3. Decrease of labeled DNA in cells of the adrenal medulla after intermittent exposure to cold

Brain Metabolic DNA: A Long Story and Some Conclusions

We have previously outlined the main properties of brain metabolic DNA (BMD) and its involvement in circadian oscillations, learning, and post-trial sleep. The presence of BMD in certain subcellular fractions and their behavior in cesium gradients have suggested that BMD originates from cytoplasmic reverse transcription and subsequently acquires a double-stranded configuration. More recently, it has been reported that some DNA sequences of cytoplasmic BMD in learning mice are different from that of the control animals. Furthermore, BMD is located in vicinity of the genes involved in different modifications of synaptic activity, suggesting that BMD may contribute to the brain's response to the changing environment. The present review outlines recent data with a special emphasis on reverse transcription of BMD that may recapitulate the molecular events at the time of the "RNA world" by activating mitochondrial telomerase and generating RNA templates from mitochondrial transcripts. The latter unexpected role of mitochondria is likely to promote a better understanding of mitochondrial contribution to cellular interactions and eukaryotic evolution. An initial step regards the role of human mitochondria in embryonic BMD synthesis, which is exclusively of maternal origin. In addition, mitochondrial transcripts involved in reverse transcription of BMD might possibly reveal unexpected features elucidating mitochondrial involvement in cancer events and neurodegenerative disorders.

Brain metabolic DNA: recent evidence for a mitochondrial connection

This review highlights recent data concerning the synthesis of brain metabolic DNA (BMD) by cytoplasmic reverse transcription and the prompt acquisition of the double-stranded configuration that allows its partial transfer to nuclei. BMD prevails in the mitochondrial fraction and is present in presynaptic regions and astroglial processes where it undergoes a turnover lasting a few weeks. Additional data demonstrate that BMD sequences are modified by learning, thus indicating that the modified synaptic activity allowing proper brain responses is encoded in learning BMD. In addition, several converging observations regarding the origin of BMD strongly suggest that BMD is reverse transcribed by mitochondrial telomerase.

DNA in Squid Synaptosomes

The synthesis of brain metabolic DNA (BMD) is modulated by learning and circadian oscillations and is not involved in cell division or DNA repair. Data from rats have highlighted its prevalent association with the mitochondrial fraction and its lack of identity with mtDNA. These features suggested that BMD could be localized in synaptosomes that are the major contaminants of brain mitochondrial fractions. The hypothesis has been examined by immunochemical analyses of the large synaptosomes of squid optic lobes that are readily prepared and identified. Optic lobe slices were incubated with 5-bromo-2-deoxyuridine (BrdU) and the isolated synaptosomal fraction was exposed to the green fluorescent anti-BrdU antibody. This procedure revealed that newly synthesized BrdU-labeled BMD is present in a significant percent of the large synaptosomes derived from the nerve terminals of retinal photoreceptor neurons and in synaptosomal bodies of smaller size. Synaptosomal BMD synthesis was strongly inhibited by actinomycin D. In addition, treatment of the synaptosomal fraction with Hoechst 33258, a blue fluorescent dye specific for dsDNA, indicated that native DNA was present in all synaptosomes. The possible role of synaptic BMD is briefly discussed.

Brain Metabolic DNA in Rat Cytoplasm

Brain metabolic DNA (BMD) is not involved in cell division or DNA repair but is modulated by memory acquisition, sleep processing, and circadian oscillations. Using routine methods of subcellular fractionation, newly synthesized BMD from male rats is shown to be localized in crude nuclear, mitochondrial, and microsomal fractions and in two fractions of purified nuclei. Sub-fractionation of the mitochondrial fraction indicates the prevalent localization of BMD in free mitochondria and to a lesser degree in synaptosomes and myelin. Cesium density profiles of homogenate, subcellular fractions, and purified nuclei obtained after incorporation periods from 30 min to 4 h indicate that BMD synthesis takes place by reverse transcription in cytoplasmic organelles. Following the acquisition of the double-stranded structure, BMD is transferred to nuclei. Kinetic analyses lasting several weeks highlight the massive BMD turnover in subcellular fractions and purified nuclei and its dependence on age. Data are in agreement with the role of BMD as a temporary information store of cell responses of potential use in comparable forthcoming experiences.

Brain metabolic DNA in memory processing and genome turnover

Sophisticated methods are currently used to investigate the properties of brain DNA and clarify its role under physiological conditions and in neurological and psychiatric disorders. Attention is now called on a DNA fraction present in the adult rat brain that is characterized by an elevated turnover and is not involved in cell division or DNA repair. The fraction, known as brain metabolic DNA (BMD), is modulated by strain, stress, circadian oscillations, exposure to enriched or impoverished environment, and notably by several training protocols and post-trial sleep. BMD is frequently localized in glial cells but is also present in neurons, often in the perinucleolar region. Its distribution in repetitive and non-repetitive DNA fractions shows that BMD differs from native DNA and that in learning rats its profile differs from that of control rats. More detailed knowledge of the molecular, cellular, and time-dependent BMD features will be necessary to define its role in memory acquisition and processing and in the pathogenesis of neurologic disorders.

Sleep memory processing: the sequential hypothesis

According to the sequential hypothesis (SH) memories acquired during wakefulness are processed during sleep in two serial steps respectively occurring during slow wave sleep (SWS) and rapid eye movement (REM) sleep. During SWS memories to be retained are distinguished from irrelevant or competing traces that undergo downgrading or elimination. Processed memories are stored again during REM sleep which integrates them with preexisting memories. The hypothesis received support from a wealth of EEG, behavioral, and biochemical analyses of trained rats. Further evidence was provided by independent studies of human subjects. SH basic premises, data, and interpretations have been compared with corresponding viewpoints of the synaptic homeostatic hypothesis (SHY). Their similarities and differences are presented and discussed within the framework of sleep processing operations. SHY's emphasis on synaptic renormalization during SWS is acknowledged to underline a key sleep effect, but this cannot marginalize sleep's main role in selecting memories to be retained from downgrading traces, and in their integration with preexisting memories. In addition, SHY's synaptic renormalization raises an unsolved dilemma that clashes with the accepted memory storage mechanism exclusively based on modifications of synaptic strength. This difficulty may be bypassed by the assumption that SWS-processed memories are stored again by REM sleep in brain subnuclear quantum particles. Storing of memories in quantum particles may also occur in other vigilance states. Hints are provided on ways to subject the quantum hypothesis to experimental tests.

Apparent anomalies in nuclear feulgen-DNA contents. Role of systematic microdensitometric errors

The Feulgen-DNA contents of human leukocytes, sperm, and oral squames were investigated by scanning and integrating microdensitometry, both with and without correction for residual distribution error and glare. Maximally stained sperm had absorbances which at lambdamax exceeded the measuring range of the Vickers M86 microdensitometer; this potential source of error could be avoided either by using shorter hydrolysis times or by measuring at an off-peak wavelength. Small but statistically significant apparent differences between leukocyte types were found in uncorrected but not fully corrected measurements, and some apparent differences disappeared when only one of the residual instrumental errors was eliminated. In uncorrected measurements, the apparent Feulgen-DNA content of maximally stained polymorphs measured at lambdamax was significantly lower than that of squames, while in all experimental series uncorrected measurements showed apparent diploid:haploid ratios significantly greater than two. In fully corrected measurements no significant differences were found between leukocytes and squames, and in four independent estimations the lowest diploid:haploid ratio found was 1.99 +/- 0.05, and the highest 2.03 +/- 0.05. Discrepancies found in uncorrected measurements could be correlated with morphology of the nuclei concerned. Glare particularly affected measurements of relatively compact nuclei such as those of sperm, polymorphs and lymphocytes, while residual distribution error was especially marked with nuclei having a high perimeter:area ratio (e.g. sperm and polymorphs). Uncorrected instrumental errors, especially residual distribution error and glare, probably account for at least some of the previously reported apparent differences between the Feulgen-DNA contents of different cell types. On the basis of our experimental evidence, and a consideration of the published work of others, it appears that within the rather narrow limits of random experimental error there seems little or no reason to postulate either genuine differences in the amounts of DNA present in the cells studied, or nonstoichiometry of a correctly performed Feulgen reaction.

Autoradiographic studies of DNA synthesis in lymphoid tissues

Using long exposures of stripping film autoradiographs before processing, mixtures of weakly and strongly labelled nuclei were seen in different areas of the mouse spleen. Previous results (Harris et al., 1973) led to the conclusion that many cells, not in division cycle, were labelling with (3H) thymidine and that this process was important for the development of specific antibody-producing cells following stimulation with an antigen such as sheep red cells (SRC). The present data are an analysis of the (3H) thymidine labelling kinetics in the spleens of mice reared in conventional or germ-free conditions. The labelling seen in the 24 h following an injection of (3H) thymidine could best be interpreted on the basis of synthesis of unstable DNA. The changes in the pattern, and distribution of labelled nuclei as well as the intensity of their labelling was not compatible with cell division only, but was also the result of movement of labelled material between the lymphoid cells of the organ. Germ-free mice were followed for 24 days following a single injection of (3H) thymidine. The rate of uptake of label into the spleen was much slower than has been found previously in mice reared in conventional conditions. When SRC were injected 2 h after giving (3H) thymidine the labelling of lymphoid cells in the spleen and blood was quite different to controls given (3H) thymidine alone. Detailed analysis indicated that turnover of labelled material, presumably DNA, as well as cells was involved. This turnover of DNA could be considered to be metabolic in the sense that renewal, increase in amount, loss, and transfer to other cells were involved. These, and other studies, in vivo (Harris & Olsen, 1973) and in vitro (Harris et al. 1975) indicate that such processes, involving DNA, are highly relevant to the development of antibody-producing capacity by cells responding to antigenic challenge.

Metabolic DNA in the hepatocyte nuclei in newborn rats

The behaviour in time of labelled nuclear DNA in the hepatocytes of newborn rats was studied using autoradiographic and biochemical techniques in two groups of experiments. In the first group H-3-thymidine was injected to the mothers at the 16th day of pregnancy and the amount of labelled DNA was evaluated in the newborns after delivery. In the second group H-3-thymidine was injected to the newborns two hours after birth and the labelled DNA was studied at the same time intervals as the first group. The amount of labelled thymidine incorporated into the first group of animals remains constant for the first three days of life, thereafter a reduction in specific activity of DNA is observed concomitant with an increase of the percentage of labelled nuclei and a decrease of the number of grains per nucleus. These results show that mitotic divisions, which are absent during the first three days of life, take place between the third and sixth days of life. The decrease of the specific activity is therefore due to dilution and not to loss of labelled DNA. In the second group of experiments the DNA labelled with H-3-thymidine shows a decrease by about 30--40% per day during the first three days of life accompanied by a decrease in the number of grains per nucleus without changes in the percentage of labelled nuclei. These data show that DNA synthesized during the first day after birth is metabolically unstable, unlike that synthesized during foetal life.

Uptake and release of DNA by lymphoid tissue and cells

Newly synthesized DNA has been shown to be released by immunized rabbit spleen tissue cultured in vitro. This DNA was mainly doublestranded, showed a large spread in buoyant density, and was in the molecular weight range 50,000–500,000. Rabbit spleen tissue and human peripheral blood lymphocytes in active DNA synthesis, also took up bacterial DNA into their nuclei. After short periods of culture this DNA had a buoyant density of the bacterial DNA employed. Upon prolonged incubation, the DNA was of mammalian density. Evidence was obtained, in the case of the blood lymphocytes, for the appearance in the cells of a DNA of intermediate buoyant density. The possible relevance of the loss and reutilization of DNA to lymphoid cell function is discussed.

Incorporation of [3H] thymidine into the spleens of intact mice during the immune response to sheep erythrocytes (SRC)

The incorporation of [³H]thymidine, administered shortly before killing, into the spleens of intact mice during the primary immune response to SRC has been studied, using autoradiography with exposure periods of 184 days before development.

A mixture of weakly and heavily labelled nuclei were situated in well-defined areas of the follicles. A marked increase of heavily labelled nuclei coincided with an increased uptake of [³H]thymidine into the spleen DNA during the first 3 days of the immune response. At this time the number of lightly labelled nuclei in the follicles was reduced. The heavily labelled nuclei were first apparent in the periarteriolar zone, then in germinal centres spreading out into the red pulp. After the peak of [³H]thymidine incorporation was over (day 3–4) weakly-labelled nuclei accumulated in the red pulp and persisted until day 9 after the injection of SRC.

It was concluded that many non-dividing cells in mouse spleen were incorporating small amounts of [³H]thymidine into their nuclear DNA. In view of the accumulation of such cells in the red pulp during the course of the immune response to SRC, it was considered that this evidence of DNA synthesis was a manifestation of metabolic turnover of this molecule and relevant to the immune process. From the data presented it was also concluded that only a small proportion of the total spleen population engaged in DNA synthesis and proliferation were actually induced to produce specific antibodies. This preliminary investigation showed the complex nature of the immune process leading to antibody synthesis, which requires much further detailed study.