Modulation of the oligomerization of myelin proteolipid protein by transmembrane helix interaction motifs

Ameliorative effects of acetyl-L-carnitine on corpus callosum and functional recovery in demyelinated mouse model

AIM

Multiple sclerosis (MS) is the most common chronic inflammatory demyelinating disease of the central nervous system. Oxidative stress via distinct pathobiological pathways plays a pivotal role in the formation and persistence of MS lesions. Acetyl-L-carnitine (ALC) facilitates the uptake of acetyl coenzyme-A into the mitochondria by a fatty acid oxidation process. ALC could be a therapeutic antioxidant in the myelin repair process. This study explored the potential neuroprotective effects of ALC in cuprizone (CPZ) intoxicated mice.

MATERIALS AND METHODS

Thirty male C57BL/6 mice were divided into three groups. The control animals received a normal diet. The CPZ and CPZ + ALC groups were fed with a 0.2% cuprizone diet for 12 weeks. In the CPZ + ALC group, animals received ALC (300 mg/kg/day) from the 10th -12th weeks. Animals were evaluated functionally by beam walking test (BWT) weekly. Eventually, the corpus callosum (CC) was extracted for histological, biochemical, and molecular studies.

RESULTS

BWT data showed ALC significantly improves balance and gait in the demyelinating mouse model. Histological staining represented ALC effectively increased remyelination in the CC. Biochemical evaluations demonstrated ALC decreased the malondialdehyde level with a parallel increase in the reduced glutathione and catalase activity levels in the CC. Molecular analysis revealed that ALC significantly increased the expression of oligodendrocyte transcription-2 (Olig-2) and Poly lipoproteins (Plp) genes in the CC.

CONCLUSIONS

ALC improved balance and motor coordination in the demyelinated mouse model. It may be by reducing the levels of free radicals and increasing the expression of Olig-2 and Plp as myelin-related genes.

MutTMPredictor: Robust and accurate cascade XGBoost classifier for prediction of mutations in transmembrane proteins

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Prediction of mutations in transmembrane proteins is of significance for diseases diagnosis.

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Building on the evolutionary information, proposed the Gaussian WAPSSM algorithm.

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Based on WAPSSM and sequence and structure-based features, proposed the cascade XGBoost algorithm.

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Webserver is freely at (http://csbio.njust.edu.cn/bioinf/ffmsresmutp/).

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Implement MutTMPredictor to predict mutations in transmembrane proteins.

Transmembrane proteins have critical biological functions and play a role in a multitude of cellular processes including cell signaling, transport of molecules and ions across membranes. Approximately 60% of transmembrane proteins are considered as drug targets. Missense mutations in such proteins can lead to many diverse diseases and disorders, such as neurodegenerative diseases and cystic fibrosis. However, there are limited studies on mutations in transmembrane proteins. In this work, we first design a new feature encoding method, termed weight attenuation position-specific scoring matrix (WAPSSM), which builds upon the protein evolutionary information. Then, we propose a new mutation prediction algorithm (cascade XGBoost) by leveraging the idea learned from consensus predictors and gcForest. Multi-level experiments illustrate the effectiveness of WAPSSM and cascade XGBoost algorithms. Finally, based on WAPSSM and other three types of features, in combination with the cascade XGBoost algorithm, we develop a new transmembrane protein mutation predictor, named MutTMPredictor. We benchmark the performance of MutTMPredictor against several existing predictors on seven datasets. On the 546 mutations dataset, MutTMPredictor achieves the accuracy (ACC) of 0.9661 and the Matthew’s Correlation Coefficient (MCC) of 0.8950. While on the 67,584 dataset, MutTMPredictor achieves an MCC of 0.7523 and area under curve (AUC) of 0.8746, which are 0.1625 and 0.0801 respectively higher than those of the existing best predictor (fathmm). Besides, MutTMPredictor also outperforms two specific predictors on the Pred-MutHTP datasets. The results suggest that MutTMPredictor can be used as an effective method for predicting and prioritizing missense mutations in transmembrane proteins. The MutTMPredictor webserver and datasets are freely accessible at http://csbio.njust.edu.cn/bioinf/muttmpredictor/ for academic use.

Intrinsic Structural Features of the Human IRE1α Transmembrane Domain Sense Membrane Lipid Saturation

Activation of inositol-requiring enzyme (IRE1α) is an indispensable step in remedying the cellular stress associated with lipid perturbation in the endoplasmic reticulum (ER) membrane. IRE1α is a single-spanning ER transmembrane protein possessing both kinase and endonuclease functions, and its activation can be fully achieved through the dimerization and/or oligomerization process. How IRE1α senses membrane lipid saturation remains largely unresolved. Using both computational and experimental tools, we systematically investigated the dimerization process of the transmembrane domain (TMD) of IRE1α and found that, with help of the serine 450 residue, the conserved tryptophan 457 residue buttresses the core dimerization interface of IRE1α-TMD. BiFC (bimolecular fluorescence complementation) experiments revealed that mutation on these residues abolished the saturated fatty acid-induced dimerization in the ER membrane and subsequently inactivated IRE1α activity in vivo. Therefore, our results suggest that the structural elements of IRE1α-TMD serve as a key sensor that detects membrane aberrancy.

In Silico Perspectives on the Prediction of the PLP's Epitopes involved in Multiple Sclerosis

BACKGROUND

Multiple sclerosis (MS) is the most common autoimmune disease of the central nervous system (CNS). The main cause of the MS is yet to be revealed, but the most probable theory is based on the molecular mimicry that concludes some infections in the activation of T cells against brain auto-antigens that initiate the disease cascade.

OBJECTIVES

The Purpose of this research is the prediction of the auto-antigen potency of the myelin proteolipid protein (PLP) in multiple sclerosis.

MATERIALS AND METHODS

As there wasn't any tertiary structure of PLP available in the Protein Data Bank (PDB) and in order to characterize the structural properties of the protein, we modeled this protein using prediction servers. Meta prediction method, as a new perspective in silico, was performed to fi nd PLPs epitopes. For this purpose, several T cell epitope prediction web servers were used to predict PLPs epitopes against Human Leukocyte Antigens (HLA). The overlap regions, as were predicted by most web servers were selected as immunogenic epitopes and were subjected to the BLASTP against microorganisms.

RESULTS

Three common regions, AA58-74, AA161-177, and AA238-254 were detected as immunodominant regions through meta-prediction. Investigating peptides with more than 50% similarity to that of candidate epitope AA58-74 in bacteria showed a similar peptide in bacteria (mainly consistent with that of clostridium and mycobacterium) and spike protein of Alphacoronavirus 1, Canine coronavirus, and Feline coronavirus. These results suggest that cross reaction of the immune system to PLP may have originated from a bacteria or viral infection, and therefore molecular mimicry might have an important role in the progression of MS.

CONCLUSIONS

Through reliable and accurate prediction of the consensus epitopes, it is not necessary to synthesize all PLP fragments and examine their immunogenicity experimentally (in vitro). In this study, the best encephalitogenic antigens were predicted based on bioinformatics tools that may provide reliable results for researches in a shorter time and at a lower cost.

Screening for transmembrane association in divisome proteins using TOXGREEN, a high-throughput variant of the TOXCAT assay

TOXCAT is a widely used genetic assay to study interactions of transmembrane helices within the inner membrane of the bacterium Escherichia coli. TOXCAT is based on a fusion construct that links a transmembrane domain of interest with a cytoplasmic DNA-binding domain from the Vibrio cholerae ToxR protein. Interaction driven by the transmembrane domain results in dimerization of the ToxR domain, which, in turn, activates the expression of the reporter gene chloramphenicol acetyl transferase (CAT). Quantification of CAT is used as a measure of the ability of the transmembrane domain to self-associate. Because the quantification of CAT is relatively laborious, we developed a high-throughput variant of the assay, TOXGREEN, based on the expression of super-folded GFP and detection of fluorescence directly in unprocessed cell cultures. Careful side-by-side comparison of TOXCAT and TOXGREEN demonstrates that the methods have comparable response, dynamic range, sensitivity and intrinsic variability both in LB and minimal media. The greatly enhanced workflow makes TOXGREEN much more scalable and ideal for screening, since hundreds of constructs can be rapidly assessed in 96 well plates. Even for small scale investigations, TOXGREEN significantly reduces time, labor and cost associated with the procedure. We demonstrate applicability with a large screening for self-association among the transmembrane domains of bitopic proteins of the divisome (FtsL, FtsB, FtsQ, FtsI, FtsN, ZipA and EzrA) belonging to 11 bacterial species. The analysis confirms a previously reported tendency for FtsB to self-associate, and suggests that the transmembrane domains of ZipA, EzrA and FtsN may also possibly oligomerize.

Filopodia formation driven by membrane glycoprotein M6a depends on the interaction of its transmembrane domains

Membrane glycoprotein M6a, which belongs to the tetraspan proteolipid protein family, promotes structural plasticity in neurons and cell lines by unknown mechanisms. This glycoprotein is encoded by Gpm6a, a stress-regulated gene. The hippocampus of animals chronically stressed by either psychosocial or physical stressors shows decreased M6a expression. Stressed Gpm6a-null mice develop a claustrophobia-like phenotype. In humans, de novo duplication of GPM6A results in learning/behavioral abnormalities, and two single-nucleotide polymorphisms (SNPs) in the non-coding region are linked to mood disorders. Here, we studied M6a dimerization in neuronal membranes and its functional relevance. We showed that the self-interaction of M6a transmembrane domains (TMDs) might be driving M6a dimerization, which is required to induce filopodia formation. Glycine mutants located in TMD2 and TMD4 of M6a affected its dimerization, thus preventing M6a-induced filopodia formation in neurons. In silico analysis of three non-synonymous SNPs located in the coding region of TMDs suggested that these mutations induce protein instability. Indeed, these SNPs prevented M6a from being functional in neurons, owing to decreased stability, dimerization or improper folding. Interestingly, SNP3 (W141R), which caused endoplasmic reticulum retention, is equivalent to that mutated in PLP1, W161L, which causes demyelinating Pelizaeus-Merzbacher disease. In this work we analyzed the functional contribution of transmembrane domains (TMDs) of the neuronal membrane glycoprotein M6a. We determined that certain glycines present in TMD2 and TMD4 are critical for filopodia induction in neurons. In addition, three nsSNPs located in the coding region of TMD2 and TMD3 of GPM6A impair M6a function by affecting its stability, folding and dimer formation.

The safety dance: biophysics of membrane protein folding and misfolding in a cellular context

Most biological processes require the production and degradation of proteins, a task that weighs heavily on the cell. Mutations that compromise the conformational stability of proteins place both specific and general burdens on cellular protein homeostasis (proteostasis) in ways that contribute to numerous diseases. Efforts to elucidate the chain of molecular events responsible for diseases of protein folding address one of the foremost challenges in biomedical science. However, relatively little is known about the processes by which mutations prompt the misfolding of α-helical membrane proteins, which rely on an intricate network of cellular machinery to acquire and maintain their functional structures within cellular membranes. In this review, we summarize the current understanding of the physical principles that guide membrane protein biogenesis and folding in the context of mammalian cells. Additionally, we explore how pathogenic mutations that influence biogenesis may differ from those that disrupt folding and assembly, as well as how this may relate to disease mechanisms and therapeutic intervention. These perspectives indicate an imperative for the use of information from structural, cellular, and biochemical studies of membrane proteins in the design of novel therapeutics and in personalized medicine.

Terminal residue hydrophobicity modulates transmembrane helix-helix interactions

Central to the formation of tertiary structure in membrane protein folding is the presence of amino acid sequence motifs (such as "small-XXX-small" segments) in the TM segments that promote interaction-compatible surfaces through which the TM α-helices interact. Here, we sought to elucidate additional factors that may work in tandem to dictate the ultimate interaction fate of TM-embedded segments. In this context, we used proteolipid protein (PLP), the major protein from central nervous system myelin for which mutant-dependent non-native oligomerization has been implicated in neurological disorders, to explore the specific effects of TM boundary residues (the membrane entry and exit points), keying on the secondary structure and self-association of peptides corresponding to the PLP TM2 α-helix (wild-type sequence ⁶⁶AFQYVIYGTASFFFLYGALLLAEGF⁹⁰). Using gel electrophoresis, circular dichroism, and Förster resonance energy transfer in the membrane-mimetic detergent sodium dodecyl sulfate (SDS), we found that mutation of F90 to residues such as A, I, L, or V maintains the onset of TM2-TM2 dimerization, whereas mutation to E, G, Q, N, S, or T abrogates dimer formation. We attribute this sensitivity to changes in local hydrophobicity, viz., a decrease in hydrophobicity reduces local lipid-peptide interactions, which in turn disrupts peptide α-helicity and hence the effectiveness of an incipient interaction-compatible surface. Our results show that the secondary structure and oligomeric state of PLP TM2 Lys-tagged peptides are significantly modulated by the specific nature of their C-terminal boundary residue, thus providing insight as to how point mutations, particularly where they produce disease states, can compromise the folding process.

Pelizaeus-Merzbacher disease, easily misdiagnosed as cerebral palsy: a report of a three-generation family

Pelizaeus-Merzbacher disease (PMD) is an X-linked recessive disorder affecting myelination of the central nervous system, and is caused by mutations of the proteolipid protein 1 (PLP1) gene. Clinical manifestations of PMD are variable and major features include progressive nystagmus, spasticity, tremor, ataxia, and psychomotor delay. We describe a classical PMD patient who had been misdiagnosed as cerebral palsy. He had nystagmus and psychomotor delay since infancy and tremor with ataxia developing gradually. Brain MRI revealed demyelination over white matter of the cerebral hemispheres and posterior limbs of the internal capsules. Positive family history led to subsequent mutation analysis, which identified a novel mutation (c.88G>C) in PLP1 in the proband, as well as his affected brother and maternal uncle, and asymptomatic maternal grandmother, mother and two sisters. Therefore, PMD should be considered in a cerebral palsy-like patient with or without positive family history. Mutation analysis is crucial for early diagnosis and further genetic counseling.

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

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

HIV-1 gp41 transmembrane domain interacts with the fusion peptide: implication in lipid mixing and inhibition of virus-cell fusion

Fusion of the human immunodeficiency virus (HIV) with target cells is mediated by the gp41 subunit of the envelope protein. Mutation and deletion studies within the transmembrane domain (TMD) of intact gp41 influenced its fusion activity. In addition, current models suggest that the TMD is in proximity with the fusion peptide (FP) at the late fusion stages, but there are no direct experimental data to support this hypothesis. Here, we investigated the TMD focusing on two regions: the N-terminal containing the GxxxG motif and the C-terminal containing the GLRI motif, which is conserved among the TMDs of HIV and the T-cell receptor. Studies utilizing the ToxR expression system combined with synthetic peptides and their fluorescent analogues derived from TMD revealed that the GxxxG motif is important for TMD self-association, whereas the C-terminal region is for its heteroassociation with FP. Functionally, all three TMD peptides induced lipid mixing that was enhanced significantly upon mixing with FP. Furthermore, the TMD peptides inhibited virus-cell fusion apparently through their interaction with their endogenous counterparts. Notably, the R2E mutant (in the GLRI) was significantly less potent than the two others. Overall, our findings provide experimental evidence that HIV-1 TMD contributes to membrane assembly and function of the HIV-1 envelope. Owing to similarities between functional domains within viruses, these findings suggest that the TMDs and FPs may contribute similarly in other viruses as well.

Misalignment of PLP/DM20 transmembrane domains determines protein misfolding in Pelizaeus-Merzbacher disease

A large number of genetic diseases have been associated with truncated or misfolded membrane proteins trapped in the endoplasmic reticulum (ER). In the ER, they activate the unfolded protein response, which can trigger cell death. Hence, a better understanding of protein misfolding features might help in developing novel therapies. Here, we have studied the molecular basis of Pelizaeus-Merzbacher disease, a leukodystrophy defined by mutations of the PLP1 gene and ER retention of two encoded tetraspan myelin proteins, PLP and DM20. In mouse oligodendroglial cells, mutant isoforms of PLP/DM20 with fewer than all four transmembrane (TM) domains are fully ER retained. Surprisingly, a truncated PLP with only two N-terminal TM domains shows normal cell-surface expression when coexpressed with a second truncated PLP harboring the two C-terminal TM domains. This striking ability to properly self-align the TM domains is disease relevant, as shown for the smaller splice isoform DM20. Here, the increased length of TM domain 3 allows for compensation of the effect of several PLP1 point mutations that impose a conformational constraint onto the adjacent extracellular loop region. We conclude that an important determinant in the quality control of polytopic membrane proteins is the free alignment of their TM domains.

Mutations in or near the transmembrane domain alter PMEL amyloid formation from functional to pathogenic

PMEL is a pigment cell-specific protein that forms physiological amyloid fibrils upon which melanins ultimately deposit in the lumen of the pigment organelle, the melanosome. Whereas hypomorphic PMEL mutations in several species result in a mild pigment dilution that is inherited in a recessive manner, PMEL alleles found in the Dominant white (DW) chicken and Silver horse (HoSi)--which bear mutations that alter the PMEL transmembrane domain (TMD) and that are thus outside the amyloid core--are associated with a striking loss of pigmentation that is inherited in a dominant fashion. Here we show that the DW and HoSi mutations alter PMEL TMD oligomerization and/or association with membranes, with consequent formation of aberrantly packed fibrils. The aberrant fibrils are associated with a loss of pigmentation in cultured melanocytes, suggesting that they inhibit melanin production and/or melanosome integrity. A secondary mutation in the Smoky chicken, which reverts the dominant DW phenotype, prevents the accumulation of PMEL in fibrillogenic compartments and thus averts DW-associated pigment loss; a secondary mutation found in the Dun chicken likely dampens a HoSi-like dominant mutation in a similar manner. We propose that the DW and HoSi mutations alter the normally benign amyloid to a pathogenic form that antagonizes melanosome function, and that the secondary mutations found in the Smoky and Dun chickens revert or dampen pathogenicity by functioning as null alleles, thus preventing the formation of aberrant fibrils. We speculate that PMEL mutations can model the conversion between physiological and pathological amyloid.

Membrane protein misassembly in disease

Helix-helix interactions play a central role in the folding and assembly of integral α-helical membrane proteins and are fundamentally dictated by the amino acid sequence of the TM domain. It is not surprising then that missense mutations that target these residues are often linked to disease. In this review, we focus on the molecular mechanisms through which missense mutations lead to aberrant folding and/or assembly of these proteins, and then discuss pharmacological approaches that may potentially mitigate or reverse the negative effects of these mutations. Improving our understanding of how missense mutations affect the interactions between TM α-helices will increase our capability to develop effective therapeutic approaches to counter the misassembly of these proteins and, ultimately, disease. This article is part of a Special Issue entitled: Protein Folding in Membranes.