PLP overexpression perturbs myelin protein composition and myelination in a mouse model of Pelizaeus-Merzbacher disease

Duplication of PLP1, an X‐linked gene encoding the major myelin membrane protein of the human CNS, is the most frequent cause of Pelizaeus‐Merzbacher disease (PMD). Transgenic mice with extra copies of the wild type Plp1 gene, a valid model of PMD, also develop a dysmyelinating phenotype dependant on gene dosage. In this study we have examined the effect of increasing Plp1 gene dosage on levels of PLP/DM20 and on other representative myelin proteins. In cultured oligodendrocytes and early myelinating oligodendrocytes in vivo, increased gene dosage leads to elevated levels of PLP/DM20 in the cell body. During myelination, small increases in Plp1 gene dosage (mice hemizygous for the transgene) elevate the level of PLP/DM20 in oligodendrocyte soma but cause only minimal and transient effects on the protein composition and structure of myelin suggesting that cells can regulate the incorporation of proteins into myelin. However, larger increases in dosage (mice homozygous for the transgene) are not well tolerated, leading to hypomyelination and alteration in the cellular distribution of PLP/DM20. A disproportionate amount of PLP/DM20 is retained in the cell soma, probably in autophagic vacuoles and lysosomes whereas the level in myelin is reduced. Increased Plp1 gene dosage affects other myelin proteins, particularly MBP, which is transitorily reduced in hemizygous mice but consistently and markedly lower in homozygotes in both myelin and naïve or early myelinating oligodendrocytes. Whether the reduced MBP is implicated in the pathogenesis of dysmyelination is yet to be established. © 2006 Wiley‐Liss, Inc.     

Modification of Schwann cell phenotype with Plp transgenes: Evidence that the PLP and DM20 isoproteins are targeted to different cellular domains

The X-linked proteolipid protein (Plp) gene encodes PLP, the major protein of central nervous system myelin, and its alternative RNA splice product, termed DM20. Schwann cells also express the Plp gene but, in contrast to oligodendrocytes, neither protein is incorporated into peripheral myelin. In the present study, we use different transgenes encoding PLP and DM20 to modify the expression of these proteins in myelin-forming Schwann cells of wild-type and jimpy mice. Increasing the level of PLP, either singly or in combination with DM20, leads to the incorporation of PLP into the compacted myelin sheath; however, DM20 always remains restricted to cytoplasmic regions of the Schwann cell. The insertion of PLP into the membrane does not appear to depend on a cooperativity of the two isoproteins. The presence of PLP does not visibly alter the ultrastructure and periodicity of peripheral nervous system (PNS) myelin. The results indicate that the absence of PLP in the peripheral myelin of normal animals most probably reflects the very low amounts of this isoprotein synthesised by Schwann cells. The preferential incorporation of PLP, as opposed to DM20, in peripheral myelin may indicate that a myelin targeting signal is present in the PLP-specific region of the molecule. J. Neurosci. Res. 50:13–22, 1997. © 1997 Wiley-Liss, Inc.     

Neuronal expression of the proteolipid protein gene in the medulla of the mouse

The proteolipid protein (PLP) gene (Plp) encodes the major myelin proteins, PLP and DM20. Expression of Plp occurs predominantly in oligodendrocytes, but evidence is accumulating that this gene is also expressed in neurons. In earlier studies, we demonstrated that myelin‐deficient (MD) rats, which carry a mutation in the Plp gene, exhibit lethal hypoxic ventilatory depression. Furthermore, we found that, in the MD rat, PLP accumulated in neuronal cell bodies in the medulla oblongata. In the current study, we sought to determine which neurons expressed the Plp gene in the medulla oblongata and whether Plp gene expression changed in neurons with maturation. A transgenic mouse expressing the Plp promoter driving expression of enhanced green fluorescent protein (Plp‐EGFP) was used to identify neurons expressing this gene. Plp expression in neurons was confirmed by immunostaining EGFP‐positive cells for NeuN and by in situ hybridization for PLP mRNA. The numbers of neurons expressing Plp‐EGFP and their distribution increased between P5 and P10 in the medulla. Immunostaining for surface receptors and classes of neurons expressing Plp‐EGFP revealed that Plp gene expression in brainstem neurons was restricted to neurons expressing specific ligand‐gated channels and biosynthetic enzymes, including glutamatergic NMDA receptors, GABA_A receptors, and ChAT in defined areas of the medulla. Plp gene expression was rarely found in interneurons expressing GABA and was never found in AMPA receptor‐ or tyrosine hydroxylase‐expressing neurons. Thus, Plp expression in the mouse caudal medulla was found to be developmentally regulated and restricted to specific groups of neurons. © 2009 Wiley‐Liss, Inc.     

Age-related axonal and myelin changes in the<Emphasis Type="Italic"> rumpshaker</Emphasis> mutation of the<Emphasis Type="Italic"> Plp</Emphasis> gene

The PLP1/Plp gene encodes proteolipid protein (PLP) and DM20, the major central nervous system myelin proteins. Mutations in the PLP1/Plp gene cause dysmyelinating disorders in man and animals. The rumpshaker mutation was first identified in mice and later linked to a family diagnosed with neurological deficits akin to spastic paraplegia. The dysmyelination in the young rumpshaker mouse is well characterised. Here we report evidence for an age-related increase in myelin due mainly to the myelination of small axons, many large axons remain dysmyelinated. Levels of PLP/DM20 and myelin basic protein are considerably greater in myelin fractions from older compared with younger mutants. Myelin in sheaths of larger axons remains poorly compacted and may account for levels of 2,3-cyclic nucleotide 3-phosphodiesterase and myelin-associated glycoprotein being elevated over wild type in older mutant mice. A late-onset distal degeneration of the axons of the longest spinal tract, the fasciculus gracilis, is also noted. This is the first report of Wallerian-type degeneration in mice with spontaneous mutations of the Plp gene.The first two authors contributed equally to the study     

Proteolipid protein modulates preservation of peripheral axons and premature death when myelin protein zero is lacking

Protein zero (P0) is the major structural component of peripheral myelin. Lack of this adhesion protein from Schwann cells causes a severe dysmyelinating neuropathy with secondary axonal degeneration in humans with the neuropathy Dejerine‐Sottas syndrome (DSS) and in the corresponding mouse model (P0^null‐mice). In the mammalian CNS, the tetraspan‐membrane protein PLP is the major structural myelin constituent and required for the long‐term preservation of myelinated axons, which fails in hereditary spastic paraplegia (SPG type‐2) and the relevant mouse model (Plp^null‐mice). The Plp‐gene is also expressed in Schwann cells but PLP is of very low abundance in normal peripheral myelin; its function has thus remained enigmatic. Here we show that the abundance of PLP but not of other tetraspan myelin proteins is strongly increased in compact peripheral myelin of P0^null‐mice. To determine the functional relevance of PLP expression in the absence of P0, we generated P0^null*Plp^null‐double‐mutant mice. Compared with either single‐mutant, P0^null*Plp^null‐mice display impaired nerve conduction, reduced motor functions, and premature death. At the morphological level, axonal segments were frequently non‐myelinated but in a one‐to‐one relationship with a hypertrophic Schwann cell. Importantly, axonal numbers were reduced in the vital phrenic nerve of P0^null*Plp^null‐mice. In the absence of P0, thus, PLP also contributes to myelination by Schwann cells and to the preservation of peripheral axons. These data provide a link between the Schwann cell‐dependent support of peripheral axons and the oligodendrocyte‐dependent support of central axons. GLIA 2016;64:155–174     

Maintenance of high proteolipid protein level in adult central nervous system myelin is required to preserve the integrity of myelin and axons

Proteolipid protein (PLP) is the most abundant integral membrane protein in central nervous system (CNS) myelin. Expression of the Plp-gene in oligodendrocytes is not essential for the biosynthesis of myelin membranes but required to prevent axonal pathology. This raises the question whether the exceptionally high level of PLP in myelin is required later in life, or whether high-level PLP expression becomes dispensable once myelin has been assembled. Both models require a better understanding of the turnover of PLP in myelin in vivo. Thus, we generated and characterized a novel line of tamoxifen-inducible Plp-mutant mice that allowed us to determine the rate of PLP turnover after developmental myelination has been completed, and to assess the possible impact of gradually decreasing amounts of PLP for myelin and axonal integrity. We found that 6 months after targeting the Plp-gene the abundance of PLP in CNS myelin was about halved, probably reflecting that myelin is slowly turned over in the adult brain. Importantly, this reduction by 50% was sufficient to cause the entire spectrum of neuropathological changes previously associated with the developmental lack of PLP, including myelin outfoldings, lamellae splittings, and axonal spheroids. In comparison to axonopathy and gliosis, the infiltration of cytotoxic T-cells was temporally delayed, suggesting a corresponding chronology also in the genetic disorders of PLP-deficiency. High-level abundance of PLP in myelin throughout adult life emerges as a requirement for the preservation of white matter integrity.     

PLP/DM20 Expression and turnover in a transgenic mouse model of pelizaeus‐merzbacher disease

The most common cause of Pelizaeus‐Merzbacher (PMD) is due to duplication of the PLP1 gene but it is unclear how increased gene dosage affects PLP turnover and causes dysmyelination. We have studied the dynamics of PLP/DM20 in a transgenic mouse model of PMD with increased gene dosage of the proteolipid protein gene (Plp1). The turnover of PLP/DM20 were investigated using an ex‐vivo brain slice system and cultured oligodendrocytes. Homozygous mice have reduced PLP translation, markedly enhanced PLP degradation, and markedly reduced incorporation of PLP into myelin. Proteasome inhibition (MG132) prevented the enhanced degradation. Numerous autophagic vesicles are present in homozygous transgenic mice that may influence protein dynamics. Surprisingly, promoting autophagy with rapamycin decreases the degradation of nascent PLP suggesting autophagic vacuoles serve as a cellular storage compartment. We suggest that there are multiple subcellular fates of PLP/DM20 when overexpressed: the vast majority being degraded by the proteasome, a proportion sequestered into autophagic vacuoles, probably fused with endolysosomes, and only a small proportion entering the myelin sheath, where its association with lipid rafts is perturbed. Transgenic oligodendrocytes have fewer membrane sheets and this phenotype is improved with siRNA‐mediated knockdown of PLP expression that promotes the formation of MBP+ myelin‐like sheets. This finding suggests that RNAi technology is in principle applicable to improve CNS myelination when compromised by PLP/DM20 overexpression. © 2010 Wiley‐Liss, Inc.     

Conservative amino acid substitution in the myelin proteolipid protein of jimpymsd mice.

The capacity for synthesizing and maintaining a compact myelin sheath is destroyed in a number of inborn errors of myelin metabolism. One class of hypomyelinating mutations, which displays an X-linked pattern of inheritance, is distinguished by marked disturbances in oligodendrocyte differentiation. We have defined the molecular defect in one such mutant that lacks mature oligodendrocytes, the X-linked jimpy myelin synthesis deficient (jpmsd) trait in mice. The structure of the gene encoding the most abundant myelin protein, proteolipid protein (PLP), was determined by mapping and partially sequencing genomic clones from jpmsd and wild-type mice. Jpmsd mice have a single base change in PLP, a C----T transition in exon 6 that would substitute a valine for alanine in both PLP and its alternatively spliced isoform, DM20. The mutation was confirmed by polymerase chain reaction-amplifying exon 6 from genomic DNA and then either sequencing the amplified DNA or directly probing exon 6 with oligonucleotides designed to detect a single base mismatch. The conservative amino acid replacement in PLP/DM20 of jpmsd mice results in a pleiotropic phenotype similar to that observed for the allelic mutation jimpy, in which a splicing defect has radically altered the PLP/DM20 protein. The accelerated turnover of oligodendrocytes in both mouse mutants suggests a function for PLP/DM20 in oligodendrocyte differentiation distinct from the role of these proteolipid proteins as structural components of the myelin sheath.     

His36Pro point-mutated proteolipid protein retained in the endoplasmic reticulum of oligodendrocytes in the shaking pup

The shaking pup (shp) is a canine mutation that affects the myelin protein proteolipid protein (PLP) and its smaller and less abundant isoform, DM20, with proline replacing histidine(36), resulting in a severe myelin deficiency in the central nervous system. We present evidence that the mutation leads to disrupted trafficking of the shp PLP/DM20 within oligodendrocytes. Immunohistochemical studies revealed significantly reduced levels of PLP/DM20 and other major myelin components such as myelin basic protein (MBP), myelin associated glycoprotein (MAG), and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) in shp myelin. The distribution of shp PLP/DM20 proteins were altered and mostly retained in perinuclear cytoplasm and proximal processes, which co-localized with distended rough endoplasmic reticulum (RER) within oligodendrocytes. No abnormal accumulation of MAG, MBP, or CNP in the cell body was found. These results suggest that mutated PLP/DM20 in the shp could be selectively retained in RER, causing disruption of their translocation to the periphery to myelinate axons.     

Myelin proteolipid proteins promote the interaction of oligodendrocytes and axons

Although proteolipid protein (PLP) and its DM20 isoform are the major membrane proteins of CNS myelin, their absence causes surprisingly few developmental defects. In comparison, missense mutations of the X-linked Plp gene cause severe dysmyelination. Previous studies have established roles for PLP/DM20 in the formation of the intraperiod line and in maintaining axonal integrity. We now show that a normal number of oligodendrocytes are present in mice lacking PLP/DM20. However, in heterozygous females, which are natural chimeras for X-linked genes, oligodendrocytes lacking PLP/DM20 are in direct competition with wild-type oligodendrocytes that have a distinct advantage. PLP+ oligodendrocytes and PLP+ myelin sheaths make up the greater majority, and this feature is generalised in the CNS throughout life. Moreover, in the absence of PLP/DM20, a proportion of small-diameter axons fails to myelinate, remaining ensheathed but lacking a compact sheath, or show delayed myelination. These findings suggest that PLP/DM20 is also involved in the early stages of axon-oligodendrocyte interaction and wrapping of the axon.     

Hypomyelinated mutant mice. II. Myelination in vitro

Organotypic cultures of cerebellum from hypomyelinated mutant mice provide a powerful experimental system for studying the cell biology of the mutant diseases. We have examined the extent to which the culture system reproduces the diseases of three well-known mutants,qk,jp^msd, andjp.Quantitation of myelin profiles per sq. mm of section demonstrates that in vitro, as in situ,qk produces the most myelin,jp^msd an intermediate amount, andjp the least. Myelin inqk cultures is unique in being invisible by light microscopy of the living culture. Hypomyelination ofjp may be more severe in vitro than in situ. Cultures ofjp^msd exhibit many of the ultrastructural features of cerebellar abnormalities that occur in situ: degree of hypomyelination, clustering of myelin segments, scarcity of oligodendrocytes, absence of nodes of Ranvier but presence of heminodes, and apparent structural integrity of the myelin sheaths. Correspondence between in vitro and in situ ultrastructure is more difficult to assess forjp, because the available sample ofjp myelin in vitro is too small, and forqk, because the abnormalities observed in situ resemble nonspecific abnormalities of normal myelin in vitro.     

Hypomyelinated mutant mice IV: Peripheral myelin injp

This study compares peripheral myelination in a specific subdivision of the sciatic nerve ofjp^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 axon 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 thejp^msd mutation affects only oligodendrocytes, distinguishes mutations at this locus from others producing CNS hypomyelination in which PNS myelin is also affected.     

Early ultrastructural defects of axons and axon–glia junctions in mice lacking expression of Cnp1

Most axons in the central nervous system (CNS) are surrounded by a multilayered myelin sheath that promotes fast, saltatory conduction of electrical impulses. By insulating the axon, myelin also shields the axoplasm from the extracellular milieu. In the CNS, oligodendrocytes provide support for the long‐term maintenance of myelinated axons, independent of the myelin sheath. Here, we use electron microscopy and morphometric analyses to examine the evolution of axonal and oligodendroglial changes in mice deficient in 2′,3′‐cyclic nucleotide 3′‐phosphodiesterase (CNP) and in mice deficient in both CNP and proteolipid protein (PLP/DM20). We show that CNP is necessary for the formation of a normal inner tongue process of oligodendrocytes that myelinate small diameter axons. We also show that axonal degeneration in Cnp1 null mice is present very early in postnatal life. Importantly, compact myelin formed by transplanted Cnp1 null oligodendrocytes induces the same degenerative changes in shiverer axons that normally are dysmyelinated but structurally intact. Mice deficient in both CNP and PLP develop a more severe axonal phenotype than either single mutant, indicating that the two oligodendroglial proteins serve distinct functions in supporting the myelinated axon. These observations support a model in which the trophic functions of oligodendrocytes serve to offset the physical shielding of axons by myelin membranes. © 2009 Wiley‐Liss, Inc.     

Myelination ofjp,jp andqk axons by normal glia in vitro: Ultrastructural and autoradiographic evidence

Normal optic nerve glia were ‘injected’ into hypomyelinated mutantjp,jp^msd, andqk 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 intojp^msd 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 mutationsjp,jp^msd, andqk are inherent in their glial cells, probably although not definitely in the oligodendrocytes.     

Modulation of rumpshaker phenotype with wild‐type PLP/DM20 suggests several pathogenic mechanisms

The rumpshaker mutation of the murine myelin proteolipid protein 1 (Plp1) gene generates misfolded PLP/DM20 protein, resulting in dysmyelination, increased oligodendrocyte apoptosis, and death prior to P40 when expressed on the C57 BL/6 background. In this study, we used transgenic complementation to normalize the levels of PLP/DM20 in myelin with wild‐type protein to determine whether loss of normal PLP function or gain of toxic function is responsible for dysmyelination in the rumpshaker. Restoring myelin PLP/DM20 levels extended the survival time to at least P60, significantly reduced the density of apoptotic cells, increased myelin volume, and restored normal periodicity of myelin. Biochemical analysis found that several myelin proteins that are reduced in rumpshaker, including MAG, CNP, and SirT2, are markedly elevated at peak myelination (P20) in the rumpshaker transgenic mouse. Myelin basic protein, however, remained low at peak myelination but was restored at P60 when myelin had matured and entered into a maintenance phase. Markers of the unfolded protein response (UPR), BiP and XBP1, remained activated with the introduction of wild‐type PLP. These data demonstrate that restoring wild‐type PLP/DM20 levels in rumpshaker improves the phenotype and the integrity of myelin, but hypomyelination persists and stress pathways remain activated. This suggests that both gain‐ and loss‐of‐function mechanisms are involved in the pathogenesis of the rumpshaker. © 2010 Wiley‐Liss, Inc.     

Trafficking of PLP/DM20 and cAMP signaling in immortalized jimpy oligodendrocytes

The synthesis, transport, and insertion of jimpy proteolipid protein and DM20 were studied in normal (158N) and jimpy (158JP) immortalized oligodendrocyte lines. Four different expression vectors encoding fusion proteins composed of native PLP and DM20 or jimpy PLP or DM20 were linked to enhanced green fluorescent protein (EGFP). All four transfected fusion proteins had similar distributions in the cell bodies and processes of the two cell types. Both normal and jimpy PLP-EGFP and DM20-EGFP were detected in both cell lines as far as 200 microM from the cell body, indicating synthesis and transport of mutated PLP and DM20 toward the plasma membrane. Immunocytochemistry of fixed normal and jimpy cells with the O10 antibody, which recognizes a conformationally sensitive PLP/DM20 epitope, confirmed that normal and jimpy PLP and DM20 were transported to the plasma membrane. Live staining of normal and jimpy cells transiently transfected with the native PLP showed positive staining, indicating PLP was correctly inserted into the membrane of both normal and jimpy oligodendrocytes. However, live staining of normal and jimpy cells transiently transfected with jimpy PLP showed no positive staining, indicating the mutated protein is abnormally inserted into the plasma membrane. Electrophysiological recordings of the resting membrane potential measured in the whole cell mode of the patch-clamp technique showed the absence of a developmentally regulated negative shift in the membrane potential in jimpy cells compared to normal native or immortalized oligodendrocytes. Treatment of 158N cells and native oligodendrocytes with dibutyryl cAMP (dbcAMP) caused morphological and biochemical differentiation, but failed to do so in 158JP cells, suggesting an abnormal signaling pathway in jimpy. The defect in cAMP signaling in jimpy oligodendrocytes was associated with the suppression of increase in mRNA level of the inducible cAMP early repressor (ICER). When the jimpy oligodendrocyte line was transfected with normal PLP or DM20 and exposed to dbcAMP, the cells failed to differentiate. This finding suggests that improper insertion of jimpy protein into the plasma membrane alters the membrane in such a way that certain signaling pathways are permanently altered. The abnormal insertion of jimpy PLP/DM20 into the plasma membrane may be the basis for the lack of cell signaling and abnormal resting potential in jimpy oligodendrocytes.     

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

Mutations of the proteolipid protein (Plp) gene cause a generalized central nervous system (CNS) myelin deficit in Pelizaeus-Merzbacher disease of man and various tremor syndromes in animal models. X-linked spastic paraplegia is also due to Plp gene mutations but has a different clinical profile and more restricted pathology involving specific tracts and regions. We have shown previously that PLP overexpression in mice homozygous for a Plp transgene results in premature arrest of CNS myelination and premature death. Here, we demonstrate that a low-level increase in Plp gene expression in transgenic mice causes significant axonal degeneration and demyelination with predilection for specific tracts. Following normal motor development, aged mice develop progressive myelin loss, axonal swellings with resultant Wallerian degeneration, and marked vacuolation of the neuropil associated with ataxia, tremor, and seizures. The age of onset and severity of the phenotype is a function of Plp gene dosage. The corticospinal tracts, optic nerve, fasciculus gracilis cerebellum, and brainstem are particularly involved. Although oligodendrocyte cell bodies show little abnormality, their inner adaxonal tongue is often abnormal, suggesting a perturbation of the axon/glial interface that may underlie the axonal changes. We conclude that abnormal expression of an oligodendrocyte-specific gene can cause axonal damage, a finding that is relevant to the pathogenesis of PLP-associated disorders and probably to other myelin-related diseases. J. Comp. Neurol. 394:506–519, 1998. © 1998 Wiley-Liss, Inc.     

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. © 1994 Wiley-Liss, Inc.     

Myelination in the absence of galactolipids and proteolipid proteins.

The galactolipids galactocerebroside and sulfatide and the proteolipid protein (PLP) and its splice variant DM20 are the most abundant lipid and protein components of central nervous system myelin. Recent studies have found that mice lacking either the galactolipids or PLP are able to form myelin sheaths with apparently normal periodicity and near normal compaction. Here, we have generated galactolipid/proteolipid double mutants to examine the possibility that these molecules have overlapping functions. We show that the absence of the galactolipids and PLP has pleotropic effects on myelin formation. While oligodendrocytes in the postnatal day 20 galactolipid/proteolipid-deficient mouse are able to elaborate myelin with close to normal intraperiod lines, there is an increased frequency of uncompacted myelin sheaths as well as unmyelinated axons. Moreover, the double mutants display extensive white matter vacuolization of the cerebellum that initiates around postnatal day 16, which correlates with the onset of a severe ataxic phenotype and an increased percentage of apoptotic nuclei in the cerebellar internal granule cell layer. These data indicate that the galactolipids and PLP/DM20 are not required for intraperiod line formation, but they suggest a role for these molecules in mediating myelin compaction and in maintaining the integrity of the cerebellum.