Interaction between human myelin basic protein and lipophilin

Single cell transcriptome analysis of the THY-Tau22 mouse model of Alzheimer's disease reveals sex-dependent dysregulations

Alzheimer's disease (AD) progression and pathology show pronounced sex differences, but the factors driving these remain poorly understood. To gain insights into early AD-associated molecular changes and their sex dependency for tau pathology in the cortex, we performed single-cell RNA-seq in the THY-Tau22 AD mouse model. By examining cell type-specific and cell type-agnostic AD-related gene activity changes and their sex-dimorphism for individual genes, pathways and cellular sub-networks, we identified both statistically significant alterations and interpreted the upstream mechanisms controlling them. Our results confirm several significant sex-dependent alterations in gene activity in the THY-Tau22 model mice compared to controls, with more pronounced alterations in females. Both changes shared across multiple cell types and cell type-specific changes were observed. The differential genes showed significant over-representation of known AD-relevant processes, such as pathways associated with neuronal differentiation, programmed cell death and inflammatory responses. Regulatory network analysis of these genes revealed upstream regulators that modulate many of the downstream targets with sex-dependent changes. Most key regulators have been previously implicated in AD, such as Egr1, Klf4, Chchd2, complement system genes, and myelin-associated glycoproteins. Comparing with similar data from the Tg2576 AD mouse model and human AD patients, we identified multiple genes with consistent, cell type-specific and sex-dependent alterations across all three datasets. These shared changes were particularly evident in the expression of myelin-associated genes such as Mbp and Plp1 in oligodendrocytes. In summary, we observed significant cell type-specific transcriptomic changes in the THY-Tau22 mouse model, with a strong over-representation of known AD-associated genes and processes. These include both sex-neutral and sex-specific patterns, characterized by consistent shifts in upstream master regulators and downstream target genes. Collectively, these findings provide insights into mechanisms influencing sex-specific susceptibility to AD and reveal key regulatory proteins that could be targeted for developing treatments addressing sex-dependent AD pathology.

Characterization and identification of the protein partners of Fn3 domain in FnTm2

Recently, a novel transmembrane protein was found to be up-regulated in the auditory learning pathway of birds and mammals. The protein, FnTm2, was predicted to have an extracellular fibronectin III (Fn3) domain and a single transmembrane domain. By contrast to other studied Fn3 domains the extracellular domain of FnTm2 bears several cysteine residues, which are predicted to form disulfide bonds. The Fn3 domain of the FnTm2 protein was expressed in DH5-α Escherichia coli (E. coli) cells, purified and characterized by circular dichroism (CD). In order to identify binding partners to Fn3, the isolated protein was incubated with bird brain lysate for a pull down treatment. Of the proteins recognized, myelin basic protein (MBP) was identified as a bona fide partner; it was further characterized for binding to Fn3 in vitro via fluorescence spectroscopy and confirmed via isothermal calorimetry (ITC).

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.

Myelin proteolipid protein, basic protein, the small isoform of myelin-associated glycoprotein, and p42MAPK are associated in the Triton X-100 extract of central nervous system myelin

To further our understanding of the functions of the major myelin proteins, myelin basic protein (MBP) and proteolipid protein (PLP), and other myelin proteins, such as 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNP) and myelin-associated glycoprotein (MAG), bovine brain myelin was extracted with Triton X-100, and protein complexes in the detergent-soluble fraction were isolated by coimmunoprecipitation and sucrose density gradient sedimentation. MBP, PLP, and the small isoform of MAG (S-MAG) were coimmunoprecipitated from the detergent-soluble fraction by anti-PLP, anti-MBP or anti-MAG monoclonal antibodies. Additionally, a 30 kDa phosphoserine-containing protein and two phosphotyrosine-containing proteins (M(r) 30 and 42 kDa) were found in the coimmunoprecipitates. The 42 kDa protein is probably p42MAPK, in that MAPK was shown also to be present in the immunoprecipitated complex. CNP, the small PLP isoform DM20, the large MAG isoform L-MAG, MOG, CD44, MEK, p44MAPK, and actin were not present in the immunoprecipitates, although they were present in the detergent-soluble fraction. Lipid analysis revealed that the PLP-MBP-S-MAG coimmunoprecipitated with some phospholipids and sulfatide but not cholesterol or galactosylceramide. However, the complex had a high density, indicating that the lipid/protein ratio is low, and it was retained on a Sepharose CL6B column, indicating that it is not a large membrane fragment. Given that MAG is localized mainly in the periaxonal region of myelin, where it interacts with axonal ligands, the PLP-MBP-S-MAG complex may come from these regions, where it could participate in dynamic functions in the myelin sheath and myelin-axonal interactions.

Isolation of myelin basic protein from whole tissue extracts by selective pH-dependent solubilization

A previous study of the selective solubility of myelin basic protein (MBP) of tissue extracts at pH 9.0 has raised issues of its quantitative recovery, and the differential solubility of its charge isomers. The pH-dependent solubility of proteins of acid extracts of delipidated tissue of bovine spinal cord was therefore reexamined. MBP of whole extracts was completely soluble up to pH 8.0 only, and less so by 25% at pH 9.0, and 43% at pH 10.0. The proteins other than MBP were virtually insoluble between pH 5.0 to 6.0, and 9.0 to 10.0. The solubility of the main charge isomers I to III of MBP of 18.5 kDa was found not to be affected by pH. Either pH 5.0 or 9.0 is therefore suitable for the selective isolation of MBP from whole tissue extracts, only pH 5.0 providing for the complete recovery of MBP. The pH-dependent solution behaviour was also examined following the separation of proteins of whole extracts by anion exchange chromatography at pH 10.4. Purified MBP and several related minor cationic components of lower molecular weight were soluble throughout. In contrast, the anionic proteins were only partly soluble between pH 4.0 to 10.0, i.e. by 4 to 20%. The results are consistent with specific protein-protein interactions of the proteins of whole extracts, either enhancing the solubility of non-MBP proteins, e.g. at pH 7.0, or impairing that of MBP between pH 8.0 to 10.0.

The basic protein of CNS myelin: its structure and ligand binding

Consideration of the evidence presented in this review leads to the following conclusions: (a) Isolated MBP in aqueous solution has little ordered secondary or tertiary structure. (b) In this state, the protein can associate with a wide range of hydrophobic and amphiphilic compounds, these interactions involving limited sections of the protein. (c) The strength of binding to bilayers and the accompanying conformational changes in the protein are greatest for systems containing acidic lipids, presumably because of the involvement of ionic interactions. (d) When bound to bilayers of acidic lipids, MBP will have substantially more ordered secondary structure than it manifests in aqueous solution, and it is likely to be oligomeric (possibly hexameric). (e) MBP does affect the organization of lipid aggregates. It influences strongly the separation of bilayers in multilayers of purified lipids, and at present this must be viewed as its prime role within myelin. The greatest impediment to our understanding of MBP is the lack of an assayable biological activity. In contrast to the situation with enzymes, for example, we have no functional test for changes in protein structure or changes accompanying interactions with other molecules. Current evidence suggests that the protein has a structural role within myelin and that its own three-dimensional structure is strongly dependent on the molecules with which it is associated. If this picture is correct, studies of the isolated protein or of the protein in reconstituted lipid systems may yield, at best, a rough guide to the structure within its biological environment. Further clarification of the structure and function of MBP may have to await development of more powerful techniques for studying proteins bound to large molecular aggregates, such as lipid bilayers. The paucity of generally applicable methods is reflected in the fact that even low resolution structures are known for only a handful of intrinsic membrane proteins, and even more limited information exists for proteins associated with membrane surfaces. However, the increasing use of a combination of electron microscopy and diffraction on two-dimensional arrays of proteins formed on lipid bilayers (Henderson et al., 1990) offers the hope that it may not be too long before it will be possible to study at moderate resolution the three-dimensional structure of MBP bound to a lipid membrane.

Interaction of myelin basic protein and proteolipid protein

The interaction of myelin basic protein (MBP) and proteolipid protein (PLP) was studied using a microtitre well binding assay and the ligand-blot overlay technique. The binding of iodinated PLP to MBP that was immobilized on microtitre wells was saturable and reversible. Its selectivity was investigated by the ligand-blot overlay technique. Iodinated PLP was found to bind MBP but not any other CNS myelin proteins. This interaction was not dependent on the phosphoryl moiety of MBP. Binding of PLP to histone H4 also occurred, but the amount of PLP bound per unit MBP was greater than for this histone.

Zinc ions stabilise the association of basic protein with brain myelin membranes

Myelin basic protein (MBP) dissociated from brain myelin membranes when they were incubated (37 degrees C; pH 7.4) at physiological ionic strength. Zinc ions inhibited, and calcium promoted, this process. Protease activity in the membrane preparations cleaved the dissociated MBP into both small (less than 4 kilodaltons) and large (greater than 8 kilodaltons) fragments. The latter were detected, together with intact MBP, by gel electrophoresis of incubation media. Zinc ions appeared to act in two distinct processes. In the presence or absence of added CaCl2, zinc ions in the range 0.1-1 mM inhibited MBP-membrane dissociation. This process was relatively insensitive to heat and Zn2+ could be substituted by either copper (II) or cobalt (II) ions. A second effect was evident only in the presence of added calcium ions, when lower concentrations of Zn2+ (less than 0.1 mM) inhibited MBP-membrane dissociation and the accumulation of intact MBP in incubation media. This process was heat sensitive and only copper (II), but not cobalt (II), ions could replace Zn2+. To determine whether endogenous zinc in myelin membranes is bound to MBP, preparations were solubilised in buffers containing Triton X-100/2 mM CaCl2 and subjected to gel filtration. Endogenous zinc, as indicated by a dithizone-binding method, eluted with fractions containing both MBP and proteolipid protein (PLP). Thus, one means whereby zinc stabilises association of MBP with brain myelin membranes may be by promoting its binding to PLP.

Possible hydrophobic region in myelin basic protein consisting of an orthogonally packed beta-sheet

Theoretical analysis was carried out to determine how the approximately 20% of beta-structure observed in the 18.5 kilodalton (kDa) myelin basic protein (MBP) could be organized into a relatively stable beta-sheet. The beta-sheet is presumed to consist of the five most hydrophobic segments of polypeptide chain, which have beta-structure potential. These correspond approximately to sequences 15-21, 37-45, 84-92, 106-112, and 148-154 (rabbit MBP sequence numbering) and constitute beta-strands a, b, c, d, and e, respectively. A number of constraints are imposed upon the sheet; e.g., it should have the same topology in all MBP forms (21.5, 18.5, 17, and 14 kDa); strand e should lie at the sheet edge; strands b, c, and d should be ordered sequentially; the sheet formed by strands a, b, c, and d should be antiparallel; a maximum of the nonpolar surface area should be removed from the aqueous milieu; and charged side chains should be solvent-accessible. On the basis of these constraints it is possible to propose six orthogonally packed beta-sheets having different topologies. If strand e is restricted to an antiparallel alignment, the number of different sheets is reduced to four. Each of these sheets can form a relatively compact hydrophobic globular region. Two of the strands (a and e) can undergo transitions to alpha-helix without disrupting the structure of the remaining sheet bcd or producing major topologic rearrangements of the polypeptide chain.