What Is the NG2 Marker and What Does It Identify?

A biomarker is a measurable substance or process that acts as an indicator, revealing what is happening inside an organism. These molecular clues can signal the presence of a disease, track its progression, or show how the body is responding to a treatment. In neuroscience, these markers are used to understand both normal function and disease.

One marker used by scientists is a protein known as Neuron-Glial Antigen 2, or NG2. This marker is an important tool for studying a unique population of cells within the central nervous system. By tagging these cells, the NG2 marker allows researchers to isolate and observe them, uncovering their distinct roles in the brain.

Identifying NG2-Expressing Cells

The NG2 marker identifies a large molecule on the surface of certain cells called a proteoglycan, which is used to label a specific class of brain cells known as NG2-glia. For many years, these cells were more commonly called Oligodendrocyte Precursor Cells (OPCs), and the two terms are often used interchangeably. These cells are now considered the fourth major type of glial cell, a group of non-neuronal support cells that also includes astrocytes, microglia, and oligodendrocytes.

NG2-glia are distributed throughout the adult brain and spinal cord, present in both the gray matter, which contains neuronal cell bodies, and the white matter, which is composed of nerve fibers. They are distinguished by their shape; each cell has a small cell body with numerous fine, highly branched processes extending outward. The presence of the NG2 proteoglycan on their surface is their most definitive feature, enabling scientists to track their development and behavior.

Function in Brain Development and Myelination

During the development of the central nervous system, the primary function of NG2-expressing cells is their role as progenitors for oligodendrocytes. As oligodendrocyte precursor cells (OPCs), they divide and then differentiate to become the mature oligodendrocytes responsible for myelination.

Myelination is the process where oligodendrocytes wrap their membranes around the axons of neurons, forming a fatty, insulating layer called the myelin sheath. The myelin sheath prevents the dissipation of the electrical signal and allows it to move much more rapidly. It does this by enabling a type of signal propagation known as saltatory conduction, where the signal “jumps” between gaps in the myelin. This rapid transmission of nerve impulses is necessary for everything from simple reflexes to complex cognitive processes.

Roles in the Mature Central Nervous System

A significant population of NG2-glia persists throughout the mature central nervous system. Unlike their developmental counterparts, the majority of these adult NG2-glia do not differentiate into oligodendrocytes but remain in a precursor-like state. Research has revealed they are active participants in the mature brain.

One discovery is that NG2-glia form direct synaptic connections with neurons, meaning they can receive signals from nerve cells, much like other neurons do. This feature is unusual for a glial cell and places NG2-glia in a position to “listen in” on communication within neural circuits. While they receive these inputs, the exact way they respond is still a subject of investigation.

This ability to monitor neural activity suggests that NG2-glia play a part in brain plasticity, the ability of the brain to adapt and change over time. By sensing the activity levels of surrounding neurons, these cells may help regulate and support the function of synapses, contributing to the overall stability of the neural network.

Response to Injury and Disease

In response to damage or disease in the central nervous system, NG2-glia become highly reactive and mobile. When the myelin sheath is damaged, a process that occurs in demyelinating diseases like Multiple Sclerosis (MS), these cells are triggered to respond. They divide and migrate to the site of injury, where they can differentiate into new oligodendrocytes to replace those that were lost. This process of creating new myelin, known as remyelination, is a natural repair mechanism that can help restore nerve function.

However, the response of NG2-glia to injury is not always beneficial. In severe physical trauma, such as a spinal cord injury, these cells contribute to the formation of a glial scar. While the scar helps to contain the initial damage, it also creates a barrier that inhibits the regrowth of severed axons, limiting functional recovery.

The reaction of NG2-glia depends heavily on the specific context of the injury or disease. Scientists are researching how to encourage the regenerative functions of these cells while limiting their contribution to inhibitory scar formation, a promising area for developing new therapies.