Visualization of single neurofilaments by immunofluorescence microscopy of splayed axonal cytoskeletons

Treatment of cultured neurons with non-ionic detergents under certain conditions causes the axonal microtubules to splay apart from each other, allowing individual microtubules to be visualized by immunofluorescence microscopy [Brown et al., 1993, J. Cell Sci. 104: 339-352]. I have investigated whether axonal neurofilaments separate from each other under similar conditions. Cultures of dissociated dorsal root ganglion (DRG) neurons from fetal rats were treated with non-ionic detergent and fixed with formaldehyde. Neurofilaments were visualized by immunofluorescence microscopy using a polyclonal antiserum specific for NF-L. Treatment of the neurons with Triton X-100 or saponin caused filamentous structures to splay apart from each other along the entire length of the axon. Quantitative analysis of fluorescence intensity along the filamentous structures indicated that many of them represent single neurofilaments and that single and bundled neurofilaments can be distinguished based on their fluorescence intensity. The extent of this splaying phenomenon was dependent on time and detergent concentration. Temporal analysis indicated that short portions of single neurofilaments initially loop out from the axonal bundle and then subsequently splay apart further along their length and adhere to the polylysine/laminin coated substrate. The maximum observed length for a single axonal neurofilament was 183 microm in neurons after only 1 day in culture, which indicates that neurofilaments can attain remarkable lengths in these young cultured neurons. The splayed axonal cytoskeleton preparation described here allows individual axonal neurofilaments to be visualized by immunofluorescence microscopy, which is not possible in conventional preparations due to the dense packing of these polymers in axons.