Cell-cycle markers in a transgenic mouse model of human tauopathy: increased levels of cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1

Alzheimer's disease and related tauopathies: disorders of disrupted neuronal identity

Postmitotic neurons require persistently active controls to maintain terminal differentiation. Unlike dividing cells, aberrant cell cycle activation in mature neurons causes apoptosis rather than transformation. In Alzheimer's disease (AD) and related tauopathies, evidence suggests that pathogenic forms of tau drive neurodegeneration via neuronal cell cycle re-entry. Multiple interconnected mechanisms linking tau to cell cycle activation have been identified, including, but not limited to, tau-induced overstabilization of the actin cytoskeleton, consequent changes to nuclear architecture, and disruption of heterochromatin-mediated gene silencing. Cancer- and development-associated pathways are upregulated in human and cellular models of tauopathy, and many tau-induced cellular phenotypes are also present in various cancers and progenitor/stem cells. In this review, I delve into mechanistic parallels between tauopathies, cancer, and development, and highlight the role of tau in cancer and in the developing brain. Based on these studies, I put forth a model by which pathogenic forms of tau disrupt the program that maintains terminal neuronal differentiation, driving cell cycle re-entry and consequent neuronal death. This framework presents tauopathies as conditions involving the profound toxic disruption of neuronal identity.

Melatonin as a Harmonizing Factor of Circadian Rhythms, Neuronal Cell Cycle and Neurogenesis: Additional Arguments for Its Therapeutic Use in Alzheimer's Disease

The synthesis and release of melatonin in the brain harmonize various physiological functions. The apparent decline in melatonin levels with advanced aging is an aperture to the neurodegenerative processes. It has been indicated that down regulation of melatonin leads to alterations of circadian rhythm components, which further causes a desynchronization of several genes and results in an increased susceptibility to develop neurodegenerative diseases. Additionally, as circadian rhythms and memory are intertwined, such rhythmic disturbances influence memory formation and recall. Besides, cell cycle events exhibit a remarkable oscillatory system, which is downstream of the circadian phenomena. The linkage between the molecular machinery of the cell cycle and complex fundamental regulatory proteins emphasizes the conjectural regulatory role of cell cycle components in neurodegenerative disorders such as Alzheimer's disease. Among the mechanisms intervening long before the signs of the disease appear, the disturbances of the circadian cycle, as well as the alteration of the machinery of the cell cycle and impaired neurogenesis, must hold our interest. Therefore, in the present review, we propose to discuss the underlying mechanisms of action of melatonin in regulating the circadian rhythm, cell cycle components and adult neurogenesis in the context of AD pathogenesis with the view that it might further assist to identify new therapeutic targets.

The Cellular Senescence Stress Response in Post-Mitotic Brain Cells: Cell Survival at the Expense of Tissue Degeneration

In 1960, Rita Levi-Montalcini and Barbara Booker made an observation that transformed neuroscience: as neurons mature, they become apoptosis resistant. The following year Leonard Hayflick and Paul Moorhead described a stable replicative arrest of cells in vitro, termed "senescence". For nearly 60 years, the cell biology fields of neuroscience and senescence ran in parallel, each separately defining phenotypes and uncovering molecular mediators to explain the 1960s observations of their founding mothers and fathers, respectively. During this time neuroscientists have consistently observed the remarkable ability of neurons to survive. Despite residing in environments of chronic inflammation and degeneration, as occurs in numerous neurodegenerative diseases, often times the neurons with highest levels of pathology resist death. Similarly, cellular senescence (hereon referred to simply as "senescence") now is recognized as a complex stress response that culminates with a change in cell fate. Instead of reacting to cellular/DNA damage by proliferation or apoptosis, senescent cells survive in a stable cell cycle arrest. Senescent cells simultaneously contribute to chronic tissue degeneration by secreting deleterious molecules that negatively impact surrounding cells. These fields have finally collided. Neuroscientists have begun applying concepts of senescence to the brain, including post-mitotic cells. This initially presented conceptual challenges to senescence cell biologists. Nonetheless, efforts to understand senescence in the context of brain aging and neurodegenerative disease and injury emerged and are advancing the field. The present review uses pre-defined criteria to evaluate evidence for post-mitotic brain cell senescence. A closer interaction between neuro and senescent cell biologists has potential to advance both disciplines and explain fundamental questions that have plagued their fields for decades.

Evidence and perspectives of cell senescence in neurodegenerative diseases

Increased life expectancies have significantly increased the number of individuals suffering from geriatric neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD). The financial cost for current and future patients with these diseases is overwhelming, resulting in substantial economic and societal costs. Unfortunately, most recent high-profile clinical trials for neurodegenerative diseases have failed to obtain efficacious results, indicating that novel approaches are desperately needed to treat these pathologies. Cell senescence, characterized by permanent cell cycle arrest, resistance to apoptosis, mitochondrial alterations, and secretion of senescence-associated secretory phenotype (SASP) components, has been extensively studied in mitotic cells such as fibroblasts, which is considered a hallmark of aging. Furthermore, multiple cell types in the senescent state in the brain, including neurons, microglia, astrocytes, and neural stem cells, have recently been observed in the context of neurodegenerative diseases, suggesting that these senescent cells may play an essential role in the pathological processes of neurodegenerative diseases. Therefore, this review begins by outlining key aspects of cell senescence constitution followed by examining the evidence implicating senescent cells in neurodegenerative diseases. In the final section, we review how cell senescence may be targeted as novel therapeutics to treat pathologies associated with neurodegenerative diseases.

Aberrant Accumulation of BRCA1 in Alzheimer Disease and Other Tauopathies

BRCA1 plays an important roles in several biological events during the DNA damage response (DDR). Recently, some reports have indicated that BRCA1 dysfunction is involved in the pathogenesis of Alzheimer disease (AD). Furthermore, it has also been reported that BRCA1 accumulates within neurofibrillary tangles (NFTs) in the AD brain. In this study, we examined the immunohistochemical distribution of BRCA1 and another DDR protein, p53-Binding Protein 1 (53BP1), in AD, Pick disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration, and frontotemporal dementia with parkinsonism linked to chromosome 17. In control subjects, neither BRCA1 nor phosphorylated BRCA1 (pBRCA1; Ser1524) immunoreactivity was observed in neurons or glial cells; and that for pBRCA1 (Ser1423) and 53BP1 were slightly detected in neuronal nuclei. The immunoreactivity for both BRCA1 and pBRCA1 (Ser1423) was localized within phosphorylated tau inclusions in all tauopathies, whereas that for pBRCA1 (Ser1524) was mainly associated with Pick bodies in PiD and to a lesser extent with NFTs in AD. On the other hand, 53BP1-immunoreactive deposits tended to be increased in the nucleus of neurons in AD and PSP compared with those in control cases. Our results suggest that DDR dysfunction due to cytoplasmic sequestration of BRCA1 could be involved in the pathogenesis of tauopathies.

Role of tumor suppressor molecules in genomic perturbations and damaged DNA repair involved in the pathogenesis of cancer and neurodegeneration (Review)

Genomic perturbations due to inaccurate DNA replication, including inappropriate chromosomal segregation often underlie the development of cancer and neurodegenerative diseases. The incidence of these two diseases increases with age and exhibits an inverse association. Therefore, elderly subjects with cancer exhibit a reduced risk of a neurodegenerative disease, and vice versa. Both of these diseases are associated with aging and share several risk factors. Cells have multiple mechanisms to repair DNA damage and inaccurate replication. Previous studies have demonstrated that tumor suppressor proteins serve a critical role in the DNA damage response, which may result in genomic instability and thus induction of cellular apoptosis. Tumor suppressor genes, such as phosphatase and tensin homologue deleted on chromosome 10 (PTEN), breast cancer susceptibility gene 1 (BRCA1) and TP53 reduce genomic susceptibility to cancer by repairing the damaged DNA. In addition, these genes work cooperatively to ensure the inhibition of the development of several types of cancer. PTEN, BRCA1 and TP53 have been recognized as the most frequently deleted and/or mutated genes in various types of human cancer. Recently, tumor suppressor genes have also been shown to be involved in the development of neurodegenerative diseases. The present review summarizes the recent findings of the functions of these tumor suppressors that are associated with genomic stability, and are involved in carcinogenic and neurodegenerative cell signaling. A summary is presented regarding the interactions of these tumor suppressors with their partners which results in transduction of downstream signals. The implications of these functions for cancer and neurodegenerative disease-associated biology are also highlighted.

Role of miRNAs in Alzheimer's Disease and Possible Fields of Application

miRNAs (or microRNAs) are a class of single-stranded RNA molecules, responsible for post-transcriptional gene silencing through binding to the coding region as well as 3' and 5' untranslated region of target genes. About 70% of experimentally detectable miRNAs are expressed in the brain and some studies suggest that miRNAs are intimately involved in synaptic function and in specific signals during memory formation. More and more evidence demonstrates the possible involvement of miRNAs in Alzheimer's disease (AD). AD is the most common form of senile dementia, a disease that affects memory and cognitive functions. It is a neurodegenerative disorder characterized by loss of synapses, extracellular amyloid plaques composed of the amyloid-β peptide (Aβ), and intracellular aggregates of hyperphosphorylated TAU protein. This review aims to provide an overview of the in vivo studies of the last 5 years in the literature describing the role of the different miRNAs involved in AD. miRNAs hold huge potential as diagnostic and prognostic biomarkers and, at the same time, their modulation could be a potential therapeutic strategy against AD.

Unknown fates of (brain) oxidation or UFO: Close encounters with neuronal senescence

Oxidative stress has long been considered a key component contributing to pathologies associated with brain aging and age-related neurodegenerative diseases. The proposed mechanisms involved are varied, but recently have been suggested to include induction of cellular senescence, a cellular growth arrest state characterized by the secretion of pre-inflammatory senescence-associated secretory phenotype (SASP) factors. The post-mitotic status of neurons has been traditionally considered to prohibit cellular senescence, however recent studies have provided compelling evidence that neurons may be capable of undergoing senescence in response to oxidative stress and other factors. Development of senolytics, small molecules that selectively induce senescent cell death, could represent a paradigm change for the treatment of neurodegenerative diseases including Alzheimer's and Parkinson's disease (AD, PD). However, their use depends on unequivocal validation that neurons can senesce and that they do not have detrimental off-target effects in other cell types in the brain and elsewhere.

Aberrant Neuronal Cell Cycle Re-Entry: The Pathological Confluence of Alzheimer's Disease and Brain Insulin Resistance, and Its Relation to Cancer

Aberrant neuronal cell cycle re-entry (CCR) is a phenomenon that precedes and may mechanistically lead to a majority of the neuronal loss observed in Alzheimer's disease (AD). Recent developments concerning the regulation of aberrant neuronal CCR in AD suggest that there are potential intracellular signaling "hotspots" in AD, cancer, and brain insulin resistance, the latter of which is characteristically associated with AD. Critically, these common signaling nodes across different human diseases may represent currently untapped therapeutic opportunities for AD. Specifically, repurposing of existing US Food and Drug Administration-approved pharmacological agents, including experimental therapeutics that target the cell cycle in cancer, may be an innovative avenue for future AD-directed drug discovery and development. In this review we discuss overlapping aspects of AD, cancer, and brain insulin resistance from the perspective of neuronal CCR, and consider strategies to exploit them for prevention or therapeutic intervention of AD.

Ageing, Cellular Senescence and Neurodegenerative Disease

Ageing is a major risk factor for developing many neurodegenerative diseases. Cellular senescence is a homeostatic biological process that has a key role in driving ageing. There is evidence that senescent cells accumulate in the nervous system with ageing and neurodegenerative disease and may predispose a person to the appearance of a neurodegenerative condition or may aggravate its course. Research into senescence has long been hindered by its variable and cell-type specific features and the lack of a universal marker to unequivocally detect senescent cells. Recent advances in senescence markers and genetically modified animal models have boosted our knowledge on the role of cellular senescence in ageing and age-related disease. The aim now is to fully elucidate its role in neurodegeneration in order to efficiently and safely exploit cellular senescence as a therapeutic target. Here, we review evidence of cellular senescence in neurons and glial cells and we discuss its putative role in Alzheimer's disease, Parkinson's disease and multiple sclerosis and we provide, for the first time, evidence of senescence in neurons and glia in multiple sclerosis, using the novel GL13 lipofuscin stain as a marker of cellular senescence.

Adenylyl cyclase activating polypeptide reduces phosphorylation and toxicity of the polyglutamine-expanded androgen receptor in spinobulbar muscular atrophy

Spinobulbar muscular atrophy (SBMA) is an X-linked neuromuscular disease caused by polyglutamine (polyQ) expansion in the androgen receptor (AR) gene. SBMA belongs to the family of polyQ diseases, which are fatal neurodegenerative disorders mainly caused by protein-mediated toxic gain-of-function mechanisms and characterized by deposition of misfolded proteins in the form of aggregates. The neurotoxicity of the polyQ proteins can be modified by phosphorylation at specific sites, thereby providing the rationale for the development of disease-specific treatments. We sought to identify signaling pathways that modulate polyQ-AR phosphorylation for therapy development. We report that cyclin-dependent kinase 2 (CDK2) phosphorylates polyQ-AR specifically at Ser96 Phosphorylation of polyQ-AR by CDK2 increased protein stabilization and toxicity and is negatively regulated by the adenylyl cyclase (AC)/protein kinase A (PKA) signaling pathway. To translate these findings into therapy, we developed an analog of pituitary adenylyl cyclase activating polypeptide (PACAP), a potent activator of the AC/PKA pathway. Chronic intranasal administration of the PACAP analog to knock-in SBMA mice reduced Ser96 phosphorylation, promoted polyQ-AR degradation, and ameliorated disease outcome. These results provide proof of principle that noninvasive therapy based on the use of PACAP analogs is a therapeutic option for SBMA.

Downregulated microRNA-222 is correlated with increased p27Kip¹ expression in a double transgenic mouse model of Alzheimer's disease

MicroRNAs (miRNAs) are a group of endogenous non-coding small RNAs that regulate protein expression by binding to the 3' untranslated region (UTR) of target genes. miRNAs are abundantly expressed in the central nervous system and participate in neuronal differentiation and synaptic plasticity. However, the possible roles and associated target genes of miRNAs in Alzheimer's disease (AD) are largely unknown. In the current study, miR‑222 was observed to be downregulated in APPswe/PSΔE9 mice (a model for AD) compared with age‑matched controls. Furthermore, the downregulation of miR‑222 was correlated with increased p27Kip1 protein levels. Bioinformatic analysis showed that there was one highly‑conserved putative binding site for miR‑222 in the 3'‑UTR of p27Kip1. Luciferase reporter assays confirmed that p27Kip1 was a direct target of miR‑222. Consistently, there was an inverse correlation between p27Kip1 and miR‑222 expression levels in SH‑SY5Y cells. In conclusion, these results suggest that the abnormal expression of miR‑222 may contribute to dysregulation of the cell‑cycle in AD, at least in part by affecting the expression of p27Kip1.

Dysregulated expression of lipid storage and membrane dynamics factors in Tia1 knockout mouse nervous tissue

During cell stress, the transcription and translation of immediate early genes are prioritized, while most other messenger RNAs (mRNAs) are stored away in stress granules or degraded in processing bodies (P-bodies). TIA-1 is an mRNA-binding protein that needs to translocate from the nucleus to seed the formation of stress granules in the cytoplasm. Because other stress granule components such as TDP-43, FUS, ATXN2, SMN, MAPT, HNRNPA2B1, and HNRNPA1 are crucial for the motor neuron diseases amyotrophic lateral sclerosis (ALS)/spinal muscular atrophy (SMA) and for the frontotemporal dementia (FTD), here we studied mouse nervous tissue to identify mRNAs with selective dependence on Tia1 deletion. Transcriptome profiling with oligonucleotide microarrays in comparison of spinal cord and cerebellum, together with independent validation in quantitative reverse transcriptase PCR and immunoblots demonstrated several strong and consistent dysregulations. In agreement with previously reported TIA1 knock down effects, cell cycle and apoptosis regulators were affected markedly with expression changes up to +2-fold, exhibiting increased levels for Cdkn1a, Ccnf, and Tprkb vs. decreased levels for Bid and Inca1 transcripts. Novel and surprisingly strong expression alterations were detected for fat storage and membrane trafficking factors, with prominent +3-fold upregulations of Plin4, Wdfy1, Tbc1d24, and Pnpla2 vs. a −2.4-fold downregulation of Cntn4 transcript, encoding an axonal membrane adhesion factor with established haploinsufficiency. In comparison, subtle effects on the RNA processing machinery included up to 1.2-fold upregulations of Dcp1b and Tial1. The effect on lipid dynamics factors is noteworthy, since also the gene deletion of Tardbp (encoding TDP-43) and Atxn2 led to fat metabolism phenotypes in mouse. In conclusion, genetic ablation of the stress granule nucleator TIA-1 has a novel major effect on mRNAs encoding lipid homeostasis factors in the brain, similar to the fasting effect.

Expression of frontotemporal dementia with parkinsonism associated to chromosome 17 tau induces specific degeneration of the ventral dentate gyrus and depressive-like behavior in mice

When bearing certain frontotemporal dementia with parkinsonism (FTDP) mutations, overexpression of human tau resulted in a decrease of the dentate gyrus ventral blade, apparently due to a reduction in the proliferation of neuronal precursors and an increase in neuronal cell death. This degenerative process was accompanied by a dramatic increase in behavioral despair, as evident in the Porsolt swim test. Interestingly, we observed an increase in GABAergic innervation in the molecular layer of the dorsal dentate gyrus but not in the ventral domain. We suggest that this increase in GABAergic innervation reflects a compensatory neuroprotective response to the overexpression of toxic tau, which may prevent or delay degeneration in the dorsal blade of the dental gyrus. Finally, we suggest that this transgenic mouse, which overexpresses human FTPD tau, may serve as a useful model to study specific functions of the ventral dentate gyrus.

Transcriptome analysis of a tau overexpression model in rats implicates an early pro-inflammatory response

Neurofibrillary tangles comprised of the microtubule-associated protein tau are pathological features of Alzheimer's disease and several other neurodegenerative diseases, such as progressive supranuclear palsy. We previously overexpressed tau in the substantia nigra of rats and mimicked some of the neurodegenerative sequelae that occur in humans such as tangle formation, loss of dopamine neurons, and microgliosis. To study molecular changes involved in the tau-induced disease state, we used DNA microarrays at an early stage of the disease process. A range of adeno-associated virus (AAV9) vector doses for tau were injected in groups of rats with a survival interval of 2 weeks. Specific decreases in messages for dopamine-related genes validated the technique with respect to the dopaminergic cell loss observed. Of the mRNAs upregulated, there was a dose-dependent effect on multiple genes involved in immune response such as chemokines, interferon-inducible genes and leukocyte markers, only in the tau vector groups and not in dose-matched controls of either transgene-less empty vector or control green fluorescent protein vector. Histological staining for dopamine neurons and microglia matched the loss of dopaminergic markers and upregulation of immune response mRNAs in the microarray data, respectively. RT-PCR for selected markers confirmed the microarray results, with similar changes found by either technique. The mRNA data correlate well with previous findings, and underscore microgliosis and immune response in the degenerative process following tau overexpression.

Tau potentiates nerve growth factor-induced mitogen-activated protein kinase signaling and neurite initiation without a requirement for microtubule binding

Microtubule-associated protein Tau is known to bind to and stabilize microtubules, thereby regulating microtubule dynamics. However, recent evidence has indicated that Tau can also interact with various components of intracellular signaling pathways, leading to the possibility that Tau might have a role in signal transduction. Here we provide evidence that during growth factor stimulation of neuronal cells, Tau has functions in advance of the neurite elongation stage. Using Tau-depleted neuronal cell lines, we demonstrate that Tau is required for neurite initiation in a manner that does not involve its microtubule binding function. In addition, we demonstrate that Tau potentiates AP-1 transcription factor activation in response to nerve growth factor (NGF). The effect of Tau on AP-1 activation is mediated through its ability to potentiate the activation of mitogen-activated protein kinase (MAPK), which occurs in response to both NGF and epidermal growth factor. Phosphorylation of Tau at Thr-231 also occurs in response to NGF and is required for Tau to impact on MAPK signaling, whereas the ability of Tau to bind to microtubules is not required. Together, these findings indicate a new functional role for Tau in early neuronal development independent of its established role in microtubule stabilization.

Misfolded tau protein and disease modifying pathways in transgenic rodent models of human tauopathies

Human tauopathies represent a heterogeneous group of neurodegenerative disorders such as Alzheimer's disease (AD) that are characterized by the presence of intracellular accumulations of abnormal filaments of protein tau. Presently, AD poses an increasing public health concern, because it affects nearly 2% of the population in industrialized countries and the number of patients is expected to increase threefold within the next 50 years. Therefore, the identification of disease modifying pathways that will lead to the development of novel therapeutic approaches targeting downstream molecular events of the tauopathy is of paramount importance. In order to identify factors that may exacerbate or inhibit the disease phenotype a number of genetically modified rodent models reproducing key clinical, histopathological and molecular hallmarks of human tauopathies were developed. Current tau transgenic rodent models express as a transgene either an individual or all six human wild-type tau isoforms, mutant tau linked to FTDP-17, or structurally modified tau species derived from AD. In this review we will provide an up-to-date account of various facets of the tau neurodegenerative cascade with a special emphasis on the evolution of neurofibrillary tangles, neuronal death and neuroinflammation.

Neurogenesis and cell cycle-reactivated neuronal death during pathogenic tau aggregation

The aim of the present study was to investigate the relation between neurogenesis, cell cycle reactivation and neuronal death during tau pathology in a novel tau transgenic mouse line THY-Tau22 with two frontotemporal dementia with parkinsonism linked to chromosome-17 mutations in a human tau isoform. This mouse displays all Alzheimer disease features of neurodegeneration and a broad timely resolution of tau pathology with hyperphosphorylation of tau at younger age (up to 6 months) and abnormal tau phosphorylation and tau aggregation in aged mice (by 10 months). Here, we present a follow-up of cell cycle markers with aging in control and transgenic mice from different ages. We show that there is an increased neurogenesis during tau hyperphosphorylation and cell cycle events during abnormal tau phosphorylation and tau aggregation preceding neuronal death and neurodegeneration. However, besides phosphorylation, other mechanisms including tau mutations and changes in tau expression and/or splicing may be also involved in these mechanisms of cell cycle reactivation. Altogether, these data suggest that cell cycle events in THY-Tau22 are resulting from neurogenesis in young animals and cell death in older ones. It suggests that neuronal cell death in such models is much more complex than believed.

Analysis of tau phosphorylation and truncation in a mouse model of human tauopathy

Recent evidence has suggested that truncation of tau protein at the caspase cleavage site D421 precedes hyperphosphorylation and may be necessary for the assembly of tau into filaments in Alzheimer's disease and other tauopathies. Here we have investigated the time course of the appearance of phosphorylated and truncated tau in the brain and spinal cord of mice transgenic for mutant human P301S tau protein. This mouse line recapitulates the essential molecular and cellular features of the human tauopathies, including tau hyperphosphorylation, tau filament formation, and neurodegeneration. Soluble tau was strongly phosphorylated at 1 to 6 months of age. Low levels of phosphorylated, sarkosyl-insoluble tau were detected at 2 months, with a steady increase up to 6 months of age. Tau truncated at D421 was detected at low levels in Tris-soluble and detergent-soluble tau at 3 to 6 months of age. By immunoblotting, it was not detected in sarkosyl-insoluble tau. However, by immunoelectron microscopy, a small percentage of tau in filaments from brain and spinal cord of transgenic mice was truncated at D421. Similar findings were obtained using dispersed filaments from Alzheimer's disease and FTDP-17 brains. The late appearance and low abundance of tau ending at D421 indicate that it is unlikely that truncation at this site is necessary for the assembly of tau into filaments.

Changes in neurogenesis in dementia and Alzheimer mouse models: are they functionally relevant?

Alzheimer's disease and related dementias are devastating disorders that lead to the progressive decline of cognitive functions. Characteristic features are severe brain atrophy, paralleled by accumulation of beta amyloid and neurofibrillary tangles. With the discovery of neurogenesis in the adult brain, the hopes have risen that these neurodegenerative conditions could be overcome, or at least ameliorated, by the generation of new neurons. The location of the adult neurogenic zones in the hippocampus and the lateral ventricle wall, close to corpus callosum and neocortex, indicates strategic positions for potential repair processes. However, we also need to consider that the generation of new neurons is possibly involved in cognitive functions and could, therefore, be influenced by disease pathology. Moreover, aberrant neurogenic mechanisms could even be a part of the pathological events of neurodegenerative diseases. It is the scope of this review to summarize and analyze the recent data from neurogenesis research with respect to Alzheimer's disease and its animal models.

Conditional neuronal simian virus 40 T antigen expression induces Alzheimer-like tau and amyloid pathology in mice

A large body of evidence has shown the activation of a cohort of cell cycle regulators and the duplication of DNA in degenerating neurons of Alzheimer's disease (AD) brain. Activation of these regulators and duplication of chromosomes precede neurodegeneration and formation of neurofibrillary tangles (NFTs), one of the diagnostic lesions of AD. These findings, in combination with evidence for cell cycle regulation of amyloid precursor protein and tau, has led to the hypothesis that reentry into the cell cycle underlies AD pathogenesis. To test this hypothesis directly, we have created transgenic mice with forced cell cycle activation in postmitotic neurons via conditional expression of the simian virus 40 large T antigen (TAg) oncogene. We show that TAg mice recapitulate the cell cycle changes seen in AD and display a neurodegenerative phenotype accompanied by tau pathology and NFT-like profiles. Moreover, plaque-like amyloid deposits, similar to those seen in AD, are also observed in the brains of TAg mice. These data provide support for an essential role of ectopic cell cycle activation in the generation of the characteristic pathological hallmarks of AD. Furthermore, our TAg mice are the first model to develop NFTs and amyloid pathology simultaneously and in the absence of any human transgenes. These mice will be useful for further defining the nongenetic mechanisms in AD pathogenesis and for the development of cell cycle-based therapies for AD.

Connecting cell-cycle activation to neurodegeneration in Drosophila

Studies in cell-culture systems and in postmortem tissue from human disease have suggested a connection between cell-cycle activation and neurodegeneration. The fruit fly Drosophila melanogaster has recently emerged as a powerful model system in which to model neurodegenerative diseases. Here we review work in the fly that has begun to address some of the important questions regarding the relationship between cell-cycle activation and neurodegeneration in vivo, including recent data implicating cell-cycle activation as a downstream effector of tau-induced neurodegeneration. We suggest how powerful research tools in Drosophila might be utilized to approach fundamental questions that remain.

A decade of tau transgenic animal models and beyond

The first tau transgenic mouse model was established more than a decade ago. Since then, much has been learned about the role of tau in Alzheimer's disease and related disorders. Animal models, both in vertebrates and invertebrates, were significantly improved and refined as a result of the identification of pathogenic mutations in Tau in human cases of frontotemporal dementia. They have been instrumental for dissecting the cross-talk between tau and the second hallmark lesion of Alzheimer's disease, the Abeta peptide-containing amyloid plaque. We discuss how the tau models have been used to unravel the pathophysiology of Alzheimer's disease, to search for disease modifiers and to develop novel treatment strategies. While tau has received less attention than Abeta, it is rapidly acquiring a more prominent position and the emerging view is one of a synergistic action of Abeta and tau in Alzheimer's disease. Moreover, the existence of a number of neurodegenerative diseases with tau pathology in the absence of extracellular deposits underscores the relevance of research on tau.

Alzheimer disease, the two-hit hypothesis: an update

Given the relative modality of single-insult models to accurately reflect Alzheimer disease pathogenesis, based on studies on mitogenic and oxidative stress signaling pathways, we proposed a two-hit hypothesis 2 years ago stating that both oxidative stress and mitogenic dysregulation are necessary and sufficient to cause the disease and suggested that it may be a common mechanism for other neurodegenerative diseases as well (X. Zhu, A.K. Raina, G. Perry, M.A. Smith, Alzheimer's disease: the two-hit hypothesis, Lancet Neurol. 3 (2004) 219-226). Recent developments in the field confirm some important predictions of the hypothesis and shed new lights on potential mechanisms regarding how steady state may be achieved in sporadic AD cases and therefore, in our opinion, strengthen the hypothesis, which will be the focus of this review.