NeuN expression in health and disease: A histological perspective on neuronal heterogeneity.

Neuronal nuclei (NeuN), also known as Rbfox3, is a widely used neuronal marker for identifying postmitotic neurons in both basic neuroscience and diagnostic neuropathology. Since its discovery, NeuN immunoreactivity has enabled accurate neuron counting, injury assessment, and anatomical mapping. However, accumulating evidence demonstrates that NeuN is not universally expressed across all mature neurons. Specific neuron types, such as olfactory mitral cells, cerebellar Purkinje cells, and retinal photoreceptors, consistently lack NeuN immunostaining despite functional maturity. Moreover, NeuN expression is dynamically regulated under pathological conditions. In ischemia-reperfusion (I/R) injury, neurodegenerative diseases (e.g., Alzheimer's and Parkinson's), and traumatic brain injury or epilepsy-selected for their representative diversity in pathophysiological stress (ischemic, degenerative, and mechanical)-NeuN downregulation may reflect functional compromise, stress responses, or reversible transcriptional changes rather than irreversible cell loss. These findings highlight the limitations of interpreting NeuN negativity as neuronal death. This review synthesizes recent findings on NeuN expression patterns, molecular mechanisms regulating its presence or absence, and the implications for research and diagnosis. We propose that NeuN should not be regarded as a binary marker but rather as a dynamic indicator of neuronal state. Multi-marker strategies and molecular tools such as spatial transcriptomics and RNA sequencing are suggested to improve the resolution of neuronal analysis. As histological and transcriptomic approaches converge, NeuN's role will likely expand from a structural identifier to a contextual readout of neuronal integrity and function.