Axonal transport declines with age in two distinct phases separated by a period of relative stability.
Bottom Line: Axonal transport also declines during normal aging, but little is known about the timing of these changes, or about the effect of aging on specific cargoes in individual axons.We also find that after tibial nerve regeneration, even in old animals, neurons are able to support higher transport rates of each cargo for a prolonged period.Thus, the age-related decline in axonal transport is not an inevitable consequence of either aging neurons or an aging systemic milieu.
Affiliation: Signalling ISP, The Babraham Institute, Babraham Research Campus, Cambridge, UK.Show MeSH
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Mentions: Most of the existing studies that have reported fluorescence live imaging of axonal transport have, presumably at least in part because of technical difficulties, focused on the peripheral nervous system. However, many age-associated neurodegenerative conditions affect the CNS (Adalbert and Coleman, 2013; Millecamps and Julien, 2013), highlighting the need to understand how aging affects the function of CNS neurons, including their axonal transport. Thus, we aimed to use NMNAT2-Venus mice to investigate age-associated changes in CNS axonal transport. The first tissue that we studied in this way was the optic nerve. In addition to technical advantages (easy accessibility, rapid dissection), degeneration of retinal ganglion cells and their axons, which constitute the optic nerve, contributes critically to pathology in glaucoma (Beirowski et al., 2008; Chidlow et al., 2011; Howell et al., 2007). Bidirectional fast axonal transport of NMNAT2-Venus particles was readily and reproducibly detectable in optic nerve explants. Individual axons were identified in time-lapse recordings and straightened, and quantification of axonal transport was performed in the same way as for sciatic nerve axons, above (Fig 3A). However, the dissection and imaging procedures used here mean that anterograde and retrograde transport were not analyzed separately and, instead, only overall transport rates were measured. Interestingly, we observed an overall similar profile of transport changes from 1.5 to 24 months of age as for sciatic nerve axons. However, the reduction in the number of moving particles at a young age occurred earlier, from 1.5 to 3 months, with a stable plateau from 3 to 18 months and a further significant drop at 24 months of age (Fig 3B). Average and maximal transport velocities were more variable overall than for sciatic nerve, but no consistent trends or significant changes were observed (Fig 3C). Although the average fluorescence intensity of labeled axons in the optic nerve varied somewhat with age (Table 3), there is no decline relative to young mice and no consistent relationship between increases or decreases in label intensity and the number of moving particles detected. Thus, simple changes in expression level are unlikely to account for the observed differences. Instead, these results indicate that, as for sciatic nerve axons above, 2 phases of reductions in axonal transport rates in young and old animals are separated by a stable plateau in adults.
Affiliation: Signalling ISP, The Babraham Institute, Babraham Research Campus, Cambridge, UK.