Ephrin-A1-EphA4 signaling negatively regulates myelination in
the central nervous system
Lisbeth S. Laursen
Department of Molecular Biology and
Genetics, Aarhus University, Gustav Wieds
Vej 10C, Aarhus C, 8000, Denmark
Lisbeth S. Laursen, Department of
Molecular Biology and Genetics, Aarhus
University, Gustav Wieds Vej 10C, 8000
Aarhus C, Denmark.
During development of the central nervous system not all axons are myelinated, and axons may
have distinct myelination patterns. Furthermore, the number of myelin sheaths formed by each oli-
godendrocyte is highly variable. However, our current knowledge about the axo-glia
communication that regulates the formation of myelin sheaths spatially and temporally is limited.
By using axon-mimicking microfibers and a zebrafish model system, we show that axonal ephrin-
A1 inhibits myelination. Ephrin-A1 interacts with EphA4 to activate the ephexin1-RhoA-Rock-
myosin 2 signaling cascade and causes inhibition of oligodendrocyte process extension. Both in
myelinating co-cultures and in zebrafish larvae, activation of EphA4 decreases myelination,
whereas myelination is increased by inhibition of EphA4 signaling at different levels of the path-
way, or by receptor knockdown. Mechanistically, the enhanced myelination is a result of a higher
number of myelin sheaths formed by each oligodendrocyte, not an increased number of mature
cells. Thus, we have identified EphA4 and ephrin-A1 as novel negative regulators of myelination.
Our data suggest that activation of an EphA4-RhoA pathway in oligodendrocytes by axonal
ephrin-A1 inhibits stable axo-glia interaction required for generating a myelin sheath.
axo-glia interaction, EphA4, myelin, oligodendrocyte, RhoA
The velocity by which signals from nerve cells are propagated is dramati-
cally enhanced by the formation of myelin sheaths. In the central nervous
system, these lipid-rich membrane sheaths are formed by the oligoden-
drocytes, which extend multiple processes that wrap around up to 50 dif-
ferent axons. However, not all axons within the central nervous system
are myelinated (Sturrock, 1980), and recent data suggest that even on
myelinated axons, the myelin sheaths are not evenly distributed (Tomassy
et al., 2014). Local regulation of axonal myelination patterns may there-
fore be important for fine-tuning signaling velocities within neuronal cir-
cuits. The formation of such variable patterns requires a tight regulation
to control which axons are myelinated and the number and length of the
myelin sheaths that are formed, implicating that communication between
nerve cells and oligodendrocytes plays an important role. Oligodendro-
cyte precursors cells (OPCs) are evenly distributed throughout the brain,
but not all differentiate into myelinating cells (Tomassy et al., 2014), and
the number of myelin sheaths formed by individual mature cells is highly
variable (Chong et al., 2012; Dumas et al., 2015; Osanai et al., 2017;
Weruaga-Prieto, Eggli, & Celio, 1996). Therefore, what determines
whether a given axon is myelinated, and what regulates the number of
myelin sheaths formed by each oligodendrocyte?
Oligodendrocytes have been reported to ensheath fixated neurons
(Rosenberg, Kelland, Tokar, De la Torre, & Chan, 2008) and inert micro-
fibers with a diameter above 0.4 mm (Lee et al., 2012), leading to the sug-
gestion that the default action of the oligodendrocyte is to ensheath
axons above a certain diameter. However, microfibers coated with the
extracellular matrix protein laminin causes an enhancement of the num-
ber of myelin sheaths formed by each oligodendrocytes (Bechler, Byrne,
& Ffrench-Constant, 2015), and JAM2 was with the use of a micropillar
system identified as an inhibitor of oligodendrocyte interaction with the
somatodendritic compartment of neurons (Redmond et al., 2016). This
implicates that contact formation with neurons and the myelinating
capacity of the oligodendrocyte are subject to modulation by signals
associated with different parts of the neuronal surface, but there are no
reports on inhibition of axonal ensheathment in these simplified systems.
Mette Harboe and Julie Torvund-Jensen contributed equally to this work.
2018 Wiley Periodicals, Inc. wileyonlinelibrary.com/journal/glia Glia. 2018;66:934–950.
Received: 8 July 2016
Revised: 28 December 2017
Accepted: 2 January 2018