Characterization of proteins transported at different rates by axoplasmic flow in the dorsal root afferents of rats
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Proteins synthesized by soma located in L4 dorsal root ganglia and supplied to the axonal branches extending centrally in the dorsal root and peripherally towards the sciatic nerve were analyzed for radioactivity following injections of [3H] leucine into the L4 dorsal root ganglia. All proteins located in the dorsal root and sciatic nerve were analyzed by SDS acrylamide gel electrophoresis at various times post injection. The differences in radioactivity between the dorsal root and sciatic nerve proteins were mainly quantitative and not qualitative, with many proteins of various molecular weight ranges being transported into both segments. Generally, it appears that in both axonal branches the high molecular weight proteins are transported at the highest rate, medium weights slower and low molecular weight proteins slowest. More proteins of high and low molecular weights are transported into the dorsal root whereas more of those of medium molecular weight are transported towards the sciatic nerve.Keywords:
Axoplasmic transport
Axonal transport of labelled protein was studied in rat sciatic nerve by analyzing nerve segments at intervals after injection of L-[ 3 H]leucine into the lumbar spinal cord. Some nerves were sectioned before injection so that material in transit accumulated proximal to the section. The segments distal to the section served as controls for incorporation into the nerve of blood-borne label. An analysis of TCA-soluble and TCA-insoluble activity in cut and intact nerve segments was also made. No evidence was found for the existence of a 'superfast' component of axonal transport (velocity 2000 mm/day). Results showed that the most rapidly transported protein derived from the neuron soma had a conventional 'fast' velocity of 350–420 mm/day. There was no transport of TCA-soluble material. It is suggested that 'superfast' transport, detected in mice by other investigators, is an artefact resulting from failure to control for incorporation of circulating label into the sciatic nerve.
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Abstract Axoplasmic transport of free 3 H‐leucine has been studied in vivo in the pike olfactory nerve following application of labeled leucine to the olfactory mucosa. A considerable amount of free 3 H‐leucine is transported at constant velocity along the axon in the form of a distinct peak. The maximum transport velocity for free 3 H‐leucine is the same as for rapidly transported 3 H‐protein (130 and 135 mm/day, respectively, at 19°C). Microtubule inhibitors block or significantly reduce the amount of free 3 H‐leucine transported, but do not influence the transport velocity. Disruption of the oxygen supply abolishes free 3 H‐leucine transport, so that this phenomenon cannot be explained by diffusion. The amount of free leucine in the rapidly moving peak decreases with time and distance along the axon and is not detectable after 5 h or more. The transported 3 H‐leucine is not derived from the circulation or from proteolysis of rapidly transported proteins. This study may help to resolve the controversy over the axoplasmic transport of free amino acids since it shows that free 3 H‐leucine is transported rapidly but does not travel by rapid axoplasmic transport to the end of axons longer than about 30 mm.
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Maple syrup urine disease (MSUD) is an inborn metabolic disease. The nutritional treatment with restricted intake of branched chain amino acids and prevention of leucine toxicity are crucially important for a favorable outcome. The aim of this study is to analyze the relation of blood leucine levels at diagnosis with future leucine tolerances, to determine whether any prediction about the future leucine tolerances or plasma leucine levels is possible by evaluating blood leucine levels at diagnosis.The study group consisted of 45 MSUD patients. Leucine levels at diagnosis were compared with age at diagnosis, leucine tolerances, maximum leucine levels/ages, and average blood leucine levels.The mean plasma leucine level at diagnosis was 2,355.47 ± 1,251.7 μmol/L (ref: 55-164 μmol/L). The median age at diagnosis was 17 days. Leucine tolerances per kg body weight declined until the age of 8 years and stabilized subsequently. The average age of maximum leucine level during follow-up was 3.14 ± 1.92 years, and the mean maximum lifetime plasma leucine level on follow-up was 1,452.13 ± 621.38 μmol/L. The leucine levels at diagnosis did not have any significant relationship with lifetime leucine tolerances, maximum plasma leucine levels or mean plasma leucine levels.The plasma leucine levels at diagnosis did not have a predictive value for later leucine tolerances or plasma leucine levels. The maximum lifetime leucine level is likely to happen within the first 3 years of life, underlining the importance of good metabolic control and compliance to dietary treatment at early ages.
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