Myelin P0: new knowledge and new roles

Complete Loss of Myelin protein zero (MPZ) in a patient with a late onset Charcot-Marie-Tooth (CMT)

Charcot-Marie-Tooth (CMT) comprises a group of hereditary neuropathies with clinical, epidemiological, and molecular heterogeneity in which variants in more than 80 different genes have been reported. One of the important genes which cause 5% of all CMT cases is Myelin protein zero (P0, MPZ). Variants in this gene have been reported in association with different forms of CMT including classical CMT1, severe DSS (CMT3B), DI-CMT, CMT2I and CMT2J with autosomal dominant (AD) inheritance. To our knowledge, MPZ variants have not been described in autosomal recessive (AR) form of CMT in previous studies. Moreover, its complete deletion has not been reported in human. Here, we described clinical characteristics of a patient with CMT symptoms who demonstrated manifestations of the disease late in his life. We performed exome sequencing for identifying CMT subtype and its associated gene, and follow that co-segregation analysis has been done to characterize inheritance pattern of the disorder. Through using exome sequencing, we identified a novel 4074 bp homozygote deletion which encompasses all 6 exons of the MPZ gene in this patient. After identifying the alteration, variant confirmation and co-segregation analysis have been performed by using specific primers. Our result revealed that the patient's parents were heterozygous for the alteration and they did not show any symptoms of CMT. Although most MPZ variants have been described with early onset CMT with AD pattern of inheritance, the reported patient in our study had late onset form and his parents did not show any symptoms. Considering substantial role of MPZ protein in the biogenesis of peripheral nervous system (PNS) myelin, we proposed that there should be another protein in PNS that compensates for lack of MPZ protein. Taken together, our finding is the first report of MPZ association with AR form of CMT with late onset features. Moreover, our results propose the presence of another protein in PNS myelin biogenesis and its assembly. However, functional studies alongside with other molecular studies are needed to confirm our results and identify the proposed protein.

The intrinsically disordered protein glue of the myelin major dense line: Linking AlphaFold2 predictions to experimental data

Numerous human proteins are classified as intrinsically disordered proteins (IDPs). Due to their physicochemical properties, high-resolution structural information about IDPs is generally lacking. On the other hand, IDPs are known to adopt local ordered structures upon interactions with e.g. other proteins or lipid membrane surfaces. While recent developments in protein structure prediction have been revolutionary, their impact on IDP research at high resolution remains limited. We took a specific example of two myelin-specific IDPs, the myelin basic protein (MBP) and the cytoplasmic domain of myelin protein zero (P0ct). Both of these IDPs are crucial for normal nervous system development and function, and while they are disordered in solution, upon membrane binding, they partially fold into helices, being embedded into the lipid membrane. We carried out AlphaFold2 predictions of both proteins and analysed the models in light of experimental data related to protein structure and molecular interactions. We observe that the predicted models have helical segments that closely correspond to the membrane-binding sites on both proteins. We furthermore analyse the fits of the models to synchrotron-based X-ray scattering and circular dichroism data from the same IDPs. The models are likely to represent the membrane-bound state of both MBP and P0ct, rather than the conformation in solution. Artificial intelligence-based models of IDPs appear to provide information on the ligand-bound state of these proteins, instead of the conformers dominating free in solution. We further discuss the implications of the predictions for mammalian nervous system myelination and their relevance to understanding disease aspects of these IDPs.

The effects of melatonin in the treatment of acute brachial plexus compression injury in rats

Introduction

Brachial plexus injury (BPI) is one of the most destructive peripheral nerve injuries and there is still a lack of effective treatment.

Methods

This study was conducted to evaluate the effects of melatonin in the treatment of acute brachial plexus compression injury in rats using histopathological, histomorphometric, immunohistochemical and electrophysiological methods. Forty-eight adult male Sprague Dawley rats were randomly allocated into three groups: sham, melatonin and vehicle groups. The brachial plexus compression injury model was performed by a vascular clamp. Melatonin group received intraperitoneal injection of melatonin at doses of 10 mg/kg for 21 days after crush injury. The conduction velocity and amplitude of compound muscle action potential (CAMP) in the regenerated nerve, and nerve histomorphometry, as well as levels of myelin protein zero (P0) protein of the crush region were assessed.

Results

Compared with the vehicle group, the melatonin group which reported significant increased CMAP conduction velocity and amplitude also showed thicker myelin sheath and lower levels of P0 protein.

Discussion

Our results suggest that melatonin effectively promotes nerve regeneration and improves the function of damaged nerves. Melatonin treatment is a promising strategy for the treatment of acute brachial plexus compression injury.

Mechanisms and Treatments in Demyelinating CMT

Demyelinating forms of Charcot-Marie-Tooth disease (CMT) are genetically and phenotypically heterogeneous and result from highly diverse biological mechanisms including gain of function (including dominant negative effects) and loss of function. While no definitive treatment is currently available, rapid advances in defining the pathomechanisms of demyelinating CMT have led to promising pre-clinical studies, as well as emerging clinical trials. Especially promising are the recently completed pre-clinical genetic therapy studies in PMP-22, GJB1, and SH3TC2-associated neuropathies, particularly given the success of similar approaches in humans with spinal muscular atrophy and transthyretin familial polyneuropathy. This article focuses on neuropathies related to mutations in PMP-22, MPZ, and GJB1, which together comprise the most common forms of demyelinating CMT, as well as on select rarer forms for which promising treatment targets have been identified. Clinical characteristics and pathomechanisms are reviewed in detail, with emphasis on therapeutically targetable biological pathways. Also discussed are the challenges facing the CMT research community in its efforts to advance the rapidly evolving biological insights to effective clinical trials. These considerations include the limitations of currently available animal models, the need for personalized medicine approaches/allele-specific interventions for select forms of demyelinating CMT, and the increasing demand for optimal clinical outcome assessments and objective biomarkers.

The Transcription Factor TFCP2L1 is Associated with Myelination via miR708-5p Regulation in the Peripheral Nerve System

MicroRNAs (miRNAs) have been implicated in nerve injury and demyelination; however, their functions in peripheral nerves remain unclear. To determine the potential functions of miRNAs, an miRNA array was carried out. Here, miRNA array analysis of neuregulin-treated Schwann cells revealed 18 upregulated (> 2-fold) and 13 downregulated (> 2-fold) miRNAs. After sciatic nerve injury, miR708-5p was highly expressed in neuregulin-treated Schwann cells, whereas it was downregulated during postnatal development. A predicted functional interaction was found between miR708-5p and transcription factor CP2-like protein 1 (TFCP2L1) using a bioinformatics tool. This finding suggested that miR708-5p may regulate TFCP2L1. During sciatic nerve development, TFCP2L1 was upregulated on postnatal days 1 and 4, while it was downregulated after nerve axotomy and crush injury. Notably, TFCP2L1 was upregulated in cAMP-treated Schwann cells. We also found that activity of the myelin protein zero promoter was downregulated in TFCP2L1 siRNA-treated Schwann cells, whereas it was upregulated in TFCP2L1-overexpressing cells. Immunofluorescence analysis showed that TFCP2L1 was localized in Schwann cells. In addition, miR708-5p overexpression promoted migration of Schwann cells, while miR-708-5p inhibitor inhibited migration. miR708-5p inhibitor also blocked the migration of TFCP2L1 siRNA-treated Schwann cells. These findings indicate the functions of miR708-5p in TFCP2L1 regulation in the peripheral nervous system occur via regulation of Schwann cell migration.

Schwann-Cell-Specific Deletion of Phosphatidylinositol 4-Kinase Alpha Causes Aberrant Myelination

Active membrane remodeling during myelination relies on phospholipid synthesis and membrane polarization, both of which are known to depend on inositol phospholipids. Here, we show that sciatic nerves of mice lacking phosphatidylinositol 4-kinase alpha (PI4KA) in Schwann cells (SCs) show substantially reduced myelin thickness with grave consequences on nerve conductivity and motor functions. Surprisingly, prolonged inhibition of PI4KA in immortalized mouse SCs failed to decrease plasma membrane phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) levels or PI 3-kinase (PI3K) activation, in spite of large reductions in plasma membrane PI4P levels. Instead, it caused rearrangements of the actin cytoskeleton, which was also observed in sciatic nerves of knockout animals. PI4KA inactivation disproportionally reduced phosphatidylserine, phosphatidylethanolamine, and sphingomyelin content in mutant nerves, with similar changes observed in SCs treated with a PI4KA inhibitor. These studies define a role for PI4KA in myelin formation primarily affecting metabolism of key phospholipids and the actin cytoskeleton.

Neuropathy-related mutations alter the membrane binding properties of the human myelin protein P0 cytoplasmic tail

Schwann cells myelinate selected axons in the peripheral nervous system (PNS) and contribute to fast saltatory conduction via the formation of compact myelin, in which water is excluded from between tightly adhered lipid bilayers. Peripheral neuropathies, such as Charcot-Marie-Tooth disease (CMT) and Dejerine-Sottas syndrome (DSS), are incurable demyelinating conditions that result in pain, decrease in muscle mass, and functional impairment. Many Schwann cell proteins, which are directly involved in the stability of compact myelin or its development, are subject to mutations linked to these neuropathies. The most abundant PNS myelin protein is protein zero (P0); point mutations in this transmembrane protein cause CMT subtype 1B and DSS. P0 tethers apposing lipid bilayers together through its extracellular immunoglobulin-like domain. Additionally, P0 contains a cytoplasmic tail (P0ct), which is membrane-associated and contributes to the physical properties of the lipid membrane. Six CMT- and DSS-associated missense mutations have been reported in P0ct. We generated recombinant disease mutant variants of P0ct and characterized them using biophysical methods. Compared to wild-type P0ct, some mutants have negligible differences in function and folding, while others highlight functionally important amino acids within P0ct. For example, the D224Y variant of P0ct induced tight membrane multilayer stacking. Our results show a putative molecular basis for the hypermyelinating phenotype observed in patients with this particular mutation and provide overall information on the effects of disease-linked mutations in a flexible, membrane-binding protein segment. Using neutron reflectometry, we additionally show that P0ct embeds deep into a lipid bilayer, explaining the observed effects of P0ct on the physical properties of the membrane.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.

Molecular structure and function of myelin protein P0 in membrane stacking

Compact myelin forms the basis of nerve insulation essential for higher vertebrates. Dozens of myelin membrane bilayers undergo tight stacking, and in the peripheral nervous system, this is partially enabled by myelin protein zero (P0). Consisting of an immunoglobulin (Ig)-like extracellular domain, a single transmembrane helix, and a cytoplasmic extension (P0ct), P0 harbours an important task in ensuring the integrity of compact myelin in the extracellular compartment, referred to as the intraperiod line. Several disease mutations resulting in peripheral neuropathies have been identified for P0, reflecting its physiological importance, but the arrangement of P0 within the myelin ultrastructure remains obscure. We performed a biophysical characterization of recombinant P0ct. P0ct contributes to the binding affinity between apposed cytoplasmic myelin membrane leaflets, which not only results in changes of the bilayer properties, but also potentially involves the arrangement of the Ig-like domains in a manner that stabilizes the intraperiod line. Transmission electron cryomicroscopy of native full-length P0 showed that P0 stacks lipid membranes by forming antiparallel dimers between the extracellular Ig-like domains. The zipper-like arrangement of the P0 extracellular domains between two membranes explains the double structure of the myelin intraperiod line. Our results contribute to the understanding of PNS myelin, the role of P0 therein, and the underlying molecular foundation of compact myelin stability in health and disease.

DHTKD1 Deficiency Causes Charcot-Marie-Tooth Disease in Mice

DHTKD1, a part of 2-ketoadipic acid dehydrogenase complex, is involved in lysine and tryptophan catabolism. Mutations in DHTKD1 block the metabolic pathway and cause 2-aminoadipic and 2-oxoadipic aciduria (AMOXAD), an autosomal recessive inborn metabolic disorder. In addition, a nonsense mutation in DHTKD1 that we identified previously causes Charcot-Marie-Tooth disease (CMT) type 2Q, one of the most common inherited neurological disorders affecting the peripheral nerves in the musculature. However, the comprehensive molecular mechanism underlying CMT2Q remains elusive. Here, we show that Dhtkd1^-/- mice mimic the major aspects of CMT2 phenotypes, characterized by progressive weakness and atrophy in the distal parts of limbs with motor and sensory dysfunctions, which are accompanied with decreased nerve conduction velocity. Moreover, DHTKD1 deficiency causes severe metabolic abnormalities and dramatically increased levels of 2-ketoadipic acid (2-KAA) and 2-aminoadipic acid (2-AAA) in urine. Further studies revealed that both 2-KAA and 2-AAA could stimulate insulin biosynthesis and secretion. Subsequently, elevated insulin regulates myelin protein zero (Mpz) transcription in Schwann cells via upregulating the expression of early growth response 2 (Egr2), leading to myelin structure damage and axonal degeneration. Finally, 2-AAA-fed mice do reproduce phenotypes similar to CMT2Q phenotypes. In conclusion, we have demonstrated that loss of DHTKD1 causes CMT2Q-like phenotypes through dysregulation of Mpz mRNA and protein zero (P₀) which are closely associated with elevated DHTKD1 substrate and insulin levels. These findings further indicate an important role of metabolic disorders in addition to mitochondrial insufficiency in the pathogenesis of peripheral neuropathies.

Endoplasmic Reticulum Protein Quality Control Failure in Myelin Disorders

Reaching the correct three-dimensional structure is crucial for the proper function of a protein. The endoplasmic reticulum (ER) is the organelle where secreted and transmembrane proteins are synthesized and folded. To guarantee high fidelity of protein synthesis and maturation in the ER, cells have evolved ER-protein quality control (ERQC) systems, which assist protein folding and promptly degrade aberrant gene products. Only correctly folded proteins that pass ERQC checkpoints are allowed to exit the ER and reach their final destination. Misfolded glycoproteins are detected and targeted for degradation by the proteasome in a process known as endoplasmic reticulum-associated degradation (ERAD). The excess of unstructured proteins in the ER triggers an adaptive signal transduction pathway, called unfolded protein response (UPR), which in turn potentiates ERQC activities in order to reduce the levels of aberrant molecules. When the situation cannot be restored, the UPR drives cells to apoptosis. Myelin-forming cells of the central and peripheral nervous system (oligodendrocytes and Schwann cells) synthesize a large amount of myelin proteins and lipids and therefore are particularly susceptible to ERQC failure. Indeed, deficits in ERQC and activation of ER stress/UPR have been implicated in several myelin disorders, such as Pelizaeus-Merzbacher and Krabbe leucodystrophies, vanishing white matter disease and Charcot-Marie-Tooth neuropathies. Here we discuss recent evidence underlying the importance of proper ERQC functions in genetic disorders of myelinating glia.

Glycans of myelin proteins

Human P0 is the main myelin glycoprotein of the peripheral nervous system. It can bind six different glycans, all linked to Asn(93) , the unique glycosylation site. Other myelin glycoproteins, also with a single glycosylation site (PMP22 at Asn(36) , MOG at Asn(31) ), bind only one glycan. The MAG has 10 glycosylation sites; the glycoprotein OMgp has 11 glycosylation sites. Aside from P0, no comprehensive data are available on other myelin glycoproteins. Here we review and analyze all published data on the physicochemical structure of the glycans linked to P0, PMP22, MOG, and MAG. Most data concern bovine P0, whose glycan moieties have an MW ranging from 1,294.56 Da (GP3) to 2,279.94 Da (GP5). The pI of glycosylated P0 protein varies from pH 9.32 to 9.46. The most charged glycan is MS2 containing three sulfate groups and one glucuronic acid; whereas the least charged one is the BA2 residue. All glycans contain one fucose and one galactose. The most mannose rich are the glycans MS2 and GP4, each of them has four mannoses; OPPE1 contains five N-acetylglucosamines and one sulfated glucuronic acid; GP4 contains one sialic acid. Furthermore, human P0 variants causing both gain and loss of glycosylation have been described and cause peripheral neuropathies with variable clinical severity. In particular, the substitution T(95) →M is a very common in Europe and is associated with a late-onset axonal neuropathy. Although peripheral myelin is made up largely of glycoproteins, mutations altering glycosylation have been described only in P0. This attractive avenue of research requires further study.

Lipid membrane association of myelin proteins and peptide segments studied by oriented and synchrotron radiation circular dichroism spectroscopy

Myelin-specific proteins are either integral or peripheral membrane proteins that, in complex with lipids, constitute a multilayered proteolipid membrane system, the myelin sheath. The myelin sheath surrounds the axons of nerves and enables rapid conduction of axonal impulses. Myelin proteins interact intimately with the lipid bilayer and play crucial roles in the assembly, function, and stability of the myelin sheath. Although myelin proteins have been investigated for decades, their structural properties upon membrane surface binding are still largely unknown. In this study, we have used simplified model systems consisting of synthetic peptides and membrane mimics, such as detergent micelles and/or lipid vesicles, to probe the conformation of peptides using synchrotron radiation circular dichroism spectroscopy (SRCD). Additionally, oriented circular dichroism spectroscopy (OCD) was employed to examine the orientation of myelin peptides in macroscopically aligned lipid bilayers. Various representative peptides from the myelin basic protein (MBP), P0, myelin/oligodencrocyte glycoprotein, and connexin32 (cx32) were studied. A helical peptide from the central immunodominant epitope of MBP showed a highly tilted orientation with respect to the membrane surface, whereas the N-terminal cytoplasmic segment of cx32 folded into a helical structure that was only slightly tilted. The folding of full-length myelin basic protein was, furthermore, studied in a bicelle environment. Our results provide information on the conformation and membrane alignment of important membrane-binding peptides in a membrane-mimicking environment, giving novel insights into the mechanisms of membrane binding and stacking by myelin proteins.

Myelin-specific proteins: a structurally diverse group of membrane-interacting molecules

The myelin sheath is a multilayered membrane in the nervous system, which has unique biochemical properties. Myelin carries a set of specific high-abundance proteins, the structure and function of which are still poorly understood. The proteins of the myelin sheath are involved in a number of neurological diseases, including autoimmune diseases and inherited neuropathies. In this review, we briefly discuss the structural properties and functions of selected myelin-specific proteins (P0, myelin oligodendrocyte glycoprotein, myelin-associated glycoprotein, myelin basic protein, myelin-associated oligodendrocytic basic protein, P2, proteolipid protein, peripheral myelin protein of 22 kDa, 2',3'-cyclic nucleotide 3'-phosphodiesterase, and periaxin); such properties include, for example, interactions with lipid bilayers and the presence of large intrinsically disordered regions in some myelin proteins. A detailed understanding of myelin protein structure and function at the molecular level will be required to fully grasp their physiological roles in the myelin sheath.

Developmental regulation of insulin receptor gene in sciatic nerves and role of insulin on glycoprotein P0 in the Schwann cells

In view of the observations that Schwann cells contain insulin receptors, in the present study, we have investigated the developmental regulation of insulin receptor gene in the sciatic nerves of different postnatal age group rats. We have also investigated the role of insulin in the expression of the major PNS myelin glycoprotein P zero (P0) in normal as well as high glucose conditions in primary rat Schwann cells. The expression of insulin receptor gene in sciatic nerves appeared to be differentially regulated. The steady-state levels of insulin receptor mRNA increased remarkably during development and after postnatal day 10, when the peak of myelin structural gene (P0) expression occur and slowly increased further until at least postnatal day 90 in parallel with the growth of the myelin sheath. By employing immunofluorescence and RT-PCR, we observed significant increase in the P0 protein and mRNA levels in Schwann cells in response to the insulin than in insulin deprived counterparts. The presence of insulin in the high glucose medium ameliorated the altered protein and mRNA of P0 in Schwann cells compared to the insulin deprived counterparts. These studies demonstrate the importance of insulin and its receptor as possible regulatory factors in the PNS and also emphasizes their novel therapeutic applications in demyelinating diseases, especially in diabetic poly-neuropathy.

Gain of glycosylation: a new pathomechanism of myelin protein zero mutations

We report the first case of a missense mutation in MPZ causing a gain of glycosylation in myelin protein zero, the main protein of peripheral nervous system myelin. The patient was affected by a severe demyelinating neuropathy caused by a missense mutation, D32N, that created a new glycosylation sequence. We confirmed that the mutant protein is hyperglycosylated, is partially retained into the Golgi apparatus in vitro, and disrupts intercellular adhesion. By sequential experiments, we demonstrated that hyperglycosylation is the main mechanism of this mutation. Gain of glycosylation is a new mechanism in Charcot-Marie-Tooth type 1B.

Progesterone synthesis in the nervous system: implications for myelination and myelin repair

Progesterone is well known as a female reproductive hormone and in particular for its role in uterine receptivity, implantation, and the maintenance of pregnancy. However, neuroendocrine research over the past decades has established that progesterone has multiple functions beyond reproduction. Within the nervous system, its neuromodulatory and neuroprotective effects are much studied. Although progesterone has been shown to also promote myelin repair, its influence and that of other steroids on myelination and remyelination is relatively neglected. Reasons for this are that hormonal influences are still not considered as a central problem by most myelin biologists, and that neuroendocrinologists are not sufficiently concerned with the importance of myelin in neuron functions and viability. The effects of progesterone in the nervous system involve a variety of signaling mechanisms. The identification of the classical intracellular progesterone receptors as therapeutic targets for myelin repair suggests new health benefits for synthetic progestins, specifically designed for contraceptive use and hormone replacement therapies. There are also major advantages to use natural progesterone in neuroprotective and myelin repair strategies, because progesterone is converted to biologically active metabolites in nervous tissues and interacts with multiple target proteins. The delivery of progesterone however represents a challenge because of its first-pass metabolism in digestive tract and liver. Recently, the intranasal route of progesterone administration has received attention for easy and efficient targeting of the brain. Progesterone in the brain is derived from the steroidogenic endocrine glands or from local synthesis by neural cells. Stimulating the formation of endogenous progesterone is currently explored as an alternative strategy for neuroprotection, axonal regeneration, and myelin repair.

Lysosomal exocytosis in Schwann cells contributes to axon remyelination

Myelin biogenesis is a complex process involving coordinated exocytosis, endocytosis, mRNA transport, and cytoskeletal dynamics. Although abnormalities of myelin are common in lysosomal storage diseases, our understanding of the role of lysosomes in the formation and maintenance of myelin is still limited. Here, we show that late endosomes/lysosomes in Schwann cells contain abundant myelin protein P0, which accounts for over half the total protein of compact myelin in the peripheral nervous system and exhibit Ca(2+) -dependent exocytosis in response to various stimuli. Downregulation of Rab27a, a small GTPase required for the trafficking of the secretory lysosomes to the plasma membrane, largely blocked lysosomal exocytosis in Schwann cells and reduced the remyelination of regenerated sciatic nerve. These findings highlight a novel role for lysosomes in Schwann cells and suggest that the regulated lysosome exocytosis in Schwann cells may have important physiological and pathological significance in the peripheral nervous system.

A new long term in vitro model of myelination

Besides in vivo models, co-cultures systems making use of Rat dorsal root ganglion explants/Schwann cells (SC) are widely used to essentially study myelination in vitro. In the case of animal models of demyelinating diseases, it is expected to reproduce a pathological process; conversely the co-cultures are primarily developed to study the myelination process and in the aim to use them to replace animals in experiences of myelin destruction or functional disturbances. We describe (in terms of protein expression kinetic) a new in vitro model of sensory neurons/SC co-cultures presenting the following advantages: both sensory neurons and SC originate from the same individual; sensory neurons and SC being dissociated, they can be co-cultured in monolayer, allowing an easier microscope observation; the co-culture can be maintained in a serum-free medium for at less three months, allowing kinetic studies of myelin formation both at a molecular and cellular level. Optimizing culture conditions permits to use 96-well culture plates; image analyses conducted with an automatic image analyzer allows rapid, accurate and quantitative expression of results. Finally, this system was proved by measuring the apparition of myelin protein to mimic in vitro the physiological process of in vivo myelination.

Conformations of peptides derived from myelin-specific proteins in membrane-mimetic conditions probed by synchrotron radiation CD spectroscopy

Myelin is a tightly packed membrane multilayer in the nervous system, which harbours a specific set of quantitatively major proteins. All these proteins interact with the lipid bilayer, being either peripheral or integral membrane proteins. In this study, we examined the conformational properties of peptides from the myelin proteins P0, CNPase, MOBP, P2 and MOG, using trifluoroethanol and micelles of different detergents as membrane-like mimics. The peptides showed significant differences in their folding under the employed conditions, as evidenced by synchrotron radiation circular dichroism spectroscopy. Our experiments provide new structural information on the interactions between myelin proteins and membranes, using a simplified model system of synthetic peptides and micelles.

Mpz gene suppression by shRNA increases Schwann cell apoptosis in vitro

We investigated the effects of short hairpin RNA (shRNA) on myelin protein zero (MPZ) gene expression in Schwann cells (SCs) in vitro and determined the effects of the MPZ gene suppression on the survival of SCs. The MPZ-specific shRNA was introduced into a lentiviral vector for expression under the U6 promoter, and the viral vector-based shRNAs were used to infect cultured SCs. The efficiency of MPZ knockdown was analyzed by real time-PCR (RT-PCR) and western blotting. Flow cytometric analysis and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) were used to determine the cell cycle and the amount of apoptosis of SCs. We found that MPZ shRNAs significantly inhibited the expression of the MPZ gene and induced SC apoptosis in vitro. These results provided interesting experimental evidence for understanding the mechanism of demyelinating neuropathies caused by MPZ gene malfunction.

Clinical features and molecular modelling of novel MPZ mutations in demyelinating and axonal neuropathies

Mutations in the myelin protein zero (MPZ) gene have been associated with different Charcot-Marie-Tooth disease (CMT) phenotypes, including classical demyelinating CMT1B and the axonal form of the disease (CMT2). The MPZ role in the pathogenesis of both demyelinating and axonal inherited neuropathies was evaluated in the Italian population by screening a cohort of 214 patients with CMT1 or CMT2. A MPZ mutation frequency of 7.9% in demyelinating cases and of 4.8% in axonal cases was observed. In the total cohort (264 patients), including those with mutations in other genes, a mutation frequency of 5.8% (7/121) in demyelinating cases and 4.2% (6/143) in axonal cases was found. Three novel MPZ mutations, two missense (p.Ser111Cys, p.Thr124Ala) and one frameshift (p.Tyr145fs) were found, and a molecular modelling approach was used to test the effects of these mutations on the protein structure. Electrostatic distribution changes within the protein, caused by the amino acid substitution, fit in with phenotypes presented by patients herein described. Our findings suggest that the clinical features associated with MPZ mutations depend partly on the nature of amino acid change and that molecular modelling may provide useful support, based on effects on secondary and tertiary protein structure, to predict the phenotype associated with MPZ mutations.

Phosphorylation and localization of protein-zero related (PZR) in cultured endothelial cells

Protein-zero related (PZR) is an immunoglobulin V (IgV)-type immunoreceptor with two immunoreceptor tyrosine-based inhibitory motifs (ITIMs). PZR interacts with Src homology 2 domain-containing tyrosine phosphatase (SHP-2) via its tyrosine-phosphorylated ITIMs, for which c-Src is a putative kinase. Towards elucidating PZR function in endothelial cells (ECs), the authors cloned PZR from bovine aortic endothelial cells (BAECs) and characterized it. Mature bovine PZR had 94.8% and 92.7% sequence identity with canine and human proteins, respectively, and the two ITIM sequences were conserved among higher vertebrates. PZR was expressed in many cell types and was localized to cell contacts and intracellular granules in BAECs and mesothelioma (REN) cells. Coimmunoprecipitation revealed that PZR, Grb-2-associated binder-1 (Gab1), and platelet endothelial cell adhesion molecule-1 (PECAM-1) were three major SHP-2-binding proteins in BAECs. H(2)O(2) enhanced PZR tyrosine phosphorylation and PZR/SHP-2 interaction in ECs in a dose-and time-dependent manner. To see if tyrosine kinases other than Src are also capable of phosphorylating PZR, the authors cotransfected HEK293 cells with PZR and one of several tyrosine kinases and found that c-Src, c-Fyn, c-Lyn, Csk, and c-Abl, but not c-Fes, phosphorylated PZR and increased PZR/SHP-2 interaction. These results suggest that PZR is a cell adhesion protein that may be involved in SHP-2-dependent signaling at interendothelial cell contacts.

Different cellular and molecular mechanisms for early and late-onset myelin protein zero mutations

Mutations in the gene MPZ, encoding myelin protein zero (MPZ), cause inherited neuropathies collectively called Charcot-Marie-Tooth type 1B (CMT1B). Based on the age of onset, clinical and pathological features, most MPZ mutations are separable into two groups: one causing a severe, early-onset, demyelinating neuropathy and a second, causing a late-onset neuropathy with prominent axonal loss. To investigate potential pathomechanisms underlying the two phenotypes, we transiently transfected HeLa cells with two late-onset (T95M, H10P) and two early-onset (H52R, S22_W28 deletion) mutations and analyzed their effects on intracellular protein trafficking, glycosylation, cell viability and intercellular adhesion. We found that the two late-onset mutations were both transported to the cell membrane and moderately reduced MPZ-mediated intercellular adhesion. The two early-onset mutations caused two distinct abnormalities. H52R was correctly glycosylated and trafficked to the plasma membrane, but strongly affected intercellular adhesion. When co-expressed with wild-type MPZ (wtMPZ), a functional dominant negative effect was observed. Alternatively, S22_W28 deletion was retained within the cytoplasm and reduced both adhesion caused by wtMPZ and cellular viability. Since the same trafficking patterns were observed in transfected murine Schwann cells, they are not an artifact of heterologous cell expression. Our results suggest that at least some late-onset mutations cause a partial loss of function in the transfected cells, whereas multiple abnormal gain of function pathways can result in early-onset neuropathy. Further characterization of these pathways will lead to a better understanding of the pathogenesis of CMT1B and a rational basis for treating these debilitating inherited neuropathies.

Molecular Characterization of Myelin Protein Zero in Xenopus laevis Peripheral Nerve: Equilibrium between Non-covalently Associated Dimer and Monomer

Myelin protein zero (P0), a glycosylated single-pass transmembrane protein, is essential in the formation and maintenance of peripheral nervous system (PNS) compact myelin. P0 in Xenopus (xP0) exists primarily as a dimeric form that remains stable after various physical and chemical treatments. In exploring the nature of the interactions underlying the dimer stability, we found that xP0 dimer dissociated into monomer during continuous elution gel electrophoresis and conventional SDS-PAGE, indicating that the dimer is stabilized by non-covalent interactions. Furthermore, as some of the gel-purified monomer re-associated into dimer on SDS-PAGE gels, there is likely a dynamic equilibrium between xP0 dimer and monomer in vivo. Because the carbohydrate and fatty acyl moieties may be crucial for the adhesion role of P0, we used sensitive mass spectrometry approaches to elucidate the detailed N-glycosylation and S-acylation profiles of xP0. Asn92 was determined to be the single, fully-occupied glycosylation site of xP0, and a total of 12 glycans was detected that exhibited new structural features compared with those observed from P0 in other species: (1) the neutral glycans were composed mainly of high mannose and hybrid types; (2) five of twelve were acidic glycans, among which three were sialylated and the other two were sulfated; (3) none of the glycans had core fucosylation; and (4) no glucuronic acid, hence no HNK-1 epitope, was detected. The drastically different carbohydrate structures observed here support the concept of the species-specific variation in N-glycosylation of P0. Cys152 was found to be acylated with stearoyl (C18:0), whereas palmitoyl (C16:0) is the corresponding predominant fatty acyl group on P0 from higher vertebrates. We propose that the unique glycosylation and acylation patterns of Xenopus P0 may underlie its unusual dimerization behaviour. Our results should shed light on the understanding of the phylogenetic development of P0's adhesion role in PNS compact myelin.

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.

Congenital hypomyelinating neuropathy, a long term follow-up study in an affected family

Congenital hypomyelinating neuropathy is a rare condition characterized by prenatal, neonatal or early infantile onset of hypotonia, paresis and areflexia. Most of the few patients described in literature die within the first years of life. Histopathologically there are no or thin myelin sheaths. Mutations have been described in the following genes, MPZ, EGR2, PMP22, and MTMR2. Here we describe a family with a heterozygous mutation in MPZ, confirmed in two generations.

Myelin protein zero/P0 phosphorylation and function require an adaptor protein linking it to RACK1 and PKC alpha

Point mutations in the cytoplasmic domain of myelin protein zero (P0; the major myelin protein in the peripheral nervous system) that alter a protein kinase Cα (PKCα) substrate motif (198HRSTK201) or alter serines 199 and/or 204 eliminate P0-mediated adhesion. Mutation in the PKCα substrate motif (R198S) also causes a form of inherited peripheral neuropathy (Charcot Marie Tooth disease [CMT] 1B), indicating that PKCα-mediated phosphorylation of P0 is important for myelination. We have now identified a 65-kD adaptor protein that links P0 with the receptor for activated C kinase 1 (RACK1). The interaction of p65 with P0 maps to residues 179–197 within the cytoplasmic tail of P0. Mutations or deletions that abolish p65 binding reduce P0 phosphorylation and adhesion, which can be rescued by the substitution of serines 199 and 204 with glutamic acid. A mutation in the p65-binding sequence G184R occurs in two families with CMT, and mutation of this residue results in the loss of both p65 binding and adhesion function.

Dichloroacetate causes reversible demyelination in vitro: potential mechanism for its neuropathic effect

Dichloroacetate (DCA) is an investigational drug for genetic mitochondrial diseases whose use has been mitigated by reversible peripheral neuropathy. We investigated the mechanism of DCA neurotoxicity using cultured rat Schwann cells (SCs) and dorsal root ganglia (DRG) neurons. Myelinating SC-DRG neuron co-cultures, isolated SCs and DRG neurons were exposed to 1-20 mm DCA for up to 12 days. In myelinating co-cultures, DCA caused a dose- and exposure-dependent decrease of myelination, as determined by immunolabeling and immunoblotting for myelin basic protein (MBP), protein zero (P0), myelin-associated glycoprotein (MAG) and peripheral myelin protein 22 (PMP22). Partial recovery of myelination occurred following a 10-day washout of DCA. DCA did not affect the steady-state levels of intermediate filament proteins, but promoted the formation of anti-neurofilament antibody reactive whirls. In isolated SC cultures, DCA decreased the expression of P0 and PMP22, while it increased the levels of p75(NTR) (neurotrophin receptor), as compared with non-DCA-treated samples. DCA had modest adverse effects on neuronal and glial cell vitality, as determined by the release of lactate dehydrogenase. These results demonstrate that DCA induces a reversible inhibition of myelin-related proteins that may account, at least in part, for its clinical peripheral neuropathic effects.

Myelin protein zero is one of the components of the detergent-resistant membrane microdomain fraction prepared from rat pituitary

Pituitary gland is a well-known endocrine tissue. The hypothalamo-neurohypophysial system, containing arginine vasopressin and oxytocin, shows a reversible morphological reorganization of both neurons and glial cells during chronic physiological stimulations. Since many signal transducing and cell adhesion molecules (CAMs) are recovered in membrane microdomain (MD) fractions, MDs are considered as signaling platforms of cells. In order to know the molecular background for these endocrine systems, we characterized MD-components derived from rat pituitary and found specific enrichment of several proteins in the fraction. One of them was identified as myelin protein zero (P0) with mass analysis and this result was further confirmed by a result that a specific antibody to this protein reacted to the authentic P0 protein in the myelin fraction of rat sciatic nerve. P0 is one of type-I transmembrane CAMs and a major structural component of mammalian peripheral nerve myelin. In mammals, expression of P0 has been considered to be restricted to peripheral nervous system. This result however indicates that P0 expresses more widely and its enrichment in the MD-fraction from rat pituitary suggests the participation in cell-cell communications.

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.

Heterozygous P0 deficiency protects mice from vincristine-induced polyneuropathy

Patients with hereditary neuropathies are more susceptible to vincristine (VIN)-induced neuropathy than patients without this comorbidity. The heterozygous P0(+/-) mouse is an animal model of a distinct form of inherited neuropathies. These mice produce only 50% of the major myelin protein protein zero (P0) and display signs of demyelination in motor nerves at 4 months of age. Here we investigated the development of neuropathic signs in P0(+/-) and wild-type (wt) mice after VIN treatment. Neuropathy was induced by daily intraperitoneal injections of VIN (0.5 mg/kg body weight) over 10 days. Behavioral and electrophysiological tests were performed at regular time points. Wt mice developed significant hypersensitivity to heat and mechanical stimuli between days 7 and 38 after the first VIN injection. Surprisingly, P0(+/-) mice did not show sensory or motor signs of neuropathy over the whole testing period. Immunohistochemical analysis showed an increase in macrophage numbers in sciatic nerve sections of wt mice after VIN, whereas P0(+/-) mice had higher baseline levels of macrophages without changes after VIN treatment. Semithin sections revealed a decrease in the number of small-diameter myelinated fibers in the sciatic nerves of wt mice after VIN application, whereas P0(+/-) mice had higher baseline values of this fiber subtype that did not change under treatment. Dorsal root ganglion neurons of both genotypes showed an up-regulation of voltage-gated sodium channel immunoreactivity after VIN application without differences between the genotypes. Thus, the P0(+/-) phenotype seems to be protected against VIN-induced neuropathy. The mechanism of this neuroprotection remains elusive.

Myelin protein zero: mutations in the cytoplasmic domain interfere with its cellular trafficking

The cytoplasmic domain of myelin protein zero (MPZ), the principal protein of peripheral myelin, undergoes phosphorylation on several serine residues and a tyrosine group that is maximal during peak nerve myelination. Mutations that could affect MPZ phosphorylation cause the inherited neuropathy, Charcot-Marie-Tooth disease Type 1B. To investigate a possible role for phosphorylation in regulation of MPZ trafficking within the cell, we expressed wild-type and mutated MPZ-enhanced green fluorescent protein (GFP) fusion proteins in cultured Schwann-like cells. Whereas wild-type protein is present almost entirely at the cell surface, mutation of serine 204 to alanine or at a nearby presumed PKC substrate motif (198RSTK201) causes 40-60% of protein to be retained in the cytoplasm. Mutation of S204 to aspartate, which introduces a permanent negative charge, also impairs MPZ movement to the plasma membrane. In contrast, tyrosine 191 mutation has no effect on MPZ cellular distribution. Simultaneous alteration of S204 and Y191 produces much less perturbation of MPZ trafficking than mutation of S204 alone. Colocalization studies showed that mutated MPZ-EGFP trapped in the cytoplasm associates with all organelles in the secretory pathway. Previous studies have shown that cytoplasmic mutations at serine, but not tyrosine phosphorylation sites, abolish MPZ adhesive properties. Our results suggest that this loss of adhesion may be due, at least in part, to a failure of sufficient MPZ to reach the cell surface and that this impaired trafficking is associated with deficient serine phosphorylation in the cytoplasmic domain.

Molecular reconstruction of nodes of Ranvier after remyelination by transplanted olfactory ensheathing cells in the demyelinated spinal cord

Myelin-forming glial cells transplanted into the demyelinated spinal cord can form compact myelin and improve conduction properties. However, little is known of the expression and organization of voltage-gated ion channels in the remyelinated central axons or whether the exogenous cells provide appropriate signaling for the maturation of nodes of Ranvier. Here, we transplanted olfactory ensheathing cells from green fluorescent protein (GFP)-expressing donor rats [GFP-olfactory ensheathing cells (OECs)] into a region of spinal cord demyelination and found extensive remyelination, which included the development of mature nodal, paranodal, and juxtaparanodal domains, as assessed by ultrastructural, immunocytochemical, and electrophysiological analyses. In remyelinated axons, Nav1.6 was clustered at nodes, whereas Kv1.2 was aggregated in juxtaparanodal regions, recapitulating the distribution of these channels within mature nodes of uninjured axons. Moreover, the recruitment of Nav and Kv channels to specific membrane domains at remyelinated nodes persisted for at least 8 weeks after GFP-OEC transplantation. In vivo electrophysiological recordings demonstrated enhanced conduction along the GFP-OEC-remyelinated axons. These findings indicate that, in addition to forming myelin, engrafted GFP-OECs provide an environment that supports the development and maturation of nodes of Ranvier and the restoration of impulse conduction in central demyelinated axons.

JNK1 activation mediates C5b-9-induced P0 mRNA instability and P0 gene expression in Schwann cells

The protein zero (P0) glycoprotein is an important component of compact peripheral nerve myelin produced by the glial cells of the mammalian peripheral nervous system. P0 mRNA expression is reduced following exposure of Schwann cells to sublytic C5b-9, the terminal activation complex of the complement cascade. Sublytic complement treatment decreased P0 mRNA by 81% within 6 h and required C5b-9 assembly. C5b-9 induced a threefold increase in both JNK1 activity and c-jun mRNA within 20 and 30 min, respectively, compared with cells treated with either human serum depleted of complement component C7 (C7dHS) or medium alone. Sublytic C5b-9 stimulation, in the presence of the transcription inhibitor Actinomycin D, decreased P0 mRNA expression by 52%, indicating that mRNA was selectively destabilized. This effect was prevented by pretreatment with L-JNK inhibitor 1 (L-JNKI1). To study a potential inhibition of P0 gene transcription, we transfected Schwann cells with a P0 promoter-firefly luciferase construct. Sublytic C5b-9 stimulation of the transfected cells decreased luciferase activity by 82% at 6 h, and this effect was prevented by pretreatment with L-JNKI1 inhibitor. Our results indicate that the ability of C5b-9 in vitro to affect P0 gene expression is mediated via JNK1 activation that leads to enhanced mRNA decay and transcriptional repression of P0.

Comparison of continuous and intermittent vibration effects on rat-tail artery and nerve

Hand-transmitted vibration from powered-tools can cause peripheral vasospasm and neuropathy. A rat-tail model was used to investigate whether the pattern of vibration influenced the type and severity of tissue damage. The tails of awake rats were vibrated continuously or intermittently for a total of 4 hours at 60 HZ, 49 m/s(2). Nerves and arteries were harvested immediately or 24 hours after treatment. Tails subjected to intermittent vibration showed transiently increased sensitivity to thermal stimuli. Intermittent vibration caused the most nerve injury immediately and 24 hours after vibration. Continuous vibration invoked a persistent reduction in vascular lumen size. Compared to epinephrine-induced transient vacuolation in vascular smooth muscle cells, both continuous and intermittent vibration caused greater persistence of vacuoles, indicating a vibration-induced pathological process. All vibration groups exhibited elevated nitrotyrosine immunoreactivity indicative of free-radical damage. Pattern of vibration exposure may exert a major influence on the type of vibration injury.

Marked phenotypic variation in a family with a new myelin protein zero mutation

Myelin protein zero (MPZ) is a member of the immunoglobulin gene superfamily, which has a role in myelin compaction. MPZ gene mutations cause mostly demyelinating neuropathies of the Charcot-Marie-Tooth 1B type (CMT1B), but axonal CMT have been described as well. There is a broad spectrum of phenotypic manifestation of neuropathies caused by MPZ mutations. Some mutations of MPZ cause severe early-onset neuropathies such as Dejerine-Sottas disease, while others cause the classical CMT phenotype with normal early milestones but development of disability during the first two decades of life. We describe a family in which five members of three consecutive generations had a heterozygous mutation in nucleotide position 143 with a T-C transition in exon 2 of the MPZ gene. The resulting substitution of Leu48 with proline has not been previously described. The age of onset of symptoms varied from 8 months to 41 years. The marked variation of the age of disease onset and clinical phenotype in this one family, related to the same MPZ mutation, suggests that in addition to the type and intragenic location of the mutation, other putative modifying gene(s) are regulating MPZ gene expression, mRNA stability and posttranslational protein modification and may have an important effect on the ultimate clinical phenotype.

Neural adhesion molecules L1 and CHL1 are survival factors for motoneurons

Many neurotrophic factors with survival activity for motoneurons in vivo were first identified using cultures of purified embryonic motoneurons. The L1 neural cell adhesion molecule has multiple roles in brain development. We showed by in situ hybridization and RT-PCR that L1 mRNA was expressed at significant levels in motoneurons of embryonic and postnatal spinal cord. We therefore cultured purified motoneurons from E14 rat embryos in the absence of trophic factors but with L1-Fc and CHL1-Fc fusion proteins. L1-Fc prevented the death of approximately half of the motoneurons that were saved by BDNF in a dose-dependent manner (EC50 = 10 pM). CHL1-Fc saved the same number of motoneurons as did L1-Fc, whereas P0-Fc had little neurotrophic activity at the same concentrations. Survival induced by L1 and CHL1 was completely inhibited by 20 microM LY294002 and PD98059, indicating that both MEK and PI3K pathways are required for signaling by these molecules. L1 can signal in other cell types through the FGF receptor FGFR1. In cultures of motoneurons, effects of suboptimal concentrations of L1 and suboptimal concentrations of FGF-2 were additive, but the effects of optimal concentrations of FGF-2 (50 ng/ml) were not further increased in the presence of L1-Fc. Thus, in this system, too, FGF and L1 may use similar signaling pathways.

Peripheral nerves: a target for the action of neuroactive steroids

Peripheral nervous system possesses both classical and non-classical steroid receptors and consequently may represent a target for the action of neuroactive steroids. The present review summarizes the state of art of this intriguing field of research reporting data which indicate that neuroactive steroids, like for instance progesterone, dihydroprogesterone, tetrahydroprogesterone, dihydrotestosterone and 3alpha-diol, stimulate the expression of two important proteins of the myelin of peripheral nerves, the glycoprotein P0 (P0) and the peripheral myelin protein 22 (PMP22). Interestingly, the mechanisms by which neuroactive steroids exert their effects involve classical steroid receptors, like for instance progesterone and androgen receptors, in case of P0 and non-classical steroid receptors, like GABA(A) receptor, in case of PMP22. Moreover, neuroactive steroids not only control the expression of these specific myelin proteins, but also influence the morphology of myelin sheaths and axons suggesting that these molecules may represent an interesting new therapeutic approach to maintain peripheral nerve integrity during neurodegenerative events.

Efficient recognition of protein fold at low sequence identity by conservative application of Psi-BLAST: validation

A substantial fraction of protein sequences derived from genomic analyses is currently classified as representing 'hypothetical proteins of unknown function'. In part, this reflects the limitations of methods for comparison of sequences with very low identity. We evaluated the effectiveness of a Psi-BLAST search strategy to identify proteins of similar fold at low sequence identity. Psi-BLAST searches for structurally characterized low-sequence-identity matches were carried out on a set of over 300 proteins of known structure. Searches were conducted in NCBI's non-redundant database and were limited to three rounds. Some 614 potential homologs with 25% or lower sequence identity to 166 members of the search set were obtained. Disregarding the expect value, level of sequence identity and span of alignment, correspondence of fold between the target and potential homolog was found in more than 95% of the Psi-BLAST matches. Restrictions on expect value or span of alignment improved the false positive rate at the expense of eliminating many true homologs. Approximately three-quarters of the putative homologs obtained by three rounds of Psi-BLAST revealed no significant sequence similarity to the target protein upon direct sequence comparison by BLAST, and therefore could not be found by a conventional search. Although three rounds of Psi-BLAST identified many more homologs than a standard BLAST search, most homologs were undetected. It appears that more than 80% of all homologs to a target protein may be characterized by a lack of significant sequence similarity. We suggest that conservative use of Psi-BLAST has the potential to propose experimentally testable functions for the majority of proteins currently annotated as 'hypothetical proteins of unknown function'.

Comparative proteomics of the Mycobacterium leprae binding protein myelin P0: its implication in leprosy and other neurodegenerative diseases

Mycobacterium leprae, the causative agent of leprosy invades Schwann cells of the peripheral nerves leading to nerve damage and disfigurement, which is the hallmark of the disease. Wet experiments have shown that M. leprae binds to a major peripheral nerve protein, the myelin P zero (P0). This protein is specific to peripheral nerve and may be important in the initial step of M. leprae binding and invasion of Schwann cells which is the feature of leprosy. Though the receptors on Schawann cells, cytokines, chemokines and antibodies to M. leprae have been identified the molecular mechanism of nerve damage and neurodegeneration is not clearly defined. Recently pathogen and host protein/nucleotide sequence similarities (molecular mimicry) have been implicated in neurodegenerative diseases. The approach of the present study is to utilise bioinformatic tools to understand leprosy nerve damage by carrying out sequence and structural similarity searches of myelin P0 with leproma and other genomic database. Since myelin P0 is unique to peripheral nerve, its sequence and structural similarities in other neuropathogens have also been noted. Comparison of myelin P0 with the M. leprae proteins revealed two characterised proteins, Ferrodoxin NADP reductase and a conserved membrane protein, which showed similarity to the query sequence. Comparison with the entire genomic database (www.ncbi.nlm.nih.gov) by basic local alignment search tool for proteins (BLASTP) and fold classification of structure-structure alignment of proteins (FSSP) searches revealed that myelin P0 had sequence/structural similarities to the poliovirus receptor, coxsackie-adenovirus receptor, anthrax protective antigen, diphtheria toxin, herpes simplex virus, HIV gag-1 peptide, and gp120 among others. These proteins are known to be associated directly or indirectly with neruodegeneration. Sequence and structural similarities to the immunoglobin regions of myelin P0 could have implications in host-pathogen interactions, as it has homophilic adhesive properties. Although these observed similarities are not highly significant in their percentage identity, they could be functionally important in molecular mimicry, receptor binding and cell signaling events involved in neurodegeneration.

Mifepristone (RU 38486) influences expression of glycoprotein Po and morphological parameters at the level of rat sciatic nerve: in vivo observations

The observations here reported indicate that, in vivo, the expression of an important protein of peripheral myelin, the glycoprotein Po, is influenced by mifespristone (RU 38486), that is, an antagonist of progesterone (PR) and glucocorticoid (GR) receptor. In our experimental model, male rats have been treated at the first day of life with this antagonist and after repeated treatments, we have analyzed in the sciatic nerve of 20- (20d) and 30-day-old rats (30d) the mRNA and protein levels of Po. Moreover, expression of Po has also been analyzed in the sciatic nerve of animals treated during the first 30 days of postnatal life and then sacrificed at 90th day of life (90d). The results obtained have indicated that both mRNA and protein levels of Po decrease at 20d. Apparently, these effects seem to be transient because no changes are evident at the other two times of analysis. As shown by morphometric analysis, the treatment with RU 38486 is also able to induce morphological changes at the level of sciatic nerve. However, at variance to what is expected by an alteration of an important component of the myelin membranes like Po, no changes are evident at the level of the myelin compartment. On the contrary, a significant reduction of axon diameter in parallel to an increase in neurofilament (NF) density occurs since 30d. In conclusion, the present data seem to suggest that progestin and/or glucocorticoid signals are not only involved in the control of myelin compartment but also on the axon maintenance.

Neuroactive steroids influence peripheral myelination: a promising opportunity for preventing or treating age-dependent dysfunctions of peripheral nerves

The process of aging deeply influences morphological and functional parameters of peripheral nerves. The observations summarized here indicate that the deterioration of myelin occurring in the peripheral nerves during aging may be explained by the fall of the levels of the major peripheral myelin proteins [e.g., glycoprotein Po (Po) and peripheral myelin protein 22 (PMP22)]. Neuroactive steroids, such as progesterone (PROG), dihydroprogesterone (5alpha-DH PROG), and tetrahydroprogesterone (3alpha,5alpha-TH PROG), are able to stimulate the low expression of these two myelin proteins present in the sciatic nerve of aged male rats. Since Po and PMP22 play an important physiological role in the maintenance of the multilamellar structure of PNS myelin, we have evaluated the effect of PROG and its neuroactive derivatives, 5alpha-DH PROG and 3alpha,5alpha-TH PROG, on the morphological alterations of myelinated fibers in the sciatic nerve of 22-24-month-old male rats. Data obtained clearly indicate that neuroactive steroids are able to reduce aging-associated morphological abnormalities of myelin and aging-associated myelin fiber loss in the sciatic nerve.