Multiple cyclin-dependent kinases signals are critical mediators of ischemia/hypoxic neuronal death in vitro and in vivo

Mouse model of diffuse brain damage following anoxia, evaluated by a new assay of generalized arousal

Diffuse brain damage following anoxia due to cardiac failure, drowning, carbon monoxide exposure or other accidents constitutes a major medical problem. We have created a novel mouse model using the breathing of pure nitrogen, followed by a recently developed assay that reflects an operational definition of generalized arousal. The operational definition is precise, complete, and leads to quantitative, physical measures in a genetically tractable animal. Exposure to pure nitrogen for controlled periods had a surprising bifurcate effect: about half the mice survived with neurological measures that were virtually normal while the other half died. The new assay detected behavioral deficits unrevealed by neurological screening. Two important features of the results were that (i) deficits were not equal across the circadian cycle, and (ii) deficits were not equal across all the measures within the operational definition of arousal. Specific voluntary motor measurements were decreased in a manner that depended on the phase of the circadian cycle. Sensory responses were also decreased, with an emphasis on vertical movement responses; but, interestingly, fear learning was not damaged. This study establishes the first useful approach to diffuse brain damage in a genetically tractable animal. The model and its outcome measurements will be useful during future attempts at amelioration of acquired neurological disabilities following hypoxic-ischemic injuries.

Cyclin D1, cdk4, and Bim are involved in thrombin-induced apoptosis in cultured cortical neurons

Thrombin, a multifunctional serine protease, is neurotoxic in vitro and in vivo. Thrombin has been shown to be increased in Alzheimer's disease (AD) and other neuropathological conditions and could be a mediator of pathological neuronal cell death in the brain. The mechanisms of thrombin-induced neuronal cell death are incompletely understood. The objective of this study is to explore mechanisms that contribute to thrombin-induced neuronal apoptosis focusing on the role of cell cycle regulators and the pro-apoptotic protein Bim (Bcl-2-interacting mediator of cell death) in this process. Our data show that thrombin treatment of primary cerebral cortical cultures results in dose-dependent apoptotic cell death. Exposure of neuronal cultures to thrombin leads to induction of cell cycle proteins cyclin D1 and cyclin E, at both mRNA and protein levels. In addition, thrombin treatment causes the appearance of cyclin-dependent kinase 4 (cdk4) and expression of the pro-apoptotic protein Bim. Inhibition of cdk4 prevents both induction of Bim expression and thrombin-induced neuronal apoptosis. These data demonstrate that thrombin-induced apoptosis proceeds via cell cycle activation involving cdk4 resulting in induction of Bim. Thus, cell cycle proteins could be therapeutic targets in diseases such as AD where thrombin has been implicated.

Cell cycle inhibition attenuates microglia induced inflammatory response and alleviates neuronal cell death after spinal cord injury in rats

The spinal cord is well known to undergo inflammatory reactions in response to traumatic injury. Activation and proliferation of microglial cells, with associated proinflammatory cytokines expression, plays an important role in the secondary damage following spinal cord injury. It is likely that microglial cells are at the center of injury cascade and are targets for treatments of CNS traumatic diseases. Recently, we have demonstrated that the cell cycle inhibitor olomoucine attenuates astroglial proliferation and glial scar formation, decreases lesion cavity and mitigates functional deficits after spinal cord injury (SCI) in rats [Tian, D.S., Yu, Z.Y., Xie, M.J., Bu, B.T., Witte, O.W., Wang, W., 2006. Suppression of astroglial scar formation and enhanced axonal regeneration associated with functional recovery in a spinal cord injury rat model by the cell cycle inhibitor olomoucine. J. Neurosci. Res. 84, 1053-1063]. Whether neuroprotective effects of cell cycle inhibition are involved in attenuation of microglial induced inflammation awaits to be elucidated. In the present study, we sought to determine the influence of olomoucine on microglial proliferation with associated inflammatory response after spinal cord injury. Tissue edema formation, microglial response and neuronal cell death were quantified in rats subjected to spinal cord hemisection. Microglial proliferation and neuronal apoptosis were observed by immunofluorescence. Level of the proinflammatory cytokine interleukin-1beta (IL-1beta) expression in the injured cord was determined by Western blot analysis. Our results showed that the cell cycle inhibitor olomoucine, administered at 1 h post injury, significantly suppressed microglial proliferation and produced a remarkable reduction of tissue edema formation. In the olomoucine-treated group, a significant reduction of activated and/or proliferated microglial induced IL-1beta expression was observed 24 h after SCI. Moreover, olomoucine evidently attenuated the number of apoptotic neurons after SCI. Our findings suggest that modulation of microglial proliferation with associated proinflammatory cytokine expression may be a mechanism of cell cycle inhibition-mediated neuroprotections in the CNS trauma.

Cell cycle molecules define a pathway required for neuron death in development and disease

We review here evidence defining a molecular pathway that includes cell cycle-related molecules and that appears to play a required role in neuron death during normal development as well as in disease and trauma. The pathway starts with inappropriate activation of cyclin dependent kinase 4 (Cdk4) in neurons which leads to hyper-phosphorylation of the pRb family member p130. This in turn results in dissociation of p130 and its associated chromatin modifiers Suv39H1 and HDAC1 from the transcription factor E2F4. Dissociation of this complex results in de-repression of genes with E2F binding sites including those encoding the transcription factors B- and C-Myb. Once elevated in neurons, B- and C-Myb proteins bind to the promoter for the pro-apoptotic BH3-only protein Bim and promote its induction. Bim then interacts with the core cellular apoptotic machinery, leading to caspase activation and apoptotic death. This pathway is supported by a variety of observations and experimental findings that implicate it as a required element for neuron loss in development and in many nervous system traumas and disorders. The components of this pathway appear to represent potential therapeutic targets for prevention of disease-associated neuron death.

The Chk1/Cdc25A pathway as activators of the cell cycle in neuronal death induced by camptothecin

Cell cycle regulators appear to play a paradoxical role in neuronal death. We have shown previously that cyclin-dependent kinases (CDKs), along with their downstream effectors, Rb (retinoblastoma) and E2F/DP1 (E2 promoter binding factor/deleted in polyposis 1), regulate neuronal death evoked by the DNA damaging agent camptothecin. However, the mechanism by which CDKs are activated in this model is unclear. The cell division cycle 25A (Cdc25A) phosphatase is a critical regulator of cell cycle CDKs in proliferating cells. In cortical neurons, we presently show that expression of Cdc25A promotes death even in the absence of DNA damage. Importantly, Cdc25A activity is rapidly increased during DNA damage treatment. Inhibition of Cdc25A blocks death and reduces cyclin D1-associated kinase activity and Rb phosphorylation. This indicates that endogenous Cdc25A activity is important for regulation of cell cycle-mediated neuronal death. We also examined how Cdc25A activity is regulated after DNA damage. Cultured embryonic cortical neurons have a significant basal activity of checkpoint kinase 1 (Chk1), a kinase that regulates cell cycle arrest. During camptothecin treatment of neurons, this activity is rapidly downregulated with a concomitant increase in Cdc25A activity. Importantly, expression of wild-type Chk1, but not kinase-dead Chk1, inhibits the camptothecin-induced increase in Cdc25A activity. In addition, Chk1 expression also promotes survival in the presence of the DNA-damaging agent. Together, our data suggest that a Chk1/Cdc25A activity participates in activation of a cell cycle pathway-mediated death signal in neurons. These data also define how a proliferative signal may be abnormally activated in a postmitotic environment.

p35/p25 is not essential for tau and cytoskeletal pathology or neuronal loss in Niemann-Pick type C disease

Hyperactivation of the cyclin-dependent kinase 5 (cdk5), triggered by proteolytic conversion of its neuronal activator, p35, to a more potent byproduct, p25, has been implicated in Alzheimer's disease (AD), amyotrophic lateral sclerosis, and Niemann-Pick type C disease (NPC). This mechanism is thought to lead to the development of neuropathological hallmarks, i.e., hyperphosphorylated cytoskeletal proteins, neuronal inclusions, and neurodegeneration, that are common to all three diseases. This pathological ensemble is recapitulated in a single model, the npc-1 (npc(-/-)) mutant mouse. Previously, we showed that pharmacological cdk inhibitors dramatically reduced hyperphosphorylation, lesion formation, and locomotor defects in npc(-/-) mice, suggesting that cdk activity is required for NPC pathogenesis. Here, we used genetic ablation of the p35 gene to examine the specific involvement of p35, p25, and hence cdk5 activation in NPC neuropathogenesis. We found that lack of p35/p25 does not slow the onset or progression or improve the neuropathology of NPC. Our results provide direct evidence that p35/p25-mediated cdk5 deregulation is not essential for NPC pathology and suggest that similar pathology in AD may also be cdk5 independent.

When good Cdk5 turns bad

The cyclin-dependent kinase-5 (Cdk5) is critical to normal mammalian development and has been implicated in synaptic plasticity, learning, and memory in the adult brain. But Cdk-5 activity has also been linked to neurodegenerative diseases. Could a single protein have opposing effects? A new study shows that production of a neuronal protein capable of regulating Cdk-5 activity can turn Cdk-5 from "good" to "bad." The findings may have implications for the development and treatment of conditions like Alzheimer's disease.

Cdh1-APC/C, cyclin B-Cdc2, and Alzheimer’s disease pathology

The anaphase-promoting complex/cyclosome (APC/C) is a key E3 ubiquitin ligase complex that functions in regulating cell cycle transitions in proliferating cells and has, as revealed recently, novel roles in postmitotic neurons. Regulated by its activator Cdh1 (or Hct1), whose level is high in postmitotic neurons, APC/C seems to have multiple functions at different cellular locations, modulating diverse processes such as synaptic development and axonal growth. These processes do not, however, appear to be directly connected to cell cycle regulation. It is now shown that Cdh1-APC/C activity may also have a basic role in suppressing cyclin B levels, thus preventing terminally differentiated neurons from aberrantly re-entering the cell cycle. The result of an aberrant cyclin B-induced S-phase entry, at least for some of these neurons, would be death via apoptosis. Cdh1 thus play an active role in maintaining the terminally differentiated, non-cycling state of postmitotic neurons--a function that could become impaired in Alzheimer's and other neurodegenerative diseases.