fetal calf serum, accumulation of myelin basic protein polypeptides was delayed relative to in vivo in cultures derived from C57BL/6J, BALB/cJ and DBA/2J inbred mice, but not in cultures derived from C3H/HeJ and AKR/J inbred mice. In medium containing serum from other species or in serum substitute, the temporal expression of myelin basic protein polypeptides in cultures from all the inbred strains was contemporaneous with that in brain. Northern hybridization analysis indicated that the steady-state level of myelin basic protein-specific mRNA in all cultures was regulated similarly to in vivo suggesting that the delayed expression of myelin basic protein polypeptides in some cultures was due to translational and/or post-translational regulation. Analysis of myelin basic protein expression in cultures from informative hybrid and recombinant inbred strains indicated that translational or post-translational expression of myelin basic protein requires trans-acting factors, the inducibility of which is controlled by multiple genetic determinants which segregate independently and are expressed additively.(ABSTRACT TRUNCATED AT 250 WORDS)

Aberrant splicing of proteolipid protein mRNA in the dysmyelinating jimpy mutant mouse

cDNA clones encoding proteolipid protein (PLP) were isolated from a mouse brain library and sequenced. We describe two transcripts arising from the PLP locus by alternative splicing: the major one encodes the 277-amino acid PLP protein and the minor one corresponds to the DM-20 protein, a PLP-like protein of 20,000 Mr that shares both amino and carboxyl regions with PLP. These two transcripts lack approximately 70 bases in PLP mRNA from the dysmyelinating jimpy mutant. The deletion spans amino acids 208-232; however, this region is present in the jimpy PLP-encoding gene. We propose that the jimpy mutant suffers a point mutation or the deletion of a few bases in the PLP gene that alters the normal splicing pattern and generates partially deleted PLP transcripts.

Shiverer jimpy double mutant mice. II. Morphological evidence supports reciprocal intergenic suppression

Mice which carry both the shiverer (shi) and the jimpy (jp) mutations have a morphological phenotype with features of each single mutation by itself but in milder form: the number of myelin sheaths is increased relative to jp, the thickness of sheaths and amount of major dense line is increased relative to shi, and the abnormal, lipid-filled cells characteristic of jp are not seen. However, the abnormal bundles of oligodendrocyte microprocesses and errors in the targeting of myelination which characterize shi are not altered by the presence of the jp mutation. This morphological evidence suggests partial reciprocal intergenic suppression in shiverer jimpy double mutant mice and therefore agrees with conclusions based on biochemical data presented by Kerner and Carson (Brain Research, 374 (1986) 45-53).

Glial and axonal development in optic nerve of myelin deficient rat mutant

Development of glial cell lines and axons is reported for the optic nerve of the myelin deficient rat mutant, md, 3-46 days postnatally. In mutants, optic nerves do not increase in area after 16 days of age whereas, in normal rats, they enlarge through 46 days of postnatal life. The density of glial cells, determined in cross-sections, is similar in md and normal littermates through 19 days postnatally. Thereafter, glial densities are greater in the mutant. Nonetheless, total glial counts are reduced in md as compared to the normal, because cross-sectional areas and lengths of mutant nerve 30-46 days after birth are smaller than those of age-matched, normal littermates. Differential counts of glial cells, made by ultrastructural criteria, show that md optic nerves contain abnormal, vacuolated, immature oligodendroglia from the third postnatal day. Furthermore, oligodendrocytes are reduced in number in older mutants; they constitute 1% of optic nerve neuroglia at 46 days. Astrocytic numbers are increased in relative, not in absolute, terms from 19 days, and microglial numbers are greater than normal in the oldest mutants. Reactive microglia, containing large cytoplasmic lipid droplets, constitute 4-8% of the glia of md nerve 19-46 days postnatally. Mean axonal areas are similar in normal rats and mutants at 19 and 43-46 days of age. However, mitochondrial density is greater in md axons 19 days after birth and mean areas of axonal mitochondria are significantly larger in 43-46 day mutants than in age-matched, normal littermates. Additionally, the percent area of axoplasm occupied by mitochondria is increased in md at both 19 and 43-46 days of age. The myelination defect in md appears to be due primarily to an oligodendroglial abnormality which precedes the normal age of onset of myelination. Astrocytic and microglial changes are secondary. Axonal enlargement proceeds normally over 46 days of postnatal life. Overall, the data do not provide definitive support for an axonal basis for the myelination defect, although measurable differences in axonal mitochondria between mutants and normals are demonstrable and qualitative abnormalities do occur in the axons of the mutant.

Altered postnatal ontogeny of alpha 1- and alpha 2-adrenoceptor binding sites in the brain of a convulsive mutant mouse (quaking)

Binding assays of [3H]dihydroalprenolol ([3H]DHA), [3H]prazosin and [3H]clonidine have been performed on whole brain (minus cerebellum) homogenates of the convulsive mutant mice quaking (qk) and the controls of the same strain (C57BL/6J:B6). In 70-day-old mutants (which fully exhibit the qk convulsive phenotype), the binding of [3H]DHA to beta-adrenoceptor binding sites was not different from the controls, whereas the binding capacities of [3H]prazosin and [3H]clonidine to alpha 1-and alpha 2-adrenoceptor sites, respectively, were greatly enhanced. The biphasic ontogenic pattern of alpha 2-adrenoceptors had a greater amplitude in the brain of 30- to 90-day-old mutants than in the corresponding B6 controls. In mutants younger than 30 days or older than 90 days, the number of alpha 2-adrenoceptor sites was not modified. The number of alpha 1-adrenoceptor binding sites was increased in the brain of the mutants, only in animals older than 70 days. In younger mice, the postnatal modulation of alpha 1-adrenoceptor sites was identical to the controls. Regional studies were performed in 70-day-old mice. [3H]clonidine binding was increased in the brainstem of the mutants, and to a lesser extent in the cerebral cortex, while it was slightly diminished in the hypothalamic area. [3H]prazosin binding was also increased in the brainstem of the mutants, and decreased in the olfactory bulbs. Our results suggest that the convulsions of the qk mutants are selectively associated with modifications of alpha- and not beta-adrenoceptor binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased number of locus ceruleus noradrenergic neurons in the convulsive mutant quaking mouse

Noradrenergic cell bodies in the locus ceruleus of the convulsive mutant quaking mouse and the control of the same strain were visualized using histofluorescence and tyrosine hydroxylase-like immunoreactivity. Cell counts performed with the two techniques gave closely similar results within each strain, indicating a 50% increase in the number of noradrenergic neurons in the midportion of the mutants' locus ceruleus when compared to the controls. This result gives histological support to the increased noradrenergic neurotransmission previously described in the brain of this mutant. Thus, the abnormally high activity of the noradrenergic system appears to be a primary effect of the mutation, associated with the convulsions of this animal model of epilepsy.

Myelin basic protein gene expression in quaking, jimpy, and myelin synthesis-deficient mice

Jimpy (jp), myelin synthesis-deficient (jpmsd), and quaking (qk) are mutations which affect myelination to different degrees in the mouse central nervous system (CNS). Total messenger RNA (mRNA) and myelin basic protein (MBP)-specific mRNA from brains of these three mutants have been analyzed by in vitro translation and immunoprecipitation with antibody to MBP. The results indicate that the three mutations do not affect the level of total MBP-specific mRNA in the CNS but do affect the relative proportions of the various MBP-related translation products encoded in vitro. In each case the proportions of 14K and 12K Mr MBP-related translation products are reduced and the proportions of 21.5K, 18.5K, and 17K Mr MBP-related translation products are increased relative to wild type. This effect is most pronounced in jp, less so in jpmsd, and least pronounced in qk animals. The MBP-related polypeptides that accumulate in vivo have also been analyzed in the three mutants by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) followed by immunoblotting with antibody to MBP. The levels of all the major MBP-related polypeptides that accumulate in vivo are reduced in all three mutations. The reduction is most pronounced in jp, less in jpmsd, and least pronounced in qk animals. These results indicate that the jp, jpmsd, and qk mutations exhibit qualitatively similar phenotypic effects on MBP gene expression but the magnitude of the effect is proportional to the extent of hypomyelination in each mutant.

Brain DNA metabolism in myelin deficient mutant jp, jpmsd and qk mice

We studied metabolism of brain DNA in three myelin deficient mutants qk, jp and jpmsd mice. The DNA content, the in vivo incorporation of [14C]thymidine in DNA and the activity of acid DNase in tissues (cerebellum and cerebrum) from normal littermates and affected mice were compared. The results showed that neither the DNA content, the incorporation of [14C]thymidine in DNA nor the activity of acid DNase in brain were altered in qk affected mice. In jpmsd mice, however, the DNA content as well as the incorporation of thymidine in DNA were reduced in both cerebellum and cerebrum, but the activity of acid DNase was reduced in cerebrum only. In jp mice, although the DNA content was reduced in both cerebellum and cerebrum, the incorporation of thymidine in DNA and the activity of acid DNase were reduced in cerebrum only. The data suggest a) that in qk mutants DNA metabolism and hence cell (glial) proliferation is not affected; b) that in jpmsd mutants DNA synthesis, and thus the cell proliferation is reduced in cerebellum as well as in cerebrum of the affected mice and c) that in jp mutants the synthesis of DNA and the cell proliferation is reduced in cerebrum but not in cerebellum.

Hypomyelinated mutant mice. V. Relationship between jp and jpmsd re-examined on identical genetic backgrounds

jp and jpmsd, two allelic mutations in the mouse that sharply reduce the amount of CNS myelin, produce diseases that can be distinguished morphologically only by their severity. This has raised the question of whether the two mutations are truly distinguishable. Since the two mutations have never been maintained on the same genetic background, correct quantitative and morphological comparison have not been possible. We have prepared a B6C3H stock of jp on the same genetic background as the available stock of jpmsd. In this jp stock, behavioral abnormalities, relative proportion of myelinated axons, and major morphological characteristics of the disease in situ are unchanged from the previous jp stock. The jp disease continues to be more severe than that of jpmsd. However, tissue from the new B6C3H stock myelinates better in organotypic culture than previous jp stocks. The increase in myelination is advantageous, not only for accurate comparison of the two alleles but for all culture studies of jp. Strictly comparable strains or stocks should be utilized in any comparative studies of closely related mutations such as jp and jpmsd.

Myelination of jp,jpmsd, and qk axons by normal glia in vitro: ultrastructural and autoradiographic evidence

Normal optic nerve glia were 'injected' into hypomyelinated mutant jp,jpmsd, and qk cerebellum by co-culturing explants in direct physical contact. Quantitative light microscopic studies demonstrated that such glial injection significantly increased the number of myelin profiles counted in cultures, suggesting that axons in all 3 mutants can accept myelination from competent glia when they are made available. In each mutant, the observed increase in myelination was independent of the ages of donor optic nerves and recipient cultures, but absolutely required positioning of the optic nerve so that direct contact occurred with the mutant cerebellar explants. The additional myelin found near the zone of fusion with the optic nerve morphologically resembled normal, not mutant myelin. Autoradiographs made after [3H]thymidine-labeled normal optic nerve was injected into jpmsd cultures showed that labeled cells had colonized the nearby mutant tissue. Labeled cells identified as oligodendrocytes by ultrastructural criteria were found adjacent to myelin segments near the fusion zone, but direct continuity between processes of these oligodendrocytes and myelin sheaths was not demonstrated. The astrocytes and phagocytic cells which were also labeled had no obvious relationship to myelinated axons. These results provide experimental evidence that the primary abnormalities produced by the three mutations jp,jpmsd, and qk are inherent in their glial cells, probably although not definitely in the oligodendrocytes.

Cholesterol ester content and activities of the ester metabolizing enzymes in jpmsd mouse brain

The free and esterified cholesterol content, and the activities of cholesterol esterifying and the ester hydrolyzing enzyme in brain tissue from myelin synthesis-deficient mutant jpmsd mice and normal littermates were determined. Results showed that cholesterol ester content was high in the brain of jpmsd-affected mice and that the esterification of cholesterol in vitro, with or without added fatty acids was also high in the brain tissue of the affected mutant mice. The data suggest that the increase in cholesterol ester concentrations in the brain of jpmsd-affected mutant mice occurs as a result of increased synthesis rather than decreased hydrolysis.

Ultrastructure of the central nervous system in a myelin deficient rat

Myelin deficiency (md) is a new mutant in the Wistar rat caused by an X-linked recessive lethal gene. One-half of the male offspring develop tremor and ataxia at 10-12 days of age and seizures at 16-21 days. Usually, the animals die 24-28 days postnatally unless survival is prolonged by anticonvulsants. Light microscopic examination of the C.N.S. shows a complete lack of myelin. The P.N.S. is normally myelinated, however. Frontal cortex, corpus callosum, optic nerves, cerebellum and spinal cord were studied routinely in affected animals aged 3-46 days. Abnormal males were identified three days after birth by the absence of myelinated axons from the ventral funiculus of the cervical cord. In mutants aged 3-16 days, axons had the usual ultrastructural features but were either entirely non-myelinated or, rarely, were invested by poorly organized, non-compacted, myelin-like loops of membranes, 2 to 4 in number. In mutants aged 17-20 days, axonal swellings appeared. These increased in number with longer survival times and contained large numbers of microtubules, neurofilaments, mitochondria and dense bodies. Normal C.N.S. myelin was not observed at any age. Two types of abnormal glial cell occur in md. The first, present in white matter at three days of age, is an abnormal oligodendrocyte. The cytoplasm contains dilatation of the rough-surfaced endoplasmic reticulum and the nuclear envelope is widened. A second cell-type, conspicuous by 10 days, has an electron-dense nucleus with prominently clumped chromatin and large cytoplasmic lipid droplets. This second cell type is believed to be a microgliacyte. The number of cytologically-normal oligodendrocytes decreases as mutants age while hypertrophied, filament-rich astrocytes occur in increasing numbers. The myelin defect in md C.N.S. is probably due to an abnormality of oligodendrocytes. Axonal alterations are probably secondary. Myelin deficiency resembles the murine mutant, Jimpy (jp), although ultrastructural changes in oligodendrocytes appear to be dis-similar and md, in contrast to jp, contains no normal-appearing C.N.S. myelin.

Noradrenergic neurotransmission in the brain of a convulsive mutant mouse, differences between the cerebral cortex and the brain stem

The Quaking mouse is a genetically determined model of convulsive disorders. We investigated the modulation of noradrenergic neurotransmission through alpha 2-adrenoceptors in the occipital cortex and the brain stem of this mutant. The endogenous levels of noradrenaline were similar in the cerebral cortex of the Quaking mice and their corresponding controls, while a significant increase of endogenous noradrenaline was found in the brain stem of the mutants. The rate of disappearance of noradrenaline in the cerebral cortex and the brain stem after injection of FLA 63 was identical in control and Quaking mice. The calcium-dependent electrically evoked overflow of 3H-noradrenaline from slices of occipital cortex was inhibited by clonidine and enhanced by yohimbine in Quaking as well as in normal mice. The negative feed-back mechanism mediated by presynaptic alpha 2-adrenoceptors operates to a similar extent in both strains of mice. In contrast to the occipital cortex, in the brain stem, the amount of neurotransmitter released by electrical stimulation was significantly increased in Quaking mice when compared with controls. However, in the brain stem, the negative feed-back regulation of noradrenaline release operates to a similar extent in both strains of mice. When the endogenous levels of MOPEG were determined in the brain stem, they were found to be significantly higher in the Quaking mice when compared to the controls. The results suggest that an increase in noradrenergic neurotransmission in the brain stem, rather than in the cerebral cortex, could contribute to the behavioural abnormalities exhibited by the Quaking mice.

Increased axonal proteolysis in myelin-deficient mutant mice

Protein degradation within retinal ganglion cell axons in vitro is 50 to 110 percent faster than normal in mutant mice exhibiting deficiencies of myelin in the central nervous system. Proteolysis is increased proximally and distally within retinal ganglion cell axons of mice carrying the jumpy mutation or its allele, myelin synthesis deficiency, and is increased distally within those axons of quaking mice. The proteolytic defect is axon (neuron)-specific since the rate of protein degradation within glial cells is normal. Increased axonal proteolysis does not bear a simple relation to hypomyelination since shiverer, another mouse mutant deficient in central myelin, displayed normal rates of axonal protein degradation under the same conditions. These observations suggest an abnormal axon-glial interaction in mice with primary glial defects and raise the possibility that the functioning of histologically normal axons (neurons) may be altered in dysmyelinating diseases.

Hypomyelinated mutant mice IV: peripheral myelin in jp msd

This study compares peripheral myelination in a specific subdivision of the sciatic nerve of jp msd and unaffected littermate mice. No significant differences are found in numbers of myelinated and unmyelinated axons, diameters of axons, thickness of myelin sheaths relative to axon diameter, extent of unmyelinated axons segregation by Schwann cell processes, or in the ultrastructure of myelin and Schwann cells. By contrast, jp msd mutant mice show severe CNS hypomyelination. This evidence, that the jp msd mutation affects only oligodendrocytes, distinguishes mutations at this locus from others producing CNS hypomyelination in which PNS myelin is also affected.