A cell cycle alteration precedes apoptosis of granule cell precursors in the weaver mouse cerebellum

Cyclin D1 controls development of cerebellar granule cell progenitors through phosphorylation and stabilization of ATOH1

During development, neural progenitors are in proliferative and immature states; however, the molecular machinery that cooperatively controls both states remains elusive. Here, we report that cyclin D1 (CCND1) directly regulates both proliferative and immature states of cerebellar granule cell progenitors (GCPs). CCND1 not only accelerates cell cycle but also upregulates ATOH1 protein, an essential transcription factor that maintains GCPs in an immature state. In cooperation with CDK4, CCND1 directly phosphorylates S309 of ATOH1, which inhibits additional phosphorylation at S328 and consequently prevents S328 phosphorylation-dependent ATOH1 degradation. Additionally, PROX1 downregulates Ccnd1 expression by histone deacetylation of Ccnd1 promoter in GCPs, leading to cell cycle exit and differentiation. Moreover, WNT signaling upregulates PROX1 expression in GCPs. These findings suggest that WNT-PROX1-CCND1-ATOH1 signaling cascade cooperatively controls proliferative and immature states of GCPs. We revealed that the expression and phosphorylation levels of these molecules dynamically change during cerebellar development, which are suggested to determine appropriate differentiation rates from GCPs to GCs at distinct developmental stages. This study contributes to understanding the regulatory mechanism of GCPs as well as neural progenitors.

Cell Cycle Deficits in Neurodegenerative Disorders: Uncovering Molecular Mechanisms to Drive Innovative Therapeutic Development

Cell cycle dysregulation has been implicated in the pathogenesis of neurodegenerative disorders. Specialised function obligates neuronal cells to subsist in a quiescent state of cell cycle once differentiated and therefore the circumstances and mechanisms underlying aberrant cell cycle activation in post-mitotic neurons in physiological and disease conditions remains an intriguing area of research. There is a strict requirement of concurrence to cell cycle regulation for neurons to ensure intracellular biochemical conformity as well as interrelationship with other cells within neural tissues. This review deliberates on various mechanisms underlying cell cycle regulation in neuronal cells and underscores potential implications of their non-compliance in neural pathology. Recent research suggests that successful duplication of genetic material without subsequent induction of mitosis induces inherent molecular flaws that eventually assert as apoptotic changes. The consequences of anomalous cell cycle activation and subsequent apoptosis are demonstrated by the increased presence of molecular stress response and apoptotic markers. This review delineates cell cycle events under normal physiological conditions and deficits amalgamated by alterations in protein levels and signalling pathways associated with cell-division are analysed. Cell cycle regulators essentially, cyclins, CDKs, cip/kip family of inhibitors, caspases, bax and p53 have been identified to be involved in impaired cell cycle regulation and associated with neural pathology. The pharmacological modulators of cell cycle that are shown to impart protection in various animal models of neurological deficits are summarised. Greater understanding of the molecular mechanisms that are indispensable to cell cycle regulation in neurons in health and disease conditions will facilitate targeted drug development for neuroprotection.

Understanding Aspects of Aluminum Exposure in Alzheimer's Disease Development

Aluminum is a ubiquitously abundant nonessential element. Aluminum has been associated with neurodegenerative diseases such as Alzheimer's disease (AD), amyotrophic lateral sclerosis, and dialysis encephalopathy. Many continue to regard aluminum as controversial although increasing evidence supports the implications of aluminum in the pathogenesis of AD. Aluminum causes the accumulation of tau protein and Aβ protein in the brain of experimental animals. Aluminum induces neuronal apoptosis in vivo and in vitro, either by endoplasmic stress from the unfolded protein response, by mitochondrial dysfunction, or a combination of them. Some, people who are exposed chronically to aluminum, either from through water and/or food, have not shown any AD pathology, apparently because their gastrointestinal barrier is more effective. This article is written keeping in mind mechanisms of action of aluminum neurotoxicity with respect to AD.

The Proteome Profiles of the Cerebellum of Juvenile, Adult and Aged Rats--An Ontogenetic Study

In this study, we searched for proteins that change their expression in the cerebellum (Ce) of rats during ontogenesis. This study focuses on the question of whether specific proteins exist which are differentially expressed with regard to postnatal stages of development. A better characterization of the microenvironment and its development may result from these study findings. A differential two-dimensional polyacrylamide gel electrophoresis (2DE) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis of the samples revealed that the number of proteins of the functional classes differed depending on the developmental stages. Especially members of the functional classes of biosynthesis, regulatory proteins, chaperones and structural proteins show the highest differential expression within the analyzed stages of development. Therefore, members of these functional protein groups seem to be involved in the development and differentiation of the Ce within the analyzed development stages. In this study, changes in the expression of proteins in the Ce at different postnatal developmental stages (postnatal days (P) 7, 90, and 637) could be observed. At the same time, an identification of proteins which are involved in cell migration and differentiation was possible. Especially proteins involved in processes of the biosynthesis and regulation, the dynamic organization of the cytoskeleton as well as chaperones showed a high amount of differentially expressed proteins between the analyzed dates.

Role of MXD3 in proliferation of DAOY human medulloblastoma cells

A subset of medulloblastomas, the most common brain tumor in children, is hypothesized to originate from granule neuron precursors (GNPs) in which the sonic hedgehog (SHH) pathway is over-activated. MXD3, a basic helix-look-helix zipper transcription factor of the MAD family, has been reported to be upregulated during postnatal cerebellar development and to promote GNP proliferation and MYCN expression. Mxd3 is upregulated in mouse models of medulloblastoma as well as in human medulloblastomas. Therefore, we hypothesize that MXD3 plays a role in the cellular events that lead to medulloblastoma biogenesis. In agreement with its proliferative role in GNPs, MXD3 knock-down in DAOY cells resulted in decreased proliferation. Sustained overexpression of MXD3 resulted in decreased cell numbers due to increased apoptosis and cell cycle arrest. Structure-function analysis revealed that the Sin3 interacting domain, the basic domain, and binding to E-boxes are essential for this activity. Microarray-based expression analysis indicated up-regulation of 84 genes and down-regulation of 47 genes. Potential direct MXD3 target genes were identified by ChIP-chip. Our results suggest that MXD3 is necessary for DAOY medulloblastoma cell proliferation. However, increased level and/or duration of MXD3 expression ultimately reduces cell numbers via increased cell death and cell cycle arrest.

Silymarin, a natural antioxidant, protects cerebral cortex against manganese-induced neurotoxicity in adult rats

Manganese (Mn) is an essential element for biological systems, nevertheless occupational exposure to high levels of Mn can lead to neurodegenerative disorders, characterized by serious oxidative and neurotoxic effects with similarities to Parkinson's disease. The aim of this study was to investigate the potential effects of silymarin (SIL), an antioxidant flavonoid, against manganese chloride induced neurotoxicity both in vivo (cerebral cortex of rats) and in vitro (Neuro2a cells). Twenty-eight male Wistar rats were randomly divided into four groups: the first group (C) received vehicle solution (i.p.) served as controls. The second group (Mn) received orally manganese chloride (20 mg/ml). The third group (Mn + SIL) received both Mn and SIL. The fourth group (SIL) received only SIL (100 mg/kg/day, i.p.). Animals exposed to Manganese chloride showed a significant increase in TBARS, NO, AOPP and PCO levels in cerebral cortex. These changes were accompanied by a decrease of enzymatic (SOD, CAT, GPx) and non-enzymatic (GSH, NpSH, Vit C) antioxidants. Co-administration of silymarin to Mn-treated rats significantly improved antioxidant enzyme activities and attenuated oxidative damages observed in brain tissue. The potential effect of SIL to prevent Mn induced neurotoxicity was also reflected by the microscopic study, indicative of its neuroprotective effects. We concluded that silymarin possesses neuroprotective potential, thus validating its use in alleviating manganese-induced neurodegenerative effects.

Gene expression signature of cerebellar hypoplasia in a mouse model of Down syndrome during postnatal development

BACKGROUND

Down syndrome is a chromosomal disorder caused by the presence of three copies of chromosome 21. The mechanisms by which this aneuploidy produces the complex and variable phenotype observed in people with Down syndrome are still under discussion. Recent studies have demonstrated an increased transcript level of the three-copy genes with some dosage compensation or amplification for a subset of them. The impact of this gene dosage effect on the whole transcriptome is still debated and longitudinal studies assessing the variability among samples, tissues and developmental stages are needed.

RESULTS

We thus designed a large scale gene expression study in mice (the Ts1Cje Down syndrome mouse model) in which we could measure the effects of trisomy 21 on a large number of samples (74 in total) in a tissue that is affected in Down syndrome (the cerebellum) and where we could quantify the defect during postnatal development in order to correlate gene expression changes to the phenotype observed. Statistical analysis of microarray data revealed a major gene dosage effect: for the three-copy genes as well as for a 2 Mb segment from mouse chromosome 12 that we show for the first time as being deleted in the Ts1Cje mice. This gene dosage effect impacts moderately on the expression of euploid genes (2.4 to 7.5% differentially expressed). Only 13 genes were significantly dysregulated in Ts1Cje mice at all four postnatal development stages studied from birth to 10 days after birth, and among them are 6 three-copy genes. The decrease in granule cell proliferation demonstrated in newborn Ts1Cje cerebellum was correlated with a major gene dosage effect on the transcriptome in dissected cerebellar external granule cell layer.

CONCLUSION

High throughput gene expression analysis in the cerebellum of a large number of samples of Ts1Cje and euploid mice has revealed a prevailing gene dosage effect on triplicated genes. Moreover using an enriched cell population that is thought responsible for the cerebellar hypoplasia in Down syndrome, a global destabilization of gene expression was not detected. Altogether these results strongly suggest that the three-copy genes are directly responsible for the phenotype present in cerebellum. We provide here a short list of candidate genes.

A novel role of the Mad family member Mad3 in cerebellar granule neuron precursor proliferation

During development, Sonic hedgehog (Shh) regulates the proliferation of cerebellar granule neuron precursors (GNPs) in part via expression of Nmyc. We present evidence supporting a novel role for the Mad family member Mad3 in the Shh pathway to regulate Nmyc expression and GNP proliferation. Mad3 mRNA is transiently expressed in GNPs during proliferation. Cultured GNPs express Mad3 in response to Shh stimulation in a cyclopamine-dependent manner. Mad3 is necessary for Shh-dependent GNP proliferation as measured by bromodeoxyuridine incorporation and Nmyc expression. Furthermore, Mad3 overexpression, but not that of other Mad proteins, is sufficient to induce GNP proliferation in the absence of Shh. Structure-function analysis revealed that Max dimerization and recruitment of the mSin3 corepressor are required for Mad3-mediated GNP proliferation. Surprisingly, basic-domain-dependent DNA binding of Mad3 is not required, suggesting that Mad3 interacts with other DNA binding proteins to repress transcription. Interestingly, cerebellar tumors and pretumor cells derived from patched heterozygous mice express high levels of Mad3 compared with adjacent normal cerebellar tissue. Our studies support a novel role for Mad3 in cerebellar GNP proliferation and possibly tumorigenesis, and they challenge the current paradigm that Mad3 should antagonize Nmyc by competition for direct DNA binding via Max dimerization.

Determination of key aspects of precursor cell proliferation, cell cycle length and kinetics in the adult mouse subgranular zone

Neurogenesis studies on the adult mouse hippocampal subgranular zone (SGZ) typically report increases or decreases in proliferation. However, key information is lacking about these proliferating SGZ precursors, from the fundamental--what dose of bromodeoxyuridine (BrdU) is appropriate for labeling all S phase cells?--to the detailed--what are the kinetics of BrdU-labeled cells and their progeny? To address these questions, adult C57BL/6J mice were injected with BrdU and BrdU-immunoreactive (IR) cells were quantified. Initial experiments with a range of BrdU doses (25-500 mg/kg) suggested that 150 mg/kg labels all actively dividing precursors in the mouse SGZ. Experiments using a saturating dose of BrdU suggested BrdU bioavailability is less than 15 min, notably shorter than in the developing mouse brain. We next explored precursor division and maturation by tracking the number of BrdU-IR cells and colabeling of BrdU with other cell cycle proteins from 15 min to 30 days after BrdU. We found that BrdU and the Gap2 and mitosis (G2/M) phase protein pHisH3 maximally colocalized 8 h after BrdU, indicating that the mouse SGZ precursor cell cycle length is 14 h. In addition, triple labeling with BrdU and proliferating cell nuclear antigen (PCNA) and Ki-67 showed that BrdU-IR precursors and/or their progeny express these endogenous cell cycle proteins up to 4 days after BrdU injection. However, the proportion of BrdU/Ki-67-IR cells declined at a greater rate than the proportion of BrdU/PCNA-IR cells. This suggests that PCNA protein is detectable long after cell cycle exit, and that reliance on PCNA may overestimate the length of time a cell remains in the cell cycle. These findings will be critical for future studies examining the regulation of SGZ precursor kinetics in adult mice, and hopefully will encourage the field to move beyond counting BrdU-IR cells to a more mechanistic analysis of adult neurogenesis.

Nigrostriatal dopaminergic neurodegeneration in the weaver mouse is mediated via neuroinflammation and alleviated by minocycline administration

The murine mutant weaver (gene symbol, wv) mouse, which carries a mutation in the gene encoding the G-protein inwardly rectifying potassium channel Girk2, exhibits a diverse range of defects as a result of postnatal cell death in several different brain neuron subtypes. Loss of dopaminergic nigrostriatal neurons in the weaver, unlike cerebellar granule neuronal loss, is via a noncaspase-mediated mechanism. Here, we present data demonstrating that degeneration of midbrain dopaminergic neurons in weaver is mediated via neuroinflammation. Furthermore, in vivo administration of the anti-inflammatory agent minocycline attenuates nigrostriatal dopaminergic neurodegeneration. This has novel implications for the use of the weaver mouse as a model for Parkinson's disease, which has been associated with increased neuroinflammation.

Potential Roles of Electrogenic Ion Transport and Plasma Membrane Depolarization in Apoptosis

Apoptosis is characterized by the programmed activation of specific biochemical pathways leading to the organized demise of cells. To date, aspects of the intracellular signaling machinery involved in this phenomenon have been extensively dissected and characterized. However, recent studies have elucidated a novel role for changes in the intracellular milieu of the cells as important modulators of the cell death program. Specially, intracellular ionic homeostasis has been reported to be a determinant in both the activation and progression of the apoptotic cascade. Several apoptotic insults trigger specific changes in ionic gradients across the plasma membrane leading to depolarization of the plasma membrane potential (PMP). These changes lead to ionic imbalance early during apoptosis. Several studies have also suggested the activation and/or modulation of specific ionic transport mechanisms including ion channels, transporters and ATPases, as mediators of altered intracellular ionic homeostasis leading to PMP depolarization during apoptosis. However, the role of PMP depolarization and of the changes in ionic homeostasis during the progression of apoptosis are still unclear. This review summarizes the current knowledge regarding the causes and consequences of PMP depolarization during apoptosis. We also review the potential electrogenic ion transport mechanisms associated with this event, including the net influx/efflux of cations and anions. An understanding of these mechamisms could lead to the generation of new therapeutic approaches for a variety of diseases involving apoptosis.

Proliferation and migration of granule cells in the developing rat cerebellum: cisplatin effects

We evaluated the relationship among proliferation, death and migration of granule cells in lobules VI-VIII of vermis, in comparison with lobule III, during cerebellar development. To this aim, a single injection of cisplatin, i.e., a cytostatic agent that is known to induce death of proliferating granule cells, was given to 10-day-old rats. Histochemical markers of proliferating (PCNA immunoreaction) and apoptotic (TUNEL staining) cells were used; the variations of the external granular layer (EGL) thickness were evaluated in parallel. After PCNA and TUNEL reactions, evident changes of the whole EGL were found on PD11 (1 day after treatment), when a reduction of the thickness of this layer was found in treated rats, mainly in consequence of the high number of apoptotic cells in all the cerebellar lobules. On PD17 (7 days after treatment), a thick layer of proliferating cells was observed in lobules VI-VIII of treated rats, while the peculiar pattern of the normal development showed a thin EGL. At the same time, in treated rats, the number of apoptotic cells in EGL was low. In all developmental stages of treated rats, after GFAP immunoreaction, glial fibers appeared twisted, thickened, and with an irregular course; intensely labeled end-feet were present. The damage of radial glia suggests an alteration of migratory processes of granule cells, which is also evidenced by the decreased thickness of the premigratory zone of the EGL. Injured radial glia fibers were restricted to lobules VI-VIII and they persisted at PD30, leading to the presence of ectopic granule cells in the molecular layer, as we previously described.

PBK/TOPK, a proliferating neural progenitor-specific mitogen-activated protein kinase kinase

We performed genomic subtraction coupled to microarray-based gene expression profiling and identified the PDZ (postsynaptic density-95/Discs large/zona occludens-1)-binding kinase/T-LAK (lymphokine-activated killer T cell) cell originating protein kinase (PBK/TOPK) as a gene highly enriched in neural stem cell cultures. Previous studies have identified PBK/TOPK as a mitogen-activated protein kinase (MAPK) kinase that phosphorylated P38 MAPK but with no known expression or function in the nervous system. First, using a novel, bioinformatics-based approach to assess cross-correlation in large microarray datasets, we generated the hypothesis of a cell-cycle-related role for PBK/TOPK in neural cells. We then demonstrated that both PBK/TOPK and P38 are activated in a cell-cycle-dependent manner in neuronal progenitor cells in vitro, and inhibition of this pathway disrupts progenitor proliferation and self-renewal, a core feature of progenitors. In vivo, PBK/TOPK is expressed in rapidly proliferating cells in the adult subependymal zone (SEZ) and early postnatal cerebellar external granular layer. Using an approach based on transgenically targeted ablation and lineage tracing in mice, we show that PBK/TOPK-positive cells in the SEZ are GFAP negative but arise from GFAP-positive neural stem cells during adult neurogenesis. Furthermore, ablation of the adult stem cell population leads to concomitant loss of PBK/TOPK-positive cells in the SEZ. Together, these studies demonstrate that PBK/TOPK is a marker for transiently amplifying neural progenitors in the SEZ. Additionally, they suggest that PBK/TOPK plays an important role in these progenitors, and further implicates the P38 MAPK pathway in general, as an important regulator of progenitor proliferation and self-renewal.

Apoptosis in the mammalian CNS: Lessons from animal models

It is generally assumed that about half of the neurons produced during neurogenesis die before completion of maturation of the central nervous system (CNS). Neural cell death is also relevant in aging and several neurodegenerative diseases. Among the modalities by which neurons die, apoptosis has very much attracted the interest of investigators because in this type of cell death neurons are actively responsible for their own demise by switching on a number of genes and activating a series of specific intracellular pathways. This review focuses on the cellular and molecular mechanisms of apoptosis in normal and transgenic animal models related to naturally occurring neuronal death within the CNS. We will also consider some examples of apoptotic cell death in canine neuropathologies. A thorough analysis of naturally occurring neuronal death in vivo will offer a basis for parallel and future studies involving secondary neuronal loss such as those in neurodegenerative disorders, trauma or ischaemia.

Altered ultrasonic vocalization in mice with a disruption in the Foxp2 gene

Neurobiology of speech and language has previously been studied in the KE family, in which half of the members have severe impairment in both speech and language. The gene responsible for the phenotype was mapped to chromosome 7q31 and identified as the FOXP2 gene, coding for a transcription factor containing a polyglutamine tract and a forkhead DNA-binding domain. Because of linkage studies implicating 7q31 in autism, where language impairment is a component of the disorder, and in specific language impairment, FOXP2 has also been considered as a potential susceptibility locus for the language deficits in autism and/or specific language impairment. In this study, we characterized mice with a disruption in the murine Foxp2 gene. Disruption of both copies of the Foxp2 gene caused severe motor impairment, premature death, and an absence of ultrasonic vocalizations that are elicited when pups are removed from their mothers. Disruption of a single copy of the gene led to modest developmental delay but a significant alteration in ultrasonic vocalization in response to such separation. Learning and memory appear normal in the heterozygous animals. Cerebellar abnormalities were observed in mice with disruptions in Foxp2, with Purkinje cells particularly affected. Our findings support a role for Foxp2 in cerebellar development and in a developmental process that subsumes social communication functions in diverse organisms.

The cfy mutation disrupts cell divisions in a stage-dependent manner in zebrafish embryos

The zebrafish curly fry (cfy) mutation leads to embryonic lethality and abnormal cell divisions starting at 12-14 h postfertilization (hpf) during neural tube formation. The mitotic defect is seen in a variety of tissues including the central nervous system (CNS). In homozygous mutant embryos, mitoses are disorganized with an increase in mitotic figures throughout the developing neural tube. One consequence of aberrant mitoses in cfy embryos is an increase in cell death. Despite this, patterning of the early CNS is relatively unperturbed with distribution of the early, primary neurons indistinguishable from that of wild-type embryos. At later stages, however, the number of neurons was dramatically decreased throughout the CNS. The effect on neurons in older cfy embryos but not young ones correlates with the time of birth of neurons: primary neurons are born before the action of the cfy gene and later neurons after. Presumably, death of neuronal progenitors that divide beginning at the neural keel stage or death of their neuronal progeny accounts for the diminution of neurons in older mutant embryos. In addition, oligodendrocytes, which also develop late in the CNS, are greatly reduced in number in cfy embryos due to an apparent decrease in oligodendrocyte precursors. Genetic mosaic analysis demonstrates that the mutant phenotype is cell-autonomous. Furthermore, there are no obvious defects in apical/basal polarity within the neuroepithelium, suggesting that the cfy gene is not critical for epithelial polarity and that polarity defects are unlikely to account for the increased mitotic figures in mutants. These results suggest that the cfy gene regulates mitosis perhaps in a stage-dependent manner in vertebrate embryos.

Strategies to investigate gene expression and function in granule cells

Studying gene expression in granule cells is a major route to understanding the factors required for many key cellular processes such as specification, proliferation, migration, differentiation, apoptosis, tumour formation and neurodegeneration. A greater understanding of these processes will not only provide insight into cerebellum development, but also diseases of the cerebellum. Granule cells can be readily grown in culture and both viral and non-viral strategies have been optimised to allow gene transfer and expression in cultured cells. However, granule cell migration and maturation are inherent parts of cerebellum development and these rely on interactions with other cells. Hence, a true picture of gene function in these cells can only be obtained when tissue context is maintained. Studies of gene function in this context can be achieved by creation of mouse models. Conditional mouse models, where loss of gene expression is restricted as far as possible to granule cells, are by far the most informative resource in this respect. Despite their obvious benefits, the production of mouse models is both costly and time-consuming and this may be further compounded by a potential lack of phenotype due to redundancy of gene function. Organotypic slice cultures, on the other hand, are a comparatively cheap and accessible model for studies of gene function where tissue context is maintained. Recent technologies have provided the means to manipulate gene expression in such systems and are beginning to yield valuable insights into the molecular regulation of cerebellum development.

Very low intensity alternating current decreases cell proliferation

Electric fields impact cellular functions by activation of ion channels or by interfering with cell membrane integrity. Ion channels can regulate cell cycle and play a role in tumorigenesis. While the cell cycle may be directly altered by ion fluxes, exposure to direct electric current of sufficient intensity may decrease tumor burden by generating chemical products, including cytotoxic molecules or heat. We report that in the absence of thermal influences, low-frequency, low-intensity, alternating current (AC) directly affects cell proliferation without a significant deleterious contribution to cell survival. These effects were observed in normal human cells and in brain and prostate neoplasms, but not in lung cancer. The effects of AC stimulation required a permissive role for GIRK2 (or K(IR)3.2) potassium channels and were mimicked by raising extracellular potassium concentrations. Cell death could be achieved at higher AC frequencies (>75 Hz) or intensities (>8.5 microA); at lower frequencies/intensities, AC stimulation did not cause apoptotic cellular changes. Our findings implicate a role for transmembrane potassium fluxes via inward rectifier channels in the regulation of cell cycle. Brain stimulators currently used for the treatment of neurological disorders may thus also be used for the treatment of brain (or other) tumors.

Bromodeoxyuridine administered during neurogenesis of the projection neurons causes cerebellar defects in rat

Bromodeoxyuridine (BrdU) is broadly used in neuroscience to study embryonic development and adult neurogenesis. The potential toxicity of this halogenated pyrimidine analogue is frequently neglected. In this study, we administered BrdU in small doses by the progressively delayed cumulative labeling method to immunocytochemically tag different cerebellar cell types with antibodies to specific markers and BrdU in the same section. The well-known structure of the cerebellum made it possible to ascertain several toxic effects of the treatment. Time-pregnant rats were given five or six injections of 5 or 6 mg of BrdU ( approximately 12-20 mg/kg) at 8-hour intervals over 2 successive days between day 11 and 21 of pregnancy (E11-E12 to E20-E21), and the adult progeny was processed by immunocytochemistry. We demonstrate that this treatment effectively labeled distinct cerebellar cell populations but produced striking defects in the proliferation, migration, and settling of the Purkinje cells; reduced the size of the cerebellar cortex and nuclei; produced defects in the patterning of foliation; and also affected litter size, body weight, and mortality of the offspring. The observed toxic effects were consistent within individual treatment groups but varied between different treatment groups. Treatment with BrdU at the peak of neurogenesis of cerebellar projection neurons (E14) produced the most severe malformations. We observed no overt effects on the timing of neurogenesis for cerebellar neurons and glia across experimental groups. In conclusion, BrdU is a useful tool to study neural development, but its cytotoxicity represents a serious pitfall particularly when multiple doses are used to label cells.

Cell cycle regulation of neuronal apoptosis in development and disease

Apoptosis of neurons is indispensable to the normal development of the nervous system and contributes to neuronal loss in neurologic injury and disease. Life and death decisions are imposed upon neurons by extracellular and intracellular stimuli including the lack of trophic support, exposure to neurotoxins, oxidative stress, and DNA damage. These stimuli induce signaling pathways that are integrated at the mitochondrial apoptotic machinery culminating in cell survival or death. Growing evidence suggests that cell cycle proteins are expressed in dying neurons in the developing and adult brain. However, the role and mechanisms by which re-activation of cell cycle pathways in postmitotic neurons propagates an apoptotic signal to the cell death machinery are just beginning to be characterized. Here, we will review the molecular mechanisms of neuronal cell death and survival with a focus on recent findings on cell cycle regulation of neuronal apoptosis in primary cultures of neurons, mouse models of neuronal diseases, and human neurodegenerative diseases.

In vivo analysis reveals different apoptotic pathways in pre- and postmigratory cerebellar granule cells of rabbit

Naturally occurring neuronal death (NOND) has been described in the postnatal cerebellum of several species, mainly affecting the cerebellar granule cells (CGCs) by an apoptotic mechanism. However, little is known about the cellular pathway(s) of CGC apoptosis in vivo. By immunocytochemistry, in situ detection of fragmented DNA, electron microscopy, and Western blotting, we demonstrate here the existence of two different molecular mechanisms of apoptosis in the rabbit postnatal cerebellum. These two mechanisms affect CGCs at different stages of their maturation and migration. In the external granular layer, premigratory CGCs undergo apoptosis upon phosphorylation of checkpoint kinase 1 (Chk1), and hyperphosphorylation of retinoblastoma protein. In postmigratory CGCs within the internal granular layer, caspase 3 and to a lesser extent 7 and 9 are activated, eventually leading to poly-ADP-ribose polymerase-1 (PARP-1) cleavage and programmed cell death. We conclude that NOND of premigratory CGCs is linked to activation of DNA checkpoint and alteration of normal cell cycle, whereas in postmigratory CGCs apoptosis is, more classically, dependent upon caspase 3 activation.

Gene profiling in spinal cord injury shows role of cell cycle in neuronal death

Spinal cord injury causes secondary biochemical changes leading to neuronal cell death. To clarify the molecular basis of this delayed injury, we subjected rats to spinal cord injury and identified gene expression patterns by high-density oligonucleotide arrays (8,800 genes studied) at 30 minutes, 4 hours, 24 hours, or 7 days after injury (total of 26 U34A profiles). Detailed analyses were limited to 4,300 genes consistently expressed above background. Temporal clustering showed rapid expression of immediate early genes (30 minutes), followed by genes associated with inflammation, oxidative stress, DNA damage, and cell cycle (4 and 24 hours). Functional clustering showed a novel pattern of cell cycle mRNAs at 4 and 24 hours after trauma. Quantitative reverse transcription polymerase chain reaction verified mRNA changes in this group, which included gadd45a, c-myc, cyclin D1 and cdk4, pcna, cyclin G, Rb, and E2F5. Changes in their protein products were quantified by Western blot, and cell-specific expression was determined by immunocytochemistry. Cell cycle proteins showed an increased expression 24 hours after injury and were, in part, colocalized in neurons showing morphological evidence of apoptosis. These findings suggest that cell cycle-related genes, induced after spinal cord injury, are involved in neuronal damage and subsequent cell death.

In vivo cellular and molecular mechanisms of neuronal apoptosis in the mammalian CNS

Apoptosis has been recognized to be an essential process during neural development. It is generally assumed that about half of the neurons produced during neurogenesis die before completion of the central nervous system (CNS) maturation, and this process affects nearly all classes of neurons. In this review, we discuss the experimental data in vivo on naturally occurring neuronal death in normal, transgenic and mutant animals, with special attention to the cerebellum as a study model. The emerging picture is that of a dual wave of apoptotic cell death affecting central neurons at different stages of their life. The first wave consists of an early neuronal death of proliferating precursors and young postmitotic neuroblasts, and appears to be closely linked to cell cycle regulation. The second wave affects postmitotic neurons at later stages, and is much better understood in functional terms, mainly on the basis of the neurotrophic concept in its broader definition. The molecular machinery of late apoptotic death of postmitotic neurons more commonly follows the mitochondrial pathway of intracellular signal transduction, but the death receptor pathway may also be involved.Undoubtedly, analysis of naturally occurring neuronal death (NOND) in vivo will offer a basis for parallel and future studies aiming to elucidate the mechanisms of pathologic neuronal loss occurring as the result of conditions such as neurodegenerative disorders, trauma or ischemia.

Kir channels in the CNS: emerging new roles and implications for neurological diseases

Inwardly rectifying potassium (Kir) channels have long been regarded as transmembrane proteins that regulate the membrane potential of neurons and that are responsible for [K(+)] siphoning in glial cells. The subunit diversity within the Kir channel family is growing rapidly and this is reflected in the multitude of roles that Kir channels play in the central nervous system (CNS). Kir channels are known to control cell differentiation, modify CNS hormone secretion, modulate neurotransmitter release in the nigrostriatal system, may act as hypoxia-sensors and regulate cerebral artery dilatation. The increasing availability of genetic mouse models that express inactive Kir channel subunits has opened new insights into their role in developing and adult mammalian tissues and during the course of CNS disorders. New aspects with respect to the role of Kir channels during CNS cell differentiation and neurogenesis are also emerging. Dysfunction of Kir channels in animal models can lead to severe phenotypes ranging from early postnatal death to an increased susceptibility to develop epileptic seizures. In this review, we summarize the in vivo data that demonstrate the role of Kir channels in regulating morphogenetic events, such as the proliferation, differentiation and survival of neurons and glial cells. We describe the way in which the gating of Kir channel subunits plays an important role in polygenic CNS diseases, such as white matter disease, epilepsy and Parkinson's disease.

Essential role of citron kinase in cytokinesis of spermatogenic precursors

During spermatogenesis, the first morphological indication of spermatogonia differentiation is incomplete cytokinesis, followed by the assembly of stable intercellular cytoplasmic communications. This distinctive feature of differentiating male germ cells has been highly conserved during evolution, suggesting that regulation of the cytokinesis endgame is a crucial aspect of spermatogenesis. However, the molecular mechanisms underlying testis-specific regulation of cytokinesis are still largely unknown. Citron kinase is a myotonin-related protein acting downstream of the GTPase Rho in cytokinesis control. We previously reported that Citron kinase knockout mice are affected by a complex neurological syndrome caused by cytokinesis block and apoptosis of specific neuronal precursors. In this report we show that, in addition, these mice display a dramatic testicular impairment, with embryonic and postnatal loss of undifferentiated germ cells and complete absence of mature spermatocytes. By contrast, the ovaries of mutant females appear essentially normal. Developmental analysis revealed that the cellular depletion observed in mutant testes is caused by increased apoptosis of undifferentiated and differentiating precursors. The same cells display a severe cytokinesis defect, resulting in the production of multinucleated cells and apoptosis. Our data indicate that Citron kinase is specifically required for cytokinesis of the male germ line.

Effects of prenatal exposure to ethanol on the cyclin-dependent kinase system in the developing rat cerebellum

Prenatal exposure to ethanol inhibits neurogenesis in the developing cerebellum. Cyclin-dependent kinases (CDKs) are a family of protein kinases that play multiple roles in the regulation of cell proliferation, differentiation and survival. The activity of CDKs is positively regulated by CDK activators, cyclins, and negatively regulated by CDK inhibitors (CDKIs). We hypothesize that impaired cerebellar development induced by gestational ethanol exposure is mediated by disruption of the CDK system. Pregnant rats were fed ad libitum with an ethanol-containing liquid diet (Et) or pair-fed an isocaloric control diet (Ct). Cerebella were collected from pups (postnatal day (P) 0 through P21) and examined for CDK, cyclin, or CDKI expression using a quantitative immunoblotting procedure. In Ct-treated rats, the expression of CDK2 and its activator, cyclin A, paralleled the pattern of granule cell proliferation. Prenatal ethanol exposure produced a significant down-regulation of CDK2/cyclin A expression. Although the amounts of CDK4/CDK6 and their activator, cyclin D2, did not oscillate during postnatal development, their expression in Et-treated pups was significantly (P<0.05) higher than in controls. The expression of a CDK inhibitor, p27(Kip), was inversely correlated to proliferation of cerebellar granule progenitors. Prenatal ethanol exposure caused the down-regulation of p27(Kip) between P0 and P21. Thus, prenatal exposure to ethanol disturbed the expression of cell cycle machineries in the postnatal cerebellum. This may account for the teratogenic effects of ethanol on the developing cerebellum.

Inhibition of caspases protects cerebellar granule cells of the weaver mouse from apoptosis and improves behavioral phenotype

The homozygous mouse mutant weaver exhibits a massive loss of cerebellar granule neurons postnatally. The death of these cells is associated with a single amino acid mutation in the G protein-activated inwardly rectifying potassium channel, Girk2. Evidence suggests that both the mutated Girk2 channel and the calcium channel-associated N-methyl-d-aspartate receptor play important roles in the apoptotic death of weaver cerebellar granule cells, but the downstream events associated with this process are unknown. In this study, we demonstrate that the consequences of the mutation result in caspase activation. In addition, our results show that caspase inhibition in vivo decreases caspase activation and granule cell apoptosis and significantly improves behavioral deficits associated with the weaver's phenotype.

Molecular analysis of gene expression in the developing pontocerebellar projection system

As an approach toward understanding the molecular mechanisms of neuronal differentiation, we utilized DNA microarrays to elucidate global patterns of gene expression during pontocerebellar development. Through this analysis, we identified groups of genes specific to neuronal precursor cells, associated with axon outgrowth, and regulated in response to contact with synaptic target cells. In the cerebellum, we identified a phase of granule cell differentiation that is independent of interactions with other cerebellar cell types. Analysis of pontine gene expression revealed that distinct programs of gene expression, correlated with axon outgrowth and synapse formation, can be decoupled and are likely influenced by different cells in the cerebellar target environment. Our approach provides insight into the genetic programs underlying the differentiation of specific cell types in the pontocerebellar projection system.

The harlequin mouse mutation downregulates apoptosis-inducing factor

Harlequin (Hq) mutant mice have progressive degeneration of terminally differentiated cerebellar and retinal neurons. We have identified the Hq mutation as a proviral insertion in the apoptosis-inducing factor (Aif) gene, causing about an 80% reduction in AIF expression. Mutant cerebellar granule cells are susceptible to exogenous and endogenous peroxide-mediated apoptosis, but can be rescued by AIF expression. Overexpression of AIF in wild-type granule cells further decreases peroxide-mediated cell death, suggesting that AIF serves as a free radical scavenger. In agreement, dying neurons in aged Hq mutant mice show oxidative stress. In addition, neurons damaged by oxidative stress in both the cerebellum and retina of Hq mutant mice re-enter the cell cycle before undergoing apoptosis. Our results provide a genetic model of oxidative stress-mediated neurodegeneration and demonstrate a direct connection between cell cycle re-entry and oxidative stress in the ageing central nervous system.

Synapse-independent and synapse-dependent apoptosis of cerebellar granule cells in postnatal rabbits occur at two subsequent but partly overlapping developmental stages

It has long been known that cells in the external granular layer die during postnatal development of the cerebellum. More recent findings indicate that at certain developmental stages, cell death occurs upon activation of an apoptotic program. We show that cerebellar granule cells in rabbits undergo programmed cell death at two different stages of maturation. At postnatal day 5 (P5), granule cell precursors and pre-migratory granule cells in the external granular layer incorporate the S-phase markers 5-bromo-2'-deoxyuridine and 5-iodo-2'-deoxyuridine with a pattern that is dependent upon the interval between the administration of the two tracers. Within 12-24 h after proliferation a significant number of labeled cells show typical ultrastructural alterations of apoptosis. DNA electrophoresis and cleavage of poly-ADP-ribose polymerase confirm the activation of the apoptotic machinery. After Southern blotting and immunodetection, incorporated 5-bromo-2'-deoxyuridine is present at the level of low size DNA oligomers as soon as 12 h after cell division. Therefore, this apoptotic phase is intrinsic to external granular layer neurons and independent of synaptic interactions with targets.Apoptotic cells, although fewer in number, are detected also in the internal granular layer and tend to increase from P5 to P10. It seems unlikely that these cells undergo DNA fragmentation in the external granular layer and subsequently migrate to their final destination, considering the data on cell cycle kinetics and the rapid tissue clearance by the glia. Parallel fiber-Purkinje spine synapses are already present in the molecular layer at P5. Therefore, the post-migratory granule cells likely undergo apoptosis as a failure to make proper synaptic contacts in the forming molecular layer. We conclude that the massive apoptosis of pre-migratory cells likely has a role in regulating the size of this rapidly expanding population of pre-mitotic neurons. The less tumultuous cell death of post-mitotic granule cells in the internal granular layer appears to be linked to the formation of the mature synaptic circuitry of the developing cerebellar cortex.

Explant-induced reactivation of herpes simplex virus occurs in neurons expressing nuclear cdk2 and cdk4

Herpes simplex virus (HSV) establishes productive (lytic) infections in nonneuronal cells and nonproductive (latent) infections in neurons. It has been proposed that HSV establishes latency because quiescent neurons lack cellular factors required for productive infection. It has been further proposed that these putative factors are induced following neuronal stress, as a requirement for HSV reactivation. To date, the identity of these putative cellular factors remains unknown. We have demonstrated that cyclin-dependent kinase (cdk) 1, 2, or 7 is required for HSV replication in nonneuronal cells. Interestingly, cdks 1 and 2 are not expressed in quiescent neurons but can be induced in stressed neurons. Thus, cdks may be among the cellular proteins required for HSV reactivation whose neuronal expression is differentially regulated during stress. Herein, we determined that neuronal expression of nuclear cdk2, cdk4, and cyclins E and D2 (which activate cdks 2 and 4, respectively) was induced following explant cultivation, a stressful stimulus that induces HSV reactivation. In contrast, neuronal expression of cdk7 and cytoplasmic cdk4 decreased during explant cultivation, whereas cdk3 was detected in the same small percentage of neurons before and after explant cultivation and cdks 1, 5, and 6 were not detected in neuronal cell bodies. HSV-1 reactivated specifically in neurons expressing nuclear cdk2 and cdk4, and an inhibitor specific for cdk2 inhibited HSV-1 reactivation. We conclude that neuronal levels of cdk2 are among the factors that determine the outcome of HSV infections of neurons.

Stage-dependent activation of cell cycle and apoptosis mechanisms in the right ventricle by pressure overload

The molecular basis of the intrinsic vulnerability of the compliant right ventricle to chronic pressure overload is poorly understood. Extensive apoptosis, possibly coupled with aberrant cell cycle reentry, in response to unrestrained biomechanical stress may account for this phenotypic flaw. To address this issue we have studied changes in expression of the cell cycle and apoptosis regulators in the right ventricle following induction of pulmonary hypertension in the rat by injection of monocrotaline. Hypertrophy, apoptosis and cell cycle events, as well as expression of their regulator genes were documented during a period of 31 days. The hypertrophy index reached 127% at day 31. At the early stage both apoptosis and cell proliferation pathways were coincidentally activated. The level of cyclin A and E transcripts steadily increased, the labeling index was 4.8% at day 31, and expression of the caspase-3 gene peaked at day 14. Until day 21 execution of apoptosis was prevented, probably by a high level of Bcl-2. At this time point Bcl-2 collapsed, cyclin D1 was upregulated, the differentiation gatekeeper p27Kip1 was downregulated, pro-caspase-3 was activated and extensive apoptosis developed. These results indicate that the right ventricle is especially vulnerable to apoptotic pressure-dependent stimuli, and that the cell cycle and apoptosis pathways were co-activated in this experimental model.

Developmental mechanisms in the pathogenesis of neurodegenerative diseases

Cellular genes that are mutated in neurodegenerative diseases code for proteins that are expressed throughout neural development. Genetic analysis suggests that these genes are essential for a broad range of normal neurodevelopmental processes. The proteins they code for interact with numerous other cellular proteins that are components of signaling pathways involved in patterning of the neural tube and in regional specification of neuronal subtypes. Further, pathogenetic mutations of these genes can cause progressive, sublethal alterations in the cellular homeostasis of evolving regional neuronal subpopulations, culminating in late-onset cell death. Therefore, as a consequence of the disease mutations, targeted cell populations may retain molecular traces of abnormal interactions with disease-associated proteins by exhibiting changes in a spectrum of normal cellular functions and enhanced vulnerability to a host of environmental stressors. These observations suggest that the normal functions of these disease-associated proteins are to ensure the fidelity and integration of developmental events associated with the progressive elaboration of neuronal subtypes as well as the maintenance of mature neuronal populations during adult life. The ability to identify alterations within vulnerable neuronal precursors present in pre-symptomatic individuals prior to the onset of irrevocable cellular injury may help foster the development of effective therapeutic interventions using evolving pharmacologic, gene and stem cell technologies.

Activation of cell-cycle-associated proteins in neuronal death: a mandatory or dispensable path?

Cell-cycle-related proteins, such as cyclins or cyclin-dependent kinases, are re-expressed in neurons committed to death in response to a variety of insults, including excitotoxins, hypoxia and ischemia, loss of trophic support, or beta-amyloid peptide. In some of these conditions events that are typical of the mid-G1 phase, such as cyclin-dependent kinase 4/6 activation, are required for the induction of neuronal death. In other cases, the cycle must proceed further and recruit steps that are typical of the G1/S transition for death to occur. Finally, there are conditions in which cell-cycle proteins might be re-expressed, but do not contribute to neuronal death. We hypothesize that cell-cycle signaling becomes a mandatory component of neuronal demise when other mechanisms are not enough for neurons to reach the threshold for death. Under this scheme, the death threshold is set by the extent of DNA damage. Whenever the extent of DNA damage is below this threshold, a cell-cycle signaling becomes crucial for the induction of neuronal death through p53-dependent or -independent pathways.

Defective neurogenesis in citron kinase knockout mice by altered cytokinesis and massive apoptosis

Citron-kinase (Citron-K) has been proposed by in vitro studies as a crucial effector of Rho in regulation of cytokinesis. To further investigate in vivo its biologic functions, we have inactivated Citron-K gene in mice by homologous recombination. Citron-K-/- mice grow at slower rates, are severely ataxic, and die before adulthood as a consequence of fatal seizures. Their brains display defective neurogenesis, with depletion of specific neuronal populations. These abnormalities arise during development of the central nervous system due to altered cytokinesis and massive apoptosis. Our results indicate that Citron-K is essential for cytokinesis in vivo but only in specific neuronal precursors. Moreover, they suggest a novel molecular mechanism for a subset of human malformative syndromes of the CNS.

Is there a role for potassium channel openers in neuronal ion channel disorders?

Malfunction in ion channels, due to mutations in genes encoding channel proteins or the presence of autoantibodies, are increasing being implicated in causing disease conditions, termed channelopathies. Dysfunction of potassium (K(+)) channels has been associated with the pathophysiology of a number of neurological, as well as peripheral, disorders (e.g., episodic ataxia, epilepsy, neuromyotonia, Parkinson's disease, congenital deafness, long QT syndrome). K(+) channels, which demonstrate a high degree of diversity and ubiquity, are fundamental in the control of membrane depolarisation and cell excitability. A common feature of K(+) channelopathies is a reduction or loss of membrane potential repolarisation. The identification of K(+) channel subtype specific openers will allow the recovery of the mechanism(s) responsible for counteraction of uncontrolled cellular depolarisation. Synthetic agents that demonstrate K(+) channel opening properties are available for a variety of K(+) channel subtypes (e.g., K(ATP), BK(Ca), GIRK and M-channel). This study reviews the realistic therapeutic potential that may be gained in a broad spectrum of clinical conditions by K(+) channel openers. K(+) channel openers would therefore identify dysfunctional K(+) channel as therapeutic targets for clinical benefit, in addition being able to modulate normally functioning K(+) channels to gain clinical management of pathophysiological events irrespective of the cause.