Nuclear size and total RNA synthesis were compared in single lumbar motoneurons isolated from the grass frog . Transcription was found to correlate significantly, but not exclusively, with nuclear area or volume over a wide range of nuclear size, the largest nuclei having the highest mean transcriptional activity . Flow cytometric analysis of propidium iodide-stained nuclei excluded polyploidy or polyteny as an explanation for the increased transcription, but left open the possibility of a small increase in DNA with increasing nuclear size . Alternatively, motoneurons may increase transcription and nuclear size without increasing their DNA content, possibly by increasing the proportion of dispersed chromatin (euchromatin) . These two mechanisms for size-related changes in RNA synthesis in motoneurons present an interesting contrast to mechanisms used by many other large animal cells .Abbreviations used: SDS, sodium dodecyl sulfate; sucrose CG, sucrose in 1.7 mM sodium citrate buffer (pH 5.0) containing 15 mM glucose.

Changes in the amounts of tubulin, actin, and neurofilament polypeptides were found in regenerating motoneurons of grass frogs during the period of axonal elongation. Ventrat roots 9 and 10 were transected unilaterally about 7 mm from the spinal cord. 35 d later, [3H]colchicine binding had decreased in the proximal stumps to approximately one-half of contralateral control values, well before the regenerating motor axons had reinnervated skeletal muscles of the hind limb.[3H]colchicine binding did not change significantly in the operated halves of the 9th and 10th spinal cord segments over a 75-d period. The relative amounts of actin, tubulin, and neurofitament polypeptides in the operated ventral roots were measured by quantitative densitometry of stained two-dimensional electrophoretic gels. AIpha-tubulin, beta-tubulin, and the 68,000 molecular weight subunit of neurofilaments (NF68) decreased within the transected ventral roots to 78%, 57%, and <15% of control values, respectively. The amount of actin increased to 132% of control values within the operated ventral roots, although this change was not statistically significant. Opposite changes were found within motoneuronal cell bodies isolated from the spinal cord. The relative amounts of alpha-tubulin, beta-tubulin and NF68 within axotomized perikarya increased, respectively, to 191%, 146%, and 144% of that in control perikarya isolated from the contralateral side of the spinal cord. Thus, the changes in NF68 and tubulin did not occur uniformly throughout the injured cells. The possible structural and functional consequences of these changes are discussed.Cytoskeletal proteins, including actin, tubulin, and neurofilameat polypeptides, are particularly abundant within axons and play important roles in axonal structure and function (35,36,67). Axons receive a continuous supply of cytoskeletal proteins from their nerve cell bodies by slow axonal transport (20). When an axon in the peripheral nervous system is transected (axotomy), the isolated distal segment degenerates and is regenerated from the proximal segment (23). Whether or not the normal continuous supply of cytoskeletal proteins from the perikaryon is sufficient for axonal regeneration is unclear at present (21, 30). A prevailing hypothesis states that the synthetic priorities of axotomized neurons become reordered to favor the increased production and delivery of structural proteins to the regenerating axons (19, 49).Altered axonal amounts of cytoskeletal proteins can arise from changes in their synthesis, degradation, and/or transport. Previous studies of cytoskeletal proteins within regenerating neurons have focused on changes in their synthesis and transport. Several laboratories have reported increased rates of tubulin and actin synthesis in the region of axotomized retinal ganglion cell bodies (6,17,26), although such increases were not statistically significant in autonomic and sensory ganglia (24,46). Increased amounts of radioactivity associated with newly synthesized tubulin and actin...

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In this study we asked whether growth hormone (GH) and one of its key mediators, insulin-like growth factor I (IGF-I), influence spinal motoneuron size in conjunction with whole body size. We present evidence that GH has such a role, possibly without the mediation of IGF-I. Both lumbar motoneuron and body size were found to be increased relative to littermate controls in transgenic mice overexpressing GH, while body size, but not motoneuron size, was increased in mice overexpressing IGF-I. GH overexpression coordinately increased nucleolar, nuclear, and cell body size in lumbar spinal motoneurons, so that their normal size relationships were preserved in the transgenic mice. In addition, spinal cord and brain weights were significantly increased in both types of transgenic animal. We conclude that GH can regulate motoneuron, central nervous system, and body size in the same animal, and that IGF-I can mimic the effects of GH on at least two of these three parameters.

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