Myelin basic protein is a zinc-binding protein in brain: possible role in myelin compaction

Associations of Maternal Prenatal Zinc Consumption with Infant Brain Tissue Organization and Neurodevelopmental Outcomes

BACKGROUND/OBJECTIVES

While studies in rat pups suggest that early zinc exposure is critical for optimal brain structure and function, associations of prenatal zinc intake with measures of brain development in infants are unknown. This study aimed to assess the associations of maternal zinc intake during pregnancy with MRI measures of brain tissue microstructure and neurodevelopmental outcomes, as well as to determine whether MRI measures of the brain mediated the relationship between maternal zinc intake and neurodevelopmental indices.

METHODS

Forty-one adolescent mothers were recruited for a longitudinal study during pregnancy. Maternal zinc intake was assessed during the third trimester of pregnancy using a 24 h dietary recall. Infant MRI scans were acquired at 3 weeks postpartum using a 3.0 Tesla scanner to measure fractional anisotropy (FA) and mean diffusivity (MD). Cognitive, language, and motor skills were assessed at 4, 14, and 24 months postpartum using the Bayley Scales of Infant Development.

RESULTS

Greater prenatal zinc intake was associated with reduced FA in cortical gray matter, particularly in the frontal lobe [medial superior frontal gyrus; β (95% CI) = -1.0 (-1.5, -0.5)], in developing white matter, and in subcortical gray matter nuclei. Greater prenatal zinc intake was associated with reduced MD in cortical gray matter and developing white matter [superior longitudinal fasciculus; -4.4 (-7.1, -1.7)]. Greater maternal zinc intake also was associated with higher cognitive development scores at 14 [0.1 (0.0, 0.1)] and 24 [0.1 (0.0, 0.2)] months of age; MRI indices of FA and MD did not mediate this relationship.

CONCLUSIONS

Maternal prenatal zinc intake was associated with more favorable measures of brain tissue microstructural maturation and cognitive development during infancy.

Multiple Sclerosis Patient Macrophages Impaired Metabolism Leads to an Altered Response to Activation Stimuli

BACKGROUND AND OBJECTIVES

In multiple sclerosis (MS), immune cells invade the CNS and destroy myelin. Macrophages contribute to demyelination and myelin repair, and their role in each process depends on their ability to acquire specific phenotypes in response to external signals. In this article, we assess whether defects in MS patient macrophage responses may lead to increased inflammation or lack of neuroregenerative effects.

METHODS

CD14+CD16- monocytes from patients with MS and healthy controls (HCs) were activated in vitro to obtain homeostatic-like, proinflammatory, and proregenerative macrophages. Macrophage activation profiles were assessed through RNA sequencing and metabolomics. Surface molecule expression of CD14, CD16, and HLA-DR and myelin phagocytic capacity were evaluated with flow cytometry. Macrophage supernatant capacity to influence oligodendrocyte precursor cell differentiation toward an astrocytic or oligodendroglia fate was also tested.

RESULTS

We observed that MS patient monocytes ex vivo recapitulate their preferential activation toward the CD16+ phenotype, a subset of proinflammatory cells overrepresented in MS lesions. Functionally, MS patient macrophages display a decreased capacity to phagocytose human myelin and a deficit of processing myelin after ingestion. In addition, MS patient macrophage supernatant favors astrocytes over oligodendrocyte differentiation when compared with HC macrophage supernatant. Furthermore, even when exposed to homeostatic or proregenerative stimuli, MS patient macrophages uphold a proinflammatory transcriptomic profile with higher levels of cytokine/chemokine. Of interest, MS patient macrophages exhibit a distinct metabolic signature with a mitochondrial energy metabolism blockage. Transcriptomic data are further substantiated by metabolomics studies that reveal perturbations in the corresponding metabolic pathways.

DISCUSSION

Our results show an intrinsic defect of MS patient macrophages, reminiscent of innate immune cell memory in MS, lifting macrophage importance in the disease and as potential therapeutic targets.

Developmental regulation of zinc homeostasis in differentiating oligodendrocytes

Oligodendrocytes develop through sequential stages and understanding pathways regulating their differentiation remains an important area of investigation. Zinc is required for the function of enzymes, proteins and transcription factors, including those important in myelination and mitosis. Our previous studies using the ratiometric zinc sensor chromis-1 demonstrated a reduction in intracellular free zinc concentrations in mature MBP+ oligodendrocytes compared with earlier stages (Bourassa et al., 2018). We performed a more detailed developmental study to better understand the temporal course of zinc homeostasis across the oligodendrocyte lineage. Using chromis-1, we found a transient increase in free zinc after O4+,O1- pre-oligodendrocytes were switched from proliferation medium into terminal differentiation medium. To gather other evidence for dynamic regulation of free zinc during oligodendrocyte development, qPCR was used to evaluate mRNA expression of major zinc storage proteins metallothioneins (MTs) and metal regulatory transcription factor 1 (MTF1), which controls expression of MTs. MT1, MT2 and MTF1 mRNAs were increased several fold in mature oligodendrocytes compared to oligodendrocytes in proliferation medium. To assess the depth of the zinc buffer, we assayed zinc release from intracellular stores using the oxidizing thiol reagent 2,2'-dithiodipyridine (DTDP). Exposure to DTDP resulted in ∼ 100% increase in free zinc in pre-oligodendrocytes but, paradoxically more modest ∼ 60% increase in mature oligodendrocytes despite increased expression of MTs. These results suggest that zinc homeostasis is regulated during oligodendrocyte development, that oligodendrocytes are a useful model for studying zinc homeostasis in the central nervous system, and that regulation of zinc homeostasis may be important in oligodendrocyte differentiation.

Developmental regulation of zinc homeostasis in differentiating oligodendrocytes

Oligodendrocytes develop through well characterized stages and understanding pathways regulating their differentiation remains an active area of investigation. Zinc is required for the function of many enzymes, proteins and transcription factors, including those important in myelination and mitosis. Our previous studies using the ratiometric zinc sensor chromis-1 demonstrated a reduction in intracellular free zinc concentrations in mature oligodendrocytes compared with earlier stages (Bourassa et al., 2018). We performed a more detailed developmental study to better understand the temporal course of zinc homeostasis across the oligodendrocyte lineage. Using chromis-1, we found a transient increase in free zinc after developing oligodendrocytes were switched into differentiation medium. To gather other evidence for dynamic regulation of free zinc during oligodendrocyte development, qPCR was used to evaluate mRNA expression of the major zinc storage proteins metallothioneins (MTs), and metal regulatory transcription factor 1 (MTF-1) which controls expression of MTs. MT-1, MT-2 and MTF1 mRNAs were all increased several fold in mature oligodendrocytes compared to developing oligodendrocytes. To assess the depth of the zinc buffer, we assayed zinc release from intracellular stores using the oxidizing thiol reagent 2,2'-dithiodipyridine (DTDP). Exposure to DTDP resulted in a ∼100% increase in free zinc in developing oligodendrocytes but, paradoxically more modest ∼60% increase in mature oligodendrocytes despite the increased expression of MTs. These results suggest that zinc homeostasis is regulated during oligodendrocyte development, that oligodendrocytes are a useful model for studying zinc homeostasis in the central nervous system, and that regulation of zinc homeostasis may be important in oligodendrocyte differentiation.

Metal Profiles in Autism Spectrum Disorders: A Crosstalk between Toxic and Essential Metals

Since hundreds of years ago, metals have been recognized as impacting our body's physiology. As a result, they have been studied as a potential cure for many ailments as well as a cause of acute or chronic poisoning. However, the link between aberrant metal levels and neuropsychiatric illnesses such as schizophrenia and neurodevelopmental disorders, such as autism spectrum disorders (ASDs), is a relatively new finding, despite some evident ASD-related consequences of shortage or excess of specific metals. In this review, we will summarize past and current results explaining the pathomechanisms of toxic metals at the cellular and molecular levels that are still not fully understood. While toxic metals may interfere with dozens of physiological processes concurrently, we will focus on ASD-relevant activity such as inflammation/immune activation, mitochondrial malfunction, increased oxidative stress, impairment of axonal myelination, and synapse formation and function. In particular, we will highlight the competition with essential metals that may explain why both the presence of certain toxic metals and the absence of certain essential metals have emerged as risk factors for ASD. Although often investigated separately, through the agonistic and antagonistic effects of metals, a common metal imbalance may result in relation to ASD.

On 2D-FTIR-XRF microscopy - A step forward correlative tissue studies by infrared and hard X-ray radiation

Correlative Fourier Transform Infra-Red (FTIR) and hard X-Ray Fluorescence (XRF) microscopy studies of thin biological samples have recently evolved as complementary methods for biochemical fingerprinting of animal/human tissues. These are seen particularly useful for tracking the mechanisms of neurological diseases, i.e., in Alzheimer/Parkinson disease, in the brain where mishandling of trace metals (Fe, Cu, Zn) seems to be often associated with ongoing damage to molecular components via, among others, oxidative/reductive stress neurotoxicity. Despite substantial progress in state-of-the-art detection and data analysis methods, combined FTIR-XRF experiments have never benefited from correlation and co-localization analysis of molecular moieties and chemical elements, respectively. We here propose for the first time a completely novel data analysis pipeline, utilizing the idea of 2D correlation spectrometry for brain tissue analysis. In this paper, we utilized combined benchtop FTIR - synchrotron XRF mapping experiments on thin brain samples mounted on polypropylene membranes. By implementing our recently developed Multiple Linear Regression Multi-Reference (MLR-MR) algorithm, along with advanced image processing, artifact-free 2D FTIR-XRF spectra could be obtained by mitigating the impact of spectral artifacts, such as Etalon fringes and mild scattering Mie-like signatures, in the FTIR data. We demonstrated that the method is a powerful tool for co-localizing and correlating molecular arrangements and chemical elements (and vice versa) using visually attractive 2D correlograms. Moreover, the methods' applicability for fostering the identification of distinct (biological) materials, involving chemical elements and molecular arrangements, is also shown. Taken together, the 2D FTIR-XRF method opens up for new measures for in-situ investigating hidden complex biochemical correlations, and yet unraveled mechanisms in a biological sample. This step seems crucial for developing new strategies for facilitating the research on the interaction of metals/nonmetals with organic components. This is particularly important for enhancing our understanding of the diseases associated with metal/nonmetal mishandling.

Metal Imbalance in Neurodegenerative Diseases with a Specific Concern to the Brain of Multiple Sclerosis Patients

There is increasing evidence that deregulation of metals contributes to a vast range of neurodegenerative diseases including multiple sclerosis (MS). MS is a chronic inflammatory disease of the central nervous system (CNS) manifesting disability and neurological symptoms. The precise origin of MS is unknown, but the disease is characterized by focal inflammatory lesions in the CNS associated with an autoimmune reaction against myelin. The treatment of this disease has mainly been based on the prescription of immunosuppressive and immune-modulating agents. However, the rate of progressive disability and early mortality is still worrisome. Metals may represent new diagnostic and predictive markers of severity and disability as well as innovative candidate drug targets for future therapies. In this review, we describe the recent advances in our understanding on the role of metals in brain disorders of neurodegenerative diseases and MS patients.

Flexible Players within the Sheaths: The Intrinsically Disordered Proteins of Myelin in Health and Disease

Myelin ensheathes selected axonal segments within the nervous system, resulting primarily in nerve impulse acceleration, as well as mechanical and trophic support for neurons. In the central and peripheral nervous systems, various proteins that contribute to the formation and stability of myelin are present, which also harbor pathophysiological roles in myelin disease. Many myelin proteins have common attributes, including small size, hydrophobic segments, multifunctionality, longevity, and regions of intrinsic disorder. With recent advances in protein biophysical characterization and bioinformatics, it has become evident that intrinsically disordered proteins (IDPs) are abundant in myelin, and their flexible nature enables multifunctionality. Here, we review known myelin IDPs, their conservation, molecular characteristics and functions, and their disease relevance, along with open questions and speculations. We place emphasis on classifying the molecular details of IDPs in myelin, and we correlate these with their various functions, including susceptibility to post-translational modifications, function in protein–protein and protein–membrane interactions, as well as their role as extended entropic chains. We discuss how myelin pathology can relate to IDPs and which molecular factors are potentially involved.

Characterization of Ionic and Lipid Gradients within Corpus Callosum White Matter after Diffuse Traumatic Brain Injury in the Rat

There is increased recognition of the effects of diffuse traumatic brain injury (dTBI), which can initiate yet unknown biochemical cascades, resulting in delayed secondary brain degeneration and long-term neurological sequela. There is limited availability of therapies that minimize the effect of secondary brain damage on the quality of life of people who have suffered TBI, many of which were otherwise healthy adults. Understanding the cascade of biochemical events initiated in specific brain regions in the acute phase of dTBI and how this spreads into adjacent brain structures may provide the necessary insight into drive development of improved therapies. In this study, we have used direct biochemical imaging techniques (Fourier transform infrared spectroscopic imaging) and elemental mapping (X-ray fluorescence microscopy) to characterize biochemical and elemental alterations that occur in corpus callosum white matter in the acute phase of dTBI. The results provide direct visualization of differential biochemical and ionic changes that occur in the highly vulnerable medial corpus callosum white matter relative to the less vulnerable lateral regions of the corpus callosum. Specifically, the results suggest that altered ionic gradients manifest within mechanically damaged medial corpus callosum, potentially spreading to and inducing lipid alterations to white matter structures in lateral brain regions.

Zinc homeostasis and zinc signaling in white matter development and injury

Zinc is an essential dietary micronutrient that is abundant in the brain with diverse roles in development, injury, and neurological diseases. With new imaging tools and chelators selectively targeting zinc, the field of zinc biology is rapidly expanding. The importance of zinc homeostasis is now well recognized in neurodegeneration, but there is emerging data that zinc may be equally important in white matter disorders. This review provides an overview of zinc biology, including a discussion of clinical disorders of zinc deficiency, different zinc pools, zinc biomarkers, and methods for measuring zinc. It emphasizes our limited understanding of how zinc is regulated in oligodendrocytes and white matter. Gaps in knowledge about zinc transporters and zinc signaling are discussed. Zinc-induced oligodendrocyte injury pathways relevant to white matter stroke, multiple sclerosis, and white matter injury of prematurity are reviewed and examples of zinc-dependent proteins relevant to myelination highlighted. Finally, a novel ratiometric zinc sensor is reviewed, revealing new information about mobile zinc during oligodendrocyte differentiation. With a better understanding of zinc biology in oligodendrocytes, new therapeutic targets for white matter disorders may be possible and the necessary tools to appropriately study zinc are finally available.

The Divalent Metal Transporter 1 (DMT1) Is Required for Iron Uptake and Normal Development of Oligodendrocyte Progenitor Cells

The divalent metal transporter 1 (DMT1) is a multimetal transporter with a primary role in iron transport. Although DMT1 has been described previously in the CNS, nothing was known about the role of this metal transporter in oligodendrocyte maturation and myelination. To determine whether DMT1 is required for oligodendrocyte progenitor cell (OPC) maturation, we used siRNAs and the Cre-lox system to knock down/knock out DMT1 expression in vitro as well as in vivo Blocking DMT1 synthesis in primary cultures of OPCs reduced oligodendrocyte iron uptake and significantly delayed OPC development. In vivo, a significant hypomyelination was found in DMT1 conditional knock-out mice in which DMT1 was postnatally deleted in NG2- or Sox10-positive OPCs. The brain of DMT1 knock-out animals presented a decrease in the expression levels of myelin proteins and a substantial reduction in the percentage of myelinated axons. This reduced postnatal myelination was accompanied by a decrease in the number of myelinating oligodendrocytes and a rise in proliferating OPCs. Furthermore, using the cuprizone model of demyelination, we established that DMT1 deletion in NG2-positive OPCs lead to less efficient remyelination of the adult brain. These results indicate that DMT1 is vital for OPC maturation and for the normal myelination of the mouse brain.SIGNIFICANCE STATEMENT To determine whether divalent metal transporter 1 (DMT1), a multimetal transporter with a primary role in iron transport, is essential for oligodendrocyte development, we created two conditional knock-out mice in which DMT1 was postnatally deleted in NG2- or Sox10-positive oligodendrocyte progenitor cells (OPCs). We have established that DMT1 is necessary for normal OPC maturation and is required for an efficient remyelination of the adult brain. Since iron accumulation by OPCs is indispensable for myelination, understanding the iron incorporation mechanism as well as the molecules involved is critical to design new therapeutic approaches to intervene in diseases in which the myelin sheath is damaged or lost.

Metabolic defects in multiple sclerosis

Brain injuries in multiple sclerosis (MS) involve immunopathological, structural and metabolic defects on myelin sheath, oligodendrocytes (OLs), axons and neurons suggesting that different cellular mechanisms ultimately result in the formation of MS plaques, demyelination, inflammation and brain damage. Bioenergetics, oxygen and ion metabolism dominate the metabolic and biochemical pathways that maintain neuronal viability and impulse transmission which directly or indirectly point to mitochondrial integrity and adenosine triphosphate (ATP) availability indicating the involvement of mitochondria in the pathogenesis of MS. Loss of myelin proteins including myelin basic protein (MBP), proteolipid protein (PLP), myelin associated glycoprotein (MAG), myelin oligodendrocyte glycoproetin (MOG), 2, 3,-cyclic nucleotide phosphodiestarase (CNPase); microglia and microphage activation, oligodendrocyte apoptosis as well as expression of inducible nitric oxide synthase (i-NOS) and myeloperoxidase activities have been implicated in a subset of Balo's type and relapsing remitting MS (RRMS) lesions indicating the involvement of metabolic defects and oxidative stress in MS. Here, we provide an insighting review of defects in cellular metabolism including energy, oxygen and metal metabolism in MS as well as the relevance of animal models of MS in understanding the molecular, biochemical and cellular mechanisms of MS pathogenesis. Additionally, we also discussed the potential for mitochondrial targets and antioxidant protection for therapeutic benefits in MS.

Iron prevents demyelination of frog sciatic nerves

Metal ions are of particular importance in nervous system function, notably iron. However, very little has been done to investigate its physiological role in frog peripheral nervous system. The present research aim to evaluate i) the time-effect of sciatic nerve ligation and/or ii) iron sulphate (1.50mg/kg, in lymphatic sac) on frog myelin sheaths. Histological sections following ligation shows degeneration of some fibres with axonal and myelin breakdown associated to a decrease of Schwann cells number following 2h (45.00±0.30, p<0.0001), 24h (28.00±0.020, p<0.0001). Interestingly, iron administration reduces the degeneration of myelin sheaths classically observed in frog ligated sciatic nerve associated with an increase of Schwann cells number (139.00±0.50, p<0.0001). Thus, iron could prevent degeneration or promote regeneration induced by ligation in frog sciatic nerve.

Pathogenic implications of distinct patterns of iron and zinc in chronic MS lesions

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system (CNS) in which oligodendrocytes, the CNS cells that stain most robustly for iron and myelin are the targets of injury. Metals are essential for normal CNS functioning, and metal imbalances have been linked to demyelination and neurodegeneration. Using a multidisciplinary approach involving synchrotron techniques, iron histochemistry and immunohistochemistry, we compared the distribution and quantification of iron and zinc in MS lesions to the surrounding normal appearing and periplaque white matter, and assessed the involvement of these metals in MS lesion pathogenesis. We found that the distribution of iron and zinc is heterogeneous in MS plaques, and with few remarkable exceptions they do not accumulate in chronic MS lesions. We show that brain iron tends to decrease with increasing age and disease duration of MS patients; reactive astrocytes organized in large astrogliotic areas in a subset of smoldering and inactive plaques accumulate iron and safely store it in ferritin; a subset of smoldering lesions do not contain a rim of iron-loaded macrophages/microglia; and the iron content of shadow plaques varies with the stage of remyelination. Zinc in MS lesions was generally decreased, paralleling myelin loss. Iron accumulates concentrically in a subset of chronic inactive lesions suggesting that not all iron rims around MS lesions equate with smoldering plaques. Upon degeneration of iron-loaded microglia/macrophages, astrocytes may form an additional protective barrier that may prevent iron-induced oxidative damage.

MyelStones: the executive roles of myelin basic protein in myelin assembly and destabilization in multiple sclerosis

The classic isoforms of myelin basic protein (MBP, 14–21.5 kDa) are essential to formation of the multilamellar myelin sheath of the mammalian central nervous system (CNS). The predominant 18.5-kDa isoform links together the cytosolic surfaces of oligodendrocytes, but additionally participates in cytoskeletal turnover and membrane extension, Fyn-mediated signalling pathways, sequestration of phosphoinositides and maintenance of calcium homoeostasis. All MBP isoforms are intrinsically disordered proteins (IDPs) that interact via molecular recognition fragments (MoRFs), which thereby undergo local disorder-to-order transitions. Their conformations and associations are modulated by environment and by a dynamic barcode of post-translational modifications, particularly phosphorylation by mitogen-activated and other protein kinases and deimination [a hallmark of demyelination in multiple sclerosis (MS)]. The MBPs are thus to myelin what basic histones are to chromatin. Originally thought to be merely structural proteins forming an inert spool, histones are now known to be dynamic entities involved in epigenetic regulation and diseases such as cancer. Analogously, the MBPs are not mere adhesives of compact myelin, but active participants in oligodendrocyte proliferation and in membrane process extension and stabilization during myelinogenesis. A central segment of these proteins is pivotal in membrane-anchoring and SH3 domain (Src homology 3) interaction. We discuss in the present review advances in our understanding of conformational conversions of this classic basic protein upon membrane association, including new thermodynamic analyses of transitions into different structural ensembles and how a shift in the pattern of its post-translational modifications is associated with the pathogenesis and potentially onset of demyelination in MS.

Zinc-induced structural changes of the disordered tppp/p25 inhibits its degradation by the proteasome

Tubulin Polymerization Promoting Protein/p25 (TPPP/p25), a neomorphic moonlighting protein displaying both physiological and pathological functions, plays a crucial role in the differentiation of the zinc-rich oligodendrocytes, the major constituent of myelin sheath; and it is enriched and co-localizes with α-synuclein in brain inclusions hallmarking Parkinson's disease and other synucleinopathies. In this work we showed that the binding of Zn(2+) to TPPP/p25 promotes its dimerization resulting in increased tubulin polymerization promoting activity. We also demonstrated that the Zn(2+) increases the intracellular TPPP/p25 level resulting in a more decorated microtubule network in CHO10 and CG-4 cells expressing TPPP/p25 ectopically and endogenously, respectively. This stabilization effect is crucial for the differentiation and aggresome formation under physiological and pathological conditions, respectively. The Zn(2+)-mediated effect was similar to that produced by treatment of the cells with MG132, a proteasome inhibitor or Zn(2+) plus MG132 as quantified by cellular ELISA. The enhancing effect of zinc ion on the level of TPPP/p25 was independent of the expression level of the protein produced by doxycycline induction at different levels or inhibition of the protein synthesis by cycloheximide. Thus, we suggest that the zinc as a specific divalent cation could be involved in the fine-tuning of the physiological TPPP/p25 level counteracting both the enrichment and the lack of this protein leading to distinct central nervous system diseases.

Myelin architecture: zippering membranes tightly together

Rapid nerve conduction requires the coating of axons by a tightly packed multilayered myelin membrane. In the central nervous system, myelin is formed from cellular processes that extend from oligodendrocytes and wrap in a spiral fashion around an axon, resulting in the close apposition of adjacent myelin membrane bilayers. In this review, we discuss the physical principles underlying the zippering of the plasma membrane of oligodendrocytes at the cytoplasmic and extracellular leaflet. We propose that the interaction of the myelin basic protein with the cytoplasmic leaflet of the myelin bilayer triggers its polymerization into a fibrous network that drives membrane zippering and protein extrusion. In contrast, the adhesion of the extracellular surfaces of myelin requires the down-regulation of repulsive components of the glycocalyx, in order to uncover weak and unspecific attractive forces that bring the extracellular surfaces into close contact. Unveiling the mechanisms of myelin membrane assembly at the cytoplasmic and extracelluar sites may help to understand how the myelin bilayers are disrupted and destabilized in the different demyelinating diseases.

Protein nanopore-based, single-molecule exploration of copper binding to an antimicrobial-derived, histidine-containing chimera peptide

Metal ions binding exert a crucial influence upon the aggregation properties and stability of peptides, and the propensity of folding in various substates. Herein, we demonstrate the use of the α-HL protein as a powerful nanoscopic tool to probe Cu(2+)-triggered physicochemical changes of a 20 aminoacids long, antimicrobial-derived chimera peptide with a His residue as metal-binding site, and simultaneously dissect the kinetics of the free- and Cu(2+)-bound peptide interaction to the α-HL pore. Combining single-molecule electrophysiology on reconstituted lipid membranes and fluorescence spectroscopy, we show that the association rate constant between the α-HL pore and a Cu(2+)-free peptide is higher than that of a Cu(2+)-complexed peptide. We posit that mainly due to conformational changes induced by the bound Cu(2+) on the peptide, the resulting complex encounters a higher energy barrier toward its association with the protein pore, stemming most likely from an extra entropy cost needed to fit the Cu(2+)-complexed peptide within the α-HL lumen region. The lower dissociation rate constant of the Cu(2+)-complexed peptide from α-HL pore, as compared to that of Cu(2+)-free peptide, supports the existence of a deeper free energy well for the protein interaction with a Cu(2+)-complexed peptide, which may be indicative of specific Cu(2+)-mediated contributions to the binding of the Cu(2+)-complexed peptide within the pore lumen.

Mapping brain metals to evaluate therapies for neurodegenerative disease

The brain is rich in metals and has a high metabolic rate, making it acutely vulnerable to the toxic effects of endogenously produced free radicals. The abundant metals, iron and copper, transfer single electrons as they cycle between their reduced (Fe(2+) , Cu(1+) ) and oxidized (Fe(3+) , Cu(2+) ) states making them powerful catalysts of reactive oxygen species (ROS) production. Even redox inert zinc, if present in excess, can trigger ROS production indirectly by altering mitochondrial function. While metal chelators seem to improve the clinical outcome of several neurodegenerative diseases, their mechanisms of action remain obscure and the effects of long-term use are largely unknown. Most chelators are not specific to a single metal and could alter the distribution of multiple metals in the brain, leading to unexpected consequences over the long-term. We show here how X-ray fluorescence will be a valuable tool to examine the effect of chelators on the distribution and amount of metals in the brain.

Copper uptake induces self-assembly of 18.5 kDa myelin basic protein (MBP)

Myelin basic protein (MBP) is predominantly found in the membranes of the myelin sheath of the central nervous system and is involved in important protein-protein and protein-lipid interactions in vivo and in vitro. Furthermore, divalent transition metal ions, especially Zn(2+) and Cu(2+), seem to directly affect the MBP-mediated formation and stabilization of the myelin sheath of the central nervous system. MBP belongs to the realm of intrinsically disordered proteins, and only fragmentary information is available regarding its partial structure(s) or supramolecular arrangements. Here, using standard continuous wave and modern pulse electron paramagnetic resonance methods, as well as dynamic light scattering, we demonstrate the uptake and specific coordination of two Cu(2+) atoms or one Zn(2+) atom per MBP molecule in solution. In the presence of phosphates, further addition of divalent metal ions above a characteristic threshold of four Cu(2+) atoms or two Zn(2+) atoms per MBP molecule leads to the formation of large MBP aggregates within the protein solution. In vivo, MBP-MBP interactions may thus be mediated by divalent cations.

Divalent cations induce a compaction of intrinsically disordered myelin basic protein

Central nervous system myelin is a dynamic entity arising from membrane processes extended from oligodendrocytes, which form a tightly-wrapped multilamellar structure around neurons. In mature myelin, the predominant splice isoform of classic MBP is 18.5kDa. In solution, MBP is an extended, intrinsically disordered protein with a large effective protein surface for myriad interactions, and possesses transient and/or induced ordered secondary structure elements for molecular association or recognition. Here, we show by nanopore analysis that the divalent cations copper and zinc induce a compaction of the extended protein in vitro, suggestive of a tertiary conformation that may reflect its arrangement in myelin.

Structural analysis of the complex between calmodulin and full-length myelin basic protein, an intrinsically disordered molecule

Myelin basic protein (MBP) is present between the cytoplasmic leaflets of the compact myelin membrane in both the peripheral and central nervous systems, and characterized to be intrinsically disordered in solution. One of the best-characterized protein ligands for MBP is calmodulin (CaM), a highly acidic calcium sensor. We pulled down MBP from human brain white matter as the major calcium-dependent CaM-binding protein. We then used full-length brain MBP, and a peptide from rodent MBP, to structurally characterize the MBP-CaM complex in solution by small-angle X-ray scattering, NMR spectroscopy, synchrotron radiation circular dichroism spectroscopy, and size exclusion chromatography. We determined 3D structures for the full-length protein-protein complex at different stoichiometries and detect ligand-induced folding of MBP. We also obtained thermodynamic data for the two CaM-binding sites of MBP, indicating that CaM does not collapse upon binding to MBP, and show that CaM and MBP colocalize in myelin sheaths. In addition, we analyzed the post-translational modifications of rat brain MBP, identifying a novel MBP modification, glucosylation. Our results provide a detailed picture of the MBP-CaM interaction, including a 3D model of the complex between full-length proteins.

Synchrotron X-ray fluorescence reveals abnormal metal distributions in brain and spinal cord in spinocerebellar ataxia: a case report

For the first time, synchrotron rapid-scanning X-ray fluorescence (RS-XRF) was used to simultaneously localize and quantify iron, copper, and zinc in spinal cord and brain in a case of spinocerebellar ataxia (SCA). In the normal medulla, a previously undescribed copper enrichment was seen associated with spinocerebellar fibers and amiculum olivae. This region was virtually devoid of all metals in the SCA case. Regions with neuronal loss and gliosis in the cerebellar cortex, inferior olivary, and dentate nuclei and areas showing loss of myelinated fibers were also low in all metals in SCA compared to control. In contrast, the ventral columns of the spinal cord that exhibited only moderate myelin pallor had increased metal levels. Iron and zinc were also elevated in the globus pallidus pars externa in SCA relative to control. We hypothesize that metals increase as part of the initial neurodegenerative process, but once degeneration is advanced, the metal levels drop. This implies a role for multiple metals in SCA neurodegeneration, but further study is required to establish a causative role. We suggest that if these findings are generally true of at least some cases of SCA, not only iron but also copper and zinc should be considered as possible therapeutic targets.

The peripheral neuropathy-linked Trembler and Trembler-J mutant forms of peripheral myelin protein 22 are folding-destabilized

Dominant mutations in the tetraspan membrane protein peripheral myelin protein 22 (PMP22) are known to result in peripheral neuropathies such as Charcot-Marie-Tooth type 1A (CMT1A) disease via mechanisms that appear to be closely linked to misfolding of PMP22 in the membrane of the endoplasmic reticulum (ER). To characterize the molecular defects in PMP22, we examined the structure and stability of two human disease mutant forms of PMP22 that are also the basis for mouse models of peripheral neuropathies: G150D ( Trembler phenotype) and L16P ( Trembler-J phenotype). Circular dichroism and NMR spectroscopic studies indicated that, when folded, the three-dimensional structures of these disease-linked mutants are similar to that of the folded wild-type protein. However, the folded forms of the mutants were observed to be destabilized relative to the wild-type protein, with the L16P mutant being particularly unstable. The rate of refolding from an unfolded state was observed to be very slow for the wild-type protein, and no refolding was observed for either mutant. These results lead to the hypothesis that ER quality control recognizes the G150D and L16P mutant forms of PMP22 as defective through mechanisms closely related to their conformational instability and/or slow folding. It was also seen that wild-type PMP22 binds Zn(II) and Cu(II) with micromolar affinity, a property that may be important to the stability and function of this protein. Zn(II) was able to rescue the stability defect of the Tr mutant.

Solution NMR and CD spectroscopy of an intrinsically disordered, peripheral membrane protein: evaluation of aqueous and membrane-mimetic solvent conditions for studying the conformational adaptability of the 18.5 kDa isoform of myelin basic protein (MBP)

The stability and secondary structure propensity of recombinant murine 18.5 kDa myelin basic protein (rmMBP, 176 residues) was assessed using circular dichroic and nuclear magnetic resonance spectroscopy (1H-15N HSQC experiments) to determine the optimal sample conditions for further NMR studies (i.e., resonance assignments and protein-protein interactions). Six solvent conditions were selected based on their ability to stabilise the protein, and their tractability to currently standard solution NMR methodology. Selected solvent conditions were further characterised as functions of concentration, temperature, and pH. The results of these trials indicated that 30% TFE-d2 in H2O (v/v), pH 6.5 at 300 K, and 100 mM KCl, pH 6.5 at 277 K were the best conditions to use for future solution NMR studies of MBP. Micelles of DPC were found to be inappropriate for backbone resonance assignments of rmMBP in this instance.

Structural properties of proteins specific to the myelin sheath

The myelin sheath is an insulating membrane layer surrounding myelinated axons in vertebrates, which is formed when the plasma membrane of an oligodendrocyte or a Schwann cell wraps itself around the axon. A large fraction of the total protein in this membrane layer is comprised of only a small number of individual proteins, which have certain intriguing structural properties. The myelin proteins are implicated in a number of neurological diseases, including, for example, autoimmune diseases and peripheral neuropathies. In this review, the structural properties of a number of myelin-specific proteins are described.

A Tale of Two Citrullines—Structural and Functional Aspects of Myelin Basic Protein Deimination in Health and Disease

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive post-translational modifications of MBP is dynamic during normal central nervous system (CNS) development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and with other molecules. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That the degree of MBP deimination is also high in early CNS development indicates that this modification plays major physiological roles in myelin assembly. In this review, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.