High-resolution structural model of porcine P2 myelin membrane protein with associated fatty acid ligand: fact or artifact?

On the synergy between myelin proteins P0, MBP, and P2 in peripheral nerve major dense line formation

The proper formation and function of the myelin sheath, a proteolipid membrane multilayer, relies on the coordinated action of several key myelin proteins. We studied how proteins from the peripheral myelin cytoplasmic apposition-myelin basic protein (MBP), the cytoplasmic tail of myelin protein zero (P0ct), and peripheral myelin protein 2 (P2)-interact with each other and with myelin-like membranes using various techniques, such as small-angle X-ray diffraction, differential scanning calorimetry (DSC), surface plasmon resonance (SPR), and electron and live epifluorescence microscopy. DSC revealed changes in lipid interactions depending on the protein combination, with altered membrane fluidity and stability. These results were supported by SPR, which indicated that the myelin proteins may compete for membrane surface binding. Analysis of the Bragg peaks induced by the myelin proteins in lipidic environments showed both lamellar and nonlamellar phases in protein-lipid complexes, indicating the formation of nanoscale structures that may be relevant for myelin assembly. Microscopy experiments showed the formation of new membrane structures with each of the proteins separately and together. Our data indicate both synergy and competition between the three main proteins residing in the peripheral nervous system myelin major dense line. The observed direct effects of myelin proteins on lipid membrane structure and properties may be relevant to their function in myelinating cells as well as their role in myelin disorders.

Study on fatty acid binding protein in lipid metabolism of livestock and poultry

Fatty acid binding proteins (FABPs) are key proteins in lipid transport, and 12 family members have been documented in the literature. In recent years, new insights have been gained into the structure and function of FABPs, which are important regulators of lipid metabolic processes in the body and play a central role in coordinating lipid transport and metabolism in various tissues and organs across species. This paper provides a brief overview of the structure and biological functions of FABPs and reviews related studies on lipid metabolism in livestock and poultry to lay the foundation for research on the mechanism underlying the regulatory effect of FABPs on lipid metabolism in livestock and poultry and for the genetic improvement of livestock and poultry.

Early onset demyelinating Charcot-Marie-Tooth disease caused by a novel in-frame isoleucine deletion in peripheral myelin protein 2

Peripheral myelin protein 2 (PMP2) is a small protein located on the cytoplasmic side of compact myelin, involved in the lipids transport and in the myelination process. In the last years few families affected with demyelinating Charcot-Marie-Tooth neuropathy (CMT1), caused by PMP2 mutations, have been identified. In this study we describe the first case of a PMP2 in-frame deletion. PMP2 was analyzed by direct sequencing after exclusion of the most frequent CMT-associated genes by using a next generation sequencing (NGS) genes panel. Sanger sequencing was used for family's segregation analysis. Molecular modeling analysis was used to evaluate the mutation impact on the protein structure. A novel PMP2: p.I50del has been identified in a child with early onset CMT1 and in three affected family members. All family members show an early onset demyelinating neuropathy without other distinguish features. Molecular modeling analysis and in silico evaluations do not suggest a strong impact on the overall protein structure, but a most likely altered protein function. This study suggests the importance to add PMP2 in CMT NGS genes panels or, at most, to test it after major CMT1 genes exclusion, due to the lack of diagnostic-addressing additional features.

Myelin Fat Facts: An Overview of Lipids and Fatty Acid Metabolism

Myelin is critical for the proper function of the nervous system and one of the most complex cell–cell interactions of the body. Myelination allows for the rapid conduction of action potentials along axonal fibers and provides physical and trophic support to neurons. Myelin contains a high content of lipids, and the formation of the myelin sheath requires high levels of fatty acid and lipid synthesis, together with uptake of extracellular fatty acids. Recent studies have further advanced our understanding of the metabolism and functions of myelin fatty acids and lipids. In this review, we present an overview of the basic biology of myelin lipids and recent insights on the regulation of fatty acid metabolism and functions in myelinating cells. In addition, this review may serve to provide a foundation for future research characterizing the role of fatty acids and lipids in myelin biology and metabolic disorders affecting the central and peripheral nervous system.

Identification of a pathogenic PMP2 variant in a multi-generational family with CMT type 1: Clinical gene panels versus genome-wide approaches to molecular diagnosis

Charcot-Marie-Tooth (CMT) disease type 1 is an inherited peripheral neuropathy characterized by demyelination and reduced nerve conduction velocities. We present a multi-generational family with peripheral neuropathy in whom clinical CMT panel testing failed to conclude a molecular diagnosis. We found a PMP2 pathogenic variant c.155T > C, p.(Ile52Thr) that segregates with disease suggesting that PMP2 variants should be considered in patients with neuropathy and that it may be prudent to include in clinical CMT gene panels.

The Role of Peripheral Myelin Protein 2 in Remyelination

The protein component of the myelin layer is essential for all aspects of peripheral nerves, and its deficiency can lead to structural and functional impairment. The presence of peripheral myelin protein 2 (P2, PMP2, FABP8, M-FABP) in Schwann cells has been known for decades and shown recently to be involved in the lipid homeostasis in the peripheral neural system. However, its precise role during de- and remyelination has yet to be elucidated. To this end, we assessed remyelination after sciatic nerve crush injury in vivo, and in an experimental de/remyelination ex vivo myelinating culture model in P2-deficient (P2 -/- ) and wild-type (WT) animals. In vivo, the nerve crush paradigm revealed temporal structural and functional changes in P2 -/- mice as compared to WT animals. Concomitantly, P2 -/- DRG cultures demonstrated the presence of shorter internodes and enlarged nodes after ex vivo de/remyelination. Together, these data indicate that P2 may play a role in remyelination of the injured peripheral nervous system, presumably by affecting the nodal and internodal configuration.

Molecular mechanisms of Charcot-Marie-Tooth neuropathy linked to mutations in human myelin protein P2

Charcot-Marie-Tooth (CMT) disease is one of the most common inherited neuropathies. Recently, three CMT1-associated point mutations (I43N, T51P, and I52T) were discovered in the abundant peripheral myelin protein P2. These mutations trigger abnormal myelin structure, leading to reduced nerve conduction velocity, muscle weakness, and distal limb atrophy. P2 is a myelin-specific protein expressed by Schwann cells that binds to fatty acids and membranes, contributing to peripheral myelin lipid homeostasis. We studied the molecular basis of the P2 patient mutations. None of the CMT1-associated mutations alter the overall folding of P2 in the crystal state. P2 disease variants show increased aggregation tendency and remarkably reduced stability, T51P being most severe. In addition, P2 disease mutations affect protein dynamics. Both fatty acid binding by P2 and the kinetics of its membrane interactions are affected by the mutations. Experiments and simulations suggest opening of the β barrel in T51P, possibly representing a general mechanism in fatty acid-binding proteins. Our findings demonstrate that altered biophysical properties and functional dynamics of P2 may cause myelin defects in CMT1 patients. At the molecular level, a few malformed hydrogen bonds lead to structural instability and misregulation of conformational changes related to ligand exchange and membrane binding.

A Mutation in PMP2 Causes Dominant Demyelinating Charcot-Marie-Tooth Neuropathy

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of peripheral neuropathies with diverse genetic causes. In this study, we identified p.I43N mutation in PMP2 from a family exhibiting autosomal dominant demyelinating CMT neuropathy by whole exome sequencing and characterized the clinical features. The age at onset was the first to second decades and muscle atrophy started in the distal portion of the leg. Predominant fatty replacement in the anterior and lateral compartment was similar to that in CMT1A caused by PMP22 duplication. Sural nerve biopsy showed onion bulbs and degenerating fibers with various myelin abnormalities. The relevance of PMP2 mutation as a genetic cause of dominant CMT1 was assessed using transgenic mouse models. Transgenic mice expressing wild type or mutant (p.I43N) PMP2 exhibited abnormal motor function. Electrophysiological data revealed that both mice had reduced motor nerve conduction velocities (MNCV). Electron microscopy revealed that demyelinating fibers and internodal lengths were shortened in both transgenic mice. These data imply that overexpression of wild type as well as mutant PMP2 also causes the CMT1 phenotype, which has been documented in the PMP22. This report might expand the genetic and clinical features of CMT and a further mechanism study will enhance our understanding of PMP2-associated peripheral neuropathy.

Isolation of causative mutation is still challenging in genetic diseases with a variety of genetic causes. We discovered a mutation in a novel gene from a family exhibiting a peripheral neuropathy by virtue of next-generation sequencing. Although the family shows characteristic clinical features of hereditary motor and sensory neuropathy, we could not find a mutation from well-known genes. To demonstrate the clinical relevance of the novel gene, we generated transgenic mice, which carry the patients’ mutation within their chromosome. The transgenic mice exhibited the same phenotype as the patients including peripheral neuropathic symptoms and reduced locomotor function. We also observed the affected peripheral nervous system through electron microscopy. It seems that the expression of the mutant protein impairs the myelin of peripheral nervous system. These data might expand the genetic, clinical, and pathophysiological features of the peripheral neuropathy and a further investigation will enhance our understanding of disease in the peripheral nervous system.

A role of peripheral myelin protein 2 in lipid homeostasis of myelinating Schwann cells

Peripheral myelin protein 2 (Pmp2, P2 or Fabp8), a member of the fatty acid binding protein family, was originally described together with myelin basic protein (Mbp or P1) and myelin protein zero (Mpz or P0) as one of the most abundant myelin proteins in the peripheral nervous system (PNS). Although Pmp2 is predominantly expressed in myelinated Schwann cells, its role in glia is currently unknown. To study its function in PNS biology, we have generated a complete Pmp2 knockout mouse (Pmp2(-/-) ). Comprehensive characterization of Pmp2(-/-) mice revealed a temporary reduction in their motor nerve conduction velocity (MNCV). While this change was not accompanied by any defects in general myelin structure, we detected transitory alterations in the myelin lipid profile of Pmp2(-/-) mice. It was previously proposed that Pmp2 and Mbp have comparable functions in the PNS suggesting that the presence of Mbp can partially mask the Pmp2(-/-) phenotype. Indeed, we found that Mbp lacking Shi(-/-) mice, similar to Pmp2(-/-) animals, have preserved myelin structure and reduced MNCV, but this phenotype was not aggravated in Pmp2(-/-) /Shi(-/-) mutants indicating that Pmp2 and Mbp do not substitute each other's functions in the PNS. These data, together with our observation that Pmp2 binds and transports fatty acids to membranes, uncover a role for Pmp2 in lipid homeostasis of myelinating Schwann cells.

Sequence motifs of myelin membrane proteins: towards the molecular basis of diseases

The shortest sequence of amino acids in protein containing functional and structural information is a "motif." To understand myelin protein functions, we intensively searched for motifs that can be found in myelin proteins. Some myelin proteins had several different motifs or repetition of the same motif. The most abundant motif found among myelin proteins was a myristoylation motif. Bovine MAG held 11 myristoylation motifs and human myelin basic protein held as many as eight such motifs. PMP22 had the fewest myristoylation motifs, which was only one; rat PMP22 contained no such motifs. Cholesterol recognition/interaction amino-acid consensus (CRAC) motif was not found in myelin basic protein. P2 protein of different species contained only one CRAC motif, except for P2 of horse, which had no such motifs. MAG, MOG, and P0 were very rich in CRAC, three to eight motifs per protein. The analysis of motifs in myelin proteins is expected to provide structural insight and refinement of predicted 3D models for which structures are as yet unknown. Analysis of motifs in mutant proteins associated with neurological diseases uncovered that some motifs disappeared in P0 with mutation found in neurological diseases. There are 2,500 motifs deposited in a databank, but 21 were found in myelin proteins, which is only 1% of the total known motifs. There was great variability in the number of motifs among proteins from different species. The appearance or disappearance of protein motifs after gaining point mutation in the protein related to neurological diseases was very interesting.