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

Salla Ruskamo,Ulrich Bergmann,Guro H Vatne,Erik I Hallin,Arne Raasakka,Anne Baumann,Ilpo Vattulainen,Tuomo Nieminen,Päivi Joensuu,Petri Kursula,Cecilie K Kristiansen

doi:10.1038/s41598-017-06781-0

Abstract

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.

Highlights

A multilamellar membrane structure, myelin, enwraps selected axons in the nervous system, enabling the prompt transmission of nerve impulses along the axonal membrane
We used a range of experimental techniques together with atomistic simulations to study the effects of the mutations on P2 structure, function, and dynamics
The hydrodynamic radius (Rh) of monomeric T51P was larger compared to the other variants (Table 1)

Summary

Introduction

A multilamellar membrane structure, myelin, enwraps selected axons in the nervous system, enabling the prompt transmission of nerve impulses along the axonal membrane. In the peripheral nervous system (PNS), Schwann cells envelop axons and form insulating myelin sheaths. Defects in this fundamental structure result in chronic, severe neuropathological conditions, affecting nerve conduction velocity and neuron viability. The I43N mutation was discovered in two families[4,5] and T51P and I52T each in one family[6] All patients with these mutations have reduced motor and sensory nerve conduction velocities, thinner myelin sheaths, and demyelinating axons with onion bulb formation. P2 is a small 15-kDa protein with a β barrel structure covered by an α-helical lid[9,10] It belongs to the conserved family of fatty acid-binding proteins (FABPs) and can transfer fatty acids from and to lipid membranes using a collision transfer mechanism, indicating a functional role in the lipid homeostasis of myelin[11]. The mutations may cause defects in protein folding and the regulation of functional conformational changes, and they provide clues into ligand binding dynamics in the FABP family

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Results

Conclusion