Structural Biology of the Myelin Membrane

Project Leader: Petri Kursula, Ph.D.
Faculty of Biochemistry and Molecular Medicine, University of Oulu and Department of Chemistry, University of Hamburg

 

Background and Significance

The myelin sheath is a specialized membrane structure in the vertebrate nervous system, enabling the fast ‘saltatory’ conduction of nerve impulses. Myelin is formed by the differentiated plasma membrane of a myelinating glial cell (Schwann cell or oligodendrocyte), which wraps itself tightly around the axon, forming an ordered, compact, multilayered membrane complex with a very low solvent content. As a biochemical membrane, myelin is unique. Essentially all of the myelin-specific proteins interact intimately with lipid bilayers, being either integral or peripheral membrane proteins. Despite a large volume of literature on myelin proteins, little is still known about their 3D structures and their complexes with other molecules. In addition to being specific to myelin, myelin proteins, in general, share little homology with proteins from other tissues or lower organisms.

We will answer long-standing questions in myelin biology, specifically related to the relationships between structure, dynamics, and function of individual myelin proteins and multilayered membranes, as well as the factors contributing to these properties. Specific points of interest include how these molecules interact with membranes and the cytoskeleton, how they are arranged on the membrane, and how they contribute to the formation and maintenance of the compact structure of myelin.

Detailed structure–function information will be crucial to understanding the physiological function of myelin proteins. Neurological demyelinating diseases, including multiple sclerosis and peripheral neuropathies, occur upon autoimmune attack against myelin or because of inherited mutations in myelin protein genes. Understanding of such diseases will be enhanced by accurate 3D structural data on myelin molecules and their interactions with each other and with ligands, including lipid membranes.
 

Recent Progress

In 2014 we made significant progress on various fronts, including determination of novel myelin protein crystal structures, characterization of myelin protein function and imaging of lipid bilayer membrane stacking induced by myelin-specific proteins. Some of the highlights are discussed below.

Periaxin is an abundant protein in myelinating Schwann cells in the peripheral nervous system. Mutations in periaxin cause peripheral neuropathies in humans. We obtained the first structural information on periaxin by crystallizing its N-terminal domain, which has weak homology to PDZ domains. The structure showed a surprising, fully intertwined dimerization mode, which involves a large degree of domain swapping. The structure explains the mode of dimerization of periaxin in protein scaffolds linking the plasma membrane to the cytoskeleton and it paves the way for further structural studies on other domains of periaxins.

The myelin cell adhesion molecule gliomedin is crucial for the clustering of sodium channels on the axonal membrane at the nodes of Ranvier. The extracellular domain of gliomedin consists of a collagen-like segment and an olfactomedin domain; structural data on olfactomedin domains have been lacking thus far. We solved the crystal structure of the gliomedin olfactomedin domain at high resolution; the structure presents a novel type of β-propeller fold. The olfactomedin β-propeller has 5 blades, but, surprisingly, its inherent symmetry is hexagonal. The structure allows us, for the first time, to obtain a detailed molecular insight into the interactions between gliomedin and its ligands on the neuronal surface. Our data will also be relevant in understanding structure–function relationships in other cell adhesion molecules containing an olfactomedin domain.

Figure 1. Crystal structure of the domain-swapped, intertwined dimer of the PDZ-like domain from periaxin.

Figure 2. A novel β-propeller structure in the olfactomedin domain from gliomedin.
 

Future Goals

We will continue to obtain high-resolution structural data from various myelin proteins and their complexes, and by combining the structures with complementary experiments, we will get detailed information on the structure–function relationships in myelin proteins. This information can be used to understand myelin protein function in normal and diseased myelin. Together with international collaborators, the structural data will further be linked to in vivo functions.

We will also use myelin proteins as general models for protein–membrane interactions. Detailed analyses of protein and membrane structure and dynamics will be carried out. The results will not only be relevant to myelination, but also to peripheral membrane protein function in general. Imaging of the molecular organization of a multilayered myelin-mimicking membrane is one major goal of our research in the future.

The project will also expand in the direction of obtaining understanding at the structural level of mutations and sequence variants in proteins and complexes involved in neurological disease conditions, including psychiatric disorders.
 

Publications 2014-

Piirainen H., Hellman M., Tossavainen H., Permi P., Kursula P. & Jaakola V.-P. (2015) Human adenosine A2A receptor binds calmodulin with high affinity in a calcium-dependent manner. Biophys. J., in press.

Han H. & Kursula P. (2015) The olfactomedin domain from gliomedin is a β-propeller with unique structural properties. J. Biol. Chem., in press.

Onwukwe G.U., Kursula P., Koski M.K., Schmitz W. & Wierenga R.K. (2015) Human Δ3, Δ2-enoyl-CoA isomerase, type-2: a structural enzymology study on the catalytic role of its ACBP-domain and helix-10. FEBS J., in press.

Laulumaa S., Kursula P. & Natali F. (2015) Neutron scattering studies on protein dynamics using the human myelin peripheral membrane protein P2. EPJ Web of Conferences, in press.

Knoll W., Peters J., Kursula P., Gerelli Y. & Natali F. (2014) Influence of myelin proteins on the structure and dynamics of a model membrane with emphasis on the low temperature regime. J. Chem. Phys. 141: 205101. 

Han H. & Kursula P. (2014) Expression, purification, crystallization, and preliminary X-ray crystallographic analysis of the extracellular olfactomedin domain from gliomedin. Acta Cryst. F 70: 1536-1539. 

Kursula P. (2014) The many structural faces of calmodulin - a multitasking molecular jackknife. Amino Acids 46: 2295-2304.


Zenker J., Stettner M., Ruskamo S., Domènech-Estévez E., Baloui H., Médard J.-J., Verheijen M.H.G., Brouwers J.F., Kursula P., Kieseier B.C. & Chrast R. (2014) A role of peripheral myelin protein 2 in lipid homeostasis of myelinating Schwann cells. Glia 62: 1502-1512. 


Han H. & Kursula P. (2014) Periaxin and AHNAK nucleoprotein 2 form intertwined homodimers through domain swapping. J. Biol. Chem. 289: 14121-14131.


Raasakka A. & Kursula P. (2014) The myelin membrane-associated enzyme 2′,3′-cyclic nucleotide 3′-phosphodiesterase: on a highway to structure and function. Neurosci. Bull. 30: 956-966.


Kursula P. (2014) Crystallographic snapshots of initial steps in the collapse of the calmodulin central helix. Acta Cryst. D 70: 24-30.


Ruskamo S., Yadav R.P., Sharma S., Lehtimäki M., Laulumaa S., Aggarwal S., Simons M., Bürck J., Ulrich A.S., Juffer A.H., Kursula I. & Kursula P. (2014) Atomic-resolution view into structure-function relationships of the human myelin peripheral membrane protein P2. Acta Cryst. D 70: 165-176.

Szambowska A., Tessmer I., Kursula P., Usskilat C., Prus P., Pospiech H. & Grosse F. (2014) DNA binding properties of human Cdc45 suggest a function as a molecular wedge for DNA unwinding. Nucleic Acids Res. 42: 2308-2319.

Knoll W., Peters J., Kursula P., Gerelli Y., Ollivier J., Demé B., Telling M., Kemner E. & Natali F. (2014) Structural and dynamical properties of reconstituted myelin sheaths by myelin proteins MBP and P2 studied by neutron scattering. Soft Matter 10: 519-529.
 

Research Group Members

Project Leader:
Petri Kursula, PhD (University of Bergen, Norway)

Senior and Post-doctoral Investigators:
Salla Ruskamo, PhD (Biocenter Oulu, Academy of Finland)
Matti Myllykoski, PhD (Foundations, Biocenter Oulu)
Huijong Han, PhD (Academy of Finland)
Weisha Luan, PhD (Foundations)

Ph.D. Students:
Arne Raasakka, MSc (Foundations)
Saara Laulumaa, MSc (European Spallation Source)
Maryna Chukhlieb, MSc (Biocenter Oulu)
Srinivas Kumar Ponna, MSc (Biocenter Oulu)

Foreign Scientists, 4
 

National and International Activities

Group Members Who Spent More Than Two Weeks in Foreign Laboratories During 2014

Petri Kursula, DESY, Hamburg, Germany & University of Bergen, Norway
Huijong Han, DESY, Hamburg, Germany
Saara Laulumaa, DESY, Hamburg, Germany & ILL, Grenoble, France
Matti Myllykoski, University of Bremen, Germany
Salla Ruskamo, University of Basel, Switzerland

Visiting Researchers in 2014 (over two weeks)

Saray Gonzalez, University of Barcelona, Spain
Marja Kornhuber, Freie Universität Berlin, Germany

Co-operation With Finnish and Foreign Companies

European Spallation Source, Lund, Sweden (PhD training grant)

Last updated: 23.3.2016