NeuN



NeuN is a neuronal nuclear antigen and was first described in 1994 by Mullen et al., who raised a series of monoclonal antibodies to mouse antigens with the original intent of finding mouse species specific immunological markers for use in transplantation experiments. In the event they isolated a hybridoma line, called mAb A60, which proved to bind an antigen expressed only in neuronal nuclei and to a lesser extent the cytoplasm of neuronal cells, and which appeared to work on all vertebrates. This unknown antigen was therefore known as NeuN for "Neuronal Nuclei". A few neuronal cell types were not recognized by the NeuN antibody, such as cerebellar Purkinje cells, olfactory Mitral cells and retinal photoreceptors. However the vast majority of neurons are strongly NeuN positive, and NeuN immunoreactivity has been widely used to identify neurons is tissue culture and in sections and to measure the neuron/glial ratio in brain regions. NeuN immunoreactivity becomes obvious as neurons mature, typically after they have downregulated expression of Doublecortin, a marker seen in the earliest stages of neuronal development. The NeuN protein was not at the time characterized, though the molecular weight was shown to be closely spaced bands running at 46kDa and 48kDa on SDS-PAGE. However the exact identity of the NeuN protein remained a mystery for many years. Despite this the mAb A66 antibody has become very widely used as a robust marker of neurons, and a May 2011 Medline search using the keyword "NeuN" produced over 1,100 hits. Recently Kim et al. used proteomic methods to show that NeuN corresponds to Fox-3. Fox-3 is one of a family of mammalian homologues of the Fox-1, originally discovered in the nematode worm Caenorhabditis elegans as a gene involved in sex determination. Fox is in fact an acronym of "Feminizing locus on X". The mammalian genome contains three genes homologous to C. elegans Fox-1, called Fox-1, Fox-2 and Fox-3. All these Fox proteins are about 46kDa in size, and each includes a central highly conserved so called RRM or RNA recognition motif. This motif corresponds to a small ~70 amino acid module consisting of 4 strands forming a beta sheet structure and two alpha helices. RRM domains are one of the most common domains in the human genome and are found in numerous proteins which bind RNA molecules. An alternate name for Fox-3 is hexaribonucleotide binding protein 3, and the Fox proteins are believed to have a role in the regulation of mRNA splicing. For a review of the Fox family of proteins see this reference.