CNS myelination and PLP gene dosage

Human myelin proteolipid protein structure and lipid bilayer stacking

The myelin sheath is an essential, multilayered membrane structure that insulates axons, enabling the rapid transmission of nerve impulses. The tetraspan myelin proteolipid protein (PLP) is the most abundant protein of compact myelin in the central nervous system (CNS). The integral membrane protein PLP adheres myelin membranes together and enhances the compaction of myelin, having a fundamental role in myelin stability and axonal support. PLP is linked to severe CNS neuropathies, including inherited Pelizaeus-Merzbacher disease and spastic paraplegia type 2, as well as multiple sclerosis. Nevertheless, the structure, lipid interaction properties, and membrane organization mechanisms of PLP have remained unidentified. We expressed, purified, and structurally characterized human PLP and its shorter isoform DM20. Synchrotron radiation circular dichroism spectroscopy and small-angle X-ray and neutron scattering revealed a dimeric, α-helical conformation for both PLP and DM20 in detergent complexes, and pinpoint structural variations between the isoforms and their influence on protein function. In phosphatidylcholine membranes, reconstituted PLP and DM20 spontaneously induced formation of multilamellar myelin-like membrane assemblies. Cholesterol and sphingomyelin enhanced the membrane organization but were not crucial for membrane stacking. Electron cryomicroscopy, atomic force microscopy, and X-ray diffraction experiments for membrane-embedded PLP/DM20 illustrated effective membrane stacking and ordered organization of membrane assemblies with a repeat distance in line with CNS myelin. Our results shed light on the 3D structure of myelin PLP and DM20, their structure-function differences, as well as fundamental protein-lipid interplay in CNS compact myelin.

IKAP deficiency in an FD mouse model and in oligodendrocyte precursor cells results in downregulation of genes involved in oligodendrocyte differentiation and myelin formation

The splice site mutation in the IKBKAP gene coding for IKAP protein leads to the tissue-specific skipping of exon 20, with concomitant reduction in IKAP protein production. This causes the neurodevelopmental, autosomal-recessive genetic disorder - Familial Dysautonomia (FD). The molecular hallmark of FD is the severe reduction of IKAP protein in the nervous system that is believed to be the main reason for the devastating symptoms of this disease. Our recent studies showed that in the brain of two FD patients, genes linked to oligodendrocyte differentiation and/or myelin formation are significantly downregulated, implicating IKAP in the process of myelination. However, due to the scarcity of FD patient tissues, these results awaited further validation in other models. Recently, two FD mouse models that faithfully recapitulate FD were generated, with two types of mutations resulting in severely low levels of IKAP expression. Here we demonstrate that IKAP deficiency in these FD mouse models affects a similar set of genes as in FD patients' brains. In addition, we identified two new IKAP target genes involved in oligodendrocyte cells differentiation and myelination, further underscoring the essential role of IKAP in this process. We also provide proof that IKAP expression is needed cell-autonomously for the regulation of expression of genes involved in myelin formation since knockdown of IKAP in the Oli-neu oligodendrocyte precursor cell line results in similar deficiencies. Further analyses of these two experimental models will compensate for the lack of human postmortem tissues and will advance our understanding of the role of IKAP in myelination and the disease pathology.

Quantifying the carrier female phenotype in Pelizaeus-Merzbacher disease

PURPOSE

Pelizaeus-Merzbacher disease and spastic paraplegia type 2 are allelic X-linked disorders that principally affect males and are caused by mutations in the proteolipid protein 1 gene. Neurologic symptoms are occasionally observed in carrier females, and anecdotal evidence suggests that these clinical signs are more likely in families with affected males. We analyze 40 pedigrees to determine whether such a link exists.

METHODS

From a chart review of patients from Wayne State University, we categorize patients according to disease severity and type of genetic lesion within the proteolipid protein 1 gene. We then analyze the clinical data using nonparametric t tests and analyses of variance.

RESULTS

Our analyses formally demonstrate the link between mild disease in males and symptoms in carrier female relatives. Conversely, mutations causing severe disease in males rarely cause clinical signs in carrier females. The greatest risk of disease in females is found for nonsense/indel or null mutations. Missense mutations carry moderate risk. The lowest risk, which represents the bulk of families with Pelizaeus-Merzbacher disease, is associated with proteolipid protein 1 gene duplications.

CONCLUSIONS

Effective genetic counseling of Pelizaeus-Merzbacher disease and spastic paraplegia carrier females must include an assessment of disease severity in affected male relatives.

Multiplex ligation-dependent probe amplification for rapid detection of proteolipid protein 1 gene duplications and deletions in affected males and carrier females with Pelizaeus-Merzbacher disease

BACKGROUND

Pelizaeus-Merzbacher disease is a rare X-linked neurodegenerative disorder caused by sequence variations in the proteolipid protein 1 gene (PLP1). PLP1 gene duplications account for approximately 50%-75% of cases and point variations for approximately 15%-20% of cases; deletions and insertions occur infrequently. We used multiplex ligation-dependent probe amplification (MLPA) to detect PLP1 gene alterations, especially gene duplications and deletions.

METHODS

We performed MLPA on 102 samples from individuals with diverse PLP1 gene abnormalities and from controls, including 50 samples previously characterized for the PLP1 gene by quantitative PCR but which were anonymized for prior results and sex.

RESULTS

All males with PLP1 gene duplications (n = 13), 1 male with a triplication, 2 males with whole gene deletions, and all controls (n = 72) were unambiguously assigned to their correct genotype. Of 4 female carriers tested by MLPA and quantitative PCR, 3 were duplication carriers by both methods, and 1 was a triplication carrier by MLPA and a duplication carrier by quantitative PCR. For 1 sample with a partial deletion, MLPA showed exon 3 deleted but PCR showed exons 3 and 4 deleted. Sequence analysis of 2 samples with reduced dosage for exons 3 and 5 revealed point variations overlapping the annealing site for the corresponding MLPA probe. The precision of MLPA analysis was excellent and comparable to or better than quantitative PCR, with CVs of 4.3%-9.8%.

CONCLUSIONS

MLPA is a rapid and reliable method to determine PLP1 gene copies. Samples with partial PLP1 gene dosage alterations require confirmation with a non-MLPA method.

High-resolution identification of chromosomal abnormalities using oligonucleotide arrays containing 116,204 SNPs

Mutation of the human genome ranges from single base-pair changes to whole-chromosome aneuploidy. Karyotyping, fluorescence in situ hybridization, and comparative genome hybridization are currently used to detect chromosome abnormalities of clinical significance. These methods, although powerful, suffer from limitations in speed, ease of use, and resolution, and they do not detect copy-neutral chromosomal aberrations--for example, uniparental disomy (UPD). We have developed a high-throughput approach for assessment of DNA copy-number changes, through use of high-density synthetic oligonucleotide arrays containing 116,204 single-nucleotide polymorphisms, spaced at an average distance of 23.6 kb across the genome. Using this approach, we analyzed samples that failed conventional karyotypic analysis, and we detected amplifications and deletions across a wide range of sizes (1.3-145.9 Mb), identified chromosomes containing anonymous chromatin, and used genotype data to determine the molecular origin of two cases of UPD. Furthermore, our data provided independent confirmation for a case that had been misinterpreted by karyotype analysis. The high resolution of our approach provides more-precise breakpoint mapping, which allows subtle phenotypic heterogeneity to be distinguished at a molecular level. The accurate genotype information provided on these arrays enables the identification of copy-neutral loss-of-heterozygosity events, and the minimal requirement of DNA (250 ng per array) allows rapid analysis of samples without the need for cell culture. This technology overcomes many limitations currently encountered in routine clinical diagnostic laboratories tasked with accurate and rapid diagnosis of chromosomal abnormalities.

Proteolipid plasmolipin: localization in polarized cells, regulated expression and lipid raft association in CNS and PNS myelin

The proteolipid plasmolipin is member of the expanding group of tetraspan (4TM) myelin proteins. Initially, plasmolipin was isolated from kidney plasma membranes, but subsequent northern blot analysis revealed highest expression in the nervous system. To gain more insight into the functional roles of plasmolipin, we have generated a plasmolipin-specific polyclonal antibody. Immunohistochemical staining confirms our previous observation of glial plasmolipin expression and proves plasmolipin localization in the compact myelin of rat peripheral nerve and myelinated tracts of the CNS. Western blot analysis indicates a strong temporal correlation of plasmolipin expression and (re-) myelination in the PNS and CNS. However, following axotomy plasmolipin expression is also recovered in non-regenerating distal nerve stumps. In addition, we detected plasmolipin expression in distinct neuronal subpopulations of the CNS. The observed asymmetric distribution of plasmolipin in compact myelin, as well as in epithelial cells of kidney and stomach, indicates a polarized cellular localization. Therefore, we purified myelin from the CNS and PNS and demonstrated an enrichement of phosphorylated plasmolipin protein in detergent-insoluble lipid raft fractions, suggesting selective targeting of plasmolipin to the myelin membranes. The present data indicate that the proteolipid plasmolipin is a structural component of apical membranes of polarized cells and provides the basis for further functional analysis.

A PLP splicing abnormality is associated with an unusual presentation of PMD

We report that a deletion of 19 base pairs (bp) in intron 3 of the proteolipid protein (PLP/DM20) gene causes a neurological disease characterized by mild developmental delay, followed by progressive decline of acquired motor and cognitive milestones. The clinical features are associated with mild delay in myelination demonstrated by magnetic resonance imaging studies and with ongoing demyelination and axonal loss demonstrated by magnetic resonance spectroscopy. We demonstrate that the purine-rich 19bp element regulates PLP-specific splice site selection in transient transfections of chimeric constructs into cultured oligodendrocytes. Runs of 4 and 5 Gs centered in the 19bp element are critical for efficient PLP-specific splicing. The intronic element is sequence specific in oligodendrocytes and is not a repressor of PLP-specific splicing in nonglial cells. These data support the conclusion that deletion of the 19bp purine-rich region in PLP intron 3 causes a reduction in PLP message and protein, which affects myelin stability and axonal integrity.

Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) can now be defined as an X-linked recessive leukodystrophy that is caused by a mutation in the proteolipid protein (PLP) gene on chromosome Xq22. The most common mutation is gene duplication followed in frequency by missense mutations, insertions, and deletions. The clinical spectrum ranges from severe neonatal cases to relatively benign adult forms and X-linked recessive spastic paraplegia type 2. The lack of PLP is accompanied by deficits in the other myelin proteins of the central nervous system, including myelin basic protein, myelin-associated glycoprotein, and cyclic nucleotide phosphodiesterase. Surprisingly, the total absence of PLP due to gene deletion or a null allele causes a relatively benign form of PMD. Abnormal PLP is thought to impair protein trafficking and to induce apoptosis in oligodendroglia. Immunocytochemistry with specific antibodies reveals the PLP deficiency and insufficient generation of myelin sheaths with the remaining proteins. Both excessive biosynthesis of PLP, as in gene duplications, or conformational change of the protein, as in missense mutations, are detrimental to myelination. Several naturally occurring and transgenic animal models with PLP gene mutations or deletions have contributed to our understanding of dysmyelination in PMD and the general knowledge of myelination and myelin repair.