Neurons are cells in the brain that govern signaling, and therefore are fundamental in sensory perception, emotional response, the regulation of bodily functions such as heart rate, digestion, and breathing, as well as higher-level cognitive processes such as language and reasoning.
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Understanding how the brain works, and locating the neural correlates of its various functions relies heavily on the investigation of functionally distinct neuronal cell types. For this reason, neuronal markers have established themselves as an incredibly valuable tool for all kinds of scientists, in particular, neuroscientists.
The development of immunohistochemistry has allowed for various antibodies to be used against specific cell components to give data on the neuronal phenotype of a cell, as well as information on their expression of certain proteins and their morphological characteristics.
The unique set of enzymes, transcription factors, cytoskeletal proteins, secreted factors, cell surface proteins and receptors that neurons express can be defined by using neuronal markers.
Neuronal markers are part of a set of nervous system markers that encompass neuronal markers, microglia markers, astrocyte markers, and oligodendrocyte markers.
Under the umbrella of neuronal markers, there are general, dendritic, axonal, presynaptic, active zone, postsynaptic, growth cone, cholinergic neuron, dopaminergic neuron, GABAergic neuron, glutamatergic neuron, glycinergic neuron, and serotonergic neuron markers.
Below, we discuss how immunohistochemistry has emerged as a reliable tool for investigating neuronal populations, which are the commonly used neuronal markers, and which have specific clinical value.
Exploring neurons with immunohistochemistry
Golgi staining was the first method used to identify the morphological features of individual neurons, and while it is still a useful technique, these days immunohistochemical methods are most common for identifying neuronal cell phenotypes, protein expression, and morphological features.
Although more modern molecular methods have been established that can identify characteristics of neurons, immunohistochemistry has maintained its popularity over alternative methods because of its many benefits in being relatively low-cost, and its reliance on simple equipment and reagents that are easy to access.
Immunohistochemistry techniques allow neurons in neural tissue or cultures to be visualized and identified by immunodetection of cell-specific antigenic markers, which are discussed below.
Many proteins are used as markers of neurons, each of which has different targets and functions in research. The cytosolic protein neuron-specific enolase, known as NSE, is expressed by mature neurons and cells of neuronal origin. The glycolytic enzyme that is specific to the brain is vital in intracellular energy metabolism.
The presence of Neuronal nuclei (NeuN) has been found to correspond to the withdrawal of neuronal cells from the cell cycle. It is detectable in embryonic and adult neurons, but not in Purkinje cells, retinal photoreceptor cells, olfactory bulb mitral cells, or dopaminergic neurons in the substantia nigra.
Microtubule-associated protein 2, known as MAP-2, is a neuron-specific cytoskeletal protein expressed in the nervous system of embryonic and adult tissues, and it is used as a marker of neuronal phenotype.
Tubulin beta III is suspected to be involved in the differentiation of neuronal cell types. Known also as TUBB3 and TuJ1, it acts as a building block of microtubules, making them fundamental to the cytoskeleton and roles such as cell structure maintenance, meiosis, mitosis, and intracellular transport.
Doublecortin, or DCX, is considered to be a marker of neurogenesis in the central nervous system. It is expressed by migrating neurons and can be observed at the beginning stages of neuronal development.
C-fos is used as a marker of neuronal activation, it is expressed in specific brain regions following vagal sensory neuron stimulation.
Clinically significant neuronal markers
Two markers of interest have become established markers of neuronal cell phenotype used in investigating the pathology of various diseases.
The primary role of choline acetyltransferase (ChAT) is in synthesizing acetylcholine. It is most commonly used as a biomarker of cognitive decline, which is an important factor to monitor in numerous neurodegenerative disorders.
It has played a significant role in exploring the neurological changes that occur in Alzheimer’s Disease (AD), which is characterized by a loss of cholinergic neurons in the basal forebrain.
Tyrosine hydroxylase (TH) has also proven to be invaluable to the investigation of neurodegenerative disorders. Specifically, it has been used to determine the dopaminergic cell loss within the substantia nigra of those with Parkinson’s disease (PD).
Studies have used the TH markers to investigate this loss and to understand whether interventions could prevent the loss of these cells that are fundamental to the development and progression of the disease. Studies continue to use TH to explore various potential therapeutic interventions.
Finally, some studies have identified potential neuronal markers of human gliomas (a type of brain tumor that begins in the brain’s glial cells). CD15 was initially thought to be a cancer stem-like marker in glioblastoma, however, more recent evidence has undermined earlier findings.
Other research has implicated Nestin and Olig2 as markers of neural stem cells, the expression of which helps to identify cancer stem cells because these stem cells are not expressed in fully differentiated mature cells.
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