The myelin proteolipid protein gene modulates apoptosis in neural and non-neural tissues

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.

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.

Protective effect of 4-Phenylbutyrate against proteolipid protein mutation-induced endoplasmic reticulum stress and oligodendroglial cell death

Proteolipid protein (PLP) mutation causes oligodendrocyte degeneration and myelin disorders including Pelizaeus-Merzbacher Disease (PMD). As the pathophysiological mechanisms involved in PMD are poorly known, the development of therapies remains difficult. To elucidate the pathogenic pathways, an immortalized oligodendroglial cell line (158JP) expressing PLP mutation has been generated. Previous investigations revealed that 158JP oligodendrocytes exhibit several abnormalities including aberrant PLP insertion into the plasma membrane, cAMP, plasmalogen and cell cycle deficits. However, further clarifications of abnormal PLP-induced oligodendrocyte degeneration are required in order to identify relevant mechanisms to target for efficient protection against oligodendrocyte death. Because PLP overexpression may lead to its accumulation inside the endoplasmic reticulum (ER) and cause ER-stress, we explored whether ER-stress may pivotally determine 158JP cell survival/death. Viability assays, RT-qPCR, western blot and flow cytometry were combined to compare cell survival, ER-stress and apoptotic markers in 158JP and control (158N) oligodendrocytes. We observed a significant decreased viability/survival of 158JP compared to 158N cells. Consistently, ER-stress markers (BiP, caspase-12) increased in 158JP (+30%) compared to the controls. mRNA and protein ratios of apoptotic modulators (Bax/Bcl2) are higher in 158JP oligodendrocytes which are also more vulnerable than 158N cells to tunicamycin-induced ER-stress. Interestingly, 4-Phenylbutyrate (ER-stress inhibitor), which decreased ER-stress and apoptotic markers in 158JP cells, significantly increased their survival. Our results, which show a direct link between the viability and endogenous levels of ER-stress and apoptotic markers in 158JP cells, also suggest that 4-Phenylbutyrate-based strategy may contribute to develop effective strategies against oligodendrocyte dysfunctions/death and myelin disorders.

Insertion of proteolipid protein into mitochondria but not DM20 regulates metabolism of cells

Proteolipid protein (PLP), besides its adhesive role in myelin, has been postulated to have multiple cellular functions. One well-documented function of PLP is regulation of oligodendrocyte (Olg) apoptosis. In contrast, DM20, an alternatively spliced product of the PLP1/Plp1 gene, has been proposed to have functions that are unique from PLP but these functions have never been elucidated. Here, we compare metabolism of PLP and DM20, and show that oxidative phosphorylation (OxPhos) was significantly decreased in Plp1 but not DM20 or EGFP expressing cells. The reserve OxPhos capacity of Plp1 expressing cells was half of control cells, suggesting that they are very vulnerable to stress. ATP in media of Plp1 expressing cells is significantly increased more than two-fold compared to controls; markers of apoptosis are increased in cells over-expressing Plp1, indicating that abnormal metabolism of PLP is most likely the direct cause leading to Olg apoptosis. We hypothesize that abnormal metabolism, mediated by increased insertion of PLP into mitochondria, underlies demyelination in Pelizaeus-Merzbacher Disease (PMD) and in models of PMD. To understand why PLP and DM20 function differently, we mutated or deleted amino acids located in the PLP-specific region. All these mutations and deletions of the PLP-specific region prevented insertion of PLP into mitochondria. These findings demonstrate that the PLP-specific region is essential for PLP's import into mitochondria, and now offer an explanation for deciphering unique functions of PLP and DM20.

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.

Insertion of proteolipid protein into oligodendrocyte mitochondria regulates extracellular pH and adenosine triphosphate

Proteolipid protein (PLP) and DM20, the most abundant myelin proteins, are coded by the human PLP1 and non-human Plp1 PLP gene. Mutations in the PLP1 gene cause Pelizaeus-Merzbacher disease (PMD) with duplications of the native PLP1 gene accounting for 70% of PLP1 mutations. Humans with PLP1 duplications and mice with extra Plp1 copies have extensive neuronal degeneration. The mechanism that causes neuronal degeneration is unknown. We show that native PLP traffics to mitochondria when the gene is duplicated in mice and in humans. This report is the first demonstration of a specific cellular defect in brains of PMD patients; it validates rodent models as ideal models to study PMD. Insertion of nuclear-encoded mitochondrial proteins requires specific import pathways; we show that specific cysteine motifs, part of the Mia40/Erv1 mitochondrial import pathway, are present in PLP and are required for its insertion into mitochondria. Insertion of native PLP into mitochondria of transfected cells acidifies media, partially due to increased lactate; it also increases adenosine triphosphate (ATP) in the media. The same abnormalities are found in the extracellular space of mouse brains with extra copies of Plp1. These physiological abnormalities are preventable by mutations in PLP cysteine motifs, a hallmark of the Mia40/Erv1 pathway. Increased extracellular ATP and acidosis lead to neuronal degeneration. Our findings may be the mechanism by which microglia are activated and proinflammatory molecules are upregulated in Plp1 transgenic mice (Tatar et al. (2010) ASN Neuro 2:art:e00043). Manipulation of this metabolic pathway may restore normal metabolism and provide therapy for PMD patients.

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.

Increased Plp1 gene expression leads to massive microglial cell activation and inflammation throughout the brain

PMD (Pelizaeus-Merzbacher disease) is a rare neurodegenerative disorder that impairs motor and cognitive functions and is associated with a shortened lifespan. The cause of PMD is mutations of the PLP1 [proteolipid protein 1 gene (human)] gene. Transgenic mice with increased Plp1 [proteolipid protein 1 gene (non-human)] copy number model most aspects of PMD patients with duplications. Hypomyelination and demyelination are believed to cause the neurological abnormalities in mammals with PLP1 duplications. We show, for the first time, intense microglial reactivity throughout the grey and white matter of a transgenic mouse line with increased copy number of the native Plp1 gene. Activated microglia in the white and grey matter of transgenic mice are found as early as postnatal day 7, before myelin commences in normal cerebra. This finding indicates that degeneration of myelin does not cause the microglial response. Microglial numbers are doubled due to in situ proliferation. Compared with the jp (jimpy) mouse, which has much more oligodendrocyte death and hardly any myelin, microglia in the overexpressors show a more dramatic microglial reactivity than jp, especially in the grey matter. Predictably, many classical markers of an inflammatory response, including TNF-α (tumour necrosis factor-α) and IL-6, are significantly up-regulated manyfold. Because inflammation is believed to contribute to axonal degeneration in multiple sclerosis and other neurodegenerative diseases, inflammation in mammals with increased Plp1 gene dosage may also contribute to axonal degeneration described in patients and rodents with PLP1 increased gene dosage.

The multiple roles of myelin protein genes during the development of the oligodendrocyte

It has become clear that the products of several of the earliest identified myelin protein genes perform functions that extend beyond the myelin sheath. Interestingly, these myelin proteins, which comprise proteolipid protein, 2′,3′-cyclic nucleotide 3′-phosphodiesterase and the classic and golli MBPs (myelin basic proteins), play important roles during different stages of oligodendroglial development. These non-myelin-related functions are varied and include roles in the regulation of process outgrowth, migration, RNA transport, oligodendrocyte survival and ion channel modulation. However, despite the wide variety of cellular functions performed by the different myelin genes, the route by which they achieve these many functions seems to converge upon a common mechanism involving Ca²⁺ regulation, cytoskeletal rearrangements and signal transduction. In the present review, the newly emerging functions of these myelin proteins will be described, and these will then be discussed in the context of their contribution to oligodendroglial development.

Neuronal loss in Pelizaeus-Merzbacher disease differs in various mutations of the proteolipid protein 1

Mutations affecting proteolipid protein 1 (PLP1), the major protein in central nervous system myelin, cause the X-linked leukodystrophy Pelizaeus-Merzbacher disease (PMD). We describe the neuropathologic findings in a series of eight male PMD subjects with confirmed PLP1 mutations, including duplications, complete gene deletion, missense and exon-skipping. While PLP1 mutations have effects on oligodendrocytes that result in mutation-specific degrees of dysmyelination, our findings indicate that there are also unexpected effects in the central nervous system resulting in neuronal loss. Although length-dependent axonal degeneration has been described in PLP1 null mutations, there have been no reports on neuronal degeneration in PMD patients. We now demonstrate widespread neuronal loss in PMD. The patterns of neuronal loss appear to be dependent on the mutation type, suggesting selective vulnerability of neuronal populations that depends on the nature of the PLP1 disturbance. Nigral neurons, which were not affected in patients with either null or severe misfolding mutations, and thalamic neurons appear particularly vulnerable in PLP1 duplication and deletion patients, while hippocampal neuronal loss was prominent in a patient with complete PLP1 gene deletion. All subjects showed cerebellar neuronal loss. The patterns of neuronal involvement may explain some clinical findings, such as ataxia, being more prominent in PMD than in other leukodystrophies. While the precise pathogenetic mechanisms are not known, these observations suggest that defective glial functions contribute to neuronal pathology.

Different proteolipid protein mutants exhibit unique metabolic defects

PMD (Pelizaeus–Merzbacher disease), a CNS (central nervous system) disease characterized by shortened lifespan and severe neural dysfunction, is caused by mutations of the PLP1 (X-linked myelin proteolipid protein) gene. The majority of human PLP1 mutations are caused by duplications; almost all others are caused by missense mutations. The cellular events leading to the phenotype are unknown. The same mutations in non-humans make them ideal models to study the mechanisms that cause neurological sequelae. In the present study we show that mice with Plp1 duplications (Plp1tg) have major mitochondrial deficits with a 50% reduction in ATP, a drastically reduced mitochondrial membrane potential and increased numbers of mitochondria. In contrast, the jp (jimpy) mouse with a Plp1 missense mutation exhibits normal mitochondrial function. We show that PLP in the Plp1tg mice and in Plp1-transfected cells is targeted to mitochondria. PLP has motifs permissive for insertion into mitochondria and deletions near its N-terminus prevent its co-localization to mitochondria. These novel data show that Plp1 missense mutations and duplications of the native Plp1 gene initiate uniquely different cellular responses.

Separate proteolipid protein/DM20 enhancers serve different lineages and stages of development

The gene encoding DM20 emerged in cartilaginous fish, descending from a bilaterian ancestor of the M6 proteolipid gene family. Its proteolipid protein (PLP) isoform appeared in amphibians, contains an additional 35 amino acids, and, in the mammalian CNS, is the dominant myelin protein in which it confers an essential neuroprotective function. During development, the DM20 isoform is prominent in a number of tissues, and plp/DM20 transcripts are detected in multiple progenitor populations, including those that continue to express plp/DM20 as they differentiate into myelinating oligodendrocytes. The locus also encodes isoforms with extended leader sequences that accumulate in the cell bodies of several types of neurons. Here, to locate and characterize regulatory sequences controlling the complex plp/DM20 transcription program, putative regulatory sequences, suggested by interspecies conservation, were ligated individually to a minimally promoted eGFPlacZ reporter gene. These constructs were inserted in single copy at a common site adjacent to the hypoxanthine-guanine phosphoribosyltransferase locus in embryonic stem cells and their in vivo expression programs were compared in transgenic mice. Most expressed developmental and cell-specific subprograms accommodated within the known expression phenotype of the endogenous plp/DM20 locus, thus defining multiple components of the combinatorial mechanism controlling its normal temporal and cell-specific program. Along with previously characterized nervous system enhancers, those described here should help expose the content and configuration of elements that are operational in multiple glial and neuronal lineages. The transgenic lines derived here also provide effective markers for multiple stages of glial and neuronal lineage progression.

Glutamate stimulates oligodendrocyte progenitor migration mediated via an alphav integrin/myelin proteolipid protein complex

In the mammalian CNS, oligodendrocyte precursor cells (OPCs) express most neurotransmitter receptors, but their function remains unclear. The current studies suggest a physiological role for glutamate (AMPA and/or kainate) receptors in OPC migration. AMPA stimulated alphav integrin-mediated OPC migration by increasing both the rate of cell movement and the frequency of Ca2+ transients. A protein complex containing the myelin proteolipid protein (PLP) and alphav integrin modulated the AMPA-stimulated migration, and stimulation of OPC AMPA receptors resulted in increased association of the AMPA receptor subunits themselves with the alphav integrin/PLP complex. Thus, after AMPA receptor stimulation, an alphav integrin/PLP/neurotransmitter receptor protein complex forms that reduces binding to the extracellular matrix and enhances OPC migration. To assess the extent to which PLP was involved in the AMPA-stimulated migration, OPCs from the myelin-deficient (MD) rat, which has a PLP gene mutation, were analyzed. OPCs from the MD rat had a normal basal migration rate, but AMPA did not stimulate the migration of these cells, suggesting that the PLP/alphav integrin complex was important for the AMPA-mediated induction. AMPA-induced modulation of OPC migration was abolished by pertussis toxin, although baseline migration was normal. Thus, G-protein-dependent signaling is crucial for AMPA-stimulated migration of OPCs but not for basal OPC migration. Other signaling pathways involved in this AMPA-stimulated OPC migration were also determined. These studies highlight novel signaling determinants of OPC migration and suggest that glutamate could play a pivotal role in regulating integrin-mediated OPC migration.