A genetic stimulation with production of adenic-uracil rich RNA in neurons and glia in learning. The question of transfer of RNA from glia to neurons

Protein synthesis in nerve terminals and the glia-neuron unit

The progressive philogenetic lengthening of axonal processes and the increase in complexity of terminal axonal arborizations markedly augmented the demands of the neuronal cytoplasmic mass on somatic gene expression. It is proposed that in an adaptive response to this challenge, novel gene expression functions developed in the axon compartment, consisting of axonal and presynaptic translation systems that rely on the delivery of transcripts synthesized in adjacent glial cells. Such intercellular mode of gene expression would allow more rapid plastic changes to occur in spatially restricted neuronal domains, down to the size of individual synapses. The cell body contribution to local gene expression in well-differentiated neurons remains to be defined. The history of this concept and the experimental evidence supporting its validity are critically discussed in this article. The merit of this perspective lies with the recognition that plasticity events represent a major occurrence in the brain, and that they largely occur at synaptic sites, including presynaptic endings.

Local Gene Expression in Axons and Nerve Endings: The Glia-Neuron Unit

Neurons have complex and often extensively elongated processes. This unique cell morphology raises the problem of how remote neuronal territories are replenished with proteins. For a long time, axonal and presynaptic proteins were thought to be exclusively synthesized in the cell body, which delivered them to peripheral sites by axoplasmic transport. Despite this early belief, protein has been shown to be synthesized in axons and nerve terminals, substantially alleviating the trophic burden of the perikaryon. This observation raised the question of the cellular origin of the peripheral RNAs involved in protein synthesis. The synthesis of these RNAs was initially attributed to the neuron soma almost by default. However, experimental data and theoretical considerations support the alternative view that axonal and presynaptic RNAs are also transcribed in the flanking glial cells and transferred to the axon domain of mature neurons. Altogether, these data suggest that axons and nerve terminals are served by a distinct gene expression system largely independent of the neuron cell body. Such a local system would allow the neuron periphery to respond promptly to environmental stimuli. This view has the theoretical merit of extending to axons and nerve terminals the marginalized concept of a glial supply of RNA (and protein) to the neuron cell body. Most long-term plastic changes requiring de novo gene expression occur in these domains, notably in presynaptic endings, despite their intrinsic lack of transcriptional capacity. This review enlightens novel perspectives on the biology and pathobiology of the neuron by critically reviewing these issues.

Macromolecular translocation--a possible function of astrocytes

We have used dense astrocytic cultures, which display a multilamellar geometry, to determine whether translocation of macromolecules can occur across astrocytic processes. Colloidal gold-labelled transferrin and serum albumin were allowed to bind to the most superficial of the astrocytic cell membranes at 4 degrees C, the temperature was then increased to 37 degrees C and the fate of these gold-labelled macromolecules was observed using the electron microscope. The gold-labelled macromolecules appeared to undergo receptor-mediated endocytosis followed by translocation and exocytosis, with the colloidal gold-labelled macromolecules moving from the apical to the more basal processes. After 1 h of incubation at 37 degrees C, colloidal gold became accumulated within secondary lysosomes of the basal-most layer of astrocyte processes. Since the extracellular space of the central nervous system (CNS) consists of narrow tortuous channels formed to a great extent by the multitude of processes extending from fibrous and protoplasmic astrocytes, these observations suggest to us that one function of astrocytes may be to facilitate transport of macromolecules from one cell to another.

Axoplasmic RNA species synthesized in the isolated squid giant axon

Isolated squid stellate nerves and giant fiber lobes were incubated for 8 hr in Millipore filtered sea water containing [3H]uridine. The electrophoretic patterns of radioactive RNA purified from the axoplasm of the giant axon and from the giant fiber lobe (cell bodies of the giant axon) demonstrated the presence of RNA species with mobilities corresponding to tRNA and rRNA. The presence of labeled rRNAs was confirmed by the behavior of the large rRNA component (31S) which, in the squid, readily dissociates into its two constituent moyeties (17S and 20S). Comparable results were obtained with the axonal sheath and the stellate nerve. In all the electrophoretic patterns, additional species of radioactive RNA migrated between the 4S and the 20S markers, i.e. with mobilities corresponding to presumptive mRNAs. Chromatographic analysis of the purified RNAs on oligo(dT)cellulose indicated the presence of labeled poly(A)+ RNA in all tissue samples. Radioactive poly(A)+ RNA represented approximately 1% of the total labeled RNA in the axoplasm, axonal sheath and stellate nerve, but more than 2% in the giant fiber lobe. The labeled poly(A)+ RNAs of the giant fibre lobe showed a prevalence of larger species in comparison to the axonal sheath and stellate nerve. In conclusion, the axoplasmic RNAs synthesized by the isolated squid giant axon appear to include all the major classes of axoplasmic RNAs, that is rRNA, tRNA and mRNA.

Motion Variability—its Definition, Quantification, and Origin

The concept of motion variability is discussed and a normalized measure for its quantification is introduced. An example demonstrates that this new measure constitutes a global indicator of the current state of a motor learning process. The causes of motion variations are briefly discussed. They include initial perturbations of the skeletal, muscular, and neural systems as well as perturbations due to incremental changes, during motion execution, of external forces, muscular parameters (fatigue), afferent sensory inputs, and of the motor programs controlling the execution of the motion.

Topochemical aspects of nucleic-acid and protein metabolism within the neuron-neuroglia unit of the hypothalamic supraoptic nucleus

The RNA and both the total and basic protein content of individual cells were determined by cytospectrophotometry in neurons and perineuronal oligodendroglia of the hypothalamic supraoptic nucleus in rats subjected to various stresses, as well as in ground squirrels during natural hibernation. Barbiturate narcosis and deep cooling, which induced a decrease in body temperature in rats and hibernation in squirrels, caused a marked decrease of all macromolecular constituents in neurons. A similar decrease was found in the perineuronal oligodendroglia in rats, but an increase was observed in ground squirrels. After cessation of cooling, while the body temperature of the animals returned to normal, the neurons, but not the oligodendroglia, of rats showed a significant accumulation of RNA, while RNA accumulated in both neurons and perineuronal oligodendroglia in ground squirrels. Milder cooling of rats, which did not lower their body temperature, induced reciprocal changes in basic-protein content in neuronal and glial cell nuclei, with the accumulation of protein occurring initially in neurons, and subsequently in glia. When cold adaptation was accomplished, the basic protein content of neurons and glial cells returned to the control level. Four days after adrenalectomy in rats, the RNA content decreased in oligodendroglia but not in neurons of the supraoptic nucleus. This effect was completely abolished by daily injections of cortisol in the adrenalectomized animals. The data obtained indicate the existence of differences in metabolic responses to stress between neurons and glial cells of the supraoptic nucleus of the hypothalamus.

A quantitative study of the glia of the Purkinje cell layer of the cerebellum in mammals

The cerebella of fourteen mammals have been examined and the number of each DNA class of glial cell, within the Purkinje cell layer, counted. Diploid glial cells were present in all species and related in number to the surface area of the Purkinje cell. It is likely that they assist in the maintenance of the physiology of this latter cell type. Tetraploid glial cells, however, occur in significant numbers only in the human and chimpanzee and possibly play a part in the establishment of certain learned patterns of co-ordinated movement peculiar to these species.

[Plasticity of the nervous system]

The most recent knowledge about the phenomenon of the nervous system plasticity are revised, as much in morphological as in physiological and molecular levels. The neuron morphological and physiological changes opposite to the experience are studied. The nervous system molecular adaptation to the information it receives as the base of all type of plasticity is also considered.

Distribution of 3H-uridine-5 into brain RNA species of rats exposed to various training tasks - an electrophoretic analysis

Operant schedules were used to isolate component parts of a training task and specific activities were determine for nuclear and cytoplasmic RNA species separated by polyacrylamide disc gel electrophoresis. Rats exposed to a stimulus or schedule change incorporated more radioactivity into nuclear rRNA and mRNA and cytoplasmic rRNA and tRNA than littermates exposed to no change from baseline training. Rats developing a change in response probability to a stimulus in the environment incorporated more radioactivity into cytoplasmic mRNA.

Fine structural localization of acetylcholinesterase in the olfactory mucosa in guinea pig and rabbit

Using histochemical procedures for electron microscopy, acetylcholinesterase (AChE) activity was demonstrated in the olfactory mucosa of the guinea pig and rabbit. This activity was found in two specific locations, one intracellular and one extracellular. The intracellular localization was limited to the cisternae of whorls of smooth endoplasmic reticulum within the cytoplasm of supporting cells. The extracellular localization was limited to the intercellular space between the limiting membranes of the receptor dendrite and the supporting cell. A role for the ACh-AChE system in olfactory transduction is suggested, and comparison made with other receptor systems.