Extra DNA in forebrain cortical neurons

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.

Changes in neuronal DNA content variation in the human brain during aging

The human brain has been proposed to represent a genetic mosaic, containing a small but constant number of neurons with an amount of DNA exceeding the diploid level that appear to be generated through various chromosome segregation defects initially. While a portion of these cells apparently die during development, neurons with abnormal chromosomal copy number have been identified in the mature brain. This genomic alteration might to lead to chromosomal instability affecting neuronal viability and could thus contribute to age-related mental disorders. Changes in the frequency of neurons with such structural genomic variation in the adult and aging brain, however, are unknown. Here, we quantified the frequency of neurons with a more than diploid DNA content in the cerebral cortex of normal human brain and analyzed its changes between the fourth and ninth decades of life. We applied a protocol of slide-based cytometry optimized for DNA quantification of single identified neurons, which allowed to analyze the DNA content of about 500 000 neurons for each brain. On average, 11.5% of cortical neurons showed DNA content above the diploid level. The frequency of neurons with this genomic alteration was highest at younger age and declined with age. Our results indicate that the genomic variation associated with DNA content exceeding the diploid level might compromise viability of these neurons in the aging brain and might thus contribute to susceptibilities for age-related CNS disorders. Alternatively, a potential selection bias of "healthy aging brains" needs to be considered, assuming that DNA content variation above a certain threshold associates with Alzheimer's disease.

Cell cycle activation and aneuploid neurons in Alzheimer's disease

Alzheimer's disease (AD) is a chronic neurodegenerative disorder, characterized by synaptic degeneration associated with fibrillar aggregates of the amyloid-ß peptide and the microtubule-associated protein tau. The progression of neurofibrillary degeneration throughout the brain during AD follows a predictive pattern which provides the basis for the neuropathological staging of the disease. This pattern of selective neuronal vulnerability against neurofibrillary degeneration matches the regional degree of neuronal plasticity and inversely recapitulates ontogenetic and phylogenetic brain development which links neurodegenerative cell death to neuroplasticity and brain development. Here, we summarize recent evidence for a loss of neuronal differentiation control as a critical pathogenetic event in AD, associated with a reactivation of the cell cycle and a partial or full replication of DNA giving rise to neurons with a content of DNA above the diploid level. Neurons with an aneuploid set of chromosomes are also present at a low frequency in the normal brain where they appear to be well tolerated. In AD, however, where the number of aneuploid neurons is highly increased, a rather selective cell death of neurons with this chromosomal aberrancy occurs. This finding add aneuploidy to the list of critical molecular events that are shared between neurodegeneration and oncogenesis. It defines a molecular signature for neuronal vulnerability and directs our attention to a failure of neuronal differentiation control as a critical pathogenetic event and potential therapeutic target in AD.

Problem of size dependence in fluorescence DNA cytometry

The relationship among fluorescence intensity, area, thickness, and DNA concentration of DNA-bearing polyacrylamide film, stained with 50 ng/ml DAPI, was studied using an epi-illumination cytofluorometer. Measurements and theoretical analysis suggest the inevitability of the size-dependent effect on human cell nuclei in which the order of the DNA concentration is approximately 10 mg/ml, and that this effect can be virtually negated if the nuclei are stretched and flattened to less than 0.33 microns in thickness on the smears. Alternative practical ways to minimize this effect are to decrease the dye concentration of a staining solution and to make smears as thin and uniform as possible by some means such as hypotonic treatment and/or using an automatic centrifuge smearing apparatus. The size-dependent effect, nevertheless, should be taken into account particularly when determining the DNA content of different cell types and the DNA aneuploidy of tumor cells, regardless of the technique used for measurement.

A cytophotometric analysis of the structure of hypothalamic cell populations in the frog, Rana temporaria (L.), with special reference to seasonal changes in chromatin status

We used cytophotometry after the Feulgen reaction and UV cytophotometry to measure the DNA content of quiescent cells of the hypothalamic preoptic region (HPR) of adult and juvenile frogs (Rana temporaria) that had been caught in their natural habitat in winter, spring and summer. The histone-to-DNA ratio in cell nuclei was cytophotometrically determined using a combined Feulgen, heparine and alcian-blue staining procedure. The vast majority of HPR cells studied had nuclei with a diploid DNA content. However, we observed great variability in the Feulgen-DNA content of the HPR cell population, which was not detected in the diploid standard (hepatocytes). This heterogeneity in the diploid sample of the HPR cell populations was always greater in prespawning frogs and may have been due to differences in the chromatin arrangement in nuclei. About 1% of cells had a DNA content either ranging between diploid and tetraploid levels (H2C cells) or at the tetraploid level (4C and 2C x 2 cells). The proportion of these cells was not affected by the age of the animals or the annual cycle, thus suggesting that there is no age-related increase in the mean DNA content in the frog HPR. The mean DNA contents of H2C and 4C cells were much higher than those in the standard (hepatocytes). This cannot be simply attributed to the presence of different amounts of nuclear proteins, but rather indicates that at least a certain proportion of the highest DNA contents may be due to a real extra-DNA synthesis.

Effect of tetraploidy on dendritic branching in neurons and glial cells of the frog, Xenopus laevis

Morphological aspects of four different groups of Golgi impregnated brain cells from a tetraploid strain of Xenopus laevis frogs were compared to analogous cells in comparably sized diploid frogs. The cells examined included neurons from the telencephalon, caudal hypothalamus, and optic tectum, and radial glial cells from the optic tectum. The brains of tetraploid frogs appeared grossly normal and were the same size and contained similar cell types as diploid brains. As observed in previous studies on polyploid amphibia, somal diameters increased significantly in tetraploid cells for each of the four groups of cells examined. Also, the total length of the dendritic arbors in tetraploid brain cells increased significantly by factors ranging from 1.4 to 2.4 times the total length of the analogous processes in diploid cells. Tetraploid neurons in the telencephalon and hypothalamus increased their arbor lengths predominantly by increasing the number of dendritic branches, while maintaining the average distance between branch points in the dendritic segments. In contrast, the tetraploid large pear-shaped neurons in the optic tectum had significantly longer terminal dendritic segments than the analogous diploid neurons, although these tetraploid neurons maintained their average number of dendritic segments per cell. Tetraploid tectal radial glial cells appeared to increase both their number of branches and the lengths of their terminal segments. Thus, the mode by which tetraploid brain cells achieved longer dendritic arbors varied from cell type to cell type. These results suggest a hypothetical basis for possible effects of genomic size on vertebrate brain structure and evolution at the cellular level.

DNA content in nerve-cell nucleus. A biochemical and cytophotometric study of the rat cerebrum

The cerebral nuclei of 30-day-old rats were separated by a two-step gradient centrifugation into the fractions of large and small nuclei. The DNA content per nucleus was determined biochemically and cytophotometrically in these fractions as well as in the non-separated cerebellar and liver-cell nuclei used as reference cells. The DNA content of large and/or mainly neuronal, and the small and/or glial cell-enriched nuclei were 6.74 +/- 0.51 and 6.21 +/- 0.30 pg, respectively, and in the cerebellum 6.10 +/- 0.28 pg per nucleus. The DNA-content values determined cytophotometrically in Feulgen-stained samples of large and small bulk isolated cortical nuclei ranged within the diploid limits, indicated by a part of a liver-cell population measured in parallel. No evidence for the large scale existence of an "extra-DNA" reported earlier [Bregnard, Knuesel and Kuenzle (1975) Histochemie 43, 59-61; Bregnard, Kuenzle and Ruch (1977) Expl Cell Res. 107, 151-157; Kuenzle, Bregnard, Hübschner and Ruch (1978) Expl Cell Res. 113, 151-160] in cortical neurons has thus been obtained.

Cytophotometric comparisons of DNA levels in neuronal and glial cells of the cerebellum: a comparative study

Several cytochemical studies of the DNA content and ploidy status of neuronal cell nuclei in the central nervous system have reported the occurrence of hyperdiploid amounts of DNA in Purkinje cells and suggest the existence of some type of 'extra' DNA, the biological significance of which is, as yet, unknown. To explore this phenomenon further, the DNA content of glial and Purkinje cell nuclei was determined in several vertebrate species, using the DNA-specific fluorochrome 4',6-diamidino-2-phenylindole (DAPI) to stain isolated cerebellar nuclei for analysis with a single parameter flow cytometer. The Feulgen reaction for DNA was used to stain liver and cerebellar tissue imprints for the measurement of individual nuclei with a Vickers M86 integrating microdensitometer. In both types of analyses, chicken erythrocyte nuclei served as an internal reference standard of 2.5 pg DNA per cell. The mean DNA content of Purkinje cells and glial or granule cells was essentially the same as that found for diploid (2C) non-neuronal cells, such as hepatocytes, in rainbow trout, Amazon molly fish, salamander (Plethodon), mouse, rat, rabbit, cat, dog, monkey and human. Although Purkinje cell nuclei with 4C DNA levels were found in all of these species, except salamander and rabbit, the frequency of such cells was low (1-7%) and varied with the species. There was a low incidence of Purkinje cell nuclei with interclass DNA amounts in all species examined. Our data show that most neuronal cell nuclei in the cerebellum contain 2C levels of DNA.

The DNA content of cerebral cortex neurons. Determinations by cytophotometry and high performance liquid chromatography

Previous work from our laboratories has indicated that the DNA content of rat cerebral cortex neurons increases postnatally to a level of slightly above 3c, where 2c denotes the diploid DNA complement. We have re-evaluated this concept by using various cytophotometric assays and a novel high performance liquid chromatography (HPLC) technique. The latter consists of digesting the DNA in isolated neuronal nuclei by a mixture of DNA-degrading enzymes followed by analysis of the resulting deoxynucleosides by HPLC. We find that the various methods fall into two groups. The first gives evidence of a postnatal DNA (or histone) increase, while the second does not. The first group (DNA increase) comprises cytofluorometry for DNA following Schiff-type staining with fluorochromes 2,5-bis-(4-aminophenyl)-1,3,4-oxadiazole (BAO) and pararosaniline, ultraviolet absorption scanning for DNA and cytofluorometry for histones following staining with sulfaflavine at pH 8. The second group (no DNA increase) consists of cytofluorometry for DNA following staining with the DNA-complexing agents mithramycin, chromomycin A3, 4',6-diamidino-2-phenylindole (DAPI) and bisbenzimide (Hoechst 33258), as well as the newly developed HPLC technique. Since the HPLC technique measures DNA by a direct chemical approach without interference from other nuclear constituents or from higher order packaging in the chromatin, and detects at least 94-95% of the total DNA contained in neuronal nuclei independent of the developmental stage, we infer that the HPLC technique and, by consequence, the cytochemical assays of the second group reflect true DNA values. Therefore, we propose that cerebral cortex neurons retain a diploid DNA level throughout development.

DNA-polymerases in neuron and glial cells of developing and aging mouse brain

DNA-polymerases alpha and beta were studied in neuron and glial cells of mouse during developing and aging brain. Maximum activity of alpha-enzyme was found to be prenatal, though a very low but significant level could be detected in aging brain. Furthermore, this pattern was found to be dependent upon cell types that shift in peak activity just before birth in glial cells in contrast to neurons. DNA-polymerase-beta remained high throughout the period studied though a second peak of activity was also observed at day 30 in both cell types, suggesting a possible role in DNA-replication in addition to DNA-repair. It was found that beta-enzyme from glial cells behaves differently than the same enzyme from neurons.

Developmental changes in chromatin organization in rat cerebral hemisphere neurons and analysis of DNA reassociation kinetics

Previous reports have demonstrated that neuronal nuclei of rabbit, mouse and rat cerebral hemispheres exhibit a short DNA repeat length of 160 bp compared to the more typical repeat size of 200 bp found in glial nuclei and other cell types of higher eukaryotes. In this study we report that the conversion of chromatin to a short DNA repeat length in rat cerebral hemisphere neurons is a gradual process which begins between the first and second day after birth and is complete by 8 days. In these neurons, histone H1 appears to be less accessible to degradation by trypsin in the newborn rat brain compared to the 8 day old rat. This suggests that the developmental shift to a short DNA repeat length may be accompanied by a dispersal or decondensation of neuronal chromatin which results in an increased accessibility of neuronal histone H1 to degradation by trypsin. The increase in nuclear DNA content to 3.5C which has been reported in rat cortical neurons during early postnatal development does not appear to be associated with a selective amplification of a subset of DNA sequences as determined by DNA reassociation kinetics.

Cytophotometric evidence of non-S-phase extra-DNA in human neuronal nuclei

After Feulgen staining with acriflavine-Schiff, the DNa content of glial and neuronal nuclei from various sites of the human CNS (pre- and post-central gyrus, cerebellar cortex and spinal cord) were determined by fluorescence cytophotometry. The specimens were obtained from twelve adult human autopsy cases. Glial cell nuclei always revealed a biomodal DNA distribution pattern with a large 2c and a smaller 4c peak. The 4c peak was most prominent in the cerebellum. A few 8c glial nuclei were found. Neuronal cell nuclei disclosed DNA histograms with hyperdiploid means in the range 2.2-2.5c (1.8-2.9c for the individual populations). Tetraploid 4c DNA values were not observed, neither in Purkinje cells, nor in pyramidal cells. In eleven out of a total of forty-four slides the higher DNA means of neuronal nuclei were found to be statistically significant (P greater than 0.05) when compared with a population of 2c hepatocytes on the same slide. The results indicate the existence of some 'extra DNA' in human neuronal cell nuclei, the biological significance of which has still to be elucidated. It is however, suggested that it may play an important role in the functional activity of the CNS.

Influence of prenatal hypoxia on brain development: effects on body weight, brain weight, DNA, protein, acetylcholinesterase, 3-quinuclidinyl benzilate binding, and in vivo incorporation of [14C]lysine into subcellular fractions

The influence of prenatal hypoxia on subsequent brain development in the young rat was investigated by examining body and brain weight, cerebral cortex wet weight, protein and DNA concentrations, acetylcholinesterase (AChE) activity, 3-quinuclidinyl benzilate (QNB)-binding levels, the relative amounts of protein in various subcellular fractions, and the in vivo incorporation of [14C]lysine into the protein of homogenate and subcellular fractions. Exposure of pregnant females to a mild hypoxia (9.1% O2, 10 h per day for the 9--11 days preceding birth) resulted in a reduced body weight in the pups and days 1 and 5 after birth; total cortical DNA was reduced but brain weight and protein content were unaffected, leading to a higher protein/DNA ratio in prenatally hypoxic pups. By 10 days of age these differences between prenatally hypoxic and control animals were no longer apparent. There were no differences between prenatally hypoxic and control animals in AChE and QNB binding per milligram cortex protein. The relative amount of synaptic membrane protein from the cerebral cortex was reduced at day 1 in prenatally hypoxic animals and the synaptic membrane fraction showed a higher level of incorporation of [14C]lysine on days 1, 5, and 10. The developmental profile of [14C]lysine incorporation showed a peak on day 10 which was higher in prenatally hypoxic rats. By 46 days after birth little difference could be found between prenatally hypoxic and control animals.

Cytochemical study of chromatin changes in Purkinje cell population as markers of rat cerebellar histogenesis

In the population of rat cerebellum Purkinje cells, Feulgen-DNA contents higher than 2c are present starting from the 9th to the 12th day of postnatal life. This stage of histogenesis corresponds to the time when other histological and cytochemical parameters suggest that important functional changes are taking place in the cerebellum. Hyperdiploid values (H2c) gradually increase during the subsequent histogenesis, involving also the physico-chemical state of chromatin. The key steps of histogenesis suggested by our investigations might be a starting point of reexamine the problem of tritiated thimidine incorporation, in order to demonstrate the possible synthesis of extra-DNA.

Histone H1o: its location in chromatin

Histone H1o is an H5-like protein found in mammals. Its location in chromatin from animal tissues has been studied by micrococcal nuclease digestion and by quantitation. It was found that H1o occurs in the linker region of chromatin, and replaces H1 there as it does so. As much as a third of the H1 becomes replaced, at least in some tissues. The abundance of H1o was similar in bulk chromatin and in a mononucleosome population which was putatively enriched in transcribed DNA sequences. It was concluded that H1o probably does not suppress transcription.

Brain nucleic acids and protein in various neurological mutant mice

A study was made to compare alterations in the cerebral contents of nucleic acids and protein of several mouse strains affected by different neurological mutations: jimpy, msd, quaking, reeler, weaver, and dwarf. In normal and affected jimpy and msd mice the brain components analyzed were very similar. On the other hand, the cerebral hemispheres of quaking mice showed significant decreases in total RNA and DNA, when compared with those of normal littermates. In the affected reeler and weaver mice, total protein, RNA, and DNA in the cerebellum differed markedly from controls. Protein decreased slightly, whereas nucleic acids showed no significant variation in the cerebral hemispheres of the same mutants. The cerebella and cerebral hemispheres of affected dwarf mice had wet weights and total protein contents that were about 20% lower than those of their controls; DNA did not vary significantly in the various brain regions analyzed. The decrease of DNA we report in reeler and weaver mutant cerebellum in toto quantifies the lack of cell number, in contrast to histological studies which give only semiquantitative information.

Quantitative analysis of sex differences in hormone accumulation in the zebra finch brain: methodological and theoretical issues

The autoradiographic method was used to compare the numbers of hormone accumulating cells in several brain regions in male and female zebra finches (Poephila guttata) after injection of tritiated testosterone. The brain regions examined were the caudal nucleus of the hyperstriatum ventrale (HVc), magnocellular nucleus of the anterior neostriatum (MAN), robust nucleus of the archistriatum (RA), nucleus intercollicularis of the midbrain (ICo), the tracheosyringeal hypoglossal motor nucleus (nXIIts), and periventricular magnocellular nucleus of the anterior hypothalamus (PVM). All but the last of these regions are thought to be involved in the control of vocalizations in passerine song birds. Males have significantly more labelled cells in HVc and MAN. In RA, there is no difference in total percentage of labelled cells, but there is a sex difference in size distribution of labelled cells. No sex difference was detected in other brain regions. These differences are found when using a criterion for cell labelling which is based on the Poisson distribution, and the relative merits are evaluated of various quantitative criteria used in the analysis of steroid autoradiograms. The magnitude of the observed sex difference may be influenced by several biasing factors, yet the sex difference persists when corrections are applied to eliminate the biases, indicating that the sex difference is not an artifact of autoradiographic procedure. The magnitude of the sex difference in hormone accumulation has certain implications for the process of sexual differentiation of the brain.

Cytophotometric re-investigation of DNA content in Purkinje cells of the rat cerebellum

Stage scanning cytophotometry of Feulgen-stained cell nuclei of the cerebellum of young adult rats revealed that the absorbance values of the majority of the Purkinje cells show a Gaussian distribution with a low coefficient of variance. The peak absorbance of this population is the same as that of the granule cells. About 1% of the Purkinje cells measured, were found to have a stain content which indicates a 4C amount of DNA. For both the granular and the Purkinje cell population, a very small number of nuclei possesses absorbance values intermediate between 2C and 4C. The present data suggest prevalent diploidy of the Purkinje cells, and are at variance with those postulating a 'tetraploid and/or hyperdiploid' status of this population.

DNA polymerase beta from brain neurons is a repair enzyme

DNA polymerase beta was isolated from rat cortex neurons and characterised. Its properties were strikingly similar to those of other mammalian beta-polymerases. In adult rats, this was the major DNA polymerase occurring in neuronal nuclei, which contained no alpha-polymerase, 99.2% beta-polymerase and only 0.8% gamma-polymerase. Isolated neuronal nuclei of this developmental stage were shown to perform ultraviolet-induced repair DNA synthesis in vitro. Since beta-polymerase was virtually the exclusive DNA polymerase in these nuclei it was concluded that the beta enzyme was responsible for the observed DNA repair. This was further substantiated by demonstrating a virtually complete suppression of DNA repair in irradiated nuclei by 2',3'-dideoxyribosylthymine 5'-triphosphate (d2TTP), a potent beta-polymerase inhibitor. However, the presence of minute amounts of gamma-polymerase in neuronal nuclei and its susceptibility to d2TTP did not allow one to rule out an ancillary role of DNA polymerase gamma in DNA repair. In view of the similarity of the neuronal DNA polymerase beta with all other mammalian beta-polymerases it may be speculated that the ability to perform repair DNA synthesis is not unique to the neuronal enzyme but is a general function of all beta-polymerases.