Conditional NF-L transgene expression in mice for in vivo analysis of turnover and transport rate of neurofilaments

We generated mice with doxycycline control of a human neurofilament light (NF-L) transgene in the context of the absence (tTA; hNF-L; NF-L+/-) or presence (tTA; hNF-L; NF-L+/-) of endogenous mouse NF-L proteins. Doxycycline treatment caused the rapid disappearance of human NF-L (hNF-L) mRNA in tTA; hNF-L mice, but the hNF-L proteins remained with a half-life of 3 weeks in the brain. In the sciatic nerve, the disappearance of hNF-L proteins after doxycycline treatment occurred in synchrony along the sciatic nerve, suggesting a proteolysis of NF proteins along the entire axon. The presence of permanent NF network in tTA; hNF- L; NF-L+/- mice further stabilized and extended longevity of hNF-L proteins by several months. Surprisingly, after cessation of doxycycline treatment, there was no evidence of leading front of newly synthesized hNF-L proteins migrating into sciatic nerve axons devoid of NF structures. The hNF-L proteins detected at weekly intervals reappeared and accumulated in synchrony at similar rate along nerve segments, a phenomenon consistent with a fast hNF-L transport into axons. We estimated the hNF-L transport rate to be of similar to 10 mm/d in axons devoid of NF structures based on the use of an adenovirus encoding tet-responsive transcriptional activator to transactivate the hNF-L transgene in hypoglossal motor neurons. These results provide in vivo evidence that the stationary NF network in axons is a key determinant of half-life and transport rate of NF proteins.