A Guide to Oligodendrocyte Markers

Oligodendrocytes are one of the major glial cell types of the central nervous system (CNS). Their primary function is to support CNS neurons, and they do so by ensheathing their axons in myelin. In addition to providing neurons with a protective layer of fat and protein, myelin sheaths speed up the conduction of electrical signals, enabling neurons to communicate quickly and efficiently. Loss of oligodendrocytes or myelin can have profound neurological consequences, ranging from cerebral palsy to multiple sclerosis. 

Oligodendrocytes originate from the same precursor cell as neurons and astrocytes: the neural stem cell. These precursors differentiate into oligodendrocyte progenitor cells (OPC), then premyelinating oligodendrocytes, and finally myelinating oligodendrocytes. This guide summarizes the most commonly used markers of each of these cell types, beginning with OPCs.

Oligodendrocyte Progenitor Cells (OPC) Markers

The first stage in oligodendrocyte development is the OPC. These bipolar, self-renewing cells originate in several waves in the developing spinal cord and forebrain. Although the transcriptional control of each OPC wave differs somewhat from the last, they share some commonalities. SOX10, NKX2.2, OLIG1, and OLIG2 are early transcriptional regulators of OPC differentiation. The upregulation of these transcription factors is followed by platelet-derived growth factor α receptor (PDGFRA) expression, a key determinant of OPC fate. The expression of this cell surface receptor is perhaps the best known marker of OPCs, enabling isolation of these cells from mouse and human brains. It should be noted that PDGFRα-negative OPCs exist, but their OPC-likeness—their ability to proliferate, migrate, and differentiate into myelinating cells—is diminished.

Image: This figure highlights the general developmental progression of oligodendrocytes and associated markers.

OPCs also express CSPG4 (more commonly known as NG2) as well as gangliosides recognized by the A2B5 antibody. However, these markers are not specific to the oligodendrocyte lineage, as they also mark neuronal, other glial progenitors, and pericytes. PDGFRA is, therefore, more useful than the NG2 marker and the A2B5 antibody for isolating OPCs.

The myelin-related proteins cyclic nucleotide phosphodiesterase (CNP) and myelin-associated glycoprotein (MAG) are expressed early during the OPC stage of the oligodendrocyte differentiation. The expression of these proteins is maintained or increased as oligodendrocytes mature.

Premyelinating Oligodendrocyte Markers

Upon transitioning to premyelinating oligodendrocytes, OPCs exit the cell cycle, express more myelin-associated proteins like proteolipid protein 1 (PLP1), upregulate lipid biosynthesis, and develop a highly branched morphology in support of their emerging myelination program.

Premyelinating oligodendrocytes express many of the same transcriptional regulators as OPCs, including OLIG1, OLIG2, SOX10, and NKX2.2. In addition to these markers, they also express other regulators such as ZFP191, ZFP488, ZFP536, SOX17, and NKX6.2, all of which help to further restrict the differentiating cells to the oligodendrocyte lineage. Indeed, the expression of SOX10, OLIG2, and NKX6.2, or ZFP536 is sufficient to convert fibroblasts into myelogenic oligodendroglia in vitro.

SMARCA4 (also known as BRG1) is highly transcribed during oligodendrocyte lineage commitment, where it participates in the remodeling of chromatin and inhibition of non-oligodendrocyte fates.

Myelinating Oligodendrocyte Markers

The transcriptional activation of myelin regulatory factor (MYRF) instructs oligodendrocytes to myelinate by upregulating the proteins used in myelin formation: myelin basic protein (MBP), MAG, myelin oligodendrocyte protein (MOG), PLP1, and CNP. SOX10 helps to maintain the expression of MYRF and its targets. Although these myelin markers label oligodendrocyte cell bodies during myelination, they only label myelin sheaths once myelination is complete. Thus, other markers are required to label oligodendrocyte cell bodies outside periods of myelination. A monoclonal antibody called “CC1”, which was recently found to bind to the protein Quaking 7 (QKI7), labels mature oligodendrocytes without labeling myelin.

Other myelinating oligodendrocyte markers include transferrin (TFR), reflecting the importance of iron as a cofactor in the production of myelin; myelin and lymphocyte protein (MAL), which regulates the patterning of myelination; CD82, a tetraspanin that regulates oligodendrocyte migration and myelination; and ENPP4, which participates in lipid (phosphatidylcholine) biosynthesis; ASPA; the cytokine BMP4; and TMEM10.

A change in membrane lipid composition also accompanies the initiation of myelination. Premyelinating oligodendrocytes express the surface lipid sulfatide, which can be detected using the O4 antibody. Initiation of the myelination program, however, results in a switch from sulfatide to galactocerebroside (also known as GalC), which can be detected using the O1 antibody. Importantly, the O1, O4, and A2B5 (OPC-specific, discussed above) antibodies are generally used for in vitro applications where the targeted antigens are less likely to be distorted by certain fixatives.

Oligodendrocyte Immunohistochemical Markers

As cells of the CNS, oligodendrocytes are frequently detected immunohistochemically. Yet, only some of the markers discussed in this article are available for immunohistochemistry. Further, many oligodendrocyte markers span multiple developmental stages in the oligodendrocyte lineage, complicating the identification or isolation of cells from each stage. This section highlights the most useful pan-oligodendrocyte and stage-specific immunohistochemical markers.

All oligodendrocytes, including OPCs, express SOX10 and OLIG2. Not only is SOX10 a pan-oligodendrocyte marker, it is also highly specific to the oligodendrocyte lineage, at least within the CNS. Unlike SOX10, OLIG2 also labels non-oligodendrocyte cells (namely, astrocytes) in the embryonic and early prenatal brain. Therefore, OLIG2 is a good pan-oligodendrocyte marker after the early postnatal period.

OPCs can be detected using the PDGFRA, NG2, and A2B5 markers. But of these, only PDGFRA is oligodendrocyte-specific. NG2 and the A2B5 antibody, as mentioned above, also label glial progenitors, neurons, and pericytes. In the absence of a highly specific marker, the characteristic bipolar morphology of OPCs can serve as a “marker” of OPCs.

Premyelinating and myelinating oligodendrocytes can be identified using the CC1 antibody. However, MYRF labeling may be better for counting oligodendrocyte numbers, given its better specificity and nuclear localization. (CC1 also labels other progenitors in the developing CNS.) Of course, mature oligodendrocytes can also be identified by their highly branched morphology, but oligodendrocyte-specific markers are still needed. The O4 antibody has great specificity and efficiency for oligodendrocyte labeling but is less suitable for cell counting than MYRF. ASPA labels more mature oligodendrocytes, as its expression starts later than the myelin-related proteins.

Myelin itself can be labeled immunohistochemically using the following markers: CNP, MBP, PLP, MAG, and MOG.

Table of Oligodendrocyte Markers

The table below lists characteristic oligodendrocyte proteins as described by review literature. The list includes a variety of marker types, including transcription factors, membrane proteins, secreted factors, signaling proteins, and structural proteins. Accompanying each marker are links to relevant antibodies and ELISA kits that can be used to detect oligodendrocytes in vitro and in vivo. The associated products are offered by a variety of manufacturers and can serve as a useful reference for oligodendrocyte characterization.

Note: Some markers are lipids (*) or names of antibodies (**). Information on Protein Type, Localization, and Size (kDa) obtained from UniProt.org (for human genes only). 

References

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