Neurofilament severing and annealing
Actin filaments and microtubules lengthen and shorten by addition and loss of subunits at their ends, but several studies suggest that this is not the case for intermediate filaments and that these polymers may lengthen principally by joining ends, a process called end-to-end annealing.
We have demonstrated end-to-end annealing of neurofilaments and vimentin filaments in non-neuronal cell lines, and also end-to-end annealing of neurofilaments in nerve cells. One way that we have shown this in cultured nerve cells is by using a photo-convertible neurofilament fusion protein to create distinct populations of red and green fluorescent neurofilaments. Over time, the red and green filaments intermix due to their axonal transport and we observe the gradual appearance of chimeric filaments composed of alternating red and green fluorescent segments. After 24 hours, about two thirds of the neurofilaments are chimeric, with some filaments exhibiting as many as a dozen red-green junctions along their length. Thus end-to-end annealing appears to be a robust and dynamic mechanism in neurons.
Recently, we have also discovered that there is a severing mechanism for neurofilaments in axons. This is the first description of such a mechanism for intermediate filaments. Using time-lapse imaging of fluorescent neurofilaments in cultured nerve cells, we have captured severing and end-to-end annealing events live on camera. Our observations indicated that there is a dynamic cycle of severing and annealing of neurofilaments that regulates the length of these polymers. We are currently working to understand the mechanism and regulation of neurofilament severing and annealing and the significance of neurofilament length for neurofilament function.