No abstract

A growing body of metabolic and molecular evidence of an endogenous protein-synthesizing machinery in the mature axon is a challenge to the prevailing dogma that the latter is dependent exclusively on slow axoplasmic transport to maintain protein mass in a steady state. However, evidence for a systematic occurrence of ribosomes in mature vertebrate axons has been lacking until recently, when restricted ribosomal domains, called "periaxoplasmic plaques," were described in goldfish CNS myelinated axons. Comparable restricted RNA/ribosomal "plaque" domains now have been identified in myelinated axons of lumbar spinal nerve roots in rabbit and rat on the basis of RNase sensitivity of YOYO-1-binding fluorescence, immunofluorescence of ribosome-specific antibodies, and ribosome phosphorus mapping by electron spectroscopic imaging (ESI). The findings were derived from examination of the axoplasm isolated from myelinated fibers as axoplasmic whole mounts and delipidated spinal nerve roots. Ribosomal periaxoplasmic plaque domains in rabbit axons were typically narrow ( approximately 2 microm), elongated ( approximately 10 microm) sites that frequently were marked by a protruding structure. The domain complexity included an apparent ribosome-binding matrix. The small size, random distribution, and variable intermittent axial spacing of plaques around the periphery of axoplasm near the axon-myelin border are likely reasons why their systematic occurrence has remained undetected in ensheathed axons. The periodic but regular incidence of ribosomal domains provides a structural basis for previous metabolic evidence of protein synthesis in myelinated axons.

No abstract

In neurons, localized RNAs have been identified in dendrites and axons; however, RNA transport in axons remains poorly understood. Here we analyzed axonal RNA transport in goldfish Mauthner neurons in vivo. BC1 RNA, a noncoding RNA polymerase III transcript that is targeted to dendrites in neurons of the rodent nervous system, was used as a probe for axonal RNA transport. Somata of Mauthner neurons were microinjected with various RNAs. Full-length BC1 RNA, but not control RNAs of similar length, was targeted to both axons and dendrites of Mauthner neurons. BC1 RNA was transported in the form of a rapidly advancing wave front that progressed along axons, in a microtubule-dependent manner, at a rate of 2 micrometer/sec. Whereas a BC1 5' segment of 65 nucleotides was transported to axons and dendrites in a way indistinguishable from full-length BC1 RNA, a BC1 3' segment of 60 nucleotides did not enter Mauthner cell processes to any significant extent. In the wake of the wave advancing through the axon, BC1 RNA was found localized to discrete, spatially delimited domains at the axonal surface. Such demarcated cortical concentrations of BC1 RNA could not be observed after disruption of F-actin organization in the axon. It is concluded that the specific delivery of BC1 RNA to spatially defined axonal target sites is a two-step process that requires the sequential participation of microtubules for long-range axial transport and of actin filaments for local radial transfer and focal accumulation in cortical domains.

The specialized jumping muscle of the locust, the metathoracic extensor tibiae (ETi), is innervated by four physiologically different motoneurons, including FETi, a phasic excitor, SETi, a tonic excitor, and CI, a tonic common inhibitor. FETi neuromuscular junctions were examined in three phasic ETi bundles innervated by FETi. FETi terminals were characterized by patchy contacts on to granular sarcoplasm. The ETi accessory extensor, innervated by both SETi and CI, contains two morphologically different types of axon ending. When this muscle was soaked in horseradish peroxidase, stimulation of SETi led to selective uptake in vesicles in terminals similar to those of FETi axons but containing smaller vesicles, while stimulation by CI caused increased uptake into terminals with more extensive contact directly on to fibrillar sarcoplasm. As has been observed in excitatory and inhibitory synapses in some crustacean and vertebrate nervous systems, the synaptic vesicles in the locust excitatory endings are round and electron-lucent while those in the inhibitory endings are more irregular in shape. The tonic neuromuscular junctions, SETi and CI, are more densely packed with vesicles, larger in cross-sectional area and appear to be of more complex shape than the smaller, vesicle-sparse, phasic FETi terminals. Following long duration stimulation at 10 Hz, the tonic neuromuscular junctions showed little morphological change. FETi endings, which fatigue within minutes at the same stimulation frequency, showed a 20% decrease in synaptic vesicle density and an increase in irregularly shaped membrane inclusions.