Novel neuronal proteolipid protein isoforms encoded by the human myelin proteolipid protein 1 gene

Characterizing cell type specific transcriptional differences between the living and postmortem human brain

Single-nucleus RNA sequencing (snRNA-seq) is often used to define gene expression patterns characteristic of brain cell types as well as to identify cell type specific gene expression signatures of neurological and mental illnesses in postmortem human brains. As methods to obtain brain tissue from living individuals emerge, it is essential to characterize gene expression differences associated with tissue originating from either living or postmortem subjects using snRNA-seq, and to assess whether and how such differences may impact snRNA-seq studies of brain tissue. To address this, human prefrontal cortex single nuclei gene expression was generated and compared between 31 samples from living individuals and 21 postmortem samples. The same cell types were consistently identified in living and postmortem nuclei, though for each cell type, a large proportion of genes were differentially expressed between samples from postmortem and living individuals. Notably, estimation of cell type proportions by cell type deconvolution of pseudo-bulk data was found to be more accurate in samples from living individuals. To allow for future integration of living and postmortem brain gene expression, a model was developed that quantifies from gene expression data the probability a human brain tissue sample was obtained postmortem. These probabilities are established as a means to statistically account for the gene expression differences between samples from living and postmortem individuals. Together, the results presented here provide a deep characterization of both differences between snRNA-seq derived from samples from living and postmortem individuals, as well as qualify and account for their effect on common analyses performed on this type of data.

Plp1 in the enteric nervous system is preferentially expressed during early postnatal development in mouse as DM20, whose expression appears reliant on an intronic enhancer

Recently, the myelin proteolipid protein gene (Plp1) was shown to be expressed in the glia of the enteric nervous system (ENS) in mouse. However, beyond this, not much is known about its expression in the intestine. To address this matter, we investigated Plp1 expression at the mRNA and protein levels in the intestine of mice at different ages (postnatal days 2, 9, 21, and 88). In this study, we show that Plp1 expression preferentially occurs during early postnatal development, primarily as the DM20 isoform. Western blot analysis indicated that DM20 migrated according to its formula weight when isolated from the intestine. However, mobilities of both PLP and DM20 were faster than expected when procured from the brain. The 6.2hPLP(+)Z/FL transgene, which uses the first half of the human PLP1 gene to drive expression of a lacZ reporter gene, recapitulated the developmental pattern observed with the native gene in the intestine, indicating that it can be used as a proxy for Plp1 gene expression. As such, the relative levels of β-galactosidase (β-gal) activity emanating from the 6.2hPLP(+)Z/FL transgene suggest that Plp1 expression is highest in the duodenum, and decreases successively along the segments, toward the colon. Moreover, removal of the wmN1 enhancer region from the transgene (located within Plp1 intron 1) resulted in a dramatic reduction in both transgene mRNA levels and β-gal activity in the intestine, throughout development, suggesting that this region contains a regulatory element crucial for Plp1 expression. This is consistent with earlier studies in both the central and peripheral nervous systems, indicating that it may be a common (if not universal) means by which Plp1 gene expression is governed.

PLP1-lacZ transgenic mice reveal that splice variants containing "human-specific" exons are relatively minor in comparison to the archetypal transcript and that an upstream regulatory element bolsters expression during early postnatal brain development

Much of what is known about the mechanisms that control the developmental expression of the myelin proteolipid protein gene (PLP1) has been attained through use of transgenic animal models. In this study, we analyzed expression of related transgenes which utilize PLP1 genomic DNA from either human or mouse to drive expression of a lacZ reporter. Human PLP1 (hPLP1) sequence span either the proximal 6.2 or 2.7 kb of 5'-flanking DNA to an internal site in Exon 2, while those from mouse comprise the proximal 2.3 kb of 5'-flanking DNA to an analogous site in Exon 2. Transgenes with hPLP1 sequence were named, in part, to the amount of upstream sequence they have [6.2hPLP(+)Z/FL and 2.7hPLP(+)Z]. The transgene containing mouse sequence is referred to here as mPLP(+)Z, to denote the species origin of PLP1 DNA. Mice which harbor the 6.2hPLP(+)Z/FL transgene were used as a model system to investigate the developmental expression of splice variants that incorporate supplementary exons from what is classically defined as PLP1 intron 1. While expression of the splice variants were detected in brain through RT-PCR analysis, they are present at much lower levels relative to the archetypal (classic) transcript. Additionally, we show that mice which harbor the 6.2hPLP(+)Z/FL transgene demonstrate wide-ranging expression throughout brain at P2, whereas expression of mPLP(+)Z is quite limited at this age. Therefore, we generated new transgenic mouse lines with the 2.7hPLP(+)Z transgene, which contains hPLP1 sequence orthologous to just that in mPLP(+)Z. Of the seven lines analyzed, six showed higher levels of 2.7hPLP(+)Z expression in brain at P21 compared to P2; the other line expressed the transgene, only weakly, at either age. This trend, coupled with the robust expression observed for 6.2hPLP(+)Z/FL at P2, suggests that the distal 3.5 kb of 5'-flanking PLP1 DNA specific to 6.2hPLP(+)Z/FL contains regulatory element(s) important for promoting early postnatal expression in brain.

Mutation of Proteolipid Protein 1 Gene: From Severe Hypomyelinating Leukodystrophy to Inherited Spastic Paraplegia

Pelizaeus–Merzbacher Disease (PMD) is an inherited leukodystrophy affecting the central nervous system (CNS)—a rare disorder that especially concerns males. Its estimated prevalence is 1.45–1.9 per 100,000 individuals in the general population. Patients affected by PMD exhibit a drastic reduction or absence of myelin sheaths in the white matter areas of the CNS. The Proteolipid Protein 1 (PLP1) gene encodes a transmembrane proteolipid protein. PLP1 is the major protein of myelin, and it plays a key role in the compaction, stabilization, and maintenance of myelin sheaths. Its function is predominant in oligodendrocyte development and axonal survival. Mutations in the PLP1 gene cause the development of a wide continuum spectrum of leukopathies from the most severe form of PMD for whom patients exhibit severe CNS hypomyelination to the relatively mild late-onset type 2 spastic paraplegia, leading to the concept of PLP1-related disorders. The genetic diversity and the biochemical complexity, along with other aspects of PMD, are discussed to reveal the obstacles that hinder the development of treatments. This review aims to provide a clinical and mechanistic overview of this spectrum of rare diseases.

Hypomyelinating leukodystrophies - unravelling myelin biology

Hypomyelinating leukodystrophies constitute a subset of genetic white matter disorders characterized by a primary lack of myelin deposition. Most patients with severe hypomyelination present in infancy or early childhood and develop severe neurological deficits, but the clinical presentation can also be mild with onset of symptoms in adolescence or adulthood. MRI can be used to visualize the process of myelination in detail, and MRI pattern recognition can provide a clinical diagnosis in many patients. Next-generation sequencing provides a definitive diagnosis in 80-90% of patients. Genes associated with hypomyelination include those that encode structural myelin proteins but also many that encode proteins involved in RNA translation and some lysosomal proteins. The precise pathomechanisms remain to be elucidated. Improved understanding of the process of myelination, the metabolic axonal support functions of myelin and the proposed contribution of myelin to CNS plasticity provide possible explanations as to why almost all patients with hypomyelination experience slow clinical decline after a long phase of stability. In this Review, we provide an overview of the hypomyelinating leukodystrophies, the advances in our understanding of myelin biology and of the genes involved in these disorders, and the insights these advances have provided into their clinical presentations and evolution.

Structural myelin defects are associated with low axonal ATP levels but rapid recovery from energy deprivation in a mouse model of spastic paraplegia

In several neurodegenerative disorders, axonal pathology may originate from impaired oligodendrocyte-to-axon support of energy substrates. We previously established transgenic mice that allow measuring axonal ATP levels in electrically active optic nerves. Here, we utilize this technique to explore axonal ATP dynamics in the Plp^null/y mouse model of spastic paraplegia. Optic nerves from Plp^null/y mice exhibited lower and more variable basal axonal ATP levels and reduced compound action potential (CAP) amplitudes, providing a missing link between axonal pathology and a role of oligodendrocytes in brain energy metabolism. Surprisingly, when Plp^null/y optic nerves are challenged with transient glucose deprivation, both ATP levels and CAP decline slower, but recover faster upon reperfusion of glucose. Structurally, myelin sheaths display an increased frequency of cytosolic channels comprising glucose and monocarboxylate transporters, possibly facilitating accessibility of energy substrates to the axon. These data imply that complex metabolic alterations of the axon–myelin unit contribute to the phenotype of Plp^null/y mice.

Imaging of ATP dynamics in the optic nerve axons of mice lacking the major myelin protein PLP (a model of spastic paraplegia) reveals complex alterations in the metabolic interaction between oligodendrocytes and axons, associated with structural deficits of myelin.

Transcriptomic Landscape of von Economo Neurons in Human Anterior Cingulate Cortex Revealed by Microdissected-Cell RNA Sequencing

The von Economo neurons (VENs) are specialized large bipolar projection neurons with restricted distribution in the human brain, and they are far more abundant in humans than in non-human primates. However, VEN functions remain elusive due to the difficulty of isolating VENs and dissecting their connections in the brain. Here, we combined laser-capture-microdissection with RNA sequencing to describe the transcriptomic profile of VENs from human anterior cingulate cortex (ACC). Using pyramidal neurons as reference cells, we identified 344 genes with VEN-associated expression differences, including 215 higher and 129 lower expression genes. Functional enrichment and protein–protein interaction network analyses showed that these genes with VEN-associated expression differences are involved in VEN morphogenesis and functions, such as dendrite branching and axon myelination, and many of them are associated with human social-emotional disorders. With the use of in situ hybridization and immunohistochemistry assays, we validated four novel VEN markers (VAT1L, CHST8, LYPD1, and SULF2). Collectively, we generated a full-spectrum expression profile of VENs from human ACC, greatly enlarging the pool of genes with VEN-associated expression differences that can help researchers to understand the role of VENs in normal and disordered human brains.

Multi-Omic Analyses of Growth Cones at Different Developmental Stages Provides Insight into Pathways in Adult Neuroregeneration

Growth cones (GCs) are structures associated with growing neurons. GC membrane expansion, which necessitates protein-lipid interactions, is critical to axonal elongation in development and in adult neuritogenesis. We present a multi-omic analysis that integrates proteomics and lipidomics data for the identification of GC pathways, cell phenotypes, and lipid-protein interactions, with an analytic platform to facilitate the visualization of these data. We combine lipidomic data from GC and adult axonal regeneration following optic nerve crush. Our results reveal significant molecular variability in GCs across developmental ages that aligns with the upregulation and downregulation of lipid metabolic processes and correlates with distinct changes in the lipid composition of GC plasmalemma. We find that these processes also define the transition into a growth-permissive state in the adult central nervous system. The insight derived from these analyses will aid in promoting adult regeneration and functional innervation in devastating neurodegenerative diseases.

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Simultaneous proteomics and lipidomics analyses of developmental growth cones

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Combined multi-omics analyses of regenerating optic nerves and growth cones

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Integrating protein-protein with protein-lipid interactions in growth cones

The wmN1 Enhancer Region of the Mouse Myelin Proteolipid Protein Gene (mPlp1) is Indispensable for Expression of an mPlp1-lacZ Transgene in Both the CNS and PNS

The myelin proteolipid protein gene (PLP1) encodes the most abundant protein in CNS myelin. Expression of the gene must be strictly regulated, as evidenced by human X-linked leukodystrophies resulting from variations in PLP1 copy number, including elevated dosages as well as deletions. Recently, we showed that the wmN1 region in human PLP1 (hPLP1) intron 1 is required to promote high levels of an hPLP1-lacZ transgene in mice, using a Cre-lox approach. The current study tests whether loss of the wmN1 region from a related transgene containing mouse Plp1 (mPlp1) DNA produces similar results. In addition, we investigated the effects of loss of another region (ASE) in mPlp1 intron 1. Previous studies have shown that the ASE is required to promote high levels of mPlp1-lacZ expression by transfection analysis, but had no effect when removed from the native gene in mouse. Whether this is due to compensation by another regulatory element in mPlp1 that was not included in the mPlp1-lacZ constructs, or to differences in methodology, is unclear. Two transgenic mouse lines were generated that harbor mPLP(+)Z/FL. The parental transgene utilizes mPlp1 sequences (proximal 2.3 kb of 5'-flanking DNA to the first 37 bp of exon 2) to drive expression of a lacZ reporter cassette. Here we demonstrate that mPLP(+)Z/FL is expressed in oligodendrocytes, oligodendrocyte precursor cells, olfactory ensheathing cells and neurons in brain, and Schwann cells in sciatic nerve. Loss of the wmN1 region from the parental transgene abolished expression, whereas removal of the ASE had no effect.

The wmN1 enhancer region in intron 1 is required for expression of human PLP1

The myelin proteolipid protein gene (PLP1) encodes the most abundant protein present in myelin from the central nervous system (CNS). Its expression must be tightly controlled as evidenced by mutations that alter PLP1 dosage; both overexpression (elevated PLP1 copy number) and lack thereof (PLP1 deletion) result in X-linked genetic disorders in man. However, not much is known about the mechanisms that govern expression of the human gene. To address this, transgenic mice were generated which utilize human PLP1 (hPLP1) sequences (proximal 6.2 kb of 5'-flanking DNA to the first 38 bp of exon 2) to drive expression of a lacZ reporter cassette. LoxP sites were incorporated around a 1.5-kb section of hPLP1 intron 1 since it contains sequence orthologous to the wmN1 region from mouse which, previously, was shown to augment expression of a minimally-promoted transgene coincident with the active myelination period of CNS development. Eight transgenic lines were generated with the parental, 6.2hPLP(+)Z/FL, transgene. All lines expressed the transgene appropriately in brain as evidenced by staining with X-gal in white matter regions and olfactory bulb. Removal of the "wmN1" region from 6.2hPLP(+)Z/FL with a ubiquitously expressed Cre-driver caused a dramatic reduction in transgene activity. These results demonstrate for the first time that the wmN1 enhancer region: (1) is functional in hPLP1; (2) works in collaboration with its native promoter-not just a basal heterologous promoter; (3) is required for high levels of hPLP1 gene activity; (4) has a broader effect, both spatially and temporally, than originally projected with mPlp1.

Effects of Intron 1 Sequences on Human PLP1 Expression: Implications for PLP1-Related Disorders

Alterations in the myelin proteolipid protein gene ( PLP1) may result in rare X-linked disorders in humans such as Pelizaeus-Merzbacher disease and spastic paraplegia type 2. PLP1 expression must be tightly regulated since null mutations, as well as elevated PLP1 copy number, both lead to disease. Previous studies with Plp1-lacZ transgenic mice have demonstrated that mouse Plp1 ( mPlp1) intron 1 DNA (which accounts for slightly more than half of the gene) is required for the mPlp1 promoter to drive significant levels of reporter gene expression in brain. However not much is known about the mechanisms that control expression of the human PLP1 gene ( hPLP1). Therefore this review will focus on sequences in hPLP1 intron 1 DNA deemed important for hPLP1 gene activity as well as a couple of "human-specific" supplementary exons within the first intron which are utilized to generate novel splice variants, and the potential role that these sequences may play in PLP1-linked disorders.

Time-course of myelination and atrophy on cerebral imaging in 35 patients with PLP1-related disorders

AIM

Brain magnetic resonance imaging (MRI) motor development score (MDS) correlations were used to analyze the natural time-course of hypomyelinating PLP1-related disorders (Pelizaeus-Merzbacher disease [PMD] and spastic paraplegia type 2).

METHOD

Thirty-five male patients (ranging from 0.7-43.5y at the first MRI) with PLP1-related disorder were prospectively followed over 7 years. Patients were classified according to best motor function acquired before 5 years (MDS) into five categories (from PMD0 without motor acquisition to PMD4 with autonomous walking). We determined myelination and atrophy scores and measured corpus callosum area, volume of cerebellum, white matter and grey matter on 63 MRI.

RESULTS

Age-adjusted multivariate analysis revealed that patients with PMD0-1 had higher-severity atrophy scores and smaller corpus callosum area than did patients with PMD2 and PMD3-4. Myelination score increased until 12 years. There was evidence that the mean myelination differed in frontal white matter, arcuate fibres, and internal capsules among the groups. Most patients showed worsening atrophy (brain, cerebellum, corpus callosum), whereas grey matter and white matter proportions did not change.

INTERPRETATION

Brain atrophy and myelination of anterior cerebral regions appear to be pertinent biomarkers of motor development. The time-course of inter- and intra-individual cerebral white matter and grey matter atrophy suggests that both oligodendrocytes and neurons are involved in the physiopathology of PLP1-related disorders.

Regulation of DM-20 mRNA expression and intracellular translocation of glutathione-S-transferase pi isoform during oligodendrocyte differentiation in the adult rat spinal cord

We previously demonstrated that NG2-positive oligodendrocyte precursor cells (OPCs) do not express DM-20 mRNA and identified a distinct DM-20 mRNA-positive cell population expressing glutathione-S-transferase pi isoform (GST-pi) in the nucleus (GST-pi(Nuc)) of the adult rat spinal cord. As GST-pi intranuclear localization correlates with progenitor cell properties, we examined the differentiation status of this cell population under the intensive 5-bromo-2'-deoxyuridine (BrdU) administration method, consisting of intraperitoneal BrdU injections every 2 h for 48 h. We observed that a certain population of proliferating/proliferated cells expressed DM-20 mRNA, and sometimes two proliferating/proliferated cells were observed still attached to each other. We performed triple staining for BrdU, DM-20 mRNA, and NG2 and found pairs of neighboring BrdU-positive cells, which were considered to originate from the same progenitor cells and where both cells expressed DM-20 mRNA. Triple staining for BrdU, DM-20 mRNA, and GST-pi detected proliferating/proliferated cells exhibiting the GST-pi(Nuc)/DM-20 mRNA-positive expression pattern. These findings suggested the presence of a GST-pi(Nuc)/DM-20 mRNA-positive oligodendrocyte-lineage progenitor cell population in the adult rat spinal cord. However, we did not find any pair of neighboring BrdU-positive cells with this expression pattern. These observations collectively support the idea that GST-pi(Nuc)/DM-20 mRNA-expressing cells are the progeny of NG2-positive OPCs rather than a novel type of oligodendrocyte-lineage progenitor cells and that DM-20 mRNA expression is dynamically regulated during differentiation of OPCs into oligodendrocytes.

The expression of PLP/DM-20 mRNA is restricted to the oligodendrocyte-lineage cells in the adult rat spinal cord

Proteolipid protein (PLP) is the major component of myelin; its gene encodes two major splicing variants: PLP and DM-20. Compared with PLP, DM-20 lacks the amino acids encoded by exon IIIb. The expression of PLP/DM-20 in cells outside the oligodendrocyte-lineage is unclear. To address this issue, we analyzed the detailed expression pattern of PLP/DM-20 mRNA in the adult rat spinal cord by in situ hybridization (ISH) with a cRNA probe complementary to DM-20 mRNA, which has been used to detect both PLP and DM-20 both mRNA. ISH did not label the cells expressing NeuN nor glial fibrillary acidic protein but detected those expressing Olig2, indicating that PLP/DM-20 mRNA are expressed only in oligodendrocyte-lineage cells. This cell population was expected to contain NG2-expressing oligodendrocyte precursor cells (OPCs), because some exhibited the expression of glutathione S-transferase pi isoform in the nucleus. A recent publication showed that OPCs express PLP but not DM-20 mRNA. However, no OPCs were detected. We performed ISH with a cRNA probe that specifically recognizes PLP mRNA to successfully detect some OPCs. Additionally, OPCs were detected by ISH with a cRNA probe complementary to DM-20 mRNA that was digested via alkaline hydrolysis prior to ISH. These findings collectively demonstrate that PLP and DM-20 mRNA expression is restricted to oligodendrocyte-lineage cells, and imply that the undigested cRNA probe complementary to the full-length DM-20 mRNA sequence only recognizes DM-20 mRNA and not the PLP counterpart when applied to ISH without denaturation/digestion methods.

Control of human PLP1 expression through transcriptional regulatory elements and alternatively spliced exons in intron 1

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These authors contributed equally to this work.

Although the myelin proteolipid protein gene (PLP1) encodes the most abundant protein in central nervous system (CNS) myelin, not much is known about the mechanisms that govern expression of the human gene (hPLP1). Much more is known about the processes that regulate Plp1 gene expression in rodents. From studies with Plp1-lacZ transgenic mice, it was determined that the first intron of mouse Plp1 (mPlp1) is required to attain high levels of expression in brain, concurrent with the active myelination period. Other studies have suggested that within mPlp1 intron 1 (>8 kb) lie several regions with enhancer-like activity. To test whether these sequences (and possibly others) in hPLP1 intron 1 are functional, deletion-transfection analysis was performed with hPLP1-lacZ constructs that contain various portions of the intron, or lack it altogether. Results presented here demonstrate the importance of hPLP1 intron 1 in achieving maximal levels of expression in the immortalized oligodendroglial cell line, Oli-neu. Deletion analysis indicates that the intron contains multiple positive regulatory elements which are active in Oli-neu cells. Some of these elements appear to be functionally conserved between human and mouse, while others are not. Furthermore, our studies demonstrate that multiple splice variants can be formed due to inclusion of extra (supplementary) exons from what is classically thought of as hPLP1 intron 1. Thus, splicing of these novel exons (which are not recognized as such in mPlp1 due to lack of conserved splice sites) must utilize factors common to both human and mouse since Oli-neu cells are of mouse origin.

Transcallosal Projections Require Glycoprotein M6-Dependent Neurite Growth and Guidance

The function of mature neurons critically relies on the developmental outgrowth and projection of their cellular processes. It has long been postulated that the neuronal glycoproteins M6a and M6b are involved in axon growth because these four-transmembrane domain-proteins of the proteolipid protein family are highly enriched on growth cones, but in vivo evidence has been lacking. Here, we report that the function of M6 proteins is required for normal axonal extension and guidance in vivo. In mice lacking both M6a and M6b, a severe hypoplasia of axon tracts was manifested. Most strikingly, the corpus callosum was reduced in thickness despite normal densities of cortical projection neurons. In single neuron tracing, many axons appeared shorter and disorganized in the double-mutant cortex, and some of them were even misdirected laterally toward the subcortex. Probst bundles were not observed. Upon culturing, double-mutant cortical and cerebellar neurons displayed impaired neurite outgrowth, indicating a cell-intrinsic function of M6 proteins. A rescue experiment showed that the intracellular loop of M6a is essential for the support of neurite extension. We propose that M6 proteins are required for proper extension and guidance of callosal axons that follow one of the most complex trajectories in the mammalian nervous system.

Fluorescent mRNA labeling through cytoplasmic FISH

RNA in situ hybridization (ISH) has been widely used in cell and developmental biology research to study gene expression. Classical ISH protocols use colorimetric staining approaches, such as the assay with nitro blue tetrazolium/5-bromo-4-chloro-3-indolyl-phosphate (NBT/BCIP), which do not allow the implementation of multiple probe analyses and do not enable investigators to achieve cellular resolution. Here we describe a protocol to determine the presence of target cytoplasmic RNA via cytoplasmic fluorescence ISH (cFISH), an approach that renders possible the visualization of specific RNA strands from the whole tissue down to the cell. This fluorescence technique, adapted here for use in mouse embryos, enables researchers to implement multiple labeling by combining several RNA probes and/or antibodies in immuno-cFISH. Depending on the options chosen, the protocol can be completed within 2 or 3 d.

Developmental activation of the proteolipid protein promoter transgene in neuronal and oligodendroglial cells of neostriatum in mice

Prior studies suggest that non-canonical proteolipid protein (PLP) gene expression occurs during development in non-myelinating neurons as well as myelinating oligodendroglia in mammalian brain. To assess this possibility in neostriatum, a region of uncertain PLP gene expression in neurons, morphological and electrophysiological tools were used to determine phenotypes of cells with activation of a PLP promoter transgene during the early postnatal period in mice. PLP gene expression is evident in both neuronal and oligodendroglial phenotypes in developing neostriatum, a conclusion based on three novel observations: (1) An enhanced green fluorescent protein (EGFP) reporter of PLP promoter activation was localized in two distinct populations of cells, which exhibit collective, developmental differences of morphological and electrophysiological characteristics in accord with neuronal and oligodendroglial phenotypes of neostriatal cells found during the early postnatal period in both transgenic and wild-type mice. (2) The EGFP reporter of PLP promoter activation was appropriately positioned to serve as a regulator of PLP gene expression. It colocalized with native PLP proteins in both neuronal and oligodendroglial phenotypes; however, only soma-restricted PLP protein isoforms were found in the neuronal phenotype, while classic and soma-restricted PLP protein isoforms were found in the oligodendroglial phenotype. (3) As shown by EGFP reporter, PLP promoter activation was placed to regulate PLP gene expression in only one neuronal phenotype among the several that constitute neostriatum. It was localized in medium spiny neurons, but not large aspiny neurons. These outcomes have significant implications for the non-canonical functional roles of PLP gene expression in addition to myelinogenesis in mammalian brain, and are consistent with potentially independent pathologic loci in neurons during the course of human mutational disorders of PLP gene expression.

The proteolipid protein promoter drives expression outside of the oligodendrocyte lineage during embryonic and early postnatal development

The proteolipid protein (Plp) gene promoter is responsible for driving expression of one of the major components of myelin – PLP and its splice variant DM-20. Both products are classically thought to express predominantly in oligodendrocytes. However, accumulating evidence suggests Plp expression is more widespread than previously thought. In an attempt to create a mouse model for inducing oligodendrocyte-specific gene deletions, we have generated transgenic mice expressing a Cre recombinase cDNA under control of the mouse Plp promoter. We demonstrate Plp promoter driven Cre expression is restricted predominantly to mature oligodendrocytes of the central nervous system (CNS) at postnatal day 28. However, crosses into the Rosa26^LacZ and mT/mG reporter mouse lines reveal robust and widespread Cre activity in neuronal tissues at E15.5 and E10.5 that is not strictly oligodendrocyte lineage specific. By P28, all CNS tissues examined displayed high levels of reporter gene expression well outside of defined white matter zones. Importantly, our study reinforces the emerging idea that Plp promoter activity is not restricted to the myelinating cell lineage, but rather, has widespread activity both during embryonic and early postnatal development in the CNS. Specificity of the promoter to the oligodendrocyte cell lineage, as shown through the use of a tamoxifen inducible Plp-CreER^t line, occurs only at later postnatal stages. Understanding the temporal shift in Plp driven expression is of consequence when designing experimental models to study oligodendrocyte biology.

Axon-glial interaction in the CNS: what we have learned from mouse models of Pelizaeus-Merzbacher disease

In the central nervous system (CNS) the majority of axons are surrounded by a myelin sheath, which is produced by oligodendrocytes. Myelin is a lipid-rich insulating material that facilitates the rapid conduction of electrical impulses along the myelinated nerve fibre. Proteolipid protein and its isoform DM20 constitute the most abundant protein component of CNS myelin. Mutations in the PLP1 gene encoding these myelin proteins cause Pelizaeus-Merzbacher disease and the related allelic disorder, spastic paraplegia type 2. Animal models of these diseases, particularly models lacking or overexpressing Plp1, have shed light on the interplay between axons and oligodendrocytes, and how one component influences the other.

Ethanol down regulates the expression of myelin proteolipid protein in the rat hippocampus

It is well known that chronic ethanol treatment affects the synthesis of RNA and protein in the brain and the maintenance and function of nervous system. The changes in myelination-related genes are most prominent in human alcoholics. Previously, our cDNA microarray study showed altered Proteolipid protein (PLP), a major protein of central myelin. The present study aimed to gain more understanding of the expression of PLP after chronic ethanol treatment. Male Sprague-Dawley rats were daily treated with ethanol (15% in saline, 3 g/kg, i.p.) or saline for 14 days. Messenger RNAs from hippocampus of each group were subjected to cDNA expression array hybridization to determine the differential gene expressions. Among many ethanol responsive genes, PLP was negatively regulated by ethanol treatment, which is one of the most abundant proteins in the CNS and has an important role in the stabilization of myelin sheath. Using northern blot and immunohistochemical analysis, we showed the change in expression level of PLP mRNA and protein after ethanol treatment. PLP mRNA and protein were decreased in hippocampus of rat with chronic ethanol exposure, suggesting that ethanol may affect the stabilization of myelin sheath through the modulation of PLP expression and induce the pathophysiology of alcoholic brain.