The molecular basis of the specificity and cross-reactivity of the NeuN epitope of the neuron-specific splicing regulator, Rbfox3

Ever since its description and the generation of its defining antibody, some 20 years ago, neural nuclei (NeuN) have been an invaluable tool for developmental neuroscientists and neuropathologists to identify neurons and follow their normal or malignant development. The recent identification of the splicing factor Rbfox3 as the molecule constituting the genuine NeuN epitope has opened up a novel perspective on NeuN immunostaining and its interpretation. Here, we briefly review these recent developments, and we provide a series of data that allow to rationalize the specificity of the NeuN/A60 antibody on aldehyde-fixed tissues on the one hand, and its cross-reactivity with Synapsin I and R3hdm2 on Western blots on the other. We argue that rather than being considered as a mere marker for mature neurons, Rbfox3-mediated NeuN/A60 immunoreactivity may provide a window onto neuronal biology. Specifically, we hypothesize that the phosphorylation-dependent antigenicity of the Rbfox3/NeuN epitope should allow to visualize neuronal physiology realized through Rbfox3, including splicing, on the single-cell level.