Core cell cycle machinery is crucially involved in both life and death of post-mitotic neurons

Uncovering Cell Cycle Dysregulations and Associated Mechanisms in Cancer and Neurodegenerative Disorders: A Glimpse of Hope for Repurposed Drugs

The cell cycle is the sequence of events orchestrated by a complex network of cell cycle proteins. Unlike normal cells, mature neurons subsist in a quiescent state of the cell cycle, and aberrant cell cycle activation triggers neuronal death accompanied by neurodegeneration. The periodicity of cell cycle events is choreographed by various mechanisms, including DNA damage repair, oxidative stress, neurotrophin activity, and ubiquitin-mediated degradation. Given the relevance of cell cycle processes in cancer and neurodegeneration, this review delineates the overlapping cell cycle events, signaling pathways, and mechanisms associated with cell cycle aberrations in cancer and the major neurodegenerative disorders. We suggest that dysregulation of some common fundamental signaling processes triggers anomalous cell cycle activation in cancer cells and neurons. We discussed the possible use of cell cycle inhibitors for neurodegenerative disorders and described the associated challenges. We propose that a greater understanding of the common mechanisms driving cell cycle aberrations in cancer and neurodegenerative disorders will open a new avenue for the development of repurposed drugs.

Effect of eIF6 on the development of silk glands and silk protein synthesis of the silkworm, Bombyx mori

The silkworm is a lepidopteran domesticated from the wild silkworm, mostly valued for its efficient synthesis of silk protein. This species' ability to spin silk has supported the 5500-year-old silk industry and the globally known "Silk Road", making the transformation of mulberry leaves into silk of great concern. Therefore, research on the silk-related genes of silkworms and their regulatory mechanisms has attracted increasing attention. Previous studies have revealed that domestic silk gland cells are endoreduplication cells, and their high-copy genome and special chromatin conformation provide conditions for the high expression of silk proteins. In this study, we systematically investigate the expression pattern of eukaryotic initiation factors (eIFs) and identified the eIF6 as a eukaryotic translation initiation factor involved in the synthesis of silk proteins. We generated an eIF6 gene deletion mutant strain of silkworm using the CRISPR/Cas9 system and investigated the function of eIF6 in silk gland development and silk protein synthesis. The results showed that deletion of eIF6 inhibited the individual development of silkworm larvae, inhibited the development of silk glands, and significantly reduced the cocoon layer ratio. Therefore, we elucidated the function of eIF6 in the development of silk glands and the synthesis of silk proteins, which is important for further elucidation of the developmental process of silk glands and the mechanism underlying the ultra-high expression of silk proteins.

Re-Evaluating the Relevance of the Oxygen-Glucose Deprivation Model in Ischemic Stroke: The Example of Cdk Inhibition

Previous research has shown that cyclin-dependent kinases (Cdks) that play physiological roles in cell cycle regulation become activated in post-mitotic neurons after ischemic stroke, resulting in apoptotic neuronal death. In this article, we report our results using the widely used oxygen-glucose deprivation (OGD) in vitro model of ischemic stroke on primary mouse cortical neurons to investigate whether Cdk7, as part of the Cdk-activating kinase (CAK) complex that activates cell cycle Cdks, might be a regulator of ischemic neuronal death and may potentially constitute a therapeutic target for neuroprotection. We found no evidence of neuroprotection with either pharmacological or genetic invalidation of Cdk7. Despite the well-established idea that apoptosis contributes to cell death in the ischemic penumbra, we also found no evidence of apoptosis in the OGD model. This could explain the absence of neuroprotection following Cdk7 invalidation in this model. Neurons exposed to OGD seem predisposed to die in an NMDA receptor-dependent manner that could not be prevented further downstream. Given the direct exposure of neurons to anoxia or severe hypoxia, it is questionable how relevant OGD is for modeling the ischemic penumbra. Due to remaining uncertainties about cell death after OGD, caution is warranted when using this in vitro model to identify new stroke therapies.

String/Cdc25 phosphatase is a suppressor of Tau-associated neurodegeneration

Tau pathology is defined by the intracellular accumulation of abnormally phosphorylated Tau (MAPT) and is prevalent in several neurodegenerative disorders. The identification of modulators of Tau abnormal phosphorylation and aggregation is key to understanding disease progression and developing targeted therapeutic approaches. In this study, we identified String (Stg)/Cdc25 phosphatase as a suppressor of abnormal Tau phosphorylation and associated toxicity. Using a Drosophila model of tauopathy, we showed that Tau dephosphorylation by Stg/Cdc25 correlates with reduced Tau oligomerization, brain vacuolization and locomotor deficits in flies. Moreover, using a disease mimetic model, we provided evidence that Stg/Cdc25 reduces Tau phosphorylation levels independently of Tau aggregation status and delays neurodegeneration progression in the fly. These findings uncover a role for Stg/Cdc25 phosphatases as regulators of Tau biology that extends beyond their well-characterized function as cell-cycle regulators during cell proliferation, and indicate Stg/Cdc25-based approaches as promising entry points to target abnormal Tau phosphorylation.

Model scenarios for cell cycle re-entry in Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent neurodegenerative disease. Aberrant production and aggregation of amyloid beta (Aβ) peptide into plaques is a frequent feature of AD, but therapeutic approaches targeting Aβ accumulation fail to inhibit disease progression. The approved cholinesterase inhibitor drugs are symptomatic treatments. During human brain development, the progenitor cells differentiate into neurons and switch to a postmitotic state. However, cell cycle re-entry often precedes loss of neurons. We developed mathematical models of multiple routes leading to cell cycle re-entry in neurons that incorporate the crosstalk between cell cycle, neuronal, and apoptotic signaling mechanisms. We show that the integration of multiple feedback loops influences disease severity making the switch to pathological state irreversible. We observe that the transcriptional changes associated with this transition are also characteristics of the AD brain. We propose that targeting multiple arms of the feedback loop may bring about disease-modifying effects in AD.

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Developed mathematical models of cell cycle re-entry in Alzheimer's disease (AD)

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Integration of multiple feedback loops drives irreversible transition to AD

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Predicted transcriptional dysregulation is validated using AD gene expression data

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Inhibition of self-amplifying feedback loops brings about disease-modifying effects

Unraveling the links between neurodegeneration and Epstein-Barr virus-mediated cell cycle dysregulation

The Epstein-Barr virus is a well-known cell cycle modulator. To establish successful infection in the host, EBV alters the cell cycle at multiple steps via antigens such as EBNAs, LMPs, and certain other EBV-encoded transcripts. Interestingly, several recent studies have indicated the possibility of EBV's neurotrophic potential. However, the effects and outcomes of EBV infection in the CNS are under-explored. Additionally, more and more epidemiological evidence implicates the cell-cycle dysregulation in neurodegeneration. Numerous hypotheses which describe the triggers that force post-mitotic neurons to re-enter the cell cycle are prevalent. Apart from the known genetic and epigenetic factors responsible, several reports have shown the association of microbial infections with neurodegenerative pathology. Although, studies implicating the herpesvirus family members in neurodegeneration exist, the involvement of Epstein-Barr virus (EBV), in particular, is under-evaluated. Interestingly, a few clinical studies have reported patients of AD or PD to be seropositive for EBV. Based on the findings mentioned above, in this review, we propose that EBV infection in neurons could drive it towards neurodegeneration through dysregulation of cell-cycle events and induction of apoptosis.

Cell maturation: Hallmarks, triggers, and manipulation

How cells become specialized, or "mature," is important for cell and developmental biology. While maturity is usually deemed a terminal fate, it may be more helpful to consider maturation not as a switch but as a dynamic continuum of adaptive phenotypic states set by genetic and environment programing. The hallmarks of maturity comprise changes in anatomy (form, gene circuitry, and interconnectivity) and physiology (function, rhythms, and proliferation) that confer adaptive behavior. We discuss efforts to harness their chemical (nutrients, oxygen, and growth factors) and physical (mechanical, spatial, and electrical) triggers in vitro and in vivo and how maturation strategies may support disease research and regenerative medicine.

Down-regulation of cyclin D2 in amyloid β toxicity, inflammation, and Alzheimer's disease

In the current study, we analyzed the effects of the systemic inflammatory response (SIR) and amyloid β (Aβ) peptide on the expression of genes encoding cyclins and cyclin-dependent kinase (Cdk) in: (i) PC12 cells overexpressing human beta amyloid precursor protein (βAPP), wild-type (APPwt-PC12), or carrying the Swedish mutantion (APPsw-PC12); (ii) the murine hippocampus during SIR; and (iii) Alzheimer’s disease (AD) brain. In APPwt-PC12 expression of cyclin D2 (cD2) was exclusively reduced, and in APPsw-PC12 cyclins cD2 and also cA1 were down-regulated, but cA2, cB1, cB2, and cE1 were up-regulated. In the SIR cD2, cB2, cE1 were found to be significantly down-regulated and cD3, Cdk5, and Cdk7 were significantly up-regulated. Cyclin cD2 was also found to be down-regulated in AD neocortex and hippocampus. Our novel data indicate that Aβ peptide and inflammation both significantly decreased the expression of cD2, suggesting that Aβ peptides may also contribute to downregulation of cD2 in AD brain.

Genetic Predisposition to Mosaic Chromosomal Loss Is Associated With Functional Outcome After Ischemic Stroke

Background and Objectives

To test the hypothesis that a predisposition to acquired genetic alterations is associated with ischemic stroke outcome by investigating the association between a polygenic risk score (PRS) for mosaic loss of chromosome Y (mLOY) and outcome in a large international data set.

Methods

We used data from the genome-wide association study performed within the Genetics of Ischemic Stroke Functional Outcome network, which included 6,165 patients (3,497 men and 2,668 women) with acute ischemic stroke of mainly European ancestry. We assessed a weighted PRS for mLOY and examined possible associations with the modified Rankin Scale (mRS) score 3 months poststroke in logistic regression models. We investigated the whole study sample as well as men and women separately.

Results

Increasing PRS for mLOY was associated with poor functional outcome (mRS score >2) with an odds ratio (OR) of 1.11 (95% confidence interval [CI] 1.03–1.19) per 1 SD increase in the PRS after adjustment for age, sex, ancestry, stroke severity (NIH Stroke Scale), smoking, and diabetes mellitus. In sex-stratified analyses, we found a statistically significant association in women (adjusted OR 1.20, 95% CI 1.08–1.33). In men, the association was in the same direction (adjusted OR 1.04, 95% CI 0.95–1.14), and we observed no significant genotype-sex interaction.

Discussion

In this exploratory study, we found associations between genetic variants predisposing to mLOY and stroke outcome. The significant association in women suggests underlying mechanisms related to genomic instability that operate in both sexes. These findings need replication and mechanistic exploration.

Regulatory mechanism of cyclins and cyclin-dependent kinases in post-mitotic neuronal cell division

Neurodegenerative diseases (NDDs) are the most common life-threatening disease of the central nervous system and it cause the progressive loss of neuronal cells. The exact mechanism of the disease's progression is not clear and thus line of treatment for NDDs is a baffling issue. During the progression of NDDs, oxidative stress and DNA damage play an important regulatory function, and ultimately induces neurodegeneration. Recently, aberrant cell cycle events have been demonstrated in the progression of different NDDs. However, the pertinent role of signaling mechanism, for instance, post-translational modifications, oxidative stress, DNA damage response pathway, JNK/p38 MAPK, MEK/ERK cascade, actively participated in the aberrant cell cycle reentry induced neuronal cell death. Mounting evidence has demonstrated that aberrant cell cycle re-entry is a major contributing factor in the pathogenesis of NDDs rather than a secondary phenomenon. In the brain of AD patients with mild cognitive impairment, post miotic cell division can be seen in the early stage of the disease. However, in the brain of PD patients, response to various neurotoxic signals, the cell cycle re-entry has been observed that causes neuronal apoptosis. On contrary, the contributing factors that leads to the induction of cell cycle events in mature neurons in HD and ALS brain pathology is remain unclear. Various pharmacological drugs have been developed to reduce the pathogenesis of NDDs, but they are still not helpful in eliminating the cause of these NDDs.

Microtubule disruption upon CNS damage triggers mitotic entry via TNF signaling activation

Repair after traumatic injury often starts with mitotic activation around the lesion edges. Early midline cells in the Drosophila embryonic CNS can enter into division following the traumatic disruption of microtubules. We demonstrate that microtubule disruption activates non-canonical TNF signaling by phosphorylation of TGF-β activated kinase 1 (Tak1) and its target IkappaB kinase (Ik2), culminating in Dorsal/NfkappaB nuclear translocation and Jra/Jun expression. Tak1 and Ik2 are necessary for the damaged-induced divisions. Microtubule disruption caused by Tau accumulation is also reported in Alzheimer's disease (AD). Human Tau expression in Drosophila midline cells is sufficient to induce Tak1 phosphorylation, Dorsal and Jra/Jun expression, and entry into mitosis. Interestingly, activation of Tak1 and Tank binding kinase 1 (Tbk1), the human Ik2 ortholog, and NfkappaB upregulation are observed in AD brains.

When Good Kinases Go Rogue: GSK3, p38 MAPK and CDKs as Therapeutic Targets for Alzheimer's and Huntington's Disease

Alzheimer's disease (AD) is a mostly sporadic brain disorder characterized by cognitive decline resulting from selective neurodegeneration in the hippocampus and cerebral cortex whereas Huntington's disease (HD) is a monogenic inherited disorder characterized by motor abnormalities and psychiatric disturbances resulting from selective neurodegeneration in the striatum. Although there have been numerous clinical trials for these diseases, they have been unsuccessful. Research conducted over the past three decades by a large number of laboratories has demonstrated that abnormal actions of common kinases play a key role in the pathogenesis of both AD and HD as well as several other neurodegenerative diseases. Prominent among these kinases are glycogen synthase kinase (GSK3), p38 mitogen-activated protein kinase (MAPK) and some of the cyclin-dependent kinases (CDKs). After a brief summary of the molecular and cell biology of AD and HD this review covers what is known about the role of these three groups of kinases in the brain and in the pathogenesis of the two neurodegenerative disorders. The potential of targeting GSK3, p38 MAPK and CDKS as effective therapeutics is also discussed as is a brief discussion on the utilization of recently developed drugs that simultaneously target two or all three of these groups of kinases. Multi-kinase inhibitors either by themselves or in combination with strategies currently being used such as immunotherapy or secretase inhibitors for AD and knockdown for HD could represent a more effective therapeutic approach for these fatal neurodegenerative diseases.

A Novel Methodology Using Dexamethasone to Induce Neuronal Differentiation in the CNS-Derived Catecholaminergic CAD Cells

The Cath.a-differentiated (CAD) cell line is a central nervous system-derived catecholaminergic cell line originating from tyrosine hydroxylase (TH)-producing neurons located around the locus coeruleus area of the mouse brain. CAD cells have been used as an in vitro model for cellular and molecular studies due to their ability to differentiate under serum-free media conditions. However, the lack of serum-derived survival factors, limits the longevity for differentiated CAD cells to be maintained in healthy conditions; thereby, limiting their use in long-term culture studies. Here, we present a novel differentiation method that utilizes dexamethasone (Dex), a synthetic glucocorticoid receptor agonist. Specifically, we discovered that the addition of 100 µM of Dex into the 1% fetal bovine serum (FBS)-supplemented media effectively induced neuronal differentiation of CAD cells, as characterized by neurite formation and elongation. Dex-differentiated CAD cells exited the cell cycle, stopped proliferating, extended the neurites, and expressed neuronal markers. These effects were dependent on the glucocorticoid receptors (GR) as they were abolished by GR knockdown. Importantly, Dex-differentiated CAD cells showed longer survival duration than serum-free differentiated CAD cells. In addition, RNA-sequencing and qPCR data demonstrate that several genes involved in proliferation, neuronal differentiation, and survival pathways were differentially expressed in the Dex-differentiated cells. This is the first study to reveal Dex as a novel differentiation methodology used to generate postmitotic neuronal CAD cells, which may be utilized as an in vitro neuronal model for cellular and molecular neurobiology research.

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.

Neuronal Cell Cycle Re-Entry Enhances Neuropathological Features in AppNLF Knock-In Mice

BACKGROUND

Aberrant cell cycle re-entry is a well-documented process occurring early in Alzheimer's disease (AD). This is an early feature of the disease and may contribute to disease pathogenesis.

OBJECTIVE

To assess the effect of forced neuronal cell cycle re-entry in mice expressing humanized Aβ, we crossed our neuronal cell cycle re-entry mouse model with AppNLF knock-in (KI) mice.

METHODS

Our neuronal cell cycle re-entry (NCCR) mouse model is bitransgenic mice heterozygous for both Camk2a-tTA and TRE-SV40T. The NCCR mice were crossed with AppNLF KI mice to generate NCCR-AppNLF animals. Using this tet-off system, we triggered NCCR in our animals via neuronal expression of SV40T starting at 1 month of age. The animals were examined at the following time points: 9, 12, and 18 months of age. Various neuropathological features in our mice were evaluated by image analysis and stereology on brain sections stained using either immunofluorescence or immunohistochemistry.

RESULTS

We show that neuronal cell cycle re-entry in humanized Aβ plaque producing AppNLF KI mice results in the development of additional AD-related pathologies, namely, pathological tau, neuroinflammation, brain leukocyte infiltration, DNA damage response, and neurodegeneration.

CONCLUSION

Our findings show that neuronal cell cycle re-entry enhances AD-related neuropathological features in AppNLF mice and highlight our unique AD mouse model for studying the pathogenic role of aberrant cell cycle re-entry in AD.