Originally identified by morphology, cells of the central (CNS) and peripheral nervous system (PNS) now are identified by proteins specific to each cell type under normal or diseased conditions. Delineating these markers is complicated by spatial and temporal variations in expression, which are further regulated by genetic and epigenetic cascades. Despite these intricacies, numerous biomarkers have been discovered for CNS and PNS cells during embryogenesis, adult neurogenesis, and the pathogenesis of neurodegenerative disorders.
All neurons of the mammalian nervous system originate from neural stem cells, which are characterized by self-renewal and multipotency. Before neurogenesis, these stem cells form a single layer of neuroepithelium constituting the neural plate and neural tube. Driven primarily by Notch1 signaling, the neuroepithelial cells give rise to related cell types known as radial glial and Schwann cells. Radial glial cells (RGCs) maintain neuroepithelial properties, including expression of the intermediate filament protein Nestin and SRY sex-determining region-box 2 (Sox2). They also show several astroglial properties, including expression of astrocyte-specific glutamate transporter (GLAST), the Ca2+-binding protein S100β, and brain lipid-binding protein (BLBP). Located in the PNS, immature Schwann cells exhibit upregulation of neural cell adhesion molecule (NCAM) and glial fibrillary acidic protein (GFAP). Upon further maturation, Schwann cells demonstrate increased expression of myelin-associated genes (e.g. myelin basic protein, MBP) to support their role as sheath-producing glial cells for PNS motor and sensory neurons.
Arising from radial glial cells are intermediate progenitors, which act as a source of most mature neurons in the CNS. Intermediate progenitors are characterized by their expression of paired box 6 (Pax6), a multifunctional transcription factor regulating NSC proliferation and differentiation. During later stages of neurogenesis, these cells upregulate doublecortin (DCX) and neurogenic differentiation factor 1 (NeuroD1), the latter serving as a neuronal fate determinant. Upon reaching maturity, mammalian neuronal cell bodies are defined by expression of neuron-specific enolase (NSE), microtubule-associated protein-2 (MAP2), and neuron-specific nuclear protein (NeuN). It should be noted that certain neuronal populations do not express NeuN, including Golgi cells, Purkinje cells, olfactory bulb mitral cells, retinal photoreceptor cells, inferior olivary and dentate nucleus neurons, and sympathetic ganglion cells. Mature neurons can further be subdivided based on direction (afferent, efferent, interneurons), action on other neurons and targets (e.g. motor neurons), discharge patterns, and neurotransmitter production. Interneurons express several calcium-binding proteins, calbindin and calretinin, while motor neurons express homeobox gene HB9. Neurotransmitter-secreting neurons can be distinguished by the enzymes/transporters required for chemical synthesis and secretion: vesicular glutamate transporter 1/2 (VGLUT1/2) for glutamatergic, glutamate decarboxylase 1/2 (GAD1/2) for GABAergic, aldehyde dehydrogenase 1 family member A1 (ALDH1A1) and tyrosine hydroxylase (TH) for dopaminergic, tryptophan hydroxylase for serotonergic, and choline acetyltransferase (ChAT) for cholinergic neurons.
In addition to neurons, the adult CNS contains other cells that contribute to the proper functioning of the brain in both normal and diseased states. These cells include microglia, oligodendrocytes, and astrocytes. Microglia originate from the yolk sac and are the resident macrophages of the CNS. Their mesodermal origin shares similarities with other myeloid cells, notably expression of integrin alpha M (ITGAM/CD11b) and the surface glycoprotein F4/80. Oligodendrocytes are a specialized glial cell type and are commonly identified by myelin proteins, including myelin oligodendrocyte glycoprotein (MOG), myelin-associated glycoprotein (MAG), and MBP. Astrocytes, a group of non-myelin-forming neuroglia, have comparable temporal and spatial patterns with oligodendrocytes. The most specific markers for mature astrocytes are GFAP and expression of S100β.
Specific biomarkers can also be used to indicate neurodegenerative diseases such as Alzheimer’s Disease (AD), Parkinson’s Disease (PD), and Amyotrophic Lateral Sclerosis (ALS). Extensive research has implicated amyloid-β (Aβ) plaques and hyper-phosphorylated tau-rich tangles with the pathogenesis of AD. Lewy bodies are a pathological hallmark of PD, characterized by the presence of α-synuclein protein inclusions, resulting in loss of dopaminergic neurons. PTEN-induced putative kinase 1 (PINK1) and leucine-rich repeat kinase 2 (LRRK2) are kinases mutated in familial PD. ALS, impacting upper and lower motor neurons, is linked to several specific gene mutations in superoxide dismutase 1 (SOD1), TAR DNA-binding protein-43 (TDP-43), and the RNA-binding protein FUS. Under pathological conditions, microglia can be stimulated to reveal molecular signatures that define distinct microglial identities, recently identified as disease-associated microglia (DAMs). DAMs undergo a two-stage maturation process. In the first step, homeostatic microglia mature into Stage 1 DAM microglia by upregulation of triggering receptor expressed on myeloid cells 2 (TREM2), ApoE, and Dap12, with a concomitant downregulation of homeostatic signatures including P2ry12 and transmembrane protein 119 (TMEM119). Maturation to Stage 2 DAMs occurs in a TREM2-dependent manner by upregulating osteopontin (Spp1) and cystatin 7 (CST7). While DAMs have been characterized primarily in AD, the contribution of DAMs in other neurodegenerative diseases is currently an intense area of research.
