Review: cell cycle aberrations and neurodegeneration.

The cell cycle is a highly regulated and fundamental cellular process that involves complex feedback regulation of many proteins, and any compromise to its integrity elicits dire consequences for the cell. For example, in neurodegenerative diseases such as Alzheimer disease (AD), evidence for abnormal cell cycle re-entry precedes other hallmarks of disease and as such, implicates cell cycle aberrations in the aetiology of AD. The mechanism(s) for cell cycle re-entry in AD, however, remain unclear. Current theory suggests it to be part of a combination of early events that together elicit the degenerative pathology and cognitive phenotype consistent with the disease. We propose a 'Two-Hit Hypothesis' that highlights the concerted interaction between cell cycle alterations and oxidative stress that combine to produce neurodegeneration. Here, we review the evidence implicating cell cycle mechanisms in AD and how such changes, especially in combination with oxidative stress, would lead to a cascade of events leading to disease. Based on this concept, we propose new opportunities for disease treatment.

Neuroprotection by c-Jun NH2-terminal kinase inhibitor SP600125 against potassium deprivation-induced apoptosis involves the Akt pathway and inhibition of cell cycle reentry.

Increasing evidence implicates the c-Jun NH(2)-terminal kinase (JNK) pathway in the regulation of apoptosis in neurodegenerative diseases. In this study, we examined the neuroprotective effect of SP600125, a selective JNK inhibitor, in cerebellar granule cells (CGNs) deprived of serum and potassium (S/K withdrawal). S/K withdrawal-induced apoptosis occurs via activation of multiple pro-apoptotic pathways, including re-entry into the cell cycle, activation of glycogen synthase kinase-3 beta (GSK-3beta), cyclin-dependent kinase 5 (cdk5/p35) breakdown, formation of cdk5/p25 and JNK activation. Here we demonstrate that SP600125 is able to inhibit all these pro-apoptotic pathways via the inhibition of JNK. Further, we found that JNK inhibition maintains the phosphorylation/activation of Akt after S/K withdrawal. For further confirmation of this result, we studied several targets downstream of Akt including GSK-3beta, p-FOXO1, p-CREB and p35. In addition, the specific PI3K/Akt inhibitor LY294002 greatly diminished the antiapoptotic effects of SP600125 upon S/K withdrawal, confirming that Akt is involved in the neuroprotection achieved by SP600125. These results suggest that the maintenance of the PI3-kinase/Akt pathway by inhibition of JNK contributes to the prevention of apoptosis in rat cerebellar granule neurons mediated by S/K withdrawal. Furthermore, we propose that JNK may regulate the cell cycle re-entry by a novel mechanism that involves Akt, GSK-3beta and Rb phosphorylation.

Synaptic dysfunction and oxidative stress in Alzheimer's disease: emerging mechanisms.

In this paper, we review experimental advances in molecular neurobiology of Alzheimer's disease (AD), with special emphasis on analysis of neural function of proteins involved in AD pathogenesis, their relation with several signaling pathways and with oxidative stress in neurons. Molecular genetic studies have found that mutations in APP, PS1 and PS2 genes and polymorphisms in APOE gene are implicated in AD pathogenesis. Recent studies show that these proteins, in addition to its role in beta-amyloid processing, are involved in several neuroplasticity-signaling pathways (NMDA-PKA-CREB-BDNF, reelin, wingless, notch, among others). Genomic and proteomic studies show early synaptic protein alterations in AD brains and animal models. DNA damage caused by oxidative stress is not completely repaired in neurons and is accumulated in the genes of synaptic proteins. Several functional SNPs in synaptic genes may be interesting candidates to explore in AD as genetic correlates of this synaptopathy in a "synaptogenomics" approach. Thus, experimental evidence shows that proteins implicated in AD pathogenesis have differential roles in several signaling pathways related to neuromodulation and neurotransmission in adult and developing brain. Genomic and proteomic studies support these results. We suggest that oxidative stress effects on DNA and inherited variations in synaptic genes may explain in part the synaptic dysfunction seen in AD.

Brain and brawn: parallels in oxidative strength.

Neuronal oxidative stress occurs early in the progression of Alzheimer disease (AD), significantly before the development of the pathologic hallmarks, neurofibrillary tangles, and senile plaques. Study of Down syndrome, cases with autosomal dominant mutation, and sporadic AD all suggest amyloid-beta deposition and hyperphosphorylated tau function as compensatory responses and downstream adaptations to ensure that neuronal cells do not succumb to oxidative damage. Amyloid-beta and tau hyperphosphorylation also define vulnerable muscle cells in sporadic inclusion-body myositis (s-IBM). The role of the structural changes of s-IBM, as in AD, remains to be determined but may mark a critical response yielding a novel balance in oxidant homeostasis.

Oxidative stress and inflammation in brain aging: nutritional considerations.

Aging can be defined as the condition where stressors are not counteracted by protective functions, leading to a dysregulation in development. These changes can be translated into decrements in neuronal functioning accompanied by behavioral declines, such as decreases in motor and cognitive performance, in both humans and animals. When coupled with genetic alterations, the ultimate expression of these changes is seen in diseases such as Alzheimer disease (AD). This association will be discussed in the last section of this chapter. In this review we will describe motor and cognitive deficits in behavior due to aging, and show how these deficits are related to increased vulnerability to oxidative stress, inflammation or signaling. Importantly, using muscarinic receptors as examples, we will also try to show that the sensitivity to these insults may be differentially expressed among neurotransmitter receptor subtypes.

Evidence for the role of gonadotropin hormones in the development of Alzheimer disease.

Differences in the prevalence and age of onset of Alzheimer disease (AD) in men and women, and observations that hormone replacement therapy (HRT) may prevent the development of AD, caused many to hypothesize that estrogen deficiency contributes to AD. However, recent trials using estrogen failed to show any benefit in preventing or alleviating the disease. To address this and other inconsistencies in the estrogen hypothesis, we suspect that another hormone of the hypothalamic-pituitary-gonadal axis, luteinizing hormone (LH), as a major factor in AD pathogenesis. Individuals with AD have elevated levels of LH when compared with controls, and both LH and its receptor are present in increased quantities in brain regions susceptible to degeneration in AD. LH is also known to be mitogenic, and could therefore initiate the cell cycle abnormalities known to be present in AD-affected neurons. In cell culture, LH increases amyloidogenic processing of amyloid-beta protein precursor, and in animal models of AD, pharmacologic suppression of LH and FSH reduces plaque formation. Given the evidence supporting a pathogenic role for LH in AD, a trial of leuprolide acetate, which suppresses LH release, has been initiated in patients.

Mitotic and gender parallels in Alzheimer disease: therapeutic opportunities.

In this review, we discuss the role of cell cycle dysfunction in the pathogenesis of Alzheimer disease and propose that such mitotic catastrophe, as one of the earliest events in neuronal degeneration, may, in fact, be sufficient to initiate the neurodegenerative cascade. The question as to what molecule initiates cell cycle dysfunction is now beginning to become understood and, in this regard, the gender-predication, age-related penetrance and regional susceptibility of specific neuronal populations led us to consider luteinizing hormone as a key mediator of the abnormal mitotic process. As such, agents targeted toward luteinizing hormone or downstream sequelae may be of great therapeutic value in the treatment of Alzheimer disease.

The effects of heavy particle irradiation on exploration and response to environmental change.

Free radicals produced by exposure to heavy particles have been found to produce motor and cognitive behavioral toxicity effects in rats similar to those found during aging. The present research was designed to investigate the effects of exposure to 56Fe particles on the ability of male Sprague-Dawley rats to detect novel arrangements in a given environment. Using a test of spatial memory previously demonstrated to be sensitive to aging, open field activity and reaction to spatial and non-spatial changes were measured in a group that received a dose of 1.5 Gy (n=10) of 56Fe heavy particle radiation or in non-radiated controls (n=10). Animals irradiated with 1.5 Gy of 56Fe particles exhibited some age-like effects in rats tested, even though they were, for the most part, subtle. Animals took longer to enter, visited less and spent significantly less time in the middle and the center portions of the open field, independently of total frequency and duration of activity of both groups. Likewise, irradiated subjects spend significantly more time exploring novel objects placed in the open field than did controls. However, irradiated subjects did not vary from controls in their exploration patterns when objects in the open field were spatially rearranged. Thus, irradiation with a dose of 1.5 Gy of 56Fe high-energy particle radiation elicited age-like effects in general open field exploratory behavior, but did not elicit age-like effects during the spatial and non-spatial rearrangement tasks.

Cognitive deficits induced by 56Fe radiation exposure.

Exposing rats to particles of high energy and charge (e.g., 56Fe) disrupts neuronal systems and the behaviors mediated by them; these adverse behavioral and neuronal effects are similar to those seen in aged animals. Because cognition declines with age, and our previous study showed that radiation disrupted Morris water maze spatial learning and memory performance, the present study used an 8-arm radial maze (RAM) to further test the cognitive behavioral consequences of radiation exposure. Control rats or rats exposed to whole-body irradiation with 1.0 Gy of 1 GeV/n high-energy 56Fe particles (delivered at the alternating gradient synchrotron at Brookhaven National Laboratory) were tested nine months following exposure. Radiation adversely affected RAM performance, and the changes seen parallel those of aging. Irradiated animals entered baited arms during the first 4 choices significantly less than did controls, produced their first error sooner, and also tended to make more errors as measured by re-entries into non-baited arms. These results show that irradiation with high-energy particles produces age-like decrements in cognitive behavior that may impair the ability of astronauts to perform critical tasks during long-term space travel beyond the magnetosphere.

Effect of age on object exploration, habituation, and response to spatial and nonspatial change.

To measure their ability to detect novel arrangements in a given environment, young (6 months old) and senescent (22-24 months old) male F344 rats were repeatedly exposed to a given spatial configuration of objects contained in an open field. After the rats were habituated to the novel environment (1 trial with no objects, followed by 3 trials with 5 salient objects), the spatial arrangement of the objects was modified (2 trials), and object novelty was tested (2 trials) by substituting a familiar object with a new one at the same location (nonspatial change). The results indicated that the senescent rats explored old objects less than young rats, particularly on Trial 2. On the 1st trial with displaced objects (Trial 5), the senescent rats explored the displaced objects less than the young rats. However, when a new object was placed in the field (Trials 7-8), there were no age differences in new object exploration. These results suggest that senescent rats have decrements in the ability to build spatial representations of the environment and to use this information to detect such changes, even though object recognition is not impaired with age.

Automated measurement of age-related changes in the locomotor response to environmental novelty and home-cage activity.

The likelihood to explore in an open-field environment decreases with age. Older animals tend to be less active and explore less both in novel and home-cage environments. The locomotor performance (fine movements, ambulatory movements, and rearing) of male Fischer 344 (F344) rats that were 6 (n=6) or 22 (n=6) months of age was evaluated by continuous automated counting of photobeam interruptions, every 30 min, during 60 consecutive hours, in standard polycarbonate cages. Novel environment performance was determined by photobeam interruption counting during the first hour in the new cage. The remaining 59 h were evaluated as home-cage activity. A significant age-related decrease in ambulatory and fine motor activity was seen during the first hour of testing (novel environment). In addition, aged rats showed a decreased number of ambulatory and fine movements in home-cage activity, predominantly during the dark portion of the light cycle and during or around both light-switch periods (05:00 and 17:00). No differences were seen in rearing behavior. These findings provide a more detailed analysis and additional evidence of the activity decreases and rhythmic changes seen in aged F344 rats under uninterrupted testing conditions.

Spatial learning and memory deficits induced by exposure to iron-56-particle radiation.

It has previously been shown that exposing rats to particles of high energy and charge (HZE) disrupts the functioning of the dopaminergic system and behaviors mediated by this system, such as motor performance and an amphetamine-induced conditioned taste aversion; these adverse behavioral and neuronal effects are similar to those seen in aged animals. Because cognition declines with age, spatial learning and memory were assessed in the Morris water maze 1 month after whole-body irradiation with 1.5 Gy of 1 GeV/nucleon high-energy (56)Fe particles, to test the cognitive behavioral consequences of radiation exposure. Irradiated rats demonstrated cognitive impairment compared to the control group as seen in their increased latencies to find the hidden platform, particularly on the reversal day when the platform was moved to the opposite quadrant. Also, the irradiated group used nonspatial strategies during the probe trials (swim with no platform), i.e. less time spent in the platform quadrant, fewer crossings of and less time spent in the previous platform location, and longer latencies to the previous platform location. These findings are similar to those seen in aged rats, suggesting that an increased release of reactive oxygen species may be responsible for the induction of radiation- and age-related cognitive deficits. If these decrements in behavior also occur in humans, they may impair the ability of astronauts to perform critical tasks during long-term space travel beyond the magnetosphere.