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

Cell cycle inhibition without disruption of neurogenesis is a strategy for treatment of aberrant cell cycle diseases: an update

Since publishing our earlier report describing a strategy for the treatment of central nervous system (CNS) diseases by inhibiting the cell cycle and without disrupting neurogenesis (Liu et al. 2010), we now update and extend this strategy to applications in the treatment of cancers as well. Here, we put forth the concept of "aberrant cell cycle diseases" to include both cancer and CNS diseases, the two unrelated disease types on the surface, by focusing on a common mechanism in each aberrant cell cycle reentry. In this paper, we also summarize the pharmacological approaches that interfere with classical cell cycle molecules and mitogenic pathways to block the cell cycle of tumor cells (in treatment of cancer) as well as to block the cell cycle of neurons (in treatment of CNS diseases). Since cell cycle inhibition can also block proliferation of neural progenitor cells (NPCs) and thus impair brain neurogenesis leading to cognitive deficits, we propose that future strategies aimed at cell cycle inhibition in treatment of aberrant cell cycle diseases (i.e., cancers or CNS diseases) should be designed with consideration of the important side effects on normal neurogenesis and cognition.

Nemo-like kinase (NLK) involves in neuronal apoptosis after traumatic brain injury

Traumatic brain injury (TBI) consists of two phases: an immediate phase in which damage is caused as a direct result of the mechanical impact: and a late phase of altered biochemical events that results in delayed tissue damage and is therefore amenable to therapeutic treatment. Because the molecular mechanisms of delayed post-traumatic neuronal cell death are still poorly understood, we investigated whether nemo-like kinase (NLK), an evolutionarily conserved serine/threonine kinase involved in neuronal apoptosis following TBI. In the model of TBI, western blot analysis, double immunofluorescent staining and immunohistochemistry were used to analyze the role of NLK in the process. The results showed a significant down-regulation of NLK and a concomitant up-regulation of caspase-3 during the early stage of TBI. In the model of glutamate inducing PC12 apoptosis, we analyzed the effect of over-expression of NLK on the neuronal cell line PC12 apoptosis by cck-8, western blot and TUNEL assays. Together with previous reports. We hypothesize NLK was related to the down-regulation of caspase-3 expression after TBI, and such an event may be associated with neuronal apoptosis.

Cell cycle proteins in brain in mild cognitive impairment: insights into progression to Alzheimer disease

Recent studies have demonstrated the re-emergence of cell cycle proteins in brain as patients progress from the early stages of mild cognitive impairment (MCI) into Alzheimer's disease (AD). Oxidative stress markers present in AD have also been shown to be present in MCI brain suggesting that these events occur in early stages of the disease. The levels of key cell cycle proteins, such as CDK2, CDK5, cyclin G1, and BRAC1 have all been found to be elevated in MCI brain compared to age-matched control. Further, peptidyl prolyl cis-trans isomerase (Pin1), a protein that plays an important role in regulating the activity of key proteins, such as CDK5, GSK3-β, and PP2A that are involved in both the phosphorylation state of Tau and in the cell cycle, has been found to be oxidatively modified and downregulated in both AD and MCI brain. Hyperphosphorylation of Tau then results in synapse loss and the characteristic Tau aggregation as neurofibrillary tangles, an AD hallmark. In this review, we summarized the role of cell cycle dysregulation in the progression of disease from MCI to AD. Based on the current literature, it is tempting to speculate that a combination of oxidative stress and cell cycle dysfunction conceivably leads to neurodegeneration.

Early defect of transforming growth factor β1 formation in Huntington's disease

A defective expression or activity of neurotrophic factors, such as brain- and glial-derived neurotrophic factors, contributes to neuronal damage in Huntington’s disease (HD). Here, we focused on transforming growth factor-β (TGF-β₁), a pleiotropic cytokine with an established role in mechanisms of neuroprotection. Asymptomatic HD patients showed a reduction in TGF-β₁ levels in the peripheral blood, which was related to trinucleotide mutation length and glucose hypometabolism in the caudate nucleus. Immunohistochemical analysis in post-mortem brain tissues showed that TGF-β₁ was reduced in cortical neurons of asymptomatic and symptomatic HD patients. Both YAC128 and R6/2 HD mutant mice showed a reduced expression of TGF-β₁ in the cerebral cortex, localized in neurons, but not in astrocytes. We examined the pharmacological regulation of TGF-β₁ formation in asymptomatic R6/2 mice, where blood TGF-β₁ levels were also reduced. In these R6/2 mice, both the mGlu2/3 metabotropic glutamate receptor agonist, LY379268, and riluzole failed to increase TGF-β₁ formation in the cerebral cortex and corpus striatum, suggesting that a defect in the regulation of TGF-β₁ production is associated with HD. Accordingly, reduced TGF-β₁ mRNA and protein levels were found in cultured astrocytes transfected with mutated exon 1 of the human huntingtin gene, and in striatal knock-in cell lines expressing full-length huntingtin with an expanded glutamine repeat. Taken together, our data suggest that serum TGF-β₁ levels are potential biomarkers of HD development during the asymptomatic phase of the disease, and raise the possibility that strategies aimed at rescuing TGF-β₁ levels in the brain may influence the progression of HD.

Beta-amyloid increases the expression level of ATBF1 responsible for death in cultured cortical neurons

Background

Recently, several lines of evidence have shown the aberrant expression of cell-cycle-related proteins and tumor suppressor proteins in vulnerable neurons of the Alzheimer's disease (AD) brain and transgenic mouse models of AD; these proteins are associated with various paradigms of neuronal death. It has been reported that ATBF1 induces cell cycle arrest associated with neuronal differentiation in the developing rat brain, and that gene is one of the candidate tumor suppressor genes for prostate and breast cancers in whose cells overexpressed ATBF1 induces cell cycle arrest. However, the involvement of ATBF1 in AD pathogenesis is as yet unknown.

Results

We found that ATBF1 was up-regulated in the brains of 17-month-old Tg2576 mice compared with those of age-matched wild-type mice. Moreover, our in vitro studies showed that Aβ1-42 and DNA-damaging drugs, namely, etoposide and homocysteine, increased the expression ATBF1 level in primary rat cortical neurons, whereas the knockdown of ATBF1 in these neurons protected against neuronal death induced by Aβ1-42, etoposide, and homocysteine, indicating that ATBF1 mediates neuronal death in response to these substances. In addition, we found that ATBF1-mediated neuronal death is dependent on ataxia-telangiectasia mutated (ATM) because the blockage of ATM activity by treatment with ATM inhibitors, caffeine and KU55933, abolished ATBF1 function in neuronal death. Furthermore, Aβ1-42 phosphorylates ATM, and ATBF1 interacts with phosphorylated ATM.

Conclusions

To the best of our knowledge, this is the first report that Aβ1-42 and DNA-damaging drugs increased the ATBF1 expression level in primary rat cortical neurons; this increase, in turn, may activate ATM signaling responsible for neuronal death through the binding of ATBF1 to phosphorylated ATM. ATBF1 may therefore be a suitable target for therapeutic intervention of AD.

Differential effect of oxidative or excitotoxic stress on the transcriptional profile of amyotrophic lateral sclerosis-linked mutant SOD1 cultured neurons

Amyotrophic lateral sclerosis (ALS) is a progressive, lethal, degenerative disorder of motor neurons. The causes of most cases of ALS are as yet undefined. In a previous study, it was shown that N-methyl-D-aspartate (NMDA) and H(2)O(2) stimuli reduce neuronal survival in cortical neurons in culture (Boutahar et al., 2008). To identify variations in gene expression in response to these neurotoxins in transgenic vs. control cortical neurons cultures, both microarray and RT-PCR analysis were performed. High-density oligonucleotide microarrays showed changes in the expression of about 600 genes involved in protein degradation, neurotrophic factors pathway, cell cycle, inflammation, cytoskeleton, cell adhesion, transcription, or signalling. The most up-regulated genes following H(2)O(2) treatment were involved in cytoskeletal organization and axonal transport, such as ARAP2, KIF17, and DKK2, or in trophic factors pathways, such as insulin-like growth factor-binding protein 4 (IGFBP4), FGF17, and serpin2. The most down-regulated genes were involved in ion transport, such as TRPV1. After NMDA treatment, the most up-regulated genes were involved in protein degradation, such as ubiquitin-conjugating enzyme E2I and cathepsin H, and the most down-regulated genes were involved in ion transport, such as SCN7A. We conclude that these neurotoxins act through different transcriptional inductions, and these changes may reflect an adaptative cellular response to the cellular stress induced by the neurotoxins involved in ALS in the presence of mutant human SOD1.

Cell cycle activation and spinal cord injury

Traumatic spinal cord injury (SCI) evokes a complex cascade of events with initial mechanical damage leading to secondary injury processes that contribute to further tissue loss and functional impairment. Growing evidence suggests that the cell cycle is activated following SCI. Up-regulation of cell cycle proteins after injury appears to contribute not only to apoptotic cell death of postmitotic cells, including neurons and oligodendrocytes, but also to post-traumatic gliosis and microglial activation. Inhibition of key cell cycle regulatory pathways reduces injury-induced cell death, as well as microglial and astroglial proliferation both in vitro and in vivo. Treatment with cell cycle inhibitors in rodent SCI models prevents neuronal cell death and reduces inflammation, as well as the surrounding glial scar, resulting in markedly reduced lesion volumes and improved motor recovery. Here we review the effects of SCI on cell cycle pathways, as well as the therapeutic potential and mechanism of action of cell cycle inhibitors for this disorder.

Activation of ataxia telangiectasia muted under experimental models and human Parkinson's disease

In the present study we demonstrated that neurotoxin MPP(+)-induced DNA damage is followed by ataxia telangiectasia muted (ATM) activation either in cerebellar granule cells (CGC) or in B65 cell line. In CGC, the selective ATM inhibitor KU-55933 showed neuroprotective effects against MPP(+)-induced neuronal cell loss and apoptosis, lending support to the key role of ATM in experimental models of Parkinson's disease. Likewise, we showed that knockdown of ATM levels in neuroblastoma B65 cells using an ATM-specific siRNA attenuates the phosphorylation of retinoblastoma protein without affecting other cell-cycle proteins involved in the G(0)/G(1) cell-cycle phase. Moreover, we demonstrated DNA damage, in human brain samples of PD patients. These findings support a model in which MPP(+) leads to ATM activation with a subsequent DNA damage response and activation of pRb. Therefore, this study demonstrates a new link between DNA damage by MPP(+) and cell-cycle re-entry through retinoblastoma protein phosphorylation.

G93A SOD1 alters cell cycle in a cellular model of Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative multifactorial disease characterized, like other diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) or frontotemporal dementia (FTD), by the degeneration of specific neuronal cell populations. Motor neuron loss is distinctive of ALS. However, the causes of onset and progression of motor neuron death are still largely unknown. In about 2% of all cases, mutations in the gene encoding for the Cu/Zn superoxide dismutase (SOD1) are implicated in the disease. Several alterations in the expression or activation of cell cycle proteins have been described in the neurodegenerative diseases and related to cell death. In this work we show that mutant SOD1 can alter cell cycle in a cellular model of ALS. Our findings suggest that modifications in the cell cycle progression could be due to an increased interaction between mutant G93A SOD1 and Bcl-2 through the cyclins regulator p27. As previously described in post mitotic neurons, cell cycle alterations could fatally lead to cell death.

A relationship between p27(kip1) and Skp2 after adult brain injury: implications for glial proliferation

S-phase-associated kinase protein-2 (Skp2) is involved in ubiquitination and proteasome-mediated degradation of p27(kip1), which plays an important role in mammalian cell-cycle regulation and neurogenesis in the developing central nervous system. To investigate their expression and function in central nervous system injury and repair, we used a brain-penetrating injury model in adult rats. Western blot analysis showed a significant downregulation of p27(kip1) and a concomitant upregulation of Skp2 following brain injury, and their expression profiles were temporally correlative (r = -0.910, p = 0.037). Immunofluorescence double-labeling revealed that p27(kip1) was highly expressed in neurons (51%), astrocytes (72%), and microglia (76%) in the sham group, while its expression was decreased prominently in microglia (26%) and astrocytes (32%) at 3 days after injury. Meanwhile, Skp2 expression was very low in all cell types in the sham group; however, 3 days after injury, its expression was increased significantly in microglia (51%) and astrocytes (31%) (p < 0.001), and less significantly in neurons (8%) (p = 0.038), and the astrocytes and microglia had proliferated. We also examined the expression profiles of CDK2, threonine-187 phosphorylated p27(kip1), proliferating cell nuclear antigen (PCNA), and Ki67, and their changes correlated with the expression profiles of p27(kip1) and Skp2. Moreover, co-immunoprecipitation data suggested that the protein-protein interactions between p27(kip1) and Skp2 were enhanced after injury. Taken with results of previous reports, we hypothesize the Skp2 is related to the downregulation of p27(kip1) expression after brain injury, and such an event may be associated with glial proliferation, including that of astrocytes and microglia.

Cell cycle inhibition without disruption of neurogenesis is a strategy for treatment of central nervous system diseases

Classically, the cell cycle is regarded as the process leading to cellular proliferation. However, increasing evidence over the last decade supports the notion that neuronal cell cycle re-entry results in post-mitotic death. A mature neuron that re-enters the cell cycle can neither advance to a new G0 quiescent state nor revert to its earlier G0 state. This presents a critical dilemma to the neuron from which death may be an unavoidable but necessary outcome for adult neurons attempting to complete the cell cycle. In contrast, tumor cells that undergo aberrant cell cycle re-entry divide and can survive. Thus, cell cycle inhibition strategies are of interest in cancer treatment but may also represent an important means of protecting neurons. In this review, we put forth the concept of the "expanded cell cycle" and summarize the cell cycle proteins, signal transduction events and mitogenic molecules that can drive a neuron into the cell cycle in various CNS diseases. We also discuss the pharmacological approaches that interfere with the mitogenic pathways and prevent mature neurons from attempting cell cycle re-entry, protecting them from cell death. Lastly, future attempts at blocking the cell cycle to rescue mature neurons from injury should be designed so as to not block normal neurogenesis.

A sporadic Parkinson disease model via silencing of the ubiquitin-proteasome/E3 ligase component SKP1A

The aim of this study was to develop a new model of sporadic Parkinson disease (PD) based on silencing of the SKP1A gene, a component of the ubiquitin-proteasome/E3 ligase complex, Skp1, Cullin 1, F-box protein, which was found to be highly decreased in the substantia nigra of sporadic PD patients. Initially, an embryonic mouse substantia nigra-derived cell line (SN4741 cells) was infected with short hairpin RNA lentiviruses encoding the murine transcript of the SKP1A gene or with scrambled vector. SKP1A silencing resulted in increased susceptibility to neuronal damages induced by the parkinsonism-inducing neurotoxin 1-methyl-4-phenylpyridinium ion and serum starvation, in parallel with a decline in the expression of the dopaminergic markers, dopamine transporter and vesicular monoamine transporter-2. SKP1A-deficient cells presented a delay in completion of the cell cycle and the inability to arrest at the G(0)/G(1) phase when induced to differentiate. Instead, the cells progressed through S phase, developing rounded aggregates with characteristics of aggresomes including immunoreactivity for gamma-tubulin, alpha-synuclein, ubiquitin, tyrosine hydroxylase, Hsc-70 (70-kDa heat shock cognate protein), and proteasome subunit, and culminating in a lethal phenotype. Conversely, stably enforced expression of wild type SKP1A duplicated the survival index of naïve SN4741 cells under proteasomal inhibition injury, suggesting a new structural role of SKP1 in dopaminergic neuronal function, besides its E3 ligase activity. These results link, for the first time, SKP1 to dopamine neuronal function and survival, suggesting an essential role in sporadic PD. In summary, this new model has reproduced to a significant extent the molecular alterations described in sporadic PD at the cellular level, implicating Skp1 as a potential modifier in sporadic PD neurodegeneration.

Parkin protects dopaminergic neurons from excessive Wnt/beta-catenin signaling

Parkinson's disease (PD) is caused by degeneration of the dopaminergic (DA) neurons of the substantia nigra but the molecular mechanisms underlying the degenerative process remain elusive. Several reports suggest that cell cycle deregulation in post-mitotic neurons could lead to neuronal cell death. We now show that Parkin, an E3 ubiquitin ligase linked to familial PD, regulates beta-catenin protein levels in vivo. Stabilization of beta-catenin in differentiated primary ventral midbrain neurons results in increased levels of cyclin E and proliferation, followed by increased levels of cleaved PARP and loss of DA neurons. Wnt3a signaling also causes death of post-mitotic DA neurons in parkin null animals, suggesting that both increased stabilization and decreased degradation of beta-catenin results in DA cell death. These findings demonstrate a novel regulation of Wnt signaling by Parkin and suggest that Parkin protects DA neurons against excessive Wnt signaling and beta-catenin-induced cell death.

Gene and protein signatures in sporadic Parkinson's disease and a novel genetic model of PD

High-throughput gene-based platform studies in human post-mortem substantia nigra from sporadic Parkinson's disease (PD) cases have revealed significant dysregulation of genes involved in biological processes linked to previously established neurodegenerative mechanisms both in sporadic and hereditary PD. These include protein aggregation, mitochondrial dysfunction, oxidative stress, cell cycle, vesicle trafficking, synaptic transmission, dopamine metabolism and cell adhesion/cytoskeleton maintenance. These observations have extended our current view on the molecular pathways underlying the etio-pathology of the disease and provided a basis for the development of a novel genetic model of sporadic PD, centered on gradual silencing/over-expression of the candidate genes. The uncovered signatures may serve as future predictive biomarkers for early PD diagnosis, disease progression and drug development.

Molecular pathology of neuro-AIDS (CNS-HIV)

The cognitive deficits in patients with HIV profoundly affect the quality of life of people living with this disease and have often been linked to the neuro-inflammatory condition known as HIV encephalitis (HIVE). With the advent of more effective anti-retroviral therapies, HIVE has shifted from a sub-acute to a chronic condition. The neurodegenerative process in patients with HIVE is characterized by synaptic and dendritic damage to pyramidal neurons, loss of calbindin-immunoreactive interneurons and myelin loss. The mechanisms leading to neurodegeneration in HIVE might involve a variety of pathways, and several lines of investigation have found that interference with signaling factors mediating neuroprotection might play an important role. These signaling pathways include, among others, the GSK3beta, CDK5, ERK, Pyk2, p38 and JNK cascades. Of these, GSK3beta has been a primary focus of many previous studies showing that in infected patients, HIV proteins and neurotoxins secreted by immune-activated cells in the brain abnormally activate this pathway, which is otherwise regulated by growth factors such as FGF. Interestingly, modulation of the GSK3beta signaling pathway by FGF1 or GSK3beta inhibitors (lithium, valproic acid) is protective against HIV neurotoxicity, and several pilot clinical trials have demonstrated cognitive improvements in HIV patients treated with GSK3beta inhibitors. In addition to the GSK3beta pathway, the CDK5 pathway has recently been implicated as a mediator of neurotoxicity in HIV, and HIV proteins might activate this pathway and subsequently disrupt the diverse processes that CDK5 regulates, including synapse formation and plasticity and neurogenesis. Taken together, the GSK3beta and CDK5 signaling pathways are important regulators of neurotoxicity in HIV, and modulation of these factors might have therapeutic potential in the treatment of patients suffering from HIVE. In this context, the subsequent sections will focus on reviewing the involvement of the GSK3beta and CDK5 pathways in neurodegeneration in HIV.

Phosphorylation of ATM by Cdk5 mediates DNA damage signalling and regulates neuronal death

The phosphatidylinositol-3-kinase-like kinase ATM (Ataxia – telangiectasia mutated) plays a central role in coordinating the DNA damage responses including cell cycle checkpoint control, DNA repair, and apoptosis. Mutations of ATM cause a spectrum of defects ranging from neurodegeneration to cancer predisposition. However, the mechanism by which DNA damage activates ATM is poorly understood. We show that Cdk5 (cyclin-dependent kinase 5), activated by DNA damage, directly phosphorylates ATM at serine 794 in postmitotic neurons. Phosphorylation at serine 794 precedes and is required for ATM autophosphorylation at serine 1981, and activates ATM kinase activity. Cdk5-ATM signal regulates phosphorylation and function of ATM targets p53 and H2AX. Interruption of Cdk5-ATM pathway attenuates DNA damage-induced neuronal cell cycle reentry and expression of p53 targets PUMA and Bax, protecting neurons from DNA damage-induced death. Thus, activation of Cdk5 by DNA damage serves as a critical signal to initiate ATM response and regulate ATM-dependent cellular processes.

Swedish amyloid precursor protein mutation increases cell cycle-related proteins in vitro and in vivo

Reactivation of the cell cycle, including DNA replication, might play a major role in Alzheimer's disease. In this study, we report that the expressions of Swedish double mutation of amyloid precursor protein (Swe-APP) or of the APP intracellular domain (AICD) into nerve growth factor (NGF)-differentiated PC12 cells or rat primary cortical neurons increased mRNA and protein levels of cyclin D1 and cyclin B1. Treatment with lithium chloride (a glycogen synthase kinase-3beta inhibitor) down-regulated cyclin B1 induced by Swe-APP expression but up-regulated cyclin D1 expression induced by Swe-APP, suggesting that glycogen synthase kinase-3beta activity is involved in these expression changes of cyclins D1 and B1. Swe-APP, which is a prevailing cause of familial Alzheimer's disease, is well known to increase amyloid beta peptide production both in vitro and in vivo, but the underlying molecular means whereby it leads to the pathogenesis of AD remains unknown. The finding that cyclin D1 and B1 expressions were up-regulated by Swe-APP in in vitro cultured cells was substantiated in the brain tissues of Tg2576 mice, which harbor the Swe-APP mutation. These results suggest that some disturbances in cell cycle regulation may be involved in Swe-APP or AICD-induced neurodegeneration and that these contribute to the pathogenesis of AD.

HDAC4 inhibits cell-cycle progression and protects neurons from cell death

HDAC4 is a Class II histone deacetylase (HDAC) that is highly expressed in the brain, but whose functional significance in the brain is not known. We show that forced expression of HDAC4 in cerebellar granule neurons protects them against low potassium-induced apoptosis. HDAC4 also protects HT22 neuroblastoma cells from death induced by oxidative stress. HDAC4-mediated neuroprotection does not require its HDAC catalytic domain and cannot be inhibited by chemical inhibitors of HDACs. Neuroprotection by HDAC4 also does not require the Raf-MEK-ERK or the PI-3 kinase-Akt signaling pathways and occurs despite the activation of c-jun, an event that is generally believed to condemn neurons to die. The protective action of HDAC4 occurs in the nucleus and is mediated by a region that contains the nuclear localization signal. HDAC4 inhibits the activity of cyclin-dependent kinase-1 (CDK1) and the progression of proliferating HEK293T and HT22 cells through the cell cycle. Mice-lacking HDAC4 have elevated CDK1 activity and display cerebellar abnormalities including a progressive loss of Purkinje neurons postnatally in posterior lobes. Surviving Purkinje neurons in these lobes have duplicated soma. Furthermore, large numbers of cells within these affected lobes incorporate BrdU, indicating cell-cycle progression. These abnormalities along with the ability of HDAC4 to inhibit CDK1 and cell-cycle progression in cultured cells suggest that neuroprotection by HDAC4 is mediated by preventing abortive cell-cycle progression.

PACAP38 protects rat cortical neurons against the neurotoxicity evoked by sodium nitroprusside and thrombin

Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 is a multifunctional anti-inflammatory and anti-apoptotic neuropeptide widely distributed in the nervous system. The objective of this study is to determine whether PACAP38 is neuroprotective against sodium nitroprusside (SNP) and thrombin, two mechanistically distinct neurotoxic agents. Treatment of primary cortical neuronal cultures with 1 mM SNP for 4 h causes neuronal cell death that is significantly reduced by 100 nM PACAP38. PACAP38 down-regulates SNP-induced cell cycle protein (cyclin E) expression and up-regulates p57(KIP2), a cyclin-dependent kinase inhibitor as well as the anti-apoptotic protein Bcl-2. Similarly, neuronal death induced by 100 nM thrombin or the thrombin receptor activating peptide (TRAP 6) is reduced by PACAP38 treatment. Thrombin-stimulated cell cycle protein (cdk4) expression is decreased by PACAP38 while PACAP38 inhibits thrombin-mediated reduction of p57(KIP2). However, the decrease in Bcl-2 evoked by thrombin is not affected by PACAP38. Finally, both SNP and thrombin (or TRAP) increase caspase 3 activity, an effect that is decreased by PACAP38. These data show that PACAP38 supports neuronal survival in vitro suppressing cell cycle progression and enhancing anti-apoptotic proteins. Our results support the possibility that PACAP could be a useful therapeutic agent for reducing neuronal cell death in neurodegenerative diseases.

Mutations in String/CDC25 inhibit cell cycle re-entry and neurodegeneration in a Drosophila model of Ataxia telangiectasia

Mutations in ATM (Ataxia telangiectasia mutated) result in Ataxia telangiectasia (A-T), a disorder characterized by progressive neurodegeneration. Despite advances in understanding how ATM signals cell cycle arrest, DNA repair, and apoptosis in response to DNA damage, it remains unclear why loss of ATM causes degeneration of post-mitotic neurons and why the neurological phenotype of ATM-null individuals varies in severity. To address these issues, we generated a Drosophila model of A-T. RNAi knockdown of ATM in the eye caused progressive degeneration of adult neurons in the absence of exogenously induced DNA damage. Heterozygous mutations in select genes modified the neurodegeneration phenotype, suggesting that genetic background underlies variable neurodegeneration in A-T. The neuroprotective activity of ATM may be negatively regulated by deacetylation since mutations in a protein deacetylase gene, RPD3, suppressed neurodegeneration, and a human homolog of RPD3, histone deacetylase 2, bound ATM and abrogated ATM activation in cell culture. Moreover, knockdown of ATM in post-mitotic neurons caused cell cycle re-entry, and heterozygous mutations in the cell cycle activator gene String/CDC25 inhibited cell cycle re-entry and neurodegeneration. Thus, we hypothesize that ATM performs a cell cycle checkpoint function to protect post-mitotic neurons from degeneration and that cell cycle re-entry causes neurodegeneration in A-T.

Src kinase inhibition decreases thrombin-induced injury and cell cycle re-entry in striatal neurons

Since Src kinase inhibitors decrease brain injury produced by intracerebral hemorrhage (ICH) and thrombin is activated following ICH, this study determined whether Src kinase inhibitors decrease thrombin-induced brain injury. Thrombin injections into adult rat striatum produced focal infarction and motor deficits. The Src kinase inhibitor PP2 decreased thrombin-induced Src activation, infarction in striatum and motor deficits in vivo. Thrombin applied to cultured post-mitotic striatal neurons caused: injury to axons and dendrites; many TUNEL positive neuronal nuclei; and re-entry into the cell cycle as manifested by cyclin D1 expression, induction of several other cell cycle genes and cyclin-dependent kinase 4 activation. PP2 dose-dependently attenuated thrombin-induced injury to the cultured neurons; and attenuated thrombin-induced neuronal cell cycle re-entry. These results are consistent with the hypotheses that Src kinase inhibitors decrease injury produced by ICH by decreasing thrombin activation of Src kinases and, at least in part, by decreasing Src induced cell cycle re-entry.

Pathological apoptosis in the developing brain

More than half of the initially-formed neurons are deleted in certain brain regions during normal development. This process, whereby cells are discretely removed without interfering with the further development of remaining cells, is called programmed cell death (PCD). The term apoptosis is used to describe certain morphological manifestations of PCD. Many of the effectors of this developmental cell death program are highly expressed in the developing brain, making it more susceptible to accidental activation of the death machinery, e.g. following hypoxia-ischemia or irradiation. Recent evidence suggests, however, that activation and regulation of cell death mechanisms under pathological conditions do not exactly mirror physiological, developmentally regulated PCD. It may be argued that the conditions after e.g. ischemia are not even compatible with the execution of PCD as we know it. Under pathological conditions cells are exposed to various stressors, including energy failure, oxidative stress and unbalanced ion fluxes. This results in parallel triggering and potential overshooting of several different cell death pathways, which then interact with one another and result in complex patterns of biochemical manifestations and cellular morphological features. These types of cell death are here called "pathological apoptosis," where classical hallmarks of PCD, like pyknosis, nuclear condensation and caspase-3 activation, are combined with non-PCD features of cell death. Here we review our current knowledge of the mechanisms involved, with special focus on the potential for therapeutic intervention tailored to the needs of the developing brain.

Class IIA HDACs in the regulation of neurodegeneration

Neurodegenerative diseases affect millions of patients annually and are a significant burden on the health care systems around the world. While there are symptomatic remedies for patients suffering from various neurodegenerative diseases, there are no cures as of today. Cell death by apoptosis is a common hallmark of neurodegeneration. Therefore, deciphering the molecular pathways regulating this process is of significant value to scientists' endeavor to understand neurodegenerative disorders. Efforts along these lines have uncovered a number of molecular pathways that regulate neuronal apoptosis. Recently, a family of proteins known as histone deacetylases (HDACs) has been linked to regulation of cell survival as well as death. The focus of this review is to summarize our current understanding of the role of HDACs and in particular a subgroup of proteins in this family classified as class IIa HDACs in the regulation of neuronal cell death. It is apparent based on the information presented in this review that although very similar in their primary sequence, members of this family of proteins often have distinct roles in orchestrating apoptotic cell death in the brain.

The cell-selective neurotoxicity of the Alzheimer's Abeta peptide is determined by surface phosphatidylserine and cytosolic ATP levels. Membrane binding is required for Abeta toxicity

Measurement of Abeta toxicity of cells in culture exposes a subpopulation of cells with resistance to Abeta, even at high concentrations and after long periods of treatment. The cell-selective toxicity of Abeta resembles the selective damage observed in cells of specific regions of the Alzheimer's disease (AD) brain and suggests that there must be particular characteristics or stages of these cells that make them exceptionally sensitive or resistant to the effect of Abeta. Using flow cytometry and cell sorting, we efficiently separated and analyzed the Abeta-sensitive and the Abeta-resistant subpopulations within a variety of neuronal cell lines (PC12, GT1-7) and primary cultured neurons (hippocampal, cortex). We found that this distinctive sensitivity to Abeta was essentially associated with cell membrane Abeta binding. This selective Abeta binding was correlated to distinctive cell characteristics, such as cell membrane exposure of the apoptotic signal molecule phosphatidyl serine, larger cell size, the G1 cell cycle stage, and a lower than normal cytosolic ATP level. The response to Abeta by the cells with high Abeta binding affinity was characterized by a larger calcium response and increased mortality, lactate dehydrogenase release, caspase activation, and DNA fragmentation. The distinctive sensitivity or resistance to Abeta of the different subpopulations was maintained even after multiple cell divisions. We believe that these distinctive cell characteristics are the determining factors for the selective attack of Abeta on cells in culture and in the AD brain.

Sequential expression of cell-cycle regulators and Alzheimer's disease-related proteins in entorhinal cortex after hippocampal excitotoxic damage

Growing evidence suggests that one of the earliest events in the neuronal degeneration of Alzheimer's disease (AD) is aberrant cell-cycle activation in postmitotic neurons, which may, in fact, be sufficient to initiate the neurodegenerative cascade. In the present study we examined whether cyclins and cyclin-dependent kinases, molecules normally associated with cell-cycle control, may be involved in delayed expression of altered Alzheimer's proteins in two interconnected areas, the entorhinal cortex (EC) and the dentate gyrus (DG), after a hippocampal excitotoxic lesion. Several cell-cycle proteins of the G1 and S phases and even of the G2 phase were found to be up-regulated in the EC after kainic acid evoked neuronal death in the hippocampus. In addition, we describe the progressive expression of two Alzheimer's-related proteins, PHF-1 and APP, which reached higher levels immediately after the increase in G1/S-phase markers. Hence, the results of the present study support the participation of cell-cycle dysregulation as a key component of the process that may ultimately lead to expression of AD proteins and neuronal death in a brain area when the target site for synaptic inputs in that area is damaged by an excitotoxic insult.

Amyloid precursor protein and presenilin involvement in cell signaling

To date the most relevant role for the amyloid precursor protein (APP) and for the presenilins (PSs) on Alzheimer's disease (AD) genesis is linked to the 'amyloid hypothesis', which considers an aberrant formation of amyloid-beta peptides the cause of neurodegeneration. In this view, APP is merely a substrate, cleaved by the gamma-secretase complex to form toxic amyloid peptides, PSs are key players in gamma-secretase complex, and corollary or secondary events are Tau-linked pathology and gliosis. A second theory, complementary to the amyloid hypothesis, proposes that APP and PSs may modulate a yet unclear cell signal, the disruption of which may induce cell-cycle abnormalities, neuronal death, eventually amyloid formation and finally dementia. This hypothesis is supported by the presence of a complex network of proteins, with a clear relevance for signal transduction mechanisms, which interact with APP or PSs. In this scenario, the C-terminal domain of APP has a pivotal role due to the presence of the 682YENPTY687 motif that represents the docking site for multiple interacting proteins involved in cell signaling. In this review we discuss the significance of novel findings related to cell signaling events modulated by APP and PSs for AD development.

HMG-CoA reductase inhibitor simvastatin inhibits cell cycle progression at the G1/S checkpoint in immortalized lymphocytes from Alzheimer's disease patients independently of cholesterol-lowering effects

Recent work has suggested that statins may exert beneficial effects on patients suffering from Alzheimer's disease (AD). The pharmacological effects of statins extend beyond their cholesterol-lowering properties. Based on the antineoplastic and apoptotic effects of statins in several cell types, we hypothesized that statins may be able to protect neurons by controlling the regulation of cell cycle. A growing body of evidence indicates that neurodegeneration involves the activation of cell cycle machinery in postmitotic neurons. We and others have presented direct evidence to support the hypothesis that the failure of cell cycle control is not restricted to neurons in AD patients, but that it occurs in peripheral cells as well. For these reasons, we found it worthy to study the role of simvastatin on cell proliferation in immortalized lymphocytes from AD patients. We report here that simvastatin (SIM) inhibits the serum-mediated enhancement of cell proliferation in AD by blocking the events critical for G(1)/S transition. SIM induces a partial blockade of retinoblastoma protein phosphorylation and inhibition of cyclin E/cyclin-dependent kinase (CDK)2 activity associated with increased levels of the CDK inhibitors p21(Cip1) and p27(kip1). These effects of SIM on AD lymphoblasts are dependent on inhibition of the proteasome-mediated degradation of p21 and p27 proteins. The antiproliferative effect of this natural statin may provide a therapeutic approach for AD disease.

Protocols for studying adult neurogenesis: insights and recent developments

The first evidence that neurogenesis occurs in the adult brain was reported in rodents in the early 1960s, using [(3)H]-thymidine autoradiography. In the 1980s and 90s, the advent of new techniques and protocols for studying cell proliferation in situ, and particularly bromodeoxyuridine labeling, helped to confirm that neurogenesis occurs in the adult brain and neural stem cells reside in the adult CNS, including in humans. Bromodeoxyuridine labeling is currently the method most commonly used for studying neurogenesis in the adult brain. However, this procedure is not without limitations, and controversies. In this article, I will review recent protocols for studying adult neurogenesis, particularly new protocols for studying cell kinetics and cell proliferative history, using halopyrimidines. I will review these techniques, and discuss their implications for the field of adult neurogenesis.

Implication of the transcription factor E2F-1 in the modulation of neuronal apoptosis

Neurodegenerative diseases as Alzheimer's disease, Parkinson's disease and other neurological disorders remain major problem worldwide since is currently no effective treatment. Thus, studying the mechanisms involved in neuronal apoptotic pathways is imperative if drugs that might stop or delay these disease processes are to be synthesized. In recent years it has become evident that mitochondria are key component of the neuronal apoptotic route. In addition to mitochondria, other intracellular components have been implicated in this process. Thus, DNA damage and re-entry into the cell cycle may constitute a common pathway in apoptosis in neurological diseases. The implication of cell cycle in neurodegenerative disorders is supported by data on the brain of patients who showed an increase in cell cycle protein expression. Indeed, studies performed in neuronal cell preparations indicate that re-entry into the cell cycle and, more specifically, an increase in the expression of E2F-1 transcription role of DNA damage/repair as a potential mechanism in cell cycle re-entry. In this context, ataxia telangiectasia mutated protein could be the enzyme responsible for neuronal apoptosis activation. Furthermore, the potential routes involved in E2F-1 induced apoptosis, p53-dependent and p53-independent, are similarly reviewed. Under this hypothesis, multiple pathways have been suggested, including the route of caspases. Finally, given the increasing experimental data on the neuroprotective and antiapoptotic effects of cyclin dependent kinase CDK inhibitory drugs, including flavopiridol, their application for the treatment of neurological disorders is proposed.

Over-expression of amyloid precursor protein in HEK cells alters p53 conformational state and protects against doxorubicin

Here we show that human embryonic kidney (HEK) cells stably transfected with amyloid precursor protein (HEK-APP), expressed a conformational mutant-like and transcriptionally inactive p53 isoform, and turned out to be less sensitive to the cytotoxin doxorubicin in comparison with untransfected cells. Treatment of HEK-APP cells with gamma- and beta-secretase inhibitors prevented generation of unfolded, mutant-like p53 isoform and made the cells vulnerable to doxorubicin as untransfected cells. Changes in p53 conformational state and reduced sensitivity to doxorubicin were also found in untransfected HEK cells after exposure to nanomolar concentrations of beta-amyloid (Abeta) and these effects were antagonized by vitamin E. The modulator effects of Abeta on p53 conformational state were, at least in part, due to the intracellular peptides as (i) treatment of HEK-APP cells with an antibody that sequestered extracellular Abeta did not modify the capability of the cells to express the mutant-like p53 isoform; (ii) in the presence of 1% serum exogenous Abeta peptide crossed the plasma membrane, as demonstrated by confocal analysis and ELISA, and induced p53 conformational change; and (iii) in the presence of 10% serum Abeta did not enter the cells and consequently did not influence the p53 conformational state.

Association of Trp53 polymorphic variants at codon 72 with nonsyndromic mental retardation

Mental retardation is the most common developmental disability affecting 2-3% of the population, a consequence of a wide range of genetic or nongenetic etiologic factors. The cause of mental retardation remains unknown in about 50% of cases. Trp53 (transformation related protein 53, also known as p53) is a tumor suppressor gene that activates the expression of genes involved in inducing growth arrest of cells in response to multiple forms of cellular stress and it plays a significant role in apoptotic cell death during the early development of the nervous system. In this study, we examined 246 children with nonsyndromic mental retardation from three Italian populations and 213 healthy children from the same populations. We observed that the Pro72/Pro72 genotype of p53 is much less represented in children with nonsyndromic mental retardation than in controls (6.5% versus 14.08%) (OR=0.42; 95% CI 0.21-0.83). These data suggest that subjects carrying the Pro allele are protected from this disease.

Abeta treatment and P301L tau expression in an Alzheimer's disease tissue culture model act synergistically to promote aberrant cell cycle re-entry

Microarrays enable the observation of gene expression in experimental models of Alzheimer's disease (AD), with implications for the human pathology. Histopathologically, AD is characterized by Abeta-containing plaques and tau-containing neurofibrillary tangles. Here, we used a human SH-SY5Y neuroblastoma cell system to assess the role of P301L mutant human tau expression, and treatment with or without Abeta on gene regulation. We found that Abeta and P301L tau expression independently affect the regulation of genes controlling cell proliferation and synaptic elements. Moreover, Abeta and P301L tau act synergistically on cell cycle and DNA damage genes, yet influence specific genes within these categories. By using neuronally differentiated P301L tau cells, we can show that Abeta treatment induces an early upregulation of cell cycle control and synaptic genes. At the protein level, by using Kinetworks multi-immunoblotting and BrdU labelling, we found that although P301L tau and Abeta both affected levels of cell cycle proteins, their effects were distinct, in particular concerning DNA damage proteins. Moreover, DNA synthesis was observed only when SH-SY5Y cells overexpressed human wild-type or P301L tau and were incubated with Abeta. Thus, our study shows that Abeta treatment and human tau overexpression in an AD cell culture model act synergistically to promote aberrant cell cycle re-entry, supporting the mitosis failure hypothesis in AD.

The CDC2 I-G-T haplotype associated with the APOE ɛ4 allele increases the risk of sporadic Alzheimer's disease in Sicily

The cell division cycle 2 (CDC2) gene is a candidate susceptibility gene for Alzheimer's disease (AD). We investigated the CDC2 genotype, and allele and haplotype frequencies in AD patients and matched controls, distinguishing between apolipoprotein E (APOE) epsilon4 allele carriers and non-carriers. APOE epsilon4 is an established predictor of AD risk. APOE and CDC2 genotypes were examined in 109 sporadic AD patients and in 110 healthy age- and sex-matched controls from Sicily. The epsilon4 allele of APOE was predictive of AD risk in our study group (odds ratio: 5.37, 95% CI 2.77-10.41; P<0.0001). Genotype and allele frequencies of the three tested CDC2 polymorphisms (Ex6+7I/D, Ex7-15 G>A, Ex7-14 T>A) were not significantly different between AD patients and controls. However, a significant different distribution of a specific CDC2 haplotype (I-G-T) was found between AD patients and controls when analyzing APOE epsilon4-positive subjects (P=0.0288). Moreover, the combined presence of the I-G-T haplotype and the epsilon4 allele almost doubled the risk of AD (odds ratio: 10.09, 95% CI 3.88-26.25; P<0.0001) compared to carriers of epsilon4 alone. This study suggests that the I-G-T haplotype of the CDC2 gene increases the risk of AD in APOE epsilon4 carriers.

Transcription factor p53 in degenerating spinal cords

The causes of spinal cord cell loss in motor neuron disorders such as amyotrophic lateral sclerosis (ALS) are currently unknown. A role can be postulated for the transcription factor p53, which can induce apoptosis via upregulation of proapoptotic genes (e.g., Bax) and inhibition of antiapoptotic genes (e.g., Bcl-2). A model of motor neuron loss is the wobbler mouse that exhibits rapid motor neuron cell death as well as motor deficit from 21 days after birth. Affymetrix microarray data from wobbler mice demonstrate a 2.2-fold increase in p53 signal compared with their normal littermates, whereas qRT-PCR of RNA from laser capture microdissected ventral horns of normal and wobbler mice reveals a larger 6.6-fold increase in gene expression and this was supported by western blotting. Human ventral horns obtained from ALS and age-matched normal spinal cords also demonstrated an increase (2.7-fold) in p53 expression as determined by qRT-PCR. Evidence of a causative role for p53 in spinal cord cell death was provided by use of a p53 inhibitor, pifithrin-alpha, in organotypic slice cultures of mouse spinal cord. A 24-h pretreatment with pifithrin-alpha (and continuing in the presence of insult), significantly reduced the toxicity of a 48-h treatment with FeSO(4), tested with the MTT viability assay. These results indicate that p53 plays a functional role in oxidative stress-induced cell death and supports the possibility that elevated p53 could be involved in motor neuron death in ALS and the wobbler mouse.

Enhanced proteasome-dependent degradation of the CDK inhibitor p27(kip1) in immortalized lymphocytes from Alzheimer's dementia patients

Cyclin-dependent kinase inhibitor p27(kip1) (p27), a critical determinant for cell cycle progression, is an important regulation target of mitogenic signals. We have recently reported the existence of a molecular link between decreased p27 levels and enhanced phosphorylation of pRb protein and proliferation of immortalized lymphocytes from Alzheimer's disease (AD) patients. These cell cycle disturbances might be considered systemic manifestations, which mirror changes thought to occur in the brain, where post-mitotic neurons have been shown to display various cell cycle markers prior to degeneration. This work was undertaken to delineate the molecular mechanisms underlying the p27 down-regulation associated with AD. To this end, we evaluated the p27 protein stability in control and AD lymphoblasts. Half-life of p27 protein was markedly reduced in lymphoblasts from AD patients compared with that in control cells. The increased phosphorylation of p27 at Thr187, rather than changes in the 26S proteasome activity, is likely responsible for the enhanced degradation of p27 in AD cells. The serum-induced enhanced proliferation of AD lymphoblasts and decreased levels of p27 were abrogated by calmodulin (CaM) antagonists. The findings presented here suggest that Ca(2+)/CaM-dependent overactivation of PI3K/Akt signaling cascade in AD cells, plays an important role in regulating p27 abundance by increasing its degradation in the ubiquitin-proteasome pathway.