Are all the neuronal nuclei polyploid?

Cell Cycle Re-entry in the Nervous System: From Polyploidy to Neurodegeneration

Terminally differentiated cells of the nervous system have long been considered to be in a stable non-cycling state and are often considered to be permanently in G0. Exit from the cell cycle during development is often coincident with the differentiation of neurons, and is critical for neuronal function. But what happens in long lived postmitotic tissues that accumulate cell damage or suffer cell loss during aging? In other contexts, cells that are normally non-dividing or postmitotic can or re-enter the cell cycle and begin replicating their DNA to facilitate cellular growth in response to cell loss. This leads to a state called polyploidy, where cells contain multiple copies of the genome. A growing body of literature from several vertebrate and invertebrate model organisms has shown that polyploidy in the nervous system may be more common than previously appreciated and occurs under normal physiological conditions. Moreover, it has been found that neuronal polyploidization can play a protective role when cells are challenged with DNA damage or oxidative stress. By contrast, work over the last two and a half decades has discovered a link between cell-cycle reentry in neurons and several neurodegenerative conditions. In this context, neuronal cell cycle re-entry is widely considered to be aberrant and deleterious to neuronal health. In this review, we highlight historical and emerging reports of polyploidy in the nervous systems of various vertebrate and invertebrate organisms. We discuss the potential functions of polyploidization in the nervous system, particularly in the context of long-lived cells and age-associated polyploidization. Finally, we attempt to reconcile the seemingly disparate associations of neuronal polyploidy with both neurodegeneration and neuroprotection.

Endopolyploidy as a potential driver of animal ecology and evolution

Endopolyploidy - the existence of higher-ploidy cells within organisms that are otherwise of a lower ploidy level (generally diploid) - was discovered decades ago, but remains poorly studied relative to other genomic phenomena, especially in animals. Our synthetic review suggests that endopolyploidy is more common in animals than often recognized and probably influences a number of fitness-related and ecologically important traits. In particular, we argue that endopolyploidy is likely to play a central role in key traits such as gene expression, body and cell size, and growth rate, and in a variety of cell types, including those responsible for tissue regeneration, nutrient storage, and inducible anti-predator defences. We also summarize evidence for intraspecific genetic variation in endopolyploid levels and make the case that the existence of this variation suggests that endopolyploid levels are likely to be heritable and thus a potential target for natural selection. We then discuss why, in light of evident benefits of endopolyploidy, animals remain primarily diploid. We conclude by highlighting key areas for future research such as comprehensive evaluation of the heritability of endopolyploidy and the adaptive scope of endopolyploid-related traits, the extent to which endopolyploid induction incurs costs, and characterization of the relationships between environmental variability and endopolyploid levels.

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.

Selective cell death of hyperploid neurons in Alzheimer's disease

Aneuploidy, an abnormal number of copies of a genomic region, might be a significant source for neuronal complexity, intercellular diversity, and evolution. Genomic instability associated with aneuploidy, however, can also lead to developmental abnormalities and decreased cellular fitness. Here we show that neurons with a more-than-diploid content of DNA are increased in preclinical stages of Alzheimer's disease (AD) and are selectively affected by cell death during progression of the disease. Present findings show that neuronal hyperploidy in AD is associated with a decreased viability. Hyperploidy of neurons thus represents a direct molecular signature of cells prone to death in AD and indicates that a failure of neuronal differentiation is a critical pathogenetic event in AD.

Neuronal aneuploidy in health and disease: a cytomic approach to understand the molecular individuality of neurons

Structural variation in the human genome is likely to be an important mechanism for neuronal diversity and brain disease. A combination of multiple different forms of aneuploid cells due to loss or gain of whole chromosomes giving rise to cellular diversity at the genomic level have been described in neurons of the normal and diseased adult human brain. Here, we describe recent advances in molecular neuropathology based on the combination of slide-based cytometry with molecular biological techniques that will contribute to the understanding of genetic neuronal heterogeneity in the CNS and its potential impact on Alzheimer's disease and age-related disorders.

Chromosomal variation in mammalian neuronal cells: known facts and attractive hypotheses

Chromosomal mosaicism is still a genetic enigma. Although the mechanisms and consequences of this phenomenon have been studied for over 50 years, there are a number of gaps in our knowledge concerning causes, genetic mechanisms, and phenotypic manifestations of chromosomal mosaicism. Neuronal cell-specific chromosomal mosaicism is not an exception. Originally, neuronal cells of the mammalian brain were assumed to possess identical genomes. However, recent studies have shown chromosomal variations, manifested as chromosome abnormalities in cells of the developing and adult mammalian nervous system. Here, we review data obtained on the variation in chromosome complement in mammalian neuronal cells and hypothesize about the possible relevance of large-scale genomic (i.e., chromosomal) variations to brain development and functions as well as neurodevelopmental and neurodegenerative disorders. We propose to cover the term "molecular neurocytogenetics to cover all studies the aim of which is to reveal chromosome variations and organization in the mammalian brain.

The amount of DNA in cells of the granular layer of the cerebellum and their susceptibility to hypoxia

Purkinje cells of the cerebellum are particularly susceptible to hypoxia. In these cells tetraploidy has been demonstrated. Therefore, a link between the susceptibility of cells of the cerebellum to hypoxia and the amount of DNA seems probable.

DNA content in neurons of Auerbach's plexus under experimental conditions in adult rats

Some nerve cells of the Auerbach's myenteric plexus of the intestine of the adult rat, which hypertrophied following a surgically induced stenosis, began DNA synthesis unrelated to mitotic division. The cytophotometric analysis confirmed and quantified the amount of synthesis revealed by autoradiography with tritiated thymidine uptake. Numerous nerve cells show a DNA content exceeding the diploid level. Only a few of these show twice the diploid content. The significance of the DNA synthesis is discussed.

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.

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.

Are CNS neurons polyploid? A critical analysis based upon cytophotometric study of the DNA content of cerebellar and olfactory bulbar neurons of the bat

A cytophotometric study of the nuclear DNA content of bat cerebellar and olfactory bulbar neurons was performed with particular attention to corrections for distributional error and non-specific light loss and to selection of appropriate control nuclei for the establishment of reliable haploid and diploid DNA values. Feulgen stained sections were measured with an integrating, scanning microdensitometer for correction of distributional error. The values thus obtained were further corrected in 3 different ways to subtract the contribution of background absorbance at 546 nm. Bat haploid (1c) DNA values were derived from spermatozoa, and diploid (2c) values from pancreatic acinar cells, hepatocytes and non-hepatocyte liver cells. Microglia of olfactory bulb also had 2c values. After correction, all neurons measured, except Purkinje cells, had DNA values more closely approximating the tetraploid (4c) than the diploid (2c) level. Purkinje neurons had mean DNA values closer to 2c than to 4c, but inherent technical difficulties in measuring these very large, light staining nuclei has created less confidence in the values obtained than for those of the other neurons. This uncertainty, plus the apparent existence of two populations of Purkinje neurons, one 2c and the other hyperdiploid, suggested by the DNA distribution curves, make it difficult to eliminate the possibility of polyploidization of Purkinje neurons. A critical analysis of various cytophotometric, radioautographic and biochemical approaches to the problem of CNS neuron polyploidization has revealed potentially serious flaws in many of them, rendering virtually impossible interpretation of the numerous contradictory results in the literature. Standardization of the cytophotometric technique and improvement of the radioautographic approach seem to be prerequisite to the resolution of the existing dilemma.

Morphological identification and biochemical characterization of isolated brain cell nuclei from the developing rat cerebellum

Cell nuclei from developing rat cerebellum were isolated and the various types of nuclei were characterized and quantified. Nuclear pellets appeared to be both quantitatively and qualitatively representative of the entire cerebellum, and of sufficient purity to perform biochemical studies as well as morphological comparison with histological sections. Isolated nuclei were classified into 6 groups based on nuclear size and shape, heterochromatin aggregations, and nucleoplasmic density. The total population of cerebellar cells primarily consisted of two types of nuclei after day 10. One group of nuclei, resembling those of internal granule neurons or external germinal cells, contributed at least 70% of the total isolated cell nuclei from day 1 to day 90, whereas another nuclear group that was identified as dark oligodendrocytes constituted 8-9% of the total population on days 45 and 90. Nuclear DNA, RNA, and protein content of the cerebellum also were determined throughout postnatal development. DNA concentration markedly declined after day 15, while the RNA/DNA ratio increased until day 3 and remained constant to day 90. The nuclear protein/DNA ratio increased from birth to day 3, decreased to its lowest value on day 10, and increased to day 90. Utilizing DNA values, the total cell population as well as contributions of different cell types were calculated. At birth the cerebellum was estimated to contain 5.9 million cells, increasing to 94 million by day 21. By day 90, 107 million cells were present, of which 8.6 million oligodendrocytes and 93.6 million internal granule cells were estimated.

Excess DNA in the nuclei of the subesophageal region of octopus brain

Cytophotometric analyses of Feulgen-stained nuclei present in homogenates of vertical and subesophageal lobes of octopus brain have shown that the latter region contains larger nuclei with up to several times the amount of DNA present in vertical nuclei. No obvious relationship was found between DNA content and nuclear size. Except for a rather small minority, nuclei of the vertical lobe have a uniform size and the expected diploid amount of DNA. These parameters are not substantially dependent on body weight. In contrast, the DNA content of subesophageal nuclei increases progressively with body weight. The amount of DNA found in subesophageal nuclei does not seem to be a simple multiple of the diploid or haploid value.

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.

Constancy and variability in the content of DNA in cerebellar Purkinje cell nuclei. A cytophotometric study

A cytophotometric study of DNA content in Purkinje cells of the cerebellum of rats, cats, chicken and humans (Feulgen staining) revealed that in a certain number of cells the amount of NDA ranged between the diploid and tetraploid level (H2C cells). The incidence of H2C Purkinje cells varied among the species studied. In rats, which were studied most thoroughly, these cells amounted on average to 3%. In some rats, as well as in some cats and chickens H2C Purkinje cells were entirely absent. In the group of animals possesing H2C Purkinje cells, great interindividual differences were observed. In rats for instance, the incidence of these cells varied from 1 to 23 per cent. Topographic analyses carried out in rat and human cerebellum revealed that H2C Purkinje cells occurred more frequently in the hemispheres than in the vermis. No significant differences were found in the number of H2C Purkinje cells in healthy and Kilham-DNA-virus infected rats. Densitometric analysis of the distribution of nuclear chromatin showed that H2C Purkinje cells were richer in condensed chromatin, especially in the region of the nucleolus, which apparently contains the hyperploid surplus of DNA. It is proposed that the phenomenon of DNA hyperdiploidy arises as a result of either incomplete S-phase in some immature Purkinje cell precursors or the amplification of some DNA sequences particularly those localized in the nucleolar region.