Chemical deacylation reduces the adhesive properties of proteolipid protein and leads to decompaction of the myelin sheath

New insights into butyrylcholinesterase: Pharmaceutical applications, selective inhibitors and multitarget-directed ligands

Butyrylcholinesterase (BChE), also known as pseudocholinesterase and serum cholinesterase, is an isoenzyme of acetylcholinesterase (AChE). It mediates the degradation of acetylcholine, especially under pathological conditions. Proverbial pharmacological applications of BChE, its mutants and modulators consist of combating Alzheimer's disease (AD), influencing multiple sclerosis (MS), addressing cocaine addiction, detoxifying organophosphorus poisoning and reflecting the progression or prognosis of some diseases. Of interest, recent reports have shed light on the relationship between BChE and lipid metabolism. It has also been proved that BChE is going to increase abnormally as a compensator for AChE in the middle and late stages of AD, and BChE inhibitors can alleviate cognitive disorders and positively influence some pathological features in AD model animals, foreboding favorable prospects and potential applications. Herein, the selective BChE inhibitors and BChE-related multitarget-directed ligands published in the last three years were briefly summarized, along with the currently known pharmacological applications of BChE, aiming to grasp the latest research directions. Thereinto, some emerging strategies for designing BChE inhibitors are intriguing, and the modulators based on target combination of histone deacetylase and BChE against AD is unprecedented. Furthermore, the involvement of BChE in the hydrolysis of ghrelin, the inhibition of low-density lipoprotein (LDL) uptake, and the down-regulation of LDL receptor (LDLR) expression suggests its potential to influence lipid metabolism disorders. This compelling prospect likely stimulates further exploration in this promising research direction.

Deficiency of MicroRNA-23-27-24 Clusters Exhibits the Impairment of Myelination in the Central Nervous System

Several microRNAs (miRNAs), including miR-23 and miR-27a have been reportedly involved in regulating myelination in the central nervous system. Although miR-23 and miR-27a form clusters in vivo and the clustered miRNAs are known to perform complementary functions, the role of these miRNA clusters in myelination has not been studied. To investigate the role of miR-23-27-24 clusters in myelination, we generated miR-23-27-24 cluster knockout mice and evaluated myelination in the brain and spinal cord. Our results showed that 10-week-old knockout mice had reduced motor function in the hanging wire test compared to the wild-type mice. At 4 weeks, 10 weeks, and 12 months of age, knockout mice showed reduced myelination compared to wild-type mice. The expression levels of myelin basic protein and myelin proteolipid protein were also significantly lower in the knockout mice compared to the wild-type mice. Although differentiation of oligodendrocyte progenitor cells to oligodendrocytes was not inhibited in the knockout mice, the percentage of oligodendrocytes expressing myelin basic protein was significantly lower in 4-week-old knockout mice than that in wild-type mice. Proteome analysis and western blotting showed increased expression of leucine-zipper-like transcription regulator 1 (LZTR1) and decreased expression of R-RAS and phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) in the knockout mice. In summary, loss of miR-23-27-24 clusters reduces myelination and compromises motor functions in mice. Further, LZTR1, which regulates R-RAS upstream of the ERK1/2 pathway, a signal that promotes myelination, has been identified as a novel target of the miR-23-27-24 cluster in this study.

Focused ion beam-scanning electron microscopy links pathological myelin outfoldings to axonal changes in mice lacking Plp1 or Mag

Healthy myelin sheaths consist of multiple compacted membrane layers closely encasing the underlying axon. The ultrastructure of CNS myelin requires specialized structural myelin proteins, including the transmembrane-tetraspan proteolipid protein (PLP) and the Ig-CAM myelin-associated glycoprotein (MAG). To better understand their functional relevance, we asked to what extent the axon/myelin-units display similar morphological changes if PLP or MAG are lacking. We thus used focused ion beam-scanning electron microscopy (FIB-SEM) to re-investigate axon/myelin-units side-by-side in Plp- and Mag-null mutant mice. By three-dimensional reconstruction and morphometric analyses, pathological myelin outfoldings extend up to 10 μm longitudinally along myelinated axons in both models. More than half of all assessed outfoldings emerge from internodal myelin. Unexpectedly, three-dimensional reconstructions demonstrated that both models displayed complex axonal pathology underneath the myelin outfoldings, including axonal sprouting. Axonal anastomosing was additionally observed in Plp-null mutant mice. Importantly, normal-appearing axon/myelin-units displayed significantly increased axonal diameters in both models according to quantitative assessment of electron micrographs. These results imply that healthy CNS myelin sheaths facilitate normal axonal diameters and shape, a function that is impaired when structural myelin proteins PLP or MAG are lacking.

Pathology of myelinated axons in the PLP-deficient mouse model of spastic paraplegia type 2 revealed by volume imaging using focused ion beam-scanning electron microscopy

Advances in electron microscopy including improved imaging techniques and state-of-the-art detectors facilitate imaging of larger tissue volumes with electron microscopic resolution. In combination with genetic tools for the generation of mouse mutants this allows assessing the three-dimensional (3D) characteristics of pathological features in disease models. Here we revisited the axonal pathology in the central nervous system of a mouse model of spastic paraplegia type 2, the Plp-/Y mouse. Although PLP is a bona fide myelin protein, the major hallmark of the disease in both SPG2 patients and mouse models are axonal swellings comprising accumulations of numerous organelles including mitochondria, gradually leading to irreversible axonal loss. To assess the number and morphology of axonal mitochondria and the overall myelin preservation we evaluated two sample preparation techniques, chemical fixation or high-pressure freezing and freeze substitution, with respect to the objective of 3D visualization. Both methods allowed visualizing distribution and morphological details of axonal mitochondria. In Plp-/Y mice the number of mitochondria is 2-fold increased along the entire axonal length. Mitochondria are also found in the excessive organelle accumulations within axonal swellings. In addition, organelle accumulations were detected within the myelin sheath and the inner tongue. We find that 3D electron microscopy is required for a comprehensive understanding of the size, content and frequency of axonal swellings, the hallmarks of axonal pathology.

Anillin facilitates septin assembly to prevent pathological outfoldings of central nervous system myelin

Myelin serves as an axonal insulator that facilitates rapid nerve conduction along axons. By transmission electron microscopy, a healthy myelin sheath comprises compacted membrane layers spiraling around the cross-sectioned axon. Previously we identified the assembly of septin filaments in the innermost non-compacted myelin layer as one of the latest steps of myelin maturation in the central nervous system (CNS) (Patzig et al., 2016). Here we show that loss of the cytoskeletal adaptor protein anillin (ANLN) from oligodendrocytes disrupts myelin septin assembly, thereby causing the emergence of pathological myelin outfoldings. Since myelin outfoldings are a poorly understood hallmark of myelin disease and brain aging we assessed axon/myelin-units in Anln-mutant mice by focused ion beam-scanning electron microscopy (FIB-SEM); myelin outfoldings were three-dimensionally reconstructed as large sheets of multiple compact membrane layers. We suggest that anillin-dependent assembly of septin filaments scaffolds mature myelin sheaths, facilitating rapid nerve conduction in the healthy CNS.

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.

Three dimensional electron microscopy reveals changing axonal and myelin morphology along normal and partially injured optic nerves

Following injury to the central nervous system, axons and myelin distinct from the initial injury site undergo changes associated with compromised function. Quantifying such changes is important to understanding the pathophysiology of neurotrauma; however, most studies to date used 2 dimensional (D) electron microscopy to analyse single sections, thereby failing to capture changes along individual axons. We used serial block face scanning electron microscopy (SBF SEM) to undertake 3D reconstruction of axons and myelin, analysing optic nerves from normal uninjured female rats and following partial optic nerve transection. Measures of axon and myelin dimensions were generated by examining 2D images at 5 µm intervals along the 100 µm segments. In both normal and injured animals, changes in axonal diameter, myelin thickness, fiber diameter, G-ratio and percentage myelin decompaction were apparent along the lengths of axons to varying degrees. The range of values for axon diameter along individual reconstructed axons in 3D was similar to the range from 2D datasets, encompassing reported variation in axonal diameter attributed to retinal ganglion cell diversity. 3D electron microscopy analyses have provided the means to demonstrate substantial variability in ultrastructure along the length of individual axons and to improve understanding of the pathophysiology of neurotrauma.

Novel pathologic findings in patients with Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is an X-linked inherited hypomyelinating disorder caused by mutations in the gene encoding proteolipid protein (PLP), the major structural protein in central nervous system (CNS) myelin. Prior to our study, whether hypomyelination in PMD was caused by demyelination, abnormally thin sheaths or failure to form myelin was unknown. In this study, we compared the microscopic pathology of myelin from brain tissue of 3 PMD patients with PLP1 duplications to that of a patient with a complete PLP1 deletion. Autopsy tissue procured from PMD patients was embedded in paraffin for immunocytochemistry and plastic for electron microscopy to obtain highresolution fiber pathology of cerebrum and corpus callosum. Through histological stains, immunocytochemistry and electron microscopy, our study illustrates unique pathologic findings between the two different types of mutations. Characteristic of the patient with a PLP1 deletion, myelin sheaths showed splitting and decompaction of myelin, confirming for the first time that myelin in PLP1 deletion patients is similar to that of rodent models with gene deletions. Myelin thickness and g-ratios of some fibers, in relation to axon diameter was abnormally thin, suggesting that oligodendrocytes remain metabolically functional and/or are attempting to make myelin. Many fibers showed swollen, progressive degenerative changes to axons in addition to the dissolution of myelin. All three duplication cases shared remarkable fiber pathology including swellings, constriction and/or transection and involution of myelin. Characteristic of PLP1 duplication patients, many axons showed segmental demyelination along their length. Still other axons had abnormally thick myelin sheaths, suggestive of continued myelination. Thus, each type of mutation exhibited unique pathology even though commonality to both mutations included involution of myelin, myelin balls and degeneration of axons. This pathology study describes findings unique to each mutation that suggests the mechanism causing fiber pathology is likewise heterogeneous.

Electron microscopy of myelin: Structure preservation by high-pressure freezing

Electron microscopic visualization of nervous tissue morphology is crucial when aiming to understand the biogenesis and structure of myelin in healthy and pathological conditions. However, accurate interpretation of electron micrographs requires excellent tissue preservation. In this short review we discuss the recent utilization of tissue fixation by high-pressure freezing and freeze-substitution, which now supplements aldehyde fixation in the preparation of samples for electron microscopy of myelin. Cryofixation has proven well suited to yield both, improved contrast and excellent preservation of structural detail of the axon/myelin-unit in healthy and mutant mice and can also be applied to other model organisms, including aquatic species. This article is part of a Special Issue entitled SI: Myelin Evolution.

Myelination of the nervous system: mechanisms and functions

Myelination of axons in the nervous system of vertebrates enables fast, saltatory impulse propagation, one of the best-understood concepts in neurophysiology. However, it took a long while to recognize the mechanistic complexity both of myelination by oligodendrocytes and Schwann cells and of their cellular interactions. In this review, we highlight recent advances in our understanding of myelin biogenesis, its lifelong plasticity, and the reciprocal interactions of myelinating glia with the axons they ensheath. In the central nervous system, myelination is also stimulated by axonal activity and astrocytes, whereas myelin clearance involves microglia/macrophages. Once myelinated, the long-term integrity of axons depends on glial supply of metabolites and neurotrophic factors. The relevance of this axoglial symbiosis is illustrated in normal brain aging and human myelin diseases, which can be studied in corresponding mouse models. Thus, myelinating cells serve a key role in preserving the connectivity and functions of a healthy nervous system.

MALDI imaging and in situ identification of integral membrane proteins from rat brain tissue sections

Transmembrane proteins are greatly underrepresented in data generated by imaging mass spectrometry (IMS) because of analytical challenges related to their size and solubility. Here, we present the first example of MALDI IMS of two highly modified multitransmembrane domain proteins, myelin proteolipid protein (PLP, 30 kDa) and DM-20 (26 kDa), from various regions of rat brain, namely, the cerebrum, cerebellum, and medulla. We utilize a novel tissue pretreatment aimed at transmembrane protein enrichment to show the in situ distribution of fatty acylation of these proteins, particularly of post-translational palmitoylation. Additionally, we demonstrate the utility of protease-encapsulated hydrogels for spatially localized on-tissue protein digestion and peptide extraction for subsequent direct coupling to LC-MS/MS for protein identification.

Cysteine thioesters as myelin proteolipid protein analogues to examine the role of butyrylcholinesterase in myelin decompaction

Multiple sclerosis (MS) is a neuroinflammatory and neurodegenerative disorder involving demyelination, axonal transection, and neuronal loss in the brain. Recent studies have indicated that active MS lesions express elevated levels of butyrylcholinesterase (BuChE). BuChE can hydrolyze a wide variety of esters, including fatty acid esters of protein. Proteolipid protein (PLP), an important transmembrane protein component of myelin, has six cysteine residues acylated, via thioester linkages, with fatty acids, usually palmitic, that contribute to the stability of myelin. Experimental chemical deacylation of PLP has been shown to lead to decompaction of myelin. Because of elevated levels of BuChE in active MS lesions and its propensity to catalyze the hydrolysis of acylated protein, we hypothesized that this enzyme may contribute to deacylation of PLP in MS, leading to decompaction of myelin and contributing to demyelination. To test this hypothesis, a series of increasing chain length (C2-C16) acyl thioester derivatives of N-acetyl-l-cysteine methyl ester were synthesized and examined for hydrolysis by human cholinesterases. All N-acetyl-l-cysteine fatty acyl thioester derivatives were hydrolyzed by BuChE but not by the related enzyme acetylcholinesterase. In addition, it was observed that the affinity of BuChE for the compound increased the longer the fatty acid chain, with the highest affinity for cysteine bound to palmitic acid. This suggests that the elevated levels of BuChE observed in active MS lesions could be related to the decompaction of myelin characteristic of the disorder.

Functional central nervous system myelin repair in an adult mouse model of demyelination caused by proteolipid protein overexpression

Two types of interventions to remyelinate the adult demyelinated central nervous system were investigated in heterozygous transgenic mice overexpressing the proteolipid protein gene. 1) A cocktail of trophic factors, "TS1," was directed toward the activation of the endogenous pool of neural progenitors to increase the number of myelinating oligodendrocytes (OL) in the brain. 2) A combinatorial approach in which OL progenitors were coinjected with TS1 into the corpus callosum of wild-type and He4e transgenic mice that displayed hindlimb paralysis. The levels of locomotor ability in these mice were evaluated after a single treatment. The data showed that a single administration of either one of the interventions had similar therapeutic effects, alleviating the symptoms of demyelination and leading to the recovery of hindlimb function. Histological and immunofluorescent examination of brain sections showed extensive remyelination that was sufficient to reverse hindlimb paralysis in transgenic mice. When the interventions were administered prior to hindlimb paralysis, He4e mice were able to walk up to 1 year of age without paralysis.

Alzheimer's disease as homeostatic responses to age-related myelin breakdown

The amyloid hypothesis (AH) of Alzheimer's disease (AD) posits that the fundamental cause of AD is the accumulation of the peptide amyloid beta (Aβ) in the brain. This hypothesis has been supported by observations that genetic defects in amyloid precursor protein (APP) and presenilin increase Aβ production and cause familial AD (FAD). The AH is widely accepted but does not account for important phenomena including recent failures of clinical trials to impact dementia in humans even after successfully reducing Aβ deposits. Herein, the AH is viewed from the broader overarching perspective of the myelin model of the human brain that focuses on functioning brain circuits and encompasses white matter and myelin in addition to neurons and synapses. The model proposes that the recently evolved and extensive myelination of the human brain underlies both our unique abilities and susceptibility to highly prevalent age-related neuropsychiatric disorders such as late onset AD (LOAD). It regards oligodendrocytes and the myelin they produce as being both critical for circuit function and uniquely vulnerable to damage. This perspective reframes key observations such as axonal transport disruptions, formation of axonal swellings/sphenoids and neuritic plaques, and proteinaceous deposits such as Aβ and tau as by-products of homeostatic myelin repair processes. It delineates empirically testable mechanisms of action for genes underlying FAD and LOAD and provides "upstream" treatment targets. Such interventions could potentially treat multiple degenerative brain disorders by mitigating the effects of aging and associated changes in iron, cholesterol, and free radicals on oligodendrocytes and their myelin.

Phylogeny of proteolipid proteins: divergence, constraints, and the evolution of novel functions in myelination and neuroprotection

The protein composition of myelin in the central nervous system (CNS) has changed at the evolutionary transition from fish to tetrapods, when a lipid-associated transmembrane-tetraspan (proteolipid protein, PLP) replaced an adhesion protein of the immunoglobulin superfamily (P0) as the most abundant constituent. Here, we review major steps of proteolipid evolution. Three paralog proteolipids (PLP/DM20/DMalpha, M6B/DMgamma and the neuronal glycoprotein M6A/DMbeta) exist in vertebrates from cartilaginous fish to mammals, and one (M6/CG7540) can be traced in invertebrate bilaterians including the planktonic copepod Calanus finmarchicus that possess a functional myelin equivalent. In fish, DMalpha and DMgamma are coexpressed in oligodendrocytes but are not major myelin components. PLP emerged at the root of tetrapods by the acquisition of an enlarged cytoplasmic loop in the evolutionary older DMalpha/DM20. Transgenic experiments in mice suggest that this loop enhances the incorporation of PLP into myelin. The evolutionary recruitment of PLP as the major myelin protein provided oligodendrocytes with the competence to support long-term axonal integrity. We suggest that the molecular shift from P0 to PLP also correlates with the concentration of adhesive forces at the radial component, and that the new balance between membrane adhesion and dynamics was favorable for CNS myelination.

Palmitoylation of membrane proteins (Review)

S-Palmitoylation is a reversible post-translational modification that results in the addition of a C16-carbon saturated fatty acyl chain to cytoplasmic cysteine residues. This modification is mediated by Palmitoyl-acyl Transferases that are starting to be investigated, and reversed by Protein Palmitoyl Thioesterases, which remain enigmatic. Palmitoylation of cytoplasmic proteins has been well described to regulate the interaction of these soluble proteins with specific membranes or membrane domains. Less is known about the consequences of palmitoylation in transmembrane proteins not only due to the dual difficulty of following a lipid modification and dealing with membrane proteins, but also due to the complexity of the palmitoylation-induced behavior. Moreover, possibly because the available data set is limited, the change in behavior induced by palmitoylation of a transmembrane protein is currently not predictable. We here review the various consequences reported for the palmitoylation of membrane proteins, which include improper folding in the endoplasmic reticulum, retention in the Golgi, inability to assemble into protein platforms, altered signaling capacity, premature endocytosis and missorting in the endocytic pathway. We then discuss the possible underlying mechanisms, in particular the ability of palmitoylation to control the conformation of transmembrane segments, to modify the affinity of a membrane protein for specific membrane domains and to control protein-protein interactions.

Effects of osmolality on PLP-null myelin structure: implications re axon damage

In order to test the adhesiveness of PLP-null compact myelin lamellae we soaked aldehyde-fixed CNS specimens from PLP-null and control mice overnight in distilled water, in Ringer's solution or in Ringer's solution with added 1 M sucrose. Subsequent examination of the tissue by EM showed that both PLP-null and control white matter soaked in Ringer remained largely compact. After the distilled water soak, control myelin was virtually unchanged, but PLP-null myelin showed some decompaction, i.e., separation of myelin lamellae from one another. After the sucrose/Ringer soak, normal myelin developed foci of decompaction, but the great majority of lamellae remained compact. In the PLP-null specimens, in contrast, many of the myelin sheaths became almost completely decompacted. Such sheaths became thicker overall and were comprised of lamellae widely separated from one another by irregular spaces. Thus, in normal animals, fixed CNS myelin lamellae are firmly adherent and resist separation; PLP-null myelin lamellae, in contrast, are poorly adherent and more readily separated. Mechanisms by which impaired adhesiveness of PLP-null myelin lamellae and fluctuations in osmolality in vivo might underlie slowing of conduction and axon damage are discussed.

The molecular and cellular defects underlying Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease (PMD) is a recessive X-linked dysmyelinating disorder of the central nervous system (CNS). The most frequent cause of PMD is a genomic duplication of chromosome Xq22 including the region encoding the dosage-sensitive proteolipid protein 1 (PLP1) gene. The PLP1 duplications are heterogeneous in size, unlike duplications causing many other genomic disorders, and arise by a distinct molecular mechanism. Other causes of PMD include PLP1 deletions, triplications and point mutations. Mutations in the PLP1 gene can also give rise to spastic paraplegia type 2 (SPG2), an allelic form of the disease. Thus, there is a spectrum of CNS disorder from mild SPG2 to severe connatal PMD. PLP1 encodes a major protein in CNS myelin and is abundantly expressed in oligodendrocytes, the myelinating cells of the CNS. Significant advances in our understanding of PMD have been achieved by investigating mutant PLP1 in PMD patients, animal models and in vitro studies. How the different PLP1 mutations and dosage effects give rise to PMD is being revealed. Interestingly, the underlying causes of pathogenesis are distinct for each of the different genetic abnormalities. This article reviews the genetics of PMD and summarises the current knowledge of causative molecular and cellular mechanisms.

Peripheral myelin of Xenopus laevis: role of electrostatic and hydrophobic interactions in membrane compaction

P0 glycoprotein is the major structural protein of peripheral nerve myelin where it is thought to modulate inter-membrane adhesion at both the extracellular apposition, which is labile upon changes in pH and ionic strength, and the cytoplasmic apposition, which is resistant to such changes. Most studies on P0 have focused on structure-function correlates in higher vertebrates. Here, we focused on its role in the structure and interactions of frog (Xenopus laevis) myelin, where it exists primarily in a dimeric form. As part of our study, we deduced the full sequence of X. laevis P0 (xP0) from its cDNA. The xP0 sequence was found to be similar to P0 sequences of higher vertebrates, suggesting that a common mechanism of PNS myelin compaction via P0 interaction might have emerged through evolution. As previously reported for mouse PNS myelin, a similar change of extracellular apposition in frog PNS myelin as a function of pH and ionic strength was observed, which can be explained by a conformational change of P0 due to protonation-deprotonation of His52 at P0's putative adhesive interface. On the other hand, the cytoplasmic apposition in frog PNS myelin, like that in the mouse, remained unchanged at different pH and ionic strength. The contribution of hydrophobic interactions to stabilizing the cytoplasmic apposition was tested by incubating sciatic nerves with detergents. Dramatic expansion at the cytoplasmic apposition was observed for both frog and mouse, indicating a common hydrophobic nature at this apposition. Urea also expanded the cytoplasmic apposition of frog myelin likely owing to denaturation of P0. Removal of the fatty acids that attached to the single Cys residue in the cytoplasmic domain of P0 did not change PNS myelin structure of either frog or mouse, suggesting that the P0-attached fatty acyl chain does not play a significant role in PNS myelin compaction and stability. These results help clarify the present understanding of P0's adhesion role and the role of its acylation in compact PNS myelin.

Proteolipid protein is required for transport of sirtuin 2 into CNS myelin

Mice lacking the expression of proteolipid protein (PLP)/DM20 in oligodendrocytes provide a genuine model for spastic paraplegia (SPG-2). Their axons are well myelinated but exhibit impaired axonal transport and progressive degeneration, which is difficult to attribute to the absence of a single myelin protein. We hypothesized that secondary molecular changes in PLP(null) myelin contribute to the loss of PLP/DM20-dependent neuroprotection and provide more insight into glia-axonal interactions in this disease model. By gel-based proteome analysis, we identified >160 proteins in purified myelin membranes, which allowed us to systematically monitor the CNS myelin proteome of adult PLP(null) mice, before the onset of disease. We identified three proteins of the septin family to be reduced in abundance, but the nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase sirtuin 2 (SIRT2) was virtually absent. SIRT2 is expressed throughout the oligodendrocyte lineage, and immunoelectron microscopy revealed its association with myelin. Loss of SIRT2 in PLP(null) was posttranscriptional, suggesting that PLP/DM20 is required for its transport into the myelin compartment. Because normal SIRT2 activity is controlled by the NAD+/NADH ratio, its function may be coupled to the axo-glial metabolism and the long-term support of axons by oligodendrocytes.

Evolution of myelin proteolipid proteins: Gene duplication in teleosts and expression pattern divergence

The coevolution of neurons and their supporting glia to the highly specialized axon-myelin unit included the recruitment of proteolipids as neuronal glycoproteins (DMbeta, DMgamma) or myelin proteins (DMalpha/PLP/DM20). Consistent with a genome duplication at the root of teleosts, we identified three proteolipid pairs in zebrafish, termed DMalpha1 and DMalpha2, DMbeta1 and DMbeta2, DMgamma1 and DMgamma2. The paralogous amino acid sequences diverged remarkably after gene duplication, indicating functional specialization. Each proteolipid has adopted a distinct spatio-temporal expression pattern in neural progenitors, neurons, and in glia. DMalpha2, the closest homolog to mammalian PLP/DM20, is coexpressed with P0 in oligodendrocytes and upregulated after optic nerve lesion. DMgamma2 is expressed in multipotential stem cells, and the other four proteolipids are confined to subsets of CNS neurons. Comparing protein sequences and gene structures from birds, teleosts, one urochordate species, and four invertebrates, we have reconstructed major steps in the evolution of proteolipids.

Disease-associated mutations cause premature oligomerization of myelin proteolipid protein in the endoplasmic reticulum

Pelizaeus-Merzbacher disease (PMD) is a dysmyelinating disease caused by mutations, deletions, or duplications of the proteolipid protein (PLP) gene. Mutant forms of PLP are retained in the endoplasmic reticulum (ER), and the resulting accumulation of mutant protein is thought to be a direct cause of oligodendrocyte cell death, which is the primary clinical feature of PMD. The molecular mechanisms underlying the toxicity of mutant PLP are however currently unknown. We report here that PMD-linked mutations of PLP are associated with the accelerated assembly of the protein into stable homooligomers that resemble mature, native PLP. Thus although WT PLP forms stable oligomers after an extended maturation period, most likely at the cell surface, mutant forms of PLP rapidly assemble into such oligomers at the ER. Using PLP mutants associated with diseases of varying severity, we show that the formation of stable oligomers correlates with the development of PMD. Based on these findings, we propose that the premature oligomerization of PLP in the ER of oligodendrocytes contributes to the pathology of PMD.

Myelination and motor coordination are increased in transferrin transgenic mice

Myelin deficiency in the central nervous system (CNS) can cause severe disabling conditions. Most of the transgenic mice models overexpressing myelin components have limitations for investigators of myelin deficiency and myelin therapy as they severely alter CNS architecture. It has been postulated that transferrin (Tf) is involved in oligodendrocyte (OL) maturation and myelinogenesis. Because Tf is not an intrinsic myelin constituent, we decided to investigate if its overexpression could have an impact on the myelination process without affecting myelin integrity. We generated transgenic mice containing the complete human Tf gene specifically overexpressed in OLs. This overexpression leads to more than a 30% increase in myelin components, such as galactolipids, phospholipids, and proteins. Electron microscopy showed that myelin is structurally normal in terms of thickness and compaction. Behavior analysis showed that mice do not display significant modifications in their locomotion and cognitive and emotional abilities. Furthermore, in one of the genetic background, animals presented a significant increase in motor coordination. We did not find any modification in OL number during early postnatal development, suggesting that Tf does not act on OL proliferation. In addition, the levels of iron and ferritin remained unchanged in the brain of transgenic mice compared to control mice. Our findings indicate that, besides its known iron transport function, Tf is able to influence myelination process and induce behavioral improvements in mice.

Effect of 2-fluoropalmitate, cerulenin and tunicamycin on the palmitoylation and intracellular translocation of myelin proteolipid protein

We have investigated the effect of documented protein palmitoylation inhibitors on the fatty acylation and intracellular transport of myelin proteolipid protein (PLP). To this end, brain slices from 20-day-old rats were incubated with either [3H]palmitate or [3H]leucine in the presence or absence of various concentrations of 2-fluoropalmitate (FP), cerulenin (CER), or tunicamycin (TM). FP (> or = 10 microM) decreased the cellular uptake of [3H]palmitate and consequently reduced the labeling of palmitoyl-CoA, glycerolipids and PLP. CER (> or = 1 mM) reduced the palmitoylation of PLP with a concomitant decline in protein thiols. Consistent with being a fatty acyl-CoA analogue, TM (> or = 200 microM) diminished the palmitoylation of PLP and lipids while increasing the amount of [3H]palmitoyl-CoA. Although both CER and TM decreased protein palmitoylation, only the latter affected the appearance of newly synthesized PLP into myelin. Because TM, but not CER, also reduced the formation of lipids, it is concluded that palmitoylation is not required for intracellular transport. Finally, comparison of the effect of TM in brain slices and in a cell-free system suggests that palmitoylation of PLP in whole cells may be an enzymatic process.

Myelin proteolipid protein-induced aggregation of lipid vesicles: efficacy of the various molecular species

The different molecular species that form the myelin proteolipid protein family were isolated by size-exclusion and ion-exchange chromatography in organic solvents and their adhesive properties were tested using a vesicle aggregation assay. Addition of the major proteolipid (PLP) to phosphatidylcholine-cholesterol vesicles caused their clustering as determined by increase in O.D.(450 nm) and by transmission electron microscopy. A small fraction of the aggregated vesicles underwent fusion as determined by resonance energy transfer experiments. Vesicle aggregation by PLP, but not the dissociation of the aggregates, was influenced by pH suggesting that electrostatic interactions are important only during cluster formation. Cleavage of disulfide bonds and methylation of carboxyl groups in PLP greatly reduced the aggregating activity, indicating that the process is dependent on the protein's conformation. Unexpectedly, the proteolipid DM-20 was also effective at inducing the clustering of neutral lipid vesicles. In contrast, three protein fractions comprising the naturally-occurring PLP fragments 1-107/112, 113/125-276 and 129/131-276, bearing different net charges, displayed a much lower activity. In addition, trypsin digestion of PLP resulted in a progressive decrease in the protein's ability to induce vesicle aggregation which coincided with the disappearance of the full-length molecule. Together, these results suggest that even large PLP fragments cannot fulfill the adhesive function of the intact protein.

Mass-spectrometric analysis of myelin proteolipids reveals new features of this family of palmitoylated membrane proteins

In this study, we have investigated the structure of the native myelin proteolipid protein (PLP), DM-20 protein and several low molecular mass proteolipids by mass spectrometry. The various proteolipid species were isolated from bovine spinal cord by size-exclusion and ion-exchange chromatography in organic solvents. Matrix-assisted laser desorption ionization-time of flight-mass spectrometry (MALDI-TOF-MS) of PLP and DM-20 revealed molecular masses of 31.6 and 27.2 kDa, respectively, which is consistent with the presence of six and four molecules of thioester-bound fatty acids. Electrospray ionization-MS analysis of the deacylated proteins in organic solvents produced the predicted molecular masses of the apoproteins (29.9 and 26.1 kDa), demonstrating that palmitoylation is the major post-translational modification of PLP, and that the majority of PLP and DM-20 molecules in the CNS are fully acylated. A series of myelin-associated, palmitoylated proteolipids with molecular masses raging between 12 kDa and 18 kDa were also isolated and subjected to amino acid analysis, fatty acid analysis, N- and C-terminal sequencing, tryptic digestion and peptide mapping by MALDI-TOF-MS. The results clearly showed that these polypeptides correspond to the N-terminal region (residues 1-105/112) and C-terminal region (residues 113/131-276) of the major PLP, and they appear to be produced by natural proteolytic cleavage within the 60 amino acid-long cytoplasmic domain. These proteolipids are not postmortem artifacts of PLP and DM-20, and are differentially distributed across the CNS.

Nitric oxide reduces the palmitoylation of rat myelin proteolipid protein by an indirect mechanism

Brain slices from 20-day-old rats were incubated with [3H]palmitate for 2 hours in the absence or presence of the NO-donors S-nitroso-N-acetyl-penicillamine (SNAP), ethyl-2-[hydroxyimino]-5-nitro-3-hexeneamide (NOR-3), 4-phenyl-3-furoxan carbonitrile (PFC) and sodium nitroprusside (SNP). Each of these drugs reduced the incorporation of [3H]palmitate into myelin proteolipid protein (PLP) in a concentration-dependent manner, SNP being the most active. The effect of SNAP was prevented by the NO-scavenger PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). Furthermore, decayed-SNAP, sodium nitrite and N- nitrosopyrrolidine were inactive, suggesting that free NO and/or some of its direct oxidation products are the active molecular species. The amount of fatty acids bound to PLP and the rate of deacylation were unaffected by NO. Although NO diminished the number of thiols in brain and myelin proteins, with the formation of both nitrosothiols and disulfides, these changes did not parallel those in PLP acylation. In contrast, NO was effective at reducing the palmitoylation of brain and myelin lipids, and this effect along with that of PLP, was ascribed to a decrease in palmitoyl-CoA levels. The NO-induced reduction in acyl-CoA concentration was due to the decline in ATP levels, while the amount of [3H]palmitate incorporated into the tissue, the activity of palmitoyl-CoA ligase and palmitoyl-CoA hydrolase, and the concentration of CoASH were unaltered by the drugs. Experiments with endogenously-synthesized [18O]fatty acids confirmed that NO affects predominantly the ATP-dependent palmitoylation of PLP. In conclusion, the inhibitory action of NO on the fatty acylation of PLP is indirect and caused by energy depletion.

Remyelination of the adult demyelinated mouse brain by grafted oligodendrocyte progenitors and the effect of B-104 cografts

The 4e transgenic mouse is characterized by overexpression of the PLP gene. Heterozygous littermates containing three PLP gene copies develop and myelinate normally. However, a progressive CNS demyelination begins at 3-4 months of age. Despite focal demyelination, these animals survive for one year with hind limb paralysis. We used this CNS demyelination model to determine if grafts of CG4 oligodendrocyte progenitors would survive and myelinate the adult CNS. Either CG4 cells, or co-grafts of CG4/B 104 cells 11:1 ratio respectively) were performed. Grafted cells survived and migrated in the normal and transgenic brain. Non-treated transgenic animals revealed extensive lack of myelin. Three months post-transplant hosts with CG4 or co-transplants displayed a near normal myelin pattern. Double immunofluorescence for neurofilament and myelin basic protein revealed the presence of many naked axons in non-grafted transgenic animals. Those grafted with progenitor CG4 cells or cografts displayed a clear increase in remyelination. This data provides a new direction for the development of cell replacement therapies in demyelinating diseases.