NMDA receptors expressedin oligodendrocytesRichard Wong

SummaryOligodendrocytes are known to express Ca2þ-permeableglutamate receptors and to have low resistance tooxidative stress, two factors that make them potentiallysusceptible to injury. Oligodendrocyte injury is intrinsic tothe loss of function experienced in conditions rangingfrom cerebral palsy to spinal cord injury, focal ischaemiaand multiple sclerosis. NMDA receptors, a subtype ofglutamate receptors, arevital to the remodelingofsynapticconnections during postnatal development and associa-tive learning abilities in adults and possibly in improve-ments in oligodendrocyte function. Previous studies hadfailed to detect NMDA receptor mRNA or current inoligodendrocytes but three new papers(1–3) demonstrateNMDA receptor expression in oligodendrocytes anddiscuss its implications for ischaemia therapy. BioEs-says 28:460–464, 2006.� 2006 Wiley Periodicals, Inc.

Oligodendrocytes

Oligodendrocytes generate and maintain central nervous

system (CNS) myelin and regulate axon function. In spinal

cord or brain injury, damage to these cells is important in a

growing list of acute and chronic conditions.(4)

Much of our current knowledge about the oligodendrocyte

concern their role in myelinated axons in the CNS. As with

neurons, oligodendrocytes are highly sensitive to injury

mediated by trophic factor deprivation, oxidative stress,

excitatory amino acids and launching of apoptotic pathways.

Hypoxic-ischemic damage to oligodendrocytes is a frequent

feature in global ischemia (cardiac arrest), focal ischemia

(stroke), cyanide intoxication and vascular dementia,(4) and

oligodendrocytes are also injured in brain and spinal

cord trauma, multiple sclerosis and even Alzheimer’s disease.

During the perinatal period, damage to oligodendrocyte

progenitor cells results in periventricular leukomalacia and

long-term demyelination, a key etiology of cerebral palsy.

We know that oligodendrocytes are responsible for myelinat-

ing axons, and that loss of myelination under such conditions

contributes to brain dysfunction.(5,6) Unlike neurons, which are

susceptible to NMDA receptor-mediated damage, white matter

oligodendrocytes were previously thought to be damaged by

glutamate acting on AMPA and kainate receptors alone.

However, several researchers have focused on a possible

role of NMDA receptors in oligodendrocytes. Recently, three

groups demonstrated that NMDA receptor is expressed in

oligodendrocytes(1–3) andNMDA receptorsmight playa role in

ischaemia, providing new clues in myelination field.

NMDA receptors

NMDA receptors, a subtype of glutamate receptors, are

oligomeric ligand-gated ion channel complexes formed by

the assembly of different subunits. NMDAR consists of an

essential subunit, NR1, and various modulatory NR2 subunits

(NR2A, NR2B, NR2C, and NR2D) and NR3 subunits (NR3A

and NR3B) (Table 1). The NMDAR channel has been found to

be particularly important for synaptic plasticity, circuit devel-

opment, learning and memory.(7–9) When activated, NMDA

receptors conduct calcium, sodium and potassium ions, and

are thus ionotropic. Calcium conducted through NMDARs

activates numerous intracellular signaling cascades, giving

the NMDAR a metabotropic character (Fig. 1). NMDA

responses contribute little to the rising phase or the peak

amplitude of the EPSP or EPSC.(7,8,10)

NMDA receptors are expressed

in oligodendrocytes

Oligodendrocyte injury is a critical element in the loss of

function experienced in conditions ranging from cerebral palsy

to spinal cord injury and multiple sclerosis.(5,6,11) Damage

to oligodendrocytes is also a crucial secondary factor

in neurological disorders such as stroke and Alzheimer’s

disease.(1,11)

Oligodendrocytes express Ca2þ-permeable glutamate

receptors and have low resistance to oxidative stress, two

factors thatmake thempotentially susceptible to injury.(1,11–13)

They were thought to express mainly non-NMDA glutamate

receptors, and this expression was developmentally regu-

lated.(14,15)

Laboratory of Cell Biology, Howard Hughes Medical Institute, The

Rockefeller University, 1230 York Avenue, New York, NY 10021, USA.

E-mail: rwong@rockefeller.edu

DOI 10.1002/bies.20402

Published online in Wiley InterScience (www.interscience.wiley.com).

460 BioEssays 28.5 BioEssays 28:460–464, � 2006 Wiley Periodicals, Inc.

Abbreviations: AMPA, amino-3-hydroxy-5-methyl-4-isoxazole propio-

nic acid; CNS, central nervous system; EPSP, excitatory postsynaptic

potential; EPSC, excitatory postsynaptic current; NMDA N-methyl-D-

aspartate; PVL, periventricular leukomalacia.

What the papers say

Moreover, high expression of non-NMDA receptors in im-

mature oligodendrocytes and low expression of the calcium-

impermeable GluR2 subunit at the point when they initiate

myelination may increase their sensitivity to an excitotoxic

cascade mediated by ischaemic glutamate release and

subsequent intracellular Ca2þ([Ca2þ]i) overload.(6,14,15)

Salter and Fern in their recent paper(1) suggested that it

might explain the selective injury of precursor oligodendro-

cytes and subsequent hypomyelination in PVL. PVL is the

main injury associated with cerebral palsy, the most-common

human birth disorder, and clinical and experimental studies

indicate that hypoxia/ischemia is a major underlying cause of

PVL. Experimental models of ischemia in immature animals

link glutamate as a vital factor in the pathogenesis of brain

injury. The long-term consequences of PVL can engage either

focal oligodendrocyte loss (associated with early loss of cell

processes in animal models) or diffuse disruption of myelina-

tion, associated with abnormal oligodendrocyte process

morphology.(1)

They showed NMDA receptor subunit expression on

oligodendrocyte processes and the presence of NMDA

receptor subunit messenger RNA in isolated white matter.

NR1, NR2A,NR2B,NR2C,NR2DandNR3A subunits showed

clustered expression in cell processes, but NR3B was

absent.(1)

Previous studies have failed to detectNR1mRNA in the rat

optic nerve(16) and were unsuccessful in detecting NMDA

receptor currents in cultured oligodendrocytes.(17)

Karadottir and colleagues have also shown that oligoden-

drocyte NMDA receptor currents occur in both the cerebellum

and corpus callosum, and they may represent a general

property of white matter oligodendrocytes.(2) Electrophysiolo-

gical recordings have been made from precursor, immature

and mature oligodendrocytes in postnatal day (P)7–14

Figure 1. Aschemedepicting theactivationofNMDAreceptors signalingpathways.PKAphosphorylatesNMDAreceptor subunits,which

alters receptor conformation to enhance sensitivity to glutamate. Alternatively, cAMP activates MEK1/2 in a PKA-independent manner

(dashedarrow).NMDA receptor activation results in an increase inCa2þ influx,which, in turn, activatesMAPKssignaling-related cascades.

MK-801 blocks the NMDA receptor in the phosphorylated state. Activation of MAPKERK via the upstream MEK1/2, which is regulated

by cAMP/PKA. MAPKp38 blocks the phosphorylation of Elk-1 and CREB, which regulates the phosphorylation of the downstream cofactor,

c-Fos. (Modified from Haddad JJ 2005 Prog Neurobiol 77:252–282.).23

Table 1. Different subunits of NMDA receptor

NMDAreceptor

Aminoacids(Human)

NCBI ACCESSIONNumber

Glycinesite

NR1-1 885 aa NP_000823 Yes

NR1-2 901 aa NP_067544 Yes

NR1-3 938 aa NP_015566 Yes

NR2A 1464 aa AAN75825

NR2B 1484 aa NP_000825

NR2C 1233 aa NP_000826

NR2D 1336 aa O15399

NR3A 1115 aa NP_597702

NR3B 1043 aa NP_619635

What the papers say

BioEssays 28.5 461

rats and adult; NMDA receptor subunits are present in

oligodendrocytes.(2)

Consistently, Salter and Fern used fluorescence polymer-

ase chain reaction with reverse transcription (RT–PCR),

coupled toahigh-stringencyRNA-extractionprotocol involving

three sequential purifications steps.(1) They found NR1

transcript in the optic nerve, which when quantified was at

1–2% of the abundance found in the whole brain.(1)

To examine further the origin of NR1 mRNA in the optic

nerve, they carried out RT-PCR for intron regions of NR1 and

Thy1, which exist only in the nucleus. This revealed robust

expression of the NR1 intron in whole brain and optic nerve,

whereas expression of the Thy1 intron was detected in whole

brain but not in the optic nerve. Optic nerve Thy1 mRNA is

therefore produced in somata that are not present in the

nerve (that is, retinal ganglion cells), whereasNR1 is produced

in optic nerve glial somata. PCR analysis of all known NMDA

receptor subunits revealed the presence of mRNA for the

subunits detected by antibody staining in oligodendrocytes.

Quantification suggests that NR1, NR2C and NR3A

mRNA are the most-abundant subunits in whole optic nerve;

NR2B mRNA was present at low abundance and NR3B

was absent.

In addition to cerebral palsy, oligodendrocyte process injury

is also relevant to adult diseases such as stroke, spinal cord

injury and multiple sclerosis.(1,5,6) They therefore further

examined process loss in P25 CNP-GFP mouse optic nerve,

a stage by which all precursor cells have progressed to the

mature oligodendrocyte phenotype. Antibody staining re-

vealed a similar pattern of NR1, NR2A and NR2B expression

in oligodendrocytes at this stage, with abundant GluR2/3

expression mainly in somata.

Consistent with this idea, Karadottir and colleagues

also found NMDA receptor currents in oligodendrocytes

in several brain regions and at various developmental

stages.(2) These NMDA receptor-mediated currents show a

low degree of voltage-dependent Mg2þ block, and immunos-

taining results imply that NR1, NR2C and NR3 are the

major NMDA receptor subunits, but that NR2A and NR2B

are present.

Twoearlier reports suggest lowCa2þpermeability inNMDA

receptors that incorporate the NR1A, NR2A/NR2B and NR3A

subunits. However, no information is available regarding the

Ca2þ permeability of receptors that include the NR2C

subunit, which may not share this feature when incorporated

with NR3A.(18,19) In addition, the dimensions of oligodendro-

cyte processes are small, and even NMDA receptors with low

Ca2þ permeability may raise intracellular Ca2þ to toxic levels

within such a confined space. These new data clearly suggest

the Ca2þ-dependence of NMDA receptor-mediated process

loss, and imply that sufficient Ca2þ influx occurs through

the NMDA receptors on oligodendrocyte processes to result

in injury.(1)

NMDA receptors are activated in ischaemia

Interestingly, in adult rodents, a similar pattern of abnormal

activity after ischemia has been shown to be mediated by

activation of the NMDA receptor and to play a key role in post-

ischemic injury. During modelled ischaemia, NMDA receptor

activation resulted in rapid Ca2þ-dependent detachment and

disintegration of oligodendroglial processes in the white

matter of mice expressing green fluorescent protein (GFP)

specifically in oligodendrocytes (CNP-GFP mice). This effect

occurred at mouse ages corresponding to both the initiation

and the conclusion of myelination. NR1 subunits were found

primarily in oligodendrocyte processes, whereas AMPA or

kainate receptor subunits were largely found in the somata.

Consistent with this observation, Salter and Fern also found

that injury to the somata was prevented by blocking AMPA/

kainate receptors, and preventing injury to oligodendroglial

processes required the blocking of NMDA receptors.(1) The

existence of NMDA receptors in oligodendrocyte processes

accounts for why previous studies that have focused on the

somata have not detected a role for NMDA receptors in

oligodendrocyte injury. These NMDA receptors give a high

sensitivity to acute injury and represent a vital new target for

drug development in a variety of brain disorders.(1)

Future perspectives: what is the exact role of

NMDA receptors in oligodendrocytes?

There is an indication of the presence of NMDA receptors on

mature astrocytes, Muller cells and Bergman glia,(14) and

someevidence for their presenceon oligodendrocytes in other

preparations.(20,21) Activation of the AMPA/kainate receptors

expressed by developing oligodendroglia can influence gene

transcription and cell proliferation, survival and fate. The

functional significance of NMDA receptor expression in

immature oligodendroglial processes is still unclear. It might

involve axon–glial signalling during myelinogenesis.

Myelin is an important structure that has been observed

histopathologically to degenerate in a broad range of CNS

disorders. Many disorders promote damage to and eventual

loss of the myelin sheath, which often results in significant

neurological morbidity. However, little is known about the

fundamentalmechanisms that initiatemyelin damage,with the

assumption being that its fate follows that of the parent

oligodendrocyte.(3)

Myelin initiation begins with the extension of multiple

processes from the somata that make contact with axons.

Oligodendrocyte NMDA receptors are likely to have a role in

controlling oligodendrocyte development and myelination(22)

and in damaging oligodendrocytes under pathological condi-

tions. They show only weak block by Mg2þ at the cells’ resting

potential, and mediate part of the inward current generated in

oligodendrocytes in response to simulation of the energy

deprivation that occurs in periventricular leukomalacia, in

stroke, and after ischaemia in spinal cord injury.(2) Remarkably,

What the papers say

462 BioEssays 28.5

NMDA receptors are present in the myelinating processes of

oligodendrocytes, where the intracellular volume is small and

receptor-mediated ion influx may produce large increases in

intracellular ion concentration and osmotic water flux, which

could disrupt myelination. The higher glutamate affinity of

NMDA receptors relative toAMPA receptorsmakes themmore

likely to be activated in neurodegenerative disorders that

involve a small but prolonged increase in extracellular

glutamate concentration, as can occur in multiple sclerosis.

Thus, oligodendrocyte NMDA receptors could contribute to

causing the white matter damage that occurs when the

extracellular glutamate concentration is increased in periven-

tricular leukomalacia, spinal cord injury, multiple sclerosis and

stroke.(1,2,5,6) Indeed, in the optic nerve, activation of NMDA

receptors on oligodendrocyte processes when glutamate is

released during ischaemia leads to the disintegration of those

processes.(1)

More recently, Micu and colleagues showed that NMDA

receptors mediate Ca2þ accumulation in central myelin in

response to chemical ischaemia in vitro.(3) Using two-photon

microscopy, Micu et al found that imaged fluorescence of the

Ca2þ indicator X-rhod-1 loaded into oligodendrocytes and the

cytoplasmic compartment of the myelin sheath in adult rat

optic nerves. Ca2þ increase inmyelin was abolished by broad-

spectrum NMDA receptor antagonists (MK-801, 7-chloroky-

nurenic acid, d-AP5), but not by more selective blockers of

NR2A andNR2B subunit-containing receptors (NVP-AAM077

and ifenprodil). In vitro ischaemia causes ultrastructural

damage to both axon cylinders and myelin. NMDA receptor

antagonism greatly reduced the damage to myelin.(3) NR1,

NR2 and NR3 subunits were detected in myelin by immuno-

histochemistry and immunoprecipitation, indicating that all

necessary subunits are present for the formation of functional

NMDA receptors.(3) They also showed that the mature myelin

sheath can respond independently to injurious stimuli. As

axons are known to release glutamate, their finding that the

Ca2þ increase was mediated in large part by initiation of

myelinic NMDA receptors suggests a mechanism of axo-

myelinic signalling. A more in-depth understanding of such an

NMDA-receptor-dependent ‘axo–myelinic’ communication

could guide the design of more effective treatments for

disorders in which demyelination of central white matter tracts

is a prominent and clinically devastating phenomenon.(3) This

mechanism may represent a important therapeutic target in

disorders in which demyelination is a prominent feature, such

as neurotrauma, infections, multiple sclerosis and aspects of

ischaemic brain injury.(3)

There are other unanswered questions. Oligodendroglial

processes will either proceed with myelination or retract from

the axon. How this decision is controlled is unclear, but Ca2þ

influx through activated NMDA receptors would affect cytos-

keletal elements, such as microtubules, actins or motor

protein, kinesin, dynein and myosin; KIF17(Kinesin-2) has

shown trafficking of NMDA receptor subunit NR2B and

involvement in higher brain function such as learning and

memory.(9) Regardless of the function of NMDA receptors on

developing oligodendrocyte processes, their pathophysiologi-

cal relevance is high, as they bestow a sensitivity to injury that

is likely to have a significant impact on variety of neurological

diseases.(1)

In addition, it is encouraging that these results show that

NMDA receptor blockade alone can be sufficient to protect

against injury. The unusual subunit composition of the

receptors (which include mainly NR2C in addition to NR3A

subunits) also raises the prospect of developing targeted

interventions with fewer side effects than those experienced

with non-selectiveNMDAantagonists.(1) These results point to

NMDA receptors of unusual subunit composition as a potential

therapeutic target for preventing white matter damage in a

variety of diseases.

In a word, identifying functional NMDA receptors in

oligodendrocytes,(1–3) at the very least has broadened our

horizons and may provide new insights for devising potential

therapeutic targets for many brain-devastating diseases,

especially myelination injury or disease.

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