No abstract

The ultrastructural view of the axonal cytoskeleton as an extensively cross-linked network of neurofilaments (NFs) and other cytoskeletal polymers contrasts with the dynamic view suggested by axonal transport studies on cytoskeletal elements. Here we reconcile these perspectives by showing that neurons form a large NF network along axons which is unequivocally stationary, metabolically stable, and maintained by NFs and nonfilamentous subunit assemblies undergoing slow transport by intermittent rapid movements and pauses. In mouse primary cortical neurons transfected with EGFP-NFL, formation of this stationary NF network requires a critical level of NFs, which explains its absence in NF-poor developing neurons studied previously. Most NFs at proximal axon regions were in a stationary structure coexisting with a smaller pool of moving EGFP-NFL assemblies that were mainly nonfilamentous. Distally along the same axon, EGFP-labeled NFL was much less abundant, and we detected only short filaments moving bidirectionally by slow transport (rapid movements and pauses) as previously described. In living mice, Ͼ25% of radiolabeled newly synthesized NFs remained in optic axons after slowly transported NFs had exited. Retained NF remained fixed over several months in a nonuniform distribution and exhibited exceptionally slow turnover (t 1/2 Ͼ2.5 months), implying that, at steady state, Ͼ90% of NFs in mature optic axons comprise the stationary cytoskeleton and Ͻ10% are undergoing slow transport. These findings reconcile in vitro and in vivo axonal transport observations, showing that slowly transported NFs or subunit oligomers are precursors to a highly stable stationary cytoskeletal network that supports mature axons.

No abstract

Abstract— The injection of large quantities of radioactive amino acid precursor is proposed as a technique for determining rates of cerebral protein synthesis in vivo. In this way the specific radioactivity of the amino acid precursor in the brain is maintained at a relatively constant level for at least 2 h. Injections of 10–15 μ mol of valine per g body weight result in nearly constant rates of incorporation of radioactivity and do not appear to inhibit cerebral protein synthesis in adult or young (2–6 day old) rat brain. Similar rates were obtained in young rat brain with lysine and histidine. Rates of protein synthesis in cerebral hemisphere were for 2‐day‐olds 2·1 per cent replacement of protein bound amino acid per h and for adult 0·62 per cent per h. Advantages and disadvantages of the procedure are discussed.

No abstract