| NeuN, also known as RBFOX3 or Fox‑3, belongs to the Rbfox family of RNA‑binding proteins that regulate neuronal alternative splicing and is expressed almost exclusively in post‑mitotic neurons across the central and peripheral nervous systems. The protein localizes predominantly to nuclei and perinuclear cytoplasm, with enrichment in regions of low chromatin density, and appears as distinct isoforms that differ in phosphorylation state and subcellular distribution. The Rbfox family shares a conserved RNA recognition motif that binds the UGCAUG element within pre‑mRNAs, and NeuN/RBFOX3 follows this pattern, placing it in a regulatory network that couples sequence‑specific RNA binding with control of exon inclusion or skipping in neuronal transcripts. Through this splicing activity, NeuN participates in shaping neuron‑specific isoform profiles that support maturation, maintenance of neuronal identity, and functional specialization, including pathways that influence neuronal excitability, synaptic organization, and survival. Expression of NeuN emerges during neuronal differentiation and is maintained in most mature neurons, while being absent or very low in progenitors and certain defined neuronal subtypes, which creates a sharp molecular distinction that aligns with the transition to a stable neuronal phenotype. The antigen recognized by classic NeuN antibodies corresponds to multiple phosphorylated forms of RBFOX3, and the pattern of these isoforms varies between nuclear and cytoplasmic compartments, linking post‑translational modification of the protein to compartment‑specific splicing and RNA‑processing functions. NeuN expression serves as a reliable indicator of neuronal differentiation status and allows assessment of neuronal integrity under physiological and pathological conditions, including neurodegenerative, ischemic, and epileptic contexts where changes in NeuN immunoreactivity accompany alterations in neuronal viability and functional state. Variable or lost NeuN staining in some diseases and specific physiological states reflects context‑dependent regulation of RBFOX3 expression, splicing, or phosphorylation, and this modulation correlates with shifts in neuronal gene expression programs controlled at the level of alternative splicing. Across brain regions, NeuN‑positive neurons form characteristic patterns that align with neuronal layering and circuit architecture, while defined populations such as Purkinje cells and a subset of other specialized neurons remain NeuN‑negative, illustrating that RBFOX3‑dependent splicing represents one major but not universal strategy for implementing neuronal transcriptome specialization. |
